What is an Optical Delay Line?

Optical Delay Line (ODL) is an optical device that is used to introduce a time delay for a light beam. It is an electric-optic instrument that produces fixed time delay(s), between nanoseconds to microseconds, for signals from 10MHz up to 40GHz and more. Frequencies below 10MHz are mostly optical noise, hence no information can be drawn out from them. The low-frequency ODL ranges from 10MHz to 6GHz with a time delay of microseconds and the high-frequency ODLs range from 6GHz to 40GHz with a time delay of nanoseconds. The time delay of microseconds for low-frequency ODL and nanoseconds for high-frequency ODL is due to the fact that time and frequency are inversely proportional.

Construction of an Optical Delay Lines

For the generation of optical delays, at first, pulses are sent through an optical arrangement (e.g., a beam splitter, that splits the incoming pulses into two equal parts simultaneously as shown in the figure above). This setup resembles an intensity autocorrelator (devices for measuring the intensity as a function of light) which contains a simple optical delay line on the left side represented as ODL. The ODL consists of two moving retroreflective mirrors mounted on a movable part (i.e., a mirror surface that reflects radiation with minimum scattering). 

By simply moving the retroreflective mirrors along with the mount we can increase or decrease the path length, which brings a time delay at the output pulse detector. We can adjust the time delay in order to bring constructive interference to the desired order in the output pulse. When the path lengths are simply in the air, then the retroreflective mirrors imply a delay of ≈3.34 ps per millimeter or ≈1 ns for 30 cm. The delay is manually adjusted in some cases, with a micrometer screw. 

What are the different types of ODL?

In general, ODLs fall into two primary categories based on their construction – passive or active.  

Passive ODLs

As the name implies, passive ODLs do not require any power and it consists of optical fiber that is cut and coiled to a precise length to provide a specific time delay as shown in the figure below.  Depending on the length and delay needed, coils are placed inside the portable component for short delays to larger rack-mount framework for longer delays. As the length of the fiber increases the delay time increases. All fiber Mach-Zehnder Interferometer can be made of two fiber couplers (i.e., an optical device used to split the input signals into two or more outputs, they are called splitters in this case) and a fiber-optic delay line as shown in the figure above.

Active ODLs

Active ODLs, are electronically powered devices that perform conversion of a radio frequency (RF) signal i.e, frequency range from around 20 kHz to around 300 GHz, to optical and then converts the output signal again as RF. The device first converts the incoming RF signal into an optical signal by using an optical transmitter, i.e, a device that accepts an electrical signal as input, and converts it into an optical signal.  The optical signal is then transmitted through a single-mode fiber coil of a particular length that provides a delay.  After passing through the fiber coil, the signal is then converted back to an RF signal using an RF modulator and collected as output by the device.

Applications of ODLs

In interferometers, i.e, device that uses the technology of splitting light into two beams that travel different optical paths and are then combined to produce constructive and destructive interference, here tunable delay line are created by making very small changes to the propagation distance of the optical beam. 

Variable time delays are required in optical fiber communications, for transmitting and receiving independent signals over a common signal path. To introduce a fixed time delay, the optical fiber communications contain a particular length of optical single-mode fiber which is wound up to a coil and kept dirt free. Since propagation losses in fibers are low, at particular telecom wavelengths, even fiber lengths of multiple kilometers, corresponding to delay times of microseconds, can be understood without any loss.

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