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
  • H01S3/1106—Mode locking
  • H01S3/1112—Passive mode locking
• • 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
  • H01S2301/00—Functional characteristics
  • H01S2301/08—Generation of pulses with special temporal shape or frequency spectrum
  • H01S2301/085—Generation of pulses with special temporal shape or frequency spectrum solitons
• • 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
  • H01S3/05—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

Definitions

• the subject of the present invention is a laser generator with modes locked in phase, comprising a resonant cavity delimited by a rear reflecting mirror, and a semi-reflecting output mirror.
• FIG. 1 schematically shows in a conventional laser source the arrangement of a rear mirror 1 and of an exit mirror 2 which define between them a path of length L during which the light passes through the amplifying active medium 3.
• the nature and the geometry of the active medium 3 are not indicated in a limiting manner in FIG. 1.
• the shape of the light intensity i ( t) as a function of time is represented in FIG. 3.
• the correlation time dt which represents the inverse of the spectral width f of the transmitted signal, is indicated in FIG. 3.
• the frequency spectrum I (f) of the periodic noise i (t) of FIG. 3 is represented in FIG. 4.
• the frequency spectrum corresponding to a conventional laser emission comprises several groups of equidistant lines whose periodicity df is the inverse of that (T) of the time signal i (t).
• df c / 2L (2) where L represents the distance between the two mirrors 1 and 2 and c is The speed of Light.
• the intensities and the phases of the groups of equidistant lines are distributed randomly. It is therefore through a certain abuse of language that the frequency groups on which the laser energy is concentrated are called "modes".
• modes In a conventional laser emission, of the noise type The amplitude of the power fluctuations in Noise is equal to the average power P of the radiation.
• the laser emission regime represented in FIGS. 5 and 6 is generally designated by the terms "synchronization, phase lock or lock of the oscillator modes". In this type of laser emission, the energy is concentrated in narrow pulses much more powerful than the average power and being able to present a maximum power PN P. (T / dt) C3) where T ax —— represents The period 2L / c of the relation (1 ) and dt represents the inverse of the spectral width k f.
• a blocking in so-called “passive” phase is carried out by introducing into the cavity a medium whose transparency changes as a function of the light intensity. It is a mixture of dye and solvent (chlorobenzene, dichloroethane, etc.) called “saturable absorbent”.
• This Liquid contained in a cell, has a low transparency at low lighting level, then a transparency which becomes very high when the incident intensity exceeds a characteristic value: the intensity of saturation. A light pulse whose intensity exceeds this threshold is transmitted with negligible attenuation.
• the emission of pulsed lasers in blocked modes is in the form of a series of fifteen brief pulses (30 ps) spaced about 10 ns and whose amplitude is modulated by an envelope of Gaussian time profile .
• the most powerful impulse carries energy of the order of mJ.
• the saturable absorbent solution wears out and must be renewed frequently (every 40 hours of use approximately),. these absorbents exist only for a few emission wavelengths in a limited number, and cannot be suitable for all types of laser,
• a device if the pumping repetition rate exceeds 1 Hz, a device must be provided to circulate the dye.
• the method of synchronizing the modes of the laser emission consists in periodically modulating the losses of the cavity at a frequency equal to c / 2L according to the above-mentioned relation C2), by means of an acoustical modulator. optical. Only The light signal in phase with this modulation can be amplified, the emerging laser beam then consists of a continuous series of brief light pulses, for example of the order of 100 picoseconds for YAG / Nd and Argon lasers, which are spaced for example 10 nanoseconds. The energy of each of these
• phase locking of the modes of a continuous laser generator also has drawbacks.
• each pulse emitted results from a large number of back and forth movements in the laser cavity.
• the modulation frequency of the acousto-optical element C (which is generally between 80 and 100 MHz) must always correspond exactly to the inverse of a time of back and forth in the oscillator. It follows that the stability of the modulator must be extremely stable, remaining accurate to about ten hertzs.
• the microwave sources used must then be stabilized by quartz and the modulation frequency must also be periodically readjusted to take account of variations in length of the cavity due to variations in temperature.
• the technique of acousto-optics and its modulation source is expensive.
• a device called “soliton laser” has been proposed.
• a laser generator with synchronized modes with colored centers comprising conventional acousto-optical means of additional active modulation, is coupled with a second cavity called “soliton pulse” to effect regulation external to the main laser cavity of the generator.
• the external regulation loop comprises a non-modal nonlinear dispersive fiber in which propagates a picosecond pulse.
• iL is necessary that the basic laser generator is of the continuous type with a wavelength greater than 1.3 j ⁇ i.
• Such a device has all the aforementioned drawbacks of continuous laser generators with synchronized modes, in particular the need to use an additional active modulation of the acousto-optical type, and involves the implementation of a second cavity external to the cavity of the. basic laser generator, using an optical fiber non-linear dispersive. The device is therefore complex, bulky, delicate to develop and can only be applied to a well-defined type of laser generator.
• the object of the present invention is to remedy the abovementioned drawbacks and to produce a laser generator with locked modes in the simple and safe design phase, which does not require frequent readjustments after the initial settings, which can operate over a very wide range of lengths. wave and improves power stability between successive pulses, due to its operating principle.
• a laser generator of the type mentioned at the head of the description characterized in that it further comprises a device for locking in phase the modes of the laser generator, placed between the amplifying active medium and the rear reflecting mirror. and comprising on the one hand, a non-linear non-dispersive medium placed in contact with said rear mirror and on the other hand, means for transforming a Gaussian beam with symmetry of revolution from the amplifying medium into a beam of morphology adapted to La soliton propagation in said nonlinear medium.
• the present invention is applicable to laser generators of the continuous type as well as to laser generators of the pulsed type, whatever the wavelength.
• Intermodal synchronization is ensured by the soliton propagation itself carried out inside the cavity of the basic laser generator and iL is not necessary to add to the basic laser generator neither additional modulator nor additional laser cavity.
• the pulsed quasi-monochromatic beam is propagated in a non-linear non-dispersive medium.
• the means for transforming a Gaussian beam comprise an interference device for dividing the amplitude of the laser beam to form two secondary waves, a spatial filter and an afocal system arranged on the path of the two secondary waves, to form on the face input of the non-linear medium opposite the rear mirror a reduced image of the interference figure created in the interference device for amplitude division.
• the interference device, the afocal system and the spatial filter can themselves be produced in various ways.
• the means for transforming a Gaussian beam comprise a WoLLaston biprism, a half-wave blade arranged on the path of one of the two secondary waves formed by the Wollaston biprism, a first converging lens, a spatial filter and a second converging lens.
• the means for transforming a Gaussian beam comprise a WoLLaston biprism, a half-wave plate disposed on the path of one of the two secondary waves formed by WoLLaston biprism, first and second mirrors arranged on the paths of the two secondary waves to make them convergent and a spatial filter placed in front of the non-linear medium.
• the means for transforming a Gaussian beam comprise an interference device constituted by a network or a hologram creating two secondary waves, a first converging lens, a spatial filter and a second converging lens.
• the means for transforming a Gaussian beam further comprise a cylindrical lens whose focal point coincides with the entry face of the non-linear medium to form on this face an interference zone having the shape of a very rectangular elongate.
• the non-linear place can be constituted by a tank containing a homogeneous non-linear material such as carbon disulphide and having a transparent entry face as well as a face opposite to the entry face which is closed by the reflecting mirror. back.
• a homogeneous non-linear material such as carbon disulphide
• the non-linear medium includes a transverse single-mode non-linear planar guide consisting of two layers of a transparent dielectric material of low refractive index enclosing a non-linear material of higher refractive index, and the rear mirror is in contact with the planar non-linear guide perpendicular to the different layers making up this guide.
• the amplifying active medium can consist of a semiconductor laser diode having an anti-reflection treated output face and the means for transforming a Gaussian beam include a converging lens interposed between the laser diode and the non-reflective medium. linear plane single transverse mode.
• the amplifying medium can also comprise a network of laser diodes implanted on the same substrate and having an anti-reflection treated output face, and the means for transforming a Gaussian beam include an optical system consisting of an afocal device and 'A spatial filter to produce a simple image of the output face of the diode array on the input face of the non-linear guide, so as to excite only a higher order mode.
• FIG. 1 symbolically represents a conventional laser generator with independent modes
• FIG. 2 symbolically represents a conventional laser generator with synchronized modes
• FIGS. 3 and 4 respectively represent the temporal structure and the spectral structure of a laser emission in independent modes, that is to say show the curves giving the intensity i (t) of the laser beam emitted as a function of time t and
• Figures 5 and 6 respectively represent La time structure and spectral structure of a laser emission in synchronized modes and show curves giving Intensity i (t) as a function of time t and Intensity I (f) as a function of frequency f in a parallel manner to the curves of FIGS. 3 and 4, FIG.
• FIG. 7 shows in perspective the soliton self-confinement of a laser beam in a medium with non-linearity Kerr
• FIG. 8 shows in perspective an example of a laser generator whose modes are locked in phase by soliton beam in accordance with the present invention
• FIG. 9 shows an example of the form of electrical signal delivered by a fast response photodetector from a laser emission with phase locked modes produced in accordance with the invention.
• FIG. 10 shows in perspective an example of a flat non-linear guide usable in a laser generator according to the invention
• FIGS. 11 to 13 schematically show three exemplary embodiments of phase-locking devices for the modes of a laser generator, using soliton propagation in a non-linear medium, and
• FIGS. 14 and 15 schematically show laser generators with modes locked in phase by soliton beam, using respectively a laser diode and an array of laser diodes.
• non-linear material means a material whose refractive index varies as a function of the intensity of the light beam passing through this material (optical Kerr effect). Optical non-linearity is also often referred to as "self-induced variation in the refractive index”.
• An essential means of the present invention lies in the implementation of a soliton propagation for a particular purpose which makes it possible to achieve phase locking of the modes of a laser generator.
• a soliton propagation for a particular purpose which makes it possible to achieve phase locking of the modes of a laser generator.
• Figure 8 shows an example of a device for implementing the present invention.
• FIG. 8 thus shows a laser generator with an output mirror 2, the transmission rate of which may for example be 35%, which is crossed by an output laser beam 8 in synchronized modes.
• An active amplifying medium 3 for example a neodymium-doped glass rod pumped by flashes, which can be operated for example on a diameter of 5 mm, is conventionally interposed between the output mirror 2 and a fully reflecting rear mirror 1.
• a cell of a non-dispersive linear medium 5 for example a tank containing a Kerr liquid such as carbon disulfide is placed in such a way that the rear mirror 1 of the resonant cavity is in contact with the non-linear cell.
• the mirror 1 can thus directly constitute one of the faces of the tank containing the Kerr Liquid 5.
• Different optical elements conventional in themselves, are interposed between the amplifying medium 3 and the non-linear medium 5 to transform the main laser beam passing through the amplifying medium 3, so that this beam, which usually exhibits symmetry of revolution and substantially Gaussian, or transformed into a beam of morphology conducive to soliton propagation on the entry face of the non-linear medium 5 opposite the rear mirror 1.
• FIG. 8 a bi-prism of WoLLaston 11 which creates an interference figure and divides the main laser beam 20 into two secondary beams 21, 22.
• a half-wave plate 12 is placed on one of the secondary beams to make parallel The orthogonal polarizations created by The Wollaston prism 11.
• Two convergent circular lenses 13, 15 produce an afocal system which forms a reduced image (for example of a factor of four) of the interference figure created in the bi-prism of WoLLaston 11.
• a cylindrical lens 16, the focus of which coincides with the plane of the entry face of cell 5 confines the light beam along the horizontal dimension so as to give the interference zone the shape of a very elongated rectangle (as in FIG. 7 ) .
• a spatial filter 14 placed between the lenses 13, 15 contributes to filtering the power of the laser beam.
• the soliton beam does not exist: the rear mirror 1 receives a divergent cylindrical wave which does not correspond to a mode of the resonant cavity. At soliton power, the rear mirror 1 receives a parallel beam capable of going back and forth without distortion between the mirrors 1 and
• the oscillator produces a sequence of periodic pulses of powers equal to the soliton power, at the transmission factor near the output mirror 2. If the illumination is too weak The beam diverges. If the illumination is higher than that desired, there is the appearance of a chaotic process, l 1 autofocusing, which destroys the spatial and temporal coherence of the radiation. There is therefore a limit both upper and lower than the power necessary to generate the self-guided propagation of a geometric soliton structure and therefore to the power of the pulses emitted by the laser generator.
• the proposed technique consisting in introducing into the laser generator a device based on lenses and polarization components in order to transform a Gaussian beam with symmetry of revolution into a beam of adequate morphology, as well as a tank containing Kerr's liquid 5 and one of the mirrors 1 of the cavity has all of the following advantages:
• the device is adjusted once and for all and should not require frequent intervention.
• the device can operate over a very wide range of wavelengths (visible-near infrared). - The stability in power between successive pulses is improved due to the very principle of operation.
• FIG. 10 represents an example of an electrical signal delivered by a nanosecond response photodetector receiving a laser emission with phase locked modes according to the invention.
• Different conventional optical combinations can be used to transform a usual beam with symmetry of revolution into a beam of morphology suitable for soliton propagation as it should appear at the entrance to the non-linear medium 5.
• Each of these combinations can give rise to a different configuration of laser cavity.
• a first configuration which corresponds to the embodiment of FIG. 8, has been shown in enlarged form in FIG. 11.
• the lenses 13, 15 of FIG. 11 are replaced by two mirrors 113, 115 intended to make the secondary beams 21, 22 converging formed by a Wollaston bi-prism 111 similar to the corresponding element 11 in FIG. 11.
• the half-wave plate 112 placed on one of the secondary beams 21, 22 aims to make the polarizations of the beams 21, 22 parallel.
• the cylindrical lens 116 plays the same role as the lens 16 in FIG. 11.
• a filtering slot 114 is placed in front of the non-linear medium 5.
• a network or a hologram 211 is placed on the main beam 20 to form a network of interference fringes, and replaces the whole of the bi-prism of Wollaston 11 and La Lame demi -wave 12 of FIG. 11.
• the elements 213 to 216 can be similar to the elements 13 to 16 of FIG. 11.
• a homogeneous non-linear medium constituted for example by a filled tank Kerr liquid 5.
• a planar non-linear guide 50 as shown in FIG. 10, can replace the homogeneous non-linear material located in contact with the rear mirror 1 in the diagram of FIGS. 8 or 11 to 13.
• soliton autoguiding in a plane is associated with guidance in the perpendicular plane, through the walls (horizontal in this example) of a plane guide of light waves.
• This planar optical guide consists of two layers 51, 52 of transparent dielectric of low index n., Which serves as a sheath, and which encloses the non-linear medium 52 (Kerr) of index higher than n, forming the core.
• the characteristic dimensions of the incident beam are chosen so that only one mode of the guide (higher order mode or fundamental mode) is excited.
• the index n ⁇ of the layers 51, 53 is greater than the index no of the ambient medium in which the guide 50 is placed but in all cases remains less than the variable index of the layer 52.
• the use of a non-linear guide plane 50 is particularly advantageous When it is necessary to lower the threshold for formation of the soliton beam, for example in the case where the amplifying active medium 3 is pumped continuously and therefore generates lower powers.
• the non-linear material used to cooperate with the rear mirror 1 of the resonant cavity must have good transparency at the emission wavelength, have a high coefficient of variation of the refractive index. with the intensity and have a response time of the Kerr effect less than the reciprocal duration of the amplification band of the amplifying medium.
• the present invention is also applicable to laser generators of the semiconductor laser type. In this case, it is necessary to use a flat non-linear guide similar to that of figure 10.
• a laser diode 30 In the case of a laser diode 30, this must have a face 31 treated with anti-reflection and a lens 314, for example sphero-cylindrical is disposed between the diode 30 and the non-linear planar guide 50 associated with the rear mirror 1 to adapt the beam to optimal injection into the non-linear transverse single-mode plane guide 50.
• the diode 30 has a cleaved face 32 opposite the face 31 treated with anti-reflection.
• FIG. 15 shows a laser generator in which an array of laser diodes 130 are installed on the same substrate and having a face 131 treated with anti-reflection.
• An optical system 13, 14, 15 made up of elements similar to the corresponding elements in FIG. 11 produces a simple image of the face 131 of the array of laser diodes 130 on the input face of the non-linear guide 50, so as to excite only a higher order mode.

Landscapes

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

Applications Claiming Priority (2)

Publications (1)

Family

ID=9348239

Family Applications (1)

Country Status (5)

Families Citing this family (9)

Family Cites Families (3)

• 1987
  • 1987-02-24 FR FR8702411A patent/FR2611320B1/fr not_active Expired
• 1988
  • 1988-02-23 US US07/276,448 patent/US4928282A/en not_active Expired - Fee Related
  • 1988-02-23 EP EP88902148A patent/EP0303667A1/de not_active Withdrawn
  • 1988-02-23 WO PCT/FR1988/000100 patent/WO1988006811A1/fr not_active Application Discontinuation
  • 1988-02-23 JP JP63502216A patent/JPH01503505A/ja active Pending

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events