1. Introduction

Recent years have seen the development of a broad range of applications of terahertz radiation in astronomy, physics, engineering, and medicine [1]. To date, most time-domain THz sources depend on large open-cavity solid state lasers such as a Ti:sapphire laser working at ~0.8 μm, and are thus not convenient for in-field applications for which a compact and portable THz system is desired. In addition, the scan time for THz imaging and sensing can be prohibitively long for many applications, thus motivating the development of sources with significantly higher average power.

Among many candidate pump sources to drive THz radiation, an ultrafast fiber laser is particularly promising due to its high optical efficiency, good beam quality, and power scalability as well as compactness and robustness. Recently, erbium-doped-fiber laser systems have been used to generate THz through optical rectification in GaAs [2], or quasi-phase matching in orientation-patterned GaAs [3], producing THz powers of up to 3.3 μW [3]. However, the power scalability of Er-doped fibers is limited, and avoiding two-photon-absorption in GaAs requires shifting of the pump wavelength from 1.55-μm to beyond 1.7-μm [3]. In contrast, Yb-doped ultrashort-pulse fiber laser systems have much better power scaling potential [4]. Recently an Yb-doped fiber amplifier has been used for InAs-surface-emitted THz generation [5]. However, the conversion efficiency and THz power achieved have not been reported. Furthermore, such semiconductor-based emitter configurations are limited by saturation effects in the laser pulse fluence as well as by thermal effects due to the absorbed pump average power. Consequently, these factors could significantly impede achievable THz power scaling.

In this paper, we demonstrate the generation of high power single-cycle THz pulses through optical rectification in GaP pumped by an ultrafast high power Yb-doped fiber amplifier. Using 10-W average power optical pulses from a parabolic fiber amplifier we have generated THz pulses with an average power of 6.5 μW. The advantage of using optical rectification is the inherent power scalability of the approach, since it is not limited either by THz emission saturation or by heat dissipation. In particular, the choice of GaP for ~1-μm pumping has many advantages such as a broad phase-matching bandwidth, absence of two-photon absorption at the pump wavelength, negligible nonlinear refractive index effect, and room temperature operation [6]. Our results indicate that combining the Yb-fiber system with a GaP emitter is a promising path for achieving high average power and high peak power single-cycle THz pulses in a compact and robust source. It should be noted that the observation of optical rectification in GaP can be dated back to 1970s for investigations of the photon-drag effect [7]. However, as other zincblende structured crystals, such as GaAs and ZnTe, have become well-established optical-rectification based THz emitters, GaP has only been employed for a broadband THz wave sensor [8] or a THz emitter through difference frequency generation (DFG) between two input optical nanosecond pulses with different wavelengths [9–11]. This work presents the first demonstration of high power THz generation using optical rectification in GaP pumped by femtosecond pulses.

2. Experimental setup and results

The laser system employed is shown in Fig. 1. It consists of a diode-pumped, Nd:glass fs-oscillator, an ytterbium-doped fiber amplifier, and a 1.2 meter hollow core photonic bandgap fiber (PBGF) to compress the chirp of the amplified pulse. The seed to the fiber amplifier is a passively mode-locked Nd:glass oscillator producing 300-fs pulses at a 120-MHz repetition rate, with a center wavelength of 1.055 μm and an average power of 15 mW. An optical isolator is used to prevent feedback from the fiber amplifier into the oscillator. The high-power fiber amplifier is based on parabolic pulse amplification [12]. In parabolic pulse amplification, the interplay of normal dispersion, self phase modulation, and gain leads to an amplified, linearly chirped pulse with a parabolic temporal profile. One advantage over chirped pulse amplification is that the absence of a stretcher makes the amplifier system more compact and robust [13]. The fiber amplifier is constructed using a standard 8-m polarization-maintaining 30-μm LMA core Yb-doped fiber from NUFERN, 0.06-NA ytterbium-doped core and a 400-μm 0.46-NA hexagonal-shaped inner cladding. A pigtailed diode laser emitting at 976 nm is employed to pump the amplifier. The PBGF has a diameter of 8.1 μm and a dispersion of 100 ps/nm/km at 1.055 μm. Using PBGF, a compact all fiber system becomes possible.

 

Fig. 1. Experimental setup for THz wave generation and detection. HDPE: high density polyethylene, PBGF: photonic bandgap fiber.

Download Full Size | PDF

 

Fig. 2. Autocorrelation width of the optical pulse after compression. (inset) Autocorrelation trace at 10W pump power

Download Full Size | PDF

During parabolic pulse amplification, the pulse spectrum broadens as the amplified power grows, which means that the compressed pulse from the PBGF becomes shorter than the initial seed pulse. As shown in Fig. 2, the autocorrelation of the compressed pulse reaches a duration of ~210 fs at 10 W output power. The compressed pulse is weakly focused into a 1 mm thick <110> GaP crystal with ~300 μm beam diameter.

Coherent detection of the THz signal is achieved using a receiver fabricated by Picometrix, consisting of a bowtie antenna that is integrated with a photoconductive sampling gate and is activated with a portion of the optical pulse. The photoconductive gate is based on an ultrafast-lifetime semiconductor specifically designed for this wavelength. The antenna is coupled to a preamplifier circuit that amplifies and conditions the low-voltage, quasi-dc electrical signal. The optical pulse is routed to the antenna via an optical fiber and is focused onto the photoconductive gate, located at the midpoint of the antenna. As the single-cycle THz waveform impinges on the antenna, a time-varying electric field of the same shape establishes itself across the gate. The optical pulse switches on the ultrafast sampling gate for a duration closely matching the optical pulse width and causes charge to flow proportional to the amplitude of the THz waveform. A delay line allows the complete waveform to then be sequentially sampled.

 

Fig. 3. THz pulse (inset) and power spectrum obtained with 8.5 W optical pump power.

Download Full Size | PDF

Figure 3 shows the measured single-cycle THz pulse waveform (inset) and its spectrum at 8.5W of optical pump power. The THz spectrum is centered at 0.7 THz and extends out to 3.5 THz. The strong modulation dips in the spectrum are attributed to absorption from atmospheric water vapor in the path of the THz radiation [14].

For the power measurement, high-density polyethylene (HDPE) and germanium are used to separate the collinear optical and THz pulses. The THz power is measured using a silicon bolometer and a lock-in amplifier. In this measurement, the PBGF length is fixed at 1.2 m. As the optical power (i.e., the pump power for THz generation) from the output PBGF goes up, the pulse duration continuously decreases and, therefore, the pulse peak power scales super-linearly. Since the THz power conversion efficiency of optical rectification is proportional to the peak power of the optical pulse, a faster than quadratic scaling of THz power versus incident optical power is obtained as shown in Fig. 4(a).

3. Discussion and conclusion

Further scaling of the THz power in the present system is currently limited by the maximum available parabolic pulse energy from the fiber amplifier, which is determined by stimulated Raman scattering and the gain bandwidth [15] in the fiber. Since the parabolic pulse energy scales with the mode field area of the Yb-doped fiber, use of larger core fibers should be a promising avenue for further power scaling [16]. It is achievable to scale our current fiber amplifier configuration to have several tens of watts output. At this power level, the effect of three photon absorption in GaP is still negligible [6].

 

Fig. 4. (a) THz power scaling with fixed compression. (b) Coherence length of GaP versus THz frequency given different pump wavelengths.

Download Full Size | PDF

Additional THz power may be obtained by increasing the thickness of the GaP, owing to the broad phase-matching bandwidth when pumped at ~1 μm. As is well known, THz generation via optical rectification relies on DFG within the optical pulse. The phase-matching condition for DFG requires the group velocity of the optical pulse to match the terahertz phase velocity. In general, these two velocities differ from one another. To describe the walk-off between the optical pulse and the THz pulse, a coherence length is introduced as L_c = λ/(2|n_THz - n_oPt|) , where λ, n_THz, and n_opt denote the THz wavelength, the phase index for the THz pulse, and the group index for the optical pump pulse, respectively. To make efficient use of the crystal, the thickness of the crystal is selected to be the coherence length. Figure 4(b) shows the coherence length of GaP corresponding to the generated THz frequency given different optical pump wavelengths. In the calculation, the Sellmeier coefficients are taken from Ref 17. It is apparent that the THz spectrum obtained from a 4 mm thick GaP in the current setup still covers about 1 THz bandwidth while the power of the THz pulse may be improved by an order of magnitude. Furthermore, the phase-matching bandwidth can be increased significantly by blue-shifting the optical pump wavelength toward 1 μm. It is worth noting that the THz absorption in GaP will degrade the quadratic power dependence on GaP thickness for a much thicker crystal [18]. As a matter of fact, the THz power tends to scale linearly beyond 1 cm thickness [17].

In conclusion, a high power THz source based on a high power parabolic pulse amplifier is demonstrated. We believe that the power scalability of both the Yb-doped fiber amplifier and the optical rectification of GaP will enable our system to scale the THz radiation power to multi mW levels. It is expected that our compact power scalable THz source will facilitate portable THz systems (such as 3D imaging) suitable to work in situ.