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Abstract
Compact high-power yellow laser is a critical part for sodium beacon adaptive optical systems. A narrow-linewidth quasi-continuous-wave (QCW) solid-state 589 nm laser with high-power and high beam quality simultaneously is investigated here, operating in hundreds-microsecond pulse duration with a tunable repetition rate of 400 to 1 kHz, which is flexible to allow the telescope to move in observing direction. The laser source is based on employing sum-frequency generation between 1319 and 1064 nm QCW Nd:YAG amplifiers. For a 100 µs pulse duration and 400 Hz repetition rate, the yellow laser provides a highest output power of 86.1 W with beam quality M2 = 1.37. The central wavelength can be precisely tuned to sodium-D2a line at 589.159 nm with a ∼440 MHz linewidth. This is the maximum power-reported for all-solid-state sodium guide star laser demonstrated to date. The result represents a key step toward solving the requirement of multi-conjugate adaptive optics for large adaptive optical telescopes.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Use of laser guide stars (LGS) along with adaptive optics (AO) allows large ground-based telescopes to significantly improve resolution by correcting image aberrations induced by atmospheric turbulence in the entire sky [1–3]. Artificial sodium LGS, generated by exciting the sodium layer in the mesosphere of ∼90 km altitude via a yellow wavelength of 589 nm, is regarded as the best choice of synthetic beacons [4–6]. It has been widely mounted on the current class of 8-10 m astronomical telescopes, including the 5-mter Hale Telescope, the 8.2-m Very Large Telescope (VLT) and the largest 10-m Keck telescope [7–9]. With sodium LGS-AO-assisted the upcoming European Extremely Large Telescope (E-ELT) and the Thirty Meter Telescope (TMT) for providing near diffraction-limited images capacity, many outstanding open questions could be probed in astronomy such as universe evolution and composition, supermassive black hole, stars and planets formation, and extrasolar planets [10–12]. However, the development of AO on astronomical telescopes demands compact sodium beacon lasers at 589 nm with high power and good beam quality simultaneously.
Four effective approaches have been developed to obtain 589 nm yellow lasers, including dye lasers, fiber laser, solid-state laser, and semiconductor laser. The dye lasers are the first sources to generate 589 nm directly [13], but they are gradually washed out due to its big scale, low stability and dependability, and high maintenance. Over the past decade, optically pumped semiconductor lasers is a attractive path to emit yellow radiation taking into account high efficiency, small volume, low cost, and wavelength coverage [14–16], but suffer from low power to be limited in engineering. The frequency doubling of Raman fiber amplifier is another technique for sodium beacon laser and has generated a 589 nm laser of more than 50 W [17–20]. However, most of Raman fiber-based 589 nm lasers operate at continuous-wave (CW) format. Compared with CW lasers, the pulsed guide star lasers can provide a gateable pulse format to eliminate the noise interference from low-layer atmospheric Rayleigh scattering and the fratricide phenomenon in multiple LGS systems, which is conducive to improving AO detection and overall performance [21–23]. Thus, the sum-frequency generation (SFG) of 1064 and 1319 nm Nd:YAG solid-state lasers has shown well promise and become the main mean for the new-generation sodium lasers sources with the advantages of high power, high stability, flexible pulse format, more compactness and robustness [24–30]. For instance, Lockheed Martin Coherent Technologies has produced 30 W and 55 W commercial CW mode-locked picosecond (ps) pulse solid-state sodium guidestar lasers for the Keck I and Gemini South telescopes, respectively [28]. However, the ps short pulsed lasers with high peak power are more easily to cause the sodium atoms absorption saturation, thereby decreasing the photon return. The fact that the quasi-continuous-wave (QCW) microsecond (µs) long pulse format is especially suitable for practical use in LGS AO system has been theoretically and experimentally demonstrated [22]. With a 100 µs level pulse width, an 81 W sodium laser is achieved with a 250 Hz pulse repetition rate (PRR) [29]. Unfortunately, this relatively low PRR could not detect and correct high-order atmospheric distortions for AO system. Recently, our group has delivered a 500 Hz QCW 589 nm laser with 65 W level output power and 120 µs pulse duration for Xinglong observatory [30].
Up to now, most large-aperture telescopes are equipped with classical single-conjugate adaptive optics (SCAO) systems, which only relay on a single guide star and a single deformable mirror to sense (WFS) and compensate (WFC) the wavefront aberrations. However, the corrected field of view is limited to a few tens of arcseconds because of the focus anisoplanatism, and this puts a severe restriction on performance. A concept known as multi-conjugate adaptive optics (MCAO) was proposed as early as 1989 to address this problem, and a graphic representation is given in Fig. 1. By using several guide stars together with several deformable mirrors, MCAO systems can potentially extend the AO-corrected over a field of view many times larger than the ones achievable with SCAO [31–34]. The Gemini MCAO system installed on Gemini South telescope is the first demonstration of atmospheric compensation with the use of multiple synthetic beacons, where a 50 W ps laser is split in five-way 10-Watt beams to produce five LGS distributed on a 60 arcsec square constellation [35]. Then, it uses two deformable mirrors to adjust the atmospheric distortions, delivering a uniform, near-diffraction-limited image over a 120 arcsec large field of view. Nowadays, the Keck, VLT, TMT and E-ELT are also adopting or planning to adopt the MCAO technology in the near future [36–39], which greatly promotes the development of a more powerful sodium laser. Owing to different position observed object with different isoplanatic patch, an inevitable difference exists for the going over distances and exciting sodium layer thickness of the yellow laser beam, as shown in Fig. 1(b). To better gate out Rayleigh scattering light in the lower atmosphere and efficiently exciting sodium atoms, the laser repetition rate R can be expressed by R = c(cosθ)/2H, and the optimal pulse width W is given by W = D/c(cosθ), where c is the speed of the laser in air, H is the altitude of the mesospheric sodium layer, θ is the zenith angle of the telescope pointing, and D is the sodium layer thickness. Generally, the angle θ will change from 0° to 60° for observing object. In this case, the sodium laser providing a tunable repetition rate around hundreds-Hz and a tunable pulse width around hundreds-µs is more desirable as well as versatile to match the need of the application [22]. All of the above-mentioned effects result in specific requirements on the performance of sodium laser source. Consequently, it is essential and urgent to develop reliable, higher-power, µs long pulse sodium beacon laser source operating at hundreds of Hertz for MCAO system on these new generation large telescopes.
In this paper, we present a latest milestone result, where we efficiently scale up the average power levels of a µs pulse sodium beacon laser to 86.1 W at 400 Hz for the first time. Compared to traditional ring lasers, the laser is generated via SFG of two laser diode (LD) pumped Nd:YAG twisted-mode lasers with lower loss and simpler configuration. The PRR of the laser system could be adjustable from 400 Hz to 1 kHz with 100 µs level pulse duration, which allows the telescope to operate in a versatile orientation. The measured M2 value is 1.37 for 86.1 W output power at 400 Hz. With the real-time controlling for temperature and angle of the etalon in the 1319 nm laser, the yellow wavelength is precisely tuned to 589.159 nm sodium D2a line with a 440 MHz linewidth. Such a high power LGS laser is also an enabling technology for MCAO system by dividing the beam into four 20 W-class beamlets with µs pulse duration, especially for future extremely large ground-based telescopes worldwide.
2. Experimental setup
The 589 nm laser source is achieved by extra-cavity single-pass sum-frequency mixing of 1064 and 1319 nm pulse trains emitted from QCW LD side-pumped Nd:YAG solid-state lasers. The experimental setup is depicted in Fig. 2, mainly including a 1319 nm master- oscillator power-amplifier (MOPA) subsystem, a 1064 nm MOPA subsystem, a SFG subsystem, and a frequency feedback control subsystem. Here, we employ the twisted-mode cavity laser, instead of the ring cavity laser in our previous work [30], as the narrow-linewidth seed source for the two fundamental beams, respectively. Compared with ring lasers, it generally only consists of two cavity-mirrors with a more compact configuration, which makes it easier to adjust and induces lower loss. The normalized curve for the influence of cavity-mirror tilting angles on laser output power is given in Fig. 3. The feedback insensitivity of twisted-mode cavity detuning is more two times than ring laser cavity, which is a crucial improvement for developing a reliable sodium laser source. Because the 1319 and 1064 nm MOPA has similar structure in the layout, only one is presented in detail. Development of a 1319 nm laser should be prior to that of a 1064 nm laser due to the fact that it has smaller gain and bigger quantum defect. In the 1319 nm MOPA laser system, the oscillator includes two homemade identical Nd:YAG laser heads LHs, where Nd:YAG crystal rod has 0.6% Nd3+-doped with a dimension of Φ3 × 82 mm2 and anti-reflectivity (AR) coating at 1319 nm. Three QCW 808 nm linear LD arrays are symmetrically distributed around the rod with highest available pump power of 240 W, providing a tunable repetition rate of 400 to 1 kHz with pulse width of hundreds microsecond. A 90° quartz rotator QR1 is located between LH1 and LH2 for compensating the thermally induced birefringence. A 45° polarizer P1 is high-reflectance (HR) coating at the vertical direction (s) beam and high-transmittance (HT) coating at the parallel direction (p) beam. Two cross-axis quarter-wave plates QWs are inserted the cavity to establish the twisted-mode within the crystals. The resonator is composed of two plano-convex mirrors (R=1000 mm). The mirror M1 is covered with HR coating at 1319 nm and AR coating at 1064 nm, and the output mirror M2 has a transmittance of 30% at 1319 nm. An etalon FP with specific thickness and reflectivity is introduced for the tunable wavelength with narrow linewidth. The etalon is glued on a well-designed optical bench with PZT, which adjusts the angle to guarantee the laser wavelength stability. The relaxation oscillation of solid-state pulsed laser is effectively suppressed by inserting a LBO frequency doubling crystal, which is described in details in Ref. [40]. To get high power output and high beam quality, the twisted-mode oscillator has a cavity length of approximately 1060 mm, operating at a thermally near-unstable cavity. Figure 4 gives the seed power stability and the beam profile comparison of the twisted-mode operation and the ring cavity for 2 hours, bringing ∼17.7% power increases and ∼1.3 time stability enhancement. By eliminating spatial hole-burning, such designs are usually used to obtain single longitudinal mode with only watts-level output power. Under high-power pumping, multi-longitudinal mode operation occurs for the two resonators, where the twisted-mode configuration may have more laser modes due the incomplete elimination of spatial hole-burning effects caused by the polarization degradation. The seed linewidth of the twisted-mode cavity is measured to be to ∼0.28 GHz, and the broadened compared with ∼0.25 GHz of the ring laser is acceptable in consideration of the increasing benefit in output power and stability.
Fig. 2. Diagram of the overall experimental configuration for the sodium beacon laser. LH1-LH10, laser modules; QR, quartz rotator; QW, quarter-wave plate; P, polarizer; FP, etalon; LBO, lithium triborate; M1, M3-M6, M7, M9-M11, and M15, high-reflection mirrors; M2 and M8, output couplers; M12, combiner; M13, filter; M14, splitter; f1 and f9, concave lens; f2-f8 and f10-f15, convex lens; AP, aperture/pinhole; HW, half-wave plate; EOM, electro-optic modulator; CCD: charge coupled device.
To solve the low gain restriction of Nd:YAG at 1319 nm line, a double-stage double-pass amplifier chain is adopted with four identical amplification modules LH3-LH6, similar to LH1 and LH2. The oscillator laser beam is injected into the first-stage amplifier by high reflections M3 and M4, shaping lenses f1 and f2. The amplifier comprises two laser heads LH3 and LH4, and a quartz rotator QR2, a spatial filter, a quarter-wave plate QW3, and a reflected mirror M5. The spatial filter is made up of a pinhole and a telescope, denoted as f3, AP, and f4, together to improve the beam quality and extraction efficiency. The beam after the first-pass amplification would rotate its polarization state by 90° through the QW3, and is reflected by the polarizer P2 after the second-pass amplification. Then, the laser beam is again double-passing amplified by the second-stage amplifier operated almost the same as first-stage amplifier. Eventually, the amplified beam is output with p-polarization from a polarizer P3. In the high gain 1064 nm oscillator, a KTP crystal with large nonlinear coefficient is used as the doubler, and the cavity length is designed to be about 1340 mm. Due to large extracted efficiency for 1064 nm line, only single-stage single-pass amplifier is adopted.
To increase the SFG efficiency, the spatial and temporal overlap of 1319 and 1064 nm laser is optimized. The diode pumping sources of two fundamental beams are controlled by a two-channel digital generator with a 20 µs time delay, as so to achieve fine pulse overlap. Then, the 1319 and 1064 nm laser beams are reshaped by lenses f7, f8, f11 and f12 respectively, and synthesized to be one beam by a mirror M12. A half-wave plate (HW) is utilized to change the polarization state of the 1319 nm beam to the vertical direction (s-polarized), which is accordant with that of 1064 nm. The two fundamental beams are designed by 1:1.24 power ratio and focused into LBO crystal for the SFG by an achromatic lens of f9. LBO with AR coating at 1064 nm, 1319 nm and 589 nm for both facets has an optical aperture of 4 mm × 4 mm. Type-I noncritical phase matching operates at a temperature of 41.2°C. Following, the 589 nm SFG output beam is collimated by a lens f14 and split from two infrared lasers with a filter M13.
A fraction of 589 nm yellow beam enters into the heated sodium-vapor cell by the mirrors M14 and M15, where resonance fluorescence can be observed. A CCD is mounted on the side of the cell to detect the intensity changes of sodium transition with tuning the sum radiation. The 589 nm beam passing through the cell is delivered to a precision wavemeter (High Finesse GmbH, WS7) for wavelength measurement. The wavemeter cooperated with the above- mentioned optical bench serves as the servo to control the sum-frequency light exactly to the sodium D2a absorption line. To further improve the sodium brightness, sodium D2b-repumping can be provided by an electro-optic modulator (EOM) with the right D2a-D2b frequency offset. Moreover, a circular polarized yellow beam is achieved by a quarter-wave plate (QW7). As well-known, the photo return flux would be better based on the circular polarized Na beacon lasers.
3. Results and discussion
In our experiment, a PRR adjustable 589 nm laser up to 1 kHz is obtained with hundreds-µs pulse by changing the PRR of the laser pumping source. As aforementioned, this unique parameter space is more preferable for LGS-AO application to avoid the Rayleigh scattering obstacle. The pulse temporal characterization is measured by an oscilloscope. From top to bottom in Fig. 5, the PRR of 589 nm beam are 400 Hz with 100 µs, 600 Hz with 130 µs, 800 Hz with 150 µs and 1 kHz with 180 µs, respectively. The maximum average output powers of 1064, 1319 and 589 nm as well as corresponding conversion efficiency are summarized in Table 1. Under a 400 Hz repetition rate, with injecting the pump power of 126.7 W at 1319 nm and 159.4 W at 1064 nm into LBO, an average output power at 589 nm is measured to be as high as 86.1 W with the highest sum-frequency efficiency of 30.1%. This is the highest solid-state laser source for sodium guide star as we know so far. The output power of 62.4, 45.4 and 34.2 W is also obtained at the PRR of 600, 800 and 1000 Hz, respectively. With an Ophir power meter, the measured power fluctuation at 86.1 W is less than 5% for 2 hours in the lab, with an enhancement compared with 6.6% of the ring laser. The linear-polarization ratio is about 110:1.
Fig. 5. Observed oscilloscope traces of 589 nm beam, from top to bottom, repetition rate and pulse width: 400 Hz with 100 µs, 600 Hz with 130 µs, 800 Hz with 150 µs and 1000 Hz with 180 µs, respectively.
Table 1. Maximum average output powers and the corresponding conversion efficiency for different PRR.
As shown in Figs. 6(a) and 6(b), with tuning the wavelength to the sodium absorption line, the resonant fluorescence scattering signal from sodium cell is observed and registered by a CCD (BASLER, ACA2000-50gm, NIR), and the signal would be disappeared with wavelength detuning. The precise wavelength tuning is done by means of a temperature and angle controls of etalons in each oscillator. One wavelength is set at 1064.655 nm. While the other wavelength increases linearly from 1319.132 to 1319.170 nm in about 0.75 pm steps. Figure 6(c) shows the corresponding sum wavelength changes from 589.1552 to 589.1627 nm with an increment of 0.15 pm and without significant power variation, which can cover the sodium Doppler-broadened spectrum. Figure 6(d) exhibits the fluorescence intensity measured by CCD versus the wavelength tuning. The highest fluorescence intensity is obtained at 589.1590 nm, and another maximum is slightly observed around 589.157 nm, which accords well with sodium D2a and D2b double-peak profile. Through controlling the wavelength of 1319 nm seed in close loop by a PID module, the yellow wavelength can be accurately tuned to be 589.159 nm of sodium D2a resonance radiation. Figure 7 expresses the average values of the laser frequency deviation are monitored to be ± 200 MHz for 2 h.
Fig. 6. (a) Resonant fluorescence scattering measured by a CCD; (b) wavelength detune over sodium absorption line; (c) wavelength tuning around 589.159 nm versus 1.3 µm wavelength; (d) sodium fluorescence intensity as a function of the wavelength from Na cell.
The linewidth of the 1319 and 1064 nm seed laser are measured to be ∼280 MHz and ∼230 MHz. Based on the resonators’ lengths, the longitudinal mode spacing of the 1319 and 1064 nm cavity is ∼120 MHz and ∼100 MHz, respectively, which means that there are three modes for 1319 nm spectrum and also three modes for 1064 nm spectrum. It can be deduced that the sum-mixing yellow laser operates at the multi-longitudinal modes. Figure 8 shows the 589 nm output spectrum detected with a scanning confocal Fabry-Perot interferometer (ThorLabs SA200-5B, free spectral range of 1.5 GHz, frequency resolution of 7.5 MHz). In consideration of the interferometer resolution and ∼10 MHz linewidth broaden of a single longitudinal mode, five longitudinal modes for 589 nm laser can be observed by the interferometer and the laser linewidth is estimated to be about 440 MHz full width at half maximum of the envelope of the modes, which were well in agreement with the sum of two fundamental beams modes. The sum-frequency linewidth is a little broadened compared with ∼300 MHz of the ring laser, which is acceptable for the AO application. The laser modes can be further decreased by means of multiple etalons in conjunction with the birefringent filters. Moreover, the beam quality factor M2 and two-dimensional intensity distribution are achieved with an optical analyzer, as displayed in Fig. 9. The beam profile is measured to be diffraction-limited TEM00 mode with M2=1.37.
Fig. 9. (a) Beam caustic measurement data (blue solid squares) at 86.1 W, and the corresponding fit (red curve). The measured value is M2<1.4. (b) Two-dimensional beam spatial profile, indicating a perfect Gaussian mode.
Various physical mechanisms can aid or inhibit the generation of ground-state atomic polarization. Reference [22] shows the optimal laser line width depends on the angle between the light propagation direction and the geomagnetic field direction. Due to the physical effects of Larmor precession of the sodium atoms, a broader line width is advantageous when this angle is small, and narrower laser lines are optimal when the angle is larger. Sodium D2a line absorption spectrum in mesospheric layer has a Doppler broadened to about 1.2 GHz, and a smaller fraction of sodium atomic can be excited using a single-frequency laser compared with a multi-longitudinal mode laser. The atoms cycle on the transition from 32S1/2 (F=2, M=2) to the 32P3/2 (F=3, M=3) has the strongest transition cross section. The intensity of atomic polarization depends on pumping power of the sodium laser. Under a low power, compared to a multi-longitudinal mode laser, a single-frequency laser of linewidth less than 10 MHz has a stronger spectral concentration and can efficiently provide the creation of atomic polarization in at the (F=2, M=2) ground state, which may lead to higher photon return flux. However, a multi-longitudinal mode laser with sufficient power makes it possible to achieve enough spectral concentration for each laser mode, result in creating as good atomic polarization as the single-frequency laser. Also, a laser with multi-longitudinal mode can make the best of sodium atom numbers, which will lead to better photon return flux. We think that it is the prospect for further power scaling of 589 nm laser while maintaining a multi-longitudinal mode operation to better match the sodium broadband spectrum.
4. Conclusion
We have introduced a relatively robust and practical high-power µs pulse solid-state yellow laser with PRR from 400 to 1000 Hz and pulse duration around 100 µs for sodium guide star systems, which will be of value for the guidestar laser community in astronomy. At the operation condition of 100 µs and 400 Hz, the yellow laser delivers an output power of 86.1 W with M2 = 1.37 and a linewidth of 440 MHz. Based on a servo control system, the yellow wavelength is precisely locked to sodium D2a absorption line at 589.159 nm. Meanwhile, the power and frequency fluctuations are measured to be about 5% and ± 200 MHz, respectively, which meet all specifications required in LGS AO system. Moreover, such an 80 W level high power pulse beam could be divided into four 20 W beams to generate four sodium beacon LGS for MCAO system on large-aperture telescopes.
Funding
National Key Research and Development Program of China (2016YFB0402003); National Natural Science Foundation of China (11504389, 11504390, 61505226).
Acknowledgments
We thank Prof. Bo for the help and the technical support, Dr. Zuo and Dr. Yuan for their discussions in the experiment, Prof. Chen, Prof. Peng and Prof. Xu for their valuable advice on the letter.
Disclosures
The authors declare no conflicts of interest.
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