We have proposed a digitally implemented QPSK system for Free-Space Optics (FSO) systems for future satellite missions. We have used a Laser source of 1550nm wavelength and data rate of 2.5Gbps.The system consists of a modulating and a demodulating block and an advantage of filters added to improve the performance and optimize errors like noise. The best suited modulator for FSO is Mach-Zehnder modulator which is tuned for high performance. The LiNB03 demodulator demodulates the signal which in turn passed to a filter to attenuate the demodulated output and then through an amplifier to increase the signal strength. Digital approach for implementation of QPSK Modulation is attempted here and compared with existing systems. Simulation and characterization is done to freeze design parameters. The system mainly concentrates on parameter like performance, security, minimum size and cost saving which will be the future of satellite communications.