All-fiber coherent Doppler LIDAR for wind sensing

Coherent Doppler LIDAR is being utilized to develop a mobile wind speed measuring station. We at CCNY are building an all fiber based eye safe laser system to measure wind speed in urban areas. A 1.5µ.m polarization maintained fiber optics master oscillator power amplifier system is used, which utilizes components from the telecommunication industry. We chose a heterodyne balanced detection to suppress the RIN noise. We have calculated the optimum local oscillator power for maximum optical detector's efficiency. A/D conversion is performed at 400 MHZ by using a data acquisition card with FPGA on board, which can be programmed to perform autocorrelation and/or FFT onboard for faster performance. This system can be used along with other units on top of high buildings in New York City as a way of detecting wind speed profile for Homeland security.

The system consists of the following components: 1) Laser source 2) Modulator 3) Fiber Amplifier 4) Optical Antenna 5) Detector 6) Signal Processor as shown in fig. 1. In our system, a fiber coupled 1545.2 nm laser is used for the master oscillator. This source is split using a fiber coupler. One signal is used as a local oscillator (LO), while the other signal is modulated and frequency shifted using an AOM (acousto-optic modulator). The modulated signal is then amplified and transmitted through an optical antenna. The scattered signal will be received by the optical antenna and mixed with the LO signal through a 50/50 coupler. The mixed signal will be detected by a balanced detector, which generates a RF electrical signal. The RF signal is then processed using a signal processor to extract information about frequency shift and signal strength as a function of time delay.

Fig. 1 Coherent Doppler Lidar system's configuration

The following analysis was performed to determine the SNR vs. distance at different focal lengths.

Signal to noise ratio general formula is given by the following equation:

η: system efficiency  

β: scattering coefficient

B: band width

λ: wave length

K: one way atmospheric transmittance

E: pulse energy

 

Fig. 2 SNR vs. distance at different focal lengths

Wind speed measurement results:

Returned signal was detected at ~ 1800m when operating the system using 6" lens and ~ 4µJ/pulse (final design will operate at 12 µJ/pulse.) The following figure shows the returned signal power spectrum at different ranges:

Fig. 3 Returned signal power spectrum at different ranges