QUASI-WAVEGUIDE ANGLED-FACET ELECTROABSORPTION MODULATOR

Future CMOS technology is clearly moving towards digital voltage levels of 1 V. The ITRS Roadmap indicates an expected off-chip speed of 9.5 GHz and a voltage of

1.0 V in 2010 [1].   By 2016, the ITRS predicts the voltage will only have dropped to

0.8 V while the off-chip speed will have increased to around 36 GHz [1].  In addition, current long-haul and medium-haul optical networks utilize WDM fiber-based systems operating at a wavelength around 1550 nm and at bit rates of 10 Gbps.  If the future of both personal computing and telecommunications involves optical networks, it seems natural to expect these two types of networks to seamlessly integrate.

In order to design a surface-normal modulator for optical interconnects, several factors must be taken into consideration, as described in Chapter 3.   The AlGaAs modulator  from  Chapter  4  achieved  a  good  contrast  ratio  of  ~  6 dB  for  a  5-nm- wavelength range around 850 nm, but required 2.5 V of voltage drive.   Lowering the voltage drive to 1 V reduced the contrast ratio to ~3 dB for 5 nm.  This standard surface- normal design is subject to a tradeoff between contrast ratio, wavelength range, and low voltage operation, discussed below.  In order to achieve a contrast ratio of 3 dB over a wider wavelength range without dramatically increasing the voltage drive, some new strategy must be employed.

These considerations, very low voltage drive and compatibility with larger C- band WDM optical networks, led us to design a high-speed, low-voltage modulator operating  at  1550 nm.    This  novel  modulator  architecture,  called  a  quasi-waveguide angled-facet electroabsorption modulator (QWAFEM), and its advantages will be presented in this chapter, followed by a description of the methods of its design, fabrication, and testing.

LOW VOLTAGE OPERATION

A surface-normal modulator that operates on only 1 V of digital voltage swing must be extremely thin in order to achieve a high electric field change across the MQW

region.  However, a thin MQW region results in a low contrast ratio, unless an optical resonator is implemented surrounding it, as in the AFPM discussed in Chapter 4.  As the quality factor of the AFPM resonator is increased, the contrast ratio improves, but the wavelength range is limited.  A high contrast ratio for a low voltage swing is possible at the expense of the wavelength range, as in [2] where a contrast ratio of 10 dB for 1 V was achieved across less than 1 nm of wavelength range.  Note that, for a given quality factor, the wavelength range increases for shorter cavities

It seems the ideal modulator would have a thin MQW region (in order to allow a large change in the electric field for a small voltage swing) and a short Fabry-Perot optical cavity with highly reflective mirrors (in order to achieve a high contrast ratio while  maintaining  the  maximum  possible  wavelength  range).    Unfortunately,  in  a surface-normal geometry (i.e. the light propagation axis perpendicular to the epitaxial layers), the penetration depth of dielectric mirrors can be many wavelengths on each side, making a very short optical cavity impossible.  Metallic mirrors can be utilized instead, but they suffer from absorption.

This dilemma, the tradeoff between contrast ratio, low voltage, and wavelength range, seems inherent to surface-normal modulators, but it is avoided in waveguide modulators.