We present Terahertz (THz) imaging with a 1D multichannel time-domain spectroscopy (TDS) system which operates with a photoconductive array of 15 detection channels excited by a 1030 nm femtosecond fiber laser. The emitter and detector are photoconductive antennas based on InGaAs/InAlAs multi-layer heterostructures (MLHS). We characterized the THz optics and the resolution of the system. The performance is demonstrated by the multichannel imaging of two samples. A simultaneous measurement of 15 THz pulses with a pixel pitch of 1 mm increases the measurement speed of the TDS system by factor 15.
© 2014 Optical Society of America
Terahertz (THz) radiation gives an opportunity to perform non-destructive spectroscopy, imaging or tomography measurements in a large variety of the applications fields such as quality control, security technologies, biosensor or medical imaging [1,2]. The advantages of THz time-domain spectroscopy (TDS) systems have led to a rapid proliferation of researchers studying properties and phenomena in the frequency range between infrared and microwave radiation [3–5]. The increased industrial interest in THz imaging or tomography measurements has pushed the development of imaging systems based on the principle of THz TDS [6–10]. Nevertheless, all commercially available systems are limited to single pixel detection, e.g. with a photoconductive antenna. Thus, THz TDS imaging measurements often imply sample scanning in the focus of a THz beam path and a measurement requires form several minutes up to hours. The measurement time also depends on the sample size, scan parameters and data acquisition techniques [11,12]. Therefore, the development of multichannel systems to speed up the measurement in THz TDS systems provides an opportunity for significant improvement.
Herrmann et al. presented an electronic circuit for the simultaneous readout of 8 photoconductive antenna signals (low temperature grown (LT)-GaAs). They performed a multichannel measurement after placing the detector array in the expanded THz beam behind the THz optics  which was rather configured for a one channel system. Pradarutti et al. presented the functionality of a 16 channel detector antenna made of LT-GaAs operating at 530 nm wavelength excited with the frequency doubled radiation of an ultrashort pulse fiber laser . The antenna was also placed in an expanded THz beam behind the THz optics, but additionally illuminated with the use of a micro lens array. The development of a multichannel emitter array with horn-type antennas on photoconductive switches was shown in .
In  we presented a complex concept of a multichannel TDS system based on a femtosecond fiber laser (1060 nm) and specially designed THz optics which generates and images a THz line focus to overcome optical aberrations in the THz beam path. Despite the successful proof-of-principle operation, the system at the time suffered from the lack of suitable semiconductor material optimized for 1030 nm operation. In two recent publications, the THz emitters and detectors based on LT-InGaAs/InAlAs multi-layer heterostructures (MLHS) have been shown to be suitable for 1030/1060 nm central wavelengths excitation and successfully demonstrated in single-pixel systems [17,18].
In this paper we present the first 1D multichannel THz TDS system with 15 detection channels excited by 1030 nm wavelength of a femtosecond fiber laser. The experimental setup of the THz system and single components of the THz and infrared optics are described in section 2. In section 3 we show the characterization of the beam profile in the THz line focus and 15 THz pulses, which were measured simultaneously with the detector array. Furthermore, we determined the resolution and the beam quality of the detector channels with a knife edge method along the optical axis in the THz focus and a second measurement of a metallic sample with different gaps up to 100 µm. In section 4 we demonstrate the results of the first THz multichannel imaging measurements where we used a metallic Siemens star and a plastic pump wheel as samples. The conclusion is presented in section 5.
2. Experimental setup
The experimental setup of the multichannel THz system is shown in Fig. 1. It consists of an ytterbium (Yb)-doped ultrashort pulses fiber laser system with a passively mode locked oscillator from Active Fiber Systems GmbH. The amplifier is an Yb-doped rod-type fiber with a length of 80 cm and a mode field diameter of 50 µm. The wavelength of the pump laser is 976 nm. The pulse compression is realized after the amplification. Thus, the fiber laser system receives an average output power of about 4.5 W and generates laser pulses with 94 fs width, 20 MHz repetition rate and energies of about 223 nJ.
The laser radiation is split into a pump and probe beam which illuminates the THz emitter and THz detector, respectively. A beam splitter and filters are used to reduce the laser power of 4.5 W and to avoid the destruction of the emitter and detector material. A motorized stage is used as an optical delay for the time resolved measurement of the THz pulses. The photoconductive emitter based on a molecular beam epitaxy grown high mobility InGaAs/InAlAs MLHS. The MLHS consists of 100 periods of a 8 nm thick InAlAs layer sandwiched between 12 nm InGaAs layers grown lattice matched on an InP substrate at a growth temperature of approximately 400°C [19,20]. The antenna structure is a 400 µm gap mesa-structured stripline antenna . The emitter is illuminated with 80 mW focused by an aspheric lens (f = 15.29 mm), and biased with ± 125 V at 1.1 kHz. The silicon lens on the THz emitter is a hyper hemispherical lens made of high-resistive float zone (HRFZ) silicon with a diameter of 6 mm and thickness of 3.45 mm. The THz optics consists of a 90° off-axis parabolic mirror with an effective focal length of 127 mm, which collimates the radiation of the THz emitter. A cylindrical mirror with an effective focal length (EFL) of 127 mm generates a line focus (LF) in the THz beam path. Two aspheric lenses made of Zeonex480R® image the THz line focus to the detector. The material Zeonex480R® has a refractive index of about 1.515 (from 0.2 to 1.5 THz) and an absorption coefficient of about 0.2 cm−1 at 1 THz . The lenses were designed with the optical design software Zemax and manufactured at the Fraunhofer IOF by ultra-precision turning. The lenses are diffraction limited for wavelengths down to 600 µm and a field up to 63.5 mm. A numerical aperture of 0.36 is realized. The effective focal length of one lens is about 95.8 mm in horizontal direction. The optimal arrangement and the working distances of the THz optics are shown in Fig. 2.A cylindrical HRFZ silicon lens was placed in front of the THz detector to focus the THz radiation on the photoconductive antenna array. The lens has a radius of 6.3 mm and a thickness of 6 mm. A detailed description of the imaging part of the THz optics was presented in , the quasi optical design of THz optics is shown in [22,24].
A telescope in the detector beam path, which features two cylinder lenses with focal lengths of f1 = 12.7 mm and f2 = 100 mm, forms about 350 mW of the laser power into a line focus of a length of about 20 mm. A third cylinder lens (f3 = 12.7 mm) focuses the laser radiation on the detector gaps with a focus height of about 0.5 µm. Because of the gap size [Fig. 3] only about 2.6 mW of the laser radiation illuminates the gap.
The THz detector material is made of Be-doped LT-grown (130°C) InGaAs/InAlAs MLHS . The detector consists of 15 mesa-type dipole antennas with a total length of 15 mm [Fig. 3]. The distance between each channel is scalable and depends on the optical design (IR and THz optics). For this experiment it was fixed to 1 mm. The THz detector is placed in horizontal direction (x-direction) and the output is connected to transimpedance amplifiers. A scalable multichannel lock-in amplifier (LIA), which is in-house technology of the Fraunhofer IOF , is used for the measurements. Several channels can be readout almost simultaneously due to the multiplexing technology. Here, every 8 detection channels are connected to one electronic board in a parallel bus system. The multiplexer devices work with triggering and a data acquisition program in LabVIEW, which separates the signals of 8 channels.
THz imaging and tomography measurements can be performed by placing a sample on motorized stages into the line focus of the THz optics.
3. Characterization of the THz system
3.1 THz line focus
In order to characterize the beam profile of the THz line focus we used a fiber coupled photoconductive antenna  to analyze the optical design of the THz beam path. The maximum THz pulse amplitude values and the position of the THz pulses were extracted to characterize the full width at half maximum (FWHM) of the line focus and the optical delay of the pulses. Figure 4 shows the results of the measurements at a distance of 106 mm from the cylindrical mirror. The distance was chosen to be shorter than the effective focal length (127 mm) of the cylinder mirror due to the silicon lens placed in front of the fiber coupled detector. The lens shortens the focal length and therefore, we calculated the optimal position with an optical design software (Zemax). Furthermore, the values of the measured THz focus must be understood as an approximation, since the silicon lens on the detector averages the THz radiation over a defined solid angle.
The FWHM of the THz focus was measured to be about 37 mm in the x direction (horizontal) and 10 mm in y direction (vertical) relatively to the optical axis of the THz beam path. Figure 4(b) shows the temporal deviations of the THz pulses with the range in the whole measurement area of about ± 0.39 ps ( ± 0.117 mm) and in the middle horizontal direction of about ± 0.13 ps ( ± 0.039 mm). The time shift is introduced by the optical aberrations of the parabolic and cylindrical mirror as well as the position errors of the emitter in front of the 90° off-axis parabolic mirror. Thus, a normalization step for the image calculation has to be applied for each multichannel measurement [section 4].
3.2 THz pulse measurement
Figure 5 shows a THz measurement over the time window of 40 ps, performed with 15 detection channels. The scan was executed with an optical delay of 1 ps/s and LIA averaging time of 10 ms. The pulses are shifted by 2 ps for clarity. Due to the imaging optics each THz pulse represents a point at the horizontal THz line focus with a lateral shift of 1 mm. The maximum amplitude values and positions of the THz pulses vary between the THz detector channels, which was expected after the optical design and system characterization in 3.1 [Fig. 4]. The signal to noise ratio of the maximum THz pulse amplitude in the middle of the detector (channel 8 and 9) was estimated to be about 39 dB, whereas in the edge regions of the detector array we measured a signal to noise ratio of 35 dB, both for a LIA average time of 30 ms.
Figure 6 demonstrates the amplitude maxima and its positions (time) of 15 measured THz pulses compared to simulation data, which was obtained with the optical design software (Zemax). First, we simulated the intensity profile behind the cylindrical silicon lens in horizontal direction [Fig. 6(a)]. The simulation was normalized to the maximum amplitudes of the pulses for a better comparison. The measurement and simulation match well together and exhibit their maximum values in the middle of the detector antenna. The amplitude values decrease to the edge region by about 34%, which depends on the irradiation pattern of the emitter. Further, we compared the position of the maximum pulse amplitudes with the simulation of the field of curvature. Channel 1 and 15 are delayed by about 1.5 ps relative to channel 8 in the middle. The simulated curve agrees quite well with the measured values and the delay between channel 1 and 15 compared to channel 8 is only about 0.93 ps. The reasons for the slight discrepancy between measurement and simulation are most likely the optical aberrations introduced by the THz optics, the IR telescope optics or adjustment errors of the THz system.
Figure 7 presents the THz signal measured with channel 8 (black line) in more detail. It was scanned with a time window of 40 ps, an optical delay velocity of 1 ps/s and LIA averaging time of 10 ms. The main pulse at 9 ps is followed by a second pulse at 36 ps, which is introduced by the THz optics of the system [Fig. 7(a)]. We compared the THz signal of channel 8 to an earlier measurement (red line) with a standard one channel TDS system at 1030 nm, which was presented in . The emitter and detector in the one channel TDS system were made of the same material. The THz optics consisted of two 90° off-axis parabolic mirrors (EFL = 101.6 mm), as well as hyper hemispherical silicon lenses that are placed in front of the emitter and detector antennas. According to the graph, the multichannel signal is 500 times lower compared to the one channel TDS system. The normalization refers to the sensitivity range of a standard single channel LIA (SR830, Stanford Research Systems) and was necessary to compare the initial one channel TDS measurement with the multichannel experiments, which were measured with a multichannel LIA. Figure 7(b) shows the spectra of both pulse measurements. The spectrum of channel 8 was calculated with the Fourier transform of the time window from 0 to 30 ps to avoid oscillations caused by the second pulse. The spectral bandwidth reaches up to 0.8 THz instead of 2.5 THz of the one channel TDS measurement, which depends on the broadening of the THz pulse.
There are various reasons for the relative low THz signals and the spectral bandwidth limitations in the multichannel TDS system. For example, the emitter generates a THz line focus with a FWHM diameter of about 37 mm [section 3.1], but each detector channel can only capture a small part of it [section 3.3]. Furthermore, the two Zeonex480R® lenses absorb the THz radiation and the refractive index of about 1.52 generates reflection losses at the surfaces. The cylindrical silicon lens, which is placed in front of the detector array, limits the focusing of the THz radiation to the vertical direction [see Fig. 2] and parts of the THz radiation are reflected because of the high refractive index step between air and HRFZ silicon (3.417) .
3.3 Resolution of the multichannel TDS system
The THz optics image only a part of the THz line focus on each detector channel. We characterized the THz beam path with the knife edge method  to define the local full width at half maximum (FWHM) beam waists and depth of focus for each detector channel. Figure 8 shows the maxima of the THz pulse amplitudes in the line focus of the THz optics. During the measurement, a metal blade was shifted vertical and horizontal with a step width of 0.3 mm. The amplitudes of the pulses decrease, if the metal blade hits the focus. With the adaptation of the integration of the Gaussian beam function  it is possible to calculate the beam waists at the measured focus positions to the measured curvatures. As expected, the vertical knife edge measurement shows equal curvatures for the detector channels. The amplitude differences are caused by the THz beam profile [Fig. 4]. In contrast, the horizontal measurements show the curvatures shifted by 1mm in horizontal direction, which corresponds to the geometrical distances of 1 mm between the detector channels. Again the maximum amplitude values are varying because of the THz beam profile.
The value of the FWHM beam diameter in vertical direction (y) is about 3.3 mm for the maximum THz pulse amplitude in the focus of the THz optics and is uniform for all detector channels. The vertical depth of focus is about 27 mm. In the horizontal direction (x) the part of the THz line focus, which is detected by each detector channel, has a FWHM beam diameter of about 2.7 mm and is also uniform for each detector. The horizontal depth of focus is about 25 mm. Figure 9 shows the beam profiles recorded by the channel 8 representative for the multichannel detector array along the optical axis of the THz beam path. The negative axis is in the emitter and the positive axis in the detector direction. The focus between vertical and horizontal beam profile is shifted by about 5 mm, which is caused by the astigmatism of the THz optics.
Despite the FWHM beam diameters greater than 2 mm, the time resolved measurement of THz pulses enables the detection with remarkable resolutions. To provide a proof, we measured a metallic sample with gaps of different distances from 5 mm to 100 µm [Fig. 10(a)]. The sample was placed in the focus of the THz beam path and scanned with a step width of 100 µm in the horizontal (x) and vertical orientation (y) compared to the THz beam path. The results of the maximum amplitudes are shown in Fig. 10(b) recorded with channel 8, which is representative for the multichannel detector. It can be shown, that the resolution in the horizontal scan direction (x) is much better than in the vertical direction (y), because the 100 µm gap is still visible, whereas the vertical direction only resolves a gap up to 500 µm. This can be caused by the smaller focus diameter in horizontal direction, the astigmatism, as well as effects of sub-wavelengths phenomena on small structures in combination with the polarization of the electromagnetic field, which is vertical orientated .
4. THz Multichannel Imaging
In a first experiment we measured a metallic Siemens star (thickness 250 µm) which is shown in Fig. 11.
The results of the multichannel measurements are presented in Fig. 12.. each detector. That overcomes inconsistencies of the changing beam profile [Fig. 6(a)], which would disturb the image quality. Firstly, we scanned an image area of 60 x 50 mm2 with a pixel resolution of 1 mm. In x direction 4 measurement steps with a step width of 15 mm were used because of the 15 detection channels. The THz pulses with a time window of 8 ps were scanned with an optical delay of 3.3 ps/s and LIA averaging time of 10 ms. Further, the net measurement time, which depends only on the measurement of the time window with a certain velocity and on no processing time, was reduced to 13 minutes instead of almost 2 hours with conventional one channel detection [Fig. 12(a)]. Secondly, we measured the same area with a higher pixel resolution of 250 µm in x and y direction, which amounts to 48000 measurement points. A time window of 7 ps was scanned with 3.3 ps/s and LIA averaging time of 10 ms. The measurement time took about 1.8 hours instead of 28 hours and the image was used to determine the resolution of the THz imaging system. The Siemens star has an outer diameter of about 50 mm and consists of a pattern of 32 metallic spokes which become wider towards the outer rim of the star. The structures can be resolved up to the inner diameter of about 24 mm in x direction and 26 mm in y direction. Thus, the spatial resolution of the multichannel system is about 1.05 mm in horizontal (x) and 1.13 mm in vertical (y) measurement direction, which corresponds to a spatial frequency of about 0.47 Line pairs per mm (LP/mm) (x) and 0.44 LP/mm (y). Within the middle of the Siemens star the vertical orientated spokes show a higher transmission (70-80%), whereas the horizontal sectors show an absorption behavior. This effect is based upon the polarization dependency of the transmission at sub-wavelength structures , which has already been observed at the metallic resolution sample of Fig. 10(a).
A pump wheel made of plastics was the second sample measured with the multichannel THz TDS system [Fig. 13]. An image area of 60 x 45 mm2 was scanned with a pixel resolution of 1 mm. The x axis was scanned in 4 steps with a width of 15 mm provided by the 15 detection channels. We measured each THz pulse with a time window of 40 ps, optical delay of 3.3 ps/s and LIA averaging time of 10 ms. Overall the measurement time took about 36 minutes instead of about 9 hours due to a one channel detector with identical measurement parameters.
Figure 14 shows the normalized THz absorption image obtained from the maximum THz pulse amplitudes [Fig. 14(a)] and the image of the time values extracted from their positions in the time domain [Fig. 14(b)]. The time positions were also normalized to a reference measurement to overcome time shifts in the THz image caused by the field curvature which was shown in Fig. 6(b). In the middle of the pump wheel there is a metallic ring part, where no transmission takes place. Therefore, the absorption is zero and the time image shows noise values. More importantly, the parts of the blades and their cavity can be resolved and thus, provide us information about the sample and production process. For example, on the left side the blades have a higher absorption, which could be determined from the greater time shift at those positions. Whereas, the absorption in the space between the blades on the left side is lower than in the blades on the right side.
We presented the first multichannel THz TDS system which operates with a photoconductive antenna array of 15 detection channels excited by 1030 nm wavelength of an Yb-doped ultra-short pulse fiber laser system. The THz optics consists of parabolic and cylindrical mirrors and creates a THz line focus with a FWHM diameter of 37 mm in horizontal and 10 mm in vertical direction. Aspheric lenses made of Zeonex480R® image the pulses onto a 15 mm long detector array with a cylindrical HRFZ silicon lens for the focusing. We measured 15 channels simultaneously with a lateral resolution of 1 mm and a signal to noise ratio of about 39 dB in the middle and 35 dB at the edge of the detector array with multichannel lock-in technology. Compared to a one pixel TDS system the measurement time has been decreased by a factor of 15. The spectrum of the THz pulses ranges from 0.1 to 0.8 THz, which offers the potential for further improvements, e.g. by applying antireflection structures on THz optics, as well as by the development of more efficient THz emitter and detector material for 1030 nm. It is also possible to adapt the THz optics to only one aspheric Zeonex® lens in combination with the development of a spherical silicon lens array. Thus the bandwidth and THz signals could be increased similar to one channel TDS systems.
The spatial resolution of the multichannel TDS system was estimated during the multichannel measurement of a Siemens star to be 1.05 mm in horizontal and 1.13 mm in vertical direction, which corresponds to the spatial frequencies of about 0.47 LP/mm (x) and 0.44 LP/mm (y).
The combination of a high-power fiber lasers at 1030 nm and photoconductive detector arrays made of InGaAs/InAlAs based on MLHS offers the potential for faster and powerful multichannel TDS imaging systems. Furthermore, only 10% of the available laser power was used in this work, which provides the opportunity to use much more than 15 detector channels, e.g. in two dimensions or to exited an array of emitter structures. Thus, the measurement time for THz imaging or tomography could be shortened even more drastically.
This work was supported by the FhG internal program under grant no. MEF 825468/620033. The authors would like to thank Dr. T. Schreiber (Fraunhofer IOF) and Prof. J. Limpert (IAP Jena) for the support of the fiber laser system, as well as W. Buß and M. Müller (Fraunhofer IOF) for developments at the electronic part of the THz system.
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