半導體量子結構雷射元件之相對雜訊強度研究

Abstract

本篇論文目的在藉由量測半導體雷射之相對雜訊強度的方法，分析多層量子井分佈回饋型雷射 (DFB) 與自行設計之啁啾式堆疊多層量子點雷射的高頻操作特性。過去研究量子井雷射的文獻已趨完整，本論文延伸此觀念對多層堆疊量子"點"雷射做進一步探討。 首先討論DFB雷射的微分增益值與K係數變溫量測之下的結果。當溫度從10 ℃升高至40 ℃，增益頻譜受熱延展，微分增益值隨溫度升高而下降了1.5倍 ( 1.66×10-15 cm2 降至1.1×10-15 cm2)。然而，我們的實驗中最大調變頻寬在同樣溫度範圍內幾乎保持定值27 GH，符合文獻記載多層量子井雷射的特性 。 接下來針對自行設計的多層堆疊啁啾式量子點雷射進行量測分析。一般建議待測量子點雷射的腔長小於2mm，在我們的量測之中腔長750 μm的元件經過校正RIN頻譜顯示了最低強度值為 -160 dB/Hz，並且必須是激發態發光，增益值才足夠克服總耗損而達到閾值條件；另外希望直接藉由探針點測自然劈裂的雷射元件，但是此量測方式控溫不易，導致電流密度上升的過程接面溫度快速上升，微分增益值由8.2´10-16 cm2 下降至 3.0´10-16 cm2。儘管如此，因為載子傳輸受堆疊多層量子結構限制反而保護K係數不受溫度影響，最大調變頻寬為14 GHz，相當於Stevens等人於2009年八月直接調變激發態量子點雷射的最大頻寬值。 據了解，我們首次取代直接調變以量測相對雜訊強度的方式預測最大頻寬。 另外當共振腔長更短時，意外觀察到RIN頻譜出現雙共振頻率的現象。但是造成多重共振頻率的機制仍具爭議，亟需更進一步的研究與分析。

Relative Intensity Noise (RIN) of multiple quantum well (MQW) DFB laser and chirped multilayer quantum dot (CMQD) lasers have been measured and analyzed. The carrier dynamics in multilayer quantum structure are therefore determined. The temperature-dependent RIN measurement of MQW DFB laser was undertaken to evaluate the K-factor and differential gain. Carrier transport is limited by multiple layer structure in the DFB laser as the values of K-factor remain almost constant in the temperature range of 10 - 40 ℃. Therefore, the intrinsic fmax is evaluated to be 27 GHz. However, differential gain reflects the nature of gain spectrum broadening which decreases by a factor of approximately 1.5 (from 1.66×10-15 cm2 down to 1.1×10-15 cm2) over the measured temperature range. In general, cavity length for RIN measurement is suggested to be within 2 mm. The characteristics of chirped multilayer quantum dot (CMQD) lasers has been presented with different cavity lengths of 750 μm, 1000 μm, and 1500 μm at ambient temperature of 20 ℃. For cavity length of 750 μm, the highly damped RIN spectra have calibrated level of -160dB/Hz. In addition, excited state lasing is essential in our device in order to overcome the total loss and therefore reaches the lasing condition. The differential gain is estimated to be in the range of 3.0-8.2´10-16 cm2, which is subject to junction heating in as-cleaved devices. However, the K-factor limited bandwidth , which is temperature insensitive, is as large as 14 GHz, shows excellent agreement with Stevens et al., who firstly demonstrated direct modulation of excited state QD lasers in August 2009. To the best of our knowledge, we have successfully demonstrated RIN spectrum of excited state quantum dot lasers for the first time. Another unexpected observation is the double-resonance RIN spectra in even shorter cavity length. However, the mechanism is still a controversial issue. Therefore, this thesis has thrown up some questions for further investigation.