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An improved erbium-doped fiber laser configuration for achieving single-polarization, switchable dual-wavelength of orthogonal polarizations oscillations at room temperature is proposed. For the first time,two fiber Bragg gratings (FBGs) directly written in a polarization-maintaining (PM) and photosensitive erbium-doped fiber (PMPEDF) as the wavelength-selective component are used in a linear laser cavity. Due to the polarization hole burning (PHB) enhanced by the polarization-maintaining FBG (PMFBG), the laser can be designed to operate in stable dual-wavelength or wavelength-switching modes with a wavelength spacing of 0.336nm at room temperature by adjusting a polarization controller (PC). Each lasing line shows a single polarization with a polarization extinction ratio of >25dB under different pump levels. The optical signal-to-noise ratio (OSNR) is greater than 50 dB. The amplitude variation with 16 times scans in nearly one and half an hour is less than 0.5dB at both operating wavelength.
©2008 Optical Society of America
1. Introduction
Multi-wavelength fiber lasers are useful in wavelength-division-multiplexing (WDM) optical communication systems, fiber sensors, and optical testing instruments. Fiber Bragg grating (FBG) is an ideal wavelength-selective component for fiber lasers due to various merits such as wavelength-selective nature, fiber compatibility, ease of use and low cost. Erbium-doped fiber is the primary homogeneous gain medium at room temperature, which leads to strong mode competition and unstable lasing, thus it is difficult to obtain simultaneous multi-wavelength oscillations in erbium-doped fiber lasers (EDFLs). Various techniques have been proposed to realize multi-wavelength oscillations by utilizing FBG with multiple phase shifts in a linear cavity [1] and in a ring cavity [2], symmetrical FBG structure in a linear cavity [3],a sampled FBG in a ring cavity [4], a multimode FBG in a linear cavity [5], and polarization-maintaining FBG (PMFBG) in a linear cavity [6] and in a ring cavity [7]. Multi-wavelength fiber lasers based on the PMFBG have been deeply studied and various configurations have been proposed [6-10]. However, the fibers which are used to form the laser cavity and the fibers with the PMFBG written in these configurations are of different types and needed to be spliced together [6-10]. Thus the cavity loss is increased and the overall robustness of the laser is also diminished. Further more, the polarizations of the two lasing lines of the orthogonal linear polarization modes corresponding to the reflection peaks of the PMFBG can not be well maintained in the laser cavity; especially, when different fibers are utilized, this will induce the mode coupling by the splice joints.
In this paper, we demonstrate an improved single-polarization, switchable dual-wavelength with orthogonal polarizations erbium-doped fiber laser at room temperature based on the polarization hole burning (PHB). For the first time, two FBGs directly written in a polarization-maintaining (PM) and photosensitive erbium-doped fiber (PMPEDF) are utilized as the wavelength-selective component in a linear laser cavity. The fiber laser is structurally simple and integrative, has low cavity loss and is easy to be fabricated (no splicing point in the laser cavity; no need to adjust the polarization-axis when splicing different polarization-maintaining fibers; the fiber don’t need to be hydrogen-loaded to increase the photosensitivity) compared with the previous reported configurations [6-10]. The polarizations of the two lasing lines of the orthogonal linear polarization modes which are separated by the reflection peaks of the PMFBG can be well maintained in the laser cavity as the whole laser cavity is formed by the PMPEDF. The laser can be designed to operate in stable dual-wavelength or wavelength-switching modes with a wavelength spacing of 0.336nm at room temperature by adjusting a polarization controller (PC). Each lasing line shows a single polarization with a polarization extinction ratio of >25dB under different pump levels. The optical signal-to-noise ratio (OSNR) is greater than 50 dB. The amplitude variation with 16 times scans in nearly one and half an hour is less than 0.5dB at both operating wavelength.
2. System configuration and principle
The configuration of the proposed laser is shown schematically in Fig. 1. The laser consists of a uniform PMFBG and a broadband polarization-maintaining linearly chirped fiber Bragg grating (PMCFBG) directly written in a PANDA polarization-maintaining and photosensitive erbium-doped fiber (PMPEDF) with the spatial spacing of 1.5-m (there are no splice joints in the whole laser cavity), a 980/1550 nm wavelength division multiplexer (WDM), a polarization controller (PC) and a PM optical isolator (PMISO). The 1.5-m PMPEDF has an absorption coefficient of 23dB/m at 1530nm and is pumped by a laser diode with a maximum output of 110mW at 980nm through the WDM. The beat length of the PMPEDF is about 4.98-mm at 1550nm which corresponds to a wavelength separation of about 0.336nm in the reflection spectra of PMFBG for the orthogonally polarized modes. The PC is used to adjust the birefringence within the cavity and to balance the gain and loss.
Fig. 2. The overlap reflection spectra (a) and transmission spectra (b) of the PMFBG and the PMCFBG under ambient-temperature conditions.
Due to the different modal refractive index along the fast and slow axis of the fiber, both of the PMFBG and PMCFBG exhibit two reflection peaks, one for each polarization. The PMFBG, which has two reflection peaks within the two reflection bands of the PMCFBG respectively under ambient temperature conditions, functions as the wavelength-selective component of the laser. The slight mode coupling between the two orthogonal linear polarization modes is unavoidable in the PMF because of the imperfect property of the PMF introduced during the fabrication process. Thus, we think that it is better to separate the two reflection peaks of the PMCFBG to reduce the mode coupling, which is different from that in the Ref. 6 (the two reflection peaks of the PMCFBG are overlapped in the Ref. 6). The PMFBG and PMCFBG were both fabricated with the phase mask method by using KrF excimer laser. The PMFBG about 5-cm long was fabricated with a uniform phase mask (with a phase mask period of 1075 nm). The two reflection peak wavelengths of the PMFBG are 1554.576nm and 1554.912nm, and each of the reflection bands has a 3-dB bandwidth of 0.03nm and a reflectivity of 82.7% for the corresponding polarizations (here, we have the same definition for the reflectivity as the Ref. 9). The 3-cm long PMCFBG was fabricated with a chirped phase mask which has a chirp rate of 0.0292nm/cm and a period of 1075.082nm. The two reflection peak wavelengths of the PMCFBG are 1554.585nm and 1554.921nm, and each of the reflection bands has a 3-dB bandwidth of 0.110nm and a reflectivity of 46.7% for the corresponding polarizations. The overlap reflection and transmission spectra of the PMFBG and PMCFBG at room temperature measured with an unpolarized amplified spontaneous emission (ASE) source are shown in Fig. 2(a) and Fig. 2(b) respectively. The spectral characteristic was measured by an ANDO AQ6317 optical spectrum analyzer (OSA) with 0.01nm resolution.
Since the whole laser cavity is all PM and there is no splicing point in the laser cavity, the polarizations of the light within the laser cavity are well maintained. Feedback from the PMFBG and PMCFBG in the laser cavity results in two linearly orthogonally polarized modes which are separated both in wavelength and polarization. Lasers operating on different linearly polarized modes greatly enhance the polarization hole-burning (PHB) in the cavity and reduce the homogeneous linewidth of the EDF [6-13]. Due to the PHB enhanced by the PMFBG, the laser can be designed to operate in stable dual-wavelength or wavelength-switching modes of single polarization with good polarization extinction ratio by adjusting the PC at room temperature.
3. Experimental results and discussion
The pump threshold of the laser is 23mW. Figure 3 shows the dual-wavelength operation of the laser with a pump power of 95mW. The two lasing wavelengths are 1554.576nm and 1554.912nm respectively corresponding to the two reflection peak wavelengths of the PMFBG. The optical signal-to-noise ratio (OSNR) is measured to be greater than 50 dB, as shown in Fig. 3(a). The 3-dB bandwidth of each lasing line is 0.014nm, 20-dB bandwidth is 0.056nm and the 40-dB bandwidth is 0.160nm respectively. 16 times repeated scans at 5-min intervals in nearly one and half an hour is shown in Fig. 3(b). The amplitude variation of the two lasing wavelengths is less than 0.5dB and the output powers are about 1mW respectively. As can be seen from the Fig. 3(b), the stability of the dual-wavelength laser at room temperature is very good.
The single-wavelength operation can be obtained by adjusting the PC. Figure 4 shows the single-wavelength operation of the proposed laser with a pump power of 95mW. Figure 4(a) and Fig. 4(b) indicate the operation at 1554.576nm and 1554.912nm respectively. The 3-dB bandwidth of each lasing line is 0.014nm, 20-dB bandwidth is 0.054nm and the 40-dB bandwidth is 0.162nm respectively. 16 times repeated scans of each single-wavelength operation at 5-min intervals in nearly one and half an hour is shown in Fig. 5(a) and Fig. 5(b) respectively. The amplitude variation is measured to be less than 0.5 dB and the OSNR is greater than 52.5 dB in each single-wavelength operation. The output powers are about 1.7mW respectively.
In the experiment, the dual-wavelength and single-wavelength operation were very stable under pump variations and no significant drift in wavelength or amplitude variation was discovered under the invariant pump. As described in section II, this is because the PHB enhanced by the PMFBG reduced the homogeneous gain broadening of EDF and then reduced the wavelength competition. Adjusting the PC led to different laser outputs. This is because of the perturbation-induced birefringence, which was used to change the polarization states of different wavelength in the laser cavity. That also verified the principle of the wavelength-switching.Note that higher output power can be obtained by increasing the pump power or using a master oscillator power amplifier (MOPA) configuration [14]. Higher efficiency and OSNR can be achieved by optimizing the length of PMPEDF and the reflectivity of the PMFBG and PMCFBG.
The polarization of the fiber laser output has been measured with a polarization multimeter.Each lasing line shows a single polarization with a polarization extinction ratio of >25dB under different pump levels. The two polarizations at the dual-wavelength are orthogonal to each other, as expected from the PMFBG.
Fig. 5. Single-wavelength operation of the laser at about 95mW pumped with 16 times repeated scans in nearly one and half an hour.
The performances of the laser demonstrated above are better than the previous reports [6, 10], higher output power and OSNR are obtained under the same pump power level. It is mainly due to the highly-doped PMPEDF and the integrative laser configuration. The fiber laser we proposed is structurally simple and integrative, has low cavity loss and is easy to be fabricated (there is no splicing point in the laser cavity; there is no need to adjust the polarization-axis when splicing different polarization-maintaining fibers; the fiber do not need to be hydrogen-loaded to increase the photosensitivity) compared with the previous reports [6-10]. Further more, the polarizations of the two lasing lines of the orthogonal linear polarization can be well maintained in the laser cavity as the whole laser cavity is formed by the PMPEDF.
Obviously, the laser is not single-longitudinal-mode laser. In fact, we have reduced the length of the laser cavity to about 3-cm (the mode-spacing depends on the length of the laser cavity) and tried to make it work at single-longitudinal-mode [15], but the laser didn’t work. It is mainly due to the low gain of the EDF (compared with the Er-Yb co-doped fiber) and the short laser cavity. But the single-longitudinal-mode lasing in each wavelength is still expected in our configuration through incorporating phase-shift fiber Bragg grating filter [2] or using the Er-Yb co-doped high gain PMPF [16]. Thus, it is possible to obtain the single-polarization, single-longitudinal-mode fiber laser with narrow linewidth. Such kind of fiber laser has potential application in the distributed long distance fiber-optic micro-strain location sensing system based on Mach-Zehnder interferometer [17]. Further more, the dual-wavelength, single-longitudinal-mode fiber laser with ultra-narrow wavelength spacing and orthogonal polarizations has potential applications in microwave or millimeter-wave signal generation and modulation of data on the microwave sub-carrier [1].
A change in the temperature of the laser will shift the two wavelengths simultaneously, but it will not affect the wavelength spacing and stability. Different lasing wavelengths can be obtained by tuning the temperature of the PMFBGs or changing the period of the custom gratings. And different wavelength spacing can be achieved by controlling the birefringence of the PMFBGs or using different PMPEDF.
4. Conclusion
We propose an improved erbium-doped fiber laser configuration for achieving single-polarization, switchable dual-wavelength oscillations with orthogonal polarizations at room temperature. For the first time, two FBGs directly written in a PMPEDF as the wavelength-selective component are utilized in a linear laser cavity. The fiber laser is structurally simple and integrative, has low cavity loss and is easy to be fabricated compared with the previous reports. Due to the PHB enhanced by the PMFBG, the laser can be designed to operate in stable dual-wavelength or wavelength switching modes with a wavelength spacing of 0.336nm at room temperature by adjusting the PC. Each lasing line shows a single polarization with a polarization extinction ratio of >25dB under different pump levels. The OSNR is greater than 50 dB. The amplitude variation with 16 times scans in nearly one and half an hour is less than 0.5dB at both operating wavelength. The potential applications of the fiber laser are also discussed.
Acknowledgments
The authors would like to thank the Optical Fiber Group of Institute of Lightwave Technology for supplying the PMPEDF with high quality and thank Dr. Yan Wei for helpful discussions about the PMFBG. Suchun Feng would like to thank Dr. Yuchun Lu for the help in the revision of this paper. This work is jointly supported by the National High Technology Research and Development Program (“863” Program) of China under Grant No. 2007AA01Z258, the National Natural Science Foundation of China, Program for New Century Excellent Talents in University, Beijing Natural Science Foundation and the Beijing Jiaotong University Foundation.
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