Optical isolator based on nonlinear absorption and self-phase modulation

Abstract

The invention discloses an optical isolator based on nonlinear absorption and self-phase modulation, which sequentially comprises a first aperture diaphragm, a first lens, a nonlinear medium, a second lens and a second aperture diaphragm along the forward propagation direction of an optical path, and is characterized in that the focal length of the first lens is greater than that of the second lens; the focus of the first lens coincides with the focus of the second lens, and the nonlinear medium is arranged between the coincided focus and the second lens. The embodiment of the invention can reduce the insertion loss, improves the isolation of the reverse optical signal, is simple in structure, and can be widely applied to the technical field of optical devices.

Description

Technical Field

[0001] The invention relates to the technical field of optical devices, and in particular to an optical isolator based on nonlinear absorption and self-phase modulation. Background Art

[0002] Optical isolators can eliminate the various disadvantages that reflected light and scattered light bring to the optical path system, effectively improve the stability of optical power output, and are key components in high-quality optical path systems. The key to preparing optical isolators is to achieve non-reciprocal transmission of light. Traditional optical isolators generally use magneto-optical materials to achieve non-reciprocal light transmission, but the strong magnetic field required for the operation of the device can easily cause sensitive components such as semiconductors in the integrated optical path to fail to work properly, resulting in instability of the entire integrated optical system, and it has a strong dependence on the polarization of light. In addition, there are a variety of non-magnetic schemes for implementing optical isolators, among which the nonlinear effect scheme is based on nonlinear optics, and uses the nonlinear effect between the medium and the light beam to achieve non-reciprocal transmission of light. The existing technology for achieving optical isolation through nonlinear effects has the following problems: there is a high attenuation of the forward incident signal, resulting in a high insertion loss, and the system structure is often very complex. Summary of the invention

[0003] In view of this, in order to solve one of the above problems, an embodiment of the present invention aims to provide an optical isolator based on nonlinear absorption and self-phase modulation, which can reduce insertion loss, improve the isolation of reverse optical signals, and has a simple structure.

[0004] On the one hand, an embodiment of the present invention provides an optical isolator based on nonlinear absorption and self-phase modulation, which includes a first aperture stop, a first lens, a nonlinear medium, a second lens, and a second aperture stop in sequence along the forward propagation direction of the optical path, the focal length of the first lens is greater than the focal length of the second lens, the focus of the first lens coincides with the focus of the second lens, and the nonlinear medium is arranged between the coincident focus and the second lens.

[0005] Optionally, both the first lens and the second lens are positive focus lenses.

[0006] Optionally, the first lens comprises a plano-convex lens, and the second lens comprises a plano-convex lens.

[0007] Optionally, the focal length of the first lens is in the range of 100-500 mm.

[0008] Optionally, the focal length of the second lens is in the range of 25-100 mm.

[0009] Optionally, the material of the nonlinear medium includes a metal material, an inorganic non-metal material, an organic material or a composite material.

[0010] Optionally, the nonlinear refractive index of the nonlinear medium is greater than 10 -8 m 2 / W.

[0011] Optionally, the position of the nonlinear medium is determined according to a normalized transmittance of the nonlinear medium.

[0012] Optionally, the clear diameter range of the first aperture stop is 0.8-25 mm.

[0013] Optionally, the light clearance diameter range of the second aperture stop is 0.8-25 mm.

[0014] The implementation of the embodiment of the present invention includes the following beneficial effects: an optical isolator based on nonlinear absorption and self-phase modulation includes a first aperture diaphragm, a first lens, a nonlinear medium, a second lens and a second aperture diaphragm in sequence along the forward propagation direction of the optical path, the focal length of the first lens is greater than the focal length of the second lens, the focus of the first lens coincides with the focus of the second lens, and the nonlinear medium is arranged between the coincident focus and the second lens; at the same time, asymmetric nonlinear absorption and self-phase modulation effects are used to achieve isolation of reverse light, and the regulation of the light phase by self-phase modulation causes far-field diffraction to produce a hollow light beam, and the nonlinear absorption strengthens the hole in the center of the hollow light beam, which can reduce insertion loss and improve the isolation of the reverse light signal; the optical isolator includes two aperture diaphragms, two lenses and a nonlinear medium, and has a relatively simple structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 It is a structural schematic diagram of an optical isolator based on nonlinear absorption and self-phase modulation provided by an embodiment of the present invention;

[0016] Figure 2 is a forward optical path diagram of an optical isolator based on nonlinear absorption and self-phase modulation provided by an embodiment of the present invention;

[0017] Figure 3 It is a reverse optical path diagram of an optical isolator based on nonlinear absorption and self-phase modulation provided by an embodiment of the present invention;

[0018] Figure 4 is a diagram of an experimental device for determining the position of a nonlinear medium provided by an embodiment of the present invention;

[0019] Figure 5 It is a schematic diagram of a transmittance curve provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0020] The present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. The step numbers in the following embodiments are only provided for the convenience of explanation and description, and the order between the steps is not limited in any way. The execution order of each step in the embodiment can be adaptively adjusted according to the understanding of those skilled in the art.

[0021] Nonlinear absorption: When the light intensity is extremely high, due to the nonlinear polarizability of the material, the absorption coefficient of the material to light is no longer a constant, but an optical phenomenon that changes with the increase of light intensity. For a saturated absorber, the absorption coefficient will decrease with the increase of light intensity; for an anti-saturated absorber, the absorption coefficient will increase with the increase of light intensity.

[0022] Self-phase modulation: When a light beam with a certain intensity enters a medium, the refractive index of the medium changes due to the optical Kerr effect, and the phase of the light beam is modulated by its own light intensity, causing a radial phase shift of the light beam. This is usually manifested as a far-field diffraction pattern after the light beam passes through the medium.

[0023] Optical isolator: also known as optical diode, is an optical device that only allows one-way light to pass through, and has a non-reciprocal characteristic at its dual ports. It has a small attenuation for optical signals transmitted in the forward direction, but a large attenuation for optical signals transmitted in the reverse direction, forming a one-way path for light. Optical isolators can eliminate the various disadvantages of reflected light and scattered light on the optical path system, and effectively improve the stability of optical power output. The main performance parameters are optical isolation and insertion loss. Optical isolation indicates the ability of the optical isolator to isolate the reverse signal, and insertion loss indicates the degree of attenuation of the optical isolator to the forward signal. Generally, it is hoped that the greater the optical isolation, the better, and the lower the insertion loss, the better. When it is called an optical diode, the performance of concern is the non-reciprocal factor.

[0024] Hollow beam: a ring-shaped beam with weak light intensity distribution in the center and strong light intensity distribution in the surrounding area in the cross section.

[0025] Closed-aperture Z scanning: The sample to be measured is placed on the optical axis (Z axis) of the converging Gaussian beam, and the sample moves along the Z axis near the focus, and a screen with a small hole is placed in the far field. The transmittance of the beam after passing through the sample is measured through the small hole. Based on the relationship between the transmittance and the sample position, the nonlinear refractive index and nonlinear absorption coefficient of the nonlinear medium can be obtained, and the sign of the nonlinear refractive index (self-focusing or self-defocusing) can be determined.

[0026] like Figure 1As shown, an embodiment of the present invention provides an optical isolator based on nonlinear absorption and self-phase modulation, which comprises a first aperture stop (1), a first lens (2), a nonlinear medium (3), a second lens (4) and a second aperture stop (5) in sequence along the forward propagation direction of the optical path, the focal length of the first lens (2) being greater than the focal length of the second lens (4), the focus of the first lens (2) coincides with the focus of the second lens (4), and the nonlinear medium (3) is arranged between the coincident focus (M) and the second lens (4).

[0027] according to Figure 1 The sequence of the light rays when incident in the forward direction is as follows: the first aperture stop (1) is set as the head end for limiting the light beam; the first focal lens (2) is placed behind the first aperture stop (1); the nonlinear medium (3) is placed behind the first telephoto lens (2); the second lens (4) is placed behind the nonlinear medium (3); the first aperture stop (5) is the tail end of the light path and is placed behind the second lens (4).

[0028] The focal lengths of the first lens (2) and the second lens (4) are F1 and F2 respectively, and the focal positions of the two coincide to form a confocal lens system for scaling parallel light beams and controlling the size of the light spot at the focal point.

[0029] In a specific embodiment, when a Gaussian beam is incident, the specific electric field distribution of the hollow beam at the second aperture stop (5) can be calculated according to the following formula (1):

[0030]

[0031] Wherein, J0 is the zero-order Bessel function, k0=2π / λ is the wave vector, λ is the wavelength, z is the position of the nonlinear medium (3) on the optical axis, r is the radial coordinate of the Gaussian beam at the z position, θ=ρ / F1 is the diffraction angle at the diffraction screen. ρ is the radial coordinate at the diffraction screen, and F1 is the distance between the first focal lens (2) (diffraction screen) and the nonlinear medium (3), that is, the focal length of the first lens (2). is the waist radius of the Gaussian beam at the optical axis z position (z = 0 is the focus of the first lens and the second lens), ω0 is the waist radius of the Gaussian beam at the z = 0 position, is the wavefront curvature radius of the Gaussian beam at the optical axis z position, is the Rayleigh length of the Gaussian beam. is the phase change of the Gaussian beam at the z position, as shown in formula (2):

[0032]

[0033] in, is the phase change value of the Gaussian beam at z = 0, as shown in formula (3):

[0034]

[0035] in, is the effective thickness of the nonlinear medium (3), n2 is the nonlinear refractive index of the nonlinear medium (3), L is the thickness of the nonlinear medium (3), and α0 is the linear absorption coefficient of the nonlinear medium (3). At the same time, the maximum value of F1 / F2 is limited by the incident light beam and the size of the element. If the minimum light diameter of the first lens (2) and the second lens (4) is D a , the incident beam diameter is D b , and when the maximum optical path size is required to be F1+F2=L, then F1 / F2 should be less than or equal to D a / D b and the minimum value among F1 / (L-F1).

[0036] Figure 2 It represents the forward light path diagram when the optical isolator allows incident light to pass through. Figure 2 (a1) is a schematic diagram of the intensity distribution of the incident light, and (a2) is a schematic diagram of the intensity distribution of the outgoing light. Figure 3 The reverse light path diagram represents the reverse light isolator isolating the incident light. (b1) is a schematic diagram of the intensity distribution of the reverse incident light. (b2) is a schematic diagram of the intensity distribution at the cross section of the parallel light beam between the first aperture stop and the first lens. (b3) is a schematic diagram of the intensity distribution of the reverse outgoing light.

[0037] See also Figure 2 and Figure 3 The working principle of the present invention is as follows: according to the characteristics of Gaussian beam, the wavefront curvature radius of the Gaussian beam focused by the lens is opposite before and after the focus, and the phase is different. When the same Gaussian beam is incident from the positive and negative directions respectively, the light passing diameters of the first aperture diaphragm (1) and the second aperture diaphragm (5) should be pre-adjusted to allow the incident beam to just pass through. Figure 2 As shown, the Gaussian beam is incident parallel to the positive direction of the Z axis (optical axis), is focused by the first lens (2) and irradiates the nonlinear medium (3), and the nonlinear medium (3) is placed at the Z direction relative to the focus. m Position, Z m The value is determined by the nonlinear properties of the nonlinear medium (3) (such as a closed-aperture Z-scan experiment). According to the Fraunhofer far-field diffraction formula, the light intensity distribution generated by the nonlinear medium through self-phase modulation is concentrated at the center. And because the focal length of the first lens (2) is relatively long, the spot area irradiated on the nonlinear medium (3) is relatively large, and the average light intensity is relatively weak, which cannot cause strong nonlinear absorption of the medium. Finally, the light beam can be emitted with relatively low attenuation (a2). Figure 3As shown, the Gaussian beam (b1) is incident in parallel along the reverse direction of the Z axis, is focused by the second lens (4) and irradiates the nonlinear medium (3). The nonlinear medium (3) is at the -Z relative to the focus of the reverse incident beam. m Position. According to the Fraunhofer far-field diffraction formula, at this time, the light beam produces a hollow light beam with weak central intensity distribution and strong peripheral intensity distribution in the far field due to self-phase modulation. At the same time, since the focal length of the second lens (4) is short, the spot area irradiated on the nonlinear medium (3) is small, and the average light intensity is strong, which can cause strong nonlinear absorption of the medium in the high-intensity part of the Gaussian beam, so that the light intensity in the center of the hollow light beam is attenuated to a greater extent. Under the action of nonlinear absorption and self-phase modulation, the light beam forms a Figure 3 The light intensity distribution in the middle (b2) is then blocked by the first aperture stop (1) to block the surrounding high-intensity part of the light beam, allowing only the low-intensity part in the center to be emitted (b3). Most of the light intensity of the reverse incident light is absorbed by the nonlinear medium or blocked by the aperture stop, thereby achieving isolation of the reverse incident light.

[0038] Optionally, both the first lens and the second lens are positive focus lenses.

[0039] The focal length (F1) of the first lens and the focal length (F2) of the second lens are both positive numbers, and F1 / F2 should at least satisfy the requirement of being greater than 1. By controlling the light beam expansion, when the light beam is incident in the reverse direction, the diameter of the central hollow area of ​​the hollow light beam arriving at the first aperture stop (1) is greater than the light passing diameter of the first aperture stop (1) and the second aperture stop (5), better performance can be obtained.

[0040] Optionally, the first lens includes a plano-convex lens, and the second lens includes a plano-convex lens.

[0041] Optionally, the focal length of the first lens is in the range of 100-500 mm.

[0042] Optionally, the focal length of the second lens is in the range of 25-100 mm.

[0043] If both the first lens (2) and the second lens (4) are plano-convex lenses, F1 can generally be in the range of 100-500 mm, and F2 can be in the range of 25.4-100 mm. When microlens and other processes are used for production, F1 and F2 can be further reduced.

[0044] Optionally, the material of the nonlinear medium includes a metal material, an inorganic non-metal material, an organic material or a composite material.

[0045] The specific material of the nonlinear medium (3) can be a sheet, film, liquid or solution of a metal material, an inorganic non-metallic material, an organic material or a composite material, such as a zinc selenide sheet or a fluorescein dye solution. If the nonlinear medium (3) has a substrate or requires a container, the influence of the properties of the substrate and the container on the hollow beam should also be calculated according to formulas (1) to (3). At the same time, the total thickness L of the sample and its substrate or container should be less than the Rayleigh length of the Gaussian beam (same as z0 in formula (1)), so that an obvious change in the transmittance curve can be generated in the open-aperture Z scan.

[0046] Optionally, the nonlinear refractive index of the nonlinear medium is greater than 10 -8 m 2 / W.

[0047] Nonlinear media have a certain nonlinear refractive index (greater than 1×10 -18 m 2 / W) of a solid or liquid, the size of the nonlinear refractive index will affect the specific form of the hollow light beam through formulas (1) to (3). At the same time, the nonlinear medium 3 may have certain nonlinear absorption properties (no specific requirements, just appropriate), such as the two-photon absorption coefficient β TPA and saturation light intensity I s , and affects the attenuation of the Gaussian beam in the nonlinear medium when it is incident from the forward and reverse directions through the following equation (4).

[0048]

[0049] Where I is the intensity of the Gaussian beam in the nonlinear medium, and z′ is the distance the light travels in the nonlinear medium. The relationship between the intensity I and the electric field E is I = |E| 2 .

[0050] Optionally, the position of the nonlinear medium is determined according to a normalized transmittance of the nonlinear medium.

[0051] The nonlinear medium (3) is placed between the first lens (2) and the second lens (4), and is located behind the common focus of the first lens (2) and the second lens (4). m When light is incident in the reverse direction, the self-phase modulation effect of the nonlinear medium (3) can be used to generate a hollow light beam with weak central light intensity distribution and strong peripheral light intensity distribution.

[0052] like Figure 4 As shown, Z m The value should be Figure 3 The reverse light path of is predetermined by experiment. The experimental setup is similar to Figure 3, including a first aperture diaphragm (1), a first lens (2), a nonlinear medium (3), a second lens (1), a second aperture diaphragm (5), and a power detector (6). The experimental process is similar to the closed-aperture Z scan. The light-passing diameters of the first aperture diaphragm (1) and the second aperture diaphragm (5) are also adjusted to be equal to twice the radius of the incident beam waist so that the incident beam can just pass through. The nonlinear medium (3) is placed on the optical axis (Z axis) of the converging Gaussian beam and moves along the Z axis near the focus. The power detector (6) is arranged after the first aperture diaphragm (1) to receive the transmitted light power curve. When When x=z / z0, the normalized transmittance received by the power detector (6) can be approximately expressed by the following formula (5):

[0053]

[0054] For a nonlinear medium (3) with a positive nonlinear refractive index (n2>0), under appropriate light intensity, the normalized transmittance curve is generally as follows: Figure 5 As shown, the Z coordinate corresponding to the lowest point of normalized transmittance is -Z m , whose Z position symmetric about the zero point is Z m The coordinate zero point is the focal point of the laser. The light beam has the maximum average light intensity and the minimum waist radius at the focal point. Its position is determined when the optical path system is constructed. The focus is generally located between the first lens (2) (focal length F1) and the second lens (4) (focal length F2), and is located on the optical axis at a distance of F1 to the first lens (2) and a distance of F2 to the second lens (4). In addition, the specific position of the focus can also be obtained by capturing the spot image by the CCD imaging unit to find the minimum waist radius position of the light beam, or by performing an open hole Z scan (taking the nonlinear medium (3)) or other standard samples (with a certain nonlinear property and satisfying the sample or container thickness less than the Rayleigh length) Figure 4 The first aperture diaphragm (1) and the second aperture diaphragm (5) are fully opened to allow the incident and outgoing light beams to completely pass through and be received by the detector, and the position corresponding to the valley bottom or peak top of the transmittance curve obtained by the aperture Z scan is the zero point position.

[0055] Optionally, the clear diameter range of the first aperture stop is 0.8-25 mm.

[0056] Optionally, the clear diameter range of the second aperture stop is 0.8-25 mm.

[0057] The light passing diameters of the first aperture diaphragm (1) and the second aperture diaphragm (5) should be pre-adjusted to allow the incident light beam to just pass through, so as to block the stray light from other angles from entering. The light passing diameters of the first aperture diaphragm (1) and the second aperture diaphragm (5) are adjusted from small to large, and the light power changes before and after passing through the diaphragms are detected by an optical power meter. When the light powers detected before and after the same Gaussian light beam passes through the first aperture diaphragm (1) and the second aperture diaphragm (5) in parallel are just the same, the adjustment is stopped. At this time, it can be determined that the incident light beam just passes through the first aperture diaphragm (1) and the second aperture diaphragm (5). The light passing diameters of the two should eventually be adjusted to be equal. The specific light passing diameter is related to the incident laser size of the laser system, and is generally in the range of 0.8-25mm.

[0058] The implementation of the embodiment of the present invention includes the following beneficial effects: an optical isolator based on nonlinear absorption and self-phase modulation includes a first aperture diaphragm, a first lens, a nonlinear medium, a second lens and a second aperture diaphragm in sequence along the forward propagation direction of the optical path, the focal length of the first lens is greater than the focal length of the second lens, the focus of the first lens coincides with the focus of the second lens, and the nonlinear medium is arranged between the coincident focus and the second lens; at the same time, asymmetric nonlinear absorption and self-phase modulation effects are used to achieve isolation of reverse light, and the regulation of the light phase by self-phase modulation causes far-field diffraction to produce a hollow light beam, and the nonlinear absorption strengthens the hole in the center of the hollow light beam, which can reduce insertion loss and improve the isolation of the reverse light signal; the optical isolator includes two aperture diaphragms, two lenses and a nonlinear medium, and has a relatively simple structure.

[0059] The above is a specific description of the preferred implementation of the present invention, but the invention is not limited to the embodiments. Those skilled in the art may make various equivalent modifications or substitutions without violating the spirit of the present invention. These equivalent modifications or substitutions are all included in the scope defined by the claims of this application.

Claims

1. An optical isolator based on nonlinear absorption and self-phase modulation, It is characterized in that The optical path includes a first aperture stop, a first lens, a nonlinear medium, a second lens, and a second aperture stop in sequence along the forward propagation direction of the optical path. The focal length of the first lens is greater than the focal length of the second lens. The focus of the first lens coincides with the focus of the second lens. The nonlinear medium is arranged between the coincident focus and the second lens.

2. The optical isolator according to claim 1, It is characterized in that The first lens and the second lens are both positive focus lenses.

3. The optical isolator according to claim 1, It is characterized in that The first lens includes a plano-convex lens, and the second lens includes a plano-convex lens.

4. The optical isolator according to claim 3, It is characterized in that The focal length of the first lens is in the range of 100-500 mm.

5. The optical isolator according to claim 3, It is characterized in that The focal length of the second lens is in the range of 25-100 mm.

6. The optical isolator according to claim 1, It is characterized in that The material of the nonlinear medium includes metal material, inorganic non-metal material, organic material or composite material.

7. The optical isolator according to claim 1, It is characterized in that The nonlinear refractive index of the nonlinear medium is greater than 10 -8 m 2 / W.

8. The optical isolator according to claim 1, It is characterized in that The position of the nonlinear medium is determined according to the normalized transmittance of the nonlinear medium.

9. The optical isolator according to claim 1, It is characterized in that The clear diameter of the first aperture stop is in the range of 0.8-25 mm.

10. The optical isolator according to claim 1, It is characterized in that The light clearance diameter of the second aperture stop ranges from 0.8 to 25 mm.

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