Experimental comparison of autodyne and heterodyne laser interferometry using an Nd:YVO₄ microchip laser

Using an Nd:YVO? microchip laser with a relaxation frequency in the megahertz range, we have experimentally compared a heterodyne interferometer based on a Michelson configuration with an autodyne interferometer based on the laser optical feedback imaging (LOFI) method regarding their signal-to-noise ratios. In the heterodyne configuration, the beating between the reference beam and the signal beam is realized outside the laser cavity, while in the autodyne configuration, the wave beating takes place inside the laser cavity, and the relaxation oscillations of the laser intensity then play an important part. For a given laser output power, object under investigation, and detection noise level, we have determined the amplification gain of the LOFI interferometer compared to the heterodyne interferometer. LOFI interferometry is demonstrated to show higher performance than heterodyne interferometry for a wide range of laser powers and detection levels of noise. The experimental results are in good agreement with the theoretical predictions.     

Resolution of a synthetic aperture laser optical feedback imaging using a galvanometric scanning

We have improved the resolution of our laser optical feedback imaging (LOFI) setup by using a galvanometric scanning and two-dimensional angular synthetic aperture (SA) process. The experimental resolution of the classical LOFI images and of the SA LOFI images are compared for different working distances, and we show that the SA LOFI method is able to balance the degradation of the images resolution with increasing distance. We also show that the resolution of the SA LOFI images can be controlled by choosing the position of the galvanometric scanner between the laser and the target under investigation. Theoretical and experimental SA resolutions are compared.     

Comparative study of autodyne and heterodyne laser interferometry for imaging

For given laser output power, object under investigation, and photodiode noise level, we have theoretically compared the signal-to-noise ratios of a heterodyne scanning imager based on a Michelson interferometer and of an autodyne setup based on the laser optical feedback imaging (LOFI) technique. In both cases, the image is obtained point by point. In the heterodyne configuration, the beating between the reference beam and the signal beam is realized outside the laser cavity (i.e., directly on the detector), while in the autodyne configuration, the wave beating takes place inside the laser cavity and therefore is indirectly detected. In the autodyne configuration, where the laser relaxation oscillations play a leading role, we have compared one-dimensional scans obtained by numerical simulations with different lasers' dynamical parameters. Finally, we have determined the best laser for LOFI applications and the experimental conditions for which the LOFI detection setup (autodyne interferometer) is competitive compared to a heterodyne interferometer.     

Orthogonal linear polarization tunable-beat ring laser with a superluminescent diode

An orthogonal linear polarization operated ring laser with a superluminescent diode has been demonstrated to generate a tunable optical beat signal. The ring cavity contains a superluminescent diode as the optical gain medium, Faraday rotators, and a variable phase retarder (Babinet-Soleil compensator). By controlling the retarder, we changed the beat frequency in the range from a few tens of megahertz to 100 MHz.     

Laser optical feedback imaging insensitive to parasitic optical feedback

We present an optical architecture for the laser optical feedback imaging (LOFI) technique that makes it possible to avoid the effect of the optical parasitic reflections introduced by the optical components located between the laser source and the studied object. These reflections damage phase and amplitude information contained in the images. This phenomenon is a leading problem that strongly limits the LOFI performance for weak feedback detection. Consequently, it is essential to be able to limit or avoid the effect of these parasitic reflections to reach the optimal LOFI performance.     

Sensitivity of synthetic aperture laser optical feedback imaging

In this paper, we compare the sensitivity of two imaging configurations, both based on laser optical feedback imaging (LOFI). The first one is direct imaging, which uses conventional optical focalization on target, and the second one is made by a synthetic aperture (SA) laser, which uses numerical focalization. We show that SA configuration allows us to obtain good resolutions with high working distance and that the drawback of SA imagery is that it has a worse photometric balance in comparison to a conventional microscope. This drawback is partially compensated by the important sensitivity of LOFI. Another interest of SA relies on the capacity of getting three-dimensional information in a single x-y scan.     

Amplitude-stabilized frequency-modulated laser diode and its interferometric sensing applications

A direct frequency-modulated (FM) laser diode light source without light power variation is developed. The amplitude variation of the FM laser diode is compensated by means of a feedback system with use of a superluminescent diode as an external light power controller. Output power greater than 1 mW is obtained at the modulation frequency to 5 kHz with a >10 stabilization factor. By use of the amplitude-stabilized FM laser diode, we measured subfringes with high accuracy in FM continuous wave interferometry, increased the dynamic range of the displacement measurement, and improved the stabilization factor in the laser diode feedback interferometer.     

Two-dimensional synthetic aperture laser optical feedback imaging using galvanometric scanning

We have improved the resolution of our laser optical feedback imaging (LOFI) setup by using a synthetic aperture (SA) process. We report a two-dimensional (2D) SA LOFI experiment where the unprocessed image (i.e., the classical LOFI image) is obtained point by point, line after line using full 2D galvanometric scanning. The 2D superresolved image is then obtained by successively computing two angular SA operations while a one-dimensional angular synthesis is preceded by a frequency synthesis to obtain a 2D superresolved image conventionally in the synthetic aperture radar (SAR) method and their corresponding laser method called synthetic aperture ladar. The numerical and experimental results are compared.     

Noise characteristics of a Nd:YVO(4) laser pumped by laser diode modulated at high frequency

We report, for the first time to our knowledge, on the noise characteristics of a Nd:YVO(4) laser pumped by a laser diode modulated at high frequency. We have investigated noise characteristics of a Nd:YVO(4) laser pumped by a laser diode that is oscillating in a stable multilongitudinal mode because of modulation by a high-frequency (several hundred megahertz) current. As a result, low-noise operations of -130 dB/Hz above frequencies of 1 MHz have been achieved. This noise level is comparable to the level obtained when pumping a Nd:YVO(4) laser with a laser diode that is oscillating in a single longitudinal mode. However, a noise peak corresponding to relaxation oscillations of the Nd:YVO(4) laser has appeared around a frequency of several hundred kilohertz.     

Three-dimensional sensing based on a dynamically focused laser optical feedback imaging technique

A new method analogous to three-dimensional confocally based sensing is proposed. This method uses the technique of laser optical feedback imaging, which takes advantage of the resonant sensitivity of a short-cavity laser to frequency-shifted optical feedback for highly sensitive detection, making it ideal for surface and volume measurements of noncooperative targets. Rapid depth scanning is made possible by use of an electrically controlled variable-focus lens. The system is able to detect height discontinuities, and because detection occurs along the axis of projection the system does not have problems of shadow. Preliminary results for a depth range of 15 mm and a resolution of 100 mum are presented.     

Design and stability analysis of a CMOS feedback laser driver

A feedback laser driver circuit has been developed to control the average optical output power of a double heterostructure CW laser diode and also to operate with a wideband amplitude modulation. The average light power monitoring is realized by measuring the photoelectric current of a photodiode integrated inside the package of the laser diode. Safety features, including transient signal suppression, protect the laser against excessive light power. To obtain responses that are not out of the absolute maximum ratings of the laser diode light power, a study of the feedback loop stability is necessary. Two transconductance structures, using inverting and noninverting circuits are compared using the dominant pole compensation method. Then, the more stable driver circuit is analyzed and integrated using CMOS technology.     

Wavelength modulation detection of water vapor with a vertical cavity surface-emitting laser

A vertical cavity surface-emitting laser was studied for gas-sensing applications. Properties of the 962-nm laser that were measured include side-mode suppression, wavelength tuning with temperature and current, power versus injection current, and the amplitude noise spectrum. With wavelength modulation spectroscopy, a rms noise level of 2 x 10(-4) absorbance units was achieved. The large current tuning range (25 cm(-1)) and smaller amplitude modulation (11%/cm(-1)) of the vertical cavity laser compare favorably with Fabry-Perot and distributed feedback diode lasers for spectroscopic gas sensing, especially at atmospheric pressure.     

Electronic subtracter for trace-gas detection with InGaAsP diode lasers

An electronic noise-cancellation scheme has been developed and tested for second-harmonic (2f) detection with short-external-cavity and distributed-feedback InGaAsP diode lasers and wavelength modulation. The 2f background signal and noise from, e.g., optical feedback, optical fringes, and power-supply pickup are effectively reduced by subtraction of a measure of the signal-beam photocurrent from a measure of the reference-beam photocurrent. The dynamic range required for the lock-in amplifier is also reduced because the signal owing to modulation of the laser output at the first harmonic is canceled. Reduction of the 2f background and dynamic range are important for atmospheric-pressure detection where a large wavelength modulation is necessary. The detector noise was minimized by the use of zero-biased detectors in the subtraction circuit. A beam-noise level (defined as 2× the rms value) equivalent to a line-center absorption of 1.6 × 10(-6) was achieved with an equivalent-noise bandwidth of 1.25 Hz for 2f detection at 10 kHz. The electronic circuit is easy to construct and low cost.     

Disturbance-free distributed Bragg reflector laser-diode interferometer with a double sinusoidal phase-modulating technique for measurement of absolute distance

A new range-finding technique that uses both double sinusoidal phase modulation and quasi-two-wavelength interferometry is described. Two independent interference signals are generated with respect to two different wavelengths on a time-sharing basis. We clarify that external disturbances of these interference signals are eliminated by both feedback control and differential detection and that the feedback control does not affect the distance measurement. A single distributed Bragg reflector laser diode allows us to simplify the optical setup and to improve the measurement accuracy. After discussing a measurement range, we estimate a measurement error by making several measurements.     

Extending the continuous tuning range of an external-cavity diode laser

The continuous tuning range of an external-cavity diode laser can be extended by making small corrections to the external-cavity length through an electronic feedback loop so that the cavity resonance condition is maintained as the laser wavelength is tuned. By maintaining the cavity resonance condition as the laser is tuned, the mode hops that typically limit the continuous tuning range of the external-cavity diode laser are eliminated. We present the design of a simple external-cavity diode laser based on the Littman-Metcalf external-cavity configuration that has a measured continuous tuning range of 1 GHz without an electronic feedback loop. To include the electronic feedback loop, a small sinusoidal signal is added to the drive current of the laser diode creating a small oscillation of the laser power. By comparing the phase of the modulated optical power with the phase of the sinusoidal drive signal using a lock-in amplifier, an error signal is created and used in an electronic feedback loop to control the external-cavity length. With electronic feedback, we find that the continuous tuning range can be extended to over 65 GHz. This occurs because the electronic feedback maintains the cavity resonance condition as the laser is tuned. An experimental demonstration of this extended tuning range is presented in which the external-cavity diode laser is tuned through an absorption feature of diatomic oxygen near 760 nm.     

Glass-fiber self-mixing intra-arterial laser Doppler velocimetry: signal stability and feedback analysis

We have developed a blood velocimeter based on the principle of self-mixing in a semiconductor laser diode through an optical fiber. The intensity of the light is modulated by feedback from moving scattering particles that contain the Doppler-shift frequency. Upon feedback the characteristics of the laser diode change. The threshold current decreases, and an instable region may become present above the new threshold. The amplitude of the Doppler signal turns out to be related to the difference in intensity between situations with and without feedback. This amplitude is highest just above feedback. The suppression of reflection from the glass-fiber facets is of paramount importance in the obtaining of a higher signal-to-noise ratio. Using an optical stabilization of the feedback, we optimized the performance of the laser-fiber system and the Doppler modulation depth and clarified its behavior with a suitable physical model. We also investigated the effect of the finite coherence length of the laser. We tested the efficiency of the self-mixing velocimeter in vivo with the optical glass fiber inserted in the artery with endoscopic catheters, both in upstream and in downstream blood flow conditions. For the latter we used a special side-reflecting device solution for the fiber facet to allow downstream measurements.     

Phase-shift-locked interferometer with a wavelength-modulated laser diode

A phase-shift-locked interferometer has been constructed for distance measurement. A phase shift produced by sawtooth-current modulation of a laser diode is locked to a phase difference preset by polarization optics that consists of a quarter-wave plate and polarizers through an electrical feedback technique. An optical path difference can be measured from the locked sawtooth-wave current amplitude in real time. The sensitivity of the interferometer is discussed.     

Three-dimensional imaging laser radar with a photon-counting avalanche photodiode array and microchip laser

We have developed a threedimensional imaging laser radar featuring 3-cm range resolution and single-photon sensitivity. This prototype direct-detection laser radar employs compact, all-solid-state technology for the laser and detector array. The source is a Nd:YAG microchip laser that is diode pumped, passively Q-switched, and frequency doubled. The detector is a gated, passively quenched, two-dimensional array of silicon avalanche photodiodes operating in Geigermode. After describing the system in detail, we present a three-dimensional image, derive performance characteristics, and discuss our plans for future imaging three-dimensional laser radars.     

Metal Speciation in the ppt Range by HPLC and Diode Laser Atomic Absorption Spectrometry in a Flame

A simple, compact, and powerful instrument for metal speciation in the ppt range is described. The instrument includes a HPLC module for separation and a diode laser for element-selective detection by wavelength modulation absorption spectrometry in an analytical flame. The high detection power for metal species is due to a two-beam arrangement with logarithmic amplification of the normalized signal, which compensates the laser residual amplitude modulation noise, the offset, and its fluctuation. The analytical figures of merit are demonstrated by measurements of very low concentrations of Cr(VI) in tap water.     

Ultrasensitive laser measurements without tears

Several easily implemented devices for doing ultrasensitive optical measurements with noisy lasers are presented. They are all-electronic noise cancellation circuits that largely eliminate excess laser intensity noise as a source of measurement error and are widely applicable. Shot-noise-limited optical measurements can now easily be made at baseband with noisy lasers. These circuits are especially useful in situations where strong intermodulation effects exist, such as current-tuned diode laser spectroscopy. These inexpensive devices (parts cost approximately $10) can be optimized for particular applications such as wideband or differential measurements. Although they cannot eliminate phase noise effects, they can reduce amplitude noise by 55-70 dB or more, even in unattended operation, and usually achieve the shot-noise limit. With 1-Hz signal-to-noise ratios of 150-160 dB, they allow performance equal or superior to a complex heterodyne system in many cases, while using much simpler dual-beam or homodyne approaches. Although these devices are related to earlier differential and ratiometric techniques, their noise cancellation performance is much better. They work well at modulation frequencies from dc to several megahertz and should be extensible to approximately 100 MHz. The circuits work by subtracting photocurrents directly, with feedback applied outside the signal path to continuously adjust the subtraction for perfect balance; thus the excess noise and spurious modulation ideally cancel at all frequencies, leaving only the shot noise. The noise cancellation bandwidth is independent of the feedback bandwidth; it depends only on the speeds of the photodiodes and of the bipolar junction transistors used. Two noise-canceled outputs are available; one is a high-pass filtered voltage proportional to the signal photocurrent and the other is a low-pass filtered voltage related to the log ratio of the signal and comparison photocurrents. For reasonable current densities, the noise floors of the outputs depend only on the shot noise of the signal beam. Four variations on the basic circuit are presented: low noise floor, high cancellation, differential high power, and ratio-only. Emphasis is placed on the detailed operation and design considerations, especially performance extension by compensation of the nonideal character of system components. Experience has shown that some applications advice is required by most users, so that is provided as well.