Matthew S. Leigh

Compact, single-frequency all-fiber Q-switched laser at 1 μm

We demonstrate a unique, all-fiber, actively Q-switched laser operating in the 1 microm region. The laser is compact, single mode, single frequency, highly polarized, and exhibits high peak power. The laser cavity is constructed without external coupling, utilizing fiber Bragg gratings that permit feedback at only a single polarization. By using a piezoelectric to press the fiber and modulate the fiber birefringence, the cavity is switched between high and low loss states, permitting Q-switching. We demonstrate this Q-switching at repetition rates up to 700 KHz.

In vivo size and shape measurement of the human upper airway using endoscopic long-range optical coherence tomography

We describe a long-range optical coherence tomography system for size and shape measurement of large hollow organs in the human body. The system employs a frequency-domain optical delay line of a configuration that enables the combination of high-speed operation with long scan range. We compare the achievable maximum delay of several delay line configurations, and identify the configurations with the greatest delay range. We demonstrate the use of one such long-range delay line in a catheter-based optical coherence tomography system and present profiles of the human upper airway and esophagus in vivo with a radial scan range of 26 millimeters. Such quantitative upper airway profiling should prove valuable in investigating the pathophysiology of airway collapse during sleep (obstructive sleep apnea).

Evaluation of pharyngeal shape and size using anatomical optical coherence tomography in individuals with and without obstructive sleep apnoea

This study compared shape, size and length of the pharyngeal airway in individuals with and without obstructive sleep apnoea (OSA) using a novel endoscopic imaging technique, anatomical optical coherence tomography (aOCT). The study population comprised a preliminary study group of 20 OSA patients and a subsequent controlled study group of 10 OSA patients and 10 body mass index (BMI)‐, gender‐ and age‐matched control subjects without OSA. All subjects were scanned using aOCT while awake, supine and breathing quietly. Measurements of airway cross‐sectional area (CSA) and anteroposterior (A‐P) and lateral diameters were obtained from the hypo‐, oro‐ and velopharyngeal regions. A‐P : lateral diameter ratios were calculated to provide an index of regional airway shape. In all subjects, pharyngeal CSA was lowest in the velopharynx. Patients with OSA had a smaller velopharyngeal CSA than controls (maximum CSA 91 ± 40 versus 153 ± 84 mm2; P < 0.05) but comparable oro‐ (318 ± 80 versus 279 ± 129 mm2; P = 0.48) and hypopharyngeal CSA (250 ± 105 versus 303 ± 112 mm2; P = 0.36). In each pharyngeal region, the long axis of the airway was oriented in the lateral diameter. Airway shape was not different between the groups. Pharyngeal airway length was similar in both groups, although the OSA group had longer uvulae than the control group (16.8 ± 6.2 versus 11.2 ± 5.2 mm; P < 0.05). This study has shown that individuals with OSA have a smaller velopharyngeal CSA than BMI‐, gender‐ and age‐matched control volunteers, but comparable shape: a laterally oriented ellipse. These findings suggest that it is an abnormality in size rather than shape that is the more important anatomical predictor of OSA.

Single-frequency terahertz source pumped by Q-switched fiber lasers based on difference-frequency generation in GaSe crystal

We demonstrate a unique terahertz (THz) source that is compact, utilizes recently developed all-fiber Q-switched lasers, and is based on difference-frequency generation in a GaSe crystal. A single piezo simultaneously Q switched the two fiber lasers by using stress-induced birefringence, to achieve the temporal overlap of pulses from the two fiber lasers. These correlated pulses then combine in the GaSe crystal to produce coherent and highly monochromatic THz pulses. The peak power for this THz source can reach 0.53 mW, corresponding to an average power of 0.43 microW and a conversion efficiency of 4.75 x 10(-7). The estimated linewidth of this THz source can be as narrow as approximately 35 MHz or 1.17 x 10(-3) cm(-1).

Needle-based refractive index measurement using low-coherence interferometry

We present a novel needle-based device for the measurement of refractive index and scattering using low-coherence interferometry. Coupled to the sample arm of an optical coherence tomography system, the device detects the scattering response of, and optical path length through, a sample residing in a fixed-width channel. We report use of the device to make near-infrared measurements of tissues and materials with known optical properties. The device could be used to exploit the refractive index variations of tissue for medical and biological diagnostics accessible by needle insertion.

Anatomical Optical Coherence Tomography for Long-Term, Portable, Quantitative Endoscopy

In this paper, we report on anatomical optical coherence tomography, a catheter-based optical modality designed to provide quantitative sectional images of internal hollow organ anatomy over extended observational periods. We consider the design and performance of an instrument and its initial intended application in the human upper airway for the characterization of obstructive sleep apnea (OSA). Compared with current modalities, the technique uniquely combines quantitative imaging, bedside operation, and safety for use over extended periods of time with no cumulative dose limit. Our experiments show that the instrument is capable of imaging subjects during sleep, and that it can record dynamic changes in airway size and shape.

High SBS-threshold single-mode single-frequency monolithic pulsed fiber laser in the C-band

We report a high SBS-threshold, single-frequency, single-mode, polarization maintaining (PM) monolithic pulsed fiber laser source in master oscillator and power amplifier (MOPA) configuration that can operate over the C-band. In order to achieve a narrow transform-limited linewidth for pulses longer than 100 ns, we use a single-frequency Q-switched fiber laser seed, which itself can be seamlessly tuned up to 1.24 micros. The Q-switched pulses are amplified in the power amplifier stage of MOPA using a high SBS threshold single-mode PM large core highly Er/Yb co-doped phosphate glass fiber (LC-EYPhF). This seed and amplifier combination represents the first monolithic, all-fiber implementation of a single-frequency pulsed laser with the highest pulse energy of 54 microJ and peak power of 332 W for 153-ns pulses at 1538 nm.

Measurement, Reconstruction, and Flow-Field Computation of the Human Pharynx With Application to Sleep Apnea

Repetitive closure of the upper airway characterizes obstructive sleep apnea. It disrupts sleep causing excessive daytime drowsiness and is linked to hypertension and cardiovascular disease. Previous studies simulating the underlying fluid mechanics are based upon geometries, time-averaged over the respiratory cycle, obtained usually via MRI or CT scans. Here, we generate an anatomically correct geometry from data captured in vivo by an endoscopic optical technique. This allows quantitative real-time imaging of the internal cross section with minimal invasiveness. The steady inhalation flow field is computed using a k- shear-stress transport (SST) turbulence model. Simulations reveal flow mechanisms that produce low-pressure regions on the sidewalls of the pharynx and on the soft palate within the pharyngeal section of minimum area. Soft-palate displacement and side-wall deformations further reduce the pressures in these regions, thus creating forces that would tend to narrow the airway. These phenomena suggest a mechanism for airway closure in the lateral direction as clinically observed. Correlations between pressure and airway deformation indicate that quantitative prediction of the low-pressure regions for an individual are possible. The present predictions warrant and can guide clinical investigation to confirm the phenomenology and its quantification, while the overall approach represents an advancement toward patient-specific modeling.

Needle-probe system for the measurement of tissue refractive index

Needle-based devices, which are in wide clinical use for needle biopsy procedures, may be augmented by suitable optical techniques for the localization and diagnosis of diseased tissue. Tissue refractive index is one optical contrast mechanism with diagnostic potential. In the case of mammary tissue, for example, recent research indicates that refractive index variations between tissue types may be useful for the identification of cancerous tissue. While many coherence-based forward-sensing devices have been developed to detect scattering changes, none have demonstrated refractive index measurement capabilities. We present a novel needle-based device that is capable of simultaneously measuring refractive index and scattering. Coupled to the sample arm of an optical coherence tomography system, the needle device detects the scattering response and optical pathlength through tissue residing in a fixed-width channel. Near-infrared measurements of tissues and materials with known optical properties using a prototype device will be presented. This work demonstrates the feasibility of integrated in vivo measurement of refractive index and scattering in conjunction with existing clinical needle-based devices.

Anatomical optical coherence tomography: a safe and effective tool for quantitative long-term monitoring of upper airway size and shape

Anatomical optical coherence tomography (aOCT) is an endoscopic optical technique that enables continuous, quantitative assessment of hollow organ size and shape in three dimensions. It is a powerful alternative to X-ray computed tomography, magnetic resonance imaging, and video endoscopy for the assessment of gross hollow-organ anatomy. This paper reviews our instrument and its application to the upper and lower airway, and includes a number of new results.