Sagnac loop interferometer

Details Sagnac loop interferometer Sagnac loop interferometer

1993-03-30

1996-01-09

Limanek, Robert P.

Optics: measuring and testing

By particle light scattering

With photocell detection

356351, G01B 902

Patent

active

054833400

DESCRIPTION:

BRIEF SUMMARY
FIELD OF INVENTION

This invention relates to interferometers and in particular to Sagnac loop interferometers in which a four port optical coupler having a first and a second input port and a first and a second output port has an optical coupling means coupling the first and second output ports.
An optical input signal coupled to an input port of such an interferometer is split into two portions by the optical coupler which portions counterpropagate round the coupling means, for example an optical fibre, to return to, and recombine at, the coupler. For an optically linear optical coupling means the optical path along the coupling means is the same for the two portions. For 50:50 splitting, the portions recombine such that the input signal emerges from the port to which it was originally input. The input signal is said to be "reflected" by the Sagnac interferometer. For this reason this configuration is often described as a loop mirror.


BACKGROUND OF THE INVENTION

The applicants co-pending application, publication number WO 88/02875, describes a Sagnac interferometer in which the symmetry of the two counterpropagating directions along the coupling means is broken so as to obtain a relative phase shift in the counterpropagating portions of the input signal. This can be achieved by, for example, providing a coupling ratio of other than 50:50 and an optically non-linear optical fibre waveguide constituting the coupling means. In this case the intensities of the signal portions coupled into the ends of the waveguide are not equal. If input signals are of sufficient intensity to produce self-phase modulation of the optical portions as they propagate around the optical fibre loop, the signal portions propagating in opposite directions around the waveguide will experience different refractive indices due to the Kerr effect. This results in the signals experiencing different phase shifts so that when the signals return back to the coupling means they have an intensity dependent relative phase shift.
The intensity dependence of the relative phase shift results in a device whose output at an input port is an oscillatory function of the intensity of the input signal. Any signal exiting the second input port (ie the port to which the input signal is not coupled) is said to be "transmitted" by the interferometer. This property can be used in a variety of applications including logic elements, optical amplifiers, optical switches and the like although complete switching is not obtained due to the non 50:50 splitting.
A disadvantage of this configuration is that the small Kerr effect coefficient of currently available optical fibre materials limits its use since a large optical power loop-length product is required to produce the necessary phase shifts.
A known approach to obviating this disadvantage is to incorporate an asymmetrically located optical amplifier in the loop with the Sagnac loop having a 50:50 coupler as disclosed in an article entitled "Nonlinear Amplifying Loop Mirror" by M E Fermann, F H Haberl, M Hofer and H Hochreiter, Optics Letters vol 15 no 13, Jul. 1, 1990. The position of the amplifier provides that the counterpropagating portions of an input signal have different intensities for a large proportion of their transit around the loop. If the amplified portions are of sufficient intensity to activate the non-linear regime of the optical fibre there will be an intensity dependent relative phase shift between the counterpropagating portions resulting in the required intensity dependent switching.
In the prior art embodiment just described the fibre loop was 306 m in order to achieve a sufficiently large intensity-loop length product.


BRIEF DESCRIPTION OF THE INVENTION

According to the present invention a Sagnac interferometer of the kind according to the preamble of claim 1 is characterised in that the coupling means includes an non-linear optical amplifier and that there are optical attenuation means for providing that the counterpropagating portions have different intensities on reaching the am

REFERENCES:
Optics Letters, vol. 15, No. 13, 1 Jul. 1990, New York, N.Y., US, pp. 752-754; Ferman et al: "Nonlinear Amplifying Loop Mirror".
Applied Physics Letters, vol. 55, No. 1, 3 Jul. 1989, New York, N.Y., pp. 25-26; Farries et al: "Optical Fiber Switch Employing a Sagnac Interferometer".
Optics Letters, vol. 14, No. 14, 15 Jul. 1989, New York, pp. 754-765; Blow et al: "Experimental Demonstration of Optical Soliton Switching in All-Fiber Nonlinear Sagnac Interferometer".
Electronics Letters, vol. 26, No. 21, 11 Oct. 1990, Stevenage Herts., GB, pp. 1779-1781; Richardson et al: "Very Low Threshold Sagnac Switch Incorporating An Erbium Doped Fibre Amplifier".
Electronics Letters, vol. 26, No. 24. 22 Nov. 1990, Stevenage, Herts., GB, pp. 2008-2009, Webb et al: "All-Optical Loop Mirror Switch Employing An Asymmetric Amplifier/Attenuator Combination".
PCT International Search Report.

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