High-power Diode Laser Bars Research Articles

Thermal Performance of Cryogenic Micro-Pin Fin Coolers with Two-Phase Liquid Nitrogen Flows

This study experimentally explores the thermofluidic performance of a cryogenic micro-pin fin cooler with two-phase liquid nitrogen flows. The liquid nitrogen cooling system is introduced to investigate the performance of the micro-pin cooler in a cryogenic condition. The result reveals that the nominal value of the base heat transfer coefficients of the micro-pin fin cooler with liquid nitrogen flows, 240 kW/m2-K at a mass flow rate of 2.23 g/s, is an order of magnitude greater than that with FC-72 flows. The result also demonstrates that the base heat transfer coefficient of the micro-pin fin cooler is nearly three times greater than that of the micro-gap cooler, not containing any fins. This study shows the feasibility of the cryogenic micro-pin fin cooler for thermally controlling very high heat density devices such as high-power laser diode bars, of which the heat density can reach 2000 kW/m2.

Fiber-coupling optical system for high-power and multi-wavelength diode laser bars oriented to integrated biomedical imaging systems

The use of pulsed high-power diode lasers (HPDLs) in the near infrared (NIR) range as multispectral sources has attracted much interest in biomedical applications due to their relatively low cost and small size compared to nanosecond Nd:YAG optical parametric oscillator (OPO) lasers. One of the main limitations of these sources is the availability to combine different wavelengths with high power density in the same beam. Various works have shown that the use of linear arrays of emitters as diode laser bars (DLBs) or stacked arrays of emitters as diode laser stacks (DLSs) allows the combination of multiple wavelengths while maintaining high power densities. Nevertheless, the highly asymmetric beam profile emitted by such laser sources between fast and slow axes implies the need for suitable beam shaping for efficient fiber coupling. In this work, we investigate a novel beam shaping technique to homogenize the beam quality of six DLBs in the wavelength range between 790 nm and 980 nm.We consider fast-axis collimating lenses (FAC) and beam twisters to reduce the beam asymmetry of the individual bars. The beams from the DLBs are then combined into a single multispectral beam using reflective mirrors, dichroic mirrors, and a polarizing beam splitter cube (PBC), and effectively coupled into a 400 µm core-diameter/N.A. = 0.22 optical fiber using a pair of cylindrical lenses. Simulation shows high coupled power densities with ~ 1.8 MW/cm2 at the output of the fiber. The coupling efficiency reaches 89.4%. The use of submillimeter fiber optic probes is particularly promising for photoacoustic endoscopy (PAE) applications requiring minimally invasive examination of internal organs.

Thermal and optical performance of cryogenically cooled laser diode bars mounted on pin-finned microcoolers

This study investigates the thermal performance of cryogenic micro-pin fin coolers for high-power laser diode (LD) bars. An open-loop liquid nitrogen cooling system, used to operate LD bars at cryogenic temperatures, is developed and characterized. The comparison study demonstrates that the total thermal resistance value of the micro-pin fin cooler, ranging from 0.03 to 0.04 °C/W, is only 1/3 of that of the micro-gap cooler and, thus, contributes significantly to reducing the LD operating temperature and enhancing its efficiency. In this study, the peak optical power of 68.8 W was observed at an LD bar package base temperature of − 100 °C, providing a 20% increase relative to the optical power of 57.3 W for a nominal operating condition with an LD bar package base temperature of 41 °C. This result clearly illustrates the enhancement in optical performance made possible by cryogenic cooling with a pin fin microcooler.

Thermal hydraulic performance of a microchannel heat sink for cooling a high-power diode laser bar.

Numerical and analytical methods are employed to investigate the thermal and fluid flow performance of a microchannel heat sink for cooling a high-power diode laser bar. Heat transfer characteristics and pressure drop in the microchannel under different flow rates are studied. A thermal resistance network, which is proved to have less than 5.4% error, is proposed to characterize the resistance components for the microchannel heat sink. Both numerical modeling and thermal resistance network analysis are verified by experimental results based on the wavelength shift method. Two new heat sinks with more uniform temperature distribution for laser emitters compared with the existing design are presented, and their performance is validated by numerical modeling and spatially resolved spectrum measurements.

传导冷却高功率半导体激光器单巴器件CW工作模式下的热加速寿命试验

传导冷却高功率半导体激光器单巴器件CW工作模式下的热加速寿命试验

Three-dimensional thermal model of a high-power diode laser bar.

An analytical, three-dimensional, steady-state thermal model of a high-power diode laser bar is presented in this paper. The heat spreading angle in a laser bar heat sink, subjected to several convective conditions on the bottom-side, was calculated with this model. Thermal design curves for the heat sink and submount are also presented. Special discussion is presented for two kinds of our conduction-cooled laser bars. Finite element simulation and experimental results based on the wavelength shift method are compared with this analytical solution. The familiar 45° angle in thermal design for a commercial hard solder conduction-cooled laser bar was found to lead to a 12% increase in thermal resistance relative to a free lateral diffusion heat sink.

Design of novel geometries for microchannel heat sinks used for cooling diode lasers

High power diode laser bars require thermal management packages with high cooling efficiencies and long-term stability. At a constant size, the power output of the diode laser is being increased rapidly; therefore, an efficient heat removal strategy has become even more important. High efficiency microchannel heat sinks are usually used to remove heat from the system. In this study, the fluid flow and heat transfer of an existing diode laser micro-channel heat sink is numerically simulated. To reduce the maximum temperature and obtain a uniform temperature distribution across the diode laser, which leads to extended lifetimes, the simulations are repeated for three different channel geometries. The hydrodynamic and thermal behavior of the heat sinks are then compared. The results show that among all geometries, the sinusoidal structure provides the best thermal performance. Although the system pressure drop increases by 18%, this configuration increases the diode lifetime by 44%.

Bypass Heat Sink Analysis for a Laser Diode Bar with a Top Canopy

With the increasing use of high-power laser diode bars (LDBs) and stacked LDBs, the issue of thermal control has become critical, as temperature is related to device efficiency and lifetime, as well as to beam quality. To improve the thermal resistance of an LDB set, we propose and analyze a bypass heat sink with a top canopy structure for an LDB set, instead of adopting a thick submount. The thermal bypassing in the top-canopy structure is efficient, as it avoids the cross-sectional thermal saturation that may exist in a thick submount. The efficient thickness range of the submount in a typical LDB set is guided by the thermal resistance as a function of thickness, and the simulated bypassing efficiency of a canopy is higher than a simple analytical prediction, especially for thinner canopies.

High power and high efficiency 1.8-kW pulsed diode laser bar

There is a great interest in the development of high-power, high-efficiency, and low-cost quasicontinuous wave (QCW) diode laser bars and arrays for pumping solid state lasers. We report on the development of kW-class 88x-nm diode laser bars that are based on a bipolar cascade design, in which multiple lasers are epitaxially grown in electrical series on a single substrate and separated by low-resistance tunnel junctions with resistance as low as 8.0×10−6 Ω-cm2. QCW power of 630 W was demonstrated in a 3-mm-wide minibar with 3-mm cavity length. Peak efficiency of 61% was measured with 200 μs and 14 Hz pulses, at 10°C. Further power scaling was demonstrated in a 1-cm-wide bar with 3-mm cavity length, where a record peak power of 1.8 kW was measured at 1-kA drive current. Ongoing work for further power scaling includes development of triple-junction diode laser bars.

Parametric second Stokes Raman laser output pulse shortening to 300 ps due to depletion of pumping of intracavity Raman conversion

A new effect of the pulse shortening of the parametrically generated radiation down to hundreds of picosecond via depletion of pumping of intracavity Raman conversion in the miniature passively Q-switched Nd:\(\hbox {SrMoO}_{4}\) parametric self-Raman laser with the increasing energy of the shortened pulse under pulsed pumping by a high-power laser diode bar is demonstrated. The theoretical estimation of the depletion stage duration of the convertible fundamental laser radiation via intracavity Raman conversion is in agreement with the experimentally demonstrated duration of the parametrically generated pulse. Using the mathematical modeling of the pulse shortening quality and quantity deterioration is disclosed, and the solution ways are found by the optimization of the laser parameters.

Method for predicting junction temperature distribution in a high-power laser diode bar.

A hybrid experimental/numerical method is proposed for predicting the junction temperature distribution in a high-power laser diode (LD) bar with multiple emitters. A commercial water-cooled LD bar with multiple emitters is used to illustrate and validate the proposed method. A unique experimental setup is developed and implemented first to measure the average junction temperatures of the LD bar emitters. After measuring the heat dissipation of the LD bar, the effective heat transfer coefficient of the cooling system is determined inversely from the numerical simulation using the measured average junction temperature and the heat dissipation. The characterized heat dissipation and effective heat transfer coefficient are used to predict the junction temperature distribution over the LD bar numerically under high operating currents. The results are presented in conjunction with the wall-plug efficiency and the center wavelength shift.

Cycled Thermomechanical Failure in 808-nm High-Power AlGaAs/GaAs Laser Diode Bars

Thermomechanical cycle failure was analyzed in 808-nm high-power AlGaAs/GaAs laser diode bars (LDBs) in detail. Two thermal stress cycle experiments were carried out in these devices with peak thermal stresses of 19.4 and 33 MPa. The threshold current increase demonstrated a square-root dependence on the cycle number, and a tail and split in lasing spectrum were observed. This result was attributed to nonradiative recombination increasing with cycle number due to the diffusion of defects that accompanies band-structure renormalization. Furthermore, from X-ray diffraction measurements we found that the epitaxial layer remains monocrystalline during thermal stress cycling. We calculated the out-of-plane strain and in-plane stress induced by plastic deformation in the epitaxial layer based on the interplanar spacing in the crystal. Our results suggest that thermomechanical strain and stress were induced by pulsed operation, which led to degradation of the high-power LDBs.

Working Thermal Stresses in AlGaAs/GaAs High-Power Laser Diode Bars Using Infrared Thermography

An interesting approach is proposed for investigating the thermal stress of AlGaAs/GaAs laser diode bars using infrared thermography. We obtained horizontal and perpendicular profiles of the working thermal stress through the active region in one emitter (emitter 5) at operating currents 0.5 and 1.0 A. The thermal stress at the center of emitter 5 is found to quickly rise to values 2.1 and 3.4 MPa under the operating currents 0.5 and 1.0 A. The thermal stress gradient in the epitaxial layer is also larger than that in the substrate. In addition, if emitter 5 operates individually, a thermal stress spike at a current of about 0.2 A appears, with a sharper rise in slope between 0.1 and 0.2 A than between 0.2 and 1.0 A. Furthermore, with regard to the properties of transient working thermal stress, the profile of the thermal crosstalk between emitters was obtained at different operating currents.

High-power diode laser bars and shear strain

The emitters at the edges of high-power laser bars tend to produce less power than emitters that are near the center of the bar. We suggest that shear strain, which owes to strain induced by bonding, creates through a photoelastic effect a weak birefringence that rotates the plane of polarization of the light. A rotation of the plane of polarization reduces the net gain for the lasing modes and hence leads to a lower output power for the emitters at the edges of the bars, where the shear strain is dominant.

AlGaAs/GaAs laser diode bars with improved thermal stability

Two series of AlGaAs/GaAs laser heterostructures have been grown by metal-organic vapour phase epitaxy, and laser diode bars fabricated from the heterostructures have been investigated. The heterostructures differed in waveguide thickness and quantum well depth. It is shown that increasing the barrier height for charge carriers in the active region has an advantageous effect on the output parameters of the laser sources in the case of the heterostructures with a narrow symmetric waveguide: the slope of their power – current characteristics increased from 0.9 to . Thus, the configuration with a narrow waveguide and deep quantum well is better suited for high-power laser diode bars under hindered heat removal conditions.

Efficient and Reliable High-Power Laser Diode Bars With Low-Smile Implementation

We report on our recent development of high power 940 nm laser diodes (LDs) bars. The optimization of the laser structure and thermal management allow over 200-W continuous wave (CW) operation with 58% conversion efficiency from the 1-cm LD bar. We have also developed an assembly technique, and have suppressed the smile of LD bars mounted even on Cu heatsinks with hard solder to less than 1 μm. Long term stability have also be confirmed under 200-W CW operation.

Silica-taper array for beam shaping of high power laser diode bar

A low-cost, easy producing and novel beam shaping system for concentration and homogenization of a high power laser diode bar (LDB) beam is demonstrated by experiment and simulation. It consists of a silica-taper array (beam shaper) and a silica-taper (beam integrator) which are fabricated by using chemical etching technique. By using this device, the output beam of LDB is reshaped to a small spot with uniform intensity and angular distributions, and could be coupled into a 200 μm silica fiber.

Spectroscopic analysis of packaging concepts for high-power diode laser bars

Double-side cooled high-power diode laser bars packaged by different techniques on different types of passive heat sinks are analyzed in terms of packaging-induced strain. Reference data from standard devices being single-side cooled only and packaged by conventional soft and hard soldering are also presented. Thermal profiling across the devices complements the results. The most suitable packaging architecture and technique for double-side cooled bars is identified. Measurements of the laser emission near field and electroluminescence pattern provide direct reference to the functionality of the devices. Furthermore, a type of cross calibration of the methods used for strain analysis is made, since all techniques are applied to the same set of bars. This involves micro photoluminescence, micro Raman, and degree-of-polarization electroluminescence spectroscopy.

808 nm大功率半导体激光器阵列的优化设计

808 nm大功率半导体激光器阵列的优化设计

Packaging-Induced Strain Measurement Based on the Degree of Polarization in GaAsP–GaInP High-Power Diode Laser Bars

The strain caused by device packaging was studied in high-power semiconductor laser bars by measuring the degree of polarization. Polarization measurement with intentionally GaAsP-GaInP strained laser bar packaged on a Cu heat sink for 799-nm emission revealed the variation of band edges between the conduction band and heavy-hole or light-hole bands in the active region. This served as a method for evaluating the strain. In the packaging process, a maximum of 1800-ppm strain was transmitted to the active region. It was found that the defect density of 14.3% was induced.