Double-comb laser

Abstract

The invention relates to a double-comb laser. According to the double-comb laser, a six-port device integrated based on a four-port circulator is connected to a part of two semiconductor saturable absorption mirrors in a non-common way; mode-locked pulses in two directions can be independently adjusted by adjusting the lengths of two sections of optical fibers between two ports of the six-port device integrated based on the four-port circulator and two semiconductor saturable absorption mirrors, and the six-port device integrated based on the four-port circulator supports to divide incident light into two beams of light with different polarization directions. One beam is directly output as a mode locking pulse, and the other beam participates in a mode locking process, so that full utilization of energy is realized, the use of coupling devices is reduced, the cavity structure is simplified, the cavity structure is more compact, and mode locking and structure compactness are both considered.

Description

Technical Field

[0001] The present application belongs to the field of laser design, and specifically relates to a dual-comb laser. Background Art

[0002] Optical frequency comb, referred to as optical frequency comb, is an effective light source that can be used for precision measurement, which appears as equally spaced comb lines in the frequency domain and as periodic pulse trains in the time domain. Compared with single-comb measurement technology, dual-comb technology can significantly reduce the requirements for bandwidth and response speed of signal measurement equipment, and expand the application of frequency comb technology. Dual-comb technology has shown great application potential in the fields of ranging, gas sensing, spectroscopy, etc.

[0003] Traditional dual-comb technology uses two independently operated optical frequency combs with slightly different repetition frequencies, and requires a feedback control system to keep the two combs coherent. The system is complex and costly. In recent years, single-cavity dual-comb technology has attracted the attention of more and more researchers due to its inherent coherence between the two combs, convenience, and low cost.

[0004] Mode locking is a major method for generating optical combs. Based on a certain mode locking mechanism, a constant phase difference is maintained between different longitudinal modes in the cavity to generate stable pulse output. Dual combs can be generated in a single optical fiber cavity by using wavelength multiplexing, polarization multiplexing, direction multiplexing, waveform multiplexing and other multiplexing methods in the cavity. Direction multiplexing uses the characteristic that pulses can oscillate bidirectionally in the optical fiber cavity to generate mode-locked pulses in both CW (Clockwise, clockwise) and CCW (Counter Clockwise, counterclockwise) directions.

[0005] In traditional research on dual-comb generation based on directional multiplexing, a fully common-path cavity structure is used. However, when two counter-propagating pulses meet at the saturated absorber, strong nonlinear interactions may occur, affecting mode locking. And because the counter-propagating pulses go through the same path, it is impossible to adjust one of the pulses separately, which lacks flexibility. Summary of the invention

[0006] Based on this, it is necessary to provide a dual-comb laser that can achieve both mode locking and compactness.

[0007] The present application provides a dual-comb laser, comprising:

[0008] Connect optical fiber;

[0009] A pump source, a wavelength division multiplexer, a gain medium, a six-port device based on a four-port circulator and a polarization controller connected by the optical fiber closed loop are used to form a laser cavity; wherein the six-port device based on the four-port circulator supports splitting the incident light into two beams of light with different polarization directions, one beam is directly output as a mode-locked pulse, and the other participates in the mode-locking process;

[0010] Two semiconductor saturable absorber mirrors are respectively connected to two ports of the six-port device based on the four-port circulator integration through optical fibers to support mode-locked pulse output in both clockwise and counterclockwise directions.

[0011] In one embodiment, the six-port device based on the four-port circulator integration includes:

[0012] A four-port circulator, connected to the polarization controller and the gain medium respectively through the connecting optical fiber;

[0013] Two polarization beam splitters are symmetrically placed on the connecting optical fiber relative to the four-port circulator, and the output ends of the two polarization beam splitters are used to output mode-locked pulses in clockwise and counterclockwise directions respectively.

[0014] In one embodiment, the output ends of the two polarization beam splitters both output the mode-locked pulses through single-mode optical fibers.

[0015] In one embodiment, the four-port circulator and the polarization beam splitter are integrated into one device.

[0016] In one embodiment, the semiconductor saturable absorber mirror is connected to two ports of the six-port device based on the four-port circulator integration through a polarization-maintaining optical fiber.

[0017] In one embodiment, the length of the polarization-maintaining optical fiber between two ports of the six-port device based on the four-port circulator integration and the semiconductor saturable absorber mirrors respectively connected thereto is adjustable.

[0018] In one embodiment, the polarization beam splitter is connected to the ports of the four-port circulator via polarization-maintaining optical fiber.

[0019] In one embodiment, the polarization controller adopts an extrusion structure.

[0020] In one embodiment, the gain medium is erbium-doped optical fiber.

[0021] In one embodiment, the pump source is a 980 nm light source.

[0022] The dual-comb laser provided in the embodiment of the present application is connected to a partial non-common path of two semiconductor saturable absorber mirrors from a six-port device based on a four-port circulator integration. By adjusting the lengths of two sections of optical fiber between the two ports of the six-port device based on the four-port circulator integration and the two semiconductor saturable absorber mirrors, the mode-locked pulses in two directions can be independently adjusted. The six-port device based on the four-port circulator integration supports splitting the incident light into two beams of light with different polarization directions, one beam is directly output as a mode-locked pulse, and the other participates in the mode-locking process, thereby achieving full energy utilization, reducing the use of coupling devices, simplifying the cavity structure, making the cavity structure more compact, and taking into account both mode locking and structural compactness. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the conventional technology, the drawings required for use in the embodiments or the conventional technology descriptions are briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0024] Figure 1 is a structural diagram of a dual-comb laser provided in one embodiment;

[0025] Figure 2 for Figure 1 Schematic diagram of the structure of the six-port device based on the integration of four-port circulators. DETAILED DESCRIPTION

[0026] In order to facilitate understanding of the present application, the present application will be described more fully below with reference to the relevant drawings. Embodiments of the present application are provided in the drawings. However, the present application can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the disclosure of the present application more thorough and comprehensive.

[0027] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which this application belongs. The terms used herein in the specification of this application are only for the purpose of describing specific embodiments and are not intended to limit this application.

[0028] When used herein, the singular forms "a", "an", and "said / the" may also include plural forms, unless the context clearly indicates otherwise. It should also be understood that the terms "include / comprise" or "have" and the like specify the presence of stated features, wholes, steps, operations, components, parts, or combinations thereof, but do not exclude the possibility of the presence or addition of one or more other features, wholes, steps, operations, components, parts, or combinations thereof. At the same time, in this specification, the term "and / or" includes any and all combinations of the relevant listed items.

[0029] The present application provides a dual-comb laser, comprising:

[0030] Connect optical fiber;

[0031] A pump source (Pump Source), a wavelength division multiplexer (WDM), a gain medium, a six-port device based on a four-port circulator integration and a polarization controller (Polarization Controller, PC) connected by the optical fiber closed loop are used to form a laser cavity; wherein the six-port device based on the four-port circulator integration supports splitting the incident light into two beams of light with different polarization directions, one beam is directly output as a mode-locked pulse, and the other participates in the mode-locking process;

[0032] Two semiconductor saturable absorber mirrors (SESAMs) are respectively connected to two ports of the six-port device based on the four-port circulator integration through optical fibers to support mode-locked pulse output in both clockwise and counterclockwise directions.

[0033] Among them, the connecting optical fiber is used to connect various parts to form a complete laser cavity.

[0034] The six-port device based on the four-port circulator integration refers to a device with two additional ports on the basis of the four-port circulator to form a six-port device. The six-port device supports polarization splitting of optical signals and the output of mode-locked pulses, so no additional coupler is required. A circulator is a non-reciprocal optical device that allows optical signals to be transmitted in one direction and blocks signal transmission in the opposite direction. A four-port circulator has four ports, which can usually be marked as 1, 2, 3 and 4. The six-port device based on the four-port circulator integration is responsible for guiding the optical signal to circulate in the laser cavity.

[0035] A mode-locked pulse is an extremely short laser pulse generated by mode-locking technology. By introducing a fixed phase relationship between different modes in the laser cavity, the interference between these modes causes the laser to produce a series of pulses. Depending on the properties of the laser, these pulses may have an extremely short duration, even up to the order of femtoseconds. Since ultrashort pulses have a certain bandwidth, dual-comb lasers used for short pulses (especially in the sub-picosecond region) require a gain medium with a large gain bandwidth, that is, the dual-comb laser provided in the embodiment of the present application can support the output of ultrashort pulses by introducing a gain medium. The polarization controller participates in the nonlinear polarization rotation mode-locking process.

[0036] Specifically, the dual-comb laser provided in the embodiment of the present application is connected to a partial non-common path of two semiconductor saturable absorber mirrors from a six-port device based on a four-port circulator integration. By adjusting the lengths of two sections of optical fiber between the two ports of the six-port device based on the four-port circulator integration and the two semiconductor saturable absorber mirrors, the mode-locked pulses in two directions can be independently adjusted. The six-port device based on the four-port circulator integration supports splitting the incident light into two beams of light with different polarization directions, one beam is directly output as a mode-locked pulse, and the other participates in the mode-locking process, thereby achieving full energy utilization (avoiding waste of optical power), reducing the use of coupling devices, simplifying the cavity structure, making the cavity structure more compact, and taking into account both mode locking and structural compactness.

[0037] In one embodiment, the six-port device based on the integration of a four-port circulator includes: a four-port circulator and a polarization beam splitter.

[0038] Among them, Figure 2 As shown, the four-port circulator is connected to the polarization controller and the gain medium respectively through the connecting optical fiber. Two polarization beam splitters are symmetrically placed on the connecting optical fiber relative to the four-port circulator, and the output ends of the two polarization beam splitters are used to output mode-locked pulses in clockwise and counterclockwise directions respectively.

[0039] Among them, the polarization beam splitter (PBS) is an optical device that separates optical signals according to polarization direction. Figure 2 As shown, two polarization beam splitters can be marked as PBS1 and PBS2, respectively, which are connected to different ports of the four-port circulator and split the optical signal into two light beams with different polarization directions.

[0040] Specifically, when an optical signal is transmitted in a laser cavity from one direction (for example, from polarization beam splitter PBS1 to polarization beam splitter PBS2), it is firstly split into two beams of light with different polarization directions by polarization beam splitter PBS1, one of which is used as a mode-locked output (for example, Figure 1The other beam of light enters port 1 of the four-port circulator through the optical fiber (as shown in Figure 2 as shown).

[0041] Inside the four-port circulator, the light entering from its port 1 is output from port 2 and transmitted to the semiconductor saturable absorber mirror (SESAM) through the optical fiber connected to port 2. SESAM is a key component for passive mode locking, which can reflect light signals of a specific intensity, thereby achieving mode-locked output of the laser.

[0042] The light beam reflected from the SESAM returns to the four-port circulator through port 2 again, then exits from the four-port circulator through port 3, and then passes through a section of optical fiber, and the light beam returns to the laser cavity from the polarization beam splitter PBS2.

[0043] The pulse in the other direction enters from the polarization beam splitter PBS2 and is also split into two beams with different polarization directions. One of the beams is used as the mode-locked output in that direction (for example, Figure 2 The other light beam enters port 3 of the four-port circulator and is emitted from port 4 through the inside. The light beam emitted from port 4 is transmitted to the SESAM connected to it through the optical fiber, and is reflected back to port 4 after being modulated by the SESAM reflection, and returns to the four-port circulator, and then is emitted from port 1 of the four-port circulator. After passing through a section of optical fiber, the light beam returns to the laser cavity from the polarization beam splitter PBS1 to complete the cycle.

[0044] The dual-comb laser provided in the example of this application realizes bidirectional mode-locked laser by adopting a mode-locking mechanism combining SESAM (semiconductor saturable absorber mirror) and NPR (nonlinear polarization rotation), combining passive mode-locking and active mode-locking. And the dual-comb laser under this mode-locking mechanism can produce a stable and uniformly distributed frequency comb.

[0045] In one embodiment, the output ends of the two polarization beam splitters both output the mode-locked pulses through single-mode optical fibers (SMF). The output ends of the polarization beam splitters output through single-mode optical fibers, based on the relatively stable transmission characteristics of single-mode optical fibers. Figure 2 For example, the output port of PBS1 and the output port of PBS2 are both connected to SMF. When the output port of PBS1 and the output port of PBS2 are connected to other optical fibers in the laser cavity during cavity formation, no additional polarization control element is required, thereby simplifying the system structure of the dual-comb laser.

[0046] In addition, the manufacturing cost of single-mode optical fiber is relatively low, which helps to reduce the cost of the entire dual-comb laser.

[0047] In one embodiment, the four-port circulator and the polarization beam splitter are integrated into one device. By integrating the PBS and the four-port circulator into one device, no additional coupler is needed to achieve output, and both beams separated by the PBS can be used, simplifying the structure of the dual-comb laser and improving the compactness of the structure.

[0048] In one embodiment, the semiconductor saturable absorber mirror is connected to two ports of the six-port device based on the four-port circulator integration through a polarization-maintaining fiber. Connecting the SESAM and the six-port device based on the four-port circulator integration through a polarization-maintaining fiber can ensure that the polarization state of the polarized light beam generated by the six-port device based on the four-port circulator integration is maintained, reducing the loss caused by the change of the polarization state, which helps to improve the efficiency and stability of the dual-comb laser.

[0049] In one embodiment, the length of the polarization-maintaining fiber between two ports of the six-port device based on the four-port circulator integration and the semiconductor saturable absorber mirrors respectively connected thereto is adjustable. By respectively adjusting the length of the polarization-maintaining fiber of the part where the two ports of the six-port device based on the four-port circulator integration are connected to the SESAM, independent adjustment of the bidirectional dual comb can be achieved.

[0050] In one embodiment, the polarization beam splitter is connected to the port of the four-port circulator through a polarization-maintaining fiber. The polarization beam splitter is connected to the port of the four-port circulator through a polarization-maintaining fiber, which can ensure that the polarized light separated by the polarization beam splitter maintains its polarization state, which helps to improve the efficiency and stability of the dual-comb laser.

[0051] In one embodiment, except for the polarization-maintaining fiber portion constrained in the above embodiment, the rest of the connecting optical fiber can adopt single-mode optical fiber. For example, the optical fiber between PBS1 and the polarization controller via the polarization controller PC, the pump source, the WDM, the gain medium to PBS2 adopts single-mode optical fiber.

[0052] In one embodiment, the polarization controller adopts an extrusion structure. Since the optical fiber in the laser cavity does not have a plastic protective jacket, the extrusion structure can more effectively adjust the polarization state of the light in the optical fiber and has a certain protective effect on the optical fiber in the active area.

[0053] In one embodiment, the gain medium is erbium-doped fiber (EDF).

[0054] In one embodiment, the pump source uses a 980nm light source. When the pump source uses a 980nm light source and the gain medium uses EDF, the 980nm pump source can excite erbium ions in the EDF, thereby generating effective light amplification and laser emission near 1550nm.

[0055] In one embodiment, the wavelength division multiplexer uses a 980nm / 1550nm wavelength division multiplexer, which is a three-port integrated device. The pump source enters from the 980nm port, and the other two ports are connected to the laser cavity.

[0056] It should be noted that in addition to supporting mode-locked pulses in the 1550nm band, according to actual needs, the output of mode-locked pulses in other optical bands can be achieved by adjusting the pump source, optical fiber and wavelength division multiplexer. For example, in the 1um band, dual-comb generation in the corresponding band can also be achieved.

[0057] In order to better illustrate the implementation process of the dual-comb laser provided in the present application, the generation of mode-locked pulses in the 1550nm band is taken as an example for illustration.

[0058] The pump source adopts a 980nm laser, which is connected to the laser cavity through a 980nm / 1550nm WDM. The gain medium adopts EDF. A section of EDF is connected to the laser cavity and connected to a port of the WDM. The 980nm pump source excites the erbium ions in the EDF to generate laser emission near 1550nm.

[0059] First, the propagation of CCW mode-locked pulses in the laser cavity is introduced. The other end of the EDF is connected to the 1550nm SMF, and the other end of the SMF is connected to the SMF connected to the PBS2 port of the six-port device based on the four-port circulator. After the light passes through PBS2, it will be divided into two beams of light with different polarization directions. One beam is used as the output of the CCW mode-locked pulse, and the other beam of light is transmitted through the polarization-maintaining fiber in the laser cavity, enters the four-port circulator through port 3 of the four-port circulator, rotates to the left along the four-port circulator, leaves the four-port circulator through port 4, and is transmitted along the polarization-maintaining fiber. The light is incident on the SESAM, and after the saturable absorption effect of the SESAM, a mode-locked pulse is generated. After passing through the reflector, the light returns to the four-port circulator from port 4 along the polarization-maintaining fiber again, and then rotates to port 1 to leave the four-port circulator. After that, it is transmitted to PBS1 through the polarization-maintaining fiber and returns to the fiber laser cavity to continue the cycle.

[0060] Next, the propagation of CW pulses is described. The other port of WDM is connected to an external optical fiber of 1550nm SMF, which can be directly connected to the SMF connected to PBS1. The direction of motion of the CW mode-locked pulse is opposite to that of the CCW direction. In the CW direction, the light passes through PBS1 and is divided into two beams with different polarization directions. One beam is used as the CW output, and the other beam is transmitted through the polarization-maintaining fiber and enters the four-port circulator through port 1, propagates rightward along the four-port circulator, leaves the four-port circulator from port 2, and is transmitted to the SESAM along the polarization-maintaining fiber. After that, it returns through the reflector, returns to the four-port circulator through port 2, and finally leaves the four-port circulator through port 3, enters PBS2 along the polarization-maintaining fiber, and returns to the cavity for the next cycle. A squeezed polarization controller (PC) is installed on the single-mode fiber in the laser cavity to adjust the polarization state of the pulse propagating in the fiber.

[0061] The dual-comb laser provided in the embodiment of the present application adopts a mode-locking mechanism combining NPR and SESAM, and through the cyclic propagation of reverse-propagating pulses in the cavity, stable mode-locked pulse outputs in the CW and CCW directions can be obtained at the output end.

[0062] In the description of this specification, the description with reference to the terms "some embodiments", "other embodiments", "ideal embodiments", etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic descriptions of the above terms do not necessarily refer to the same embodiment or example.

[0063] The technical features of the above-mentioned embodiments can be combined arbitrarily. In order to make the description concise, not all possible combinations of the technical features of the above-mentioned embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0064] The above-described embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be construed as limiting the scope of the patent application. It should be pointed out that, for a person of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the patent application shall be subject to the attached claims.

Claims

1. A dual-comb laser, characterized in that: include: Connect optical fiber; A pump source, a wavelength division multiplexer, a gain medium, a six-port device based on a four-port circulator and a polarization controller connected by the optical fiber closed loop are used to form a laser cavity; wherein the six-port device based on the four-port circulator supports splitting the incident light into two beams of light with different polarization directions, one beam is directly output as a mode-locked pulse, and the other participates in the mode-locking process; Two semiconductor saturable absorber mirrors are respectively connected to two ports of the six-port device based on the four-port circulator integration through two sections of optical fiber to support mode-locked pulse output in both clockwise and counterclockwise directions.

2. The dual-comb laser according to claim 1, characterized in that: The six-port device based on the four-port circulator integration includes: A four-port circulator, connected to the polarization controller and the gain medium respectively through the connecting optical fiber; Two polarization beam splitters are symmetrically placed on the connecting optical fiber relative to the four-port circulator, and the output ends of the two polarization beam splitters are used to output mode-locked pulses in clockwise and counterclockwise directions respectively.

3. The dual-comb laser according to claim 2, characterized in that: The output ends of the two polarization beam splitters both output the mode-locked pulses through single-mode optical fibers.

4. The dual-comb laser according to claim 2, characterized in that: The four-port circulator and the polarization beam splitter are integrated into one device.

5. The dual-comb laser according to claim 1, characterized in that: The semiconductor saturable absorber mirror is connected to two ports of the six-port device integrated based on the four-port circulator through a polarization-maintaining optical fiber.

6. The dual-comb laser according to claim 5, characterized in that The length of the polarization-maintaining optical fiber between two ports of the six-port device based on the four-port circulator integration and the semiconductor saturable absorber mirrors respectively connected thereto is adjustable.

7. The dual-comb laser according to claim 2, characterized in that: The polarization beam splitter is connected to the ports of the four-port circulator through polarization-maintaining optical fiber.

8. The dual-comb laser according to claim 1, characterized in that: The polarization controller adopts an extrusion structure.

9. The dual-comb laser according to claim 1, characterized in that: The gain medium is erbium-doped optical fiber.

10. The dual-comb laser according to claim 1, characterized in that The pump source adopts a 980nm light source.

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