Simplified Homodyne Detection For Linear FM Lidar
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1076-K04-07
Simplified Homodyne Detection For Linear FM Lidar Peter Adany1, Chris Allen2, and Rongqing Hui3 1 ITTC, University of Kansas, 247N Nichols Hall, 2335 Irving Hill Rd., Lawrence, KS, 66045 2 ITTC, University of Kansas, 321 Nichols Hall, 2335 Irving Hill Rd., Lawrence, KS, 66045 3 ITTC, University of Kansas, 222 Nichols Hall, 2335 Irving Hill Rd., Lawrence, KS, 66045 ABSTRACT A fiber based lidar system is developed which simplifies the processing of linear FM pulses by using a modulated local oscillator power in the coherent detector. Experiments were conducted on lidar systems with direct, heterodyne and simplified homodyne detection to compare receiver sensitivity. A field experiment using the homodyne system verified the sensitivity estimation on a building target at 370-m range. INTRODUCTION The need to better understand and predict climate change requires more efficient and compact high resolution instruments to map ice topography from satellites and small aircraft. Compared to classic radars, fiber based lidar systems offer narrower radiating angle, higher target contrast and higher resolution. Combined with pulse compression techniques, lidar can provide high performance using off-the-shelf components from the communications industry. In traditional pulsed lidar systems, a simple transmit and receive scheme is used that achieves range resolution proportional to the pulse duration as ∆R = cτ 2
(1)
where τ is the pulse duration and c is the speed of light. To achieve fine range resolution, short pulses are used and thus high pulse peak powers are needed to preserve the signal-to-noise ratio in the receiver. The maximum transmit power is limited by the hardware, causing limited range resolution in these systems. Furthermore, in q-switched lasers the emitter contains damagingly high optical fields that limit the system’s lifespan to less than 5 years. In previous research at the University of Kansas a lidar system was demonstrated that uses linear FM pulse compression to achieve fine range resolution with low peak power [1]. This system applies RF modulation to the optical pulse, in the form of a linear frequency sweep (chirp). A unique feature of this method is that recovering the range information simply requires multiplying the received RF chirp signal with a local reference chirp. This system achieved heterodyne detection using a wavelength-tuned laser as a local oscillator at an intermediate frequency of several GHz. In the heterodyne system, high fidelity analog RF circuitry is required after the optical receiver to process the linear FM chirp, resulting in a hybrid system with a relatively complex architecture. However, a shortcut is possible because both linear FM and homodyne detection use time domain multiplication of signals. In the simplified homodyne design the coherent receiver is supplied with a modulated local oscillator so that the photodetector directly produces a dechirped RF signal. The block diagram of this system is shown in Figure 1 below.
Optical amplifier
Modulator Laser Op
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