Optical Phase Change Research Articles

Determination of the Absorption Coefficient of Thin Absorbing Films from Measurements on the Change of Phase in Transmission

Optical phase changes of visible radiation in transmission through evaporated gold and tellurium thin films were determined using the Rayleigh-Lowe refractometer. The variation of the phase change in transmission with thickness was utilized to determine the absorption coefficient, ku, for both films as well as the refractive index, nu.

Thermal phase noise in coherent optical-fibre systems

Coherent optical-fibre systems are sensitive to optical phase changes along the length of the fibre link. Thermal energy in the optical fibre can give rise to acoustic waves propagating along it and these result in optical phase changes. Thus, there is the possibility of a noise signal induced by thermal acoustic energy propagating along the fibre. This letter postulates a theoretical model for this interaction and estimates the magnitude of the effect. Conditions under which it may be a significant noise source are discussed.

Static pressure sensitivity amplification in interferometric fiber-optic hydrophones

The induced optical phase change produced when a static pressure is applied to the test arm of an interferometric single-mode fiber optic hydrophone is examined in terms of hydrostatic and radial mechanical models. The expressions for the models are given in terms of a 3-D solution to the equations of elastostatics for multilayered cylinders. The induced phase change is calculated using both models for various values of the diameter and elastic properties of fiber jacket materials. It is shown that the phase change predicted from the 3-D approach for each model can be adequately described in terms of much simpler 2-D plane strain models. Calculations show that the hydrophone sensitivity of a jacketed fiber is amplified compared with a bare fiber. The largest increase in sensitivity is predicted with the radial model. Calculated sensitivities for the hydrostatic model are shown to correspond closely in value with static pressure sensitivity measurements for the experimental arrangement used here.

Subnanosecond optical free-induction decay

A novel form of laser frequency switching is devised which extends coherent optical transient studies to a 100-psec time scale. The technique incorporates a traveling-wave electro-optic element which imposes with unit efficiency a uniform time-varying phase and thus a frequency shift on a cw laser beam. In contrast to earlier optical traveling-wave modulators which are driven by a microwave oscillator, here the optical phase change is induced rapidly and easily by a dc electric field pulse that propagates in a microwave guide either parallel or antiparallel to the light wave. This advance enables optical free-induction decay (FID) studies on a subnanosecond time scale and reveals such new features as a rapid first-order FID that dephases with the inhomogeneous dephasing time ${T}_{2}^{*}$. The well-known nonlinear FID can interfere with the first-order component at short times and decays over the much longer period $\frac{{T}_{2}}{[1+{(1+{\ensuremath{\chi}}^{2}{T}_{1}{T}_{2})}^{\frac{1}{2}}]}$, where $\ensuremath{\chi}$ is the Rabi frequency. A complete analytical expression is derived for optical FID of a transition subject to homogeneous and inhomogeneous broadening and supports detailed observations of the sodium ${D}_{1}$ line.

Pressure sensitivity amplification in interferometric fiber optic hydrophones

The induced optical phase change when a pressure is applied to a fiber optic hydrophone is examined both theoretically and experimentally. The induced phase change is calculated both with and without a plastic jacket around the glass fiber. It is shown that the phase change predicted from a detailed three‐dimensional analysis can be adequately described in terms of the much simpler two‐dimensional generalized plane strain approximation. Sensitivities calculated assuming hydrostatic boundary conditions are shown to correspond closely in value with static pressure‐sensitivity measurements. The application of a plastic jacket can substantially increase the pressure sensitivity of the fiber. The increased phase shift is due primarily to the tendency of the jacket to increase the axial strain experienced by the glass fiber. It is predicted that maximum pressure sensitivity (up to 30 times that of the bare fiber) will be achieved by jacketing with Teflon.

Assessment of thin film modulator performance by homodyne detection of optical phase change

The phase change in a phase modulator can be simply and readily measured by means of homodyne detection and a display arrangement. The static optical bias inherent in the interferometer can be varied by applying a DC voltage in series with the AC voltage across the electro-optic structure. The method is ideally suited to thin film modulators where the small voltages required can be derived from an inexpensive signal generator.

Hologram–Moiré Interferometry for Transparent Objects

A modification of hologram interferometry applicable to the study of transparent objects of moderate optical quality has been devised which combines hologram and moiré techniques and allows the observation of high contrast, relatively unlocalized, interferometric fringes under conditions less critical than those of the conventional hologram interferometer. By spatially modulating one beam of the interferometer during exposure, moiré fringes may be obtained in the reconstruction which distort and move with optical phase changes in either beam of the interferometer. An analysis of the origin and nature of these distortions is given and experimental verification of the analysis is presented.