Wednesday 17 February 2010

Optical fibre dispersion compensation with a fibre Bragg grating

Though I've largely left my optical fibre research behind, occasionally I spot things of interest. This, reported in Lightwave, is really nice, partly because it is easy to understand and uses a couple of neat optical fibre devices.

Dispersion is the (optical) frequency-dependent variation of propagation speed which causes pulse-spreading in optical fibre. In single-mode fibre it is often the factor that limits the bit rate that can be transmitted over long distances. A way to overcome it is through 'dispersion compensation': adding a device that does the opposite of the dispersion in the fibre. So if the fibre causes the longer wavelengths to be take longer than the shorter wavelengths, the device needs to delay the shorter wavelengths more than the longer wavelength.

The device reported here does that using a fibre Bragg grating, which is a length of optical fibre with a regular variations in the refractive index, on a scale similar to the wavelength of light. (Incidentally, the wavelength of light used in optical comms, of the order of 1.5 micro-metres, is large compared to the 'feature size' of electronic devices, so doing things on that scale is not unreasonable.) Interference effects mean that the grating reflects light which has a wavelength that matches the grating, and by careful control of the grating spacing - varying the spacing over the length of the grating, a technique known as 'chirp' - the light can be manipulated in various ways dependent upon the wavelength. You can, for example, arrange for light of different wavelengths to be reflected at different locations, thereby making the path-length different for different wavelengths.

The fibre Bragg grating (one of the 'neat' optical devices I referred to above) is combined with an optical circulator (the other 'neat' device). An optical circulator is a three- or four-port device, which I picture as like a roundabout in road, but with the rule that you are only allowed to turn left.

Have a look at picture in the Lightwave article to see how they are combined, but what happens is that light enters the circulator from the incoming fibre, turns left down to the fibre Bragg grating (a cul-de-sac) and gets reflected back to the circulator where it turns left to the output fibre. Having taken the detour down the Bragg grating, the light propagation time between input and output is controlled by the details of the grating dimensions.

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