Peter J. de Groot

Ranging and velocimetry signal generation in a backscatter-modulated laser diode

A compact self-aligning laser radar has been constructed for coheren ranging and velocimetry using a laser diode modulated by feedback from light scattered from a diffusing target. The phenomenology of beat-signal generation in the device is discussed from both experimental and theoretical points of view. The ac-coupled modulation waveform is asymmetric (similar to a sawtooth) and different for the two propagation directions of the light leaving the diode. Atheoreticalmodel, based on the mode structure ofathree-mirrorFabry-Perot cavity, describes signal generation in these experiments and accounts for the asymmetric waveform.

Three-color laser-diode interferometer

The combined optical spectrum of a pair of multimode laser diodes is composed of a large number of well-defined wavelengths. This work reports the use of three of these wavelengths in a phase-modulated interferometer to measure absolute distance over 360-μm intervals with a resolution of 0.5 nm. The laboratory demonstration system is composed of a three-wavelength source coupled by single-mode fiber to a compact interferometric probe. This system has been used for displacement measurement and profiling of optical surfaces.The combined optical spectrum of a pair of multimode laser diodes is composed of a large number of welldefined wavelengths. This work reports the use of three of these wavelengths in a phase-modulated interferometer to measure absolute distance over 360-microm intervals with a resolution of 0.5 nm. The laboratory demonstration system is composed of a three-wavelength source coupled by single-mode fiber to a compact interferometric probe. This system has been used for displacement measurement and profiling of optical surfaces.

Synthetic wavelength stabilization for two-color laser-diode interferometry

The phase ambiguity in conventional interferometers can be removed by using two laser diodes of different optical frequencies to generate a synthetic wavelength. However, the stability requirements for a two-color interferometric laser gauge that must provide unambiguous determination of the optical fringe order over a large distance can be severe. We derive upper limits on the optical wavelength uncertainty and express them as a function of optical path difference between the object and reference beams, phase measurement errors, and the synthetic wavelength. A simple stabilization arrangement is proposed, involving simultaneous servo control of both lasers with a single Fabry-Perot étalon. The experimental implementation of the proposed system demonstrates its effectiveness for long-term (16-h) stabilized two-color interferometry over a distance of 250 mm, with a 15-mm synthetic wavelength and a repeatability of 40 nm. For periods of < 1000 s, the repeatability was 8 nm.

Backscatter-modulation velocimetry with an external-cavity laser diode

Feedback phenomena in lasers induce modulation signals that can be used for ranging and velocimetry. The modulation depth of a backscatter-modulated semiconductor laser diode is considerably enhanced by operating the diode near the lasing threshold. In the present work an external cavity is used to maintain a spectral linewidth of less than 1 MHz in a diode running at 2.5% of its rated power of 30 mW, resulting in a modulation depth enhancement by a factor of 30 over the high-power limit. Coherent velocimetry has been performed for targets at a distance of 50 m, or 20 times farther than the previously published operational range for backscatter-modulated laser diodes. Light power of less than 10 pW at the collection aperture produces useful modulation signals.

Chromatic dispersion effects in coherent absolute ranging.

Multiple-wavelength interferometers and chirped-frequency coherent laser radars use the variation of interferometric phase as a function of wavelength to measure optical path lengths. Since these instruments generally include a number of dispersive components, the group-velocity index must be included in the path-length calculation. A simple experiment is performed to show that neglecting first-order chromatic dispersion can lead to measurement errors of the order of 1% of the distance.

Chirped synthetic-wavelength interferometry

We demonstrate distance measurement by varying, or chirping, the synthetic wavelength in a two-color laser diode interferometer. An absolute accuracy of 3 microm is achieved over a 200-mm range, without the phase ambiguities or optical complexities normally associated with synthetic-wavelength interferometry. The fibercoupled experimental system uses a 25-microW, 0.25-mm-diameter collimated object beam and can be used for dimensional gauging and rough-surface profiling.

Range-dependent optical feedback effects on the multimode spectrum of laser diodes

Abstract Weak optical feedback from diffusely-reflecting objects can dramatically affect the optical spectrum of homogeneously-broadened laser diodes operated near the lasing threshold. Switching between two or more oscillation modes can be controlled by as little as 1 nW of optical feedback power. The modulation effects on the optical spectrum and total power output depend on the phase of the returned light, and the strength of the effect has a range dependency that is useful for metrology applications. In the present work, experimental and theoretical analyses of these phenomena are presented, and a simple metrology tool for position sensing and remote surface profiling is proposed.

Interferometric laser profilometer for rough surfaces

A simple synthetic-wavelength interferometer is reported that uses two wavelengths of a multimode laser diode. The difference in interferometric phase measurements at the two wavelengths provides absolute range information to an accuracy of 0.75 μm over an ambiguity interval of 310 μm. The instrument can be used to profile surfaces too rough to be measured by conventional interferometry.

Three-Dimensional Imaging Coherent Laser Radar Array

We have constructed a small-scale laboratory demonstration of a laser-diode-based radar array for simultaneous coherent ranging and velocimetry of multiple target points. Signals are produced by an array of laser/detector packages in a self-detection geometry with common imaging optics for both the source and backscattered light. The radar has been used with a microcomputer to find the orientation and rotation speed of a spinning disk at an arbitrary position in the field of view.

Laser gage using chirped synthetic wavelength interferometry

An absolute-ranging interferometer will be described that is suitable for dimensional gaging and surface profiling in manufacturing applications. The interferometer has a two-frequency source and a continuously tuned or chirped synthetic wavelength. The fiber-coupled experimental system uses an eye-safe 25 (mu) W, 0.25 mm diameter collimated probe beam; and has an absolute distance measurement accuracy of 3 micrometers over a 150 mm dynamic range.