RADAR | Optical Delay Line Applications
An Optical Delay Line 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.
Optical Delay Line Methods
The Optical Delay line method is the most accurate and reliable method for time domain measurement for delay times of a few nano seconds to hundreds of microseconds. An Optical Delay line is a method of wave guide where the media is fiber with a fixed index of refraction and relative constant group delay variation. The main advantages of this method as compared to other methods are:
- Delay Length – long achievable delay line due to the extremely low loss of the fiber (~0.25dB/Km), which is not achieved in any other methods. There are methods than can measure in range of picoseconds such as light reflection but do not cover the typical range of Radars or EW systems. There are also methods for very long delay lines in the order of milliseconds, which are not accurate for practical lengths of delays. Therefore, the Optical Delay line is the suitable method for length range from a few nano seconds to hundreds of microseconds. Moreover, utilizing switching or progressive system architectures, it is possible to include several different delays in the same system, which saves space, weight and
- Bandwidth – Optical Delay Line can support bandwidths from the MHz range to tenth of Giga Hertz. This allows to use the ODL in various applications which require high bandwidth, where other waveguide methods are limited in allowed
bandwidth and applications. For example SAW is used for a bandwidth of a few tenths of kilohertz.
- Group Delay Variation - One of the most important issues for Radar Designers is that the delay will be equal in the entire bandwidth. Thanks to the fiber the group delay is constant and very small in compression to the delay length.
- Spurious – the spurious level of Optical Delay line is small supporting Doppler shift measurements/applications, where the noises which are caused due to the circuit boards are cleaned by the system.
- Phase Noise – an important parameter in the performance of airborne radars is the phase noise of the radar's carrier frequency. Low phase noise is important for an accurate long range detection of a target. Many phase noise tests sets utilize waveguide delay lines as part of the test circuit. Because of its size, weight, and signal attenuation, typical waveguide delay line has length limitation. Replacing the waveguide with fiberoptic delay line allows for a major reduction in size and weight, as well as an added ability to improve the sensitivity of the test set in measuring phase noise close to the radar's carrier frequency. A laser diode with low RIN can provide at 0 delay length a phase noise less than -130 dBc (input of).
Optical Delay Line Applications
There are various applications that can use ODL systems, including:
- Radar range calibration; (ii) MTI (moving target indication); (iii) Clutter Canceller; (iv) BIT; (v) Ground Based System Test; (vi) Radar Warning Receiver; (vii) Jammers for EW Systems; (viii) Timing Control; (iv) Path Delay Simulation; and (x) Phase Shift Discriminator.
Optical Delay Line main Features:
- Support transmission of RF and Microwave analogue signals, covering
L, S, C, X, and Ku bands, for various applications.
- Supports width bandwidth analogue signals.
- Supports various delay lines ranging from few ns up to hundreds of m
- High dynamic range.
- Excellent delay repeatability and phase linearity.
- Small Group Delay Variation.
- Easy operation – manually or remotely through RS-232 or Ethernet.
Optical Delay Line Block Diagram and main Configurations
Fixed Delay Line System
The basic ODL system configuration consists of Transceiver and one fixed Delay Line modules, which are integrated, in one enclosure (see below in Figure 2). ODL versions where the Transceiver and Delay Line units are separated into two modules are optional (This option provides flexibility to the user to use one ODL Transceiver unit with several passive Delay Line units Line units. On the other hand, the ODL in one enclosure is more robust as the Delay line fiber is fused to the system, where in the two modules configuration there is a need of a connection between the two modules by at least two external fibers (for a single delay line) connected to the optical connectors on the two modules.
Variable Delay Line Systems
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.
A second approach is of Switched Fiber delay system which is more cost effective, it consists of an ODL system with include several different delay lines, where two optical matrixes (e.g. 1:2, 1:4 or 1:8) that selects (either manually or through PC) the desired delay line (i.e. DL 1 to DL 8) - see below in Figure 4 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 in the order of milliseconds.
A third approach for a variable delay system is an ODL system configuration which includes cascaded 1:2 and 2:2 optical matrixes with several different delay lines in between (replacing the above two optical switch matrix 1:8). The cascaded switch matrix - Progressive Delay Configuration which is shown in Figure 5 below, selects the desired combination of delay lines to define the desired delay. See below a schematic picture of a four progressive delay lines cascaded switches matrixes. With such configuration the user may select any of the 16 combinations of possible delay values (16=24): for example can select a Delay which is equivalent to Dtot= D1+D2 +D4, or Dtot= D3+D4 etc.)