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- Coating High-Power Semiconductor Lasers (Laser Diode Bars) Facets Via Ion Beam Deposition

 Introduction

Laser diode bars come under the category of high-power semiconductor lasers and are constructed by joining several single emitters having a width of 100-200 µm in an array. Each diode bar normally consists of 20 and 50 emitters. A typical commercial device can generate tens or even several hundreds of Watts of output power. These high power diode arrays are used in pumping of solid state laser systems for medical, military, commercial and industrial applications in addition to material processing applications such as surface treatment, cutting and welding.

Significance of Optical Coating

Laser diode bars operate over a narrow range of wavelengths, thus necessitating the application of a suitable optical coating to increase or reduce the reflectivity of facets, which are partially reflecting ends created by cleaving, so as to deliver the required optical feedback within the lasing cavity and greatly affect the value of the threshold current of the operating laser.

Types of Optical Coating

There are three categories of optical coatings, namely partial reflection coatings, high reflection (HR) coatings, and anti-reflection (AR) coatings. AR coatings significantly decrease the reflectivity of the facets through which the optical signal enters and exists a laser cavity by equaling their thickness to one quarter of the light wavelength, causing destructive interference of light being reflected from the front and back surfaces of the optical coating.

AR Coatings

AR coatings are typically composed of a single-layer or dual-layer of materials. Very low reflectance is not possible with single-layer coatings, but dual layer coatings provide very low reflectance and are typically composed of several repeating thin oxide layers, with each bi-layer composed of a pair of low and high refractive index materials.

Figure 1. Laser bar holder

The typical low index material is SiO₂, and TiO₂-SiO₂ is one among the most widely used dual-layer multilayers, yielding broad reflectance bands owing to the large difference in refractive indices. Ta₂O₅ is often used as a replacement for TiO₂ in multilayer systems with very high specifications regarding optical losses, thermal stability and precision.
The thickness tolerances of the dual-layer are crucial in the determination of the reflectance bands of the coatings. Besides reflection properties, the damage threshold of AR coatings is also crucial particularly for application in Q-switched and high-power lasers where extreme temperature could affect the mechanical and optical performance.

Advantages of Ion Beam Deposition

AR coatings through ion beam deposition provide numerous benefits. Since the deposition occurs in a high vacuum environment, the method yields high quality optical coatings with low absorption and scatter losses. The energetic sputtered target materials produce densely packed films with better longevity and environmental stability than traditional or ion-assisted evaporated coatings.

Figure 2. Laser bar coating

Another advantage is the possibility of decoupling ion energy and ion flux, thus enabling each parameter to be varied independently. This, in turn, provides better control over the resulting film properties such as the refractive index of the deposited thin films so as to obtain the correct reflectivity.
In addition, it is possible to introduce assist gas such as oxygen either directly into the process chamber or through the deposition and/or assist ion source. This assist gas enables depositing stoichoimetric dielectrics either from a metal target in reactive mode where the oxidation of the sputtered metal atoms takes place during the deposition process, or from a stoichiometric dielectric target, where replacement of oxygen depletion takes place during sputtering.
The transmission spectrum of a dual wavelength AR coating at 532 and 1064 nm deposited utilizing a lonfab300Plus ion beam system is illustrated in Figure 3. The coating comprises eight repeating multilayers of Ta₂O₅/SiO₂ with superior transmission of 99.815% and 99.390% at 532nm and 1064 nm, respectively.

Figure 3. Transmission spectrum of a dual-wavelength AR coating for 532 and 1064 nm

Another key factor of the AR coating performance is the thickness error tolerance for each of the dual-layer pairs. The repeatability of the ion beam deposition process has been improved by deploying in-situ monitoring such as a white light optical monitor (WLOM). The process repeatability results of five consecutive runs of a 115 nm thick SiO₂ film utilizing an Optofab3000 system is depicted in Figure 4. The ion beam deposition process reliably applies the appropriate AR coating thicknesses with reduced facet reflectivity, thanks to its stability.

Figure 4. Repeatability study for a 150 nm thick SiO₂ film by IBD

Conclusion

Ion beam deposited optical coatings are suitable for laser diode bar facets thanks to their superior optical properties with low scatter and absorption. The use of sophisticated in-situ monitoring such as WLOM creates highly reproducible films with precise control over thickness, making the technique suitable for optimized production of coatings.