FR2785098A1 - Laser doubler modular laser architecture comprises folded construction active laser regions single pump beam fed using progressive reflections - Google Patents

Abstract

The laser beam architecture has several active laser regions (5a,5b) placed in an optical cavity. The laser architecture is folded, a single pump beam is progressively reflected to cover each of the active laser regions.

Description

SOLID LASER PUMP BY LASER DIODE
The invention relates to a solid laser pumped by laser diode and more particularly to a modular laser comprising several solid lasers and its application to lasers doubled in frequency.

 Solid lasers pumped by laser diodes have experienced significant development in recent years due to many advantages over the other laser families (efficiency, compactness, lifetime, all solid state laser, etc.). They can operate in continuous mode or in impulse mode.

 The laser diodes transfer energy to the active medium by emitting light at a wavelength corresponding to an absorption line of the active medium (YAG: Nd for example). This resonant pumping by laser diodes is thus more efficient than pumping by lamp. In addition, the beam of the laser diodes being directional, it makes it possible to carry out a pumping located spatially in the active medium. The most effective consists in making the pump beam and the laser beam propagate collinearly (longitudinal pumping). This gives good spatial overlap between the two beams, resulting in a significant energy efficiency. If, in addition, the size of the pump beam is adapted to the size of the fundamental mode of the laser cavity, the laser emission is preferably done in this mode.

 For a given diode beam size, the available power is imitated (a few tens of watts maximum in usual cases). The object of the invention relates to an architecture of solid lasers pumped by laser diodes longitudinally and making it possible to obtain a large output power.

 The invention therefore relates to a pumped laser comprising several active laser media placed in series in an optical cavity and at least one optical pump source emitting a pump beam towards each active laser medium; in each active laser medium, the axis of the optical cavity being quasi-collinear or forming an acute angle with the direction of the pump beam, said optical cavity being folded back inside each active laser medium by reflection on a reflecting face of said active laser medium.

The various objects and characteristics of the invention will appear more clearly in the description given by way of example and in the appended figures which represent:
FIG. 1, a basic module of a laser according to the invention;
FIG. 2, an example of a folded laser according to the invention;
FIGS. 3a and 3b, alternative embodiments of the laser according to
the invention;
FIG. 4, a multi-pass laser using only two
active environments;
FIGS. 5a and 5b, an exemplary embodiment in which the
active laser media are arranged in a circle;
FIG. 6, a control of the active media allowing
increase the pulse frequency;
FIG. 7, an application comprising a non-linear medium.

 FIG. 1 represents a basic laser module according to the invention.

This laser module is composed of an active laser medium 1, a pump laser diode 3a and means 3b for coupling the beam coming from the laser diode to the active medium. The cavity is closed by two mirrors 2a and 2b. The mirror 2b allows transmission at the laser wavelength. By active laser medium is meant solid medium having one or more laser transitions (YAG: Nd, YV04: Nd, sapphire: Ti, etc.). By pumping laser diode is meant a unit laser diode, any assembly of laser diodes (strip, stack of strips, surface emission plates, etc.) or any assembly of diodes or unit diode whose beam has been reshaped (cylindrical lens, micromirrors, mirrors, holographic lenses, injection into one or more optical fibers, etc.). By means 3b, to couple the beam coming from the laser diode and the active medium is meant means for focusing the beam 3a in the active medium 1 by a lens or a set of lenses; focusing the pump beam 3a in the active medium by a conical light concentrator; injection of the pump beam 3a by placing the exit face of the laser diode (or of the beam fitness system) very close to the active medium 1 etc.

 The pump beam 3c enters the active medium 1 through the face 1a supporting a dichroic treatment of very large reflection coefficient at the laser wavelength (for example R> 99.5%) and of great transmission cl the length d pump wave (for example T> 95%). The laser beam 4 enters the active medium 1 through the face 1b and is reflected on the face 1a before coming out through the face 1b.

 The laser beam 4 and the pump beam 3c have one. angle of incidence on side 1a zero or small (<15). One thus realizes a longitudinal pumping.

 The face 1a is flat or curved to compensate for the thermal lens induced in the active medium 1 by its heating under pumping. So that the laser beam 4 does not undergo reflection losses on the face 1 b, the latter is either anti-reflective treatment at the laser wavelength or tilted so that the laser beam is at Brewster incidence (polarized beam ). In the first case, the face 1a is flat or curved for example to compensate for the thermal lens.

 Figure 2 shows an example of laser architecture. It is made up of several modules, preferably identical, placed one after the other in the laser cavity. Figure 2 shows a possible embodiment of a cavity composed for example of four pumping modules 5a to 5d. The laser cavity is in this case closed by one of the modules 5a and a mirror 6b having a non-zero transmission at the laser wavelength through which the laser beam exits. The laser media 5a, 5c on the one hand and 5b, 5d on the other hand are arranged along lines L1, L2 arranged on either side of an axis OX. The faces of the laser media furthest from the axis OX with respect to the laser medium are reflective at the laser wavelength so that the laser beam is reflected from a line L1 of laser media to the other line L2 of laser media.

 The addition of intermediate lenses 7a to 7c or of any other beam control system between the pumping modules may prove to be necessary so that the laser beam has the same size in each module regardless of their number. One can also consider the use of active mirrors compensating for phase disturbances and replacing some of the laser modules.

 The advantage of the invention is to be able to combine the efficiency of longitudinal pumping with high laser powers. Modularity has the advantage that it suffices to define a module and then to make the scale factor by multiplying them. Hence, among other things, reductions in design and production costs.

 FIG. 3a represents an alternative embodiment of the architecture of FIG. 2.

 In this architecture, all of the active laser media 5a to 5d are arranged in parallel planes. It is therefore necessary to provide a mirror 6a perpendicular to the laser beam 4 '. All the pump beams emitted by the laser diodes 3aa to 3ad towards the active media are therefore mutually parallel, including that directed towards the active medium 5a.

 FIG. 3b represents a folded structure in which the active laser media 5a to 5f are arranged on two lines parallel to a direction OX and are in the same number on these two lines. The active media 5c to 5f have their reflection face parallel to the direction OX. The active media 5c to 5f have their reflection face parallel to the direction OX. The active media 5a and 5b are placed opposite and return the laser beam. The mirrors 6b and 6b of the optical cavity of the assembly are both arranged on the same side with respect to the set of active laser media.

This produces a ring cavity. By providing a unidirectional optical component in the cavity, a unidirectional cavity is produced.

 FIG. 4 represents a laser architecture derived from that represented in FIG. 3a. It only comprises two active laser media 17a and 17b. The medium 17a includes the media 5a and 5c of Figure 3a and the medium 17b includes the media 5b and 5d.

 FIG. 5a represents an alternative embodiment in which the active laser media 9a to 9g are arranged in a circle. Preferably, it is the reflecting faces of these media which are arranged in a circle. At the laser beam exit location, an exit mirror 19 is placed. A ring cavity has thus been produced. A unidirectional optical element 18 is also provided as in FIG. 3b.

 FIG. 5b represents a variant of FIG. 5a in which the cavity is closed by two mirrors 29 and 39. Instead of producing a ring cavity, the equivalent of a linear cavity is thus produced.

 FIG. 6 represents a control mode making it possible to increase the pulse frequency of the system by time multiplexing. For example, if we consider the architecture of FIG. 3b, the active laser media 5a, 5c, 5e are controlled by pump pulses A as shown in the pulse diagram A and the active laser media 5b, 5d, 5f are controlled by pump pulses B out of phase with respect to pulses A.

 In the various configurations which have just been described, provision has been made to associate with each active laser medium a pump laser diode.

However, it is also possible to arrange the active media so that they are as close as possible to one another so as to be able to pump several active media using the same laser beam. In the context of the configuration of FIG. 4a, in particular, the pump beams 3c1 to 3c3 can be the same beam and the beams 3c4 to 3c6 can also be the same beam.

 A cavity was made (see Figure 3a) with one or two laser modules (5a, 5b, ...); a concave mirror 6a with a radius of curvature 20 cm inclined at an angle of 15 and with a maximum reflectivity of 1.06 µm; a concave mirror 6b with a 10 cm radius of curvature and with a reflectivity of 5% at 1.06 μm. A laser comprising basic modules thus designed typically emits 6W at 1.06 μm (for 12 W of pump). By placing 2 modules in series in the cavity, we obtained 11.5 W, thus demonstrating the advantage of the modular approach.

 The laser diodes can operate in continuous or pulse mode. Likewise, the laser can operate in relaxed mode (continuous or long pulses), or in short pulse mode by adding, for example, a trigger cell in the cavity.

 One can add in the cavity a frequency conversion operation exploiting the non-linear properties of optical materials (doubling, OPO, etc.). Thus, Figure 7 shows a laser having two active media 1a, 1b arranged as described above.

In addition, a non-linear medium 8 is placed in the cavity produced by the mirror 13 and the active medium 1a.

Claims (11)

 1. Pumped laser comprising several active laser media (5a to 5d) placed in series in an optical cavity (6a, 6b) and at least one pump optical source (3a to 3c) emitting at least one pump beam towards the active media lasers; in each active laser medium, the axis of the optical cavity (6a, 6b) being quasi-collinear or forming an acute angle with the direction of the pump beam, said optical cavity being folded back inside each active laser medium by reflection on a reflecting face (1a) of said active laser medium.  2. Laser according to claim 1, characterized in that each active laser medium is pumped by optical pump sources independent of each other.  3. Laser according to one of claims 1 or 2, characterized in that it comprises between the laser media control devices (7a to 7c) of the size and divergence of the laser beam.  4. Laser according to one of claims 1 or 2, characterized in that the laser media are arranged along two parallel lines (L1, L2) located on either side of an axis (OX), the reflecting face ( 1a) of each laser medium being the side of the laser medium furthest from the axis (OX) with respect to the laser medium).  5. Laser according to one of claims 1 or 2, characterized in that the two laser media (5a, 5b) facing each other and located at one end of the two lines (L1, L2) are oriented to transmit the laser beam from so as to constitute a ring cavity.  6. Laser according to one of claims 4 or 5, characterized in that it comprises two laser media (17a, 17b) each located along a line (L1, L2), the laser beam making multiple reflections between the two medialasers .  7. Laser according to one of claims 5 or 6, characterized in that the laser medium (s) of one line (L1) are pumped by a pump beam and the laser medium (s) of the other line (L2) are pumped by another pump harness.  8. Laser according to claim 1, characterized in that the active laser media (9a to 9g) are arranged in a circle.  9. The laser as claimed in claim 1, characterized in that part of the active media is pumped by first pump pulses having a determined phase and another part of the active media is pumped by second pump pulses out of phase with respect to at the first impulses.  10. Laser according to claim 9, characterized in that half of the active media are pumped by the first pulses and that The other half of the active media are pumped by the second pulses, the second pulses being phase shifted by 7c with respect to the first pulses.  11. Laser according to one of the preceding claims, characterized in that it comprises a medium of non-linear material included in said optical cavity.