Converting an electrical signal to optic pulses has been a costly option that consumes too much PCB space and power. That’s changing now, as mid-board optical transceivers light up the datacom market.
Demand for faster internal and external system interconnects has begun to push the limits of copper, especially in lengths of more than a few meters. Converting an electrical signal to optic pulses has been a costly option that consumes too much PCB space and power. That picture has begun to change now as new generations of optical transmitters and receivers have entered the market.
Huge strides in the science of silicon photonics are bringing closer the long-anticipated day when high-speed electro-optic conversion will be economically practical in short links. The introduction of active optical cable assemblies several years ago offered designers a seamless method of extending the range of length-constrained standard copper I/O interfaces while reducing cable bulk and susceptibility to EMI. Short-reach embedded optic transmitters and receivers from Avago Technologies, Finisar, and Reflex Photonics have recently been joined by full duplex mid-board optical transceivers from FCI Electronics, Molex, and TE Connectivity, which bring the advantages of optic transmission to applications inside the box.
These transceivers offer several major advantages over conventional copper I/O interconnects:
- Located adjacent to a high-performance processor or ASIC, the copper signal path between devices can be kept very short, which minimizes degradation of high-speed signals, simplifies board layout, and raises the possibility of utilizing lower-cost PCB laminate material.
- I/O panel density can be greatly increased as compared to traditional and even newer small form factor interfaces such as QSFP. A 1U panel fully configured with MPO optic connectors could deliver up to 4.8 Tb of I/O.
- Advances in silicon photonics allow the integration of all but the laser on a single chip or substrate, which greatly reduces the device’s PCB footprint. Conventional CMOS fabrication technology keeps costs under control.
- Power consumption as well as resulting heat generation is also minimized. Both are critical issues in densely packaged products.
Optical engines in a variety of forms and performance levels have been on the market for many years. They include:
- SNAP12 transmitter and receiver modules from Finisar
- MiniPOD™ and MicroPOD™ from Avago Technologies
- LightABLE™ transceivers from Reflex Photonics
All of these devices are designed to provide a fatter data pipe off the daughtercard for termination to a backplane or the front I/O panel. They can also be used in chip-to-chip and board-to-board internal applications.
Each of the devices in this segment share some common characteristics, but also demonstrate some differentiators. For instance, nearly all feature:
- Minimized footprint to conserve precious PCB real estate
- A solution to increased I/O panel density
- Capability for fiber to be brought to the backplane for distribution to other daughtercards (especially important in larger high-speed backplane systems)
- Low-power electro-optic conversion
- Data protocol agnostic
- 12 channels (most of them) and data rates are scalable
They differ in construction and performance.
Some, such as the Avago MicroPOD and MiniPOD, are either a transmitter or receiver module, while others such as the Molex, TE, and FCI mid-board devices are full duplex transceivers.
The optical engines from Finisar, Samtec, TE, and Avago utilize 850nm sources, while the Molex transceiver is based on 1490nm laser.
Most devices are designed to link with multimode fiber, while Molex has settled on the use of single-mode fiber.
Some transceivers directly modulate the laser while others utilize a secondary shutter to encode the data.
The Samtec FireFLY Flyover System features a choice of integrated heat sinks. The manufacturer of at least one device claims low power consumption may allow operation without the use of a heat sink.
The maximum data rates per channel vary between 1 and 28 Gb/s.
The maximum length of an optic link can vary from 10 to 600 meters.
Many transceivers are designed so the fiber is pluggable to the module via an MT-style connector. Avago MiniPOD and MicroPOD devices use the top mounted PRIZM® LightTurn® connector from US Conec, which allows tiling of optical modules for increased packaging density. Others such as the FireFLY System permanently attach the fiber to the module, which is sold as an assembly at a customer-defined length.
Some transceivers, such as the TE Coolbit, are constructed using highly integrated flip-chip technology on a glass substrate while others consist of components mounted on PCB material.
Although the Light Peak optical I/O link from Intel did not prove successful, it got designers thinking about what advantages could be achieved with a cost-effective short-range optical I/O. The investments made by leading connector manufacturers in optical technology over the past few years are beginning to pay off as interest in fiber optic interconnects continues to expand.
Robert Hult, Market Director, Bishop & Associates, Inc.
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