Presented By,
Supreetha S R
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SILICON PHOTONICS
INTRODUCTION
Silicon photonics can be defined as the utilization of silicon-based materials for the generation, guidance, control, and detection of light to communicate information over distance.
It is an evolving technology in which data is transferred among computer chips by optical rays. Optical rays can carry far more data in less time than electrical conductors.
It aims to determine how to use silicon and standard silicon processing techniques to build optical devices. The concept is based on developing optical building blocks that give active functionality, rather than simple, passive optical wave-guiding. These tiny silicon building blocks can be selectively placed into optical modules, reducing cost and size.
Why silicon photonics?
The presence of a single material, silicon, which is widely available, can be purified to an unprecedented level, is easy to handle and to manufacture and shows very good thermal and mechanical properties which render the processing of devices based on it easy.
The presence of a single dominating processing technology, CMOS, with integration of more and more devices on larger wafers.
Limitations of operating speed due to interconnects.
The success and capacity of optical communication.
Properties of silicon
Transparent in 1.3 – 1.6 µm region
CMOS compatibility
Low cost
It has a very high refractive index
It exhibits stronger Raman effect
Very good thermal and mechanical properties
Hybrid integration is considered to achieve electrically pumped light emission, known as Electro Luminescence (EL), from silicon.
In this approach, a simple gain element is coupled to a silicon based bragg filter and used to form an ECL.
Silicon wave guide based Bragg grating can be used in an external cavity to alter the lasing properties of a III-V gain chip to produce a useful source for optical communications. The strong thermo-optic effect in silicon can be used to tune the lasing wavelength by heating the silicon grating.
The ECL is formed by butt coupling a Single Angled Facet (SAF) gain chip to a waveguide containing the polycrystalline/crystalline silicon Bragg grating.
The laser cavity is formed between the Bragg grating as one end mirror, and a 90% high-reflection coating of the gain chip as the other mirror. The 8º angled facet between the two chips decreased the effective reflectivity of that facet to ~10-3.Combining an angled facet with a 1% antireflection coating resulted in an effective facet reflectivity of ~10-5.
The output of the laser was taken from the 90% high-reflectivity coated side of the laser diode with a conical polished (140º) lensed single-mode fiber.
CONTINUOUS SILICON LASER
RAMAN EFFECT:
The Raman Effect is widely used today to make amplifiers and lasers in glass fiber. These devices are built by directing a laser beam known as the pump beam into a fiber.
As the light enters, the photons collide with vibrating atoms in the material and, through the Raman Effect; energy is transferred to photons of longer wavelengths. If a data beam is applied at the appropriate wavelength, it will pick up additional photons. After traveling several kilometers in the fiber, the beam acquires enough energy to cause a significant amplification of the data signal (Figure 1a).
By reflecting light back and forth through the fiber, the repeated action of the Raman Effect can produce a pure laser beam. However, fiber-based devices using the Raman Effect are limited because they require kilometers of fiber to provide sufficient amplification.
The process of building a Raman amplifier or laser in silicon begins with the creation of a waveguide – a conduit for light in silicon. This can be done using standard CMOS techniques to etch a ridge or channel into a silicon wafer (Figure 1b).
Light directed into this waveguide will be contained and channeled across the chip. In any waveguide, some light is lost through absorption by the material, imperfections in the physical structure, roughness of the surfaces and other optical effects.
The amplification provided by the Raman effect exceeds the loss in silicon waveguide.
The reason was a physical process called two-photon absorption which absorbs a fraction of the pump beam and creates free electrons. These electrons build up over time and collect in the waveguide.
The problem is that the free electrons absorb some of the pump and signal beams, reducing the net amplification. The higher the power density in the waveguide, the higher the loss incurred.
Intel’s Breakthrough Laser
Change the design of the waveguide so that it contains a semiconductor structure, technically called a PIN (P-type – Intrinsic – N-type) device. When a voltage is applied to this device, it acts like a vacuum and removes the electrons from the path of the light.
Figure 3 is a schematic of the PIN device. The PIN is represented by the p- and n- doped regions as well as the intrinsic (undoped) silicon in between. This silicon device can direct the flow of current in much the same way as diodes and other semiconductor devices.
To create the breakthrough laser, the ends of the PIN waveguide were coated with mirrors to form a laser cavity (Figure 4). After applying a voltage and a pump beam to the silicon, a steady beam of laser light of a different wavelength exiting the cavity was observed – the first continuous silicon laser.
SILICON MODULATOR
Silicon optical intensity modulator with a modulation bandwidth of 2.5 GHz at optical wavelengths of around 1.55 μm.
The high-speed modulation is achieved by using a novel phase shifter design based on a metal-oxide-semiconductor (MOS) capacitor embedded in a passive silicon waveguide Mach- Zehnder Interferometer (MZI).
Light wave coupled into the MZI is split equally into the two arms, each of which may contain an active section which converts an applied voltage into a small modification in the propagation velocity of light in the waveguide. Over the length of the active section(s), the velocity differences result in a phase diff. in 2 waves. Depending on the relative phase of 2 waves after passing through the arms, the recombined wave will experience an intensity modulation.
A ~1.4 μm n-type doped crystalline silicon slab and a p-type doped poly-silicon rib with a 120 Å gate oxide .
The poly-silicon rib and the gate oxide widths are ~2.5 μm and total polysilicon thickness is ~0.9 μm.
To minimize metal contact loss- a wide (~10.5 μm) top polysilicon layer on top of the oxide layers. Aluminum contacts on top of polysilicon layer.
The oxide regions maintain horizontal optical confinement and prevent the optical field from penetrating into the metal contact areas.Vertical optical confinement is provided by buried oxide (~0.375 μm) and an oxide cover.
The n-type silicon in the MOS capacitor phase shifter is grounded and a positive drive voltage, VD, is applied to the p-type poly-silicon causing a thin charge layer to accumulate on both sides of the gate oxide. The voltage-induced charge density change ΔNe (for electrons) and ΔNh (for holes) is related to the drive voltage by
where ε 0 and ε r are the vacuum permittivity and low frequency relative permittivity of the oxide, e is the electron charge, tox is the gate oxide thickness, t is the effective charge layer thickness, and VFB is the flat band voltage of the MOS capacitor.
Due to the free carrier plasma dispersion effect, the accumulated charges induce a refractive index change in the silicon. The index changes obtained through Kramers-Kronig analysis, are given by
Δne=−8.8×10−22ΔNe
Δnh=−8.5×10−18(ΔNh)0.8
The change in refractive index results in a phase shift Δφ in the optical mode given by
where L is the length of the phase shifter, λ is the wavelength of light in free space, and Δneff is the effective index change in the waveguide, which is the difference between the effective indices of the waveguide phase shifter before and after charge accumulation.
SI-BASED PHOTODETECTORS
Silicon is a poor detector in infrared region. So it is combined with germanium to reduce the bandgap and extend the maximum detectable wavelength. The two main factors to be considered in photodetectors are absorption coefficient or penetration depth of the light; responsivity and bandwidth.
The responsivity is the ratio of collected photocurrent to the optical power incident on the detector. The bandwidth of a photodetector can be limited by the transit time required for the photocarriers to travel to the contacts.
Maximizing the light absorption by making the layers thicker results in a reduction of bandwidth due to transit time issues. The way around this problem is to illuminate the device from the side. By doing this, the transit time can be kept low while the effective length of the detector is increased from a few micrometers to as long as a few millimeters. This is the approach used for waveguide-based photodetectors.
ADVANTAGES
Low cost/high volume production
Faster data transmission
Silicon Photonics interfaces and medium remain constant
Low power consumption
High thermal conductivity
Robustness of device
APPLICATIONS
Fiber to the home
High speed interconnects
Multi-core processors and Supercomputers
Microwave systems and Biometrics
Optical sensors
CONCLUSION
Silicon photonics is a generic technology with a wide range of high volume applications for which the industrial technology base largely exists today. The development of silicon photonics has seen enormous progress over the last decade.
Silicon photonics is being used to make commercially competitive devices that provide the modulation and detection functions needed in data communications, telecommunications systems and optical interconnects. Exciting new applications are emerging in sensing, mid-infrared optics and optomechanical systems.
Silicon photonics will likely eventually make a revolutionary impact in the high-volume data communication and computing industries.