Norbert Lichtenstein

Further development of high-power pump laser diodes

AlGaAs/InGaAs based high power pump laser diodes with wavelength of around 980 nm are key products within erbium doped fiber amplifiers (EDFA) for todays 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.

9xx high-power 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.

325 Watts from 1-cm wide 9xx laser bars for DPSSL and FL applications

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.

Recent developments for BAR and BASE: Setting the Trends

Bars with high and low filling factors serve the different schemes for beam transformation and fiber coupling. We report on highly efficient 8xx bars for operation in excess of 100 W and reliable broad-area single-emitter lasers (BASE) with 90 um aperture being capable to deliver in excess of 10 W from a 105 um core fiber. For 9xx bars we present solutions with power levels per device ranging from 60 W to 300 W corresponding to linear power levels beyond 8.5 W per 100 um stripe width indicating convergence of BAR and BASE devices. Life test results for these devices will be shown and high brightness fiber coupled solutions will be discussed.

Broad area single emitter (BASE) modules with improved brightness

High power pump modules are used for industrial as well as telecom applications such as for fiber lasers and cladding pumped erbium doped fiber amplifiers (EDFA) [Lichtenstein, N., et al., 2004]. Key element of a pump module is a single emitter laser diode, which normally has an aperture of around 90 mum, in order to provide best coupling into an optical multi-mode fiber with a 105 mum core diameter. Increasing the brightness of the pump module lends additional flexibility in the design of fiber lasers and amplifiers and enables higher output power. In this communication we report on two approaches to increase the brightness of our latest generation of high power pump modules. Within the first approach the electro-optical performance of the single emitter laser diode is improved by optimizing Bookhams mature chip technologies such as InGaAs/AlGaAs structures grown by MBE, the ridge-waveguide manufacturing process, the E2 facet passivation and the mounting technology. The combination of improved chip design and new coupling scheme leads to reliable broad area modules with outstanding brightness.

Improved brightness on broad-area single emitter (BASE) modules

In this communication we report on the approaches to increase the brightness of Bookhams latest generations of high power pump modules. Since the single-emitter laser diode is the essential building block in all module designs, the optimization of the device design towards higher wall-plug efficiency, higher brightness and better reliability is one focus of the ongoing development efforts at Bookham. By using an analytical simulation tool critical parameters for efficient emitter-fiber coupling as the beam divergence and coupling scheme could be identified.

Brightness Scaling of High Power Laser Diode Bars

High power laser diode bars used in pumping units of solid state or fibre lasers or as a direct source in laser diode systems have in the last years become more and more important for industrial laser applications such as welding, brazing, hardening or marking. Increasing the brightness of the laser diode source offers the possibility to use fibres with smaller numerical apertures and smaller core diameters, and therefore to increase the brightness of the laser beam emitted by the laser system used for the materials processing step. With Bookhams new very high brightness (VHB) devices described in this paper, the next significant increase in brightness by a factor of up to 4 depending on the beam shaping technique has been achieved.

CW to QCW power scaling of high-power laser bars

We report on laser diode bars with wavelengths ranging from 793 to 1080 nm and optimized for high power and high temperature operation. For 808 nm bars output power values of 300 W at 300 A drive current and 200 μs pulse length have been recorded at a cooler temperature of 75°C. Extending our wavelength range to 1080nm we report on bars with >65% power conversion efficiency in CW operation and more than 500 W output power for a wide range of qCW modes. Finally, the properties of a 6-bar stack with 3 kW output power at 460 A drive current and 200 μs pulse width will be discussed.

High-brightness 9xx and 14xx single-mode emitter array laser bars

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.

Laser diodes with distributed feedback for application as subnanosecond fiber laser seeder

We report on high power wavelength stabilized single-mode lasers operating at ~1060 nm. Due to their capability of fast gain switching (<1 ns) and internal wavelength stabilization, distributed feedback (DFB) lasers are attractive for utilization as an ultrafast seeder for MOPA fiber laser systems. We successfully developed narrow band single mode DFB laser emitting at the wavelength of ~1060 nm and providing >1.5W of peak power in pulse mode. Time response of the DFB lasers was analyzed using both small and large signal modulation techniques. Furthermore, we present the results of integration of DFB lasers into subnanosecond fiber laser system. We obtained spectrally narrow (~50 pm) optical pulses as short as 170 ps with peak power of ~7 kW.