Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B17/00—Systems with reflecting surfaces, with or without refracting elements
  • G02B17/02—Catoptric systems, e.g. image erecting and reversing system
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/01—Arrangements or apparatus for facilitating the optical investigation
  • G01N21/03—Cuvette constructions
  • G01N21/031—Multipass arrangements
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00

Definitions

• the present invention relates to an optical system with extended beam propagation.
• An optical system is defined by a series of optical elements through which a light beam propagates.
• a system is developed comprising a minimum number of optical elements distributed over a small surface but allowing a very large beam propagation length.
• FIG. 1 gives the principle in the case of an extended cavity.
• the output beam of a laser 1 is coupled to an optical fiber 2 by means of collimation optics 3.
• a Bragg grating 4 provides reflection at the end of the cavity.
• the optical fiber is wound into a coil, which allows a large propagation distance between the laser and the array in a relatively small space.
• This solution widely used today, however requires the passage in guided optics with coupling losses.
• the radius of curvature of the fibers must also be of the order of a centimeter minimum, which limits the integration.
• Figure 2 shows a first embodiment of beam propagation. It is simply a matter of placing mirrors 10 facing each other, so as to fold the beam 11. For comparison, an identical beam 12 is shown without extension. If the use of the extension makes it possible to increase the distance traveled by the beam compared to a straight path
• FIG. 3 gives a known example of using optics 13 to correct the divergence of a system equivalent to that of FIG. 2.
• the increase in the number of lenses considerably increases the alignment adjustment constraints.
• the present invention overcomes the disadvantages of the prior art.
• an optical system with an optical beam propagation extension having an input and an output of the optical beam comprising at least two optical elements of beam reflection arranged to extend the propagation of the beam by reflection on the reflection elements, the optical system also comprising at least one optical beam transmission element, characterized in that the reflection optical elements have non-collinear optical axes, the transmission optical element being arranged to be traversed at least twice by said next optical beam different directions during its propagation in the optical system, the optical transmission element ensuring an optical transformation of the beam at each crossing so as to correct its divergence.
• the transmission optical element is a ball lens and the optical reflection elements are plane mirrors. In the case of a plane mirror, the optical axis is perpendicular to the plane of the mirror.
• the transmission optical element is a lens cylindrical having an axis of symmetry, the reflection optical elements being cylindrical mirrors of axis of symmetry perpendicular to the axis of symmetry of the cylindrical lens.
• the optical axis is perpendicular to the axis of symmetry of the mirror.
• the optical transmission element is at least partially made of an optically active material.
• at least one of the optical reflection elements comprises an optically active material.
• the optical reflection elements may be arranged to propagate the optical beam first in a direction of reflection, then in the opposite direction, the input and the output of the optical beam being merged.
• the optical system may consist of at least two elementary optical systems cascaded.
• one of the optical reflection elements is semi-reflective to serve as an output to the optical beam.
• the invention also relates to a gas sensor of the absorption measurement type, comprising an optical system with extended beam propagation as described above.
• FIG. 1 represents a first optical system with extended beam propagation according to the known art
• FIG. 2 represents a second optical system with beam propagation extension according to the known art
• FIG. 3 represents an optical system of the type of FIG. 2 provided with corrective elements of the divergence of the beam
• FIG. 4 illustrates a first variant of an optical system according to the invention
• FIG. 5 illustrates a second variant of optical system according to the invention
• FIG. 6 illustrates a third variant of optical system according to the invention
• FIG. 7 illustrates a fourth variant of optical system according to the invention
• FIG. 8 illustrates a fifth variant of an optical system according to the invention
• FIG. 9 illustrates a sixth variant of an optical system according to the invention.
• FIG. 10 illustrates a seventh variant of optical system according to the invention
• FIG. 11 illustrates the geometrical principle of propagation of the beam in a system according to the invention
• FIG. 12 is an equivalent propagation diagram of the unfolded system of the invention.
• FIG. 13 is an explanatory diagram
• FIG. 14 illustrates a method of manufacturing the invention
• FIG. 15 illustrates a particular application of an optical system according to the invention.
• the principle of the beam propagation extension system according to the invention is based on the use of transmission optics having symmetries in order to correct the divergence of the beam, and folding mirrors in order to extend the propagation distances.
• the symmetries of the optics allow a beam to cross it several times in different directions and thus guarantee the compactness of the system.
• FIG. 4 is illustrative of the principle used by the present invention.
• the optics 20 used has a circular symmetry with respect to its center.
• a beam 21 first passes through the lens 20, then is reflected a first time by a first mirror 22.
• the mirror 22 orients the beam to a second mirror 23 which directs the beam towards the center of the lens 20. This being symmetrical, the second crossing is equivalent to the first.
• the optical axes of the mirrors 22 and 23 are not collinear.
• FIG. 5 represents a three-dimensional view of another variant of the system according to the invention. This variant uses two planar mirrors 32 and 33 of non-collinear optical axes and a spherical lens 30. In this case, the lens 30 alone allows the divergence correction of the beam 31.
• FIG. 6 represents a three-dimensional view of yet another variant of the system according to the invention.
• This variant uses two cylindrical mirrors 42 and 43 with non-collinear optical axes and a cylindrical lens 40.
• the lens 40 corrects the divergence in a first direction and the mirrors 42 and 43 correct the divergence in the other perpendicular direction. while reflecting the beam 41.
• FIG. 7 shows a mirror and single lens system.
• the system comprises a part 50 having a cavity equipped with fifteen mirrors referenced 51 to 65 non-collinear optical axes.
• the cavity also allows the housing, in the central part, a lens (spherical or cylindrical) 66.
• Spherical lenses will be preferred because they allow to correct the complete divergence of the beam (according to the two axes).
• a cylindrical lens which corrects only the divergence along an axis (that perpendicular to the axis of the lens), can be used for systems that are not constrained in dimension along the axis of this cylinder.
• a light beam 67 enters the cavity being directed towards the mirror 51 after passing through the lens 66. It is returned successively to the other mirrors in the increasing order of their references through the lens 66 after two reflections. When the beam reaches the last mirror, the mirror 65, it is reflected on itself and in the opposite direction the path it has done previously. The beam then emerges from the cavity by the place where it had entered. It is readily apparent that the propagation distance between the input and the output of the beam can become large despite a reduced component area and the use of a single optic. This example typically corresponds to the production of an extended cavity.
• FIG. 8 represents a system comprising two subsystems 71 and 72 of the type of FIG. 7 cascaded in a room 70.
• a beam 73 entering the subsystem 71 emerges from this subsystem to be directed into the sub-system. and 72 to exit the subsystem 72.
• the system of Figure 8 can be used in the case of a transmission delay line.
• Figure 9 shows another application of the present invention.
• the transmission optics and divergence correction element 80 (a spherical lens) is constructed of an active material.
• active material is meant a material capable of emitting an optical wave in stimulated or spontaneous emission under the effect of pumping.
• the system shown comprises four mirrors referenced 81 to 84 non-collinear optical axes.
• This propagation extension system is closed so as to produce a stable cavity.
• One of the reflection elements, the mirror 83 has a lower reflection coefficient than the others to allow the output of the laser beam 85.
• An optical source 86 serves as a pump to excite the active material.
• An additional mirror 87 optionally makes it possible to increase the confinement of the pump beam 88 in the spherical lens 80.
• the beam is amplified because of the interaction of the pump beam with the active material.
• FIG. 10 shows a four-mirror system comprising three reflecting mirrors 91, 92 and 93 and a mirror semi-reflective 94 for extracting the optical wave.
• the mirrors 91 to 94 are covered with a layer of active material 101 to 104 respectively.
• the pumping of the layers of active material 101 to 104 is obtained by means of pumping diodes 111 to 114, respectively.
• the mirrors 91 to 94, non-collinear optical axes, transmit the pump beam.
• the beam passes twice through the ball lens 90.
• FIG. 11 describes the geometrical principle of propagation of the beam in a system according to the invention.
• the beam F is divided into main segments M n M ' n and secondary M' n M n + 1 .
• Mirrors, with non - collinear optical axes, are positioned at the ends of these segments on the circle C of radius R.
• the center O of the circle C is the point of origin of the orthonormal coordinate system x, O, y.
• the radius R may correspond to a beam incident to a mirror, of angle ⁇ .
• the secondary segment must not cross the LS spherical lens. We can therefore define a geometrical condition:
• the optical system according to the invention is based mainly on the use of Gaussian beams often encountered in integrated optics.
• a simple case is to consider the following conditions:
• - first condition the propagation distances remain equal between each crossing of the lens
• - second condition the "waist" (that is to say the minimum radius of the beam) is positioned in the middle of each secondary segment.
• the second condition implies that the distances separating the positions of "waists" object and image at the focal points object and image are equal.
• the "waists" object and image are the same size and the optical systems are growing.
• FIG. 12 gives an equivalent propagation diagram of the unfolded system, in which the beam is rectilinear. Mirrors are represented by dotted lines, spherical lenses by circles.
• the focal length of a ball lens of index ni and diameter Di is:
• FIG. 13 gives the different values of minimum angles 0C min as a function of the diameter Di of the lens for different sizes of "waists"
• the glass index is 1.5 and the wavelength considered is 1.55 ⁇ m).
• N ⁇ R / (2 W M ) (13) This last equation is to be calculated with the value ⁇ min in the definition of W M. It must also be ensured that the inclination of the mirrors and their size do not cause the occultation of the beams reflected by the neighboring mirrors. For this, N must remain rather weak.
• a ball lens with a diameter of 4 mm and a refractive index of 1.5 is considered.
• Equations (8), (9) and (10) give a minimum deflection angle of 40.8 °.
• Equations (7), (8) and (12) give for this minimum angle value a support radius of the mirrors:
• a surface component smaller than 1 cm 2 allows a propagation distance of 32 cm.
• the number of mirror reflections can be high, it is important that each angular deflection be performed with the greatest precision.
• an error of 0.01 degree on the angle of the mirrors leads to a shift of about 5 microns in the position of the output beam.
• a preferred embodiment will therefore be one that implements the production of mirrors by lithography.
• the mirrors can then be made directly by deep etching of a substrate according to the mirror planes or by molding techniques.
• Figure 14 schematically shows a mold 121 and a molded substrate 122.
• the mold has negative cavity formed by the succession of mirror planes.
• Another solution is to position the mirrors one by one on a substrate by gluing. The positioning must then be very precise.
• the above description has been directed to an optical system for propagation of a beam over a large distance, generally using a single optical transmission element. To do this, this transmission element must have particular properties of symmetry and the beam must be directed frequently by mirrors arranged appropriately.
• the particular feature of the invention is to allow, in a small space, a significant propagation of a wave in the air (as opposed to propagation in an optical fiber where the electromagnetic wave is confined in silica). This structure can therefore be used for absorption sensor applications.
• a typical example is the gas sensor.
• the presence of a gas in the atmosphere results in the increase of the absorption coefficient ⁇ for specific wavelengths specific to the gas in question.
• Measuring the absorption, and therefore the gas concentration is by measuring the factor p, root of the ratio of the intensity of the transmitted signal t by the intensity of the original signal I 0 to the length of characteristic wave:
• Equation (14) shows that if ⁇ is very small, large propagation lengths are required to measure a significant p-factor.
• the present invention provides a solution that meets both of these requirements.
• Ltot 2 NR (l +
• This value also includes the propagation in the lens.
• FIG. 15 illustrates an example of application as absorption sensor of the optical system according to the invention.
• An optical source 131 is first filtered by a filter 132 to select the significant wavelength (s) and the emitted light beam is injected into an optical fiber 50/50 coupler 133.
• One half of the signal is received by the optical detector 134 and serves as a reference.
• the other half of the signal goes to the sensor 135 constituted by an optical system according to the invention.
• the sensor 135 is placed in the gas 136 whose absorption value is to be measured.
• the signal After propagation in the optical system 135, the signal is reflected in the input fiber and, after passing through the coupler 133, is received by the detector 137.
• the ratio of the signals of the detectors 134 and 137 allows the measurement of the value of the absorption and therefore the concentration of gas.

Landscapes

• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Optics & Photonics (AREA)
• Health & Medical Sciences (AREA)
• Life Sciences & Earth Sciences (AREA)
• Chemical & Material Sciences (AREA)
• Analytical Chemistry (AREA)
• Biochemistry (AREA)
• General Health & Medical Sciences (AREA)
• Immunology (AREA)
• Pathology (AREA)
• Investigating Or Analysing Materials By Optical Means (AREA)
• Optical Couplings Of Light Guides (AREA)

Applications Claiming Priority (2)

Publications (1)

Family

ID=34948882

Family Applications (1)

Country Status (4)

Families Citing this family (2)

Family Cites Families (4)

• 2004
  • 2004-10-07 FR FR0452298A patent/FR2876461B1/fr not_active Expired - Fee Related
• 2005
  • 2005-10-06 US US11/576,845 patent/US7554738B2/en not_active Expired - Fee Related
  • 2005-10-06 WO PCT/FR2005/050821 patent/WO2006037932A1/fr active Application Filing
  • 2005-10-06 EP EP05810795A patent/EP1797413A1/de not_active Withdrawn

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events