|Publication No: 1298||Search all ORC publications|
All fibre 2x2 polarisation insensitive switch
D.O.Culverhouse, R.I.Laming, S.G.Farwell, T.A.Birks* and M.N.Zervas
(*) School of Physics, University of Bath, Claverton Down, Bath BA2 7AY, UK
We report the development of a polarisation insensitive, low loss, broad band all fibre 2×2 acousto-optic switch based on twisting the waist of a null taper coupler. The best polarisation insensitivity observed so far is 0.2 dB.
Recently a new type of acousto-optic device based on four port null taper couplers has been demonstrated, and has been shown to be efficient as an optical switch, frequency shifter and tunable filter [1-3]. The devices demonstrated so far, however, have been shown to be strongly polarisation dependent and this effect has to be suppressed for the device to be practical. Here we show that polarisation can be overcome by twisting the waist of the null coupler. The effect is attributed to a combination of ellipticity of the taper waist and the requirement that the polarisation beat length between the higher order modes exceeds the twist pitch [4, 5]. The fact that both of these conditions are violated for taper waists having a diameter < 6 μm means that the technique is not suitable for narrow band filters and frequency shifters, but is suitable when used as a broadband optical switch.
The coupler is a special type called a null coupler. It is made from two fibres with diameters mismatched to the extent that the resultant coupler does not actually couple any light. It can be made by pretapering one of two identical fibres along a short length before both fibres are fused and elongated together to form the coupler. When an appropriate flexural wave is excited along the waist it causes light to couple between the fibres. The switch is in the "cross" state when the acoustic wave is on, and in the "bar" state when the acoustic wave is off. The device has an interaction length of 8 mm and for an optical wavelength of 1550 nm has an acousto-optic resonance of 1 Hz, a conversion efficiency of ~ 99 %, an insertion loss of ~ 0.2 dB and requires a drive power of only 1 mW.
|Figure 1 Normalised output spectra of the untwisted device for throughput light and coupled light. The polarisation states correspond to peaks at 1550 nm and 1490 nm, and where the sidelobes are attributed to non-uniformity along the taper length.|
For the untwisted device the output spectra versus wavelength are shown in Figure 1. The throughput spectrum reveals two principal dips, and the coupled spectra two peaks at the same corresponding wavelength. The spacing of 60 nm between the main peaks implies a beat length of 9 mm between the polarisation states.
|Figure 2 Normalised output spectra for 2 revolutions of twist: (i) throughput and (ii) coupled output light. The single peak at 1530 nm corresponds to an intermediate value given by the two peaks in Figure 1.|
Consequently when twisting the waist by a number of turns, the twist pitch becomes short compared to the beat length. For this case the output spectra, Figure 2, reveal a single 50 nm bandwidth dip for the throughput and a corresponding single peak for the coupled light at an intermediate value in relation to the polarisation states in Figure 1. Laser light at 1550 nm was then launched through a polariser and bulk optic half-wave plate prior to input to the device. Output from both the throughput and coupled ports was monitored. For the untwisted device, rotating the half-wave plate by 45 degrees reduces the coupled output light by 17 dB on account of exciting the orthogonal polarisation state.
|Figure 3 Throughput and coupled optical outputs of the null coupler versus half-wave plate rotation for two revolutions of twist.|
However, for the twisted device, Figure 3, light monitored at the coupled output port is reduced by a maximum of 0.2 dB. The switching time of the device was measured as 40 μs and any fluctuation in the output due to rotating the half-wave plate was similarly < 0.2 dB in all cases. We have demonstrated for the first time a broad band, polarisation independent all fibre switch. It is based on twisting the waist of a null taper coupler. The polarisation sensitivity is reduced from 17 dB to 0.2 dB. The device has a low drive power requirement, low insertion loss and a conversion efficiency of ~ 100 %. The null coupler is fabricated from standard telecommunications fibre and is, therefore, compatible with any existing network. This work was funded by the UK EPSRC through the LINK initiative.
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IEEE Photonics Technology Letters (1997) Vol.9(4) pp.455-457
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