Erbium-Doped Fiber Amplifiers Stimulate Optoelectronics Growth

	Leo M. Higgins III, Ph.D., Siemens Dematic EAS, Austin, Texas

A long-haul network segment of several hundred kilometers, or more, may use a number of EDFAs and one repeater/regenerator since optical amplifiers generally amplify both signal and noise, leaving required signal reconditioning to the regenerator. Signal regeneration in wavelength division multiplexed (WDM) systems places extreme demands on the electronics due to the high frequency of the optical signals, and because each WDM wavelength requires dedicated circuitry. This problem will intensify with emerging 40 Gb/s WDM transmission systems. EDFAs do not require such high-performance electronics, and will concurrently amplify all the propagating WDM wavelengths in the fiber core.

EDFA Schematic (adapted from ADVA Optical Services Inc.)

EDFAs use a length of a special optical fiber where rare earth (RE) erbium ion dopants provide the actual amplification. Work on doped silicate fibers began in the early 1960s using other RE elements. Diode laser pumping of RE doped fibers was reported in the early 1970s. Diode pumped EDFAs were demonstrated in England in the late 1980s, and early commercial EDFAs were announced by AT&T in the early 1990s. Erbium became the RE element of choice in these systems since erbium dopants allow amplification in the C-Band (1530 - 1570nm) and L-Band (1570 - 1610nm) used in today's high bandwidth WDM systems. Er-doped fibers also may contain small amounts of ytterbium, another rare earth. Other RE fiber amplifiers are less common than EDFAs, but R&D with other RE dopants, such as neodymium (Nd), continues.

A possible construction of an EDFA (ref. ADVA Optical Services Inc., EDFA brochure) is shown in the figure. Elements are (1) fiber delivering input signal, (2) fiber coupler diverting 1-5 percent of input power to a photodiode power monitor, (3) optical isolator, (4) WDM coupler, (5) diode pump laser linked by standard fiber to the input fiber at the WDM coupler, (6) length of erbium doped fiber connected to the output of the WDM coupler, (7) optical isolator, (8) fiber coupler diverting 1-5 percent of the signal to an output power monitoring photodiode and (9) the output fiber.

Commercial EDFAs may be comprised of several cascaded EDFA stages, using different pump wavelengths and EDF. The 1480nm pump photons excite the Er3+ ions from ground state to a first excited state. The higher energy 980nm laser excites the erbium ions to a second excited state that is higher than the first state, and these ions decay to the first excited state. A signal laser photon interacts with these first state ions, stimulating their decay to ground state. This causes stimulated emission of a photon of the same wavelength, and in phase with the initiating signal laser photon. The stimulated emission thus amplifies the initiating signal. While EDFAs simultaneously amplify all signal wavelengths, the gain for each wavelength will generally be different, and this wavelength-gain relation is also a function of pump power. Gain flattening filters may be used on the output side of EDFAs to restore each of the multiplexed wavelengths to desired power levels.

The pump laser wavelength is generally much lower than the signal-carrying wavelength to minimize pump power and optical noise. The two most common WDM pumps for C and L-band systems are 980nm and 1480nm lasers, typically InGaAs GaInAsP laser diodes, respectively. One or both may be used in an EDFA system. Recently, EDFAs using laser pumps with wavelengths (1530-1545nm) in the C-band, for use with L-band amplification, have been reported. L-band (±1570nm signal) gain was enhanced, but these systems require optical filters to remove noise from the signal. Pump wavelengths
>1550nm showed rapid gain decreases, with virtually no gain using (≥1557nm wavelengths (ref. Choi, B. H., et al., "Performances of Erbium Doped Fiber Amplifier Using 1530nm Band Pump for Long Wavelength Multichannel Application," ETRI Journal, Vol. 23, No. 1, March 2001).

The EDFA gain generally increases with fiber length, pump laser power and Er3+ concentration in the core. Fiber lengths of 10 - 100m, or more, are common. The Er3+ -doped core typically contains < 100ppm of erbium, and the cladding is erbium-free. Typical single-mode transmission fiber has a germanium-silicate core, making the core refractive index slightly higher than the silica cladding, a requisite for signal propagation via total internal reflection. The EDF core chemistry may be Er-germanium-silicate, or newer Er-aluminum-silicate, and Er-Ge-Al-silicate.

Even more complex chemistries, such as Er-F-zirconate glass, have been reported. The excitation of the Er3+ to the necessary higher energy state is enhanced if the Er3+ exists as isolated ions. The more complex core chemistries may allow higher dopant level, but erbium oxide precipitates, or Er3+ - rich regions in the microstructure must be avoided, or else the laser efficiency and power output will be degraded due to reduced excited ion populations. Silica dopants also increase the core refractive index, decreasing the numerical aperture, which can affect the pump-to-fiber coupling efficiency. The design of the radial Er3+ gradient in the core can have a significant impact on EDFA gain efficiency.

The use of such an EDFA system continues to grow, and new configurations and chemistries emerge regularly. The proliferation of the optical network demands significant infrastructure cost reduction, while continuing to meet insatiable bandwidth demands. EDFAs will continue to be extremely important elements of this network expansion.