Timing synchronization is critical in digital demodulation systems such as intradyne optical receivers. In this paper, a novel timing recovery method for all-digital coherent receivers is proposed. With the help of a parallel architecture, the new method can be implemented on ASIC or FPGA platform, especially when the symbol rate is much higher than the clock rate of FPGAs. Through adjusting the frame structure in the parallel interpolator, the receiver synchronizes its symbol rate with the transmitter. Different from existing parallel timing recovery methods, the proposed method does not adjust the period between adjacent frames of samples, which can be beneficial to the subsequent processing in hardware. The performance is tested by simulation in QPSK modulation. Under relative clock offset between ±50 ppm and jitter noise with 0.3% standard deviation, the proposed method shows almost no degradation compared with its serial equivalent. Combined with different timing error detection algorithms, this method can be used in kinds of modulation formats like MPSK and QAM.
The performance of fiber nutation tracking system based on coherent demodulation is constrained by the nonlinearity of devices in the signal coherent demodulation module. With the influence of this non-linear factors, the reconstruction accuracy of intensity envelope fluctuation signal depends on the input optical power. The influence of optical amplifiers and detectors on the signal intensity calculation in the range of our interest input optical power is analyzed theoretically and verified experimentally.
By rotating the half-wave plate, the variable communication bit rate between 5.12Gbps and 2.56Gbps has been verified at satellite-to-ground optical communication linkage, the communication link used Differential Phase Shift Keying(DPSK) modulation format and the wavelength is 1549.731nm. Without error correcting code and adaptive optics, an average bit error rate of 1.9E-9 was achieved while the link distance exceeded 1500km.
Narrow-linewidth high-power fiber pulsed sources have many applications, such as laser remote sensing and
spectroscopy. In this article, an all fiber 1064nm single-frequency pulsed laser in Master Oscillator Power Amplifier
(MOPA) configuration and the corresponding experimental results are presented. The laser consists of four
Polarization-Maintaining (PM) fiber amplification stages. In the final stage, 1.8m long large mode area (LMA)
double-clad gain fiber with core diameter of 25μm and numerical aperture (NA) of 0.06 is utilized. In the system,
considering the low power of the seed laser, a continuous-wave (cw) seed laser is externally modulated by an
acousto-optic modulator (AOM). The modulated pulse repetition frequency (PRF) of the first and the second
amplification stage are chosen to be 10KHz to minimize the Amplified-Spontaneous-Emission (ASE). Then another
AOM is inserted to change the PRF from 10KHz to 100Hz, which also eliminates build-up of optical power emitted
within the same bandwidth of the ASE filter. Besides, in order to prevent the pulse steepening in the amplification stage,
the modulated pulse in the first AOM is pre-shaped and last pulsed shape with almost flat top are demonstrated. The
pulse energy is increased by more than 80% through that special pulse shaped method. In the end, pulse energy of 100μJ
with 500ns pulse duration and 100Hz repetition rate is obtained. Near diffraction limited beam quality (M<sup>2</sup> ~ 1.1) is also
proved by coiling the fiber at a diameter of 10-cm.