We present a novel technique to generate a continuous, combless broadband Terahertz spectrum with conventional low-cost laser diodes. A standard time-domain spectroscopy system using photoconductive antennas is pumped by the output of two tunable diode lasers. Using fine tuning for one laser and fine and coarse tuning for the second laser, difference frequency generation results in a continuous broadband THz spectrum. Fast coarse-tuning is achieved by a simple spatial light modulator introduced in an external cavity. The results are compared to multi-mode operation for THz generation.
© 2011 OSA
Since the first demonstration of coherent generation and detection of terahertz (THz) radiation using photoconductive antennas in conjunction with continuous wave or pulsed lasers  and its promising wide-spread field of applications [2, 3], the search for cost-effective systems has been an ongoing process. The major component dominating the price of THz time-domain spectroscopy (TDS) systems has always been the pump laser. Several types of lasers have been proven to be suitable for THz TDS, reaching from mode-locked Titanium-Sapphire  to modern compact femtosecond fiber lasers . At the beginning of the past decade, the applicability of multi-mode diode lasers for TDS-like, so-called cross-correlation spectroscopy (CCS) was demonstrated  and recently recalled into the focus of attention [7, 8]. This technique provides in principle the same time-domain signal as femtosecond pumped systems, based on the mixing of the laser modes in a photoconductive antenna (emitter) and cross-correlating the generated THz radiation with the same multi-mode laser pattern in a second photoconductive antenna (detector). The disadvantage of these systems, when driven by a standard non AR-coated diode laser, is the inherent discrete THz spectrum consisting of spectral lines typically spread by tens of GHz. This is because the length of the laser cavity of the pump source is only in the range of some millimeters. In principle, this can be overcome by AR-coating the laser-active medium and using a larger external cavity, resulting in a decrease of the spectral spacing of the laser modes, leading to a tightened THz frequency comb. However, the generated THz spectrum is still not continuous. In this paper we propose and demonstrate a cost-effective technique based on standard, non AR-coated laser diodes, resulting in a continuous THz spectrum. The output of two external cavity diode lasers, which are driven in different spectral modulation modes, is combined on a photoconductive antenna for difference frequency generation.
2. Experimental setup
2.1. External cavity diode laser
The experimental setup of the used Fourier transform external cavity diode lasers is shown in Fig. 1.
A commercially available laser diode (Thorlabs Inc., Model L785P100) mounted on a temperature stabilized holder was used as laser source. This laser diode is specified to be a multimode source, capable of delivering at least 90 mW of optical power at a center wavelength of 785 nm. The divergent radiation was collimated by a standard AR-coated lens with a focal length of 4.5 mm. A variable attenuator, consisting of a half-wave plate and a polarizing beam splitter cube, was used to adjust the power level directed to the external cavity to the minimum required power for spectral control. The external cavity consists of a blazed grating with 1200 rules per millimeter, followed by a lens with a focal length of 400 mm. This lens parallelizes the spectral components (modes) spread by the grating and focuses each beam of a specific mode. So in the Fourier plane of this lens, spatially separated foci of different modes of the diode laser are accessible. Finally, a dielectric end mirror was used to feed the radiation back. In front of it, several types of apertures serve as spatial light modulators (SLM), enabling a selective spectral feedback to the laser diode (see Fig. 2). The overall length of the external cavity is about 1 m.
2.2. Terahertz cross-correlation spectroscopy system
The TDS-like THz CSS system is shown in Fig. 3. The radiation of the two lasers is directly fiber coupled into the branches of a fiber-optic 50:50 coupler. The use of fibers ensures a perfect overlap of the laser radiation and reduces the complexity of the system drastically. In contrast to the use of femtosecond pulses with fibers, no special dispersion compensation techniques have to be applied here. The collimator in the emitter arm is mounted on a standard motor driven linear stage serving as a delay line. A standard 60 μm dipole antenna based on low-temperature gallium arsenide (LT-GaAs) serves as emitter and is biased by a 2.7 kHz modulated 40 V RMS voltage, enabling Lock-In technique as detection method. Another 20 μm dipole antenna on LT-GaAs is used as detector and connected to a standard transimpedance amplifier followed by a Lock-In amplifier, whose time constant is set to 300 ms in the measurements presented here. The emitter and detector antennas are illuminated with optical powers of 25 mW and 40 mW respectively. The THz path consists of two off-axis parabolic mirrors enabling the measurement of samples in a collimated THz beam.
2.3. Spectral modulation of the lasers
The key principle of the proposed method to generate a continuous broadband THz spectrum is the simultaneous modulation of the two lasers used. Generally, a THz CCS system delivers the same type of signal, when either pumped by a single multi-mode laser diode (see Fig. 4 a) or pumped by two lasers, where one is single-mode and the other one is coarse (but not modehop-free) tuned (Fig. 4 b). As long as the tuning frequency is higher than the corresponding cutoff frequency set by the Lock-In integration time, the signals are very similar and both consist of discrete THz frequencies. As non AR-coated diode lasers cannot be tuned modehop-free over a wide spectral range, one has to introduce a continuous tuning of the second laser (Fig. 4 c). The modes of a laser diode can be fine-tuned by modulation of the injection current up to a frequency shift of +/− half the mode separation before a modehop occurs. Typical tuning sensitivities are about 3 GHz/mA. The laser diodes used in this work have a mode spacing of approx. 40 GHz, resulting in the need of a current modulation of about 13 mA of one of the lasers. The current modulation of only one laser results in a modulation of the optical power in the experiment. To minimize this effect and to reduce the amplitude of the modulation for one laser, the two lasers are modulated in opposite directions. So the optical power in the experiment, which consists of the sum of the two lasers is nearly constant.
The result of these combined modulation techniques can be described either in the frequency or time domain. In the frequency domain, the coarse modulation generates a THz frequency comb, which is shifted during the integration time of the Lock-In steadily by one FSR of the laser diode by the injection current modulation, resulting in a continuous overlap. In the time domain, the cross-correlation peaks adjacent to the center peak are modulated in their phase, resulting in a destructive interference during the Lock-In integration time. So the proposed method is based on the integration over an appropriate fast modulation of the difference frequency. While the instantaneous THz spectrum is not continuous, the finite integration time used in the measurement results in the measurement of a combless spectrum.
In the left column of Fig. 5, the THz time signal from the system driven by only one of the lasers run with broadband spectral feedback (no inserted SLM) is shown. As known well by literature [6–8], this results in a periodic time-domain signal (upper plot) corresponding to discrete frequencies in the spectrum (lower plot). The spreading of these frequencies in this case is approx. 40 GHz, in agreement with the FSR of the laser diode. The spectrum directly shows the disadvantage of such a system driven by a non AR-coated diode. In spectroscopic applications, narrow (below 40 GHz) absorption features of potential samples cannot be resolved or even detected.
When applying the method already described, the repetitions of the THz pulse vanish, resulting in a truly continuous, combless spectrum (right column in Fig. 5). The modulation frequencies for the coarse and fine modulation in these measurements are 924 Hz and 17 Hz, respectively, but do not need to be very stable or referenced. A slightly mismatched modulation depth of the injection current results in a small remaining echo about 25 ps after the main pulse here. The steep cutoff at about 1 THz originates from the bandwidth of the used SLM blade.
A first spectroscopic application of this THz CCS system utilizing the generation of a continuous THz spectrum is shown in Fig. 6. A lactose pellet is inserted in the THz beam path between the two off-axis parabolic mirrors. The prominent absorption line around 530 GHz (see e.g. ) is clearly detected. Application to narrow absorption measurement tasks have not yet been performed, but are expected to be possible with the proposed setup.
A measurement of two silicon (Si) wafers of different thicknesses is shown in Fig. 7 and proves the principle applicability of the introduced method to layer thickness measurement tasks. The main cross-correlation peak is shifted in time and additional echoes occur in agreement with results from femtosecond pumped THz TDS systems.
The method introduced here and used in the measurements of Fig. 5, Fig. 6 and Fig. 7 was performed with the fine tuned laser spectrally emitting at one edge of the spectrum of the SLM-modulated laser. However, the used principle in general allows for the generation of an arbitrary THz spectrum, if appropriate photoconductive emitters and detectors are available. To demonstrate the flexibility of our simple system, the center frequency of the fine modulated laser was shifted by translating the slit-SLM of its external cavity. The result of eight measurements is shown in Fig. 8. One can clearly see the potential of our proposed method. By increasing the frequency separation of the spectra of the tuned lasers, the THz spectrum is shifted to higher frequencies. In our experiment the low efficiency of our photoconductive switches for high frequencies causes the small THz signal for widely separated laser spectra. Nevertheless, this measurement proofs the feasibility to generate predefined THz spectra using more sophisticated spectral control techniques.
We have proposed and demonstrated a THz cross-correlation spectroscopy (THz CCS) system based on conventional non AR-coated laser diodes. A combination of two tuning methods results in the generation of a broadband, truly continuous THz spectrum. The applicability of the system was demonstrated by a spectroscopy measurement of lactose, reproducing a prominent absorption feature well known from measurements with conventional THz TDS systems as well as a measurement of Si wafers of different thicknesses. The pump lasers and additional equipment needed for this setup are very cost-effective, compared to systems pumped by femtosecond lasers or high accuracy frequency stabilized lasers. Further improvements by tailoring the emitter and detector characteristics to the frequency range to be generated and optimizing the tuning process of the two diode lasers are expected.
References and links
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