週期性區域極化反轉鉭酸鋰晶體光纖之研製

Study and Fabrication of Periodically Poled Lithium Tantalate Crystal Fiber

孫得皓(De-Hao Sun)

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(2007年)

https://doi.org/10.6342/NTU.2007.01328

摘要

在非線性光學領域中，波長轉換器為高效率光傳輸系統中最重要的一項技術。於光纖傳輸中，當傳輸訊號量超過10 Giga Bytes時，全光波長轉換器比起傳統之電光-光電轉換器有著更大的優勢；此外，利用波長轉換器產生的藍綠光光源，可應用在密度光學儲存、影像顯示、生物分析與水下通訊方面，皆為其最重要的技術之一。利用非線性晶體配合準相位匹配週期設計，可達到諸如上述各種不同之應用，其中關鍵則是非線性晶體之優劣性以及準相位匹配週期之準確與均勻性。

本論文探討以雷射加熱基座法(LHPG)生長出鉭酸鋰晶體光纖，外加以週期性變換之高壓電場，製作出波長轉換用的週期性區域極化反轉之鉭酸鋰晶體光纖。在製作的過程中，我們藉以改變外加電場之參數，以利於增進極化反轉之形成，並分析找出最佳化之製程條件，用於日後自動化流程之晶體生長。在週期檢測方面，我們利用倍頻顯微術觀察實際上極化反轉之週期，設計週期為20.67 um的PPLTCF其掃描後分析後，其週期為21.33 um，與理論值僅有3.2%的誤差。並針對倍頻掃描之影像，由實驗結果建立理論模型，假設在極化反轉交界處之偶極矩極化為正負隨機分布，進而利用Fortran程式來模擬，可成功地解釋倍頻影像下極化反轉邊界處有較強倍頻的成因。
而在光學實驗上我們以平均功率100 mW，波長1524.24 nm之基頻光入射時，可量測到0.77 uW的倍頻輸出，成功地實現了以PPLTCF作為倍頻轉換元件的初步目標。

文中更介紹以串接式倍頻/和頻理論設計可調式藍綠光之波長轉換器，由我們新提出之非均勻式漸變週期法設計出元件，其基頻光有著225.52 nm的3 dB頻寬，對應至藍綠光輸出3 dB頻寬為75.17 nm，範圍由460.25 nm至535.42 nm，且比起相同晶體長度下的傳統均勻式漸變週期設計方式有著更寬以及更平坦的頻寬。

關鍵字

鉭酸鋰 ； 非線性光學 ； 波長轉換 ； 頻率轉換 ； 極化反轉 ； 共焦 ； 倍頻顯微 ； 週期漸變

並列摘要

Wavelength converters based on nonlinear optics is one of the most important techniques in high efficienct optical communication systems. For fiber communication, especially when the capability of transmission is over 10 Gb/s, all-optical wavelength converters have advantages than traditional electrical-optical-electrical converters. Besides, blue/green light generation due to wavelength conversion plays a great role in many applications, such as high-density-optical storage, display, biomedical analysis, and under-water communications. Various applications can be achieved by designing quasi-phase-matching (QPM) period no nonlinear-optical crystals. The key factors to achieve high conversion efficiency are crystal quality and pitch uniformity.

In this thesis, periodically poled Lithium Tantalate crystal fiber (PPLTCF) for wavelength conversion was grown by laser heated pedestal (LHPG) growth method with additional high-electric field bias. During the process of fabrication, parameters of applied E-field were optimized in order to achieve domain inversion with high uniformity and process automation. To examine domain pitch of PPLTCF, a confocal second harmonic (SH) microscopy was used. The analyzed SH pattern showed that the 21.33-um domain pitch had only a 3.2% deviation from the 20.67-um designed pitch. It was unexpected that the SH sigal at the domain interface is stronger than that in the +Z and –Z domains. To examine this, a randomly distributed polarization model was established, and compared with the experimental result with good agreement.

In the optical experiment, the SH signal was measured to be 0.77 uW at 1524.24-nm fundamental wavelength with an 100-mW pump power. Accordingly, a PPLTCF SHG device made by LHPG method was experimentally demonstrated. Besides, a wavelength-conversion design for generating tunable blue/green light was proposed by means of self-cascaded SH generation and Sun frequency generation effect. Based on the nonlinear-chirped-grating design, the simulation showed an extended 225.25 nm 3-dB bandwidth for fundamental wavelength. It corresponds to a blue/green bandwidth of 75.17 nm in the range from 460.25 nm to 535.42 nm. With the same crystal length, the spectrum width was broader and more flattened by nonlinear chirp than that by linear chirp.

並列關鍵字

Litao3 ； PPLT ； Nonlinear optics ； wavelength conversion ； frequency conversion ； domain inverse ； poling ； SH microscopy ； confocal ； second harmonic ； fiber grating ； chirp

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