Can you see me now?
The first thing to do when defining any optical network is determine the link budget and link loss.
Optical networks are made up of incredibly advanced elements such as semiconductor lasers, flexible glass fibers, diamond polished termini, and precision metal connectors. While the materials and detail could merit calling them jewelry, the beauty of an optical network is not in the component’s complexities but in the overall simplicity. We can take all the advanced technology that companies like Finisar and JDSU can throw at us, and reduce its practice to a simple question: what’s the link budget?
Light is both a wave and particle and we have to wrestle this immaterial state of existence to several absolute ones we can manage: output power and receiver sensitivity measured in dBm, and the delta, or dB. Link budgets are based on these measurements coupled with the loss that occurs along the path.
A dB is an abbreviation for a decibel (it stands for one tenth of a bel). 10s are a nice base unit for how we see light as our perception of it at one power and our perception of it at 10x that power is twice as bright. This is helpful to keep in mind as we go from millwats (mW) to Watts which as the prefix shows, is done in powers of ten. Much as we might regret giving engineers control, there are reasons things work the way they do in our world and math is at the heart of much of it. We’re going to use watts behind the scenes much as a transceiver does in taking electrical power in and putting optical power out.
The simple question of whether an optical network or link will work is based on staying within the link budget, again, measured in dB. It’s a relative measurement to the two dBm measurements…which are absolute of output power and receiver sensitivity. We switch from milliwats (very small amounts of light compared to your lightbulbs and in our cases, not visible) to dBm because we can get to a nice linear counting scheme. Since light transmission is all about loss, we have a measurement point at Zero dBm which is also 1mW. And here we have one more transition: going from one to two milliwatts is the same as increasing 3 dB or 1 dBm to 4 dBm. Every 3 db up or down is a doubling or halving of optical power.
Transceivers have largely abstracted the end user from the milliwatts portion of this problem solving but it’s important to remember it because transceivers like cotsworks’ are designed onto a board and they have to have clean power supplies. If not, you can see how the campbell’s soupcan effect can ruin your end link…milliwatts to dBms to dBs to changes of just 3 dB reducing network efficiency by ½. Thankfully, the loss on glass is very low and the transceivers can operate from 1 to 1/1000th of their rated light received. And even that number isn’t enough as we add connectors and expanded beam or rotary joints or just increase our speed which reduces the efficiency of the receiver. One of the things that make our transceivers rugged is that they exceed industry standards at room and extended temperature. Our SX parts, for example are nearly 2x what the industry standard calls out. This doesn’t mean you can avoid calculating link budgets…just that we have a little extra pad to help out in harsh environments.
