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A solid-state sodium (Na) Doppler lidar developed at YanQing Station, Beijing, China (40°N, 116°E) aiming to simultaneous wind and temperature measurement of mesopause region was reported. The 589 nm pulse laser was produced by two injection seeded 1064 nm and 1319 nm Nd:YAG pulse lasers using the sum-frequency generation (SFG) technique. A fiber amplifier is implemented to boost the seed power at 1064 nm, enabling a robust, all-fiber-coupled design for seeding laser unit, absolute laser frequency locking, and cyclic three-frequency switching necessary for simultaneous temperature and wind measurements. The all-fiber-coupled injection seeding configuration together with the solid-state Nd:YAG lasers make the Na Doppler lidar more compact and greatly reduce the system maintenance, which is conducive to transportable and unattended operation. A preliminary observational result obtained with this solid-state sodium Doppler lidar was also reported in this paper.
© 2017 Optical Society of America
1. Introduction
Based on probing the spectral Doppler broadening and shift of the laser-excited sodium (Na) D2 resonance line with a narrowband 589 nm pulsed laser [1, 2], Na Doppler lidar can obtain the temporal and spatial structure of temperature and wind of the mesopause region (80-105 km), which plays an important role in studying space physics and atmospheric dynamics and thermodynamics [2–6].
The technical realization of Na Doppler lidar is challenging [7–11]. Simultaneous wind and temperature detection in the mesopause with narrowband Na lidar was firstly realized by She et al. using three-frequency-ratio technique at Colorado State University (CSU) [7, 8]. The CSU Na lidar system continues for temperature and wind measurements to date and can achieve 24-hour continuous measurement [12]. A similar narrowband Na lidar was built at the University of Science and Technology of China (USTC) in Hefei, China, in which a more stable pulse dye amplifier (PDA) was constructed and new designs for automation control was implemented [11]. Significant contributions on Na lidar development have been made by Chu’s group from University of Colorado Boulder [13–16] and Gardner’s group from University of Illinois at Urbana-Champaign (UIUC) [13, 15, 17] with many technical and scientific innovations, such as the large aperture lidar measurements [13,17] and high-efficiency lidar design [14, 16], which greatly enhanced our understanding of mesopause region including its thermal structure [13], gravity wave dynamics [16], fluxes [15, 17], etc.
The laser transmitter of conventional narrowband Na Doppler lidar is based on continuous wave (CW) tunable single-mode ring dye laser combining with a PDA. However, the ring cavity is sensitive to the ambient temperature variations and vibrations, resulting that the running of the Na lidar system requires extensive maintenance and is only appropriate for a well-controlled laboratory environment. As the solid state laser technology has achieved rapid progress in the past few decades, solid-state Na lidar laser transmitter source based on non-linear frequency conversion has attracted considerable research interest [9]. An all-solid-state narrowband Na lidar system based on sum-frequency generation (SFG) technique with two injection seeded Nd:YAG pulse lasers at 1319 nm and 1064 nm was developed successfully for temperature measurements at the Syowa Station (69°S, 39°E), Antarctica [18, 19] and at INPE, Sao Jose dos Campos (23°S,46°W) [20]. More recently, the Japanese group developed an upgraded all-solid-state narrowband Na Doppler lidar at EISCAT radar site in Tromsø, Norway (69.6°N, 19.2°E) [6] with an average output power of 1.4~1.8 W at a repetition rate of 1 KHz, and started observations of neutral temperature and sodium density in the mesosphere and lower thermosphere (MLT) region since 2010. Though they utilized diode-pumped Nd:YAG lasers, with high system thermal stability, they did not employ fiber coupling between subsystems. They have collected a valuable data set on sporadic Na layers and temperature variation in the polar mesosphere atmosphere. However, simultaneous mesopause wind and temperature measurements have not been reported yet [21], to the best of our knowledge.
In this paper, we will present a solid-state Na Doppler lidar with an all-fiber-coupled seeding unit developed at YanQing lidar station in Beijing, China. The 589 nm pulse laser is produced by two injection-locked Nd:YAG lasers of 1064 nm and 1319 nm based on SFG technique. In injection seeding unit, absolute frequency locking and rapid three-frequency switching are implemented, allowing simultaneously probing the Doppler broadening and shift of Na D2 spectrum. In particular, a fiber amplifier is implemented to boost the seed power at 1064 nm along with the injection seeding, enabling all-fiber-coupled delivery and control of laser light, making the lidar system compact and easy to maintain. The detailed design of the lidar system is described in section 2 and its initial experimental results are presented in section 3.
2. Lidar system
In order to achieve wind and temperature measurements, three-frequency ratio technique is adopted, which requires the narrowband 589 nm laser frequency absolutely locked to Na D2a peak transition frequency (fa) and alternately tuned with several hundred MHz to allow cycling measurement among three different operating frequencies within the Doppler-broadened Na D2 absorption spectrum [2, 8]. Figure 1 gives the schematic setup of the newly designed narrowband Na Doppler lidar system. This section will focus on the design of the all-fiber-coupled injection seeding unit of the lidar transmitter, which is the key technical contribution of this solid-state Na lidar system. The 589 nm pulse laser generation based on SFG technique (Pulsed-SFG), lidar receiver and detection, whose technical details have been discussed previously in [2, 11, 19], will be briefly described. The main parameters of the lidar transmitter and receiver systems are summarized in Table 1.
Table 1. System parameters of the lidar system.
2.1 All-fiber-coupled seed laser unit
Seed injection is employed to achieve single longitudinal mode operation of pulse lasers. A Yb doped fiber laser at 1064 nm (Continuum, SI 2000) and a diode-pumped solid state laser at 1319 nm (JDSU NPRO 125), are respectively used as the seeding laser sources of the two Nd:YAG pulse lasers. The main parameters of the two CW seed laser are shown in Table 2. Both seed lasers have thermal tuning and piezoelectric tuning elements for frequency scanning. In our system, the sum-generated laser frequency tuning is accomplished in two ways: fine frequency tuning by changing the piezoelectric transducer (PZT) voltage of 1064 nm seeder through RS232 communication and coarse frequency tuning by varying the voltage applied to the thermal tuning BNC port of the 1319 nm seeder controller. The continuous tuning range without mode hop is about 2 GHz for 1064 nm seeder PZT and up to tens of gigahertz for 1319 nm seeder thermal control, which sufficiently cover the Na D2 resonance line. The fast frequency tuning of 1319 nm laser with PZT can only be tuned over tens of megahertz, and thermal tuning of 1064 nm laser also allows for a large tuning range of >20 GHz with mode-hop free, but temperatures of two fiber Bragg gratings (FBG) that established the laser cavity should be changed synchronously, which is more complex. Therefore they are not used for frequency control.
Table 2. The main parameters of the seed laser sources.
With fiber-coupled outputs, about 40 mW of 1064 nm laser light and 200 mW of 1319 nm laser light (after collimator and isolator) are available. In order to allow absolute frequency locking and cyclical three-frequency switching, a single-mode polarization maintaining ytterbium-doped fiber amplifier (YDFA, Amonics) with a output power of 500 mW (the maximum output power is 2W) is used after the 1064 nm seed laser to provide sufficient narrow-linewidth seed light power. The outputs of the 1064 nm seed amplifier and 1319 nm seed laser are coupled into two fiber-optic beam splitters (1064 FOBS and 1319 FOBS) with a split ratio of 25:25:25:25 and 20:80, respectively. The major output portion of 1319 FOBS (component 2) and one output of 1064 FOBS (component 3) are used to implement absolute frequency locking, and the other three outputs of 1064 FOBS (component 4-6) are used for three-frequency switching, whose output and the remaining 1319 nm seed laser component (component 1) are coupled into the Pulse-SFG unit. The details of the absolute frequency locking and three-frequency switching are presented below.
A. Absolute frequency locking
The pulsed 589 nm laser cannot provide stable saturated spectrum features to lock the laser frequency. In order to accurately determine the operating frequency and ensure long-term frequency stability for achieving high-precision wind and temperature measurements, we specially designed a laser frequency control and locking subunit, as illustrated in Fig. 2. It mainly includes a SFG of CW 589 nm laser radiation (CW-SFG), a Doppler-free saturation-absorption setup to provide accurate D2a transition peak feature as absolute frequency locking reference, and frequency control electronics and software for presetting the frequency of each seed laser and compensating the long-term frequency drift.
Fig. 2 Absolute laser frequency control and locking subunit. WG-PPLN: PPLN waveguide mixer. F: 589 nm optical filter to separate the sum-generated 589 nm laser light from the unconverted part of the infrared laser beams. C: collimating lens. HWP: half-wave plate to control the ratio of the pumping beam and probing beam. M1, M2 and M3: reflecting mirrors. PBS1 and PBS2: polarizing beam splitting cubes. BS1 and BS2: beam splitters. PD1 and PD2: photodiodes. FC: fiber coupler. DAQ: Data Acquisition Card. TC: temperature control. The blue, yellow and green lines in Na Doppler-free saturation-absorption setup designate the strong pump beam, the weak probe beam and the reference beam, respectively.
CW-SFG of Na resonance radiation using infrared 1319 nm and 1064 nm laser beams have been demonstrated with periodically poled LiNbO3 or KTiOPO4 (PPLN/PPKTP) crystals using single-pass schemes or doubly resonator designs [22, 23]. The single-pass SFG reported by J. Yue et al. generated more than 10 mW power of CW Na D2 resonance radiation at 589nm with 800 mW of 1064 nm and 350 mW of 1319 nm, which has been used as the CW laser source of a narrowband Doppler wind/temperature lidar in ALOMAR Observatory, Norway to improve its reliability. Compared with the single-pass configuration, doubly resonant SFG system can achieve a greater optical power conversion efficiency, but it is sensitive to environment and need complex adjustment and maintenance, resulting its frequency locking is less robust, which is not suitable for mobile or remote deployed lidar system.
In our lidar system, the CW-SFG is basically similar to what have been done by She et al. [9, 22] and by Kawahara et al. [24]. A commercially available PPLN waveguide frequency mixer (WG-PPLN, HCPhotonics) which is highly integrated and configured with two fiber-coupled inputs and a free-space output is utilized, making the CW-SFG flexible and no special optical adjustment is needed. The two infrared seed light (component 2 and component 3) from the two FOBSs are directly coupled into the frequency mixer. By adjusting the crystal temperature, we can optimize its phase matching to obtain high nonlinear frequency conversion efficiency. In our case, it is adequate to control the temperature of the crystal to 66 °C with a precision of ± 0.1 °C using a semiconductor thermo-electric cooler (TEC) module. On the condition of 120 mW 1064 nm and 150 mW 1319 nm CW laser components, the measured power of yellow light after the collimating lens is about 4 mW. Considering the optical power loss caused by the optical filter and the collimating lens, 7 mW of 589 nm laser light is actually generated through the WG-PPLN.
A small part of the output 589 nm laser beam is split out and coupled into a wavelength meter (High Finesse, WS 6), which facilitate coarse wavelength tuning. Most of the 589 nm CW light is guided into the Doppler-free saturation-absorption setup, as illustrated in Fig. 2. The Na vapor cell is temperature controlled to 130 °C to optimize the frequency discriminating sensitivity. All the optical components are mounted with specifically designed mini optic adapters and assembled into a black box (45 cm × 30 cm × 15 cm), making the setup more compact and depressing the interference of surrounding environment.
The most common method to lock CW lasers to a saturation feature is using wavelength modulation in conjunction with a proportional-integral-derivative (PID) feedback servo loop [25–27]. In our system, we designed a software-based digital feedback control program, to conveniently control and lock the sum-generated 589 nm laser frequency by monitoring the Doppler-free signal and controlling both seed lasers coordinately. The procedure of how the frequency stabilization works is illustrated in Fig. 3 via a block diagram.
The control program firstly scan the frequency of the sum-generated 589 nm radiation around Na D2 transition by thermally tuning the 1319 nm seeder. The measured Na D2 absorption signals versus thermal tuning voltage (Vt) of 1319 nm seed laser is shown in Fig. 4(a). The red line shows the saturation-absorption signal, and the blue line shows the Doppler-limited absorption signal. The Doppler-free features of D2a, crossover, and D2b are denoted, respectively. The differential signal of saturation-absorption is illustrated in the Fig. 4(b). By finding the maximum value of the differential spectrum signal, we can determine the required bias voltage Vt0 of thermal control port to preset the sum radiation frequency to be around the Na D2a transition. When the ambient temperature did not change significantly, Vt0 is usually around −8.3 V. After the 1319 nm seeder temperature reaches a steady state, we scan the PZT voltage Vp of 1064 nm seeder to obtain the Doppler-free hyperfine structure at Na D2a.
Fig. 4 (a) Na D2 absorption spectrum. Blue line: Doppler-limited absorption spectrum; red line: saturation-absorption spectrum. (b) The differential signal of saturation-absorption and Doppler-limited absorption. (c) D2a transition signal (solid red line) with higher spectral resolution by tuning the voltage of PZT that changes the cavity length of 1064 nm laser, providing distinct Doppler-free features for absolute frequency discrimination. The spectrum data near peak is linear fitted (black dotted line).
Figure 4(c) illustrated the D2a hyperfine spectrum signal with amplitude rescaled to 1. Thus Vp can also be preset to make the output frequency exactly tuned to the largest D2a transition peak, which is used as the absolute frequency reference. The black dotted line is the linear fitted spectrum data near peak, whose slope is used for frequency stability estimation in this experiment. As the spectrum signal is normalized using background absorption, deducting the influence of laser intensity fluctuations and electronic noise, we can compensate the laser frequency shift by maximizing the spectrum intensity. Then a wavelength automatic correction module executes to keep track of the D2a transition peak. The frequency compensation principle is illustrated in Fig. 5. In this process, the normalized Doppler-free signal Si corresponding to the PZT voltage Vp is sampled, and the next value Si + 1 at a new voltage equal to Vp + d is also monitored and compared with the previous value Si. If Si + 1 is less than Si, the program perceives that the adjusting direction is wrong and the laser frequency is deviating from the resonant transition peak further. Then the voltage Vp is updated to a new value Vp-Kd, otherwise it is updated to Vp + Kd. K and d are the positive gain coefficient and the correcting step value, respectively, which both influence the performance of the feedback control. Thus by making the piezo voltage adjusted continuously to control the 1064 nm seed laser cavity length according to signal changes, the sum-generated laser frequency drift can be compensated. In order to avoid over-limit of PZT adjustment (Vpmax = 15 V, Vpmin = −15 V), the procedure can automatically reset the Vp to 0 V if it has reached to within ~1 V of its limit. Simultaneously the preset 1319 nm thermal control voltage Vt0 is updated with an offset voltage D to counteract the frequency change due to Vp reset. Then continuous locking at the central frequency can be achieved. D can be calculated by:
Where Vp0 is the recorded PZT voltage before reset, Sp and St are the conversion coefficients of laser frequency tuning from PZT scan voltage and thermal control voltage, respectively, which were measured to be St = 13.67GHz/V, Sp = 82 MHz/V.To evaluate the locking stability, the normalized Doppler-free saturation-absorption signal Si was recorded for a period of two hours with the frequency drift compensation procedure engaged. The signal samples after amplitude rescaling were within approximately e = 0.1 Error (arbitrary unit) from peak as shown in the Fig. 6(a). According to the D2a spectrum curve in Fig. 4(c), the frequency discriminant near peak can be approximated as a linear function shown with the black dotted fitting curve, whose slope is = 4.28 Error/V. Then we can convert the error signal e to the frequency drift amount according to Eq. (2). The frequency stability of the sum-frequency-generated 589 nm laser is calculated to be, which could satisfy the requirement of Na Doppler lidar application. The PZT voltage of the 1064 nm seeder laser is simultaneously recorded during the locking periods, as shown in Fig. 6(b). From the figure we can see how the cavity length of the 1064 nm seed laser changes via time to keep the frequency on the Doppler-free D2a peak. The maximum laser frequency tuning range is estimated from the change of the PZT voltage to be about, which can be attributed to variations in ambient temperature.
Fig. 6 (a) Normalized Doppler-free saturation-absorption signal recorded over time when the seed laser is locked to Na D2a peak. (b) PZT voltage of 1064 laser cavity simultaneously recorded over time.
B. Three-frequency switching
The existing Na Doppler lidar system implement cyclical operation of the 589 nm emission at fa and fa ± δf, whereδf = 630 MHz by inserting a tandem and double-pass frequency shifting unit after the frequency stabilized 589 nm laser. It is based on two free-space acousto-optic modulators (AOMs) with modulation frequency of ± 315 MHz, respectively [28]. The double-pass configuration lowered the required modulation frequency of AOMs by half. However, in order that the laser beams at each operating frequency exit the shifting unit with the same direction, its optical design is complex and the optical path is long, needing professional operators to make the critical optical alignments. In addition, a customized chopper wheel should be utilized to separate the frequency-shifted and the un-shifted beams, which may reduce the flexibility in timing sequence of frequency switching. In this work, a simple and compact all-fiber-configured frequency switching subunit is designed, which is schematically depicted in Fig. 7.
Fig. 7 All-fiber-coupled three-frequency switching subunit for cyclical measurement among fa, f+ = fa + δf and f- = fa - δf, where δf = 585 MHz.
With the sum-generated CW laser permanently locked to fa, the 1064 nm laser components 4 and 6 from the outputs of the FOBS are connected to two fiber pigtailed acousto-optic frequency shifters (AOFSs, manufactured by Brimose), respectively, so that their frequencies are precisely up and down shifted. The frequency shifting amount δf depends on the space of longitudinal modes of the 1064 nm pulsed laser cavity to ensure the optimized seed injection for all three operating frequencies. The frequency spacing between longitudinal modes is given by:
Where c is the speed of light, and nL is the optical path length of the pulsed resonator. Considering both the mechanical cavity length and the optical path difference of Nd:YAG oscillator rod, Pockel cells and mirrors, the total optical resonator length of 1064 nm pulsed oscillator is calculated to be 2303.19 mm, yielding the longitudinal mode spacing = 65 MHz. The Nd:YAG can support a large number of longitudinal modes as its gain curve is approximately 120 GHz (FWHM). In order to minimize the wind and temperature measurement errors caused by photon noise, δf in our system is chosen to be 585 MHz, which is the integral multiple of the longitudinal mode spacing (exactly 9 times) to make all three working frequencies match a longitudinal mode of the pulsed laser, and also close to 630 MHz used in the traditional Na Wind/Temperature lidar system. The outputs of the two AOFSs are respectively coupled into the inputs of a fiber optical switch (OSW, 14-980E-FC/APC, manufactured by Thorlabs) with a typical insertion loss of 0.7dB. The remaining 1064 nm laser component 5 is directly coupled into another input of the OSW without frequency change. The OSW can be controlled conveniently by periodic TTL sequence signals or operated remotely by control computer, allowing the output sequence and the working time of each frequency to be flexibly specified to balance the signal-to-noise ratio (SNR) of echo signals. In our experiment, the laser frequency is switched every 30 pulses. Meanwhile, the OSW state signals are used as the tagging inputs of the data acquisition boards to make the photon counts corresponding to three operating frequencies recorded into separate arrays.2.2 Pulse-SFG for high power 589 nm laser pulse generation
The 1064 nm laser with frequency unshifted, upshifted and downshifted can be alternatively outputted and injected into the Nd:YAG pulse laser (Continuum) cyclically, and the unshifted 1319 nm seed light (component 1) is directly delivered to the 1319 nm Nd:YAG pulse laser (as illustrated in Fig. 1). After going through a set of 9-mm amplification rods, the two pulse light beams go to a KTP crystal to generate the desired high-power narrowband 589 nm laser pulses at fa, fa + 585 MHz and fa-585 MHz. In order to achieve stable single-mode operation, Q-switch build-up time minimization technique is utilized, where the seeder system output feedback voltages to adjust the pulsed laser cavity length to match the pulsed wavelength to the seed laser wavelength. Both oscillators use Gaussian output couplers, and an etalon is used in the 1064nm oscillator to assist in single longitudinal mode locking. The etalon is optimized to achieve efficient injection seeding for each operating frequency. To ensure the frequency conversion efficiency of the Pulse-SFG, the spatial and temporal overlap of the two pulse laser beams are critically required. Thus the delay time of the Q-switch trigger and the angle of the KTP crystal are carefully adjusted.
After optimization, an averaged output power of 1.8 W at 15 Hz pulse repetition rate for 589 nm pulse laser radiation was measured. The measured temporal pulse width is about 23 ns (FWHM). The laser linewidth is estimated to be 60 MHz (FWHM) based on a CCD recorded interference fringes by sending the pulsed laser through an etalon. During data retrieval, we assume that the laser lineshape was close to a Gaussian shape. In order to increase the wind and temperature measurement accuracy, measurement of the laser lineshape is necessary.
2.3 Lidar receiver and detection
The sum-generated Na resonance radiation laser beam is divided into three branches emitting in three different direction: one is pointed toward the zenith for temperature measurement as well as vertical wind monitor for line of sight (LOS) winds calibration [2, 29]; while the other two pointed 30 °off-zenith to the north and east, respectively, for horizontal winds measurement. The backscattered echo signals are collected by three Cassegrain telescopes with diameters of 800 mm. Three multimode fibers (MF) with a core diameter of 1.5 mm and a numerical aperture (NA) of 0.37 are used to guide the collected signals to the detection unit. Finally, the time resolved laser backscatter signal from the atmosphere are recorded with two dual-channel range-gated photon counting boards (Fast comtec, MCA-3). The board can acquire the echo signal with a minimum bin width of 100 ns (200 ns in dual input mode), corresponding to a vertical resolution of 15 m. In our system, the raw data files are stored with a range bin length of 90 m and integration time of 80 s.
3. Preliminary observation results
We conducted simultaneous wind and temperature measurements utilizing the new Doppler lidar system at Yanqing Lidar Station, Beijing (116.0°E, 40.0°N). In order to reduce the impact of saturation on the measurement results [2], the laser pulse energy in each beam direction was reduced to be no more than 20 mJ in our current observation campaigns. The typical measurement uncertainty induced by photon noise for temperature and wind with 1.8 km spatial resolution and 1 hour integration are estimated to be 0.5 K and 1 m/s at the Na peak (about 91 km), and 5 K and 7 m/s at the edges (82 and 103 km) of the Na layer. Figures 8(a) and 8(b) show two typical temperature profiles with 1.8 km spatial and 16 min temporal resolutions measured by the new lidar system (black solid curves) at UT 14:30 on 19 September, and UT 16:50 on 31 October 2016, respectively. The temperature data matching the time of the lidar near lidar location measured by the Sounding of Atmosphere Broadband Emission Radiometer (SABER) onboard Thermosphere Ionosphere and Mesosphere Electric Dynamics (TIMED) satellite are also plotted in Fig. 8 (red asterisk) for comparison. It can be seen that the measured temperature profiles from both instruments are basically consistent, and obvious temperature wavy structures can be seen, which may be connected to the gravity wave perturbations.
Fig. 8 Measured temperature structures of the mesopause region at (a) UT 14:30 on 19 September, and (b) UT 16:50 on 31 October, 2016. The black solid lines with error bars denote the results obtained by the Na lidar and the red asterisks indicate the SABER temperature. The lidar data were integrated using ~16 min temporal resolution and 1.8 km spatial resolution.
As for horizontal wind measurement, the vertical wind is simultaneously measured to monitor the possible laser frequency error (chirp or shift) for improving the LOS wind measurement accuracy [2, 12, 29]. An example of the measured vertical wind profiles averaged over an hour (colors) and the whole night (black) has been shown in Fig. 9. The nightly-mean vertical wind from 85 km to 100 km is about −1.5 ± 1 m/s, thus the frequency offset is estimated to be about 2 to 3 MHz. This detected value is used to calibrate the LOS wind. Horizontal wind observation results after chirp correcting obtained at UT 16:00 on 19 September and UT 13:00 on 11 October 2016 are presented in Figs. 10(a) and 10(b) and Figs. 10 (c) and 10(d), respectively (solid lines with error bars). A meteor radar which can provide temporally continuous atmosphere winds from 70 km to 110 km was deployed in Beijing Ming tombs (116°E, 40°N), which is very close to our lidar location (a distance of about 40 km). Therefore the corresponding radar wind results (solid lines marked with circles) are also plotted in Fig. 10 for comparison. The radar profiles has a spatial resolution of 2 km and temporal resolution of 1 hour, the lidar results are averaged with 1.8 km in height and 1 hour in time. All the profiles have been oversampled at a 1 km height interval.
Fig. 9 Vertical wind measurements on Oct.31, 2016. The color curves represent the 1 hour integrated profiles from 16:30 UT to 22:30 UT, and the black curve is the nightly-mean result.
Fig. 10 Comparison of meridional and zonal wind profiles measured by the new lidar and meteor radar over Beijing at UT 16:00 on 19 September (a-b) and UT 13:00 on 11 October (c-d) 2016. The solid lines with error bars represent the lidar profiles and the radar results are marked with circles.
In Fig. 10(a), the meridional wind direction is mainly southward, and the maximum meridional wind speed appeared at about 90 km for both radar and lidar. Above 98 km and below 83 km, the lidar measurement results are unreliable due to not enough SNR at these altitudes. As shown in Fig. 10(b), the lidar zonal wind direction changed at about 88 km, basically in agreement with the changing tendency of the radar zonal wind. The main difference occurs at around 93 km, where lidar measured the maximum westward wind velocity of about 70 m/s, which is about 35 m/s larger than that of the radar measured result. The horizontal wind profiles measured by lidar in Figs. 10(c) and 10(d) are basically in accordance with that of the radar. However, more wave features can be seen in lidar wind profiles, which might be caused by the presence of atmospheric gravity waves. While the meteor radar measures almost the entire sky, the Na lidar measures a smaller volume of the atmosphere, therefore the lidar winds are more sensitive to small-scale wave perturbations [30, 31].
Figure 11 presents the contours of meridional (a-b) and zonal (c-d) winds measured simultaneously by lidar and radar on the night of 31 October 2016. It can be seen that the temporal evolutions of either meridional or zonal winds measured by the two instruments is similar. For meridional wind results shown in Figs. 11(a) and 11(b), both instruments detect a downward propagating wave structure between 88 and 100 km, indicating an tidal variability, which can be easily detected by both Na lidar and meteor radar as it has horizontal wavelength much larger than the scale of the atmospheric volume measured by both instruments [30, 31]. For zonal winds in Figs. 11(c) and 11(d), both the lidar and radar measurement results show eastward wind below 90 km and westward wind above 90 km.
Fig. 11 (a-b) Meridional and (c-d) zonal winds simultaneously measured by lidar and meteor radar on the night of 31 October 2016.
4. Discussion and summary
A solid-state narrowband Na Doppler lidar system with an all-fiber-coupled seeding unit was reported in this paper. Two pulsed Nd:YAG lasers seeded by two CW lasers at 1064 nm and 1319 nm were employed to generate the single-mode pulsed laser at 589 nm. The power of seeder laser at 1064 nm was boosted with a fiber amplifier, which enables SFG of 589 nm CW laser for lock the laser frequency of the lidar system to Na D2a Doppler-free feature and a rapid three-frequency switching based on fiber optical switch. In order to minimize the optimization time of pulsed laser cavity, three operating laser frequencies were chosen by considering the cavity modes matching requirement of the pulsed Nd:YAG laser. The all-fiber-coupled injection seeding configuration together with the solid-state Nd:YAG lasers make the Na Doppler lidar system more robust and compact, which facilitate remotely or automatically controlled operation.
Observational experiments were conducted using this new lidar system in YanQing, China (116°E, 40.0°N) and the preliminary measurement results of mesopause region temperature, meridional wind and zonal wind were presented and compared with the TIMED/SABER temperature and meteor radar wind. It should be noticed that the beam divergence of the transmitted laser is very small and the single pulse energy is relatively large in our current lidar system, which caused the Na layer saturation. The saturation effect will affect the measurement accuracy of not only Na density but also temperature and wind, as pointed out in [2, 14, 32–34]. In order to reduce the saturation effect of sodium layer, the laser energy density should be decreased in the future lidar observation. The beam divergence and repetition rate of pulsed Nd:YAG laser could be increased [14], and the transmitted laser could be divided into more beams in multi-direction observations. This solid-state Na Doppler lidar system with the all-fiber-coupled injection seeding unit could be a new option for the realization of Na Doppler lidar for easy operation thus potentially more data collection. In order to derive accurate temperature and wind measurement results for future scientific research, the lidar configuration should be further optimized and the saturation effect should be taken into account for data retrieval calibration. In addition, to calibrate the lidar system, some parameters of lidar system should be measured and calculated precisely in the future, such as nonlinear effect of photomultipler tubes and the lineshape of pulsed laser. After the lidar configuration optimized and data retrieval carefully calibrated, this system will be used to measure wind and temperature of mesopause for atmospheric research.
Continued observations using this lidar system are also required to study the mesopause atmosphere dynamics and seasonal variation in North China. For future research, simultaneous wind and temperature measurements of mesosphere and lower thermosphere (MLT) will be performed along with a Rayleigh Doppler lidar built in YanQing in 2015 [35]. Together, they could enhance the understanding of dynamics and interactions between the middle and upper atmosphere. In addition, by making use of narrowband Faraday filters [2, 14, 36], daytime measurement capability of this system could be available for investigation of the chemical and radiative processes and their interactions in the mesopause.
Funding
National Natural Science Foundation of China (NSFC) (41127901, 41505027, 11403085, 41627804, 2016YFC1400300).
Acknowledgments
The Data Center for Geophysics, Data Sharing Infrastructure of Earth System Science, National Science & Technology Infrastructure of China (http://geospace.geodata.cn/) are gratefully acknowledged for providing the meteor radar wind data. The satellite temperature data used in this studies is from the TIMED/SABER (http://saber.gats-inc.com/), which is also greatly appreciated. This research was also supported by the Chinese Meridian Project (http://www.meridianproject.ac.cn/).
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