Electric field sensing system using coherence modulation of light

This paper describes an experimental setup for detecting electric fields using electrooptic lithium niobate (LiNbO/sub 3/) sensors and coherence modulation of light. In this detection scheme, the sensed electric fields modulate optical delays, which are introduced by LiNbO/sub 3/ coherence modulators when associated to low-coherence optical sources. The optical delays act as coherence-multiplexed carriers of the sensed electric fields and are transmitted through an optical fiber channel. At the receiver, the electric fields are detected by measuring the auto-correlation of the received light by using two-wave interferometers, which are matched to the introduced optical delays on the sensing devices.     

Reference-beam storage for long-range low-coherence pulsed Doppler lidar

We present a laser Doppler velocimeter that stores and delays the reference beam to preserve coherence with a long-path-length measurement beam. Our storage and delay technique relaxes the strict coherence requirements associated with lidar laser sources, permitting the use of low-coherence lasers. This technique potentially could reduce the cost and size of lidar systems for commercial applications. Experiments that use fiber-optic ring resonators to store the reference beams and generate reference pulse trains validated the concept. We obtained results at several simulated distances by beating each usable reference pulse with a delayed Doppler-shifted measurement beam reflected off a rotating mirror.     

Measurement of up- and down-lead fiber sensitivity caused by the lead in a multimode fiber in an interferometric system

The results of a comparative experimental study on the effect of the modal noise induced by lead-in fibers in an extrinsic interferometric system, illuminated by high- and low-coherence light sources, respectively, are reported. When the up-lead fiber was subject to a perturbation, the sensitivity of the system was reduced by 20.9 dB through the use of a high-coherence source, and by 1.8 dB through the use of a low-coherence source. When the down-lead fiber was perturbed, the sensitivity dropped by 30.3 dB and 4.9 dB for high- and low-coherence sources, respectively. The results from the experimental analysis supported qualitatively by simple theory show that the use of a low-coherence light source can greatly suppress the modal noise induced in both the up- and down-lead fibers, if the coherence length of the light source used is less than the optical path difference between two adjacent fiber modes. This shows the practicality of the use of niultimode fibers in an interferometric system with a suitable lig t source.     

Fiber-optical sensor with miniaturized probe head and nanometer accuracy based on spatially modulated low-coherence interferogram analysis

Fiber-optical sensors have some crucial advantages compared with rigid optical systems. They allow miniaturization and flexibility of system setups. Nevertheless, optical principles such as low-coherence interferometry can be realized by use of fiber optics. We developed and realized an approach for a fiber-optical sensor, which is based on the analysis of spatially modulated low-coherence interferograms. The system presented consists of three units, a miniaturized sensing probe, a broadband fiber-coupled light source, and an adapted Michelson interferometer, which is used as an optical receiver. Furthermore, the signal processing procedure, which was developed for the interferogram analysis in order to achieve nanometer measurement accuracy, is discussed. A system prototype has been validated thoroughly in different experiments. The results approve the accuracy of the sensor.     

Electric Field Sensing Scheme Based on Matched $ hbox{LiNbO}_{3}$ Electro-Optic Retarders

In this paper, a wideband-electric-field-sensing scheme that uses optically matched integrated optics electrooptic devices and coherence modulation of light is described. In a coherence modulation scheme, the integrated optics sensor detects the electric field and imprints it around an optical delay. The optical delay is generated by a birefringent optical waveguide in a lithium niobate (LiNbO₃) integrated optics two-wave interferometer. The modulated optical delay, acting as an information carrier, is transmitted through an optical fiber channel. At the receiver, light is demodulated by a second integrated optics two-wave interferometer, which also introduces a second optical delay. The optical delays on the sensor and demodulator are matched at the same value. The integrated optics demodulator measures the autocorrelation of light around the optical delay value, and the imprinted electric field is recuperated as a linear variation of the received optical power. The matching of the sensor and demodulator allows a direct detection of the electric field, giving a unique feature to this fiber-integrated optics scheme. The experimental setup described here uses two pigtailed LiNbO₃ electrooptic crystals: one acting as the electric field sensor and the other acting as the optical demodulator. The wideband sensing range on the experimental setup corresponds to frequencies between 0 and 20 kHz.     

Coherence-multiplexed acousto-optic correlator for signal processing: theory and applications

A method of multichannel processing to compute correlation products simultaneously is introduced. The signals that process are encoded on a single light beam through the use of an electro-optic modulators that induce optical delays greater than the coherence length of light. The coherence-modulated light beam thus obtained is then spatially and temporally modulated through an acousto-optic Bragg cell. The potential number of channels is estimated to be approximately 5-10. The method can be combined with other existing systems, such as time-, space-, or frequency-multiplexed correlators, to increase the channel number. The method also applies to high-resolution spectral analysis.     

High-speed path-length scanning with a multiple-pass cavity delay line

Techniques for high-speed delay scanning are important for low-coherence interferometry, optical coherence tomography, pump probe measurements, and other applications. We demonstrate a novel scanning delay line using a multiple-pass cavity. Differential delays are accumulated with each pass so that millimeter delays can be generated with tens of micrometer mirror displacements. With special design criteria, misalignment sensitivity can be dramatically reduced. The system is demonstrated to scan 6 m/s at 2-kHz repetition rates. Real-time optical coherence tomography imaging with 500 pixel images at four frames/s is performed. Using a Cr:forsterite laser source, we obtained axial image resolutions of 6 microm with 92-dB sensitivity.     

A Practical Implementation of IEEE 1588-2008 Transparent Clock for Distributed Measurement and Control Systems

This paper addresses issues with time synchronization using the IEEE 1588-2008 for distributed measurement and control systems. A practical implementation of the transparent clock is presented with the overall system architecture and detailed operation of each building block. To verify the submicrosecond accuracy using the implemented devices, an experimental setup that was analogous to a practical distributed system has been built. Measured results from the experiment show that the time error is limited below 30 ns for nodes that were connected by three switches. It is remarkable that the results are observed in spite of large packet queuing delays that were introduced by a traffic generator. The discussion on sources of time error that was outlined here provides technical considerations to designing IEEE 1588 systems.      

Optical coherence tomography system with no high-precision scanning stage and stage controller

Optical coherence tomography (OCT) is a novel technique for noninvasive imaging based on the use of a low-coherence interferometer. Conventionally, obtaining high-resolution images requires the use of high-precision sample and scanning stages and a stage controller for simultaneous measurement of the refractive index and the thickness of an optical sample. However, in this study a novel optical-fiber-type OCT system is developed that does not need both a high-precision scanning stage and a stage controller. Additionally, two signal demodulation processes are described. Compared with that of conventional OCT systems, the current configuration eliminates the high-precision scanning stage and stage controller and is therefore cheaper and less complex. Also, this new technique could be applied to conventional OCTs in biotissue scanning.     

Coherent coupling of vertical-cavity surface-emitting laser arrays and efficient beam combining by diffractive optical elements: concept and experimental verification

A phase-locked diode-laser system based on master-slave coupling of two-dimensional vertical-cavity surface-emitting laser (VCSEL) arrays by injection locking is presented. Frequencies and phases are adjusted by laser-trimmed microresistors. Additional beam-transformation optics consisting of two diffractive optical elements (DOEs) and a Fourier lens concentrates most of the far-field power in a nearly diffraction-limited beam. Both the VCSEL array and the microlens array are monolithically integrated and mounted in a compact module. With an array of 21 slave lasers a system coherence of 95% (for several hours) and of nearly 90% (for several months) has been demonstrated without any active phase control. The scalability of the output power has been verified by locking of an array of 77 slave lasers with a system coherence of 78%. The optical system efficiency is 20-23%; with beam-transformation optics this efficiency could be improved to 44%.     

Adaptive optics in microscopy

The imaging properties of optical microscopes are often compromised by aberrations that reduce image resolution and contrast. Adaptive optics technology has been employed in various systems to correct these aberrations and restore performance. This has required various departures from the traditional adaptive optics schemes that are used in astronomy. This review discusses the sources of aberrations, their effects and their correction with adaptive optics, particularly in confocal and two-photon microscopes. Different methods of wavefront sensing, indirect aberration measurement and aberration correction devices are discussed. Applications of adaptive optics in the related areas of optical data storage, optical tweezers and micro/nanofabrication are also reviewed.     

Low-coherence interferometry with synthesis of coherence function

Synthesis of a coherence function by manipulation of the spectral phase of low-coherent light with a segmented liquid-crystal phase modulator and its application in a low-coherence interferometry are described. Effects of space-time coupling caused at diffractive gratings and second-order dispersion at the spatial light modulator on the coherence function synthesis are theoretically and experimentally verified. Various coherence functions can be shaped with phase-only masks designed by simulated annealing optimization algorithm. We utilized this technique for a novel optical low-coherence reflectometry without any mechanical movement for scanning optical delay.     

Dielectric-Parameter Measurements of SiC at Millimeter and Submillimeter Wavelengths

High-precision continuous spectra of the absorption coefficient, refractive index, complex dielectric permittivity, and loss tangent for several silicon carbide (SiC) specimens are reported in this paper over a broad millimeter- and submillimeter-wave range for the first time. Measurements have been successfully performed using three different types of specially constructed spectrometer systems: a dispersive Fourier transform spectroscopy, an automated 60-GHz open resonator, and a free-space quasi-optical millimeter-wave spectrometer equipped with high-power backward-wave oscillator sources and their associated state-of-the-art dielectric-measurement techniques. Data are presented as continuous functions of frequency from 30 to 600 GHz. The employment of various measurement systems and techniques ensured the measurement of polycrystalline SiC specimens with various degrees of absorption and dispersion characteristics over an extended frequency range with high precision. Results presented here provide comprehensive information of SiC on its optical and dielectric behavior as a function of frequency and purity for its potential application in semiconductor and radio-frequency devices and circuits. An error analysis of measured dielectric-parameter results is also provided.     

Semiconductor line source for low-coherence interferometry

The suitability for low-coherence interferometry of a high-power, semiconductor laser line source operated at a forward bias current below threshold is demonstrated. Measurements of the important characteristics of the source are presented. For example, the source produces an output power of 1.3 mW and a spatially uniform coherence length of 16 mum at a bias current of 86% of threshold (250 mA) at 20 degrees C. The usefulness of the source is verified by measurement of the line profile of a contact lens.     

Influence of geometry on polychromatic speckle contrast

Understanding speckle behavior is very important in speckle metrology application. The contrast of a polychromatic speckle depends not only on surface roughness and the coherence length of a light source, as shown in previous works, but also on optical geometry. We applied the Fresnel approach of diffraction theory for the free-space geometry and derived a simple analytical relationship between contrast, coherence length, size of illuminated spot, and distances between source, object, and observation plane. The effect of contrast reduction is found to be significant for low-coherence light sources.     

Dispersion-insensitive measurement of thickness and group refractive index by low-coherence interferometry

We describe a low-coherence interferometric technique for simultaneous measurement of geometric thickness and group refractive index of highly dispersive samples. The technique is immune to the dispersion-induced asymmetry of the interferograms, thus overcoming limitations associated with some other low-coherence approaches to this simultaneous measurement. We use the experimental configuration of a tandem interferometer, with the samples to be characterized placed in an air gap in one arm of the measurement interferometer. Unambiguous, dispersion-insensitive measurements of critical group-delay imbalances in the measurement interferometer are determined from the optical frequency dependence of interferogram phases, by means of dispersive Fourier transform spectrometry. Sample thickness and group refractive index are calculated from these group delays. A thickness measurement precision of 0.2 mum and group index measurement accuracy of 5 parts in 10(5) across a wavelength range of 150 nm have been achieved for BK7 and fused-silica glass samples in the thickness range 2000 to 6000 mum.     

Monte Carlo modeling of spatial coherence: free-space diffraction

We present a Monte Carlo method for propagating partially coherent fields through complex deterministic optical systems. A Gaussian copula is used to synthesize a random source with an arbitrary spatial coherence function. Physical optics and Monte Carlo predictions of the first- and second-order statistics of the field are shown for coherent and partially coherent sources for free-space propagation, imaging using a binary Fresnel zone plate, and propagation through a limiting aperture. Excellent agreement between the physical optics and Monte Carlo predictions is demonstrated in all cases. Convergence criteria are presented for judging the quality of the Monte Carlo predictions.     

Use of PC mouse components for continuous measuring of human heartbeat

A new technology for remote measuring of vibration sources was recently developed for industrial, medical, and security-related applications [Int. Appl. Patent No: PCT/IL2008/001008]. It requires relatively expensive equipment, such as high-speed complementary metal oxide semiconductor (CMOS) sensors and customized optics. In this paper, we demonstrate how the usage of a simple personal computer (PC) mouse as an optical system composed of a low-power laser and a CMOS circuitry on the same integrated circuit package, can be used to monitor heartbeat from the wrist. The method is based on modifying the mouse optical system in such a way that it will recognize temporal change in skin's vibration profile, generated due to the heart pulses, as mouse movement. The tests that were carried out show a very good correlation between the heartbeat rate measured from human skin and the reference values taken manually.     

Aberration evaluation of alignment optics in lithographic tools by use of a step-height structure highly sensitive to the asymmetry of an optical image

To satisfy the increasing demand for extremely tight overlay accuracy in semiconductor manufacturing processes, all the measurement error factors in alignment systems and overlay measurement tools need be identified and eliminated. The principle of most alignment systems is based on image processing of target marks on the wafer under bright-field illumination. Although the phenomenon that the sensitivity to the alignment error varies with the step height (SH) of the mark has been known and used for evaluating the performance of the alignment optics, no investigation has been made into the origin and the physical mechanism of the phenomenon. We propose a simplified optical model that can account for the origin of the asymmetric image and clarify its relation to the SHs. The model is validated with simulation and experimental results. The improved performance of an alignment system using marks with optimally designed SHs is demonstrated.     

Low-coherence optical tomography in turbid tissue: theoretical analysis

On the basis of white-light interferometry and statistical optics, a theoretical model for low-coherence optical tomography is presented that establishes the relation of interference modulation with path-length-resolved reflectance and that can provide analytical expressions and numerical solutions by means of a Fourier transform. The Monte Carlo technique is used to simulate the path-length-resolved reflectance from different multilayer tissue phantoms. Theoretical analyses and preliminary experimental results suggest that, unlike time-resolved spectroscopy, low-coherence optical tomography detects the local relative variations of path-length-resolved reflectance from the turbid tissues.