Julia Rentz Dupuis

Considerations for DMDs operating in the infrared

The evolution of the DMD has enabled the development of a broad range of technologies from sensors to various types of devices such as projectors and displays. The low cost and excellent reliability has made the DMD an ideal choice for most applications requiring a spatial light modulator. The aluminum micromirrors can be used over a broad spectral range with an appropriate window; DMD-based systems have therefore been realized across the ultraviolet through and including the longwave infrared. Of particular interest for scene projector, compressive imaging, and spectrometer applications is the use of the DMD in the infrared where diffraction, instrument radiance, and optical resolution impose performance limits. Diffraction and instrument radiance, among other factors, impact the highest achievable contrast, and constraints on the lowest practical illumination and projection f/# limit the ability to resolve a single micromirror at longer wavelengths. In this paper, we present analytical models addressing these issues as well as demonstrated solutions in a DMD-based midwave infrared (MWIR) scene projector as well as a MWIR compressive imaging camera.

Time-domain surface profile imaging via a hyperspectral Fourier transform spectrometer.

We describe a method for time-domain surface profile measurements via white-light reflection spectroscopy using a hyperspectral Fourier transform spectrometer (HS-FTS). This technique measures the frequency of the spectral modulation of reflected light from a multilayer optical surface and reports the spatially resolved optical thickness. Owing to the Fourier relationship, the Fourier transform spectrometer manifests this spectral modulation as temporal satellites in interferogram space. We show that measurement of the positions of these satellites can be used to reconstruct the optical thickness profile over a surface using the HS-FTS.

Hyperspectral Fourier transform spectrometer for reflection spectroscopy and spectral self-interference fluorescence microscopy

A hyperspectral Fourier transform spectrometer has been developed for studying biological material bound to optically reflecting surfaces. This instrument has two modes of operation: a white-light reflection mode and a spectral self-interference fluorescence mode. With the combined capability, information about the conformation of an ensemble of biomolecules may be determined. To the best of our knowledge, ours is the first report of this hybrid white-light reflection, spectral self-interference fluorescence measurement with any type of hyperspectral imager. The measurement technique is presented along with a full description of the system, including theoretical performance projections. Proof-of-principle measurements of artificial samples are shown, and the results are discussed.

Compact, wide field DRS explosive detector

OPTRA is developing a compact, wide field standoff diffuse reflectance spectrometer for trace explosive detection from a safe standoff. This system is comprised of two key components: a Risley scanner and an infrared tunable laser based spectrometer. The Risley scanner is a mature technology, which uses a pair of matched prisms to steer a laser beam anywhere inside a cone. The compact size, low operating power, and large field of view of the Risley scanner make it the ideal solution for rapidly scanning the laser over the field. The infrared tunable laser spectrometer utilizes a low-cost quartz crystal tuning fork (QCTF) in place of a traditional infrared detector. The large Q-factor of the QCTF enables high sensitivity, low noise detection of explosive signatures even for low concentrations and large standoffs. By coupling this demonstrated technology with a mature Risley scanner design, the field can be scanned both spatially and spectrally. Pairing this data with sophisticated algorithms results in a map of explosives in the field. This paper presents OPTRAs breadboard spectrometer design along with the TNT and RDX spectra it produced.

High-dynamic range DMD-based IR scene projector

OPTRA is developing a next-generation digital micromirror device (DMD) based two-band infrared scene projector (IRSP) with infinite bit-depth independent of frame rate and an order of magnitude improvement in contrast over the state of the art. Traditionally DMD-based IRSPs have offered larger format and superior uniformity and pixel operability relative to resistive and diode arrays, however, they have been limited in contrast and also by the inherent bitdepth / frame rate tradeoff imposed by pulse width modulation (PWM). OPTRA’s high dynamic range IRSP (HIDRA SP) has broken this dependency with a dynamic structured illumination solution. The HIDRA SP uses a source conditioning DMD to impose the structured illumination on two projector DMDs – one for each spectral band. The source conditioning DMD is operated in binary mode, and the relay optics which form the structured illumination act as a low pass spatial filter. The structured illumination is therefore spatially grayscaled and more importantly is analog with no PWM. In addition, the structured illumination concentrates energy where bright object will be projected and extinguishes energy in dark regions; the result is a significant improvement in contrast. The projector DMDs are operated with 8-bit PWM, however the total projected image is analog with no bit-depth / frame rate dependency. In this paper we describe our progress towards the development, build, and test of a prototype HIDRA SP.

High dynamic range digital micromirror device-based infrared scene projector

Abstract. OPTRA is developing a next generation digital micromirror device (DMD) based two-band infrared scene projector (IRSP) with infinite bit depth independent of frame rate and an order of magnitude improvement in contrast over the state of the art. Traditionally, DMD-based IRSPs have offered larger format, superior uniformity, and pixel operability relative to resistive and diode arrays. However, they have been limited in contrast and also by the inherent bit depth/frame rate tradeoff imposed by pulse width modulation (PWM). OPTRA’s high dynamic range IRSP (HIDRA SP) has broken this dependency with a dynamic structured illumination solution. The HIDRA SP uses a source-conditioning DMD to impose the structured illumination on two projector DMDs—one for each spectral band. The source-conditioning DMD is operated in binary mode, and the relay optics that form the structured illumination act as a low-pass spatial filter. The structured illumination is, therefore, spatially grayscaled and more importantly is analog with no PWM. In addition, the structured illumination concentrates energy where bright objects will be projected and extinguishes energy in dark regions; the result is a significant improvement in contrast. The projector DMDs are operated with 8-bit PWM; however, the total projected image is analog with no bit depth/frame rate dependency. We describe our progress toward the development, building, and testing of a prototype HIDRA SP.

Cooperative use of standoff and UAV sensors for CBRNE detection

The defense of the US armed forces against chemical and biological (CB) attack is transitioning from a focus on standoff detection of these threats to the concept of Early Warning (EW). In this approach an array of dual-use and low-burden dedicated use sensor capabilities are used to replace longer-range single use sensors to detect a CB attack. In this paper we discuss the use of passive broadband thermal imaging to detect chemical vapor clouds as well as a developing suite of compact UAV-borne chemical and radiological sensors for the investigation of threats detected by these indirect approaches. The sensors include a colorimetric ammonia sensor, a chemical sensor based on ion mobility spectrometry, and a radiation detector based on gamma ray scintillation. The implementation and initial field tests of each of these sensor modalities is discussed and future plans for the further development of the capability is presented.

Fiber Optic Snapshot Hyperspectral Imager

OPTRA is developing a snapshot hyperspectral imager (HSI) employing a fiber optic bundle and dispersive spectrometer. The fiber optic bundle converts a broadband spatial image to an array of fiber columns which serve as multiple entrance slits to a prism spectrometer. The dispersed spatially resolved spectra are then sampled by a two-dimensional focal plane array (FPA) at a greater than 30 Hz update rate, thereby qualifying the system as snapshot. Unlike snapshot HSI systems based on computed tomography or coded apertures, our approach requires only the remapping of the FPA frame into hyperspectral cubes rather than a complex reconstruction. Our system has high radiometric efficiency and throughput supporting sufficient signal to noise for hyperspectral imaging measurements made over very short integration times (< 33 ms). The overall approach is compact, low cost, and contains no moving parts, making it ideal for unmanned airborne surveillance. In this paper we present a preliminary design for the fiber optic snapshot HSI system.

Contrast analysis for DMD-based IR scene projector

OPTRA has developed a two-band midwave infrared (MWIR) scene projector based on digital micromirror device (DMD) technology; the projector is intended for training various IR tracking systems that exploit the relative intensities of two separate MWIR spectral bands. Next generation tracking systems have increasing dynamic range requirements (on the order of 12-bits) which current DMD-based projector test equipment is not capable of meeting. While sufficient grayscale digitization can be achieved with drive electronics, commensurate contrast is not currently available. In this paper we present a detailed analysis of the contrast of our MWIR DMD-based scene projector. A series of factors which affect the overall contrast are modeled and design approaches to address the worst offenders are presented. In addition, we present methods for meeting the grayscale digitization requirements through the drive electronics.

Imaging Open-Path Fourier Transform Infrared Spectrometer For 3D Cloud Profiling

OPTRA is developing an imaging open-path Fourier transform infrared (I-OP-FTIR) spectrometer for 3D profiling of chemical and biological agent simulant plumes released into test ranges and chambers. An array of I-OP-FTIR instruments positioned around the perimeter of the test site, in concert with advanced spectroscopic algorithms, enables real time tomographic reconstruction of the plume. The approach is intended as a referee measurement for test ranges and chambers. This Small Business Technology Transfer (STTR) effort combines the instrumentation and spectroscopic capabilities of OPTRA, Inc. with the computed tomographic expertise of the University of North Carolina, Chapel Hill.