PIERS ONLINE, VOL. 6, NO. 4, 2010 390 Continuous-wave Terahertz Spectroscopy System Based on Photodiodes Tadao Nagatsuma 1, 2, Akira Kaino 1, Shintaro Hisatake 1, Katsuhiro Ajito 2, Ho-Jin Song 2, Atsushi Wakatsuki 3, Yoshifumi Muramoto 3, Naoya Kukutsu 2, and Yuichi Kado 2 1 Graduate School of Engineering Science, Osaka University 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan 2 NTT Microsystem Integration Laboratories, NTT Corporation 3-1 Morinosato Wakamiya, Atsugi, Kanagawa 243-0198, Japan 3 NTT Photonics Laboratories, NTT Corporation 3-1 Morinosato Wakamiya, Atsugi, Kanagawa 243-0198, Japan Abstract Photodiodes have been commonly used for generation of continuous terahertz (THz) waves. In this paper, we proposed the use of photodiodes also for detection of THz waves in order to realize CW THz spectroscopy system based on 1.55-µm fiber-optics. We experimentally demonstrated two detection schemes based on the square-law detection and downconversion, and compared them with respect to sensitivity and dynamic range at 260 420 GHz. 1. INTRODUCTION Terahertz (THz) waves, which cover the frequency range from 100 GHz to 10 THz, have been actively applied to sensing, radars, spectroscopy, measurement and communications. THz pulses based on femto-second pulse lasers have proven to be useful for imaging of objects, and spectroscopy of gas, liquid and solid materials [1, 2]. In particular, the time-domain spectroscopy (TDS) system based on THz pulses has been established as a laboratory standard for the THz spectroscopy, and is commercially available. In the THz-TDS system, frequency characteristics are obtained by Fourier transforming the time-domain data as shown in Fig. 1. Recently, spectroscopy systems based on continuous wave (CW) technology (Fig. 2), which uses monochromatic sources, have attracted great interest [3]. The CW source-based system provides a higher signal-to-noise ratio (SNR) and spectral resolution. When the frequency band of interest is targeted for the specific absorption line of the objects being tested, the CW system with the selected frequency-scan length and resolution is more practical in terms of data acquisition time as well as system cost. Pulsed THz Wave Optical Pulse Optical Source Modulator Generator Object Detector FFT Movable Delay Line Optical Pulse Time Figure 1: Block diagram of THz time-domain spectroscopy system. Continuous Wave (CW) CW THz Wave Optical Source Modulator Generator Object Detector Sweep Fixed Delay Line CW Figure 2: Block diagram of THz frequency-domain spectroscopy system, based on homodyne detection.
PIERS ONLINE, VOL. 6, NO. 4, 2010 391 1.55-µm telecom-wavelength technology is essential in universal instrumentation of CW systems, since low-loss/low-dispersion optical fiber cables can be employed similar to the use of coaxial cables in the conventional systems, and optical components are highly reliable and matured. At 1.55 µm, high-power THz photodiodes such as uni-traveling-carrier-photodiodes (s) [4] are superior to photoconductors based on, for example, low-temperature grown InGaAs in terms of output power as THz signal generators or emitters, while only photoconductors have been used as THz detectors in the CW spectroscopy system [5, 6]. In this paper, we propose and demonstrate the use of photodiodes for both generation and detection in the CW spectroscopy system. First, we experimentally show two kinds of operation modes in photodiodes at 260 420 GHz; one is a square-law detector under forward bias, and the other is a down converter under reverse bias. Then, we compare them with respect to sensitivity and dynamic range. 2. THz PHOTODIODE TECHNOLOGIES Figure 3 shows the band diagram of the photodiode optimized for the operation at 300 400 GHz [7]. This structure is a modification of the uni-traveling-carrier photodiode (). The UTC- PD has a feature of both high-speed and high-output power operation owing to its unique carrier transport mechanism [8]. The photodiode chip was packaged into the module with a rectangular waveguide (WR-3) output port [7]. The frequency dependence of the output power was evaluated by heterodyning the two wavelengths of light from the wavelength-tunable light sources at around 1.55 µm. The THz output power was measured by thermo-coupled power meter. Fig. 3 shows the frequency dependence of the output power generated from the module. The 3-dB bandwidth is 140 GHz (from 270 to 410 GHz). The peak output power was 110 µw at 380 GHz for a photocurrent of 10 ma with a bias voltage of 1.1 V. The output power could be further increased to over 400 µw ( 4 dbm) with increasing the photocurrent up to 20 ma. 3. PRINCIPLE OF THz-WAVE DETECTION WITH PHOTODIODE Figure 4 shows the operation principle of the photodiodes as detectors. There are two operation modes with different voltage-bias conditions; one is a square-law detector under the forward bias condition, and the other is a downconverter under the reverse bias. In case of down-conversion (Fig. 4), the origin of the nonlinearity of the can be explained by Diffusion Block p-contact p-doped Absorption un-doped Collection un-doped Absorption n-contact Detected Power (µw) 120 100 80 60 40 20 0 270 410 GHz 10 ma 6 ma 260 300 340 380 420 (GHz) Figure 3: Band diagram of the photodiode. dependence of output power from the photodiode module for photocurrents of 6 ma and 10 ma. Homodyne Detection Photodiode I Diode V Square-law Detection f = f LO + f THz wave Photonic LO Figure 4: Operation points of photodiode as a THz detector. Schematic representation of photodiode as a down-converter. f f LO
PIERS ONLINE, VOL. 6, NO. 4, 2010 392 the dynamic capacitance associated with charge storage in the photo-absorption layer [9]. Mixing between the input THz wave, f, and the local oscillator (LO) signal, f LO, photonically generated in the photodiode leads to the intermediate frequency, f. 4. EXPERIMENTS AND DISCUSSION Figure 5 shows a block diagram of the CW THz spectrometer consisting of the transmitter (Tx) and the receiver (Rx) based on the square-law detection. can be changed by tuning a difference in the wavelengths of two frequency (wavelength)-tunable lasers. The frequency was modulated at an intermediate frequency (10 khz) using an optical chopper, and the output signal from the Rx at 10 khz was measured by the spectrum analyzer. Figure 6 plots the relationship between the input THz power and the detected () power at 350 GHz. Bias voltages were 1 V and 0.68 V for Tx and Rx, respectively, which were chosen to make the Tx output power and the Rx sensitivity maximum. In the experiment, Tx and Rx were directly connected by the rectangular waveguide. As shown in Fig. 6, the slope almost fits to the square-law relationship. Figure 7 shows a block diagram of the CW THz spectrometer consisting of the transmitter and the receiver based on the down-conversion. This coherent system is often referred to as homodyne detection system [5]. Optical delay line was used to maximize the intermediate frequency signal at 10 khz. Figure 8 plots the relationship between the detected () power and the optical delay length at 350 GHz. Bias voltages were 1 V and 1 V for Tx and Rx, respectively. In the experiment, Tx and Rx were directly connected by the rectangular waveguide. Two periods corresponds to the wavelength of 0.86 mm at 350 GHz, which confirms the proper homodyne detection. By changing the wavelength difference between two wavelength-tunable lasers, frequency dependence of the power was measured as shown in Fig. 8. 6-dB bandwidth ranges from 280 GHz to 410 GHz, which corresponds to 3-dB bandwidth of the output power of the (Fig. 3). 350 GHz Coupler Spectrum Analyzer 10 khz Chopper THz Wave Bias -tee Bias Voltage1 Tx Rx P 2-35 -30-25 -20-15 -10 Power (dbm) Bias Voltage 2 Figure 5: Block diagram of CW THz spectrometer using the receiver based on the square-law detection. Figure 6: Relationship between input THz () power and the detected () power measured at 350 GHz. 350 GHz 10 khz Chopper Bias Voltage1 Tx THz Wave Coupler Spectrum Analyzer Optical Delay Line LO Rx Bias -tee Bias Voltage 2 Figure 7: Block diagram of CW THz spectrometer using the receiver based on the down-conversion.
PIERS ONLINE, VOL. 6, NO. 4, 2010 393 Figure 9 shows the dependence of the power on the photocurrent, which corresponds to the optical LO power, at 350 GHz. As is the case of usual electrical mixers, the power increases with the LO power, and saturates at certain LO level, or the photocurrent of 4 5 ma. Finally, dependence of the power on the input PF power was compared between the homodyne detection and the square-law detection as shown in Fig. 10. For the homodyne detection, the photocurrent was set to 4 ma, which is an optimum condition experimentally confirmed (Fig. 9). Maximum S/N (ratio of power to noise level) is 39 db for the square-law detection, while it is 58 db for the homodyne detection. This corresponds to the difference in the maximum conversion efficiency is 19 db. Since the available output power from the is more than 4 dbm, about 20-dB loss in the transmission between transmitter and receiver and as well as in the object under test is still allowable for the actual spectroscopy. In addition, since the slope in the relationship between the power and the power is smaller for the homodyne detection, loss caused in the object and transmission has less effect, which may be a merit of the homodyne detection scheme. -75-85 -95 0.86 mm 0 0.2 0.4 0.6 0.8 1 1.2 Delay Length (mm) Figure 8: Dependence of power on delay length. system. 280 300 320 340 360 380 400 420 (GHz) characteristics of the homodyne Detected Power ( dbm) -40-50 Saturated against LO power 0 2 4 6 Photocurrent ( ma ) Figure 9: Relationship between the photocurrent and the detected () power measured at 350 GHz. -40-50 Homodyne Detection Square-Law Detection -40-35 -30-25 -20-15 -10 Power ( dbm) Figure 10: Comparison of receiver performance between down-conversion and square-law detection. 5. CONCLUSION In the conventional CW THz spectroscopy system, photoconductor and/or photodiode have been used for the transmitter, while only photoconductors have been employed for the receiver. In this paper, we proposed the use of photodiode as the receiver for the CW terahertz spectroscopy system in addition to the transmitter, and conducted proof-of-concept experiments at 300 400 GHz. We compared two detection schemes; square-law detection with forward diode bias, and downconversion with reverse bias. The conversion efficiency of the down-conversion was 19 db higher than that of the square-law detection, and was comparable to those of InGaAs photoconductors. The components used are all 1.55-µm telecom-fiber-optic ones, which may lead to the cost-effective and versatile spectroscopy system. Currently, the bandwidth is limited by waveguide-structure of the transmitter and receiver modules, and it can be enhanced by using a broadband antenna
PIERS ONLINE, VOL. 6, NO. 4, 2010 394 integrated with the photodiode. Use of the same photodiodes for both transmitter and receiver will also promising for integrated spectrometer chip in bio-sensor applications [10]. ACKNOWLEDGMENT The authors wish to thank Drs. K. Iwatsuki, N. Shigekawa, T. Enoki, and M. Kitamura for their support and encouragement. REFERENCES 1. Cooke, M., Filling the THz gap with new applications, Semiconductor Today, Vol. 2, No. 1, 39 43, 2007. 2. Zhang, X.-C. and J. Xu, Introduction to THz Wave Photonics, Springer, 2009. 3. Deninger, A., A. Roggenbuck, S. I. Schindler, C. Mayorga, H. Schmitz, J. Hemberger, R. Güsten, and M. Grüninge, CW THz spectrometer with 90 db SNR and MHz frequency resolution, Proc. 2009 Infrared, Millimeter and Terahertz Waves (IRMMW-THz 2009), T4A03.0193, September 2009. 4. Nagatsuma, T., Generating millimeter and terahertz waves, IEEE Microwave Magazine, Vol. 10, No. 4, 64 74, 2009. 5. Ducournau, G., A. Beck, K. Blary, E. Peytavit, M. Zaknoune, T. Akalin, J.-F.Lampin, M. Martin, and J. Mangeney, All-fiber continuous wave coherent homodyne terahertz spectrometer operating at 1.55 µm wavelengths, Proc. 2009 Infrared, Millimeter and Terahertz Waves (IRMMW-THz 2009), T4A02.0362, September 2009. 6. Stanze, D., H.-G. Bach, R. Kunkel, D. Schmidt, H. Roehle, M. Schlak, M. Schell, and B. Sartorius, Coherent CW terahertz systems employing photodiode emitters, Proc. 2009 Infrared, Millimeter and Terahertz Waves (IRMMW-THz 2009), T4A01.0176, September 2009. 7. Wakatsuki, A., T. Furuta, Y. Muramoto, T. Yoshimatsua, and H. Ito, High-power and broadband sub-terahertz wave generation using a J-band photomixer module with rectangularwaveguide output port, Proc. 2008 Infrared, Millimeter and Terahertz Waves (IRMMW-THz 2008), M4K2.1199, September 2008. 8. Nagatsuma, T., H. Ito, and T. Ishibashi, High-power photodiodes and their applications, Laser & Photonics Review, Vol. 3, No. 1 2, 123 137, January 2009. 9. Fushimi, H., T. Furuta, T. Ishibashi, and H. Ito, Photoresponse nonlinearity of a uni-travelingcarrier photodiode and its application to optoelectronic millimeter-wave mixing in 60 GHz band, Jap. J. Appl. Phys., Vol. 43, No. 7B, L966 968, 2004. 10. Nagel, M., P. H. Bolivar, M. Brucherseifer, and H. Kurz, Integrated THz technology for label-free genetic diagnostics, Appl. Phys. Lett., Vol. 80, No. 1, 154 156, 2002.