Coherent light generators – Laser logic system
Patent
1990-11-28
1992-07-28
Epps, Georgia Y.
Coherent light generators
Laser logic system
372 6, 372 19, 372 25, 372703, 359333, 359195, 359181, H01S 330
Patent
active
051346214
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to high gain semiconductor laser amplifiers, and more particularly to the combination of two such laser amplifiers.
BACKGROUND AND SUMMARY OF THE INVENTION
Travelling wave semiconductor laser amplifiers have been proposed for a number of optical communications applications. They have been used, for example, as repeaters, preamplifiers, power boosters, switches and filters. Many of these applications rely on high gain to achieve optimum performance and a critical factor in achieving high saturated output power is minimising residual facet reflectivity. Up to the present time, only limited gains of around 20 dB have been demonstrated, and it has not yet proved possible to improve on this due to the difficulty in reducing residual facet reflectivity.
The present invention is directed in a first aspect towards tackling both of these problems, and producing a laser amplifier package having higher gain than previously achieved.
When a pulsed optical signal is propagated along an optical fibre link there is usually pulse spreading due to dispersion and thus it is necessary to regenerate the signal at repeaters spaced along the link. However it is possible to propagate optical pulses along an optical fibre without significant dispersion, if the pulses are optical solitons, and in that case very long transmission distances without regeneration becomes possible. Optical solitons are relatively high peak power (e.g. 100 mW), short duration (e.g. 3 ps) pulses of smooth profile. Generation of optical solitons is at present achieved utilising cryogenic lasers and the equipment is large and expensive. See, for example, U.S. Pat. No. 4,635,263, in which a colour centre laser is used as a pulse source.
The present invention in a second aspect is directed towards the production of optical solitons utilising semiconductor laser amplifiers.
Another aspect of the invention is directed towards modulation techniques for use in the first and second aspects of the invention.
Accordingly a first aspect of the present invention provides a laser amplifier package comprising two semiconductor laser amplifiers coupled in series via two lenses forming a collimated beam section and an optical isolator.
In its second aspect the first amplifier may be driven as a pulse source as part of a long-external-cavity (LEC) or gain switched DFB laser assembly. The pulses may be made near transform limited (see chapter 9 of `Lasers`, by Anthony E. Siegman, Oxford University Press, 1986, in particular pages 334-335) at generation or by compression and the amplification sufficient to produce optical solitons.
In the third aspect the second laser amplifier is switched to modulate the output of the package.
BRIEF DESCRIPTION OF THE DRAWINGS
A more detailed description of a specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which:
FIG. 1 shows a schematic diagram of an embodiment of the invention;
FIG. 2 shows a schematic diagram of an experimental apparatus used for testing the embodiment of the invention shown in FIG. 1; and
FIG. 3 is a modification of the package of FIG. 1 suitable for generating solitons.
DETAILED DESCRIPTION OF THE DRAWINGS
Very surprisingly, we have discovered that when two laser amplifiers are aligned in series, as little as 20 dB of optical isolation may be sufficient to prevent the backward amplified wave (signal and spontaneous) from the second amplifier affecting the first. Previously it was believed that, if one were to put two laser amplifiers in series, in order to prevent interaction one would need to reduce the level of back spontaneous emission which would typically be about 1 mW) to about 10 percent of the signal level. To do this would typically require 40 dB of isolation.
What we have discovered is that the critical factor is not back-spontaneous emission, but is the returned signal level. Because of the influence of the amount of gain provided by the second device on the level of reflected
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British Telecommunications public limited company
Epps Georgia Y.
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