以反射式體積型全像片研製雙向波長區分多工/解多工元件

Abstract

由耦合波理論可知，反射式體積型全像片的繞射效率與感光材料厚度、平均折射率、調變折射率及重建波長等有關。在適當的條件下，可讓某一波長繞射效率最高而成為反射式繞射光，而另一波長繞射效率為最低，而成為透射光。基於此原理，我們設計一種可用於光纖通信中，具有「波長區分」及「雙向傳輸」功能的雙向波長區分多工元件。為了證明此設計的可行性，我們以感光乳劑厚度為5.7 μm的鹵化銀作為記錄材料，以短波長記錄，長波長重建的方式來研製。曝光波長為532nm，為實驗方便起見，重建波長設為532nm及632.8nm。首先，測量鹵化銀感光材料在重建波長的平均折射率及調變折射率與曝光量之間的關係。由此可決定可用的平均折射率及調變折射率的範圍。其次，將雙向波長區分多工元件中，入射光、反射光及透射光均與法線成45o的條件代入K-vector圖中，以求出曝光時的兩記錄光幾何關係。最後，根據所訂出的幾何關係及曝光量，製作出所設計的元件。理論上來說，此元件對532nm的反射光及632.8nm的穿透光分別有-3.979 dB及-1.549dB的傳輸率。對於1.3/1.55微米光纖通訊用的雙向波長區分多工元件，亦可依照此種設計與方法研製出。全像元件具有製作容易、體積小、重量輕及成本低等之優點。

From the coupled-wave theory, it is obvious that the diffraction efficiency of a reflection-type volume hologram depends on the thickness, the average refraction index, the modulation refraction index of the recording material, and the reconstructed wavelength. Under optimal conditions, a reflection volume holographic element with maximum diffraction efficiency for a chosen wavelength, and minimum diffraction efficiency for another chosen wavelength is designed. Consequently, it can be used as a bi-directional wavelength-division multiplexer/demultiplexer (BIDI WDM) for optical communications. In order to demonstrate its feasibility, silver halide emulsions with a 5.7 m thickness and a diode-pumped solid-state laser with a 532nm wavelength are used. For experimental conveniences, 532nm and 632.8nm are chosen as two reconstructed wavelengths. Firstly, the average refraction index and the modulation refraction index of the silver halide emulsions with various exposures are measured with these reconstructed wavelengths. And their relation curves are depicted. Based on these results, the optimal exposures and its corresponding average refraction index and modulation refraction index can be obtained. Since this element is designed for BIDI WDM, any of the incident angle, the reflected angle and the diffracted angle equals 45o. Substituting these conditions into K-vector diagram, the recording geometry can be derived. Finally, an element is fabricated with the derived recording geometry and the optimal exposure. Its performance is estimated that it has transmission rates 3.979 dB and 1.549 dB for wavelengths 532nm and 632.8nm, respectively. And the elements for 1.3/1.55 m can be fabricated similarly.