Optical Frequency Comb Generation Based on a Current-modulated Weak-resonant-cavity Fabry-Perot Laser Diode Subject to Optical Injection

Optical Frequency Combs(OFC)is a special type of laser source, for which the time series is a series of narrow pulses and meanwhile the spectrum presents a series of discrete, equally spaced, strongly coherent comb lines. OFC effectively establish the connection between optical frequency and microwave frequency, and promote the development of many fields such as precision metrology, microwave photonics, optical fiber communication, etc. Since the first generation of OFC based on mode-locking technology in 2000, a variety of generation schemes have been reported based on microcavity, electro-optical modulator, Semiconductor Lasers (SLs) and other devices. Among them, the scheme for generating OFC based on current-modulated SLs under optical injection has attracted much attention due to its advantages such as simple system structure, low cost, integrability, and flexible adjustment of comb spacing. At present, SLs utilized in such a scheme are mostly single longitudinal mode SLs including Distributed Feedback Semiconductor Lasers (DFB-SLs), Vertical-Cavity Surface-Emitting Lasers (VCSELs), etc. Constrained by a limited range of wavelength tuning for such single longitudinal mode SLs, it is difficult to obtain the OFC whose central wavelength can be tuned within a wide range. In order to meet the demand for OFC with tunable center wavelength in some special applications, some schemes based on Fabry-Perot laser diode(FP-LD) have been proposed successively for generating OFC with tunable center wavelength. For FP-LD, there exist multi-longitudinal modes in cavity, which can be oscillated simultaneously. Through varying the wavelength of injection light, different mode with different wavelength in gain-switched FP-LD can be become the main oscillation mode, and therefore the central wavelength of generated OFC can be detuned easily. However, for FP-LD, there exist power fluctuation among the longitudinal modes due to mode selection characteristics of the laser cavity and the gain depended on the wavelength. At the same time, the power of the current-modulated sideband for each longitudinal mode is related to the order of the sideband. These two factors will undoubtedly lead to large power fluctuation among come lines in OFC generated by gain-switched FP-LD. By introducing appropriate optical injection, the powers of the comb teeth can be balanced within a certain wavelength range, and therefore the number of comb lines with power fluctuations within a certain amplitude(10 dB in this work) can be improved. Obviously, for generating OFC with a specific comb spacing, high-order modulation sideband in gain-switched FP-LD with smaller mode spacing are more easily compensating, which is beneficial to obtain OFC with more comb lines. Weak-Resonant-Cavity Fabry-Perot Laser Diode(WRC-FPLD)are a new type of FP-LD, which have been applied in dense wavelength division multiplexing, chaotic secure communication, random number generation and other fields. Compared with traditional FP-LD, the front face of WRC-FPLD possess lower reflectivity, which is not only conducive to improving the efficiency of external optical injection, but also helps to increase the output share of modes. In particular, compared with traditional FP-LD, the resonant cavity of WRC-FPLD is longer, resulting in smaller mode spacing. Therefore, WRC-FPLD are more suitable for producing OFC with more comb lines. Based on the above considerations, in this paper, we propose a scheme for generating OFC with a much more comb lines based on current-modulated WRC-FPLD under external single (and dual) optical injection, and the influence of some key parameters such as injection light wavelength λ and injection power P on the performances of the OFC have been investigated experimentally. In this work, the performances of the OFC are characterized by the number of comb lines NC and Carrier to Noise Ratio(CNR). Starting from the maximum value of output comb line, the number of continuous comb lines within a 10 dB range below the maximum value is taken as the value of NC. Considering different line possesses different CNR, we choose the minimum value of CNR to standardized the CNR of OFC. Firstly, a sinusoidal signal with frequency of 1.6 GHz and power of 19 dBm is utilized to drive the WRC-FPLD into the gain-switched state, and then an external optical injection is further introduced into the gain-switched WRC-FPLD for generating OFC. The experimental results show that, under single optical injection with a given P, the NC of the OFC varies periodically between 20 and 49 with a period of 0.28 nm (35 GHz), which corresponds to the longitudinal mode interval of the WRC-FPLD, and the CNR of the OFC fluctuates between 26.43 dB and 33.78 dB. Under a given λ, with the increase of P, the value of NC oscillates around a high level, then drops sharply, and finally stabilizes, and meanwhile the CNR increases first and then tends to saturation. Under optimized injection parameter, an OFC with 49 comb lines has been obtained. Through introducing an extra optical injecting to construct dual-optical injection, an OFC including 92 comb lines has been generated. Finally, the coherence between the comb lines of the OFC generated under single optical injection and double optical injection has also analyzed respectively. The results show that, the single sideband phase noise of the fundamental frequency for the beat signal is lower than - 125.0 dBc/Hz @ 10 kHz for two injection methods, which demonstrates that the coherence between the comb lines of the OFC is strong. Through adding the number of injection light, it can be expected that an OFC with much more comb lines can be obtained.