Download Progress in joule-class diode laser bars and high brightness

Progress in joule-class diode laser bars and high brightness modules
for application in long-pulse pumping of solid state amplifiers
P. Crump*, M. M. Karow*, S. Knigge*, A. Maaßdorf* and G.Tränkle*,
J. Lotz‡, W. Fassbender‡ and J. Neukum‡,
J. Körner¥, R. Bödefeld¥ and J. Hein¥
*
Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik, 12489 Berlin, Germany
‡
DILAS Diodenlaser GmbH, Galileo-Galilei-Str. 10, 55129 Mainz, Germany
¥
Lastronics GmbH, Winzerlaer-Str. 2, 07745 Jena, Germany
ABSTRACT
Progress in the development of highly brilliant pump modules based on novel high-energy-class diode laser
bars is presented, targeting long-pulse (2 ms) application in pumping Yb:CaF2 amplifier crystals. Bars with
long resonators and high fill factors are used for low resistance and efficient cooling. Advanced extremedouble asymmetric epitaxial structures are used for high efficiency at high power. Facet passivation is used
for long life at high powers, enabling operation at 600 W per bar (1.2 J). Performance of bars and passively
cooled stacks is summarized and prospects for further performance enhancements are reviewed.
Keywords: diode laser, pump sources, bars, joule-class, high efficiency, monolithic stacked arrays
1. INTRODUCTION
High-energy-class diode pumped solid state laser (HEC-DPSSL) systems are a rapidly developing technology that
delivers up to kJ-class energies within nanosecond pulses with high average powers, to address applications in
fundamental science and for emerging industrial applications [1,2]. The laser design depends on the target application
and needed performance level, but are typically constructed using a seed laser and power amplifier, with the amplifier
playing a key role in regulating the overall efficiency and cost of the system. The amplifier commonly uses Yb and Nd
doped crystals, ceramics, or glasses, which are optically pumped with high brightness diode laser modules, whose
properties are tailored to match the gain media in use. High power pump modules are preferred for higher solid state
laser performance and for lower overall system cost in €/W. In addition, pump sources with narrow far field and narrow
spectral width are preferred for effective coupling into the amplifier and high absorption in the targeted transition. The
performance of the pump modules is itself limited by the properties of the high power diode lasers used in its
construction.
Yb-doped CaF2 crystals are a promising material for future HEC-DPSSLs, and show a long upper state lifetime of
> 2 ms, enabling them to integrate pump energy over a longer period than YAG or FAP crystals, with equivalent
lifetimes of < 1 ms [3]. It is therefore beneficial to make diode lasers available that deliver high peak optical power Popt
within long pulses with width τ = 2 ms, whilst maintaining high conversion efficiency ηE(Popt), narrow spectral width
∆λ95%(Popt) (95% power content) and narrow vertical and lateral far field angles, ΘV95% and ΘL95%(Popt) (95% power
content), all at the operating point. We present here updates from a development study seeking to achieve these goals,
leveraging recent work at the Ferdinand-Braun-Institut (FBH) in high power and efficiency diode laser bars for use as
pump sources [4-8]. We focus here on diode lasers with a centroid operating wavelength λc(Popt) ~ 940 nm, at
Popt = 600 W and τ = 2 ms. We summarize first the results of an optimization study to identify the highest performance
bar configuration. We next show initial test data on equivalent bars integrated into an eight-bar passively cooled stack,
packaged by DILAS. We then conclude with a discussion.
2. LASER BAR PERFORMANCE
In previous studies at the FBH, diode lasers and associated pump modules were developed, that were tailored for high
pulse power operation at long-duty-cycle D = τf (for repetition rate f ). Extreme double asymmetric epitaxial structures
and device designs were developed that enable wide-stripe single emitters with aperture W = 1200 µm and λc = 940 nm
to deliver Popt = 120 W at ηE(120 W) = 60% [4], for quasi-continuous wave operation with high duty cycle D = 10…20%
(τ = 1 ms, f = 100…200 Hz). The single emitters operated with ΘL95%(120 W) = 12° and ΘV95% = 43° [6]. In parallel
studies [7], bars with large resonator length L = 4 mm were integrated in low-cost passively cooled QCW stacks,
enabling maximum per-bar Popt = 800 W for D = 1% (τ = 1 ms f = 10 Hz), corresponding to a pulse energy
Ep = Poptτ = 0.8 J. Pulse energy was shown to increase to Ep = 1.7 J per bar for lower D = 0.3% (τ = 2.8 ms f = 1 Hz). In
this previous study, bars with low F = 45%, and low ηE(800 W) ~ 40% were used. High fill factor bars with higher ηE
are anticipated to deliver increased Popt and hence to be good candidates for application as pump sources for Yb:CaF2
amplifiers.
Therefore, in the study presented here, efficient epitaxial designs taken from [4,6] were fabricated into bars with high fill
factor F ~ 70%, and assessed for use in QCW applications. The epitaxial structure was grown using MOVPE and
fabricated using standard techniques following [7,8] into high power 1-cm laser bars with L = 4 mm. Three different
lateral geometries were compared:
(1) W = 1200 µm and n = 6 (F = 72%) following [4,6],
(2) W = 400 µm and n = 18 (F = 72%) following [5,8] and
(3) W = 186 µm and n = 36 (F = 67%) following [8].
In each case, index-guiding trenches were etched on either side of each laser stripe, to stabilize the near and far field
under high heat loads. Devices with W = 400 µm and W = 1200 µm also included lateral sub-structure in the electrical
contact, introduced via implantation, as a further measure to stabilize the near and far field, following previous studies
[4-6]. Facets were passivated following normal FBH practice [9] and coated with nominal front and back facet
reflectivities of 2% and 96% respectively. In an initial quality control test, bars were mounted between two CuW
submounts using AuSn, then subsequently soldered onto high-current-compatible, low electrical resistance (~ 10 µΩ)
conductively cooled carriers (CCP), as in [8]. These bars were then characterised in QCW pulsed mode at heatsink
temperature THS = 25°C, with D = 2% (τ = 2 ms f = 10 Hz), up to Popt = 600 W, corresponding to Ep = 1.2 J. All bars
were burnt-in for 8 hours at Ep = 1.2 J (Popt = 600 W, τ = 2 ms, f = 10 Hz, THS = 25°C). Post-burn-in test results for
exemplary bars using the three lateral configurations are collected in Fig. 1, and are broadly comparable, with
ηE(600 W) = 60…62% at an operating current of I(600 W) = 630…650 A. Centroid wavelength and spectral width are
λC(600 W) = 937 nm and ∆λ95%(600 W) = 10 nm respectively, suitable for pump application. The light-current-voltage
performance is broadly comparable (on a per-emitter basis) to reference single emitters. Therefore, from the perspective
of power and efficiency, there is no clear highest performer in the lateral configuration. It should be noted that (in
addition to advanced epitaxy and chip designs), the long resonator and low resistance packaging are both critical for
sustaining high efficiency to high per-bar powers, especially for long pulse operation.
In contrast, the integrated polarisation purity of the bars does show clear differences with bar configuration, as
summarized in Fig. 2. Here, DOP is reported as the ratio of TE intensity (in-plane electrical field) to total intensity at
Popt = 120 W, again with all values quoted post burn-in. As the stripe width is increased, DOP is seen to also increase,
from 94% (W = 186 µm) to 98% (W = 1200 µm). We note that the low DOP is often associated with mechanical stressrelated effects stemming from the trenches at the edges of the stripes [10], and there are many fewer trenches in bars with
wide stripes (12 for W = 1200 µm, 72 for W = 186 µm). For reference, we show also equivalent DOP results for identical
single emitters mounted p-side down onto CuW carriers, with n-side contact made via wirebonds. Single emitters were
measured in QCW mode (τ = 1 ms, f = 10 Hz, THS = 25°C) and results are reported for single emitters at Popt = 600/36 =
16.7 W for W = 186 µm, Popt = 600/18 = 33.3 W for W = 400 µm and Popt = 600/6 = 100 W for W = 1200 µm. In all
cases, DOP was > 97%. Therefore, wide stripes are beneficial as a measure to increase polarisation purity in 1-cm bars,
and in particular bars with the very widest (1200 µm) stripes showing equivalent results to reference single emitters.
Fig. 1. Optical output power, bias voltage and conversion efficiency as a function of QCW drive current (τ = 2 ms, f = 10 Hz), for
single 1-cm bars with 4 mm resonator length mounted on low resistance CCPs. Three different geometries are compared, all with fill
factor close to 70% (a) 36 emitters with W = 186 µm, (b) 18 emitters with W = 400 µm and (c) 6 emitters with W = 1200 µm.
Spatially integrated wavelength is shown at Popt = 600 W in each case as an inset.
Fig. 2. Degree of polarization as a function of stripe width for single emitters and bars with fill factor close to 70%, measured in
QCW mode. Single emitters measured at Popt = 16.7 W, 33.3 W and 100 W for W = 186 µm, 400 µm and 1200 µm respectively
(τ = 1 ms, f = 10 Hz). Bars measured at Popt = 600 W (τ = 2 ms f = 10 Hz)
3
BEAM QUALITY OF SINGLE EMITTERS AND BARS
The optical energy provided by the bars for pumping must be collected efficiently into the solid state amplifier, and this
reqires narrow lateral propagation angles, ΘL95% < 12°. Therefore, lateral far field was measured as a function of bias, up
to Popt = 600 W for bars (τ = 1.25 ms, f = 10 Hz, THS = 20°C) and for single emitters to the same equivalent power
density, namely 16.7 W, 33.3 W and 100 W for W = 186 µm, 400 µm and 1200 µm respectively (τ = 1 ms, f = 10 Hz,
THS = 25°C). Single emitter measurements were performed at the FBH and bar measurements at Lastronics. Results are
presented in Fig. 3 for all bar and single emitter configurations (including exemplary lateral intensity profiles) and the
evolution of ΘL95% as a function of Popt (scaled to equivalent bar power for single emitters) and as a function of average
active region temperature increase over THS, as determined from the shift in λc with bias (following [10]).
Fig. 3. Spatially integrated lateral far field of single emitters and bars, measured in QCW mode (τ = 1 ms, f = 10 Hz). Lateral field
profile at power equivalent to Popt = 600 W per bar, for (a) W = 186 µm, (b) W = 400 µm and (c) W = 1200 µm. Integrated lateral far
field width with 95% power content, ΘL95%, as a function of (d) increase of active region temperature over THS and (b) bar equivalent
power. Results are shown for two devices of each type.
In all cases, ΘL95% increases monotonically with operating power and active region temperature. Otherwise the behavior
of single emitters and bars is quite different. Single emitters show large and rapidly growing lateral far field angles for
the widest stripes, and small and slowly varying far fields for the narrowest stripes, with (for example) ΘL95% being two
times wider for devices with W = 1200 µm than for devices with W = 186 µm. In contrast, far field angles for laser bars
are much more similar (all results within 2…3° range), with the narrowest stripes showing in all cases the widest far field
angles. The source of the large difference between single emitters and bars must still be identified, and may be related to
(a) differences in packaging, (b) the influence of neighbor-to-neighbor heating in bars on the thermal lens profile or (c)
packaging related stress (seen earlier to behave differently for single emitters and bars, via its influence on DOP).
However, as a quality metric for bar applications, it is clear that bars with eighteen stripes with W = 400 µm show the
highest performance, being the only configuration where all bars operate consistent with the nominal target of ΘL95% ≤
12° to Popt = 600 W. However, differences remain small.
4
STACK PERFORMANCE
Fig. 4. Photographs of efficient, long-resonator EDAS bars integrated into DILAS 8 bar passively cooled stacks with attached fastaxis collimation lenses.
As a next step, exemplary 8-bar passively (macro-channel) cooled QCW stacks were assembled and tested at DILAS,
using sibling bars to those presented in section 3. All bars were fast axis collimated. Photographs of an example stack are
shown in Fig. 4, and exemplary test data is shown in Fig. 5, here for bars that contain 36 stripes, each with W = 186 µm.
In these initial (quality control) tests, current was supply-limited to 300 A (τ = 1 ms, f = 10 Hz), and testing was
performed at THS = 22°C. For comparison, single bar test data is also included Fig. 5, with power and voltage scaled
eightfold (τ = 2 ms, f = 10 Hz, THS = 25°C). Comparable performance is observed in all cases, indicating that the bars are
suitable for integration into high brightness modules, for application in pumping of Yb:CaF2 solid state amplifiers.
Longer pulse and higher dity cycle testing of stacks will follow.
Fig. 6. (a) Optical output power, bias voltage and conversion efficiency as a function of QCW drive current (τ = 1 ms, f = 10 Hz),
for an example 8 bar stack of 1-cm bars, each with 4 mm resonator length, compared to results of single bar testing (τ = 2 ms,
f = 10 Hz), with power and voltage scaled eightfold. (b) Spatially integrated optical intensity as a function of wavelength at 300 A
operation current.
5
SUMMARY AND OUTLOOK
Advanced, long resonator bars with high (F = 70%) fill factor were shown to enable QCW bars to operate at high power
(Popt = 600 W) in long pulse mode (τ = 2 ms, f = 10 Hz), corresponding to high pulse energy Ep = 1.2 J. The polarisation
purity of mounted bars was shown to be a strong function of stripe width, with the very widest stripes eliminating
polarisation penalty over single emitters. Strongly different performance was seen in lateral far field of single emitters
compared to bars, potentially due to differences in packaging, in-bar emitter to emitter thermal cross-talk or stress related
effects. Bars containing 18 stripes with W = 400 µm showed the narrowest lateral far field, although differences between
bars were small. Exemplary bars were assembled into fast-axis collimated 8-bar stacks, and no performance degradation
was observed compared to single bars. Such bars and associated stacks and pump engines are highly attractive as pump
sources for Yb:CaF2, due to their high power density, resulting low cost in € per Joule (and € per Watt) and high
efficiency (ηE > 60%) to high powers (Popt > 600 W per bar) in long-pulse operation. Further increases in optical output
power and conversion efficiency are sought, as are measures for narrower far field. Transfer to other wavelengths and the
implementation of spectral stabilization (for example via extrernal volume Bragg gratings) would allow other transitions
and gain media to be pumped. Stacks and bars will (in follow-on studies) be integrated into high brightness pump
engines for use in the pumping of high-energy-class solid state lasers.
ACKNOWLEDGEMENTS
This work was funded as part of the ZIM-cooperation project “DoppelPump” by the Federal Ministry for Economic
Affairs and Energy (BMWi) under Grant No. KF2194510DF4.
REFERENCES
[1] Lucianetti, A., Divoky, M., Sawicka, M., Sikocinski, P., Jambunathan, V., Pilar, J., Slezak, O., Kmetik, V., Novak,
J., Fibrich, M., Rus, B., Körner, J., Hein, J., and Mocek, T., “HiLASE cryogenically-cooled diode-pumped laser
prototype for inertial fusion energy,” Proc. SPIE, vol. 8602, 860208-1 (2013).
[2] Wölz, M., Pietrzak, A., Kindsvater, A., Meusel, J., Stolberg, K., Hülsewede, R., Sebastian, J. and Loyo-Maldonado,
V., “Laser diode stacks: pulsed light power for nuclear fusion,” High Power Laser Science and Engineering 4, e14
(2016).
[3] Siebold, M., Bock, S., Schramm, U., Xu, B., Doualan, J.L., Camy, P., and Moncorgé, R., “Yb:CaF2—a new old
laser crystal,” Appl Phys B 97, 327 (2009).
[4] Pittroff, W., Eppich, B., Erbert, G., Platz, R., Tyralla, D., and Tränkle, G., “Simple design for fiber coupled 9xx nm
kW-QCW pump module with high duty cycle based on customized chips and lateral heat removal,” Proc. SPIE
8965, 896515-1 (2014).
[5] Platz, R., Eppich, B., Crump, P., Pittroff, W., Knigge, S., Maaßdorf, A., and Erbert, G., "940-nm Broad Area Diode
Lasers Optimized for High Pulse-Power Fiber Coupled Applications," IEEE Photonics Technol. Lett. 26(6), 625
(2014).
[6] Platz, R., Eppich, B., Rieprich, J., Pittroff, W., Erbert, G., and Crump, P., "High duty cycle, highly efficient fiber
coupled 940-nm pump module for high-energy solid-state lasers," High Power Laser Sci. Eng. 4, e3 (2016).
[7] Crump, P., Frevert, C., Ginolas, A., Knigge, S., Maaßdorf, A., Lotz, J., Fassbender, W., Neukum, J., Körner, J.,
Töpfer, T., Pranovich, A., Divoky, M., Lucianetti, A., Mocek, T., Ertel, K., De Vido, M., Erbert, G., and Tränkle,
G., “Joule-Class 940-nm Diode Laser Bars for Millisecond Pulse Applications,” IEEE Photon. Techn. Lett. 27(15),
1663 (2015).
[8] Frevert, C., Bugge, F., Knigge, S., Ginolas, A., Erbert, G., and Crump, P., "940 nm QCW diode laser bars with 70%
efficiency at 1 kW output power at 203 K: analysis of remaining limits and path to higher efficiency and power at
200K and 300K," Proc. SPIE 9733, 97330L (2016).
[9] Ressel, P., Erbert, G., Zeimer, U., Häusler, K., Beister, G., Sumpf, B., Klehr, A., and Tränkle, G., “Novel
Passivation Process for the Mirror Facets of Al-Free Active-Region High-Power Semiconductor Diode Lasers,”
IEEE Photon. Techn. Lett. 17(5), 962 (2005).
[10] Winterfeldt, M., Crump, P., Wenzel, H., Erbert, G., and Tränkle, G., “Experimental investigation of factors limiting
slow axis beam quality in 9xx nm high power broad area diode lasers,” J. Appl. Phys. 116, 063103-1 (2014).