Research On Circuit Control System Of Optical Fibre Pulse Accumulation

Inertial confinement fusion (ICF) is to approach the problem of controlled fusion by means of laser. To reach the demands of various physical experiments, it is necessary to get the arbitrary shaping of the front ultrashort laser pulse, which will dramatically increase the utilization ratio of laser pulse power. Up to now, several methods of nanoseconds laser pulse shaping have received the theoretical discussion and were developed, such as electrooptic method, space-time transformation method, whole-optical method and integrated optics method. At the present two techniques are widely adopted. One is regulating the laser intensity by the shaped ultrashort electrical pulse, the other is accumulating pulses by means of optical fibre. In China, we still has long way to go under the technologic conditions in these days, in order to realize the production, transportation, nestification and measurement. However, if optical fibre pulse accumulation adopted, the control will be shifted from time of lOOps to the length of the optical fibre in millimeters. In addition, using the special polarization technique to control the angle of related optical elements can make the intensity controlling possible, which also provides a feasible and practical method to the problem of ICF pulse shaping .This research is designed for the circuit controlling system in the optical fibre pulse accumulator, which is the front part of the Shenguang III. With the 32-route network of equal optical path difference constructed by PMF couplers, this system divides the pre-input lOOps ultrashort laser pulse into 32 sub-pulses, after the intensity of which are adjusted through the PMF polarizer, then accumulates them as the 3.2ns shaping pulse. In the practical usage, the time resolution is lOOps, amplitude resolution is 8-bits. As for the partial research of the circuit controlling, two questions need answer:i. Precise controlling of the polarizers' separation angle. To some single fiber VOA ,the output laser intensity indicates a non-linear variation with the angle of the polarizer pare. To obtain the required intensity-regulating precision in the whole adjustable range, precision on the angle controlling reach a high standard up to 0.2mrad.ii. System correction. Due to the inconsistency of the components, and the sharpdifferences in the light intensity of one-way output after the light beam dividing, waveform regulation is necessary in normalization processing which demands the system can revise the zero point and the culminating point.By the end of October this research has produced the sample machine and conducted a correlative experiment on it. Optical accumulation effects were obviously observed while the circuit controlling system operates well as designed.