In order to study the polarization characteristics of the hybrid resonator, we first measured the polarization dependence of the LRSPP waveguide coupler at 1550 nm by an infrared CCD. Figure 2(h) shows part of the LRSPP waveguide coupler. Figure 2(g) shows the top view of the sliver strip under optical microscope. A trapezoidal groove structure is very helpful for the latter lift-off process. Figure 2(f) shows the cross-section of the photoresist pattern after process (b). The detail fabrication processes of the LRSPP waveguide coupler was given in the method section. The silver strip was embedded in the polymer cladding. Additionally, the total thickness of the polymer cladding was designed to be about 40 μm. Low-loss polymer material, photo-active UV curable fluorinated resins based on acrylate, was used to make the waveguide cladding (n = 1.44 at 1550 nm). In order to ensure the low propagation loss, silver stripe with a width of about 4 μm and a thickness of about 10 nm was used as the core of the LRSPP waveguide. This hybrid resonator provides a very efficient solution to simultaneously inhibit polarization noise and temperature noise in RFOG, and promotes the rate-grade and tactical-grade RFOGs to be practical.įigure 2(a–e) show the main fabrication processes of LRSPP waveguide coupler, including spin coating, exposing, developing, electron beam evaporation (EBE) and lift-off. The TDWS of the resonator is heavily reduced by the temperature insensitive characteristic. The unwanted ESOP is effectively eliminated by the single-polarization LRSPP waveguide. The resonator was formed by a polymer-based LRSPP waveguide coupler and a silica fiber. This letter reports a hybrid resonator with special properties of single polarization and temperature insensitive to overcome the bottlenecks of RFOG. These advantages render LRSPP waveguides an outstanding platform for specialized applications in high-precision optics 13, 21, 22, 23, 24, 25. LRSPPs show good mode-matching to conventional optical fibers 18, as well as simultaneous guiding of optical and electrical signals 19, 20. The guided mode is a mixed mode of electrons and photons propagating along a metal-dielectric interface, and is always transverse-magnetic (TM) polarized 15, 16, 17. This waveguide consists of nanometer-thin metal strips embedded in homogeneous dielectric environment. Long-range surface plasmon polaritons (LRSPP) waveguides is a new research hotspot of integrated optics 12, 13, 14. Therefore, a novel resonator scheme with characteristics of single ESOP, athermality and low cost is required to overcome the above bottlenecks of RFOG. In addition, these schemes are unable to remove the influence of temperature fluctuation on resonant wavelength, even the resonator formed by hollow-core PBF 11. In practice, almost all the proposed schemes can only reduce rather than completely estimate the polarization noise.
The PBF-based resonator is usually high cost. The resonators based on 90° polarization-axis rotated splice and two portions of oppositely twisted single-mode fiber have unavoidable fabrication error. For example, the added polarization controller or fiber polarizer always requires high precision, and also seriously enlarges the gyroscope system. However, these schemes have some shortcomings. Many resonator schemes have been proposed to reduce the polarization noise, including polarization-maintaining fiber (PMF) resonator employing polarization controller (PC) or fiber polarizer, PMF resonator with 90° polarization-axis rotated splice 7, hollow-core photonic bandgap fiber (PBF) resonator 8, 9, single-mode fiber resonator composed of two parts of oppositely twisted fiber 10 and etc.
To overcome these limits, efforts have been made during the past decades. In addition, resonant wavelength of resonator is extremely sensitive to the fluctuation of ambient temperature, which makes it difficult to obtain a high-performance RFOG 5, 6.
Among them, the polarization-fluctuation induced drift, which mainly caused by the existence of dual eigen-states of polarization (ESOP), has been regarded as the dominant non-reciprocal noise of RFOG 4. In practice, its performance to date is still below expectation due to various effects that produce noises. Resonant fiber optic gyroscope (RFOG) based on Sagnac effect has been considered as a promising candidate for the new-generation inertial navigation system 1, 2, 3.