1106 Terahertz Photonics for Imaging Peter R. Herczfeld' and Yifei Li' -Invited Abstract: This paper concerm the application of microrvuw photonic techniques for terahertz imaging. The system under investigation comprises of two high power, low noise electro-optic microchip lasers that mix in a traveling wave detector to produce a 200 GHz millimeter wave signal that is multiplied to.8 or 1.2 THz. The traveling wave detector, the multiplier and the THz radiating elements are to be integrated on an MMIC chip. The paper will focus on the development of the laser subsystem. The THz micro-system The detection of concealed weapons and explosive hidden under clothing carried by terrorists is one of the most daunting problems facing the military and the civilian law enforcement personnel. Terahertz (THz) imaging, by vime of its ability to penetrate various materials and short wavelength (high resolution) is the most promising approach to address this problem. Terahertz frequencies fall in between the optical and nillimeter wave domains of the electromagnetic spectrum, and there are no readily available techniques to generate low noise, frequency agile signals as required for imaging. To exploit the inherent advantages of THz imaging significant scientific and engineering breakthroughs are needed for the generation of THz signals. The extension of microwave photonics offers an ideal avenue for THz system development. This paper concerns the design and related research for the development of optically generated THz sources for active illumination and for local oscillator applications, including a low loss optical distribution network. The specific aim is to meet the technical requirements specified for an effective imaging system. The proposed approach is based on a sophisticated merging of optical and millimeter wave techniques. The fundamental approach of the evolving THz micro-system is depicted in figure 1. It comprises of three critical components; an efficient, high fidelity electro-optic microchip laser, a low loss fiberoptic/integrated optic distribution network and an optical to THz (O/THz) conversion module that includes 25 radiating elements on a chip. Each OiTHz module is designed to radiate 50 mw of average (CW) power at.8 THz with an overall DC to THz efficiency of 2%. Alternatively, the OiTHz module can generate 25 mw at 1.2 THz with an efficiency of 1%. The micro -system is compact, lightweight, robust and scalable and suitable for man-portability. The illuminator will be able to operate either in coherent or non-coherent mode. The frequency agility of the system is designed to he 10% of the carrier (THz) signal, but can he expended to 40% or higher, and will have the ability to wave shaping. The phase noise of the THz signal is designed to he -9OdBciHz or lower, which is significant for coherent detection schemes. Finally, the micro-system is flexible and can be tailored to different applications. Table I summarize the performance specifications of the current project. The electro-optic module has two active components, a pump diode operating at the optical wavelength 808 nanometer and two.5mm1 electro-optic microchip lasers. The outputs of the lasers are combined, routed via a fiberoptic cable to the O/THz module. Each O/THz InP chip contains an integrated optic routing network, an array of traveling wave photodetectors, multipliers and radiating elements. The optical frequencies of the lasers are set 200 GHz apart In the photoconductor the two optical signals heterodyne and produce a strong 200 GHz millimeter wave signal that is multiplied by 4 (.8THz) or by 6 (I.2THz) in a Schottky diode. The output of the diode is fed through a dielectric guide into a THz radiating element. Alternatively, the output of the photodetector-multiplier may be utilized as a strong, clean local oscillator signal. ' Center for Microwave-lightwave Engineering, Drexl University, Philadelphia, PA 191 04, Email: Herczfeld@ece.drexel.edu
1107 Fig. I. The basic layout of the THz source micro-system. The combination of the small, efficient microchip laser, low loss optical distribution network and OiTHz' chip modules provide for a lightweight and robust subsystem that can be easily scaled and integrated into a manportable imaging system. We performed a careful energy audit, namely the DC to THz conversion efficiency. Furthermore, we considered the step by step optical lo THz photon transfer efficiency. This analysis enabled us to distinguish fundamental scientific limitations from 'engineering limitations', and thus predict with confidence that the approach will lead to efficient, viable THz imaging systems. This approach has three significantly novel elements representing scientific and technological breakthroughs. These are: i. the imaging subsystem, as depicted in Fig. 1, that provides for high performance, flexibility and future improvements, ii. the high power, single mode microchip laser that generates the tunable, clean, low noise optical signals, and iii. the traveling photodetector that converts very efficiently the photons into electrons and millimeter waves. Pi" PO", Efficiency PhaseBoise@IOkHz (mw) (mw) offset (dbc/hz).8thz 2000 50 2% -90 1.2THz 2000 25 1% -90 Tuning range 10% 10% Table I. Performance specifications to be achieved under of this project. The laser subsystem In this section we review the salient results of prior work on the tunable laser and identify the required modificationslimprovements to satisfy the specific requirements of a THz illuminator. Two identical optical cavities are formed by depositing dielectric mirrors on opposite ends of a composite Nd:WO4/Mg0LiNb0, crystal assembly, as shown in Fig. 2. The two side-by-side lasers are each pumped by independently driven 808 nm high power laser diode sources. The extremely short pump absorption length in the composite cavity, which significantly reduces the spatial hole burning effect, assures single mode operation. The outputs of the two single mode lasers are combined, coupled into a single mode fiber, and transmitted to a high-speed photodiode, where the optical signals self-heterodyne resulting in a microwave or millimeter-wave signal. The monolithic configuration gives the device simplicity, compactness, stability, and reduced sensitivity to external temperature fluctuations.
1108 Fig. 2. Dynamically tunable optical transmitter configuration Ideally, if the two lasers are at the same temperature, pumped at the same level and subjected to no applied voltage their output is expected to he identical in optical frequency and intensity. However, if the output frequency of either laser is shifted by temperature or voltage bias (Vl), then a heterodyned beat frequency is produced at the photodetector. Specifically, we use temperature or voltage biasing to set the initial frequency, Q to the desired millimeter-wave frequency, in this case 200 GHz. Then by applying a time dependent voltage, the millimeter heterodyned frequency is tuned. A simple phased lock loop (PLL) is utilized to provide stability. The implementation of the rapidly tunable transmitter, discussed earlier, is shown in Fig. 3. This configuration consists of a0.3 mm long NdW04 crystal (the gain medium) and a 1.2 mm long Mg0:LiNbq crystal (the tuning section). Electrodes are deposited on the top and the bottom of the 1.2 mm Mg0:LiNbO.i tuning section for the tuningimodulation input signals. Fig. 3. Implementation of the tunable laser assembly The laser was evaluated in both chirped lidar [l] and communication experiments [2]. Table I1 summarizes the results obtained. In this same table we also listed the proposed results we wish to attain for the THz illuminator, and the improvement and changes required to achieve these results.
. 1109 Parameters -I, Current result ~I Proposed value Optical wavelength.., 1.06pm., 1.06 pm.. Required improvernent ~ Maximmrheterodyned 150 GHz 200 GHz New gain medium frequency (Nd:LSB) Dvnamic odtical 3-16-GHz.1-30 GHz Upgrade PLL.,.. TABLE 11. Tunable laser current and expected results As the table reveals it to attain high efficiency, high power and a 200 GHz beat frequency the original laser needs to he redesigned. The laser must still operate in a single mode for the heterodyning to work, which means that the optical cavity must be short. More significantly to attain high power and high efficiency spatial hole burning must be eliminated or kept to a minimum. In the subsequent paragraphs a new laser, the dual polarization, dual frequency microchip laser depicted in Fig. 4 will be described. This configuration comprises of two co-propagating single mode lasers within the optical cavity. It eliminates the need for beam combining and makes it easier to couple into fibers. Polarization 0th order quarterwave plate: 0.050mm 25% Nd:LSB : LiNb03: 0.2mrn 0.15rnm Figure 4: Dual polarization two frequency microchip laser
1110 The dual polarization, dual frequency microchip laser manipulates the position of the two sets of spatial holes generated via the two lasing waves. Ideally, the two sets of spatial holes will be set to have identical intensities but their relative phases will be set apart by the angle 7C. As a result, the spatial hole burning effect common to F-P cavities is diminished by mutual compensation of the two sets of holes and an efficient, stable, two-frequency output is assured at high pump levels. This process can he best illustrated in the following diagram: Spatial holes for x polarization Spatial holes for y polarization.. Caiii medium;.,.,..... t Spatial hole free 1 Gain medium Figure 5: Principle for spatial-hole buming free operation As depicted in Fig. 5, in order to obtain a complete spatial hole burning free operation, the following three requirements must be met: The wave numbers (k) for both polarizationsmust be the same The two set of spatial holes must be offset by a phase 7C The two sets of spatial holesmust have identical amplitudes In the following, we explain how the cavity configuration shown Fig. 4 is capable of satisfying those conditions. 1. Identical wave numbers The wave numbea for the two lasing waves with orthogonal polarizations are: For the NdLSB gain medium, n,= 1.8280 and ny= 1.8272. Therefore if fx-fy=130 GHz, k,=k,. If fx-fy is not equal to 130 GHz, the two sets of spatial holes will walk off withrespect to each other. This case will be discussed later. ii. Initial 7t phase difference Boundaly condition requires the two waves have the same phase at the mirror. Therefore, by placing a A/ 4 plat between the gain medium and the mirror, we can set the phase of the spatial holes to be differed by 7C. In addition, since the thickness of the wave plate is very small (.05mm), the phase difference is practically independent of the frequency difference.... 111. Equal amplitudes for spatial holes The emission cross-sections for x and y polarizations are 13e-I9 CA and 9e-19 c d, respectively. To assure equal amplitudes by the spatial holes, the cavity loss for x and y polarizations has to he set to a ratio of 9 to 13, which can be accomplished by careful design of the laser output coupler.
1111 In the ideal situation, the spatial hole huming is completely diminished. However, when frequency tuned away from 130 GHz, a small amount of spatial hole burning may be introduced that can lead to the onset of other unwanted modes. To maintain stable dual mode operation, the following relation has to he satisfied: 1 P(n8 1 sn(6k z 60) SUI( kn z) N where p(n,l) is mode discrimination between the n-th and the first two mode, 6k is the wave number mismatched between two polarizations, 60 is the phase mismatch, and k, is the wave number difference between the n-th mode and the first mode. Extensive calculations show that for laser structure depicted in Fig. 4 stable single mode operation is assured up to 25 times the laser threshold if the frequency tuning is limited from 130 GHz to 200 GHz. Table III summarizes the predicted performance of the new dual polarization, dual frequency laser. Laser threshold 130 t0 200GHz 5hW Table 111. Expected performance of the new dual polarization, dual frequency electro-optic microchip laser. Summary The THr range of the electromagnetic spectrum is one of the last remaining frontiers of to he explored and exploited. The main bottleneck in the development of THz system for imaging and other applications is the generation of efficient, strong and spectrally clean signals. The THz region is bounded on the low frequency side by millimeter wave electronics and on the high frequency side by photonics. It is therefore natural to seek ways to merge these two technologies to create new THz systems. This paper described one such approach. References 1. Li Y., Goldwasser, S., Herczfeld, P., Rapidly Tunable Millimeter-Wave Optical Transmitter for lidarhadar, IEEE Transactions on Microwave Theory and Techniques, special issue on microwave and millimeter-wave photonics, Vol 49, No. 10, pp. 2048-2054, Oct. 2001. 2. Li Y., Goldwasser, S., Bystrom, M., Herczfeld, P., Generation ofmsk Modulated Millimeter Wave Subcarrier for Radio Over Fiber Applications, Proceedings of the Microwave Photonics 2001 Conference, Long Beach, CA, 2002, pp. 33-36, Jan. 7-9.