Implementation of Optoelectronics in High-Speed Computation

1. Micro-Ring Resonators in
Optical Computation
by Shreyan Datta

2. Contents
1) Micro-Ring Resonator
2) The basic working principle
3) Switching Mechanisms-----> 1X1 and 2X2 switching mechanisms
4) Examples of the different mechanisms
5) Multi-Spectral Logic and its significance in OR operation.
6) Translation of MZIM based full adder design to MRR based design
7) Conclusion

3. Micro-Ring Resonator
1)An optical microring resonator is an integrated optic traveling wave
resonator constructed by bending an optical waveguide to form a closed loop,
typically of a circular or racetrack shape.
2) Light propagating in the microring waveguide interferes with itself after
every trip around the ring.
3) When the roundtrip length is exactly equal to an integer multiple of the
guided wavelength, constructive interference of light occurs which gives rise
to sharp resonances and large intensity buildup inside the microring.

4. Schematic of a microring resonator coupled to two bus
(or access) waveguides

5. Basic Working Principle
1) Whenever Resonance occurs in the circular micro-ring waveguide there
is a drop in the transmission in the bus waveguide. The ring works as a
spectral filter.
2) We find the ring to be on resonance when the phase φ is a multiple of 2π,
or when the wavelength of the light fits a whole number of times inside
the optical length of the ring.
3) We can tune the resonance characteristics of the the Micro-Ring
Resonator by applying electric fields across the waveguide or by
providing heat using microheaters and sometimes both.
4) Tuning the characteristic wavelength for the resonance to take place
opens up avenues for multiple types of switching operations that can be
carried out.

6. Switching Mechanisms.
We make use of the basic principle that at the resonant frequency the light is
coupled into the ring resonator from the bus waveguide.
We generally require one bus waveguide for a switching operation and
coupled with it lies the ring resonator.

7. Schematic of a Micro-Ring Resnonator (MRR)

8. Optical Transmission Spectra showing the transmission through a MRR
arrangement and thus its variation with wavelength.

9. (a):The Schematic of an 1X1 switch used in
logical operations.
(b): A diagram of a silicon microring
modulator with a p-i-n junction built
across the ring. This is one of the
mechanisms that can be used to tune the
resonance of the ring with high speed.
The 1X1 Switching Mechanism

10. Pass and Block refer to the operations done on the light when it passes
through a resonator. For ex: Pass/Pass mode allows the light to pass through in
both logical High and Low states applied to the resonator.
We consider a Laser source of certain wavelength λ and we tune the ring
resonators in such a way that we get three basic modes of operation.
The Nomenclature represents the states in the form 1/0.
State 1 0
Pass/Pass Pass Pass
Pass/Block Pass Block
Block/Pass Block Pass

11. 0V denotes Logical ‘0’.
0.89V denoted Logical ‘1’.

12. The Notations used for the three different types of Switching operation.

13. While the OR function is hard to implement directly, we can take advantage of
this relationship between OR and NAND functions.
*The Techniques of realizing an OR function will be dealt with further into the
slides.

14. The 2X2 Switching Mechanism
2X2 Mircro-Ring
Resonator used in the
switching mechanism
2X2 MRR along with a reconfigurable
1X1 MRR switch.

15. 1) Resonance state ------> CROSS state
1) Off-Resonance state -->BAR state

16. The Nomenclature represents the states in the form 1/0.
State 1 0
Cross/Bar Cross Bar
Bar/Cross Bar Cross
Bar/Bar Bar Bar
The 1X1 MRR in the system will bring another set of pass and blocking
possibilities thus increasing the number of operational modes.

17. The 6 possibilities and their representations in circuits

18. Truth Table for 8bit Priority Encoder
Example: 1X1 switching mechanism and realizing a 8 bit priority encoder

19. To tackle the OR functions we express the inverted version of the outputs as
sum of the products

20. The directed logic implementation of the encoder. Black lines: optical
waveguides; Red lines: electric logic signal controlling the optical switches;
Solid squares: optical switches in the pass/block mode, i.e. it passes light when
the control logic signal is ‗1‘ and blocks light when the control logic signal is
‗0‘; Hollow squares: optical switches in the block/pass mode; Dashed circles,
optical switches in the pass/pass mode.

21. Implementation of the 2X2 switching mechanism in an 4 bit comparator.

22. A simpler implementation of the XOR gate using the 2X2 Switching
mechanism.

23. The broadband photodetector at the end of an output waveguide will absorb
photons at all source wavelengths and create a photocurrent that sums the
optical outputs from the horizontal waveguides that couple to that vertical
output waveguide. If the photocurrent is higher than that expected when one
of the horizontal waveguide has high output, it will trigger the receiver circuit
to define the output logic as =1‘. This performs the OR (sum) function of the
products calculated by the coupled horizontal waveguides.
Multi-Spectral Logic and its significance in OR operation.

24. The diagram of a four-input multi-wavelength DL circuit that performs sum
operation by collecting optical output at different wavelengths into one
waveguide with the tunable microring resonators.

25. Initial design using Mach-Zehnder Interferometer Modulators.
Design of Optical Full Adder using MRR elemental blocks

26. Electrode configuration followed in the previous architecture and will be
constant in the MRR based design

27. Translation of the Optical full adder circuit initially designed by MZIM, to MRR design.

28. The working of an MRR and its applications in switching and logic circuits and
the implementations have been discussed in this presentation. The discussed
architectures can be used in the design of common logic circuits.
Implementation of these minimizes the latency in calculating a complicated
logic function by taking advantage of fast and low-loss propagation of light in
a highly integrated, waveguided, on-chip photonic system and can also
provide ultrafast network routing functions that enable highly efficient
packet-switched interconnections for high-performance computing. Thus
immense potential of such logic architectures need to be tapped into for
advantage like mentioned above.
Conclusion