Mode-locking by nonlinear polarization rotation in a semiconductor optical amplifier

Zhonggui Li, Xuelin Yang, Eduward Tangdiongga, Heongkyu Ju, Giok Djan Khoe, Harm J.S. Dorren, Daan Lenstra

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We numerically investigate a passive mode-locking system based on nonlinear polarization rotation in a semiconductor optical amplifier (SOA), which has been experimentally demonstrated [1].The principle of polarization-dependent gain saturation induced by the high-intensity part of the optical pulse in a SOA makes it possible to reshape the pulse in such a way that the low-intensity part is cut away. This will result in pulse narrowing and, when realized in a loop with sufficiently high gain, will lead to mode locking. The reshaping is realized by simple polarization controlling elements. Extensive numerical simulations have been done for commercially available SOA with a consistent pulse propagation model based on [2]. Our results show that the linewidth enhancement factor α plays a crucial role in the pulse built-up process. With fixed current, α should be larger than a critical value to obtain net roundtrip gain. Meanwhile, with a fixed α, there is a minimum current for the system to build up. On the other hand, if the current is increased beyond some critical value, instabilities instead of mode locking occur. The system normally takes several tens of roundtrips to build up to a stable state. In Fig.1, one typical built-up process from a super Gaussian pulse with white noise is shown. In the unrealistic case of dispersion-free operation, there is no fundamental limit to the pulse narrowing; finally nothing is left in the cavity after several hundred of roundtrips. However, in practice, the pulse width in stable state is determined by cavity dispersion and ultra-fast gain dynamics, where the latter is current dependent. The pulse root mean squared width (RMS) is shown in Fig.2, where the pulse shapes for two currents (152mA and 175 mA) are shown in the inset. The repetition rate of the system is limited by the carrier recovery time. Upon increasing the repetition rate, the intensity of the pulses decreases due to gain depletion until the net roundtrip gain falls below zero and the laser is switched off. For a fixed carrier recovery time, the repetition rate can be increased by increasing injection current. We numerically obtained a highest possible repetition rate of 28 GHz at 200 mA and at a carrier lifetime of 300 ps. Instabilities occur when the injection current is increased further.

Original languageEnglish
Title of host publication2005 European Quantum Electronics Conference, EQEC '05
Place of PublicationPiscataway
PublisherInstitute of Electrical and Electronics Engineers
Number of pages1
ISBN (Print)0780389735, 9780780389731
Publication statusPublished - 1 Dec 2005
Event2005 European Quantum Electronics Conference, EQEC '05 - Munich, Germany
Duration: 12 Jun 200517 Jun 2005


Conference2005 European Quantum Electronics Conference, EQEC '05


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