Helge E. Engan

All-fiber acousto-optic frequency shifter

An all-fiber-optic frequency shifter is demonstrated that uses mode coupling between the LP(01) and LP(11) modes by a traveling acoustic flexural wave guided along the optical fiber. The input and output leads of this frequency shifter are single-mode fibers. Unity mode-conversion efficiency for cw operation is achieved at 8-MHz frequency shift with about 0.25 W of electrical input power. Carrier and image sideband suppression of 15 and 35 dB, respectively, are demonstrated.

Excitation of elastic surface waves by spatial harmonics of interdigital transducers

The electric field distribution from a surface wave interdigital transducer is investigated. Expressions for the various spatial harmonics and the capacitance of a transducer applied on an anisotropic, small-coupling propagation medium are derived. Experiments are consistent with the calculations.

Analysis of intermodal coupling in a two-mode fiber with periodic microbends

Simple expressions for the coupling between the LP(01) and LP(11) modes of a two-mode optical fiber with a periodic microbending structure are developed. Implementation of the microbend structure using a flexural acoustic wave is described. The dependences of the acoustic frequency and power requirements on the pertinent fiber parameters are presented.

Wide-band operation of horns for excitation of acoustic modes in optical fibers

Acousto-optic interaction in fibers between optical modes and acoustic waves has been investigated recently by several groups. Coupling has been demonstrated between spatial modes in two-mode fibers as well as between polarization modes of high birefringence fibers. Most publications have reported results for a single acoustic frequency without discussing the frequency response of such devices. This paper reports an experimental investigation on the frequency behavior of horns, in particular aimed at wide band operation of devices coupling optical spatial modes by acoustic flexural waves in the fiber. This is provided by longitudinal waves impinging on the horn tip. Results for fused silica horns as well as aluminum horns are reported.

Fiber-optic frequency shifter with no mode change using cascaded acousto-optic interaction regions.

A new method for performing frequency shifting in two-mode elliptical-core optical fibers is described. We report the use of cascaded acousto-optic interaction regions in analogy with techniques already used in LiNbO3 technology. The interaction region is divided in two. The first section consists of a contradirectional acoustic flexural wave, and the second section consists of a codirectional acoustic flexural wave. With this scheme, a frequency shifter has been built that maintains the light in the fundamental mode of the optical fiber. Experimentally, we have obtained carrier and spurious sideband suppression of better than 40 dB.

New SAW Velocity Measurements on Y-Cut LiNbO 3

Recent measiirements on SAW velocities are reported. The results are based on scanning of a laser probe across the propagation beam at certain intervals. Previously reported measurements on YZ LiNb03 have been extended to include the complete Y surface. For t.his orientation, results for both a free surface and a metallized surface are reported. A noticeable digference has been found between the beam steering properties of free and met.alized surface around Z propagation direction. Effects from va r i a t ion (I) f me t a 1 1 i z a t i on t h i c kne s s have been studied also and are presented. Many SAW devices use parallel metal strips on the substrate surface. Measurements on the angular variation of SAW velocities in such st.ructiires are reported.

All-fiber acousto-optic frequency shifter excited by focused surface acoustic waves.

A fiber-optic frequency shifter that couples light between two spatial modes is discussed. The acoustic flexural wave that provides the coupling is excited in the fiber by pressing it against the surface of a LiNbO(3) substrate at a location where a surface acoustic wave is focused by a curved interdigital transducer. This results in an excitation arrangement that is not destructive and provides high efficiency.

Analysis of polarization-mode coupling by acoustic torsional waves in optical fibers

Coupling of polarization modes of the LP01 spatial mode by acoustic torsional waves in an optical fiber is considered in detail. Analytic expressions of the coupling coefficients based on the combined effects of birefringence and photoelastic contributions are found. The results show that for regular fibers the birefringence component is the larger of the two. By the use of higher radial orders of the acoustic torsional modes, the coupling efficiency can be increased.

Spectral flattening by an all-fiber acousto-optic tunable filter

An acousto-optic notch filter based on modal coupling in a two-mode optical fiber is used to flatten the spectrum of a fiber laser. The FWHM bandwidth is optimized so that only one acoustic frequency is needed. At 1090 nm the spectral variation is reduced from 4 dB to less than 0.3 dB within a bandwidth of 6 nm.

Spectral stability and smoothness of a phase-modulated fiber laser

A fiber laser is phase modulated in the presence of a weak external reflection, leading to a strong intensity modulation at the laser output. The modulation causes the optical spectrum to be substantially more stable and smooth. The usefulness of the improved spectrum is illustrated by two different experiments, one for the characterization of an optical filter by measurement of its influence on the laser spectrum and the other for the measurement of the spatial-mode group-delay difference in a two-mode fiber by use of white-light interferometry. In both experiments the noise level is substantially reduced when the phase-modulated source is used.