Introduction to RF over fiber System

Introduction to RF over fiber System

RF over fiber is one of the important applications of microwave photonics and shows unparalleled advantages in advanced fields such as microwave photonic radar, astronomical radio telephoto, and unmanned aerial vehicle communication.

The RF over fiber ROF link is mainly composed of optical transmitters, optical receivers and optical cables. As shown in Figure 1.

Optical transmitters: Distributed feedback lasers (DFB laser) are applied in low-noise and high-dynamic range applications, while FP lasers are used in applications with lower requirements. These lasers have wavelengths of 1310nm or 1550nm.

Optical receiver: At the other end of the optical fiber link, the light is detected by the PIN photodiode of the receiver, which converts the light back into current.

Optical cables: In contrast to multimode fibers, single-mode fibers are used in linear links due to their low dispersion and low loss. At a wavelength of 1310nm, the attenuation of the optical signal in the optical fiber is less than 0.4dB/km. At 1550nm, it is less than 0.25dB/km.

The ROF link is a linear transmission system. Based on the characteristics of linear transmission and optical transmission, the ROF link has the following technical advantages:

• Extremely low loss, with fiber attenuation less than 0.4 dB/km

• Optical fiber ultra-bandwidth transmission, optical fiber loss is independent of frequency

The link has a higher signal carrying capacity/bandwidth, up to DC to 40GHz

• Anti-electromagnetic interference (EMI) (No signal impact in bad weather)

• Lower cost per meter • Optical fibers are more flexible and lightweight, weighing approximately 1/25 of waveguides and 1/10 of coaxial cables

• Convenient and flexible layout (for medical and mechanical imaging systems)

According to the composition of the optical transmitter, the RF over fiber system is divided into two types: direct modulation and external modulation. The optical transmitter of the direct-modulated RF over fiber system adopts the direct-modulated DFB laser, which has the advantages of low cost, small size and easy integration, and has been widely used. However, limited by the direct-modulated DFB laser chip, the direct-modulated RF over fiber can only be applied in the frequency band below 20GHz. Compared with direct modulation, the external modulation RF over fiber optical transmitter is composed of a single-frequency DFB laser and an electro-optic modulator. Due to the maturity of electro-optic modulator technology, the external modulation RF over fiber system can achieve applications in the frequency band greater than 40GHz. However, because of the addition of the electro-optic modulator, the system is more complex and not conducive to application. ROF link gain, noise figure and dynamic range are important parameters of ROF links, and there is a close connection among the three. For instance, a low noise figure means a large dynamic range, while high gain is not only required by every system, but also has a greater impact on other performance aspects of the system.