A Wideband Microwave Optical Link for Use in Hostile Airborne Environments

Improvements in optical component technologies and the adoption of advanced manufacturing techniques have allowed the deployment of broadband optical links into harsh airborne environments to become a reality.

In an airborne radar receiver, high frequency signals are usually down converted as close as possible to the antenna and fed to the receiver to minimize de-sensitization associated with running wide bandwidth, high frequency signals through high loss coaxial cable. Even with this technique, across the 20m half  wingspan of a C130 Hercules it will still be possible to observe 15dB  loss with 6dB  variation over  a typical operating bandwidth (500 MHz at 1GHz IF), however  this is an improvement on more than 50dB loss at 40GHz.

Additionally, down-conversion hardware typically requires a reference signal, and potentially a local oscillator (LO) signal, to be routed alongside a high speed data connection within the aircraft exposing the reference and LO to noise.

Co-axial cables in Direction Finding (DF) systems connect multiple antennas to several receivers located in the aircraft. Current systems compensate for amplitude and phase imbalances in the link with equalizers and attenuators, resulting in the practice of de-sensitizing all the receivers to compensate for the path with the highest loss.

As an alternative to co-axial distribution Micreo has developed a fibre based link permitting the positioning of down conversion hardware and receivers in a less environmentally stressful location on the aircraft and closer to the mission computer. Additionally the installation of fibre cabling within an aircraft frame is possible at a substantially lower cost than the equivalent installation of co-axial cable with the added potential to route all control and reference signals on a single fibre.

Despite the obvious advantages of replacing coaxial cable with optical fibre in airborne systems, widespread adoption has been dampened due to laser specifications and optical components which are not specified for operation over the required temperature range and harsh vibration environment.

COMPONENT SELECTION AND MODULE DESIGN

An externally modulated link was chosen and so the optical link comprises of a laser, a  Mach-Zehnder Modulator (MZM), fiber link and photodiode detector.  The laser is a 1550nm 100mW DFB laser specially selected for low RIN performance and survivability over the wide  40°C to +85°C operating temperature range required for the link.

The LNA in the transmitter is bare-die MMIC-based with the amplifier and its associated bias circuitry all located within a hermetically sealed compact housing.  This LNA is connected directly to the unit input through SMA connector panel feedthroughs, and also to the following MZM via another SMA connector.

Micreo’s design and development of the MZM and photodetector was conducted entirely in house so as to meet the demands for compact, low power units that would survive the rugged airborne uninhabited environment. 

Micreo has previously developed modulators with RF amplifiers built into the same housing, however in this case the mechanical layout restrictions meant that a separate MZM housing provided a better approach.  The MZM is conventional LiNbO3 chip housed within a fully hermetic housing with all seals either glass to metal or metal to metal through laser welding.  Amplitude modulation is used.

The transmitter module is shown in Figure 1, and in the current format has external dimensions of 150 x 95 x 30mm.  To fit within this volume, bend insensitive fibre is used extensively.

The receiver is a top-illuminated photodiode connected to a wideband MMIC amplifier.  All associated bias circuitry and RF attenuators are included in a single hermetic housing with the photodiode.

The MMIC, MZM and photodetector packaging, along with all the associated circuitry is designed using approaches previously qualified by Micreo for other airborne platforms including rotary wing and fast jet.  The laser, RF amplifier, MZM and voltage regulators are all mounted on the base of the housing to provide a good thermal path and minimize vibration resonances to these critical components.

THE MODELED SYSTEM

Extensive modeling of the system during the initial design phase allowed for all of the typical RF EW performance parameters to be easily evaluated and the effect of variations in the performance of the surrounding components to be incorporated easily in the design.  This approach was taken to allow EW system designers to consider only parameters that they are familiar with rather than having to become acquainted with new terminology.

A block diagram for the Optical link is shown in Figure 2. The link is partitioned into the following sections.

1) An RF Tx section - Incorporating RF gain and level control within the transmit module.

2) An RF Rx Section, consisting of a final gain stage in the receiver.

3) An Optical Link Section consists of the MZM, optical fiber and the detector.

To allow for a complex fibre routing with multiple connector interfaces a high optical loss of 7dB is allowed for in the design even though the physical length of the link will be less than 100m.

LINK TEST RESULTS

The measured results for the optical link at hot, cold and ambient temperatures are shown in Figures 4 and 5.  The comparison confirms good correlation between measured results and those that were modeled during the design phase.  Note that for comparison with measured results the reader is reminded of the 7dB optical loss included between the transmitter and receiver leading to a high link loss.

CONCLUSIONS

A wideband analogue microwave optical link has been developed and qualified by Micreo for integration into a rugged, airborne environment and is now in full production. The Airborne Optical Link (AOL) is a compact design developed for an extreme environment with wide thermal variations, high vibration and shock level.

The performance achieved through the careful selection of components and a commercially available low RIN laser makes this system competitive against coaxial cable based systems.  The production of this system offers integrators a pair of compact module for integration into EW systems and finally allows photonics to move into this domain.