Microwave photonic integrator based on a multichannel fiber Bragg grating

Jiejun Zhang, Jianping Yao
2016 Optics Letters  
We propose and experimentally demonstrate a microwave photonic integrator based on a multichannel fiber Bragg grating (FBG) working in conjunction with a dispersion compensating fiber (DCF) to provide a step group delay response with no in-channel dispersion-related distortion. The multichannel FBG is designed based on the spectral Talbot effect, which provides a large group delay dispersion (GDD) within each channel. A step group delay response can then be achieved by cascading the
more » ... ing the multichannel FBG with a DCF having a GDD opposite the in-channel GDD. An optical comb, with each comb line located at the center of each channel of the FBG, is modulated by a microwave signal to be integrated. At the output of the DCF, multiple time-delayed replicas of the optical signal, with equal time delay spacing are obtained and are detected and summed at a photodetector (PD). The entire operation is equivalent to the integration of the input microwave signal. For a multichannel FBG with an in-channel GDD of 730 ps/nm and a DCF with an opposite GDD, an integrator with a bandwidth of 2.9 GHz and an integration time of 7 ns is demonstrated. Photonic signal processing is considered to be a solution to alleviating the bandwidth bottleneck of electronic signal processing and has applications where high speed and wideband signal processing are needed. Among the numerous signal processing functions, a temporal integrator is a fundamental building block that has been widely investigated in the past few years. A photonic integrator, used to implement the temporal integration of an electronic signal, can operate at a much higher speed and wider bandwidth, compared to an electronic integrator. So far, photonic integrators have been demonstrated using a fiber Bragg grating (FBG) [1-4], a microring resonator [5, 6] , an active Fabry-Perot filter [7] , or an optical dispersive device [8] [9] [10] [11] . For example, a photonic integrator was demonstrated with a weak uniform FBG [1] . For an FBG with a physical length of 5 mm, the integration time window was only around 50 ps, which is very small. To increase the integration time window, two superimposed FBGs fabricated in an Er-Yb co-doped optical fiber were used to form an active optical resonator [2] . A temporal window of 2.9 ns was demonstrated. In [3, 4] , a high-order photonic integrator using a specially designed FBG with its spectral response corresponding to the spectrum of a higher order integrator was demonstrated. A photonic integrator can also be implemented using a microring resonator [5, 6] . In [5], a passive microring resonator on a silica platform was used to perform the integration of an input signal with a time feature as fast as 8 ps. An integration time window of 800 ps was achieved. To realize an even longer integration time window, a photonic integrator based on an active InP-InGaAsP integrated ring resonator was proposed [6] . Due to the active nature of the ring resonator, the Q-factor can be controlled to be large, which leads to a significantly increased integration time of 6.3 ns. However, the larger diameter of the active ring resonator, compared to that of a passive ring resonator [5], resulted in a reduced bandwidth of 15 GHz. In [7], an active Fabry-Perot filter incorporating a semiconductor optical amplifier (SOA) was proposed as a photonic integrator with a simulated integration time window of 160 ns and an operation bandwidth of 180 GHz. However, in an experimental implementation, the integration time window and the operation bandwidth may be smaller since the SOA may not operate in conditions as ideal as assumed in the simulations. A photonic integrator can also be realized with an optical dispersive device, such as a dispersion compensating fiber (DCF) or simply a single-mode fiber (SMF). Due to their passive nature, such integrators are simpler and have smaller noise figures. For example, an integrator was demonstrated using a length of SMF as a dispersive device to generate time-delayed replicas of a signal that was carried by an incoherent broadband source with a rectangular spectrum. The time-delayed replicas are summed at a photodetector (PD). The entire operation is equivalent to an integrator [8] . The integrator in [8] allows the processing of an electrical signal with a time feature as fast as 7 ps, but the integration time window of 0.5 ns was small. To increase the integration time window, in [9], an integrator consisting of a low-speed integrator and a high-speed integrator that are cascaded was proposed. The integration time window was increased to 4 ns. However, for both integrators in [8, 9] ,
doi:10.1364/ol.41.000273 pmid:26766692 fatcat:u5kbnawutzc2bbqw3ts5nesb6i