Extended continuous-wave supercontinuum generation in a low-water-loss holey fiber

We report on the development of a 2.5 microm core photonic crystal fiber with a substantially reduced water-peak loss around 1.38 microm, which allows extended Raman-soliton supercontinuum generation up to 1.55 microm with a cw ytterbium fiber laser pump source. The resulting broadband, high-spectral-power-density, low-coherence light source can be employed for advanced, submicrometer resolution optical coherence tomography.     

Optical phase-space distributions for low-coherence light

Using a novel heterodyne technique, we measure optical phase-space distributions in momentum and position for low-coherence light. Quantitative information is obtained simultaneously about the longitudinal and the transverse coherence properties as well as the wave-front curvature of the light field. This method can be used to monitor these optical parameters directly for signal fields scattered from samples of interest, for tomographic imaging.     

Measurement of optical constants for dense media by low-coherence dynamic light scattering

We perform the optical constants measurements for different absorption dense media by low-coherence dynamic light scattering (DLS) technique. The estimated particle size is used to calculate the scattering coefficient of particles suspended in dense media. The path-length resolved intensity distributions of light backscattered from the absorbing dense media are investigated experimentally by virtue of path-length resolved performance in low-coherence DLS measurements. The absorption coefficient can be obtained by applying the measured path-length resolved intensity distributions to the modified Lambert-Beer law. As a result, we proposed a new low-coherence DLS technique in simultaneous measurement of the scattering and absorption coefficients of absorbing dense media.     

Phase-dispersion optical tomography

We report on phase-dispersion optical tomography, a new imaging technique based on phase measurements using low-coherence interferometry. The technique simultaneously probes the target with fundamental and second-harmonic light and interferometrically measures the relative phase shift of the backscattered light fields. This phase change can arise either from reflection at an interface within a sample or from bulk refraction. We show that this highly sensitive (~5 degrees ) phase technique can complement optical coherence tomography, which measures electric field amplitude, by revealing otherwise undetectable dispersive variations in the sample.     

光纤零色散波长的精确测量

提出了一种测量单模光纤零色散波长的新方法,即让可调谐的连续波（波长λｓ）和波长λｐ位于光纤正常色散区的泵浦脉冲串在光纤中共同传输,交叉相位调制与群速度色散的相互作用可使得连续波的强度得到调制。对连续波λｓ进行调谐,若连续波被调制成亮脉冲,则表明λｓ位于光纤负色散区;若连续波被调制成暗脉冲,则表明λｓ位于光纤正色散区;若连续波的强度不能被调制,则表明λｓ位于光纤的零色散波长λ０附近。利用这一方法对长度为０．９８ｋｍ的色散位移光纤的零色散波长进行了测量,测量精度可达０．５ｎｍ。     

High-efficiency 60 W TEM(00) Nd:YVO(4) oscillator pumped at 888 nm

We propose a technique for pumping Nd:YVO(4) with high optical power at 888nm while making absorption independent of the pump light polarization state. This is especially suitable for systems end pumped by high-power, high-brightness fiber-coupled diode sources associated with long vanadate crystals to effectively spread the heat load in a large volume. A compact 60 W output, 55% optical efficiency cw TEM(00) oscillator was demonstrated.     

Determination of the depth-resolved Stokes parameters of light backscattered from turbid media by use of polarization-sensitive optical coherence tomography

Polarization-sensitive optical coherence tomography (PS-OCT) was used to characterize completely the polarization state of light backscattered from turbid media. Using a low-coherence light source, one can determine the Stokes parameters of backscattered light as a function of optical path in turbid media. To demonstrate the application of this technique we determined the birefringence and the optical axis in fibrous tissue (rodent muscle) and in vivo rodent skin. PS-OCT has potentially useful applications in biomedical optics by imaging simultaneously the structural properties of turbid biological materials and their effects on the polarization state of backscattered light. This method may also find applications in material science for investigation of polarization properties (e.g., birefringence) in opaque media such as ceramics and crystals.     

Optical low-coherence reflectometry characterization of cladding modes excited by long-period fiber gratings

The group refractive-index difference of cladding modes excited by a long-period fiber grating is characterized by use of the technique of optical low-coherence reflectometry, with a precision of <10(-4) . Very good agreement between theoretical and experimental results is demonstrated.     

Spectral-domain optical coherence phase microscopy for quantitative phase-contrast imaging

We describe a novel microscopy technique for quantitative phase-contrast imaging of a transparent specimen. The technique is based on depth-resolved phase information provided by common path spectral-domain optical coherence tomography and can measure minute phase variations caused by changes in refractive index and thickness inside the specimen. We demonstrate subnanometer level path-length sensitivity and present images obtained on reflection from a known phase object and human epithelial cheek cells.     

Design of a subnanometer resolution beam position monitor for dielectric laser accelerators

We present a new concept for a beam position monitor with the unique ability to map particle beam position to a measurable wavelength. Coupled with an optical spectrograph, this beam position monitor is capable of subnanometer resolution. We describe one possible design, and through finite-element frequency-domain simulations, we show a resolution of 0.7 nm. Because of its high precision and ultracompact form factor, this device is ideal for future x-ray sources and laser-driven particle accelerators "on a chip."     

Precision phase control of an ultrawide-bandwidth femtosecond laser: a network of ultrastable frequency marks across the visible spectrum

We demonstrate that the stability of the current optical frequency comb generated by a Kerr-lens mode-locked femtosecond laser is limited by the microwave reference used for phase locking the comb spacing. Hence we implement precision frequency/phase control of the entire comb to the fundamental and second-harmonic frequencies of a stable cw laser without any external microwave reference. The stability of a cw iodine-stabilized laser is transferred to millions of comb lines (with an instability of 3 x 10(-13)) covering more than one octave of the optical frequency spectrum. In addition, the mode spacing of the comb can be used as a stable microwave frequency derived directly from a stable optical oscillator.     

Two-dimensional birefringence imaging in biological tissue by polarization-sensitive optical coherence tomography

Using a low-coherence Michelson interferometer, we measure two-dimensional images of optical birefringence in bovine tendon as a function of depth. Polarization-sensitive detection of the signal formed by interference of backscattered light from the sample and a mirror in the reference arm give the optical phase delay between light that is propagating along the fast and slow axes of the birefringent tendon. Images showing the change in birefringence in response to laser irradiation are presented. The technique permits rapid noncontact investigation of tissue structural properties through two-dimensional imaging of birefringence.     

Dual-axes confocal microscopy with post-objective scanning and low-coherence heterodyne detection

We present a dual-axes confocal microscope that employs postobjective scanning and low-coherence heterodyne detection to collect vertical cross-sectional images from biological tissue with high axial resolution, reduced noise from scattered light, deep tissue penetration, and a large dynamic range. This architecture can be scaled down to millimeter dimensions with microelectromechanical systems technology for performance of in vivo optical biopsy.     

Tunable,dual-wavelength interferometrically coupled continuous-wave parametric source

Generation of dual-wavelength continuous-wave (cw) radiation with independent and arbitrarily tuning, and indefinitely close spacing, using two cw optical parametric oscillators (OPOs) coupled with an anti-resonant ring interferometer is reported. The singly resonant OPOs, based on identical 30-mm-long MgO:sPPLT crystals, are pumped by a single cw laser at 532 nm. Two pairs of signal and idler wavelengths can be independently and arbitrarily tuned, with each signal (idler) pair tuned through degeneracy and beyond. Frequency separation between two distinct resonant signal waves from 7 down to 0.8 THz is demonstrated, and their overlap at 951 nm providing a frequency difference as small as ~220 MHz is shown. The OPOs independently provide a signal (idler) wavelength coverage across 870–1,000 nm (1,040–1,370 nm) and simultaneously generate idler powers of >1 W.     

En-face coherence imaging using galvanometer scanner modulation

We introduce a novel optical path-modulation technique for a low-coherence interferometric imaging system based on transverse scanning of the target with a galvanometric scanning-mirror pair. The path modulation arises when the beam that is incident upon one of the scanning mirrors does not fall on its axis of rotation. The method is demonstrated by the production of en-face low-coherence images of different objects such as a fiber-optic tip and a human retina invivo .     

Laser power noise detection at the quantum-noise limit of 32 A photocurrent

The power noise of a cw Nd:YAG laser system was measured at radio frequencies using the optical ac coupling technique. An additional mode cleaner in the setup allowed a high optical ac coupling amplification of 62.3. For the first time, to our knowledge, a sensitivity of 1.1×10?1? Hz(-1/2) relative power noise was achieved corresponding to an equivalent detected photocurrent of 32 A. High precision optics experiments can utilize this scheme to improve the sensitivity of their photodetectors.     

Low-coherence self-referencing velocimetry

Low-coherence self-referencing velocimetry optically measures the relative velocity between a point in a particle-laden fluid and a (potentially moving) reference surface. Low-coherence light scattered off the particles and off the reference surface is coupled into an interferometer with variable optical delay in one arm and an acousto-optical modulator in the second arm. The measurement location is set relative to the reference surface. Its location can be scanned along a line by adjusting the optical delay in the interferometer. The spatial resolution is typically tens of micrometers. Only one low-coherence light beam is required for each component of the velocity vector. Proof-of-principle measurements in Taylor-Couette flow are presented.     

Wavelength dependence of the phase retardation of a quarter-wave plate

We describe an optical heterodyne polarization interferometer that can be efficiently used for the precision measurement of the change in the state of polarization of a light wave induced by polarizing optical devices. This technique is used to measure the change with wavelength of the phase retardation of a quarter-wave plate. A theoretical derivation is presented to permit computation of the wavelength dependence of the phase shift induced by a quarter-wave plate.     

Data storage in optical fibers and reconstruction by use of low-coherence spectral interferometry

We demonstrate optical data storage in optical fibers and reconstruction by use of low-coherence spectral interferometry. The information was stored by means of writing fiber Bragg gratings with different central wavelengths at different locations of the fiber. We need only a single short pulse is needed to read all the stored data. The maximum theoretical reconstruction rate that can be obtained with our technique is 10 Tbits/s. Our storage technique can be useful for identifying users in optical communication networks.