Improvement and characterization of high-reflective ...

Optics Communications 355 (2015) 94–102

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Optics Communications journal homepage: www.elsevier.com/locate/optcom

Improvement and characterization of high-reflective and anti-reflective nanostructured mirrors by ion beam assisted deposition for 944 nm high power diode laser A. Ghadimi-Mahani n, E. Farsad, A. Goodarzi, S. Tahamtan, S.P. Abbasi, M.S. Zabihi Iranian National Center for Laser Science and Technology (INLC), P.O. Box 14665-576, Tehran, Iran

art ic l e i nf o

a b s t r a c t

Article history: Received 31 December 2014 Received in revised form 20 May 2015 Accepted 9 June 2015 Available online 11 June 2015

Single-layer and multi-layer coatings were applied on the surface of diode laser facets as mirrors. This thin film mirrors were designed, deposited, optimized and characterized. The effects of mirrors on facet passivation and optical properties of InGaAs/AlGaAs/GaAs diode lasers were investigated. High-Reflective (HR) and Anti-Reflective (AR) mirrors comprising of four double-layers of Al2O3/Si and a single layer of Al2O3, respectively, were designed and optimized by Macleod software for 944 nm diode lasers. Optimization of Argon flow rate was studied through Alumina thin film deposition by Ion Beam Assisted Deposition (IBAD) for mirror improvement. The nanostructured HR and AR mirrors were deposited on the front and back facet of the laser respectively, by IBAD system under optimum condition. Atomic Force Microscope (AFM), Vis-IR Spectrophotometer, Field Emission Scanning Electron Microscopy (FESEM) and laser characterization Test (P–I) were used to characterize various properties of mirrors and lasers. AFM images show mirror’s root mean square roughness is nearly 1 nm. The Spectrophotometer results of the front facet transmission and the back facet reflection are in good agreement with the simulation results. Optical output power (P) versus driving current (I) characteristics, measured before and after coating the facet, revealed a significant output power enhancement due to optimized AR and HR optical coatings on facets. & 2015 Elsevier B.V. All rights reserved.

Keywords: Thin film mirror design Ion Beam Assisted Deposition Catastrophic Optical Mirror Damage Nanostructure mirror High power diode laser

1. Introduction High power, high efficiency diode lasers with 944 nm or a similar wavelength have come into wide use across the industrial and medical applications such as diode pumping solid-state laser (DPSSL), material processing, varicose vein removal and Laser Therapy and laser surgery [1–6]. In order to enhance the optical power of the laser diode and to protect the diode facet from degradation due to gradual oxidation, front and back facets are passivated with Anti-Reflective (AR) and High-Reflective (HR) coatings, respectively [7,8]. Moreover, these coatings were used to protect the lasers from Catastrophic Optical Damage (COD) or mirrors from Catastrophic Optical Mirror Damage (COMD) which are the consequence of colossal optical intensity on the facet mirrors [2,9]. Asymmetric facet coating is one of the methods to obtain high optical power. The asymmetric facet coating amplifies the stimulated emission by feed backing of light in laser cavity and also n

Corresponding author. Fax: þ98 2188007286. E-mail address: [email protected] (A. Ghadimi-Mahani).

http://dx.doi.org/10.1016/j.optcom.2015.06.021 0030-4018/& 2015 Elsevier B.V. All rights reserved.

completion of the resonator structure of laser and improves the differential quantum efficiency at the front facet [10]. In addition, at the front facet, where the emitted light concentration increases, the non-radiative recombination of the injected carriers (electrons and holes) occurs on the facet surface. This recombination increases the facet temperature by several hundred degrees. Higher optical densities, about 6–9 MW/cm2 for AlGaAs lasers, may lead to COMD. Therefore, thermal runaway results in COD, burning the facet and effectively limiting the laser power available from the diode [11,12]. In order to overcome such difficulties, Electron Beam Evaporation (also known as E-Beam Evaporation) system is being used for fabrication of non-absorbing mirrors [13]. In addition, in E-Beam coating, Ion Beam Assisted (IAD) is used to improve the Quality of mirrors [14]. The mirror is used to increase the COMD threshold to several megawatts per square centimeter by passivating the front and back facets with AR and HR coatings, respectively [15]. The front and back mirrors are essential parts of these lasers those act as a powerful tool for controlling the lasing threshold current, external quantum efficiency, maximum output optical power, longitudinal mode behavior, lasing wavelength and spectral

A. Ghadimi-Mahani et al. / Optics Communications 355 (2015) 94–102

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bandwidth. Dielectric coating is also used for laser facet passivation to improve the device reliability [10,16,17]. Recently, nanostructure dielectric porous films for anti-Reflective mirror and coating were reported [18]. In this paper, mirrors of 944 nm InGaAs/AlGaAs/GaAs Single Quantum Well Separate-Confinement Heterostructure Laser Diode (SQW-SCH LD) were designed, synthesized and improved. The mirrors were characterized and then, the results were presented. In Section 2, the theoretical analysis, design and simulation of the diode laser mirrors are discussed. The ion beam assisted optimization, then improvement and synthesized of non-absorbing mirrors for the front and back facet of diode laser by Ion Beam Assisted Deposition (IBAD) are described in Section 3. Properties of thin film mirrors were characterized, and the performance of the diode laser was discussed in Section 4. AFM and FESEM were used to investigate nanostructure and surface morphology of mirrors. Optical output power (P) versus driving current (I) characteristics are measured before and after coating the facet; a significant output power enhancement is observed due to the optimized AR and HR optical coatings on facets.

2. Theoretical modeling An important application of dielectric thin films is the production of facets with specific reflectivity. These films are applied to increase the front-facet intensity, maximize the front-to-back optical power ratio and increase the overall device efficiency [12]. Dielectric materials have negligible absorption; therefore, they are suitable for mirror coating, specifically front mirror. Front mirror or AR mirror consists of a single dielectric layer e.g. Al2O3, SiO2 or MgF2 [18,19], and back mirror or HR mirror consists of a few double-layers e.g. Al2O3/Si, Al2O3/TiO2, or Mo/SiO2 called Bragg mirrors [20]. Optical thickness of each layer must be a quarter of the operational wavelength of laser. The greater the difference between the refractive index of the layers, the fewer the number of the double-layers are needed in order to achieve high reflection [14]. For example; The higher index ratio Si/Al2O3 requires fewer layers (only 6) which has a much broader high reflection region than the lower index ratio TiO2/Al2O3 (requiring 14 layers). Al2O3, Si3N4 and sometimes SiO2 are utilized as optical coatings with low refractive index. Si, ZnSe, TiO2 or Ta2O5 are common coating materials with high reflective index [15]. Alumina combines many useful properties such as high dielectric constant, high thermal conductivity, good stability, relatively low refractive index and transparency over wide range of wavelength [21,22]. These properties causes alumina film has various applications such as microelectronic devices, optoelectronics, resistive coatings, corrosion protective coating material and surface passivation of diode lasers and solar cells because of high dielectric constant, wide band gap, and high temperature operation [22]. Alumina (Al2O3) films are among the widely used ultra-violet optical film materials because of their excellent optical transparency within a wide spectral range. Al2O3 is used for the preparation of high-reflective mirrors, antireflective coatings, optical filters and other laser mirrors for laser systems. Because of its excellent properties, for the AR mirror fabrication, usually a layer of Al2O3 was coated on the facet of the laser, as shown in figure 1. There are a variety of material choices available for HR mirrors or rear mirror. Before mirror coating, reflection of each InGaAs/AlGaAs/GaAs laser facet is about 30%. After mirror coating reflection of front and back facets of InGaAs/AlGaAs/GaAs laser is in the range of 3–20% and 90–100%, respectively [15]. In mirror design and coating, refraction index and thickness of each layer must be precisely noticed. Also, the refraction index and

Fig. 1. A schematic illustration of AR and HR coatings for laser diodes.

thickness of each layer in design must be as much as possible closed and adopted to refraction index and thickness of corresponding layer in coating. Designing multilayer optical coatings is a difficult optimization problem because of the huge size of the search space [23]. The high reflectivity of Bragg mirrors is the result of constructive interference of electromagnetic waves reflected at the consecutive interfaces of a multilayer structure. To obtain constructive interference, all interfaces should be parallel. The distance between subsequent interfaces should be one quarter of the optical wavelength, considering the refractive indices of the media surrounding the Bragg mirror [16]. In practice, stacks of two alternative layers with different refractive indices n1 and n2 and a Quarter-Wave Optical Thickness (QWOT) are used. The mirror characteristics are then governed by the refractive index contrast (n2/n1), and the number of λ/4 pairs (N). At the target wavelength, the reflectivity R of a Bragg mirror consisting of N pairs with λ/4 layers is given by:

⎛ 1 − n /n (n /n )2N ⎞2 t i 1 2 ⎟⎟ R = ⎜⎜ ⎝ 1 + ni /nt (n1/n2)2N ⎠

(1)

where ni and nt are the refractive indices of the materials in front and back of the multilayer stack, respectively. Eq. (1) shows clearly that apart from the refractive index contrast between the mirror materials, the reflectivity is determined by the total number (N) of pair layers as well as by the refractive indices nt and ni outside the mirror [14,16]. Transfer matrix method was also used to calculate the whole transmission and reflection spectra of an arbitrary arrangement of dielectric layers [14,24]. Mirrors for diode lasers were designed by Macleod software. In Macleod software, glass was selected for substrate material and then 4 pairs of Al2O3/Si layers were placed on the substrate, each layer with QWOT at 944 nm. Finally, air was regarded as external region of 4 pairs of Al2O3/Si layers. Therefore, Back mirror design as S|(LH)4|Air, in which L stands for a QWOT of a low index material i.e. Al2O3, H stands for a QWOT of a high index material i.e. Si, and S presents the glass substrate. By Macleod software, the reflection spectra

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Fig. 2. Simulation and experimental reflection of back facet.

The aluminum oxide thin films were made by various methods such as atomic layer deposition (ALD), Chemical Vapor Deposition (CVD), sol–gel deposition method, Laser Assisted Spray Pyrolysis (LASP), Ion Beam Sputtering and so on [8,26,27]. Among these techniques, Electron Beam Evaporation is one of the most common methods for optical thin film deposition [8,28], and is a powerful technique to prepare amorphous and even well-crystallized oxide thin films with high deposition rate and ease of control for a wide range of evaporation [29,30]. This method produces uniform dense coatings strongly bonded to the substrate [20]. Si thin films have been prepared by various methods such as pulsed laser deposition (PLD) and high-frequency (HF) sputtering [31–33]. Electron Beam Evaporation is one of the methods to deposit Si thin films [34], and is efficient technique because Si has very high melting and boiling point. Electron Beam Evaporation with IAD (Ion-Assist Deposition) is the preferable method to optimize optical thin film quality [14]. Films synthesized by Ion Beam Assisted Deposition technique have improved adhesion to the substrate, smooth morphology of coating surface, high density thin film and corrosion resistance compared with coatings deposited without ion bombardment [13,35]. Also, Facet-passivation techniques have been reported by

Fig. 3. Simulation and experimental transmission of front facet.

from designed sample were measured in normal incident as high reflection mirror. To design the front mirror, 154 nm Al2O3 thin films were placed on glass slides as substrate. Also, air was regarded as the external region of the Al2O3 layer. The dotted lines in Figs. 2 and 3 show the simulated front and back mirrors spectra. The front facet transmission and back mirror reflection for simulated samples are nearly 85% and 99%, respectively at 944 nm. The solid line show the experimental results of the samples which will be explained in the next section.

3. Experimental setup A number of schemes are used for facet passivation. The choice of scheme depends on the application [9,12]. Facet-passivation schemes were performed through cleaning of facets by uncreative ions (e.g., Argon) before coating and then HR and LR mirror coating on facet with optimized Ar ion beam simultaneously. Improving the properties of Al2O3 thin films is of great importance for further development of mirrors and devices. Therefore, great attention must be paid to Al2O3 thin films and their properties as front mirror [25].

Fig. 4. Gridless end hall ion source working condition (a) and RMS roughness of alumina thin film versus Ar flow rate (b).


employing ablation with nonreactive ions (e.g. Argon) [12,36]. Hence, mirrors were coated by Ion-Assisted E-Beam Evaporation system [37,38]. End Hall Girdless Ion Source was used to produce Ar ion current aligned to rotational substrates holder. Production range of stable Ar ion beam was extracted and investigated. Then, at optimum operation voltage and current, the effect of Ar flow rate variation was studied via preparation of Al2O3 thin films by IBAD. The ion beam current is an important parameter for the deposition and is constant during the deposition process [39]. By increasing Ar flow rate, stable working condition of Gridless End Hall Ion Source is achieved at lower voltages (as shown in Fig. 4a). In V¼70 V, Gridless End Hall Ion Source is stable in all Ar

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flow rates. Therefore, V¼ 70 V was considered as Alumina film coating with various flow rates. Fig. 4b shows that when Ar flow rate is 4 sccm, Alumina film have the highest quality and the lowest roughness. Resultantly, 70 V and 1.75 A with 4 sccm Ar input flow rate was considered as the best condition for mirror coating. Final designed mirrors for InGaAs/AlGaAs/GaAs diode laser were coated in the optimized condition with IBAD system as shown in Fig. 5. In order to coat the mirrors on the facets, laser bars were loaded in a bar holder and placed together with glass slide and GaAs (to analyze Surface morphology) substrates as witness samples in the chamber of the E-Beam Evaporation system. After mounting the holder on the substrate manipulator, one side of facets can be coated. The coating chamber was vacuumed and simultaneously, the temperature increased up to 120 °C. Then, when the pressure reached to 2 10 3 Pa, first, the substrates holder rotation system was turned on with velocity of 12 rpm and then the Ion Beam Assisted system (see Fig. 5) was turned on. The laser bar holder must be rotate to coat the other side of facets. After pre-cleaning the substrates with Ar ion flow for 5 min, coating of the layers was started. The deposition rate is 10 Å/s for Al2O3 and 3 Å/s for Si layers. Ar ions impact the particles of the evaporated material and stick them to the facet of laser bars and substrates more tightly. It causes adhesion to the substrates and therefore, the reflection index of the layers increases and the optical properties of layer improve. On the other side, the films prepared using Ion-Assisted Deposition technique exhibits high refraction index [40]. Also, Al2O3 has good adherence to the substrate and good humidity durability [41]. Thickness of the nine coated layer are controlled by optical and crystal thickness measurement simultaneously. Optical thickness was merely used for measuring turning point at the QWOT of back mirror. Refraction index and the thickness of layers were measured using Spectrophotometer and Dektak surface profiler. Specifications of 8 layers, which form the back mirror stack, are listed in Table 1. The single layer of Al2O3 for the front mirror has a thickness of 154 nm and refraction index of 1.75. Materials (Al2O3, Si) evaporated during coating cause the chamber pressure to increase up to 3 10 2 Pa. Temperature also increases up to 220 °C due to the heat produced in Al2O3 and Si target as a consequence of accelerated electron beam. Al2O3 and Si thin film synthesized by IBAD at this temperature, which is lower than 500 °C, are amorphous [13]. Reflection and transmission spectra of the back and front mirrors, which were coated on glass slides substrate, were measured using spectrophotometer. The results are shown in Figs. 2 and 3 by solid lines. The results show that the front mirror sample transmission and back mirror sample reflection are nearly 83% and 98% at 944 nm, which are approximately adapted with results of Table 1 Specifications of the 8-layer S|(LH)4|Air mirror which coated on the back facet of diode laser bar.

Fig. 5. Schematic (a) and picture of the ion-assisted E-Beam Evaporation System (b).

Number of layers

Material

Reflection Index

Thickness (nm)

Substrate 1 2 3 4 5 6 7 8 Medium

Glass/InGaAs Al2O3 Si Al2O3 Si Al2O3 Si Al2O3 Si Air

1.52 1.75 3.61 1.75 3.61 1.75 3.61 1.75 3.61 1.0

– 139 71 137 67 138 68 138 68 –

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design mirror on glass slide by Macleod software. It should be noted that both simulation and experimental results obtained for the front and back mirrors, designed and coated on glass substrate, differ from spectral results of mirrors coated on InGaAs/AlGaAs/GaAs laser bars. In the experiments, Al2O3 and Si were coated on the glass slide and GaAs substrates, extra to InGaAs/AlGaAs/GaAs laser bars, simultaneously. Spectroscopic analysis was performed on the glass slide sample coated with front and back mirrors. A glass slide was used as the substrate. At first, in Macleod software, the substrate was considered as glass to compare the results (see Figs. 2 and 3). The results are in good agreement. Then, substrate was switched to InGaAs/AlGaAs/GaAs (or GaAs) in Macleod software to calculate

real transmission and reflection of the mirror coated on InGaAs/ AlGaAs/GaAs laser bars. By this procedure, front facet transmission and back facet reflection at 944 nm laser bars are 97.1% and 99.4% respectively.

Table 2 Rrms and Ra values of Al2O3/Glass, 4LH/Glass and 4LH/GaAs mirrors Layer

Ra (nm)

Rrms (nm)

Al2O3/Glass 4LH/Glass 4LH/GaAs

0.74 1.4 0.97

0.93 1.31 1.23

Fig. 6. AFM images of from Al2O3/Glass, 4LH/Glass and 4LH/GaAs samples in sweep area of 5 5 mm2.


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After facet coating, the InGaAs/AlGaAs/GaAs laser bars were scribed again to obtain standard cm-bars. Then, the single bars and single emitters underwent a preliminary test to check their Power–Current curves.

4. Results and discussion Varian Spectrophotometer in the Vis-IR region was used for spectral analysis of front and back mirrors on glass slide substrate. It was known that a front mirror with high transmission and a back mirror with low transmission are appropriate for a 944 nm InGaAs/AlGaAs/GaAs Quantum Well laser diode (see Figs. 2 and 3). AFM images of Al2O3/Glass, 4LH/Glass and 4LH/GaAs samples in the sweep area of 5 5 mm2 are shown in Fig. 6, respectively. Surface quality and layer morphology were analyzed with Atomic Force Microscope (AFM) images in DME-SPM software. The Arithmetic Mean Deviation Roughness (Ra) and the Root Mean Square Roughness (Rrms) of the mirrors were tabulated in Table 2. The surface quality of the single-layer front mirror is better than that of the multilayer back mirror. Table 2 also shows that the surface of mirrors is of excellent quality and smaller Rrms than those reported by Xi et. al. [34], in the case of Si, and by Zhang et. al and Peitao et. al in the case of Al2O3 [13,28]. FESEM image of front and back mirrors surface, demonstrating nanostructure and morphology of samples, are shown in Fig. 7-a and b, respectively. Fig. 7-b shows the grain size of back mirror in the order of approximately 20 nm. Fig. 7-a demonstrates that grains of front mirror are obscure and irregular in the order of 30 nm. Comparison between Fig. 7-a and -b shows that in back mirror, grains size are smaller and boundary of grains are apparent related to front mirror. The reason is that the surface layer in front mirror is Al2O3 and in back mirror it is Si. Fig. 8 shows cross-sectional FESEM image of front and back facet mirror. Fig. 8-b and -d shows magnification of selected area in Fig. 8-a and -c, respectively. Layer interfaces in some parts can be realize easily and considered as the base for thickness measurements of the layers. In Fig. 8-b, thickness of Al2O3 layer is about 154 nm and the growth pattern (model) seems to be columnar. Fig. 8-d shows the thickness of layers. The thicknesses of layers are obtained from optical and crystal thickness measurements (Table 1) and all interfaces seem to be distinct, straighten and align compared to the results obtained by Zhang et. al. [42]. From Fig. 8-d, it can concluded that the grain sizes of Si layers are smaller than that of Al2O3 layers. Also, pair Bragg layer has good and precise repeatability. By decreasing deposition rate, mean grain size of Alumina and Si thin films decreases [13,43]. At low deposition rates the number of particles arriving onto the substrate surface is low and the formation of alumina nuclei is not favored. This implies a low nucleation density, which results in a surface with small grains. When the deposition rate is high, the number of particles arriving into the surface is high, resulting in a higher nucleation rate and bigger grain size. Using lower deposition rate (10 Å/s for Al2O3 and 3 Å/s for Si) and optimize Gridless End Hall Ion Source, grains with 20–30 nm size achieved, as shown in Fig. 7a and b. On the other side, surface roughness decrease with diminishing deposition rate and grain size [44]. Lower surface roughness alleviates light scattering loss at the interface. This accounts for the observed increment in the transmittance with reducing grain size and surface roughness. Furthermore, an increase in the scattering coefficient would reduce the optical transmittance [45]. Void, porosity and moisture absorption diminish with alleviating surface roughness [46]. Therefore, output power of diode laser enhances due to decreasing absorption and scattering of light, the reduction in defects and voids formation and improving surface morphology.

Fig. 7. FESEM image of front (a) and back (b) mirror surfaces.

Furthermore, optical mirror performance and reliability and then laser life time improves leading to the enhancement of COMD level. The electro-optical characteristics of unmounted laser chip samples before and after deposition of AR and HR coatings on front and back facets are listed in Table 3. According to Table 3, threshold current (Ith) and slop efficiency of laser chip samples increases after coating compared with their values before facet coating. A reduction in the front facet reflectivity after coating increases the mirror loss of cavity and subsequently, increases the threshold current of laser chip. Effect of mirror coating can be attained by comparison between results of before and after laser characteristic tests. Comparison between parameters in Table 3 shows that the laser power increased from 264 to 325 mW for uncoated or bare facets chips to 630 mW for facet coating chips. By mirror coating on the facets, slope efficiency is also increase from 0.39–0.47 mW/ mA to 1.01–1.02 mW/mA. Therefore, mirror coating, not only provides a passivated barrier for atmospheric oxygen and moisture, but also produces a specified facet reflectivity which can be implemented to increase

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Fig. 8. Cross-sectional FESEM image of the cleaved edge of Al2O3 layer mirror on GaAs substrate (a), (b) and 4-pair Al2O3/Si Bragg mirror on GaAs substrate (c), (d).

Table 3 The characteristic test results of unmounted laser chip before and after mirror coating. Sample Mirror coating

Po (mW) I (mA) Ith (mA) ηs (W/A) Ep (%) λ (nm)

1

Before After

325 630

800 800

110 184

0.47 1.02

19 44.8

946.2 944.6

2

Before After

264 631

800 800

124 177

0.39 1.01

14.4 36.3

944.6 944.4

the front-facet intensity, thus maximizing the front-to-back optical-power ratio. Fig. 9 shows the effect of optimized mirror coating on power– current characterizations and power efficiency for an unmounted

diode laser bar. Significant enhancement of output power can be seen after mirror coating. The application of high reflection coating on the back facet blocks the light emission from the rear facet and improves the effective efficiency of the laser diode. The AR coating increases the output power and protects the facet from COD. Thus, the cumulative effect of AR and HR coating boosts the optical power and increases power efficiency by almost 129% at current of 20 A, as seen in Fig. 9. The burn-in screening is one of the reliability test to screen out diode laser bar that are likely to have unacceptably short lives and to ensure that the remaining population of laser bar will have a statistically acceptable level of reliability. Due to the impact of burn-in on manufacturing cost and cycle times, burn-in times of less than 100 h are common [47]. The unmounted laser bar after mirror coating is directly bonded onto CS copper block using indium solder. Fig. 10 shows the CS package fabricated in this study. Also, the position of laser bar and front mirror surface can be seen


in this figure. CS package configuration is studied for rapid assessment of the laser bar performance during the burn in test. The surface of facet seems homogeneous and relatively uniform light blue color, without any visual defect and contamination. Fig. 11 shows the typical behavior of the normalized optical power versus burn in time. The burn in test was performed at constant current of 28 A, CW mode and T¼ 20 °C, for 58 h. The decrease in the optical power over the burn in time was about 5%. Therefore, these lasers passed the burn-in test successfully [48].

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Multilayer HR coating and single-layer AR coating on the back and front facets of high power single quantum well InGaAs/AlGaAs/GaAs diode lasers were designed, optimized and deposited.

Spectrophotometer's results show that the mirror reflection values are in favorable range. The front facet transmission and back facet reflection at 944 nm was 97.1% and 99.4%, respectively. The AFM images show the mirrors to have root mean square Roughness (Rrms) nearly 1 nm which proves that the surface of mirrors has excellent quality. The surface FESEM images demonstrate that the nanostructure and homogeneous mirrors form. The cross-sectional FESEM images demonstrate the nano-grain sizes of Si layers are smaller than the nano-grain sizes of Al2O3 layers. Also, Bragg pair layers have good and precise repeatability, the layers and interfaces in the most positions can realize easily and is the base for thickness recognize of layers. The (P–I) characteristics were measured before and after facet coating for an unmounted diode laser bar. Significant output power and efficiency enhancement for laser bar due to AR and HR coatings were observed. The decrease in the optical power of the selected laser after facet coating was about 5%

Fig. 9. P–I curves and efficiency of an unmounted laser bar before and after mirror coating.

Fig. 11. Burn-in test result of InGaAs/AlGaAs/GaAs laser bar performed at I¼ 28 A, CW, T ¼ 20 °C, for 58 h.

5. Conclusion

Fig. 10. The pictures of chip, bar, CS package and front mirror surface of produced diode laser.

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at burn-in test. Also COMD in laser facets after burn-in test was not observed.

Acknowledgment We would like to thank the staff of semiconductor lasers group, who designed and fabricated the diode laser systems.

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