In this paper the possibility of improvement of emitted beam quality of high-power laser diodes by stabilization of an optical field distribution in the junction plane, forced by lateral periodicity built into their wide-stripe waveguide is described. Current flow control by such periodic structure prevents the lateral current crowding, carrier accumulation at the stripe edges and optical far-field blooming typical for conventional wide-stripe laser diodes. As a result, stabilization of the lateral emitted beam divergence in wide drive current range of CW operation has been evidenced.
The quality of the die bonding is critical to the operation and reliability of the laser diodes since it can affect the
electrical, thermal, and optical properties of the device. We investigated the effect of mounting induced strain and
defects on the performance of high power laser. In this paper measurements of the temperature distribution, the
spontaneous emission spectrum and the electroluminescence along the cavity of quantum well lasers are presented. The
electro-optical parameters of the high output power laser diodes, such as emission wavelength, output power, threshold
current, slope efficiency, and operating lifetime are presented too. In the experiment, high power diode lasers emitting in
808 nm and 880 nm- range are investigated. We have observed that defect lines tend to create in areas where temperature
gradients were observed in thermovision measurements.
The quality of the beam emitted by high-power laser diodes is still the main disadvantage of these devices. One of the
ways to improve it is to design diode as a matrix of narrow active stripes – so called: phase-locked arrays. The optical
coupling which is occurs in such devices causes the emission in the form of a few almost diffraction limited beams
(lobes). Unfortunately, because of temperature dependence of refractive indices this coupling often disappears at high
drive currents. In this paper the CW operation (up to 4Ith) of the phase-locked semiconductor laser arrays is reported. The
devices are based on asymmetric heterostructure which is designed for improving thermal and electrical resistances. The
single supermode operation is obtained and the lasers are emitted up to 1 W of the optical power in CW.
The laser diodes (LD) have numerous applications and promise to become key elements for next generation laser
technologies. LD are usually operated under conditions of heavy thermal load. As a result, the devices are affected by
aging processes leading to changes of the operation parameters, degradation and, eventually, complete failure.
Degradation of high power semiconductor lasers remains a serious problem for practical application of these devices. We
investigated the effect of mounting induced strain and defects on the performance of high power laser. In this paper
measurements of the temperature distribution and the electroluminescence along the cavity of InGaAs quantum well
lasers before and after accelerated aging processes are presented. The electro-optical parameters of the high output power
laser diodes, such as emission wavelength, output power, threshold current, slope efficiency, and operating lifetime are
presented too.
Low quality of the optical beam emitted by high-power laser diodes is the main disadvantage of these devices. The two
most important reasons are highly non-Gaussian beam profile with relatively wide divergence in the junction plane and
the filamentation effect. Designing laser diode as an array of narrow, close to each other single-mode waveguides is one
of the solutions to this problem. In such devices called phase locked arrays (PLA) there is no room for filaments
formation. The consequence of optical coupling of many single-mode waveguides is the device emission in the form of
few almost diffraction limited beams. Because of losses in regions between active stripes the PLA devices have,
however, somewhat higher threshold current and lower slope efficiencies compared to wide-stripe devices of similar
geometry. In this work the concept of the high-power laser diode resonator consisted of joined PLA and wide stripe
segments is proposed. Resulting changes of electro-optical characteristics of PLA are discussed. The devices are based
on the asymmetric heterostructure designed for improvement of the catastrophic optical damage threshold as well as
thermal and electrical resistances. Due to reduced distance from the active layer to surface in this heterostructure, better
stability of current (and gain) distribution with changing drive level is expected. This could lead to better stability of
optical field distribution and supermodes control. The beam divergence reduction in the direction perpendicular of the
junction plane has been also achieved.
Ceramic materials are interesting alternative to single crystals for various optoelectronic applications including high-power
lasers and phosphors. Main advantages of ceramics compared to their single crystal counterparts are lower costs
of production, ability to incorporate higher dopant concentrations and possibility to manufacture larger elements. In the
present work, the spectroscopic properties of ceramics obtained by two different methods are compared. First method
relies on solid-state reaction of nanometric oxide powders, i.e. Al2O3, Y2O3 and Nd2O3. The oxides with addition of
tetraethyl orthosilicate were sintered under vacuum and anealled. Second method is the synthesis of neodymium-doped
aluminium garnet (Nd:YAG) nanocrystalline powders prepared by coprecipitation technique. The powders were calcined
and vacuum-sintered in optimized process conditions. For all ceramic samples fluoresce and decay data is presented.
Presented results indicate that the ceramic samples obtained by reactive sintering method have superior spectroscopic
properties compared to the samples synthesized from Nd:YAG nanocrystalline powders. The optimization of
manufacturing process allowed to demonstrate ceramics having the properties comparable to single crystal counterparts.
Optical quality and luminescent properties make the ceramics manufactured at the Institute of Electronic Materials
Technology an interesting candidate for laser applications.
High brightness, high power, semiconductor lasers have many potential applications such as: free space communications,
printing, material processing, pumping etc [1]. Such applications require lasers, which are characterized by reliability
and long lifetime.
Catastrophic optical mirror damage (COMD) process is one of the major mechanisms, which drastically limits laser
lifetime and emitted optical power [2]. Mirror degradation and eventually destruction of lasers is caused by facet heating
due to nonradiative surface recombination of carriers. Facet heating reduces the band gap energy, consequently
increasing the absorption coefficient at the facet. The absorbed light and photo-induced electron-hole pair are increased
by the increase in the absorption coefficient. Both effects lead to further nonradiative recombination of carriers which
induces heating and so on, up to degradation of mirror or even destruction of laser. We see that this effect is very
undesirable and knowledge of the temperature dissipation on the surface is very important for improving
semiconductor lasers design. In this work we present the analysis of temperature distribution at the front facet of the
broad area GaAsP/AlGaAs lasers by means of micro-Thermoreflectance (μTR) Spectroscopy.
Several methods proved to be useful in determining the temperature of the laser surface. These are micro-probe band-to-band
photoluminescence, thermoreflectance spectroscopy and Raman spectroscopy [2, 3, 4, 5]. We have used μTR
because it is contactless, non-destructive technique which enables us to obtain temperature distribution in real time.
The application of multi-spectral microthermography to the monitoring of aging processes in diode lasers is reported.
We have found that an intensity of the luminescence in near IR (1.5 -2 micron range) increases with the operational
time, which tentatively is correlated with increased concentration of point defects. This effect is monitored with a
specially configured thermographic camera. The set-up provides spatially resolved information about the luminescence
that originates from radiative recombination at defect centers as well as the pure thermal emission.
In order to elucidate the role of point defects in the aging process of diode laser complementary spectroscopic
measurements are performed. Photocurrent spectroscopy is used to examine the absorption properties of laser structure
for fresh as well for aged devices. The results of low-current I-V characterization are presented. A correlation between
measurement results obtained using different methods is found, discussed and interpreted in detail.
KEYWORDS: Semiconductor lasers, Heterojunctions, Near field optics, Near field, Waveguides, High power lasers, Quantum wells, Silicon, Spectroscopy, Absorption
An analysis of the mode structure of high-power double-barrier separate confinement DB SCH diode lasers is presented. The devices are characterized by very low vertical beam divergence (13 - 22°, depending on the design version). Modelling of the fundamental mode distribution for three different design versions of DB SCH diode lasers is discussed and the results are compared to a macroscopic characterization of the devices (far-field directional characteristics and photocurrent spectra). Microscopic measurements of the near field distribution of these diode lasers with subwavelength spatial resolution are performed by employing a Near-field Photocurrent (NPC) technique. The mode structure of diode lasers is directly visualized giving indications about the interplay between the heterostructure design and the emission characteristics.
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.
We report on novel evaluation methodology of high-power diode lasers that potentially will increase the reliability level of these devices. The study is carried out for wide-stripe, 808 nm diode lasers with low fast-axis beam divergence that base on a double-barrier single quantum well separate confinement heterostructure. The diodes are assembled in standard packages with base diameter &slasho; = 9 mm. Degradation of diode lasers is a result of the interaction between internal and external factors. Thus, insight into degradation mechanisms is only possible with a complex characterization of the devices. In our analysis we involved standard measurements such as current-voltage, light-current characterizations, as well as advanced methods such as high-resolution thermography. The latter one allows for investigations of thermal properties of diode lasers including fast temperature profiling and defect recognition. We discuss the usefulness of above techniques for screening purposes. Finally we present results of reliability tests of the diode lasers. A correlation between initial tests and lifetest results is shown.
KEYWORDS: Semiconductor lasers, Thermography, Temperature metrology, Heatsinks, Spatial resolution, High power lasers, Micro raman spectroscopy, Data modeling, Finite element methods, Thermal modeling
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.
KEYWORDS: Semiconductor lasers, High power lasers, Diodes, Waveguides, Heterojunctions, Near field optics, CCD cameras, Optical filters, Cameras, Laser systems engineering
A low-cost system for quick measurements of laser diodes far-field characteristics is demonstrated. The measurement results of various types of laser diodes are presented. The usefulness of the system for high-power laser diode testing is proven.
This paper presents results of the investigated levels of Catastrophic Optical Damage of the SQW-SCH lasers mirrors coated with AlN (λ/4n) layer. These results have been compared with COD levels of this same type of the lasers with mirros coated by Si3N4 (λ/4n), thick AlN (3λ/4n) layers, and without coatings.
This work presents the possibility of modification of semiconductor laser characteristics by a superposition of stresses introduced during technological process. A short description of the most often introduced stress mechanisms and their theoretically expected influence on laser diode emission characteristics are outlined. Experimental results of combination of stress (or strain) mechanisms introduced both by a MOVE growth of lattice-mismatched GaAsP active layer in AlGaAs heterostructure and by H+ and He+ implantation used for stripe definition are presented. It has been shown that He+ implantation-induced strain can be considerably stronger with respect to lattice-mismatch originated strain then it has been evaluated so far in literature, giving possibilities of new solutions in optoelectronics design.
A method of high power (AlGa)As laser mirrors passivation with thin AIN layers deposited by low temperature reactive sputtering is presented. In SQW-SCH laser diodes (LDs) with high optical confinement the optical power densities at the mirrors are very high, leading at some power level to so-called catastrophic optical damage (COD) of the mirrors (and lasers). Thus the COD level limits the LD’s optical output and to improve it, mirrors are usually protected with dielectric coatings, such as SiO2, Si3N4 and Al2O3 layers. Here, the AIN layers are proposed as a LD mirror coatings because of their unique features including high smoothness and thermal conductivity (3.19 Wcm-1K-1) which is close to that of copper (4.01 Wcm-1K-1). Moreover thermal expansion coefficient of AIN matches well coefficient of GaAs. Single AIN layers (approx. λ/4 thick) have been deposited as front, low reflectivity LD coatings and for the rear, high reflectivity mirrors the AIN-Si λ/4 layer stacks (4 pairs) have been used. LDs with such coatings exhibit twofold external quantum efficiency and good stability during CW life test.
This paper presents method for facets passivation of high power lasers GaAl As using thin films of AIN and GaN formed by reactive sputtering at low temperature. GaN has been chosen due to its exceptional stability at high temperatures and resistance against attack. GaN is insoluble in acids. Layers of GaN protect AIN coating against influence of chemical and physical factors. We report some problems concerning technology of GaN thin films deposition using RF Sputtering from Ga target in Ar-N2 gas mixtures. Observation of N+ peaks in plasma spectrum provides information on optimal conditions for synthesis of GaN. The quality of GaN layers was examined by X-ray Diffraction (XRD), SIMS method, optical absorption, measurement of refractive index and density. Lasers diodes with coated mirrors of AIN-GaN exhibit good stability during CW life-test.
To meet the requirements for the narrow emission line of ht laser diode pumping system exactly at 808 nm wavelength, an optimization of the laser (AlGa)As heterostructure because of the composition and thickness of the constituent layers is necessary. In the paper the design principles, the characteristics of the manufactured laser diodes based on this design and intrinsic limitations of the possibility of 'tuning' the lasers to desired wavelength in the MOVPE growth process are presented.
We present comprehensive numerical modeling of graded-index separate-confinement-heterostructure single-quantum well (GRIN SCH SQW) lasers. In particular, we present for the first time a modified drift-diffusion model of the laser, which correctly takes into account quantum nature of carriers in the active region. The drift-diffusion equations are solved self- consistently together with Schrodinger and Poisson equation. The analysis is carried out for the general case of MQW active region. We employ our model to predict the behavior of the devices fabricated by molecular beam epitaxy (MBE) technique. The broad contact, test lasers produced in our laboratory exhibited threshold current density Jth equals 350 A/cm2, intrinsic mode loses as low as 7 cm-1 and internal quantum efficiency near unity. External quantum efficiency above 80% was achieved. The emitted power per uncoated facet just before degradation was equal to 5 W for pulsed operation with 0.1% filing factor and 140 mW for cw operation. We also discuss the development of the technology in terms of growth temperature and V/III beam fluxes ratio optimization.
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