High-purity microwave signal generation using a low-order harmonic-suppressed rational harmonic mode-locked fiber laser incorporating a Mach-Zehnder interferometer

Abstract

phase of two paths, the IMD3 can be simultaneously enhanced by more than 10 dB for a frequency range of 510 MHz (1.98-2.49 GHz) without any change in electrical circuitry. As the linearization process that covers broad RF band would be very desirable, we measured the IMD3 for various channel bandwidths from the inset of Figure 3. As encouraging results, the IMD3 enhancements of more than 10 dB in 100 MHz channel spacing are achieved compared to free running operation. These results indicate that the proposed predistortion scheme is available for various frequency bands and their broad channel bandwidths. Figure 4 shows output powers of the fundamental and IM3 signals against input powers fed to LD1. The SFDR under free running is measured to 85.8 dB Hz 2/3 and is increased to 93.0 dB Hz 2/3 with predistortion. We can achieve 7.2 dB dynamic range enhancement.The proposed optoelectrical predistorter can suppress distortions of optical sources such as a DFB-LD as well as a FabryPerot LD with the same epi-structure. Also, this method has effectiveness for external environments even if the distortion characteristics of the LD are functions of external parameters. CONCLUSIONSA laser diode has been used as the key component of radioover-fiber system. Our linear optical transmitter using the novel opto-electrical predistorter is a strong candidate for broadband RoF systems. We obtained the broadly and significantly enhanced IMD3 without any electrical change. These results indicate that the proposed broadband optical transmitter is very suitable for future high-capacity and broadband multimedia wireless services. ABSTRACT: A simple approach to generate high-purity microwave signals based on a rational harmonic mode-locked fiber laser is proposed and demonstrated by incorporating an external cavity MachZehnder interferometer filter to suppress the low-order harmonics. A pulse train of equal amplitude with a repetition rate of about 10 GHz, which is four times of the frequency of the microwave drive signal ($2.5 GHz), is obtained. A high-spectral purity microwave signal at 10 GHz can be achieved by beating the filtered rational harmonics at a photodetector. The signal-to-noise ratio of the generated microwave signal is about 20 dB enhanced compared with the filter-free case.