In an all-polarization-maintaining-fiber master oscillator power amplifier system at 1064 nm under all normal dispersion, intense nanosecond emission was generated with spectral broadening from 980 to 1600 nm. In such a fiber nonlinear power amplifier, efficient power scaling is able to be free from significant depletion because both laser amplification and nonlinear conversion are simultaneously employed. As a result, output peak power up to 117 kW with a pulse energy of 1.2 mJ is generated with a maximum core intensity of 30 GW cm-2. In addition, the conversion efficiency is 66% for a pulse duration of 6.1 ns at the moderate repetition of 20 kHz. The output level is close to the damage threshold for long-term operation. The onset and interplay of constituted fiber nonlinearities can be addressed, especially from single mode to a few modes, stage by stage. Furthermore, the seeding influence on the spectral broadening reveals its versatility for enabling many potential applications. For seeding by a highly controlled diode laser at the nanojoule level, a double-pass preamplifier significantly improves the energy extraction, resulting in a high input level for an efficient nonlinear power amplifier. Such a linearly polarized light source composed of an intense 1064 nm pump and a broad sideband seed is beneficial for efficiently driving broadband tunable optical parametric amplification.