A Delay Line Interferometer is an optical device that incorporates a delay line in one of its arms to introduce a controlled path length difference between the interfering beams. By adjusting the delay line, the interferometer provides precise control over the interference pattern. It also enables flexible measurements in various positions or depths within a sample. The commonly used and well-known configurations of delay line interferometers are the Mach-Zehnder interferometer and Michelson interferometer, both of which are based on two-beam interference. Sagnac interferometer, Twyman-Green interferometer, and Common Path interferometer are other configurations and variations of delay line interferometers.

The delay line can be implemented using different techniques:

• Length of optical fiber - By adjusting the length of the fiber, the path length difference can be controlled. The fiber can be straight or coiled, depending on the desired delay and space constraints.
• Optical Components - Delay lines can also be implemented using specific optical components. For example, a delay line can be created using mirrors or retro-reflectors that introduce a physical displacement in the optical path. These components redirect the beam, effectively increasing the path length.
• Moving Components - In some cases, the delay line may involve moving components, such as a translation stage or a scanning mechanism. These components can be used to physically shift the position of one beam relative to the other, creating a controlled delay.

Both Mach-Zehnder and Michelson interferometer configurations involve splitting an incoming optical signal into two beams. One of these beams is intentionally delayed in time relative to the other by a desired interval. while the other beam follows the direct path without any delay. The delay element such as an optical fiber or waveguide, air gap, retroreflectors, etc. is placed in the delay path of one of the arms of the interferometer. The beam passing through the delay line experiences a time delay due to the longer path length it travels compared to the undelayed beam. By controlling the length of the delay line, the precise time delay can be adjusted.

The delayed and undelayed beams are recombined at the beam splitter, and their interference pattern generates an output signal with an intensity pattern dependent on the introduced time delay. This interference pattern depends on the relative phase between the two beams. To control the phase relationship between the beams, the delay length can be adjusted. Furthermore, manipulating the interference pattern allows for the extraction of useful information or modulation of the signal. Both of these interferometers differ in their optical configurations and their operation.

In the above figure, the term "n" refers to the number of passes or round trips that the optical signal makes through the delay line before interference occurs. It represents the number of times the signal is delayed and recombined within the interferometer.

The value of "n" can vary depending on the specific configuration and requirements of the DLI. For example, an “n = 3” pass delay line involves three passes. Higher values of "n" generally provide increased sensitivity, improved signal-to-noise ratio, and enhanced dispersion compensation.

Advantages of Delay Line Interferometers

• Adjustable Time Delay: By introducing a controlled and adjustable time delay between the interfering beams, it is useful for time-domain measurements, where precise control over the relative timing of the signals is crucial.
• Coherence Length Manipulation: The effective path length difference in the interferometer can be controlled by adjusting the delay line. This manipulation allows for extending the coherence length of the interferometer.
• Dispersion Compensation: The interferometer can be tuned to correct for dispersion-induced broadening or distortion of optical signals, improving signal quality and preserving signal integrity.
• Precision Measurement Capabilities: Delay line interferometers offer high precision in measuring parameters such as time delay, phase, frequency, or other relevant parameters. 
• Signal Processing and Modulation: The phase relationship between interfering beams can be controlled, enabling techniques such as frequency shifting, signal mixing, or waveform generation.
• Versatility and Flexibility: The interferometer can be implemented using different delay line technologies, such as optical fibers, coiled fibers, or other optical components. This flexibility allows for customization and adaptation to different experimental setups or specific requirements

Applications of Delay Line Interferometer

• DLIs are commonly utilized as optical DPSK (Differential Phase-Shift Keying) demodulators. Their purpose is to convert a phase-keyed signal into an amplitude-keyed signal. In this application, an incoming DPSK optical signal is split into two equal-intensity beams within the two arms of a Mach-Zehnder or Michelson interferometer. One of the beams is then delayed by an optical path difference equivalent to a 1-bit time delay. Upon recombination, the two beams interfere with each other, resulting in constructive or destructive interference. The resulting intensity pattern represents the amplitude-keyed signal.
• This interferometer is also employed as optical delay lines in various systems. By precisely controlling the delay length, DLIs enable time-domain manipulations of optical signals. They find applications in optical signal processing, optical coherence tomography (OCT), optical imaging, and other fields where controlled time delays are required.
• Its ability to introduce time delays makes it valuable in interferometric sensing applications. By incorporating a sensing element within the delay line, minute changes in the environment can be detected. DLIs have been utilized in sensing applications such as displacement measurements, vibration analysis, and strain sensing.