CN108036726B - Device for measuring nanowire displacement by using polarization maintaining optical fiber and double micro lenses - Google Patents
Device for measuring nanowire displacement by using polarization maintaining optical fiber and double micro lenses Download PDFInfo
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- CN108036726B CN108036726B CN201711268230.6A CN201711268230A CN108036726B CN 108036726 B CN108036726 B CN 108036726B CN 201711268230 A CN201711268230 A CN 201711268230A CN 108036726 B CN108036726 B CN 108036726B
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B11/00—Measuring arrangements characterised by the use of optical techniques
- G01B11/02—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B9/00—Measuring instruments characterised by the use of optical techniques
- G01B9/02—Interferometers
- G01B9/02001—Interferometers characterised by controlling or generating intrinsic radiation properties
- G01B9/02011—Interferometers characterised by controlling or generating intrinsic radiation properties using temporal polarization variation
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01H—MEASUREMENT OF MECHANICAL VIBRATIONS OR ULTRASONIC, SONIC OR INFRASONIC WAVES
- G01H9/00—Measuring mechanical vibrations or ultrasonic, sonic or infrasonic waves by using radiation-sensitive means, e.g. optical means
- G01H9/004—Measuring mechanical vibrations or ultrasonic, sonic or infrasonic waves by using radiation-sensitive means, e.g. optical means using fibre optic sensors
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Abstract
The invention discloses a device for measuring the displacement of a nanowire by using a polarization maintaining optical fiber and double microlenses, wherein linearly polarized light generated by a laser diode enters an optical fiber coupler from one arm of the input end of the optical fiber coupler, 10% of output laser forms a Gaussian beam with a small beam waist radius after sequentially passing through one arm of the output end of the optical fiber coupler and a double microlens structure, the nanowire in the Gaussian beam is positioned at the beam waist radius to obtain the best detection effect, a photoelectric detector is connected with the other arm of the input end of the optical fiber coupler, and the other arm of the output end of the optical fiber coupler is immersed in a liquid immersion device. The invention utilizes the polarization maintaining fiber and the double-micro-lens structure, greatly improves the signal-to-noise ratio and the signal stability of detection, simultaneously reduces the beam waist radius of the emergent Gaussian beam and improves the spatial resolution of detection by benefiting from the parameter matching of the double-micro-lens.
Description
Technical Field
The invention relates to the technical field of quantum optics, in particular to a device for measuring the displacement of a nanowire by using a polarization maintaining optical fiber and double micro lenses.
Background
The interference effect of laser can be used for precisely measuring the displacement and vibration of micro samples such as nanowires and the like, so that the micro-nano-scale micro-nano-scale micro-electro-mechanical system has been widely. Compared with a free space, the laser can be transmitted by utilizing the total internal reflection of the optical fiber, a better emergent Gaussian beam mode and a smaller spot diameter (about a few microns) can be obtained, and the method is particularly suitable for researching micro-nano size samples. In addition, the optical fiber has the advantages of being bendable, small in loss, anti-electromagnetic interference and the like, and is beneficial to obtaining stable and reliable experimental conditions with low maintenance cost.
The existing common optical fiber is mainly divided into a single mode optical fiber and a multimode optical fiber, wherein the single mode optical fiber is used as a waveguide of light, and only a single mode in which a poynting vector is parallel to the axial direction of the optical fiber is allowed to transmit in the single mode optical fiber, so that the single mode optical fiber has smaller dispersion compared with the multimode optical fiber, which means higher bandwidth and longer transmission distance, and therefore, the single mode optical fiber is widely applied in the communication field, and the existing optical fiber interferometer also adopts the single mode optical fiber for measurement. However, with the continuous and deep research, the performance of the existing single-mode fiber has become more and more difficult to meet the detection requirement, and the design of the fiber interferometer with higher sensitivity has become an urgent need.
The measurement sensitivity can be significantly improved by using the polarization state of the laser. Ordinary lasers produce unpolarized light, single-mode fibers allow a single mode to pass through, but are insensitive to the polarization direction, and random birefringence in the fiber or bending of the fiber changes the polarization direction of the light, and thus the polarization direction of the emerging light is also random. The light collected by the fiber optic interferometer includes ambient light signals, i.e., background noise, in addition to the light reflected by the surface of the nanowire. Due to the characteristics of single mode fibers, these signals of different polarization directions can be received through the fiber, thereby increasing the noise level, and the measured signals can also become unstable, which all affect the detection sensitivity of the fiber optic interferometer. More importantly, when the diameter of the nanowire is smaller than or comparable to the size of the light spot, the scattering efficiency of the laser with the polarization direction perpendicular to the nanowire becomes very low, which affects the intensity of the measurement signal, while the laser with the polarization direction parallel to the nanowire is hardly affected, so that the detection of the displacement of the nanowire using the linearly polarized laser will significantly improve the sensitivity.
Another characteristic parameter related to the detection sensitivity is the beam waist radius of the emergent gaussian beam, which is directly related to the spatial resolution of the fiber optic interferometer, and is a basic parameter for detecting micro-nano-scale samples, and it becomes feasible to detect the displacement or vibration of the micro-nano-scale samples only when the spatial resolution parameter is close to or smaller than the size of the nano-wires. The radius of the waist of the Gaussian beam directly emitted from the single-mode fiber is larger, and the waist can be further reduced by using a micro lens matched with parameters. In addition, the micro lens is additionally arranged at the front end of the optical fiber, so that the numerical aperture of the system can be obviously increased, and the micro lens is an important means for improving the detection light intensity and the signal-to-noise ratio.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provides a device for measuring the displacement of a nanowire by using a polarization maintaining optical fiber and a double micro lens.
The invention is realized by the following technical scheme:
a device for measuring the displacement of a nanowire by using a polarization maintaining optical fiber and a double micro lens comprises the nanowire in a Gaussian beam, a double micro lens structure, a liquid immersion device, a polarization maintaining optical fiber coupler, a Fabry-Perot laser diode with a polarization maintaining tail fiber and a photoelectric detector, wherein four arms of the optical fiber coupler, an emergent tail fiber of the laser diode and an input end optical fiber of the photoelectric detector are all the polarization maintaining optical fibers, linearly polarized light generated by the laser diode enters the optical fiber coupler from one arm of the input end of the optical fiber coupler, 10% of output laser forms the Gaussian beam with a small beam waist radius after passing through one arm of the output end of the optical fiber coupler and the double micro lens structure in sequence, the nanowire in the Gaussian beam is positioned at the beam waist radius to obtain the best detection effect, and the photoelectric detector is connected with the other arm of the input end of the optical fiber coupler, the other arm of the output end of the optical fiber coupler is immersed in a liquid immersion device. By controlling the nanometer precision displacement table contained in the nanowire fixing part, the relation between the light intensity measured by the photoelectric detector and the position of the nanowire can be obtained, and therefore the relative displacement of the nanowire can be determined. In addition, the power spectral density function obtained by performing spectral analysis on the measured light intensity can give information on the vibration amplitude and angle of the nanowire.
The central wavelength of the laser generated by the laser diode is from 785nm to 2000 nm; the polarization-maintaining optical fiber coupler is of a 2 x 2 type, has central wavelengths of 780nm, 1064nm, 1310nm and 1550nm, has a light splitting ratio of 50:50, 75:25, 90:10 or 99:1, and has a bidirectional coupling characteristic and a 2.0mm narrow key FC/APC joint; the working wavelength range of the polarization maintaining fiber is 350nm-2200nm, and the polarization maintaining fiber is provided with a panda type or bow-tie type stress rod.
The central wavelength of the laser generated by the laser diode is 1550nm, the central wavelength of the polarization-maintaining optical fiber coupler is 1550nm, the light splitting ratio is 90:10, the working wavelength of the polarization-maintaining optical fiber is 1550nm, and the polarization-maintaining optical fiber coupler is provided with a panda type stress rod.
The double-microlens structure is fixed in front of the end face of the optical fiber by using the supporting member, the center of the first microlens, the center of the second microlens and the center of the polarization-maintaining optical fiber in the double microlenses are collinear, and the convex surfaces of the first microlens and the second microlens are opposite.
The numerical aperture of the first micro lens is 0.15, the focal length is 5.00mm, the outer diameter is 2.00mm, and the working distance is 4.37 mm; the numerical aperture of the second micro lens is 0.58, the focal length is 1.45mm, the outer diameter is 2.40mm, and the working distance is 0.81 mm.
The distance between the end face of the optical fiber and the first micro lens is 4.44mm, and the distance between the first micro lens and the second micro lens is 3.3 mm.
Aiming at the characteristics of a detected object, the polarization-maintaining optical fiber and the double-microlens structure are used, the stability and the contrast of a detection signal are obviously improved by means of the characteristics of linearly polarized light and the parameter design of the double microlenses, and meanwhile, the spatial resolution of the displacement of the detection nanowire is improved, so that the detection sensitivity of the system is greatly improved, and the system can be applied to occasions with higher measurement requirements.
Experimental data show that the signal-to-noise ratio of the measurement result is improved by at least one order of magnitude, and the spatial resolution is improved by about 5 times, so that the optical fiber interferometer device provided by the invention can obviously improve the working performance of the conventional detector and realize the detection of the displacement of the nanowire with higher sensitivity.
The invention has the advantages that: the invention utilizes the polarization maintaining fiber and the double-micro-lens structure, greatly improves the signal-to-noise ratio and the signal stability of detection, simultaneously reduces the beam waist radius of the emergent Gaussian beam and improves the spatial resolution of detection by benefiting from the parameter matching of the double-micro-lens.
Drawings
Fig. 1 is a light path diagram of the present invention.
FIG. 2 is a cross-sectional view of a polarization maintaining fiber.
Fig. 3 shows a double microlens structure according to the present invention.
Fig. 4 is a diagram showing the relationship between the detection signal and the position of the nanowire in a two-dimensional plane in example 4 of the present invention.
FIG. 5 is a comparison of the spectral density of the thermal noise measured before and after the use of the apparatus of the present invention under the same conditions.
Detailed Description
As shown in fig. 1, a device for measuring the displacement of a nanowire by using a polarization maintaining fiber and a double microlens, comprises a nanowire 1 in a gaussian beam, a double microlens structure 2, a liquid immersion device 3, a polarization maintaining fiber coupler 4, a fabry-perot laser diode 5 with a polarization maintaining tail fiber, and a photodetector 6, wherein four arms of the fiber coupler 4, an emergent tail fiber of the laser diode 5, and an input end fiber of the photodetector 6 are all polarization maintaining fibers 7, linearly polarized light generated by the laser diode 5 enters the fiber coupler 4 from one arm of the input end of the fiber coupler 4, 10% of output laser forms the gaussian beam with a very small beam waist radius after passing through one arm of the output end of the fiber coupler 4 and the double microlens structure 2 in sequence, and the nanowire 1 in the gaussian beam is positioned at the beam waist radius to obtain the best detection effect, the photoelectric detector 6 is connected with the other arm of the input end of the optical fiber coupler 4, and the other arm of the output end of the optical fiber coupler 4 is immersed in the liquid immersion device 3.
The central wavelength of the laser generated by the laser diode 5 is from 785nm to 2000 nm; the polarization-maintaining optical fiber coupler 4 is of a 2 x 2 type, has central wavelengths of 780nm, 1064nm, 1310nm and 1550nm, has a light splitting ratio of 50:50, 75:25, 90:10 or 99:1, and has a bidirectional coupling characteristic and a 2.0mm narrow key FC/APC joint; the working wavelength range of the polarization maintaining optical fiber 7 is 350nm-2200nm, and the polarization maintaining optical fiber is provided with a panda type or bow-tie type stress rod.
The central wavelength of the laser generated by the laser diode 5 is 1550nm, the central wavelength of the polarization-maintaining optical fiber coupler 4 is 1550nm, the splitting ratio is 90:10, the working wavelength of the polarization-maintaining optical fiber 7 is 1550nm, and the polarization-maintaining optical fiber coupler is provided with a panda-shaped stress rod.
The double-microlens structure 2 is fixed in front of the end face of the optical fiber by using a supporting member, the center of a first microlens 8, the center of a second microlens 9 and the center of a polarization-maintaining optical fiber 7 in the double microlenses are collinear, and the convex surfaces of the first microlens 8 and the second microlens 9 are opposite.
The numerical aperture of the first microlens 8 is 0.15, the focal length is 5.00mm, the outer diameter is 2.00mm, and the working distance is 4.37 mm; the numerical aperture of the second microlens 9 is 0.58, the focal length is 1.45mm, the outer diameter is 2.40mm, and the working distance is 0.81 mm.
The distance between the end face of the optical fiber and the first micro lens 8 is 4.44mm, and the distance between the first micro lens 8 and the second micro lens 9 is 3.3 mm.
The polarization maintaining fiber 7 has a panda-type structure, and its cross section is shown in fig. 2. The optical fiber working in the wavelength range of 1440-1625nm, the diameter of a fiber core is 8.5 mu m, the diameter of a cladding layer is 125 mu m, the nominal mode field diameter and the numerical aperture are respectively 10.1 mu m and 0.125, and the attenuation at 1550nm is lower than 1.0 dB/km. The panda type stress structure can keep a polarization state, has ultralow loss in long-distance transmission and has good anti-interference capability on radiation induced damage.
The structure of the present invention with two micro-lenses designed to improve spatial resolution and numerical aperture is shown in fig. 3. FIG. 3(a) shows the spatial positions of the polarization maintaining fiber and two microlenses, where the distance between the fiber end face and the microlens 1 is denoted as l1The distance between two microlenses is denoted by l2，l1And l2The values of (c) can be adjusted within a certain range according to the principle of numerical aperture matching, and fig. 3(b) and 3(c) are a preferred set of geometrical parameters of two microlenses.
One way to characterize the detection system response to nanowire displacement is to perform a scanning measurement in the plane perpendicular to the nanowire, as shown in fig. 4. Fig. 4(a) and 4(b) are the scanning measurement results measured before and after the device of the present invention is used, respectively, and it can be seen that the signal-to-noise ratio and the spatial resolution of the test are both greatly improved, and the instability of the background signal is effectively suppressed.
As shown in fig. 5, the thermal noise spectral density data obtained under the same conditions without (as in fig. 5(a)) and with (as in fig. 5(b)) the inventive device is seen to increase the peak intensity of the vibration modes by 2 to 3 orders of magnitude.
Claims (2)
1. An apparatus for measuring displacement of a nanowire using a polarization maintaining fiber and a double microlens, comprising: comprises a nano wire in a Gaussian beam, a double micro-lens structure, a liquid immersion device, a polarization-maintaining optical fiber coupler, a Fabry-Perot laser diode with a polarization-maintaining tail fiber and a photoelectric detector, wherein, the four arms of the optical fiber coupler, the emergent tail fiber of the laser diode and the input end optical fiber of the photoelectric detector are all polarization maintaining optical fibers, linearly polarized light generated by the laser diode enters the optical fiber coupler from one arm of the input end of the optical fiber coupler, 10 percent of output laser forms a Gaussian beam with extremely small beam waist radius after passing through one arm of the output end of the optical fiber coupler and the double micro-lens structure in sequence, the nano wire in the Gaussian beam is positioned at the beam waist radius, the photoelectric detector is connected with the other arm of the input end of the optical fiber coupler, and the other arm of the output end of the optical fiber coupler is immersed in the liquid immersion device;
the central wavelength of the laser generated by the laser diode is from 785nm to 2000 nm; the polarization-maintaining optical fiber coupler is of a 2 x 2 type, has central wavelengths of 780nm, 1064nm, 1310nm and 1550nm, has a light splitting ratio of 50:50, 75:25, 90:10 or 99:1, and has a bidirectional coupling characteristic and a 2.0mm narrow key FC/APC joint; the working wavelength range of the polarization maintaining optical fiber is 350nm-2200nm, and the polarization maintaining optical fiber is provided with a panda type or bow-tie type stress rod;
the double-microlens structure is fixed in front of the end face of the optical fiber by using a supporting member, the center of a first microlens, the center of a second microlens and the center of the polarization-maintaining optical fiber in the double microlenses are collinear, and the convex surfaces of the first microlens and the second microlens are opposite;
the numerical aperture of the first micro lens is 0.15, the focal length is 5.00mm, the outer diameter is 2.00mm, and the working distance is 4.37 mm; the numerical aperture of the second micro lens is 0.58, the focal length is 1.45mm, the outer diameter is 2.40mm, and the working distance is 0.81 mm;
the device for measuring the displacement of the nanowire by using the polarization maintaining fiber and the double microlenses is characterized in that: the distance between the end face of the optical fiber and the first micro lens is 4.44mm, and the distance between the first micro lens and the second micro lens is 3.3 mm.
2. The apparatus of claim 1, wherein the apparatus for measuring the displacement of the nanowire comprises a polarization maintaining fiber and a double micro-lens, wherein: the central wavelength of the laser generated by the laser diode is 1550nm, the central wavelength of the polarization-maintaining optical fiber coupler is 1550nm, the light splitting ratio is 90:10, the working wavelength of the polarization-maintaining optical fiber is 1550nm, and the polarization-maintaining optical fiber coupler is provided with a panda type stress rod.
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