Lightwave Logic Inc (LWLG)

[Photonics_Guy]

What are the advantages of a "driverless" or direct drive modulator?

I was asked this question recently and since it's such a compelling advantage of LWLG's modulators, I felt it was beneficial to put on my tinfoil hat and share my response.

Here is a good App Note from ixBlue that describes what a Modulator Driver is in fairly good detail.

The modulator Driver does two things, (1) it amplifies the incoming RF signal (the datastream) to the level that is optimized for the modulator (dependent upon the Vpi of the modulator). (2) it tracks the optimum DC bias point of the modulator (modulation dependent) and maintains the bias by tracking the output in a feedback loop to address operating point drift w.r.t. temperature, signal levels, electrode capacitance, etc.

So the driver needs to have a very linear high bandwidth RF amplifier, DACs/ADCs, op-amps and a processor to do all these fine functions.

The most power-hungry component in the Driver will be the RF Amp, typically using GaAs transistors. Linear amplifiers are notoriously inefficient (Class A). And - to be linear - need to be 'backed off' from their 1dB compression point (P1dB) by several dB. It is difficult (read expensive) to build a wideband RF amplifier that has linear characteristics, a flat passband and low noise.

The difficulty in building a high-performance RF Amplifier is compounded as the bandwidth is increased. With LWLG's modulators approaching 100 GHz, this becomes an extremely challenging (expensive) endeavor.

If the Vpi of the modulator is low enough, the incoming signal does not require amplification, which obviates the need for this expensive, power-hungry Driver component. Most Modulator Drivers require 12V to operate and consume prodigious amounts of power. Removing the need for the driver saves a LOT of power and expense.

This is true regardless of whether the modulator is intensity or phase modulated (amplitude or phase modulation).

Consequently, Direct Drive Modulators such as LWLG is marketing are a really, really big deal to the industry.

PG