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
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 , which could meet the requirement of high resolution radio frequency (RF) signal processing. In , a ring assisted MZI tunable filter based on hybrid silicon platform was proposed. In , 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 , limiting their application for fine processing for broadband signal processing, where an additional optical RF channelizer is always required to enlarge the FSR , introducing extra loss and complexity to the whole system.
In addition, with the development of new types of analog photonic link (APL) , more advanced functionalities are required to be explored such as high selectivity and tunable filtering, frequency conversion and high fidelity transport . The silicon based MWP signal processors are believed to be a promising solution to these challenges in the complex APL , 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 K-band utilizing the proposed filter is presented as well and an SFDR of 104.1dB-Hz2/3 is observed.
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
Totally 2N couplers are needed for N stages filter. In order to minimize transfer-loss in passband, the coupling ratio of the coupler is set to be , so that all the zeros are on the unit circle . A simply phase shift is introduced for each stage in our design to align their resonance, i.e. Therefore, the Eq. (1) can be simplified as
The zeros diagram in z-plane of three-stage cascaded MZIs filter is shown in Fig. 2 , in which zeros distribute around the circle uniformly. One can see from the zeros diagram, the FSR of the filter is determined by the last stage of the filter shown in Fig. 1, which can also be derived from Eq. (2)Fig. 2, i.e. the bandwidth of the filter is mainly determined by the first stage MZI, meanwhile, the bandwidth will become narrower a little as the number of the stages increases. In addition, the zeros will rotate around the unit circle when the phase difference of each stage MZI changes, which performs a tunable and reconfigurable filter.
The simulation results of the filter response when N = 1, 3, 5 are shown in Fig. 3 . For N = 1, the FSR is nearly 2 times the and the extinction ratio is about 6.5dB as shown in Fig. 3(a). For N = 3, the FSR of the filter is enlarged by 4 times to 16GHz, nonetheless the decreased a little to 2.02GHz and the extinction ratio is about 17dB. When N = 5, the FSR is enlarged to 64GHz and the extinction ratio is about 30dB. The phase response is linear as for its FIR features as shown in Fig. 3. In this simulation, we assumed L is about 2.98cm,is about 3dB/cm , and a fixed coupling ratio of 0.5 considering the narrow wavelength range, which is in correspondence with the practical devices. The results also indicate that as N increases, the FSR can be enlarged by 2N-1 times, while the almost does not change and the extinction ratio of the filter can also be significantly improved.
3. Fabrication and device test
The proposed filter with three-stage cascaded MZIs is fabricated on SOI wafer, considering the current fabrication tolerance, phase matching requirement and the insertion loss as shown in Fig. 4(a) . The SOI platform applied is based on 220nm thick silicon layer laid on top of 2um buried-oxide layer by LETI foundry using 193nm DUV lithography. The width and height of the etched silicon waveguide used in this filter is about 450nm and 220nm, meanwhile the gap between the coupler waveguides is 200nm and coupling length is about 10um. The heaters and electrical contact pads, which consist of 110nm Ti/TiN are deposited on a buried channel waveguides by means of 600nm thick silica layer grown on the structure.
Due to the limitation of the current SOI technology, the characteristics, for instance, the feather size and propagation loss, of the waveguiding structure may be not identical as that of our designed ones, which would result in negative influence on the performance of the fabricated devices. There are two methods can be adopted to mitigate these limitations. Firstly, gradual changed exposure and oxidation process are adopted in the optimization procedure to compensate the exposure dose variation and smooth the waveguide surface, respectively. Secondly, considering the inevitable technological fabrication tolerances, micro-heaters covered on the isolation oxide layer are employed to tune the effective parameters of the device to modify the filtering response of the device.
By independently tuning the micro heaters covered on the MZI arms, we can achieve the phase matching between different stages, and the amplitude and phase response of the device measured by an optical vector analyzer (OVA) are shown in Fig. 4(b). The bandwidth of the filter is about 1.536GHz and the FSR is enhanced to 13.5GHz, which meets the processing bandwidth of from X- to Ka-band, while the phase response is linear which agrees well with the theoretical results. The extinction ratio of the measured results is a little decrease mainly due to the fabrication error and mismatching between different MZI stages. Vertical coupling based on grating coupler is used for coupling light from single mode fiber to the silicon waveguide where the fiber to waveguide loss is about 10dB and insertion loss of the filter itself is about 9dB. Since the silicon waveguide and the coupling rate of the grating couplers are both polarization sensitive, a polarization controller (PC) has to be used to control the light polarization before the light coupling into the chip.
4. Application in downconverting APL
In our experiment, a downconverting APL utilizing our proposed multi-stage MZIs filter as a key component for microwave signal processing is demonstrated as shown in Fig. 5 . A continuous wave (CW) is sent to a null-biased Mach-Zehnder modulator (MZM), which is modulated by a strong local oscillator (LO) RF signal. Two coherent optical sideband generated from MZM is then separated by an interleaver. The upper sideband is used as an optical LO for downconversion and lower sideband is served as the optical carrier and phase-modulated by the input broadband RF signal . The optical signal is then processed using our proposed narrow-passband silicon filter. Thanks to the narrow-passband and large FSR of the filter, the processor can precisely filter out the interesting signal over a wide RF frequency band of from X-band to Ka-band. To maintain phase correlation of the upper and lower paths, their path lengths and polarization states are carefully matched using optical delay line and the PCs. The filtered signal and the optical LO are then combined in an optical coupler and detected by balanced photodetector (BPD).
The parameters of the system are listed as follows: the optical power from the CW laser is 12.8 dBm, with center wavelength of 1550.018nm and 1 kHz linewidth, the output power of the tunable LO is about 26 dBm, the bandwidth of the MZM and PM are 20 GHz and 40 GHz, respectively the half-wave voltage of the MZM and PM are about 5V and 7V, the gain of the first erbium-doped fiber amplifier (EDFA1) is about 9dB and the gain of EDFA2 is about 22 dB to compensate the whole link loss, the channel space of the interleaver is about 25 GHz, the BPD has a responsivity of ~0.62 A/W. The electrical spectrum is measured by an electrical spectrum analyzer (ESA).
In order to investigate the performance of the downconverting APL and filtering features of the proposed narrow bandwidth filter, an APL works at K-band is presented and analyzed. A RF LO centered at 12GHz drives the null-biased MZM to get the carrier suppression of about 30dB and a two-tone signal at frequency of 24.0085 GHz and 24.0095 GHz are injected to the PM. By carefully tuning the micro-heaters on the phase-shifter, the desired response can be achieved and by heating the chip we can move the resonance frequency and select first modulated band perfectly and then the filtered signal is coupled with the upper sideband optical LO. As ones can see from Fig. 6(a) both the strong optical carrier and even-order optical sidebands are significantly suppressed thanks to the carrier-suppressed configuration and optical filtering, which are more than 30 dB lower relative to the signal and LO and hence ignorable. The signals are detected by the BPD and downconverted to 8.5MHz and 9.5MHz as well as the third-order intermodulation distortion (IMD3) at 7.5MHz and 10.5 MHz, as the results for the nonlinear behavior of the transfer response of whole APL link due to the phase modulation and the photo-detection scheme as shown in Fig. 6(b). The SFDR of the APL in K-band has been measured and plotted in Fig. 6(c). The measured noise floor is at about −127.7 dBm/Hz and the SFDR is about 104.1dB-Hz2/3. The high noise floor is mainly due to the ASE noise induced by the EDFAs in the link, and the number of EDFAs can be reduced by lowering the fiber-waveguide coupling loss and waveguide transmission loss utilizing higher coupling ratio couplers  and advanced fabrication technology .
An SOI multi-stage cascaded MZIs based filter was proposed, theoretically analyzed and experimentally demonstrated. Considering the application and the state of art in practice, a three-stage cascaded MZIs bandpass filter with a bandwidth of about 1.536GHz and FSR nearly 13.5GHz was realized. A downconvering APL in K-band based on the proposed SOI processor is also successfully implemented and an SFDR of the link as large as 104.1dB-Hz2/3 has been achieved.
This work was partially supported by National Program on Key Basic Research Project (973) under Contract 2012CB315703, NSFC under Contract 61120106001, 61271134, 61090391 and the Program for New Century Excellent Talents in University (NCET-10-0520), and the Open Fund of Key Laboratory of Optical Communication and Lightwave Technologies (BUPT), China
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