Ruixin Wang

Microwave photonics: radio-over-fiber links, systems, and applications [Invited]

Microwave photonics (MWPs) uses the strength of photonic techniques to generate, process, control, and distribute microwave signals, combining the advantages of microwaves and photonics. As one of the main topics of MWP, radio-over-fiber (RoF) links can provide features that are very difficult or even impossible to achieve with traditional technologies. Meanwhile, a considerable number of signal-processing subsystems have been carried out in the field of MWP as they are instrumental for the implementation of many functionalities. However, there are still several challenges in strengthening the performance of the technology to support systems and applications with more complex structures, multiple functionality, larger bandwidth, and larger processing capability. In this paper, we identify some of the notable challenges in MWP and review our recent work. Applications and future direction of research are also discussed.

Dissipative soliton in actively mode-locked fiber laser

A dissipative soliton in an all-normal-dispersion actively mode-locked ytterbium-doped fiber laser is reported for the first time. Pulses with 10-ps duration and edge-to-edge bandwidth of 9 nm are generated, and then extra-cavity compressed down to 560 fs due to the large chirp. Widely wavelength tuning between 1031 and 1080 nm is achieved by adjusting the driving frequency only. Our simulation shows that the proposed laser operates in the dissipative soliton shaping regime.

Digital joint compensation of IMD3 and XMD in broadband channelized RF photonic link

Based on forward distortion information acquisition and post digital signal processing (DSP), we propose and demonstrate a novel scheme to effectively suppress the third-order intermodulation distortion (IMD3) and cross-modulation distortion (XMD) in a channelized RF photonic link. The simultaneous distortion compensation capacity is studied numerically, and suppression of XMD and IMD3 by about 28 dB and 25 dB, respectively, is achieved experimentally. The scheme principle and the digital compensation procedure are discussed, which shows a simple hardware implementation and algorithm.

Coherent photonic RF channelizer by optical frequency combs and I/Q demodulation

Two-optical-frequency-comb-based RF channelizer is proposed to slice RF band seamlessly and downconvert to multiple IF bands, with rectangular response in each channel. Without special optical filters, 7-channel 500-MHz-step channelizer is reported.

Sidemode suppression for coupled optoelectronic oscillator by optical pulse power feedforward.

Multiple sidemodes have been observed in a coupled optoelectronic oscillator (COEO) when the contained actively mode-locked fiber ring laser employs erbium-doped fiber (EDF). We propose that such sidemodes can be suppressed significantly by an optical pulse power feedforward scheme, through which the mode-locked optical pulse is reversely intensity-modulated by itself, resulting in a fast power limiting. Experimentally we show that sidemodes are suppressed as much as 40 dB in a 10-GHz COEO. The additional noise induced by the power feedforward technique is analyzed numerically. We show that for a COEO with a typical cavity length, the feedforward contribution to final single-side band (SSB) noise is minor and neglectable.

Digital nonlinearities compensation based on forward distortion information acquisition in channelized RF photonic links

A novel technique to suppress nonlinearities, including third-order intermodulation (IMD3) distortion and channel cross modulation (XM) distortion, in a channelized RF photonic link based on post digital signal processing and forward distortion information acquisition is proposed and demonstrated. The IMD3 and XM nonlinearity compensation function is obtained theoretically. Furthermore, we demonstrate the linearization technique using a polarization modulator. In experiment, the XM distortion and IMD3 distortion are suppressed simultaneously with about 25 dB and 19 dB, respectively.

Generation of high-speed broadband optical frequency comb in actively mode-locked fiber laser based on stretch-lens effect

We propose and experimentally demonstrate a method to generate 10-GHz and broadband optical frequency comb by novel stretch-type time-lens rather than nonlinear effects in actively mode-locked fiber laser. Injection locking and feedback loop have been used to suppress super-mode noise and help stabilize the optical spectrum. As a result, a comb that 20-dB bandwidth equals to 8nm and phase noise lower than -120 dBc/Hz in beat 10GHz microwave signal is achieved.

Actively mode-locked fiber laser with pulse intensity feed-forward

We report a novel actively mode-locked fiber laser with a pulse intensity feed-forward path. In the novel scheme, the optical pulse is firstly converted to electrical signal, and then fed forward to a dual-drive Mach-Zehnder modulator, where the optical pulse is modulated by the intensity profile of itself. The laser is modulated by the 10-GHz sinusoidal signal, but by setting the sign of the feed-forward signal, the laser can work at totally different mode-locking regimes (10-GHz mode locking or fundamentally mode locking). So the feed-forward path in the cavity can be used for different functions, i.e., supermode noise suppression in 10-GHz mode locking and saturable absorption in fundamental mode-locking regime. Due to the simple synchronization to external clock and stable operation, many applications would benefit from the novel laser.

Linear dissipative soliton in an anomalous-dispersion fiber laser

We report on the generation of linear dissipative soliton (LDS) from an erbium-doped actively mode-locked fiber laser. We show that depending on the down-chirping effect of quadratic phase modulation, instead of the fiber nonlinear Kerr effect in an all-normal-dispersion (ANDi) cavity, stable LDS can be realized in the linear dissipative system. The DS operation of ANDi laser and LDS operation of anomalous dispersion laser are experimentally investigated and compared, and the formation mechanisms of the DS and LDS are discussed. Finally, optical frequency comb generated by the LDS laser is demonstrated.