VCSELs (vertical-cavity surface-emitting lasers) are commercially available infrared semiconductor lasers with λ850 nm. Short-cavity-length, high-quality Bragg mirrors impart properties to VCSELs that differ from those that earlier Fabrey-Perot lasers impart. The emission characteristics—optical power, P_λ, versus diode current—shows threshold and operating currents of approximately 3 and 13 mA, respectively, for the Lasermate TSC-M85A416 VCSEL. Such values are typical for this type of laser (Figure 1). The current characteristics are nearly independent of temperature. Thanks to their low operating current, effective coupling to multimode fibers, high-speed switching, and low price—about one-third that of a Fabrey-Perot laser—VCSELs may replace LED light sources in short fiber-optic links. The MC10EP89 ECL coaxial VCSEL driver allows the circuit to switch current with less-than-1-nsec rise and fall times (Figure 2). The IC delivers a voltage swing of 1.6V minimum to a 75Ω load (Figure 3).

The 120Ω resistor, R_M, limits the amplitude of the pulse current, and R_P determines the initial polarization current of the laser. For R_P=∞, the laser practically switches off during the low state of the driver. You must load the MC10EP89 with resistor R_L for proper operation of the IC. You can optimize the values of R_M and R_P for each type and characteristic (individual emission parameters) of the VCSEL you select. Because of the dynamic resistance of the laser, the voltage drop across the junction varies from approximately 1.9 to 2.2V within the full current range. Therefore, you should consider this variation when you calculate R_M and R_P. In turning off the optical power of the VCSEL, you observe some residual emission (tail) in the optical-pulse response (Figure 4). If the current falls to zero (R_P=∞), the tail is shorter and smaller, but the optical-power amplitude also decreases.

The oscillogram represents the response of a 155-Mbps laser (Figure 4). VCSELs for 622 and 1250 Mbps are also available from Lasermate. The MC10EP89 needs symmetrical drive; the input OR/NOR gate works as an asymmetrical/symmetrical ECL converter. The laser's good thermal properties eliminate the need to provide a complicated stabilization loop for the optical power. The entire circuit is dc-coupled and operates with constant optical power amplitude for each binary code, and the circuit is insensitive to bit patterns. A slow-start circuit is unnecessary. Because of the operating speed, you must carefully design the pc board according to high-frequency rules: Keep connections as short as possible, use surface-mount components, and carefully perform decoupling, for example. You must ground the metal case of the laser and isolate it from the chip.