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Self-written waveguide (SWW) trajectories fabricated inside a dry photopolymer bulk material, acrylamide/polyvinyl alcohol (AA/PVA), are studied. Their production using both Gaussian and Laguerre-Gauss exposing (writing) light beams, output from optical fibers, is explored. The formation of the primary and secondary eyes is also discussed. Furthermore, the interactions that take place when two counterpropagating beams pass through the photopolymer material (both Gaussian and Laguerre-Gauss) are examined. In all cases experimental and theoretical results are presented. Good agreement between the predictions of the proposed model and experimental observations are demonstrated.

We have developed a broadly tunable diode laser system by employing custom-designed asymmetric multiple-quantum-well (AMQW) InGaAsP lasers in an external cavity configuration. Feedback is provided by a diffractive optical element with high coupling efficiency and wavelength selectivity, allowing for single-mode tuning of the output wavelength by varying the external cavity length. This tunable laser system was used experimentally to perform absorption spectroscopy on weak CO2 lines over a broad wavelength region in the near infrared. An experimental tuning range of 80 nm has been observed for a laser cavity length of 600 micron, which is double the tuning range found with conventional, uncoated quantum-well lasers. We achieved a detection sensitivity of 5 x 10(-6) at 95% confidence over the wavelength range of 1.54-1.62 micron by employing a second-harmonic detection technique. The theoretical predictions of a broad gain profile from an ambipolar rate equation model are found to correspond to the experimentally observed increased tunability of the uncoated AMQW lasers.

A liquid detection system consisting of a diode laser with multiple short external cavities (MSXC's) is reported. The MSXC diode laser operates single mode on one of 18 distinct modes that span a range of 72 nm. We selected the modes by setting the length of one of the external cavities using a piezoelectric positioner. One can measure the transmission through cells by modulating the injection current at audio frequencies and using phase-sensitive detection to reject the ambient light and reduce 1/f noise. A method to determine regions of single-mode operation by the rms of the output of the laser is described. The transmission data were processed by multivariate calibration techniques, i.e., partial least squares and principal component regression. Water concentration in acetone was used to demonstrate the performance of the system. A correlation coefficient of R(2) = 0.997 and 0.29% root-mean-square error of prediction are found for water concentration over the range of 2-19%.

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.

A technique to map the spectral uniformity of luminescent semiconducting materials at room temperature is described. This technique is based on spatially resolved and polarization-resolved measurements of the photoluminescence and requires a polarizing beam splitter with a splitting ratio that has a linear dependence on wavelength. Measurements on a quantum-well sample that was patterned by intermixing with a focused ion beam are used to demonstrate the technique. With a spectral resolution of better than 1 nm and a spatial resolution of ≊1 µm, as well as the ability to map concurrently the strain field through the measurement of the degree of polarization of the photoluminescence and the photoluminescence yield, this technique provides a simple, nondestructive method of assessing luminescent materials.

Interference fringes in the far field of short-external-cavity InGaAsP diode lasers are discussed and explained. The interference occurs between light emitted from the front laser facet and light emitted from the rear laser facet that is reflected forward by the external mirror. The change in the relative phase of light emitted from each facet caused by a change in the longitudinal mode results in a shift of position of a portion of the fringes in the far field. This shift can be detected and processed to produce a discrimination signal that is used in a feedback control system to detect or prevent longitudinal mode hops. The control system has a response time as short as 15 ms and can prevent mode hops caused by drift, mechanical shock, or laser tuning; the technique permitted complete frequency tuning over the range of operation of the laser. The behavior of the control system was analyzed with the help of a model that can be used to optimize the control system.

A correlation between the spectral output of 1.3 microm InGaAsP semiconductor diode lasers and the distribution along the length and width of the active region of strain and scattering centers is reported. The strain and scattering center distributions in the active region were obtained by measuring and analyzing the spatially resolved and polarization resolved electroluminescence along the active region of the lasers. Measurements were made on gain-guided, planar buried heterostructure, and arrowhead buried crescent lasers. The results suggest that the material properties of the laser structure affect the longitudinal mode spectrum.

Second harmonic detection at audio frequencies of isolated absorption lines with a laser absorption spectrometer employing AlGaAs and InGaAsP diode lasers, short external cavity (SXC) AlGaAs and InGaAsP diode lasers, and InGaAsP distributed feedback (DFB) diode lasers was investigated and compared. Noise levels equivalent to line center absorptions of ~3 x 10(-6) were achieved with each source. Single mode tuning ranges of 20-50 cm(-1) were obtained with both the SXC and DFB sources. The contamination of absorption spectra by suppressed laser side modes was identified and investigated as it relates to the identification of weak lines in the presence of strong lines.

High sensitivity detection of O(2) in the near infrared with short external cavity (SXC) AlGaAs semiconductor diode lasers is reported. The mode control provided by the SXC enhances the performance of these diode lasers by allowing a number of single laser modes (up to ten) to be individually selected and continuously scanned over extended frequency intervals. At a constant laser heat sink temperature, this provides nearly complete spectral coverage of up to approximately 40 cm(-1) for overlapping modes. When using second harmonic detection techniques, these SXC controlled lasers were found to provide high sensitivity detection of O(2) at atmospheric pressure. A minimum detectable absorbance of approximately 4 x 10(-6) has been achieved with a SNR of 1. The stability of these laser systems has also allowed the continuous monitoring of an absorption signal over extended time intervals. Absorbances of 1 x 10(-2) have been continuously monitored for durations of up to 15 h with an rms uncertainty of +/-6 x 10(-5). These results are reproducible for measurements made on all the laser modes that could be selected with the SXC.

The detection of small absorptions using a laser absorption spectrometer (LAS) based on InGaAsP diode laser transmitter modules has been investigated. The modules are normally employed in optical communication systems and as such operate at 1.3 microm and have single-mode-fibor pigtails to couple the light out. The minimum detectable absorption of the LAS wasfound tobe ~5 x 10(-5) with harmonic detection and ~1 x 10(-4) with sweep integration. The dominant noise source was caused by reflections off the cleaved end of the fiber pigtail. The strength and number of absorption lines in the 0.7-1.6-microm spectral region which are free from interference is considered for the major constituents of the atmosphere. It is found that there arc sufficient strong isolated lines for trace gas detection and monitoring purposes usinga LAS based on InGaAsP lasers and the reported minimum detectable absorption.

A technique for obtaining spatially and polarization resolved maps of the electroluminescence along the length of 1.3-microm InGaAsP semiconductor diode lasers is described. The technique yields valuable information on the mechanical strain and uniformity of material in the active region. Measurements on thirty lasers of two different types-planar buried heterostructure (PBH) and gain guided-are summarized. This technique provides a means of correlating operating characteristics of the lasers with the physical properties of the laser material. Large differences between PBH and gain guided lasers are found. PBH exhibit typically a larger average value of strain and more scattering centers than gain guided lasers. A laser with a bent active region exhibited spectral bistability with respect to temperature. The strain distribution in the active region was observed to change during the first few hours of high temperature aging.

Detection of CO(2) and CO at 1.58 microm (6322 cm(-1)) using an InGaAsP diode laser and mode control is described.Mode control is a technique whereby a short cavity, external to the laser, is used to force the laser to operate in a single mode. By monitoring the voltage across the terminals of the laser and using electronic feedback it is possible to optimize continually the external cavity so that the laser operates reliably in the mode of choice for scans >3.5 cm(-1). This technique offers the possibility of high-sensitivity detection over a region of ~30 cm(-1) with continuous coverage in overlapping sections of ~3 cm(-1) with conventional (and hence inexpensive) multimode diode lasers.

The side-mode suppression ratio (SMSR) and single-mode continuous tuning range were measured for planar short-external-cavity semiconductor diode lasers to examine the positional and angular alignment tolerances of the external cavity mirror relative to the laser facet. For inverted rib waveguide lasers, the SMSR was found to be