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Abstract
We demonstrated a scheme to spectrally combine two high power, broad-linewidth single beams using a steep edge filter as the combining element. 10.12 kW combined output power is achieved with a beam quality of M2x = 11.4 and M2y = 10.4. To the best of our knowledge, this is the highest output power ever reported for the filter based SBC system. Despite the broad emission spectrum of the single channel, the combining efficiency is measured to be 98.9%, which proves the high efficiency of the filter for both the reflection and transmission cases. A detail analysis of the thermal behavior is carried out to aid in the optimization of the beam quality. This SBC system permits the efficient combining of the beams with broad linewidth and provides another approach to achieve a combined output beam beyond 10 kW level.
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
With the rapid development of laser technology, high-power and high-brightness Yb-doped fiber laser (YDFL) has been widely applied in material processing, scientific research, laser weapons, 3D printing and other aspects. These lasers are unrivaled for their high conversion efficiency, high beam quality and excellent heat dissipation, which makes them a real competitor to canonical laser systems like CO2 lasers or Nd: YAG lasers [1–4]. Owing to the development of large-mode-area (LMA) fibers and high brightness semiconductor-based pump sources, the output power of fiber laser has been improved obviously [5–7]. However, limited by the stimulated Brillouin scattering (SBS) and thermal effects, the scaling of the output power can’t meet the demand of modern and technologically advanced industry [8–11]. In this case, laser beam combining technology which mainly includes coherent beam combining (CBC) and spectral beam combining (SBC) has become the most promising way with respect to future scaling of the output power [12–18]. For coherent combining, a radiation from the master oscillator is split into a number of beams, which are individually amplified and then superposed in the far field. This approach has the advantage of high combining efficiency and excellent beam quality, but requires a sophisticated phase control for all amplifiers so as to achieve stable constructive interference over long times [19–21]. Unlike the CBC, SBC has lower requirements for phase control and the coherent property of laser, making it a promising alternative for power scaling.
The basic principle of SBC is to multiplex the beams of different wavelength sources in the near and far fields by means of a wavelength selective element, such as a diffraction grating, a prism or a filter. Up to now, diffraction grating which was firstly proposed by Lincoln Laboratory has been considered as the first choice for SBC [12] and some breakthroughs have been obtained. For instance, C. Wirth et al report a spectral beam combining scheme of four narrow-linewidth photonic crystal fiber amplifier chains using a highly efficient reflective diffraction grating, which produces 2 kW combined power with good beam quality (M2x = 2.0, M2y = 1.8) [4]. Furthermore, a polarization independent multilayer dielectric diffraction grating is designed to diffract laser beams in an SBC experiment, wherein a 10.8 kW laser beam was generated with an optical-to-optical conversion efficiency of 94% [22]; Along similar lines, Eric Honea et al. combined 96 individual fiber lasers into a single high brightness beam yielding an output power of 30 kW with an M2 of 1.6 (horizontal direction) and 1.8 (vertical direction), which is the highest power fiber SBC demonstration [23]. Generally, grating-based combining devices are sensitive to position tolerance and the quality of the combined beam is correlated with a narrow spectral linewidth (∼gigahertz) as well as small (∼1 mm) beam sizes [24]. In this case, narrowband laser source has gained great attraction in SBC and gradually been in a hot research. However, due to the nonlinearity effect, the broadening of the spectrum for high power laser is inevitable, which seriously limits the output power of each individual channel. One of the viable solution is to suppress the nonlinearity such as increasing the fiber modal effective area, using highly doped and LMA fibers to reduce the effective length of the fiber, phase modulation and dispersion compensation [25–28]. Even so, the narrow-linewidth is difficult to sustain for high power laser, especially for the laser at a power level exceeding 3 kW.
An alternative approach to relax the requirement for narrow-linewidth of the individual beams is filter-based SBC, which use steep-edge interference filters instead of diffraction gratings as the combining element so as to superpose higher power beams. This scheme was firstly demonstrated by Kestutis Regelskis et al, reaching 52 W of average power and 1.9 mJ of pulse energy with a combining efficiency of 90% [24]; Via increasing the number of individual channels, O. Schmidt et al achieved 208 W of average output power and 4.2 mJ of pulse energy at a 50 kHz repetition frequency. When the repetition frequency is reduced down to 10 kHz, 6.3 mJ pulse energy could be extracted [29]. However, the individual channel in the research mentioned above is pulse laser with narrow linewidth and the edge filters haven’t yet been reported to be applied in beam combining of continuous-wave (CW), especially for kilowatt-level beam combining system.
In this contribution, we demonstrate spectral beam combination of two high power, broad-linewidth fiber lasers, running at different wavelengths and each delivering around 5 kW. It yields an output power of 10.12 kW with an M2 of 11.4 (horizontal direction) and 10.4 (vertical direction). Although the spectrum of each channel is wide (>4 nm), the combining efficiency is measured to be 98.9%, which proves the high efficiency of the filter for the broad-linewidth laser for both the reflection and transmission cases. To the best of our knowledge, this is the highest output power ever reported for the filter based SBC system. Based on the uncorrelated surface roughness model, the structure of the edge filter is conducted, achieving good performance with high damage threshold (>20 MW/cm2) and steep rising edge (<2 nm). In this case, this approach permits the efficient combining of broader spectrum and arbitrarily large beams. With the increasing of output power, the beam quality of the combined output beam shows obvious signs of deterioration and the reasons of this phenomenon have been discussed in details.
2. Spectral beam combination experimental setup and measurement system
The optical layout of the high power SBC experiment and measurement system is shown in Fig. 1, in which two 5 kW-level Yb-doped fiber lasers operating at different wavelengths are combined by means of an edge filter. The combined laser beam is sampled by a highly reflectivity (HR-2) mirror (with a reflectivity of 99.9%). The reflected beam, which contains ∼99% of the incident power, is sent to a power meter and the transmitted beam that contains ∼1% of the incident power is used for beam quality diagnosis. Meanwhile, the scattered light of the combined beam is focused onto the detection surface of a spectrometer. For the sake of eliminating the influence of the pump light, a HR-3 mirror which can reflect the combined beam and transmit the pump light, is located behind the HR-2. Two wedge prisms with a reflectivity of 5% are applied to further attenuate the power of sampling laser.
Fig. 1 Experimental configuration of filter-based SBC scheme. HR, high reflectivity mirror; YDCF, ytterbium-doped double-cladding fiber; WP, wedge prism.
2.1 5 kW-level Yb-doped fiber laser
The schematic diagram of the 5 kW-level Yb-doped fiber laser is shown in Fig. 2(a). It consists of three identical MOPA systems, each yielding 2 kW output power. The pumping source of each channel is composed of 14 laser diodes (LDs), whose maximum power is 220W. A polarization maintaining ytterbium-doped large-mode-area (LMA) double-clad fiber (YDCF) with 20 μm core diameter and 400 μm cladding diameter is employed as the gain fiber and the cladding absorption coefficient was 1.2 db/m at 975nm. Highly reflective fiber Bragg grating (HR FBG) (with reflectivity R>99.5% and spectral width FWHM = 1nm) in combination with the output coupler fiber Bragg grating (OC FBG) (with peak reflectivity R = 10%) form the oscillator cavity, which is applied to sustain the spectral bandwidth narrow and ensure the central wavelength of each channel identically. For the purpose of efficiently improving the output power, three output beams are coupled into a 50 μm core output fiber by means of a fiber optic coupler. To minimize back reflections, the coupler was terminated with a quartz block head (QBH).
Fig. 2 (a). Scheme of the 5 kW-level Yb-doped fiber laser. LD, laser diode; HR FBG, highly reflective fiber Bragg grating; OC FBG, output coupler fiber Bragg grating; YDCF, ytterbium-doped double-cladding fiber; QBH, quartz block head. (b) The measured normalized laser spectrum for various power levels. (c) The output power of the high power Yb-doped fiber laser with respect to the input current.
Figure 2(b) shows the measured normalized laser spectrum for various power levels, and the nonlinear spectral broadening of the Yb-doped fiber laser with increasing power is evident. The output power of the 5 kW-level Yb-doped fiber laser with respect to the input current is shown in Fig. 2(c). The maximum output power of these two incident channels is 5.3 kW and 4.9 kW respectively, in which situation the FWHM linewidth reaches 4 nm and the beam quality factor (M2) at full power was measured to be 5.55, 5.46 at the x, y direction.
2.2 Structure and characteristics of the edge filter
The edge filter in this system plays a major role in combining the beams in the near and far fields, and its characteristics significantly influence the combining efficiency and the maximum output power, especially the steepness of the rising edge and the damage threshold. The structure of the edge filter is shown in Fig. 3(a), in which SiO2 (n = 1.433@1070nm) and ZrO2 (n = 1.97@1070nm) films are deposited alternately on fused silica substrate (n = 1.5@1070nm). On the consideration of increasing the transmittance of the filter, a layer of antireflection coating is deposited on surface B. In this case, the edge filter is capable of sustaining 20 MW/cm2 average power density in theory, which offers superior energy and power scalability for combining high power CW lasers. On the other hand, the optical and physical properties of the edge filter, such as reflectance, damage threshold and steepness of the rising edge, are greatly influenced by the surface roughness. In this case, the paper quoted the uncorrelated surface roughness model developed by Eastman [30] to investigate the relation between the surface roughness and the filter’s reflectance curve so as to develop the optimal structure.
Fig. 3 (a) Structure of the edge filter used in the SBC system. (b) The reflectance curves of the edge filter under different surface roughness. The inset shows the edge filter.
Based on the uncorrelated surface roughness model, the amplitudes of the incident wave E0+, the reflected wave E0- and the transmitted wave Es+ are related by [31]:
where P is a matrix of the form:With regard to the multilayer coating, matrix P can be given as the product:The matrix Iij and Tj are expressed as: where rij = (ni-nj)/(ni + nj) and tij = 2ni/(ni + nj) represent the Fresnel reflectance and transmittance of the ij interface. ϕj = 2πnjdj/λ is the phase thickness of the layer. The matrix Sij that represents the effects of the surface roughness is given in terms of the Fresnel coefficients by:where a = exp[ik(ni-nj)fij] and b = exp[ik(ni + nj)fij]. fij is the function of surface profile, whose associated probability density is assumed to be a random function with zero mean and Gaussian, or normal distribution. The associated probability density w(z) for the function fij is expressed as:where σ is the root mean square (RMS) roughness of the surface. Hence, the amplitude reflectance of the edge filter can be computed in terms of the elements of P using:The radiant reflectance can be expressed as:The simulations of the filter’s reflectance curves under different surface roughness are shown in Fig. 3(b). With the increase in the surface roughness, the reflectance curve of the edge filter shifted to the short wavelength direction and the rising edge tends to be more gentle, which has a negative effect on the combining efficiency. For the sake of high combining efficiency, the roughness of both surfaces should be less than 1 nm. Thus, we experimentally measured the surface roughness (σ = 0.89 nm) by white light scanning profiler and the actual reflectance curve (red line) shows good agreement with the simulated ones. In this case, the edge filter (shown in the inset graph) has a high damage threshold (>20 MW/cm2) and steep rising edge (<2 nm) in theory, which expands the scaling potential of the presented combining scheme [32].
3. Experimental results of high power spectral beam combination
The power of the combined beam with respect to the incident power is shown in Fig. 4(a). An optical power of 10.12 kW is reached and the combining efficiency, determined by the property of the edge filter, is measured to be 98.9%. Figure 4(b) shows the relation between the reflectance curve of the edge filter and the emitted optical spectrum of the combined output beam at 10.12 kW. Although the spectrum of each channel is wide, the edge filter is able to transmit the 1090 nm laser and reflect the 1070 nm laser with high combining efficiency. That proves the high efficiency and high damage threshold of the self-designed edge filter. Accordingly, the SBC scheme based on edge filters permits the efficient combining of spectrally broad and arbitrarily large beams, which is the significantly advantageous over grating based SBC schemes [24].
Fig. 4 (a) Left scale: The measured combined output power as a function of incident power. Right scale: The optical-to-optical conversion efficiency of the combined output beam. (b) The reflectance curve of the edge filter and the emitted optical spectrum of the combined output beam at 10.12 kW.
Figure 5 shows the beam quality characterization of the combined output beam with respect to the rising current in horizontal (Fig. 5(a)) and vertical (Fig. 5(b)) directions. The beam quality factor M2 of each individual channel remains constant with the increasing output power. However, the combined beam quality shows obvious signs of deterioration, especially when the current is above 14 A (with a combined output power of 3.5 kW, Fig. 5(c)). The beam quality factor M2 at the power of 10.12 kW is characterized to M2x = 11.4 and M2y = 10.4 (Fig. 5(d)) and the significant deterioration of the combined beam quality compared to the quality of the individual channels can be linked to an increased filter temperature and the thermal induced deformation of the surface. This dependency will be discussed in detail in the fourth chapter of this paper.
Fig. 5 The measured beam quality of the combined output beam versus the rising current. (a) M2x; (b) M2y. Near-field beam profile of the combined output beam at optical power of 3.5 kW (c) and 10.12 kW (d).
4. The influence of the edge filter on beam quality
Beam quality is one of the classical parameters for evaluation of the SBC system [33–36]. Due to the spectral broadening induced by the nonlinearity effect, a grating always adds divergence to the combined output beam and hence decreases beam quality, which can be avoided by the edge filter [29]. However, Fig. 5 shows an increase of the beam quality factor M2 versus rising current and the obvious beam quality degradation is observed at maximum power of 10.12 kW. The reason is supposed to be related to the filter’s residual absorption, which not only increases the temperature and induces a change in refractive index but causes also the deformation of the surface. Therefore, we applied a dynamic laser interferometer and a thermal imaging camera to observe the filter’s surface distortion and temperature rise.
The setup of the measurement system is shown in Fig. 6(a). The probing laser beam from the interferometer is incident on the surface of the filter, which is irradiated by high power laser. A high reflectivity mirror is set to reflect the probe beam back to the detection surface of the interferometer. The measured result of the deformation and temperature of the filter with respect to the rising current is shown in Fig. 6(b), in which a growth of temperature on the filter can be observed at higher output powers. This heat results in a thermal lens and the bulges on the filter are obvious, which attributes to the wave-front distortions of the non-combined beams and then lead to the deterioration of the combined beam quality. The temperature of the filter at full power is measured to be 197.7 °F (Fig. 6(c)), corresponding to the beam quality of M2x = 11.4 and M2y = 10.4. Therefore, an air cooling system for the filter is essential and a self-adaptive system to achieve a combined beam with better beam quality will be the subject of further investigations.
Fig. 6 (a) Test setup for measuring the deformation and temperature of the filter under high brightness optical load. (b) The measured result of the deformation and temperature of the filter versus the input current. (c) The temperature of the filter at the power of 10.12 kW.
5. Conclusion
In summary, we have demonstrated a spectral beam combining scheme of two ytterbium-doped fiber lasers using an edge filter as the combining element. Although the individual beams have spectral widths of about 4 nm and output powers of 4.91 kW and 5.31 kW respectively, 10.12 kW combined output power is achieved with an efficiency of 98.9%, illustrating the effectiveness and high laser induced damage threshold of the filter. The beam quality of the combined output beam at the full power is measured to be M2x = 11.4 and M2y = 10.4 in horizontal and vertical directions. An obvious degradation of combined beam quality can be obtained with the increasing incident power and the reason mainly attribute to the absorption of the filter. In this case, the surface deformation and the thermal behavior are monitored during the combining process and the temperature of the filter at full power is measured to be 197.7 °F. To restrict the thermal-induced wave-front distortions of each incident channel, an air cooling system could be added to this system for further power scaling and higher beam quality. Via increasing the number and power of the channel, a further scaling appears to be feasible and this system has been proved to be a prospective way for high brightness spectral beam combination.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (NSFC) (No. 61575095); the preresearch Foundation of the CPLA General Equipment Department (No. 9140A2103021513Q02325); Fundamental Research Funds for the Central Universities (No.30916014112-009); Young Elite Scientist Sponsorship Program by the Chinese Association for Science and Technology (CAST) (2015QNRC001)
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