Low Frequency Intensity Noise Studies in Erbium-ytterbium Fiber Lasers

Book excerpt: ABSTRACT: In this thesis we characterize the low frequency intensity noise spectrum of an erbium-ytterbium co-doped fiber laser. Under normal detection the intensity noise is found to be far above the quantum noise limit, although predictions suggest fiber lasers pumped by quiet diode laser sources may exhibit sub-shot noise performance. The excess of noise at low frequency under polarization insensitive detection indicates additional noise processes related to the pump laser or the fiber laser that inhibit the possibility of observing amplitude squeezing. Polarization sensitive detection reveals a resonance below the normal relaxation resonance that indicates anti-correlated intensity fluctuations on orthogonal linearly polarized modes. The frequency of the resonance varies linearly with pump power and leads to an increase in low frequency noise of 45-50 dB. Our studies confirm that polarization fluctuations alone as characterized by the spin-flip model can not account for the resonance, which is not predicted by application of the usual Langevin rate equations for coupled modes. Alternative models are investigated. Reduction in the intensity noise of the fiber laser by ̃5 dB is achieved through polarization selective self-injection locking, resulting in excess noise (above the shot noise level) of approximately 15 dB.