Research On The Design And Application Of A New Type Of Fiber Polarizer And Interferometer

Fiber polarizer is a kind of basic fiber passive device.Since the 1980s,many high-performance fiber polarizers have been developed.However,the conventional fiber polarizers have fixed polarization direction and extinction ratio.They are usually used as an auxiliary device for a fiber optic system.The report of using fiber polarizer as sensitive device is very rare.It will be of great significance to design a fiber polarizer with adjustable polarization degree and polarization directions,since this kind of fiber polarizer can be applied as a sensitive device.Magnetic fluid is a new type of functional material with a variety of magneto-optical properties that enable the dynamic modulation of the polarization state of light.This paper designs a tunable fiber polarizer based on magnetic fluids and attempts to explore its application areas.The fiber interferometers exploit optical fiber as the transmission medium to build an optical path.It has merits of simple structure,flexible operation,and very sensitive detection capability.They are widely utilized for designing fiber sensors.However,its production and application still face many obstacles,such as complex transmission spectrum,low resolution,poor reproducibility,high operational difficulty,and expensive signal acquisition systems.In order to solve these problems,this paper designs a wave front spliting fiber interferometer that is construsted based on broadband light source,and realizes high precision refractive index measurement.Additionally,a real time orthogonal demodulation scheme for laser interferometer is proposed for phase unwrapping.High-precision distance measurement is realized.The detailed research contents are as follows:1.Design of the fiber polarizer based on magnetic fluid and microfiber.On the surface of the microfiber,there exists a strong three-dimensional axisymmetric evanescent wave,which can be easily modulated from all directions.In this paper,a microfiber is immersed in a magnetic fluid,and a magnetic field is applied perpendicular to the direction of the fiber.An in-line tunable fiber polarizer is designed.The principle of the light polarization modulation is based on the magneto-optical dichroism of the magnetic fluid,and the non-circular symmetric distribution of the magnetic nanoparticles around the microfiber.This tunable fiber polarizer allows for the arbitrary modulation of the degree of polarization and polarization directions.We have found the following experimental results.A thinner microfiber has a larger insertion loss,but it can generate a large polarization degree.After applying a magnetic field with a certain intensity,the increase of the light polarization degree will last for a long time before reaching magnetic saturation.To define a saturation magnetic field for the magnetic fluid is meaningless.High concentration magnetic fluids,as well as strong magnetic fields,all have strong polarization modulation capability.However,at the same time,greater optical power loss can be induced.When the direction of the magnetic field changes from 0^o to 360^o,the fiber polarizer exhibits a good orientation response.2.Design of the fiber polarizer based on magnetic fluid and D-shaped fiber.Due to the fragility of the microfiber,it is difficult to operate,and cannot be reused.D-shaped fiber is more robust than microfiber.It also has strong evanescent wave on the polished surface,which facilitates the modulation of the light polarization state.In this paper,by immersing a D-shaped fiber in the magnetic fluid and applying a magnetic field parallel to the polished surface of the fiber,an in-line robust fiber polarizer is designed.Both theoretical analysis and experimental results demonstrate that the magneto-optical dichroism of magnetic fluid plays a key role in the D-shaped fiber polarization modulation.When a magnetic fluid film is solidified in the external magnetic field,regular lines composed of clusters of magnetic nanoparticles can be observed with a microscope.In the process of optical polarization modulation,the evanescent field polarized parallel to the magnetic field has a greater loss than its orthogonal component.By researching into the factors that affect the light polarization modulation,it is found that the polarization of the D-shaped fiber with a wide polished surface is easy to modulate,the high-concentration magnetic fluid and the large magnetic field have great ability to modulate the polarization of D-shaped fiber.Additionally,the time response of light polarization modulation is also obtained in our experiments.3.Design of vector magnetic field sensor based on magnetic fluid and microfiber.By immersing a microfiber in the magnetic fluid,a vector magnetic field sensor is designed based on the detection of the light polarization state.It is found that,if the magnetic field intensity is large,the response time of the light polarization modulation will be very long.It is not suitable to measure the time-varying magnetic field.Light polarization modulation will have a good transient response only when the intensity of the magnetic field satisfies a certain threshold condition.Under the detection threshold of magnetic field,the influence of hysteresis can be neglected.The magnetic field sensor has a good orientation response to the direction of the magnetic field.However,it should be pointed out that there is an uncertainty of 180^o in the direction detection of the magnetic field.Because the magnetic field sensor cannot distinguish the sign of the direction of the magnetic field.Unlike the sensing of the magnetic field intensity,if the analyzer rotates fast enough,the measurement of the direction of the magnetic field is not limited by the detection threshold of the magnetic field intensity.The signal analysis system of the vector magnetic field sensor consists of a polarizing beam splitter and two photodetectors.The sophisticated and expensive spectral analysis equipment is not required.The magnetic field sensor has a simple structure and low cost.4.Design of label-free biosensor based on magnetic fluid and microfiber.Protein can induce self-assembly of the functionalized magnetic nanoparticles to form large clusters.In the base solution,the movement speed of a magnetic nanoparticle cluster is smaller than that of the single magnetic nanoparticles.When the external magnetic field is applied or removed,the response time of polarization modulation will become longer.In this paper,a label-free fiber biosensor is designed based on the response time variation of the microfiber polarization modulation.Bio-sensing experiments were carried out by taking bovine serum albumin as an example.The maximum polarization degree of p_max,and the difference?p between the maximum and minimum degrees of polarization,obtained by polarization modulation,monotonically decrease as the concentration of bovine serum albumin increases.Therefore,the concentration of bovine serum albumin can be obtained based on the detection of p_max and?p.Since the functionalization of the microfiber is no need,the experimental operation is greatly reduced,and the bio-sensing process is simplified.The system of signal acquisition does not need expensive spectral analysis equipment.It is inexpensive and has a fast time response.5.Research of the wave front splitting robust fiber Mach–Zehnder interferometer?MZI?.A cavity is constructed between two aligned Green lens fiber collimators.After inserting a piece of polyethylene terephthalate?PET?film in the cavity,the wave front of the Gaussian beam emitted from the first fiber collimator is split into two parts.Then,the Gaussian beam is focused into a single mode fiber by another collimator,constituting a stable and robust fiber MZI.The effective spectral width is larger than 390 nm,which is wide enough for constructing a sensitive fiber MZI.The COMSOL Multiphysics simulation and the experimental results demonstrate that this MZI is based on the multi-beam interference.Thanks to the low insertion loss of the fiber MZI,two types of stable and repeatable interference fringes are obtained when the sampling accuracy of the optical spectrum analyzer is merely 0.1 nm.It ensures that the fiber sensor based on this MZI has a high resolution.The two adjacent resonant wavelengths respectively shift–0.78 and–0.81 nm when salinity increases one part per thousand,showing high sensitivities.It can be widely utilized in detecting physical,chemical,or biological quantities.6.Research of laser range finder based on inflection point retrieval for phase unwrapping of interference signal.A real-time wavelength scanning laser range finder is proposed.The interference signal generated by the light wavelength modulation is transformed into preliminary conversion signal with the two summarized waveform transformation laws.Inflection points of the interference signal are retrieved with two complementary methods that are discontinuity point judgment and the adjacent null points distance judgment.Based on the preliminary conversion signal and the retrieved inflection points,the orthogonal signal is constructed.According to the central wavelength and the wavelength modulation depth,distance can be calculated after demodulating the phase shift with the orthogonal demodulation algorithm.When distance of several millimeters is measured,the fluctuation of the measured result is around micrometers in one hour.Compared with the PGC demodulation scheme,the measurement complexity and cost of our system are all decreased.While,ideal measurement results can still be obtained.