Application Prospects of Optical Fiber Structured Light Field

Due to its special phase and polarization state distribution characteristics, spatially structured light fields have important research value and broad application prospects in the fields of precision measurement, super-resolution imaging, particle manipulation, nonlinear optics, quantum optics, and large-capacity optical communications.

Generally, space structured light fields can be divided into two categories: one is free space structured light fields, and the other is fiber structured light fields.

Compared with free space structured light fields, the generation of fiber structured light fields is strictly constrained by boundary conditions, and the types of structured light fields are very limited, but fiber structured light fields also have some incomparable advantages. For example, when the structured light fields with different wavelengths and intensity/polarization state distributions are naturally coaxially transmitted in the optical fiber, the coaxiality and environmental stability of the stimulated emission depletion (STED) fluorescence microscopy illumination light source can be greatly improved;

The optical fiber transmission loss is low, and the optical fiber structured light field is used as a new degree of freedom of the optical fiber communication system, which can greatly improve the information transmission capacity of the long-distance optical fiber communication system; the optical fiber structured light field is transmitted inside the optical fiber, and when it is used to excite the fiber end face When the metal tip is integrated, it can not only realize the generation of high-power density nano-focusing light source, but also eliminate the background noise of far-field excitation, which has very important application value in high-resolution spectroscopy measurement.

Optical Communication

STED imaging

STED fluorescence microscopy uses two lasers with different wavelengths, different energy distributions and strictly coaxial, one is a Gaussian beam, which is used as excitation light; the other is a ring beam, which is used as loss light.

The excitation light causes the fluorescent molecules in the Airy disk range to be excited, and their electrons transition from the ground state to the excited state. The loss light makes the excited state molecules around the excitation spot return to the ground state by stimulated radiation, while the excited state molecules located in the inner region of the excitation spot are not affected by the loss light and still return to the ground state by autofluorescence. This coaxial illumination method can limit the autofluorescence area to an area smaller than the Airy disk, which greatly improves the imaging resolution and can be widely used in biomedicine and other fields.

Conventional STED illumination sources are built in free space using bulk optics. In order to ensure that the excitation light and the loss light are highly coaxial, a strict coaxial calibration process of two beams is required, and the system stability is easily affected by the external environment. The two beams of light with different wavelengths and different intensity distributions in the fiber are naturally coaxial, without the need for a harsh double-beam coaxial calibration process, and are not easily affected by the external environment, which is very convenient in constructing STED lighting sources.

Long-distance, high-capacity optical fiber OAM communication

At present, OAM communication has attracted extensive attention of researchers at home and abroad. The amplitude, phase, polarization, wavelength and time dimensions of light waves, as the degrees of freedom of optical fiber communication systems, have been widely used to increase the information transmission capacity of optical fiber communication systems. The optical fiber OAM beam is used as a new degree of freedom in the optical fiber communication system, which can further improve the information transmission rate of the optical fiber communication system.

Using the vortex fiber that can achieve high separation of degenerate vector modes, the OAM beam is used as a new degree of freedom in the optical fiber communication system, and the OAM optical fiber communication system is constructed. Information transmission, this research work provides an experimental basis for the use of OAM beams for long-distance, high-capacity optical fiber communication. Subsequently, many researchers at home and abroad have carried out a lot of research work on optical fiber OAM and optical fiber column vector optical communication, such as Li SH of Huazhong University of Science and Technology and Li J P of Sun Yat-sen University, etc., which increased the transmission distance and capacity of optical fiber communication systems.

Plasma Tip Nanofocusing

The plasmonic tip nanofocusing can break the diffraction limit of conventional optical systems and localize the light energy in the nanoscale range of the tip of the tip. It has important application value in the fields of tip-enhanced Raman spectroscopy and nano-nonlinear spectroscopy. Usually, the far-field excitation light is used to illuminate the metal tip to achieve nano-focusing at the tip of the tip. However, the focused spot size of the excitation light is much larger than that of the metal tip. When the metal tip is excited to achieve nano-focusing, a large amount of background noise will also be generated. , which will reduce the sensitivity and resolution of the tip Raman/nano nonlinear spectroscopy detection system.

Using the radial polarization vector light field of the optical fiber to excite the metal-coated fiber tip can not only realize the nano-focusing of the plasma tip, but also eliminate the background noise of the far-field excitation. The results show that the metal-coated needle tip nano-focusing light source has radial polarization distribution characteristics.

Nonlinear frequency conversion

Stimulated Raman scattering (SRS) effect in optical fibers is one of the important research hotspots in the field of nonlinear fiber optics. Using the SRS effect, fiber Raman lasers and fiber Raman amplifiers can be constructed, both of which have important application value in the field of fiber optic communications. In addition, the new frequency components generated by the SRS effect in the optical fiber communication system will cause the channel power distribution imbalance of the optical fiber communication system, thereby reducing the information carrying capacity of the communication system.

In the optical fiber communication system, although the optical fiber structured light field, as a new degree of freedom, can greatly improve the information carrying capacity of the system, however, the SRS effect caused by the structured light field during long-distance transmission in the optical fiber has no impact on the data transmission of the communication system. It is not clear, so it is crucial to study the SRS effect caused by the transmission of spatially structured light fields in optical fibers.

A nanosecond radially polarized vector light field (1064 nm, 22 ns, 10 Hz) was generated in a vortex fiber using an experimental setup, and the nanosecond radial vector light field was measured when it propagated in a 100 m long vortex fiber Relationship between SRS spectral intensity and pump pulse power. With increasing pump power, Stokes lines with a frequency separation of about 13 THz can be clearly observed. The polarization state detection results of the spectra of the pump light, the first- and second-order Stokes lines, and the corresponding transverse mode field intensity distributions show that the transverse ring mode field intensities of the first and second-order Stokes lines The distribution still maintains the radial polarization distribution characteristics.

Outlook

Optical fiber structured light field, as an important branch of light field regulation, has attracted extensive attention in many fields. Up to now, researchers have carried out a lot of work on the generation mechanism and methods of optical fiber structured light fields, and have made a series of research progress; The researchers have carried out a series of explorations in STED imaging, fiber vortex optical communication, background-free nanofocus light source generation, and nonlinear frequency conversion. After more than ten years of unremitting efforts, optical fiber structured light field regulation has been developed by leaps and bounds, but there are still many problems to be solved urgently.

Optical fiber structured light field splitting/combining. The splitting/combining of energy in optical fibers is the key to constructing an all-fiber communication system. Conventional fiber couplers are prepared by using single-mode fibers, and based on the evanescent wave coupling of tapered fibers, fiber energy splitting can be realized. However, the fiber-structured light field is a high-order vector mode, which can only be transmitted in few-mode fibers. To realize the splitting/combining of the fiber-structured light field, it is necessary to realize the energy splitting/combining and at the same time. To ensure that the energy coupled into other fibers still maintains the same phase and polarization distribution as the original optical field, this part of the research work still lacks theoretical and experimental support.

Preparation of few-mode fibers supporting steady-state transmission of structured light fields. Most of the optical fields of optical fiber structures are generated by conventional few-mode fibers. Such fibers increase the number of vector modes that can support transmission by increasing the core diameter, but they cannot achieve effective separation of degenerate vector modes. Although this type of fiber can be used to generate a structured light field, the phase/polarization distribution of the fiber structured light field is easily affected by external disturbances because the effective refractive indices of the same group of higher-order vector modes are very close. In addition, although there have been reports of long-distance, high-capacity optical communication using vortex fibers, such fibers can only achieve effective separation of the first group of higher-order degenerate vector modes, and do not support higher-order degenerate vector modes. Mode transmission and efficient separation. The preparation of few-mode fibers that support effective separation of higher-order degenerate vector modes and steady-state transmission will greatly improve the information transmission capacity of fiber-optic mode division multiplexing systems.

Precise wavelength manipulation of fiber-structured light fields. The generation of optical fiber structured light fields is mainly concentrated in the communication band. Although the wave vector matching conditions of gratings can be met by tuning the laser wavelength to realize the generation of optical fiber structured light fields, many subsequent applications, such as STED imaging, background-free nano-focusing light sources, etc., All need to achieve efficient generation of optical fiber structured light fields at specific wavelengths.

At present, the existing preparation methods can only write grating structures with a fixed period, and lack the ability to generate light fields with specific wavelength structures. Therefore, it is necessary to find a more suitable method or improve the existing grating fabrication process to realize the precise wavelength manipulation of the optical fiber structured light field.

In conclusion, there are still many problems in the regulation of optical fiber structured light fields that need to be solved urgently. Breakthroughs in related theories and technologies can not only enrich the means of optical fiber structured light field regulation, but also greatly promote its application in biophotonics, high-capacity optical fiber communication, nanophotonics applications in the fields of science and nonlinear spectroscopy.