1. Introduction

The development of coherent terahertz (THz)-wave sources is of great interest in applications to solid-state physics, molecular analysis, and so on. THz-wave generation with photoconductive switching or optical rectification by use of femtosecond optical pulses has been applied in THz time-domain spectroscopy, taking advantage of its high temporal resolution [1]. Such ultrashort pulses consist of an ultrabroad bandwidth spectral component from dc to the THz region. In contrast, to access the excited states of matter of interest for high-resolution spectroscopy, ultrahigh-speed optoelectronics, and other applications, it is necessary for the THz-wave source to be tunable in frequency and to have a narrow spectral bandwidth.

Difference frequency generation (DFG) is a promising technique for the realization of widely tunable, narrow-bandwidth THz-wave generation. In the early stage of this research, THz oscillation and amplification were expected by utilization of the resonant frequency of a crystal lattice or of the molecule itself [2], and different ferroelectric materials and semiconductors were explored for efficient and tunable THz-wave generation [3–6]. The recent interest in THz-wave applications has stimulated the development of DFG THz-wave sources [7–11]. In general, however, nonlinear optical materials have high absorption coefficients in the THz-wave region [9,11], which prevents efficient THz-wave generation from these materials. Furthermore, it is necessary to satisfy phase-matching conditions for efficient THz-wave generation. One method to overcome this problem, THz-wave generation from near the surface of periodically poled LiNbO₃ (PPLN), has been proposed to obtain difference-frequency mixing in a PPLN waveguide [13]. This surface-emitted method can generate a THz wave perpendicular to the direction of an optical beam by use of a PPLN with an appropriately designed domain structure. The absorption loss is minimized because the THz wave is generated from near the PPLN surface. Moreover, the phase-matching condition can be designed by use of an appropriate grating period of PPLN. As described theoretically, an optical waveguide should enable the realization of efficient interaction of dual-wavelength optical waves, but most proton-exchanged waveguides are made on a +Z or a −Z crystal surface. To avoid this problem, we proposed using bulk slanted PPLN, which can be used to realize a quasi-phase-matched (QPM) interaction in the direction of both launched optical pulses and a perpendicularly generated THz-wave. A detailed explanation of surface-emitted THz-wave generation has been given elsewhere [14].

For efficient THz-wave conversion, in addition to preparation of an appropriate nonlinear material, the generation of high peak power synchronized dual-wavelength lasers is also important. The dual-wavelength optical pulses must have temporal overlap and a tunable frequency interval. The generation of such dual-wavelength optical pulses with a repetition rate from several hertz to a kilohertz has been reported by use of solid-state lasers or optical parametric oscillation [3–9]. We used dual-wavelength optical pulses based on a gain-switched semiconductor laser at 1.5-µm, which makes it easy to achieve high-repetition-rate operation up to the gigahertz order. This feature makes a semiconductor laser a promising candidate for obtaining high-repetition-rate THz-wave generation. Furthermore, 1.5-µm semiconductor lasers have the potential for highly stable operation and a compact setup compared with large-sized solid-state lasers. Kilowatt-level peak power pulses for efficient DFG can be obtained by use of the appropriate repetition rate and subsequent optical amplification. The typical pulse duration of a laser from a gain-switched semiconductor laser is 100 ps; the corresponding bandwidth is several gigahertz.

Here we report on 1-MHz repetition-rate synchronized dual-wavelength optical pulses using a combination of semiconductor lasers and a four-wave mixing (FWM) device. The 1-W average power of dual wavelengths with a 1-MHz repetition rate and a 100-ps duration means that the peak power of the optical pulses reached approximately 10 kW, and, based on such high-repetition-rate laser sources, a 10-GHz narrow-bandwidth THz-wave generation with wide tunability was demonstrated by use of slanted PPLN.

2. Operating principle of dual-wavelength optical pulses for difference-frequency generation

The dual-wavelength optical pulses used for DFG must be synchronized with each other for an efficient nonlinear optical process. The complete timing synchronization of optical pulses from two individual lasers is quite difficult, especially with a short pulse duration. Tunability of the frequency interval between two wavelengths is also needed for tunable DFG. Figure 1 shows the operating principle of our scheme, which can be used to achieve timing synchronization and frequency tunabililty simultaneously. Laser pulses and a continuous-wave (cw) laser light are coinjected into an optical fiber with high nonlinearity. This generates synchronized new wavelength optical pulse components by means of a nonlinear FWM process. Here, the pulse laser light is used as the pump, and the cw laser light serves as a signal. If the dispersion of optical fiber is sufficiently small, the pump and generated pulses can be synchronized automatically. Then the cw laser component is removed by spectral filtering, which enables efficient amplification of the synchronized dual-wavelength optical pulses in the next optical amplification stage. Using an Er-doped fiber amplifier (EDFA), we can obtain dual-wavelength optical pulses with high peak powers. The frequency interval between pump frequency ω_p and FWM frequency ω _FWM equals that between ω_p and signal frequency ω_s, which corresponds to the frequency of the THz wave generated by difference-frequency mixing. We can tune the frequency interval between the two wavelength components by varying the wavelength of either the incident cw or the pulsed laser light.

 

Fig. 1. Schematic of the generation of timing synchronized dual-wavelength optical pulses.

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3. Dual-wavelength optical source experiment

Optical pulses with a 1-MHz repetition and a 100-ps duration at 1554 nm were generated from a gain-switched InGaAsP distributed-feedback (DFB) diode laser. An external cavity laser diode was also used to generate cw laser light with wavelength tunability from 1520 to 1575 nm. The optical pulses (5-µW average) and cw (2-mW) laser light were coupled by a fiber coupler and amplified to a level of 20 mW. They were coinjected into a 100-m-long high-nonlinearity, dispersion-shifted optical fiber (Sumitomo Electric Industries, Ltd.). The zero-dispersion wavelength and dispersion slope of this fiber were 1550 nm and 0.026 ps/nm²/km, respectively. Figure 2(a) shows the output spectrum from this fiber. The new wavelength component of 1566 nm was generated through FWM between optical pulses at 1554 nm and the cw input at 1542 nm. The conversion efficiency from the pump to FWM pulses was 10% to suppress unexpected mixing between the pump pulses and the FWM output. Figure 2(b) shows the spectrum after it passed through a 15-nm (FWHM) bandpass filter. Spectral filtering removed only the cw laser light, and the power relation between the dual-wavelength optical pulses was not changed. This made possible efficient amplification of the synchronized dual-wavelength optical pulses at the next optical amplification stage. By using an EDFA we obtained dual-wavelength optical pulses of nearly 1-W average optical power. Therefore, the peak optical power was almost 10 kW. This result was obtained in the optical fiber components instead of a conventional large solid-state laser. In this experiment we tuned the frequency interval of two wavelength components by varying the wavelength of incident cw laser light.

 

Fig. 2. (a) Optical spectrum from a highly nonlinear fiber. The component at 1566 nm was newly generated by means of the FWM process. (b) The optical spectrum after spectral filtering. Only the cw laser light was removed.

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Figure 3 shows the second harmonic generation intensity correlation profile for dualwavelength optical pulses. Instead of the entire 100-ps pulse envelope, the beating of the 1.5-THz frequency period in this 12-ps measurement range clearly indicates the timing synchronization between the pump and the FWM optical pulses.

 

Fig. 3. Second harmonic generation correlation trace for dual-wavelength optical pulses. The beating at 1.5 THz is in a pulse envelope.

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4. Terahertz-wave difference-frequency generation experiment

A dual-wavelength optical pulse source was applied to demonstrate its usefulness with a combination of PPLN. Avetisyan et al. proposed and demonstrated surface-emitted THz-wave generation [13,14]. We prepared a slanted PPLN 1-mm-thick sample with a grating period of 25–52 µm and a 40-mm interaction length. The amplified optical pulses were launched into a slanted PPLN, and THz-wave generation was successfully detected by use of a 4-K Si bolometer set perpendicular to the slanted PPLN. The average power of THz-wave output exceeded 10 nW, and the estimated peak power of a pulse was greater than 100 µW. The THz wavelength was measured with a scanning Fabry–Perot interferometer consisting of two metal-mesh plates. When the free spectral range (FSR) and the resolution of the interferometer were 33 and 3.3 GHz at FWHM (finesse of 10), respectively, the measured bandwidth of this THz wave was 10 GHz. Figure 4 shows wavelength tuning between 1.05 and 2.1 THz by use of four PPLNs with different grating periods. Although this figure shows the measured wavelength as a function of grating period Λ of PPLNs, each PPLN has a different angle α [14]. The wavelength of a cw laser was varied from 1563 to 1572 nm whereas that of a DFB laser diode was fixed at 1554 nm. Each measured THz frequency coincided with the frequency interval of the generated optical pulses. These results also indicate that the THz wavelength is determined by the grating period of slanted PPLN.

 

Fig. 4. THz-wavelength tuning curve of a PPLN DFG. The measured THz wavelength (dots) is in good agreement with the calculated value (solid line).

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5. Conclusion

In conclusion, we have reported 1-MHz repetition-rate dual-wavelength optical pulses based on laser diodes and THz-wave generation using slanted PPLN. The combination of a gain-switched DFB laser diode and FWM easily made possible the generation of automatically synchronized optical pulses. We generated THz-wave output from 1.05 to 2.1 THz with a 10-GHz bandwidth by using slanted PPLNs. Although the average THz-wave power was still 10 nW, more efficient generation will achieve a high average output by use of optical waveguide PPLN. With its high average power, this configuration is a promising candidate for a quasi-cw THz-wave source with a megahertz or higher repetition rate and a bandwidth of several gigahertz.