Proc. SPIE. 10514, High-Power Diode Laser Technology XVI
KEYWORDS: Optical amplifiers, Polarization, Semiconductor lasers, Collimation, High power fiber lasers, High power fiber amplifiers, Connectors, Fiber couplers, High power diode lasers, High power fiber coupled lasers
In the most developed fiber amplifiers, optical pump power is introduced into the ~400μm-diameter, 0.46NA first cladding of the double-clad, Yb-doped, gain fiber, using a (6+1):1 multi-mode fiber combiner. For this configuration, the core diameter and numerical aperture of the pump delivery fibers have maximum values of ~225μm and ~0.22, respectively. This paper presents the first fiber-coupled laser-diode pump module emitting more than 1kW of claddingmode- stripped power from a detachable 225μm, 0.22NA delivery fiber at 976nm. The electrical-to-optical power conversion efficiency at 1kW is ~50%. The FWHM spectral width at 1kW output is ~4nm and has an excellent overlap with the narrow absorption spectrum of ytterbium in glass. Six of these pump modules attached to a (6+1):1 multimode combiner enable a 5-6kW, single-mode, Yb-doped fiber amplifier.
Copper-based micro-channel coolers (Cu-MCC) are the lowest thermal-resistance heat-sinks for high-power laserdiode
(LD) bars. Presently, the resistivity, pH and oxygen content of the de-ionized water coolant, must be actively
controlled to minimize cooler failure by corrosion and electro-corrosion. Additionally, the water must be constantly
exposed to ultraviolet radiation to limit the growth of micro-organisms that may clog the micro-channels. In this
study, we report the reliable, care-free operation of LD-bars attached to Cu-MCCs, using a solution of distilledwater
and ethanol as the coolant. This coolant meets the storage requirements of Mil-Std 810G, e.g. exposure to a
storage temperature as low as -51°C and no growth of micro-organisms during passive storage.
Recent advances in thermal management and improvements in fabrication and facet passivation enabled extracting unprecedented optical powers from laser diodes (LDs). However, even in the absence of thermal roll-over or catastrophic optical damage (COD), the maximum achievable power is limited by optical non-linear effects. Due to its non-linear nature, two-photon absorption (TPA) becomes one of the dominant factors that limit efficient extraction of laser power from LDs. In this paper, theoretical and experimental analysis of TPA in high-power broad area laser diodes (BALD) is presented. A phenomenological optical extraction model that incorporates TPA explains the reduction in optical extraction efficiency at high intensities in BALD bars with 100μm-wide emitters. The model includes two contributions associated with TPA: the straightforward absorption of laser photons and the subsequent single photon absorption by the holes and electrons generated by the TPA process. TPA is a fundamental limitation since it is inherent to the LD semiconductor material. Therefore scaling the LDs to high power requires designs that reduce the optical intensity by increasing the mode size.
Dense array slab-coupled optical waveguide lasers (DASCOWLs) consist of several hundred single-mode SCOWL
lasers on a monolithic bar. Near diffraction-limited output of the SCOWLs is preserved with spacing down to 40μm.
Greater than 200W CW operation of a 4% FF, 100-element, 100μm-pitch, centimeter wide DASCOWL bar has
been demonstrated, corresponding to <2W/emitter in array format. We have also demonstrated near 500W
continuous wave (CW) operation from a 10% fill factor (FF) 1-cm wide, 1cm long DASCOWL bar which contains
250 emitters, with a 40μm pitch. The goal of 2W/emitter, 500W/bar represents a 5X increase above the conventional
10-emitter, 10% FF broad area laser diode bar that operates at 10W/100μm-emitter. Some of the reported
DASCOWL performance benefits from SRL’s low thermal resistance EPIC heat sinks.
We present a novel, high-power stack of 20% fill-factor, 976nm, laser-diode bars, each directly attached to an enhanced lateral-flow (ELF), copper-based, water-cooled heat-sink. The heat-sinks contain mounting screws that form a kinematic mount to minimize detrimental mechanical-stress on the diode bars while also providing beneficial, double-side cooling of the bars. A stack of 18-bars, emitting 2.54kW, was constructed to validate the technology. Using standard optics and a polarization multiplexer, a 320μm diameter, 0.3NA focus is achieved with a 6-bar stack that robustly couples 450W, with a ~67% coupling efficiency, from a passive, 400μm, 046NA doubleclad fiber.
The slow axis (SA) divergence of 20% fill-factor, 980nm, laser diodes (LDs) have been investigated under short pulsed
(SP) and continuous (CW) operation. By analyzing the data collected under these two modes of operation, one finds that the SA divergence can be separated into two components: an intrinsic divergence and a thermally induced divergence. At low injected current and power, the intrinsic SA divergence is dominant while at high power their magnitudes are approximately equal. The thermal gradient across the broad stripe is negligible under SP operation and, the SA divergence increased at a much slower rate as a function of injected current, thereby increasing the brightness of the LD by 2X. SRL has redesigned microchannel coolers that remove the thermal gradient under CW operation thereby eliminating the thermally induced SA divergence resulting in LDs that are 2X brighter at 300W/bar.
High brightness, laser-diode bars are required for efficient coupling into small-core optical-fibers. Record power and
brightness results were achieved using 20% fill-factor, 980nm, 1cm-wide, 4mm cavity-length bars. Lifetimes of single
bars, operated CW at 200W and 20°C, exceed 1000hr. Due to superb thermal management, the power conversion
efficiency (PCE) exceeds 60% at 200W output power. Similar lifetime and PCE were obtained for a 3-bar stack
emitting 600W output power.
A record, 940W, CW output-power has been achieved for a single, 1cm-wide, 5mm cavity-length, 77% fill-factor,
940nm, laser-diode bar operated at 900A and 20°C heat-sink temperature. The slope efficiency below 400A is 1.2W/A
and the peak power-conversion efficiency is 70%. The laser bar was attached to a novel EPIC (Enhanced Performance
Impingement Cooler) heat-sink which has a heat removal capacity exceeding 3kW/cm<sup>2</sup>. Constant current operation at
580A (~600W), 20°C over a period of 100hrs was also demonstrated. These record results are, in large part, due to the
record low thermal resistance of 0.060K/W, about a third that of micro-channel coolers.
A record, 250W, CW output-power has been achieved for a single, 1cm-wide, 3.5mm cavity-length, 20% fill-factor,
976nm, laser-diode bar operated at 20°C. The remarkable laser-bar performance was in part the result of a novel
EPIC (Enhanced Performance Impingement Cooler) heat-sink with a thermal resistance of 0.16K/W. The superb
thermal management resulted in record brightness for a laser bar, i.e. a slow-axis divergence of 10° (95% power
containment angle) was achieved at 200W output-power. A coupling efficiency of ~74% into a 200μm core, 0.22NA
fiber was achieved.
The present model of formation and propagation of catastrophic optical-damage (COD), a random failure-mode in laser
diodes, was formulated in 1974 and has remained substantially unchanged. We extend the model of COD phenomena,
based on analytical studies involving EBIC (electron-beam induced current), STEM (scanning transmission-electron
microscopy) and sophisticated optical-measurements. We have determined that a ring-cavity mode, whose presence has
not been previously reported, significantly contributes to COD initiation and propagation in broad-area laser-diodes.
Detailed reliability studies of high-power, CW, broad-area, GaAs-based laser- diodes were performed. Optical and
electrical transients occurring prior to device failure by catastrophic optical-damage (COD) were observed. These
transients were correlated with COD formation as observed in laser diodes with an optical window in the n-side
electrode. In addition, custom electronics were designed to fault-protect the laser diodes during aging tests, i.e. each time
a transient (fault) was detected, the operating current was temporarily cut off within 4μs of fault detection. The lifetime
of fault-protected 808-nm laser-diode bars operated at a constant current of 120A (~130W) and 35°C exceeded similar
unprotected devices by factors of 2.