High-Reliability Spatial Output Blue Laser Based on COS Package

 

Abstract

The application of spatial output blue laser in the desktop engraving machine market is experiencing rapid growth. Given the constraints of end-user application conditions, air-cooling is the mainstream method for thermal dissipation. With the continuous increase in output power, the COS package exhibits lower thermal resistance compared to the TO-package structure, provide a more advantageous solution.

Due to the high photon energy of blue laser, volatile organic and inorganic gas introduced by the outgassing of adhesive can adsorbed and deposited onto the high-power density region of the laser chip's front cavity during operation, affecting both the laser's output power and long-term operational reliability. Through the analysis of typical blue laser failure mode, we have implemented module package design and process control methods specifically developed for the COS blue laser package. These spatial output blue lasers power range from 10W to 60W, with a shaped and collimated residual divergence angle of less than 0.1°, making them ideally suited for desktop engraving machine. After continuous full-power operation at high temperatures for 3000 hours, power drop less than 10% . These blue lasers has successfully passed multiple environmental reliability tests, 85℃ high temperature storage, -40℃ low temperature storage, -40℃~85℃ temperature cycling test, vibration and mechanical shock test, provide a highly reliable light source for desktop engraving machines.

 

Keywords: blue laser, high reliability, spatial output laser diodes, high temperature operation, desktop engraving machine, blue laser failure mode, laser engraver

 

1. Introduction

With the price drop and power rapid increase of the laser engraver light source, the consumer-grade laser engraver market has witnessed a rapid growth. Laser engraver can assist creative handicraft creators in completing DIY processing on materials such as wood, acrylic, stainless steel, leather, and even leaves.

Figure1.Wood carving with blue laser engraver

Blue laser devices are the main light sources for consumer-grade laser engraver due to their excellent focused energy density and high absorption rates in materials like wood. The power of the laser determines the processing effect and speed of the engraver. In recent years, the power of laser light sources for engraving machines has increased rapidly. In 2021, 5W power was the mainstream product in the market. From 2023 to 2024, the power continued to increase, and many new products with powers such as 10W, 20W, and 40W have emerged in the market.

The early consumer-grade laser engraving machines used blue laser devices with TO-packaged blue laser and CoreXY motion structures. The laser devices and air-cooled heat sinks were fixed on the mechanical structures and moved at high speed during the processing. As the power of the light sources increased, the waste heat also rose. The bulky air-cooled heat dissipation structures and the increased weight restricted the processing speed of the CoreXY structures. In order to improve the engraving speed, laser engraving machines with galvanometer and field lens structures emerged. During the engraving process, the lasers remained fixed, and based on the rapid response of the galvanometer system, large-format processing could be completed at high speed.

Due to the high photon energy of blue laser, packaging can affect the long-term reliability of lasers. TO-packaged blue laser from manufacturers such as Nichia and Osram have been verified for long-term operational reliability in markets such as displays and automobiles. In the application of laser engraving machines, the beam quality and spot size at the imaging point will directly affect the processing effect. The chip underhang of TO-packaged blue laser will have an impact on optical collimation and shaping, resulting in unilateral optical speckle, as shown in Figure 2. Moreover, as the power increases, the problem of high thermal resistance of lasers devices based on TO-packaged blue laser is gradually exposed.

Figure 2. Chip underhang of TO-packaged blue laser impact on optical collimation

Blue laser modules base on COS-packaged laser chip have the advantages of low thermal resistance and excellent optical collimation and focusing effects. In this article, we will introduce our achievements in failure analysis, reliability improvement and verification in blue laser modules base on COS package.

 

2.    Blue Laser Failure Mode and Process Control Methods

For diode laser in the short wavelength range, high energy photons may chemically react with the package atmosphere, package-induced failure is the main failure mode of GaN-based blue diode laser. Previous report have observed carbon deposits on the surface of 405nm GaN diode laser during ther study of mirrior degradation^[1]. In 2021, Xiaowei Wang reported black strip on the unsealed LD facet after 30h of aging, which is logically consistent with the decrease in the optical output power. EDS and SEM results showed the SiO₂ deposition on the facet, due to the micrometer-scale Si particles folating in the air^[2].

In order to achieve miniaturization of laser modules, in addition to the laser chips, module package also contains optical lens, lens fixing epoxy. Although the overall module package is sealed, we have observed a similar failure mode of SiO₂ deposition in the early aging experiments, as shown in Figure 3 to Figure 6.

Figure 3. Optical micrograph of front facet after module aging

Figure 4. SEM image of front facet deposition area

Figure 5. FIB result of front facet deposition area

Figure 6. EDS result of front facet deposition area

In response to the failure mode of SiO₂ deposition, we conducted multiple sets of Design of Experiments (DOE) to test the influence of various factors. Process adjustments and optimizations were carried out for the factors with significant impacts. The main process control methods are as follows.

A.       Low outgassing epoxy

B.       Sufficient pre-curing and post-curing of epoxy

C.       The ratio of sealing gas components

D.       Cleanliness of the process environment and cleanliness of the sealing gas

E.       Moisture of the sealing gas

The bulk failure of blue laser chip caused by electro-static-discharge, transient overcurrent, short-circuit overcurrent and etc. is another typical failure mode. The typical failure phenomenon as Figure 7.

Figure 7(a). Side view of failed chip, melted material

Figure 7(b). Top view of failed chip after removed N-side metal layer, bulk damage area

The main process control methods to avoid electrical damage are as follows.

A.       Electrostatic protection during the manufacturing process, such as using ionizing blower and wearing anti-static wrist straps.

B.       Use Transient Voltage Suppressor or Zener Diode as a protection circuit at COS level or module level

 

3.    High-Reliability Blue Laser Design and Reliability Test

For COS blue laser chips, collimation is carried out for the fast axis and the slow axis respectively. After that, spatial beam combining is performed in the direction of the fast axis. With polarization combination, BWT has completed 10-60W spatial output high-reliability blue laser modules design and manufacturing. The typical 20W&40W module power-voltage (PIV) curves and spectrum are shown in Figure 8.

    

Figure 8(a). PIV of 20W spatial output laser module  (b).PIV of 40W spatial output laser module

Figure 8(c). Spectrum of spatial output blue laser module

For different structures of laser engraving machines, we can provide customized solutions. For laser engraver with a CoreXY motion structure, we can provide spatial output laser with the fixed focus working distance. For laser engraver with galvanometer and field lens structures, we can provide spatial output laser with collimation and fine-tuning of the divergence angle. Schematics are shown in Figure 9.

Figure 9(a). Schematic of laser module for CoreXY laser engraver

Figure 9(b). Schematic of laser module for galvanometer and field lens laser engraver

The high brightness blue laser base on COS package has passed various reliability tests, including 85℃ high temperature storage, -40℃ low temperature storage, -40℃~85℃ temperature cycling test, vibration and mechanical shock test, and 3000 hours high temperature accelerated aging test. Test results are shown in Figure 10.

Figure 10(a). Reliability test plan of 20W blue laser module

Figure 10(h). Accelerated aging test results

Base on the experience in COS-packaged blue laser chips and module process control methods, 80W laser output power is obtained through a 50μm core diameter 0.22NA fiber, PIV data as shown in Figure 11. The coupling efficiency exceeds 90%, and the 0.15NA/0.22NA power ratio exceeds 95% for high brightness.

Figure 11. PIV of 80W high brightness fiber ourput blue laser    

 Figure 12.Schematic of 80W laser module

4.    Conclusion

The rapid growth of application markets such as consumer-grade laser engraving machines has brought more opportunities for blue laser devices. Through the analysis of typical blue laser failure mode, we have implemented module package design and process control methods. The spatial output blue lasers power range from 10W to 60W, with a shaped and collimated residual divergence angle of less than 0.1°, passed multiple environmental reliability tests, making them ideally suited for desktop engraver.

This work was supported by National Key R&D Program of China (Grant No. 2023YFB4604400).

 

Reference

[1] C. C. Kim, Y. Choi, Y. H. Jang, M. K. Kang, M. Joo, and M. S. Noh, “Degradation modes of high power InGaN/GaN laser diodes on low-defect GaN substrates,” Proc. SPIE 6894, 689400–689401 (2008).

[2] Xiao-Wei Wang, Zong-Shun Liu, De-Gang Zhao, Ping Chen, FengLiang, JingYang, “New mechanisms of cavity facet degradation for GaN-based laser diodes” Journal of Applied Physic 129,223106(2021)