Tunable radio frequency generation using optical injection locked single mode Fabry-Pérot laser diode

Abstract

Modern communication systems are divided into wired communication based on optical communication and wireless communication based on microwaves. However, electronic-based microwave engineering has limitations, such as low frequency coverage, low bandwidth, and high transmission loss. To overcome these disadvantages, microwave photonics, which interacts between microwave engineering and photonics, has emerged. Microwave technology based on photonics has the advantages of high frequency coverage, wide bandwidth, and low transmission loss, and it is free from electromagnetic interference. In this dissertation, a novel principle of injection locking in a single mode FP-LD (SMFP-LD) is proposed and verified by demonstrating its application on a microwave photonics and radar system. Various optical technologies and optical active components for tunable microwave generation are considered, and their advantages, disadvantages, and limitations are discussed. The SMFP-LD possesses good characteristics in terms of power consumption, cost, and simplicity of implementation. Basic injection locking with non-linear dynamics is discussed theoretically, including important parameters, such as wavelength detuning, injection ratio, and phase for the stable and unstable operation of the SMFP-LD. Various methods of photonic generation and microwave modulation using an external injected SMFP-LD are introduced. First, microwaves and millimeter waves are generated by injection locking of the SMFP-LD. The range of the generated microwaves and millimeter waves is measured from 5 GHz to 402 GHz with a signal-to-noise ratio of more than 25 dB. The signals generated by injection locking has the advantages of being able to tune the frequency with wavelength detuning and generate multiple signals simultaneously. However, the external injected beam not only takes time to change the wavelength but also has limited the wavelength tuning resolution since a change of the laser physical properties is generally required to change the wavelength of the external injection beam. Second, the frequencies of the generated microwaves and millimeter waves are controlled by unstable injection locking in the period-one (P1) oscillation state of the SMFP-LD. Every self-injected mode has a 3 GHz continuous tuning range with mode hopping of 15 GHz or 150 GHz depending on the mode-matching condition. Since the external beam power can be easily changed using an optical amplifier or an optical modulator, not only the generated signal can be easily tuned, but also wavelength modulation is possible. Third, multi-band linear frequency modulation (LFM) in the P1 oscillation state is experimentally demonstrated by modulating the external injected beam intensity with a sawtooth signal. The generated multi-band LFM signal has a bandwidth of 3 GHz in four center frequencies (35, 31, 27, and 25 GHz) with a total effective bandwidth of 12 GHz. The multi-band LFM has the advantages of improving the performances of radar, such as a range resolution, the maximum measurable and velocity. The wider bandwidth of the multi-band LFM radar system provides higher range resolution. The various different chirp rates of each frequency band can overcome a trade-off of the maximum measurable range and velocity. Compared to a conventional single-band LFM radar system, it has a wider total effective bandwidth, in which 6 times improved the range resolution (1.25 cm) was achieved. The maximum measurable velocity (49.8km/h), which is 3.2 times improved, was achieved with the fixed maximum measurable range of 15km. The above three methods of the microwave generation and modulation based on the SMFP-LD can be applied to various applications, such as radar systems, radio-over-fiber, biochemical sensors, satellite communication, warfare systems, and terahertz photonics.