3424
OPTICS LETTERS / Vol. 34, No. 21 / November 1, 2009
High signal-to-noise-ratio electro-optical terahertz imaging system based on an optical demodulating detector array Gunnar Spickermann,1,* Fabian Friederich,2 Hartmut G. Roskos,2 and Peter Haring Bolívar1 1
Institute of High Frequency and Quantum Electronics, Siegen University, Hölderlinstr. 3, 57076 Siegen, Germany 2 Physikalisches Institut, Johann Wolfgang Goethe Universität Frankfurt, Max-von-Laue-Strasse 1, 60438 Frankfurt, Germany *Corresponding author:
[email protected] Received July 16, 2009; revised September 25, 2009; accepted October 2, 2009; posted October 6, 2009 (Doc. ID 114399); published October 29, 2009 We present a 64⫻ 48 pixel 2D electro-optical terahertz (THz) imaging system using a photonic mixing device time-of-flight camera as an optical demodulating detector array. The combination of electro-optic detection with a time-of-flight camera increases sensitivity drastically, enabling the use of a nonamplified laser source for high-resolution real-time THz electro-optic imaging. © 2009 Optical Society of America OCIS codes: 110.6795, 190.7110.
The terahertz (THz) frequency range has been demonstrated to be an extremely interesting range of the electromagnetic spectrum with a multitude of applications. For many applications real-time imaging capabilities are desired. However, most imaging systems are based on single-pixel detector concepts that investigate a sample or scene by measuring THz properties sequentially at each position of interest in order to compose a THz image [1]. This serial acquisition is associated with a mechanical scanning of a scene. Imaging with a reasonable pixel resolution is therefore extremely slow. Hence there is a huge interest in accelerating high-resolution image acquisition by parallelizing the pixel recording. A promising approach is to use an optical charge-coupled device (CCD) or a complementary metal on semiconductor (CMOS) camera to image the THz-induced birefringence in an electro-optic (EO) crystal, in order to record all pixels of a full THz scene simultaneously [2]. Several real-time THz systems have been developed that build on this concept. However, to date, real-time EO THz imaging has been possible only by using amplified laser systems that allow one to generate a large enough THz field amplitude and thereby a sufficient THz-induced birefringence in an EO detector crystal to reach an acceptable signal-tonoise ratio (SNR) despite the intrinsically large optical background associated with EO imaging. Typical optical generation pulse intensities are in the 4 – 140 J range, generating EO modulation depths in the 1% to 30%–50% range [3,4]. At THz intensities reachable with standard (unamplified) femtosecond oscillator pulse energies (typically in the 10 nJ regime) [5] or using cw-laser systems [6], the small amplitude of the THz-induced birefringence inhibits practical application of the concept. In these cases, imaging has been possible only at the expense of extremely long integration times. E.g., in [6] a 40 min acquisition time for one image is reported. In this paper we propose the use of a time-of-flight (TOF) camera [7] comprising an optical demodulation detector array to measure the THz-induced modula0146-9592/09/213424-3/$15.00
tion of the optical probe beam much more efficiently in comparison with a standard CCD or CMOS camera configuration. The TOF camera has the advantage of measuring the amplitude and the phase of modulated light simultaneously while effectively suppressing background light. This gives us the possibility to realize a THz imaging setup that produces high-resolution images within seconds without the need of an amplified laser source. Our system is set up as a classical THz timedomain spectroscopy system using a 76 MHz repetition rate commercial femtosecond laser (Coherent Mira 900) as described, e.g., in [8], except for the geometry of the THz path. Instead of focusing all generated THz radiation to a single spot on the sample, the diverging THz beam generated by the emitter is collimated by the Teflon condenser lens and illuminates an area of the sample (see Fig. 1). The semilarge-aperture THz emitter consists of two sharp gold electrodes separated by a 1.2 mm gap photolithographically defined on an undoped GaAs sub-
Fig. 1. (Color online) Scheme of experimental setup for 2D EO THz imaging. © 2009 Optical Society of America
November 1, 2009 / Vol. 34, No. 21 / OPTICS LETTERS
strate forming a photoconductive structure. It is biased at 300 V with a 1 MHz sinusoid and optically pumped with an average power of approximately 1.3 W. This emitter delivers modulated THz pulses, and its time-averaged power was estimated to be around 2 W by using a Golay cell. The sample located in the object plane of a second Teflon lens is imaged on a 20 mm⫻ 20 mm⫻ 1 mm 具110典 ZnTe detector crystal. The electrical field of the THz radiation induces a birefringence distribution in the large-area ZnTe EO crystal in proportion to the THz image. This THz-induced birefringence is sampled by an expanded femtosecond laser probe beam (time-averaged power, 260 mW), which leads to an intensity modulation of the laser light after transmitting a polarization analyzer. This modulation carries all THz image information and is synchronous to the electrical modulation of the THz emitter. Therefore the THz scene image on the EO crystal is imaged to the demodulating optical detector array of the TOF camera, which is sensitive only to light modulation and rejects the background light very effectively. In our system a modified PMDtec PMD[vision] 3k-S TOF ranging camera comprising a PhotonICs PMD 3k-S sensor with a resolution of 64⫻ 48 pixels was used. Every pixel acts as a lock-in-detector that samples the incoming signal at four different phase shifts. The camera is fed with the fourth harmonic of the THz emitter modulation frequency to allow the internal generation of four 90° phase-shifted 1 MHz reference signals. The sensitivity for modulated light is enhanced through a circuit for the suppression of background illumination, which is integrated in every pixel of the demodulating array. This circuit drains the two integration capacities in every pixel from carriers photogenerated by uncorrelated background light to prevent saturation over an extended range of illumination and preserving the whole dynamic range of the readout circuit for the measurement of light modulation [7]. The detector is gated with femtosecond probe pulses, which makes it possible to sample the THz waveform as a function of time: the time position of the probe pulse relative to the THz waveform can be adjusted by moving a delay stage to sample the temporal evolution of the THz scene. The EO technique in combination with a demodulating detector array is intrinsically coherent, enabling the measurement of both amplitude and phase information. First we present an image of the beam profile generated by the photoconductive emitter. The upper
Fig. 2. Electric field distribution of the illuminating THz pulse without (top) and with time correction (bottom). The vertical dimension of each image is 12 mm at the detector crystal position and 17 mm at the THz object plane.
3425
half of Fig. 2 shows the resulting distribution of the electric field of the THz pulse penetrating the EO crystal from its first touch on the detector crystal at t = 1.6 ps to the intersection of the hemispherical wavefront of this half-cycle THz pulse with the active volume of the crystal. The total integration time for each frame is 1 s. The formation of rings results from a different path length PL between pixels in the object plane of the THz beam path and their respective pixels on the detector crystal, depending on their lateral distance to the optical axis. This dependency is simply given by PL = 冑共LD − f兲2 − AD2 .
共1兲
Here LD is the distance between the imaging lens and the detector crystal, f the focal length of the imaging lens, and AD the distance of the respective pixel on the EO detector crystal to the optical axis. This formula can be used to correct the relative time difference of the pixels, as depicted in the lower part of Fig. 2. A comparison of these corrected images to the measured field distribution in the direct collimated beam (measured by removing the imaging lens) shows the good consistency of the method. The dark regions in the upper half of the frame result from inhomogeneities in the EO crystal, giving less sensitivity. Some individual pixels in the image appear dark, as they are oversaturated by probe beam leakage and therefore drained by the suppression-ofbackground-illumination circuit on the detector chip. The circular active region, which results from the circular illumination of the detector crystal by the probe beam, has a diameter of 12 mm, and the FWHM diameter of the THz beam on the crystal is determined to be approximately 6 mm. The time dependence is found by looking at one individual pixel signal within the maximum THz amplitude area in Fig. 3, showing a typical half-cycle THz pulse. The inset shows the spectrum of this pulse, illustrating the bandwidth of the system. The
Fig. 3. (Color online) Measured electrical field transient and noise signal (measured by blocking the THz beam path) of a pixel in the center of the scene. The noise signal is amplified 共 ⫻ 10兲 and shifted vertically for clarity. Inset, power spectrum of the field transient.
3426
OPTICS LETTERS / Vol. 34, No. 21 / November 1, 2009
noise floor was measured while the THz emitter was blocked. All results shown are an average of 10 frames integrated for 100 ms, yielding an average SNR of 44 dB/ 冑Hz. After this initial system characterization we image a steel sheet with an array of holes as an exemplary sample with a strong signal contrast to demonstrate the lateral imaging resolution. Figure 4 shows a composition of two THz images, the steel sheet sample in the background, and a ruler at the bottom. The steel sheet has a hexagonal hole pattern with a minimum distance of 10 mm between the hole centers, which have a diameter of 5 mm. The right-hand THz image shows the distribution of the electrical field amplitude at the time of the maximum pulse amplitude after correction of the lateral position dependency of the time axis. In addition, the THz image on the left was normalized to the beam shape (see lower part of Fig. 2, image at t = 2.0 ps) to compensate for the low THz intensity in the border regions. This image is inserted as a transparent overlay to show the correspondence between the THz image and the optical image. In the region between the holes the high electric field amplitude with inverted polarity arises from interference of the diffracted wave after passing the holes. These images use the same linear scale and palette as shown in Fig. 2 but magnified by a factor of three. By moving the sample by a known distance it is determined that the imaging Teflon lens causes a downscaling of the THz image on the EO crystal by a factor of ⬇0.706, leading to a diameter of 17 mm of the circular actively used area in the THz object plane. The real FWHM diameter of the illuminating THz beam is therefore 8.5 mm. The acquisition of 100 ms integrated THz images was realized by aver-
Fig. 4. Overlay of the THz image of a hole punched steel sheet. Right, amplitude after temporal correction; left, halftransparent THz image with laterally normalized amplitude; background, optical image of sample.
aging 100 individual frames integrated for 1 ms on the detector array each. The present prototype of our TOF camera uses an outdated embedded computer to preprocess the data and for the interconnection to the data acquisition computer via a Firewire (IEEE1394) interface. This leads to large readout delay 共3.8 s兲 between the individual frames. However, an updated readout circuit is under construction to allow an image acquisition time closer to the 100 ms integration time. In conclusion, we present a new THz imaging approach combining an EO detector with a TOF camera comprising an optical demodulating detector array to enable THz imaging with significantly enhanced efficiency. This combination is highly attractive for realtime THz imaging, as demonstrated by the experimental data. Using a simple femtosecond oscillator and a time-averaged THz power around 2 W only, a THz image acquisition system with 3072 pixels and a SNR⬎ 30 dB within 100 ms integration time is demonstrated. The detected peak EO modulation depth is ⬇7 ⫻ 10−5 (referenced to the unmodulated optical intensity at the detector position). Sensitive EO imaging is demonstrated at much lower THz field amplitudes in comparison with previous real-time EO imaging demonstrations [3,4]. A precise intercomparison is difficult, but an estimate of the improvement can be attained from the 250–8800 times lower optical excitation pulse requirement or the 140–5700 times lower EO modulation values. We greatly appreciate financial support by the Deutsche Forschungsgemeinschaft (DFG) and the German Research Ministry BMBF in the framework of the LYNKEUS project. References 1. B. B. Hu and M. C. Nuss, Opt. Lett. 20, 1716 (1995). 2. Q. Wu, T. D. Hewitt, and X.-C. Zhang, Appl. Phys. Lett. 69, 1026 (1996). 3. Z. G. Lu, P. Campell, and X.-C. Zhang, Appl. Phys. Lett. 71, 593 (1997). 4. F. Miyamaru, T. Yonera, M. Tani, and M. Hangyo, Jpn. J. Appl. Phys. Part 1 43, L489 (2004). 5. D. Crawley, S. Withington, and J. Obradovic, Rev. Sci. Instrum. 77, 053106 (2006). 6. A. Nahata, J. T. Yardley, and T. F. Heinz, Appl. Phys. Lett. 81, 963 (2002). 7. T. Ringbeck, T. Möller, and B. Hagebeuker, Adv. Radio Sci. 5, 135 (2007). 8. A. Nahata, D. H. Auston, T. F. Heinz, and C. Wu, Appl. Phys. Lett. 68, 150 (1996).
