A reconfigurable optoelectronic oscillator based on cascaded coherence-controllable recirculating delay lines

Xinkai Liu; Wei Pan; Xihua Zou; Bin Luo; Lianshan Yan; Bing Lu

doi:10.1364/OE.20.013296

1. Introduction

Optoelectronic oscillator (OEO) [1,2] has been extensively employed in the fields of the optical and microwave signal generation and processing [3–5], radar system and wireless communication [6,7], for the advantages of low phase noise, tunable frequency, high quality factor(Q), etc. The key part of a conventional OEO is a positive feedback loop which includes an optical source, an electro-optic modulator (EOM), a photodetector (PD), an electrical amplifier (EA), and an electrical band pass filter (EBPF). Generally, in order to get a lower phase noise, a longer delay line is needed to increase the Q value of the OEO. Moreover, an EBPF is usually required to suppress the side modes [8]. But an EBPF with a high Q value and a large tunable range is difficult to be manufactured.

Recently, photonic microwave filters (PMFs) have been regarded as a promising solution for the mode selection in OEOs, which are characterized by wide instantaneous bandwidth, low loss, and immunity to electromagnetic interference. Generally, large tunable range and high Q value are two key parameters of PMF in OEOs. Thus several approaches have been reported to meet this requirement. For instance, a microwave signal with tunable frequency is generated using a spectrum sliced PMF [9]. As the number of taps depends on the number of spectrum slices, a high Q value is obtained to ensure a generated microwave signal low phase noise. Another method to improve the Q value is the use of a two-port optical phase modulator (PM) which provides one more tap for PMF [10]. The Q value can also be further improved by cascading infinite impulse response (IIR) filters [11,12]. However, the optical wavelength shift is essentially required to avoid the self-interference of each RDL and the mutual interference between two RDLs, which makes the system complex.

In this paper, we propose a novel OEO based on cascaded IIR PMFs for the generation of microwave signals. In the proposed OEO, no electronic microwave ðlter and no optical wavelength shift are needed, resulting in a simple reconfigurable operation. Moreover the Q factor of the OEO can be up to million with a short fiber. The significant novelty lies in the combination of an amplified spontaneous emission (ASE) source and two cascaded RDLs. Since the coherence time of the ASE source is much smaller than the delay of the RDLs [13], the self-interference of each RDL can be neglected or removed. The use of two RDLs with approximately equal gain can maintain a stable spectral response of the cascaded IIR PMFs, although the mutual interference between two RDLs exists. Therefore, the FSR of the cascaded structure can be considered as the least common multiple of two IIR filters’ FSRs. In this way, a microwave signal, whose frequency corresponds to the least common multiple of the two PMFs’ FSRs, can be generated from the proposed OEO. In addition, the oscillating frequency can be tuned and even could be extended to millimeter-wave band by changing the lengths of the RDLs [14,15].

2. Experimental setup and theoretical model

The schematic diagram of the proposed OEO is shown in Fig. 1 . The light wave from an ASE source is modulated by the oscillating signal at a Mach-Zehnder modulator (MZM) that is biased at the quadrature transmission point. After being amplified by an erbium doped fiber amplifier (EDFA), the modulated light wave is sent to two RDLs, i.e., two cascaded IIR filters. Finally a PD is connected to convert the light wave into microwave signals, part of which is fed back to the RF port of the MZM. Here an electronic amplifier (EA) provides suitable gain to keep oscillating in the loop.

Fig. 1 Schematic diagram of the proposed reconfigurable OEO. ASE: amplified spontaneous emission source diode; PC: polarization controller; MZM: Mach-Zehnder modulator; EDFA: erbium doped fiber amplifier; OC: optical coupler; RDL: recirculating delay line; PD: photodetector; EA: electronic amplifier.

Download Full Size | PDF

Firstly, the case without feedback is taken into account. The MZM is driven by a microwave signal at the angular frequency of $Ω$ . Thus the open-loop response of the proposed OEO can be calculated. As the MZM is quadraturely biased, the output optical intensity, $P_{o u t} (t)$ , of the MZM can be expressed as

P_{o u t} (t) \propto V_{b i a s} + V_{R F} \cos (Ω t)

where

V_{b i a s}

is the direct-current bias voltage applied to the MZM, and

V_{R F} \cos (Ω t)

is the applied microwave signal. It can be seen that the intensity at the output of the MZM is proportional to the intensity of the RF signal. The overall impulse response of the cascaded RDLs is given by

\begin{array}{l} H (t) = \sum_{m = 0}^{\infty} \frac{1}{2} h^{m} δ (t - m τ_{1}) * \sum_{n = 0}^{\infty} \frac{1}{2} g^{n} δ (t - n τ_{2}) \\ = \frac{1}{4} \sum_{m = 0}^{\infty} \sum_{n = 0}^{\infty} h^{m} g^{n} δ [t - (m τ_{1} + n τ_{2})] \\ τ_{i} = \frac{1}{F S R_{i}} = \frac{L_{i} \times n}{c} (i = 1, 2) \end{array}

where

1 / 2

is coupling coefficient of the two

2 \times 2

couplers,

δ (t)

is the Dirac delta function,

h

and

g

are the net gain of the two RDLs,

τ_{i}

and

L_{i}

are the time delays and the lengths of the two RDLs,

n

is the refractive index of the RDLs, and

c

is speed of light in vacuum.

Owing to the use of the ASE source, self-interference caused by each RDL can be neglected and associated details are shown as follows. When the condition $h = g = 1 / 2$ holds, the impulse response of the cascaded PMFs can be expressed as [13,16]

H (t) \propto \frac{1}{4} \sum_{k = 0}^{\infty} {(k + 1)}^{2} {(\frac{1}{2})}^{2 k} δ [t - x (k) τ_{1} τ_{2}]

where

k = m + n

and

x (k) = (m / τ_{2} + n / τ_{1})

. Compared to the standard expression of the IIR filter’s impulse response, the increased term

{(k + 1)}^{2}

can be regarded as the additional taps of the cascaded IIR PMFs.

The overall spectral response of the generated microwave signal can be expressed as

\begin{array}{l} I_{0} (f) \propto \frac{π}{4} ℜ G P_{0} V_{R F} {δ [(ω - Ω) t] + δ [(ω + Ω) t]} \\ \times \sum_{k = 0}^{\infty} {\frac{(2 k + 1) \times {(\frac{1}{2})}^{2 k} \exp [- k j ω (τ_{1} + τ_{2})]}{[1 - \frac{1}{2} \exp (- j ω τ_{1})] [1 - \frac{1}{2} \exp (- j ω τ_{2})]}} \end{array}

where

ℜ

is responsivity of the PD,

G

is the gain of the EDFA, and

P_{0}

is the peak of optical intensity.

When the cascaded IIR PMFs are inserted into the OEO, specified oscillating modes which match the FSR of the cascaded IIR PMFs are selected. By controlling the bandwidth of the PD or the EA, only the first-order oscillating frequency can be reserved. Also, from (4), self-interference has been avoided, and mutual interference has been used to form new taps of PMFs. In addition, the frequency of the generated microwave signal is determined by the least common multiple of the two IIR filters’ FSRs. Therefore, a high Q value is obtained, compared to that of the OEO using a single IIR filter. Meanwhile, the frequency of the microwave signal generated from the OEO can be tuned by changing the lengths of the two RDLs.

3. Experimental results

To verify the effectiveness of the proposed approach, an experiment is carried out based on the system shown in Fig. 1. In the experiment, the total length of the OEO loop is about 130 meter and the resulting FSR is 1.65MHz. An ASE source with the output power level at 10dBm and a MZM biased at the quadrature transmission point are adopted. The cascaded IIR PMFs are realized by using two RDLs which are fabricated via two $2 \times 2$ 3-dB couplers with a coupling coefficient 50%, and the loss in each RDL is estimated as 3.1-dB. A PD is then used to perform the opto-electronic conversion and an EA is adopted to amplify the generated microwave signal. After passing through a power divider, part of the microwave signal is fed back to MZM and the other is sent to electrical spectrum analyzer (ESA).

The experiment results are shown in Fig. 2 . Firstly, a single RDL is inserted into the OEO loop. The resulting mode spacing among the generated microwave signals is 97.4MHz or 65.6MHz, when the length of the RDL is specified as 2.20m or 3.27m. It can be seen that the ratio of the two FSRs is close to $3 : 2$ . When the two RDLs are connected to form the cascaded PMFs, only the matched modes can be selected while others are suppressed. Therefore, oscillating modes at frequencies located at the least common multiple of the two FSRs of the two PMFs, are generated. For instance, an oscillating frequency at 194.1MHz is achieved which is the least common multiple of the two FSRs of 97.4MHz and 65.6MHz. As the 3-dB bandwidth of the cascaded PMFs is very narrow and the overall FSR can be further increased via the cascaded configuration, a lower phase noise is ensured for the generated microwave signal.

Fig. 2 Principle illustration of principle for the proposed OEO. The FSR of two cascaded RDLs is the least common multiple of their FSRs.

Download Full Size | PDF

The reconfigurable frequency of the proposed OEO is also investigated. In our experiment, the OEO is reconfigured by changing different fiber loops. The reconfiguration can be also implemented quickly via thermo-optic or electro-optic effect based length tuning over small step. When the lengths of the two RDLs are adjusted, the microwave signals at higher frequency are generated. In Figs. 3(a) and 3(b), two electrical spectra of the generated microwave signals are shown. The corresponding mode spacings are 218.7MHz and 129.9MHz respectively, with a ratio close to $5 : 3$ .While for the cascaded structure of the two RDLs above, the frequency of the generated signal is 648.5MHz and its power level is 40dB higher than that of the second harmonic components, as shown in Fig. 3(c). Figure 4(c) shows several spectra of the generated microwave signals by reconfiguring cascaded RDLs. Furthermore, another microwave signal at 2.99GHz is generated by changing the lengths of the two RDLs, as shown in Fig. 4(a). To clarify the quality of the generated signals, the phase noise of the signal at 2.99GHz is shown in Fig. 4 (b). The measured values are −95.5826 dBc/Hz at 10-kHz offset frequency and −110.1033 dBc/Hz at 100-kHz offset frequency, which are comparable to the phase noise obtained using laser source. Consequently, the experimental results shown in Figs. 3(c), 4(a) and 4(c) are in accordance with the theoretical prediction that the generated signal has a frequency corresponding to the least common multiples of the two FSRs.

Fig. 3 Spectra of the generated microwave signals: (a) RDL having a length of 0.98m; (b) RDL having a length of 1.65m; (c) cascaded IIR PMFs. (RBW: resolution bandwidth)

Download Full Size | PDF

Fig. 4 (a) Spectrum and (b) phase noise of the generated microwave signal at 2.99 GHz in the experiment. (c) Spectra of the generated microwave signals with different cascaded RDLs. (d) Spectrum of the generated microwave signal at 40GHz in the simulation.

Download Full Size | PDF

Next, the stability of the generated signal is investigated. During a period of half an hour, no frequency drift are observed from the figures of the ESA, showing a stable oscillation frequency. In addition, the proposed approach would be able to generate microwave signals with a frequency up to tens of GHz while two compact RDLs are used, such as micro-ring filters. For instance, a microwave signal at 40GHz is generated in a simulation experiment when two micro-rings, which are 4.286cm and a 2.679cm, in length are employed, as shown in Fig. 4(d).

4. Conclusions

In conclusion, a novel OEO have been proposed and experimentally demonstrated. The key novelty of the proposed OEO was the combination of an ASE source and two cascaded RDLs which perform mode selection, instead of an electronic microwave ðlter. Moreover, without using optical wavelength shift, self-interference has also been avoided and mutual interference can be effective controlled in the proposed OEO, with the OEO structure greatly simplified. As the structure of RDLs can be reconfigurable, the oscillating frequency can be tuned simply. Experiments were performed to verify the effectiveness of the proposed approach, with microwave signals at 194.1MHz, 648.5MHz, and 2.99GHz generated. Meanwhile, the low phase noise and the frequency stability of the generated signals were also experimentally demonstrated. The RDLs can be further reduced to generate microwave signals at higher frequency.

Acknowledgments

The work was supported by Research Fund for the Doctoral Program of Higher Education of China (20110184130003,20100184120007), National Natural Science Foundation of China (61101053), and “973” Project (2012CB315704), Fok Ying-Tong Education Foundation for Young Teachers (132033), and Young Innovative Research Team of Sichuan Province (2011JTD0007).

References and links

1. X. S. Yao and L. Maleki, “Optoelectronic microwave oscillator,” J. Opt. Soc. Am. B 13(8), 1725–1735 (1996). [CrossRef]

2. X. S. Yao and L. Maleki, “Multiloop optoelectronic oscillator,” IEEE J. Quantum Electron. 36(1), 79–84 (2000). [CrossRef]

3. M. Shin and P. Kumar, “Optical microwave frequency up-conversion via a frequency-doubling optoelectronic oscillator,” IEEE Photon. Technol. Lett. 19(21), 1726–1728 (2007). [CrossRef]

4. S. L. Pan and J. P. Yao, “Optical clock recovery using a polarization-modulator-based frequency-doubling optoelectronic oscillator,” J. Lightwave Technol. 27(16), 3531–3539 (2009). [CrossRef]

5. L. X. Wang, N. H. Zhu, W. Li, and J. G. Liu, “A frequency-doubling optoelectronic oscillator based on a dual-parallel Mach–Zehnder modulator and a chirped fiber bragg grating,” IEEE Photon. Technol. Lett. 23(22), 1688–1690 (2011). [CrossRef]

6. L. Huo, Y. Dong, C. Y. Lou, and Y. Z. Gao, “Clock extraction using an optoelectronic oscillator from high-speed NRZ signal and NRZ-to-RZ format transformation,” IEEE Photon. Technol. Lett. 15(7), 981–983 (2003). [CrossRef]

7. H. Tsuchida and M. Suzuki, “40-Gb/s optical clock recovery using an injection-locked optoelectronic oscillator,” IEEE Photon. Technol. Lett. 17(1), 211–213 (2005). [CrossRef]

8. T. Sakamoto, T. Kawanishi, and M. Izutsu, “Optoelectronic oscillator employing reciprocating optical modulator for millimetre-wave generation,” Electron. Lett. 43(19), 1031–1033 (2007). [CrossRef]

9. M. Li, W. Z. Li, and J. P. Yao, “A tunable optoelectronic oscillator based on a high-Q spectrum sliced photonic microwave transversal filter,” Proc. MWP 2011, 304–307 (2011).

10. W. Z. Li and J. P. Yao, “An optically tunable optoelectronic oscillator,” J. Lightwave Technol. 28(18), 2640–2645 (2010). [CrossRef]

11. E. M. Xu, X. L. Zhang, L. N. Zhou, Y. Zhang, Y. Yu, X. Li, and D. X. Huang, “Ultrahigh-Q microwave photonic filter with Vernier effect and wavelength conversion in a cascaded pair of active loops,” Opt. Lett. 35(8), 1242–1244 (2010). [CrossRef] [PubMed]

12. E. H. W. Chan, “High-order inðnite impulse response microwave photonic filters,” J. Lightwave Technol. 29(12), 1775–1782 (2011). [CrossRef]

13. J. Capmany, B. Ortega, and D. Pastor, “A tutorial on microwave photonic filters,” J. Lightwave Technol. 24(1), 201–229 (2006). [CrossRef]

14. P. Rabiei, W. H. Steier, C. Zhang, and L. R. Dalton, “Polymer micro-ring filters and modulators,” J. Lightwave Technol. 20(11), 1968–1975 (2002). [CrossRef]

15. C. Caspar and E. J. Bachus, “Fiber-optic micro-ring-resonator with 2mm diameter,” Electron. Lett. 25(22), 1506–1508 (1989). [CrossRef]

16. J. Capmany, “On the cascade of incoherent discrete-time microwave photonic filters,” J. Lightwave Technol. 24(7), 2564–2578 (2006). [CrossRef]

A reconfigurable optoelectronic oscillator based on cascaded coherence-controllable recirculating delay lines

Abstract

1. Introduction

2. Experimental setup and theoretical model

3. Experimental results

4. Conclusions

Acknowledgments

References and links

Cited By

Figures (4)

Equations (4)

Optics Express