Silicon nitride (Si3N4) waveguides with high confinement and low loss have been widely used in integrated nonlinear photonics. Indeed, state-of-the-art ultralow-loss Si3N4 waveguides are all fabricated using complex fabrication processes, and all of those reported that high Q microring resonators (MRRs) are fabricated in laboratories. We propose and demonstrate an ultralow-loss Si3N4 racetrack MRR by shaping the mode using a uniform multimode structure to reduce its overlap with the waveguide. The MRR is fabricated by the standard multi project wafer (MPW) foundry process. It consists of two multimode straight waveguides (MSWs) connected by two multimode waveguide bends (MWBs). In particular, the MWBs are based on modified Euler bends, and an MSW directional coupler is used to avoid higher-order mode excitation. In this way, although a multimode waveguide is used in the MRR, only the fundamental mode is excited and transmitted with ultralow loss. Meanwhile, thanks to the 180 deg Euler bend, a compact chip footprint of 2.226 mm perimeter with an effective radius as small as 195 μm and a waveguide width of 3 μm is achieved. Results show that based on the widely used MPW process, a propagation loss of only 3.3 dB / m and a mean intrinsic Q of around 10.8 million are achieved for the first time.
As a new pulse compression signal, the phase-coded linearly frequency modulated (LFM) signal is proposed to faciliate multi-purpose radars. Here, we propose and demonstrate a phase-coded LFM signal generator, whose center frequency and bandwidth can be both adjusted. The phase-coded LFM signal generator is realized by feeding a phase-modulated optical signal into an optoelectronic oscillator (OEO). When the Fourier domain mode-locking is achieved, a phasecoded LFM signal is successfully generated. The center frequency and bandwidth of the generated microwave signal can be adjusted by changing the wavelength of the tunable laser source (TLS) and the peak-to-peak voltage of the driving signal, respectively. Experiment results show that a phase-coded LFM microwave signal with a center frequency of 9.5 GHz and a bandwidth of 5 GHz is successfully generated, whose TBWP is 5.6×105. To demonstrate the flexibility of the proposed scheme, the center frequency and bandwidth of the generated phase-coded LFM signal are adjusted to 12.4 and 2.6 GHz, respectively. The signal generator can be used in pulse compression radar in the future.
KEYWORDS: Optoelectronics, Microwave radiation, Oscillators, Waveguides, Frequency response, Silicon, Signal attenuation, Photonics, Signal generators, Solids
Parity‐time (PT) symmetry breaking offers mode selection capability for facilitating single‐mode oscillation in the optoelectronic oscillator (OEO) loop. However, most OEO implementations depend on discrete devices, which impedes proliferation due to size, weight, power consumption, and cost. In this work, we propose and experimentally demonstrate an on-chip tunable PT‐symmetric OEO. A tunable microwave photonic filter, a PT‐symmetric mode‐selective architecture, and two photodetectors are integrated on a silicon‐on‐insulator chip. By exploiting an on‐chip Mach–Zehnder interferometer to match the gain and loss of two mutually coupled optoelectronic loops, single‐mode oscillation can be obtained. In the experiment, the oscillation frequency of the on-chip tunable PT‐symmetric OEO can be tuned from 0 to 20 GHz. To emulate the integrated case, the OEO loop length is minimized, and no extra-long fiber is used in the experiment. When the oscillation frequency is 13.67 GHz, the single‐sideband phase noise at 10-kHz offset frequency is −80.96 dBc / Hz and the side mode suppression ratio is 46 dB. The proposed on-chip tunable PT‐symmetric OEO significantly reduces the footprint of the system and enhances mode selection.
We present a silicon-on-insulator (SOI) based device that exhibits Fano resonance with high extinction slope rate (SR) and extinction ratio (ER). It is constructed by using two cascaded tunable Mach–Zehnder interferometers (MZIs). The first MZI is used to adjust the power splitting ratio of the second MZI. In the second MZI, two add-drop microring resonators (MRRs) are located in each arm of the second MZI, respectively. The MRRs are used to generate a high-Q and low-Q resonance respectively. Due to the interference between these two resonances, a Fano resonance could be implemented. Considering that the optical power splitting ratio and phase difference between the two resonances can be finely adjusted, the ER can be greatly increased. In the experiment, the measured Fano resonance of the fabricated device exhibits simultaneous ER of 41.5 dB and SR of 3388.1 dB/nm. To the best of our knowledge, this is the first time to achieve a Fano resonance with such high ER and SR simultaneously. By adjusting the bias voltage in the fabricated device, a pair of complementary Fano resonance line shapes can be achieved.
Microwave photonic systems have huge potential for both existing and future applications, including radar, radiofrequency sensing and modern wireless communications due to their distinct advantages in terms of ultra-wide bandwidth, flexible tunability, and immunity to electromagnetic interference. There is a strong research trend in microwave photonic systems towards integration and miniaturization, resulting in multiple radio frequency functions on a single chip which is both compact and light weight. Thus integrated microwave photonics has attracted a lot of attentions and achieves significant improvements in last ten years. In this paper, we will review some research progresses on silicon-based integrated microwave photonics in our group, including highly efficient micro heater on silicon photonic chip, chip-scale microwave waveform generation, on-chip true time delay, and microwave photonic processing and measurement. Our schemes are all fabricated on silicon-on-insulator chips and have advantages of compactness and capability to integrate with electronic units. These chips may motivate the great application potentials in silicon-based integrated microwave photonics.
The high incidence and mortality of breast cancer requires an effective, rapid, and cost-effective method for its diagnosis. Here, visible and near-infrared spectroscopy in the wavelength range of 400 to 2200 nm is utilized for distinguishing the malignant tumor tissue from benign tumor and normal breast tissues. Based on the absorption and scattering spectra of fixed samples, three spectral analysis methods are proposed which include an absorption spectral analysis, a scattering spectral analysis, and a combined spectral analysis of the two. By comparison with the histopathological examination, the sensitivity, specificity, and accuracy of the three analysis methods are calculated. The results showed that the combined spectral analysis method can significantly enhance the effectiveness when compared with the sole absorption or scattering spectral analysis method. The sensitivity, specificity, and accuracy of the combined spectral analysis method are 100%, 87.82%, and 87.50% for the benign tumor tissue and 81.82%, 100%, and 87.5% for malignant tumor tissue, respectively. All of the three values are 100% for normal breast tissue. This study demonstrates that the combined spectral analysis method has better potential for in vitro optical diagnosis for breast lesions.
The high incidence and mortality of breast cancer require an effective method for early breast diagnosis. In order to
investigate the optical differences among malignant tumor, benign tumor and normal human breast tissue, a commercial
spectrophotometer combined with single integrating sphere was used to measure the optical properties of different types
of breast tissue in the wavelength range of 400 nm to 2200 nm in vitro. The hematoxylin and eosin staining (H&E
staining) are used as the standard, and to find the find possible optical markers from the corresponding absorption or
scattering spectra. This work is not only used for in vitro rapid optical diagnosis, but very helpful to develop innovative
optical diagnosis of breast tumor in vivo.
A new cascaded microwave photonic filter consisting of two or more infinite impulse response (IIR) filters based on
active loops. is presented. Owing to wavelength conversion, the interference between the modulated optical signals of
different taps from different active loops can be avoided and the stable transmission characteristic of the cascaded filter
can then be achieved. The cascaded filter can increase the free spectral range (FSR) and the Q value significently by
designing the FSR differences of the IIR filters. The cascaded filter with two IIR filters is demonstrated, and the
measured results of a high Q of 3338 and rejection ratio of about 40 dB are obtained. The tunability can also be realized.
In this paper, a tunable microwave photonic filter with one complex coefficient is proposed and experimentally
demonstrated. The complex coefficient is generated using a tunable optical RF phase shifter that consists of a
semiconductor optical amplifier (SOA) and a tunable band-pass filter (TBPF). The phase of the RF signal is shifted by
the differentiation process in semiconductor optical amplifier (SOA) where both Cross-Phase Modulation (XPM) and
Cross-Gain Modulation (XGM) effects are exploited simultaneously. A two-tap photonic microwave filter with one
tunable complex coefficient and a tuning range of a full free spectral range (FSR), is experimentally demonstrated.
We propose and experimentally demonstrate a novel all-optical microwave filter with high quality factor (Q). It is based
on a recirculating delay line (RDL) loop in which a semiconductor optical amplifier (SOA) is followed by a tunable
narrow-band optical filter and a 1x2 10:90 optical coupler. Converted signal used as a negative tap is generated through
wavelength conversion employing the cross-gain modulation (XGM) of the amplified spontaneous emission (ASE)
spectrum of the SOA. The converted signal can circulate in the RDL loop so that the proposed filter realizes a high Q
factor response after photo-detection. The 1x2 10:90 coupler is employed to extract 10% optical power from the loop as
output. A frequency response with a high Q factor of 543, a rejection ratio of 40 dB is experimentally demonstrated.
A novel photonic generation of power-efficient ultra-wideband (UWB) pulse by combining two asymmetric monocycle
pulses with inverted polarities is experimentally demonstrated. The principle lies in cross-phase modulation (XPM) in a
single semiconductor optical amplifier (SOA) and phase modulation to intensity modulation conversions in an arrayed-waveguide
grating (AWG). The Federal Communications Committee (FCC) compliant UWB pulse gains 24.3 dB and
20.8 dB improvements compared to positive and negative monocycle pulses after power attenuation to respect the FCC
spectral mask, respectively. The generated power-efficient UWB with pulse duration of about 310 ps has potential to
achieve high speed transmission and modulation without overlapping and distortion.
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