We demonstrate a tunable continuous-wave (CW) terahertz (THz) homodyne system with a novel detuned dual-mode laser diode (DML) and low-temperature-grown (LTG) InGaAs photomixers. The optical beat source with the detuned DML showed a beat frequency tuning range of 0.26 to over 1.07 THz. Log-spiral antenna integrated LTG InGaAs photomixers are used as THz wave generators and detectors. The CW THz radiation frequency was continuously tuned to over 1 THz. Our results clearly show the feasibility of a compact and fast scanning CW THz spectrometer consisting of a fiber-coupled detuned DML and photomixers operating in the 1.55-μm range.
©2011 Optical Society of America
Terahertz (THz) technologies have various applications due to their unique properties . Many materials such as cloth, paper, and plastic are transparent to THz waves, and the waves are safe for the human body because of their low photon energy. Furthermore, many chemical and organic materials have their unique absorption spectra in the THz range. Owing to these unique properties, THz technologies are now being applied to imaging, sensing, and spectroscopy fields for security, agriculture, environment, and medicine [1–3].
Over the past several decades, significant progress has been made in the field of THz time-domain spectroscopy (THz-TDS) . However, the THz-TDS system has some limitations such as size, cost, and frequency resolution. In many THz applications, a low-cost and compact continuous-wave (CW) THz system is preferred. The portable and widely tunable CW THz emitter consisting of a photomixer and optical beat source is a prerequisite for the realization of a compact and cost-effective system [5–7]. In commercial CW THz photomixer systems, optical beat sources are typically composed of two tunable distributed feedback (DFB) laser diodes (LDs) with different wavelengths . However, the use of two independent lasers still requires considerable effort to stabilize each frequency and match the two beams spatially. A remarkable simplification can be achieved by using a monolithically integrated semiconductor laser emitting two selected modes simultaneously. The adoption of the integrated device ensures that the beam path and polarization of the two modes are automatically matched, and the beating frequency is insensitive to the temperature variation of surroundings . We have already reported on monolithically integrated DFB LD for CW THz generation . However, its tuning range is limited to a maximum of 0.49 THz, and the THz output signal is measured using a helium-cooled bolometer. To realize a compact CW THz system operating in 1550 nm region for various applications such as spectroscopy, wireless communication and so on, a high-performance optical beat source and efficient photomixer are required.
Studies on THz photomixers for tunable CW generation have mainly used low-temperature-grown (LTG) GaAs as the photoconductive material because of its short carrier lifetime and high resistivity [5,7]. Recently, several reports have shown that LTG-InGaAs and ion-implanted (IM) InGaAs can act as the photoconductive material by focusing on the connection between THz and well-developed InP-based 1550-nm communication technologies [10–14]. However, InGaAs-based photomixers have the problem of low dark resistance (high dark current), which decreases the efficiency of THz emission because of background thermal radiation and low thermal-damage threshold. Therefore, more studies on active optimization are needed for achieving more efficient and broadband InGaAs-based photomixers.
Various CW THz systems operating in the 1.55-μm region and using InP-based photomixers and photonics technology have been reported. An optical-fiber-based dual-wavelength laser was used to generate CW THz radiation . However, dual-wavelength fiber lasers suffer from the longitudinal multi-mode problem; they also need complex configurations including additional components such as a saturable absorber, fiber Bragg grating, and Fabry-Perot filter . Monolithic integrated semiconductor lasers only have single-frequency operation or limited tuning range [17,18]. For spectroscopic applications, the typical CW THz system uses two independent external cavity lasers to generate the optical beat signal . Therefore, to the best of our knowledge, widely tunable CW THz systems consisting of a monolithically integrated dual-mode laser and InP-based photomixers have not been reported thus far.
In this paper, we present a fiber-coupled CW THz system composed of LTG-InGaAs based photomixers and a widely tunable detuned dual-mode DFB LD (DML). The developed 1.55-μm detuned DML shows precise and wide optical beat frequency tuning from 0.26 to 1.07 THz. The log-spiral antenna integrated LTG-InGaAs photomixers are used for CW THz generation and homodyne detection. The completely fiber-based CW THz system consisting of our detuned DML and an InGaAs-based photomixer showed a wide frequency tuning range from 0.315 to over 1 THz.
2. Tunable dual-mode laser
Figure 1 shows the wavelength-tuning characteristics of the detuned DML (Fig. 1(a)) and the spectra amplified by an erbium-doped fiber amplifier (EDFA) (Fig. 1(b)). The detuned DML consists of one 50-μm-long phase and two 300-μm-long DFB sections. The μ-heaters are used to independently tune the wavelengths of the two laser modes of the detuned DML. Wavelength tuning with the μ-heaters is preferable to optical beat sources because it maintains the spectral linewidth of lasing modes and the optical beat frequency can be continuously tuned without mode hopping.
Both the device structure and the shape of the μ-heaters were carefully designed for thermal isolation and easy fabrication. Moreover, the detuned DML structure does not have a passive waveguide because an all-active structure was adopted. The all-active structure of our detuned DML enables easy fabrication steps and stable operation of the DFB LDs. Furthermore, besides playing the role of the typical phase section controlling the interaction between the DFB LD sections by changing the optical phase, the phase section with the active layer can be used as an absorption layer that suppresses compound cavity modes with a reverse bias. The operation wavelengths of each DFB LD section are detuned by 5 nm to increase the beat frequency tuning range. Therefore, the continuous tuning of wavelength differences from 2.1 to 8.5 nm was successfully demonstrated in the detuned DML. Details on the device structure, fabrication, and basic characteristics of the detuned DML were reported elsewhere .
Figure 2 shows the autocorrelation traces. The efficient mode beating signals are measured throughout the whole tuning range. It should be noted that the measured autocorrelation traces are obtained under fixed operating conditions of the detuned DML. We did not change any operating conditions of the detuned DML such as operating current, temperature and reverse bias of the phase section. Optical beat frequency is tuned by adjusting only the μ-heater currents. This clearly proves the feasibility of the detuned DML as a fast and compact optical beat source.
3. Experiments of tunable CW THz generation
LTG-InGaAs was adopted for the photoconductive active layer in this study. For fabrication of the LTG-InGaAs photomixers, 1-μm-thick Be-doped InGaAs layers were grown on semi-insulating InP substrates at 250°C using a molecular beam epitaxy (MBE) system. The wafers were then annealed in situ in the same MBE chamber at 550°C in an AsH3 environment. Figure 3(a) shows the electrical properties of the LTG-InGaAs layers after the annealing process. The electron concentration monotonically decreased from 2.3 × 1016 cm−3 to 1.3 × 1015 cm−3 as the Be doping level increased from 5 × 1016 cm−3 to 1 × 1018 cm−3. Type conversion from n- to p-type occurred at a Be doping level in between 1 × 1018 and 5 × 1018 cm−3. These properties are roughly consistent with already reported data in . Therefore, the Be doping level of 1 × 1018 cm−3 is the optimal condition for the lowest dark current in our case. The electron mobility also decreased with the Be doping level due to the increase in the dopant-scattering mechanism. The very low mobility (~30 cm2/Vs) at the Be doping level of 5 × 1018 cm−3 may be related to the hole-transport property. The carrier lifetime of the LTG-InGaAs sample was measured by using time-resolved pump-probe reflection spectroscopy shown in Fig. 3(b), and its value was 7.8 ps.
The log spiral antenna and interdigitated-finger patterns were transferred onto the LTG-InGaAs layer by a stepper system and defined by evaporation of 10-nm-thick Ti and 250-nm-thick Au, as shown in Fig. 4(a) . Finally, a 200-nm-thick SiNx layer was deposited for anti-reflection coating. The log spiral antenna consisted of two three-turn spirals, and its designed length and width were 850 and 650 μm, respectively.
Figure 4(b) shows the input impedance of the log spiral antenna on the LTG-InGaAs by the commercial High Frequency Structural Simulator (HFSS) software. As expected, the input impedance of the log spiral antenna had broadband characteristics varying between 60 and 80 Ω for the frequency range of 0.1 to 3 THz. Figure 4(c) shows the radiation pattern of the antenna on the substrate with the collimating Si lens at 100 GHz. The radius of the lens was 2.5 mm, and the distance from the photomixer to the tip of the lens was 3.6 mm. The directivity of the antenna was 12.7 dB in the forward direction at 100 GHz and increased rapidly with increasing frequency.
To prove the feasibility of our detuned DML as a compact optical beat source for a tunable CW THz system, a fiber-coupled CW THz system was fabricated as shown in Fig. 5 . The optical beat signal emitted from the detuned DML package was amplified by an EDFA. The amplified spontaneous emission (ASE) noise generated from the EDFA was filtered out by optical band-pass filters. An emitter and receiver made from our LTG-InGaAs photomixer were used to generate and detect THz signals, respectively. In order to enhance the detection sensitivity, we used a lock-in amplifier and sine-wave function generator.
The emitter (or receiver) module in Fig. 5 consists of a hyper-hemispherical Si lens, log-spiral antenna-integrated LTG-InGaAs photomixer chip electrically contacted on a printed circuits board (PCB), and fiber assembly. The fiber assembly is adjusted by a micromanipulator to couple the optical beat signal to the active area of the photomixer chip.
Figure 6 shows the continuous frequency tuning of the CW THz wave emitted from the fabricated LTG-InGaAs photomixers illuminated by the detuned DML. The number of interdigitated fingers, dark resistance of the photomixer, total optical input power, and operation bias voltage on the photomixer were 3, ~10 kΩ, ~20 mW, and 3.2 V, respectively. The CW THz frequency was smoothly tuned from 0.315 to over 1 THz as shown in Fig. 6. The THz output power was estimated to be about 10 nW at 0.3 THz by comparison with our previous results . This value is somewhat low compared to that of typical InGaAs-based photomixers ; this may have been due to the non-optimized photoconductive material properties, such as the low dark resistance and long carrier lifetime of 7.8 ps.
THz radiations in the low-frequency region from 0.315 THz to 0.56 THz were generated by reducing the wavelength difference between lasing modes of the detuned DML via current injection into μ-heater1 only. The high-frequency region from 0.56 THz to 1.01 THz was measured by increasing the wavelength difference between lasing modes of the detuned DML. The operating conditions of the detuned DML were fixed to 0.4–1.01 THz except for the μ-heater currents to tune the optical beat frequency. At the low-frequency region, the operating current of the DFB1 section was changed slightly to maintain the power balance. The origin of the slight deviation of THz power near 0.5 THz is still not clear, but it is under investigation. The inset of Fig. 6 shows the THz waveforms of 0.315 and 0.99 THz. The THz signal at 0.99 THz seems to be unstable. However, this result is from the low signal-to-noise ratio at the high-frequency region. We also measured the thermal tuning speed of the detuned DML. It shows fast tuning speed of about 370 GHz/10 ms. This result is comparable to the experimental result in previous literature . In CW-THz measurement, integration time of 300 ms for each point was used.
The THz output power fell to the thermal radiation level of the photomixer beyond 1 THz; this was possibly due to the long carrier lifetime (~7 ps) of the LTG-InGaAs layer as well as the background amplified spontaneous emission (ASE) from the EDFA. Although the bandwidth and output power were not enough for practical applications, we believe that if the structure of the detuned DML and material properties of LTG-InGaAs were carefully optimized, the THz output power and bandwidth would be greatly enhanced.
We successfully demonstrated a continuous-wave (CW) THz system with a detuned dual-mode laser (DML) and low-temperature-grown (LTG) InGaAs-based photomixers. In the monolithic detuned DML, each wavelength of the two modes can be independently tuned by adjusting the currents in the micro-heaters. The detuned DML showed a beat frequency tuning range of up to 1 THz. Continuous frequency tunings of the CW THz emissions from 0.315 THz to over 1 THz were also successfully achieved by using the LTG-InGaAs photomixers. Our results show that photomixing using the detuned DML is very promising for the realization of compact and cost-effective tunable CW THz systems.
This work was supported by Joint Research Project of ISTK and the Public Welfare & Safety Research program through the National Research Foundation of Korea (NRF), by the Ministry of Education, Science and Technology—grant #2010-0020822 and also by the Global Research Laboratory Program of NRF (No. 2007-00011).
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