EP2021827A1

EP2021827A1 - Laser source for lidar application

Info

Publication number: EP2021827A1
Application number: EP07729531A
Authority: EP
Inventors: Simon Richard; Arnaud Brignon
Original assignee: Thales SA
Current assignee: Thales SA
Priority date: 2006-05-30
Filing date: 2007-05-25
Publication date: 2009-02-11
Also published as: FR2901923A1; US20100034222A1; WO2007138013A1; FR2901923B1

Abstract

This application concerns a laser source comprising a principal self-adaptive laser cavity comprising at least a principal gain medium (MA1) set about a principal direction and several mirror (HR) permitting the creation of a gain hologram in the center of the above mentioned principal gain medium by interference of a first optical wave (A1) set about the principal direction and a second optical wave (A3) set about a direction different from the principal direction, the waves being generated by the principal gain medium whose main feature is that it comprises a secondary laser source (SLs) delivering photons at a frequency which these impose on the principal cavity as well as the means for inserting the said photons in the center of the cavity of the principal laser.

Description

Laser source for Lidar application

The field of the invention is that of high energy laser sources for LIDAR systems, especially for optronic, industrial and scientific applications. For example, these systems are widely used in the atmospheric sciences (detection of pollutants and aerosols, dynamic measurements of the movements of air masses and clouds), the sciences of planets (cartography of the reliefs of planets, embedded Lidars on satellites for meteorological measurements). These systems can also be used in aeronautics (airborne Lidars or Lidars in airports) to detect turbulence and increase air traffic while providing increased security.

In general, the spatial and spectral quality of the laser used in a Lidar system as well as its energy and power are crucial and directly determine the overall performance of the system. However, it becomes difficult to maintain a good beam quality when the energy or power of the laser increases. Indeed, the thermal effects within the laser crystal used as amplifying medium, bring strong phase aberrations that contribute to distort the wavefront and reduce the beam quality. Conventional laser sources are therefore often limited in energy / power because of these problems.

In addition, laser sources for LIDAR systems use a number of critical, complex and expensive components to create pulses and to spectrally refine the laser emission:

• To obtain pulses of a few tens of nanoseconds, conventional sources use an active trigger system such as a Pockels cell or an acousto-optic cell.

• To spectrally refine the laser emission, the laser cavity must be enslaved to another low-power single-frequency continuous laser. To achieve this control, it is necessary to mount one of the mirrors of the laser cavity on a piezoelectric wedge to adjust by means of an electronic feedback loop the length of the cavity. The length of the laser cavity must thus be monitored in real time by means of an electronic feedback loop. FIG. 1 illustrates this type of laser source: the cavity comprises between 2 mirrors R1 and R2, an amplifying medium MA1 can typically be a laser bar in Nd ³⁺ Υ3Al5O1 ₂ (Nd: YAG) pumped by lamps or diodes, the length L of the cavity is thus defined between the mirrors R1 and R2. In order to benefit from a pulsed source of high energy, the cavity also comprises a trigger, not shown, which acts as a switch enabling, after a certain time_accumulation of energy within said cavity, to release the beam laser. To force the laser cavity to oscillate in a single longitudinal mode corresponding to a single frequency, this laser cavity is enslaved by a small so-called secondary laser cavity SL with respect to the previously defined main cavity. The small secondary laser having a cavity length I, is a single-frequency laser that can inject into the primary laser cavity photons hv at a single frequency v. The laser beam in the main cavity oscillates preferentially on this frequency provided that this frequency corresponds to a resonant frequency of the primary cavity. For this condition to be satisfied it is necessary that the respective lengths of the cavities satisfy the equation: L / l = Nλ with N integer and λ the laser wavelength

This condition is satisfied by introducing into the primary laser cavity a piezoelectric shim C1 making it possible to adjust the length L of the primary laser cavity for any operating frequency.

In order to spatially control (correction of laser crystal aberrations), temporally (pulse generation) and spectrally (single frequency operation) the laser, it has also been proposed another type of source architecture that uses the four-wave mixing in the laser medium as illustrated in Figure 2 and which is described in particular in the following articles: Bel'dyugin et alii, Solid-state lasers with self-pumped phase-conjugate mirrors in an active medium, Sov. J. Quantum Electron., Vol. 19, pp. 740-742 (1989); Damzen, Green and Syed, Self-adaptive solid-state laser oscillator formed by dynamic gain-grating holograms, Optics Letters, vol. 20, pp. 1704-1706 (1995); Sillard, Brignon and Huignard, Gain- grating analysis of a self-starting self-pumped phase-conjugate Nd: YAG loop resonator, IEEE J. Quant. Electron, vol. 34, pp. 465-472 (1998). Such an architecture makes it possible to obtain a single-frequency emission without the use of a secondary laser.

This ring laser is formed of an output mirror having a low reflectivity R1 (typically 4% -10%) and an amplifying medium MA1 (laser head 1) in which the waves inscribe a dynamic gain hologram.

For this, the mirrors are arranged in such a way that it is possible to interfere with waves in different directions. The amplifying medium generates waves in all directions, only some of which can be amplified in the laser cavity. In FIG. 2, the interference phenomenon is represented diagrammatically by the interference of the waves A1,

A3. The waves A1 and A3 register a transmission gain network, also called amplitude hologram. The A2 wave reads the network and generates a diffracted A4 wave.

The waves A2 and A3 also inscribe a reflection network which is read by the wave A1.

The wave A1 is thus called pump wave because registering a network in transmission. Wave A2 is also referred to as a pump wave because it inscribes a grating in reflection.

The A3 wave is a signal wave

The wave A4 is a conjugate wave of playback of the networks inscribed in the amplifying medium. The hologram corresponding to the amplitude networks inscribed, is under certain conditions a phase conjugation mirror, ie the wave A4 is the conjugate wave in phase of the signal wave A3. If the A3 wave has undergone phase distortions during its propagation in the cavity, the phase conjugated A4 wave will correct its aberrations during its reverse propagation in the cavity. . Such a phase conjugation mirror will thus make it possible to compensate for the phase aberrations of the laser media and thus to create an output beam of good spatial quality.

For the gain hologram to be effective, the contrast of the interference fringes must be high. It is therefore important that the waves A1 and A3 have amplitudes of the same order of magnitude. To favor this phenomenon is introduced into the cavity a non-reciprocal element ENR, to introduce losses in the clockwise direction shown in Figure 2 and not in the opposite direction. The non-reciprocal element may typically consist of a Faraday rotator, two polarizers, and a half-wave plate.

Initially, the process is initiated by the spontaneous emission from the MA1 amplifying medium. The waves A1, A2, A3 and A4 from this noise start to register the gain hologram. This hologram has a diffraction efficiency η. It is also possible to introduce other lasers having a gain G (MA2 shown in Figure 2) to increase the efficiency of the system. The losses of the cavity for an oscillating wave in the counterclockwise direction in FIG. 2 are denoted by the letter T. When ηx G χ T> 1, the oscillation condition is verified and the 4 waves at inside the cavity become more and more intense at each turn in the cavity. The output intensity of the laser increases in proportion. In a few tens of nanoseconds, the amplification of the beam extracts all the energy stored in the amplifying media and the oscillation stops. The laser thus provides a light pulse.

The laser emission is naturally mono-frequency, the gain hologram realizing a spectral filter of great fineness.

Nevertheless, although this type of self-adaptive laser cavity is single frequency, from one pulse to another this frequency can vary.

To solve this problem, the present invention proposes a new laser source of the same type with audo-adaptive cavity with four-wave mixing and having a secondary source for forcing the main source to operate on the frequency imposed by this small auxiliary source.

More specifically, the subject of the invention is a laser source comprising a main self-adaptive laser cavity comprising at least one main amplifying medium in a main direction and several mirrors making it possible to create a gain hologram within said main amplifying medium by means of interference. a first optical wave in the main direction and a second optical wave of different direction, said waves being generated by the main amplifying medium. The source further comprises a secondary laser source delivering photons at a frequency they impose on the main cavity and means for introducing said photons into the main laser cavity.

Advantageously, the secondary laser source is placed in the direction of the second optical wave. Advantageously, the laser source comprises a non-reciprocal element making it possible to create losses in a non-reciprocal manner on waves circulating in one direction or the other within the main laser cavity.

This element can be composed of a Faraday rotator, two polarizers and a half-wave phase plate.

According to a variant of the invention, the mirrors are highly reflective, the laser beam being extracted from the main laser cavity by the loss path generated by the non-reciprocal element.

According to a variant of the invention, the laser source further comprises optical means for creating a homothety on the first and second waves so as to compensate for the divergence which affects the waves after propagation within the main laser cavity.

The optical means may be of the pair type convergent lens and divergent lens. They can be placed near the main amplifier medium.

According to a variant of the invention, the main cavity comprises at least a second amplifying medium for increasing the gain of amplification within the main laser cavity. According to a variant of the invention, this second amplifying medium can be advantageously replaced by two amplifying media between which is placed a 90 ° polarization rotator to compensate for the effects of depolarization introduced by thermal effect in these two amplifying media.

The invention will be better understood and other advantages will appear on reading the description which follows and with reference to the appended figures among which:

FIG. 1 illustrates an example of a laser source for Lidar according to the known art; FIG. 2 illustrates an example of a laser source for Lidar comprising a self-adaptive cavity with four waves according to the known art;

FIG. 3 illustrates a first example of a laser source for Lidar according to the invention;

FIG. 4 illustrates a second example of a laser source for Lidar according to the invention;

FIG. 5 illustrates a third example of a laser source according to the invention comprising a split amplifying medium and a 90 ° polarization rotator.

In general, the laser source according to the invention comprises an amplifying medium within which is created a transmission gain network as described in a self-adaptive cavity according to the prior art. To solve the problem of the frequency of the laser that can change from one pulse to the next, which can be of the order of 1 GigaHertz according to the prior art, the invention proposes to use a small laser of low power to to inject photons into the main laser cavity, these photons are amplified and impose their frequency on the main laser cavity. In conventional injection systems, in order for the injected photons to be amplified, their frequency must be resonant with the natural frequencies of the cavity, as has been explained in the preamble of the invention. One of the mirrors of the cavity to be injected must be placed on a piezoelectric wedge to adjust the length of the cavity by means of an electronic feedback loop.

According to the invention, there is no longer any need to control the length of the cavity to be injected since the cavity is self-adaptive and closed by a non-linear mirror formed by the four-wave mixing of the beam present in the cavity. The dynamic gain hologram that forms in the main amplifier medium is therefore automatically adapted to the frequency of the continuous laser.

FIG. 3 illustrates a first example of a laser according to the invention advantageously using 3 amplifying media. Indeed, it will generally be preferred to use a plurality of gain media to obtain a maximum gain (equal to the sum of the gains of each amplifying medium) within the cavity. laser: a first amplifying medium MA1 in which is generated the interference gain hologram of the A1 and A3 waves and A2 and A3 waves, a second amplifying medium MA2 and a third amplifying medium M A3. The so-called secondary low frequency single-frequency laser source SLs is injected into the main laser cavity via a non-reciprocal element of Faraday isolator type Is to avoid beam returns from the main cavity towards said secondary source. A set of high reflectivity HR mirrors makes it possible to constitute the main laser cavity as in the configuration of the known art comprising a self-adaptive cavity. Advantageously, however, the mirror R1 corresponding to the output mirror of the prior art is replaced by a highly reflective mirror Rmax to reduce losses in the cavity and the output laser beam Fs is recovered at the non-reciprocal RF element which can typically be composed of a Faraday rotator and a half-wave phase plate, inserted between two polarizers PoH and Pol2. Typically, it has been experimentally validated that the uses of the mirror Rmax and the output at the non-reciprocal element allowed an increase of the output energy by a factor between 2 and 3.

According to a variant of the invention, it is also proposed to use optical means to compensate for the divergence that is created on the amplified intra-cavity laser beam. These optical means may advantageously be of the telescope type.

FIG. 4 illustrates such a configuration in which a convergent and divergent lens assembly is introduced near the amplifying medium MA1 in which interference is produced. This telescope makes it possible to adapt the size of the beam to that of the laser bars. By calculating in particular the evolution of the beam size in the cavity, it appears that without a telescope, the beam could naturally reach very large dimensions (maximum diameter of 10 mm at the A1 and A2 waves). However, the laser bars generally have a smaller dimension (typically 4 - 7 mm in diameter). This results in a very large vignetting effect which greatly degrades the beam quality and the stability of the laser. The telescope makes it possible to reduce the size of the beam so that it always remains adapted to the size of the laser bars. This Tel telescope can have a typical magnification value of 1.5 (for example a divergent focal lens -100 mm associated with a convergent focal lens +150 mm).

The telescope thus allows an improvement of the beam quality and a better stability of the overall performances of the laser.

In general, lasers delivering high power pulses undergo heating problems generating depolarization problems. However, most laser applications require a polarized output beam in particular to be able to perform frequency conversion operations in nonlinear crystals. On the other hand depolarization directly affects the beam quality and can decrease the output energy of the laser. When lasers are used at a high rate (typically> 100 Hz), depolarization effects become extremely troublesome. In order to partially compensate for this effect, a variant of the invention proposes to split the amplifying medium which is just before the laser output as shown in FIG. 5. The use of 2 identical amplifying media MA2, MA2 'and of FIG. a 90 ° polarization rotator between the two makes it possible to compensate for the depolarization of these two amplifying media. This figure also illustrates lens positioning f1, f2, f3, f4 for adapting the beam within the cavity.

Example of realization

The laser source according to the invention comprises:

- a small continuous secondary laser source, delivering a few hundred μW

the self-adaptive cavity comprises 3 Nd: YAG amplifying media pumped by a flash lamp or by laser diodes at 100 Hz. 2 amplifying media corresponding to the media FIG. 6 may be laser bars of diameter equal to 6 mm and each having a gain g ₀ L of 3.5, the factor exp (g ₀ L) corresponding to the amplification factor of the beam. laser within the cavity. The output energy obtained can thus be greater than 300 mJ delivering pulses of 20 ns with a beam quality 1, 5 times the diffraction limit.

With bar diameters of 10 mm, in a configuration identical to the previous one, the output energy delivered becomes of the order of the Joule.

Claims

A laser source comprising a self-adapting main laser cavity comprising at least one main amplifying medium (MA1) in a main direction and several mirrors (HR) for creating a gain hologram within said main amplifying medium by interference of a first wave optical (A1) in the main direction and a second optical wave (A3) in a direction different from the main direction, said waves being generated by the main amplifying medium characterized in that it further comprises: a secondary laser source (SLs ) delivering photons at a frequency that they impose on the main cavity and means for introducing said photons (Is) into the main laser cavity; a non-reciprocal element (RF) for non-reciprocally creating losses on waves flowing in one direction or the other within the main laser cavity; the mirrors being highly reflective and the laser beam being extracted from the main laser cavity from the non-reciprocal element which generates losses.

2. laser source according to claim 1, characterized in that the secondary laser source is placed in the direction of the second optical wave.

3. laser source according to one of claims 1 or 2, characterized in that the non-reciprocal element is composed of a Faraday rotator.

4. laser source according to one of claims 1 to 3, characterized in that it further comprises a polarization rotator to compensate for the depolarization effects introduced by the thermal effects in the amplifying media introduced into the cavity.

5. laser source according to one of claims 1 to 4, characterized in that it further comprises optical means (Tel) to create a homothety on the first and second waves so as to adapt the diameter of the beams to the diameter of the or amplifying media of the cavity.

6. laser source according to claim 5, characterized in that the optical means are of the pair type convergent lens and divergent lens.

7. laser source according to one of claims 5 or 6, characterized in that the optical means are placed in the vicinity of the main amplifying medium.

8. laser source according to one of claims 1 to 7, characterized in that the main cavity comprises at least a second amplifying medium (MA2, MA2 ') for increasing the gain of amplification within the main laser cavity.