Laser device

Abstract

The present invention relates to a laser device (10) comprising a housing (12), wherein the housing (12) comprises a laser resonator (20) partitioned by a reflective mirror (M1) and a semi-reflective mirror (M2), the semi-reflective mirror (M2) forming an outlet for the laser beam amplified within the laser resonator (20), an optical fiber (22) capable of transmitting the laser beam between the outlet of the laser resonator (20) and the outside of the housing (12), the optical fiber (22) being fixed to the housing (12) at at least one fixed point (P1, P2), and a selection element (24) capable of selecting the wavelength of the laser beam at the outlet of the laser device (10), the selection element (24) being located within the optical fiber (22) at the outlet of the laser resonator (20) and located upstream of the fixed point (P1, P2).

Description

Technical Field

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

Background Art

[0002] In the prior art, a laser device is known that includes a laser resonator capable of generating a horizontally single-mode laser beam and a single-mode optical fiber that guides the laser beam generated by the resonator outside the housing within the housing.

[0003] In order to select and stabilize the wavelength of the laser beam generated by the laser resonator, a selection element such as a Bragg grating, particularly a Bragg grating (FBG) written in an optical fiber, has been used, and conventionally, it has been arranged in an optical fiber outside the housing.

Summary of the Invention

Problems to be Solved by the Invention

[0004] In recent years, particularly, integrating external components including FBGs within the housing has become an issue for miniaturization.

[0005] However, when integrating FBGs within the housing, abrupt jumps in wavelength and output occur along with fluctuations in the laser diode current, and the spectral stability and output stability at the optical fiber exit deteriorate. This problem is particularly prominent in optical measurement applications such as EDFA pumping applications, frequency doubling applications, and optical clock experiments.

[0006] Therefore, there is a need for a laser device that can achieve miniaturization while maintaining good spectral stability and output stability at the optical fiber exit.

Means for Solving the Problems

[0007] For this purpose, the present disclosure targets a laser device including a housing, and within the housing, A laser resonator partitioned by a reflective mirror and a semi-reflective mirror, wherein the semi-reflective mirror forms the outlet for the laser beam amplified within the laser resonator, An optical fiber capable of transmitting a laser beam between the exit of a laser resonator and the outside of the housing, wherein the optical fiber is fixed to the housing at at least one fixed point, A selection element capable of selecting the wavelength of a laser beam at the exit of a laser device, wherein the selection element is located in an optical fiber at the exit of a laser resonator and is positioned upstream of a fixed point. The elements are arranged.

[0008] According to a particular embodiment, the apparatus comprises one or more of the following features, either individually or in any technically possible combination. The selection element is a Bragg grid. The housing has an exit opening, and the optical fiber is fixed to the housing at a first fixed point and a second fixed point, the first fixed point being located between the exit of the laser resonator and the exit opening of the housing, the second fixed point being coincident with the exit opening of the housing, and the selection element being positioned between the exit of the laser resonator and the first fixed point. The selection element is positioned at a distance of less than 1 millimeter from the exit of the laser resonator. The surface of the semi-reflective mirror facing the optical fiber is optimized to transmit the laser beam of the selected wavelength from the selection element into the laser resonator. The reflection spectrum of the selection element has a bandwidth selected to retain only one or two laser modes of the laser beam, preferably only one mode of the laser beam. The reflection spectrum of the selected element has a bandwidth of 5 times or less the free spectral range (FSR) of the laser resonator, preferably 0.5 nanometers or less, and more preferably 0.3 nanometers or less. The reflection spectrum of the selected element is apodized. The optical fiber includes an optical element capable of focusing a laser beam within the optical fiber, and the optical element is positioned within the optical fiber between the exit of the laser resonator and the selection element, preferably a lens formed within the optical fiber. A laser resonator is a linear resonator. A laser resonator is a curved resonator. The length of the laser housing is no more than the length of the laser resonator plus 8 millimeters.

[0009] Other features and advantages of the present invention will become apparent below from the description of embodiments of the invention, which are described by illustration only with reference to the drawings. [Brief explanation of the drawing]

[0010] [Figure 1] This is a schematic cross-sectional view showing an example of a laser device. [Figure 2] This graph shows an example of the modes of the laser beam generated by the laser resonator, and superimposed on it the reflection spectra of a first select element and a second select element (Bragg grating), where the first and second select elements have different bandwidths (1.2 nm vs. 0.25 nm). [Figure 3] This graph shows an example of the change in optical output and the derivative of the optical output with respect to the laser current in a laser device where a selected FBG element is positioned less than 1 mm from the exit of the laser resonator and upstream of the fixing point of the optical fiber to the housing. [Figure 4] This graph shows an example of wavelength variation with respect to laser current in a laser device where a selected FBG element is positioned less than 1 mm from the exit of the laser resonator and upstream of the fixing point of the optical fiber to the housing. [Modes for carrying out the invention]

[0011] Figure 1 shows an example of a laser device 10.

[0012] The laser device 10 includes a housing 12 in which a laser resonator 20, an optical fiber 22, and a selection element 24 are arranged.

[0013] The housing 12 is a sealed housing and includes at least one opening 26 for passing the optical fiber 22.

[0014] The laser resonator 20 can generate and amplify a laser beam. The laser resonator 20 forms a Fabry - Perot resonator. The laser resonator 20 is, for example, a laser chip such as a laser diode.

[0015] As shown in the example of FIG. 1, the laser resonator 20 includes a waveguide 30 having an active medium disposed between a reflective mirror M1 and a semi - reflective mirror M2. The semi - reflective mirror M2 forms an exit of the laser beam generated and amplified within the laser resonator 20.

[0016] Particularly, the surface of the reflective mirror M1 facing the laser resonator 20 (the inner side of the laser resonator 20) is called the rear surface of the laser resonator 20. On the other hand, the surface of the semi - reflective mirror M2 facing the optical fiber 22 (the outer side of the laser resonator 20) is called the front surface of the laser resonator 20.

[0017] Preferably, the surface (front surface) of the semi - reflective mirror M2 facing the optical fiber 22 is optimized to transmit a laser beam having a wavelength selected by the selection element 24 (this will be described later). Here, the "optimization" means that the reflectivity of the semi - reflective mirror M2 is less than 0.5% at the wavelength of the selection element 24.

[0018] Thereby, the laser beam at the wavelength of interest (the wavelength of the selection element 24) is re - introduced into the laser resonator 20 for re - amplification, and an additional laser resonator is formed between the rear surface of the laser resonator 20 and the selection element 24.

[0019] Preferably, the laser resonator 20 is a linear resonator, that is, the reflection mirror M1 and the semi-reflection mirror M2 are parallel to each other. In other words, the laser resonator 20 is not bent. With this configuration, it is possible to increase the output power and reduce the power consumption.

[0020] Alternatively, the laser resonator 20 may be a curved resonator.

[0021] The optical fiber 22 can transmit the laser beam between the exit of the laser resonator 20 and the outside of the housing 12.

[0022] The optical fiber 22 is fixed to the housing 12 at at least two fixed points P1, P2.

[0023] In the embodiment shown in FIG. 1, the optical fiber 22 is fixed to the housing 12 at the first fixed point P1 and the second fixed point P2. The first fixed point P1 is located between the exit of the laser resonator 20 and the exit opening 26 of the housing 12. The second fixed point P2 fixes the optical fiber 22 to the exit opening 26 of the housing 12. In particular, the fixing at the second fixed point P2 can fix the optical fiber 22 airtightly to the housing 12. Therefore, the portion of the optical fiber 22 between the first fixed point P1 and the second fixed point P2 is affected by the temperature change of the housing 12 and the deformation of the housing 12 due to the difference in the thermal expansion coefficients of the respective materials constituting the laser device 10.

[0024] For example, as the fixing means, an adhesive or a low melting point glass solder is used.

[0025] As an example, the outer length of the housing 12 excluding the second fixed point P2 does not exceed the length obtained by adding 8 mm to the length of the laser resonator 20. In this case, the laser housing 12 is called a "mini pump". With this configuration, in the prior art, miniaturization is possible without being restricted by the distance between the fixed points P1 and P2 that required the selection element 24 to be sufficiently separated.

[0026] In one embodiment, the optical fiber 22 includes an optical system 32 capable of focusing a laser beam within the optical fiber 22. The optical system 32 is positioned within the optical fiber 22 between the exit of the laser resonator 20 and the selection element 24. For example, the optical system 32 is a (focusing) lens fabricated and formed within the optical fiber 22.

[0027] The selection element 24 can select the wavelength of the laser beam at the output of the laser device 10. That is, the laser resonator 20 initially generates and amplifies a laser beam over a wide wavelength band of several nanometers, and the selection element 24 selects a specific wavelength, which is then re-amplified to obtain a laser beam of a controlled and stabilized wavelength at the output of the laser device 10.

[0028] The selection element 24 is positioned within the optical fiber 22 at the exit of the laser resonator 20 and upstream of the fixed points P1 and P2. Therefore, the selection element 24 is not affected by the mechanical deformation of the housing 12 caused by temperature changes. That is, when the selection element 24 (FBG) is positioned between the two fixed points P1 and P2, the center wavelength of the selection element 24 changes according to its temperature and the mechanical stress caused by the difference in thermal expansion between the housing 12 and the optical fiber 22. On the other hand, in the present invention, by positioning the selection element 24 outside the space between the fixed points P1 and P2, these mechanical constraints can be avoided, and the wavelength fluctuation of the selection element 24 can be attributed solely to the temperature change of the selection element 24 itself.

[0029] More specifically, when the optical fiber 22 is fixed by two fixed points P1 and P2, the selection element 24 is positioned between the exit of the laser resonator 20 and the first fixed point P1. That is, the selection element 24 is not positioned between the two fixed points P1 and P2.

[0030] Preferably, the selection element 24 is a Bragg grid. More specifically, the selection element 24 is a Bragg grid formed on the optical fiber 22 (fiber core).

[0031] Preferably, the selection element 24 is positioned at a distance of 1 millimeter or less from the exit of the laser resonator 20, thereby reducing wavelength jumps and power jumps compared to when the selection element 24 is positioned further away from the exit of the laser resonator 20. That is, when the selection element 24 is positioned very close (less than 1 mm) to the exit of the laser resonator 20, the free spectral range (FSR) of the laser resonator 20 and the FSR of the additional resonator formed between the rear surface of the laser resonator 20 and the selection element 24 become very close. This proximity reduces the amplitude of wavelength and power jumps, even when mode jumps occur due to, for example, laser current fluctuations.

[0032] Preferably, the reflection spectrum of the selection element 24 has a bandwidth selected to retain only one or two laser modes of the laser beam, and more preferably, retain only one mode of the laser beam.

[0033] For example, in Figure 2, A shows the transmission curve of the laser modes in the laser resonator 20, B shows the reflection curve of a broadband (1.2 nm) FBG, and C shows the reflection curve of a narrowband (0.25 nm) FBG. As is clear from this figure, the narrowband FBG can select only a smaller number of modes and reduces wavelength jumps by "forcing" the laser resonator 20 to amplify only a limited number of modes.

[0034] Preferably, the reflection spectrum of the selection element 24 has a bandwidth of 5 times or less the free spectral range (FSR) of the laser resonator (defined by the full width at half maximum of the reflectance spectrum). More preferably, the bandwidth of the reflection spectrum of the selection element 24 is 0.5 nanometers or less, and even more preferably 0.3 nanometers or less.

[0035] Preferably, the reflection spectrum of the selection element 24 is apodized. That is, in the case of FBG, the refractive index modulation of the optical fiber core 22 has an envelope shape without ends, such as a Gaussian envelope.

[0036] In other words, in the case of a non-apodized (linear) selection element 24 (FBG), the front and rear ends of the selection element 24 are perceived as refractive index steps by light propagating within the optical fiber core, causing reflections over a wide range of wavelengths. In a non-apodized FBG, all incident wavelengths are reflected by the FBG envelope with the same optical phase, whereas in an apodized FBG, the optical phase changes continuously according to the shape of the FBG envelope. As a result, the external laser resonator 20 preferentially selects a small number of modes located near the center of the apodization envelope, and a quasi-continuous optical output transition is obtained in the apodized FBG compared to the discrete optical output transition with significant mode jumps in conventional FBGs. Therefore, the main advantage of making the selection element 24 apodized (e.g., Gaussian shape) is that reflections at the ends of the selection element 24 can be avoided. Furthermore, the outer edge of the apodization envelope is preferably as small and "smooth" as possible, so that the light is mainly reflected near the top of the envelope (the top of the Gaussian hat) where the refractive index modulation of the selection element 24 is strongest.

[0037] The following describes an example of the operation of the laser device 10.

[0038] First, the laser resonator 20 generates a laser beam at the exit of the semi-reflective mirror M2. This laser beam is focused into the optical fiber 22 by the optical system 32.

[0039] The selection element 24 transmits a portion of the laser beam at the wavelength selected by the selection element 24. This laser beam is transmitted by the optical fiber 22 at the exit of the laser device 10.

[0040] Furthermore, the selection element 24 returns a portion of the laser beam at the selected wavelength (for example, less than 10%, specifically 2-3%) into the optical resonator 20.

[0041] The laser beam of interest, which has been returned in this manner, is amplified again within the laser resonator 20 and then emitted again from the exit of the semi-reflective mirror, and this process is repeated.

[0042] Therefore, the selection element 24 in the optical fiber 22, positioned at the exit of the laser resonator 20, enables the selection and stabilization of the laser beam wavelength at the exit of the laser device 10. Furthermore, since the selection element 24 is positioned upstream of the fixed points P1 and P2, it is not affected by temperature changes in the optical fiber 22 and the housing 12, and wavelength fluctuations are limited to those caused solely by temperature changes in the selection element 24 itself. As a result, wavelength jumps are suppressed, and the spectral stability and output stability of the laser beam at the exit of the laser device 10 are maintained.

[0043] In particular, the inventors' tests confirmed that when the selection element 24 is placed upstream of the fixed points P1 and P2, the wavelength jump is reduced by approximately half compared to a configuration where the selection element 24 is placed between the fixed points P1 and P2. Specifically, the wavelength drift was reduced from 0.016 nm / °C to 0.009 nm / °C.

[0044] This laser device 10 has a simpler configuration compared to conventional technologies because it does not require changing the waveguide shape from a straight line to a curve or writing a Bragg grid (DFB) into the gain waveguide of the laser resonator 20. Furthermore, since the gain waveguide is not changed, the power conversion efficiency of the laser is maintained, and energy consumption can be reduced compared to other conventional technologies.

[0045] Furthermore, Figures 3 and 4 show the output stability and wavelength stability of the laser beam generated by the laser apparatus 10 of the present invention, in which the Bragg grating is positioned less than 1 mm from the exit of the laser resonator 20 and upstream of the fixed points P1 and P2, respectively. In particular, as shown in Figure 3, the number of mode jumps has been reduced to only 4, which is a significant reduction compared to the 50 mode jumps in the conventional configuration in which the selection element is positioned between the fixed points P1 and P2 and 5 mm away from the exit of the laser resonator 20. The amplitude of the output jump has also been reduced by approximately half. In addition, the peak wavelength jump across the entire current range has been suppressed to a maximum of 0.3 nm, and the maximum amplitude of the wavelength jump has also been reduced to 0.12 nm, which is about one-quarter of that of the conventional technology.

[0046] Those skilled in the art will understand that the embodiments described above can be combined with each other to the extent technically possible.

[0047] Furthermore, in another embodiment, the optical fiber 22 may be fixed to the housing 12 by a single fixed point (a configuration in which fixed points P1 and P2 are integrated). In this case, this single fixed point serves both the function of fixing the optical fiber 22 to the housing 12 and the function of sealing the housing 12. In this case as well, the above description applies, assuming that fixed points P1 and P2 are integrated. In particular, the selection element 24 is located upstream of this single fixed point.

Claims

1. A laser device (10) comprising a housing (12), Inside the housing (12), A laser resonator (20) is partitioned by a reflective mirror (M1) and a semi-reflective mirror (M2), wherein the semi-reflective mirror (M2) forms the outlet for the laser beam amplified within the laser resonator (20), An optical fiber (22) capable of transmitting a laser beam between the outlet of the laser resonator (20) and the outside of the housing (12), wherein the optical fiber (22) is fixed to the housing (12) at at least one fixed point (P1, P2), A selection element (24) capable of selecting the wavelength of the laser beam at the exit of the laser device (10), wherein the selection element (24) is located in the optical fiber (22) at the exit of the laser resonator (20) and is located upstream of the fixed points (P1, P2), The elements are arranged Laser device.

2. The laser apparatus according to claim 1, wherein the selection element (24) is a Bragg grating.

3. The housing (12) has an outlet opening (26), The optical fiber (22) is fixed to the housing (12) at the first fixed point (P1) and the second fixed point (P2). The first fixed point (P1) is located between the outlet of the laser resonator (20) and the outlet opening of the housing (12), The second fixed point (P2) coincides with the exit opening (26) of the housing (12), The selection element (24) is positioned between the outlet of the laser resonator (20) and the first fixed point (P1). The laser apparatus according to claim 1 or claim 2.

4. The laser apparatus according to any one of claims 1 to 3, wherein the selection element (24) is arranged at a distance of 1 millimeter or less from the outlet of the laser resonator (20).

5. The laser apparatus according to any one of claims 1 to 4, wherein the surface of the semi-reflective mirror (M2) facing the optical fiber (22) is optimized to transmit a laser beam of a wavelength selected by the selection element (24) into the laser resonator (20).

6. The laser apparatus according to any one of claims 1 to 5, wherein the reflection spectrum of the selection element (24) has a bandwidth selected to retain only one or two laser modes of the laser beam, preferably retaining only one mode of the laser beam.

7. The laser apparatus according to any one of claims 1 to 6, wherein the reflection spectrum of the selection element (24) has a bandwidth of five times or less the free spectral range (FSR) of the laser resonator (20), preferably 0.5 nanometers or less, and more preferably 0.3 nanometers or less.

8. The laser apparatus according to any one of claims 1 to 7, wherein the reflection spectrum of the selection element (24) is apodized.

9. The optical fiber (22) includes an optical element (32) capable of focusing the laser beam within the optical fiber (22), The optical element (32) is disposed within the optical fiber (22) between the outlet of the laser resonator (20) and the selection element (24), and is preferably a lens fabricated and formed within the optical fiber (22). A laser apparatus according to any one of claims 1 to 8.

10. The laser apparatus according to any one of claims 1 to 9, wherein the laser resonator (20) is a linear resonator.

11. The laser apparatus according to any one of claims 1 to 9, wherein the laser resonator (20) is a curved resonator.

12. The laser apparatus according to any one of claims 1 to 11, wherein the length of the laser housing (12) is less than or equal to the length of the laser resonator (20) plus 8 millimeters.

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