Square optical pulse generator

Details Square optical pulse generator Square optical pulse generator

1992-03-25

1993-05-04

Nelms, David C.

Radiant energy

Photocells; circuits and apparatus

Optical or pre-photocell system

356350, H01J 516

Patent

active

052084558

DESCRIPTION:

BRIEF SUMMARY
This invention relates to square optical pulse generators.
Considerable effort has been employed in generating square optical pulses for all-optical switching purposes using spatial transform techniques as discussed for example in an article by A. M. Weiner et al entitled "Femtosecond Pulse Tailoring" published in Optics Letters 13, 300 (1988).
This method requires suitable mask to be made which is an involved procedure and the transformation must be carried out in bulk optics which is inconvenient when the square pulses produced are to be used in optical fibre devices or networks.
According to the present invention an optical pulse generator comprises a first optical coupler having a first and a second pair of optical communication ports in which substantially equal first signal portions of an optical signal at a first wavelength received at a port of one pair are coupled to the two ports of the other pair of ports;
an optical waveguide coupling together the second pair of ports having an interaction section which includes a material having a non-linear refractive index;
a cw optical source for providing a cw optical signal at the first wavelength optically coupled to a first port of the first pair of ports;
a pulsed optical source for providing a pulsed optical signal at a second wavelength optically coupled to the interaction section so the pulsed optical signal propagates along it in substantially one direction;
the intensity of the pulsed optical signal being sufficient to provide a relative phase shift in the first signal portions as they propagate round the optical waveguide and the interaction section being longer than the inverse of the absolute difference in group delays of the cw and pulsed optical signals.
In this specification by "non-linear" we means that the refractive index of a material varies with the intensity of the transmitted signal. Typically the refractive index n is given by the formula n=n.sub.o +n.sub.2 /E/.sup.2 where n.sub.o is the linear refractive index, n.sub.2 is the Kerr coefficient and /E/.sup.2 the intensity of the transmitted signal.
In the absence of the pulsed optical signal the first optical coupler and the optical waveguide, which form a Sagnac antiresonant interferometer, act as a loop mirror to the cw optical signal in that the signal entering the coupler at the first port will be reflected i.e. it will exit form the same port. This is because the two counter-propagating portions maintain the same relative phase. When the pulsed optical signal propagates along the interaction section of the waveguide-so inducing a phase shift in the first portion which co-propagates with it in the same direction-the condition for reflection is broken and some of the cw optical signal will exit the second port. The present invention relies on the realisation that the group delays of the cw and pulsed signals are different so that for a sufficiently long interaction section the pulse can effect switching to the other port pulse of cw optical signal wider than the width of the pulse from the pulsed source. Thus a short pulse at the second wavelength can be used to provide a square optical pulse at the first wavelength which will have a rise and fall time of the order of the short pulse and a width dependant on the length of the interaction section.
Preferably, the first optical coupler is a dichroic optical fibre coupler substantially all of the pulsed optical signal received at one port of one pair to one port of the other pair and the optical waveguide is an optical fibre. The pulsed optical source being optically coupled to the first port as this provides in a simple manner both the two counter propagating cw portions at the first wavelength and propagation of the pulsed signal in a single direction around the optical fibre.
The pulsed optical source can be coupled to the interaction portion by other arrangements, for example by means of dichroic couplers in the optical fibre loop.
Other waveguides providing the necessary non-linearity may also be employed within the scope of the

REFERENCES:
patent: 5046848 (1991-09-01), Udd
patent: 5056919 (1991-10-01), Arditty et al.
Conference on Lasers and Electro-Optics, Technical Digest Series, Ahaheim, Calif., vol. 7, Apr. 25-29, 1988, abstract No. TUP5, Optical Society of America, (Washington, US), L. F. Mollenauer et al: "Demonstration of soliton transmission over >1000 km in fiber with loss with loss compensated by Raman gain", pp. 132-133.
Optics Letters, vol. 11, No. 3, Mar. 1986, Optical Society of America, (Washington US), A. M. Weiner et al: "Synthesis of phase-coherent, picosecond optical square pulses", pp. 153-155.
Optics Letters, vol. 13, No. 4, Apr. 1988, Optical Society of America, (Washington, US), A. M. Weiner et al: "Encoding and decoding of femtosecond pulses", pp. 300-302.
Optics Letters, vol. 13, No. 1, Jan. 1988, Optical Society of America, (Washington, US), N. J. Doran et al: "Nonlinear-optical loop mirror", pp. 56-58.
Journal of the Optical Society of America B/Optical Physics, vol. 5, No. 8, Aug. 1988, Optical Society of America, (Woodbury, N.Y., US), A. M. Weiner et al: "High-resolution femtosecond pulse shaping":, pp. 1563-1572.

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