Coherent beam combining of high power broad-area laser diode array with a closed-V-shape external Talbot cavity

High power diode lasers have been widely used in many fields. To meet the requirements of high power and high reliability, passively cooled single bar CS-packaged diode lasers must be robust to withstand thermal fatigue and operate long lifetime. In this work, a novel complete indium-free double-side cooling technology has been applied to package passively cooled high power diode lasers. Thermal behavior of hard solder CS-package diode lasers with different packaging structures was simulated and analyzed. Based on these results, the device structure and packaging process of double-side cooled CS-packaged diode lasers were optimized. A series of CW 200W 940nm high power diode lasers were developed and fabricated using hard solder bonding technology. The performance of the CW 200W 940nm high power diode lasers, such as output power, spectrum, thermal resistance, near field, far field, smile, lifetime, etc., is characterized and analyzed.

Due to the narrow absorption peak of Yb in glass at 975nm, a wavelength stabilisation is necessary for pump laser diodes. Our original approach is to use a Distributed Feedback broad area structure which is made possible since we use an Aluminium free active region laser structure. The target is to obtain high optical power of 10W with both a reduce wavelength evolution with temperature and a low spectral width. On a 3mm x 100 μm BA Fabry Perot laser, we have obtained a high power of 10W. Design, fabrication and results on BA DFB lasers will be presented, showing a low spectral width at 976nm at 3W output power.

We report results of multi-cell life tests performed on nearly (500) laser diodes representing our new generation of very efficient high power broad area 9xx nm lasers. The acceleration model showed a steep power dependence of the failure rate with an exponent of nearly 6. Improvement in the facet passivation process resulted in significantly less power acceleration of failures. Analysis of the life test on upgraded lasers showed median lifetime of 1,500,000 hours at operating conditions of 8W and 350°C. Optical powers as high as 17.8W for thermally limited CW operation and 32W for 20 μs pulsed operation were recorded. The CW life test was complemented by a life test performed at power cycling conditions (1Hz repetition rate, 50% duty cycle).

Laser-based displays have gathered much attention because only the displays can express full color gamut of Ultra-HDTV, ITU-R BT.2020. There are many types of laser-based displays. A projector with a single spatial light modulator is mainstream in the displays so far. This projector uses the laser light sources under pulse with duty of 20% to 50%. We improve a triple-emitter red color 638-nm high-power broad area laser diode (LD) for this application. Countermeasures are adopted to improve the output power characteristics and strengthen the facet to the fatal degradation. The improved triple emitter shows the output of 5.0 W at the injection current of 5 A under pulse with duty of 30%, case temperature of 25°C. The output is increased by 4.1% compared to our former one. The peak wall plug efficiency reaches to 41.6% under 25°C. The long-term aging test is also performed. Mean time to failure of the improved LD is estimated 72,000 h under the output of 3.5 W, pulse condition with duty of 30%. The value is ∼3.3 times longer than that of the former.

Military, industrial, and medical applications have expressed interest in using ~1550nm laser diodes as efficient laser sources for reduced eye safety concerns, especially where free-space propagation is concerned. In addition, covert military applications, including pointers and illuminators, benefit from the spectral insensitivity of common sensor types to ~1.5um wavelengths. While more efficient than other sources, high-power broad-area laser diodes can sometimes require complex packaging and beam shaping/combining optics in order to meet requirements for brightness, uniformity, stray light, and environmental insensitivity. This paper will highlight and discuss a range of developments exploring laser diode components near 1.5um for free-space military applications, including broadarea high-power illuminators and highly-collimated pointers. Options for tailored beam conditioning and performance results will also be presented.

We present high-power GaAs-based broad-area diode lasers with a novel variant of the enhanced self-aligned lateral structure ‘eSAS’, having a strongly reduced lasing threshold and improved peak conversion efficiency and beam quality in comparison to their standard gain-guided counterparts. To realize this new variant (eSAS-V2), a two-step epitaxial growth process involving in situ etching is used to integrate current-blocking layers, optimized for tunnel current suppression, within the p-Al0.8GaAs cladding layer of an extreme-triple-asymmetric epitaxial structure with a thin p-side waveguide. The blocking layers are thus in close proximity to the active zone, resulting in strong suppression of current spreading and lateral carrier accumulation. eSAS-V2 devices with 4 mm resonator length and varying stripe widths are characterized and compared to previous eSAS variant (eSAS-V1) as well as gain-guided reference devices, all having the same dimensions and epitaxial structure. Measurement results show that the new eSAS-V2 variant eliminates an estimated 89% of lateral current spreading, resulting in a strong threshold current reduction of 29% at 90 μm stripe width, while slope and series resistance are broadly unchanged. The novel eSAS-V2 devices also maintain high conversion efficiency up to high continuous-wave optical power, with an exemplary 90 μm device having 51.5% at 20 W. Near-field width is significantly narrowed in both eSAS variants, but eSAS-V2 exhibits a wider far-field angle, consistent with the presence of index guiding. Nonetheless, eSAS-V2 achieves higher beam quality and lateral brightness than gain-guided reference devices, but the index guiding in this realization prevents it from surpassing eSAS-V1. Overall, the different performance benefits of the eSAS approach are clearly demonstrated.

Injection-locking of high power diode laser arrays1) is a powerful means to improve the spectral and spatial quality of high power diode lasers. In our experiments the 45mW output of a single stripe diode laser (SDL 5422) was focussed, for example, into a 20 stripe 1W diode laser array (Siemens 480401). The substantial improvement in beam quality is shown in Fig. 1. The injection-locked array emitts about 80% of the 800mW output power of the free running array in a nearly diffraction limited beam. While, free running, the diode laser array is highly multi-mode with a spectral width of 2-3nm, the injection-locked radiation is single-mode with a spectral width of less than 20MHz. Injection-locking thus improved the spatial power density by a factor of 50 and the spectral power density by 5 to 6 orders of magnitude.

We report on the high-power high-temperature long-pulse performance of the 8XX-nm diode laser bars and arrays, which were recently developed at Lasertel Inc. for diode laser pumping within high-temperature (130 °C) environment without any cooling. Since certain energy in each pulse is required, the diode laser bars have to provide both high peak power and a nice pulse shape at 130 °C. Optimizing the epi-structure of the diode laser, the laser cavity and the distribution of waste heat, we demonstrate over 40-millisecond long-pulse operation of the 8XX-nm CS bars at 130 °C and 100 A. Pumping the bar with 5-Hz frequency 15-millisecond rectangular current pulses, we generate over 60 W peak power at 100 A and 130 °C. During the pulse duration, the pulse shape of the CS bars is well-maintained and the power almost linearly decays with a rate of 1.9% peak power per millisecond at 130 °C and 100 A. Regardless of the pulse shape, this laser bar can lase at very high temperature and output pulse can last for 8 ms/2ms at 170 °C/180 °C (both driven by 60 A current pulses with 5-Hz frequency, 10 millisecond pulse width), respectively. To the best of our knowledge, this is the highest operating temperature for a long-pulse 8XX-nm laser bar. Under the condition of 130 °C and 100 A, the laser bars do not show any degradation after 310,000 10-millisecond current pulse shots. The performance of stack arrays at 130 °C and 100 A are also presented. The development of reliable high-temperature diode laser bar paves the way for diode laser long-pulse pumping within a high-temperature environment without any cooling.

“Smile” in high power diode laser arrays is mainly caused by thermal stress generated during packaging processes and operation. By numerical modeling, a three-dimensional finite element model of a conduction-cooled packaged 60W high power diode laser array was established. Finite element method (FEM) was conducted to simulate and analyze the thermal and mechanical characteristic of the diode laser bar in different scenarios. According to the variation of thermal stress and displacement across the laser bar, the influence of temperature and driving current on “smile” was analyzed and discussed. Numerical simulation results show that “smile” will decrease with the increasing heatsink temperature and increase with the increasing driving current. In this paper, the corresponding quantitative relationships of smile with heatsink temperature and driving current were obtained. The work provides theoretical guidance for optimizing high power diode laser array design, packaging process and reducing “smile”.

Laser based displays have gathered much attention because only the displays can express full color gamut of Ultra- HDTV, ITU-R BT.2020. There are many types of laser based displays. A projector with single spatial light modulator (SLM) is a main stream in the displays so far. This projector uses the laser light sources under pulse with duty of 20- 50 %. We improved a triple-emitter 638-nm high-power broad area (BA) laser diode (LD) for this application. Countermeasures were adopted to improve the output power characteristics and strengthen the facet to the fatal degradation. The improved triple emitter showed the output of 5.0 W at the injection current of 5 A under pulse with duty of 30%, case temperature of 25°C. The output was increased by 4.1% compared to the former. The peak wall plug efficiency reached to 41.5% under 25°C. The long term aging test was also performed. Mean time to failure of the improved LD was estimated 57,000 hours under the output of 3.5 W, pulse condition with duty of 30%. The value was approximately 2.6 times longer than that of the former.

High power diode laser arrays have found increasing applications in the field of pumping solid-state lasers and fiber lasers. Due to the thermal crosstalk across diode laser arrays and non-uniformity of local flow rate within microchannel cooler, junction temperature distribution becomes inhomogeneous, consequently leading to spectrum broadening and large beam divergence of diode laser pumping sources. In this work, an analytical method and numerical heat transfer based on finite volume method were employed to optimize the inner structure of microchannel cooler so as to obtain low thermal resistance and uniform junction temperature distribution for the diode laser arrays. Three-dimensional numerical models were developed to study the fluid flow and heat transfer of copper stacked microchannel coolers with different dimensions and arrangements of inner channels and fins. More uniform junction temperature distribution of diode laser array package could be achieved by self-heating compensation with specific coolant covering width. These results could provide significant guidance for the design of microchannel coolers of high power diode laser arrays for better performance.

A yellow green solid-state light source with luminous power output of 969 lumen is demonstrated by high power laser diode pumped static YAG:Ce transparent ceramic plate. The 445-nm blue laser diode array with a full-width-half-maximum (FWHM) of <1.5 nm and with maximum output power of 15 W is designed to attain high spectral absorption efficiency. High power laser diode array beam is collimated, shaped, despeckled and focused to display a Gaussian-like profile with 1.5 mm beam waist (1/e2 radius) to stimulate 1.03 mm thick YAG:Ce transparent ceramic plate with emission spectra at 500–650 nm.

Summary form only given. Packaging of high-power diode lasers represents a crucial issue regarding device lifetime and reliability. In a series of papers we characterized packaging-induced strains in such devices quantitatively. These measurements were based on photocurrent (PC) data. The spectral positions of the optical transitions of quantum-confined carriers were used to get information about the sign, the absolute value and spatial distributions of packaging-induced strains in high-power laser arrays. Now new systematic studies show that packaging sometimes is accompanied by the creation of additional defects in the laser chip resulting in below-bandgap absorption bands. So far the spatial location of these defects is unknown. In the paper we believe we give the first experimental evidence for packaging-induced defects located in the active region of the high-power lasers. A total of 21 devices InGaAlAs/GaAs 2 W single chip devices (from the same wafer) are investigated.

High-power broad area diode lasers emitting at 980 nm with a maximum output power of 20 W at 15degC and with reliable operation over 2000 h at 15 W and 25degC will be presented.

We outline our project to develop a circulating polarized helium-3 system for developing of large, quasi-continuously operating neutron analyzers. The project consisted of four areas: 1) Development of robust external cavity narrowed diode laser output with spectral line width < 0.17 nm and power of 2000 W. 2) Development of large glass polarizing cells using cell surface treatments to obtain long relaxation lifetimes. 3) Refinements of the circulation system with an emphasis on gas purification and materials testing. 4) Design/fabrication of a new polarizer system. 5) Preliminary testing of the new polarizer. 1. Developed Robust High-Power Narrowed Laser The optical configuration of the laser was discussed in the proposal and will be reviewed in the body of this report. The external cavity is configured to mutually lock the wavelength of five 10-bar laser stacks. All the logistical milestones were been met and critical subsystems- laser stack manifold and power divider, external laser cavity, and output telescope- were assembled and tested at low power. Each individual bar is narrowed to ~0.05 nm; when combined the laser has a cumulative spectral width of 0.17 nm across the entire beam due to variations of the bars central wavelength by +/- 0.1 nm, which is similar to that of Volume Bragg Grating narrowed laser bars. This configuration eliminates the free-running “pedestal” that occurs in other external cavity diode lasers. The full-scale laser was completed in 2016 and was used in both the older and newer helium polarizers. This laser was operated at 75% power for periods of up to 8 hours. Once installed, the spectrum became slightly broader (~.25 nm) at full power; this is likely due to very slight misalignments that occurred during handling. 2. Developed the processes to create uniform sintered sol-gel coatings. Our work on cell development comprised: 1) Production of large GE180 cells and explore different means of cell preparation, and 2) Development of apply sol-gel coatings to the interior of both borosilicate and aluminosilicate cells. We applied six sol-gel coatings. By modifying the mixture and developing procedures to drain and dry the cell, we produced visually uniform coatings on the interior of the cells. We now have perfected that process as described below in our report. We were able to accelerate the testing of cells using an ex situ method that avoids installing each cell into a polarizer. In the project’s last year, we conducted 38 external tests of 8 different cells. We also installed two sol-gel coated cells in our polarizers. We created cell with long ex situ relaxation lifetimes, one of which exceeded 40 hours. However, when installed in the polarizer the measured lifetime is 8 hours or less. 3. Demonstrated cycling of polarized gas and ex situ cell testing We are now cycling polarized gas from the polarizer to glass vessels and back. This has allowed us, for the first time, to make ex situ T1 measurements of polarizing cells without installing them into the polarizer itself. This has greatly improved our productivity in producing cells and evaluating our cell preparation processes. We continued development of the gas handling system in parallel with fabricating new polarizer. The integrated system was tested by the end of 2016. We now regularly cycle gas into and out of the polarizer. 4. Completed new polarizer infrastructure and control systems. We completed the new polarizer infrastructure in November 2016. The polarizer subsystems are 1) the frame, 2) the oil flow system, 3) the gas handling system, 4) the pressure vessel, with embedded solenoid, 5) cell mounting hardware with heat spreaders, and 6) electrical power and instrumentation. 5. Carried out initial tests of polarizer. We completed initial testing of the polarizer in April and May of 2017. These tests were carried out for periods up to 6 hours with laser power between 750 and 1300 Watts. The laser performed well and the polarization with asymptotic to 45 percent, which was below expectations. This low value resulted from a stationary thermal inversion in the cell that caused most of the laser power to be absorbed near the laser inlet window and deprived the lower portions of the cell of pumping laser light. Possible solutions to this problem include enhanced cooling of the cell near the laser entry and slight detuning of the laser. 6. Ongoing work. Our polarizer development efforts are ongoing to pursue our interest in neutron analyzers, nuclear targets, and providing helium for medical imaging. Current tests in the pipeline include: 1. Testing cooling enhancements to improve laser penetration of spectrally narrow lasers; 2. Testing of a cell with isolation valves that minimizes diffusive contact with gas handling hardware during polarization; 3. Testing of smaller hybrid cells with reduced alkali loading; 4. Producing polarized helium-3 for MRI imaging at the University of Missouri.