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High power CEP-stable OPCPA at800nm

Golz, T., Buß, J., Schulz, M., Grguras, I., Prandolini, M., etal.

T. Golz, J. H. Buß, M. Schulz, I. Grguras, M. J. Prandolini, R. Riedel, "Highpower CEP-stable OPCPA at 800nm," Proc. SPIE 11259, Solid State LasersXXIX: Technology and Devices, 112591L (21 February 2020); doi:10.1117/12.2546254

Event: SPIE LASE, 2020, San Francisco, California, United States

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High Power CEP-stable OPCPA at 800 nm

T. Golza, J. H. Bußa, M. Schulza, I. Grgurasa, M. J. Prandolinia,b, and R. Riedela

aClass 5 Photonics GmbH, Notkestr. 85, 22607 Hamburg, GermanybInstitut fur Experimentalphysik, Universitat Hamburg, Luruper Chaussee 149, 22761

Hamburg, Germany

ABSTRACT

High power and high repetition rate femtosecond lasers are crucial tools furthering the scientific developmentacross many fields. So far, these systems have been realized by Ti:Sapphire lasers at 800 nm, being limited inpower scaling. A novel optical parametric chirped-pulse amplifier (OPCPA), pumped by high-power Yb-dopedsolid state lasers, and combined with bulk crystal white-light-generation seeding (WLG) allows to circumventthe limitation in average power. The presented laser system features carrier-envelope phase (CEP) stable sub20 fs pulses centered at 800 nm with millijoule pulse energies and average power of 20 W while remaining on acompact footprint. Such systems have recently become commercially available from Class 5 Photonics and allowfor scalability beyond millijoule pulse energies at up to 100 W average power.

Keywords: optical parametric chirped-pulse amplifier, CEP-stable pulse generation and amplification

1. INTRODUCTION

The key to future experiments in strong-field physics, enabling the electronic response of matter to be controlledand measured within one optical cycle,1 will be provided by high power, CEP-stable pulses. Previously, CEP-stable laser sources were driven by Ti:sapphire lasers at 800 nm with limited bandwidth (Fourier limited pulse of∼20 fs), and more importantly limited power levels; power levels ∼40 W and above require large complex coolingsystems. In addition, considerable effort has been carried out to actively stabilize the CEP of mode-lockedTi:sapphire oscillators.2 However, most of the CEP noise is introduced during the amplification, dominated bythermal, beam-pointing and mechanical fluctuations.3 In particular, Ti:sapphire amplifiers require large a gratingstretcher and compressor; this is usually the main source of CEP jitter.3 In comparison, optical parametricchirped-pulse amplification (OPCPA) together with bulk crystal white-light-generation (WLG) opens up thepossibility of high power lasers (well above 100 W), with wavelength tunable and broadband pulses (for example,<10 fs at 800 nm), requiring no complex cooling with a compact design; for recent examples see.4–7 In thesesystems, CEP-stable pulses are generated passively8 and because optical parametric amplification has a very highsingle pass gain, path lengths are short compared to conventional multilevel laser gain material. Thus allowingOPCPA systems to provide high power with CEP stable pulses.

Further author information: Send correspondence to R. R.R. R.: E-mail: [email protected], Tel. +1 650 353 9700

Solid State Lasers XXIX: Technology and Devices, edited by W. Andrew Clarkson, Ramesh K. Shori, Proc. of SPIE Vol. 11259, 112591L · © 2020 SPIE

CCC code: 0277-786X/20/$21 · doi: 10.1117/12.2546254

Proc. of SPIE Vol. 11259 112591L-1Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 12 Mar 2020Terms of Use: https://www.spiedigitallibrary.org/terms-of-use

2. SYSTEM OVERVIEW

High power picosecond Yb-doped solid state lasers offer pulse energies within the multi mJ level and repetitionrate scalability to about 100 kHz. This class of laser systems can be paired with optical parametric chirped pulseamplifiers to achieve mJ level femtosecond laser pulses ranging from the UV to the mid infrared.

Figure 1. Schematic set-up for the Supernova OPCPA laser system. SHG: second harmonic stage, converting 1030 nm to515 nm. WLG: white light generation stage, providing a broadband spectrum via filamentation. DFG: difference frequencygeneration, providing the difference frequency between 515 nm and 680 nm. Stretcher: Dispersive element to match pumpand seed pulse duration. OPA1,2,3: Optical parametric amplifier stages pumped. Compressor: static chirped mirroraround to compensate existing second and third order dispersion in order to compress the pulse to less than 20 fs. CEPcontrol: delay unit that allows the compensation of CEP offset drifts with up to 1 kHz.

A schematic set-up of the Supernova OPCPA system is depicted in figure 1. The Supernova OPCPA lasersystem consists out of 3 main parts, a picosecond Yb-YAG Innoslab pump laser system, the White Dwarf 800CEP OPCPA, and the Supernova 800 CEP OPCPA, with the latter two being housed on a 1200× 800 mmfootprint. The composition of each part will be explained in the following sections.

3. PUMP LASER SYSTEM

The picosecond pump laser is a commercial Amphos A3000 providing 300 W at repetition rate of 20 kHz anda pulse energy of 15 mJ. It consists out of an amplifier module and a separate compressor providing a pulseduration of about 1.2 ps FWHM. The pump laser system offers long term upgrade options to higher repetitionrates of up to 100 kHz at similar output energies.

Figure 2. Amphos A3000 beam profile at 1.2, 4, and 8 m respectively (f.l.t.r).

Figure 2 shows the collimated beam size of about 6mm (1/e2) at 1.2, 4, and 8 m, from left to right. The collimation


is achieved using telescope systems within the A3000 compressor in order to minimize the nonlinear integralaccumulated by the pulse.

Figure 3. A3000 power stability over 60 min after warmup. (courtesy of Amphos GmbH)

The power stability of the A3000 shown in figure 3 is about 0.2 % rms over 60 minutes after a warmup periodof about 2 hours. When reconsidering the schematic set-up shown in figure 1 and taking into account all 8non-linear frequency conversion stages throughout the White Dwarf OPCPA pre-amplifier and the SupernovaOPCPA main amplifier it becomes apparent, that a high degree of stability from the pump laser system is ofutter importance, as nonlinear frequency conversion will amplify any power instabilities if not designed carefully.

4. OPCPA PRE-AMPLIFIER

The White Dwarf 800 CEP OPCPA module converts a small portion of the pump pulse that provides 15 mJenergy at 1030 nm into the passively CEP stable Supernova seed with about 1.5 uJ energy, 800 nm central wave-length and a broadband spectrum that supports sub 20 fs Fourier limited pulse duration. In order to generatesuch pulses five different nonlinear frequency conversion stages are necessary. As depicted in figure 1, a smallamount of the pump pulse is frequency doubled in a second harmonic stage utilizing a 1 mm thick beta bariumborate (BBO) crystal creating 515 nm light pulses. These pulses are split 3 way, simultaneously pumping thefirst white light generation stage (WLG1), the difference frequency generation stage (DFG), and the first opticalparametric amplifier stage (OPA1).The white light generation stage is utilizing a yttrium-alluminium-garnet (YAG) crystal in order to generate abroadband red shifted spectrum reaching 610 to 760 nm.

Figure 4 shows the overlay of 4600 individual spectra emphasizing the spectral stability and the broad red shiftedspectrum achieved. Special interest lies in the spectral range around 680 nm, as these components will be mixedwith 515 nm in order to generate the CEP stabilized pulse. The spectral stability can be evaluated by integratingover a spectral bandwidth of about 20 nm, centered at 680 nm. The spectral stability is shown in Figure 5.

A spectral stability of about 2.2 % rms is achieved over a measurement of 4600 samples. The generated whitelight seed is consecutively used in the collinear difference frequency generation stage DFG, where the componentsaround 680 nm are frequency mixed with a part of the 515 nm pump pulse in order to generate a narrow bandpulse at about 2100 nm. When using a portion of the same pump pulse for white light generation and differencefrequency generation the random carrier envelope phase offset is eliminated and a passively CEP offset stabilizedpulse is created. The generated spectrum can be seen in Figure 6. It spans from 2000 to 2260 nm and has aFourier limit of about 60 fs.


Figure 4. Overlay of 4600 individual white light spectra generated in the first white light stage. The spectrum spansabout 150 nm from 610 to 760 nm.

Figure 5. Integrated spectral stability for a 20 nm spectral band centered at 680 nm displayed for 4600 consecutive spectralmeasurements.

Figure 6. Difference frequency generation spectrum centered at 2080 nm spanning about 250 nm allowing for a Fourierlimited pulse duration of 60 fs.


The generated infrared pulse is compressed using bulk silicon and focused into a YAG crystal in order to generatea second white light filament. Hence, a passively CEP offset stabilized two octave spanning white light rangingfrom 570 nm to beyond 2500 nm shown in Figure 7, can be achieved. It is to be noted that the driver pulse isnot filtered out of the presented data, which causes the large amount of spectral content around 2000 nm.

Figure 7. White light spectrum generated with 2100 nm driving wavelength spanning from 570 nm to beyond the detectionlimit of 2500 nm. The visible spectrum (bright red) is measured using a visible/NIR spectrometer and the infraredspectrum (dark red) is measured using an infrared spectrometer.

With the combination of a suitable amplifier crystal and fine control of the applied dispersion after the YAGcrystal any part of this spectrum can be amplified and tuned in spectral bandwidth, allowing for a truly gaplessamplifier between 570 and 2000 nm. In the presented system the spectral range between 700 and 900 nm hasbeen chosen. The spectral stability is evaluated by integrating the individual spectral components of about 5000consecutive spectral measurements taken over the course of 90 minutes. Figure 8 shows the resulting rms stabilityof 1.4 %.

Figure 8. Spectral stability of the second white light integrated over the spectral range of 700 to 900 nm. The displayed5000 samples are taken over the course of 90 minutes.

The selected spectral components are stretched to match the pump window and overlapped with the remainderof the pump pulse to be amplified in the first optical parametric amplifier stage. This stage uses a 5 mm thickBBO crystal achieving a gain of about 10000, amplifying the nanojoule level seed to 1.5 uJ. Figure 9 shows theamplified spectrum (left) and the power stability (right) of OPA2. The spectrum spans the desired 200 nm footto foot and allows for a Fourier limited pulse duration of about 10 fs FWHM. The power stability is taken over2 hours and shows a rms fluctuation of 0.87 % with an average power of 30.6 mW or about 1.5 uJ. The thereby


generated seed beam concludes the White Dwarf CEP OPCPA part of the Supernova laser system.

Figure 9. Amplified spectrum of OPA2 (left) spanning 700 to 900 nm and power stability measurement (right) achieving0.87 % rms fluctuations at 30.6 mW average power.

5. OPCPA MAIN AMPLIFIER

The Supernova amplifier uses most of the 15 mJ incoming laser pulse energy in order to amplify the generatedseed beam from 1.5 uJ to 1 mJ. The amplification is done in two separate stages utilizing BBO crystals of 4and 2 mm thickness, respectively. A 1 mm thick BBO crystal is used to generate the pump pulse by frequencydoubling the incoming laser pulse from 1030 nm to 515 nm, required in the amplification process. A conversionefficiency of about 61 % is achieved, resulting in 8.5 mJ pulse energy at 515 nm. After separation the pump pulseis overlapped in time and space with seed pulse from the OPCPA pre-amplifier module in OPA2 and consec-utively in OPA3. Each stage is optimized to amplify the seed spectrum from 700 to 900 nm. OPA2 achievesan amplification factor of about 460 resulting in an amplified pulse with about 700 uJ pulse energy and OPA3achieves the final amplification with a factor of 1.8 generating an uncompressed 1.25 mJ pulse energy.

Figure 10. Uncompressed spectrum (left) and spectral stability plot (right) of the Supernova 800 CEP OPCPA.

Figure 10 shows the uncompressed amplified spectrum of the Supernova 800 CEP OPCPA after OPA2 and 3(left) and the spectral stability integrated from 680 to 930 nm. The spectrum spans 680 to 930 nm and is thereforebroader than the White Dwarf seed spectrum. The broadening to both spectral edges can be explained with the


high conversion efficiency in the Supernova stages. The spectral stability is plotted over 18 hours and shows aslight drift over the course of the first 10 hours and a stabilization of the system thereafter. The rms fluctuationof the integrated spectral intensity over the full time period is 1.47 %. The output power of the Supernova 800CEP OPCPA is monitored using a Ophir 50(150)A−BB − 26 power meter head. The resulting power stabilityplot is shown in Figure 11. An average power of 24.47 W and an rms power fluctuation of 0.35 % is shown.

Figure 11. Output power measurement of the Supernova 800 CEP OPCPA. The average recorded power over the time of15 hours is 24.47 W exhibiting rms power fluctuations of 0.35 % .

In the final step the beam is propagated through the compressor unit comprised of an array of chirped mirrorsoptimized for the wavelength range of 680 to 960 nm. The pulse is thereby compressed to about 15 fs FWHM.Figure 12 shows the raw FROG trace as well as the autocorrelation and the beam profile of the Supernova lasersystem in a collimated and focused geometry using a lens with a focal length of 1800 mm.

The presented FROG trace as well as the correlating autocorrelation in figure 12 show a distinct central peak withside wings smaller 10 % of the intensity. The autocorrelation width is 21 fs FWHM which can be deconvolutedto a pulse duration of about 15 fs. The collimated beam is adjusted to a beam size of about 5.6 by 6.1 mm (1/e2)using a telescope at the exit of the Supernova laser system. The focus quality is evaluated using a f = 1800 mmlens in order to match the experimental requirements at the customers site. The resulting focus shows a Gaussiandistribution with a beam size of 185 by 286 um. Due to spectral filtering and reflective losses the output powerafter the compressor is reduced by about 3.9 W comparing to the uncompressed pulse, setting the final outputpower of the system to 20.6 W or about 1 mJ. Figure 13 shows the power stability of the compressed output pulseof the Supernova laser system. It can be seen that the compressor has no negative influence on the stability ofthe output power with the measurement exhibiting rms power fluctuations of 0.3 %.

6. CONCLUSION

A novel optical parametric chirped-pulse amplifier (OPCPA), pumped by a high-power Yb:YAG InnoSlab laser,and combined with bulk crystal white-light-generation seeding (WLG) is presented. The OPCPA system featurespassively stable carrier-envelope phase (CEP) with sub 20 fs pulses centered at 800 nm. Pulse energies above1 mJ at 20 kHz repetition rate within a corresponding average power of 20 W are achieved with a power stabilityof < 0.5% rms. This system will be combined with a hollow-core fiber based nonlinear spectral broadening set-upto drive attosecond experiments.


Figure 12. The raw FROG trace (top left) with the corresponding autocorrelation (top right) and the collimated beamprofile after the compressor (bottom left), as well as the focus using f = 1800 mm (bottom right) of the Supernova CEPOPCPA laser system.

Figure 13. Power stability measurement of the compressed Supernova OPCPA Laser system output beam. An average of20.6 W with rms power fluctuations of 0.3 % is realized.


ACKNOWLEDGMENTS

We acknowledge the contribution of our customer and research collaborator Prof. Mohammed Hassan (Universityof Arizona), and the colleagues from our industry partner Amphos GmbH, who provided the Yb:YAG pumpLaser.

REFERENCES

[1] Krausz, F. and Stockman, M. I., “Attosecond metrology: from electron capture to future signal processing,”Nat. Photonics 8, 205–213 (2014).

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[4] Mecseki, K., Windeler, M. K. R., Miahnahri, A., Robinson, J. S., Fraser, J. M., Fry, A. R., and Tavella,F., “High average power 88W OPCPA system for high-repetition-rate experiments at the LCLS x-ray free-electron laser,” Opt. Lett. 44, 1257–1260 (2019).

[5] Mecseki, K., Windeler, M. K. R., Robinson, J. S., Fraser, J. M., Fry, A. R., and Tavella, F., “Thermal effectsin a high repetition rate 88 W average power OPCPA system at 800 nm,” in [High Power Lasers for FusionResearch V ], Awwal, A. A. S. and Haefner, C. L., eds., 10898, 67 – 73 (2019).

[6] Windeler, M. K. R., Mecseki, K., Miahnahri, A., Robinson, J. S., Fraser, J. M., Fry, A. R., and Tavella,F., “100 W high-repetition-rate near-infrared optical parametric chirped pulse amplifier,” Opt. Lett. 44,4287–4290 (2019).

[7] Mecseki, K., Windeler, M. K. R., Prandolini, M. J., Robinson, J. S., Fraser, J. M., Fry, A. R., and Tavella,F., “High-power dual mode IR and NIR OPCPA,” in [High-Power, High-Energy, and High-Intensity LaserTechnology IV ], Hein, J. and Butcher, T. J., eds., 11033, 65–75 (2019).

[8] Baltuska, A., Fuji, T., and Kobayashi, T., “Controlling the carrier-envelope phase of ultrashort light pulseswith optical parametric amplifiers,” Phys. Rev. Lett. 88, 133901 (2002).


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