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Abstract
What we believe to be a new hybrid-polarization diversity scheme which can eliminate the polarization state variation caused by wavelength tuning of laser in optical frequency domain reflectometry is proposed in the paper. In the scheme, a 45° polarizer is used to maintain the polarization of signals. It decreases the polarization angle fluctuation to 2.81° and realizes a −145 dB test sensitivity with a 32 dB Rayleigh scattering signal-to-noise ratio in a 10 m fiber single test. The polarization fading suppression is achieved for tests with a large wavelength tuning range from 1480 nm to 1640 nm. Meanwhile, a 6 µm spatial resolution is also achieved. The proposed scheme can be applied to the structure measurement of high-precision optical fiber devices with high spatial resolution and sensitivity.
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1. Introduction
In recent years, distributed optical fiber measurement and sensing have been widely applied in monitoring the structure of civil infrastructures such as dams and bridges [1,2]. Distributed optical fiber sensing technology based on Rayleigh scattering can be divided into optical time domain reflectometry (OTDR) [3,4] and optical frequency domain reflectometry (OFDR) [5,6] technology. Compared with OTDR, OFDR can overcome the limitation of pulse width in lasers [7]. Its spatial resolution depends on the frequency tuning range [8] and can provide high spatial resolution and sensitivity [9]. Scholars have widely studied it in recent years due to these advantages.
However, researchers prefer suppressing nonlinearity noise in OFDR systems to achieve a more extended measurement range and higher spatial resolution. There is also a severe polarization fading problem in the OFDR system [10,11]. The polarization state of a signal varies with the random change of birefringence in single-mode fiber under the influence of the external environment [12] and the tuning of wavelength [13–15]. This effect will lead to random intensity modulation of interference signals, and the intensity of some interference signals may decrease or disappear completely. It will reduce the signal-to-noise ratio, the sensitivity of test results and the loss of some effective signals happened [10]. Therefore, it is necessary to propose an effective method to suppress polarization fading problem.
In order to suppress the influence of polarization-induced fading on coherent detection, several anti-polarization fading methods have been developed, including Faraday rotation mirror method [16], polarization-perturbation method [17], polarization state control method [18], polarization preserving technique [19] and polarization diversity detection [15,20]. Among the above methods, the Faraday rotation mirror method only applies to the Michelson interferometer [10] and is not conducive to work in the extreme environment of strong electromagnetic. The polarization-perturbation scheme needs us to test repeatedly, which is time-consuming [17]. The polarization preserving technique method will bring the crosstalk of polarization-maintaining fiber into the result.
The polarization diversity detection scheme is a commonly used polarization fading suppression scheme in OFDR. It simplifies the sensing unit and suppresses the polarization fading phenomenon caused by the random variation of birefringence in the fiber [21]. However, the frequency-modulated continuous wave (FMCW) technique is used in the OFDR system, and polarization states will vary in both the reference arm and measurement arm as the wavelength modulation [12,13,22]. Aiming to decrease the influence of wavelength change on the polarization state of OFDR, a scheme combining single-mode fiber, polarization-maintaining fiber and polarization state controller has been proposed [23]. This method only adapts to the application scenarios with a narrow wavelength tuning range and can not satisfy the needs of high spatial resolution with a wide wavelength tuning range. Therefore, a scheme that applies to both narrow and wide wavelength tuning ranges needs to be proposed.
In this paper, a new polarization diversity scheme is demonstrated. A 45$^{\circ }$ polarizer controls the polarisation of the reference light injected from the single-mode fiber into the polarization-maintaining fiber. Then, the orthorhombic polarization components of the beat signal will be separated by polarization beam splitters (PBS) and detected. Besides, an algorithm is proposed to correct the optical path difference introduced by the birefringence of the polarization-maintaining fiber and the angle difference of the 45$^{\circ }$ polarizer in the fast and slow axes. The experimental results show that this design successfully eliminates the change of the reference ratio polarization state due to wavelength variation and eliminates the polarization fading phenomenon in the optical path when the OFDR structure is tuned to a wide range of wavelengths.
2. Principles
According to [23], interference signals can be obtained as below during the transmission of signals in OFDR:
where ${I_0}$ is intensity of incident light, $M$ is the intensity modulation coefficient, ${\phi _0} = 2\pi {f_b}t + {\phi _c}$ is the phase information, ${f_b}$ is the beat frequency of interference signal, $\tau$ is the transmission time delay between the reference and signal light due to their optical path, $t$ is the transmission time of input light signal, ${\phi _c}$ is the phase constant, ${R(\tau )}$ is the reflectivity of the device under test. $\Delta {\phi _0} = 2\pi \Delta \tau c/\lambda$ is the birefringent phase difference which is an important factor acting to $M$, where $c$ is the velocity of light, $\lambda$ is the wavelength of the interference light, $\Delta \tau$ is the fiber differential group delay.In OFDR optical path, the intensity of the system beat interference signal will be modulated with the random fluctuation of the polarization state in signals. Besides, the tuning of signal wavelength is also one of the reasons for intensity modulation. The two kinds of modulation is the polarization fading phenomenon in the OFDR system, which will decrease the signal-to-noise ratio and sensitivity of the system. In order to suppress this phenomenon, we optimize the polarization diversity scheme to reduce the impact of the intensity modulation caused by random polarization variation and the influence of wavelength change on polarization.
The traditional polarization diversity scheme adopts the idea of detecting polarization interference along x and y orthogonal directions respectively to avoid direct superposition of x and y polarization direction components. When the polarization angle of reference arm in OFDR interferometer is adjusted to 45$^{\circ }$, the light intensity of reference signal separated by PBS in x and y directions is equal. The interference signals of polarization diversity structure in x and y directions can be expressed as follows:
As the wavelength is tuned, the polarization of the beat signal will be distorted, and the suppression of the polarization attenuation will be affected. To avoid this problem, we propose a new polarization diversity structure for OFDR by combining single-mode fibers or devices with polarization-maintaining fibers or devices in Fig. 1.
Fig. 1. Hybird-polarization diversity structure. (TSL: tunable laser, SMC: single-mode fiber coupler, Cir: single-mode fiber circulator, PMC: polarization maintaining fiber coupler, PBS: polarized light beam splitter, PD: photodetector)
The 45$^{\circ }$ polarizer replaces the polarization controller in the traditional polarization diversity structure on the polarization state of the reference arm so that the energy of the reference signal separated by PBS is the same in the x and y directions. After the 45$^{\circ }$ polarizer, the optical path adopts a full polarization-preserving structure, which avoids the phenomenon that the phase difference in x and y directions changes with the wavelength by using the fixed birefringence of the polarization maintaining fiber. After passing through the polarization diversity structure in Fig. 1, the interference signal can be expressed as follows in the x and y directions, respectively:
In Eq. (4), ${\phi _k} = 2\pi {f_k}t$ is the phase difference between ${I_x}$ and ${I_y}$ introduced by the fixed birefringence of the polarization maintaining fiber. The Fourier transform of Eq. (4) can be obtained as follows:
After the Fourier transform, the phase information of the OFDR system in the time domain will be translated to frequency information in the frequency domain and related to the length of fiber under test through wavelength or frequency tuning rate ${\gamma }$ [5,6]. So, the fixed phase difference in Eq. (4) will be translated as a beat frequency difference ${f_k}$ between x and y in the frequency domain. The beat frequency difference ${f_k}$ can correspond to the optical path difference through ${\gamma }$. Based on the theory, the influence of fixed birefringence of polarization-maintaining fiber on polarization diversity effect can be eliminated by frequency shift only in the frequency domain. The frequency shift range can be obtained by performing a peak search in both signals of the axis. In order to avoid the problem of the peak search results indicating different reflection peaks, it is possible to limit the peak search range during the search of the y-axis light based on the search results of the x-axis. The final frequency domain signal can be expressed as:
3. Experiments and results
3.1 Polarization states changing in diversity system with frequency tuning
In order to verify the variation of polarization in different structures, we set up two optical path systems, as shown in Fig. 2. The optical path shown in Fig. 2(a) demonstrates the structure proposed in this paper. It mainly contains a 45$^{\circ }$ polarizer, polarization-maintaining fiber coupler (PMC) and polarization-maintaining PBS. The detector performs single-ended detection of the x and y orthogonal polarization directions, respectively. The fluctuation of the proportion of optical power in the total power of the two orthogonal direction polarization states was observed to calculate the polarization angle variation with continuous wavelength tuning. Figure 2(b) shows the optical path structure of the OFDR interferometer reference optical signal in the traditional polarization diversity scheme, which uses a polarization controller (PC) to adjust the polarization angle. The fluctuation law of the proportion of optical power to the total power of the two orthogonal directional polarization states was observed and compared with the scheme in Fig. 2(a). TSL-770(SANTEC) was used as the light source in both schemes in the experiment. We used a balanced photodetector (Newport PDB-1020) to achieve this function at the receiver. A multi-channel high-speed data acquisition card (M4i.4471-x8) with a sampling rate 11.25 MHz was used to achieve signal acquisition and analog-to-digital conversion.
Fig. 2. Structures of verification optical path of two polarization diversity schemes. (a) Verification optical path of hybrid-polarization scheme. (b) Verification optical path of traditional polarization diversity scheme.
According to Eq. (3), the polarization angle of ideal final signals depends on the polarization angle of signal light from fiber under test when the influence of wavelength variation is suppressed. However, the parameter $\theta _s$ is random. Besides, we need to modulate the polarization angle of the reference signal to 45$^{\circ }$ for polarization fading suppression. The results in Fig. 3 show the polarization angle variation measurement data in the two schemes. Theoretically, it should always equal 45 $^{\circ }$ in traditional and hybrid-polarization schemes. The hybrid-polarization scheme can decrease the polarization angle variation to 2.81$^{\circ }$ while the traditional scheme can only achieve 40.93$^{\circ }$. The result of the hybrid-polarization scheme is close to the target that the polarization angle of the reference signal is controlled to 45$^{\circ }$ in theory. Compared with the 45$^{\circ }$ polarization angle in theory, the angle offset of the new scheme is caused by the insertion loss of the fiber device in the optical path. The phenomenon that the polarization state of the reference arm signal changes with the optical wavelength is suppressed in our new scheme.
Fig. 3. The 45$^{\circ }$ polarization angle fluctuation actual measurement results in traditional and hybrid-polarization schemes.
3.2 Comparison of final results within two scheme
To verify the effectiveness of the proposed mixed polarization diversity scheme, we built OFDR test principle devices based on both schemes. The two schemes are similar, as shown in Fig. 4. The linearly tuned laser is injected into the measurement and auxiliary interferometer through a single-mode coupler. In the interferometer, a single-mode coupler (SMC) is connected to inject the incident light into the measurement and reference arms. The light in the measuring arm is injected into the device under test through a circulator (Cir). The Rayleigh scattering and Fresnel reflection signal to carry the information under test in the device to be measured enter the polarization-maintaining fiber coupler in the interferometer through the circulator. The reference signal is biased through the 45$^{\circ }$ polarizer and evenly injected into the fast and slow axes of the polarization maintaining fiber in our new scheme. In contrast, a polarization controller is used to achieve it in a traditional scheme. The reference and measurement light interfere in the polarization-maintaining coupler and enter two polarized beam splitters (PBS), respectively. The two orthogonal direction signals of PBS are differentially detected to improve the test signal strength and eliminate the influence of the signal DC component. The laser and detectors are the same as in Fig. 2.
Fig. 4. Experimental setup for OFDR test based on traditional and new polarization diversity schemes. (a) The new polarization diversity scheme. (b) The traditional polarization diversity scheme. (FUT: the fiber under test, BPD: balanced photodetector, DAQ: data acquisition)
In the two OFDR systems, we set the frequency sweep range of the tunable laser to 160 nm (1480 nm-1640 nm) and the frequency sweep speed to 40 nm/s. The optical path difference between the two arms of the auxiliary interferometer is 35 m. The length of the fiber under test is 30 m. Figure 5(a) shows the frequency domain results of the signal obtained by the difference between the two output ports of the PBS device in the main interferometer structure. In order to obtain this result, the phase information of the auxiliary interferometer is used as the clock to suppress the frequency sweep nonlinearity of the laser. After signal correction, intensity jitter can be found in the scattering background signals. The mechanism of this jitter is that the random fluctuation of the polarization state in measurement light causes the amplitude modulation of the beat interference signal in a specific polarization direction. Besides, there is an intensity difference between the two orthogonal beat signals. It comes from the polarization of signal light and the insert loss difference between the output channel of PBS. The factor should be corrected according to the result of the insert loss measurement.
Fig. 5. Results of new polarization diversity scheme. (a) Frequency domain test results of x and y polarization directions in hybrid-polarization scheme. (b) Point spread function of frequency domain test results in x and y polarization directions.
Figure 5(b) shows a point spread function of 30m-fiber measurement result at 14.52 m. The optical path difference between the two axes of the polarization-maintaining fiber can be expressed as $\Delta z= (n_p-n_s)z$, where $n_p$ and $n_s$ is the refractive index of the two axes, $z$ is the length of the polarization-maintaining fiber. $n_p-n_s$ is $5\times 10^{-4}$ in the hybrid-polarization scheme. The positional offset $\Delta z$ shown in Fig. 5(b) is about 0.5 mm, so the length of the polarization-maintaining fiber corresponding to it is 1 m. This corresponds precisely to the polarisation-maintaining fibre’s length in Fig. 1 of the experimental optical path system. The position offset due to the birefringence is reflected in the frequency domain by the fixed frequency shift between the two beat interference signals in the orthogonal direction. Therefore, the effect of birefringence caused by the polarization-maintaining fiber can be eliminated by only shifting the two signals in the frequency domain. The distance of frequency shifting can be obtained by peak searching in the orthogonal signals. The result after both intensity and position correction is shown in Fig. 6.
Fig. 6. Results after correction of new polarization diversity scheme. (a) Frequency domain test results of x and y polarization directions in hybrid-polarization scheme. (b) Point spread function of frequency domain test results in x and y polarization directions.
After eliminating the influence of birefringence of the polarization maintaining fiber, vector synthesis of signals in the x and y directions is carried out. Figure 7 shows the final test result obtained in the hybrid-polarization scheme before and after correction. The dramatic jitter in Fig. 7(a) decreases apparently in Fig. 7(b). The polarization fading phenomenon in the interferometer is successfully suppressed, and the jitter still existing in the scattering background signals is Rayleigh scattering information contained in the fiber to be tested.
Fig. 7. Comparison results of hybrid-polarization diversity scheme before and after correction. (a) Before correction. (b) After correction.
In order to observe the improvement of the new scheme, we tested 10 m fiber with both the hybrid-polarization scheme and the traditional scheme. The results are shown in Fig. 8. The spatial resolution of 6 µm and the loss test sensitivity of −145 dB are achieved in the test of 10 m fiber through our proposed hybrid-polarization optical path scheme. However, under the influence of polarization fading, the result of the traditional scheme shown in Fig. 8 demonstrates a lower sensitivity and a scattering background signal with drastic fluctuation.
Fig. 8. 10 m fiber measurement results of traditional scheme and hybrid-polarization scheme. (a) Hybrid-polarization result. (b) Traditional scheme result.
The OFDR system composed of full-polarization-maintaining fibers can decrease polarization fading. However, the introduction of polarization crosstalk is an unavoidable drawback of polarization-maintaining fiber fabrication techniques. Polarization crosstalk can make the final results suffer more significantly. The results of the the hybrid-polarization scheme versus full-polarization-maintaining scheme are shown in Fig. 9.
Fig. 9. Measurement results of the hybrid-polarization scheme and full-polarization-maintaining scheme with their details. (a) Hybrid-polarization scheme. (b) Full-polarization-maintaining scheme.
It can be noticed that many parasitic peaks near the reflect peak are shown in the result of an OFDR system with full-polarization-maintaining fiber. Meanwhile, the parasitic peaks are much less due to a hybrid-polarization OFDR system for less polarization crosstalk from the polarization-maintaining fiber. Considering the advantages and disadvantages of all polarization-maintaining fiber and traditional OFDR schemes used in this paper, we proposed a hybrid-polarization OFDR system to suppress polarization fading and decrease the influence of polarization crosstalk from polarization-maintaining fiber.
4. Conclusion
This paper proposes an OFDR system scheme based on hybrid-polarization diversity. The scheme suppresses the phenomenon that the polarization state of the reference arm in the OFDR interferometer constantly changes with the change in the wavelength of the light source. It also avoids the phenomenon that the wavelength change in the test process of the extensive tuning range of the light source leads to the deterioration of the system polarization fading suppression effect. The frequency sweep of the tunable laser is set from 1480 nm to 1640 nm, and we achieved an ultra-high spatial resolution of 6 µm and a loss test sensitivity of −145 dB in a 10 m fiber single test. This scheme is of great significance for OFDR testing and sensing research with micron-level spatial resolution requirements.
Funding
National Key Research and Development Program of China (2022YFB3205200); National Science Fund for Distinguished Young Scholars (61925501); National Natural Science Foundation of China (61975040, 62005062, 62305081); Guangdong Province Introduction of Innovative R&D Team (2019ZT08X340); Guangdong Provincial Pearl River Talents Program[China] (2019CX01X010).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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