GB2231169A - Optical fibres - Google Patents

Abstract

Optical fibres with a high non-linear response, such as Kerr coefficient, are produced by incorporating small quantities of rare earth dopants, typically 3 to 300 ppm or erbium, therein which spreads a signal of 1.5 mu m to 1.3 mu m.

Description

OPTICAL FIBRES.

This invention relates to optical fibres.

According to the present invention there is provided an optical fibre which has high Kerr coefficient and in which is incorporated small amounts of rare earth dopants, for example of the order of 3 to 300pom.

It has been observed in experiments that the addition of small amounts of dopant, in particular rare earths, to silica based fibres, results in the Kerr coefficients or effects which depend on the Kerr coefficient being enhanced by dramatic amounts. The small amounts of dopant concerned to date are of the order of 3 to 300pom. It is not fully understood why such small amounts result in the observed large enhancements. If the enhancements effects were due purely to the Kerr coefficient then it would be expected that a lot more dopant would have been necessary than that which has been employed. In general, the more dopant and the heavier the ion employed the larger would be the expected effect. For example, lead increases the Kerr coefficient by some forty times.

It has been found that when a signal at 1.5rum, or thereabouts is transmitted through a short length (of the order of 3 metres) of erbium doped fibre (300ppm), the output spectrum is spread out either side of the central wavelength to about 1.3cm. This is typical of the effect which is obtained with self-phase-modulation, which is itself a Kerr related effect (Kerr effect in a single pulse).

It is considered that the effect is not related to the lasing properties of a fibre which are known to arise when such dopants are incorporated. It is certain that it is not related to exited energy levels of material because their lifetime is so long and these enhancement effects happen at a sub-picosecond scale.

Possible uses of such low level doped fibre are included in diode pumped modulators and tunable sources.

Such devices would be more practicable to produce than hitherto.

Such a modulator may be comprised by a length of polarisation maintaining fibre doped with such small amounts of a rare earth, erbium, for example, and a means to input a signal at one wavelength to the fibre and means to input a pump signal at another wavelength.

If there is sufficient pump power to cause the Kerr effect in the fibre then that will modulate the signal at the one wavelength. Since the Kerr effect is so strong with these doped fibres the pump source can be a much lower power device than hitherto, such as a semiconductor diode. Thus one diode can be used to modulate another diode.

A tunable source may be comprised by a length of fibre doped with small amounts of a rare earth, such as erbium. An input signal with a narrow line width will, as a result of the self-phase-modulation due to the enhanced Kerr effect, be broadened in bandwidth and thus a broader spectral width will be output from the fibre. By means of a filter at the output, the output wavelength can be tuned to a preselected value.

Claims (7)

CLAIMS:

1. . An optical fibre which has a high non-linear response and in which is incorporated a small amount of one or more rare earth dopants.

2. An optical fibre as claimed in claim 1 wherein the amount is of the order 3 to 300 ppm.

3. An optical fibre as claimed in claim 1 or claim 2 wherein the non-linearity is the Kerr coefficient.

4. An optical fibre as claimed in any one of the preceding claims wherein the rare earth is erbium.

5. An optical fibre with a high non-linear response and substantially as herein described.

6. A diode pumped modulator including a length of optical fibre according to any one of the preceding claims and which is also polarisation maintaining.

7. A tunable source including a length of optical fibre according to any one of claims 1 to 5 and a filter at an output end of the fibre for tuning the output wavelength to a preselected value.