Wanjun Wang

Conversion between EIT and Fano spectra in a microring-Bragg grating coupled-resonator system

A conversion between the electromagnetically induced transparency (EIT) transmission and Fano transmission is theoretically and experimentally demonstrated in an all-pass microring-Bragg grating (APMR-BG) coupled-resonator system. In this work, the coupling between the two resonators (the microring resonator and the Fabry-Perot resonator formed by two Bragg gratings) gives rise to the EIT and Fano transmissions. The resonant status strongly depends on the round-trip attenuation of the microring and the coupling strength. By tuning the coupling strength, the EIT and Fano transmissions can be controlled and converted. The device performance has been theoretically calculated and analyzed with a specially developed numerical model based on the transfer matrix method. The APMR-BG coupled-resonator systems with different gap widths were designed, fabricated, and characterized on a silicon-on-insulator (SOI) platform. The conversion of resonance was experimentally observed and verified. In addition, this on-chip...

Compact microring resonators integrated with grating couplers working at 2 μm wavelength on silicon-on-insulator platform

Compact all-pass and add-drop microring resonators (radius=10u2009u2009μm) integrated with grating couplers working at 2 μm wavelength are designed, fabricated, and characterized on a commercial 340-nm-thick-top-silicon silicon-on-insulator platform. They are suitable for high-volume integrated optical circuits at 2 μm wavelength as the fabrication process involved are uncomplicated and complementary metal-oxide-semiconductor (CMOS)-process compatible, thus making them more convenient to be utilized. The performance of the grating couplers, based on four most important parameters, has been simulated and optimized. The simulation and experimental results of grating couplers show the lowest coupling loss of 4.5 dB and 6.5 Db, respectively. By utilizing the grating couplers to couple light in and out from the chip, the designed microring resonators have been tested. The experimental results of microring resonators show that an extinction ratio of 12 dB and a quality factor of 11,200 can be achieved. To the best of our knowledge, this is thus far the smallest microring resonator ever demonstrated at this wavelength.

Investigation of regime switching from mode locking to Q-switching in a 2 µm InGaSb/AlGaAsSb quantum well laser

A two-section InGaSb/AlGaAsSb single quantum well (SQW) laser emitting at 2 μm is presented. By varying the absorber bias voltage with a fixed gain current at 130 mA, passive mode locking at ~18.40 GHz, Q-switched mode locking, and passive Q-switching are observed in this laser. In the Q-switched mode locking regimes, the Q-switched RF signal and mode locked RF signal coexist, and the Q-switched lasing and mode-locked lasing happen at different wavelengths. This is the first observation of these three pulsed working regimes in a GaSb-based diode laser. An analysis of the regime switching mechanism is given based on the interplay between the gain saturation and the saturable absorption.

All silicon approach to modulation and detection at λ = 2 µm

Silicon photonics has traditionally focused on near infrared wavelengths, with tremendous progress seen over the past decade. However, more recently, research has extended into mid infrared wavelengths of 2 μm and beyond. Optical modulators are a key component for silicon photonics interconnects at both the conventional communication wavelengths of 1.3 μm and 1.55 μm, and the emerging mid-infrared wavelengths. The mid-infrared wavelength range is particularly interesting for a number of applications, including sensing, healthcare and communications. The absorption band of conventional germanium photodetectors only extends to approximately 1.55 μm, so alternative methods of photodetection are required for the mid-infrared wavelengths. One possible CMOS compatible solution is a silicon defect detector. Here, we present our recent results in these areas. Modulation at the wavelength of 2 μm has been theoretically investigated, and photodetection above 25 Gb/s has been practically demonstrated.

Characteristic temperature of a 2 µm InGaSb/AlGaAsSb mode-locked quantum well laser

Mode locking is achieved in a 2 μm GaSb-based laser up to 60 °C. The laser has a T0 of ~82 K at room temperature, and the absorber bias voltage has little effect on T0.

Dual-band optical filter based on a microracetrack resonator embedded with Bragg gratings

In the last decades, dual-band optical filters (DBOFs) have attracted great attentions as they play important roles in various optical applications, such as optical modulation [1], sensing [2], fast/slow light [3], etc. Multiple microring resonators have been utilizing in various methods to realize the dual-band filtering spectrum [4-6]. However, the multiple-microring-resonators structure is not suitable for high volume integration due to its large form factor. In this work, we firstly developed a novel design based on single-microracetrack-resonator structure embedded with Bragg gratings, which is more compact and feasible for large-scale integration. Besides, through changing the dimensional sizes of the Bragg gratings, the reflectivity of the Bragg grating can be tuned and the transmission spectrum can be engineered as well. nxa0 nThe schematic of the microracetrack resonator embedded with Bragg gratings is shown in Fig. 1. Two embedded Bragg gratings act as partial reflective elements to form a Fabry-Perot resonator with a part of the microracetrack. The coupling between the F-P resonator and the microracetrack gives rise to the dual-band filtering transmission spectrum. In the simulation, we regard the Bragg grating as a periodic structure consisting of wide waveguide segments, narrow segments and reflective interfaces [7]. As shown in Fig. 2, the transmission spectrum is calculated with a specially designed numerical model based on the transfer matrix method. In this simulation, the parameters are chosen as: width of the waveguides = 500 nm, dw = 20 nm, pitch = 324 nm, the number of pitches = 20, radius of the microracetrack = 10.2 μm. The effective indices of the waveguides under different wavelengths around 1.55 µm are calculated with the BeamPROP module of Rsoft software by taking the dispersion into account. nxa0 nAs shown in Fig. 3, the microracetrack resonator embedded with Bragg is fabricated on a SOI platform with a 220-nm top silicon slayer and a 2um buried oxide (BOX) layer. The grating layer and the waveguide layer both are patterned with electron beam lithography. The grating layer is etched with RIE to the depth of 70 nm. The waveguide layer is etched down to the BOX layer. Then the sample is coated with a 1 um SiO2 cladding layer. The characterizations are carried out at room temperature. The experimental results are plotted in Fig. 4. As it can be seen, the extinction ratios (ERs) of the dual-band filtering spectrum are ~ 4.5 dB. The separation between two notches is ~ 0.5 nm. The full-width-at-half-maximums (FWHMs) are 0.05 nm and 0.045 nm, which corresponds to the Q factors of 30900 and 34400. The insertion loss is < 0.5 dB. The uneven transmission may be caused by the F-P resonator formed by the two end facets of the bus waveguide. By reducing the roughness of the waveguide sidewalls, the ER can be further increased. nxa0 nThis compact dual-band filter is expected to have good potential for applications in optical modulation, optical signal processing, and biological/chemical sensing.