industry VCSELs
Fig.1. Sumitomo’s successor to its 10 Gbit/s VCSEL is designed to operate at 25 Gbit/s and has a superior 3 dB bandwidth at 25°C and 85°C (a).The slope of curves of relaxation oscillation frequency (ROF) as a function of bias indicate how fast bandwidth ramps with bias (b)
reliability improvements to our 10 Gbit/s VCSEL, the Ultralase, we have been the leading provider for several standards of active optical cables.
Our efforts at developing 25 Gbit/s VCSELs started just over 18 months ago, and our design is now in the advanced stage, with prototypes producing respectable performance. To reach these very high speeds we have had to develop devices with a very high bandwidth. In designs such as the Ultralase, bandwidth saturates at around 10 to 11 GHz at high temperature, which is more than enough for operation at 10 Gbit/s, and sufficient for even 14 Git/s (see Fig. 1(a)). But to operate at 25 Gbit/s, we need a minimum bandwidth of 16-18 GHz – drastically higher than the current device capability. Bridging this gap is the biggest challenge to making really high speed VCSELs.
Back to basics To understand the limits to reaching higher speeds, you have to understand some of the basic operating principles of a VCSEL. Like every class of semiconductor laser, this device reaches its threshold – the point at which it starts to emit laser light – when the carrier density in the gain medium
hits a level where gain fully compensates for cavity loss. Increasing the drive current further causes light output to rise linearly with injection current for relatively low bias current.
Crank up the current to higher values and the device enters a new regime: Light output increases with current in a sub-linear manner, and eventually rolls over at increased bias. This rollover results from a rise in junction temperature due to self-heating – that’s why it is often referred to as ‘thermal rollover’.
At a higher junction temperature, maintaining the same gain requires more carriers, so an increasing fraction of the injection current is consumed to balance the loss, lowering the light emission efficiency. When the increase in current can no longer make up for the increase in threshold, the laser power starts to drop.
Self-heating severely limits the performance of VCSELs, due to their intrinsic high-current density and large thermal impedance, which stems from a far smaller size compared with edge-emitting cousins. This heating not only hampers the device’s efficiency to produce light – it lowers its speed. In a directly modulated laser, gain governs the modulation of light. If the laser is to respond rapidly to the modulating current, gain must change sharply in response to adjustments in carrier density.
Fig.2. A circuit diagram for the VCSEL pad capacitance; Li from mirror stack; Ca
’s parasitic equivalent circuit.Cp
is inductance of interconnect metal; Rm is aperture capacitance; Ra
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www.compoundsemiconductor.net October 2012 is is resistance is aperture resistance A significant downside of the VCSEL is that its
The rate that gain changes with carrier density is called the differential gain, and it must be high to ensure high-speed operation. Unfortunately, gain rises sub- linearly with carrier density and it decreases with rising temperature, so a high junction temperature lowers differential gain. The upshot is that bandwidth, like output power, is a victim of thermal rollover.
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