Introduction to Silicon Photonic Devices (Part 2)
Silicon Photonics Overview
In this section, we explore silicon photonic devices, starting with the waveguide, which is crucial for silicon photonics. The waveguide's height is a key parameter:
- Height of 3 micrometers in some configurations.
- Height of 220 nanometers in others.
These waveguides are commonly used in the industry today. The advantage is their similar behavior for both TE and TM polarizations. However, the polarization behavior for the 220-nanometer waveguide differs, as seen in the effective index values:
- TE polarization: 2.55
- TM polarization: 1.55
Trends and Innovations
The current trend is to decrease the length of silicon devices. One approach is to utilize nonlinearity to reduce the length of photonic components:
- Using silicon nanowires to increase nonlinearity for all-optical devices.
- Photonic crystal waveguides, which offer a grain-like fiber structure.
For instance, silicon nanowires range from millimeters to centimeters in length, while photonic crystal waveguides are around 100 micrometers. The latter offers an order of magnitude higher nonlinearity than silicon nanowires.
Nonlinear Effects and Applications
Silicon photonics exhibit several nonlinear effects:
- Self-phase modulation
- Two-photon absorption
- Third harmonic generation
- Four-wave mixing for multiple wavelength conversion
These effects are useful for photonic chip components. For example, an 80-micrometer photonic crystal waveguide can emit green light through third harmonic generation when illuminated with 1.55 micrometer light.
Challenges and Solutions
One challenge in silicon photonics is coupling light through the chip. Edge coupling is a common solution:
- Using a lens fiber to couple light to the waveguide.
- Implementing a tapered waveguide and silicon photonic crystal waveguide.
The group index of around 40 to 50 poses a coupling challenge, addressed by using tapered waveguides and modifying photonic crystal waveguide holes.
Passive Components and Coupling Techniques
Silicon photonic integrated circuits use various passive components:
- Wavelength filters
- Resonators
- Multiplexers (MOX, DMOX)
- Couplers and splitters
- Fiber interfaces
Coupling techniques include:
- Edge couplers
- Grating couplers
- Metamaterial waveguides
Optical Links and Modulation
An optical link consists of:
- A laser source
- A modulator that converts electrical signals to optical signals
- A photodetector that extracts the electrical signal
In a typical setup, multiple modulators (e.g., lambda 1, lambda 2, lambda 3, lambda 4) are used to modulate electrical signals into optical signals, with detectors for signal extraction.
Introduction to Silicon Photonic Devices (Part 2) Electronics. So now we want to see silicon photonic devices. So the first item for silicon photonics is a waveguide. So this waveguide is very important.
For instance, in this picture, as you can see here, we can see that the height of our waveguide, the height of the waveguide is three micrometers in this picture. And in these two pictures, the height of the waveguide is around 220 nanometers.
So these are two waveguides that are commonly used with companies these days. The advantage of this waveguide is that it's similar for TE and TM polarization. But as you can see, the polarization behavior for the 220-nanometer waveguide is different.
For instance, for effective index, we see a 2.55 for TE, and 1.55 for TM. I will discuss how we can model different components. I will review some of the components here. The trend is that we want to decrease the length of silicon devices. One idea is to use nonlinearity.
By using nonlinearity, we can decrease the length of our photonic component. One idea is to use a silicon nanowire. In this paper, they use a silicon nanowire and increase the nonlinearity, which can be used for photonic chips in terms of all-optical devices.
The idea I used before is a photonic crystal waveguide. With this waveguide, you see a grain-like fiber. In silicon, pour a America-microcombustion mass. The pseudo-inequalities with this waveguide introduce specific characteristics.
We can see the grid pattern, but also that when overload is diminished, the nonlinearity is increased. For example, the length of a silicon nanowire is around a millimeter to a centimeter. And the length of a photonic crystal waveguide is around 100 micrometers.
But the order of nonlinearity is one order of magnitude more than a silicon nanowire. However, for a single input pulse in silicon, you have self-phase modulation, two-photon absorption, third harmonic generation, and four-way mixing. And these are very useful for photonic chip components.
For instance, for just 80 micrometers, we can see a green light emission in the silicon waveguide. This is an experimental one. If you can see here, this is a green light emission. And this shows how a silicon 80-micrometer waveguide can be used to create a third harmonic generation.
It means that you give a light with 1.55 micrometers and you see a green light emission because of the third order. Actually, third harmonic generation. However, there are some challenges for silicon photonics. For instance, coupling the light through the chip is a challenge.
One idea is edge coupling. For instance, for my case, I use a lens fiber to couple the light to the waveguide. And then we have a tapered waveguide. And then we have a silicon photonic crystal waveguide.
This part is similar to the input, but the group index is around 50 or 40. And coupling the light to the group index of 40 is very difficult. For instance, this is one challenge. And so, for this reason, we use a tapered waveguide.
We change some holes through the photonic crystal waveguide that can be used in a photonic chip. For instance, here's an example of four-way mixing. We put a pump in the middle of the flat band and probe here, and we can see a very nice idler in the silicon photonic waveguide.
This can be used for regeneration, wavelength conversion, and different applications. Regarding other passive components, we have a wavelength filter, resonator, MUX, DMUX, coupler, splitter, and fiber interface. These can be used for silicon photonic and photonic integrated circuits.
We need a silicon spot size converter, as you can see in this picture. For instance, in this picture, we want to couple the light from the fiber to the waveguide. We can see there are different strategies for this case. Or for optical coupling technology, we have an edge coupler here.
We have a grating coupler in this figure. We have a metamaterial waveguide, as you can see in this picture. These are some common methods that can be used to couple the light to the silicon photonic chips. For instance, this is an optical link. We have a laser, then we have a modulator.
We have an electrical signal here. And this electrical signal is modulated to the optical signal. And then, at the photodetector side, we have a photodetector. And then, we can extract our electrical signal. This picture also shows that this is an optical input. And this is an electrical signal.
And we can see how the electrical signal is modulated to the optical. For instance, in this picture, we can see that we have a meeting board here.
We have different wavelengths, and we have four modulators here, lambda 1, lambda 2, lambda 3, lambda 4. And these drivers actually create electrical signals, and this modulator modulates the electrical signal to the optical signal. And we have a detector here. This is the structural air lab.
