Reduction of beam divergence angle in laser-diode arrays with large smiles using a dual-beam transformation system

Reduction of beam divergence angle in laser-diode arrays with large smiles using a dual-beam transformation system

Journal Pre-proof Reduction of beam divergence angle in laser-diode arrays with large smiles using a dual-beam transformation system Bin Liu, Hui Liu,...

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Journal Pre-proof Reduction of beam divergence angle in laser-diode arrays with large smiles using a dual-beam transformation system Bin Liu, Hui Liu, Fenning Chen, Haiyan Li, Lei Gao, Pengfei Zhu, Xingsheng Liu

PII: DOI: Reference:

S0030-4018(20)30025-0 https://doi.org/10.1016/j.optcom.2020.125279 OPTICS 125279

To appear in:

Optics Communications

Received date : 16 October 2019 Revised date : 17 December 2019 Accepted date : 7 January 2020 Please cite this article as: B. Liu, H. Liu, F. Chen et al., Reduction of beam divergence angle in laser-diode arrays with large smiles using a dual-beam transformation system, Optics Communications (2020), doi: https://doi.org/10.1016/j.optcom.2020.125279. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

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Reduction

of

beam

divergence

angle

in

of

laser-diode arrays with large smiles using a dual-beam transformation system

pro

Bin Liu1,2,*, Hui Liu1, Fenning Chen3, Haiyan Li3, Lei Gao3, Pengfei Zhu3, Xingsheng Liu3 1

State Key Laboratory of Transient Optics and Photonics, Xi'an Institute of Optics and Precision

Mechanics, Chinese Academy of Sciences, Xi'an, Shaanxi 710119, P.R. China 2

Center of Materials Science and Optoelectronics Engineering, University of Chinese

Academy of Sciences, Beijing,100049, P.R. China

Focuslight Technologies Inc, Xi'an, Shaanxi 710077, P.R. China

*

[email protected]

re-

3

Abstract: Two new angled half-beam transformation systems (BTSs) for the reduction of the beam-divergence problem in laser-diode arrays with large smiles are investigated. Both simulations and experiments show that the angle of divergence of laser-diode arrays with the

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parabola-shaped smile and the S-shaped smile can be reduced by more than 37% and 23% respectively, when two angled half-BTSs are used in front of the laser-diode array. Furthermore, a fiber-coupling module, with a 400 μm core-fiber and two angled half-BTSs, is fabricated. The optical/optical coupling efficiency increases to more than 85%, which is an improvement by more than 4% over laser diodes that use a full-BTS. Keywords: Diode laser array, Smile correction, Beam shaping, Optical system design 1. Introduction

The application of semiconductor laser-diode arrays for, e.g., materials processing and pumping of solid-state lasers, is limited by the large “smile” often found in commercial laser-diode arrays [1]-[4]. A typical laser-diode array consists of a linear array of 19-50 diode emitters (50-200 μm wide and 100-400 μm pitch, the distances are measured from emitter to emitter) [5]-[7]. The so called “smile”, can occur if the emitters do not follow an exact line. This is often caused by a mismatch of the thermal expansion coefficients between the bar and heat sink in die bonding, which leads to an increase of the fast axis far angle of divergence

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after the fast-axis collimator (FAC) [8]. The divergence of the collimated fast axis increases due to smile [9] – [10]. In addition, the reduction in beam quality does not only depend on the smile but on the smile in relation to the near field fast axis height before collimation. A diode laser with a smile of 5 μm (peak-to-valley) may reduce the beam quality by a factor of 6 compared to a diode laser without smile. This makes the reduction of the angle of divergence in laser-diode arrays with large smiles even more important. Several techniques were suggested to reduce the angle of divergence for large smile

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diode-arrays in the fast axes direction [11-15]. One technique is to select a heat sink that matches the thermal expansion coefficient of the chip. Other techniques use tilted cylindrical lenses, custom refractive plates [13, 16], rotating slow-axis cylindrical lenses after fast-axis

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collimation, and telescopes array of micro-optic. Adding external optical components increases the complexity of the system which is a problem for commercial production lines. In this paper, two angled half-BTSs are proposed to reduce the angle of divergence of

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laser-diode arrays with large smiles. A BTS consists of a fast-axis collimator and a beam transformation unit, which can rotate the fast and slow beams by 90°. The beam of full bar located on the left and the right side can be shaped by two angled half-BTS, respectively. Compared with that used by full-BTS, this setup can compensate the smile. This technique enables the reduction of the angle of divergence in a slow axis direction for large smile laser-diode bars. Using two angled half-BTSs, a fiber-coupling module with a 400 μm core

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fiber is fabricated. The optical-optical coupling efficiency increased to at least 85%, which is an improvement by more than 4% compared to a diode laser with a full-BTS. The divergence improvement in laser-diode arrays with large smiles can also increase the yield of suitable laser-diode bars.

Simulation analysis of dual half-BTSs

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2.

The smile of a laser-diode array is due to a significant displacement of the individual emitters of the diode bar along the fast-axis (y-axis). Depending on the emitter positions, the diode bars are grouped into different smile-shapes such as the prominent smile resembling a parabola or an S-shape. Compared to a full-BTS, the residual angle of divergence decreases, when two angled half-BTSs are used. The key parameters for full-BTS and two half-BTSs are shown in Table 1.

Table 1 Key parameters of BTS (manufactured by LIMO GmbH). Parameter

Numerical value

Material

S-TIH53

Effective focal length

365μm

Back focal length

0.095mm

Thickness

2.05mm

Pitch

0.5mm

The diode-laser bar is collimated by two angled half-BTSs. The dual half-BTSs are used to

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reduce the angle of divergence of the diode laser with large smile in the fiber coupling system. The linear-displacement relationship between full-BTS and dual half-BTSs for the parabola-shaped smile and the S-shaped, in rectangular coordinates, is shown in Fig. 1 and Fig. 2. The x and y axis represent the fast and slow axis of the laser-diode array, respectively. Generally, the smile of a laser-diode array generates a vertical deflection angle of the FAC lens – see Fig. 3. In geometric optics, the deflection angle θ is defined as [17]

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

y f ,

(1)

of

where Δy is the linear displacement of the FAC in y-direction, f is the focal length of the FAC. In Fig. 1, the linear displacements Δy1 and Δy2 of the FAC of laser-diode array with parabola-shaped smile are reduced by titling two half-BTSs by a small angle, compared to the linear displacement Δy when using a full-BTS. In Fig. 2, the residual angle of divergence is

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reduced by two angled half-BTSs based on the form of S-shaped smile, because Δy1 and Δy2 are respectively less than Δy. As a result, the deflection angle in the slow axis direction of diode laser bar after two angled half-BTSs is reduced according to equation (1).

y Δy2

Δy1

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Δy

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Emitter

x

0

Fig. 1. Linear-displacement relationship between full-BTS and half-BTS for the parabola-shaped smile.

y

Δy2

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Δy1 Δy x

0

Emitter

Fig. 2. Linear-displacement relationship between full-BTS and half-BTS of S-shaped smile.

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FAC

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Δy

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θ

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Fig. 3. The deflection angle induces the vertical point error of the FAC.

In the ZEMAX simulation, a 60W laser-diode bar (typical values are shown in Table 2) has 19 emitters with a width of 150 μm and a pitch of 500 μm. The emitter positions of the laser-diode bar with a 2 μm smile (parabola- and S-shaped) are shown in Fig. 4. Reduction of the divergence angle depends on the decrease of the linear displacement. For parabola- and

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S-Shaped smile, the minimum of divergence angle can be obtained by two tilted half-BTSs. In the simulation, all measurements of the residual angle of divergence are modeled using a setup based on the Gaussian-beam propagating theory [18] – see Fig. 5. The diameter of spot in CCD is divided by the focal length (f=250mm) to yield the full angular subtense of the input radiation [18]. Therefore, its divergence is defined

d

(2)

f

where f is the focal length of lens, d is the spot diameter in CCD, and Φ is the full angle divergence in radians.

Table 2 Parameters for the laser diode bar.

Parameter

Numerical value

Number of emitters

19

Pitch

500μm

Emitter height

1μm

Emitter width

150 μm

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Fast axis divergence (full width at 1/e2) 2

Slow axis divergence (full width at 1/e )

40° 8°

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Parabola-Shaped-Smile S-Shaped-Smile

2.5

2.5 2.0

1.5

1.5 1.0

0.5 0.0

-5

-4

-3

-2

-1

0

pro

1.0

1

2

3

4

0.5

S-Shaped-Smile(μm)

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Parabola-Shaped-Smile (μm)

2.0

-0.5

0.0 -0.5 5

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Emitter-Position (mm)

Fig. 4. Emitter positions of the laser-diode bar with a 2 μm smile (parabola- and S-shaped).

Spherical lens

Diode

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FAC/BTS

Heatsink

f=250mm

Fig. 5 Simulation setup for the residual-divergence measurement.

As the 2 μm smile with parabola form, the beam profiles along the fast axis after the full-FAC and dual half-FACs are shown in Fig. 6. As shown in Fig. 6, the residual angle of divergence for beam transmission after full-FAC and half-FAC are respectively (0.797+0.876) ×1000/250=6.35 mrad and (0.159+0.398)×1000/250=2.23 mrad according to equation (2). Compared to the full-BTS, the beam profiles after two angled half-BTS are collimated well see Fig. 7. In Fig. 7, the residual angles of divergence for beam transmission after a full-BTS

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and dual half-BTS are respectively (1.12+1.19)×1000/250=9.21 mrad and (0.468+0.972)× 1000/250=5.47 mrad according to equation (2). Fig. 7 indicates that the residual angle of divergence can be reduced by 40.6 %, when two angled half-BTSs are used.

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Full-FAC

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Half-FAC

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Fig. 6. Comparison of the beam profiles after the full-FAC and the dual half-FAC with a parabola-shaped smile.

Full-BTS

Half-BTS

Fig. 7. Comparison of the beam profiles after the full-BTS and dual half-BTS with a parabola-shaped smile.

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The beam profiles for a full-FAC and a half-FAC are shown in Fig. 8 for the 2 μm

S-shaped smile. In Fig. 8, the residual angles of divergence, after full-FAC and after dual half-FACs, are respectively (0.765+0.765) × 1000/250=6.12 mrad and (0.382+0.382) × 1000/250 =3.06 mrad according to equation (2). Compared to the full-BTS, the beam profiles after two angled half-BTSs are collimated well - see Fig. 9. Fig. 9 shows the residual angle of divergence for a full-BTS and a two angled half-BTSs, which are respectively (1.15+1.19)×

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1000/250=9.36 mrad and (0.89+0.87)×1000/250=7.04 mrad according to equation (2). Fig. 9 indicated that the residual angle of divergence can be reduced by more than 24.7% using two angled half-BTSs, which is much smaller than the reduction obtained after using dual

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half-BTSs with the parabola-shaped smile.

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Full-FAC

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Half-FAC

Fig. 8. Beam profiles for the full-FAC and the dual half-FACs with the S-shaped smile.

1800

Full-BTS-Indensity Half-BTS-Indensity

Intensity

1600 1400

Full-BTS

1200

Half-BTS

1000 800 600 400 200

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0 -2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

Slow-axis-diameter (mm)

Fig. 9. Beam profiles for the full-BTS and dual half-BTSs with the S-shaped smile.

3. Experiment with the half-BTS The residual angles of divergence of three diodes bars are obtained using the test setup.

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The 60 W laser-diode bars in the experiment have 19 emitters, with a width of 150 μm and a pitch of 500 μm. The half-BTS designed for the 880 nm laser-diode bars is used to reduce the angle of divergence of the large-smile diode-laser, and the effective focal-length lens of the two half-BTSs, are shown in Fig. 10.

(1)

(2)

of

FAC is 365 μm. The BTS beam-shaping setups for the laser-diode bar, with a full-BTS and

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pro

θ0

Fig. 10. BTS beam-shaping structures with a full-BTS and two half-BTSs: (1) Full-BTS beam-shaper. (2) Half-BTS

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beam-shaper.

As shown in Fig. 11, the measured smile (peak-to-valley) values for the three diode-bars are 2.28 μm, 2.26μm, and 1.9μm. The residual angle of divergence in the slow axis direction, after the full-BTS and two angled half-BTSs, and the collimated beam spot (enclosing 95% of the beam energy) after the full-BTS and two angled half-BTSs, are shown in Fig. 11. The angles for bar 1 and bar 3 with the parabola-shaped smile after the two angled half-BTSs are reduced by more than 37.5% compared to the full-BTS. The angles for bar 2 with the S-shaped smile after the two angled half-BTSs are reduced by more than 23.7% compared to the full-BTS. This shows that the residual angle of divergence for the same smile diode-bar can be effectively reduced with a half-BTS. According to the smile value and the smile shape, the amount of divergence-reduction with the two angled half-BTSs relative to the full-BTS varies - see Fig. 11. Because the smiles of bar 1 and bar 3 resemble a parabola, their residual angles of divergence are much smaller with two half-BTSs than using the full-BTS, which is consistent with the simulation.

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The residual angle The residual angle Collimated Collimated of divergency after of divergency after beam after beam after full-BTS two half-BTS full-BTS half-BTS (95% energy (95% energy enclosure) enclosure)

Bar 1

Smile (P-V):2.26μm Bar 2

Bar 3

7.4 mrad

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9.7mrad

Smile (P-V):1.9μm

6.7 mrad

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10.65mrad

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Smile (P-V):2.28μm

10.06 mrad

6.28 mrad

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Fig. 11. Residual angles of divergence along the slow axis direction of diode laser after the full-BTS and two angled half-BTSs, and collimated beam spot after the full-BTS and two angled half-BTSs.

The coupling of 19-emitters into a fiber is typically done with optical elements that collimate the emitted beam and subsequently focus the beam into the fiber - see Fig. 12. Both full-BTS and the two angled half-BTSs are used in fiber-coupling systems. Because the divergence and beam diameter of the emitted light of diode laser array differ for both axes, the beam-parameter products of both axes are different after the collimation. To symmetrize the beam for both axes, a BTS is used. SAC is used to collimate the light in the slow axis. The collimated light is focused into the fiber with a FAC focus lens as well as a SAC focus lens. As mentioned above, while the two angled half-BTSs can reduce the residual angle of divergence, this effect is clearly reduced for the parabola-shaped smile. It is reduced less if the smile is the S-shaped. The results are shown in Table 3 for the fiber-coupling system. In Table 3, the power changes of three bars before coupling used full-BTS and dual half-BTSs are less than 0.2 W, that can be negligible. Table 3 shows that the obtained fiber-coupling power was higher by dual half-BTSs. The optical-optical efficiency of the three bars is increased by more

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than 85%, which is an improved of more than 4%.

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(1)

BTS

FAC focus SAC lens collimation lens

SAC focus lens

BTS

FAC focus lens

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Fiber

SAC collimation lens

SAC focus lens

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Diode bar

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(2)

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Diode bar

Fiber

Fig. 12. Fiber coupling system of laser-diode array. (1) View of the fast axis (2) View of the slow axis. Table 3. Results for fiber-coupling of the laser-diode arrays with full-BTS and two angled half-BTSs. With full-BTS

With two half-BTSs

Smile

Power

Current Bar

(p-v)

FA div

Coupling

in A

in

in

power in

mrad

W

Bar2 Bar3

div

before

in

coupling

mrad

in W

efficiency

in W

Bar1

Power

coupling

coupling

μm

FA

Optical-optical

before

Coupling

Optical-optical

power in

coupling

W

efficiency

42

2.28

10.65

40.1

31.2

77.7%

6.7

40

34.2

85.5%

56

2.26

9.7

55.1

45.4

82.3%

7.4

55

47.8

86.9%

10.06

55

45.1

82%

6.28

54.9

48

87.3%

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56

1.9

The fiber coupling system with two angled half-BTSs is characterized to ensure reliable

operating conditions in typically encountered in real-life applications. Fig. 13 shows the power changes for a range of endurance conditions: 180 g mechanical shock, 50 g vibration, high-temperature storage (55℃), and low-temperature storage (-5℃). The test results indicate that the power range of the fiber-coupling systems is within 4%, for all stress conditions. In other words, the fiber system should operate reliably.

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6%

Bar1-Power change Bra2-Power change Bar3-Power change

of

2%

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Power change(%)

4%

0%

-2%

-6% High temperature storage

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-4%

Low temperature storage

Mechanical shock and vibration

Endurance conditions

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Fig. 13. Reliability of optical power for a range of endurance-conditions.

4. Conclusion

Using our residual angle of divergence measuring setup, the diode-array smiles of three different bars are measured and analyzed. With the help of two angled half-BTSs, it is possible to shape the beam so that it produces significantly higher brightness. According to our ZEMAX simulation, the angle of divergence of a large smile diode array with a parabola-shaped smile is substantially reduced (by more than 40%) when two angled half-BTSs are used.

In conclusion, the angle of divergence of a diode array with a large (almost parabolic) smile can be significantly reduced such, that it no longer represents a limiting problem for diode-laser applications like material processing or optical pumping of solid state lasers. We believe that this approach can significantly increase the brightness of commercial laser-diode arrays that rely on fiber coupling. Funding

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Support by the National Key R&D Program of China (2018YFB1107302 and

2018YFB1107303). References 1.

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H.G. Treusch and R. Pandey, High-power diode laser arrays, in High-Power Laser Handbook (2011) 133-159.

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K. Price, S. Karlsen, P. Leisher, and R. Martinsen, High-brightness fiber-coupled pump laser development, SPIE 7583 (2010) 7583-7583. Nicola Coluccelli, Nonsequential modeling of laser diode stacks using Zemax: simulation, optimization, and experimental validation, Appl. Opt. 49 (2010) 4237-4245.

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bar, Opt. Express 15 (2007) 17038-17043. 7.

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S. Hengesbach, U. Witte, M. Traub, D. Hoffmann, Simulation and analysis of volume holographic gratings

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integrated in collimation optics for wavelength stabilization, SPIE 79180A (2011) 1-13. 10. B. Liu, Y. Liu, and Y. Braiman, Linewidth reduction of a broad-area laser diode array in a compound external cavity, Appl. Opt. 48 (2009) 365-370.

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12. C. L. Talbot, M. E. J. Friese, D. and Wang, I. Brereton, Linewidth reduction in a large-smile laser diode array, Appl. Opt. 44 (2005) 6264-6268.

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14. Gabriel Pelegrina-bonillal, Thomas Mitral, Compensation of the laser diode smile by the use of micro optics, Appl. Opt. 51 (2018) 3329-3333.

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*Author Contributions Section

Journal Pre-proof Author contribution Manuscript title: Reduction of beam divergence angle in laser-diode arrays with large smiles using a dual-beam transformation system.

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Author: Bin Liu; Hui Liu; Fenning Chen; Haiyan Li; Lei Gao; Pengfei Zhu; Xingsheng Liu;

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Contribution:

Bin Liu conceived of the presented idea and developed the theory. Bin liu wrote the manuscript with support from Hui Liu, Xingsheng Liu and Lei Gao. Bin Liu performed the simulations. Bin Liu, Fenning Chen, Haiyan Li, and Pengfei Zhu

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conceived and carried out the experiments. Hui Liu contributed to the interpretation of the results. Bin Liu, Fengning Chen and Haiyan Li fabricated the fiber coupling system of diode lasers. Lei Gao and Xingsheng Liu helped supervise the project. Both

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Xingsheng Liu and Hui Liu contributed to the final version of the manuscript. All

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authors discussed the results and commented on the manuscript.