Linear cavity mode-locked fiber laser based on phase bias and control method

Abstract

The present invention discloses a linear cavity mode-locked fiber laser based on phase bias and a control method. The laser includes: a pump source, a wavelength division multiplexer connected to the pump source, a polarization-maintaining gain fiber connected to the common end of the wavelength division multiplexer, and a collimator connected to the transmission end of the wavelength division multiplexer; a birefringent crystal; a Faraday rotator; a half-wave plate; a Faraday rotator connected to the polarization-maintaining gain fiber; a polarization beam splitter prism, which is used to generate interference to achieve mode locking for the orthogonal pulse light returned by the birefringent crystal and then split it into two paths; a high reflector, which is used to reflect one path of transmitted light output by the polarization beam splitter prism back into the linear cavity. The present invention generates interference at the polarization beam splitter prism to achieve mode locking, which greatly reduces the threshold for mode locking startup, increases the repetition rate, and improves the overall stability and reliability.

Description

Technical Field

[0001] The present invention relates to the field of optical fiber lasers, and in particular to a linear cavity mode-locked optical fiber laser based on phase bias and a control method thereof. Background Art

[0002] This section only provides background information related to the present application to help those skilled in the art understand the present application more thoroughly and accurately, and it is not necessarily prior art.

[0003] Compared with solid lasers, ultrafast pulsed mode-locked fiber lasers have the advantages of low price, compact structure, high conversion efficiency, and good beam quality. They are widely used in medical surgery, laser communication, optical frequency comb, material micro-processing and other fields. These fields have put forward high requirements on the operation stability of ultrafast lasers in different environments, while the stability and reliability of industrial-grade ultrafast lasers, especially seed sources, have not been well solved. Using fiber technology to replace solid seed source technology itself has many advantages. In fiber technology, lasers using full polarization-maintaining fibers are considered to be an effective method to resist environmental changes.

[0004] The commonly used technology at present is to use saturable absorber mode-locking technology to make full polarization-maintaining fiber lasers. However, saturable absorption elements such as semiconductor saturable absorbers (SESAM), carbon nanotube saturable absorbers, and graphene saturable absorbers all have the disadvantages of low damage thresholds and decay over time. Therefore, these weaknesses limit their use out of the laboratory and into the broader industrial market. Traditional nonlinear polarization rotation (NPR) fiber laser technology cannot fully utilize the full polarization-maintaining system. Without the application of polarization-maintaining fiber, NPR fiber laser technology is easily affected by environmental interference. In order to obtain a more stable NPR laser, people have tried to replace the non-polarization-maintaining fiber in the laser cavity with a polarization-maintaining fiber to improve its stability.

[0005] The key problem to be solved in realizing NPR mode locking in full polarization-maintaining fiber is to compensate for the group velocity mismatch (GVM) between the two orthogonal polarization components. The temporal separation of the orthogonal polarization components caused by GVM will cause the pulse quality to deteriorate. In order to solve this problem, technicians in the field have been constantly exploring and proposing different solutions: Nielsen et al. proposed the use of Faraday rotators as end mirrors of the laser cavity in 2007, and proposed a linear cavity polarization-maintaining fiber laser based on nonlinear polarization rotation mode locking, which can generate pulses with a repetition rate of 5.96 MHz and a pulse width of 5.6 ps; Jiaqi Zhou et al. proposed a full polarization-maintaining fiber laser based on nonlinear polarization rotation mode locking in 2008, using Faraday rotators to compensate for group velocity mismatch and linear phase shift in the laser. The laser can generate dissipative solitons and achieve a single pulse energy of 2.9 nJ and a pulse width of 5.9 ps; Szczepanek et al. proposed in 2017 to fuse three sections of polarization-maintaining fiber at a special angle as an artificial saturable absorber to achieve NPR mode locking. The developed full polarization-maintaining fiber laser can achieve ultrashort pulse output with a pulse width of 150 fs; Wenchao In 2018, Zhang et al. proposed to fuse some welding points at a certain angle in the ring cavity laser to achieve NPR mode locking, thereby increasing the repetition frequency of this type of laser to 111MHz. However, these solutions have not completely solved the technical problems such as the instability of conventional mode-locked fiber lasers, the difficulty of implementing NPR mode locking technology in polarization-maintaining fibers, and the difficulty of increasing the laser repetition frequency. Summary of the invention

[0006] The present invention aims to solve at least one of the technical problems existing in the prior art. To this end, the present invention proposes a linear cavity mode-locked fiber laser based on phase bias and a control method, which reduces the mode-locking threshold and increases the repetition rate.

[0007] The present invention provides a linear cavity mode-locked fiber laser based on phase bias, comprising a pump source, a wavelength division multiplexer connected to the pump source, a polarization-maintaining gain fiber connected to the common end of the wavelength division multiplexer, and a collimator connected to the transmission end of the wavelength division multiplexer; the laser also includes:

[0008] A birefringent crystal, used for projecting a small pulse light randomly formed by laser oscillation onto a fast axis and a slow axis of the birefringent crystal for phase biasing, so that the small pulse light is converted into an orthogonal pulse light including an orthogonal first polarized light and a second polarized light;

[0009] Faraday rotator, used to rotate the polarization direction of orthogonal pulse light 45° clockwise;

[0010] A half wave plate, used for adjusting the polarization state of the orthogonal pulse light, so as to project the first polarized light and the second polarized light onto the fast axis and the slow axis of the collimator respectively;

[0011] The Faraday rotator connected to the polarization-maintaining gain fiber is used to rotate the orthogonal pulse light by 90 degrees, so that the first polarized light and the second polarized light of the orthogonal pulse exchange the transmission paths, and then transmit through the polarization-maintaining gain fiber, the wavelength division multiplexer, the collimator, the half-wave plate, the Faraday rotator and the birefringent crystal in sequence;

[0012] Polarization beam splitter, used to generate interference and achieve mode locking of the orthogonal pulse light returned from the birefringent crystal and then split it into two paths;

[0013] The high reflective mirror is used to reflect the transmitted light output by the polarization beam splitter prism back into the linear cavity.

[0014] In a preferred embodiment, the wavelength division multiplexer and the optical fiber collimator are replaced by an optical fiber wavelength division multiplexing collimator, and the pump source is connected to the optical fiber wavelength division multiplexing collimator.

[0015] In a preferred embodiment, a dispersive element is disposed between the polarization beam splitter prism and the high reflective mirror.

[0016] In a preferred embodiment, the dispersive element is a grating pair.

[0017] In a preferred embodiment, the grating density of the grating pair ranges from 150 lines / mm to 2000 lines / mm.

[0018] In a preferred embodiment, the polarization-maintaining gain optical fiber is a large mode area polarization-maintaining optical fiber, a doped gain polarization-maintaining optical fiber, a large mode area double-clad polarization-maintaining optical fiber or a polarization-maintaining photonic crystal fiber.

[0019] In a preferred embodiment, the Faraday rotator is a thin-film Faraday rotator, or a Faraday rotator formed by inserting a magneto-optical crystal into a permanent magnet.

[0020] In a preferred embodiment, the phase offset produced by the birefringent crystal is It is determined by the properties and thickness of the birefringent crystal, that is, ,in, is the refractive index of e-light, is the refractive index of o light, l is the thickness of the birefringent crystal and λ is the wavelength.

[0021] In a preferred embodiment, the length of the polarization-maintaining gain fiber is adjustable so that the cavity length of the laser can be adjusted by adjusting the length of the polarization-maintaining gain fiber.

[0022] The present invention also discloses a control method of a linear cavity mode-locked fiber laser based on phase bias, which comprises the steps of:

[0023] Step S1, coupling the pump light of the pump source into the linear cavity of the laser through a wavelength division multiplexer, and increasing the pump power of the pump source to above the threshold of the laser;

[0024] Step S2, aligning the light collimator with the high reflective mirror to make the laser oscillate and randomly form small pulse light;

[0025] Step S3, projecting the small pulse light in a linear polarization state onto the fast axis and the slow axis of the birefringent crystal to adjust the phase bias, so that the small pulse light is converted into an orthogonal pulse light including an orthogonal first polarization light and a second polarization light;

[0026] Step S3, rotating the polarization direction of the orthogonal pulse light by 45° clockwise using a Faraday rotator;

[0027] Step S4, projecting the polarization direction of the orthogonal pulse light onto the fast axis and the slow axis of the collimator through a half-wave plate, and allowing the orthogonal pulse to enter the polarization-maintaining gain fiber to be amplified to generate laser gain;

[0028] Step S5, rotating the orthogonal pulse light by 90° by a Faraday rotator, so that the first polarized light and the second polarized light of the orthogonal pulse light exchange the transmission paths, and then sequentially transmitted through the polarization-maintaining gain fiber, the wavelength division multiplexer, the collimator, the half-wave plate, the Faraday rotator and the birefringent crystal;

[0029] Step S6, when the orthogonal pulse light returns to the polarization beam splitter prism, interference is generated to achieve mode locking. The polarization beam splitter prism splits the interference light into two paths, one of which is transmitted light and reflected back into the linear cavity by the high reflector, while the other is reflected light coupled to the output of the laser.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] 1. The present invention adopts a half-wave plate to project two orthogonal polarized light beams onto the fast and slow axes of a polarization-maintaining gain optical fiber, and transmits them along the fast and slow axes of the polarization-maintaining gain optical fiber respectively, and adopts a Faraday rotator to realize the compensation of group velocity mismatch and linear phase shift; at the same time, a phase shift unit is formed by a Faraday rotator and a birefringent crystal, and a phase difference of π / 2 is generated between the light transmitted along the fast axis and the light transmitted along the slow axis in the orthogonal polarization state, and interference is generated at the return polarization beam splitter prism to realize mode locking. After the orthogonal pulse light is phase-shifted by the phase shift unit, the threshold value for starting mode locking is greatly reduced, so that mode locking can be realized at a high repetition rate in this structure, the repetition rate is improved, and the overall stability and reliability are improved at the same time.

[0032] 2. Using fiber wavelength division multiplexing collimator to replace conventional wavelength division multiplexer and fiber collimator greatly shortens the length of the fiber in the fiber laser, simplifies the laser system, improves the coupling power and efficiency, and increases the repetition frequency.

[0033] 3. The present invention solves the problems of instability of conventional mode-locked fiber lasers, difficulty in implementing NPR mode-locked technology in polarization-maintaining fibers, and difficulty in increasing the laser repetition frequency, making it an ideal seed light source for future industrial Burst mode ultrafast industrial lasers.

[0034] 4. The present invention utilizes ultra-thin space elements to compress the length of the linear cavity to the extreme. The shorter the length of the linear cavity, the higher the repetition frequency. Usually, mode locking can be achieved on a short cavity when the mode locking threshold is low, that is, mode locking is achieved when the repetition frequency is high. Therefore, the present invention obtains a higher repetition rate than that using discrete optical fiber polarization-maintaining elements, which can reach the order of hundreds of megahertz or above. Therefore, the laser of the present invention plays a very important role as a high-repetition-rate seed source in future femtosecond amplification and pulse train mode frequency selection, and has extremely broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is a schematic structural diagram of Embodiment 1 of the linear cavity mode-locked fiber laser disclosed in the present invention.

[0036] Figure 2 It is a schematic structural diagram of the second embodiment of the linear cavity mode-locked fiber laser disclosed in the present invention.

[0037] Figure 3 It is a schematic structural diagram of the third embodiment of the linear cavity mode-locked fiber laser disclosed in the present invention.

[0038] Figure 4 It is a schematic structural diagram of Embodiment 4 of the linear cavity mode-locked fiber laser disclosed in the present invention. DETAILED DESCRIPTION

[0039] In order to further explain the technical means and effects adopted by the present application to achieve the predetermined purpose, the specific implementation methods, structures, features and effects of the present application are described in detail below in conjunction with the accompanying drawings and preferred embodiments. In the following description, different "one embodiment" or "embodiment" does not necessarily refer to the same embodiment. In addition, specific features, structures, or characteristics in one or more embodiments may be combined in any suitable form.

[0040] Embodiment 1

[0041] like Figure 1As shown, this embodiment discloses a linear cavity mode-locked fiber laser based on phase bias (hereinafter referred to as "laser" or "light laser"), comprising: a linear cavity, a Faraday rotator 2 and a high reflector 11 respectively arranged at two opposite end faces in the linear cavity; a wavelength division multiplexer 4, a collimator 5, a half wave plate 6, a Faraday rotator 7, a birefringent crystal 8 and a polarization beam splitter prism 9 arranged in sequence between the Faraday rotator 2 and the high reflector 11, and the wavelength division multiplexer 4 is connected to the Faraday rotator 2 through a polarization-maintaining gain fiber 3, and the transmission end of the wavelength division multiplexer 4 is connected to the collimator 5; a high reflector 11 arranged opposite to the polarization beam splitter prism 9; a pump source 1 connected to the wavelength division multiplexer 4, the pump source 1 provides pump light to the wavelength division multiplexer 4, and the wavelength division multiplexer 4 is used to couple the pump light to the polarization-maintaining gain fiber 3 to generate laser gain.

[0042] The birefringent crystal 8 is used to adjust the phase bias of the small pulse light randomly formed by the laser oscillation, so as to project the small pulse light in a linear polarization state onto the fast axis and the slow axis of the birefringent crystal 8, so that the small pulse light is divided into an orthogonal first polarized light and a second polarized light. The intensity ratio between the first polarized light and the second polarized light is determined by the angle between the small pulse light and the fast axis of the birefringent crystal 8 when the small pulse light is incident on the birefringent crystal 8.

[0043] The fast axis and the slow axis of the birefringent crystal 8 introduce an asymmetric phase shift, which results in a phase shift difference, i.e., a phase offset, between the polarized light propagating parallel to different axes. Determined by the properties and thickness of the birefringent crystal, i.e. ,in, is the refractive index of e-light, is the refractive index of o light, l is the thickness of the birefringent crystal and λ is the wavelength.

[0044] The Faraday rotator 7 is a thin-sheet Faraday rotator or a Faraday rotator formed by inserting a magneto-optical crystal into a permanent magnet. The Faraday rotator 7 is used to rotate the polarization direction of the orthogonal pulse light by 45° clockwise.

[0045] The half wave plate 6 is used to adjust the polarization state of the orthogonal pulse light, and project the first polarized light and the second polarized light of the orthogonal pulse light onto the fast axis and the slow axis of the collimator 5 respectively.

[0046] The orthogonal pulses enter the polarization-maintaining gain fiber 3 and are transmitted along the fast axis and the fast-slow axis of the polarization-maintaining gain fiber 3; at this time, the pump source 1 injects the pump light into the linear cavity through the wavelength division multiplexer 4, so that the orthogonal pulses enter the polarization-maintaining gain fiber 3 to be amplified to generate laser gain, and finally reach the Faraday rotator 2 at one end of the linear cavity.

[0047] The polarization-maintaining gain fiber 3 is one of a large mode area polarization-maintaining fiber, a doped gain polarization-maintaining fiber, a large mode area double-clad polarization-maintaining fiber, and a polarization-maintaining photonic crystal fiber. One end of the polarization-maintaining gain fiber 3 is connected to the common end of the wavelength division multiplexer 4, and the other end of the polarization-maintaining gain fiber 3 is connected to the Faraday rotator 2. Therefore, by adjusting the length of the polarization-maintaining gain fiber 3, the cavity length of the linear cavity can be correspondingly adjusted, and the shorter the cavity length of the linear cavity, the higher the repetition frequency of the laser generated by the laser.

[0048] The Faraday rotator 2 rotates the orthogonal pulse light by 90°, so that the first polarized light and the second polarized light of the orthogonal pulse exchange the transmission paths, and then are transmitted toward the polarization splitter prism 9 in sequence through the polarization-maintaining gain fiber 3, the wavelength division multiplexer 4, the collimator 5, the half wave plate 6, the Faraday rotator 7 and the birefringent crystal 8.

[0049] When the orthogonal pulse light is transmitted back to the polarization beam splitter prism 9, interference will occur to form interference light, which is split into two paths by the polarization beam splitter prism 9. One interference light is reflected back into the linear cavity by the high reflector 11, and the other interference light is coupled as the output of the laser.

[0050] For example, in this embodiment, the pump source 1 adopts a 976nm single-mode pump. Correspondingly, the Faraday rotator 2 has an operating wavelength of 1550 nm, the wavelength division multiplexer 4 has a built-in 976nm high-reflection 1550 high-transmittance filter, the collimator 5 is coated with a 1550nm anti-reflection film, the front and rear surfaces of the half-wave plate 6 are coated with a 1550nm anti-reflection film, the front and rear surfaces of the birefringent crystal 8 are coated with a 1550nm anti-reflection film, the transmission-reflection ratio of the polarization beam splitter prism 9 is 50:50, and the high reflector 11 is coated with a 1550nm total reflection film.

[0051] For ease of understanding, the control process of the laser disclosed in the present invention is as follows:

[0052] Step S1, coupling the pump light of the pump source 1 into the linear cavity through the wavelength division multiplexer 4, and increasing the pump power of the pump source 1 to above the threshold of the laser, so that the laser can generate small pulse random oscillation to generate laser.

[0053] Step S2: Align the light collimator 5 with the high reflective mirror 11 to make the laser oscillate and randomly form small pulse light.

[0054] After the laser is able to produce small pulse random oscillations, the random small pulse light generated will oscillate multiple times in the linear cavity to form a stable ultrashort pulse light, that is, small pulse light.

[0055] Step S3, the small pulse light starts from the polarization splitter prism 9, and when passing through the birefringent crystal 8, the birefringent crystal 8 performs phase bias adjustment on the pulse light, specifically, the small pulse light in a linear polarization state is projected onto the fast axis and the slow axis of the birefringent crystal 8, so that the small pulse light is converted into orthogonal pulse light, and the orthogonal pulse light includes a first polarized light and a second polarized light orthogonal to the first polarized light.

[0056] Step S3: The orthogonal pulse light enters the Faraday rotator 7, and the Faraday rotator 7 rotates the polarization direction of the orthogonal pulse light by 45° clockwise.

[0057] Step S4, the orthogonal pulse light is polarized by the half wave plate 6, and the half wave plate 6 projects the polarization direction of the orthogonal pulse light onto the fast axis and the slow axis of the collimator 5; then, the orthogonal pulse enters the polarization-maintaining gain fiber 3 and is transmitted along the fast axis and the fast and slow axes of the polarization-maintaining gain fiber 3; at this time, the pump source 1 injects the pump light into the linear cavity through the wavelength division multiplexer 4, so that the orthogonal pulse enters the polarization-maintaining gain fiber 3 to be amplified to generate laser gain, and finally reaches the Faraday rotator 2 at one end of the linear cavity.

[0058] Step S5, the Faraday rotator 2 rotates the orthogonal pulse light by 90°, so that the first polarized light and the second polarized light of the orthogonal pulse light exchange the transmission paths, and then are transmitted toward the polarization splitter prism 9 in sequence through the polarization-maintaining gain fiber 3, the wavelength division multiplexer 4, the collimator 5, the half wave plate 6, the Faraday rotator 7 and the birefringent crystal 8.

[0059] The first polarized light and the second polarized light of the orthogonal pulse light exchange transmission paths, which means that if the first polarized light is transmitted along the fast axis of each device on the transmission path during transmission toward the Faraday rotator 2, then the second polarized light is transmitted along the slow axis of each device on the transmission path during transmission toward the Faraday rotator 2; correspondingly, after the orthogonal pulse light is rotated 90° by the Faraday rotator 2, the orthogonal pulse light is transmitted from the Faraday rotator 2 toward the polarization splitter prism 9, at which time, the first polarized light is transmitted along the slow axis of each device on the transmission path, and the second polarized light is transmitted along the fast axis of each device on the transmission path.

[0060] Step S6, the orthogonal pulse light then passes through the half wave plate 6, the Faraday rotator 7, and the birefringent crystal 8 in sequence. When the orthogonal pulse light returns to the polarization beam splitter prism 9, interference is generated to achieve mode locking. The polarization beam splitter prism 9 divides the interference light into two paths. One path of transmitted light is reflected by the high reflector 11 back into the linear cavity as a small pulse light formed by the next laser oscillation, and the other path of reflected light is coupled to the output laser of the laser.

[0061] When the first polarized light and the second polarized light of the orthogonal pulse light enter the collimator 5 and propagate along the fast axis and the slow axis respectively, due to the different refractive indices between the fast axis and the slow axis, a group velocity mismatch and a linear phase shift difference will occur between the first polarized light and the second polarized light; when the Faraday rotator 2 rotates the orthogonal pulse light by 90°, the first polarized light and the second polarized light exchange transmission paths, so that the transmission paths of the two polarized component lights from entering the collimator 5 to returning to the collimator 5 are exactly the same, and the group velocity mismatch and the linear phase shift caused by the dispersion of the optical fiber material are equal, so that the group velocity mismatch and the linear phase shift can be perfectly compensated when the orthogonal pulse light returns to the collimator 5, so that only the nonlinear phase shift characteristics remain when the orthogonal pulse light returns to the collimator 5.

[0062] Orthogonal pulse light is generated by small pulse light after the polarization direction is changed. Therefore, the orthogonal pulse light has the same characteristic of uneven light intensity distribution as the small pulse light. The light intensity in the center is stronger and the light intensity on both sides is weaker. Therefore, the transmittance in the center is large and the transmittance on both sides is small. When the orthogonal pulse light passes through the polarization beam splitter prism 9, its pulse is narrower. The orthogonal pulse light interferes at the polarization beam splitter prism 9. The stronger the light intensity of the orthogonal pulse light, the greater the nonlinear phase difference between the two polarization component lights, and the greater the transmittance of the polarization beam splitter prism 9. On the contrary, the weaker the light intensity of the orthogonal pulse light, the lower the transmittance of the polarization beam splitter prism 9 relative to it. Instead, the orthogonal pulse light is mainly coupled through the reflected light to output the linear cavity as the output laser of the laser, thereby achieving the effect of saturable absorption to achieve mode locking.

[0063] Since the two polarized components of the orthogonal pulse light pass through the Faraday rotator 7 and the birefringent crystal 8 twice, the orthogonal pulses produce non-reciprocal nonlinear phase shifts. The nonlinear phase shift and non-reciprocal phase shift produced in the polarization-maintaining gain fiber 3 constitute a phase bias, which changes the reflectivity curve, and acts as a fast saturable absorber in the laser. The reflectivity of the orthogonal pulse light in the polarization beam splitter 9 is related to the nonlinear phase shift of the two polarized components. The reflectivity is a function of the phase shift difference of the two polarized components. The greater the phase shift introduced by the phase shift bias, the higher the reflectivity, and the lower the phase shift, the lower the reflectivity. The central part of the orthogonal pulse light is strong, and the phase shift produced in the linear cavity of the laser is large, so the reflectivity is high, and the pulse is easier to oscillate, thereby realizing the laser mode locking and forming ultra-short pulse output.

[0064] Embodiment 2

[0065] like Figure 2 As shown. Figure 1Compared with the first embodiment shown, this embodiment adopts a fiber wavelength division multiplexing collimator 5' to replace the wavelength division multiplexer 4 and the fiber collimator 5, and the pump source 1 is connected to the fiber wavelength division multiplexing collimator 5'. The use of the fiber wavelength division multiplexing collimator 5' is conducive to greatly shortening the length of the optical fiber in the fiber laser, simplifying the laser system, and is conducive to improving the coupling power and efficiency of the laser, and increasing the repetition frequency.

[0066] Embodiment 3

[0067] like Figure 3 As shown. Figure 1 Compared with the first embodiment shown, in this embodiment, a dispersion element is arranged between the polarization beam splitter prism 9 and the high reflector 11, and the dispersion element is used to perform dispersion compensation so that the laser can obtain a shorter pulse width, thereby improving the performance of the laser. For example, the dispersion element is a grating pair 10, and the grating density of the grating pair 10 ranges from 150 lines / mm to 2000 lines / mm.

[0068] For example, in the third embodiment, the pump source 1 adopts a 976nm single-mode pump, the Faraday rotator 2 has an operating wavelength of 1030nm, the wavelength division multiplexer 4 has a built-in 976nm high-reflection 1030nm high-transmittance filter, the collimator 5 is coated with a 1030nm anti-reflection film, the half wave plate 6 is coated with a 1030nm anti-reflection film on the front and back, the birefringent crystal 8 is coated with a 1550nm anti-reflection film on the front and back, the transmission-reflection ratio of the polarization beam splitter prism 9 is 50:50, the grating pair 10 adopts a 1600-line transmission grating, and the high reflector 11 is coated with a 1030nm total reflection film.

[0069] Embodiment 4

[0070] like Figure 4 As shown. Figure 3 Compared with the first embodiment shown, this embodiment uses a fiber wavelength division multiplexing collimator 5' to replace the wavelength division multiplexer 4 and the fiber collimator 5. The use of the fiber wavelength division multiplexing collimator 5' is beneficial to greatly shorten the length of the optical fiber in the fiber laser and simplify the laser system.

[0071] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present application should be included in the protection scope of the present application.

Claims

1. A linear cavity mode-locked fiber laser based on phase bias, comprising a pump source, a wavelength division multiplexer connected to the pump source, a polarization-maintaining gain fiber connected to the common end of the wavelength division multiplexer, and a collimator connected to the transmission end of the wavelength division multiplexer; characterized in that: The laser also includes: A birefringent crystal, used for projecting a small pulse light randomly formed by laser oscillation onto a fast axis and a slow axis of the birefringent crystal for phase biasing, so that the small pulse light is converted into an orthogonal pulse light including an orthogonal first polarized light and a second polarized light; Faraday rotator, used to rotate the polarization direction of orthogonal pulse light 45° clockwise; A half wave plate, used for adjusting the polarization state of the orthogonal pulse light, so as to project the first polarized light and the second polarized light onto the fast axis and the slow axis of the collimator respectively; The Faraday rotator connected to the polarization-maintaining gain fiber is used to rotate the orthogonal pulse light by 90 degrees, so that the first polarized light and the second polarized light of the orthogonal pulse exchange the transmission paths, and then transmit through the polarization-maintaining gain fiber, the wavelength division multiplexer, the collimator, the half-wave plate, the Faraday rotator and the birefringent crystal in sequence; Polarization beam splitter, used to generate interference and achieve mode locking of the orthogonal pulse light returned from the birefringent crystal and then split it into two paths; A high reflective mirror, used for reflecting one path of transmitted light outputted from the polarization beam splitter prism back into the linear cavity; The magnitude of the phase offset produced by the birefringent crystal Determined by the properties and thickness of the birefringent crystal, i.e. Among them, n e is the refractive index of e-light, n o is the refractive index of o light, l is the thickness of the birefringent crystal, and λ is the wavelength; The length of the polarization-maintaining gain fiber is adjustable so as to adjust the cavity length of the laser by adjusting the length of the polarization-maintaining gain fiber.

2. The linear cavity mode-locked fiber laser based on phase bias according to claim 1, characterized in that: The wavelength division multiplexer and the optical fiber collimator are replaced by an optical fiber wavelength division multiplexing collimator, and the pump source is connected to the optical fiber wavelength division multiplexing collimator.

3. The linear cavity mode-locked fiber laser based on phase bias according to claim 1, characterized in that: A dispersion element is arranged between the polarization beam splitter prism and the high reflective mirror.

4. The linear cavity mode-locked fiber laser based on phase bias according to claim 3, characterized in that: The dispersive element is a grating pair.

5. The linear cavity mode-locked fiber laser based on phase bias according to claim 4, characterized in that: The grating density of the grating pair ranges from 150 lines / mm to 2000 lines / mm.

6. The linear cavity mode-locked fiber laser based on phase bias according to claim 1, characterized in that: The polarization-maintaining gain fiber adopts large mode area polarization-maintaining fiber, doped gain polarization-maintaining fiber, large mode area double-clad polarization-maintaining fiber or polarization-maintaining photonic crystal fiber.

7. The linear cavity mode-locked fiber laser based on phase bias according to claim 1, characterized in that: The Faraday rotator adopts a thin-film Faraday rotator or a Faraday rotator composed of a magneto-optical crystal inserted into a permanent magnet.

8. The control method of a linear cavity mode-locked fiber laser based on phase bias according to any one of claims 1 to 7, characterized in that: Includes steps: Step S1, coupling the pump light of the pump source into the linear cavity of the laser through a wavelength division multiplexer, and increasing the pump power of the pump source to above the threshold of the laser; Step S2, aligning the light collimator with the high reflective mirror to make the laser oscillate and randomly form small pulse light; Step S3, projecting the small pulse light in a linear polarization state onto the fast axis and the slow axis of the birefringent crystal to adjust the phase bias, so that the small pulse light is converted into an orthogonal pulse light including an orthogonal first polarization light and a second polarization light; Step S3, rotating the polarization direction of the orthogonal pulse light by 45° clockwise using a Faraday rotator; Step S4, projecting the polarization direction of the orthogonal pulse light onto the fast axis and the slow axis of the collimator through a half-wave plate, and allowing the orthogonal pulse to enter the polarization-maintaining gain fiber to be amplified to generate laser gain; Step S5, rotating the orthogonal pulse light by 90° by a Faraday rotator, so that the first polarized light and the second polarized light of the orthogonal pulse light exchange the transmission paths, and then sequentially transmitted through the polarization-maintaining gain fiber, the wavelength division multiplexer, the collimator, the half-wave plate, the Faraday rotator and the birefringent crystal; Step S6, when the orthogonal pulse light returns to the polarization beam splitter prism, interference is generated to achieve mode locking. The polarization beam splitter prism splits the interference light into two paths, one of which is transmitted light and reflected back into the linear cavity by the high reflector, while the other is reflected light coupled to the output of the laser.

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