We present a novel scheme of reducing the spectral broadening effects of self phase modulation (SPM) in high-peak-power,
narrow-band fiber lasers by preconditioning the input pulses of a master oscillator seeded high-peak-power fiber
amplifier with a commercially available fiber-coupled phase modulator. We demonstrate an output emission linewidth
reduction in a 1064 nm nanosecond pulse-width, 10 kW peak power fiber laser system. Modeling and measurements are
presented for this system using a directly modulated diode seed laser and the resulting practical limitations imposed on
SPM mitigation are discussed.