Abstract

This thesis describes the design, realization and characterization of densely integrated optical components based on thermally tunable microring resonators fabricated in Si₃N₄/SiO₂.

Chapter 1 “Introduction”

In this chapter a brief introduction and overview are given of current broadband communication networks to provide a background for the work presented in this thesis. Current copper based networks are unable to meet future bandwidth demands and will therefore be slowly replaced with optical networks. A promising technology for these networks is WDM-PON. Currently, however, this technology is too expensive. The Broadband Photonics and NAIS projects within which the presented work was carried out both seek to lower the cost of WDM-PON implementations through dense integration of reconfigurable optical components based on optical microring resonators.

Chapter 2 “The micro-resonator”

In the second chapter the operating principle of a microring resonator is explained and the basic parameters that govern its operation are introduced. The filter frequency-domain responses for single as well as serial higher order systems based on two resonators are derived. Solutions for typical problems that occur when designing resonators such as a Free Spectral Range (FSR) that is too small or a filter shape that does not meet the desired specifications are also given.

Chapter 3 “Design”

In the third chapter the design of microring resonator based devices is discussed in general terms. Several performance parameters are introduced that can be used to translate the requirements of a certain application into specific values of the basic microring resonator parameters.

For microring resonators with a radius of 50 µm (FSR≈4.2 nm) , which is the case for most of the devices presented in this thesis, it is shown that for telecom applications a good target for the field coupling coefficients is between 0.4 and 0.6 when reasonable losses of 2 dB/cm are assumed for the resonator. The methodologies for creating an actual resonator design from these basic parameters are also given. In addition design aspects on a device level (the whole device layout) are discussed. Here it is shown that for these resonators the miniaturization of devices that incorporate these resonators is limited by the spacing of the fibers in the fiber array used for pigtailing rather than the size of the individual resonators.

Chapter 4 “Simulation and analysis”

In Chapter 4 some of the tools that were created to aid in the design and characterization of microring resonator based devices are presented. In particular the analytical and numerical methods used to fit measured resonator responses are examined in detail. Another tool that is discussed is Aurora, a tool that was created to perform simulations on complex optical circuits containing resonators. The simulations performed by Aurora are time-domain based. Although the simulation principle, based on the delayed forwarding of signals between optical components, is fairly simple, it is nonetheless very powerful and allows for a very fast simulation of highly complex optical circuits.

Chapter 5 “Fabrication”

In Chapter 5 three distinct fabrication processes are described. Each process was designed for a specific resonator type. The simplest process was designed for laterally coupled resonators. This process does not suffer from resonator misalignment but is critical where the resolution of the lithography is concerned. A more complex process was designed for vertically coupled resonators. The lithographic requirements of this process are less important although the process is highly susceptible to resonator misalignment. The most elaborate fabrication process is based on stepper lithography. This allows for very small feature sizes as well as a high alignment accuracy which is very important from a device yield perspective. The only downside is that the maximum size of devices is limited to 22 by 22 mm. However, devices made in a materials system with a high index contrast such as Si₃N₄/SiO₂ will often be smaller than this. The stepper process also included chemical mechanical polishing of the separation layer between the ring resonator and the port waveguides in order to reduce the losses in the resonator caused by an abrupt “lifting” of the resonator on top of the port waveguides.

Chapter 6 “Microring-resonator building blocks”

In Chapter 6 the design of a basic resonator building block is given. This building block is based on a 2.0 x 0.14 µm port waveguide from where the light is coupled into a ring resonator that has waveguide dimensions of 2.5 x 0.18 µm and a radius of 50 µm. On top of the resonator a heater is placed to be able to shift its resonance wavelength. Depending on the resonator radius (25 or 50 µm) and the thickness of the cladding layer on top of the resonator (3 or 4 µm) resonance shifts between 7 pm/mW and 21 pm/mW have been observed. By using an overshoot in the electrical signal that drives the heater, thermal modulation frequencies up to 10 KHz could be observed. Also demonstrated in this chapter is a wavelength selective optical switch based on two cascaded resonators. The switch measures only 200 µm x 200 µm. The “on/off” attenuation of the switch is 12 dB. When the switch is “on” the crosstalk with the adjacent channels is ≈-20 dB (channel spacing of 0.8 nm). The on chip insertion loss of the switch is around 5 dB. A Vernier resonator based on two resonators with a radius of 46 µm and 55 µm is also demonstrated. The combined FSR is ≈28 µm.

Chapter 7 “Densely integrated devices for WDM-PON”

In Chapter 7 the design and characterization of two different types of OADM, for use at 1310 nm or at 1550 nm, and a Router are discussed. The 1550 nm OADM could be fully tuned and could be configured to drop one or more channels. In addition system level measurements were performed in this OADM. A 40 Gbit/s could be dropped to a single channel without a significant penalty in BER. In addition multicasting was demonstrated. The same reconfigurability was also shown for the 1300 nm OADM. Finally the 1300 nm router is discussed and basic functionality of the router, dropping one, two or three channels to a single output is demonstrated.

Chapter 8 “Polarization independent devices”

A major problem of microring resonator based devices is that it is often very difficult to make them polarization independent. Although this can by solved by introducing polarization diversity in the devices this also doubles the number of resonators and creates a number of new problems. In Chapter 8 a method is described where a single microring resonator is used bi-directionally so that a single resonator effectively operates as if two resonators are present. The number of resonators that is required to implement polarization diversity in a device is therefore more or less the same (there is a minor overhead) as the number of resonators in an implementation that uses polarization independent resonators.

Chapter 9 “Discussion and conclusions”

Finally, in Chapter 9, conclusions are drawn based on the results presented in this thesis.

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Home News Events Strategic Orientations research groups Publications PhD students Education MESA+ NanoLab Starting entrepreneurs Tech Park Industrial Partners Vacancies Links Sitemap Search	Abstract This thesis describes the design, realization and characterization of densely integrated optical components based on thermally tunable microring resonators fabricated in Si₃N₄/SiO₂. Chapter 1 “Introduction” In this chapter a brief introduction and overview are given of current broadband communication networks to provide a background for the work presented in this thesis. Current copper based networks are unable to meet future bandwidth demands and will therefore be slowly replaced with optical networks. A promising technology for these networks is WDM-PON. Currently, however, this technology is too expensive. The Broadband Photonics and NAIS projects within which the presented work was carried out both seek to lower the cost of WDM-PON implementations through dense integration of reconfigurable optical components based on optical microring resonators. Chapter 2 “The micro-resonator” In the second chapter the operating principle of a microring resonator is explained and the basic parameters that govern its operation are introduced. The filter frequency-domain responses for single as well as serial higher order systems based on two resonators are derived. Solutions for typical problems that occur when designing resonators such as a Free Spectral Range (FSR) that is too small or a filter shape that does not meet the desired specifications are also given. Chapter 3 “Design” In the third chapter the design of microring resonator based devices is discussed in general terms. Several performance parameters are introduced that can be used to translate the requirements of a certain application into specific values of the basic microring resonator parameters. For microring resonators with a radius of 50 µm (FSR≈4.2 nm) , which is the case for most of the devices presented in this thesis, it is shown that for telecom applications a good target for the field coupling coefficients is between 0.4 and 0.6 when reasonable losses of 2 dB/cm are assumed for the resonator. The methodologies for creating an actual resonator design from these basic parameters are also given. In addition design aspects on a device level (the whole device layout) are discussed. Here it is shown that for these resonators the miniaturization of devices that incorporate these resonators is limited by the spacing of the fibers in the fiber array used for pigtailing rather than the size of the individual resonators. Chapter 4 “Simulation and analysis” In Chapter 4 some of the tools that were created to aid in the design and characterization of microring resonator based devices are presented. In particular the analytical and numerical methods used to fit measured resonator responses are examined in detail. Another tool that is discussed is Aurora, a tool that was created to perform simulations on complex optical circuits containing resonators. The simulations performed by Aurora are time-domain based. Although the simulation principle, based on the delayed forwarding of signals between optical components, is fairly simple, it is nonetheless very powerful and allows for a very fast simulation of highly complex optical circuits. Chapter 5 “Fabrication” In Chapter 5 three distinct fabrication processes are described. Each process was designed for a specific resonator type. The simplest process was designed for laterally coupled resonators. This process does not suffer from resonator misalignment but is critical where the resolution of the lithography is concerned. A more complex process was designed for vertically coupled resonators. The lithographic requirements of this process are less important although the process is highly susceptible to resonator misalignment. The most elaborate fabrication process is based on stepper lithography. This allows for very small feature sizes as well as a high alignment accuracy which is very important from a device yield perspective. The only downside is that the maximum size of devices is limited to 22 by 22 mm. However, devices made in a materials system with a high index contrast such as Si₃N₄/SiO₂ will often be smaller than this. The stepper process also included chemical mechanical polishing of the separation layer between the ring resonator and the port waveguides in order to reduce the losses in the resonator caused by an abrupt “lifting” of the resonator on top of the port waveguides. Chapter 6 “Microring-resonator building blocks” In Chapter 6 the design of a basic resonator building block is given. This building block is based on a 2.0 x 0.14 µm port waveguide from where the light is coupled into a ring resonator that has waveguide dimensions of 2.5 x 0.18 µm and a radius of 50 µm. On top of the resonator a heater is placed to be able to shift its resonance wavelength. Depending on the resonator radius (25 or 50 µm) and the thickness of the cladding layer on top of the resonator (3 or 4 µm) resonance shifts between 7 pm/mW and 21 pm/mW have been observed. By using an overshoot in the electrical signal that drives the heater, thermal modulation frequencies up to 10 KHz could be observed. Also demonstrated in this chapter is a wavelength selective optical switch based on two cascaded resonators. The switch measures only 200 µm x 200 µm. The “on/off” attenuation of the switch is 12 dB. When the switch is “on” the crosstalk with the adjacent channels is ≈-20 dB (channel spacing of 0.8 nm). The on chip insertion loss of the switch is around 5 dB. A Vernier resonator based on two resonators with a radius of 46 µm and 55 µm is also demonstrated. The combined FSR is ≈28 µm. Chapter 7 “Densely integrated devices for WDM-PON” In Chapter 7 the design and characterization of two different types of OADM, for use at 1310 nm or at 1550 nm, and a Router are discussed. The 1550 nm OADM could be fully tuned and could be configured to drop one or more channels. In addition system level measurements were performed in this OADM. A 40 Gbit/s could be dropped to a single channel without a significant penalty in BER. In addition multicasting was demonstrated. The same reconfigurability was also shown for the 1300 nm OADM. Finally the 1300 nm router is discussed and basic functionality of the router, dropping one, two or three channels to a single output is demonstrated. Chapter 8 “Polarization independent devices” A major problem of microring resonator based devices is that it is often very difficult to make them polarization independent. Although this can by solved by introducing polarization diversity in the devices this also doubles the number of resonators and creates a number of new problems. In Chapter 8 a method is described where a single microring resonator is used bi-directionally so that a single resonator effectively operates as if two resonators are present. The number of resonators that is required to implement polarization diversity in a device is therefore more or less the same (there is a minor overhead) as the number of resonators in an implementation that uses polarization independent resonators. Chapter 9 “Discussion and conclusions” Finally, in Chapter 9, conclusions are drawn based on the results presented in this thesis.