High-precision Tunable Narrowband Rectangular Optical Filter Design And Its Applications

With the development of high precision optical signal processing and the prosperity of the microwave photonics, the narrowband signal filtering technique is highly desired. The narrowband optical filter with bandwidth of ~GHz or ~MHz has become one of the indispensable key components.An ideal optical filter has three principal characteristics: first an ultraflat passband, second very steep edges and third wavelength and bandwidth tunability. This kind of filter will not distort the signal at all and can suppress crosstalk from adjacent frequency components at the extreme. However, it is still challenging to make this narrowband rectangular filter because when the filter bandwidth is narrow, the steep edges can hardly be realized.In this paper, we realize a narrowband rectangular optical filter based on stimulated Brillouin scattering(SBS). The rectangular Brillouin gain shape has been well controlled by controlling the electrical comb precisely, which then modulates the light to generate optical comb as the SBS pump. The filter bandwidth can be changed precisely from 10 MHz to 4 GHz. We propose an effective feedback compensation algorithm and nonlinearity management in the single Brillouin peak fiber, and the passband ripple is reduced to less than 1 dB. The selectivity of the filter reaches larger than 40 dB(within 2-GHz bandwidth) by using cascaded scheme.Then we quantitatively analyze the Brillouin noise performance for single-and dual-stage schemes. The experimental results show that the noise performance of the dual-stage approach is even better than the single-stage method especially when the Brillouin gain is large. Afterwards, we apply the proposed filter to a reconfigurable optical add and drop multiplexer(ROADM) system to amplify and absorb Multi-band orthogonal frequency division multiplexing(MB-OFDM) signals at the same time. The signal to noise ratio penalty induced by the proposed filter is less than 1 dB when amplifying the OFDM signals with quadrature phase shift keying(QPSK) format. The performance of amplifying high-order modulation format signal is also acceptable.The proposed tunable narrowband rectangular optical filter can realize high-fidelity flexible filtering and will find its versatile applications in high speed optical communication systems, optical signal processing and microwave photonics.