Crosstalk analysis of 1 m to 10 m laser phase-shift range finder

A heterodyne phase-shift laser range finder has been developed for the measurement of distances of 1 m to 10 m. An analysis of the crosstalk between the current source and the photoelectric receiver of this device permits prediction of the accuracy of the setup. For example, when the amplitude of the crosstalk is proportional to the modulation frequency and for a given photoelectric signal-to-induction ratio of 30 at 10 m, the maximum corresponding error is about 5 cm. To determine the magnitude of the crosstalk, we propose a model taking into account that the amplitude and the phase-shift of the induced signal depend on whether the coupling is inductive or capacitive. Crosstalk modeling shows that very slight inductive or capacitive coupling is sufficient to affect distance measurement     

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.     

Saturated semiconductor optical amplifier phase modulation for long range laser radar applications

We investigate the use of a semiconductor optical amplifier operated in the saturation regime as a phase modulator for long range laser radar applications. The nature of the phase and amplitude modulation resulting from a high peak power Gaussian pulse, and the impact this has on the ideal pulse response of a laser radar system, is explored. We also present results of a proof-of-concept laboratory demonstration using phase-modulated pulses to interrogate a stationary target.     

Quantitative characterization of the x-ray imaging capability of rotating modulation collimators with laser light

We developed a method for making quantitative characterizations of bi-grid rotating modulation collimators (RMC's) that are used in a Fourier transform x-ray imager. With appropriate choices of the collimator spacings, this technique can be implemented with a beam-expanded He-Ne laser to simulate the plane wave produced by a point source at infinity even though the RMC's are diffraction limited at the He-Ne wavelength of 632.8 nm. The expanded beam passes through the grid pairs at a small angle with respect to their axis of rotation, and the modulated transmission through the grids as the RMC's rotate is detected with a photomultiplier tube. In addition to providing a quantitative characterization of the RMC's, the method also produces a measured point response function and provides an end-to-end check of the imaging system. We applied our method to the RMC's on the high-energy imaging device (HEIDI) balloon payload in its preflight configuration. We computed the harmonic ratios of the modulation time profile from the laser measurements and compared them with theoretical calculations, including the diffraction effects on irregular grids. Our results indicate the 25-in. (64-cm) x-ray imaging optics on HEIDI are capable of achieving images near the theoretical limit and are not seriously compromised by imperfections in the grids.     

Tunable Diode Laser Spectroscopy for Industrial Process Applications: System Characterization in Conventional and New Approaches

Tunable diode laser spectroscopy (TDLS) can only be successfully implemented if a number of system characterization procedures and critical parameter measurements can be made accurately. These include: application of a wavelength/frequency scale to the signals recovered in time; measurement of the frequency dither applied to the laser; measurement of the relative phase between the laser power modulation and frequency modulation; determination of the background amplitude modulation for normalization purposes and measurement of required cross broadening coefficients for the host/target gas mixtures. Easy to implement, accurate and low-cost systems and procedures for achieving these are described and validated below. They were developed for two new approaches to TDLS measurements, viz the residual amplitude modulation (RAM) technique and the phasor decomposition (PD) method, but are equally applicable to all forms of TDLS. Following full system characterization using the new techniques, measurements of the absolute transmission function of the 1650.96 nm absorption line of methane over a wide range of concentration and pressure were made using the RAM technique. The close agreement with theoretical traces derived from HITRAN data validated the entire approach taken, including the system characterization procedures. In addition, measurements of a wide range of gas concentration and pressure were made by curve fitting theoretical traces to the measured transmission functions obtained using a variety of operating conditions. Again, the low errors confirmed the validity of the new methods and the system characterization/measurement procedures described here.     

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.     

Polarized differential-phase laser scanning microscope

A polarized differential-phase laser scanning microscope, which combines a polarized optical heterodyne Mach-Zehnder interferometer and a differential amplifier to scan the topographic image of a surface, is proposed. In the experiment the differential amplifier, which acts as a PM-AM converter in the experiment, converting phase modulation (PM) into amplitude modulation (AM). Then a novel, to our knowledge, phase demodulator was proposed and implemented for the differential-phase laser scanning microscope. An optical grating (1800 lp/mm) was imaged. The lateral and the depth resolutions of the imaging system were 0.5 mum and 1 nm, respectively. The detection accuracy, which was limited by the reflectivity variation of the test surface, is discussed.     

Simultaneous real-time imaging of surface and subsurface structures from a single space-frequency multiplexed photodisplacement interferogram

A new parallel photodisplacement technique has been developed that achieves simultaneous real-time imaging of surface and subsurface structures from a single space-frequency multiplexed interferogram, which greatly simplifies the system and the optical alignment. A linear region of photodisplacement is excited on the sample for subsurface imaging by use of a line-focused intensity-modulated laser beam, and the displacement and surface information on reflectivity and topography are detected by a parallel heterodyne interferometer with a charge-coupled device linear image sensor used as a detector. The frequencies of three control signals for excitation and detection, that is, the heterodyne beat signal, modulation signal, and sensor gate pulse, are optimized such that surface and subsurface information components are space-frequency multiplexed into the sensor signal as orthogonal functions, allowing each to be discretely reproduced from Fourier coefficients. Preliminary experiments demonstrate that this technique is capable of simultaneous imaging of reflectivity, topography, and photodisplacement for the detection of subsurface lattice defects at a remarkable speed of only 0.26 s per 256 x 256 pixel area. This new technique is promising for use in nondestructive hybrid surface and subsurface inspection and other applications.     

Linearization of the frequency sweep of a frequency-modulated continuous-wave semiconductor laser radar and the resulting ranging performance

The performance of a frequency-modulated continuous-wave (FMCW) semiconductor laser radar has been examined. Frequency modulation (linear chirp) has been studied experimentally in detail. To create a linear frequency sweep, we modified the modulating function according to the measured frequency response of the laser, using an arbitrary function generator. The measurements indicate the possibility of achieving a spectral width of the signal peak that is transform limited rather than limited by the frequency modulation response of the laser, which permits the use of a narrow detection bandwidth. The narrow width results in a relatively high signal-to-noise ratio for low output power and thus also in relatively long-range and high-range accuracy. We have performed measurements of a diffuse target to determine the performance of a test laser radar system. The maximum range, range accuracy, and speed accuracy for a semiconductor laser with an output power of 10 mW and a linewidth of 400 kHz are presented. The influence of the laser's output power and coherence length on the performance of a semiconductor-laser-based FMCW laser radar is discussed.     

Direct observation of transient fluorine atoms with 25-µm wavelength-stabilized diode laser absorption

Through the use of continuous diode laser absorption, detection of transient fluorine atoms with an initial number density in the range of 10(14) cm(-3) has been demonstrated. A crucial part of the continuous-detection technique was laser frequency stabilization with a reference cell of atomic fluorine with Zeeman modulation of the absorption lines to generate a feedback signal. Long-term wavelength stability was demonstrated with second-harmonic phase-sensitive detection of the second-derivative signal for periods up to several hours. For determination of the short-term wavelength stability in the range of microseconds to seconds, a transient signal was generated by photolysis of F(2) with an excimer laser at 308 nm. The initial diode laser absorption was compared to a calculated value obtained from the measured excimer laser fluence, the known dissociation cross section of F(2), and the atomic fluorine absorption cross section, which included a statistical population distribution, the finite bandwidth of the laser dode, and the effects of pressure broadening. The observed absorption was approximately 33% less than the calculated value, possibly because of the diode laser's wavelength instability on the time scale of a few seconds, which is consistent with an observed amplitude instability from pulse to pulse when pulsed at 1-10 Hz.     

Frequency-shifted optical feedback in a pumping laser diode dynamically amplified by a microchip laser

Compared with conventional optical heterodyne detection, laser optical feedback imaging (LOFI) allows for a several orders of magnitude higher intensity modulation contrast. The maximum contrast amplification is typically 10(3) for a diode laser in the gigahertz range and 10(6) for a microchip laser in the megahertz range. To take advantage of the wavelength tunability of a laser diode and of the lower resonant detection frequency of a microchip laser, we used LOFI modulation induced by the frequency-shifted optical feedback in a laser diode as a modulated pumping power for a microchip laser for resonant dynamic amplification. In this way, we were able to transfer the optical feedback sensitivity of the laser diode to the megahertz range. Application to telemetry is also reported.     

Iodine stabilized dye laser system for frequency measurements in the visible and near IR region of the spectrum

An iodine stabilized dye laser system is described that provides traceable measurement of reference frequencies in the visible spectrum from 540 to 670 nm and in the near infrared at 1.15 /spl mu/m. The system allows calibration of the widely used 633 nm, 612 nm, and 543 nm HeNe laser systems. Also, frequency measurements of a polarization stabilized 1153 nm HeNe laser have been performed via frequency doubling and comparison with the dye system operating on the corresponding 576 nm lines. Studies of the shift sensitivities of the system at various wavelengths of interest are described for variation of iodine cell pressure, laser modulation amplitude, and optical saturation power. The dye system was also stabilized to hyperfine components associated with the 6-3 P(33) iodine transition and compared with a 633 nm iodine stabilized HeNe standard.     

Radial modulation of single microbubbles

Radial modulation imaging is a new promising technique to improve contrast-enhanced ultrasound images. The method is based on dual-frequency insonation of contrast agent microbubbles. A low-frequency (LF) pulse is used to modulate the responses of the microbubbles to a high-frequency (HF) imaging pulse. Inverting the LF pulse induces amplitude and phase differences in the HF response of contrast agent microbubbles, which can be detected using Doppler techniques. Although the technique has been successfully implemented, no consensus persists on parameter choice and resulting effects. In a separate study, "compression-only" behavior of coated microbubbles was observed. Compression-only behavior could be beneficial for radial modulation imaging. This was investigated using high-speed camera recordings and simulations. We recorded the vibrations of 78 single microbubbles in a dual-frequency ultrasound field. The results showed that the LF pulse induced significant compression-only behavior, which for microbubble sizes below and at HF resonance resulted in high radial amplitude modulation. It, however, also appeared that, for radial modulation imaging, microbubble size is more important than resonance and compression-only effects.     

Amplitude noise and linewidth reduction of an Ar+ laser

Frequency modulation spectroscopy is used to frequency stabilize an Ar⁺ laser at 515 nm with respect to a commercial passive cavity. Two transducers, a slow and a fast one, are used. The fast transducer is also used to perform amplitude stabilization of a laser beam. Measured amplitude and frequency spectral noise densities are reported. A simplified scheme of the circuitry used to reduce amplitude noise is shown. Noise characteristics of the realized system are adequate to perform spectroscopy of hyperfine-structure transitions of molecular iodine     

Performance analysis of a time-division-multiplexed fiber-optic gas-sensor array by wavelength modulation of a distributed-feedback laser

The results of an investigation of the performance of a time-division-addressed fiber-optic gas-sensor array by means of wavelength modulation of a distributed-feedback (DFB) laser are reported. The system performance is found to be severely limited by the extinction ratio of the optical switch used for pulse amplitude modulation. Formulas that relate the cross-talk level to the extinction ratio of the switch, the modulation parameters of the DFB laser, and the optical path differences among sensing channels are derived. Computer simulation shows that an array of 20 methane gas sensors with a detection sensitivity of 2000 parts in 10(6) (ppm) (10-cm gas cell) for each sensor may be realized with a commercially available single Mach-Zehnder amplitude modulator (-35-dB extinction ratio). An array of 100 sensors with a 100-ppm detection sensitivity for each sensor may be realized if a double Mach-Zehnder amplitude modulator is used.     

Iodine-stabilized extended cavity diode laser at λ=633 nm

An extended cavity diode laser at 633 nm has been frequency stabilized to I₂-Doppler-free absorption signals of the P(33)6-3 transition using a third-harmonic detection technique. The frequency was measured by the beat-frequency technique with an iodine-stabilized He-Ne laser as reference. A minimum value for the two-probe relative standard uncertainty of 1× 10^-11 (5 kHz) is reached after 100 s. We also report measurements of the hyperfine splittings of the P(33)6-3 transition and laser frequency dependence on modulation amplitude and iodine pressure     

Fast, all-optical Rayleigh wave microscope: imaging on isotropic and anisotropic materials

A fast, non-contact Rayleigh wave scanning microscope is demonstrated, which is capable of scan rates of up to a maximum of 1000 measurements/s with typical speeds of up to 250 measurements/s on real samples. The system uses a mode-locked, Q-switched Nd:YAG laser operating at a mode-locked frequency of 82 MHz and a Q-switch frequency of 1 kHz. The Q-switch frequency determines the upper limit of the scanning rate. The generating laser illumination is delivered and controlled by a computer-generated hologram (CGH). The generating laser produces around 30 pulses at 82 MHz and additional harmonics at 164 and 246 MHz and above. The microscope can operate at these harmonics provided the spatial bandwidth of the optics and the temporal bandwidth of the electronics are suitable. The ultrasound is detected with a specialized knife-edge detector. The microscope has been developed for imaging on isotropic materials. Despite this, the system can be used on anisotropic materials, but imaging and interpreting images can be difficult. The anisotropy and grain structure of the material can distort the Rayleigh wavefront, leading to signal loss. A model has been developed to simulate polycrystalline-anisotropic materials; this is discussed along with possible solutions that would overcome the problems associated with anisotropy. Rayleigh wave amplitude images are demonstrated on silicon nitride at 82 and 164 MHz and on polycrystalline aluminium at 82 MHz.     

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.     

Building blocks for a two-frequency laser lidar-radar: a preliminary study

A new principle of lidar-radar is theoretically and experimentally investigated. The proposed architecture is based on the use of an rf modulation of the emitted light beam and a direct detection of the backscattered intensity. Use of a radar-processing chain allows one to obtain range and Doppler measurements with the advantages of lidar spatial resolution. We calculate the maximum range of this device, taking into account different possible improvements. In particular, we show that use of a pulsed two-frequency laser and a spatially multimode optical preamplification of the backscattered light leads to calculated ranges larger than 20 km, including the possibility of both range and Doppler measurements. The building blocks of this lidar-radar are tested experimentally: The radar processing of an rf-modulated backscattered cw laser beam is demonstrated at 532 nm, illustrating the Doppler and identification capabilities of the system. In addition, signal-to-noise ratio improvement by optical pre-amplification is demonstrated at 1.06 microm. Finally, a two-frequency passively Q-switched Nd:YAG laser is developed. This laser then permits two-frequency pulses with tunable pulse duration (from 18 to 240 ns) and beat frequency (from 0 to 2.65 GHz) to be obtained.     

Diode laser-frequency stabilization by use of frequency modulation by a vibrating mirror

Modulation of a laser frequency with a negligibly small residual amplitude modulation component has been demonstrated with a piezoelectric transducer-driven mirror. The vibrating-mirror method has been successfully applied to high-resolution spectroscopy of saturated absorption and to laser-frequency stabilization. The bandwidth of the stabilization feedback loop as wide as 15 kHz has been achieved to suppress acoustic noise in ordinary laboratory environment.