Milliwatt Power Research Articles

Recent advances in coherent detection imaging (CDI) in biomedicine: laser tomography of human tissues in vivo and in vitro

We review the advantages of the optical heterodyne detection-based coherent detection imaging system, for transillumination laser computed tomography (CT) in biomedicine using CW and single-frequency lasers as light sources. The unique properties of the coherent detection imaging system such as excellent directionality, selectivity, and high sensitivity are exploited to differentiate and detect the minimally deviated on-axis/near-axis photons emerging from a tissue enabling the reconstruction high-resolution laser CT images. Our recent progress on the applications of the coherent detection imaging system to laser CT of human fingers in vivo and calcified tissues in vitro at different wavelengths in the visible and near-infrared regions are described. The laser CT images are obtained with low incident power of a few milliwatts and are comparable to conventional imaging techniques. The internal layers of the imaged tissues could be clearly differentiated and identified with submillimeter resolution. We propose that further refinements in the coherent detection imaging system could lead to a novel and potential diagnostic tool in dentistry, osteology, and bone and joint related diseases and disorders.

Optically switched volume holographic elements

We optically switched the diffraction efficiency of volume holographic elements recorded in DMP-128 photopolymer by use of a thermal nonlinearity of the liquid crystal 5CB. The liquid crystal was incorporated into the volume grating. The diffraction efficiency of small areas of the grating was switched with a control beam power of several milliwatts. The intensity of a signal beam at a different wavelength and arbitrary power was switched with time constants below 1 ms.

The performance of the hollow-cathode discharge continuous-wave multicolour He-Cd+ laser

We report on the performance of a hollow-cathode discharge continuous wave He-Cd+ laser module, which is capable of simultaneously delivering stable, milliwatt output power at the three primary spectral lines: blue ( lambda =441.6 nm), green ( lambda =533.7 and 537.8 nm), and red ( lambda =635.5 and 636.0 nm). In particular, the laser output characteristics for single- and multi-line operation and the power interaction between the laser lines are presented. The results show that the most efficient simultaneous laser oscillations at the red, green and blue lines occur for the He pressure (8 mbar at an inner diameter of the hollow cathode of 4 mm) at which the hollow-cathode effect is strongest. We find strong power interaction between the green and red lines, showing experimentally that, apart from the already known increase in the He-Cd+ laser output power in the green owing to the radiative cascade transitions in the red, also the red lines gain in their output power under simultaneous operation with the green lines, which de-populate the lower laser levels of the red transitions. Under some conditions the de-population of the lower laser levels of the red transitions by the green lines' oscillations can be the dominant mechanism for the build-up of the population inversion between the upper and lower levels of the red transitions.

A hollow-cathode discharge cw multicolour He-Cd+ laser module

We report on the design and performance of a hollow-cathode discharge continuous wave He-Cd+ laser module, which is capable of simultaneously delivering stable, milliwatt power output at the three primary spectral lines blue, green and red. A mixture of these lines can result in a wide band of colours, including white light. The laser can be formed from one or more laser modules. A module consists of two anodes with a hollow cathode located between them. Two long, narrow cylinders (50 mm long, 4 mm in diameter) separated by a wider cylinder (10 mm long, 10 mm in diameter) are made in the hollow cathode. The narrow cylinders serve as hollow cathodes, while the wider cylinder efficiently stabilizes the discharges in the narrow cathode cylinders, making them spatially similar. Owing to this particular cathode design the laser module, with a 10 cm active length, stably and effectively lased at seven wavelengths in blue ( lambda =441.6 nm, 3 mW), green ( lambda =533.7 nm, 0.3 mW; lambda =537.8 nm, 0.4 mW), red ( lambda =635.5 and 636.0 nm, total 0.2 mW), and infrared ( lambda =723.8 and 728.4 nm, total 0.1 mW). The optimum He pressure, Cd vapour pressure and cathode current were 8 mbar, 0.01 mbar (corresponding to a cathode temperature of 533 K), and 260 mA, respectively. The short- and long-term laser output power variations were less than 1% (peak-to-peak). The laser has exhibited stable operation for 300 h without discharge deterioration. The above allows us to claim that the presented hollow-cathode discharge continuous wave He-Cd+ laser module should be useful as a simple, short, long-lived, multicolour laser source, operating at milliwatt output power levels. However, for higher output power level demands (tens of milliwatts) three or four laser modules must be used.

Low-power and broadband optical bistability by excitation of surface plasmons in doped polymer film

The low-power and broadband optical bistability in an acceptor-donor-substituted azobenzene-doped polymethyl methacrylate thin film, which relies on the excitation of surface plasmons with a cw laser of several tens of milliwatt power, is studied experimentally and theoretically. The observations on the characteristics of the bistability are essentially consistent with the theoretical predictions.

Photodiode nonlinearity measurement with an intensity stabilized laser as a radiation source

A new facility for the nonlinearity measurement of the spectral responsivity of photodiodes has been developed by using an intensity stabilized laser as radiation source. This device enables an automated measurement of the photodiode nonlinearity from the nanowatt to the milliwatt radiant power range at wavelengths between 250 and 800 nm with uncertainties of better than 0.01% in the visible and better than 0.05% in the UV at the 1σ level.

Low temperature continuous operation of vertical-cavity surface-emitting lasers with wavelength below 700 nm

Low-temperature continuous operation of vertical-cavity surface-emitting lasers with wavelength below 700 nm is discussed. For heat sink temperatures below -25 degrees C, threshold currents less than 4 mA and optical power of several milliwatts are obtained. The possibility of achieving efficient room temperature continuous operation is discussed. >

BACKWARD SCATTERING AND OPTICAL DIODE EFFECT IN Mn DOPED KNSBN CRYSTAL

When a He-Ne laser beam (632.8 nm) with the power of milliwatts or submilliwatts penetrates through a Mn doped KNSBN crystal, a phenomenon of strong backward scattering is observed. At the same time, an optical diode effect which is associated with the backward scatering is also observed. In a certain range, the response time constant T of the optical diode effect is inversely propotional to the incident intensity on the surface of the crystal. The τ is decreased by one order of magnititude when a K9 glass plate which serves as a feedback mirror is introduced. A time constant of 1.5s and a ratio of the forward transmissivty to the backward one, which is greater than 15.3, have been obtained. A theoretical explanation is given on the basis of the experimental results

Ultra-low DC power consumptions in monolithic L-band components

A set of monolithic L-band components operating at milliwatt and sub-milliwatt DC power consumptions have been designed and fabricated. A maximum gain/power quotient of 19.1 dB/mW was recorded for a monolithic amplifier at a frequency of 1.25 GHz with a cascade of 2 MMIC amplifiers yielding a total gain of 15.3 dB on a total power consumption of just 800 mu W. This is believed to be the highest gain/power quotient ever reported for a monolithic circuit at L-band. A four-pole voltage controlled filter with low-power amplifier gain stages showed a loss of 1.6 dB with 15% 3-dB bandwidth on a power consumption of 6.75 mW at 1.575 GHz. A subsystem containing the chips was assembled and tested. The ultra-low power consumptions were obtained with a standard foundry process using an enhancement-mode MESFET with a variety of design techniques. Yields obtained on two 4-in GaAs wafers were 96-100%.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

611.8 nm single-mode He-Ne laser

The conditions for optimum 611.8 nm laser operation are obtained and a convenient method for indicating 3.39 µm suppression is proposed. Single longitudinal mode 611.8 nm generation is obtained using higher gas-mixture pressure and application of an axial magnetic field. Several milliwatt monomode output power and 0.75 multimode-to-monomode power conversion coefficients are obtained without an internal iodine cell. A comparison of the experimental results with a single-mode laser theory is given.

Practical small-scale hollow-cathode CW metal-ion lasers

Design and output characteristics are reported for sealed-off self-heated miniature hollow-cathode laser tubes (active volume 2-3 mm ID * 60-75 mm long) operating on the blue (441.6 nm) and green (533.7/537.8 nm) transitions of Cd+ and blue-green (491.2/492.4 nm) and near-infrared (747.9 and 758.8 nm) transitions of Zn+ at milliwatt power levels. Continuous operating lifetimes of 1000 h have been observed for the He-Cd system. Technical problems involved in achieving long operating lifetimes are discussed.

Estrogen fluoroimmunoassay with a fluorimeter designed for low-intensity light detection

A small, highly sensitive, inexpensive fluorimeter for the detection of very low fluorescence intensities for analytical and clinical purposes is described. High sensitivity is achieved by use of an inexpensive continuous wave laser with a power of a few milliwatts. The optical arrangement of the instrument has been thoroughly designed so that the effects of Rayleigh and Raman scattering are minimized. Fluorescence is dected through optical filters with a side-on photomultiplier. Concentrations of rhodamine B in ethanol as low as 10 −11 M can be measured. To use this instrument for a fluoroimmunoassay of urinary estrogens (10 −9−10 −7 M), a purified monoclonal IgG, raised against estradiol, was labelled with RITC (rhodamine isothiocyanate) in a 1:1 molar ratio. It was possible to measure IgG-RITC concentrations between 10 −9 and 10 −10 M with good accuracy and reproducibility.

High-speed characteristics at high input optical power of GaInAsP electroabsorption modulators

High-speed characteristics at high input optical power levels of GaInAsP double heterostructure electroabsorption modulators are investigated. It is shown that the bandwidth degradation due to high input optical power is strongly dependent on the absorption coefficient. A design to maintain highspeed characteristics under milliwatt range output power operation, and experimental results including a 100km single-mode fibre pulse transmission experiment, are demonstrated.

RF powering of millimeter- and submillimeter-sized neural prosthetic implants.

The size of the transducers for neural stimulation has shrunk steadily with application of thin-film techniques to electrode design. The feasibility is examined of designing millimeter- and submillimeter-sized power sources based on RF coupling that could be integrated into these implants to provide power without a tethering power cable. The coupling between a transmitter coil and receiver coil when the coil diameters are markedly different is analyzed, and for these circumstances, a simple Thevenin equivalent model is developed to describe the power transmission between the transmitter and receiver. The equivalent circuit developed gives insight into the way that coil diameters, frequency, and turns affect coupling between large and small coils. Several examples demonstrate that milliwatt range power sources can be implemented with millimeter- and submillimeter-diameter receivers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Room-temperature single-wavelength optical latching circuits using GaAs bistable devices as logic gates

Room-temperature two-element latching circuits are experimentally demonstrated using GaAs multiple-quantum-well (MQW) nonlinear etalons under single-wavelength bistable operation. Signal power gains of up to 4 and contrast ratios of 10 have been achieved at milliwatt power levels with 1-μs pulses, which will allow the construction of more complicated digital optical circuits using diode lasers as the light sources. The problems of applying these nonlinear etalons to fast optical signal processing are also discussed. Numerical simulations show that the gain vanishes as the pulse duration decreases and approaches the carrier lifetime.

INVESTIGATION OF FLUORESCENT CHARACTERISTIC IN Nd:YAG SINGLE CRYSTAL FIBERS

The fluorescent characteristic in Nd: YAG single crystal fibers has been investigated. One rod and four claded fibers produced with MPG process were tested, of which the diameters ranged from 41 micron to 120 micron with variations less than 0.5%. Excited by 300 milliwatt power laser at 0.6 micron, the fluorescence amplification at 1.06 micron was observed. The analysis predicted the condition and capability of superfluorescence.

The photorefractive effect—a review

Photorefractive materials have been assuming ever increasing prominence in the non-linear optics field due to their unique capability for displaying strong non-linear effects at milliwatt power levels. In this review we describe in some detail the two currently accepted models of the photorefractive process. The “band transport” model assumes that photo excited electrons (or holes) are ejected from filled donor (or acceptor) sites to the conduction (or valence) band where they migrate to dark regions in the crystal before recombining into empty traps. The charge separation results in a space charge field which modulates the refractive index via the linear electro-optic effect. The “hopping” model assumes that carrier transport occurs via light induced hopping from a filled donor site to a neighbouring empty trap. In particular, we examine the timeand spatial frequency dependence of grating formation and discuss the validity of the simplified model of a material response linear in modulation ratio (fringe contrast). In addition we show that the “hopping” model although physically distinct from the band transport model is described by equations which can be considered as a special case of the band transport equations. Results of a complete numerical simulation of the band transport model are presented to illustrate the properties of the photorefractive effect and to test the validity of the simplified model. The two models are normally formulated for an isotropic material illuminated by two plane wave beams which are assumed to be unaffected by the material. Photorefractive materials, however, are naturally anisotropic and in applications one is often interested in non-plane wave beams. We present for the first time an extension of the model to non-plane wave beams and explicity take full account of the vectorial nature of light and the crystal anisotropy. Barium titanate is used as an example to illustrate the extended model and graphs are presented which show the dependence of the nonlinearity on beam angles, polarisation and crystal orientation. Finally we discuss some applications is non-linear optics which exploit some of the unique features of the photorefractive effect. The photorefractive effect is illustrated throughout the paper by experimental results performed in our own laboratory.

Giant nonlinearities and low power optical bistability in cadmium sulphide platelets

Both whole-beam and transverse continuous wave optical bistability are observed at milliwatt power levels near the band gap in thin uncoated cadmium sulphide platelets. The whole-beam bistability is free from cavities and requires an absorption coefficient that increases with intensity.

Cavityless optical bistability due to light-induced absorption in cadmium sulfide

Cavityless, whole-beam optical bistability in thin, uncoated cadmium sulfide platelets is observed at milliwatt power levels when a cw laser is tuned near a sharp extrinsic excitonic resonance in cadmium sulfide (the I2 bound exciton). Very large contrast ratios between the on and off states are measured. The detuning and temperature dependence of the bistability results are reported. Preliminary transient measurements are also discussed.

Four-wave mixing in semi-insulating InP and GaAs using the photorefractive effect

The photorefractive effect has been observed for the first time in semi-insulating InPe:Fe and GaAsCr. These materials are sensitive and versatile recording media for high bit rate parallel optical processing in the 0.8–1.8-μm spectral region using injection lasers of milliwatt power levels.