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A three-stage high conversion efficiency optical parametric amplifier (OPA) with passively stabilized carrier-envelope phase (CEP) is presented. After passing through an argon-filled hollow fiber and the dispersion compensator for pulse compression, CEP stabilized 0.7 mJ, 1.5 cycle laser pulses at 1.75 μm center wavelength are obtained. Terahertz (THz) emission spectroscopy is adopted to determine the value of CEP, indicating an excellent long-term CEP stability.
©2011 Optical Society of America
1. Introduction
Ultrashort laser pulses are powerful tools for the investigation of the ultrafast dynamics process in various fields including physics, chemistry, biology, and material science research [1]. To investigate these processes, the laser pulses are required to have shorter pulse duration than the time range of phenomena of interest. The advances in ultrashort pulse laser technology make it possible to observe ultrafast dynamics process in femtosecond resolution [2]. Isolated bursts of attosecond extreme ultraviolet (XUV) radiation make it possible to probe ultrafast dynamics on a previously unexplored timescale (~100 as) [3,4].
Such attosecond pulses are produced via high harmonic generation (HHG) in noble gases, driven by intense carrier-envelope phase (CEP) stabilized few-cycle laser pulses [5]. Usually, the driving sources for HHG are the compressed few-cycle laser pulses from a CEP stabilized Ti:sapphire laser at wavelength of 800 nm. To obtain higher photon energy harmonic supercontinuum for the shorter attosecond pulses, the driving laser pulses at longer wavelength is suggested [6]. In addition, the longer wavelength laser also has an advantage to improve the isolated attosecond pulse contrast due to higher nonlinearity. Especially the laser pulses with center wavelength lies in the range of 1.5-3 μm, as a balance between high cut-off energy and acceptable conversion efficiency of HHG, is regarded as a suitable driver for attosecond pulse generation [7,8]. Using 2 μm laser pulses, fully phase-matched HHG spanning the water window spectral region has been realized recently [9]. Isolated attosecond pulses with wavelength lies in the water window spectral region may be obtained in the near future.
In order to obtain the CEP stabilized pulses in longer wavelength region, the difference-frequency generation (DFG) technique has been developed [10]. Compared with the active CEP stabilization technique, the passive stabilization technique is relatively insensitive to the environmental disturbance. Moreover, as the phase drift is naturally eliminated in the DFG process, excellent long-term CEP stability can be expected. There are two methods for CEP stabilized pulse generation by the DFG technique: (i) the CEP stabilized seed is generated in the DFG process within a single pulse, then amplified in the following optical parametric amplifier (OPA) or optical parametric chirped-pulse amplifier (OPCPA) stages. 1.2 mJ, 17 fs at 1.5 μm [8] and 740 μJ, 15.6 fs at 2.1 μm [11] CEP stabilized pulses were generated by using this method; (ii) the CEP stabilized idler pulse is generated in a OPA process directly, if the pump and the signal pulses are from the same laser source with the same carrier phase offset. 1.5 mJ, 19.8 fs at 1.5 μm [12] and 0.4 mJ, 11.5 fs at 1.8 μm [13,14] CEP stabilized pulses were obtained in this way.
In this paper, we report the generation of CEP stabilized 8.4 fs, 0.7 mJ pulses at 1.75μm center wavelength by an OPA system followed by a hollow fiber compressor. Firstly, we obtained CEP stabilized 1.6 mJ, 57 fs pulses at 1.8 μm from a three-stage near-IR OPA pumped by a commercial 8 mJ, 40 fs Ti:sapphire laser amplifier at 1 kHz repetition rate. The obtained 1.8 μm pulses are then spectrally broadened by the nonlinear propagation in an argon-filled hollow fiber and subsequently compressed to 8.4 fs (less than 1.5 cycles) with energy of 0.7 mJ simply using a pair of thin fused silica wedges. The compressed pulses are CEP stabilized with ~547 mrad root-mean-squared (rms) CEP fluctuations, making this source a suitable driver for attosecond pulse generation. Nonlinear spectral interferometry and terahertz (THz) emission spectroscopy are employed to test the short-term and long-term CEP stability.
2. Experimental setup
2.1 Optical parametric amplifier
The configuration of the three-stage OPA is shown in Fig. 1 . The pump laser pulses are from a commercial Ti:sapphire laser amplifier (Coherent LEGEND-HE-Cryo) which provides 40 fs pulses with pulse energy up to 8 mJ at 1 kHz repetition rate. The pump pulses are divided into four parts using three beam splitters. The smallest part of the laser pulses (<30 μJ) is focused into a 2-mm-thick sapphire plate to generate a single-filament white light continuum (WLC), which is used as the seed pulses for the following OPA stages.
Fig. 1 Experimental setup for the generation of high energy self-phase-stabilized pulses: VND, variable neutral density filter; HWP, half-wavelength waveplate; Sp, sapphire plate; TD, time delay crystal.
A fraction of the pump laser pulses with ~120 μJ pulse energy is used to pump the first near-collinear OPA stage (OPA1) consisting of a 2.5-mm-thick BBO crystal cut for type II phase matching (θ = 27.2°,φ = 30°). The intersection angle between the pump and seed beams is ~1°. The WLC is amplified to ~4 μJ in OPA1 with the center wavelength at 1.44 μm. Although OPA with type I BBO crystal in this spectral region has broader phase matching bandwidth [15], in the experiment, we found a lower efficiency and strong parasitic self-diffraction.
The amplified pulses at 1.44 μm from OPA1 are collimated and injected into the second collinear OPA stage (OPA2) consisting of a 2-mm-thick BBO crystal cut for type II phase matching (θ = 27.2°,φ = 30°). About 1.2 mJ laser pulse with a diameter of ~4 mm is used to pump OPA2, corresponding to a pump intensity of about 250 GW/cm2. In order to get high efficiency and prevent wave front tilt [16], both the signal and the pump beams are well collimated and the two beams are collinearly injected into the BBO crystal. The seed pulses are amplified to ~90 μJ in this stage.
The amplified signal pulses from OPA2 are enlarged and collimated to ~9 mm in diameter with a Galilean telescope. A 1.5-mm-thick a-cut YVO4 crystal is employed as a time delay crystal to separate the signal and the idler pulses in time. Then the laser beam is injected into the third collinear OPA stage (OPA3) consisting of a 2-mm-thick BBO crystal cut for type II phase matching (θ = 27.2°,φ = 30°). The remaining ~6.4 mJ pump laser with diameter of ~9 mm is used to pump OPA3 corresponding to pump intensity of about 250 GW/cm2. The same as OPA2, the pump and the signal beams are firstly collimated and then collinearly injected into the BBO crystal. This collinear setup is quite necessary to avoid angular dispersion in the generated idler beam. The signal pulses are further amplified to ~2 mJ at 1.44 μm, corresponding to the idler pulses with energy of ~1.6 mJ at 1.8 μm. The energy conversion efficiency is approximately 55% in OPA3. Usually, it is necessary to optimize the grating-based compressor (both the angle of incidence and the separation distance) in the Ti:sapphire laser system to optimize the chirp of the pump laser to obtain this high conversion efficiency. The pulse duration of the output signal pulses and the idler pulses are measured when the highest conversion efficiency is obtained by a home-built single-shot autocorrelator. The signal pulse duration is ~40 fs at 1.44 μm and the idler pulse duration is ~57 fs at 1.8 μm without any further pulse compression.
In fact, both the signal and the idler pulses from OPA2 can be used to seed OPA3 and the energy conversion efficiency is almost the same. The near-field beam profiles are measured in these two conditions using an IR beam profiler (Spiricon, PY-III-C-A). The results are shown in Fig. 2 . Although the near-field beam profiles show significant difference, they are all about 2 times diffraction limited according to our measurement. In the experiment, the signal pulse from OPA2 is chosen to seed OPA3 to get a relatively better near-field beam profile.
Fig. 2 Near-field beam profile of 1.44 μm (a) and 1.8 μm (b) laser when the signal pulses of OPA2 are used to seed OPA3; Near-field beam profile of 1.44 μm (c) and 1.8 μm (d) laser when the idler pulses of OPA2 are used to seed OPA3.
2.2 Pulse compression
The 1.8 μm idler beam from above OPA system is coupled into a 1-m long hollow fiber filled with argon (400 μm inner diameter, 0.5-mm-thick fused silica window) using a f = 0.75 m plano-convex lens. The output beam is collimated with an R = 2 m silver-coated concave mirror. We measured the pulse duration when the hollow fiber is evacuated, it is reduced from ~57 fs before the hollow fiber to ~45 fs.
Significant spectral broadening appears when argon gas is filled in the hollow fiber, as shown in Fig. 3 . Normally, the pressure of argon is set to be ~400 mbar, as a balance between broad spectrum and single mode purity of the output beam. In this case, the supercontinuum in the range of 1200-2100 nm is obtained which support the Fourier transform-limited pulse duration of about 8 fs. Only a pair of fused silica wedge is used to compensate the pulse dispersion due to its negative group delay dispersion (GDD) in this spectral range [13].
Pulse duration characterization is carried out with a home-built second harmonic generation frequency resolved optical grating (SHG-FROG). No transmissive optics elements are used in the SHG-FROG which makes it suitable to measure the pulse duration of few- cycle laser pulses. A 20-μm-thick type I BBO crystal (θ = 20.2°) is used in the SHG-FROG which ensures broad enough SHG bandwidth. The best pulse compression is achieved when the total thickness of fused silica after hollow fiber is ~2 mm, the corresponding SHG-FROG trace and the reconstructed electric field are as shown in Fig. 4 , with a retrieval error of 0.009.
Fig. 4 (a) Measured SHG-FROG trace of the compressed pulses. (b) Measured (shaded) and retrieved (black) spectral intensity and retrieved phase (blue), (c) Retrieved temporal intensity (black) and phase (blue). (d) The black curve is the temporal intensity profile corresponding to the measured spectrum and retrieved spectral phase, the red curve is the ideal Fourier-limit intensity profile corresponding to the measured spectrum.
It can be seen from Fig. 4(b) that the reconstructed spectrum (black) is not well in agreement with the spectrum measured directly using spectrometer (Ocean Optics, NIR 256), which could be caused by frequency filtering effect [17] and different spectral sensitivity of the instruments we used, because all these factors tend to decrease the intensity in the longer wavelength region. According to consistency of the fine structure of the retrieved spectrum and directly measured spectrum around 1600-1800 nm, we believe the reconstructed spectral phase [blue line in Fig. 4(b)] is reasonable. The temporal intensity is calculated based on the directly measured spectrum and the retrieve spectral phase, as shown in Fig. 4(d), the pulse duration is 8.4 fs, which is ~1.5 optical cycles considering the 1.75 μm center wavelength.
2.3 Carrier envelope phase
The CEP stability is characterized with a home-built collinear f-to-2f interferometer, as shown in Fig. 5 . The layout is similar to Ref. [18], the only difference is a 5-mm-thick c-cut LiNbO3 crystal is introduced between the WLC and SHG crystal. Because the group delay is almost the same for 900 nm and 1800 nm laser in glass, the fringe period cannot be observed if the time delay crystal is not introduced. Frequency overlap between the spectral broadened fundamental frequency and the second harmonic frequency is achieved in the 800-900 nm spectral range. Figure 6 shows the CEP fringes and phase stability for the 1.8 μm idler pulses before the hollow fiber and the 8.4 fs pulses after compression over a 30 min observation time (the exposure time and the delay time of the spectrometer are 2 ms and 0.2 s respectively). The rms CEP drift before and after the hollow fiber is ~401 mrad and ~547 mrad, respectively.
Fig. 5 Setup for CEP characterization by nonlinear spectral interferometry: VND, variable neutral density filter; WLC, white light continuum; SHG, second harmonic generation; TD, time delay crystal.
Fig. 6 CEP fringes (a) and phase drift (b) of the 1.8 μm pulses before the hollow fiber. CEP fringes (c) and phase drift (d) of the compressed pulses.
Similar to Ref. [19], terahertz (THz) emission spectroscopy is also adopted in the experiment to determine the value of CEP. Using the cross polarizer electro-optic sampling scheme [20], the THz waveforms from air filamentation of our few-cycle IR pulses are recorded at different CEP in a step of 0.2π radian, as seen in Fig. 7 . It can be seen that the measured THz field reverse its polarity as the CEP change from 0 to π, and from π to 2π as well. The cosinoidal dependence of THz intensity on CEP lasts over 2 periods of 2π in 4 hours data acquisition time, which indicates an excellent long-term CEP stability.
Fig. 7 Temporal THz waveforms measured by adjusting the CEP of few-cycle IR pulses in a step of 0.2πusing thin fused silica wedges.
4. Conclusion
In summary, we have developed a CEP stabilized 1.5 cycle, sub-millijoule level laser system at 1.75 μm center wavelength at 1 kHz repetition rate. Benefit from the character of passive CEP stabilization of the idler pulse during the OPA process, 1.6 mJ, 57 fs pulses at 1.8 μm with rms CEP fluctuation of ~401 mrad are obtained by three-stage OPA system. After passing through a hollow fiber and block material for dispersion compensation, the pulses are finally compressed to 8.4 fs and 0.7 mJ, with rms CEP fluctuation of ~547 mrad. The system provides an excellent laser source for isolated attosecond pulse generation and other HHG experiments.
Acknowledgments
This work was supported by Chinese Academy of Science, National Natural Science Foundation of China (NSFC) (Grant No. 10734080, 60921004, 60908008, 61078037), National 973 Program (Grant No. 2011CB808100), National Basic Research Program of China (Grant No. 2011CB808101), and Shanghai Commission of Science and Technology (Grant No. 09QA1406500). The author is grateful to B. E. Schmidt for helpful advice.
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