We have developed a 1 kHz repetition picosecond laser system dedicated for intense terahertz (THz) pulse generation. The system comprises a chirped pulse amplification laser equipped with a Yb:YAG thin-disk amplifier. At room temperature, the Yb:YAG thin-disk regenerative amplifier provides pulses having energy of over 10 mJ and spectral bandwidth of 1.2 nm. The pulse duration achieved after passage through a diffraction grating pair compressor was 1.3 ps. By employing this picosecond laser as a pump source, THz pulses having a peak frequency of 0.3 THz and 4 µJ of energy were generated by means of optical rectification in an Mg-doped LiNbO3 crystal.
© 2015 Optical Society of America
Terahertz (THz) pulses generated using short-pulse infrared lasers are useful in a variety of fields. THz time-domain spectroscopy has been an indispensable technique for material characterization, for example, to determine the dynamic properties of free carriers in condensed matter [1,2]. Imaging and tomography using intense THz pulses are of great importance in material, biomedical, and industrial applications [3,4]. Furthermore, interaction between intense THz pulse and molecule creates significant possibilities of controlling molecular vibration, rotation, spin precession, and so on . For generating intense THz pulses, optical rectification in a nonlinear crystal has been proposed  and remarkably developed in the current decade [7–9]. The most important invention in this regard is the tilted-pulse-front pumping scheme  by which the velocity matching between the pump pulse and the generated THz pulse in a LiNbO3 (LN) crystal can be fulfilled. After then, it was suggested that the use of Fourier limited pulses of sub-picosecond to picosecond durations is more effective than using 100 fs pulses . Furthermore, the generation of 125 µJ THz pulses using a 1.3 ps pump pulse with 50 mJ energy  and 436 µJ THz pulses using a 0.785 ps pump pulse with 58 mJ energy  delivered from a Yb:YAG chirped pulse amplification (CPA) laser with 10 Hz repetition rate  has been demonstrated. For increasing THz pulse energy, a contact-grating scheme, in which the diffraction grating is fabricated directly on the LN crystal, was proposed . In recent times, a contact-grating device with high diffraction efficiency was developed and THz pulse generation was demonstrated using this device .
In terms of the practical use of intense THz pulses in application research, picosecond pump laser systems with high peak power and high repetition rate are of considerable significance. Yb:YAG is one of the most promising materials for producing high repetition and high power picosecond laser systems . Its broad absorption band at around 940 nm matches that of the InGaAs laser diode. The long fluorescence lifetime (~1 ms) is suitable for continuous wave pumping. The small quantum defect because of close energy gaps between the absorption (940 nm) and emission (1030 nm) wavelengths limit heat generation. The thermal conductivity of YAG is high (~11 W/mK). Furthermore, thin-disk geometry is preferable for high average power operation because this geometry affords good cooling efficiency by facilitating large volume heat transfer through the contacted disk surface [17,18]. Using the Yb:YAG thin-disk regenerative amplifier, 165 mJ output at 100 Hz repetition rate  and 45 mJ output at 1 kHz repetition rate  have been demonstrated. As for the compressed short pulse, an energy output of 25 mJ with a duration of 1.6 ps at a repetition rate of 3 kHz was reported . More recently, a Yb:YAG CPA laser system providing 800-fs and 330-µJ pulses at a repetition rate of 300 kHz was developed and THz pulse generation using this laser was demonstrated .
In this study, we characterize our picosecond laser system developed for THz pulse generation. The system is a CPA laser equipped with a Yb:YAG thin-disk regenerative amplifier having a repetition rate of 1 kHz. After describing the experimental setup of THz pulse generation and detection, we present the specifications of the obtained THz pulses, such as waveform of the electric field strength, frequency spectrum, and output power as a function of pump power.
2. Laser system development
The laser system used herein is a CPA laser comprising a master oscillator, pulse stretcher, fiber pre-amplifier, Yb:YAG thin-disk regenerative amplifier, and pulse compressor. The master oscillator is a mode-locked Ti:Sapphire laser (Coherent: Mira) operated at a center wavelength of 1030 nm with full width at half maximum (FWHM) bandwidth of 6 nm, shown by a red line in Fig. 1. The oscillation frequency is 80 MHz, and the typical output power is 160 mW. Pulse duration of the seed laser is stretched by a Martinez-type pulse stretcher composed of a diffraction grating with 1740 grooves/mm, a spherical lens (plano-convex) with a focal length of 1000 mm, a flat end mirror, and roof mirrors for folding. The incident angle of the seed laser to the grating is set to 61° from the normal. A distance between the grating and the spherical lens is set to 625 mm. And the flat end mirror is put at 1000 mm from the spherical mirror. Consequently the effective length for spectral dispersion becomes 6000 mm with an 8-pass configuration, leading to a group delay dispersion (GDD) of 2.2 × 108 fs2, corresponding to a chirp of 0.4 ns/nm. The spectrum after the pulse stretcher, shown by a yellow line in Fig. 1, indicates that this pulse stretcher covers a spectral range of 1029.5 ± 5 nm. The FWHM bandwidth and typical power after the pulse stretcher are 6 nm and 30 mW, respectively. The stretched seed laser is first amplified in a 3.5 m Yb-doped fiber (Thorlabs: Yb1200-10/125DC-PM) pumped by a 976 nm laser diode. At a pump power of 6 W, the seed laser is amplified to 2.3 W. The spectrum after the fiber amplifier is shown by a blue line in Fig. 1. Here weak amplification on the short-wavelength side is because of a gain feature of this fiber. As a result, the FWHM bandwidth changes to 4 nm. After the fiber amplifier, 1 kHz pulses are picked from 80 MHz by a KD*P Pockels cell optical shutter and inserted into the Yb:YAG thin-disk regenerative amplifier.
A schematic view of the thin-disk amplifier is shown in Fig. 2(a). In our system, we use a thin-disk made from a 7at% Yb-doped YAG ceramic produced by Konoshima Chemical Co., Ltd. The outer diameter and the thickness are 10 mm and 220 µm, respectively. The front surface is wedged to the rear surface at an angle of 0.1° to avoid the interference inside the disk. The front and rear surfaces are coated with an anti-reflection (AR) coating and a high-reflection (HR) coating, respectively, for both the 940 nm pump light and the 1030 nm seed laser. The damage threshold fluence of the coating is one of the critical restrictions for high energy extraction. Therefore, we developed highly resistant coating using Al2O3/SiO2 multilayers, of which the damage threshold fluence for 500 ps pulses is 75 J/cm2 for AR coating . The thin-disk is mounted on a water-cooled copper heat sink. The pump light extracted from the fiber-coupled laser diode is imaged on the thin-disk by collimation lenses and a parabolic mirror (D + G GmbH: TDM 1.0 HP Lab). Unabsorbed light is re-collimated on the opposite side of the parabolic mirror and then bounced back by deflecting prisms. Thus, the pump light is guided into the thin-disk multiple times. In our case, over 90% pump power can be absorbed by 24-pass absorption. Figure 2(b) shows a spatial profile of the pump spot measured with a 940 nm scattering light. The FWHM diameter (Dpump) is 2.4 mm. At a pump power of 100 W, which corresponds to a pump flux of 2.2 kW/cm2, the small signal gain by one thin-disk (i.e., 2-pass amplification) is measured to be 1.13, which leads to a gain coefficient of 2.78 cm−1. In consideration of amplified spontaneous emission (ASE), the effective length of the thin-disk becomes Dpump/sinϑt (~0.437) because of the total reflection at an angle (ϑt) of 33.3°. Consequently, the evaluated gain length product is 1.21, which is smaller than the critical value of 3 .
Schematics layout of the regenerative amplifier and calculated beam diameters in the cavity are shown in Figs. 3(a) and 3(b), respectively. We used the software WinLase for designing the cavity. The seed laser is inserted and extracted from the thin film polarizer (TFP-1) placed 1000 mm from the end mirror (EM1). For the Pockels cell, a BBO crystal with an aperture diameter of 7 mm and a length of 38 mm is used. In the cavity, we set the seed laser to go through the thin-disk four times per round trip because the single thin-disk gain is small. The radius of curvature of the thin-disk used in this cavity becomes larger with increasing pump power from the initial value of 4.3 m. Therefore, the cavity is designed to ensure that the beam diameter of the seed laser at the thin-disk is 2 mm; this covers 80% of the pump region and ensures insensitivity to variations in the thin-disk’s curvature. Moreover, the beam diameter at the Pockels cell should not be smaller than half the diameter of the Pockels cell aperture when the thin-disk curvature changes, which is important from the viewpoint of suppressing the B-integral.
Figure 4(a) shows pulse energy amplification at a pump flux of 2.2 kW/cm2 as a function of the number of round trips. Build-up of the energy amplification is monitored using the leakage light behind the EM2 by a Si photo diode detector (PIN), which was calibrated using a power meter (PM; Ophir: 30A-P), see Fig. 3(a). The small signal gain per round trip investigated from the energy build-up is 1.52 (~exp(0.42)). By comparing this with the fourth power of the single thin-disk gain of (1.13)4 ~1.63, the loss per round trip is estimated to be 0.07 ( = 1 − 1.52/1.63), most of which is attributed to the Pockels cell. As a result, the amplified pulse energy increases to over 10 mJ after 40 round trips. Figure 3(b) shows a spatial profile of the output beam near the TFP-1, as observed by a charged coupled device (CCD) camera with imaging lens. The beam diameter at 1/e2 intensity is 3.6 mm, which is consistent with the design value shown in Fig. 3(b).
The spectrum of the output pulse after 41 round trips is shown in Fig. 5(a) together with the initial spectrum. The output spectrum has its center wavelength at 1030 nm and an FWHM bandwidth of 1.24 nm, down from 4 nm at the input because of multiple amplification steps. The amplified pulse is compressed by the pulse compressor comprising two gold-coated diffraction gratings with 1740 groves/mm and roof mirrors. The distance between the two gratings is set to 3000 mm to compensate for the GDD caused in the pulse stretcher. A second-harmonic autocorrelation trace of the compressed pulse is shown in Fig. 5(b). The FWHM pulse duration of 1.29 ps is obtained by assuming Gaussian pulse shape. The time–bandwidth product of 0.452 is close to the Gaussian shape pulse’s Fourier transform limit of 0.441.
3. THz pulse generation
By employing this 1.3 ps pulse as a pump, we performed THz pulse generation based on optical rectification in an Mg-doped stoichiometric LN crystal at room temperature. The experimental setup is shown in Fig. 6. To satisfy the velocity matching between the pump pulse and the THz pulse, the pulse front of the pump pulse was tilted by the diffraction grating with 1480 grooves/mm and then imaged on the prism-shaped LN crystal. The pump spot size on the LN crystal was 6 mm in the horizontal direction and 5 mm in the vertical direction. The maximum pump energy in this experiment was 7 mJ, corresponding to the pump intensity of 24 GW/cm2. The THz pulse output from the LN crystal was collimated by an aspheric lens made of a plastic (Pax co.; Tsurupica) with a focal length of 50 mm. Then, the output was focused on a 1-mm-thick cadmium telluride (CdTe) crystal using an off-axis parabolic mirror having a focal length of 50 mm.
The THz waveform of the electric field strength was obtained using electro-optic (EO) sampling measurement. The EO sampling measurement was based on rotation of the probe pulse’s polarization in the CdTe crystal, which depended on the THz pulse electric field strength. To obtain a precise THz waveform, synchronization between the probe pulse and the pump pulse was required. Furthermore, the pulse duration of the probe pulse should be shorter by one order of magnitude than that of the THz pulse. To ensure that the probe pulse satisfies these requirements, we first picked up a small fraction of the pump pulse (~100 µW) and injected it into a single mode optical fiber (Thorlabs: 1060XP) with a length about 15 cm. In the optical fiber, the spectral shape was broadened by means of self-phase-modulation (SPM), as shown in Fig. 7(a). The spectral bandwidth could be adjusted by changing the input pulse energy. After passing through the optical fiber, the broadened pulse was recompressed by a pair of transmission gratings with 1000 lines/mm (LightSmyth technology: LSFSG-1000). The FWHM duration of the recompressed pulse was determined as 0.2 ps from the second-harmonic autocorrelation trace assuming Gaussian pulse shape, as shown in Fig. 7(b).
The THz waveform of the electric field strength obtained by EO sampling and its Fourier-transformed spectrum are shown in Figs. 8(a) and 8(b), respectively. A single-cycle THz pulse with a peak frequency of 0.3 THz and a frequency range of over 1 THz is successfully obtained with a repetition rate of 1 kHz. Figure 9 shows the THz output energy measured at a position of the CdTe crystal by a thermopile detector (Ophir: 3A-P-THz) as a function of the pump energy. The data are in good agreement with a second-order polynomial fitting. This is consistent because optical rectification is a second-order nonlinear process. A THz pulse energy of 4 µJ is obtained with a pump pulse energy of 7.4 mJ. The obtained pump-to-THz energy conversion efficiency of 0.05% is comparable with that in the previous work using 1.3 ps pump pulse (0.08% at 4 mJ pump energy) . As mentioned above, generated THz energy (ETHz) is proportional to the square of the pump energy (Epump2). Therefore the conversion efficiency, i.e. ETHz /Epump, is expected to increase with the pump energy. In fact, 0.25% conversion efficiency was achieved with 50 mJ pump energy . Further increase of the conversion efficiency can be achieved by using sub-picosecond pump pulse. The efficiency of 0.77% was demonstrated by 785-fs pump pulse with 58 mJ energy .
We have developed a dedicated CPA laser system equipped with a Yb:YAG ceramic thin-disk regenerative amplifier for THz pulse generation. The system provided 1030 nm laser pulses with an energy of over 10 mJ, FWHM bandwidth of 1.24 nm, and repetition rate of 1 kHz. The pulse was compressed to 1.29 ps, close to the ideal theoretical value. Using these pulses, THz pulses were generated by means of optical rectification in an Mg-doped LN crystal and single-cycle THz pulses having a peak frequency of 0.3 THz with energy on the order of a few microjoules were obtained at a repetition rate of 1 kHz. The pump-to-THz energy conversion efficiency, 0.05%, was comparable with that of the previous work with 1.3 ps pump pulse .
Further increases of the conversion efficiency and the THz pulse energy are expected by using pump pulses with higher energy and sub-picosecond duration. For developing a more intense pump laser (100 mJ pulse energy at kHz repetition rate), the use of a thin-disk multi-pass amplifier  is a promising technique. However, the THz pulse generation efficiency under large-area pumping will be limited because of an imaging error of the tilted pulse front. Moreover, the generated THz pulse may be spatially inhomogeneous based on the difference of the optical path in the LN crystal. A contact grating  with a large aperture is the most promising device for eliminating these problems and realizing intense and practical THz pulses for a variety of applications.
This work was supported by the “Consortium for Photon Science and Technology Program” of MEXT, Japan.
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