Diode Laser Bars Research Articles

Optical trapping, manipulation, and sorting of cells and colloids in microfluidic systems with diode laser bars

We demonstrate a new technique for trapping, sorting, and manipulating cells and micrometer-sized particles within microfluidic systems, using a diode laser bar. This approach overcomes the scaling limitations of conventional scanned laser traps, while avoiding the computational and optical complexity inherent to holographic optical trapping schemes. The diode laser bar enables us to control a large trapping zone, 1 microm by 100 microm, without the necessity of scanning or altering the phase of the beam.

Characteristics of Laser Diode Bar and Stack with Jet-Type, Water-Cooled Heatsink

Our group has developed a funryu (“fountain flow” in Japanese) heatsink structure for a 1 cm laser diode (LD) bar. The funryu heatsink is only 1.1 mm in thickness and is designed in stackable form. With the help of a good indium soldering technique and detailed thermal analysis, this device achieved a thermal resistance of only 0.25°C/W, defined by the temperature increase per unit of heat dissipation. This thermal resistance is one of the lowest ever attained by a laser diode bar. By stacking 30 LD bars equipped with these funryu heatsinks, we achieved a CW output power of 2.3 kW from a 10 mm×45.8 mm laser-emitting area within the stacked-LD module. This power intensity corresponds to 502 W/cm2.

Nd:glass regenerative amplifier for CPA (chirped-pulse amplification) lasers

We demonstrate a highly stable, diode-pumped regenerative amplifier for CPA laser systems that utilizes Nd:glass as the active medium. When end pumped with a 120-W quasi-cw (QCW) diode laser bar, a maximum pulse energy of 270μJ has been achieved at an excellent pulse-to-pulse energy stability of 0.56% rms.

Simultaneous quantitative determination of strain and defect profiles within the active region along high-power diode laser bars by micro-photocurrent mapping

We present microscopically-resolved photocurrent spectroscopy as a new powerful analytical tool for the simultaneous detection of packaging-induced strain and defects in GaAs-based high-power laser diode arrays (cm-bars). Using the Fourier-transform (FT) technique we measure photocurrent (PC) spectra with a high spatial resolution of better than 50 μm at the active layer of the device. By analyzing this data, spatially resolved distributions of strain as well as of defects are obtained. So far, PC measurements at cm-bars have only provided an overview of the distribution of strain and defects in the device on an emitter-by-emitter scale. The high spatial resolution now allows examination of the distribution of strain and defects even within one single emitter. This is essential for obtaining insights into device failure mechanisms and also makes a substantial contribution for improving device performance and reliability.

Experimental study of high-power pulse side-pumped Nd:YAG laser

The paper reports on the characterization of a compact and simple side-pumped 0.538 J×100 Hz pulse Nd:YAG laser. A side-pumping configuration with 100 laser diode bars is used in the laser head. We also experimentally studied the laser performance of the diode-pumped Nd:YAG laser head in the free running and Q-switched operation under different repetition rates.

Effective suppression of beam divergence for a high-power laser diode bar by an external-cavity technique.

We describe effective suppression of beam divergence for a high-power laser diode bar by use of an external-cavity technique. Nineteen off-axis external-cavity laser diodes of the high-power laser diode are formed by feedback with a stripe mirror. At three times the threshold current, the diverging angle (1/e2) of the external-cavity laser diode bar is reduced to 1.5 degrees from 6.6 degrees (free running) with 14.1-W peak output power and 70.4% of the radiated power of the free-running state without the external cavity. This technique effectively improves the beam quality of the high-power laser diode bar.

Jet-type, water-cooled heat sink that yields 255-W continuous-wave laser output at 808 nm from a 1-cm laser diode bar

A newly designed jet-type, water-cooled heat sink (the funryu heat sink, meaning fountain flow in Japanese) yielded 255-W cw laser output at 808 nm from a 1-cm bar made from InGaAsP/InGaP quantum-well active layers with a 67% fill factor [70 quantum-well laser diode (LD) array along the 1-cm bar]. A funryu heat sink measuring 1.1 mm in thickness gave the LD 0.25 degrees C/W thermal resistance, one of the lowest values achieved with a 1-cm LD bar. Over a short period of operation, the device reached a maximum cw power of 255 W. To the best of our knowledge, this is the highest power ever achieved in 808-nm LD operation. In the future, the funryu heat sink may be capable of 80-W cw operation over an extended lifetime of several thousand hours.

Study of a Nd3+:KGW crystal laser transversely pumped by laser diode bars

A Nd3+:KGW crystal laser transversely pumped by laser diode bars is studied experimentally. The optimisation of the laser parameters provided the maximum slope efficiency of ~50 % at 1.067 μm in the free running regime. Using the SRS self-conversion in a Nd3+: KWG laser, lasing was obtained in the eye-safe spectral region at 1.538 nm with the energy up to 5 mJ and a pulse repetition rate up to 20 Hz.

Reliable high-efficiency high-brightness laser diode bars at [formula omitted

High-quality InGaAs/AlGaAs laser diode bars emitting at 940 nm have been fabricated by low-pressure metal-organic chemical vapor deposition (LP-MOCVD). Two hundred and ten Watts maximum continuous-wave output power and a maximum power conversion efficiency of 60% at an output power of 72 W have been demonstrated for a single 1-cm-wide laser bar. These bars exhibit a very good beam quality of 5.7°×27.2° (full-width at half-maximum). Reliability test have been carried out for over 2000 h at ∼58 W at room temperature. Under these conditions, the extrapolated lifetime is ∼100,000 h , which suggests that AlGaAs-based lasers of proper designs could have similar long-term reliability as their Al-free counterpart.

Spatially resolved spectroscopic strain measurements on high-power laser diode bars

We compare a number of strain-sensitive spectroscopic techniques, namely, micro-Raman, micro-photoluminescence, photocurrent, and electroluminescence by applying them to two AlGaAs/GaAs high-power diode laser arrays, so-called “cm-bars.” Both devices are fabricated from the same bar batch and considered “identical,” but experienced two different packaging procedures that resulted in different intentionally adjusted “packaging-induced” compressive strains along the array, namely, about −(3.0±0.5)×10−4 and −(5.0±0.7)×10−4 strain difference between edges and centers of the bars. The methods are primarily evaluated with respect to their ability to monitor and quantify the different spatial strain distributions along the devices. In addition their potential for defect detection is demonstrated. Pros and cons of the techniques are summarized and discussed.

Investigation of reliability issues in high power laser diode bar packages

As applications continue to demand increasingly higher optical output power and longer lifetime, thermo-mechanical stresses on dissimilar materials interfaced for packaging pose an ever-growing challenge for the realization of a durable system. Particularly important for an epitaxy-down configuration is the die-attachment interface, which is desired to be defect free and stress managed for reliable optical alignment. A knowledge of the changes in the physical defect density and magnitude of the thermo-mechanical stress present in the active region as a function of the fabrication process and aging is crucial to an understanding of the influence of the process parameters and operating conditions on device performance and reliability. In this study, we investigated high power laser diode array packages aged under various conditions. Microscopic defect analyses of the die attachment interface and device stress were carried out using primarily metallography, scanning electron microscopy, scanning acoustic microscopy, microhardness, and micro-Raman spectroscopy. It was noted that the intermetallic compounds and microscopic physical defects at the die attach interface are detrimental to transient heat transfer, and thus, overall package reliability. Using micro-Raman spectroscopy, we found that tensile stress near the bar-package interface increases with aging for the first few hundred hours and then decreases with further aging.

Additional possibilities offered by using porous metallic materials for cooling laser diode bars

A new scheme of using porous metallic materials (PMs) for cooling laser diode bars is proposed, which provides for a two-dimensional regime of heat removal and reduces the effective filtration length of a cooling agent. The values of characteristic limiting thermal fluxes are calculated for the heat sink made of a permeable PM, operating under conditions of a maximum permissible temperature of the active layer in a laser diode bar.

High-average-power Nd:YAG planar waveguide laser that is face pumped by 10 laser diode bars

A planar waveguide Nd:YAG laser is pumped with 430 W of power from 10 laser diode bars to produce a multimode output power of 150 W at an optical efficiency of 35%. Use of a hybrid resonator of the positive-branch confocal unstable type for the lateral axis and of one of the near-case I waveguide type for the transverse axis increased the laser brightness by a factor of ~26 with only 12% less power than in the multimode case.

Quasi-CW 110 kW AlGaAs Laser Diode Array Module for Inertial Fusion Energy Laser Driver

We have successfully demonstrated a large aperture 803 nm AlGaAs diode laser module as a pump source for a 1053 nm, 10 J output Nd:glass slab laser amplifier for diode-pumped solid-state laser (DPSSL) fusion driver. Detailed performance results of the laser diode module are presented, including bar package and stack configuration, and their thermal design and analysis. A sufficiently low thermal impedance of the stack was realized by combining backplane liquid cooling configuration with modular bar package architecture. Total peak power of 110 kW and electrical to optical conversion efficiency of 46% were obtained from the module consisting of a total of 1000 laser diode bars. A peak intensity of 2.6 kW/cm2 was accomplished across an emitting area of 418 mm×10 mm. Currently, this laser diode array module with a large two-dimensional aperture is, to our knowledge, the only operational pump source for the high output energy DPSSL.

Q-switched, TEM 00-mode, diode-pumped Yb 3+:YAG laser with extended tunability

The results of a comprehensive investigation of the Yb:YAG laser pumped by a single InGaAs laser diode bar stack with the maximum output peak power of 420 W at 940 nm are presented. The maximum laser output pulse energy in a free running multimode operation at a pulse repetition rate of 10 Hz was 94 mJ/pulse at the pump pulse duration of 1 ms and pump pulse energy of 420 mJ. Tunability in a Q-switched regime was demonstrated in the wavelength range of 1023–1054 nm with the laser output bandwidth of 0.5 nm. This is to the best of our knowledge the widest tunability range obtained for the Q-switched, diode-pumped TEM 00-mode Yb:YAG laser. The maximum Q-switched TEM 00 laser output was 6.4 mJ/pulse at 1030 nm and 2.1 mJ/pulse at 1047 nm. The highest laser efficiency obtained in a free running operation was 29% with respect to the absorbed pump power and 22.3% with respect to the incident pump power. An advanced model of the laser performance has been developed and found to be in a good agreement with the obtained experimental results.

A planar waveguide Nd:YAG laser with a hybrid waveguide–unstable resonator

Mode selection processes are presented for a 200 μm active thickness Nd:YAG planar waveguide laser, pumped by 10 laser diode bars in a multi-pass, slotted mirror pump chamber. A hybrid waveguide–unstable resonator is shown to be effective at mode control, producing a single, edge-coupled output beam of up to 5 W average power for quasi-CW pulsed pumping. A lateral M2 factor of 1.5 and a low order waveguide mode with a transverse M2 factor of 4.3 is obtained with 4 W output. The resonator configuration is shown to be unaffected by transverse or lateral thermal lens effects.

Micro thermal management of high-power diode laser bars

Lifetime and reliability of high-power diode laser bars are sensitively related to operating temperature, mounting stress, and solder electromigration. These three factors have been taken into account for the development of a new packaging technology for 1 cm laser bars of gallium arsenide. We examine the use of chemical-vapor-deposited (CVD) diamond as heatspreaders in order to reduce thermal resistance of a microchannel cooler for liquid cooling. We show that it is possible to perform hard soldering on a CVD-diamond with a new technique. Additionally, we present a controlled water cooling system fit to the flow characteristics of the cooler. It permits one to adjust the emission wavelength of the diode lasers by changing the water flux.

Three-fold effective brightness increase of laser diode bar emission by assessment and correction of diode array curvature

An optical arrangement is used to characterize diode bar curvature which is known to limit the effective brightness achievable with such devices. By introducing an inclined, cylindrical collimating lens in front of the diode bar, the curvature of the diode's beam can be reduced by more than 60%. The correct inclination angle for this method is derived. Using a beam shaper together with this correction mechanism we achieve a homogeneous beam profile with low M 2 values in orthogonal directions and an effective brightness increase of more than 200%.

Coupling from Laser Diode Bar to Single-mode Fiber Using a Stubbed Waveguide

We demonstrate a simple method for coupling from a laser diode bar to a single-mode fiber using a stubbed waveguide. The stubbed waveguide serves as a mode homogenizer that converts the rectangular multimode distribution of the emission of the laser diode bar into a circular beam with a smooth profile. Then the output beam is coupled to the lensed single-mode fiber. With this method, a laser diode bar operating at 795 nm is coupled into a 4 km single-mode silica fiber and the first order of stimulated Raman scattering of 100 mW in the fiber is observed.

100-Watt laser bars based on phase-locked arrays

High-power diode laser bars (DLBs) based on InGaAlAs quantum-confinement heterostructures with an output optical power of no less than 100 W in a quasi-continuous wave regime (pulse duration, 200–400 μs; repetition rate, 50–100 Hz) were fabricated and investigated. A bar of this type consists of a set of strip emitters, each representing an optimized phase-locked array of single-mode emitters. This design provides for a considerably improved temporal and spatial stability of the DLB emission, with a simultaneous decrease in the optical noise level.