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Abstract
A robust 20-W continuous-wave single frequency 589 nm laser is developed to aim for sodium guide star in astronomy. The source is based on applying π-depth binary phase modulation to a single frequency seed laser along with 3 steps of strain in the gain fiber to suppress the stimulated Brillouin scattering in the high power 1178 nm amplifier and realizing the recovery of single frequency after frequency doubling in a periodically poled LiTaO3 crystal. The efficiency of frequency doubling reaches up to 41.6%. To the best of our knowledge, it is the highest power reported for continuous-wave 589 nm laser generation by single-pass frequency doubling. The approach significantly simplifies the sodium guide star laser design and improves robustness.
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1. Introduction
High-power 589 nm yellow lasers have applications in sodium laser guide star (LGS) [1,2], laser radar [3,4], and medicine [5]. Sodium LGS is used in astronomical adaptive optics as an artificial guide star. To generate a guide star at high altitude, 589 nm lasers are used to excite a layer of sodium atoms at about 90 km in the mesosphere. The application requires the 589 nm laser to be narrow-linewidth and resonant with the sodium D2 transition. At present, most guide star lasers are based on sum-frequency-generation (SFG) of 1064 nm and 1319 nm solid-state Nd:YAG lasers [6,7] or second-harmonic-generation (SHG) of a 1178 nm Raman fiber amplifier (RFA) [8,9]. The latter fiber based approach, because of compactness, robustness, high efficiency and good beam quality, has been developed into commercial laser products and implemented in major astronomical telescopes.
However, power scaling of single frequency (SF) or narrow linewidth Raman fiber amplifiers is limited by stimulated Brillouin scattering (SBS) [10,11]. Previously, SBS suppression has been achieved effectively by applying dozens of strain steps along the gain fiber, which however is complicated to implement. In recent years, spectral broadening by phase modulation has been widely investigated for SBS suppression in high power narrow linewidth rare earth doped fiber amplifiers [12–14]. Nevertheless, the amplifier output is not single-frequency anymore. Recently, we introduced a passive spectral compressing method utilizing the phase doubling effect in SHG [15]. A discrete phase modulation of π difference is applied to a single frequency fundamental seed laser, which allows SBS suppression and high power amplification. The second harmonic of the laser will return to single frequency, because the π-difference phase modulation is doubled to 2π in SHG.
With the Raman fiber amplification of a narrow bandwidth 1178 nm seed laser and sequential frequency doubling in an external resonant cavity containing Lithium triborate (LBO) crystal, more than 50 W continuous-wave (CW) single-frequency 589 nm laser has been achieved [9]. However, the frequency doubling cavity needs to be actively locked, which adds complexity and instability to the laser systems. Developing sodium guide star laser based on single pass frequency doubling is desirable, because it significantly improves the robustness of the laser system. And single pass scheme is advantageous for developing guide star laser of advanced spectral or temporal format, for example, frequency chirping [16,17]. Periodically poled near-stoichiometric LiTaO3 crystal (PPSLT) has higher effective nonlinear coefficient than LBO, which allows efficient single pass frequency doubling at tens of watts of the fundamental laser. We had previously reported a 7 W yellow laser generation by single-pass frequency doubling of a Raman fiber amplifier in PPSLT crystal. The corresponding conversion efficiency is 20% [18]. Recently PPSLT has shown improved performance in optical damage threshold, which makes it suitable for high power applications.
In this contribution, we report a robust 20 W CW narrow linewidth 589 nm laser by single pass frequency doubling of an 1178 nm Raman fiber amplifier. A $\{{0,\pi } \}$ binary phase modulation is applied to the 1178 nm single frequency diode seed laser before the fiber amplifier to broaden the laser linewidth and suppress SBS. 3 steps of strain are applied to the Raman gain fiber to further suppress SBS. A 49 W laser of linewidth 800 MHz at 1178 nm is produced with an optical conversion efficiency of 53.6% from 1120 nm to 1178 nm. After single-pass frequency doubling in a PPSLT crystal, 20 W 589 nm laser is obtained with a doubling efficiency of 41.6%. Notably, the 589 nm laser spectrum is compressed back to single frequency in the SHG process with a linewidth less than 16 MHz. And to the best of our knowledge, it is the highest power reported for CW 589 nm laser generation by single-pass frequency doubling.
2. Experimental setup
The experimental configuration of the 589 nm yellow fiber laser is shown in Fig. 1. A high-power linearly polarized 1120 nm pump source is built in-house, which is constructed by an integrated ytterbium-Raman fiber amplifier scheme [19]. Two stages of Raman fiber amplifiers are constructed to boost an 1178 nm single-frequency laser. The 1178 nm seed laser is a fiber-pigtailed linearly polarized distributed feedback diode laser with an output power of 10 mW and a linewidth of less than 5 MHz. An electro-optic phase modulator (EOM) of 2 GHz bandwidth @3dB is inserted between the seed and pre-amplifier to broaden the spectrum. Pseudo-random binary sequence with shift register length of 7 (PRBS7) phase modulation was applied to broaden the laser linewidth. The modulation depth is carefully adjusted to realize the required {0, π} phase modulation, which maximizes the spectral broadening and allows the spectral compression in SHG to work. The output power of the pre-amplifier is 0.65 W. The main amplifier is backward pumped by the in-house-built high power 1120 nm fiber laser via wavelength division multiplexers (WDM) and the residual 1120 nm laser is extracted out of the RFA by another two WDMs. The gain fiber is a 88 m-long piece of PM1310 fiber. The backward-propagating light is monitored at the circulator, which is inserted between the two amplifiers.
Fig. 1. The experimental setup consists of a single frequency seed laser, a phase modulator, two stages of 1178 nm Raman amplifiers and single-pass frequency doubling unit. EOM: electro-optic modulator; CIR: circulator; ISO: isolator; WDM: wavelength division multiplexer; L: lens.
In the main RFA, 3 steps of longitudinally varied strain along the gain fiber are applied to further suppress SBS, which can be avoided if EOM of higher bandwidth is used. The strain introduces a proportional shift of the SBS gain spectrum. Therefore, with a strain distribution, SBS light from different portions of the gain fiber is spectrally isolated and cannot be amplified in other portions of the fiber. This technique has been investigated intensively [20] and used in our previous works on single-frequency Raman amplifiers [9]. Starting from the pre-amplifier side of the gain fiber, the length of the 1st and 2nd steps are 70 m and 13 m. The applied strain introduced a calculated Brillouin spectrum shift of about 900 MHz and 1.8 GHz, respectively. The 3rd step is unstrained.
The collimated and optically isolated 1178 nm RFA output is then coupled with a 60 mm lens into a 30-mm-long PPSLT crystal. The transmittance of the isolator is 95% and the output polarization is parallel to the poling direction of the crystal. The crystal is housed in a homemade oven with a temperature stability of +/-0.01℃. The antireflection coating of the PPSLT has a reflectivity of R < 0.5% at 1178 nm and 589 nm according to the data sheet. The generated yellow light and the residual fundamental light are separated by a dichroic mirror, which has a high transmittance at 589 nm (98.15%) and a high reflection at 1178 nm (99.98%).
3. Results and discussions
Suppression of SBS in the 1178 nm Raman fiber amplifier while maintaining narrow linewidth 589 nm output is the main challenge in power scaling of the sodium guide star laser. We apply the recently invented technique of passive spectral compressing in SHG [15]. The effect of linewidth broadening under PRBS phase modulation is measured firstly. PRBS7 phase modulation of π difference at 600 Mbps, 800 Mbps and 1000 Mbps bit rates is applied in the experiments to the single frequency seed laser. The spectra are measured with a Fabry-Perot Interferometer (FPI) of 4 GHz free spectral range for single-frequency laser and modulated laser respectively, as shown in Fig. 2. The measured linewidth of the SF laser is 8 MHz, which is wider than the actual value due to the resolution of the FPI. As the bit rate increases, the linewidth broadens accordingly with the corresponding bit rate. Therefore, the peak spectral density of the fundamental laser is reduced significantly, which is critical for SBS suppression and power scaling of the amplifier output. Obviously, the broadened linewidths are still far within the spectral acceptance of the PPLST crystal used in the experiment (28 GHz), which will not affect the frequency doubling efficiency. It is worth noting that a few discrete sidebands are also generated at both sides of the carrier wave, which is caused by the non-ideal two-value function modulation. The sidebands are symmetrically distributed at both sides and the interval is the bit rate.
Fig. 2. The spectra of the 1178 nm laser. (a) without modulation. (b) with PRBS7 modulation of 600 Mbps, 800 Mbps and 1000 Mbps bit rates respectively.
The peak spectral density of the 1178 nm laser can be further reduced by phase modulating at higher frequency, which is favorable for the SBS suppression. However, higher bandwidth phase modulator and related electronic devices are not available in our lab. Therefore, instead of further broadening of laser linewidth, 3 steps of strain is applied on the gain fiber to suppress the SBS to generate sufficient higher power 1178 nm laser. It should be noted that the strain is not necessary in principle, because there are modulation devices with higher bandwidth on the market.
To characterize the SBS suppression effect of the phase modulation and stepwise strain in the main amplifier, a variety of experiments are carried out. Figure 3(a) shows the output power and backward-propagating light from the main Raman amplifier as a function of pump power under three different situations of 3 steps strain only, 800 Mbps PRBS 7 modulation only, and a combination of the two measures. With 3 steps of strain, the output power of the RFA is only 1.46 W, while the power of the backward propagating light is already 238 mW. If only phase modulation is applied, the power can reach 20.65 W, while the backward light power increases to 183 mW. When these two measures are combined, the 1178 nm output power is scaled to 49.2 W at an 1120 nm pump power of 91.7 W, corresponding to 53.6% optical to optical conversion efficiency. The total power of the backward light is about 170 mW even at the highest output power, indicating that SBS is effectively suppressed with the combined techniques. The effects of different bit rates for PRBS 7 modulation on output power and backward power are also compared and the results are shown in Fig. 3(b). The maximum achievable output power increases with higher bit rate, because more pump power can be applied, indicating higher SBS suppression capability. At a bit rate of 1000 Mbps, the amplifier output can go up to 56.6 W. However, due to the limited bandwidth of the phase modulation setup, the demodulation rate decreases gradually with respect to bit rate in the second harmonic generation process [15]. Therefore, we finally chose 800 Mbps modulation and 3 steps of variable strain for the frequency doubling experiment. Compared with the previously reported 44 W 1178 nm laser obtained by applying 30 steps strain distribution along the fiber [21], this scheme is much more convenient to implement and has a higher maximum output power. Please also note that the strain steps can be avoided completely when sufficiently higher bandwidth phase modulation is implemented.
Fig. 3. Output and backward powers at different cases. (a) 3 Steps strain only, 800 Mbps PRBS 7 modulation only, and combination of the two measures, (b) PRBS7 modulation at 600Mbps, 800 Mbps and 1000 Mbps bit rates, with 3 steps strain.
The spectrum of the 1178 nm amplifier output at the highest power is measured with an optical spectral analyzer (AQ6370) with a resolution of 0.02 nm, as shown in Fig. 4(a). The measured center wavelength is 1178.29 nm and the signal-to-noise ratio is found to be better than 53 dB. The spectrum of the backward light is measured, which contains not only backward 1178 nm laser but also a part of residual 1120 nm pump laser, as shown in the inset of Fig. 4(b). The zoom-in spectra of the backward 1178 nm light at different output powers are measured, as shown in Fig. 4(b). It can be seen that the Rayleigh scattering light and the SBS light separate about 0.07 nm spectrally. With the amplifier output increasing, the peaks of the Rayleigh scattering light and SBS light increase at the same time. When the output power is 23 W, the intensity of the SBS light exceeds the Rayleigh light. However, the total power remains small.
Fig. 4. (a) Spectrum of the amplifier at the maximum output, (b) Fine spectra of the backward laser at different output powers. Inset: wide spectrum at the maximum output.
The polarization extinction ratio (PER) of the amplifier output is measured to be >30 dB, owing to the all-polarization-maintaining configuration and the polarization-dependent gain of stimulated Raman scattering.
The amplified 1178 nm laser is then frequency doubled in a PPSLT crystal. The focal length of the lens is 60 mm. The optimum position of the lens is fine-tuned experimentally. In the experiment, the beam waist radius inside the PPSLT crystal is ∼36 µm and the optimum crystal temperature is found to be 46.4℃ at the maximum output power, which is slightly lower than the matching temperature at low power (47℃). This is primarily because the crystal absorbs a small amount of fundamental and second-harmonic lasers and thus accumulates heat inside itself. The dependence of the SHG output power and conversion efficiency on the incident pump power are shown in Fig. 5(a). At lower power, the SHG output increases nonlinearly. At higher power, the frequency doubled output becomes linear with respect to the fundamental laser. Up to 20.2 W of SHG output is obtained when 48.6 W 1178 nm laser is incident on the PPSLT crystal, corresponding to a SH efficiency of 41.6%.
Fig. 5. (a) SH output power and conversion efficiency as a function of 1178 nm laser power, (b) Spectrum of the SH output laser at the highest output power (20 W). Inset: fine spectra of output laser.
The spectrum of the generated 589 nm laser is measured with a FPI of 4 GHz free spectral range, as shown in Fig. 5(b). A single frequency narrowband 589 nm laser is generated as expected [15]. The linewidth of the 589 nm laser is ∼16 MHz, limited by the FPI resolution. There is a small spectral pedestal which is due to the unideal binary phase modulation. There are also a few of other components besides the main frequency component, corresponding those of the fundamental laser. By spectral integration, it is found that the main frequency component accounts for 98.8% of all the frequency components. In fact, even the uncompressed part of the laser is useful for the LGS application, because the sodium D2 line is about 3 GHz wide.
At maximum output power of about 20 W, the optical spectrum is measured. The signal-to-noise ratio is found to be over 54 dB, as shown in Fig. 6. The center wavelength is 589.14 nm, and the 3 dB spectral width is 0.03 nm, which is limited by the resolution of the optical spectral analyzer. Since the fundamental laser is generated with a single-mode fiber, and the frequency doubling is achieved with a PPSLT crystal, the beam quality of the 589 nm laser is expected to be good, according to reported studies of similar frequency doubled fiber lasers. An output beam profile is shown in the inset of Fig. 6. The elliptical profile is mainly due to the thermal lensing effect in the crystal at high power.
Fig. 6. Spectrum of the generated 589 nm laser at 20 W. Inset is the corresponding output beam profile.
4. Conclusion
In conclusion, we have developed a 20 W 589 nm single frequency laser with simple structure. It is based on a simple and compact single-pass SHG in a 30 mm-long PPSLT crystal of a Raman fiber amplifier. The SBS effect in the 1178 nm RFA is effectively suppressed by applying a PRBS7 binary phase modulation to the single frequency seed laser and 3 steps of strain on the Raman gain fiber. We’d like to note that the strain steps are not necessary when sufficiently higher bandwidth phase modulation is implemented. Such devices are available on the market. The phase modulation broadened spectrum of the fundamental laser is recovered to single frequency in the frequency doubled output. Up to 20 W CW 589 nm radiation has been achieved with an optical conversion efficiency of 41.6% from 1178 nm to 589 nm. The yellow laser has a linewidth of less than 16 MHz and excellent beam quality.
The presented simple-structure 589 nm laser is an efficient and reliable alternative solution for laser-guide-star adaptive optics. The single pass scheme significantly improves the robustness of the laser system. And it is also advantageous for developing guide star laser of advanced spectral or temporal format for better sodium return, for example, frequency chirping. To further scale the output power with the technology, the primary challenge is the power-handing capability of the WDMs in the Raman fiber amplifier and the damage threshold of the PPSLT crystal.
Funding
National Key Research and Development Program of China (2020YFB0408300, 2020YFB1805900, 2018YFB0504600); National Natural Science Foundation of China (62075226); Science and Technology Commission of Shanghai Municipality (19441909800).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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