Optical frequency domain polarimeter: distributed polarization extinction ratio measurement technology spanning 110dB

 01 Introduction

Polarization extinction ratio (PER) is an important performance indicator of polarization-maintaining optical fiber and its devices, which reflects the ability to maintain polarization characteristics of optical fibers and devices. In applications such as high-precision fiber optic gyroscopes that require extremely high polarization purity, the PER of devices such as multi-functional integrated waveguide modulators (referred to as Y-waveguides) exceeds 80dB, which is better than the performance of ordinary fiber polarizers (PER generally does not exceed 40dB). The requirements are 4 orders of magnitude higher; in addition, with the introduction of anti-resonant hollow-core photonic crystal fibers and the rapid improvement of performance, PER performance has reached more than 100dB/m. Therefore, there is an urgent need to overcome ultra-high performance distributed polarization measurement methods and technologies above 100dB to achieve performance evaluation, defect analysis and fault diagnosis of the above-mentioned polarization-maintaining fibers and polarization devices. At present, although the optical coherence domain polarimetry (OCDP) based on the principle of white light interference can achieve a distributed polarization measurement performance of 100dB, it uses a low-power-density wide-spectrum light source and a mechanical scanning light. The measurement performance of the process delay line has reached the theoretical limit, and it also has shortcomings such as slow measurement speed and complex device structure. It is difficult to meet the ultra-high-precision testing requirements and requires an urgent breakthrough.

In response to the demand for ultra-high distributed polarization extinction ratio measurement, the Institute of Advanced Photonics Technology of Guangdong University of Technology recently proposed an optical frequency domain polarimetry (OFDP) measurement method, aiming to break through the existing distributed polarization extinction ratio measurement method PER100dB. the theoretical limit. OFDP measurement technology uses a laser light source with narrow linewidth and wide-range tuning. With the help of non-equilibrium time delay interferometer and coherent detection technology, it achieves distributed polarization extinction ratio measurement by suppressing the nonlinearity and phase noise of the light source sweep. The capability has been increased from the current OCDP optimal 100dB to 110dB, and has the potential to exceed 120dB. The research results were published in Optics Letters under the title "Optical Frequency Domain Polarimetry for Distributed Polarization Crosstalk Measurement beyond 110 dB Dynamic Range". Associate Professor Yu Zhangjun of Guangdong University of Technology is the first author of the paper, and Professor Yang Jun is the corresponding author of the paper.

 02 Research background

Polarization crosstalk caused by internal defects or external stress can degrade the transmission performance of photonic integrated chips or polarization-maintaining optical fibers. Polarization crosstalk not only increases transmission loss, changes polarization-related loss, leads to unnecessary interference in the optical path, but also causes additional nonlinear effects. Distributed polarization extinction ratio measurement can locate the location where polarization crosstalk occurs and determine its intensity. It can also further extract the distributed extinction ratio characteristics, thereby helping us evaluate device quality and improve device preparation processes. The emergence of high-performance devices and optical fibers has put forward high requirements for the dynamic range performance of distributed polarization extinction ratio measurements. It is necessary to develop an ultra-wide range distributed polarization extinction ratio measurement method to meet the growing number of devices. Testing requirements.  

03 Innovative Research 3.1 The polarization extinction ratio measurement capability of the optical frequency domain polarimeter The optical frequency domain polarimeter mainly consists of three parts: a tunable laser light source, a device under test, and a time delay interferometer, as shown in Figure 1. After the linear frequency sweep laser emitted by the light source passes through different transmission paths and finally merges at the detector, an interference beat signal with a frequency proportional to the optical path difference will be generated . After the interrogating light is injected into a polarization main axis of the polarization-maintaining device under test, the polarization crosstalk light occurring at different positions can produce such an interference beat signal with the interrogating light. The frequency of the beat signal can be used to determine the location where crosstalk occurs, and the strength of the beat signal can be used to determine the intensity of crosstalk. Through interference frequency shifting, the time-delay interferometer plays a role in regulating the dynamic range of the measurement. After theoretical analysis and experimental verification, we found that the measurement dynamic range of the optical frequency domain polarimeter can be expressed as,

Among them, Δν, ΔF, and γ are the line width, frequency sweep range, and frequency sweep rate of the light source respectively, c is the speed of light in vacuum, and L0 is the optical path difference of the time-delay interferometer. It can be seen from the experimental results in Figure 2 that by adjusting the light source sweep rate and the optical path difference of the time delay interferometer, a polarization extinction ratio measurement of 110dB dynamic range can be achieved.

Figure 1 The generation process of polarization crosstalk and delayed coherent detection principle of optical frequency domain polarimeter Source: Optics Letters (2022). https://doi.org/10.1364/OL.468893 (Fig. 1)

Figure 2 The relationship between the dynamic range of the optical frequency domain polarimeter and the sweep range of the light source, the sweep rate, and the optical path difference of the time-delay interferometer. Source: Optics Letters (2022). https://doi.org/10.1364 /OL.468893 (Fig. 5) 3.2 Ultra-high polarization extinction ratio Y-waveguide test application Using optimized scanning parameters and interferometer optical path difference, an OFDP optical path system as shown in Figure 3 was built to test an ultra-high extinction ratio The distributed polarization crosstalk of the lithium niobate Y waveguide chip was tested, and the test results are shown in Figure 4. Thanks to the OFDP system with a dynamic range of more than 110 dB, we can even clearly see the polarization crosstalk characteristics (Cy1) with a polarization extinction ratio of 105dB inside the waveguide.

Figure 3 Optical path structure diagram of optical frequency domain polarimeter Source: Optics Letters (2022). https://doi.org/10.1364/OL.468893 (Fig. 3)

Figure 4 Distributed polarization crosstalk test results of ultra-high extinction ratio lithium niobate Y waveguide Source: Optics Letters (2022). https://doi.org/10.1364/OL.468893 (Fig. 6)  

04 Application and Prospects

In summary, we propose a novel distributed polarization extinction ratio measurement method—optical frequency domain polarimeter (OFDP). The polarization crosstalk distribution in the device under test is measured using a wavelength scanning interferometer composed of a tunable laser and a delayed interference optical path. First, the relationship between the dynamic range of a phase-noise-limited OFDP system and the wavelength/frequency scanning range of the light source, the scanning speed of the light source, and the optical path difference of the delay interferometer was studied. Secondly, using the optimized scanning speed and optical path difference, achieving a dynamic range of 110.52dB; further, the distributed polarization crosstalk of an ultra-high polarization extinction ratio lithium niobate waveguide modulator was measured. Thanks to the ultra-large dynamic range of OFDP, the polarization crosstalk characteristic of -105dB in the waveguide was observed for the first time. In the future, OFDP will play a huge role in the development and manufacturing of high-performance polarized optical fibers and devices.