Novel dual-loop optoelectronic oscillator based on self-polarization-stabilization technique

Linbojie Huang; Quan Yu; Lei Deng; Songnian Fu; Ming Tang; Mengfan Cheng; Minming Zhang; Michael Choi; Donald Chang; Gordon K. P. Lei; Deming Liu

doi:10.1364/OE.25.021993

1. Introduction

Optoelectronic oscillator (OEO) has been widely considered to have good ability to produce microwave and millimeter-wave signals with very high spectral purity and low phase noise [1, 2]. This feature is extremely useful in radar, high speed signal processing and radio over fiber (RoF) systems [3, 4]. The classical single-loop OEO is composed of an intensity modulator (IM), single mode fiber (SMF) with length of several hundred meters, a photodiode (PD) and an electrical bandpass filter [5]. The optical fiber is the energy-storage component, and longer fiber will generate higher spectral purity signals. Nevertheless, longer fiber will introduce smaller mode spacing, resulting in the requirement of electrical bandpass filter with very narrow bandwidth [6]. To address this issue, OEO systems based on multi-loop configuration have been proposed [7–9]. In [7], the optical signals in dual loops are detected by two independent PDs and then combined in electrical domain to drive a dual-drive Mach-Zehnder modulator (MZM). A dual-loop OEO based on wavelength multiplexing technique is reported in [8], and only one PD is used. In this scheme, two lasers with different wavelengths and a wavelength division multiplexer are still required. To reduce the system complexity, another scheme based on polarization multiplexing technique is proposed in [9] and the two loops are directly combined in optical domain without any additional active device. However, it is not easy to keep the polarization states of optical signals stable in these two loops due to the fact that the characteristic of each fiber is easily influenced by some environment factors such as temperature and mechanical vibration.

On the other hand, in the classical OEO, the drifting DC bias of IM will significantly influence system stability [10]. OEO using phase modulation can be considered as an alternative to avoid this problem [11]. Nevertheless, the phase modulated signals cannot be recovered directly by using a PD, thus phase modulation to intensity modulation (PM-IM) conversion is required before the PD [12]. Optical filter [13, 14] and a dispersive device [15] could be used to break the phase balance of the sidebands and realize the PM-IM conversion. However, the effectiveness of these methods are reduced by either non-ideal cut-off slope of the filters or the dispersion-induced power penalty.

In this paper, we introduce a novel dual-loop OEO using self-polarization-stabilization technique. In this scheme, two pairs of Faraday rotator (FR) and Faraday rotator mirror (FRM) are used to maintain the polarization states orthogonal to each other and eliminate the influence from polarization fluctuations. Compared with the traditional dual-loop OEO based on polarization multiplexing technique [9], no polarization maintaining fiber (PMF) or polarization controller (PC) is required before the polarization-beam combiner (PBC), and the polarization states of optical signals in two loops are always stable even if the fibers are perturbed by the mechanical vibration or temperature. The experimental results show that in a MZM-based OEO system, this scheme has better phase noise performance than the traditional polarization multiplexing method, and it needs only half the length of optical fiber in each loop. Furthermore, based on this scheme, a novel dual-loop OEO using phase modulation is proposed and a polarization-dependent phase modulator and a polarizer are used to realize PM-IM conversion without the need of optical filter or dispersive device. The phase noise performance of −114.1 dBc/Hz at 10 kHz away from the carrier (10 GHz) is experimentally achieved and the side mode suppression ratio is improved to 63 dB.

2. Principle

2.1 Dual-loop OEO based on the proposed self-polarization-stabilization technique

Figure 1(a) shows the schematic diagram of the dual-loop OEO based on the proposed self-polarization-stabilization technique. In our scheme, the OEO includes a distributed feedback (DFB) laser, a Mach-Zehnder intensity modulator, a high-speed photodetector, a RF amplifier (AMP) and an electrical bandpass filter (BPF). Continuous-wave (CW) light from DFB laser is injected into the MZM, and PC controls the power distribution in the two optical paths. After optical circulator (OC) and polarization beam splitter (PBS), the intensity modulated light is split into two orthogonal polarization states. In each loop, the input light will retrace its path through a 45° Faraday rotator (FR) and single mode fiber with different length after reflecting off a 45° Faraday rotator mirror (FRM). After that, these two-loop optical signals are combined in PBS and then captured by PD. Subsequently, the generated RF signal is used to drive the MZM after AMP and BPF. In order to eliminate the interference of polarization state in each polarization loop, it is essential to keep the polarization states of optical signals stable and orthogonal to each other when two optical signals in two paths are coupled.

Fig. 1 The structure of the dual-loop OEO based on the self-polarization-stabilization technique (a), and the principle of the self-polarization-stabilization technique (b).

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To solve this problem, a self-polarization-stabilization technique is proposed and shown in Fig. 1(b). It could be observed that the input light will retrace its path through a 45° FR and single mode fiber after reflecting off a 45° FRM in each loop. The Jones matrix of the input optical signal at point (A) could be expressed as

P_{A} = [\begin{matrix} E_{x} \\ E_{y} \end{matrix}] .

The Jones matrix indicating the rotation of coordinates by $φ$ in FR could be defined as

R (φ) = [\begin{matrix} \cos φ & \sin φ \\ - \sin φ & \cos φ \end{matrix}] .

Assuming that the polarization-dependent loss of the used fiber is sufficiently small, the Jones matrix from point (B) to the input of FRM in Fig. 1(b) could be defined as a unitary matrix $U$ [16–19]. Due to the fact that the polarization fluctuations in SMF is much slower than the round-trip time of the used fiber, the transfer matrix from the output of FRM to point (B′) could be expressed by the transpose matrix $U^{T}$ [17, 18]. Thus, the Jones matrix from point (A) to point (A′) could be given as

T_{A B} = α^{2} R (45^{\circ}) U^{T} R (45^{\circ}) [\begin{matrix} 1 & 0 \\ 0 & 1 \end{matrix}] R (45^{\circ}) U R (45^{\circ}),

where

α

is the link loss between (A) and FRM, and the unity matrix

U

can be expressed as

U = [\begin{matrix} e^{- i θ_{1} / 2} \cos ϕ & e^{- i θ_{2} / 2} \sin ϕ \\ - e^{i θ_{2} / 2} \sin ϕ & e^{i θ_{1} / 2} \cos ϕ \end{matrix}],

where

θ_{1}

,

θ_{2}

and

ϕ

describe the changes of birefringence in the used fiber [17, 18]. Substituting Eq. (2) and Eq. (4) into Eq. (3), the Jones matrix of the round-trip loop could be rewritten as

T_{A B} = α^{2} [\begin{matrix} - 1 & 0 \\ 0 & - 1 \end{matrix}] = α^{2} R (180^{\circ}) .

Therefore, the polarization state of the output light could be given as

P_{A^{'}} = T_{A B} P_{A} = - α^{2} [\begin{matrix} E_{x} \\ E_{y} \end{matrix}] .

It could be clearly observed that the polarization state of the output signal at point (A′) is always rotated by 180° from the polarization state of the input signal at point (A), and it is independent of all details of the birefringence of the round-trip fiber. The similar phenomenon has been first found by M. Martinelli in 1989 [18] and has been used in WDM-PON based on reflective semiconductor optical amplifier (RSOA) [16, 19]. To our best of knowledge, it is the first time to introduce this self-polarization-stabilization technique in dual-loop OEO. However, by using this scheme, the two output signals of these two paths are always orthogonal to each other and could be perfectly combined in PBS regardless of the birefringence in the round-trip fibers. Therefore, there is no need to use PMF or PCs before PBS to keep the polarization states of optical signals stable in two paths. Moreover, the required fiber length is cut in half due to the round-trip transmission in each path.

2.2 OEO based on a polarization-dependent phase modulator and a polarizer

As mentioned above, the DC bias point of IM is sensitive to the temperature and mechanical vibration, thus OEO using phase modulation is preferred. To avoid using expensive optical filters or dispersive devices for PM-IM conversion, a simple PM-IM convertor based on a polarization-dependent phase modulator and a polarizer (POL) is proposed and shown in Fig. 2. In the polarization-dependent phase modulator, if the input signal is aligned with the extraordinary axis (EA) of PM crystal, the output signal will undergo a pure phase shift and the polarization state will not be changed. Conversely, if the polarization of the input signal is orthogonal to the extraordinary axis, a phase shift will also occur but with less modulation efficiency [20]. It means that if the polarization state of the input signal is at 45°, the polarization state of the output signal will undergo a change from linear to elliptical, as shown in the inset (A) and (B) of Fig. 2. The optical input signal at point A could be expressed as $E_{c} = \sqrt{P_{c}} \exp (- j (ω_{c} t + ϕ_{c} (t)))$ , here $P_{c}$ , $ω_{c}$ and $φ_{c}$ represent the optical power, angular frequency and phase of the optical carrier respectively. The Jones matrix of the optical signal at point B could be described as

E_{P M} = [\begin{matrix} \cos θ \exp (- j φ_{s} (t)) E_{c} \\ \sin θ \exp (- j α φ_{s} (t)) E_{c} \end{matrix}],

where

θ

is the angle between the input polarization and the extraordinary axis of PM crystal.

φ_{s} (t) = \sin (π V_{s} (t) / V_{π})

, here

V_{s} (t)

is the electrical input signal and

V_{π}

is the half-wave voltage of the phase modulator.

α

is the modulation efficiency ratio between two polarization states in PM.

Fig. 2 The principle of phase modulation to intensity modulation conversion based on the proposed scheme.

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A polarizer is placed at the output of PM, and the angle between the extraordinary axis of PM crystal and polarizer is $β$ . Thus, the optical signal at point C could be given by

E_{P O L} = \cos β \cos θ \exp (- j φ_{s} (t)) E_{c} + \sin β \sin θ \exp (- j α φ_{s} (t)) E_{c} .

The combined signal beats at PD, and the responsivity of the used PD is $R$ . So the output signal $I (t)$ could be expressed as

\begin{matrix} I (t) = R {| E_{P O L} |}^{2} \\ = R \cdot P_{c} \cdot [\cos {}^{2}β \cos {}^{2}θ + \sin {}^{2}β \sin {}^{2}θ + 2 \cos β \sin β \cos θ \sin θ \cos ((1 - α) φ_{s} (t))] . \end{matrix}

If $θ = β = 45^{\circ}$ , $I (t) = 1 / 2 \cdot R \cdot P_{c} \cdot (1 + \cos ((1 - α) φ_{s} (t)))$ . In this way, PM-IM conversion could be effectively achieved.

3. Experimental setup and results

3.1 Comparison of dual-loop structures based on traditional polarization multiplexing and self-polarization-stabilization technique

As mentioned above, the dual loop structure based on self-polarization-stabilization technique is stable, and it only need half length of the fiber comparing with the traditional polarization multiplexing technique. To investigate the system stability of dual-loop structures based on traditional polarization multiplexing technique and the proposed self-polarization-stabilization technique, experimental setups are built up as shown in Figs. 3(a) and 3(b). In Fig. 3(a), CW light with the wavelength of 1550 nm from DFB laser is injected into an OC, and PC is used to control the power distribution in the two optical paths. After OC and PBS, the input optical signal is split into two orthogonal polarization states. In each path, the input light will retrace its path through a 45°FR and single mode fiber with different length (100 m and 1 km for these two paths) after reflecting off a 45°FRM. After that, these two round-trip optical signals are combined in the same PBS. In Fig. 3(b), three PCs, one PBS and one PBC are used to build up a traditional polarization multiplexing structure. The fiber length is 200 m and 2 km in two paths respectively, and PC2 and PC3 are used to align the input polarization state to the extraordinary axis of PBC. In these two experiments, the optical power of the combined optical signals are measured while randomly shaking the fiber spool by an air fan for large polarization fluctuations emulation.

Fig. 3 Experimental setup of the dual loop structure based on self-polarization-stabilization technique (a), and the dual loop structure based on traditional polarization multiplexing technique (b).

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The measured results are shown in Fig. 4, and it could be clearly observed that in traditional polarization multiplexing structure, the measured optical power is seriously fluctuated due to the temperature and mechanical vibration. However, by using the proposed self-polarization-stabilization technique, the residual power fluctuation is about 0.05 dB which could be completely neglected.

Fig. 4 The measured optical power at the output of dual loop structure based on self-polarization-stabilization technique and the traditional polarization multiplexing techniques.

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3.2 Demonstration of MZM-based OEO by using the proposed scheme

We have experimentally demonstrated a MZM-based OEO by using the proposed self-polarization-stabilization technique as shown in Fig. 5. CW light from DFB laser with wavelength of 1550 nm and optical power of 13 dBm is injected into a MZM (EOSPACE, AX-0MSS-20) with half-wave voltage of 3.6 V. PC1 is used to obtain the maximum modulation depth, and PC2 controls the power distribution in the two optical paths. A dual-loop structure mentioned above is adopted in our experiment, and fiber length of two paths is 100 m and 1 km respectively. Erbium doped fiber amplifier (EDFA) is used to compensate the loop loss. After a PD (Discovery Semiconductors, DSC-R401HG) with 3 dB bandwidth of 20 GHz, an electrical bandpass filter with center frequency of 10 GHz and bandwidth of 20 MHz, and an electrical amplifier (Photline, DR-AN-20-MO) with gain of 29 dB, the generated RF signal is used to drive the MZM. In our experiment, the electrical spectrum and phase noise performance of the generated RF signal are measured by a phase noise analyzer (PNA, R&S FSWP). Meanwhile, traditional single-loop OEO with different fiber length (200 m and 2 km), dual-loop OEO based on polarization multiplexing technique and dual-loop OEO using optical coupler instead of PBS are all tested for comparison.

Fig. 5 The experimental setup of the MZM-based OEO by using the proposed scheme.

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Figures 6(a) and 6(b) show the measured electrical spectrum of the generated RF signal in single-loop OEOs with 200 m and 2 km SMF respectively. In the test, the SPAN and RBW of PNA is set to 10 MHz and 3 kHz respectively. The measured side mode suppression ratio is 61 dB and the mode spacing is 800 kHz for single-loop OEO with 200 m SMF. When 2 km SMF is used, the side mode suppression ratio is 25 dB and the mode spacing is 100 kHz. As shown in Figs. 6(c) and 6(d), the side modes are effectively suppressed to 65 dB and 69 dB in dual-loop OEO based on traditional polarization multiplexing technique and the proposed self-polarization-technique respectively. 4 dB improvement is achieved by using the proposed scheme. The single-sideband (SSB) phase noise of the single-loop and dual-loop OEOs are measured in Fig. 7. In each test, the output power of DFB and the input power of PD is kept the same. It could be clearly observed that the phase noise of the generated RF signal at frequency offset of 10kHz is −105.5 dBc/Hz, −117.3 dBc/Hz, −115.3 dBc/Hz, and −116 dBc/Hz in single-loop OEOs with 200 m and 2 km SMF, dual-loop OEOs based on traditional polarization multiplexing technique, and dual-loop OEOs based on the proposed scheme, respectively. For non-ideal PBS emulation, the PBS in OEO is replaced by an optical coupler, and two PCs in two loops are adjusted for emulation different non-ideal extinction ratio of PBS. As shown in Fig. 7, we try our best to maximize the extinction ratio but the achieved phase noise performance is still obviously worse than that of other OEO systems due to the beating noises. Moreover, the phase noise of the proposed dual-loop OEO is slightly worse than the result of single-loop OEO with 2 km SMF. This could be attributed to the imperfection of PBS’s extinction ratio, resulting in the conversion of DFB’s phase noise to OEO’s phase noise [21]. However, our scheme outperforms the traditional dual-loop OEO based on polarization multiplexing technique.

Fig. 6 The measured power spectrum of the generated RF signals in single-loop OEO with 200 m SMF (a), single-loop OEO with 2 km SMF (b), dual-loop OEO based on traditional polarization multiplexing technique (c), and dual-loop OEO based on the proposed self-polarization-stabilization technique (d).

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Fig. 7 The SSB phase noise curves of the single-loop OEOs with 200 m and 2 km SMF, dual-loop OEOs based on traditional polarization multiplexing technique, and dual-loop OEOs based on self-polarization-stabilization technique.

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3.3 Demonstration of dual-loop OEO based on a polarization-dependent phase modulator and a polarizer

To solve the DC bias drifting of IM, a simple PM-IM convertor based on a polarization-dependent phase modulator and a polarizer is proposed and the experimental setup of the dual-loop OEO based on this scheme is shown in Fig. 8. MZM is replaced by a polarization-dependent phase modulator (EOSPACE, PM-0S5-20) with half-wave voltage of 4.6 V and a fixed optical polarizer compared with Fig. 5. As mentioned in Section 2.2, PC1 and PC2 are used to adjust $θ$ and $β$ to maximize the PM-IM conversion efficiency. In out experiment, the electrical spectrum and phase noise performance of the generated RF signals are measured by PNA and the measurement parameter cross correlation factor (XCORR Factor) is set to 1. Meanwhile, single-loop OEO by using this PM-IM convertor with different fiber length are also tested for comparison.

Fig. 8 The experimental setup of the dual-loop OEO based on a polarization-dependent phase modulator and a polarizer.

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Figures 9(a)-9(c) illustrate the measured electrical spectrum of the generated RF signal in single -loop PM-IM-based OEO with 200 m fiber, single-loop PM-IM-based OEO with 2 km fiber and dual-loop PM-IM-based OEO by using the self-polarization-stabilization technique respectively. 29 dB side mode suppression ratio is observed in single-loop PM-IM-based OEO with 2 km fiber. This value could be improved to 63 dB when the proposed self-polarization-stabilization technique is used. The SSB phase noise of the single-loop and dual-loop PM-IM-based OEOs are measured in Fig. 10. Similarly, the output power of DFB and the input power of PD is kept the same in each test. Obviously, the phase noise of the generated RF signal at frequency offset of 10 kHz is −101.2 dBc/Hz, −118.1 dBc/Hz, and −114.1 dBc/Hz in single-loop PM-IM-based OEOs with 200m and 2 km fiber, and dual-loop PM-IM-based OEOs by using the proposed scheme, respectively. The phase noise of our dual-loop OEO is 4 dB worse than the single-loop scheme, and the reason could also be explained by the imperfection of PBS’s extinction ratio. There exists minimal discrepancy between the achieved phase noise performance of the PM-IM-based OEO in Fig. 10 and the MZM-based OEO in Fig. 7, but PM-IM-based OEO has no DC bias drifting problem. Nevertheless, such a little different phase noise performance is attributed to different loop gain which is caused by different half wave voltage of modulators. Furthermore, it is also proved that the dual-loop structure based on the proposed self-polarization-stabilization technique could be effective in different OEO structures.

Fig. 9 The measured power spectrum of the generated RF signals in single-loop PM-IM-based OEO with 200 m SMF (a), single-loop PM-IM-based OEO with 2 km SMF (b), and dual-loop PM-IM-based OEO by using self-polarization-stabilization technique (c).

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Fig. 10 The SSB phase noise curves of single-loop PM-IM-based OEO with 200m SMF, single-loop PM-IM-based OEO with 2km SMF, and dual-loop PM-IM-based OEO by using self-polarization-stabilization technique.

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4. Conclusions

We have experimentally demonstrated a novel dual-loop OEO based on self-polarization-stabilization technique. Due to the use of a 45° Faraday rotator and a 45° Faraday rotator mirror in each loop, the polarization state of the output signal is always rotated by 180° from the polarization state of the input signal in each loop even if the round-trip fiber is perturbed by the mechanical vibration or temperature. Moreover, the required fiber length is cut in half due to the round-trip transmission in each path, resulting in higher system stability and compacter structure. This scheme is used in a MZM-based OEO system, and better phase noise performance of −116 dBc/Hz at 10 kHz away from carrier (10 GHz) is achieved compared with the traditional polarization multiplexing method. Furthermore, a simple phase modulator based OEO by using a polarization-dependent phase modulator and a polarizer is proposed not only to replace the expensive optical filters or dispersive devices but also to solve the DC drifting problem. The experimental results show that the achieved phase noise at 10 kHz away from carrier (10 GHz) is −114.1 dBc/Hz and the side mode suppression ratio is 63 dB. The proposed schemes provide a promising solution to realize higher stability, compacter structure and better performance OEO for different applications.

Funding

Program 863 of China (2015AA016904); National Nature Science Foundation of China (NSFC) (61675083, 61505061); Fundamental Research Funds for the Central Universities HUST (2017KFKJXX010); National Key Research and Development Program of China (2016YFE0121300); Science and Technology Development Fund of Macau SAR (012/2015/AMJ).

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Novel dual-loop optoelectronic oscillator based on self-polarization-stabilization technique

Abstract

1. Introduction

2. Principle

2.1 Dual-loop OEO based on the proposed self-polarization-stabilization technique

2.2 OEO based on a polarization-dependent phase modulator and a polarizer

3. Experimental setup and results

3.1 Comparison of dual-loop structures based on traditional polarization multiplexing and self-polarization-stabilization technique

3.2 Demonstration of MZM-based OEO by using the proposed scheme

3.3 Demonstration of dual-loop OEO based on a polarization-dependent phase modulator and a polarizer

4. Conclusions

Funding

References and links

Cited By

Figures (10)

Equations (9)

Optics Express