## Abstract

A tunable dual-frequency optoelectronic oscillator (OEO) based on dual-parallel Mach-Zehnder modulator (DPMZM) with low intermodulation is proposed and experimentally demonstrated. Two tunable electronic band-pass filters are used to select the oscillation modes of the two oscillating frequencies to achieve tunable dual-frequency OEO (TDF-OEO). Each of the two selected modes are modulated onto one sub-modulator of the DPMZM. By choosing proper bias conditions of the DPMZM, the intermodulation components generated by the dual-frequencies are suppressed. The tuning range of the TDF-OEO is from 1 GHz to 15 GHz in the experiment. We also discussed and experimentally demonstrated the tuning limitation of this dual frequency system.

© 2016 Optical Society of America

## 1. Introduction

Optoelectronic oscillator (OEO) has attracted many attentions in numerous field due to its high spectral purity and low phase noise [1]. Numerous researchers were always working on its improvement and applications, for instance, optical clock recovery based on OEO [2], self-oscillating optical frequency comb generator based on an OEO employing cascaded modulators [3], the injection locking method for single mode operation [4], and so on [5–10]. Thanks to their outstanding achievements mentioned above, OEO would have broad prospects of application in both photonics and microwave technology.

Meanwhile, due to the fast development of wireless radio communication in recent years, multi-frequency signal generators (MFSGs) have been widely used [11,12]. As one kind of MFSGs, dual-frequency generator sees is applications in radar system, global positioning system (GPS), wireless local area network (WLAN) system, Bluetooth, and so on. A basic requirements of dual radio-frequency (RF) system is its lower phase noise at high frequency band, which is helpful to reduce radio signal distortion and enhance the efficiency of weak signal detection [11,13,14]. Currently, electronic signal with the high frequency and wide bandwidth is typically obtained by multiplying a narrow baseband signal with a low-frequency reference together with several stages of frequency doubling to the required frequency and bandwidth [16], which deteriorates phase noise greatly, and brings about nonlinear distortion to RF signal as well [15]. Therefore, an OEO that can simultaneously generate two frequencies of signal with low phase noise is extremely demanded by RF systems mentioned above.

Many researchers have proposed their dual-frequency OEO systems. Yao’s group has proposed a dual-frequency OEO structure, where a phase-shifted fiber Bragg grating (PS-FBG) was applied to create two orthogonally polarized notches to generate two oscillating frequencies [17]. Jiang’s system used two filters to obtain two frequencies, with the phase noise measured to be −108 dBc/Hz and −113 dBc/Hz@10kHz at 20 GHz and 9 GHz, respectively [18]. Pan’s group also demonstrated a structure of multi-frequency OEO system, which used a multi-channel optical notch filter to generate U-band signal. The phase noise was −100.62 dBc/Hz and −84.64 dBc/Hz @ 10kHz offset at 10 GHz and 40 GHz, respectively [19]. Zhang got a triangular pulse train with two frequency 4.44- and 13.32-GHz generated by using a polarization multiplexed optoelectronic oscillator, and the two tones have phase noises of −100.6 dBc/Hz and −99.2 dBc/Hz at 10 kHz offset, respectively [20]. On the other hand, tunable OEO has been widely reported [21–25]. Electronic band-pass filter (EBPF) as one of the frequency tuning solution to the OEO is first used in the [25].

In this paper, we proposed and experimentally demonstrated a DPMZM-based OEO with tunable dual-frequency output. The DPMZM isolates physically the two RF components fed back to its RF input ports. Furthermore, by adjusting bias voltages of its sub-modulators, the DPMZM can suppress effectively the intermodulation components mentioned in [18]. Finally, the OEO realizes dual-frequency output independently with their tunable range from 1GHz to 15GHz; and the two outputs have similar performance in terms of phase noise which are measured to be experimentally −128dBc/Hz and −120dBc/Hz at 10 kHz offset at 1GHz and 15GHz, respectively.

## 2. Principle and experimental setup

The configuration of the proposed dual-frequency signal generator is shown in Fig. 1. A continuous wave (CW) light from a laser diode is sent into a dual parallel Mach-Zehnder modulator (DPMZM), and then divided by an optical coupler (OC2) into two branches with branches having different lengths of optical fiber with each other, for example, one branch has 1 km of optical fiber and the rest 4 km, as labeled in the Fig. 1. Here the two branches of different lengths of optical fibers are to suppress the spurious peaks of OEO output [26,27]. A balanced photo detector (BPD) is used to convert the two optical signals out of the two branches into RF signal, which is then divided into two parts by an electrical coupler (EC) with each part being amplified by an electronic amplifier (EA1/EA2) and filtered with a narrowband EBPF centered at different frequencies. Finally, the two RF signals are fed back to the different RF input ports of the DPMZM, forming two closed OEO loops (loop1 and loops2 in the Fig. 1). An Optical coupler (OC1) is used to get light from the loops and a PD converts it into dual-frequency output. By tuning the central frequencies of the two EBPFs, the output frequencies of the OEO are changed accordingly and independently.

The DPMZM is composed of two sub-MZMs (MZM-1 and MZM-2), embedded in a parent MZM (MZM-3) show in the Fig. 1. When the two RF signals ${V}_{1}(t)$ and${V}_{2}(t)$are applied to the DPMZM, and MZM1 and MZM2 operate at the chirp-free configuration, its output light field${E}_{out}(t)$ can be written as [28]:

After the PD, the output optical current of the OEO ${I}_{out}(t)$ can be expressed as:

From Eq. (2) it can be seen that the output of the OEO contains dc component, the desired dual-frequency signals, and the intermodulation between the dual-frequency signals. And it also can be found from the Eq. (2) that if static working points of three sub-MZMs of DPMZM are biased such that $\mathrm{cos}\varphi =0$ is satisfied then the intermodulation vanishes, where$\varphi =\frac{\pi {V}_{b1}}{2{V}_{\pi 1}}-\frac{\pi {V}_{b2}}{2{V}_{\pi 2}}-\phi $. And in this case, the Eq. (2) can be simplified to

## 3. Experimental results

The experiment setup is same as the Fig. 1. In experiments, the central wavelength of the CW laser diode (Teraxion PS-NLL) is controlled at 1550nm. The EBPF (YIG filter, Watkins Johnson, WJ-795-4D) has a tunable range from 1GHz to 18.2GHz and a 3dB bandwidth around 40MHz in its operating band. The gain of the EA (JiTai, LNA0015) is about 45dB ranging from dc to 15GHz and noise figure is less than 3.5dB. The lengths of the two optical fibers are 1 kilometer (km) and 4 km. And Fujitsu DPMZM (FTM7961EX) has a 3-dB bandwidth around 25GHz and its insertion loss is about 9.5 dB. BPD of U^{2}T (V2150RM) are used in experiments. The spectrum of the OEO output signals and their phase noises are all measured with Agilent spectrum analyzer (N9030A).

During experiments, the static working points of MZM1, MZM2 and MZM3 are adjusted carefully so that the condition$\mathrm{cos}\varphi =0$is satisfied.

As a comparison, we also set up an OEO system, in which a Mach-Zehnder modulator (EOspace, AZ-DK5-20) replaces the DPMZM. And the biased voltage of MZM is set at its linear modulation point.

#### 3.1 Suppression of intermodulation

Experiment results shows the proposed dual-frequency OEO can suppress both even order and odd order of intermodulation between dual-frequency signals. To verify this point, two sets of experiments were made.

We set the two EBPFs at 2.4 GHz and 5 GHz, respectively, to generate dual-frequency signals at 2.4 GHz and 5 GHz. The generated dual-frequency spectra are shown in Fig. 2(a) and 2(b), respectively. Figure 2(a) were obtained from the OEO with MZM and (b) from OEO with DPMZM. It can be seen from them that OEO with MZM has a Carrier-Noise-Ratio (CNR) of about 45dB where the noise comes from second order intermodulation between 2.4GHz and 5GHz, whereas the OEO with DPMZM has a CNR more than 60dB with CNR improvement of more than 15dB.

We set the two EBPFs at 4.89 GHz and 5 GHz, respectively, to generate dual-frequency signals at 4.89GHz and 5GHz. The generated dual-frequency spectra are shown in Fig. 2(c) and 2(d), respectively. Figure 2(c) were obtained from the OEO with MZM and (d) from OEO with DPMZM. It can be seen from them that OEO with MZM has a Carrier-Noise-Ratio (CNR) of about 30dB where the noise comes from third order intermodulation between 4.89GHz and 5GHz, whereas the OEO with DPMZM has a CNR more than 55dB with CNR improvement of more than 25dB.

#### 3.2 Phase noise of the tunable dual-frequency signals

Experimental results showed that the tunable dual-frequency OEO has a good performance in terms of the phase noise within the full tuning range. Figure 3 shows two sets phase noise values measured in case of different dual-frequency outputs. One set is obtained where the dual-frequency OEO were adjusted at 2.4GHz and 5GHz, and their phases noise at 10 kHz offset were measured to be −123.1 dBc/Hz and −122.7 dBc/Hz respectively. The another set is obtained where the dual-frequency OEO were adjusted at 1GHz and 15GHz, and Their phase noises at 10 kHz were measured to be −128.1 dBc/Hz @10kHz and −120.7 dBc/Hz@10kHz, respectively. The slight changes between two sets of phase noises comes mainly from the variation of EA gain response to the frequency.

#### 3.3 Tuning range of the OEO

The tunable range of the dual-frequency OEO system mainly depends on the tuning range of EBPFs and the gain range of EAs. In our experiment, the tuning range of the EBPF is from 1GHz to 18.2 GHz and the gain range of EA is from dc to 15 GHz, resulting the tuning range of the dual-frequency OEO are limited from 1GHz to 15GHz, as follows in Fig. 4.

## 4. Discussion: mode competition and frequency hopping

There exists mode competition and frequency hopping in dual-frequency OEO in the case that the dual frequencies are very close (within 3dB bandwidth of EBPF). This phenomenon is illustrated in the Fig. 5(a).

From the Fig. 5(a), we can see that when we tune the two RF signals frequency so that they are close and fall together into the 3-dB bandwidth of EBPF, the side modes around both signals will have probability to share two amplifiers gain due to EBPFs bandwidth overlap. When one side mode gets the gain enough, it can compete over two main modes of the OEO, as a result RF1 and RF2 signals disappeared. In this case the OEO will generate a new RF signal which is fall into the band of two EBPFs bandwidth, and frequency hopping occurs.

In our experiment, we fixed one output of dual-frequency OEO output at 5GHz, and tuned the other frequency from 2.4GHz to 5GHz. When its frequency approach closely to the 4.93GHz, both two signals disappeared and a new RF signal at 4.96GHz occurred, as shown in the Fig. 5(b).

Experiments showed the frequency hopping of the proposed dual-frequency OEO only occurs in the case where the two output frequencies are very close. In practice, the two frequencies are typically separated far than the 3dB bandwidth of EBPF, and the proposed OEO can operates in a stable state.

## 5. Conclusion

A tunable dual-frequency OEO based on DPMZM is proposed and demonstrated experimentally. By controlling the working points of three sub-MZMs of DPMZM the intermodulation between two frequencies can be successfully suppressed, and phase noises of dual-frequency outputs are better than −120dBc/Hz@10KHz frequency offset with the full tuning range. Although there exists mode completion and frequency hopping, it occurs only with a limited frequency range, and doesn’t have negative impact on practice application.

## Funding

National Natural Science Foundation of China (NSFC) under grant No. 6169190011, 61420106003, 61621064, 61321004, 61307081, and Chuanxin Funding.

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