Investigation Of Wavelength Sensing And Goos-hnchen Effect Based On Optical Waveguide

Symmetrical metal-cladding waveguide (SMCW) is a special type of optical waveguide. Compared to traditional dielectric waveguide, it has the stronger confining effect and the wider range of effective refractive index of guided modes which begins from zero. According to this character, without other coupling techniques, such as prism coupling and grating coupling, a so called free space coupling technique is developed to transfer energy into waveguide. Because the transmission loss of the waveguide will be extremely large if metal films are used as cladding layers, which is bad for producing useful devices. We prefer to the reflection manner rather than transmission manner, so the transmission loss of energy is avoided. Many opti-electronic devices are introduced based on SMCW, including sensor, filter and modulator. In the dissertation, the enhanced Goos-H?nchen (GH) effect of a reflected beam from SMCW, and the dispersion characteristics of SMCW are extensively discussed. Then a new method is proposed to stabilize and monitor wavelength.Firstly, the principle of the Surface Plasma Resonance (SPR) and dielectric waveguide are described. Their reflectivity and dispersion equation are derived. Because the incident mode must coincide with the synchronous condition, Prism is commonly used to match longitudinal wavenumber. The range of effective refractive index of guided modes on SMCW is much larger than other dielectric waveguide. Free space coupling technique is developed to transfer energy into SMCW. This characteristic is useful for the small size of devices. We demonstrate that the ultra-high order modes in SMCW with submillimeter scale are polarization independent and sensitive to the refractive index, thickness of the guided layer and the incident wavelength. Some typical applications of the SMCW, such as narrow band filter, comb filter, oscillating wave sensor, displacement sensor and electro-optical modulator, are also mentioned.Secondly, the loss characteristic of SMCW, including the intrinsic and radiative damping, is discussed and derived based on the first order perturbation theory. The intrinsic damping results from imaginary part of metal dielectric constant and represents absorption loss of the guided wave due to the materials. The radiative damping represents the leakage loss of the guided mode back into free space and is inversely proportional to the exponential function of thickness of upper metal layer. Using stationary-phase approach and weak coupling condition, a concise equation of GH shift is obtained when incident wavelength coincides with the synchronous condition. From this equation, we can obtain that the magnitude of the beam shift is closely related to the intrinsic and radiative dampings of the resonant mode. When the intrinsic damping is larger than the radiative damping, negative GH shift occurs. The positive GH shift corresponds to the reverse case. Larger GH shift can be obtained when intrinsic damping approaches the radiative damping. This conclusion is also proved by Gaussian beam model. The shifts can be enhanced to as large as hundreds of micrometers.Then we present an elegant experimental approach to directly measure the GH shift for a single total reflection of the light beam from SMCW. A one-dimensional position-sensitive detector (PSD) that can provide small spatial resolution is used to detect the large GH shift of the reflected beam when the ultrahigh-order mode is excited in the SMCW. After passing through two apertures and a splitter, a large part of the Gaussian beam from a tunable laser is introduced onto the SMCW with upper metal layers of different thicknesses. Another part of the beam, which is reflected from the first splitter, irradiates the second splitter and is detected by a wavemeter connected to a computer. In the experiment, ultrahigh-order modes are excited. Because of the polarization independence of the ultrahigh-order modes, TE and TM incidence have nearly the same characteristics. The reflected light from the SMCW is first detected by a photodiode. An angular scan is performed by rotating the goniometer and the spectrum is generated. We select the operation angle to be located at the maximum reflectivity near a certain dip of the spectrum. The GH effect is not remarkable at this position due to the deviation of the resonance condition. The position of the reflected beam is set as the reference at this point. Then we move the photodiode out of the light path without changing any position of the incident beam and let the reflected light beam cast onto the center of the PSD perpendicularly. Then by changing the wavelength through temperature tuning, the variation of GH shift can be measured. The positive and negative shifts about 480 and 180μm can be observed, respectively. The trend of experimental results shows good agreement with the numerical results. Furthermore, a new sensor is proposed by measuring the ultrahigh-order mode enhanced GH shift. The resolution of this sensor can reach 10-9 RIU theoretically, which is higher than SPR-GH sensors.Finally, a novel wavelength sensor based on ultrahigh-order mode from SMCW is also proposed. The sensitivity of the ultrahigh-order mode and the optimal thickness of the upper gold thin films are discussed. Owing to the very narrow resonance dip, the proposed device exhibits unusual sensitivity enhancement. Its sensitivity is much higher than traditional wavelength probes and can be obtained as high as 5×1010m-1 experimentally. This sensor has quite excellent application prospect owing to its many advantages, such as high sensitivity, simple structure and independence of polarization, and can be widely used to stabilize and monitor wavelength.