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A novel and simple multi-wavelength linear-cavity tunable fiber laser source using a chirped fiber Bragg grating (CFBG) and a few-mode fiber Bragg grating (FMFG) is demonstrated using erbium-doped fiber (EDF) as the gain medium. In our linear-cavity configuration, the FMFG acts as full-reflecting mirror and wavelength selector while the CFBG with the 3-dB bandwidth of over 30 nm acts as a broadband partially reflecting mirror. The number of lasing wavelengths can be controlled by changing the state of polarization inside the cavity using a polarization controller. The large bandwidth of the CFBG enables continuous tuning of the lasing wavelengths by application of mechanical strain or thermal heating on the FMFG.
©2005 Optical Society of America
1. Introduction
Multi-wavelength fiber lasers are cost-effective sources in wavelength division multiplexed (WDM) fiber communication systems, fiber sensors, and optical instrument testing. Various techniques have been proposed to realize multi-wavelength oscillation in erbium-doped fiber lasers (EDFLs) by using a fiber grating Sagnac loop [1], a high birefringence fiber loop mirror [2], and a co-doped fiber [3] to mention a few. Erbium-doped fiber (EDF) is primarily homogeneous gain medium at room temperature, which leads to unstable lasing and strong mode competition. In order to achieve stable multi-wavelength oscillation, several techniques have been proposed to reduce wavelength competition, such as cooling EDF at cryogenic temperature [4], using an elliptical core EDF [5], and utilizing acoustic-optic frequency shifter [6].
Ring-cavity and linear-cavity are two types of operating fiber lasers [7]. Ring-cavity fiber lasers are relatively complex and susceptible to mode-hopping due to the length of fiber [8–9].
On the other hand, linear-cavity fiber lasers using fiber Bragg gratings (FBGs) have advantages of being simpler and cheaper [10–11].
FBGs are ideal wavelength-selective components for fiber lasers. Various methods have been demonstrated to realize switchable multi-wavelength fiber laser by using cascaded FBG cavities [12], an FBG written in Hi-Bi fiber [13–14], a sampled FBG [15], and few-mode fiber grating (FMFG) [16–17]. In General, FBGs written in multimode or few-mode optical fibers have multiple resonance peaks in the transmission and reflection spectra. The characteristics and applications of few-mode fiber gratings (FMFGs) and multimode fiber gratings (MMFGs) have been described in Refs. 18 and 19.
A linear-cavity fiber laser using a chirped fiber Bragg grating (CFBG) and the FBG written in single-mode fiber for wideband tunability has been previously reported [20–21]. For multi-wavelength fiber laser configuration, multi-wavelength selector needs to be inserted in the cavity and we recently investigated the characteristics of multi-wavelength oscillation in erbium-doped fiber laser using an FMFG and a Sagnac fiber loop, which acts as a broad band reflector [17]. In this paper, a novel and simple multi-wavelength fiber laser source using a CFBG, FMFG and EDF is proposed and demonstrated for the first time to the best of our knowledge. In this configuration, the CFBG with a large bandwidth of over 30 nm acts as a partially reflecting mirror while the FMFG acts as a full-reflecting mirror and wavelength selector. Moreover, the number of lasing wavelengths can be controlled by changing the state of polarization inside the cavity using a polarization controller (PC).
In addition, we will discuss continuous tuning of the lasing wavelengths by mechanical straining and thermal heating of the FMFG and by utilizing the large reflection bandwidth of the CFBG. The measurement results indicate that the output power level was quite stable for single-wavelength oscillation largely due to the relatively uniform reflection spectrum of the CFBG, while it showed large variation (∼20 dB) for multi-wavelength oscillation.
2. System configuration and experiments
Modified chemical vapor deposition (MCVD) was used for fabrication of the few-mode fiber, and Ge-B was co-doped in the core region in order to facilitate inscription of fiber Bragg gratings [22–23]. The amount of germanium and boron was 300 SCCM and 20 SCCM, respectively. The heating temperature of boron was about 40°C. The drawing temperature was 1930°C and the capstan speed was 30 m/min. The relative index difference Δ of the few-mode fiber was 1.3 % and the core and cladding diameters were 9.64 μm and 125 μm, respectively.
Prior to fabrication of the FMFG, the few-mode fiber was loaded with hydrogen under 100 bars and 100°C for 5 days. The FMFG was fabricated by the interferometric method with a frequency-doubled argon-ion laser. The period of the phase mask was 1060 nm, and the length of FMFG was 1.5 cm, the output power of the laser was 80 mW, and the UV exposure time was about 2 minutes. The CFBG was fabricated using step-chirped phase mask (4.5 nm/cm) and Fig. 1 shows a reflection spectrum of the CFBG. The length of the CFBG was 5 cm, the center wavelength was about 1555 nm, the full-width at half-maximum (FWHM) bandwidth was over 30 nm, and the reflectivity was about -2.5 dB (56%) across the reflection bandwidth.
The schematic of the multi-wavelength linear-cavity fiber laser is shown in Fig. 2. The laser cavity consists of a WDM coupler, a CFBG, an EDF, a PC, and an FMFG. The 10 m-long EDF is pumped by a 980 nm laser diode with the output power of 110 mW through a WDM coupler. The CFBG with large reflection bandwidth is employed as a broadband partially reflecting mirror and the FMFG acts as a full-reflecting mirror and wavelength selector.
In contrast to FBG with one resonance wavelength written in single-mode fiber, FBG written in FMFG can have several resonance wavelengths due to the presence of multiple core modes that satisfy the phase matching condition. In our linear-cavity fiber laser, the resonance wavelengths of FMFG should be in the reflection band of CFBG in order to achieve multi-wavelength oscillation. The threshold pumping power for multi-wavelength oscillation was 22 mW. The fibers in the cavity were all single-mode fibers (SMF) except the section of the FMFG. We used a mechanical splicer instead of the fusion splicer to facilitate adjustment of the transverse offset between the SMF and the few-mode fiber for better control of the mode excitation condition [24–25].
3. Experimental results
With the output power of the pump laser diode set at 63 mW and with the PC properly adjusted, simultaneous triple-wavelength lasing oscillations could be achieved as shown in Fig. 3. The resolution of the optical spectrum analyzer (OSA) was 0.08 nm. The lasing wavelengths are 1546.96 nm, 1548.43 nm, 1550.23 nm, respectively. Therefore, the wavelength separations among the adjacent lasing wavelengths are 1.47 nm and 1.8 nm, respectively. The 3-dB bandwidth is about 0.1 nm, and the side-mode suppression ratio (SMSR) is over 27 dB.
In general, FMFG has strong polarization dependence due to the damage cracks formed on one side of the core during the process of grating fabrication, and the multiple resonance wavelengths have different polarization states from one another [13]. We could switch between the modes with different number of lasing wavelengths by adjusting the state of polarization inside the cavity using a PC.
Fig. 4. Output spectra of linear-cavity fiber laser with different state of polarization.(a) single-wavelength oscillation, (b) dual-wavelength oscillation.
Figure 4(a) shows single-wavelength operation at 1550.20 nm with the 3-dB bandwidths of 0.05 nm, and the SMSR is over 32 dB. Figure 4(b) shows dual-wavelength laser operation at 1548.48 nm and 1550.23 nm with 3-dB bandwidths of 0.1 nm and the side-mode suppression ratio (SMSR) of about 30 dB. At the switching among the single-, dual- and triple-wavelength lasing modes, the shift of the lasing wavelengths was measured to be less than 0.03 nm. Since this is smaller than the measurement resolution (0.08 nm), it indicates that the polarization-dependent wavelength shift is negligibly small.
Figure 5 shows the results of repeated output power measurements of the dual-wavelength oscillation. A total of six measurements were made at two minutes’ interval. The peak power variation is less than 0.5 dB, which is sufficiently stable compared with the previously reported results [26–28].
Fig. 6. Output spectra of the single-wavelength oscillation with different strain range from 0 με to 1050 με.
The large reflection bandwidth of the CFBG enables continuous tuning of the lasing wavelengths by applying mechanical strain or thermal heating on the FMFG. In our experiments, application of the mechanical strain was done by moving the translation stage with a micrometer in steps of 150 με. Figure 6 shows the output spectra of the single-wavelength oscillation with application of different strains to the FMFG between 0 με and 1050 με. The lasing wavelength shifted to longer wavelength with increase of the applied strain. The tuning range of the wavelength was over 12 nm between 1550.25 nm and 1562.43 nm. The strain sensitivity of the resonance wavelength shift was 11.5 pm/με.
Fig. 7. Output spectra of the dual-wavelength oscillation with different temperature settings of 30°C to 200°C at the FMFG. The PC was adjusted to obtain the same output power level for both temperatures.
We also performed the wavelength tuning of the laser by thermal heating of the FMFG. Figure 7 shows the output spectra of the dual-wavelength oscillation with different temperature settings of 30°C and 200°C. The wavelength shift in this case was about 1.8 nm. With the increase of the temperature, the lasing wavelengths shifted toward longer wavelength due to the thermo-optic effect of the FMFG. The shift of the two resonance wavelengths of the FMFG versus temperature is shown in Fig. 8. The linear fitting of the resonance wavelength shift gives
where T is the temperature at the FMFG. Equation (1) shows that the temperature sensitivities of the resonance wavelengths are 10.79 pm/°C and 10.55 pm/°C, respectively, which are the typical values for FBGs. The measured temperature and strain sensitivities imply approximately 0.9 με for the strain measurement error per 1°C change in the ambient temperature.
The thermal heating of FMFG can also accompany output power variation of the laser due to the resonance wavelength shift of FBG and the uneven gain profile of EDF [21]. Even though the output power variation was small for single-wavelength oscillation, it was rather large (∼20 dB) for multi-wavelength oscillation due to the competition among the different lasing wavelengths. Nevertheless, it is possible to obtain the almost same output power level for different temperatures as shown in Fig. 7 by adjusting the PC.
4. Summary
In this work, a simple and novel multi-wavelength linear-cavity fiber laser using a chirped fiber Bragg grating (CFBG), a few-mode fiber grating (FMFG) and erbium-doped fiber has been demonstrated for the first time to the best of our knowledge. In a linear-cavity configuration, the FMFG acts as full-reflecting mirror and wavelength selector. The CFBG, which has the reflection bandwidth of over 30 nm, acts as a broadband partially reflecting mirror.
The lasing wavelengths for the triple-wavelength operation were 1546.96 nm, 1548.43 nm, 1550.23 nm, respectively and the 3-dB bandwidths were 0.1 nm. The wavelength separations among the adjacent lasing wavelengths were 1.47 nm and 1.8 nm, and the side-mode suppression ratio (SMSR) was over 27 dB. By properly adjusting the polarization controller, the laser operation could be switched between the single-, dual- and triple-wavelength laser operations.
The large reflection bandwidth of the CFBG enabled continuous tuning of the lasing wavelengths by applying mechanical strain or thermal heating to the FMFG. The tuning range in case of the strain application was over 12 nm. With thermal heating, the wavelength shift with the temperature change from 30°C to 200°C was 1.8 nm. The fiber laser proposed in this work is expected to find various applications as a tunable multi-wavelength light source for optical communication and sensing.
Acknowledgments
This work was performed under the partial support from the Brain Korea-21 (BK-21) Project, Ministry of Education, Korea.
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