Laser multi-spectral polarimetric diffuse scattering (LAMPODS) imaging is an approach that maps object intrinsic optical
scattering properties rather than the scattered light intensity like in conventional imaging. The technique involves
comprehensive measurements to parameterize object optical responses with respect to wavelength, polarization, and
diffuse scattering. The derived parametric properties constitute LAMPODS images, which are more fundamental than
conventional images. The application is to uncover and discriminate features that are not obvious or obtainable with
conventional imaging. The experiments were performed for a number of targets, using near-infrared lasers. A system
architecture design configured similarly to optical wireless network is described, which can be used as a design for a
LAMPODS "camera". The results for a number of targets indicate unique LAMPODS capabilities to distinguish and
classify features based on optics principles rather than phenomenological image processing. Examples of uncovering,
enhancing, and interpreting target features, which are unseen or ambiguous in conventional images, are described.
The Houston Ship Channel (HSC) is a 50-mile long shipping channel that contains many private ports including the Port
of Houston Authority. It has a uniquely critical role with respect to the US petroleum energy supply. The HSC security
is currently planned for significant enhancement under the auspices of the Harris County and the Houston-based Port
Strategic Security Council. The ultimate objective is to comprehensively address the HSC threat matrix. This paper describes
the technical effort in support of this program. The HSC security is a complex system of systems that includes
the physical control access system, the command, control, communication, and information (C4I) system, and the telecom
infrastructure. The strategy is to coordinate the improvements of different components to achieve a high-impact net
effectiveness. A key element is a planned high-capacity backbone optical network for integrating the C4I of many different
HSC administrative-jurisdictional authorities, which will allow unified situational awareness for a more effective
cooperation and coordination. Enhancement of surveillance and intrusion protection is crucial. A feasibility study was
conducted for the HSC assuming common surveillance technologies including visible/IR camera, radar, and sonar. The
method includes survey and theoretical modeling to detect threats of concern in the HSC natural environment. The result
indicates that the HSC unique river-like geography offers both advantages and challenges. The narrow channel allows
ease of waterside surveillance, but likely incurs high cost for its great length. In addition, landside security is also important
owing to its location amidst urban-industrial zone. Lastly, limitation of the various technologies is discussed by
considering the broader challenges of the intrusion problem.
A theoretical design and simulation of a 3D ladar system concept for surveillance, intrusion detection, and access control
is described. It is a non-conventional system architecture that consists of: i) multi-static configuration with an arbitrarily
scalable number of transmitters (Tx's) and receivers (Rx's) that form an optical wireless code-division-multiple-access
(CDMA) network, and ii) flexible system architecture with modular plug-and-play components that can be deployed
for any facility with arbitrary topology. Affordability is a driving consideration; and a key feature for low cost is
an asymmetric use of many inexpensive Rx's in conjunction with fewer Tx's, which are generally more expensive. The
Rx's are spatially distributed close to the surveyed area for large coverage, and capable of receiving signals from multiple
Tx's with moderate laser power. The system produces sensing information that scales as NxM, where N, M are the
number of Tx's and Rx's, as opposed to linear scaling ~N in non-network system. Also, for target positioning, besides
laser pointing direction and time-of-flight, the algorithm includes multiple point-of-view image fusion and triangulation
for enhanced accuracy, which is not applicable to non-networked monostatic ladars. Simulation and scaled model experiments
on some aspects of this concept are discussed.
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.
LO-phonons in the active region of interband cascade lasers based on type-II multi-quantum well are investigated in detail. The dispersion relation and phonon potential for interface LO-phonon modes are calculated by the macroscopic dielectric continuum model. The expression of electron- phonon overlap with multiband k*p approximation is obtained and energy separations of hole subbands for fast LO-phonon assisted depopulations of the lower lasing state are proposed.
The electronic structures of (InAs)m/(GaSb)n short- period superlattices are investigated by the empirical pseudo potential method and multiband kp envelope function approximation with the same underlying bulk band structures. The calculated result are compared to each other and with experiment. Generally, the results of the empirical pseudo potential method are in better agreement with the experimental measurement. The superlattice active region of an optically-pumped type-II laser are investigated by the empirical pseudo potential method. The calculated result predict that its lasing wavelength is about 3.5 micrometers at 80K and internal losses due to intervalence absorption have been suppressed.
We report the recent progress of interband cascade (IC) lasers based on InAs/Ga(In)Sb/AlSb type-II quantum wells. For the 4.5-micrometers IC lasers, the internal loss was 11.6 cm-1 and the internal quantum efficiency was 460% at 90 K. When mounted epi-side down on diamond, cw operation was observed with an external quantum efficiency (EQE) of 193%, a cw output power over 500 mW, and a threshold current density as low as 35 A/cm2 at 80 K. Dual-wavelength IC laser was also demonstrated. The device lased simultaneously at 4.482 and 4.568 micrometers . At 110 K, a peak output power of 150 mW per facet was achieved with 5-microsecond(s) pulses at 1-KHz repetition rate. The threshold current density, average EQE, and peak output power of a 0.4-mm long device were 119 A/cm2, 278%, and 150 mW per facet, respectively.
The first demonstration of an epi-down mounted type-II optically pumped (OP) mid-infrared (IR) laser grown on an InGaAs-GaAs bonded substrate is reported. The device consisted of 60 periods of InAs/InGaSb/InAs/AlSb quantum wells, and was mounted epi-side down on a Cu heat sink. The InGaAs-GaAs bonded substrate allowed the device to be pumped from the substrate side of the laser with a 980 nm diode laser array. The laser emitted at 4.6-micrometers at 80 K, and peak power output was 300-mW per (uncoated) facet for 10 microsecond(s) pulses, and 200-mW per facet for 500-microsecond(s) pulse duration, 500-Hz-repetition rate. For comparison, a low- filled-factor mid-IR OP laser grown on a GaSb substrate was also studied. This device ((lambda) equals 3.67 micrometers ) showed improved external quantum efficiency (approximately 26%) compared with previously type-II OP lasers, and high peak output power (> 0.45 W per facet 500-microsecond(s) pulse).
High power and high quantum efficiency operation of both diode-pumped and interband cascade lasers based on InAs/Ga(In)Sb type-II quantum well with a broken gap band alignment have been demonstrated. The interband cascade laser yielded 0.5 W peak and 16 mW average output per facet under 1- and 5-microsecond(s) long pulses at 80K, while the optically pumped 4-micrometers devices yielded 0.9-1.6 W peak and 90-150 mW average output per facet for 0.1- to 1-ms long pulses at 71K. These output powers are among the highest long-pulse results reported from any semiconductor laser at these wavelengths.
Laser efficiency is an important issue for mid-IR Sb-based semiconductor lasers. It has been the key limiting factor in the efforts to develop high power lasers in recent years. This paper reviews the basic aspects of the problem and discusses some recent results. A number of factors affect the efficiency, one of which common to many materials was a high internal loss that increases rapidly versus temperature. A major contribution to this internal loss is the large intervalence band carrier absorption that occurs in all mid- to-low-gap III-V semiconductors. Recent studies of both types of Sb-based laser materials with InAs-like valence band (InAsSb) and GaSb-like valence band (InAs/GaInSb/AlSb type-II quantum wells) have showed such a strong absorption. Coupled with the thermal effects, the internal loss behavior results in an efficiency roll-off that limits the power performance. Issues for the improvement of the efficiency are discussed.
GaSb-based and InAs-based semiconductor gain media with band-edge wavelengths between 3.3 to 4 micrometers were used in grating-tuned external cavity configuration. Output wavelength was tuned up to approximately 9.5% of the center wavelength; and power from few tens of mW to 0.2-W peak, 20- mW average was achieved at 80 K operation. The tuning range is approximately 2 - 3 times wider than those of near-IR semiconductor lasers, as expected for mid-IR semiconductors which have smaller electron masses. The external cavity laser had a multimode linewidth of 1 - 2 nm, which was approximately 10 to 20 times narrower than that of a free running laser. Analysis of the gain/loss spectral properties indicates that the tuning range is still severely limited by facet anti-reflection coating and non-optimal wafer structure. Model calculation indicates a tuning range a few times larger is possible with more optimal wafer design.
A number of double heterostructure and quantum well lasers with wavelengths approximately 3.1, 3.2, 3.4, 3.85 - 4.1, and 4.5 micrometers have been realized in InAsSb/GaSb and HgCdTe/CdZnTe material systems. Peak powers at the few W level and average power at the few hundred mW-level were obtained from optically pumped broad-area lasers at >= 80 K. Threshold, efficiency, internal loss, and gain saturation studies are reported. A compact laser package was built, using a high-power diode array for pumping and a Stirling pump for cooling. Its performance with a 4-micrometers laser is described.