All-fiber non-magnetic isolator and preparation method thereof

Abstract

The embodiment of the present invention discloses an all-fiber non-magnetic isolator and a preparation method thereof, wherein the isolator comprises an optical fiber, on which a 45° tilted fiber grating and a 45° incident λ / 4 fiber wave plate are formed. The present invention utilizes precise ultraviolet light control to complete the production of the tilted fiber grating and the fiber wave plate online on the same optical fiber, realizes the control of polarization conversion, thereby achieving the goal of low cost and miniaturization of reflected light isolation, and solves the problems of high cost, large volume, low reliability, etc., which require the introduction of magneto-optical materials in traditional reflected light isolation.

Description

Technical Field

[0001] The present invention relates to the field of optical fiber sensing and optical fiber communication technology, and in particular to an all-optical fiber non-magnetic isolator and a preparation method thereof. Background Art

[0002] An optical isolator is an optical device that allows light to be transmitted in one direction. Traditional optical isolators are based on the magneto-optical effect (Faraday effect). Specifically, when light passes through a magneto-optical material in a magnetic field, its polarization plane rotates with the propagation direction of the light. With a polarization element or polarization separation element of the corresponding angle, it is possible to allow light in only one direction to pass through, while blocking light in the opposite direction, thereby achieving unidirectional light isolation.

[0003] Optical isolators are widely used in the fields of fiber amplifiers, fiber lasers, fiber sensors, etc., to protect and stabilize the system. Their main purpose in the system is to protect the light source from interference and damage caused by reverse light. For example, in a fiber optic communication system, an optical isolator can prevent reverse light generated by reflection, scattering, etc. from entering the light source, thereby protecting the laser from damage and improving the stability of the system.

[0004] Traditional magneto-optical isolators introduce new magneto-optical materials in addition to optical fibers, and often require optical fiber collimators and other collimated input and output, which increases the complexity of the device and reduces the reliability of the device. In addition, such devices are large in size and high in cost. In particular, the shorter the working wavelength, the longer the magneto-optical material required. The large size and high price limit the wide application of isolators. Summary of the invention

[0005] The technical problem to be solved by the embodiments of the present invention is to provide an all-fiber non-magnetic isolator and a preparation method thereof, so as to reduce the volume, reduce the cost and improve the reliability.

[0006] In order to solve the above technical problems, an embodiment of the present invention proposes an all-fiber non-magnetic isolator, including an optical fiber on which a 45° tilted fiber grating and a 45° incident λ / 4 fiber wave plate are formed.

[0007] Furthermore, the grating constant of the 45° tilted fiber Bragg grating is Λ g satisfy:

[0008]

[0009] in, n is the refractive index of the optical fiber, m is the order of the 45° tilted fiber Bragg grating, λ It is the operating wavelength of the all-fiber non-magnetic isolator.

[0010] Furthermore, it is applied to the isolation of light sources in one or more wavelength bands of 850nm, 1310nm, and 1550nm.

[0011] Accordingly, an embodiment of the present invention further provides a method for preparing an all-fiber non-magnetic isolator, comprising:

[0012] Step 1: Use ultraviolet laser to write on the optical fiber to make a 45° tilted fiber Bragg grating;

[0013] Step 2: Control the intensity of ultraviolet laser irradiation, and then write and make a λ / 4 fiber wave plate in the area next to the 45° inclined fiber grating on the same optical fiber. After completion, an all-fiber non-magnetic isolator is obtained.

[0014] Furthermore, in step 1 and step 2, a 193 nm or 248 nm excimer laser, or a frequency-doubled 244 nm argon ion laser is used to emit ultraviolet laser.

[0015] Furthermore, in step 1, the ultraviolet laser is modulated by a mask to diffract a light and dark writing pattern, and the optical fiber absorbs the modulated ultraviolet laser by utilizing the photosensitivity of the optical fiber to form a tilted grating structure with a periodic change in refractive index. Λ p Satisfy the following formula:

[0016] ;

[0017] in, n is the refractive index of the optical fiber, λ It is the operating wavelength of the all-fiber non-magnetic isolator.

[0018] Furthermore, the linear polarized light after 45° tilted fiber Bragg grating filtering of the all-fiber non-magnetic isolator is λ / 4 The main axis of the fiber wave plate is 45°.

[0019] Furthermore, in step 2, the photosensitivity of the optical fiber is utilized to control the intensity of ultraviolet laser irradiation so that the refractive index on the side facing the ultraviolet laser is higher than the refractive index on the side facing away from the ultraviolet light, thereby generating ultraviolet induced birefringence. The birefringence and the writing length are controlled so that the phase delay caused by the birefringence is π / 2, thus completing the production of the λ / 4 optical fiber wave plate.

[0020] The beneficial effects of the present invention are:

[0021] 1. The present invention does not require the use of magneto-optical materials, thereby reducing the size of the optical isolator and lowering the cost of the isolator.

[0022] 2. The present invention uses ultraviolet laser to perform contactless production on the same optical fiber online without interrupting the optical fiber, thus greatly improving the reliability of the optical isolator.

[0023] 3. The present invention can match various wavelength systems by changing the period of the grating and the length of the wave plate, and the solution is flexible.

[0024] 4. The present invention is an all-fiber device that can be bent and coiled, greatly reducing the volume of the sensor. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 1 is a light path diagram of an all-fiber non-magnetic isolator according to an embodiment of the present invention.

[0026] Figure 2 It is a schematic diagram of a 45° tilted fiber Bragg grating according to an embodiment of the present invention.

[0027] Figure 3 It is a schematic diagram of the manufacturing principle of a λ / 4 optical fiber wave plate based on ultraviolet light induction according to an embodiment of the present invention.

[0028] Figure 4 It is a schematic diagram of the variation of the induced birefringence with the writing time according to an embodiment of the present invention.

[0029] Figure 5 It is a schematic diagram of the writing structure of the tilted fiber grating according to an embodiment of the present invention.

[0030] Figure 6 It is a schematic diagram of the writing structure of the λ / 4 optical fiber wave plate according to an embodiment of the present invention.

[0031] Description of Figure Numbers

[0032] Optical fiber 1, 45° tilted fiber grating 2, λ / 4 fiber wave plate 3, laser 4, beam expander 5, cylindrical lens 6, mask 7. DETAILED DESCRIPTION

[0033] It should be noted that, in the absence of conflict, the embodiments in the present application and the features in the embodiments may be combined with each other. The present invention is further described in detail below in conjunction with the drawings and specific embodiments.

[0034] In the embodiments of the present invention, if there are directional indications (such as up, down, left, right, front, back, etc.), they are only used to explain the relative position relationship, movement status, etc. between the components under a certain specific posture (as shown in the accompanying drawings). If the specific posture changes, the directional indication will also change accordingly.

[0035] In addition, in the present invention, the descriptions of "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or implying their relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" or "second" may explicitly or implicitly include at least one of the features.

[0036] Please refer to Figure 1The all-fiber non-magnetic isolator of the embodiment of the present invention comprises an optical fiber. A 45° inclined fiber grating and a 45° incident λ / 4 optical fiber wave plate are formed on the optical fiber. The present invention reduces the size of the optical isolator, reduces the cost of the optical isolator, and improves the reliability of the optical isolator.

[0037] The present invention intends to utilize the photosensitivity of optical fiber to make optical fiber devices by absorbing spatially modulated ultraviolet lasers, and to complete a non-magnetic optical isolator on the same optical fiber without interrupting the optical fiber to introduce new materials. The device has the characteristics of small size, low cost, and high reliability, which is of great significance to the development of new optical systems.

[0038] The method for preparing the all-fiber non-magnetic isolator according to the embodiment of the present invention comprises steps 1 and 2.

[0039] Step 1: Use ultraviolet laser to write on the optical fiber to make a 45° tilted fiber Bragg grating.

[0040] Step 2: Control the intensity of ultraviolet laser irradiation, and make a λ / 4 fiber wave plate in the area next to the 45° tilted fiber grating on the same optical fiber, complete the conversion of polarization state on the same complete optical fiber, and obtain a full-fiber non-magnetic isolator, and use a polarizer (45° tilted fiber grating) to complete the extinction of reflected light, thereby achieving isolation of reflected light without introducing magneto-optical materials. The present invention is made by connecting on one optical fiber, and there is no melting point between the devices.

[0041] The present invention utilizes the photosensitivity of the refractive index slightly increased after the optical fiber core absorbs ultraviolet light, spatially modulates the ultraviolet light used for writing, and then manufactures a 45° tilted fiber grating as a polarizer on the same optical fiber, and rotates the wave plate birefringence axis 45° relative to the optical axis of the polarizer to manufacture a 45° incident λ / 4 optical fiber wave plate to complete the conversion of the polarization state. Utilizing the opposite chirality of the reflected light, the polarization state of the reflected light is perpendicular to the optical axis of the fiber grating, thereby achieving the goal of isolating the reflected light.

[0042] As an implementation method, in step 1, the ultraviolet laser is modulated by a mask to diffract a light and dark alternating engraving pattern, and the photosensitivity of the optical fiber is used to make the optical fiber absorb the modulated ultraviolet laser to form a tilted grating structure with a periodic change in refractive index.

[0043] As an implementation method, in step 2, the photosensitivity of the optical fiber is used to control the intensity of ultraviolet laser irradiation so that the refractive index on the side facing the ultraviolet laser is higher than the refractive index on the side facing away from the ultraviolet light, thereby generating ultraviolet induced birefringence, and the birefringence and writing length are controlled so that the phase delay caused by the birefringence is π / 2, thus completing the production of the λ / 4 optical fiber wave plate.

[0044] The working principle of the all-fiber non-magnetic isolator of the present invention is as follows: Figure 1As shown in the figure, the elliptically polarized light emitted by the light source passes through the 45° tilted fiber Bragg grating, the S polarization component is filtered out, and the P polarization component is retained and passed through without loss, thus being converted into linearly polarized light. The linearly polarized light is converted into right-handed circularly polarized light through the λ / 4 fiber wave plate with a principal axis of 45°, and is transmitted in the optical fiber. After encountering the reflection point, the light is reflected. The chirality of the reflected light is opposite to that of the incident light. The right-handed circularly polarized light is converted into left-handed circularly polarized light, and is converted into S linearly polarized light through the λ / 4 fiber wave plate. The linearly polarized light is filtered out by the 45° tilted fiber Bragg grating, achieving the purpose of isolating the reflected light.

[0045] The working principle of 45° tilted fiber Bragg grating is shown in Figure 2. The photosensitive optical fiber absorbs periodic ultraviolet light that alternates between light and dark to produce a periodic photochemical reaction, thereby producing a periodic refractive index modulation, thereby producing a grating structure. The periodic ultraviolet light pattern is rotated to produce a tilted fiber Bragg grating in the optical fiber. When the tilt angle of the grating is the Brewster angle, the reflected light radiates out of the optical fiber in the form of S-polarized light, retaining the P-polarized light to propagate losslessly in the optical fiber. Brewster angle θ Calculation is as follows:

[0046] (1)

[0047] in, n 1, n 0 are the refractive indices formed by the ultraviolet light and dark modulation on the optical fiber. The refractive index contrast formed, Δ n 1= n 1− n 0, about 10 −5 ~10 −3 , such a slight refractive index modulation makes n 1≈ n 0, so θ ≈ 45°. Grating constant Λ g With grating period Λ The relationship is as follows:

[0048] (2)

[0049] When the grating constant satisfies the wave vector matching condition shown in equation (3), the radiation mode is resonantly amplified and the 45° tilted fiber Bragg grating has the best polarization correlation.

[0050] (3)

[0051] in, n is the refractive index of the optical fiber. Without considering the dispersion, it is simplified to 1.46. m is a positive integer, λ is the working wavelength. For the most commonly used 1st order tilted grating (m= 1) Taking the working wavelength as 1310nm, the grating constant should be designed to be 634.5nm.

[0052] The present invention utilizes ultraviolet light to induce the production of optical fiber wave plates. It also utilizes the photosensitivity of the optical fiber when the refractive index increases after absorbing ultraviolet light. The production principle is as follows: Figure 3 As shown, ultraviolet light irradiates the optical fiber from the side of the optical fiber. The side of the optical fiber core facing the light is directly irradiated by the ultraviolet light, while the ultraviolet light absorbed by the side facing away from the light is attenuated. Therefore, the refractive index of the side of the optical fiber core facing the light is n 3Slightly higher than the refractive index of the side facing away from the light n 4. The electric field vibrations of S light and P light are differentially modulated, resulting in UV-induced birefringence Δ n 2. The relationship between this birefringence and writing time is as follows Figure 4 As shown in the figure, at the beginning of irradiation, the birefringence increases with the increase of irradiation time. When the absorption of the side facing the light approaches saturation, the refractive index of the side facing away from the light increases faster than that of the side facing the light. The birefringence gradually decreases with the increase of writing time until it approaches 0. The optimal irradiation time t is obtained by designing process experimental tests based on process conditions such as laser intensity and optical fiber photosensitivity.

[0053] When writing birefringence and length l When the condition shown in equation (4) is satisfied, the phase difference of the wave plate is π / 2, that is, λ / 4 fiber wave plate.

[0054] (4)

[0055] in, k A non-negative integer.

[0056] Since both the tilted fiber grating and the wave plate need to use ultraviolet light to induce the refractive index change, the two devices can be connected and made online on the same optical fiber. After the 45° tilted fiber grating is made, the optical fiber is rotated 45° to ensure that the linear polarized light after filtering is consistent with the λ / 4 The main axis of the fiber wave plate is 45°.

[0057] The present invention is based on the photosensitivity of optical fiber to ultraviolet light, which can be derived from the doping of the optical fiber itself, or hydrogen-carrying sensitization. The ultraviolet laser for writing gratings and wave plates can be a 193 nm or 248 nm excimer laser, or a frequency-doubled 244 nm argon ion laser.

[0058] 45° tilted fiber Bragg grating writing method Figure 5As shown in the figure, the laser emits ultraviolet laser, which is expanded to the length to be written by the beam expander, generally 30mm~60mm. The laser is focused radially in the optical fiber using a cylindrical lens, and the focal position is at the optical fiber, so that the laser energy for writing is higher. The laser is modulated by the phase mask and diffracts a light and dark writing pattern. In view of the cylindrical lens effect of the optical fiber, the pattern inclination angle of the mask is designed to be 33.8°, and the mask period is Λ p It can be calculated using formula (5).

[0059] (5)

[0060] The writing method of the fiber wave plate of the present invention is similar to the writing method of the grating, except that no phase mask is required. Figure 6 As shown, the optimal writing time can be determined by specific process experiments.

[0061] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing an all-fiber non-magnetic isolator, characterized in that: include: Step 1: Use ultraviolet laser to write on the optical fiber to make a 45° tilted fiber Bragg grating; Step 2: Control the intensity of ultraviolet laser irradiation, and continue to write and make a λ / 4 fiber wave plate in the area next to the 45° inclined fiber grating on the same optical fiber, and after completion, an all-fiber non-magnetic isolator is obtained; In step 1, the ultraviolet laser is modulated by a mask plate to diffract a light and dark alternating engraving pattern, and the optical fiber absorbs the modulated ultraviolet laser by utilizing the photosensitivity of the optical fiber to form the 45° inclined grating structure with a periodic change in refractive index; In step 2, the photosensitivity of the optical fiber is used to control the intensity of ultraviolet laser irradiation so that the refractive index on the side facing the ultraviolet laser is higher than the refractive index on the side facing away from the ultraviolet light, thereby generating ultraviolet induced birefringence, and the birefringence and writing length are controlled so that the phase delay generated by the birefringence is π / 2, thus completing the production of the λ / 4 optical fiber wave plate; When elliptically polarized light passes through a 45° tilted fiber Bragg grating, the S polarization component is filtered out, and the P polarization component is retained to pass through intact, thereby converting it into linearly polarized light. The linearly polarized light is converted into right-handed circularly polarized light through a λ / 4 fiber wave plate with a principal axis of 45°.

2. The method for preparing the all-fiber non-magnetic isolator according to claim 1, characterized in that: In step 1 and step 2, a 193nm or 248nm excimer laser, or a frequency-doubled 244nm argon ion laser is used to emit ultraviolet laser.

3. The method for preparing the all-fiber non-magnetic isolator according to claim 1, characterized in that: The mask period Λp satisfies the following formula: Where n is the refractive index of the optical fiber, and λ is the operating wavelength of the all-fiber non-magnetic isolator.

4. The method for preparing the all-fiber non-magnetic isolator according to claim 1, characterized in that: The linear polarized light after 45° tilted fiber Bragg grating filtering of the all-fiber non-magnetic isolator is at 45° to the main axis of the λ / 4 fiber wave plate.

5. An all-fiber non-magnetic isolator, characterized in that: The all-fiber non-magnetic isolator obtained based on the preparation method of the all-fiber non-magnetic isolator according to claim 1 comprises an optical fiber on which a 45° tilted fiber grating and a 45° incident λ / 4 fiber wave plate are formed; elliptically polarized light passes through the 45° tilted fiber grating, the S polarization component is filtered out, and the P polarization component is retained to pass through losslessly, thereby being converted into linearly polarized light, and the linearly polarized light is converted into right-handed circularly polarized light through the λ / 4 fiber wave plate with a main axis of 45°.

6. The all-fiber non-magnetic isolator according to claim 1, characterized in that: The grating constant Λg of the 45° tilted fiber Bragg grating satisfies: Wherein, n is the refractive index of the optical fiber, m is the order of the 45° tilted optical fiber grating, and λ is the operating wavelength of the all-fiber non-magnetic isolator.

7. The all-fiber non-magnetic isolator according to claim 1, characterized in that: Applicable to the isolation of light sources in one or more wavelength bands including 850nm, 1310nm, and 1550nm.

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