高速超寬頻掃頻式摻鈦藍寶石晶體光纖雷射之研究

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 cm-1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。掃頻光源式OCT相比於全場OCT可運用於血管結構，不需要將樣本攤平，可直接插入生物體內進行活體掃描。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，利用其顯著的克爾效應(10 fm2/V2)，折射率變化正比於電場的二次方(折射率變化20倍於鈮酸鋰晶體)，以及非常快速的響應時間(高達500 MHz)，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 782 nm時，在該波長輸入端鍍膜為3.9%穿透率和光柵第零階(正向)繞射效率為12.4%，其背向(輸入端)與正向雷射斜線效率分別為1.36%和4.92%，雷射閥值為421 mW，而單一背向的可調輸出頻寬可達190.86 nm，可調範圍為719.56 nm~910.42 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，其可調頻寬在10 kHz的掃描頻率下可達193.6 nm，在100 kHz仍然可達161.2 nm的可調頻寬。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100 kHz的掃描頻率且仍保有161.2 nm可調頻寬的掃頻雷射技術。未來，若能使用單模光纖作為增益介質，預計可使可調波長雷射的連續可調範圍更大，降低多模態干涉，使掃頻雷射的譜線根數增加許多，更有效率的運用幫浦光功率。

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 laser. The tunable laser can have 300 nm tunning bandwidth. 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 cm-1 made by laser-heated pedestal growth method was used to our researches. Compared with the full-field OCT, swept-source OCT can be used in blood vessel. Somene doesn’t need to flat the sample and can be used in a human artery in vivo. 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 782 nm, the input end coatings had around 3.9% transmittance and the 0th order (forward output) diffraction efficiency of blazed grating was 12.4%. The backward (input end) and forward laser slope efficiency were 1.36% and 4.92%, respectively. The threshold power was 421 mW. A unidirectional backward tunable laser output had 190.86-nm tuning bandwidth from 719.56~910.42 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 bandwidth 193.6 nm with 10 kHz of repetition rate and 161.2 nm with 100 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 100 kHz and up to 161.2 nm simultaneously. In the future, if we can use single mode fiber as our gain medium, we can increase the continuous tuning range of the tunable laser and decrease multimode interference to let the peak number of the swept source laser increase. We can use the pump power more efficiently.

並列關鍵字

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

參考文獻

[1] J. H. Wang, “The study of Ti:sapphire crystal fiber based wavelength swept laser,” Master Thesis, National Taiwan University, Taiwan, 2018.

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[2] http://www.litecure.com/about-photobiomodulation/photobiomodulation-glossary/

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[3] W. Drexler and J. G. Fujimoto, “Optical Coherence Tomography Technology and Applications,” 2nd edition, Springer International Publishing Switzerland, pp. 3–94, pp. 865–911, 2015.

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[4] https://en.wikipedia.org/wiki/Optical_coherence_tomography

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[5] https://www.ntt-at.com/product/ktn_oct/

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高速超寬頻掃頻式摻鈦藍寶石晶體光纖雷射之研究

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