1. Introduction

In the continuous-wave (cw) regime, fiber lasers have shown extraordinary progress in power level. A ytterbium- (Yb)-doped double-clad (DC) fiber laser with an output power of 1000 W and M ²=3.4 [1], and an 810-W single transverse mode Yb-doped fiber laser [2] have been reported recently. These results were achieved with large mode area (LMA) fibers end pumped with high-power laser diodes coupled with bulk optics.

Fiber lasers can be built as monolithic systems [3], with optical pump and signal beams completely in fiber. All-fiber components that replace the bulk-optic interface in fiber laser configurations exhibit low loss. These components mainly include multimode fused fiber bundle combiners and Bragg gratings. Fiber-pigtailed pumps can be coupled into a DC fiber cladding by use of multimode fused fiber bundle combiners. So far such couplers can sustain high average powers up to 200 W [4]. The maximum power coupled through such a combiner into a DC fiber is typically smaller than that achieved by use of free-space end coupling. So power scaling of monolithic fiber lasers is one of the hot topics in high-power fiber lasers around the world. One typical monolithic fiber laser is the IPG Photonics 300-W fiber laser with M ²=1.1 by use of two fused fiber bundle combiners.

To scale up the output powers in a monolithic fiber laser, a good choice appears to be a multicoupler side-pumped fiber laser [5]. Alhough several side-pumped methods have been studied, such as fiber bundle combiners [6], V-groove side-coupling combiners [7], angle-polished combiners [8], embedded-mirror combiners [9], and diffraction grating combiners[10], a side-pumped fiber laser with more than three couplers has not actually been developed because of the sophisticated fabrication process necessary for a multicoupler in one DC fiber.

Here we propose a new type of fiber laser with multicouplers, but these couplers are not in one fiber. It is called a distributed pumping multifiber series fiber laser with the following advantages: easy to manufacture, high efficiency, and power scaling. This kind of fiber laser will lead to a monolithic fiber laser with kilowatt output without the requirement of further development of high-brightness diodes. We also provide the simulated results of the fiber laser.

2. Structure of the fiber laser

The proposed distributed pumping multifiber series fiber laser is shown in Fig. 1. The fiber laser is composed of several side-pumped fibers, a Bragg grating, and spliced spots. The side-pumped fibers are connected in series by fusion splicing. The fiber Bragg gratings (FBGs) set on the ends of the connected fiber are used as reflectors for the laser cavity.

In each side-pumped gain fiber there are fused fiber bundles, a LMA DC gain fiber and pumping laser diodes (LDs). Pumping power is coupled into the gain fiber through the fused fiber bundle. The fused fiber bundle combiner consists of several relatively large diameter pump-fiber leads fused together with a signal-carrying fiber, which contains a core matching that of a LMA DC gain fiber. The signal-carrying fiber is not the DC gain fiber itself but a non-rare-earth-doped version with a matching index profile. To avoid the pumping light leakage at combiners, the length of the individual side-pumped doped DC fiber should be long enough to absorb the pumping powers efficiently. Usually two fused fiber bundles in each side-pumped fiber are used to inject more pump power to the doped DC fiber.

 

Fig. 1. Structure of the distributed pumping multifiber series fiber laser.

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To make high efficiency for laser oscillation, it is important to decrease the splicing insert loss in the laser cavity and to suppress the amplified spontaneous emission (ASE) among individual fiber. It is known that the insert loss of the fused splice has been as low as 0.01dB and the backreflection is more than -60 dB for the laser signal in a standard 125-µm telecom fiber. With the development of fusion splicing technology for large area DC fiber, the splicing quality is close to that of a standard 125-µm telecom fiber. Because of the weak back reflections of splicing it is impossible to produce laser oscillation within any individual fiber before the whole fiber laser operates. The threshold for the whole fiber laser is lower than that for any individual fiber. Certainly to achieve high transfer efficiency and to decrease the cavity loss, all the fibers connected by fusion splicing should be the same core size and have the same numerical aperture (NA).

With this laser configuration, more pump powers can be coupled into doped DC fiber by easily connecting more side-pumped fibers. So output powers of the all-fiber laser would be scaled up before reaching the facet destroy threshold of the DC fiber itself or the power limiting caused by nonlinearity of the fiber. In the meantime the output beam quality, which is decided mainly by a DC fiber, is as good as for an individual fiber. The beam quality is not worse with the increase of output power in the laser.

Although pump and signal fused fiber combiners are becoming commercially available for use with 20-µ m core fibers, as an example we constitute a four-fiber series fiber laser with a 30-µ m core for higher output power. The Yb-doped DC fiber has a 30-µ m core, a 400-µ m octagonal-shaped inner cladding, a 0.06 core NA, and a 0.46 cladding NA. The (6+1)×1 combiner (six pumps plus one signal) transfers pumping power from both ends of an individual DC fiber. The input fibers of the (6+1)×1 combiner have a 200/220-µ m (core/cladding) and a 0.22-NA fiber, which are connected to a pigtailed (200-µ m core, 220-µ m cladding, 0.22 NA) pumping LD. The signal-carrying fiber of the (6+1)×1 combiner has nondoped DC fiber with a 30-µ m core, a 400-µ m octagonal-shaped inner cladding, a 0.06- core NA, and a 0.46 cladding NA. The pumping LD is a 25-W fiber coupled at the center wavelength of 974 nm. So the total pumping power for one (6+1)×1 combiner is 150 W. The length of the DC fiber is 10 m, which is long enough to absorb the pumping light sufficiently (2 dB/m).

The above four side-pumped fibers are connected in series. The total pumping powers for the whole fiber laser are 1200 W because the distributed eight-side couplers are used. The FBG is carved in 30/400-µ m fiber with >99% reflectivity (a 1100-nm center wavelength and a 0.5-nm wavelength width) as a high reflection mirror. A flat fiber cleave that provides 3.5% Fresnel reflection is on the other end of the fiber as the output mirror.

3. Theoretical model

For high-power Yb-doped fiber lasers with Bragg grating reflectors and strongly pumped conditions by LDs, the pump light and output laser are narrow spectra that can be seen as a single wavelength. The Yb ion is a three-level or quasi-three-level system that avoids pump and laser excited-state absorption (ESA). We assume that strong pumping conditions are high enough to saturate the gain medium and to suppress the spontaneous emission. We consider scattering losses for both the laser and the pump. The schematic illustration and parameters are shown in Fig. 2. For cw lasers the time-independent steady-state rate equations can be expressed as [11,12]

 

Fig. 2. Schematic of the distributed-pumping DC fiber laser.

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Equation (1) describes the upper level Yb ³⁺ population density N ₂ (z) (averaged over the core cross section) at position z along the fiber length that varies with the positive and the negative directions of laser powers $P_s^{+}$ (z), $P_s^{-}$ (z) as well as forward and backward pump powers $P_p^{+}$ (z), $P_p^{-}$ (z) at the same location. Equations (2), (3), (4), and (5) express the relations of the forward and backward pump powers, the positive and the negative directions of laser powers as well as the upper level Yb ³⁺ population density, where N is theYb ³⁺ dopant concentration in the core whose cross-sectional area is A; Γ _p and Γ _s are power filling factors; Γ _p illustrates the fraction of the pump power actually coupled to the active core; Γ _p can be expressed approximately by the ratio between the area of the core and that of the multimode inner cladding; Γ _s represents the contribution of laser power in the core; σ _ep and σ _ap are the emission and absorption cross sections of pump light, whereas σ _es and σ _as are the emission and absorption cross sections of the laser; ν_p is the pump frequency; ν_s is the laser frequency; τ is the spontaneous lifetime; parameter h is Planck’s constant; coefficient α_p and α_s represent scattering losses of the pump light and the laser, which are independent of z; and P ₀=2hν_s Δν_s is the laser contributed by spontaneous emission in the gain of the Δν_s frequency range and is neglected because of its low value.

For the above distributed-pumping four-fiber series fiber laser, each side-pumped fiber length is L ₁, L ₂, L ₃, and L ₄, respectively. The two pumping combiners in each side-pumped fiber are set on the two ends of the fiber at positions of L_{i 1}, L_{i 2} (i represents the ith side-pumped fiber) and provide the fiber with $p_p^{+}$ (L_{i 1}) and $p_p^{-}$ (L_{i 2}), respectively. The pump power in each side-pumped fiber is P_{i 1} and P_{i 2} respectively. The length of the Yb-doped DC fiber is long enough for the pump power to absorb adequately, so the total loss of pump leakage between combiners is neglected. The boundary conditions for $p_p^{+}$ (z) and $p_p^{-}$ (z) are as follows:

where η ₂ is the pumping transmission efficiency of the fused-fiber combiner. $p_s^{+}$ (z) and $P_s^{-}$ (z) are enforced by R ₁ and R ₂ to comply with boundary conditions of the linear cavity. At each splicing spot, $p_s^{+}$ (z) and $P_s^{-}$ (z) should obey the continuous conditions with the signal transmission efficiency of η ₁ caused by the insertion loss of the splice. So we can write the boundary conditions for $p_s^{+}$ (z) and $P_s^{-}$ (z) as follows:

With Eqs. (1)–(11), the distributed pumping four-fiber series fiber laser is simulated. The parameters used for the simulation are listed in Table 1.

Table 1. Parameters Used for Simulation of a Distributed-Pumped Four-Fiber Series Fiber Laser

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4. Simulation results and discussion

Figure 3 shows the simulated results of the positive and the negative directions of laser powers $P_s^{+}$ (z), $P_s^{-}$ (z) as well as the forward and backward pump powers $P_p^{+}$ (z), P^-_p(z) along the fiber. The laser output power from the distributed pumping four-fiber series fiber laser is up to 932 W when the LD pump power is 1200 W. The transfer efficiency of the fiber laser is approximately 77.7%. From Fig. 3 the positive direction of the laser power is added up along the fiber because each part of the side-pumped fiber provides gain and contributes to the laser output power. With the distributed pumping, the increased output power is approximately linear by optimized laser design. Compared with the results of a fiber laser that consists of a laser oscillator and a booster amplifier [13], a higher transfer efficiency is obtained with our laser system.

 

Fig. 3. Calculated pump and laser powers as functions of position along the fiber.

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From the calculation, the threshold of the distributed-pumped four-fiber series fiber laser is 3.61 W if we neglect the splicing insertion loss and other transmission losses in nondoped short fiber. With the splicing insertion loss, the threshold of the fiber is increased. The increment of pump threshold is shown in Fig. 4 with different splicing insertion losses. Here the splicing insertion loss refers to the loss caused by one splice. In the four-fiber series system the total splicing insertion loss is twelve times that of one splice because there are twelve splice spots in the laser. Figure 4 shows that even the splicing insertion loss is up to 0.1dB, the increment of the threshold is only 374 mW, and the fiber laser is easy to lase.

 

Fig. 4. Increment of pump threshold with different splicing insertion loss of a single splice in the distributed-pumped four-fiber series fiber laser.

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With the distributed-pumped multifiber series fiber laser, the temperature distribution along the fiber is uniform compared with that of a double-end-pumped fiber laser as shown in Fig. 5. The highest temperature from a distributed-pumped multifiber series fiber laser is 119°C, whereas the highest temperature from a double-end-pumped fiber laser is 303°C at the same total input power of 1200 W. The highest temperature in the fiber is decreased significantly in the kilowatt power domain with the distributed-pumped fiber laser.

 

Fig. 5. Temperature distribution along the fiber with a distributed-pumped and a double-end-pumped fiber laser at a total of 1200-W input power.

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Before reaching the cw power limit of fiber itself, a distributed-pumped multifiber series fiber laser structure gives a good solution for power scaling when the injection power is limited by the output powers of the pigtailed LD or the maximum sustainable powers of couplers. The method can provide enough pump powers by connecting more side-pumped fibers in series. The individual fiber length and doping concentration can be designed by optimization. Moreover if there is additional scaling of laser output powers, a larger core DC fiber should be taken.

5. Conclusions

We have proposed a new type of high-power fiber laser with a multifiber series and distributed pumping. The fiber laser is composed of several side-pumped gain fibers, Bragg gratings, and spliced spots. The side-pumped gain fibers are connected in series by fusion splicing. The fiber Bragg gratings set on the two ends of the connected fiber are used as reflectors for the laser cavity. This kind of laser has the advantages of a monolithic structure, is easy to manufacture, and has power scaling.

From the cw lasers time-independent steady-state rate equations, a simulation of the distributed-pump multifiber laser has been given. It is shown that high transfer efficiency can be obtained in the laser because an individual part of the side-pumped fiber provides gain and contributes to the laser output power efficiently. The increment of the threshold caused by the insertion loss of splicing is not big; moreover the monolithic fiber laser is easy to lase because the weak backreflections of splicing suppress the amplified spontaneous emission among individual fibers. With the distributed pumping the lower temperature distribution can be obtained in the laser system.

The distributed-pumped multifiber series fiber laser provides a good solution for power scaling when the injection power is limited by the output power of the pigtailed laser diode or the maximum sustainable powers of couplers. Before reaching the cw power limit of doped fused silica fiber, the output power of a monolithic laser would be scaled up by connecting more side-pumped fibers in series within the laser resonator.