Open Access
Add to BibSonomyAbstract
Frequency doubling of conventional fiber lasers in the near-infrared remains the most promising method for generating integrated high-peak-power lasers in the visible, while maintaining the benefits of a fiber geometry; but since the shortest wavelength power-scalable fiber laser sources are currently restricted to either the 10XX nm or 15XX nm wavelength ranges, accessing colors other than green or red remains a challenge with this schematic. Four-wave mixing using higher-order fiber modes allows for control of dispersion while maintaining large effective areas, thus enabling a power-scalable method to extend the bandwidth of near-infrared fiber lasers, and in turn, the bandwidth of potential high-power sources in the visible. Here, two parametric sources using the LP0,7 and LP0,6 modes of two step-index multi-mode fibers are presented. The output wavelengths for the sources are 880, 974, 1173, and 1347 nm with peak powers of 10.0, 16.2, 14.7, and 6.4 kW respectively, and ~300-ps pulse durations. The efficiencies of the sources are analyzed, along with a discussion of wavelength tuning and further power scaling, representing an advance in increasing the bandwidth of near-infrared lasers as a step towards high-peak-power sources at wavelengths across the visible spectrum.
© 2017 Optical Society of America
1. Introduction
Fiber lasers at near-infrared wavelengths have become a ubiquitous technology; compact monolithic sources in the visible portion of the spectrum, by comparison, remain substantially challenging to realize – mainly due to the lack of suitable gain materials with transitions in this spectral regime. Development of high-peak-power, quasi-continuous wave lasers that leverage the benefits of a fiber geometry (compact, robust, spliceable) while operating at visible wavelengths would enable many applications: for example, underwater sensing and communications using high-power blue lasers [1], sodium guide stars for adaptive optics calibration with yellow lasers [2], or next generation semiconductor lithography [3]. The most successful approach for developing visible fiber lasers thus far has been to use second harmonic generation (SHG) in χ(2) crystals to convert the output of a traditional laser to the visible. While this technique has been employed extensively with ytterbium-doped fiber lasers to cover the green (5XX nm) portion of the spectrum, and with erbium to cover 7XX nm, there are substantial gaps – notably at yellow and blue wavelengths – due to the finite bandwidth of traditional rare-earth dopants.
There has therefore been significant interest in extending the bandwidth of fiber lasers in the near-infrared (800–1400 nm) to span the entire visible spectrum via SHG. One approach is to inhibit the dominant lasing transitions in typical rare earth elements to allow for lasing at lower-gain wavelengths. For example, in ytterbium-doped fibers, researchers have employed ring-doping [4], spectral filtering [5,6], micro-structuring [7], or heating [8] to favor gain at wavelengths outside the typical band (1030—1060 nm), allowing for operation as low as 975 nm, and as high as 1180 nm. Alternatively, other dopants, such as neodymium (9XX nm), or bismuth (12XX nm), have been explored to extend this bandwidth [9—11]. While there have been many examples of fiber lasers operating outside the typical ytterbium band, including some demonstrations with significant peak power (notably [6]); these amplifiers generally suffer from unwanted ASE noise from the suppressed transitions, broad linewidths (which in turn limit SHG efficiency), and relatively low gain.
Another approach has been to use various nonlinear frequency conversion schemes to convert conventional fiber lasers to other wavelengths in the near-infrared, followed by SHG to access the visible spectral range. For example, frequency-quadrupling of thulium fiber lasers has been employed to reach blue wavelengths with considerable peak power [12]. However, the bandwidth of thulium limits this technique’s applicability over the whole visible spectrum, and the need for two separate free space crystal stages may limit the alignment tolerance of the system. Alternatively, in-fiber stimulated Raman scattering has been employed to frequency convert ytterbium lasers, which in turn can be frequency-doubled to reach the visible [13]; however, Raman gain can only be achieved for wavelengths greater than the pump limiting wavelength coverage in the visible. Sources based on fiber optical parametric amplification (FOPA) are perhaps more attractive in this respect, as the gain bandwidth is dictated by phase-matching rather than the material composition of the fiber [14]. Through high confinement of the light (such as in photonic crystal fibers, PCFs), the group velocity dispersion can be tailored to provide phase-matching at wavelengths across the near-infrared, while using pumps in the ytterbium band [15]. However, the tight confinement of the light necessary for achieving the desired dispersion in PCFs reduces the effective area to << 10 μm2, limiting power handling; hence, PCF based FOPA sources in the 800-1400 nm band have been limited to sub-kW peak powers [16,17].
In this work, we use higher-order fiber modes (HOMs) to subvert the dispersion vs. effective-area (Aeff) tradeoff imposed by fundamental fiber-mode architectures. HOMs can be selectively excited with high purity [18], and propagated stably over length scales significantly exceeding those required for nonlinear (or doped) amplifiers [19]. Furthermore, dispersion at wavelengths in the ytterbium band can be tuned to achieve the desired phase-matching and parametric gain while maintaining large effective areas (>500 μm2 in this work) – allowing for high-power nonlinear frequency conversion across the near-infrared portion of the spectrum [20,21]. Here we illustrate the versatility of this approach by demonstrating two FOPA sources (henceforth referred to as FOPA sources A and B) each operating in a single, higher-order mode: source A is pumped with the LP0,7 mode and has output wavelengths of 974 and 1173 nm, corresponding to the blue/green and yellow portions of the visible spectrum (via SHG), with 16.2 and 14.7 kW, ~300 ps output pulses respectively; source B is pumped with the LP0,6 mode and operates at 880 and 1347 nm, corresponding to the blue and red portions of the visible spectrum, with 10.0 and 6.2 kW, ~300 ps output pulses.
2. Experimental setup
A schematic of the setup used to create both of the FOPAs is shown in Fig. 1(a). The pump laser is a Nd:YAG microchip laser (Teem Photonics SNP-20F-100), amplified by a 1.5 m segment of ytterbium-doped fiber, emitting 0.5 ns Q-switched pulses at a repetition rate of 19.5 kHz, and a wavelength of 1064 nm [pump spectrum and pulse profile shown in Figs. 1(b) and 1(c) respectively]. For FOPA source A, polarization-maintaining gain fiber was used (Nufern PLMA-YDF-25/250-VIII), with a maximum power of 87 kW incident at the spatial light modulator (SLM). For FOPA source B, non-polarization maintaining gain fiber was employed (Nufern LMA-YDF-25/250-VIII), whose lower polarization extinction ratio reduced the maximum peak power incident on the SLM to 63 kW.
Fig. 1 (a) Setup schematic; QWP = quarter-wave plate, HWP = half-wave plate, BPF = bandpass filter, PBS = polarizing beam splitter, SLM = spatial light modulator; (b) Pump spectrum, measured prior to SLM1; (c) Pump pulse temporal profile, measured prior to SLM1.
The SLM (Hamamatsu X10468-07) was programmed with a binary phase plate designed to target a given mode in the fiber, along with a Fresnel lens allowing for highly selective excitation of desired modes in the test fibers (this technique is capable of ~15 dB mode purity, described in detail in [18]). The pump beam was coupled through a dichroic mirror (Chroma T1020PXR-BCM) into the fiber under test (FUT) with ~50-60% coupling efficiency. In this experiment, SLMs were employed to allow for rapid characterization of all the guided LP0,m modes in the test fibers – in systems which target a single, known mode, coupling loss could be reduced to < 1 dB by using an axicon instead [18].
A parallel arm was constructed for the seed, comprising a second SLM with a similar phase pattern as SLM1, albeit designed for the seed rather than pump wavelength. The reflective port of the dichroic mirror was used to route the beam in the FUT with ~30-35% coupling efficiency. The specifics of the seed lasers employed for FOPA sources A and B are discussed in the relevant sections below.
The output of the FUT was characterized using spectral bandpass filters to separate the pump wave from the nonlinearly generated Stokes and anti-Stokes features (i.e. the desired wavelength-converted sources). A thermal power meter (Coherent PowerMax USB), silicon camera (Thorlabs DCC1645C), high speed detector and oscilloscope (Thorlabs DET08C, Agilent 86100A), and an optical spectrum analyzer (Ando AQ6317) were used to analyze the dynamics of each FOPA source.
3. Results
3.1 Parametric source A: 974 and 1173 nm operation
Parametric source A comprises a 1.5 m segment of 47.5-μm core diameter step-index multi-mode fiber [facet image and refractive index profile shown in Figs. 2(a) and 2(b) respectively] manufactured to design specifications by OFS-Fitel LLC. The core region of the fiber is up-doped with germanium, and the cladding is down-doped with fluorine leading to an index step of 4.1 × 10−2. The fiber guides the first fourteen LP0,m modes at the pump wavelength, however, the FOPA source is operated in the LP0,7 mode, far from cutoff. Thus we expect that similar performance could have been obtained from a fiber with a significantly smaller index step, such as a commercial step-index multimode fiber; our choice of this fiber was dictated by currently available optical fibers in our lab.
Fig. 2 (a) Facet image and (b) measured refractive index profile of FUT for FOPA source A; (c) Simulated dispersion for the LP0,7 mode of FUT; dashed vertical line designates pump wavelength; (d) Simulated Aeff for the LP0,7 mode of FUT with simulated mode image inset; (e) Simulated phase-matching for LP0,7 pumping at 1064 nm; (f) Simulated small signal gain for LP0,7 pumping of a 1.5-m FUT at 1064 nm vs. pump peak powers ranging from 5 to 40 kW.
Figures 2(c) and 2(d) show the dispersion and effective area of the pump mode (LP0,7) as a function of wavelength. At the pump wavelength (dashed, black vertical lines), the LP0,7 mode has near-zero normal dispersion of −0.1 ps/nm-km and a 537 μm2 Aeff. For four-wave mixing (FWM) interactions degenerately pumped within a single, transverse fiber mode, the dispersion of the pump beam dictates the phase-matching condition. Figure 2(e) shows a simulated phase-matching curve given by Δβ = 2βp – βas - βs, where βp, βas and βs are the propagation constants for the pump, anti-Stokes, and Stokes waves respectively. FWM gain is expected for the region −4γP < Δβ < 0 [shaded in yellow on Fig. 2(e)], where γ is the nonlinear coefficient, and P is the peak power of the pump [P = 40 kW for Fig. 2(e)].
The dashed black line in Fig. 2(e) corresponds to maximum parametric gain given by the condition Δβ = −2γP, thus the frequency for which peak small signal gain occurs is proportional to pump power. Figure 2(f) shows the simulated small signal parametric gain (calculated for a 1.5 m segment of fiber) as a function of peak pump power. As pump power decreases, the gain regions move closer to the pump wavelength. In the course of an efficient FWM process, the pump is depleted; therefore we expect the peak gain wavelength to move toward the pump wavelength as light propagates down the fiber. Note that this approximation is not strictly valid in the depleted pump regime where self-phase modulation of the Stokes and anti-Stokes waves will also play a role in phase matching. However, the small signal gain regime does provide the intuition that in order to achieve the highest gain on average, it therefore follows that the seed wavelength should be detuned to the pump wavelength side of the gain peak [22]. Optimizing this detuning is easily achieved by tuning the wavelength of either the pump or seed lasers. For FOPA source A, the wavelengths of the seed and pump lasers were fixed, so efficiency was maximized by using different sample fibers with slightly different diameters (and thus slightly different dispersion and phase-matching) to maximize efficiency. As noted earlier, a simpler implementation would involve a pump whose wavelength is tunable, but we were constrained by available pump lasers in our lab.
FOPA source A is seeded on the anti-Stokes line with a Bragg-grating-stabilized fiber-pigtailed diode laser (3SP 3CN01178EA) which emits at 974 nm with a continuous wave (CW) output power of ~500 mW, and a spectral full-width at half-maximum (FWHM) of 0.4 nm [spectrum shown in Fig. 3(a)]. The seed is coupled into the LP0,7 mode of the FUT with 34% coupling efficiency, with the loss primarily due to the linear polarizer in the beam path and the intrinsic loss of the SLM.
Fig. 3 (a) Seed spectrum and (b) output spectra for FOPA source A; spectrally filtered mode images for the anti-Stokes, pump, and Stokes waves are shown inset – all in LP0,7 as expected.
Figure 3(b) shows the output spectra for FOPA source A. The red curve corresponds to the case where the pump is coupled to the FUT without the seed (peak power in the sample is 41 kW). There is clear evidence of spontaneous parametric fluorescence at ~950 and ~1200 nm, in good agreement with the simulated small signal gain curves in Fig. 2(f). The seed wavelength [Fig. 3(a)] is detuned from the small signal gain peak (as determined from the spontaneous parametric fluorescence, red curve) by 18 nm, which was found, experimentally, to correspond to the maximum conversion efficiency for the FOPA source.
The blue curve in Fig. 3(b) corresponds to the output spectrum of the seeded FOPA source. The seed experiences parametric amplification, and a Stokes wave at 1173 nm is generated on the long wavelength side of the pump. The FWHM of the amplified anti-Stokes line is 0.4 nm, consistent with the bandwidth of the seed laser; therefore the nonlinear converter does not significantly broaden the seed. The FWHM of the generated Stokes line is 0.5 nm. Spectrally filtered images of the output of FOPA source A are inset above the corresponding laser lines in Fig. 3. As expected for a monomode FWM process, each wave’s intensity profile corresponds to the LP0,7 mode. Furthermore, the mode purity of the Stokes and anti-Stokes waves appears to increase relative to the pump (as discerned from the fact that the regions between the intensity rings are significantly darker). This phenomenon, referred to as “nonlinear beam clean up” [20] occurs because only the LP0,7 mode is phase matched and therefore any light generated at this wavelength must be residing in this mode only.
The average power for each peak was measured to determine the FOPA source’s efficiency; the pump wave was depleted by 32.0% and the power transfer efficiencies to the anti-Stokes and Stokes waves were 17.9% and 15.3% respectively. The slight discrepancy in transfer efficiencies is due to the relative energy of photons in the anti-Stokes and Stokes waves; normalizing by the frequencies of each of the waves, the resulting photon-to-photon efficiencies for the anti-Stokes and Stokes (16.4 and 16.9%) are in good agreement with each other and the measured pump depletion. Given that the power in the system is well accounted for, FOPA source A does not seem to suffer from any parasitic nonlinear processes such as stimulated Raman scattering or intermodal FWM.
Figure 4(a) shows the temporal profile of the measured pump pulse with (blue curve) and without (red curve) the seed present. The center portion of the pump pulse is largely depleted when the seed is present; however, wings remain – limiting the overall efficiency. This is a well-known consequence of using rapidly varying pump pulse shapes (such as from a Q-switched laser). Despite the limitation in overall efficiency, the instantaneous depletion of the pump pulse approaches 73%, allowing for high-peak-powers for the output pulses. The measured anti-Stokes and Stokes pulses are shown in Figs. 4(b) and 4(c) (measurements correspond to a 64-point running average on a sampling oscilloscope). The pulses have been reshaped and are shorter in duration (304 ps for anti-Stokes, 287 ps for Stokes) than the initial pump pulse (~550 ps), consistent with the region of measured depletion for the pump pulse [Fig. 4(a)].
Fig. 4 (a) Pump pulse temporal profiles with (blue) and without (red) the seed; Measured anti-Stokes (b) and Stokes (c) pulse with 64-point running average.
The calculated peak powers for the pulses are 16.2 kW and 14.7 kW for the anti-Stokes and Stokes waves respectively, where once again the discrepancy is due to the relative energy per photon at the different wavelengths. The instantaneous parametric gain approaches 50 dB, indicating that the parametric conversion process has high fidelity. The dispersion of the fiber, and thus the detuning of the seed wavelength from the spontaneous gain peak, was optimized for the available pump power (<45 kW); were more pump power available, the sample could be optimized to scale the output powers even more, since HOMs in multimode fibers are uniquely scalable in Aeff while maintaining desired dispersive characteristics.
3.2 Parametric source B: 880 and 1343 nm operation
FOPA source B comprises a 3 m segment of standard off-the-shelf 50-μm core step-index multi-mode fiber [Thorlabs FG050LGA, facet image and refractive index profile shown in Figs. 5(a) and 5(b) respectively]. The core is un-doped silica glass, and the cladding is fluorine-down-doped, resulting in an index step of 1.7 × 10−2. The fiber guides the first ten LP0,m modes at the pump wavelength, and FOPA is operated in the LP0,6 mode.
Fig. 5 (a) Facet image and (b) measured refractive index profile of FUT for FOPA source B; (c) Simulated dispersion for the LP0,6 mode of FUT; dashed vertical line designates pump wavelength; (d) Simulated Aeff for the LP0,6 mode of FUT with simulated mode image inset; (e) Simulated phase-matching for LP0,6 pumping at 1064 nm; (f) Simulated small signal gain for LP0,6 pumping of a 3-m FUT at 1064 nm vs. pump peak powers ranging from 5 to 40 kW.
Plots of dispersion and Aeff versus wavelength for the LP0,6 mode are shown in Figs. 5(c) and 5(d) respectively; the Aeff (577 μm2) is similar to that for FOPA source A (537 μm2), however the dispersion (−1.5 ps/nm-km) is significantly more normal than that for FOPA source A (−0.1 ps/nm-km). Accordingly, phase-matching for FOPA source B occurs at wavelengths farther from the pump than in FOPA source A, as shown in Fig. 5(e) for a pump peak power of 40 kW. The slope of the phase-matching curve is large, leading to narrower gain regions than the previous case [Fig. 5(f), L = 3 m]; however, the wavelength for which peak gain is achieved is much more stable with change in pump power. As a result, wavelength detuning between the gain peak and seed wavelength is less critical and easier to optimize compared to FOPA source A.
The seed laser for FOPA source B is a wavelength-tunable Ti:Sapphire laser (Spectra-Physics Mai Tai) operated in CW mode at a wavelength of 880 nm (optimized for maximum conversion efficiency). The output power of the laser, measured after the polarizer in the seed path, Fig. 1(a), was 230 mW, with 115 mW coupled to the FUT in the LP0,6 mode. The bandwidth of the seed laser is fairly large (0.8 nm), since the laser is designed for mode-locked operation [spectrum shown in Fig. 6(a)]; the choice of seed laser for the experiment was limited by the equipment on-hand, however, a narrow-band fiber-coupled diode laser could easily be employed as was the case in FOPA source A.
Fig. 6 (a) Seed spectrum for FOPA source B; (b) Output spectra; spectrally filtered mode images for the anti-Stokes, pump, and Stokes waves are shown inset – all in LP0,6 as expected.
Figure 6(b) shows the output spectra for FOPA source B. The red trace corresponds to the case where the pump is coupled into the FUT (~33 kW peak power in the fiber) and the seed is blocked. Narrowband parametric fluorescence is observed, in good agreement with the simulations in Fig. 5(f). The peak of the spontaneous fluorescence is coincident with the wavelength of the seed laser.
The blue curve in Fig. 6(b) is the output spectrum showing parametric gain for the seeded anti-Stokes wave at 880 nm, and generation of a Stokes wave at 1347 nm. The parametrically amplified anti-Stokes wave appears to broaden relative to the input seed spectrum, mirroring similar broadening which occurs at the pump wavelength – possibly due to self-phase modulation in the fiber. However, the broadening is only significant ~10 dB down from the peak of the laser line, and the spectral FWHM remains 0.9 nm, comparable to that of the seed laser. Spectrally filtered mode images are inset above the plot – all of the images correspond to the target LP0,6 mode, as expected for the mono-mode FWM process.
The measured pump depletion for FOPA source B was 30.4%, with power transfer efficiencies to the anti-Stokes and Stokes of 18.9% and 11.5% respectively. The corresponding photon-to-photon efficiencies, 15.7% and 14.8%, are approximately equal as expected for an efficient FWM process. The depleted pump power is completely accounted for in the Stokes and anti-Stokes waves, therefore there is no evidence of any parasitic nonlinear process.
The measured output pulses for FOPA source B are shown in Figs. 7(a) and 7(b) for the anti-Stokes and Stokes waves respectively (measurements correspond to a running average of 64 points on the sampling oscilloscope). As with FOPA source A (Fig. 4), there is evidence of pulse-shaping due to the Q-switched profile of the pump pulse, leading to shorter output pulse durations (386 ps for the anti-Stokes pulse, and 363 ps for the Stokes pulse). The efficiency of the wavelength conversion process is limited, as in the previous case (FOPA source A) due to the temporally long tails characteristic of pump pulses from a Q-switched laser, indicating significant scope for improvement with the use of rectangular pump pulses. The calculated peak powers for the pulses are 6.2 kW for the Stokes pulse, and 10.0 kW for the anti-Stokes pulse, corresponding to an instantaneous parametric gain of 49 dB. Further scaling of the output power was limited by the power available from the Q-switched laser pump.
4. Summary and conclusions
We have demonstrated two high-peak-power fiber parametric sources using higher-order fiber modes. The operation wavelengths are 880, 974, 1173, and 1347 nm – all wavelengths within the second harmonic range of the visible portion of the spectrum. Furthermore, by adjusting the wavelength of the pump (and thus the dispersion) we expect that the outputs could be tuned to particular wavelengths of interest – for example 976 nm, the harmonic of the Fraunhofer F line, of use for underwater communications and sensing; or 1178 nm, the harmonic of the sodium D2 line used in sodium guide star applications. Currently, the peak output powers for the parametric sources are on the order of ~10 kW, which to the best of our knowledge, represent the highest values reported for fiber parametric sources in this wavelength range. We expect that the pulses should be compatible with frequency doubling to yield kW-peak-power pulses in the visible portion of the spectrum.
The parametric conversion process is currently limited by only ~30% (rather than full) pump depletion, which we believe is due to the temporal profile of the pump pulse. However, the instantaneous depletion near the peak of the pump pulses is significantly higher; therefore we expect that the efficiency of the FOPA sources could be increased by pumping with flat top, rectangular pump pulses. It is worth noting however, that the peak power extraction is already very high for these sources (73% for the case of FOPA source A), so further optimization of the efficiency would most likely lead to less pulse shaping, but not necessarily more peak power in the output pulses. Instead, further peak power scaling should be possible by increasing the pump peak power. Both sources are currently limited by available pump power, however, simulations suggest that these fibers should be able to support peak powers of ~300 kW for pulse durations of ~0.5 ns prior to the onset of dielectric breakdown, allowing for a 7-fold increase in peak power levels.
Future experiments will investigate frequency doubling of the output of these FOPA sources to achieve high-peak-power, ~100-ps pulsed sources in the visible portion of the spectrum. There are two avenues for exploration in this regard. We have previously demonstrated low loss (0.85 dB) reconversion of LP0,m higher-order fiber modes to Gaussian beams [18], allowing for the use of conventional nonlinear crystals for frequency doubling. Alternatively, and perhaps more attractively, we have theoretically predicted [23] that non-collinear phase matching with nonlinear crystals can, in addition to offering conventional frequency doubling, also enable mode conversion from the HOM of the FOPA source to a Gaussian beam with 90% efficiency, implying that even the output mode conversion step may be avoided. Thus, we envisage that higher-order mode fiber parametric sources could lead to an attractive means of achieving high-peak-power monolithically integrated compact source across the visible spectral range.
Funding
Air Force Office of Scientific Research (AFOSR) BRI program (FA9550-14-1-0165); Office of Naval Research (ONR) MURI (N00014-13-1-0627).
Acknowledgements
The authors wish to thank P. Kristensen (OFS-Fitel LLC.) for kindly manufacturing the fibers used in FOPA source A, and acknowledge M. Sander and A. Akosman for loaning laboratory equipment.
References and links
1. I. F. Akyildiz, D. Pompili, and T. Melodia, “Underwater acoustic sensor networks: Research challenges,” Ad Hoc Netw. 3(3), 257–279 (2005). [CrossRef]
2. C. E. Max, K. Avicola, J. M. Brase, H. W. Friedman, H. D. Bissinger, J. Duff, D. T. Gavel, J. A. Horton, R. Kiefer, J. R. Morris, S. S. Olivier, R. W. Presta, D. A. Rapp, J. T. Salmon, and K. E. Waltjen, “Design, layout, and early results of a feasibility experiment for sodium-layer laser-guide-star adaptive optics,” J. Opt. Soc. Am. A 11(2), 813–824 (1994). [CrossRef]
3. W. Haske, V. W. Chen, J. M. Hales, W. Dong, S. Barlow, S. R. Marder, and J. W. Perry, “65 nm feature sizes using visible wavelength 3-D multiphoton lithography,” Opt. Express 15(6), 3426–3436 (2007). [CrossRef] [PubMed]
4. J. Nilsson, J. D. Minelly, R. Paschotta, A. C. Tropper, and D. C. Hanna, “Ring-doped cladding-pumped single-mode three-level fiber laser,” Opt. Lett. 23(5), 355–357 (1998). [CrossRef] [PubMed]
5. J. Boullet, Y. Zaouter, R. Desmarchelier, M. Cazaux, F. Salin, J. Saby, R. Bello-Doua, and E. Cormier, “High power ytterbium-doped rod-type three-level photonic crystal fiber laser,” Opt. Express 16(22), 17891–17902 (2008). [CrossRef] [PubMed]
6. J. Boullet, R. Dubrasquet, C. Médina, R. Bello-Doua, N. Traynor, and E. Cormier, “Millijoule-class Yb-doped pulsed fiber laser operating at 977 nm,” Opt. Lett. 35(10), 1650–1652 (2010). [CrossRef] [PubMed]
7. A. Shirakawa, H. Maruyama, K. Ueda, C. B. Olausson, J. K. Lyngsø, and J. Broeng, “High-power Yb-doped photonic bandgap fiber amplifier at 1150-1200 nm,” Opt. Express 17(2), 447–454 (2009). [CrossRef] [PubMed]
8. A. S. Kurkov, “Oscillation spectral range of Yb-doped fiber lasers,” Laser Phys. Lett. 4(2), 93–102 (2007). [CrossRef]
9. J. Kim, P. Dupriez, D. B. S. Soh, J. K. Sahu, J. Nilsson, and D. Payne, “Nd:Al-doped depressed clad hollow fiber laser at 930nm,” in Advanced Solid-State Photonics, Technical Digest (Optical Society of America, 2005), paper MC5.
10. A. Wang, A. K. George, and J. C. Knight, “Three-level neodymium fiber laser incorporating photonic bandgap fiber,” Opt. Lett. 31(10), 1388–1390 (2006). [CrossRef] [PubMed]
11. E. M. Dianov, A. V. Shubin, M. A. Melkumov, O. I. Medvedkov, and I. A. Bufetov, “High-power cw bismuth-fiber lasers,” J. Opt. Soc. Am. B 24(8), 1749–1755 (2007). [CrossRef]
12. E. Honea, M. Savage-Leuchs, M. S. Bowers, T. Yilmaz, and R. Mead, “Pulsed blue laser source based on frequency quadrupling of a thulium fiber laser,” Proc. SPIE 9728, 860111 (2013). [CrossRef]
13. D. Georgiev, V. P. Gapontsev, A. G. Dronov, M. Y. Vyatkin, A. B. Rulkov, S. V. Popov, and J. R. Taylor, “Watts-level frequency doubling of a narrow line linearly polarized Raman fiber laser to 589nm,” Opt. Express 13(18), 6772–6776 (2005). [CrossRef] [PubMed]
14. J. Hansryd, P. A. Andrekson, M. Westlund, J. Li, and P. O. Hedekvist, “Fiber-based optical parametric amplifiers and their applications,” IEEE J. Sel. Top. Quantum Electron. 8(3), 506–520 (2002). [CrossRef]
15. J. C. Knight, J. Arriaga, T. A. Birks, A. Ortigosa-Blanch, W. J. Wadsworth, and P. S. J. Russell, “Anomalous dispersion in a photonic crystal fiber,” IEEE Photonics Technol. Lett. 12(7), 807–809 (2000). [CrossRef]
16. P. J. Mosley, S. A. Bateman, L. Lavoute, and W. J. Wadsworth, “Low-noise, high-brightness, tunable source of picosecond pulsed light in the near-infrared and visible,” Opt. Express 19(25), 25337–25345 (2011). [CrossRef] [PubMed]
17. R. T. Murray, E. J. R. Kelleher, S. V. Popov, A. Mussot, A. Kudlinski, and J. R. Taylor, “Widely tunable polarization maintaining photonic crystal fiber based parametric wavelength conversion,” Opt. Express 21(13), 15826–15833 (2013). [CrossRef] [PubMed]
18. J. Demas, L. Rishøj, and S. Ramachandran, “Free-space beam shaping for precise control and conversion of modes in optical fiber,” Opt. Express 23(22), 28531–28545 (2015). [CrossRef] [PubMed]
19. S. Ramachandran, J. W. Nicholson, S. Ghalmi, M. F. Yan, P. Wisk, E. Monberg, and F. V. Dimarcello, “Light propagation with ultralarge modal areas in optical fibers,” Opt. Lett. 31(12), 1797–1799 (2006). [CrossRef] [PubMed]
20. P. Steinvurzel, J. Demas, B. Tai, Y. Chen, L. Yan, and S. Ramachandran, “Broadband parametric wavelength conversion at 1 μm with large mode area fibers,” Opt. Lett. 39(4), 743–746 (2014). [CrossRef] [PubMed]
21. S. Ramachandran, J. M. Fini, M. Mermelstein, J. W. Nicholson, S. Ghalmi, and M. F. Yan, “Ultra-large effective-area, higher-order mode fibers: a new strategy for high-power lasers,” Laser Photonics Rev. 2(6), 429–448 (2008). [CrossRef]
22. H. Steffensen, J. R. Ott, K. Rottwitt, and C. J. McKinstrie, “Full and semi-analytic analyses of two-pump parametric amplification with pump depletion,” Opt. Express 19(7), 6648–6656 (2011). [CrossRef] [PubMed]
23. O. Shatrovoy, L. Rishøj, and S. Ramachandran, “Simultaneous mode and nonlinear-frequency conversion of HOMs,” Proc. SPIE 9728, 9728 (2016).
