Shasha Liao

Comparison analysis of optical frequency comb generation with nonlinear effects in highly nonlinear fibers

We compare several schemes for broadband optical frequency comb (OFC) generation based on several nonlinear effects in highly nonlinear fiber (HNLF). Cascaded four wave mixing (CFWM) and self-phase modulation (SPM) processes in HNLF are proved to be effective ways for spectrum broadening. We investigate some parameters affecting the performance of the output OFC in detail. When only CFWM occurs in the HNLF, broadband OFC can be generated with poor power flatness. When only SPM occurs in the HNLF, we obtain a 10 GHz OFC of 103 comb lines within 5-dB power deviation. When both CFWM and SPM simultaneously occur in the HNLF, we obtain a 10 GHz OFC of 143 comb lines within 4.5-dB power deviation. All the OFC generation schemes have the advantages of tunability of central wavelength and repetition frequency.

High-order photonic differentiator employing on-chip cascaded microring resonators.

We propose and experimentally demonstrate a high-order photonic differentiator using on-chip complementary metal oxide semiconductor-compatible cascaded microring resonators, including first-, second-, and third-order differentiators. All the microring resonator units have a radius of 150 μm and a free spectral range of 80 GHz. The microring resonator can implement the first-order derivative of the optical field near its critical coupling region. Hence higher-order differentiation can be obtained by cascading more microring units on a single chip. For the periodical Gaussian optical pulse injection, the average deviations of all differentiators are less than 6.2%. The differentiation of pseudo-random bit sequence signals at 5 Gbit/s is also demonstrated. Our scheme is a compact and low-power-consumption solution since the cascaded microring units are fabricated with compact size on the silicon-on-insulator substrate.

Experimental observation of optical differentiation and optical Hilbert transformation using a single SOI microdisk chip

Optical differentiation and optical Hilbert transformation play important roles in communications, computing, information processing and signal analysis in optical domain offering huge bandwidth. Meanwhile, silicon-based photonic integrated circuit is one of the most promising candidates for all-optical signal processing due to its intrinsic advantages of low power consumption, compact footprint, ultra-high speed and compatibility with electronic integrated circuits. In this study, we analyze the interrelation between first-order optical differentiation and optical Hilbert transformation and then experimentally demonstrate a feasible integrated scheme which can simultaneously function as first-order optical differentiation and optical Hilbert transformation based on a single microdisk resonator. This finding may motivate the development of integrated optical signal processors.

Arbitrary waveform generator and differentiator employing an integrated optical pulse shaper.

We propose and demonstrate an optical arbitrary waveform generator and high-order photonic differentiator based on a four-tap finite impulse response (FIR) silicon-on-insulator (SOI) on-chip circuit. Based on amplitude and phase modulation of each tap controlled by thermal heaters, we obtain several typical waveforms such as triangular waveform, sawtooth waveform, square waveform and Gaussian waveform, etc., assisted by an optical frequency comb injection. Unlike other proposed schemes, our scheme does not require a spectral disperser which is difficult to fabricate on chip with high resolution. In addition, we demonstrate first-, second- and third-order differentiators based on the optical pulse shaper. Our scheme can switch the differentiator patterns from first- to third-order freely. In addition, our scheme has distinct advantages of compactness, capability for integration with electronics.

Route-asymmetrical optical transmission and logic gate based on optical gradient force

We put forward a route-asymmetrical optical transmission scheme employing optical gradient force, which means that forward and backward propagation of an optical device have different transmittance provided they are not present simultaneously. The device is based on optical gradient force between two single-mode waveguides followed by a Mach-Zehnder interferometer. Our numerical investigation shows that the forward transmittance is about -6 dB while the backward transmittance is suppressed below -20.5 dB in C + L bands. The proposed device is passive, wideband, and compatible with complementary metal-oxide semiconductor (CMOS) process. Furthermore, we demonstrate the applications of route-asymmetrical transmission such as an all-optical switch and all-optical AND gate for all-optical information processing.

Integrated programmable photonic filter on the silicon-on-insulator platform

We propose and demonstrate a silicon-on-insulator (SOI) on-chip programmable filter based on a four-tap finite impulse response structure. The photonic filter is programmable thanks to amplitude and phase modulation of each tap controlled by thermal heaters. We further demonstrate the tunability of the filter central wavelength, bandwidth and variable passband shape. The tuning range of the central wavelength is at least 42% of the free spectral range. The bandwidth tuning range is at least half of the free spectral range. Our scheme has distinct advantages of compactness, capability for integrating with electronics.

Operation bandwidth optimization of photonic differentiators

We theoretically investigate the operation bandwidth limitation of the photonic differentiator including the upper limitation, which is restrained by the device operation bandwidth and the lower limitation, which is restrained by the energy efficiency (EE) and detecting noise level. Taking the silicon photonic crystal L3 nano-cavity (PCN) as an example, for the first time, we experimentally demonstrate that the lower limitation of the operation bandwidth does exist and differentiators with different bandwidths have significantly different acceptable pulse width range of input signals, which are consistent to the theoretical prediction. Furthermore, we put forward a novel photonic differentiator scheme employing cascaded PCNs with different Q factors, which is likely to expand the operation bandwidth range of photonic differentiator dramatically.

Photonic arbitrary waveform generator based on Taylor synthesis method

Arbitrary waveform generation has been widely used in optical communication, radar system and many other applications. We propose and experimentally demonstrate a silicon-on-insulator (SOI) on chip optical arbitrary waveform generator, which is based on Taylor synthesis method. In our scheme, a Gaussian pulse is launched to some cascaded microrings to obtain first-, second- and third-order differentiations. By controlling amplitude and phase of the initial pulse and successive differentiations, we can realize an arbitrary waveform generator according to Taylor expansion. We obtain several typical waveforms such as square waveform, triangular waveform, flat-top waveform, sawtooth waveform, Gaussian waveform and so on. Unlike other schemes based on Fourier synthesis or frequency-to-time mapping, our scheme is based on Taylor synthesis method. Our scheme does not require any spectral disperser or large dispersion, which are difficult to fabricate on chip. Our scheme is compact and capable for integration with electronics.

125-GHz Microwave Signal Generation Employing an Integrated Pulse Shaper

We propose and experimentally demonstrate an on-chip pulse shaper for 125-GHz microwave waveform generation. The pulse shaper is implemented based on a silicon-on-insulator (SOI) platform that has a structure with eight-tap finite impulse response (FIR) and there is an amplitude modulator on each tap. By controlling the thermal heaters on the amplitude modulators, we obtain several signals centered at 125 GHz with typical envelopes, such as square envelope, triangular envelope, sawtooth envelope, Gaussian envelope, etc. Our scheme has some significant advantages, such as the central frequency of the generated microwave waveforms is larger than 100 GHz, and it has wide bandwidth when changing the time delay of the adjacent taps and compactness, capability for integration with electronics and small power consumption are also its merits.

Optical true time delay based on contradirectional couplers with single sidewall-modulated Bragg gratings

We propose and demonstrate optical true time delay using tapered SOI contradirectional couplers with single sidewallmodulated Bragg gratings. The contradirectional couplers consist of two tapered rib waveguides with different width, and the Bragg gratings are modulated in the inner sidewall of the wider one. The optical signal is launched from the wide waveguide and coupled to the narrow waveguide through the Bragg gratings structure. Along the direction of light propagation, the waveguide width varies linearly, so the reflection wavelength is different at different positions. Therefore, linear delay line can be realized within the grating passband using the present structure. In the simulation, grating period is 310nm and grating number is 2400, corresponding to the grating length of 744μm. Using 2.5D FDTD simulation, the current structure can realize optical group delay of 20ps within bandwidth of 18nm. The proposed device is fabricated on a 220nm SOI chip with Electron Beam Lithography (EBL) and Inductively Coupled Plasma (ICP) etching. In the experiment, continuous light is modulated by 10GHz radio-frequency signal and travel through the chip, which is finally detected by the oscilloscope. By adjusting the wavelength of input light, group delay of different wavelength are recorded by the oscilloscope. The experimental results show that group delay of 28ps is realized within the bandwidth of 20nm. In the end, the drift of the reflection spectrum and delay lines under different temperature are analyzed. The reflection spectrum drifts 0.1nm/°C and causes redshift of the corresponding delay line.