半導體雷射於不同外部光回饋條件時的線寬增益係數

Linewidth enhancement factor in semiconductor lasers subject to various external optical feedback conditions

莊朝富(Chao-Fu Chuang)

指導教授 ： 林凡異

國立清華大學/電機資訊學院/光電工程研究所/碩士(2013年)

摘要

近年來，線寬增益係數α的量測一直是研究半導體雷射特性及應用的重要課題，較低的α可使半導體雷射在光通訊上的訊號傳輸更為穩定。由於傳統上，視α為固定常數。因此，如何利用外在的因素改變α是一個很重要的課題。有鑒於此，本研究提出利用不同光回饋的條件，包含改變光回饋強度、光回饋距離、以及光回饋相位來達到調製α的目標。
藉由光注入的方式量測半導體雷射穩定鎖頻區域Hopf bifurcation曲線最低調製頻率，我們可以得到不同光回饋條件下的α。實驗上，我們利用薄膜分光鏡產生光回饋，將元件裝載於PZT上作精準的光回饋相位調製，透過線性平移台改變光回饋距離，於薄膜分光鏡與半導體雷射間架設光衰減片以改變光回饋強度。根據
實驗所得之結果顯示，α的原始值為3.46，若藉由光回饋相位的調製可以使α 在
不同光回饋強度及光回饋距離時呈現2π週期的連續性變化。我們以半導體雷射
的鬆弛震盪頻率4.1GHz、光回饋距離3.66公分作為分界點，劃分長共振腔區域
及短共振腔區域。過程中，不論光的回饋距離位於長共振腔區域或是短共振腔區
域，α都會隨著光回饋強度的增加而上升，且受光回饋相位影響時的可調製範圍
也會增寬。不同於光回饋強度對α受光回饋相位影響時所呈現的正比關係，當光
回饋距離越長時，光回饋相位對α的可調製範圍越窄；反之，當光回饋距離越短時，則會越寬，尤其是當光回饋距離於短共振腔區域內縮短至1.5公分時，α受
光回饋相位的可調製範圍達到了21.09%，以理論模擬分析的方式則可於光回饋距離為0.5公分時，讓α的可調製範圍達到了63.41%，且最低的α僅只有2.25， 明顯地低於α原始值3.46。有鑑於此，只要能夠在極短光回饋距離下適當地調變光
回饋的相位，就可降低α，使半導體雷射的行為上更加穩定。最後，我們利用理論模擬分析的方式求得α在受光回饋時的變化情形，並將其與實驗結果比較，發現兩者間皆具有相同的趨勢，藉此確認了此研究的準確性。

關鍵字

半導體雷射 ； 線寬增益係數 ； 光回饋 ； 光注入

並列摘要

The linewidth enhancement factor α of a semiconductor laser has been widely studied in recent years. In optical communications, the stable signal transmission is attributed
to the lower α. In tradition, α is considered as a constant value. Thus, we propose to use the external control to modulate α under the influences of optical feedback with different feedback strengths, external cavity lengths, and feedback phases. By optically injecting the laser, the Hopf bifurcation curve corresponding to the upper frequency boundary of the stable locking region can be obtained. In the experiment, a pellicle beamsplitter mounted on a PZT stage placed on a linear translation stage is used as the reflector, where the external cavity length can be adjusted continuously from the long cavity regime to the short cavity regime with phase accuracy. The boundary between the long cavity regime to the short cavity regime is determined by the relaxation oscillation frequency.
α is found to be strongly affected by the feedback strength both in the long and short cavity regimes. Moreover, while α is insensitive to the feedback phase in the long cavity
regime, it can be tuned continuously when varying the phase in the short cavity regime. With a moderate feedback strength, the α is found to increase as the feedback strength increases. Moreover, while the α is insensitive to the feedback phase in the long cavity regime, it can be tuned continuously in the short cavity regime when varying the phase. A normalized variation range of 21.59% is obtained experimentally at an external cavity length of 1.5 cm, which can be further enhanced with a shorter cavity. With an external cavity length of 0.5 cm, a normalized variation range of 63.41% is predicted with the simulation. Under such condition, α as low as 2.25 is found which is notably lower than its free-running value.

並列關鍵字

semiconductor laser ； linewidth enhancement factor ； optical feedback ； optical injection

參考文獻

[1] C. H. Henry, “Theory of the linewidth of semiconductor lasers,” IEEE J. Quantum electron. 18, 259–264 (1982).

[2] H. Li, J. Ye, and J. G. McInerney, “Detailed analysis of coherence collapse in semiconductor lasers,” IEEE J. Quantum electron. 29, 2421–2432 (1993).

[3] T. Zhang, N. H. Zhu, B. H. Zhang, and X. Zhang, “Measurement of chirp parameter and modulation index of a semiconductor laser based on optical spectrum analysis,” IEEE Photon. Technol. Lett. 19, 227–229 (2007).

[4] M. Osinski and J. Buus, “Linewidth broadening factor in semiconductor lasers–An overview,” IEEE J. Quantum electron. 23, 9–29 (1987).

[5] U. Kruger and K. Kruger, “Simultaneous measurement of the linewidth enhancement factor α, and FM and AM response of a semiconductor laser,” J. Lightw. Technol. 13, 592–597 (1995).