Optimization of High and Anti-reflective Facet Coating for...

Optimization of High and Anti-reflective Facet Coating

for Near Infrared High Power Laser Diode

Seung Jin Lee1,1,Hee Jo An1, Taeksoo Ji*

1 Department of Electronics and Computer Engineering,

Chonnam National University, Gwangju 500-757, Korea, *tji@chonnam.ac.kr

Abstract. The output and efficiency of laser diode can be changed by single or

multi-layer thin film coating having different refractive indices. In this research,

laser diode is coated with single layer Anti-reflection (AR) coating of Al2O3

and Multi-layer High-reflection (HR) coating of SiO2/Ta2O5. Thin film was

deposited by ion beam assisted deposition (IAD) equipment after design

optimization. For the wavelength range of 850~1,100nm (near infrared

wavelength range), the transmittance of thin film coated front facet is 95% and

reflectance of the back facet is 90%. The output of laser diode is increased by

100% after facet AR/HR coating.

Keywords: Laser diode, Facet coating, Reflection, Al2O3, SiO2/Ta2O5

1 Introduction

Laser diode (LD) is used for a many applications in optical communication, medical

field, industry, printing and many other fields [1]. Recently, high power LD research

has received wide attention [2, 3]. One of important factor in fabrication technology

of high power LD is thin film coating. It is possible to make high quality thin film

coating by using Ion beam deposition technology. This improves the optical and

mechanical properties of the laser diode [4, 5]. While selecting the material for

AR/HR coating, many parameters such as wavelength band, transmittance, absorption

coefficient, deposition rate, refractive index must be considered [6].

In a typical experiment GaAs wafer based LD was used. Al2O3 (Aluminium oxide)

was found to be suitable material for front facet coating(AR). Al2O3 refractive index

is similar to commonly used SiO2 and does not cause the absorption of wavelength

300~5,000nm range. So, this material is suitable to be used as AR Coating. Back facet

HR coating is done using SiO2(Silicon dioxide)/Ta2O5(Tantalum pentoxide). Using

the multi-layer structure of different materials having different refractive indices, like

high refractive index of Ta2O5 and low refractive index of SiO2, the reflectance

increases [7, 8]. The comparison of output of LD before and after coating for

wavelength range 850~1,100nm has been done and report. In this paper, designed and

optimized single layer anti-reflection/high-reflection coating on laser diode.

Advanced Science and Technology Letters Vol.139 (EEC 2016), pp.418-421

http://dx.doi.org/10.14257/astl.2016.139.83

ISSN: 2287-1233 ASTL Copyright © 2016 SERSC

2 Experimental Details

As shown in Fig. 1 (b), light is reflected from Back facet, and emitted from front facet

to improve output efficiency. For simulation design in 850~1,100nm, Essential

Macleod software (Thin Film Center) is used. Software is many used for simulating

different types of thin films. Many simulations were carried out for reference

wavelength to design thin film layer and to optimize the thickness. Simulations were

carried out to design LD with 95% transmittance (front facet) and 90% reflectance

(back facet).

Fig.1. Laser diode (a) before coating, (b) after coating

Deposition was performed using the IAD (Ion Assisted Deposition) equipment. Ion

Assisted Deposition (IAD) is used to improve the Quality of coating. Thin film

coating by Ion Beam Assisted improved adhesion to the substrate, smooth coating

surface, high density thin film [9]. Deposition conditions were fixed. The chamber

temperature 150℃, and oxygen and argon gas was use for generating plasma ions. 50

and 10 sccm of Oxygen and argon gas respectively were injected.

Fig. 2. A schematic image of AR and HR coatings for laser diodes.

Advanced Science and Technology Letters Vol.139 (EEC 2016)

Copyright © 2016 SERSC 419

As shown in Fig. 2, for AR coating, single-layer of Al2O3 150.52nm thickness for

deposited. For back facet multi-layer of SiO2/Ta2O5 of 163.4/112.5nm thickness were

deposited for 7 times, i.e. total 14 layers and 1,931.3nm thickness.

3 Result and Discussion

As shown in Fig. 3 (a) and (b), AR/HR graph of software simulation output and

experimentally measured transmittance/reflectance data is compared. At 850~ 1,100

nm wavelength range, the simulation output shows more than 95% and 90% of

transmittance and reflectance respectively. The experimental data deviates slightly

from the simulation output due to a little difference in physical thickness, density,

refractive index from simulation design.

800 850 900 950 1000 1050 1100 1150 1200

0

20

40

60

80

100R

efle

cta

nce

(%

)

Wavelength (nm)

Experiment Reflectance (%)

Design Reflectance (%)

(a) (b)

0 200 400 600 800 1000 1200 1400 1600 1800 2000

0

200

400

600

800

1000

1200

1400

Po

we

r(m

W)

Current(mA)

Before coating

After Coating

(c)

Fig. 3. (a) Design and experimental transmission of front facet (b) Design and experimental reflection of back facet (c) P-I Curve measurement of before/after coating LD

800 850 900 950 1000 1050 1100 1150 1200

0

20

40

60

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Tra

nsm

itta

nce

(%)

Wavelength (nm)

Design Transmittance(%)

Experiment Transmittance(%)

Advanced Science and Technology Letters Vol.139 (EEC 2016)

420 Copyright © 2016 SERSC

The output of LD was measured before and after thin film deposition to compare

the change in output. As shown in Fig. 3 (c), at reference current of 2A, before

coating optical output is 654mW and after coating it is 1,242mW. The output has

improved by almost twice the amount.

4 Conclusion

In this experiment, for near infrared wavelength laser diode, an attempt was made to

optimize and deposit anti-reflection and high-reflection facet coating. Optical output

change was measured before and after coating at 850~1,100nm. The simulation

prediction of front facet 95% transmittance and back facet 90% reflectance using

Al2O3, SiO2/Ta2O5 material was confirmed. The actual result shows that the output

power and efficiency increased by twice. This result suggests that AR/HR coating

laser diode with a high transmissivity and high reflectivity can be useful for high

performance optical device application.

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Advanced Science and Technology Letters Vol.139 (EEC 2016)

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