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Jiaotong University, 100044 Beijing, China. 2Physics ..... the Fundamental Research Funds for the Central Universities (Beijing Jiaotong University. No.
DFB laser based on single mode large effective area heavy concentration EDF Qi Li,1 Fengping Yan,1,* Wanjing Peng,1 Ting Feng,1 Suchun Feng,1 Siyu Tan,1 Peng Liu,2 and Wenhua Ren1 1

Key Lab of All Optical Network and Advanced Telecommunication, Institute of Lightwave Technology, Beijing Jiaotong University, 100044 Beijing, China 2 Physics Department of xingtai college, 0054001 Xingtai, China * [email protected]

Abstract: A  phase shifted distributed feedback (DFB) laser based on single mode large effective area heavy concentration erbium-doped fiber (EDF) has been demonstrated. The homemade EDF was fabricated by the modified chemical-vapor deposition (MCVD) technique, and the 13cm long  phase shifted fiber grating was written in the intracore of the EDF. The erbium-doped concentration is 4.19 × 10 25 ions/m3, the mode field diameter of the fiber is 12.2801 um at 1550 nm, the absorption coefficients of the fiber are 34.534 dB/m at 980 nm and 84.253 dB/m at 1530 nm. The threshold of the DFB laser is 66 mW, and the measured maximum output power is 43.5 mW at 450 mW pump power that corresponding to the slope efficiency of 11.5%. The signal-to-noise ratio (SNR) of the operating laser at 200 mW input power is 55 dB, and the DFB laser has a Lorentz linewidth of 9.8 kHz at the same input pump power. ©2012 Optical Society of America OCIS codes: (050.5080) Phase shift; (060.2280) Fiber design and fabrication; (060.2410) Fibers, erbium; (140.3490) Lasers, distributed-feedback; (140.3510) Lasers, fiber.

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Received 23 Jul 2012; revised 19 Sep 2012; accepted 19 Sep 2012; published 1 Oct 2012 8 October 2012 / Vol. 20, No. 21 / OPTICS EXPRESS 23684

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1. Introduction Single frequency, narrow line bandwidth optical fiber lasers are of considerable interest for a variety of important applications for coherent telecommunications, sensor systems, LIDAR, spectroscopy, and generation of optical microwave signal [1–5]. Moreover, the single frequency narrow linewidth fiber lasers can be coherently beam-combined [6] or amplified by master oscillator power amplifier [7] to increase the output power. As a result, stable high power, single frequency, and narrow linewidth fiber lasers are highly desirable for more perfect performance fiber laser sources. Various techniques have been proposed to realize narrow bandwidth fiber laser at 1.5 um. Stable single-frequency narrow linewidth lasers were realized by making using of saturable absorber as narrow band filter [8, 9]. However, because of the malleable components in the complex structure of the lasers, these lasers couldn't be operating stably when the surrounding conditions were changed. Some narrow linewidth stimulated Brillouin fiber lasers have been demonstrated [10–12]. The free-running spectral linewidth of single-frequency Brillnouin fiber lasers could potentially be only a few hertz that can be several orders of magnitude narrower than the narrow linewidth lasers by using other technologies. Although the Brillnouin lasers could realize narrow linewidth and high-power (approximately 100mW) [11], there are some practically difficulties to obtain stable operation in Brillouin lasers. To make an efficient practical single frequency narrow linewidth fiber laser, the most efficient way is to lessen the employ of the malleable components in the fiber laser cavity. Narrow bandwidth fiber lasers based on Fabry-Perot (F-P) linear cavity were reported [13, 14]. The short cavity length of the laser thus to suppress the longitude mode hopping is of confinement to the highly pump absorption of the active fiber. Distributed feedback (DFB) fiber lasers can overcome these problems above [15–18]. They have simple, robust, and compact structure providing operation without longitude mode hopping, as well as surrounding insensitivity that compare to other complex structure fiber lasers. However, the DFB lasers have a biggest disadvantage: the output power is typically smaller than 10 mW that limited by the pump absorption, heat dissipation and saturation of the gain medium owing to high intracavity powers concentrated around the phase shift region [18]. In order to overcome this disadvantage, the feasible improvements are enlarging the effective area and increasing the concentration of the active fiber core. In this paper, we propose a  phase shifted DFB laser based on single mode large effective area heavy concentration erbium doped fiber (EDF). The homemade EDF we used was fabricated by the modified chemical-vapor deposition (MCVD) technique. The erbium-doped concentration is 4.19 × 1025 ions/m3, the mode field diameter of the fiber is 12.2801 um at 1550 nm, the absorption coefficients of the fiber are 34.534 dB/m at 980 nm and 84.253 dB/m at 1530 nm. The 13 cm long  phase shifted fiber grating was written in the intracore of the EDF. The threshold of the DFB laser is 66 mW, the measured maximum output power is 43.5 mW at 450 mW pump power that corresponding to the slope efficiency of 11.5%, and the modified slope efficiency is 12.9% after including the insertion loss of the mode field mismatch. The signal-to-noise ratio (SNR) of the operating laser at 200 mW input power is 55

#173068 - $15.00 USD (C) 2012 OSA

Received 23 Jul 2012; revised 19 Sep 2012; accepted 19 Sep 2012; published 1 Oct 2012 8 October 2012 / Vol. 20, No. 21 / OPTICS EXPRESS 23685

dB, and the DFB laser has a Lorentz linewidth of 9.8 kHz at the same input pump power. As far as we know, the simple, robust, and compact DFB laser based on single mode large effective area heavy concentration EDF is proposed and demonstrated for the first time, and the output power of the DFB laser is the highest. 2. Fabrication and Operation of distributed feedback fiber laser The erbium-doped single mode large effective area high concentration fiber we used was made by institute of lightwave technology and key lab of all optical network and advanced telecommunication of Beijing Jiaotong University of China which the authors work for, and was fabricated by the MCVD technique. The erbium-doped core of the fiber was multicompound doped of Bi3+, Ga3+, Er3+, and Al3+. Meanwhile, to ensure the EDF is under the condition of single-mode transmission, we adopted fluorine-doped technique to reduce the relative index difference (n), the cross section of the fiber is shown as the inserted picture in Fig. 1. The erbium-doped concentration is 4.19 × 1025 ions/m3, the mode field diameter of the fiber is 12.2801 um at 1550 nm, the absorption coefficients of the fiber are 34.534 dB/m at 980 nm and 84.253 dB/m at 1530 nm.

Fig. 1. Experimental setup of the proposed DFB laser. (The inserted picture in the top right corner is the cross section of the homemade EDF under microscope.)

The experimental setup of the proposed laser is shown schematically in Fig. 1. The operating fiber laser consists of a 15 centimeters homemade EDF with 13 centimeters  phase shifted grating, a 980/1550 nm wavelength division multiplex (WDM), a 976 nm pump source with maximum output power of 500 mW, an isolator used to isolate the laser that reflect from the fiber end face. The laser is monitored by the ANDO AQ6317C optical spectrum analyzer (OSA) with resolution of 0.01 nm, and the Agilent 9010 electrical spectrum analyzer (ESA) with resolution of 10 kHz.

Fig. 2. Transmission spectrum of the FBG (solid line) and phase shifted grating (dashed line).

The  phase shifted point was induced by exposing one point of a uniform fiber Bragg grating (FBG) directly with a UV-excimer laser operated at 248 nm, the uniform FBG is written in the hydrogen-loaded homemade EDF with phase-mask method. The central of the  phase shifted point locates at 2 centimeters location of the phase shifted grating from the

#173068 - $15.00 USD (C) 2012 OSA

Received 23 Jul 2012; revised 19 Sep 2012; accepted 19 Sep 2012; published 1 Oct 2012 8 October 2012 / Vol. 20, No. 21 / OPTICS EXPRESS 23686

pump source input port in order to achieve higher output laser power [19]. The transmission spectrum of the FBG and  phase shifted grating as shown in Fig. 2. To eliminate the influence of the hydrogen molecule ion on the radiation of the erbium ion, the hydrogen-loaded homemade EDF, in which the phase shifted grating is written, couldn't be used before the anneal treatment. During the experiment, the DFB laser was fixed on a glass plate and covered with a transparent plastic box to reduce the effects of air currents and acoustic vibrations of the surrounding environment. 3. Experimental results and discussion The threshold of the laser is 135 mW, and the measured optical spectrum of the proposed laser is shown in Fig. 3(a) with input pump power of 200 mW. The central wavelength of the laser is 1544.768 nm, the optical SNR of the lasers is approximately 55 dB, and the 3 dB bandwidth is 0.013 nm. To study the stability of the laser, we measured the optical spectrum with 16 times repeated scans at an interval of 5 mins as shown in Fig. 3(b). As the input pump power is fixed at 200 mW, the peak power and central wavelength variations of the laser are