Dark pulse generation

Details Dark pulse generation Dark pulse generation

1997-02-11

1998-07-21

Bovernick, Rodney B.

Coherent light generators

Particular beam control device

Modulation

372 18, 372 25, H01S 310

Patent

active

057843953

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

This invention relates to an optical pulse generator for generating dark pulses such as dark soliton pulses.


BACKGROUND

An optical pulse is usually considered to comprise a burst of optical carrier radiation with a given modulation envelope shape. When the pulse has a particular initial envelope shape e.g. U(t)-N sech(t), where N is an integer, the pulse can be transmitted as a soliton in an optical fibre. For such particular envelope shapes, the wavelength dispersion produced in the pulse by the fibre, or so-called chirp, is counterbalanced by the fibre's non-linear dependence of refractive index on amplitude, which produces a self phase modulation (SPM by which the phase of the pulse is modulated by its own intensity. This counterbalance results in a self maintaining pulse or soliton, which tends to maintain its envelope shape with time as it is transmitted along the fibre. The non dispersive nature of solitons makes them attractive for data transmission through optical fibres over long distances.
A pulse having the characteristics just described is known as a bright pulse. It is also possible to generate so called dark pulses such as dark solitons, which occur when an essentially continuous burst of optical radiation contains temporal gaps or regions of reduced intensity radiation. Such gaps are known as dark pulses. It can be shown that for the particular case of solitons, dark solitons may have a general envelope shape given by U(t)-N tanh(t), where N is an integer. For a fuller discussion, reference is directed to Nonlinear propagation effect in optical fibres: numerical studies--K. J. Blow & N. J. Doran Chapter 4, Optical Solitons--Theory and Experiment, edited by J. R. Taylor, Cambrigde University Press 1992.
As used herein, the term dark pulse includes both a black pulse in which the intensity drops to zero a grey pulse in which the intensity drops only partially towards zero.
Dark solitons have been produce experimentally for example as described on pages 394-396 of "Optical Solitons-Theory Experiment" Supra. In this arrangement, pulses from a dye laser have their frequency components spatially dispersed by means of a grating and then individually weighted by means of a mask. The resultant weighted amplitude components are then recombined by another grating. The pulse is accordingly imparted with a desired temporal profile according to a fourier transform of the desired pulse shape. Using this technique, dark pulses closely resembling the expected black and grey solitons have been generated.
However, a problem with this prior arrangement is that the fourier transform performed by mask and grating is not readily controlable.
The use of a modulator, responsive to input modulating signals, to produce bright solitons is described in D. M. Pataca et al: "Actively Mode-locked Pr.sup.3+ -doped fluoride fibre laser" Electronics Letters 9th Jun. 1994, Vol. 30, No. 12, pp. 964-5. In this arrangement, the praseodymium (Pr.sup.3+)-doped fibre is included in a resonant cavity pumped by a Nd:YAG pump laser. The cavity is defined by a semi-reflective mirror at one end of the fibre and a filly reflective mirror at the other. The cavity also includes a electro-optical modulator. In use, the modulator is driven by a sinusoidal waveform which produces positive and negative sinusoidal variations in the refractive index of the modulator, so as to phase modulate light resonating in the cavity. If the period of the modulation is selected to correspond to the transit time for light resonating in the cavity, the cavity is said to be mode-locked. The sinusoidal phase modulation produced by the modulator causes positive and negative going chirp for successive half cycles of the modulation frequency. When the resulting chirp is negative going, it compensates for the dispersive characteristics of the Pr.sup.3+ fibre so that bright solitons are produced during successive negative half cycles of the modulating waveform. For the other half cycles, the positive going chirp that is produced, adds to the

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patent: 5508845 (1996-04-01), Frisken
Zhao et al, "Generation of Dark Solitons Under a CW Background Using Waveguide Electro-Optic Modulator", Optics Letters, vol. 15, No. 8, 15 Apr. 1990, New York US, pp. 405-407.
Greer et al, "All-Optical FM Mode-Iocking of Fibre Laser", Electronics Letters, vol. 28, No. 18, 27 Aug. 1992, Engage GB, pp. 1741-1743, XP 000309486.
Pataca et al, "Actively Modelocked Pr3+-Doped Fluride Fibre Laser", Electronics Letters, vol. 30, No. 12, 9 Jun. 1994, Engage GB, pp. 964-965, XP 000459784.
Pataca et al, "Bright and Dark Pulse Generation in an Optically Modelocked Fibre Laser at 1.3.mu.m", Electronics Letters, vol. 31, No. 1, 5 Jan. 1995 Stevenage, GB, pp. 35-36.
Peter et al, "570 fs Pulses for a Pr3+ Doped Fibre Laser Modelocked by Pump Pulse Induced Cross-Phase Modulation", Electronics Letters, vol. 30, No. 19, 15 Sep. 1994, Stevenage GB, pp. 1595-1596, SP 000466461.

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