Christopher N. Pannell

Transient response in high-resolution Brillouin-based distributed sensing using probe pulses shorter than the acoustic relaxation time

We perform numerical simulations on a model describing a Brillouin-based temperature and strain sensor, testing its response when it is probed with relatively short pulses. Experimental results were recently published [e.g., Opt. Lett. 24, 510 (1999)] that showed a broadening of the Brillouin loss curve when the probe pulse duration is reduced, followed by a sudden and rather surprising reduction of the linewidth when the pulse duration gets shorter than the acoustic relaxation time. Our study reveals the processes responsible for this behavior. We give a clear physical insight into the problem, allowing us to define the best experimental conditions required for one to take the advantage of this effect.

Heat transfer modelling in CO2 laser processing of optical fibres

The diameter self-regulation of optical fibre tapers produced using a CO2 laser has been examined theoretically and modelled numerically. Results of the simulation show that for small diameters, where the geometric model of absorption cannot be used, the Mie theory of absorption derived from the Maxwell equations exhibits structure due to resonances of the CO2 laser radiation in the tapered fibre. At small diameters, the polarisation state of the CO2 laser is important, and resonance effects cause the equilibrium temperature/diameter graph to exhibit oscillations.

Four-port fiber frequency shifter with a null taper coupler

A prototype of a new design of fiber frequency shifter, based on a special four-port fused taper coupler with a null maximum coupling ratio, is reported. In this coupler the constituent fibers are so dissimilar that the maximum coupling is (ideally) zero. In the presence of a flexural acoustic wave, light is coupled between the fibers, acquiring a frequency shift in the process. Near 100% conversion with 30-dB sideband and carrier suppression is achieved, in an unoptimized device, for just 1 mW of electrical drive power.

Low power acousto-optic device based on a tapered single-mode fiber

A new type of fiber acousto-optic device, based on tapered single-mode fiber, is described. Unlike earlier designs, the overlap between the acoustic and optical waves is complete, so very little acoustic power is required. Maximum coupling in an experimental device was attained for an electrical drive power of just 0.5 mW.<<ETX>>

In situ monitoring of sputtered zinc oxide films for piezoelectric transducers

Abstract We report the application of in situ ellipsometry to monitor the growth by RF reactive sputtering of zinc oxide thin film piezoelectric transducers on silicon substrates. To establish the effect of substrate temperature on film quality and piezoelectric activity, films were grown at substrate temperatures of 100–300°C. Films of refractive index as high as 1.940 are achieved compared with refractive index of 1.99 for bulk zinc oxide crystals. Under optimum deposition conditions, transparent homogenous films of high piezoelectric activity are formed from the initial stage of growth. A simple single layer model is used to predict the in situ ellipsometry measurement data for films deposited under optimum conditions, while a two layer model (duplex model) is needed to model the ellipsometry measurement data for rough films formed under conditions away from the optimum. Films grown at the optimum substrate temperature of 200°C were smooth and of reproducibly good quality, having high piezoelectric activity. Although the refractive index remained constant during growth, the rate of growth was not quite linear, and was found to increase slowly with time according to a weakly quadratic law. X-ray diffraction and pulse echo techniques were used to establish that films deposited close to 200°C have good oriented structure and high piezoelectric activity. We have thus shown that a strong correlation exists between the optical properties measurable with a simple in situ ellipsometer, and the appearance of highly oriented films of zinc oxide having excellent piezoelectric activity.

ULTRASONIC HYDROPHONE BASED ON SHORT IN-FIBER BRAGG GRATINGS

We investigate the feasibility of using in-fiber Bragg gratings for measuring acoustic fields in the megahertz range. We found that the acoustic coupling from the ultrasonic field to the grating leads to the formation of standing waves in the fiber. Because of these standing waves, the system response is complex and, as we show, the grating does not act as an effective probe. However, significant improvement in its performance can be gained by use of short gratings coupled with an appropriate desensitization of the fiber. A noise-limited pressure resolution of approximately 4.5 x 10(-3) atm/ radicalHz was found.

Effective piezoelectric activity of zinc oxide films grown by radio‐frequency planar magnetron sputtering

We present a study of the effective piezoelectric activity of thin ZnO films produced by radio‐frequency (rf) planar magnetron sputtering. The energetic plasma particles which bombard the substrate in the above deposition system increase the substrate temperature, thus causing a gradual variation in film structure during the beginning of the film growth. As a result, a precursor layer is formed which consists of small randomly oriented crystallites, exhibiting poor piezoelectric activity. Hence, the film thickness responsible for piezoelectric activity is generally less than the physical thickness of the film and is adjacent to a layer having different acoustic impedance. This leads to an increase in the resonant frequency of the film. For example, a film designed to have a half‐wave resonance at 288 MHz, was found to be resonant at 332 MHz. The poorly structured initial layer meant in this typical case that only 87% of this film volume exhibited piezoelectric activity. Investigations based on the substra...

Nanosecond response of Bragg deflectors in periodically poled LiNbO 3

For the first time, transients are measured for a bulk-optical Bragg-deflector based on an electrooptically (EO) induced grating in periodically poled LiNbO/sub 3/ (PPLN). Optical pulse risetimes of 1.2 ns are reported with a first-order diffraction efficiency of 76% for circular Gaussian beams. The fast risetime is accounted for by a substantially suppressed excitation of acoustic thickness modes and careful electrode design. For many applications, the device could replace both acoustooptical Bragg-deflectors and conventional EO amplitude modulators due to a fivefold reduction in risetime and fourfold reduction in drive power, respectively.

Stimulated Brillouin scattering in optical fibers: the effects of optical amplification

Stimulated Brillouin scattering in optical fibers is carefully reassessed for the case of distributed optical gain (achievable in optical fibers lightly doped with rare-earth ions) instead of the usual loss. The effects of gain and amplified spontaneous emission on the threshold power are discussed, together with treatments of the effects of amplification on the line-narrowing process normally associated with stimulated Brillouin scattering in a lossy medium and the appearance of strong higher-order Stokes waves. The implications for erbium-doped fiber amplifier systems are explored.

Broadband monolithic acousto-optic tunable filter.

Broadband monolithic acousto-optic tunable filters that combine a piezoelectric transducer array and an acousto-optic interaction medium in a single crystal have been investigated. A linearly chirped acoustic superlattice with an optical tuning range of lambda = 1.3-1.6 mum was formed by domain inversion in LiNbO(3) . X-propagating longitudinal acoustic waves are excited in a crossed-field scheme by a rf E(y) field applied to the superlattice and couple collinearly propagating e- and o-polarized optical modes. At mu = 1.319 mum and mu = 1.55 mum the spectral bandwidths (FWHM) were 1.54 and 2.3 nm, respectively. A relative conversion efficiency of 43%/W and a maximum conversion efficiency of 51% were measured at 1.319 mum.