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
  • H01S3/11—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
• • 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
  • H01S3/08018—Mode suppression
  • H01S3/0804—Transverse or lateral modes
  • H01S3/0805—Transverse or lateral modes by apertures, e.g. pin-holes or knife-edges
• • 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
  • H01S3/09—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
  • H01S3/09408—Pump redundancy
• • 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
  • H01S3/094084—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light with pump light recycling, i.e. with reinjection of the unused pump light, e.g. by reflectors or circulators
• • 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
  • H01S3/10038—Amplitude control
• • 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
  • H01S3/107—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 electro-optic devices, e.g. exhibiting Pockels or Kerr effect
• • 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
  • H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
  • H01S3/16—Solid materials
  • H01S3/1601—Solid materials characterised by an active (lasing) ion
  • H01S3/1603—Solid materials characterised by an active (lasing) ion rare earth
  • H01S3/1611—Solid materials characterised by an active (lasing) ion rare earth neodymium
• • 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
  • H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
  • H01S3/16—Solid materials
  • H01S3/163—Solid materials characterised by a crystal matrix
  • H01S3/164—Solid materials characterised by a crystal matrix garnet
  • H01S3/1643—YAG

Definitions

• the present invention relates to the field of pulsed laser oscillators.
• Pulse laser oscillators comprising a laser cavity, said laser cavity comprising a laser medium capable of being pumped by pump radiation emitted by at least one pump radiation source and shutter means capable of closing said cavity.
• Such lasers are known as triggered laser (or "Q-switch laser” in English).
• the shutter means are commonly referred to as quality factor switches, or Q-switches ("Q-switches” in English).
• Q-switches Quality factor switches
• a laser crystal is pumped by a pump diode, while the shutter means are closed and prevent the return of the laser wave into the crystal. This produces a population inversion within the crystal, but lasing of the laser medium does not occur since there is no return of the wave.
• the sealing means still close the cavity, the laser crystal is charged with energy by pumping.
• the sealing means are then open to allow the return of the wave after reflection on one end of the cavity.
• the stimulated emission amplification process can then begin. Because of the large amount of energy stored in the laser medium, the generated laser signal is very short, and a short pulse is obtained at the output of the oscillator.
• the duration of the laser pulse output of the oscillator is a priori constant.
• the adjustment of the pulse duration is interesting to adapt the characteristics of the pulses to the type of phenomenon to be studied.
• pulsed lasers with variable pulse duration.
• a first known solution for making such an adjustment of the pulse duration is to vary the pumping power of the laser medium. Indeed, when this pumping power is varied, the amount of energy stored in the laser crystal varies, and the pulse duration also varies.
• this solution has significant disadvantages. Indeed, a variation of the pumping power produces a variation of the thermal regime in the laser medium and consequently a modification of the thermal lens that it produces inside the cavity. Even though the laser cavity is configured to be insensitive to thermal lens changes in the form of a dynamically stable cavity, a large amplitude of variation of the pulse duration, controlled by the pumping power, causes a change in spatial characteristics of the beam and a variation of the energy emitted up to the stop of the laser emission. In laser sources with a high repetition rate (from 1 kHz to 100 kHz) this disadvantage is overcome by adjusting the pulse duration by varying the repetition rate. If the laser medium is pumped continuously and the interval between two pulses is less than the life of the upper level of the laser transition, the change in the repetition rate changes the stored energy and consequently the duration of impulse.
• the pumping power is constant, but such a device has the disadvantage that the repetition rate is not constant.
• Another solution for achieving such an adjustment of the pulse duration is to control the switch Q within the cavity.
• a pulsed laser oscillator comprising a laser cavity, said laser cavity comprising a laser medium capable of being pumped by a pump radiation emitted by less a source of pump radiation and closure means capable of closing said cavity.
• This application teaches to use within the laser resonator, an acousto-optical Q switch connected to an electronic unit generating a high-frequency wave that can be modulated.
• the switch Q and in particular its duration of opening and closing, is then controlled by this high-frequency wave.
• a first drawback is that the duration of opening, which is related to the size of the beam in the switch, is generally long and short pulse times are therefore difficult to obtain.
• a second disadvantage is that if one seeks to increase the stored energy in order to reduce the duration of the pulses, the relaxed operating regime appears very easily because the closing rate of the acousto-optic Q switch is not good. .
• the present invention intends to overcome these disadvantages.
• a first object of the invention is therefore to provide a pulsed laser with variable pulse duration.
• Another object of the invention is to provide a pulsed laser with variable pulse duration without requiring modification of the pumping power of the laser crystal.
• Another object of the invention is to provide a pulsed laser oscillator not using an acousto-optic Q switch. At least one of these objects is achieved by the invention, which according to a first aspect concerns a pulsed laser oscillator for emitting a laser pulse, comprising a laser cavity, said laser cavity comprising a laser medium capable of being pumped by pump radiation emitted by at least one pump radiation source and to emit laser radiation, said laser cavity comprising shutter means able to close said cavity during a shutter duration, characterized in that said shutter means are means for closing said cavity, electro-optical shutter, and in that said shutter means are adapted to be powered by a supply voltage, so that the duration of the transmitted pulse is changed when the value of the supply voltage is changed .
• the sealing means comprise, for example, electro-optical crystals.
• closure rate of an electro-optical switch is much better than that of an electro-acoustic switch as described for example in US-A-2001/0021205.
• said laser cavity comprises a coupling polarizer able to reflect said laser radiation with reflectivity, the coupling polarizer being arranged so that said reflectivity is changed when the value of the supply voltage is changed.
• the reflectivity of the coupling polarizer can vary depending on the power supply, which has the effect of varying the laser pulse duration.
• said shutter means can comprise a first crystal of RbTiOPO 4 having a first axis, and a second crystal of RbTiOPO 4 having a second axis, the first axis and the second axis being crossed.
• said laser cavity is closed by a first mirror and a second mirror, said first mirror and said second mirror defining a parameter stability parameter, said stability parameter being between 0.4 and 0.6 and preferably 0.5.
• the invention also relates to a device comprising a pulsed laser oscillator as described above, power supply means capable of supplying said variable supply voltage to said shutter means, and means for controlling the shut-off power supply of the means shutter power control circuit capable of modifying said supply voltage so as to modify the duration of the laser pulse emitted by the oscillator.
• the supply means comprise for example a voltage generator, and the shutter supply control means comprise for example a potentiometer.
• the invention also aims to provide a pulsed laser oscillator with variable pulse duration, while maintaining a substantially constant energy.
• one of the disadvantages of pulsed oscillators with variable pulse duration is that the variation in the duration of the pulse involves a variation of the energy of the laser. This is particularly the case when the variation of the pulse duration is achieved by varying the pumping power since this pumping power affects both the pulse duration and the energy emitted.
• the aforementioned device may comprise pump supply means capable of supplying a pump current to said source of pump radiation, said pump radiation having an energy, said energy being a function of said pump current, said device comprising pump control means adapted to vary said pump current.
• the pulsed laser oscillator according to the invention comprises two independently modifiable adjustment parameters, which makes it possible to adjust the pulse duration by means of the shutter power control means and to adjust the pump energy by the pump control means.
• a suitable adjustment of these two parameters now independently modifiable makes it possible to keep the energy of the emitted laser pulse substantially constant.
• said pulsed laser oscillator is able to emit a laser signal, and wherein said supply voltage and said pump current are selected so that said laser energy is substantially constant.
• the voltage to be applied is not too high. This makes it possible in particular to avoid the use of complex and expensive feeding means.
• Another object of the invention is therefore to provide a pulsed laser oscillator with variable pulse duration by switching a Q switch supplied with voltage, without the voltage to be applied to the switch Q is too high.
• the gain of the laser medium be as high as possible, while preventing the oscillator from operating in a relaxed regime which would be detrimental to the quality of the beam and to the the flow resistance of the optical components.
• the aforementioned laser cavity has a laser threshold
• said laser cavity may comprise biasing means able to modify a polarization state of said laser radiation before said coupling polarizer, said biasing means being arranged so as to place said laser cavity just below said laser threshold, at a limit of disappearance of the relaxed regime.
• the voltage to be applied to the switch Q may be low and therefore the supply means can be simple and inexpensive.
• the invention also relates to a method for varying the duration of a pulse emitted by a pulsed laser oscillator comprising a laser cavity, said laser cavity comprising a laser medium capable of being pumped by pump radiation emitted by at least one pump radiation source and emitting laser radiation and electro-optical sealing means, said method being characterized by comprising steps of:
• said laser cavity may comprise a coupling polarizer able to reflect said laser radiation with reflectivity and wherein said reflectivity is changed when said power supply voltage is changed.
• said pump radiation has a pump energy
• said method comprises steps of:
• said pulsed laser oscillator is able to generate a laser signal having a laser energy, and said shutter duration and said pump energy being chosen so that said laser energy is substantially constant.
• said laser cavity has a laser threshold, said laser cavity comprising polarization means able to modify a polarization state of said laser radiation before said coupling polarizer, said method being able to understand steps consisting of:
• FIG. 1 is a diagram illustrating an example pulsed laser oscillator according to the invention
• FIG. 2 is a graph showing the evolution of the tripping voltage as a function of the pulse duration of the pulsed laser oscillator of FIG. 1;
• FIG. 3 is a graph showing the evolution of the energy emitted by the laser cavity before adjustment in voltage and current
• FIG. 4 is a graph illustrating the reflection coefficient of the coupling polarizer as a function of the voltage applied to the shutter means of the pulsed laser oscillator of FIG. 1 for an orientation of the quarter-wave plate of 0.4 rad;
• FIG. 5 is a graph illustrating the reflection coefficient of the coupling polarizer as a function of the voltage applied to the shutter means of a pulsed laser oscillator with usual adjustment of the quarter-wave plate of ⁇ / 4 radian;
• FIG. 6 is a graph illustrating, with a constant energy of 300 microjoules, the pulse duration emitted when the current injected to the pump diodes and the voltage applied to the shut-off means are varied.
• a device 1 comprises a pulsed laser oscillator 15 in the form of a laser cavity 15. It also comprises supply means 12 of the elements of the pulsed oscillator.
• the supply means 12 comprise current supply means 11 and voltage supply means 10.
• the laser cavity 15 comprises a laser medium 4.
• the laser medium 4 is a crystal commonly called YAG, or yttrium aluminum garnet of composition Y3AI5O12 doped with neodymium.
• the laser cavity 15 has an effective length 170 mm. It is closed by two totally reflective mirrors 2, 8 for the laser radiation at the wavelength of 1064 nm.
• the first mirror 2 is plane and the second mirror 8 is concave with a radius of curvature of 2000 mm so that the cavity 15 is stable with a beam diameter of about 0.9 mm on the plane mirror 2.
• a diaphragm of diameter 1, 2 mm can be placed just in front of the mirror 2 so as to select the TEMoo Gaussian mode of the cavity 15.
• the laser cavity 15 if the thermal stresses of the laser medium 5 are high, it is also possible to configure the laser cavity 15 to satisfy the criterion of insensitivity to thermal lens variations, that is to say with a stability parameter between 0.4 and 0.6, preferably close to 0.5.
• the Nd: YAG laser crystal 4 is cut in the form of a half-cylinder of length 15 mm and diameter 4.4 mm.
• a stack of three laser diode arrays 5 fed by a current generator 1 1 pumps this crystal through the cylindrical face.
• the unabsorbed pump radiation during a first pass through the laser crystal is reflected by the reflective processed planar back face for the pumping wavelength at 808 nm.
• the Laser beam that forms in the cavity is amplified in the pumped area following a path parallel to the axis of the half-cylinder.
• the device for coupling the laser light towards the outside of the cavity consists of a polarizing plate 3, inclined at the Brewster angle.
• the reflectivity of this polarizing plate depends on the polarization state of the incident light.
• a quarter wave plate 6 having a means of rotation about the axis of the cavity, allows the cavity to operate either in the relaxed regime or in the triggered mode according to the orientation of the axes of the blade.
• the cavity is triggered by a pair of electro-optical crystals 7 commonly referred RTP or RbTjOPO 4 ).
• the crystals 7 of RTP are matched in length and their axes are crossed such that, without applied voltage, their overall birefringence is zero. In this configuration, they are simply equivalent to a phase plate and their polarization properties are almost insensitive to temperature.
• the X and Z axes of the RTP crystals are oriented at 45 ° with respect to the polarization plane defined by a coupling polarizer 3.
• the triggering electric field is applied along the Z axis of each crystals by means of gold electrodes 13 deposited on the orthogonal faces at Z.
• a pulse generator 10 delivers an adjustable pulse voltage between 0 and 500V, synchronized with the falling edge of the pumping current of the diodes. The value of the voltage is controlled, for example, controlled by a potentiometer 10.
• the concave mirror 8 is mounted on a piezoelectric ceramic 9 used to enslave the optical length of the cavity. In this way, the injected frequency can remain in resonance with a mode of the cavity.
• the optimization of the cavity is performed in several steps so as to obtain pulses of variable duration with a low trigger voltage.
• the cavity mirrors 2 and 8 are conventionally adjusted, without voltage applied to the electrooptical crystals 7, by acting on means for adjusting the rotation of the mirrors 2 and 8.
• a current slot of a duration of 100 microseconds is injected into the pump diodes 5.
• the optical axis of the quarter wave plate 6 is oriented so as to obtain a maximum of laser energy in relaxed mode, that is to say in optimization coupling in the cavity. Indeed, by rotating the quarter wave plate 4, the polarization state of the incident beam on the coupling polarizer 3 varies and consequently the effective reflectivity of the coupling polarizer also varies.
• the current of the diodes is increased to the maximum allowed by the supply 1 1 or up to the maximum recommended by the manufacturer of the diodes, that is to say, for example to 80 amperes.
• the quarter-wave plate is rotated by an angle ⁇ such that the laser passes just below the laser threshold, here.
• the angle is for example here set at 0.4 radian.
• the angular coordinate system that determines ⁇ corresponds to the alignment of the axis 5
• optical waveguide 4 in the plane of polarization defined by the plane of incidence of the coupling polarizer 3.
• FIG. 4 illustrates this behavior in which without applied voltage, the laser threshold corresponds to a neighboring coupling reflectivity of 44%.
• the voltage that is applied to the RTP 7 crystals has the effect of reducing the reflectivity and thus the loss by coupling.
• the laser cavity 15 can then emit.
• the pulse duration is adjustable between more than 50 ns and 17 ns when the supply voltage varies between 50 volts and 220 volts.
• the supply voltage is set according to the pulse duration to be obtained. Once this supply voltage is fixed, possibly following a modification, depending on the pulse duration to be obtained, the supply voltage remains constant as a function of time during each pumping and emission cycle. After remission, the supply voltage can be changed again.
• This configuration is illustrated FIG. 5.
• FIG. 6 where is represented the current in the pump diodes and the duration of the transmitted pulse as a function of the voltage applied to the switch Q 7 for a constant energy of 300 microjoules. These two functions are represented with a good approximation by polynomials of degree three for the pulse duration and of degree five for the current of the diodes 5.
• the Applicant has determined a polynomial of 2.10 "10 ⁇ 5 -1 .10 " 7 x 4 + 4.10 "5 x 3 - 0.0048 x 2 + 0, 1059 x + 77.213 for the current curve, and a polynomial of 6.10 " 7 x 3 - 0.0005 x 2 + 0.0018 x + 51, 255 7

Landscapes

• Physics & Mathematics (AREA)
• Electromagnetism (AREA)
• Engineering & Computer Science (AREA)
• Plasma & Fusion (AREA)
• Optics & Photonics (AREA)
• Lasers (AREA)

Priority Applications (3)

Applications Claiming Priority (2)

Publications (1)

Family

ID=36693753

Family Applications (1)

Country Status (5)

Families Citing this family (2)

Citations (4)

Family Cites Families (11)

• 2006
  • 2006-05-03 FR FR0651575A patent/FR2900771B1/fr not_active Expired - Fee Related
• 2007
  • 2007-05-03 EP EP07765990A patent/EP2013950A1/fr not_active Ceased
  • 2007-05-03 JP JP2009508435A patent/JP2009535832A/ja active Pending
  • 2007-05-03 WO PCT/FR2007/051208 patent/WO2007125269A1/fr not_active Ceased
  • 2007-05-05 US US12/299,208 patent/US20090196315A1/en not_active Abandoned

Patent Citations (4)

Non-Patent Citations (2)

Also Published As

Similar Documents

Legal Events