What is Optical Delay Line - ODL?

The ODL is an electric-optic-electric instrument. It performs fixed time delay(s), between a few nanoseconds up to several hundred microseconds, for RF signals from 10 MHz up to 20 GHz and more (there are low-frequency ODL versions from 10 MHz to 5 GHz, and high-frequency ODLs versions up to 8 GHz, 15 GHz, 18 GHz and 20 GHz. The RF input signal is converted into an optical modulated signal. The optical signal is transmitted into a long single mode fiber, usually at a 1.55 micron wavelength or similar. Passing the fiber, 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 tuning by the operator.

An Optical Delay Lines system (ODL) 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.

What is Light Coefficient?

Light coefficient refers to light traveling at a different velocity in glass fiber and its index of refraction is 1.5. That means that 10 kilometer range in the air is equivalent to 6.666 kilometers in fiber and vice versa. Usually, RFOptic requests data from the user in order to provide microseconds from the fiber so it will be easy to prepare the spools.

Can an ODL Support More than 1 Delay Line?

Yes, RFOptic standard Optical Delay Lines can support up to 8 distinct delay lines.

RFOptic has a range of products that support more than one delay line. The customer can also extend it later by adding additional spools. Eight (8) delays lines that can create 256 combinations of delay lines are standard.

(ODL) Optical Delay Line Investment Protection

The most expensive portion of an Optical Delay Line (ODL) is the transceiver. It is therefore important to design an ODL that captures the customer needs in the future and/or with different applications. If the customer is not sure of the number of delay lines that may be required, we provide specific ODLs that allow for the possibility to add more delay lines as needed in the future.

What is a Progressive Optical Delay Line?

Our progressive ODLs can combine distinct delay line to create additional delay lines.  Progressive delay lines are used if the customer needs more than eight (8) delay lines.  Progressive Optical Delay Lines can be used to provide up to 255 delay lines. One classic application is for phased radar arrays where on top of a base delay line, the delays have to be entered in equal steps.  For example, this solution would allow for creating delay lines from 1 microsecond to 255 microseconds with 1 microsecond steps.

What are Variable Delay Lines?

Variable delay lines are of considerable interest in a variety of applications including radar range simulation and signal processing. There are two basic techniques to consider; Switched RF and Switched Fiber.

Switched RF uses multiple delay lines and RF switches to select various delay values. This technique has a good performance but is relatively expensive because multiple delay lines are required.

Switched Fiber delay system is more cost effective. It consists of  an ODL system with includes several different delay lines. Two optical matrices (e.g., 1:2, 1:4 or 1:8) select (either manually or through PC) the desired delay line (i.e. DL 1 to DL 8). The diagram shows a ODL system with up to 8 delays that can be selected by optical switches matrix. The disadvantage of this approach is that the switches are relatively slow, with switching time of milliseconds.

 

ODL Block Diagram

ODL Block Diagram including two Optical switched for multiple delay lines.

 

A third 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 which is shown below. The desired combination of delay lines is selected to define the desired delay. In the diagram below, there are 4 progressive delay lines cascaded switch matrices. In such a configuration, the user may 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 Delay Configuration

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

 

Progressive ODL is also used when adding small increments on top of a base delay line.

What are the Minimum and Maximum Delay Amounts?

RFOptic has created optical delay lines solutions that range from a few nanosecond up to 1000 nanosecond and more.

When is Dispersion Compensation Needed?

As the signal frequency and the delay line length increase, the optical signal can be dispersed and weaken significantly. As needed, RFOptic incorporates DCM (Dispersion Compensation Module) in its ODL solutions.

  1. Optical dispersion of long fibers at high RF frequencies causes additional insertion loss at specific frequency ranges per defined delay line length(s), where the insertion loss deep can reach 20 dB and more. The optical dispersion loss can be eliminated by using an optical dispersion unit connected to the long delay line for compensating the undesired dispersion loss (See the optical dispersion effect below).
ODL System Gain

System Gain (S21) of a 100 μsec delay ODL system up to 20 GHz. The “deep” around 15 GHz is due to the ~20.7 km SM fiber dispersion effect at 1.55 mm wavelength. The dispersion effect can be eliminated by adding a DCM unit with negative dispersion.

ODL System Gain

Since ODL typically involves fairly long delay lines, the link budget calculation becomes important.  Based on the requirements, pre- and post LNA are used.

Automatic Gain Control

When the customer requires an ODL solution with delay lines that are significantly vary in length from each other, the output signal will also vary significantly.  While a 5 km delay line will introduce 2.5 dB RF loss, a 100 km delay line will introduce 50 dB loss.  An AGC mechanism can be added to ensure that the output signal strength is steady, regardless the delay line used. Gain control is used in very long and short delay lines in the same ODL; the output signal will therefore be the same.

How Do I Control the ODL?

The delay line can be controlled manually using the push button on the unit, or remotely through an RS-232 or Ethernet connection.

What is the Delivery Time of the Purchased Items?

Usually, all RFoF products are sold from stock which means that the delivery time is about 2 weeks. An ODL (which is a customized product) can be less than 4 weeks depending on the complexity of the system.

We support customized solutions and are flexible regarding in the system architectures as per customer request.