A simple and robust interferometer using a laser diode subject to optical feedback is presented. Fringes phase can be locked by the optical feedback within less than 0.2(pi) (peak to valley value) even when the interferometer is placed on a wooden table where the fringe movement caused by vibration amounts to about 6(pi) (peak to valley value) in the absence of optical feedback. The locked fringe pattern with spatial carriers can be analyzed by a fringe analyzer at video rate and the measured results of a spherical mirror showed the same result and repeatability as on an optical bench. We developed an optical feedback interferometric head which is easily to set both vertically and horizontally corresponding to situation of sample.
We have developed an optical coherence tomographic system which uses a high power low-coherence laser source and a Michelson interferometer. A heterodyne detection method has been used for detecting feeble reflection from object. We discuss the application of this imaging system for neuron activities and in vivo imaging of tissues.
A simple and robust interferometer using a laser diode subject to optical feedback is presented. The mechanism of fringe locking is first explained in terms of the compound cavity composed of the laser and the interferometer. Our theoretical analysis showed that the fringe locking is caused by compensation for a change in the path difference by mode hopping between external cavity modes of the laser diode. Fringe phase can be locked by the optical feedback within less than 0.2 (pi) (peak to valley) even when the interferometer is placed on a wooden table where in the absence of optical feedback whereas the fringe fluctuation amounts to about 6 (pi) (peak to valley value). The locked fringe pattern with spatial carriers can be analyzed by a fringe analyzer at the video rate and the measured results of a spherical error showed the same values of spherical mirror and repeatability as on an optical bench.
A simple robust interferometer using a laser diode subject to optical feedback from the interferometer is presented. Fringe phase can be locked by the optical feedback within less than 0.2(pi) (peak to valley value) even when the interferometer is placed outside optical benches where the fringe movement caused by vibration amounts to about 6(pi) (peak to valley value) in the absence of optical feedback. The fringe locking is caused by the change of lasing wavelength that suppresses the net phase change to be much less than 2(pi) . The locked fringe pattern with spatial carries can be analyzed by a fringe analyzer at video rate and the measured results of a spherical mirror showed the same accuracy as on an optical bench.
A fringe locking phenomenon in a two beam interferometer using a semiconductor laser subject to optical feedback has been observed, whose injection current is modulated. When a path difference of the interferometer is sufficient, fringes taken by a CCD camera are seen to be stationary and the rms fluctuations of fringe phase is reduced to as low as 0.2 (pi) radians from more than 9 (pi) radians that is observed without the optical feedback. The rms phase fluctuation is independent of frequency and amplitude of the current modulation. The fringe locking has also been observed in the presence of both injection current modulation and PZT mirror vibration of the interferometer. A theoretical analysis has been performed that explains the observed phenomenon. It has been shown that the wavelength change due to injection current is controlled by the interferometer. The dependence of wavelength change on the injection current variation is calculated using a model of coupled resonators consisting of the laser cavity and the interferometer. The fringe phase change caused by modulation of injection current is derived from it. The calculated phase fluctuation agrees well with those observed in experiments.
We describe a fringe locking phenomenon that is observed in a two beam interferometer using a laser diode. We found that the fringes were locked even when one mirror of the interferometer was vibrated at an amplitude of several wavelengths. The phenomenon is caused by optical feedback from the interferometer to the laser diode so that oscillation wavelength of the laser can be changed by mirror movement to compensate for the resultant change in the path difference. The phenomenon occurs if a fraction at least 0.1% of LD output power is returned. The degree of fringe locking depends on optical path difference periodically with a period of optical laser cavity length. The locking has also been observed for the mirror vibration at two wavelengths in amplitude and 10 kHz in frequency limited by the PZT driver. The wavelength shift caused by the mirror movement has been calculated using an equivalent Fabry-Perot cavity model for the laser diode interferometer and showed good agreement with experimental results.
An active two-beam interferometer has been developed which locks fringes in the presence of external perturbations such as vibration and air flow and enables measurement on an unstabilized table. Movement of carriered fringes caused by the perturbations are detected by a spatial filtering detector whose output signal is fed back to injection current of a laser diode to compensate for path variations by the resulting wavelength shift. The video image of the fringes is supplied to a real time fringe analyzer which delivers the phase distribution at the video rate. The analyzer is based on electronic moire principle for simultaneous generation of three phase shifted fringe patterns. The results from a spherical mirror of 130 mm- diameter have shown good coincidence in spherical errors with that obtained from measurement on an optical bench. Only its repeatability proved to be worse by 1.7 times.
It is generally difficult to perform interferometric measurements of surface shape or deformation outside optical benches because fringes are blurred by external perturbations such as mechanical vibration or air turbulence. For overcoming this we have developed active interferometers, in which fringes are stabilized by detecting their movement induced by the perturbations and feeding back the signal to a piezoelectric mirror of the interferometer. We also extend the system to an active phase-shifting speckle interferometer by which the correlation fringes are shifted under feedback control for automatic analysis. Examples of deformation measurement under air turbulence are presented.