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
  • H01S3/106—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
  • H01S3/109—Frequency multiplication, e.g. harmonic generation
• • 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
  • H01S3/08022—Longitudinal modes
  • H01S3/08031—Single-mode emission
  • H01S3/08036—Single-mode emission using intracavity dispersive, polarising or birefringent elements
• • 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
  • H01S3/02—Constructional details
  • H01S3/04—Arrangements for thermal management
  • H01S3/0401—Arrangements for thermal management of optical elements being part of laser resonator, e.g. windows, mirrors, lenses
• • 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
  • H01S3/02—Constructional details
  • H01S3/04—Arrangements for thermal management
  • H01S3/0405—Conductive cooling, e.g. by heat sinks or thermo-electric elements
• • 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
  • H01S3/08022—Longitudinal modes
  • H01S3/08027—Longitudinal modes by a filter, e.g. a Fabry-Perot filter is used for wavelength setting
• • 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
  • H01S3/08086—Multiple-wavelength emission
• • 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
  • H01S3/09415—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light of a laser diode the pumping beam being parallel to the lasing mode of the pumped medium, e.g. end-pumping
• • 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
  • H01S3/1671—Solid materials characterised by a crystal matrix vanadate, niobate, tantalate
  • H01S3/1673—YVO4 [YVO]

Definitions

• the present invention relates to a continuous laser device tripled in frequency in intra-cavity, pumped by diode; and comprising an amplifying medium, a non-linear birefringent medium for frequency doubling, and a non-linear birefringent medium for frequency tripling.
• UV ultraviolet
• CD mastering or the semiconductor inspection
• the frequency tripling of a diode pumped continuous laser requires two non-linear conversion stages ( ⁇ + ⁇ and 2 ⁇ + ⁇ ) and can only be effective within one or at least two resonant cavities.
• the resonant frequency doubling is possible intra-cavity or in an external cavity, slaved to the frequency of the emission of the laser. In both cases, a single frequency fundamental emission is necessary. In the first case (intra-cavity) it is necessary to eliminate the noise. In the second case it is necessary because the strongly resonant cavities (great fineness) are very narrow spectrally.
• the second external cavity stage is very complex if one seeks a double resonance with the fundamental wave and the harmonic wave, because two optical paths (fundamental wave and harmonic wave) must be controlled.
• the present invention more particularly relates to intra-cavity tripling which is easier to implement because the resonance of the fundamental wave is automatic.
• the laser cavity is elongated by the insertion of nonlinear crystals and it is much more difficult to make the laser single frequency.
• the object of the present invention is to design a continuous wave (CW) laser for frequency-tripled intra-cavity frequency and operating in single frequency.
• Another object of the invention is the design of such a laser operating in a stable manner, ie by limiting, if necessary, the phenomenon of birefringence interference.
• At least one of the aforementioned objectives is achieved with a continuous laser device tripled in frequency in intra-cavity, pumped by diode; said device comprising: an amplifying medium, a non-linear birefringent medium for frequency doubling, and a non-linear birefringent medium for frequency tripling; these media are usually crystals.
• the laser device further comprises a polarizing medium arranged so as to constitute, with at least one of the birefringent crystals, a birefringent filter or Lyot filter in an intra-cavity, said Lyot filter being adapted to allow an emission single frequency output of said laser device.
• the birefringence axes of the non-linear crystals are not parallel to the axes of the polarizing medium. If they are parallel, a birefringent crystal is inserted between the amplifying medium and the polarizing medium, this birefringent crystal having its birefringence axes preferentially oriented at 45 ° to the axes of the polarizing medium.
• the output emission wavelength is in the ultraviolet (UV) range. It is the whole of the resonant cavity that can constitute a Lyot filter.
• the polarizing medium is advantageously arranged between the amplifying medium and the frequency doubling medium. More specifically, these media are crystals such that: for the amplifying medium: Nd: YAG and Nd: YVO 4 or any other crystal or glass doped by any rare earth or globally any doped glass or crystal having a transition that may oscillate in a laser cavity, - for the frequency doubling medium: KTP, KNbO 3 , BBO,
• BiBO, and LBO or any other nonlinear crystal adapted to frequency doubling for the frequency tripling medium BBO, BiBO, LBO or any other nonlinear crystal suitable for frequency tripling.
• the other advantage of the Lyot filter is that the emitted wavelength is the one with the lowest losses and therefore it is the one whose polarization at the output of the polarizer is parallel to the axis of least loss.
• the power distribution between the two axes of the doubling and tripling crystals is therefore perfectly controlled and stable.
• the axes of the doubling and frequency tripling medium are oriented substantially between 30 and 60 ° with respect to the axes of the polarizing medium.
• the orientation is 45 °.
• the doubling and tripling crystals can be cut and arranged so as to achieve phase matching ("phase matching" in English ") type I and / or II, without the device becoming unstable.
• the polarizing medium comprises one or two Brewster interfaces (interfaces at an angle between two refractive index medium ni and n 2 such that the tangent of the angle is equal to the ratio of indices).
• all other media are preferably parallel-faced crystals.
• the device according to the invention constitutes a monolithic linear resonant cavity.
• Linear cavities are usually the shortest. This small size allows a separation of the axial mode as wide as possible, which is beneficial for the efficiency of a single frequency operation.
• the design of the device may be such that each medium comprises an inlet face and an exit face parallel to each other and to the other faces of the others. environments; these faces being orthogonal to the output direction of the triplated laser beam.
• the amplifying medium, the polarizing medium and the doubling and tripling frequency mediums are optically contacted with each other, which greatly facilitates obtaining a single frequency emission and also reduces the manufacturing costs. It is therefore not necessary to insert focusing elements to adjust the mode size in the non-linear elements as is done in the prior art.
• the length of the nonlinear crystals is generally optimized according to the output power of the UV. If the ISL obtained is not of the order of magnitude of the emission width, it can be adjusted by an additional birefringent crystal. Indeed, it can further provide a second birefringent element disposed after the polarizing medium, the second birefringent medium being adapted to adjust the Free Spectral Interval (ISL) of the Lyot filter if necessary.
• / _sz where ⁇
• the laser device comprises means for controlling the temperatures of non-linear media.
• the filter is tuned by an agreement of the temperature of the crystals.
• the change in the temperature of the birefringent crystals induces a slight displacement of the modes of the cavity and a variation in general more fast of the central wavelength of the peak ⁇ m .
• a finer positioning of the wavelength of the mode in the center of the filter can be obtained by modifying the temperature of the amplifying medium for example. Thus, it is possible to tune the laser to a wavelength and to center the transmission mode on the filter.
• the laser device comprises: a highly reflective mirror (HR) at the fundamental wavelength, this mirror being disposed on the input face of the amplifying medium; and a highly reflective output mirror (HR) at the fundamental wavelength, which mirror is optionally disposed on the output face of the frequency-tripling non-linear birefringent medium.
• HR highly reflective mirror
• HR highly reflective output mirror
• the laser device may also comprise: a highly reflective mirror (HR) at the frequency-tripled wave, this mirror being arranged between the two nonlinear birefringent doubling and tripling frequency mediums, this makes it possible to preserve the crystals arranged in upstream of the tripler crystal against UV waves and increase the UV power at the output of the laser; and a highly reflective mirror (HR) at the frequency doubled wave, this mirror being disposed between the polarizing medium and the non-linear frequency doubling birefringent medium.
• HR highly reflective mirror
• FIG. 1 is a simplified diagram of a first UV laser according to the invention
• FIG. 2 is a simplified diagram of a second UV laser according to the invention.
• FIG. 1 shows a laser according to the invention for an emission of 7mW of single-frequency power at 355nm with a 2.4W pump.
• This laser device comprises a pump diode D associated with a focusing element F for guiding the beam emitted by the diode to
• the crystal doubler 808nm to an input side of a crystal amplifier A.
• the X2 is disposed between the polarizing element P and the tripler crystal X3.
• the amplifier crystal, the polarizing element, and the doubling and tripling crystals are optically contacted in this order and in a linear fashion. We took care to insert on each side four mirrors.
• the first Peltier element Pl is in contact with the pump diode assembly D and focusing element F. This first Peltier element makes it possible in particular to control the emission wavelength of the diode and to cool the diode.
• the second Peltier element P2 is in contact with the amplifying crystal and the polarizing element P. Its function is to cool the amplifier and can be used to finely adjust the wavelength of the cavity mode.
• the third Peltier element P3 is in contact with the doubling crystal X2.
• the fourth Peltier element P4 is in contact with the X3 tripler crystal.
• the set is fixed on the same support S.
• the fundamental beam is at its "waist" (focal point) on the mirror.
• the beam is thus well focused in the crystal tripler, but it may have strongly diverged in the doubling crystal. It is generally preferable to use a length of crystal tripler a little shorter than the length optimal so as not to degrade the conversion of the fundamental to the second harmonic.
• the frequency tripled wave generation is in both directions as soon as part of the harmonic wave is reflected by the mirror M2. It is desirable to prevent this wave (usually located in the UV) from propagating in the other crystals of the laser, as many crystals age in the presence of UV. On the other hand, by adjusting the propagation phase in the tripler crystal (by a temperature adjustment), it is possible to increase the output power of the tripled wave by the insertion of the mirror M3.
• the power of the second harmonic in the cavity is increased by inserting the mirror M4, reflector to the harmonic wave and ensuring that the mirror M2 is also reflective at the harmonic wavelength.
• the cavity between the mirrors M2 and M4 becomes resonant when the propagation phase in a round trip is close to 0 modulo 2% radians. This phase can be regulated by the temperature of the doubling crystal, but especially by the choice of the wavelength emitted.
• FIG. 2 It is possible to have a single temperature control for the two nonlinear crystals according to FIG. 2.
• a laser is shown in a very schematic manner for which the non-linear doubler 3 and the tripler 5 crystals are not not directly contiguous " to the amplifier 1.
• the Brewster 2 blade serves as a polarizing element
• the 1064nm amplifying crystal is a Nd: YV0 4 doped at 1.1% and 1mm long
• the input side of this crystal amplifier 1 is HR treated (highly reflective) at 1064nm (> 99.8%)
• Brewster 2 is a flat, highly melted silica plate of 1mm, the nonlinear group has 4 elements 3 to 6 which are optically bonded.
• the second crystal 5 is a frequency-tripling crystal
• the LBO crystals are sandwiched between two fused silica plates 4 and 6.
• the output blade 6 is HR treated at 1064 nm (99.65%) and the transmission at 532 nm and 355 nm are respectively 2 to 7% (depending on the mirror ) and 95%.
• Input blade 4 is HR treated at 355 nm (98%) to prevent UV emission from entering the KTP crystal.
• the total cavity length is about 20mm.
• the polarizing medium which may be the combination of Nd: YVO 4 and Brewster's slide, in combination with the birefringent crystals rotated at 45 ° allows to obtain a Lyot filter or birefringent filter.
• the set is temperature controlled by a three Peltier effect of 2W. This allows tuning the peak of the filter wavelength that can be achieved in a temperature range of 1 to 2K. These two crystals tolerate large temperature variations in phase agreement, which makes it possible to maintain the nonlinear frequency conversion.
• the laser is pumped by a diode 3W 1 * 100 ⁇ m 808 nm.
• the focusing element F is a GRIN lens.
• the diode is also temperature controlled by a Peltier effect.
• the Nd: YVO 4 amplifier crystal is controlled by a Peltier effect.
• Type II frequency doubling is generally discouraged because it causes a problem of birefringence interference.
• the laser device of FIG. 2 overcomes this problem by proposing a solution for operation in single frequency.
• the axes of the frequency-doubling crystal 3 of type II in FIG. 2 and the axes of the tripler crystal 5 are aligned at 45 ° with respect to the Brewster angle.
• the polarization of the Nd: YVO 4 is aligned with the Brewster polarization so that the whole of the cavity constitutes a birefringent filter or Lyot filter.
• the wavelength with 100% transmission is linearly polarized in the Brewster blade and also separated on the two polarization axes of the frequency doubling crystal (maximum frequency doubling efficiency).
• the table below shows a set of possible crystal configurations.
• the doubling or tripling efficiency can be 100% when the polarization is optimal.
• Preferred configurations are not necessarily optimized for maximum frequency conversion, but for better stability and simplicity.

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

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