Optoelectronic chromatic dispersion in photodiodes

  Rita Abramov  ,  Ziv Glasser   ,  Shmuel Sternklar  
Department of Electrical and Electronic Engineering, Ariel University, Ariel 40700 Israel

Chromatic dispersion is a fundamental mechanism that occurs as a result of the interaction of light with any medium. As described by the Drude-Lorentz model, this effect is due to a wavelength-dependent time response of the bound and free charges to the illumination, and can be described as a dependence of the medium's real and imaginary dielectric constants on the wavelength. It leads to an effective group index of refraction that is also wavelength dependent, and is the source of pulse spreading, posing a fundamental bandwidth limitation in fiber-optic communications.

In this work we show that the wavelength-dependent time response of photodiodes can be recast as an effective source of chromatic dispersion, which we call optoelectronic chromatic dispersion (OED). It is well understood that the photodiode dependence on wavelength is manifested in the responsivity as well as the RF bandwidth. The former describes the efficiency of current formation in units of A/W, and the latter determines the photodiode response to a short light pulse. After over more than half a century of work, the theoretical models that describe this dependence are well established, and form the basis for our work as well. The source of the electronic bandwidth dependence on optical wavelength originates with the absorption coefficient dependence on wavelength.

The physics of OED is fundamentally different from that of chromatic dispersion. In optically dispersive media, such as an optical fiber, it is ultimately the optical response of the media that is monitored: the output light carries the  information on its phase. In OED, it is the electrical response of the photodiode that displays the effective chromatic dispersion.  As we describe below, it results from a multi-stage optoelectronic process: light absorption, exciton formation, and various mechanisms for current generation. Surprisingly, we show that the size of the OED effect can be significantly larger than that of a standard telecommunication optical fiber. For example, for a germanium PN photodiode in the c-band, we demonstrate that the OED is equivalent to 210 km of standard optical fiber. This new approach for describing the optoelectronic interaction also leads to new methods for characterizing the physics of the photodiode response, and new techniques for wavelength monitoring, spectroscopy, and other types of optical sensing.