可調波長摻鉻釔鋁石榴石晶體光纖雷射之研究

Abstract

摻鉻釔鋁石榴石具有寬頻放射頻譜，中心波長落在1.4 μm附近，其3 dB增益頻寬可達250 nm，涵蓋常用光通訊、眼球安全波段以及水吸收波段。對於頻域式光學同調斷層掃描(Fourier domain OCT)而言，例如掃頻式同調斷層掃描術(SS-OCT)，其寬頻特性可提供良好的縱向解析度。 在本論文中我們利用雷射加熱基座長晶法製成玻璃雙纖衣摻鉻釔鋁石榴石晶體光纖，並以其作為雷射增益介質利用反射式光柵將波長從1384 nm調變至1479 nm共95 nm的調變範圍。在晶體光纖的兩端使用電子槍蒸鍍方式，將TiO2和SiO2做為原料沉積高品質的介電質膜層做為共振腔元件。為求雷射斜線效率最佳化，幫浦光源的偏振方向調整到與晶軸方向一致，在3 W幫浦功率下能達到200 mW的雷射輸出，而在以1064-nm LD做為幫浦激發下達到12.9%的斜線效率和51.6 mW的低閥值，其產生的雷射偏振消光比最高可達12:1。 為了達到更高的增益，我們嘗試減少長晶過程中加熱的步驟，以保留更高的四價鉻離子濃度。實驗量測結果顯示，減少單次加熱步驟使單位離子濃度從3.4×10^17 〖cm〗^(-3)增加至5.6×10^17 〖cm〗^(-3)。我們藉由模擬結果顯示在越高濃度下，晶體光纖所能產生的淨增益更能隨輸入幫浦功率大小成線性提升，相較於一般製程下的正常濃度的晶體光纖，其增益大小更不易因幫浦功率上升而飽和。我們也在模擬中除去加熱帶來的離子擴散影響，當晶體光纖在原始晶棒的濃度下配合雙向幫浦，其所產生之淨增益有望達到25 dB以上。

Cr4+:YAG crystal is a material for broadband light source. The central wavelength is around 1.4 μm and its 3dB gain spectrum can reach 250 nm. The emission spectrum covered the E, S, C band of optical communication, eyes safe wavelength range and water absorption window. It’s also a ideal candidate for Fourier domain OCT light source since the axial resolution depends on the bandwidth, the widely wavelength tuning range can supply better axial resolution for OCT image. In this thesis, we use laser-heated pedestal growth (LHPG) to growth Cr4+:YAG crystal fiber, and then cladded with glass using co-drawing LHPG process to make double cladding crystal fiber (DCF). The Cr4+:YAG crystal fiber is used as the laser gain media, the wavelength tuning bandwidth is 95 nm, tuning range from 1384.8 nm to 1479.4 nm by using reflective grating as wavelength selector. The both endface of crystal fiber are coated with high quality dielectric coating. The collimated external cavity Cr4+:YAG fiber laser can reach 200 mW output under 3 W pumping. The 12.9% laser efficiency and 51.6 mW low threshold are reach by hemispherical external cavity Cr4+:YAG fiber laser cavity structure. To increase the gross gain in the cavity, a higher Cr4+ ion concentration crystal fiber was made by reducing the heated step of LHPG process. The higher ASE power and less residual pump power implies that more Cr4+ ion remain in crystal fiber than the DCF under usual LHPG process. The measurement result also shows that the concentration of Cr4+ ion remain 40% more in crystal fiber through this process. The gain simulation also shows that if the Cr4+ ion in crystal fiber can recover to original concentration, the gross gain in cavity can reach over 25 dB under double pumping system.