Optical waveguides – With optical coupler – Particular coupling function
Patent
1994-07-13
1996-07-02
Bovernick, Rodney B.
Optical waveguides
With optical coupler
Particular coupling function
385 24, 385122, G02B 626, G02B 642
Patent
active
055331548
DESCRIPTION:
BRIEF SUMMARY
This invention relates to an optical memory.
BACKGROUND OF THE INVENTION
A major or goal of all-optical processing is speed, which requires rapid access to high capacity, relatively short term memory. An optical memory is a basic element which is essential for more complex data processing, including pattern recognition applications.
The use of a fibre loop as a recirculating delay line, that is to say as an "optical memory", has been discussed for several years and demonstrated in conjunction with optical amplifiers. The potential high storage capacity of such a configuration was first realised in the all-optical, long distance soliton transmission experiments which employed distributed Raman amplification. A known optical memory configuration is shown in FIG. 1. Its basic elements consist of a long loop (25 km in this example) of single mode fibre 2, a fibre coupler 4 having a 3 dB coupling ratio, and a single erbium-doped fibre amplifier (EDFA) 6. The fibre is chosen to be suitable for soliton propagation over ultra-long fibre distances. To this end, the fibre used is dispersion-shifted, so that the dispersion is low, and soliton supporting (D.about.+1 ps
m.km) at the operating wavelength. The signal pulses to be stored are then gated, via a first gate 8, into the loop 2 via a port 10 of the coupler 4, a 120 .mu.s pulse train just filling the 25 km long loop. As long as unity loop gain is maintained, the signal continues to circulate without decrease in pulse energy. A port 12 of the coupler 4 acts as a tap for the memory, and a second gate 14 selects the stored data after the desired time interval.
A disadvantage of this type of optical memory is that optical processing cannot be carried out on the stored data circulating in the loop, other than by gating in additional data pulses, or by erasing the memory and gating in a new stream of data pulses. It is an object of the present invention to provide an optical memory with improved optical processing capabilities.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention provides an optical memory for storing optical signals of a first wavelength, the optical memory comprising a first, non-linear loop mirror switch having first and a second optical ports, and an optical delay line having one end coupled to one port of the first non-linear loop mirror switch and its other end coupled to a reflector reflective at the first wavelength.
Non-linear optical loop mirror switches have been very successful in demonstrating non-linear optical switching in all-fibre configurations. In particular, when used in two-wavelength mode, the device can be employed as a high-speed modulator or logic gate. In this mode of operation, a strong pump wavelength is used to switch a signal of another wavelength. See K. J. Blow, N. J. Doran, B. K. Nayar and B. P Nelson; "Two-wavelength operation of the non-linear fibre loop mirror", Opt. Lett., 15, p. 248 (1990).
In the present invention, the first non-linear loop mirror switch is configured to be reflective to signals of the first wavelength, until the strong pump signal of appropriate intensity at a second wavelength is coupled to the loop. This permits single pulses to be inserted into, or extracted from, the delay line, thereby achieving single pulse processing.
The reflector may comprise a second non-linear loop mirror switch operable in the same manner as the first. This configuration allows both insertion and extraction of data from either end of the delay line. In particular, it may be convenient to insert data at one end of the delay line and extract data from the other end.
The delay line preferably includes an optical amplifier to provide gain at the first wavelength, since this compensates for the loss which would otherwise restrict the long term storage time. Preferably, soliton pulses are used as the data storage bits in the optical delay line. In this way, the essentially distortionless propagation properties of solitons over thousands of kilometers of total fibre path length are exploited. The storage time is,
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Bovernick Rodney B.
British Telecommunications plc
Wise Robert E.
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