Optical frequency modulated continuous wave (FMCW) range and velocity measurements

Abstract: Today a number of different optical techniques capable of measuring range, velocity or both exist. With recent advancements in laser diode manufacturing, small tunable sources have become available. By modulating current supplied to the laser the optical output frequency can be modulated. If the outgoing modulated light is divided into two parts, a transmitted and a local oscillator part, and a photodiode is used as detector, the frequency difference between the two parts can be obtained directly from the photodiode current. This frequency difference is often referred to as the beat frequency. If triangular frequency modulation is used, both range and radial velocity can be determined from the measured beat frequency. The common name for this technique, which was first implemented using regular radar, is frequency modulated continuous wave (FMCW). The thesis consists of an introduction and five papers. Paper one is a theoretical investigation where contributions from different noise sources are discussed. We have investigated how the signal to noise ratio for the photodiode is affected by how the optical power is divided between the transmitted part and the local oscillator part. Paper number two introduces a new modulation scheme that avoids ambiguity problems resulting from a Doppler shift larger than the frequency shift associated with the range. As a result of this new modulation scheme other benefits are also gained. The modulation scheme was tested and verified in our lab system built with a tunable laser diode and a fiber optic coupler. Paper number three presents a single stage OP-amp solution suited for an FMCW system. Our circuit combines a high gain in the desired frequency region with minimal gain at dc, without using any inductors. The risk of saturation or clipping due to the local oscillator can thereby greatly be reduced. Inserting a high pass filter between the photodiode and the first amplifier stage, to remove the dc level, is often not practical when using a regular current to voltage converter. A cascade of two stages, with a high pass filter between the two, is therefore commonly used. Our solution has similar performance as the cascade solution, but since it uses only one OP-amp, it is less sensitive to external disturbances. Paper number four and five deal with a common problem in an FMCW systems where the optical frequency is used as the carrier wave, and the modulation is obtained by modulating a current to the laser. For optimum performance, the frequency sweep should be as linear as possible, but due to thermal effects, a linear current ramping seldom results in a linear frequency ramping. At lower modulation frequencies, below 1 MHz, the temperature behaviour of the laser has a large influence on the frequency behaviour. To model the frequency behaviour we hence started by investigating the thermal behaviour. This work is presented in paper four. The goal is to obtain a model for the frequency behaviour with respect to changes in the laser drive current. In paper five, this model has been tweaked a bit and is used to obtain modulation currents that gives a linear frequency ramping.