What Is a Fiber Optic Delay Line?

Fiber optic delay line is a new type of microwave frequency signal processor, whose primary function is to achieve delay control of the signal. The fiber delay line should have a flat amplitude-frequency characteristic and certain phase shift characteristics (or delay-frequency characteristics) within the passband, appropriate matching impedance, and low attenuation. In digital and analog signal processing technologies, surface acoustic waves, charge-coupled devices, coaxial cables, and helical lines are usually used as signal processing devices.

Applications of  Fiber Optic Delay Line

Due to the advantages of fiber optic delay line, such as small size, light weight, high bandwidth, minimal loss, resistance to electromagnetic interference, and delay-independent frequency (true delay), they have replaced traditional bulky and inferior-performance metal waveguides and coaxial cable delay lines. These are widely used in radar, electronic computers, color television systems, communication systems, and measuring instruments (e.g., oscilloscopes). They also have numerous applications in the military field, where controlling the delay of enemy radar signals could disrupt the enemy's real-time judgment of war information.

Classification of  Fiber Optic Delay Line

Given the importance of fiber optic delay lines, many documents explicitly mention the term "fiber optic delay line" for such devices. Based on the application fields and systems, they come in various structures. Structurally, they can be categorized into two major types:

Simple Fixed Delay Fiber Optic Delay Lines

Simple fixed delay fiber optic delay lines include single-fiber type, loop-coupled type, tapped type, and fiber Bragg grating (FBG) type; 

Complex Programmable Delay Fiber Optic Delay Line Arrays

Complex programmable delay fiber optic delay line arrays include forward type (further divided into parallel and series types), feedback type, and hybrid type. The complex programmable delay fiber optic delay line arrays are built upon simple fixed delay fiber optic delay lines as basic delay units.

Basic Principle of  Fiber Optic Delay Line

The RF signal is input into a laser diode (LD), and the LD converts the input RF electrical signal into an optical signal modulated by the RF signal, which is coupled into the optical fiber through an optical connector. The photodetector (PD) converts the RF-modulated optical signal back into the original RF signal. The output RF signal has the same spectrum as the input RF signal, but the optical fiber serves as a medium to delay the signal by a certain amount of time. That is, the RF signal is instantaneously stored in the fiber optic delay line unit, and the storage time 't' is proportional to the length 'L' of the optical fiber. Without considering dispersion, when an optical pulse signal propagates in the fiber at group velocity 'v', △t = L/v = Lng/C  (1)

In equation (1), 'ng' is the group refractive index of the fiber, and 'C' is the speed of light in a vacuum. By changing the length of the optical fiber, different delay times can be achieved. The accuracy of the fiber optic delay line is mainly determined by the fiber cutting precision, and the delay range is approximately from picoseconds to milliseconds. Based on the current level of technology, the fiber cutting length error can be controlled within 0.8~1.0mm, and the delay accuracy is generally between 1~10ps.