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Abstract
A narrowband sodium lidar for measuring mesospheric temperature and wind has been established at YangBaJing, Tibet (90°E, 30°N, 4300 m a.s.l), China. The system is designed and optimized based on important upgrades using new technology. In the lidar system, single-mode 589 nm seed laser is produced by frequency doubling of 1178 nm diode laser with a periodically poled lithium niobate (PPLN) waveguide. The output power of 589 nm continuous-wave laser is up to 1.5 W with the help of a seed-injected Raman fiber amplifier. Furthermore, fast three-frequency switcher is designed with a couple of fiber magneto optical switches (FMOS) for measuring wind and temperature, simultaneously, which greatly reduces the system maintenance. These improvements greatly simplify the lidar system, thus, achieve robust operation with minimum maintenance requirements.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The mesopause region over Tibetan Plateau is an ideal region of atmospheric interactions and coupling between the high, middle and lower layers. Measurements of this area contribute to the holistic understanding of atmospheric activities in response to solar activities [1,2], and advance researches on atmospheric wave propagation, energy and momentum circulations [3–5]. Narrowband sodium lidar can detect temperature structure and wind structure with high spatio-temporal resolution in the mesopause region, therefore plays an important role in studies of atmospheric dynamics and thermodynamics [6–10].
Simultaneous measurements of mesopause wind and temperature can be realized by a narrowband sodium lidar with a three-frequency single-mode pulsed 589 nm laser. The single-mode pulsed 589 nm laser generation is the key to sodium lidar, the corresponding lasers are important components of the system. Therefore, improvement of the performance of lidar system is closely relied on the development of 589 nm laser system. Classic narrowband sodium lidar is fabricated on continuous-wave (CW) single-mode ring cavity dye laser injection-seeding pulsed dye amplifier (PDA), which has been widely used around the world [11–14]. However, the ring cavity is sensitive to vibrations and temperature variations, which brings great burden to operation and maintenance, especially under the abominable environment in Tibetan Plateau. Recently, both CW 589 nm seeder and pulsed 589 nm laser have been trending towards solidification and compactification. 589nm CW laser sum-frequency generation (SFG) using two CW lasers at 1064 and 1319 nm with a periodically poled lithium niobate (PPLN) crystal was employed in the sodium lidar at ALOMAR station, Norway [15]. However, the power of CW 589 nm laser is not very sufficient for the PDA. Shinshu University, Japan, is reported to build a solid-state temperature sodium lidar based on SFG of two diode-pumped pulsed Nd:YAG lasers at 1064 and 1319 nm which are injection-seeded [16], and upgraded it to simultaneously detecting of wind, temperature by three frequencies switching at 1064 nm [17]. National Space Science Center, in corporation with Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, has developed a solid-state Doppler lidar for simultaneous wind and temperature measurements at YanQing Station, Beijing, China [18]. However, there exists some difficulty for fast three frequencies switching of cavity-enhanced pulsed Nd:YAG laser.
In this paper, we report an all solid-state of CW 589 nm laser for sodium lidar in Tibetan Plateau. A fast three-frequency switcher is designed utilizing two fiber acousto-optic modulators (AOM) and two fiber magneto optic switches (F-MOS), without any moving component. A two-stage Raman fiber amplifier (RFA) is employed to boost sufficient 1178 nm laser. A PPLN with waveguide structure is applied to increase the conversion efficiency of second harmonic generation (SHG) for enough CW 589 nm laser. The CW laser is injecting the PDA for pulsed 589 nm laser with a power of 1.5 W. The system applies a lot of fiber-coupled structures, which are facilitated for ingenious design and small structure with fiber’s flexibility and winding, thus greatly reduce adjustment and maintenance.
The CW 589 nm laser has been employed in the sodium lidar in Tibet. With all the conveniences, the lidar system has well adapted to the severe environment in Tibet. In the second part, the newly designed transmitter of the sodium lidar is discussed in details. In the third part, preliminary observations of temperature and wind measured by the sodium lidar at YangBaJing, Tibet are reported.
2. Lidar transmitter and improvements
The seed laser of the sodium lidar has been redesigned to deal with the severe environmental challenges in Tibet. Figure. 1 shows the schematic of newly designed transmitter of the sodium lidar. The 1178 nm diode laser, from Toptica Photonics, generates CW laser up to ~1.5W, linearly polarized fiber coupled laser at frequency tunable around 1178 nm. The light beam is split by a 50/50 fused fiber splitter. One output of the splitter is coupled to a PPLN waveguide to produce light at 589 nm, which is then sent into a Na cell for Doppler-free spectroscopy. The other output is sent through an all-fiber three-frequency switcher. The output of the switcher is amplified by a two-stage RFA. Then it is focused into a Quasi-phase-matched PPLN waveguide for CW 589 nm laser with a power of 1.5 W. The CW laser with three-frequency components is injected into the cavity-free PDA to produce 589 nm pulsed laser beam for lidar emitting.
2.1 Frequency stabilization
Free-running laser has frequency drift mainly due to environmental variations. For absolute stabilization and drift compensation required by long-term lidar operation, a Na vapor cell is used to provide Doppler-free saturated spectroscopy of the NaD2 line as a reference for frequency locking. As shown in Fig. 1, a closed proportional–integral–derivative (PID) feedback loop is formed between the diode laser and the Na vapor cell. A PPLN waveguide is applied for frequency doubling of the 1178 nm laser to generate 589 nm CW laser with a phase-matching temperature of 31.5°C. The CW 589 nm laser is coupled into the Na vapor cell, which is heated to 130°C, providing Doppler-free spectroscopy to Digilock controller. The Digilock uses a modulation frequency smaller than the characteristic resonance width to obtain a derivative signal by demodulation, which is utilized to stabilize the laser via a PID regulator.
For long-term frequency stabilization, auto tracking and locking function is needed. A relock function is used to actively track the laser frequency. The laser frequency could be tuned back and fast relocked even if an occasional interruption of lock out occurs during observing. In a test, given a few minutes loss of laser by a shading plate, the spectroscopy signal is still latched, and once the plate is removed, it will start scanning to find the locking point, and quickly relock the laser. The frequency locking accuracy is less than 2.0 MHz, resulting an error of about 1.2 m/s in wind. The error is taken into account in the retrieving process.
The frequency stabilization brings great convenience for temperature/wind measurements in Tibet, laser frequency can be stabilized throughout the whole night observing without human assistance.
2.2 Three-frequency switching
Based on previous works [19,20], in this system, the center operating frequency is(589.158 nm), two flanking frequencies areMHz andMHz, respectively. Generally, two flanking frequencies are obtained using two acousto-optic modulators (AOM) in a free space optical path, the 589 nm laser passes through the AOM twice, and a mechanical shutter is used to switch the frequency between, MHz. Another approach is using fiber splitter and optical switch to switch the frequency. However, the fiber splitter would lose a lot of the laser power. In this system, a three-frequency switcher is designed with two F-MOSs and two fiber AOMs, as shown in Fig. 2. Among them, the F-MOS1 has one input port and three output ports, and the F-MOS2 has three input ports and one output port. In terms of structure, the switcher has one port of input and one port of output, with three optic paths between them. The 1178 nm laser is coupled into F-MOS1 (1-3). The OUT3 port of F-MOS1 is directly connected to the IN2 port of F-MOS2, (3-1). The OUT1 and OUT2 ports of F-MOS1 are connected to the AOM- and AOM + , respectively, of which the output ports are connected to the IN3 and IN1 ports of F-MOS2.
Due to the second harmonic generation (SHG) technique, the modulation frequency of the AOM is reduced by half, toMHz, which produces a shift of MHz at . Each F-MOS has two pins (the Ctr0 and Ctrl1) for switching control in binary logic, which provides flexible working time of each frequency and fast switching speed by electrical control. We designed a timing control for the switcher, as shown in Fig. 3, the switcher is driven by synchronized Transistor-Transistor Logic (TTL) signals. In actual observation, each frequency lasts for two seconds within a cycle. At the first 2s interval, AOM- down shifts the laser frequency by −315MHz. At the second 2s interval, AOM + up shifts the laser frequency by + 315 MHz. At the third 2s interval, neither AOM- nor AMO + works, and the output frequency is (1178.3190 nm). Figure. 4 shows three frequencies flexibly switching at the control of TTL signals.
Because the coupling efficiencies at three ports of F-MOS are different, and AOM has an insertion loss, output power of three frequencies is not identical. To balance the laser power, efficiencies of two F-MOSs and two AOMs were checked, therefore, the connecting between them is optimized, as shown in Fig. 2. Table 1 gives the checked efficiencies of three frequencies, when the input 1178 nm laser has power of 200 mW. The output power of three frequencies is sufficiently enough for the RFA.
Table 1. The efficiencies of three channels of the switcher.
2.3 Raman fiber amplifier
With the rapid development of Raman fiber laser technology, CW 589 nm laser with high power up to more than 50 W is achieved based on Raman fiber amplifier (RFA) [21]. In this system, a RFA is constructed and applied to boost the linearly polarized 1178 nm seed laser with a power up to10 W. The two stages of RFA are backward pumped by 50W 1120nm Yb doped fiber laser via the 1178/1120 wave division multiplexer (WDM). The remaining 1120nm laser is extracted out by another two WDMs. The output power is controllable by current of the pump laser. Since the input seeder power is limited to 5 mW to 100 mW, a power supervisor is designed to monitor the input power of RFA for seeder protection.
The spectrum of the 1178 nm RFA output has been checked in some studies [21,22]. However, at a pump source of 1120 nm, the wavelength range of the spectrum is not quantitatively determined. To our knowledge, optical frequency response of the RFA is still unknown. Therefore, an experiment is designed to check the performance of the RFA at three-frequency switching. The experimental setup is illustrated in Fig. 5. Two PPLN waveguides are employed to generate 589 nm laser. And two Na Doppler-free Spectroscopy devices are applied in the experiment. One of them is used to detect the saturated absorption spectrum before the RFA as the reference, the other one is used to detect the saturated absorption spectrum after the RFA.
The diode laser scanned over the NaD2 line. Figure. 6 shows the saturated absorption spectrum before and after RFA with three frequencies switching in a cyclic manner. The blue line is the reference saturated absorption spectrum, and the red line is the saturated absorption spectrum after the RFA. In Fig. 6(a), it is found that peaks of red line D2a, D2b, and Crossover follow closely to that of the blue line. The error of frequency measurement is less thanMHz, within a reasonable range. Figure. 6(b) shows the saturated absorption spectrum when it is working at MHz, it is observed that there is a clear frequency lag relative to the reference blue line. Figure. 6(c) shows the saturated absorption spectrum when it is working at MHz, and it is ahead of the reference line. Therefore, this experiment demonstrates that the possible frequency shift due to RFA is acceptable for the sodium lidar.
The SHG of 589 nm laser is realized either by LiB3O5 (LBO) crystal in a cavity-resonator SHG with high efficiency, or PPLN crystal or waveguide. Due to the high stability, a PPLN waveguide is used in single-pass optical path. The temperature of PPLN is optimized to 64.3with a precision of to produce 589 nm laser. The output power of CW 589 nm laser is up to 1.5 W, which is sufficient for PDA.
The cavity-free PDA is pumped by Nd:YAG laser that is injection-seeded, and produced single-mode pulsed laser with a power of 2.0 W for lidar observation.
3. Preliminary observation results
The pulsed 589 nm laser is divided to three beams, one of which is pointed to zenith, while other two are pointed 30° off-zenith to east and 22° off-zenith to south, respectively. The optical reception of the system comprises three telescopes. The echo signal is focused on the focal plane of the telescope and coupled into photomultiplier tube (PMT) via optical fiber. Then the optical signal is converted to electrical signal and two dual channel photon counting cards (P7882) undertake synchronous data acquisition of signals from three different directions. The received signal is stored in the form of photon count profile, with a spatial resolution of 150 m and a temporal resolution of 50 seconds, which contains 500 pulses at each frequency. Figure 7 shows the photon profiles at three different frequencies from the south and east channels with 100 seconds integration obtained at 14:50 (UT), on October 18th 2017.
Fig. 7 Typical photon profiles at three different frequencies obtained on Oct 18th 2017, with 150m spatial resolution and 100s integration. (a) From East channel. (b) From South channel.
The vertical wind is measured, as shown in Fig. 8. The color curves are vertical wind profiles averaged at 1 hour from 13:00 to 18:00 (UT) on October 18th 2017, and the black curve is the nightly mean profile. The nightly mean vertical wind from 82 to 100 km is −3.0 ± 0.6 m/s, corresponding frequency offset is about 4 to 6 MHz, which is most likely resulted from chirp effect caused by the imperfect pulse line shape. Therefore, the value of vertical wind is used to calibrate the lidar measurement bias.
Fig. 8 Vertical wind profiles on October 18th 2017, the color curve is 1 hour averaged profile, the black curve is nightly mean profile.
Figure. 9 presents mesospheric temperature profiles measured by the sodium lidar (90°E, 30°N) and the Sounding of Atmosphere Broadband Emission Radiometer (SABER) (84°E, 32°N) onboard Thermosphere Ionosphere and Mesosphere Electric Dynamics (TIMED) satellite near the location of the lidar, at the same time period on Oct 18th. Blue line with a small circle describes temperature profile in NRLMSISE-00 atmosphere model. It can be seen that the profiles agreed in general trend, while differ from each other within small range.
The original photon files are integrated by temporal resolution of 30 min, to subtract background noise, and smoothed by spatial resolution of 2 km. The de-noised photon files at three frequencies are then normalized respectively to Rayleigh backscattering photon counts at heights around the reference range, 35km. Figure 10 shows the contours of (a) zonal and (b) meridional wind, and (c) temperature, on the night of October 18th, 2017. It can be inferred that the maximum and minimum values of zonal wind and meridional wind are showing a trend of downward propagation. This structure is also found in the temperature contours. The results have demonstrated the lidar reliability preliminarily, and in the future, regular data is needed to do further advancements in this area of research.
Fig. 10 The observational results of sodium lidar on Oct 18th 2017, with a 30 min temporal resolution and 2 km vertical resolution. (a) contour of zonal wind. (b) contour of meridional wind. (c) contour of temperature.
4. Summary
This paper has demonstrated a robust and adaptable sodium lidar for measuring mesopause temperature and wind in Tibetan Plateau. The seed laser is of all-fiber-coupled single-pass structure, with a complete electronic control system. By using a two-stage RFA, the seed laser generates up to 1.5W 589 nm CW laser. With all these improvements, the lidar system allowed less human interventions, long-term frequency stabilization, easy operation, and good environmental adaptability particularly in Tibetan Plateau. A recorded frequency stabilization is up to 8 hours without human assistance during observing. It shows a promising future for sodium lidar’s popularization and application in abominable environment.
Based on the newly designed sodium lidar, observational experiments were conducted at YangBaJing, Tibet, China (90°E, 30°N, 4300 m a.s.l). And preliminary measurement results of wind and temperature have been obtained, which demonstrate the stability and detection capability of the lidar system at high altitude region. To improve the retrieval accuracy of lidar measurements, the vertical wind is measured to calibrate the frequency chirp induced by the imperfect line shape of the pulsed laser. To further optimize the lidar system, the pulse lineshape should be measured in the future work.
Funding
The National Natural Science Foundation of China, the Grant nos. 41127901, 41505027, 41627804, 2016YFC1400301.
Acknowledgments
The filed work is conducted at Yangbajing National Cosmic Ray Observatory in Tibet, the authors are thankful to the managers and staff for hospitability, time and opinions. The TIMED/SABER (http://saber.gats-inc.com/) is greatly acknowledged for providing satellite temperature data. The authors also thank the editors and reviewers for their useful suggestions in improving quality of this paper.
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