Homodyne Detection of Microwaves Using Low-Temperature-Grown GaAs at 1.55 µm Shintaro Hisatake1, Kyohei Shimahashi1, Genki Kitahara1, Yuta Morimoto1, Ho-Jin Song2, Katsuhiro Ajito2, and Tadao Nagatsuma1 1
Graduate School of Engineering Science, Osaka University 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan
NTT Microsystem Integration Laboratories, NTT Corporation 3-1 Morinosato Wakamiya, Atsugi, Kanagawa 243-0198, Japan e-mail: [email protected]
Table 1. Summary of recent work for photoconductive mixer operating at 1.55 µm.
Abstract—We demonstrate homodyne detection of microwaves using low-temperature-grown (LTG) GaAs with 1.55-µm optical local oscillator (LO). The sensitivity of the detection is almost proportional to the average power of the optical LO, which indicates that the photoresponse of the LTG-GaAs is linear process in our case. The dynamic range of the detection is more than 50 dB. About -22 dBm of 14.6 GHz emitted from a patch antenna is detected. The possibility for terahertz wave homodyne detection has been discussed.
Keywords—LTG-GaAs; homodyne detection; photoconductive mixer; microwave; millimeter wave; terahertz wave
Tani, 2000 
Sartorius, 2007 
Kataoka, 2010  This work
Song , 2007 
I. INTRODUCTION Time-domain techniques based on pulsed terahertz (THz) wave have proven to be useful for wide-band spectroscopy of gas, liquid and solid materials [1,2]. Recently, frequencydomain techniques based on a continuous wave (CW) technology, which uses monochromatic sources, have extensively studied because it provides a higher signal-tonoise ratio (SNR) and resolution in spectroscopic applications. CW spectrometer with MHz resolution  has been commercially available in which frequency stabilized 850 nm DFB lasers are used for the THz wave generation and the detection. On the other hand, 1.55 µm telecom-wavelength technology is essential in universal instrumentation of such systems, because low-loss/low-dispersion optical fiber cables can be employed and optical components are highly reliable and matured. Recently, generation and detection of microwaves (MW) or THz waves using 1.55 µm telecom components have extensively studied.
GaAs is usually not sensitive at 1.55 µm wavelength because the band gap is 1.43 eV at room temperature. In spite of this, there have been several reports in which LTG-GaAs has been used as a photoconductive mixer in the pulse system with 1.55 µm femto-second laser [6,7]. To the best of our knowledge, however, there have been no reports in which LTG-GaAs has been used as a photoconductive mixer in the CW system even in the MW region. In this paper, we demonstrate homodyne detection of the MW using LTG-GaAs based photoconductive mixer with 1.55-µm optical local oscillator (LO).
HOMODYNE DETECTION USIGN LTG-GAAS
The LTG-GaAs we will use is THz wave detection module, in which a LTG-GaAs based photoconductive antenna (PCA) chip and a silicon lens for THz wave focusing are integrated (Hamamatsu: G10620). Figure 1(a) shows the PCA. The antenna was a 2-mm-long Bow-Tie antenna with a 6 µm gap. Figure 1(b) shows the spectral response of the PCA . The spectrum was measured by using two Bow-Tie PCAs as an emitter and a detector based on time-domain spectroscopy. The PCAs were exited by 800 nm, 80 fs Ti:sapphire laser. Two antennas are designed for the THz frequency range. The spectral response is flat from 250 GHz
At 1.55 µm, high-power THz photodiodes such as unitraveling-carrier-photodiodes (UTC-PDs)  are strong candidate for the THz generator. For the detectors, InGaAsbased photoconductive mixers are usually used at 1.55 µm both in pulse and CW system. Table 1 shows a summary of recent work for the photoconductive mixers operating at 1.55 µm. Although InGaAs can be exited directly by 1.55 µm laser, the resistivity is about 1 kΩ cm . On the other hand, lowtemperature-grown (LTG) GaAs has significantly large resistivity (107 Ω cm), which leads the higher SNR. However,
This research was partially supported by a grant from the Global COE Program, ”Center for Electronic Devices Innovation,” from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
978-1-61284-718-4/11/$26.00 ©2011 IEEE
Takazato, 2007 
Deninger , 2010 
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Figure 3. Simple model of the photoconductive mixer.
Figure 2 shows the optical response of the PCA. The conductance was measured by illuminating 1.55 µm CW laser diode. The bias voltage was 10 V. The linear line is well fitted to the data, suggesting that the photoresponse of our PCA is almost linear at such a low optical power level. On the other hand, THz detection using LTG-GaAs with 1.55 µm pulse laser exhibited nonlinear response such that both the signal n amplitude and the photoconductance increase as P with n = 1.35 for the incident power P . We will proof the operation of the photoconductive mixer in the CW system by comparing theoretically predicted response and measured data. In the remainder of this section, € we will derive relationships between detected IF signal power and optical LO power and modulation depth of the optical LO signal. Figure 3 shows a model of the photoconductive mixer. When the optical LO signal is illuminated to the photoconductive materials, the photo carrier is generated which leads the increase in conductivity of the photoconductor as shown in Fig. 2. Here we assume nonlinear response of optically modulated conductance, Δg, written as,
Figure 1. (a) LTG-GaAs based PCA. The antenna structure is Bow-Tie type. (b) Spectral response of the PCA.
100 80 60
Δg(t) ∝ P n (t)
, where P(t) is the power of the optical LO signal. In our CW case, the power of the optical LO signal can be written as,
€ P(t) = P0 (1+ m cos(ω LO t + δφ ))
Optical power (mW) Figure 2. Photocurrent of the LTG-GaAs.
to 1 THz. We will conduct experiments at 14.6 GHz. In our case, the Bow-Tie antenna is sufficiently larger than the wavelength, therefore this antenna acts as a lumped element.
, where P0 is average power, m ( 0 ≤ m ≤ 1) is the modulation depth, ωLO is the angular frequency of the optical LO signal, and δφ is the phase deference between LO and RF signal. At €the photoconductor, the RF signal induces an AC voltage € proportional to the incoming electric field. Together with the modulated conductance, this generates an AC current written as,
iIF ∝ E RF P n 0 m cos[(ω RF − ω LO )t + δφ ]
, where ERF and ωRF are electric field and the angular frequency of the RF signal, respectively. Therefore, the power of the detected IF signal is proportional to the power of the RF € signal. The sensitivity is proportional to P0 2n and m 2 .
Proceedings of the 2011 IEEE MWP
Figure 6 shows the relationship between the average power of the optical LO signal (P0) and the SNR of the detected IF signal. The RF power was about 25 dBm and the modulation depth was about 0.72. The white circles are measured data and the solid line is fitted to the data. The fitted slope of the relationship is 2n = 1.8 and the measured data almost fit to the square-law relationship. In our case, n = 0.9 and this suggests that the primary mechanism of photoresponse is not based on the nonlinear process at this level of the optical LO signal. Two-step photoabsorption mediated by the high-density midgap states in LTG-GaAs seems to be responsible. Increase of the defect density will enhance the detection sensitivity. On the other hand, it is worth to note that Kataoka et al. reported that the detection efficiency can be improved by reducing the spot size of the optical LO and gap length of the antenna .
III. EXPERIMENTS AND DISCUSSION Figure 4 shows a block diagram of the experimental setup. The 1557 nm CW laser is modulated by the Mach-Zehnder intensity modulator (Photoline technologies: MX-LN-20) to produce optical LO signal. The LO frequency was 14.6 GHz. An Er-doped fiber amplifier (EDFA) and a variable optical attenuator were employed to adjust optical LO power. The optical LO signal was coupled to the free space via the pigtailed fiber collimator and focused onto the LTG-GaAs via the objective lens. Note that the power of the optical LO signal was measured just before the fiber collimator. The frequency of the RF signal was set to be 14.6 GHz + 1 kHz. The RF signal was amplified by small signal amplifier (R&K: AA380) and coupled to the free space via patch antenna. The resonance frequency of the patch antenna was 14.6 GHz. Note that the RF power was measured just before the patch antenna. The IF current (1 kHz) was amplified by a transimpedance amplifier and measured by a spectrum analyzer.
In our experiment, the detected IF power linearly increases with the optical LO power, whereas the noise level does not depend on the optical LO power. The noise level of our system is dominated by amplifier noise. The very low noise level of photoconductive material is based on the large resistivity of the LTG-GaAs which is very big benefit for the signal detection.
Figure 5 shows the relationship between the modulation depth and the normalized sensitivity. The RF power was 25 dBm and optical LO power was 20 mW. The modulation depth were calculated from temporal waveform of the optical LO signal. The white circles are measured data and the solid curve is a quadratic curve fitted to the data. The quadratic curve is well fitted to the data and this indicates that the sensitivity of the homodyne detection is almost proportional to the square of the modulation depth. The square of the modulation depth of the optical LO can be regarded as the LO power in conventional electric mixers.
Figure 5. Relationship between modulation depth and the normalized sensitivity.
Figure 4. Experimental setup. Figure 6. Relationship between optical LO power and the SNR of the detected IF signal.
Proceedings of the 2011 IEEE MWP
process. The dynamic range of the detection was more than 50 dB and -22 dBm of RF signal (14.6 GHz) could be detected. Our results suggest that the LTG-GaAs with 1.55-µm optical LO can be used as a homodyne/heterodyne receiver in the THz region with the SNR of more than 55 dB. ACKNOWLEDGEMENT The authors would like to thank Dr. F. Ishikawa, Graduate school of engineering, Osaka University, Osaka, Japan, for helpful discussion.
Figure 7. Relationship between RF power and the SNR of the detected IF signal.
Figure 7 shows relationship between the RF power and the SNR of the detected IF signal. The optical LO power and the modulation depth were 20 mW and 0.72, respectively. The fitted slope of the relationship is 0.98. The achieved SNR for the optical LO power of 20 mW was about 50 dB. The dynamic range of the detection is more than 50 dB, and we could detect -22 dBm of RF signal.
The noise level of current system is now limited by the excess noise of our transimpedance amplifier. The input noise current density of our amplifier is estimated to be 0.42 pA/ √ Hz. By replacing the amplifier to low-noise one, for example LI-76 (NF corporation), whose input current noise density is 0.013 pA/√Hz, the SNR of the detection system will improve about 30 dB. On the other hand, the sensitivity of the PCA at THz region increases more than 10 dB compared to the sensitivity at the MW region, as shown in Fig. 1(b). Considering the output power of the UTC-PD at 300 GHz (more than -10 dBm), it could be estimated that SNR for the CW THz wave homodyne detection using LTG-GaAs with 1.55-µm optical LO will be more than 55 dB.
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IV. CONCLUSION We demonstrated homodyne detection of microwaves using LTG-GaAs with 1.55-µm optical LO signal. We confirmed that the photoresponse is based on the linear