Silicon-on-insulator narrow-passband filter based on cascaded MZIs incorporating enhanced FSR for downconverting analog photonic links Hongchen Yu, Minghua Chen,* Pengxiao Li, Sigang Yang, Hongwei Chen, and Shizhong Xie Department of Electronic Engineering, Tsinghua University, Tsinghua National Laboratory for Information Science and Technology (TNList), Beijing 100084, China *
[email protected]
Abstract: A silicon-on-insulator (SOI) narrow-passband filter based on cascaded Mach-Zehnder interferometers (MZIs) is theoretically simulated and experimentally demonstrated, indicating that the free spectral range (FSR) of the proposed filter can be significantly enlarged by increasing the number of the MZI stages. A filter using three-stage cascaded MZIs structure is successfully realized in the experiment and a 3-dB bandwidth of about 1.536 GHz and FSR about 13.5 GHz have been achieved. The performance of a downconverting analog photonic link (APL) employing the designed filter for microwave signal processing is also measured and a spurious free dynamic range (SFDR) as high as 104.1dB-Hz2/3 is observed. ©2013 Optical Society of America OCIS codes: (230.3120) Integrated optics devices; (060.5625) Radio frequency photonics; (070.1170) Analog optical signal processing.
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#184240 - $15.00 USD (C) 2013 OSA
Received 28 Jan 2013; revised 4 Mar 2013; accepted 5 Mar 2013; published 11 Mar 2013 25 March 2013 / Vol. 21, No. 6 / OPTICS EXPRESS 6749
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1. Introduction Silicon-on-insulator (SOI) integration based devices have been a hot topic in the area of integrated microwave photonics for signal processing as the state of art is mature, compatible with CMOS technology and immunity to electromagnetic interference, and the devices are small for it has high index contrast between Si and SiO2 etc [1–4]. Microwave photonic (MWP) signal processors based on reconfigurable filters with narrow bandwidth, wide FSR and linear phase response have attracted a lot of research interest [5,6]. Recently, several integrated microwave photonics filters have been reported [6–12]. Generally, there are three kinds of integrated filters: 1) infinite impulse response (IIR) filters, which usually have the feedback loop, a phase jump in the filter band and is sensitive to fabrication tolerances, 2) finite impulse response (FIR) filters, which are usually formed by the MZIs and the phase response are linear, 3) coupled hybrid unit cell filter having both zero and pole. Accordingly, FIR filters have attracted a lot of attention, for their stability, simplicity and linear phase response. An optical filter unit cell with the bandwidth of only 1-2 GHz and reconfigurable for both FIR and IIR filter was proposed by the Telcordia research group in [6], which could meet the requirement of high resolution radio frequency (RF) signal processing. In [8], a ring assisted MZI tunable filter based on hybrid silicon platform was proposed. In [9], a four stage lattice based filter with fully configurable was demonstrated. However, all these proposed filters mainly rely on the traditional MZI scheme, whose FSRs are inversely proportional to their differential path length and proportional to their bandwidth [13], limiting their application for fine processing for broadband signal processing, where an additional optical RF channelizer is always required to enlarge the FSR [12], introducing extra loss and complexity to the whole system. In addition, with the development of new types of analog photonic link (APL) [14], more advanced functionalities are required to be explored such as high selectivity and tunable filtering, frequency conversion and high fidelity transport [15]. The silicon based MWP signal processors are believed to be a promising solution to these challenges in the complex APL [16], especially in terms of cost, power consumption and reliability. However, more recent efforts have focused on single silicon-based device function or subsystem performance, as far as we know there are still few reports on their applications in the APL. In this paper, an SOI multi-stage cascaded MZIs based narrow-bandwidth FIR filter with enhanced processing range is proposed and experimentally demonstrated, where the FSR of the filter can be enlarged by increasing the number of MZI stages. A three-stage cascaded MZIs based filter with a bandwidth of 1.536GHz and FSR nearly 13.5GHz has been successfully achieved, which can meet the requirement of RF signal processing bandwidth of from X- to Ka-band. Moreover, an intermediate frequency (IF) downconversion APL in Kband utilizing the proposed filter is presented as well and an SFDR of 104.1dB-Hz2/3 is observed.
#184240 - $15.00 USD (C) 2013 OSA
Received 28 Jan 2013; revised 4 Mar 2013; accepted 5 Mar 2013; published 11 Mar 2013 25 March 2013 / Vol. 21, No. 6 / OPTICS EXPRESS 6750
2. Device design and simulation Schematic diagram of the proposed filter based on multi-stage MZIs with enhanced processing range and negligible change in the 3-dB bandwidth is shown in Fig. 1, where the input- and output-ports on the upper paths (port1 to port2N) are combined for each stage MZI. In our design, the differential path length of each stage MZI satisfies the condition by ∆L1 = L, ∆L2 = 2−1L,…, ∆LN = 2-(N-1)L, and the phase shifters are realized by the micro heaters covered on the upper arms of the MZIs. The transfer function of the filter can be written as N
H 11 = ∏ ( 1 − κ 2i −1 1 − κ 2i e j φi − κ 2i −1 κ 2i γ i e − j β (2
− ( i −1)
L)
(1)
),
i =1
where κi (i
