Optical fiber quarter-wave plate phase delay calibration system and phase delay calibration method based on laser beat frequency

Abstract

The invention discloses an optical fiber quarter-wave plate phase delay calibration system and a phase delay calibration method based on laser beat frequency, relates to the field of optical fiber quarter-wave plate phase delay calibration, and aims to solve the problems of complex equipment, tedious operation and poor precision in the prior art. A channel port of the wavelength division multiplexer is connected with a light source, an output port of the wavelength division multiplexer is connected with an analysis assembly, a line port of the wavelength division multiplexer is connected with a Bragg grating, and the rear end of the Bragg grating is sequentially connected with an erbium-doped fiber, a phase delay fiber, a wave plate position and a reflection assembly. And an included angle is formed between the optical path of the phase delay optical fiber and the optical path of the optical fiber quarter-wave plate to be measured. The method has the advantages of fewer used optical devices, simple equipment, simplicity and convenience in operation, more intuitive optical path debugging and convenience in quantification, avoids the complex operation that the traditional polarization state detection depends on a polarization analyzer, and improves the system debugging efficiency.

Description

Technical Field

[0001] This invention relates to the field of fiber quarter-wave plate phase delay calibration technology, specifically to a fiber quarter-wave plate phase delay calibration system and phase delay calibration method based on laser beat frequency. Background Technology

[0002] The phase delay of a quarter-wave plate in optical fiber is easily affected by factors such as material uniformity, processing precision, residual stress from packaging, and environmental factors such as temperature and humidity fluctuations, vibration, and strong electromagnetic interference. This can cause the phase delay to deviate from the ideal λ / 4 (π / 2 phase difference) value, resulting in incomplete conversion between linearly polarized and circularly polarized light, producing elliptically polarized light components, and consequently leading to signal distortion and decreased measurement accuracy. While methods for measuring this phase delay have been reported, such as optical rotation extinction, electro-optic modulation phase comparison, and phase compensation, existing technologies have the following limitations:

[0003] The optical extinction method relies on a polarizer, analyzer, standard quarter-wave plate, and precision rotary stage. It depends on the accuracy of the rotation angle reading, requires repeated fine adjustments, and has a cumbersome process. Mechanical hysteresis and bearing wobbling can introduce errors; fluctuations in the light source can also reduce repeatability.

[0004] The electro-optic modulation phase comparison method, in addition to a polarizer and analyzer, also requires an electro-optic modulator, a reference optical path, a lock-in amplifier, and a high-voltage driver, making it the most complex and costly system. It is extremely sensitive to modulation frequency, temperature drift, and optical axis alignment, resulting in a large workload for debugging.

[0005] The phase compensation method requires a known high-precision compensator, whose phase delay error will be directly transmitted to the measurement result.

[0006] Therefore, establishing a simple and high-precision phase delay measurement method is of great theoretical significance and practical application value for improving the overall performance of optical systems and promoting technological innovation in related fields. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to overcome the existing defects and provide a phase delay calibration system and method for fiber quarter-wave plate based on laser beat frequency, which can effectively solve the problems in the background art.

[0008] To achieve the above objectives, this invention discloses a fiber quarter-wave plate phase delay calibration system based on laser beat frequency. The technical solution includes a wavelength division multiplexer (WDM), with a light source connected to its channel port, an analysis component connected to its output port, and a Bragg grating connected to its line port. Erbium-doped fiber, a phase delay fiber, a waveplate position, and a reflection component are sequentially connected to the rear end of the Bragg grating. The waveplate position is used to mount the fiber quarter-wave plate under test, and the optical paths of the phase delay fiber and the fiber quarter-wave plate under test form an angle. In the system, the WDM, Bragg grating, erbium-doped fiber, and other fiber optic devices are connected by fusion splicing.

[0009] As a preferred embodiment of the present invention, the phase delay fiber is a panda polarization-maintaining fiber, and the optical path of the phase delay fiber and the optical path of the quarter-wave plate of the fiber under test are perpendicular. Angle.

[0010] As a preferred embodiment of the present invention, the light source is a 980nm pump light source.

[0011] As a preferred embodiment of the present invention, the analysis component includes a photodetector, the two ends of which are connected to the wavelength division multiplexer and the spectrum analyzer.

[0012] As a preferred embodiment of the present invention, the reflecting component is a plane mirror.

[0013] This invention also discloses a phase delay calibration method based on the aforementioned fiber quarter-wave plate phase delay calibration system. The technical solution involves mounting the fiber quarter-wave plate to be calibrated in the waveplate position of the aforementioned fiber quarter-wave plate phase delay calibration system based on laser beat frequency, turning on the light source to output light energy, monitoring the frequencies of adjacent longitudinal mode beat frequency and polarization mode beat frequency through the analysis component, calculating the phase delay of the fiber quarter-wave plate, thereby obtaining the theoretical length of the fiber quarter-wave plate, and then adjusting the center wavelength of the Bragg grating to calibrate the phase delay of the fiber quarter-wave plate.

[0014] As a preferred embodiment of the present invention, the phase delay of the fiber-optic quarter-wave plate... The calculation formula is as follows:

[0015]

[0016] in, The effective refractive index of the optical fiber. The length of the laser resonant cavity, The interval between the polarization mode beat frequency and the longitudinal mode beat frequency. It is the speed of light.

[0017] This formula can be used to obtain the phase delay of the current fiber quarter-wave plate under test.

[0018] As a preferred embodiment of the present invention, the theoretical length of the fiber-optic quarter-wave plate (6) is... The calculation method is as follows:

[0019]

[0020] in, The wavelength of the incident light before adjustment. The wavelength of the incident light after adjustment. The refractive index difference between the fast and slow axes of the phase-delay fiber. This is the wavelength adjustment amount, and its value is... and The absolute value of the difference.

[0021] The phase delay of a fiber quarter-wave plate can be calibrated by cutting a phase-delay fiber to a calculated length.

[0022] Compared with existing technologies, the advantages of this invention are as follows: This invention connects fiber optic devices such as wavelength division multiplexers (WDM), fiber optic gratings (FBG), and erbium-doped fiber (EDF) using fusion splicing, eliminating the need for free-space optical elements (such as traditional block waveplates and lenses). This results in low optical path loss, strong resistance to vibration interference, and fewer optical components used in the entire system, making the equipment simple, easy to operate, and quick to debug. Furthermore, using panda polarization-maintaining fiber with controllable beat length as a quarter-wave plate, a dual-control mechanism of "polarization-maintaining fiber cutting length adjustment + fiber Bragg grating (FBG) wavelength compensation" is designed to accurately calibrate the phase delay of the fiber quarter-wave plate. Addressing the pain points of high precision requirements and operational difficulty in cutting 1 / 4 beat length (1.25mm), a cutting scheme of "n times beat length + 1 / 4 beat length" simplifies the processing flow. Simultaneously, when a slight deviation in the cutting length causes the phase delay to deviate from π / 2, the polarization state change can be compensated by adjusting the center wavelength of the FBG, ensuring the phase delay accuracy of the quarter-wave plate. This design directly solves the problems of high cutting and splicing loss and irreversible phase adjustment in traditional quartz waveplates, significantly improving the accuracy and flexibility of polarization state control in the optical path. A polarization mode beat frequency monitoring link is constructed using a photodetector (PD) and a spectrum analyzer. By utilizing the "correspondence between polarization mode beat frequency changes and phase delay," it is possible to determine in real time whether the phase delay of the fiber-type quarter-waveplate reaches the ideal state of π / 2. When the polarization mode beat frequency signals of the two longitudinal modes merge into a single peak, phase delay matching can be determined. This characteristic provides an intuitive and quantitative basis for optical path debugging, avoiding the complex operation of traditional polarization state detection relying on polarization analyzers, and improving system debugging efficiency. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the structure of the present invention;

[0024] Figure 2 This is a diagram showing the frequency shift of the polarization mode in this invention.

[0025] Figure 3 This is a fitting graph of the experimental data for this invention.

[0026] In the diagram: 1. Light source; 2. Wavelength division multiplexer; 3. Bragg grating; 4. Erbium-doped fiber; 5. Phase delay fiber; 6. Fiber-type quarter-wave plate; 7. Plane mirror; 8. Photodetector; 9. Radio frequency line; 10. Spectrum analyzer. Detailed Implementation

[0027] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0028] Example 1

[0029] like Figure 1 As shown, this invention first discloses a fiber quarter-wave plate phase delay calibration system based on laser beat frequency. The technical solution includes a 980 / 1550nm wavelength division multiplexer 2, which is connected to a light source 1, a photodetector 8 (response bandwidth 2.5GHz), and a Bragg grating 3 (center wavelength 1550nm, reflectivity >90%). The light source 1 outputs 980nm pump light. The photodetector 8 is connected to a spectrum analyzer 10 via an RF line 9. An erbium-doped fiber 4 is fused to the end of the Bragg grating 3 furthest from the wavelength division multiplexer 2. A phase delay fiber 5 is fused to the erbium-doped fiber 4. The phase delay fiber 5 is a 10cm long, horizontally oriented panda polarization-maintaining fiber. At a 45-degree angle in the optical path, its rear end is connected to a flange via an FC / APC connector to a panda polarization-maintaining fiber that acts as a fiber-type quarter-wave plate 6. The beat length of the panda polarization-maintaining fiber acting as the fiber-type quarter-wave plate 6 is 5mm, and the refractive index difference between the fast and slow axes is approximately... The rear end of the fiber optic quarter-wave plate 6 is connected to the plane reflector 7 via an FC / APC connector, and the connector types at both ends of the flange are consistent.

[0030] This embodiment also discloses a phase delay calibration method based on the above-mentioned fiber quarter-wave plate phase delay calibration system. The technical solution adopted is as follows: the light source 1 is turned on, and the 980nm pump light enters the Bragg grating 3 through the wavelength division multiplexer 2. After passing through the Bragg grating 3, it excites the erbium-doped fiber 4 to generate gain light. After the optical signal passes through the phase delay fiber 5, the orthogonal polarization modes generate a fixed phase difference and enter the fiber-type quarter-wave plate 6. The fiber-type quarter-wave plate 6 converts the linearly polarized light into circularly polarized light, which is reflected by the plane mirror 7 and returns along the original path. It is converted back into linearly polarized light through the fiber-type quarter-wave plate 6, thus completing the polarization state modulation of the resonant cavity. The laser output from the resonant cavity is coupled to the photodetector 8 through the 1550nm port of the wavelength division multiplexer 2. The photodetector 8 converts the optical signal into an electrical signal and transmits it to the spectrum analyzer 10 through the radio frequency line 9. The spectrum analyzer 10 monitors the polarization mode beat frequency signal between two adjacent longitudinal mode beat frequencies.

[0031] At this point, the interval between adjacent longitudinal mode beat frequencies can be expressed as:

[0032]

[0033] In the formula, At the speed of light, Where is the effective refractive index of the optical fiber, and L is the length of the laser resonant cavity;

[0034] Because optical fibers are not perfectly circular and possess inherent birefringence, the interval between the polarization mode beat frequency and the longitudinal mode beat frequency is as follows:

[0035]

[0036] In the formula, The refractive index difference of orthogonally polarized light is converted into a phase difference. :

[0037]

[0038] in, λ is the wavelength of the laser.

[0039] After adding a quarter-wave plate to the laser, when the phase delay of the fiber-type quarter-wave plate 6 is precisely π / 2, the light travels one round trip within the resonant cavity. At this point, the equivalent effect of the fiber-type quarter-wave plate 6 becomes that of a half-wave plate, resulting in a reversal of the fast and slow axes, thus canceling out the inherent phase delay contribution of the fiber. Therefore, the ideal calibration condition should be simplified to depend only on the phase delay of the fiber-type quarter-wave plate 6. When the light travels one round trip through the fiber-type quarter-wave plate 6 twice, with a single-trip phase delay of π / 2, the cumulative phase delay for the round trip is π, equivalent to a half-wave plate. In this case, the interval between the polarization mode beat frequency and the longitudinal mode beat frequency is:

[0040]

[0041] At this point, the beat frequency signals are combined.

[0042] When the phase delay of the fiber-type quarter-wave plate 6 cannot be precisely π / 2, let the phase delay of the actual fiber-type quarter-wave plate 6 be... ,but:

[0043]

[0044] in, For circularly polarized phase difference, This represents the linear polarization phase difference;

[0045] At this point, the interval between the polarization mode beat frequency and the longitudinal mode beat frequency is:

[0046]

[0047] Therefore, the phase delay of the actual fiber-optic quarter-wave plate 6 is It can be represented as:

[0048]

[0049] Therefore, the actual phase delay of the fiber-optic quarter-wave plate 6 can be calculated.

[0050] Then, the phase delay of the fiber-optic quarter-wave plate is calibrated.

[0051] The beat length of the Panda polarization-maintaining fiber used in the fiber-optic quarter-wave plate 6 It can be represented as:

[0052]

[0053] in, The wavelength of the light passing through the fiber-optic quarter-wave plate 6; The refractive index difference between the fast and slow axes is determined by the fiber itself and does not change with wavelength. When the wavelength changes from... Change to At that time, the long shot was taken from Change to ,satisfy:

[0054]

[0055] Fiber-optic quarter-wave plate, 6 fiber length Phase difference with fast and slow axes The relationship is:

[0056]

[0057] If we require the phase difference change to correspond to a 1 / 4 beat length change, that is:

[0058]

[0059] Therefore, the fiber length of the fiber-type quarter-wave plate 6 satisfies:

[0060]

[0061] in, For wavelength adjustment, satisfying ;

[0062] In this embodiment, 2nm, wavelength It is 1550nm. 5×10 -4 To achieve the effect of adjusting a 1 / 4 beat length, i.e., a phase difference adjustment of π / 2, then... At least approximately 600mm, therefore, the cutting length of the fiber-optic quarter-wave plate 6 is calculated as n times the beat length + 1 / 4 beat length, which is 120×5mm + 1.25mm = 601.25mm.

[0063] Because accurately cutting a 601.25mm long panda polarization-maintaining fiber is quite difficult, if the beat frequency signals are not merged due to cutting errors, the center wavelength of the FBG is adjusted. The two polarization modes gradually approach each other until the spectrum analyzer displays a single beat frequency peak. At this point, the phase delay reaches the ideal π / 2 state. Figure 2 As shown. The sensitivities were adjusted to 3405.91 kHz / nm and -3659.56 kHz / nm, with goodness-of-fit of 0.99443 and 0.98034, respectively. Figure 3 As shown.

[0064] Furthermore, utilizing the characteristic in Example 1 that the change in the center wavelength of the FBG causes a change in the phase delay of the fiber 1 / 4 waveplate, which in turn causes a change in the polarization mode beat frequency, the FBG can be used as a grating sensor. When the external physical quantity to be measured causes a change in the center wavelength of the FBG, the change in the polarization mode beat frequency signal between two adjacent longitudinal mode beat frequencies can be monitored by a spectrum analyzer to realize the sensing of the physical quantity to be measured. Thus, this system can be used as a detection sensor for the physical quantity to be measured.

[0065] It should be further noted that on July 12, 2024, the applicant filed an invention patent application with publication number CN118670561A entitled "Temperature Sensor Based on Polarization-Maintaining Fiber". This application discloses a method of connecting a wavelength division multiplexer to a light source and a single-mode fiber, with the single-mode fiber connected to a Bragg grating, a polarization-maintaining fiber, an erbium-doped fiber, and a fiber ring mirror. The inlet port of the fiber ring mirror is connected to a monitoring demodulation module, which uses the polarization-maintaining fiber for temperature sensing.

[0066] Chinese patent CN103278310B, "A Device and Method for Measuring the Temperature Characteristics of Phase Delay of a Fiber Optic Quarter-Waveplate," discloses a correlation between the phase delay and temperature of a fiber optic quarter-waveplate.

[0067] The applicant considered the possibility of combining two existing technologies for detecting the phase delay of a quarter-wave plate in optical fiber. However, the applicant found that patent CN103278310A focuses on measuring the temperature characteristics of the phase delay of a quarter-wave plate in optical fiber, using the principle of calculating the ratio of interference light intensity; patent CN118670561A achieves temperature sensing based on the birefringence effect of polarization-maintaining fiber, relying on the monitoring of polarization mode beat frequency signals. Although combining the two technologies can achieve indirect measurement of phase delay to a certain extent, it still has the inherent limitations of "dependence on intermediate medium (temperature), error accumulation, and lack of active calibration capability". When measuring phase delay by combining the technologies of patent CN118670561A and patent CN103278310A, there is an indirect conversion path: "polarization mode beat frequency → temperature (patent CN118670561A polarization mode beat frequency-temperature model) → interference light intensity ratio → phase delay (patent CN103278310A temperature-phase model)", and this indirect conversion path depends on the intermediate physical quantity (temperature). Because temperature measurement errors and light intensity detection noise accumulate with each other, high-precision measurement is difficult to achieve. Furthermore, the core principle of patent CN103278310A is to detect the ratio of interference light intensity, which is easily affected by optical power, noise, and environmental interference. The balanced detector can only cancel some common-mode noise and cannot eliminate differential-mode noise. Moreover, the interference light intensity varies cosinely with the phase delay, making it suitable for measurement near the ideal operating point when the phase delay is close to π / 2. If the phase deviation is too large, the rate of change of light intensity decreases, and the response speed slows down.

[0068] Example 1 utilizes the phase delay variation of the quasi-quarter waveplate to directly measure the phase delay, which causes an increase or decrease in the splitting of orthogonal longitudinal modes in the laser, leading to a shift in the polarization mode beat frequency. This method avoids error accumulation during conversion, resulting in high measurement accuracy. Furthermore, the response speed is unaffected by factors such as optical power and noise. By controlling the polarization-maintaining fiber cutting length and the center wavelength of the FBG, the phase delay of the laser wave is actively controlled. When the polarization mode beat frequencies between two adjacent longitudinal modes merge into a single peak, it can be determined that the quarter waveplate is operating at a phase delay of π / 2, thus achieving calibration of the quarter waveplate's phase delay. Although the FBG is sensitive to the environment, whether the environment affects the FBG wavelength or the wavelength is stretched manually by adjusting the micro-displacement stage, the wavelength at which the polarization mode beat frequency peaks merge is the wavelength corresponding to the quarter waveplate, and its environmental influence can be overcome.

[0069] The circuits and fiber optic connections involved in this invention are conventional methods used by those skilled in the art, and technical inspiration can be obtained through a limited number of experiments; they are common knowledge.

[0070] Components not described in detail in this article are existing technologies.

[0071] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A fiber quarter-wave plate phase delay calibration system based on laser beat frequency, characterized in that: The device includes a wavelength division multiplexer (2), whose channel port is connected to a light source (1), whose output port is connected to an analysis component, and whose line port is connected to a Bragg grating (3). The Bragg grating (3) is connected in sequence to an erbium-doped fiber (4), a phase delay fiber (5), a waveplate position, and a reflection component. The waveplate position is used to mount a quarter-wave plate of the fiber under test, and the optical path of the phase delay fiber (5) and the optical path of the quarter-wave plate of the fiber under test are at an angle.

2. The fiber quarter-wave plate phase delay calibration system based on laser beat frequency according to claim 1, characterized in that: The phase delay fiber (5) is a panda polarization-maintaining fiber, and the optical path of the phase delay fiber (5) and the optical path of the quarter-wave plate of the fiber under test are perpendicular. Angle.

3. The fiber quarter-wave plate phase delay calibration system based on laser beat frequency according to claim 1, characterized in that: The light source (1) is a 980nm pump light source.

4. The fiber quarter-wave plate phase delay calibration system based on laser beat frequency according to claim 1, characterized in that: The analysis component includes a photodetector (8), which is connected at both ends to the wavelength division multiplexer (2) and the spectrum analyzer (10).

5. The fiber quarter-wave plate phase delay calibration system based on laser beat frequency according to claim 1, characterized in that: The reflective component is a plane mirror (7).

6. A phase delay calibration method, characterized in that: The fiber-type quarter-wave plate (6) to be calibrated is installed in the waveplate position of the fiber-type quarter-wave plate phase delay calibration system based on laser beat frequency as described in claim 1. The light source (1) is turned on to output light energy. The frequencies of adjacent longitudinal mode beat frequency and polarization mode beat frequency are monitored by the analysis component. The phase delay of the fiber-type quarter-wave plate (6) is calculated, and then the theoretical length of the fiber-type quarter-wave plate (6) is obtained. The center wavelength of the Bragg grating (3) is then adjusted to calibrate the phase delay of the fiber-type quarter-wave plate (6).

7. The fiber quarter-wave plate phase delay calibration method based on laser beat frequency according to claim 6, characterized in that, Phase delay of fiber-optic quarter-wave plate (6) The calculation formula is as follows: in, The effective refractive index of the optical fiber. The length of the laser resonant cavity, The interval between the polarization mode beat frequency and the longitudinal mode beat frequency. It is the speed of light.

8. The fiber quarter-wave plate phase delay calibration method based on laser beat frequency according to claim 6 or 7, characterized in that, The theoretical length of the fiber-optic quarter-wave plate (6) The calculation method is as follows: in, The wavelength of the incident light before adjustment. The wavelength of the incident light after adjustment. The refractive index difference between the fast and slow axes of the phase-delay fiber (5) This is the wavelength adjustment amount, and its value is... and The absolute value of the difference.

Images