Researchers at Universiti Putra Malaysia in Serdang have conceived and implemented a double-pass amplifier that incorporates a narrow-bandpass filter between the first and second passes to filter out amplified spontaneous emission. The amplifier has high gain, low noise and good sensitivity to low input signals, important characteristics of these critical components of optical communications systems.Figure 1. Radiation at 1550 nm passes through the erbium-doped fiber amplifier, a narrowband filter and back through the fiber amplifier.In the setup, a 980-nm laser diode capable of output powers up to 100 mW pumped a 7-m length of fiber doped at 440 ppm Er3+ (Figure 1). An optical circulator at the input side of the amplifier separated the low-level, 1550-nm incoming signal from the outgoing amplified signal, and another circulator provided the feedback for the second pass through the amplifier. A Fabry-Perot filter at the top of the figure removed amplified spontaneous emission before it made a second pass through the amplifier. This is important because amplified spontaneous emission can contribute to gain saturation during a second pass and can reduce the total gain seen by the signal.In experiments with a low input signal, the scientists saw a gain improvement as great as 9.5 dB and a noise improvement of 3 dB when comparing results with and without the Fabry-Perot filter. When pumping the amplifier with approximately 90 mW, they observed gain of 37 dB when the 1550-nm input signal was –50 dBm. The bit error rate at this level, however, was approximately 10–7. In most systems, the highest acceptable bit error rate is 10–9. To reduce it to this value, the researchers increased the input signal to –43 dBm without observing an appreciable decrease in gain. When the input signal rises above approximately –20 dBm, there are so many signal photons in the amplifier that they overwhelm the amplified spontaneous emission photons, and the advantage of placing the narrowband filter in the amplifier diminishes. For this reason, the scientists believe that the amplifier design is better suited for application to in-line preamplifiers, and not to power amplifiers.Figure 2. By substituting a multiplexer/demultiplexer for the Fabry-Perot bandpass filter, the amplifier can be adapted for use in a multichannel dense wavelength division multiplexing system.A limitation of the design is that it works only for a single frequency -- that passed by the narrowband filter -- and thus is not suitable for the multichannel signal of a wavelength division multiplexed system. This can be overcome by substituting a multichannel filter for the Fabry-Perot (Figure 2). Such a filter could be constructed by cascading a dense wavelength division multiplexing demultiplexer and multiplexer.