Millimeter-Wave Photonics: Signal Sources and Transmission Links

Licentiate thesis, 2005

With the use of higher and higher frequencies in a number of current and future wireless millimeter-wave applications it is beneficial to use photonics for several reasons. The use of optical fibers is an advantageous way to transport millimeter-waves in terms of loss, mechanical flexibility, weight, and insensitivity to electromagnetic interference. Photonics can also be used for analog applications such as signal generation and signal processing of millimeter-waves, as well as in high-capacity wireless communication networks, referred to as radio-over-fiber. In this thesis, novel techniques for signal generation and data transmission using photonics are presented. The techniques for millimeter-wave signal generation include optical phase modulators for harmonic frequency generation and the optical nonlinear phenomenon four-wave-mixing for frequency multiplication in the optical domain. Different setups are used for generation of millimeter-wave carriers at 40 GHz utilizing the polarization properties of an optical phase modulator. The millimeter-wave carrier is created from signals at 10 GHz and the quality of the 40 GHz carrier is mainly determined by the original source. This principle is then further refined into an opto-electronic oscillator. Harmonic frequencies were extracted and carriers up to 60 GHz were created. A six-fold frequency multiplication is presented based on carrier suppression technique and optical four-wave mixing. This technique is scalable to very high frequencies, and the potential is demonstrated with a 6.7 GHz input and 40 GHz output. Furthermore, analog processing of millimeter-waves such as filtering or phase delaying is investigated in the optical domain with the use of fiber Bragg gratings. The concept of radio-over-fiber is demonstrated in this thesis using a high bit-rate data pattern modulating a 40 GHz millimeter-wave carrier downlink from central station to base station. The benefits of using a centralized approach are that expensive and sensitive equipment could be located in the central station, in a less hostile environment, and also be shared by a number of connected base stations. The overall cost can thereby be reduced. Aspects of millimeter-waves transported over fiber such as the impact of chromatic dispersion are investigated and successful data transmission with data rates of up to 2.5 Gbit/s is demonstrated over 44 km fiber.