Laser system design for table-top X-ray light source Download: 926次
1 Introduction
Several laser facilities for strong-field physics experiments such as generation of attosecond pulses[1], electron acceleration[2], proton acceleration[3, 4] or X-ray generation[5], have been or are currently being built around the world. These facilities contribute to numerous advances in science, including physics, chemistry, biology, medicine and material science. Their development constantly faces technological and engineering challenges to reach the desired peak power, pulse energy, average power and lifetime.
The ‘table-top’ free-electron laser (FEL) powered by terahertz (THz) radiation proposed in the project AXSIS (Frontiers in attosecond X-ray science: imaging and spectroscopy) in 2016[5] is part of the current intense research on alternative technologies to traditional, radio frequency (RF) based FELs[6–8], which are very powerful but complex, large and expensive structures. These devices are thus few in number, resulting in very high demand and very limited access. The scientific community greatly benefits from alternative light sources with reduced size, cost and infrastructure needs. This demand has instigated extensive research efforts aiming the realization of ‘table-top’ light sources[9–15]. Nonetheless, all the proposed solutions, either directly or indirectly, take advantage from the recent progress in high-energy optical lasers, which are complex facilities.
Building an X-ray light source or other types of light sources entirely based on optical lasers requires a system capable of producing a multitude of diverse laser beams with various characteristics like center wavelength, spectrum, pulse duration and energy to serve as both drivers and diagnostic tools. In particular, for the AXSIS project, ultra-short ultraviolet (UV) pulses are required for electron generation, multiple beams of high-energy picosecond pulses are required for single-cycle THz generation and high-energy nanosecond (ns) pulses, with tailorable, tunable spectral and temporal profiles, are required for multi-cycle THz generation. In addition, the facility should run at high repetition rates with femtosecond level synchronization and stabilization. Such a system is highly complex and requires a combination of different laser technologies, some of which must be developed or customized. The right choice of technologies is crucial to its successful implementation and operation.
In this paper, we will first present the AXSIS project and derive the requirements of the driving laser system. Second, the available laser technologies will be reviewed. The third section presents possible modules of the laser system and discusses the first experimental results. Discussion of the laser chains constituted by the described modules tailored for the application within the AXSIS machine is the focus of the fourth section. The main engineering challenges in e.g., synchronization, diagnostics and controls, long-term stability or facility design, will be described and the current status of the implementation will be reported in Section
2 Overall layout of the THz-driven light source
Figure
Fig. 1. Schematic representation of the THz-driven light source with the driving laser system. SC: single-cycle; MC: multi-cycle, ICS: inverse Compton scattering.
A key technology of this scheme is the use of THz radiation to accelerate electrons. The high frequency allows acceleration gradients that are 10 to 100 times stronger than in a conventional RF accelerator[5], resulting in a proportional decrease of the accelerator size and opening the possibility to achieving relativistic electrons within centimeter-scale devices.
To reach kilo electron volt (keV) X-ray photon energies using the OU, electrons with energies in the tens of mega electron volt (MeV) range should be produced, which in turn requires tens of millijoule (mJ) of THz radiation. Such THz pulse energies exceed the current state of the art technology by several orders of magnitude. Although various methods[16] exist to generate THz radiation, laser-based nonlinear optical conversions via difference frequency generation (DFG) or optical rectification (OR) are the only ones suitable for short, high-energy pulses. Both processes allow for THz generation with intrinsic synchronization and high conversion efficiencies above 1%[17–22]. Thus, laser pulse energies in the range of 0.1 to 1 J are anticipated for the generation of multi-10-mJ THz pulses. A primary technological challenge in this approach is the development of laser sources with sufficient pulse energy and optical bandwidth to support the required efficiencies in the DFG or OR process.
The design of the AXSIS gun and LINAC calls for THz frequencies in the range of 0.1–0.6 THz in order to balance the benefits of high operation frequencies with the drawbacks of small wavelengths[23]. These THz frequencies require optical bandwidths in the range of 1–2 nm or more at
Also, the OU stage of the scheme presented in Figure
X-ray diffraction investigations of nano-crystals of organic macro-molecules like photosystem II[25], to be pursued in the AXSIS project, require acquisition and evaluation of a large number of shots, which dictates a high repetition rate for the machine. Based on the current laser technology and possible advances during the time frame of this project, the design of the 1-J laser sources aims at a repetition rate in the range from 100 Hz up to 1 kHz, implying laser average powers in the kW range.
The AXSIS architecture as in Figure
Considering the conversion efficiency from IR to THz pulses and of the OU process, those laser systems should generate 100 mJ to 1 J IR laser pulses. Because of the gain typically reachable within a single laser amplifier, a chain of amplifiers (henceforth the ‘laser chain’) must be developed. The requirements of each laser chain are summarized in Table
Table 1. Summary of the requirements of each laser chain. The THz energy takes into account the transport losses (for single-cycle THz pulses, twice the required energy within the gun is accounted for, and ${\sim}1.5$ for multi-cycle THz pulses).
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The arrival time of each pulse at its interaction region is critical; consequently, a common oscillator, stabilized in repetition rate, seeds all laser chains to achieve an intrinsically stable synchronization. Furthermore, optical locks between all laser chains are implemented.
The spatial beam profile of the IR lasers is critical for the efficient conversion to THz radiation, in order to achieve a Gaussian THz beam (required by the coupling into the gun and LINAC). Similarly, for the OU process, the spatial beam profile is the dominant property affecting the spot size of the high-energy IR beam at the interaction point, which should be as small as few micrometers in diameter. Consequently, the spatial profile required in every laser chain, independent of the concept chosen, should be homogeneous and diffraction limited. Therefore, we utilize amplifiers able to deliver Gaussian and super-Gaussian beam profiles. The required pointing stability is 1% of the diffraction limit (DL) – the DL expressed in mm-mrad is defined as
For the accelerator, the energy of the THz pulses has a direct relationship with the final energy of the accelerated electrons and the one of the IR pulses with the energy of the X-ray photons emitted during the industrial control system (ICS) process. The energy stability of the laser chain determines the energy stability of the THz pulses. For both applications, the energy stability of the IR pulses is set to
3 Available laser technologies
High power laser systems rely on a master-oscillator power-amplifier (MOPA) configuration wherein a frontend generates a low power pulse train with the required spectrum and stretching ratio and a power amplifier provides the gain necessary to reach high pulse energy. Since master oscillators which can generate the pulse shapes required for our application are readily available (either commercially or home-built – see Section
3.1 State of the art
Suitable gain media for amplification of short pulses at high average power are Yb-doped materials, such as Yb:YAG and Yb:YLF[27, 28]. The small quantum defect of ytterbium minimizes the heat load while the availability of high-brightness pump diodes allows for scaling to high average power[27]. Efficient heat removal and mitigation of thermo-optic effects have been the key to the continued performance improvements in Yb diode-pumped solid-state lasers.
Delivering sub-picosecond pulses with energy up to 1 J at high repetition rates and high average power represents a challenge on the laser source which can be addressed in multiple ways. One solution could be cryogenically cooled ytterbium based laser amplifiers.
Amplification up to 1 J at 500 Hz with 5 ps pulse duration has been demonstrated by Reagan
The Innoslab geometry has demonstrated its advantage for high average powers with limited pulse energies[37]. The gain medium is a slab cooled in one direction, perpendicular to the plane of beam propagation; efficient cooling is achieved by keeping this dimension as thin as possible. The beam profile is elliptical due to the minimized crystal height, given by the heat removal along this direction. During propagation through the crystal, the ellipticity increases, and consequently complicates beam shaping and transport after the amplifier. At high energies and short pulses, the B-integral increases and modulation instability starts to appear, limiting the achievable energy. Having said this, a demonstration of 0.5 J has been made in 2017 with two Innoslabs amplifiers in a row, with Nd:YAG as active medium and
RT Yb:YAG thin-disk lasers have shown tremendous potential for high average power operation in the continuous wave regime. In this geometry, the heat removal also occurs in the direction perpendicular to the crystal, but the extraction is implemented in the same direction. The beam size scales then with the surface of the gain material, preventing the increase of nonlinear effects. The challenge here resides in the pump absorption through the thin disk and the extraction of the energy. Recent developments toward developing this technology for operation in the 100 mJ energy range with picosecond duration pulses have been significant enough to qualify these laser systems as potential drivers for our electron acceleration facility. These lasers would either directly power THz generation setups or be used as seeds for 1-J-class cryogenic amplifier modules. The demonstrated energy, average power and beam profile agree with the range of our required parameters. The narrow spectral bandwidth, limiting the frequency of THz possibly generated, however might be a bottleneck that necessitates additional investigations to overcome.
Several independent research groups or companies have demonstrated and commercialized RT Yb:YAG-based thin-disk amplifiers with operation above 100 mJ energy and repetition rate up to 5 kHz. For instance, Jung
From the considerations on thermal issues and achievable beam qualities, we decided in the design of the AXSIS laser system to use either the thin-disk geometry laser amplifiers or the traditional rod architecture[27].
3.2 Discussion and comparison between Yb:YAG and Yb:YLF lasers
The electron gun and LINAC may operate with 0.1–0.6 THz frequency radiation[24], which can be obtained via DFG between two near-IR (NIR) frequencies separated by the frequency of the generated THz wave[42]. It corresponds to a separation of 0.3 nm at 0.1 THz to 1.7 nm at 0.5 THz for wavelengths close to
Yb:YAG and Yb:YLF laser amplifiers exhibit different spectroscopic, optical and thermal properties (summarized in Table
Table 2. Summary of the spectroscopic and thermo-optic properties of Yb:YAG at RT and CT and Yb:YLF at cryogenic temperature.
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RT-Yb:YAG is a 3-level system with considerable thermal population of the ground state. In addition it shows a small emission cross-section (
Yb:YLF and Yb:YAG amplifiers differ also in their scalability to high energies and high average powers. Average powers of 1 kW are feasible and have been demonstrated with Yb:YAG at room temperature[37, 45]. A significant advantage is realized when Yb:YAG is operated at liquid nitrogen temperature[44], because of the improved thermo-optic and thermo-mechanical properties. At cryogenic temperature, the thermal conductivity of Yb:YAG increases to
In the AXSIS laser system, we consider laser chains based on (i) cryo-Yb:YAG, (ii) cryo-Yb:YLF, and (iii) RT Yb:YAG. In what follows, we will discuss the characteristics of the modules available for building laser amplification chains, while describing in some detail the performance obtained with the technology demonstrators we have already built and are using.
4 Description of modules
4.1 Frontend
The main function of the frontend is to provide multiple laser seeds for the principal laser systems. The main outputs of the frontend are summarized in Table
Table 3. Description of the main outputs of the frontend.
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Frontend output #1 consists of two locked single frequency lasers with center wavelengths in the vicinity of
Frontend outputs #2–#4 deliver ultra-short pulsed signals. These signals are generated from the frontend’s master oscillator, which is a home-built ultra-short pulse fiber laser. The ytterbium-doped fiber oscillator is mode-locked by nonlinear-polarization rotation (NPR). NPR offers experimentally the highest reliability, largest spectrum and longest operating lifetime compared with other mode-locking technologies. The parameters at the oscillator’s output are: energy of 1.5 nJ, repetition rate of 70 MHz, spectral bandwidth (at 10 dB) of 25 nm, compressed pulse duration of 120 fs. The repetition rate of the master oscillator is stabilized to 70 MHz, chosen so that the temporal pulse-to-pulse spacing of about 14 ns is larger than twice the rise time of the Pockels cell of any of the subsequent regenerative amplifiers (which is typically around 5 ns). The RF reference clock for repetition-rate stabilization is based on an oven controlled crystal oscillator (OCXO) and can be further disciplined by referencing to a GPS signal.
The energy of the repetition-rate stabilized master oscillator is further amplified in polarization-maintaining, core-pumped single mode fiber to
4.2 Yb:KYW regenerative amplifier
The reason for using Yb:KYW in the regenerative amplifier is to provide a larger bandwidth than achievable with Yb:YAG or Yb:YLF at RT or CT during the six decades of amplification. Large bandwidth from this seed source counteracts spectral gain narrowing in the high-energy amplifier stages and gives a sufficiently short pulse to drive the optical parametric amplifiers (OPAs) for the UV generation. The design of the Yb:KYW regenerative amplifier is based on the laser demonstrated in Ref. [49]: the cavity is symmetric around two laser crystals sharing the heat load equally and allowing amplification to high energies. The two 2% Yb:KYW, 3 mm long crystals are pumped by a fiber-coupled laser diode operating in quasi-continuous wave regime where 120-W pump power is split equally between the two crystals.
The design of the cavity, with the crystal located at one Rayleigh length distance from the focus of the laser beam, allows for insensitivity of the output beam parameters to the thermal lens. Hence, stable operation is enabled over a large range of repetition rates limited only by the Pockels cell driver.
The stretching ratio implemented in the demonstration in Ref. [49] was designed to be adapted for amplification up to the 1 J level; the minimum stretched pulse duration for operation of the regenerative amplifier is however 300 ps according to B-integral calculations.
The first version of this regenerative amplifier has been implemented with standard opto-mechanics and fine-adjustable mirror holders, and operates trouble-free and alignment-free since four years on a daily basis in our lab. For AXSIS, it is engineered to remove degrees of freedom and minimize the beam height on the breadboard to enhance the mechanical stability and lifetime. In this revised version, a water-cooled
4.3 RT Yb:YAG amplifiers
As a consequence of the material parameters detailed in Section
All RT Yb:YAG-based regenerative amplifiers reported to-date have been focused on optimizing output energy, average power, beam profile and long-term stability. In our case, we are in addition interested in maximizing the amplified spectral bandwidth to match the THz radiation generation requirements. The regenerative amplifier architecture in conjunction with the wide emission bandwidth of Yb:YAG at room temperature offers opportunities to maximize emission bandwidth via either injection of high-energy broadband pulses or via intra-cavity spectral shaping. Considering the empirical formula established by Rouyer
Fig. 2. Computed amplified spectral bandwidth as a function of seed energy in a Yb:YAG thin-disk regenerative amplifier ($\unicode[STIX]{x0394}\unicode[STIX]{x03BB}_{\text{Fluo}}=5$ nm).
Maintaining a wide spectral band within the regenerative amplifier is also beneficial to minimize the accumulation of nonlinear phase typically plaguing thin-disk regenerative amplifiers as the stretched pulse duration increases linearly – in first approximation – with the spectral width of the amplified pulse. Considering a stretching ratio as large as 650 ps/nm – a parameter we have safely demonstrated in our laboratory and have been operating for several years – and an amplified bandwidth inside the cavity as wide as 2 nm, pulses as temporally stretched as 1.3 ns could be employed.
The damage fluence of a typical optical coating or material lays in the
Fixing the mode size at the gain medium determines much of the energetics behavior of the amplifier as well as the thermal loading of the gain medium. Our current numerical simulations of the energetics are in agreement with the reported results: several tens of passes are required in the gain medium in order to reach the 100 mJ level. Operation at 100 Hz repetition rate is preferable to avoid the regime of period doubling observed in regenerative amplifiers operated at a period close to the inverse of the excited lifetime of the dopant ion in the gain medium. It is nonetheless possible to operate at 1 kHz repetition rate, yet care must be taken to operate in the stable regime of period doubling or on the edge of bifurcation.
4.4 100 mJ and 1 J cryo-Yb:YAG amplifiers
We are in the process of constructing several engineered prototypes based on various types of the high power thin-disk amplifier: a first stage of the composite-thin-disk multi-pass amplifier[31, 32] was first reduced to practice at 100 mJ per pulse, demonstrated recently. It will serve as the pre-amplifier for the 1 J scaled-up amplifier which is under development.
The Yb:YAG 100 mJ and 1 J amplifiers are based on a cryogenic composite-thin-disk and multi-pass, relay-imaging architecture. The gain-element geometry is resistant to amplified spontaneous emission (ASE), which enables operation at high gain. The strict image relayed multi-pass architecture allows spatial filtering with every pass to avoid damaging intensity ripples, whereby the laser input efficiently and smoothly saturates the stored energy to deliver high-energy ultrafast pulses with diffraction limited beam quality at high repetition rates.
The enhancements of the composite-thin-disk approach over the traditional approach are detailed in Figure
Fig. 3. The cryogenic composite thin disk: in our approach, a thin Yb:YAG gain sheet is diffusion bonded to a thicker index-matched cap on one face while the other face is HR coated and soldered to a backplane high-performance cooler. See text for details.
Considerably facilitating the pumping by cryogenic operation: A large amount of diode power can be absorbed in a single ‘bounce’ avoiding the multi-passing necessary at room temperature, taking advantage of the inherent brightness of modern diode stacks.Maximizing aperture size: ASE limits the aperture. Compared to the traditional thin disk, the composite thin disk dilutes spontaneous emission into the undoped cap where it is guided toward the edges and ejected. This dramatically reduces ASE loss of gain due to extending the aperture dimensions.Minimizing deformations: The undoped cap adds strength. Stiffness increases in proportion to the cube of the thickness, minimizing wavefront errors.
In the one-dimensional (1D) thin-disk geometry, the temperature rises uniformly and constantly. The diffusion-bonded undoped cap does not compromise this thermal advantage and does not affect the 1D thermal distribution in the gain sheet.
This composite-thin-disk gain-element geometry is being scaled to store higher energy offering the possibility to reach 1 J output energy. To extend the results of our 100 mJ multi-pass amplifier to the 1-J level, the aperture diameter must increase from 4 mm to 16 mm while the aspect ratio (diameter to thickness) remains the same in order to scale the ASE rejection. The shape of the edges is also maintained to avoid recirculation and the undoped cap thickness increases proportionately. Calculations show a B-integral value of
The 100 mJ pulses of the first amplifier (shown in Figure
Fig. 5. (a) Measured output spectrum (black line) at the 10 mJ energy level along with seed spectrum (grey shaded region). (b) Measured output energy versus pump input fluence characteristics showing an output energy ${\sim}$ 90 mJ at full pump power.
Yb:LuAG emerged recently and is a suitable alternative to Yb:YAG, with higher thermal conductivity at high doping levels and increased emission cross-section but narrower spectral bandwidth[51]. We envisage further development using Yb:LuAG.
4.5 Cryo-YLF laser amplifiers
After stretching, the cryo-YLF laser chain comprises a regenerative amplifier and a booster amplifier[33, 34]. The stretcher and compressor are described in Section
The regenerative amplifier features a ring-type cavity consisting of four mirrors, a Pockels cell and half-wave plate, two thin-film polarizers (TFPs) and a cryo-Yb:YLF crystal[33, 34]. The pump power is provided by a fiber-coupled 300 W diode array operated in the pulsed regime. The cavity length of the regenerative amplifier is set to 1.5 m as a compromise between a small footprint and a large enough spot size. A small footprint enables long enough propagation time to ensure full switching of the Pockels cell high voltage during a round trip in the cavity, whereas a large enough spot size in the gain medium and Pockels cell leads to damage free operation at the 10–15 mJ output energy level. The Pockels cell is chosen to be a dual crystal rubidium titanyle phosphate (RTP) cell with a 6 mm aperture allowing a low (
In order to reach the targeted 100 mJ output energy, we designed and implemented an additional 4-pass amplifier, which relies on polarization switching and features two
4.6 Stretchers and compressors
The stretchers and compressors, though having different specifications depending on the amplifying medium, exhibit the same engineering challenges. The 0.3 nm or 2.1 nm wide pulses at the output of the Yb:YAG or Yb:YLF amplifiers respectively have to be stretched to 0.8 ns/nm and 0.33 ns/nm (to 240 ps and 800 ps stretched pulses after amplification) in order to minimize nonlinear effects and reduce the intensity of the pulses well below damage threshold.
Different technologies make such stretching ratios achievable, including the traditional Offner stretcher, chirped fiber and volume Bragg gratings (CFBG and CVBG), which are now commercially available. The strong benefit of CFBG and CVBG over the grating based Offner stretcher is the compactness of the stretcher for a fixed stretching ratio and the possible compensation of higher order dispersion. Due to manufacturing constraints, the CFBGs can be written over a longer length (up to 300 mm) compared with CVBGs (up to 75 mm). As a result, the net dispersion added to a pulse is higher using CFBGs than CVBGs in a single-pass architecture. The net dispersion can be increased by multi-passing the chirped Bragg grating (CBG), but insertion losses and imperfection of the CBGs limit the amount of possible passes in practice. Temperature tuning on the CFBGs allows for fine-tuning of the dispersion curve, i.e., fine adjustment of high-order dispersion after compression. Our desired stretching ratios and pulse bandwidth demand a cascade of 4–5 CVBGs. Therefore, it is more convenient to implement CFBGs as stretcher for this laser system.
Fig. 6. CAD modeling of (a) the grating compressor currently in use after a Yb:YAG high-energy amplifier and (b) the holder of the large grating in the first compressor built in our lab after the Yb:KYW regenerative amplifier[47]. (c) A newer version of the grating holder, implemented for the Yb:YLF laser system.
The ultra-short pulse outputs from the splitter of the frontend should be stretched before being used in any subsequent regenerative amplifiers of the ICS or electron gun laser systems. We employ a stretcher module that is completely fiber-based ensuring long-term stability. It consists of two CFBGs. CFBG 1 is followed by an ytterbium-doped fiber amplifier (YDFA) 1 and another CFBG 2 and YDFA 2. To monitor the long-term system performance and to detect degradation of components, a 1% fiber tap coupler after each amplifier stage is inserted and directly coupled to a photodiode and permanently monitored. All these fiber components are polarization-maintaining (PM) to ensure environmentally independent performance of the stretcher modules. The operation of the CFBG can be adjusted to the center laser wavelength of the subsequent amplifiers, i.e., for 1018 nm in case of Yb:YLF or for 1030 nm in case of Yb:YAG. A high isolation isolator is placed behind the output of the stretcher modules. The output energy of approximately 15 nJ serves as input for the regenerative amplifiers.
On the compressor side though, technological alternatives reduce to the grating based Treacy compressor[52, 53], as the energy is too high for the achievable aperture of CFBG or CVBGs. The larger the stretching ratio, the longer the distance between the gratings, i.e., the larger the footprint of the compressor. For the Yb:YAG laser chain, the proposed compressor with 1740 lines/mm,
The prevalent use of large optics in our setup calls for special care to their specifications. The turning mirrors are thick to prevent bending along their length, which would be detrimental to the beam quality and pulse compression particularly at the regions, where the beam is highly spatially chirped (after the second grating, on the roof mirrors to change the beam height). The surface quality (polishing and scratch/digs) is specified in agreement with the effective size of the beams on the mirrors: for example, on the roof mirrors located after the second grating, where the beam is highly spatially chirped, the flatness is defined as
During propagation, spatially quasi-flat-top or super-Gaussian beams develop ripples, which might cause damage on the optics in the compressor. There exist some solutions to prevent this: one is to implement the compressor so that the beam remains over the full distance in the compressor in the near field; for this, one can use a configuration with relay imaging, similar to an Offner stretcher. In the case of a double pass compressor, another solution is to relay image the beam between the two passes when it remains over one pass within the near-field range. With these two solutions, the footprint of the compressor increases a lot, due to the very long focal lengths of the lenses required. A third solution is to let the beam propagate so that the ripples diffract and use the far field of the beam; the inconvenience of this method, that we are currently evaluating, is the loss of 10%–20% of the energy, depending on the exact super-Gaussian order of the beam.
Fig. 7. Schematic of the two-stage OPA system to drive the UV generation setup. In the prism compressor located between the two OPA stages, a pulse shaper is implemented: knifes block the highest and lowest spectral components. WL: white-light generation, SHG: second harmonic generation, Comp: compressor.
At high average powers, even the residual absorption of the coatings might lead to heating with impact on the alignment or on the wavefront. The multi-layer dielectric gratings we currently use have an efficiency of 97%–98% and the remaining energy is mainly reflected into the 0th order. Thermal effects can also be minimized by using Zerodur substrates. A kW-class compressor has also been demonstrated with dielectric gratings in Ref. [41].
4.7 UV generation
In conventional electron guns, electron bunches are generated on a photocathode exposed to ultrafast UV pulses. The UV pulse is generated via third harmonic generation of the output from a Ti:sapphire amplifier delivering 10–150 fs (FWHM) pulses in the NIR. In that case, the generated UV can support an electron bunch shorter than 90 fs. As described in the previous sections, we are planning to use a Yb-based pump laser system to run the THz based accelerator, but its bandwidth is limited to
Figure
The generated seed is amplified in the first OPA stage (OPA1) to
Fig. 8. (a) Spectra of the first and second OPA stages (OPA1 and OPA2). (b) Autocorrelation trace of the second OPA stage after the prism compressor and the corresponding Gaussian fit.
Table 4. Summary of the pulse parameters after each module of the CT Yb:YAG laser chain.
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After compression, the pulse energy dropped to
In order to generate UV pulses via third harmonic generation, the compressed output of the second OPA is focused inside types I and II phase matched BBO crystals, respectively. The first crystal with 100-
The following section will make use of these presented modules and build possible laser chains, before discussing the achievable pulse characteristics with each laser chain and place this in light of the requirements for driving each stage of the light source.
5 Laser chains
5.1 Possible laser chains
5.1.1 Yb:YAG cryogenically cooled laser chain
A cryo-Yb:YAG laser chain starts from a dedicated output of the frontend delivering nJ level pulses centered at 1029.5 nm. After stretching, the pulses are amplified to the mJ level in a Yb:KYW regenerative amplifier, then to the 100 mJ level in a first cryo-Yb:YAG amplifier and finally to the 1 J level in a second cryo-Yb:YAG amplifier. The pulses are finally compressed with a Treacy compressor.
Between the amplifier modules, telescopes allow for adaptation of the beam size while relay-imaging the output of one module to the input of the next one. After the first booster amplifier, up to 100 mJ, it is critical to strictly image the super-Gaussian near field from the output of one module to the entrance of the next one. After amplification to 1 J, the beam profile is highly super-Gaussian. We are evaluating whether the beam will be propagated over a long distance to the compression stage to use the bell-shape far field, or whether relay-imaging telescopes will be used before and after the compressor.
Table
5.1.2 RT Yb:YAG laser chain
The RT-Yb:YAG regenerative amplifier described in Section
Table 5. Summary of the pulse parameters after each module of the RT-Yb:YAG laser chain.
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5.1.3 Yb:YLF cryogenically cooled laser chain
The cryo-Yb:YLF detailed in Section
Table 6. Summary of the pulse parameters after each module of the CT-Yb:YLF laser chain.
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5.2 Comparison of the achievable specifications
The cryo-Yb:YAG laser chain will deliver pulses with 0.3 nm bandwidth sustaining the generation of 100 GHz radiation, whereas the pulses out of the cryo-Yb:YLF and RT-Yb:YAG laser chains keep enough bandwidth to sustain the generation of 300 GHz radiation.
From these considerations on the achievable pulse width and pulse energy with different laser media, from the trade-off on the efficiency of the THz conversion (Section
Using the 1 J Yb:YAG cryogenically cooled laser chain allows for generating 5 to 15 mJ of THz radiation for driving the LINAC, which can realize an energy gain to relativistic energies in the range of 5–15 MeV. With all laser chains, the highest photon energy achievable from the ICS interaction is in the range of 6 keV[55]. The difference between the laser drivers occurs in the photon yield. In the ICS driven by the Yb:YLF line, we expect a yield of
The data acquisition constraints during scientific experiments make a repetition rate of 1 kHz desirable. A long-term stable and reliable 1 J, 1 kHz laser system and a 100 mJ level cryo-Yb:YLF are however challenging to engineer. One remedy is the operation in a first step at 100 Hz repetition rate, which decreases the heat load on the crystals by a factor of 10. At the 100 mJ level, Yb:YAG and Yb:YLF laser amplifiers are demonstrated in our group operating at CT[32, 33] and in other groups at RT[41, 44, 45]. Yb:YLF at CT and Yb:YAG at RT are both good candidates for experimenting the THz generation and exploring electron acceleration while a 1 J laser system is being developed. As the conversion efficiency into THz radiation increases with the THz frequency, it is advantageous to implement the cryo-Yb:YLF or RT-Yb:YAG laser chains during the time frame of our project, while the 1 J laser systems are being developed for further upgrade. With laser chains on the level of 100 mJ pulse energy, it is possible to demonstrate electron acceleration and X-ray photon emission, though at lower photon energies than expected in the final machine.
The three possible laser chains exhibit an energy stability of 1%–2% rms, which is one order of magnitude larger than the requirement. For ultimate stability of the X-ray pulses, active stabilization of the IR-pulse energy will be implemented.
6 Challenges
6.1 Synchronization
Synchronization between the individual laser lines will have a significant influence on the achievable performance of the light source. Specifically, the laser systems need to be synchronized in such a way that the light–electron interactions along the machine are synchronized, too. For example, these interaction points include photo-electron generation at photocathode, terahertz electron acceleration in the gun and LINAC as well as electron interaction with the ICS laser pulse. Ideally, synchronization between the electrons and the individual THz or laser beams will be employed. Methods to directly observe the arrival time of electron bunches are under development[56–58] but will be discussed elsewhere. In the following, we will describe our approach to lock the individual laser beams prior to THz or laser–electron interaction.
Considering the multiple round trips within the regenerative amplifiers and the multi-pass amplifiers a total optical path in excess of 300 m is expected for each beam line, despite the fact that the physical separation of the seed source and the accelerator is only about 10 m. Assuming a typical thermal expansion of the materials used to house the laser of approximately
In order to achieve a manageable engineering problem, multiple levels of accuracy are considered. The first level provides timing on a timescale of the seed laser repetition rate. By electronically selecting a pulse for separate beam lines, path length differences can be corrected on multiples of the seed repetition rate.
Fig. 9. (a) Simultaneous measurement of the energy at the output of the Yb:KYW regenerative amplifier, pointing measured after the regenerative amplifier, and stretched spectrum. Only a fraction of the energy of the regenerative amplifier is measured without rescaling to the total energy. An rms value for the relative energy fluctuations of 0.8% is measured. The stretched spectrum was measured with a 12.5 GHz photodiode and a 4 GHz oscilloscope. (b) Long term measurement of the Yb:KYW regenerative amplifier output.
In the second synchronization level, optical path lengths must be length matched for all beam lines derived from the same low-repetition-rate source, e.g., a regenerative amplifier. Optical pulses with a length on the order of a few picoseconds can be well synchronized using balanced optical cross-correlators (BOCs) as timing jitter detectors[59, 60]. To employ this method, one pulse is declared a master pulse, preferentially the pulse arriving at the electron gun. Co-propagating this pulse with the electron bunch and adding slight delays using high precision delay lines creates a timescale on the same total length as the accelerator. BOCs can then detect timing deviations and corrections can be made at the beginning of the individual beam lines.
A third level of locking can be made after the THz generation by combining techniques from the standard optical BOC and electro-optical sampling of THz waveforms. The optical gun pulse is used as master, and that pulse is used to determine the arrival time of a THz pulse close to the accelerator. This is especially advantageous for the multi-cycle THz pulses where the phase between the THz and the electron bunch is of higher importance than the envelope and the bunch[61].
Independent of the method to achieve synchronization, the low final repetition rate results in a severe limitation of achievable performance. The laser pulses sample the acoustic and thermal noise with 100 to 1000 Hz, down-folding higher frequencies into the Nyquist spectral range. To minimize this impact, specific attention must be paid to the noise sources during facility construction and thermal stabilization of the laser system aiming to damp vibrations with higher frequency components.
6.2 Long-term stability
The scientific experiments intended with the table-top X-ray source require laser operation for the time of alignment and of data acquisition, both of which may last for several hours. This sets a strong constraint on the long-term stability of the laser system and THz generators.
Efforts have been already set in engineering the Yb:KYW regenerative amplifier and the cryo-Yb:YAG amplifiers. Measurements on the currently available amplifiers, shown in Figure
The Yb:YLF laser amplifier chain operating daily in our lab exhibits a pulse-to-pulse stability below 1% after the regenerative and booster amplifiers. Figure
Fig. 10. (a) Measured 1-h stability of the regenerative amplifier output at the 10 mJ energy level. The computed shot-to-shot instabilities are less than $\pm 0.75\%$ rms over 1-h. In inset, the measured spatial intensity profile at 10 mJ output energy. (b) Measured output energy stability recorded over 3.5 h at ${\sim}$ 75 mJ output energy. The observable slow drift is attributed to a minor drift in seed energy of the current frontend. Energy instabilities less than $\pm 0.7\%$ over 3.2 h are routinely achieved.
In all cryogenically cooled modules, automatic refill systems will be implemented to ensure long-term operation. Implementing pointing stabilizers after each module will enhance long-term operation by minimizing the influence of misalignments between stages.
The output of the OPAs for UV generation, already operating for electron acceleration experiments, demonstrates a good day-to-day stability. The pulse energy measurement exhibits very good stability over 15 h as shown in Figure
6.3 Controls and diagnostics
The complex laser system requires controls and monitors to check the correct operation of each module. Table
Table 7. Diagnostics for the modules.
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6.4 Cryogenic cooling
The cryogenic cooling of the CT laser chain is enforced with liquid nitrogen for the 100 mJ Yb:YAG and Yb:YLF amplifier modules. The LN2 consumption, measured empirically, of the 100 mJ and 1 J stage cryo-Yb:YAG amplifiers together is 132 L per 12 h at 1 kHz, whereas the cryo-Yb:YLF regenerative amplifier requires 1.2 L per 6 h and the following 4-pass amplifier
6.5 Beam transport
For radiation protection purposes, the accelerator must be located in a shielded room, separated from the laser system. The high-energy laser beams will then be transported to the accelerator setup via chicanes needed to avoid direct line of sight from one room into the other. In principle, either the IR beams or the THz beams could be transported. Optics for THz radiation exhibit high losses and are not adequate for transport over long distances requiring multiple optics. The IR pulses, even though highly energetic, are easier to transport, whether stretched or unstretched. Considering 1 J, 5 ps compressed pulses with 20 mm flat-top beam diameter, a transport in air would accumulate 0.03 rad/m B-integral (assuming
7 Overall description of the laser system
7.1 Presentation of three concepts
We have developed three concepts for the laser system driving our X-ray light source, two operating with cryogenically cooled laser crystals: the first one is based on Yb:YAG whereas the second one takes advantage from both Yb:YAG and Yb:YLF. A third concept relies on RT Yb:YAG amplifiers, which are not easily scalable to the joule level, but partially commercially available.
Fig. 12. Schematic representation of the laser system based on cryo-Yb:YAG laser systems.
7.1.1 Design solution 1
Figure
Fig. 13. Schematic representation of the laser system based on cryo-Yb:YLF and cryo-Yb:YAG laser systems.
7.1.2 Design solution 2
Figure
7.1.3 Design solution 3
Figure
In all three concepts, the synchronization of the pulses occurs at the low-energy stages, while the delay between two pulses is detected just before the THz frequency conversion stages.
7.2 Discussion
These three concepts are able to deliver pulses serving the different stages of the table-top X-ray light source. Obviously, variants of these possibilities can be thought of.
The first version is advantageous because it delivers enough optical pulse energy in all stages to achieve the full specification of the table-top X-ray light source. One regenerative amplifier could seed the two cryo-Yb:YAG lines for the gun in order to minimize the jitter between the two gun drivers.
The second and third versions mitigate this risk by requiring only 1-J class amplifier, whose output pulses do not require compression, thus avoiding the issues coming along the compression of high-energy, flat-top beams. The cryo-Yb:YLF laser amplifiers have not yet been demonstrated at 100 Hz, compared to the RT-Yb:YAG amplifiers. However, the performances of the RT-Yb:YAG amplifiers in terms of pulse energy, repetition rate, beam quality and daily operation and maintenance (without use of liquid nitrogen) make the third version highly attractive. Scaling cryogenic Yb:YLF laser systems to high average power has been demonstrated up to 100 W (10 kHz, 10 mJ) average power when operated in a CPA architecture[64]. These systems could operate at high average power since the relatively low energy enables tight focusing of the pump therefore enabling high gain. In the high-energy regime, the pump area must be increased in order to ensure that the signal fluence remains below the damage fluence, which ultimately reduces the gain or demands increased pump levels (with a direct impact on thermal loading of the gain medium). There are two solutions to untie this dilemma which consists in increasing the signal pulse duration to the ns to multi-ns regime – a measure we have already implemented and that allows maintaining relatively small pump area and therefore acceptable gain levels – or resorting to regenerative type amplification where low gain can be compensated for by increasing the number of passes in the gain medium. Operation in the kHz regime will require merging these two approaches and operating any regenerative type amplifier at one of the stability points where period doubling can be avoided[65].
In all versions, the multi-cycle, multi-mJ level THz stage for the LINAC is driven by a 1-J-class laser, here presented with a sequence of a Yb:KYW regenerative amplifier and cryo-Yb:YAG amplifier seeding the high-energy cryo-Yb:YAG amplifier. The Yb:KYW amplifier restricts the variations in bandwidth and the spectral narrowing during six decades of amplification. One could replace the first cryo-Yb:YAG amplifier with an RT-Yb:YAG amplifier, for ease of maintenance, similarity with the other laser chains and decrease of engineering effort; however, the spectral peak of amplification shifts slightly between RT and CT operations.
Concerning the UV generation, the first version is advantageous for the bandwidth offered by the Yb:KYW regenerative amplifier. However stable WLG has been demonstrated with ps-long pulses[63, 66, 67].
One last point to discuss is the requirements induced by the laser system on the facility.
7.3 Facility requirements
Figure
Fig. 15. Layout of two Yb:YAG laser chains on one optical table. The seed pulses are fiber delivered. The delay stage (dt) is followed by the Yb:KYW regenerative amplifier (REG), followed by the two CTD amplifiers with a relay imaging telescope (R.Tel) in between. After the regenerative amplifier and the first CTD amplifier, there is a pointing stabilizer. The spatial profile of the beam is measured after each stage. The alignment laser for first alignment of the 100 mJ CTD is represented.
In total, the laser system necessitates a lab of roughly 190
8 Conclusion
The conceptual design of a laser system driving a table-top, THz-driven X-ray source was discussed. The laser system generates several outputs to drive successive stages of the light source. First, the specifications for these outputs were inferred from each specific task. After discussing the achievable laser performances with cryo-Yb:YAG, cryo-Yb:YLF and RT-Yb:YAG, laser amplifier modules are presented, with the available experimental results. These performances and results are then discussed in light of each task. We then envisage and compare three architectures, one based on all cryo-Yb:YAG amplifiers, the other one based on cryo-Yb:YAG and cryo-Yb:YLF laser chains and the third one based on RT-Yb:YAG laser chains. The cryo-Yb:YLF based one has for advantage that the lasers have been demonstrated in our laboratory, though at lower repetition rate over long time periods. The RT-Yb:YAG, though not yet demonstrated in our group, seems to be the most promising and straightforward solution in terms of daily maintenance and scaling to high repetition rates. In all cases, a J-class amplifier currently under development will drive the multi-cycle THz stage for LINAC. Engineering challenges such as synchronization, long-term operation, stability, beam transport and facility requirements are discussed.
The authors declare no conflict of interest.
[1]
[2]
[3]
[4]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[23]
[24]
[25]
[27]
[28]
[29]
[30]
[32]
[33]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42]
[45]
[47]
[48]
[49]
[50]
[51]
[52]
[53]
[55]
[56]
[57]
[58]
[59]
[60]
[62]
[63]
[64]
[65]
[66]
[67]
Article Outline
Anne-Laure Calendron, Joachim Meier, Michael Hemmer, Luis E. Zapata, Fabian Reichert, Huseyin Cankaya, Damian N. Schimpf, Yi Hua, Guoqing Chang, Aram Kalaydzhyan, Arya Fallahi, Nicholas H. Matlis, Franz X. Kärtner. Laser system design for table-top X-ray light source[J]. High Power Laser Science and Engineering, 2018, 6(1): 01000e12.