In recent years, the broad application prospects of higher harmonics (HHG) and attosecond science have greatly promoted the development of high average power, short pulse width, and mid-infrared wavelength laser systems. On the one hand, since longer-wavelength laser driving pulses have higher plasmodynamic potential and can drive the generation of higher-energy photons, the use of longer-wavelength infrared or even mid-infrared ultra-short and ultra-strong laser pulses to interact with matter can expand the cutoff frequency for generating higher harmonics (HHG) and generate higher-energy and shorter-wavelength ultraviolet and even X-ray band photons [1], which is of great significance in cutting-edge research fields such as water window band X-ray lasers and even shorter wavelength lasers. On the other hand, due to the increase in the laser driving pulse wavelength, the transit time increases and the quantum scattering mechanism of the electron wave packet reduces the generation efficiency of higher harmonics. Therefore, the mid-infrared high-power ultrashort pulse laser of about 1-5 microns is considered to be an ideal light source that interacts with matter to generate broadband high-energy X-band higher harmonics. It is also an ideal driving source to further obtain a single ultraviolet and soft It is of great significance in cutting-edge application research such as ray coherent radiation.
The so-called Red refers to the relatively short-wavelength part, which can output near-infrared pulses with an average power of 20W, a tunable central wavelength range of 1.4-1.75um, stable CEP and a high repetition frequency of 100kHz.
The so-called Black refers to the relatively long-wavelength mid-infrared pulse that can output an average power of 15W, a tunable central wavelength range of 2.5-4.0um, stable CEP and a high repetition frequency of 100kHz [2].
This laser system has the following outstanding advantages:
Industrial grade Yb laser is used as pump source and seed source. Unlike titanium sapphire lasers, industrial-grade semiconductor-pumped Yb lasers have the characteristics of high repetition frequency and high stability, and are especially suitable for OPCPA pump sources.
Passive light synchronization. The seed pulse of OPCPA is first generated by the infrared supercontinuum (SC) generated by the pump pulse in the solid medium, and then generated by the difference frequency process (DFG) with the pump light (2.5-4.0um). Therefore, the signal light and the pump light are of the same source, which can eliminate the synchronous jitter of the signal light and the pump light, and obtain CEP stable idle light.
Structurally stable. The signal pulse generation, amplification and compression parts of the OPCPA device all use bulk solid materials. The device and its output energy and spectrum are very stable.
Dispersion is precisely controlled. The front end of the OPCPA device uses an ultra-high-speed 100kHz programmable acousto-optic modulator (LN-AOPDF), which can regulate the spectral phase of the SC, obtain the best dispersion control and compensation in the entire OPCPA system, and obtain the best compressed pulse width.
CEP precise control. In this OPCPA device, the above-mentioned LN-AOPDF can be combined with the CEP measurement device to form a CEP feedback stabilization system (DAZZLER--Fringeezz) to achieve long-term active stabilization of CEP (65 mrad rms @8h). So far, the stability of this CEP remains the best in the world among similar products.
Below we introduce the laser system in detail:
The basic principle of Red & Black Starzz is based on the optical parametric chirped pulse amplification (OPCPA) technology proposed by A. Dubeitis et al. in 1992 [3], as shown in Figure 1 below. The ultrashort pulse (fs) first passes through a stretcher to stretch the pulse in time to the picosecond (ps) or nanosecond (ns) level, and then amplifies the broadened pulse energy based on optical parametric amplification technology (OPA). Finally, a compressor is used to compress the broadened pulse back to an ultrashort pulse (fs). This can avoid damage to optical components such as nonlinear crystals and optical lenses during pulse amplification.
Figure 1. Basic principle block diagram of OPCPA
Figures 2 and 3 are the system structure block diagram and optical path diagram of Red & Black Starzz respectively. The Red & Black Starzz system mainly consists of 4 parts: external pump source (Dira-200, 200W, 100kHz, 1030nm), OPCPA front-end amplifier (4W, “front-end”) [4], OPCPA main amplifier (15W, “booster”) [2] and pulse comprehensive diagnostic system.
Figure 2. Starzz system structure diagram
Figure 3. Starzz system optical path diagram
Pump source: Any high repetition frequency Yb amplifier with pulse width <2ps, representative products are Trumpf's Dira series and Amplitude's Tangerine series. Here we take Dira-200 as an example. Its output average power is 200W@100kHz, the center wavelength is 1030nm (Dira-200, TRUMPF Scientific Lasers), and the pulse half-height width is 1.1ps;
Seed source: In order to obtain accurate time synchronization and eliminate delay jitter, infrared supercontinuum generation technology is used here, and a pump light close to 1W is focused on the YAG crystal to obtain a broadband and stable supercontinuum (SC) of 1.3-1.9um;
LN-AOPDF: Core component for dispersion and CEP control. The phase of SC can be controlled to perfectly match the pulse width of signal light and pump light; the bandwidth and waveform of SC can be selected; the high-order dispersion introduced by later optical components can be pre-compensated; it can be combined with 2f-to-f interferometer and Fringeezz to form an active CEP control and stabilization system, etc.
DFG: (MgO:PPLN, quasi-phase matched). Generate tunable broadband mid-infrared light 2.5-4.0um; and achieve its passive CEP stabilization ("self-seeded DFG");
OPA2&3: front-end amplifier, 4W@3.2um, 100kHz;
OPA4: main amplifier, 16.5W@3.2um, 100kHz;
Compressor: Dispersion compensation is performed in a 20mm silicon plate to obtain 15.2W, 38.2fs@3.2um, 100kHz;
Environment: The entire OPCPA system is filled with dry air or nitrogen to prevent water vapor and CO2 in the mid-infrared band from absorbing the specific infrared spectrum;
Due to the entire OPCPA process, all dispersion control elements use bulk solid materials. Therefore, the OPCPA system is extremely stable. For example, if it runs continuously for 8 hours, its energy stability is 0.6% RMS (1s).
Figure 4. CEP locked Starzz
In addition, by using the 2f-to-f method with a fast fringe detector (Fringeezz, Fastlite) to measure the CEP of the output pulse, and forming a closed loop with LN-AOPDF (Dazzler), CEP active control stability (@10kHz) can be achieved. As shown in Figures 4 and
5. Without CEP active control, the CEP stability is 344 mrad RMS; with CEP active control, the CEP stability is 76 mrad RMS; and no CEP drift occurs for more than 8 hours. So far, this record has not been broken by other amplification systems of the same type.
Figure 5. CEP passive stability (without feedback loop) and CEP active control stability (with feedback loop)
Summary: Starzz series products, with their high repetition frequency of 100kHz, long wavelength of 3.2um and tunable, high power of 15W, short pulse width of 4-cycle, long-term power/energy stability, excellent CEP stability and no drift, the output spot focus peak intensity is 〖10〗^15 W⁄〖cm〗^2 (Focused spot 20um, FWHM), can be widely used in HHG, XUV spectroscopy and imaging, attosecond science and other fields. It has been successfully used in systems such as the European ELI-ALPS attosecond science system, the British Rutherford National Laboratory, the German MBI, the Canadian NRC-Corkum Group, the Japanese accelerator and the synchrotron radiation system Spring-8, and has received unanimous praise from industry insiders.
The output indicators of Red & Black Starzz are summarized as follows [5]:
References
【1】T.Popmintchev, et.al. “Bright coherent ultrahigh harmonics in the keV x-ray regime from mid-infrared femtosecond lasers,” Science 336(6086), 1287-1291 (2012).
【2】N. Thiré, et.al "Highly stable, 15 W, few-cycle, 65 mrad CEP-noise mid-IR OPCPA for statistical physics," Opt. Express 26, 26907-26915 (2018).
【3】A. Dubietis, G. Jonušauskas, A. Piskarskas, “Powerful femtosecond pulse generation by chirped and stretched pulse parametric amplification in BBO crystal, ” Optics Communications, Volume 88, Issues 4–6, 1992,
【4】N. Thiré, et.al "4-W, 100-kHz, few-cycle mid-infrared source with sub-100-mrad carrier-envelope phase noise," Opt. Express 25, 1505-1514 (2017).
【5】http://www.fastlite.com/en/ar824685-823533-Starzz.html