Eliminating Beat Noise in Ring Lasers

Breck Hitz

Single-frequency ring lasers utilizing erbium-doped fiber amplifiers as the gain medium are attractive in a number of applications, from optical sensors to telecommunications. A serious barrier to their adoption into these technologies, however, is the intensity noise that is generated when the lasing longitudinal mode beats against amplified spontaneous emission in suppressed longitudinal modes. Researchers at Princeton University in New Jersey have suppressed this noise by inserting a semiconductor optical amplifier into the ring resonator. This amplifier acts as a high-pass filter, effectively blocking the lower-frequency beating noise.

Because a fiber ring laser's resonator is typically several meters in length, the beat frequency between adjacent longitudinal modes is relatively low, on the order of 10 MHz. The phenomenon of self-gain modulation in a semiconductor optical amplifier prevents it from amplifying these low frequencies and their harmonics.

Figure 1. Researchers inserted a semiconductor optical amplifier into a fiber ring laser to suppress intensity noise caused by mode beating.

Self-gain modulation occurs because the low-frequency intensity fluctuation on the incoming light depletes the carrier density in the semiconductor optical amplifier, causing its gain to saturate. As a practical matter, this carrier depletion causes gain saturation at frequencies into the gigahertz range. Limited carrier lifetime prevents self-gain modulation from occurring at higher frequencies.

In one of the experimental arrangements investigated at Princeton, the polarization controller, rotating polarizer and polarization-maintaining fiber comprise a Lyot filter that forces the laser to oscillate in a single longitudinal mode, but cannot eliminate amplified spontaneous emission at other modes (Figure 1). The RF spectrum on the left in Figure 2 shows the beat note and its harmonics when the amplifier is not in the resonator. The spectrum on the right shows that this noise is suppressed when the semiconductor optical amplifier is added to the resonator.

Figure 2. The laser's RF spectra without (left) and with (right) the semiconductor optical amplifier in the resonator demonstrate how the amplifier suppresses the beat notes.

To demonstrate that the suppression technique is not limited to a single resonator design, the investigators applied it to a different fiber ring laser, which employed a fiber Bragg grating rather than a Lyot filter to limit the bandwidth. The results were the same, with the RF spectrum showing a dramatic decrease in beat noise when the semiconductor optical amplifier was inserted.