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Abstract
A 208 W all-solid-state modulated-longitudinal-mode quasi-continuous-wave sodium guide star (SGS) laser was developed by sum-frequency of a 1064 nm laser and a 1319 nm laser. The laser contained two spectral lines separated by 1.72 GHz for re-pumping the sodium atoms. To suppress absorption saturation effect of the sodium atoms induced by the high light intensity, we used a white noise generator to modulate the 1064 nm single frequency seed laser in the frequency domain. The line width of the modulated-longitudinal-mode 589 nm laser was maximally broadened to 0.74 GHz compared to the initial line width of ~0.30 GHz. A bright SGS with photon return flux of 56800 photons/s/cm2 during the pulse length was obtained.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Reference light sources are always necessary to get clear images of the observed objects in astronomical adaptive optics [1–4]. The natural stars were first used as the reference sources to detect and adjust the wavefront aberration caused by atmospheric turbulence. However, due to the poor sky coverage of natural guide stars, artificial laser guide stars were introduced as important tools in adaptive optics in astronomy since 1980s [2,5]. Sodium guide star (SGS) is regarded as an ideal artificial guide star in the mesospheric sodium layer at the ~90 km altitude, compared to a Rayleigh guide star which is much lower in the atmosphere [1,2,6]. SGS is generated by a 589 nm yellow laser, of which the wavelength is matched to the D2 absorption line of the sodium atoms. Dye lasers, which could provide enormous power (over 1000 W at the Lawrence Livermore National Lab), were the primary excitation light sources of SGS with high return flux [7]. By launching multiple wavelength dye lasers, polychromatic guide stars were also developed to correct the tip and tilt aberrations [8,9]. However, with the advantages of compactness, efficiency and robustness, all-solid-state lasers are more frequently used as SGS lasers nowadays, including both bulk-solid lasers and Raman fiber lasers [10–14]. Among all these lasers, microsecond-pulse format laser has its own advantages [14–18], since this format is useful to avoid temporal overlap with backwards Rayleigh scattering light in the lower atmosphere [19] and mitigate spot elongation problem [20]. In addition, the pulsed SGS lasers can efficiently match the pulsed working mode of the adaptive optics system with higher return flux during the pulse length compared with continuous-wave (CW) lasers at the same average-power.
Since the atom column density of mesosphere sodium layer changes dramatically throughout the year, high-brightness lasers with more than one hundred watt average power is required to get enough photons return flux [21]. However, there are two significant factors restricting the resonance fluorescence intensity of sodium atoms when pumped by high average-power pulsed lasers. One is the down-pumping effect in which atoms are pumped from the upper F = 2 ground state but decay to the F = 1 ground state [22]. As demonstrated by many researchers, the down-pumping effect can be well solved by the method of re-pumping [15,23,24], which uses a D2b-line laser to pump the F = 1 ground state. The other limitation is the transition saturation of the sodium atoms because the peak intensity of the pulsed lasers is tens or even hundreds of times of the D2-line saturation threshold (~64 W/m2 for circularly polarized light laser [25]). Under this condition, the vapor medium becomes transparent and the returned fluorescence flux is far from the optimal. The principle to solve the problem is to broaden the laser spectral bandwidth to excite more atoms within different velocity groups [26–28], for example, to adopt a multi-longitudinal-mode spectra structure [29]. However, the type of spectra is not continuous but comb-like, thus the impact is so limited that only a few more velocity groups are excited than the single-longitudinal-mode laser. Therefore, the spectra structure needs further improvement and optimization for high power SGS lasers.
In this work, we developed an all-solid-state modulated-longitudinal-mode microsecond-pulse SGS laser with high average power of 208 W and near-diffraction-limit beam quality. The laser has a dual-spectral structure for re-pumping. Moreover, we used white noise modulating technology to convert the comb-like multi-longitudinal mode into continuous modulated multi-longitudinal mode to increase the returned fluorescence flux in SGS lasers. A high-brightness SGS was observed with the return flux of more than 56800 photons/s/cm2 during the pulse length with a comparatively small saturation effect at the launched average-power of 120 W.
2. Experimental setup
The SGS laser system was based on single pass sum-frequency generation of 1064 nm and 1319 nm master oscillator-power amplifier (MOPA) laser sources, as shown in Fig. 1.
Fig. 1 Schematic of the laser system. Seed1, 1064 nm laser seed; Nd1-Nd4, diode-pumped Nd:YAG module of 1064 nm; Seed2, 1319 nm laser seed;Nd5-Nd10,diode-pumped Nd:YAG module of 1319 nm. EOPM, electro-optic phase modulator; FAMP, fiber amplifier; EOAM, electro-optic amplitude modulator; FI Faraday isolator; PP, 56° polarizer plate; S1-S6, beam shaper. W1, 1064 nm quarter-wave plate; M1, 0° high-reflection (HR) at 1064 nm; M2, 34° HR at 1064 nm; M3-M4, 45° HR at 1064 nm; M5-M7, 45° HR at 1319 nm; M8, 45° anti-reflection (AR) at 1064 nm and HR at 1319 nm; M9, 45° HR at 1064 nm & 1319 nm and AR at 589 nm; M10, 45° AR at 589 nm. R1-R2, 1064 nm quartz rotator; R3-R5, 1319 nm quartz rotator; L1, f = 240 mm; L2, f = 300 mm.
The 1319 nm MOPA subsystem was composed of a 1319 nm oscillator, a pre-amplifier and a main amplifier. The 1319 nm oscillator generated stable pulsed seed laser with 100 μs level pulse length. It was a plano-plano cavity with an LBO crystal inserted to suppress the relaxation oscillation spikes. The diode-pumped Nd:YAG modules worked at a repetition rate of 250 Hz with the diode pump pulse length of 200 μs. Two F-P etalons with thickness of 1.5 mm (with the free spectra range ~69 GHz and the finesse ~9.1) and 10 mm (with the free spectra range ~10 GHz and the finesse ~4.9), were introduced to narrow the line width and tune the central wavelength precisely. Temperatures of the etalons and the LBO crystal were accurately controlled with a precision of ± 0.01°C. As a result, we obtained 1319 nm seed laser with a line width of ~0.3 GHz. The output average power from the oscillator was 4.76 W with M 2<1.1 (all the M 2 factors are measured by the Spiricon M2-200 beam propagation analyzer in this paper). The 1319 nm power-amplifiers included three pairs of Nd:YAG modules. Each Nd:YAG module worked at the repetition rate of 250 Hz with the diode pump pulse length of 250 μs. After 3-stage amplification, the output power was as high as 115 W with the beam quality of M2 = 3.0. The optical-to-optical efficiency was about 9.8% and the pulse length was 150 μs (in FWHM), as shown in Fig. 2(a). Since the 1319 nm’s emission cross section is only about 1/3 of that of 1064 nm in the Nd:YAG crystal, and its Stokes efficiency is ~61.3% from 808 nm pump, which is also smaller than ~75.9% at 1064 nm, then the optical-to-optical efficiency of 1319 nm laser is lower with more serious thermal effect and worse beam quality compared with 1064 nm laser.
In 1064 nm MOPA subsystem, a 10 mW CW single frequency laser (Koheras Adjustik Y10) was used as the laser seed. The 1064 nm seed laser was injected into an electro-optic phase modulator (EOPM) driven by a noise source (white noise generator), and then was boosted to 10 W by a fiber amplifier (FAMP). An electro-optic amplitude modulator (EOAM) was used to chop the CW laser into pulsed mode with a rectangle pulse shape and a 150 μs pulse length at 250 Hz repetition rate. With an energy of 0.1 mJ per pulse and near-diffraction-limited beam quality (M 2 <1.1), the pulsed laser was then amplified by two stages of Nd:YAG rod amplifiers. To maximize the extraction energy, the laser was in double-pass propagation, and a quarter-wave plate W1 was used to adjust the polarization of the laser to the orthogonal direction. The output average power of the first stage amplifier was 22.5 W and the beam quality M 2 was about 1.2, and the final average-power was 203 W with the beam quality of M 2 ~1.8. The corresponding optical-to-optical efficiency was about 18.8%. As the 1064 nm pulsed laser was amplitude-modulated from a CW seed laser, there was no relaxation oscillation in pulse profile and the pulse shape was trapezoidal with a pulse length of 150 μs in FWHM, as shown in Fig. 2(b).
The sum-frequency generation (SFG) unit was accomplished with the single-pass frequency mixing of the 1064 nm and 1319 nm beams through the LBO crystal. The 3 × 12 × 50 mm3 LBO SFG crystal was AR coated for 1064 nm, 1319 nm and 589 nm. Its temperature was controlled within ± 0.01°C by an oven working at 40.30°C to enable noncritical phase-matched operation. The SFG average-power was measured while the total power of the fundamental lasers injected to the LBO crystal ranges from 25 W to 319 W as shown in Fig. 3(a). While increasing the incident power, the power ratio between the 1064 nm and the 1319 nm average power was kept about 1.5:1. At the highest total incident fundamental laser power of 319 W, the average power of the 589 nm laser reached to 106 W and the SFG efficiency was as high as 33.2%. The SFG efficiency became higher due to the increasing power density of the fundamental lasers. The beam quality M 2 factor at the highest output power is measured to be less than 1.4 (Mx2 = 1.38, My2 = 1.20), as shown in Fig. 3(b), which is quite better than both the fundamental lasers. The reason can be expressed that the mode-matched part of the fundamental beams makes much more contribution in the nonlinear process, and the sum frequency generation process can be regarded as a beam quality cleaner.
Fig. 3 (a) Curves of SFG average power and conversion efficiency versus total average power of 1064 nm and 1319 nm; (b) beam quality of 589 nm laser measured at 106 W.
3. Spectral control
3.1 Wavelength locking and modulating
To lock the central wavelength of the sodium beacon laser to the sodium D2a line, we used a wavelength locking system with a wavelength meter (HighFinesse WS7). The central wavelength of the yellow laser was monitored in real-time by the wavelength meter, from which the ‘error signal’ between the measured result and the targeted value (e.g. 589.1591 nm) was obtained. The operating central wavelength was precisely adjusted by controlling the tensile strain of piezo-electric transducer (PZT) to the fiber grating of the 1064 nm seed laser. As a result, the wavelength range and accuracy of the locked 589 nm laser are ~3.6 pm and ± 0.02 pm, respectively.
The line width of 589 nm laser was determined by both fundamental lasers. The 1319 nm laser had a multi-longitudinal mode with a measured line width of ~0.3 GHz, while the line width of the 1064 nm single-longitudinal mode laser was less than 70 kHz before modulating. As shown in Fig. 1, the 1064 nm seed laser was injected into an electro-optic phase modulator (EOPM) driven by a noise source (white noise generator), so that the actual line width of the yellow laser can be broadened by phase modulating of the 1064 nm laser.
Figure 4 is the measured results of the central wavelength and the line width of the D2a-line laser. In Fig. 4(a) the wavelength was accurately matched to 589.1591 nm after locking with the initial line width of about 0.3 GHz (340 fm). Then we modulated the phase of the 1064 nm laser, the line width of the D2a-line laser was broadened to be ~0.7 GHz (as shown in Fig. 4(b)), when the driving radio-frequency power of the white noise source was tuned to about 0.4 W. It was verified that the line width could be broadened by phase-modulating, in accord with the theoretical results reported in [30].
Fig. 4 Line width of the 589 nm laser (a) before and (b) after phase-modulation measured by HighFinesse WS7. The x-axis scale is the CCD arrays number with the unit of pixel. The y-axis scale is the voltage of the CCD chip with the unit of mV.
3.2 Double-spectral structure
Sodium D2b line laser is necessary for further increasing the photon return by mitigating the down-pumping effect. Different from transferring power to D2b-line by phase modulating on a D2a-line laser [15,23,24], we produced two individual SFG lasers locked at the central wavelength of 589.1591 nm (the sodium D2a-line) and 589.1571 nm (the sodium D2b-line) respectively, and combined them together.
The schematic of the double-spectral-line SGS laser was shown in Fig. 5. The D2a-line laser output was ~106 W, and the power of D2b-line laser was ~102 W. They were both p-polarized initially, so we converted the polarization of the D2a-line laser into s-polarization by a λ/2 wave-plate before the beam combination. The two lasers were controlled coaxially and combined through a polarizer after beam-expanding into the same size of ~40 mm. The total power of the combined lasers was up to ~208 W with the regardless combining loss, and their polarization was tuned into circular polarization by a λ/4 wave-plate. Although the volume of the laser system was increased with this method, the complexity of the system was reduced. Furthermore, it avoided the energy loss from the undesired frequency generated opposite to the D2b-line occurred by phase modulating.
4. Sodium guide star laser performance
A laser prototype based on the former introduced technologies was manufactured for the SGS detecting experiment. The experimental parameters for the system are summarized in Table 1. The laser beam was expanded into the size of 28 cm and then launched to the atmosphere sodium layer at the height of ~90 km. In the sky the laser spot size was about 216 cm, which was defined as 1/e2 of the maximum intensity and calculated based on the measurement results of Shack-Hartmann wavefront sensor. The picture of the SGS laser system and the on-sky test is shown in Fig. 6. The total transmission efficiency of the telescope and the atmosphere were 49.7%. The SGS return flux was ~1.9 times of that pumped only by a D2a-line laser at the same average-power. On this basis, the return flux was further enhanced to ~1.3 times with a laser line width of 0.7 GHz compared with that obtained without laser phase modulation. Finally, the return flux at the telescope primary mirror was 2116 photons/s/cm2 at the average laser power of 120 W with the power ratio of about 1.2:1 between the D2a-line and the D2b-line. It was verified that re-pumping and line width broadening was of great benefit to increase the return flux for high-power SGS lasers.
Table 1. Summary of experimental parameters
Due to the modulated-longitudinal-mode of our laser, the saturation effect was greatly restrained as the power increased, so that the return flux could reach more than 56800 photon/s/cm2 during the pulse length. Figure 7 shows the return flux versus the corresponding laser peak power density in the mesosphere. Due to the saturation effect, the return flux would be less than 2 times of the return flux at the threshold intensity as the power density increased, while the saturation threshold of the single frequency circular polarization laser was about 64 W/m2 [25]. In our experiment, it was shown that when the power density changed from ~80 W/m2 to ~434 W/m2, the return flux increased by ~3.5 times, which indicated that the saturation intensity was larger than 80 W/m2. Furthermore, the experimental data can be simply fit to y = C·Isat/(1 + Isat/x) [22], in which the saturation intensity Isat is determined to be about 1400 W/m2, which are more than 3 times higher than our maximum power density. As we can see, the saturation effect of the SGS laser was greatly suppressed with this modulated spectral structure, which is efficient and promising in the future higher average-power SGS lasers.
Fig. 7 Return flux during the pulse at different mesospheric peak power density. The experimental data are fit to y = C·Isat/(1 + Isat/x).
5. Conclusions
In summary, we demonstrated a 208 W all-solid-state modulated-longitudinal-mode SGS laser with double spectral lines (D2a-line and D2b-line) and near-diffraction-limit beam quality (M 2<1.4). The laser line width was broadened from ~0.3 GHz to ~0.7 GHz by white noise source phase modulating to restrain the saturation effect of the mesospheric sodium atoms. A high brightness SGS was observed with the return flux of more than 56800 photons/s/cm2 during the pulse length at the launched average-power of 120 W. With optimized laser power ratio of the D2a-line and the D2b-line, better atmosphere transmission and launching system, further improvement in the return flux of SGS is feasible.
Funding
National Natural Science Foundation of China (61505189, 61705208); Key Laboratory of Science and Technology on High Energy Laser of China Academy of Engineering Physics (2017HEL06).
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