Systems are under development using transferred electron photocathode technology for military and industrial applications.
Verle Aebi and Peter Vallianos
The basic concept for laser-illuminated viewing and ranging (LIVAR) has been around since the development of the laser. But a recent range-gated, laser-illuminated, two-dimensional imaging system developed at Intevac operates at novel "eye-safe" wavelengths. The system is based on a transferred electron (TE) photocathode that allows operation in the 1500- to 1600-nm band where the eye's tolerance to high intensity illumination is substantially greater than at shorter wavelengths where traditional photocathodes have sensitivity.
The eye safety of this system along with the relatively high availability of lasers operating in this band opens up a broader range of applications for LIVAR than in the past. When included in a surveillance sensor system, LIVAR provides long-range high-resolution images of specific objects that have been detected using a wide field-of-view night surveillance system such as a forward-looking infrared radar (FLIR) or synthetic aperture radar (SAR; see Fig. 1).
Targeting and surveillance
As a result, laser-gated imaging technology is being extended to an eye-safe, and exceptionally long range (>20 km), tactical targeting and target identification (ID) capability that can be incorporated into existing and new FLIR-based night targeting systems for airborne, surface-vehicle, and human portable applications. This represents a factor-of-seven increase in identification range over the best fielded 10-µm-band FLIR systems and a factor-of-three increase over planned 3- to 5-µm-band staring systems.
The sensor also has applicability in general long-range surveillance markets related to law enforcement or other security requirements. In operation, this capability is achieved by using laser pulses to illuminate the area of interest (FLIR or SAR detected target, or cued coordinates). The reflected laser energy from the object under observation is then collected and imaged onto a high quantum efficiency short-wave IR photocathode that is "gated on" to collect only the radiation reflected from the target area of interest. In this way, only the objects in a volume defined by the area covered by the laser beam and a depth of field set by the gate time width are presented in the image.
FIGURE 2. An uncooled FLIR system detects a target at a distance of 2 km, which the LIVAR image identifies as a vehicle and person.
Specific additional advantages of this mode of operation are higher contrast due to reduction of veiling glare caused by scatter from haze or other particulates or aerosols in the atmospheric line of light to the target; the ability to eliminate background to achieve greater target contrast; the ability to "walk" through a target in small increments to get structural detail; the ability to provide a high-contrast silhouette of a target against background; a reflective image providing feature-shadow detail not seen in FLIR imagery; and no need for precision stabilization when operated in "snapshot mode."
A prototype system was constructed to demonstrate this concept. System features utilized in initial testing are summarized in Table 1. This simple system allowed target ID at ranges out to 5 km. Sensor limiting resolution was 35 line pairs/mm at the faceplate of the TE photocathode sensor. Sample imagery (single frames with no shot-to-shot averaging or image processing) has been obtained with this system using an uncooled FLIR to detect the target of interest (see Fig. 2).
Enabling technology
The enabling technology for the LIVAR sensor is the TE Photocathode (TEP) developed by Intevac in 1996. This photocathode has a demonstrated quantum efficiency of 20% or higher over the spectral range between 0.95 and 1.7 µm, allowing it to be used for imaging with either the currently proliferated 1.06-µm Nd:YAG lasers or with Er:glass and Raman lasers, or with optical-parametric-oscillator-shifted laser sources operating in the "eye-safe" 1.54- to 1.57-µm band (see Fig. 3).
The TEP is coupled directly with a CCD chip in an electron bombarded CCD (EBCCD) configuration, which was developed under a cost-shared technology development agreement with the Defense Advanced Research Projects Agency between 1995 and 1997. In the EBCCD camera photoelectrons from the photocathode are accelerated to and imaged directly in the back-illuminated CCD (see Fig. 4). Gain is achieved by electron multiplication resulting when the high velocity electron beam dissipates its energy in the silicon of the CCD chip to produce electron-hole pairs by the electron-bombarded semiconductor gain process. Noise is generated in all the multiplier chain elements in the conventional ICCD, particularly in the micro-channel plate (MCP).
In contrast, very little noise is generated in the EBCCD camera, in which the multiplication occurs by electron-hole pair formation as the accelerated electron beam travels through the silicon so the overall noise figure is close to one, approximately half that of a standard Generation-III image intensifier. The EBCCD eliminates the MCP, phosphor screen, and fiber optics, and as a result both improved image quality and increased sensitivity can be obtained in a smaller sized camera.
Active pixel sensors
Similar technology also is being developed using complimentary metal oxide semiconductor (CMOS) active pixel sensors (APS) for two surveillance and targeting projects that have significant industrial as well as dual use military applications.
The first project involves an integrated camera for surveillance and targeting that will be developed in collaboration with the US Army Communications-Electronics Command (CECOM).1 The project uses an electron bombarded active pixel sensor (EBAPS) technology that is being developed under a contract with the National Institutes of Standards and Technology (NIST; Boulder, CO). The NIST effort is focused on development of a low-light-level video camera, which requires development of a new imaging sensor and an automated assembly process for the manufacture of low-cost, high-performance, low-light-level cameras for security systems and law enforcement markets.
Intevac is collaborating with the National Semiconductor Corporation (Santa Clara, CA) in the NIST effort. The military CECOM project, which also depends on this technology, has three primary aspects. They include development of a transferred electron EBAPS camera, development of the CMOS APS chips for the camera, and thermal hardening of the TE photocathode to meet military and industrial storage requirements.
The second project involves development of a manufacturing technology for reducing the fabrication cost of the transferred electron photocathode tube, which is the primary camera component; investigations into manufacturing processes aimed at increasing the yield of component parts, particularly the finished photocathodes; improving the component assembly process; increasing component fabrication and assembly throughput. The lower cost sensors will allow broader use by military and non-military users in general targeting and surveillance applications.
REFERENCE
- F. A. Milton, F. A. Klager, and T. R. Bowman Jr., Proc. SPIE 4024, 24 (April 2000).
VERLE AEBI is president and PETER VALLIANOS is business development manager of the Photonics Technology Division of Intevac Inc., 3560 Bassett St., Santa Clara, CA 95054-2704; e-mail: [email protected].