Uniformization and collimation of a high-power blue EESLD

doi:10.21203/rs.3.rs-3133774/v1

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Uniformization and collimation of a high-power blue EESLD

https://doi.org/10.21203/rs.3.rs-3133774/v1

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Delivering the highest power among the visible diodes, semiconductor lasers at 450 nm have been extensively applied in biomedical and micro-processing fields. However, the edge-emitting inherence that gives birth to the divergent and ununiform profile largely prohibits the practice due to the fiber coupling and pumping difficulty. To address these challenges, we propose and demonstrate a scheme that is both robust and cost-effective. A laser spot with 3×3 mm2 profile, 0.86 mrad divergence and 87.28% uniformity is experimentally achieved. Further simulation reveals that the coupling efficiency into a S460-HP single-mode fiber by a 20-mm focus-lens can reach 58.35%.

EESLD

beam shaping

collimated

flat-top beam

Due to the energy structure of the GaN material [Coldren et al. 2000], the edge-emitting semiconductor laser diode (EESLD) at 450 nm features the highest efficiency among the visible counterparts, and is able to deliver an output power as high as 5 W [Masui et al. 2017]. This facilitates the emerging applications e.g. solid-state laser pumping [Han et al. 2014], metal processing [Wang et al. 2017] and underwater communication [Chen et al. 2020; Huang et al. 2017]. However, the inherent disadvantages i.e. large emitting angle and un-uniform energy distribution, have prohibited its efficient use to a large extent. While a collimated beam is highly valuable in long distance transmission [Moruzzi et al. 2020; Ning et al. 2019] and optical manipulation, a uniform profile is of high demand in the field of precision machining [Kim et al. 2018; Nakata 2019] and skin therapy [Aslam and Alster 2014; Harfouche and Aït-Ameur 2013]. These surging applications poses a requisite to reshape the 450-nm EESLD to a high-quality beam that is both well-collimated and distributed with high-uniformity.

Although numerous ways for the improvement of light beam uniformity exist, i.e. using fly’ eye [Jia et al. 2011; Lindlein et al. 2010], diffractive optical element (DOE) [Qu et al. 2015; Yao and Chen 2012], and spatial light modulator (SLM) [Yu et al. 2016; Chen et al. 2021; Ackermann and Roider 2021], they are either large-volume or high-cost, and are thus not suitable for shaping the compact high-power EESLDs. To name a few, the fly’s eye method appeals to the segmentation and refocusing of the original beam, resulting in a flat-top pattern only in the vicinity of the focus point, thus is contradictory to light collimation. For DOE method [Yao and Chen 2012], transmission masks with high-fineness etching are essential. The masks possess elaborate structures that are not coexisting with the high-power delivery. Though SLM [Ackermann and Roider 2021] aims to change the amplitude and phase of the incident light, thus raising the uniformity, the device is costly and the volume is huge that is not considered in our compact design.

In this paper, based on the simulation in ZEMAX, we propose a shaping scheme for the 450-nm EESLD. The achieved laser beam features a 3×3 mm2 flat-top pattern with collimated output of 3.5 W power. The divergence is 0.86 mrad and the uniformity is measured to be 87.28%. Simulation indicates that the coupling efficiency into a piece of S460-HP single-mode fiber can reach as high as 58.35%.

Aspheric lens is equipped with a function of redistributing the beam energy, transforming the incident beam to a flat-top emergent and compressing the divergence angle. Galileo-type shaping method redistributes and re-collimates the incident collimated beam. This shaping system includes an aspherical lens and a Galileo system with two spherical cylindrical lenses, optimizing the beam quality in two directions, parallel and perpendicular to the luminous surface respectively. The design method of homogenization is mainly established a mapping formula between the input and the output [Möhl et al. 2019; Möhl and Fuchs 2016]. The shaping scheme is shown in Fig. 1, the light intensity distribution function f(r) and g(R) correspond for input and output beam respectively, which satisfy the law of conservation of energy.

$$2\pi \int_{0}^{\infty } f (r)rdr=2\pi \int_{0}^{\infty } g (R)RdR=1$$

To reduce computational calculation, the Fermi-Dirac (F-D) type function is adopted [Shealy and Chao 2003; Li et al. 2014]. For a given propagation direction, the input and output beams are symmetrical about the optical axis z. The input energy distribution f(r) follows the Gaussian distribution shows in Fig. 1(b),

$$f(r)=\frac{2}{{\pi \omega _{0}^{2}}} \cdot \exp \left( { - \frac{{2{r^2}}}{{\omega _{0}^{2}}}} \right)$$

The output energy distribution g(R) follows the F-D distribution shows in Fig. 1 (c),

$$g(R)={g_0}{\left\{ {1+\exp \left[ {\beta \left( {\frac{R}{{{R_0}}} - 1} \right)} \right]} \right\}^{ - 1}}$$

In the formula, R₀ is the beam radius when the light intensity is reduced to half of its on-axis value, β is the shape factor. According to the conservation of energy law and integrate it, the mapping relation between r and R said as R = h(r) can be obtained.

In Ref [Borghi 2001; Kai et al. 2011], different β and R₀ values will affect the state of the emergent beam spot. The larger the β is, the closer the beam cross-section is to a rectangle. However, the excessive β value will increase the diffraction effect in the system. The value of R0 determines the half-height width of the flattop beam. The higher the R₀ value, the more easily the light energy is dissipated. Therefore, depending on the specific LD parameters and simulation results, a suitable energy distribution state is selected to achieve the ideal aspherical mirror.

Investigating the surface parameters of the aspheric lens by exploiting the ray-tracing function. The optical path along the optical axis is equal to the optical path with an incident angle to the optical axis. The incident angle and refraction angle in an aspheric lens satisfy Snell's law. The surface parameter equation Z(R) of the aspherical lens and the refraction angle θ_r at the emergent plane follow a function like this

$$\tan {\theta _r}=\frac{{dZ(R)}}{{dR}}$$

By calculating the above equations with the help of simulation software, the surface parameters of the aspheric lens can be determined. The numerical solution of Z(R) can be solved by substituting the relationship between R and r into the equation.

The fast-axis divergence angle of EESLD is much larger than the slow-axis divergence angle. In this paper, a system of inverted Galilean telescope spherical lenses is used to homogenize the slow-axis beam. The first spherical lens redistributes the incident beam to change its intensity distribution, while the second lens collimates the beam, improves the uniformity of the output beam and compresses the divergence angle. After passing through the telescope system, the beam in the slow axis direction will be a flattop and collimated beam [Frieden 1965; Hoffnagle and Jefferson 2000].

We have studied the output characteristics of the EESLD and found that, its emitting region is very elongated and the aperture constraints in the fast and slow axis directions are different. The far field of the output beam shows a Gaussian elliptical distribution, with inconsistent beam quality levels and divergence angles in both directions. In the fast axis direction, the divergence angle is typically 40°~50°, the beam quality is close to the diffraction limit and the energy distribution is consistent with a single-Gaussian-mode. In the slow axis direction, the divergence angle is generally 10°~15°, and the width of the luminous area is often greater than the minimum width of the fundamental mode, showing a horizontal multi-mode [Shang et al. 2010].

In this work, a blue EESLD (NUBM 08) at wavelength 450 nm with 3.5 W output power is selected. We first measure the basic luminous parameters of NUBM 08. To fast axis direction, the divergence angle and the waist radius in are 46° and 10.2 µm. To slow axis direction, the divergence angle and the waist radius in are 10° and 2.3 µm. There is a certain aberration between fast and slow axis direction.

According to the emission characteristics of the beam in the fast axis direction, with the help of the optical software ZEMAX, an even-order aspheric lens is used. After selecting the initial structure of the aspherical lens, the REAY operand is used to control the deflection of the beam for different incident radii, adjusting the weights to optimize the surface parameters and achieve a uniform distribution of energy [Coluccelli 2010].

The surface parameters could be determined after selecting emergent light spot diameter as 3 mm, the aspheric lenses have a refractive index of 1.595. The operand instructions in ZEMAX concluded that the slow-axis homogenizing process completed with a plano-concave negative lens with a focal length of -20 mm and a plano-convex positive lens with a focal length of + 100 mm.

In order to determine the effectiveness of the shaping scheme for highly divergent EESLD beam spot, the optical system of the flattop beam shaper was tested with optics design software ZEMAX. As shown in Fig. 2(a), ZEMAX simulates the beam propagation process of the laser diode NUBM 08 in the shaping system. Figure 2(b) shows the light spot diagram very close to the light-emitting surface of the EESLD. Due to the divergence angle of the fast and slow axes, the light beam spreads rapidly thereafter. It can be seen from the image that the energy concentrates in the center of the beam with an unevenly distribution. The emergent light spot is rectangular when all the lenses are placed confocal, owing to the special structure of the EESLD light-emitting surface. After the light passes through the shaping system, as shown in Fig. 2(c), the light spot at 1.2 meters presents a very regular square, and the size of the light spot is 2.97×2.99 mm2. The uniformity of the spot energy distribution is greatly improved. Figure 2(d) shows the state of the beam without shaping system. Expanding the detector area 500 times in Fig. 2(d) to capture diffused light spot. The spot diagram at 1.8 m is shown in Fig. 2(e), and the size of the light spot is 2.98×3.01 mm2. It can be concluded that the spot size does not spread compared with the spot size at 1.2 m. The shaping system in this paper is supposed to be very effective in compressing the divergence angle of the outgoing beam of the EESLD.

Comparing the beam intensity distribution in fast-axis and slow-axis direction, respectively, and analyzing the energy distribution of the EESLD after propagating in the shaping system. Figure 3(a) and Fig. 3(b) are cross-sectional views of the light intensity before shaping in the fast axis and slow axis directions, respectively. The beam in the fast axis direction exhibits a standard single-Gaussian-mode. The slow-axis beam has a multi-mode distribution, which can be understood as the superposition of multiple Gaussian fundamental mode beams at the FWHM. After the shaping system, the energy distribution of the fast-axis beam is significantly improved compared to the unshaped beam uniformity. As shown in Fig. 3(c), the energy fluctuation at the edge is attributed to the diffraction effect at the edge of the optical system. As shown in Fig. 3(d), the uniformity of the energy distribution in the slow axis direction is also improved.

According to the Bessel formula, the parameter V for evaluating the uniformity of light intensity,

$$V=1 - \frac{{\sqrt {{{\sum\limits_{{i=1}}^{N} {{{\left( {{E_i} - {E_0}} \right)}^2}} } \mathord{\left/ {\vphantom {{\sum\limits_{{i=1}}^{N} {{{\left( {{E_i} - {E_0}} \right)}^2}} } N}} \right. \kern-0pt} N}} }}{{{E_0}}}$$

Where E_i is the light intensity value of the flattop area, E₀ is the average energy value of the flattop site, and N is the number of sampling points in the flattop area. The light intensity uniformity V describes the deviation of the overall light intensity in the profile from the average light intensity. When V > 0.9, it can be considered that the beam homogenization work is practical. The shaping scheme adopted in this paper has a calculated uniformity V = 98.6%, which shows that the homogenization scheme designed in this paper is reasonable.

For investigating uniform pump is beneficial for fiber coupling, shaped pump source coupling into the fiber is simulated with ZEMAX, as shown in Fig. 4(a). The coupling model consists of a focusing mirror with a focal length of 20 mm and a piece of fiber S460-HP with a length of 20 cm. Here, single-mode fiber S460-HP with core/ cladding parameters of 3/125 µm and a numerical aperture (NA) of 0.13 was used to estimate the pump-launching efficiency. An annular surface is placed in order to prevent light from entering the cladding structure. By arranging a detector at each end of the fiber, the beam spot energy distribution on the two detectors is shown in Fig. 4(b) and Fig. 4(c). Varying the EESLD output power to test the coupling efficiency. Figure 5 shows the typical pump-launching coupling properties. According to the EESLD output characteristics, the maximum outgoing power is 3.5 W and the efficiency is 58.35%.

Experiments with commercial lenses have been executed to verify the simulation results. The measured beam profile is shown in Fig. 6, where the emergent beam is captured by the CCD shown in Fig. 6(a). Performing information processing on the captured spot in Fig. 6(b), and the beam energy distribution in the fast and slow axis is shown in the Fig. 6(c) and Fig. 6(d), respectively. The emergent beam presents a uniform square spot sized 3.1 × 2.9 mm2. By observing the power distribution in the fast axis and slow axis directions, the uniformity of the beam spot energy can be intuitively found that is greatly improved after shaping. The normalized parameter V = 87.28%, which is used to characterize the energy uniformity. Differences between experimental and theoretical values can be attributed to inevitable errors in the actual test.

We recorded the spot state in the range of 20 m and measured the spot size. The spot status has been stable, and the 20 m away spot is 1.017 times the size of the spot at shaping system exit surface, proved that the divergence angle is compressed to 0.86 mrad.

The output characteristics of the NUBM 08 fast axis and slow axis are inconsistent, and the energy distribution is uneven. In this paper, we perform reshaping operations on the fast axis and slow axis respectively, which can achieve a collimated flattop beam. The system consists of an aspherical lens and an inverted Galileo-type cylindrical lens group. Improved uniformity and high collimation are accomplished in both simulation calculation and experimental measurement. This method effectively optimizes the beam energy distribution and pump output quality, which promotes the laser coupling process and expands cognition for solving the gain medium depletion defect.

Funding

National Natural Science Foundation of China (62105153); Natural Science Foundation of Jiangsu Province (BK20180460); Fundamental Research Funds for the Central Universities (30920021126).

Ethical Approval

(applicable for both human and/ or animal studies. Ethical committees, Internal Review Boards and guidelines followed must be named. When applicable, additional headings with statements on consent to participate and consent to publish are also required)

Competing interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Authors' contributions

Han Gu: Conceptualization, Investigation, Data curation, Methodology, Visualization, Writing – original draft. Xiang Geng: Data Curation, Investigation, Validation. Xiao Tang: Software, Validation. Saiyu Luo: Funding acquisition, Resources. Li Li: Supervision, Writing – review & editing.

Funding

This research was funded by the National Natural Science Foundation of China (No. 62105153).

Availability of data and materials

Data will be made available on request.

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No competing interests reported.

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Uniformization and collimation of a high-power blue EESLD

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Abstract

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1. Introduction

2. The basic theoretical analysis

3. The simulations of flattop beam shaping and fiber coupling

4. The experimental verification of flattop beams

5. Conclusion

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