單模寬頻晶體光纖光源之製備與量測

Abstract

我們過去已成功生長出摻鉻釔鋁石榴石雙纖衣晶體光纖，雖然可以成功的將纖心直徑縮小至10 μm，但因纖心與內纖衣折射率差為0.17，仍為多模輸出，為了降低折射率差，我們採用在晶纖上側鍍二氧化鈦與使用高折射率玻璃為纖衣材料兩種方式。 藉由濺鍍二氧化鈦於單晶晶纖上，再使用共抽絲雷射加熱基座長晶法，生長出雙纖衣晶體光纖，以提升內纖衣折射率。在二氧化鈦薄膜厚度1μm下，纖心與內纖衣折射率差由0.17降至0.09。 以高折射率玻璃SF57作為披覆摻鉻釔鋁石榴石晶體光纖的材料，我們先將SF57製作成毛細管，再利用共抽絲雷射加熱基座長晶法，成功生長出單纖衣結構晶體光纖。折射率差可以降為7x10-3，晶纖數值孔徑與傳統單模光纖接近，僅為0.16，在纖心直徑40 μm、波長1.4 μm下，V參數為14.35，晶纖傳輸模態數約為42個。 我們成功生長單纖衣摻鈦藍寶石晶體光纖，並藉由直接雷射退火法，解決鈦離子氧化的問題，提高Ti3+濃度，並以硼玻璃披覆藍寶石晶體纖心。以綠光雷射為幫浦光源，寬頻輸出功率為875 μW，使用藍光二極體直接幫浦，可以得到100 μW的寬頻光源輸出，其輸出光譜之半高寬為180 nm，應用在OCT系統寬頻光源，縱向解析度可達1.44 μm，又因波形相當接近完美的高斯分布，使得干涉訊號旁瓣極小，鄰近與第二鄰近縱向像素互擾指數僅為-26.1 dB與-43.4 dB。

We successfully fabricated Cr4+:YAG double-clad crystal fibers with co-drawing laser heated pedestal growth method to achieve 10-um-core Cr4+:YAG double-clad crystal fiber. This is a multimode fiber due to the high refractive index difference between the core and the inner-clad. In this study, TiO2 side-deposition method and high-index-glass capillary were used to reduce the index difference. Via the sputter method, the TiO2 thin film was coated on the Cr4+:YAG single crystal fiber. To observe double-clad crystal fiber, the refractive index of inner-clad near the interface between the core and the inner-clad was increased from 1.66 to 1.74. Utilizing the SF57 high-index-glass as the surrounding material to fabricate the Cr4+:YAG single-clad crystal fiber, the refractive index difference is about 0.007. The corresponding numerical aperture of the crystal fiber is 0.16, and it is very close to the conventional single-mode fiber. Furthermore, the V-value (normalized frequency) is 14.35 and the propagation mode number of SF57-clad Cr4+:YAG single-clad crystal fiber is 42 for the 40-μm-core diameter. Besides, we successfully fabricated the Ti:sapphire crystal fiber by means of laser heated pedestal growth method to generate broadband amplified spontaneous emission centered at 760 nm with 180 nm bandwidth and 875 μW power pumped by a 532-nm laser. On the other hand, 100 μW output power of this crystal fiber pumped by 446-nm diode laser was also demonstrated. Using this broadband fluorescence as the optical coherence tomography light source, -26.1 dB 1st and -43.4 dB 2nd cross talk factors with 1.44-um-axial resolution were calculated.