A. A. Friesem

Resonant grating–waveguide structures for visible and near-infrared radiation

A new ray picture model based on the multiple interference of light waves in dielectric resonant grating–waveguide structures is presented. The model clearly elucidates the phase relationship between the incident plane wave and the waves diffracted from the resonant grating structure that is responsible for the interference of these waves. As a result of this interference process the incident wave can be totally reflected at a certain wavelength and orientation angle. The model is used to describe and analyze this resonance behavior of the grating–waveguide structures as a function of wavelength and incidence angle. The analysis is verified experimentally with semiconductor (InGaAsP/InP) structures at wavelengths of 1.55 μm and also with dielectric (silicon nitride/SiO2) structures at wavelengths of 0.6 μm. All of the structures were formed by electron beam lithography and chemical vapor deposition. The measured results reveal that subnanometer resonance bandwidths and finesses as large as 6000 can be achieved at contrast ratios of 50 with relatively compact structures.

Light modulation with resonant grating–waveguide structures

Resonant grating-waveguide structures formed with InP/InGaAsP semiconductor materials were tested to show light modulation at a wavelength of 1.55 microm. Narrow, subnanometer resonant spectral bandwidths and a ratio of ref lected intensities between resonance and away from resonance of greater than 50 were measured. For a resonant structure with an area of 3 mm x 3 mm, the modulation frequency reached 5 MHz.

Narrow spectral bandwidths with grating waveguide structures

Under certain conditions grating/waveguide structures have a resonant behavior with narrow spectral bandwidths. Several such structures were designed, fabricated, and evaluated, demonstrating resonances with narrow spectral bandwidths ranging to as low as 0.035 nm at full width at half maximum. These can be exploited as compact spectral filters having high finesse.

Stimulated Brillouin scattering excited by a modulated pump wave in single-mode fibers

The effects of modulation on the gain of stimulated Brillouin scattering (SBS) are discussed. Three different modulation techniques, i.e. frequency shift keying (FSK), amplitude shift keying (ASK), and phase shift keying (PSK) are considered. The results show that the SBS threshold depends on the ratio between the spontaneous Brillouin linewidth and the bit rate. The SBS threshold for ASK and FSK is 6 dB higher at high bit rates, whereas the SBS threshold for PSK increases by 10dB for each factor of 10 increase in the bit rate. >

Visor-display design based on planar holographic optics.

A method for designing and recording visor displays based on planar holographic optics is presented. This method can deal with the problem of recording-readout wavelength shift. The display system is composed of two holographic optical elements that are recorded on the same substrate. One element collimates the waves from each data point in the display into a plane wave that is trapped inside the substrate by total internal reflection. The other diffracts the plane waves into the eye of an observer. Because the chromatic dispersion of the first element can be corrected by the dispersion of the second, this configuration is relatively insensitive to source wavelength shifts. The method is illustrated by the design, recording, and testing of a compact holographic doublet visor display. The recording was at a wavelength of 458 nm, and readout was at 633 nm. The results indicate that diffraction-limited performance and relatively low chromatic dispersion over a wide field of view can be obtained.

Holographic recording and all-optical modulation in photochromic polymers

Polymers doped with photochromic spirooxazine and spiropyran dyes have been investigated for real-time holographic recording. The temporal holographic response was found to depend strongly on optical recording configurations and on the recording beam intensities. The exposure sensitivities that were required for maximal diffraction efficiency were 250 mJ/cm2 for the polymers doped with spirooxazine and 650 mJ/cm2 for those doped with spiropyran. All-optical modulation of the holographic gratings with 2000 lines/mm is possible by modulation of a separate excitation beam of incoherent UV radiation. The modulated gratings can be stored for relatively long times or erased immediately.

Nonlinear effects in coherent multichannel transmission through optical fibers

Various nonlinear optical interactions in single-mode fibers that are used in coherent FDM (frequency division multiplexed) transmission systems are examined. It is these nonlinearities that lead to crosstalk between channels, power losses, and deleterious fluctuations, which in turn limit the power of the transmitted light and the number of allowed channels, and dictate the channel allocations. It is shown that, for long-haul transmission systems with fiber lengths exceeding 100 km, typical channel separation of 10 GHz, and few channels, the maximum allowed input power per channel, P/sub max/, is limited by SBS (stimulated Brillouin scattering) to about 5 dBm. As the number of channels increases, FWM (four wave mixing) becomes the limiting process with P/sub max/ of about -5 dBm, whereas above several hundred channels SRS (stimulated Raman scattering) becomes dominant with P/sub max/ of about -5 dBm. For local area networks with shorter lengths, the results are similar, except that the values of P/sub max/ are uniformly higher by about 5 dB. >

Highly concentrating Fresnel lenses

A new type of convex Fresnel lens is discussed capable of concentrating solar radiation very near the ultimate concentration limit. The differential equations that describe the lens are solved to provide computed solutions, which are then checked by ray tracing techniques. The performance (efficiency and concentration) of the lens is investigated and compared with that of a flat Fresnel lens, showing that the new lens is preferable for concentrating solar radiation.

Efficient Fresnel lens for solar concentration

Abstract The development effort in optical components for optimally concentrating solar energy has thus far emphasized reflecting elements, such as cylindrical and compound parabolic mirrors. In this paper we consider transmission elements, particularly a new design for an efficient linear Fresnel lens capable of high concentration for a given acceptance angle. The predicted performance of the lens is comparable to that of the “ideal” reflector, while providing greater reliability at a lower cost.

Stimulated Brillouin scattering excited by two pump waves in single-mode fibers

Stimulated Brillouin scattering (SBS), excited by two pump waves in single-mode fibers, is investigated both theoretically and experimentally. Steady-state calculations, supported by experimental results, show that the SBS gain depends on the ratio of the pump coherence length to the characteristic gain length of the SBS as well as on the ratio of the frequency separation between the two pump waves to the SBS linewidth. These dependences are fully analyzed by following the evolution of the pressure wave that is generated by the SBS interaction. The competition between four-wave mixing and SBS is also considered.