Laser device

Abstract

Laser Device The present invention relates to a laser device (10) comprising a housing (12) in which the following elements are arranged: a laser cavity (20) delimited by a reflective mirror (M1) and a partially reflective mirror (M2), the partially reflective mirror forming the output of the laser cavity for a laser beam amplified in the laser cavity, an optical fiber (22) for carrying the laser beam between the output of the laser cavity and the outside of the housing, the optical fiber being fixed to the housing at at least one attachment point (P1, P2), and a selective component (24) for selecting the wavelength of the laser beam at the output of the laser device, the selective component being positioned in the optical fiber at the output of the laser cavity and upstream of the attachment point(s). Figure for the abstract: Figure 1

Description

Title of the invention: Laser device

[0001] The present invention relates to a laser device.

[0002] Known from the state of the art are laser devices comprising, in a housing, a laser cavity capable of generating a transverse single-mode laser beam and a single-mode optical fiber capable of conveying the laser beam generated by the cavity to the output of the housing.

[0003] To select and stabilize the wavelength of the laser beam generated by the laser cavity, it is known to use a component, such as a Bragg grating, in particular a Bragg grating inscribed in an optical fiber (in English "Fiber Bragg Grating", abbreviated to FBG). Such a component is traditionally positioned in the optical fiber, outside the housing.

[0004] Currently, one challenge is to gain compactness by integrating external components into the housing, in particular the FBG.

[0005] However, the integration of the FBG in the package causes significant wavelength jumps and power jumps when the laser diode current varies, which degrade the spectral and power stability of the laser beam at the fiber output. This is problematic for certain envisaged applications, in particular pumping applications of Erbium-doped fiber amplifiers (EDFA), but also frequency doubling and optical metrology applications, such as optical clock experiments.

[0006] There is therefore a need for a laser device which is compact while maintaining good spectral stability and power of the laser beam at the fiber output.

[0007] For this purpose, the present description relates to a laser device comprising a housing in which the following elements are arranged: - a laser cavity delimited by a reflecting mirror and a partially reflecting mirror, the partially reflecting mirror forming the output of the laser cavity for a laser beam amplified in the laser cavity, - an optical fiber capable of conveying the laser beam between the exit of the laser cavity and the exterior of the housing, the optical fiber being fixed to the housing at at least one fixing point, and - a selective component capable of selecting the wavelength of the laser beam at the output of the laser device, the selective component being positioned in the optical fiber at the output of the laser cavity and upstream of the fixing point(s).

[0008] According to particular embodiments, the device comprises one or more of the following characteristics, taken individually or in all combinations: technically possible: - the selective component is a Bragg grating; - the housing has an exit port, the optical fiber being fixed to the housing in one first and a second attachment point, the first attachment point being between the outlet of the laser cavity and the outlet port of the housing, the second attachment point coinciding with the outlet port of the housing, the selective component being positioned between the outlet of the laser cavity and the first attachment point; - the selective component is positioned at a distance less than or equal to 1 millimeter from the output of the laser cavity; - the face of the partially reflecting mirror facing the optical fiber is optimized to transmit, into the laser cavity, a laser beam at the selection wavelength of the selective component; - the reflection spectrum of the selective component has a bandwidth chosen to retain only one or two laser modes of the laser beam, advantageously a single mode of the laser beam; - the reflection spectrum of the selective component has a bandwidth less than or equal to five times the free spectral interval of the laser cavity, preferably less than or equal to 0.5 nanometers, preferably less than or equal to 0.3 nanometers; - the reflection spectrum of the selective component is apodized; - the optical fiber comprises an optic suitable for focusing the laser beam in the optical fiber, the optic being positioned in the optical fiber, between the output of the laser cavity and the selective component, the optic preferably being a lens machined in the optical fiber; - the laser cavity is a rectilinear cavity; - the laser cavity is a curved cavity; and - the laser housing has a length less than or equal to the length of the laser cavity plus 8 millimeters.

[0009] Other characteristics and advantages of the invention will appear on reading the following description of embodiments of the invention, given by way of example only and with reference to the drawings which are:

[0010] - [Fig. 1][Fig. 1], a schematic sectional view of an example of a laser device,

[0011] - [Fig.2][Fig.2], a graphical representation of an example of the modes of a beam laser generated by a laser cavity, and superimposed the reflection spectrum of a first selective component and a second selective component (Bragg gratings), the first and second selective components having different bandwidths (1.2 nm versus 0.25 nm),

[0012] - [Fig.3][Fig.3], a graphical representation of an example of the evolution of the optical power and the derivative of the optical power as a function of the laser current for a laser device having a selective FBG component positioned less than 1 mm from the laser cavity output and upstream of the optical fiber attachment points to the housing, and

[0013] - [Fig.4][Fig.4], a graphical representation of an example of the evolution of the wavelength as a function of laser current for a laser device having a selective FBG component positioned less than 1 mm from the laser cavity output and upstream of the optical fiber attachment points to the housing.

[0014] A laser device 10 is illustrated in [Fig.l].

[0015] The laser device 10 comprises a housing 12 in which the following elements are positioned: a laser cavity 20, an optical fiber 22 and a selective component 24.

[0016] The housing 12 is a hermetic housing, provided with at least one orifice 26 for the passage of the optical fiber 22.

[0017] The laser cavity 20 is capable of generating and amplifying a laser beam. The laser cavity 20 forms a Fabry-Pérot cavity. The laser cavity 20 is, for example, a laser chip, such as a laser diode.

[0018] As illustrated in the example of [Fig.l], the laser cavity 20 comprises a waveguide 30 with an active medium between a reflecting mirror M1 and a partially reflecting mirror M2. The partially reflecting mirror M2 forms the output of the laser cavity 20 for the laser beam generated and amplified in the laser cavity 20.

[0019] In particular, the face of the reflecting mirror M1 facing the laser cavity 20 (towards the inside of the laser cavity 20) is called the rear face of the laser cavity 20. The face of the partially reflecting mirror M2 facing the optical fiber 22 (towards the outside of the laser cavity 20) is called the front face of the laser cavity 20.

[0020] Preferably, the face of the partially reflecting mirror M2 facing the optical fiber 22 (front face) is optimized to transmit a laser beam at the selection wavelength of the selective component 24 (as will be described in the remainder of the description). By “optimization”, it is understood that the reflectivity of the partially reflecting mirror M2 is less than 0.5% at the wavelength of the selective component 24.

[0021] Thus, this allows the laser beam at the wavelength of interest (wavelength of the selective component 24) to be reintroduced into the laser cavity 20 to be reamplified, thus forming an additional laser cavity between the rear face of the laser cavity 20 and the selective component 24.

[0022] Preferably, the laser cavity 20 is a rectilinear cavity (straight cavity), i.e. the reflecting mirror M1 and the partially reflecting mirror M2 are pa ralles. In other words, the laser cavity 20 is not curved. This allows for higher laser power and lower energy consumption.

[0023] Alternatively, the laser cavity 20 is curved.

[0024] The optical fiber 22 is capable of conveying the laser beam between the output of the laser cavity 20 and the exterior of the housing 12.

[0025] The optical fiber 22 is fixed to the housing 12 at at least two fixing points PI, P2.

[0026] In an exemplary implementation illustrated by [Fig. 1], the optical fiber 22 is fixed to the housing 12 at a first fixing point PI and a second fixing point P2. The first fixing point PI is between the outlet of the laser cavity 20 and the outlet orifice 26 of the housing 12. The second fixing point P2 fixes the optical fiber 22 to the outlet orifice 26 of the housing 12. In particular, the fixing made at the second fixing point P2 is suitable for hermetically securing the optical fiber 22 to the housing 12. The part of the optical fiber 22 between the fixing points PI and P2 is thus subject to deformations of the housing 12 due to changes in temperature of the housing 12 and differences in thermal expansion coefficients of the different materials constituting the laser device 10.

[0027] For example, the fixing means used are glue or low melting point glass solder.

[0028] In one example, the housing 12 has an external length, excluding the attachment point P2, which does not exceed that of the laser cavity 20 plus 8 mm. In this case, the laser housing 12 is called a “mini-pump”. This makes it possible to gain in compactness because the distance between the points PI and P2 is not constrained by the length of the selective component 24, which must be sufficiently far from the points PI and P2 in the state of the art.

[0029] In an exemplary implementation, the optical fiber 22 comprises an optic 32 suitable for focusing the laser beam in the optical fiber 22. The optic 32 is positioned in the optical fiber 22 between the output of the laser cavity 20 and the selective component 24. For example, the optic 32 is a (converging) lens machined in the optical fiber 22.

[0030] The selective component 24 is capable of selecting the wavelength of the laser beam at the output of the laser device 10. Indeed, the laser cavity 20 initially generates and amplifies a laser beam over a broad spectral band of several nanometers in wavelength, and the selective component 24 makes it possible to select a specific wavelength for the laser beam, which is then reamplified so as to obtain a laser beam of controlled and stabilized wavelength at the output of the laser device 10.

[0031] The selective component 24 is positioned in the optical fiber 22 at the output of the laser cavity 20 and upstream of the fixing points PI, P2. Thus, the selective component 24 is not subject to mechanical deformations of the housing 12, induced by changes temperature. Indeed, when the selective component 24 (FBG) is positioned between two fixing points PI, P2, the central wavelength of the selective component 24 moves according to its temperature and the mechanical stress induced by the difference in thermal expansion of the housing 12 and the fiber 22. By positioning the selective component 24 outside the space between two fixing points PI, P2, these mechanical stresses induced by the difference in thermal expansion of the housing 12 and the fiber 22 are overcome and the variations in wavelength of the selective component 24 are only due to the variations in temperature of said selective component 24.

[0032] More particularly, when the optical fiber 22 is fixed by two fixing points PI, P2, the selective component 24 is positioned between the output of the laser cavity 20 and the first fixing point PI. Thus, the selective component 24 is not located between the two fixing points PI, P2.

[0033] Preferably, the selective component 24 is a Bragg grating. More particularly, the selective component 24 is a Bragg grating inscribed in the optical fiber 22 (fiber core).

[0034] Preferably, the selective component 24 is positioned at a distance less than or equal to 1 millimeter from the output of the laser cavity 20. Thus, the selective component 24 is positioned very close to the output of the laser cavity 20, which makes it possible to reduce wavelength jumps and power jumps compared to a selective component 24 further from the output of the laser cavity 20. Indeed, when the selective component 24 is very close to the output of the laser cavity 20 (less than 1 mm), the free spectral range (in English "Free Spectral Range", abbreviated to FSR) of the laser cavity 20 is very close to the FSR of the additional cavity formed between the rear face of the laser cavity 20 and the selective component 24. Due to this proximity, when the mode jumps occur (for example due to variations in the laser current), this involves wavelength jumps. wave and reduced amplitude power.

[0035] Preferably, the reflection spectrum of the selective component 24 has a bandwidth chosen to retain only one or two laser modes of the laser beam, advantageously a single mode of the laser beam.

[0036] For example, [Fig. 2] illustrates in A the transmission curve of the laser modes of the laser cavity 20, in B the reflection curve of an FBG having a wide bandwidth (1.2 nm) and in C the reflection curve of an FBG having a narrow bandwidth (0.25 nm). As visible in this figure, the narrowband FBG makes it possible to select a smaller number of modes, which makes it possible to reduce the wavelength jumps by “forcing” the laser cavity 20 to amplify only a reduced number of modes.

[0037] Preferably, the reflection spectrum of the selective component 24 has a band bandwidth (defined by the half-width of the reflectivity spectrum) less than or equal to five times the free spectral range (FSR) of the laser cavity. Advantageously, the reflection spectrum of the selective component 24 has a bandwidth less than or equal to 0.5 nanometers, preferably less than or equal to 0.3 nanometers.

[0038] Preferably, the reflection spectrum of the selective component 24 is apodized. In other words, for an FBG, the modulation of the refractive index of the core of the optical fiber 22 has an edgeless envelope, such as a Gaussian envelope.

[0039] Indeed, in the case of a non-apodized (straight) selective component 24 (FBG), the two edges of the selective component 24 (front and rear) are perceived by the light traveling in the fiber core as index steps giving rise to reflections with a wide range of wavelengths. In the case of a non-apodized FBG, all incoming wavelengths are reflected by the FBG envelope with the same optical phase, whereas for the apodized FBG, the optical phase changes continuously depending on the shape of the FBG envelope. This has the effect that the external laser cavity 20 favors the few modes located in the center of the apodization envelope, which gives rise to a quasi-continuous transfer of optical power with an apodized FBG compared to a singular power transfer with a conventional FBG inducing a significant mode hop.Thus, the main advantage of the apodized shape (Gaussian shape for example) of the selective component 24 is that we avoid the light being reflected on the edges of the selective component 24. Note that the outer edges of the apodization envelope are preferably as small as possible and "smoothed" so that the light is preferably reflected around the top of the envelope (of the Gaussian hat), where the selective component 24 has the strongest index modulation.

[0040] An example of operation of the laser device 10 will now be described.

[0041] Initially, the laser cavity 20 generates a laser beam at the output of the semi-reflecting mirror M2. This laser beam is focused by the optics 32 in the optical fiber 22.

[0042] The selective component 24 transmits a portion of the laser beam at the selection wavelength of the selective component 24. This laser beam is conveyed by the optical fiber 22 at the output of the laser device 10.

[0043] The selective component 24 also returns into the optical cavity 20 a part (less than 10%, for example 2-3%) of the laser beam at the selection wavelength of the selective component 24.

[0044] The laser beam returned at the wavelength of interest is then amplified in the laser cavity 20, before being transmitted again at the output of the semi-reflecting mirror, and so on.

[0045] Thus, the selective component 24 placed in the optical fiber 22 at the output of the cavity laser 20 makes it possible to select and stabilize the wavelength of the laser beam at the output of the laser device 10. The selective component 24 being positioned upstream of the fixing points PI, P2, it is not subject to the temperature variations of the optical fiber 22 and the housing 12, but only to the possible temperature variations of the selective component 24. This thus limits the wavelength jumps, and preserves the spectral and power stability of the laser beam at the output of the laser device 10.

[0046] In particular, tests have been carried out by the inventors showing that the wavelength jumps decrease by a factor of two when the selective component 24 is positioned upstream of the fixing points PI, P2, compared to a configuration where the selective component 24 would be positioned between the two fixing points PI, P2. In particular, in these tests, the wavelength drift decreases from 0.016 nm / °C to 0.009 nm / °C.

[0047] The laser device 10 is furthermore simpler than those of the prior art since it does not involve changing the shape of the waveguide from a straight design to a curved design, or writing a Bragg grating on the gain waveguide (DFB) of the laser cavity 20. Furthermore, the power conversion efficiency of the laser is unchanged because the gain waveguide has not been modified, which allows for reduced power consumption compared to other prior art solutions.

[0048] In addition, Figures 3 and 4 illustrate respectively the stability in power, and in wavelength, of a laser beam generated by a laser device 10 as described previously with a Bragg grating positioned at a distance of less than 1 mm from the output of the laser cavity 20 and upstream of the fixing points PI, P2. In particular, the number of mode jumps, see [Fig. 3], has been reduced to only 4, compared to 50 in the case of a Bragg grating positioned between the fixing points PI, P2 and 5 mm from the output of the laser cavity 20. The amplitude of the power jumps is reduced by a factor of 2. In addition, the peak wavelength jumps have a maximum variation of 0.3 nm over the entire duration of the current over the entire current range. The amplitude of wavelength jumps is also reduced, with a maximum amplitude of 0.12 nm, which is 4 times less than in the state of the art.

[0049] Those skilled in the art will understand that the embodiments described above are capable of being combined with each other when such combinations are compatible.

[0050] Furthermore, in an alternative embodiment, the optical fiber 22 is fixed to the housing 12 by a single fixing point (fixing points PI and P2 combined). In this case, this single fixing point ensures both the functions of fixing the optical fiber 22 to the housing 12 and of sealing the housing 12. The previous description applies in this case considering the fixing points PI and P2 combined. In particular, the selective component 24 is positioned upstream of the single fixing point.

Claims

Claims

1. A laser device (10) comprising a housing (12) in which the following elements are arranged: - a laser cavity (20) delimited by a reflecting mirror (M1) and a partially reflecting mirror (M2), the partially reflecting mirror (M2) forming the output of the laser cavity (20) for a laser beam amplified in the laser cavity (20), - an optical fiber (22) suitable for conveying the laser beam between the output of the laser cavity (20) and the outside of the housing (12), the optical fiber (22) being fixed to the housing (12) at at least one fixing point (PI, P2), and - a selective component (24) suitable for selecting the wavelength of the laser beam at the output of the laser device (10), the selective component (24) being positioned in the optical fiber (22) at the output of the laser cavity (20) and upstream of the fixing point(s) (PI, P2).

2. The laser device (10) of claim 1, wherein the selective component (24) is a Bragg grating.

3. A laser device (10) according to claim 1 or 2, wherein the housing (12) has an outlet (26), the optical fiber (22) being attached to the housing (12) at a first and a second attachment point (PI, P2), the first attachment point (PI) being between the outlet of the laser cavity (20) and the outlet of the housing (12), the second attachment point (P2) coinciding with the outlet (26) of the housing (12), the selective component (24) being positioned between the outlet of the laser cavity (20) and the first attachment point (PI).

4. A laser device (10) according to any one of claims 1 to 3, wherein the selective component (24) is positioned at a distance less than or equal to 1 millimeter from the output of the laser cavity (20).

5. Laser device (10) according to any one of claims 1 to 4, in which the face of the partially reflecting mirror (M2) facing the optical fiber (22) is optimized to transmit, into the laser cavity (20), a laser beam at the selection wavelength of the selective component (24).

6. A laser device (10) according to any one of claims 1 to 5, wherein the reflection spectrum of the selective component (24) has a bandwidth chosen to retain only one or two laser modes of the laser beam, advantageously a single mode of the laser beam.

7. A laser device (10) according to any one of claims 1 to 6, wherein the reflection spectrum of the selective component (24) has a bandwidth less than or equal to five times the free spectral range (FSR) of the laser cavity (20), preferably less than or equal to 0.5 nanometers, preferably less than or equal to 0.3 nanometers.

8. A laser device (10) according to any one of claims 1 to 7, wherein the reflection spectrum of the selective component (24) is apodized.

9. A laser device (10) according to any one of claims 1 to 8, wherein the optical fiber (22) comprises an optic (32) suitable for focusing the laser beam in the optical fiber (22), the optic (32) being positioned in the optical fiber (22), between the output of the laser cavity (20) and the selective component (24), the optic (32) preferably being a lens machined in the optical fiber (22).

10. A laser device (10) according to any one of claims 1 to 9, wherein the laser cavity (20) is a rectilinear cavity.

11. A laser device (10) according to any one of claims 1 to 9, wherein the laser cavity (20) is a curved cavity.

12. The laser device (10) of any one of claims 1 to 11, wherein the laser housing (12) has a length less than or equal to the length of the laser cavity (20) plus 8 millimeters.

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