Balanced detection spectral domain optical coherence tomography with a multiline single camera for signal-to-noise ratio enhancement

In this study, the use and advantages of balanced detection (BD) in spectral domain optical coherence tomography (SD-OCT) are demonstrated. A-scans are calculated as a combination of two phase-opposed interferometric spectra acquired simultaneously by using a multiline single camera spectrometer. Not only does this system suppress artifacts due to autocorrelation, but also the signal of interest is increased by a factor of 2 as experimentally verified. Our BD-based SD-OCT gives a signal-to-noise ratio improvement of 8-14?dB for the peak within 1?mm compared to standard SD-OCT using a single detection scheme. This method is validated by experimental measurement of a glass plate.     

Molecular kinetics and multiline model for the E-beam pumped XeF(B-X) laser

A physical model of the time-, temperature- and wavelength-dependent behavior of an E-beam pumped XeF(B-X) laser is developed. Correlations with published laser- and fluorescence-efficiency data, laser spectra, gain, and absorption data are discussed.     

Programmable color tuning of a multiline laser by means of a twisted nematic liquid crystal display

An optical system useful to tune in a controlled way the color of a triline argon krypton (Ar-Kr) laser by means of a twisted nematic liquid crystal display (TNLCD) is presented. The optical setup employs a 4f system and two blazed gratings to first separate and then recombine the spectrum of the light beam. The TNLCD is included in the intermediate focal plane operating in the amplitude modulation mode to control the relative transmission of each spectral line. The resulting color is accurately predicted by using a previously developed physical model of the spectral and voltage dependence of the TNLCD birefringence. By simply changing the gray level image addressed to the display, the Ar-Kr laser is color modulated at a video rate, thus becoming an interesting, reconfigurable, coherent light source for applications such as multicolor holography or color inspection.     

Electromagnetic coherence theory of laser resonator modes

A theory of open laser resonators is formulated within the framework of the electromagnetic coherence theory. It is shown that if only one Fox-Li mode contributes to the field at a given frequency, then the field at that frequency is necessarily completely coherent in view of the space-frequency counterpart of the recently introduced degree of coherence of electromagnetic fields [Opt. Express 11, 1137 (2003)]. It is also shown that the relation between the number of Fox-Li modes and the new degree of coherence is analogous to the relation established in the scalar theory of laser resonator modes. Difficulties that arise with the formerly introduced visibility-based definition of the electromagnetic degree of coherence are briefly discussed.     

Performance comparison of an all-fiber-based laser Doppler vibrometer for remote acoustical signal detection using short and long coherence length lasers

All-fiber laser Doppler vibrometer systems have great potential in the application of remote acoustic detection. However, due to the requirement for a long operating distance, a long coherence length laser is required, which can drive the system cost high. In this paper, a system using a short coherence length laser is proposed and demonstrated. Experimental analysis indicates that the multi-longitudinal modes of the laser cause detection noise and that the unequal length between two paths (local oscillator path and transmission path) increases the intensity and the frequency components of the noise. In order to reduce the noise, the optical length of the two paths needs to be balanced, within the coherence length of the source. We demonstrate that adopting a tunable optical delay to compensate the unequal length significantly reduces the noise. In a comparison of the detection results by using a short coherence laser and a long coherence laser, our developed system gives a good performance on the acoustic signal detection from three meters away.     

Quantitative temperature measurements in high-pressure flames with multiline NO-LIF thermometry

An accurate temperature measurement technique for steady, high-pressure flames is investigated using excitation wavelength-scanned laser-induced fluorescence (LIF) within the nitric oxide (NO) A-X(0, 0) band, and demonstration experiments are performed in premixed methane/air flames at pressures between 1 and 60 bars with a fuel/air ratio of 0.9. Excitation spectra are simulated with a computational spectral simulation program (LIFSim) and fit to the experimental data to extract gas temperature. The LIF scan range was chosen to provide sensitivity over a wide temperature range and to minimize LIF interference from oxygen. The fitting method is robust against elastic scattering and broadband LIF interference from other species, and yields absolute, calibration-free temperature measurements. Because of loss of structure in the excitation spectra at high pressures, background signal intensities were determined using a NO addition method that simultaneously yields nascent NO concentrations in the postflame gases. In addition, fluorescence emission spectra were also analyzed to quantify the contribution of background signal and to investigate interference in the detection band-width. The NO-LIF temperatures are in good agreement with intrusive single-color pyrometry. The proposed thermometry method could provide a useful tool for studing high-pressure flame chemistry as well as provide a standard to evaluate and validate fast-imaging thermometry techniques for practical diagnostics of high-pressure combustion systems.     

Partial spatial coherence effects in digital holographic microscopy with a laser source

We investigate a digital holographic microscope that permits us to modify the spatial coherence state of the sample illumination by changing the spot size of a laser beam on a rotating ground glass. Out-of-focus planes are refocused by digital holographic reconstruction with numerical implementation of the Kirchhoff-Fresnel integral. The partial coherence nature of the illumination reduces the coherent artifact noise with respect to fully coherent illumination. The investigated configuration allows the spatial coherence state to be changed without modifying the illumination level of the sample. The effect of the coherence state on the digital holographic reconstruction is theoretically and experimentally evaluated. We also show how multiple reflection interferences are limited by the use of reduced spatial coherent illumination.     

Noise-induced coherence in an excitable system with frequency-dependent feedback

The influence of the properties of an excitable system on the characteristics of the coherence resonance effect have been studied by experimental and numerical methods. The transition from monostable to bistable regime is accompanied by the appearance of a false maximum in the regularity parameter of the power spectrum plotted as a function of the noise intensity. An increase in the dimensionality of the system activates a mechanism whereby the noise reveals oscillatory dynamics, also resulting in the appearance of an additional maximum in the regularity parameter.     

Parallel optical coherence tomography system

We present the design and procedures for implementing a parallel optical coherence tomography (POCT) imaging system that can be adapted to an endoscopic format. The POCT system consists of a single mode fiber (SMF) array with multiple reduced diameter (15 microm) SMFs in the sample arm with 15 microm center spacing between fibers. The size of the array determines the size of the transverse imaging field. Electronic scanning eliminates the need for mechanically scanning in the lateral direction. Experimental image data obtained with this system show the capability for parallel axial scan acquisition with lateral resolution comparable to mechanically scanned optical coherence tomography systems.     

A procedure for measuring the coherence length of the sea texture

In this paper, we deal with the analysis of the temporal coherence properties of the signal backscattered from the sea surface, in the hypothesis that it can be modeled as a compound-Gaussian process. Precisely, we address the problem of measuring to what extent a compound-Gaussian process, namely the product of a nonnegative, slowly-varying modulating signal times a rapidly varying Gaussian component, can be approximated by a Gaussian process with random variance. The problem is restated in terms of a binary hypothesis test, wherein the null hypothesis of constant modulating sequence is contrasted to any alternative. The test is calibrated resorting to pseudorandom sequences and, finally, the procedure is applied to measuring the coherence length of sea data collected by scatterometers mounted on the “Forschungsplatform Nordsee” during the SAXON experiment in November 1990     

Finite size compression gratings in a large aperture chirped pulse amplification laser system

A simple model is presented to calculate the effects of the finite size of the diffraction gratings in the compressor of a chirped pulse amplification laser system. A wavelength-dependent clipping at the second grating alters the spectral distribution of the compressed pulses, affecting their time domain as well as their spatial distribution in the focal plane. Laser parameters of paramount importance to laser/plasma interaction experiments such as peak intensity, pulse duration, and prepulse levels are affected by the compressor design. Calculations of the effect of spectral clipping on these parameters for Gaussian, sech(2), and top-hat input spectra are discussed, and the benefit of double-pass compared with single-pass compression is also investigated. As an example, with 300-mm gratings and single-pass compression, for a sech(2) spectrum the pulse length of a 400-fs pulse increases to 459 fs, the peak intensity decreases by 25%, and the focal spot size increases by 8% because of the finite size of the gratings.     

Near-threshold spectral modes and coherence of a semiconductor laser and a diode-pumped neodymium laser

The spectral, coherence, and polarization characteristics of a semiconductor injection laser and a diode-pumped Nd laser operating on second harmonic have been thoroughly traced for the first time during the transition via the generation threshold and on the passage from one mode to another with increasing pump current. It is shown that the behavior of the intensity, coherence, and noise near the threshold is indicative of the possible analogy between laser generation and phase transitions of the first or second order.     

Changes in longitudinal spatial coherence length of optical field in image space

The laws of variation in the length of spatial longitudinal coherence (LC) and the width of the LC interference pulse of the optical field of an extended spatially incoherent source are established in the image space behind a collecting lens that occurs in the output arm of the scanning (oscillating-mirror) Michelson interferometer. The LC length (and the interference pulse width) varies nonmonotonically and exhibits a local minimum, the magnitude and position of which on the longitudinal axis of the system are determined by the source size and position and depend on the lens aperture diameter.     

Low temperature raman study of the electron coherence length near graphene edges

We developed a novel optical defocusing method for studying spatial coherence of photoexcited electrons and holes near edges of graphene. Our method is applied to measure the localization l(D) of the disorder-induced Raman D band (～1350 cm(-1)) with a resolution of a few nanometers. Raman scattering experiments performed in a helium bath cryostat reveal that as temperature is decreased from 300 to 1.55 K, the length l(D) increases. We found that the localization of the D band varies as 1/T(1/2), giving strong evidence that l(D) scales with the coherence length of photoexcited electrons near graphene edges.     

Finite temporal measurements of the statistical characteristics of the atmospheric coherence length

Based on the theory of stationary random processes, the probability density function of the atmospheric coherence length measured in a finite time period is derived analytically and verified experimentally. An iteration method is proposed to obtain Fried's coherence length within a finite time measurement.