Optical parametric oscillator

Abstract

The invention relates to an optical parametric oscillator (2) comprising: - a laser source system (10) producing a pump light beam (100); - a dichroic mirror (20) receiving the pump light beam and transmitting a first combination light beam (102); - a focusing optical system (30) receiving the first combination light beam; - a non-linear medium (40), the non-linear medium producing as output a second combination light beam (104); - a focusing optical system (32) receiving the second combination light beam; - a dichroic mirror (22) receiving the second combination light beam, the dichroic mirror reflecting, in a first direction, the signal light beam (108); and - a hollow optical fiber (70) receiving the signal light beam, the hollow optical fiber being wound onto itself.

Description

DESCRIPTION TITLE: Optical Parametric Oscillator TECHNICAL FIELD OF THE INVENTION

[0001] The technical field of the invention is that of light sources and more particularly parametric light sources.

[0002] In particular, the invention relates to an optical parametric oscillator.

[0003] The present invention is particularly advantageously applicable to imaging in the biomedical field or in the field of spectroscopy. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0004] Light sources based on optical parametric interaction are particularly advantageous because they make it possible to provide laser wavelengths that cannot be obtained from conventional light sources based on electronic transitions.

[0005] An optical parametric oscillator (OPO) forms such a parametric light source, mainly used to produce wavelengths unattainable by the use of conventional laser sources.

[0006] In general, an optical parametric oscillator comprises a nonlinear medium for frequency conversion and an optical cavity. The optical cavity includes two reflecting mirrors located on either side of the nonlinear medium. This cavity allows the radiation to oscillate, thereby amplifying it through successive passes through the nonlinear medium.

[0007] Furthermore, to achieve the desired wavelengths, the optical cavity used must be at least a few meters long. Therefore, the size of the optical parametric oscillator is significant, and consequently, the overall size of such a device is considerable.

[0008] Furthermore, the higher the power of the incident beam, the more the spatial distribution of the light beam at the output of the optical parametric oscillator degrades. The efficiency of the optical parametric oscillator is therefore also degraded. SUMMARY OF THE INVENTION

[0009] The present invention therefore proposes a compact optical parametric oscillator, allowing the spatial quality of the beam obtained at the output to be maintained and with improved efficiency.

[0010] One aspect of the invention relates to an optical parametric oscillator comprising:

[0011] - a laser source system generating a pump light beam,

[0012] - a first dichroic mirror receiving the pump light beam and transmitting a first combination light beam resulting from a combination of said pump light beam and a signal light beam,

[0013] - a first optical focusing system receiving the first combination light beam,

[0014] - a non-linear medium receiving the first combination light beam focused by the first optical focusing system, the non-linear medium generating at its output a second combination light beam resulting from the combination of the first combination light beam and a complementary light beam generated by the non-linear medium,

[0015] - a second optical focusing system receiving the second combination light beam,

[0016] - a second dichroic mirror receiving the second collimated combination light beam via the second focusing optical system, the second dichroic mirror reflecting, in a first direction, the signal light beam and transmitting, in a second direction, the pump light beam, and

[0017] - a hollow optical fiber receiving at input the signal light beam reflected by the second dichroic mirror, the hollow optical fiber allowing the propagation of the signal light beam in air, the signal light beam emerging from the hollow optical fiber being directed towards the first dichroic mirror, the hollow optical fiber being wound on itself.

[0018] Thus, advantageously according to the invention, the use of an optical fiber in the cavity of the optical parametric oscillator makes it possible to obtain a compact parametric oscillator operating over a wide range of repetition frequencies for the pump light beam.

[0019] Furthermore, the hollow nature of the optical fiber eliminates dispersion phenomena during the propagation of the signal light beam. This improves the spatial quality of the light beam propagating through the optical parametric oscillator, and therefore also improves its efficiency.

[0020] Furthermore, winding the hollow optical fiber around itself improves the compactness of the optical parametric oscillator regardless of the repetition frequency of the pump light beam, and therefore reduces its size when used for a intended application (for example in biomedical imaging).

[0021] Advantageously, the present invention thus makes it possible not only to reduce the size, but also to considerably broaden the application possibilities of optical parametric oscillators by offering greater flexibility and adaptability.

[0022] In addition to the characteristics mentioned above, the optical parametric oscillator according to the invention may have one or more additional characteristics from the following, considered individually or in all technically possible combinations:

[0023] - the second dichroic mirror reflects, along the first direction, the signal light beam and transmits, along the second direction, a third combination light beam resulting from the combination of the pump light beam and the complementary light beam;

[0024] - the second dichroic mirror reflects, along the first direction, a fourth combination light beam resulting from the combination of the signal light beam and the complementary light beam and transmits, along the second direction, the pump light beam;

[0025] - The optical parametric oscillator also includes:

[0026] a1) a third optical focusing system positioned at the input of the hollow optical fiber so as to focus the signal light beam at the input of the hollow optical fiber, and

[0027] a2) a fourth optical focusing system positioned at the output of the hollow optical fiber so as to collimate the signal light beam at the output of the hollow optical fiber;

[0028] - the third focusing optical system and the fourth focusing optical system have the same focal length, a first distance between the third focusing optical system and the inlet of the hollow optical fiber being on the order of said focal length, a second distance between the fourth focusing optical system and the outlet of the hollow optical fiber being on the order of said focal length;

[0029] - The optical parametric oscillator also includes:

[0030] b1) a half-wave plate positioned at the output of the second dichroic mirror, and

[0031] b2) a beam splitter positioned between the half-wave plate and the hollow optical fiber input so as to separate the signal light beam into a first signal portion and a second signal portion, the second signal portion being directed towards the hollow optical fiber;

[0032] - the beam separation device includes a polarizing beam splitter cube;

[0033] - The optical parametric oscillator also includes:

[0034] c1) a first quarter-wave plate positioned at the input of the hollow optical fiber, and

[0035] c2) a second quarter-wave plate positioned at the output of the hollow optical fiber;

[0036] - the non-linear medium is formed by a lithium niobate crystal;

[0037] - a radius of curvature of the hollow optical fiber wound on itself is greater than 20 centimeters;

[0038] - the distance traveled by the signal light beam between the second focusing optical system and the input of the hollow optical fiber is on the order of the distance traveled by the signal light beam between the output of the hollow optical fiber and the first focusing optical system;

[0039] - Hollow optical fiber is a single-mode optical fiber for a signal light beam wavelength between 1.2 and 1.8 micrometers;

[0040] - a diameter of the mode field of the hollow optical fiber is greater than 30 micrometers;

[0041] - the signal light beam emerging from the hollow optical fiber is directed straight to the first dichroic mirror without passing back through the hollow optical fiber; and

[0042] - the optical parametric oscillator also includes a mirror for reflecting the signal light beam back to the hollow optical fiber, the signal light beam being directed towards the first dichroic mirror by passing back through the hollow optical fiber. BRIEF DESCRIPTION OF THE FIGURES

[0043] Other features and advantages of the invention will become apparent from the description, which can be read in conjunction with the figures. These figures are provided for illustrative purposes only and are not intended to limit the scope of the invention.

[0044] Figure 1 schematically and functionally represents a first embodiment of an optical parametric oscillator according to the invention,

[0045] Figure 2 schematically represents a first example of an embodiment of the first mode of the optical parametric oscillator according to the invention,

[0046] Figure 3 schematically represents a second example of an embodiment of the first embodiment of the optical parametric oscillator according to the invention.

[0047] Figure 4 schematically represents a third example of an embodiment of the first embodiment of the optical parametric oscillator according to the invention,

[0048] Figure 5 schematically represents a fourth example of an embodiment of the first embodiment of the optical parametric oscillator according to the invention,

[0049] Figure 6 schematically represents a fifth example of an embodiment of the first embodiment of the optical parametric oscillator according to the invention,

[0050] Figure 7 schematically and functionally represents a second embodiment of an optical parametric oscillator according to the invention,

[0051] Figure 8 schematically represents a first example of an embodiment of the second embodiment of the optical parametric oscillator according to the invention,

[0052] Figure 9 schematically represents a second example of the second embodiment of the optical parametric oscillator according to the invention,

[0053] Figure 10 schematically represents a third example of the second embodiment of the optical parametric oscillator according to the invention,

[0054] Figure 11 schematically represents a fourth example of the second embodiment of the optical parametric oscillator according to the invention,

[0055] Figure 12 schematically represents a fifth example of the second embodiment of the optical parametric oscillator according to the invention,

[0056] Figure 13a represents an example of the spatial distribution of the electric field of a near-field light beam at the output of the optical parametric oscillator according to the invention,

[0057] Figure 13b represents a cross-section of the electric field intensity of Figure 13a along a first direction,

[0058] Figure 13c represents a cross-section of the electric field intensity of Figure 13a along a second direction,

[0059] Figure 14a represents an example of the spatial distribution of the electric field of a far-field light beam at the output of the optical parametric oscillator according to the invention,

[0060] Figure 14b shows a cross-section of the electric field strength of Figure 14a along one direction, and

[0061] Figure 14c represents a cross-section of the intensity of the electric field of Figure 14a along a second direction.

[0062] For clarity, identical or similar elements are identified by identical reference symbols across all figures. DETAILED DESCRIPTION

[0063] The present invention aims to provide a more compact optical parametric oscillator, ensuring the spatial quality of a light beam propagating within this oscillator in order to achieve high efficiency. In other words, it specifically proposes a compact, efficient, simple, and practical optical parametric oscillator that is inexpensive to implement and manufacture.

[0064] Figure 1 represents, schematically and functionally, a first embodiment of an optical parametric oscillator 1 according to the invention.

[0065] As shown in Figure 1, in this first embodiment, this optical parametric oscillator 1 comprises a laser source system 10, a first dichroic mirror 20, a first focusing optical system 30, a nonlinear medium 40, a second focusing optical system 32, a second dichroic mirror 22 and a hollow optical fiber 70. Optionally, and represented by dashed boxes in Figure 1, the optical parametric oscillator 1 also comprises a half-wave plate 50, a beam splitter 60, a first quarter-wave plate 52, a third focusing optical system 34, a fourth focusing optical system 36 and a second quarter-wave plate 54.

[0066] It should be noted here that all elements other than the nonlinear medium form the cavity of the optical parametric oscillator. Within the framework of this first embodiment, and as will be apparent from the rest of the description with reference to figures 1 to 6, the cavity of the optical parametric oscillator has an annular configuration.

[0067] These different elements included in the optical parametric oscillator 1; 2; 3; 4; 5; 6 according to this first embodiment are described in detail in the remainder of this description within the framework of the different examples of embodiments of the invention illustrated in figures 2 to 6.

[0068] Figure 2 schematically represents a first example of the realization of the optical parametric oscillator 2 according to the first embodiment of the invention.

[0069] This optical parametric oscillator 2 comprises, firstly, the laser source system 10. This laser source system 10 is configured to generate a pump light beam 100. The laser source system 10 generates laser pulses with an associated energy below one microjoule. The wavelength emitted by the laser source is, for example, 1035 nanometers (nm). Preferably, the energy of the laser pulses generated by the laser source system 10 is between a few nanojoules and a few hundred nanojoules.

[0070] The frequency of the pump light beam is denoted here as fp.

[0071] The laser source system is configured, for example, to generate a pump light beam with a fixed frequency or a variable frequency.

[0072] The optical parametric oscillator 2 then includes the first dichroic mirror 20. This first dichroic mirror 20 receives the pump light beam 100. It also receives an output portion 120 of a signal light beam (described below), which it reflects. The first dichroic mirror 20 transmits a first combination light beam 102. This first combination light beam 102 results from the combination of the pump light beam 100 (transmitted by the first dichroic mirror 20) and the output portion 120 of the signal light beam (reflected by the first dichroic mirror 20).

[0073] For this, the first dichroic mirror 20 has a transmittance greater than 99.5% at 1 micrometer (pm) and a reflectance greater than 99.5% between 1.35 and 1.7 pm.

[0074] In practice, as can be seen in Figure 2, the first dichroic mirror 20 forms a non-right angle with the propagation direction of the pump light beam 100. This allows, in particular, the reception of the output portion 120 of the signal light beam, which propagates in a different direction (than that of the pump light beam). This arrangement of the first dichroic mirror 20 makes it possible to obtain the annular configuration for the cavity of the optical parametric oscillator 2.

[0075] As shown in Figure 2, the optical parametric oscillator 2 comprises the first focusing optical system 30. This first focusing optical system 30 receives the first combination light beam 102 (transmitted from the output of the first dichroic mirror 20). This first focusing optical system 30 is adapted to focus the first combination light beam 102 onto the nonlinear medium 40.

[0076] In practice, the first focusing optical system 30 includes, for example, a converging lens with a focal length f1. Preferably, the distance between the first focusing optical system 30 and the nonlinear medium 40 is on the order of the focal length f1.

[0077] The optical parametric oscillator 2 then includes the nonlinear medium 40. This nonlinear medium 40 is responsible for the frequency conversion performed by the optical parametric oscillator 2. This nonlinear medium 40 is particularly suitable for generating a complementary light beam (generally called an "idler") of frequency f c and a beam of light signal at frequency fs. Each of the frequencies f c of the complementary light beam and fs of the signal light beam is less than the frequency f Pof the pump light beam. Conservation of energy then imposes the following relationship on the frequencies: f p = f s + f c The non-linear medium 40 then allows the frequency conversion from the pump light beam to two generated light beams (complementary and signal) through non-linear optical interaction.

[0078] In practice, the nonlinear medium 40 is formed by a nonlinear crystal. Preferably, this is a periodically polarized lithium niobate crystal (usually denoted MgO:PPLN). Advantageously, according to the invention, the gain associated with such a MgO:PPLN crystal allows obtaining a gain of the optical parametric oscillator greater than 1000.

[0079] The size (in particular the length, dimension along the optical axis A) of the nonlinear medium 40 (for example the size of the periodically polarized lithium niobate crystal) is advantageously chosen as a function of the duration of the pump light beam 100 and the difference in group velocities between the desired frequencies of the pump light beam, the signal light beam and the complementary light beam.

[0080] For example, in the case of a laser pulse duration of the pump light beam (for a wavelength of 1035 nm) on the order of picoseconds, the length of the nonlinear medium is greater than 10 mm, preferably greater than 20 mm. In the case of a pulse duration on the order of femtoseconds, the length of the nonlinear medium is on the order of 3 mm.

[0081] At the output, the nonlinear medium 40 generates a second combination light beam 104. This second combination light beam 104 results from the combination of the first combination light beam 102 (with the pump light beam and the signal light beam) and the complementary light beam (generated by the nonlinear medium 40).

[0082] As shown in Figure 2, the parametric oscillator 2 then includes the second focusing optical system 32. This second focusing optical system 32 receives the second combination light beam 104 (transmitted from the output of the nonlinear medium 40). This second focusing optical system 32 is adapted to collimate the second combination light beam 104 towards the second dichroic mirror 22.

[0083] In practice, the second focusing optical system 32 includes, for example, a converging lens with a focal length of f2. Preferably, the distance between the second focusing optical system 32 and the nonlinear medium 40 is on the order of the focal length f2.

[0084] Preferably, the focal length f2 of the second focusing optical system 32 is of the order of the focal length f1 of the first focusing optical system 30.

[0085] As can be seen in Figure 2, the optical axis of the second focusing optical system 32 is substantially coincident with the optical axis A of the first focusing optical system 30.

[0086] The optical parametric oscillator 2 also includes the second dichroic mirror 22. This second dichroic mirror 22 receives the second combination light beam 104 collimated by the second focusing optical system 32. The second dichroic mirror 22 reflects, on the one hand and in a first direction, the portion of the second combination light beam 104 corresponding to the signal light beam (denoted as the "input portion 108 of the signal light beam"). On the other hand, and in a second direction (different from the first direction), it transmits a third combination light beam 106, comprising the pump light beam 100 and the complementary light beam (generated by the nonlinear medium 40).

[0087] To achieve this, the second dichroic mirror 22 exhibits a transmittance greater than 99.5% at 1 pm and greater than 98% for wavelengths above 2.4 pm. Furthermore, the second dichroic mirror 22 exhibits a reflectance greater than 99.5% between 1.35 and 1.7 pm.

[0088] Advantageously according to the present invention, the propagation (between the laser source system 10 and the second dichroic mirror 22) of the pump light beam 100, the first combination light beam 102 and the second combination light beam 104 takes place on the same optical axis A. This optical axis A corresponds here to the optical axis of the first focusing optical system 30 and to the optical axis of the second focusing optical system 32.

[0089] This arrangement is particularly advantageous because it avoids astigmatism problems during the propagation of the different light beams. This allows for optimized coupling and propagation within the optical fiber (described below), thereby improving the efficiency of the optical parametric oscillator and the spatial quality of the signal light beam.

[0090] Advantageously, according to the invention, the optical parametric oscillator 2 comprises the hollow optical fiber 70. As its name indicates, the optical fiber in question is hollow, meaning that the light beam propagating inside it propagates through air. This is particularly advantageous because propagation in the air helps to limit the dispersion phenomena during the propagation of the light beam in question inside.

[0091] Here, the hollow optical fiber 70 is a single-mode optical fiber for a light beam wavelength between 1.2 and 1.8 pm. Preferably, the wavelength considered is between 1.45 and 1.8 pm. In other words, the hollow optical fiber 70 behaves like a spatial filter, selecting only one spatial mode that can propagate through the hollow optical fiber.

[0092] In practice, the mode field diameter of hollow 70 optical fiber is greater than 30 pm. Preferably, this mode field diameter is greater than 40 pm. Even more preferably, this mode field diameter is on the order of 42 pm. In other words, hollow 70 optical fiber exhibits a large core, meaning that the diameter of the portion of the optical fiber containing only air is large compared to conventional optical fibers.

[0093] Furthermore, such a hollow optical fiber allows the polarization of the light beam propagating through it to be preserved.

[0094] Advantageously, the hollow optical fiber 70 is wound upon itself. In other words, the hollow optical fiber 70 is folded upon itself into a circular shape. Preferably, the hollow optical fiber 70 is wound in such a way as to present a shape comprising a plurality of circular portions superimposed on one another. This arrangement is particularly advantageous because it ensures the compactness of the optical parametric oscillator, as the length of the light beam propagation cavity is reduced.

[0095] In practice, the radius of curvature of the hollow optical fiber 70 wound on itself is greater than 20 cm.

[0096] It should be noted here that the length of the hollow optical fiber is not important in obtaining the aforementioned benefits.

[0097] The hollow optical fiber 70 receives, at its input, the input portion 108 of the signal light beam reflected by the second dichroic mirror 22 and provides, at its output, the output portion 120 of the signal light beam. This output portion 120 of the signal light beam is then directed directly to the first dichroic mirror 20. In other words, the hollow optical fiber 70 enables the propagation of the signal light beam in air. In this description, "directly directed" means that the light beam in question does not pass back through the hollow optical fiber to reach the first dichroic mirror.

[0098] The propagation of the light beams then resumes in the optical parametric oscillator, causing the radiation to oscillate in such a way as to induce its amplification through successive passages in the non-linear medium.

[0099] Advantageously, the distance traveled by the signal light beam between the second focusing optical system 32 and the entrance of the hollow optical fiber 70 is on the order of the distance traveled by the signal light beam between the exit of the hollow optical fiber 70 and the first focusing optical system 30. This also makes it possible to improve the compactness of the optical parametric oscillator by reducing the length of the cavity in which the light beams propagate.

[0100] Thus, the hollow nature of the optical fiber eliminates dispersion phenomena during the propagation of the signal light beam. This improves the spatial quality of the light beam propagating through the optical parametric oscillator, and therefore also improves its efficiency.

[0101] Furthermore, winding the hollow optical fiber around itself improves the compactness of the optical parametric oscillator regardless of the repetition frequency of the pump light beam, and therefore reduces its size when used for a intended application (for example in biomedical imaging).

[0102] Figure 3 schematically represents a second embodiment of the optical parametric oscillator 3 according to the first embodiment of the invention. Only the differences compared to the first embodiment are described in detail below.

[0103] The main difference lies in the fact that, in this second embodiment, the optical parametric oscillator 3 includes a third optical focusing system 34 and a fourth optical focusing system 36.

[0104] The third focusing optical system 34 is positioned at the input of the hollow optical fiber 70. This third focusing optical system 34 receives the input portion 108 of the signal light beam (reflected by the second dichroic mirror 22). This third focusing optical system 34 is adapted to focus the input portion 108 of the signal light beam onto the input of the hollow optical fiber 70.

[0105] In practice, the third focusing optical system 34 includes, for example, a converging lens with a focal length of f3. Preferably, the distance between the third focusing optical system 34 and the entrance of the hollow optical fiber 70 is on the order of the focal length f3.

[0106] The fourth focusing optical system 36 is positioned at the output of the hollow optical fiber 70. This fourth focusing optical system 36 receives the output portion 120 of the signal light beam emerging from the hollow optical fiber 70. This fourth focusing optical system 36 is adapted to collimate the output portion 120 of the signal light beam towards the first dichroic mirror 20.

[0107] In practice, the fourth focusing optical system 36 includes, for example, a converging lens with a focal length of f4. Preferably, the distance between the fourth focusing optical system 36 and the output of the hollow optical fiber 70 is on the order of the focal length f4.

[0108] Preferably, the focal length f4 of the fourth focusing optical system 36 is of the order of the focal length f3 of the third focusing optical system 34.

[0109] Moreover, the distance traveled by the signal light beam between the second focusing optical system 32 and the third focusing optical system 34 is on the order of the distance traveled by the signal light beam between the fourth focusing optical system 36 and the first focusing optical system 30. This allows the size of the signal light beam to be adapted to that of the pump light beam during their propagation in the non-linear medium, thus ensuring efficient coupling.

[0110] Advantageously, this second embodiment of the optical parametric oscillator according to the invention, with the third and fourth optical focusing systems, makes it possible to guarantee satisfactory propagation of the signal light beam in the hollow optical fiber by ensuring good alignment of the entire signal light beam with respect to the hollow optical fiber.

[0111] Figure 4 schematically represents a third embodiment of the optical parametric oscillator 4 according to the first embodiment of the invention. Only the differences from the first embodiment are described in detail below.

[0112] The main difference lies in the fact that, in this third embodiment, the optical parametric oscillator 4 includes a half-wave plate 50 and a beam separation device 60.

[0113] As shown in Figure 4, the half-wave plate 50 is positioned at the output of the second dichroic mirror 22. The half-wave plate 50 receives the signal light beam 108a reflected by the second dichroic mirror 22. A first polarized portion 110 of the signal light beam emerges from the half-wave plate 50. This half-wave plate 50 is of zero order. Furthermore, it has an anti-reflective coating to ensure optimal transmission and precise control of the signal light beam over a wavelength range between 1.35 and 1.7 pm.

[0114] The beam splitter 60 is positioned between the half-wave plate 50 and the entrance of the hollow optical fiber 70. The beam splitter 60 is adapted to separate the first polarized part 1 10 of the signal light beam into a first signal portion 1 12 and a second signal portion 1 14. The second signal portion 114 is directed towards the hollow optical fiber 70 (and corresponds to the part of the signal light beam which enters the hollow optical fiber 70).

[0115] In practice, the beam splitter 60 is, for example, a polarizing beam splitter (also denoted PBS for "Polarizing Beam Splitter" or TF-PBS for "Thin Film Polarizing Beam Splitter," according to commonly used Anglo-Saxon acronyms). The beam splitter 60 exhibits a polarization extinction rate greater than 97% for s or p polarizations. Furthermore, it exhibits a transmission rate greater than 98% for light beams in the spectral range associated with the signal light beam (with a frequency fs).

[0116] The assembly formed by the half-wave plate 50 and the beam splitter 60 allows the intensity of the portion of the signal light beam that is subsequently coupled into the hollow optical fiber 70 to be varied, while maintaining its polarization. This assembly thus allows for better control of the signal light beam entering the hollow optical fiber 70. Furthermore, this arrangement improves the conversion efficiency of the pump light beam to the signal light beam by at least 30%.

[0117] Figure 5 schematically represents a fourth embodiment of the optical parametric oscillator 5 according to the first embodiment of the invention. Only the differences from the first embodiment are described in detail below.

[0118] The main difference lies in the fact that, in this fourth embodiment, the optical parametric oscillator 5 includes a first quarter-wave plate 52 and a second quarter-wave plate 54.

[0119] As shown in Figure 5, the first quarter-wave plate 52 is positioned at the input of the hollow optical fiber 70. The second quarter-wave plate 54 is positioned at the output of the hollow optical fiber 70.

[0120] The use of these two quarter-wave plates makes it possible to compensate for birefringence phenomena that could be induced by the twisting of the hollow optical fiber 70.

[0121] It has been observed that the degree of polarization of the signal beam output from the hollow optical fiber (evaluated according to the PER, or Polarization Extinction Ratio, in Anglo-Saxon terminology), in an optical parametric oscillator according to the first embodiment, is on the order of 97%. With the quarter-wave plates (in an optical parametric oscillator according to this fourth embodiment), the result is optimal (on the order of 100%). The use of these quarter-wave plates therefore further improves the efficiency of the optical parametric oscillator.

[0122] Figure 6 schematically represents a fifth example of an embodiment of the optical parametric oscillator 6 according to the first embodiment of the invention.

[0123] This fifth example of an implementation of the optical parametric oscillator corresponds to a combination of the implementation examples described previously. The various elements included in the optical parametric oscillator 6 are not described again in detail here.

[0124] As shown in Figure 6, the optical parametric oscillator 6 comprises the laser source system 10 generating the pump light beam 100. It also includes the first dichroic mirror 20, which receives the pump light beam 100 and the final portion 120a of the signal light beam. The first combining light beam 102 is obtained at the output of the first dichroic mirror 20.

[0125] As can be seen in Figure 6, the optical parametric oscillator 6 includes the first optical focusing system 30 which allows the first combination light beam 102 to be focused onto the nonlinear medium 40.

[0126] The nonlinear medium 40 generates, in particular, the complementary light beam and the signal beam. At the output of the nonlinear medium 40, the second combining light beam 104 is collimated by the second focusing optical system 32.

[0127] The optical parametric oscillator 6 then includes the second dichroic mirror 22. This second dichroic mirror 22 transmits, on the one hand, in a first direction, the third combination light beam 106 comprising the pump light beam and the complementary light beam (which are rejected from the cavity of the optical parametric oscillator). On the other hand, the second dichroic mirror 22 reflects the signal light beam 108a.

[0128] As shown in Figure 6, the optical parametric oscillator 6 includes the half-wave plate 50 which modifies the polarization state of the reflected signal light beam 108a so as to obtain the first polarized part 1 10 of the signal light beam.

[0129] The optical parametric oscillator 6 then includes the beam splitting device 60 which separates the first polarized part 110 of the signal light beam into a first signal portion 1 12 and a second signal portion 1 14.

[0130] The second portion of signal 1 14 is then directed towards the first quarter-wave plate 52 which forms, at the output, a second polarized part 1 16. This second polarized part 1 16 (elliptically) is then focused by means of the third focusing optical system 34. The input part 108 of the signal light beam is directed towards the hollow optical fiber 70 and propagates there.

[0131] At the output, the hollow optical fiber 70 provides the output part 120 of the signal light beam.

[0132] As shown in Figure 6, the optical parametric oscillator 6 also includes the fourth focusing optical system 36, positioned at the output of the hollow optical fiber 70. The output part 120 of the signal light beam is then collimated by the fourth focusing optical system 36 (at the output of the hollow optical fiber 70).

[0133] The optical parametric oscillator 6 then includes the second quarter-wave plate 54 so as to compensate, in the output portion 120 of the collimated signal light beam, for the elliptical polarization induced during propagation in the hollow optical fiber 70 and introduced, at the input of this hollow optical fiber 70, by the first quarter-wave plate 52. A final portion 120a of the signal light beam is obtained at the output of the second quarter-wave plate 54. As shown in Figure 6, the final portion 120a of the signal light beam is then directed directly to the first dichroic mirror 20.

[0134] The propagation of the light beams then resumes in the optical parametric oscillator, causing the radiation to oscillate in such a way as to induce its amplification through successive passages in the non-linear medium.

[0135] Advantageously, according to the invention, the hollow nature of the optical fiber eliminates dispersion phenomena during the propagation of the signal light beam. This improves the spatial quality of the light beam propagating in the optical parametric oscillator, and therefore also improves its efficiency.

[0136] Furthermore, winding the hollow optical fiber around itself improves the compactness of the optical parametric oscillator and therefore reduces its size when used for a intended application (for example in biomedical imaging).

[0137] Furthermore, thanks to the various components of the optical parametric oscillator, propagation in the hollow optical fiber is optimized by ensuring good alignment of the signal beam at the fiber input and by limiting birefringence phenomena. The arrangement used also optimizes coupling within the optical fiber, thus improving the efficiency of the optical parametric oscillator.

[0138] Furthermore, the use of optical focusing systems in the form of lenses is more advantageous than the use of mirrors (as is the case in a conventional optical parametric oscillator) because it limits, or even prevents, astigmatism within the cavity. This, in turn, increases the coupling within the hollow optical fiber (which is greater than 98%).

[0139] In an alternative embodiment not shown (and which applies regardless of the example described above), the first dichroic mirror transmits the pump light beam and reflects the complementary light beam, while the second dichroic mirror can reflect, along the first direction, a fourth combination light beam resulting from the combination of the pump light beam and the complementary light beam. In this case, it then transmits, along the second direction, both the signal light beam and the pump light beam.

[0140] Thus, this fourth combined light beam, comprising the pump light beam and the complementary light beam, propagates within the optical parametric oscillator 2, and specifically within the hollow optical fiber 70. In other words, the pump light beam and the complementary light beam propagate within the cavity of the optical parametric oscillator. This configuration optimizes the efficiency of the complementary light beam.

[0141] In a variant not shown (and which applies regardless of the example described above), the optical parametric oscillator may include a stage configured to allow movement of the output of the hollow optical fiber and the first dichroic mirror. This stage allows, for example, translational movement of the hollow optical fiber and the first dichroic mirror. This arrangement then allows a delay to be introduced in order to vary the length of the optical path in the optical cavity of the optical parametric oscillator.

[0142] In an alternative embodiment not shown, the optical parametric oscillator may include a control system configured to control a parameter of the nonlinear medium. The control system is, for example, configured to control the temperature or the polarization period of the nonlinear medium.

[0143] Figure 7 represents, schematically and functionally, a second embodiment of an optical parametric oscillator 207 according to the invention.

[0144] The elements common to the first embodiment of the optical parametric oscillator 1; 2; 3; 4; 5; 6 have the same references and are not described again in detail in the following.

[0145] As shown in Figure 7, in this second embodiment, the optical parametric oscillator 207 comprises a laser source system 10, a first dichroic mirror 20, a first focusing optical system 30, a nonlinear medium 40, a second focusing optical system 32, a second dichroic mirror 22, a hollow optical fiber 70, and a reflecting mirror 80. Optionally (as in the first embodiment described above), and represented by dashed boxes in Figure 7, the optical parametric oscillator 207 also comprises a half-wave plate 50, a beam splitter 60, a first quarter-wave plate 52, a third focusing optical system 34, a fourth focusing optical system 36, and a second quarter-wave plate 54. All elements common to the first embodiment described above have the same characteristics and are not described again here.

[0146] It should be noted here that all elements other than the nonlinear medium form the cavity of the optical parametric oscillator. In this second embodiment, the cavity of the optical parametric oscillator has a linear configuration.

[0147] In this second embodiment, the optical parametric oscillator 207 more specifically includes the reflecting mirror 80 positioned at the output of the hollow optical fiber 70. This reflecting mirror 80 is adapted to reflect the beam The signal light 220, emerging from the hollow optical fiber 70, is directed towards this same hollow optical fiber 70. In other words, the reflecting mirror 80 is adapted to reflect the signal light 220 from the hollow optical fiber 70 back towards the same hollow optical fiber 70. The signal light 220 then propagates in the opposite direction to the first dichroic mirror 20 (passing back through all the elements of the optical parametric oscillator). It is then combined with the pump light beam 100 to form the first combination light beam 102. The reverse propagation is represented by a double arrow in Figure 7.

[0148] In practice, the reflecting mirror 80 is, for example, a flat mirror. It exhibits, for example, a reflectance greater than 99.5% between 1.35 and 1.7 pm.

[0149] The examples shown in Figures 2 to 6 for the first embodiment can then be adapted within the framework of this second embodiment. Figures 8 to 12 respectively represent equivalent embodiments to those shown in Figures 2 to 6, here within the framework of the second embodiment of the invention. Only the elements that differ from those in Figures 2 to 6 are described in detail below.

[0150] In all of these figures 8 to 12, the cavity of the optical parametric oscillator is presented in a linear configuration. The main difference compared to figures 2 to 6 lies in the presence of the reflecting mirror 80, which allows this linear configuration to be implemented for the cavity of the optical parametric oscillator.

[0151] Figure 8 schematically represents a first example of the realization of the optical parametric oscillator 208 according to the second embodiment of the invention.

[0152] As in the example of Figure 2, the optical parametric oscillator 208 here includes the laser source system 10, the first dichroic mirror 20, the first focusing optical system 30, the nonlinear medium 40, the second focusing optical system 32, the second dichroic mirror 22 and the hollow optical fiber 70. In addition to these elements, the optical parametric oscillator 208 also includes the reflecting mirror 80 allowing the implementation of a linear cavity.

[0153] Furthermore, and as can be seen in Figure 8, the first dichroic mirror 20 forms a right angle with the direction of beam propagation The pump light beam 100 allows, in particular, the reception of the output portion 220 of the signal light beam propagating in a different direction (than that of the pump light beam). This arrangement of the first dichroic mirror 20 makes it possible to obtain the linear configuration for the cavity of the optical parametric oscillator 208.

[0154] Figure 9 schematically represents a second embodiment of the optical parametric oscillator 209 according to the second embodiment of the invention. Only the differences compared to the first embodiment of the second embodiment are described below.

[0155] The main difference lies in the fact that, in this second embodiment, the optical parametric oscillator 209 includes the third optical focusing system 34 and the fourth optical focusing system 36 (as introduced previously, in particular in Figure 3, for the first embodiment).

[0156] The third optical focusing system 34 and the fourth optical focusing system 36 here have similar characteristics to those described for the first embodiment (with reference to Figure 3).

[0157] Figure 10 schematically represents a third embodiment of the optical parametric oscillator 210 according to the second embodiment of the invention. Only the differences compared to the first embodiment of this second embodiment of the invention are described below.

[0158] The main difference lies in the fact that, in this third embodiment, the optical parametric oscillator 210 includes the half-wave plate 50 and the beam separation device 60.

[0159] The half-wave plate 50 and the beam separation device 60 have similar characteristics to those described for the first embodiment (with reference to figure 4).

[0160] Figure 11 schematically represents a fourth embodiment of the optical parametric oscillator 21 1 according to the second embodiment of the invention. Only the differences compared to the first example of The implementation of this second embodiment of the invention is described below.

[0161] The main difference lies in the fact that, in this fourth embodiment, the optical parametric oscillator 211 includes the first quarter-wave plate 52 and the second quarter-wave plate 54.

[0162] The first quarter-wave plate 52 and the second quarter-wave plate 54 here exhibit characteristics similar to those described for the first embodiment (with reference to figure 5).

[0163] Figure 12 schematically represents a fifth embodiment of the optical parametric oscillator 212 according to the second embodiment of the invention. This fifth embodiment of the optical parametric oscillator 212 corresponds to a combination of the embodiments of the second embodiment described previously. The various components of the optical parametric oscillator 212 are not described in detail again here.

[0164] As shown in Figure 12, the optical parametric oscillator 212 comprises the laser source system 10 generating the pump light beam 100. It also includes the first dichroic mirror 20, which receives the pump light beam 100 and the final portion 220a of the signal light beam. The first combining light beam 102 is obtained at the output of the first dichroic mirror 20.

[0165] As can be seen in Figure 12, the optical parametric oscillator 212 includes the first optical focusing system 30 which allows the first combination light beam 102 to be focused onto the nonlinear medium 40.

[0166] The nonlinear medium 40 generates, in particular, the complementary light beam and the signal beam. At the output of the nonlinear medium 40, the second combining light beam 104 is collimated by the second focusing optical system 32.

[0167] The optical parametric oscillator 212 then includes the second dichroic mirror 22. This second dichroic mirror 22 transmits, on the one hand, in a first direction, the third combination light beam 106 comprising the pump light beam and the complementary light beam (which are rejected of the cavity of the optical parametric oscillator). On the other hand, the second dichroic mirror 22 reflects the signal light beam 108a.

[0168] As shown in Figure 12, the optical parametric oscillator 212 includes the half-wave plate 50 which modifies the polarization state of the reflected signal light beam 108a so as to obtain the first polarized part 110 of the signal light beam.

[0169] The optical parametric oscillator 212 then includes the beam splitting device 60 which separates the first polarized part 110 of the signal light beam into a first signal portion 112 and a second signal portion 114.

[0170] The second portion of signal 1 14 is then directed towards the first quarter-wave plate 52 which forms, at the output, a second polarized part 1 16. This second polarized part 1 16 (elliptically) is then focused by means of the third focusing optical system 34. The input part 108 of the signal light beam is directed towards the hollow optical fiber 70 and propagates there.

[0171] At the output, the hollow optical fiber 70 provides the output portion 220 of the signal light beam.

[0172] As shown in Figure 12, the optical parametric oscillator 212 also includes the fourth focusing optical system 36, positioned at the output of the hollow optical fiber 70. The output part 220 of the signal light beam is then collimated by the fourth focusing optical system 36 (at the output of the hollow optical fiber 70).

[0173] The optical parametric oscillator 212 then includes the second quarter-wave plate 54 so as to compensate, in the output portion 220 of the collimated signal light beam, for the elliptical polarization induced during propagation in the hollow optical fiber 70 and introduced, at the input of this hollow optical fiber 70, by the first quarter-wave plate 52. A final portion 220a of the signal light beam is obtained at the output of the second quarter-wave plate 54. As shown in Figure 12, the final portion 220a of the signal light beam is directed towards the reflecting mirror 80. The final portion 220a of the signal light beam is then reflected back towards the hollow optical fiber 70 so that the The signal light beam is directed towards the first dichroic mirror 20 by passing back through the hollow optical fiber 70.

[0174] The propagation of the light beams then resumes in the optical parametric oscillator, causing the radiation to oscillate in such a way as to induce its amplification through successive passages in the non-linear medium.

[0175] Advantageously, according to the invention, the hollow nature of the optical fiber eliminates dispersion phenomena during the propagation of the signal light beam. This improves the spatial quality of the light beam propagating in the optical parametric oscillator, and therefore also improves its efficiency.

[0176] Furthermore, winding the hollow optical fiber around itself improves the compactness of the optical parametric oscillator and therefore reduces its size when used for a intended application (for example in biomedical imaging).

[0177] Furthermore, thanks to the various components of the optical parametric oscillator, propagation in the hollow optical fiber is optimized by ensuring good alignment of the signal beam at the fiber input and by limiting birefringence phenomena. The arrangement used also optimizes coupling within the optical fiber, thus improving the efficiency of the optical parametric oscillator.

[0178] Furthermore, the use of optical focusing systems in the form of lenses is more advantageous than the use of mirrors (as is the case in a conventional optical parametric oscillator) because it limits, or even prevents, astigmatism within the cavity. This, in turn, increases the coupling within the hollow optical fiber (which is greater than 98%).

[0179] Experimental results:

[0180] Figure 13a represents a two-dimensional map showing the distribution of the electric field intensity |E| 2 measured in the near field (according to the commonly used Anglo-Saxon terminology) in the output plane of the optical parametric oscillator according to the present invention. This electric field intensity |E| 2 is represented according to the spatial coordinates x and y.

[0181] Figures 13b and 13c respectively represent the variations in the intensity of the electric field |E| 2 along the y and x directions. These cross-sections of the electric field intensity profile |E| 2 They allow, in particular, the extraction of characteristic quantities of the light beam. They allow, in particular, the extraction of widths A y(1 / e) and Ax(1 / e), defined as the distances separating the points for which the intensity is equal to lo / e, with lo corresponding to the maximum value of the intensity of the electric field |E| 2 .

[0182] Here, A y (1 / e) = 0.825 mm (millimeters) and A x (1 / e)=0.750 mm.

[0183] As can be seen in Figures 13a, 13b, and 13c, advantageously according to the invention, the light beam exhibits a high-quality Gaussian profile with excellent symmetry properties and an absence of optical aberrations. This demonstrates the efficiency of the guidance in the hollow-core optical fiber (here in the near field).

[0184] Figure 14a represents a two-dimensional map showing the distribution of the electric field intensity |E| 2measured in the far field (according to the commonly used Anglo-Saxon terminology) at the output of the optical parametric oscillator according to the present invention. This intensity of the electric field |E| 2 is represented according to the spatial coordinates x and y.

[0185] Figures 14b and 14c respectively represent the variations in the intensity of the electric field |E| 2 along the y and x directions. These cross-sections of the electric field intensity profile |E| 2 They allow, in particular, the extraction of characteristic quantities of the light beam. They allow, in particular, the extraction of widths A y (1 / e) and A x (1 / e), defined as the distances separating the points for which the intensity is equal to lo / e, with lo corresponding to the maximum value of the intensity of the electric field |E| 2 .

[0186] Here, A y (1 / e) = 1.028 mm and A x(1 / e)=1.006 mm.

[0187] As can be seen in Figures 14a, 14b and 14c, advantageously according to the invention, the light beam exhibits a high-quality Gaussian intensity distribution with excellent symmetry properties (illustrated by the width values ​​A). y (1 / e) and A x (1 / e), as well as an ellipticity ratio of approximately 0.98) and an absence of optical aberrations. The light beam exhibits a uniform intensity profile characteristic of the far field, without structure Tl multi-peak nor secondary hot spots. This then confirms the optical quality and efficiency of the optical parametric oscillator according to the invention.

Claims

1. 28 DEMANDS

1. Optical parametric oscillator (1; 2; 3; 4; 5; 6; 207; 208; 209; 210; 211; 212) comprising: - a laser source system (10) generating a pump light beam (100), - a first dichroic mirror (20) receiving the pump light beam (100) and transmitting a first combination light beam (102) resulting from a combination of said pump light beam (100) and a signal light beam (120; 120a), - a first optical focusing system (30) receiving the first combination light beam (102), - a non-linear medium (40) receiving the first combination light beam (102) focused by the first optical focusing system (30), the non-linear medium (40) generating at the output a second combination light beam (104) resulting from the combination of the first combination light beam (102) and a complementary light beam generated by the non-linear medium (40), - a second optical focusing system (32) receiving the second combination light beam (104), - a second dichroic mirror (22) receiving the second combination light beam (104) collimated by the second focusing optical system (32), the second dichroic mirror (22) reflecting, in a first direction, the signal light beam (108; 108a) and transmitting, in a second direction, the pump light beam, and - a hollow optical fiber (70) receiving at input the signal light beam (108; 108a) reflected by the second dichroic mirror (22), the hollow optical fiber (70) allowing the propagation of the signal light beam in air, the signal light beam (120) emerging from the hollow optical fiber (70) being directed towards the first dichroic mirror (20), the hollow optical fiber (70) being wound on itself.

2. Optical parametric oscillator (1; 3; 6; 207; 209; 212) according to claim 1, wherein the second dichroic mirror (22) reflects, along the first direction, the signal light beam (108; 108a) and transmits, along the second direction, a third combination light beam (106) resulting from the combination of the pump light beam and the complementary light beam.

3. Optical parametric oscillator according to claim 1, wherein the second dichroic mirror (22) reflects, along the first direction, a fourth combination light beam resulting from the combination of the signal light beam and the complementary light beam and transmits, along the second direction, the pump light beam.

4. Optical parametric oscillator (1; 3; 6; 207; 209; 212) according to any one of claims 1 to 3, also comprising: - a third optical focusing system (34) positioned at the input of the hollow optical fiber (70) so as to focus the signal light beam (108; 116) at the input of the hollow optical fiber (70), and - a fourth optical focusing system (36) positioned at the output of the hollow optical fiber (70) so as to collimate the signal light beam (120) at the output of the hollow optical fiber (70).

5. Optical parametric oscillator (1; 3; 6; 207; 209; 212) according to claim 4, wherein the third focusing optical system (34) and the fourth focusing optical system (36) have the same focal length, a first distance between the third focusing optical system (34) and the inlet of the hollow optical fiber (70) being on the order of said focal length, a second distance between the fourth focusing optical system (36) and the outlet of the hollow optical fiber (70) being on the order of said focal length.

6. A parametric optical oscillator (1; 4; 6; 207; 210; 212) according to any one of claims 1 to 5, also comprising: - a half-wave plate (50) positioned at the exit of the second dichroic mirror (22), and - a beam splitter device (60) positioned between the half-wave plate (50) and the entrance of the hollow optical fiber (70) so as to separate the signal light beam (1 10) into a first signal portion (1 12) and a second signal portion (1 14), the second signal portion (1 14) being directed towards the hollow optical fiber (70).

7. Optical parametric oscillator (1; 4; 6; 207; 210; 212) according to claim 6, wherein the beam splitting device (60) comprises a polarizing beam splitter cube.

8. Optical parametric oscillator (1; 5; 6; 207; 211; 212) according to any one of claims 1 to 7, also comprising: - a first quarter-wave plate (52) positioned at the input of the hollow optical fiber (70), and - a second quarter wave blade (54) positioned at the output of the hollow optical fiber (70).

9. Optical parametric oscillator (1; 2; 3; 4; 5; 6; 207; 208; 209; 210; 211; 212) according to any one of claims 1 to 8, wherein the nonlinear medium (40) is formed by a lithium niobate crystal.

10. Optical parametric oscillator (1; 2; 3; 4; 5; 6; 207; 208; 209; 210; 211; 212) according to any one of claims 1 to 9, wherein a radius of curvature of the hollow optical fiber (70) wound on itself is greater than 20 centimeters. [Claim 1 1 ] Optical parametric oscillator (1; 2; 3; 4; 5; 6; 207; 208; 209; 210; 211; 212) according to any one of claims 1 to 10, wherein a distance traveled by the signal light beam between the second focusing optical system (32) and the inlet of the hollow optical fiber (70) is on the order of a distance traveled by the signal light beam between the outlet of the hollow optical fiber (70) and the first focusing optical system (30).

12. Optical parametric oscillator (1; 2; 3; 4; 5; 6; 207; 208; 209; 210; 211; 212) according to any one of claims 1 to 11, wherein the hollow optical fiber (70) is a single-mode optical fiber for a signal light beam wavelength between 1.2 and 1.8 micrometers.

13. Optical parametric oscillator (1; 2; 3; 4; 5; 6; 207; 208; 209; 210; 211; 212) according to any one of claims 1 to 12, wherein a mode field diameter of the hollow optical fiber (70) is greater than 30 micrometers.

14. Optical parametric oscillator (1; 2; 3; 4; 5; 6) according to any one of claims 1 to 13, wherein the signal light beam (120) emerging from the hollow optical fiber (70) is directed directly towards the first dichroic mirror (20) without crossing back through the hollow optical fiber (70).

15. Optical parametric oscillator (207; 208; 209; 210; 211; 212) according to any one of claims 1 to 13, also comprising a reflecting mirror (80) of the signal light beam (220) to the hollow optical fiber (70), the signal light beam (220) being directed to the first dichroic mirror (20) by passing back through the hollow optical fiber (70).

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