All-polarization-maintaining optical fiber mode-locked laser

Abstract

The invention provides an all-polarization-maintaining optical fiber mode-locked laser which comprises a linear cavity and an annular cavity. Wherein the annular cavity consists of a wavelength division multiplexer, an optical fiber coupler, a phase bias device, a gain optical fiber and a dispersion compensation optical fiber; the linear cavity is composed of an optical fiber coupler and an optical fiber reflector. As a function multiplexing device, an external electric field is applied to the phase bias device, so that the functions of continuous tuning of the phase difference in the positive direction and the negative direction in the annular cavity of the 9-shaped cavity mode-locked fiber laser and rapid phase tuning are realized, and the realization of a mode-locked state and a high-precision phase locking state is promoted. In addition, the composition structure of the resonant cavity can be simplified, and the repetition frequency and the system stability of the mode-locked laser can be improved. The technology for constructing the 9-shaped cavity mode-locked fiber laser by adopting the active phase bias device has originality in the field of ultrafast optics, and a better choice is provided for the all-polarization-maintaining fiber mode-locked laser.

Description

Technical Field

[0001] This invention belongs to the field of ultrafast optics technology, specifically relating to a fully polarization-maintaining fiber mode-locked laser. Background Technology

[0002] Mode-locked fiber lasers, with their advantages of compact structure, high stability, and moderate price, have become the most widely used type of ultrafast laser. Common mode-locking methods for fiber mode-locked lasers include nonlinear polarization rotation mode-locking, saturable absorber mode-locking, and nonlinear optical amplification ring mirror mode-locking (including figure-8 and figure-9 cavities). Among these, the figure-9 cavity mode-locking technology has enormous application potential due to its advantages such as 100% self-starting, full polarization maintenance, and high stability. Mode-locked fiber lasers and optical frequency comb systems built based on this technology have even been deployed on spacecraft and are operating stably for extended periods.

[0003] The core component of a fiber laser based on figure-nine cavity mode-locking is the phase biaser, a passive fiber device that introduces different phase differences when laser light passes through it in opposite directions, thus aiding in mode-locking of the nonlinear optical amplification ring mirror. Currently, common phase biasers mainly consist of two Faraday rotators and a waveplate. The phase difference introduced in the opposite directions is mainly utilized by the fast and slow axis characteristics of the waveplate, resulting in a fixed phase difference after fabrication. Therefore, during mode-locking adjustment, it is often necessary to select a phase biaser with a different phase difference. Although mode-locking can be achieved through repeated experiments, further fine-tuning to optimize the mode-locking state is not possible. Furthermore, existing phase biasers have a single function in the system, not only increasing the system's structure but also increasing the cavity length due to the added pigtail, which is a significant obstacle to improving the repetition rate of the mode-locked fiber laser. Summary of the Invention

[0004] The technical problem solved by this invention is to overcome the shortcomings of the prior art and provide a fully polarization-maintaining fiber mode-locked laser that can finely adjust the mode-locking state while reducing the complexity of the resonant cavity, which is beneficial to improving the repetition frequency and system stability of the mode-locked laser.

[0005] The technical solution of this invention is: a fully polarization-maintaining fiber mode-locked laser, comprising a pump laser and a resonant cavity; the resonant cavity is composed of two sub-resonant cavities, including a linear cavity and a ring cavity; wherein, the ring cavity is composed of a wavelength division multiplexer, an optical fiber coupler, a phase biaser, a gain fiber, and a dispersion compensation fiber; the linear cavity is composed of an optical fiber coupler and an optical fiber mirror;

[0006] The pump laser emitted by the pump laser enters the ring cavity through a wavelength division multiplexer, causing the gain fiber to generate laser light. The laser light generated by the gain fiber enters the linear cavity through port 1 of the fiber coupler, and after being reflected by the fiber mirror, it is injected back into the fiber coupler through port 1. Then, it passes through ports 3 and 4 of the fiber coupler and is split into two laser beams that propagate in opposite directions according to the set optical power ratio before entering the ring cavity again. The phase biaser is installed in the ring cavity using fiber fusion splicing to compensate for the phase difference introduced by the two laser beams. The dispersion compensation fiber is installed in the ring cavity using fiber fusion splicing to control the net dispersion of the full polarization-maintaining fiber mode-locked laser to near zero, thereby maximizing the spectral width of the mode-locked pulse laser output from port 2 of the fiber coupler.

[0007] The phase bias is a bidirectional optical transmission device, including a first Faraday rotator, a second Faraday rotator, an electro-optic modulator, a first collimator, and a second collimator;

[0008] After the forward-propagating laser beam is collimated by the first collimator, it is incident on the first Faraday rotator. The polarization state of the laser output from the first Faraday rotator is rotated 45° to the right and then incident on the electro-optic modulator for phase modulation. Subsequently, the laser output from the electro-optic modulator passes through the second Faraday rotator, and the laser polarization state is rotated 45° to the right again before being coupled into the second collimator.

[0009] After the reverse-propagating laser beam is collimated by the second collimator, it is incident on the second Faraday rotator. The polarization state of the laser output from the second Faraday rotator is rotated 45° to the right and then incident on the electro-optic modulator for phase modulation. Subsequently, the laser output from the electro-optic modulator passes through the first Faraday rotator, and the laser polarization state is rotated 45° to the right again before being coupled into the first collimator.

[0010] Both the first and second collimators are laser collimation output ports, and both pigtails are polarization-maintaining fibers, and both operate on a slow axis.

[0011] The laser polarization states output by the first collimator and the second collimator are orthogonal to each other.

[0012] The electro-optic modulator is made of electro-optic crystal material, cut along the Z-axis, and the electro-optic crystal operates in phase modulation mode.

[0013] Continuous tuning of the phase delay of the incident laser is achieved by applying an external electric field.

[0014] The direction of the applied electric field is parallel to the polarization state of the laser output from the first collimator after passing through the first Faraday rotator, and perpendicular to the polarization state of the laser output from the second collimator after passing through the second Faraday rotator.

[0015] The phase biaser continuously adjusts the phase difference between the laser beams in both directions by changing the electric field strength applied to the electro-optic modulator, and assists the laser in initiating mode-locking. After the fully polarization-maintaining fiber laser is mode-locked, the mode-locking state is optimized by adjusting the applied electric field strength.

[0016] The advantages of this invention compared to the prior art are:

[0017] This invention utilizes an active phase biaser to achieve continuous tuning of the phase difference between the forward and reverse directions within the ring cavity of a figure-9 cavity mode-locked fiber laser by changing the electric field intensity applied to its internal electro-optic modulator. This not only facilitates mode-locking but also allows for fine-tuning of the mode-locking state. Furthermore, as a functional multiplexer, it not only enables high-precision phase locking of the mode-locked laser but also reduces the complexity of the resonant cavity, thus improving the repetition rate and system stability of the mode-locked laser. Attached Figure Description

[0018] Figure 1 A schematic diagram of a fully polarization-maintaining fiber mode-locked fiber laser structure;

[0019] Figure 2 This is a schematic diagram of the phase biaser structure; Detailed Implementation

[0020] like Figure 1 As shown, the present invention proposes a fully polarization-maintaining fiber mode-locked laser, including a pump laser 1, a wavelength division multiplexer 2, a gain fiber 3, a dispersion compensation fiber 4, a phase biaser 5, a fiber coupler 6, and a fiber mirror 7.

[0021] The resonant cavity of a fully polarization-maintaining fiber-locked laser consists of two sub-cavities: a linear cavity on the left and a ring cavity on the right. The ring cavity comprises a wavelength division multiplexer, fiber coupler, phase biaser, gain fiber, and dispersion compensation fiber. The linear cavity consists of a fiber coupler and fiber mirrors. All fiber optic devices within both sub-cavities operate on the slow axis, and both pigtails are polarization-maintaining fibers.

[0022] The pump laser emitted by the pump laser enters the ring resonant cavity through a wavelength division multiplexer and is used to pump the gain fiber to generate laser light.

[0023] The fiber coupler has a 2×2 structure. The laser generated by the gain fiber is reflected by the fiber mirror and injected into the ring cavity on the right side of the fiber coupler through port 1. It is then split into two laser beams propagating in opposite directions by ports 3 and 4 according to their respective optical power ratios. The gain fiber is arranged asymmetrically to introduce different nonlinear phases for the laser beams propagating in the two directions.

[0024] Dispersion-compensating fiber is used to control the net dispersion of a fully polarization-maintaining fiber mode-locked laser system to near zero, thereby maximizing the spectrum of the mode-locked pulsed laser output from port 2 of the fiber coupler, while also reducing the phase noise of the mode-locked laser.

[0025] The phase biaser is a bidirectional optical transmission device used to additionally compensate for the phase difference introduced by lasers propagating in both directions, thus promoting mode locking. Unlike existing phase biasers, the phase polarizer in this invention is an active fiber optic device, and its composition is as follows... Figure 2 As shown, the system comprises a first Faraday rotator 8, a second Faraday rotator 9, an electro-optic modulator 10, a first collimator 11, and a second collimator 12. The forward-propagating laser beam is collimated by the first collimator and then enters the first Faraday rotator. After exiting the first Faraday rotator, the laser's polarization state is rotated 45° to the right and then enters the electro-optic modulator for phase modulation. The laser output from the electro-optic modulator then passes through the second Faraday rotator, where its polarization state is again rotated 45° to the right before coupling into the second collimator. The reverse-propagating laser beam is collimated by the second collimator and then enters the second Faraday rotator. After exiting the second Faraday rotator, the laser's polarization state is rotated 45° to the right and then enters the electro-optic modulator for phase modulation. The laser output from the electro-optic modulator then passes through the first Faraday rotator, where its polarization state is again rotated 45° to the right before coupling into the first collimator.

[0026] Both the first collimator 11 and the second collimator 12 use polarization-maintaining fibers, with the laser polarization state parallel to the slow axis direction. The first Faraday rotator 8 and the second Faraday rotator 9 work together to ensure that the polarization states of the lasers propagating in both directions are orthogonal at the electro-optic modulator. The overall polarization state is rotated 90°, aligning perfectly with the slow axis of the corresponding receiving fiber collimators, thus achieving polarization-maintaining slow-axis operation for the entire system. By changing the electric field strength applied to both ends of the electro-optic modulator, continuous tuning of the phase difference between the lasers propagating in both directions can be achieved. Simultaneously, by applying an additional modulation signal to the external electric field, the phase biaser can act as a fast brake, and in conjunction with a phase-locked loop, high-precision phase locking of the mode-locked laser can be achieved.

[0027] In summary, existing phase biasers primarily introduce phase delay by adjusting the fast and slow axes of the waveplate, resulting in a fixed value that hinders fine-tuning of the mode-locked state. Furthermore, their function is limited to simply introducing phase bias. This invention proposes an active phase biaser composed of two Faraday rotators, an electro-optic modulator, and fiber collimators at both ends. A laser beam output from the first collimator first passes through the first Faraday rotator, where its polarization state is rotated 45° to the right before entering the electro-optic modulation crystal from the left. The electro-optic crystal is Z-cut and operates in phase modulation mode. Its electric field strength is parallel to the polarization state of the laser beam entering from the left. According to the electro-optic effect, the introduced phase delay is related to the electric field strength. After modulation by the electro-optic crystal, the left-incident beam passes through the second Faraday rotator, where its polarization state is rotated 45° to the right before coupling into the second collimator. The polarization state of the laser beam output from the first collimator is rotated 90° to the right after passing through the two Faraday rotators and the electro-optic modulator. Another laser beam, output from the second collimator, propagates in the opposite direction. Its initial polarization state is orthogonal to that of the left incident beam. After passing through the second Faraday rotator, its polarization state is rotated 45° to the right before entering the electro-optic modulation crystal. At this point, the polarization state of the right incident laser is perpendicular to the electric field direction. Therefore, the phase delay after passing through the electro-optic crystal is independent of the electric field strength, thus introducing a phase difference between the two incident beams. After passing through the electro-optic crystal, the right incident laser passes through the first Faraday rotator and is rotated 45° to the right again before coupling into the first collimator. Its polarization state is parallel to the initial polarization state of the left incident beam. By changing the electric field strength, the phase difference between the laser beams in both directions can be continuously adjusted, assisting in initiating mode-locking. After mode-locking, the mode-locking state can be optimized by finely adjusting the electric field strength. Furthermore, this invention innovatively employs an active phase biaser to construct a fully polarization-maintaining fiber mode-locked laser. As a functional multiplexing device, by applying an external electric field to the phase biaser, continuous tuning and rapid phase tuning of the phase difference in both directions within the ring cavity of a figure-9 cavity mode-locked fiber laser are achieved, facilitating the realization of mode-locked state and high-precision phase-locked state. Furthermore, it simplifies the resonant cavity structure, which is beneficial for improving the repetition rate and system stability of the mode-locked laser. This technique of constructing a figure-9 cavity mode-locked fiber laser using an active phase biaser is original in the field of ultrafast optics.

[0028] Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make possible changes and modifications to the technical solutions of the present invention based on the above-disclosed technical content without departing from the spirit and scope of the present invention. Therefore, any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solutions of the present invention shall fall within the protection scope of the technical solutions of the present invention.

Claims

1. A fully polarization-maintaining fiber mode-locked laser, characterized in that, It includes a pump laser and a resonant cavity; the resonant cavity consists of two sub-resonant cavities, including a linear cavity and a ring cavity; wherein, the ring cavity consists of a wavelength division multiplexer, an optical fiber coupler, a phase biaser, a gain fiber, and a dispersion compensation fiber; the linear cavity consists of an optical fiber coupler and an optical fiber mirror; The pump laser emitted by the pump laser enters the ring cavity through a wavelength division multiplexer, causing the gain fiber to generate laser light. The laser light generated by the gain fiber enters the linear cavity through port 1 of the fiber coupler, and after being reflected by the fiber mirror, it is injected back into the fiber coupler through port 1. Then, it passes through ports 3 and 4 of the fiber coupler and is split into two laser beams that propagate in opposite directions according to the set optical power ratio before entering the ring cavity again. The phase biaser is installed in the ring cavity using fiber fusion splicing to compensate for the phase difference introduced by the two laser beams. The dispersion compensation fiber is installed in the ring cavity using fiber fusion splicing to control the net dispersion of the full polarization-maintaining fiber mode-locked laser to near zero, thereby maximizing the spectral width of the mode-locked pulse laser output from port 2 of the fiber coupler.

2. The fully polarization-maintaining fiber mode-locked laser according to claim 1, characterized in that, The phase bias is a bidirectional optical transmission device, including a first Faraday rotator, a second Faraday rotator, an electro-optic modulator, a first collimator, and a second collimator; After the forward-propagating laser beam is collimated by the first collimator, it is incident on the first Faraday rotator. The polarization state of the laser output from the first Faraday rotator is rotated 45° to the right and then incident on the electro-optic modulator for phase modulation. Subsequently, the laser output from the electro-optic modulator passes through the second Faraday rotator, and the laser polarization state is rotated 45° to the right again before being coupled into the second collimator. After the reverse-propagating laser beam is collimated by the second collimator, it is incident on the second Faraday rotator. The polarization state of the laser output from the second Faraday rotator is rotated 45° to the right and then incident on the electro-optic modulator for phase modulation. Subsequently, the laser output from the electro-optic modulator passes through the first Faraday rotator, and the laser polarization state is rotated 45° to the right again before being coupled into the first collimator.

3. The fully polarization-maintaining fiber mode-locked laser according to claim 2, characterized in that, Both the first and second collimators are laser collimation output ports, and both pigtails are polarization-maintaining fibers, and both operate on a slow axis.

4. A fully polarization-maintaining fiber mode-locked laser according to claim 2, characterized in that, The laser polarization states output by the first collimator and the second collimator are orthogonal to each other.

5. A fully polarization-maintaining fiber mode-locked laser according to claim 1, characterized in that, The electro-optic modulator is made of electro-optic crystal material, cut along the Z-axis, and the electro-optic crystal operates in phase modulation mode.

6. A fully polarization-maintaining fiber mode-locked laser according to claim 1, characterized in that, Continuous tuning of the incident laser phase delay is achieved by applying an external electric field and using a phase biaser.

7. A fully polarization-maintaining fiber mode-locked laser according to claim 2, characterized in that, The direction of the applied electric field is parallel to the polarization state of the laser output from the first collimator after passing through the first Faraday rotator, and perpendicular to the polarization state of the laser output from the second collimator after passing through the second Faraday rotator.

8. A fully polarization-maintaining fiber mode-locked laser according to any one of claims 1-7, characterized in that, The phase biaser, by changing the electric field intensity applied to the electro-optic modulator, enables continuous adjustment of the laser phase difference in both positive and negative directions and assists the laser in initiating mode-locking. After mode-locking of a fully polarization-maintaining fiber laser, the mode-locking state can be optimized by adjusting the intensity of the applied electric field.

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