March 07, 2005
Silicon Laser and Cell Processor
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Jack Horgan - Contributing Editor

by Jack Horgan - Contributing Editor
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In the past few weeks there have been two announcement of technical significance. On February 17, 2005 Intel published an article in the prestigious scientific journal Nature disclosing the development of the first continuous wave all-silicon laser on a single silicon chip using the Raman effect. On February 7, 2005 IBM, Sony Group and Toshiba announced at the International Solid State Circuit Conference (ISSOC) their jointly developed microprocessor code-named Cell.

In general when such announcements are made, it does not mean that one can rush down to the corner store and buy one. Considerable time may elapse before these discoveries become part of any commercial offerings. Why then do firms make these technical achievement announcements? Sometimes they are made as an "atta boy" of appreciation for the people involved, to promote the firm with existing and prospective customers and shareholders and to scare off potential competitors in the same way that patent pending notices do. Firms like IBM and Intel have significant research operations who publish their findings in respected journals just like University professors.

Silicon Laser

Intel's announcement on the continuous wave laser was a rapid follow up to earlier papers. The silicon photonics research team has achieved a number of breakthroughs, starting in 2004 with the first silicon-based optical modulator to encode data at 1GHz, an increase of over 50 times the previous research record of about 20MHz. In October 2004 Drs. Ozdal Boyraz and Bahram Jalali of the University of California Los Angeles reported the first demonstration of a pulsed silicon Raman laser in Optics Express. Haisheng Rong was the lead researcher at Intel.

Building a Raman laser in silicon begins with etching a waveguide, a conduit for light on a chip. Silicon is transparent to infrared light so that when light is directed into a waveguide it can be contained and channelled across a chip. An external light source is used to 'pump' light into its chip. As light is pumped in, the natural atomic vibrations in silicon amplify the light as it passes through the chip due to the Raman effect. Because of its crystalline structure, silicon atoms readily vibrate when hit with light and the resulting Raman effect is more than 10,000 times stronger in silicon than in glass fibres.

Raman lasers and amplifiers are used today in the telecoms industry and rely on miles of fibre to amplify light. By using silicon, Intel said it could achieve similar results using a silicon chip just a few centimetres in size.

Dr. Mario Paniccia, director, Intel's Photonics Technology Lab., said about the announcement that "Fundamentally, we have demonstrated for the first time that standard silicon can be used to build devices that amplify light. The use of high-quality photonic devices has been limited because they are expensive to manufacture, assemble and package. This research is a major step toward bringing the benefits of low-cost, high-bandwidth silicon based optical devices to the mass market." This brings Intel closer to realizing its vision of "siliconizing" photonics. In the past lasers that drive optical communications have been made of more exotic materials such as indium phosphide (InP)
and gallium arsenide (GaAs).

What is attractive about the silicon laser is that semiconductor fabs have considerable investment in and experience with making semiconductors. The demand for semiconductors is cyclic leading to boom and bust stages. Using the facilities for a different application would help even out capacity utilization.

Laser Background

The term laser stands for light amplification by stimulated emission of radiation. The basic components of a laser are a medium and a cavity or housing to contain the medium. The medium can be a solid, gas, liquid or semiconductor. Let us look at the basic mechanism of a ruby laser, the first laser that was invented in 1960. Ruby (CrAlO3) is an aluminum oxide crystal in which some of the aluminum atoms have been replaced with chromium atoms. Chromium gives ruby its characteristic red color and is responsible for the lasing behavior of the crystal.

An electron is typically found in its ground state but can be raised to one or more higher specific energy levels or quantum levels by a variety of means. The electron will in a small fraction of a second return to its ground state or cascaded down the energy levels to the ground state by giving up the difference in energy levels as electromagnetic energy in the form of a photon. This is spontaneous emission. The possible energy levels differ from atom to atom. In fact this mechanism is the basis of atomic spectroscopy that identifies the presence of various elements in a sample. If many electrons or molecules in the case of some lasers could be excited creating an inversion population
of excited versus unexcited electrons, then the possibility arises of creating a laser beam. This process is sometimes referred to as "pumping". The spontaneously emitted photon however will go in all directions and are not be "in step". Laser light is monochromatic, directional and coherent. If one of these spontaneous photons passes by an electron in an excited state, it stimulates the electron to give off its energy in the form of a photon that is in phase, in the same direction and of the same wavelength and frequency as the original photon. As these two photons move through the medium they can stimulate additional emissions with the same properties. If the cavity has mirrors at
both ends, these photons can pass back and forth through the medium stimulating more and more photons thereby amplifying the light intensity. If one of the mirrors is only partially reflecting, the coherent laser beam can escape. Unlike a flashlight beam this coherency property prevents the beam from spreading out.

In the case of the early ruby laser, a cylindrical crystal of ruby was used. A high-intensity lamp spiraled around the ruby cylinder provided a flash of white light that triggers population inversion. The ruby laser is a three-level laser and therefore is somewhat more complicated than described above. Light in the green and blue regions of the spectrum is absorbed by chromium ions, raising the energy of electrons of the ions from the ground state level to a broad band of levels which rapidly undergo non-radiative transitions to the two metastable levels. In this case of non-radiative transition the energy released in the transition is dissipated as heat in the ruby crystal. The
metastable levels are unusual in that they have a relatively long lifetime of about 4 milliseconds which aids the creation of an inversion population. The process now proceeds as describe above emitting red light of wavelength 694.3 nm.

Lasers differ in the medium used, the method to activate and the frequency and wavelength of the emitted beam. Frequency, wavelength and energy of electromagnetic waves are related by the well known formulas

λ = c/f and E = hc/ λ

E is energy measured in ergs

c is the speed of light in a vacuum, 29,979,245,800 centimeters per second

f is the frequency of light measured in cycles per seconds or Hertz

λ is the wavelength of light measured in centimeters. The wavelength of light is often presented in terms of Angstroms (10-10 meters)
The wavelength of the laser beam determines the range of applications and degree of potential danger to humans.

Laser are routinely used in consumer products (printers, scanners, CD, DVD, ..), medicine (dental, eye and cosmetic surgery), military (guidance, navigation, ..), industry (cutting, annealing, …)

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-- Jack Horgan, Contributing Editor.


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