Multi-spectral imaging with mid-infrared semiconductor lasers

Abstract

Multi-spectral laser imaging can be a useful technology for target discrimination, classification, and identification based on object spectral signatures. The mid-IR region (~3-14 m) is particularly rich of molecular spectroscopic fingerprints, but the technology has been under utilized. Compact, potentially inexpensive semiconductor lasers may allow more cost-effective applications. This paper describes a development of semiconductor-laser-based multi-spectral imaging for both near-IR and mid-IR, and demonstrates the potential of this technology. The near-IR study employed 7 wavelengths from 0.635-1.55 m, and used for system engineering evaluation as well as for studying the fundamental aspects of multi-spectral laser imaging. These include issues of wavelength-dependence scattering as a function of incident and receiving angle and the polarization effects. Stokes vector imaging and degree-of-linear-polarization were shown to reveal significant information to characterize the targets. The mid-IR study employed 4 wavelengths from 3.3-9.6 m, and was applied to diverse targets that consist of natural and man-made materials and household objects. It was shown capable to resolve and distinguish small spectral differences among various targets, thanks to the laser radiometric and spectral accuracy. Colorless objects in the visible were shown with "colorful" signatures in the mid-IR. An essential feature of the study is an advanced system architecture that employs wavelength-division-multiplexed laser beams for high spectral fidelity and resolution. In addition, unlike conventional one-transmitter and one receiver design, the system is based on a scalable CDMA network concept with multiple transmitters and receivers to allow efficient information acquisition. The results suggest that multi-spectral laser imaging in general can be a unique and powerful technology for wide ranging applications.

Laser Multi-Spectral Polarimetric Diffuse-Scatter Imaging

“…Each wavelength is independently modulated and the Rx can simultaneously decode to receive signals for all wavelengths. More details of the experimental approaches can be found elsewhere [6][7][8].…”

Semiconductor Laser Multi-Spectral Sensing and Imaging

“…A variety of thin-film filters were employed as needed. Polarization optics for Stokes parameter measurements were also available for the vis-near-IR setup, but the results [ 26 , 27 ] are not relevant to the results discussed in this paper. The detectors include Si and InGaAs for near-IR, and a combination of InSb and HgCdTe with a bandpass beam-splitter for M/LWIR.…”

“… Example of spectral images with features that allow strong discrimination [ 27 ]. (a) Legitimate US currency (top left 4 images) vs. its color photocopy (bottom left 4 images).…”

“…This paper describes some recent studies [ 22 – 27 ] in laser multi-spectral sensing and imaging with semiconductor lasers ranging from near-IR (NIR) to midwave- and longwave-IR (M/LWIR), showing the technique capability and potential for spectroscopic discrimination of objects. The essence of this work is imaging, in the same spirit of passive spectral imaging and is not limited to spectroscopic sensing in the conventional sense of those works mentioned above [ 4 – 17 ].…”