被動式鎖模半導體量子點雷射之研究

Abstract

本篇論文主旨在研究適用於光纖通訊及生醫影像等應用中的超短雷射脈衝源，內容包含整合式與外腔式被動鎖模量子點雷射元件的研究成果，主要分為三個部分：(1)利用量子點基態躍遷發光的被動鎖模半導體雷射；(2)利用量子點激發態發光的被動鎖模雷射特性研究；(3)利用外腔架構實現波長與重複率可調的多功能短脈衝光源，並具有目前文獻上被動式鎖模半導體雷射可達到最低的重複率。 在鎖模量子點雷射基態發光的研究當中，我們利用砷化鎵基板上成長的砷化銦量子點，在整合的兩截式波導結構中，成功製作出具有高重複率20 GHz與40 GHz、小於2 ps的脈衝及在1.3微米發光波段的被動式鎖模雷射元件。藉由架設一整合式的自動化量測分析系統，我們探討元件在不同操作條件下對脈衝特性的影響，此外在40 GHz的雷射中，我們並觀察到量子點鎖模雷射從基態轉換至激發態的現象。 承續在基態鎖模雷射中的經驗，我們萃取在量子點雷射在連續波輸出時的特性參數，設計出適合在激發態操作的被動鎖模量子點雷射，預期利用激發態發光可得到較基態更短及更高輸出功率的脈衝。在雷射輸出功率與注入電流的特性上我們觀察到與基態相反的遲滯現象，及脈衝重複率隨著與操作電流下降，後者我們歸究其原因為在激發態中折射率對載子具有負的斜率(負的微分折射率)，此特性將有助於得到較短的光脈衝及更好的光侷限特性，此外，利用外部的色散補償，我們將脈衝壓縮至780 fs，此亦為目前利用量子點激發態發光所得的最短脈衝。 在外腔式雷射的架構中，我們利用光柵耦合兩截式的Ｊ型波導量子點元件，實現多功能的連續波或短脈衝雷射源。當兩截波導在相同順向偏壓操作時其輸出為連續波，利用光柵選擇，我們在1.3微米波段成功達成140奈米的連續調變，其波長範圍涵蓋量子點基態與激發態的發光頻譜，雷射光譜半高寬皆小於0.1奈米，且具有大於30分貝的旁模抑制比；當其中一段波導被施以逆向偏壓做為飽和吸收體、另一段仍為順向偏壓增益區時，其輸出為被動式鎖模的雷射脈衝，利用光柵選擇，其波長在基態與激發態可調變波長範圍分別為33 nm與30 nm，脈衝寬度在10 ~ 30 ps範圍；最後，藉由調整外部共振腔的長度，其脈衝重複率可由79.3 MHz到2 GHz連續調變，其中79.3 MHz為目前文獻上被動鎖模半導體雷射所能達到的最低重複率。

In this dissertation, passive mode locking via ground-state or excited-state transitions of quantum dots (QDs) are studied in monolithic diode lasers or in external-cavity configurations, which generate ultrashort pulses near the 1.3-μm wavelength range and are potential for applications in optical fiber communication and bio-photonics. In the investigation of ground-state mode locking in monolithic lasers, self-assembled InAs QDs were grown on GaAs substrate by molecular beam epitaxy as the active media and were incorporated into two-section ridge-waveguide diode lasers. Passive mode locking was demonstrated with the repetition rate up to 40 GHz, pulse width smaller than 2 ps and lasing wavelength in the 1.3-μm range. An integrated measurement system was setup, which helps for characterizing the mode-locked lasers under different operating conditions. In addition, mode locking with a transition of the lasing state from the ground state to the excited state was observed in the 40-GHz device. Following in the footsteps of the ground-state mode-locked lasers, we designed the passively mode-locked lasers via excited-state transition which were expected for generating relatively short pulses. The optical and electrical characteristics were studied in detail at different levels of injection current and absorber bias. The minimum pulse widths were 3.3 ps and 780 fs before and after an external compression, respectively. Interesting behaviors in the evolution of the hysteresis, the optical spectra and the evolution of repetition frequency were observed and different from that for conventional ground-state mode-locked lasers. Possible mechanisms behind these observed phenomena were proposed and discussed. In the external-cavity configurations, two-section J-shaped superluminescent diodes were used. The laser acted as a multi-functional light source. When the two sections were shorted, continuous-wave output with a narrow spectral linewidth and a wavelength tuning range of over 140 nm were achieved by rotating the diffraction grating. The spectral linewidths were all smaller than 0.1 nm with the side mode suppression ratio (SMSR) larger than 30 dB. Passive mode locking was achieved when one of the two sections was reversely biased as a saturable absorber. Under the mode-locked condition, the wavelength tuning ranges were 33 nm and 30 nm for the ground-state and the excited-state transitions, respectively. The obtained pulse widths were in the range from 10 ps to more than 30 ps. Finally, the repetition rate of pulses can be continuously tuned from 2 GHz to 79.3 MHz by changing the external cavity length. To the best of our knowledge, the repetition rate of 79.3 MHz was the lowest frequency achieved to date for any passively mode-locked semiconductor laser.