Abstract
Based on the simplified model of the tunable fiber laser system, the tuning performance of the laser was analyzed. Two kinds of tunable setups were established, which are the configurations with an external cavity and the configuration of the Littrow cavity. The tuning output characteristics experimentally were analyzed by means of setups. The simulation gives the output efficiency of two tunable lasers as 40% and 30%. In the experiment, the measured slope efficiency of the two lasers was 24% and 18.3%, and the tunable range of the two lasers was 32 nm and 40 nm, respectively. Both lasers could achieve laser output with good beam quality.
1. Introduction
The fiber laser is widely used in many fields, including optical communication, industrial manufacturing, and fiber sensing systems [1–4], which require that the output beam of the laser can be tuned to one or more specific wavelengths. Therefore, tunable fiber lasers have become important optical devices. A tunable external cavity fiber laser can achieve tunable wavelengths by tuning the optical element [5, 6]. Various external cavity tunable fiber lasers system with the grating have been reported, which include the configuration with an output coupled mirror in an external cavity (the laser is called configuration); and the configuration with a Littrow cavity (the laser is called configuration); the maximum continuous tunable range can be up to several tens of nanometers; and the linewidth of the output beam is effectively narrowed [7–11]. However, when we compare the external cavity tunable lasers with different configurations, the output performances of lasers are different. Most researchers only reported their experimental results in tunable lasers but did not carry out a theoretical and experimental comparison of the performance between the different external cavity tunable lasers.
In this study, two kinds of tunable lasers with different external cavity configurations are studied. First, the output effect of the two lasers is analyzed theoretically. Then, using a diffraction grating with a diffraction efficiency of 70% as a wavelength selector, we build two experimental systems of the external cavity tunable fiber lasers and compare their experimental results. The results show that configuration 1 can realize the tunable range of 32 nm with the maximum output power of 560 mW and for configuration 2, can achieve the maximum output power of 427 mW with the tunable range of 40 nm. Both two tunable fiber lasers can produce single-mode output with a beam quality factor (M2) < 1.35.
2. Theoretical Analysis
2.1. Simplification of Tunable Systems
Figures 1(a) and 1(b) show the schematic diagram of configurations 1 and 2, respectively. According to the generation principle of laser, the system can be simplified, as shown in Figure 2 [12].
(a)
(b)
In Figure 2, the inserted devices in configuration 1 are the collimating lens and grating and those in configuration 2 are the collimating lens and DM2. Assuming the length of the gain fiber is L, that of the resonator is L′, launched pump power is , and the output power is . According to the framework shown in Figure 2, if the transmissivity of the front-cavity mirror to the pump light equals 1, then the following equations can be obtained [12]:where K(λ) is the diffraction efficiency of a blazed grating, and η(λ) is the coupling efficiency that the signal light is coupled back into the fiber core.
For K(λ), it can be derived aswhere K0 is a constant with respect to the grating structure, d is the grating period, b is the lateral size of the diffractive facet, m is the diffraction order, and θ is the blazed angle.
If d = 2.43 μm, b = 5 μm, λc = 1550 nm, K0 = 0.70, and m = 1, the diffraction efficiency of the grating at different wavelengths could be obtained, as shown in Figure 3. It can be seen that the diffraction efficiency of the grating gradually increases from 1500 nm to 1545 nm, the diffraction efficiency gradually decreases from 1545 nm to 1600 nm, the highest diffraction efficiency is 0.70 at 1545 nm, and the lower diffraction efficiency is 0.598 at 1600 nm.
In the tuning performance analysis, another parameter of η must be used. For the parameter, Bochove has carried out the derivation [9], which can be written aswhere θ0 = λ/πw0 is the far-field divergence angle of the Gaussian mode field, σ is the coupling bandwidth, and ωx and ωy are the wavefront curvature parameters, which are [13]where ZR is the Rayleigh length of the mode field, ε is a defocus value, X is the transverse distance of the emitter from the system axis, Y is the emitter lateral offset out of the array plane, α and β are the parameters determined by transform lens, α = 3 and β = 1.33 [13].
If Y = 0, θx,y = 0, and ε = 0, for different focal lengths of transformed lenses, the coupling efficiency at different transverse distances from the system axis can be obtained according to Equation (3), as shown in Figure 4. It can be seen that when the transverse distance increases, the coupling efficiency of the laser decreases gradually, and the coupling efficiency gradually increases with the increase of the focal length of the transform lens.
2.2. Analysis of Tuning Performance
2.2.1. Relevant Parameters
In the theoretical analysis, if the used gain fiber is Er3+/Yb3+ codoped double-clad fiber with a fiber length of 4 m. The relevant parameters include the spectrum absorption-emission cross-sections (σEra and σEre) of Er3+, as shown in Figure 5 [14].
According to the parameters of the fiber given by the Nufern Company, the core diameter of the fiber is 25 μm, the clad diameter is 300 μm, and the coating diameter is 450 μm. In addition, the concentration of Er3+ and Yb3+ doped in the fiber is 4.8 × 1025 m−3 and 3.7 × 1026 m−3, respectively, and the lifetime of Er3+ is 11 × 10−3 s. Assuming the wavelength of the pump is 980 nm, the transition cross-sections σEr13 = 2.0 × 10−25 m2 and σYb65 = 5.0 × 10−25 m2. Other relevant parameters involved are given in Table 1 [15, 16].
2.2.2. Simulation Results
Assuming the tunable range is 1500–1600 nm, the tuning step is 5 nm, Y = 0, θx,y = 0, ε = 0, and R1 = 0.90, and the power of the tuning laser at different wavelengths was calculated, as shown in Figure 6.
(a)
(b)
For configuration 1, maximum output power is 800 mW, at a wavelength of 1547 nm, the minimum power is 210 mW at 1600 nm, and the calculated maximum output efficiency is 40%. Furthermore, the laser output power increases gradually in the range of 1500–1547 nm, and the output power gradually decreases in the range of 1547–1600 nm. For configuration 2, the maximum power is 600 mW at 1555 nm, the minimum power is 345 mW at 1500 nm, and the maximum output efficiency is 30%.
The output power of the laser changes greatly at different wavelengths. The reason is that the diffraction efficiency of the grating at different wavelengths is different, and the absorption-emission cross-section of the optical fiber at different wavelengths is different. The difference in output efficiency between the two configurations is mainly caused by the difference in feedback intensity (R3) at the backend of the external cavity. As the increase of the backend feedback intensity, the output efficiency gradually decreases [12].
3. Experiment
3.1. Experimental Setup
In two kinds of laser configurations, the pump was provided by a laser diode with a central wavelength of 976 nm through a coupled system. The maximum power coupled into the fiber was 3.0 W. The fiber used was LMA Er3+/Yb3+ codoped double-clad fiber named PLMA-EYDF-25P/300-HE with a length of 4 m, whose core had a diameter of 25 μm with a numerical aperture (NA) of 0.09. The absorption coefficient was 2.9 dB/m at 980 nm. A dichroic mirror (DM1, high transmission to pump, and high reflectivity to signal light) was placed at the input end of the fiber, which served as the front-cavity mirror of the resonator. The output end of the fiber was angle cleaved to suppress Fresnel reflection. A collimating lens (f = 50 mm) was placed at the output end of the fiber. The experimental setups are shown in Figure 7.
For configuration 1, the first-order diffracted beam of the grating was incident on the outcoupling mirror (OCM), which has a reflectivity of 15% to provide feedback for laser oscillating and achieve laser output. For configuration 2, to improve the conversion efficiency and output power of the fiber laser, DM2 (high transmission to signal light and high reflectivity to pump) is placed behind the collimating lens. The grating was placed according to the Littrow angle, the first-order diffracted beam was reflected back, and the zeroth-order diffracted beam was used as the output beam.
In addition, the grating has a frequency of 1200 lines/mm with a blazing wavelength of 1550 nm, and the first-order diffraction efficiency of the grating can reach 70%. The laser beam quality was measured using a beam quality analyzer (M2MS BP209-IR), and the spectral performance was analyzed by an optical spectra analyzer (model: MS9710B).
3.2. Comparison of Experimental Results
3.2.1. Comparison of Tunable Ranges
First, using configuration 1, the experiment was carried out. When the coupled pump power was 3 W, the tuned laser at the range of 32 nm from 1532 nm to 1564 nm was achieved, as shown in Figure 8(a). For each spectrum of the tuned laser, the measured 3 dB linewidth was less than 0.08 nm. The suppression of the ASE background was better than 46 dB. Next, the experiment of configuration 2 was conducted, as shown in Figure 8(b). At the coupled pump power of 3 W, the measured maximum tuning range is 40 nm from 1530 nm to 1570 nm. For each spectrum of the tuned laser, the measured 3 dB linewidth was less than 0.1 nm. The suppression of the ASE background was better than 41 dB.
(a)
(b)
Compared to Figures 8(a) and 8(b), the tunable range of configuration 2 is wider than that of configuration 1. This is because the backend of the external cavity of configuration 1 is OCM, which has low feedback for the signal light. According to the curve in Figure 6, for configuration 1, the weak feedback signal light cannot induce a more population of Er3+ transitions. For configuration 2, the backend of the external cavity is the grating, which has higher feedback for the signal light, and the higher feedback intensity can induce more population of Er3+ to finish the energy level transition, realizing the tuned laser output in a wider band.
Furthermore, we also compared the tuning range of the simulation and the experiment. It can be found that the tuning range in the experiment is narrower than that in simulation. This is because the oscillation threshold of two configurations is different at different wavelengths; it can also be found in Figure 9 (Section 3.2.2). However, the oscillation threshold is set to a small constant in the simulation, which makes laser oscillation be produced at lower power, resulting in a wider tuning range.
3.2.2. Comparison of Output Power
In the experiment, the measurements of the laser output power of the two configurations at different coupled pump power were made, as shown in Figure 9. When the coupled pump power was 3 W, the maximum laser output of 501 mW at 1546 nm was achieved by configuration 1 with a slope efficiency of 24%. For configuration 2, the maximum output power is 409 mW at 1550 nm with a slope efficiency of 18.3%. The output efficiency of the tuned output laser in the experiment is lower than that in the simulation. This is because the angle-cleaved facet of the fiber is not polished, and the lenses used in this experiment do not have an antireflection coating of a 1550 nm band, which reduces the efficiency of the lasers. In addition, with the increase of the pump power, the output laser power increases approximately linearly, and saturation gain does not occur, which means that the pump power can be continuously increased to obtain higher output power.
Furthermore, the output power of the tunable laser at different wavelengths is measured, which is shown in Figure 10. It can be seen that the wavelength of maximum power in the experiment is slightly different from that in the calculation because of the different lengths of the resonators. The output power of configuration 1 is higher than that of configuration 2; this is because considering all external cavity loss factors, the total feedback level of first-order diffraction light is over 50% in configuration 2, and the grating diffraction efficiency of zero-order light is 25%, which lead to a lower outcoupling ratio of the resonator, and cause large cavity loss and limit the effective increase of the output power and slope efficiency of the tunable fiber laser. However, in configuration 1, the feedback intensity of the external cavity is about 15%, which causes a higher outcoupling ratio and increases the efficiency of the resonator. Furthermore, the slope efficiency and output power of the laser can be improved by reducing the external cavity loss and adopting the backward output configuration [15].
3.2.3. Comparison of Beam Quality
When the wavelength of the output laser was 1548 nm, the output beam quality factor(M2) of the two lasers was measured, as shown in Figure 11. For configuration 1, the measured Mx2 is 1.22, and for configuration 2, Mx2 = 1.31. Compared with the ideal Gaussian beam, the beam quality is degraded; this is due to the aberration of the transform lens and the etching error of the grating.
(a)
(b)
Figure 12 shows the measurement results of the M2 in the tuning range. The M2 of configuration 1 is kept at the range of 1.22–1.26, and M2 of the configuration 2 is kept in the range of 1.31–1.34. The output beam quality of configuration 1 is better than that of configuration 2; the reason is that the direction of propagation of the output beams is normal to the OCM [17] in configuration 1, which causes the divergence angle to be smaller than that in configuration 2. In addition, the lower NA of the Er3+-Yb3+ codoped double-clad fiber also makes it easier to achieve laser output with good beam quality. Furthermore, with relatively simple modifications to the cavity design (with a shorter focal-length collimating lens or a diffraction grating with a larger pitch), it should be possible to improve the beam quality M2 to <1.1 [18–21].
4. Conclusion
In this study, according to the principle of laser generation, we simplify the two external cavity tunable systems into a fiber laser system. Based on the rate equation of Er/Yb codoped fiber laser and the established boundary conditions, the changes of the output powers with the wavelength were analyzed. The result shows that assuming the pump power is 2 W, the maximum tuning power of configuration 1 is 800 mW at 1545 nm. The maximum tuning power of configuration 2 is 600 mW at 1555 nm. Furthermore, the experimental setups of two kinds of external cavity tunable fiber lasers were also built, and their differences in output effects of them were compared and analyzed. For configuration 1, the measured maximum output power is 501 mW with a maximum slope efficiency of 24%, the tuning range is 32 nm from 1532 to 1564 nm, and M2 is kept in the range of 1.22–1.24; the suppression of the signal background caused by ASE was better than 46 dB. For configuration 2, the maximum output power is 409 mW with a slope efficiency of 18.3%, the tuning range is 40 nm from 1530 to 1570 nm, and M2 is kept in the range of 1.31–1.33. The suppression of the signal background caused by ASE was better than 41 dB. The backend feedback (R3) of the resonator has a great influence on the tunable range and output power. This research can provide potential value for the DWDM and fiber sensing system.
Data Availability
The (DATA TYPE) data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (41805014) and Natural Science Foundation of Anhui Province, China (1808085MF189, gxbjZD2021063, KJ2019A0565, and KJ2019A0576).