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.
In this paper, we present hybrid assembly technology to maximize coupling efficiency for spatially combined laser systems. High quality components, such as center-turned focusing units, as well as suitable assembly strategies are necessary to obtain highest possible output ratios. Alignment strategies are challenging tasks due to their complexity and sensitivity. Especially in low-volume production fully automated systems are economically at a disadvantage, as operator experience is often expensive. However reproducibility and quality of automatically assembled systems can be superior. Therefore automated and manual assembly techniques are combined to obtain high coupling efficiency while preserving maximum flexibility. The paper will describe necessary equipment and software to enable hybrid assembly processes. Micromanipulator technology with high step-resolution and six degrees of freedom provide a large number of possible evaluation points. Automated algorithms are necess ary to speed-up data gathering and alignment to efficiently utilize available granularity for manual assembly processes. Furthermore, an engineering environment is presented to enable rapid prototyping of automation tasks with simultaneous data ev aluation. Integration with simulation environments, e.g. Zemax, allows the verification of assembly strategies in advance. Data driven decision making ensures constant high quality, documents the assembly process and is a basis for further improvement. The hybrid assembly technology has been applied on several applications for efficiencies above 80% and will be discussed in this paper. High level coupling efficiency has been achieved with minimized assembly as a result of semi-automated alignment. This paper will focus on hybrid automation for optimizing and attaching turning mirrors and collimation lenses.
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.
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, 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.