3x3 coupler optimized self-starting o-band dogleg cavity mode-locked laser

Abstract

The application provides a 3*3 coupler optimized self-starting O-band eight-character cavity mode-locked laser, relates to the technical field of fiber communication and nonlinear optics, and comprises a pump light source, a wavelength division multiplexer, a bismuth-doped optical fiber, a 1*2 coupler, a 3*3 coupler, an isolator and a polarization controller; the wavelength division multiplexer, the bismuth-doped optical fiber, the 1*2 coupler and the polarization controller form a main ring, the isolator forms a mirror ring, and the main ring and the mirror ring are connected through the 3*3 coupler. The laser constructed by the application utilizes the 120-degree or 2pi / 3 phase shift provided by the 3*3 coupler, significantly reduces the self-starting laser threshold and improves the output power, has a higher power damage threshold compared with a conventional saturable absorbing material, and is a high-efficiency and stable solution provided for the technical field of optical communication and laser.

Description

Technical Field

[0001] This invention relates to the fields of optical fiber communication technology and nonlinear optics technology, and in particular to an O-band figure-eight cavity mode-locked laser with optimized self-starting 3×3 coupler. Background Technology

[0002] In the fields of optical communication and laser technology, nonlinear optical ring cavity mirrors, as a technique for realizing all-fiber mode-locked lasers, have received widespread attention from researchers since their initial proposal in 1988. Nonlinear optical ring cavity mirrors typically use a 2×2 coupler to connect the main ring and mirror ring. If the 2×2 coupler has a symmetrical splitting ratio (50:50), the nonlinear optical ring cavity mirror requires an attenuator to compensate for asymmetric losses, for example, by bending the fiber or inserting a tunable optical attenuator. This causes the optical transmittance of the nonlinear optical ring cavity mirror to depend on the attenuator's transmission and its position in the loop, thus requiring a higher mode-locking self-starting power threshold to balance the negative consequences of asymmetric losses.

[0003] In recent years, 3×3 couplers have been used as a novel optical device in nonlinear optical ring mirrors. The integration of 3×3 couplers with nonlinear optical ring mirrors is also being employed in current research to realize low-threshold mode-locked self-starting fiber lasers. 3×3 couplers possess an almost symmetrical power splitting ratio (approximately 33:33:33), meaning the transmission curve of a symmetrical 3×3 coupler has a phase shift of 120 degrees or 2π / 3. Compared to the 90-degree or π / 2 phase shift provided by a symmetrical 2×2 coupler, its transmission is not zero at a nonlinear phase difference of 0 rad. Because the 3×3 coupler has a certain transmission value at a nonlinear phase difference of 0 rad, this allows the pulse to be generated from noise; therefore, compared to a symmetrical 2×2 coupler, a symmetrical 3×3 coupler has a lower mode-locked self-starting power threshold.

[0004] However, although research papers on O-band mode-locked pulsed fiber lasers were published in 2013 and 2018, the mode-locking devices used to generate pulsed lasers are semiconductor saturable absorber mirrors or single-walled carbon nanotube saturable absorber materials. Both of these saturable absorber materials need to be embedded within the fiber laser cavity to form a mode-locked laser. In other words, because the laser cavity uses saturable absorber materials, it cannot form an all-fiber laser, and these materials are easily damaged under high power density focusing, which is detrimental to the fabrication of laser products.

[0005] Furthermore, although low-threshold mode-locked self-starting fiber lasers have been achieved in the 1-micron and 2-micron bands using 3×3 couplers integrated with nonlinear optical ring cavity mirrors, no related research demonstrations have been conducted in the widely used communication bands, particularly the O-band (approximately 1.31 microns) and C-band (approximately 1.55 microns). These two bands offer advantages such as low dispersion and low loss, making them suitable for inexpensive and long-distance (O-band) and long-distance (C-band) optical signal transmission. Therefore, applying 3×3 couplers integrated with nonlinear optical ring cavity mirrors to O-band and C-band mode-locked pulsed fiber lasers has significant research and application value. Summary of the Invention

[0006] To address this issue, this invention provides a 3×3 coupler-optimized self-starting O-band figure-eight cavity mode-locked laser, which solves the problem that in existing O-band mode-locked pulsed fiber lasers, semiconductor saturable absorber mirrors or single-walled carbon nanotube saturable absorber materials are typically used. This not only increases the complexity and cost of the laser system, but also limits high-power applications due to the fragility of the materials, failing to achieve an all-fiber structure and affecting the stability and reliability of the laser system.

[0007] To address the aforementioned issues, this invention provides a 3×3 coupler-optimized self-starting O-band figure-eight cavity mode-locked laser, comprising a pump source, a wavelength division multiplexer, a bismuth-doped fiber, a 1×2 coupler, a 3×3 coupler, an isolator, and a polarization controller.

[0008] The main ring is composed of the wavelength division multiplexer, bismuth-doped fiber, 1×2 coupler and polarization controller, and the mirror ring is composed of the isolator. The main ring and the mirror ring are connected by a 3×3 coupler.

[0009] The pump source is used to generate pump light, which provides energy to the bismuth-doped fiber in the laser cavity, causing population inversion between the upper and lower energy levels of the laser.

[0010] One end of the wavelength division multiplexer is connected to the pump light source and polarization controller, and the other end is connected to the bismuth-doped fiber. It is used to separate or combine the pump light and the laser signal, and to excite the gain and nonlinear effects in the bismuth-doped fiber.

[0011] One end of the bismuth-doped fiber is connected to a wavelength division multiplexer, and the other end is connected to a 1×2 coupler. It is used as a laser gain medium to generate population inversion by pump light and produce a laser signal.

[0012] One end of the 1×2 coupler is connected to a bismuth-doped fiber to receive the laser signal generated by the bismuth-doped fiber. Part of the laser signal is used to measure the output laser power and characteristics, and the other part of the laser signal is fed back to the main loop to form a closed loop and maintain laser oscillation.

[0013] One end of the 3×3 coupler is connected to the 1×2 coupler and the polarization controller, and the other end is connected to the isolator. It is used to connect the main loop and the mirror loop, allowing the laser signal to propagate in the two loops and perform coherent interference.

[0014] Both ends of the isolator are connected to 3×3 couplers to ensure that the laser signal propagates unidirectionally in the reflection loop of the ring mirror, reducing interference and instability caused by the reverse propagation of the laser signal.

[0015] The polarization controller is connected to a 3×3 coupler at one end and a wavelength division multiplexer at the other end. It is used to adjust the polarization state of the back-propagating beam to ensure that the two back-propagating laser signals have appropriate polarization matching during nonlinear interference.

[0016] Preferably, the pump light source is a 1240 nm semiconductor laser diode.

[0017] Preferably, the maximum power of the pump light source is 561 milliwatts.

[0018] Preferably, the wavelength division multiplexer is a 1240 / 1310 nm wavelength division multiplexer.

[0019] Preferably, the bismuth-doped optical fiber is a single-mode bismuth-doped phosphosilicate silica optical fiber.

[0020] Preferably, the bismuth-doped optical fiber is 40 meters long.

[0021] Preferably, the splitting ratio of the 1×2 coupler is 80 to 20.

[0022] Preferably, the 1×2 coupler is used to receive the laser signal generated by the bismuth-doped fiber, wherein 20% of the laser signal is used to measure the output laser power and characteristics, and 80% of the laser signal is fed back to the main loop to form a closed loop and maintain laser oscillation.

[0023] Preferably, the splitting ratio of the 3×3 coupler is 33:33:33.

[0024] Preferably, the main ring and the mirror ring are connected by a 3×3 coupler in a figure-eight shape.

[0025] As can be seen from the above technical solutions, this invention application has the following beneficial effects:

[0026] This invention provides a 3×3 coupler-optimized self-starting O-band figure-eight cavity mode-locked laser. Compared to saturable absorbers, mode-locked (ultrafast pulsed fiber) lasers fabricated entirely in fiber optics possess a higher power damage threshold. This is because the power damage threshold of fiber optics is higher than that of saturable absorbers when light is transmitted solely through it, thus resulting in a higher damage threshold for mode-locked (ultrafast pulsed fiber) lasers. This invention integrates a nonlinear optical ring cavity mirror using a 3×3 coupler in the practical O-band of optical communication and optimizes the self-starting laser threshold of the fabricated O-band figure-eight cavity mode-locked laser. The 3×3 coupler provides a 120-degree or 2π / 3 phase shift, which reduces the self-starting threshold of the mode-locked laser by 45% and doubles the output power. Attached Figure Description

[0027] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the embodiments will be briefly described below. Referring to the accompanying drawings will provide a clearer understanding of the features and advantages of the present invention. The drawings are illustrative and should not be construed as limiting the present invention in any way. For those skilled in the art, other drawings can be obtained based on these drawings without any creative effort. Wherein:

[0028] Figure 1 This is a schematic diagram of a 3×3 coupler-optimized self-starting O-band figure-eight cavity mode-locked laser provided in the embodiment;

[0029] Figure 2 This is a schematic diagram of a figure-eight cavity mode-locked laser integrated with a 2×2 coupler provided in the embodiment;

[0030] Figure 3 The data for the figure-eight cavity mode-locked laser integrated with a 2×2 coupler in the embodiment are shown, where (a) is the spectrum observed at the highest pump power and (b) is the power development curve of the laser output.

[0031] Figure 4 The data for the O-band figure-eight cavity mode-locked laser with 3×3 coupler optimization and self-starting are shown in the embodiment. (a), (b), and (c) are the spectra of the output terminals (output-1, output-2, and output-3), respectively; (d), (e), and (f) are the power development curves of the laser output at the output terminals (output-1, output-2, and output-3), respectively; and (g), (h), and (i) are the pulse array diagrams of the output terminals (output-1, output-2, and output-3), respectively. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0033] Example 1

[0034] like Figure 1 As shown, this embodiment of the invention proposes a 3×3 coupler-optimized self-starting O-band figure-eight cavity mode-locked laser, including a pump source, a wavelength division multiplexer, a bismuth-doped fiber, a 1×2 coupler, a 3×3 coupler, an isolator, and a polarization controller.

[0035] The main ring is composed of the wavelength division multiplexer, bismuth-doped fiber, 1×2 coupler and polarization controller, and the mirror ring is composed of the isolator. The main ring and the mirror ring are connected by a 3×3 coupler.

[0036] The pump source is used to generate pump light, which provides energy to the bismuth-doped fiber in the laser cavity, causing population inversion between the upper and lower energy levels of the laser.

[0037] One end of the wavelength division multiplexer is connected to the pump light source and polarization controller, and the other end is connected to the bismuth-doped fiber. It is used to separate or combine the pump light and the laser signal, and to excite the gain and nonlinear effects in the bismuth-doped fiber.

[0038] One end of the bismuth-doped fiber is connected to a wavelength division multiplexer, and the other end is connected to a 1×2 coupler. It is used as a laser gain medium to generate population inversion by pump light and produce a laser signal.

[0039] One end of the 1×2 coupler is connected to a bismuth-doped fiber to receive the laser signal generated by the bismuth-doped fiber. Part of the laser signal is used to measure the output laser power and characteristics, and the other part of the laser signal is fed back to the main loop to form a closed loop and maintain laser oscillation.

[0040] One end of the 3×3 coupler is connected to the 1×2 coupler and the polarization controller, and the other end is connected to the isolator. It is used to connect the main loop and the mirror loop, allowing the laser signal to propagate in the two loops and perform coherent interference.

[0041] Both ends of the isolator are connected to 3×3 couplers to ensure that the laser signal propagates unidirectionally in the reflection loop of the ring mirror, reducing interference and instability caused by the reverse propagation of the laser signal.

[0042] The polarization controller is connected to a 3×3 coupler at one end and a wavelength division multiplexer at the other end. It is used to adjust the polarization state of the back-propagating beam to ensure that the two back-propagating laser signals have appropriate polarization matching during nonlinear interference.

[0043] As can be seen from the above technical solution, this invention provides a 3×3 coupler-optimized self-starting O-band figure-eight cavity mode-locked laser. By integrating key instruments and optical components such as a wavelength division multiplexer, bismuth-doped fiber, 1×2 coupler, 3×3 coupler, isolator, and polarization controller, it achieves efficient mode-locked laser output in the O-band of optical communication. This laser utilizes the 120-degree or 2π / 3 phase shift provided by the 3×3 coupler to significantly reduce the self-starting laser threshold and increase the output power. Compared to traditional saturable absorber materials, the all-fiber laser has a higher power impairment threshold, providing an efficient and stable solution for the fields of optical communication and laser technology.

[0044] Figure 1 A 3×3 coupler-optimized self-starting O-band figure-eight cavity mode-locked laser is demonstrated. This laser comprises a main ring and a mirror ring in a figure-eight shape. The main ring consists of a wavelength division multiplexer, bismuth-doped fiber, a 1×2 coupler (coupler-1), and a polarization controller. The mirror ring is formed by isolators and connected via a 3×3 coupler (coupler-2). This allows coherent interference between the backpropagating laser signals of loop-1 and loop-2. The mirror ring is connected to loop-4 via loop-3. In the reflection loop of the mirror ring, the isolator ensures unidirectional propagation of the laser signal. Output-1 is obtained through one end of the 1×2 coupler (coupler-1), output-3 through loop-5, and output-2 through loop-6.

[0045] The pump source is a 1240 nm semiconductor laser diode with a maximum power of 561 mW, which is used to generate pump light to provide energy to the bismuth-doped fiber in the laser cavity, thereby causing population inversion between the upper and lower energy levels of the laser.

[0046] One end of the wavelength division multiplexer is connected to the pump light source and polarization controller, and the other end is connected to the bismuth-doped fiber. It is used to separate or combine the pump light and laser signal, and to excite the gain nonlinearity effect in the bismuth-doped fiber. In this embodiment, the wavelength division multiplexer is a 1240 / 1310 nm wavelength division multiplexer.

[0047] One end of the bismuth-doped fiber is connected to a wavelength division multiplexer, and the other end is connected to a 1×2 coupler (coupler-1). This fiber serves as a laser gain medium, generating population inversion through pump light excitation and producing a laser signal. In this embodiment, the bismuth-doped fiber is a single-mode bismuth-doped phosphosilicate silica fiber, and its length is 40 meters.

[0048] Specifically, in this embodiment, a 40-meter-long single-mode bismuth-doped phosphosilicate silica fiber is used to provide asymmetric and sufficient nonlinear effects at the highest pump power through a 1240 / 1310 nm wavelength division multiplexer to realize a stimulated emission ultrafast pulsed mode-locked laser.

[0049] One end of the 1×2 coupler (coupler-1) is connected to the bismuth-doped fiber to receive the laser signal generated by the bismuth-doped fiber. A portion of the laser signal is used to measure the output laser power and characteristics, while the other portion is fed back to the main loop to form a closed loop and maintain laser oscillation. In this embodiment, the splitting ratio of the 1×2 coupler (coupler-1) is 80 to 20.

[0050] Specifically, a 1×2 coupler (coupler-1) is used to receive the laser signal generated by the bismuth-doped fiber, of which 20% of the laser signal is used to measure the output laser power and characteristics, and 80% of the laser signal is fed back to the main loop to form a closed loop and maintain laser oscillation.

[0051] One end of the 3×3 coupler (coupler-2) is connected to the 1×2 coupler (coupler-1) and the polarization controller, while the other end is connected to the isolator. This connects the main loop and the mirror loop, allowing the laser signal to propagate and coherently interfere in both loops, resulting in a higher nonlinear phase shift, i.e., a 2π / 3 or 120-degree phase shift. In this embodiment, the splitting ratio of the 3×3 coupler is approximately 33:33:33.

[0052] Both ends of the isolator are connected to a 3×3 coupler (coupler-2) to ensure that the laser signal propagates unidirectionally in the reflection loop of the ring mirror, reducing interference and instability caused by the reverse propagation of the laser signal.

[0053] One end of the polarization controller is connected to a 3×3 coupler (coupler-2), and the other end is connected to a wavelength division multiplexer. It is used to adjust the polarization state of the back-propagating beam to ensure that the two back-propagating laser signals have proper polarization matching during nonlinear interference.

[0054] Figure 2A figure-eight cavity mode-locked laser integrated with a 2×2 coupler is demonstrated. This laser comprises a main ring and a mirror ring, connected via a 2×2 coupler (coupler-4) with a splitting ratio of approximately 50:50. The laser signal propagates in opposite directions within the laser, producing a π / 2 or 90-degree nonlinear phase shift. The pump source is a 1240 nm semiconductor laser diode with a maximum power of 561 mW. This embodiment utilizes approximately 40 meters of single-mode bismuth-doped phosphosilicate silica fiber, delivered via a 1240 / 1310 nm wavelength division multiplexer, to provide asymmetric and sufficient nonlinear effects at maximum pump power to achieve stimulated emission pulsed laser. A polarization controller is used to adjust the polarization state of the backpropagating beam, while the 1×2 coupler (coupler-3) has a splitting ratio of 70:30. 30% of the laser signal (output-1) is used to measure the output laser power and characteristics, while the remaining 70% is fed back to the main ring, forming a closed loop. The main ring and mirror ring of the laser are integrated via a 2×2 coupler (coupler-4), allowing coherent interference between the back-propagating laser signals of loop-1 and loop-2. The mirror ring is connected to loop-4 via loop-3. In the reflection loop of the ring mirror, an isolator ensures unidirectional propagation of the laser signal.

[0055] To further compare the laser performance of the figure-eight cavity mode-locked laser integrated with 2×2 and 3×3 couplers, the following explanation is based on specific experiments.

[0056] Experimental apparatus:

[0057] The experiment used a fiber optic fusion splicer (model: Fujikura 87C+) to splice all single-mode fibers, including all optical components and bismuth-doped fibers. The loss at each fiber fusion splice was less than 0.02 dB.

[0058] The spectrum of this invention was measured using a near-infrared spectrometer (model: Yokogawa AQ6370D), with the center wavelength and full width at half maximum (FWHM) measured using the spectrometer's built-in settings. The power development curve of the laser output was measured using a power amplifier (model: Solvay PM100D, probe model: Solvay S122C).

[0059] The laser output measurement points (output-1, output-2, and output-3) use APC type fiber optic patch cords. This is because, compared to using traditional UPC type fiber optic patch cords, APC type fiber optic patch cords can reduce Fresnel reflections and lower losses.

[0060] Figure 3 (a) shows spectral data measured from a 2×2 coupler-integrated figure-eight cavity mode-locked laser at a pump power of 300 mW. The spectrum is shown as the output of the guided continuous laser observed at a center wavelength of approximately 1326.75 nm, with a spectral half-width of approximately 0.03 nm. Figure 3 Figure (b) shows the power development curve of the laser output. The start-up pump power threshold of the continuous laser is approximately 37 mW, and its maximum output power at the highest pump power is approximately 3.16 mW, thus deriving a laser slope of approximately 1.19%. Even under the highest pump power (561 mW), the figure-eight cavity mode-locked laser integrated with the 2×2 coupler (coupler-4) did not reach the self-start-up power threshold of the mode-locked laser, therefore it is inferred that this laser requires a pump power higher than 561 mW to achieve self-start-up operation of the mode-locked laser.

[0061] Figure 4 Tables (a), (b), and (c) show spectral data measured from a 3×3 coupler-integrated figure-eight cavity mode-locked laser (a 3×3 coupler-optimized self-starting O-band figure-eight cavity mode-locked laser) at a pump power of 300 mW. Compared to Figure 3 (a) Figure 4 The spectra measured in (a), (b), and (c) have broader half-widths (FWHMs), typically ranging from several to tens of nanometers, but in this embodiment, approximately 5 to 13 nanometers are observed, and the Kelly lateral bands represent the generation of mode-locked lasers. Kelly lateral bands (e.g.) Figure 4 (b) and (c) show that the laser is operating in soliton mode-locked mode, confirming the generation of mode-locked laser. Figure 4 The Kelly sideband in (a) is suppressed due to the different polarization states, but still represents a broadened mode-locked laser form. The laser center wavelengths measured at all outputs remain stable at approximately 1340 nm, while the largest half-width at half-maximum (FWHM) (13.05 nm) is obtained from output-1, followed by output-3 (6.52 nm) and output-2 (4.91 nm). Figure 4 Figures (d), (e), and (f) show the power development curves of the laser output. A low pump power threshold (36.9 mW) is observed at output-2, while outputs-1 and-3 both show a pump power threshold of 57.5 mW for continuous laser output. At a pump power of 300 mW, all outputs (output-1, output-2, and output-3) show a mode-locked self-starting laser output threshold, and the laser output power development curves exhibit a clear linear trend. Stable mode-locked laser output is still observed even when the pump power is reduced to approximately 120 mW. Under mode-locked pulsed laser operation, the laser slopes at outputs-1, output-2, and output-3 are approximately 1.02%, 0.51%, and 1.24%, respectively. At a pump power of 300 mW, the maximum pulse energies of the mode-locked laser measured at outputs-1, output-2, and output-3 are approximately 0.63 nanojoules, 0.44 nanojoules, and 0.79 nanojoules, respectively. Figure 4(g), (h), and (i) show pulse arrays of a 3×3 coupler-integrated figure-eight cavity mode-locked laser, tested separately by an oscilloscope at outputs -1, -2, and -3 at a pump power of 300 mW. The oscilloscope shows that the repetition frequency of the mode-locked pulses remains consistent at approximately 3.5 MHz.

[0062] Table 1 compares the laser parameters of figure-eight cavity mode-locked lasers with 2×2 couplers and 3×3 integrated couplers. Even under the highest pump power, the figure-eight cavity mode-locked laser with 2×2 couplers cannot achieve self-starting mode-locked laser operation. In contrast, the figure-eight cavity mode-locked laser with 3×3 couplers requires at least 45% less pump power threshold for self-starting mode-locked laser operation. Furthermore, at a pump power of 300 mW, the total output power of the figure-eight cavity mode-locked laser with 3×3 couplers is 6.47 mW, which is 2.05 times that of the figure-eight cavity mode-locked laser with 2×2 couplers.

[0063] Table 1. Comparison of laser parameters for figure-eight cavity mode-locked lasers with 2×2 couplers and 3×3 integrated couplers.

[0064]

[0065] Note that the above description is merely a preferred embodiment of the present invention and the technical principles employed. Those skilled in the art will understand that the present invention is not limited to the specific embodiments described herein, and various obvious changes, readjustments, and substitutions can be made without departing from the scope of protection of the present invention. Therefore, although the present invention has been described in detail through the above embodiments, the present invention is not limited to the above embodiments, and may include many other equivalent embodiments without departing from the concept of the present invention, the scope of which is determined by the scope of the appended claims.

Claims

1. A 3x3 coupler optimized self-starting O-band dogleg cavity mode-locked laser, characterized in that, The pump light source, the wavelength division multiplexer, the bismuth-doped fiber, the 1x2 coupler, the 3x3 coupler, the isolator, and the polarization controller are included. The wavelength division multiplexer, the bismuth-doped fiber, the 1x2 coupler, and the polarization controller form a main ring, and the isolator forms a mirror ring, and the main ring and the mirror ring are connected through the 3x3 coupler. The pump light source is used to generate pump light, and provides energy for the bismuth-doped fiber in the laser cavity, so that particle number inversion is generated between the upper and lower energy levels of the laser. One end of the wavelength division multiplexer is connected with the pump light source and the polarization controller, and the other end is connected with the bismuth-doped fiber, which is used to separate or combine the pump light and the laser signal, and excite the gain and nonlinear effect in the bismuth-doped fiber. One end of the bismuth-doped fiber is connected with the wavelength division multiplexer, and the other end is connected with the 1x2 coupler, which is used as a laser gain medium, and generates particle number inversion and laser signal through pump light excitation. One end of the 1x2 coupler is connected with the bismuth-doped fiber, which is used to receive the laser signal generated by the bismuth-doped fiber, and part of the laser signal is used to measure the output laser power and characteristics, and the other part of the laser signal is fed back to the main ring to form a closed loop and maintain laser oscillation. One end of the 3x3 coupler is connected with the 1x2 coupler and the polarization controller, and the other end is connected with the isolator, which is used to connect the main ring and the mirror ring, and allow the laser signal to propagate in the two loops and interfere coherently. Both ends of the isolator are connected with the 3x3 coupler, which is used to ensure that the laser signal propagates unidirectionally in the reflection loop of the mirror ring, and reduces the interference and instability caused by the reverse propagation of the laser signal. One end of the polarization controller is connected with the 3x3 coupler, and the other end is connected with the wavelength division multiplexer, which is used to adjust the polarization state of the reverse propagation beam, and ensure that the two reverse propagation laser signals have appropriate polarization matching when nonlinearly interfering.

2. The 3x3 coupler optimized self-starting O-band dogleg cavity mode-locked laser of claim 1, wherein, The pump light source is a 1240 nanometer semiconductor laser diode.

3. The 3x3 coupler optimized self-starting O-band dogleg cavity mode-locked laser of claim 1, wherein, The maximum power of the pump light source is 561 milliwatts.

4. The 3x3 coupler optimized self-starting O-band dogleg cavity mode-locked laser of claim 1, wherein, The wavelength division multiplexer is a 1240 / 1310 nanometer wavelength division multiplexer.

5. The 3x3 coupler optimized self-starting O-band dogleg cavity mode-locked laser of claim 1, wherein, The bismuth-doped fiber is a single-mode bismuth-doped phosphosilicate quartz optical fiber.

6. The 3x3 coupler optimized self-starting O-band dogleg cavity mode-locked laser of claim 1, wherein, The length of the bismuth-doped fiber is 40 meters.

7. The 3x3 coupler optimized self-starting O-band dogleg cavity mode-locked laser of claim 1, wherein, The splitting ratio of the 1x2 coupler is 80 to 20.

8. The 3x3 coupler optimized self-starting O-band dogleg cavity mode-locked laser of claim 7, wherein, The 1x2 coupler is used to receive the laser signal generated by the bismuth-doped fiber, and 20% of the laser signal is used to measure the output laser power and characteristics, and 80% of the laser signal is fed back to the main ring to form a closed loop and maintain laser oscillation.

9. The 3x3 coupler optimized self-starting O-band dogleg cavity mode-locked laser of claim 1, wherein, The splitting ratio of the 3x3 coupler is 33 to 33 to 33.

10. The 3x3 coupler optimized self-starting O-band dogleg cavity mode-locked laser of claim 1, wherein, The main ring and the mirror ring are connected through the 3x3 coupler, and are in the shape of an eight.

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