C. Frevert

Cryolaser: Innovative cryogenic diode laser bars optimized for emerging ultra-high power laser applications

“Cryolaser” diode laser designs exploit the improvement in semiconductor material properties at sub-zero temperatures to increase efficiency and power. Optimized single 9xx-nm laser bars demonstrate record peak pulse energy of 2 J (1.7 kW, 1.2 ms, -50°C).

Cryogenic ultra-high power infrared diode laser bars

GaAs-based high power diode lasers are the most efficient source of optical energy, and are in wide use in industrial applications, either directly or as pump sources for other laser media. Increased output power per laser is required to enable new applications (increased optical power density) and to reduce cost (more output per component leads to lower cost in

Progress in high-energy-class diode laser pump sources

/W). For example, laser bars in the 9xx nm wavelength range with the very highest power and efficiency are needed as pump sources for many high-energy-class solid-state laser systems. We here present latest performance progress using a novel design approach that leverages operation at temperatures below 0°C for increases in bar power and efficiency. We show experimentally that operation at -55°C increases conversion efficiency and suppresses thermal rollover, enabling peak quasi-continuous wave bar powers of Pout > 1.6 kW to be achieved (1.2 ms, 10 Hz), limited by the available current. The conversion efficiency at 1.6 kW is 53%. Following on from this demonstration work, the key open challenge is to develop designs that deliver higher efficiencies, targeting > 80% at 1.6 kW. We present an analysis of the limiting factors and show that low electrical resistance is crucial, meaning that long resonators and high fill factor are needed. We review also progress in epitaxial design developments that leverage low temperatures to enable both low resistance and high optical performance. Latest results will be presented, summarizing the impact on bar performance and options for further improvements to efficiency will also be reviewed.

Study of waveguide designs for high-power 9xx-nm diode lasers operating at 200 K

A new generation of diode-pumped high-energy-class solid-state laser facilities is in development that generate multijoule pulse energies at around 10 Hz. Currently deployed quasi-continuous-wave (QCW) diode lasers deliver average inpulse pump powers of around 300 W per bar. Increased power-per-bar helps to reduce the system size, complexity and cost per Joule and the increased pump brilliance also enables more efficient operation of the solid state laser itself. It has been shown in recent studies, that optimized QCW diode laser bars centered at 940…980 nm can operate with an average in-pulse power of > 1000 W per bar, triple that of commercial sources. When operated at pulsed condition of 1 ms, 10 Hz, this corresponds to > 1 J/bar. We review here the status of these high-energy-class pump sources, showing how the highest powers are enabled by using long resonators (4…6 mm) for improved cooling and robustly passivated output facets for high reliability. Results are presented for prototype passively-cooled single bar assemblies and monolithic stacked QCW arrays. We confirm that 1 J/bar is sustained for fast-axis collimated stacks with a bar pitch of 1.7 mm, with narrow lateral far field angle (< 12° with 95% power) and spectral width (< 12 nm with 95% power). Such stacks are anticipated to enable Joule/bar pump densities to be used near-term in commercial high power diode laser systems. Finally, we briefly summarize the latest status of research into bars with higher efficiencies, including studies into operation at sub-zero temperatures (-70°C), which also enables higher powers and narrower far field and spectra.

Low-temperature optimized 940 nm diode laser bars with 1.98 kW peak power at 203 K

Currently, a new generation of ultra-high-energy laser systems (ELI, HILASE) is in development that requires huge amounts of pump power. Their diode laser pump sources should be low cost (

940nm QCW diode laser bars with 70% efficiency at 1 kW output power at 203K: analysis of remaining limits and path to higher efficiency and power at 200K and 300K

/W) and operate with the highest power conversion efficiency E. One way to increase both output power and efficiency is to lower the operating temperature. We present latest results of detailed design investigations into 9xx nm, AlGaAs based broad-area devices, specifically targeting low temperature (200K) operation. We show experimentally that decreasing temperature reduces threshold current and increases internal efficiency. However, the series resistance RS increases, limiting the net benefit especially at high powers. To address this limitation, the impact of aluminum content in the diode laser AlGaAs waveguide has been studied using 100 μm wide devices mounted p-up on CuW. Near room temperature, structures with low Al-content in the waveguide have poor optical performance, due to high carrier leakage. However at temperatures around 200K, carrier leakage is shown to be strongly suppressed, eliminating the low-aluminum performance penalty of a higher threshold and lower slope efficiency. Simultaneously RS is strongly decreased for low Al-content structures. Overall, we show that optimized designs should enable single 100 μm broad-area lasers to operate at 200K with E = 72% at 20 W output power, corresponding to about 1.5 kW from a bar with 75% fill-factor.

High power diode lasers for pumping high energy solid state lasers

Passively cooled 1-cm wide QCW diode laser bars with low-temperature optimized vertical designs reach output powers of 1.98 kW at 203 K, with conversion efficiency of 64% at 1 kW and 57% at 1.9 kW.

Progress in high duty cycle, highly efficient fiber coupled 940-nm pump modules for high-energy class solid-state lasers

Both high-energy-class laser facilities and commercial high-energy pulsed laser sources require reliable optical pumps with the highest pulse power and electro-optical efficiency. Although commercial quasi-continuous wave (QCW) diode laser bars reach output powers of 300…500 W further improvements are urgently sought to lower the cost per Watt, improve system performance and reduce overall system complexity. Diode laser bars operating at temperatures of around 200 K show significant advances in performance, and are particularly attractive in systems that use cryogenically cooled solid state lasers. We present the latest results on 940 nm, passively cooled, 4 mm long QCW diode bars which operate under pulse conditions of 1.2 ms, 10 Hz at an output power of 1 kW with efficiency of 70% at 203 K: a two-fold increase in power compared to 300 K, without compromising efficiency. We discuss how custom low-temperature design of the vertical layers can mitigate the limiting factors such as series resistance while sustaining high power levels. We then focus on the remaining obstacles to higher efficiency and power, and use a detailed study of multiple vertical structures to demonstrate that the properties of the active region are a major performance limit. Specifically, one key limit to series resistance is transport in the layers around the active region and the differential internal efficiency is closely correlated to the threshold current. Tailoring the barriers around the active region and reducing transparency current density thus promise bars with increased performance at temperatures of 200 K as well as 300 K.

Assessment of high-power kW-class single-diode bars for use in highly efficient pulsed solid state laser systems

Summary form only given. The optical energy used in high-energy-class solid state laser facilities is generated by high power diode lasers. This tutorial reviews and summarizes progress in ongoing efforts worldwide to improve the performance of these critical components.

Corrections to “Efficient High-Power Laser Diodes” [Jul/Aug 13 1501211]

Diode lasers pump sources for future high-energy-class laser systems based on Yb-doped solid state amplifiers must deliver high optical intensities, high conversion efficiency (ηE = > 50%) at high repetition rates (f = 100 Hz) and long pulse widths (τ = 0.5…2 ms). Over the last decade, a series of pump modules has been developed at the Ferdinand-BraunInstitut to address these needs. The latest modules use novel wide-aperture single emitter diode lasers in passively side cooled stacks, operate at τ = 1 ms, f = 100…200 Hz and deliver 5…6 kW optical output power from a fiber with 1.9 mm core diameter and NA of 0.22, for spatial brightness BΩ > 1 MW/cm2 sr. The performance to date and latest developments in these high brightness modules are summarized here with recent work focusing on extending operation to other pumping conditions, as needed for alternative solid state laser designs. Specifically, the electro-optic, spectral and beam propagation characteristics of the module and its components are studied as a function of τ for a fixed duty cycle DC = 10% for τ = 1...100 ms, and first data is shown for continuous wave operation. Clear potential is seen to fulfill more demanding specifications without design changes. For example, high power long-pulse operation is demonstrated, with a power of > 5 kW at τ = 100 ms. Higher brightness operation is also confirmed at DC = 10% and τ = 1 ms, with > 5 kW delivered in a beam with BΩ > 4 MW/cm2 sr.