Coherence function control of a multi-mode laser diode by the frequency modulation and its low coherence interferometric application

• We propose a low coherence interferometry using a frequency modulated multi-mode laser diode, suitable for spatially coherent measurements.
• The frequency modulation of a multi-mode LD can improve the contrast of the coherence peak.
• In the experiments, LDs with 635 nm was frequency-modulated with the current modulation and the coherence length was 200 μm.
• The possibility to measure surface profile of a sample was experimentally confirmed with step height samples.

In this investigation, a frequency-modulated multi-mode laser diode (LD) is introduced as a light source of low coherence scanning interferometry (LCSI) and its applicability is verified with experiments. The frequency modulation of a multi-mode LD can improve the contrast of the coherence peak corresponding to zero OPD and overcome the ambiguity to find the exact coherence peak appeared in frequency modulated single-mode LD or a multi-mode LD. As the experimental results, the coherence function of a multi-mode LD was changed with the frequency modulation and the expected coherence peak was obviously determined. As the result, the coherence length was 200 μm. Also, the possibility to measure surface profile of a sample was experimentally confirmed with step height samples. Although the proposed technique cannot be guaranteed for high accuracy in the lack of the position accuracy of the scanning stage, it is suitable for the demand of spatially coherent measurements and high speed measurements adopting the sub-sampling technique because of its low cost, high available power and better spatial coherence than other white light sources.