Wavelength and bandwidth-tunable silicon comb filter based on Sagnac loop mirrors with Mach-Zehnder interferometer couplers

We propose and experimentally demonstrate a wavelength and bandwidth-tunable comb filter based on silicon Sagnac loop mirrors (SLMs) with Mach-Zehnder interferometer (MZI) couplers. By thermally tuning the MZI couplers in common and differential modes, the phase shift and reflectivity of the SLMs can be changed, respectively, leading to tunable wavelength and bandwidth of the comb filter. The fabricated comb filter has 93 comb lines in the wavelength range from 1535 nm to 1565 nm spaced by ~0.322 nm. The central wavelength can be red-shifted by ~0.462 nm with a tuning efficiency of ~0.019 nm/mW. A continuously tunable bandwidth from 5.88 GHz to 24.89 GHz is also achieved with a differential heating power ranging from 0.00 mW to 0.53 mW. ©2016 Optical Society of America OCIS codes: (130.3120) Integrated optics devices; (130.7408) Wavelength filtering devices; (230.5750) Resonators. References and links 1. Z. Luo, W. Cao, A. Luo, and W. Xu, “Polarization-independent, multifunctional all-fiber comb filter using variable ratio coupler-based Mach-Zehnder interferometer,” J. Lightwave Technol. 30(12), 1857–1862 (2012). 2. I. Giuntoni, P. Balladares, R. Steingrüber, J. Bruns, and K. Petermann, “WDM multi-channel filter based on sampled gratings in silicon-on-insulator,” in Optical Fiber Communication Conference (Optical Society of America, 2011), paper OThV3. 3. M. W. Pruessner, T. H. Stievater, M. S. Ferraro, and W. S. Rabinovich, “Thermo-optic tuning and switching in SOI waveguide Fabry-Perot microcavities,” Opt. Express 15(12), 7557–7563 (2007). 4. M. W. Pruessner, T. H. Stievater, and W. S. Rabinovich, “Integrated waveguide Fabry-Perot microcavities with silicon/air Bragg mirrors,” Opt. Lett. 32(5), 533–535 (2007). 5. P. Dong, S. F. Preble, and M. Lipson, “All-optical compact silicon comb switch,” Opt. Express 15(15), 9600– 9605 (2007). 6. B. G. Lee, A. Biberman, P. Dong, M. Lipson, and K. Bergman, “All-optical comb switch for multiwavelength message routing in silicon photonic networks,” IEEE Photonics Technol. Lett. 20(10), 767–769 (2008). 7. R. Soref, “The past, present, and future of silicon photonics,” IEEE J. Sel. Top. Quantum Electron. 12(6), 1678– 1687 (2006). 8. X. Sun, L. Zhou, J. Xie, Z. Zou, L. Lu, H. Zhu, X. Li, and J. Chen, “Tunable silicon Fabry-Perot comb filters formed by Sagnac loop mirrors,” Opt. Lett. 38(4), 567–569 (2013). 9. X. Jiang, J. Wu, L. Jiang, Y. Yang, P. Cao, X. Hu, T. Pan, and Y. Su, “Variable bandwidth comb filter based on tunable silicon Sagnac-loop reflectors,” in Proceedings of IEEE Conference on Group IV Photonics (IEEE, 2014), pp. 217−218. 10. A. Yariv, “Critical coupling and its control in optical waveguide-ring resonator systems,” IEEE Photonics Technol. Lett. 14(4), 483–485 (2002). 11. A. Zhang, L. Ding, T. Zhang, and Z. Guo, “Tunable bandwidth and wavelength liquid crystal optical filter,” Proc. SPIE 3936, 66–73 (2000). 12. W. Vogel and M. Berroth, “Tuneable liquid crystal Fabry-Perot filters,” Proc. SPIE 4944, 293–302 (2003). 13. T. J. Kippenberg, R. Holzwarth, and S. A. Diddams, “Microresonator-based optical frequency combs,” Science 332(6029), 555–559 (2011). #255179 Received 14 Dec 2015; revised 17 Jan 2016; accepted 19 Jan 2016; published 27 Jan 2016 © 2016 OSA 8 Feb 2016 | Vol. 24, No. 3 | DOI:10.1364/OE.24.002183 | OPTICS EXPRESS 2183 14. J. Wu, P. Cao, X. Hu, X. Jiang, T. Pan, Y. Yang, C. Qiu, C. Tremblay, and Y. Su, “Compact tunable silicon photonic differential-equation solver for general linear time-invariant systems,” Opt. Express 22(21), 26254– 26264 (2014). 15. F. Gan, T. Barwicz, M. A. Popović, M. S. Dahlem, C. W. Holzwarth, P. T. Rakich, H. I. Smith, E. P. Ippen, and F. X. Kärtner, “Maximizing the thermo-optic tuning range of silicon photonic structures,” in Proceedings of IEEE Conference on Optical Switch (IEEE, 2007), pp.67−68. 16. J. Song, L. W. Luo, X. Luo, H. Zhou, X. Tu, L. Jia, Q. Fang, and G. Q. Lo, “Loop coupled resonator optical waveguides,” Opt. Express 22(20), 24202–24216 (2014). 17. X. Dong, P. Shum, N. Q. Ngo, and C. C. Chan, “Multiwavelength Raman fiber laser with a continuously-tunable spacing,” Opt. Express 14(8), 3288–3293 (2006). 18. Z. Luo, A. Luo, and W. Xu, “Tunable and switchable multiwavelength passively mode-locked fiber laser based on SESAM and inline birefringence comb filter,” IEEE Photonics J. 3(1), 64–70 (2011).


Introduction
Optical comb filters, which perform data filtering and blocking of multi-wavelength channels, are key components in wavelength-division multiplexed (WDM) optical networks [1][2][3][4][5][6].In practical applications, dynamic tuning of central wavelengths and bandwidths of comb filters is desired to meet the requirements of flexible WDM optical networks.Comb filters based on silicon photonics can offer competitive advantages including compact device footprint, complementary metal oxide semiconductor (CMOS) compatible fabrication, and low power consumption [7].Various schemes have been proposed to realize on-chip comb filters based on Bragg gratings [2][3][4], microring resonators [5,6], and cascaded Sagnac loop mirrors (SLMs) [8].However, the bandwidths of the above comb filters cannot be tuned without changing the central wavelength.In our previous work [9], we proposed and experimentally demonstrated a variable bandwidth comb filter based on cascaded SLMs with Mach-Zehnder interferometer (MZI) couplers.The variable bandwidth is achieved by thermally tuning one arm of the MZIs, but not in a differential mode [10], which changes the central wavelength.Thus, independent bandwidth and wavelength tuning cannot be realized.
In this paper, we experimentally demonstrate a wavelength and bandwidth independently tunable comb filter based on cascaded SLMs with MZI couplers.The directional couplers of the SLMs are replaced by MZI couplers.By controlling the MZI couplers in common and differential modes with micro-heaters, the phase shift and reflectivity of the SLMs can be changed, leading to tunable wavelength and bandwidth of the comb filter.By thermally tuning the device, the central wavelength can be red-shifted by ~0.462 nm with a tuning efficiency of ~0.019 nm/mW.The bandwidth can be tuned from 5.88 GHz to 24.89 GHz with a differential heating power ranging from 0.00 mW to 0.53 mW.These experimental demonstrations of wavelength and bandwidth tuning verify the feasibility of the proposed tunable comb filter.Figure 1(a) shows a Fabry-Perot interferometer (FPI) comb filter [8] consisting of two cascaded SLMs.By replacing the directional couplers of the SLMs with MZI couplers in Fig. 1(b), the phase shift and reflectivity of the SLMs can be controlled by the phase shifters along the MZI arms.Consequently, the effective cavity length and reflectivity of the FPI can be tuned accordingly, leading to tunable central wavelength and bandwidth of the comb filter [11,12].Note that comb filters do not generate new frequencies as comb generators [13].

Device structure and operation principle
As shown in Fig. 1(c), the central wavelength can be tuned in a common mode, which varies the phase shift of the MZI arms symmetrically.Therefore, a varied cavity length and a constant reflectivity of the FPI are achieved, resulting in a tunable central wavelength with an unchanged bandwidth.To obtain different bandwidths at the same central wavelength, the MZI arms are tuned in a differential mode, which changes the phase shifts of two MZI arms l 1 and l 3 asymmetrically, as depicted in Fig. 1(d).Based on transfer matrix method [10], the field transmission function of the structure can be given as follows: (2) where t s and r s are the transmission and reflection functions of a SLM with MZI coupler, respectively.t and k (t 2 + k 2 = 1) are the transmission and coupling coefficients of the directional couplers, respectively.a i = exp(−αl i − j2πn g /λl i ) (i = 1, 2, 3, 4) are the transmission factors of the waveguides, with l i (i = 1, 2, 3, 4) denoting the lengths of the waveguides depicted in Fig. 1(b).α and n g are the loss factor and the group index of the silicon waveguides, respectively.The structural parameters are chosen as follows.The lengths of the waveguides l 1,3 = 287.965μm, l 2 = 146.842μm, and l 4 = 134.998μm are used to achieve a narrow channel spacing.The cross sections of the waveguides are 450 nm × 220 nm.The coupling length and gap of the directional couplers are l c = 20 μm and 300 nm, respectively.The directional couplers are designed to be 3-dB couplers (t = ~0.707) to realize independent tuning of central wavelength and bandwidth of the comb filter.Therefore, t s and r s can be simplified to t s = −a 2 (a 1 2 + a 3 2 )/2 and r s = ja 2 (a 1 2 − a 3 2 )/2, respectively.

Simulation results
Figure 2 shows the simulated results of central wavelength and bandwidth tuning of the proposed comb filter.In the simulations, the wavelength and bandwidth tuning are realized by changing the refractive indexes of the phase shifters along the MZI arms in common and differential modes, respectively.The simulation parameters are α = 10.16 dB/cm, t = 0.707, and n g = 4.31.The length of the phase shifters along the MZI arms is 236.549μm.The transmission function of the comb filter in Eq. ( 1) can be simplified to t FP = a 2 2 a 1 4 a 4 if the MZIs have equal arm lengths (l 1 = l 3 ), leading to a constant transmission amplitude.Thus, a small length difference of 0.1 μm is introduced to the two arms of the MZIs to avoid constant transmission amplitude.
Figure 2(a) shows the transmission spectra of wavelength tuning if the device operates in a common mode.The refractive indexes of the phase shifters are set to be n g + ∆n with ∆n denoting the refractive index change.The central wavelength is tuned from 1548.713 nm to 1548.871 nm with ∆n ranging from 3.6 × 10 −3 to 4.4 × 10 −3 , which can be achieved by the thermo-optic effect in silicon.We also investigated the dependence of the ER on the transmission coefficients of the directional couplers and the cavity length.The cavity length can be controlled by varying the lengths l 1,3 of the MZI arms.As shown in Fig. 3, the ER decreases from 18.43 dB to 14.08 dB by changing t from 0.707 to 0.880.The ER decreases from 23.01 dB to 18.43 dB with l 1,3 increasing from 57.965 μm to 287.782 μm.

Device fabrication and measured transmission spectra
The device was fabricated on an 8-inch silicon-on-insulator (SOI) wafer.248-nm deep ultraviolet (DUV) photolithography was used to define the pattern and an inductively coupled plasma (ICP) etching process was used to etch the top silicon layer.Grating couplers for TE polarization were employed at the two ends of the device to couple light into and out of the chip with single mode fibers.Four TiN micro-heaters [14] were fabricated along the MZI arms to tune the phase shifts of the SLMs.A micrograph of the fabricated device is shown in Fig. 4(a).Finally, the device was wire-bonded to a printed circuit board (PCB) for electrical connections, as shown in Fig. 4(b).
The transmission spectrum of the fabricated device is measured using a tunable laser (Keysight 81960A) scanning from 1535 nm to 1565 nm with a step size of 1 pm.The measured transmission spectrum is shown in Fig. 4(c).The total insertion loss of the chip is ~25 dB, including a ~19-dB coupling loss introduced by a fiber coupling system.There are 93 comb lines in the wavelength range from 1535 nm to 1565 nm with a channel spacing of ~0.322 nm.The channel spacing can be designed to fit the International Telecommunications Union (ITU) grids by varying the waveguide lengths.The peak-transmission variation of the comb lines can be attributed to the wavelength-dependent coupling coefficients of the directional couplers and the ripples in the transmission spectrum of the grating couplers.The measured transmission spectrum in a 2-nm spectral range is shown in Fig. 4(d), which is fitted by the theoretically calculated transmission spectrum (red-dot curve) obtained from Eq. ( 1).The fitting parameters are α = 10.16 dB/cm, t = 0.88, and n g = 4.31.

Experimental demonstration of central wavelength and bandwidth tuning
To realize wavelength tuning, the MZI arms are tuned in a common mode.The resonance redshifts as the cavity length increases.Figure 5(a) shows the measured transmission spectra of central wavelength tuning.The resonance redshifts from 1548.628 nm to 1548.803 nm with the heating power P of each micro-heater tuned from 4.97 mW to 7.53 mW.The bandwidth also changes from 6.13 GHz to 4.38 GHz, which can be attributed to the small change of the transmission coefficients of the MZI couplers arising from the small difference of heating powers applied to the two MZI arms.
The measured and fitted central wavelength shift from 1548.402 nm is shown in Fig. 5(b).The central wavelength shifts ~0.462 nm with the heating powers of the four micro-heaters tuned from 2.38 mW to 8.50 mW, which covers a channel spacing of ~0.322 nm.Thus, the wavelength tuning efficiency is ~0.019 nm/mW, which can be doubled by tuning the phase shift of the waveguide connecting the two SLMs [8].Multi-wire structure micro-heaters [15] can also be used to improve the tuning efficiency.
To obtain tunable bandwidths without changing the central wavelength, the MZI couplers are tuned in a differential mode.Firstly, the four micro-heaters are biased at a heating power P of 4.60 mW.Then heating powers are changed according to Fig. 1(c) with a differential heating power ∆P, i.e. heating powers P + ∆P/2 and P − ∆P/2 are applied to the MZI arms l 1 and l 3 , respectively.The bandwidth of the comb line at 1548.633 nm ranges from 5.88 GHz to 13.63 GHz with ∆P tuned from 0.00 mW to 0.24 mW, as shown in Fig. 5(c).Due to the imperfect 3-dB directional coupler, the central wavelength also shows a slight change, which can be reduced by fine-tuning the heating powers.Meanwhile, the ER decreases from 14.34 dB to 4.66 dB due to the changed transmission coefficients of the MZI couplers, which determine the reflectivity of the SLMs.The ER can be improved by using more precise 3-dB couplers and shorter cavity length, as discussed in Section 3. Improved ER and flat-top passband can also be realized by cascading more SLMs [16].Then the change of ER would have a smaller impact on the filtering performance.

Conclusion
In conclusion, a silicon comb filter implemented by Sagnac loop mirrors with MZI couplers has been proposed and experimentally demonstrated.The central wavelength and bandwidth of the comb lines can be tuned by controlling the MZI arms in common and differential modes, respectively.By thermally tuning the device, the central wavelength can be redshifted by ~0.462 nm with a tuning efficiency of ~0.019 nm/mW.The bandwidth ranges from 5.88 GHz to 24.89 GHz with a differential heating power changing from 0.00 mW to 0.53 mW.The proposed device can be used for flexible filtering to suppress out-of-band noises or define the lasing wavelengths of multi-wavelength lasers [17,18] in WDM optical communication systems.

Fig. 1 .
Fig. 1.(a) A FPI comb filter consists of two loop mirrors.(b) Schematic of the proposed comb filter based on SLMs with MZI couplers.(c) Common and (d) differential tuning of the MZI couplers for wavelength and bandwidth tuning of the comb filter, respectively.SLM: Sagnac loop mirror.MZI: Mach-Zehnder interferometer.FPI: Fabry-Perot interferometer.

Figure 2 (
Figure2shows the simulated results of central wavelength and bandwidth tuning of the proposed comb filter.In the simulations, the wavelength and bandwidth tuning are realized by changing the refractive indexes of the phase shifters along the MZI arms in common and differential modes, respectively.The simulation parameters are α = 10.16 dB/cm, t = 0.707, and n g = 4.31.The length of the phase shifters along the MZI arms is 236.549μm.The transmission function of the comb filter in Eq. (1) can be simplified to t FP = a 2 2 a 1 4 a 4 if the MZIs have equal arm lengths (l 1 = l 3 ), leading to a constant transmission amplitude.Thus, a small length difference of 0.1 μm is introduced to the two arms of the MZIs to avoid constant transmission amplitude.Figure2(a) shows the transmission spectra of wavelength tuning if the device operates in a common mode.The refractive indexes of the phase shifters are set to be n g + ∆n with ∆n denoting the refractive index change.The central wavelength is tuned from 1548.713 nm to 1548.871 nm with ∆n ranging from 3.6 × 10 −3 to 4.4 × 10 −3 , which can be achieved by the thermo-optic effect in silicon.Figure2(b) depicts the resonance shift in central wavelength for various ∆n.One can see that the resonance shift increases linearly with ∆n.

Fig. 2 .
Fig. 2. (a), (c) Simulated transmission spectra of (a) central wavelength and (c) bandwidth tuning by changing the refractive indexes of the phase shifters along the MZI arms according to Figs. 1(c) and 1(d), respectively.(b) Central wavelength shift versus ∆n.(d) Bandwidth and extinction ratio versus ∆n.∆n: refractive index change.R: reflectivity.

Figure 2 (
Figure 2(c) shows the bandwidth tuning effect obtained by operating the device in a differential mode.The refractive indexes of the phase shifters along the two MZI arms are set to be n g + ∆n/2 and n g − ∆n/2, respectively.The 3-dB bandwidth of the resonance at 1548.792 nm can be tuned from 3.13 GHz to 19.64 GHz by varying ∆n from 5.1 × 10 −3 to 5.9 × 10 −3 .The reflectivity R of the SLM with MZI coupler is also shown in Fig. 2(c).The bandwidth and maximum transmission increase as R decreases.As shown in Fig. 2(d), for an increased ∆n, the bandwidth and extinction ratio (ER) increases and decreases, respectively.

Fig. 3 .
Fig. 3. Simulated transmission spectra of the comb filter with different t and l 1,3 .

Fig. 4 .
Fig. 4. (a), (b) Micrograph of (a) the fabricated device and (b) the device after wire-bonding to a PCB.(c) Measured transmission spectrum of the comb filter in the wavelength range from 1535 nm to 1565 nm.(d) Measured (blue-solid curve) and fitted (red-dot curve) transmission spectra in the wavelength range from 1548 nm to 1550 nm.SLM: Sagnac loop mirror.MZI: Mach-Zehnder interferometer.PCB: printed circuit board.

Fig. 5 .
Fig. 5. (a), (c) Measured transmission spectra of (a) central wavelength and (c) bandwidth tuning versus heating power P of each micro-heater and differential heating power ∆P, respectively.(b) Central wavelength shift versus heating power P of each micro-heater.(d) Bandwidth and extinction ratio versus differential heating power ∆P.

Figure 5 (
Figure 5(d) shows the bandwidth and extinction ratio in bandwidth tuning.The 3-dB bandwidth changes from 5.88 GHz to 24.89 GHz with ∆P tuned from 0.00 mW to 0.53 mW.Meanwhile, the ER decreases from 14.34 dB to 4.66 dB due to the changed transmission coefficients of the MZI couplers, which determine the reflectivity of the SLMs.The ER can be improved by using more precise 3-dB couplers and shorter cavity length, as discussed in Section 3. Improved ER and flat-top passband can also be realized by cascading more SLMs[16].Then the change of ER would have a smaller impact on the filtering performance.