High Resolution Cr4+:YAG Based Optical Coherence Tomography System

在光學同調掃描系統之中，縱向解析度由光源的中心波長與頻寬決定；而訊雜比則受光源的不穩度所影響。由本實驗室所製造之摻鉻雙纖衣晶體光纖光源，具備高頻寬及高穩定度，因此可將之架設成高縱向解析度與高訊雜比之近紅外光學同調斷層掃描系統(OCT)。 藉由雷射加熱基座生長法，我們生長出雙纖衣結構的摻鉻釔鋁石榴石(Cr4+:YAG)光纖，當此光纖以波長1064 nm之摻鐿光纖雷射作為幫浦光時，可產生中心波長為1.4 μm、頻寬為233 nm、功率為1.3 mW、功率變異數為2.4*10^-8 W、不穩度為0.24%的自發輻射光源。將其應用於光學同調斷層掃描系統中，可以得到此縱向畫素互擾之第一互擾指數及第二互擾指數分別為26.5 dB、37.1 dB。亦同時擁有3.7 μm縱向解析度、18 μm橫向解析度及76 dB的訊雜比。 利用雙纖衣結構的Cr4+:YAG光纖、縱向與橫向步進馬達及一軸掃描鏡，我們成功架構出一套具三維掃描能力的光學同調斷層掃描系統。在非生物樣本中，掃描並分析了積體電路電感元件之結構得到金屬層厚度，基板與其上下金屬層的光學厚度分別為1.4 μm、2.5 μm及6.8 μm，此結果與IC之設計值吻合；而在生物樣本中，則檢測出單層癌細胞厚度為7.4 μm，並利用頻譜之水吸收特性對皮膚之二維影像進行了水分含量的分析。For optical coherence tomography (OCT), axial resolution is determined by center wavelength and bandwidth of the light source, whereas signal-to-noise ratio (SNR) is affected by instability of the light source. The Cr4+:YAG double-clad fiber made by our own is not only broadband in spectrum but also highly stable; therefore, it is certainly suitable for the development of high axial resolution and high SNR near infrared OCT scanning systems. By means of laser heated pedestal growth (LHPG) method, we have successfully fabricated a Cr4+:YAG double-clad fiber, generating broadband amplified spontaneous emission (ASE) centered at 1.40 μm with a bandwidth of 233 nm, a variance 2.4*10^-8 W of power and 0.24% instability by an 1064-nm Yb:fiber laser pump. Using the ASE as light source of the OCT system, we have shown a 26.5 dB 1st cross talk factor and 37.1 dB 2nd cross talk factor of axial image cross talk, we also have 3.7 μm axial resolution, 18 μm transversal resolution and 76-dB SNR. We successfully built up a 3D OCT system by a Cr4+:YAG double-clad fiber, axial and transversal stepping motor and a scanning mirror. We scanned and analyzed the inductor part of IC as a non-biological sample and found out the thickness of metal layer is 1.4 μm and the optical path length of substrate is 6.8 μm which matches the design of IC. On the other hand, we scanned biological samples and demonstrated a result of 7.4-μm-thick single-layer human pulmonary adenocarcinoma cells, we also demonstrated the moisture difference in skin with ASE spectrum deformation caused by the water absorption