Time lens based on cross phase modulation can realize accurate quadratic phase modulation. However, its application in optical pulse compression is restricted by the pump pulse with a high peak power. The formula of the peak power is deduced. It is proposed to use an output fiber with positive dispersion to lower the high peak power requirement for the pump pulse. It is also proposed to use a 4f system which consists of two time lenses to realize optical pulse compression, thereby more effectively lowering the peak power of the pump pulse. The formulas of the peak power and the resolution of the 4f system are deduced. The optical pulse compression is simulated and analysed. The research results demonstrate that in a 4f system based on cross phase modulation, femtosecond pulses can be generated by using pump pulses with small peak power; With compression factor increasing, the pulse width of the output pulse is restricted mainly by the solution of the 4f system. The method of improving the solution of the 4f system is discussed.