Optical Delay Line Converter

Optical Delay Line (ODL)

An Optical Delay Line (ODL) is an electric-optic-electric instrument. It performs fixed time delay(s), between 0.01 and 250 μsec for RF signals  up to 40GHz and more. The optical signal is transmitted into a long single-mode fiber, usually at a 1.55 micron wavelength or similar. Passing through the fiber and a series of optical switches, the optical signal is converted back into an electrical RF signal. The electrical control on the ODL elects the optical system automatically, with no need for any tuning by the operator.

An ODL is required as a measurement tool to determine the signal distance from the target in Radar systems. Since the speed of light in the fiber is slowed down by the fiber (index of reflection), 100 μsec equals about 20 km.  Since the propagation of the light is equal for all RF frequencies, an optical delay line is suitable for a wide range of frequencies, in contrast to a coax delay line.

In general, there are three types of Optical Delay Lines:

  1. Any type of fixed or progressive ODL up to 6.0GHz with direct modulation (L, S, & C bands)
  2. Any type of fixed or progressive ODL up to 8.0GHz – 18GHz with indirect modulation (L, S, C, X, & Ku bands).
  3. Any type of fixed or progressive ODL up to 20GHz, up to 30GHz and up to 40GHz upon request.

For some radar calibration & testing, signal & phase noise processing, and extension of radar range site applications, a Mini ODL in a ruggedized enclosure is required, available up to 6 GHz with direct modulation, and from 6 GHz up to 18 GHz with indirect modulation.

When hundreds of delay combinations are needed, variable ODL is required, which supports up to 12 distinct delay lines.

An Optical Delay Lines system (ODL) also incorporates high-performance lasers such as DFBs, optical modulators for high operation frequencies, photodiodes, and optionally other components such as optical dispersion compensators, optical switches, optical amplifiers, and pre- and post RF amplifiers to provide exceptionally high performance. The ODL optical system supports very high bandwidths of analog signals, high sensitivity with wide dynamic range, for various delays.

ODL solutions consist of the following components:

  • Transmitters (Tx), receivers (Rx), and optimal switches
  • Optical amplifiers (to amplify the optical data signal without changing it into its electrical form)
  • RF amplifiers (to amplify a low power RF signal to one with a larger amplitude)
  • Dispersion compensators (to compensate for loss due to dispersion)

The most common practical approach for a variable delay system is an ODL system configuration, which includes cascaded 1:2 and 2:2 optical matrices with several different delay lines in between (replacing the above two optical switch matrix 1:8). This cascaded switch matrix is a Progressive Delay Configuration, as shown below.

The desired combination of delay lines is selected to define the required delay. In the diagram below, there are 4 progressive delay lines with cascaded switch matrices. In such a configuration, the user can select any of the 16 combinations of possible delay values (16=24). For example, a delay can be selected which is equivalent to Dtot= D1+D2 +D4, or Dtot= D3+D4, etc.)

Progressive Optical Delay Line configuration consisting of four 2:2 optical switch, providing 16 different delay lengths.

Sometimes, dispersion compensation is needed when the signal frequency and the delay line length increase, resulting in the optical signal to be dispersed and weakened significantly. As RFOptic, we solve this problem by incorporating DCM (Dispersion Compensation Module) in our Optical Delay Line (ODL) solutions.

For very long delay lines, RFOptic also uses Optical Amplifiers (EDFAS) to compensate for the optical loss. In some cases, also Pre and Post amplifiers are required.

Any changes of the delay lines and the attenuation are made through a user-friendly software interface with easy to use GUI (Graphic User Interface).

ODL features include:

  • RF delay accuracy: 0.5%
  • Software: Local and remote through USB
  • Max. delays: From 0.01 to 250 μsec for all frequencies except Ka, which is upon request
  • Number of states: Up to 255 delays
  • High Dynamic Range

Applications include:

  • Radar testing
  • Remote antenna & Radar calibration
  • Electronic warfare (EW) and defense applications
  • Telecommunications testing and simulating channel delay
  • Radar for the automotive industry

For more information about Optical Delay Lines, contact us at info@rfoptic.com