February 15, 2015 / Vol. 40, No. 4 / OPTICS LETTERS
605
CW hollow-core optically pumped I2 fiber gas laser A. V. V. Nampoothiri,1,* B. Debord,2 M. Alharbi,2 F. Gérôme,2 F. Benabid,2 and W. Rudolph1 1
Department of Physics and Astronomy, University of New Mexico, Albuquerque, New Mexico 87131, USA 2
GPPMM group, Xlim Research Institute, UMR CNRS 7252, Université de Limoges, Limoges, France *Corresponding author: [email protected] Received December 8, 2014; accepted January 6, 2015; posted January 14, 2015 (Doc. ID 229269); published February 11, 2015
Continuous wave lasing of a hollow-core fiber gas laser (HOFGLAS) is achieved with molecular iodine in the 1280–1340 nm region when optically pumped at 532 nm. © 2015 Optical Society of America OCIS codes: (140.4130) Molecular gas lasers; (140.3510) Lasers, fiber; (060.5295) Photonic crystal fibers. http://dx.doi.org/10.1364/OL.40.000605
Optically pumped gas-filled hollow-core photonic-crystal fiber (HC-PCF) lasers [1] combine advantages of fiber lasers such as long interaction length between pump and active medium and high thresholds for detrimental nonlinear optical effects, such as stimulated Brillouin scattering and stimulated Raman scattering, compared to solid core fibers [1]. Because of the inherently narrow gain transitions in gases, long coherence length can be achieved without additional bandwidth-limiting elements. Such a hybrid Hollow-core Optical Fiber Gas LASer (HOFGLAS) based on population inversion was reported for the first time in 2010 [2] in the 3.1-μm region by optical pumping of C2 H2 [2,3] and HCN [4] with nanosecond pulses from an optical parametric oscillator. Because of the high gain, these pulsed lasers did not require an external cavity and a few tens of centimeters of gas-filled fiber were sufficient. While continuous wave (CW) Raman lasing in H2 -filled HC-PCFs was demonstrated with pump powers as low as 600 mW [5], HOFGLAS CW lasing based on population inversion remained elusive until now. The CW HOFGLAS to be reported here consists of inhibited-coupling (IC) guiding hypocycloid core-contour (i.e., negative curvature) kagome-structured HC-PCF [6] filled with molecular iodine, 127I2, and is pumped at 532 nm with a frequency-doubled Nd:Vanadate laser. The kagome HC-PCF [7] contrasts with the photonic bandgap (PBG)-guiding HC-PCF [8,9] in its intrinsic guidance mechanism and in its optical properties. It was first introduced in 2002 [7] and has the striking feature of guiding with relatively low loss over a large bandwidth. The guidance mechanism relies on the photonic analog of a bound or quasi-bound state in a continuum, and is coined IC optical guidance [10]. In PBG-guiding HC-PCFs, the core guided-mode is confined by the absence of phasematched cladding modes prohibiting light to be coupled out of the core. In contrast, in IC-guiding HC-PCFs, the required suppression of coupling between the core mode and the cladding is achieved by a cladding mode continuum whose transverse field components are strongly phase-mismatched with that of the core mode, and are strongly localized in the silica, thus minimizing the optical overlap integral of core and cladding modes. Cladding structures can be engineered to have IC guidance over a bandwidth larger than what is possible with PBG HCPCFs. In addition, for gas-phase photonic applications, IC HC-PCFs allow one to handle greater optical powers 0146-9592/15/040605-04$15.00/0
due to their larger core sizes and ultra-low optical overlap with the silica core-surround. These are the attributes that we exploited in developing acetylene HOFGLAS [2,3]. Absorption lines of molecular iodine in the visible spectral regions exhibit peak absorption cross-sections of the order of ∼10−16 cm2 [11]. The emission spectrum extends from ∼500 nm to 1.34 μm, distributed over 106 discrete lines depending on the excitation wavelength. Because of the abundance of narrow absorption lines in the visible to near-IR region, molecular iodine has been used for absolute frequency stabilization of lasers [12]. Lasing of I2 has been observed in conventional cells— pulsed [13] and CW [14–19]. The CW systems were pumped with Ar-ion lasers at 514.5 nm (lasing from 583 to 1338 nm [14] and from 570 to 1027 nm [15]), with Kr-ion lasers at 521 and 531 nm (laser emission between 540 and 650 nm [16]), with a frequency-doubled Nd:YAG laser at 532 nm (lasing between 780 and 1330 nm [17,18]), and by tunable dye lasers [19]. The latter work indicated the possibility of I2 laser emission on more than 104 transitions between 670 nm and 1.24 μm for pump wavelengths between 570 and 620 nm. The experimental layout for the cw optically pumped I2 fiber gas laser is shown in Fig. 1. A 20-cm-long HC-PCF was contained in a gas cell filled with approximately 300 mTorr of I2 (vapor pressure at room temperature). This fiber exhibits a hypocycloid contour core with an inner diameter of ∼85 μm, and negative curvature of b 0.4 (see Ref. [20] for the definition of b). Figure 2 shows the normalized transmission spectrum of the fiber. The fiber has been designed to provide acceptable loss at the pump wavelength of 532 nm and low-loss guidance at the targeted emission wavelength range of 1200–1350 nm. The ends of the gas cell were sealed with windows antireflection coated for both pump and lasing wavelengths. The output from a tunable (over ∼8 GHz around 532.1 nm), single-frequency, frequency-doubled CW Nd:YVO4 laser was expanded, collimated, and focused onto the input facet of the hollow-core fiber. The focal length and position of lens L1 was chosen for optimum coupling of the pump-free space TEM00 mode to the hollow fiber mode [21]. The frequency stability and singlefrequency operation of the pump laser were ensured by monitoring pump absorption through a 10-cm-long reference I2 cell (300 mTorr) and a scanning Fabry–Perot © 2015 Optical Society of America
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OPTICS LETTERS / Vol. 40, No. 4 / February 15, 2015
Fig. 1. Schematic diagram of a Hollow-core Optically pumped Fiber Gas LASer (HOFGLAS) with I2 as active medium. The laser is pumped by a tunable CW Nd:Vanadate laser (Coherent, Verdi) at 532.1 nm. Curved mirrors M1 and M2 form the fiber laser cavity. FI—Faraday isolator; L1, L2, and L3 are lenses used for focusing and collimation, D1, D2—photodiodes. Upper right corner: crosssection of the kagome hollow-core photonic-crystal fiber (HC-PCF) used.
interferometer, respectively. The fiber propagation loss in the 1.3-μm lasing region was previously measured to be ∼30 dB∕km using a cut-back technique on a 100-mlong section. Near our pump wavelength, the fiber transmission spectrum exhibits sharp resonances (see Fig. 2) due to the coupling between the core mode and the cladding struts (thickness t ≈ 490 nm), which occur at q
wavelengths λj 2t n2g − 1∕jj 1; 2; 3… [10]. It is noteworthy that despite this, the fiber still guides at these wavelengths but with much higher transmission loss. An upper figure of the loss at 532 nm was estimated to be about 42 dB/m by measuring the pump transmission coefficient before filling in the gas and considering a coupling efficiency of 100%. The small signal absorption of the pump in the I2-filled HC-PCF was more than 99.5%. An external resonator was built with two focusing mirrors (radius of curvature 15 cm) serving as end mirror (M1, high-reflecting at the expected laser wavelength 1.33 μm) and outcoupler (M2), respectively. Both mirrors
Fig. 2. Normalized (to source) transmission of the 3-m-long HC-PCF used in the experiment. Y axis has been scaled to reflect the previously measured loss ∼30 dB∕km in the fundamental transmission band. The lack of long enough fiber prevented us to resolve this low loss by performing cut-back measurements.
had high transmission at the pump wavelength. Outcoupling losses from 1% to 45% were used in the experiments. For a few outcoupling values, an antireflection coated lens of focal length 15 cm and a plane mirror of suitable reflectivity was used as a substitute to the curved mirror outcoupler. The initial alignment of the passive cavity was optimized by sending a diode probe laser (New Focus model 6248) through the fiber and minimizing the round trip losses. The latter were found to be as low as 0.5%. Figure 3 shows the output power from the I2 HOFGLAS as a function of outcoupling losses. The pump (power 0 140 mW) was tuned to the P(50) line of the B3 Π 0u v 1 00 32←X Σg v 0 absorption transition. The laser performance is near its optimum for a 15% loss (R 85%) outcoupler. Taking into account the passive losses from the cell windows, the total optimum outcoupling loss was estimated to about 22%. A pump threshold of 7 mW corresponding to a pump intensity of about 290 W∕cm2 was observed at optimum outcoupling. The laser spectrum for the R 85%-outcoupler and a pump power of 140 mW is shown in Fig. 4 and a simplified energy level diagram of I2 with the relevant
Fig. 3. Laser output power as a function of outcoupler reflectivity obtained for a pump power of 140 mW.
February 15, 2015 / Vol. 40, No. 4 / OPTICS LETTERS
Fig. 4. Spectrum of the CW HOFGLAS I2 laser output for the 85% reflectivity outcoupler. The pump power was 140 mW.
transitions is shown in Fig. 5. There are three lasing transitions in the 1.3-μm region; each transition is a doublet with corresponding R and P branches. All observed lasing lines originate from the terminal pumped state. From the known spectroscopic constants [22–24], the lines are identified as R(48) and P(50) transitions from the pumped (v0 32) level to v00 72, 71, and 69 levels. Each R and P transition of I2 exhibits 15 or 21 hyperfine lines for even or odd j00 transition [25]. No attempt was made to resolve the hyperfine structure. As can be seen in Fig. 4, the line at 1328.4 nm has the highest intensity. This is due to the more favorable Franck–Condon factor for the v0 32 → v00 72 transition compared to the other lasing transitions; see for example [24]. Figure 6 shows the output power as a function of the input power for different outcoupler reflectivities. Above the threshold, the laser output varies linearly with input pump power before it starts to level off. This is likely due to saturation of pump as well as reduction in gain due to dissociation from upper [11,13] and lower laser levels (v00 72) because of their close proximity (
605
CW hollow-core optically pumped I2 fiber gas laser A. V. V. Nampoothiri,1,* B. Debord,2 M. Alharbi,2 F. Gérôme,2 F. Benabid,2 and W. Rudolph1 1
Department of Physics and Astronomy, University of New Mexico, Albuquerque, New Mexico 87131, USA 2
GPPMM group, Xlim Research Institute, UMR CNRS 7252, Université de Limoges, Limoges, France *Corresponding author: [email protected] Received December 8, 2014; accepted January 6, 2015; posted January 14, 2015 (Doc. ID 229269); published February 11, 2015
Continuous wave lasing of a hollow-core fiber gas laser (HOFGLAS) is achieved with molecular iodine in the 1280–1340 nm region when optically pumped at 532 nm. © 2015 Optical Society of America OCIS codes: (140.4130) Molecular gas lasers; (140.3510) Lasers, fiber; (060.5295) Photonic crystal fibers. http://dx.doi.org/10.1364/OL.40.000605
Optically pumped gas-filled hollow-core photonic-crystal fiber (HC-PCF) lasers [1] combine advantages of fiber lasers such as long interaction length between pump and active medium and high thresholds for detrimental nonlinear optical effects, such as stimulated Brillouin scattering and stimulated Raman scattering, compared to solid core fibers [1]. Because of the inherently narrow gain transitions in gases, long coherence length can be achieved without additional bandwidth-limiting elements. Such a hybrid Hollow-core Optical Fiber Gas LASer (HOFGLAS) based on population inversion was reported for the first time in 2010 [2] in the 3.1-μm region by optical pumping of C2 H2 [2,3] and HCN [4] with nanosecond pulses from an optical parametric oscillator. Because of the high gain, these pulsed lasers did not require an external cavity and a few tens of centimeters of gas-filled fiber were sufficient. While continuous wave (CW) Raman lasing in H2 -filled HC-PCFs was demonstrated with pump powers as low as 600 mW [5], HOFGLAS CW lasing based on population inversion remained elusive until now. The CW HOFGLAS to be reported here consists of inhibited-coupling (IC) guiding hypocycloid core-contour (i.e., negative curvature) kagome-structured HC-PCF [6] filled with molecular iodine, 127I2, and is pumped at 532 nm with a frequency-doubled Nd:Vanadate laser. The kagome HC-PCF [7] contrasts with the photonic bandgap (PBG)-guiding HC-PCF [8,9] in its intrinsic guidance mechanism and in its optical properties. It was first introduced in 2002 [7] and has the striking feature of guiding with relatively low loss over a large bandwidth. The guidance mechanism relies on the photonic analog of a bound or quasi-bound state in a continuum, and is coined IC optical guidance [10]. In PBG-guiding HC-PCFs, the core guided-mode is confined by the absence of phasematched cladding modes prohibiting light to be coupled out of the core. In contrast, in IC-guiding HC-PCFs, the required suppression of coupling between the core mode and the cladding is achieved by a cladding mode continuum whose transverse field components are strongly phase-mismatched with that of the core mode, and are strongly localized in the silica, thus minimizing the optical overlap integral of core and cladding modes. Cladding structures can be engineered to have IC guidance over a bandwidth larger than what is possible with PBG HCPCFs. In addition, for gas-phase photonic applications, IC HC-PCFs allow one to handle greater optical powers 0146-9592/15/040605-04$15.00/0
due to their larger core sizes and ultra-low optical overlap with the silica core-surround. These are the attributes that we exploited in developing acetylene HOFGLAS [2,3]. Absorption lines of molecular iodine in the visible spectral regions exhibit peak absorption cross-sections of the order of ∼10−16 cm2 [11]. The emission spectrum extends from ∼500 nm to 1.34 μm, distributed over 106 discrete lines depending on the excitation wavelength. Because of the abundance of narrow absorption lines in the visible to near-IR region, molecular iodine has been used for absolute frequency stabilization of lasers [12]. Lasing of I2 has been observed in conventional cells— pulsed [13] and CW [14–19]. The CW systems were pumped with Ar-ion lasers at 514.5 nm (lasing from 583 to 1338 nm [14] and from 570 to 1027 nm [15]), with Kr-ion lasers at 521 and 531 nm (laser emission between 540 and 650 nm [16]), with a frequency-doubled Nd:YAG laser at 532 nm (lasing between 780 and 1330 nm [17,18]), and by tunable dye lasers [19]. The latter work indicated the possibility of I2 laser emission on more than 104 transitions between 670 nm and 1.24 μm for pump wavelengths between 570 and 620 nm. The experimental layout for the cw optically pumped I2 fiber gas laser is shown in Fig. 1. A 20-cm-long HC-PCF was contained in a gas cell filled with approximately 300 mTorr of I2 (vapor pressure at room temperature). This fiber exhibits a hypocycloid contour core with an inner diameter of ∼85 μm, and negative curvature of b 0.4 (see Ref. [20] for the definition of b). Figure 2 shows the normalized transmission spectrum of the fiber. The fiber has been designed to provide acceptable loss at the pump wavelength of 532 nm and low-loss guidance at the targeted emission wavelength range of 1200–1350 nm. The ends of the gas cell were sealed with windows antireflection coated for both pump and lasing wavelengths. The output from a tunable (over ∼8 GHz around 532.1 nm), single-frequency, frequency-doubled CW Nd:YVO4 laser was expanded, collimated, and focused onto the input facet of the hollow-core fiber. The focal length and position of lens L1 was chosen for optimum coupling of the pump-free space TEM00 mode to the hollow fiber mode [21]. The frequency stability and singlefrequency operation of the pump laser were ensured by monitoring pump absorption through a 10-cm-long reference I2 cell (300 mTorr) and a scanning Fabry–Perot © 2015 Optical Society of America
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OPTICS LETTERS / Vol. 40, No. 4 / February 15, 2015
Fig. 1. Schematic diagram of a Hollow-core Optically pumped Fiber Gas LASer (HOFGLAS) with I2 as active medium. The laser is pumped by a tunable CW Nd:Vanadate laser (Coherent, Verdi) at 532.1 nm. Curved mirrors M1 and M2 form the fiber laser cavity. FI—Faraday isolator; L1, L2, and L3 are lenses used for focusing and collimation, D1, D2—photodiodes. Upper right corner: crosssection of the kagome hollow-core photonic-crystal fiber (HC-PCF) used.
interferometer, respectively. The fiber propagation loss in the 1.3-μm lasing region was previously measured to be ∼30 dB∕km using a cut-back technique on a 100-mlong section. Near our pump wavelength, the fiber transmission spectrum exhibits sharp resonances (see Fig. 2) due to the coupling between the core mode and the cladding struts (thickness t ≈ 490 nm), which occur at q
wavelengths λj 2t n2g − 1∕jj 1; 2; 3… [10]. It is noteworthy that despite this, the fiber still guides at these wavelengths but with much higher transmission loss. An upper figure of the loss at 532 nm was estimated to be about 42 dB/m by measuring the pump transmission coefficient before filling in the gas and considering a coupling efficiency of 100%. The small signal absorption of the pump in the I2-filled HC-PCF was more than 99.5%. An external resonator was built with two focusing mirrors (radius of curvature 15 cm) serving as end mirror (M1, high-reflecting at the expected laser wavelength 1.33 μm) and outcoupler (M2), respectively. Both mirrors
Fig. 2. Normalized (to source) transmission of the 3-m-long HC-PCF used in the experiment. Y axis has been scaled to reflect the previously measured loss ∼30 dB∕km in the fundamental transmission band. The lack of long enough fiber prevented us to resolve this low loss by performing cut-back measurements.
had high transmission at the pump wavelength. Outcoupling losses from 1% to 45% were used in the experiments. For a few outcoupling values, an antireflection coated lens of focal length 15 cm and a plane mirror of suitable reflectivity was used as a substitute to the curved mirror outcoupler. The initial alignment of the passive cavity was optimized by sending a diode probe laser (New Focus model 6248) through the fiber and minimizing the round trip losses. The latter were found to be as low as 0.5%. Figure 3 shows the output power from the I2 HOFGLAS as a function of outcoupling losses. The pump (power 0 140 mW) was tuned to the P(50) line of the B3 Π 0u v 1 00 32←X Σg v 0 absorption transition. The laser performance is near its optimum for a 15% loss (R 85%) outcoupler. Taking into account the passive losses from the cell windows, the total optimum outcoupling loss was estimated to about 22%. A pump threshold of 7 mW corresponding to a pump intensity of about 290 W∕cm2 was observed at optimum outcoupling. The laser spectrum for the R 85%-outcoupler and a pump power of 140 mW is shown in Fig. 4 and a simplified energy level diagram of I2 with the relevant
Fig. 3. Laser output power as a function of outcoupler reflectivity obtained for a pump power of 140 mW.
February 15, 2015 / Vol. 40, No. 4 / OPTICS LETTERS
Fig. 4. Spectrum of the CW HOFGLAS I2 laser output for the 85% reflectivity outcoupler. The pump power was 140 mW.
transitions is shown in Fig. 5. There are three lasing transitions in the 1.3-μm region; each transition is a doublet with corresponding R and P branches. All observed lasing lines originate from the terminal pumped state. From the known spectroscopic constants [22–24], the lines are identified as R(48) and P(50) transitions from the pumped (v0 32) level to v00 72, 71, and 69 levels. Each R and P transition of I2 exhibits 15 or 21 hyperfine lines for even or odd j00 transition [25]. No attempt was made to resolve the hyperfine structure. As can be seen in Fig. 4, the line at 1328.4 nm has the highest intensity. This is due to the more favorable Franck–Condon factor for the v0 32 → v00 72 transition compared to the other lasing transitions; see for example [24]. Figure 6 shows the output power as a function of the input power for different outcoupler reflectivities. Above the threshold, the laser output varies linearly with input pump power before it starts to level off. This is likely due to saturation of pump as well as reduction in gain due to dissociation from upper [11,13] and lower laser levels (v00 72) because of their close proximity (
