Applied science is a very large class of disciplines that apply existing scientific knowledge to practical systems. Basic science develops a wealth of fundamental information to predict, explain, and understand phenomena in the natural world. Whereas applied science is the use of scientific processes and knowledge as a means to obtain specific practices or useful results. This includes a wide range of fields related to applied science, including engineering and medicine.

Now the Internet industry is so active, it seems that when we talk about "applications", we are talking about mobile APPs. So what applications can our polarization-maintaining fibers be used for?

 (1) Interferometer

Applications for polarization-maintaining fibers include telecommunications, medicine, and sensors. A very typical application is to use interference for measurement to ensure that the light propagating in the signal arm and reference arm of the interferometer is always recombined with the same polarization state, and the optical fiber is used to ensure optical constructive interference to prevent the ability of signal attenuation. If a conventional single-mode fiber were used, the polarization state of the light propagating in each arm would vary independently with time, resulting in a recombined signal between maximum and zero as the relative polarization states of the two waveforms varied over 360 degrees decay.

In a nutshell, the fundamentals of polarization-maintaining fiber unleash its capabilities for interferometers. Therefore, in the main application fields, it is related to interferometric measurement technology.

(2) Fiber Optic Gyroscope

Fiber Optical Gyroscope (FOG) is an interferometric fiber optic sensor that has achieved great commercial success. Essentially, a FOG is a rotational and rotational speed sensor that generally consists of three sensing rings of polarization-maintaining fibers, each corresponding to a desired degree of freedom (in aircraft terms: roll, pitch and yaw yaw). Light is simultaneously launched to the fiber ends (both ends) of each sensing ring and recombined at the detector. If the sensor ring is rotated, the travel distance of the light in the two directions transmitted inside will show a certain difference, and a Doppler frequency shift (Sagnac effect) occurs. As a result, the phase of the forward and backward propagating beams appears. Different, the two paths of light that were originally coherent caused interference. It can be analyzed to determine the extent and rate of disturbance, slew.

The basic design of the FOG is a perfect illustration of the main benefit of using optical fibers as intrinsic optical sensing elements; optical fibers have the ability to guide and bend light, thus confining ultra-long optical paths into small physical volumes. These longer path lengths amplify relatively weak optical effects, allowing the fabrication of very compact, high-precision sensors. A typical FOG sensing ring consists of 200 to 5000 meters of polarization-maintaining fiber, depending on the required precision performance, which is now high enough to challenge the accuracy of laser gyroscopes (Boeing aircraft use laser gyroscopes).

On the other hand, the size of the gyroscope is also shrinking. In 1920, the first demonstration of the basic principles of FOG was carried out using free-space optics, which were deployed over an area of several square kilometers. In stark contrast, the same measurements can now be made in a sensing ring smaller than the mouth of a teacup.

(3) Coherent Optical Communication

Polarization-maintaining fibers have been used in communications since the earliest days of special fiber technology. Humans continue to pursue high bandwidth, and technological development has led to higher symbol rates, more parallel channels, and higher-order complex modulation technologies. Today's coherent communications (Coherent Communications) has developed into a very large system, modulation, transmission, Coherent reception and so on have a lot of cutting-edge technology.

Communication technology is mainly about the transmission and reception of signals. The basic principle of coherent optical communication: at the sending end. The signal is modulated on the optical carrier by means of amplitude modulation, phase modulation and frequency modulation by means of external light modulation, and is sent out through the back-end processing. After reaching the receiving end, it first undergoes equalization processing, and then enters the optical mixer, where it is coherently mixed with the optical signal generated by the local optical oscillator, and finally received by the detector. A more efficient approach emerged in the early 1990s with the advent of erbium-doped fiber amplifiers (EDFAs) combined with dense wavelength division multiplexing (DWDM). Universal solutions for high-bandwidth repeater-less transmission are also maturing. To understand coherent optical communication in detail, you need to know many related technologies and terms, such as I/Q demodulation, OOK modulation, BPSK modulation, constellation diagram, etc. The technology is still developing, and this part of the content needs to be shared with you. study.

As a specialized technology, coherent communication is also often used in applications that require real-time processing of large amounts of data, especially in military phased array radar deployments that enable antenna remote processing.

(4) Integrated optics

Signal processing in interferometric sensors and transmission or detection in conventional and coherent communications use polarization-maintaining fibers. Another important technology is Integrated Optics (IO).

IO is most often encountered in lithium niobate (LiNbO3) modulators used in telecom transmitters. A typical modulator consists of a lithium niobate chip in which a waveguide doped with titanium dioxide is diffused, flanked by gold electrodes. The pigtail of the PM fiber provides a stable polarization state and is aligned with the birefringence axis of the chip. The functionality of the device is based on the Pockels effect. When a voltage is applied to the electrodes, the refractive index of the substrate changes in proportion to the voltage. The resulting change in the effective optical path length can be used to generate interference, which, depending on the precise design of the titania-doped waveguide, can be manipulated to provide modulation of phase, frequency or amplitude, or even switch optical power between channels.

Supplement: The Pockels effect was discovered by German physicist Friedrich Pockels in 1893. The birefringence effect of light in an optical medium under a constant or alternating electric field is a linear electro-optical effect in which the change in refractive index is proportional to the magnitude of the applied electric field. But this effect only exists in crystals that lack inversion symmetry (geometry), such as lithium niobate (LiNbO3), lithium tantalate (LiTaO3), barium borate (BBO), and gallium arsenide (GaAs), etc., or Other non-centrosymmetric media exist, such as occur in electric field polarized polymers and glasses. Electric field polarized polymers contain specially designed organic molecules that have nonlinear coefficients 10 times higher than those of highly nonlinear crystals. The difference between the Pockels effect and the Kerr effect is that the Pockels effect is proportional to the magnitude of the electric field, while the Kerr effect is proportional to the square of the magnitude of the electric field.

 (5) Doppler Laser Anemometer and Velocimeter

In many cases, the function performed by polarization-maintaining fibers is to provide a flexible transmission system that enables the processing of weak optical signals. For example, Laser Doppler Anemometry (LDV) and Laser Doppler Anemometer (LDV, Velocimetry) are non-contact technologies for flow velocity measurement. This technique is applied to air flow in wind tunnels, and blood flow in veins and arteries, where flow velocity is determined by measuring the Doppler shift of light scattered from the fluid. To perform the measurement, the linearly polarized light from the laser source is split into two equal components and transmitted to the measurement location through two polarization-maintaining fibers of the same length.

At the output of the polarization-maintaining fiber, a lens focuses the two beams onto a small spot within the moving fluid. At this time, the two light beams converge to form interference fringes. Small particles in the fluid scatter light from each beam at slightly different Doppler frequencies as they move relative to the two beam directions. Some of this scattered light is then collected by a multimode fiber with a larger core diameter and transmitted to a photodetector. Here, the two frequencies combine to form an instantaneous beat frequency. This beat frequency is linearly related to the difference between the Doppler frequencies produced by each laser beam, which determines a linear relationship with particle velocity, forming an overall test setup.

（6） More application scenarios (EDFA pump combiners, reflection suppression schemes, current sensors and optical coherence tomography)

· The use of polarization-maintaining fibers enables long-range transmission of polarized light, extending to various other applications throughout the industry. The evolution of telecom system architectures requires EDFAs to continuously increase power output, in some designs, through polarization multiplexing of 980 or 1480 nm pump diodes. Similarly, the pump diodes are also pigtailed in polarization-maintaining fibers to achieve a polarization-based scheme to suppress back reflections.

· Among the sensors, the Faraday effect current sensing industry has gradually developed. As polarization devices, current sensors rely on delivering a stable and known polarization state to the sensor head, and are typically implemented with polarization-maintaining fibers.

· In medical terms, patients with coronary heart disease are called "chronic total occlusion" (CTO), that is, the blood vessels are completely blocked. Doctors are making the diagnosis with the help of a special catheter or "guide wire" known as OCT. The origin of OCT technology can be traced back to Optical Low Coherent Reflectometry (OLCR) in the telecommunications industry in the late 1980s. OCT uses low coherence (broadband) light. Polarization-maintaining fibers also play an important role, enabling surgeons to distinguish the relationship between vessel wall and self-occlusion by optical coherence reflectometry (OCR), facilitating safe resection.

The application of polarization-maintaining fibers is becoming more and more extensive. Taking advantage of the advantages of optical fiber, driven by the Internet of Things, there will be more meaningful applications. As mentioned in the fiber optic gyroscope technology chapter, the optical fiber has both light transmission and bending properties, which can limit the ultra-long optical path to a small physical volume and amplify relatively weak optical effects, so that very compact of high-precision sensors.