A recirculating optical delay line 30 has a laser 32 controlled by wavelength control 33 so as to vary the wavelength of radiation over time to provide a first input 34 to an optical modulator 35. The tooth units include a low-torque tooth unit that convey a relatively low torque, and a high-torque-tooth unit that conveys a relatively high torque. The optical radiation processing unit directs to a detector polarized optical radiation that received from the object.