Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
  • H01S3/06—Construction or shape of active medium
  • H01S3/0627—Construction or shape of active medium the resonator being monolithic, e.g. microlaser
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
  • H01S3/06—Construction or shape of active medium
  • H01S3/0602—Crystal lasers or glass lasers
  • H01S3/0604—Crystal lasers or glass lasers in the form of a plate or disc
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
  • H01S3/08—Construction or shape of optical resonators or components thereof
  • H01S3/08004—Construction or shape of optical resonators or components thereof incorporating a dispersive element, e.g. a prism for wavelength selection
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
  • H01S3/106—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity
  • H01S3/108—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity using non-linear optical devices, e.g. exhibiting Brillouin or Raman scattering
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
  • H01S3/11—Mode locking; Q-switching; Other giant-pulse techniques, e.g. cavity dumping
  • H01S3/1123—Q-switching
  • H01S3/113—Q-switching using intracavity saturable absorbers

Definitions

• the present invention relates to solid lasers, in particular microlasers, intended to emit light pulses.
• a solid laser, or laser in the solid state is a laser whose amplifying medium is a solid. It is often a single crystal doped or not, a glass, or more rarely a polymer. With a view to its optical pumping, this solid is often excited by a light source such as a lamp, a flash, a laser diode, or even a light-emitting diode.
• a light source such as a lamp, a flash, a laser diode, or even a light-emitting diode.
• the amplifying medium is placed in a resonant cavity.
• resonant cavities There are several types of resonant cavities, the most used being Fabry-Perot cavities with two mirrors or ring cavities with three or four mirrors.
• the mirrors used are flat or spherical. Sometimes these mirrors are replaced by prisms. Lenses or deflection mirrors are possibly used in the cavity. The whole is aligned by means of precise mechanical mounts.
• One of the mirrors often constitutes the output coupler: the laser beam is extracted by this mirror which is partially reflecting. Other output coupling systems exist.
• a microlaser is a particular solid state laser. Its amplifying medium is made up of a thin laser material (generally of the order of 150 to 2000 ⁇ m) and of small dimensions (generally a few square millimeters). The mirrors of the resonant cavity are generally directly deposited on the polished optical faces of the laser material. In addition, the microlaser is generally pumped by a laser diode, which is either directly hybridized on the microlaser, or coupled thereto via an optical fiber.
• the microlaser has several components, all of these components are assembled in various ways to form a single monolithic element.
• the microlaser is polished on its two end faces (these two faces are perpendicular to the axis of the laser).
• the mirrors which form the resonant cavity are deposited directly on the polished faces in the form of thin layers.
• One of the faces has a maximum reflectivity, close to 100%, and the other is semi-reflecting in order to transmit part of the light outside the cavity.
• the amplifying medium can constitute the substrate of the microlaser.
• thin sheets a few hundred micrometers
• large sheets severe centimeters
• a solid laser or a microlaser
• Q s itched in an active or passive way, by introducing, into the cavity, a loss modulator that is to say a means of modulating the overvoltage factor ("Q factor") of this cavity.
• the value of the losses introduced into the cavity (resonator) is controlled externally for example by means of an acousto-optical modulator which diffracts light out of the resonator or an electro-optical modulator which switches the polarization of the laser beam in this resonator.
• Such modulators require electronic control means capable of supplying radio frequency signals for the acousto-optical modulator, or else high-voltage signals for the electro-optical modulator.
• FIG. 1 schematically represents a known microlaser, actively activated by an electro-optical modulator.
• This microlaser successively comprises an input mirror 2, an amplifying medium 4, an intermediate mirror 6, an electro-optical modulator 7, which is provided with two lateral control electrodes 8 and 9, and an output mirror 10.
• the medium amplifier 4 is fixed to the intermediate mirror 6 by means of an optical adhesive or a bead of resin 12.
• the optical pumping beam 14 has also been shown, coming for example from a laser diode (not shown) and intended to excite the amplifying medium 4 through the input mirror 2.
• We also see the impulse laser beam 16 which results therefrom and which leaves the microlaser through the mirror 10. These beams 14 and 16 propagate along the same axis X. The operation of such an active trigger laser is described below.
• the gain in the amplifying medium is zero and the losses due to the modulator are maximum, the threshold is therefore very high and the laser does not oscillate.
• the energy stored in the amplifying medium increases.
• the level of pumping and the losses are such that the laser does not oscillate.
• the level of losses induced by the modulator is suddenly reduced. Consequently, the gain of the amplifying medium becomes much greater than the level of the total losses, therefore the net gain in the laser cavity is very high.
• the stored energy is released in the form of a giant light pulse, in a very short time.
• non-linear modulators are used without external power supply and, more specifically, saturable absorbents, materials which are highly absorbent when illuminated with a beam of low power density and which become almost transparent when this density exceeds a certain threshold (called saturation intensity).
• FIG. 2 schematically represents a known microlaser, triggered passively.
• This microlaser successively comprises the input mirror 2, the amplifying medium 4, a saturable absorbent 18 and the output mirror 10.
• the pumping beam 14 and the beam can also be seen laser 16 generated by the microlaser.
• the saturable absorbent 18 is for example a sol-gel or semiconductor layer, or a solid material, doped with active ions. The operation of such a passive trigger laser is described below.
• the gain in the amplifying medium is zero and the losses (absorption) due to the saturable absorbent are maximum, the threshold is therefore high and the laser does not oscillate.
• the few laser photons present in the cavity are absorbed by the saturable absorbent: the transmission thereof increases slightly, so the losses decrease. Consequently, the net gain is increased, so the number of photons in the cavity increases. This “loop” behavior is gradually increasing. Very quickly, the absorbent saturates and the losses reach their minimum level. The net gain in the cavity is then very significant.
• the stored energy is released in the form of a giant light pulse, in a very short time.
• the pulse ends.
• a certain number of laser applications require pulses whose duration is of the order of 100 nanoseconds to 1 millisecond with significant peak powers (greater than 1 watt).
• gain-switch gain modulation
• the amplifying medium is pumped by a light pulse from the pumping source.
• the laser then emits a series of damped light pulses corresponding to the transient laser start-up regime. This phenomenon is undesirable when a long pulse (i.e. a long duration pulse) is desired.
• no known microlaser is capable of emitting pulses whose duration is greater than one hundred nanoseconds.
• the object of the present invention is to remedy these drawbacks. It relates to a solid laser comprising a resonant cavity and, in this cavity, an amplifying medium and a loss modulator, in order to generate light pulses after optical pumping of the amplifying medium, this laser being characterized in that it comprises in addition, in the cavity, a means of lengthening the duration of the light pulses by the photorefractive effect which is a non-linear optical effect which does not transform the wavelength of the laser.
• this means of lengthening the duration and the amplifying medium (or the loss modulator) are formed from the same material.
• the duration extension means, the amplifying medium and the loss modulator are formed from different materials.
• this laser is a microlaser.
• the means for lengthening the duration of the light pulses is for example an element comprising at least one layer of a material chosen from the group comprising semiconductor crystals, oxide crystals and polymers, having photorefractive properties.
• the material constituting such an element known for its photorefractive properties can be:
• a semi-insulating semiconductor crystal such as indium phosphide iron doped (InP: Fe), gallium arsenide (AsGa), cadmium tellurium doped vanadium or titanium (CdTe: V or CdTerTi), or others semiconductors known for their photorefractive properties, or
• a crystal of oxide material such as BaTi0 3 , Bi ⁇ 2 SiO 20 , LiNb0 3 : Fe, or
• this polymer being for example a mixture of a polymer chromatophore and a conductive polymer in a polymer matrix (see document [10]).
• the element may be between two electrodes provided to create, when an electric voltage is applied between them, an electric field parallel to the axis of propagation of the light pulses generated in the cavity.
• the loss modulator is a saturable absorbing medium, the laser being thus triggered passively.
• This saturable absorbent medium is for example chosen from the group comprising:
• the semiconductors comprising one or a plurality of quantum wells (width) adapted to the wavelength of the laser.
• the amplifying medium (the laser material) is monocrystalline
• a monocrystalline layer forming the saturable absorbent medium can be deposited by epitaxy in liquid phase on this amplifying medium and can be given to the latter from the point of view of the crystal meshes or, on the contrary, constraint.
• a ternary or quaternary semiconductor forming the saturable absorbent medium is deposited by molecular beam epitaxy.
• Molecular beam epitaxy or by chemical vapor deposition with organometallics (“metalorganic chemical vapor deposition”) on a semiconductor substrate which is then assembled with the amplifying medium.
• the composition of the latter is preferably further adjusted to obtain a material which is matched to that of the corresponding substrate from the point of view of crystalline meshes and which is therefore unconstrained.
• the quantum well (s) mentioned above are for example formed on a semiconductor substrate which is assembled with the amplifying medium.
• the loss modulator is a voltage-controlled modulator, the laser being thus actively triggered.
• the loss modulator in particular when it is a saturable absorbent
• the means for lengthening the duration of the light pulses are associated with the amplifying medium. by a process chosen from the group comprising:
• the thin layer it is for example a sol-gel layer or an epitaxial layer, doped with saturable absorbent ions.
• the latter can then be removed but it may already contain a mirror constituting the exit mirror of the resonant cavity in which case this substrate is preserved.
• the resonant cavity is delimited by an input mirror and an output mirror and the means for lengthening the duration of the light pulses is arranged between the loss modulator and the mirror. output or between this loss modulator and the amplifying medium.
• the lengths are arranged between the loss modulator and the mirror. output or between this loss modulator and the amplifying medium.
• 'Particularly advantageous basic wave are 0.946 microns, 1.06 microns, 1.3 microns, 1.55 microns and 2 microns.
• other wavelengths can be obtained by using a non-linear optical element to obtain harmonics of these basic wavelengths (for example 473 nm, 532 nm, 466 nm) or to generate lengths of higher wave, for example by optical parametric oscillation or optical parametric amplification (OPO or OPA).
• OPO or OPA optical parametric amplification
• it is possible to find a saturable absorbent suitable for wavelength for example a semiconductor material).
• a microlaser according to the invention retains the advantages of a microlaser of the continuous or triggered type (compactness, reduced cost, collective manufacturing, reliability due to its monolithic and foolproof nature, reduced maintenance, design of optical microsystem through the use of micro-optical components).
• the pulses produced are long and respond to all applications where a known solid-state laser, with triggering and gain switching does not provide a solution (see below).
• the saturable absorbent case of passive triggering
• the parameters of the non-linear element intended to lengthen the pulses one can play on the shape of the pulses. emitted: triangular or Gaussian-looking pulses, for example.
• Long pulses can be time modulated.
• a code can be associated with each pulse emitted by the laser using a modulator outside the laser cavity.
• the loss modulator material is also photorefractive.
• LiNb0 3 is an electro-optical material usable for the active triggering of lasers. When this material is doped with iron, it becomes photorefractive. In any case, in general, a photorefractive material has -. a significant electro-optical coefficient. Equipped with electrodes, it can therefore play the role of voltage-controlled modulator for the active triggering of a laser according to the invention.
• FIG. 1 is a schematic longitudinal section view of a known microlaser, actively activated, and has already been described
• FIG. 2 is a schematic longitudinal section view of a known microlaser, triggered passively, and has already been described
• FIG. 3A is a schematic longitudinal section view of a microlaser according to the invention, comprising a saturable absorbent and a photorefractive element,
• FIG. 3B is a schematic longitudinal section view of a solid laser according to the invention, comprising a saturable absorbent and a photorefractive element,
• FIG. 4 is a schematic longitudinal section view of another microlaser according to the invention, comprising a saturable absorbent deposited in a thin layer, and
• Figure 5 is a schematic longitudinal sectional view of another microlaser according to the invention, comprising a photorefractive element between two electrodes.
• the microlaser according to the invention which is schematically represented in longitudinal section in FIG. 3A, successively comprises an input mirror 20, an amplifying medium or gain medium 22, a saturable absorbent 24 comprised between two intermediate layers 26 and 27 used to control the reflection and transmission coefficients at each interface between two distinct materials, and for example produced by a set of layers in Si0 2 and Ti0 2 alternating, a photorefractive element 28 and an output mirror 30.
• a beam 32 of optical pumping of the amplifying medium sent towards the latter along an axis X, through the input mirror, as well as a pulsed laser beam, or laser pulse 34, which is generated by the amplifying medium. thus excited and which also propagates along the X axis.
• the solid laser according to the invention which is shown diagrammatically in longitudinal section in FIG. 3B, successively comprises an input mirror 35, an amplifying medium 36, a saturable absorbent 38, a photorefractive element 40 and an output mirror 42
• the beam 44 or 45 pumping the amplifying medium 36 and the generated pulsed laser beam 46 which propagates along the same axis X as the beam 44 (this beam 44 corresponding to a longitudinal pumping and the beam 45 to a pumping transverse).
• the components 35, 36, 38, 40 and 42 are spaced from each other while the components of the microlaser of FIG. 3A are in contact with each other (and so on even for the microlasers of Figures 4 and 5).
• the microlaser according to the invention which is schematically represented in longitudinal section in FIG. 4, successively comprises an input mirror 48, an amplifying medium 50, a thin layer 52 of a saturable absorbent, a photorefractive substrate 54 and a exit mirror 56.
• An anti-reflection layer 58 (for example made of MgF 2 ) or a set of anti-reflection layers (for example in Si0 2 / Ti0 2 ) is optionally interposed between the amplifying medium and the thin layer 52.
• the substrate 54 is semiconductor (for example made of Fe doped InP, undoped AsGa or V or Ti doped CdTe) and the thin layer 52, also semiconductor, is deposited on the latter.
• the amplifying medium 50 is fixed to the layer 52 (or to the layer 58 when it exists) by a bead of resin 60 or by a layer of optical glue.
• a beam 62 for pumping the amplifying medium and the generated pulsed laser beam 64 which propagate along the same axis X.
• the microlaser according to the invention which is shown diagrammatically in longitudinal section in FIG. 5, successively comprises an input mirror 66, an amplifying medium 68, a photorefractive layer 70, a saturable absorbent 72 and an output mirror 74.
• a beam 76 for optical pumping of the amplifying medium and the generated pulsed laser beam 78 which propagate along the same axis X.
• the photorefractive layer 70 can be placed between two electrodes 80 and 82 to create an electric field (direct or alternating) parallel to the X axis by applying an appropriate electric voltage
• the saturable absorbent for example 24 in FIG. 3A or 38 in FIG. 3B
• an electro-optical modulator 25 in the FIG. 3A and 39 in FIG. 3B
• MOCVD metal-organic chemical vapor deposition
• Microchip lasers with a diamond circular saw (identical to that used in microelectronics).
• microlasers in accordance with the invention. Individually, by assembling small samples (a few millimeters) of the different materials.
• a laser according to the invention can be produced using a glass doped with erbium and ytterbium, an LMA crystal doped with cobalt (see document [11]) and a semi-insulating InP crystal doped with iron.
• Microlasers were produced with a gain medium consisting of phosphate glass doped at 0.8% by mass with erbium oxide (Er 2 0 3 ) and at 20% by mass with oxide of ytterbium (Yb 2 0 3 ).
• This genre glass: Er, Yb is commercially available from Kigre in the USA. It has an Er and Yb composition specially adapted for the operation of a 1.55 ⁇ m microlaser. A 0.75 mm or 1 mm thick glass slide is used. It has been polished to obtain two flat and parallel faces.
• the role of the saturable absorbent is held by a layer of semiconductor material whose forbidden bandwidth is fixed by the desired wavelength (1.55 ⁇ m).
• the saturable absorbent layers are either thick layers constrained in InGaAs, or layers in InGaAIAs, or structures with several quantum wells in 9 nm inGaAs in InAlAs barriers.
• the entrance mirror is placed on one side of the glass slide: Er, Yb. It is a stack of Si0 2 / Ti0 2 layers produced by ion sputtering.
• the anti-reflective layer is deposited, for example on the other face of the glass slide and the output mirror on the semiconductor.
• the strips of semiconductor materials are assembled mechanically with glass strips: Er, Yb.
• the cavity is pumped by a laser diode emitting around 975 nm (commercially available from Spectra Diode Labs in the USA).
• microlasers according to the invention make it possible to deliver long pulses of strong energy, at high rate of fire, which is interesting for many applications: 1) generation and detection of ultrasound using short pulses and long pulses (coded pulse trains for application to non-destructive testing) ultrasound), 2) numerous medical applications:
• optical data storage which uses pulses of determined lengths to erase, write or read data
• solid lasers for example those whose amplifying medium is made of sapphire doped with titanium
• laser diodes cannot currently be pumped by laser diodes.
• the use of solid lasers pumped by laser diodes and emitting sufficiently long pulses to obtain effective pumping of the preceding is a solution which can prove to be very profitable.
• US 5,321,709 A US 5,321,709 and US 5,805,622 relate to lasers triggered with a pulse extender. This device, in both cases, includes a non-linear material for converting optical frequency (frequency doubling, frequency summation).
• phase tuning there is an interaction between coherent waves whose optical frequencies have a simple relationship (for example, one being twice the other, or one being the sum of two other distinct frequencies).
• This interaction can be modeled by a named coefficient. 2 for example. The higher the coefficient, the more effective the nonlinear effect.
• Another condition is necessary: a phase relationship must be respected between the waves since they do not have the same wavelength, this condition is known as phase tuning. When it is satisfied, there is effectively frequency conversion, for example frequency doubling.
• the photorefractive effect is obtained in a material in which at least two waves propagate. These two waves must be coherent to be able to produce interference in the material. This is necessarily only possible if the two waves have equal wavelengths or optical frequencies.
• This interference consists in the modulation in space of the intensity of the light. Some areas are brightly lit and other areas receive no radiation. In the zones illuminated, the light excites the free carriers of the material. These excited carriers move and are trapped in the material. This displacement phenomenon causes a local modification of the index of the material. An index network conforming to the location of the interference fringes is therefore created in the material. This grating would diffract a third beam (always at the same wavelength) which would cross the material.
• the photorefractive crystal works whatever its crystallographic orientation.
• the effects are easily distinguished by two aspects: the equal or different wavelengths (for example double, triple, in a sum relation, ...); the need to orient the non-linear element relative to the axis of the beams (nonexistent for the photorefractive element).
• lithium niobate (LiNb03) is both non-linear and photorefractive. It is possible, in view of the above criteria, to know whether this crystal is used in frequency converter or in photorefractive material.
• the lithium niobate in a phase-matched configuration.
• SHG that is to say of generation of second harmonic and therefore of frequency doubling.
• SFM for “sum-frequency-mix”

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