Timothy E. Dimmick

Miniature all-fiber devices based on CO(2) laser microstructuring of tapered fibers.

A focused carbon dioxide laser beam is used to microstructure fibers that have already been narrowed by conventional fiber tapering. We describe three new miniature devices made with this technique: a fused fiber microcoupler with an interaction length of 200 mum, a long-period grating made from a periodic chain of microtapers, and a new type of prolate whispering-gallery mode microcavity.

Carbon dioxide laser fabrication of fused-fiber couplers and tapers

We report the development of a fiber taper and fused-fiber coupler fabrication rig that uses a scanning, focused, CO(2) laser beam as the heat source. As a result of the pointlike heat source and the versatility associated with scanning, tapers of any transition shape and uniform taper waist can be produced. Tapers with both a linear shape and an exponential transition shape were measured. The taper waist uniformity was measured and shown to be better than +/-1.2%. The rig was also used to make fused-fiber couplers. Couplers with excess loss below -0.1 dB were routinely produced.

Compact all-fiber acoustooptic tunable filters with small bandwidth-length product

All-fiber acoustooptic tunable filters (AOTFs) with bandwidth as small as 2.8 nm were fabricated from highly uniform, tapered optical fiber. These filters have typical excess loss of 0.02 dB, greater than 20 dB extinction, are polarization insensitive, and have the smallest bandwidth-length products reported to date.

Narrow-band acousto-optic tunable filter fabricated from highly uniform tapered optical fiber

A 2.9 nm bandwidth AOTF was fabricated from highly uniform, tapered optical fiber. The filter had an excess loss of 0.02 dB, >20 dB extinction, was polarization insensitive, and had the smallest bandwidth-length product reported to date.

Simple, inexpensive wavemeter implemented with a fused fiber coupler

A simple, inexpensive wavemeter for measuring the wavelength of optical fiber coupled light sources is described. We determined the wavelength by measuring the wavelength-dependent coupling coefficient of a fused fiber coupler. A prototype device covering the wavelength range of 1500-1600 nm was assembled and tested in the laboratory. The wavemeter had a wavelength uncertainty of less than 0.087 nm over a wavelength range of 1530-1570 nm. The dominant source of error in the test was found to be due to the presence of broadband amplified spontaneous emission (ASE) in the laser output. It is expected that the accuracy of the device can be increased significantly by the introduction of a band-limiting filter to reduce the ASE level.

Simple and accurate wavemeter implemented with a polarization interferometer

A simple and accurate wavemeter for measuring the wavelength of monochromatic light is described. The device uses the wavelength-dependent phase lag between principal polarization states of a length of birefringent material (retarder) as the basis for the measurement of the optical wavelength. The retarder is sandwiched between a polarizer and a polarizing beam splitter and is oriented such that its principal axes are 45 deg to the axis of the polarizer and the principal axes of the beam splitter. As a result of the disparity in propagation velocities between the principal polarization states of the retarder, the ratio of the optical power exiting the two ports of the polarizing beam splitter is wavelength dependent. If the input wavelength is known to be within a specified range, the measurement of the power ratio uniquely determines the input wavelength. The device offers the advantage of trading wavelength coverage for increased resolution simply through the choice of the retarder length. Implementations of the device employing both bulk-optic components and fiber-optic components are described, and the results of a laboratory test of a fiber-optic prototype are presented. The prototype had a wavelength accuracy of +/-0.03 nm.

Fabrication of high performance fibre tapers and couplers using a CO/sub 2/ laser rig

This paper presents the first example of a CO/sub 2/ laser rig that can make fibre tapers and couplers with a precisely controlled longitudinal profile and shape. Fibre tapers and directional couplers are very important components for all-fibre devices. Although the fabrication processes of such components are well established, high resolution of taper formation has not been demonstrated. Theoretical analysis has shown that precise control of taper shape can in principal be achieved using a scanning point-like heat source. If the source is scanned and the taper elongated at constant speed, every section of the fibre receives the same treatment as each neighbour, ensuring a uniform profile. By varying the scanning distance of the heat source, different taper shapes can be produced. Various types of flames are commonly used and heat the fibre quickly enough to be scanned as discussed above but their performance is limited by instabilities and fluctuations induced by irregularities of the gas supply and air currents in the laboratory. Most importantly, their actual physical size is fixed and large limiting the minimum hot-zone length, and the inertia of the flame inhibits fast scanning. Instead, the use of a stabilised CO/sub 2/ laser can prove ideal.

Imaging detector of temporally coherent radiation

We present an optical architecture and image processor capable of detecting and locating temporally coherent radiation that may be dominated by incoherent background radiation. The optical architecture makes use of a coherent light modulator that modulates light of sufficient coherence length while it leaves light of short coherence length unmodulated. The design of the coherent light modulator offers a substantially wider field of view than did past designs, permitting its application within an imaging system. The image processor synchronously detects the modulation imposed on coherent light while it rejects incoherent light fluctuations. Results of a laboratory test are presented. The system tested in the laboratory had a 26 degrees field of view and was able to detect and locate coherent radiation >30 dB below the background incoherent light level.

Fused taper fibre microcoupler

Single-mode fibre couplers considerably less than 1 mm long are formed by fusing together two tapered fibres using a carbon dioxide laser. This will allow a miniature version of many all-fibre devices to be made.

Impact of beam weighting on dynamic range and resolution of optical spectrum analyzers

Acousto-optic spectrum analyzers have found wide application in signal processing as a result of their ability to provide wide bandwidth, rapid transforms with relative ease. In the past the dynamic range of acousto-optic spectrum analyzers has been limited to approximately 30 dB by the dynamic range of the CCD sensor array. Recently CCD sensors with non-linear response have been developed with dynamic range in excess of 60 dB. In order to fully utilize the dynamic range of these sensors, it is necessary to properly weight the beam profile illuminating the Bragg cell. We examine the effect of beam weighting on the resolution and dynamic range of optical spectrum analyzers. Because of its ease of implementation, particular attention is paid to the truncated Gaussian beam shape. The effects of acoustic attenuation in the Bragg cell and amplitude and phase variations on the beam profile are examined. We find that beam weightings which result in increased dynamic range do so at the expense of decreased frequency resolution.