Tran Quoc Tien

Observation of deep level defects within the waveguide of red-emitting high-power diode lasers

The waveguides of 650nm emitting high-power laser diodes are analyzed regarding the presence of deep level defects by photoelectrical techniques, namely, photocurrent spectroscopy, laser beam induced current, and near-field optical beam induced current (NOBIC). Deep level configurations in pristine devices and the kinetics of defect creation during device operation are monitored and discussed. The localization of the defects within the epitaxial layer sequence is done by NOBIC. We show that light, which is confined within the laser waveguide, interacts with the deep level defects detected. This demonstrates that the presence of deep level defects directly affects the device properties.

Thermal properties and degradation behavior of red-emitting high-power diode lasers

The thermal properties and the degradation behavior of high-power broad-area diode lasers emitting at 650nm are analyzed. Imaging thermography is applied to assess the bulk temperature while the facet temperature is measured by micro-Raman spectroscopy. Although no visible facet alteration is observed, power degradation is found to be accompanied by increased temperatures at the facets. The immediate vicinity of them also turns out to be the starting point for the creation of defect networks within the quantum well seen in cathodoluminescence images. The observed behavior is compared to that known for near-infrared emitting devices.

Microexternal cavity tapered lasers at 670 nm with 5 W peak power and nearly diffraction-limited beam quality

Wavelength-stabilized compact laser systems at 670 nm on a micro-optical bench are presented. The resonator concept consists of a tapered semiconductor gain medium and a reflection Bragg grating as a wavelength selective resonator mirror. In pulse operation mode with 100 ns pulses, an optical peak power of 5 W with a spectral width below 150 pm was achieved. Nearly diffraction-limited beam quality at optical output powers up to 1 W is obtained. Such laser systems can be used, e.g., for Raman spectroscopy and as pumping sources for frequency conversion toward UV spectral range.

Mode transitions in distributed Bragg reflector semiconductor lasers: experiments, simulations and analysis

The performance of a multisection distributed Bragg reflector (DBR) semiconductor laser emitting around 1.06 µm is experimentally and theoretically investigated. A thermal-induced change of the refractive index implied by an increase in the injection current yields nearly periodic transitions between neighbouring cavity modes. These transitions are explained by means of a modal analysis and by numerical simulations based on the travelling wave model.

Mechanical strain and defect distributions in GaAs-based diode lasers monitored during operation

We monitor the mechanical strain and the defect concentration in AlGaAs–GaAs-based high-power diode laser arrays. This allows studying the interplay between these extrinsic parameters in dependence on device operation. There are two parameters, which contribute to the spread of the mechanical strain, the local position at the device, and, the device operation time that substantially enhances the strain. For midgap levels as well as shallower defect levels, which are due to physically different defects, very different creation scenarios are observed. The concentration of shallow defects and band-tail states is strongly correlated with compressive strain in their vicinity, no matter how the strain is created. For midgap levels, there is no direct correlation; however, an increase by a factor of 3 after 1500h of operation time is observed. The knowledge on defect creation scenarios is extensible to other GaAs-based devices.

Spectroscopic strain measurement methodology: Degree-of-polarization photoluminescence versus photocurrent spectroscopy

A methodological approach to strain analysis in semiconductor devices is presented. Two methods, degree-of-polarization of photoluminescence and photocurrent spectroscopy, are compared by analyzing a spatially inhomogeneous strained test sample, namely, a high-power diode laser array that is affected by packaging-induced stress. Both methods concordantly reveal a −0.1% uniaxial compression in the vicinity of the midpoint of the active region of the device, demonstrating the compatibility of and justifying the assumptions involved in the two different approaches. Furthermore, we discuss some distinctive details of the processing-induced strains observed in the vicinities of metallized contacts and grooves involved in the device design.

Gradual degradation of red-emitting high-power diode laser bars

The authors analyze early stages of gradual degradation in highly reliable 650nm emitting high-power diode laser arrays with continuous wave emission powers of 2.5W (facet load of 4mW∕μm). In all cases the edges of the metallized emitter stripes are identified as the starting points of gradual degradation. The magnitude of the observed degradation signatures, however, is highly correlated with the bar-specific packaging-induced strain at each emitter. We find a bar-specific effect, namely, the presence of packaging-induced strain, to be the driving force of gradual degradation. Our findings point to the significance of proper strain management in advanced device structures.

Relaxation of packaging-induced strains in AlGaAs-based high-power diode laser arrays

We monitor the mechanical strain evolution with typical use in the quantum well of AlGaAs∕GaAs-based high-power diode laser arrays (cm bars) by spectroscopic means. We show experimentally that pristine devices are essentially uniaxially compressed along the 110-direction with a strain maximum of −0.16% at the center of the device. At the device edges, almost no packaging-induced strain is detectable. After 500h of continuous wave operation at a current of I=80A, the strain is reduced by 50%. Furthermore, we observe the growth of a localized region of compressive strain, of hydrostatic symmetry, in one emitter of a particular cm-bar. A compression of about −0.017% is observed, and is most likely caused by point defect accumulation. Our results demonstrate information about absolute strain values and, at least in part, about strain symmetry as well can be obtained by spectroscopic means even within packaged complex optoelectronic devices.

Spatially resolved and temperature dependent thermal tuning rates of high-power diode laser arrays

Thermal tuning properties of passively cooled 808nm emitting high-power diode laser bars are analyzed. Data from standard devices packaged on Cu heat sinks and identical devices mounted on expansion-matched Cu–W heat sinks are compared. For a standard device, we find up to one-fifth of the thermal tuning rate of −(0.56±0.04)meVK−1 to be caused by pressure tuning driven by the relaxation of compressive packaging-induced stress for increasing temperatures. For devices packaged on expansion-matched heat sinks the observed tuning rate of −(0.46±0.01)meVK−1 represents almost the genuine thermal tuning rate of the semiconductor device structure. Thus this technology potentially leads to improved device properties.

Screening of high-power diode laser bars by optical scanning

High-power diode laser bar arrays (808 nm) with very uniform emission properties are inspected by the laser-beam-induced current technique (LBIC). Setting the excitation energy to 50 meV below the lasing energy, we observe distinctive signatures within the LBIC scans at certain locations on the devices. After 1000 h of high-power operation, we observe degradation at exactly those positions that previously showed a characteristic LBIC signature. A cathodoluminescence analysis reveals that the quantum wells of these anomalous device sections suffer from the existence of spots with reduced luminescence efficiency. Additionally, a concomitant 2.3 meV redshift of the quantum-well cathodoluminescence spectrum is also observed. Our measurements demonstrate the efficiency of the LBIC approach as a screening tool as well as its capacity for predicting device failure well before any degradation of the emission properties is observed.