CN112415789A - All-fiber coupling single-frequency light output GHz-level frequency shift method and system - Google Patents
All-fiber coupling single-frequency light output GHz-level frequency shift method and system Download PDFInfo
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Abstract
The invention provides a frequency shift method and a frequency shift system for all-fiber coupling single-frequency light output in GHz magnitude, wherein the method comprises the following steps: inputting single-frequency output light with the frequency v as incident light to an electro-optical phase modulator, and applying the frequency f to the electro-optical phase modulator0To obtain a frequency v + -nf0And (n-0, 1, 2..) the multi-frequency modulated light is input to a narrow-bandwidth fiber grating filter matched with the central wavelength of the target frequency, and sideband components with the frequency of v + delta are screened out by the narrow-bandwidth fiber grating filter, so that the single-frequency light is shifted from the frequency v to v + delta. The method uses the electro-optical modulation of optical fiber coupling and the narrow bandwidth fiber grating filterThe fundamental frequency and the harmonic component which are irrelevant after the electro-optic modulation are inhibited or filtered by skillfully combining the components, only the target frequency component is reserved, and the pure and clean modulation frequency shift effect is achieved.
Description
Technical Field
The invention relates to the field of laser frequency conversion, in particular to a frequency shift method and a frequency shift system for all-fiber single-frequency output in GHz level.
Background
In the technical fields of modern heterodyne and coherent detection, laser manipulation of atomic molecules, microwave photon conversion and the like, in order to improve the measurement precision, the frequency manipulation efficiency and the accuracy of frequency conversion, the frequency of laser is often required to be changed, so that output light has a stable frequency difference relative to input light, and the method is specifically characterized in that single-frequency laser generates microwave-level frequency shift by applying a driving signal of microwave frequency, for example, the frequency difference between two ground states of cesium atoms is 9.19GHz, and the two-photon raman transition interference of rubidium atoms requires 6.83GHz frequency shift and other existing frequency shift methods mainly include acousto-optic frequency shift, electro-optic frequency shift, magneto-optic frequency shift and the like. The magneto-optical frequency shift is generally based on the Zeeman effect of light under a magnetic field, so that the frequency shift is generally difficult to be large, generally can only be in the level of dozens of MHz, is generally completed based on free space, and has no commercial product specially used for frequency shift at present; the electro-optical frequency shift is mainly based on an electro-optical modulator, although the frequency shift can reach dozens of GHz, a multi-order sideband can be generated while the frequency shift is modulated, so that the frequency shift is accompanied by parasitic harmonic frequency, and pure single-frequency output cannot be achieved; the traditional acousto-optic frequency shift method utilizes the principle that when laser is diffracted through an acousto-optic crystal, diffraction light can generate frequency shift, and the frequency shift is realized by inputting the frequency of a radio frequency signal through a driver, has the characteristics of pure frequency spectrum, high frequency shift precision, reliable stability, convenience in use and the like, and is the most widely applied frequency shift method. Most commercial all-fiber acousto-optic frequency shifter (AOM) products today typically operate over a frequency band of around 20MHz to 300MHz, and the conversion efficiency deteriorates when the driving frequency is up to several GHz. In order to realize frequency shift of GHz or even higher by using a single AOM, one method is to directly use a high-frequency AOM, the currently commercialized all-fiber AOM can achieve 1.0GHz (such as the American Brimrose product IPF-1000-; the other method is to utilize ingenious light path design of free space AOM multi-loop, documents of Laser frequency shift up to 5GHz with a high-efficiency 12-pass 350-MHz acousto-optical modulator, Rev.Sci.Instrum.91 and 033201(2020) make light beams reciprocate AOM for 12 times through fine adjustment of light paths, finally realize frequency shift of 4.2GHz, and the total diffraction efficiency is up to 11 percent.
In order to realize high frequency shift and high output efficiency and simultaneously meet the requirements of stable and reliable practicability, the development of an all-fiber frequency shift technology is urgently needed, the frequency shift with low insertion loss and above GHz magnitude is realized, and meanwhile, the all-fiber frequency shift technology has large modulation bandwidth so as to meet the requirements of various laser frequency conversion from GHz to hundreds of GHz magnitude.
Disclosure of Invention
The invention aims to provide a GHz magnitude frequency shift method for all-fiber coupling single-frequency light output, which realizes low insertion loss and frequency shift above the GHz magnitude and has large modulation bandwidth so as to meet various laser frequency conversion requirements from GHz to hundreds of GHz magnitude.
In order to achieve the above object, the present invention provides a frequency shift method for all-fiber coupled single-frequency light output in GHz order, comprising:
a frequency shift method for all-fiber coupling single-frequency light output in GHz level is characterized by comprising the following steps:
single-frequency output light with the frequency v is input to the electro-optical phase modulator as incident light, and the frequency f is applied to the electro-optical phase modulator through a microwave local oscillator0To obtain a frequency v + -nf0The multi-frequency modulated light of (1), wherein n is 0,1, 2; the multi-frequency modulation light is input to a narrow-bandwidth fiber grating filter matched with the center wavelength of the target frequency, and sideband components with the frequency of v + delta are screened out by the narrow-bandwidth fiber grating filter, so that the single-frequency light is shifted from the frequency of v to v + delta.
Further, when the target frequency shift amount delta is lower than 20GHz, the electro-optic phase modulator is directly driven by a microwave signal with the frequency delta, and frequency shift light is generated by a first-order sideband.
Further, when the target frequency shift quantity delta is larger than 20GHz and smaller than 100GHz, a microwave signal with the frequency of 10 GHz-20 GHz is used as a driving signal of the electro-optical phase modulator, and frequency shift light is generated by using a high-order sideband component of electro-optical modulation.
Furthermore, a temperature control device is arranged on the narrow-bandwidth fiber grating filter and used for enabling the refractive index and the grating period parameter to be matched for use, and the central wavelength can be adjusted as required.
The invention also provides a frequency shift system of all-fiber coupling single-frequency light output in GHz level, which comprises an electro-optic phase modulator, a narrow-bandwidth fiber grating filter and a microwave local oscillation source;
single-frequency output light with the frequency v is input to the electro-optical phase modulator as incident light, and the frequency f is applied to the electro-optical phase modulator through a microwave local oscillator0To obtain a frequency v + -nf0The multi-frequency modulated light of (1), wherein n is 0,1, 2; the multi-frequency modulation light is input to a narrow-bandwidth fiber grating filter matched with the center wavelength of the target frequency, and sideband components with the frequency of v + delta are screened out by the narrow-bandwidth fiber grating filter, so that the single-frequency light is shifted from the frequency of v to v + delta.
Further, the narrow bandwidth fiber grating filter is a reflective narrow bandwidth fiber grating filter or a transmissive narrow bandwidth fiber grating filter.
Further, the reflective narrow bandwidth fiber grating filter comprises a circulator, a uniform fiber bragg grating, an input end, a reflection output end and a transmission output end, the circulator is respectively connected with the input end, the transmission output end and the reflection output end, and the uniform fiber bragg grating is arranged between the circulator and the transmission output end.
The temperature control device comprises a temperature sensor and a heating sheet coated around the uniform fiber Bragg grating, and is connected with the temperature control end, and the temperature of the uniform fiber Bragg grating is controlled through the temperature control end.
Further, the transmission type narrow bandwidth fiber grating filter comprises an input end, a phase shift fiber bragg grating and an output end which are connected in sequence.
The temperature control device comprises a temperature sensor and a heating sheet coated around the phase-shift fiber Bragg grating, and is connected with the temperature control end, so that the temperature of the phase-shift fiber Bragg grating can be controlled through the temperature control end.
The invention has the following beneficial effects:
the invention provides a hundred GHz-level frequency shift method for all-fiber coupling single-frequency laser output, which skillfully combines electro-optic modulation of fiber coupling with a narrow-bandwidth fiber grating filter, inhibits or filters irrelevant fundamental frequency and harmonic components after the electro-optic modulation, only retains target frequency components, and achieves the pure and clean modulation frequency shift effect. The method can realize pure single-frequency laser output with optical frequency shift of GHz to hundreds of GHz level, has high conversion efficiency and large modulation bandwidth, is an integrated input-output structure of all-fiber coupling, and is easy for engineering practicability.
The Raman light generation method designed by the invention has the advantages of simple device structure, low cost, high integration level, easy realization, high maturity, good stability and important application value to atomic physics, microwave photonics and high-coherence detection.
In addition to the objects, features and advantages described above, other objects, features and advantages of the present invention are also provided. The present invention will be described in further detail below with reference to the drawings.
Drawings
The accompanying drawings, which are incorporated in and constitute a part of this application, illustrate embodiments of the invention and, together with the description, serve to explain the invention and not to limit the invention. In the drawings:
FIG. 1 is a schematic diagram of the GHz-level frequency shift method for all-fiber coupled single-frequency laser output according to the present invention;
FIG. 2 is a graph of the transmission and reflection of a spectral representation of a reflective uniform fiber Bragg grating according to the present invention;
fig. 3 is a structural contrast diagram of a reflection-type uniform fiber bragg grating and a transmission-type pi-phase shift fiber bragg grating according to the present invention, wherein fig. 3(a) is a structural schematic diagram of the reflection-type uniform fiber bragg grating, and fig. 3(b) is a structural schematic diagram of the transmission-type pi-phase shift fiber bragg grating;
FIG. 4 is a transmission/reflection diagram of a spectral diagram of a transmission type π phase shift fiber Bragg grating according to the present invention;
FIG. 5 is a schematic diagram of a time-shift system employing a reflective narrow bandwidth fiber grating filter according to a preferred embodiment of the present invention (the transmission output is covered by a temperature control device and not shown);
FIG. 6 is a schematic diagram of a time-shift system employing a transmissive narrow bandwidth fiber grating filter according to a preferred embodiment of the present invention.
Detailed Description
Embodiments of the invention will be described in detail below with reference to the drawings, but the invention can be implemented in many different ways, which are defined and covered by the claims.
As shown in fig. 1, the principle of the hundred GHz shift method for all-fiber coupled single-frequency laser output is as follows: the frequency v of the single-frequency laser is input into the electro-optical phase modulator corresponding to the wavelength lambda, and the frequency f is applied to the electro-optical phase modulator through a microwave local oscillator0Corresponding to the modulation frequency f of the electro-optical phase modulator0The invention can meet the requirement of large frequency shift, so the frequency is GHz and adjustable. Due to the first-order electro-optic effect of the nonlinear crystal, the frequency v +/-nf can be obtained after passing through the electro-optic phase modulator0(n ═ 0,1, 2.) multifrequency outputs light, the relative size of each frequency component being determined by the electro-optic modulation depth. Assuming that the target frequency shift amount of the wavelength λ laser is Δ, it is necessary to select an appropriate f0The frequency difference between the m-order sideband component modulated by the electro-optical phase modulator and the fundamental frequency is just delta, namely delta-mf is satisfied0. The modulation light containing the target frequency shift component passes through a narrow-bandwidth fiber grating filter which corresponds to the center wavelength matching and has a certain bandwidth, and because of the filtering capability of the narrow-bandwidth fiber grating filter on the target light, the sideband component with the frequency of v + delta is screened out by the filter, so that the single-frequency light is shifted from the frequency of v to v + delta.
The frequency shift efficiency is related to the modulation depth, the applied order and the transmittance of the filter, the modulation bandwidth is determined by the bandwidth of the narrow bandwidth fiber grating filter, and the size is generally f0/3～f0Between/2; the wavelength of the filter is generally far smaller than the wavelength range of the electro-optic phase modulator, the main reason is that the central wavelength adjusting range of the narrow-bandwidth fiber grating filter is very limited, hundreds of pm can be achieved by adopting temperature control adjustment (the optical frequency difference corresponding to 8pm in a 1550nm wave band is close to 1GHz), and only dozens of pm can be achieved by adopting piezoelectric ceramic PZT adjustment. To improve the frequency shift efficiency, when the target frequency shift amount Δ is lower than 20GHz, direct advantages can be consideredThe electro-optic phase modulator is directly driven by a microwave signal with the frequency delta, a first-order sideband generates a frequency shift component, namely m is +/-1, and therefore the conversion efficiency of frequency shift light can be improved by utilizing the first-order sideband; when the target frequency shift quantity delta is larger than 20GHz and smaller than 100GHz, a microwave signal with the frequency of 10 GHz-20 GHz is taken as a driving signal of the electro-optical phase modulator, a high-order sideband component of the electro-optical modulation is used for generating frequency shift light, and the driving signal f is taken when the delta is equal to 100GHz as an example0At 20GHz, the frequency-shifted light should be filtered and extracted in the fifth order of the electro-optical modulation, i.e., m is 5, and the modulation depth of the electro-optical phase modulator needs to be adjusted to achieve the maximum output of the fifth order light.
Based on the above principle, the invention provides a frequency shift method for all-fiber coupling single-frequency light output in GHz level, which comprises the following steps:
single-frequency output light with the frequency v is input to the electro-optical phase modulator as incident light, and the frequency f is applied to the electro-optical phase modulator through a microwave local oscillator0To obtain a frequency v + -nf0And (n-0, 1, 2..) the multi-frequency modulated light is input to a narrow-bandwidth fiber grating filter matched with the central wavelength of the target frequency, and sideband components with the frequency of v + delta are screened out by the narrow-bandwidth fiber grating filter, so that the single-frequency light is shifted from the frequency v to v + delta.
When the target frequency shift quantity delta is lower than 20GHz, the microwave signal with the frequency delta is used for directly driving the electro-optic phase modulator, and the first-order sideband generates frequency shift light. And when the target frequency shift quantity delta is larger than 20GHz and smaller than 100GHz, taking a microwave signal with the frequency of 10 GHz-20 GHz as a driving signal of the electro-optical phase modulator, and generating frequency shift light by using a high-order sideband component of the electro-optical modulation.
In addition, in order to expand the use wavelength range of the frequency shifter, the refractive index and the grating period need to be modulated, and the refractive index and the grating period parameters are matched for use by arranging the temperature control device on the narrow-bandwidth fiber grating filter, so that the central wavelength can be adjusted as required.
The invention also provides a frequency shift system with full optical fiber coupling single-frequency light output in GHz level, which comprises an electro-optical phase modulator, a narrow-bandwidth fiber grating filter and a microwave local oscillation source,
single-frequency output light with the frequency v is input to the electro-optical phase modulator as incident light, and the frequency f is applied to the electro-optical phase modulator through a microwave local oscillator0To obtain a frequency v + -nf0And (n-0, 1, 2..) the multi-frequency modulated light is input to a narrow-bandwidth fiber grating filter matched with the central wavelength of the target frequency, and sideband components with the frequency of v + delta are screened out by the narrow-bandwidth fiber grating filter, so that the single-frequency light is shifted from the frequency v to v + delta.
The narrow bandwidth fiber grating filter is an important device in the frequency shift system of the invention, and the core functional component is a Fiber Bragg Grating (FBG), which is a grating formed in the fiber core of the fiber and with periodically distributed spatial phases, and the essence of the function is to form a narrow band filter in the fiber core. The narrow bandwidth fiber grating filter can be a reflection type narrow bandwidth fiber grating filter or a transmission type narrow bandwidth fiber grating filter, and is determined by specific requirements and bandwidth.
The reflection-type narrow-bandwidth fiber grating filter comprises a circulator, a uniform fiber Bragg grating, an input end, a reflection output end and a transmission output end, wherein the circulator is respectively connected with the input end, the transmission output end and the reflection output end, and the uniform fiber Bragg grating is arranged between the circulator and the transmission output end. Since the filter component of a uniform fiber bragg grating will be reflected from the incident direction of the fiber, a circulator must be used in conjunction to separate the outputs. The main parameters of the uniform fiber bragg grating are: initial refractive index n1The grating comprises a light-induced refractive index perturbation value delta n, a grating pitch lambda and a grating region length L. The central wavelength λ of the reflective FBG is determined by the grating pitch Λ, and the two relations are:
λ＝2n1Λ
reflection spectrum full width at half maximum delta lambda and reflection type FBG photoinduced refractive index perturbation value delta n1The positive correlation is formed, and the positive correlation is inversely proportional to the length L of the grating area, and the expression is as follows:
by designing the initial refractive index n1The value of the perturbation Δ n of the refractive index1Parameters such as grating pitch lambda, grating zone length L and the like enable only the carrier wave of the electro-optic phase modulator to enable a certain order sideband to be in the reflection spectrum bandwidth of the reflection type FBG, and the rest sidebands are filtered. Fig. 2 is a transmission/reflection curve diagram of a spectral diagram of a reflection-type uniform fiber bragg grating, in fig. 2, a solid line is a reflection curve of the FBG, and a dotted line is a transmission curve of the FBG, where λ is a reflection center wavelength of the FBG, and Δ λ is a full width at half maximum of the reflection curve, that is, a corresponding curve bandwidth when a loss is-3 dB. In the wavelength range of
The internal laser is reflected by FBG to obtain a reflection spectrum with a wavelength from the center of the FBG of lambda-lambdasAt |, the spectrum is not substantially reflected, at which time the optical cut-off wavelength is λsAnd because the influence of the laser refractive index is fixed, when the grating pitch Lambda and the grating region length L are changed, the bandwidth of the reflected light is adjustable. Meanwhile, because lambda is 2n1And Λ, the refractive index and the grating period need to be modulated in order to enlarge the use wavelength range of the frequency shifter, and the refractive index and the grating period parameters are matched for use by arranging the temperature control device on the uniform fiber Bragg grating, so that the central wavelength can be adjusted as required. The temperature control device comprises a temperature sensor and a heating sheet coated around the uniform fiber Bragg grating, and is connected with the temperature control end, and the temperature of the uniform fiber Bragg grating is controlled through the temperature control end.
The transmission type narrow bandwidth fiber grating filter comprises an input end, a phase shift fiber Bragg grating and an output end which are connected in sequence. Fig. 3 is a structural contrast diagram of a reflection-type uniform fiber bragg grating and a transmission-type pi-phase shift fiber bragg grating in the present invention, wherein fig. 3(a) is a structural schematic diagram of the reflection-type uniform fiber bragg grating, and fig. 3(b) is a structural schematic diagram of the transmission-type pi-phase shift fiber bragg grating. As shown in fig. 3(b), the phase-shifted fiber bragg grating is a grating that introduces a certain phase shift (preferably pi phase shift) at a specific position of a commonly used uniform Fiber Bragg Grating (FBG) to generate two mutually out-of-phase gratings, and the combined effect of the two mutually out-of-phase gratings is to open a transmission window with extremely narrow bandwidth in the transmission spectrum stop band (as shown in fig. 4). The phase-shifted fiber Bragg grating has high-quality wavelength selectivity, low insertion loss and no relation to polarization state, and is especially used for manufacturing an extremely narrow-bandwidth optical filter.
Therefore, the transmissive narrow-bandwidth fiber grating filter is generally suitable for a case where the modulation bandwidth is relatively narrow, that is, when the target frequency shift amount Δ is relatively small, for example, 1GHz to 3GHz, and in this case, single-frequency light having a frequency shift amount of 1GHz to 3GHz can be filtered out from the modulated light with high quality by using the transmissive phase-shift fiber grating.
Example 1
A frequency shift method of full-fiber coupling single-frequency light output GHz level by adopting reflection type FBG (fiber Bragg Grating), the system implementation scheme is shown in figure 5, single-frequency output light with the frequency v is used as incident light, and the frequency f is applied to an electro-optic phase modulator (EOM)0To obtain a frequency v + -nf0(n-0, 1, 2..) multi-frequency modulated light, wherein the desired target frequency is m-order sideband light v + mf0. The output of the frequency shift light is mainly to filter the other sideband lights except the carrier light and the positive first-order sideband light by the uniform fiber Bragg grating matched with the target frequency light through a circulator with three ports of the optical fiber. Meanwhile, the used wavelength also needs to be controlled by a temperature control device, so that the adjustable range of the target used wavelength is realized. The temperature control device comprises a temperature sensor and a heating sheet coated around the uniform fiber Bragg grating, and is connected with the temperature control end, and the temperature of the uniform fiber Bragg grating is controlled through the temperature control end.
Example 2
A frequency shift method of full-fiber coupling single-frequency light output in GHz order using transmissive FBG (system implementation is shown in fig. 6), which is different from embodiment 1 in that: by applying a frequency f to an electro-optic phase modulator (EOM)0By a modulated signal ofThe frequency shift light is directly transmitted from the phase shift fiber Bragg grating by adopting the phase shift fiber Bragg grating, the m-order sideband light is screened out by a target component in the multi-frequency modulation light through the temperature control device, and at the moment, the transmission type narrow bandwidth fiber grating filter is the phase shift fiber Bragg grating, a transmission window appears in the transmission spectrum, the optical wave bandwidth is generally very narrow, and the optical filter is suitable for the optical filtering requirements of large frequency shift and ultra-narrow bandwidth.
Finally, the invention provides a hundred GHz-level frequency shift method for all-fiber coupling single-frequency laser output, which skillfully combines the electro-optical modulation of fiber coupling and a narrow-bandwidth fiber grating filter, inhibits or filters irrelevant fundamental frequency and harmonic components after the electro-optical modulation, only keeps target frequency components, and achieves the pure and clean modulation frequency shift effect. The method can realize pure single-frequency laser output with optical frequency shift of GHz to hundreds of GHz level, has high conversion efficiency and large modulation bandwidth, is an integrated input-output structure of all-fiber coupling, and is easy for engineering practicability.
The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (10)
1. A frequency shift method for all-fiber coupling single-frequency light output in GHz level is characterized by comprising the following steps:
single-frequency output light with the frequency v is input to the electro-optical phase modulator as incident light, and the frequency f is applied to the electro-optical phase modulator through a microwave local oscillator0To obtain a frequency v + -nf0The multi-frequency modulated light of (1), wherein n is 0,1, 2; the multi-frequency modulation light is input to a narrow-bandwidth fiber grating filter matched with the center wavelength of the target frequency, and sideband components with the frequency of v + delta are screened out by the narrow-bandwidth fiber grating filter, so that the single-frequency light is shifted from the frequency of v to v + delta.
2. The method of claim 1, wherein when the target frequency shift amount Δ is less than 20GHz, the electro-optic phase modulator is directly driven by a microwave signal with a frequency Δ, and the first-order sideband is used to generate the frequency-shifted light.
3. The frequency shift method for the all-fiber coupled single-frequency light output in the GHz level according to claim 1, wherein when the target frequency shift amount Δ is greater than 20GHz and less than 100GHz, the frequency shift light is generated by using the high-order sideband component of the electro-optical modulation as the electro-optical phase modulator driving signal with the microwave signal having the frequency of 10 GHz-20 GHz.
4. The all-fiber coupling single-frequency optical output GHz-level frequency shift method according to claim 1, wherein a temperature control device is disposed on the narrow-bandwidth fiber grating filter for matching the refractive index with the grating period parameter, so as to ensure that the central wavelength can be adjusted as required.
5. A frequency shift system with all-fiber coupling single-frequency light output in GHz magnitude is characterized by comprising an electro-optical phase modulator, a narrow-bandwidth fiber grating filter and a microwave local oscillation source;
single-frequency output light with the frequency v is input to the electro-optical phase modulator as incident light, and the frequency f is applied to the electro-optical phase modulator through a microwave local oscillator0To obtain a frequency v + -nf0The multi-frequency modulated light of (1), wherein n is 0,1, 2; the multi-frequency modulation light is input to a narrow-bandwidth fiber grating filter matched with the center wavelength of the target frequency, and sideband components with the frequency of v + delta are screened out by the narrow-bandwidth fiber grating filter, so that the single-frequency light is shifted from the frequency of v to v + delta.
6. The all-fiber coupled single-frequency optical output frequency shift method in the order of GHz according to claim 5, wherein the narrow-bandwidth fiber grating filter is a reflective narrow-bandwidth fiber grating filter or a transmissive narrow-bandwidth fiber grating filter.
7. The method of claim 6, wherein the reflective narrow bandwidth fiber grating filter comprises a circulator, a uniform fiber Bragg grating, an input port, a reflective output port, and a transmissive output port, the circulator being connected to the input port, the transmissive output port, and the reflective output port, the uniform fiber Bragg grating being disposed between the circulator and the transmissive output port.
8. The frequency shift method of an all-fiber coupled single-frequency optical output GHz level according to claim 7, further comprising a temperature control device for adjusting the temperature of the uniform fiber Bragg grating, wherein the temperature control device comprises a temperature sensor and a heating sheet coated around the uniform fiber Bragg grating, and the temperature control device is connected to the temperature control end, and the temperature of the uniform fiber Bragg grating is controlled through the temperature control end.
9. The all-fiber coupled single-frequency optical output frequency shift method in the GHz level according to claim 6, wherein the transmission type narrow bandwidth fiber grating filter comprises an input end, a phase shift fiber Bragg grating and an output end which are connected in sequence.
10. The all-fiber coupled single-frequency optical output GHz-level frequency shifting method according to claim 9, further comprising a temperature control device for adjusting the temperature of the phase-shifted fiber Bragg grating, wherein the temperature control device comprises a temperature sensor and a heating plate coated around the phase-shifted fiber Bragg grating, and the temperature control device is connected to the temperature control end, and the temperature of the phase-shifted fiber Bragg grating is controlled by the temperature control end.
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