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 a wide dynamic range for various delays. In addition, the Altimeter Emulator is also considered as an ODL type.
An Optical Delay Line method is the most accurate and reliable method for time-domain measurement for delay times of a few nanoseconds to hundreds of microseconds. 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, bandwidth, group delay variation, spurious, and phase noise.
Optical Delay Line applications include range calibration, MTI, ground-based system test, radar warning receiver, jammers for EW systems, time control, path delay situation and (x) phase shift discriminator.
The main Optical Delay Line features include:
- Supporting transmission of RF and Microwave analog signals, covering L band, S band, C band, X band, and Ku band, for various applications.
- Supporting width bandwidth analog signals.
- Supporting various delay lines ranging from few ns up to hundreds of μsec.
- High dynamic range
- Excellent delay repeatability and phase linearity
- Small Group Delay Variation
- Easy operation – manually or remotely through Management & Control
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 0.1 up to 40 GHz and more. There are low-frequency Optical Delay Lines up to 6GHz and high-frequency Optical Delay Lines of up to 40GHz.
Fixed Delay Line System
The basic ODL system configuration consists of a Transceiver and fixed Delay Line modules that are integrated in one enclosure (See Figure 2 below). ODL versions where the Transceiver and Delay Line units are separated into two modules are optional for providing the user with flexibility when using one ODL Transceiver unit with several passive Delay Line units. The ODL in one enclosure is robust since the Delay line fiber is fused to the system.
Progressive Delay Line systems
Progressive Delay Line is another approach for variable delay systems. It consists of an ODL system configuration which includes cascaded 1:2 and 2:2 optical matrixes with several different delay lines in between (replacing the two optical switch matrixes 1:8 in Figure 3). The cascaded switch is shown in Figure 4 below, where the desired combination of delay lines is defined for the desired delay. It shows four progressive delay line-cascaded switches matrixes. With such a configuration, the user can select any of the 16 combinations of possible delay values (16=24) e.g., a delay which is equivalent to Dtot= D1+D2 +D4 etc.)
Optical Dispersion of long fibers at high RF frequencies causes additional insertion loss at the specific frequency range 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 Figure 6). The ‘deep’ around 15GHz 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.
The basic ODL system configuration consists of one Transceiver and one fixed Delay Line module that are integrated in one enclosure configuration. Depending on the length of the delay, such an ODL is typically packaged in 2U enclosure (short delay) or in 3Uenclosure in case of long delays (e.g., > 50 μsec). Mini ODL enclosures are optional, depending on the required ODL configuration and specifications.
Other ODL versions where the Transceiver and Delay Line unit(s) are separated into two (or more) modules are optional. Due to the flexibility and immunity of the RFI and EMI properties of optical fibers, ODL systems could be built with the delay spool removed from the Transceiver. In such a case, the Transceiver unit (including optical switches if required) is connected to the Delay lines through SM short fibers connecting the ODL optical input and output ports to the passive Delay units.
Phase Noise: ODL Phase noise is smaller than -130 dB/Hz at 10Khz from the carrier for various operating frequencies and delay lines. Typical phase noise is shown in Figure 7 below. The measurement is limited by the Measuring Equipment noise: PN<-127 dB at 1 MHz from the carrier, PN <-113 dB at 100 KHz from the carrier, and PN<-105 dB at 10 KHz from the carrier.
RF Amplifiers considerations: Pre and/or Post RF amplifiers can compensate for ODL Insertion Loss and for the optical loss in case of long delay lines that is translated into RF loss in the ODL’s photo-detector unit. The advantage of using Pre-Amp is, that it also improves the system Noise Figure and the SNR. On the other hand, it reduces the Input P1dB (typically less important for most of the ODL applications). Alternatively, adding a Post Amp will improve the ODL system gain and will not affect the system Input P1dB, but will not improve the system Noise Figure. Adding RF amplifiers will increase the ODL system Gain Flatness, where in case of requirement for better Flatness, either EDFA could be used instead (in case of long delay lines), or RF amps with special low gain flatness can be selected.
Environmental and Reliability: The basic optical transceiver units, including DFB laser, optical modulator, photodiode, optical switches, EDFA, and Optical Dispersion compensator as applicable, are all packages in rugged packages and capable of withstanding considerable shock and vibration without damage.