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Abstract
We report on kW-class dense wavelength beam combining of a laser diode module consisting of ten broad-area laser diode bars by using a novel multi-laser cavity approach based on a thin-film filter (TFF) as a dispersive optical element. The wavelength-stabilized output of the TFF cavity is beam combined upon a −1st order transmission grating. Hereby, a cylindrical telescope is used for linear dispersion-matching between the TFF and the combiner grating. On the basis of simulations of the resulting beam quality deterioration, we are able to optimize the cavity and the combiner setup for optimal beam quality preservation. We demonstrate a highly efficient direct diode laser with 1.1-kW output power and a symmetrical beam parameter product of about 6mm × mrad (95 % power content) in both beam axis.
© 2017 Optical Society of America
1. Introduction
High-power direct diode lasers in the wavelength band between 900 and 1000 nm are of great interest in many fields of today’s industrial laser material processing. The striking advantage of such lasers compared to optically pumped solid-state lasers consists of a higher compactness and an enhanced electrical-to-optical (e-o) conversion efficiency of up to 50 %. Dense wavelength beam combining (DWBC) of broad-area laser (BAL) diode bars in external resonators has been proven to be a suitable technique for building efficient, high-brightness kW-class direct diode lasers which are able to serve all kinds of high-brilliance laser applications as for instance flat sheet metal cutting or remote welding [1–3]. Typically, these applications require several kilo-watts of output power and a beam parameter product (BPP) in the range of 2 to 10mm × mrad. Furthermore, the spectral bandwidth has to be smaller than 50 nm in order to allow for good imaging quality without chromatic aberrations in laser processing optics with few optical elements. Commercially available high-brightness direct diode laser turn-key systems, which are based on DWBC, already provide an output power of 4 kW with an e-o conversion efficiency of 44 % and a BPP of 4mm × mrad [4]. However, competitive costs and equivalent reliability, compared to well established high-power solid-state disk or fiber lasers, and a sufficient lifetime of the laser diodes under external optical feedback [5–7] will be basic prerequisites for a successful market penetration in the future.
In this paper, we report on DWBC of a laser diode module in an external thin-film filter (TFF) multi-laser cavity. This technique represents an alternative approach to intra-cavity diffraction grating based DWBC systems [8–10] and external cavity architectures using volume Bragg gratings (VBGs) [11–13] or dichroic steep-edge filters [3,14] for spectral beam combining. Using a TFF as wavelength selective element inside the external resonator has major advantages. First of all, TFFs exhibit increased spectral-angular dispersions compared to conventional diffraction gratings which allows for a high spectral denseness of the stabilized wavelength channels even at moderate transform focal lengths. Furthermore, the TFF cavity provides an intrinsic suppression of spectral emitter cross-talk in the optical feedback between adjacent emitters. Compared to commercial VBGs, which are recorded into photo-thermo-refractive glass, TFFs exhibit much lower thermo-optical-induced wavelength shift which ensures a stable wavelength stabilization with respect to the beam-combining element. Finally, wavelength stabilization of hundreds of individual gain elements can be achieved by only one individual optical component inside the resonator. This is the reason why this approach is suited to realize robust and low-cost optical designs of the external laser cavity.
We recently demonstrated the feasibility of this approach by individual wavelength locking the emitters of a BAL diode bar with 50 % filling factor (FF) and proposed a grating combiner setup for subsequent beam combination of the wavelength-stabilized cavity output [15]. We now apply the cavity structure to a laser diode module consisting of ten horizontally stacked actively cooled 150-W BAL diode bars. In order to perform beam combining in the fundamental mode fast axis (FA), the diode bars exhibit commercial micro-optic beam transformation systems. FA-DWBC is favorable in terms of the aperture of the resonator optics, the achievable spectral denseness of the stabilized emitters, external cavity induced BPP deterioration, smile induced imaging errors in the optical feedback and lifetime of the diode bars [16–18]. We achieve an output power of the wavelength-combined laser diode module of 1.1 kW at 180-A diode current corresponding to an electrical-to-optical (e-o) conversion efficiency of about 40 %. The beam quality is about 6.6mm×mrad in the beam-combining axis and 5.7mm×mrad in slow axis (SA) corresponding to a power content of 95 %. The 230 BAL emitters of the laser diode module are spectrally stabilized within a bandwidth of 43 nm.
2. Experimental setup
2.1. Laser diode module
The setup of the laser diode module is shown in Fig. 1. The module consits of a horizontal stack of ten BAL diode bars (23 emitters, 100-μm contact opening, emitter pitch pe = 400μm, 25 % FF, FA divergence angle 2θFA,em = 40° (95 % power content), 4-mm cavity length, front facet reflectivity < 0.5 %) which are mounted on a shared copper heat sink. The diode bar chips are hard soldered on an active isolated laser cooler [19] and operated at 25 °C cooling water temperature. The free-running central operation wavelength of respectively two diode bars of the module is 938 nm, 946 nm, 954 nm, 962 nm and 970 nm at a diode operation current of 180 A. Figure 2(a) shows a generic measurement of the output power characteristic of an individual free-running 954-nm diode bar. The output power is about 150 W at 180-A diode current with a corresponding e-o conversion efficiency of > 50 %. The degree of TE-polarization is approximately 93 % at 180 A. The anti-reflection coating of the front facet allows for a spectral locking-range of the diode bar emitters of 35 nm at an optical feedback ratio of 10 % [see Fig. 2(b)]. A sufficiently large locking-range is needed in order to compensate for the spectral shift of the gain spectrum of 13 nm over the whole diode operation current range, whereby a stable wavelength locking is ensured. Furthermore, it reduces the amount of required different epitaxial designs of the diode bar chips since two bar slots of the module, respectively, can be assembled with bars exhibiting identical free-running operation wavelengths. Each diode bar exhibits a 300-μm FA collimation lens (FAC) and a commercial beam transformation system (BTS). The BTS consists of an array of tilted cylindrical micro-lens telescopes which rotate the beam of every individual emitter by 90°, which means that FA and SA direction are interchanged. Thus, the BTS enables FA-DWBC along the direction of the horizontally stacked diode bars (x-axis). Subsequent to the BTS, a cylindrical lens of 40-mm focal length (SAC) collimates the emitter sub-beams of every diode bar in SA. The collimated outputs of the diode bars are directed towards the front of the module by highly reflective mirrors (HR) to create an ensemble of coaxially propagating sub-beams. The architecture ensures a maximized spatial FF at the front of the module for a high spectral denseness and equal propagation distances of the sub-beams of the diode bars with respect to each other. The pitch between the output beams of two adjacent diode bars of the module at the front is pb = 12mm. The inset of Fig. 1 shows a ray tracing simulation of the intensity distribution of the collimated output beams at the front of the laser diode module.
Fig. 1 Setup of the laser diode module. The inset shows a ray tracing simulation of the intensity distribution of the collimated output beams at the front of the laser diode module.
Fig. 2 (a) Output power characteristic of an individual free-running 954-nm diode bar of the laser diode module. (b) Spectral shift of an individual free-running 954-nm bar and locking-range measurement.
2.2. TFF multi-laser cavity
A schematic setup of the external wavelength-stabilizing multi-laser cavity is shown in the lower part of Fig. 3. The cylindrical Fourier transform lens with a focal length of fTL = 565mm images the far field of the coaxially propagating beam ensemble of every diode bar of the module upon an ultra-narrowband TFF. The customized TFF is made up of a Ta2O5/SiO2 quarter-wave layer stack exhibiting a half-wave low-index SiO2 spacer in the middle. The filter is designed for 100 % transmission at a wavelength of λTFF = 1037nm for 0° angle of incidence (AOI) and provides a transmission bandwidth of 50 pm (FWHM) for TE-polarized light. A zero-order half-wave plate (HWP) (not shown in Fig. 3) in front of the TFF is used to rotate the TE-polarized laser output of the diode bar emitters into TE-polarization with respect to the plane of incidence at the TFF. The Fourier transform lens creates a fan of angled rays which overlap at the location of the TFF. Each sub-beam strikes the TFF under an unique AOI. Mathematically, the Fourier lens transforms the lateral position xk,n of the nth emitter (n = 1,...,23) of the diode bar k of the module (k = 1,...,10) with respect to the optical axis into an unique angle θk,n with respect to the surface normal of the TFF, given by:
Here, θ0 denotes the angle between the optical axis and the surface normal of the TFF. Assuming that the output beams of the laser diode module symmetrically propagate with respect to the optical axis, the lateral position of the emitters at the front of the diode laser module can be described as: According to the spectral-angular dispersion characteristic of the TFF the transmission of the incoming sub-beams of the emitters is fixed to a specific wavelength λk,n. In Eq. (3), neff ≈ 1.63 is the effective index of refraction of the filter which is deduced by use of an effective index model [20]. Behind the TFF, a cylindrical lens with a focal length of fFB = 200mm in conjunction with a highly reflective (HR) mirror complete the external resonator and image the transmitted frequency-filtered sub-beams back into the originating emitters providing wavelength-selective optical feedback. Consequently, the external laser cavity stabilizes each emitter on an individual wavelength λk,n, given by Eq. (3), realizing a stable position-to-wavelength mapping for each emitter of the laser diode array, which is required for subsequent spectral beam combining by means of a diffraction grating. The corresponding stabilized spectral bandwidth Δλ of the diode laser module, using a paraxial approximation, is given by where Δx = |x1,23 − x10,1| is the lateral spatial separation of the outer emitters of the outer diode bars of the module along the FA and being the angular dispersion of the TFF at the corresponding angle θk,n. According to Eq. 4, the theoretical value of the stabilized spectral bandwidth is Δλ = 42.9nm for the experimental parameters Δx = 116.8mm, fTL = 565mm and θ0 = 39.5°, which corresponds to a central wavelength λc = 955nm of the stabilized spectrum. Two effects mainly impact the optical feedback provided by the TFF multi-laser cavity. First, the residual spectral linewidth of the stabilized emitters is larger than the spectral transmission bandwidth of the TFF due to the residual far-field divergence of the emitters in FA in the focal plane of the Fourier transform lens. Furthermore, the spectral multi-mode character of the BAL diode emitter in SA prevents narrowband spectral operation. The resulting excess spectrum of the stabilized emitters can not be transmitted through the TFF and is reflected out of the cavity. Second, the TFF exhibits a lateral inhomogeneous spacer thickness which results from the manufacturing coating process of the filter by magnetron sputtering. Due to this fact, originating from the center of the filter, the resonant transmission wavelength at the corresponding AOI shifts by about 0.1 % to shorter wavelengths at a lateral position of 10 mm. Consequently, the transmitted power at the TFF depends, beside the residual spectral emitter linewidth, on the beam diameter of the emitter sub-beam upon the filter. The experimental beam diameter of the emitter sub-beams upon the filter is about 4 mm, which is the reason why each beam experiences a significant variation of the spacer thickness. As a result of both mentioned effects, only a small amount of the intra-cavity power is transmitted at the TFF and used for optical feedback, whereas the major portion is coupled out of the resonator at the location of the filter. Not shown in the external cavity setup of Fig. 3 are the imaging optics in the vertical SA direction subsequent to the SAC of the laser diode module. Two additional cylindrical lenses are used to image the beam waist of the emitter sub-beams in SA onto the HR mirror for feedback re-imaging.Fig. 3 Schematic setup of the TFF multi-laser cavity and the grating combiner. The imaging in the vertical SA direction is explained in the text and not shown here.
2.3. Grating combiner
In order to perform beam combining of the wavelength-locked emitter sub-beams emerging from the TFF multi-laser cavity, a grating combiner setup is used [15]. A schematic of the setup is shown in the upper part of Fig. 3. It consists of a cylindrical lens telescope (focal lengths fC1 = 100mm and fC2 = 194mm) and a −1st order transmission grating which is placed in Littrow configuration with respect to an imaginary central emitter ray of the module exhibiting the central wavelength λc of the stabilized spectrum. Specifically, the grating is a dielectric transmission grating [21] with Λ−1 = 1600 lines/mm and an optimized diffraction efficiency of 98 % for TE-polarized light at a Littrow angle θL(λc = 955nm) of 50°.
Since the wavelength-to-AOI dependency and the spectral-angular dispersion characteristic of the TFF, governed by Eqs. (3) and (5), and the transmission grating are different and additionally nonlinear across the relevant wavelength band of the TFF cavity, an exact dispersion-matching upon the combiner grating is required to preserve beam quality after beam combining [22]. The cylindrical telescope is used for linear dispersion-matching between the TFF and the combiner grating across the stabilized bandwidth Δλ of the laser diode module, as shown in Fig. 4(a). The telescope images the individual sub-beams of the wavelength-stabilized cavity output upon the combiner grating. According to the magnification of the telescope, given by M = fC2/fC1, the relative angles βk,n of Eq. (1) are transformed as , in order to match the angular dispersion characteristic of the combiner grating. As a result, the incident emitter sub-beams are diffracted into one single combined output beam. The magnification of the telescope is given by the absolute value of the dispersion ratio of the TFF and the combiner grating at the central wavelength λc of the stabilized spectrum (M = 1.94 for λc = 955nm). The residual nonlinear part of the dispersion, which cannot be matched by the telescope, results in beam pointing errors of the emitter sub-beams with respect to each other after beam combining. Consequently, the far-field divergence of the combined output beam is increased which can, beside diode bar smile and the residual spectral linewidth of the stabilized emitters, significantly deteriorate the beam quality in the beam-combining axis [18]. By use of the grating equation, the beam pointing angles Δθk,n of the diffracted emitter sub-beams, with reference to the Littrow angle θL(λc), are given by
where Λ denotes the groove spacing of the grating. Figure 4(b) shows the calculated beam pointing angles Δθk,n as a function of wavelength for different magnifications M of the telescope. From the plot one can see that the curve for M = 1.94 lies within the region of the emitter beam divergence 2θG in the beam-combining axis behind the grating. That implies that beam quality is sufficiently preserved. For magnifications differing from the optimal value, the beam pointing angles above and below the central wavelength λc increase in amount and become larger than the beam divergence of a diffracted individual emitter. As a result, the far-field divergence of the combined beam is increased and the beam quality significantly differs from the BPP of an individual emitter sub-beam in the beam-combining axis.Fig. 4 Spectral beam combining of the TFF wavelength-locked cavity output. (a) Linear dispersion-matching by use of a telescope. Left axis, measured (symbols) and calculated (lines) relative angle with respect to the TFF AOI θ0 and Littrow angle θL(λc) of the combiner grating at the central wavelength λc of the stabilized spectrum vs. wavelength. Right axis, dispersion ratio of the TFF to the combiner grating vs. wavelength. (b) Calculated beam pointing angles Δθk,n of the diffracted emitter sub-beams, with reference to the Littrow angle θL(λc) vs. wavelength for different magnifications M of the telescope.
3. Simulation of beam quality preservation
In order to quantify the beam quality deterioration in the beam-combining FA, due to the dispersion mismatch between the TFF and the combiner grating, the far-field intensity distribution ICB of the combined beam has to be calculated, which is given by the sum of the normalized Gaussian intensity distributions Ik,n = exp[−2(θ/θG)2] of the diffracted emitter sub-beams:
Each emitter sub-beam has an angle offset Δθk,n according to Eq. (6) and a far-field divergence of: In Eq. (8), the FA beam divergence of the uncollimated diode bar emitter is denoted by 2θFA,em and 2θG,em represents the beam divergence of an individual monochromatic (δλ = 0) emitter sub-beam in the beam-combining axis behind the grating. The factor Δlinewidth accounts for the beam quality deterioration by the residual spectral linewidth of the wavelength-stabilized emitters [18,23]. Due to the diffraction at the grating, the residual spectral emitter linewidth δλ results in an increased far-field divergence of the emitter sub-beam compared to a monochromatic emitter which will consequently deteriorate the BPP of the combined output beam in the beam-combing axis. The factor is given by where δλ is the linewidth of each stabilized emitter and being the angular dispersion of the combiner grating at the Littrow angle θL(λc). The beam quality deterioration factor Δlinewidth in Eq. (9) describes the ratio of the width 2θG of the enlarged far-field intensity distribution of the diffracted emitter sub-beam to the beam divergence 2θG,em of a monochromatic emitter behind the grating. Hereby, it is assumed that the far-field intensity distribution of the diffracted emitter sub-beam is given by the convolution of two Gaussian far-field intensity profiles with respective 4σ-widths 2θG,em and DG · δλ.Figure 5(a) shows the calculated far-field intensity distribution ICB of the combined beam for two different magnifications M of the dispersion-matching telescope. As a comparison, the far-field intensity distribution for perfect matching (Δθk,n = 0) is plotted and the beam divergence 2θG,em of an individual monochromatic emitter sub-beam in the beam-combining axis behind the grating is marked. As input parameter for the linewidth, we used the experimentally observed value of δλ = 129pm (4σ). In case of good matching (M = 1.94), the widths of both distributions are comparable (2θCB ≅ 2θG). If the magnification is slightly changed to M = 2, the simulation shows a significant far-field broadening (2θCB ≫ θG). In fact, the ratio of the 4σ-width 2θCB of the far-field intensity distribution ICB to the beam divergence 2θG,em of an individual monochromatic emitter sub-beam is a measure for the deterioration factor Δ of the BPP in the beam-combining axis:
The width 2θCB has to be numerically deduced by means of Eq. (7), whereas θG,em can directly be calculated from Eq. (8). The factor Δ in Eq. (11) accounts for both the BPP deterioration due to the residual spectral emitter linewidth and the dispersion mismatch. The impact of diode bar smile on the resulting BPP in the beam-combining axis is not included. Our simulations yield a deterioration factor Δ = 2.4 for the ideal magnification (M = 1.94). For M = 2 the factor is significantly higher (Δ = 15.2). We further simulated the dependency of the factor Δ on the magnification M of the telescope for different groove spacings Λ of the combiner grating. Figure 5(b) shows that the system sensibly reacts on both parameters demanding for a precisely adjusted magnification of the telescope and a stable performance of the grating at high power levels for sufficient beam quality preservation.Fig. 5 (a) Far-field intensity distribution ICB of the combined beam for two different magnifications M of the dispersion-matching telescope. (b) Simulations of the beam quality deterioration factor Δ as a function of the magnification M of the dispersion-matching telescope for different groove spacings Λ of the grating.
4. Experimental results and discussion
4.1. Stabilized spectrum of the cavity output
Figure 6 shows the spectrum of the output beams out of the TFF multi-laser cavity at a diode current of 40 A. The spectrum was measured using a high-resolution spectrometer (HighFinesse HDSA; 40-pm spectral resolution @ 960 nm). The 230 emitters of the laser diode module are stabilized within a spectral bandwidth of Δλ = 43nm around a the central wavelength of about λc = 955nm. This value is in good agreement with the theoretical prediction of 42.9 nm resulting from Eqs. (4) and (5) for Δx = 116.8mm, fTL = 565mm and the corresponding angular dispersion of the TFF (DTFF = 4.8mrad/nm @ λc = 955nm). The spectral channel spacing of adjacent emitters is approximately 153 pm. Each emitter is stabilized at a unique wavelength. The spectrum shows a high modulation depth and the spectral lines of the individually wavelength-locked emitters are completely resolved in the spectrum without any appearance of spectral emitter cross-talk. The spectral linewidth of the stabilized emitters is δλ = 129pm (4σ). The gradually reduction of the spectral intensity of the respective bars is related to a measurement artifact of the spectrometer and no real effect, since the measured power of the cavity output for the individual bars is comparable and shows no such significant variations as indicated by the measured spectrum shown in Fig. 6. With increasing diode current we observed a thermo-optical-induced wavelength shift of the stabilized spectrum. The extract in the lower right part of Fig. 6 shows the spectrum of three bars of the laser diode module at a diode current of 40 and 180 A, respectively. The absolute wavelength shift is about 550 pm corresponding to a relative shift of 0.6 pm/W. The reason for this observation is a heating of the TFF to a peak temperature of 53 °C of the resulting lateral temperature distribution at high power levels due to the residual absorption of the incident laser power by the substrate of the filter. Furthermore, the spectra show a significant broadening of the stabilized emitter linewidth with increasing diode current accompanied by a reduced modulation depth of the spectral emitter lines. The linewidth at 180 A is δλ = 202pm (4σ). This effect is potentially related to higher order lateral modes of the radiated electric field in SA with increasing emitter current [18,24, 25]. In order to determine the optical feedback ratio inside the external cavity, we replaced the HR mirror by a partially reflective mirror (PRM) with a reflectivity of 90 %. From the measured power behind the PRM and at the location of the beam block, we were able to deduce a feedback ratio of 5 % for the central bars of the module. The feedback ratio for the outer bars is about 10 %. This deviation can be explained by an incomplete rotation of the polarization of the outer bars, due to larger AOIs upon the HWP, compared to the central bars. Consequently, the outer bars exhibit larger TM-polarized power contents for which the TFF has a significantly broader transmission bandwidth of 440 pm (FWHM) resulting in an increased optical feedback strength.
Fig. 6 Spectrum of the output beams of the TFF multi-laser cavity at a diode current of 40 A. The extract in the lower right part shows the spectrum of three bars of the laser diode module at a diode current of 40 and 180 A, respectively.
4.2. Output power and beam quality
Figure 7(a) shows the output power characteristic and the related e-o conversion efficiencies of the free-running laser diode module, the wavelength-stabilized cavity output and the combined output beam as a function of diode current. We achieved a combined output power of 1.1 kW at a diode current of 180 A. The corresponding e-o conversion efficiency is 40 %. The power losses of 10 % of the stabilized cavity output compared to free-running laser diode module operation can be related to the measured power ratio of about 8 % which is reflected out of the cavity towards the beam block at the location of the TFF in the feedback branch. The power losses of about 12 % of the combined beam compared to the wavelength-stabilized cavity output are due to transmission losses of depolarized power contents at the combiner grating and the restricted diffraction efficiency into the −1st order.
Fig. 7 (a) Left axis, optical output power vs. diode current for the free-running laser diode module, the wavelength-stabilized cavity output and the combined output beam. Right axis, corresponding e-o conversion efficiencies vs. diode current. (b) FA- and SA-BPP of the combined cavity output vs. diode current. (c) Intensity distribution and extracted beam profiles along the FA- and SA direction of the combined output beam at a diode current of 140 A in the focal plane of a spherical lens with 300-mm focal length.
The BPP of the combined output beam as a function of diode current is depicted in Fig. 7(b). The presented BPP values correspond to a power content of 95 % and were measured using a camera-based automatic laser beam profiler (Ophir Photonics; M2−200s). By way of example, Fig. 7(c) shows the intensity distribution and the extracted beam profiles along the FA- and SA direction of the combined cavity output at a diode current of 140 A in the focal plane of a spherical lens with 300-mm focal length which is used by the laser beam profiler to generate the beam caustic for the BPP measurements. The SA-BPP of the combined cavity output lies within a range of 3 to 5.7mm × mrad and shows a typical linear dependency on the diode current. At a diode current of 20 A the FA-BPP is about 3.2mm × mrad and is consequently deteriorated by a factor Δ = 10.7 compared to the diffraction-limited BPP in FA of an individual free-running emitter (0.3mm × mrad @ 955 nm). This value is much larger than the factor Δ = 2.4 predicted by the simulations of Fig. 5 for the optimal configuration of the grating combiner (M = 1.94; Λ−1 = 1600 lines/mm) and the presented experimental parameters. An explanation for this deviation is a slightly detuned telescope magnification and smile of the diode laser bars. As the simulations show, the deterioration factor sensibly reacts on the magnification. A variation of the ideal magnification value by ±0.04 leads to an increase of the deterioration factor to a value of Δ ≥ 10. Such a deviation is very likely to occur since it lies in between the manufacturing tolerances of the telescope lenses and the alignment errors of the grating combiner setup. Besides, smile of the laser diode bars has been excluded so far in the calculation of the beam quality deterioration factor. Smile induced beam pointing errors in the beam-combining axis significantly affect the BPP of an individual bar [18]. Figure 8(a) shows the FA-BPP as a function of diode current of the combined beam of selected individual bars of the laser diode module. The smile measurement of the corresponding bars is shown in Fig. 8(b). For low-smile bars (≈ 1μm), as for instance bar 10, the FA-BPP is around 1.5mm × mrad. For bars with larger smile, the BPP is significantly increased. For example bar 6, exhibiting 2.2-μm smile, has a FA-BPP of 3mm × mrad. In summary, the interaction of diode bar smile and an imperfect telescope magnification cause the FA-BPP to be larger than the expected theoretical minimum for the optimal configuration.
Fig. 8 (a) FA-BPP vs. diode current for the combined cavity output of the laser diode module and selected individual bars. (b) Smile measurements of the corresponding laser diode bars.
4.3. Influence of thermo-optics
Furthermore, the measurements of Figs. 7(b) and 8(a) show that the FA-BPP of the combined output beam has a linear dependency on the diode current and reaches a value of 6.6mm × mrad at 180 A. Since the BPP of an individual bar stays constant over the whole diode operation current range, the increasing BPP of the complete laser diode module must be related to a thermo-optical effect. Both the observed spectral shift of the stabilized spectrum due to the heating of TFF and the increased emitter linewidth cannot explain the strongly increased BPP at large operation currents. The spectral shift results in a deviation from the Littrow condition at the combiner grating and a changed diffraction angle of the combined output beam. The resulting beam pointing deviation is about 1 mrad at 180 A which corresponds to only 5 % of the far-field divergence of the combined beam in FA direction. The increased linewidth of the stabilized emitters results in a higher beam divergence of the emitter sub-beams behind the grating, governed by Eqs. (8) and (9). If we include both effects into our simulation, we get a deterioration factor of Δ = 3.1, which is only a factor of 1.3 larger than the value Δ = 2.4 for the optimal configuration and the experimental parameters at low currents. The experimentally observed increase of the FA-BPP from 3.2 to 6.6mm×mrad corresponds to a factor of 2.1 which is much larger than the theoretical prediction.
A possible explanation for the increasing FA-BPP with increasing diode current are thermo-optical-induced wavefront aberrations [26,27] due to the heating of the TFF which deteriorate the beam quality in beam-combining FA. In order to validate this hypothesis, we investigated the dependency of the FA-BPP of an individual bar of the laser diode module on the peak temperature of the TFF. For this experiment, we used a larger transform focal length of fTL = 1060mm to further decrease the stabilized spectral bandwidth of the individual bar to about 2 nm. Hence, we can completely neglect the influence of the dispersion mismatch on the beam quality of the combined output beam. Furthermore, the beam diameter in FA at the location of the TFF is larger which results in an increased sensitivity of the emitter sub-beams to aberrations. We used two experimental configurations which are depicted in Fig. 9(a). In configuration A, we blocked the output beams of nine of the ten diode bars in front of the laser diode module. In this case, only one bar is present inside the external cavity and the thermal heat load of the TFF is correspondingly small. In configuration B, we blocked the diode bars behind the TFF. Consequently, the complete output power of the module is incident on the TFF. The measured FA-BPP as a function of diode current and the corresponding peak temperature of the TFF for both configurations are shown in Fig. 9(b). The data shows that the FA-BPP of the individual bar correlates to the peak temperature of the TFF in both cases. The results prove that thermo-optical-induced effects are the reason for the dependency of the FA-BPP on the diode current.
Fig. 9 (a) Experimental configurations A and B for the investigation of the influence of thermo-optical effects in the TFF on the FA-BPP. (b) Left axis, FA-BPP vs. diode current for the combined cavity output of an individual bar of the laser diode module vs. diode current for the experimental configurations A and B. Right axis, corresponding peak temperature of the TFF vs. diode current.
5. Conclusion
In summary, we have demonstrated dense wavelength beam combining (DWBC) of a laser diode module consisting of ten actively cooled 150-W broad-area laser diode bars with 25 % filling factor (FF) and 4-mm resonator length, which exhibit commercial micro-optic beam transformation systems in order to perform beam combining in the fundamental mode fast axis (FA). A novel multi-laser cavity approach based on an ultra-narrowband thin-film filter (TFF) is used for spectral stabilization of the 230 diode bar emitters of the laser diode module within a bandwidth of 43 nm, each at a unique wavelength. Spectral emitter cross-talk is intrinsically suppressed and not present inside the external resonator. The spectrum of the stabilized diode bar emitters shows a low wavelength shift of 0.6 pm/W which results from a heating of the TFF due to material absorption by the substrate of the filter. Beam combining of the wavelength-locked output of the TFF cavity is performed by use of a −1st order transmission grating. In order to compensate the linear spectral-angular dispersion mismatch between the TFF and the combiner grating, a cylindrical telescope is used. Regarding this, we have presented simulations of the beam quality deterioration in the beam-combining axis in order to judge the performance of the grating combiner for different experimental configurations. The combined output beam of the laser diode module has an output power of 1.1 kW at 180-A diode current corresponding to an electrical-to-optical (e-o) conversion efficiency of about 40 %. The beam parameter product (BPP) for a power content of 95 % is about 3.2mm×mrad in the beam-combining FA and 3.3mm×mrad in slow axis (SA) at 20-A diode current. The theoretical simulation of beam quality preservation for the used optimal configuration of the grating combiner setup yields a significantly smaller value for the FA-BPP. The discrepancy between the theoretically calculated BPP deterioration in the beam-combining axis and the experimental results can be explained by an imperfect telescope alignment and diode bar smile. The FA-BPP shows a linear dependency on the diode current due to thermo-optical-induced wavefront aberrations in the TFF. The observed wavelength shift of the stabilized spectrum has a minor impact on the BPP in the beam-combining axis, but results in an beam pointing deviation of 5 % of the far-field divergence of the combined beam in FA direction. At a diode current of 180 A, the BPP is 6.6mm × mrad in FA and 5.7mm × mrad in SA yielding an overall BPP of the combined laser diode module of 8.7mm × mrad (95 % power content). The demonstrated beam quality is sufficient for fiber coupling into a 200-μm, 0.14 NA fiber with high efficiency (> 95 %). Regarding beam coupling into a fiber with a core diameter of 100 μm and comparable NA, a power loss of about 20 % is expected based on the measured beam quality of the combined output beam. Consequently, the estimated fiber-coupled output power is approximately 880 W at a diode current of 180 A.
In a next step, we will use laser diode bars with a higher FF of 50 % in order to increase the combined output power of the laser diode module to a level of about 2 kW. An increased e-o conversion efficiency of the laser is expected by using diode bars with enhanced degree of TE-polarization (> 93 %) which will significantly reduce transmission losses of depolarized power contents at the combiner grating. Furthermore, we will improve the laser performance by using TFF substrates with lower absorption, in order to reduce the thermo-optical effects inside the filter and thereby solving the issues of BPP deterioration and beam pointing errors of the combined beam which. In this context, a shift of the stabilized spectrum above the OH absorption band at a wavelength of 940 nm is beneficial to further reduce absorption of the filter. Additionally, we hope to achieve better beam quality preservation in future by using a cylindrical zoom lens telescope for dispersion-matching which enables a more precise adjustment of the magnification. A lower BPP in the beam-combining axis is a basic requirement in terms of efficient fiber coupling of the combined output beam into high-brilliance fibers and brightness scaling of multiple wavelength-combined laser diode modules towards the multi-kW regime by polarization beam combining and spatial beam stacking.
References and links
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