Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
  • H01S3/08—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
  • H01S3/083—Ring 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
  • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
  • H01S3/106—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity
  • H01S3/108—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity using non-linear optical devices, e.g. exhibiting Brillouin or Raman scattering
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
  • H01S5/14—External cavity lasers
  • H01S5/141—External cavity lasers using a wavelength selective device, e.g. a grating or etalon
• • 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
  • H01S3/10076—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating using optical phase conjugation, e.g. phase conjugate reflection

Definitions

• the present invention relates to a source of coherent light radiation of the laser type. It relates more particularly to the reduction in the number of longitudinal and / or transverse modes (spectral and / or spatial refinement) of such a source.
• European patent EP284908 describing a device for controlling or adjusting an emission wavelength ⁇ e and an optical power emitted from a semiconductor laser is known.
• the optical power emitted by the laser is sent at least in part to an optoelectric detector device and at least to an optical filtering device which is selective from the point of view of wavelengths. Part of the power sent to this filtering device is transmitted to another opto-electric detector device.
• the semiconductor laser, the filter device and the detectors are integrated on a common substrate.
• the integrated filter device is constituted by a Bragg grating or by a directional coupler or an interference filter or by an assembly formed by two or more Bragg grids arranged one behind the other in the direction of propagation of the power sent and / or by directional optical couplers and / or by interference filters.
• Wavelength selective devices have been proposed in the prior art laser cavity but not self-adjusting.
• the article "82 nm of continuous tunability for an external cavity semiconductor laser” appeared in Electron. Lett. 27, 183 (1991) describes a method of refining and stabilizing a laser diode in an extended cavity using a conventional diffraction grating.
• the diffraction grating is a selective element of the extended cavity.
• the two factors which contribute to the refinement are the dispersion of the network and the length of the extended cavity which reduces the width of Shalo-Towns of the emitted line.
• DFB distributed feedbac
• DBR distributed bragg reflector
• Patent EP433122 is also known which describes a ring cavity laser device. It's about injecting light from a master laser to a slave laser which thus inherits the coherence properties of the master laser. The presence of the photosensitive material does not modify the spectral properties of the master laser since there is no return from the slave cavity to the master cavity.
• the devices of the prior art have various drawbacks.
• the characteristics of the cavity vary, for example by a modification of the length due to thermal variations, the effectiveness of the interference filters proposed in certain devices of the prior art deteriorates.
• the invention relates in its most general sense to a source of coherent light radiation formed by a resonant cavity (laser, optical parametric oscillator called OPO, etc.) comprising an amplifying medium placed inside. of the cavity, and characterized in that a dynamic photosensitive material forms, with the other elements of the cavity, a self-adapting spectral and / or spatial filter.
• a resonant cavity laser, optical parametric oscillator called OPO, etc.
• OPO optical parametric oscillator
• the invention is therefore characterized by the incorporation into the coherent light radiation source of an intracavity optical device achieving a self-adapting and stable spectral and / or spatial refinement.
• Such a device makes it possible to select one or more longitudinal or transverse modes existing in the cavity.
• the invention is based on the inscription, in the dynamic photosensitive material, of a network by the wave structure of the cavity.
• the selectivity of this network associated with one or more mirrors of the cavity suitably chosen, achieves an automatically adapted filter and therefore allows, by using the competition between modes, to improve the temporal and spatial coherence of the emitted light, while preserving the pre-existing possibilities of wavelength change.
• This filter automatically modulates in a different way the losses for each of the possible modes of the cavity and the force to oscillate only on one or a small number of these modes.
• the adaptation is carried out through the inscription by the oscillating beam in the cavity of a network or a hologram in a dynamic photosensitive material. It results from the particular response of the chosen dynamic photosensitive material which only responds to interference networks (spatial variation of the intensity or state of polarization of the light).
• the dynamic photosensitive material consists of a photorefractive material. These materials are sensitive to the gradient of illumination.
• the dynamic photosensitive material is sensitive to spatial variations in the illumination characteristics (intensity or polarization).
• the dynamic photosensitive material is formed by a plurality of plates and placed in the resonant cavity to form a self-adapting spectral and / or spatial filter.
• the cavity is a linear cavity and in that the dynamic photosensitive material is formed by a photorefractive material operating in diffusion mode, oriented to form with the exit mirror of the cavity a Fabry interferometer -Perot which presents a maximum of reflectivity for the mode which registered the hologram.
• the cavity is a linear cavity and in that the dynamic photosensitive material is a material of induced transparency suitably placed.
• the cavity is a ring laser cavity and in that the dynamic photosensitive material is placed at the intersection of two crossed beams.
• the dynamic photorefractive material consists of a barium titanate crystal.
• the proposed invention advantageously avoids the use of Fabry-Perot interferometers to finely select the wavelength. It is applicable to a large number of lasers, including laser diodes and micro-lasers, both pulsed and continuous at all wavelengths where dynamic photosensitive materials are found which are suitable for their sensitivity, resistance to flux and the like. properties necessary for the operation of the device.
• FIG. 1 shows a schematic view of a first alternative embodiment
• FIG. 2 shows a schematic view of a second alternative embodiment
• FIG. 3 shows a diagram of the assembly of an installation comprising a device 1 according to the invention
• - Figure 4 shows the evolution of the ratio between the intracavity energy and the laser output energy as a function of time for different output energies in the stationary state
• - Figure 5 shows the energy stability curve of the S2 / S1 ratio over time, from the stationary state (left curve) and after an operating period of 10 minutes (right curve);
• FIG. 8 shows an interferogram showing the biode mode of the laser.
• FIG. 9 shows a schematic view of a third alternative embodiment.
• the device is developed around a dynamic material (1) photosensitive to the wavelength used.
• this material should only be sensitive to spatial variations in the characteristics of the illumination figure (intensity, local polarization, etc.) and not to its uniform components. This is typically the case in materials whose variation in index or absorption is mainly a function of the modulation rate m t of the interference pattern to which they are exposed:
• the intensity I of mode i as well as the diffraction efficiency of the hologram or associated network increases with. This decreases the other modulation rates mj and the diffraction efficiency of the holograms associated with them.
• the dynamic photosensitive material is the heart of the device which reacts so that its self-adaptation to a mode no i, or to a small number of modes, increases the ratio:
• the reduction in losses for this mode, and / or the increase in losses for the other modes is defined by the position of this dynamic photosensitive material in the device which is made up of this photosensitive material and of usual optical components of the cavities such as mirrors.
• holograms entered absorption or index
• different devices can be envisaged.
• multiple structures can be produced according to the geometry of the cavity (linear, ring, etc.). Examples include the following devices:
• the induced hologram is an index hologram which is in quadrature with respect to the illumination figure.
• the device is shown in the gray rectangle. It is composed of the photorefractive material (1) and the exit mirror (2) of the cavity.
• the network inscribed in the photorefractive material by the standing waves of the cavity constitutes a Bragg mirror.
• the photorefractive material is placed so that the direction of the offset between the illumination network and the index network gives this Fabry-Perot filter maximum reflectivity for the mode having entered the hologram.
• the reflectivity of the device increases for this mode, therefore the losses decrease and the oscillation is favored.
• the selectivity introduced is a function of the distance between the material and the mirror, and of the thickness of the material.
• the mirror is preferably glued against the amplifying medium, the assembly forming a microlaser.
• the material is inserted between the amplifying medium (3) and the output mirror (2).
• the hologram registered by the standing waves corresponding to a mode of the cavity is a Bragg mirror which reflects partially each of the two counter propagating waves.
• the holographic material (1) consists of a photorefractive crystal of barium titanate.
• Self-adaptation is achieved here by writing within a suitable dynamic photosensitive material, a hologram induced by one or a few laser modes.
• FIG. 2 An alternative embodiment in which the laser cavity is in a ring is illustrated in the diagram in FIG. 2.
• the device in the gray rectangle, is formed of the material and of two mirrors including the exit mirror.
• the network or hologram is, as in the previous case, inscribed by the interference which exists in the cavity.
• the network is registered by all the waves which interfere in the photorefractive material. This is oriented so that the reflectivity of the device increases for the mode having registered the hologram.
• the device can also fulfill the function of an optical diode (the laser only oscillates in one direction).
• the device For a hologram of induced transparency, the device consists of the dynamic photosensitive material and of any of the mirrors of the cavity.
• the two counterpropagating beams create a network of transparency which is in phase with the figure of illumination, which decreases the losses for this mode.
• the absorbent centers are preferably in the dark fringes of the illumination figure of the favored mode, which in turn increases the losses for the other modes.
• FIG. 3 represents a diagram of the assembly of an installation comprising a device according to the invention and the control systems which have made it possible to verify its operation.
• the laser is a titanium doped sapphire laser pumped at a rate of 10Hz by an Nd: YAG nanosecond laser doubled in frequency. It emits pulses of duration 50ns in triggering mode by gain. Its tunability is ensured by two adjustable prisms. There is no Fabry-Perot filter in the cavity so that the spectrum obtained is spread over approximately 0.7nm. The homogeneity of the line allows competition between modes.
• the laser cavity of commercial origin has a length of 60 cm.
• the output mirror (mirror B) has a reflection coefficient of 60% and transmits the output S0.
• the dynamic photosensitive material (1) was placed at a distance which could vary from 1mm to a few centimeters from the exit mirror (B), due to the space available in this particular laser.
• the mirror (B) could have been deposited on the crystal, according to an alternative embodiment. Due to the emission wavelength of this laser, a photorefractive crystal of barium titanate doped with cobalt, cut at 45 ° from the optical axis, having an absorption of 0.2cm ⁇ l around 750nm and 2mm thick. This made it possible to postpone the problems of optical damage while taking advantage of the maximum gain of the sapphire laser crystal doped with titanium (around 780nm). This sets the working wavelength to 760nm. To test the operation of the device according to the invention, three systems have been implemented:
• an LS2 glass slide placed in front of the prisms extracts part of the intracavity beam (output S2).
• a second LSI blade extracts part of the beam at output S0 (signal SI).
• Two fast photodiodes PI and P2 on the outputs SI and S2 make it possible to observe the accumulation of energy in the laser cavity, resulting from the increase in reflectivity due to the self-adjusting filter;
• a plane Fabry-Perot interferometer makes it possible to visualize the multiple interference rings and therefore to observe the spectral refinement of the laser,
• a Michelson interferometer makes it possible to assess the coherence length of the laser source.
• a Michelson interferometer shown in FIG. 3 and the path difference ⁇ of which is varied between the two arms, makes it possible to verify that this energy transfer is accompanied by an increase in the coherence length of the source.
• photorefractive crystal Without photorefractive crystal in the laser cavity, it is of the order of 1 mm, in agreement with a spectral width of the order of 0.7 nm.
• a coherence length greater than 60 cm was measured.
• the spectral refinement is observable using a plane Fabry-Perot interferometer with a thickness of 3 mm (free spectral interval of 50 GHz) and the spectral refinement is visualized by a reduction in the number of rings.
• Figure 7 On the same image ( Figure 7), on the left 1 interferogram obtained without photorefractive crystal in the laser cavity and right, that obtained when the photorefractive crystal is placed 1 mm from the output mirror.
• the spectral refinement is clearly demonstrated. This result confirms that the line obtained at a width of 1 GHz.
• the proposed invention reduces the spectral width of the laser. In addition, it does not require any fine adjustment. You just have to place approximately the photorefractive crystal so that the output mirror crystal assembly increases its reflectivity.
• the device according to the invention has a fineness comparable to that of the usual adjustable devices but also has self-adaptability properties which give it its advantage.
• the interest of the device is great for sources tunable in wavelength.
• the wavelength adjustment by the usual devices can be carried out according to the invention; the line width is automatically reduced by the device whatever the wavelength chosen. It suffices for this that the material is photosensitive over a sufficiently wide wavelength range.
• the laser emission is generally multimode.
• Fabry-Perot interferometers coupled to birefringent filters or prisms. These devices are complex to use and are not self-adapting.
• the resonant cavity is a linear cavity and in that the dynamic photosensitive material is formed by a photorefractive material located inside the main cavity placed at the intersection of the beams oscillating in the cavity main and all or part of the beam emitted by this cavity and returned to the photorefractive material by a set of optical components.
• Figure 3a shows a schematic view of a third alternative embodiment.
• FIG. 9 A third possible geometry, adapted to a linear cavity, is shown in FIG. 9.
• the resonant cavity consists of the mirror 31 and the coupler 22.
• the mirrors 3 and 4 located outside this cavity make it possible to return the beam emitted on the crystal. photorefractive.
• This beam interferes with the beams present in the resonant cavity to register a hologram.
• This hologram reinjects light from the beam 35 into the main cavity.
• a spatial or spectral filter can be inserted into the cavity formed by the mirrors 32, 33, 34 and the photoreactive material for provide additional filtering or selection of wavelength or spatial mode.
• the applications relate in particular to the field of telecommunications for transmission in continuous regime, for the production of a single-mode source avoiding mode jumps.

Landscapes

• Physics & Mathematics (AREA)
• Electromagnetism (AREA)
• Optics & Photonics (AREA)
• Engineering & Computer Science (AREA)
• Plasma & Fusion (AREA)
• Condensed Matter Physics & Semiconductors (AREA)
• General Physics & Mathematics (AREA)
• Nonlinear Science (AREA)
• Lasers (AREA)
• Holo Graphy (AREA)

Priority Applications (5)

Applications Claiming Priority (2)

Related Child Applications (1)

Publications (1)

Family

ID=9532094

Family Applications (1)

Country Status (7)

Cited By (2)

Families Citing this family (11)

Citations (1)

Family Cites Families (7)

• 1998
  • 1998-10-28 FR FR9813525A patent/FR2785459B1/fr not_active Expired - Fee Related
• 1999
  • 1999-10-28 EP EP99950876A patent/EP1125347B1/fr not_active Expired - Lifetime
  • 1999-10-28 JP JP2000578881A patent/JP4372354B2/ja not_active Expired - Fee Related
  • 1999-10-28 DE DE69905188T patent/DE69905188T2/de not_active Expired - Lifetime
  • 1999-10-28 CA CA002347995A patent/CA2347995A1/fr not_active Abandoned
  • 1999-10-28 WO PCT/FR1999/002644 patent/WO2000025396A1/fr not_active Ceased
• 2001
  • 2001-04-25 US US09/841,723 patent/US6674782B2/en not_active Expired - Fee Related

Patent Citations (1)

Non-Patent Citations (5)

Also Published As

Similar Documents

Legal Events