Research Article
Vol. 55, No. 4 / February 1 2016 / Applied Optics
783
Imaging of various optical fiber Bragg gratings using differential interference contrast microscopy: analysis and comparison CLAIRE M. ROLLINSON,1 SCOTT A. WADE,2 GREG W. BAXTER,1
AND
STEPHEN F. COLLINS1,*
1
Optical Technology Research Laboratory, College of Engineering and Science, Victoria University, P.O. Box 14428, Melbourne, VIC 8001, Australia CQOS, Swinburne University of Technology, Hawthorn, VIC 3122, Australia *Corresponding author: [email protected]
2
Received 9 September 2015; revised 25 November 2015; accepted 30 November 2015; posted 30 November 2015 (Doc. ID 249775); published 26 January 2016
Differential interference contrast images of various optical fibers and optical fiber Bragg gratings (FBGs), written with the phase mask technique, are presented to provide information about the resultant refractive index variations present in each case. Use of different fiber types using two distinct phase masks producing four Type I FBGs and a Type In FBG allowed similarities and differences in these FBG images due to variations in the Talbot diffraction patterns produced to be studied. © 2016 Optical Society of America OCIS codes: (060.3735) Fiber Bragg gratings; (060.3738) Fiber Bragg gratings, photosensitivity; (070.6760) Talbot and self-imaging effects; (050.2770) Gratings. http://dx.doi.org/10.1364/AO.55.000783
1. INTRODUCTION Optical fiber Bragg gratings (FBGs) have received much attention for over 20 years and have been reported for a plethora of applications in telecommunications and sensing, as reviewed by Canning [1]. Typically, they operate as an in-line filter, in which a strong reflection of a narrow wavelength band occurs when a propagating core mode experiences resonance with the periodic variation of refractive index (RI) along the fiber core. Many FBG types have been reported due to the existence of various fiber types and the use of different fabrication techniques. Type I gratings are generally formed in photosensitive fiber types when exposed to continuous wave or relatively low pulse energy UV radiation and show a monotonic increase in the amplitude of the RI modulation with UV exposure. Those known as Type In (previously known as IIa) evolve from Type I grating growth. Properties of FBGs may be studied via analysis of grating reflection or transmission, or mathematical modelling (e.g., coupled mode theory [2]). As such methods give little information about the RI structure within a FBG, measurement of such profiles along a grating has been undertaken by various authors, for instance [3–5]. Standard microscopes are unable to detect the small RI changes in the transparent core of a FBG. More detail has been revealed in a diffraction-limited photographic technique [6], scanning near-field optical microscopy (SNOM) [7] has been used to examine both the structure and internal field distribution of a polished FBG by accessing its evanescent 1559-128X/16/040783-08$15/0$15.00 © 2016 Optical Society of America
field, and a side-diffraction interference technique has been reported [8]. Recently, quantitative phase microscopy was used to show the three-dimensional RI distribution of a FBG [9]. The SNOM technique was also used to study the irradiance field in free space behind an illuminated phase mask [10], verifying the existence of a periodicity in the diffraction field along the direction of the incident beam, as expected for FBGs fabricated using the phase mask technique [11] and referred to as the Talbot effect [12]. The repeat length of the pattern, known as the Talbot length [13], is Z T m; n
2π ; k2 − m2 G 2 1∕2 − k2 − n2 G 2 1∕2
(1)
where m and n are integers representing diffraction orders, k 2πnw ∕λw , nw is the RI of the medium behind the phase mask at the writing wavelength λw , and G 2π∕Λpm is the unit reciprocal lattice vector of the phase mask. If the pattern behind the phase mask is formed by first-order diffraction alone, the two symmetrically crossing components generate uniform fringes with half the period of the phase mask and with no Talbot beating [14]. Actual phase masks, however, produce some zeroth and higher diffraction orders and thus exhibit a Talbot length, for which measurements agree well with Eq. (1) for the interaction between the dominant first and second orders [10]. The differential interference contrast (DIC) [15,16] technique is ideal for optical fiber imaging as it is nondestructive.
784
Vol. 55, No. 4 / February 1 2016 / Applied Optics
A laser beam is split into two orthogonal components that are slightly spatially separated (the “shear”), and after traversing the system their subsequent recombination provides high resolution RI variation information with the spatial resolution determined by the shear [17]. It was used to image a phase-mask-written FBG in which the intensity corresponds to the RI in the fiber core [18]. Slices through the fiber center, in which the sample was oriented at 0° and 90°, demonstrated the formation of a Talbot diffraction pattern as described above. The Talbot length, Z T , was consistent with the calculated value of ∼4.65 μm for beating between the first and second diffraction orders of the phase mask [18], and subsequently the complex pattern was replicated based on the strength of the orders of the phase mask [19]. The Bragg wavelength of the imaged FBG was ∼1535 nm, resulting from a phase mask period of Λpm 1.059 μm, although the planes of interleaved fringes indicate that the grating exhibits two periodicities, namely ∼0.53 and ∼1.06 μm, corresponding to Λpm ∕2 and Λpm , respectively. Indeed it has been shown that the Bragg wavelength arises from both periodicities [20], while various Bragg wavelength harmonics arise from one or both of these periodicities [6,21,22]. FBGs fabricated with ultrafast lasers have been imaged using an optical microscope, in which the spatial resolution is poor compared with the DIC technique [23]; similar structures were evident but only as their phase masks were of a larger periodicity. This paper presents use of the DIC technique to investigate the RI structures of five FBGs that were fabricated using both standard and custom-made phase masks in several types of optical fiber. This enabled the role of the actual fiber type used in determining the details of RI changes induced in the fiber to be appreciated, along with a comparison of Type I and Type In gratings. Images of four plain fiber types are also provided. 2. EXPERIMENTAL METHODS The setup depicted in Fig. 1(a) was used to fabricate FBGs; 244-nm CW radiation from a Coherent Innova FreD Ar laser of maximum power 120 mW was used. Two different phase masks were used: a “standard” phase mask (Lasiris) had contributions of about 40% from the 1 orders and less than 5% from the 2 orders, and a custom-made phase mask (Ibsen Photonics) had approximately equal contributions from each of the 1 and 2 orders, while keeping the zeroth order low. Both have been used previously to investigate the role of phase
Fig. 1. Schematic of (a) FBG fabrication using a phase mask, and the pertinent FBG structure expected in DIC imaging planes for fiber orientations (b) perpendicular and (c) parallel to the UV writing beam showing interleaving planes of RI modulation exhibiting two periods, namely Λpm and Λpm ∕2.
Research Article mask orders on FBG spectra, while their expected intensity distributions were simulated using the actual strengths of phase mask diffraction orders [20]. Five FBGs were imaged, the properties of which are summarized in Table 1. Four of them, designated A–D, having Ge-doped cores, were loaded with H 2 prior to UV irradiation (their growth characteristics have been analyzed elsewhere [20]), while the other had Ge:B core dopants and was Type In. Additionally, four different plain fibers were imaged. A modified form of DIC microscopy was used based on an Olympus IX FL microscope that was infinity-corrected, equipped with high-resolution Nomarski optics and an Ar laser (488 nm) [19]. The resolution of the system, 0.58 0.01 μm, was governed by a lateral shear of a sliding Wollaston prism. The intensity recorded within a DIC image is related to the gradient of the phase function of the object and can be approximated as the square of the differential of the optical path length through the specimen [15]. Once the argon-ion laser was stabilized, images were recorded using an Olympus UPlanApo 40× infinity-corrected objective, which is corrected for the 0.17-mm-thick cover slips (22 mm × 22 mm × 0.17 mm), and the glass cover slides were 22 mm× 22 mm × 1 mm. Each sample was mounted and located through the microscope oculars, and the centering of the condenser and the extinction of the two polarizers were optimized. The focus conditions for each image were determined qualitatively by adjusting the objective lens height until the sharpest edges at the core or cladding boundaries were obtained. Samples were scanned and the images were recorded using FluoView software (Olympus). The setting of the sliding Wollaston prism has been discussed elsewhere [24]. Optimal contrast in images of the fiber core, requiring a high degree of index matching between the immersion oil and the fiber cladding of the sample, was achieved using a Cargille Series AA oil (RI 1.4580 0.0002 at 25°C; temperature dependence −3.73 × 10−4 ∕°C) with samples mounted on a temperature-controlled rotation stage to ensure optimal index-matching conditions for the various fibers used, as depicted in Fig. 2. The sample was placed on a cover slip along with extra pieces of the same fiber (“spacers”) placed either side of the test sample to form a “well” with the index-matching oil and also to prevent tilting in the microscope slide that is then placed on top of the sample. Prepared samples were mounted in the central groove of a temperature-controlled rotation stage, and one end of the fiber was fastened to allow imaging at different rotational orientations about the fiber axis, controlled electronically with a mechanical stepper with a precision of 0.01°. The optimum index-matching conditions were achieved by examining the intensity difference between the index-matching oil and fiber cladding from averaged line profiles across images measured at various temperatures until the cladding boundary was no longer distinguishable. Linear fits were applied to the variation of intensity extrema at the cladding boundaries for each temperature to determine the zero intercept. To remove all sources of unwanted intensity variation from the imaging system, background images, with the fiber sample moved just out of the field of view, were measured at each temperature and subtracted from the corresponding image.
Research Article Table 1.
Vol. 55, No. 4 / February 1 2016 / Applied Optics
785
Details of the FBGs Imaged in This Work and Summary of Their Properties Determined from Images A I
B I
C I
D I
In
Standard 1.0668 0:1.8 ±1:39 ±2:3.5 ±3:3.3 ±4:3.3
Standard—see FBG “A”
Custom 1.07: 0:3.2 ±1∶15 ±2:19.6 ±3:13.5 ±4:0.4
Custom—see FBG “C”
See “A”
Fiber type Core diameter (μm) λcutoff (nm) Numerical aperture
Optix 7.4 ∼1300 0.12
Corning HI1060 3.6 930 40 0.2
See FBG “A”
See FBG “B”
Fibre-core 8 1260 0.13
Total UV fluence (kJ∕cm2 ) Bragg wavelength, λB (nm) Peak reflectance (%)
18 3 1546.3 99.99
51 1546.8 99.99
31 6 1551.1 95.8
10 2 1551.2 99.99
60 13 1544.5 99.93
1.09 1.0 0.1π 4.6
1.09 1.02 0.02π 4.6
FBG Sample (type) Phase mask Pitch (μm) Diffraction order: efficiency (%)
FBG period, Λ (0.05 μm) Parallel image phase difference (rad) Talbot length, Z T 0.5 μm
1.08 1.0 0.1π 4.6
Results from analyses of images 1.09 1.09 1.0 0.1π 1.03 0.01π 4.6 4.6
The mounting of each grating sample was first tested by checking that the expected structure changes were observed when the sample was rotated at a fixed image depth. Then images were recorded in a fixed plane after the sample was rotated in 5° increments from 0° to 180° (usually), and the corresponding background image was subtracted. Images were analyzed to identify the best two orthogonal images, of which one exhibited uniform parallel grating lines [i.e., as in Fig. 1(b)] and the other a Talbot diffraction pattern [i.e., as in Fig. 1(c)], with a precision of 5°. To determine periodicities of image features, linescans of the images (using ImageJ) that had been averaged over a particular area and normalized with respect to the maximum and minimum intensity values were obtained. For determining periodicity along the fiber, two adjacent regions were selected (size 30 μm × 2 μm in the standard core size FBGs [A, C, and In] and 30 μm × 1 μm in the smaller core size FBGs [B and D]). To study the pattern extending across the core, linescans across the fiber axis from 10 regions were averaged (size 0.5 μm × 7 μm in FBGs A, C, and In, and 0.4 μm × 12 μm to include the core and depressed cladding regions in FBGs B and D). These linescans were fitted using sinusoidal functions along the fiber direction, z, and parallel to the FBG-writingbeam direction, x, respectively, using
I z A sin2πz∕Λ φ I off δz;
(2a)
I x A sin2πx∕Z T φ I off δx;
(2b)
where A is the amplitude, φ is the phase, and I off is an intensity offset. The intensity gradient factor, δ, accounted for linear intensity gradients exhibited across some images arising from slight differences in conditions during FBG and background measurements. The period of observed RI perturbations along the fiber is Λ, while RI perturbations across the gratings were examined in terms of possible Talbot lengths, Z T , which by using Eq. (1) for the beating between the pair of strongest orders (i.e., 1 and 2) of each phase mask is ∼4.6 μm. 3. RESULTS AND DISCUSSION A. Plain Fibers
Images of four fiber types (i.e., non-UV exposed with the acrylate coating removed) are shown in Fig. 3, and all have cladding diameters of ∼125 μm for which the outer edges are visible along with the core (in the center of the images) while their depressed cladding boundaries are also faintly visible on either side of the core. The smaller core diameter of the Corning HI 1060 fiber, of 3.6 μm, is evident in Fig. 3(c). Depressed claddings, usually introduced using boron or fluorine during manufacture to lower the RI surrounding the core and reduce bending losses [25,26], are most evident for the Corning HI 1060 and Fibrecore PS-Series fibers (use of boron has been shown to reduce losses to cladding modes in FBGs [27]). The fibers in Figs. 3(b) and 3(d) have a RI dip in the ridge along the center of the core, due to the collapse of the preform when using the chemical vapor deposition technique [28]. B. Bragg Gratings
Fig. 2. Schematic illustration of sample preparation and positioning used for DIC imaging of fibers at different rotational orientations.
DIC images of all five FBGs, taken in both image planes, are shown in Fig. 4, where Figs. 4(a), 4(c), 4(e), 4(g), and 4(i) were acquired in the image plane perpendicular to the writing beam while Figs. 4(b), 4(d), 4(f ), 4(h), and 4(j) were acquired in the image plane parallel to the writing beam. These images,
786
Vol. 55, No. 4 / February 1 2016 / Applied Optics
Research Article
Fig. 3. DIC images of fibers, with the background subtracted: (a) Optix fiber, (b) Fibrecore, (c) Corning HI1060, and (d) Corning SMF28. The dimensions of each image are about 140 μm × 140 μm.
as expected, show clearly that in each case a grating was written uniformly across the core (perpendicular images) while showing a Talbot diffraction pattern for the parallel images. Linescans of averages of indicated regions of these images were fitted with Eqs. (2a) and (2b) for perpendicular and parallel images, respectively. 1. Standard Telecommunications Phase Mask (FBG A)
Fiber
and
Standard
FBG A, shown in Figs. 4(a) and 4(b), exhibited clear Type I growth characteristics, with a strong transmission dip observed at λB and smaller dips observed near 2λB ∕3 [20]. The apparent tilt in Fig. 4(b) is due to a residual intensity gradient arising from slight differences in imaging conditions during measurement of the FBG and the corresponding background image. The linescans in Figs. 5(a) and 5(b), along the fiber axis from the perpendicular image [from Fig. 4(a)], exhibit periodic features on either side of the core that are in phase to within 0.02π rad. The linescans in Figs. 5(c) and 5(d) along the fiber axis from the parallel image [from Fig. 4(b)] have periodic features that are out of phase by 1.0 0.1π rad, that is, half a period. The period of each of the four linescans in Figs. 5(a)– 5(d) was determined to be 1.08 0.05 μm, in excellent agreement with the period of the phase mask, 1.0668 μm. Linescans across the fiber axis in Fig. 4(b) resulted in Fig. 5(e), for which a sinusoidal function [Eq. (2b)] with an expected Z T 4.56 μm compares well with the data. The period of the data (∼5 μm) is slightly greater but with 0.5 μm uncertainty as it is difficult to distinguish the Talbot pattern from the large intensity differences at the edges of the core. 2. Smaller Core Fiber and Standard Phase Mask (FBG B)
FBG B, shown in Figs. 4(c) and 4(d), also exhibited clear Type I growth characteristics and strong transmission dips at both λB
Fig. 4. DIC images of various FBGs with top and bottom row images recorded at fiber orientations perpendicular and parallel to the writing beam direction, respectively, after subtraction of nonexposed fiber image: (a), (b) standard phase mask and telecom fiber (FBG A); (c), (d) standard phase mask and smaller core fiber (FBG B); (e), (f ) custom phase mask and telecom fiber (FBG C); (g), (h) custom phase mask and smaller core fiber (FBG D); (i), (j) Type In FBG. The dimensions of each image are about 47 μm × 47 μm.
and 2λB ∕3 [20]. The RI variations in Fig. 4(c) are distributed uniformly across the core with a slight tilt in the grating planes, and the apparent nonuniformity of the grating planes at the edges of the core is due to misalignment during image subtraction. Despite the smaller fiber core diameter the structure in Fig. 4(d) exhibits the expected complex RI structure. Smaller RI perturbations evident in the cladding on either side of the central core region in both images are UV-induced due to low concentrations of germanium or fluorine/boron in the depressed cladding region [27]. The linescans in Figs. 6(a) and 6(b), taken along the fiber axis from the perpendicular image [Fig. 4(c)], have periodic features on either side of the core that are in phase, to within 0.04π rad. Similarly, the linescans in Figs. 6(c) and 6(d) along the fiber axis
Research Article
Fig. 5. Analyses of image features of FBG A, through normalized averages of linescans, obtained from the indicated regions of the perpendicular and parallel images in Figs. 4(a) and 4(b): along the fiber axis taken from (a) the left side and (b) the right side of the core in Fig. 4(a); along the fiber axis taken from Fig. 4(b), taken from (c) the left side of the core and (d) the right side of the core; and (e) comparison between the expected and measured Talbot profiles from Fig. 4(b).
from the parallel image [Fig. 4(d)] have periodic features in the two regions that are out of phase, and fitting resulted in a phase shift of 1.0 0.1π rad. The periods of the four linescans in Figs. 6(a)–6(d) were determined to be 1.09 0.05 μm, in excellent agreement with the phase mask period of 1.0668 μm. The profile across the fiber in Fig. 6(e) shows large differences in measured intensities due to the index perturbations in the core and the depressed cladding regions; this made it difficult to estimate Z T . The largest maximum and minimum, at ∼4.5 and 7 μm, respectively, are much larger than the RI changes in the cladding as the core is more photosensitive. The intensity dip at 8.6 μm is due to the ridge along the right hand side of the core in Fig. 6(e) and is likely due to misalignment during the subtraction of the nonexposed fiber image, since the process involves a pixel-by-pixel subtraction of the intensity values, and consequently small deviations from parallel alignment between the fibers in the two images have a significant impact on the final image. The sinusoidal function [Eq. (2b)] with the expected Z T 4.56 μm reproduces the periodicity of the data in Fig. 6(e) that was estimated to be 4.6 0.5 μm.
Vol. 55, No. 4 / February 1 2016 / Applied Optics
787
Fig. 6. Analyses of image features of FBG B, through normalized averages of linescans, obtained from the indicated regions of the perpendicular and parallel images in Figs. 4(c) and 4(d): along the fiber axis taken from (a) the left side and (b) the right side of the core in Fig. 4(c); along the fiber axis taken from Fig. 4(d), taken from (c) the left side of the core and (d) the right side of the core; and (e) comparison between the expected and measured Talbot profiles from Fig. 4(d).
3. Standard Telecommunications Fiber and Custom-Made Phase Mask (FBG C)
This FBG, for which the DIC images are shown in Figs. 4(e) and 4(f ), exhibited clear Type I growth characteristics; a strong transmission dip was observed at λB , and smaller dips were observed in the region of 2λB ∕3 [20]. Despite the use of a nonoptimized phase mask, there is still clear evidence of the expected orthogonally distinguishable RI structure. The apparent tilt in the data is possibly due to a residual intensity gradient as discussed above. The linescans in Figs. 7(a) and 7(b), along the fiber axis [from Fig. 4(e)], had periodic features on either side of the core that are in phase; the determined phase shift was 0.00 0.01π rad. The linescans in Figs. 7(c) and 7(d), along the fiber axis from the parallel image [Fig. 4(f )], show periodic features in the pair of regions that are out of phase by half a period; the phase shift was 1.03 0.01π rad. The period of each of the four linescans in Figs. 7(a)–7(d) was determined to be 1.09 0.05 μm, which is in excellent agreement with
Research Article
z
z
Vol. 55, No. 4 / February 1 2016 / Applied Optics
z
788
Fig. 7. Analyses of image features of FBG C, through normalized averages of linescans, obtained from the indicated regions of the perpendicular and parallel images in Figs. 4(e) and 4(f ): along the fiber axis taken from (a) the left side and (b) the right side of the core in Fig. 4(e); along the fiber axis taken from Fig. 4(f ), taken from (c) the left side of the core and (d) the right side of the core; and (e) comparison between the expected and measured Talbot profiles from Fig. 4(f ).
the period of the phase mask, 1.07 μm. The pattern extending across the core shown in Fig. 7(e) has a measured period of 4.6 0.5 μm and compares well with Eq. (2b) for the expected Z T 4.59 μm. 4. Smaller Core Fiber and Custom-Made Phase Mask (FBG D)
This FBG [images in Figs. 4(g) and 4(h)] exhibited clear Type I growth characteristics and strong transmission dips at both λB and 2λB ∕3 [20]. These images show evidence of a slight tilt in the grating planes (∼3°) that may be due to a slight rotational misalignment of the phase mask relative to the fiber during fabrication. Smaller RI perturbations are evident in the depressed cladding on either side of the central core region, as seen for FBG B (written in the same fiber type). The linescans in Figs. 8(a) and 8(b) along the fiber axis [from Fig. 4(g)] have periodic features in both regions that are in phase; the phase shift of 0.01π rad was negligible. The linescans in Figs. 8(c) and 8(d), taken along the fiber axis from Fig. 4(h), exhibit periodic features that are out of phase by half
Fig. 8. Analyses of image features of FBG D, through normalized averages of linescans, obtained from the indicated regions of the perpendicular and parallel images in Figs. 4(g) and 4(h): along the fiber axis taken from (a) the left side and (b) the right side of the core in Fig. 4(g); along the fiber axis taken from Fig. 4(h), taken from (c) the left side of the core and (d) the right side of the core; and (e) comparison between the expected and measured Talbot profiles from Fig. 4(h).
a period between the two regions; a phase difference of 1.0 0.1π rad was found. The period of each of the four linescans in Figs. 8(a)–8(d) was 1.09 0.05 μm, in excellent agreement with that of the phase mask, 1.07 μm. The profile across the fiber core shown in Fig. 8(e) exhibits large differences in measured intensities due to the index perturbations in the core and depressed cladding regions, and, as for FBG B, it is difficult to measure Z T . The maximum and minimum at 4.5 and 6.9 μm, respectively, are likely to be due to the higher UV-induced changes in the more photosensitive core. The intensity dip near 10 μm is due to the dark ridge along the right hand side of the core in Fig. 4(h) and arises from the subtraction process. A sinusoidal function [Eq. (2b)] with the expected Z T (4.59 μm) compares well with the data, but the data show a slightly longer period than the expected Talbot length due to the difficulty in distinguishing the Talbot patterns from the large intensity differences at the edges of the core. Thus the measured Z T 4.6 0.5 μm is consistent with that determined for FBG C (made with the same phase mask).
Research Article 5. Type In FBG (FBG In)
z
z
The Type In FBG exhibited an initial reflectance increase, partial erasure, and subsequent regrowth (over an extended period), giving a strong transmission dip at λB and small dips in the region of 2λB ∕3 [29], and the images shown in Figs. 4(i) and 4(j) are as expected. The linescans in Figs. 9(a) and 9(b) are taken along the fiber axis from Fig. 4(i), and, like those for the Type I FBGs, have periodic features on either side of the core in phase, verified by the determined phase difference of approximately 0 rad. The linescans in Figs. 9(c) and 9(d), taken along the fiber axis from Fig. 4(j), are also like those for the Type I FBGs having periodic features on either side of the core out of phase by half a period, that is, π rad; the fit gave this as 1.02 0.02π rad. The average period of each of the four linescans in Figs. 9(a)–9(d) was determined to be 1.09 0.05 μm, in excellent agreement with the period of the phase mask, 1.0668 μm. The pattern extending across the core, Fig. 9(e), was obtained from Fig. 4(j). Unfortunately the subtraction process has not removed the RI dip in the center of the fiber due to slight misalignments.
Fig. 9. Analyses of image features of a Type In FBG, through normalized averages of linescans, obtained from the indicated regions of the perpendicular and parallel images in Figs. 4(i) and 4(j): along the fiber axis taken from (a) the left side and (b) the right side of the core in Fig. 4(i); along the fiber axis taken from Fig. 4(j), taken from (c) the left side of the core and (d) the right side of the core; and (e) comparison between the expected and measured Talbot profiles from Fig. 4(j).
Vol. 55, No. 4 / February 1 2016 / Applied Optics
789
A calculated sinusoidal function with Z T 4.56 μm is also shown, and the observed value (4.6 0.5 μm) is in excellent agreement. 4. CONCLUSIONS DIC imaging was performed on plain optical fibers and various phase-mask-written FBGs, including Type I FBGs (samples A to D) and a Type In FBG fabricated in standard and smaller core fibers with the standard and custom-made phase masks. Analyses of various linescans of these FBG images, as summarized in Table 1, confirmed the expected RI structure within each FBG. Images recorded in the plane parallel to the direction of the UV writing beam had RI perturbations which were distributed uniformly across the core, while those recorded in the perpendicular plane all exhibited interleaved grating planes that were π phase shifted with respect to each other, with a period consistent with that of the phase mask. All measured Talbot lengths of the RI patterns extending across the core in the perpendicular images were consistent with those expected for beating between the dominant 1 and 2 orders of each mask. The imaged structures of the larger core FBGs written with the standard (A) and custom phase masks (C) revealed full periods of the Talbot diffraction patterns in the core. However, the imaged structures of the smaller core FBGs written with the standard (B) and custom (D) phase masks demonstrated how the smaller core prevents a full Talbot length from being formed across the core. FBGs written in this fiber required less integrated fluence to achieve saturation compared with standard fiber [20], owing to the reduction in UV-absorbing volume. While the RI variations in the FBGs made with the custom-made phase mask (C and D) appear stronger than those in the standard phase mask images (A and B), it is possible that this is due to the larger exposure fluences required to obtain high reflectances in the custom-made phase mask samples [20], and caused by the diminished 1 orders that produce the usual peak or dip at the Bragg wavelength, λB . Spectrally, the use of the custom-made phase mask enhanced features at 2λB ∕3, as expected, as this arises from other combinations of phase mask orders, for instance, the third harmonic of grating features having the periodicity of the phase mask, which arises from the combination of 0 and 1 orders [11,14]. This work forms part of a wider body of work that has investigated the relationship between fiber type (core diameter and RI profile), phase mask features (periodicity and relative strength of the diffraction orders), and the resulting spectral features in reflection at the Bragg wavelength and other grating harmonics. The DIC imaging technique clearly demonstrates how the complex Talbot field of a phase mask produces corresponding RI variations within a photosensitive fiber irrespective of the grating type or fiber core diameter, even if the phase mask is not optimized for FBG writing. Exploration of the relationship of FBG images and grating spectra is the subject of ongoing work, including the determination of actual RI variations within DIC images [17] and the understanding of double peaks (at certain harmonics of the Bragg wavelength) that arise from the Talbot planes not being perfectly aligned with the fiber core [22,30].
790
Research Article
Vol. 55, No. 4 / February 1 2016 / Applied Optics
Funding. Australian Research Council. Acknowledgment. C. M. Rollinson thanks Victoria University for a postgraduate scholarship.
17.
REFERENCES 1. J. Canning, “Fibre gratings and devices for sensors and lasers,” Laser Photon. Rev. 2, 275–289 (2008). 2. T. Erdogan, “Fiber grating spectra,” J. Lightwave Technol. 15, 1277– 1294 (1997). 3. P. A. Krug, R. Stolte, and R. Ulrich, “Measurement of index modulation along an optical fiber Bragg grating,” Opt. Lett. 20, 1767–1769 (1995). 4. E. Brinkmeyer, “Simple algorithm for reconstructing fiber gratings from reflectometric data,” Opt. Lett. 20, 810–812 (1995). 5. J. Canning, M. Janos, and M. G. Sceats, “Rayleigh longitudinal profiling of optical resonances within waveguide grating structures using sidescattered light,” Opt. Lett. 21, 609–611 (1996). 6. B. Malo, D. C. Johnson, F. Bilodeau, J. Albert, and K. O. Hill, “Singleexcimer-pulse writing of fiber gratings by use of a zero-order nulled phase mask: grating spectral response and visualization of index perturbations,” Opt. Lett. 18, 1277–1279 (1993). 7. J. D. Mills, C. W. J. Hillman, W. S. Brocklesby, and B. H. Blott, “Evanescent field imaging of an optical fiber Bragg grating,” Appl. Phys. Lett. 75, 4058–4060 (1999). 8. F. El-Diasty, A. Heaney, and T. Erdogan, “Analysis of fiber Bragg gratings by a side-diffraction interference technique,” Appl. Opt. 40, 890–896 (2001). 9. X. M. Goh, S. S. Kou, B. P. Kouskousis, N. M. Dragomir, S. F. Collins, G. W. Baxter, and A. Roberts, “Non-destructive three-dimensional optical imaging of a fiber Bragg grating,” IEEE Photon. J. 6, 6900207 (2014). 10. J. D. Mills, C. W. J. Hillman, B. H. Blott, and W. S. Brocklesby, “Imaging of free-space interference patterns used to manufacture fiber Bragg gratings,” Appl. Opt. 39, 6128–6135 (2000). 11. P. E. Dyer, R. J. Farley, and R. Giedl, “Analysis of grating formation with excimer laser irradiated phase masks,” Opt. Commun. 115, 327–334 (1995). 12. H. Talbot, “Facts relating to optical science,” London Edinburgh Philos. Mag. J. Sci. 9, 401–407 (1836). 13. Lord Rayleigh, “XXV. On copying diffraction-gratings, and on some phenomena connected therewith,” Philos. Mag. Ser. 5 11(67), 196–205 (1881). 14. B. P. Kouskousis, D. J. Kitcher, S. F. Collins, and G. W. Baxter, “Numerical investigation on the effects of fabrication conditions on fiber Bragg grating spectra using the phase mask technique,” Appl. Opt. 52, 3338–3344 (2013). 15. M. Pluta, Advanced Light Microscopy (Elsevier, 1989), Vol. 2. 16. C. Preza, S. King, N. Dragomir, and C. Cogswell, “Phase imaging microscopy: beyond dark-field, phase contrast, and differential
18.
19.
20.
21.
22.
23.
24. 25.
26. 27.
28. 29.
30.
interference contrast microscopy,” in Handbook of Biomedical Optics, D. Boas, C. Pitris, and N. Ramanujam, eds. (CRC Press, 2011), pp. 483–517. B. Kouskousis, D. J. Kitcher, S. Collins, A. Roberts, and G. W. Baxter, “Quantitative phase and refractive index analysis of optical fibers using differential interference contrast microscopy,” Appl. Opt. 47, 5182–5189 (2008). N. M. Dragomir, C. Rollinson, S. A. Wade, A. J. Stevenson, S. F. Collins, G. W. Baxter, P. M. Farrell, and A. Roberts, “Nondestructive imaging of a type I optical fiber Bragg grating,” Opt. Lett. 28, 789–791 (2003). B. P. Kouskousis, C. M. Rollinson, D. J. Kitcher, S. F. Collins, G. W. Baxter, S. A. Wade, N. M. Dragomir, and A. Roberts, “Quantitative investigation of the refractive-index modulation within the core of a fiber Bragg grating,” Opt. Express 14, 10332–10338 (2006). C. M. Rollinson, S. A. Wade, B. P. Kouskousis, D. J. Kitcher, G. W. Baxter, and S. F. Collins, “Variations of the growth of harmonic reflections in fiber Bragg gratings fabricated using phase masks,” J. Opt. Soc. Am. A 29, 1259–1268 (2012). C. M. Rollinson, S. A. Wade, N. M. Dragomir, G. W. Baxter, S. F. Collins, and A. Roberts, “Reflections near 1030 nm from 1540 nm fibre Bragg gratings: evidence of a complex refractive index structure,” Opt. Commun. 256, 310–318 (2005). S. A. Wade, W. G. A. Brown, H. K. Bal, F. Sidiroglou, G. W. Baxter, and S. F. Collins, “Effect of phase mask alignment on FBG spectra at harmonics of the Bragg wavelength,” J. Opt. Soc. Am. A 29, 1597–1605 (2012). C. W. Smelser, S. J. Mihailov, D. Grobnic, P. Lu, R. B. Walker, H. Ding, and X. Dai, “Multiple-beam interference patterns in optical fiber generated with ultrafast pulses and a phase mask,” Opt. Lett. 29, 1458–1460 (2004). B. P. Kouskousis, “Microscopic characterisation of fibre Bragg gratings,” Ph.D. dissertation (Victoria University, 2009). W. W. Morey, G. Meltz, J. D. Love, and S. J. Hewlett, “Mode-coupling characteristics of UV-written Bragg gratings in depressed-cladding fiber,” Electron. Lett. 30, 730–732 (1994). R. Kashyap, Fiber Bragg Gratings, Optics and Photonics (Academic, 1999). L. Dong, L. Reekie, J. L. Cruz, J. E. Caplen, J. P. deSandro, and D. N. Payne, “Optical fibers with depressed claddings for suppression of coupling into cladding modes in fiber Bragg gratings,” IEEE Photon. Technol. Lett. 9, 64–66 (1997). H. O. Pierson, Handbook of Chemical Vapor Deposition: Principles, Technology, and Applications, 2nd ed. (Noyes, 1999), p. 420. C. M. Rollinson, “Microscopic and spectral characterisation of optical fibre Bragg gratings,” Ph.D. dissertation (Victoria University, 2012). K. Tarnowski and W. Urbanczyk, “Origin of Bragg reflection peaks splitting in gratings fabricated using a multiple order phase mask,” Opt. Express 21, 21800–21810 (2013).
Vol. 55, No. 4 / February 1 2016 / Applied Optics
783
Imaging of various optical fiber Bragg gratings using differential interference contrast microscopy: analysis and comparison CLAIRE M. ROLLINSON,1 SCOTT A. WADE,2 GREG W. BAXTER,1
AND
STEPHEN F. COLLINS1,*
1
Optical Technology Research Laboratory, College of Engineering and Science, Victoria University, P.O. Box 14428, Melbourne, VIC 8001, Australia CQOS, Swinburne University of Technology, Hawthorn, VIC 3122, Australia *Corresponding author: [email protected]
2
Received 9 September 2015; revised 25 November 2015; accepted 30 November 2015; posted 30 November 2015 (Doc. ID 249775); published 26 January 2016
Differential interference contrast images of various optical fibers and optical fiber Bragg gratings (FBGs), written with the phase mask technique, are presented to provide information about the resultant refractive index variations present in each case. Use of different fiber types using two distinct phase masks producing four Type I FBGs and a Type In FBG allowed similarities and differences in these FBG images due to variations in the Talbot diffraction patterns produced to be studied. © 2016 Optical Society of America OCIS codes: (060.3735) Fiber Bragg gratings; (060.3738) Fiber Bragg gratings, photosensitivity; (070.6760) Talbot and self-imaging effects; (050.2770) Gratings. http://dx.doi.org/10.1364/AO.55.000783
1. INTRODUCTION Optical fiber Bragg gratings (FBGs) have received much attention for over 20 years and have been reported for a plethora of applications in telecommunications and sensing, as reviewed by Canning [1]. Typically, they operate as an in-line filter, in which a strong reflection of a narrow wavelength band occurs when a propagating core mode experiences resonance with the periodic variation of refractive index (RI) along the fiber core. Many FBG types have been reported due to the existence of various fiber types and the use of different fabrication techniques. Type I gratings are generally formed in photosensitive fiber types when exposed to continuous wave or relatively low pulse energy UV radiation and show a monotonic increase in the amplitude of the RI modulation with UV exposure. Those known as Type In (previously known as IIa) evolve from Type I grating growth. Properties of FBGs may be studied via analysis of grating reflection or transmission, or mathematical modelling (e.g., coupled mode theory [2]). As such methods give little information about the RI structure within a FBG, measurement of such profiles along a grating has been undertaken by various authors, for instance [3–5]. Standard microscopes are unable to detect the small RI changes in the transparent core of a FBG. More detail has been revealed in a diffraction-limited photographic technique [6], scanning near-field optical microscopy (SNOM) [7] has been used to examine both the structure and internal field distribution of a polished FBG by accessing its evanescent 1559-128X/16/040783-08$15/0$15.00 © 2016 Optical Society of America
field, and a side-diffraction interference technique has been reported [8]. Recently, quantitative phase microscopy was used to show the three-dimensional RI distribution of a FBG [9]. The SNOM technique was also used to study the irradiance field in free space behind an illuminated phase mask [10], verifying the existence of a periodicity in the diffraction field along the direction of the incident beam, as expected for FBGs fabricated using the phase mask technique [11] and referred to as the Talbot effect [12]. The repeat length of the pattern, known as the Talbot length [13], is Z T m; n
2π ; k2 − m2 G 2 1∕2 − k2 − n2 G 2 1∕2
(1)
where m and n are integers representing diffraction orders, k 2πnw ∕λw , nw is the RI of the medium behind the phase mask at the writing wavelength λw , and G 2π∕Λpm is the unit reciprocal lattice vector of the phase mask. If the pattern behind the phase mask is formed by first-order diffraction alone, the two symmetrically crossing components generate uniform fringes with half the period of the phase mask and with no Talbot beating [14]. Actual phase masks, however, produce some zeroth and higher diffraction orders and thus exhibit a Talbot length, for which measurements agree well with Eq. (1) for the interaction between the dominant first and second orders [10]. The differential interference contrast (DIC) [15,16] technique is ideal for optical fiber imaging as it is nondestructive.
784
Vol. 55, No. 4 / February 1 2016 / Applied Optics
A laser beam is split into two orthogonal components that are slightly spatially separated (the “shear”), and after traversing the system their subsequent recombination provides high resolution RI variation information with the spatial resolution determined by the shear [17]. It was used to image a phase-mask-written FBG in which the intensity corresponds to the RI in the fiber core [18]. Slices through the fiber center, in which the sample was oriented at 0° and 90°, demonstrated the formation of a Talbot diffraction pattern as described above. The Talbot length, Z T , was consistent with the calculated value of ∼4.65 μm for beating between the first and second diffraction orders of the phase mask [18], and subsequently the complex pattern was replicated based on the strength of the orders of the phase mask [19]. The Bragg wavelength of the imaged FBG was ∼1535 nm, resulting from a phase mask period of Λpm 1.059 μm, although the planes of interleaved fringes indicate that the grating exhibits two periodicities, namely ∼0.53 and ∼1.06 μm, corresponding to Λpm ∕2 and Λpm , respectively. Indeed it has been shown that the Bragg wavelength arises from both periodicities [20], while various Bragg wavelength harmonics arise from one or both of these periodicities [6,21,22]. FBGs fabricated with ultrafast lasers have been imaged using an optical microscope, in which the spatial resolution is poor compared with the DIC technique [23]; similar structures were evident but only as their phase masks were of a larger periodicity. This paper presents use of the DIC technique to investigate the RI structures of five FBGs that were fabricated using both standard and custom-made phase masks in several types of optical fiber. This enabled the role of the actual fiber type used in determining the details of RI changes induced in the fiber to be appreciated, along with a comparison of Type I and Type In gratings. Images of four plain fiber types are also provided. 2. EXPERIMENTAL METHODS The setup depicted in Fig. 1(a) was used to fabricate FBGs; 244-nm CW radiation from a Coherent Innova FreD Ar laser of maximum power 120 mW was used. Two different phase masks were used: a “standard” phase mask (Lasiris) had contributions of about 40% from the 1 orders and less than 5% from the 2 orders, and a custom-made phase mask (Ibsen Photonics) had approximately equal contributions from each of the 1 and 2 orders, while keeping the zeroth order low. Both have been used previously to investigate the role of phase
Fig. 1. Schematic of (a) FBG fabrication using a phase mask, and the pertinent FBG structure expected in DIC imaging planes for fiber orientations (b) perpendicular and (c) parallel to the UV writing beam showing interleaving planes of RI modulation exhibiting two periods, namely Λpm and Λpm ∕2.
Research Article mask orders on FBG spectra, while their expected intensity distributions were simulated using the actual strengths of phase mask diffraction orders [20]. Five FBGs were imaged, the properties of which are summarized in Table 1. Four of them, designated A–D, having Ge-doped cores, were loaded with H 2 prior to UV irradiation (their growth characteristics have been analyzed elsewhere [20]), while the other had Ge:B core dopants and was Type In. Additionally, four different plain fibers were imaged. A modified form of DIC microscopy was used based on an Olympus IX FL microscope that was infinity-corrected, equipped with high-resolution Nomarski optics and an Ar laser (488 nm) [19]. The resolution of the system, 0.58 0.01 μm, was governed by a lateral shear of a sliding Wollaston prism. The intensity recorded within a DIC image is related to the gradient of the phase function of the object and can be approximated as the square of the differential of the optical path length through the specimen [15]. Once the argon-ion laser was stabilized, images were recorded using an Olympus UPlanApo 40× infinity-corrected objective, which is corrected for the 0.17-mm-thick cover slips (22 mm × 22 mm × 0.17 mm), and the glass cover slides were 22 mm× 22 mm × 1 mm. Each sample was mounted and located through the microscope oculars, and the centering of the condenser and the extinction of the two polarizers were optimized. The focus conditions for each image were determined qualitatively by adjusting the objective lens height until the sharpest edges at the core or cladding boundaries were obtained. Samples were scanned and the images were recorded using FluoView software (Olympus). The setting of the sliding Wollaston prism has been discussed elsewhere [24]. Optimal contrast in images of the fiber core, requiring a high degree of index matching between the immersion oil and the fiber cladding of the sample, was achieved using a Cargille Series AA oil (RI 1.4580 0.0002 at 25°C; temperature dependence −3.73 × 10−4 ∕°C) with samples mounted on a temperature-controlled rotation stage to ensure optimal index-matching conditions for the various fibers used, as depicted in Fig. 2. The sample was placed on a cover slip along with extra pieces of the same fiber (“spacers”) placed either side of the test sample to form a “well” with the index-matching oil and also to prevent tilting in the microscope slide that is then placed on top of the sample. Prepared samples were mounted in the central groove of a temperature-controlled rotation stage, and one end of the fiber was fastened to allow imaging at different rotational orientations about the fiber axis, controlled electronically with a mechanical stepper with a precision of 0.01°. The optimum index-matching conditions were achieved by examining the intensity difference between the index-matching oil and fiber cladding from averaged line profiles across images measured at various temperatures until the cladding boundary was no longer distinguishable. Linear fits were applied to the variation of intensity extrema at the cladding boundaries for each temperature to determine the zero intercept. To remove all sources of unwanted intensity variation from the imaging system, background images, with the fiber sample moved just out of the field of view, were measured at each temperature and subtracted from the corresponding image.
Research Article Table 1.
Vol. 55, No. 4 / February 1 2016 / Applied Optics
785
Details of the FBGs Imaged in This Work and Summary of Their Properties Determined from Images A I
B I
C I
D I
In
Standard 1.0668 0:1.8 ±1:39 ±2:3.5 ±3:3.3 ±4:3.3
Standard—see FBG “A”
Custom 1.07: 0:3.2 ±1∶15 ±2:19.6 ±3:13.5 ±4:0.4
Custom—see FBG “C”
See “A”
Fiber type Core diameter (μm) λcutoff (nm) Numerical aperture
Optix 7.4 ∼1300 0.12
Corning HI1060 3.6 930 40 0.2
See FBG “A”
See FBG “B”
Fibre-core 8 1260 0.13
Total UV fluence (kJ∕cm2 ) Bragg wavelength, λB (nm) Peak reflectance (%)
18 3 1546.3 99.99
51 1546.8 99.99
31 6 1551.1 95.8
10 2 1551.2 99.99
60 13 1544.5 99.93
1.09 1.0 0.1π 4.6
1.09 1.02 0.02π 4.6
FBG Sample (type) Phase mask Pitch (μm) Diffraction order: efficiency (%)
FBG period, Λ (0.05 μm) Parallel image phase difference (rad) Talbot length, Z T 0.5 μm
1.08 1.0 0.1π 4.6
Results from analyses of images 1.09 1.09 1.0 0.1π 1.03 0.01π 4.6 4.6
The mounting of each grating sample was first tested by checking that the expected structure changes were observed when the sample was rotated at a fixed image depth. Then images were recorded in a fixed plane after the sample was rotated in 5° increments from 0° to 180° (usually), and the corresponding background image was subtracted. Images were analyzed to identify the best two orthogonal images, of which one exhibited uniform parallel grating lines [i.e., as in Fig. 1(b)] and the other a Talbot diffraction pattern [i.e., as in Fig. 1(c)], with a precision of 5°. To determine periodicities of image features, linescans of the images (using ImageJ) that had been averaged over a particular area and normalized with respect to the maximum and minimum intensity values were obtained. For determining periodicity along the fiber, two adjacent regions were selected (size 30 μm × 2 μm in the standard core size FBGs [A, C, and In] and 30 μm × 1 μm in the smaller core size FBGs [B and D]). To study the pattern extending across the core, linescans across the fiber axis from 10 regions were averaged (size 0.5 μm × 7 μm in FBGs A, C, and In, and 0.4 μm × 12 μm to include the core and depressed cladding regions in FBGs B and D). These linescans were fitted using sinusoidal functions along the fiber direction, z, and parallel to the FBG-writingbeam direction, x, respectively, using
I z A sin2πz∕Λ φ I off δz;
(2a)
I x A sin2πx∕Z T φ I off δx;
(2b)
where A is the amplitude, φ is the phase, and I off is an intensity offset. The intensity gradient factor, δ, accounted for linear intensity gradients exhibited across some images arising from slight differences in conditions during FBG and background measurements. The period of observed RI perturbations along the fiber is Λ, while RI perturbations across the gratings were examined in terms of possible Talbot lengths, Z T , which by using Eq. (1) for the beating between the pair of strongest orders (i.e., 1 and 2) of each phase mask is ∼4.6 μm. 3. RESULTS AND DISCUSSION A. Plain Fibers
Images of four fiber types (i.e., non-UV exposed with the acrylate coating removed) are shown in Fig. 3, and all have cladding diameters of ∼125 μm for which the outer edges are visible along with the core (in the center of the images) while their depressed cladding boundaries are also faintly visible on either side of the core. The smaller core diameter of the Corning HI 1060 fiber, of 3.6 μm, is evident in Fig. 3(c). Depressed claddings, usually introduced using boron or fluorine during manufacture to lower the RI surrounding the core and reduce bending losses [25,26], are most evident for the Corning HI 1060 and Fibrecore PS-Series fibers (use of boron has been shown to reduce losses to cladding modes in FBGs [27]). The fibers in Figs. 3(b) and 3(d) have a RI dip in the ridge along the center of the core, due to the collapse of the preform when using the chemical vapor deposition technique [28]. B. Bragg Gratings
Fig. 2. Schematic illustration of sample preparation and positioning used for DIC imaging of fibers at different rotational orientations.
DIC images of all five FBGs, taken in both image planes, are shown in Fig. 4, where Figs. 4(a), 4(c), 4(e), 4(g), and 4(i) were acquired in the image plane perpendicular to the writing beam while Figs. 4(b), 4(d), 4(f ), 4(h), and 4(j) were acquired in the image plane parallel to the writing beam. These images,
786
Vol. 55, No. 4 / February 1 2016 / Applied Optics
Research Article
Fig. 3. DIC images of fibers, with the background subtracted: (a) Optix fiber, (b) Fibrecore, (c) Corning HI1060, and (d) Corning SMF28. The dimensions of each image are about 140 μm × 140 μm.
as expected, show clearly that in each case a grating was written uniformly across the core (perpendicular images) while showing a Talbot diffraction pattern for the parallel images. Linescans of averages of indicated regions of these images were fitted with Eqs. (2a) and (2b) for perpendicular and parallel images, respectively. 1. Standard Telecommunications Phase Mask (FBG A)
Fiber
and
Standard
FBG A, shown in Figs. 4(a) and 4(b), exhibited clear Type I growth characteristics, with a strong transmission dip observed at λB and smaller dips observed near 2λB ∕3 [20]. The apparent tilt in Fig. 4(b) is due to a residual intensity gradient arising from slight differences in imaging conditions during measurement of the FBG and the corresponding background image. The linescans in Figs. 5(a) and 5(b), along the fiber axis from the perpendicular image [from Fig. 4(a)], exhibit periodic features on either side of the core that are in phase to within 0.02π rad. The linescans in Figs. 5(c) and 5(d) along the fiber axis from the parallel image [from Fig. 4(b)] have periodic features that are out of phase by 1.0 0.1π rad, that is, half a period. The period of each of the four linescans in Figs. 5(a)– 5(d) was determined to be 1.08 0.05 μm, in excellent agreement with the period of the phase mask, 1.0668 μm. Linescans across the fiber axis in Fig. 4(b) resulted in Fig. 5(e), for which a sinusoidal function [Eq. (2b)] with an expected Z T 4.56 μm compares well with the data. The period of the data (∼5 μm) is slightly greater but with 0.5 μm uncertainty as it is difficult to distinguish the Talbot pattern from the large intensity differences at the edges of the core. 2. Smaller Core Fiber and Standard Phase Mask (FBG B)
FBG B, shown in Figs. 4(c) and 4(d), also exhibited clear Type I growth characteristics and strong transmission dips at both λB
Fig. 4. DIC images of various FBGs with top and bottom row images recorded at fiber orientations perpendicular and parallel to the writing beam direction, respectively, after subtraction of nonexposed fiber image: (a), (b) standard phase mask and telecom fiber (FBG A); (c), (d) standard phase mask and smaller core fiber (FBG B); (e), (f ) custom phase mask and telecom fiber (FBG C); (g), (h) custom phase mask and smaller core fiber (FBG D); (i), (j) Type In FBG. The dimensions of each image are about 47 μm × 47 μm.
and 2λB ∕3 [20]. The RI variations in Fig. 4(c) are distributed uniformly across the core with a slight tilt in the grating planes, and the apparent nonuniformity of the grating planes at the edges of the core is due to misalignment during image subtraction. Despite the smaller fiber core diameter the structure in Fig. 4(d) exhibits the expected complex RI structure. Smaller RI perturbations evident in the cladding on either side of the central core region in both images are UV-induced due to low concentrations of germanium or fluorine/boron in the depressed cladding region [27]. The linescans in Figs. 6(a) and 6(b), taken along the fiber axis from the perpendicular image [Fig. 4(c)], have periodic features on either side of the core that are in phase, to within 0.04π rad. Similarly, the linescans in Figs. 6(c) and 6(d) along the fiber axis
Research Article
Fig. 5. Analyses of image features of FBG A, through normalized averages of linescans, obtained from the indicated regions of the perpendicular and parallel images in Figs. 4(a) and 4(b): along the fiber axis taken from (a) the left side and (b) the right side of the core in Fig. 4(a); along the fiber axis taken from Fig. 4(b), taken from (c) the left side of the core and (d) the right side of the core; and (e) comparison between the expected and measured Talbot profiles from Fig. 4(b).
from the parallel image [Fig. 4(d)] have periodic features in the two regions that are out of phase, and fitting resulted in a phase shift of 1.0 0.1π rad. The periods of the four linescans in Figs. 6(a)–6(d) were determined to be 1.09 0.05 μm, in excellent agreement with the phase mask period of 1.0668 μm. The profile across the fiber in Fig. 6(e) shows large differences in measured intensities due to the index perturbations in the core and the depressed cladding regions; this made it difficult to estimate Z T . The largest maximum and minimum, at ∼4.5 and 7 μm, respectively, are much larger than the RI changes in the cladding as the core is more photosensitive. The intensity dip at 8.6 μm is due to the ridge along the right hand side of the core in Fig. 6(e) and is likely due to misalignment during the subtraction of the nonexposed fiber image, since the process involves a pixel-by-pixel subtraction of the intensity values, and consequently small deviations from parallel alignment between the fibers in the two images have a significant impact on the final image. The sinusoidal function [Eq. (2b)] with the expected Z T 4.56 μm reproduces the periodicity of the data in Fig. 6(e) that was estimated to be 4.6 0.5 μm.
Vol. 55, No. 4 / February 1 2016 / Applied Optics
787
Fig. 6. Analyses of image features of FBG B, through normalized averages of linescans, obtained from the indicated regions of the perpendicular and parallel images in Figs. 4(c) and 4(d): along the fiber axis taken from (a) the left side and (b) the right side of the core in Fig. 4(c); along the fiber axis taken from Fig. 4(d), taken from (c) the left side of the core and (d) the right side of the core; and (e) comparison between the expected and measured Talbot profiles from Fig. 4(d).
3. Standard Telecommunications Fiber and Custom-Made Phase Mask (FBG C)
This FBG, for which the DIC images are shown in Figs. 4(e) and 4(f ), exhibited clear Type I growth characteristics; a strong transmission dip was observed at λB , and smaller dips were observed in the region of 2λB ∕3 [20]. Despite the use of a nonoptimized phase mask, there is still clear evidence of the expected orthogonally distinguishable RI structure. The apparent tilt in the data is possibly due to a residual intensity gradient as discussed above. The linescans in Figs. 7(a) and 7(b), along the fiber axis [from Fig. 4(e)], had periodic features on either side of the core that are in phase; the determined phase shift was 0.00 0.01π rad. The linescans in Figs. 7(c) and 7(d), along the fiber axis from the parallel image [Fig. 4(f )], show periodic features in the pair of regions that are out of phase by half a period; the phase shift was 1.03 0.01π rad. The period of each of the four linescans in Figs. 7(a)–7(d) was determined to be 1.09 0.05 μm, which is in excellent agreement with
Research Article
z
z
Vol. 55, No. 4 / February 1 2016 / Applied Optics
z
788
Fig. 7. Analyses of image features of FBG C, through normalized averages of linescans, obtained from the indicated regions of the perpendicular and parallel images in Figs. 4(e) and 4(f ): along the fiber axis taken from (a) the left side and (b) the right side of the core in Fig. 4(e); along the fiber axis taken from Fig. 4(f ), taken from (c) the left side of the core and (d) the right side of the core; and (e) comparison between the expected and measured Talbot profiles from Fig. 4(f ).
the period of the phase mask, 1.07 μm. The pattern extending across the core shown in Fig. 7(e) has a measured period of 4.6 0.5 μm and compares well with Eq. (2b) for the expected Z T 4.59 μm. 4. Smaller Core Fiber and Custom-Made Phase Mask (FBG D)
This FBG [images in Figs. 4(g) and 4(h)] exhibited clear Type I growth characteristics and strong transmission dips at both λB and 2λB ∕3 [20]. These images show evidence of a slight tilt in the grating planes (∼3°) that may be due to a slight rotational misalignment of the phase mask relative to the fiber during fabrication. Smaller RI perturbations are evident in the depressed cladding on either side of the central core region, as seen for FBG B (written in the same fiber type). The linescans in Figs. 8(a) and 8(b) along the fiber axis [from Fig. 4(g)] have periodic features in both regions that are in phase; the phase shift of 0.01π rad was negligible. The linescans in Figs. 8(c) and 8(d), taken along the fiber axis from Fig. 4(h), exhibit periodic features that are out of phase by half
Fig. 8. Analyses of image features of FBG D, through normalized averages of linescans, obtained from the indicated regions of the perpendicular and parallel images in Figs. 4(g) and 4(h): along the fiber axis taken from (a) the left side and (b) the right side of the core in Fig. 4(g); along the fiber axis taken from Fig. 4(h), taken from (c) the left side of the core and (d) the right side of the core; and (e) comparison between the expected and measured Talbot profiles from Fig. 4(h).
a period between the two regions; a phase difference of 1.0 0.1π rad was found. The period of each of the four linescans in Figs. 8(a)–8(d) was 1.09 0.05 μm, in excellent agreement with that of the phase mask, 1.07 μm. The profile across the fiber core shown in Fig. 8(e) exhibits large differences in measured intensities due to the index perturbations in the core and depressed cladding regions, and, as for FBG B, it is difficult to measure Z T . The maximum and minimum at 4.5 and 6.9 μm, respectively, are likely to be due to the higher UV-induced changes in the more photosensitive core. The intensity dip near 10 μm is due to the dark ridge along the right hand side of the core in Fig. 4(h) and arises from the subtraction process. A sinusoidal function [Eq. (2b)] with the expected Z T (4.59 μm) compares well with the data, but the data show a slightly longer period than the expected Talbot length due to the difficulty in distinguishing the Talbot patterns from the large intensity differences at the edges of the core. Thus the measured Z T 4.6 0.5 μm is consistent with that determined for FBG C (made with the same phase mask).
Research Article 5. Type In FBG (FBG In)
z
z
The Type In FBG exhibited an initial reflectance increase, partial erasure, and subsequent regrowth (over an extended period), giving a strong transmission dip at λB and small dips in the region of 2λB ∕3 [29], and the images shown in Figs. 4(i) and 4(j) are as expected. The linescans in Figs. 9(a) and 9(b) are taken along the fiber axis from Fig. 4(i), and, like those for the Type I FBGs, have periodic features on either side of the core in phase, verified by the determined phase difference of approximately 0 rad. The linescans in Figs. 9(c) and 9(d), taken along the fiber axis from Fig. 4(j), are also like those for the Type I FBGs having periodic features on either side of the core out of phase by half a period, that is, π rad; the fit gave this as 1.02 0.02π rad. The average period of each of the four linescans in Figs. 9(a)–9(d) was determined to be 1.09 0.05 μm, in excellent agreement with the period of the phase mask, 1.0668 μm. The pattern extending across the core, Fig. 9(e), was obtained from Fig. 4(j). Unfortunately the subtraction process has not removed the RI dip in the center of the fiber due to slight misalignments.
Fig. 9. Analyses of image features of a Type In FBG, through normalized averages of linescans, obtained from the indicated regions of the perpendicular and parallel images in Figs. 4(i) and 4(j): along the fiber axis taken from (a) the left side and (b) the right side of the core in Fig. 4(i); along the fiber axis taken from Fig. 4(j), taken from (c) the left side of the core and (d) the right side of the core; and (e) comparison between the expected and measured Talbot profiles from Fig. 4(j).
Vol. 55, No. 4 / February 1 2016 / Applied Optics
789
A calculated sinusoidal function with Z T 4.56 μm is also shown, and the observed value (4.6 0.5 μm) is in excellent agreement. 4. CONCLUSIONS DIC imaging was performed on plain optical fibers and various phase-mask-written FBGs, including Type I FBGs (samples A to D) and a Type In FBG fabricated in standard and smaller core fibers with the standard and custom-made phase masks. Analyses of various linescans of these FBG images, as summarized in Table 1, confirmed the expected RI structure within each FBG. Images recorded in the plane parallel to the direction of the UV writing beam had RI perturbations which were distributed uniformly across the core, while those recorded in the perpendicular plane all exhibited interleaved grating planes that were π phase shifted with respect to each other, with a period consistent with that of the phase mask. All measured Talbot lengths of the RI patterns extending across the core in the perpendicular images were consistent with those expected for beating between the dominant 1 and 2 orders of each mask. The imaged structures of the larger core FBGs written with the standard (A) and custom phase masks (C) revealed full periods of the Talbot diffraction patterns in the core. However, the imaged structures of the smaller core FBGs written with the standard (B) and custom (D) phase masks demonstrated how the smaller core prevents a full Talbot length from being formed across the core. FBGs written in this fiber required less integrated fluence to achieve saturation compared with standard fiber [20], owing to the reduction in UV-absorbing volume. While the RI variations in the FBGs made with the custom-made phase mask (C and D) appear stronger than those in the standard phase mask images (A and B), it is possible that this is due to the larger exposure fluences required to obtain high reflectances in the custom-made phase mask samples [20], and caused by the diminished 1 orders that produce the usual peak or dip at the Bragg wavelength, λB . Spectrally, the use of the custom-made phase mask enhanced features at 2λB ∕3, as expected, as this arises from other combinations of phase mask orders, for instance, the third harmonic of grating features having the periodicity of the phase mask, which arises from the combination of 0 and 1 orders [11,14]. This work forms part of a wider body of work that has investigated the relationship between fiber type (core diameter and RI profile), phase mask features (periodicity and relative strength of the diffraction orders), and the resulting spectral features in reflection at the Bragg wavelength and other grating harmonics. The DIC imaging technique clearly demonstrates how the complex Talbot field of a phase mask produces corresponding RI variations within a photosensitive fiber irrespective of the grating type or fiber core diameter, even if the phase mask is not optimized for FBG writing. Exploration of the relationship of FBG images and grating spectra is the subject of ongoing work, including the determination of actual RI variations within DIC images [17] and the understanding of double peaks (at certain harmonics of the Bragg wavelength) that arise from the Talbot planes not being perfectly aligned with the fiber core [22,30].
790
Research Article
Vol. 55, No. 4 / February 1 2016 / Applied Optics
Funding. Australian Research Council. Acknowledgment. C. M. Rollinson thanks Victoria University for a postgraduate scholarship.
17.
REFERENCES 1. J. Canning, “Fibre gratings and devices for sensors and lasers,” Laser Photon. Rev. 2, 275–289 (2008). 2. T. Erdogan, “Fiber grating spectra,” J. Lightwave Technol. 15, 1277– 1294 (1997). 3. P. A. Krug, R. Stolte, and R. Ulrich, “Measurement of index modulation along an optical fiber Bragg grating,” Opt. Lett. 20, 1767–1769 (1995). 4. E. Brinkmeyer, “Simple algorithm for reconstructing fiber gratings from reflectometric data,” Opt. Lett. 20, 810–812 (1995). 5. J. Canning, M. Janos, and M. G. Sceats, “Rayleigh longitudinal profiling of optical resonances within waveguide grating structures using sidescattered light,” Opt. Lett. 21, 609–611 (1996). 6. B. Malo, D. C. Johnson, F. Bilodeau, J. Albert, and K. O. Hill, “Singleexcimer-pulse writing of fiber gratings by use of a zero-order nulled phase mask: grating spectral response and visualization of index perturbations,” Opt. Lett. 18, 1277–1279 (1993). 7. J. D. Mills, C. W. J. Hillman, W. S. Brocklesby, and B. H. Blott, “Evanescent field imaging of an optical fiber Bragg grating,” Appl. Phys. Lett. 75, 4058–4060 (1999). 8. F. El-Diasty, A. Heaney, and T. Erdogan, “Analysis of fiber Bragg gratings by a side-diffraction interference technique,” Appl. Opt. 40, 890–896 (2001). 9. X. M. Goh, S. S. Kou, B. P. Kouskousis, N. M. Dragomir, S. F. Collins, G. W. Baxter, and A. Roberts, “Non-destructive three-dimensional optical imaging of a fiber Bragg grating,” IEEE Photon. J. 6, 6900207 (2014). 10. J. D. Mills, C. W. J. Hillman, B. H. Blott, and W. S. Brocklesby, “Imaging of free-space interference patterns used to manufacture fiber Bragg gratings,” Appl. Opt. 39, 6128–6135 (2000). 11. P. E. Dyer, R. J. Farley, and R. Giedl, “Analysis of grating formation with excimer laser irradiated phase masks,” Opt. Commun. 115, 327–334 (1995). 12. H. Talbot, “Facts relating to optical science,” London Edinburgh Philos. Mag. J. Sci. 9, 401–407 (1836). 13. Lord Rayleigh, “XXV. On copying diffraction-gratings, and on some phenomena connected therewith,” Philos. Mag. Ser. 5 11(67), 196–205 (1881). 14. B. P. Kouskousis, D. J. Kitcher, S. F. Collins, and G. W. Baxter, “Numerical investigation on the effects of fabrication conditions on fiber Bragg grating spectra using the phase mask technique,” Appl. Opt. 52, 3338–3344 (2013). 15. M. Pluta, Advanced Light Microscopy (Elsevier, 1989), Vol. 2. 16. C. Preza, S. King, N. Dragomir, and C. Cogswell, “Phase imaging microscopy: beyond dark-field, phase contrast, and differential
18.
19.
20.
21.
22.
23.
24. 25.
26. 27.
28. 29.
30.
interference contrast microscopy,” in Handbook of Biomedical Optics, D. Boas, C. Pitris, and N. Ramanujam, eds. (CRC Press, 2011), pp. 483–517. B. Kouskousis, D. J. Kitcher, S. Collins, A. Roberts, and G. W. Baxter, “Quantitative phase and refractive index analysis of optical fibers using differential interference contrast microscopy,” Appl. Opt. 47, 5182–5189 (2008). N. M. Dragomir, C. Rollinson, S. A. Wade, A. J. Stevenson, S. F. Collins, G. W. Baxter, P. M. Farrell, and A. Roberts, “Nondestructive imaging of a type I optical fiber Bragg grating,” Opt. Lett. 28, 789–791 (2003). B. P. Kouskousis, C. M. Rollinson, D. J. Kitcher, S. F. Collins, G. W. Baxter, S. A. Wade, N. M. Dragomir, and A. Roberts, “Quantitative investigation of the refractive-index modulation within the core of a fiber Bragg grating,” Opt. Express 14, 10332–10338 (2006). C. M. Rollinson, S. A. Wade, B. P. Kouskousis, D. J. Kitcher, G. W. Baxter, and S. F. Collins, “Variations of the growth of harmonic reflections in fiber Bragg gratings fabricated using phase masks,” J. Opt. Soc. Am. A 29, 1259–1268 (2012). C. M. Rollinson, S. A. Wade, N. M. Dragomir, G. W. Baxter, S. F. Collins, and A. Roberts, “Reflections near 1030 nm from 1540 nm fibre Bragg gratings: evidence of a complex refractive index structure,” Opt. Commun. 256, 310–318 (2005). S. A. Wade, W. G. A. Brown, H. K. Bal, F. Sidiroglou, G. W. Baxter, and S. F. Collins, “Effect of phase mask alignment on FBG spectra at harmonics of the Bragg wavelength,” J. Opt. Soc. Am. A 29, 1597–1605 (2012). C. W. Smelser, S. J. Mihailov, D. Grobnic, P. Lu, R. B. Walker, H. Ding, and X. Dai, “Multiple-beam interference patterns in optical fiber generated with ultrafast pulses and a phase mask,” Opt. Lett. 29, 1458–1460 (2004). B. P. Kouskousis, “Microscopic characterisation of fibre Bragg gratings,” Ph.D. dissertation (Victoria University, 2009). W. W. Morey, G. Meltz, J. D. Love, and S. J. Hewlett, “Mode-coupling characteristics of UV-written Bragg gratings in depressed-cladding fiber,” Electron. Lett. 30, 730–732 (1994). R. Kashyap, Fiber Bragg Gratings, Optics and Photonics (Academic, 1999). L. Dong, L. Reekie, J. L. Cruz, J. E. Caplen, J. P. deSandro, and D. N. Payne, “Optical fibers with depressed claddings for suppression of coupling into cladding modes in fiber Bragg gratings,” IEEE Photon. Technol. Lett. 9, 64–66 (1997). H. O. Pierson, Handbook of Chemical Vapor Deposition: Principles, Technology, and Applications, 2nd ed. (Noyes, 1999), p. 420. C. M. Rollinson, “Microscopic and spectral characterisation of optical fibre Bragg gratings,” Ph.D. dissertation (Victoria University, 2012). K. Tarnowski and W. Urbanczyk, “Origin of Bragg reflection peaks splitting in gratings fabricated using a multiple order phase mask,” Opt. Express 21, 21800–21810 (2013).
