The All-Solid-State Narrowband Lidar Developed by Optical Parametric Oscillator/Amplifier (OPO/OPA) Technology for Simultaneous Detection of the Ca and Ca⁺ Layers

Abstract

We report an all-solid-state narrowband lidar system for the simultaneous detection of Ca and Ca⁺ layers over Yanqing (40.41°N, 116.01°E). The uniqueness of this lidar lies in its transmitter, which is based on optical parametric oscillation (OPO) and optical parametric amplification (OPA) techniques. The injection seeded OPO and the OPA are pumped by the second harmonic of an injection-seeded Nd:YAG laser, which can generate coherent light at the wavelength of 786 nm or 846 nm lasers, whose second harmonics in turn generate the 393 nm or 423 nm pulses, respectively, for the detection of thermospheric and ionospheric Ca⁺ and Ca layers. Compared to the conventional dye-based system, this lidar transmitter is a narrowband system (bandwidth < 200 MHz), which has produced a factor of two more output power with higher stability and reliability. The lidar system in Yingqing demonstrated Ca⁺ detection sensitivity of 0.1 atoms-cm⁻³ for long-term observation and reached a height of ~300 km. Potential applications and further improvements in this lidar technique are also discussed in this paper.

1. Introduction

A large number of metal atoms (Na, K, Fe, Ni, Mg, Ca, etc.) have existed in the mesosphere and lower thermosphere (MLT) region for a long time, which is generally believed to be generated by the ablation of interplanetary dust particles into the Earth’s atmosphere, and the peak height is around 90 km in the atmosphere [1,2,3]. These metallic layers can be detected directly by ground-based lidar or space-borne spectrometers, and can be used as an effective tracer to reflect atmospheric chemical and physical parameters (atmospheric temperature, wind, gravity waves, etc.) in the region [4,5,6].

Ca is currently the only element that can be detected by ground-based lidar at the same time as atoms and ions, because the resonant wavelengths of other metal ions are in the extreme ultraviolet range (shorter than 300 nm) and are absorbed by the stratospheric ozone, and the resonance wavelength of Ca⁺ is longer than 300 nm. In addition, the ablation rate and peak height of Ca are lower than those of other metals, so there are certain difficulties in detection. Simultaneous detection of Ca and Ca⁺ can independently obtain the corresponding ion data and is not limited to detection via ionospheric equipment. Currently, there are few studies on the simultaneous detection of Ca and Ca⁺, and most of the existing results are based on dye lidar systems. Granier et al. [7] were the first to implement lidar for Ca and Ca⁺ detection at Haute Provence (44°N, 6°E): they used two Nd-YAG pumped dye lasers, and then the 393 nm and 423 nm measurements were obtained by the mixing of three waves in a non-linear crystal. The detection results show that the integrated abundance of the atomic form was low and the Ca⁺ layers appeared between 85 and 115 km altitude. Gardner et al. [8] adopted an excimer-dye laser system using Exciton QUI laser dye dissolved in p-dioxane to achieve Ca⁺ detection; they found that sporadic E, Fe, Na, and Ca⁺ layers formed simultaneously near 100 km. Later, in 1995, Qian and Gardner [9] adopted the same system to achieve lidar observation of Ca, located at Urbana Atmospheric Observatory (40°10′N, 88°10′W) in the United States, but Ca and Ca⁺ were not detected simultaneously. Alpers et al. [10] once again achieved simultaneous observation of Ca and Ca⁺ with lidar in Juliusru, Germany (54°38′N, 13°24′E). Their dual-wavelength lidar system used a single excimer pump laser and two dye lasers to create the two resonance wavelengths. They found that sporadic Ca⁺ layers occurred frequently in the altitude range of 90–120 km. Gerding et al. [11], upgrading the Double Lidar of the Institute of Atmospheric Physics (IAP), moved it to Kuhlungsborn (54°N, 12°E) and made long-term observations. Their results showed that atmospheric Ca was severely depleted compared to other metals, and they constructed a one-dimensional steady-state chemistry model of the nighttime Ca and Ca⁺ layers. Tepley et al. [12] first realized the observation of Ca⁺ at Arecibo (18°35′N, 66°75′W); later, Raizada et al. [13] achieved Ca observations. Yi et al. [14] achieved Ca and Ca⁺ lidar detection using a YAG-pumped dye laser and fundamental frequency optical mixing in Wuhan (30.5°N, 114.4°E). Their research showed a regular altitude relationship, in which Na_s is highest, Fe_s is a few to tens of hundreds of meters lower than Na_s, Ca_s is a few hundred meters lower than Fe_s, and Ca_s⁺ is the lowest. Wu et al. [15] reported a dual-wavelength tunable lidar system to detect Ca and Ca⁺ at Yanqing Station (40.41°N, 116.01°E). Their lidar system implemented a pulsed Nd:YAG laser that simultaneously pumps two dye lasers, and completes simultaneously to detect Ca and Ca⁺. They reported three nights of simultaneous observations and the nocturnal evolution diversity of Ca and Ca⁺ layers.

Most of the existing Ca and Ca⁺ lidar systems are dye-based laser emission systems. For the dye laser emission system, due to the use of dye as the excitation medium, when a high-energy or high-frequency pump laser is used, the dye absorption energy reaches saturation and can easily boil. As a result, the uniformity, energy, and stability of the output spot are greatly reduced, and the life of the dye is also sharply shortened and therefore needs to be replaced frequently, so the laser frequency and energy used by the dye laser are relatively low. Moreover, dyes have certain toxicities, which should be taken into account in safety assessments.

Based on the above characteristics of dye-based laser emission systems, we proposed the application of OPO and OPA technologies in a laser emission system. In 2018, an all-solid-state narrowband of Ca and Ca⁺ dual-wavelength lidar was constructed at Yanqing Station in Beijing (40.41°N, 116.01°E). (In the paper, we refer to this lidar as the OPO Lidar.) The design of this lidar system was authorized by a Chinese invention patent (ZL 20191060260.3). In this paper, the transmitting system of the device and the data obtained are described in detail.

2. Lidar Configuration

Figure 1 shows the schematic setup of the newly designed OPO Lidar system. The OPO Lidar consists of a laser emission system and a receiving and acquisition system. The main parameters of the OPO Lidar are summarized in

Table 1. Parameters of the lidar system.

Parameter	Ca	Ca+
Wavelength (nm) (in the air)	422.6728	393.3663
Pulse energy (mJ)	31	30
Repetition rate (Hz)	15	15
Linewidth (MHz)	169.3	154.6
Telescope aperture (m)	1.23
Focal length (m)	2.4
Fiber diameter (mm)	1.5
Optical filter FWHM (nm)	1
Count rate (MHz)	150
Time resolution(s)	66.7
Spatial resolution(m)	96

.

This section will focus on the laser emission system, which is the key part of the OPO Lidar design. The receiving and acquisition system will be briefly described, whose technical details have been discussed previously in the dual-wavelength tunable lidar system of a Ca and Ca⁺ layer detector [15].

2.1. Laser Emission System and the Key Technologies

The laser emission system uses a seed-injected Nd:YAG laser to pump two OPO/OPA lasers, and output 786 nm and 846 nm lasers, simultaneously. These two lasers are then converted to 393 nm and 423 nm by the SHG Module.

2.1.1. Pump Laser

The pumped laser is 532 nm, produced by the SHG of a seed-injected Nd:YAG laser. In order to avoid the local hot spot generated by the pumping laser damaging the OPO crystal, the spot requires a uniform distribution of multi-transverse mode energy similar to the flat-top beam. In this system, we used Continuum DPL9015 single-longitudinal-mode as a pumped laser, which can output an OPO level flat-topped beam. The central 80% region modulation is less than 15%, and the single-longitudinal-mode output probability can reach more than 99.9% under the condition of seeder injection, which improves the stability of the OPO oscillator cavity. The single-longitudinal-mode-pumped laser also ensures the linewidth of the OPO laser. The 532 nm energy output of the Nd:YAG laser is 1 J, which is divided into two beams using 50% of the beamsplitter, and two OPO lasers are pumped, respectively.

2.1.2. 786 nm/846 nm Seeder Laser

The injected seeder laser originates from an external cavity diode laser using the DLC DL PRO series from Toptica, which is injected into the OPO oscillating cavity to produce a signal laser. There are two seed lasers: one of them is tunable between 765–805 nm; the other is tunable between 840–875 nm, using wavelengths of 786 nm and 846 nm for seeder injection wavelengths, respectively. The mode-hop free tuning is 30–50 GHz, the typical short-term linewidth at 5 µs is less than 100 kHz and the peak power is greater than 15 mw. The seed injection also ensures that the output laser of the OPO/OPA laser is a narrow linewidth laser.

2.1.3. OPO Module and OPA Module

The OPO Module and OPA Module are the key units for the generation of the emitted laser. The entire OPO Module and OPA Module are placed in an anodized and black-plated aluminum box, isolated from the outside world to prevent external dust from falling on the optical components. This reduces the interference of external temperature change and airflow to the system. All optical components are directly installed on the same bottom plate, effectively increasing the stability of the system and making it easy to handle. Moreover, the stray light produced in the laser system is also closed inside the system and directly enters the light receiver.

The OPO cavity adopts the conventional ring cavity structure, allowing the photon to be generated and oscillate repeatedly in the cavity, and obtains gain amplification through the KTP crystal many times, finally forming a laser pulse. The OPO cavity is composed of four cavity mirrors and two nonlinear crystals, and the nonlinear crystals are KTP crystals. As shown in Figure 2, mirror No. 1 is the output mirror, which not only leads the signal laser and idler laser generated into the resonant cavity, but also introduces the seeder laser into the cavity. The function of cavity mirror 2 is to draw the pump and idler laser out of the cavity. A piezoelectric ceramic module (PZT) is attached to the No. 3 cavity lens, which moves back and forth within a certain range to fine-tune the longitudinal mode of the cavity. The function of the No. 4 cavity mirror is to introduce the pump laser. For each pulse, the sweep-hold-fire methods are adopted through PZT to ensure that the seeder laser is injected into the optical cavity most effectively, and that the wavelength of the output signal laser is exactly the same as that of the seeder laser. At this time, the efficiency of the OPO cavity reaches its maximum.

The OPA Module consists of two lenses (mirror No. 5 and mirror No. 6) and a nonlinear crystal BBO. The signal laser output from the cavity mirror of the OPO Module 1 passes through a bicolor mirror and a 532 nm pumped photosynthetic beam, and then enters a BBO crystal together to further amplify the energy of the signal laser. The idler laser produced with the signal laser does not pass through the BBO crystal under the effect of this cavity mirror, but is directly exported to the outside of the module. In addition to the main cavity mirrors described above, there are other optical devices and optical systems. For example, before entering an OPA crystal, it uses a half-wave plate to change the polarization direction of the signal laser by 90 degrees. Therefore, the directions of crystal departure in OPA and OPO are perpendicular to each other, which effectively improves the far-field beam quality. Before the pump laser enters the OPO cavity, a telescope group is added to ensure that the incident pump laser enters the nonlinear crystal more efficiently, and so on.

2.1.4. SHG Module

The amplified signal laser output from the OPA Module is at the same wavelength as the seeder of 786 nm and 846 nm, passed through an SHG Module to convert to 393 nm and 423 nm, the internal schematic diagram of the SHG Module is shown in Figure 3. The fundamental frequency laser with a wavelength of 786 nm or 846 nm is emitted from the laser inlet on the left side of the SHG Module. After passing through the dichroic mirror 7, the light path turns 90° and enters the nonlinear crystal (LBO, SHG efficiency is about 20%), at which point the remaining pump laser is introduced into the beam dump. After passing through the nonlinear crystal, part of the fundamental frequency laser is transformed into the frequency-doubling laser. After passing through the dichroic mirror 8 and reflection mirror 9, the mixed untransformed fundamental frequency laser is removed. Then, the frequency-doubling lasers of 423 nm and 393 nm are reflected and output from the reflection mirror 10.

2.2. Receiving and Acquisition System

The receiving and acquisition system has a 1.23 m telescope with a focal length of 2.4 m; its coating has been specially designed to enhance the reflectivity of the ultraviolet band, as shown in the Figure 4. The reflectance at 393 nm and 423 nm was greater than 95.1% and 94.2%, respectively. By using dual-optical fiber focal plane splitting technology, the back-scattered photon signals of the 393 nm and 423 nm were coupled to their respective receiving optical fibers [16]. Considering the loss of light in optical fiber transmission, the fiber length was made the shortest in terms of spatial distance, and here the fiber had a length of 1 m and the core of the optical fiber was 1.5 mm. After the backscattered fluorescence photons pass through the fiber, they are fed into the collimating lens, then they pass through the optical interference filter, the OD6 UV narrowband filter (0.058 min diameter) with an FWHM of 1 nm, and central transmittance greater than 90%, and then under the beam focusing system and eventually into the photomultiplier tube. The electrical signals converted by the two photomultiplier tubes enter the data acquisition device. The acquisition card was an ORTEC’ model Easy-MCS with a sampling rate of 150 MHz.

3. Advantages of Lidar Using OPO/OPA Technology

3.1. Gain Higher Emission Energy

OPO technology is one of the means of tunable laser generation; it can convert a laser into a coherent output of signal and idle laser. Using OPO technology, it can produce wavelengths from ultraviolet to infrared, covering a large range of bands, and more importantly, it can produce high-energy lasers. Increasing the laser energy can improve the signal-to-noise ratio, and the observation signal is better. In our system, the laser energy output of the Ca and Ca⁺ OPO lidar were 30 mJ and 31 mJ, and the laser repetition rate was 15 Hz, respectively. Compared with the Ca and Ca⁺ dye lidar system at Yanqing Station, which is more than double the transmitter power [15], and with others reported Ca and Ca⁺ lidar [7,10,11], it is more than double the transmitter power.

The more important point is that the system has the possibility of further improving the output energy, which is used to make lidar able to detect metal layers and more conducive to realizing high-precision detection. For example, we can take the following measures to improve the output energy of the signal laser: (1) Increase the number of nonlinear crystals in the OPA Module, which will increase the conversion efficiency of the pump laser. Compared with our experimental results (a 488 mJ @ 532 nm laser was pumped to produce a 135.6 mJ signal laser with an opto-optical conversion efficiency of 27.8%) and similar systems at other wave lengths that have been reported [17], it is possible to double the efficiency of the pumping laser (a 490 mJ @ 532 nm laser was pumped to produce a 265 mJ signal laser with an opto-optical conversion efficiency of 54%). (2) At the same time, increasing the pump laser energy also increases the amount of OPA Modules, which will undoubtedly increase the output laser energy.

3.2. With a Narrower Bandwidth

In general, dye lidar is a broadband system, and for the detection of metal components of the lidar, if the use of narrow bandwidth laser detection will be more advantageous. In this system, the WS7-60 wavemeter of Highfinesse was used to measure the linewidth of both 423 nm and 393 nm, its wavelength measurement absolute accuracy is 0.04 pm @ 330–420 nm, and linewidth measurement accuracy is 200 MHz. The result found that the linewidths of each of them were less than 200 MHz, which was also the measurement limit of this wavemeter. In order to further determine the laser linewidth, an air gap etalon with a free spectral range of 1 GHz was used to measure. According to the multi-beam interference method, the interferometric ring was imaged in the image plane and the stored interferometric ring was photographed in real-time, as shown in Figure 5. The linewidths (with the full-width-at-half-maximum) of 423 nm and 393 nm were calculated to be 169.3 MHz and 154.6 MHz, respectively. According to the lidar detection equation, the resonance count rate is proportional to the resonance fluorescence scattering cross-section of metal atomic ions. The linewidth of the detection laser has an effect on the resonance fluorescence scattering cross-section of metal atomic ions. It is found that except for Na, the effective backscattering cross-section is larger when the laser linewidth is narrower [18]. Therefore, the narrow linewidth is beneficial for lidar detection.

3.3. System Stability and Reliability

All the devices of the OPO system are in solid form, which increases the stability and reliability of the system [19]. Compared with traditional dye systems, it completely avoids frequent dye changes during routine lidar observations and avoids excessive manual intervention, which greatly improves the stability of laser output energy and the stability of the overall performance of the system.

In addition, the output laser spot can be realized by designing laser components according to the demand, and the laser spot shape is controllable and the uniformity is better, Figure 6b shows the burn patterns of the Ca⁺ OPO lidar system. However, the working substance of a traditional dye laser is a flowing liquid, and the pumping mode is generally side pumping, so that the side of the dye near the pump absorbs more pump laser, and the corresponding dye laser output is strong. Therefore, the output laser spot of the dye laser is gradually weakened in the radial direction. Because the frequency doubling efficiency of the dye laser is proportional to the square of the fundamental frequency intensity of the dye laser, the radial intensity of the dye laser frequency doubling laser spot changes more obviously, showing a crescent-shaped light spot rather than a circular light spot, Figure 6a shows the burn patterns of the dye laser system. For high-altitude detection lidar, the beam quality in the far field is better if the laser emission spot is uniform, and the laser divergence angle is easier to match with the field angle of the telescope, so as to obtain a high-quality raw photon counts.

4. Initial Observation Results

4.1. Original Echo Signal Data Analysis

The lidar system has been operating steadily since September 2019, when it began to detect Ca and Ca⁺. It has been switched on in fine weather and has accumulated certain data. Figure 7 shows the Ca layer (a) and Ca⁺ layer (b) raw photon counts on 3 October 2020, with a time resolution of 10 min and a spatial resolution of 96 m. The peak photon counts (C) near 95 km and background photon counts (averaged between 120 and 140 km) (B) are, respectively, 2942 and 92 for the Ca layer and 9003 and 44 for the Ca⁺ layer, giving rise to the peak C/B ratio of ~32 and ~204, respectively. The corresponding peak signal-to-noise ratio (SNR), S/N = (C-B)/√C, are, respectively, ~52.5 and 94.4, or 1.87% and 1.06% uncertainty. Compared with the Ca and Ca⁺ dye-based lidar system [15] at the same location, the emitting laser energy is 18 mJ and 9 mJ, the photon counts of Ca is 500 and the uncertainty is 5%, and the photon count of Ca⁺ is 800 and the uncertainty is 4% under the same resolution. Because only the emissions differ between the dye-based emission lidar and the OPO emission lidar, the uncertainties of the Ca layer of the OPO system should be 1.71 (31 mJ/18 mJ) times that of the dye-based emission lidar, and the Ca⁺ layer 3.33 (30 mJ/9 mJ) times that of the dye-based emission lidar, but the actual uncertainty results were far less than these two values. It can be concluded that the OPO solid-state system has a better signal-to-noise ratio and lower photon number error, which is more conducive to studying the fine structures of metal layers.

4.2. Density Evolution of the Ca Layer and the Ca⁺ Layer

Figure 8 shows the density evolution of the Ca layer and the Ca⁺ layer on 15 July 2020 and 21 October 2020, respectively. From these observations, some interesting observations can be seen, such as multiple layers of Ca and Ca⁺, and very low Ca and Ca⁺ layers. As can be seen in Figure 8a, the density of the Ca layer is stable all night and the altitude is higher than normal Ca layers, appearing at 98 km. At 17:10, this Ca layer exhibits a downward trend, and eventually reaches 90 km. From this point on, a second Ca layer appears at 100 km, and a third Ca layer at 108 km. Figure 8b shows a double Ca⁺ layer. The evolution trend of the Ca⁺ layer is consistent with that of the Ca layer. The appearance time of the second layer of Ca⁺ is 14:50, and the initial altitude is 110 km. Figure 8c,d show that on the night of 21 October 2021, the Ca⁺ layer and the Ca layer appeared simultaneously at an altitude be-low of 90 km, and the altitude and time of their appearance were almost perfectly synchronized. From a chemical model point of view, usually below 90 km, the combination plays a dominant role, and the ions disappear because of the rapid combination reaction. Where do these ions come from, which are concentrated for such a long time?

4.3. Up to 300 km Ca⁺ Layer

Since 1985, France, Germany, the United States, Japan, and other countries have used lidar to observe Ca⁺ layers at 90–120 km [20,21,22,23]; in 2020, the Arecibo Station reported lidar detection of a Ca⁺ layer at an altitude of 110–180 km (lower F layer of the ionosphere). Fortunately, our lidar at Yanqing station also found a number of Ca⁺ layers in the F region, with an altitude of more than 200 km, and the peak region of altitude up to 300 km. Figure 9 shows Ca⁺ number density measurements versus local time and altitude from 80 to 300 km, but there is no corresponding altitude Ca layer at all at the same time. As can be seen from the figure, the Ca⁺ layers have a long duration; from the peak height of the E zone to the F zone, there is a magnificent phenomenon, presenting a fine, rising and falling movement of the complex structure.

The Ca or Ca⁺ densities at altitude z are calculated using the standard lidar equation:

$$n_{Mt}\left(z\right)=n_A\left(z_R\right)\frac{N_{Mt}\left(z\right)-N_B}{N_R\left(z_R\right)-N_B}\frac{z^2}{{z_R}^2}\frac{4\pi {\sigma }_R}{{\sigma }_{Mt}},Mt=Caor{Ca}^{+}$$

(1)

where $N_{Mt}\left(z\right)$ is the resonance count rate of Ca atoms or Ca⁺ ions, $N_R\left(z_R\right)$ is the Rayleigh count rate at the reference altitude, $N_B$ is the background count rate, $n_A\left(z_R\right)$ is the air density at the reference altitude taken from NRLMSISE-00 [24], and $z_R$ is the reference altitude chosen as 30 km for the Ca and Ca⁺ layer. The effective differential backscatter cross sections are calculated as described by Chu and Papen [4]. We selected the background photon counts at 350–390 km as the noise signal, and the data at the original lidar resolution of 96 m and 33 s in Figure 9, and a Hamming window with a full-width-at-half-maximum of 960 m and 165 s was used to smooth.

The Ca⁺ layer in the F region over Yanqing is not a very random phenomenon. In the observation data of January to December 2021, there were a total of 114 observation days, and a Ca⁺ layer in the F region was detected on 38 days. We compared the total observation days of the Ca⁺ layer in the F region with the total monthly observation days, and concluded that the probability of high calcium occurrence was the largest in May, June, and July, all of which were greater than 60%, and that the occurrence rate of this phenomenon is very high in summer, especially in May, in which almost every observation occurred. Through comparison with geomagnetic data, it was also found that the occurrence of the Ca⁺ layer in the F region in Yanqing area had little correlation with space weather events [25]. It is precisely because Yanqing Station is located in the mid-latitude region that the origin and formation mechanism of such frequent occurrence of metal ions in the F region is a very interesting research topic.

5. Discussion

The improvement of laser technology has greatly improved the detection ability of lidar systems, a finer metal layer structure can be obtained by increasing the laser energy and time resolution appropriately. We can use the concept of detection limits [26] to discuss the advanced use of OPO technology in this lidar, which is the standard deviation of the background photon noise.

Taking the raw photon counts of Ca⁺ on 1 May 2021, the noise of 120–140 km is 10.8 and the standard deviation is 4.6. Therefore, when the raw photon counts obtained are greater than 17.7 (the detection limit is the background noise plus 1.5 times its standard deviation), the Ca⁺ signal can be detected, and the corresponding density detection limit is 0.0746/cm³, as shown in Figure 10. Conventional lidar systems have been reported, most of which are several atoms/cm³, such as Ejiri [27] set alexandrite ring laser at Tachikawa (35.7°N, 139.4°E) with a detection limit of 2 atoms/cm³, and Raizada1’s resonance lidars [28] has a detection limit of 2 atoms/cm³. With the increase in detection limits, the information of lower density metal ion layer can be obtained, for example, the Ca⁺ layer with an altitude higher than 200 km mentioned in Figure 8. It is difficult to detect the Ca⁺ layer at this altitude. Due to the increase in the detection height, the atmospheric temperature will also increase correspondingly, so the Doppler broadening will lead to a decrease in the scattering cross-section. According to the parameters of the lidar system, the altitude of the Ca⁺ layer is 80–100 km, the model temperature is 200 K, and the effective backscattering cross section is 1.09 × 10⁻¹⁵ m²/sr, When the high layer Ca⁺ altitude is 300 km, the model temperature is 1064 K, and the effective backscattering cross section is 4.84 × 10⁻¹⁷ m²/sr. We can calculate that the effective backscattering cross-section of 100 km is 22.5 times that of 300 km. Therefore, the detection of 300 km of the Ca⁺ layer proves that the system has excellent detection performance, and such a wide distribution range, long duration, and fine Ca⁺ layer structure can only be due to high-precision lidar.

For a long time, conventional dye-based lidar systems have reported bursts of Ca and Ca⁺ layers with density as low as several atoms/cm³. With a narrowband all-solid state lidar transmitter, we demonstrated the detection of a background Ca⁺ layer with a density of ~0.1 atoms/cm³, as shown in Figure 11 below.

Moreover, we plan to upgrade the OPO/OPA Ca and Ca⁺ lidar transmitter to a higher output pulse energy, from ~30 mJ to ~100 mJ per pulse, for the Meridian Project to be installed in Mohe station. Upon the improvement of the receiving system, even higher precision detection with higher sensitivity is expected. Since the narrowband OPO/OPA is a tunable system, the all-solid-state lidar systems can also be applied to the detection of other metal atomic ions, including some of the more difficult-to-detect metals.

6. Conclusions

The OPO/OPA technology was applied to metal resonance scattering lidar and realized the simultaneous detection of Ca and Ca⁺ at a previously undetected low density. Compared with the conventional dye lidar system, the most significant characteristics of the all-solid-state system are high pulse energy, narrow bandwidth, and uniformity in illuminated profile. In addition, unlike the dye-based system, it does not have fluorescence at unwanted wavelengths. Its stable performance minimizes the need for frequent intervention, essential for a lidar undertaking continuous observations. In time, the lidar will provide data for the study of the formation mechanism of the thermosphere metal layer and of the coupling between the neutral and ionized atmosphere.