We present a continuous-wave terahertz (THz) vector spectroscopy and imaging system based on a 1.5-µm fiber optic uni-traveling-carrier photodiode and InGaAs photo-conductive receiver. Using electro-optic (EO) phase modulators for THz phase control with shortened optical paths, the system achieves fast vector measurement with effective phase stabilization. Dynamic ranges of 100 dB·Hz and 75 dB·Hz at 300 GHz and 1 THz, and phase stability of 1.5° per minute are obtained. With the simultaneous measurement of absorbance and relative permittivity, we demonstrate non-destructive analyses of pharmaceutical cocrystals inside tablets within a few minutes.
© 2014 Optical Society of America
Terahertz (THz) waves have received much attention due to their interesting features such as spectral fingerprints of biological or chemical materials and a relatively long penetration depth for spectroscopy and imaging . Concurrently, many potential applications, such as non-destructive inspection of pharmaceuticals, drug detection, material testing, and safety screening, are being investigated [2–5]. Many of these practical applications need high-frame-rate imaging close to real time and adaptive spectroscopy in the test protocol. Fast measurement is also important for reducing instabilities of samples and the setup with time.
One potential advantage of a continuous-wave (CW) system is frequency selectivity with high and easily scalable spectral resolution, which has stimulated a lot of active research using electrical MMICs , quantum cascade lasers , or photo-mixing [7, 8]. Among them, the photo-mixing THz generation method can support the widest frequency range for phase-sensitive spectroscopy and imaging simply with free-running CW lasers by using homodyne detection [9, 10]. In addition, in the 1.55-µm-wavelength range, the well-developed electro-optic (EO) devices, such as compact dual-mode tunable lasers, have merits in cost and size .
A homodyne detection scheme requires phase rotation of THz waves for acquisition of a complete waveform, which has been achieved with a mechanical delay stage [9, 10]. However, the low speed of mechanical stages limits the acquisition time and requires additional magnitude chopping to avoid the flicker noise of the detector. Moreover, the bulky delay stage prevents compact all-fiber implementation. An alternative approach is to utilize an EO phase modulator. Because the frequency or phase modulation of a lightwave before the photo-mixing is directly proportional to that of the resulting THz wave, EO phase modulation enables fast THz phase control  and synchronized lock-in detection [13, 14]. In addition, the phase-sensitive detection can enhance the contrast by using depth information [15, 16]. However, in the early versions of the system, the use of a long-fiber setup caused environmental phase instability which prevented the practical use of the phase information.
In this paper, we present a CW-THz vector spectroscopy system using EO phase modulation which enables fast and simultaneous measurement of absorbance and relative permittivity for spectroscopy and imaging applications at up to 2 THz. To stabilize the system, the environmental phase changes have been suppressed by reducing the optical path length for THz phase control. By combining an advanced InGaAs photoconductive receiver  and a wideband uni-traveling-carrier photodiode (UTC-PD) , high dynamic ranges of 100 and 75 dB·Hz are obtained at 300 GHz and 1 THz. Thus, fast vector measurement with millisecond-order data acquisition is achieved for both spectroscopy and raster-scan imaging in a frequency range of more than 2 THz. The short measurement time also reduces errors caused by environmental instability of the system. Using the THz spectroscopy and imaging system, a complex-domain characterization of pharmaceutical cocrystal inside a tablet  has been demonstrated.
2. CW-THz homodyne system for fast vector measurement
Figure 1 shows the configuration of the CW-THz homodyne spectroscopy system . For THz wave generation, two individual lightwaves from a fixed-wavelength DFB laser and a tunable laser (TLS) are combined and then photo-mixed in a UTC-PD. The EDFAs are used for both input lightwaves to compensate for the power loss throughout the fiber setup. In practical implementation, the two lighwave sources can be replaced with other combinations like two tunable DFB lasers or a dual-mode laser. For homodyne detection, the identical lightwaves are also delivered to the InGaAs-based photoconductive antenna (PCA). When the THz wave from the UTC-PD arrives at the PCA through free space, the PCA generates the detected signal by mixing the received THz-wave and the photonic THz wave propagating through the other path. In the free space, the THz wave emitted from the UTC-PD module integrated with a silicon lens is guided with lenses for beam focusing on the sample and the PCA. Both of the emitter and detector have a bow-tie antenna for broadband mode matching with polarization control of the radiated beam to minimize path loss.
For coherent detection, the phase of the photonic THz wave is linearly rotated by using EO phase modulators driven by saw-tooth signals. Then, a sine wave is formed in the detected signal by the continuous changing of the phase difference between the two THz waves. Both the phase and intensity responses of the THz wave passing through the sample directly appear in the detected signal. When the differential saw-tooth signals drive the two EO phase modulators, the frequency of THz phase rotation and the detected signal is an integer multiplication of the saw-tooth frequency. The balanced configuration also has a merit of balancing the path length from the source to detector, which suppresses the phase noise caused by the unlocked laser sources.
Because the detected signal at fixed frequency contains the phase and intensity responses, simple lock-in detection using the saw-tooth control signal as a reference clock enables fast and continuous readout of the THz responses in each period of the E/O phase control. In this system, the lock-in integration time for measuring a data point can be freely selected depending on the desired signal-to-noise ratio (S/N) or measurement speed. With the continuous data readout, spectroscopy and imaging can be done by a laser sweep or raster scan of the sample position. In both of these operations, the phase and intensity responses are always obtained individually.
The frequency response of the CW-THz homodyne system was measured from 0.2 to 2 THz with linear wavelength tuning of the TLS and synchronized lock-in detection of the output signal as shown in Figs. 2(a) and 2(b). The integration time of the lock-in detection was 3 ms. Because the lock-in detector used in the experiment needs a guide interval for each data point, the 64 data points were obtained in 1 s so that the total sweep time was about 15 s. for 900 frequency points with about 2-GHz steps. The measured signal linearly decreases as frequency increases due to the frequency response of the UTC-PD and PCA. At the same time, there is linear shift of the phase response due to the fixed offset of the travel lengths for the two THz waves.
Because the THz wave path contains about 30-cm-long free space between the emitter and detector, the fingerprint of water vapor appears in both the intensity and phase responses as line absorption and discontinuity due to resonance, respectively. The water vapor absorption coincides well with a previous report . To reveal the system’s dynamic range, the noise level was measured with 1-s integration time in the same frequency range without THz emission from the UTC-PD. The measured dynamic ranges are about 100 dB·Hz and 75 dB·Hz at 0.3 and 1 THz, respectively. The noise level is inversely proportional to the integration time. The obtained high dynamic range can support a high S/N even in a short integration time, e.g., 50 dB at 1 THz with 3-ms integration, enabling fast spectroscopy and imaging.
In a spectroscopy and imaging system, measurement stability is another important figure of merit. This is especially true in a homodyne system with EO phase modulator: The separated lightwave propagates through individual paths from the power dividers after the EDFA to the power combiners before the emitter and detector. Because the lightwave has very short wavelength, environmental temperature ambiguity can induce very sensitive phase variation. To enhance the stability, one potential solution is to miniaturize the optical path by photonic integration or by using short fiber components, especially in the critical section from the optical power divider to combiner. Another is to achieve fast measurement, because the stability of the setup only needs to be maintained during the measurement time.
For the phase stability, short fiber components were used in the optical phase control and distribution section from the power divider to combiner, of which total length is about 40 cm. The fiber section was insulated in a box for thermal stability. Compared to our earlier version, in which the total fiber length was about 5 m , the setup size is reduced by ten times, resulting in a corresponding stabilization of the phase response. Figures 3(a) and 3(b) shows the variation of the measured phase and intensity responses for 1 h in the open space of the experimental room. Although the phase response is largely changed as about ± 20° for 1 h, the expected phase variation inside 1 min is only about 1.5°. Therefore, in a high-speed measurement for less than a minute, the effect of environmental phase variation is negligible. The measured intensity stability is dominated by short-term variation of about 0.67%, which is equivalent to relative intensity noise of about −50 dB.
3. Fast vector spectroscopy and imaging demonstration for pharmaceutical tablet
The CW-THz homodyne system was used for vector spectroscopy and imaging of a pharmaceutical multi-component tablet. Figure 4(a) shows a photograph of the tablet. The tablet is composed of three sections, which cannot be well identified with a visual photograph. The sections are commonly based on polyethylene (PE) binder with different caffeine:oxalic-acid (Caf:Oxa) cocrystal concentrations of 0, 20 and 40%, respectively. Cocrystallization, a method of manipulating the crystal structure, can change the physicochemical properties of the active pharmaceutical ingredient, such as bioavailability. Previously, we demonstrated the advantages of THz spectroscopy for distinguishing the crystal structures of pharmaceuticals in a tablet . The diameter and thickness of the tablet are 10 and 1 mm, respectively.
Figures 4(b) and 4(c) show the intensity and phase images of the tablet at several different frequencies, measured with a raster scan of the sample position by continuously moving the translation stage. The imaging resolution is 64 × 40 and the measurement time for a pair of intensity and phase images at a fixed frequency point is about 40 s. For calibration, the reference intensity and phase response of the air path were measured at the frequency before each imaging was taken. The intensity images are plotted as relative transmittance, which was obtained by subtracting the reference intensity of the air path from the measured raw data. In the phase images, the relative phase differences of each pixel from the center of the tablet are plotted. In the intensity images, we can see the high absorption property in both side areas of the tablet at around 1.4 THz and the losses at the borderlines. In the phase images, the changes of the phase response between the areas more directly show that there are different materials having different refractive indexes. With the phase images, the intensity losses at the borderline can be identified as a scattering effect. From 1.2 to 1.6 THz, the phase responses do not increase linearly, which means that the refractive index is distorted in this frequency region. Therefore, from the intensity and phase images, we can identify the divided areas of the tablet and the different concentrations of the Caf:Oxa cocrytal in both side areas, which has absorption peak at 1.4 THz.
Figures 5(a) and (b) show THz absorbance and relative permittivity calculated from measured THz intensity and phase spectra, respectively. The spectroscopic measurements were performed only at a sampled spatial point for each area of the tablet shown in Fig. 4(a). The spectroscopy results were obtained by subtracting the response of the air path as a reference from the measured sample responses. With integration time of 3 ms, measurement of single pair of intensity and phase spectra from 0.2 to 2 THz takes about 15 s for 900 frequency points with about 2-GHz steps. The absorbance is defined as the power loss of the sample on a logarithmic scale, and the relative permittivity was calculated from the measured phase spectra by using a simple wave equation.
In Fig. 5(a), the absorption peaks around 1.4 and 1.2 THz show linear dependence on the concentration of the Caf:Oxa cocrystal over the nearly transparent response of the PE. These results agree well with our previous result obtained using THz time-domain spectroscopy. In Fig. 5(b), the relative permittivity based on phase spectra also clearly shows the resonance effect around 1.4 and 1.2 THz. Furthermore, an increase of the relative permittivity from the pure PE to the mixture with the concentration of Caf:Oxa cocrystal can be observed. This suggests that, in the vector spectroscopy, the absorbance and relative permittivity can serve as complementary markers for material specification.
A continuous-wave THz vector spectroscopy and imaging system was implemented using 1.55-µm fiber optics, including EO phase modulators, a UTC-PD and an InGaAs PCA. The reduced size of optical fiber setup enhances the phase stability of the system by more than ten times, resulting in phase error of 1.5° per minute. High dynamic ranges of 100 and 75 dB·Hz obtained at 300 GHz and 1 THz allow fast millisecond-order data acquisition and both the spectroscopy and imaging to be performed inside 1 min. Using the vector spectroscopy and imaging system, we demonstrated non-destructive characterization, including absorbance and relative permittivity for pharmaceutical cocrystals inside tablets at up to 2 THz, emphasizing the flexibility of the continuous-wave system for practical THz applications.
The authors thank Dr. T. Ishibashi and Dr. A. Wakatsuki of NTT, for providing the UTC-PDs, and Dr. O. Kagami of NTT for his encouragement.
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