Researchers at Lucent Technologies (Holmdel, NJ, and Murray Hill, NJ) have demonstrated that a 1-W cladding-pumped fiber amplifier can deliver analog video efficiently to 256 fiber nodes, each serving roughly 500 homes over coax, or to 512 fiber nodes in either a fiber-to-the-curb (passive coax) or fiber-to-the-home architecture.
According to Lucent's Gordon Wilson and colleagues, as new interactive services such as Internet-Protocol data via cable modems, video-on-demand, and telephony are introduced to cable television systems (CATv), optical fiber must be pushed closer to the home to facilitate upstream transmissions, increase bandwidth, and improve system reliability. Not only must more optical power be produced, it must be delivered cost-effectively. The good news, say the researchers, is that the cost/mW of power drops as the saturation output power of high-power erbium-doped fiber amplifiers increases. The new class of fiber amplifiers—with a cladding-pump design that offers scalable output power and pump redundancy—may help pull down the price of optical power even further.1
Cladding-pumped fiber amplifiers use low-cost broad-area diodes to either pump a fiber laser, which in turn pumps an erbium-ytterbium (Er/Yb)-doped fiber amplifier, or to directly pump the cladding of an Er/Yb-doped fiber amplifier. To produce a low-noise figure, the amplifier includes a preamplifier stage comprising a 980-nm-pumped stage concatenated with a 1480-nm pumped stage. The pumps are copropagated with the signal, and the preamplifier output is fed through an isolator into the Er/Yb-doped single-mode core of a double-clad fiber spliced to a tapered fiber bundle. The outputs from six ~1-W facet-power, 920-nm multimode diode pumps are injected into each of six multimode fibers that are fused together and tapered to form a transition to the inner cladding of the Er/Yb-doped fiber. The counter-propagating light pumps the Yb atoms in the core, which then transfer energy to the Er atoms that provide gain at 1550 nm.
The Lucent researchers identified the amplifier noise factor by measuring the noise power at the output of an optical receiver as a function of input power with and without an amplifier between the laser and the receiver. A tunable 4-MHz RF bandpass filter was placed between the receiver and the noise meter to measure the noise factor in a particular CATv channel. The results included the noise factor of the post-amplifier alone and the noise factor of the pre- and post-amplifier combination in a channel centered at 535.25 MHz—which was roughly 0.5 dB higher.
To achieve low-noise performance in CATv systems, the researchers report that input power of an Er-doped fiber amplifier is typically greater than 3 dBm. In their experiments, the inclusion of the preamplifier reduces the noise factor from 7.5 dB to 5.6 dB at 1557 nm and at +4-dBm input. Although the saturated output power is greater than 1 W from 1535 to 1565 nm, the gain peak is at ~1558 nm, and the noise factor begins to degrade at shorter wavelengths. The noise factor is approximately 1 dB higher at 1544.5 nm than at 1557 nm.
REFERENCE
- Gordon Wilson et al., "Low-noise 1-W Er/Yb fiber amplifier for CATv distribution in HFC and FTTH/C systems," Proc. OFC 2000, OSA (March 2000).