ScienceWeek

APPLIED PHYSICS: ON OPTICAL SILICON CHIPS

The following points are made by Graham T. Reed (Nature 2004 427:595):

1) Research into optical circuits for communications began in the 1970s. Early visions of optical circuits were as "optical superchips", containing light emitters, modulators, amplifiers, optical isolators, detectors and, latterly, electronic intelligence(1). However, there are still differing views about the optimum material for such components. This is sometimes articulated in the phrase "optical circuits have yet to find their silicon" -- a reference to silicon's dominance in the microelectronics industry. Despite its success as an electronic material, silicon has received rather modest attention as an optical material. But that may be about to change: Liu et al(2) of the Intel Corporation have fabricated the first silicon-based optical modulator with a bandwidth that exceeds 1 gigahertz (GHz).

2) In technology terms, research into silicon as an optical material has been under way for a long time -- since the mid-1980s. However, relatively little progress has been made in comparison with more exotic materials such as III-V compounds (indium phosphide, gallium arsenide and related compounds), the insulator lithium niobate, or even silica (which typically uses silicon as a substrate material). That is not to say that little has been achieved, but the global technical effort has largely been concentrated on materials other than silicon.

3) There are two main reasons for this. First, silicon does not have an inherent mechanism for the emission of light: it is an indirect bandgap material, which means that its crystal structure makes it impossible to fabricate an efficient light-emitting device, such as a laser, from this material in the conventional way. The III-V compounds, in contrast, are direct bandgap materials and for this reason are often used in semiconductor lasers.

4) Second, silicon does not exhibit an electro-optic effect known as the "Pockels effect", the traditional characteristic for fast modulation of light (that is, encoding data onto light by selectively changing its intensity). In other materials, such as lithium niobate, modulation is typically achieved via the Pockels effect, which causes a linear change in a material's refractive index with an applied electric field. Other optical modulators use electric-field effects known as electro-absorption and electro-refraction, but these are weak in silicon. Instead, modulation in silicon is achieved through thermal mechanisms, which are relatively slow (typically kilohertz), or through the introduction of free carriers (electrons or their positively charged counterparts, holes), which in turn results in both absorption and a change in refractive index. This latter mechanism, known as the "free-carrier dispersion effect", is still also relatively slow, as it is associated with the physical movement of charge within the device. Nevertheless, it has been predicted that silicon-based modulators using this effect could achieve bandwidths exceeding 1 GHz (3), although in practice working devices(4,5) have been limited to about 20 MHz.

References (abridged):

1. Soref, R. A. Proc. IEEE 81, 1687-1706 (1993)

2. Liu, A. et al. Nature 427, 615-618 (2004)

3. Png, C. E., Reed, G. T., Atta, R. M. H., Ensell, G. J. & Evans, A. G. R. Proc. SPIE 4997, 190-197 (2003)

4. Tang, C. K. & Reed, G. T. Electron. Lett. 31, 451-452 (1995)

5. Dainesi, P. et al. IEEE Photon. Technol. Lett. 12, 660-662 (2000)

Nature http://www.nature.com/nature

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ON THE LIMITS OF QUANTUM OPTICAL COMMUNICATION CHANNELS

The following points are made by J. Tworzydlo and C.W. Beenakker (Phys. Rev. Lett. 2002 89:043902):

1) To faithfully transmit information through a communication channel, the rate of transmission should be less than the capacity of the channel [1,2]. Although current technology is still far from the quantum limit, there is an active scientific interest in the fundamental limitations to the capacity imposed by quantum mechanics [3,4]. Ultimately, these limitations originate from the uncertainty principle, which is the source of noise that remains when all external sources have been eliminated.

2) An important line of investigation deals with strategies to increase the capacity. One remarkable finding of recent years has been the beneficial role of multiple scattering by disorder, which under some circumstances can increase the capacity by increasing the number of modes that effectively carry the information [5]. Quite generally, the capacity increases with increasing signal-to-noise ratio, so that amplification of the signal is a practical way to increase the capacity. When considering the quantum limits, however, one should include not only the amplification of the signal (e.g., by stimulated emission), but also the excess noise (e.g., due to spontaneous emission). The two are linked at a fundamental level by the fluctuation-dissipation theorem, which constrains the beneficial effect of amplification on the capacity.

3) While the effects of disorder and amplification on communication rates have been considered separately in the past, their combined effects are still an open problem. Even the basic question, "Does the capacity go up or down with increasing gain?", has not been answered. The authors were motivated to look into this problem by the recent interest in so-called "random lasers". These are optical media with gain, in which the feedback is provided by disorder instead of by mirrors. Below the laser threshold, these materials behave similar to linear amplifiers with strong intermode scattering, and this results in some unusual noise properties. As the authors demonstrate, the techniques developed in connection with random lasers can be used to predict under what circumstances the capacity is increased by amplification.

4) In summary: The authors report a study of the competing effects of stimulated and spontaneous emission on the information capacity of an amplifying disordered waveguide. At the laser threshold the capacity reaches a "universal" limit, independent of the degree of disorder. Whether or not this limit is larger or smaller than the capacity without amplification depends on the disorder, as well as on the input power. Explicit expressions are obtained for heterodyne detection of coherent states, and generalized for an arbitrary detection scheme.

References (abridged):

1. C.E. Shannon, Bell Syst. Tech. J. 27, 379 (1948); 27, 623 (1948)

2. T. M. Cover and J. A. Thomas, Elements of Information Theory (Wiley, New York, 1991)

3. C. M. Caves and P. D. Drummond, Rev. Mod. Phys. 66, 481 (1994)

4. M.A. Nielsen and I.L. Chuang, Quantum Computation and Quantum Information (Cambridge University, Cambridge, England, 2000)

5. G.J. Foschini, Bell Labs Tech. J. 1, 41 (1996)

Phys. Rev. Lett. http://prl.aps.org

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ON OPTICAL COMMUNICATIONS

The term "optical communications" refers to the transmission of speech, data, or other information by light. The essential idea is that an information-carrying light wave signal originates in a transmitter, passes through an optical channel, and enters a receiver that reconstructs the original information. The ensemble transmitter-channel-receiver constitutes an optical communication system.

In general, the advantages in using light waves instead of longer electromagnetic waves for communication are as follows:

a) Because of its shorter wavelength, light can be focused into narrower beams or confined in smaller waveguides than radiation of longer wavelengths.

b) The information-carrying capacity of light is potentially greater than that of longer-wavelength electromagnetic radiation.

c) The highest transparency for electromagnetic radiation in any solid material is that of silica glass [SiO(sub2) glass] in the wavelength range 1 to 2 microns. This transparency is orders of magnitude higher than that of any other solid material in any other part of the spectrum.

Before the 1960s, the only available optical communication sources generated light from a multitude of independent atomic radiators, and such light cannot be focused effectively to form a narrow beam or to propagate along the axis of a waveguide. The invention in the 1960s of the first laser removed this shortcoming and made feasible in the optical wavelength region all the communication techniques formerly limited to microwaves and other electromagnetic waves of longer wavelengths.

The following points are made by G.A. Thomas et al (Physics Today September 2000):

1) Alexander Graham Bell began the age of optical communications when he patented an optical telephone system in 1880, but the field lay dormant for nearly a century. The fundamentals of digital optical communications are straightforward: To transmit something as simple as a phone message or as complicated as an image, the message or image is broken up into a series of binary bits, the bits transmitted, and the bits decoded at the other end to create the message or image. The ones or zeroes in the bits are encoded by turning some signal on or off. In the past, the signal has been electrical, but increasingly it is composed of light pulses. A laser is used to produce the light, then information is added with a modulator, the modulated light transmitted through optical fibers, amplified if necessary, received with a photodetector, and the message is recreated with a demodulator. An optical signal is better than an electrical signal, with less attenuation, faster switching, and more signals traveling together.

2) The physics of an optical communication system involves such fundamental concepts as light scattering, superposition of waves, and optical excitations of electrons in semiconductor crystals and in glasses. The clarity of the transmitter medium is the most important factor in determining the practicality of optical communications, and new optical fibers are now near the clarity limit. The development of silica glass fiber over the past 30 years has made possible the transformation of Bell's old invention into a powerful technology.

3) Researchers are responding to the bandwidth challenge by developing advanced light sources that provide the needed bright, multi-wavelength light combined with rapid modulation. One new multi-wavelength architecture is called "dense wavelength division multiplexing" (DWDM). In dense wavelength division, the optical window is divided into as many closely packed wavelength bands (channels) as possible, while still preserving the information content. The term "multiplexing" refers to the transmission of many signals together. Ideally, the optical signal in each small band should be clean and stable and have minimal chromatic content, i.e., it should be nearly a single wavelength and remain such. Transmitter sources must be good lasers that are tunable versions of those used now.

4) The authors conclude: "Compared to a military revolution, the changes in communications are relatively peaceful, but compared to the status quo, the changes are closer to a whirlwind."

Physics Today http://www.physicstoday.org

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