A passively mode-locked Nd:glass fiber laser pumped by a single-stripe laser diode was developed and stable, ultra-short pulses with the pulse duration of 300 fs were generated. The passive mode-locking was started with a slowly moving mirror, and the adjustment of lasing spectrum using an intra-cavity slit was found to be crucial for a low start-up threshold of mode-locking in a Nd:glass fiber laser. A four-pass grating stretcher was constructed to stretch the mode-locked pulses to 400 ps and the amplification of a stretched pulse through a Nd:phosphate glass ring regenerative amplifier was tested.
Passive mode-locking in the fiber laser is based on nonlinear birefringence due to self-phase modulation and cross-phase modulation. The total linear and nonlinear birefringence in the fiber was adjusted by a polarization controller made of two λ/4 fiber loops. The intra-cavity chirp induced in the fiber was compensated for by using two sets of three Brewster-cut SF10 prisms. A slit installed between the prism sets and the end mirror was used to control the lasing wavelength and bandwidth. The mode-locking could be started by adjusting the polarization controller and the slit while the end mirror, attached to an ordinary audio speaker, was vibrating at 60 Hz. A relatively long 2 m fiber was used to lower the mode-locking threshold by increasing the Kerr effect in the fiber. The shortest pulse duration obtained without any pedestal was 290 fs and the maximum energy per pulse was about 180 pJ. The minimum absorbed pumping power for the start-up of mode-locking was as low as 43 mW, and the mode-locking, once started, could be sustained even at the absorbed pumping power of 15 mW after the mirror stopped moving. The mode-locking was found to be very stable with the measured r.m.s. noise of as low as 0.3%.
The variation of the start-up threshold was investigated by changing the length of fiber from 0.74 to 2 m. It was found that the equation for the self-starting condition could be applied to the laser started with a moving mirror by introducing a proportional constant, about 0.04, to take into account the reduction of the build-up time of mode-locking due to the moving mirror. It was found that an intra-cavity slit helped the start-up and maintenance of the mode-locking. The lasing wavelength can be tuned from 1.053㎛ to 1.070㎛ by adjusting the position of the slit and the pulse duration can be controlled from 250 fs to 650 fs by adjusting the width of the slit.
As the lasing wavelength of the Nd:glass fiber laser can be matched well with the gain peak of the Nd:phosphate glass, the Nd:glass fiber laser pumped by an LD is well suited as a cost effective, femtosecond front-end oscillator for a chirped pulse amplification(CPA) Nd:glass laser. In order to amplify the laser pulse by CPA method through Nd:phosphate glass amplifiers, a four-pass grating pulse stretcher was constructed. The pulse stretcher consisted of one 1700 lines/mm grating and a lens with the focal length of 75 cm, and was designed to stretch the femtosecond pulses to 400 ps pulses. Nd:phosphate glass amplifiers consisted of a ring regenerative amplifier, two 8 mm amplifiers, a 20 mm amplifier and a 45 mm amplifier. For the maintenance of good spatial beam quality, spatial filters should be set between the amplifiers. The first spatial filter between the ring regenerative amplifier and 8 mm amplifier was designed as an air-spatial filter and the other two spatial filters was designed as vacuum-spatial filters, and one of them was prepared. One of the stretched pulses was selected and amplified through the ring regenerative amplifier with 22 round-trips. With a further refinement of the Nd:glass fiber laser, especially the elimination of cw level contained in the mode-locked fiber laser, the Nd:glass fiber laser pulse will be amplified subsequently through the Nd:glass rod ampifier chain.
