TRANSMITTER DEVICES FOR OPTICAL INTERCONNECTS PART 3

Contrast ratio and insertion loss

A significant figure of merit for transmitter devices is the contrast ratio.   This value is calculated by dividing the power in the “1” bit by the power in the “0” bit. Ultimately the system should be judged by its bit error rate (BER), the fraction of bits that are incorrectly received.  A relatively simple analysis of the relationship of the BER to the transmitter device demonstrates that a higher contrast ratio is directly related to a lower BER [3].  This calculation depends on the quantum nature of light and the Poisson nature of the shot noise of photons.  A higher contrast ratio is always preferred, of course,

but a contrast ratio of 3 dB is sufficient for short distance interconnects.  Improvements in BER can also be achieved using a low contrast ratio transmitter by using differential signaling [4-6].

High contrast ratio is achieved primarily by sending a very low optical power in the “0” bit.   For a modulator, it is also important to reduce the insertion loss which corresponds to the loss of the device when sending the “1” bit.   A large insertion loss corresponds to a low transmitted power in the “1” bit state.  The energy of the laser must be deposited in the modulator structure due to the large amount of absorption, and this generates unwanted heating.   (The absorption in the “0” state is unavoidable in a modulator structure that achieves high contrast ratio.)  In addition, this generally requires a higher power continuous wave (CW) laser source, simply because the modulator itself absorbs a lot of the light.  Ideally, insertion loss should be minimized in the modulator design process.

3.1.2.3  2D arrays of devices

CMOS  chips  are  planar  structures  with the  electronics  designed  into  the  top surface.   Extracting the signals in a direction normal (perpendicular) to that surface greatly simplifies the geometry of both the electrical and optical systems.  Furthermore, the high aggregate bit rate of the entire set of interconnects can be more easily achieved if the devices can be accessed in a surface-normal fashion and can be fabricated in 2D arrays.  Many lasers and modulators have been designed in 1D arrays using a waveguide geometry which makes sense for the telecom applications where only a few high speed signals are needed.

For optical interconnects in internet routers, so much data must be transmitted that

2D arrays are, at the least, highly desirable, if not necessary.   The design of surface- normal lasers and modulators is challenging because the interaction volume between the light and the active layers is small, relative to a waveguide geometry.  (The active layers are grown epitaxially in a relatively slow and costly manner, precluding the growth of thick layers.   Thus, the total epitaxial thickness is typically kept under a few microns. Furthermore, thick active layers in surface-normal devices are likely to necessitate high

drive voltages.)  Conquering this geometrical limitation has been a focus of our research and our strategies will be described in Chapters 4 and 5.

 Optical bandwidth/wavelength range

WDM systems require that specific wavelengths be utilized as the carrier waves. In a WDM interconnect, the wavelengths of the laser sources must be tightly controlled. An optical system that uses diffractive optics, such as in Refs. [7, 8], may also be sensitive to changes in wavelength.   This requirement restricts both the designs of the laser sources and the allowable temperature variations.  As a result, WDM systems utilize lasers designed with stringent wavelength-dependent feedback elements and they must be kept under tight temperature control.   A modulator in a WDM system would ideally operate over a wide wavelength range.   Such a device would modulate any incoming laser wavelength within the allowed band without stringent temperature control.   The laser sources would still have to be temperature controlled, but they would exist in a separate location and would not be heated by the power dissipated in the CMOS chip.  In addition, one laser source might be used by many modulators by splitting a single laser beam into many channels.  Controlling this one laser source for n modulators is simpler than controlling n lasers separately.   The use of a modelocked “comb laser” [9] may allow a single laser to generate a multiwavelength spectrum with fixed spacing and a single thermal load to stabilize.

The optical system may be designed to use WDM or diffractive elements, so a modulator with a wide wavelength range of operation is highly preferable.