本論文介紹一種使半導體雷射線寬變窄的系統。使用外部光纖雷射共振腔來回授光到半導體雷射裡的方式，將原本線寬為MHz級的半導體雷射變窄至<1kHz，並實現單縱模窄線寬雷射。外部光纖雷射共振腔包含雙耦合光纖環形共振器(DCFR)，藉由跳接線的端面反射，讓DFB(Distributed Feedback)雷射光不斷經過DCFR，並使部分的光回授到DFB雷射腔中，窄化半導體雷射的線寬。
在本研究中，DCFR扮演很重要的角色，提供其窄帶通濾波器的特性，是使線寬變得更窄的關鍵。接著探討不同耦合比DCFR對雷射線寬的影響，隨著耦合比增加，線寬會越窄，透過自延遲外差法量測，此雷射最窄線寬可來到300Hz。我們認為這是獲得超窄線寬雷射之具有成本效益的方法。

In this paper, we provide a system for narrowing the linewidth of a semiconductor laser. The external fiber laser resonator is used to feedback the light to the semiconductor laser, so that the original MHz linewidth of the semiconductor laser is reduced to<1kHz, becoming a narrow linewidth laser. The external fiber laser resonator contains a dual-coupler fiber ring resonator (DCFR), which allows the DFB laser light to pass through the DCFR in a narrow band. The laser light is then reflected back inside the DFB laser by the end face of the fiber connector through the DCFR. Through this external fiber laser resonator, we observed the narrowing of the linewidth of the laser light.
In this study, DCFR plays an important role in providing its narrow bandpass filter characteristics, which is the key to making the linewidth narrower. Then, the effect of different coupling ratios of DCFR on the laser linewidth is investigated. As the coupling ratio increases, the linewidth becomes narrower. The narrowest linewidth of this laser was measured to be 300 Hz by using the delayed self-heterodyne method. We believe this is the cost-effective method for obtaining super narrow linewidth lasers.

[1] B. Pedersen, A. Bjarklev, J.H. Povlsen, K. Dybdal, and C.C. Larsen, “The design of erbium-doped fiber amplifiers,” Journal of Lightwave Technology, vol. 9 no. 9 pp. 1105-1112, 1991.
[2] H. Takashashi, “Temperature stability of thin-film narrow-bandpass filters produced by ion-assisted deposition,” Applied Optics, vol. 34, no. 4, pp. 667-675, 1995.
[3] K.O. Hill and G. Meltz, “Fiber bragg grating technology fundamentals and overview,” Journal of Lightwave Technology, vol. 15, no. 8, pp. 1263-1276, 1997.
[4] T. Matsui, T. Sakamoto, K. Tsujikawa, S. Tomita, and M. Tsubokawa, “Single-mode photonic crystal fiber design with ultralarge effective area and low bending loss for ultrahigh-speed WDM transmission,” Journal of Lightwave Technology,
vol. 29, no. 4, pp. 511-515, 2010.
[5] D.M. Baney, B. Szafraniec, and A. Motamedi, “Coherent optical spectrum analyzer,” IEEE Photonics Technology Letters, vol. 14, no. 3, pp. 355–357, 2002.
[6] K. Numata, J. Camp, M.A. Krainak, and L. Stolpner, “Performance of planar-waveguide external cavity laser for precision measurements,” Optics Express, vol. 18 , pp. 22781-22788, 2010.
[7] M. Morin, S. Ayotte, C. Latrasse, M. Aube, M. Poulin, Y. Painchaud, N. Gagnon, and G. Lafrance, “What narrow-linewidth semiconductor lasers can do for defense and security,” Proceedings of SPIE, 2010.
[8] E. Luvsandamdin, C. Kurbis, M. Schiemangk, A. Sahm, A. Wicht, A. Peters, G. Erbert, and G. Trankle, “Micro-integrated extended cavity diode lasers for precision potassium spectroscopy in space,” Optics Express, vol. 22, pp.7790-7798, 2014.
[9] B. Lu, F. Wei, Z. Zhang, D. Xu, Z. Pan, D. Chen, and H. Cai, “Research on tunable local laser used in ground-to-satellite coherent laser communication,” Chinese Optics Letters, vol. 13, no. 9, 2015.
[10] B. Dahmani, L. Hollberg, and R. Drullinger, “Frequency stabilization of semiconductor lasers by resonant optical feedback,” Optics Letters, vol. 12, pp. 876-878, 1987
[11] W.H. Loh, B.N. Samson, and L. Dong, “High performance single frequency fiber grating-based erbium:ytterbiumco doped fiber lasers,” Lightwave Technology, vol. 16, no. 1, pp. 114-118, 1998.
[12] C. Spiegelberg, J. Geng, Y. Hu, Y. Kaneda, S. Jiang, and N. Peyghambarian, “Low-noise narrow-linewidth fiber laser at 1550 nm,” Journal of Lightwave Technology, vol. 22, no. 1, pp. 57-62, 2004.
[13] X.P. Cheng, P. Shum, C.H. Tse, J.L. Zhou, M. Tang, W.C. Tan, R.F. Wu, and J.Zhang, “ Single-longitudinal-mode erbium-doped fiber ring laser based on high finesse fiber Bragg grating Fabry-Perot etalon, ” IEEE Photon Technology Letter, vol. 20, no. 12 , pp. 976-978, 2008.
[14] S. Feng, Q. Mao, Y. Tian, Y. Ma, W. Li , and L. Wei, “Widely tunable single longitudinal mode fiber laser with cascaded fiber-ring secondary cavity,” IEEE Photonics Technology Letters, vol. 25, no. 4, pp. 323-326, 2013.
[15] C.A. López-Mercado, V.V. Spirin, J.L. Bueno Escobedo, A. Márquez Lucero, P. Mégret, I.O. Zolotovskii, and A.A. Fotiadicde, “Locking of the DFB laser through fiber optic resonator on different coupling regimes,” Optics Communications, vol. 359, pp. 195-199, 2016.
[16] D.A. Korobko, I.O. Zolotovskii, K. Panajotov, V.V. Spirin, and A.A. Fotiadi, “Self-injection-locking linewidth narrowing in a semiconductor laser coupled to an external fiber-optic ring resonator,” Optics Communications, vol. 405, pp. 253-258, 2017.
[17] J.L. BuenoEscobedo, V.V. Spirin, C.A. López-Mercado, A. Márquez Lucero, P. Mégret, I.O. Zolotovskii, and A.A. Fotiadi, “Self-injection locking of the DFB laser through an external ring fiber cavity: application for phase sensitive OTDR acoustic sensor,” Results in Physics, vol. 7, pp. 641-643, 2017.
[18] Y. Lu, T. Zhu, L. Chen, and X. Bao, “Distributed vibration sensor based on coherent detection of phase-OTDR,” Journal of Lightwave Technology, vol. 28, pp. 3243-3249, 2010.
[19] D. Lu, Q. Yang, and Y. He, “Review of semiconductor distributed feedback lasers in the optical communication band,” Chinese Journal of Lasers, vol. 47, no. 7, 2020.
[20] Jianluo Zhang and J.W.Y. Lit, “Erbium-doped fiber compound-ring laser with a ring filter,” IEEE Photonics Technology Letters, vol. 6, no. 5, pp. 588-590, 1994.
[21] D.G. Rabus and C. Sada, Ring resonators: theory and modeling, Springer International Publishing, pp. 3-46, 2020.
[22] 許立原，1.55m DFB半導體雷射頻寬窄化及穩頻研究，東海大學物理學系碩士班碩士論文，2002。
[23] A.S. Arnold, J.S. Wilson and M.G. Boshier, “A simple extended-cavity diode laser,” Review of Scientific Instruments, vol. 69, no. 3, pp.1236, 1998.
[24] 蔡宗衡，高穩定度超窄線寬光纖布拉格光柵外腔半導體雷射之設計，國立臺北科技大學製造科技研究所碩士班碩士論文，2014。
[25] F.S. Pavone, P. Cancio, C. Corsi, M. Inguscio, R.U. Martinelli , and R.J. Menna, “Linewidth and tuning characteristics of a mirror-extended cavity distributed-feedback 1.65μm diode laser,” Apply Physics, vol. 60, pp. 249-253, 1995.
[26] K.Y. Liou, Y.K. Jhee, G. Eisenstein, R.S. Tucker, R.T. Ku, T.M. Shen, U.K. Chakrabarti, and P.J. Anthony, “Linewidth characteristics of fiber-extended- cavity distributed-feedback lasers,” Apply Physics Letter, vol. 48, pp.1039, 1986.
[27] M.J.F. Digonnet, Rare earth doped fiber lasers and amplifiers, Marcel, 1993.
[28] https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=9021
[29] L.E. Richter, H.I. Mandelberg, M.S. Kruger, and P.A. McGrath, “Linewidth determination from self-heterodyne measurements with subcoherence delay times,” IEEE Journal of Quantum Electronics, vol. 22, no. 11, 1986.
[30] H. Ludvigsen, M. Tossavainen, and M. Kaivola, “Laser linewidth measurements using self-homodyne detection with short delay,” Optics Communications, vol. 155, no. 1, pp.180-186, 1998.
[31] S. Huang, T. Zhu, Z. Cao, M. Liu, M. Deng, J. Liu, and X. Li, “Laser linewidth measurement based on amplitude difference comparison of coherent envelope,” IEEE Photonics Technology Letters, vol. 28, no. 7, pp. 759-762, 2016.