A Coherent Wind Lidar with Frequency-Modulated and Long-Duration Pulse: Principles and Experiments for Feasibility Study

Traditional wind lidars radiate pulsed coherent laser, and receive signals which are scattered from aerosol particles flowing with wind. A delay time, and Doppler frequency shift of the received signal can be translated to a distance and lidar-radial velocity of the aerosol particles. A pulse length is designated to hundreds of nanoseconds or a few microseconds in order to accomplish range resolution of tens or hundreds of meters which are useful for typical wind measurements. As a result, velocity resolution is determined by Δv=λ/(2T), where λ is wave length, and 2T is pulse length. That is, range resolution and velocity resolution are in relation of trade-off. Velocity resolution broadens (deteriorates) by shrinking (enhancing) range resolution, and vice versa.

Yoshikawa and Ushio proposed a concept of wind lidars with frequency-modulated and long-duration pulse, where its range resolution is determined by band width of the frequency modulation. On the other hand, its velocity resolution is determined by wave length and pulse length as well as traditional wind lidars. Thus, the concept enables wind lidars to independently designate range and velocity resolution. In other words, it can realize wind velocimetry with very high range resolution, or can uses very long-duration pulses to increase signal-to-noise ratio of received signals.

We have been developing a prototype wind lidar in order to validate feasibility of the concept. Figure 1 shows an optical setup of the prototype. A laser source is 1550-nm coherent continuous wave (CW) laser. The CW laser is pulsed and frequency modulated through an acousto-optic modulator by using a microwave input from an arbitrary waveform generator. The pulsed laser is amplified by an Erbium-doped fiber amplifier (EDFA), and then radiated vertically to the air. Received signals are introduced to a receiver side via a circulator, and detected by a heterodyne detection system. Figure 2 shows a received signal from a cloud bottom, where x- and y-axes are distance (m) and radial velocity (m/sec), respectively. A signal from the cloud bottom appeared at distances of 300—400 m high and radial (vertical) velocities of -2—0 m/sec. Received signals from aerosol particles have not yet been received because a mean radiation power of the prototype is very weak at 20 mW. The prototype is going to be upgraded to have sufficient power, and to measure signals from aerosols.

In the presentation, principles of the wind lidar with frequency-modulated and long-duration pulse will be explained. Furthermore, the prototype and its observation results will be shown. Observation results by a power-upgraded prototype may be included.

[1] Eiichi Yoshikawa, Tomoo Ushio: Wind ranging and velocimetry with low peak power and long-duration modulated laser, Optics Express, Vol 25, No.8, 2017.