The Path to 100G Single Lambda in the Data Center - Part 4

One merit of moving to externally modulated lasers (and leaving behind intensity modulation schemes) is that a single "continuous wave" operated VCSEL can supply sufficient light power through an optical splitter to four external optical modulators saving power, cost and space. The modulators likely make up the difference (and possibly more) from a budget perspective.

Optical modulators are probably the most baffling element of the transceiver signal chain for people without a degree in optical engineering. The device structure used in these applications is called a Mach-Zehnder modulator and is constructed typically with lithium niobate or more recently with polymer-based materials. Lithium niobate is also the magical material used to build the narrowband SAW and BAW filters used in cellular RF front-ends for smartphones. Recently, Ranovus has introduced silicon photonic modulators based on a ring resonator structure, BrPhotonics has announced products based on thin film polymers on silicon, and LightWave Logic has revealed work on high speed optical modulators using organic chromophores.

The theory of operation of MZMs is somewhat obscure. Mach-Zehnder interferometers use the electro-optic effect in which the index of refraction of a material is altered by the electric field applied to it, slightly and precisely changing the speed of light passing through the material. Specifically, to perform the amplitude modulation needed for PAM-4 based 100G single wavelength transmission, the laser light is divided into two paths with a beam splitter, with one path subjected to the variable electric field (signals coming from the upstream linear driver). The two beams are then recombined. The relative phase delay between the two paths then leads to constructive or destructive interference, yielding either an increase in amplitude or a decrease in amplitude at the MZM output. Oclaro and NeoPhotonics are the high volume leaders in MZM device design and manufacturing.

Mach-Zehnder Modulator (MZM) functional diagram and physical implementation
(Image courtesy of Optics Express)

The MZM output is then coupled to the outgoing single-mode or multimode fiber aligned using V-grooves, either free space or using micro lenses. This completes the 100 Gbps Tx signal chain. We'll look at the 100G per lambda Rx signal chain in the next installment later this fall.