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 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. 1. RIN noise can be reduced by a factor of RB (RIN suppression ratio.) through the use of a balanced detection. Assuming we operate below the detector saturation power, the 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. 1. Therefore, operating with about 10 dBm of LO power on each detector of our balanced detector units should be optimum (Rin ≈ -150 dB for our laser).
Fig. 1 Effect of Local Oscillator Power on Efficiency Power Penalty