Optical probe using eccentric optics for optical coherence tomography

We propose and demonstrate an OCT optical probe using eccentric optics. This probe enabled both forward imaging and side imaging by dividing a circular scanning area into two semicircular scanning areas using an external motor to rotate the flexible tube. The outer diameter of the probe was 2.6 mm, and its rigid portion length was 10 mm. The lateral resolution was 23 μm, and the eccentric radius was 1.1 mm. The circumferential length in scanning was 6.9 mm, and the working distance was 5 mm. OCT images of 1.5 mm × 6.9 mm (in tissue, axial × circumference), including forward image and side image, were measured with the axial resolution of 19 μm in air and a frame rate of one frame per second. The epidermis, dermis, and sweat gland of in vivo human ventral finger tips were observed.     

Interference effects in low coherence interferometry and optical coherence tomography

Optical coherence tomography (OCT) relies on the construction of two-dimensional images from a series of depth scans. These depth scans are made up of a series of interferograms relating to reflections from individual interfaces within a sample. The theoretical resolution of an OCT system is given as half the coherence length of the light source used and therefore interfaces between layers with different refractive indices, which are separated by less than this distance, cannot be resolved. We consider the occurrence of interference between adjacent interferograms and its consequence in signal, and image interpretation. Computational simulations were created to model two or three interfaces in close proximity such that interference would occur between component interferograms. Further to this, images were acquired of an air wedge between two glass slides that corresponded to the simulations. The results of both the simulated OCT signals and those from the air wedge showed the presence of interference effects that could influence the interpretation of the final (OCT) image.     

Optical interferometry method for the study of density variation in the nanosecond pulsed Nd:YAG laser interaction with aluminum

An interferometer-based technique has been used to determine the change in the refractive index induced by a Q-switched Nd–YAG laser (λ=532 nm and pulse duration 6 ns) on an aluminum target under atmospheric air pressure. The average density changes in front of the target irradiated by two laser pulse energy were observed for different focusing conditions. An Abel inversion technique is also presented for calculating three-dimensional density distributions in cylindrical coordinates.     

Spectral shaping of femtosecond pulses in aperiodic quasi-phase-matched gratings

We analyze the use of cascading second harmonic interactions in quadratic nonlinear crystals to mould the spectral characteristics of broadband near-infrared femtosecond pulses. Using a genetic algorithm, we optimize the design of the aperiodically poled ferroelectric crystal capable of generating the desired femtosecond infrared pulsed radiation.     

Temporal shaping of femtosecond solitary pulses in photoionized media

Femtosecond light pulses interacting with solids are studied. With a power close to the self-focusing threshold, an optical pulse evolves like a solitary wave with a temporal duration shrunk to a few femtoseconds through the defocusing action of multiphoton ionization. Self-steepening and space-time focusing are shown to form shock profiles, which do not prevent the pulse from keeping a shorter duration over one Rayleigh length. Theoretical estimates justify the key mechanisms leading to pulse compression, whose influence on the power spectra is finally discussed.     

Measurements of the vertical coherence length in neutron interferometry

The study and use of macroscopic quantum coherence requires long coherence lengths. Here we describe an approach to measuring the vertical coherence length in neutron interferometry, along with improvements to the NIST interferometer that led to a measured coherence length of 790 A. The measurement is based on introducing a path separation and measuring the loss in contrast as this separation is increased. The measured coherence length is consistent with the momentum distribution of the neutron beam. Finally, we demonstrate that the loss in contrast with beam displacement in one leg of the interferometer can be recovered by introducing a corresponding displacement in the second leg.     

Spectral shaping for non-Gaussian source spectra in optical coherence tomography

We present a digital spectral shaping technique to reduce the sidelobes (ringing) of the axial point-spread function in optical coherence tomography for non-Gaussian-shaped source spectra. The spectra of two superluminescent diodes were combined to generate a spectrum with significant modulation. Images of onion cells demonstrate the improved image quality in a turbid biological sample. A quantitative analysis of the accompanying penalty in signal-to-noise ratio is given.     

Ultrasensitive pulsed, balanced homodyne detector: application to time-domain quantum measurements

A pulsed, balanced homodyne detector has been developed for precise measurement of the electric field quadratures of pulsed optical quantum states. A high level of common mode suppression (>85 dB) and low electronic noise (730 electrons per pulse) provide a signal-to-noise ratio of 14 dB for measurement of the quantum noise of individual pulses. Measurements at repetition rates as high as 1 MHz are possible. As a test, quantum tomography of the coherent state was performed, and the Wigner function and the density matrix were reconstructed with 99.5% fidelity. The detection system can be used for ultrarsensitive balanced detection in cw mode, e.g., for weak absorption measurements.     

Improved signal-to-noise ratio in spectral-domain compared with time-domain optical coherence tomography

A signal-to-noise ratio (SNR) analysis is presented for optical coherence tomography (OCT) signals in which time-domain performance is compared with that of the spectral domain. A significant SNR gain of several hundredfold is found for acquisition in the spectral domain. The SNR benefit is demonstrated experimentally in a hybrid time-domain-spectral-domain OCT system.     

Spectrum control and shaping of optical pulses in optical fibre interferometers

This paper discusses the use of single-mode fibre ring interferometers for injection laser line narrowing and for enhancement of optical emission power. The efficient launching into fibre ring interferometers of multi-frequency external cavity injection laser emission, in both the c.w. and mode-locking regime, has been achieved. Matching the optical lengths of the external cavity and fibre interferometer allowed the measurement of mode width of such a laser. A method is proposed for shaping optical pulses in fibre ring interferometers from constant intensity frequency modulated emission. The mode-locking regime of all-fibre external cavity injection lasers has been achieved.     

Femtosecond pulse shaping using counter-propagating pulses in a semiconductor optical amplifier

An efficient pulse shaping method using counter-propagating pulses in the femtosecond regime is proposed and investigated. The effects of pump pulse power and pulsewidth on probe pulse are studied in counter-propagation scheme. It is shown that, with the proposed method, output probe pulse temporal and spectral peak shift due to femtosecond nonlinearities can be compensated, while the output pulse is amplified sufficiently. Furthermore, in relatively high power regime, the probe pulsewidth and time-bandwidth product are improved using counter-propagating pump pulse.     

Optical interferometry in phase separated He---He mixture films

We report on interferometric studies of ³He---⁴He liquid mixtures. Anomalous wetting phenomena were observed in phase separated mixture films: the upper ³He-rich phase did not wet the lower ⁴He-rich phase under a small continuous feed of ⁴He atoms to the vapor phase. Instead, under this dynamic situation, floating pools of the ³He-rich phase were stabilized with a contact angle on the order of 10 mrad. In equilibrium, the concentrated ³He-rich phase was found to nucleate uniformly on the free surface, consistent with Antonow's rule.     

From femtosecond infrared to picosecond visible pulses: temporal shaping with high-efficiency conversion

We present a single-pass frequency doubler that efficiently converts broadband near-infrared femtosecond pulses into narrow-spectrum visible picosecond pulses. An optimal control technique is introduced for properly designing aperiodically poled nonlinear crystals to generate visible pulses of the desired amplitude, phase profile, and carrier wavelength.     

Mutual coherence of supercontinuum pulses collinearly generated in bulk media

We report the collinear generation of white light pulses in bulk media by phase-locked and time-delayed intense laser pulses. The mutual phase coherence of the pump pulses is fully transferred to the two supercontinua, which thus show a highly-modulated two-pulse spectrum extending from all of the visible down to the near-infrared. We have investigated the effects of changing the relative delay between the pulses as well as the role of imbalances in the pump energy, and we have found that, at least in a limited energy interval, the mutual phase coherence of the two white-light pulses is surprisingly robust.     

Optical polarizabilities of large molecules measured in near-field interferometry

We discuss a novel application of matter wave interferometry to characterize the scalar optical polarizability of molecules at 532 nm. The interferometer presented here consists of two material absorptive gratings and one central optical phase grating. The interaction of the molecules with the standing light wave is determined by the optical dipole force and is therefore directly dependent on the molecular polarizability at the wavelength of the diffracting laser light. By comparing the observed matter-wave interference contrast with a theoretical model for several intensities of the standing light wave and molecular velocities, we can infer the polarizability in this first proof-of-principle experiment for the fullerenes C₆₀ and C₇₀, and we find a good agreement with literature values. PACS 03.75.Dg; 03.65.-w; 33.15.Kr     

Picosecond coherence coupling in the pump and probe technique

A general expression for the coherent artifact in photoinduced absorption measured by coherent polarized pump and probe beams is derived. The conditions for its reduction in isotropic media by using cross-polarized beams is discussed. Examples of experimental data illustrating the conclusions of the theory are shown.     

Spectral shaping to improve the point spread function in optical coherence tomography

We demonstrate inhibition of the sidelobes of the axial point spread function in optical coherence tomography by shaping the power spectrum of the light source with a remaining power of 4.54 mW. A broadband amplified spontaneous emission source radiating at 1565 +/- 40 nm is employed in a free-space optical coherence tomography system. The axial point spread functions before and after optical spectral shaping are presented. Results show that spectral shaping of the source can inhibit sidelobes of the point spread function up to 12.9 dB, with an associated small increase of 2.2 dB in noise floor in the far field. The effect of spectral shaping on axial resolution is demonstrated according to three metrics. Image quality improvement is also illustrated with optical coherence tomography images of an onion before and after spectral shaping.     

Imagery of local defects in multilayer components by short coherence length interferometry

We propose to image local defects inside multidielectric optical components by using a special configuration Linnik interference microscope, along with a CCD camera and a dedicated detection, to extract the amplitude scattered by the defects in the interference image. The use of a short coherence length source allows one to obtain interference only from a thin slice (~1microm) within the observed object. The object is tilted to permit the use of a dark-field configuration to enhance the defect contrast. We describe the experimental setup and the detection scheme. Images that exhibit local point defects on the interfaces of various multilayer optical components (laser mirrors) are presented.