Optical up-conversion enables capture of millimeter-wave video with an IR camera

Millimeter waves (mmWs) are electromagnetic signals with frequencies between 30GHz (10mm wavelength) and 300GHz (1mm wavelength). They are longer in wavelength than IR and terahertz signals but shorter than radio waves. Most objects naturally emit weak amounts of mmWs, much like IR radiation or heat. In addition, the atmosphere provides high thermal contrast for reflective objects at mmWs in all weather conditions, day and night. Signals at these long wavelengths are able to penetrate many materials, including clothing, most building materials, and atmospheric conditions such as fog (see Figure 1). However, unlike x-rays mmWs are completely safe and non-ionizing to human tissue. These advantages have been driving the desire to develop a real-time imaging system that operates at mmWs for surveillance and security applications.¹

Figure 1. Atmospheric attenuation curves from 10GHz to 1THz under various levels of relative humidity (RH) and fog. Plot was generated using atmospheric codes developed for the North Atlantic Treaty Organization and notes some of the atmospheric ‘windows’ used for imaging.²

Our eyes cannot ‘see’ mmW signals. To detect them, we at Phase Sensitive Innovations (PSI) Inc.³ and the University of Delaware (UD) have developed sensitive imaging systems capable of capturing imagery in the Q-band (33–50GHz) and W-band (75–110GHz) mmW regimes. These systems can be used for all-weather navigation, situational awareness in degraded visual environments, and stand-off detection of contraband and improvised explosive devices. As an example, Figure 2 shows a picture of a UD laboratory taken with a W-band, single-pixel scanning imager. The imagery is intuitive and the modality can be completely covert, unlike radar or laser detection and ranging images.

Figure 2. Visible (top) and passive millimeter wave (mmW, bottom) images taken with a single-pixel scanning imager using optical up-conversion.

The diffraction-limited resolution of an imaging system is proportional to λ/D, where λ is the wavelength of the radiation collected and D is the width of the imaging aperture. In mmW imaging, the wavelength of the radiation is more than a thousand times larger than that of visible light. Therefore, the imager aperture needs to be larger than a traditional camera to obtain suitable resolution. Traditional imaging approaches use optics and a focal plane array (FPA) to produce the imagery. An FPA requires a lens, larger volume (particularly with the larger aperture size needed for an mmW imager), and an expensive mmW detector for each pixel. In contrast, the new approach we developed at PSI uses a distributed aperture imaging system to capture the complex (amplitude and phase) mmW signal at discrete points. These complex signals are then recombined using simple optics to create the overall image. Because a radio frequency lens is not required, the imager can be flat or conformal, work around existing infrastructure, and achieve higher resolution without the volumetric scaling of size and weight inherent in an FPA imager. In addition, a lower number of expensive mmW components are needed to generate a large pixel count image.

The signal strength of the black body emission of a terrestrial object (300K) at mmWs is more than eight orders of magnitude lower than its IR peak at roughly 10μm. Consequently, the mmW detectors must be very sensitive, with low noise, to generate high-quality imagery. As previously noted, mmW radiation has frequencies that are above 30GHz, which limits the availability of amplifiers and increases the cost of the associated electronics. Many distributed aperture systems, such as those used by the radio astronomy community, mix the incoming signal with a distributed local oscillator to down-convert the frequency and make it easier to work with. They then correlate the different signals from all of the channels to re-create the mmW image. At PSI we have chosen to up-convert the complex (amplitude and phase) mmW signals to near-IR optical wavelengths (1.55μm, or ≈193THz) using a standard telecommunications laser optical carrier and ultra-high-performance optical modulators developed at UD.⁴ This enables the multi-GHz signals to be easily routed on lightweight, low-loss optical fibers. The up-converted signals are launched from an optical fiber array that mimics the geometry of the mmW antenna array. One of the mmW sidebands is optically filtered from the carrier on each channel, and then all channels are coherently recombined onto a standard shortwave IR camera to generate a real-time, video-rate image of the mmW scene.

Figure 3. Picture of the 220-channel front face of the passive mmW imager (left). Visible (middle) and mmW (right) images of spoke and cross targets taken in the laboratory.

We developed technology demonstration systems using this technique at 35GHz⁵ (λ≈8.6mm) and recently at 77GHz (λ≈3.9mm). Figure 3 shows a 220-channel, 77GHz, 20×20° instantaneous field of view passive imaging system housed in a 0.6×0.6m aperture size. Also shown are visible and mmW images of both a spoke and cross-shaped target taken in the laboratory. The visible images are captured with a small USB visible camera that is mounted in the center of the array. The targets consist of a patterned, room-temperature absorber sheet through which the radiation from a liquid nitrogen bath is reflected. The target distance is roughly 3m from the imager. This passive imaging system is designed to be used by the helicopter community to provide situational awareness in degraded visual environments such as ‘brownout’ conditions, which occur when landing rotary craft in dusty landing zones.⁶ The system can generate passive mmW video at 30 frames per second with an angular resolution of less than 7mrad, or 0.4°, and a sensitivity, or noise equivalent temperature delta, of a few kelvins. The 77GHz frequency band was chosen to leverage the availability of low-cost, low-noise amplifiers used in automotive adaptive cruise control systems. The total power consumption is on the order of 400W, and the next-generation imager will use composite housing materials to lower the overall weight to less than 30 pounds. The imager is scheduled for several helicopter flight tests in 2012 to determine its effectiveness as a navigational aid in brownout conditions.

We would like to thank Elizabeth Twarog, Dave Dowgiallo, and Peter Gaiser of the Naval Research Laboratory and their colleagues, as well as Michael Duncan and Antti Makinen at the Office of Naval Research – Information, Electronics, and Surveillance Division for supporting the development of a real-time W-band imaging system for use in brownout mitigation. Finally, we would like to thank Bruce Wallace of the Defense Advanced Research Projects Agency and MMW Concepts LLC for generously supplying the atmospheric attenuation model.