Acousto-optic reconfigurable filter based on vector mode fusion in dispersion-compensating fiber.

An acousto-optic reconfigurable filter (AORF) is proposed and demonstrated based on vector mode fusion in dispersion-compensating fiber (DCF). With multiple acoustic driving frequencies, the resonance peaks of different vector modes in the same scalar mode group can be effectively fused into a single peak, which is utilized to obtain arbitrary reconfiguration of the proposed filter. In the experiment, the bandwidth of the AORF can be electrically tuned from 5 nm to 18 nm with superposition of different driving frequencies. The multi-wavelength filtering is further demonstrated by increasing the interval of the multiple driving frequencies. The bandpass/band-rejection can also be electrically reconfigured by setting the combination of driving frequencies. The proposed AORF gains the feature of reconfigurable filtering types, fast and wide tunability, and zero frequency shift, which is advantageous for high-speed optical communication networks, tunable lasers, fast optical spectrum analyzing and microwave photonics signal processing.

Ultra-narrow linewidth vertical-cavity surface-emitting laser based on external-cavity weak distributed feedback.

We demonstrate an ultra-narrow linewidth vertical-cavity surface-emitting laser (VCSEL) based on external-cavity weak distributed feedback from Rayleigh backscattering (RBS). A single longitudinal mode VCSEL with the linewidth as narrow as 435 Hz and a contrast of 55 dB are experimentally achieved by RBS fiber with a feedback level of RBS signal of -27.6 dB. By adjusting the thermal resistance of the VCSEL from 4.5 kΩ to 7.0 kΩ, the laser wavelength can be tuned from 1543.324 nm to 1542.06 nm with a linear tuning slope of -0.506 nm/kΩ. In the tuning process, the linewidth fluctuates in the range of 553-419 Hz.

Tens of hertz ultra-narrow linewidth fiber ring laser based on external weak distributed feedback.

We suggest and demonstrate a single-frequency fiber ring laser with an ultra-narrow linewidth based on an external weak distributed feedback. A π phase-shifted fiber Bragg grating (PSFBG) is used to improve mode selection and enable single-longitudinal mode (SLM) laser operation. The linewidth is then further strongly compressed using a signal generated by a weak distributed feedback structure (WDFS) and injected into the main laser cavity to suppress spontaneous emission. The resulting ultra-narrow linewidth fiber ring laser achieves a side-mode suppression ratio (SMSR) of ∼72 dB, and low white frequency noise of ∼10.3 Hz2/Hz, which correspond to an instantaneous linewidth of ∼32.3 Hz in the normal operating condition of the laser. Our linewidth compression mechanism not only solves the problems associated with deep linewidth compression in long-cavity fiber laser, but also fosters the development of practical and reliable all-fiber structures. Our laser source is characterized by low cost, high coherence, and low noise, which are highly desirable features in coherent optical detection, high-resolution spectrometers, microwave photonics, and optical sensing.

Ultra-narrow-linewidth DFB laser array based on dual-cavity feedback.

Herein, we propose a structure to simultaneously compress the distributed feedback (DFB) laser array's linewidth. The proposed structure is meticulously designed to ensure single longitudinal mode operation via the interference phenomenon between the laser's primary cavity and the dual-cavity feedback. Given the weak feedback effect for each wavelength in the laser array, the proposed structure could realize the intense compression of the laser linewidths. The study results show that the side-mode suppression ratios of each DFB laser are over 40 dB, and the linewidths have been compressed from 3 MHz to ∼800 Hz. Thus, we believe the idea of an overall compression linewidth scheme in the present study can be adopted for integrated laser arrays.

Control of polarization switching in a VCSEL via resonant feedback from a whispering-gallery-mode cavity.

We report a method for flexibly switching the dominant polarization of a vertical-cavity surface-emitting laser (VCSEL) by introducing polarization-resolved resonant optical feedback from a whispering-gallery-mode (WGM) cavity to the lasing cavity. Switching between the originally dominant mode and a side mode is experimentally demonstrated under different bias currents once one of them is locked to the resonance mode of the WGM cavity. In addition to a controllable polarization state, the reported VCSEL also demonstrates a linewidth as narrow as tens of kilohertz, which is highly desirable for many applications, including high-speed data communication, light detection and ranging (lidar), and absorption spectroscopy.

Simultaneous self-injection locking of two VCSELs to a single whispering-gallery-mode microcavity.

Simultaneous self-injection locking of two vertical-cavity surface-emitting lasers (VCSELs) to a single whispering-gallery-mode (WGM) microcavity is experimentally demonstrated. The linewidths of the two VCSELs are compressed from 3.5 MHz and 5 MHz to 20.9 kHz and 24.1 kHz, which is on the same order of magnitude as that of locking each VCSEL to the microcavity separately. Moreover, the frequency noises of the two simultaneously locked VCSELs are suppressed by more than 60 dB below the offset frequency of 100 kHz compared to that of the free-running VCSELs. The method demonstrated here might be used in the multi-wavelength laser array with low phase and frequency noises, especially the VCSELs with the unique architecture of a two-dimensional array.