A. B. Buckman

Position dependence of the transient response of a position-sensitive detector under periodic pulsed light modulation

A model based on sinusoidal steady-state analysis is developed for the position-sensitive detector (PSD), which is approximated as a distributed RC transmission line. This model is used to study the output response of the lateral effect PSD (i) when only one pulse-modulated light beam is incident and (ii) when two light beams of different wavelengths, with one beam modulated at a frequency f and the other beam modulated at a frequency 2f, are incident at the same time. The simulation shows, and experiments confirm, that the transient response of the PSD consists of a position-dependent dead time (time for the output current to sense the change in the induced photocurrent) and a position-independent exponential rise decay time. For the application of resolving the positions of two or more pulsed-modulated light spots, it is shown that there is a position-dependent upper limit on the usable modulation frequency. With the per-unit-length values chosen for the sheet resistance as R=315 Omega /mm and the junction capacitance as C=330 pF/mm, the modulation frequency limit is determined to be 20 kHz. >

Position detection of multiple light beams using phase detection

Earlier work on lateral-effect position-sensitive detectors (PSD) has used an amplitude detection signal-processing scheme to determine the position of the centroid of the incident light beam on the PSD surface. In this work we introduce a phase detection method of position measurement which is based on detecting the phase difference between the sinusoidal currents flowing through the metal electrodes. A distributed transmission line model for the PSD and a one-pole model for the transimpedance preamplifier are used in the analysis of the sensor. Experimental results are presented and are found to be in close agreement with simulation results. It is seen that the spatial resolution is proportional to the modulation frequency. With a light modulation frequency of 50 kHz and the introduction of frequency multiplication (/spl times/8) after the transimpedance preamplifier, a spatial resolution of 2 /spl mu/m is demonstrated. For application in multi-degree-of-freedom position sensors, we demonstrate the simultaneous detection of centroids of multiple light beams using frequency division multiplexing. Application of the phase method of position detection to a two dimensional PSD shows a maximum deviation from linearity of 1% over the working range of the PSD. >

Modulated ellipsometry for band structure studies of solids and films

Abstract The use of modulated ellipsometry to measure the very small changes in the optical or dielectric constants of surfaces under the influence of external electric fields or applied stresses can provide more experimental information than the normal incidence electro- or piezoreflectance experiments generally used in optical studies of the energy band structure of solids. The experiment provides enough independent measured parameters to directly calculate the complex dielectric constant and the modulation-induced change in it from measurements taken at only the photon energy of interest. Thus, the Kramers-Kronig analysis of the reflectance and the modulation-induced change in reflectance, required to obtain the dielectric constants and the induced changes from the normal incidence data, is not required with modulated ellipsometry. Extrapolation of the Kramers-Kronig integral to experimentally inaccessible photon energies can lead to errors in the dielectric constant calculations in certain cases. The change δe1 in the real part e1, and the change δe2 in the imaginary part e2, of the dielectric constants of opaque, evaporated films of Au and Ag, have been measured using this method. The modulation is a high (10 5 –10 7 V cm ) electric field applied normal to the sample surface across the dipole layer of a 1 N KCl electrolyte. It is apparent that the electronic structure is shown more clearly in δe1 and δe2 than in δR R alone, since δe1 and δe2 can evidently combine in δR R such as to mask certain structure in the curves. Much of the energy band structure in the range of photon energies measured can be readily identified in the lineshapes of δe1 and δe2.

Noise analysis for position-sensitive detectors

A study of the different noise sources in a position sensitive detector (PSD)-transimpedance amplifier (TIA) sensing system is presented and the dominant noise sources are identified. The effect of these noise sources on the position detection capability of the sensing system is analyzed. An expression derived for the position resolution of the phase method of position detection reveals the position resolution depends inversely on the modulation frequency of the light source and the square of the amplitude of the currents flowing through the metal electrodes, and is dependent on the position of the incident light beam. Simulation results show that the best achievable position resolution is at the center of the PSD and becomes worse as one moves away from the center toward the edges. Compared to the 4 nm//spl radic/Hz position resolution that is achievable using the amplitude method of position detection, the phase method of position detection provides a resolution of 9 nm//spl radic/Hz and 6 nm//spl radic/Hz corresponding to a modulation frequency of 50 kHz and 70 kHz respectively.

Effective electro-optic coefficient of multilayer dielectric waveguides: Modulation enhancement

A matrix method for calculating the effective refractive index of guided modes on multilayer dielectric waveguides is developed and employed to calculate the effective electro-optic coefficient of such waveguide structures when one of the media composing them is electro-optic. In properly specified three-layer waveguides, enhancement of the effective electro-optic coefficient above the bulk value by as much as a factor equal to the square of the highest refractive index in the structure is possible. The maximum modulation enhancement for the three-layer guide is attainable with loosely, as well as with tightly, confined waveguide modes.

Coupled surface plasmons in structures with thin metallic layers

If the negative-permittivity region in a layered structure is very thin (≲ 300 A for Ag at 6328 A excitation), the two surface plasmons at its interfaces couple in its interior to yield a coupled-surface-plasmon mode whose propagation constant increases with decreasing layer thickness. These modes were observed by measuring reflectance versus angle of incidence through a high-index prism for TM-polarized light. For very thin Ag films, the reflectance minimum diverges from the phase-matching condition, as determined from independently measured refractive indices and thicknesses. This divergence apparently arises from separation of the pole and the zero in the reflectance versus propagation vector as the Ag thickness is decreased.

Analysis of a novel optical fiber interferometer with common mode compensation

An optical fiber interferometer formed when the unused output of a fiber Mach-Zehnder interferometer is fed back through a length of fiber to the unused input port is discussed. For sensor applications, the resulting optical circuit combines many features of a Mach-Zehnder with the high measured sensitivity of a resonant fiber recirculating delay line. This analysis shows that suitable choice of path lengths preserves high measurand sensitivity, yet it permits complete common-mode compensation in which the effects of all perturbations which uniformly affect the interferometer vanish. >

Large refractive-index change in PbI 2 films by photolysis at 150–180 °C*

The refractive index of evaporated PbI2 films, in the red region of the visible spectrum, can be reduced up to 18% by exposure to 488 nm light at film temperatures of 150–180 °C. Absorption in the same region remains very small throughout the process, until a temperature-dependent exposure threshold is reached, then rises rapidly due to scattering by photolysis-produced Pb agglomerates. For exposures below this threshold, localized band-gap states and/or bounded-plasma effects in the small Pb agglomerates give rise to an absorption band at 520 < λ < 580 nm, which causes the large refractive-index change. The photolysis in the polycrystalline films is uniform through the film rather than concentrated near the free surface, as in single crystals.

Polarization-selective lateral waveguiding in layered dielectric structures

A riblike waveguide structure consisting of three isotropic transparent layers on an isotropic transparent substrate is shown to be capable of selectively guiding only the TE-like (TM-like) mode if the waveguide axis is defined by a trough (ridge) in the low-index middle layer. Guidance in the film plane arises from the joint dependence of the planar waveguide mode-propagation constants on the strength of evanescent coupling between high-index layers and on the difference between the propagation constant for the three-layer structure and that for a single, isolated layer on the same substrate. The mode with polarization opposite the guided one radiates away from the guide axis in the film plane. Because the insertion loss arises only from scattering and mode conversion at boundaries, it is expected that high extinction ratios, accompanied by tolerable insertion losses, will be realized.

Theory of an Efficient Electronic Phase Shifter Employing a Multilayer Dielectric-Waveguide Structure

Multilayer dielectric-waveguide theory is applied to the design of a submillimeter phase shifter driven by a p-i-n diode. In a three-layer structure, proper choice of layer thicknesses can yield a predicted phase shift/unit length about ten times that possible from a single slab at the same frequency. In a properly specified four-layer structure, still larger shifts, limited primarily by the required excursion in effective refractive index, are shown to be possible. A further advantage of the four-layer structure is that it can be designed to have a very thin active region, thus lowering diode power/unit phase shift, which is proportional to the cross section of the intrinsic region of the p-i-n diode.