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We report an 8-element spectral beam combination of Yb-doped all fiber superfluorescent sources around 1070 nm wavelength. Each source consists of a 60 mW front-end and a 1.5 kW three-stage fiber amplifier chain. The eight output beamlets are spectrally combined using a home-made polarization-independent multilayer dielectric reflective diffraction grating. 10.8 kW output power is achieved with an efficiency of 94%. Besides, both theoretical and experimental studies of dual grating dispersion compensation scheme have been performed, which is proved to be a prospective way for high brightness spectral beam combination.
© 2016 Optical Society of America
1. Introduction
Yb-doped fiber amplifier (YDFA) has been established as reliable and promising high-power laser architecture owing to its advantages like compactness, high conversion efficiency and excellent heat dissipation [1]. The increasing demand for high brightness laser sources has led to a significant power scaling of fiber lasers [2,3]. However, the power scaling limitation of a monolithic fiber amplifier is set by damage, thermal issues, nonlinear effects, and modal instability [4–6].
For the purpose of further scaling the output power while maintaining excellent beam quality, beam combining technology has been proposed which mainly includes coherent beam combining (CBC) and spectral beam combining (SBC) [7–10]. In the case of CBC, several beams with identical wavelength are phase controlled, and superposed in the far-field to accomplish the brightness improvement [11–16]. Comparing to CBC, SBC is a promising alternative concept which relaxes the requirement for phase control of the individual beams. Narrowband laser source has gained great attraction in SBC because the combined beam quality strongly depends on the signal linewidth [17–21]. However, for high power narrowband fiber amplifiers stimulated Brillouin scattering (SBS) is always a primary limiting factor. Lots of effort has been made to suppress SBS in the past few years, such as increasing the fiber modal effective area, reducing overlap between optical and acoustic fields, phase or intensity modulation on the distributed feedback (DFB) laser, and gain competition between two signals in a monolithic co-pumped amplifier [22–27]. The superfluorescent source has advantages on suppressing SBS due to its features of no longitudinal modes and equally distributed photons within the spectral range, which could be of great interest in spectral beam combining area [28,29].
In this contribution we demonstrate spectral beam combination of eight all fiber superfluorescent sources around 1070 nm wavelength by a home-made polarization independent multilayer dielectric (MLD) diffraction grating, and produce 10.8 kW combined power. The combined efficiency of the non-polarized beamlets array is 94%. The beam quality optimization is achieved through dispersion compensation by the dual grating structure. To the best of our knowledge, this approach is the first to introduce all fiber superfluorescent sources into the high brightness incoherent beam combination area.
2. All fiber superfluorescent laser array
The all fiber superfluorescent laser source contains an amplified spontaneous emission (ASE) seed and a three-stage amplifier chain, which is shown in Fig. 1. The upper part of Fig. 1 illustrates the schematic diagram of the ASE seed. A 250 mW, all-fiber ASE source with a FWHM of 30 nm is realized by a counter-pumped configuration by means of a pump combiner. The fiber end is angle-cleaved to provide effective suppression of parasitic lasing. Back reflections can be avoided by a polarization-maintaining (PM) isolator. PM fiber Bragg gratings (FBG) accompany with the circulators (CIRC) form a two-stage filter to confine the broad emission spectrum to a narrowband one. Finally, a 70 pm narrow linewidth ASE seed source with 60 mW power is obtained. The other PM isolator is used to protect the seed from potential back reflection from the subsequent amplifier system. All the fibers used in the seed are 10/125 double cladding fibers in order to accomplish a single-mode output. 1055.7 nm, 1058.5 nm, 1064.8 nm, 1068.0 nm, 1071.9 nm, 1075.9 nm, 1081.0 nm and 1084.5 nm center wavelength ASE seeds are produced by eight FBG pairs with corresponding wavelength.
Fig. 1 Scheme of the narrowband superfluorescent source. AC, angle-cleaved; YDF, Yb-doped double-cladding fiber; ISO, isolator; PS, pump stripper; LD, laser diode; PA, preamplifier; FBG, fiber Bragg grating; CIRC, circulator; Co., Collimator.
Narrowband super fluorescent seed is amplified by a three-stage YDFA chain, which is shown in the bottom of Fig. 1. The YDFA boosts the front-end from 60 mW to 10 W in stage 1, 10 W to 100 W in stage 2, and 100 W to 1500 W in stage 3. The maximal backward power is 280 mW, which accounts for 0.019% of the total output power 1500 W. This ratio is far less than 0.1%, which indicates the SBS threshold has not been reached. Stage 1 and stage 2 use 10/125 fiber to make sure a single-mode seed for stage 3. The main amplifier is based on a 10 m large-mode-area Yb-doped double-cladding fiber (LMA-YDF-20/400, Nufern) for high-power operation, which is pumped by six 350 W 976 nm laser diodes through a (6 + 1) × 1 combiner. This active fiber is placed in an aluminum water-cooled heatsink and coiled properly to introduce losses for the high-order modes. The output end of the pump stripper is spliced to a fiber optic cable (QBH, Optoskand). The diameter of the laser beam output from a collimator is 12 mm, corresponding to the 0.06 NA of the output fiber and 100 mm focal length of the collimator.
3. Spectral beam combining experiment
A home-made MLD grating is used as the combining element in the eight-channel SBC system. The grating has a period of 1040 nm (960 lines/mm line density) with an optimized design for non-polarization application. Figure 2(a) illustrates the 50 mm × 50 mm MLD grating in the SBC system, which is fixed in a multidimensional mount. The measured diffraction efficiency versus incident beam wavelength from 1040 nm to 1090 nm is over 95% for both TE-polarization and TM-polarization, as is shown in Fig. 2(b).
Fig. 2 (a) MLD reflective diffraction grating and (b) measured efficiency of both TE and TM polarization.
The optical layout of the high power SBC experiment is shown in Fig. 3. The eight beamlets emitting from the collimators are combined by the MLD grating. The minimum spectral spacing among the eight channels is about 3 nm. A distance of 4 m between the final folding mirrors and the MLD grating is arranged to provide sufficient spatial separation between adjacent beams. The grating is aligned in first-order Littrow configuration at an angle of 30.9° (corresponding to a wavelength of 1070 nm) in the dispersive plane and tipped 1.5° vertically. The four channels with shorter wavelength than 1070 nm irradiate on the grating from the right direction and the other four channels with longer wavelength than 1070 nm irradiate on the grating from the left direction. The wavelength dependent angle of incidence can be adjusted by means of a group of folding mirrors (HR 1050~1090 nm @ 45°) so that the beamlets of the eight channels are geometrically overlapped in both near and far field. The combined output beam is separated into two parts by a beam splitter which removes about 99.8% of the combined beam for power monitor and leaves the rest for beam quality and spectrum measurement. The sample beam propagates through a Fourier lens with a focal length of 1000 mm. Then a pair of folding mirrors (HR 1050~1090 nm @ 20°) directs the beam into a CCD camera which is located on the focal plane of the Fourier lens.
Fig. 3 Spectral beam combining setup including ASE seed source, YDFA chain, collimator, steering mirrors, multilayer dielectric diffraction grating with a line density of 960 lines/mm. The grating is aligned in first-order Littrow configuration at an angle of 30.9° in the horizontal plane and tipped 1.5° vertically.
The total combined power reaches up to 10.8 kW when all the incident amplifiers work on full power at 1.5 kW level. The diffracted power in zero order of the grating is measured to be 687 W. Therefore, the diffracted efficiency of this SBC system, defined as the combined beam power divided by the sum of all the diffracted power in zero and first order, is calculated to be 94%. This efficiency is a little less than the grating efficiency 95% mentioned above. It can be attributed to the thermal distortion of the grating at high power operation. The emission spectrum at 10.8 kW combined power is measured by an optical spectrum analyzer (YOKOGAWA AQ6370B) with a resolution of 20 pm, which is shown in Fig. 4. Eight distinct central wavelengths have been monitored corresponding to 1055.7 nm, 1058.5 nm, 1064.8 nm, 1068.0nm, 1071.9 nm, 1075.9 nm, 1081.0 nm, and 1084.5 nm. The peak heights reveal relative contributions of the acquired signal, and the ASE background is suppressed by a factor of >25 dB. There are several gaps in the wavelength range corresponding to the locations where additional fiber lasers could be added to this system for further power scaling.
Fig. 4 Emission spectrum at 10.8 kW SBC measured by an optical spectrum analyzer with a resolution bandwidth of 20 pm.
As a key element in the SBC system, the MLD grating needs to suffer all the laser radiation from each incident channel. Obtaining high-quality SBC combined beam requires that the MLD grating remains heat distortion free when irradiated by the full output power from the incident fiber laser arrays. Therefore, we perform an Michelson interferometer to observe the grating surface distortion introduced by high power laser irradiance. The basic test setup is shown in Fig. 5(a). A 632.8 nm He-Ne laser is introduced to our SBC system with an incident angle of 11°, corresponding to its Littrow reflection, which serves as the sample arm of this interferometer. The optical load of the grating is provided by the eight channel beams. Figure 5(b) shows interferograms for the MLD grating when the heating beam is off (left image), and when the total eight fiber lasers irradiate on the grating surface (right image). The right image in Fig. 5(b) illustrates that the 10 kW/cm2 optical load generate ~0.1 wave distortion at 632.8 nm, or roughly 0.05 wave at 1-um, which indicates negligible beam quality degradation is introduced by the MLD grating during the 10kW power radiation.
Fig. 5 (a) Test setup for measuring distortion of the multi-layer dielectric grating under high brightness optical load. (b) Interferograms for the grating with and without the laser radiation.
The far field pattern of the combined beam is captured by a CCD camera, which is shown in Fig. 6. Figure 6(a) illustrates the far field beam pattern in low power situation, which provides assistance for the precise adjustment of all the incident beams to guarantee the perfect overlap. The FWHM linewidth of the individual beam in this case is 0.07nm, which will not cause obvious beam quality degradation of the combined beam. Then each amplifier works on full power gradually and the far field pattern of the combined beam becomes what is shown in Fig. 6(b). Several attenuating filters are placed in front of the CCD camera to achieve the proper signal to noise ratio. It is shown that the beam size spreads obviously in the dispersive axis (x-axis) and remains unchanged in the non-dispersive axis (y-axis). The main reason for this phenomenon is that the FWHM linewidth of the individual amplifier is broadened to 0.5 nm at full power. The spectral broadening can be attributed to the intensity noise of the ASE front-end of our superfluorescent source, which modulates the effective refractive index of the fibers in the kilowatt-level final stage and introduces the self-phase modulation (SPM) effect. The variation in optical intensity generates a time-dependent phase and brings in new frequency components. According to the theory proposed in the work of Kablukov, et al. [30], the SPM induced spectral broadening is a linear function of laser output power in Yb-doped fiber lasers. Such broad linewidth is beyond the resolution of the SBC system that the beam quality of the dispersive plane will degrade severely. The beam quality degradation is estimated to be over 5, according to the formula (see Eq. (5) in [18]).
Fig. 6 Far field pattern of the combined beam in low power case (a) and full power of 10.8 kW case (b).
4. Dispersion compensation by dual-grating spectral beam combining
The dual grating configuration has the ability of dispersion compensation and relaxes the requirement of the FWHM linewidth of the incident beam [31]. We can give a simple analysis of the beam quality evolution in both single grating and dual grating SBC system. Assuming that the Rayleigh length of the beam is long enough in the range interested, the beam quality of the single grating diffracted beam can be expressed as
where, are the beam radius of the incident and diffracted beam and,are the divergence of the incident and diffracted beam in single grating SBC system, respectively. Taking the angular spread introduced by the finite linewidth () of the beamlet into account, the diffracted beam divergence becomeswhereandare the angles of incidence and diffraction, respectively,is the grating groove density. Then the beam quality of the single grating diffracted beam can be expressed asIn the case of dual grating architecture, the diffracted beam radius becomeswhereis the perpendicular distance between the two gratings. Then the dual grating diffracted beam quality can be expressed asWe take a set of specific parameters = 30°, = 1064 nm, = 960 l/mm, = 20 mm, = 4 nm to illustrate the effect of FWHM linewidth and beam radius on the diffracted beam quality in both single grating and dual grating SBC system, which is shown in Fig. 7. The results reveal that the beam quality in the dual-grating system increase slowly as the FWHM linewidth goes up, comparing to the sharp increase in the single-grating system. Therefore, the dual grating approach allows much larger linewidth, providing a significant advantage in power scaling of the individual fiber laser channels. In addition, the diffracted beam quality of the dual grating structure has an inversely proportional relationship with beam radius, which means higher beam quality and lower power density on the grating can be guaranteed simultaneously.Fig. 7 Calculated beam quality versus FWHM linewidth and beam radius for (a) single and (b) dual grating architecture. The calculation assumes a beam diameter of 12 mm, center wavelength of 1064 nm, a channel spacing of 20 mm and wavelength interval of 4 nm for the dual-grating design, and a grating groove density of 960 l/mm operating at the Littrow angle of 30°.
Therefore, we carry out a proof-of-principle experiment to verify the feasibility of the dual grating structure to achieve high beam quality combining beam by employing our linewidth broadening superfluorescent laser sources. As the size of the grating is not large enough to support all the eight incident beams, we use only two of them, corresponding to the central wavelength of 1081.0 nm and 1084.5 nm. The experimental setup is illustrated in Fig. 8, in which a second grating is added in the SBC system and aligned parallel to the first grating. The second grating has the same properties with the first one. The 1081.0 nm channel and 1084.5 nm channel irradiate on the first MLD grating by the same incident angle by adjusting the folding reflectors and the distance between the two beams is controlled to be 20 mm. The perpendicular distance between the two gratings is about 5 m.
Fig. 8 Experimental setup of dual grating SBC of two fiber amplifiers. MOPA, master oscillator power amplifier; MLD grating, multilayer dielectric grating; LBP, laser beam profiler.
The two incident fiber amplifiers both operate at full power of 1.5 kW, in which situation the FWHM linewidth reaches 0.5 nm. The beam quality of the combined beam through dual grating architecture is measured, which is shown in Fig. 9. It is obvious that the horizontal spreading of the combined beam is disappeared. The combined beam quality is measured to be M2 = 1.9, which is improved dramatically comparing to Fig. 6(b). The experiment result shows that it is possible to achieve 10 kW combined beam with high beam quality by the dual grating SBC system. The extension of the amounts of incident channels on the first grating is the most important issue to be solved, which means that a larger size MLD grating is necessary.
Fig. 9 Far field pattern of the combined beam in low power case (a) and full power of 3kW case (b) in the dual grating SBC system.
5. Conclusion
In summary, we have established eight 1.5 kW-level SBS-free all fiber superfluorescent sources with distinct wavelengths around 1070 nm, which have stable output power and compact structure. Based on these laser sources array, we have achieved 10.8 kW combined power by a polarization independent MLD reflective diffraction grating through spectral beam combination. The combined efficiency is 94%. To the best of our knowledge, this is the highest combined power of all fiber superfluorescent sources. The spectral broadening of the single channel (from 70 pm to 500 pm), which can be attributed to the self-phase modulation effect introduced by the inherent power noise of the seed source in high power operation, leads to the degradation of the combined beam quality. In order to restrict the linewidth broadening effect, we can increase the temporal stability of the ASE source by fixing the FBG in an apparatus with strong disturbance resisting ability in further study or employ FBGs with narrower linewidth to reduce the linewidth of the ASE seed source. Based on the theoretical analysis, a two channel dual grating SBC proof-of-principle experiment is conducted, achieving a combined beam with high beam quality factor (M2 = 1.9) when the FWHM linewidth of the incident beams reaches 0.5 nm. Due to the relaxed linewidth requirement on the incident beams, the dual grating configuration is a prospective way to achieve tens of kilowatts combining beam with high beam quality. Further study should be focused on the fabrication of larger size MLD grating which can support more incident channels.
Acknowledgments
This research is sponsored by the National High Technology Research and Development Program of China (NO.2014AA041901), NSAF Foundation of National Natural Science Foundation of China (No.U1330134), and National Natural Science Foundation of China (NSFC) (No. 61308024).
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