Instrumentation for Beam Profiling in the Terahertz Regime

Martin S. Heimbeck and Henry O. Everitt

Department of the Army AMRDEC WSD

Redstone Arsenal, AL, 35898, USA

Kent Taylor, Carys Davis, Eric Hamilton, and David E. Thomas The AEgis Technologies Group, Inc.

Huntsville AL, 35806, USA

Patrick J. Reardon The University of Alabama in Huntsville

Center for Applied Optics Huntsville, AL 35899, USA

ABSTRACT We report on the Terahertz (THz) Imaging Profiler Array (TIPA). TIPA is an innovative standalone and portable THz instrument that provides a system capable of operating as a THz beam profiler and multispectral imager for continuous wave (CW) THz sources. It integrates a solid-state detector technology that can be configured in an array to cover the frequency range from 0.60 to 0.90 THz along with imaging and scanning mirror modules and system control hardware and software. The system is presently being expanded to cover a total frequency range from 0.17 – 0.90 THz by the end of 2010. Images of THz source profiles are presented along with THz images of relevant targets. Potential applications are discussed.

1. INTRODUCTION THz radiation is recognized as an increasingly important region for directed energy (DE) applications. THz wavelengths are shorter than microwaves, therefore, they do not diffract as much as energy generated by high power microwave (HPM) systems. When compared to visible and near infrared (NIR) lasers, THz energy can couple better with electronics systems (much like HPM systems). The interaction between various materials and terahertz energy is also unique. Since THz frequencies exist between radio frequency (RF) and IR regions, they can penetrate certain materials (paint, clothing, paper, wood, etc.). Applications include non-destructive evaluation of components for fatigue or tampering, and imaging for security purposes. In free space communication applications, where atmospheric attenuation is not an issue, THz offers significant improvement in bandwidth over RF without the requirement for extreme pointing accuracy required by laser communication systems. We are currently witnessing the development of THz technologies in applications that combines the best attributes of microwave and IR spectral regions. The wealth of new science and technology opportunities found in applications of THz has fostered rapid advancement in sources, with higher power levels being demonstrated in the laboratory. Most THz sources currently in use are semiconductor-based sources: photoconductive switches pumped by a Ti:Sapphire laser, quantum cascade lasers and multiplier chains driven by microwave oscillators. These sources typically generate up to a few milliwatts of power. The DARPA Terahertz Imaging Focal Plane Array Technology (TIFT) program is currently investing improved designs for THz sources operating at 650 GHz that offer higher efficiencies and power levels that could reach 50 mW.[1] Scientists have demonstrated tens of Watts of broadband THz radiation produced by electrons in an accelerator.[2] There are three types of vacuum-based sources under development (backward wave oscillator, desktop free electron laser, optically pumped far infrared laser) that may offer THz power from 1 to 100 Watts.

These power levels are of interest to the DE community for their potential to enhance their capabilities. Therefore, for the first time, there is a need to develop a tool that allows us to characterize important beam characteristics and to quantify and standardize THz source performance. This project is a timely recognition of the growing importance of THz technology, and the need for a THz beam imager capable of accurately characterizing the beam properties of new THz sources. TIPA is a standalone system that incorporates custom hardware and software components with COTS products to deliver a test system capable of profiling a wide variety of continuous wave THz sources. Some of TIPA’s main components are depicted in Figure 1.

Figure 1: Graphical representation of TIPA components

2. OPTICAL DESIGN In order to maintain initial beam quality parameters of the profiled THz source, all-reflective optics were chosen to relay a THz beam onto a linear detector array via a scanning mirror. An effort of minimizing the necessity for THz lenses prevents the spatial content of the THz beam to be influenced by unknown inhomogeneities and birefringence of the lens material. The optical system is a derivative of the Offner Relay system. Figure 2 shows a layout of the beam profiler’s optical train designed with ZEMAX®. The total length of the optical train is approximately 4 meters. The first mirror (M1) functions as the radiation collecting optic and system aperture stop. M1 is a 13” diameter all aluminum spherical mirror that is tilted slightly off axis to project the THz beam onto the scanning mirror (M2). M2 is a 150 mm diameter slightly convex spherical aluminum mirror, which relays the THz beam onto mirror M3, which functions as the collimating optic. M3 can produce a collimated THz beam at the detector array (D) that is up to 14” in diameter. Therefore the relay system not only serves to relay a THz beam across a scanning mirror but also to expand and collimate it at the detector plane for high spatial resolution THz beam profiles. All this is accomplished without affecting the quality of the THz beam. The relatively long wavelength of THz radiation allows us to use spherical mirrors in this off-axis configuration. This three mirror system assumes an incident beam having a divergence angle of 3 degrees (NA= 0.052). However, TIPA can potentially accommodate any source if an additional source lens coupler is employed at location S in Figure 2. The source lens coupler, which can be a simple and inexpensive Teflon lens, changes the divergence angle of the THz source to a 5 degree divergence angle. Slight resetting of the optics allows us to turn the beam profiler configuration into a THz imaging system with conjugate object and image points. Refer to Figure 3 for the ZEMAX® layout representing the imager configuration. The total optical path length of this configuration is approximately 5 meters. This system can be used in a configuration where the object is illuminated broadly from an arbitrary source location or in a transceiver setup. The system has been tested with

the object located at O1 while the source and detectors are located at S1 as well as in a reversed configuration (O2, S2). While the O1,S1 configuration results in higher resolution images than configuration O2,S2, the reversed configuration allows for a larger scanning field. The total visible field in the O1,S1 configuration is approximately 1000cm2 while the O2,S2 configuration allows us to scan an area of approximately 500cm2. This three mirror system allows for a variety of relative mirror separations which provides the advantage of being able to adjust this system to image objects at various distances. Specific THz images are shown in Section 7.

Figure 2: Beam Profiling Configuration

Figure 3: Imaging Configuration

S

D

M1 M2

M3

O1, S2

O2, S1

M1

M2

M3

3. DETECTOR ARRAY The detector array consists of 4 custom detector blocks, each housing 4 detectors responding in the 0.60 – 0.90 THz range. TIPA will be expanded with an additional 3 1x16 detector arrays to cover a total frequency range from 0.17 – 0.9 THz by the end of 2010. This 0.60 – 0.90 THz array of 16 detectors was manufactured by Virginia Diodes Inc. The detector spacing is maintained at 0.2 inches throughout the array. This spacing was chosen to allow the detector spacing to be maintained across the full 0.17 – 0.90 THz region. Each block incorporates the housing of 4 detectors, custom detector horns, as well as some detector voltage filtering and amplification. A motherboard attaches to the rear of the housings which serves as the interface between the detectors and an ADC and provides biasing circuitry as well as some additional signal amplification. The 4 detector blocks can be configured as a 1x16 or 4x4 detector array. Refer to Figure 4 for pictures of both array configurations. The 1x16 array is employed in the beam profiler depicted in Figure 2 whereas the 4x4 array can be placed into the THz beam to provide high temporal, low spatial resolution beam profiles.

Figure 4: Linear array configuration (left) and 4x4 staring configuration (right)

Some of the risk factors were potential losses in coupling efficiency due to the unusually long detector horns for this frequency range as well as potential crosstalk between detectors sharing the same housing. However, detector tests show excellent coupling and no presence of detector crosstalk. The average measured noise equivalent power (NEP) of the detectors is 140pW/Hz½, which is comparable to the stated NEP of COTS detectors by the manufacturer. [3] The main challenge for integrating the detector array successfully is their unique spectral response curves. The spectral responsivity of the detectors can vary by more than a factor of 4 as can be seen in Figure 5, which shows the responsivity curves of 4 of the 16 detectors. Therefore, a calibration routine has to be employed immediately before each beam profile measurement in order to account for responsivity differences as well as other contributing factors that may affect detector performance such as temperature fluctuations.

Figure 5: Responsivity curves of four 0.60 – 0.90 THz detectors

4. ACQUISITION AND CALIBRATION Although some THz sources are able to emit THz radiation of larger than 1W, common laboratory and turnkey THz sources emit in the milliwatt range and less. While high power sources can be attenuated with polymer based materials, another challenge lies in the detection and profiling of low power sources within reasonable time. A relatively quick scan will not only result in a more efficient test range system but also be immune from source output instabilities on a longer time scale (10+ minutes). The fact that we are limited to 16 detectors to achieve a 4096 pixel beam profile (64x64 pixels) requires us to acquire from all 16 detectors in parallel as well as perform intelligent post processing of the acquired data. Many single detector experiments utilize acquisition techniques that incorporate source modulation in conjunction with hardware lock-in amplifiers such as the SR530 from Stanford Research Systems. We also utilized lock in detection but on a software level. NATIONAL INSTURMENTS® provides a basic software lock-in capabilities with its NI4472 signal acquisition board. We used three NI4472 PXI board as well as the LabVIEW based software lock-in code in conjunction with other processing techniques such as signal averaging to maximize the signal to noise ratio (SNR) while minimizing the acquisition time. Depending on source output power, desired SNR and consequential post processing techniques, the scan rate can vary between more than 200 pixels/sec and less than 16 pixels/sec. As mentioned in Section 3, each detector has a unique responsivity curve. The first task of the calibration process is to subtract the dark current, which is a simple offset in output voltage of the detectors when not illuminated with a THz signal. After this de-biasing process the data for flat fielding is acquired. For this all detectors have to be illuminated by an identical irradiance. This is achieved by scanning the beam across the 16 detectors in 16 steps such that an identical portion of the beam illuminates each detector. The responsivity difference can be described with a unique detector voltage output vs. irradiance slope. The detectors were calibrated with respect to the average slope of all 16 detectors. Therefore the calibrated individual detector signal Ical can be obtained through

(1)

where Idark is the dark signal of the detector, Iflat is the flat field signal of the detector, Mflat is the average value of 16 flat field detector signals, and Iraw is the uncalibrated detector signal. Ical is not an absolute power measurement. In order to calibrate Ical for absolute power, an absolute power meter is required at the detector plane. TIPA will be expanded with this capability in 2010 to allow for absolute power beam profiling.

The following figures are beam profile plots used to compare the calibration routines to un-calibrated beam profiles as well as beam profiles acquired with one detector, which do not require any calibration but take longer to acquire. The relative acquisition times are displayed in the corner of each figure. Refer to Figure 6 for 3 beam profiles acquired with no further processing. Profile 1 was taken with TIPA and one detector, which is the baseline for comparison. Profile 2 is an un-calibrated beam profile acquired with TIPA and the detector array. Strong streaking shows that calibration is absolutely necessary. Profile 3 shows the calibrated beam profile. This calibration only adds approximately 2 seconds to the acquisition compared to the un-calibrated beam profile and is significantly faster than the scan using one detector. We also note that this test was performed using no additional processing such as signal averaging or lock-in detection. The acquisition time difference between one detector and the detector array would increase dramatically if a processing type such as lock-in detection were used. The rings in the beam profile are coherence effects between the highly coherent source and the detectors and are not caused by the optics.

Figure 6: High resolution beam profiles: 1) One detector, 2) uncalibrated array, 3) calibrated array

5. HIGH SPATIAL RESOLUTION BEAM PROFILING The optical train expands the THz beam to a 12.8” diameter beam for profiling. This allows us to acquire a 32 x 32 pixel beam profile considering that the detector/pixel separation is 0.4”. However, testing of the system shows that acquiring beam profile information at half the detector spacing (0.2”) significantly increases the resolved content of the THz beam. We therefore acquire a 64 x 64 pixel beam profile of a 12.8” diameter THz beam. Refer to Figure 7 for a picture of the optical setup. The picture shows the 2 large mirrors (M1 and M3) as well as the scanning mirror mounted on two rotation stages. The 1x16 detector array can be seen in the bottom right. The black material is Eccosorb® HR, which is used to baffle the system from stray THz light. The acquisition program allows us to acquire a beam profile using a series of different post processing methods. Depending on the desired signal to noise ratio of the beam profile more time consuming processing methods such as lock-in processing can be chosen. Refer to Figure 8 for a series of normalized beam profiles acquired from a Virgnia Diodes, Inc. 660 – 760 GHz source that has output power between a few microwatts at 0.60 THz and a few hundred microwatts at 0.74 THz. The output frequencies as well as processing techniques and total acquisition times are listed in Table 1.

Figure 7: Picture of optical setup with detector array

1 300 sec 2 80 sec 3 83 sec

Table  1:  Beam  profiles  

Freq.  (GHz)   Processing     Scan  time  (sec)    660   lock-­‐in   133  670   none   79  680   none   79  690   none   79  700   none   79  710   none   79  720   none   79  740   none   79  750   lock-­‐in   151  

660GHz 670GHz 680 GHz

690GHz 700GHz 710 GHz

720GHz 740 GHz 750GHz

Figure 8: Various beam profiles of a 0.660 – 0.760 THz source

6. HIGH TEMPORAL RESOLUTION BEAM PROFILING The detector array can be configured into a 4x4 staring array for low spatial resolution, high temporal resolution beam profiling. This setup does not require a scanning mirror. Nevertheless, the detectors still require calibration which is performed by stepping the detector into the same portion of the THz beam with help of two linear stages set up in a x-y configuration. Refer to Figure 9 for the setup showing the THz source in the bottom right corner, collimating lens in the center and 4x4 detector array mounted on linear stages. This setup allows us to acquire a 4x4 pixel beam THz profiles in real time. The individual snapshots can be stitched together to shoot a beam profile movie that can provide low spatial resolution information on the temporal evolution of a THz source. For example, the warm up phase of the 0.660 – 0.760 THz source as well as changes of the ambient temperature or humidity levels and their affect on the shape and relative power of the THz beam can be recorded. Moreover, the array is capable of detecting obscurations that are introduced into the beam. In particular, the array can provide an estimate of the shape of the obscurant and is able to provide information on from which direction the obscurant was introduced into the beam. Refer to Figure 9 for the front panel of a MATLAB® algorithm used to playback the THz movie.

Figure 9: Picture of staring array profiling a THz source (left); MATLAB® program to playback THz movies (right)

7. THZ IMAGING Today THz imaging is identified as the main application besides THz spectroscopy. Common imaging setups utilize a large scanning mirror which is the closest optic to the object in conjunction with a heterodyne setup to maximize the system’s dynamic range. [4] In the TIPA setup, the scanning mirror is sandwiched between two stationary mirrors allowing us to use the imager in a forward or reversed configuration as described in Section 2. The scanning mirror also serves as the aperture stop. The 0.660 – 0.760 THz source only allows for a single detector transceiver setup due to the limited available dynamic range in this direct irradiance setup. Future plans are to test the imaging capabilities of TIPA with a source outputting several milliwatts of THz power, which will be sufficient to utilize the detector array for imaging. One single detector of the 1x16 array was chosen. The source and detector are configured in a transceiver setup with a wire grid polarizer used to separate the beam path of the source and the return immediately in front of the source. In order to test the imaging configuration, two metallic objects were imaged in both configuration. Refer to Figures 10 and 11 as comparison of the forward and backward configurations. Figure 10 shows the THz image of a plane aluminum mirror at 708 GHz. Figure 11 shows the image of a knife. The red lines indicate the location of the plotted scan line.

Figure 10: THz image of 3” diameter flat Al mirror in forward configuration (right) and reserve configuration (center) at 708 GHz

Figure 11: THz image of knife (5” blade) in forward configuration (bottom) and reverse configuration (above) at 708 GHz

8. CONCLUSION TIPA is a versatile tool allowing for high quality beam profiling and imaging in the THz region below 1THz. The dual capability of serving as a beam profiler and imager in conjuction with is portability allow the system to be used in many scenarios from a instrument at a test range for beam profiling high power THz sources to the research of THz imaging of inside and outside the laboratory. The fact that it will be able to span the THz region from 0.17 – 0.90 THz will allow the

0   1   2   3   4   5   6   7   8   9   10  

cm   0   1   2   3   4   5   6   7   8   9   10  cm  

0   1   2   3   4   5  cm  

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characterization of atmospheric effects on THz imaging and beam profiling. Furthermore, its ability to be used with highly coherent sources opens up opportunities in interferometric profiling and imaging such as wave front sensing and optical coherence tomography. Furthermore, the incorporated detector array and its consequences on detector design and optical system design is an important step from a single detector system towards a THz focal plane array.

ACKNOWLEDGEMENTS This work was supported by….

REFERENCES

[1] Assessment of Millimeter-Wave and Terahertz Technology for Detection and Identification of Concealed Explosives and Weapons. Committee on Assessment of Security Technologies for Transportation, National Materials Advisory Board, Division on Engineering and Physical Sciences, National Research Council of the National Academies: http://books.nap.edu/ openbook.php?record_id=11826&page=R1.

[2] G. Carr, M. Martin, W. McKinney, K. Jordan, G. Nell, and G. Williams, “High-power terahertz radiation from relativistic electrons.” Letters to Nature, Vol. 420, 153-156, 2002.

[3] Jeffrey L. Hesler and Thomas W. Crowe, "Responsivity and Noise Measurements of Zero-Bias Schottky Diode Detectors." 18th Intl. Symp. Space Terahertz Tech., Pasadena, CA, March 2007.

[4] W. von Spiegel, C. am Weg, R. Henneberger, R. Zimmermann, T. Loeffler and H. G. Roskos, “Active THz-imaging system with improved frame rate” Proc. Of SPIE, Vol. 7311, 2009.