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
  • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
  • H01S3/105—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling the mutual position or the reflecting properties of the reflectors of the cavity, e.g. by controlling the cavity length
• • 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
• • 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
  • H01S3/08054—Passive cavity elements acting on the polarization, e.g. a polarizer for branching or walk-off compensation
• • 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
  • H01S3/08059—Constructional details of the reflector, e.g. shape
• • 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
  • H01S3/081—Construction or shape of optical resonators or components thereof comprising three or more reflectors
  • H01S3/082—Construction or shape of optical resonators or components thereof comprising three or more reflectors defining a plurality of resonators, e.g. for mode selection or suppression
• • 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
  • H01S3/09—Processes or apparatus for excitation, e.g. pumping
  • H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
  • H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
  • H01S3/0941—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light of a laser diode
• • 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
  • H01S3/115—Q-switching using intracavity electro-optic devices

Definitions

• the present invention relates to a solid-state laser source. More particularly, the invention relates to a pulsed solid-state laser source producing brief and energetic pulsations.
• Solid state lasers or semiconductor lasers
• Solid state lasers are widely used in industrial applications, such as material marking, cutting and welding, or in scientific applications.
• Solid state lasers consist of a non-conductive solid gain medium optically pumped by diodes or arc lamps (also known as flash lamps).
• the triggered solid-state laser sources produce short and energetic pulses, thus finding applications, for example, in material processing processes.
• Such lasers comprise an active or passive variable attenuator for modulating the losses in the laser cavity, by switching between an opaque state and a transparent state.
• the attenuator is opaque, the losses in the laser resonator, also called the laser cavity, are very high, the light emitted by the gain medium can not circulate in the resonator and return to the gain medium.
• the laser emission process can not begin.
• the energy supplied by the pump source then accumulates in the gain medium.
• the attenuator is transparent, the losses in the resonator are very small and the radiation can flow in it.
• variable attenuator can be active and therefore controlled by an external signal, such as electro-optical modulators or acousto-optical.
• the variable attenuator can also be passive, as a saturable absorber.
• the pumping is typically performed by flash lamp.
• the pulse rate does not generally exceed about 30 Hz.
• MOPA Anglo-Saxon master oscillator power amplifier
• a pump laser the oscillator
• at least one optical amplifier for amplifying the output power of the laser. pump.
• Additional optical elements for example for the modulation of phase, intensity or wavelength, can thus be placed in the oscillator or between the oscillator and the amplifier so as not to expose them to excessive intensities.
• two pump modules are used in series to avoid depolarization due to birefringence induced by thermal effects that can occur in case of heavy pumping.
• Such systems are however very bulky, expensive and complex, and they do not allow to generate pulses with a rate of more than about 200 Hz.
• changing the gain in the amplifier may alter the timing of the pulse, which may be undesirable for some applications.
• the object of the present invention is to remedy all or part of the aforementioned drawbacks.
• the invention aims to provide a compact solid-state laser source generating short pulses, very energetic and at a high rate.
• the invention relates to a pulsed laser source comprising:
• a first resonator formed by a first reflecting mirror and a partially reflecting output mirror
• a laser module placed between the mirrors of the first resonator comprising:
• a laser rod made of solid material with a gain adapted to generate a laser beam
• a laser pumping source adapted to emit a pump beam into the laser bar.
• the laser bar is adapted to amplify the pump beam to generate the laser beam
• the output mirror is adapted to partially transmit the laser beam.
• the pulsed solid-state laser source further comprises:
• variable attenuation device that can switch between an opaque state and a transparent state for the laser beam to enable the laser source to operate in a triggered mode, the device being placed between the laser module and the reflecting mirror,
• variable attenuation device disposed between the variable attenuation device and the laser module
• phase retarding element disposed between the output mirror and the laser module.
• the laser source according to the present invention has many advantages. Since the laser module is used as the only oscillator of the laser source to produce the laser radiation, the laser source is particularly compact, while delivering very energetic pulses. Additional devices such as amplifier stages or flash lamps are not needed, thus making the laser source much more robust and relatively inexpensive compared to prior art solid state laser sources.
• the phase retardation element has the effect of improving the homogeneity of the laser beam coming out of the laser source.
• the profile of the laser beam is smoother due to the decrease of losses due to depolarization in the gain medium. Depolarization can have place due to thermal effects in the gain medium, leading to the birefringence of the material, this introduced birefringence being all the stronger as the pumping power is high.
• the phase retarding element alone compensates for the birefringence introduced into the laser bar, even in case of high pumping power.
• the output energy of the laser source is significantly increased by the phase retarding element with respect to the same laser source without this element.
• the retarder element also contributes to the protection of the rod of gain material against parasitic reflections coming from outside the laser cavity.
• the phase-retarding element is a quarter-wave plate whose fast axis is parallel to the direction of linear polarization. Such a blade does not increase the size of the cavity.
• the laser source according to the invention further comprises a second reflecting mirror adapted to form a second resonator with the output mirror, the second mirror being placed on the opposite side of the laser module with respect to the output mirror when the second resonator is formed, the second resonator does not contain a variable attenuation device.
• the second resonator is adapted to allow the laser source to operate in a quasi-continuous wave (QCW) mode and the second mirror is mobile to allow to switch between the first and the second resonator.
• QCW quasi-continuous wave
• the laser source according to this embodiment makes it possible to easily adapt its mode of operation to the needs of the desired applications, by using the same laser module and without complex mechanical intervention.
• the laser source can thus be used for a multitude of very diverse applications requiring energies and cadences of the different laser pulsations, while remaining compact and robust.
• variable attenuation device comprises an electro-optical modulator, an acousto-optical modulator, a rotary modulator or a passive variable attenuator.
• the laser source according to the invention further comprises an adjustable frequency conversion module adapted to receive a beam emitted by the laser bar and emit a beam whose wavelength is modified with respect to the wavelength of the beam emitted by the laser bar.
• the frequency conversion module can double or triple the frequency and deliver beams with a wavelength of 532 nm or 355 nm, respectively.
• the pulse rate is between about 1 Hz and about 1000 Hz. According to other embodiments, the pulse rate may be less than 1 Hz or greater than 1 kHz.
• the laser rod is garnet yttrium e t aluminum (YAG) doped with neodymium (Nd 3+) (Nd: YAG) and the wavelength of the laser beam emitted from the bar is 1064 nm.
• YAG yttrium e t aluminum
• Nd 3+ neodymium
• the laser pump source comprises at least one pulsed or continuous laser diode.
• the first reflecting mirror is a deformable mirror or a variable reflectivity mirror, these two types of mirrors being adapted to modify or shape the profile of the laser beam.
• the polarizer is a Brewster polarizer, a polarizer bi-prism or a thin film polarizer.
• the invention relates to a laser beam emission system, comprising:
• a laser source according to the first aspect of the invention, at least one optical fiber in which the laser beam transmitted by the output mirror is injected.
• the system comprises a laser source according to the first aspect of the invention, and the system further comprises at least one additional optical fiber, so that at least one laser beam not transmitted by the output mirror can be injected into the additional optical fiber.
• this embodiment makes it possible to recover light that is not transmitted by the output mirror. It may be, for example, beams reflected on an optical element of the cavity in axes other than that of the cavity. Thus, the energy of these beams can be recombined with the energy of the laser beam exiting through the output mirror.
• FIG. 1 shows, in a schematic sectional view, a laser source according to one embodiment of the invention.
• FIG. 2 schematically shows a laser emission system according to one embodiment of the invention.
• variants of the invention comprising only a selection of characteristics described or illustrated isolated from the other characteristics described or illustrated (even if this selection is isolated within a sentence including these other features), if this selection of features is sufficient to confer a technical advantage or to differentiate the invention from the state of the prior art.
• This selection comprises at least one preferably functional characteristic without structural details, and / or with only a part of the structural details if this part alone is sufficient to confer a technical advantage or to differentiate the invention from the state of the art. earlier.
• FIG. 1 an embodiment of the laser source 1 according to the first aspect of the invention will be described.
• the laser source 1 comprises a first optical resonator R1 which is formed by a first mirror 2 and an output mirror 3 aligned with respect to each other.
• the first mirror 2 is a reflecting mirror with an ideal reflection ratio of 100%. In the embodiment shown, the first mirror 2 is concave. This makes it possible to avoid the thermal effects inside the cavity and to obtain a laser beam at the output whose energy is more stable compared to a flat reflecting mirror.
• the output mirror 3 is partially reflective, allowing both to form the first resonator with the first mirror 2 and to transmit radiation at the wavelength of the laser beam generated by the resonator. For example, the reflection rate of the output mirror 3 may be 20% and the transmission rate 80%.
• the laser source 1 comprises, inside the first resonator R1, a laser module 4.
• the laser module 4 comprises a laser bar made of a solid material with a gain adapted to generate a laser beam.
• the laser module 4 also includes a pump source for emitting laser radiation from the pump radiation and pumping the laser rod. The laser bar then amplifies the pump radiation to generate the laser beam.
• the laser module comprises, as a gain medium, an Nd: YAG crystal, whose emitted laser radiation has a wavelength of 1064 nm.
• the pumping source is preferably constituted by a plurality of semiconductor laser diodes.
• the pump source comprises at least one laser diode array arranged axially around the laser bar or on a longitudinal side of the bar, for transverse pumping of the bar.
• the arrangement of the diodes on the side or around the bar makes it possible to obtain an integrated and particularly compact laser module.
• the laser diodes may, for example, be surface-emitting vertical cavity laser diodes, or VCSELs (for vertical-cavity surface-emitting laser terminology).
• the laser diodes may also be standard laser diodes.
• the laser diodes can, for example, emit at a wavelength of 808 nm.
• the peak pumping power is relatively high.
• the peak pump power can be of the order of 10 kW, and the average power can be of the order of 200 W.
• the peak pump power can be of the order of 1.5 kW, and the average power can be of the order of 30 W.
• the laser module 4 may also include cooling means for cooling the pump source.
• a cooling means may advantageously be a heat sink, for example made of copper.
• Another means may be circuitry adapted to circulate cooling water around the pump source.
• the laser source 1 according to the present invention emits pulsed laser radiation.
• the laser diodes of the pump source are pulsed with a pulse duration of between 10 ⁇ s and 1 ms. Since this mode of operation is close to a continuous excitation operation, it is called quasi-continuous wave mode (QCW) according to English terminology.
• QCW quasi-continuous wave mode
• the laser diodes of the pump source may be continuous laser diodes (CW, for the Anglo-Saxon term continuous wave).
• the laser source 1 further comprises a variable attenuation device QS (formed by the elements referenced 6 and 7) disposed between the laser module 4 and the first mirror 2.
• the device QS variable attenuation is able to switch between an opaque state and a transparent state for the laser beam.
• the QS device switches from opaque state to transparent state, the laser source 1 is triggered.
• the mode of operation in triggered mode of the laser source 1 allows the formation of large short pulsations and energy. This technique is called Q-switching, or Q-switching according to English terminology, because of the switching of the quality factor of the resonator.
• the switching between the opaque state and the transparent state of the attenuation device QS is controlled by an external signal; it is then active switching-Q.
• the signal can be mechanical (for example, a chopper) or electric.
• the QS device may be an acousto-optic or electro-optical modulator.
• the variable attenuator can also be passive in nature.
• the laser source 1 according to the invention further comprises a polarizing element 5 for polarizing the light linearly.
• This polarizer 5 may be, for example, a Brewster polarizer, a polarizing bi-prism, or a thin-layer polarizer.
• the polarizer 5 is disposed between the laser module 4 and the variable attenuation device QS. The light traveling between the laser module 4 and the attenuation device QS is then polarized linearly.
• the variable attenuation device QS comprises a Pockels cell 6 combined with a quarter-wave plate 7.
• the delay plate 7 is arranged between the Pockels cell 6 and the first mirror 2.
• the Pockels cell 6 is controlled with an electric field applied thereto.
• the polarizer 5 is arranged between the Pockels cell 6 and the laser module 4.
• each element is oriented in a certain way with respect to the optical axis 10. By this, the assembly can be switched between the opaque state and the transparent state by means of the electric field signal.
• the polarizer 5 polarizes the light linearly along the axis lying in the board plane, and the fast axis of the quarter-wave plate 7 is oriented at 45 ° with respect to the plane of the board. two axes being perpendicular to the optical axis 10. With the applied electric field, the Pockels cell also acts as a quarter wave plate.
• the laser source 1 further comprises a phase-retarding element 8.
• This element The phase retarder 8 is disposed between the output mirror 3 and the laser module 4.
• the element 8 is arranged so as not to alter the linear polarization of the light propagating in the cavity.
• Element 8 has the effect of improving the homogeneity and the profile of the laser beam by compensating for the birefringence introduced by the gain material due to thermal effects. The birefringence introduced is even stronger than the pumping power is high.
• the element 8 also makes it possible to protect the laser bar against parasitic reflections coming from outside the cavity, thus making it possible to avoid the use of an external insulator which would make the laser source 1 more cumbersome and expensive.
• the output energy of the laser source 1 is significantly increased by virtue of the phase-retarding element 8 with respect to the same laser source without this element 8.
• the increase in the output energy is, for example, of the order of 10-15% depending on the orientation of the crystallographic axes of the element 8 in the plane of rotation perpendicular to the optical axis 10.
• the phase retarding element 8 is preferably a quarter-wave plate.
• the fast axis of the quarter-wave plate coincides with the plane of the plate, and the polarizer and the variable attenuation device are arranged in such a way that the light propagating between the polarizer 5 and the laser module 4 is polarized linearly in the plane of the board.
• the direction of linear polarization of the light coincides with the fast axis of the quarter wave plate 8.
• the polarization of the laser beam leaving the laser source 1 is circular.
• the laser source 1 according to the invention is adapted to operate either with the first resonator R1 or with a second resonator R2.
• the second resonator R2 is formed by the output mirror 3 and a second reflecting mirror 9 (shown in dashed line in FIG. 1).
• the second mirror 9 is placed on the opposite side of the laser module 4 with respect to the output mirror 3.
• the second resonator R2 does not contain a switch-Q, the duration and the rate of the pulses are determined by the pulsations delivered by the pump source of the laser module 4.
• the second resonator R2 allows the laser source 1 to operate in a QCW regime, also called relaxed mode.
• the laser pulses emitted may have a duration of between 10 ps and about 1000 ps, depending on the duration of the pulsations of the pump source, for example QCW laser diodes.
• the delay element 8 retains its effects of homogenization and increase of output energy as in the case of the triggered mode. In the relaxed mode, however, this increase is about 50% less than that achieved in triggered mode.
• the second reflecting mirror 9 is movably mounted in the laser source 1.
• the second mirror 9 is pivotable in the plane perpendicular to the optical axis 10 of the laser source 1.
• the second reflecting mirror 9 must be implemented accurately, for example, mechanical stop, and repeatable at the position desired, to ensure the performance of the cavity thus obtained.
• the second mirror 9 can be mounted, for example, on a pivotable mounting element in the plane perpendicular to the optical axis 10. The pivoting of the mounting element, preferably motorized, can be controlled by a external signal, automatically or manually.
• the laser source 1 can operate either with the first resonator R1 in a triggered regime delivering short pulses (of the order of a few ns to a few tens of ns), or with the second resonator R2 in a QCW regime delivering longer pulsations (of the order of a few tens to a few hundred ps).
• the laser source 1 according to this embodiment makes it possible to easily adapt its operating mode to the needs of the desired applications, by using the same laser module 4 and without complex mechanical intervention.
• the radiation emitted by the laser source 1 according to the invention is preferably multimode.
• the radiation can be rendered monomode, for example, by placing a pinhole of suitable size near the output mirror 3 of the cavity, at the size of the beam coming out of the laser module 4.
• the pulse rate is between about 1 Hz and 1 kHz for the triggered mode and the QCW mode.
• the pulse rate determines the maximum energy of a pulse for a given pulse duration.
• the laser source according to the present invention may also comprise a frequency conversion module (not shown). Frequency conversion techniques, such as frequency doubling, are known. The frequency conversion module is adjustable in order to obtain frequency doubling or tripling according to the application of the desired laser source 1. Thus, for a transmission wavelength of the laser module 4 of 1064 nm, the frequency doubling makes it possible to obtain an emission wavelength of the laser source 1 of 532 nm, and the frequency tripling allows to obtain a wavelength of 355 nm.
• the first reflecting mirror 2 of the cavity is a deformable mirror or a mirror with variable reflectivity. The profile of the laser beam can thus be modulated or shaped and adapted according to the desired application of the laser source 1.
• FIG. 2 shows a laser beam emission system 11 according to one embodiment.
• the laser system 11 comprises the laser source 1 according to one of the embodiments described above, and at least one optical fiber 12 in which the laser beam transmitted by the output mirror is injected.
• the laser beam emitted by the output mirror of the laser source 1 is coupled in the optical fiber 12 by a coupling means 13.
• This coupling means 13 is known to those skilled in the art.
• the coupling means 13 may be, for example, a lens or a lens system.
• the laser beam is coupled in a plurality of optical fibers, for example three identical fibers. This makes it possible to distribute the output energy on these fibers and thus to be able to increase the output energy without damaging the optical fibers.
• the at least one optical fiber 12 is a multimode fiber.
• the laser system 11 also comprises two additional optical fibers 14, 15. These additional fibers 14, 15 are arranged laterally with respect to the longitudinal axis of the cavity, for example on the sides of a housing surrounding the cavity. Thus, lateral reflections on optical elements in the cavity can be recovered and injected into the additional fibers 14, 15 by known means.
• the polarizer used is a Brewster polarizer, producing lateral reflections on either side of the glass plate constituting the polarizer. This makes it possible to minimize the losses due to these reflections.

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• Physics & Mathematics (AREA)
• Electromagnetism (AREA)
• Engineering & Computer Science (AREA)
• Plasma & Fusion (AREA)
• Optics & Photonics (AREA)
• Chemical & Material Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Lasers (AREA)

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• 2017
  • 2017-10-26 FR FR1760099A patent/FR3073098B1/fr not_active Expired - Fee Related
• 2018
  • 2018-10-18 WO PCT/EP2018/078529 patent/WO2019081334A1/fr unknown
  • 2018-10-18 EP EP18796599.1A patent/EP3701602A1/de not_active Withdrawn

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