There are the key factors to consider when choosing a manual adjustable fiber delay line：

Delay Range

• The maximum and minimum delay that the device can introduce.
• This is crucial for applications requiring specific delay values.

Delay Resolution

• The smallest increment of delay that can be accurately adjusted.
• A higher resolution is essential for precise timing control.

Fiber Type

• The type of optical fiber used (single-mode or multi-mode) should match the application's requirements.
• Single-mode fibers are ideal for long-distance and high-bandwidth applications, while multi-mode fibers are suitable for shorter distances and lower bandwidths.  

Polarization Maintaining (PM) Capability

• Whether the device maintains the polarization state of the input light.
• PM fibers are necessary for applications that are sensitive to polarization, such as interferometry and coherent communication systems.

Environmental Factors

• The device's tolerance to temperature, humidity, and vibration.
• These factors can affect the stability and accuracy of the delay.