Patent ID: 12199399

DESCRIPTION OF EMBODIMENTS

Figures and the following detailed description contain, essentially, some exact elements. They can be used to enhance understanding the invention and, also, to define the invention if necessary.

As represented onFIG.1, it is provided a laser device for LiDAR with circularly polarized pumping beam. The laser device comprises:a set of elements acting functionally as a laser chip30for emitting a laser beam B4at a laser wavelengtha set of elements acting functionally as a laser beam amplifier9;a set of elements acting for functionally pump the laser chip30as well as the laser beam amplifier9.

The elements of the laser device are optically connected in free space. In other words, the elements of the laser device are not connected by optical fibers.

For the sake of clarity, the following description of the laser device ofFIG.1is divided into two sub-parts referring respectively toFIG.2andFIG.3.FIG.2describes more specifically the set of elements acting functionally as a laser chip30for emitting a laser beam B4at a laser wavelength and the set of elements acting functionally as a laser beam amplifier9.FIG.3describes more specifically the set of elements acting for functionally pump the laser chip30as well as the laser beam amplifier9. It will be understood that the laser ofFIG.1comprises all the set of elements acting together.

Laser Chip

The laser chip30is described below with reference toFIG.2. The laser chip30is configured to emit a laser beam B4. For the sake of clarity, the laser beam B4along the laser path is referred to as the laser beam B41at the output of the laser chip30, to as the laser beam B42at the input of the laser amplifier9, and to as the laser beam B43at the output of the laser device. The laser direction X1is the laser path, in the direction of transmission of the laser beam B4.

As pictured onFIG.2, the laser chip30mainly comprises a solid state oscillator material4, a saturable absorber medium3, and two reflective elements31,32which define an optical resonator, also referred to as a laser cavity. The elements31,3,4and32are sequentially stacked as described below such as to form together a monolithic body also referred to as “microchip”.

The solid state oscillator material4causes stimulated emission of photons at the laser wavelength when pumped by an optical pump beam B2.

The saturable absorber medium3causes losses until saturation, when it does almost not cause losses anymore. Therefore a brutal increase of the gain of the laser is observed when the loss in the absorber medium becomes brutally negligible.

The reflective element32is partially reflective at the laser wavelength (between 75% and 95%) whereas the reflective element31is highly reflective (almost 100%). Therefore the laser beam B41gets out the laser cavity through the reflective element32.

The solid state oscillator material4and the saturable absorber medium3are bonded together such as to form the monolithic body. For instance, the bonding is performed by diffusion bonding or by deposition of the saturable absorber on the laser medium by liquid phase epitaxy.

For instance, the materials for the solid state oscillator material4is a doped uniaxial crystal, such that the laser beam B4at the output of the laser is polarized. For instance, the uniaxial crystal may have the chemical formula AVO4 with A selected in the list consisting in Y, Lu and Gd. For instance the doping ions are neodymium, such as Nd3+. Preferably, but not limitatively, the materials for the solid state oscillator material4is Nd:YVO4

Therefore, in absence of any major additional polarization selective element in the laser cavity, the laser light will be polarized along the vertical crystallographic axis, also referred to as “c-axis”, of the crystal. Indeed, the laser gain is much higher along the c-axis than the horizontal crystallographic axis passing from front to back, also referred to as “a-axis”, for such materials.

The material for the saturable absorber medium3is a semiconductor saturable absorber (SESAM).

The laser cavity between the two reflective elements31,32is of length L=0.5 mm, and the emitted laser wavelength is 1064 nm.

The reflective elements31and32may be formed by multiple stacks of dielectric and/or semiconductor layers.

Laser Beam Amplifier

As represented onFIG.2, the laser beam B41is emitted by the laser chip30along the optical axis of the laser chip30. A laser beam amplifier9is disposed on the path of the laser beam B42, along the optical axis of the laser chip30.

It is possible the laser beam amplifier9is composed from the same material, for instance YVO4, as the solid state oscillator material4. However, the active ion, for instance Nd3+, doping level may be different. For instance, the doping level of the laser beam amplifier9may be 0.5% whereas the doping level of the solid state oscillator material4may be 1%.

The laser beam amplifier9causes stimulated emission of photons at the laser wavelength when pumped by an optical pump beam B3. Therefore the laser beam amplifier9amplifies the laser beam B42on its path and transmits an amplified laser beam B43.

Advantageously, the amplified laser beam B43has a power which is about 10 times the power of the laser beam B42.

FIG.2further represents a pumping unit10on the path of the laser beam B4. The pumping unit10comprises a polarizer62and a polarizing beamsplitter7.

Travelling through the polarizer62, the laser beam B4is p-polarized as represented by the arrows. The polarizing beamsplitter7does not impact the travel of the p-polarized laser beam B4, as it will be further described below.

Functional Double Pumping

FIG.3represents how to functionally achieve to pump both the laser chip30and the laser beam amplifier9using the same unique polarized laser diode1.

In general, the pumping unit10is configured for receiving a pump beam B1from the unique linearly polarized laser diode1. The linearly polarized laser diode1is arranged such that its optical axis is perpendicular to the optical axis of the laser chip30and that the direction of emission intersects the pumping unit10, and in particular the polarizing beamsplitter7. The pumping unit10further comprises a quarter wave plate61disposed between the laser chip30and the beamsplitter7along the laser path.

For the sake of clarity, following the pump beam travel on theFIG.3, the pump beam B2along the laser path in the opposite direction X2to the laser direction X1is referred to as:the pump beam B21at the input of the quarter wave plate61, which originates from the pump beam B1deviated by the beamsplitter7,the pump beam B22at the output of the quarter wave plate61, which originates from the pump beam B23having travelled through the quarter wave plate61.

The pump beam B3is referred to as:the pump beam B33, at the input of the quarter wave plate61, wherein the pump beam B33originates from a partial reflection of the pump beam B22on the laser chip30, as it will be further described below,the pump beam B32at the output of the quarter wave plate61, which originates from the pump beam B31having travelled through the quarter wave plate61, andthe pump beam B33at the output of the beamsplitter7, which originates from the pump beam B32having travelled through the beamsplitter7.

As previously stated, the respective functions of the pump beams B2and B3are respectively pumping the laser chip30and the laser beam amplifier9.

The light travel of the pump beam B1emitted from the linearly polarized laser diode1through each component is explained more in details below. The beam B1is s-polarized.

The polarizing beamsplitter7is configured to reflect a linearly-polarized part of a received light in a different direction than its optical axis, and to transmit the perpendicularly-polarized part of the received light.

In the example of theFIG.2, the polarizing beamsplitter7is a dielectric polarizing beamsplitter cube. The polarizing beamsplitter cube is configured to transmit the p-polarized received light and to steer the s-polarized received light to 90°.

Therefore, receiving the s-polarized beam B1, the polarizing beamsplitter7reflects it perpendicularly of the optical axis of the linearly polarized laser diode1, and parallel to the optical axis of the laser chip30and in the opposite direction X2to the laser direction X1. The resulting beam is referred to as the pump beam B21. At first, the pump beam B21is s-polarized.

The quarter wave plate61is disposed on the optical axis of the laser chip30such that its slow and fast axis are arranged at 45° with the s-polarization of the pump beam B21. The slow axis of a retarder is the axis through which the light travels slower, whereas the fast axis of a retarder is the axis through which the light travels faster. In the case of a quarter wave plate, the retardation describes the phase shift (a quarter of the wavelength) between the polarization component projected along the fast axis and the component projected along the slow axis.

The pump beam B22passes through the quarter wave plate61which results in changing the polarization from a s-polarization to a circular polarization, as represented on theFIG.3.

A first part of the pump beam B22is reflected by the reflective element32and a second part of the pump beam B22enters the laser chip30through the reflective element32in order to pump the solid state oscillator material4. The ratio between the first and the second part is for example lower than 50%, preferably lower than 30%, and even preferably sensibly equal to 20%.

The second part of the beam B22which is reflected is referred to as pump beam B31. At first, due to the reflexing on the reflective element32, pump beam B31is inversely circularly polarized compared to pump beam B22which arrives on the reflective element32.

The pump beam B32goes through the quarter wave-plate61in the laser direction X1. The polarization is converted by the quarter wave-plate61into a p-polarization.

The pump beam B33is transmitted through the polarizing beamsplitter7without any changes due to the fact that the polarization of the beam B32is a p-polarization.

The pump beam B33arrives on the laser beam amplifier9and therefore enables to amplify the laser beam B4by optically pumping the laser amplifier9.

The quarter wave plate61and the polarizer62may be a unique optical component6acting differently for the laser wavelength and for the pumping wavelength, such as for example a dual wavelength multi-order wave plate.

FIG.1further represents a pair of lenses5and8whose purpose is to collimate the pump beams B22and B33respectively on the laser chip30and on the laser beam amplifier9along the pump beam paths on both side of the beamsplitter7.

As represented onFIG.1, the beam B1further passes through a lens2between the linearly laser diode1and the beamsplitter7, in order to be collimated on the beamsplitter7.

One of the specificities of the laser schematically represented onFIG.1is that both the laser chip30and the laser beam amplifier9are pumped by the same linearly polarized laser diode1emitting a pumping beam B1at a pumping wavelength. For example the pumping wavelength is of 808 nm and the laser beam is of 1064 nm. For instance the power of the pumping beam B1is split in 20% and 80% by reflecting on the reflective surface32of the laser oscillator4, such that the 20% of the power may pump the laser oscillator4whereas the 80% of the power may pump the laser beam amplifier9.

One of the specificities of the laser schematically represented onFIG.1is that the pump beams B2, B3are collinear to the laser beam B4.

One of the specificities of the laser schematically represented onFIG.1is that the polarized laser diode1is arranged such that its optical axis is not collinear to the optical axis of the laser chip30. More specifically, the optical axis of the laser chip30and the optical axis of the polarized laser diode1are perpendicular to each other.

With reference now toFIG.4, a schematic graph of power40over time41is represented for the different optical beams B22, B33and B4of the laser device ofFIG.1.

Namely, the power44of the laser beam B4is represented at the output of the laser device, namely after the laser beam B4having being amplified by the laser amplifier9. The laser beam B4is a power pulsed beam whose bursts43are periodically emitted according to a period Tcyc=1 μs.

Such a laser device is advantageously powerful and the energy of each burst may reach values over 1 μJ, for instance equal to 1 or 2 μJ. For instance, the time lapse T1of the bursts is 1 ns and the peak PW1of the power44is 2000 W. The laser chip30emits the bursts at the period Tcyc having the time lapse T1and the laser amplifier9amplifies the power of the bursts until the peak PW1.

The power44of the laser beam B4is generated thanks to a power42of the pump beam B1emitted by the pumping diode1.

The power42of the pump beam B22is represented at the input of the laser chip30and the power46of the pump beam B33is represented at the input of the laser amplifier9. As represented, the ratio of the reflection of the pumping beam B22on the reflective surface32is 20%.

When, the pumping diode1is turned on such that the pumping diode1emits a pump beam B1, the power42(and the power46) are maximum and continuous and respectively feed the laser chip30and the laser amplifier9.

The foregoing discussion disclosed and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.