We present a photoconductive terahertz transceiver based on a modulation of the optical pulses used for generation and detection at different rates. External modulation of the THz pulses is not required as opposed to previously reported approaches. Devices from fiber-optic technology are used, providing flexibility and stability to the system. Imaging and thickness measurement experiments are carried out to demonstrate the performance of the transceiver.
© 2014 Optical Society of America
Terahertz time-domain spectroscopy (THz-TDS) is a well-established technique which relies on the time-resolved detection of ultrashort pulses the bandwidth of which extends into the THz range . Despite of having become an essential tool not only in basic sciences but also in industrial environments  cost and size postpone the extensive exploitation of this measurement scheme. A large part of the cost of photonic THz TDS setups comes from the optical source, often a femtosecond Ti:Sapphire laser emitting at 800 nm. Photoconductive antennas (PCAs) excitable with cheaper optical sources from the telecom market have been developed to circumvent this problem . Furthermore, fiber-coupled PCAs reduce long-term alignment instabilities and simplify angular measurements  and allow for building compact sensors for analysis of liquids  or medical applications .
Typical THz TDS systems use two separate antennas to generate and detect THz radiation. However, it is also possible to use only one antenna for generation and detection of THz pulses. In this case the antenna unit is called a THz transceiver . In 2000 X.-Z. Zhang and M. Tani demonstrated THz transceivers based on time multiplexing in electro-optical crystals  and PCAs , respectively. Later, a fiber-coupled photoconductive transceiver based on the same concept but operated at 800 nm was demonstrated and used in reflection measurements under normal incidence .
Photoconductive THz transceivers reduce the number of THz antenna units and, hence, the cost-level of an optoelectronic THz system. Besides, they allow for 0° reflection measurements without the need of frequency-dependent beam splitters . Yet, approaches based on photoconductive antennas face an important limitation: the large difference in magnitude between the bias and terahertz electrical fields. This difference, in the order of 106, leads to a severe masking of the THz field during detection. Typically, mechanical chopping of the THz radiation is performed to remove the continuous bias field via lock-in detection. However, this solution is not compatible with the idea of compact optoelectronic THz heads without external elements.
In this paper, we demonstrate a fiber-coupled THz transceiver which does not require mechanical chopping of the radiation. Instead, the femtosecond pulses used for generation and detection are modulated at different rates and the THz trace is sampled at the difference frequency. This approach is convenient to implement compact and stable transceiver heads and allows for increased flexibility since the whole solution is based on fiber-optic components at the telecom wavelength.
2. Principle of operation
The proposed THz transceiver is modeled taking into account the electrical field present in the electrodes and the optically gated conductance of the substrate. Generation and detection are assumed to be independent because the delay introduced between consecutive optical pulses used for THz generation and detection is in the ns range, largely exceeding the carrier lifetime which is in the order of a picosecond. The fundamental difference with respect to typical THz TDS configurations is the biasing of the PCA during detection.
This scenario is schematically illustrated in Fig. 1. A train of ultra-short optical pulses, emitted by a fiber laser at a repetition rate T, is split into two parts. A fixed delay τFix between both trains, which matches the time that the THz pulse takes to propagate from the PCA to the sample and back, is introduced by any means before combining the trains of optical pulses. Both pulse trains propagate through an optical fiber and arrive at the PCA, which is biased with Ubias = UDC. For the generation and detection of each THz pulse, two optical pulses separated by τFix are involved. The first pulse arrives to the PCA at t = 0 while the second one arrives at t = τFix + τODL, where τFix is the fixed delay mentioned above and τODL is constantly varied by an optical delay line, in our case a fiber stretcher. Optical pulses trigger the emission of THz pulses (see  for details of the physical processes involved), as shown in Fig. 1(a) for the first arriving pulse. Although both pulses generate THz radiation the purpose of the second optical pulse is to sample the THz radiation generated by the first optical pulse through variation of τODL (see  for details on the detection of THz pulses). To enable lock-in detection, the amplitude of both pulse trains is modulated at two different rates, f1 and f2, respectively, before they are combined in one fiber using a 50/50 optical coupler. Assuming that each THz pulse propagates through free-space, is reflected from the sample and propagates back to the PCA introducing a potential difference in the electrodes equal to UTHz pulse(t) the total potential difference between the electrodes of the PCA can be expressed as
On the other hand, the injection of optical pulses reduces the resistance of the PCA. This mechanism is used to sample THz waveforms by temporally increasing the photocurrent induced by the difference of potential existent between the PCA’s electrodes. The time-domain response function for the conductance of the antenna, known as the gating function g(t), can be modeled as in . Because up to two optical pulses coexist for each repetition period of the laser the total gating function becomes
Finally, the photocurrent flowing through the electrodes can be expressed as the convolution of Eqs. (1) and (2). Several terms appear, although most of them can be neglected due to the difference between the free-space propagation delay and standard values for the decay of g(t). The relevant terms of the photocurrent are
The last term of the equation provides information on the THz pulse. Although DC, f1 and f2 components appear, detection in these frequency bands becomes difficult due to the large interference introduced by the first two terms of Eq. (3), as shown in Fig. 1(b). Accordingly, lock-in detection must be performed at the sum or difference frequencies, f1 + f2 or f2-f1, respectively.
3. Experimental results
To demonstrate the performance of the proposed THz receiver we built an experimental setup as shown in Fig. 2. A commercially available fiber laser (Menlo T-Light) delivers dispersion-controlled pulses with a duration of 100 fs at a repetition rate of 100 MHz. The laser pulses exit the laser via two outputs ports with an average power of 25 mW each. A pair of fiber-coupled electro-optic modulators with a modulation depth of 25 dB (KOTURA UltraVOA) is operated at two different frequencies in the 100 kHz-500 kHz band. The difference between both modulation tones serves as reference signal for the lock-in amplifier (LIA, Stanford Research 830). A fiber stretcher (TEM Messtechnik) with a maximum delay of 180 ps and a scan rate of up to 3 Hz is employed. To ensure a good signal, measurements were done at a very low scan rate, which lead to an acquisition time of 3 minutes per THz pulse. Emitter and detector pulses are combined using a 50/50 coupler. A fiber-coupled bowtie antenna module is used as transceiver head . The distance separating transceiver and sample is 18 cm. Therefore, the emission and detection optical pulses were separated by τFS = 1.2 ns. A computer controlled, ac-coupled and biased trans-impedance pre-amplifier is used as both voltage source for the antenna and reference for the LIA. The pre-amplifier is equipped with a third-order low-pass filter designed with a cut-off frequency slightly above the difference frequency f2-f1 which introduces more than 60 dB of attenuation for components at f1 and f2, 300 kHz and 321 kHz, respectively. All of the components in the setup (fiber stretcher, modulators, preamp and lock-in) are controlled by a computer. For imaging applications the sample was mounted on an x-y-stage to perform a raster scan whereas for measuring the thickness of pipe walls a rotary table was used.
Reflection measurements were performed on a metallic mirror to determine the performance of the transceiver. The upper part of Fig. 3(a) shows a THz waveform obtained with the fiber stretcher. A time-dependent and approximately linear component indicated by the dashed red line is clearly noticeable in the trace. We attribute this linear component to the fact that the antenna which has been excited by the first pulse has not returned fully to its ground state when the second pulse arrives. The antenna is made of a LT-grown InGaAs/InAlAs heterostructre, with As-Antisite defects as the major electron trap. Even though trapping into these defects is extremely fast, the recombination of electrons from the trap states with holes from the valence band is much slower (~100 ps). Electrons in these states might contribute slightly to the conductivity of the antenna due to hopping conductivity. More importantly, they block electrons generated by the second pulse from being trapped and, hence, the antenna cannot relax to its ground state. This leads to the time-dependent linear component visible in the signal. Anyway, the signal can be easily corrected by subtracting this linear component. In the lower part of the figure, the corrected waveform is shown. The spectrum of the corrected THz pulse is plotted in the inset. We obtain a bandwidth of 500GHz with a signal to noise ratio of 20dB. If the above explanation for the time-dependent linear component is correct one would expect that it is not present in a z-scan where the distance between PCA and sample is varied but the time-delay between the two optical pulses remains fixed. To check this we performed a z-scan using stripline and bowtie antennas. Figure 3(b) shows the THz traces obtained with the z-scan, it can be clearly seen that for both antennas the time-dependent linear component is not observed. Furthermore, one can notice that the signal amplitude of the bowtie (black curve) is by a factor four larger than the stripline signal amplitude (red curve). The inset of Fig. 3(b) shows the corresponding spectra of both signals. Although the bandwidth increased drastically by using the stripline, from 500 GHz to 1.2 THz, the signal-to-noise ratio drops drastically, from 40 dB to 20 dB respectively.
To demonstrate the imaging capabilities of the proposed transceiver an image of the aluminum letters PUM was acquired through an x-y-raster scan of 25 x 60 points with a step of 1 mm in both axes. Results are shown in Fig. 4(a), where we plot a THz image of the reflected intensity at one selected frequency as well as the reflected intensity integrated up to 1 THz. The letters can be clearly resolved.
The setup was also employed to determine the thickness of pipe walls. A pipe was placed at the focal point of the radiation on a rotary table, as shown in Fig. 4(b). It was rotated in 10° steps by 360°. Through analysis of the time of flight between reflections from the first and second interfaces (Fig. 4(c)) a thickness profile of the pipe was obtained. It is shown in Fig. 4(d). Variations in the thickness are obvious. The measured thickness of the pipe wall is 3.9 ± 0.2 mm.
A fiber-coupled terahertz transceiver operated in the telecom band is demonstrated. The electro-optical modulation of emission and detection pulses at different frequencies allows for compact THz transceiver heads which do not require external chopping elements as previous approaches. The presented concept paves the way for compact THz heads with increased flexibility and full optical control. The performance of the transceiver is satisfactory in different applications such as imaging and thickness determination of samples.
The authors would like to thank Graham Town for helpful discussions and Carsten Schindler for his aid designing and building the pre-amplifier. This work has been founded by the German Federal Ministry of Economics and Technology (promotional reference 395ZN).
References and links
1. M. Exter, C. Fattinger, and D. Grischkowsky, “Terahertz time-domain spectroscopy of water vapor,” Opt. Lett. 14(20), 1128–1130 (1989), http://www.opticsinfobase.org/ol/abstract.cfm?URI=ol-14-20-1128. [CrossRef] [PubMed]
2. P. Jepsen, D. G. Cooke, and M. Koch, “Terahertz spectroscopy and imaging – Modern techniques and applications,” Laser Photon. Rev. 5(1), 124–166 (2011).
3. R. J. B. Dietz, M. Gerhard, D. Stanze, M. Koch, B. Sartorius, and M. Schell, “THz generation at 1.55 µm excitation: six-fold increase in THz conversion efficiency by separated photoconductive and trapping regions,” Opt. Express 19(27), 25911–25917 (2011). [CrossRef] [PubMed]
4. C. D. Robiné, C. Wiegand, K. Rühle, F. Ellrich, T. Weinland, and R. Beigang, “Angle-resolved THz time domain reflection spectroscopy of rough surfaces,” in Conference on Lasers and Electro-Optics 2010, OSA Technical Digest (CD) (Optical Society of America, 2010), paper JWA114.
5. D. Molter, G. Torosyan, J. Klier, C. Matheis, C. Petermann, S. Weber, F. Ellrich, J. Jonuscheit, and R. Beigang, “Handheld miniature THz ATR module,” in 36th International Conference on Infrared, Millimeter and Terahertz Waves (IRMMW-THz), 2–7 Oct. (2011), pp. 1–2.
7. M. Tani, Z. Jiang, and X.-C. Zhang, “Photoconductive terahertz transceiver,” Electron. Lett. 36(9), 804–805 (2000). [CrossRef]
8. Q. Chen, Z. Jiang, M. Tani, and X.-C. Zhang, “Electro-optic terahertz transceiver,” Electron. Lett. 36(15), 1298–1299 (2000). [CrossRef]
9. C. Jördens, N. Krumbholz, T. Hasek, N. Vieweg, B. Scherger, L. Bähr, M. Mikulics, and M. Koch, “Fibre-coupled terahertz transceiver head,” Electron. Lett. 44(25), 1473–1475 (2008). [CrossRef]
10. P. U. Jepsen, R. H. Jacobsen, and S. R. Keiding, “Generation and detection of terahertz pulses from biased semiconductor antennas,” J. Opt. Soc. Am. B 13(11), 2424–2436 (1996). [CrossRef]
11. D. Grischkowsky and N. Katzenellenbogen, “Femtosecond pulses of terahertz radiation: physics and applications,” in Picosecond Electronics and Optoelectronics, T. C. L. G Stollner and J. Shah, eds., Vol. 9 of OSA Proceedings (Optical Society of America, Washington, D.C. 1991), pp. 9–14.
12. B. Sartorius, H. Roehle, H. Künzel, J. Böttcher, M. Schlak, D. Stanze, H. Venghaus, and M. Schell, “All-fiber terahertz time-domain spectrometer operating at 1.5 µm telecom wavelengths,” Opt. Express 16(13), 9565–9570 (2008). [CrossRef] [PubMed]