Mid-IR semiconductor lasers of two wavelength bands, 5.4 and 9.6 µm, are applied to measure aqueous glucose concentration ranging from 0 to 500 mg/dL with Intralipid® emulsion (0 to 8%) added as a fat simulator. The absorption coefficient µa is found linear with respect to glucose and Intralipid® concentrations, and their specific absorption coefficients are obtained via linear regression. These coefficients are subsequently used to infer the concentrations and compare with known values. The objective is to evaluate the method accuracy. Glucose concentration is determined within ±21 mg/dL with 90% confidence and ±32 mg/dL with 99% confidence, using <1-mJ laser energy. It is limited by the apparatus mechanical error and not the photometric system noise. The expected uncertainties due to photometric noise are ±6 and ±9 mg/dL with 90 and 99% confidence, respectively. The uncertainty is fully accounted for by the system intrinsic errors, allowing rigorous inference of the confidence level. Intralipid® is found to have no measurable effect on glucose determination. Further analysis suggests that a few mid-IR wavelengths may be sufficient, and that the laser technique offers advantages with regard to accuracy, speed, and sample volume, which can be small, ~0.4×10−7 mL for applications such as microfluidic or microbioarray monitoring.
This paper describes an application-centric development of broadly tunable and multi-spectral mid/long-wave IR semi-conductor lasers. Examples of various external-cavity lasers capable of broad, continuous wavelength tuning with type-I and type-II quantum cascade lasers are discussed. Laser configurations studied include conventional Littman-Metcalf, Littrow, multi-segment and Bragg-grating-coupled surface-emitting. All were capable of single-mode continuous tuning with high side-mode-suppression ratio. The lasers were evaluated with spectroscopic applications, which include wave-length-modulation spectroscopic imaging and multi-wavelength decomposition of a gas mixture. The results showed that these lasers were capable of maintaining wavelength accuracy and stability over the entire tuning range. Multi-spectral imaging with discrete wavelengths over a wide spectral range was also studied. The results with a modest 4-wavelength system demonstrated the potential application for target discrimination, detection, and identification. These results suggest potential value for broadly tunable, wide-band M/LWIR laser technology.
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 μm) 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 μm, 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 μm, 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.
Mid-wave/Long-wave IR (3-14 μm) semiconductor lasers such as QC and Sb can be used for standoff chemical agent sensing in a network architecture that is different from conventional absorption lidars. Compact, potentially inexpensive semiconductor lasers may allow using them in a large number that form a cooperative network in which, the integrated sensing information is much more than the sum of its parts. This paper presents a study of system architecture based on CDMA, similarly to a CDMA optical wireless network, which allows a system of many distributed units to plug-and-play and cooperate with each other for N2 information scaling, rather than interfering with each other in non-networked architecture. This paper describes experimental studies with this system architecture, conducted with M/LWIR lasers, near-IR lasers, using wavelength-division-multiplexing (WDM) technique for high spectral fidelity, optical scanner for multi-spectral imaging, and simulated spatially distributed transmitters and receivers for sensor network. Specifically, the use of advanced lasers capable of broad and continuous wavelength tuning and modulation for WMS imaging is described. The experimental results suggest that M/LWIR spectral imaging with WDM multi-spectral transmitters is highly promising for chemical agent detection and visualization.
Infrared micro-spectroscopy is a useful tool for basic research and biomedical applications. Conventional microspectroscopic imaging apparatuses use thermal sources for sample illumination, which have low brightness, low optical spectral intensity, and high noise. This work evaluates the system engineering advantages of using mid-infrared semiconductor lasers that offer orders-of magnitude higher brightness, spectral intensity, and lower noise. A laser-based microscopic spectral imaging system with focal plane array detectors demonstrated a high signal-to-noise ratio (>20 dB) at video frame rate for a large illuminated area. Microscopic spectral imaging with fixed-wavelength and tunable lasers of 4.6, 6, and 9.3-μm wavelength was applied to a number of representative samples that consist of biological tissues (plant and animal) and solid material (a stack of laminated polymers). Transmission spectral images with ~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.
A system is described for production-line mail packaging inspection and validation. This system utilizes optical character recognition to achieve real-time performance. A computationally efficient technique is employed for binary image noise reduction. Another effective method has been developed for fast and concise text segmentation. An improved character feature extraction technique has been adopted for robust numeral recognition. A data structure is implemented to enable the system to simultaneously track and process multiple targets. High recognition rate and real-time performance is demonstrated by a prototype system.