SAPs

What Are the SAPs？

The stress-applying parts (SAPs) are components inserted into the cladding of a stress-based polarization-maintaining optical fiber. These SAPs are positioned on opposite sides of the fiber core and serve the purpose of inducing birefringence in the core. SAPs are typically made from materials with a thermal expansion coefficient that differs from that of fused silica, which is the common cladding material used in optical fibers. During the manufacturing process, as the fiber cools, the varying thermal expansion coefficient of the SAPs generates stress in one axis of the core, resulting in a highly birefringent fiber known as HiBi Fiber. The distinctive shapes of these SAPs have given rise to the two most well-known polarization-maintaining fibers in the optical market: Bow-tie and Panda.

How Many Types of Commercial PM Fibers Include SAPs?

1. PANDA Fiber

The PANDA fiber incorporates two boron-doped glass cylinders arranged longitudinally in the cladding, positioned on opposite sides of the core. The borosilicate glass used in the cylinders has a lower refractive index and a higher coefficient of thermal expansion (CTE) compared to the core and cladding compositions. As a result, stress regions are formed during the drawing and cooling process. The fabrication process involves several intricate procedures, which include:

• Precise positioning and sizing of the holes without any cracks, imperfections, or impurities.

• Acquiring or manufacturing accurately doped and uniformly sized boron-doped boro-silicate rods that fit perfectly into the holes.

• Managing the induced stresses by processing the ends of the preform. Ensuring the cylindrical shape of the stress-applying parts (SAPs) is maintained throughout the drawing process.

• Implementing various other meticulous steps to ensure the overall quality and performance of the fiber.

While it may seem like a simplified process, the fabrication of PANDA fiber involves numerous complex procedures, demanding careful attention to detail and precision at each stage.

2. Bow-tie Fiber

Bow-tie fiber also utilizes boron dopants in two longitudinal stress-applying parts (SAPs), which are fabricated in the cladding on both sides of the core, similar to PANDA fiber. However, there are significant differences between the two. In the case of bow-tie fibers, the SAPs have a wedge-shaped or trapezoidal design. Additionally, the fabrication of bow-tie SAPs takes place while the preform is on the MCVD (Modified Chemical Vapor Deposition) lathe.

Instead of drilling holes and inserting rods into the cladding, the bow-tie SAPs are created through a multi-step process. Initially, a layer of boron-doped glass is deposited near the core of the cladding. Then, specific areas of that layer, opposite the core, are selectively etched away. Subsequently, glass with other dopants is deposited to refill those etched regions.

This approach allows for the SAPs to be positioned closer to the core, resulting in higher birefringence with less induced stress. However, similar to the PANDA process, fabricating the bow-tie SAPs involves numerous precise steps and techniques. Essentially, the complexities associated with drilling and inserting boron rods are replaced with the intricacies of etching and re-doping, which include:

• Coordinating the flow of etchant, typically a fluorine compound, with the size and position of the etching burners.

• Achieving accurate volume and positioning of etching in the two regions along the length of the preform.

• Controlling the burner position and dopant flow during multiple steps to deposit the appropriate core and cladding glasses in the etched areas.

• Managing the collapse of the MCVD preform to achieve the desired bow-tie SAP shape and position.

Ensuring proper processing of the preform ends to avoid issues with the stress regions.The bow-tie process allows for flexibility in adjusting the size, position, and shape of the stress regions, enabling the manufacturer of polarization-maintaining fibers to customize the level of birefringence for different applications while balancing optical and mechanical properties. However, due to the limitations of the MCVD deposition tube diameter, the bow-tie process lacks flexibility in terms of the amount of fiber that can be drawn from a single preform.

3. Elliptical-stress-layer Fiber

Elliptical-stress-layer fiber preforms, similar to bow-tie preforms, are manufactured using MCVD lathes. In the case of elliptical-stress-layer PM fiber, a boron-doped glass ring is also incorporated into the cladding near the core. However, the elliptical-stress-layer fiber employs a machining process to remove a portion of the cladding layer, as opposed to the chemical etching utilized in the bow-tie process. During this step, the initially round or circularly symmetric preform is machined to create two flat sides positioned opposite each other with respect to the core.

The resulting preform, which now possesses flat sides and is roughly rectangular in shape, is carefully drawn while controlling the temperature and tension. This drawing process ensures that the resultant fiber becomes round, effectively eliminating the flat surfaces. Simultaneously, the borosilicate layer, previously in a ring shape, transforms into an elliptical configuration, generating an asymmetric stress region within the cladding. With precise drawing, the core can maintain its round shape. The complexities associated with this method involve:

• Ensuring meticulous performance deposition and machining to prevent stress mismatches.

• Achieving precise machining of the flat sides with uniformity along the length of the preform.

• Preparing the machined preform for drawing to minimize surface defects.Controlling the drawing temperature to achieve the correct viscosity and desired fiber shape.

While it may seem like a simplified process, the fabrication of PANDA fiber involves numerous complex procedures, demanding careful attention to detail and precision at each stage.

Why Are There Numerous Trade-offs Among the Three Types of SAP?

There are several trade-offs to consider when comparing the three types of stress-applying parts (SAPs) in polarization-maintaining (PM) fibers. These trade-offs include:

• Level of Birefringence: The attainable level of birefringence in the fiber is influenced by factors such as the proximity of the SAPs to the core. Different SAP types offer varying levels of birefringence.

• Size and Asymmetry of Stress Regions: The size and extent of asymmetry required to achieve high birefringence can impact the complexity of manufacturing and the strength of the fiber. Different SAP designs result in different stress region characteristics.

• Uniformity of Stress Regions: The ability to fabricate stress regions uniformly throughout the fiber is an important consideration. Variations in stress distribution can affect fiber's performance and consistency.

• Preform Size and Fiber Length: The choice of SAP type can influence the size and length of the preform and resulting fiber. Bow-tie and elliptical-stress-layer PM fibers rely on the MCVD process, while PANDA fibers can utilize outside-deposition methods like OVD (Outside Vapor Deposition) or VAD (Vapor Axial Deposition).

• Mechanical Properties: The strength, crack resistance, and other mechanical properties of the fiber are crucial factors to consider. Different SAP designs can impact these properties differently.

• Complexity of Preform Processing: The complexity of processing the preform, particularly when SAPs are located close to the core, is an important consideration. Ensuring the preservation of SAP shapes during the drawing process can be challenging and may vary depending on the SAP type.

Considering these trade-offs, the choice of SAP type depends on the specific requirements of the application, balancing factors such as birefringence level, manufacturing complexity, mechanical properties, and performance uniformity.