Diode Laser Bars Research Articles

Micro-structured lateral thermal lens for increased output power and narrowed beam divergence in kilowatt-class 9xx nm diode laser bars

Local temperature non-uniformity is a critical limit to power in large-area semiconductor lasers, playing a larger role than the conversion efficiency and temperature sensitivity in the most efficient modern devices. For the specific case of kilowatt-level edge-emitting diode laser bars, we demonstrate that laterally re-distributing current locally within each emitter using a customized micro-structuring of the electrical contact can flatten the thermal profile, based on the thermal design using COMSOL. The concept is demonstrated experimentally in adapted bars that contain eight broad-area emitters, each having wide (∼1100 μm) stripes and a 4 mm long resonator. Each emitter in the laser bar has an electrical contact layer that is electrically structured into parallel sub-contacts using implantation, essential to prevent lateral lasing, implemented here with a period of 29 μm. The width of the individual sub-contacts is narrowed in the device center and then monotonically increased toward the emitter edges to collect a higher proportion of heat at the edges, for a fivefold reduction in the thermal lens, despite a significant (20%) overall increase in electrical and thermal resistance. Two performance benefits are observed. First, the slope varies more slowly with average temperature, recovering (here) around half of the efficiency penalty from in-stripe temperature non-uniformity, and hence increasing the power conversion efficiency at 800 W optical output power by around 5%. Second, the lateral far field (95% power content) is narrowed by around 2° at 800 W optical output power, corresponding to a reduction of around half of the thermal contribution.

Experimental and Computational Investigation of Flow Boiling in a 52 μm Hydraulic Diameter Microchannel Evaporator With Inlet Restrictions and Heat Spreading

Abstract Microchannel flow boiling presents an effective thermal management strategy for high heat flux (>1 kW/cm2) devices. Fundamental mechanisms of microchannel flow boiling behaviors are difficult to determine due to macroscopic limitations of experimental hardware. In addition, flow stabilizing features of microchannel evaporators such as inlet restrictions and heat spreading further complicate fluid flow and heat transfer dynamics. Computational models, when utilized with experiments, can provide a more detailed understanding of behaviors which cannot be determined experimentally. The present study developed a computational model for flow boiling heat transfer in a 52 μm silicon microchannel evaporator designed to cool a laser diode bar, with inlet restrictions and a nonuniform heating profile at the channel level. A conjugate heat transfer model along with a coupled level set and volume of fluid (CLSVOF) model was created in ansysfluent and compared with experimental flow boiling data to gain further insights into the performance of a realistic microdevice. Heat spreading in the channel outside of the heater footprint was observed due to the high thermal conductivity of the silicon substrate. The inlet orifices impacted local flow patterns by creating a large pressure drop and forming a recirculation zone immediately downstream. This behavior resulted in pressure recovery zones and regions of separated flow boiling behavior. Bubbly, slug, and churn flows were seen to be dominant flow regimes. The heat transfer coefficient was found to be dependent on heat flux and flow regime, and more weakly on mass flux and outlet vapor quality.

The SMILE Effect in the Beam Propagation Direction Affects the Beam Shaping of a Semiconductor Laser Bar Array

Near-field bending of a laser diode bar (i.e., the SMILE effect) degrades the laser beam brightness, adversely affecting optical coupling and beam shaping. Previous reports mainly focused on the two-dimensional near-field bending of a laser diode bar. However, the near-field bending of a laser diode bar not only occurs in the laser bar growth direction, but also in the beam propagation direction. The present article proposes the three-dimensional near-field bending of a laser diode array, which is commonly known as the three-dimensional spatial SMILE effect. Through theoretical and simulated investigations, it has been found that a laser bar array not only deforms in the fast axis direction to cause the traditional two-dimensional SMILE effect but also experiences an additional deformation of approximately 2 μm in the laser emission direction simultaneously. Due to the SMILE effect in the beam propagation direction, not all emitters are aligned in a straight line, and some emitters experience defocusing during collimation. Consequently, there is an increase in the residual divergence angle and beam width, resulting in a degradation of the laser bar array’s beam quality. According to the theoretical calculations, ZEMAX simulations, and experimental results, for a FAC (fast axis collimation) with a focal length of 300 μm, the divergence angle of single emitter after collimating in the fast axis increases from 4.95 mrad to 6.46 mrad when the offsetting of the working distance between the incident beam waist and FAC lens increases from 0 μm to 2 μm.

Source of High-Power, High-Repetition-Rate, Pulsed Laser Radiation (1060 nm) Based on a Hybrid Stack of a Laser Diode Bar and a 2D Array of Optothyristors as a High-Speed Current Switch

Source of High-Power, High-Repetition-Rate, Pulsed Laser Radiation (1060 nm) Based on a Hybrid Stack of a Laser Diode Bar and a 2D Array of Optothyristors as a High-Speed Current Switch

Beam Shaping Technique for 5-mm Fiber-coupled Laser Diode Bars with Lens Group

In the last few years, fiber-coupled diode lasers have shown massive applications in many fields of communication and scientific research. Particularly, the pumping of solid-state lasers is a key application for more powerful diode lasers enabling good solutions in various laser micro methods like metal cutting, sintering, structuring as well as drilling. In this work, a simple beam shaping method is demonstrated for coupling a high-power semiconductor laser diode into multi-mode fiber optic using optical lenses. The optical lenses as beam transformation components are utilized to reshape the asymmetrical irradiation of the diode laser bar and to circularize the laser beam. Using this simple method, compact, high-output-power, and high-brightness fiber-coupled laser diode systems are achieved. The obtained results show that the entire optical coupling efficiency of the system is more than 65%.

High-efficiency spectral beam combining of an individual laser diode bar with polarization multiplexing.

In this Letter, a new strategy for the spectral beam combining (SBC) of an individual laser diode (LD) bar based on a polarization multiplexing external cavity is proposed and demonstrated. The maximum combining efficiency is up to 95.51%, which leads to an output power of 76.6 W and an electro-optic conversion efficiency of 48.33% under continuous wave operation at a current of 100 A. Compared to the conventional SBC, the combining efficiency, the output power, and the electro-optical conversion efficiency present improvements of 12%, 10W, and 6%, respectively. The results show that this novel SBC method is a prospective technique for increasing the combining efficiency of LD bars.

Kinetics and wavelength dependence of thermal and excited-state population on lens effect induced in a Nd:YAG rod amplifier.

Using a wavefront sensor, we have measured the temporal evolution of the lens induced in a Nd:YAG rod amplifier under side pumping by laser diode bars centered around 808 nm in a quasi-continuous wave regime. The evolution of the induced lens is drastically different when measured with a probe pulse centered at 532 nm or 1064.5 nm. To explain this evolution, we developed a model that accounts for both the excited state population of the Nd3+ ions and the thermal contribution to the refractive index of the amplifier. This model, which takes into account amplification and wavelength shift of the probe pulse at 1064.5 nm, makes it possible to quantitatively describe the spectral and temporal evolution of the amplifier focal length. It also shows that the excited state population contribution is more important around 1064.5 nm and can partly compensate for the thermal lens induced in the amplifier.

Wavelength‐stabilized ns‐pulsed 2.2 kW diode laser bar with multiple active regions and tunnel junctions

The improvement of the performance of a distributed Bragg reflector laser bar emitting near 905 nm through the use of multiple epitaxially stacked active regions and tunnel junctions is reported. The bar consisting of 48 emitters (each having an aperture of 50 µm) emits an optical power of 2.2 kW in 8 ns long pulses at an injection current of 1.1 kA. This corresponds to an almost threefold increase of the pulse power compared to a bar with lasers having only a single active region. Due to the integrated surface Bragg grating, the bar exhibits a narrow spectral bandwidth of about 0.3 nm and a thermal tuning of only 68 pm/K.

Effect of thermoelastic stresses on radiation spectra of CW laser diode bars

Detailed measurements of radiation spectra of individual emitters in CW laser diode bars (LDBs) were done. For the first time, the anomalies in the measured spectral distributions over the emitting apertures were observed, which can not be explained through consideration of measured and modeled relatively smooth temperature distributions over the LDBs. Explanation of these spectral features in the measured spectral distributions along LDBs has been achieved with numerical modeling of not only the temperature profiles but also thermo-elastic stress in LDBs that can strongly affect the radiation characteristics of emitters. Three main mechanisms of the thermo-elastic stresses occurrence in operating LDBs have been considered: “initial” stresses in an unbounded bar, stresses as a result of the assembling process, and stresses as results of working regime of LDB. Understanding of demonstrated experimentally and theoretically effect of thermo-elastic stresses on spectral behavior of emitters in LDBs will promote development of new generation of high-power laser systems, in particular, with beam summation via spectral combining method.

Determination of the Limiting Characteristics of a Laser-Diode Bar by Analyzing Emission in the Near Field

Determination of the Limiting Characteristics of a Laser-Diode Bar by Analyzing Emission in the Near Field

Spectral linewidth narrowing of broad-area blue diode bar in V-shape external Talbot cavity.

A 1-D linear array of 23 high-power broad-area laser diode (BALD) beams in the blue spectral region (447 nm) is combined employing a V-shape external Talbot cavity in Littrow configuration. A surface grating provides optical feedback via self-imaged diffractive coupling to the diode bar and induces all the emitters to lase at a common central wavelength. The external cavity reduces the spectral linewidth of the free-running laser diode bar from several nm to 20-50 pm (FWHM) with the power level of 11.8 W. The narrow spectrum of the external cavity stabilized laser can be tuned in the range of 3-4 nm by adjusting the tilt angle of the grating while the laser diode bar is operated in constant current mode at a temperature of 20°C.

Analysis of crosstalk in spectral beam combining of diode laser bar

Spectral beam combining (SBC) of laser diodes has been proven to be an effective method to improve beam brightness. The crosstalk between different emitters in the SBC system will significantly affect the beam-combining efficiency and output beam properties without effective control. The interfering factors of the crosstalk beam intensity in semiconductor laser SBC have been theoretically analyzed, and experiments have been built to carry out the SBC of a semiconductor laser bar with a high filling factor of 80%. It is found that by adding an inverted Kepler telescope, reducing the focal length of the transmission lens or increasing the spacing of the grating and the output coupling mirror, the crosstalk can be suppressed. In an experiment of beam combining with five emitters, the ratio of the first-order crosstalk peak energy to that of the central beam spot is about 0.34 when the spacing of the grating and the output coupling mirror is 50 mm and the focal length of the transmission lens is 200 mm. The ratio decreases to 0.035 when the spacing of the grating and the output coupling mirror is 200 mm. With the addition of an inverted Kepler telescope with a magnification of 2, the ratio decreases to 0.085. In addition, the ratio decreases to 0.07 when the focal length of the transmission lens is 80 mm.

Solid-State Pulsed Time-of-Flight 3-D Range Imaging Using CMOS SPAD Focal Plane Array Receiver and Block-Based Illumination Techniques

Full realization of a solid-state 3-D range imager based on the pulsed time-of-flight method is presented. The system uses block-based segmented illumination in the transmitter realized with a 16-element common anode laser diode bar. The receiver is based on a single chip 2-D SPAD/TDC 0.35 μm CMOS array with 32 × 128 SPAD pixels and 257 TDCs. Segmentation of the illumination improves the SN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">BG</sub> R in the detection and results in simpler receiver realization than in the commonly used flood-illumination approach. The system is capable of producing cm-accurate 3-D range images within a FOV of 40° × 10° up to a range of 15 m in normal office lighting at a frame rate of ∼30 fps using a low average illumination power of only 2.6 mW. In background illumination of ∼20 klux, 3-D range images were captured at a distance of ∼13 m using the gated SPAD approach at a frame rate of ∼1 fps.

Far field output distribution features for spectral beam combining of diode laser array stack in an external cavity

We present the design and the experimental implementation of spectral beam combining of diode laser bars, which contains six 800 nm Mini-Bars along the fast-axis direction in an external cavity. The output power of 175 W with its efficiency of 62% is achieved at 60 A of operating current. For better applications, some of output features need to be uncovered. In this work, the far field distribution of lobes is studied. The variation tendency of efficiencies with pump currents and the distribution of spectrum are measured. The distribution of the spectrum with the angle between the axis of output laser and the axis of central wavelength is simulated. The result shows that 1. the efficiency decrease slowly with current increasing. 2.it is measured that Beam Parameter Product of main lobe along the slow axis considered by one-dimension, 3.05 mm × mrad, is close to that of single emitter. 3. it is verified that mutual locked mode generate side lobes effecting both beam quality and efficiency.

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.

Beam-shaping design for multi-wavelength diode laser stack system coupled into optical fiber for biomedical applications

Multispectral diode laser sources are extensively used for a variety of applications involving the identification of small objects based on their spectral signature. Although the power scaling of single emitters is severely limited, they are easily stackable as diode laser bars and stacks, allowing the combination of wavelengths and power levels required for each application. However, a critical drawback given by this topology is the asymmetry between the fast and the slow axes in the beam profile, leading to poor beam quality and possibly poor fiber coupling efficiency. In this regard, a suitable beam shaping is required to maximize the power coupling in the smallest possible fiber core. In this work, we propose an innovative beam shaping method for the homogenization of the beam quality of six 8-bar diode laser stacks at wavelengths from 790 nm to 980 nm. We performed realistic simulations to examine the shaping method with, when possible, commercially available components. Fast-axis collimating (FAC) lenses and beam twisters are designed in Zemax to remodel the far-field beam emitted by each bar. The beam of each diode laser stack is halved in the vertical axis using polarization beam combiners, and then three quartz-plate stacks combine and rearrange the beams coming from each diode laser stack pair in the horizontal axis to eliminate the lightless regions. A single multispectral beam is then obtained by using reflective and dichroic mirrors and effectively coupled into an optical fiber with a core diameter of 1 mm and a numerical aperture (N.A.) of 0.5 using a doublet of cylindrical lenses. A maximum power density of ∼ 0.73 MW/cm2 is calculated at the output of the fiber with a fiber coupling of 89 %. A number of applications can benefit from the proposed topology, in particular biomedical applications using fiber probes are identified as potential candidates for the implementation of the proposed system.

Theoretical and experimental research on crosstalk in spectral beam combining of a laser diode bar with a high fill factor.

Spectral beam combining is an important way to improve the brightness of semiconductor laser beams. For a spectral beam combining system, crosstalk between different emitters would cause the deterioration of beam quality and the reduction of beam combining efficiency, especially for the laser diode bar with a high fill factor. In this paper, an analysis model of the spectral beam combining system with crosstalk is established. The most important advantage of this model is that it can analyze the spectral distribution of the combined beam and also give a relatively good estimation for the beam quality parameters, such as beam size and far-field divergence angle. This model is verified by the experimental results. Furthermore, based on the theoretical model, a method for eliminating crosstalk is developed. By introducing a spatial filter inside the grating external cavity, the crosstalk between different emitters is blocked in the far field, and the beam quality is improved. In the experiment of beam combining of five emitters, after crosstalk is eliminated, the divergence angle of the combined laser beam is reduced from 10.09 to 4.73 mrad, the beam parameter product is reduced from 2.95 to 0.91 mm⋅mrad, and the power of the main lobe is improved from 1.77 to 2 W.

Solid-state block-based pulsed laser illuminator for single-photon avalanche diode detection-based time-of-flight 3D range imaging

A laser diode illuminator for single-photon avalanche diode detection-based pulsed time-of-flight 3D range imaging is presented. The illuminator supports a block-based illumination scheme and consists of a 16-element custom-designed common anode quantum-well laser diode bar working in the enhanced gain switching regime and lasing at ∼810 nm. The laser diode elements are separately addressable and driven with gallium nitride drivers, which produce current pulses with a width of ∼2 ns; the current pulse amplitude was estimated from the supply voltage (90 V) as 5 to 10 A. Cylindrical optics are used to produce a total illumination field-of-view of 40 × 10 deg2 (full width at half maximum) in 16 separately addressable blocks. With a laser pulsing frequency of 256 kHz and laser pulse energy of ∼8.5 nJ, the average optical illumination power of the transmitter is 2.2 mW.

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.

High Efficiency 1.9 Kw Single Diode Laser Bar Epitaxially Stacked With a Tunnel Junction

We report on the development of a 940-nm diode laser bar based on epitaxially stacked active regions by employing a tunnel junction structure. The tunnel junction and the device parameters were systematically optimized to achieve high output and power conversion efficiency. A record quasi-continuous wave (QCW) peak power of 1.91 kW at 25 °C was demonstrated from a 1-cm wide bar with a 2-mm cavity length at 1 kA drive current (200 μs pulse width and 10 Hz repetition rate). Below the onset of the thermal rollover, the slope efficiency was as high as 2.23 W/A. The maximum power conversion efficiency of 61.1% at 25 °C was measured at 300 A. Reducing the heatsink temperature to 15 °C led to a marginal increase in the peak power to 1.95 kW.