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Ultrahigh-Q microwave photonic filter with Vernier effect and wavelength conversion in a cascaded pair of active loops Enming Xu, Xinliang Zhang,* Lina Zhou, Yu Zhang, Yuan Yu, Xiang Li, and Dexiu Huang Wuhan National Laboratory for Optoelectronics and School of Optoelectronic Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China *Corresponding author:
[email protected] Received December 21, 2009; revised March 8, 2010; accepted March 8, 2010; posted March 11, 2010 (Doc. ID 121795); published April 15, 2010 A new cascaded microwave photonic filter that can realize a high Q value is presented. It consists of two infinite impulse response filters based on two active loops. Owing to wavelength conversion employing crossgain modulation of the amplified spontaneous emission spectrum of the semiconductor optical amplifier in one loop, the interference between the modulated optical signals of different taps from two loops can be avoided, and stable transmission characteristics of the filter can then be achieved. Using this cascaded structure, the free spectral range and Q value can be increased significantly, and tunability can also be realized. Measured results of a high Q of 3338 and a rejection ratio of about 40 dB are achieved. © 2010 Optical Society of America OCIS codes: 350.4010, 060.2340.
Microwave photonic filters have attracted considerable attention in recent years because of their low loss, high bandwidth, immunity to electromagnetic interference, and potential to solve the limitations of electronic approaches [1–4]. A common objective is to increase the quality factor (Q) value and frequency selectivity of these filters. For finite impulse response filters, increasing the Q value requires increasing the tap number correspondingly; thus the number of components and the cost of the filters are increased, impairing the reliability [5–9]. Compared with finite impulse response filters, infinite impulse response (IIR) filters can achieve higher Q values with fewer components [10–12]. However, the Q values of IIR filters are limited by amplified spontaneous emission (ASE) noise [12] and the requirement to operate close to the lasing threshold point. It has been reported that an IIR filter cascaded with a finite impulse response filter can increase the Q value [13–16]. A Q value of 801 was achieved in the optical domain [14], and a Q value of 3000 was achieved with the help of an electrical filter [16]. The scheme of an IIR filter cascaded with an additional IIR filter has not been demonstrated to date, which is because of the interference between the optical signals of different taps from the different IIR filters [17]. In this Letter, we demonstrate that such interference can be avoided with wavelength conversion based on the cross-gain modulation of the ASE spectrum of a semiconductor optical amplifier (SOA) in one IIR filter. Also, owing to the Vernier effect, only those frequency components that can match both IIR filters can be filtered out, which significantly increases the free spectral range (FSR) and the Q value of the cascaded filter. A microwave photonic filter with a Q value of 3338 and a rejection ratio of about 40 dB is demonstrated. The experiment setup is schematically shown in Fig. 1. The cascaded filters are based on two active 0146-9592/10/081242-3/$15.00
loops, one with an erbium-doped fiber amplifier (EDFA) and one with an SOA. The optical source is a tunable laser diode centered at 1560 nm 共p兲, whose coherence length is far smaller than the lengths of the two active loops. The laser is externally modulated by a Mach–Zehnder modulator (MZM) driven by microwave signals from a vector network analyzer (VNA). Then the modulated optical signal is launched into the front active loop consisting of an EDFA (EDFA1), a tunable bandpass filter (TBPF), a 50:50 optical coupler (OC1) and an optical variable delay line (OVDL). An attenuator (ATT1) is used to control the input power of the front active loop. The tunable bandpass filter (BPF) centered at p with a 3 dB bandwidth of 0.3 nm is used to reduce the ASE noise of EDFA1. By properly controlling the input power, one can obtain a sharp bandpass filter response from the front active loop. The output power of the front active loop filter is further adjusted by an EDFA (EDFA2) and an attenuator (ATT2) before entering the back active loop, which consists of an SOA, an optical BPF centered at p + ⌬, a 50:50 optical coupler (OC2), and a 10:90 optical coupler (OC3). The ASE spectrum of the SOA is inversely modulated by the pump signal 共p兲 owing to the cross-gain modulation effect; therefore the modulation information at the pump wavelength p will be inversely copied into the whole ASE spectrum. The
Fig. 1. (Color online) Experiment setup. © 2010 Optical Society of America
April 15, 2010 / Vol. 35, No. 8 / OPTICS LETTERS
BPF with a 3 dB bandwidth of 1.2 nm is centered at 1557.8 nm, detuning from the pump wavelength by about 2.2 nm. The BPF here is used to extract the converted signal from the whole ASE spectrum. With OC3, 10% optical power is taken out from the back active loop, while the residual 90% power is kept in the amplified loop and delayed to obtain the subsequent recursive taps. At a given SOA current, by properly adjusting ATT2, one can obtain a high-Q frequency response for the converted signal in the back active loop. The output optical signal is detected with a photodetector (PD), and the transfer function of the filter is measured with the vector network analyzer. The interference between the signals of different taps from the cascaded IIR filter is avoided here through the wavelength conversion processed in the back IIR filter. In this way, we achieve stable transfer characteristics for the cascaded IIR filter. This is one of the main advantages of our proposed scheme. The FSR of the front IIR filter is designed to be different from that of the back one; thus the FSR of the cascaded filter is the least common multiple of that of each IIR filter. So one can significantly increase the FSR of the cascaded filter by properly choosing the FSR difference of the two IIR filters, which is another main advantage of our proposed scheme. The FSR of the front IIR filter can be adjusted by an OVDL in the front loop. With the Vernier effect, one can shift the peaks of the front IIR filter to match a desired peak of the back IIR filter to realize tunability. The measured frequency responses of the front IIR filter and the back filter are shown in Fig. 2(a). The frequency response of the cascaded filter is shown in Fig. 2(b). It can be seen that the FSR of the back IIR filter is exactly 31/ 8 times that of the front one. Therefore, only one peak of every 31 peaks of the front IIR filter can match one peak of every 8 peaks of the back IIR filter. For the cascaded filters, the matched peaks are selected and others are removed, so the FSR of the cascaded filter is increased, obviously. Owing to the cascaded structure, the peaks of the frequency response of the cascaded filter are sharpened along the leading and trailing edges. Thus the 3 dB bandwidth 共⌬f3 dB兲 of the cascaded filter is reduced. Because of the increase of the FSR and the decrease of ⌬f3 dB, the Q value of the cascaded filter is increased significantly. One of the matched peaks is enlarged and is shown in the inset of Fig. 2(b). The measured parameters for the front IIR filter, the back IIR filter, and the cascaded filter are shown in Table 1. To demonstrate the tunability of the cascaded filter, we change the loop length of the front active loop by adjusting the OVDL (the FSR and the position of the peaks of the front IIR filter will change accordingly). With the delay time of the OVDL set to 60 ps, the frequency responses of the front IIR filter and the back IIR filter are shown in the upper part of Fig. 3(a), and the frequency response of the cascaded filter is shown in lower part of Fig. 3(a). When the delay time of the OVDL increases to 150 ps, the peak of the cascaded filter shifts to the right by 14.08 MHz as shown in Fig. 3(b), which is exactly equal to the FSR
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Fig. 2. (Color online) (a) Compared frequency responses of the front IIR filter and the back IIR filter. (b) Frequency response of the cascaded filter.
of the back active loop. When the delay time of the OVDL further increases to 240 ps, the peak of the cascaded filter also shifts further to the right by 14.08 MHz as shown in Fig. 3(c). It is clearly seen that the cascaded filter can be tuned by adjusting the OVDL in the front active loop, which makes the proposed filter very flexible in practical applications. In summary, a novel microwave photonic filter consisting of two IIR filters has been proposed and demonstrated. In this structure, stable transmission characteristics are achieved by using wavelength conversion based on cross-gain modulation of the
Table 1. Measured Results of the Front IIR Filter, the Back IIR Filter, and the Cascaded Filter Filter
⌬f3dB (kHz)
FSR (MHz)
Q
Rejection (dB)
Front loop Back loop Cascaded
68.02 63.55 33.75
3.63 14.08 112.67
53.38 221.56 3338.37
28.06 44.53 40.26
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ASE spectrum of the SOA in the back loop. The FSR and the Q value are increased significantly compared with a single IIR filter in the cascade. The measured results demonstrate a high Q of 3338 and a rejection ratio of about 40 dB, and the tunability can be realized by adjusting the OVDL in the front loop filter. Theoretically, based on the scheme with Vernier effects, different Q values can be obtained without adding any new devices. Therefore, the Q value of our proposed filter can be further increased by properly optimizing the relevant parameters, especially the FSR difference of the two IIR filters. The filter scheme has the potential for being easily integrated. This work was supported by a grant from the National Basic Research Program of China (grant 2006CB302805) and the Program for New Century Excellent Talents in Ministry of Education of China (grant NCET-04-0715). References 1. J. Capmany, B. Ortega, and D. Pastor, J. Lightwave Technol. 24, 201 (2006). 2. R. Minasian, IEEE Trans. Microwave Theory Tech. 54, 832 (2006). 3. J. P. Yao, J. Lightwave Technol. 27, 314 (2009). 4. J. Capmany and D. Novak, Nat. Photonics 1, 319 (2007). 5. D. Pastor, B. Ortega, J. Capmany, S. Sales, A. Martinez, and P. Muñoz, Opt. Lett. 28, 1802 (2003). 6. W. Zhang, G. Yu, and J. A. R. Williams, Electron. Lett. 36, 1708 (2000). 7. J. Mora, S. Sales, M. D. Manzanedo, R. Garcia-Olcina, J. Capmany, B. Ortega, and D. Pastor, IEEE Photon. Technol. Lett. 18, 1594 (2006). 8. B. Vidal, V. Polo, J. L. Corral, and J. Marti, IEEE Photon. Technol. Lett. 18, 2272 (2006). 9. M. Popov, P. Y. Fonjallaz, and O. Gunnarsson, IEEE Photon. Technol. Lett. 17, 663 (2005). 10. D. B. Hunter and R. A. Minasian, in International Topical Meeting on Microwave Photonic (IEEE, 1996), pp. 273–276. 11. D. B. Hunter and R. A. Minasian, IEEE Trans. Microwave Theory Tech. 45, 1463 (1997). 12. L. N. Zhou, X. L. Zhang, E. M. Xu, and D. X. Huang, Acta Phys. Sin. 58, 1036 (2009). 13. E. M. Xu, X. L. Zhang, L. N. Zhou, Y. Zhang, and D. X. Huang, Chin. Phys. Lasers 26, 094208 (2009). 14. N. You and R. A. Minasian, IEEE Trans. Microwave Theory Tech. 47, 1304 (1999). 15. N. S. You and R. A. Minasian, Electron. Lett. 35, 2125 (1999). 16. B. Ortega, J. Mora, J. Capmany, D. Pastor, and R. Garcia-Olcina, Electron. Lett. 41, 1133 (2005). 17. J. Capmany, J. Lightwave Technol. 24, 2564 (2006). Fig. 3. (Color online) (a) Frequency response of the cascaded filter for an OVDL delay time of 60 ps, (b) increased to 150 ps, and (c) increased to 240 ps.
