A linearly polarized single-transverse-mode Yb-doped double clad polarization-maintaining fiber amplifier at 1064nm is reported, which delivers 53.1W average power at 40kHz repetition rate, with pulse energy >1mJ, duration 30ns, polarization extinction ratio 13dB, and M2<1.2. Stimulated Brillouin scattering (SBS) phenomenon during amplification is discussed, and a new SBS suppression approach using linewidth broadening induced by self-phase modulation (SPM) is demonstrated.
© 2006 Optical Society of America
A wide variety of applications, such as material processing, nonlinear frequency conversion, ranging, etc., require laser output with linear polarization, high average and peak power, mJ-level pulse energy, multi-10ns duration and multi-10kHz repetition rate. Instead of conventional Q-switched diode-pumped solid-state lasers (DPSSLs), pulsed Yb-doped double clad fiber lasers and amplifiers have attracted more attention, because of their advantages such as high beam quality, high efficiency, outstanding thermal behavior, compactness and robustness.
Although output power of continuous-wave fiber lasers for random polarized and linearly polarized output is as high as 1.36KW  and 633W  respectively, the power scaling of pulsed fiber lasers is much more difficult, because of the nonlinear effects of the fiber, such as stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS), which are caused by high peak power and tight confinement in the fiber core. Limpert et al. reported a multi-10ns multi-mJ fiber amplifier with 100W average power , but the output was unpolarized because the fiber was not polarization-maintaining (PM).
The thresholds of SBS and SRS in PM fibers are much lower than in non-PM ones [4, 5]. Thus, the power scaling of linearly polarized pulsed fiber lasers and amplifiers is more challenging. For pulsed fiber amplifier in multi-10ns and multi-10kHz regime, SBS is the dominant limitation. Avila et al. found that the average power of PM fiber amplifier was limited to only 2-5W level by the onset of SBS effect . Although Liu et al. recently achieved a linearly polarized fiber amplifier with 120W average power , the result is not comparable to ours, because we focus on relatively high peak power (10KW level) and pulse energy (mJ-level) in multi-10ns and multi-10kHz regime, while Liu’s pulse duration is much shorter (5ns) and repetition rate is as high as 10MHz, which makes the peak power (2.4KW) and pulse energy (12?J) very low.
In this contribution, we report on a linearly polarized single-transverse-mode Yb-doped fiber amplifier with average power of 53.1W at 1064nm, which generates pulses of >1mJ energy, 30ns duration, and 40kHz repetition rate. The polarization extinction ratio (PER) is 13dB, and M2<1.2. SBS phenomenon during amplification is discussed, and a new SBS suppression approach using linewidth broadening induced by self-phase modulation (SPM) is demonstrated. To our knowledge, this result represents the highest average power, peak power and pulse energy achieved to date in linearly polarized multi-10ns multi-10kHz regime from Yb-doped double clad fibers.
2. Experimental setup
The experimental setup is shown in Fig. 1(a). The fiber used in the experiment is a 6-m-long Yb-doped Panda-type PM double clad fiber manufactured by Liekki Oy, which has a core diameter of 21.6 um (NA=0.08), and an inner clad diameter of 386 um (NA=0.46). Two stress rods in the round shaped inner clad generate a birefringence of 2×10-4. A microscope image of the fiber cross-section is shown in Fig. 1(b). The pump absorption at 976 nm is ~3dB/m. In order to avoid fiber facet damage and to eliminate reflection, both ends are end-capped and polished at an angle of 8 degrees. The fiber is coiled to a diameter of 8cm to filter out the high-order modes . The fundamental mode-field diameter is 18 um.
The seed source is an acousto-optic Q-switched Nd:YVO4 laser, which can provide up to 10W average power at 1064 nm. Since the output linewidth and pulse duration of the seed source varies under different output power, we fix the output power of the seed source to 10W and adjust the power injected into the fiber by rotating the half waveplate in front of the seed source (HWP1 in Fig. 1(a)). By this means, we can change only the injected seed power with constant seed linewidth and pulse duration. Although less than 0.5W is sufficient to saturate the fiber amplifier, higher seed power is used to stimulate extra linewidth broadening induced by SPM for SBS suppression, which will be discussed below. A Faraday rotator, a quartz rotator and two polarization beam splitters (PBSs) are used as the optical isolator. Another half waveplate (HWP2 in Fig. 1(a)) is applied to adjust the polarization orientation of seed light to be aligned in the slow axis direction of the PM fiber. The fiber amplifier is pumped by a collimated LD with a central wavelength of 976nm. A piece of dichroic mirror, which has high reflectivity at 1064nm and high transmissivity at 976nm, is placed by an angle of 45 degrees to separate the pump light and the amplified signal light.
3. Stimulated Brillouin scattering phenomenon and its suppression
For a wide range of repetition rates from 20kHz to 100kHz, when 0.5W seed light was injected into the fiber, SBS occurred at less than 10W level of output average power. SBS pulses appeared first in the backward direction. At higher output power, SBS pulses appeared in both forward and backward directions with higher intensity and instability. Cascaded SBS was observed at the output of the fiber amplifier by an Agilent 86140B optical spectrum analyzer (OSA), shown in Fig. 2(a). The frequency spacing between the neighboring peaks was 34GHz, corresponding to a Brillouin frequency shift of 17GHz, which is in good agreement with the theoretical value of the silica fiber at 1064nm.
The intensity and duration of SBS pulses were quite unstable, as shown in Fig. 2(b), captured with a 1.5GHz Agilent oscilloscope and a 350MHz Thorlabs DET210 photo-detector. The higher the output power the shorter the pulse duration. The duration of forward SBS pulses were shorter. When cascaded SBS occurred, forward SBS pulses were extremely sharp, with durations as short as 1ns, which may be limited by the 350MHz bandwidth of the photo-detector. Some SBS pulses split into a series of sharp peaks, each had few ns duration, and the separation of the peaks was several ns. The peak power of SBS pulses was much higher than the signal pulses, since their durations were much shorter. If pump power is increased to a high level ignoring the onset of SBS, the SBS pulses will have extremely high peak power, which will cause catastrophic damage of the fiber. In our experiment, without any suppression, the SBS effect limited the average output power to only a few watts, which coincided with previous reports [6, 9].
To achieve higher output power, SBS must be suppressed first. Since the SBS threshold is highly related to the signal linewidth , broader linewidth is expected for SBS suppression. However, for Q-switched DPSSLs, the linewidth is limited by the properties of the laser crystal; thus, it can not be broader in nature. FP laser diodes can offer broader linewidth, but the average power of single-mode laser diodes at 1064nm is low. If a FP laser diode is used as the seed source, a multi-stage fiber amplifier system must be built, of great complexity.
We find that linewidth broadening induced by SPM can be used to suppress SBS. SPM is mainly related to the peak power, pulse duration and effective interaction length. With the increase of the signal power, SPM gradually broadens the output linewidth. Generally, the linewidth broadening induced by SPM is not sufficient to suppress SBS. Consequently, SBS occurs. Once SBS occurs, linewidth broadening will not be obvious, because SBS dominates the process. This is the case we described above. However, if SPM effect is strong enough, the output linewidth will be broader, and it will increase successively with the increase of output power. In this case, SPM dominates the process and SBS is suppressed. In order to achieve this approach, the pulse duration, repetition rate and seed power should be optimized. Short pulse duration, low repetition rate and high seed power will lead to strong linewidth broadening induced by SPM. But these parameters have other limits due to SRS, etc.
The optimum parameters were determined experimentally. Fig. 3 shows the pulse duration and peak power dependence of the seed laser on the repetition rate, when the seed laser works at 10W average power. For repetition rate from 50kHz to 100kHz, with all available seed power and pulse durations, the linewidth broadening was not sufficient to suppress SBS. For repetition rate less than 50kHz, SBS suppression by SPM was feasible. If the repetition rate is set to less than 20kHz, although SBS can be suppressed, SRS will occur at high average power level, because of the high peak power. The threshold of SRS is around 30KW.
The optimum repetition rate was 40kHz, and the pulse duration was 40ns. In order to show the SBS suppression by SPM, firstly, the seed power injected into the fiber was adjusted to 0.5W, for comparison. SBS occurred at ~6W of output average power. In this case, SBS dominated, consequently, at higher output power, the output spectrum was like Fig. 2(a), and linewidth broadening by SPM effect played a minor role. In order to generate enough linewidth broadening induced by SPM, the seed power injected into the fiber was increased gradually by rotating the half waveplate (HWP1 in Fig. 1(a)), until SBS disappeared. 5W average seed power was injected when output power exceeded 50W without SBS. The output power when SBS occurred, i.e. the SBS threshold (peak power) at different seed power is shown in Fig. 4 (a).
The peak power of pulses was boosted to several tens of KW through the fiber amplifier. SPM effect dominated the process and broadened the linewidth significantly. The output spectrum around 1064nm under different output power is shown in Fig. 4(b), in linear scale. It can be seen that the linewidth broadening is obvious at high power levels. The linewidth of seed light was ~0.1nm (black line), and it was broadened to 0.3nm (blue line) at 20W, then to 0.7nm (red line) at 50W output power. By this approach, the onset of SBS was well suppressed.
4. Experimental results and discussion
With 5W seed power, 40kHz repetition rate and 40ns pulse duration, the maximum output average power of the fiber amplifier was 53.1W. Further increases in output power will cause SBS pulses to appear occasionally in the backward direction. The output power (squares) and output linewidth (circles) as a function of launched pump power is shown in Fig. 5. A linear fit provided the slope efficiency 78%. After amplification, the pulse duration was reduced to 30ns, and the pulse shape exhibited slight distortions compared to Gaussian-like seed pulses, shown in the inset of Fig. 5. The pulse-to-pulse amplitude fluctuation was <±15% at the maximum power, and much less (~±5%) at lower power. The backward direction output power, monitored by a power meter, showed no sudden increase, which implied that SBS didn’t occur. Measurements by OSA and photo-detector also confirmed this point.
Fig. 6 (a) shows the output spectrum of the fiber amplifier at the maximum power. Amplified spontaneous emission (ASE) around 1045nm was 30dB below the signal. The little peak near 1090nm was caused by another ASE peak. The first order SRS peak at ~1120nm was more than 20dB below the signal level. The linewidth of signal centered at 1064.4nm was broadened to 0.7nm (FWHM) by SPM, which was measured by the OSA with a resolution of 0.06nm, as shown in Fig. 4(b) (red line). A 10nm-bandwidth band-pass filter at 1064nm was used to measure the out-of-band power. More than 90% of power was included in the 1064nm signal peak.
PER was measured by a polarizer. PER degraded slightly with the increase of the output power, mainly caused by the increase of ASE. The inset of Fig. 6(a) shows the normalized transmitted power through the polarizer as a function of its angle, which reveals that the output was linearly polarized, with a polarization extinction ratio of 13dB.
Output beam quality was evaluated with a Spiricon M2-200 laser beam analyzer by 90/10 knife edge method. The M2 values of the output beam were 1.13 and 1.14 in x- and y-axis direction, respectively, indicating that the output beam was single transverse mode, as shown in Fig. 6(b).
Further power scaling is limited by SRS. Although the output average power can be increased at higher pump power, the SRS effect will shift a significant part of power from 1064nm to longer wavelength. The first order SRS peak around 1120nm in the output spectrum will grow apparently, and the output spectrum will spread to up to 1200nm by complex nonlinear effects.
Compared with other pulsed fiber amplifiers, our approach offers simultaneously high average power, high peak power, high pulse energy, high beam quality and linear polarization, which are needed for many applications. In addition, our approach works in multi-10ns and multi-10kHz regime, i.e. the “Q-switched regime” of DPSSLs, which exhibits the possibility to directly replace conventional DPSSLs with Yb-doped fiber amplifiers.
In conclusion, we have demonstrated a new SBS suppression approach using linewidth broadening induced by SPM. By this approach, the SBS limitation is overcome in a linearly polarized high energy Yb-doped double clad PM fiber amplifier at 1064nm, which generates 40kHz repetition rate, 30ns pulses of energy >1mJ, average power 53.1W, polarization extinction ratio 13dB and M2<1.2.
References and links
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