半導體雷射幫浦摻鈦藍寶石晶體光纖之可調波長雷射研究

Abstract

摻鈦藍寶石晶體為常見的寬頻雷射材料，其3-dB頻寬可達180 nm，廣泛地使用於寬頻可調波長雷射、鎖模飛秒雷射；而其中心波長為780 nm之螢光波段落在常見的人體組織之低吸收區域，因此被廣泛使用於生物醫學影像量測技術，如光學同調斷層掃描術(OCT)等。使用LHPG法生長纖心直徑為16 μm且低傳輸損耗(0.075 dB/cm)之玻璃包覆摻鈦藍寶石晶體光纖結構作為雷射之增益介質，可以提供高強度幫浦與訊號光，並克服摻鈦藍寶石晶體本質上的兩大缺點: 低吸收截面積與低螢光生命週期，達到低閥值且高效率之摻鈦藍寶石雷射。長度24 mm之玻璃纖衣摻鈦藍寶石晶體光纖經由端面研磨與拋光處理後，利用介電質電子槍蒸鍍系統於晶體光纖端面鍍製設計之光學薄膜後，做為雷射增益材料。以520-nm綠光半導體雷射作為激發光源，無外加散熱系統下，架設內腔式、外腔式與波長可調式雷射。 在內腔式雷射架構中，以20%輸出耦合鏡下，其斜線效率與閥值功率分別為24.9%與140.5 mW，且在1 W幫浦功率下，最高輸出功率為215 mW，其結果同時展現高效率與低閥值雷射特性。在增益波導效應下，其雷射輸出近乎為基模。在外腔式架構中，在高反射輸出耦合鏡下，雷射閥值為37.3 mW，其結果低於文獻所記載值；在17.8%輸出耦合鏡下，雷射斜線效率與雷射閥值分別為18.6%與123.2 mW。以雙折射濾波器或光柵輸出耦合鏡做為波長可調元件，達到波長可調雷射。在雙折射濾波器做為波長調變元件下，其可調範圍為710- 860 nm，共150 nm寬。而輸出功率大於50 mW之可調區間可達130 nm寬，可提供足夠之功率應用。在光柵輸出耦合鏡架構下，可達到波長連續可調，其範圍為693.4- 876.5 nm，共183.1 nm寬，而3 dB可調頻寬為143 nm。其可調雷射特性充分應用摻鈦藍寶石180 nm之寬頻頻寬，而低閥值且高效率之特性，具有相當大的潛力可取代目前的摻鈦藍寶石雷射。

Ti:sapphire crystal is a widely used material. It has a wide emission spectra with center wavelength located around 780 nm and a 180-nm, 3-dB bandwidth. Due to its broadband emission spectra, there are various applications in wavelength tunable lasers and mode-locked lasers. Since its emission wavelength also sits in the region where there is low tissue absorption, there are also applications in biological imaging technologies, such as optical coherence tomography (OCT). To solve the two drawbacks of Ti:sapphire crystal: low absorption cross section and low fluorescence lifetime, Ti3+:Al2O3 single-cladding crystal fiber was grown using the LHPG method. As the laser gain material, crystal fiber with length of 24 mm was used. The single crystal core diameter was 16 μm and the glass cladding outer diameter was 320 μm. The Ti3+ doped concentration was 0.049 wt.%, with regarding to a 0.075 dB/cm attenuation coefficient, which is the lowest value recorded for Ti:sapphire waveguide structures. After end face grinding and polishing, the crystal fiber was coating with dielectric coating using thin film E-gun deposition system to form the required optical cavity structure. By using a 520-nm LD as the pumping source, intra-cavity and external-cavity lasers were constructed. Due to a gain guiding effect, the output mode is near fundamental mode. Under intra-cavity setup, with a 20% output coupler, the laser efficiency and laser threshold were 24.9% and 140.5 mW, respectively. The maximum output power at 1W pumping was 215 mW, achieving high slope efficiency and low threshold simultaneously. Under external-cavity setup, with high reflectance output coupler, the laser efficiency and laser threshold were 18.6% and 123.2 mW, respectively. With wavelength tuning elements, such as inserting a birefringent filter or using a grating output coupler, tunable laser was achieved. Under birefringent filter setup, the tuning was not continuous. The wavelength tuning range was 710-860 nm, with a 150-nm width. Range of output power exceeding 50 mW was 130 nm, providing sufficient output power for various applications. Under grating setup, continuous wavelength tuning was achieved. Continuous range was 693.4- 876.5 nm, with a 183.1 nm width. With 3-dB tuning bandwidth of 143 nm, the broadband emission properties of Ti:sapphire crystal fiber were demonstrated .