Performance tuning of 1*7 wavelength-flattened fused fibre couplers

In order to successfully fabricate monolithic 1*7 wavelength-flattened couplers, careful control over the degree of fusion of the structure is required. The variation of coupling characteristics with the degree of fusion is discussed. Two degrees of fusion are identified where an equal power split wavelength-flattened response may be obtained.<>     

Reduction of chlorine concentration and microbial load during washing-disinfection of shredded lettuce

Disinfection of shredded lettuce by washing is an essential operation to eliminate foreign matter and cellular fluids produced by cutting, and to reduce the initial microbial contamination. The response surface methodology was used to evaluate the effect of initial chlorine concentration (50–150 ppm), water‐to‐produce ratio (20–50 L kg^?1) and time (2–8 min) on the depletion of total and available chlorine and also the microbial count reduction on shredded lettuce. The predictive models obtained, validated through additional confirmatory experiments, were shown to be adequate and could be used as a way of improving the operation.     

Fast resonant nonlinear relaxation observed in Q-switchedneodymium-doped twin-core fibre

Enhanced relaxation of resonant nonlinearities is demonstrated. The increase in the relaxation rate has been achieved by enclosing a neodymium-doped fibre in a Q-switched resonant cavity. The rapid build up of the Q-switched pulse at the lasing wavelength was used to quench the population inversion, and hence the induced index change. The decay time of the nonlinearity has been improved from ~250 μs to ~7 μs     

Low-power all-optical broad-band switching device using ytterbium-doped fiber

All-optical photonic switching has been demonstrated at 1310 nm and 1545 nm using an all-fiber Mach-Zehnder interferometer incorporating ytterbium-doped silica fiber. The switching was achieved with 980 nm excitation from a laser diode, with a power-length product of 3.7 mW/spl middot/m and 5.8 mW/spl middot/m at 1310 nm and 1545 mm respectively, thus giving low power all-optical switching at the preferred telecommunications wavelengths.     

Measuring the thickness profiles of wafers to subnanometer resolution using Fabry-Perot interferometry

The resolution of an angle-scanning technique for measuring transparent optical wafers is analyzed, and it is shown both theoretically and experimentally that subnanometer resolution can be readily achieved. Data are acquired simultaneously over the whole area of the wafer, producing two-dimensional thickness variation maps in as little as 10 s. Repeatabilities of 0.07 nm have been demonstrated, and wafers of up to 100 mm diameter have been measured, with 1 mm or better spatial resolution. A technique for compensating wafer and system aberrations is incorporated and analyzed.     

Design of multiaperture masks for subnanometer correction of ultraprecision optical components

A technique for correction of optical surfaces has recently been reported. The technique involves oscillating an optical surface back and forth behind a multiaperture mask to deposit a spatially varying dielectric layer onto the optic to create the desired surface profile. Details are reported of a modified mask design that inherently smooths the deposited layer used for these corrections. Results are also reported with regard to a recent correction that resulted in a thickness uniformity of better than lambda/2000 rms over a working aperture of 37.5 mm.     

Experimental and theoretical analysis of the resonant nonlinearityin ytterbium-doped fiber

Experimental measurements are described characterizing the nonlinear index change over the range from 500 to 1550 nm induced in an ytterbium (Yb³⁺)-doped twin-core fiber by a 980 nm pump. At 1550 nm, a phase change of π is induced with as little as 14 mW of pump power for a signal loss of only 0.2 dB, By allowing the doped fiber to lase and observing the associated clamping of the induced phase change, we show that a digital nonlinear response can be achieved in which a constant, pump-power-insensitive, phase change is induced for all pump powers above a certain threshold. This lasing induced clamping of the phase change also demonstrates that the nonlinear effect is population dependent as opposed to thermal. The pump-induced phase change is observed to increase for shorter signal wavelengths, which suggests that the effect is due principally to pump-induced changes in the strong ultraviolet (UV) absorptions of Yb³⁺. This observation is accurately predicted by a theoretical analysis that takes into account absorptions in both infrared and ultraviolet regions. This analysis shows that Yb³⁺ may be suitable for low-power all-optical switching applications in both 1300 and 1550 nm telecommunications windows when the speed of response is not a critical parameter     

An investigation of Q-switched induced quenching of the resonantnonlinearity in neodymium doped fibers

A method of reducing the decay time of the population-inversion-dependent refractive index change in rare-earth doped fibers is investigated, and its suitability for use in all-optical switching is considered. Normally, the temporal response of this type of nonlinearity is dominated by the slow fluorescent decay from the metastable level present in the doped fiber. However, in this instance, the nonlinearity is rapidly quenched by the internally generated lasing signal produced by Q-switching a lasing cavity containing the doped fiber. Using this technique the observed decay time was reduced from 370 μs to ~1 μs and a phase modulation of π-radians was observed, demonstrating that the effect could be used to achieve complete optical switching in an interferometric device. A model of the temporal behavior of this effect is developed using a rate equation analysis. The induced index change is modeled by summing the effects of individual transitions in the region of the signal wavelength, and by including the effects of assumed transitions in the VUV. An extension to this method is discussed in which the depopulating signal is generated using gain-switched techniques, obviating the need for intracavity modulation, thus making the depopulation mechanism more suitable for practical purposes     

UV tuning of coupling in twin-core optical fibres

The isolation of a wavelength-demultiplexing twin-core fibre with nominally identical cores is improved from 7 dB to 26 dB by increasing the refractive index of one core. Tuning relaxes the fabrication tolerances necessary for ⩾20 dB isolation between channels, enhancing the feasibility of using twin-cure fibres for demultiplexing 1.3 and 1.55 μm signals in communications systems     

Resonantly enhanced nonlinearity using stimulated down-pumping inneodymium doped twin-core fibre

Preliminary observations and measurements of increased relaxation speeds for optical switching in resonantly enhanced nonlinear systems are demonstrated. The effect is observed using a neodymium-doped twin-core fibre interferometer, and uses stimulated down-pumping of the ⁴F_3/2 metastable state in neodymium. The relaxation time has been reduced from ~390 μs to less than 120 μs, and it is shown that the reduction is dependent on the power of the down-pumping source