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 π 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 π 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 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 multi-compound 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 (Δn), 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.)

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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 π 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.

The π 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 π phase shifted point locates at 2 centimeters location of the phase shifted grating from the pump source input port in order to achieve higher output laser power [19]. The transmission spectrum of the FBG and π phase shifted grating as shown in Fig. 2 .

 

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

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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 <0.5 dB and <0.01 nm in nearly one and half an hour, respectively.

 

Fig. 3 Measured optical spectrum of the DFB laser.

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The variations of the central wavelength and the output power of the laser with surrounding temperature are a very important for practical application of the DFB laser. Figure 4 shows the measured variation of the optical spectrum of the DFB laser with surrounding temperature. The central wavelengths of the laser increase from 1544.768 nm to 1545.306 nm while the environment temperature increase from 30 to 90 degrees, corresponding to the variation of 0.009 nm per degree. The peak power variation with the temperature is smaller than 1.2 dB, but the variation is unrelated to the surrounding temperature from the diagram in the lower right corner of Fig. 4. The variation could be mainly attributing to the effects of air currents and acoustic vibrations that aroused by the fan rotating of the temperature control box. Because of the transmission loss of the rest part uncovered fiber of the DFB laser were affected by the vibrations, the output peak power changed randomly.

 

Fig. 4 Measured variation of the optical spectrum of the DFB laser with surrounding temperature.

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The threshold of the laser was approximately 135 mW, the maximum measured output power was 43.5 mW for a pump power of 460 mW, corresponding to the measured slope efficiency of approximately 11.5% as shown in Fig. 5 . The mode field diameter of the pig tail fiber (SMF-28) of the WDM which was used in the DFB laser is 10.5 um, however, the mode field diameter of the homemade EDF which was used is about 12.2801 um. Owing to the mode field mismatching of the two kind of fiber, the attenuation at the fusion splice point is approximately 10.61% on the basis of α = −10lg[4/(d₁/d₂ + d₂/d₁)²]. Where the d₁, d₂ mean the mode field diameter of the homemade EDF and SMF-28. The truth output power value excited from the EDF should be modified. According to the calculation result above, the modified slope efficiency is approximately 12.9%.

 

Fig. 5 Output power varies with the input power.

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According to Ref [19], the effective cavity L_eff of the π phase shifted DFB fiber laser is approximately 2.94 cm, corresponding to free spectrum range (FSR) of 3.49 GHz. The detected electrical spectrum of the DFB fiber laser at the input power of 200 mW as shown in Fig. 6(a) , the scanning range is 5 GHz that lager than 3.49 GHz. There is no conspicuous beat frequency signal in the Fig. 6(a), and this result indicates that the DFB fiber laser operating in single longitude mode.

 

Fig. 6 (a)Verification of the DFB fiber laser single longitude mode operation at the input power of 200mW; (b) Measured electrical spectrum (blue line) and Lorentz fitting curve (red line) of the DFB laser.

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The electrical spectrum of the DFB laser was measured with delayed self-heterodyne method. And the length of the fiber delay line used in the delayed self-heterodyne measurement setup is 40 km, which gives a nominal resolution of 4.8 kHz. When the input pump power is fixed at 200 mW, the red solid line in Fig. 6(b) shows the Lorentz fitting curve with linewidth of 9.8 kHz. The part of measured linewidth that larger than Lorentz linewidth mainly comes from the white noise and 1/f noise of the delayed self-heterodyne linewidth measurement setup [20].

4. Conclusion

In conclusion, a π phase shifted DFB laser based on single mode large effective area high concentration EDF has been demonstrated. The homemade EDF we used was fabricated by the MCVD technique. The erbium-doped concentration of the fiber is 4.19 × 10²⁵ ions/m³, and has a mode field diameter of 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 π phase shifted fiber grating was written in the intracore of the EDF. The threshold of the DFB laser is 135 mW, and the measured maximum output power is 43.5 mW at 450 mW pump power corresponding to the slope efficiency of 11.5%. The modified slope efficiency is 12.9% after including the insertion loss of the mode field mismatch. The 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.