DEMANDS on data communications systems are growing by leaps and bounds. Information travels faster and farther than anyone might have dreamed possible even 20 years ago, but still the Information Superhighway wants more.|
Lawrence Livermore National Laboratory's Sol DiJaili, Frank Patterson, and coworkers have developed a small, inexpensive optical amplifier. It incorporates a miniature laser to send information over fiber-optic lines at a rate of more than 1 terabit (1 quadrillion bits of information) per second. The amplifier is about the size of a dime, which is 1,000 times smaller than comparable amplifiers, and in production quantities it will cost 100 times less than the competition.
Fiber amplifiers can operate at comparable bit rates, but they are large and expensive, which limits their usefulness. For example, erbium-doped fiber amplifiers currently enable hundreds of thousands of simultaneous telephone conversations across continents and under oceans on a single fiber-optic cable. But their high cost makes them economical only for long-haul systems, and their large size means that they cannot be integrated easily with other devices.
On the other hand, conventional semiconductor optical amplifiers are inexpensive and relatively small, but crosstalk and noise at high transmission rates limit their performance to about 1 gigabit (1 billion bits of information) per second or less.
The Laboratory's new amplifier combines the best of both worlds. Its small size, low cost, and high performance make it an excellent candidate for use in wide-area networks, local-area networks, cable TV distribution, computer interconnections, and anticipated new fiber-to-the-home applications that will require multiple amplification steps and therefore many amplifiers.
A Vertical Laser at Work
In a conventional semiconductor optical amplifier (SOA), the signal passes through a waveguide that has been processed directly onto a direct bandgap semiconductor. Inside the waveguide is a gain medium through which the optical signal passes and where the signal gains in intensity. The problem with these conventional SOAs is that the gain cannot be controlled, so signals tend to fluctuate. A signal at one wavelength can deplete the gain of a signal at another wavelength. This interchannel depletion of gain allows the signal at one wavelength to modulate the signal at another, causing crosstalk among channels.
In Livermore's new amplifier, the waveguide supplying the signal gain incorporates a very small laser that operates perpendicularly to the path of the signal through the waveguide. This "vertical cavity surface emitting laser," composed of a stack of cavity mirrors that are fabricated during semiconductor crystal wafer growth, replaces the standard gain medium of a conventional SOA.
This new laser amplifier takes advantage of some basic properties of lasers to reduce crosstalk by a factor of 10,000. In a typical laser, electrical current is introduced into the gain medium, which is situated between two sets of mirrors. Much too rapidly to be seen, the photons in the gain medium bounce back and forth between the sets of mirrors, constantly gaining in intensity. Because no mirror is perfectly reflective, some of the photons are lost through the mirrors during this back-and-forth process. But once the gain is equal to the losses or, put another way, equal to the reflectivity of the mirrors, the photons will begin to "lase."
A laser's gain thus has a cap. By introducing a laser into an SOA waveguide, the signal gain can be "clamped" at a specific level. Then, when signal channels at multiple optical wavelengths pass through the waveguide, there is virtually no crosstalk across the independent optical channels.
The lasing field also affects the recovery time of signals through the waveguide. After every "bit" of the optical signal passes through the gain medium, the medium requires a short recovery time before it can accept the next bit. This gain recovery time in a conventional SOA is typically a billionth of a second. Attempts to push the amplifier to faster bit rates than the gain medium can accommodate often result in one bit depleting the gain of the subsequent bit, which is another form of crosstalk. The introduction of a lasing field prompts the medium to recover much more quickly, on the order of 20 trillionths of a second. This means that the amplifier can successfully track the amplification of a serial bit stream at very high bit rates.
A New Ubiquitous Amplifier?
This new amplifier is truly the optical analog of the electronic amplifier, the electronics industry's ubiquitous workhorse. Because the new amplifier relies on standard integrated circuit and optoelectronic fabrication technology, it can be incorporated into many different types of photonic integrated circuits.
In the near term, because this amplifier puts SOA performance on a par with fiber amplifiers, it could be used as a replacement for or complement to fiber amplifiers in long-haul communication networks.
Looking farther into the future, if tiny, inexpensive optical amplifiers provide the broad signal bandwidth needed to transmit visual images as well as computer data, many people may someday work in "virtual offices" in their homes. Via two-way video, they will be able to confer with colleagues, participate in meetings, and hear the latest company news without commuting to work. Two-way, high-resolution, panoramic video will also facilitate remote learning with a teacher in one place and one student or hundreds of students in another.
These kinds of applications, involving many individual users, will require an enormous number of amplifiers for signal propagation and distribution. Livermore's new laser optical amplifier could well become ubiquitous.