Semiconductor lasers are the most efficient man-made narrow-band light sources and convert up to three-quarters of electric energy into light High-power diode lasers are characterized by very high internal power densities in their small cavity, resulting in local heating and sometimes device degradation Catastrophic optical damage (COD) of diode lasers is a relevant degradation mechanism and limit for reaching ultra-high optical powers An overview is given on research activities targeting the mechanisms being relevant for the COD process in GaAs-based diode lasers emitting in the 630―1100 nm range The discussion of experiments, where COD is artificially provoked, represents the main topic The sequence of events and fast kinetics taking place on a nanosecond to microsecond time scale are addressed A particular emphasis is laid on recent experimental work performed in the authors' laboratories Paving the way for knowledge-based solutions towards more robust diode lasers represents the ultimate goal of this work COD diagram determined for a batch of broad-area AlGaAs diode lasers The time to COD within a single current pulse is plotted versus the actual average optical power in the moment when the COD takes place Full circles stand for clearly identified COD events (right ordinate), whereas open circles (left ordinate) represent the pulse duration in experiments, where no COD has been detected A borderline (gray) exists between two regions, i e, parameter sets, of presence (orange) and absence of COD (blue) This borderline is somewhat blurred because of the randomness in filamentation of the laser nearfield and scatter in properties of the involved individual devices

Mechanisms and fast kinetics of the catastrophic optical damage (COD) in GaAs-based diode lasers

The transient thermal properties of high-power diode laser bars with active and passive cooling are analyzed. Both thermal imaging and the analysis of the thermal wavelength tuning behavior are employed to extract the device temperature as a function of time. A steady-state thermal situation is established with rise times of about 10 and 60ms for active and passive cooling, respectively. The latter number, however, is substantially increased by the particular properties of the external heat sink. Such results are confirmed by model calculations based on the finite element method.

Transient thermal properties of high-power diode laser bars

This review focuses on theoretical foundations, experimental implementation and an overview of experimental results of the thermoreflectance spectroscopy as a powerful technique for temperature monitoring and analysis of thermal processes in semiconductor lasers. This is an optical, non-contact, high spatial resolution technique providing high temperature resolution and mapping capabilities. Thermoreflectance is a thermometric technique based on measuring of relative change of reflectivity of the surface of laser facet, which provides thermal images useful in hot spot detection and reliability studies. In this paper, principles and experimental implementation of the technique as a thermography tool is discussed. Some exemplary applications of TR to various types of lasers are presented, proving that thermoreflectance technique provides new insight into heat management problems in semiconductor lasers and in particular, that it allows studying thermal degradation processes occurring at laser facets. Additionally, thermal processes and basic mechanisms of degradation of the semiconductor laser are discussed.

https://iopscience.iop.org/article/10.1088/1361-6463/aa9812/pdf

Thermoreflectance spectroscopy—Analysis of thermal processes in semiconductor lasers

Experimental study and numerical modeling of micro-channel cooler with micro-pipes for high-power diode laser arrays

The implementation of infrared imaging for the purpose of thermal analysis of semiconductor lasers is demonstrated. The experimental results obtained in the different spectral bands ranging from near IR (1.5–2 µm) to mid IR (8–13 µm) are presented. The results show the applicability of the method for steady- and transient-state characterization of individual as well as multi-emitter laser structures. Moreover, various aspects of the defect detection are discussed. We demonstrate the detection of defects that are not only located at the front facet of the laser, but also deeper in the cavity. The effects of radiative recombination at defect centres are distinguished from purely thermal effects by transient analysis of infrared images. It is shown that the near IR band (1.5–2 µm) deep-level luminescence is responsible for these effects. This finding is independently proved by complementary measurements of the near IR emission spectrum as well as by characterization of absorption properties of the devices in the below-band edge region. The information from two distinct spectral channels of an infrared imaging system (near IR and mid IR) is used for pre-selection of potentially long-lived devices. It is shown that conventional burn-in tests do not discard all devices which have a limited lifetime. Thus, infrared imaging is considered a potential industrial screening tool with measuring times of fractions of a second only.

https://iopscience.iop.org/article/10.1088/0268-1242/22/8/R01/pdf

Infrared imaging of semiconductor lasers

Broadband imaging thermography is applied to investigate the infrared (IR) emission from high-power diode laser bars. We demonstrate the capabilities of a multispectral imaging system that operates in two optical channels, the near IR at 1.5–2μm and the mid IR at 2.4–5.5μm. In the near-IR region, deep level luminescence of the device contributes significantly to the thermo-images. A complementary measurement of the IR emission spectrum shows a broad defect band with maxima at E1=0.94eV and E2=1.05eV. Analysis of IR transients in both spectral channels makes a distinction of thermal radiation and deep level emission possible. In the mid IR, thermal radiation dominates, allowing for an analysis of thermal properties of the device with measurement times of fractions of a second only. This makes imaging thermography highly attractive for device screening.

Deep level emission from high-power diode laser bars detected by multispectral infrared imaging

Thermal properties of 808 nm emitting high-power diode lasers are investigated by means of micro-thermography. A thermo-camera equipped with a 384x288 pixel HgCdTe-detector (cut off wavelength at 5.5 micron) and IR-micro-objective is used, which allows for thermal imaging with a spatial resolution of 5 μm. A novel methodological approach for data re-calibration for absolute temperature measurements is proposed. We present steady-state thermal distributions from broad-area devices. The remarkable agreement of this data with the results of modeling work has been reached. Cross-calibration of the micro-thermographic results is obtained by complementary micro-Raman data that give information about facet temperatures with a spatial resolution of about 1 micron. Transient thermal properties are monitored with a temporal resolution of 1.4 ms. Such thermal transients illustrate the heat flow trough the device after turning on the operation current. Special experiments are done in order to detect and localize hot spots at the facet and within the devices. Moreover, we show that the analysis of thermal images can be used as a recognition method of defects hidden inside the cavity, even if they are not detectable by visual inspection. These activities are paving the way towards a novel screening methodology.

Thermal properties of high-power diode lasers investigated by microthermography

Multispectral infrared (IR) imaging is used for reliability screening of diode lasers. The signal detected by the camera in the near IR region is found to be mainly affected by midgap deep level luminescence, whereas the thermal radiation is the main signal contribution in the mid IR. The information from both spectral channels is used for preselection of potentially long-lived devices. The devices with enhanced luminescence from deep levels and elevated temperature of the active region are the candidates for faster degradation. Defects such as hot spots can be discarded extremely quickly by using thermographic inspection. The thermographic results are complemented with spectral Fourier transform photocurrent measurements and low-current I-V characterization. The results of aging experiments of diode lasers are presented. A correlation between measurements of unaged devices and lifetest results is found and discussed in detail. The proposed preselection methodology is an alternative to conventional burn-i...

Reliability screening of diode lasers by multispectral infrared imaging

We demonstrate the applicability of imaging thermography for investigations of mechanisms associated with gradual degradation in diode lasers. The introduction of two spectral channels provides the means for separate observation of deep level luminescence and thermal radiation emitted according to Planck's law. In the near IR region we found the signal detected by the camera to be mainly affected by mid-gap deep-level luminescence. An intensity increase of the luminescence signal for an aged diode laser compared to an unaged device is noticed. It can be explained by an increase of deep level defect concentration during the aging. In the mid IR, we mainly encounter thermal radiation, which can be used for the analysis of the thermal properties of devices. In present work the thermal behavior of the device subjected to an aging of 3000 hours is analyzed. A significant increase of device temperature is noticed.

Piotr Wawrzyniak

Papers

Deep level emission from high-power diode laser bars detected by multispectral infrared imaging

Thermal properties of high-power diode lasers investigated by microthermography

Reliability screening of diode lasers by multispectral infrared imaging

Studies of the degradation mechanisms in high-power diode lasers using multi-channel micro-thermography