Self-tuned mode-locked laser

Details Self-tuned mode-locked laser Self-tuned mode-locked laser

1994-06-10

1996-04-16

Scott, Jr., Leon

Coherent light generators

Particular beam control device

Mode locking

372 29, 372 32, 372 20, H01S 3098

Patent

active

055090222

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to a mode-locked laser and more particularly to a mode-locked laser incorporating a control system for producing short, stable optical pulses with known properties. The invention also relates to a method of controlling a mode-locked laser to produce such pulses.
Recently much progress has been made with lasers towards producing short optical pulses, and pulse widths down to fractions of a picosecond have been reported. A limit to the performance of systems employing short optical pulses is amplitude, timing and wavelength instability of the pulses, often known as jitter. Such amplitude, timing and spectral instabilities would result in systems being unworkable because of such problems as intersymbol interference and degraded receiver sensitivities in communications systems, accuracy limitations in instrumentation and errors in optical computers, for example.
A low jitter level is critically dependent on the operating conditions of the laser and in particular on the detuning (or offset) of the RF drive frequency from the external cavity fundamental resonance frequency or a harmonic of this frequency. Ideally, a mode-locked laser system should produce stable pulses, that is, pulses should have constant amplitudes, wavelengths, shapes and timings with respect to a reference. To obtain this stability requires tuning of the mode-locked laser cavity length for a given drive frequency. In the laboratory such tuning is usually achieved by monitoring the pulse using either an oscilloscope or a streak camera. Both the oscilloscope and the streak camera produce plots of pulse amplitude versus time, that is, they operate in the time-domain. Because the optical pulses are short (less than 100 ps) the oscilloscope and the streak camera must be capable of measurements with extremely good time resolution. Such equipment is costly and non-portable,.
Because of the high cost and non-portability of monitoring or tuning equipment, a portable, inexpensive and reliable mode-locked laser control system for producing short pulses has not previously been possible, although the many uses of such a laser source have been readily apparent.
Accordingly, it is an object of this invention to provide a relatively inexpensive and portable mode-locked laser incorporating a control system for automatically producing short stable pulses with known properties.
It is also an object to provide a method of controlling a mode-locked laser so as to automatically produce short, stable pulses with known properties in a relatively inexpensive and portable manner.
The invention therefore provides a mode-locked semiconductor laser including a tuning circuit for producing short stable pulses, characterised in that, said tuning circuit comprises a feedback loop (18,19,20) connected to the optical output (15) of the laser, said feedback loop including an optical spectrum analyser (18) for monitoring the optical spectrum of said output to obtain spectral information and a controller (19) connected to said spectrum analyser for analysing an output of said spectrum analyser and producing a control signal dependent on said spectral information, said control signal causing adjustment of a parameter of said laser so as to tune said laser and produce said short stable pulses.
Another form of the invention provides a method of controlling a mode-locked semi-conductor laser to produce short stable pulses, characterised in that, said method includes the steps of monitoring the optical spectrum of the laser output to obtain spectral information, using said spectral information to produce a control signal, and controlling the laser by said control signal to produce said short stable pulses.
In order that the invention may be more readily understood, a particular embodiment will now be described with reference to the accompanying drawings wherein:
FIG. 1 is a general block diagram of a control system for a mode-locked laser, according to the invention;
FIG. 2 is a more specific diagram similar to FIG. 1;
FIG. 3 is a schematic diagram of a

REFERENCES:
patent: 4410992 (1983-10-01), Javan
patent: 4847584 (1989-07-01), Van Dijk
patent: 5204640 (1993-04-01), Logan, Jr.
patent: 5263039 (1993-11-01), Skupsky et al.

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