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Ytterbium-Raman cascaded oscillators with linearly polarized output are designed and achieved based on polarization selection loss (PSL) mechanism for the first time. The 1120 nm laser cavity is designed with fully non polarization-maintained (NPM) fiber Bragg gratings (FBGs) and NPM active fiber while the 1080 nm laser cavity is designed based on polarization-maintained (PM) FBGs and PM active fiber. By using PSL mechanism in 1080 nm cavity, even with fully NPM 1120 nm cavity, both linear-polarized 1120 nm and 1080 nm lasers are achieved in the output port of the cascaded oscillators. Based on the new designed cascaded seeds, a high power polarization-maintained Yb-Raman hybrid nonlinear amplifier is established for further power scaling of the 1120 nm laser. In the nonlinear amplifier, only 21-meter-long active fiber and 1.5-meter-long passive fiber is used for power transferring from 1080 nm to 1120 nm. Output power of 1181 W is achieved at central wavelength of 1120 nm with the M2 factor of <1.2 and polarization-extinction ratio (PER) of 18.2 dB. As far as we known, the output power of this all fiber format is the highest one in 1120 nm with linear polarization. This type of high power Yb-Raman nonlinear amplifier design with linear polarization can be further extended to Yb-Raman amplifying the wavelength range of 1100-1200 nm.
© 2015 Optical Society of America
1. Introduction
High brightness laser sources operating in the wavelength range of 1100-1200 nm have widely applications including biomedicine, new-style pump sources and remote sensing [1–3]. As a result of broad emission wavelength range of Ytterbium-doped (Yb-doped) active fiber, generation of the laser sources by directly using Yb-doped fiber lasers and/or amplifiers seems to be the most convenient approaches [4]. By using these techniques, 322 W output power at 1120 nm was attained directly from an Yb-doped oscillator [5] and 453 W output power at 1117 nm had been reported recently [6]. However, by directly employing Yb-doped fiber lasers/amplifiers for power scaling, as a result of relative small emission cross section in Yb-doped fiber in the wavelength range of 1100-1200 nm, the amplified spontaneous emission (ASE) and parasitic lasing should be controlled carefully by gain partitioning the various amplifier stages. Consequently, normally multiple amplification stages should be required for high power scaling.
An alternative method to achieve high brightness fiber lasers in the wavelength range of 1100-1200 nm is to use Raman fiber lasers or amplifiers [7]. In the previous study, by using a high power wavelength division multiplexer (WDM) to combine the signal laser and Raman stokes into Raman gain fiber, output power of hundreds watt-level had been reported based on core-pumped Raman fiber lasers [8]. Nevertheless, power handing ability of the high power WDM will limit further power scaling of the setup. As for the power scaling limitation of the WDM, integrated Yb-Raman fiber amplifier and Yb-Raman combined nonlinear fiber amplifier was successively demonstrated by L. Zhang et al. and H. Zhang et al. [9, 10], in which the signal laser and the Raman stokes were simultaneously injected into the Yb-doped fiber amplifier (YDFA) by a high power pump-combiner. Based on these approaches, kilowatt level 1120 nm laser had been demonstrated quite recently [11, 12].
As for lasers emitting within 1100-1200 nm, in many direct or extendible applications, for example in nonlinear wavelength conversion regime to ensure high conversion efficiency [2, 3, 13] and polarization beam combining for optical pumping of Raman lasers/amplifiers, except for efforts on power scaling, linear polarization should also be particularly considered carefully. Normally, linear-polarized output in an oscillator can be directly achieved by cross-splicing a pair of spectra specially controlled and polarization-dependent FBGs [14, 15]. By using this approach, about 100 W power-level linearly polarized 1120 nm output power was demonstrated directly in Yb-doped fiber oscillator [16], and 300 W linear-polarized output power had been achieved in integrated Yb-Raman fiber amplifier [9]. However, by employing above conventional technique, in order to achieve linear-polarized output, the reflected spectra of the fast axis of high-reflective (HR) grating and the slow axis of low-reflective (LR) grating should be overlapped well in the power scaling process. According to our carefully experimental investigation, along with power scaling (tens of watts), the temperature gradient between the two FBGs will be increased and the spectra overlapped state will be destroyed, which will seriously influence the performance (output power, polarization extinction ratio, and time stability) of the output laser [17]. Our experimental results show that further power scaling by using the conventional technique should be dependent on accurate controlling the temperature of the FBGs. Besides, the manufacturing of spectra specially controlled and polarization-dependent FBGs requires controlling the central wavelengths and line-widths precisely in practice.
In this manuscript, we design linear-polarized output Yb-Raman cascaded oscillators based on polarization selection loss (PSL) mechanism. In the cascaded oscillators, the 1120nm oscillator (Raman stokes laser) is designed based on fully non-polarization-maintained (NPM) FBGs and NPM active fiber. The 1080 nm oscillator (signal laser) is designed based on PSL mechanism to ensure linear-polarized output. Because that the PSL mechanism is also functioned to Raman stokes laser when it transmitted through the signal laser oscillator, both linear-polarized Raman stokes and signal lasers are achieved in the output port of the cascaded oscillators. Then, a high power polarization-maintained Yb-Raman hybrid nonlinear amplifier seeded with our new designed cascaded seeds is demonstrated. As high as 1181 W near-diffraction-limited (M2<1.2) 1120 nm laser is obtained with optical to optical efficiency of 74.3% and PER~18.2 dB. To the best of our knowledge, the output power of this all fiber format is the highest one in 1120 nm with linear polarization. Furthermore, this design concept can be simply used for linear-polarized power scaling within 1100-1200 nm.
2. Experimental setups and results
2.1 Generation of linear-polarized Yb-Raman cascaded oscillators
Our experimental design of linear-polarized Yb-Raman cascaded oscillators based on PSL mechanism is shown in Fig. 1, which mainly includes two separate cavities. The first cavity is a Raman stokes cavity operating with central wavelength of 1120 nm. This cavity includes a 4.5 m long double clad NPM Yb-doped fiber (core diameter~10 um with NA~0.08, inner cladding diameter~125 um with NA ~0.46, cladding absorption coefficient~4.8 dB/m at 976nm) and a pair of fully NPM FBGs. The V number of the active fiber is calculated to be 2.244 (strictly single mode). The reflectivities of the high-reflectance grating (NPM-HR) and output coupler (NPM-OC) are~99.8% and 50%, respectively. After the 1120 nm cavity, the Raman stokes laser is transmitted through a (2 + 1) × 1 pump combiner and then injected into the signal laser cavity. The signal laser cavity operates with central wavelength of 1080 nm, which consists of a 6 m long double-clad and polarization-maintained active fiber (core diameter~15 um with NA~0.075, inner cladding diameter~130 um with NA~0.46, cladding absorption coefficient~5.7 dB/m at 976 nm, birefringence of ~2 × 10−4) and a pair of type-matched polarization-maintained FBGs. The V numbers of the active fiber in this cavity for 1080 nm and 1120 nm lasers are calculated to be 3.273 and 3.16, respectively. Despite that two types of LPmn modes (LP01 and LP11) are theoretically supported for 1080 nm and 1120nm lasers in the PM active fiber, the LP11 mode is not easy to excite if the splicing points are managed well in practice. In 1080 nm laser cavity, the reflectivities of the high-reflectance grating (PM-HR) and the output coupler (PM-OC) are ~99.7% and 9.4%, respectively. Each cavity is pumped by a fiber pigtailed laser diode (LD) at 976 nm pumping wavelength.
It is to be noted that the PM FBGs in the 1080 nm laser cavity is just conventional PM FBGs while not spectra specially controlled and polarization-dependent ones. Also, cross spicing is not required in the cavity. By using this type of PM FBGs, in order to obtain linear-polarized laser in 1080 nm cavity, PSL should be employed by coiling the PM active fiber in 1080 nm cavity to only preserve the polarized mode in the slow-axis direction (denoted by x-axis) while loss the polarized mode in the fast-axis direction (denoted by y-axis) [18, 19]. Further, because that the 1120 nm Raman signal laser can be transmitted through the 1080 nm cavity, so PSL can also continuously attenuate the 1120 nm laser power in the fast-axis direction. According to the previous maturely theoretical analysis [20, 21] and afore parameters of the PM active fiber used in 1080 nm cavity, the calculated relationship between power loss coefficient (Γ) and coiling radius (R) of the PM active fiber is shown in Fig. 2. From Fig. 2, we show that both linear-polarized 1080 nm laser and linear-polarized 1120 nm laser can be simultaneously obtained in the output port of PM-OC by properly selecting the coiling radius of the PM active fiber in 1080 nm cavity.
Fig. 2 The calculated relationship between power loss coefficient (Γ) of 1080 nm /1120 nm lasers and coiling radius.
In the experiment, 6 m long PM active fiber is coiled with 1.8 cm radius (R = 1.8 cm). Firstly, we investigate the characteristics (power scaling process, PER and spectra) of the separate 1080 nm cavity. In this process, the 1120 nm laser is switched off. The power scaling characteristics and PERs (measured by combination components of a half-wavelength plate and a polarization beam combiner operating at central wavelength of 1080 nm) along with the absorbed 976 nm pump power are shown in Fig. 3(a), and the spectra at 5.7 W, 14.4 W and 30 W are shown in Fig. 3(b). The power scaling process is nearly linear and optical to optical efficiency of ~50% is achieved at 30 W output power with PER of 18 dB. By using PSL mechanism, along with power scaling process, the PER just fluctuates from 16.4 dB (97.8%) to 18.8 dB (98.7%). The 3 dB spectral line-widths at 5.7 W, 14.4 W and 30 W are measured to be 0.16 nm, 0.19 nm and 0.47 nm, respectively.
Fig. 3 (a) The output power scaling characteristics and measured PER of the separate 1080 nm cavity; (b) The spectra of 1080 nm laser at 5.7 W, 14.4 W, and 30 W.
Further, we explore the characteristics of the separate 1120 nm NPM cavity in the output port of NPM-OC. The power scaling characteristics and PERs (measured by combination components of a half-wavelength plate and a polarization beam combiner operating at central wavelength of 1120 nm) along with the absorbed 976 nm pump power are shown in Fig. 4(a), and the spectra at 3.2 W, 35 W and 64 W are shown in Fig. 4(b). The power scaling process is also nearly linear-changed and optical to optical efficiency of ~67% is achieved at 64 W output power. However, as a result of NPM active fiber and FBGs, the PER just fluctuates from 0 (50%) to 0.46 dB (52.7%) in the power scaling process. At maximal output power (64 W), the PER is only measured to be 0.09 dB (50.5%). The 3 dB spectral line-widths at 3.2 W, 35 W and 64 W are measured to be 0.24 nm, 0.4 nm and 0.54nm, respectively. Then the 1120nm laser from the NPM cavity is injected into the PSL-imposed 1080 nm laser cavity and its PER purified effect is investigated when it transmitted through 1080 nm cavity. In this process, the 1080 nm laser is switched off. Figure 4(c) shows the power scaling process and PER purified results when 1120 nm NPM laser transmitted through the 1080 nm cavity, which is measured in the output port of PM-OC. From Fig. 4(c), we show that the PER is purified to be as high as 13 dB (95.2%) when 64 W 1120 nm NPM laser transmitted through 1080 nm cavity. Nevertheless, because of the functions of PSL and re-absorption in 1080 nm cavity, the 1120 nm output power is lost from 64 W to 15.5 W.
Fig. 4 (a) The output power scaling characteristic and measured PERs of the separate 1120 nm cavity; (b) The spectra of 1120 nm laser at 3.2 W, 35 W and 64 W; (c) The power boosting process and PERs purified results when 1120 nm NPM laser transmitted through PSL-imposed 1080 nm laser cavity.
2.2 Amplification of linear-polarized Yb-Raman cascaded oscillators
After achieving the linear-polarized Yb-Raman cascaded oscillators based on PSL mechanism, we establish an all fiber and PM Yb-Raman combined nonlinear fiber amplifier for further power scaling of the 1120 nm laser. The schema of experimental setup is shown in Fig. 5. After cascaded seeds, the hybrid linear-polarized 1120 nm and 1080 nm lasers are simultaneously injected into an all fiber and PM high power amplifier for Yb-Raman nonlinear amplification. As for the PM high power amplifier, six high power LDs (central wavelength~976nm) are incorporated into a (6 + 1) × 1 PM pump combiner to pump the active fiber. The active fiber is PM large mode area (PM-LMA) double clad fiber, which has a core diameter of 20 μm and an inner cladding diameter of 400 μm. The cladding absorption coefficient of the active fiber is about 1.7 dB/m at 976 nm, and 21 m long active fiber is used in the amplifier. Then, a high power end-cap with 1.5 m PM passive fiber with the same core and inner cladding diameters as active fiber is spliced to the active fiber for power delivery and about 40 cm pump dump section is made in the passive fiber for stripping out the residual pump laser. Finally, a coated collimator is used to collimate the beam into free space.
In the experiment, the output powers of separate 1120 nm NPM cavity and 1080 nm cavity is set to be 64 W and 30 W, respectively. As afore-mentioned, with just single cavity operation, the output powers of 1120 nm and 1080 nm lasers in the output port of PM-OC are 15.5 W and 30 W, which seems that the total output power of the cascaded seeds is about 45.5W. However, in the experiment, the 1120 nm laser would be nonlinear amplified in the 1080 nm cavity. As a result, the 1080 nm laser power will be gradually transferred into 1120nm laser in 1080 nm cavity. Because that PSL is imposed in 1080 nm cavity, so a portion of the transferred 1120 nm laser power will be lost. Consequently, the measured total output power of the cascaded seeds will be lower than 45.5 W in practice. In our experiment, the total output power of the cascaded seeds is 37.1 W, and the 1120 nm power ratio is about 56.5%. By collimating the output beam of the cascaded seeds and further using a polarization-independent dichromatic mirror (DCM) to separate the 1120 nm laser and 1080 nm laser, the PERs of 1120 nm and 1080 nm lasers are measured to be 13.2 dB and 16.7 dB, respectively.
In the experiment, along with increasing 976 nm pump power, by measuring the total output power and integrating the corresponding wavelength ranges, the power scaling and transferring process of different wavelength ranges can be obtained, which is shown in Fig. 6(a). In Fig. 6(a), along with power scaling process, power ratios at central wavelength of 1080nm were integrated from 1070 nm to 1095 nm and power ratios at central wavelength of 1120 nm were integrated from 1095 nm to 1150 nm. At 1561 W 976 nm pump power, the total output power of ~1279 W is achieved with the 1120 nm output power of ~1181 W, which means that the power ratio of 1120 nm laser is as high as 92.31%. The optical to optical efficiency of 1120 nm at 1181 W is calculated to be 74.3%. At maximal output power, the residual output power at central wavelength of 1080 nm and power in the wavelength range of 1150-1200 nm are measured to be just 96.3 W (power ratio~7.53%) and 2 W (power ratio~0.16%). Figure 6(b) gives the output spectra at total power-level of 25.7 W and 1279 W. From Fig. 6(b), we obtain that the 3 dB line-width of 1120 nm laser broadens from 0.8 nm to 1.1 nm in the power scaling process. This line-width broadening effect is mainly induced by the combination effects of self-phase-modulation (SPM) and four-wave-mixing (FWM) [22, 23]. Besides, at 1279 W total output power, a bit peak at 1160 nm and a side peak at 1180 nm are observed. The 1160 nm bit peak is caused by the Raman-assisted-amplified FWM effect between the 1080 nm laser and 1120 nm laser, and the 1180 nm peak is the second order Raman stokes light. Figure 6(c) shows the measured beam quality (M2 factor) at maximal total output power. Based on 4-sigma method by M2-200s, the M2 factor is measured to be M2x~1.16 and M2y~1.15.
Fig. 6 (a) The power scaling and transferring process of different wavelength ranges in Yb-Raman amplifier; (b) The output spectra at total power-level of 25.7 W and 1279 W; (c) The measured beam quality (M2 factor) at maximal output power.
An interesting issue should be concerned is the PERs of the output laser. In the experiment, by using the polarization-independent DCM to separate the 1120 nm laser and 1080 nm laser, the PERs of the two split beams are respectively investigated with different total output powers. The measured PERs results are shown in Fig. 7(a). From Fig. 7(a), it is shown that the PER of 1120 nm is gradually optimized from 13.8 dB to 18.2 dB while the PER of 1080 nm is gradually degraded from 15 dB to 9.4 dB. By switching off the 1120 nm cavity and just measuring the PERs of 1080 nm laser in the Yb-Raman amplifier, we find that 1298 W 1080 nm laser can be achieved with slope efficiency of 82.9%. More importantly, the PER just fluctuates from 15.6 dB to 14.8 dB in the power scaling process. Thus, we infer that the PERs results in Fig. 7(a) is mainly attributed to the fact that Raman gain is related to the polarization directions of the signal laser (1080 nm) and Raman stokes (1120 nm) [24]. Specifically, the Raman gain coefficient is approximately an order of magnitude smaller when the Raman stokes is orthogonally polarized to the signal laser compared with the co-polarized case [24]. Thus, the Raman gain of the 1120 nm signal is effectively polarizing on the slow-axis of the 1080 nm signal, leading to improving PERs of 1120 nm laser in the power scaling process. This also has the effect of degrading the 1080 nm PERs due to pump depletion from the strong transfer of power to the 1120 nm signal on one axis. Figures 7(b) and 7(c) respectively show the output spectra of the 1120 nm laser output port (reflected port) and 1080 nm laser output port (transmitted port) of the polarization-independent DCM at maximal output power. From the measurement, we conclude that the isolation degree of 1120 nm laser output port to 1080 nm laser is as high as 24.6 dB and the isolation degree of 1080 nm laser output port to 1120 nm laser is as high as 28 dB.
Fig. 7 (a) The measured PERs results of 1080 nm and 1120 nm lasers; (b) the output spectrum of the 1120 nm laser output port of the polarization-independent DCM at maximal output power; (c) the output spectrum of the 1080 nm laser output port of the polarization-independent DCM at maximal output power.
2.3 The influence of 1120 nm and 1080 nm seed powers on the power ratios of the nonlinear amplified 1120 nm laser
In the nonlinear Yb-Raman amplifier, in order to optimize the power transferring efficiency from 1080 nm to 1120 nm laser and comprehend the transferring process between the signal laser and Raman stokes, it is necessary to investigate the influence of 1120 nm and 1080 nm seed powers on the power ratios of the nonlinear amplified 1120 nm laser. Based on our experimental setup, the influence of 1080 nm and 1120 nm seed powers on the power ratios of the nonlinear amplified 1120 nm laser can be simply studied by respectively controlling the 976 nm pump power in the 1120 nm cavity and 1080 nm cavity.
Firstly, we just adjust the 976 nm pump power in the 1120 nm cavity and remain the output power of 1080 nm cavity operating at 30 W power level. Some representative characteristics of the cascaded seeds are shown in Table 1. In Table 1, Ps is the total power of the cascaded seeds, and Γ1120 is the power ratio of 1120 nm laser in the cascaded seeds.
Table 1. Characteristics of the cascaded seeds
The power scaling and conversion characteristics of the PM nonlinear Yb-Raman amplifier are shown in Fig. 8. Figure 8(a) shows the total power scaling characteristics as a function of 976 nm pump power at different seed powers listed in Table 1. From Fig. 8(a), we show that the slope efficiencies at different seed powers are nearly identical in the amplified process. At 1561 W 976 nm pump power, the maximal total output powers are measured to be 1282 W, 1279 W, 1281 W, 1287 W and 1279 W, respectively. Figure 8(b) shows the 1120 nm power ratios as a function of the 976 nm pump power at different seed powers in Table 1. From Fig. 8(b), we show that the ultimate 1120 nm power ratios are calculated to be 77.78%, 82.77%, 87.13%, 91.45% and 92.31%, respectively. The experimental results show that when the operating power in 1080 nm cavity is determinate, the power ratio of the nonlinear amplified 1120 nm laser can be optimized by increasing the seed power of 1120 nm laser cavity. This phenomenon is mainly attributed to the fact that higher 1120 nm Raman stokes seed power will generate stronger power conversion effect from 1080 nm laser to 1120 nm laser in the nonlinear amplifier when the 1080 nm signal seed power remains the same. However, when Γ1120 is higher than 46.9%, the optimizing effect of the power ratio of nonlinear amplified 1120 nm laser becomes not obvious in the experiment. Besides, when the 976 nm pump power is below ~0.2 kW, due to that the powers of 1080 nm and 1120 nm lasers are relatively low, the dual-wavelength (1080 nm and 1120 nm) cascaded seeds are mainly amplified based on Yb ion gain while effective Raman amplification and energy transfer between 1080 nm laser and 1120 nm laser are not obvious. In this stage, because that the emission cross section in Yb-doped fiber at 1080 nm is much larger than that in the wavelength of 1120 nm, the output power of 1080 nm laser grows rapidly while the output power of 1120 nm laser increases in a slow step. However, when the 976 nm pump power is higher than ~0.2 kW, effective Raman amplification and energy transfer between 1080 nm laser and 1120 nm laser will be gradually obvious and the output power of 1120 nm laser will gradually dominate the nonlinear amplifier. Consequently, the 1120 nm power ratio has a trough at the 976 nm pumping power of ~0.2 kW in the experiment (shown in Fig. 8(b)). These experimental results are compatible with our previous theoretical analysis of the power conversion process of this type of nonlinear Yb-Raman hybrid amplifier [10].
Fig. 8 (a) The total output power scaling characteristics as a function of 976 nm pump power at different total seed powers listed in Table 1; (b) The 1120 nm power ratio as a function of the 976 nm pump power at different total seed powers listed in Table 1.
Secondly, we gradually increase the operating power in the 1080 nm cavity and the output power of 1120 nm cavity is constant. In this circumstance, the representative characteristics of the cascaded seeds are presented in Table 2.
Table 2. Characteristics of the hybrid seed
In the nonlinear amplified process, along with increasing of the 976 nm pump power, the total output power scaling and transferring characteristics at different seed powers in Table 2 are shown in Fig. 9. From Fig. 9(a), we also conclude that the slope efficiencies at different seed powers are nearly identical in the amplified process. At maximal 976 nm pump power, the total output powers are measured to be 1279 W, 1280 W, 1276 W and 1282W, respectively. From Fig. 9(b), even Γ1120 is decreased along with increasing of the power in the 1080 nm cavity, we show that the ultimate 1120 nm power ratios are respectively calculated to be 86.75%, 87.36%, 87.9%, and 88.6%, which is gradually optimized a little in the experiment. These experimental results can be comprehensible because that the 1080 nm laser gradually offers pump power to 1120 nm laser along with the nonlinear amplifier, so higher 1080 nm seed power will also generate stronger power conversion effect from 1080 nm laser to 1120 nm laser when the 1120 nm signal seed power is certain.
Fig. 9 (a) The total output power scaling characteristics as a function of 976 nm pump power at different total seed powers listed in Table 2; (b) the 1120 nm power ratio as a function of the 976 nm pump power at different total seed powers listed in Table 2.
By separately controlling the output power in 1120 nm and 1080 nm cavities to investigate the influence of 1120 nm and 1080 nm seed powers on the ultimate power ratios of the nonlinear amplified 1120 nm laser, we show that the ultimate 1120 nm power ratio can be mainly optimized by increasing the 1120 nm seed power while the optimized effect is not very obvious by increasing the 1080 nm seed power in our experiment.
3. Conclusion
In conclusion, we perform Yb-Raman cascaded oscillators with linear-polarized output based on PSL mechanism. By employing PSL mechanism in signal laser (1080 nm) cavity, both linear-polarized Raman stokes and signal laser are obtained in the output port of the cascaded oscillators. Further, a high power PM Yb-Raman hybrid nonlinear amplifier is demonstrated based on the new designed cascaded seeds. With full 976 nm pump power, 1181 W output power is achieved at central wavelength of 1120 nm with near diffraction-limited beam quality (M2<1.2). At maximal output power, the 3 dB line-width and PER of the 1120 nm laser is measured to be 1.1 nm and 18.2 dB. The design concept of Yb-Raman cascaded seeds with linear-polarized output can be further extended to amplifying the wavelength range of 1100-1200 nm.
Acknowledgment
This research is sponsored by the National Natural Science Foundation of China under NO. 11274386.
References and links
1. S. D. Jackson, “Towards high-power mid-infrared emission from a fiber laser,” Nat. Photonics 6(7), 423–431 (2012). [CrossRef]
2. L. R. Taylor, Y. Feng, and D. B. Calia, “50W CW visible laser source at 589nm obtained via frequency doubling of three coherently combined narrow-band Raman fibre amplifiers,” Opt. Express 18(8), 8540–8555 (2010). [CrossRef] [PubMed]
3. S. Sinha, C. Langrock, M. J. Digonnet, M. M. Fejer, and R. L. Byer, “Efficient yellow-light generation by frequency doubling a narrow-linewidth 1150 nm ytterbium fiber oscillator,” Opt. Lett. 31(3), 347–349 (2006). [CrossRef] [PubMed]
4. H. M. Pask, R. J. Carman, D. C. Hanna, A. C. Tropper, C. J. Mackechnie, P. R. Barber, and J. M. Dawes, “Ytterbium-doped silica fiber lasers: versatile sources for the 1-1.2 μm region,” IEEE J. Sel. Top. Quantum Electron. 1(1), 2–13 (1995). [CrossRef]
5. H. Zhang, H. Xiao, P. Zhou, K. Zhang, X. Wang, and X. Xu, “322 W single-mode Yb-doped all-fiber laser operated at 1120 nm,” Appl. Phys. Express 7(5), 052701 (2014). [CrossRef]
6. V. R. Supradeepa and J. W. Nicholson, “Power scaling of high-efficiency 1.5 μm cascaded Raman fiber lasers,” Opt. Lett. 38(14), 2538–2541 (2013). [CrossRef] [PubMed]
7. P. Dupriez, C. Farrell, M. Ibsen, J. K. Sahu, J. Kim, C. Codemard, Y. Jeong, D. J. Richardson, and J. Nilsson, “1 W average power at 589 nm from a frequency doubled pulsed Raman fiber MOPA system,” Proc. SPIE 6102, 61021G (2006). [CrossRef]
8. Y. Feng, L. R. Taylor, and D. B. Calia, “150 W highly-efficient Raman fiber laser,” Opt. Express 17(26), 23678–23683 (2009). [CrossRef] [PubMed]
9. L. Zhang, H. Jiang, S. Cui, and Y. Feng, “Integrated Ytterbium-Raman fiber amplifier,” Opt. Lett. 39(7), 1933–1936 (2014). [CrossRef] [PubMed]
10. H. Zhang, H. Xiao, P. Zhou, X. Wang, and X. Xu, “High power Yb-Raman combined nonlinear fiber amplifier,” Opt. Express 22(9), 10248–10255 (2014). [CrossRef] [PubMed]
11. L. Zhang, C. Liu, H. Jiang, Y. Qi, B. He, J. Zhou, X. Gu, and Y. Feng, “Kilowatt Ytterbium-Raman fiber laser,” Opt. Express 22(15), 18483–18489 (2014). [CrossRef] [PubMed]
12. H. Zhang, R. Tao, P. Zhou, X. Wang, and X. Xu, “1.5-kW Yb-Raman combined nonlinear fiber amplifier at 1120 nm,” IEEE Photonics Technol. Lett. 27(6), 628–630 (2015). [CrossRef]
13. T. H. Runcorn, R. T. Murray, E. J. R. Kelleher, S. V. Popov, and J. R. Taylor, “Duration-tunable picosecond source at 560 nm with watt-level average power,” Opt. Lett. 40(13), 3085–3088 (2015). [CrossRef] [PubMed]
14. A. Shirakawa, M. Kamijo, J. Ota, K. Ueda, K. Mizuuchi, H. Furuya, and K. Yamamoto, “Characteristics of linearly polarized Yb-doped fiber laser in an all-fiber configuration,” IEEE Photonics Technol. Lett. 19(20), 1664–1666 (2007). [CrossRef]
15. A. El-Damak, J. Chang, J. Sun, C. Xu, and X. Gu, “Dual-wavelength, linearly polarized all-fiber laser with high extinction ratio,” IEEE Photonics J. 5(4), 1501406 (2013). [CrossRef]
16. J. Wang, J. Hu, L. Zhang, X. Gu, J. Chen, and Y. Feng, “A 100 W all-fiber linearly-polarized Yb-doped single-mode fiber laser at 1120 nm,” Opt. Express 20(27), 28373–28378 (2012). [CrossRef] [PubMed]
17. L. Huang, P. Ma, R. Tao, C. Shi, X. Wang, and P. Zhou, “Experimental investigation of thermal effects and PCT on FBGs-based linearly polarized fiber laser performance,” Opt. Express 23(8), 10506–10520 (2015). [CrossRef] [PubMed]
18. C. H. Liu, A. Galvanauskas, V. Khitrov, B. Samson, U. Manyam, K. Tankala, D. Machewirth, and S. Heinemann, “High-power single-polarization and single-transverse-mode fiber laser with an all-fiber cavity and fiber-grating stabilized spectrum,” Opt. Lett. 31(1), 17–19 (2006). [CrossRef] [PubMed]
19. S. Belke, F. Becker, B. Neumann, S. Ruppik, and U. Hefter, “Completely monolithic linearly polarized high-power fiber laser oscillator,” Proc. SPIE 8961, 896124 (2014). [CrossRef]
20. D. Marcuse, “Curvature loss formula for optical fibers,” J. Opt. Soc. Am. 66(3), 216–220 (1976). [CrossRef]
21. K. Okamoto, “Single-polarization operation in highly birefringent optical fibers,” Appl. Opt. 23(15), 2638–2642 (1984). [CrossRef] [PubMed]
22. J. T. Manassah, “Self-phase modulation of incoherent light revisited,” Opt. Lett. 16(21), 1638–1640 (1991). [CrossRef] [PubMed]
23. J. P. Fève, “Phase-matching and mitigation of four-wave mixing in fibers with positive gain,” Opt. Express 15(2), 577–582 (2007). [CrossRef] [PubMed]
24. G. P. Agrawal, Nonlinear Fiber Optics, 4th ed. (Academic Press, 2007).
