B. Golubovic

Single mode fiber-optic catheter/endoscope for optical coherence tomography

Summary form only given. In order to apply OCT for imaging of internal organ systems, a flexible, small diameter, catheter/endoscope, which is capable of delivering, focusing, scanning, and collecting a single-spatial-mode optical beam, must be constructed. In this summary, we describe the design and performance of a prototype single-mode fiber-optic scanning OCT catheter with a diameter of 1 mm. OCT imaging may be performed at 1.3-micron wavelengths using either a superluminescent laser diode source or a Kerr-lens mode-locked Cr:forsterite laser, which provides high powers for high-speed imaging. This device is an enabling technology for OCT and will permit micron scale, cross-sectional medical diagnostic imaging in tissues such as the vascular system, the gastrointestinal tract, the urinary tract, and the respiratory tract.

Rapid acquisition of in vivo biological images by use of optical coherence tomography

The development of techniques for high-speed image acquisition in optical coherence tomography (OCT) systems is essential for suppressing motion artifacts when one is imaging living systems. We describe a new OCT system for performing micrometer-scale, cross-sectional optical imaging at four images/s. To achieve OCT image-acquisition times of less than 1 s, we use a piezoelectric fiber stretcher to vary the reference arm delay. A Kerr-lens mode-locked chromium-doped forsterite laser is employed as the low-coherence source for the highspeed OCT system. Dynamic, motion-artifact-free in vivo imaging of a beating Xenopus laevis (African frog) heart is demonstrated.

Optical frequency-domain reflectometry using rapid wavelength tuning of a Cr4+:forsterite laser.

We present a cw chromium-doped forsterite laser that permits rapid wavelength tuning over a broad bandwidth and demonstrate the application of this source to frequency-domain ranging and optical tomography. The entire tuning range of 1200 to 1275 nm can be swept in less than 500 micros . This permits frequency-domain ranging to be performed with a scan rate of 2 kHz and an axial resolution of 15 microm .

Self-phase-modulated Kerr-lens mode-locked Cr:forsterite laser source for optical coherence tomography

An all-solid-state Kerr-lens mode-locked Cr:forsterite laser operating at 1.28 microm is demonstrated as a shortcoherence-length, high-average-power source for optical coherence tomographic (OCT) imaging. We achieve ultrahigh resolution by spectrally broadening the laser pulses, using self-phase modulation in a dispersionshifted single-mode fiber. OCT imaging with a resolution of 6 microm and a dynamic range of 115 dB is achieved.

Femtosecond investigations of spectral hole burning in semiconductor lasers

An investigation of spectral hole burning effects is reported in the gain region of InGaAs/AlGaAs strained‐layer single quantum well diode lasers, using a heterodyne nondegenerate pump‐probe technique. This technique permits simultaneous measurement of the femtosecond dynamics and spectral dependence of transient gain in semiconductor lasers. At high bias current, strong spectral hole burning effects were observed.

Millimeter–submillimeter wavelength filter system

We describe the design, fabrication, measurement, and performance of a set of cryogenic millimetersubmillimeter wavelength filters used in a balloonborne bolometric radiometer. The set contains single resonant mesh grids used as dichroic beam splitters, resonant meshes in a double quarter-wave configuration, a commercial inductive grid filter, and high-frequency blocking filters. The resultant system has passbands at λ = 1.73, 1.05, 0.61, 0.44 mm with δλ/λ = 0.23, 0.23, 0.12, 0.06. Limits on high-frequency leakage are deduced from laboratory measurements and from the analysis of flight data. The filter set response to three different sources of radiation is presented to show the method and limitations of our characterization. The key element of the filter system is a resonant periodic array of cross-shaped holes etched in thin aluminum. We give an empirical scaling law for the resonant wavelength as a function of structure parameters for aluminum on 25-µm-thick Mylar. Plots of the transmittance for normally incident radiation and the transmittance and reflectance for a 45° incident radiation are presented.

Thin crystal, room-temperature Cr(4+):forsterite laser using near-infrared pumping.

We describe a new design for chromium-doped forsterite lasers based on near-IR (700-800-nm) pumping of a thin (3-mm) crystal. In contrast with pumping at 1.06 microm, near-IR pumping permits the use of shorter crystal lengths, which enable one to develop compact and diode-pumped laser geometries. Near-IR pumping also results in an increased effective figure of merit and tuning performance. Using a Ti:Al(2)O(3) pump laser, we investigated cw laser characteristics over a range of pump wavelengths compatible with diode pumping. Room-temperature cw operation with a tuning range of 1175 to 1375 nm was achieved. Kerr-lens mode-locked operation of this laser was also demonstrated with pulse durations of ~50 fs.

Heterodyne nondegenerate pump-probe measurement technique for guided-wave devices.

We describe a new heterodyne nondegenerate pump-probe waveguide measurement technique that permits independent control of wavelengths, pulse widths, and polarizations of the pump and probe pulses in a collinear geometry. This technique provides both time- and spectral-domain information and can be applied to transmission and index measurements alike. We demonstrate this technique for the measurement of gain dynamics in a strained-layer single-quantum-well diode laser.

Optical Coherence Tomographic Imaging Using a Mode Locked Cr4+:Forsterite Laser Source

A fiber coupled KLM Cr4+:Forsterite laser spectrally broadened in a dispersion shifted fiber providing broad bandwidth light is used to enable high resolution biomedical imaging.

Hot Carrier Effects in Femtosecond Gain Dynamics of InGaAs/AlGaAs Quantum Well Lasers

Ultrafast optical nonlinearities in semiconductors play a central role in determining transient amplification and pulse-dependent gain saturation in quantum-well diode lasers. Both carrier-phonon and carrier-carrier scattering are expected to influence the nonlinearities. In this paper, we investigate hot electron effects on the semiconductor gain dynamics of strained-layer quantum well lasers. We present a relaxation time approximation model for carrier-carrier scattering in strained layer lasers. The relaxation approximation makes the problem an effective one dimensional problem which can then be solved directly using an adaptive Runge Kutta routine1. This procedure requires substantially less computational resources than a full Monte Carlo simulation. Results show that the inclusion of carrier-carrier scattering improves previous results with only carrier-phonon scattering.