可調多波長量子點外腔式雷射及其應用於產生兆赫波輻射源之研究

Abstract

本論文目的在藉由啁啾式堆疊的量子點雷射低臨界電流密度、寬廣增益頻譜的特點，來實現多波長可調的外腔式雷射，並用來作為光混頻的光源，以期達到室溫操作、電激發、便宜、簡便的兆赫波源。論文中針對兩大課題作研究：(一)可調式外腔式雷射，(二)光混頻產生兆赫波。 理論部分，在外腔式雷射中，我們說明了外腔式可調的原理，並進一步解釋使用單一增益介質，能達到多波長雷射的可能性；在光混頻的部分，我們詳細說明了脈衝操作與連續操作時的原理。 而實驗部分，我們使用啁啾式堆疊的量子點長晶結構，製作出寬廣發光波段的雷射元件，在注入電流密度1.33 kA/cm^2 下，Littrow架構下， 150 nm(1135 nm~1285nm)；在Littman架構下，調制範圍132 nm (1148 nm~1281nm)；而兩種傳統架構下，訊號對放大自發性輻射比幾乎皆達 40 dB。為實現多波長可調外腔式雷射，我們使用了兩種Littman架構的變形：FT-ECL、Double-Littman ECL。在FT-ECL架構下，實現了雙、三、四波長雷射，雙波長最大的波長間距為 86 nm，三波長最大間距為 13 nm，四波長最大間距為 4 nm；而接著使用Double-Littman的架構，可以快速地量測出可調制的雙波長雷射極限，藉由獨立地控制兩平面反射鏡反饋光的程度，達到雙波長最大的波長間距 126 nm，訊號對放大自發性輻射比達 20 dB 以上。最後利用雙波長雷射兩頻率差落在THz的特點，藉由光導天線的輔助，輻射出兆赫波，使用初步THz量測架構，量測到了輻射的訊號。

In this work, the chirped multilayer quantum-dot (QD) gain media are arranged in Fourier-transform external-cavity laser (FT-ECL) setup. Novel slit designs select 2, 3, and 4 different wavelengths that are diffracted from the grating for optical feedback. Therefore, the dual-, triple- and quadruple-wavelength ECLs are demonstrated in this study. The resulted multi-wavelength lasing emissions are achieved under injected current of 100 mA (or 1.33 kA/cm2) with signal to amplified spontaneous emission (ASE) ratio over 20 dB. Around peak-gain wavelength of 12xx-nm range, the adjacent wavelength separation is over 50 nm for dual-wavelength lasing, up to 13 nm for triple-wavelength lasing, and about 4-5 nm for quadruple-wavelength lasing emissions. To further extend the wavelength separation for dual-wavelength lasing emissions, another modified scheme with two separate external mirrors are adopted and the achieved maximum value is about 126 nm in wavelength separation or over 25 THz in frequency separation. The terahertz (THz) generation by photomixing of dual-wavelength ECLs is also discussed. Finally, by using the Golay cell detector, we measure the radiation signal from the photomixer.