Parametric Amplifier Shows Potential for Telecom
The erbium-doped fiber amplifier (EDFA) was one of the most important enabling technologies behind the fiber optic telecommunications boom of the 1990s. Because optical signals are attenuated as they travel through fiber, they must be periodically amplified, and EDFAs excel in that role. They provide gain across the necessarily wide bandwidth of tens of nanometers, typically from 1530 to 1565 nm (i.e., the telecom C-band).
But the gain EDFAs provide is uneven across their bandwidth, so complicated gain-flattening filters are required for spectrally smooth amplification. Researchers around the world have investigated alternative approaches, and one interesting candidate is the double-pumped fiber optical parametric amplifier (2P-FOPA), which operates on an entirely different principal from an EDFA.
An EDFA’s gain is dependent on a population inversion and stimulated emission, but the gain of a 2P-FOPA derives from four-wave mixing, a nonlinear effect. A 2P-FOPA is pumped at two wavelengths, and four-wave mixing couples the pump photons to the signal and idler photons generated in the amplifier (Figure 1).
Figure 1. A double-pumped fiber optical parametric amplifier (2P-FOPA) is pumped at two wavelengths, λ1 and λ2, and four-wave mixing transfers energy at those wavelengths into the signal (λs) and idler (λi) wavelengths.
One can imagine that the energy in the two pump photons reappears in the signal and idler photons, and the signal and idler wavelengths are such that the energy is conserved. To phase-match the nonlinear process, investigators constructed 2P-FOPAs with dispersion-shifted fiber whose zero-dispersion wavelength is shifted approximately to the center of the interaction region in Figure 1.
The concept, then, is to pump the 2P-FOPA on both sides of the wavelength region to be amplified so that half the incoming telecom channels are amplified as the amplifier’s signal wavelengths, and the other channels are amplified as idler wavelengths (Figure 2). An obvious advantage of this scheme over EDFAs is that the 2P-FOPA has zero quantum defect.
Figure 2. In a telecom system, the 2P-FOPA would replace an erbium-doped fiber amplifier, providing gain across a number of different wavelength channels.
In general, researchers have used highly nonlinear fiber for these devices to maximize the parametric gain. But such fiber has several drawbacks, the most important of which is the large fluctuation of the zero-dispersion wavelength along the fiber, which restricts the gain bandwidth and produces a spectrally uneven gain.
Recently, investigators at the Centro de Pesquisas em Óptica e Fotônica at Universidade Estadual de Campinas in Brazil designed and demonstrated a 2P-FOPA based on conventional (not highly nonlinear) dispersion-shifted fiber. The operating parameters of the device -- high gain (37 dB) with low ripple (±1.5 dB) across a broad (47 nm) spectral range -- make it attractive for telecom applications.
Figure 3. The apparatus exhibited flat (±1.5 dB) gain over a 47-nm-wide spectral region. The black dots are experimental data, and the curves are numerical simulations with different assumed parameters. ©2005 IEEE.
The researchers arranged their apparatus similarly to the arrangement in Figure 2 and pumped the device with a little more than 2 W of peak power at each pump wavelength -- ~1540 and ~1597 nm. They used pulsed pumps to achieve high power easily, but continuous-wave pumping would be required in any real application. They aligned 25 telecom lasers, with output of –28 dBm from each, to provide the channels to be amplified, and observed 37 dB of gain between ~1548 and ~1583 nm (Figure 3). By combining a numerical analysis with experimental data, the investigators were able to show that the gain was limited by polarization mode dispersion and that interchannel crosstalk from spurious four-wave mixing was negligible.
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