As a crucial building block of the photonic integrated circuit (PIC), a photonic differentiator (DIFF) plays one of the key roles in analog optical signal processing due to its functionality of performing temporal differentiation of optical signal waveforms. Silicon PICs have been proposed to perform photonic differentiation by utilizing micro-ring resonators, Mach–Zehnder interferometers, and directional couplers. An ultra-wide-bandwidth DIFF has long been pursued by the scientific community due to its ability to process ultra-high-speed optical signals. However, present DIFFs have lower limitations of the operation bandwidths due to the energy efficiency of the DIFF. This fact indicates that even a photonic DIFF with terahertz operation bandwidth remains to be a bandpass device because it cannot process a low speed signal. This study led by Prof. Jianji Dong has been published on the Optics Express in 2015 (Opt. Express, 23, 18925-18936, 2015).

Figure 1: Schematic of the PhCW-MZI

To address this issue, Prof. Dong proposed a novel scheme based on photonic crystal waveguide (PhCW). By optimizing the structural parameters of the photonic crystal waveguide, the slow-light effect is obtained. Two PhCWs having different lengths and placed in separate arms of a MZI structure as Fig. 1 shows. As the slow light effect is related to the wavelength, a wavelength-dependable transmission spectrum can be obtained as Fig. 2 indicates, demonstrating the possibility of PhCW-MZI to act as a photonic differentiator.

Figure 2: Transmission spectrum of the PhCW-MZI.

Based on this theoretical model, Prof. Dong’s team cooperated with Dr. Yunhong Ding’s team to fabricate the device. Optical pulses with pulse widths ranging from 2.7 to 81.4 ps have been differentiated with process accuracy higher than 87.5%, according to our experimental results. The proposed concept can effectively avoid the bandwidth limitation of the photonic DIFF; hence, it will greatly enhance the performance of photonic DIFFs and largely expand the application field of DIFFs in large-scale photonic computing circuits, as well as in integrated optical signal processing systems.

Figure 3: Experimental results.

    On the 11th April, 2017, this work is published on the Optics Letters. This work is supported by National Natural Science Foundation of China (NSFC) (61475052, 61622502); Opened Fund of the State Key Laboratory on Advanced Optical Communication System and Network (2015GZKF03004); Ministry of Education of the People’s Republic of China (MOE) (NCET-11-0168).

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