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Abstract
The polychromatic laser guide star (PLGS) is one of the solutions proposed to measure the differential atmospheric tip-tilt. A watts-level microsecond pulse all solid state laser source with two wavelengths at 589 and 819.7 nm are developed to perform a proof-of-concept on-sky test for what is believed to be the first time. By sum-frequency of 1319 and 1064 nm, a 44 W maximum average output power at 589.159 nm is generated with the pulse width of ~90 μs at 500 Hz, the linewidth of 0.46 pm, and the beam quality of M2 = 1.50. Meanwhile, a 2.4 W average output power is achieved operating at 819.710 nm with the pulse width of ~25 μs at 500 Hz, the linewidth of 0.8 pm, and beam quality factor of M2 = 1.20, which is end-pumped by a frequency-doubled 1064 nm Nd:YAG laser. Moreover, double resonant fluorescence in sodium cell with two step excitation of sodium atom from 3S1/2 to 3D5/2 via 3P3/2 level is observed clearly by tuning the wavelength of 589 and 819.7 nm beams. In the proof-of-principle experiment, it is preliminarily verified that this laser system is expected to be applied to the sky experiment.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The ground-based large-aperture telescopes, as a powerful tool for comprehensive understanding of the Universe, advanced rapidly with higher sensitivity and higher spatial resolution [1] and achieved significant observations [2]. However, atmospheric turbulence distorts light waves received from space objects and severely limits the imaging resolution of telescopes. Fortunately, these distortions can be practically eliminated in real time via a technology known as adaptive optics (AO), where AO requires a reference sources to probe atmospheric turbulence and provide feedback to deformable mirrors in order to compensate image blur effects induced by this turbulence [3–5]. It is a major limitation that the natural bright stars are too rare to cover larger fraction of sky. Accordingly, in order to increase sky coverage, monochromatic laser guide star (LGS) has been widely used on large astronomical facilities [6,7]. Unfortunately, there is a most severe problem with these LGSs: tilt referred as the [8].
Tilt problem is caused by the inverse return property of light, which makes it impossible to measure the overall wavefront tilt with a single on-axis monochromatic LGS [9]. Polychromatic laser guide stars (PLGS) [10] consisted of exciting the mesospheric sodium atom 4D5/2 level via the intermediate level 3P3/2 with two lasers at 589 and 569 nm were proposed to solve the tilt problem. Some of which have been pursued experimentally [11–13]. Another concept for PLGS were proposed, which consisted in exciting directly the 4P3/2 sodium level with one photon, using a single laser operating at 330 nm [14–16]. The corresponding energy level and the radiative cascade of the two photon excitation of the 4D5/2 sodium level at 589 & 569 nm and one photon excitation of the 4P3/2 sodium level at 330 nm are presented in Fig. 1(a) and Fig. 1(b), respectively. Recently, Wang et al proposed a new bi-chromatic sodium laser guide star concept centered at 589 nm and 819 nm, corresponding the energy diagram and the pathways of excitation and radiative shown as Fig. 1(c), which originated from 3S1/2→3P3/2→3D5/2 circular closed transition with largest cross-section and minimum branching photon loss, moreover, the system permits circular optical pumping and sodium atom polarizing [17]. Meanwhile, they have demonstrated its potential by vapor cell fluorescence experiment with single frequency lasers and expanded to multi-longitudinal mode long pulse lasers with time-dependent 48 × 48 density matrix modelling [18]. Moreover, Zhou Fan et al. has measured the transmission of atmospheric from 330 to 970 nm at Xinglong Observatory, as shown in Fig. 2. It can be seen that the transmission of 330 nm is less than half of 819.7 nm. And, the fluorescence intensity of 819.7 nm (110) is higher ~15 times than that of 569 nm (7) (the fluorescence intensity of 589 nm is 1000) [19]. Besides, as to the previous work [11–16], dye lasers have been used to generate PLGS, but they typically remain highly maintenance intensive and can be troublesome to operate at remote astronomical observatories [20]. Nowadays, the diode-pumped solid-state lasers have been developed as the new generation of LGS with the advantages of high power, compactness, more reliability, and a more robust setup. Therefore, developing 589 and 819.7 nm all solid state lasers as laser source for PLGS is of great significance.
Fig. 1 Energy diagram and relaxation pathways of: (a) two photon excitation of the 4D5/2 sodium level at 589 nm + 569 nm; (b) one photon excitation of the 4P3/2 sodium level at 330 nm; (c) two photon excitation of the 3D5/2 sodium level at 589 nm + 819.7 nm.
In this paper, we demonstrate a new PLGS all solid state laser source, which is composed of two microsecond (μs) pulse laser wavelengths operating at 589 and 819.7 nm. A 44 W average output power at 589 nm is generated with a linewidth of ~0.46 pm and the beam quality M2 = 1.5 by sum frequency generation (SFG) of 1319 and 1064 nm Nd:YAG laser, which is operated at a pulse repetition rate (PRR) of 500 Hz with the pulse width of ~90 μs. Meanwhile, a 2.4 W 819.7 nm Ti:sapphire laser is obtained operating at a PRR of 500 Hz with a pulse width of ~25 μs, linewidth of ~0.8 pm and beam quality factor M2 of 1.2, which is end-pumped by the second harmonic generation of the shared 1064 nm Nd:YAG laser. Moreover, it is the first time that 589 and 819.7 nm watts-level all solid-state laser overlap well in spectral and temporal scale by tuning the output wavelength, which will be suitable for the on-sky experiment of double resonant fluorescence of sodium atom from 3S1/2→3P3/2→3D5/2 transitions, and the permitted band-width of sodium atoms at 819.710 nm based on the μs 589 nm laser is also measured.
2. Experimental details
Sodium lasers can be categorized generally into continuous wave (CW) and quasi-continuous wave (QCW) μs pulse format. Compared with the CW sodium beacon laser, the QCW μs pulse sodium beacon laser can provide a gateable pulse format that could avoid wavefront sensor imaging contamination induced by lower altitude atmospheric Rayleigh scattering, which in turn improve AO detection and overall performance [21]. The experiment setup to observe the resonant fluorescence scattering from the 3S1/2→3P3/2 and 3P3/2→3D5/2 of sodium atoms is schematically shown in Fig. 3. There are three branches: a 589 nm laser, an 819.7 nm laser, and a double resonant fluorescence scattering device. In order to generate QCW μs 589 nm laser, the basic design is based on single-pass SFG from two Nd:YAG master oscillator power amplifier (MOPA) lasers operating at 1064 and 1319 nm, which has beendescribed in detail as our previous work [22]. For completeness, we provide a brief description here. In the 1319 nm MOPA laser system, a three-mirror unidirectional ring is employed as the oscillator, which is composed of two identical diode-side-pumped Nd:YAG laser heads, a 90° quartz rotator (QR), two reflectors, a Faraday rotator, a half-wave plate (HWP), a thin film polarizer (TFP), and a solid etalon. The output beam of the oscillator is injected into two stage double-pass amplifier via a beam shaper (BS). Each stage amplifier contains two Nd:YAG laser heads, a TFP, a QR, a BS, a quarter-wave plate (QWP), and a reflector. For 1064 nm MOPA laser system, the amplified source shares the same design with the 1319 nm oscillator but different coatings. As a result, the 1319 nm seed laser provides an output power of 28 W with the beam quality of M2 = 1.35 at 500 Hz. After amplification, the 1319 nm output power is as high as 78.5 W with the beam quality of M2 = 1.85. With 1064 nm seed-injected power of 53.4 W (beam quality M2 = 1.21, and 500 Hz PRR), the amplified 1064 nm output power is up to 238 W with beam quality of M2 = 1.65. The amplified 1319 and 1064 nm beams are expanded and collimated by BS1 and BS2. After that, the 1064 nm beam is divided two beams with the power ratio 1:1 by mirror M3. One beam is reflected by M4 and M5, and then together with 1319 nm beam is focused into a Lithium triborate (LBO1) nonlinear crystal to achieve single-pass 589 nm SFG by a suitable lens F1. HWP3 is used to adjust the polarization of the 1064 nm beam in consistent with the 1319 nm. The 589 nm SFG output beam is separated from two fundamental beams by a M6 and collimated with BS3, and reflected by M7, M8 and M9. The wavelength stability can be obtained through the PZT servo control. The attenuator comprising of a HWP1 and a TFP1 is used to control the input power of the sodium cell.
Fig. 3 Schematic diagram of the double resonant fluorescence scattering on the 3S-3D in Sodium Vapor.
In the 819.7 nm laser branch, another shared 1064 nm beam reflected by mirror M3 is focused into LBO2 crystal to generate 532 nm laser by a suitable lens F2. The dichroic flat mirrors M10 and M11 are employed to separate residual 1064 nm and reflect 532 nm. The attenuator comprising of a HWP2 and a TFP2 is used to regulate the input power of the Ti:sapphire. And the generated 532 nm beam is collimated by BS4. The Ti:sapphire crystal is cut at Brewster’s angle at both end facets with respect to the direction of c-axis for the highest absorption in the pumping light (>80%), highest emission in the infrared, and the lowest residual absorption at the laser wavelengths [23]. M12 is HR-coated at 820 nm and antireflection (AR)-coated at 532 nm. M13 is an output coupler with a transmission of 10% at 820 nm. To get the tunable output wavelength with narrow linewidth around 819.7 nm, the main broad-range tunable element, namely, three BPs tuner acting as a wavelength selection and “coarse” spectral narrowing filter, is utilized, and further spectral narrowing is achieved via inserting one Fabry-Perot (FP) etalon in the cavity of the Ti:sapphire laser. The generated 819.7 nm beam is separated from the green pump beam by a Brewster prism. The dispersed 819.7 nm beam is collimated by BS5, and then reflected by M15. M16 is used to control the input power of the sodium cell.
For the double resonant fluorescence scattering device, the 589 and 819.7 nm beams are expanded and collimated to the same size. Then they are combined by a dichroic mirror M17. The combined beam is injected into the sodium vapor sealed in a cell, which is enchased into a black-box. The resonant fluorescence scattering signal is collected by a charge coupled device (CCD) via a lens F3 with the focus length of 100 mm in the side of the sodium cell.
3. Results and discussion
Based on the SFG of 1319 and 1064 nm beams, a pulse width of ~90 μs at 589 nm is obtained at 500 Hz. The output wavelength tunability of the yellow laser can be realized by tuning the temperature of the etalon in the 1064 nm oscillator. Figure 4 shows the wavelength as a function of the temperature of the etalon. With the temperature changed from 74 °C to 76.2 °C, the wavelength is continuously tuned from 589.1565 nm to 589.1615 nm, and the slope of the tuning curve is 2.3 pm/°C. Meanwhile, using a power meter (OPHIR, FL400A-LP1-50), the power stability of 589.159 nm laser are measured over 2 h, as displayed in theupper-left inset in Fig. 4. The mean value of output power at 589 nm is 44 W, and the standard deviation of the power stability is about 1.6%. Moreover, the beam intensity distribution is measured at the maximum output power by a beam quality analyzer (M2-200, Spiricon Inc.), and calculated to be M2 = 1.5. The lower-right inset in Fig. 4 shows the two dimensional (2-D) spatial intensity profile of the laser beam, which exhibits that the beam pattern is quite smooth with an almost Gaussian profile. Besides, a wavelength meter (High Finesse WS-7) is adopted for the wavelength measurement over 2 h, as shown in Fig. 5. The central wavelength and linewidth are controlled by the etalon. It is found from Fig. 5 that the measured linewidth is about 0.46 pm at the wavelength 589.15906 nm. The wavelength fluctuation is measured to be within ± 0.5 pm over 2 h with standard deviation of 120 fm. Compered with the parameters of laser in on-sky experiment [24,25], the performance of our 589 nm laser can meet the actual needs.
Fig. 4 Wavelength of 589 nm laser as a function of the temperature of the etalon in the 1064 nm oscillator; the upper-left inset: power-stability test of 589.159 nm beam at the highest output over 120 min; the lower-right inset: 2D spatial intensity profile of the 589 nm beam.
The generated pulse at 532 nm (38 W, M2 = 1.60) is employed as a pump source for the Ti:sapphire laser at 819.7 nm. Figure 6 shows the output power of the Ti:sapphire laser as a function of the incident green pump power. We observe that the laser output power grows monotonically with the increasing input power. It can be seen from Fig. 6 that the maximum output power at 819.7 nm is 2.4 W for an incident average power of 38 W, corresponding to an optical to optical conversion efficiency of 6.3%. The Ti:sapphire laser operates at 500 Hz with a pulse width of ~25 μs. Meanwhile, the output wavelength tunability of the Ti:sapphirelaser can be realized by tuning the temperature of the etalon. The upper-left inset in Fig. 6 shows the wavelength as a function of the temperature of the etalon. When the temperature is changed from 25 °C to 27.5 °C with the temperature step-length of 0.1 °C, the wavelength is continuously tuned from 819.699 nm to 819.718 nm. The slope of the tuning curve is 7.6 pm/°C, corresponding to a tuning resolution of 0.76 pm. Moreover, under the maximum output power, the beam quality factor M2 values of the horizontal axis and the vertical axis are 1.18 and 1.21, respectively, corresponding to an average value of M2 = 1.2. The lower-right inset in Fig. 6 shows the 2-D spatial intensity distribution of the laser beam. It is suggested that the 819.7 nm beam is operating very close to Gaussian mode. Besides, the wavelength and linewidth of the Ti:sapphire laser at FP temperature of 26.3 °C is measured by a wavelength meter (WS-U High finesse GmbH) under maximum output power, as illustrated in Fig. 7. It can be seen from Fig. 7 that the measured linewidth is ~0.8 pm at the center wavelength 819.710 nm. The wavelength fluctuation is measured to be within ± 0.44 pm over 2 h with standard deviation of 124 fm.
Fig. 6 Output power at 819.7 nm versus pump power of 532 nm; the lower-right inset: 2D spatial intensity profile of the 819.7 nm beam; the upper-left inset: wavelength as a function of the temperature of the etalon.
In the double resonant experiment, a control loop with electrical signal delay is designed to provide the synchronicity of two pulse beams, in which the phase difference of the 589 nm pulse and 819.7 nm pulse could be compensated by adjusting the delay module. To validate the double resonant fluorescence effect, firstly only the 589 nm beam is injected into sodium cell. While tuning the wavelength to 589.159 nm, the resonant fluorescence scattering signalis observed and registered by a CCD (BASLER, ACA2000-50gm, NIR), as displayed in Fig. 8(a). When the power of 589.159 nm beam is up to 2W, the sodium atoms are saturated. Following, a narrow band-pass filter at 820 nm (Thorlabs, center wavelength: 820 nm; full width half maximum: 10 nm) is placed in the front of the CCD to block out the resonant fluorescence of 589.159 nm, and Fig. 8(b) shows no any signal. It suggests that the resonant fluorescence at 589.159 nm is disappeared on the CCD. When the 589.159 and 819.7 nm beams are simultaneously injected into sodium cell, the signal on the CCD appears again as presented in Fig. 8(c), which should be the resonant fluorescence at 819.7 nm, because the resonant fluorescence at 589.159 nm on the CCD has been cut out. Finally, with detune of wavelength for either 589.159 nm or 819.7 nm beams, the signal of resonant fluorescence at 819.7 nm further vanishes, as shown in Fig. 8(d). To sum up, the double resonant fluorescence scattering through 3D5/2→3P3/2→3S1/2 emissions is observed with two step excitation in sodium atoms by using our watts-level QCW μs pulse all solid state laser operating at 589 and 819.7 nm.
Fig. 8 Measurement the double resonant fluorescence scattering by CCD: (a) only the 589 nm beam is injected into sodium cell; (b) the narrow band-pass filter at 820 nm is insert to remove resonant fluorescence scattering of 589 nm beam on the CCD; (c) the 589 and 819.7 nm beams are simultaneously injected into sodium cell with the narrow band-pass filter at 820 nm; (d) the influences of wavelength detune over either 589 or 819.7 nm beams.
As mentioned above, the output wavelength tunability of the Ti:sapphire laser can be realized by changing the temperature of the etalon, and the intensity of the resonant fluorescence around 819.710 nm as function of wavelength is measured by CCD, as illustrated in Fig. 9. It is found that the peak intensity is located at the center wavelength of 819.710 nm and the permitted band-width of sodium atoms at 819.710 nm based on the μs 589 nm laser is also measured to be 2.56 pm.
4. Conclusion
In summary, we demonstrate an all solid state a QCW μs pulse PLGS laser source, which contains two wavelengths operating at 589.159 and 819.710 nm. A 44 W average output power at 589.159 nm is generated with a linewidth of ~0.46 pm, beam quality factor of M2 = 1.50, operating at a 500 Hz repetition rate with the pulse width of ~90 μs. Meanwhile, a 2.4 W average output power is achieved operating at 819.710 nm with the linewidth 0.8 pm, and beam quality factor M2 of 1.20, and a pulse width of ~25 μs at 500 Hz. Further power sealing of the laser output at 819.710 nm can be expected by employing the Ti:sapphire MOPA approach. Moreover, a proof-of-concept demonstration on the double resonant from 3S1/2-3P3/2-3D5/2 transition of sodium atoms has been carried out. This work opens the possibility of real 589 and 819.7 nm bi-chromatic sodium LGS.
Funding
National Natural Science Foundation of China (NSFC) (11504389, 61505226, and 11504390), and the Knowledge Innovation program of Chinese Academy of Science.
Acknowledgments
We acknowledge support from Prof. Xiaojun Xu and fruitful discussions with Prof. Hongyan Wang of National University of Defense Technology, China. Meanwhile, we appreciate help from Prof. Lu Feng and Prof. Zhou Fan from National Astronomical Observatories, Chinese Academy of Sciences, China.
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