Multi-Wavelength Mid-Infrared Micro-Spectral Imaging Using Semiconductor Lasers

Abstract

Infrared (IR, 3-12-microm) microscopic spectral imaging is an important analytical technique. Many current instruments employ thermal IR light sources, which suffer the problem of low brightness and high noise. This paper evaluates the system engineering merit in using semiconductor lasers, which offer orders-of-magnitude-higher power, brightness, and lower noise. A microscopic spectral imaging system using semiconductor lasers (quantum cascade) as illuminators, and focal plane array detectors demonstrated a high signal-to-noise ratio (> 20 dB) at video frame rate for a large illuminated area. The comparative advantages of laser vs. thermal light source are analyzed and demonstrated. Microscopic spectral imaging with fixed-wavelength and tunable lasers of 4.6-, 5.1-, 6-, and 9.3-microm wavelength was applied to a number of representative samples that consist of biological tissues (plant and animal), solid material (a stack of laminated polymers), and liquid chemical (benzene). Transmission spectral images with approximately 30-dB dynamic range were obtained with clear evidence of spectral features for different samples. The potential of more advanced systems with a wide coverage of spectral bands is discussed.

Infrared Spectroscopic Imaging: The Next Generation

“…Lasers have been widely used for spectroscopic analyses 108 for a number of years and, similarly, various studies have used QCLs for IR spectroscopy 109,–113 as well as microscopy and imaging. 114,115 The most widely reported studies in spectroscopy have been focused on using the narrow-band properties and high intensity of the QCL to profile specific species, aqueous environments 116,–118 or the integrated and compact instrumentation 119,120 that may be developed for specific experimental configurations 121 or applications. 122,–124 The technology has made rapid progress, specifically in spectral properties and power.…”

Hyperspectral microscopy of explosives particles using an external cavity quantum cascade laser

“…The quantum cascade (QC) laser has emerged as a high performance semiconductor laser for MIR spectroscopy, offering high output power and high spectral radiance in a compact and robust device. 11 Early uses of QC lasers in microscopic chemical imaging capitalized on their high brightness, 12 but were limited by a small wavelength tuning range. The ECQCL source overcomes this limitation by providing a tuning range of 10% to 20% of its center wavelength or greater.…”

Mid-infrared laser measurements of aqueous glucose

“…For practicality, they are robust, compact, and potentially low cost. Advanced MIR semiconductor lasers have been demonstrated for tissue imaging, 27 and application for conjunctiva glucose measurement is being developed. 24 A potential issue with MIR lasers is the lack of large spectral coverage, but some studies have shown that only a few wavelengths may be needed.…”

“…24 A potential issue with MIR lasers is the lack of large spectral coverage, but some studies have shown that only a few wavelengths may be needed. 11,13,24 Also, recent progress in tunable MIR lasers [27][28][29][30][31] promises broad wavelength coverage.…”