Model of a multiple-line distributed-feedback dye laser

The theoretical basis for simultaneous oscillation of 2N - 3 laser lines is due to interference of N (for all even N > or = 2) pump beams in a distributed-feedback dye laser is described. Multiple gratings are produced in a dye solution by interference patterns of N/2 pairs of a frequency-doubled Nd:YAG laser. N/2 pairs of mutually time-delayed pulses induce multiple gratings of different periodicities, of which 2N - 3 gratings support oscillation of 2N - 3 lines and the remaining gratings, because of their larger periods, cannot support Bragg scattering. The maximum number of laser lines depends on the mutual delay between adjacent pairs of beams, coherence, states of polarization, pulse lengths, and of course the number of pulses. For three pairs of excitation beams derived from the same source through wave-front or amplitude phase division techniques, the output lasing lines varied from a minimum of three to a maximum of nine. This research was carried out by pumping of a dye solution with two, four, and six pulses, but the principle may be extended to multiple output lines, depending on the number of pump pulses and on the gain of the dye solution.     

Threshold analysis of a chirped-grating distributed-feedback laser with the power series method

A new and efficient method for analyzing a chirped-grating distributed-feedback (DFB) laser is presented. We show that coupled-wave equations can be solved by use of the power series method. The single-mode gain margin of a linearly chirped-grating DFB laser is calculated for different chirping factors and coupling constants. We found that clearly optimum chirping exists for which the single-mode gain margin is maximum. The gain margins were also calculated for different positions of the cavity center. The effect of facet reflectivities and their phases on the gain margin was investigated. We found that the gain margin is maximum and the spatial hole burning is minimum for the cavity center at the middle of the laser cavity.     

Investigation of interferometric noise in fiber-optic gas sensors with use of wavelength modulation spectroscopy

We report on interferometric noise limitation of fiber-optic gas sensors with highly coherent lasers and wavelength modulation spectroscopy. Interference between signal wave and reflected waves causes signal fluctuation in the output, which limits the performance of the sensing system. Sensor resolution limited by interferometric noise is calculated for a fiber-optic gas sensor with the Q(6) absorption line of methane gas at approximately 1650 nm. The results are useful for system designers of this particular type of gas sensor.     

Analysis of a dephased complex-coupled distributed-feedback laser by the power-series method

A novel power-series method to solve the coupled-wave equations is introduced. The method is used to calculate the threshold gain margins of a complex-coupled distributed-feedback laser as functions of the ratio of gain coupling to index coupling (|kappa(g)|/|kappa(n)|) and of the phase difference between the index and the gain gratings. For coupling coefficient |kappa|l < ., the laser shows a mode degeneracy at specific values of the ratio |kappa(g)|/|kappa(n)| for cleaved facets. At phase differences pi/2 and 3pi/2 between the gain and the index gratings, an antireflection-coated complex-coupled laser becomes multimode, and a different mode starts to lase. The effect of facet reflectivity (both magnitude and phase) on the gain margin of a complex-coupled DFB laser is also investigated. Although the gain margin varies slowly with the magnitude of the facet's reflectivity, it shows large variations as a function of the phase. Spatial hole burning was found to be minimum at phase difference npi, n =, ..., and maximum at phase differences pi/2 and 3pi/2.     

Multianalyte single-cell analysis with multiple cell lines using a fiber-optic array

A single-cell drug screening method is described that produces rich single-cell data and discriminates between single-cell responses from clonal populations stimulated with different agonists. Ligand-induced receptor activation is commonly detected by observing intracellular Ca2+ oscillations using high-throughput screening (HTS) methods. In most cases, HTS results in an average signal from several cells and is not sensitive enough to enable the identification of population outliers or population variance. In order to obtain this information, many individual cells must be analyzed simultaneously. We have developed a novel system using a specialized fiber-optic platform and have combined it with statistical analysis, to simultaneously analyze the dynamics of Ca2+ oscillations in a large number of single cells. Mammalian cells ectopically expressing different human GPCR receptors were stimulated, and Ca2+ changes in numerous single cells were recorded over time using a fluorescent microscope and a CCD camera. We determined the percentage of live cells in a population responding to stimuli, the distribution of responses within a population of clonal cells, and the number of outliers. By employing principal component analysis and K-nearest neighbor modeling, we classified the time-resolved Ca2+ traces of single cells as a function of the stimulus type with high certainty for a population of cells. This method is potentially a powerful tool for identifying new drug targets or for investigating the single-cell behavior of an existing target or known receptor. The development of this single-cell drug screening method is presented, and fluorescent and statistical analyses of single-cell dynamic responses are discussed.     

Autodyne interferometry for range-finding under laser radiation wavelength modulation

The results of solution of the inverse problem of determining the distance to the reflector in the case of current modulation of the laser radiation wavelength are presented. It is shown that current modulation of the autodyne signal amplitude can reduce the inverse problem of finding the distance to the reflector to a state of affairs characteristic only of phase modulation. The technique that we propose provides a higher range-finding accuracy than does direct analysis of the autodyne signal.     

Experimental investigation of an optically amplified time-division-multiplexed polarization-insensitive fiber-optic michelson interferometric sensor system

We experimentally investigate optically amplified time-division-multiplexed polarization-insensitive fiber-optic Michelson interferometric (PIFOMI) sensor systems, using an erbium-doped fiber amplifier (EDFA) and a phase-generated carrier (PGC) demodulation technique. The influence of the EDFA on the extinction ratio (ER) of the light pulse and on the minimum phase-detection sensitivity (MPDS) is examined. We find that the EDFA acting as a preamplifier has limited usefulness because the highly amplified spontaneous emission (ASE) noise generated by the EDFA degrades the ER and the MPDS. However, both postamplifiers and in-line EDFA's can work successfully. The MPDS of the unamplified time-division-multiplexed PIFOMI system with an ER of 33 dB was 2.4 x 10(-5) rad/(Hz)(1/2) at ~1 kHz. For maintaining a MPDS of better than 3.4 x 10(-5) rad/(Hz)(1/2) at ~1 kHz, the worst ER's for the postamplified and in-line amplified systems were 20 and 17.8 dB, respectively. The corresponding input signal peak power should be larger than -20 and -25 dBm for the postamplifiers and in-line amplifiers, respectively. When two postamplifiers and two in-line amplifiers are used, an allowable sensor system loss of 47 dB and a link length of the input-output lead fiber of 108 km can be realized for this system with a 32-sensor array. Implementation of optically amplified time-division-multiplexed and wavelength-division multiplexed-time-division multiplexed PIFOMI subarray sensor systems are also addressed.     

Optical frequency-domain ranging using a frequency-shifted feedback distributed-feedback laser

A distributed-feedback (DFB) laser is used in a frequency-shifted external cavity configuration to generate a frequency comb. The frequency comb generates a beat frequency in a Michelson interferometer, which is proportional to the path difference of a target and reference arm. The calculated and actual readings for the beat frequency are in good agreement. A proof of principle demonstration of using a DFB laser for optical frequency-domain ranging is shown for the first time.     

Particle-size analysis by laser diffraction with a complementary metal-oxide semiconductor pixel array

Existing laser-diffraction instruments that use photodiode detectors have a limited resolution for particle sizing. We attempt the implementation of a complementary metal-oxide semiconductor pixel sensor for particle-size measurement by laser diffraction. The sensor has unique features: high resolution, no blooming, and a wide dynamic range (i.e., direct measurement of the scattering pattern). The calibration of the sensor is based on each pixel. The signal-processing and the inversion schemes for obtaining the particle-size distribution are described. The results indicate an improved size resolution and an increased flexibility of application.     

Compensation of parasitic polarization modulation in a fiber-optic gyroscope

It has been shown that the influence of parasitic polarization modulation on the precision of a fiber-optic gyroscope may be reduced substantially by incorporating reflection-type phase modulators with conversion of polarization modes. Pis’ma Zh. Tekh. Fiz. 23, 90–94 (August 12, 1997)     

Fabrication of a distributed-feedback dye laser with a grating structure in its plastic waveguide

Two approaches of fabricating grating structures for waveguided plastic dye lasers are described and compared for lasing performance. Rhodamine6G-doped poly(methyl methacrylate) (PMMA) film on a PMMA substrate was used for the waveguide, and a distributed-feedback (DFB) laser operation with a single-propagation mode was demonstrated. The performances of both types of permanent grating structured DFB dye laser were better than those of a DFB dye laser on a plain waveguide with a dynamic grating formed by the interference of two pump beams. Wide tuning range is expected by use of a multistripe DFB laser with different grating pitches.     

Low-frequency wavelength modulation spectroscopy with D2 transition of atomic cesium by use of an external-cavity diode laser

Low-frequency wavelength modulation spectroscopy is acquired with an external-cavity diode laser. The wavelength modulation is achieved with voltage tuning by means of scanning with the piezoelectric stepper motor, which rotates the end mirror in the laser cavity. With optimum 1-kHz frequency modulation and harmonic detection, direct absorption experiments for the 6S(1/2)(F = 4) --> 6P(3/2) transition of the cesium D(2) line were carried out. We found that 6f-harmonic detection is best here with a signal-to-noise voltage ratio of 460.     

Improving fiber-optic laser beam delivery by incorporating GRADIUM optics

The performance of a fiber-optic laser beam delivery system strongly depends on the fiber and the optics used to image the fiber face on the workpiece. We have compared off-the-shelf homogenous (BK7) and GRADIUM (axial-gradient) singlets to determine what improvement the GRADIUM offers in practice to the typical laser user. The realized benefit for this application, although significant, is much smaller than would be realized by a conventional imaging application. The figure of merit for laser-based materials processing is the 86% energy-enclosure radius, which is not directly supported by commercial ray-tracing software. Therefore empirical rules of thumb are presented to understand when GRADIUM (or any other well-corrected optics) will yield meaningful improvements to the beam delivery system.     

Conversion of phase modulation of light into intensity modulation by means of an external fiber-optic interferometer

A new method of detecting phase modulation, based on using an “external” optical waveguide interferometer, is considered. This “detection” is weakly dependent on the length, position, and external conditions of the propagation channel for the phase-modulated light and, in many cases may prove more efficient than conventional methods of constructing phasemodulated fiber-optic systems. Pis’ma Zh. Tekh. Fiz. 23, 9–16 (May 26, 1997)     

High-bandwidth laser frequency stabilization to a fiber-optic delay line

Stabilization of laser frequency to interferometers with a large time delay in one arm is of significant interest to space-based gravitational wave detectors such as the Laser Interferometer Space Antenna. A recently proposed technique allows a control bandwidth larger than the inverse delay time to be achieved. We present experimental results demonstrating laser frequency stabilization to an optical fiber delay line. A control bandwidth approximately 50 times the inverse delay time is demonstrated.     

Measurements of OH radical concentration in combustion environments by wavelength-modulation spectroscopy with a 1.55-microm distributed-feedback diode laser

Wavelength-modulation spectroscopy with a standard commercial 1.55-microm distributed-feedback diode laser was applied to in situ quantitative measurements of OH radical concentration in combustion environments. The second-harmonic (2f) signal was generated from absorption by the P11.5 (nu', nu") = (2, 0) overtone vibrational transition of OH at 6421.354 cm(-1). The absorption occurred in the postflame region of a two-dimensional laminar counterflow burner (Tsuji burner) with a 60-mm line-of-sight path length. The postflame region lies between propane-air premixed twin flames stabilized in the Tsuji burner at various equivalence ratios (phi = 0.65-1.0). The OH concentrations were determined by least-squares fitting of theoretical f line shapes to the experimental counterparts. The measured OH concentrations were in general agreement with adiabatic chemical equilibrium predictions. The lower limit of OH detectivity by multiline deconvolution was limited by ubiquitous unidentified high-temperature H(2)O transitions.