1. Introduction

Terahertz (THz) waves have received much interest because of their biological and chemical spectral fingerprints and relatively long penetration depth for non-destructive inspection [1–3]. Moreover, recent progress of THz is pursuing practical applications outside of the lab environment with high frame-rate imaging and spectroscopy [4, 5]. For compact and low-cost equipment, photonic-based continuous-wave (CW)-THz systems have been reported with potential advantages using well-developed fiber-optic telecom technologies [6–12]. In general, a CW-THz system has high resolution frequency selectivity in a wide frequency range and integration compatibility [7, 8]. In addition, a homodyne detection scheme using electro-optic (EO) modulators enables fast THz phase control and vector detection without physically moving parts, such as a mechanical delay or chopper, even with free-running CW lasers [9–12]. However, fiber-optic technology is still too bulky to be used in hand-held equipment and the long fiber length can seriously degrade the phase stability of systems in vector detection [10, 12].

Silicon-based photonics is drawing considerable interest as an emerging technology for high-speed optical I/O and microelectronics because almost all active and passive photonic components except lasers can be monolithically integrated with silicon electronics [13–17]. In this paper, we present the first implementation of a photo-mixing-based CW-THz vector spectroscopy and imaging system that uses silicon photonics for compactness and stability. The photonic THz phase control circuit, including electro-optic (EO) modulators, couplers, and waveguides, is monolithically integrated on 2-mm²-area of silicon-on-insulator (SOI), which replaces fiber-optic parts in previously reported systems [10, 12]. The phase control circuit uses carrier-injection EO modulators for high-speed operation, where a balanced configuration using a modulator for each lightwave reduces the serious intensity distortion from each modulator. Using an InGaAs photoconductive receiver [11], wideband uni-traveling photodiode (UTC-PD) [18], and the integrated phase control circuit, we experimentally demonstrated THz generation and fast vector measurement. The dynamic ranges of 65 and 35 dB are obtained at 300 GHz and 1 THz with 1-ms integration time, which is comparable with that of our previous report [12]. Moreover, the integrated phase control circuit on silicon photonics greatly enhances the phase stability of the system, resulting in less than ± 10 degree phase variation over a period of 2 h.

2. THz phase control using silicon photonic circuit

The photonic-based CW-THz homodyne spectroscopy system generates THz waves by photo-mixing lightwaves from two lasers [10]. For coherent detection, the relative phases of two THz waves are linearly changed by photonic phase control so that the detected signal forms a sinewave in which the THz phase and intensity responses of a sample can be identified. In this work, the photonic-THz phase control circuit was integrated on the SOI photonic platform using the reported fabrication technology [19].

Figure 1 shows a schematic diagram of the silicon photonic circuit, which consists of rib-type silicon-wire waveguides, carrier-injection EO modulators, and multimode-interferometer (MMI) couplers. Because the silicon waveguide and silica fiber have a huge difference in core size, spot-size converters (SSCs) are configured on the chip facets to achieve low-loss fiber coupling. In the SSC, the lightwave mode in a low-index waveguide is guided into a channel-type silicon wire using an inverse taper, and then the silicon wire is changed to a rib-type. Input lightwaves (λ₁, λ₂) inserted via SSCs are divided by MMI couplers and guided for two outputs of a photonic THz reference and probe. A photonic THz signal is a two-tone optical signal that can generate a THz wave by photo-mixing, where the frequency of the THz wave is the same as the wavelength difference between the two tones. Because both the photonic THz probe and reference are generated with the same laser sources, the two photonic THz signals are correlated with each other. The EO modulators are composed of p-i-n junctions along the silicon-wire waveguides [20], where the current bias can be used for high-speed modulations of the two lightwaves for the photonic THz probe. Therefore, after conversion from the photonic to the THz domain, the relative intensity and phase modulations of the THz probe compared to those of the THz reference are the same as the differential phase modulation and the product of the intensity modulations for each lightwave.

 

Fig. 1 Schematic diagram and operation principle of photonic-THz phase control circuit integrated on the silicon-on-insulator (SOI) photonic platform.

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Figure 2(a) shows static phase and intensity responses of an EO modulator as a function of applied current bias, which were measured with a fabricated device having 5-mm length. By increasing the bias current, the free carrier concentration changes the refractive index inside the two-dimensional silicon waveguide so that the phase response of the modulator increases according to the current bias and device length [21]. However, as an undesired side effect of the phase modulation, increasing the free carrier concentration in the waveguide also causes lightwave absorption.

 

Fig. 2 (a) Measured static phase shift and intensity loss of EO modulator as a function of current bias. Simulated waveforms of the THz probe signal after homodyne detection using (b) single-ended EO modulation and (c) differential EO modulation. I_π is π-radian drive current.

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Figure 2(b) shows the simulated waveform of detected signal in homodyne detection of the THz probe using the THz reference. When only a single EO modulator is driven by a saw-tooth current whose magnitude is two times that of the π-radian drive current (I_π), the phase of the THz probe rotates 360° and the detected signal is a sinewave. At the same time, the simultaneous lightwave absorption by the current modulation distorts the resulting waveform and causes about a 20% intensity error and about a 10° phase error in simple lock-in measurement. To reduce the signal distortion caused by the intensity modulation, the two EO modulators can be driven by differential current bias. When the magnitude of the differential saw-tooth current is the same as the π-radian drive current, the THz phase rotates 360° and the intensity distortion from the two EO modulators is normalized as shown in Fig. 2(c). In this simulation, the intensity and phase errors in simple lock-in measurement are under 1% and 0.5°, respectively.

Figure 3 shows a chip micrograph of the fabricated photonic-THz phase control circuit on SOI substrate. The core area of the integrated chip is about 2 × 2 mm². The insets show a MMI coupler and a rib-type silicon-wire waveguide. The MMI coupler consists of waveguide ports and a 5.7 × 2.4 μm² rectangle structure. The rib-type silicon-wire waveguide has a 600 x 200 nm core and 100-nm-thick slab. The silicon wires are extended to both side facets of the chip for optical inputs and outputs. For electrical current driving of the EO modulator, metal pads were fabricated on both sides of each 1-mm-long modulator. The measured insertion loss of an MMI coupler is about 3.3 dB, and the propagation loss of the silicon waveguide is about 0.2 dB/mm.

 

Fig. 3 Chip micrograph of fabricated photonic phase control circuit and SEMs of MMI coupler and Si rib waveguide.

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3. CW-THz vector spectroscopy

Figure 4 shows the experimental setup for the CW-THz homodyne spectroscopy system using the fabricated silicon photonic control circuit. Two individual lightwaves are generated using a fixed-wavelength DFB laser and a tunable laser (TLS). Two arrayed fiber blocks are used for optical input and output couplings for the fabricated chip with polarization controllers due to the polarization dependence of the silicon waveguide. Using an on-wafer probe, the modulators are driven by differential saw-tooth currents at a frequency of 40 kHz. As shown in the inset of Fig. 4, the spectrum of the photonic THz probe has two optical modes whose spacing is the same as the resulting THz frequency after photo-mixing in the UTC-PD. The measured total power loss of the photonic circuit is about 10 dB, including the two MMI couplers with 3.3-dB insertion loss in each, the two fiber blocks with 1.1-dB coupling loss in each, and waveguide loss. In addition, the photonic THz probe experiences additional 3-dB loss caused by the EO modulators, including the propagation loss and the absorption by free carriers. The EDFAs are used for both the photonic THz probe and reference to compensate for the power loss throughout the photonic circuit and setup.

 

Fig. 4 Experimental setup for CW-THz homodyne spectroscopy system with on-wafer optical and electrical probing of fabricated silicon photonics chip for THz phase control.

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The THz probe emitted by the UTC-PD passes through a sample and is received by an InGaAs-based photoconductive antenna (PCA), where the THz probe is mixed with the separately delivered photonic THz reference to achieve homodyne detection. Because the relative phase of the THz probe linearly rotates compared to the THz reference, the detected signal forms a sinewave at the modulation frequency of 40 kHz. At the same time, the phase and intensity responses of the THz probe from the inserted sample directly change those of the detected signal so that lock-in detection using the EO control signal as a reference enables continuous readout of the THz responses. With the continuous data readout, spectroscopy and imaging can be done by laser sweeping or raster scanning of the sample position. The measuring time for a data point can be freely selected depending upon the desired signal-to-noise(S/N) ratio or measurement speed.

Figure 5(a) shows a time-domain waveform of the detected PCA output signal when the THz probe is at 0.3 THz. As expected from the simulation, the differential current control allows sinewave output from the receiver without severe distortion of intensity. Therefore, the lock-in detector can read out the phase and magnitude values without a large error. The frequency response of the CW-THz homodyne system was obtained with linear wavelength tuning of the TLS and synchronized sampling of the magnitude values, as shown in Fig. 5(b). The measured signal intensity linearly decreases as frequency increases due to the frequency responses of the UTC-PD and PCA, including water vapor absorption through about 30-cm-long free space and the responses of the four lenses. The thermal noise level was measured with 1-ms integration time without THz emission from the UTC-PD, where the dynamic range of system in the same integration time is comparable with that of our previous report [12]. Although the thermal noise can be suppressed by using a longer integration time of over 1 ms, the parasitic intensity modulation in the EO modulators is not fully suppressed in this design so that leakage noise can limit the dynamic range [10]. This problem originates from the huge nonlinearity of the EO modulator and can be solved by enhancing the linearity of the device, e.g [22].

 

Fig. 5 Measured (a) waveform of detected signal, (b) frequency response of THz spectroscopy system with a 30-cm-long air path, and (c) variation of phase response for 1 h. (d) Comparison of mean phase variation to earlier two fiber-optic setups using 0.4- and 5-m-long optical paths. Δφ is phase variation, δn/δT is the temperature coefficient of the refractive index, ΔT is temperature variation in the photonic circuit, and L is optical path length.

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In a spectroscopy and imaging system, the measurement stability is an important figure of merit. In particular, a homodyne system using EO modulation needs separate lightwave paths for phase control, where the phase of the lightwave is very sensitive to environmental temperature ambiguity. In this work, the integrated phase control circuit with short optical path length of about 2 mm has an advantage of a stable phase response. Figure 5(c) shows the long-term variation of the measured phase response. Although the silicon photonic chip is open to air with on-wafer probing, the measured phase response is very stable with variation of less than ± 10° over a period of 2 h. Moreover, the typical measurement time using the CW spectroscopy and imaging system is less than 1 min (e.g., 15 s for a spectrum from 0.2 to 2 THz with 2-GHz steps; 40 s for a 64 × 40 image at a fixed frequency, with 3-ms integration time for each frequency or pixel [12].) Figure 5(d) shows the measured RMS phase variation of THz homodyne systems using phase control setups with different optical path lengths. The phase stability strongly depends on the setup size and the thermal package. Therefore, even though the silicon waveguide has about a ten times higher temperature coefficient than optical fiber, the very short optical path length reduces the phase variation considerably. Moreover, the RMS phase variation is expected to decrease to about 1° by applying a thermal package on the silicon chip.

4. Conclusion

The first silicon-photonic integration of photonic continuous-wave THz generation and phase control functions was demonstrated for a compact and fast THz vector spectroscopy system. The integrated silicon circuit has only a 2-mm² footprint and supports fast and linear phase control of THz waves by using differential carrier-injection EO modulators for each lightwave. Using the silicon-photonic circuit, we successfully demonstrated a THz spectrometer having dynamic ranges of 65 and 35 dB at 300 GHz and 1 THz with 1-ms integration time and stable phase response of less than ± 10° even without thermal packaging. Our results point to the possibility of a compact THz spectrometer based on photonic integration and its huge potential for out-of-laboratory applications. The utilization of InP-on-silicon hybrids comprising a laser diode, UTC-PD, SOA, and PCA is the next step towards the future development of a fully integrated THz spectrometer using monolithic silicon photonic devices.