Optical frequency comb generation by the exploitation of gain modulation phenomenon in semiconductor optical amplifier

Using mathematical modelling we have studied the conditions of low-threshold collinear optical frequency comb generation under transient (picosecond) stimulated Raman scattering (SRS) and parametric four-wave coupling of SRS components in crystals. It is shown that Raman-parametric generation of an octave-spanning optical frequency comb occurs most effectively under intermediate, transient SRS at a pump pulse duration exceeding the dephasing time by five-to-twenty times. We have found the optimal values of not only the laser pump pulse duration, but also of the Raman crystal lengths corresponding to highly efficient generation of an optical frequency comb from the second anti-Stokes to the fourth Stokes Raman components. For the (high dispersion) and (low dispersion) crystals pumped at a wavelength of and a pulse duration five or more times greater than the dephasing time, the optimum length of the crystal was 0.3 and , respectively, which is consistent with the condition of the most effective Stokes – anti-Stokes coupling , where is the wave detuning from phase matching of Stokes – anti-Stokes coupling, determined by the refractive index dispersion of the SRS medium.

We study optical frequency comb (OFC) generation using cascaded injection of semiconductor lasers in this work. The OFC generation system is operated in two cascaded optical injection stages. When a master laser optically injects into the first stage with proper injection power and frequency, period-one (P1) dynamics are invoked in an optically injected semiconductor laser of the first stage. Another semiconductor laser in the second stage is then optically injected by the P1 dynamics. With proper injection power adjusted in the second stage, the P1 dynamics are regenerated, and the semiconductor laser relaxation oscillations (ROs) become undamped so that subharmonic oscillations appear. Because a subharmonic oscillation frequency is half of an oscillation frequency of the P1 dynamics, extra optical frequency components appear in the middle of the adjacent optical frequency components of the P1 dynamics, thus signaling OFC generation. The OFC signals exhibit at least 15 comb lines, resulting in a bandwidth greater than 140 GHz. Microwave comb signals are obtained after photodetection, although the microwave linewidth is on the order of a few megahertz because of the semiconductor laser noise. Thus, we propose a cascaded injection-locking scheme to stabilize the P1 dynamics and OFC signals. We have demonstrated pure microwave generations with a linewidth of less than 3 Hz and low phase noise.

Flexible broadband optical frequency comb generation assisted by injection locking of semiconductor laser

Optical frequency combs have been explored in different applications from optical frequency measurements to optical communications. OFCs can be formed using numerous techniques such as optical modulation, laser mode locking and optical fiber recirculating loops. An optical frequency comb generator based on an optical fiber recirculation loop operating with a Mach-Zehnder modulator was experimentally investigated using different types of amplifiers as gain media. The idea is to compare the use of semiconductor optical amplifiers (SOAs) and erbium-doped fiber amplifiers (EDFAs) and their efficiency in aiding or contributing on the generation of spectral lines. The process efficiency associated with nonlinear effects, specially four-wave mixing, is the main advantage of using SOAs over EDFAs. Under the best operational conditions, an optical comb with up to 30 lines and spacing around 20 GHz was obtained by a SOA-based loop, where an ultralong SOA was used for the first time in such an application.

The precise measurement of difference frequencies between two lasers in a terahertz (THz) region is a basic technique for optical frequency standards and for many other applications, such as dense WDM communication systems. Frequency difference measurements based on the beat signal between two lasers become quite difficult when the frequency difference exceeds the detector bandwidth. An optical frequency comb (OFC) generator with an electro-optic (EO) phase modulator inside a Fabry-Perot cavity can overcome this problem. The OFC generator creates a large number of sidebands, which can be applied for frequency markers. In conventional OFC generation, modulation sidebands appears on both sides of the carrier, which is a disadvantage for the efficient use of frequency bandwidth. The limit of the frequency difference measurement with this scheme depends on the power degradation in the sidebands. We propose an alternative technique for generating OFC which employs a dual-electrode Mach-Zehnder Modulator (D-MZM) inside a fiber recirculating loop to generate optical single sideband (SSB). This enables the efficient use of the frequency bandwidth. An optical amplifier is used in the loop to compensate for the losses.

We design, fabricate and characterize first on-chip prism-like waveguide (prism-waveguide) coupler to high-Q (Q>1010) micro-resonator with record 1.1dB coupling loss at 1550nm and demonstrate an integrated optical frequency comb (OFC) generation unit based on this coupler.

We investigate an InP-based semiconductor Mach-Zehnder modulator (MZM) for the purpose of generating a flat optical frequency comb (OFC). A nine-line OFC having flatness of <;0.8 dB is experimentally obtained by using an InP MZM under an optimized operation condition. To determine the optimized drive voltages, we use a simple model that takes into account nonlinear changes in the refractive index (phase modulation) and optical absorption loss (amplitude modulation) induced in the InP MZM. We confirm that it can reproduce the measured spectra of OFCs generated by the InP MZM. Optimum drive voltages of the InP MZM for flat OFC generation are found by considering the amplitude differences of the linear and nonlinear phase modulations for each arm. It is found that nonlinear phase modulation with a moderate amplitude (0.1-0.2π) flattens the OFC. The OFC becomes less flat at larger amplitudes of the nonlinear phase modulation; however, this deterioration can be suppressed using the amplitude modulation induced by the dependence of the optical absorption loss on the applied voltage. A numerical analysis using the proposed model reveals feasible operation conditions for the InP MZM.

Nanophotonics, driven by low processing power and high density integration, is appearing as the next logical step for microwave photonics. Optical frequency combs (OFCs), which play a vital role in integrated microwave photonics, have not yet generated using nano-structured devices. In this paper, we propose OFC generation based on all-optical modulation of nanophotonic structures. A theoretical model based on temporal coupled mode theory is developed to describe cascaded all-optical photonic crystal intensity and phase modulators. Using a modulation light with a sinusoidal waveform, a separate carrier light is modulated to generate an OFC. By manipulating the modulation power and device dimensions, a flat OFC that spans over a 600 GHz is delivered. The proposed system offers OFC generation with tunable comb line spacing using devices with high density integration capabilities.

Broadband optical frequency comb (OFC) generation based on a single electro-absorption modulator (EAM) is proposed. The EAM is driven by a radio frequency (RF) multi-frequency signal generated by a multiplication coupler composed of an electrical power splitter and an arithmetic circuit. Thus the number of comb-lines of the generated OFC can be increased. A complete theoretical model of OFC generation by an EAM driven by nth power of the RF source is established, and the performance of the OFC is analyzed by using OptiSystem software. The results show that, the number of comb-lines of the OFC is positively correlated with the number of multiplication of the RF source signal. The frequency spacing of the comb-lines is twice the frequency of the RF source signal and is tunable by adjusting the frequency of the RF source signal. Increasing chirp factor and modulation index of EAM could increase the number of comb-lines of the generated OFC. The amplitude of the RF source signal had little impact on the flatness of the OFC and the average OFC power. The scheme developed is not only simple and low-cost, but also can produce a large number of comb-lines.Graphical

Flat optical frequency comb generation by using one DPMZM and superposed harmonics

The research work carried out within the framework of this PhD has focused on the generation of Optical Frequency Combs (OFC) in semiconductor lasers using the gain-switching (GS) technique. The work has been mainly experimental, although simulations have been used to contribute to a better understanding of the internal physics of the OFCs generation process. Various types of lasers have been used, specially distributed feedback (DFB) lasers and discrete mode lasers (DML). An exhaustive experimental characterisation of the properties of the OFCs generated by GS in a full range of operating conditions has been performed. Two ranges of repetition frequencies have been explored: the GHz range with applications in optical communications, and the range of hundreds of MHz for application in spectroscopy. The effect of the optical injection (OI) on the properties of the OFCs has been investigated, and interesting results have been obtained, such as the improvement or deterioration of the quality of the spectra depending on the GS and OI conditions. The optimal operating conditions to generate high quality OFCs at the different frequencies considered have been found. In order to improve the understanding of the different physical processes involved in the generation of the OFCs generated by GS, a simulation tool based on the rate equation model for semiconductor lasers has been developed, with detailed treatment of the noise sources. For a correct simulation of the OFCs the values of a set of internal laser parameters is are required. They were obtained through an original procedure based on the measurement of the relative intensity noise (RIN) spectra. Using this procedure, the uncertainty in the simulated results has been reduced, since it allows to extract more parameters than other procedures due to an adequate analytical treatment of the RIN equations. The simulations have completely reproduced the temporal and spectral response of different lasers in a wide range of electrical and optical injection conditions. From the simulations, the predominant physical processes that contribute to the generation of OFCs, dynamic chirp at high repetition frequencies, and adiabatic chirp at low frequencies, have been identified. It has been shown that it is possible to improve the characteristics of low frequency OFCs by driving the laser with electric pulses, instead of using sinusoidal driving. High quality 100 MHz OFCs (up to 866 tones in 10 dB) with good carrier-to-noise (CNR) ratio have been generated. It has also been shown that step-recovery diodes (SRDs) can be used as a source of electrical pulses to generate OFCs, thus reducing the cost and size of the equipment involved, while maintaining the quality of the OFCs. The role of the “jitter” of the electrical source in the quality of the OFCs has been studied theoretically and experimentally. Finally, the OFCs generated by gain-switching and optical injection have been used to measure the absorption spectra of different elements, using three different architectures: heterodyne detection, and dual-comb spectroscopy and a new implementation of this technique. In this implementation the dual-comb is generated from a single GS laser by alternating the repetition frequency. Promising results for the application of these OFCs in gas spectroscopy have been obtained.

The rapid growth of diverse service demands and the limited spectrum resources pose challenges to traditional wireless communication systems. Combining optical communication with radio-over-fiber (RoF) and wavelength division multiplexing (WDM) technologies offers a promising solution for high-capacity scenarios. The existing optical frequency comb (OFC) as a WDM light source has the potential to significantly enhance the capacity and flexibility of RoF systems. However, the OFC generation schemes have issues such as high complexity, poor flatness, and limited scalability. This paper proposes a new OFC generation technology based on the cascade of an electro-absorption modulator (EAM) and a frequency modulator (FM),and explores the influence of parameters such as the center frequency of the light source, the line width of the light source, the working parameters of EAM, the amplitude of the FM driving signal and the phase difference of the modulator driving signal on the performance of OFC generation. The generated OFC is used as multiwavelength carriers in the WDM-RoF system. The proposed optical frequency comb (OFC) can adjust the number and spacing of spectral lines, with a bandwidth of 615 GHz and a flatness of 0.68 dB for the central 16 lines. The WDM-RoF system supports 16-channel multiplexing, with a transmission distance of up to 70 kilometers and a bit error rate (BER) of only 10^-9. This solution provides a cost-effective and scalable option for high-bandwidth scenarios such as urban areas and transportation hubs.

A novel scheme to generate broadband high-repetition-rate optical frequency combs and low phase noise microwave signals simultaneously is proposed and experimentally demonstrated. By incorporating an optical frequency comb generator in an optoelectronic oscillator loop, more than 200 lines are generated for a 25 GHz optical frequency comb, and the single-sideband phase noise is as low as -122 dBc/Hz at 10 kHz offset for the 25 GHz microwave signal. 10 and 20 GHz optical frequency combs and microwave signals are also generated. Unlike the microwave frequency synthesizer, the phase noise of the microwave signals generated by this new scheme is frequency independent.

We report about the optical frequency comb generation by means of a bulk type phase modulator with periodic domain inversion. The phase modulator operates as a quasi-velocity-matched electro-optical phase modulator having high modulation efficiency. It is possible to generate a broadband optical frequency comb by using the modulator installed into an Fabry-Perot resonator. In this experiment, we made the quasi-velocity-matched phase modulator and then used it to generate the efficient optical frequency comb. From this result, we could confirm operation characteristics of the optical frequency comb and the utility of the phase modulator respectively.

Optical frequency combs have been widely used in spectrum analysis, coherent optical communication, and accurate distance measurement. We propose a straightforward method to improve the flatness of optical frequency combs. First, we derived the output of the optical signal for the configuration of a cascaded MZM and two PMs. Second, we identified the parameter value when the flatness was optimal after traversing different parameter spaces. The optimal flatness conditions could be automatically determined from an existing sample dataset by using neural networks and Bayesian optimization, which significantly reduced the calculation cost. Furthermore, a broad spectrum and low power consumption were also achieved. Finally, the generated optical frequency comb signal was divided into eight carriers with 50 GHz intervals, and the optical transmission system was verified by applying a 16-QAM modulation of 40 GBaud/s to each channel. The constellation diagram proved the feasibility of this optical comb generation scheme.