In this letter, a simple, switchable dual-wavelength erbium-doped fiber laser operating in L-band is successfully proposed and demonstrated. The two wavelengths are specified by a Bragg grating formed in multimode fiber and the wavelength separation is 1.7 nm. The multimode section including the multimode fiber grating selects not only the lasing wavelengths but also the polarization states of the lasing lines. Stable dual-wavelength operation can be achieved due to the spatial mode beating effect induced by a single-mode/multimode/single-mode fiber combination structure. By simple adjustment of a polarization controller, the proposed laser can operate in stable dual-wavelength or switch between two wavelengths, all at room temperature.
©2004 Optical Society of America
Multiwavelength fiber lasers are useful sources in wavelength-division-multiplexed (WDM) fiber communication systems, fiber sensors, and optical instrument testing. In many fields, especially in sensor and instrument testing applications, the requirements for switchable multiwavelength lasers are very rigorous. Various techniques have been proposed to realize switchable multiwavelength oscillations in erbium-doped fiber lasers (EDFLs) by utilizing cascaded fiber Bragg grating (FBG) cavities , a tree, inline topology FBG , polarization-dependent loss element , one or two FBGs written in a birefringent fiber [4–6], two overlapping cavities composed of two FBGs with a common gain medium , a polarization-dependent multiple-quantum-well waveguide . Multiwavelength fiber lasers are also demonstrated by incorporating a spatial mode beating filter in the laser cavity [9–11]. Some of the systems mentioned above allow control of the wavelengths, but they require a different FBG for each wavelength. In some other systems [8–11], there is no FBGs to perform simple or more elaborated filtering functions, however the switchable wavelengths can’t be precisely controlled and the lasing wavelengths are not very stable. Of course, to obtain stable multiwavelength laser emission it is necessary to introduce some mechanism to overcome the mode competition at room temperature.
FBGs are ideal wavelength selection components for fiber lasers due to the unique advantages of fiber compatibility, ease of use and low cost. Bragg gratings formed in multimode fiber (MMF) show multiple lobs in the reflectivity spectrum. Some theoretical and experimental studies have been performed on the characteristics and applications of multimode fiber gratings (MMFG) [12–16]. Recently, we have reported a simple, switchable dual-wavelength Ytterbium-doped fiber laser based on a few-mode fiber grating (FMFG) . The FMFG exhibits different reflectivities at two wavelengths and the longer-wavelength one can be stronger or weaker than the other through adjustment of a polarization controller (PC), which revealed the strong polarization-dependence of the FMFG. This strong polarization dependence may due to the damage tracks localized on one side of the core  and we consider that the FMFG is type▫ grating . But this polarization dependent reflectivity is not easily controllable or predictable during the grating fabrication process. In this letter, a MMFG is introduced into the laser cavity for wavelength selection that enables us to achieve L-band switchable dual-wavelength oscillations. The difference between our previously reported paper  and this letter is not simply in that we applied a MMFG to an L-band erbium laser. The main difference lies in that the reflectivity at the longer-wavelength is always much stronger than the other for the MMFG. Eventually, we give out a widely applicable technique to achieve switchable multiwavelength lasing in a fiber laser whatever the polarization characteristic of the MMFG is. In the proposed laser, the multimode section including the MMFG selects not only the lasing wavelengths but also the polarization states of the lasing lines. Wavelength switching can be achieved by means of induced wavelength-dependent cavity loss control. Stable dual-wavelength operation at room temperature can be achieved due to the spatial mode beating effect induced by a single-mode/multimode/singlemode fiber combination structure. We gave the result when the wavelength separation is 1.7 nm. This method has the advantages of simple design and fabrication of a multiwavelength selection element and precisely controlled stable lasing wavelength, compared with many other techniques mentioned above.
2. Performance of the multimode fiber Bragg grating
A multimode fiber carries several modes that propagate with different velocities and exhibit different mode field profiles. These modes exchange energy inside the fiber, and will propagate until they reach the grating. Reaching the grating, a few modes satisfying the phase-matching condition will be reflected back by the grating, and because they have different propagation constants, they will create some lobes in the reflectivity spectrum. The phasematching condition, or Bragg reflection condition, of a grating with the period Λ is given by , where β 1 and β 2 are the propagation constants of forward and back ward propagating modes, respectively. For reflection to the same mode, β 1=-β 2=β. Then the phase-matching condition is simply
In the case of our graded-index MMF, the propagation constant for the Nth principal mode is approximated by the equation 
V normalized frequency: V=2πaN A/λ ;
NA numerical aperture;
a core radius;
n 1 refractive index of the core;
Δ maximum relative index difference.
Fabrication of the MMFG was performed by the phase mask method. The grating period is about 1073.33 nm. The multimode fiber we use for fabrication of the MMFG is standard graded multimode fiber with a numerical aperture of 0.275,a core diameter of 62.5 µm, a refractive index of 1.47. Figure 1 shows the measured reflection spectrum of the MMFG. Only two reflection peaks are observed in the reflectivity spectrum. The calculated Bragg wavelengths from Eqs. (1) and (2) were about 1578.86 nm, and 1577.18 nm for N=0, 1, respectively. These wavelengths almost agree with those of experiment (1578.8 nm, and 1577.1 nm, respectively) in Fig.1. This agreement can reveal that the lobe centered at 1577.1 nm (left sidelobe) corresponds to the LP11 fiber mode, while the lobed centered at 1578.8 nm (right sidelobe) corresponds to the LP01 fiber mode . As can be seen from Fig.1 that the MMFG exhibits different reflectivities at those two wavelengths and the difference was about 7 dB.
We measured the polarization dependence of the MMFG using a setup shown in Fig. 2, which is similar to that used in Ref. . Amplified spontaneous emission (ASE) from an L-band erbium-doped fiber pumped with a 980 nm laser diode was used as a nonpolarized light source. The ASE was introduced through a fiber coupler with a branching ratio of 50:50 into the MMFG. The reflected light was introduced into a PC and a polarizer. The light was measured with an optical spectrum analyzer (OSA) to observe the polarization characteristic of the MMFG. Through adjustment of the PC, the reflectivities at the two wavelengths can be changed and when the longer-wavelength one decreased, the shorter-wavelength one increased. But the measured variations in the two reflected peaks are both only about 0.5dB and the reflectivity at the longer-wavelength is always much stronger than the other, as shown in Fig. 1, whatever state of PC is. From the result, we consider the MMFG in the present experiment is much less polarization dependent than the FMFG reported in .
3. Experimental setup and principle
The configuration of the proposed laser is shown schematically in Fig. 3. An L-band erbium-doped fiber amplifier (EDFA) provides gain to the fiber ring cavity and is based on a bi-directional pumping scheme, using two 90 mW optical power pumps at 980nm coupled to 20 m erbium-doped fiber (EDF) by two 980 nm/1550 nm WDM couplers. This EDFA takes advantage of high erbium concentration in the specially designed L-band fiber. An isolator is used for granting the unidirectional operation of the laser. The conjunction of a PC and a polarizer introduces wavelength-dependent cavity loss. The PC is used to rotate the polarization state and allowed continuous adjustment of the birefringence within the cavity. A L-band MMFG is introduced into the cavity by an optical circulator (OC). The fibers in the cavities are all single-mode fibers (SMF) except a section of about 12 cm MMF used for the MMFG fabrication. The laser output was taken simultaneously via a 70:30 fused fiber coupler (output1) and just after the MMFG (output2). The spectral characteristic was measured using an ADVANTEST Q8383 optical spectrum analyzer with 0.1 nm resolution.
To obtain dual-wavelength laser emission, it is necessary to balance the cavity losses with the gain of erbium-doped fiber amplifier for each one of the wavelengths. How the conjunction of a PC and a polarizer introduces wavelength-dependent cavity loss and balances the reflectivity difference of the two wavelengths can be described briefly as follows: The birefringence of the light exiting the polarizer will substantially modify its state of polarization as it completes a round trip and reapproaches the polarizer. Moreover, due to birefringence chromatic dispersion, different wavelengths will emerge from the round trip with different states of polarization. Since round-trip loss critically depends on the scalar product between the polarization of the incident wave and that of the polarizer, only those wavelengths will lase for which the loss is low enough to match the available gain. Then, the conjunction of the polarizer and the PC can introduce wavelength-dependent cavity loss and balances the reflectivity difference of the two wavelengths defined by the MMFG . Thus, switchable dual-wavelength oscillation can be generated from the proposed laser by adjustment of the PC.
On the other hand, to obtain stable dual-wavelength laser emission it is necessary not only to balance the cavity losses for each one of the wavelengths but also to overcome the mode competition at room temperature. In the proposed laser, there exists a combination structure of single-mode/multimode/single-mode fiber after the optical circulator. Many experimental results have shown that stable multiwavelength oscillations in the fiber laser can be achieved due to the spatial mode beating effect induced by a single-mode/multimode/single-mode fiber combination structure [9–11]. The principle can be described in brief as follows: Altering the fiber cavity birefringence by the PC allowed the different polarized mode components within the LP01 and LP11 spatial mode to be excited in the multimode fiber. That’s to say the two wavelength reflected are with different mode polarizations. Then, the multimode section including the MMFG selects not only the lasing wavelengths but also the polarization states of the lasing lines. The section isolates each reflected wavelength in its own state of polarization and therefore enhances the polarization hole burning (PHB). This PHB greatly increases the inhomogeneous gain broadening of EDF and accordingly reduces the wavelength competition. It is then possible to achieve stable dual-wavelength oscillations at room temperature.
In conclusion, by simple adjustment of a polarization controller (PC), the proposed laser can operate in stable dual-wavelength or switch between two wavelengths, all at room temperature.
4. Results and discussions
Figure 4 shows the results obtained from output1 when the laser is under single-wavelength operation. The lower and upper lasing spectra indicate the 1578.8 nm and 1577.1 nm wavelength operation, respectively. As described above, each single-wavelength can be obtained by adjusting the state of the PC. Amplitude variation was measured to be less than 3 dB in each single operation and the signal-to-ASE ratios were both over 35 dB. The according output powers were about 1.75 mW and 1.58 mW, respectively. The spectra under dual-wavelength operation of the laser is shown in Fig. 5. Figure 5(a) shows the result obtained from the output2, which is of 16 times repeated scans with 1-minite intervals. Amplitude variation of the two laser lines was about 13 dB and the output power was about 1.76 mW. Figure 5(b) shows the result obtained from the output1. Amplitude variation of the two laser lines was less than 2 dB and the output power was about 1.68 mW. As can be seen from Fig. 5(a) that the stability of the dual-wavelength operation of the laser at room temperature was very good.
There are many smaller dips on the short-wavelength side of the lasing spectra. They are due to the transmission losses of high order modes. As can be seen from the Fig.5(a) and Fig.5(b) that the spectral characteristics from the output1 and output2 is same. There was larger variation in the amplitude▫as large as 13 dB▫between the two wavelengths from the output2, mainly due to the unbalanced reflectivities for the different lasing wavelengths. Another point we have to note is that the wavelength and the wavelength spacing can be changed by using multimode fibers of different parameters, which can be seen from the Eq. (1) and (2). Thus, it could be expected that more wavelengths of oscillation with closer wavelength spacing could be obtained in the proposed laser.
In conclusion, a widely applicable technique to achieve a simple, switchable dual-wavelength erbium-doped fiber laser based on a multimode fiber Bragg grating is developed. In the proposed laser, the multimode section including the MMFG selects not only the lasing wavelengths but also the polarization states of the lasing lines. By simple adjustment of a polarization controller, the proposed laser can operate in stable dual-wavelength or switch between two wavelengths, all at room temperature. We gave the result when the wavelength separation is 1.7 nm. This method has the advantages of simple design and fabrication of a multiwavelength selection element, which also induces a mechanism to overcome the mode competition at room temperature, compared with the technique of using many single-mode fiber gratings.
This work is supported by the Tianjin Natural Science Foundation projects under Grant No 033800211, and the start-up Foundation of Scientific Research provided by the Personnel Department of Nankai University, P.R.China.
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