1. Introduction

Terahertz time-domain spectroscopy (THz-TDS) is a very attractive tool for diverse applications, including medical analysis and biochemical standoff detection for hazardous materials. In the past decade, coherent ultrabroad terahertz (THz) radiation has been investigated intensively, and several research groups have demonstrated the generation and detection of THz waves beyond 20 THz [1–6]. Wu et al. reported the observation of coherent ultrabroad THz radiation for the first time [1], and optical rectification in a GaAs crystal and EO sampling with a ZnTe crystal has led to the detection of a THz field extended to 37 THz. At present, the maximum frequency region reaches up to 120 THz through use of GaSe crystals and 10-fs optical pulses. Kono et al. succeeded in detecting up to 20 THz with a photoconductive (PC) antenna [3]. All of these data were obtained using ultrashort-pulse Ti:sapphire lasers.

The generation and detection of coherent THz radiation using a fiber laser instead of a Ti:sapphire laser has recently attracted a great deal of attention because fiber lasers are stable, compact, and robust. Several techniques have been reported using, for example, GaAs crystals and InGaAs PC antennas [7–11]. However, the bandwidth of THz pulses reported in these papers is limited to below 7 THz due to the longer pulse width of fiber lasers. The pulse width of conventional ultrashort-pulse fiber lasers is about 100 fs, while 10-fs Ti:sapphire lasers are commercially available.

Some experiments on pulse compression using optical fibers have been reported [12–14]. For example, Hori, et al. demonstrated the generation of short 14-fs optical pulses from a fiber laser [14]. However, the peak power of compressed optical pulses remains below the level of a few tens of kilowatts. To generate the broadband THz pulses effectively, higher peak-power pulses are required.

In this paper, we report on the broadband generation and detection of electromagnetic fields using an ultrashort-pulse fiber laser. First, we developed a 17-fs ultrashort-pulse fiber laser by controlling the nonlinear and dispersion effects occurring in optical fibers. Then, we demonstrated broadband THz-wave generation and detection using an organic crystal DAST. THz waves extended to beyond 20 THz were observed for the first time using a fiber-laser-based system.

2. Development of 17-fs fiber laser

The configuration of our fiber laser is shown in Fig. 1. The passively mode-locked Er-doped-fiber laser oscillator provided 300-fs sech²-shaped optical pulses at 1.56 µm. The average power and the repetition frequency were 5 mW and 48 MHz, respectively. The output pulses from the fiber laser oscillator were amplified and compressed using two stages as follows.

The first stage consisted mainly of the standard single-mode fiber (SMF1), the erbium-doped fiber (EDF), and the large mode-area photonic-crystal fiber (LMA-PCF). The lengths of each fiber were 4.5 m, 6.0 m, and 0.42 m, respectively. First, the pulses were stretched to about 1 ps in the SMF1 to avoid the excessive nonlinear effects caused in the subsequent EDF, such as the generation of complex chirp, induced Raman scattering, and pulse breakup. Then, the stretched pulses were amplified in the EDF pumped by three 400-mW laser diodes (LDs) through the WDM couplers. The mode-field diameter (MFD) and the second-order group-velocity dispersion (GVD) parameter of the EDF were 8.0 µm and 6.4 ps²/km, respectively. The benefit of using positive-dispersion EDF was that the optical spectrum was broadened smoothly due to the self-phase modulation (SPM) and that the pulses had the monotonic positive chirp along the dispersive delay curve of the EDF. In addition, because of the small dispersion of the EDF, the pulse width was kept practically constant and large spectral broadening was obtained. The spectral bandwidth was broadened to beyond 100 nm. These phenomena, caused by the peculiar dispersion characteristics of the EDF, were useful for shorter pulse generation with suppressing pedestals. Finally, the chirp of the amplified pulses was compensated for using the negative GVD characteristic of the LMA-PCF. The LMA-PCF had a large MFD of 26 µm [15]. The low nonlinearity of this fiber enabled linear dispersion compensation without nonlinear effects. After dispersion compensation, the pulse width and the average power were 55 fs and 280 mW, respectively. Effective spectral broadening, caused in the EDF, greatly shortened the pulse width compared to that of the oscillator pulse.

 

Fig. 1. Experimental setup for ultrashort-pulse compression: SMF1, a standard single-mode fiber; SMF2, a single-mode fiber with a small mode-field diameter; EDF, erbium-doped fiber; PBS, polarization beam splitter; LMA-PCF, large mode-area photonic crystal fiber; λ/2, half-wave plate; PC, polarization controller; WDM, wavelength-division multiplexing coupler; PBC, polarization beam combiner; LD, laser diode.

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Fig. 2. (a) Temporal waveforms and (b) optical spectra of laser pulses. The green, blue, and red curves show the waveforms of oscillator pulses, pulses after first stage, and 17-fs pulses, respectively.

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In the second stage, 15-mm-long optical fiber (SMF2) compressed the pulses further. The MFD of this fiber was 6.1 µm, whereas that of SMF1 was 10.0 µm. Since the nonlinear coefficient of optical fiber γ is proportional to MFD^-2, that of SMF2 was about three times larger than that of the SMF1. When the soliton number, which is defined by characteristics of optical pulse and optical fiber, is larger than 1, the pulse propagating in optical fiber with a negative GVD parameter is compressed by higher-order soliton compression [16]. Here, roughly stated, the effect of higher-order soliton compression is proportional to the nonlinear coefficient γ. Therefore, using fiber with high nonlinearity, the shorter pulses were successfully generated in the second stage. In this experiment, the lengths of each fiber were optimized using a cut-back method.

Figure 2 shows the temporal and spectral waveforms of the optical pulses compressed using the all-fiber system. The waveforms of the oscillator pulses, output pulses from the first stage, are also shown. These figures were obtained using the second-harmonic generation frequency-resolved optical gating (SHG-FROG) method [17]. The eventual pulse width was 17 fs, and the pedestal components were well suppressed. The spectrum was broadened widely, from 1.3 to 1.8 µm, along with the pulse compression. The average power of the compressed pulses was 200 mW, corresponding to a peak power of 170 kW. To our knowledge, this is the first demonstration of the generation of a sub-20-fs pulse with a peak power larger than 100 kW using an all-fiber system. Furthermore, characteristics of pulse width and average power comparable to conventional mode-locked Ti:sapphire lasers were successfully obtained.

3. Broadband generation and detection of THz radiation using a fiber laser

In this experiment, an organic crystal, 4-N, N-dimethylamino-4’-N’-methyl-stilbazolium tosylate (DAST), was used for THz-wave generation [18, 19]. The terahertz wave generation using the DAST crystal through optical rectification have been demonstrated and investigated using Ti:sapphire laser [20–23]. The use of commercially available fiber laser has been also reported [11]. The DAST crystal is a promising material for efficient THz-wave generation because it has a very large electro-optic coefficient (r₁₁₁=47 pm/V at 1535 nm). A remarkable additional property of the DAST crystal for THz-wave generation with a fiber laser is that it can satisfy the phase-matching condition throughout the wide wavelength range below 4.5 THz around 1.5 µm [22]. To investigate the phase-matching condition at the higher frequency region, the coherence length was calculated up to 20 THz. A color plot of the calculated coherence length of the DAST crystal is shown in Fig. 3. In this calculation, the refractive index of the optical region and THz region were taken from Pan et al. [18] and Ito et al. [24]. The x- and y-axes represent the THz frequency and optical pump wavelength, respectively. The coherence length is characterized by the tone of the graph, and the deep area in the figure denotes the region in which long coherence length is achieved. The dashed lines show the high- and low-frequency edges of the 17-fs optical pulses described above. From this figure, the long coherence length is achieved at the wavelength range of 17-fs fiber laser. Thus, the DAST crystal is suitable for broadband THz-wave generation using our laser. Especially, at the THz frequency range below 12 THz, the phase-matching condition is well satisfied using less than 0.2-mm thin DAST crystal. On the other hand, at the high THz frequency range above 12 THz, it is difficult to satisfy the phase-matching condition throughout the whole wavelength range.

 

Fig. 3. Calculated coherence length of the DAST crystal.

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Fig. 4. Experimental setup for generation and detection of broadband THz radiation.

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Fig. 5. Temporal waveforms of the detected THz pulses; (a) the entire waveform and (b) the close-up of the shortest oscillation.

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Figure 4 shows the experimental setup for broadband THz-wave generation and detection using the fiber laser. A 50:50 beam splitter divided the 17-fs pulses from the fiber laser into two beams. One of divided beams was input to the periodically poled lithium niobate (0.6 mm, 100 °C) to generate 780-nm pulses used for THz-wave detection. The pulse width and average power of the frequency-doubled pulses were 30 fs and 10 mW, respectively. The THz wave was radiated in the DAST crystal by focusing the 17-fs pulses on it using an aspheric lens. A pair of parabolic mirrors collimated and focused the emitted THz pulses through a silicon hyper-hemisphere lens onto an LT-GaAs photoconductive antenna with a 5-µm electrode gap. A metal pole 5 mm in diameter was inserted between the pair of parabolic mirrors to eliminate the optical pulses that transmitted through the DAST crystal. The delay stage, which provided the time delay between the THz pulses and the frequency-doubled pulses, was scanned over a distance of 4 mm, providing a spectral resolution of 37.5 GHz. The obtained signal was amplified by a pre-amplifier with a gain of 70 dB and detected using a lock-in amplifier referenced to the chopper frequency of 1 kHz.

Figure 5 shows a typical temporal waveform of detected THz pulses generated using the 0.1-mm DAST crystal and detected with the PC receiver. These data were obtained when the experimental setup shown in Fig. 4 was purged with dry nitrogen gas to reduce the effect of water vapor absorption. The entire temporal waveform is shown in Fig. 5(a). There is a large THz pulse centered around 3.3 ps. The shorter oscillations with several periods are superimposed on this pulse. The close-up of the shortest oscillation is shown in Fig. 5(b). The shortest oscillation period is about 40 fs. The optical rectification in the DAST crystal using 17-fs optical pulses caused this short oscillation. Figure 6 shows the corresponding spectrum obtained by Fourier transform of Fig. 5(a). The maximum SNR was about 25 dB at 1.8 THz. Even though the spectral amplitude is small and the spectral shape is not flat at the high frequency region, we can observe the widely-extended spectrum. The spectrum extended from 0.1 to beyond 25 THz. To the best of our knowledge, this frequency bandwidth is wider than any previously obtained using a fiber-laser-based system. As the LT-GaAs substrate has large phonon absorption, a wide spectral dip occurs around 8 THz. The several relatively-narrow spectral dips are observed at the frequency range above 12 THz, while the spectral shape at the low frequency range is smooth. These spectral dips are considered to arise mainly from the absorption of the DAST crystal [25,26]. The DAST crystal has the strong absorption at 15.2, 17.3, and 20.3 THz at this frequency region. These frequencies are well matched with those of the spectral dip. The possible reason of the other dips is the phase mismatching shown in Fig. 3.

 

Fig. 6. The spectrum of detected THz pulses. The noise level of the system is shown as the dotted curve.

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4. Conclusion

In conclusion, we present the generation and detection of broadband coherent terahertz (THz) radiation based on a fiber-laser system. By combining a 17-fs ultrashort-pulse fiber laser and a DAST crystal, wideband THz radiation was achieved. Our fiber laser had an all-fiber configuration, consisting mainly of optical fibers such as normal- and small-dispersion erbium-doped fiber, large mode-area photonic-crystal fiber, and small mode-field diameter fiber for effective higher-order soliton compression. By controlling the nonlinearity and dispersion in these fibers, 17-fs pulses with an average power of 200 mW were generated. The peak power of these pulses reached up to 170 kW, achieving effective optical rectification in the DAST crystal. In addition, because of the large electro-optic coefficient and the phase-matching condition around 1.5 µm, widely extended THz waves, from 0.1 to beyond 25 THz, were successfully generated. To the best of our knowledge, these THz pulses have the broadest bandwidth of all THz pulses generated using an ultrashort-pulse fiber laser.