Silicon photonics can be defined as the utilization of silicon-based materials for the generation (electrical-to-optical conversion), guidance, control, and detection (optical-to-electrical conversion) of light to communicate information over distance. The most advanced extension of this concept is to have a comprehensive set of optical and electronic functions available to the designer as monolithically integrated building blocks upon a single silicon substrate.
Within the range of fiber optic telecommunication wavelength (1.3 µm to 1.6 µm), silicon is nearly transparent and generally does not interact with the light, making it an exceptional medium for guiding optical data streams between active components. But no practical modification to silicon has yet been conceived which gives efficient generation of light. Thus it required the light source as an external component which was a drawback.
There are two parallel approaches being pursued for achieving opto-electronic integration in silicon. The first is to look for specific cases where close integration of an optical component and an electronic circuit can improve overall system performance. One such case would be to integrate a SiGe photodetector with a Complementary Metal-Oxide-Semiconductor (CMOS) transimpedance amplifier. The second is to achieve a high level of photonic integration with the goal of maximizing the level of optical functionality and optical performance. This is possible by increasing light emitting efficiency of silicon. The paper basically deals with this aspect.
Silicon photonics 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. Its overarching goal is to develop high volume, bolt-and-go optical components using silicon. While silicon is opaque in the visible spectrum, it is transparent at the infrared wavelengths used in optical transmission, hence it can guide light.
Why silicon photonics?
The big success of today’s microelectronic industry is based on various factors, among others
• 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 availability of a natural oxide of silicon, SiO2, which effectively passivates the surface of silicon, is an excellent insulator, is an effective diffusion barrier and has a very high etching selectivity with respect to Si,
• the presence of a single dominating processing technology, CMOS, which accounts for more than 95% of the whole market of semiconductor chips,
• the possibility to integrate more and more devices, 55 000 000 transistors in PENTIUM® 4 (figure), on larger and larger wafers (300 mm process and 400 mm research) with a single transistor size which is decreasing (gate lengths of 180 nm are in production while 15 nm have been demonstrated), yielding a significant reduction in cost per bit,
• the ability of the silicon industry to face improvements when the technology is hitting the so-called red brick wall, e.g. the use of SiGe for high frequency operation and the introduction of low k-materials and of Cu to reduce RC delays,
All these factors have rendered the microelectronics industry very successful. However, in recent years some concerns about the evolution of this industry have been raised which seem related to fundamental materials and processing aspects.
Silicon photonics 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. In the future, these tiny silicon building blocks can be selectively placed into optical modules, reducing cost and size.
Today, both home and business computers are limited less by processor performance than by the rate at which data can be transmitted between the processor and the outside world. Major corporations, financial institutions, and virtually all businesses and consumers demand instant and reliable transmissions, whether their data is traveling across the street or around the world. Corporate LAN and Internet traffic already exceeds telephony traffic, and Internet traffic has been doubling every year.
Copper-based networking can no longer keep up, so the telecommunications industry has turned to fiber optics to fulfill the growth demands of Internet traffic. But optical networks are arcane and expensive. Because of their high costs, their use has been limited primarily to long-haul and backbone networks.
The evolution toward faster data rates will drive the fiber-optic industry to move next to 40 Gbps, and to even higher data rates in the future. With the combination of higher data rates and DWDM capabilities, telecommunication companies will be able to transmit a trillion bits of data per second on a single fiber — a rate that would exceed the total traffic on the entire Internet today.
EXTERNAL CAVITY LASER
Silicon Light Source:
In this section the use of silicon photonics applied to the light source is discussed. While a silicon laser is still out of reach, work is being done worldwide on silicon light emitters that emit both visible and infrared radiation. A silicon emitter is the missing piece for monolithic integration as it would enable all optical elements and drive electronics to be fabricated on a common substrate. Because we are using silicon waveguides to guide light, the emitter must be in the infrared region of the wavelength spectrum (> 1.1 μm) where optical absorption loss is low.
We first summarize the different paths researchers are investigating to achieve electrically pumped light emission, known as Electro Luminescence (EL), from silicon. Until reliable and efficient silicon emitter can be produced, hybrid integration must be considered (i.e., using a non-silicon-based light source coupled to silicon waveguides). In such a hybrid integrated approach, we show how a simple gain element (III-V gain chip) coupled to a silicon-based Bragg filter can be used to form an ECL. Proof of principal of this tunable, single mode laser is discussed.
This section describes how a 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 driving force behind this is to produce an inexpensive narrow line-width source suitable for optical communications.
The Bragg grating is fabricated by etching a set of 1.2x2.3 μm, 3.4 μm deep, trenches into a 4 μm thick Silicon-on- Insulator (SOI) wafer. One thousand of these trenches are laid out in a line along the waveguide with a range of periods around 2.445 μm (although these were laid out as rectangular trenches, due to litho resolution, they were rounded after processing). These trenches are then filled with poly-silicon and annealed to reduce the loss
due to the poly-silicon. The poly-silicon is then chemically/mechanically polished to obtain a planar surface and the 3.5 μm wide, 0.9 μm deep rib is patterned using standard lithography and etching. The last step in the fabrication is to deposit a final, 0.5 μm thick, low temperature layer of oxide to provide the necessary upper cladding for the rib waveguides. A schematic of the Bragg grating is shown in Figure 1.
The novel property of this Bragg grating is that it only reflects a narrow, 0.5 nm wide range of wavelengths back through the waveguide with a reflectivity of 70%. An example of a reflection spectrum from a 1500- trench grating filter is shown in Figure. As a separate component these Bragg filters can be used in optical communication networks as channel filters for wavelength division multiplexed systems.
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 [Figure 3]. The purpose of this lensed optical fiber was to increase the coupling between the laser and the optical fiber.
CONTINUOUS SILICON LASER
The Raman Effect:
The term “laser” is an acronym for Light Amplification through Stimulated Emission of Radiation. The stimulated emission is created by changing the state of electrons – the subatomic particles that make up electricity. As their state changes, they release a photon, which is the particle that composes light. This generation of photons can be stimulated in many materials, but not silicon due to its material properties. However, an alternate process called the Raman Effect can be used to amplify light in silicon and other materials, such as glass fiber. Intel has achieved a research breakthrough by creating an optical device based on the Raman Effect, enabling silicon to be used for the first time to amplify signals and create continuous beams of laser light. This breakthrough opens up new possibilities for making optical devices in silicon.
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 (see sidebar on lasers). However, fiber-based devices using the Raman Effect are limited because they require kilometers of fiber to provide sufficient amplification.
The Raman Effect is more than 10,000 times stronger in silicon than in glass optical fiber, making silicon an advantageous material. Instead of kilometers of fiber, only centimeters of silicon are required (Figure 1b). By using the Raman Effect and an optical pump beam, silicon can now be used to make useful amplifiers and lasers.