November 1, 2014 / Vol. 39, No. 21 / OPTICS LETTERS
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Ultra low-loss hypocycloid-core Kagome hollow-core photonic crystal fiber for green spectral-range applications B. Debord,1 M. Alharbi,1 A. Benoît,1 D. Ghosh,1 M. Dontabactouny,1 L. Vincetti,2 J.-M. Blondy,1 F. Gérôme,1 and F. Benabid1,* 1 2
GPPMM group, Xlim Research Institute, UMR CNRS 7252, Université de Limoges, France
Department of Engineering “Enzo Ferrari,” University of Modena and Reggio Emilia, I-41125 Modena, Italy *Corresponding author: [email protected] Received September 10, 2014; revised October 2, 2014; accepted October 4, 2014; posted October 6, 2014 (Doc. ID 222557); published October 23, 2014
We report on the development of a hypocycloidal-core Kagome hollow-core photonic crystal fiber guiding, with low transmission loss in the 450–650 nm visible spectral range. Transmission loss records have been achieved with 70 dB∕km at 600 nm, and 130 dB∕km at 532 nm. As a demonstration of the fiber potential applications, we report on a compact 600 THz wide Raman comb generator, centered around 532 nm, and on a 10 W average power frequency-doubled Yb-fiber picosecond laser beam delivery, along with its use for organic material laser microprocessing. © 2014 Optical Society of America OCIS codes: (060.2280) Fiber design and fabrication; (060.5295) Photonic crystal fibers; (060.2270) Fiber characterization. http://dx.doi.org/10.1364/OL.39.006245
The seminal introduction in 2010, of a hypocycloid corecontour (i.e., negative curvature) inhibited coupling (IC) guiding hollow-core photonic crystal fiber (HC-PCF) [1,2], has led to a dramatic transmission loss reduction, as illustrated in the record loss figure of 70 dB∕km around 780 nm [3], and 17 dB∕km in the vicinity of the 1 μm wavelength range [4,5]. These results stem from the IC guidance mechanism model [6], which predicts that negative curvature core-contour enhances the reduction of the transverse mismatch between the fundamental core mode and the cladding continuum modes, via a strong drop in the overlap integral between the two types of modes [4,5]. Furthermore, these results set IC guiding HC-PCF as a superior alternative to photonic bandgap (PBG) guiding HC-PCF, especially for wavelengths shorter than 1 μm. Of particular timely importance is the low-loss optical guidance in the spectral range around 532 nm, coined in this Letter as green spectral range (GSR). Indeed, the advent of photonic structures that guide high-power lasers with minimum power attenuation, and in a flexible fashion over a long distance in the GSR, are of interest for a number of growing applications. For example, in photovoltaic solar micro-processing, lasers emitting in the GSR are now favorably used in the solar cell fabrication process chain, including marking, scribing, or cutting [7]. Also, powerful lasers emitting in the 400–600 nm range are very sought-after in bioinstrumentation for cytometry, DNA sequencing, or confocal microscopy. Beam transport of these lasers with an optical fiber, instead of the current free-space solutions, would add an obvious benefit in terms of easy use, precision, and cost. Finally, it can be of interest to capitalize on the continuing progress in frequency-doubled solid-state laser development (e.g., Nd:Yag and Yb-fiber lasers). To date, the reported HC-PCF transmission performance in the GSR remains too high, in both IC and PBG HC-PCF. The best-reported loss figure in PBG 0146-9592/14/216245-04$15.00/0
HC-PCF guiding, in this visible wavelength range, is 0.87 dB∕m at 557 nm [8]. This is limited by the combination of a large core-cladding mode optical overlap (>1%), and by surface roughness at the silica core-surround [9]. This is aggravated by the difficulties in the fabrication process of the required small pitch size, required to open a bandgap at short wavelengths. Equally, and despite offering a much wider optical bandwidth, the IC HC-PCF transmission loss in the visible wavelength range remains comparable to the PBG HC-PCF. For example, the loss figures in conventional Kagome HC-PCF, with either a hexagonal-shaped single-cell core or with a circular seven-cell core, are typically between 1 dB∕m and 500 dB∕km [10,11]. Conversely, the record transmission performance of a hypocycloid core IC guiding Kagome-lattice HC-PCF was so far limited to wavelengths larger than 780 nm [2–4]. This is mainly due to difficulties in drawing a HC-PCF combining a core-contour with a sufficiently strong negative curvature, and very thin silica struts (thickness t ≤ 350 nm). In this Letter, we report on fabrication and characterization of a new hypocycloid core Kagome HC-PCF, developed for high transmission in the visible domain. An ultra low-loss guidance is obtained over a broad range, between 450 and 650 nm. A record of transmission loss is demonstrated at 600 nm with 70 dB∕km. Also, record values are obtained at 532 and 515 nm, with 130 and 150 dB∕km, respectively. As a demonstration of its potential application for both bioinstrumentation and high power delivery, the fiber was used for generation of a wide Raman comb, extending from 370 to 1150 nm, pumped by a hand-held frequency-doubled Nd:Yag pulsed microchip laser, laser beam delivery of a frequency-doubled 10 W Yb-fiber picosecond laser, and its use for organic solar cell scribing. The scanning electronic micrograph (SEM) of the fabricated fiber is shown in Fig. 1. The fiber was fabricated using the well-known stack-and-draw technique, and is © 2014 Optical Society of America
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Fig. 1. (a) SEM of the fabricated Kagome HC-PCF; (b) calculated loss spectrum (the insets show the cross section of the simulated structure, and the intensity profile of the fundamental core-mode HE11 at 550 nm); (c) calculated spectrum of the effective index of the fundamental mode (solid curve, including two zooms), and mode field diameter (MFD, dashed curve).
based on a seven-cell core defect. Its design and dimensions have been chosen to target transmission on the emission wavelengths of frequency-doubled Nd:Yag and Yb-fiber lasers (i.e., 532 and 515 nm, respectively). The outer diameter was set to 125 μm, to match that of conventional optical fibers. The fiber core exhibits a hypocycloid contour, with an arc curvature b equal to 0.3 (see the definition of the arc curvature in [4]). The inner and outer diameters were 21 and 32 μm, respectively [see a zoom of the core on the left-hand side of Fig. 1(a)]. The core contour inward arcs present very thin silica struts of 140 nm. This corresponds to the first transmission band cut-off wavelength λ1 of 295 nm, which is set by the resonance condition between the core mode and theqglass thickness given by the expression
λm 1∕m2t n2gl − 1, where ngl is the refractive index of the glass forming the cladding structure, and t is the thickness of the glass web. In this Letter, m is transmission band order. Unlike previous hypocycloid core Kagome HC-PCFs [1–5], it is noteworthy that the outward
arcs are significantly thicker, with a 350 nm thickness value. Its effect on the fiber transmission properties are detailed in a subsequent paragraph. Finally, the cladding structure is formed by three concentric rings, with a pitch of approximately 10 μm. Figure 1(b) summarizes the results of the numerical mode solving of the fiber. The calculations have been carried out using the modal solver of the commercial software Comsol Multiphysics 3.5, based on the finite-element method, with an optimized anisotropic perfectly matched layer (PML) [12]. The cross section of the fiber that has been simulated is presented in the first inset of Fig. 1(b), and is extracted from the fabricated fiber SEM. Figure 1(b) shows the calculated loss spectrum of the fabricated Kagome HC-PCF, and the intensity profile of the fundamental core-mode HE11 at 550 nm (inset). One can identify the fundamental transmission band set by the inward arcs, and which corresponds to wavelengths longer than ∼300 nm, thus covering the targeted GSR. Within this band, the confinement loss (CL) was in the range of 20–40 dB∕km, for wavelengths between 400 and 600 nm. For wavelengths shorter than 400 nm, the CL increases due to the resonance with the inward arc glass, and form a wide spectral region (270–400 nm) in which the loss peaks to 10 dB∕m. For wavelengths shorter than 270 nm, the second-order IC guiding band starts. Furthermore, the CL spectrum exhibits a typical spectral structure of IC guiding HC-PCFs, with several peaks and dips, due to residual coupling with the different structural features of the cladding [6]. However, in contrast with conventional Kagome HC-PCF [1–5], we observe that the inward arcs-related fundamental band shows higher CL values, in the 720–730 nm wavelength-range, and which reaches a maximum of 4 dB∕m. For longer wavelengths than 720–730 nm, the loss figure presents a minimum averaged value of 300 dB∕km. The relatively moderated increase of the loss in 720–730 nm, compared to the strongest high-loss region caused by the resonance of the inward-arcs glass, is due to the resonance with the glass of the thicker outer arcs and the cladding struts. The effect of the glass thickness of the inward cusps and the rest of the structure are also manifested in the effective index spectrum. Figure 1(c) presents the calculated effective index of the fundamental core-mode HE11 . The evolution of the MFD is also superimposed on the same graph, with a value varying from 15 to 14 μm. Two zooms on the effective index curve, around 250–400 nm (inset 1) and 680– 750 nm (inset 2) spectral ranges, corresponding to glass resonances are shown at the bottom of Fig 1(c). A strong coupling is observed around 300 μm, due to resonance between the core mode and the 140 nm thick corecontour “inward” glass struts. This resonance affects both the effective index and the guided-mode MFD. On the other hand, the resonance of the “outward” strut glass, and that of the rest of the cladding, has a minor effect on both dispersion and the MFD. This is manifested in the effective index curve, by the presence of moderate oscillations in the wavelength range of 700– 720 nm. This behavior is due to a weaker optical overlap between the guided fundamental core-mode, which MFD is set by inward arcs [4], and the modes residing in outward arcs (and to a lesser extent with the cladding struts).
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Fig. 3. Measured output spectrum with OSA (top), and a picture of the dispersed output spectrum of the generated Raman comb (bottom).
Fig. 2. (a) Transmission spectrum through 3.5 m (red curve) and 30 m (blue curve) long piece of fiber. Both spectral traces were normalized relative to the white light source spectrum. Inset, near field intensity distribution at 600 nm wavelength; (b) measured loss spectrum of the fiber.
Figure 2(a) shows the fiber-transmitted spectrum over a length of 3.5 m (red curve), and of a 30 m piece (blue curve). The transmission spectra were both normalized to the source spectrum. The loss spectrum was measured using a supercontinuum light source, and cut-back of a 30 m long fiber to 3.5 m [Fig. 2(b)]. The different measured spectra reproduce qualitatively the calculated confinement loss and its spectral structure, including that around 720 nm. As shown by the simulations mentioned previously in this Letter, a region with higher loss is measured between 650 and 800 nm, caused by the glass resonance of the outward arcs. The inset in Fig. 2(a) presents a typical reconstructed near field of the laser beam profile, at the output of a 3.5 m long fiber, measured in the vicinity of 600 nm wavelength. Ultra-low attenuation is highlighted in the spectral range between 450 and 650 nm, with transmission loss record of 130 and 150 dB∕km, at 532 and 515 nm, respectively. A minimum value is obtained at 600 nm with 70 dB∕km. The relatively higher measured loss compared to the calculated one is due to the fact that the model does not take into account the scattering loss that results from the frozen capillary wave effect, and the surface roughness at the core silica surround during the drawing process [9]. Furthermore, and as expected, the loss spectrum shows that outside the high loss region due to the inner arc glass, the low loss figure are much lower, with an average of around 300 dB∕km. To demonstrate the potential of such an ultra-low loss visible-guiding Kagome HC-PCF, in both biophotonics and laser material micro-processing, several demonstrations have been carried out. First, we used the fiber to generate a Raman comb in the visible range, to demonstrate its relevance to cytometry, DNA sequencing, or
confocal microscopy applications [13]. A piece of 3 m length fiber has been filled with H 2 , at a 25 bar gas pressure. It was pumped using a 532 nm diode-pumped solidstate laser, emitting pulses with 3.45 ns duration, 10 kHz repetition rate, and 160 mW of maximum average power. Figure 3 shows the generated ro-vibrational Raman comb, as recorded with an optical spectrum analyzer (OSA). The generated comb spans over more than 600 THz, from 370 to 1150 nm, thus covering the whole visible range, and extends in the UV to the OSA detection limit of 350 nm. The spectrum consists of the pump line, two vibrational Stokes (V.S) and anti-Stokes (V.AS), with ∼125 THz spectral spacing. Each of these lines exhibits rotational Raman sidebands, with ∼17 THz spacing. Within this limited OSA short wavelength range, one can readily observe a total of 30 lines. The photography of the dispersed output spectrum of the Raman comb is shown below the measured spectrum. Another timely application is beam delivery of a highpower green laser for micro-processing. We used a 27 ps pulsed laser emitting at 515 nm, with a repetition rate of 30 MHz and a maximum average power of 25 W, to transport its beam through a 2 m long piece of the fabricated Kagome HC-PCF. Figure 4 shows the evolution of the fiber transmitted power in accordance with input power. A constant transmission coefficient of 60% was obtained, for input powers varying from 1 to 10 W. This transmission figure is limited by the mode field mismatch between the beam size and the fiber core, which resulted from a lack of suitable available lenses. The beam guidance occurs in a near-Gaussian fashion, as illustrated by the farfield distribution observed at 1.5 m from the fiber end (see the inset of Fig. 4). Note that no damage on the fiber has been observed. Finally, the fiber delivered a 515 nm laser beam, which has been used to realize precise micromachining on an organic solar cell, as illustrated by the picture on Fig. 4(b) with a logo scribing at the top. In conclusion, we fabricated a hypocycloidal Kagome HC-PCF, offering a low-loss transmission window in the
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OPTICS LETTERS / Vol. 39, No. 21 / November 1, 2014
Limousin”. A. Benoit acknowledges financial support from Eolite Systems.
Fig. 4. (a) Output power versus input power of a 515 nm ps laser beam along a 2 m long piece of the fabricated fiber (black curve). The coefficient of transmission is the blue curve. Inset, observed far field at 1.5 m from the fiber end; (b) HC-PCF delivered laser beam micro-scribing on an organic solar cell.
visible range. A loss record value at 532 nm has been demonstrated, with 130 dB∕km combined with a minimum transmission loss of 70 dB∕km at 600 nm. This fiber opens the way for several green spectral range applications. As a proof, an efficient 30 multiline Raman comb has been generated, over a bandwidth larger than 600 THz. The guidance of 10 W 27 ps pulses at 515 nm has been successfully demonstrated. Finally, scribing on an organic solar cell with the fiber-delivered beam was performed. This Letter was funded by “Agence Nationale de la Recherche (ANR)” (grants PHOTOSYNTH and Σ_LIM Labex Chaire, Astrid UV-Factor), and by “la région
References 1. Y. Wang, F. Couny, P. J. Roberts, and F. Benabid, in Conference on Lasers and Electro-Optics (2010), paper CPDB4. 2. Y. Y. Wang, N. V. Wheeler, F. Couny, P. J. Roberts, and F. Benabid, Opt. Lett. 36, 669 (2011). 3. T. D. Bradley, Y. Wang, M. Alharbi, B. Debord, C. FourcadeDutin, B. Beaudou, F. Gerome, and F. Benabid, J. Lightwave Technol. 31, 2752 (2013). 4. B. Debord, M. Alharbi, T. Bradley, C. Fourcade-Dutin, Y. Y. Wang, L. Vincetti, F. Gérôme, and F. Benabid, Opt. Express 21, 28597 (2013). 5. M. Alharbi, T. Bradley, B. Debord, C. Fourcade-Dutin, D. Ghosh, L. Vincetti, F. Gérôme, and F. Benabid, Opt. Express 21, 28609 (2013). 6. F. Couny, F. Benabid, P. J. Roberts, P. S. Light, and M. G. Raymer, Science 318, 1118 (2007). 7. A. Lemke, D. Ashkenasi, and H. J. Eichler, Phys. Procedia 41, 769 (2013). 8. P. J. Mosley, W. C. Huang, M. G. Welch, B. J. Mangan, W. J. Wadsworth, and J. C. Knight, Opt. Lett. 35, 3589 (2010). 9. P. Roberts, F. Couny, H. Sabert, B. Mangan, D. Williams, L. Farr, M. Mason, T. Birks, J. Knight, and P. St. J. Russell, Opt. Express 13, 236 (2005). 10. F. Couny, F. Benabid, and P. S. Light, Opt. Lett. 31, 3574 (2006). 11. F. Benabid and P. J. Roberts, J. Mod. Opt. 58, 87 (2011). 12. S. Selleri, L. Vincetti, A. Cucinotta, and M. Zoboli, Opt. Quantum Electron. 33, 359 (2001). 13. Y. Y. Wang, F. Couny, P. S. Light, B. J. Mangan, and F. Benabid, Opt. Lett. 35, 1127 (2010).
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Ultra low-loss hypocycloid-core Kagome hollow-core photonic crystal fiber for green spectral-range applications B. Debord,1 M. Alharbi,1 A. Benoît,1 D. Ghosh,1 M. Dontabactouny,1 L. Vincetti,2 J.-M. Blondy,1 F. Gérôme,1 and F. Benabid1,* 1 2
GPPMM group, Xlim Research Institute, UMR CNRS 7252, Université de Limoges, France
Department of Engineering “Enzo Ferrari,” University of Modena and Reggio Emilia, I-41125 Modena, Italy *Corresponding author: [email protected] Received September 10, 2014; revised October 2, 2014; accepted October 4, 2014; posted October 6, 2014 (Doc. ID 222557); published October 23, 2014
We report on the development of a hypocycloidal-core Kagome hollow-core photonic crystal fiber guiding, with low transmission loss in the 450–650 nm visible spectral range. Transmission loss records have been achieved with 70 dB∕km at 600 nm, and 130 dB∕km at 532 nm. As a demonstration of the fiber potential applications, we report on a compact 600 THz wide Raman comb generator, centered around 532 nm, and on a 10 W average power frequency-doubled Yb-fiber picosecond laser beam delivery, along with its use for organic material laser microprocessing. © 2014 Optical Society of America OCIS codes: (060.2280) Fiber design and fabrication; (060.5295) Photonic crystal fibers; (060.2270) Fiber characterization. http://dx.doi.org/10.1364/OL.39.006245
The seminal introduction in 2010, of a hypocycloid corecontour (i.e., negative curvature) inhibited coupling (IC) guiding hollow-core photonic crystal fiber (HC-PCF) [1,2], has led to a dramatic transmission loss reduction, as illustrated in the record loss figure of 70 dB∕km around 780 nm [3], and 17 dB∕km in the vicinity of the 1 μm wavelength range [4,5]. These results stem from the IC guidance mechanism model [6], which predicts that negative curvature core-contour enhances the reduction of the transverse mismatch between the fundamental core mode and the cladding continuum modes, via a strong drop in the overlap integral between the two types of modes [4,5]. Furthermore, these results set IC guiding HC-PCF as a superior alternative to photonic bandgap (PBG) guiding HC-PCF, especially for wavelengths shorter than 1 μm. Of particular timely importance is the low-loss optical guidance in the spectral range around 532 nm, coined in this Letter as green spectral range (GSR). Indeed, the advent of photonic structures that guide high-power lasers with minimum power attenuation, and in a flexible fashion over a long distance in the GSR, are of interest for a number of growing applications. For example, in photovoltaic solar micro-processing, lasers emitting in the GSR are now favorably used in the solar cell fabrication process chain, including marking, scribing, or cutting [7]. Also, powerful lasers emitting in the 400–600 nm range are very sought-after in bioinstrumentation for cytometry, DNA sequencing, or confocal microscopy. Beam transport of these lasers with an optical fiber, instead of the current free-space solutions, would add an obvious benefit in terms of easy use, precision, and cost. Finally, it can be of interest to capitalize on the continuing progress in frequency-doubled solid-state laser development (e.g., Nd:Yag and Yb-fiber lasers). To date, the reported HC-PCF transmission performance in the GSR remains too high, in both IC and PBG HC-PCF. The best-reported loss figure in PBG 0146-9592/14/216245-04$15.00/0
HC-PCF guiding, in this visible wavelength range, is 0.87 dB∕m at 557 nm [8]. This is limited by the combination of a large core-cladding mode optical overlap (>1%), and by surface roughness at the silica core-surround [9]. This is aggravated by the difficulties in the fabrication process of the required small pitch size, required to open a bandgap at short wavelengths. Equally, and despite offering a much wider optical bandwidth, the IC HC-PCF transmission loss in the visible wavelength range remains comparable to the PBG HC-PCF. For example, the loss figures in conventional Kagome HC-PCF, with either a hexagonal-shaped single-cell core or with a circular seven-cell core, are typically between 1 dB∕m and 500 dB∕km [10,11]. Conversely, the record transmission performance of a hypocycloid core IC guiding Kagome-lattice HC-PCF was so far limited to wavelengths larger than 780 nm [2–4]. This is mainly due to difficulties in drawing a HC-PCF combining a core-contour with a sufficiently strong negative curvature, and very thin silica struts (thickness t ≤ 350 nm). In this Letter, we report on fabrication and characterization of a new hypocycloid core Kagome HC-PCF, developed for high transmission in the visible domain. An ultra low-loss guidance is obtained over a broad range, between 450 and 650 nm. A record of transmission loss is demonstrated at 600 nm with 70 dB∕km. Also, record values are obtained at 532 and 515 nm, with 130 and 150 dB∕km, respectively. As a demonstration of its potential application for both bioinstrumentation and high power delivery, the fiber was used for generation of a wide Raman comb, extending from 370 to 1150 nm, pumped by a hand-held frequency-doubled Nd:Yag pulsed microchip laser, laser beam delivery of a frequency-doubled 10 W Yb-fiber picosecond laser, and its use for organic solar cell scribing. The scanning electronic micrograph (SEM) of the fabricated fiber is shown in Fig. 1. The fiber was fabricated using the well-known stack-and-draw technique, and is © 2014 Optical Society of America
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OPTICS LETTERS / Vol. 39, No. 21 / November 1, 2014
Fig. 1. (a) SEM of the fabricated Kagome HC-PCF; (b) calculated loss spectrum (the insets show the cross section of the simulated structure, and the intensity profile of the fundamental core-mode HE11 at 550 nm); (c) calculated spectrum of the effective index of the fundamental mode (solid curve, including two zooms), and mode field diameter (MFD, dashed curve).
based on a seven-cell core defect. Its design and dimensions have been chosen to target transmission on the emission wavelengths of frequency-doubled Nd:Yag and Yb-fiber lasers (i.e., 532 and 515 nm, respectively). The outer diameter was set to 125 μm, to match that of conventional optical fibers. The fiber core exhibits a hypocycloid contour, with an arc curvature b equal to 0.3 (see the definition of the arc curvature in [4]). The inner and outer diameters were 21 and 32 μm, respectively [see a zoom of the core on the left-hand side of Fig. 1(a)]. The core contour inward arcs present very thin silica struts of 140 nm. This corresponds to the first transmission band cut-off wavelength λ1 of 295 nm, which is set by the resonance condition between the core mode and theqglass thickness given by the expression
λm 1∕m2t n2gl − 1, where ngl is the refractive index of the glass forming the cladding structure, and t is the thickness of the glass web. In this Letter, m is transmission band order. Unlike previous hypocycloid core Kagome HC-PCFs [1–5], it is noteworthy that the outward
arcs are significantly thicker, with a 350 nm thickness value. Its effect on the fiber transmission properties are detailed in a subsequent paragraph. Finally, the cladding structure is formed by three concentric rings, with a pitch of approximately 10 μm. Figure 1(b) summarizes the results of the numerical mode solving of the fiber. The calculations have been carried out using the modal solver of the commercial software Comsol Multiphysics 3.5, based on the finite-element method, with an optimized anisotropic perfectly matched layer (PML) [12]. The cross section of the fiber that has been simulated is presented in the first inset of Fig. 1(b), and is extracted from the fabricated fiber SEM. Figure 1(b) shows the calculated loss spectrum of the fabricated Kagome HC-PCF, and the intensity profile of the fundamental core-mode HE11 at 550 nm (inset). One can identify the fundamental transmission band set by the inward arcs, and which corresponds to wavelengths longer than ∼300 nm, thus covering the targeted GSR. Within this band, the confinement loss (CL) was in the range of 20–40 dB∕km, for wavelengths between 400 and 600 nm. For wavelengths shorter than 400 nm, the CL increases due to the resonance with the inward arc glass, and form a wide spectral region (270–400 nm) in which the loss peaks to 10 dB∕m. For wavelengths shorter than 270 nm, the second-order IC guiding band starts. Furthermore, the CL spectrum exhibits a typical spectral structure of IC guiding HC-PCFs, with several peaks and dips, due to residual coupling with the different structural features of the cladding [6]. However, in contrast with conventional Kagome HC-PCF [1–5], we observe that the inward arcs-related fundamental band shows higher CL values, in the 720–730 nm wavelength-range, and which reaches a maximum of 4 dB∕m. For longer wavelengths than 720–730 nm, the loss figure presents a minimum averaged value of 300 dB∕km. The relatively moderated increase of the loss in 720–730 nm, compared to the strongest high-loss region caused by the resonance of the inward-arcs glass, is due to the resonance with the glass of the thicker outer arcs and the cladding struts. The effect of the glass thickness of the inward cusps and the rest of the structure are also manifested in the effective index spectrum. Figure 1(c) presents the calculated effective index of the fundamental core-mode HE11 . The evolution of the MFD is also superimposed on the same graph, with a value varying from 15 to 14 μm. Two zooms on the effective index curve, around 250–400 nm (inset 1) and 680– 750 nm (inset 2) spectral ranges, corresponding to glass resonances are shown at the bottom of Fig 1(c). A strong coupling is observed around 300 μm, due to resonance between the core mode and the 140 nm thick corecontour “inward” glass struts. This resonance affects both the effective index and the guided-mode MFD. On the other hand, the resonance of the “outward” strut glass, and that of the rest of the cladding, has a minor effect on both dispersion and the MFD. This is manifested in the effective index curve, by the presence of moderate oscillations in the wavelength range of 700– 720 nm. This behavior is due to a weaker optical overlap between the guided fundamental core-mode, which MFD is set by inward arcs [4], and the modes residing in outward arcs (and to a lesser extent with the cladding struts).
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Fig. 3. Measured output spectrum with OSA (top), and a picture of the dispersed output spectrum of the generated Raman comb (bottom).
Fig. 2. (a) Transmission spectrum through 3.5 m (red curve) and 30 m (blue curve) long piece of fiber. Both spectral traces were normalized relative to the white light source spectrum. Inset, near field intensity distribution at 600 nm wavelength; (b) measured loss spectrum of the fiber.
Figure 2(a) shows the fiber-transmitted spectrum over a length of 3.5 m (red curve), and of a 30 m piece (blue curve). The transmission spectra were both normalized to the source spectrum. The loss spectrum was measured using a supercontinuum light source, and cut-back of a 30 m long fiber to 3.5 m [Fig. 2(b)]. The different measured spectra reproduce qualitatively the calculated confinement loss and its spectral structure, including that around 720 nm. As shown by the simulations mentioned previously in this Letter, a region with higher loss is measured between 650 and 800 nm, caused by the glass resonance of the outward arcs. The inset in Fig. 2(a) presents a typical reconstructed near field of the laser beam profile, at the output of a 3.5 m long fiber, measured in the vicinity of 600 nm wavelength. Ultra-low attenuation is highlighted in the spectral range between 450 and 650 nm, with transmission loss record of 130 and 150 dB∕km, at 532 and 515 nm, respectively. A minimum value is obtained at 600 nm with 70 dB∕km. The relatively higher measured loss compared to the calculated one is due to the fact that the model does not take into account the scattering loss that results from the frozen capillary wave effect, and the surface roughness at the core silica surround during the drawing process [9]. Furthermore, and as expected, the loss spectrum shows that outside the high loss region due to the inner arc glass, the low loss figure are much lower, with an average of around 300 dB∕km. To demonstrate the potential of such an ultra-low loss visible-guiding Kagome HC-PCF, in both biophotonics and laser material micro-processing, several demonstrations have been carried out. First, we used the fiber to generate a Raman comb in the visible range, to demonstrate its relevance to cytometry, DNA sequencing, or
confocal microscopy applications [13]. A piece of 3 m length fiber has been filled with H 2 , at a 25 bar gas pressure. It was pumped using a 532 nm diode-pumped solidstate laser, emitting pulses with 3.45 ns duration, 10 kHz repetition rate, and 160 mW of maximum average power. Figure 3 shows the generated ro-vibrational Raman comb, as recorded with an optical spectrum analyzer (OSA). The generated comb spans over more than 600 THz, from 370 to 1150 nm, thus covering the whole visible range, and extends in the UV to the OSA detection limit of 350 nm. The spectrum consists of the pump line, two vibrational Stokes (V.S) and anti-Stokes (V.AS), with ∼125 THz spectral spacing. Each of these lines exhibits rotational Raman sidebands, with ∼17 THz spacing. Within this limited OSA short wavelength range, one can readily observe a total of 30 lines. The photography of the dispersed output spectrum of the Raman comb is shown below the measured spectrum. Another timely application is beam delivery of a highpower green laser for micro-processing. We used a 27 ps pulsed laser emitting at 515 nm, with a repetition rate of 30 MHz and a maximum average power of 25 W, to transport its beam through a 2 m long piece of the fabricated Kagome HC-PCF. Figure 4 shows the evolution of the fiber transmitted power in accordance with input power. A constant transmission coefficient of 60% was obtained, for input powers varying from 1 to 10 W. This transmission figure is limited by the mode field mismatch between the beam size and the fiber core, which resulted from a lack of suitable available lenses. The beam guidance occurs in a near-Gaussian fashion, as illustrated by the farfield distribution observed at 1.5 m from the fiber end (see the inset of Fig. 4). Note that no damage on the fiber has been observed. Finally, the fiber delivered a 515 nm laser beam, which has been used to realize precise micromachining on an organic solar cell, as illustrated by the picture on Fig. 4(b) with a logo scribing at the top. In conclusion, we fabricated a hypocycloidal Kagome HC-PCF, offering a low-loss transmission window in the
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OPTICS LETTERS / Vol. 39, No. 21 / November 1, 2014
Limousin”. A. Benoit acknowledges financial support from Eolite Systems.
Fig. 4. (a) Output power versus input power of a 515 nm ps laser beam along a 2 m long piece of the fabricated fiber (black curve). The coefficient of transmission is the blue curve. Inset, observed far field at 1.5 m from the fiber end; (b) HC-PCF delivered laser beam micro-scribing on an organic solar cell.
visible range. A loss record value at 532 nm has been demonstrated, with 130 dB∕km combined with a minimum transmission loss of 70 dB∕km at 600 nm. This fiber opens the way for several green spectral range applications. As a proof, an efficient 30 multiline Raman comb has been generated, over a bandwidth larger than 600 THz. The guidance of 10 W 27 ps pulses at 515 nm has been successfully demonstrated. Finally, scribing on an organic solar cell with the fiber-delivered beam was performed. This Letter was funded by “Agence Nationale de la Recherche (ANR)” (grants PHOTOSYNTH and Σ_LIM Labex Chaire, Astrid UV-Factor), and by “la région
References 1. Y. Wang, F. Couny, P. J. Roberts, and F. Benabid, in Conference on Lasers and Electro-Optics (2010), paper CPDB4. 2. Y. Y. Wang, N. V. Wheeler, F. Couny, P. J. Roberts, and F. Benabid, Opt. Lett. 36, 669 (2011). 3. T. D. Bradley, Y. Wang, M. Alharbi, B. Debord, C. FourcadeDutin, B. Beaudou, F. Gerome, and F. Benabid, J. Lightwave Technol. 31, 2752 (2013). 4. B. Debord, M. Alharbi, T. Bradley, C. Fourcade-Dutin, Y. Y. Wang, L. Vincetti, F. Gérôme, and F. Benabid, Opt. Express 21, 28597 (2013). 5. M. Alharbi, T. Bradley, B. Debord, C. Fourcade-Dutin, D. Ghosh, L. Vincetti, F. Gérôme, and F. Benabid, Opt. Express 21, 28609 (2013). 6. F. Couny, F. Benabid, P. J. Roberts, P. S. Light, and M. G. Raymer, Science 318, 1118 (2007). 7. A. Lemke, D. Ashkenasi, and H. J. Eichler, Phys. Procedia 41, 769 (2013). 8. P. J. Mosley, W. C. Huang, M. G. Welch, B. J. Mangan, W. J. Wadsworth, and J. C. Knight, Opt. Lett. 35, 3589 (2010). 9. P. Roberts, F. Couny, H. Sabert, B. Mangan, D. Williams, L. Farr, M. Mason, T. Birks, J. Knight, and P. St. J. Russell, Opt. Express 13, 236 (2005). 10. F. Couny, F. Benabid, and P. S. Light, Opt. Lett. 31, 3574 (2006). 11. F. Benabid and P. J. Roberts, J. Mod. Opt. 58, 87 (2011). 12. S. Selleri, L. Vincetti, A. Cucinotta, and M. Zoboli, Opt. Quantum Electron. 33, 359 (2001). 13. Y. Y. Wang, F. Couny, P. S. Light, B. J. Mangan, and F. Benabid, Opt. Lett. 35, 1127 (2010).
