A kilowatt-level Raman fiber laser is demonstrated with an integrated Ytterbium-Raman fiber amplifier architecture. A high power Ytterbium-doped fiber master oscillator power amplifier at 1080 nm is seeded with a 1120 nm fiber laser at the same time. By this way, a kilowatt-level Raman pump laser at 1080 nm and signal laser at 1120 nm is combined in the fiber core. The subsequent power conversion from 1080 nm to 1120 nm is accomplished in a 70 m long passive fiber. A 1.28 kW all-fiber Raman amplifier at 1120 nm with an optical efficiency of 70% is demonstrated, limited only by the available pump power. To the best of our knowledge, this is the first report of Raman fiber laser with over one kilowatt output.
© 2014 Optical Society of America
Recently, fiber lasers have drawn extensive attention due to their high efficiency, high power scaling capacity and wide emission spectrum. Until now, up to 10 kW continuous wave ytterbium-doped fiber (YDF) laser in close to diffraction-limited beam quality has been reported . Typically, the efficient emission wavelength of a YDF laser is between 1030 nm and 1100 nm. However, some specific wavelengths beyond this wavelength range have important applications. For example, high power 1120 nm laser could be used to pump Raman fiber amplifier at 1178 nm for laser guide star  and to pump Tm-doped fiber . Lasers with wavelength ranging from 1150 nm to 1160 nm are attractive, since their second harmonics could find wide applications in medicine, ophthalmology and dermatology . High power narrow linewidth laser at 1262 nm is required for remote sensing of atmospheric oxygen . Extending to even longer spectral range, Ytterbium-and Erbium co-doped fiber lasers operating at 1.5 µm wavelength region also have attractive features, such as eye safety and atmospheric transparency. However, their power scaling is limited by the lower quantum efficiency and the onset of 1 µm lasing . There is an increasing interest in 1645 nm laser, which is used for pumping mid-infrared optical parametric oscillator . Recent research progress shows that all these lasers can be potentially produced with a single laser technology, which is YDF laser pumped Raman fiber laser.
With the right pump source, Raman fiber lasers could lase at arbitrary wavelength across the transparency window of optical fibers. Core-pumped Raman fiber lasers with output power of up to 200 W at 1120 nm with high optical efficiency were reported in [8–10]. Codemard et al. demonstrated a 100 W continuous wave (CW) Raman fiber laser operating at 1120 nm, cladding-pumped with an YDF laser . Supradeepa et al reported a 300 W high-efficiency cascaded Raman fiber laser at 1.5 µm with a novel amplifier architecture, core-pumped by a high power YDF laser . In their experiments, the wavelength division multiplexer (WDM) used to combine the Raman pump and seed lasers needs to handle hundreds to thousands watts laser power. For even higher power, this component could be the bottle neck.
Most recently, we proposed an integrated ytterbium-Raman fiber amplifier (YRFA) architecture for power scaling of Raman fiber laser . A 300 W all-fiber linearly-polarized single mode amplifier at 1120 nm was demonstrated in a proof of principle experiment. One of the most important improvements for this architecture is the elimination of the WDM that has been used in almost all core-pumped Raman fiber laser. In the new architecture, the Raman seed lasers and pump laser are propagated and amplified in the core of the same fiber. Later, Zhang et al. reported a 730 W 1120 nm laser with a similar configuration, which proves that the new YRFA architecture has a good perspective of power scaling .
In this paper, we report a demonstration of a kilowatt level Raman fiber laser with the proposed YRFA architecture. A 1.54 kW Yb-doped fiber master oscillator power amplifier (MOPA) at 1080 nm is seeded again with a 1120 nm fiber laser. The amplified 1080 nm laser is Raman-shifted to 1120 nm in a subsequent piece of Raman gain fiber. Up to 1.28 kW Raman fiber laser at 1120 nm is achieved with an optical efficiency of 70%, limited by the available pump power, which is the first report of Raman fiber laser with over kilowatt power. A preliminary numerical simulation shows that kilowatt level Raman fiber laser covering 1.1~2 µm is feasible with the YRFA architecture and a state-of-the-art high power Yb fiber MOPA laser.
2. Experiment configuration
Figure 1 illustrates the schematic diagram of the kilowatt-level Ytterbium-Raman fiber laser. The Raman Stokes seed laser is a 1120 nm YDF fiber laser , which emits 40 W of linearly-polarized laser. Then a 1080 nm laser oscillator consists of a pair of FBGs (R>99% high reflector and R = 10% output coupler) and 20 m of Yb-doped double-clad fiber (a core diameter of 20 µm, a numerical aperture of 0.06, a cladding diameter of 400 µm, and a nominal cladding absorption of 1.2 dB/m at 976 nm), followed by a cladding mode stripper (CMS). The 1120 nm light is coupled into the 1080 nm laser cavity through a pump and signal combiner. The multimode input ends of the combiner are connected to the pigtailed 976 nm laser diodes with total power of 148 W. The 1080 and 1120 nm laser are then coupled into the main YDF booster amplifier for power amplification. The total available pump power for this stage is 1630 W. 12 m 20/400 YDF with the same fiber parameters as those of the 1080 nm oscillator is used as the gain fiber. A second CMS is spliced after the amplifier to remove the residual pump laser in the cladding. At the end of the YDF amplifier, a piece of 70 m-long germanium-doped fiber (GDF) with the matching parameters to the YDF fiber is spliced as a Raman converter (Nufern LMA-GDF-20/400, background loss <15 dB/km at 1095nm). The output end of this fiber is cleaved at an angle of 8° to suppress the parasitic oscillation.
3. Experimental results and analysis
First, the performance of the 1080 nm YDF MOPA is measured with the 1120 nm laser turned off and without the 70 m Raman gain fiber. The 1080 nm oscillator gave an output power of 110 W with a pump power of 148 W, corresponding to an optical efficiency of 75%. After the power boost amplifier, as depicted in Fig. 2, the maximum output power at 1080 nm is up to 1.54 kW, with a total coupled-in pump power of 1.63 kW. The maximum optical-to-optical conversion efficiency is around 87.7%. The output spectrum at an output power of 800 W is measured with an optical spectrum analyzer (AQ6370, YOKOGAWA) with a multimode (50μm) fiber patchcord and 0.02 nm resolution. The result is shown in the inset of Fig. 2. The M2 of the 1080nm laser is measured to be 1.4 at full output power.
Then the 1120 nm seed laser is turned on. It propagates through the 1080 nm fiber oscillator and enters the YDF boost amplifier together with the 1080 nm light. In both YDF oscillator and boost amplifier, the gain at 1080 nm is higher than that at 1120 nm. As a result, the 1120 nm laser gets amplified less than the 1080 nm laser. Figure 3(a) shows the power ratios of the 1080 nm and 1120 nm light as a function of the total output power after the YDF part of the amplifier. When the total output power is lower than 800 W, the 1120 nm laser ratio drops as expected. However, when the output power increases to above 800 W, the Raman shift between the two wavelengths becomes significant already in the YDF and, as a result, the 1120 nm ratio increases. The output spectrum of the dual-wavelength laser after the YDF part of the amplifier at full output power is depicted in Fig. 3(b), and the power ratios of 1080 nm and 1120 nm are 75.3% and 24.7%, respectively. Accordingly, the power of the 1080 nm and 1120 nm laser are calculated to be 1167 W and 383 W, respectively.
Then 70 m GDF fiber with the matched parameters is spliced to the YDF boost amplifier after a CMS to serve as the Raman converter. At full pump power, a maximum total output of 1.49 kW is achieved as shown in Fig. 4. According to the spectrum depicted in Fig. 3(b) (in dB scale) and in the inset of Fig. 4 (in linear scale), the 1120 nm power ratio is calculated to be 86%, as shown in Fig. 5.The maximum 1120 nm laser power is 1.28 kW, which is much higher than ever reported for Raman fiber lasers [13, 16]. The optical conversion efficiency from 976 nm diode laser to 1120 nm laser is 70%.
In , the 1120 nm power ratio reaches over 98% at maximum power. Here at full power, there is still about 190 W 1080 nm residual laser which could not be Raman-shifted to 1120 nm. In fact, the conversion ratio saturates already below 1 kW. Most of the residual 1080 nm laser is in the cladding mode because of the low-NA cladding mode light leaking through the CMS between the YDF and Raman fiber, which cannot be converted to 1120 nm. This explanation is confirmed by adding an extra CMS after the Raman fiber. At a total power of 830 W, the 1120 nm power ratio is already 85% and the M2 is measured to be 1.6. Since this CMS could not withstand the full power, the data shown in Fig. 4 is measured without CMS. That is why significant amount of 1080 nm laser exists in the total laser output. Nevertheless, the reading of 1120 nm output power is precise, since it is calculated by spectral integration. We had also checked the time domain characteristic of the laser, which is continuous wave.
The 1120 nm laser spectra at zero and full pump power are also depicted in Fig. 3(b). The linewidth of the 1120 nm laser broadens from 0.4 nm to 3.0 nm, which is mainly due to the four-wave-mixing between numerous longitudinal modes associated with a long fiber. Besides, at high power, two peaks at 1160 nm and 1180 nm are observed. The 1160 nm peak comes from the four-wave-mixing of the 1080 nm and 1120 nm laser and amplified by the stimulated Raman scattering. The 1180 nm peak is the amplified spontaneous Raman emission of the 1120 nm laser. As shown in Fig. 5, the proportion of light at 1150~1200 nm increases up to 1.5%, that can be mitigated by decreasing the power ratio of 1120 nm laser in the seed laser and optimizing the length of Raman gain fiber.
Having demonstrated a 1.28 kW YRFA at the first Raman Stokes wavelength, a future work would be the power scaling of this laser to kilowatt level at wavelengths ranging from 1.1 µm to 2 µm by injecting a multiple-wavelength seed laser whose wavelength separations match the Raman shifts of the consecutive order. Since the state-of-the-art YDF MOPA laser can now produce several tens of kilowatt output  and the cascaded Raman laser made the multi-wavelength seed laser available [16, 18], we think that bringing the Raman fiber laser/amplifier to kilowatt level at the wavelength not attainable by Yb and Er doped gain medium will represent a significant step in the development of high power fiber laser. In the following, we show a preliminary simulation to show the potential.
A 10-order cascaded Raman fiber amplifier from 1070 nm to 2066 nm is numerically simulated with a model similar to that in . The intermediate Raman Stokes wavelengths are 1120, 1180, 1242, 1314, 1394, 1486, 1598, 1728, and 1882 nm. The 11th-order spontaneous Raman emission at 2.29 µm is also included in simulation. Note that the YDF laser can lase efficiently from 1030 nm to 1100 nm and the Raman gain spectrum has a linewidth over 5 THz , so any wavelength between 1.1 µm and 2 µm could be generated by a Yb fiber laser pumped cascaded Raman fiber amplifier. In the simulation, a fiber with a core diameter of 20 µm and NA of 0.06 is considered. The loss of the fiber is set to be the same as that of SMF28 fiber. And the Raman gain coefficients are given by 5 × 10−14 /λ, where λ is wavelength in µm.
The high power 1070 nm YDF pump laser is set to produce up to 5.0 kW in our simulation. Figure 6(a) shows the power evolution of the pump laser and each order of the Raman Stokes laser as a function of the Raman gain fiber length. Depending on the fiber length, several kilowatts of intermediate Raman Stokes lasers can be generated. Over 2.0 kW of 2066 nm laser could be achieved by the 10th-order Raman conversion with an optimum fiber length of about 110 m. The optical efficiency is 43%, close to the quantum efficiency of 52%. Figure 6(b) shows the power evolution of each Stokes light as a function of the pump power with a 120 m long fiber. The growth of individual Stokes components is shifted to the higher Stokes order consecutively with no sign of the significant 11th-order Raman Stokes emission. The increased loss of silica fiber after 2 µm (0.0133 m−1 at 2290 nm) provides a nature filtering effect, which contributes to the suppression of the 11th-order Raman Stokes.
Of course, to realize such high power cascaded Raman fiber amplifiers, one may encounter problems of significant linewidth broadening and four wave mixing between the Raman Stokes etc due to other Kerr nonlinearity. These will be the topics of future study.
In summary, we have demonstrated a 1.28 kW Raman fiber laser based on an integrated Ytterbium-Raman fiber amplifier architecture, which is the first report of Raman fiber laser with over kilowatt output. The laser is constructed by injecting a seed laser at 1120 nm into an 1080 nm Yb doped fiber MOPA and splicing a piece of Raman fiber after MOPA. Both the Raman pump laser and seed laser propagate and are amplified in the core of same fiber, therefore, the requirement for a demanding high power WDM component is avoided. Power scaling to even higher power is feasible, considering the state-of-the-art high power Yb-doped fiber MOPAs. The YRFA architecture offers a prospect of achieving kilowatt-level Raman fiber lasers covering the whole spectral range from 1.1 µm and 2 µm by seeding a kilowatt Yb-doped fiber MOPA with a multiple-wavelength laser. Future work will concentrate on the wavelength expansion of the high power Raman fiber laser.
This work was supported by the National Natural Science Foundation of China (No. 61378026) and the Hundred Talent Program of the Chinese Academy of Sciences.
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