| |
|
July 16, 1998
 Optical amplifiers literally pump up the (photon) volume
Bill Schweber, Technical Editor
An advanced technology often languishes as "a solution looking for a problem"
until it eventually meets up with a corresponding problem that's looking for a solution.
Optical amplifiers and multiplexed fiber-optic signals are an excellent example of such a
pairing. The high-capacity fiber-optic links installed by telephone and networking
companies during the last decade had far more gigabit-per-sec capacity than most
applications needed. But now that the many demands on fiber capacity have grown at
spectacular rates and have saturated the fiber's capacity, system designers must devise
ways to increase channel capacity.
The obvious way to increase channel capacity is to install more fiber, but this
technique is undesirable because it is costly and disruptive to all but the shortest
links. Your second choice is wavelength-division multiplexing (WDM), a process that
multiplexes several independent optical signals onto the same fiber. In WDM, you combine
different optical wavelengths (essentially, different colors) using optical filters made
of in-fiber Bragg gratings, and the single fiber carries these wavelengths as independent,
noninteracting photon streams. The result is a gain in the fiber's capacity by a factor of
2, 4, 8, 16, or more (see sidebar "What color is your bit
stream?").
But there's a serious drawback to WDM that has limited the technique's application, in
addition to the inherent complexities of optically multiplexing and demultiplexing optical
signals. Amplifying optical signals to compensate for unavoidable losses in the fiber is a
multistep process (Figure 1). You need to optically demultiplex the wavelengths using
costly optical filters; go through an optical-to-electronic conversion for each signal;
use electronic regenerative amplifiers and reclocking circuitry; reconvert from
electronic-to-optical form; and then perform the complementary multiplexing operation
again. This process can easily negate the advantages of using one fiber to carry
multiplexed optical signals, and a complete amplifier stage costs $50,000 to $100,000 for
just one channel.
So, who needs electronics?
Enter the optical amplifier, also called the optical-fiber amplifier (OFA)--and no,
it's not an "op amp" for short. Using advanced and subtle principles of atomic
physics, this amplifier neatly solves the amplification problem for nonmultiplexed signals
as well as for multiplexed optical signals that have differing data rates, jitter, and
waveshapes in the same fiber. The beauty of the OFA is that it closely approximates a
photon pump, multiplying the volume of photons by a constant factor without regard to the
photon's underlying timing or frequencies. OFAs are no longer lab devices, either. Vendors
such as Lucent Technologies (www.lucent.com/micro/opto),
Ciena Corp (www.ciena.com), Pirelli Cables and Systems
(www.pirelli.com/cables), and Hewlett-Packard
(www.hp.com) are supplying them to OEMs and installing
them in long-haul optical-fiber links.
The optical amplifier is analogous to a highly linear electronic amplifier that
functions as a gain block and transparently boosts the amplitude of an input
signal--whether that signal is one pure tone, multiple tones, complex music, or random
variations--without any need for the amplifier to judge or understand the nature of the
input signal. Just as you do with a linear amplifier, you can change the signal you
present or increase the signal frequency up to the amplifier's maximum capability without
changing the OFA itself. You'll endure a very different situation when you try to upgrade
signals passing through an optical-to-electronic-to-optical regenerator.
Because of various solid-state physics constraints, OFAs currently operate effectively
in the third optical-transmission window of 1530 to 1565 nm (1.530 to 1.565 mm). This
wavelength window is commonly used for long-haul fiber links. OFAs are now less practical
for the first optical window, centered on 860 nm, and the second window, centered on 1310
nm. You use the second window for shorter distance cable TV and LANs; at this shorter
range, there is less need for amplifiers.
An OFA comprises a laser-diode pump; a WDM; a waveguide made of silica-based fiber
about 70m long and with a high concentration of erbium atoms (that's why an OFA is also
referred to as an EDFA, for erbium-doped fiber amplifier); and optical isolators (Figure
2). In operation, the fiber signal to be amplified goes through the input isolator and is
multiplexed with excited erbium ions that are coupled into the fiber at the WDM. This
meeting of the input-signal photons and the excited erbium ions causes stimulated emission
in the fiber of more photons, which are identical to the photons of the input (see sidebar "Energy state transitions yield photons"). The result is
amplification, but instead of increased voltage--the conventional measure of signal
amplitude--optical amplification increases the number of photons, which is driven by the
input wavelengths and the relative quantity of photons in the input stream.
The isolators keep internal reflection under control for two reasons. First, these
reflections can cause an undesired "lasering" effect. Second, they can
contribute to the generation of excess, undesirable photons via amplified spontaneous
emission (ASE). When there is no input signal, the OFA produces these random emissions as
the stimulated ions decay to their ground energy state. These spontaneous emissions have
an overall spectral energy density that corresponds to the statistical distribution of
energy bands in the erbium atoms, with most of the energy concentrated across a spectral
range of about 40 nm (Figure 3). The critical ASE problem is that the spontaneously
stimulated photons can travel along the fiber and create additional photons in a cascade
effect, with ASE magnitudes of 0 dBm or more at the amplifier output.
Although ASE is undesirable and is partially minimized by the isolators that shield the
amplifier from the reflections, the operation of the OFA with an input signal actually
improves the ASE situation. With an input signal applied to the OFA, many of its
potentially spontaneous emissions from the fiber are instead stimulated by photons from
the laser (this reminds me of the ancient FM "quieting" effect, in which signals
that are greater than a threshold value suddenly diminish the FM-channel background noise
to an inaudible level!). The result is both the desired signal amplification and a
reduction in ASE by about 10 dB. OFAs have typical optical gains of 30 dB and output in
the 10-mW range, although both figures are increasing. Although their cost is about 50%
higher than one electronic/optical regenerator channel, OFAs remain attractive because
they handle multiple channels and upgraded data rates without difficulty.
References
- "Understanding Dense WDM: New technologies, new test solutions," 24-pg
application note from Wandel & Goltermann, 1998.
- Baney, Douglas and Wayne Sorin, "Broadband frequency characterization of optical
receivers using intensity noise," Hewlett-Packard Journal, February 1995.
- Lucent Technologies, "Product Selector Guide," April 1998.
What color hat color is your bit stream?
Wavelength-division multiplexing (WDM) relies on the simple fact that electromagnetic
wavelengths--in this case, optical ones--that share a common medium do not interact with
each other, just as red- and green-light beams do not mix when you shine their beams
through each other. To make WDM commercially practical, you need standards defining the
operating wavelengths so various vendors' multiplexers, demultiplexers, and associated
components will be compatible and interoperable. ITU-T Recommendation G.692, "Optical
Interfaces for Multichannel Systems with Optical Amplifiers," defines WDM systems
with four or eight channels, each operating at maximum bit rates of 2.5 Gbps for the
OC-48/STM-16 standard. A revised version of this recommendation will cover 16- and
32-channel systems, each at 10-Gbps OC-192/STM-64 rates.
Because the overall optical window is fixed, you need closely spaced
channels, which is like having slightly different shadings of the same basic color optical
signal. The dense WDM spacing of G.692 uses a wavelength grid centered on 1552.52 nm,
corresponding to a frequency of 193.1 terahertz (1 THz=1000 GHz), and with channel spacing
of 20 nm/100 GHz (it's hard to believe that it's the channel spacing, not the overall
band!). There are 43 evenly spaced channel assignments over the range of 191.7 to 195.9
THz (1563.86 to 1530.33 nm). The industry has plans to halve spacing to a mere 50 GHz and
to provide twice as many channels in the optical-transmission window.
Energy state transitions yield photons
To produce more photons (optical amplification), you need to excite the erbium ions in
the optical fiber to a higher energy meta-stable state than their initial ground energy
state (Figure A). To excite the ions, couple about 20 mW of laser-diode pump light into
the erbium-doped fiber (pump wavelengths of 980 and 1480 nm are common). The ions in the
fiber then absorb this pump-light energy and are excited to their metastable state. They
transition back to their ground state either by stimulated emission, which produces the
desired photon amplification, or by random spontaneous emission after about 10 msec. |
