Novel High-Power, High Repetition Rate Laser Diode Pump Modules Suitable for High-Energy Class Laser Facilities

Abstract

The latest generation of high-energy-class pulsed laser facilities, under construction or planned, such as EuPRAXIA, require reliable pump sources with high power (many kW), brightness (>1 MW/cm2/sr) and electro-optical conversion efficiency (>50%). These new facilities will be operated at high repetition rates (around 100 Hz) and only diode lasers are capable of delivering the necessary performance. Commercial (quasi-continuous wave, QCW) diode laser pulse-pump sources are, however, constructed as low-cost passively cooled stacked arrays that are limited either in brightness, efficiency or repetition rate. Commercial continuous wave diode laser pumps constructed using microchannel coolers (as used in high-value industrial machine tools) can fulfil all requirements, but are typically not preferred, due to their cost and complexity and the challenges of preventing cooler degradation. A custom solution is shown here to fill this gap, using advanced diode lasers in a novel passive side-cooling geometry to realize 100 … 200 Hz pump modules (10%–20% duty cycle) that emit peak power of 6 kW at wavelength = 940 nm. The latest performance of these modules is summarized and compared to literature. We show that a brightness >1 MW/cm2/sr can be efficiently delivered across a wide range of laser pulse conditions with 10% duty cycle (pulse width: 100 µs … 100 ms … cw, repetition rate up to 1 kHz). Furthermore, we describe how these pumps have been used to construct and reliably operate (>109 pulses without degradation) in high-energy-class regenerative and ring amplifiers at the Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie (MBI). We also show first results on 100 Hz pumping of cryogenically cooled solid-state Yb:YAG slab amplifiers, as anticipated for use in the EuPRAXIA laser, and note that peak temperature is disproportionately increased, indicating that improved cooling and more detailed studies are needed.

Diode pumps for future laser plasma accelerators: perspectives from research and industry

“…5, where a pre-and post-amplifier enable pump-pulses with p in the nanoseconds and Ep = 100 J to be delivered at f = 10 Hz repetition rate to the frequency-doubler crystal. Diode laser pumps for these sophisticated large-scale systems must fulfill demanding specifications, as recently summarized in [18], illustrated using the anticipated requirements of the EuPRAXIA facility. Specifically, a diode laser pump beam for a system that generates Ep = 100 J from the Ti:Sa amplifier is required that is pumped with green light pulses of energy around Ep = 200…300 J.…”

“…Specifically, a diode laser pump beam for a system that generates Ep = 100 J from the Ti:Sa amplifier is required that is pumped with green light pulses of energy around Ep = 200…300 J. Allowing for losses in the system, this leads to a total optical pump power being needed of around Popt = 300…1200 kW, depending on the exact conditions, with a brightness of B = 0.8…1.0 MW•cm -2 •sr -1 (following definition in [18]). The diode pump beam must have a homogenous flat-top intensity profile, adapted to match the solid-state crystal used, typically many cm 2 in size (e.g.…”

“…As reviewed in [25,27] and discussed in briefly in [18], cost reduction in QCW stacked arrays can be achieved by using advanced automation, by scaling fabrication to larger area wafers, integrating and simplifying micro-optics and by efforts in defect minimization, amongst other measures. Cost reduction can also be achieved by scaling power per bar, with Popt = 500…600 W now available, via use of advanced diode laser designs with higher peak power (lower temperature sensitivity), and advances in device technology such as the implementation of high-quality, high throughput facet passivation.…”

“…Alternative concepts are urgently needed, to address the emerging needs. Research prototypes of stacked arrays are available that meet all performance requirements for pumping of YAG amplifiers in large-scale plasma acceleration systems [18], based on many years of applied research at the FBH, with an example shown in Fig. 8.…”

“…Using these techniques, QCW operation of  = 940 nm stacks at duty cycles up to 50% is possible (e.g. 500 µs pulses at f = 1000 Hz), with low (< 10%) degradation in power, efficiency and far field [18], at costs lower than microchannel-cooler based designs. However, the arrangement currently limits the maximum width of the diode lasers and hence the reliable operating power per stack into the few kilowatts range.…”