Modulated Laser Light Research Articles

Photoacoustic detection of nitric oxide by use of a quantum-cascade laser

A photoacoustic trace-gas sensor for the measurement of nitric oxide with a detection limit of 500 parts in 10(9) has been demonstrated. The radiation source was a thermoelectrically cooled distributed-feedback quantum-cascade laser operating in pulsed mode near 5.3 microm with an average laser power of 8 mW. A resonant photoacoustic cell was excited in its first longitudinal mode by the modulated laser light. Preliminary measurements have been performed to test the performance of our photoacoustic sensor; possible improvements to reach lower detection limits are discussed.

Generation of flat power-envelope terahertz-wide modulation sidebands from a continuous-wave laser based on an external electro-optic phase modulator

Flat power-envelope terahertz-wide modulation sidebands are generated by only electro-optic phase modulation of continuous-wave laser light. Generation and power equalization of widespread sidebands are realized simultaneously by spatial distribution of the modulation index within a laser beam cross section by use of simple domain-engineering processes in LiTaO3 electro-optic crystal. Generation of 46 sidebands spaced by 16.25 GHz within a -3-dB bandwidth (over a 1-THz span for a +/- 3-dB bandwidth) is demonstrated.

Photodynamic Effect of Modulated and Continuous LASER Radiation on Cancerous Cells Cultivated In Vitro

Photodynamic therapy (PDT) is a cancer therapy that uses nonionizing photons and a photosensitizer to treat solid tumors and surface malignancies. Experiments with cells in vitro have been useful in order to investigate cellular effects due to PDT. In this work, we study the photodynamic effect of modulated and continuous laser radiation on cancerous cells cultivated in vitro. Three experiments were performed for this study. For the first one, HeLa cells were exposed to δ-Aminolevulinic acid (ALA) at concentrations of 0, 10, 20, 40, 80 and 160 µg of ALA/ml of Dulbecco's modified Eagle's medium (DMEM) and were irradiated at total light dose of 80 J/cm2 by using continuous and modulated argon laser light. In the case of modulated radiation, at 1000 Hz, we observed more cellular death (20%) than those produced by continuous light (0%) when these cells were exposed at 20 µg of ALA/ml of DMEM. For other ALA doses cellular deaths were similar in both irradiation types. The death percentage in cancerous cell, after application of PDT with continuous and modulated irradiation, was determined by means of the neutral-red spectrophotometric assay. In the second experiment HeLa cells were exposed only at 20 µg of ALA/ml of DMEM and irradiated with argon laser at total light dose of 80 J/cm2 but using different light modulation frequencies: 100, 160, 600, 1250, 1500, 1850 Hz. Finally in the third experiment, where the HeLa cells were exposed in the same conditions of second experiment, the modulation frequencies used to irradiate the cell were: 10, 20, 40, 80 Hz. It was observed that light modulation frequency at 40 Hz produces maximum cellular death (36%). These results showed that modulated light used in PDT, for low ALA doses in HeLa cells, is more efficient than continuous light irradiation. Modulated irradiation in PDT could reduce the quantity of required ALA, and in consequence to reduce possible collateral damage, for application of this therapy.

New devices for microwave photonics in optical communications

Optical microwave interactions in photonic devices are reviewed in the perspective of applications to optical communication systems. Laser sources, light modulators and photodetectors are successively considered, with emphasis on basic concepts of microwave photonics and their recent developments in new, faster and more efficient devices. The main advances concern dual-mode lasers, as well as modulators and photodetectors in the traveling-wave configuration.

Small-volume frequency-domain oximetry: phantom experiments and first in vivo results

We describe a new method to determine the oxygen saturation and the total hemoglobin content of tissue in vivo absolutely at small source-detector separations (<10 mm). Phase and mean intensity of modulated laser light of various wavelengths was measured at several predetermined source-detector separations in the frequency domain. From these measured quantities, the absorption coefficient was derived using the modified time-integrated microscopic Beer-Lambert law (MBL). In addition, the interaction volume of the photons was determined using a multi-layer Monte-Carlo model of human skin. To evaluate the method, we employed homogenous solid phantoms (consisting of TiO2 particles embedded in resin) with mean scattering and absorbing properties comparable to those of human skin. Furthermore, in vivo measurements were performed in a healthy volunteer to demonstrate that the technique is applicable for the determination of the oxygen saturation and the total hemoglobin content in the skin in vivo. The proposed technique is especially suited for the on-line determination of the oxygen saturation and total hemoglobin content in applications where small applicators are required (e.g., fetal oxygen monitoring sub partu).

Self-sustained ultrafast pulsation in coupled vertical-cavity surface-emitting lasers

We demonstrate that high-frequency, narrow-band, self-pulsating intensity oscillation can be generated with two coupled vertical-cavity surface-emitting lasers (VCSELs) and that such oscillation is greatly enhanced by collecting one of the two field lobes. The coupled VCSELs provide an ideal source for high-repetition-rate (over 40 GHz), sinusoidallike modulated laser light with Gaussian-like near- and far-field profiles. We also show that the modulation frequency can be tuned by inter-VCSEL separation or by a dc bias level.

Determination of tissue optical parameters using HF modulation spectroscopy

Radio frequency modulation spectroscopy is a capable method to determine tissue optical parameters in-vivo. For the eventual purpose of clinical measurements we have developed and tested an rf laser spectroscopy device which enables a measurement of the spatial amplitude and phase shift profiles of backscattered modulated laser light. Spectral absorption and scattering coefficients are computed by inverse formulas derived from analytical solutions of the diffusion model of light transport in a semi-infinite geometry.

Detection of Ultrasonic Mechanical Vibration of a Solid Using Fiber Bragg Grating

Based on the intensity modulation of laser light propagating through a fiber Bragg grating (FBG) attached to a Pb(Zr, Ti)O3 (PZT) vibrator, the detection of mechanical vibration is demonstrated. The output of the sensor is stable and the involved harmonic component is small. It is considered that the sensor operation is linear. The sensitivity depends on the slope of the FBG transmission spectrum curve at the operating wavelength and is higher for a steeper slope. Since not only an FBG has little influence on the object being measured because of its smallness and light weight but also its frequency characteristics are thought to be better than those of a PZT vibration sensor, an FBG is expected to provide us with an important tool of practicality for measuring mechanical vibration.

Metallothionein—An early marker in the carcinogenesis of ulcerative colitis-associated colorectal carcinoma

A variety of imaging modalities have been developed to determine the characteristics of tumors in the colon. Endoscopic ultrasonography has been applied clinically to stage the depth of invasion of colon cancers and more recently, optical coherence tomography has been developed in the laboratory to increase imaging of the mucosal surface. We have initiated the first application of ultrasound modulated laser tomography to colonic tumors. This technique has the advantage of combining ultrasound and light to characterize tissue. Methods: The principle behind this technique is to use ultrasound waves to modulate laser light passing through a tissue. The modulated laser light can then be detected and amplified to generate a signal that is characteristic of that tissue. The system we used consists of a helium-neon laser to generate a 632-nm beam, which is focused on the tissue. A function generator is used to create a sinusoidal wave at I MHz, which is amplified to drive an ultrasound transducer placed perpendicular to the tissue. The signal is detected by a photodiode, which converts the modulated light to a current, which can be filtered using the current from the function generator. The time domain signal can then be processed and displayed. Results: The following graphs display the signal taken from 8 resected normal colon tissue and colon cancers. The X-axis represent wavelength while the Y-axis represents absorption in AU. The graph on the right represents colon cancer while the graph on the left represents the modulated signal from normal colon. The most prominant difference was noted between two samples at wave length of 350 nm. Conclusions: The signal received from ultrasound modulated optical tomography appears to differentiate between colonic cancer and normal tissue. This tool may be 6283

Development of an optical fiber hydrophone with fiber Bragg grating

A new type of optical fiber hydrophone is constructed with a fiber Bragg grating (FBG) based on the intensity modulation of laser light in an FBG under the influence of sound pressure. The FBG hydrophone shows linearity, with dynamic range about 70 dB. It can measure amplitude and phase of an acoustic field in real time, and operates in a wide range of acoustic frequency, at least from 1 kHz to 3 MHz. No signal distortion is observed in the detected signal. Because of the simplicity in its operating principle and geometry, an FBG hydrophone is expected to be an acoustic sensor of high practicality compared to a conventional optical fiber hydrophone.

Fiber-Optic Transmission of SMPTE 259M and SMPTE 292M SDI Signals

This paper, in the form of a review, introduces some technical aspects dealing with fiber-optic digital transmitters. The special case of the serial digital interface (SDI) data transmitter is considered. The paper presents the basic features of semiconductor laser structures; the problem of laser light modulation and stabilization is discussed, especially for direct modulation; and SDI signal features are briefly reviewed. The most commonly used mean-value stabilization loop is analyzed, and its limited performance for SDI signal transmission is demonstrated. Finally, the SDI designated feed-forward compensated stabilization loop is proposed, and its properties are analyzed.

A simple optical microphone

In the past, a number of different working principles have been suggested for optical microphones, but many of these require a high degree of technical sophistication. Thus some suggestions are based on the interferometric principle, whilst others would utilize frequency modulation of laser light via a variable mirror (membrane) situated in the laser resonator. Light intensity modulation is a much more simple approach: A light beam emitted from an optical fiber is aimed at a reflecting membrane. The beam reflected from the membrane is captured by a second optical fiber and fed into a photodiode. Depending on the movement of the membrane, more or less light is launched into a second optical fiber. The photoelectric current of the photodiode thus expresses the movement of the membrane, and with it the sound field, as an electrical signal. As optical fibers transmit light with very few losses, the actual optoacoustic transducer can be positioned at a long distance from the light source and the photodiode. This allows problem-free use of the optoacoustic transducer in the presence of very strong electromagnetic fields (e.g., MRI). It is possible to improve upon this principle via simple modifications. The electroacoustic characteristics achieved are discussed and a model is presented.

New Functionality in Glass

Glass serves a critical function in diverse technologies ranging from medicine to telecommunications. Fax machines and text scanners use micro-optic lens arrays made from ion-exchanged, gradedindex glass to image original documents. Data for long-distance communication by telephone or computer are transmitted in the form of modulated laser light through highly transparent glass fibers. These signals are boosted for long-haul transmission by stimulated emission in erbium-doped glass optical amplifiers. Lap-top computers use flat-panel-display glass in their liquid-crystal screens (see “Materials for Flat-Panel Displays,” MRS Bulletin, March 1996). Gall-bladder and hysterectomy surgeries have been made minimally invasive in part because of slender glass imaging endoscopes.Functionality in glass is expanding rapidly. New properties are often attributable to new defect species created by strong optical or electrical fields. For example rare-earth-doped ZBLAN glass (a commercial fluoride glass), when exposed to high peak-intensity femtosecond laser pulses, becomes phosphorescent. (See the cover of this issue.) The phosphorescence is visible to the naked eye up to 12 hours after illumination. Other compositions show similar effects. Phosphorescent glasses are transparent and may be more easily formed into contiguous films and fibers than powder or microcrystalline phosphors.

A novel fibre optic strain sensor

A novel strain sensor based on polarisation-maintaining optical fibre and FMCW laser diode technology is described. The sensor consists of a l00m length of fibre, a frequency-modulated laser diode light source and a PIN photodiode detector. The principle of operation relies on that facts that the optical frequency of the laser light is modulated by approximately 1GHz during a current modulated cycle of 1ms, and that the fibre has a twist inserted to induce coupling between the eigenmodes HEx 11 or HEy 11 due to circular birefringence. By selectively launching the modulated laser light into one of the eigenmodes and retaining the other for detection of coupled modes, the length of fibre from the launch end to the twist was rendered insensitive to changes in environmental conditions such as temperature. The system was shown to have a sensitivity of approximately 2πrad per 1000 microstrain with a resolution of 2 microstrain within a dynamic range of 5000 microstrain.

Saturation measurements of the a 3P 0–y 3D 1o Ti I transition by optogalvanic spectroscopy

The optogalvanic signal (OGS) induced in a Ti–Ar hollow-cathode discharge, stimulated by modulated cw laser light tuned to the a 3P 0–y 3D 1 o Ti I transition, was measured as a function of the light intensity. By solving analytically the rate equations of a three-level system, which include the Ti I ground state, the lower metastable a 3P 0 and the upper y 3D 1 o levels, an expression was obtained that relates the magnitude of the OGS to the laser intensity and the saturation parameter (1/ σ 0 τ). The σ 0 τ product was determined by fitting the theoretical curve to the experimental data.

Ultrasonic Modulation of Scattered Light in Turbid Media and a Potential Novel Tomography in Biomedicine

Ultrasonic modulation of scattered laser light was used to image objects buried in tissue-simulating turbid media. The ultrasonic wave focused into the turbid media modulated the laser light passing through the ultrasonic field. The modulated laser light collected by a photomultiplier tube reflected primarily the local mechanical and optical properties in the zone of ultrasonic modulation. A raster scanning over a heterogeneous turbid medium yielded an image of the medium based on the ultrasound-modulated optical signal. The detectability of modulated signal was estimated using diffusion theory. The dependence of the modulated signal on the off-axis distance of the detector from the optic axis and on the amplitude of ultrasound was studied. The mechanisms of ultrasonic modulation of scattered light are discussed. A theory based on the field autocorrelation function is reviewed as well.

Light-Heat Conversion Material for Dye Thermal Transfer by Laser Heating

In a laser thermal transfer printing using dye sublimation type medium, a high definition and continuous tone image can be obtained easily because a laser light is focused to small spot and heat energy can be controlled by the pulse width modulation of laser light. The ink donor sheet is composed by the laser light absorbing layer and the ink donor layer. We have investigated light-heat conversion efficiency for several IR light absorbing materials in the ink donor sheets. Carbon black, metal-phthalocyanine pigment and dye, metal complex organic compound, and cyanine dye were used as light-heat conversion materials. As experimental results of dye transfer from ink donor layer to receiving sheet by laser heating, each sample exhibits different dye transfer characteristics according to kind of light-heat conversion materials. By the measurement of absorption spectra of laser light absorbing layers after laser heating, it became clear that the phenomena such as the decomposition reaction and the crystal phase transformation of light-heat conversion materials were caused by laser heating. We have concluded that it is necessary for a high heat generation efficiency to have no change of IR light absorption by laser heating.

Ultrasound-modulated optical tomography of absorbing objects buried in dense tissue-simulating turbid media

Continuous-wave ultrasonic modulation of scattered laser light was used to image objects buried in tissue-simulating turbid media. The buried object had an absorption coefficient greater than the background turbid medium. The ultrasonic wave that was focused into the turbid media modulated the laser light that passed through the ultrasonic field. The modulated laser light that was collected by a photomultiplier tube reflected the local mechanical and optical properties in the zone of ultrasonic modulation. Objects buried in the middle plane of 5-cm-thick dense turbid media were imaged with millimeter resolution through the scanning and detecting alterations of the ultrasound-modulated optical signal. The optical properties of the dense turbid media included an absorption coefficient of 0.1 cm(-1) and a reduced scattering coefficient of 10 cm(-1) and were comparable with those of biological tissues in the visible and near-IR ranges. The dependence of the ultrasound-modulated optical signal on the off-axis distance of the detector from the optic axis and the area of the detector was studied as well.

Laser safety analysis of a retinal scanning display system

The Virtual Retinal Display (VRD) is a visual display that scans modulated laser light on to the retina of the viewer's eye to create an image. Maximum permissible exposures (MPE) have been calculated for the VRD in both normal viewing and possible failure modes. The MPE power levels are compared to the measured power that enters the eye while viewing images with the VRD. The power levels indicate that the VRD is safe in both normal operating mode and in failure modes.

An investigation of the 2f—wavelength modulation technique for detection of atoms under optically thin as well as thick conditions

An investigation of the 2 f-wavelength modulation (WM) technique for detection of atoms under optically thin as well as thick conditions is presented. A program that simulates the entire process of laser light modulation, atomic absorption and signal demodulation has been constructed. The dependence of the 2 f-WM signal upon frequency amplitude modulation and detuning for a variety of atomic masses (i.e. optical densities) is investigated in detail. It is shown, among other things, that the 2 f-WM ( S ¯ 2 f - r ) signal strength is either a dual-or a quadrupled-valued function of optical density. Despite this, it shown that the 2 f-WM technique can be used, by suitable choices of the frequency amplitude modulation and detuning, for detection of atoms also under optically thick conditions.