Microlaser cavity and externally controlled, passive switching, pulses solid microlaser including a saturable absorber 46 and a device (60, 62) for introducing a beam 56 into the microlaser cavity initiating or starting saturation of the saturable absorber.

FIELD OF THE INVENTION 
The invention relates to the field of switched solid microlasers. 
The main advantage of the microlaser is its structure in the form of a 
stack of multilayers, which constitutes its essential characteristic. The 
active laser medium is constituted by a material of limited thickness 
(between 150 and 1000 um) and small dimensions (a few mm.sup.2), on which 
are directly deposited dielectric cavity mirrors. This active medium can 
be pumped by a III-V laser diode, which is either directly hybridized on 
the microlaser, or is coupled to the latter by an optical fibre. The 
possibility of mass production using microelectronic means authorizes the 
production of said microlasers at a very low cost. 
Microlasers have numerous applications in fields as varied as cars, the 
environment, scientific instrumentation and telemetry. 
DISCUSSION OF BACKGROUND 
The known microlasers generally have a continuous emission of a few dozen 
mW power. However, most of the aforementioned applications require peak 
powers (instantaneous power) of a few kW supplied for 10.sup.-8 to 
10.sup.-9 seconds with an average power of a few dozen mW. 
In solid lasers, it is possible to obtain such high peak powers by making 
them function in the pulsed mode at frequencies between 10 and 10.sup.4 
Hz, for which cavity switching processes are used. A cavity can be 
actively or passively switched. 
In the case of active switching, the value of the losses is externally 
controlled by the user, e.g. with a rotary cavity mirror or intracavity 
electrooptical or acousto-optical means changing either the path of the 
beam, or its polarization state. The storage duration, the opening time of 
the cavity, and the repetition rate can be separately chosen. 
In the field of microlasers, an active switching procedure is described in 
the article by J. J. Zayhowski et al entitled "Diode-pumped microchip 
lasers electro-optically Q-switched at high pulse repetition rates", 
published in Optics Letters, vol. 17, No. 17, pp 1201-1203, 1992. In this 
document, switching takes place in a configuration of two coupled 
Fabry-Perot cavities. Such an assembly is illustrated in FIG. 1, where the 
reference 2 designates the active laser medium and the reference 4 a 
switching electrooptical material (LiTaO.sub.3). The laser active medium 2 
forms, with an input mirror 6 and an intermediate mirror 8, a first 
Fabry-Perot cavity. The switching material forms, with the intermediate 
mirror 8 and the output mirror 10, a second Fabry-Perot cavity. Switching 
takes place by modifying the optical length of the switching material 4 by 
an external action, switching electrodes 12, 14 being placed perpendicular 
to the axis of the laser beam 16 on either side of the material 4. If a 
voltage V is applied between these electrodes, an electric field E=V/e, 
where e is the distance between the electrodes (which corresponds to the 
thickness of the electrooptical material), results. The optical index 
n.sub.2 and consequently the optical length n.sub.2 L.sub.2 of the 
electro-optical material is modified by the action of the field E. This 
affects the coupling of the cavities and modifies the reflectivity of the 
Fabry-Perot cavity formed by the mirrors 8 and 10 and by the switching 
material 4, seen by the laser medium. 
For a YAG:Nd microlaser emitting at 1.06 .mu.m and a switching material 
constituted by LiTaO.sub.3 with an approximate thickness of 1 mm, we 
typically obtain: n.sub.1 =1.8, n.sub.2 =2, L.sub.1 =500 um, L.sub.2 =900 
.mu.m. The maximum reflectivity variation of the second cavity is obtained 
for approximately d.lambda./.lambda.=dL.sub.2 /L.sub.2 =dn.sub.2 /n.sub.2 
=10.sup.-4. This index variation is obtained by applying an electric field 
of approximately 10.sub.4 V/cm in the switching material. It is possible 
to liken the second, electrooptical cavity to an output mirror of the 
first cavity constituted by the laser material. The reflectivity of this 
output mirror is variable and controlled by the external control voltage 
applied to the electrodes 12, 14. FIG. 2 shows the variation of the 
reflectivity R of the second cavity as a function of the voltage V 
applied. For the case where three mirrors 6, 8, 10 have reflectivities 
respectively equal to 99%, 95% and 50%, the reflectivity of the second 
cavity will vary between 75 and 99%. For the active medium this amounts to 
varying the reflectivity of the output mirror between 75 and 99% by an 
external voltage control. On the basis of the graph of FIG. 2, it can be 
seen that it is necessary to apply several hundred volts to obtain a 
reflectivity close to 90% and approximately 1000 V to obtain a 
reflectivity of approximately 99%, in the case of an interelectrode 
distance of 1 mm. 
This type of microlaser suffers from problems preventing its practical use. 
Firstly the microlaser is manufactured manually (it involves the bonding of 
precut fragments). Therefore there is a lower limit placed on the 
geometrical dimensions which are a minimum of about 1 mm and in particular 
for the distance between the two electrodes. Another problem is the need 
to reach an adequate field E for switching. It is therefore necessary to 
apply a voltage of approximately 1000 V between the two electrodes in a 
very short time (less than 1 nanosecond) and on laser chips with a volume 
of approximately 1 mm.sup.3. This is very difficult to implement in 
practice and requires sophisticated electronics incompatible with the 
simplicity and low production cost of the microlaser. 
In the case of a passive switching, variable losses are introduced into the 
cavity in the form of a material called a saturable absorber or S.A., 
which is highly absorbent at the laser wavelength and low power density 
and which becomes virtually transparent when said density exceeds a 
certain threshold, which is called the saturation intensity of the 
saturable absorbent. 
Compared with the actively switched microlaser, the passively switched 
microlaser is a self-switched device without any supply problem. There is 
neither a high voltage or a high current, so that it is much simpler to 
manufacture. However, the time control of the laser emission is much more 
difficult to implement. Thus, certain characteristics are regulated or set 
once and for all on manufacture. In particular, the loss level of the 
saturable absorber is fixed by its characteristics (composition, 
thickness, spectroscopic characteristics). Thus, two types of time drifts 
have been observed in the passively switched microlaser: 
a drift of the operating frequency in time, which can sometimes reach 50% 
in a few minutes (drift of 7 kHz on 16 kHz after approximately 10 
minutes), said frequency drift often being accompanied by an amplitude 
fluctuation (to more than 10% in the same time period), 
a starting delay variation or jitter, which can be more than 10 
nanoseconds. 
Therefore the problem arises of finding an optically pumped, solid 
microlaser source combining the advantages of the passive switching system 
(no supplementary, complex supply electronics) and those resulting from 
the active switching system (time control and regularity in the frequency 
of the emission). 
SUMMARY OF THE INVENTION 
In order to solve this problem, the invention relates to a microlaser 
cavity having a solid active medium, characterized in that it incorporates 
a saturable absorber and means able to permit the introduction of a beam 
starting the saturation of the saturable absorber. 
Thus, it is possible to control the cavity switching time by using only 
elements having a lower complexity than those used in the case of an 
actively switched microlaser. The saturable absorber is controlled so as 
to control the microlaser pulse starts. 
The system functions in the following way. Firstly, the losses are at a 
high level in the microlaser cavity, because the saturable absorber is in 
its absorbent state and the gain rises in the amplifier medium. Secondly, 
when it has been decided to switch the microlaser, a control signal is 
supplied to the saturable absorber by the starting or initiating beam. The 
saturation of the absorber is started or initiated, the cavity losses are 
slightly reduced and are at the cavity gain. This leads to the production 
of a switched microlaser pulse. 
From the time standpoint, this novel device has all the advantages 
associated with the actively switched device, i.e. precision of the 
repetition rate, control of the laser pulse start times and 
synchronization possibility within a system. 
The laser pulse control signal is of low power. It is merely necessary to 
introduce a switching beam light pulse into the saturable absorber and the 
gain of the laser medium does the rest for completely saturating said 
saturable absorber. Therefore the power to be injected for starting 
switching is low, particularly when it is compared with the need of 
imposing a voltage of several hundred volts (up to 1000 V) in the case of 
the actively switched microlaser. In addition, it is much more economic to 
have an ancillary starting source than to have to develop a high voltage 
source for a small medium. 
Another aspect of the invention relates to the relative arrangement of the 
saturable absorber and the active medium within the microlaser cavity. 
In general terms, in the known lasers passively switched with the aid of a 
saturable absorber, the following arrangements have been proposed for the 
interior of the laser cavity. 
1. A first arrangement is illustrated in FIG. 3A, where reference 20 
represents a laser cavity and references 22, 23, 24, 25 respectively 
designate the active laser material, the saturable absorber and the input 
and output mirrors of the cavity. 
There is no contact between the saturable absorber 23 on the one hand and 
the other elements of the cavity 20 on the other. 
In this type of device, it is necessary to optically align the cavity 
elements. Moreover, optical settings may be necessary during the use of 
the laser. 
2. In the arrangements illustrated in FIGS. 3B and 3C, a contact is ensured 
between the saturable absorber 26 and a mirror 27 (FIG. 3B), or the active 
laser material 28 (FIG. 3C) with the aid of an optical adhesive 29. 
However, the adhesive introduces a residual absorption factor, as well as 
index differences at the adhesive-adhered material interface. Moreover, a 
possible parallelism deficiency between the adhered elements can also be 
the source of losses in the laser cavity. 
3. FIGS. 3D and 3E illustrate a third possible arrangement. References 30, 
31 respectively designate the laser cavity input and output mirrors. 
Reference 32 designates the active laser material, but the latter is 
codoped with active laser ions and saturable absorber ions. The same 
medium then serves as the active medium and the saturable absorber medium. 
It is therefore impossible to indepedently regulate the properties of the 
laser material and the saturable absorber. 
However, the thickness of the medium influences both the absorption of the 
saturable absorber and the absorption of the active laser ions, as well as 
the laser mode structure. 
In addition, the absorption coefficients of the active laser ions and 
saturable absorbers are directly linked with the concentrations of said 
ions, which are definitively fixed during the growth of the crystals and 
cannot be subsequently modified. Thus, a new crystal must be produced for 
each laser configuration. 
Finally, in the case of passively switched lasers where the same ion (e.g. 
Er) is used both for the laser action and as the saturable absorber, it is 
impossible to use this codoping method. Thus, the same ion could serve as 
the active ion or as the saturable absorber ion, provided that the 
concentrations differ very widely. For the saturable absorber, the 
concentration must be much higher than for the active laser material. 
To obviate the aforementioned problems, the invention proposes depositing 
the saturable absorber in the form of a thin film, directly on the active 
material of the microlaser cavity. 
One of the main advantages of the invention, according to this particular 
embodiment, is in the structure of the microlaser cavity (or the laser 
microsystem consisting of a microlaser associated with the microoptics) in 
switched form, which then consists of a stack of layers, making it 
possible to retain the possibility of low cost mass production. This 
multilayer structure causes no problems with regards to the simplicity and 
mass production at low cost of the microlasers, such as have been 
developed for continuous microlasers. It makes it possible to produce 
fool-proof, self-aligned (no optical setting), monolithic, passively 
switched microlasers. This structure requires no adhesion or bonding 
operation and no complex alignment operation. 
Another advantage of the microlaser compared with the "codoped" laser is 
that the active medium is separate from the saturable absorber, whilst 
preventing any adhesion of the two media and whilst retaining a monolithic 
structure. Thus, it is possible on the one hand to independently regulate 
the thicknesses (during the deposition of the layers or by mechanical 
thinning after the deposition of the layers) and the concentrations of the 
ions in the two media and on the other, as a result of this separation, 
implement switched lasers where the same ion (e.g. Er) can be used as the 
active ion and as the saturable absorber with different concentrations. 
According to an even more specific embodiment of the invention, the 
starting beam introduction means are arranged so as to permit a guided 
propagation of the starting beam in the plane of the saturable absorber 
film. 
Thus, the saturable absorber can have an etched microsurface so as to 
permit a guided propagation of the starting beam in the plane of the 
saturable absorber film, following the introduction of said beam into the 
microlaser cavity and the reflection of said beam against said 
microsurface. 
In addition, means can be provided permitting the introduction into the 
microlaser cavity of a starting beam, parallel to the introduction 
direction into the microlaser cavity of an active laser medium pumping 
beam. 
According to a variant, a recess can be made in at least part of the 
saturable absorber film, said recess permitting the positioning of the end 
of an optical fibre in the plane of the saturable absorber film. 
According to another embodiment of the invention the means permitting the 
introduction of a starting beam are arranged so that this beam is 
propagated, in the microlaser cavity, in a direction not contained in the 
plane of the saturable absorber film. 
More specifically, according to said embodiment, the means permitting the 
introduction of the starting beam have an off-axis microlens portion 
located on the side of the input mirror of the microlaser cavity. 
According to another aspect of the invention, the film can be formed by an 
organic dye dissolved in a polymer solvent. According to a variant, the 
film can be deposited by liquid phase epitaxy. 
Finally, the invention also relates to a microlaser incorporating a 
microlaser cavity such as has been described hereinbefore, cavity pumping 
means and means for generating a beam for starting the saturation of the 
saturable absorber.

DETAILED DESCRIPTION OF EMBODIMENTS 
In general terms, the invention firstly relates to a microlaser cavity 
having a solid active medium, between an input mirror and an output 
mirror, as well as a saturable absorber making it possible to passively 
switch the microlaser. Means are also provided for permitting the 
introduction of a beam for starting or initiating the saturation of the 
saturable absorber. 
The starting beam can e.g. be obtained by a laser diode. This laser source 
type is compact and perfectly compatible with the reduced size of the 
microlaser. Moreover, the power emitted by a laser diode can be very 
easily modulated by means of the diode supply current. In the case of the 
envisaged application (starting a saturable absorber) it will preferably 
be ensured that the diode satisfies length and power conditions. 
From the wavelength standpoint, the starting source preferably excites the 
saturable absorber on the same transition as the laser beam. The 
absorption of the latter is due to said transition and is consequently 
what is to be saturated. It is therefore possible to use a wavelength 
equal to the laser wavelength or a shorter wavelength corresponding to 
more energetic photons. In the latter case, it is preferably ensured that 
the excited centres are deexcited so as to drop to the correct energy 
level (in the case of a discreet distribution of the energy levels, or so 
that the wavelength chosen remains within the absorption band limit (in 
the case of an absorption band in the saturable absorber medium). In the 
example of a saturable absorber, whose impurities are constituted by 
Cr.sup.4+ ions, a diode emitting at 980 nanometers is suitable for 
obtaining a starting beam. 
With regards to the emitted power, only a small power quantity is necessary 
for initiating the absorption of the saturable absorber. If it is 
considered that the path in the laser medium and in the saturable absorber 
is short and that the losses on the starting beam power are limited, it is 
sufficient to have a starting or initiating source with a power of a few 
dozen milliwatts (between approximately 10 and 100 mW). This order of 
magnitude is compatible with commercially, low power diodes, such as III-V 
semiconductor diodes, which also have a low cost. When using such diodes, 
the wavelength adjustment is possible by adjusting the III-V semiconductor 
materials used in the laser diode. These adjustment processes are well 
known and will not be described in detail here. Reference can also be made 
to the article by Pocholle in SPECTRA 2000, No. 164, April 1992, p 27. 
The invention will now be described in the case where the saturable 
absorber is in the form of a film. In particular, it can be advantageous 
to deposit the saturable absorber film directly on the amplifier medium, 
as illustrated in FIGS. 4A and 4B. The reference 36 therein designates the 
active laser medium, the reference 38 a saturable absorber film and these 
two elements are located between two mirrors 42, 44 which close the laser 
cavity. Reference 40 designates the complete cavity. 
Optionally and as illustrated in FIG. 4B, it is possible to manufacture by 
a prior art method (A. Eda et Al., CLEO'92, paper CWG33, p 282 (Conf. on 
Laser and Electro-optics, Anaheim, USA, May 1992)) a microlens array 45 of 
transparent material (e.g. silica) on the surface of the laser material 
36. The typical microlens dimensions are a diameter of 100 to a few 
hundred microns and a radius of curvature of a few hundred micrometers to 
a few millimetres. 
These microlenses are used for obtaining "stable" cavities (the plane-plane 
cavity is not stable) and which are of the planoconcave type. In the case 
of optical pumping, they also make it possible to focus the pumping beam. 
The material from which the active medium 36 is made will e.g. be doped 
with neodymium (Nd) for a laser emission of around 1.06 .mu.m. This 
material could e.g. be chosen from among one of the following materials: 
YAG (Y.sub.3 Al.sub.5 O.sub..sub.12), LMA (LaMgAl.sub.11 O.sub.19), 
YVO.sub.4, YSO (Y.sub.2 SiO.sub.5), YLF (YLiF.sub.4), GdVO.sub.4, or SYS 
(SrY4(SiO.sub.4).sub.3 O), etc. This choice will be conditioned by the 
following criteria, but will obviously be dependent on the particular 
applications. 
If the laser cavity is optically pumped, preferably with one or more laser 
diodes, the material must have a high absorption coefficient at the pump 
wavelength (e.g. III-V laser diode emitting at about 800 nm) to increase 
the pumping efficiency, whilst retaining a low material thickness (&lt;1 mm). 
A wide absorption band at the wavelength of the pump, e.g. 800 nm, in order 
to satisfy the problem of wavelength stabilization of the laser diode, so 
as to simplify the choice and electrical control of the pumping laser 
diode. 
A considerable effective, stimulated emission cross-section, in order to 
obtain high efficiencies and high output powers. 
A limited emission band width so as to easily obtain a monofrequency laser, 
or conversely a wide emission band to bring about a frequency-tunable 
laser emission. 
Good thermomechanical properties in order to simplify the machining of the 
material and limit the prejudicial thermal effects by a good dissipation 
of the heat produced by the absorption of the pump (said excess heat 
depending on the energy efficiency of the laser). 
A long life in the excited state for a high energy storage, or a short life 
for a rapid switching rate. 
Large dimensions so as to be able to simultaneously mass produce the 
largest possible number of microlasers with a single laser crystal. 
Among the known materials, the most appropriate for the operation of the 
microlaser are (with comparable life periods of a few hundred 
microseconds): 
YVO.sub.4, which has a good coefficient and a wide absorption band, 
together with a good effective cross-section, 
YAG, whose absorption coefficient and effective stimulated emission 
cross-section are average and whose absorption and emission band widths 
are low, being in the form of large dimensions and with a good thermal 
conductivity, 
LMA, which offers low absorption coefficient and effective cross-section, 
the absorption and emission bands being wide, whilst it can also have 
large dimensions. 
With respect to the active ions (dopants), they are generally chosen from 
among: 
Nd for an emission at around 1.06 .mu.m, 
Er or an erbium-ytterbium Er+Yb codoping for an emission around 1.5 .mu.m, 
Tm or Ho or a thulium and holmium codoping for an emission around 2 .mu.m. 
Another decisive parameter is the thickness e of the active medium 36. The 
thickness e conditions the characteristics of the microlaser: 
on the one hand, the absorption of the pumping beam will be greater as the 
thickness e increases, 
on the other, the number of longitudinal modes of a Fabry-Perot cavity 
increases with the thickness and if it is wished to obtain a longitudinal 
monomode laser this thickness must be small. 
If dg is the width of the gain band (laser emission) of the material, the 
number of modes N will be given by: 
##EQU1## 
in which C is the speed of light and n the refractive index of the 
material. 
For a monofrequency laser, generally a minimum thickness is chosen for N=1, 
provided that said thickness is &gt;100 .mu.m. Typical thicknesses for 
obtaining a single mode are: 
YAG L=750 .mu.m, 
YVO.sub.4 L=500 .mu.m, 
LMA L=150 .mu.m. 
In practice, the thickness e will vary between 100 .mu.m and 5 mm. 
In the embodiments illustrated in FIGS. 4A and 4B, the saturable absorber 
38 is in the form of a thin film. Two types of film can be used: 
a polymer containing saturable absorber molecules and typically for a 1.06 
.mu.m microlaser use is made for the saturable absorber of an organic dye 
such as bis(4-diethylaminodithiobenzyl) nickel (BDN, Kodak, CAS No. 
51449-18-4) in a solution containing by weight 6% polymethyl methacrylate 
(PMMA) in chlorobenzene. 
Variants are described hereinafter in conjunction with the description of a 
preparation process. 
This type of solution will be deposited using a trammel directly onto the 
laser material (cf. hereinafter for the preparation process). This leads 
to films with a thickness of approximately 1 to 5 .mu.m, e.g. 2, 3 or 4 
.mu.m. Another type of film will be obtained by liquid phase epitaxy 
(LPE), directly on the laser material or any other process making it 
possible to obtain the same deposit (same material, same doping, same 
properties). Thus, the film will generally have been obtained by LPE. The 
LPE preparation process is described hereinafter and makes it possible to 
obtain, on the substrate 36 constituted by the active solid medium, a film 
of thickness between 1 .mu.m and 500 .mu.m (e.g. 100, 200, 300 and 400 
.mu.m). It is constituted by a basic material identical to that of the 
active solid medium (e.g. YAG), but it is doped with ions giving it 
saturable absorber properties, e.g. Cr.sup.4 for a 1.06 .mu.m laser or 
Er.sup.3+ for a roughly 1.5 .mu.m laser or Ho.sup.3+ for a roughly 2 .mu.m 
laser. 
The type of dopant is adapted to the laser which it is wished to switch, so 
that the epitaxied film has a saturable absorption at the emission 
wavelength of said laser. 
Therefore, in this case, the active laser material and the saturable 
absorber film have the same crystalline structure and only differ through 
the different dopants which affect the crystalline and optical properties 
of these two media. 
The properties of the film in the two cases will differ very widely. Thus, 
definition takes place for each film type of the damage threshold. Beyond 
a certain power density present in the laser cavity, it is possible to 
destroy the saturable absorber film. This limiting power density, known as 
the damage threshold, will be lower in the case of the polymer with the 
organic dye than in the case of the LPE-deposited film. Therefore in the 
first case it is necessary to operate with lower energy levels in the 
cavity than in the second case. 
Moreover, in one case the index difference between the laser material 8 and 
the polymer 12 introduces an optical interface between the two media. In 
the other case, it is only possible to carry out LPE on the same material 
(e.g. YAG on YAG, only the doping differing), which limits the extent of 
the applications, but makes it possible to adjust the index of the 
epitaxied film to that of the active laser material serving as the epitaxy 
substrate, so as to avoid the formation of an optical interface between 
the two media. 
Finally, the nature of the film will influence the time form of the 
emission or laser pulse train. In the case of an organic dye dissolved in 
a polymer, the dye decline time is very short (.about.1 ns), whereas in 
the case of the epitaxied film the ions constituting the impurities 
(Cr.sup.4+, Er.sup.3+, Ho.sup.3+) have much longer decline times of 
approximately 1 microsecond or more. These properties will obviously 
condition the choice of the film as a function of the intended use. 
In order to obtain a complete laser cavity, the active medium with its 
saturable absorber film or films will be located between two mirrors 42, 
44. The input mirror, deposited by known methods, is preferably a dichroic 
mirror having a maximum reflectivity (as close as possible to 100%) at the 
laser wavelength and the highest possible transmission (&gt;80%) at the pump 
wavelength (generally 800 nm for Nd doped materials, 980 nm for Er doped 
materials and 780 nm for Tm doped materials). The output mirror is then 
also of the dichroic type, but allows the passage of a few percent of the 
laser beam. This gives a laser cavity with structures as shown in FIGS. 4A 
and 4B. 
It is immediately clear what is the advantage of such a structure, because 
at no time does it require an optical alignment of the different 
components and also introduces no optical adhesive, whilst avoiding the 
problems associated with a structure, where the active medium is codoped 
with active laser ions and saturable absorber ions. 
The pumping of such a cavity is preferably an optical pumping. Thus, III-V 
laser diodes are particularly suitable for pumping a microlaser cavity. 
According to the invention, a microlaser of the aforementioned type can be 
switched in controlled manner by starting the saturation of the saturable 
absorber with the aid of a starting beam provided for this purpose. 
Variants will now be described in conjunction with FIGS. 5, 6, 7A and 7B. 
In FIG. 5, reference 46 designates the active laser medium, 48 a saturable 
absorber film, 50 and 52 the input and output mirrors of the microlaser 
cavity. An active laser medium pumping beam is diagrammatically 
represented by an arrow 54, whilst 56 designates a beam for starting the 
saturation of the saturable absorber 48. The configuration shown in FIG. 5 
is a so-called transverse configuration, i.e. the starting beam 56 is 
propagated in the saturable absorber film perpendicular to the axis of the 
pumping beams 54 of the laser cavity and the laser beam 58 emitted by the 
microlaser. Thus, in the case of a saturable absorber in thin film form, 
said transverse configuration is particularly advantageous to the extent 
that the film will serve as a guide for the starting beam. The latter will 
thus propagate to the level of the zone indicated by the letter A in FIG. 
5, i.e. the zone where the laser beam in the cavity encounters the 
saturable absorber. In preferred manner, within, the saturable absorber 
film, it is desirable for the starting beam propagation to take place over 
the shortest possible distance d, because the latter is absorbed 
throughout its propagation in the film. However, the distance d is 
determined by the size of the microoptical componets used for injecting 
the pumping and starting beams into the microlaser cavity. 
Moreover, the index of the saturable absorber film can be adapted to the 
guided propagation mode (codoping with gadolinium (Ga) and lutetium (Lu), 
the first serving to adapt the index and the second making it possible to 
compensate the widening of the crystal lattice due to the introduction of 
the first). 
In this embodiment, in order to inject the starting beam 56 into the 
microlaser cavity, it is possible to have recourse to microoptical 
methods. Thus, the film 48 can be etched so as to obtain a planar or 
non-planar face 60 making it possible to reflect the starting wave. The 
inclination of said etched surface 60 is preferably such that there is a 
total reflection of the starting wave. If there is no total reflection, it 
is possible in a variant to carry out a reflecting treatment of the 
surface 60. 
For a saturable absorber film of limited thickness (approximately below 10 
.mu.m), the etched surface can be obtained by photolithography and a 
variable density mask. For greater thicknesses (above 10 .mu.m) a bevel 
polishing can be used for obtaining the surface 60. 
The starting beam 56 can be focussed as from its entry into the laser 
medium, e.g. with the aid of a microlens 62 etched on the microlaser input 
face in an area adjacent to the pumping beam axis. In the configuration 
shown in FIG. 5, the laser medium pumping beam 54 and the saturable 
absorber starting beam 56 are located on the same side of the microlaser. 
The focussing function of the starting beam 56 can also be obtained by any 
other means, such as e.g. a diffractive lens, Fresnel lens, etc. 
In the embodiment of FIG. 6, the references 64, 66, 68, 70 respectively 
designate the laser amplifier medium, a saturable absorber film, the 
output and input mirrors of the microlaser cavity. The pumping, starting 
and laser beams are designated by the same references as in FIG. 5. 
The embodiment of FIG. 6 is quasi-longitudinal, i.e. the starting beam 56 
is propagated towards the saturable absorber film in a direction not 
contained in the plane of said film. This embodiment requires no etching 
of the saturable absorber film, unlike in the embodiment described in 
conjunction with FIG. 5. Once again the starting beam can be supplied 
parallel to the pumping beam 54 in the direction of the microlaser cavity 
input face. At said input face it is deflected towards an area A of the 
saturable absorber film on which the pumping beam is incident. The 
deviation can be obtained by an off-axis microlens portion 72 obtained by 
etching the active laser medium 64 with the aid of a variable density mask 
in an area close to the axis of the pumping beam. 
In the cases described hereinbefore, the starting 54 and pumping 56 beams 
are parallel to the laser beam 58 obtained at the microlaser cavity 
output. 
Another embodiment will now be described in conjunction with FIGS. 7A and 
7B. In said drawings, a thin saturable absorber film is designated by the 
reference 80 and is deposited on an amplifier medium 74, the means being 
located between an input mirror 78 and an output mirror 76 of the thus 
obtained microlaser cavity. A groove or notch 82 is made in at least part 
of the saturable absorber and optionally, as illustrated in FIGS. 7A and 
7B, in the output mirror and in part of the amplifier medium, so as to be 
able to position the end of an optical fibre 84 permitting the injection 
of the saturable absorber starting beam 80 directly into the same without 
making it pass through the amplifier medium. Here again the geometry is 
transverse and the starting beam is propagated in the film 80 serving as a 
guide for the same. An advantage of this configuration is that it also 
makes it possible to reduce the distance d over which the starting beam 
will propagate in the film 80 between the output end 86 of the fibre 84 
and the area of the saturable absorber 80 on which the pumping beam 54 is 
incident. 
The size of the groove 82 is a function of the diameter of the starting 
fibre 84, which can be a multimode fibre. It can also be a monomode fibre 
because, as explained hereinbefore, the power which has to be transmitted 
to the saturable absorber is relatively low being a few dozen milliwatts. 
The choice of a monomode fibre also makes it possible to minimize the 
overall dimensions, because it has a smaller diameter than a multimode 
fibre. Moreover, in the case of saturable absorber films with a thickness 
of a few micrometers, a monomode fibre will also be suitable, as a result 
of its small core diameter. 
For all the embodiments described hereinbefore and with respect to the 
choice of the source supplying the starting beam 56, the conditions to be 
respected for the wavelength and power are preferably the same as 
described hereinbefore. Thus, a III-V semiconductor diode, whose spectral 
emission properties are adjusted by the choice of the semiconductor 
material, can also be suitable for the different embodiments described. 
Moreover and once again for all the embodiments described hereinbefore, it 
is clear that the presence of means able to permit the introduction of a 
beam for initiating the saturation of the saturable absorber does not 
prejudice the compact character of the microlaser structure. Moreover, 
each element is not introduced into the microlaser, which requires an 
optical setting of the latter. Finally, no parasitic element of the 
optical adhesive type is required. In particular, the benefit of a 
saturable absorber structure in the form of a film directly deposited on 
the active laser medium is retained. 
The operation of a device according to the invention will now be described 
in conjunction with FIGS. 8A and 8B. These drawings show the time 
evolutions and the different operating conditions for the loss levels and 
gain of a microlaser cavity, as well as a laser pulse obtained by 
switching the cavity. 
FIG. 8A corresponds to the case where the microlaser cavity is of the 
conventional type, i.e. without any means for initiating or starting the 
saturation of the saturable absorber. It is firstly possible to see a 
phase I in which the system has no laser effect, because the saturable 
absorber imposes within the microcavity a loss level P higher than the 
gain G obtained by pumping; However, said gain increases, because the 
solid amplifier medium stores the energy of the pumping beam (with a 
saturation effect due to the reemission of the energy by fluorescence). As 
from a certain stored pumping power value, the gain G reaches and exceeds 
the total loss level of the cavity (residual loss+output transmission+high 
saturable absorber loss), i.e. phase II (cf. FIG. 8A). During phase II, 
the few photons emitted by fluorescence at the laser wavelength start to 
be amplified by the highly pumped laser medium despite the still high 
losses. Then, at the start of phase III, the absorber is saturated until 
it becomes transparent and the laser pulse I is emitted. This phenomenon 
is very fast, the absorber being suddenly saturated under the avalanche of 
amplified photons in the laser amplifier medium. The losses switch to 
their lowest level, whereas the gain has remained at a high level, so that 
a laser pulse is emitted. Thus, the gain will decrease very rapidly until 
it drops below the threshold imposed by the loss level P. Then (phase IV), 
the saturable absorber returns to its starting state, because its active 
centres are deexcited in various ways (by spontaneous emission of photons, 
phonons, etc.) and the modulator constituted by the saturable absorber 
closes again. 
In the case of a microlaser cavity according to the present invention, the 
time evolution of these same quantities (loss P, gain G, laser pulse I) is 
shown in FIG. 8B. In a first phase I', the loss level P exceeds the gain G 
within the microlaser cavity, said gain level being a rising function of 
time. The maximum loss level P is fixed by the saturable absorber 
(composition, thickness, spectral characteristics), whilst the gain level 
G is determined by the intensity of the microlaser cavity pumping beam. 
Thus, this pumping level will be regulated in such a way that the gain 
does not reach the loss level P (otherwise the saturable absorber would 
enter phase II according to the standard FIG. 8A). The introduction of the 
starting or initiating beam at time t.sub.0 (e.g. with the aid of the 
synchronization means for the laser cavity pumping source and the starting 
beam source), corresponds to the introduction of a power P.sub.a in light 
form into the saturable absorber, said power being adequate to bring the 
loss level P within the cavity beneath the gain level. As soon as the 
saturable absorber saturation is initiated in this way, it rapidly 
decreases the loss level (phase III' in FIG. 8B), which thus becomes well 
below the gain level and a laser pulse I is emitted. Finally, in phase 
IV', the gain level rapidly decreases to below the threshold imposed by 
the losses and the saturable absorber returns to its starting state, its 
active centres being deexcited, so that a cycle can recommence. The time 
at which the starting beam is switched can be chosen by the user, so that 
in FIG. 8B it can be chosen at t.sub.1, t.sub.2 or t.sub.0. 
By comparing these two diagrams, it is possible to consider the saturable 
absorber as a controlled loss modulator: 
in the conventional diagram (FIG. 8A), by a light source within the laser 
microcavity, the latter being very noisy and this high noise level leads 
to indetermination on the microcavity switching time, 
in the diagram according to the invention (FIG. 8B), by an external 
starting source, which makes it possible to obtain freedom from the random 
character inherent in the standard operating diagram. 
A process for the production of a microlaser according to the invention 
will now be described. The following stages occur in this process. 
1) The first stage consists of choosing the active laser material and 
conditioning the chosen laser crystal, the latter being oriented and cut 
into plates with a thickness between 0.5 and 5 mm. 
2) The following stage is a stage of grinding and polishing the plates and 
serves to remove the surface cold working coating due to the cutting and 
brings the thickness of the plates to a level slightly exceeding the 
microlaser specification. The ground plates close to the final thickness e 
are polished on both faces with an optical quality. The cutting, grinding 
and polishing are carried out using known processes and known machines. 
3) A saturable absorber preparation stage. 
3a) In the case of a conventional saturable absorber, various processes are 
known making it possible to obtain a switched microlaser cavity. It is in 
particular possible to carry out a codoping of the basic material of the 
active laser medium in order to give active laser medium and saturable 
absorber properties (e.g. YAG doped with neodymium Nd.sup.3+ and chromium 
Cr.sup.4+ ions). 
3b) In the case of the saturable absorber deposited in thin film form, two 
deposition types can be implemented. 
3b1) First deposition type: deposition of a saturable absorber organic dye 
dissolved in a polymer. 
Typically, for a microlaser operating at 1.06 .mu.m, it is possible to use 
as the saturable absorber an organic dye such as 
bis(4-diethylaminodithiobenzyl) nickel (BDN, Kodak, CAS No. 51449-18-4) in 
a polymethyl methacrylate (PMMA) solution. For this purpose preparation 
takes place of a solution containing 6% by weight polymethyl methacrylate 
(Polyscience mean weights) in chlorobenzene (Prolabo) stirring for 24 
hours. To it is added 0.2 wt. % BDN, followed by stirring for a further 2 
hours. The solution is then filtered and deposited on the substrate on its 
output face (opposite to the input face having the dichroic mirror), this 
taking place in dropwise manner with a circular centrifugal movement. It 
is possible to use for this purpose a trammel, which is a standard machine 
such as that used in microelectronics for the deposition of resins used in 
lithography operations. The substrate is previously cleaned with respect 
to all traces of impurities resulting from the polishing operation. It is 
rotated for 20 seconds at 2000 r.p.m. and then 30 seconds at 5000 r.p.m. 
The film is then dried for 2 hours in an oven at 70.degree. C. 
This gives a 1 .mu.m thick film containing 3% of active molecules (BDN) and 
whose optical density is 0.13 at 1.06 .mu.m (74% transmission) before 
saturation. Such a saturable absorber has a relaxation time close to 10 ns 
and is saturated at an intensity close to 1 mW/cm.sup.2. 
By varying the concentration parameters of the polymer, its molecular 
weight or solvent, the dye proportion and the trammel rotation speed, it 
is possible to adjust the saturable absorber performance characteristics. 
The specifications typically obtained are: 
film thickness:1 to 5 .mu.m (e.g. 2, 3, 4 .mu.m), 
molecule density:5 to 10 wt. %, 
dye:BDN, mm=685 g, 
glass transition point:Tg=78.degree. C., 
absorption at 1.06 .mu.m:10 to 70%, 
saturation rate:90%, 
effective cross-section:10.sup.-16 cm.sup.2, 
relaxation time:2 to 15 ns, 
saturation intensity:0.1 to 1 mW/cm.sup.2, 
non-uniformity of film:&lt;5% on 1 cm 
depolarization rate:&lt;10.sup.-5, 
losses at 800 nm:&lt;1%, 
repetition rate:10 to 10,000 Hz, 
photostability:10.sup.8 strokes, 
deposition method:trammel. 
Other polymers, such as polyvinyl alcohol or polyvinyl acetate or even 
polystyrene can be used in their respective solvents in place of PMMA. It 
is also possible to use as the dye bis(4-dimethylaminodithiobenzyl) nickel 
(BDN, Kodak, CAS No. 38465-55-3). 
The dye can also be incorporated into a silica gel or grafted to the 
polymer chain. 
Numerous other dithiene metal complexes can be used as the dye for other 
wavelengths, as described in the articles of K. H. Drexhage et al, Optics 
Communication 10(1), 19, 1974 and Mueller-Westerhoff, Mol. Cryst. Liq. 
Cryst. 183, 291, 1990. 
The method can also be used for switching lasers operating at wavelengths 
other than 1.06 .mu.m. For example switching will take place of Er or 
Er+Yb lasers (Er or Er+Yb doped materials where the active ion is Er) 
emitting at about 1.5 um using 
tetraethyloctahydrotetraazapentaphene-dithiolato-nickel (cf. article by 
Mueller-Westerhoff indicated above). 
3b2) Second deposition type:deposition of a film by liquid phase epitaxy 
(LPE). 
The saturable absorber film is obtained by soaking the substrate on which 
it is deposited in an appropriately chosen, supersaturated solution. This 
solution or epitaxy bath is a mixture of a solvent and a solute 
constituted by different elements forming the final material. The 
substrate and film have the same crystalline structure and only differ 
through the different dopants affecting the crystalline and optical 
properties of the film. The active ions such as Nd, Er and Yb make the 
material amplifying, whilst other ions (Cr and Er) give it saturable 
absorber properties, whilst certain other ions can be used for varying the 
refractive index or crystal lattice of the material (e.g. Ga, Ge, Lu, 
etc.). It is thus possible to control the properties of the films 
produced. 
This process can be used for any material in the form of monocrystals (for 
producing substrates) and which can be prepared by liquid phase epitaxy. 
This is the case of the aforementioned materials for the basic material of 
the active laser medium: Y.sub.3 A.sub.15 O.sub.12 (YAG), Y.sub.2 
SiO.sub.5 (YSO), YVO.sub.4, YLiF.sub.4 (YLF) or GdVO.sub.4 or SrY.sub.4 
(SiO.sub.3 (SY). The bath composition (choice of solvent and 
substituents), the concentrations in the solute of different oxides and 
the experimental growth conditions (temperature range, operating 
procedure, etc.) are adjusted for each material so as to obtain films 
having the optimum crystalline quality. 
In the case of garnets (YAG), the chosen solvent is a PbO/B.sub.2 O.sub.3 
mixture and the solute has an Al.sub.2 O.sub.3 excess in order to 
stabilize the garnet phase. The solute/solvent ratio is then calculated so 
as obtain growth at about 1000.degree. C. 
As a function of the bath composition, the temperature and the deposition 
time, it is possible to adjust the thickness (1.ltoreq.e.ltoreq.200 um, 
e.g.:25 .mu.m, 50 .mu.m, 75 .mu.m, 100 .mu.m, 125 .mu.m, 150 .mu.m, 175 
.mu.m, e.ltoreq.200 .mu.m also being possible) and the dopant 
concentration in the films. The growth of a film takes place at constant 
temperature, which makes it possible to obtain a homogeneous dopant 
concentration in the film thickness. The substrate is given a uniform or 
alternating rotary movement, which leads to a good thickness uniformity. 
It is possible to obtain a substrate carrying one or two S.A. films, 
depending on whether soaking takes place of one face of the active laser 
medium in the bath, at the surface thereof, or both faces, the laser 
material being completely immersed in the bath. 
The epitaxied face or faces obtained can be repolished in order to remove 
roughness which may be caused by the epitaxy process and so as to bring 
the thickness of the epitaxied film or films to the desired level for the 
operation of the microlaser. 
4) A stage of depositing in put and output mirrors. These can be dichroic 
mirrors obtained by a deposition of dielectric multilayers, which is a 
known, commercially available process. 
The deposition of the input mirror can take place before or after the 
preceding stage, in the case of S.A. polymer deposition, but must be 
performed afterwards in the case of liquid phase epitaxy, which occurs at 
high temperature and may destroy the mirror. 
5) A stage of cutting plates for obtaining microlaser chips. 
The small plates having the mirrors, the saturable absorber and the active 
laser medium, as well as optionally the microlenses are cut with a diamond 
saw of the type used in microelectronics for Si chip cutting, so as to 
obtain laser chips with a section of a few mm.sup.2. 
6) The specific stages of the embodiments illustrated in FIGS. 5 and 6 have 
already been described (etching the film 80 for forming the reflecting 
surface 60 and formation of microlenses 62, 72). Moreover, the formation 
of the groove 82 in the embodiment illustrated in FIGS. 7A and 7B is 
obtained by a conventional etching procedure. 
The microlaser source according to the invention has all the advantages of 
the microlaser, all the characteristics of actively switched pulsed lasers 
and all the advantages of passively switched pulsed lasers. 
It can also be mass produced, so that the production costs are reduced, 
because the samples are produced in batches, a good reliability of each 
laser within a batch and an absence of settings and greatly reduced laser 
maintenance. 
The production process greatly benefits from processes developed for 
passively switched microlasers. In addition, the microlaser structure 
remains very simple, very reliable and very robust. 
The time operation is identical to that of active switching, i.e. with 
precision of the repetition rate, control of the pulse starting times and 
synchronization possibility within a system. This time behaviour can be 
modelled and researched. 
The laser pulse control signal is of low power, it being sufficient to 
initiate the saturation of the absorber and the laser medium gain does the 
rest in order to completely saturate the absorber. 
Finally, the higher the control signal, the lower the noise on the starting 
time. Thus, the greater the saturation advance of the absorber, the more 
the latter is controlled by the exterior and not by the photons within the 
laser cavity, which constitute a noise source. With this switching system, 
a compromise can be made between the precision of the emission time 
characteristics and the energy consumption of the switching system. 
Among possible industrial applications of microlasers, reference is made to 
laser telemetry, laser micromachining and designation, pollutant detection 
and three-dimensional imaging. In addition, the switching device can be 
adapted to a very wide wavelength range.