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
  • 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
  • H01S5/18—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities
  • H01S5/183—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL]
• • 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
  • H01S5/1071—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
  • 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

Definitions

• the field of the invention is that of solid state gyrolasers used in particular in inertial units. This type of equipment is used, for example, for aeronautical applications.
• the laser gyro developed about 30 years ago, is widely marketed and used today. Its operating principle is based on the Sagnac effect, which induces a difference in frequency ⁇ v between the two optical transmission modes propagating in opposite directions, said counterpropagating, of a bidirectional ring laser cavity animated by a movement of rotation.
• the frequency difference ⁇ v induced between the two optical modes by the rotational movement is equal to:
• the amplifying medium is a gaseous mixture of helium and neon atoms in appropriate proportion.
• the gaseous nature of the amplifying medium is a source of technical complications during the production of the laser gyro, in particular because of the high purity of gas required and the premature wear of the cavity during its use due, in particular, to leakage. of gases and deterioration of the high voltage electrodes used to establish the population inversion.
• a solid state laser gyrolaser operating in the visible or the near infra-red using, for example, a medium an amplifier based on crystals doped with rare-earth ions such as neodymium, orbium or interbium instead of the helium-neon gas mixture; the optical pumping being then provided by laser diodes operating in the near infra-red. This removes, de facto, all the problems inherent to the gaseous state of the amplifying medium.
• the object of the invention is a solid-state gyrolaser comprising a semiconductor medium with an external cavity and consisting of assembled discrete elements, thus offering the possibility of making cavities of large size making it possible at the same time to reach the accuracies. and insert optical elements into the cavity.
• the subject of the invention is a gyrolaser comprising at least one ring optical cavity and a solid state amplifier medium arranged so that two optical waves of average wavelength ⁇ 0 can propagate in the opposite direction to the interior of the cavity, characterized in that the dimensions of the cavity are substantially greater than those of the amplifying medium and that said amplifying medium is a semiconductor medium of average optical index n, with a vertical structure comprising a stack of zones gain flat and parallel to each other.
• the semiconductor medium comprises a plane mirror disposed under the gain zones and parallel to said zones so that the two optical waves propagating inside the cavity are reflected by said mirror, after crossing the zones of gain.
• the optical waves propagating inside the cavity are reflected by the mirror at an oblique incidence i
• the mirror is a so-called Bragg stack optimized to be totally reflective at the average wavelength ⁇ o and under said oblique incidence i
• the stack of the gain zones comprises on the surface opposite to that of the mirror an antireflection treatment at the length of mean wave ⁇ 0 and under the oblique incidence i.
• the amplifying medium is arranged in such a way that the intensity maxima of the interference pattern obtained by the optical waves propagating inside the semiconductor medium are located in the planes of the gain zones, said zones of gains are then distant from each other by - ⁇ ,. .
• the laser gyro comprises means for photo-detecting the intensity of the counter-propagating waves, the intensity modulations of said waves constituting the signal for measuring the speed or the angular position of the laser gyro.
• the invention also relates to an angular measurement system or angular velocity, comprising at least one laser gyro according to the invention.
• the cavities of the gyrolasers of the measurement system are oriented so as to perform measurements in three independent directions.
• FIG. 1 represents a diagram of a laser gyro according to the invention
• FIG. 2 represents the geometry of a semiconductor laser medium in the form of a ribbon
• FIG. 3 represents the geometry of a semiconductor laser medium with a vertical structure
• FIGS. 4 and 5 show the geometry of the standing wave created in a structure by an incident wave and by its reflection on a mirror disposed under said structure;
• FIG. 6 represents the state of polarization of the incident and reflected waves in the case of FIG. 4;
• FIG. 7 represents the geometry of the standing wave created in a structure by two incident waves propagating in the opposite direction and by their reflections on a mirror disposed under said structure;
• FIG. 8 represents the variations of intensity of the standing wave in the configuration of FIG. 7.
• FIG. 1 represents the block diagram of a laser gyro according to the invention. He understands :
• a cavity 1 made of a first material and comprising a plurality of reflecting mirrors 3 and 4, and a partially reflecting mirror 5;
• a semiconductor amplifier medium 2 comprising: optical elements 6 and 7 shown in dashed lines, used, for example, to eliminate the blind zone or to introduce thermal compensations;
• the assembly being arranged so that two optical waves can propagate in two opposite directions inside the cavity. These two waves are represented by a double line in Figure 1. These waves pass through the various optical elements arranged in the cavity;
• an optoelectronic measuring device 8 represented in dashed lines making it possible to calculate the angular parameter measured from the interference pattern of the two counterpropagating waves issuing from the partially reflecting mirror 5.
• the main choices concerning the semiconductor medium are:
• FIG. 2 represents such a structure 2.
• the active zone 21 in which the stimulated emission takes place is continuous.
• the emission of the optical beam 22 is by one of the side faces 23.
• the optical mode 22 propagating in this structure can be multimode.
• the geometry of the beam is asymmetrical as indicated in FIG. 2.
• the height of the mode corresponding to its dimension along the so-called fast AR axis is then generally a few microns and its width corresponding to its dimension along the axis AL. said slow is several tens of microns.
• the optical mode propagating in this structure can also be monomode. It is then symmetrical. We then speak of so-called transverse monomode structures.
• transverse single-mode ribbon For gyrometric applications, the use of a semiconductor laser medium in the form of transverse single-mode ribbon is complicated. Indeed, it is necessary that the mode has a diameter of a few microns inside the cavity of the ribbon, and a diameter of several tens of microns outside the cavity. The propagation of the mode in the active zone must also be guided. The use of a transverse non-monomode ribbon is not easier since the mode in addition to being focused and guided on the slow axis must be highly elliptical.
• FIG. 3 shows such a structure.
• the active medium is then discontinuous. It is composed of a stack of 24 thin active zones whose thickness is typically about ten nanometers separated by thicknesses equal to ⁇ / 2n.
• the light is then emitted by the top faces 26 or below 27 and the mode propagating in this type of cavity has a symmetry of revolution.
• These structures are called VCSEL, Vertical Cavity Surface Emitting Laser's acronym, when the laser is completely monolithic, the gain zones then being sandwiched between two Bragg stacks, one totally reflective and the other, the output mirror, having a transmission of about 0.1%.
• VECSEL the English acronym for Vertical External Cavity Surface Emitting Laser.
• the totally reflective mirror can be a mirror of Bragg or a dielectric mirror attached to the structure.
• the treatment of the face of the structure opposite to the mirror may include antireflection treatment. It is also possible, by adjusting its reflection coefficient, to promote the monomode emission of these structures.
• the use of a vertical structure is more appropriate, since the gain zones may have a diameter of one hundred microns, close to the dimensions of the optical beam circulating in the cavity, which also allows propagation of the unguided wave.
• the intensity therefore evolves temporally between a maximum and a minimum with a pulsation equal to ( ⁇ + - ⁇ _), so that it seems that the wave is moving relative to this point.
• the intensity maxima may be superimposed on the gain zones.
• the standing wave is then no longer free to move under the effect of a rotation. This results in a "gain frequency lock" which renders the device unusable as a laser gyro.
• the operation in reflection of these vertical structures makes it possible to overcome the above disadvantages.
• Figure 4 shows a vertical structure 2 sectional view operating in reflection. For simplicity, it is considered that the structure is comparable to an active medium 28 of index n, on which a mirror 29 is deposited.
• the incident wave 30 and the reflected wave 31 by the mirror 29 interfere in the active medium 28.
• This interference zone 32 is shown in Figure 4 by a hatched triangular area.
• the field E + representative of the incident wave is:
• the scalar product E 0+ .E 0 + r depends on the polarization of the incident wave.
• FIG. 6 represents a base of possible states of linear polarization of the incident wave and of the reflected wave, called perpendicular and parallel states, depending on whether the representative vector of the electric field of the wave is in the plane of incidence. or he is perpendicular. These vectors are denoted E + // , E + r // , E +1 , E + ri in FIG.
• the interference pattern corresponding to the intensity It is fixed. It is composed of a network of plane interference fringes, equidistant ⁇ n and parallel to the mirror with a step of
• FIG. 5 shows the structure of the interference fringes 33 in the reference (O, Ox, Oy, Oz). Each parallelepiped represents the position of the intensity maxima.
• Figure 7 shows a vertical structure 2 sectional view operating in reflection.
• the structure is comparable to an active medium 28 of index n, on which a mirror 29 is deposited.
• Co is k, ⁇ .7 - fi> + - ⁇ - H + -t PRP -%) - ⁇ P + - ⁇ + P ⁇ Po E n
• the medium is composed of a stack of thin active zones, making these lines coincide with the active zones, the operation of the laser is optimized.
• the progressive wave introduces at most a variation of a maximum, negligible variation.
• a light beam with an average diameter of 100 microns has 140 maxima.
• the modulation of the gain is at most 1 maximum on 140 or 0.7%.
• Such low modulation does not result in gain lock. It causes a slight modulation of the output power which can advantageously be used as a read signal.
• the totally reflecting mirror is a so-called Bragg stack or a dielectric mirror reported optimized for the desired incidence. This stack or mirror achieves reflection coefficients close to 100%.
• the gain zones, made on top of this stack, ⁇ must be well positioned. For this, their step is - ⁇ - and the
• another stack can be manufactured with a greater or lesser reflection coefficient if it is desired to benefit in the gain zone of a sub-cavity effect increasing the effective gain seen by the cavity of the laser gyro.
• the stack traversed by the pump beam can also be made to be anti-reflective at the wavelength of said pump beam.

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• Physics & Mathematics (AREA)
• Condensed Matter Physics & Semiconductors (AREA)
• General Physics & Mathematics (AREA)
• Electromagnetism (AREA)
• Optics & Photonics (AREA)
• Gyroscopes (AREA)
• Semiconductor Lasers (AREA)
• Lasers (AREA)

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• 2004
  • 2004-11-05 FR FR0411816A patent/FR2877775B1/fr not_active Expired - Fee Related
• 2005
  • 2005-10-26 EP EP05804595A patent/EP1807675A2/de not_active Withdrawn
  • 2005-10-26 JP JP2007538413A patent/JP2008519251A/ja active Pending
  • 2005-10-26 US US11/718,717 patent/US7663763B2/en not_active Expired - Fee Related
  • 2005-10-26 CN CNA2005800399896A patent/CN101061369A/zh active Pending
  • 2005-10-26 WO PCT/EP2005/055574 patent/WO2006048398A2/fr active Application Filing
  • 2005-10-26 RU RU2007120755/28A patent/RU2381450C2/ru not_active IP Right Cessation

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