An end-pumped ytterbium-doped all-fiber laser with 300 W output in continuous regime was reported, which was based on master oscillator multi-stage power amplifiers configuration. Monolithic fiber laser system consisted of an oscillator stage and two amplifier stages. Total optical-optical efficiency of monolithic fiber laser was approximately 65%, corresponding to 462 W of pump power coupled into laser system. We proposed a new method to connect power amplifier stage, which was crucial for the application of end-pumped combiner in high power MOPAs all-fiber laser.
© 2008 Optical Society of America
The combination of small size, maintenance-free operation, ease of cooling, high efficiency and outstanding beam quality has made high power ytterbium-doped fiber laser be widely used in many fields, such as materials processing and military applications [1–3]. Several ytterbium-doped fiber lasers with output powers approaching the kilowatts-range in the continuous-wave (CW) regime have been reported . These demonstrations typically employed a length of gain fiber pumped via free-space coupling and bulk optics as the reflector. However, the availability of monolithic fiber laser is crucial for making this technology available for a variety of application. All-fiber laser employs all-fiber components to replace the bulk-optic interface and consequently becomes compact, rugged and reliable . There are mainly two considerations for the investigation of high power all-fiber laser.
One consideration is the selection of all-fiber pump scheme. Now there are two fiber-coupled pumping concepts for double clad fiber (DCF). They are side-pumped and end-pumped. Several side-pumped techniques [6, 7] have been developed for launching pump light into the DCF. However, only fused taper side coupling developed by IPG and Co-linear coupling investigated by SPI are applied in high power monolithic fiber laser. These two techniques are not available on the open market. Compared with side-pumped, end-pumped is the conventional method [8, 9] to couple pump power into the DCF. It is commercially available. Triggered by the progress in high-brightness pump diodes and the availability of all-fiber components with high power handling, end-pumped monolithic fiber laser with output powers approaching the kilowatt-range is possible .
The other consideration is the selection of the laser configuration. Before reaching the CW power limit of fiber itself, the structure of master oscillator multi-stage power amplifiers (MOPAs) gives a good solution for power scaling when the injection pump power is limited by the output power of the pigtailed LD or the power handling of all-fiber components. Apart from this advantage, in multi-stage fiber amplifiers the temperature distribution along the fiber is uniform, compared with that of fiber laser containing a single ytterbium-doped fiber. The highest temperature in the fiber is decreased significantly in the kilowatt power domain.
There are fewer reports of high power all-fiber laser. As example, IPG photonics have reported on significant increase of all-fiber format single-mode ytterbium-doped CW fiber laser output power to 1.96 kW with M2<1.2, demonstrated by a side-pumped multi-stage amplifiers configuration during a laboratory test in 2005 . In 2006, Stephen Norman et al. reporte for the first time on the power-scaling extension of SPI’s proprietary side coupled cladding-pumped GT-Wave technology platform with 400 W output power , which is based on a side-pumped one stage amplifier configuration.
Generally, during the investigation of high power end-pumped fiber laser with MOPAs structure, the connection of fiber amplifier stage by fusion splice is not perfect as a result of the splicing of eccentric cladding fibers. This problem would make end-pumped combiner in the amplifier stage sustain too much dissipated power and be destroyed. It limits power scaling of end-pumped fiber laser. A new method to connect power amplifier stage is demonstrated in our experiment, which is crucial for the application of end-pumped combiner in high power MOPAs all-fiber laser. To the best of our knowledge, this method is proposed for the first time during studying MOPAs laser. And using this method, we report the development of 300 W continue-wave output end-pumped ytterbium-doped all-fiber laser with master oscillator multi-stage power amplifiers structure. The end-pumped multi-stage power scaling monolithic fiber laser is under particular description in this paper.
2. Experimental setup
The developed fiber laser consisted of three stages: a laser oscillator stage and two booster amplifier stages. They were built up by fusion splice, which made the all-fiber laser system require less maintenance and no misalignments. The experimental setup was depicted in Fig.1.
Laser oscillator contained gain fiber, a 7×1 end-pumped combiner and a pair of fiber Bragg gratings. Double clad ytterbium-doped fiber (YDF) manufactured by Nufern was employed as gain fiber, which had a core diameter of 20 µm (NA=0.06) and an inner cladding diameter of 400 µm (NA=0.46). A microscope image of the inner cladding shape was shown in the inset of Fig.1, which was observed by surface mapping microscope. The absorption of ytterbium doped fiber at 975 nm was 1.7 dB/m and the fiber length was 10 m. The laser cavity was formed by a pair of fiber Bragg gratings (FBG). One of them was used as the high reflector and the other was used as the output coupler. The output fiber of 7×1 end-pumped combiner was passive fiber with core diameter of 20 µm (NA=0.06) and inner cladding diameter of 400 µm (NA=0.46). Pump delivery fiber of combiner had a diameter of 200/220 µm (NA=0.22). Seven 25 W fiber-coupled laser diodes (200/280 µm 0.22 NA delivery fiber) were employed to provide 975 nm pump power.
The first power amplifier stage consisted of gain fiber and a (6+1)×1 end-pumped combiner with signal feed-through, spliced to the delivery fiber of oscillator. The gain fiber used in amplifiers was same to that in laser oscillator and the length was 13 m. The (6+1)×1 combiner made by ITF had a structure of tapered fiber bundle. In this special configuration, six pump delivery fibers and one signal input fiber were coupled to the inner cladding and the core of the double clad output fiber respectively. The pump fibers were also standard 200/220 µm fiber (NA=0.22). Both signal input fiber and double clad output fiber were germaniumdoped fiber (GDF) with a core diameter of 20 µm (NA=0.06) and circle inner cladding diameter of 400 µm (NA=0.46), which were compatible with gain fiber. The combiner provided 0.05 dB of insertion loss for pump power and the insertion loss for signal was approximately 0.1 dB. This booster amplifier was pumped by six 25 W laser diodes of 975 nm (200/220 µm 0.22 NA delivery fiber).
The configuration of the second power amplifier stage was similar to the first amplifier stage. The length of ytterbium doped fiber was 14.5 m. The insertion loss of combiner for signal was approximately 0.3 dB. At the output of monolithic fiber laser system an end-cap with length of 1.5 mm and angle of 8 degree was employed to minimize back-reflection into the amplifier and avoid the surface damage.
In this experiment, the tolerance of wavelength of all laser diodes was ±3 nm and the spectral width was approximately 4 nm. Since the absorption of ytterbium around 975 nm was extremely wavelength dependant, depending upon optical characteristics of laser diodes gain fiber probably gave us only around 1.2 dB/m of absorption.
3. Connection between two power amplifier stages
Because of the special structure of tapered fiber bundles for end-pumped, light accepted by the input fiber of end-pumped combiner must be in the core. If the signal or the unabsorbed pump light from previous stage was propagating in the inner cladding of input fiber, the combiner would be heated up and possible to be destroyed. When connected the second power amplifier stage by splicing the delivery fiber of the first amplifier stage and the input fiber of combiner, we found that the quality of connection affected the performance of combiner significantly because of operating at multi-100 W high power level. The analysis of this phenomenon was described as follow.
Firstly, signal power in the cladding of input fiber induced by splice joint would dissipate on the combiner and may destroy it. Fusion splice between 20/400 µm circle inner cladding GDF fiber and 20/400 µm octagonal inner cladding YDF fiber was a challenging assignment. The geometric shape of inner cladding of two fibers was not symmetrical and the cores were not in the center of fiber as a result of manufacturing errors. Usually, the fiber cores could be aligned well prior to splicing, but the eccentric cladding fibers could not align to each other’s cladding. As a result of surface tension that acted to align the cladding of the two tips together, the cores were slightly mismatched during joint formation. This phenomenon often occurred in the experiment when two non-concentricity fibers were spliced. The micrograph of typical splice completed by Vytran FFS-2000 splicer was shown in Fig. 2 (a). The fusion splice loss of 20/400 µm fibers versus deviation of core position was presented in Fig. 2 (b). With 1 or 2 micron error in the core position, the splice loss would be small when the mode field diameter was large. However, since signal power was multi-100 W class, only 0.15 dB of optical loss would make end-pumped combiner sustain too much dissipated power.
Secondly, unabsorbed pump light propagating along inner cladding of input fiber would dissipate on the combiner. That was why we used a longer gain fiber length to avoid a large amount of pump light from passing through. However, too long gain fiber would lead to decreasing of laser efficiency. So pump light from previous stage was also a factor influencing the performance of combiner.
In order to connect power amplifier stages reliably in monolithic fiber laser based on end-pumped scheme, we proposed a novel method to avoid combiner from being heated up by the light in the cladding of input fiber. Before delivery fiber of previous stage (YDF 20/400 µm gain fiber) and input fiber of combiner (GDF 20/400 µm) were spliced, we put the two fiber ends into hydrogen chloride. As a result of chemical reaction between silica and hydrogen chloride, the inner claddings of two fibers were both eroded from 400 µm to 125 µm while the core diameter of 20 µm was not changed. Then two 125 µm fiber ends were spliced by Vytran FFS-2000 splicing workstation. Microscope image of fusion splice was shown in Fig. 3. This technique resulted in two significant improvements. On the one hand, smaller cladding diameter of fiber means lower splice power and much shorter splice time. Decrease of splice power and splice time would greatly weaken the negative effect of surface tension during the splicing process. No deviation of core position was observed from the micrograph of splice and optical loss induced by the splice joint could be almost negligible. On the other hand, the smaller fiber cladding could only confine higher brightness light, and more than 90% of pump light unabsorbed by 20/400 µm YDF could not propagate in 125 µm fiber. Besides, the rough transitional area between 400 µm and 125 µm also scattered some of pump light. Thus, the most of unabsorbed pump light dissipated when passing through this novel structure. The light in the inner cladding of input fiber could be attenuated sufficiently by this method.
In our experiment, as the second amplifier stage was connected by splicing 20/400 µm fibers with eccentric claddings, deviation of core position at fusion splice joint was around 1.5 µm. When around 200 W output power was produced by the oscillator and the first power amplifier, signal power in cladding induced by splice and pump power accepted by end-pumped combiner were 9.5 W and 4.3 W respectively. With our proposed method, the second amplifier stage was connected by splicing eroded 20/125 µm fibers and deviation of core position was less than 0.2 µm. Signal power in cladding induced by splice was less than 0.5 W and pump power accepted by end-pumped combiner was around 0.4 W. This novel structure reduced the optical power dissipating on the combiner significantly when the end-pumped combiner was used in high power all-fiber laser.
4. Experimental results and discussion
The output laser power from the end-cap as a function of pump power was shown in Fig.4. The maximum CW output power of this system was 300.7 W at 1085 nm with the total coupled pump power of 462 W. The optical to optical efficiency was around 65% and wall plug efficiency was around 25%. The unabsorbed pump power in this system was 7 W. The stability of the output power was less than ±1% within around 10 minutes measuring time.
The laser oscillator exhibited a slope efficiency of 61% and achieved output power of 90 W from the oscillator output end under 168 W of pump power. The first stage power amplifier produced 113 W of laser power under 134 W of pump power, compared with the 132 W laser power of the second stage power amplifier under 160 W of pump power. Note that, amplified spontaneous emission did not consume much up-level population and the extraction efficiency of two stage power amplifiers had been over 80%.
The output spectra of the system were measured by an optical analyzer (Agilent Inc.), which was shown in Fig. 5. The emission was centered at 1085 nm and the spectral line-width was less than 0.15 nm FWHM, which was almost same with the output spectra of oscillator. Although the total fiber length of the booster amplifiers was more than 25 m, we did not observe any non-linear signature on the output spectra.
The output polarization of the laser was random polarized because the fiber in this experiment was not polarization-maintaining.
The beam quality factors were measured with a Spiricon M2-200 laser beam analyzer. And M 2 factors of all-fiber laser were measured as M2x=1.86, M2y=1.95. The gain fiber was coiled to a diameter of 30 cm in our experiment. The results indicated that the output beam was made of several low-order transverse-modes in the fiber.
During operation of this fiber laser, no failure of splices or components was observed for any of power level. It should be feasible to scale to higher output power based on the end-pumped all-fiber laser with master oscillator multi-stage power amplifiers configuration.
We had demonstrated up to 300.7 W output power based on an end-pumped MOPAs configuration all-fiber laser system, with around 65% optical-optical efficiency. Both the laser oscillator stage and booster amplifier stages could convert pump energy to laser output effectively. A new method to connect power amplifier stage was demonstrated, that ensured the end-pumped MOPAs high power all-fiber laser operate reliably.
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