In this communication we present the characteristics of Bookham's MU7-9xx-01 laser module with multimode fiber output. This latest generation of our multimode modules is designed for light output power of up to 7 W in uncooled operation in the wavelength range between 915 nm and 975 nm. The key element of the module is our new SES8-9xx-01 broad area single emitter. These high power lasers in the 9xx nm wavelength range show a high slope efficiency of up to 1.2 W/A in CW room temperature operation. High efficiency combined with low threshold current and low operation voltage result in a maximum wall plug efficiency of above 65%. Almost 4000 h lifetest data at accelerated conditions are available for the laser diodes. The data give estimated reliability values of below 5 kFIT at operating conditions (between 8 A and 8.5 A injection current at up to 35°C heat sink temperature). The robustness of the new lasers is also illustrated by the fact that no catastrophic mirror damage was observed up to 22.5 W of light output power. The low divergence of the laser beam allows coupling into multimode fiber with 0.15 or 0.22 numerical aperture (NA) with a coupling efficiency above 90% at operation condition. Maximum ex-fiber light output powers of 11.5 W are shown. On module level around 2000 h lifetest data are accumulated without any failure or sign of degradation.
Based on the most recent generation of Bookham's laser diode bars in the 9xx nm wavelength range which are able to deliver in excess of 250 W of output power from 50% filling factor 2.4 mm cavity length design, we have developed low 20% fill-factor bar devices for high brightness applications. Close to 200 W of output power has been achieved in CW mode from actively cooled micro-channel cooler devices without signs of damage. Mounted on conductively cooled copper blocks, still more than 130 W (CW) has been obtained, indicating the high conversion efficiency of >60% reducing the thermal load on the mounting assembly. Based on extensive reliability testing in excess of 5000 h and at power densities ranging up to 36mW/um and beyond, highly reliable operation of 20% fill-factor bars is expected. To facilitate fiber coupling into wide-core multi-mode fibers a further reduction of the emitter aperture has been realized. From a single 3.6 mm cavity length by 800 um wide emitter design ("MaxiChip") about 50 W output power has been obtained in CW mode from devices mounted on standard conductively cooled 1x1 inch copper blocks. While CW operation has been thermally limited, extremely high peak power operation can be expected in qCW operation. Due to the narrow aperture of this MaxiChip efficient and easy coupling into wide aperture multimode fibers can be achieved.
We report on the development of a new cost-effective, small form-factor laser source at a wavelength of 980 nm. The laser module is based on proven technology commonly used for pump laser modules deployed in fiber amplifiers of telecommunication networks. The package uses a state-of-the-art 14-pin butterfly housing with a footprint of 30x15 mm2 with a Fabry-Perot AlGaAs-InGaAs pump laser diode mounted inside having an anti-reflection coating on its front facet. The light is coupled into a single-mode polarization-maintaining fiber with a mode-field diameter of 6.6 micrometer. The spectral properties of the source are defined by a fiber Bragg grating (FBG) that provides feedback in a narrow reflection band. The laser back facet and the FBG form a long resonant cavity of 1.7 m length in which laser light with a low coherence length of a few cm is generated. This configuration with the laser being operated in the coherence-collapse regime has the advantage of being robust against variations in the optical path, thus enabling stable and mode-hop free emission. The laser module has the following properties: a continuous-wave fiber output power exceeding 800 mW, a spectral bandwidth of less than 50 pm, a root-mean square power variation of less than 0.2 % from DC to 2 MHz over the entire power operating range, and a polarization extinction ratio of more than 20 dB. This is a compact, low noise, high power source for frequency conversion with nonlinear optical materials, such as blue light generation.
In this communication we report on the successful realization of Single-mode Emitter Array Laser (SEAL) bars. Various laser bars with a cavity length of 2.4 mm containing between 25 to 350 narrow stripe lateral single-mode emitters have been realized and mounted epi-side down onto expansion matched heatsinks using a stable AuSn-solder technology. Optical power in excess of 1 W per emitter has been obtained resulting in more than 200 W total output power for the highest emitter density. While these total power levels are comparable to conventional broad-area laser bars (BALB), the brightness of each of the emitters is drastically improved over the BALB approach making theses bars ideal candidates for beam-shaping concepts. Lateral farfield measurements with smooth gaussian patterns, high electro-optical conversion efficiency well above 60% and threshold currents as low as 0.5 A are presented. Similar devices realized from the InGaAsP/InP material system deliver in excess of 20 W from 100 NS emitters at wavelengths around 1480 nm.
KEYWORDS: Semiconductor lasers, High power lasers, Multimode fibers, Resistance, Continuous wave operation, Temperature metrology, Prototyping, Reliability, Broad area laser diodes
In this communication we report on the performance characteristics of Bookham’s latest generation of 915-990 nm broad area single emitter (BASE) laser diodes with around 90 μm wide aperture. Representative high power devices in the wavelength range of 950-960 nm, mounted p-side down onto expansion matched assemblies using our highly reliable AuSn-solder technology, reveal a high slope efficiency of around 1.05 W/A during CW operation at 25°C heat sink temperature. Coupling efficiency into multi-mode fiber with 0.15 or 0.22 numerical aperture exceeds 93% mainly due to the low vertical divergence of the laser beam. In addition, low laser threshold and series resistance enable more than 62% maximum wall plug efficiency of the present generation of the laser diodes. Preliminary tests of new prototypes reveal already excellent performance characteristics of the next generation device with up to 19.9 W light output power in pulsed operation and 16 W for thermally limited CW operation.
Reliable power scaling by stretching the cavity length of the laser bars ranging from 1.2 mm to 3.6 mm at constant filling factor of 50% is demonstrated. While a relatively short cavity length of 1.2 mm allows for thermally limited CW output powers in excess of 180 W, extremely high 325 W at 420 A (CW, 16°C) have been achieved by leveraging the enhanced thermal properties of a 3.6 mm cavity length on standard micro-channel coolers. A high electro-optical conversion efficiency of 62% and 51% respectively is attributed to the low internal losses from an optimized waveguide design and the excellent properties of the AlGaAs-material system accounting for low thermal and electrical resistance. Multi-cell lifetest data at various operation conditions show extremely low wear-out rates even at harsh intermittent operation conditions (1-Hz type, 50% duty-cycle, 100% modulation). At 100 W output power 300 Mshots corresponding to 64000 h mean-time-to-failure (MTTF) have been extrapolated for 20% power drop from initial 2000 h and 4000 h lifetest readouts of a 1.2 mm cavity design. Similar results have been obtained for our next generation of ultra high power laser bars enabling reliable operation at 120 W output power and beyond. From 2.4 mm cavity length bars we have obtained 250 W of CW output power at 25°C while extrapolated reliability data at 120 W and 140 W power levels of up to 2000 h duration indicates that such devices are able to fulfill the requirements for lifetimes in the 20 - 30 kh range.
Diode-pumped solid state laser (DPSSL) and fiber laser (FL) are believed to become the dominant systems of very high power lasers in the industrial environment. Today, ranging from 100 W to 5 - 10 kW in light output power, their field of applications spread from biomedical and sensoring to material processing. Key driver for the wide spread of such systems is a competitive ratio of cost, performance and reliability. Enabling high power, highly reliable broad-area laser diodes and laser diode bars with excellent performance at the relevant wavelengths can further optimize this ratio. In this communication we present, that this can be achieved by leveraging the tremendous improvements in reliability and performance together with the high volume, low cost manufacturing areas established during the "telecom-bubble." From today's generations of 980-nm narrow-stripe laser diodes 1.8 W of maximum CW output power can be obtained fulfilling the stringent telecom reliability at operating conditions. Single-emitter broad-area lasers deliver in excess of 11 W CW while from similar 940-nm laser bars more than 160 W output power (CW) can be obtained at 200 A. In addition, introducing telecom-grade AuSn-solder mounting technology on expansion matched subassemblies enables excellent reliability performance. Degradation rates of less than 1% over 1000 h at 60 A are observed for both 808-nm and 940-nm laser bars even under harsh intermittent operation conditions.
AlGaAs/InGaAs based high power pump laser diodes with wavelength of around 980 nm are key products within erbium doped fiber amplifiers (EDFA) for today's long haul and metro-communication networks, whereas InGaAsP/InP based laser diodes with 14xx nm emission wavelength are relevant for advanced, but not yet widely-used Raman amplifiers. Due to the changing industrial environment cost reduction becomes a crucial factor in the development of new, pump modules. Therefore, pump laser chips were aggressively optimized in terms of power conversion and thermal stability, which allows operation without active cooling at temperatures exceeding 70°C. In addition our submarine-reliable single mode technology was extended to high power multi-mode laser diodes. These light sources can be used in the field of optical amplifiers as well as for medical, printing and industrial applications. Improvements of pump laser diodes in terms of power conversion efficiency, fiber Bragg grating (FBG) locking performance of single mode devices, noise reduction and reliability will be presented.
For high data rate (greater than 1 Gbps) Optical Inter- Satellite Link (OISL), a compact laser transmitter with high power and good efficiency is required. A trade-off analysis between the technologies such as the mature 840 nm laser diodes, 1064 nm diode-pumped solid state laser and the more recent 1550 nm Erbium Doped Fiber Amplifier (EDFA) is used to find the optical solution. The Si-APDs are preferred for their large detector areas and good noise figures which reduce the tracking requirements and simplify optical design of the receiver. Because of significant amount of power needed to close the link distance up to 7000 km (LEO-LEO), use of 840 nm diodes is limited. In this paper, we present an alternative system based on a system concept denoted as the SLYB (Semiconductor Laser Ytterbium Booster). The SLYB uses a polarization maintaining double-clad ytterbium fiber as a power amplifier. The device houses two semiconductor diodes that are designed to meet telecom reliability: a broad-area 917 nm pump diode and a directly modulated FP laser for signal generation. The output signal is in a linearly polarized state with an extinction ratio of 20 dB. The complete module (15 X 12 X 4.3 cm3) weighs less than 0.9 kg and delivers up to 27 dBm average output power at 985 nm. Designed primarily for direct detection using Si APDs, the transmitter offers a modulation data rate of at least 1.5 Gb/s with a modulation extinction ratio better than 13 dB. Total power consumption is expected to be lower than 8 W by using an uncooled pump laser. Preliminary radiation testing of the fiber indicates output power penalty of 1.5 dB at the end of 10 years in operation. We are presently investigating the fabrication of an improved radiation-hardened Yb-fiber for the final prototype to reduce this penalty. For higher data rate the design can be extended to a Wavelength Division Multiplexing (WDM) scheme adding multiple channels.
State-of-the-art pump lasers for fiber amplifiers in optical telecommunication systems can deliver up to 250 mW optical power in single mode emission. Power approaching 500 mW is seen as the upper limit for single mode lasers without sacrificing reliability. For yet higher power, broad area laser diodes coupled into the cladding of ytterbium-erbium doped fibers have a great potential to achieve more gain. Here, Diode lasers with a ridge width of 30 micrometer mounted junction-side-up using a hard-tin solder are presented as an alternative over EDFA pumping by single-mode lasers. Reliable broad area lasers are obtained by adopting standard narrow-stripe single-mode laser technology with proven 110 FIT at 150 mW and 25 degrees Celsius varying only the ridge width. Reliability and packaging problems commonly associated with p- side down mounting are avoided. Due to the high power conversion efficiency greater than 55% of the laser chip, a cw maximum output power as high as 1.1 W can be obtained. Submicron fiber alignment tolerances are no longer required, as compared to single mode lasers. Due to low heat dissipation these 30 micrometer stripe lasers can be operated in standard butterfly packages and high coupling efficiencies are achieved above 80% into standard 50 micrometer multimode fibers. YEDFA's with 23 dBm output power using these lasers have been demonstrated.
In a first part we report on a Ti:sapphire pumped Er3+:YLF laser. The dopant concentration and the polarization of the pump beam is optimized. The highest slope efficiency is obtained with the polarization of the pump beam parallel to the c-axis and a dopant concentration of 20 at. %Erbium. Slope efficiencies up to 50% are achieved. This value clearly exceeds the Stokes limit of 35%. Further, we report on an Er3+:BYF laser pumped with two polarization-coupled diode lasers. The dependence of slope efficiency on the reflectance of the resonator mirrors, on the focus of the pump light, and on the resonator length is investigated. The best slope efficiency of (tau) equals 24% is obtained for a nearly hemiconcentric resonator with a reflectance of Rin(DOT)Rout equals 97.6% and a pump beam focused with a lens of f equals 20 mm. To obtain a higher pump power, we investigate the coupling of sixteen diode lasers, each one operated to emit a maximum power of 800 mw. The diode laser emission is collimated and then focused using aspherical lenses. The point spread function of this system is investigated. Laser action in a longitudinally pumped Er3+:YLF is already achieved using only one single diode laser.
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