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 will be 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 done to determine the optimum local oscillator power:

By increasing local oscillator power (PL), shot noise from local oscillator will dominate thermal noise on load impedance (RL). Low level of PL will cause thermal noise to dominate shot noise, and optical efficiency will suffer as shown in fig. 2. RIN noise can be reduced by a factor of RB (RIN suppression ratio.) through the use of a balanced detection. Heterodyne detection general formula that relates the local oscillator power to power efficiency is give by the following equation:

ηpp: Efficiency on power penalty

ηq: Quantum efficiency of the detector

k: Boltzmann's constant

RB: Balanced detection suppression

h: Plank's constant

RL: Load resistor

e: Electron charge

T: Temperature in degrees Kelvin

Assuming room temperature operation, a 70 Ohm load resistor, and 0.8 quantum efficiency, the efficiency on power penalty can be plotted as a function of local oscillator power for Rin values of between -140 dB/Hz and -170dB/Hz as shown in Fig. 2. As RIN is frequency dependent, we requested from our laser vendor that the RIN to be measured vs. frequency in the band of interest to us (50 to 110 MHz.)  R_in is less than -152 dBm/Hz in band. Therefore, operating with about 10 dBm of LO power on each detector of our balanced detector units should be optimum and provides us with a few dB of margin in available power. Recent reports of field operations of similar systems have inspired our development and suggested our improvements to take advantage of signal enhancements by using balanced detection as well as advanced signal processing.

Fig. 2 Effect of Local Oscillator Power on Efficiency Power Penalty