摻鉻釔鋁石榴石晶體光纖主動元件之研製

Abstract

光學同調斷層掃描技術，具備非侵入式、即時與高空間解析度之優點，未來在臨床細胞檢測具有極大的潛力。光學同調斷層掃描術的縱向掃描解析度，取決於光源的同調長度，若頻寬愈寬，中心波長愈短可得到較高的縱向解析度。Cr4+:YAG (摻鉻釔鋁石榴石)晶體可產生波長為1.3 μm~1.6 μm的能量增益，在1.4 μm波長附近有一水吸收波段，作為OCT系統掃描時，可偵測人體皮膚中的含水量分佈，對醫學應用方面具有優勢。 Cr4+:YAG的寬頻增益對於光通訊應用亦有很大的發展性。現今光纖可使用的波段為1.3 μm~1.6 μm，其傳輸損耗在1.2 μm~1.7 μm的波段可低至於0.2 dB/km甚至更小，但現有的光放大器只適用於其中的部份波段。Cr4+:YAG晶體之增益頻譜涵蓋了整個光通訊波段，若發展出光纖放大器將有助於提昇光纖通訊的整體頻寬。 本實驗使用雷射加熱基座長晶法成功生長出Cr4+:YAG DCF (雙纖衣晶體光纖)、Borosilicate-clad Cr4+:YAG SCF (單纖衣晶體光纖)以及N-LaSF9-clad Cr4+:YAG SCF。Borosilicate-clad Cr4+:YAG SCF傳輸損耗僅為0.03 dB/cm，Cr4+:YAG DCF傳輸損耗為0.04 dB/cm，以1064 nm單模二極體雷射功率400 mW幫浦Borosilicate-clad Cr4+:YAG SCF，受限於晶纖長度得到200 μW的放大自發輻射光輸出其中心波長為1382 nm，頻寬為223 nm。為了成功發展掃頻式OCT之光源，本論文呈現兩種不同架構的Cr4+:YAG DCF雷射外部共振腔系統，半球型外部共振腔雷射架構其雷射輸出斜線效率可達11.3%，閥值功率僅為44 mW，準直式外部共振腔雷射架構其雷射輸出效率可達到6.8%，閥值功率60 mW。 為了成功發展摻鉻釔鋁石榴石單纖衣晶體光纖雷射，降低纖衣與纖心之間的應力避免Cr4+螢光生命週期以及受激發射截面的下降，本論文藉由量測鉻釔鋁石榴石晶體光纖R-line偏移量，得知以高折射率玻璃包覆纖心小於27 μm之單纖衣晶體光纖具有很大潛力。 本論文也模擬分析Cr4+:YAG DCF放大器的小訊號增益放大，在總吸收功率4.6 W的雙向對稱幫浦下，以我們目前技術能生長出晶纖長度為20 cm的Cr4+:YAG DCF，在纖心直徑為20 μm可達到10 dB的毛增益。

Optical coherence tomography (OCT) plays an important role in medical applications due to high longitudinal resolution and noninvasive detection. The high longitudinal resolution of optical coherence tomography depends on the light source coherence length. Higher longitudinal resolution can be obtained with wider bandwidth or shorter center wavelength. The Cr4+:YAG crystal has a broadband emission from 1.3 μm to 1.6 μm with a water absorption band located near 1.4 μm. While applied in an optical coherence tomography system, it can be used to detect the water distribution in human tissues and has great potential in medical applications. Nowadays, available bandwidth of optical fiber is 1.3 μm~1.6 μm, and the propagation loss at 1.2 μm~1.7 μm band can be as low as 0.2 dB/km or less. Despite the large bandwidth of the transmission fiber, the existing fiber amplifiers cannot cover the whole bandwidth. The emission bandwidth of Cr4+:YAG crystal fiber covers the whole communication bandwidth and could be helpful to high-speed data communication. We have successfully fabricated Cr4+:YAG double-clad crystal fibers (DCF), borosilicate-clad Cr4+:YAG single-clad crystal fibers (SCF) and N-LaSF9-clad Cr4+:YAG SCF with co-drawing laser-heated pedestal growth method . We measured the propagation loss of different kinds of Cr4+:YAG crystal fibers by the cutback method. The propagation loss of borosilicate-clad Cr4+:YAG SCF and Cr4+:YAG DCF are only 0.03 dB/cm and 0.04 dB/cm, respectively. The borosilicate-clad Cr4+:YAG SCF generates broadband amplified spontaneous emission centered at 1382 nm with 223 nm bandwidth and 200 μW power when pumped by a 400 mW 1064-nm diode laser, and the output power is limited by the length of fiber . In order to develop swept-source OCT, we present hemispherical external-cavity laser and collimated external-cavity laser. Lasing threshold of hemispherical external-cavity laser system was 44 mW and the slope efficiency was 11.3%. In collimated external-cavity laser, the slope efficiency was 6.8% and the threshold was 60 mW. In order to successfully develop Cr4+:YAG SCF lasers, we have to lower the strain field between core and clad to avoid the decrease of Cr4+ fluorescence lifetime and stimulated emission cross sections. By measuring the shift of R-line fluorescence of the crystal fibers, we found that the N-LaSF9-clad Cr4+:YAG SCF with a core diameter less than 27 μm has great potential for the crystal fiber laser. We simulated Cr4+:YAG double-clad crystal fiber amplifier and achieved 10-dB gross gain with a 4.6 W bi-directional symmetric pumping with a length of 20 cm and a core diameter of 20 μm.