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What is a CFP2-ACO? Mar 14, 2017

What is a CFP2-ACO?

Following Article is from NeoPhotonics's Website. Just for our visitor's reference.

The CFP2-ACO pluggable coherent optics transceiver, which is gaining considerable interest for next generation 100G metro systems. System designers want to move to pluggable transceivers for 100G coherent applications to take advantage of the “pay as you grow” benefit of only adding the expensive 100G optics when traffic exists to support the additional expense. At the same time, designers are pushing for smaller transceivers to increase the bandwidth density making the small size of the CFP2 form factor attractive. These two desires created a problem in that the current technology could not fit all of the necessary elements for a 100G coherent transceiver within either the physical size or the electrical power budget of a CFP2. The solution to the problem was to define a new type of coherent transceiver, the CFP2-ACO, and to develop newer and smaller optical components.

The major optical elements required to make a 100G coherent transceiver are a narrow linewidth tunable laser to serve as a light source, a dual polarization coherent modulator to impress the data on the phase of the optical signal and an integrated coherent receiver (ICR), with a narrow linewidth local oscillator laser, to convert the optical phase information back into an electrical voltage signal. To this must be added the major electronic elements of a high speed digital to analog converter (ADC) to digitize the received signal and a high speed digital signal processor (DSP) to reconstruct the transmitted information, removing the impairments imposed by long distance transmission through the fiber.   A CFP2-ACO divides these elements in two and puts the optics into the pluggable CFP2 and puts the electronics on the circuit board into which the CFP2 plugs. This approach significantly reduces the number of components that have to be put in the CFP2 as well as the electrical power that must be dissipated, but has the significant drawback that a CFP2-ACO must be plugged into a special slot that can only accept CFP2-ACOs, and cannot be plugged into any arbitrary CFP2 slot. This greatly reduces the flexibility of using the same form factor for client and line side transceivers, but it is the only level of integration that is achievable at present. The interoperability of different vendors’ DSP is also not possible in most cases.

Even with this new approach, the CFP2-ACO is a challenging component to design and build. The CFP2 form factor has nominal dimensions of 41.5 mm wide by 91.5 mm long by 12.4 mm high. First generation coherent optical components were simply too large to all fit into the CFP2 envelop. For example, the first coherent receiver defined by the OIF, “Implementation Agreement for Integrated Dual Polarization Intradyne Coherent Receivers” OIF-DPC-RX-01.0 (known as Type 1), in 2010 specified dimensions, including the flanges and fiber boot, of 75mmx41mm. The receiver itself would take up almost the entire CFP2 envelop, an obvious non-starter. Even the Type 2 ICR defined by the OIF in 2013 was too large, having dimensions including flanges and boots of 45mmx22mm. The OIF responded this year with a new MSA agreement for a micro-ICR specifically designed to fit in a CFP2 ACO, “Implementation Agreement for Integrated Dual Polarization Micro-Intradyne Coherent Receivers,”, OIF-DPC-MRX-01.0, with a maximum dimension including flanges, boots and pins of 43mmx16mm. Multiple vendors are currently offering micro-ICRs.

In 2011 the OIF agreed on an MSA form factor for the laser assembly, termed a micro-ITLA, “Micro Integrable Tunable Laser Assembly Implementation Agreement”, OIF-MicroITLA-01.0, that allowed dimensions of 43mm long by 20 mm wide, with the length extending to 65mm when the fiber boot is included. Some overlap of the boots with other components is permitted. These dimensions can just be accommodated in a CFP2 form factor, but many vendors are electing to remove the laser “gold box” assembly itself from the micro-ITLA standard circuit board to better optimize space in the CFP2-ACO.

This brings us to the last major optical component in a CFP2-ACO, the coherent modulator. This component is more challenging to fit in a CFP2 form factor. Virtually all of the hundreds of thousands of coherent systems that have been deployed use Lithium Niobate based modulators, and the OIF duly issued an implementation agreement MSA specifying many of the parameters, including dimensions: There are several different types, but all have a total length, including boots, on the order of 100mm, and are therefore unsuitable for use in a CFP2-ACO. Therefore, many companies are exploring other types of coherent modulators which can be much more compact and are based on different materials, particularly Indium Phosphide and Silicon Photonics. Coherent modulators and these two materials were discussed in detail in the post by Dr. Milind Gokhale last week. While there has been discussion in the OIF about a suitable implementation agreement, no MSA has yet been issued. Individual vendors are currently implementing the coherent modulator for a CFP2 in their own ways.

Another approach to implementing a CFP2-ACO is to combine two or more of the optical functions in a single gold box. This approach will be discussed in future pos