A nonlinear polarization filtering method, device and application equipment

Abstract

The present invention relates to the field of optical signal processing technology, specifically disclosing a nonlinear polarization filtering method, apparatus, and application equipment. The apparatus comprises a pump source, a coupler, a birefringent medium, and several polarizers. The pump source is used to output a pump laser, causing the birefringent medium to produce a photoinduced birefringence effect. The polarizer is used to polarize the signal light according to a preset polarization angle, and the coupler is used to couple the pump laser and the signal light into the birefringent medium. The birefringent medium and the preset polarization angle of the polarizer are at an angle other than 0°. The technical solution of the present invention enables precise and continuous tuning of the modulation depth of the filter, as well as modification of the passband width and center wavelength.

Description

Technical Field

[0001] The present invention relates to the technical field of optical signal processing, and in particular to a nonlinear polarization filtering method, device and application equipment. Background Art

[0002] Optical filters are among the most common optical components, and their wavelength dependence makes them crucial for optical signal processing. Common optical filters typically take the form of lenses, typically absorptive or interferometric optical filters. However, these spatial filters typically have a single operating wavelength and passband width (bandwidth), making them difficult to tune.

[0003] A Lyot filter is a periodic filter device that relies on the linear birefringence of a crystal. It typically consists of two polarizers sandwiched between a birefringent crystal. While Lyot filters address some of the tuning difficulties of traditional optical filters, their tunability only refers to the shifting of their passband. Changing the passband width requires replacing the birefringent crystal, which is essentially the same as simply replacing the filter.

[0004] Therefore, the development of new filter devices that can be precisely and continuously tuned, with variable passband width (bandwidth) and center wavelength, is of great significance for achieving high-precision optical signal processing. Furthermore, applications based on such filters, particularly in ultrafast lasers, will significantly promote the realization and development of broadband tunable laser pulse oscillators. Summary of the Invention

[0005] One of the objectives of the present invention is to provide a nonlinear polarization filtering device that can achieve accurate and continuous tuning and modification of the filter passband width and center wavelength.

[0006] In order to solve the above technical problems, this application provides the following technical solutions:

[0007] A nonlinear polarization filtering device includes a pump source, a coupler, a birefringent medium, and a plurality of polarizers; the pump source is used to output a pump laser to cause the birefringent medium to produce a photoinduced birefringence effect; the polarizer is used to polarize signal light according to a preset polarization angle; and the coupler is used to couple the pump laser and signal light into the birefringent medium; wherein an angle other than 0° exists between the birefringent medium and the preset polarization angle of the polarizer.

[0008] The basic scheme principles and beneficial effects are as follows:

[0009] In current technology, optical filters are a common filtering device that relies on vacuum coating to alternately form high-refractive-index or low-refractive-index metal-dielectric-metal films, or all-dielectric films, with a certain thickness on a dielectric surface, forming a low-order, multi-stage cascade solid Fabry-Perot interferometer. The choice of film material, thickness, and cascade configuration determines the filter's operating center wavelength and transmission bandwidth, λ. This process is complex, expensive, and time-consuming to manufacture. In this solution, signal light is polarized by a polarizer, and pump laser light is output by controlling a pump source. The pump laser and signal light are then coupled into a birefringent medium via a coupler. The pump laser induces photoinduced birefringence in the birefringent medium, and nonlinear birefringence is used to offset or superimpose linear birefringence, thereby enabling continuous tuning and expansion of the overall birefringence of the birefringent medium, thereby changing the passband width and center wavelength.

[0010] Furthermore, there are two polarizers, the two polarizers, the coupler, and the birefringence medium are arranged on the same optical path, and the two polarizers are respectively located at two ends of the optical path; and the polarization angles of the two polarizers are consistent.

[0011] The nonlinear polarization filter device in this preferred embodiment has a single-pass structure. After the signal light passes through a polarizer, it is polarized according to a preset polarization angle. The pump source outputs the pump laser. The coupler couples the pump laser and the signal light into the birefringent medium. The birefringent medium produces a photoinduced birefringence effect. The signal light output by the birefringent medium passes through another polarizer and is output.

[0012] refer to Figure 1 The structure of the incident light (i.e., signal light), the light field, and the optical device can all be represented by the Jones matrix. Assume that the Jones matrix of the incident light is The Jones matrix of two polarizers (a specific form of polarizer) is: The Jones matrix of the intermediate birefringent medium (birefringent fiber) is in, Where B is the birefringence coefficient of the birefringent medium, including the linear birefringence coefficient and the nonlinear birefringence coefficient (B = B l +B nl ). L is the length of the birefringent medium, and λ is the wavelength of the incident light. In addition, since the birefringent medium has an angle θ with the incident light, the rotation matrix must also be considered. Then after the incident light passes through the filtering device described in this preferred solution, the Jones matrix expression of the output light is:

[0013]

[0014] Then the transmission curve expression of the filter device in the present invention is:

[0015]

[0016] The typical characteristics of this transfer function are as follows Figure 2 As shown. Obviously, the transmission function of the nonlinear polarization filter device in the present invention is modulated periodically with the wavelength. The nonlinear polarization filter device forms a multi-passband filtering effect on the incident light, and the passband period Δλ = λ 2 / BL, the modulation depth of the nonlinear polarization filter device is related to the angle θ. Therefore, by changing the birefringence coefficient B and the length L of the birefringent medium, the passband width of the nonlinear polarization filter device can be changed.

[0017] For a given birefringent medium B, B = B l +B nl Among them, the linear birefringence coefficient B l is a fixed value, the nonlinear birefringence coefficient B nl Related to the incident light intensity. For birefringent media, its birefringence coefficient B = n o -n e , n o and n e The refractive index coefficients corresponding to the optical axes of ordinary light o and extraordinary light e (for optical fiber media, they correspond to the fast and slow axes of the optical fiber respectively). For birefringent media:

[0018] n0=n 0l +Δn o

[0019] n e =n el +Δn e

[0020] Among them, n ol and n el are the intrinsic linear refractive indices of the birefringent media at different optical axes, Δn o and Δn e is the nonlinear birefringence coefficient caused by the pump light intensity

[0021] Δn o =2n2|E P | 2

[0022] Δn e =2n2b|E P | 2

[0023] |E P | 2 is the pump light intensity, n2 is the nonlinear refractive index coefficient of the birefringent medium. Usually the value of b is Then the nonlinear birefringence coefficient introduced by the pump light intensity is:

[0024]

[0025] n2 is the nonlinear refractive index coefficient of the birefringent medium. Taking quartz optical fiber as an example, the value of n2 is: (2.2~3.4)×10 -20 m 2 / w range. For optical fiber, its linear birefringence coefficient B l The value of (10 -6 ~10 -4 ) level. If the pump light intensity reaches 10 16 ~10 14 w / m 2 The magnitude of the nonlinear birefringence coefficient is comparable to the linear birefringence coefficient and cannot be ignored.

[0026] For ultrashort pulses transmitted in optical fiber (pulse width is in the order of picoseconds or even femtoseconds), when the peak power reaches 10kW, the pump intensity can reach 10 14 w / m 2 By adjusting the incident pump light intensity, the overall birefringence coefficient of the nonlinear polarization filter device of the present invention can be controlled. Furthermore, by adjusting the pump light intensity, the passband width and center position of the filter are continuously changed, thereby achieving control over the output laser spectrum bandwidth and center wavelength.

[0027] Furthermore, it further comprises a first reflector, and the number of the polarizer is one; the polarizer, the coupler, the birefringence medium and the first reflector are arranged on the same optical path;

[0028] The first reflector is used to return the signal light output by the birefringence medium to its original path.

[0029] The nonlinear polarization filtering device in this preferred embodiment has a round-trip structure. After the signal light passes through the polarizer, it is polarized according to a preset polarization angle. The pump source outputs the pump laser. The coupler couples the pump laser and the signal light into the birefringent medium. The birefringent medium produces a photoinduced birefringence effect. The signal light output by the birefringent medium is reflected back through the first reflector, and then passes through the birefringent medium, the coupler, and the polarizer again to return to the original path.

[0030] Furthermore, the included angle between the birefringent medium and the preset polarization angle of the polarizer is less than or equal to 45°.

[0031] The angle affects the modulation depth of the nonlinear polarization filter. At a 45° angle, the modulation depth is maximum, while at 0°, there is no modulation. By changing the angle, the modulation depth can be adjusted in real time.

[0032] Furthermore, the polarizer is a polarizer, an isolator, a polarization beam splitter or a fiber-coupled uniaxial working device; the polarized signal light is linearly polarized light;

[0033] The birefringent medium is a passive birefringent crystal, an active birefringent crystal, an active birefringent optical fiber or a passive birefringent optical fiber;

[0034] The pump source is a continuous laser light source or a pulsed laser light source; the pump source is used to output pump laser once or immediately;

[0035] The coupler is a spatial beam splitter, a fiber-optic wavelength division multiplexer, a fiber coupler or a combiner.

[0036] When the nonlinear polarization filtering device of the preferred embodiment is used, the pump light intensity is changed instantly, thereby instantly changing the output filtering effect, thereby achieving the purpose of instantly controllable filtering effect.

[0037] Furthermore, the pump source is used to output a pump laser, perform non-differential pumping on the ordinary light axis and the extraordinary light axis of the birefringent medium, so that the ordinary light axis and the extraordinary light axis obtain an uneven nonlinear birefringence change, or perform differential pumping on the ordinary light axis and the extraordinary light axis of the birefringent medium, and induce a nonlinear birefringence change based on the imbalance of the gain coefficients of the ordinary light axis and the extraordinary light axis.

[0038] By making the ordinary light axis and the extraordinary light axis obtain unbalanced nonlinear birefringence changes, a nonlinear birefringence coefficient that varies with light intensity can be introduced, thereby further changing the passband width of the nonlinear polarization filter device of the present invention.

[0039] A second object of the present invention is to provide a nonlinear polarization filtering method, comprising the following steps:

[0040] S1, output pump laser,

[0041] S2. polarizing the signal light according to a preset polarization angle; wherein an angle between the birefringent medium and the preset polarization angle is not 0°;

[0042] S3, coupling the pump laser and the signal light into the birefringent medium;

[0043] S4. The signal light output from the birefringent medium is polarized again according to a preset polarization angle and output; or the signal light output from the birefringent medium is returned along the original path.

[0044] In this scheme, nonlinear birefringence is used to offset or superimpose linear birefringence, thereby achieving continuous tuning and expansion of the overall birefringence of the birefringent medium. By introducing the nonlinear polarization filtering device of this scheme into the laser, a broadband tunable pulsed laser oscillator can also be realized.

[0045] Furthermore, in step S1, the intensity of the pump laser is adjusted according to the setting requirements of the passband width and the center position;

[0046] In step S2, the angle between the birefringent medium and the preset polarization angle is adjusted according to the setting requirement of the modulation depth; wherein the angle between the birefringent medium and the preset polarization angle of the polarizer is less than or equal to 45°.

[0047] Passband width Δλ=λ 2 / BL. Therefore, the passband width and center position can be changed by adjusting the length L of the birefringent medium and the birefringence index B of the medium. However, adjusting the length L of the birefringent medium means that the original birefringence medium needs to be replaced, which is too costly. For a given birefringent medium, its birefringence index B = B l +B nl , B l is the intrinsic linear birefringence coefficient of the medium, B nl is the light-induced nonlinear birefringence coefficient caused by the pump light intensity. Where n2 is the nonlinear refractive index coefficient of the medium, |E P | 2 It can be seen that adjusting the intensity of the incident pump laser can change the birefringence coefficient of the medium, and thus change the passband width and center position of the filter.

[0048] When the angle is 45°, the modulation depth is maximum, and when it is 0°, there is no modulation. The modulation depth can be adjusted in real time by changing the angle.

[0049] A third object of the present invention is to provide a nonlinear polarization filtering application device, which uses the above-mentioned nonlinear polarization filtering device and further includes an input coupler, a gain medium, a saturable absorber, an isolator, a first output coupler, and a pump laser source;

[0050] The pump laser source is used to output pump laser to make the gain medium produce photoinduced birefringence effect;

[0051] The input coupler is used to couple the pump laser and the laser and input the gain medium;

[0052] The gain medium is used to input the passed laser light into the nonlinear polarization filtering device;

[0053] The laser outputted by the nonlinear polarization filter device passes through the saturable absorber and the isolator, and is then outputted outwards through the first output coupler.

[0054] The nonlinear polarization filtering device can also be applied to a laser oscillator with a linear cavity structure, which can greatly improve the flexibility, controllability and tunability of the output parameters of the laser oscillator.

[0055] A nonlinear polarization filtering application device, using the above nonlinear polarization filtering device, further comprising a second reflector, an input coupler, a gain medium, a saturable absorber, a second output coupler, and a pump laser source;

[0056] The pump laser source is used to output pump laser to make the gain medium produce photoinduced birefringence effect;

[0057] The input coupler is used to couple the pump laser and the laser and input the gain medium;

[0058] The gain medium is used to input the passing laser into the nonlinear polarization filtering device;

[0059] The laser outputted by the nonlinear polarization filter device passes through a saturable absorber and is then inputted into a second output coupler;

[0060] The second output coupler reverses the laser light; the reversed laser light passes through the saturable absorber, inputs the nonlinear polarization filter device, the gain medium, the input coupler, and then inputs the second reflector;

[0061] The second reflector is used to reverse the laser light again, so that the laser light returns to its original path;

[0062] The second output coupler is also used to output the returned laser light outward. BRIEF DESCRIPTION OF THE DRAWINGS

[0063] Figure 1 Schematic diagram of a typical nonlinear polarization filtering device;

[0064] Figure 2 is a transmittance curve diagram of a typical nonlinear polarization filter device;

[0065] Figure 3 This is a schematic diagram of a nonlinear polarization filtering device with a single-pass structure according to Example 1;

[0066] Figure 4 Schematic diagram of the relationship between the transmittance curve and the pump laser wavelength in Example 1;

[0067] Figure 5 A schematic diagram of a nonlinear polarization filtering device with a round-trip structure according to Example 2;

[0068] Figure 6 Schematic diagram of the relationship between the transmittance curve and the pump laser wavelength in Example 2;

[0069] Figure 7 Schematic diagram of a ring cavity ultrafast laser pulse oscillator in Example 4;

[0070] Figure 8 Schematic diagram of a linear cavity ultrafast laser pulse oscillator in Example 5. DETAILED DESCRIPTION

[0071] The following is further described in detail through specific implementation methods:

[0072] The symbols in the drawings of the specification include: a first polarizer 1, an active birefringent crystal 2, a second polarizer 3, a pump source 4, a dichroic mirror 5, an angle fusion point 6, a uniaxial coupler 7, a wavelength division multiplexer 8, an active birefringent optical fiber 9, and a fiber reflector 10.

[0073] Example 1

[0074] like Figure 3 As shown, a nonlinear polarization filtering device of this embodiment includes a first polarizer 1, a birefringent medium, a second polarizer 3, a pump source 4, and a coupler. In this embodiment, the coupler used is a spatial beam splitter, specifically a dichroic mirror 5.

[0075] The first polarizer 1 , the dichroic mirror 5 , the birefringent medium, and the second polarizer 3 are sequentially arranged on the same optical path.

[0076] The first polarizer 1 is a polarizing plate, and its angle with the incident signal light is adjusted to make the polarized signal light linearly polarized. Specifically, it ensures that only vertically linearly polarized signal light can pass through. The second polarizer 3 is also a polarizing plate, and has the same parameters and is placed at the same angle as the first polarizer 1.

[0077] In this embodiment, the birefringent medium is an active birefringent crystal 2, specifically a neodymium-doped yttrium vanadate crystal, which is a typical active birefringent crystal. The optical axis of the o-ray is placed at an angle of 45° to the vertical direction, and its birefringence coefficient is 10 -4 In addition, since the yttrium vanadate crystal is doped with rare earth neodymium ions, it will generate spontaneous emission laser light in the 1064nm band after being excited by the pump laser. In other embodiments, a passive birefringent crystal can also be used.

[0078] The pump source 4 is preferably a spatially coupled output semiconductor diode (LD), which outputs a pump laser with a wavelength of 808 nm and a maximum output power of 200 W. After the pump laser is injected into the active birefringent crystal, it will stimulate it to output a spontaneously radiated laser at 1064 nm.

[0079] The dichroic mirror 5 is a 808 high reflection, 1064 anti-reflection coated lens. Figure 3 The arrangement shown is used to couple the 808nm pump laser and the 1064nm signal light into the active birefringent crystal.

[0080] After the pump laser output by the pump source 4 is incident on the active birefringent crystal, the pump laser causes its nonlinear birefringence coefficient to become prominent, which will offset or superimpose the intrinsic birefringence (linear birefringence) of the active birefringent crystal. By changing the power of the incident pump laser, the overall birefringence of the active birefringent crystal can be continuously tuned, thereby achieving accurate, continuous, and instantaneous tuning and expansion of the output bandwidth and center wavelength position of the nonlinear polarization filter device.

[0081] like Figure 4 As shown in the figure, when the pump laser power is 0, the passband width of the nonlinear polarization filter device of this embodiment is mainly determined by the medium length and its inherent linear birefringence, and the nonlinear birefringence has no effect, resulting in a passband width of approximately 3.2nm. When the pump laser power is P = 2000W, the nonlinear birefringence coefficient caused by the strong field pump laser becomes prominent, and the passband width becomes 2.5nm. Further increasing the pump laser power to P = 2P, the influence of the pump laser on the nonlinear birefringence coefficient becomes more prominent, and the passband width becomes 1.3nm. This confirms the feasibility of the solution of this embodiment.

[0082] It should be noted that this embodiment only takes the nonlinear polarization filtering device with a spatial structure as the preferred embodiment, and all changes in its form, such as changing to an all-fiber structure, a half-space half-fiber structure, etc., should fall within the scope of protection of the present invention.

[0083] Example 2

[0084] This embodiment provides a round-trip all-fiber nonlinear polarization filtering device, which includes, in order of connection, a polarizer, a pump source 4, a coupler, an angled fusion point 6, a birefringent medium, and a first reflector.

[0085] In this embodiment, the polarizer is a fiber-coupled, uniaxial device, specifically a uniaxial coupler 7. In other embodiments, an isolator or polarization beam splitter can also be used. The coupler is a fiber-optic wavelength division multiplexer 8. In other embodiments, a fiber coupler or combiner can also be used. The birefringent medium is an active birefringent fiber 9. The first reflector is a fiber mirror 10. In other embodiments, the birefringent medium can also be a passive birefringent fiber.

[0086] The uniaxial coupler 7, the wavelength division multiplexer 8, the angle fusion point 6, the active birefringent fiber 9 and the fiber reflector 10 are connected in sequence, and the output end of the pump source 4 is connected to the input end of the wavelength division multiplexer 8 to form the following structure: Figure 5 Fiber optic link shown.

[0087] The uniaxial coupler 7 is preferably a 2×2 polarization-maintaining fiber coupler, typically characterized by slow-axis operation and fast-axis isolation. When signal light enters the uniaxial coupler 7, only signal light traveling along the slow axis is allowed to pass. The uniaxial coupler 7 preferably has a splitting ratio of 50:50, which maximizes the modulation depth of the nonlinear polarization filter device formed in this embodiment. Increasing or decreasing the splitting ratio can continuously and controllably change the resulting modulation depth.

[0088] The pump source 4 is a fiber-coupled semiconductor LD, which can output a continuous laser light source or a pulsed laser light source with different peak powers (P), and its output end is connected to the wavelength division multiplexer 8.

[0089] The wavelength division multiplexer 8 is used to couple the pump laser output by the pump source 4 and the signal light output by the uniaxial coupler 7 into the active birefringent optical fiber 9 .

[0090] The active birefringent optical fiber 9 is a special optical fiber with a core doped with rare earth ions, such as ytterbium, neodymium, erbium, thulium, etc., which can generate spontaneous laser radiation when excited by the pump laser output by the pump source 4. Unlike birefringent crystals, the active birefringent optical fiber 9 has a fast and slow axis, and its birefringence coefficient B includes a linear birefringence coefficient B l and the nonlinear birefringence coefficient B nl , where B nl =4×10 -4 , length L = 0.8m.

[0091] The angled splice 6 is designed to ensure that the initial polarization state (slow axis) of the uniaxially operating coupler 7 forms a certain angle θ with the slow axis of the active birefringent fiber 9. This angle influences the modulation depth of the nonlinear polarization filter device formed in this embodiment. A 45° angle maximizes the modulation depth, while a 0° angle results in no modulation. Changing the splice angle allows for real-time adjustment of the modulation depth.

[0092] The fiber reflector 10 is a fiber-coupled reflector used to return the signal light along its original path and pass through the active birefringent fiber 9, the wavelength division multiplexer 8 and the uniaxial coupler 7 again.

[0093] For this embodiment, the transmission function of the nonlinear polarization filter device is:

[0094]

[0095] Where θ is the fiber splicing angle at the angle splicing point 6, and B is the birefringence coefficient of the active birefringent fiber 9: B = B l +B nl , where B l =4×10 -4, L is the length of the active birefringent optical fiber 9, λ is the wavelength of the signal light, which is selected as the 1030 nm band.

[0096] according to:

[0097]

[0098] Where n2 is the nonlinear refractive index coefficient of the active birefringent fiber 9, which is 2.5×10 -20 m 2 / w. |E P | 2 is the light intensity of the incident pump laser when it is transmitted in the optical fiber, |E P | 2 =P / πr 2 , P is the peak power of the pump laser, and r is the fiber core radius (r = 4 μm in this embodiment). Assuming P = 300 kW, then |E P | 2 ≈6×10 15 w / m 2 At this time, B nl =2×10 -4 .

[0099] The transmission function T of the nonlinear polarization filter device is further simplified as:

[0100]

[0101] Obviously, in this embodiment, the intensity of the transmitted light and the wavelength of the signal light change in a cosine function curve, as shown in FIG. Figure 6 As shown. When the fiber fusion angle θ at the angle fusion point 6 is 0, the nonlinear polarization filter device of this embodiment has basically no effect on the signal light, and the incident light returns along the original path. As the angle θ increases, the nonlinear polarization filter device begins to work, realizing comb filtering of the signal light. And the larger θ is, the greater the modulation depth of the filtering effect. When θ increases to 45°, the transmittance range covers 0 to 1, and the modulation depth reaches the maximum value. Continue to increase the angle θ, and the modulation depth decreases. When θ is fixed at 45°, when there is no pump laser incident, in the comb filter structure of the nonlinear polarization filter device, the comb tooth spacing Δλ≈1.6nm. When the pump laser with a peak power of P=3kW is injected, Δλ becomes Δλ≈1.1nm. This confirms the effect of introducing the nonlinear birefringence effect after the pump laser is injected into the nonlinear filter device of the present invention.

[0102] It should be noted that the structure and configuration described in this embodiment are merely conventional choices for the relevant devices of the present invention. Optimization and selection changes of the key components of this embodiment, such as replacing ordinary optical fibers with double-clad optical fibers or photonic crystal fibers, should fall within the scope of protection of the present invention even if they can bring advantages such as increased power, spectrum broadening, and mode-locked state switching.

[0103] Example 3

[0104] Based on the nonlinear polarization filtering devices of the first and second embodiments, this embodiment further provides a nonlinear polarization filtering method, comprising the following steps:

[0105] S1. Adjust the intensity of the pump laser according to the setting requirements of the passband width and center position; output the pump laser;

[0106] S2. According to the setting requirement of the modulation depth, adjusting the angle between the birefringent medium and the preset polarization angle; wherein the angle between the birefringent medium and the preset polarization angle of the polarizer is greater than 0° and less than or equal to 45°; polarizing the signal light according to the preset polarization angle;

[0107] S3, coupling the pump laser and the signal light into the birefringent medium;

[0108] S4. The signal light output from the birefringent medium is polarized again according to a preset polarization angle and output; or the signal light output from the birefringent medium is returned along the original path.

[0109] The order of the steps in this embodiment is not limited. For example, in other embodiments, step S2 may be performed first and then step S1, or step S1 and step S2 may be performed simultaneously.

[0110] Example 4

[0111] This embodiment introduces a nonlinear polarization filtering application device, specifically an oscillator implemented based on the nonlinear polarization filtering apparatus of the present invention. The oscillator can be a fully solid-state pulsed oscillator with a full-space structure, or a laser oscillator with a full-fiber or half-space / half-fiber structure. The structure shown in this embodiment is merely a preferred solution and is not intended to be limiting.

[0112] This embodiment is a ring cavity ultrafast laser pulse oscillator, comprising an input coupler, a gain medium, a nonlinear polarization filter, a saturable absorber, an isolator, a first output coupler, and a pump laser source. In this embodiment, the input coupler is specifically a wavelength division multiplexer.

[0113] Each device is as follows Figure 7 The relative position settings shown.

[0114] The wavelength division multiplexer is used to couple the laser and the pump laser into the gain medium to achieve amplification of the laser pulses. The wavelength division multiplexer can be a dichroic mirror or an optical fiber wavelength division multiplexer.

[0115] The gain medium is specifically a birefringent crystal or birefringent optical fiber medium doped with rare earth ions such as ytterbium, neodymium, erbium, and thulium, which outputs spontaneous emission laser of the corresponding wavelength band after being excited by the pump laser of the pump laser source.

[0116] The nonlinear polarization filter is the nonlinear polarization filter device described in the present invention.

[0117] The saturable absorber can be a Kerr lens mode-locking mechanism, or a physical or virtual saturable absorption mechanism, including a semiconductor saturable absorber mirror, graphene, carbon nanotubes, nonlinear polarization rotation, nonlinear amplifying ring mirror, etc.

[0118] The isolator is used to ensure the unidirectional circulation of the laser in the entire ring cavity ultrafast laser pulse oscillator.

[0119] The first output coupler is used to output the mode-locked pulse portion generated by the ring cavity ultrafast laser pulse oscillator to achieve application.

[0120] Example 5

[0121] This embodiment introduces a linear cavity ultrafast laser pulse oscillator implemented based on the nonlinear polarization filtering device of the present invention.

[0122] The linear cavity structure pulsed laser oscillator includes: a second reflector, an input coupler, a gain medium, a nonlinear polarization filter, a saturable absorber, a second output coupler, and a pump laser source. In this embodiment, the second reflector is specifically a reflector, and the input coupler is specifically a wavelength division multiplexer.

[0123] Each device is as follows Figure 8 The relative position settings shown.

[0124] Some of the components described in this embodiment have the same functions as those in the fourth embodiment, such as the wavelength division multiplexer, the gain medium, the nonlinear polarization filter, the pump laser source, and the saturable absorber.

[0125] The reflecting mirror is used to return the laser to its original path, thereby realizing back-and-forth oscillation of the laser.

[0126] The second output coupler is a semi-transparent and semi-reflective mirror. The reflective part realizes the return of the laser along the original path and forms two cavity mirrors of the linear cavity laser with the reflective mirror. The transmissive part is used for laser output to realize the application.

[0127] The above are only embodiments of the present invention. The invention is not limited to the fields involved in this implementation case. Common knowledge such as the known specific structures and characteristics in the scheme is not described in detail here. Ordinary technicians in the relevant field are aware of all common technical knowledge in the technical field to which the invention belongs before the application date or priority date, can obtain all existing technologies in the field, and have the ability to apply conventional experimental means before that date. Ordinary technicians in the relevant field can improve and implement this scheme in combination with their own abilities under the inspiration given by this application. Some typical known structures or known methods should not become obstacles for ordinary technicians in the relevant field to implement this application. It should be pointed out that for those skilled in the art, without departing from the structure of the present invention, several variations and improvements can be made, which should also be regarded as the scope of protection of the present invention. These will not affect the effect of the implementation of the present invention and the practicality of the patent. The scope of protection required by this application shall be based on the content of its claims, and the specific implementation methods and other records in the specification can be used to interpret the content of the claims.

Claims

1. A nonlinear polarization filter device, characterized in that: The invention comprises a pump source, a coupler, a birefringent medium and several polarizers; the pump source is used to output a pump laser to cause the birefringent medium to produce a photoinduced birefringence effect; the polarizer is used to polarize the signal light according to a preset polarization angle; the coupler is used to couple the pump laser and the signal light into the birefringent medium; wherein the birefringent medium and the preset polarization angle of the polarizer have an angle other than 0°; the angle between the birefringent medium and the preset polarization angle of the polarizer is less than or equal to 45°; and the modulation depth is adjusted in real time by changing the angle; There are two polarizers, and the two polarizers, the coupler, and the birefringent medium are arranged on the same optical path, and the two polarizers are respectively located at two ends of the optical path; the polarization angles of the two polarizers are consistent; Or further comprising a first reflector, the number of the polarizer is one; the polarizer, the coupler, the birefringence medium and the first reflector are arranged on the same optical path; The first reflector is used to return the signal light output by the birefringence medium to its original path.

2. The nonlinear polarization filter device according to claim 1, wherein: The polarizer is a polarizer, an isolator, a polarization beam splitter or a fiber-coupled uniaxial working device; the polarized signal light is linearly polarized light; The birefringent medium is a passive birefringent crystal, an active birefringent crystal, an active birefringent optical fiber or a passive birefringent optical fiber; The pump source is a continuous laser light source or a pulsed laser light source, and the pump source is used to output pump laser once or immediately; The coupler is a spatial beam splitter, a fiber-optic wavelength division multiplexer, a fiber coupler or a combiner.

3. The nonlinear polarization filter device according to claim 2, wherein: The pump source is used to output a pump laser, perform non-differential pumping on the ordinary light axis and the extraordinary light axis of the birefringent medium, so that the ordinary light axis and the extraordinary light axis obtain an unbalanced nonlinear birefringence change, or perform differential pumping on the ordinary light axis and the extraordinary light axis of the birefringent medium, and induce a nonlinear birefringence change based on the imbalance of the gain coefficients of the ordinary light axis and the extraordinary light axis.

4. A nonlinear polarization filtering method, characterized in that: The steps include: S1, output pump laser, S2. polarizing the signal light according to a preset polarization angle; wherein an angle between the birefringent medium and the preset polarization angle is not 0°; S3, coupling the pump laser and the signal light into the birefringent medium; S4, polarizing the signal light output from the birefringent medium again according to a preset polarization angle and outputting the signal light; or returning the signal light output from the birefringent medium to its original path; In step S1, the intensity of the pump laser is adjusted according to the setting requirements of the passband width and the center position; In step S2, the angle between the birefringent medium and the preset polarization angle is adjusted according to the setting requirement of the modulation depth; wherein the angle between the birefringent medium and the preset polarization angle of the polarizer is less than or equal to 45°.

5. A nonlinear polarization filtering application device, characterized in that: The nonlinear polarization filtering device according to claim 1 further comprises an input coupler, a gain medium, a saturable absorber, an isolator, a first output coupler, and a pump laser source; The pump laser source is used to output pump laser to make the gain medium produce photoinduced birefringence effect; The input coupler is used to couple the pump laser and the laser and input the gain medium; The gain medium is used to input the passing laser into the nonlinear polarization filtering device; The laser outputted by the nonlinear polarization filter device passes through the saturable absorber and the isolator, and is then outputted outwards through the first output coupler.

6. A nonlinear polarization filtering application device, characterized in that: The nonlinear polarization filtering device according to claim 1 further comprises a second reflector, an input coupler, a gain medium, a saturable absorber, a second output coupler, and a pump laser source; The pump laser source is used to output pump laser to make the gain medium produce photoinduced birefringence effect; The input coupler is used to couple the pump laser and the laser and input the gain medium; The gain medium is used to input the passing laser into the nonlinear polarization filtering device; The laser outputted by the nonlinear polarization filter device passes through a saturable absorber and is then inputted into a second output coupler; The second output coupler reverses the laser light; the reversed laser light passes through the saturable absorber, inputs the nonlinear polarization filter device, the gain medium, the input coupler, and then inputs the second reflector; The second reflector is used to reverse the laser light again, so that the laser light returns to its original path; The second output coupler is also used to output the returned laser light outward.

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