IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 44, NO. 1, JANUARY 2008 49 Fiber Laser Thermally Tunable by a Filter Composed of Two Sagnac Interferometers Isaac Huixtlaca-Cuatecatl, Georgina Beltrán-Pérez, Juan Castillo-Mixcóatl, and Severino Muñoz-Aguirre Abstract—Many applications, such as fiber Bragg grating char- acterization, wavelength division multiplexing, etc. require tunable laser sources with one emission line in a certain wavelength range. Generally, such sources are laser systems combined with tunable filters. In this work an all-fiber laser tunable by a filter composed of two Sagnac interferometers in series is described. The filter loops were constructed of high birefringence fibers (HiBiFi) and the filter was thermally tunable with a single transmission maximum in the Erbium gain spectrum. The results showed that for interferome- ters with 14 cm HiBiFi length a bandwidth reduction of the trans- mission peak, a single-line laser emission, a tuning sensitivity of approx. nm C and a spectral full-width at half-maximum bandwidth of 0.03 nm in the range of 1538–1580 nm were obtained. Index Terms—Fiber optic laser, Sagnac interferometer, tunable filter. I. INTRODUCTION M ANY applications such as optical sensing, photonic components characterization or wavelength-division multiplexing (WDM) systems require tunable laser sources with a single emission line in a certain wavelength range [1]. In order to achieve the tunable laser, switched multiple wavelength emission lasers combined with comb filters have been reported in some works [2]–[4]. Many of them are based on a Sagnac interferometer composed by a high birefringence fiber (HiBiFi) loop, since it has various advantages such as low insertion loss, polarization changes independence at low light power and it is useful in practically any spectral region. These and other characteristics determine the advantages over other filters for laser emission tuning. However, multiple wavelengths emission is not a suitable solution to some applications such as WDM technique since it has to be filtered again to obtain a single emission line to send information. For this reason it is more useful to have a bandpass filter with a single transmission maximum and a narrow passband to achieve a single laser emission. In the literature there have been reported various all-fiber filter configurations based on the Sagnac interferometer [5], [6], which use different specific fibers in the interferometer loop, for instance Xuewen et al. [5] employed inside the loop a fiber Bragg grating asymmetrically placed between two optic fiber with lengths and , which determine the spectral period Manuscript received February 5, 2007; revised August 25, 2007. This work was supported in part by the project CONACyT SEP-2004-C01-47121/A-1. The authors are with the Facultad de Ciencias Físico Matemáticas Benemérita Universidad Autónoma de Puebla, San Manuel, Puebla 72570, México (e-mail: ihuixtla@fcfm.buap.mx; gbeltran@fcfm.buap.mx; jcastill@fcfm.buap.mx; smunoz@fcfm.buap.mx). Digital Object Identifier 10.1109/JQE.2007.910449 of transmission maxima. Their results showed that for length differences between and of 1.05, 2.1, and 4.2 mm the period was 0.8, 0.4, and 0.2 nm, respectively. However, a dis- advantage of this experimental setup is the small tunning range which is about 11.5 pm/ -strain or 0.01 nm/ C for stretching and temperature changes, respectively. On the other hand Yong Wook et al. [6], presented an experimental system composed by a polarizing beam splitter, birefringent fiber and a polarization controller composed by two quarter wave-plates. The filter can introduce a shift of the peak separation by means of the ade- quate selection of the angle between fast and slow axis of these two quarter wave-plates. The separation between each emission line can be about 0.2–0.06 nm. However the use of the quarter wave-plates means a higher insertion loss which represents a disadvantage. In summary, the Sagnac interferometer is commonly used like a tunable comb filter whose main characteristic are multiple transmission passband peaks in the amplification range. This is due to the use of long birefringent fiber lengths (some me- ters) [7]–[8]. However, to achieve the emission of a single line in the Erbium Er gain spectrum is necessary to have a single passband peak in the Sagnac filter, which is the purpose of the present work. The single emission could be accomplished by the adequate selection of birefringence value and loop length. Bire- fringence is a fixed parameter for a certain kind of fiber; there- fore the parameter that can be varied is the length of the HiBiFi. The results of this modification would be basically the variation of the transmission passband peaks number, which means that in principle it would be possible to obtain a single transmission peak in the gain spectrum if the loop length is sufficiently re- duced. However, this makes the passband peak bandwidth wider (covering almost the whole gain spectrum), which degrades the emission quality. Therefore, it would be necessary to reduce such bandwidth. To solve this problem, the use of two Sagnac interferometers connected in series as a thermally tunable filter is proposed. To the best of our knowledge, the reduction of the filter loop length to obtain a single emission line in the Er gain spectrum and the use of two Sagnac interferometers as a filter have not been performed up to date. With the suitable selection of the filter parameters, it is possible to reduce the bandwidth of the transmission passband peak and to achieve a single laser emission line free of side modes. The single-line laser emission could be useful, for instance in communication systems where it is required to send information in one channel without the necessity of another extra device to select the transmission wavelength. On the other hand, the side modes suppression could help to eliminate crosstalk when channel separation is small (around 1 nm). 0018-9197/$25.00 © 2007 IEEE