In the past decade of development in fiber laser technology, the market share of high power full fiber lasers has expanded tremendously to replace traditional gas and solid laser systems. While fiber lasers offer advantages of compact dimensions, high conversion efficiency, and full fiber integration, cost is still a major challenge for fiber laser manufacturing. With an increasing power rating, high power diode pump lasers become the most critical component to control the cost margin of fiber laser. Oclaro offers the most complete solution of fiber coupled diode lasers for fiber laser customers with platforms of multi-mode single emitter, multi emitters and mini-bar fiber coupled, with the benefit of competitive $/W margin and Telcodia standard high reliability performance .
In this work we show that mini-bar-based 8xx products show the reliability characteristics of independent emitter
failures and "non-degrading" drift plots similar to those of their 9xx counterparts. This fact is in part an outcome of the
bonding process and heat-sink design. Multi-cell life testing gives projected reliable operation of compact, fiber-coupled
modules (200-μm-diameter, 0.15 NA) at 30 W and 808 nm, with 35 W at 880 nm.
New-generation multi-mode 9xx mini-bars used in fiber pump modules have been developed. The epitaxial designs have
been improved for lower fast-axis and slow-axis divergence, higher slope efficiency and PCE by optimizing layer
structures as well as minimizing internal loss. For 915nm mini-bars with 5-mm cavity length, maximum PCE is as high
as ~61% for 35W operation and remains above 59% at 45W.
For 808nm, a PCE of 56% at 135W CW operation has been demonstrated with 36%-fill-factor, 3-mm-cavity-length,
water-cooled bars at 50°C coolant temperature. On passive-cooled standard CS heatsinks, PCE of >51% is measured for
100W operation at 50°C heatsink temperature. Leveraging these improvements has enabled low-cost bars for high-power,
Continued advances in high power diode laser technology enable new applications and
enhance existing ones. Recently, mini-bar based modules have been demonstrated which combine
the advantages of independent emitter failures previously shown in single-stripe architectures with
the improved brightness retention enabled by multi-stripe architectures.
In this work we highlight advances in a family of compact, environmentally rugged mini-bar
based fiber coupled Orion modules. Advances in PCE (power conversion efficiency) and reliable
operating power from a 9xx nm wavelength unit are shown from such modules. Additionally,
highly reliable fiber coupled operation and performance data is demonstrated in other wavelengths
in the 780 - 980 nm range. Data demonstrating the scaling this technology to 25W and higher
power levels will be given.
Fiber combining multiple pump sources for fiber lasers has enabled the thermal and
reliability advantages of distributed architectures. Recently, mini-bar based modules have been
demonstrated which combine the advantages of independent emitter failures previously shown in
single-stripe pumps with improved brightness retention yielding over 2 MW/cm<sup>2</sup>Sr in compact
economic modules. In this work multiple fiber-coupled mini-bars are fiber combined to yield an
output of over 400 W with a brightness exceeding 1 MW/cm<sup>2</sup>Sr in an economic, low loss
High-power, packaged diode-laser sources continue to evolve through co-engineering of epitaxial design, beam conditioning and thermal management. Here we review examples of improvements made to key attributes including reliable power, brightness, power per unit volume and value.
As GaAs based laser diode reliability improves, the optimum architecture for diode pumped
configurations is continually re-examined. For such assessments, e.g. bars vs. single emitters, it is
important to have a metric for module reliability which enables comparisons that are the most
relevant to the ultimate system reliability. We introduce the concept of mean time between emitter
failures (MTBEF) as a method for characterizing and specifying the reliability of multi-emitter
pumps for ensemble applications. Appropriate conditions for an MTBEF model, and the impact of
incremental changes of certain conditions on the robustness of the model are described.
In the limit of independent random failures of individual emitters as the dominant failure mechanism
it is shown that an ensemble of multi-emitter modules can be modeled to behave like an ensemble of
single emitter modules. The impact of thermal acceleration due to failed emitters warming other
emitters on a shared heat-sink is considered. Data taken from SP built multi-emitter devices bonded
with AuSn on CTE matched heat-sinks is compared with the MTBEF model with and without
correction for the thermal acceleration effect.
Leveraging improvements to device structures and cooling technologies, ultra-high-power bars have been integrated into
multi-bar stacks to obtain CW power densities in excess of 2.8 kW/cm<sup>2</sup> near 960 nm with spectral widths of <4nm FWHM. These characteristics promise to enable cost-effective solutions for a variety of applications that demand very high spatial and/or spectral brightness. Using updated device designs, mini-bar variants have been employed to derive CW powers of several tens of Watts near 940 nm on traditional single-emitter platforms. For example, >37 W CW have been obtained from 5-emitter devices on standard CuW CT heatsinks with AuSn solder. Near 808 nm, a PCE of 65% with a slope efficiency of 1.29 W/A has been demonstrated with a 20%-fill-factor, 2-mm-cavity-length bar.
Here we present characteristic performance of laser-diode devices employing a novel CTE-matched heatsink technology
(where CTE is Coefficient of Thermal Expansion). Design variants of the composite-copper platforms include form-fit-compatible
versions of production CS (for standard 1-cm-wide bars) and CT (for single-emitter devices and mini-bars)
assemblies. Both employ single-step AuSn bonding and offer superior thermal performance to that of current production
standards. These attributes are critical to reliability at high powers in both CW and hard-pulse (e.g., 1sec on/1sec off)
The superior thermal performance of the composite-copper CS device has been verified in CW testing of bars where
85W is typically obtained at 95A (compared to 76W from production-standard, indium-bonded, solid-copper CS
devices). This result is especially significant as alternative CTE-matched bar platforms (e.g., those employing a sub-mount
bonded to a solid copper heatsink) typically compromise the effective thermal resistance in order to achieve the
CTE match (and often require two-step bonding). The close CTE match of the composite-copper CS results in relatively
narrow, single-peaked spectra. Initial step stress tests of eight devices in hard-pulse operation up to 80A has been
completed with no observed failures. Six of these devices have subsequently been operated in hard-pulse mode at 55A
for >4000 with no failures.
The CT variant of the composite-copper heatsink is predicted to offer a reduction in thermal resistance of nearly 30% for
a 5-emitter mini-bar (500-μm pitch). In first-article testing, the maximum achievable CW power increased from 20W
(standard CuW CT) to 24W (composite-copper CT). As with the CS devices, the composite-copper CT assemblies
exhibited characteristically narrower spectral profiles.
This paper gives an overview of recent product development and advanced engineering of diode laser technology at
Spectra-Physics. Focused development of device design, heat-sinking and beam-conditioning has yielded significant
improvement in both power conversion efficiency (PCE) and reliable power, leading to a family of new products. CW
PCEs of 60% to 70% have been delivered for the 880 to 980 nm wavelength range. For 780 to 810 nm, PCE are typically
between 50% and 56%. Comprehensive life-testing indicates that the reliable powers of devices based on the new
developments exceed those of established, highly reliable, production designs.
For the progress of ultra-high power bars, CW output power in excess of 1000 W and 640 W have been demonstrated
from single laser bars with doubled-side and single-side cooling, respectively. Spatial power density of greater than 2.8
kW/cm<sup>2</sup> and FWHM spectral widths of 3.5 nm have been obtained from laser stacks.
Successful thermal and stress management of edge-emitting GaAs-based diode lasers is key to their performance and
reliability in high-power operation. Complementary to advanced epitaxial structures and die-fabrication processes, next-generation
heatsink designs are required to meet the requirements of emerging applications. In this paper, we detail the
development of both active and passive heatsinks designed to match the coefficient of thermal expansion (CTE) of the
laser die. These CTE-matched heatsinks also offer low thermal resistance, compatibility with AuSn bonding and
improved manufacturability. Early data representing the performance of high-power devices on the new heatsinks are
included in the presentation.
Among the designs are a water-cooled, mini-channel heatsink with a CTE of 6.8 ppm/°C (near to the nominal 6.5
ppm/°C CTE of GaAs) and a thermal resistance of 0.43 °C/W (assuming a 27%-fill-factor diode-laser bar with a cavity
length of 2 mm). The water flow in the heatsink is isolated from the electrical potential, eliminating the possibility of
electrolytic corrosion. An additional feature of the integrated design is the reduction in required assembly steps.
Our next-generation, passive, CTE-matched heatsink employs a novel design to achieve a reduction of 16% in thermal
resistance (compared to the predecessor commercial product). CTE's can be engineered to fall in the range of
6.2-7.2 ppm/°C on the bar mounting surface. Comparisons between simulated performance and experimental data (both
in CW and long-pulse operation) will be presented for several new heat-sink designs.
Developers building high-power fiber lasers and diode pumped solid state lasers can receive significant benefits in thermal management and reliability by using single emitter multi-mode diodes in distributed pump architectures. This proposed distributed architecture relies on independent single emitter pump lasers and a modest level of pump redundancy. Driving the remaining diodes slightly harder componensates individual diode failures. A model of the ensemble lifetime based on module failure rates and power-scaling factors demonstrates that the distributed pump architecture requires random failure rates corresponding to better than 200,000h mean time between failure (MTBF), which meets typical industrial requirements. A high power, pigtailed, multi-mode pump module suitable for commercial applications is created through this model. Critical elements are based on telecom architectures, including the optical train and the fiber alignment. The module has a low thermal resistance of 4°C/W from the chip-on-sub-mount to the external heat sink, coupling efficiency of over 80% into 0.2 NA, and demonstrated reliable output power of over 5W cw with peak wavelengths near 915 nm. Individual pump modules are predicted to produce 5W cw output power with an MTBF of more than 400,000h. The relationship between anticipated MTBF requirements, test duration and test population is shown.
Multi-mode pumps based on single emitter diodes deployed in distributed pump architectures offer significant advantages in thermal management and reliability for pumping high-power fiber lasers and amplifiers. In a distributed architecture, while individual diode failures do not directly generate failures of other diodes in the distributed ensemble, failures do cause the rest of the sources to drive to higher power levels to compensate for the loss of power. A model of the ensemble lifetime based on module failure rates and power-scaling factors demonstrates that the distributed pump architecture requires random failure rates corresponding to better than 200,000 h mean time between failure (MTBF) to meet typical application requirements. A high power multi-mode pump module suitable for commercial aplications is shown. Critical elements are based on telecom architectures, including the optical train and the fiber alignment. The module has a low thermal resistance of 4 C/W from the laser diode junction to the external heat sink, couplng efficiency of over 80% into 0.2 NA, and demonstrated reliable output power of over 5W CW with peak wavelengths near 915 nm. Telecom qualified modules have random failure rates corresponding to better than 1,000,000 h MTBF. Stability of the critical fiber alignment joint for single mode packages has been demonstrated at elevated temperatures (e.g. 85 C) for thousands of hours. The reliability of the commercial multi-mode package can be estimated by similarity to the telecom package, and is verified by testing of conditions considered to be at risk based on the differences between the known telecom, and the new commercial package, designs. Test results are shown for temperature cycling, CW operation, and damp heat. The relationships between anticipated MTBF requirements, test duration and test population are shown.
A single laser diode bar design, based on the AlGaAs material system, has been developed for high power, high reliability operation at a variety of CW and QCW operating conditions. The bar has a cavity length of 750 micrometer and a fill factor of 40%. Typical CW operation has a threshold current of approximately 10A and a conversion efficiency of greater than 45% at 40W. A variety of lifetests have been conducted at both CW and QCW operating conditions from the same bar design. On- going 3000 hr CW operation at 45C and 40W shows an extrapolated median lifetime (20% current increase) of 16,500 hrs at 45C or approximately 50,000 hrs at 25C (with 0.45eV activation energy). On-going 3000 hr QCW operation at 60C/60W and 35C/100W, with a pulse width of 200 microseconds and a duty factor of 2%, shows a median lifetime of approximately 10 billion shots and approximately 5 billion shots, respectively. In addition to single bar operation, this bar design can be stacked in various 2-D configurations. A 4 bar linear stack operating at 160W CW and a 6 bar vertical stack operating at 240W CW have been developed with superior performance. Results for high duty and low duty QCW stacks will also be presented.
A low cost and highly reliable fiber coupled laser diode is demonstrated with up to 1.0 W output power in 0.14 NA out of a 60 micrometer core fiber. Package reliability with extended operation at 600 mW is shown for over 600 hours at 85 degrees Celsius, and with stringent environmental tests, including thermal cycling, high temperature storage, and two sec. on two sec. off power cycling.
Sophisticated packaging architectures have been developed that enable low cost, very high average power, long lived pumping of solid state lasers. Single water cooled manifolds now provide slab pumping of up to 2.5 kW of average optical power, while low cost yet flexible bar mounting techniques allow burn-in that enables very long lifetimes. Architecture modification allows for high peak power of up to 80 kW per water cooled pump manifold. Specialized high brightness packaging now allows approximately 20 watt cw bars to be lensed into less than 200 micrometers diameter spot sizes (approximately 54 kW/cm<SUP>2</SUP>).
A compact, broad beam diode based laser transmitter has been developed for moderate range and data rate free space laser communications. The laser transmitter spatially combines high average power AlGaAs laser diodes in the near field and overlaps several beams into a uniform beam in the far field. Individual high brightness laser diodes are lensed with a cylindrical lens along the length of the emitting aperture and projected with a short focal length lens into a broad beam of several milliradian divergence in the far field. The engineering prototype laser transmitter consists of six precision aligned diode assemblies, two fold mirrors and a six lenslet macrooptic assembly all mounted on a beryllium baseplate. Data will be presented showing that the laser transmitter is highly efficient by reserving the inherent high brightness of the individual diodes in the optical design, by the development of a pulsed switcher electrical circuit based on recent lightweight dc power converter designs for spacecraft applications and the removal of excess diode heat via the beryllium baseplate.
Methods of reformatting the output of laser diodes and maintaining much of their intrinsic brightness are discussed. A commercial, fiber-coupled package is shown with a symmetric etendue and a brightness of 15 kW/(cm<SUP>2</SUP> sr). A symmeterized beam with a brightness of 200 kW/(cm<SUP>2</SUP> sr) is demonstrated by using a combination of a micro-lensed diode array and a lens array.