industry VCSELs
seen as overshoot and jitter in the optical eye diagram. Making matters worse, a shorter photon lifetime increases cavity loss, leading to a higher threshold. In turn, this lowers the differential gain that we have tried so hard to maximize.
In our view, the correct approach is to select a photon lifetime that is sufficiently short to not unduly penalize the bandwidth, but long enough to offer low threshold and ample damping. The good news for us is that our simulations show that the photon lifetime of our 10 Gbit/s design is also a good choice for 25 Gbit/s (see Figure 4 for details).
Fig.4.Simulations show that changes to photon lifetime impact the laser’s frequency response. The bandwidth decreases with increasing photon lifetime,and so does the amount of peaking. Eye diagram simulations (Fig.4.(b) to (d)) reveal that when peaking is high (low damping),overshoot and jitter are more severe. However,when photon life is too long,rise and fall times begin to increase and the eye starts to close vertically
so that less high-frequency modulation is shunted by it. Reducing parastic elements, in particular the oxide capacitance, has a significant impact on the frequency response of the laser (see Figure 3). Cutting oxide capacitance in half is one of the goals for our 25 Gbit/s design.
More than a bandwidth game If you read through the theory section of textbooks detailing laser behaviour, somewhere you’ll likely to find a prediction for where the maximum bandwidth occurs – at the point where the relaxation oscillation is critically damped. According to theory, our current 10 Gbit/s VCSEL should reach an ultimate bandwidth of 22 to 27 GHz. In reality, however, it is less than half of this, due to thermal rollover.
A major term that defines the maximum bandwidth is inversely proportional to photon lifetime. This has led many engineers within the optoelectronic industry to believe that one of the best handles for increasing the bandwidth is cutting photon lifetime. But this is a misconception, at least for the VCSEL.
Cut photon lifetime and you get an unwanted side effect – reduced damping of the relaxation oscillation under the actual operation bias. This is limited by the thermal effect and requirements for either power consumption or reliability, or both of them. The key point to note is that insufficient damping degrades signal integrity, which can be
46
www.compoundsemiconductor.net October 2012
Putting theory to the test Based on the design strategy just detailed, we have carried out extensive simulations to narrow the range of design parameters. Since there is some uncertainty in many material characteristics, there is some trial-and-error associated with the process of designing, building, evaluating and modifying our lasers. However, we have found that after a few iterations, our device’s performance is now close to its target. The improved bandwidth characteristics compared to our 10 Gbit/s design are shown in
Fig.5. Increasing the 25 Gbit/s operating temperature from 25°C (a) to 85°C (b) leads to a slightly inferior optical eye diagram
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