FR2987179A1 - Impulse laser source for use in microlaser for e.g. laser telemetry, has unit delivering pulse between electrodes for generating medium between electrodes to provide gradient of electric charges in temporary direction perpendicular to axis - Google Patents

Abstract

In one aspect, the invention relates to an active trigger pulse laser source comprising a source for emitting a predetermined light power pump beam and a given pump wavelength for optically pumping a laser cavity (10). ) comprising an amplifying medium (13) and delimited by two mirrors (11, 12) substantially parallel, for the transmission along a preferred propagation axis (18) of the cavity of a laser pulse at a laser wavelength. mirrors have relative to each other an angular offset, of such a value that the cavity is below the laser emission threshold at the given power of the pump beam. The laser source further comprises two electrodes positioned on either side of said preferred propagation axis and means for delivering an electrical pulse between the electrodes, for generating in the medium between said electrodes a temporary electrical charge gradient in one direction substantially perpendicular to said preferred axis of propagation and the deflection of a light beam propagating in the cavity such that the laser emission threshold is reached in said optical cavity at the given power of the pump beam.

Description

STATE OF THE ART Technical Field of the Invention The present invention relates to an active trigger laser source and a method of active triggering of a laser source; it applies in particular to microlasers. State of the art Microlasers have a wide variety of applications such as laser telemetry, spectroscopy, supercontinuum generation, lidar. These miniature lasers can operate at different wavelengths, in continuous or pulsed mode. In triggered microlasers (or "Q-switched" according to the English expression), a miniature resonator whose optical cavity is less than a few centimeters, makes it possible to generate short pulses typically ranging from a few hundred picoseconds to a few nanoseconds. Microlasers can be triggered actively or passively. In passive trigger microlasers, a nonlinear semiconductor saturable absorber may be used as a cavity mirror, or alternatively a saturable absorber crystal. However, the performance of passive trigger microlasers is inherently limited by significant time jitter effects. The temporal jitter effects can be reduced by the implementation of an active trigger. Active tripping requires an actuator element such as an electro-optical modulator or an acousto-optic modulator. These elements, however, have the disadvantage of being chromatic. Moreover, they are difficult to miniaturize in the case of application to microlasers. An active trigger based on the use of an electromagnetic discharge tube ("shock tube" according to the English expression) has also been implemented in a ruby laser (M. Dembinski et al., "Refractive Qswitching of a Tuby Laser by a Plasma Plasma ", Applied Optics, Vol 21, No. 20). In this device, an index gradient induced by a charge gradient is generated by means of a discharge tube arranged in the laser cavity, initially misaligned. The index gradient is generated in a direction perpendicular to that of the light beam, causing a deflection of the beam which allows the laser to be triggered by realigning the cavity. This device however has a size not compatible with microlasers. Moreover, the temporal jitter in such a device is important because of the index gradient establishment time generated by the reflected shock wave, making this technique not compatible with the triggering of short pulse lasers. The invention proposes a method of active tripping of a laser cavity, especially a miniature laser cavity, incorporating an ultra-compact modulator, capable of operating at any wavelength and having a reduced temporal jitter, compatible with pulsed laser sources. short.

SUMMARY OF THE INVENTION According to a first aspect, the invention relates to an active trigger pulse laser source comprising a source of emission of a pump of predetermined light power pump and pump wavelength xp given, intended to optically pump a laser cavity comprising an amplifying medium and defined by two substantially parallel mirrors, for the transmission along a preferred propagation axis of the cavity of a laser pulse at a laser wavelength XL. The mirrors have an angular offset with respect to each other, of such a value that the cavity is below the laser emission threshold at the given power of the pump beam.

The laser source further comprises two electrodes positioned on either side of said preferred propagation axis and means for delivering an electrical pulse between the electrodes, for generating in the medium between said electrodes a temporary electrical charge gradient in one direction substantially perpendicular to said preferred axis of propagation and the deflection of a light beam propagating in the cavity such that the laser emission threshold is reached in said optical cavity at the given power of the pump beam. The applicants have shown that the index gradient thus obtained allows an irreversible start of the laser oscillation, the resulting laser pulse being created while the index gradient is dissipated, which allows better control of the time jitter.

Advantageously, the duration of the electric pulse is less than 10 ns, advantageously less than 1 ns, a short electric pulse making it possible to increase the time between the dissipation of the index gradient and the creation of the laser pulse.

According to one variant, the electrical pulse is bipolar, making it possible to better control the moment of dissipation of the index gradient. According to one variant, the medium between said electrodes consists of air. The index gradient results from an ionization of the air, forming an electric arc.

According to a variant, the medium between said electrodes comprises a semiconductor material at least partially transparent to the laser emission wavelength. The creation of charges in a semiconductor medium can be obtained with lower voltages than in the air. According to one variant, the active trigger laser source is a microlaser, the length of the optical cavity between the two mirrors being less than 2 cm. According to one variant, the cavity further comprises a saturable absorber crystal, making it possible to reduce the duration of the emitted laser pulse. According to one variant, the distance between the electrodes is substantially equal to 4 times the diameter of the laser beam taken at 1 / e 2 in intensity. This distance makes it possible to limit possible effects of diffraction of the laser beam on the ends of the electrodes. According to a second aspect, the invention relates to a method of active triggering of a laser source according to the first aspect comprising: - adjusting the cavity to obtain at a given power of the pump beam, the emission of a laser beam ; - the adjustment of the optical cavity so as to introduce said angular offset between the mirrors of the cavity, causing the passage of the optical cavity below the laser emission threshold to said pump power; the application of an electrical pulse between the electrodes for the generation of a laser pulse.

According to one variant, the method of active triggering of a laser source further comprises, after the step of adjusting the optical cavity: the increase of the pump power, resulting in a laser emission according to a first mode; - The application of an electrical pulse between the electrodes for switching the laser emission in a second mode.

BRIEF DESCRIPTION OF THE DRAWINGS Other advantages and characteristics of the invention will appear on reading the description, illustrated by the following figures: FIG. 1, a diagram illustrating the plasma prism principle in an exemplary embodiment of the invention; - Figure 2, the optical response of an electric discharge generated between two electrodes, in an exemplary embodiment; FIG. 3, an example of an active trigger laser source, according to a first exemplary embodiment; FIGS. 4A to 4C, the temporal shape of a laser pulse obtained by active triggering in a laser source of the type of that of FIG. 3, and the spatial distribution of light intensity of the pulse; FIG. 5, an example of an active trigger laser source, according to a second exemplary embodiment; FIGS. 6A to 6C, the temporal shape of a laser pulse obtained by active triggering in a laser source of the type of that of FIG. 5, and the spatial distribution of light intensity of the pulse; - Figures 7A and 7B two examples showing the time and frequency of electric pulses adapted to create a plasma prism in an embodiment of the invention; - Figure 8, the spatial distribution of the light intensity of laser beams obtained in an exemplary embodiment of the invention.

DETAILED DESCRIPTION FIG. 1 illustrates by a diagram the principle of producing an active trigger laser source according to an exemplary embodiment.

The laser source comprises a laser cavity 10 mainly comprising an amplifying medium 13 between two mirrors 11 and 12 forming a preferred propagation axis z for an incident pump beam. The two mirrors have relative to each other a deflection angle θ whose value is less than a few degrees, the value is for example a few fractions of degrees (the value of the angle θ represented in FIG. very largely exaggerated for better understanding). The angular disturbance of the cavity is such that for a given light power of the pump beam, the cavity is just below the stimulated laser emission threshold. Two electrodes symbolized by arrows 14 and 15 are connected to the terminals of an electric pulse generator (not shown). The rapid passage of charges between the two electrodes makes it possible to generate an ionization of the air, forming a plasma or electric arc 19, which makes it possible to temporarily modify the refractive index of the air and to obtain a deflection of the orientation of a light beam passing through said plasma ("plasma prism"). An incident light beam (16, FIG. 1) can then be deviated from its trajectory during the duration of the phenomenon. As will be described in more detail below, by limiting the plasma prism effect over time, it is possible to modify the resonator-related overvoltage coefficient and to start the laser oscillation with a very good control of the moment of oscillation. creation of the laser pulse. In FIG. 1, the effect of the plasma prism is evidenced by sending a laser beam 18 with a wavelength of 1.064 μm between the two electrodes. The beam is observed by placing an infrared camera at a given distance from the electrodes (72 cm in the example of Figure 1). In this example, the experimental device for carrying out the electric discharge comprises two electrodes connected to the terminals of a high-voltage generator equipped with a circuit breaker. The electrodes are made from two copper tips electrically insulated from the rest of the system, for example by keeping them in a hollow teflon tube. The distance between the copper tips is in this example about 7 mm. In this configuration, the appearance of an electric discharge is observed as soon as the voltage becomes higher than 6 kV, due to the ionization of the air. During the duration of the electric discharge between the two electrodes, there is an angular deviation of the laser beam in a plane perpendicular to the axis of propagation of the charges (yz plane). It is thus observed the modification of the position of the light spot formed by the laser beam on the camera (position (a) before the discharge, position (b) during the discharge). When the electric discharge stops, the light spot returns to its initial position (position (a)). The measured deviation is 4.5 mrad. As shown in the images (a) and (b) of FIG. 1, no significant spatial deformation of the beam is observed. The deflection of the light beam passing through the electric discharge can be explained by the index gradient which is established during the passage of the current, perpendicular to the x axis of the electric discharge, this index gradient being due to the modification electron density. Applicants have shown that the plasma prism effect thus demonstrated could be used to trigger a laser source with reduced temporal jitter by delivering an electric pulse across the electrodes to generate an ultra-fast response plasma prism. fast. The optical response of an electric discharge generated between two electrodes is shown in FIG. 2. Each time a plasma is created between the two electrodes, this plasma resulting from the ionization of the air, a white light pulse of high intensity is observed (electric arc), at the same frequency as the rate of electric discharges between the electrodes. By measuring the duration of these plasma-linked light pulses, the lifetime of the "plasma prism" can be determined. To measure the optical response of the plasma, a fast photodiode connected to an oscilloscope can be used. At half height, the detected optical response is about 50 ns, with a rise time of about 10 ns. The half-height optical response corresponds to the lifetime of the plasma. The visible oscillations at the tail of the signal correspond to the artifacts caused by the residual electromagnetic disturbances. FIG. 3 illustrates a first embodiment of an active trigger source, implementing the principle previously described. The optical cavity 30 is delimited by two planar mirrors 31, 32 whose reflection coefficients are respectively 100% and 95% at the wavelength of 1.064 i_tm of laser emission. The mirrors are about 50 cm apart. A 4mm Nd: YAG amplifier crystal is placed inside the cavity. The cavity further comprises a lens system 34 for collimating the beam along the length of the cavity. The cavity is pumped longitudinally through the mirror 31 by a laser diode 35 of wavelength 808 nm corresponding to the absorption band of the neodymium ion. The radiation is derived from the pump laser diode 35 by an optical fiber 36 and is focused in the amplifying medium 33 by a doublet of lenses 37. The two electrodes (not shown in FIG. 3) are placed inside the cavity so that the light oscillates between the mirrors passing between the electrodes. The beam is observed using an infrared camera placed at the cavity outlet (not shown in FIG. 3). At first, no voltage is applied between the electrodes. The cavity is then adjusted so as to obtain for a given power pump beam a Gaussian beam whose image is shown in FIG. 4C. Then the cavity is very slightly disturbed so as to be placed, at equal pump power, just below the stimulated emission threshold. Then, a voltage is applied between the electrodes. It is observed that each time an electric plasma is created 39, the image of the Gaussian beam reappears temporarily on the screen. An active trigger is thus realized.

The light emitted by the laser pulse thus triggered is detected by a fast photodiode connected to an oscilloscope. Figure 4A shows the signal recorded during the trip. We first observe the trace 41 of the radio signal associated with the electric discharge in the form of oscillations damped for about 1! As. It is noted that the acquisition of the oscillogram is triggered on the rising edge of the electromagnetic disturbance. From this moment, the time of creation of the pulse 42 (Figure 4B) is 10.8! As. The duration of the laser pulse is 730 ns. The time of creation of the pulse then depends on the characteristics of the plasma and the laser gain in the cavity. This example shows that the "closing" time of the laser cavity during which the optical cavity is aligned is small. The plasma prism thus generated allows a momentary reduction of the losses in the optical cavity and an irreversible start of the laser oscillation, the resulting laser pulse being created while the plasma prism is dissipated. The distance between the electrodes is as small as possible to obtain a plasma prism effect with a limited electrical voltage, but large enough not to induce a loss on the beam by diffraction at the electrodes. For example, the distance between the electrodes is chosen to be substantially equal to four times the diameter of the laser beam taken at 1 / e 2 in intensity. FIG. 5 illustrates a second exemplary embodiment of an active trigger laser source applied more particularly to a miniature laser ("microlaser"). The optical cavity 50 is formed of two planar mirrors 51, 52 whose reflection coefficients are respectively 100% and 95% at the wavelength of 1.064 i_tm of laser emission. The mirrors are about 1 cm apart. An Nd: YAG crystal 53 of 4 mm in length and a crystal 54 of CR4 +: YAG are placed inside the cavity. This is pumped longitudinally through the mirror 51 by a laser diode 55 of wavelength 808 nm, the radiation of which is focused in the amplifying medium 53 by a doublet of lenses 57. The two electrodes (not shown in FIG. Figure 5) inside the cavity so that the light oscillates between the mirrors passing between the electrodes. The beam is observed with the aid of an infrared camera placed at the cavity outlet (not shown in FIG. 5). As in the previous example, the optical cavity is very slightly out of adjustment so that at a given power of the pump beam sufficient for the laser emission, the stimulated emission threshold is not reached. In the example of FIG. 5, the CR4 +: YAG crystal acts as a saturable absorber whose effect combines with active plasma triggering to refine the triggered laser pulse. FIG. 6B shows an example of a laser pulse thus obtained. Alternatively, only an active trigger can be implemented. Figure 6C shows the spatial profile of the laser pulse created. FIG. 6A illustrates the delay of creation of the pulse with respect to the generation of the plasma prism and a measurement of the temporal jitter. In this example, the pump power is 3W, with a laser gain mode of 1001.1m in diameter. A delay of 3.54 μs laser pulse generation is measured with respect to the onset of the electric discharge with a rising edge of 50 ns. Time jitter of approximately 300 ns is measured. This time jitter, measured from the rising edge of the plasma trigger is then 40 to 45 times lower than that which is used only by the passive trigger. The time of creation of the laser pulse here represents more than 45 times the lifetime of the plasma prism. The temporal jitter between the plasma trigger signal and the arrival time of the laser pulse can be minimized by decreasing the duration of the electrical pulse that generates the electrical discharge between the electrodes. FIGS. 7A and 7B show, according to examples, forms of suitable electrical pulses. The example of FIG. 7A shows an electrical pulse of substantially monopolar shape, the curves 71 and 72 respectively representing the time component and the frequency component of the pulse. The example of FIG. 7B shows an electrical pulse of substantially bipolar shape, the curves 73 and 74 respectively representing the time component and the frequency component of the pulse. In the case of a bipolar electric pulse, the inversion of polarity of the electrical voltage makes it possible to better control the disappearance of the plasma prism and thus to control the closing time of the laser cavity.

In the examples illustrated in FIGS. 3 and 5, a plasma is generated in the air, between the two electrodes. Alternatively, the medium between the two electrodes may comprise a semiconductor material that is transparent or partially transparent at the laser emission wavelength. For example, this material may be silicon (Si), gallium nitride (GaN), zinc sulphide (ZnS), indium phosphide (InP), gallium arsenide (GaA). The creation of a charge density sufficient to obtain the deflection of the light beam and the exceeding of the laser emission threshold in the cavity in a semiconductor can then occur for an electric voltage between the electrodes lower than that required for the establishment of the same gradient in the air. Doping materials is adapted to control their internal resistivity. FIG. 8 illustrates an embodiment variant according to which the cavity being out of adjustment, the power of the pump beam is increased to pass above the laser emission threshold in the cavity. This results in the emission of a particular laser mode (71) different from the fundamental laser mode of the cavity. By applying an electric pulse between the electrodes, a mode switch is generated in this way. The mode represented at 71 is transformed into a circular mode 73. Although described through a certain number of detailed exemplary embodiments, the active triggering method according to the invention and the laser source triggered by said method comprise different variants, modifications and improvements which will become apparent to those skilled in the art, it being understood that these various alternatives, modifications and improvements are within the scope of the invention, as defined by the following claims.

Claims (10)

REVENDICATIONS1. An active trigger pulse laser source comprising: - a source of emission (35, 55) of a predetermined light power pump beam and a given pump wavelength xp, - a laser cavity (10, 30, 50 ) comprising an amplifying medium (13, 33, 53) arranged between two mirrors (11, 12) substantially parallel, the laser cavity having a preferred axis of propagation and being intended to receive said pump beam for emission along said axis of propagating a laser pulse at a laser wavelength XL, the mirrors having, with respect to each other, an angular offset, of such a value that the cavity is below the laser emission threshold at the power pump beam data, - two electrodes positioned on either side of said preferred propagation axis and means for delivering an electrical pulse between said electrodes, for generating in the medium between said electrodes; electrodes a temporary electrical charge gradient in a direction substantially perpendicular to said preferred axis of propagation and the deflection of a light beam propagating in the cavity such that the laser emission threshold is reached in said optical cavity at the given power pump beam. 2. Active trigger laser source according to claim 1, wherein the duration of the electric pulse is less than 10 ns, preferably less than 1 ns. An active trigger laser source according to any one of the preceding claims, wherein the electrical pulse is bipolar. An active trigger laser source according to any one of the preceding claims, wherein the medium between said electrodes is air. An active trigger laser source according to any one of claims 1 to 3, wherein the medium between said electrodes comprises a semiconductor material at least partially transparent to the laser emission wavelength. An active trigger laser source according to any one of the preceding claims, wherein the length of the optical cavity between the two mirrors is less than 2 cm. An active trigger laser source according to any one of the preceding claims, wherein the cavity further comprises a saturable absorber crystal. 8. Active trigger laser source according to any one of the preceding claims, wherein the distance between the electrodes is substantially equal to 4 times the diameter of the laser beam taken at 1 / e2 intensity. 9. A method of active triggering of a laser source according to any one of the preceding claims comprising: - adjusting the cavity to obtain at a given power of the pump beam, the emission of a laser beam; - the adjustment of the optical cavity so as to introduce said angular offset between the mirrors of the cavity, causing the passage of the optical cavity below the laser emission threshold to said pump power; the application of an electrical pulse between the electrodes for the generation of a laser pulse. The method of active triggering of a laser source according to claim 9, further comprising, after the step of adjusting the optical cavity: increasing the pump power, resulting in a laser emission according to a first mode ; the application of an electrical pulse between the electrodes for switching the laser emission according to a second mode.