This paper presents a two-modality laser diode (LD) interferometer which combine as
two-wavelength sinusoidal phase modulating (SPM) interferometer with a wavelength
scanning interferometer (WSI) for measurement of distance over long range with high
accuracy. Moreover, the intensity modulation due to power changes of LD is
suppressed by appropriately choosing the modulation amplitude of injection current
(IC) of LD. Triangle wave is used to modulate the IC of one LD with that of the other
LD being constant at first. Thus the interferometer works as a wavelength scanning
interferometer. An initial estimate of the distance can be obtained from the phase
change of the interference signal. Then sinusoidal wave is used for modulating IC of
both LDs to realize a two-wavelength SPM interferometer. However, the modulation
of the IC of two LDs results in not only the wavelength modulation but also the
intensity modulation. This intensity modulation will cause a measured phase error. To
eliminate this error, SPM depths are appropriately chosen, therefore the distance to be
measured can be accurately obtained with synthetic-wavelength algorithm.
Experimental results indicate that an absolute distance measurement accuracy of 1μm
can be achieved over the range of 40mm to 100mm.
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