Compact high-power shipborne doppler lidar based on high spectral resolution techniques

The Compact High-Power Shipborne Doppler Wind Lidar (CHiPSDWiL) based on highspectral-resolution technique has been built up at the Ocean University of China for the measurement of the wind field and the properties of the aerosol and clouds in the troposphere. The design of the CHiPSDWiL including the transceiver, the injection seeding, the locking and the frequency measurement will be presented. Preliminary results measured by the CHiPSDWiL are provided.


INTRODUCTION
Atmospheric wind field has significant impact on atmospheric circulation, atmospheric carbon cycle, marine-atmosphere circulation and aerosol activities. Aerosols in the whole troposphere play a key role in climate change and air quality due to its direct, semi-direct and indirect effects on the radiation budget. It is the main driving force of various weather phenomena. To measure the characteristics of the optical properties, dynamics and the thermodynamics in the whole troposphere, the shipborne Doppler lidar based on High Spectral Resolution techniques is developed. Shipley [1] and Sroga [2] describe one of the first HSRLs. Eloranta [3] presents the principles of high spectral resolution lidar systems. Chanin et al. [4] presents the use of a dual Fabry-Perot interferometer (FPI) for Doppler frequency analysis. Korb et al. performed analyzes wind measurements with a single FPI [5] and a dual FPI [6][7]. Liu et al. [8] proposed the use of iodine vapor frequency discriminator for frequency analysis of backscattered light from both aerosol and air molecules for tropospheric wind measurements. Later on, a mobile ground-based incoherent Doppler wind lidar (MIDWiL) with an iodine filter was developed for the winds profiles from the signals of Mie and Rayleigh backscattering together in 2000. A van-based Doppler lidar using an injection-seeded diodepumped Nd:YAG pulsed laser with a high repetition rate was established to measure the sea surface wind with high spatial and temporal resolution for the Sailing Game.
[9] The purpose of this paper is to introduce a Compact High-Power Shipborne Doppler Wind Lidar (CHiPSDWiL) based on High-Spectral-Resolution technique established at the Ocean University of China for the offshore measurement of the wind field and the properties of the aerosol and clouds in the troposphere.

METHODOLOGY
The atmospheric wind field can be retrieved from the combined Mie and Rayleigh backscattering by use of a direct-detect technology. The use of the second harmonic of Nd:YAG wavelength at 532 nm, with adequate response to both Mie and Rayleigh scattering and a molecular filter as a frequency discriminator with 2 GHz bandwidth. The wind measuring sensitivity is proportional to the slope of the filter transmission at an operating point.
Another unique feature of the CHiPSDWiL system is the capability to monitor the aerosol scattering ratio, with which the radial wind can be calculated according to the aerosol load.
With the aerosol-scattering ratio independently measured, the lidar returns passing through the frequency discriminator can be processed to retrieve Doppler wind profiles under a varied mixture of Rayleigh and Mie scattering.
EPJ Web of Conferences 176, 01020 (2018) https://doi.org/10.1051/epjconf/201817601020 ILRC 28 Figure 1 illustrates a schematic diagram of CHiPSDWiL, consisting of five major function blocks: telescope and scanner system, laser and seed injection system, frequency locking system, optical frequency discriminator system, and transceiver switch. All of the modules are integrated in a small container of 2m×2m×2.4m, excepting for the scanner, for the field experiment in a research vessel with limited spaces (Figure 2).

Figure 1
Schematic diagram of the CHiPSDWiL The lidar transmitter provides high-peak-power and frequency-stabilized laser pulses at 532 nm by a diode-pumped, frequency-doubled, pulsed Nd:YAG laser, injection seeded by a seed laser and stabilized to an iodine absorption line. The laser repetition rate is 100 Hz, and the single pulse energy is ~125 mJ with pulse width of 10 ns. The seed laser is a continuous None-Planer-Ring-Oscillator Nd:YAG laser and its 1064 nm output is injected into the pulsed Nd:YAG laser. The outgoing laser beam is firstly expanded to 250 μrad divergence by a 2X beam expander, and then directed into the transceiver telescope. A 300 mm diameter Cassegrain Dall Kirkham telescope with focal length of 3572 mm is the common part of the transmitting and the receiving optical path. Due to the structure blocking of the telescope, only the light beam in the annular space near the optical axis can be normally emitted, leading to much of the energy wasted. In order to solve this problem, for the first time to our knowledge, we use a pair of axicon lens together to produce an annular non diffractive Bessel beam to maximize the transmittance of laser beam and to minimize the unwanted back reflection and stray light. The annular beam transceiver simplifies the alignment between the transmitted beam and the receiver field of view and permits the lidar to operate with a 45-μrad FOV. Figure 3 shows the photos of transmitting annular beam at the surface of scanner and in the air.

Figure 3 Lidar Scanner and the annular laser beam
A passive polarization transmit-receive switch consisting of a polarization beamsplitter and a 1/4-wave plate is employed. The telescope directs the received photons through the quarterwave plate, which are converted into linear polarization with its axis of polarization perpendicular to that of the transmitted photons. So that the received photons pass through the polarizing cube. The received photons are then focused by lens onto the 200-μm-diameter multimode fiber that defines the receiver FOV. The fiber delivers the scattered light onto the high spectral resolution receiver system. After collimation by lens, scattered light passes through a skylight background filter comprised Atmosphere of a 0.1-nm interference filter followed by a non-polarizing beamsplitter. The signals are then split into two channels: one is detected directly by a photomultiplier tube (PMT) as a reference channel, and another goes through an iodine filter before being detected by another PMT in the measurement channel.
A small portion of leaked power from the transmitted laser beam through the polarizing beamsplitter is guided to a single mode fiber, which is directed to the second identical iodine cell and is locked to the 50% absorption level of the I2 line 1109 to stabilize the seed laser frequency. The frequency stability of the seed and amplified pulsed lasers are 200 kHz and 1 MHz, respectively. The spectral linewidth of the laser pulses is around 35 MHz.
The two-axis scanner provides coverage over an entire hemisphere (0-360⁰ azimuth and 0-180⁰ elevation). This scanner has the clear aperture of 320 mm and can accommodate co-axial transmission and reception with continuous scanning speed of 1-10°/s and directional accuracy of 0.1°. The scanning mirror has active stabilizing mechanism, which can compensate the pointing error caused by ship shake according to the information of orientation and attitude provided by the inertial measurement unit and GPS.
To measure the very large dynamic range of backscatter signal, an analog data acquisition and multi-channel scaler photon counting device is developed and used for the processes the data in real time with 10-m resolution for the altitude range from 0 m to 20 km to retrieve wind products. It also controls the laser trigger, the optical chopper, the scanner.

RESULTS
After the CHiPSDWiL system was built up, we conducted atmospheric measurements for verifying its capability. Aiming at validating the wind direction and speed measured by the CHiPSDWiL, the concurrent observation by the CHiPSDWiL and radiosonde was operated. One of the comparison results is shown in Figure 4.    Figure  6. The value of Sa of the aerosol layer below 2 km is in the range of 10-25, and δa is lower than 5%, which is in accordance with the characteristics of marine aerosols. On the other hand, at an altitude of 3 km, the value of R is 15 and δa is around 5% at the beginning of this observation, indicating a water cloud layer.

SUMMARY
We introduce a newly developed Doppler wind lidar/HSRL for measurement of wind profile and aerosol particle optical properties. The lidar transceiver transmits a unique Bessel laser beam into the atmosphere, offering a combination of advantages: (1) a high-peak-power seed injected DPSS laser and a normal aluminum coating DK telescope can be used to avoid laser damage and to improve the daytime observation capability.
(2) Transceiver greatly releases the maintaining alignment between the laser beam and the receiver field of view and permits the lidar to operate in an unstable shipborne campaign.