2-June-2015

Application Note AN-2134

Modulating Finisar Oxide VCSELs

INTRODUCTION

In the last decade, proton isolated VCSELs have become the industry standard for short

wavelength (850nm) gigabit data communications links on multimode fiber. As the speeds have

increased beyond 2Gbps, however, the oxide isolated VCSEL is increasingly the preferred source.

While proton isolated VCSELs can serve this application, oxide isolated VCSELs have proven

easier to drive at the higher speeds. Many characteristics are independent of the isolation

technology, so the reader is referred to the Finisar VCSEL website at www.Finisar.com/vcsel for

general information on VCSEL characteristics. The present note augments that more general

information with specifics related to driving oxide isolated VCSELs at high bit rates. Generally, this

information relates to TO-can packaged and connectorized VCSELs operating at speeds up to

approximately three gigabits per second. A separate application note deals with operation at even

higher speeds [1].

Oxide VCSEL Equivalent Circuit Model

Packaging of VCSELs and monitor photodiodes requires attention to details in order to deal with

electrical parasitics. These parasitics, small as they may be, can dominate the performance through

their interaction with the laser-driver interface. For this reason, the most important design

consideration in building an optical transceiver is often the electrical interface to the VCSEL

package. This interface is not just the driver chip, but includes as important elements the details of

the board layout, parasitic capacitance or inductance of any passive elements incorporated in the

drive path, and the length of lead from the VCSEL package to the solder joint on the board. It is not

always intuitive how each of these elements affects the VCSEL performance. Subtle changes to

packaging can result in dramatic performance changes in optical characteristics; a fact that was

used to good effect in many proton VCSEL based designs.

2-June-2015

Many commercial laser drivers are available that interface to Finisar oxide isolated VCSELs. Each

of these laser drivers may require a different interface circuit or set of VCSEL driving conditions. It is

considered outside the scope of this application note to address designs with specific laser driver

manufacturers, though many of them offer design assistance using Finisar VCSELs. This

application note instead provides general guidance and characterization information for the Finisar

product. The electrical equivalent circuits described below were derived from S parameter

measurements of VCSELs in TO-style packages, and are intended to be a starting point for

modeling performance. The values are shown for above-threshold operation at room temperature;

some characteristics may change due to temperature and driving conditions. The reader is referred

to the application notes VCSEL Spice Model [2], Modulating VCSELs [3], and the product data

sheets [4] which are available on the Finisar VCSEL website for more detailed information on

variations of VCSEL characteristics with temperature. Like all models, the ones detailed below are

representative, but do provide useful design guidelines. The models are intended to characterize

the electrical interface to the VCSEL, so no attempt is made to model the optical output. Two

models are given, the Common Anode (HFE4x90-xxx), and Common Cathode (HFE4x91-xxx)

configurations. While these two configurations represent the bulk of Finisar production, other

configurations are possible. The parametric values shown in the models are typical. Most do not

vary much from device to device, with the exception of the package lead inductances, which are

dependent on the user-chosen lead lengths, and the parasitic capacitances from the monitor

photodiode to the VCSEL cathode and to the package header. In different designs these latter

values can vary substantially, but within any given design they are nearly constant. In the standard

configurations, the VCSEL and PD common lead is common to the case of the TO package.