掃頻式摻鈦藍寶石晶體光纖雷射之研究

The study of Ti:sapphire crystal fiber based wavelength swept laser

王智弘(Jhih-Hong Wang)

指導教授 ： 黃升龍

國立臺灣大學/電機資訊學院/光電工程學研究所/碩士(2018年)

https://doi.org/10.6342/NTU201802142

摘要

摻鈦藍寶石以具有寬廣的放射頻譜而聞名，其螢光頻譜之半高寬可達180 nm而常使用於可調波長雷射或是鎖模雷射；且760 nm的螢光中心波長落於組織散射損耗及水的吸收較小的波段或稱該區段為診療視窗，因此亦廣泛使用於生物量測系統，除此之外，對於光學同調斷層掃描術(OCT)系統而言，寬頻的輸出特性具備了高度的縱向解析度。然而，摻鈦藍寶石因本身的短螢光生命週期以及低吸收截面積特性而難達到低閥值輸出，本實驗室使用雷射加熱基座長晶法生長出纖心直徑為16 μm且衰減係數僅為0.017 1/cm的玻璃纖衣之波導結構，建構出高效率、低閥值的雷射輸出以解決上述缺點。
近年來因掃頻雷射的發展，而掃頻光源式OCT相比於傳統時域式OCT具備了更高的敏感度以及更快的成像速度。鉭鈮酸鉀(KTN)則因近期晶體生長技術的成熟，已有相當程度的應用如超高速光偏折器或是可調焦距透鏡，利用其顯著的二次電光效應(折射率變化大於20倍於鈮酸鋰晶體)以及非常快速的響應時間(數百倍於傳統機械式偏折器技術)，可將掃頻雷射系統的尺寸縮至更小，且具有更為快速的掃描能力。
於寬頻可調波長雷射架構中，我們以閃耀光柵作為雷射系統的輸出耦合鏡及可調濾波器並且在腔內配置較低損耗的非球面準直透鏡，當雷射波長調整至805 nm 時，在該波長輸入端鍍膜為3%穿透率和光柵第零階(正向)繞射效率為9.3%，其背向(輸入端)與正向雷射斜線效率分別為1.75%和6.68%，雷射閥值為210 mW。而單一背向的可調輸出頻寬已達190 nm，可調範圍由724至914 nm。
在掃頻式雷射架構中，以高速掃描且大角度調制的KTN光偏折器取代傳統光偏折技術，並配合閃耀光柵製作出以摻鈦藍寶石晶體光纖作為雷射增益介質的掃頻式雷射，當重複率為60 kHz 時，其可調範圍由775至850.6 nm。根據我們所知，目前以非機械且電控式的摻鈦藍寶石掃頻雷射來說，這是第一個能以高速掃描頻率且仍保有近80 nm 可調頻寬的掃頻雷射技術。未來，若能提升幫浦功率至 1.26 W，在相同的可調頻寬下，掃頻摻鈦藍寶石雷射能以相同的雷射數量以100 kHz的重複率輸出；或使用高效率且色散能力較小之閃耀光柵，則掃頻雷射則有潛力朝向更為寬頻的輸出表現。

關鍵字

摻鈦藍寶石 ； 晶體光纖 ； 鉭鈮酸鉀光偏折器 ； 寬頻可調波長雷射 ； 掃頻式雷射

並列摘要

Titanium (Ti):sapphire has been well known for its broadband emission spectra with 180-nm width of 3-dB bandwidth. Due to this property, it can be applied to tunable lasers or mode-locked lasers. The center wavelength of fluorescence is 760 nm, which falls into a region called therapeutic window with low tissue scattering and low water absorption. Therefore, it is widely used in bio-measurement systems. Besides, for optical coherence tomography (OCT) system, the broadband feature means Ti:sapphire light source has high axial resolution.
To overcome short fluorescence lifetime and low absorption cross section that causes high threshold power of Ti:sapphire laser, a glass-clad and crystal-fiber structure with core diameter of 16 μm and low attenuation coefficient of 0.017 1/cm made by laser heated pedestal growth method was used to our researches.
Compared with the traditional time-domain OCT, swept-source OCT shows higher sensitivity and imaging speed features. KTa1-xNbxO3 (KTN) crystal is a brand-new technology as ultra-high speed optical deflectors or vari-focal lens in recent years. With KTN’s large eletro-optic effect (δn: >20 times than LiNbO3) and short response time (hundreds of times than conventional scanners), a small-sized and high-speed swept light source of OCT system can be expected.
Under broadband tunable laser setup, using blazed grating as Ti:sapphire crystal fiber laser’s output coupler and tunable filter and lower loss of aspheric lens as collimated lens in laser cavity, when the lasing wavelength was tuned to 805 nm, the input end coatings had around 3% transmittance and the 0th order (forward output) diffraction efficiency of blazed grating was 9.3%. The backward (input end) and forward laser slope efficiency were 1.75% and 6.68%, respectively. The threshold power was 210 mW. A unidirectional backward tunable laser output had 190-nm tuning bandwidth from 724 to 914 nm.
In wavelength swept laser system, replacing conventional optical scanners, KTN deflector could deflect light without any moving part rapidly and widely. Combining
blazed grating and KTN deflector into external-cavity laser system, we constructed a wavelength swept Ti:sapphire crystal fiber laser which had tuning range from 775 to 850.6 nm with 60 kHz of repetition rate. Recently, to our knowledge, for not mechanically but electrically tuned Ti:sapphire swept laser, this is the first time that the repetition rate and tuning bandwidth can be up to such a fast value and around 80 nm simultaneously. In the future, if we can improve the pump power to 1.26 W, under the same tuning bandwidth, swept Ti:sapphire crystal fiber laser will have 100 kHz of repetition rate with the same laser peaks as 60-kHz result. And using higher efficiency and weaker dispersion of blazed grating, swept Ti:sapphire laser will have a potential toward a wider swept laser performance.

並列關鍵字

Ti:sapphire ； crystal fiber ； KTN deflector ； broadband tunable laser ； wavelength swept laser

參考文獻

Reference

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