Magneto-optical measurement system

Abstract

The invention provides a magneto-optical measurement system which can be applied to the technical field of optical measurement. The system comprises a broadband light source, a broadband light source performance regulation and control module, a polarization-maintaining circulator, an optical phase modulation and demodulation module, a magneto-optical spectrum acquisition module and a signal detection module, an initial optical signal generated by the broadband light source is regulated and controlled by the broadband light source performance regulation and control module to obtain a stable optical signal, and the stable optical signal passes through the polarization-maintaining circulator and is transmitted to the optical phase modulation and demodulation module. The light enters the optical phase modulation and demodulation module and is modulated to generate an emergent light beam; the emergent light beam enters a magneto-optical spectrum acquisition module, and a reflected light beam on the sample is obtained after magneto-optical signal modulation; the reflected light beam passes through the optical phase modulation-demodulation module, interference is generated after demodulation, and an interference signal is generated; and the interference signal enters the signal detection module through the polarization-maintaining circulator and is recorded by the signal detection module, and a magneto-optical spectrum is generated.

Description

Technical Field

[0001] The present invention relates to the field of optical measurement technology, and in particular to a magneto-optical measurement system. Background Art

[0002] Traditional magneto-optical effect measurement technology has long relied on cross-polarization technology as its core means. However, with the rapid development of science and technology, a series of new measurement strategies such as cavity-enhanced Kerr rotation and optical balanced bridge light detection have gradually emerged, and they have shown excellent performance in resolution. The resolution ability of magneto-optical signals is usually measured by the key indicator of magneto-optical Kerr angle. At present, the magneto-optical Kerr angle resolution of commercial magneto-optical measurement systems on the market can generally reach about 10 -6 In the laboratory, the resolution of the magneto-optical Kerr angle can be increased to about 5×10 -7 radian.

[0003] However, to further improve the resolution of magneto-optical signals, we still need to face two major challenges. On the one hand, the reflectors, prisms and various polarization devices in the measurement light path will have a certain impact on the polarization state of light, thereby introducing errors. On the other hand, the characteristics of the sample itself are also factors that cannot be ignored, such as scattering on the sample surface or interface, which often brings additional noise interference.

[0004] Therefore, how to further improve the measurement resolution of the magneto-optical effect under the influence of these complex factors has become a key and difficult issue that needs to be urgently addressed in the current research field. Summary of the invention

[0005] 1. Technical issues to be resolved

[0006] In order to solve the above-mentioned problems existing in magneto-optical measurement in the prior art, an embodiment of the present invention provides an all-fiber ultra-high resolution magneto-optical measurement system based on the Sagnac effect. The basic principle of the Sagnac effect is to detect the phase difference between light propagating in different directions in the same optical fiber. The two beams of light separated by the coupler will propagate in different directions in the optical fiber loop, and finally return to the coupler and interfere. This structural characteristic determines that it can only measure phase signals with broken time reversal symmetry. In the system, except for the magneto-optical signal of the sample, the other parts of the optical path have time reversal symmetry, so this design can greatly reduce the influence of optical components on the measurement optical path. At the same time, optical phase modulation and magneto-optical signal modulation are added to the measurement optical path and the sample respectively, and dual modulation phase-locked amplification measurement is adopted, so that ultra-high resolution magneto-optical Kerr signals can be obtained.

[0007] (II) Technical solution

[0008] In view of the above problems, an embodiment of the present invention provides a magneto-optical measurement system.

[0009] According to a first aspect of the present invention, a magneto-optical measurement system is provided, comprising: a broadband light source, a broadband light source performance control module, a polarization-maintaining circulator, an optical phase modulation and demodulation module, a magneto-optical spectrum acquisition module and a signal detection module, wherein the broadband light source is used to generate an initial light signal; the broadband light source performance control module is used to control the initial light signal to obtain a stable light signal, the stable light signal passes through the polarization-maintaining circulator, enters the optical phase modulation and demodulation module, and generates an outgoing light beam after modulation; the outgoing light beam enters the magneto-optical spectrum acquisition module, and the reflected light on the sample after the magneto-optical signal modulation is collected by the same polarization-maintaining optical fiber to obtain a reflected light beam; the reflected light beam passes through the optical phase modulation and demodulation module again, and generates an interference signal after demodulation; and the interference signal passes through the polarization-maintaining circulator and enters the signal detection module, and the signal detection module records and generates a magneto-optical spectrum.

[0010] In some exemplary embodiments, the broadband light source performance control module includes an online adjustable light intensity attenuator and a polarization-maintaining isolator, wherein the online adjustable light intensity attenuator is used to adjust the intensity of the initial light signal; and the polarization-maintaining isolator is used to isolate the reflected light generated by other devices to ensure the stability of the light source working mode.

[0011] In some exemplary embodiments, the optical phase modulation and demodulation module includes an online polarizer, an electro-optical phase modulator, and an arbitrary waveform generator, wherein the online polarizer and the electro-optical phase modulator are used to modulate and demodulate the light beam; the arbitrary waveform generator is used to adjust the operating frequency of the electro-optical phase modulator and provide a clock synchronization signal.

[0012] In some exemplary embodiments, the polarization angle of the online polarizer is 45 degrees to the slow axis of the electro-optic phase modulator, and the online polarizer and the electro-optic phase modulator are used to modulate and demodulate the light beam, including: the online polarizer is used to align the polarization direction of the light beam with a direction that is 45 degrees to the fast and slow axes, so as to evenly divide the light beam propagating on the slow axis into two modes, the slow axis and the fast axis; and the electro-optic phase modulator is used to add phase modulation to the light beams propagating on the two modes respectively.

[0013] In some exemplary embodiments, the magneto-optical spectrum acquisition module includes a fiber collimator, a self-focusing lens, a quarter-wave plate, an AC current source, and a three-dimensional sample displacement stage, wherein the fiber collimator is used to collimate the light spot into parallel light; the self-focusing lens is used to focus the light spot to reduce the diameter of the light spot; the quarter-wave plate is at a 45-degree angle to the fast and slow axes of the optical fiber, and the quarter-wave plate is used to convert linearly polarized light into circularly polarized light and to convert the reflected light beam into linearly polarized light; the AC current source is used to modulate the magneto-optical signal of the sample; and the three-dimensional sample displacement stage is used to place the sample and align the position of the light beam and the sample.

[0014] In some exemplary embodiments, the operating frequency of the AC current source is controlled by an arbitrary waveform generator and synchronized with the electro-optic phase modulator clock.

[0015] In some exemplary embodiments, when measuring the magneto-optical effect of a sample, the sample is placed on a three-dimensional sample displacement stage; the magneto-optical signal of the sample is collected by a magneto-optical measurement system, and the reflected light beam of the sample is circularly polarized light, which is converted back into linearly polarized light by a quarter-wave plate; the reflected light beam converted back into linearly polarized light enters the optical phase modulation and demodulation module again; the electro-optical phase modulator performs phase demodulation on the reflected light beams propagating in two modes respectively to obtain two demodulated light beams; and the two demodulated light beams interfere at the online polarizer to generate an interference signal.

[0016] In some exemplary embodiments, the signal detection module includes an avalanche photodetector and a phase-locked amplifier, wherein the avalanche photodetector is used to collect interference signals; and the phase-locked amplifier is used to record interference signals.

[0017] In some exemplary embodiments, a phase-locked amplifier is used to measure the amplitude of a first-time frequency signal and an amplitude of a second-time frequency signal of a modulation angular frequency in a light intensity signal; and based on the amplitude of the first-time frequency signal and the amplitude of the second-time frequency signal, the magneto-optical Kerr angle is calculated.

[0018] In some exemplary embodiments, the arbitrary waveform generator is controlled by a computer; and the arbitrary waveform generator is used to provide a reference signal for the lock-in amplifier.

[0019] In some exemplary embodiments, the broadband light source includes one of a source of amplified spontaneous emission or a superluminescent diode.

[0020] (III) Beneficial effects

[0021] It can be seen from the above technical solutions that the magneto-optical measurement system provided by the embodiments of the present invention has at least one of the following beneficial effects:

[0022] (1) The structural characteristics of the all-fiber ultra-high-resolution magneto-optical measurement system based on the Sagnac effect determine that this system only responds to the phase signal with broken time reversal symmetry (i.e., the magneto-optical signal of the sample), while the other parts of the optical path have time reversal symmetry. Therefore, this design can greatly reduce the influence of optical components on the measurement optical path.

[0023] (2) Optical phase modulation and magneto-optical signal modulation are added to the measurement optical path and sample respectively, and dual modulation phase-locked amplification measurement is adopted to obtain ultra-high-resolution magneto-optical Kerr signals.

[0024] (3) In terms of performance, the magneto-optical measurement system provided by the embodiment of the present invention has the following advantages: High resolution: It can achieve better than 5×10 -8 The system has a Kerr angle resolution of 1000 radians; high stability: the system is insensitive to temperature changes and air flow; simple operation: the use of all-fiber components simplifies the alignment process and is easy to build and use; and high flexibility: it can adapt to a variety of experimental designs and can be quickly built. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The above contents and other objects, features and advantages of the present invention will become more apparent through the following description of the embodiments of the present invention with reference to the accompanying drawings, in which:

[0026] Figure 1 The schematic diagram shows the structure of a magneto-optical measurement system according to an embodiment of the present invention.

[0027] Reference numerals:

[0028] 1- broadband light source; 2- broadband light source performance control module; 3- optical phase modulation and demodulation module; 4- magneto-optical spectrum acquisition module; 5- signal detection module; 6- online adjustable light intensity attenuator; 7- polarization-maintaining isolator; 8- polarization-maintaining circulator; 9- online polarizer; 10- electro-optic phase modulator; 11- arbitrary waveform generator; 12- AC current source; 13- fiber collimator; 14- self-focusing lens; 15- quarter-wave plate; 16- three-dimensional sample displacement stage; 17- avalanche photodetector; 18- phase-locked amplifier. DETAILED DESCRIPTION

[0029] In order to make the purpose, technical scheme and advantages of the present invention more clearly understood, the present invention is further described in detail below in combination with specific embodiments and with reference to the accompanying drawings. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0030] Figure 1The schematic diagram shows the structure of a magneto-optical measurement system according to an embodiment of the present invention.

[0031] like Figure 1 As shown, a magneto-optical measurement system according to an embodiment of the present invention includes: a broadband light source 1, a broadband light source performance control module 2, an optical phase modulation and demodulation module 3, a polarization-maintaining circulator 8, a magneto-optical spectrum acquisition module 4, and a signal detection module 5, wherein the broadband light source 1 is used to generate an initial optical signal; the broadband light source performance control module 2 is used to control the initial optical signal to obtain a stable optical signal, the stable optical signal passes through the polarization-maintaining circulator 8, enters the optical phase modulation and demodulation module 3, and generates an outgoing light beam after modulation; the outgoing light beam enters the magneto-optical spectrum acquisition module 4, and the reflected light on the sample after the magneto-optical signal modulation is collected by the same polarization-maintaining optical fiber to obtain a reflected light beam; the reflected light beam passes through the optical phase modulation and demodulation module 3 again, and interference occurs after demodulation to generate an interference signal; and the interference signal passes through the polarization-maintaining circulator 8 and enters the signal detection module 5, and the signal detection module 5 records and generates a magneto-optical spectrum. Among them, the polarization-maintaining circulator 8 has the characteristics of enabling unidirectional transmission and control of optical signals, maintaining the stability of polarization states, and serving as a key component in the bidirectional transmission line of optical communication, so that the optical signal can be accurately and stably transmitted and processed in the system.

[0032] In some exemplary embodiments, the broadband light source 1 includes an amplified spontaneous emission light source or a superluminescent diode. The broadband light source 1 is used to generate a light beam with a flat spectrum output near the central wavelength, and its shorter coherence length can reduce the interference signal interference of the end face reflected light.

[0033] In some exemplary embodiments, the broadband light source performance control module 2 includes an online adjustable light intensity attenuator 6 and a polarization-maintaining isolator 7, wherein the online adjustable light intensity attenuator 6 is used to adjust the intensity of the initial light signal; and the polarization-maintaining isolator 7 is used to isolate the reflected light generated by other devices to ensure the stability of the light source working mode.

[0034] In some exemplary embodiments, the optical phase modulation and demodulation module 3 includes an online polarizer 9, an electro-optical phase modulator 10 and an arbitrary waveform generator 11, wherein the online polarizer 9 and the electro-optical phase modulator 10 are used to modulate and demodulate the light beam; the arbitrary waveform generator 11 is used to adjust the operating frequency of the electro-optical phase modulator 10 and provide a clock synchronization signal.

[0035] Preferably, the polarization angle of the online polarizer 9 is 45 degrees to the slow axis of the electro-optic phase modulator 10 to obtain the best contrast. The online polarizer 9 and the electro-optic phase modulator 10 modulate and demodulate the light beam, including: the online polarizer 9 is used to align the polarization direction of the light beam with the direction that is 45 degrees to the fast and slow axes, so as to evenly divide the light beam propagating on the slow axis into two modes, the slow axis and the fast axis; and the electro-optic phase modulator 10 is used to add phase modulation to the light beams propagating on the two modes respectively.

[0036] In some exemplary embodiments, the magneto-optical spectrum acquisition module 4 includes a fiber collimator 13, a self-focusing lens 14, a quarter-wave plate 15, an AC current source 12, and a three-dimensional sample displacement stage 16, wherein the fiber collimator 13 is used to collimate the light spot into parallel light; the self-focusing lens 14 is used to focus the light spot to reduce the diameter of the light spot; the quarter-wave plate 15 is at a 45-degree angle to the fast and slow axes of the optical fiber to obtain the optimal contrast, and the quarter-wave plate 15 is used to convert linearly polarized light into circularly polarized light and to convert the reflected light beam into linearly polarized light; the AC current source 12 is used to modulate the magneto-optical signal of the sample; and the three-dimensional sample displacement stage 16 is used to place the sample and align the position of the light beam and the sample, and the three-dimensional sample displacement stage 16 is controlled by a computer.

[0037] In the embodiment of the present invention, when measuring the magneto-optical effect of a sample, the sample is placed on a three-dimensional sample displacement stage 16; the magneto-optical signal of the sample is collected by a magneto-optical measurement system, and the reflected light beam of the sample is circularly polarized light, which is converted back into linearly polarized light again by a quarter-wave plate 15; the reflected light beam converted back into linearly polarized light enters an optical phase modulation and demodulation module 3; the electro-optical phase modulator 10 performs phase demodulation on the reflected light beams propagating in two modes respectively to obtain two demodulated light beams; and the two demodulated light beams interfere at the online polarizer 9 to generate an interference signal.

[0038] In some exemplary embodiments, the signal detection module 5 is used to measure weak magneto-optical signals, and the signal detection module 5 includes an avalanche photodetector 17 and a phase-locked amplifier 18, wherein the avalanche photodetector 17 is used to collect interference signals; and the phase-locked amplifier 18 is used to record interference signals.

[0039] In the embodiment of the present invention, the arbitrary waveform generator 11 is controlled by a computer and is used to provide a reference signal for the lock-in amplifier 18. The reference signal is the difference frequency between the modulation frequency of the electro-optic phase modulator 10 and the modulation frequency of the sample magneto-optical signal.

[0040] In an embodiment of the present invention, the amplitude of the first-time frequency signal and the amplitude of the second-time frequency signal of the modulation angular frequency in the light intensity signal are measured by the phase-locked amplifier 18; and the magneto-optical Kerr angle is calculated based on the amplitude of the first-time frequency signal and the amplitude of the second-time frequency signal.

[0041] The structural characteristics of the all-fiber ultra-high resolution magneto-optical measurement system based on the Sagnac effect in the embodiment of the present invention determine that the system only responds to the phase signal (i.e., the magneto-optical signal of the sample) with broken time reversal symmetry, while the other parts of the optical path have time reversal symmetry, so this design can greatly reduce the influence of optical components on the measurement optical path. At the same time, the embodiment of the present invention adds optical phase modulation and magneto-optical signal modulation to the measurement optical path and the sample respectively, and adopts dual modulation phase-locked amplification measurement, so as to obtain ultra-high resolution magneto-optical Kerr signals.

[0042] based on Figure 1 The measurement system shown is further specifically described below by measuring the magneto-optical Kerr signal of a cobalt / platinum (Co / Pt) film.

[0043] The broadband light source 1 uses a C+L band ("C+L band" refers to the combination of the C band (wavelength range 1530-1565nm) and L band (wavelength range 1565-1625nm) commonly used in optical communications) high-stable amplified spontaneous emission (ASE) light source with a central wavelength of 1550nm. A phase-locked amplifier 18 is used to improve the signal-to-noise ratio. The modulation frequency of the electro-optic phase modulator 10 and the modulation frequency of the sample magneto-optical signal are controlled by an arbitrary waveform generator 11 and are set at 13.191 MHz and 91 kHz respectively. The model of the phase-locked amplifier 18 is SR844, the model of the AC current source 12 that modulates the sample magneto-optical signal is Keithley 6221, the model of the arbitrary waveform generator 11 is Tektronix AFG 3252C, and the reference signal is the difference frequency of the two, 13.1 MHz.

[0044] A Co / Pt film with a thickness of about 0.8nm / 2nm was prepared on a quartz substrate by magnetron sputtering, and a metal electrode was prepared by photolithography.

[0045] Several slow-axis aligned optical fiber flanges are used to respectively connect the ASE light source, the online adjustable light intensity attenuator 6, the polarization-maintaining isolator 7, the polarization-maintaining circulator 8, the online polarizer 9, the electro-optic phase modulator 10, the optical fiber collimator 13 in the optical path, and the output end of the polarization-maintaining circulator 8 and the avalanche photodetector 17 in the return optical path.

[0046] All optical fiber components from the output end of the online polarizer 9 to the return end are in the fast-slow axis biaxial working state, and other optical fiber components are in the slow axis working state with the fast axis cut off.

[0047] In order to make the structure of the all-fiber ultra-high resolution magneto-optical measurement system more compact, the slow axis of the quarter-wave plate 15 is first aligned with the slow axis of the optical fiber, rotated 45 degrees, and then adhered to the self-focusing lens 14 with UV-curing glue.

[0048] The power of the broadband light source 1 is set at 1 mW. By manually adjusting the pitch of the sample stage and controlling the Z axis of the three-dimensional sample displacement stage 16 by computer, the collimation and focusing conditions are optimized to maximize the light intensity returned from the sample to the detector.

[0049] The phase-locked amplifier 18 is used to measure the amplitudes V1 and V2 of the first-harmonic frequency and the second-harmonic frequency signal respectively. Under the small angle approximation, V1 is related to the magneto-optical Kerr angle and the system configuration, and V2 is only related to the system configuration. Since V2 remains unchanged during the measurement process, it only needs to be measured once at the beginning of the experiment. The magneto-optical Kerr angle can be calculated by the ratio of the amplitudes V1 and V2 of the first-harmonic frequency and the second-harmonic frequency signal.

[0050] The three-dimensional sample displacement stage 16 is controlled by a computer to scan point by point in the XY plane to obtain the spatial distribution of the sample's magneto-optical Kerr signal.

[0051] The embodiments of the present invention are described above. However, these embodiments are only for the purpose of illustration, and are not intended to limit the scope of the present invention. Although each embodiment is described above, it does not mean that the measures in each embodiment cannot be used in combination advantageously. Without departing from the scope of the present invention, those skilled in the art may make various substitutions and modifications, which should all fall within the scope of the present invention.

Claims

1. A magneto-optical measurement system, characterized in that: include: Broadband light source, broadband light source performance control module, polarization-maintaining circulator, optical phase modulation and demodulation module, magneto-optical spectrum acquisition module and signal detection module, Wherein, the broadband light source is used to generate an initial light signal; The broadband light source performance control module is used to control the initial optical signal to obtain a stable optical signal. The stable optical signal passes through the polarization-maintaining circulator and enters the optical phase modulation and demodulation module to generate an outgoing light beam after modulation. The outgoing light beam enters the magneto-optical spectrum acquisition module, and the reflected light on the sample after being modulated by the magneto-optical signal is collected by the same polarization-maintaining optical fiber to obtain a reflected light beam; The reflected light beam passes through an optical phase modulation and demodulation module, and after demodulation, interference occurs to generate an interference signal; as well as The interference signal passes through the polarization-maintaining circulator and enters the signal detection module, which records and generates a magneto-optical spectrum.

2. The magneto-optical measurement system according to claim 1, characterized in that: The broadband light source performance control module includes an online adjustable light intensity attenuator and a polarization-maintaining isolator, wherein: The online adjustable light intensity attenuator is used to adjust the intensity of the initial optical signal; and The polarization-maintaining isolator is used to isolate the reflected light generated by other devices to ensure the stability of the light source working mode.

3. The magneto-optical measurement system according to claim 1, characterized in that: The optical phase modulation and demodulation module includes an online polarizer, an electro-optical phase modulator and an arbitrary waveform generator, wherein: The online polarizer and the electro-optic phase modulator are used to modulate and demodulate the light beam; The arbitrary waveform generator is used to adjust the operating frequency of the electro-optical phase modulator and provide a clock synchronization signal.

4. The magneto-optical measurement system according to claim 3, characterized in that: The polarization angle of the online polarizer is 45 degrees to the slow axis of the electro-optic phase modulator. The online polarizer and the electro-optic phase modulator are used to modulate and demodulate the light beam, including: The online polarizer is used to align the polarization direction of the light beam with a direction that is 45 degrees to the fast and slow axes, so as to evenly divide the light beam propagating in the slow axis into two modes, the slow axis and the fast axis; and The electro-optic phase modulator is used to perform phase modulation on the light beams propagating in the two modes respectively.

5. The magneto-optical measurement system according to claim 3, characterized in that: The magneto-optical spectrum acquisition module includes a fiber collimator, a self-focusing lens, a quarter wave plate, an AC current source and a three-dimensional sample displacement stage, wherein: The optical fiber collimator is used to collimate the light spot into parallel light; The self-focusing lens is used to focus the light spot to reduce the diameter of the light spot; The quarter wave plate forms an angle of 45 degrees with the fast-slow axis of the optical fiber, and the quarter wave plate is used to convert linearly polarized light into circularly polarized light and to convert the reflected light beam into linearly polarized light; The AC current source is used to modulate the magneto-optical signal of the sample; and The three-dimensional sample displacement stage is used to place the sample and align the positions of the light beam and the sample.

6. The magneto-optical measurement system according to claim 5, characterized in that: The operating frequency of the AC current source is controlled by an arbitrary waveform generator and is synchronized with the clock of the electro-optical phase modulator.

7. The magneto-optical measurement system according to claim 5, characterized in that: When measuring the magneto-optical effect of the sample, the sample is placed on the three-dimensional sample displacement stage; The magneto-optical signal of the sample is collected by a magneto-optical measurement system. The reflected light beam of the sample is circularly polarized light, which is converted back into linearly polarized light by a quarter-wave plate. The reflected light beam converted back into linearly polarized light enters the optical phase modulation and demodulation module; The electro-optic phase modulator performs phase demodulation on the reflected light beams propagating in the two modes respectively to obtain two demodulated light beams; and The two demodulated light beams interfere with each other at the online polarizer to generate an interference signal.

8. The magneto-optical measurement system according to claim 3, characterized in that: The signal detection module includes an avalanche photodetector and a lock-in amplifier, wherein: The avalanche photodetector is used to collect the interference signal; The lock-in amplifier is used to record the interference signal.

9. The magneto-optical measurement system according to claim 8, characterized in that: The lock-in amplifier is used to measure the amplitude of the first-time frequency signal and the amplitude of the second-time frequency signal about the modulation angular frequency in the light intensity signal; and The magneto-optical Kerr angle is calculated based on the amplitude of the first-harmonic frequency signal and the amplitude of the second-harmonic frequency signal.

10. The magneto-optical measurement system according to claim 8, characterized in that: The arbitrary waveform generator is controlled by a computer; and The arbitrary waveform generator is used to provide a reference signal for the lock-in amplifier.

11. The magneto-optical measurement system according to claim 1, characterized in that: The broadband light source includes one of an amplified spontaneous emission light source or a superluminescent diode.

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