Switchable and tunable microwave frequency multiplication based on a dual-passband microwave photonic filter

Hao Chen; Zuowei Xu; Hongyan Fu; Shiwei Zhang; Congxian Wu; Hao Wu; Huiying Xu; Zhiping Cai

doi:10.1364/OE.23.009835

1. Introduction

Photonic generation and processing of microwave signals have been under constant research in the last two decades. Compared with the traditional approaches which are limited by the “electronic bottleneck” and other sources of degradation such as electromagnetic interference or frequency dependent losses, photonic techniques for microwave signal generation and processing have many advantages inherent to optical fiber, such as large bandwidth, low loss, the immunity to the electromagnetic interference, and so on [1,2]. Many techniques have been reported to realize microwave signal generation with photonic methods, among which the technique of external modulation for frequency multiplication enjoys good tunability, low loss and phase noise, thus becoming one of the most important methods for generating microwave signals with high frequency [3–6]. With external modulation method, normally a low frequency microwave signal is modulated onto the optical carrier to generate high order harmonics, and filters are adopted to select the harmonics with desirable frequencies [7–9]. Microwave photonic filter (MPF) is a new type of microwave filter that has attracted much research attention recently. Many techniques have been proposed to realize functional MPFs, among which MPFs based on continuous sampling of a broadband optical source and a dispersive medium have been studied extensively, since its free spectrum range is infinite in the theory, and thus resulting in a single-passband frequency response [10–12]. Single-passband MPFs with designed sliced spectrum have been proposed, and the relationship between the sliced spectra and the frequency response of MPF have been studied, as well as the realization of a tunable MPF with flat-top frequency response [13]. Although the single-passband MPF has shown good application potential, in many practical applications, multi-passband MPFs with freely tunable central frequencies are more desirable [14].

In this paper, a tunable and switchable dual-passband MPF based on the slicing spectrum after a modified FMZI and a dispersive medium has been proposed and demonstrated experimentally. By combining the MPF with the external modulation technology, switchable and tunable microwave frequency multiplication has been realized. In the experiment, tunable microwave harmonics (double, triple times of the driving signal, and quadruple, sextuple of the driving signal) can be filtered out and switched between the two frequencies and dual frequency states. The proposed tunable and switchable MPF and microwave signal multiplication techniques show good applications in the fiber wireless communication and measurement systems.

2. Operating principle and experiment setup

The schematic diagram of the proposed switchable and tunable microwave frequency multiplication technique is shown in Fig. 1. The light from the BOS (QPHOTONICS QSDM-1550-15) is launched into the spectrum slicer after the ISO. The sliced light goes into the MZM (JDSU, 20GHz) via which a driving microwave signal with low frequency is modulated onto the lightwave to generate high order harmonics. Then, the light is coupled into a dispersive medium, which in our experiment is a coil of 25 km standard single mode fiber (SMF). After the dispersive fiber, the light is amplified by an EDFA (Amonics, AEDFA-PA-25) and sent to a PD (Optilab PD-30, 60 kHz~30 GHz), and then measured by the ESA (HP8593E, 9 k~22 GHz).

Fig. 1 Schematic diagram of the proposed switchable and tunable microwave frequency; broadband optical source: BOS; optical isolator: ISO; Mach-Zehnder modulator: MZM; photodiode: PD; electrical spectrum analyzer: ESA; optical coupler: OC; polarization controller: PC; variable optical delay line: VODL.

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The spectrum slicer in our experiment shown in the inset of Fig. 1 is a modified dual-pass FMZI, and consists of a 50:50 and a 70:30 OCs with a polarization controller (PC2) in one arm of the FMZI in order to achieve the best extinction ratio of the sliced comb spectrum, and a VODL (General Photonics MDL-002, 0~560 ps, accuracy-3 µm) in the other arm of the FMZI to tune the wavelength spacing of the spectrum slicer. One input port of the 50:50 OC is taken as the input port of the FMZI and connected to PC1, and the other input port of the 50:50 OC is taken as the output port of the FMZI. The transmission function of the spectrum slicer can be expressed as [15],

\begin{array}{l} T_{F M Z I} = {| \frac{E_{2 o u t}}{E_{1 i n}} |}^{2} = 2 a_{2} (1 - a_{2}) + {(1 - 2 a_{2})}^{2} \sin^{2} θ \sin^{2} \frac{δ}{2} \\ + 2 a_{2} (1 - a_{2}) [(\cos^{2} θ - \sin^{2} θ \cos δ) \cos 2 φ + \sin θ \sin (2 α + θ) \sin δ \sin 2 φ] \\ - 2 (1 - 2 a_{2}) \sqrt{a_{2} (1 - a_{2})} \sin^{2} θ \times \cos (2 α + θ) \sin δ \sin φ \end{array}

where

δ = 2 k (n_{x} - n_{y}) L

, n_x and n_y are the indices of the two axes of the fiber, k is the wave number, and L is the length of shorter arm of the FMZI, respectively. α and θ are the polarization angle of the input light and the rotation angle of the propagating light through the PC2, respectively; φ is the phase difference between the two arms. By carefully adjusting α and θ (PC1 and PC2 in the experiment), the wavelength spacing of the transmission spectrum can be switched from

Δ λ = λ^{2} / n Δ L

to

Δ λ = 2 λ^{2} / n Δ L

and dual-period state (

n Δ L

is the optical path difference of the two arms), which can be observed from the calculated transmission spectrum of the modified FMZI in Fig. 2. The spectrum depicted in Fig. 2(b) shows the dual-period state, while the spectra in Fig. 2(a) and 2(c) show the sliced comb spectrum with only one period, while the spectrum component of the other period is suppressed by adjusting two PCs in the modified FMZI.

Fig. 2 Calculated transmission spectra of the modified FMZI by adjusting PC1 and PC2.

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The light from the spectrum slicer, MZM and the dispersive medium actually compose of an MPF, whose frequency response can be expressed as [11],

H_{R F} (f) = \int S (ω) \cdot T_{F M Z I} (ω) [m_{1} H^{*} (ω) H (ω - 2 π f) - m_{2} H (ω) H^{*} (ω + 2 π f)] d ω

where

S (ω)

is the spectrum of the BOS,

T_{F M Z I} (ω)

is the transmission of the spectrum slicer.

H (ω) = | H (ω) | \cdot e^{- j Φ (ω)}

is the transfer function of the fiber link. The calculated frequency responses of the switchable and tunable MPF with the transmission spectra of the spectrum slicer shown as Fig. 2 are depicted in Fig. 3. One can see that by carefully adjusting the polarization state of the input light and the rotation angle of light, MPFs with dual passbands (Fig. 3(a)) and switchable single passband can be obtained (Fig. 3(b)). One can also observe that the amplitude of the second passband is not higher than that of the first one, and that is because that the polarization state of the input light and rotation angle of the light have not been optimized in our simulation. However, in this case dual-passband operation can still be achieved.

Fig. 3 Calculated frequency response of the switchable MPF.

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The central frequency of the passbands can be described as Eq. (3), where $L$ is the length of the SMF, and $Δ ω$ is given by the optical path difference $n Δ L$ as $Δ ω = 2 π c / n Δ L$ . One can find that the central frequency of the passband is related to the length of the SMF and the optical path difference of the FMZI, thus by adjusting the length of the VODL in one arm of the FMZI or changing the length of the SMF, the central frequency of the passbands can be tuned.

Ω = 2 π f_{0} = \frac{2 π}{β \cdot L \cdot Δ ω}

The microwave signal multiplication principle is shown as Fig. 4. The generated harmonics by the external modulation of the optical source can be filtered out by the tunable and switchable MPF. By adjusting PCs in the modified FMZI, switchable microwave frequency multiplication can be obtained, and by tuning the length of the VODL while changing the frequency of the driving microwave signal, tunable microwave frequency multiplication can also be achieved.

Fig. 4 Operation principle of the proposed microwave signal multiplication.

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3. Experimental results and discussions

In the experiment, by properly adjusting the PC1 and PC2, switchable MPF can be obtained, whose frequency responses when the length of VODL setting at 4.25 mm are shown as Figs. 5(a)–5(c) while the corresponding transmission spectra of the spectrum slicer are shown in Figs. 5(d)–5(f), respectively. The network analyzer (TRANSCOM T5230A) has been utilized to measure the frequency response with its port 1 connected to the RF input of the MZM and port 2 connected to the output of PD. The experimental results show good agreements with the theoretical prediction shown in Fig. 2 and Fig. 3, except for the central frequencies of the passbands are different as we have set different parameters for the dispersive filter and FMZI in the simulation. One can also see from Fig. 5 that, dual-passband state of 1 GHz and 2 GHz, single passband of 1 GHz or 2 GHz with the suppression ratio of more than 30 dB for the MPF can be obtained by simply tuning the PCs in the modified FMZI. One can see from Fig. 5(c) that, the second passband has not been totally suppressed in the experiment. It is because that apart from the polarization state of the input light and the rotation angle of the propagating light, to totally suppress the second passband the optical path difference needs to be optimized at a precise value, and due to the limitation of our devices there is a residual at the frequency of the second passband.

Fig. 5 Frequency response of the MPF (a)(b)(c) and the corresponding transmission spectra of the spectrum slicer (d)(e)(f).

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By tuning the length of the VODL in one arm of the FMZI, the wavelength spacing of the spectrum slicer can be varied, and thus the central frequencies of both passbands can be tunable. The measured tunable frequency responses of the MPF for the dual-passband state are shown as Fig. 6(a). The relationship between the central frequencies of both passbands and the length of VODL is shown in Fig. 6(b), which shows a good tuning linearity. One can see that there is a baseband resonance for both simulated and experiment results, and it is because we have used an MZM, which is an intensity modulator in the experiment. To eliminate the baseband resonance, phase modulator can be adopted instead of the MZM [12]. One can observe that the bandwidth of the second passband (higher frequency) is larger than that of the first passband (lower frequency). It is mainly caused by the dispersion characteristics of the dispersive medium, could be optimized by carefully designing the dispersion parameters of the dispersive fiber for the practical applications. One can also see that the slopes of two tuning curves are different, and it is because the central frequencies of the two passbands is determined by the wavelength spacing of the spectra components after the spectrum slicer, which for the first passband is twice as that of the second. When we tune the MPF by tuning the VODL, and thus the wavelength spacing of the spectrum slicer, the changes of the central frequencies of the MPF are different.

Fig. 6 Measured frequency responses of the MPF for the dual-passband state with different length of VODL (a) and the relationship between the central frequencies and the length of VODL (b).

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There is some distortion on both passbands of the MPF as can be seen from Fig. 6(a), and it is caused by the uneven gain spectrum of the EDFA, which can be solved by employing an equalized EDFA. The power and frequency stability of the MPF has been measured when it is tuned to the dual-passband state with central frequency of one passband locating at ~1.1 GHz while the other is around 2.2 GHz. In the experiment, we measure the power and the frequency of the peak of both passbands every 5 minutes for over one hour, and the experimental results are shown in Fig. 7(a) and 7(b), respectively. One can see from Fig. 7 that, both passbands show good power and frequency stability.

Fig. 7 Measured power and frequency stability for the proposed MPF.

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When applying the driving microwave signal onto the RF input of MZM, switchable microwave frequency multiplication can be achieved by adjusting PC1 and PC2, and tunable frequency multiplication can be implemented by tuning the frequency of the driving frequencies and the length of the VODL. In the experiment, driving microwave signal with the frequency of 1 GHz, 1.5 GHz, 2 GHz, 2.5 GHz, and 3 GHz have been applied onto the MZM, respectively. Figure 8 shows the measured microwave signal with doubled and quadruple frequencies of the driving signal. The left, middle and right column of Fig. 8 show the spectra of direct detection after the modulation, selecting the second and the fourth harmonics, and switching between the second and the fourth harmonics, respectively. One can see that, by adjusting the PCs and tuning the length of VODL, microwave signal with doubled and quadruple frequencies can be obtained and freely switched, while other harmonics are well suppressed.

Fig. 8 Measured microwave frequency multiplication with the driving signal at 1 GHz, 1.5 GHz, 2 GHz, 2.5 GHz, 3 GHz, respectively.

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Higher order harmonics can also been selected just by tuning the length of the VODL. In our experiment, triple and sextuple microwave signal can be achieved and freely switched, as shown in Fig. 9. When the driving microwave signal with the frequency of 2.5 GHz has been applied onto the MZM, the third and sixth harmonics at 7.5 GHz and 15 GHz have been filtered out and switched between dual frequency state (left) and two single frequencies (right). In the experiment, by simply changing the length of VODL from 6.10 mm to 7.2 mm, the frequencies of the generated microwave signal can be switched from double and quadruple of the driving signal to triple and sextuple of the driving signal, which is very easy to implement and shows good tunability and stability. One can see from Fig. 9 that, for triple and sextuple signal generation, there is ~15dB power difference between two generated harmonics. It is because that, external modulation have been used to generate harmonics, and the power of the generated harmonics will be affected by the power of the driving signal and as well as the bias on the MZM. Normally the power of the higher order harmonics will be lower than that of the lower harmonics. It only affects the power of the generated signals, but not other performances. To solve this problem, techniques such as ring modulation technique or cascaded modulators with carefully adjusted bias can be employed to balance the power of generated harmonics in a certain extent, as well as the power performance of the generated signals.

Fig. 9 Measured tripled and sextuple microwave signal generation when the driving microwave signal with 2.5 GHz is applied.

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4. Conclusions

In this article, we have proposed and experimentally demonstrated a new approach to implementing microwave frequency multiplication by using external modulation and a tunable and switchable MPF. The MPF is based on the sliced broadband optical source with a modified FMZI and a dispersive medium, and exhibits tunable and switchable dual-passband frequency responses. The MPF is theoretically analyzed and implemented in the experiment, which show good agreement. By incorporating the MPF with the external modulation, switchable and tunable microwave frequency multiplication can be achieved. By properly adjusting the polarization state by two PCs in the modified FMZI, high order harmonics with dual frequencies can be filtered out and switched freely between these two frequencies and dual-frequency state. In our experiment, we have generated multiplication signals with double and quadruple frequencies, and by simply changing the length of VODL, microwave signal with the frequency of triple and sextuple of the driving signal can be obtained. Also, by tuning the length of the VODL and the frequency of the driving signal, the microwave signal multiplication can be tuned freely. With further optimizing the system, even higher harmonics with higher frequencies and quality can be achieved. The proposed microwave signal multiplication technique exhibits the advantages of good tenability, stability and flexibility, and the ease to implement, thus shows good application potential in the fiber wireless communication and measurement systems, e.g. generating one or two switchable carrier frequencies in the fiber wireless communication or sensing systems, especially for the signal generation with high frequency. The proposed technique enjoys the advantage of large bandwidth, low loss, light weight and the immunity to the electromagnetic interference, and can overcome the “electronic bottleneck” compared with traditional electrical methods.

Acknowledgments

This work was supported in part by National Natural Science Foundation of China (No. 61205059, No. 61107023, and No. 61107045), Fundamental Research Funds for the Central Universities of Xiamen University (Grant No. 2010121059), and Ph.D. Programs Foundation of Ministry of Education of China (No. 20120121120037, No. 20110121120020).

References and links

1. J. Capmany and D. Novak, “Microwave photonics combines two worlds,” Nat. Photonics 1(6), 319–330 (2007). [CrossRef]

2. J. Yao, “Microwave photonics,” J. Lightwave Technol. 27(3), 314–335 (2009). [CrossRef]

3. J. Yu, Z. Jia, L. Yi, Y. Su, G. K. Chang, and T. Wang, “Optical millimeter-wave generation or up-conversion using external modulators,” IEEE Photon. Technol. Lett. 18(1), 265–267 (2006). [CrossRef]

4. Z. Jia, J. Yu, G. Ellinas, and G. K. Chang, “Key enabling technologies for optical–wireless networks: optical millimeter-wave generation, wavelength reuse, and architecture,” J. Lightwave Technol. 25(11), 3452–3471 (2007). [CrossRef]

5. W. Li and J. Yao, “Investigation of photonically assisted microwave frequency multiplication based on external modulation,” IEEE Trans. Microw. Theory Tech. 58(11), 3259–3268 (2010). [CrossRef]

6. B. Vidal, A. Bockelt, and J. Palaci, “Cascaded four-wave mixing for microwave photonic harmonic multiplication,” IEEE Photon. Technol. Lett. 25(1), 100–103 (2013). [CrossRef]

7. H. Ou, C. Ye, K. Zhu, Y. Hu, and H. Fu, “Millimeter-wave harmonic signal generation and distribution using a tunable single-resonance microwave photonic filter,” J. Lightwave Technol. 28(16), 2337–2342 (2010). [CrossRef]

8. G. J. Schneider, J. A. Murakowski, C. A. Schuetz, S. Shi, and D. W. Prather, “Radiofrequency signal-generation system with over seven octaves of continuous tuning,” Nat. Photonics 7(2), 118–122 (2013). [CrossRef]

9. L. Gao, W. Liu, X. Chen, and J. Yao, “Photonic-assisted microwave frequency multiplication with a tunable multiplication factor,” Opt. Lett. 38(21), 4487–4490 (2013). [CrossRef] [PubMed]

10. J. Capmany, J. Mora, I. Gasulla, J. Sancho, J. Lloret, and S. Sales, “Microwave photonic signal processing,” J. Lightwave Technol. 31(4), 571–586 (2013). [CrossRef]

11. J. Mora, B. Ortega, A. Díez, J. L. Cruz, M. V. Andrés, J. Capmany, and D. Pastor, “Photonic microwave tunable single-bandpass filter based on a Mach-Zehnder interferometer,” J. Lightwave Technol. 24(7), 2500–2509 (2006). [CrossRef]

12. H. Fu, K. Zhu, H. Ou, and S. He, “A tunable single-passband microwave photonic filter with positive and negative taps using a fiber Mach–Zehnder interferometer and phase modulation,” Opt. Laser Technol. 42(1), 81–84 (2010). [CrossRef]

13. T. X. H. Huang, X. Yi, and R. A. Minasian, “Single passband microwave photonic filter using continuous-time impulse response,” Opt. Express 19(7), 6231–6242 (2011). [CrossRef] [PubMed]

14. Y. Jiang, P. P. Shum, P. Zu, J. Zhou, G. Bai, J. Xu, Z. Zhou, H. Li, and S. Wang, “A Selectable Multiband Bandpass Microwave Photonic Filter,” IEEE Photon. J. 5(3), 5500509 (2013). [CrossRef]

15. A. P. Luo, Z. C. Luo, and W. C. Xu, “Tunable and switchable multiwavelength erbium-doped fiber ring laser based on a modified dual-pass Mach-Zehnder interferometer,” Opt. Lett. 34(14), 2135–2137 (2009). [CrossRef] [PubMed]

Switchable and tunable microwave frequency multiplication based on a dual-passband microwave photonic filter

Abstract

1. Introduction

2. Operating principle and experiment setup

3. Experimental results and discussions

4. Conclusions

Acknowledgments

References and links

Cited By

Figures (9)

Equations (3)

Optics Express