A direction-polarization multiplexed single-cavity dual-comb laser

Abstract

The present application relates to a dual-comb laser, and in particular to a direction-polarization multiplexing single-cavity dual-comb laser, which solves the technical problems of the prior art that pulse components in different polarization directions are difficult to separate, leading to high difficulty in dual-polarization mode locking, harsh implementation conditions, or difficulty in adjusting the pulse repetition frequency difference, and the numerical value is small, or the mutual coherence of the output pulse sequence is low, and the signal-to-noise ratio is low. The present application adopts a full polarization maintaining optical fiber resonant cavity with a polarization beam splitter as a central node and a double-linked ring cavity structure composed of ring structure A and ring structure B. The laser transmitted in the counterclockwise direction in the ring structure A is cut off, and the laser transmitted in the clockwise direction is separated by the polarization beam splitter and transmitted in the opposite direction in the ring structure B, partially output, and partially returned to the ring structure A, thereby realizing cyclic oscillation.

Description

TECHNICAL FIELD

[0001] The present application relates to dual-comb lasers, in particular to a direction-polarization multiplexed single-cavity dual-comb laser. BACKGROUND

[0002] Optical frequency comb (referred to as optical comb for short) is a kind of broadband light source composed of a large number of discrete frequency teeth with strictly equal frequency intervals. Due to its extremely high optical frequency measurement accuracy and resolution, the emergence of optical comb has brought unprecedented development to high-resolution spectroscopy, and in the past decade, many new methods of spectral analysis have been produced. Compared with other precise spectral techniques related to optical comb, dual-comb spectral measurement technology has attracted widespread attention due to its ability to fully utilize the high frequency resolution and high accuracy of the comb tooth structure, as well as its strong performance, simple and easy-to-implement structure.

[0003] The core of dual-comb spectral measurement technology lies in the dual-comb light source, and the performance of the light source directly determines the key indicators such as the accuracy, speed, measurement range, and signal-to-noise ratio of the spectral measurement. At present, the methods for generating dual-comb light sources mainly include: phase locking two independent mode-locked lasers, electro-optical modulation of a single continuous light source at different frequencies, and generation of two Kerr combs in a micro-ring resonant cavity. However, the system using two mode-locked lasers for phase locking is relatively complex; the method of using a continuous light source for electro-optical modulation produces fewer comb teeth; the fabrication process of the micro-ring resonant cavity is complex, and the generated optical combs have low mutual coherence and insufficient stability.

[0004] In addition to the above three schemes, a method of generating two groups of pulse sequences with different repetition frequencies in the same mode-locked laser can also be used to construct a dual-comb system, which is called a single-cavity dual-comb. The single-cavity dual-comb inherits the advantages of mode-locked laser comb output quality and high stability, and can effectively avoid the defects of small spectral operable range of electro-optical modulation comb and poor stability of Kerr comb. At the same time, since the two optical combs come from the same laser, common-mode noise is suppressed; and the generated dual-combs have natural intrinsic mutual coherence, so a complex phase locking system is not needed.

[0005] Without adding an isolator in the cavity to make the mode-locked laser operate bidirectionally, or introducing strong birefringence effect into the cavity to cause group velocity mismatch of the two orthogonal polarization components of the pulse, two groups of pulse sequences with slightly different repetition frequencies can be generated, and a single-cavity dual-comb light source can be constructed using these principles.

[0006] Kieu K, Mansuripur M. All-fiber bidirectional passively mode-locked ring laser [J]. Optics letters, 2008, 33(1): 64-66 discloses an all-fiber bidirectional passively mode-locked ring laser. Since there is no fiber isolator in the laser, the laser operates in both clockwise and counterclockwise directions simultaneously. Due to the different orders of the fiber and fiber devices, the modulation effects on the laser in the two directions are slightly different, so two groups of pulse sequences with different repetition frequencies are generated. However, the repetition frequency difference of the bidirectional mode-locked pulses obtained by this method depends on the inherent properties of the device, so it is difficult to control the repetition frequency difference, and the numerical value is small.

[0007] Saito S, Yamanaka M, Sakakibara Y, et al. All-polarization-maintaining Er-doped dual comb fiber laser using single-wall carbon nanotubes [J]. Optics express, 2019, 27(13): 17868-17875 discloses that two carbon nanotube saturable absorbers are respectively arranged in the non-common paths of the two arms of the cavity to realize bidirectional pulse mode-locking. Chinese patent CN 111812909A discloses that two groups of pulse sequences with different polarization directions are separated by a polarization beam splitter, and then the length of the delay fiber in the non-common path of one group is adjusted to control the pulse repetition frequency of the path, so as to obtain the repetition frequency difference of the pulse sequences. Since there are non-common paths in these two methods, common mode noise is brought, thereby reducing the mutual coherence of the bidirectional output pulses.

[0008] Zhao X, Li T, Liu Y, et al. Polarization-multiplexed, dual-comb all-fiber mode-locked laser [J]. Photonics Research, 2018, 6(9): 853-857 discloses that a strong birefringence effect is introduced by inserting a piece of polarization maintaining fiber into the cavity of an erbium-doped mode-locked laser to force the pulses to separate. Uyama K, Shirahata T, Jin L, et al. All-PM dual-comb fiber ring laser using CNT-SA [C] / / 2020 Conference on Lasers and Electro-Optics (CLEO). IEEE, 2020: 1-2 discloses that the polarization-dependent loss between the two orthogonal polarization modes is reduced by 90° splicing two pieces of polarization maintaining fiber with different lengths, and the residual polarization-dependent loss is compensated by appropriately bending a piece of polarization maintaining fiber. However, both methods make it difficult for the pulses in the two orthogonal polarization directions to separate, resulting in high difficulty in dual-polarization mode-locking and harsh conditions for implementation. SUMMARY

[0009] The purpose of the present application is to solve the technical problems of the prior art that the pulse components in different polarization directions are difficult to separate, resulting in high difficulty in dual-polarization mode-locking and harsh conditions for implementation, or the pulse repetition frequency difference is difficult to adjust and has a small value, or the mutual coherence of the output pulse sequence is low and the signal-to-noise ratio is low, and to provide a direction-polarization multiplexing single-cavity dual-comb laser.

[0010] To achieve the above purpose, the technical scheme adopted by the present application is:

[0011] A direction-polarization multiplexing single-cavity dual-comb laser, comprising a full polarization maintaining fiber resonant cavity 11, a pump source 1 providing a light source for the full polarization maintaining fiber resonant cavity 11, and a first output end 9 and a second output end 10 connected to the output port of the full polarization maintaining fiber resonant cavity 11, characterized in that:

[0012] The full polarization maintaining fiber resonant cavity 11 comprises a wavelength division multiplexer 2, a polarization maintaining doped optical fiber 3, a polarization beam splitter 4, an optical coupler 5, a delay optical fiber 6, a polarization-independent optical isolator 7, and a transmission type saturable absorber 8 connected by polarization maintaining fibers, forming a double-linked cavity structure with the polarization beam splitter 4 as the center node and composed of a ring structure A and a ring structure B;

[0013] The polarization beam splitter 4 comprises a polarization beam splitter first port 41, a polarization beam splitter second port 42, a polarization beam splitter third port 43, and a polarization beam splitter fourth port 44;

[0014] The optical coupler 5 comprises an optical coupler first port 51, an optical coupler second port 52, an optical coupler third port 53 and an optical coupler fourth port 54;

[0015] The output end of the pump source 1 is connected with the pump end 21 of the wavelength division multiplexer 2, and the optical coupler third port 53 and the optical coupler fourth port 54 are respectively connected with the first output end head 9 and the second output end head 10 through polarization maintaining optical fibers;

[0016] The wavelength division multiplexer output end 23, the polarization maintaining doped optical fiber 3 and the polarization beam splitter first port 41 are sequentially connected through polarization maintaining optical fibers, and the polarization beam splitter second port 42, the polarization independent optical isolator 7, the transmission type saturable absorber 8 and the input end 22 of the wavelength division multiplexer 2 are sequentially connected through polarization maintaining optical fibers, forming a ring structure A; the laser transmitted in the counterclockwise direction in the ring structure A is cut off by the polarization independent optical isolator 7, and the laser transmitted in the clockwise direction reaches the polarization beam splitter 4 and is separated according to the different polarization directions;

[0017] The polarization beam splitter third port 43 and the optical coupler first port 51, the optical coupler second port 52 and the delay optical fiber 6, and the delay optical fiber 6 and the polarization beam splitter fourth port 44 are all connected through polarization maintaining optical fibers, forming a ring structure B; the laser separated by the polarization beam splitter 4 according to the different polarization directions is transmitted in opposite directions in the ring structure B, a part of which is output from the first output end head 9 and the second output end head 10 through the optical coupler third port 53 and the optical coupler fourth port 54 to form two pulse sequences, and the other part returns to the ring structure A to realize cyclic oscillation, forming two groups of stable pulse sequences.

[0018] Further, the length of the polarization maintaining optical fiber in the full polarization maintaining optical fiber resonant cavity 11 is adjustable.

[0019] Further, the polarization beam splitter 4 is a 2x2 polarization beam splitter.

[0020] The optical coupler 5 is a 2x2 optical coupler.

[0021] Further, the splitting ratio of the optical coupler 5 is 50:50.

[0022] Further, the polarization maintaining doped optical fiber 3 is a polarization maintaining erbium-doped optical fiber.

[0023] Further, the pump source 1 is a single-mode fiber-coupled semiconductor laser, and the center wavelength is the same as the pump absorption peak of the polarization maintaining doped optical fiber 3.

[0024] Further, the center wavelength of the polarization beam splitter 4, the optical coupler 5, the polarization-independent optical isolator 7 and the transmissive saturable absorber 8 is the same as the working wavelength of the wavelength division multiplexer 2.

[0025] Further, the first output end 9 and the second output end 10 adopt FC-APC or FC-PC joints.

[0026] Compared with the prior art, the present application has the following beneficial technical effects:

[0027] 1. The laser provided by the present application adopts a full polarization-maintaining optical fiber resonant cavity, and all the devices in the cavity are connected by polarization-maintaining optical fibers. During the operation of the laser, the vertical polarization light and the horizontal polarization light are transmitted on the fast axis and the slow axis of the polarization-maintaining optical fiber respectively, so that the output pulse sequence has a certain repetition frequency difference. At the same time, since the transmission paths of the two polarization components in the cavity are the same, there is no non-common path, so the two output pulse sequences have good mutual coherence. In addition, the full polarization-maintaining structure can prevent the confusion of the pulse polarization state, and the reverse transmission in the ring structure B can effectively avoid the recoupling of the two polarization components.

[0028] 2. The laser provided by the present application can adjust the repetition frequency difference of the output pulse sequence by adjusting the length of the polarization-maintaining optical fiber between any devices in the full polarization-maintaining optical fiber resonant cavity. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 It is a structure schematic view of an embodiment of the direction-polarization multiplexing single-cavity dual-comb laser of the present application;

[0030] Figure 2 It is a schematic view of the transmission of light beams in different directions and polarization states in the 2x2 polarization beam splitter in the embodiment of the present application;

[0031] The following is a description of the reference signs:

[0032] 1-pump source, 3-polarization-maintaining doped optical fiber, 6-delay optical fiber, 7-polarization-independent optical isolator, 8-transmissive saturable absorber, 9-first output end, 10-second output end, 11-full polarization-maintaining optical fiber resonant cavity;

[0033] 2-wavelength division multiplexer, 21-pump end, 22-input end, 23-output end;

[0034] 4-polarization beam splitter, 41-first port of the polarization beam splitter, 42-second port of the polarization beam splitter, 43-third port of the polarization beam splitter, 44-fourth port of the polarization beam splitter;

[0035] 5-optical coupler, 51-first port of the optical coupler, 52-second port of the optical coupler, 53-third port of the optical coupler, 54-fourth port of the optical coupler. DETAILED DESCRIPTION

[0036] A direction-polarization multiplexing single-cavity dual-comb laser according to the present application is described in detail below in conjunction with the accompanying drawings and specific embodiments. Those skilled in the art should understand that these embodiments are only used to explain the technical principles of the present application, and are not intended to limit the protection scope of the present application.

[0037] A direction-polarization multiplexing single-cavity dual-comb laser, as shown in Figure 1 includes a pump source 1, a full-polarization fiber cavity 11, and a first output end 9 and a second output end 10.

[0038] The pump source 1 is a single-mode fiber-coupled semiconductor laser with a center wavelength of 980 nm. The semiconductor laser serves as the light source of the dual-comb laser provided in the embodiment, and is connected to the full-polarization fiber cavity 11. The first output end 9 and the second output end 10 are FC-APC or FC-PC connectors, which are connected to the output ports of the full-polarization fiber cavity 11.

[0039] The full-polarization fiber cavity 11 includes a wavelength division multiplexer 2, a polarization doped fiber 3, a polarization beam splitter 4, an optical coupler 5, a delay fiber 6, a polarization independent optical isolator 7, and a transmission type saturable absorber 8 connected by polarization maintaining fibers. All devices in the full-polarization fiber cavity 11 support two orthogonal polarization modes.

[0040] The wavelength division multiplexer 2 is used to couple the laser from the semiconductor laser into the full-polarization fiber cavity 11, and has a working wavelength of 980 / 1550 nm, including a pump end 21, an input end 22, and an output end 23.

[0041] The polarization beam splitter 4 includes a polarization beam splitter first port 41, a polarization beam splitter second port 42, a polarization beam splitter third port 43, and a polarization beam splitter fourth port 44. The polarization beam splitter 4 is a 2x2 polarization beam splitter with a center wavelength of 1550 nm. The transmission of light beams of different directions and different polarization states in the polarization beam splitter 4 is as shown in Figure 2 The vertically polarized light incident from the polarization beam splitter first port 41 will be output from the polarization beam splitter second port 42, and the horizontally polarized light will be output from the polarization beam splitter fourth port 44; the vertically polarized light incident from the polarization beam splitter second port 42 will be output from the polarization beam splitter fourth port 44, and the horizontally polarized light will be output from the polarization beam splitter third port 43; the light path when incident from the polarization beam splitter third port 43 and the polarization beam splitter fourth port 44 is opposite to the above path.

[0042] The optical coupler 5 comprises an optical coupler first port 51, an optical coupler second port 52, an optical coupler third port 53 and an optical coupler fourth port 54. The optical coupler 5 is a 2x2 optical coupler with a center wavelength of 1550 nm and a splitting ratio of 50:50, and is used to pass the laser from the polarization beam splitter third port 43 and the delay fiber 6 and output part of the laser from the first output end 9 and the second output end 10.

[0043] The output end of the pump source 1 semiconductor laser is connected with the pump end 21 of the wavelength division multiplexer 2, and the optical coupler third port 53 and the optical coupler fourth port 54 are respectively connected with the first output end 9 and the second output end 10 through polarization maintaining optical fibers.

[0044] The full polarization maintaining fiber resonator 11 of the dual optical comb laser provided in the embodiment comprises two ring structures, forming a double connected ring cavity structure with the polarization beam splitter 4 as the center node. The wavelength division multiplexer output end 23, the polarization maintaining doped optical fiber 3 and the polarization beam splitter first port 41 are sequentially connected through polarization maintaining optical fibers, the polarization beam splitter second port 42, the polarization independent optical isolator 7 and the transmission type saturable absorber 8 and the input end 22 of the wavelength division multiplexer 2 are sequentially connected through polarization maintaining optical fibers, forming a ring structure A. The polarization beam splitter third port 43 and the optical coupler first port 51, the optical coupler second port 52 and the delay fiber 6, and the delay fiber 6 and the polarization beam splitter fourth port 44 are all connected through polarization maintaining optical fibers, forming a ring structure B.

[0045] Among them, the polarization maintaining doped optical fiber 3 is the gain medium for the laser to circulate in the full polarization maintaining fiber resonator 11, and the polarization maintaining doped optical fiber 3 is selected as a polarization maintaining doped erbium optical fiber in the embodiment, so that the center wavelength of the pump source 1 is the same as the pump absorption peak of the polarization maintaining doped optical fiber 3. The delay fiber 6 is used to adjust the repetition frequency difference of the dual polarization pulse. The polarization independent optical isolator 7 is used to suppress the counterclockwise direction of the laser in the ring structure A, forming a clockwise unidirectional path; the transmission type saturable absorber 8, as a mode locking device in the full polarization maintaining fiber resonator 11, realizes periodic modulation of loss by interacting with the transmission optical field in the process of circulating the laser in the full polarization maintaining fiber resonator 11, absorbs part of the low intensity light, passes part of the high intensity light, and continuously narrows the pulse to realize mode locking. The center wavelength of the polarization independent optical isolator 7 and the transmission type saturable absorber 8 is the same as the working wavelength of the wavelength division multiplexer 2, which is 1550 nm.

[0046] When the semiconductor laser is turned on, the laser transmitted in the anticlockwise direction in the ring structure A is cut off by the polarization-independent optical isolator 7; the laser transmitted in the clockwise direction is separated by the polarization beam splitter 4 according to the different polarization directions, is transmitted in the opposite direction in the ring structure B, and a part of the laser is output from the first output end 9 and the second output end 10 through the third port 53 and the fourth port 54 of the optical coupler respectively, and another part of the laser returns to the ring structure A to realize the circulation oscillation, and gradually forms two groups of stable pulse sequences.

[0047] The detailed working process is as follows:

[0048] The optical signal emitted by the pump source 1 and the optical signal transmitted from the polarization-independent optical isolator 7 through the transmission type saturable absorber 8 are coupled through the wavelength division multiplexer 2, and then are amplified through the polarization maintaining doped optical fiber 3 to reach the polarization beam splitter 4 to be adjusted as vertical polarization light and horizontal polarization light; the vertical polarization light is transmitted from the third port 43 of the polarization beam splitter to the first port 51 of the optical coupler, a part of the vertical polarization light is output from the second output end 10 through the fourth port 54 of the optical coupler, and another part of the vertical polarization light is transmitted to the fourth port 44 of the polarization beam splitter through the second port 52 of the optical coupler through the delay optical fiber 6; the horizontal polarization light is transmitted from the fourth port 44 of the polarization beam splitter to the second port 52 of the optical coupler through the delay optical fiber 6, a part of the horizontal polarization light is output from the first output end 9 through the third port 53 of the optical coupler, and another part of the horizontal polarization light is transmitted to the third port 43 of the polarization beam splitter through the first port 51 of the optical coupler; the part of the horizontal polarization light and the part of the vertical polarization light received by the third port 43 of the polarization beam splitter and the fourth port 44 of the polarization beam splitter are transmitted from the second port 42 of the polarization beam splitter to the polarization-independent optical isolator 7, are intensity-modulated by the transmission type saturable absorber 8, and then return to the wavelength division multiplexer 2 to be coupled with the optical signal emitted by the pump source 1. After circulation, two groups of stable pulse sequences are gradually formed and output.

[0049] The specific discussion about the circulation of the laser with different transmission directions and different polarization states in the full polarization maintaining optical fiber resonator 11 is as follows:

[0050] (1) The light beam transmitted from the wavelength division multiplexer 2 to the transmission type saturable absorber 8, that is, the light beam transmitted in the anticlockwise direction in the ring structure A, is greatly attenuated when passing through the polarization-independent optical isolator 7, and cannot realize the circulation in the full polarization maintaining optical fiber resonator 11;

[0051] (2) The vertical polarization light from the polarization beam splitter 4 through the polarization beam splitter third port 43 to the optical coupler 5, the optical coupler 5 outputs a part of the light through the second output end 10, the remaining part transmits to the polarization beam splitter fourth port 44 after passing through the delay fiber 6, the light transmits from the polarization beam splitter second port 42 after passing through the polarization beam splitter 4 for the second time, and returns to the wavelength division multiplexer 2 through the polarization independent optical isolator 7 and the transmissive saturable absorber 8, completing a circulation in the cavity;

[0052] (3) The horizontal polarization light from the polarization beam splitter fourth port 44 transmits to the optical coupler 5 after passing through the delay fiber 6, the optical coupler 5 outputs a part of the light through the first output end 9, the remaining part transmits to the polarization beam splitter third port 43, the light transmits from the polarization beam splitter second port 42 after passing through the polarization beam splitter 4 for the second time, and returns to the wavelength division multiplexer 2 through the polarization independent optical isolator 7 and the transmissive saturable absorber 8, completing a circulation in the cavity.

[0053] From the above process, it can be seen that the pulse sequences in two polarization directions circulating in the full polarization fiber cavity 11 will transmit in opposite directions in the ring structure B, and the vertical polarization component is finally output by the second output end 10, and the horizontal polarization component is finally output by the first output end 9.

[0054] During the operation of the laser, the vertical polarization component and the horizontal polarization component transmit on the fast axis and the slow axis of the polarization maintaining fiber respectively, so that the pulse sequences output from the first output end 9 and the second output end 10 have a certain repetition frequency difference, which can be adjusted by adjusting the length of the polarization maintaining fiber connected between any devices in the full polarization fiber cavity 11. Since the transmission paths of the two polarization components in the full polarization fiber cavity 11 are the same, there is no non-common path, so the two pulse sequences output have good coherence. In addition, the full polarization structure can prevent the confusion of the polarization state of the pulse, and the reverse transmission in the ring structure B can effectively avoid the recoupling of the two polarization components.

Claims

1. A direction-polarization multiplexed single-cavity dual-comb laser, comprising a full-polarization-maintaining fiber resonant cavity (11), a pump source (1) providing incident laser for the full-polarization-maintaining fiber resonant cavity (11), and a first output end head (9) and a second output end head (10) connected with an output port of the full-polarization-maintaining fiber resonant cavity (11), characterized in that: the full-polarization-maintaining fiber resonant cavity (11) comprises a wavelength division multiplexer (2), a polarization-maintaining doped optical fiber (3), a polarization beam splitter (4), an optical coupler (5), a delay optical fiber (6), a polarization-independent optical isolator (7), and a transmission-type saturable absorber (8) connected by polarization-maintaining optical fibers, forming a double-linked cavity structure composed of ring structure A and ring structure B with the polarization beam splitter (4) as the central node; the polarization beam splitter (4) comprises a polarization beam splitter first port (41), a polarization beam splitter second port (42), a polarization beam splitter third port (43), and a polarization beam splitter fourth port (44); the optical coupler (5) comprises an optical coupler first port (51), an optical coupler second port (52), an optical coupler third port (53), and an optical coupler fourth port (54); the output end of the pump source (1) is connected with a pump end (21) of the wavelength division multiplexer (2), and the optical coupler third port (53) and the optical coupler fourth port (54) are connected with the first output end head (9) and the second output end head (10) respectively through polarization-maintaining optical fibers; the output end (23) of the wavelength division multiplexer (2), the polarization-maintaining doped optical fiber (3), and the polarization beam splitter first port (41) are sequentially connected by polarization-maintaining optical fibers, and the polarization beam splitter second port (42), the polarization-independent optical isolator (7), the transmission-type saturable absorber (8), and the input end (22) of the wavelength division multiplexer (2) are sequentially connected by polarization-maintaining optical fibers, forming ring structure A; the laser transmitted in the counterclockwise direction in the ring structure A is cut off by the polarization-independent optical isolator (7), and the laser transmitted in the clockwise direction is separated by the polarization beam splitter (4) according to the different polarization directions after reaching the polarization beam splitter (4); the polarization beam splitter third port (43) and the optical coupler first port (51), the optical coupler second port (52) and the delay optical fiber (6), and the delay optical fiber (6) and the polarization beam splitter fourth port (44) are connected by polarization-maintaining optical fibers, forming ring structure B; the laser separated by the polarization beam splitter (4) according to the different polarization directions is transmitted in opposite directions in the ring structure B, a part of which is output from the first output end head (9) and the second output end head (10) as two pulse sequences through the optical coupler third port (53) and the optical coupler fourth port (54), and the other part returns to the ring structure A, thereby realizing cyclic oscillation and forming two groups of stable pulse sequences. The length of the polarization-maintaining optical fiber in the full-polarization-maintaining fiber resonant cavity (11) is adjustable. The polarization beam splitter (4) is a 2×2 polarization beam splitter. The optical coupler (5) is a 2×2 optical coupler. The splitting ratio of the optical coupler (5) is 50:

50. ​ ​ 2. The direction-polarization multiplexed single-cavity dual-comb laser of claim 1, wherein: ​ 3. The direction-polarization multiplexed single-cavity dual comb laser of claim 2, wherein: ​ ​ 4. The direction-polarization multiplexed single-cavity dual comb laser of claim 3, wherein: ​ 5. The direction-polarization multiplexed single-cavity dual-comb laser of claim 4, wherein: The polarization maintaining doped optical fiber (3) is a polarization maintaining doped erbium optical fiber.

6. The direction-polarization multiplexed single-cavity dual-comb laser of claim 5, wherein: The pump source (1) is a single-mode fiber-coupled semiconductor laser, whose central wavelength is the same as the pump absorption peak of the polarization maintaining doped optical fiber (3).

7. The direction-polarization multiplexed single-cavity dual-comb laser of claim 6, wherein: The central wavelength of the polarization beam splitter (4), the optical coupler (5), the polarization independent optical isolator (7) and the transmissive saturable absorber (8) is the same as the working wavelength of the wavelength division multiplexer (2).

8. The direction-polarization multiplexed single-cavity dual-comb laser of claim 7, wherein: The first output end (9) and the second output end (10) adopt FC-APC or FC-PC joints.

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