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We propose a high-sweeping-speed optically synchronized dual-channel terahertz (THz) signal generator for an active gas-sensing system with a superconductor-insulator-superconductor (SIS) mixer. The generator can sweep a frequency range from 200 to 500 GHz at a speed of 375 GHz/s and a frequency resolution of 500 MHz. With the developed gas-sensing system, a gas-absorption-line measurement was successfully carried out with N2O gas in that frequency range.
©2009 Optical Society of America
1. Introduction
Terahertz (THz) waves in a frequency range from 0.1 to 10 THz are attractive for remote gas sensing because many gases have characteristic absorption lines, called fingerprint spectra, in that frequency range. In order to realize the remote gas sensing, a time-domain THz spectroscopy (TDS) will probably be paid attention as one of the candidates because its operation bandwidth is very wide [1]. However, the TDS is not suitable for sensing a narrow absorption line of gas molecules due to a poor frequency resolution. Another approach is to use a continuous-wave (CW) THz spectroscopy which can provide a narrow frequency resolution [2]. By using a CW THz-signal generation based on a photonics technology, we have been studying active gas-sensing systems for detecting toxic gases from a distance which can help assisting a rescue operation at a disaster area [3,4]. We used a room temperature Schottky-barrier diode detector in those researches. However, the sensitivity of the Schottky-barrier diode detector was not enough for extending a detection range. Recently, we reported a highly sensitive active gas-sensing system with photonics-based THz-signal generators and a superconductor-insulator-superconductor (SIS) mixer [5]. We chose the SIS mixer to build a highly sensitive THz-signal receiver. Comparing to the previous system with the room temperature Schottky-barrier diode detector, the system exhibited the superior sensitivity to extend the detection range more than 25 m. However, the sweeping speed of the system was limited because the previous THz-signal generator was controlled by a microwave synthesizer. The sweeping speed is important because the gas sensing system must quickly provide information of detected gases at an emergency situation. To overcome this limitation, we propose a high-sweeping-speed optically synchronized dual-channel THz-signal generator for driving the SIS mixer with a fast sweeping speed. The newly developed THz-signal generator synchronously generates THz transmitter (TX) and local-oscillator (LO) signals with a sweeping speed of 375 GHz/s and a frequency step of 500 MHz in a frequency range from 200 to 500 GHz. The new generator can sweep that frequency range within three seconds while the previous one can sweep that frequency range with several minutes. To test the performance of the SIS mixer receiver driven by the new generator, we investigated the stability of the intermediate frequency (IF) and the receiver’s noise temperature and dynamic range. Finally, we carried out gas-absorption-line measurements with N2O gas. The results show the excellent performance of the system.
2. Operation principle of the optically synchronized dual-channel THz-signal generator
The optically synchronized dual-channel THz-signal generator consists of three lasers operating at around 1.55 μm as shown in Fig. 1(a) . One is a wavelength-tunable laser source (TLS) and the other two are frequency-fixed distributed-feedback (DFB) laser diodes (LDs). The optical signal of the TLS is coupled to optical signals of the two DFB LDs by optical couplers, respectively. The wavelengths of two DFB LDs are fixed at around 1548.959 nm with a frequency difference of 350 MHz, while the wavelength of the TLS is swept from 1547.363 to 1544.971 nm with a sweeping speed of 375 GHz/s and frequency step of 500 MHz. Therefore, the generator can generate synchronously THz-TX and LO optical beat signals with the frequency difference of 350 MHz. Corresponding optical beat frequencies of the generated THz-TX and LO signals change from 200 to 500 GHz and from 199.65 to 499.65 GHz, respectively, as shown Fig. 1(b). Two optical beat signals are converted into THz-TX and LO signals by two uni-travelling carrier photodiode (UTC-PD) modules integrated with waveguide antennas [6]. The UTC-PD functions as a photomixer. Notably, the developed THz-signal generator can synchronously generate the transmitter and LO signals from 10 GHz to 30 THz, provided that the photomixer can cover those frequency ranges.
Fig. 1 Concept of the optically synchronized dual-channel THz-signal generator. (a) Configuration of the generator. (b) Composition of generated THz signals.
3. Heterodyne detection of THz-wave signal with the developed THz-signal generator
Figure 2 shows the configuration for the heterodyne detection of THz-wave signal with the developed THz-signal generator and SIS mixer. The frequencies of the THz-TX and LO signals are described by the optical beat signals between laser 1 and 2 and between laser 1 and 3, respectively, where f 1, f 2, and f 3 are the frequencies of laser 1, 2, and 3, respectively, and Δf 1, Δf 2, and Δf 3 are their frequency jitter, respectively. The THz-TX signal is down-converted into the signal of the IF corresponding to the frequency difference between the THz-TX and LO signals at the SIS mixer. Here, notably, the frequency f 1 and frequency jitter Δf 1 of the TLS is completely cancelled out in the IF. As a result, the frequency difference perfectly coincides with the frequency difference between the two frequency-fixed DFB LDs. The IF and system stability are determined by the frequency difference and the mutual frequency jitter between the DFB LDs, respectively.
Fig. 2 Configuration for heterodyne detection of THz-wave signal with the developed THz-signal generator.
4. Experimental results
4.1 Frequency stability of IF signal
As described above, when the THz-TX and LO signals are fed to the SIS mixer receiver, the IF is determined by the frequency difference of the two frequency-fixed DFB LDs, while the sweeping frequency of the THz signal is determined by that between the TLS and the DFB LDs. Therefore, the frequency stability of the IF does not depend on variable wavelength of the TLS but on the frequency stability of the two frequency-fixed DFB LDs. Figure 3 shows the frequency jitter of the photomixer oscillating from the two DFB LDs with the frequency difference of 350 MHz for 30 minutes. The measured result shows the frequency jitter of about 21 MHz. The measured frequency jitter is 8.4% of the receiver bandwidth of 250 MHz. The small frequency jitter of the IF makes the system stable.
4.2 Noise temperature of the receiver
We investigated the noise temperature T RX of the SIS mixer heterodyne receiver driven by the developed THz-signal generator. The SIS mixer was cooled in a liquid helium cryostat [7]. The T RX was measured by the Y-factor method, where T RX is derived from the ratio of the IF output power for hot (295 K) thermal radiation to that for cold (77 K) thermal radiation. The receiver bandwidth of 250 MHz was determined by a band-pass filter. The measured T RX is shown in Fig. 4 . The corresponding input noise power in the 250 MHz receiver bandwidth is lower than −80 dBm in a frequency range from 220 to 470 GHz. The result indicates that the generator enables us to exploit the high sensitivity of the SIS mixer.
Fig. 4 Measured noise temperature of the developed active gas-sensing system. The black- and red-dotted lines are the measured T RX and corresponding input noise power of the SIS mixer heterodyne receiver, respectively.
4.3 Dynamic range
We tested the dynamic range of the system composed of the developed THz-signal generator and SIS mixer by inserting an attenuator in the path of the THz-TX signal. The output of the IF was measured by attenuating the THz-TX signal (about −28 dBm at 300 GHz) and with a constant THz-LO power level (about −35 dBm at 300 GHz). Figure 5 shows measured IF signals corresponding to attenuation of the THz-TX signal. The black line is the IF signal corresponding to the THz-TX signal with no attenuation. The red, blue, and green lines are the IF signals corresponding to the attenuated THz-TX signal with attenuation of 10, 20, and 30 dB, respectively. The results show good linearity in a frequency range from 250 to 425 GHz, while the linearity is degraded around the outskirt frequencies due to a sharp reduction of the IF signal level. The low level of the IF signals at those frequencies is attributed to the small conversion gain of the SIS mixer and the small responsivity of the UTC-PD. When we drove the SIS mixer with a high LO power level (about −29 dBm at 300 GHz), the IF signal had a sufficient power level at those frequencies. However, the IF signal was completely saturated in a frequency range from 250 to 400 GHz. In order to obtain a wide dynamic range with a broad frequency range, we drove the SIS mixer, alternating low and high LO power levels, and exploited partially the two output spectra with regard to frequency range. The spectrum for driving the SIS mixer with low LO power (about −35 dBm at 300 GHz: spectrum A) was captured in a frequency range from 250 to 410 GHz. The spectrum for the high LO power (about −29 dBm at 300 GHz: spectrum B) was captured in frequency ranges from 200 to 250 GHz and 410 to 500 GHz, respectively. Spectrum A and B were then merged in a post process as shown Fig. 6 . As a result of alternating the low and high LO power levels, we could compensate for the small responsivity of the UTC-PD and the small conversion gain of the SIS mixer at the outskirt frequencies and obtain a wide dynamic range of about 45 dB in a frequency range from 225 to 450 GHz, taking into account the marginal TX power of about 20 dB.
4.4 Gas absorption line measurement
To demonstrate the performance of the developed system, we carried out active gas-spectrum measurements with N2O gas as shown in Fig. 7 . A 0.2-m gas cell was positioned between the THz TX and SIS mixer. The gas cell was connected with N2 and N2O gas reservoirs to produce gas with various N2O contents. The THz-TX signal passed through the gas cell and was then coupled to the SIS mixer with a lens. The THz-LO signal was directly coupled to the SIS mixer with a coupler (coupling efficiency: about 10%). The THz-TX signal was down-converted into the IF of 350 MHz with the SIS mixer and the THz-LO signal. A band-pass filter determined the receiver bandwidth of 250 MHz and the output signal was amplified with an IF amplifier. A RF-power detector detected the power of the output signal. Finally, a fast analog-to-digital converter acquired the output signal of the RF power detector and then its output was saved in a computer.
For the gas-spectrum measurement, the spectrum of the gas cell filled with N2 gas was first measured to obtain reference data. The gas cell was then purged with a vacuum pump and quickly filled with N2O gas diluted by N2 gas with the concentration of 100, 50, 25, or 15% and the pressure of ~1000 hPa. Absorption spectra of the N2O gas were obtained by normalizing them with the reference data. In this measurement, frequency sweepings from 200 to 500 GHz were carried out eight times with alternating low and high LO power levels, respectively, and all sweeps were averaged. It took about six seconds to sweep two times from 200 to 500 GHz with alternating low and high LO levels. Thus, the total of sixteen sweeps took about 48 seconds. The integration time of the signal at each frequency point is about 170 μs per scan [8]. The spectrum shown in Figs. 8 was obtained by averaging the data of the eight spectra. Therefore, the time spent for each frequency element is about 1.4 ms.
Figure 8 shows the measured and calculated absorption spectra of 100, 50, 25, and 15% N2O gases at room temperature. The positions of the absorption peaks starting at 250 GHz and appearing with 25-GHz intervals as shown in the Fig. 8 are caused by the vibration-rotation transition of N2O molecules [9]. The measured results agree quite well with the calculation results, indicating our system successfully detects N2O gas with N2O content of 100, 50, 25, and 15%.
5. Conclusion
We proposed a high-sweeping-speed optically synchronized dual-channel THz-signal generator for a fast-sweeping-speed and broadband active gas-sensing system with a SIS mixer. The developed active gas-sensing system can quickly sweep a wide frequency range. With this system, we successfully carried out measurements of N2O gas.
Acknowledgments
This work was supported in part by the National Institute of Information and Communications Technology (NICT), Japan.
References and links
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