EP1807675A2 - Semiconductor solid-state gyrolaser having a vertical structure - Google Patents

Abstract

The field of the invention concerns that of solid-state gyrolasers used in inertial navigation systems. This type of equipment is particularly used for aeronautical applications. It is possible to realize a solid-state gyrolaser from optically or electrically pumped semiconductor media. Existing gyrolasers of this last type are monolithic and have reduced sizes. They do not make it possible, on the one hand, to achieve levels of precision comparable to those of gas gyrolasers and, on the other, to implement optical methods for suppressing the frequency coupling at low rotational speeds or the temperature drifts. The invention relates to a solid-state gyrolaser comprising a semiconductor medium and constituted of assembled discrete elements thus offering the possibility to realize large-size cavities making it possible to achieve the desired levels of precision. More precisely, the gyrolaser comprises a ring optical cavity and an external cavity semiconductor amplifying medium having a vertical structure comprising a stack of gain regions that are planar and parallel to one another, the dimensions of the cavity being noticeably greater than those of the amplifying medium, said amplifying medium being used in reflection.

Description

 SEMICONDUCTOR SOLID SOLID GYROLASER WITH VERTICAL STRUCTURE

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 Δv between the two optical transmission modes propagating in opposite directions, said counterpropagating, of a bidirectional ring laser cavity animated by a movement of rotation. Conventionally, the frequency difference Δv induced between the two optical modes by the rotational movement is equal to:

Δv = 4AΩ / XL

where L and A are the length and area of the cavity respectively; λ is the Sagnac effect laser emission wavelength; Ω is the speed of rotation of the assembly. The measurement of Δv, obtained by spectral analysis of the beat of the two emitted beams, makes it possible to know the value of Ω with a very great precision.

In conventional gyrolasers, the amplifying medium is a gaseous mixture of helium and neon atoms in appropriate proportion. The gaseous nature of the amplifying medium, however, 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.

It is possible to produce 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.

It is also possible to make a solid-state laser gyro from optically or electrically pumped semiconductor media. Current gyrolasers of the latter type are monolithic and are small in size. They do not allow on the one hand to achieve accuracies comparable to those of gyrolaser gaseous and on the other hand to implement optical methods either to suppress frequency coupling at low speeds of rotation, phenomenon of the blind zone, to compensate for noise phenomena of thermal origin.

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.

More specifically, 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 λ 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. Advantageously, 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.

Advantageously, 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 λo and under said oblique incidence i and 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 λ ₀ and under the oblique incidence i.

Advantageously, 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 - ^,. .

2 "cos (/)

Advantageously, 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.

Advantageously, the cavities of the gyrolasers of the measurement system are oriented so as to perform measurements in three independent directions.

The invention will be better understood and other advantages will become apparent on reading the following description given by way of non-limiting example and with reference to the appended figures in which: 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; Optionally, 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;

And 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.

Regardless of the materials used and the wavelengths of use, the main choices concerning the semiconductor medium are:

• its structure;

• and its mode of operation in transmission or in reflection. Semiconductor media are available mainly under two types of structure:

• ribbon. 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. In this case, 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. 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.

• Vertical. Figure 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 λ / 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%. When the output mirror is a discrete element, these cavities are called VECSEL, the English acronym for Vertical External Cavity Surface Emitting Laser. In FIG. 3, only the stack of active zones is represented. 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.

For gyrometric applications, 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.

However, these vertical structures can not simply be used for transmission in a laser gyro. Indeed, two-wave against the propagating whose electric field vector are denoted IL + and E with E = O _{0+ +} e ^{i (t +} ^ ^φJ and E _^ = E ₀ e ⁱ _^ ^{(^ F "uj + φ} - ^> k classically representing the wave vector of the wave, ω its pulsation, φ its phase at the origin The + and - signs indicate the direction of propagation of the wave In the cavity of the laser gyro, the field electrical total E _t from the interference of the two waves is:

_C _ Ë _o i (kr-ω ₊ t + φ ₊ ), p -Kk.r-ω.t + φ) and the intensity

I = O _{t t} ^* .E -ω_) t + φ ₊ -φ_)

For a fixed point in the cavity, the intensity therefore evolves temporally between a maximum and a minimum with a pulsation equal to (ω ₊ - ω_), so that it seems that the wave is moving relative to this point. If a vertical structure operating in transmission is available in the cavity, 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.

When illuminated by a single wave, 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. We will indicate by the sign + the parameters related to the incident wave and by the sign + r the parameters related to the reflected wave. We also note the angle of incidence.

In the reference (O, Ox, Oy) of FIG. 4, the propagation vectors k ₊ and k _{+ r} of the incident and reflected waves are written in the active medium, respectively:

r 2π sιn (ι) -2π ^• sir k k, = n) \ and k _{+ r} = --n λ cos (ι) λ ₀ ; os (i)

The field E ₊ representative of the incident wave is:

E _ P _o i (k ₊ .r ^' ~ ω ₄ t + φ ₊ ) ^c + - ^I -o + ^ti and the field E_ representative of the reflected incident wave is written, if the reflection coefficient is VRe " ^Po : É_ = jR É _{Ç) + r C} ^{Cκ'rr → J + ψ <+ φ «)}

In the zone where the two waves are superimposed, the total electric field E _tota |, at the point r is given by:

E _ F r -ω _f t + φ ₍₎ / π ^"E _Q l (k _r .r -θ.) _(T + φ _(φ + ₀₎ tota _l - H) ₊ Vκ + t _{+ r} e _o and the total intensity of the _t which is typically Ê _ωtal .E ^* _{t otal} is equal to:

By replacing the wave vectors by their expressions according to the wavelength λo:

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.

In the case where the wave is in a state of parallel polarization, the dot product E ₀₊ .E _{0 + r} is equal to 0 0 + r co ^ π-2i). In the case where the wave is in a state of perpendicular polarization, the dot product E ₀₊ .E _{0 + 1} . worth

EΛE _{0 + r}

To simplify the expressions, we choose E ₀₊ and E _{0 + r} real and R, reflection coefficient in intensity equal to 1. The total intensity is then rewritten:

The interference pattern corresponding to the intensity It is fixed. It is composed of a network of plane interference fringes, equidistant λ _n and parallel to the mirror with a step of

2n cos (i)

Figure 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. 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. When it is illuminated by two counter-propagating waves of different frequencies, 4 waves 30, 31, 35 and 36 interfere then:

• A first incident wave 30 flowing in a first direction noted + which is, with the same notations as above, i (k ₊ .r -ω.t + φ.J

E _{0 +} e 'reflected wave 31 corresponding to the first incident wave which is VRE ₀₊ e i (- k_.r - ω _f t + q> ₍₊ φ ₀₎

A second incident wave 35 flowing in the opposite direction noted - which is equal to É ₀ _e ^{i (SF} - ° ^{) t + φ)} , • the reflected wave 36 corresponding to this second incident wave which is worth ; The total field E _tota | is written then:

E _ C αKk, r-ω.t + φj fθ C _β K-k_.r-ω ₊ t + φ _τ + φ ₀ ) total ^~ "-0 ₊ ^ + V r \ C ₀₊ C

₊ G gi (k_.r- (0-t + φ-) _ _| _ / p | gi (-k ₊ .r-ω_t + (p + φ ₀ )

As before, to simplify the calculations, one chooses to take the modules of the real electric fields and equal, and R, reflection coefficient in intensity equal to 1. The total field is written then:

- (φ ₊ + φ_ + φ ₀ -ω ₊ -ω t)

'F -total = 2 ^ώ e ^{c 2}

Co is k, τ .7 - ^fi> ₊ - ^β - _H + -t PRP -%) - <P ₊ - <+ P <Po E _n

com

So

We have :

From the expression of the total field, we calculate the value of the total intensity Itotaie which is:

Thus, when two waves circulate in the medium of index n, as is the case when one is in the presence of bidirectional emission in a laser gyro, the maximum intensity is located along fixed lines parallel to the mirror. Figure 8 illustrates this interference pattern.

If 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. There is indeed a progressive wave along these lines, but it introduces only a weak modulation of the gain saturation. Indeed, i! exist generally a large number of maxima within the interfering optical beams. The progressive wave introduces at most a variation of a maximum, negligible variation.

By way of example, for a wavelength λo equal to 1 micron, for an average incidence i of 45 degrees and an average optical index n of 3, two maxima of the traveling wave moving in the plane of the mirrors according to Ox are separated by a distance d equal to: d = \, ≈ ^λ ° ≈ Ql μm 2n sin (/ j 2 «x0.23

Therefore, a light beam with an average diameter of 100 microns has 140 maxima. Thus, and without taking into account the Gaussian profile of the mode which reinforces the role of the center of the beam with respect to its edges, 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.

To optimize this operation, 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, λ must be well positioned. For this, their step is - ^ - and the

2n cos (i) position of the first zone with respect to the stack is optimized so as to take account of any fixed phase shifts so that all the gain zones coincide with the lines parallel to the plane of the structure for which the intensity is maximum.

On the output face, 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. In the case of using a pump beam, the stack traversed by the pump beam can also be made to be anti-reflective at the wavelength of said pump beam.

We also note that the possibilities of multimode transmission by non-uniform saturation of the gain, also called in terminology ang! O ~ Saxon "spatial hole burning" are reduced. Indeed, the pitch of the interference pattern changes little as a function of the wavelength. Thus, for a semiconductor emitting at 1 micron and gain width of 1 nanometer, the pitch changes at most 0.1%. Therefore, the only way for a wave to exploit an unsaturated gain region would be to have a very different wavelength, which is impossible because of the spectral width of the gain.

Claims

1. Gyrolaser comprising at least one ring optical cavity (1) and a solid state amplifier medium (2) arranged so that two optical waves of average wavelength λo can propagate in opposite directions inside. 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, having a vertical structure comprising a stack of gain zones ( 24) flat and parallel to each other. 2. Gyrolaser according to claim 1, characterized in that the semiconductor medium comprises a plane mirror (29) disposed under the gain zones and parallel to said zones so that the two optical waves propagating inside the the cavity are reflected by said mirror after passing through the gain areas. 3. Gyro laser according to claim 2, characterized in that the optical waves propagating inside the cavity are reflected by the plane mirror (29) at an oblique incidence i. 4. Gyrolaser according to claim 3, characterized in that the mirror (29) is a so-called Bragg stack optimized to be totally reflective at the average wavelength λ ₀ and oblique incidence i. 5. Laser gyro according to claim 3, characterized in that the mirror (29) is a mirror reported under the gain zones and designed to be totally reflective at the average wavelength λ ₀ and oblique incidence i. 6. laser gyro according to one of claims 3 to 5, characterized in that the stack of the gain zones comprises on the opposite surface to that of the mirror an optical treatment at the average wavelength λo and under the oblique incidence i. 7. Gyro laser according to claim 6, characterized in that the stack of the gain zones comprises on the surface opposite to that of the mirror antireflection treatment at the average wavelength λ ₀ and under the oblique incidence i. 8. Gyrolaser according to one of claims 3 to 7, characterized in that the amplifying medium is arranged so that the intensity maxima of the interference pattern obtained by the optical waves propagating inside the semi medium -conductor are located in the plans of the gain areas. 9. Gyrolaser according to claim 8, characterized in that the gain zones are distant from each other by "^ 2n cos (z) 10 laser gyro according to one of the preceding claims, characterized in that the laser gyro comprises means for photo-detecting the intensity of the counter-propagating waves, the intensity modulations of said waves constituting the angular velocity measurement signal or angular position of the laser gyro. 11. Angular measuring system or angular velocity, characterized in that it comprises at least one laser gyro according to one of the preceding claims. 12. Measuring system according to claim 11, characterized in that it comprises three gyrolasers whose cavities are oriented so as to make measurements in three independent directions.