Shedding Light on Optical Interconnects

Demystify optical interconnects to take your connectivity to the next level

Converting Electricity to Light

Electrical data signals transmitted over copper are digital, meaning there is a ‘1’ or a ‘0’ that is transmitted at one end and received at the other end. Optical transmission is nearly identical. The electrical ‘1’ or ‘0’ is read by a chip inside our optical engine that tells the laser to output a brighter signal for a ‘1’ or a dimmer signal for a ‘0’ and a photodetector and receiver chip reverses the process and outputs the corresponding electrical ‘1’ or ‘0’.

Unlike copper cables, where higher bandwidths cause cross-talk problems with neighboring signals, optical signals are actually guided in the core of the fiber, so even laying a fiber right on top of another fiber will not cause any cross-talk, ground loops, or electromagnetic interference issues. Goodbye EMI problems!!

Inneos’ optical interconnects are even more unique in that they transmit multiple optical signals (channels) on the same fiber at the same time by using a different wavelength (color) for each channel.

Different Applications Require Different Optical Interconnects

Systems & Extender Boxes

  • System solutions employing optical interconnects include an optical subsystem to do the optical-to-electrical conversion of the data stream. Such system-level and extender boxes may use a standard SFP+ optical module to implement their optical link. The 10 Gbps SFP+ modules are inexpensive and readily available. But if the optical link is limited to 10 Gbps, then the system is also limited to 10 Gbps, which means for any solution greater than 10 Gbps, the system must first compress the signal before sending over an optical cable. In some applications, discarding data or adding latency is acceptable, but compression is unacceptable in many cases. This is where a full-rate optical interconnect is needed.
  • For example, optical interconnects that are designed for video transmit the full 18 Gbps for 4K, uncompressed HDMI video. These types of boxes will not require any compression and can provide customers with an uncompromised video solution. Inneos interconnects can support up to 24 Gbps of unidirectional data and 1.25 Gbps bi-directional data for a wide range of connectivity solutions.

Optical Adapters

  • Optical adapters are a unique optical interconnect solution for point-to-point implementations. Optical adapters use standard fiber cable that is installed in a system or building and then connect adapters with optoelectronic subsystems to each end convert the protocol-level electrical signal to optical. This allows the fiber to be pre-installed between the endpoints and the optical adapters are simply connected to begin using the optical interconnect system. As an added advantage, optical adapters are entirely upgradable – just swap out the optical subsystem ends when it is time to implement solution higher-bandwidth solution… the fiber doesn’t need to change.
  • Solutions that use standard optical fibers, either pre-terminated or field terminated, have the benefit of being low cost and readily available. These cables are used in Datacom, telecom, videocom, industrial and aerospace solutions worldwide, and they can interface with a range of systems once installed. Additionally, if there is a cable break or connector damage during installation, they can simply be re-terminated using commercially available termination kits and standard procedures.

Hybrid and Parallel Active Optical Cables

  • There are other types of optical interconnects deployed including hybrid cables and parallel fiber cables. Hybrid optical cables include both copper and optical fiber to transmit the data, generally keeping low-speed control and communication signals on copper wires to provide a simpler system-level interface and running higher-speed data signals over fiber to get the best signal integrity. Hybrid cables generally cannot be field-terminated, so solutions using hybrid cables usually are shorter distances and do not need to run through conduit or bulkheads.
  • Parallel optical cables transmit each channel over a different fiber, similar to a differential pair in an electrical cable. This approach is easy to implement because each channel can use the same transmitter laser since they are all on separate fibers. However, having 4 or more fibers in the optical interconnect prevents field termination of the cables, so the delicate optical connector must be carefully pulled through the conduit during installation, and any damage renders the cable unusable.

There’s So Many Types of Fiber, What Do I Use?

For an optical interconnect solution using industry-standard fiber cables, using the correct fiber can make all the difference. If the optical subsystem is already chosen, look to see whether it is single-mode or multimode and be sure to use the fiber corresponding to the optical engine. Inneos optical subsystems use multimode fiber. Why? Because a multimode fiber has a larger core, the part of the fiber that transmits the signal, so it is more forgiving for field termination and multimode hardware is also generally less expensive than single-mode hardware. For distances longer than ~1km, single-mode optical subsystems and single-mode fiber are typically required.

There are four primary components to an optical fiber: the core, cladding, buffer, and outer jacket. The core is the area where the light is actually transmitted. In single-mode fiber with 9μm diameter core, only a single optical mode, or light wave behavior, is supported. In multimode fiber, the core is much larger at 50μm or 62.5μm, and multiple modes are supported. So how does the light stay in this core? The cladding layer provides a refractive index contrast that causes the light to reflect off the interface and propagate along the fiber core. If the fiber is run as a single strand, it is referred to as a “simplex” fiber cable, whereas if there are two strands of fiber run side-by-side, it is referred to as a “duplex” fiber cable. Applications that use special optics to perform multiplexing, such as the Inneos WaveStacker Optic, can simultaneously transmit and receive multiple high-speed signals on just one fiber, which makes the field termination even faster since it only needs to be performed once at each end.

Other terms that are used with multimode fiber include the classifications: OM5, OM4, OM3, OM2, and OM1. OM1 has a 62.5μm core whereas OM2, OM3, OM4, and OM5 have 50μm cores. The higher OM number corresponds to higher modal bandwidth, so generally OM3 or OM4 should be installed in new installations for lengths longer than 100m, with OM4 preferred over OM3, especially for 200m+ installation links. OM5 was specifically designed for wide-bandwidth wavelength-division multiplexing applications, so while it is great for the WaveStacker optic, it is generally more expensive than OM4.

The type of connector needed for the fiber cable is dependent on the connector that is on the mating hardware. Generally, it will be SC, LC, or maybe MTP/MTRJ. SC and LC connectors can both be readily terminated in the field with no-polish, no-epoxy termination kits from multiple vendors. SC connectors are slightly larger than LC connectors. MTP/MTRJ connectors have multiple fibers in them and cannot be terminated in the field because these fibers all must be carefully polished and aligned within the connector, which must be done in a factory with specialized equipment.

Is Fiber Really Rugged Enough to Pull in an Install?

Yes, it is! Fiber cable is routinely pulled through the conduit during installation. Generally, a fiber cable includes a strength member to help with the pull strength and it can be pulled just like a typical category cable. The cable jacket material also comes in various options for riser or plenum requirements as well as the newer EU Construction Products Regulations for cables. These are standard options that are specified when ordering fiber cable, so the process is the same as any other category cable.

Fiber cable is tough enough for rugged applications too, including automotive, aerospace, and industrial environments. There is a wide range of fiber cable construction materials that are used for fiber cables that allow them to operate in ultra-wide temperature ranges applications, such are airplanes, automobiles, and factory assembly floors, high radiation environments such as satellites and nuclear power plants, and even high-pressure environments, such as down-hole sensor cables. Our team can help you spec the right fiber cable for your solution.

Field Termination – Is This Really Feasible?

Field termination is actually pretty easy with field termination kits that are available from many different vendors, and there are also training videos available for many of them. With minimal training, terminating a fiber can be just as fast and easy, if not faster, than terminating a copper cable. Plus an additional benefit is that fiber cables don’t have the challenge of maintaining the critical shielding through the termination process that can lead to hard-to-detect signal integrity issues with copper cables.

What if I just want to run a pre-terminated fiber because I’m just not comfortable with doing fiber terminations yet? No problem! Pre-terminated fiber cables in almost any length and type and are available from many different vendors. Just make sure to put the dust cap on the fiber connector(s) while pulling the cable and don’t exceed the pull strength of the connector end.

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