A system for automated fabrication of chirped fiber gratings and method thereof

Abstract

The present application relates to the technical field of fiber grating preparation, and particularly relates to a system and a method for automatically preparing a chirped fiber grating, the system comprising a laser, an electrically-controlled optical switch, an optical power attenuator, a dichroic mirror, a microscopic objective, and a three-dimensional displacement platform provided with a rotating clamp along an optical path; the system further comprises a control device electrically connected to the laser, the electrically-controlled optical switch, the optical power attenuator, and the three-dimensional displacement platform, respectively; and a CCD camera arranged on a side of the dichroic mirror away from the microscopic objective. The system is used to prepare the chirped fiber grating, and the system's optical pulse timing control and gray-scale lithography are realized through the cooperation of the system components, so that the fiber grating with different apodization modulation functions and chirp rates can be prepared at low cost and high efficiency; and the control of the laser, the electrically-controlled optical switch, the optical power attenuator, and the three-dimensional displacement platform by the controller improves the stability of the laser pulse output power and realizes the customized preparation of complex spectrum shapes.

Description

Technical Field

[0001] The present invention relates to the technical field of fiber grating preparation, and in particular to a system and method for automatically preparing chirped fiber gratings. Background Art

[0002] Chirped fiber Bragg gratings (CFBGs), a key component in chirped pulse amplification (CPA) technology, can precisely control the reflection time delay at different wavelengths, thereby achieving pulse broadening and compression. Currently, there are two main methods for fabricating CFBGs: phase masking, which uses ultraviolet light to induce a change in the fiber's refractive index; and femtosecond laser direct writing of fiber Bragg gratings (FBGs). While mature, UV phase masking technology suffers from limited flexibility, making it difficult to customize complex spectral shapes. It also requires fiber pretreatment (such as hydrogen loading), increasing fabrication complexity and cost. In contrast, femtosecond laser direct writing of fiber Bragg gratings (FBGs) is an advanced writing method applicable to various types of optical fibers and gratings. This technique leverages the multiphoton aggregation effect of femtosecond laser pulses to focus laser light onto the fiber core, creating a refractive index modulated region. The grating is then modulated to a specific period according to the Bragg grating equation.

[0003] However, existing femtosecond laser direct writing fiber Bragg grating (FBG) technology still has some limitations. First, the precision of the displacement platform limits the accuracy and consistency of grating preparation. Second, the stability of the laser pulse output power affects the uniformity and performance of the grating. In addition, existing technologies have difficulty in achieving high-precision control of the toe-cut function and flexible adjustment of the chirp rate, which limits the application of CFBG in complex optical systems.

[0004] Therefore, the existing technology still needs to be improved and developed. Summary of the Invention

[0005] In view of the above-mentioned deficiencies in the prior art, the purpose of the present invention is to provide a system and method for automatically preparing chirped fiber gratings, aiming to solve the problems of poor accuracy and laser pulse output power stability in the existing systems for preparing chirped fiber gratings.

[0006] The technical solutions of the present invention are as follows:

[0007] A system for automatically preparing chirped fiber gratings includes a laser, an electrically controlled optical switch, an optical power attenuator, a dichroic mirror, a microscope objective lens, and a three-dimensional displacement platform provided with a rotating fixture.

[0008] The system for automatically preparing the chirped fiber grating further comprises a control device electrically connected with the laser, the electrically controlled optical switch, the optical power attenuator and the three-dimensional displacement platform respectively; and a CCD camera arranged on a side of the dichroic mirror away from the microscope objective.

[0009] The system for automatically preparing the chirped fiber grating, wherein the optical power attenuator is composed of a half-wave plate and a polarizer.

[0010] A method for automatically preparing the chirped fiber grating based on the system, comprising the steps of:

[0011] inputting a grating length, a center wavelength and a apodization function setting the fiber core on the rotating clamp and focusing the fiber core position by using the laser;

[0012] outputting femtosecond laser pulses by using the laser, and passing the laser pulses through the optical power attenuator according to the apodization function controlling the output pulse energy, the laser output pulses and the frequency function of the laser output pulses varying;

[0013] controlling the uniform movement of the three-dimensional displacement platform, while the laser output pulses are focused on the fiber core position, until the grating length is reached, to obtain the chirped fiber grating.

[0014] The method for automatically preparing the chirped fiber grating, wherein the grating length is less than or equal to 50 cm; and the center wavelength is between 900 nm and 1600 nm.

[0015] The method for automatically preparing the chirped fiber grating, wherein the apodization function includes one or more of a rectangular function, a Gaussian function and a triangular function.

[0016] The method for automatically preparing the chirped fiber grating, wherein the step of focusing the fiber core position by using the laser comprises:

[0017] acquiring an illumination light image below the fiber core by using the CCD camera, and converting the image into a gray histogram through image recognition, to determine whether the fiber core center is in a horizontal position through an image processing algorithm;

[0018] if it is determined that the fiber core center is not in the horizontal position, the rotating clamp is controlled until the fiber core center is in the horizontal position;

[0019] the imaging of the CCD camera is used to determine whether the laser focus position is in the fiber core position, and if it is determined that the laser focus position is not in the fiber core position, the three-dimensional displacement platform is controlled until the laser focus position is in the fiber core position.

[0020] The method for automatically preparing chirped fiber gratings, wherein the apodization modulation function The coupled mode theory is used to simulate the apodized chirped fiber Bragg grating. The expression of the refractive index change is:

[0021] ;

[0022] Among them, n( z ) represents the refractive index of the optical fiber as it changes with position z changes; is the effective refractive index of the fiber Bragg grating; is the amplitude change of the induced refractive index modulation, i.e. the apodization modulation function; Λ( z ) is expressed as a fiber Bragg grating periodic function; L is the grating length;

[0023] ( z ) is the phase function, and its expression is: , where F is the chirp rate.

[0024] In the method for automatically preparing a chirped fiber Bragg grating, the relationship between the chirp rate F and the dispersion D is as follows:

[0025] ;

[0026] in, m is the grating diffraction order; Δλ is the full width at half maximum of the chirped fiber Bragg grating; λ ( z ) indicates that the wavelength changes with position z changes; is the entire fiber length; D is the group velocity dispersion, which represents the velocity delay between light of different frequencies per unit length, and the unit is ps / (km·nm); L is the grating length; λ is the center wavelength.

[0027] The method for automatically preparing chirped fiber gratings, wherein the effective refractive index of the fiber grating is The group delay generated when the optical signal passes through the fiber Bragg grating Related, The expression is:

[0028] ;

[0029] in, v g It represents the average group velocity of light passing through the optical fiber; c is the speed of light in a vacuum; The unit is ps.

[0030] The method for automatically preparing chirped fiber gratings, wherein the fiber grating periodic function Λ( z ) is:

[0031] ;

[0032] in, v is the moving speed of the three-dimensional displacement platform; is the frequency function of the laser output pulse.

[0033] Beneficial effects: The present invention provides a system and method for automatically preparing chirped fiber Bragg gratings. The system for automatically preparing chirped fiber Bragg gratings includes a laser, an electrically controlled optical switch, an optical power attenuator, a dichroic mirror, a microscope objective lens, and a three-dimensional displacement platform provided with a rotating clamp, which are arranged along an optical path; the system for automatically preparing chirped fiber Bragg gratings also includes a control device electrically connected to the laser, the electrically controlled optical switch, the optical power attenuator, and the three-dimensional displacement platform, respectively; and a CCD camera arranged on the side of the dichroic mirror away from the microscope objective lens. The present invention utilizes the system to realize the preparation of chirped fiber Bragg gratings. The system realizes the system's optical pulse timing control and grayscale lithography through the coordination of system components, and can prepare fiber Bragg gratings with different toe-cut modulation functions and chirp rates at low cost and high efficiency. The controller is used to control the laser, the electrically controlled optical switch, the optical power attenuator and the three-dimensional displacement platform, thereby improving the stability of the laser pulse output power. This not only solves the problems of poor precision and stability in the existing femtosecond laser direct writing technology, but also realizes the customized preparation of complex spectral shapes. In addition, the system adopts an automated preparation process, which significantly improves the repeatability and reliability of the preparation, ensuring that each batch of products meets strict technical indicators. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is a schematic diagram of the structure of a system for automatically preparing chirped fiber gratings according to the present invention;

[0035] Figure 2 This is a schematic flow chart of a method for automatically preparing chirped fiber Bragg gratings according to the present invention;

[0036] Figure 3 This is a schematic diagram of the structure of the apodized chirped fiber Bragg grating prepared in Example 1;

[0037] Figure 4 This is a graph showing the spectral reflectance test results of the apodized chirped fiber Bragg grating prepared in Example 1. DETAILED DESCRIPTION

[0038] The present invention provides a system and method for automatically preparing chirped fiber Bragg gratings. To make the objectives, technical solutions, and effects of the present invention more clear and explicit, the present invention is further described in detail below. It should be understood that the specific embodiments described herein are merely illustrative of the present invention and are not intended to limit the present invention.

[0039] It will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by those skilled in the art in the art to which the present invention belongs. It should also be understood that terms such as those defined in common dictionaries should be understood to have meanings consistent with their meanings in the context of the prior art and will not be interpreted in an idealized or overly formal sense unless specifically defined as herein.

[0040] In recent years, ultrafast fiber laser technology has made significant progress and opened up a wide range of application prospects in fields such as micromachining, materials science, and biomedical research. Among them, one of the core technologies of high-power ultrafast fiber lasers is chirped pulse amplification (CPA), which can generate high-performance ultrashort pulses and is widely used in scientific research and industry. Chirped fiber Bragg gratings (CFBGs), as a key component in CPA technology, can precisely control the reflection time delay of light of different wavelengths, thereby achieving pulse broadening and compression. However, existing femtosecond laser direct writing fiber Bragg grating technology still has some limitations. Therefore, the development of a high-performance, customizable apodized chirped fiber Bragg grating preparation technology is of great practical significance.

[0041] Based on this, Figure 1 As shown, the present invention provides a system for automatically preparing chirped fiber gratings, comprising a laser 10, an electrically controlled optical switch 20, an optical power attenuator 30, a dichroic mirror 40, a microscope objective lens 50, and a three-dimensional displacement platform 60 provided with a rotating fixture, arranged along the optical path;

[0042] The system for automatically preparing chirped fiber gratings also includes a control device 70 electrically connected to the laser 10, the electrically controlled optical switch 20, the optical power attenuator 30 and the three-dimensional displacement platform 60; and a CCD camera 80 arranged on the side of the dichroic mirror 40 away from the microscope objective lens 50.

[0043] In this embodiment, the system is used to prepare chirped fiber Bragg gratings. The system's optical pulse timing control and grayscale lithography are achieved through the coordination of system components, and fiber Bragg gratings with different toe-cut modulation functions and chirp rates can be prepared at low cost and high efficiency. The controller is used to control the laser, the electrically controlled optical switch, the optical power attenuator, and the three-dimensional displacement platform, thereby improving the stability of the laser pulse output power. This not only solves the problems of poor accuracy and stability in existing femtosecond laser direct writing technology, but also enables customized preparation of complex spectral shapes. In addition, the system adopts an automated preparation process, which significantly improves the repeatability and reliability of the preparation, ensuring that each batch of products meets strict technical indicators.

[0044] In some embodiments, the optical power attenuator is composed of a half-wave plate and a polarization analyzer.

[0045] In some embodiments, the laser is a femtosecond pulse laser; and the control device is a computer.

[0046] Specifically, the optical power attenuator utilizes a combination of a half-wave plate and a polarizer to achieve continuous, high-precision adjustment of optical power by rotating the polarization angle. The dichroic mirror adjusts the optical path, and the control device controls all optoelectronic components. A femtosecond pulse laser outputs single-polarization light pulses, which sequentially pass through an electrically controlled optical switch, an optical power attenuator, and a dichroic mirror. The pulses are then focused by a microscope objective onto the core of an optical fiber placed on a three-dimensional (i.e., three-axis) displacement platform. The control device controls the movement of the three-dimensional (3-axis) displacement platform to fabricate chirped fiber gratings. Furthermore, a CCD camera captures illumination light from beneath the optical fiber and displays it on a screen for real-time monitoring of the writing process.

[0047] In addition, the present invention also provides a method for automatically preparing a chirped fiber grating based on the system, comprising the steps of:

[0048] Step S10: Input grating length, center wavelength and apodization modulation function , placing the optical fiber core on a rotating fixture and using laser to focus the core position;

[0049] Step S20: Using a laser to output a femtosecond laser pulse, the laser is modulated by an optical power attenuator according to the apodization modulation function. Control the output pulse energy, laser output pulse and the frequency function of the laser output pulse change;

[0050] Step S30: Controlling the uniform movement of the three-dimensional displacement platform, while focusing the laser output pulse on the optical fiber core position until the grating length is reached, thereby obtaining a chirped fiber grating.

[0051] In this embodiment, by introducing the apodization modulation function By realizing the timing control of light pulses and setting the optical fiber core on a rotating fixture and combining system components to focus the laser on the core position, it is possible to prepare fiber Bragg gratings with different toe-cut modulation functions and chirp rates at low cost and high efficiency; this preparation method not only solves the accuracy and stability problems in the existing femtosecond laser direct writing technology, but also can realize the customized preparation of complex spectral shapes; moreover, this preparation method has broad application prospects in the fields of optical communications, optical sensing, etc., and can meet the diversified needs of high-performance optical systems for fiber Bragg gratings.

[0052] Specifically, the automated method for preparing chirped fiber Bragg gratings (FBGs) enables customized grating designs by adjusting the chirp rate, apodization modulation function, and dispersion as needed. Furthermore, the method utilizes an automated preparation process, significantly improving the repeatability and reliability of preparation, ensuring that each batch of products meets strict technical specifications. Furthermore, by optimizing the preparation method and reducing raw material consumption, production costs are significantly reduced and production efficiency is significantly improved, making it suitable for large-scale industrial production. This preparation method provides high-performance, low-cost fiber Bragg gratings for applications in fiber-optic communications, lasers, sensors, and other fields.

[0053] In some embodiments, the grating length is less than or equal to 50 cm, and the central wavelength is between 900 nm and 1600 nm. By adjusting the grating length and central wavelength according to actual production requirements, fiber Bragg gratings with different apodization functions and chirp rates can be produced efficiently and cost-effectively.

[0054] In some embodiments, the bandwidth of the central wavelength is less than 100 nm.

[0055] In some embodiments, the apodization modulation function Including one or more of rectangular function, Gaussian function and trigonometric function. By selecting different apodization modulation functions Fiber Bragg gratings with different apodization functions can be prepared to meet the needs of practical applications.

[0056] In some embodiments, in step S10, the step of focusing the core position with a laser includes:

[0057] Step S11: collecting an illumination light image below the optical fiber core by a CCD camera, performing image recognition and converting the image into a grayscale histogram, and determining whether the center of the optical fiber core is in a horizontal position by an image processing algorithm;

[0058] Step S12: If it is determined that the center of the optical fiber core is not in a horizontal position, the fixture is controlled to rotate until the center of the optical fiber core is in a horizontal position;

[0059] Step S13: Determine whether the laser focus position is at the fiber core position by imaging with a CCD camera. If the laser focus position is not at the fiber core position, control the three-dimensional moving platform until the laser focus position is at the fiber core position.

[0060] In this embodiment, the laser focusing on the fiber core position is achieved through the above steps, which can solve the accuracy and stability problems in the existing femtosecond laser direct writing technology and can also realize the customized preparation of complex spectral shapes.

[0061] In some embodiments, the apodization modulation function The coupled mode theory is used to simulate the apodized chirped fiber Bragg grating. The expression of the refractive index change is:

[0062] ;

[0063] Among them, n( z ) represents the refractive index of the optical fiber as it changes with position z changes; is the effective refractive index of the fiber Bragg grating; is the amplitude change of the induced refractive index modulation, i.e. the apodization modulation function; Λ( z ) is expressed as a fiber Bragg grating periodic function; L is the grating length;

[0064] ( z ) is the phase function, and its expression is: , where F is the chirp rate.

[0065] In some embodiments, the relationship between the chirp rate F and the dispersion D is as follows:

[0066] ;

[0067] in, m is the grating diffraction order; Δλ is the full width at half maximum of the chirped fiber Bragg grating; λ ( z ) indicates that the wavelength changes with position z changes; is the entire fiber length; D is the group velocity dispersion (GVD), which represents the velocity delay between light of different frequencies per unit length, in ps / (km·nm); L is the grating length; λ is the center wavelength.

[0068] In some embodiments, the effective refractive index of the fiber Bragg grating is The group delay generated when the optical signal passes through the fiber Bragg grating a group delay (GD) related, The expression of the group delay (GD) related,

[0069] ;

[0070] wherein, v g represents the average group velocity of light passing through the optical fiber; c is the speed of light in vacuum; The unit of GD is ps.

[0071] In some embodiments, the expression of the optical fiber grating period function Λ( z ) is:

[0072] ;

[0073] wherein, v is the moving speed of the three-dimensional displacement platform; is the frequency function of the laser output pulse.

[0074] Specifically, the method flow of automatically preparing the chirped fiber grating is specifically as shown in Figure 2 , comprising:

[0075] 1) setting parameters, including grating length, center wavelength, modulation function, etc.

[0076] 2) image recognition is performed on the optical fiber core arranged on the rotating clamp to determine whether the core is adjusted horizontally; if the core is not adjusted horizontally, the rotating clamp is automatically rotated and leveled through a leveling recognition algorithm, and then image recognition is performed again; if the core is adjusted horizontally, the three-dimensional displacement platform moves, the laser outputs a single-point laser, and it is determined whether the laser focal point is on the core; if the laser focal point is not on the core, the three-dimensional displacement platform moves again through a focusing core recognition algorithm, and the subsequent operation is repeated; if the laser focal point is on the core, the laser is focused on the core;

[0077] 3) then, the attenuator is used to control the pulse energy according to the apodization modulation function , and then the laser outputs an externally modulated pulse, the three-dimensional displacement platform is used to move to write the grating, and then it is determined whether the grating length reaches the set length; if yes, the preparation of the apodized chirped fiber grating is completed, and the preparation process is ended; if not, the grating is continuously written.

[0078] The following examples are further used to specifically describe the present application. It should also be understood that the following examples are only used to further illustrate the present application, and cannot be understood as limiting the protection scope of the present application. Some non-essential improvements and adjustments made by those skilled in the art according to the above content of the present application all belong to the protection scope of the present application.

[0079] Example 1

[0080] This embodiment utilizes Figure 1 The system shown in FIG. 1 (the optical power attenuator is composed of a half-wave plate and a polarization analyzer, the laser is a femtosecond pulse laser, and the control device is a computer) and the method for automatically preparing a chirped fiber grating are used to prepare an apodized chirped fiber grating. The schematic diagram of the structure is shown in FIG. Figure 3 As shown; wherein, the wavelength range of the prepared grating is 1532-1571nm, the chirp rate is 14.62nm / cm, the first-order dispersion is 6.6ps / nm, the grating length is 27.35mm, and the selected toe-cutting function type is Gaussian function.

[0081] The chirped fiber grating produced under these parameters was tested, and the test results of its spectral reflectance spectrum are as follows: Figure 4 As shown in the figure, through the reflection spectrum of the apodized chirped fiber grating, it can be seen intuitively that the reflection intensity of the chirped grating at different wavelengths is relatively consistent, the reflection spectrum is flat as a whole, and the side mode suppression ratio of the reflection spectrum is greater than 10dB, which has a good effect of suppressing side lobes.

[0082] In summary, the present invention provides a system and method for automatically preparing chirped fiber Bragg gratings. The system for automatically preparing chirped fiber Bragg gratings includes a laser, an electrically controlled optical switch, an optical power attenuator, a dichroic mirror, a microscope objective lens, and a three-dimensional displacement platform provided with a rotating fixture, arranged along an optical path. The system for automatically preparing chirped fiber Bragg gratings also includes a control device electrically connected to the laser, the electrically controlled optical switch, the optical power attenuator, and the three-dimensional displacement platform, respectively; and a CCD camera arranged on the side of the dichroic mirror facing away from the microscope objective lens. The present invention utilizes the system to realize the preparation of chirped fiber Bragg gratings. The system realizes the system's optical pulse timing control and grayscale lithography through the coordination of system components, and can prepare fiber Bragg gratings with different toe-cut modulation functions and chirp rates at low cost and high efficiency. The controller is used to control the laser, the electrically controlled optical switch, the optical power attenuator and the three-dimensional displacement platform, thereby improving the stability of the laser pulse output power. This not only solves the problems of poor precision and stability in the existing femtosecond laser direct writing technology, but also realizes the customized preparation of complex spectral shapes. In addition, the system adopts an automated preparation process, which significantly improves the repeatability and reliability of the preparation, ensuring that each batch of products meets strict technical indicators.

[0083] It should be understood that the application of the present invention is not limited to the above examples. For those skilled in the art, improvements or changes can be made based on the above description. All these improvements and changes should fall within the scope of protection of the claims attached to the present invention.

Claims

1. A method for automatically preparing chirped fiber Bragg gratings based on a system for automatically preparing chirped fiber Bragg gratings, characterized in that: Including steps: Enter grating length, center wavelength, and apodization modulation function , placing the optical fiber core on a rotating fixture and using laser to focus the core position; The laser is used to output femtosecond laser pulses, which are modulated by the apodization function through an optical power attenuator. Control the output pulse energy, laser output pulse and the frequency function of the laser output pulse change; Controlling the uniform movement of the three-dimensional displacement platform, while focusing the laser output pulse on the optical fiber core position until the grating length is reached, thereby obtaining a chirped fiber grating; The step of focusing the core position with laser light comprises: The CCD camera is used to collect the illumination light image below the optical fiber core, and the image is converted into a grayscale histogram through image recognition, and the image processing algorithm is used to determine whether the center of the optical fiber core is in a horizontal position; If it is determined that the center of the optical fiber core is not in a horizontal position, the clamp is controlled to rotate until the center of the optical fiber core is in a horizontal position; Determine whether the laser focus position is at the optical fiber core position by imaging with a CCD camera. If it is determined that the laser focus position is not at the optical fiber core position, control the three-dimensional mobile platform until the laser focus position is at the optical fiber core position; The apodization modulation function The coupled mode theory is used to simulate the apodized chirped fiber Bragg grating. The expression of the refractive index change is: ； Among them, n( z ) represents the refractive index of the optical fiber as it changes with position z changes; is the effective refractive index of the fiber Bragg grating; is the amplitude change of the induced refractive index modulation, i.e. the apodization modulation function; Λ( z ) is expressed as a fiber Bragg grating periodic function; L is the grating length; ( z ) is the phase function, and its expression is: , where F is the chirp rate; The relationship between the chirp rate F and the dispersion D is as follows: ； in, m is the grating diffraction order; Δλ is the full width at half maximum of the chirped fiber Bragg grating; λ ( z ) indicates that the wavelength varies with position z changes; is the entire fiber length; D is the group velocity dispersion, which represents the velocity delay between light of different frequencies per unit length, and the unit is ps / (km·nm); L is the grating length; λ is the central wavelength; c is the speed of light in a vacuum.

2. The method for automatically preparing a chirped fiber Bragg grating according to claim 1, wherein: The grating length is less than or equal to 50 cm; and the central wavelength is between 900 nm and 1600 nm.

3. The method for automatically preparing chirped fiber Bragg gratings according to claim 1, wherein: The apodization modulation function It includes one or more of rectangular function, Gaussian function and trigonometric function.

4. The method for automatically preparing a chirped fiber Bragg grating according to claim 1, wherein: The effective refractive index of the fiber Bragg grating The group delay generated when the optical signal passes through the fiber Bragg grating Related, The expression is: ； in, v g It represents the average group velocity of light passing through the optical fiber; c is the speed of light in a vacuum; The unit is ps.

5. The method for automatically preparing chirped fiber Bragg gratings according to claim 1, wherein: The fiber Bragg grating periodic function Λ( z ) is: ； in, v is the moving speed of the three-dimensional displacement platform; is the frequency function of the laser output pulse.

6. The method for automatically preparing a chirped fiber Bragg grating according to claim 1, wherein: The system for automatically preparing chirped fiber gratings includes a laser, an electrically controlled optical switch, an optical power attenuator, a dichroic mirror, a microscope objective lens, and a three-dimensional displacement platform provided with a rotating fixture, which are arranged along the optical path; The system for automatically preparing chirped fiber gratings also includes a control device electrically connected to the laser, the electrically controlled optical switch, the optical power attenuator and the three-dimensional displacement platform; and a CCD camera arranged on the side of the dichroic mirror away from the microscope objective lens.

7. The method for automatically preparing a chirped fiber Bragg grating according to claim 6, wherein: The optical power attenuator consists of a half-wave plate and a polarizer.

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