CN106410599B - Brillouin Single Longitudinal Mode Frequency Shifted Fiber Laser - Google Patents

Abstract

The invention provides a Brillouin single longitudinal mode frequency shift fiber laser which comprises a light source module, an optical fiber circulator, an optical amplifier, an optical fiber isolator, a first optical fiber coupler, a first frequency shift optical fiber, a second frequency shift optical fiber and a second optical fiber coupler. The unidirectional active ring resonant cavity is constructed, based on the technical scheme of the optical fiber composite cavity, the cavity longitudinal mode interval is larger than the Brillouin gain spectrum width by utilizing the mode selection characteristic of the composite cavity, so that only one cavity longitudinal mode can start vibrating in the Brillouin gain spectrum range and form laser emission, and the output of the Brillouin single longitudinal mode operation transfer frequency optical fiber laser is realized with high efficiency and low cost. The Brillouin single-frequency laser frequency always follows the pumping seed light, the broadband frequency shift effect is stable, single longitudinal mode operation is realized based on the composite cavity, the structure is simple, the complex control requirement is avoided, and the harsh application requirement of the self-Brillouin scattering-based distributed optical fiber temperature strain sensing system on the broadband frequency shift technology can be fully met.

Description

Brillouin Single Longitudinal Mode Frequency Shifted Fiber Laser

technical field

The invention relates to a broadband frequency shifting technology required by a distributed optical fiber sensing system based on spontaneous Brillouin scattering, in particular to a Brillouin single longitudinal mode frequency shifting fiber laser.

Background technique

In the distributed optical fiber sensing (BOTDR) system based on spontaneous Brillouin scattering, the frequency shift of Brillouin spontaneously scattered light relative to the incident light is affected by temperature and strain. For single-mode optical fibers in the communication band, Brillouin The frequency shift of the spontaneous scattered light relative to the incident light is about 11GHz, where the linear coefficient of the change in the frequency shift of the Brillouin scattered light caused by temperature is 1.09±0.08MHz/℃, and the linear coefficient of the change in the frequency shift of the Brillouin scattered light caused by the strain The coefficient is 0.052±0.004MHz/με, and the extraction of 10 ^-4 relative frequency variation on this 11GHz ultra-high frequency base is the key technology for the BOTDR system to realize temperature and strain sensing. Using the broadband frequency shifting scheme and selecting the appropriate local beat frequency light can reduce the difference frequency signal between the Brillouin spontaneously scattered light and the local beat frequency light from 11GHz to 100MHz, which is conducive to signal extraction and system device cost reduction .

Therefore, a variety of broadband frequency shifting technology solutions have been developed. For example, the first solution uses a laser with a frequency difference close to the Brillouin frequency shift of the seed light as the local laser (see Toshio Kurashima, et al, IEICE TRANS.COMMUN., E76- B(4) (1993)), in this technical solution, the BOTDR system needs to use two lasers, which makes the cost and structure complex, and this solution has extremely high requirements for the frequency and frequency difference stability of the two lasers; The second scheme uses the acousto-optic frequency shifting ring for broadband frequency shifting (see Kaoru Shimizu, et al., J. LightwaveTechnol., 12, 730-736 (1994)), but the acousto-optic frequency shifter usually only shifts one hundred times at a time The frequency change of 11 GHz can only be realized after hundreds of cyclic frequency shifts, all of which put forward high requirements on the performance of the acousto-optic frequency shifter. Moreover, the adoption of the acousto-optic frequency shifting loop increases the complexity of the optical part of the system, which affects the stability and measurement accuracy of the system. Scheme 3 uses electro-optic modulators for broadband electro-optic modulation and frequency shift (see Song Muping, Acta Optics Sinica, 24, 1110-1114 (2004)). The electro-optic modulator can achieve frequency shift of 11GHz at one time, which simplifies the optical path relatively, but the electro-optic modulator High requirements are placed on the polarization control characteristics of the optical path. At the same time, the energy loss of using the electro-optical modulator to achieve broadband frequency shift is too large, and the obtained frequency shifted optical power is too small.

Compared with the above-mentioned schemes, using Brillouin laser to achieve broadband frequency shift is a new technology scheme with high efficiency and low cost (see Jihong Geng, et al., Appl. Opt. 46, 5928-5932 (2007)), attracting Many researchers have carried out related research and application. Since the Brillouin gain spectrum width is about 30MHz, and the cavity length of an active ring cavity Brillouin laser is generally on the order of tens of meters (corresponding to a cavity longitudinal mode interval of 2~10MHz), it is easy to appear multi-longitudinal mode operation mode, resulting in large fluctuations in the amount of frequency shift, which directly leads to the reduction of measurement accuracy in the BOTDR system. In order to avoid multi-longitudinal mode operation, some researchers locked the frequency of the injected pump seed light and the Brillouin laser operating frequency on two cavity longitudinal modes of the resonator at the same time through a laser frequency stabilization scheme, thereby realizing a single longitudinal mode. Brillouin laser output in mode operation (see Jihong Geng, et al., IEEE Photon. Technol. Lett. 18, 1813-1815 (2006)), but this scheme involves a complex feedback control system, and the stability is not easy Done well, under external disturbances, it is easy to lose the frequency stability lock, which will reduce the stability of the entire BOTDR system and increase the cost a lot.

Contents of the invention

In order to overcome the shortcomings of the prior art and better meet the actual requirements of the BOTDR system for a broadband frequency-shifted Brillouin laser, the present invention provides a high-efficiency and low-cost Brillouin laser that realizes single longitudinal mode operation.

The basic principle of the invention is to construct an active annular cavity Brillouin laser, which is mainly composed of a pumping light source and an active annular cavity. The pump light source needs to be a single-frequency laser with a linewidth of less than 1MHz in the communication C-band. The ring cavity includes a three-port circulator, an optical amplifier, a fiber composite cavity unit composed of a frequency-shifting fiber, and a fiber coupler. Using the unidirectional characteristics of the circulator, a ring cavity that can only transmit in one direction is constructed. The optical amplifier is used to provide amplification in the cavity, and the frequency-shifting fiber is used as a nonlinear medium to provide gain for the amplification of Brillouin scattered light. The coupler is used The output of the intracavity laser.

Brillouin single longitudinal mode frequency-shifted fiber laser, including light source module, fiber circulator, optical amplifier, fiber isolator, first fiber coupler, first frequency-shifting fiber, second frequency-shifting fiber, second fiber coupler ; Wherein, the pigtail output of the light source module is connected to the first port of the optical fiber circulator, the second port of the optical fiber circulator is connected to the input port of the optical amplifier, and the output port of the optical amplifier is connected to the first port of the first optical fiber coupler; The second port and the fourth port of the first optical fiber coupler are connected to form a loop by optical fiber, and the components connected in this loop include optical fiber isolator and first frequency-shifting optical fiber, wherein the forward conduction of the optical fiber isolator The input port is connected to the second port of the first fiber coupler, the forward conduction output port of the fiber isolator is connected to the first frequency shifting fiber, and the other end of the first frequency shifting fiber is connected to the fourth port of the first fiber coupler ; One end of the second frequency-shifting fiber is connected with the third port of the first fiber coupler, and the other end is connected with the first port of the second fiber coupler; the third port of the second fiber coupler is connected with the third port of the fiber optic circulator The ports are connected and closed into a large fiber loop, and the second port of the second fiber coupler is used as the output port of the single longitudinal mode Brillouin laser; based on the technical scheme of the fiber composite cavity, the mode selection characteristic of the composite cavity is used to make The cavity longitudinal mode interval is greater than the Brillouin gain spectrum width, and within the gain spectrum range, there is only one cavity longitudinal mode that can oscillate and form laser output, and finally achieve high-efficiency and low-cost Brillouin single longitudinal mode frequency-shifted fiber laser output .

Furthermore, the light source module is a laser with a narrow linewidth, the linewidth is less than 1MHz, and the output power can reach 20mW.

Furthermore, the optical fiber circulator is a three-port optical fiber circulator with one-way conduction, and the optical fiber circulator can be replaced by connecting a fiber coupler and an isolator to play the role of the fiber optic circulator.

Further, the optical amplifier is mainly used to amplify the pump light signal and the weak Brillouin scattering signal, and needs to adopt an optical amplifier with a shorter optical action length, which can be constructed by using an erbium-doped optical fiber with a high gain coefficient as the gain medium Optical fiber amplifier, or use 1550nm band semiconductor optical amplifier (SOA).

Further, for the optical amplifier, a 1m-long erbium-doped fiber with a high gain coefficient can be selected to build an erbium-doped fiber amplifier (EDFA).

Further, the optical fiber isolator is used for unidirectional conduction to prevent reverse resonance, and a commercial optical fiber isolator with a wavelength of 1550nm can be used.

Further, the first optical fiber coupler adopts a common single-mode fiber 1550nm band, 2×2 ports, and a splitting ratio of 50:50.

Further, the first frequency-shifting optical fiber and the second frequency-shifting optical fiber adopt ordinary communication single-mode optical fiber, and commercial G652 type communication single-mode optical fiber can be used; there are two kinds of length options for the first frequency-shifting optical fiber and the second frequency-shifting optical fiber Ways: one is that the lengths of the two cavities forming the composite cavity are equal or close; the other is that the length of one cavity forming the composite cavity is many times the length of the other cavity.

Further, the second fiber coupler is a 1×2 common single-mode fiber coupler with a center wavelength of 1550 nm and a beam splitting ratio of 10:90, and 10% of the ports are used as the output ports of the Brillouin laser.

When no pump light is injected at the input, the laser is equivalent to an erbium-doped fiber laser, and the output wavelength depends on the net gain distribution in the cavity and the final lasing mode established by free oscillation. When pumping light is injected at the input end of the laser, and the pumping light intensity reaches the resonance threshold of stimulated Brillouin scattered light in the cavity, the laser will work in the Brillouin fiber laser mode. When the pump light is injected into the ring cavity from the input end of the circulator, after being amplified by the optical amplifier, it enters the frequency-shifting fiber, and the backscattered light is excited in the frequency-shifting fiber, and the pump light continues to transmit to the circulator to stop. The scattered light can be circulated in the cavity, and the scattered light will be amplified when it passes through the optical amplifier. When passing through the frequency-shifting fiber, the Brillouin signal light in the scattered light and the incident pump light will undergo nonlinear Brillouin in the process of opposite transmission. The role of nonlinear Brillouin gain amplification can be obtained, while the Rayleigh scattered light that is also transmitted backward cannot be amplified here. In such a cycle, if the comprehensive gain of the Brillouin scattered light signal in the cavity is greater than the Rayleigh scattered light and spontaneous emission noise light, it will experience multiple cycles of transmission and amplification in the cavity to establish oscillation, and finally a stable Brillouin laser output can be formed.

Compared with the prior art, the present invention has the following advantages and beneficial effects:

(1) Compared with the traditional broadband frequency shifting technical scheme, the present invention is a single longitudinal mode laser based on fiber Brillouin nonlinear effect, and its laser frequency always follows the seed light, so the broadband frequency shifting effect is stable, the structure is simple, no Complex control requirements.

(2) Compared with the frequency-stabilized Brillouin laser, the present invention utilizes the fiber composite cavity for mode selection, so that the cavity longitudinal mode is larger than the Brillouin gain spectrum spectral width, so that there is only one cavity in the Brillouin gain spectrum range The longitudinal mode can vibrate and form laser emission. The frequency-stabilized Brillouin laser needs to build a complex optical structure for error signal extraction and feedback regulation of seed light to achieve stable single longitudinal mode Brillouin laser output. This single longitudinal mode Brillouin laser technology solution has a simple principle, does not require complex feedback control, has a stable optical structure, reduces the cost by more than 90%, and can be well applied in the BOTDR sensing system as a broadband frequency-shifted local light source.

Description of drawings

A schematic diagram of a compound cavity longitudinal mode frequency selection in the example of Fig. 1;

The schematic diagram of another composite cavity longitudinal mode frequency selection in the example of Fig. 2;

The schematic diagram of the structure of the single longitudinal mode Brillouin laser based on the fiber composite cavity in the example in Fig. 3 .

Detailed ways

The implementation of the present invention will be further described below in conjunction with the examples and accompanying drawings, but the implementation and protection of the present invention are not limited thereto. Realized with reference to prior art.

In order to obtain the output of the Brillouin laser, the present invention needs to solve two problems: first, avoiding the spontaneous radiation noise of the optical amplifier to establish oscillation to form laser; second, avoiding Rayleigh scattered light resonance to form laser. The first problem is that the amplification performance of the optical amplifier needs to be set at a lower position, so that the proportion of the gain provided by the optical amplifier in the overall gain obtained by the Brillouin scattering signal is as small as possible. This problem can be solved well by setting the gain coefficient of the optical amplifier; the second problem is to increase the Brillouin nonlinear gain as much as possible so that the Rayleigh scattered light cannot compete in the cavity with the gain provided by the optical amplifier alone. Rieouin scatters light, and finally makes Rayleigh scattering unable to resonate to form laser emission.

The second problem above, in order to suppress Rayleigh scattered light to form laser output, it is necessary to increase the Brillouin nonlinear gain as much as possible, which can be achieved by increasing the power of the pump light source or increasing the length of the frequency-shifting fiber used to provide Brillouin nonlinear gain . Relying solely on increasing the power of the pump light source will result in reduced energy transfer and utilization, and the cost of narrow linewidth lasers will rise sharply with the increase in power. Therefore, the simplest and most economical way is to increase the length of the intracavity frequency-shifting fiber as much as possible. . In this case, a new problem is introduced: in the communication band single-mode fiber (the fiber with the lowest cost of frequency shifting), the Brillouin gain spectral width is about 30MHz, and the cavity of the active ring cavity Brillouin laser The length is generally on the order of tens of meters (corresponding to the longitudinal mode interval of the cavity is 2~10MHz), so it is easy to appear multi-longitudinal mode operation mode. In order to avoid multi-longitudinal mode operation, the present invention does not use frequency locking technology, but proposes a A technical solution based on a fiber composite cavity, using the mode selection characteristics of the composite cavity, the cavity longitudinal mode spacing is greater than the Brillouin gain spectrum width, and only one cavity longitudinal mode can be oscillated within the gain spectrum range and form laser output . Therefore, using the composite cavity solution, the Brillouin laser output with single longitudinal mode operation can be realized efficiently and at low cost.

The basic principle of the compound cavity to realize the selection of the longitudinal mode is shown in Fig. 1 and Fig. 2. Assuming that the cavity lengths of the compound cavity are L ₁ and L ₂ respectively, the sequence interval of the cavity longitudinal modes corresponding to the two cavities is △f ₁ =c /nL ₁ , △f ₂ =c/nL ₂ , the resonance frequency of the composite cavity laser composed of two cavities must be aligned with a certain order frequency of the above two sets of longitudinal mode intervals at the same time, in order to form an effective resonance and finally form a laser output. Therefore, according to the principle of vernier caliper, the cavity longitudinal mode sequence interval of the composite cavity is:

Δf = m ₁ c/(nL ₁ ) = m ₂ c/(n L ₂ ); ₂ / L ₁ =m ₂ /m ₁

Wherein, m ₁ and m ₂ are positive integers with no common divisor.

At this time, there are two situations that can achieve better longitudinal mode selection. One situation is that, as shown in Figure 1, the cavity length L ₁ is basically equal to the cavity length L ₂ . The cavity length difference between two cavities is inversely proportional, that is, △f≈c/ (nL ₁ -nL ₂ ) , at this time, the cavity length L ₁ and cavity length L ₂ can choose a longer optical fiber, which fully satisfies the laser's Brillouin non-linearity In order to meet the requirements of linear gain, and choose the cavity length L ₁ and cavity length L ₂ to be close in length, it can fully ensure that the cavity longitudinal mode interval of the composite cavity is much larger than the Brillouin gain spectrum width, and then within the gain spectrum range there is and only A cavity longitudinal mode can be oscillated and form laser output.

In another case, as shown in Figure 2, the length L2 of one cavity is many times longer than the _length L1 of the other cavity. At this time, _the cavity longitudinal mode interval _of the composite cavity is basically equal to the cavity longitudinal mode of the short cavity L1 , that is, △f≈c/(n L ₁ ) , the cavity length L ₁ is selected to be shorter, and the cavity length can be selected to be less than 4m , so that Δf ₁ >50MHz, which is greater than the Brillouin gain spectrum width, and the cavity length L ₂ can be selected to be longer than the cavity length L ₁ is ten times longer, which fully meets the requirements of the laser for Brillouin nonlinear gain. In this case, there is one and only one cavity longitudinal mode in the gain spectrum range that can oscillate and form laser output.

According to the above basic principles, the structural design of the single longitudinal mode Brillouin laser based on the fiber composite cavity in this example is shown in Figure 3, including the light source module (1), fiber circulator (2), optical amplifier (3), fiber isolation (4), the first fiber coupler (5), the first frequency-shifting fiber (6), the second frequency-shifting fiber (7), the second fiber coupler (8); the pigtail output of the light source module 1 and the fiber The first port 201 of the circulator 2 is connected, the second port 202 of the optical fiber circulator 2 is connected with the input port 301 of the optical amplifier 3, the output port 302 of the optical amplifier 3 is connected with the first port 501 of the fiber coupler 5, and the fiber coupler Several optical devices are connected between the second port 502 and the fourth port 504 of 5, and connected by optical fiber to form a loop. The devices connected in this loop include optical fiber isolator 4 and frequency-shifting optical fiber 6, wherein the optical fiber isolator The forward conducting input port 401 of 4 is connected with the second port 502 of the fiber coupler 5, the forward conducting output port 402 of the fiber isolator 4 is connected with the frequency-shifting optical fiber 6, and the other end of the frequency-shifting optical fiber 6 is connected with the optical fiber coupler 5 The fourth port 504 is connected. One end of another section of frequency-shifting fiber 7 is connected to the third port 503 of the fiber coupler 5, and the other end is connected to the first port 801 of the fiber coupler 8, and the third port 803 of the fiber coupler 8 is connected to the third port 503 of the fiber optic circulator. The ports 203 are connected to form a large fiber loop, and the second port 802 of the fiber coupler 8 is used as the output port of the single longitudinal mode Brillouin laser.

The specific implementation of each device module is illustrated as follows.

The light source module 1 is a pumping light source of a single longitudinal mode Brillouin laser based on a fiber composite cavity. Since the Brillouin gain spectrum is only on the order of tens of MHz, the linewidth of the pump seed light source needs to be narrow. The light source used in this example is a 1550nm narrow-linewidth single-frequency fiber laser with a linewidth of 2kHz and a laser power of 100mW; other types of narrow-linewidth lasers can also be used, but the linewidth cannot exceed 10MHz.

The optical fiber circulator 2 is a three-port optical fiber circulator with one-way conduction, and can also be connected to an optical fiber coupler and isolator to play the role of an optical fiber circulator.

Optical amplifier 3 is mainly used to amplify the pump light signal and the weak Brillouin scattering signal in the loop. In order not to affect the performance of the Brillouin laser output, it is necessary to adopt an optical amplifier with a shorter optical action length. A 2cm-long erbium-ytterbium co-doped phosphate glass fiber with high gain coefficient is used as a gain medium to build a fiber amplifier, or a commercial 1550nm band semiconductor optical amplifier (SOA) is directly used.

The optical fiber isolator 4 is mainly used for the one-way conduction function. The forward conduction input port 401 is conducted to the forward conduction output port 402, and the reverse is not conducted. Therefore, the pump light cannot pass through the loop formed by the fiber coupler 5. Medium cyclic transmission, while Brillouin backscattering can.

Fiber coupler 5, common single-mode fiber 1550nm band, 2×2 ports, coupler with a splitting ratio of 50:50.

The first frequency-shifting optical fiber 6 and the second frequency-shifting optical fiber 7, the frequency-shifting optical fiber here plays the function of providing spontaneous Brillouin scattered light and Brillouin nonlinear amplification gain, and commercial G652 type communication single-mode optical fiber can be used, According to the discussion of the principle part, there are two specific implementation methods regarding the lengths of the first frequency-shifting optical fiber 6 and the second frequency-shifting optical fiber 7 here: one is that the lengths of the two cavities forming the composite cavity are close, and the first frequency-shifting optical fiber 7 can be set here. The length of the frequency-shifting fiber is about 1m, and the difference between the lengths of the two cavities can be 1m, and there can be only one cavity longitudinal mode in the range of the Brillouin gain spectrum; the second is that the length of one cavity forming a composite cavity is the other multiple times of the length of the cavity, here the length of the first frequency-shifting optical fiber 6 can be set to be 30m, and the length of the second frequency-shifting optical fiber 7 is about 3m, then the length of the long cavity is 10 times that of the short cavity, and the longitudinal mode of the composite cavity The interval is basically the same as the interval of the longitudinal mode of the short cavity, which can fully meet the requirement that there is one and only one cavity longitudinal mode within the range of the Brillouin gain spectrum.

Fiber coupler 8, ordinary single-mode fiber 1550nm band, port 1×2, center wavelength 1550nm, beam splitting ratio 10:90, 10% of the ports are used as the output port of the Brillouin laser.

A ring cavity is constructed by a fiber optic circulator 2, and the circulator is cascaded with an optical amplifier 3, a fiber isolator 4 for isolating reverse light, a fiber coupler 5 for building a composite cavity, and a fiber coupler for coupling out 8 and frequency-shifting fibers 6 and 7 for providing Brillouin nonlinear gain. The seed laser is input through the first port of the circulator, and can only be transmitted in the clockwise direction in the cavity, and the back Brillouin scattered light can be circulated in the compound cavity, and finally there is and only one in the range of the Brillouin gain spectrum The cavity longitudinal mode oscillates to form Brillouin laser emission.

The invention constructs a unidirectional active ring resonant cavity, based on the technical scheme of the fiber composite cavity, and utilizes the mode selection characteristics of the composite cavity, so that the longitudinal mode interval of the cavity is greater than the Brillouin gain spectrum width, so that there is a range of Brillouin gain spectrum And only one longitudinal mode of the cavity can oscillate and form laser output, and then realize the Brillouin type single longitudinal mode transmission frequency transfer fiber laser output with high efficiency and low cost. The frequency of this Brillouin single-frequency laser always follows the pump seed light, and the broadband frequency shift effect is stable. Based on the composite cavity, the single longitudinal mode operation is realized. Optical fiber temperature and strain sensing system has strict application requirements for broadband frequency shifting technology.

Claims (5)

1. The Brillouin type single longitudinal mode frequency-shifted fiber laser is characterized in that it comprises: a light source module (1), an optical fiber circulator (2), an optical amplifier (3), an optical fiber isolator (4), and a first optical fiber coupler (5), the first frequency-shifting optical fiber (6), the second frequency-shifting optical fiber (7), the second optical fiber coupler (8); wherein, the pigtail output of the light source module (1) and the optical fiber circulator (2) The first port (201) is connected, the second port (202) of the optical fiber circulator (2) is connected with the input port (301) of the optical amplifier (3), and the output port (302) of the optical amplifier (3) is coupled with the first optical fiber The first port (501) of the device (5) is connected; the second port (502) and the fourth port (504) of the first optical fiber coupler (5) are connected to form a loop by optical fiber, and the loop is connected The imported device comprises a fiber isolator (4) and a first frequency-shifting fiber (6), wherein the forward conduction input port (401) of the fiber isolator (4) is connected to the second port ( 502) is connected, the forward conduction output port (402) of the optical fiber isolator (4) is connected with the first frequency-shifting optical fiber (6), and the other end of the first frequency-shifting optical fiber (6) is connected with the first optical fiber coupler (5) The fourth port (504) of the second frequency-shifting fiber (7) is connected to the third port (503) of the first fiber coupler (5), and the other end is connected to the second port of the second fiber coupler (8). One port (801) is connected; the third port (803) of the second fiber coupler (8) is connected with the third port (203) of the fiber optic circulator, and is closed into a large fiber loop, and the second fiber coupler ( 8) The second port (802) is used as the output port of the single longitudinal mode Brillouin laser; the first frequency-shifting optical fiber (6) and the second frequency-shifting optical fiber (7) adopt ordinary communication single-mode optical fiber, and the first frequency-shifting optical fiber There are two ways to select the length of the optical fiber (6) and the second frequency-shifting optical fiber (7): the one is that the lengths of the two cavities forming the composite cavity are equal or close; the second is that the length of one cavity forming the composite cavity is the length of the other cavity. multiple times; the optical fiber isolator (4) is used for unidirectional conduction to prevent reverse resonance; the first optical fiber coupler (5) adopts common single-mode optical fiber 1550nm band, port 2×2, and the splitting ratio is 50 :50 coupler. 2. The Brillouin type single longitudinal mode frequency-shifted fiber laser according to claim 1, characterized in that the light source module (1) is a narrow linewidth laser with a linewidth less than 1 MHz. 3. Brillouin type single longitudinal mode frequency-shifted fiber laser as claimed in claim 1, is characterized in that described fiber optic circulator (2), is a three-port fiber optic circulator, one-way conduction, fiber optic circulator ( 2) It can be replaced by connecting fiber coupler and isolator to play the role of fiber optic circulator. 4. Brillouin type single longitudinal mode frequency-shifted fiber laser as claimed in claim 1, is characterized in that described optical amplifier (3) selects high gain factor erbium-doped fiber for use as gain medium to build fiber amplifier, or selects 1550nm for use band semiconductor optical amplifier (SOA). 5. Brillouin type single longitudinal mode frequency-shifting fiber laser as claimed in claim 1, is characterized in that described second fiber coupler (8), is the common single-mode fiber coupler of 1 * 2, center wavelength 1550nm, the beam splitting ratio is 10:90, and 10% of the ports are used as the output port of the Brillouin laser.

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