Principle and Application of a PM Coupler

Principle of a PM Coupler

A PM (Polarization Maintaining) coupler is a device used to couple optical signals from a polarization-maintaining fiber while ensuring that the polarization state of the light signal remains unchanged. The working principle of a PM coupler is as follows:

• Characteristics of a PM Fiber: A PM fiber has a unique core structure that effectively maintains the polarization state of the light signal. This means that during transmission through the optical fiber, the polarization state does not change, which is crucial for applications requiring precise control of polarization.

  
  

• Role of Optical Elements: The optical signal undergoes the influence of optical elements, which couple the light signal either by merging or separating the signal in different ways. These optical elements can be part of a fused taper coupler, where the fiber is locally heated and stretched, causing the fiber core to become thinner and enabling the coupling of the optical signals.

  
  

• Coupling Process: In a fused taper coupler, two or more fibers are locally heated and stretched simultaneously to form a special waveguide coupling structure in the shape of a double cone. As the heating and stretching process proceeds, the fiber core gradually becomes thinner and eventually merges into a composite waveguide structure, where the cladding acts as the core, and the outer medium as the new cladding. In this process, the light signal propagates through the composite waveguide with the new cladding as the core and undergoes redistribution. Ultimately, some light exits directly from the through arm, while the rest exits from the output arm.

  
  

• Maintaining Polarization State: In coupling region I, due to the merging of the two fiber claddings and their cores being sufficiently close, the coupler forms a weak coupling structure of the two waveguides. According to weak coupling mode theory, the fields in the two coupled waveguides retain their respective field distributions and transmission coefficients as if the waveguides were independent. The coupling only affects the complex amplitude of the fields, without altering the polarization state.

  
  

Applications of a PM Coupler

A PM coupler is primarily applied in the following fields:

• Optical Fiber Sensing: A PM coupler is used in optical fiber sensing systems, where it can stably transmit two orthogonal linear polarization states and maintain their polarization unchanged. It is suitable for precision sensing devices such as fiber optic gyroscopes and fiber optic hydrophones.

  
  

• Interferometric Measurement: In applications that require high-precision measurements, such as optical interferometers, a PM coupler ensures that the polarization state of the beams remains stable, thereby improving the accuracy of measurements.

  
  

• Planar Waveguides: A PM coupler also plays a critical role in planar waveguide applications, where it helps achieve efficient signal transmission.

  
  

• Coherent Optical Communication: In the field of coherent optical communication, a PM coupler ensures the stability of the polarization state during optical signal transmission in fibers, making it suitable for wavelength-division multiplexing (WDM) systems.

  
  

• Fiber Amplifiers and Fiber Lasers: A PM coupler plays an important role in fiber amplifiers and fiber lasers, especially in applications requiring precise control over optical signal transmission.

  
  

• Optical Communication Networks: In 5G base stations and other modern optical communication networks, a PM coupler is used to connect different optical modules, enabling multi-wavelength multiplexing and enhancing the capacity of optical fibers.