1. Field of the Invention
The present invention relates to the field of switched lasers. The aim of this type of laser is to produce short duration, coherent light pulses whose emitted peak power is high compared with that used for pumping. In a standard manner there are two solutions for obtaining said switching, one is called active and the other passive, the latter being used in the present invention.
2. Discussion of the Background
More specifically, the switching of a laser cavity consists of adding therein time-variable losses, which prevent the laser effect from occurring during a certain time, during which the pumping energy is stored in the excited level of the gain material of the laser. These losses are suddenly reduced at precise times, thus releasing the stored energy in a very short time (giant pulse). Thus, high peak power is obtained.
In the case of active switching, the value of the losses is externally controlled by the user (e.g. using a rotary cavity mirror, an intercavity acousto-optical or electro-optical means changing either the path of the beam, or its polarization state). The storage time, the laser opening time and the repetition rate can be chosen independently. However, this requires adapted electronics and makes the laser system more complicated.
In the case of passive switching, variable losses are introduced into the cavity in the form of a saturable absorber or SA material, which is highly absorbent at the laser wavelength and low power density and which become substantially transparent when said density exceeds a certain threshold, which is called the saturation intensity of the SA. The enormous advantage of passive switching is that it requires no control electronics.
Known saturable absorbers often contain organic molecules which are responsible for the absorption. These materials are generally in liquid or plastic form and consequently are often of poor optical quality, which age very rapidly and have a poor resistance to the laser flux.
Solid materials are also used as saturable absorbers, e.g. for lasers emitting at around 1 .mu.m (YAG with active ions: Nd.sup.3+ or Yb.sup.3+), it is possible to use:
crystals of LiF:F2 having coloured centers responsible for the SA behaviour of the material and which have a limited life, PA1 certain Cr.sup.4+ -doped solid crystals having a saturable absorption at around 1 .mu.m.
However, for this type of solid saturable absorber, the limited absorbent ion concentration requires the use of a considerable material thickness, prevents a strong focussing of the beam and therefore increases thresholds. Moreover, it prevents applications where the source must be very compact, i.e. in the case of a microlaser with a cavity length of approximately 1 mm.
Good results have been obtained with solid crystals codoped with the active ion and the absorber ion: e.g. Nd.sup.3+ and Cr.sup.4+. The advantage of these self-switched laser materials is that no different material is introduced for the switching and therefore no supplementary losses are introduced. Their disadvantage is of linking the active ion concentration with that of the absorbent ion, which makes it difficult to optimize the laser, the adaptation of the laser to the available pumping power then requiring the growth of a new solid crystal.
For lasers emitting at around 1.5 .mu.m (active ion: Er.sup.3+), there are highly Er.sup.3+ doped solid materials, which have a saturable absorption around 1.5 .mu.m and which permit the switching of such lasers. However, once again the problems described hereinbefore in connection with solid materials are encountered.
In known lasers passively switched with the aid of said saturable absorbers, different switched cavity production methods exist and are dependent on the SA used:
1. A first method is illustrated in FIG. 1a, where it is possible to see the laser cavity 1, the solid, active laser material 2, the saturable absorber 3 and the cavity exit mirror 4 and entrance mirror 5. There is no contact between the saturable absorber 3 on the one hand and the other elements of the cavity 1 on the other. In this type of device, it is necessary to optically align the cavity elements.
2. In the arrangements illustrated in FIGS. 1b and 1c, a contact is ensured between the saturable absorber 3 and the mirror 4 (FIG. 1b) or the active laser material 2 (FIG. 1c) with the aid of an optical adhesive 6. However, the adhesive introduces a residual absorption factor, as well as index differences at the interface between the adhesive and the adhered materials. Moreover, a possible parallelism error between the adhered elements can also be the source of losses in the laser cavity.
3. FIG. 1d, where references 4 and 5 again designate the mirrors and reference 2 the active laser medium, illustrates a third possible arrangement, where one of the mirrors 4 is directly deposited on the saturable absorber 3. However, this is only possible when the saturable absorber can undergo a polishing operation prior to the deposition thereon of the mirror and this is not the case when it is essentially made from a glass or a crystal.