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A simple composite cavity structure Er3+-doped fiber laser was proposed and demonstrated experimentally. The resonant cavity consists of a pair of uniform fiber Bragg gratings (FBGs) and a π-phase shifted FBG. By introducing the π-phase shifted FBG into the cavity as the selective wavelength component, it can increase the effective length of the laser cavity and suppress the multi-longitudinal modes simultaneously. The narrow linewidth of 900Hz and low RIN of −95dB/Hz were obtained. And the lasing wavelength was rather stable with the pump power changing. The SMRS was more than 67dB. The results show that the proposed fiber laser has a good performance and considerable potential application for fiber sensor and optical communication.
© 2013 Optical Society of America
1. Introduction
Fiber lasers have attracted a great deal of interests [1–5] in recent years due to their favorable and excellent optical properties, such as single-longitudinal mode, narrow linewidth and low relative intensity noise (RIN), etc. And they have enormous potential applications [5–13] in the laser radar, laser ranging, photoelectric sensor and scientific instruments. For example, narrow linewidth and low RIN fiber lasers have been highly desirable in applications for interferometric sensors [11, 12]. And power is relates to their detection sensitivity and transmission distance of sensor systems.
The main types of fiber lasers are DBR (distributed-Bragg reflector) and DFB (Distributed-feedback) fiber laser. DFB fiber laser can work on a good performance, especially narrow linewidth [6, 8, 13] and low RIN [14, 15]. However, the length of DFB structure is limited by the ability of the Bragg grating photo-inscription method. Maximum length is about 10 cm when using a phase mask and holographic method to write the grating. The short cavity provides the necessary high frequency separation between longitudinal modes, but it limits the absorption efficiency of pump power [7, 16], which results in the output power being limited. To improve output power and efficiency of the laser, the cavity length needs to be increased [17]. DBR fiber laser can lengthen its cavity length to get a higher gaining. However, when the cavity length of DBR is beyond a certain value, multiple transmission responses operate in the reflection spectrum [18] of FBGs.
During the last decade, various techniques have been focusing on realizing high performance fiber laser [6–8, 13, 14]. A number of works have been developed to make fiber lasers operate in single polarization, for example, appropriate twisting or straining the fiber lasers [3,7], employing polarization maintaining active fiber and optical devices [19] etc. And some works have been done to narrow linewidth and suppress intensity noise of fiber laser, for instance optoelectronic feedback [13], optical injection locking [4, 5], and so on. In our former work [8, 15], we achieved that the linewidth was narrowed by half and the RIN was suppressed about 16 dB/Hz by self-injection locking technology. But the effect was not obvious in improving the efficiency and output power of fiber laser. In general, the fiber lasers can be coherently beam-combined [20] or amplified by master oscillator power amplifier [21] to increase the output power. However, in this case it will lead to linewidth broadening and RIN amplifying besides power increasing. Another way to improve the efficiency of pump is to use fibers with increased concentration of active ions [22]. Like this, problems result from clustering and concentration quenching arise [23]. In particularly clustering will decrease the laser efficiency and lead to the self-pulsing generation.
In this paper, we investigated a new configuration of fiber laser by introducing a π-phase shifted FBG (π- PSFBG) into the ordinary DBR resonant cavity as the selective wavelength component. It can improve the absorption efficiency of pump power by increasing the laser cavity length, and achieve single-frequency single polarization high-performance laser generation employing Er3+-doped fiber with low concentration of active ions. And narrow linewidth of 900Hz and low RIN of −95dB/Hz were obtained.
2. Experiment system
The composite structure fiber laser proposed in this paper is shown in Fig. 1. The fiber laser is consisted of two matched fiber Bragg gratings (FBGs) and an inserted π-phase shifted fiber Bragg grating. The fiber Bragg gratings were written on both ends of a segment Er3+- doped optical fiber (Nufern, EDFC-980-HA). The π-phase shifted fiber Bragg grating with a length of 45 mm was written in the Er3+-doped fiber.
Fig. 1 Experimental setup of the proposed laser. (FBG: fiber Bragg grating, WDM: wavelength division multiplexer; OSA: optical spectral analysis; PD: photo detector; Er3+: erbium-doped fiber)
The gratings were written by scanning 244 nm frequency-doubled harmonic Argon ion continuous wave laser across the phase mask. The argon ion laser was used as UV source and would change effective refractive index of the photosensitive fiber. The phase shift was introduced by a simple displacement of the phase mask to fiber during the beam scanning. The displacement of the phase mask for an accurate π-phase shift was a quarter of the phase mask period while the phase mask was mounted on a piezoelectric transducer (PZT) stage (PI, P-752.11c) with nanometer resolution. By introducing a π-phase, a transmission window was opened at Bragg wavelength of the grating. The reflection spectrum of the π-phase shifted fiber Bragg grating made by our equipment is shown in Fig. 2. And the wide of the window is narrower than 1 pm.
Same to the ordinary DBR fiber laser, FBG1 and FBG2 are equivalent to reflect mirrors, which compose the laser resonator. The π-phase shifted fiber Bragg grating plays the role of a wavelength selective component and is inserted into the laser resonator. Three gratings (FBG1, π-PSFBG and FBG2) compose the new fiber laser resonator, and thus the proposed laser configuration can be considered as a composite cavity. It will give possibilities to obtain single-frequency lasing with high output power provided by the long length of the laser cavity.
As shown in Fig. 1, the fiber laser was pumped by a 980 nm laser diode through a 980/1550 nm wavelength division multiplexer (WDM). When the fiber laser was pumped, it would emit stimulated emission bidirectionally. Then the outgoing light, via the WDM, an optical circulator (OC) and a fiber Bragg grating (FBG3), was measured by Optical Spectrum Analyzer.
The reflectivity of the FBG1 is 85% at 1535.639 nm with 3dB bandwidth of 0.193nm. And the reflectivity of the FBG2 is 99% at 1535.674 nm with 3dB bandwidth of 0.202nm. Except the wavelength of transmission window of the π-phase shifted fiber Bragg grating, there is a big loss in the composite cavity at other wavelengths. Tips of FBG2 and FBG3 require to be cleaved at an angle about 8 degrees off of perpendicular, which is to minimize reflection. FBG3 is used as a spectral filter to avoid the Amplified Spontaneous Emission (ASE) spectrum.
3. Experiments and results
We used a segment of 3m long Er3+-doped optical fiber and two FBGs to setup a DBR fiber laser. The theoretical longitudinal mode spacing is about 0.27pm (Δλ = λ2/2nL, where n is refractive index of the resonant cavity, L is the length of the resonant cavity, and λ is the center wavelength of fiber laser). The bandwidth at 3dB of a common FBG is about 0.2nm, which is wider than longitudinal mode spacing and will make the laser operate on multi-longitudinal modes. In order to observe the fine optical spectrum including multi-longitudinal modes, an Ultra High Resolution Optical Spectrum Analyzer (APEX ap2040a) with a resolution of 0.16pm was employed. When the 980 nm pump power was about 20mW, the DBR fiber laser operated in chaos multi-longitudinal modes as shown in Fig. 3(a). The result is consistent with [24]. It is very difficult for such a long length cavity DBR fiber laser to inhibit multi-modes effectively.
Fig. 3 Output spectrum of the fiber laser. ((a) the DBR fiber laser without π-phase shifted FBG. (b)the composite structure fiber laser with π-phase shifted FBG.)
To improve the performance of the fiber laser, we introduced a π-phase shifted fiber Bragg grating into the DBR fiber laser resonator. The parameters of the FBGs are the same as the mentioned above. Considering that the center wavelength of these gratings may not be identical, it is necessary to finely adjust the gratings by PZT tensioner. The π-phase shifted fiber Bragg grating plays the role of a spectral filter and a wavelength controlling oscillator simultaneously, to suppress the multi-longitudinal modes. The transmission window of the π-phase shifted fiber Bragg grating is narrower than 1 pm, so that many longitudinal modes which are not located in this window are inhibited, as Fig. 3(b) shown. The single-longitudinal mode generation was observed at 1535.632 nm when the π-phase shifted FBG peak wavelength was tuned to the Bragg wavelengths of FBG1 and FBG2 by PZT tensioner fine adjustment. In this case, resonator of the composite structure fiber laser proposed in this paper can inhibit most longitudinal modes because lasing window of resonator is narrowed strongly by π-phase shifted FBG. Considering that the Optical Spectrum Analyzer (AP2040) has the unique advantage to display the two polarization channels simultaneously, polarization modes of the proposed fiber laser can be measured. From Fig. 3(b) we can see, the fiber laser has a good single polarization performance, and no other polarization modes generation. When the π-phase shifted FBG is written, the refractive index of the core changes with exposure to UV light and side-exposure can introduce asymmetry or birefringence to the π-phase shifted FBG which make it be polarized. The birefringence is pertinent to UV exposure time and intensity. The polarization mode which satisfied with major polarization axis works on low loss and the orthogonal polarization mode is restrained. Hence, the proposed fiber laser can performs a single polarization laser after modes competition.
A polarization controller and a polarizer were employed to research the polarization extinction ratio (PER) of the fiber laser. The polarization extinction ratio can be measured by manually rotating the polarization controller. The outgoing light of the fiber laser, via the polarization controller and the polarizer, is measured by a power meter. The polarization extinction ratio is obtained by:
Where Pmax and Pmin is the maximum and minimum value of the output power, respectively.
When the 980 nm pump power was about 20mW, the maximum and minimum value of the output power was 680μW and 20μW, respectively. The PER of the proposed fiber laser was 15.31dB according to the Eq. (1).
The linewidth behavior of the proposed composite structure fiber laser was measured using the delayed self-homodyne method. The employed optic fiber delay line is 80Km. By fitting the frequency spectrum of the self-homodyne signal with a Lorentzian line shape function, the laser linewidth is deduced from the fitting curve. The predicted laser linewidth is considered to be 1/20 of −20dB bandwidth. When the 980 nm pump power was about 30mW, the linewidth of the fiber laser was about 900 Hz after Lorentz fitting. The result was obtained by the RF spectrum analyzer (Agilent N9340B), as Fig. 4 shown.
High concentration of active ions of the doped fiber will take high self-pulsation to fiber laser. Thus, it will make the laser work on a high RIN level. The RIN is dominated by the relaxation oscillation and is given out by relaxation oscillation peak. The relaxation oscillation arises from the dynamic energy exchange process between the injected pump field and the laser signal field. In the fiber laser, we used Er3+-doped fiber with low concentration of active ions (Nufern, EDFC980-HA) to get low RIN. The RIN spectral density of the output light was measured by launching the laser emission onto a photo detector and taking the Fourier transform of the photocurrent, as Fig. 5 shown. When the power of 980nm pump was about 20mW, the RIN of the composite cavity structure fiber laser was deduced to about −95dB/Hz around the first relaxation oscillation peak and the frequency shifted from 22.3 kHz to 30 kHz.
Fig. 5 RIN of the fiber laser(a: the usual DBR fiber laser without π-phase shifted FBG; b: the composite structure fiber laser with π-phase shifted FBG).
To study the lasing wavelength stability of the proposed composite structure fiber laser, the optical spectra were carried out as shown in Fig. 6. When the pump power was in order of 7, 8, 10, 25, 50, 80, and 140mW, the results indicated that the lasing wavelength was rather stable with the pump power changing.
Figure 7 shows the output powers against the pump powers for the proposed composite structure fiber laser measured by power meter. The output power of the fiber laser increased linearly with increasing the pump power from 3.5mW to 140 mW. The slope efficiency was calculated to be 6.84% with respect to the launched pump power even using the Er3+-doped fiber with low concentration of active ions. The inset of Fig. 7 shows that the threshold of the generation for the laser is less than 7.5 mW. No attenuation or saturation of the output power was observed even at the pump power of 140mW, which indicated that the output power could be improved further by increasing the available pump power.
It is inevitable that the fiber laser will emit ASE when using Er3+-doped fiber as gain media. Considering that the Optical Spectrum Analyzer (APEX ap2040a) has a limited measurement spectrum range (1520-1567nm), we chose another Optical Spectrum Analyzer (Yokogawa, AQ6370c, with a resolution of 0.02nm and 600-1700nm spectrum range) to look into amplified spontaneous emission (ASE) spectrum noise and side-mode suppression ratio (SMRS) of the proposed composite structure fiber laser. As Fig. 8 shown, the output light of proposed fiber laser generated a −60dBm ASE spectrum when the laser’s peak level was about 0dBm before filtered by FBG3. In the sensor system, the power of ASE should not be ignored in consideration of enormous integral energy. The red curve demonstrates that the fraction of the ASE noise background in the output laser light is well suppressed to −67dBm after filtered by FBG3.
Fig. 8 Optical spectrum of the proposed fiber laser measured by an OSA with a resolution of 0.02nm(black:without FBG3,Red:with FBG3)
4. Conclusion
We have proposed and demonstrated experimentally a simple composite cavity configuration of a single longitudinal high performance fiber laser by introducing a π-phase shifted FBG into the resonant cavity of DBR laser. The proposed fiber laser can operate on single longitudinal mode, narrow linewidth and low RIN even thought the cavity length is increased up to 3 meters. The long length Er3+-doped fiber gives a possibility to obtain high lasing power output. And the π-phase shifted fiber Bragg grating plays the role of a spectral filter and a wavelength controlling oscillator simultaneously, to suppress the multi-longitudinal modes. The proposed composite structure fiber laser obtained narrow linewidth of 900Hz and low RIN of −95dB/Hz to the presence of additional cavity. The pump threshold was about several mW, and the output power increased linearly with the pump power increasing from 3.5mW to 140mW. The SMRS was more than 67dB. The results have considerable potential application for fiber sensor and optical communication.
Acknowledgment
This work was supported by Natural Science Foundation of China (60977058, & 61205083), Independent Innovation Foundation of Shandong University (IIFSDU2012JC015), The key projects of Shandong Province (2011ZHZX1A0107), International Science and Technology Cooperation Program of China (2012DFA10730).
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