Douglas M. Bruce

Detection of oxygen using short external cavity GaAs semiconductor diode lasers

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.

Short‐external‐cavity module for enhanced single‐mode tuning of InGaAsP and AlGaAs semiconductor diode lasers

The construction and single‐mode tuning characteristics of a short‐external‐cavity (SXC) module for InGaAsP and AlGaAs semiconductor diode lasers are presented. The SXC system is a cost effective, simple, and rugged means to achieve single‐mode tuning and complete spectral coverage over large spectral regions with normally multimode lasers. With AlGaAs lasers, complete spectral coverage in overlapping ∼5‐cm−1 intervals over ∼400 cm−1 is obtained, while with InGaAsP diode lasers, complete spectral coverage in overlapping ∼5‐cm−1 intervals of 100–200 cm−1 is obtained.

Imaging the strain fields resulting from laser micromachining of semiconductors

The residual strain fields resulting from laser micromachining of grooves in indium phosphide with femtosecond and nanosecond light pulses are analyzed using a spatially resolved degree-of-polarization photoluminescence technique. Significant differences in the geometry of the strain patterns are observed in grooves machined by femtosecond and nanosecond pulses. For the specific conditions investigated, the sign of the degree of polarization signal is opposite in the two cases indicating that areas under tension in femtosecond machined samples are under compression in nanosecond machined samples and visa versa. The experimental data are compared with results from a finite element model.

Strain mapping by measurement of the degree of polarization of photoluminescence.

A technique is described for the simultaneous measurement of the difference in the normal components of strain and of the shear strain in luminescent III-V material from the degree of polarization (DOP) of photoluminescence. This technique for the measurement of shear strain and of the difference in the normal components of strain in InP was calibrated by applying known external loads on the bars of InP with V grooves etched into the bars and by fitting the experimental results to two-dimensional finite-element simulations. Fits to the difference in the normal components of strain (as opposed to stress) yielded significantly smaller residues. On this basis we conclude that the DOP of luminescence is proportional to the difference in the normal components of strain.

Facet phases and sub-threshold spectra of DFB lasers: spectral extraction, features, explanations and verification

The sub-threshold spectra of distributed feedback (DFB) lasers are heavily influenced by the phase of the internal grating with respect to the end facets. In this paper, we document features commonly observed in sub-threshold spectra and explain these features as manifestations of the facet phases. We extract estimates of facet phases by fitting a probability-amplitude transfer-matrix model to spectra from six truncated-well DFB lasers, and use the probability-amplitude model to document, isolate, and explain the sub-threshold spectral dependence on facet phase. To verify the accuracy of the approach that we have taken, we compare estimates of the facet phases from the fits to independent measurements of the facet phases using a scanning photoluminescence method. The results from the two methods are compared and are found to be in agreement. The agreement validates our use of the probability-amplitude model in this paper to explain laser facet phase phenomena.

Gain characteristics of asymmetric multiple quantum-well lasers

The spectra and gain characteristics were investigated for an asymmetric multiple quantum-well (AMQW) laser with two wide and two narrow QWs. It is reported that the laser initially lased on a long wavelength corresponding to the lowest energy transition of the wide well then switched to a short wavelength that was red-shifted more than 50 nm off the lowest energy transition of the narrow well. Correspondingly, the gain spectrum is much narrower than predicted by a free carrier gain theory. This large red wavelength shift or narrowing of the gain spectrum could not be explained by the present gain theories, while it could be explained satisfactorily by the measured gain spectra of QW lasers. This indicates that an accurate gain theory is required to predict the performance of AMQW lasers.

Thin‐film induced stress in GaAs ridge‐waveguide structures integrated with sputter‐deposited ZnO films

ZnO films were deposited on a GaAs ridge structure using radio‐frequency (rf)‐magnetron sputtering. A SiO2 thin buffer layer was introduced to alleviate a thermal mismatching problem between the ZnO film and the GaAs substrate. Deposition parameters such as rf power, distance, and gas composition/pressure were optimized to obtain highly c‐axis oriented and highly resistive ZnO films. Postdeposition anneal treatment at 430 °C for 5–10 min was found to enhance c‐axis orientation of the ZnO films dramatically and to reduce intrinsic stress significantly. Stress on the cleaved facet of the waveguide was imaged with a spatially resolved and polarization‐resolved photoluminescence technique. The results showed that the GaAs mesa is stressed up to 1×109 dyn/cm2 (10−3 strain) due to residual stress from the ZnO/SiO2/GaAs structure.

Characterization and modelling of the strain fields associated with InGaAs layers on V-grooved InP substrates

A series of lattice-matched or compressively strained InGaAs layers were grown by gas source molecular beam epitaxy on V grooved InP substrates, with grooves bounded by {211}A or {111}B facets. The stress field associated with the layers was measured by the degree of polarization technique, and compared to the predictions of analytical or finite element models. Good agreement was found for the {211}A V grooves, but both the nominally lattice-matched and compressively strained layers grown on {111}B  V grooves displayed similar degree of polarization maps. Analytical electron microscopy demonstrated that the {211}A  V-groove samples had the targeted composition, but the {111}B  samples showed much higher In/Ga ratios at the bottom of the groove than the expected values. Indium enrichment at the bottom of the groove led to defect formation there, and left the V groove of both the lattice-matched and compressively strained samples under a net compressive force.

Waveguide−detector couplers for integrated optics and monolithic optoelectronic switching arrays

An integrated waveguide-detector coupler (IWDC), in which a controlled fraction of the optical power in a waveguide is selectively coupled to a detector element, has been investigated. The device structure consists of a rib waveguide formed from a sputtered Corning 7059 glass film on an oxidized silicon substrate. A photoconductive detector is fabricated on the same substrate, and the degree of coupling is controlled by tapering the SiO(2) cladding layer thickness in the region between the electrodes and by varying the interaction length. Couplers with cladding layer thicknesses ranging from 0.15 to 0.80 microm in the detector interaction region were measured to have coupling values from 400 to 1500 dB/cm for TE modes and to 5800 dB/cm for TM modes, in good agreement with theory. The first integrated optoelectronic 2 x 2 switching matrix using IWDCs as switching cross-points has been demonstrated. We have shown that the passive power splitting in the integrated switch is nearly the ideal 50%.

Application of polarization resolved photoluminescence to the study of quantum well intermixing in InGaAsP systems

A room-temperature spatially resolved and polarization resolved photoluminescence (PL) technique is demonstrated to be a useful tool for studying the intermixing process in InGaAsP multiple-quantum-well systems disordered by focused ion beam implantation. This technique produces maps of the spectral uniformity, degree of polarization, and PL yield at room temperature. Information on spectral shift (i.e., bandgap change) and anisotropy in the implanted region can be obtained from these maps. A significant enhancement in PL yield (up to 40%) for Si+ implanted line patterns was observed.