Photon Return On-Sky Test of Pulsed Sodium Laser Guide Star with D 2b Repumping

Transcription

1 PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF THE PACIFIC, 127: , 2015 August The Astronomical Society of the Pacific. All rights reserved. Printed in U.S.A. Photon Return On-Sky Test of Pulsed Sodium Laser Guide Star with D 2b Repumping KAI JIN, 1,2,3 KAI WEI, 1,2 LU FENG, 4 YONG BO, 5 JUNWEI ZUO, 5 MIN LI, 1,2 HANCHU FU, 1,2,3 XIAOLIN DAI, 1,2,3 QI BIAN, 5 JI YAO, 5 CHANG XU, 5 ZHICHAO WANG, 5 QINGJUN PENG, 5 XIANGHUI XUE, 6 XUEWU CHENG, 7 CHANGHUI RAO, 1,2 ZUYAN XU, 5 AND YUDONG ZHANG 1,2 Received 2014 December 16; accepted 2015 June 04; published 2015 August 4 ABSTRACT. Sodium laser guide star (LGS) system has become one of the critical components in modern astronomical adaptive optics system (AOS), especially for the next-generation extremely large telescopes, such as the Thirty Meter Telescope and the European Extremely Large Telescope. Since the wavefront detection performance of AOS is directly related to the brightness of LGS, it is important for AOS to maximize its photon generation efficiency by all means. Sodium D 2b line repumping is such a technique that can greatly increase the returned photons for either sodium continuous wave (CW) laser or pulsed laser. This technique has been studied theoretically and field tested with a 20 W CW laser by European Southern Observatory team. However, field test results of a 20 W class pulsed laser with D 2b repumping have not been reported yet. In this paper, our latest field test results with theoretical comparison of D 2b repumping with a 20 W quasi-continuous wave (QCW) pulsed laser will be presented. With a linearly polarized beam, approximate 40% photon return enhancement was achieved when 10% of laser power was detuned to D 2b line, which agreed well with results from a rate equation-based Monte Carlo photon return simulation program. Both experiment and simulation results indicate that with a higher laser intensity projected at the sodium layer, the D 2b repumping will be more effective. Online material: color figures 1. INTRODUCTION Astronomical telescopes with adaptive optics systems (AOS) have been in use since early 1990s (Rousset et al. 1990). The functioning of AOS requires relatively bright stars as references for measuring the wavefront distortions induced by the atmospheric turbulence of the Earth. However, sky coverage of AOS is limited by the scarcity of suitable reference stars in close proximity to observing targets. To overcome this limitation, artificial beacons, or laser guide stars (LGS), generated either by Rayleigh backscattered or resonance fluorescence of sodium atoms, were proposed and well-tested by many observatories (Thompson & Gardner 1987; Greenwood & Primmerman 1 Institute of Optics and Electronics, Chinese Academy of Sciences, Chengdu , China; wei_kai@126.com. 2 The Key Laboratory on Adaptive Optics, Chinese Academy of Sciences, Chengdu , China. 3 University of Chinese Academy of Sciences, Beijing , China. 4 National Astronomical Observatories, Chinese Academy of Sciences, Beijing , China. 5 Research Center for Laser Physics and Technique, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing , China. 6 University of Science and Technology of China, Hefei , China. 7 Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan , China. 1992; Fugate et al. 1994). Due to higher altitude, sodium LGS offer substantially improved spatial sampling of atmospheric turbulence (Hardy 1998). Nearly all the 8+ meter telescopes are using or planning to use sodium LGS for their AOS, including the upcoming Thirty Meter Telescope (TMT) and the European Extremely Large Telescope (E-ELT) (d Orgeville et al. 1999; Wizinowich et al. 2006; Calia et al. 2006; Hayano et al. 2006; Joyce et al. 2006; Boyer et al. 2010; Diolaiti et al. 2010). These new generation telescopes have strict requirements on the laser with the prerequisite combination of volume, beam quality, spectral/pulse format, and especially a higher output power (Boyer et al. 2010). To our knowledge, only a few lasers are close to meeting such a combination of requirements. Among them, the Technical Institute of Physics and Chemistry (TIPC), Chinese Academy of Sciences (CAS) quasi-continuous wave (QCW) 20 W solid state sodium laser (Wang et al. 2014), which combines Hz pulse repetition frequency (PRF) and 100-μs pulsed format, is a better alternative to continuous wave (CW) laser for solving Rayleigh scattering and fratricide problem (Wang et al. 2010). Theoretical study of photon return efficiency of this type of laser has been reported by Rochester et al. (2012), which shows a substantial increase of flux with D 2b repumping. However, few field tests have been done to support this theoretical claim. In this paper, we will show our latest field test results to validate this claim. We will first give the 749

2 750 JIN ET AL. background of the test in 2. Our optical setup for the on-sky test will be described in 3. Experiment results will be listed in 4. A comparison between field test results and numerical simulation results is given in 5. Conclusions will be summarized in BACKGROUND Sodium LGS takes advantage of sodium D2 transitions between the 32 S1=2 and 32 P3=2 levels. The sodium 32 S1=2 ground level consists of two hyperfine multiplets due to its different atomic angular momentum F, with three magnetic substates for F ¼ 1 and five magnetic substates for F ¼ 2 ground state, corresponding to D2b and D2a lines, respectively. These two hyperfine multiplets are separated by GHz (Steck 2010). With Doppler broadening of 1 GHz full width at half maximum (FWHM) at mesospheric temperature near 200 K, a double-hump absorption profile is generated (see Milonni et al. [1998], Fig. 2). In order to maximize the photon return efficiency of the sodium beacon, study of the interaction physics between the laser light field and sodium atoms is necessary. Milonni & Thode (1992) simplified the process with a two-level Bloch model which he solved in time domain. Morris (1994) studied with a broader 3 GHz spectrum, which includes the whole absorption spectrum of the sodium D2 line. Milonni et al. (1998) classified pulsed laser into three categories based on pulse length compared to sodium D2 radiative lifetime. Spin relaxation and Larmor precession were later incorporated into the Bloch equation for CW laser simulation (Milonni et al. 1999). Holzlöhner et al. (2010) numerically optimized the CW laser format with Bloch equations by including transitions between all the 24 sodium hyperfine states, the effect of the geomagnetic field, and collision-induced spin exchange. In his work, repumping of 10 20% of the total laser power on the D2b line was suggested as an effective method of enhancing the photon return efficiency. Rochester et al. (2012) simulated the coupling efficiency of laser format similar to TIPC 20 W sodium laser with 10% D2b repumping. All of the above works utilized Bloch equations for solving the coupling efficiency. Kibblewhite (2008) simulated the photon return with rate equations, using the Monte Carlo method. The description of physics with this method is relatively simple and intuitive compared to the Bloch equations method. We collaborated with Prof. Kibblewhite using the rate equation method in our simulation for the results shown in this paper. However, the Monte Carlo method has a drawback in that it requires huge amount of calculation resources. Theoretical simulation for the effectiveness of D2b repumping have been studied in many texts; however, only a few of experimental results have been reported. Calia et al. (2012) reported the photon return increase with a narrow band CW laser in the test at Ottobeuren AVSO observatory. The results showed D2b repumping is more useful when the laser propagation direction is away from the geomagnetic field lines. While the angle between the laser propagation direction and the geomagnetic FIG. 1. Experimental configuration of the EOM. (a): schematic diagram; (b): photograph; and (c): RF connection of the EOM. See the electronic edition of the PASP for a color version of this figure. field line was 68:3deg, D2b repumping brought 43.5% and 42.5% photon return increases for linearly and circular polarized light, respectively. Li et al. (2014) demonstrated in the lab the D2b repumping effect with a vacuum sodium cell using TIPC sodium laser prototype #1. In 2011 and 2013, on-sky test of the photon return measurement with TIPC sodium laser prototypes #1 and #2 were conducted in Lijiang Observatory, China (Wei et al. 2012; Jin et al. 2014). Circular polarized light brought 30% or even more returned photons than linearly polarized light during those tests. In the latest test, an electro-optic modulator (EOM, Qubig EO-Na23-5R) was installed on TIPC laser prototype #2 that provided the laser with a D2b repumping capability. In the following sections, we will describe our setup and results of the photon return measurement for the TIPC QCW sodium laser prototype #2, and compare them with the results of rate equation-based Monte Carlo simulation.

3 LASER GUIDE STAR WITH REPUMPING SET UP OF THE ON-SKY TEST 3.1. TIPC Pulsed Laser The TIPC sodium laser prototype #2 was used for the field test. Details of the laser had been reported in Wang et al. (2014). Two side-pumped Nd:YAG master oscillator power amplifiers (MOPA) were used for amplification of the 1064 nm and 1319 nm seed lasers, respectively. The sum frequency generation (SFG) stage with Lithium triborate (LBO) crystal combined the two beams and output a beam of 589 nm wavelength (see Wang et al [2014], Fig. 1). The maximum output power of the 589 nm laser was 33 W during the test in the laboratory. Main parameters of the TIPC pulsed laser prototype #2 are listed in Table Beam Transfer Optics and Laser Launch Telescope The test was conducted at Gaomeigu Site, Lijiang Observatory, Yunnan Province, China. A 1.8-m telescope was used as LGS photometry telescope. A standard astronomical Johnson photometric V-band filter was used to do the photometry of the sodium beacon. A 300 mm-diameter laser launch telescope (LLT) was used to expand the laser beam and project it to the sky. Details of the 300 mm-diameter LLT have been described in 2.2 of Jin et al. (2014). The LLT was attached to the elevation journal of the 1.8-m telescope but always pointing to zenith during the test. The distance between the optical axes of the LLT and the 1.8-m receiver telescope was 1.46 m. There were ninefold mirrors in the Beam Transfer Optics (BTO) between the laser package and LLT. The total transmission of the BTO measured was Setup of the EOM The main purpose of the test was to validate the effectiveness of sodium D2b line repumping for the photon generation efficiency. An EOM was used to create the D2b side-band signal. As shown in Figure 1, the EOM was placed at the end of the 589 nm laser. The laser was output with horizontal polarization. A combination of a half-wave plate (HWP) and a polarizer was used as power attenuator. By changing the rotation angle of the HWP, one could change the beam output power afterward, and still maintain the horizontal polarization direction as required by the EOM. The reflected light from the polarizer was collected by a beam dump to prevent damage of the laser. A lens group formed a 2 expander system expanding the laser beam to proper size before entering LLT. A quarter-wave plate (QWP) was placed at the end to change final polarization state of the laser beam from linearly polarized to circular polarized when needed. The resonance frequency of the EOM was originally set to 1713 MHz. Figure 1c shows the radio-frequency (RF) connection of the EOM. A RF generator (Windfreak SynthNV) with a power amplifier (Mini-Circuits ZHL-5W-2G-S+) and a viable FIG. 2. Example of LGS image. Top: initial data; bottom: after subtracting the Rayleigh background. See the electronic edition of the PASP for a color version of this figure. TABLE 1 MAIN PARAMETERS Parameter Maximum output power Linewidth Beam quality Polarization Pulse length PRF OF THE TIPC PULSED LASER Typical value 32 W at 500 Hz 0.6 GHz M 2 1:5 >99% μs Hz

4 752 JIN ET AL. attenuator (Fairview Microwave SA4086) were used as the EOM s RF power driver. The driving RF signal was then passed through a RF circulator (Fairview Microwave SFC1020). While most of the power of the RF signal transmitted to the EOM, part of the signal was reflected and fed into a high bandwidth oscilloscope (Teledyne LeCroy 8 Series) for monitoring via the circulator. By adjusting the RF modulation depth, 10% of the total laser power was detuned to sodium D 2b line. At the same time, another 10% of the total power was converted to the 1713 MHz side-band, which will not interact with the sodium atoms. 4. EXPERIMENT RESULTS Before the D 2b repumping test, the laser wavelength was optimized by maximizing the returned photons of the sodium LGS. The LGS spot size was minimized by adjusting the focal stage distance of secondary mirror of LLT. A 3 (FWHM) LGS spot with an exposure time of 0.5 s was achieved. Because the sodium LGS spot was close to the Rayleigh scatting, it is necessary to remove the Rayleigh as background noise. After adjusting the wavelength of laser away from sodium resonance frequency, a series of exposures of the Rayleigh were taken and subtracted from the sodium LGS images during postprocessing (Fig. 2). For each group of measurements, 20 frames of images were taken, with 0.5 s exposure times and a 4 s interval between each frame. To test the repeatability of the EOM D 2b repumping effect, two groups of images with EOM off (without D 2b repumping) were taken before and after the group with EOM on (with 10% D 2b repumping). To keep the laser working at a stable state, the maximum laser power used during the test was 15 W. Because the laser power cannot be measured simultaneously with the data acquisition of the CCD camera, the laser power value given in this paper is the average of the two measurements before and after image acquisition. Table 2 gives the results of the EOM test during the night of 2014 April 20. Three different laser output power levels (14 W, 10 W, and 8 W) were selected. However, the acquired image data indicated that there were thin clouds in the sky during 14 W test, so this group of data is not presented in this paper. In most cases, standard deviation of the calibrated flux of sodium beacons was less than 7% while only one group of data reached 10%. This variation contained the drift of output laser power, changes of the atmospheric transmission, sodium layer activity, and the calibration error of the sodium beacon. Figure 3 shows the LGS returned flux when EOM was turned off/on when at 10 W output power. Figure 4 shows the cross section at the center of the LGS spot in both direction for all 20 frames for measurements done at 10 W. The state when EOM was on is shown in blue, and the off state is shown in red. We observed three reference natural stars near zenith (see Table 3) afterward for calculating the absolute photon return of the LGS. The total duration of all testing was about 2 hr. The average coupling efficiency s ce of the laser is calculated with following equation as listed in Holzlöhner et al. (2010), s ce ¼ ΦL 2 PT 2X a C Na X : (1) where Φ is the photon flux on the detector (in units of photons=s=m 2 ), which was calculated from the photometry results of the LGS. L is the vertical distance from the receiver telescope to the center of the sodium layer, P is the laser power at the exit of the LLT, T a is the atmospheric transmission, X ¼ 1= secðθþ and θ is the zenith angle of the launching beam, and C Na is the column density of the sodium atoms. s ce is in units of photons=s=w=ðatoms=m 2 Þ=sr. As laser power varied during the test, the effectiveness of D 2b repumping will be validated with s ce. Though the sodium column density C Na and atmospheric transmission T a are not known for sure, the assumption is made that these quantities changed little whether EOM was on or off. Then the increase of the s ce is independent of the sodium profile and atmospheric transmission. For the 10 W and 8 W level, coupling efficiency s ce increased 42.5% and 38.2% when 10% of total laser power was detuned to D 2b line. Unfortunately, in our test, the QWP was not placed at the right angle to ensure a circular polarized beam during the test of D 2b repumping. Therefore, results with linearly polarized light only are presented in this paper. 5. DISCUSSION Numerical simulation of the interaction between laser and sodium atoms with the rate equation method has been studied by Kibblewhite (2008). In this paper, we use this method for TABLE 2 ON-SKY TEST RESULTS OF THE EOM Start time (local) Power (W) EOM Fwhm-x (arcseconds) Fwhm-y (arcseconds) Flux (ADU) Flux error s ce increase 01: off E % 01: on E % 42.5% 01: off E % 01: off E % 01: on E % 38.2% 01: off E %

5 LASER GUIDE STAR WITH REPUMPING 753 TABLE 3 REFERENCED NATURAL STARS Start time (local) Star Flux (ADU) 02:46 HIP E+05 02:48 HIP E+05 02:50 HIP E+05 FIG.3. LGS flux at 10 W level, with two groups of EOM off and one group of EOM on. See the electronic edition of the PASP for a color version of this figure. simulation. Input parameters had been changed according to the local conditions, which are listed in Table 4. Three longitudinal modes of the TIPC laser were assumed, with each separated by 150 MHz. The laser pulse shape measured is shown in Figure 5. The LGS spot was assumed to have a Gaussian distribution on the sodium layer. The simulated sodium profile was based on a group of sodium density lidar data from actual measurement by University of Science and Technology of China (USTC). The data was then discretized from 80 to 110 km into total of 30 layers as our simulation input data. The total column density was assumed to be 2:0E þ 013 atoms=m 2, which is the typical seasonal value in the last few years (Xue et al. 2013). We neglected the fluorescence difference between each sodium layer, because loss by absorption is little compared to the total laser power, as described by Rochester et al. (2012). Considering the exposure time of the CCD images was relatively long compared to atmospheric turbulence change, and the imperfect imaging system would enlarge the LGS spot size, after removing the effect of laser jitter (standard deviation of 0.15 in our measurement), turbulence of atmosphere (which broadened the FWHM of two stars images in V-band for 1.1 at 00:36 and 1.4 at 01:39), and optical transfer function of the imaging system, the actual spot size should be around In our simulation, we changed the spot size according to the reasons mentioned above. The total transmission of the BTO and LLT was 0.6, attained by multiplying transmissions of premeasured BTO and LLT transmission. The atmospheric transmission T a was assumed to be 0.5, which was deduced from the photometry data of the natural stars. The zenith angle θ was set to zero as all the measurements were made at zenith. Figure 6 shows the comparison between numerical simulation and calculated experimental results with linearly polarized TABLE 4 PARAMETERS USED FOR NUMERICAL SIMULATION OF THE COUPLING EFFICIENCY FIG. 4. Cross section of the calibrated LGS spot center with D 2b repumping (blue line) and without D 2b repumping (red line). See the electronic edition of the PASP for a color version of this figure. Parameter Value PRF 500 Hz Pulse length 100 μs Total transmission of BTO and LLT 0.6 Atmospheric transmission 0.5 Geomagnetic field strength a 0.46 Gauss Geomagnetic field angle from zenith a 131 Observatory altitude 3.2 km Temperature of the sodium layer 185 K a Geomagnetic field information obtained from the International Geomagnetic Reference Field (IGRF) model:

6 754 JIN ET AL. FIG. 6. Coupling efficiency at different laser intensity for linearly polarized light, with and without D2b repumping. Solid line: Numerical simulation results with D2b repumping (solid blue line) and without D2b repumping (solid red line). Blue squares: experimental results with D2b repumping; red diamonds: experimental results without D2b repumping; purple diamonds: experimental results without repumping but measured in See the electronic edition of the PASP for a color version of this figure. bars represent mean value and standard deviation of the experimental results with D2b repumping while red diamonds with error bars represent results without repumping. Purple diamonds with error bars are measured results of 2013 testing using the same laser but different BTO and LLT (Jin et al. 2014). Average laser intensity on the x-axis is calculated by P =ðπ F W T M 2 Þ, where P is the laser power projected at the sodium layer and FWTM is the full width at tenth maximum of the LGS spot. The difference between the simulation and estimated experimental coupling efficiency is listed in Table 5. Numerical simulation indicates that an enhancement of 39.6% and 21.7% of total photon return will be achieved with 10% D2b repumping at 10 W and 8 W laser power for linearly polarized light, respectively. Both experimental and simulation results indicate that D2b will be more effective at higher laser irradiance. At the 10 W level test, D2b repumping resulted in average 65.1% more photons at the center of the images while the total returned photons were 42.5% more. Figure 7 gives another measurement result for the D2b repumping. Though the polarization of the laser TABLE 5 SIMULATION AND ESTIMATED EXPERIMENTAL RESULTS AVERAGE COUPLING EFFICIENCY FOR LINEARLY POLARIZED (LP) LIGHT FIG. 5. Input parameters for numerical simulation. (a): laser spectrum; (b): laser pulse shape; and (c): sodium profile. light. Solid lines indicate the numerical simulation results for linearly polarized light with (solid blue line) and without (solid red line) D2b repumping, respectively. Blue squares with error 7.8 W Power LP, no D2b LP + 10% D2b Increase OF 10.0 W Sim. Exp. Sim. Exp % % % %

7 LASER GUIDE STAR WITH REPUMPING 755 results indicate that for linearly polarized light, sodium laser with 10% of total power on D 2b repumping will bring 20% or even more returned photons than that without D 2b repumping. As the weather condition did not permit further tests with circular polarized light, more test will be held at Xinglong Observing Station, China, in late 2014, in cooperation with National Astronomical Observatories, Chinese Academy of Sciences (NAOC), and the TMT Observatory Corporation. FIG. 7. Total flux of the LGS for an unknown-polarization-state light, with and without D 2b repumping. See the electronic edition of the PASP for a color version of this figure. was uncertain (the QWP was not in the right rotate angle), D 2b repumping brought more returned photons with higher laser intensity. Numerical simulation indicates that at higher laser intensity levels, saturation will lead coupling efficiency dropping down quickly. However, the D 2b repumping will postpone the influence of saturation and keep the coupling efficiency at a steady level. The LGS spot size and spot shape on sodium layer would have an effect on the simulation results of average coupling efficiency of the laser. The assumption that column density would change little over time between the two measurements could also introduce errors in calculating the exact coupling efficiency. These will all cause the differences between the simulation and experimental results. However, for the experimental circumstances, both the simulation and experimental 6. CONCLUSIONS In this paper, we validate the effectiveness of D 2b repumping using TIPC pulsed laser. The experimental results indicate that for linearly polarized light and laser intensity at 1 2 W=m 2, coupling efficiency of TIPC sodium laser increases approximate 40%, with 10% of the total laser power detuned to D 2b line. Numerical simulation results using rate equations Monte Carlo method agree well with the experimental data. Both experimental data and simulation programs show D 2b repumping will be more effective at higher laser intensity. We thank Doctor XianMei Qian and ChaoLong Cui from Anhui Institute of Optics and Fine Mechanics, CAS, Doctor Yu Zhou from Yunnan Observatory, CAS, for their help during the test. We thank DingWen Zhang, FuRui He, HanLong He, and LiPing He from Yunnan Observatory, CAS, for their assistance during the data acquisition. We thank Professor Kibblewhite for explaining and helping us setting up the simulation. We are thankful for the advice, guidance, and support given by Doctor Angel Otarola from TMT Observatory Corporation and Professor Paul Hickson from University of British Columbia. Last but not least, we thank all the other members in the group for their great support. REFERENCES Boyer, C., Ellerbroek, B., Gilles, L., et al. 2010, 1st AO4ELT Conference, Calia, D. B., Allaert, E., Alvarez, J. L., et al. 2006, Proc. SPIE, 6272, Calia, D. B., Guidolin, I., Friedenauer, A., et al. 2012, Proc. SPIE, 8450, 84501R Diolaiti, E., Conan, J. M., Foppiani, I., et al. 2010, Proc. SPIE, 7736, 77360R d Orgeville, C., Chun, M. R., Sebag, J., et al. 1999, Proc. SPIE, 3762, 150 Fugate, R. Q., Spinhirne, J. M., Moroney, J. F., et al. 1994, J. Opt. Soc. Am. A, 11, 310 Greenwood, D. P., & Primmerman, C. A. 1992, Lincoln Laboratory J., 5, 3 Hardy, J. W. 1998, Adaptive Optics for Astronomical Telescopes (New York: Oxford University Press) Hayano, Y., Saito, Y., Ito, M., et al. 2006, Proc. SPIE, 6272, Holzlöhner, R., Rochester, S. M., Calia, D. B., et al. 2010, A&A, 510, A20 Joyce, R., Boyer, C., Daggert, L., et al. 2006, Proc. SPIE, 6272, 62721H Jin, K., Wei, K., Xie, S., et al. 2014, Proc. SPIE, 9148, 91483L Kibblewhite, E. 2008, Proc. SPIE, 7015, 70150M Li, L., Zhang, S., Hua, W., et al. 2014, Proc. SPIE, 9148, 91483T Milonni, P. W., & Thode, L. E. 1992, Appl. Opt., 31, 785 Milonni, P. W., Fearn, H., Telle, J. M., et al. 1999, J. Opt. Soc. Am. A, 16, 2555 Milonni, P. W., Fugate, R. Q., & Telle, J. M. 1998, J. Opt. Soc. Am. A, 15, 217 Morris, J. R. 1994, J. Opt. Soc. Am. A, 11, 832 Rochester, S. M., Otarola, A., Boyer, C., et al. 2012, J. Opt. Soc. Am. B, 29, 2176 Rousset, G., Fontanella, J. C., Kern, P., et al. 1990, A&A, 230, L29

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