CN112397995B

CN112397995B - Narrow-linewidth fixed-wavelength laser and optical module

Info

Publication number: CN112397995B
Application number: CN201910712104.8A
Authority: CN
Inventors: 涂文凯; 骆亮
Original assignee: Innolight Technology Suzhou Ltd
Current assignee: Innolight Technology Suzhou Ltd
Priority date: 2019-08-02
Filing date: 2019-08-02
Publication date: 2022-02-15
Anticipated expiration: 2039-08-02
Also published as: CN112397995A

Abstract

The application discloses a narrow linewidth fixed wavelength laser, which comprises a shell, an external resonant cavity arranged in the shell, a gain chip arranged in the external resonant cavity and a filter element, wherein the gain chip is arranged in the external resonant cavity; the shell is provided with an optical interface and an electrical interface; the filter element is used for selecting the required specific wavelength; the electrical interface is used for transmitting electrical signals, and the electrical signals comprise driving signals; the driving signal drives the gain chip to emit a light beam, the light beam resonates in the external resonant cavity to generate a laser mode, and the filter element selects a required wavelength from the laser mode to oscillate in the external resonant cavity so as to output a laser beam with a specific wavelength. The application adopts an external cavity structure with fixed wavelength, reduces the number of elements in the external cavity, and on one hand, the packaging size can be greatly smaller and the size is smaller; on the other hand, the external cavity insertion loss can be effectively reduced, the influence of the insertion loss on the laser power and the line width is further reduced, and the laser with more stable and narrower line width is output by matching with a laser mode capable of being finely adjusted.

Description

Narrow-linewidth fixed-wavelength laser and optical module

Technical Field

The application relates to the technical field of optical communication, in particular to a narrow-linewidth fixed-wavelength laser and an optical module.

Background

Coherent optical communication is an important technology for solving the problem of chromatic dispersion in long-distance transmission in the field of optical communication, a wavelength tunable laser is a core device of a coherent optical transmitter, and particularly, a tunable external cavity laser has a narrow linewidth characteristic, so that the chromatic dispersion problem in long-distance transmission is well solved.

Miniaturization of the package has been a trend of tunable laser, and it is required to be applicable to various optical modules in different package forms. The Chinese patent application 'small-sized packaged tunable laser component' (application number: 201410059069.1) discloses a miniaturized tunable laser, which comprises a shell with the volume less than 0.6 cubic centimeter, and a tunable semiconductor laser, a beam splitter, an optical isolator, a photodiode and a coupling optical system are sequentially arranged in the shell. The tunable semiconductor laser comprises a gain chip, a vernier tuning mechanism consisting of two filters and a cavity length actuator. With the higher integration of the device, the optical module is required to have a smaller volume, and thus the laser used as the core device of the coherent optical module is also required to have a smaller volume. The tunable laser assemblies disclosed in the above-mentioned patent applications, although having a smaller volume, obviously have a smaller volume requirement in some more miniaturized optical modules.

Disclosure of Invention

An object of the application is to provide a fixed wavelength laser of narrow linewidth and optical module, have advantages such as the encapsulation size is little, the external cavity insertion loss is little, laser linewidth is narrow.

In order to achieve one of the above objects, the present application provides a narrow linewidth fixed wavelength laser comprising a housing provided with an optical interface and an electrical interface; the filter also comprises an external resonant cavity arranged in the shell, and a gain chip and a filter element which are arranged in the external resonant cavity;

the filter element is used for selecting a required specific wavelength;

the electrical interface is used for transmitting electrical signals, and the electrical signals comprise driving signals;

the driving signal drives the gain chip to emit light beams, the light beams resonate in the external resonant cavity to generate a laser mode, and the filter element selects a required wavelength from the laser mode to oscillate in the external resonant cavity so as to output a laser beam with a specific wavelength.

As a further improvement of the embodiment, the electrical signal transmitted by the electrical interface further includes a fine tuning signal, and the fine tuning signal is used for fine tuning the laser mode of the external resonant cavity to align the laser mode with the specific wavelength.

As a further improvement of the embodiment, the fine tuning signal is applied to the gain chip for changing the refractive index and/or the temperature of the gain chip to fine tune the laser mode of the external resonant cavity.

As a further refinement of the embodiment, the fine tuning signal comprises a fine tuning current superimposed on a bias current of the gain chip.

As a further improvement of the embodiment, a TEC is disposed in the housing, and the gain chip is disposed on the TEC; the fine tuning signal adjusts the laser mode of the external resonant cavity by adjusting the temperature of the TEC.

As a further improvement of the embodiment, the laser further comprises an actuator, one facet of the external cavity is disposed on the actuator, and the fine tuning signal adjusts the lasing mode of the external cavity by adjusting the position of the facet of the external cavity on the actuator.

As a further refinement of the embodiment, the 3dB bandwidth of the filter element is less than or equal to 1 times the free spectral range of the lasing mode.

As a further refinement of the embodiment, the filter element comprises two superimposed cut-off filters.

As a further improvement of the embodiment, the two superimposed cut-off filters are respectively disposed on two planes opposite to a transparent flat plate.

As a further refinement of the embodiment, the filter element comprises a band-pass filter.

As a further improvement of the embodiment, the fixed-wavelength laser further includes a transparent plate located in the external cavity, the transparent plate includes two opposite planes, and the band-pass filter is disposed on one plane of the transparent plate close to the gain chip; one cavity surface of the external resonant cavity is arranged on the other plane of the transparent flat plate.

As a further improvement of the embodiment, a coupling lens is further disposed in the housing, and the coupling lens is located on an output optical path of the external resonant cavity; the coupling lens comprises a plane close to the external resonant cavity and a spherical surface or an aspherical surface far away from the external resonant cavity; and the output cavity surface of the external resonant cavity is arranged on the plane of the coupling lens.

As a further improvement of the embodiment, an isolator is further arranged in the shell, and the isolator is positioned on an output optical path of the external resonant cavity; and the output cavity surface of the external resonant cavity is arranged on the plane of the isolator close to the external resonant cavity.

As a further improvement of the embodiment, the housing is a cuboid, and the volume of the cuboid is less than or equal to 0.3 cubic centimeter.

As a further improvement of the embodiment, the fixed wavelength laser further includes an optical fiber, the optical fiber includes an optical fiber head and a tail fiber, the optical fiber head includes a fixed sleeve, a capillary is disposed in the fixed sleeve, and one end of the tail fiber is disposed in the capillary; the fixing sleeve of the optical fiber head is arranged in the optical interface of the shell.

The application also provides an optical module comprising the fixed wavelength laser in any one of the above embodiments.

The beneficial effect of this application: the external cavity structure with fixed wavelength is adopted, so that the number of elements in the external cavity is reduced, and on one hand, the packaging size can be greatly reduced, and the size is smaller; on the other hand, the external cavity insertion loss can be effectively reduced, the influence of the insertion loss on the laser power and the line width is further reduced, and the laser with more stable and narrower line width is output by matching with a laser mode capable of being finely adjusted.

Drawings

FIG. 1 is a schematic view of a narrow linewidth fixed wavelength laser package according to the present application;

FIG. 2 is a schematic diagram of an external cavity laser module in embodiment 1 of the fixed wavelength laser of the present application;

FIG. 3 is a schematic diagram of the laser mode and spectral characteristics of the filter element according to the present application;

FIG. 4 is a schematic diagram showing the superposition of spectral characteristics of two cut-off filters;

FIG. 5 is a schematic diagram of an external cavity laser module in embodiment 2 of the fixed wavelength laser of the present application;

FIG. 6 is a schematic diagram of an external cavity laser module in embodiment 3 of the fixed wavelength laser of the present application;

FIG. 7 is a schematic diagram of another package for a narrow linewidth fixed wavelength laser according to the present application;

FIG. 8 is a schematic diagram of an optical module according to the present application.

Detailed Description

The present application will now be described in detail with reference to specific embodiments thereof as illustrated in the accompanying drawings. These embodiments are not intended to limit the present application, and structural, methodological, or functional changes made by those skilled in the art according to these embodiments are included in the scope of the present application.

In the various illustrations of the present application, certain dimensions of structures or portions may be exaggerated relative to other structures or portions for ease of illustration and, thus, are provided to illustrate only the basic structure of the subject matter of the present application.

Also, terms used herein such as "upper," "above," "lower," "below," and the like, denote relative spatial positions of one element or feature with respect to another element or feature as illustrated in the figures for ease of description. The spatially relative positional terms may be intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as "below" or "beneath" other elements or features would then be oriented "above" the other elements or features. Thus, the exemplary term "below" can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. When an element or layer is referred to as being "on," or "connected" to another element or layer, it can be directly on, connected to, or intervening elements or layers may be present.

The application provides a narrow linewidth fixed wavelength laser with miniaturized package size. As shown in fig. 1 and 2, the fixed wavelength laser includes a housing 100 and an external cavity laser module disposed within the housing 100, the external cavity laser module including an external cavity and a gain chip 310 and a filter element 320 disposed within the external cavity. The housing 100 is provided with an optical interface 110 and an electrical interface 120, wherein the optical interface 110 is used for transmitting optical signals, and the electrical interface 120 is used for transmitting electrical signals. The electrical signal includes a driving signal, such as a bias current of the gain chip 310. The filter element 320 is used for selecting a specific wavelength required, for example, in optical communication, for selecting a standard wavelength of itu (international Telecommunication union) required for optical communication. In operation, the driving signal drives the gain chip 310 to emit a light beam, which resonates in the external cavity to generate a lasing mode, and the filter element 320 selects a desired wavelength from the lasing mode to oscillate in the external cavity to output a laser beam having a specific wavelength.

The electrical signals transmitted by the electrical interface 120 further include a fine tuning signal for fine tuning the laser mode of the external cavity to align the desired wavelength of the laser mode with the specific wavelength. Specifically, as shown in fig. 3, curve a is a laser mode oscillating in the external cavity, i.e., a cavity film of the cavity, and curve b is a spectral characteristic curve of the filter element 320. Here, the filter element 320 is a narrow band pass filter, and the Free Spectral Range (FSR) of the laser mode whose 3dB bandwidth of the transmission Spectrum is less than or equal to 1 times can effectively suppress the side mode, and superpose with the laser mode of the external cavity, and by mode competition in the external cavity, a single mode output with a narrow line width with a good side mode suppression ratio can be obtained. In other embodiments, the filtering element 320 may also be formed by overlapping two cut-off filters, such as the overlapping of two cut-off filter spectral lines as shown in fig. 4, for example, overlapping a high-pass filter and a low-pass filter, so that the filtering characteristic with narrow line width can be easily realized, and the filtering element has a better side mode suppression ratio, and after being overlapped with the laser mode, a single-mode output with a better side mode suppression ratio can be obtained.

The external cavity structure adopted by the fixed wavelength laser reduces the number of elements in the external cavity, and on one hand, the packaging size can be greatly smaller, and the size is smaller; on the other hand, the external cavity insertion loss can be effectively reduced, the influence of the insertion loss on the laser power and the line width is further reduced, and the laser with more stable and narrower line width is output by matching with a laser mode capable of being finely adjusted. The following embodiments will explain in detail the specific structure of the external cavity laser module in the housing with reference to the accompanying drawings.

Example 1

As shown in fig. 1 and 2, the external cavity laser module disposed in the housing 100 includes an external resonant cavity, and a gain chip 310 and a filter element 320 disposed in the external resonant cavity, a coupling lens 330 and an isolator 340 are further disposed in the housing 100, the optical interface 110 of the housing 100 is connected to the optical fiber 200, and laser light output from the external cavity laser is coupled into the optical fiber 200 through the coupling lens 330 and transmitted out of the housing 100 through the optical fiber 200. The optical fiber 200 includes a fiber head 210 and a pigtail 220, wherein the fiber head 210 includes a fixed sleeve, such as a glass sleeve, a capillary is disposed in the fixed sleeve, and one end of the pigtail 220 is disposed in the capillary; the fixed sleeve of the fiber optic head 210 is disposed within the optical interface 110 of the housing 100. The optical fiber head omits a ceramic ferrule, and the tail fiber is directly fixed by the glass sleeve, so that the length of the optical fiber head is shortened, and the overall size of a device is reduced. Of course, in other embodiments, the optical interface 110 of the housing 100 may also be connected to a pluggable connector, not necessarily an optical fiber.

In this embodiment, a TEC (thermal Electric Cooler) 360 is further disposed in the housing 100, the gain chip 310, the filter element 320 and the coupling lens 330 are disposed on the TEC 360, and the isolator 340 is disposed in the optical interface 110 of the housing 100. The first end face 311 of the gain chip 310 away from the filter element 320 is plated with a high reflection film, and the first end face 311 serves as a reflection cavity face of the external resonant cavity. The gain chip 310 is antireflection coated on the second end face 312 near the filter element 320. The coupling lens 330 includes a first plane 331 and a convex surface 332 on the optical path, the first plane 331 is close to the filter element 320, and is coated with a partial reflective film as the output cavity surface of the external resonant cavity, and the convex surface 332 is spherical or aspheric and is coated with an antireflection film. I.e. the first end face 311 of the gain chip 310 and the plane 331 of the coupling lens 330 form the two cavity faces of the external cavity. In this embodiment, a collimating lens 350 is further disposed between the gain chip 310 and the filter element 320, and collimates the light beam emitted from the gain chip 310 and enters the filter element 320, the filter element 320 selects a desired wavelength, so that the desired wavelength is competed by modes in the external resonant cavity, and resonates in the cavity to generate laser, and finally outputs a single-mode laser with a narrow line width through the output cavity surface, and the coupling lens 330 focuses and couples the output laser into the optical fiber 200 and outputs the laser through the optical fiber 200. The isolator 340 disposed in the optical interface 110 isolates return light reflected by the end face of the optical fiber and the end faces on the external optical path, thereby preventing the return light from returning to the cavity and affecting the stability of the output laser.

The structure integrates two cavity surfaces of the external resonant cavity into one end of the gain chip and one end of the coupling lens respectively, so that optical elements in the cavity are effectively reduced, optical insertion loss in the cavity is reduced, the influence of the insertion loss on laser power and line width is reduced, and the cavity length of the external cavity is effectively shortened. The shorter the cavity length is, the larger the Free Spectral Range (FSR) of the laser mode (cavity film) of the cavity resonance is, the more stable the output laser is, the mode hopping is not easy, the 3dB bandwidth of the filter element can be wider (not more than the FSR of the laser mode), the technological requirement on the filter element is relatively reduced, and the processing is easy. In addition, the shorter the cavity length, the smaller the laser package can be made, and then the isolator can be integrated into the optical interface (or optical window) of the housing, and also can be integrated into the optical fiber head of the optical fiber, so that fewer elements are arranged in the housing, and the housing can be made smaller. In this embodiment, the housing may be made to be less than or equal to 3 cubic centimeters, for example, a rectangular parallelepiped housing with a length of 10mm, a width of 5.8mm, and a height of 4.4mm, or smaller.

The filtering element 320 in this embodiment may employ the aforementioned band pass filter or a superposition of two cut-off filters. The filter element 320 may be a transparent plate, such as a glass plate, and a band-pass filter is formed by coating on a plane of the transparent plate, or two cut-off filters are formed by coating on two opposite planes of the transparent plate on the optical path, and the two cut-off filters are stacked to form a filter element with a narrow bandwidth.

In this embodiment, the trimming signal for trimming the lasing mode of the external cavity is applied to the gain chip 310 to change the refractive index and/or temperature of the gain chip to trim the lasing mode of the external cavity such that the desired wavelength in the lasing mode is aligned with the specific wavelength. For example, the trimming signal may be a trimming current superimposed on the bias current of the gain chip 310, and the refractive index and the temperature of the gain chip 310 are changed by trimming the bias current of the gain chip 310, so as to change the optical cavity length (the optical path length in the cavity) of the external resonant cavity, thereby achieving the purpose of trimming the laser mode in the cavity. Alternatively, a fine tuning signal may be applied to the TEC 360 to adjust the temperature of the TEC 360 to change the temperature of the gain chip 310, so as to achieve the purpose of fine tuning the intracavity laser mode. The gain chip 310 is disposed on the TEC 360 through a chip substrate 370, and the chip substrate 370 is provided with a thermistor 380 for feeding back the temperature of the gain chip 310, and forms a closed-loop feedback system together with the TEC 360 to control the temperature of the gain chip 310.

Alternatively, in other embodiments, the external cavity laser module may include an actuator having a cavity surface of the external cavity disposed thereon, the fine tuning signal adjusting the lasing mode of the external cavity by adjusting the position of the cavity surface of the external cavity on the actuator. Here, the actuator may be a piezoelectric element, an acousto-optic element, an electro-optic element, a liquid crystal assembly, a MEMS or other linear motor, or the like. For example, the coupling lens is disposed on a piezoelectric element (such as PZT), a fine tuning signal is applied to the piezoelectric element, and the position of an output cavity surface disposed on the coupling lens is adjusted by controlling the deformation of the piezoelectric element to fine tune the laser mode of the external cavity. Alternatively, as shown in fig. 2, a piezoelectric ceramic (PZT)390 is added under the gain chip 310, the gain chip 310 and the collimating lens 350 are both disposed on the PZT 390, the PZT 390 is further disposed on the TEC 360, a trimming signal is applied to the PZT 390, and the position of the cavity surface disposed on the gain chip 310 is adjusted by controlling the deformation of the PZT 390 to trim the laser mode of the external resonant cavity.

In other embodiments, the positions of the coupling lens and the isolator can be interchanged, the coupling lens is arranged in the optical interface of the shell, the isolator is arranged on the TEC, and a part of reflecting film is plated on the isolator, which is close to the plane of the external resonant cavity and is used as the output cavity surface of the external resonant cavity.

Example 2

As shown in fig. 5, unlike embodiment 1, in this embodiment, the cavity surface of the external cavity is not on the coupling lens or the isolator but on the filter element. In this embodiment, the filter element 320 is a band pass filter. The fixed wavelength laser further comprises a transparent plate located within the outer cavity, the transparent plate comprising two opposing planes: a second plane 321 and a third plane 322. The bandpass filter is disposed on a second plane 321 of the transparent plate near the gain chip 310, and the output cavity surface of the external cavity is disposed on a third plane 322 of the transparent plate far from the gain chip 310. The design further shortens the cavity length of the external resonant cavity, and the cavity length (optical path length in the cavity) can be shortened to 5mm under the limit condition, so that a laser mode (cavity film) with the FSR near 30GHz can be obtained, the laser mode interval is large, and mode hopping is not easy. Meanwhile, the optical plane in the cavity is reduced, and the insertion loss in the cavity is further reduced.

As in embodiment 1, the trimming signal may be applied to the gain chip or to the TEC, for changing the refractive index and/or the temperature of the gain chip, so as to trim the lasing mode of the external cavity, and align the required wavelength of the lasing mode with the specific wavelength. Or the piezoelectric ceramic can be added under the gain chip, and the position of the cavity surface arranged on the gain chip is adjusted by changing the deformation of the piezoelectric ceramic through the fine tuning signal so as to finely tune the laser mode of the external resonant cavity and align the wavelength required in the laser mode with the specific wavelength. Or an actuator is added below the filter element, and the fine tuning signal adjusts the position of the cavity surface arranged on the filter element by controlling the actuator so as to finely tune the laser mode of the external resonant cavity and align the required wavelength in the laser mode with the specific wavelength.

Example 3

As shown in fig. 6, unlike embodiments 1 and 2, in this embodiment, a partial reflective film is plated on a first end surface 311 of the gain chip 310 away from the filter element 320 as an output facet of the external cavity, and a high reflective film is plated on a third plane 322 of the filter element 320 away from the gain chip 310 as a reflective facet of the external cavity. The coupling lens 330, the isolator 340 and the optical interface 110 of the housing are located on the optical path of the output facet output of the gain chip 310. The light beam emitted from the gain chip 310 resonates between the first end surface 311 and the third plane 322 to generate a laser light mode, the external cavity is fine-tuned by a fine tuning signal to align the laser light mode with the wavelength of the filter element 320, and laser light of a desired wavelength is output from the first end surface 311 of the gain chip 310, and the output laser light is coupled into the optical fiber through the coupling lens 330. In this embodiment, the isolator 340 is disposed within the optical interface 110 of the housing 100, reducing the length of the housing. In other embodiments, the positions of the isolator 340 and the coupling lens 330 may be interchanged. A collimating lens 350 may be further disposed between the gain chip 310 and the filter element 320 to collimate the light beam emitted from the gain chip 310 to be incident on the filter element 320.

In order to avoid the influence of the reflection of the end face of the gain chip on the stability of the laser light in the cavity, in this embodiment, the gain chip is arranged at a small angle with the main optical axis of the cavity, so that the laser light output from the first end face of the gain chip also forms a small angle with the main optical axis of the cavity, and a turning element, such as a turning prism, is added between the gain chip and the coupling lens to correct the angle of the output laser light, so that the laser light incident on the coupling lens returns to the direction of the main optical axis.

As in embodiments 1 and 2, a trimming signal may be applied to the gain chip or to the TEC to change the refractive index and/or temperature of the gain chip to trim the lasing mode of the external cavity to align the desired wavelength in the lasing mode with the specific wavelength. Or the piezoelectric ceramic can be added under the gain chip, and the position of the cavity surface arranged on the gain chip is adjusted by changing the deformation of the piezoelectric ceramic through the fine tuning signal so as to finely tune the laser mode of the external resonant cavity and align the wavelength required in the laser mode with the specific wavelength. Or an actuator is added below the filter element, and the fine tuning signal adjusts the position of the cavity surface arranged on the filter element by controlling the actuator so as to finely tune the laser mode of the external resonant cavity and align the required wavelength in the laser mode with the specific wavelength.

In the above embodiments, the electrical interface 120 of the housing 100 may be disposed on an end surface of the housing 100 opposite to the optical interface 110 thereof, as shown in fig. 1, or the electrical interface 120 may be disposed on a side surface of the housing 100, as shown in fig. 7. The electrical interface 120 is disposed on the side of the sealed housing 100, so that the wiring in the sealed housing can be omitted, the space for wiring in the sealed housing can be reduced, and the volume of the sealed housing can be further reduced. The cavity surface of the external resonant cavity can also be an independent total reflection cavity mirror and/or a partial reflection cavity mirror which are respectively arranged on one side of the gain chip far away from the filter element and one side of the filter element far away from the gain chip.

Example 4

As shown in fig. 8, this embodiment 4 provides an optical module including a module case 500, a circuit board 600, a silicon optical chip 400, and a narrow-linewidth laser. Here, the narrow linewidth laser may adopt the narrow linewidth fixed wavelength laser of any of the above embodiments, and a feedback component of the narrow linewidth fixed wavelength laser is further integrated on the silicon optical chip 400, where the feedback component includes a light splitting element and an optical detector (MPD) for feeding back the optical power and the central wavelength output by the fixed wavelength laser. Of course, in other embodiments, the feedback assembly may also employ a combination of a split detector or other form of splitting element and MPD. The miniaturized narrow-linewidth fixed wavelength laser adopted by the optical module has the advantages that the size of the module can be smaller, the integration level is higher, and more stable narrow-linewidth single-mode laser output is realized.

In other embodiments, the housing of the optical module may also adopt a package housing with other structures, and is not limited to the housing structure shown in fig. 8. In this embodiment, the optical modulator and the optical receiver are integrated on a silicon optical chip, and in other embodiments, the optical modulator and/or the optical receiver may be independent devices in free space.

The narrow-linewidth fixed wavelength laser can provide a high-power narrow-linewidth light source for a coherent optical module, the output optical power can be higher than 12dBm, the linewidth can be smaller than or equal to 100kHz, the power is higher than that of a DFB laser, the linewidth is narrower than that of the DFB laser, and the narrow-linewidth fixed wavelength laser is particularly suitable for an optical module modulated by silicon light and is certainly suitable for other optical modules.

The above list of details is only for the concrete description of the feasible embodiments of the present application, they are not intended to limit the scope of the present application, and all equivalent embodiments or modifications that do not depart from the technical spirit of the present application are intended to be included within the scope of the present application.

Claims

1. A narrow linewidth fixed wavelength laser comprises a shell, wherein the shell is provided with an optical interface and an electrical interface; the method is characterized in that: the filter also comprises an external resonant cavity arranged in the shell, and a gain chip and a filter element which are arranged in the external resonant cavity;

the filter element has a fixed specific center wavelength and is used for selecting a required specific wavelength;

the electrical interface is used for transmitting electrical signals, and the electrical signals comprise driving signals and fine tuning signals;

the driving signal drives the gain chip to emit a light beam, the light beam resonates in the external resonant cavity to generate a laser mode, the filter element selects a required wavelength from the laser mode to oscillate in the external resonant cavity, and the fine tuning signal is used for fine tuning the laser mode of the external resonant cavity to align the laser mode with the specific wavelength so as to output a laser beam with the specific wavelength;

the fine tuning signal is used for fine tuning the laser mode of the external resonant cavity by changing the refractive index and/or the temperature of the gain chip.

2. The fixed wavelength laser of claim 1, wherein: the fine tuning signal includes a fine tuning current superimposed on a bias current of the gain chip.

3. The fixed wavelength laser of claim 1, wherein: a TEC is arranged in the shell, and the gain chip is arranged on the TEC;

the fine tuning signal changes the temperature of the gain chip by adjusting the temperature of the TEC so as to adjust the laser mode of the external resonant cavity.

4. The fixed wavelength laser of claim 1, wherein: the 3dB bandwidth of the filter element is less than or equal to 1 times the free spectral range of the lasing mode.

5. The fixed wavelength laser of claim 1, wherein: the filter element comprises two superimposed cut-off filters.

6. The fixed wavelength laser of claim 5, wherein: the two superposed cut-off filters are respectively arranged on two opposite planes of a transparent flat plate.

7. The fixed wavelength laser of claim 1, wherein: the filtering element comprises a band pass filter.

8. The fixed wavelength laser of claim 7, wherein: the fixed wavelength laser also comprises a transparent flat plate positioned in the external resonant cavity, the transparent flat plate comprises two opposite planes, and the band-pass filter is arranged on one plane of the transparent flat plate close to the gain chip; one cavity surface of the external resonant cavity is arranged on the other plane of the transparent flat plate.

9. The fixed wavelength laser of any one of claims 1-7, wherein: a coupling lens is further arranged in the shell and is positioned on an output light path of the external resonant cavity; the coupling lens comprises a plane close to the external resonant cavity and a spherical surface or an aspherical surface far away from the external resonant cavity; and the output cavity surface of the external resonant cavity is arranged on the plane of the coupling lens.

10. The fixed wavelength laser of any one of claims 1-7, wherein: an isolator is also arranged in the shell and is positioned on an output light path of the external resonant cavity; and the output cavity surface of the external resonant cavity is arranged on the plane of the isolator close to the external resonant cavity.

11. The fixed wavelength laser of any one of claims 1-7, wherein: the casing is the cuboid, the volume of cuboid is less than or equal to 0.3 cubic centimetre.

12. The fixed wavelength laser of any one of claims 1-7, wherein: the fixed wavelength laser also comprises an optical fiber, wherein the optical fiber comprises an optical fiber head and a tail fiber, the optical fiber head comprises a fixed sleeve, a capillary tube is arranged in the fixed sleeve, and one end of the tail fiber is arranged in the capillary tube; the fixing sleeve of the optical fiber head is arranged in the optical interface of the shell.

13. An optical module, characterized in that: comprising the fixed wavelength laser of any one of claims 1-12.