Laser generator with phase mode-locking

Details Laser generator with phase mode-locking Laser generator with phase mode-locking

1988-10-20

1990-05-22

Scott, Jr., Leon

Coherent light generators

Particular beam control device

Mode locking

372 21, 307427, H01S 3098

Patent

active

049282824

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention:
The present invention relates to a laser generator with phase mode-locking, the generator comprising a resonant cavity delimited by a reflecting rear mirror, and by a semi-reflecting outlet mirror.
2. Description of the Related Art:
FIG. 1 is a diagram showing the disposition in a conventional laser source of a rear mirror 1 and an outlet mirror 2 defining a path between them of length L with the light passing through an active amplifier medium 3 lying on the path. Given the various types of laser source that exist, FIG. 1 is not limiting in the way it indicates the nature and the geometry of the active medium 3.
A conventional laser source constitutes an oscillator, and by virtue of the multiple back-and-forth passes of the light through the resonant cavity of the oscillator, and assuming that no special precautions are taken, it provides laser emission whose time structure is that of a periodic noise of period T=2L/c (1) where L is the distance between the two mirrors 1 and 2, and c is the speed of light. The shape of the light intensity i(t) as a function of time is shown in FIG. 3. FIG. 3 shows the correlation time dt which represents the reciprocal of the spectrum width .DELTA.f of the emitted signal. FIG. 4 shows the frequency spectrum I(f) of the periodic noise i(t) shown in FIG. 3. It can be seen that the frequency spectrum corresponding to conventional laser emission comprises a plurality of groups of equidistant lines with the intervals df between the groups being the reciprocal of the period T of the time signal i(t). Thus, df=c/2L (2) where L is the distance between the two mirrors 1 and 2, and c is the speed of light.
However, in the FIG. 4 frequency spectrum, the intensities and the phases of the groups of equidistant lines are distributed randomly. It is thus something of a misuse of language for the groups of frequencies at which laser energy is concentrated to be called "modes".
In conventional laser emission of the periodic noise type, the amplitude of the power fluctuations in the noise is equal to the mean power P of the radiation.
Proposals have already been made for locking the modes of a laser emission by various different means, symbolized by the rectangle 4 in the FIG. 2 diagram of a laser source; with said means all relying on periodically modulating the loss or the gain to which the light is subjected on its back-and-forth passes in the resonant cavity between the mirrors 1 and 2, with the modulation being at the frequency df=c/2L as defined in equation (2). This periodically enhances preferential amplification of certain regions in the periodic noise i(t). Thus, a particular sequence of duration T.sub.O of pulses spaced apart in the period T=2L/c become progressively less noisy and increases to the detriment of regions which do not possess the optimum phase. The laser signal of intensity i(t) loses its random character and takes on the regular shape shown in FIG. 5. The frequency spectrum I(f) also takes on the regular shape shown in FIG. 6 with a sequence of regularly spaced-apart peaks at intervals df=c/2L, with each peak having a width 1/T.sub.O.
The laser emission regime shown in FIGS. 5 and 6 is generally designated by the terms "oscillator mode locking, phase locking, or synchronization". In this type of laser emission, the energy is concentrated into narrow pulses which are much more powerful than the mean power and which are capable of presenting a maximum power P.sub.max .apprxeq.P.(T/dt) (3) where T represents the period 2L/c in equation (1), and dt represents the reciprocal of the spectrum width .DELTA.f.
By way of example, an emission having a mean power P=1 watt as produced by a neodymium-doped YAG crystal (with .DELTA.f=10.sup.11 Hz) in a resonant cavity having a length of 1.5 m, contains pulses having peak power of about 1,000 watts.
By obtaining high instantaneous powers from much smaller mean power it is possible to enlarge the field of application of laser sources, and the use of a laser emission of the ph

REFERENCES:
patent: 4635263 (1987-01-01), Mollenauer
patent: 4637026 (1987-01-01), Liu
patent: 4835778 (1989-05-01), Kafuka et al.

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