A novel approach to generating an ultraflat and stable optical frequency comb with tunable frequency spacing is proposed and experimentally demonstrated. The proposed generator consists of a polarization modulator (PolM) and a polarizer. The joint operation of a PolM and a polarizer is equivalent to intensity modulation, but with a third controllable parameter in addition to the two controllable parameters in conventional intensity modulation. By tuning the three parameters, an ultraflat optical frequency comb with five comb lines is generated. By cascading the PolM with a second PolM, an ultra-flat optical frequency comb with 25 lines is generated. An experiment using two cascaded PolMs is performed. A 25- line frequency comb with the comb flatness within 1 dB is generated.

A wideband-tunable optoelectronic oscillator (OEO) is proposed and experimentally demonstrated based on a tunable microwave photonic filter (MPF) consisting of a polarization modulator, a chirped fiber Bragg grating, and a polarization beam splitter (PBS). By simply adjusting the polarization state of the signal before the PBS, the center frequency of the MPF is tuned. The proposed OEO is experimentally demonstrated. A high-purity microwave signal with a tunable frequency within 5.8-11.8 GHz is generated. The single-sideband phase noise of the generated signal is −104.56 dBc/Hz at 10-kHz offset.

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Frequency mixers represent one of the most essential components of microwave and millimeter-wave systems. Compared to conventional electrical mixers, photonic microwave frequency mixers are of great interest for wideband, multifunctional, and reconfigurable radio frequency (RF) systems. This is due to a number of distinct features, including wide bandwidth, good isolation, and immunity to electromagnetic interference. However, despite great efforts being devoted to the development of high-performance photonic microwave frequency mixers over the past two decades, the two intrinsic challenges of photonic mixers-undesirable mixing spurs (unnecessary frequency components produced due to nonlinearities in the mixer), which restrict the operation bandwidth and reduce the signal-to-interference ratio, and limited mixing functions-have not yet been overcome. Most traditional photonic microwave mixers are equivalent to a single-ended mixer. Single-ended mixers suffer from large mixing spurs, DC leakages, phase-relevant operation, and relatively large mixing noise values. Other highperformance mixers that are widely used in microwave and millimeter-wave systems-e.g., the double-balanced mixer, which reduces spurious responses and suppresses DC components; the in-phase/quadrature (I/Q) mixer, which enables phase-irrelevant frequency conversion and produces two quadrature signals with equal amplitudes; and the image-reject mixer, which performs single-sideband downconversion, enabling interference from spurious signals to be avoided-have rarely been implemented in the optical domain.

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Optical isolators and circulators are critical building blocks for large-scale photonic integrated circuits. Among the several methods proposed to realize such nonreciprocal devices, including heterogeneous integration with garnet-based materials or using nonlinearities, dynamic modulation of the waveguide properties is a potentially practical and easily accessible method. However, most proposals relying on this method rely on modulators with a very large footprint, limiting their practical applicability. This paper overcomes this issue by presenting a method to achieve nonreciprocal optical transmission taking advantage of compact ring modulators. We use a cascaded system of microring modulators with a footprint as small as 15 μm × 220 μm and propose that, by tuning the relative time delay between the RF driving signals and the optical delay between the modulators, nonreciprocal transmission can be achieved. We present a detailed theoretical analysis of our design and investigate the origin of the asymmetric transmission. The modulators were designed and fabricated on IMEC's Silicon-on-Insulator platform iSiPP50G. We achieve a 16 dB difference between forward and backward optical signals at a driving voltage (V pp ) of 8 V at 6 GHz. Moreover, we analyze the impact of fabrication imperfections on the device performance. Our work leads to a significant reduction in device footprint compared to formerly explored solutions using dynamic modulation and is well suited for monolithic integration with photonic integrated circuits.

The dynamics of dual-beam injection into single mode FP-LD are analyzed. With the wavelength of injected beams unchanged and injection strength changed, the dynamics are observed and determined as scenario A, B or C.