High accuracy or high speed laser ranging: a new concept
Prof. Dr. Klaas Bergmann
Fachbereich Physik and OPTIMAS Research Center, Technische Universität Kaiserslautern, Germany
Central to this presentation will be the discussion of a novel concept for laser ranging based on a frequency-shifted feedback laser (fsf-laser) suitable for industrial applications. In a fsf-laser, the typical frequency-space mode structure is overcome by shifting the frequency of the circulating laser field at each round trip via an acousto-optic frequency shifter and feeding back only the frequency-shifted part.
Injection seeding of such a device with a narrow band laser leads to a spectral combstructure in the continuous wave output spectrum (different from the well-known comb structure in the spectrum of mode-locked femto-second laser). Part of this radiation field is sent to a reference surface the other to the object. Superposition of the two coherent multi-component (reflected and scattered) fields on a fast photo diode leads to a complex response. The latter is simplified when the injection laser is phase modulated. The detector response is monitored as the modulation frequency is scanned. At a specific modulation frequency all the components contribution to the signal are in phase and a signal, orders of magnitude above the noise floor, results.
The distance of the object is easily and accurately derived from the modulation frequency which leads to the maximum signal. Depending on the layout of the fsf-laser system, an accuracy of about 1 micrometer over distances of a few meter ( with> 1 kHz of data taking) or 1 mm over the distance of several ten meters (with about 1 MHz rate of data taking) can be achieved. The focus of the seminar will be on the high accuracy system, with results shown. The limiting accuracy of that approach is in the range of < 100 nm.