掺高濃度鉻四價離子之鉻釔鋁石榴石晶體光纖之仿真數值模擬

Abstract

摻鉻釔鋁石榴(Cr4+：YAG)晶體是一種常見的雷射增益介質。該晶體經過泵浦光增益後所產生的1100-1700 nm的近紅外波段的寬頻光譜，常被使用在製作光放大器應用於光通信技術中，亦或製作高速掃頻雷射以應用於光學同調斷層掃描技術（Optical coherence tomography：OCT）中。在過去的研究發展中，摻鉻釔鋁石榴石晶體中正四價鉻離子之摻雜濃度不足這一現實條件，一直影響著該增益介質的增益能力。目前，有一種新的摻鉻釔鋁石榴石晶體製成方法得以實現。該方法將正二價鈣離子與正二價鎂離子摻雜入釔鋁石榴石中並製成為晶體光纖，能有效地增加正四價鉻離子之濃度，並使得正四價鉻離子更多的以正四面體配位存在於晶體中，以期望提升其增益效果。 本論文中主要討論了通過數值模擬的方法，模擬由有更高正四價鉻離子摻雜濃度的摻鉻釔鋁石榴石晶體光纖在作為增益介質時，光放大器和掃頻雷射之表現。經過由MATLAB作為基礎的數值模擬，該晶體光纖可以在光放大器的應用中，提升到原本四倍的離子濃度，於20 W泵浦功率下，將放大倍率提升41.4倍。離子濃度提升4到10倍後，光纖長度可縮減至7到9公分。在掃頻雷射的應用中，提升十倍的離子濃度後，掃描範圍達到196 nm，比實驗提升了至少 62 nm 的掃描範圍，掃描重複頻率達到了150 kHz，並在使用78%反射率的外腔反射鏡下達到最佳輸出功率。繼而說明該種新製成方法所製作的摻鉻釔鋁石榴石晶體光纖，可以在未來的應用中有更好的提升使用效果。

Chromium-doped yttrium aluminum garnet (Cr⁴⁺: YAG) crystal is a common laser gain medium. The broadband near-infrared spectrum (1100-1700 nm) generated by this crystal after pumping gain is widely used in optical amplifiers for optical communication technologies, or in high-speed swept-source lasers for Optical Coherence Tomography (OCT). In previous research, the insufficient doping concentration of Cr⁴⁺ ions in Cr⁴⁺: YAG crystals has long limited the gain capability of this gain medium. Currently, a new fabrication method for Cr⁴⁺: YAG crystal fibers has been developed. By co-doping divalent calcium (Ca²⁺) and magnesium (Mg²⁺) ions into yttrium aluminum garnet to form crystal fibers, this method effectively increases the Cr⁴⁺ concentration and promotes more Cr⁴⁺ ions to exist in tetrahedral coordination in the crystal, aiming to enhance the gain performance. This paper focuses on numerical simulations of optical amplifiers and swept-source lasers using Cr⁴⁺: YAG crystal fibers with higher Cr⁴⁺ doping concentrations as gain media. Based on MATLAB numerical simulations, the crystal fiber achieves a four-fold increase in ion concentration in optical amplifier applications. Under a 20 W pump power, the amplification factor is improved by 41.4 times. When the ion concentration is increased by 4 to 10 times, the fiber length can be reduced to 7-9 cm. In swept-source laser applications, with a ten-fold increase in ion concentration, the scanning range reaches 196 nm, which is at least 62 nm wider than that in previous experiments, and the scanning repetition rate reaches 150 kHz. The optimal output power is achieved when using an external cavity mirror with 58% reflectivity. These results demonstrate that Cr⁴⁺: YAG crystal fibers fabricated by this new method can provide enhanced performance in future applications.