Continuous-Wave Terahertz Spectroscopy System Based on Photodiodes

PIERS ONLINE, VOL. 6, NO. 4, 2010 390

Continuous-wave Terahertz Spectroscopy System Based on Photodiodes

Tadao Nagatsuma1, 2, Akira Kaino1, Shintaro Hisatake1, Katsuhiro Ajito2, Ho-Jin Song2, Atsushi Wakatsuki3, Yoshifumi Muramoto3, Naoya Kukutsu2, and Yuichi Kado2 1Graduate School of Engineering Science, Osaka University 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan 2NTT Microsystem Integration Laboratories, NTT Corporation 3-1 Morinosato Wakamiya, Atsugi, Kanagawa 243-0198, Japan 3NTT 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 Signal Signal

Optical THz-wave THz-wave FFT Modulator Signal Signal Generator Detector Source Object

Movable Delay Line Time Frequency Optical Pulse

Figure 1: Block diagram of THz time-domain spectroscopy system.

Continuous Wave (CW) CW THz Wave

Signal Signal Frequency Optical THz-wave THz-wave Modulator Signal Sweep Signal Generator Detector Source Object

Fixed Delay Line CW Frequency Frequency

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 RF systems, and optical components are highly reliable and matured. At 1.55 µm, high-power THz photodiodes such as uni-traveling-carrier-photodiodes (UTC-PDs) [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(a) shows the band diagram of the photodiode optimized for the operation at 300–400 GH- z [7]. This structure is a modiﬁcation of the uni-traveling-carrier photodiode (UTC-PD). 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(b) 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(a) shows the operation principle of the photodiodes as THz-wave 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(b)), the origin of the nonlinearity of the UTC-PD can be explained by

Diffusion Block Layer p-doped 270−410 GHz Absorption Layer 120 10 mA un-doped W) p-contact µ 100 Collection Layer 80 Layer 60 6 mA 40 un-doped n-contact 20 Absorption Layer Layer ( Power Detected 0 260 300 340 380 420 Frequency (GHz) (a) (b) Figure 3: (a) Band diagram of the photodiode. (b) Frequency dependence of output power from the photodiode module for photocurrents of 6 mA and 10 mA.

fRF= fLO+ fIF RF IF f Homodyne IF Detection THz wave

V Photonic LO fLO Square-law Detection

Photodiode Diode (a) (b) Figure 4: (a) Operation points of photodiode as a THz detector. (b) Schematic representation of photodiode as a down-converter. 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, fRF , and the local oscillator (LO) signal, fLO , photonically generated in the photodiode leads to the intermediate frequency, fIF .

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. Frequency can be changed by tuning a difference in the wavelengths of two frequency (wavelength)-tunable lasers. The RF frequency was modulated at an intermediate frequency (10 kHz) using an optical chopper, and the output signal from the Rx UTC-PD at 10 kHz was measured by the spectrum analyzer. Figure 6 plots the relationship between the input THz power and the detected (IF) power at 350 GHz. Bias voltages were −1 V and 0.68 V for Tx UTC-PD and Rx UTC-PD, 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(a) plots the relationship between the detected (IF) power and the optical delay length at 350 GHz. Bias voltages were −1 V and −1 V for Tx UTC-PD and Rx UTC-PD, 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 IF power was measured as shown in Fig. 8(b). 6-dB bandwidth ranges from 280 GHz to 410 GHz, which corresponds to 3-dB bandwidth of the output power of the UTC-PD (Fig. 3(b)).

350 GHz Bias Voltage1 -60 λ− 10 kHz Tunable Laser -70 2 P RF Coupler Chopper Tx λ−Tunable Laser UTC-PD -80

THz Wave -90 -100 RF Noise Level -110

Rx (dBm) Power IFDetected Spectrum Bias -tee -120 UTC-PD -35 -30 -25 -20 -15 -10 Analyzer IF RF Power (dBm) Bias Voltage 2

Figure 5: Block diagram of CW THz spectrometer Figure 6: Relationship between input THz (RF) using the receiver based on the square-law detection. power and the detected (IF) power measured at 350 GHz.

Bias Voltage1

10 kHz 350 GHz Tx Chopper UTC-PD

λ−Tunable Laser THz Wave Coupler RF Spectrum λ−Tunable Laser Analyzer

Rx Optical Delay Line UTC-PD Bias -tee LO IF 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 IF power on the photocurrent, which corresponds to the optical LO power, at 350 GHz. As is the case of usual electrical mixers, the IF power increases with the LO power, and saturates at certain LO level, or the photocurrent of 4–5 mA. Finally, dependence of the IF 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 IF 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 UTC-PD 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 RF power and the IF 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.

0.86 mm -70 -70

-75 -80

-80 -90

-85 -100 -90 Noise Level Detected IF(dBm) Power

Detected IF(dBm) Power -110 -95 0 0.2 0.4 0.6 0.8 1 1.2 280 300 320 340 360 380 400 420 Delay Length (mm) Frequency (GHz) (a) (b) Figure 8: (a) Dependence of IF power on delay length. (b) Frequency characteristics of the homodyne system.

-40 -50 -40 Homodyne Detection -60 -50 Square-Law Detection -60 -70 -70 -80 Saturated against LO power -80 -90 -90 -100 -100 Detected IF Power ( Power IF Detected dBm) Noise Level Power Detected (dBm) IF Noise Level -110 -110 -120 -120 0 2 4 6 -40 -35 -30 -25 -20 -15 -10 Photocurrent ( mA) RF Power ( dBm) Figure 9: Relationship between the photocurrent Figure 10: Comparison of receiver performance be- and the detected (IF) power measured at 350 GHz. tween 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 down- conversion 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.

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