A protocol for the operation of a high-energy, high-power optical parametric chirped pulse amplifier pump source based on an Yb:YAG thin-disk regenerative amplifier is presented here.
This is a report on a 100 W, 20 mJ, 1 ps Yb:YAG thin-disk regenerative amplifier. A homemade Yb:YAG thin-disk, Kerr-lens mode-locked oscillator with turn-key performance and microjoule-level pulse energy is used to seed the regenerative chirped-pulse amplifier. The amplifier is placed in airtight housing. It operates at room temperature and exhibits stable operation at a 5 kHz repetition rate, with a pulse-to-pulse stability less than 1%. By employing a 1.5 mm-thick beta barium borate crystal, the frequency of the laser output is doubled to 515 nm, with an average power of 70 W, which corresponds to an optical-to-optical efficiency of 70%. This superior performance makes the system an attractive pump source for optical parametric chirped-pulse amplifiers in the near-infrared and mid-infrared spectral range. Combining the turn-key performance and the superior stability of the regenerative amplifier, the system facilitates the generation of a broadband, CEP-stable seed. Providing the seed and pump of the optical parametric chirped-pulse amplification (OPCPA) from one laser source eliminates the demand of active temporal synchronization between these pulses. This work presents a detailed guide to set up and operate a Yb:YAG thin-disk regenerative amplifier, based on chirped-pulse amplification (CPA), as a pump source for an optical parametric chirped-pulse amplifier.
The generation of high-energy, few-cycle laser pulses at a high repetition rate is of great interest to applied fields, such as attosecond science1,2,3,4 and high-field physics5,6, which stand to directly benefit from the availability of such sources. OPCPA represents the most promising route to achieving high pulse energies and large amplification bandwidths that simultaneously support few-cycle pulses1. To date, OPCPA allows for ultra-broadband amplification, which generates few-cycle pulses7,8,9,10. However, a modified implementation of the OPCPA scheme, which uses short pump pulses on the picosecond scale, holds promise for making this approach scalable for even higher pulse energies and average powers in the few-cycle regime1,11,12. Due to the high pump intensity in short-pulse pumped OPCPA, the high single-pass gain allows for the use of very thin crystals to support large amplification bandwidths. Although the short-pulse pumped OPCPA has many advantages, the realizability of this approach is subject to the availability of lasers that are specially tailored for this purpose. Such pump lasers are required to deliver high-energy picosecond pulses with near-diffraction limited beam quality at repetition rates in the kHz to MHz range13,14,15.
The introduction of ytterbium-doped lasers at different geometries, capable of delivering picosecond laser pulses with high energy and high average power, are about to change the current state of the field1,13,14,15,16,17,18. Yb:YAG has good thermal conductivity and a long upper-state lifetime, and it can be pumped by cost-effective diode lasers. Its performance when used in thin-disk geometry is outstanding due to the efficient cooling of the gain medium to simultaneously scale the peak and average power. Moreover, the occurrence of self-focusing inside the gain medium during the amplification process is suppressed due to the slenderness of the thin-disk in comparison to other gain medium geometries, resulting in excellent temporal and spatial profiles of the amplified pulses. Combining this concept with CPA holds promise for generating picosecond pulses with hundreds of millijoules of energy and hundreds of watts of average power19,20.
The aim of this work is to demonstrate a turn-key Yb:YAG thin-disk regenerative amplifier with outstanding daily performance as a suitable source for pumping OPCPAs21. To achieve this goal, this study employs a Yb:YAG thin-disk oscillator22 with several microjoules of pulse energy to seed the amplifier to minimize the accumulated nonlinear phase during the amplification process. This protocol provides the recipe for building and operating the laser system, which is described elsewhere21. Details about component implementation and control software are presented, and the alignment process of the system is described.
Caution: Please be aware of all safety regulations that are relevant to lasers before using this equipment. Avoid exposure of the eyes or skin to direct or scattered laser beams. Please wear appropriate laser safety goggles throughout the process.
Figure 1: Schematic layout of the Yb:YAG thin-disk regenerative amplifier. (a) Yb:YAG thin-disk Kerr-lens mode-locked oscillator. The 13 m linear cavity of the oscillator consists of a 13% transmission output coupler, three high-dispersion mirrors with GDD of -3,000 fs2, 1 mm sapphire Kerr medium, and a copper hard aperture. A pulse picker, containing a 25 mm-thick BBO crystal, is used to reduce the repetition rate to 5 kHz. (b) CPA. First block: the pulse stretcher setup containing two antiparallel gold gratings (1,740 lines/mm), where the seed pulses are temporally stretched to approximately 2 ns. Second block: the regenerative amplifier, where the seed pulse is confined in the amplifier cavity for amplification when the high voltage of the Pockels cell, which contains a BBO crystal with a thickness of 20 mm, is applied. Third block: the pulse compressor containing two parallel dielectric gratings (1,740 lines/mm), where the amplified pulses are temporally compressed down to 1 ps. This figure has been modified from Fattahi et al., with permission from reference21. Please click here to view a larger version of this figure.
Component | ROC | Distance |
(mm) | (mm) | |
OC | ∞ | 0 |
TD | -17000 | 600 |
M1 | -1000 | 5000 |
BP | ∞ | 510 |
M2 | -1000 | 510 |
EM | ∞ | 800 |
Table 1: Cavity design of the oscillator. ROC: radius of curvature, OC: output coupler, TD: thin-disk, M: mirror, BP: Brewster plate, EM: end mirror.
Figure 2: Oscillator cavity design. Calculated mode radius on the cavity components. OC: output coupler, TD: thin-disk, M: mirror, BP: Brewster plate, EM: end mirror. Please click here to view a larger version of this figure.
Component | ROC | Distance |
(mm) | (mm) | |
EM1 | ∞ | 0 |
PC | ∞ | 200 |
M1 | -5000 | 525 |
M2 | 1500 | 1500 |
TD | -2000 | 1050 |
EM2 | -2000 | 2350 |
Table 2: Cavity design of the regenerative amplifier. ROC: radius of curvature, EM: end mirror, PC: Pockels cell, M: mirror, TD: thin-disk.
Figure 3: Regenerative amplifier cavity design. Calculated mode radius on the cavity components. EM: end mirror, PC: Pockels cell, M: mirror, TD: thin-disk. Please click here to view a larger version of this figure.
1. Oscillator
2. Pulse Picker and Pulse Stretcher
NOTE: Caution, be aware of all relevant electrical safety regulations before applying the high voltage on the pulse picker. Use appropriate high-voltage isolation. Remove the diagnostics from the beam path before proceeding with this section. If aligning the pulse picker and its setting is not required, skip steps 2.1, 2.3-2.6, 2.8-2.9, and 2.11.
3. Regenerative Amplifier
Caution; Be aware of all relevant electrical safety regulations before applying the high voltage to the Pockels cell. Use appropriate high-voltage isolation. Remove the diagnostics from the beam path before proceeding with this section. Seed pulses are delivered from the Yb:YAG thin-disk Kerr-lens mode-locked oscillator. Other seed strategies can be used to seed the amplifier, such as fiber amplifiers.
4. Pulse Compressor, Beam Alignment, and Stabilization System
NOTE: Remove the diagnostics from the beam path before proceeding with this section. If aligning the compressor and the beam stabilizer unit is not required, skip steps 4.3 and 4.6.
5. Pump Source of the OPCPA System
NOTE: Remove the diagnostics from the beam path before proceeding with this section.
The oscillator delivers 350 fs, 2 µJ, 25-W pulses at 11 MHz repetition rate, with a pulse-to-pulse stability of 1% (rms) and beam-pointing fluctuations of less than 0.6% over 1 h of measurement (Figure 4).
Figure 4: Yb:YAG thin-disk, Kerr-lens mode-locked oscillator. (a) The spectrum (red), the retrieved temporal intensity profile (blue), and the spatial profile (inset) of the oscillator pulses. (b) Measured and retrieved SHG-FROG spectrograph of the oscillator. This figure has been modified from Fattahi et al., with permission from reference21. Please click here to view a larger version of this figure.
The seed pulses are amplified in the regenerative amplifier to 125 W while being pumped with a CW fiber-coupled diode at a wavelength of 940 nm at 280 W, corresponding to an optical-to optical efficiency of 47%. The pulse-to-pulse stability of the amplifier is less than 1%, and the amplifier exhibits excellent long-term stability after 10 h of continuous operation. The amplified beam has an excellent spatial profile, with a M2 of 1 (M2x = 1.08 and M2y = 1.07) and an excellent temporal profile after compression to 1 ps (at FWHM) (Figure 5).
Figure 5: Characterization of the regenerative amplifier output and the gain-narrowing effect. (a) The stability of the regenerative amplifier average power after 10 h of continuous operation. Inset: (a-1) Normalized power to its mean value in a time window of 0.5 h; (a-2) Output beam profile of the regenerative amplifier. (b) Amplifier output spectrum (green) and the retrieved temporal intensity (blue) of the laser pulses at 100 W average power after the grating compressor. (c) Seed energy versus spectral bandwidth (FWHM) of the amplifier output and the required round trips for the same output average power at 300 W of pump power. This figure has been modified from Fattahi et al., with permission from reference21. Please click here to view a larger version of this figure.
The SHG was analyzed using the SISYFOS code25. Two different crystals with the following parameters were considered: 1) a type-I, 6 mm-thick lithium triborate (LBO), with a phase-matching angle of 13.7° and a nonlinear coefficient of 0.819 pm/V, and 2) a type-I, 3 mm-thick BBO with a phase-matching angle of 23.4° and a nonlinear coefficient of 2 pm/V26,27. 1-ps, 20-mJ pulses at 1,030 nm and a peak intensity of 100 GW/cm2 were considered as the input of the simulation. The simulation results showed that the BBO performance was superior to that of the LBO for SHG (Figure 6).
Figure 6: Second harmonic generation. (a) Simulated SHG energy for a 6 mm-thick LBO crystal and a 3 mm-thick BBO crystal. (b) Experimental SHG efficiency versus input pump peak intensity in a 1.5 mm-thick BBO crystal using 0.5 mJ (black) and 20 mJ (green) of the amplifier output. (c) The retrieved spectral intensity and (d) the group delay of XFROG measurements for different SHG efficiencies corresponding to points A, B, and C in (b). This figure has been modified from Fattahi et al., with permission from reference21. Please click here to view a larger version of this figure.
The turn-key operation of the oscillator is achieved by the optimum heat management of the different components of the laser. The output of the oscillator is reproducible on a daily basis, with no need for extra alignment or optimization. In addition, the pulse-to-pulse energy stability and spatial pointing stability of the seed laser fulfills the preconditions to achieving the stable operation of the regenerative amplifier.
Other low-energy seed sources, such as fiber amplifiers, can be used to seed the amplifier. In this study, a 2 µJ Yb:YAG thin-disk KLM oscillator was used to assist the amplification of the regenerative amplifier by reducing the growth of the accumulated nonlinear phases, since the required number of round trips is reduced for higher-input seed energy. Additionally, the higher seed energy influences the amplification process and reduces the gain narrowing. The measured spectral bandwidth of the amplified pulses for different seed energies at a fixed pump power is shown in Figure 5c. Amplified spectral bandwidth decreases for lower seed energies because of gain narrowing. For 10 pJ seed energy, the laser operates in the period doubling, and it is not possible to reach stable operation, even by increasing the number of round trips. In addition to the careful optimization of the cooling systems and the power supply of the diodes, the operation of the regenerative amplifier at saturation plays a major role in the achieved stability of the amplifier.
The fundamental or second harmonic of the laser can be used to pump an OPCPA system. For SHG, the performances of an LBO and a BBO crystal were compared, as they offer a high nonlinear coefficient and damage threshold, in spite of the larger spatial walk-off and the limited available aperture in the case of BBO. As the nonlinear coefficient of BBO is almost twice that of the LBO, a shorter crystal is sufficient to reach the saturation limit for SHG (Figure 6a). Therefore, BBO is the more suitable choice, as the accumulated nonlinear phase is smaller28.
The pulse durations of the SH pulses are characterized experimentally at different conversion efficiencies. It was observed that at high conversion efficiencies, the SHG spectrum is broadened and a higher-order spectral phase appears (Figure 6). Therefore, case B, with the conversion efficiency of 70%, is chosen where the SH and the unconverted fundamental beams maintain excellent quality.
The authors have nothing to disclose.
We would like to thank Prof. Ferenc Krausz for the discussions and Najd Altwaijry for her support for finalizing the manuscript. This work has been financed by the Centre for Advanced Laser Applications (CALA).
Electrooptics | |||
Fiber-Coupled Diode Laser Module | Dilas Diodenlaser GmbH | M1F8H12-940.5-500C-IS11.34 | |
Fiber-Coupled Diode Laser Module | Laserline GmbH | LDM1000-500 | |
Power Supply for Diode Laser | Delta Elektronika B.V. | SM 15-100 | |
Power Supply for Diode Laser | Delta Elektronika B.V. | SM 35-45 | |
Pulse Picker's Driver | Bergmann Messgeräte Entwicklung KG | N/A, customized | |
Pockels Cell's Driver | Bergmann Messgeräte Entwicklung KG | N/A, customized | |
Pulse Picker's Driver Power Supply | Bergmann Messgeräte Entwicklung KG | PCD8m7 | |
Pockels Cell's Driver Power Supply | Bergmann Messgeräte Entwicklung KG | PCD8m7 | |
Delay Generator PCI | Bergmann Messgeräte Entwicklung KG | BME_SG08p | |
Splitter Box | Bergmann Messgeräte Entwicklung KG | N/A, customized | |
Resonant Preamplifier | Bergmann Messgeräte Entwicklung KG | BME_P03 | |
Pulse Picker's crystal | Castech Inc. | N/A, customized | 12*12*20 mm³ |
Pockels Cell's crystal | Castech Inc. | N/A, customized | 12*12*20 mm³ |
Name | Company | Catalog Number | Comments |
Optics | |||
Thin-disk | TRUMPF Scientific Lasers | N/A, customized | |
Thin-disk Head | TRUMPF Scientific Lasers | N/A, customized | |
Fiber | Frank Optic Products GmbH | N/A, customized | |
Fiber Objective | Edmund Optics GmbH | N/A, customized | |
Faraday Isolator | Electro-Optics Technology, Inc | EOT.189.12231 | |
Faraday Rotator | Electro-Optics Technology, Inc | EOT.189.22040 | |
Stretcher's Grating 1 | Horiba Jobin Yvon GmbH | N/A, customized | 60*40*10 mm³ |
Stretcher's Grating 2 | Horiba Jobin Yvon GmbH | N/A, customized | 350*190*50 mm³ |
Compressor's Grating 1 | Plymouth Grating Laboratory, Inc. | N/A, customized | 40*40*16 mm³ |
Compressor's Grating 2 | Plymouth Grating Laboratory, Inc. | N/A, customized | 300*100*50 mm³ |
HR Mirror, 1" (1030nm), flat, 0° | Layertec GmbH | 108060 | |
HR Mirror, 1" (1030nm), flat, 0° | Laseroptik GmbH | B-09965, S-04484 | |
HR Mirror, 1" (1030nm), flat, 45° | Layertec GmbH | 108063 | |
HR Mirror, 1" (1030nm), flat, 45° | Laseroptik GmbH | B-09966, S-04484 | |
HR Mirror, 1" (1030nm), curved | Layertec GmbH | N/A, customized | set |
HR Mirror, 2" (1030nm), flat, 0° | Laseroptik GmbH | B-09965, S-05474 | |
HR Mirror, 2" (1030nm), flat, 45° | Laseroptik GmbH | B-09966, S-05474 | |
Thin Film Polarizer (1030nm), 2" | Layertec GmbH | 103930 | |
Waveplate L/2 (1030nm) | Layertec GmbH | 106058 | Ø=25mm |
Waveplate L/4 (1030nm) | Layertec GmbH | 106060 | Ø=25mm |
AR Window (1030nm), wedge | Laseroptik GmbH | B-00183-01, S-00988 | Ø=38mm |
Output Coupler, 1" (1030nm) | Layertec GmbH | N/A, customized | PR = 88 % |
High-dispersion Mirror (1030nm) | UltraFast Innovations GmbH | N/A, customized | GDD = -3000 fs² |
Lens, 1" (1030nm), Plano-Convex | Thorlabs GmbH | N/A, customized | set |
Lens, 1" (1030nm), Plano-Concave | Thorlabs GmbH | N/A, customized | set |
Lens, 2" (1030nm), Plano-Convex | Thorlabs GmbH | N/A, customized | set |
Lens, 2" (1030nm), Plano-Concave | Thorlabs GmbH | N/A, customized | set |
HR Mirror, 1" (515nm), flat, 0° | Layertec GmbH | 129784 | |
HR Mirror, 1" (515nm), flat, 0° | Eksma Optics | 042-0515-i0 | |
HR Mirror, 1" (515nm), flat, 45° | Layertec GmbH | 110924 | |
HR Mirror, 1" (515nm), flat, 45° | Eksma Optics | 042-0515 | |
HR Mirror, 1" (515nm), curved | Layertec GmbH | N/A, customized | set |
HR Mirror, 1" (515nm), curved | Eksma Optics | N/A, customized | set |
HR Mirror, 2" (515nm), flat, 0° | Eksma Optics | 045-0515-i0 | |
HR Mirror, 2" (515nm), flat, 45° | Eksma Optics | 045-0515 | |
Thin Film Polarizer (515nm), 2" | Layertec GmbH | 112544 | |
Waveplate L/2 (515nm) | Layertec GmbH | 112546 | Ø=25mm |
Lens, 1" (515nm), Plano-Convex | Thorlabs GmbH | N/A, customized | set |
Lens, 1" (515nm), Plano-Concave | Thorlabs GmbH | N/A, customized | set |
Kerr Medium | Meller Optics, Inc. | N/A, customized | Sapphire, 1mm |
BBO Crystal | Castech Inc. | N/A, customized | 7*7*1.5 mm³ |
Harmonic Separator, 1", 45° | Eksma Optics | 042-5135 | |
Harmonic Separator, 2", 45° | Eksma Optics | 045-5135 | |
Silver Mirror, 1", flat | Thorlabs GmbH | PF10-03-P01 | |
Silver Mirror, 1", curved | Eksma Optics | N/A, customized | set |
Filter – Absorptive Neutral Density | Thorlabs GmbH | NE##A | set |
Filter – Reflective Neutral Density | Thorlabs GmbH | ND##A | set |
Filter – Round Continuously Variable | Thorlabs GmbH | NDC-50C-4M | |
Filter – Edgepass Filter (Longpass) | Thorlabs GmbH | FEL#### | set |
Filter – Edgepass Filter (Shortpass) | Thorlabs GmbH | FES#### | set |
Wedge | Thorlabs GmbH | N/A, customized | set |
Name | Company | Catalog Number | Comments |
Optomechanics & Motion | |||
Mirror Mount 1" (small) | S. Maier GmbH | S1M4-##-1” | |
Mirror Mount 1" (large) | S. Maier GmbH | S3-## | |
Mirror Mount 1" | TRUMPF Scientific Lasers | 1" adjustable | |
Mirror Mount 2" | S. Maier GmbH | S4-## | |
Mirror Mount 2" | TRUMPF Scientific Lasers | 2" adjustable | |
Rotation Mount 1” | S. Maier GmbH | D25 | |
Rotation Mount 1” | Thorlabs GmbH | RSP1/M | |
Rotation Mount 2” | Thorlabs GmbH | RSP2/M | |
Precision Rotation Stage | Newport Corporation | M-UTR120 | |
Four-Axis Diffraction Grating Mount | Newport Corporation | DGM-1 | |
Translation Stage | OptoSigma Corporation | TADC-651SR25-M6 | |
Pockels cell stage | Newport Corporation | 9082-M | |
Pockels Cell Holder | Home-made | N/A, customized | |
Picomotor Controller/Driver Kit | Newport Corporation | 8742-12-KIT | |
Picomotor Piezo Linear Actuators | Newport Corporation | 8301NF | |
Picomotor Rotation Mount | Newport Corporation | 8401-M | |
Hand Control Pad | Newport Corporation | 8758 | |
Name | Company | Catalog Number | Comments |
Light Analysis | |||
Beam Profiling Camera | Ophir Optronics Solutions Ltd | SP620 | |
Beam Profiling Camera | DataRay Inc. | WCD-UCD23 | |
Photodiodes (solw) | Thorlabs GmbH | DET10A/M | |
Photodiodes (fast) | Alphalas GmbH | UPD-200-SP | |
Thin-disk Camera | Imaging Development Systems GmbH | UI-2220SE-M-GL | |
Oscilloscope | Tektronix GmbH | DPO5204 | |
Oscilloscope | Teledyne LeCroy GmbH | SDA 760Zi-A | |
Spectrometer | Avantes | AvaSpec-ULS3648-USB2 | |
Spectrometer | Ocean Optics, Inc | HR4C1769 | |
Spectrometer | Ocean Optics, Inc | HR4C3762 | |
Spectrometer | Ocean Optics, Inc | HR4D464 | |
Spectrometer | Ocean Optics, Inc | HR4D466 | |
Laser Thermal Power Sensor | Ophir Optronics Solutions Ltd | L50(150)A-PF-35 | |
Laser Thermal Power Sensor | Ophir Optronics Solutions Ltd | FL500A | |
Laser Thermal Power Sensor | Ophir Optronics Solutions Ltd | 3A-P-V1 | |
Power and Energy Meter | Ophir Optronics Solutions Ltd | Vega | |
Name | Company | Catalog Number | Comments |
Systems | |||
Laser Beam Stabilization System | TEM-Messtechnik GmbH | Aligna | |
Laser M² Measuring System | Ophir Optronics Solutions Ltd | M²-200s | |
FROG | Home-made | N/A, customized | |
XFROG | Home-made | N/A, customized | |
Name | Company | Catalog Number | Comments |
Miscellaneous | |||
Cooling Chiller | H.I.B Systemtechnik GmbH | 6HE-000800-W-W-R23-2-DI | |
Cooling Chiller | Termotek GmbH | P201 | |
Cooling Chiller | Termotek GmbH | P208 | |
Laser Safety Goggles | Protect – Laserschutz GmbH | BGU 10-0165-G-20 | |
Infra-red Viewer | FJW Optical Systems | 84499A | |
Laser Viewing Card | Thorlabs GmbH | VRC4 | |
Laser Viewing Card | Thorlabs GmbH | VRC5 | |
Laser Viewing Card | Laser Components GmbH | LDT-1064 BG | |
Flowmeter | KOBOLD Messring GmbH | DTK-1250G2C34P | |
Pressure Gauge | KOBOLD Messring GmbH | EN 837-1 | |
Temperature Sensor | KOBOLD Messring GmbH | TDA-15H* ***P3M | |
WinLase Software | Dr. C. Horvath & Dr. F. Loesel | WinLase Version 2.1 pro. | Laser Cavity Software |