Phase reconstruction from reflectivity in fiber Bragg gratings.

It is shown that a recently proposed phase-retrieval technique for Bragg gratings [Opt. Lett. 22, 93 (1997); J. Lightwave Technol. 15, 1314 (1997)] is not well suited for gratings with imperfections. The reconstructed group delay is in many cases not a more accurate estimate than the simulated group delay of the perfect, designed grating, independently of how small the errors in the grating structure are. The error in the group delay may be especially large near the zeros in the reflection spectrum.

Torsional rod wave scattering from tapering between regions of different cross-sections.

Previously (Engan 1998) we have studied the scattering of torsional waves in circular rods with a free surface where the rod diameter is subject to abrupt diameter changes. Here, we discuss the scattering of torsional waves from tapered regions and, in particular, transmission of the fundamental torsional mode. The calculations are based on a mode expansion technique in which propagating as well as cut-off modes are taken into account. The tapered region is approximated by numerous steps separated by uniform rod parts arranged so that they closely render the taper shape. Transfer matrices for all steps and uniform parts are combined to yield the desired transmission coefficient through the taper region. Numerical results of the frequency variation of fundamental mode transmission are presented, depending on geometrical parameters, such as taper length and the diameters of the two uniform adjacent rod parts. The dependence of the transmission on taper shape is demonstrated by calculating the response of various shapes.

Position weighting of fiber Bragg gratings for bandpass filtering.

A fiber Bragg grating bandpass filter with tailored refractive-index modulation is investigated. The writing method includes a position weighting technique, which provides flexible and accurate control of the index modulation along the fiber. This weighting is accomplished by adjusting the positions of a large number of overlapping subgratings to control the local grating strength. A 10-cm grating with a truncated sin(x)/x refractive-index modulation function was written and characterized with this method, showing good agreement with simulations. These types of filter are well suited for wavelength-division-multiplexing applications and can be made to have steep edges, low sidelobes, large bandwidth, and low stopband ripple.

Broadband spatial mode conversion by chirped fiber bending.

We solve the coupled-mode equations for linearly chirped codirectional mode couplers and use the results to develop design rules for broadband couplers. By comparing a chirped coupler with a uniform coupler having the same effective coupling length we find that high coupling efficiency can be obtained over a substantially wider bandwidth. We design a chirped device to have a 99% bandwidth of 47 nm at 1090 nm, which is 5.8 times larger than for the uniform coupler. Experimentally the bandwidth was improved by a factor of 5.4.

Intermodal beat lengths in birefringent two-mode fibers.

We use an accousto-opitc method to measure the beat lengths between the LP(01) and the LP(11) modes propagating both in an elliptical-core polarization-maintaining fiber and in a bow-tie high-birefringence (Hi-Bi) fiber for a wide range of optical wavelengths. The Hi-Bi fiber exhibits considerable polarization splitting in the measured beat lengths at all measured wavelengths. According to theory for circular-core fibers, the beat length should have a minimum at the wavelength at which the two modes have the same group velocity in the fiber. We find that for the elliptical-core fiber such a minimum does not exist in the two-mode region. Based on our measurements we discuss the use of different fiber types in various two-mode devices.

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.

Narrow-band acousto-optic tunable filtering in a two-mode fiber.

We demonstrate an optical filter that utilizes acousto-optic coupling between the spatial modes that propagate in a two-mode elliptical-core fiber. The optical bandwidth at 1090 nm is 0.85 nm, which is in agreement with predictions based on measurement of differential group delay between the modes. The filter is slightly polarization dependent, with a 0.28-nm wavelength difference between the passband peaks for the two polarization eigenstates. The optical insertion loss is dominated by 2-4-dB bending loss in nonoptimized mode strippers, and the coupling loss is negligible. Full acousto-optic mode conversion was achieved at an 8-mW electrical input power to the acoustic transducer.

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 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.

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.