ZHAO Bi-yao, JING Hong-qi, ZHONG Li, LIU Cui-cui, LIU Su-ping， MA Xiao-yu

(1. National Engineering Research Center for Optoelectronic Devices, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China；2. College of Materials Science and Optoelectronic Technology, University of Chinese Academy of Sciences, Beijing 100049, China)*Corresponding Author, E-mail: jinghq@semi.ac.cn

Abstract: In order to reduce far-field divergence angle of semiconductor laser along slow axis and improve slow axis beam quality, a new type of adiabatic package structure is proposed. With this structure, the strength of thermal lens effect of the laser chip can be reduced. We have simulated the lateral heat distribution of broad-area semiconductor laser chip with the new structure by ANSYS 18.0, the model is based on the law of heat conduction(Fourier’s law).The beam quality of 808 nm In0.08Ga0.78Al0.14As/Al0.37GaAs broad-area semiconductor laser was experimentally investigated. Measurement is based on charge coupled device(CCD) image acquisition analysis method. The results show that the adiabatic package structure can reduce slow axis divergence angle by about 40%, and the slow axis divergence angle is more stable as the working currents change. Moreover, the corresponding beam parameter product (BPP) and beam quality factor M2 are also reduced by about 33% and 30%. The narrower the contact width between chip and heat sink is, the better the effect of the improvement will be. The introduction of the air gap in the adiabatic package leads to a deterioration of photoelectric characteristics. The output power P reduces by 14%, electro-optical conversion efficiency η reduces by 8.7%. Adiabatic package method has guiding significance to the improvement of the slow axis beam quality of 808 nm broad-area semiconductor lasers.

Key words: slow axis divergence angle； thermal lens effect； adiabatic package

1 Introduction

808 nm semiconductor LDs are widely used in processing materials, pumping solid-state lasers, industrial production,etc. As they have the characteristics of high efficiency, low cost, small size and good operability, LDs are applied in various aspects of production. When LD operates, broad-area semiconductor LD has gained more medium for radiation composition, therefore they are with high efficiency. Thus high power semiconductor LDs are generally designed into a wide strip shape[1]. Beam quality directly determines spot size and limits the transmission distance and application scenarios,etc. Improving beam quality is an important investigation direction of high power semiconductor lasers. High-power semiconductor laser often suffers from far-field blooming due to self-heating, which results in the deterioration of beam quality and limits the application of laser seriously.

In 2011, Baietal. proposed a structurer of gold as heat sink material. The heat-sinkable submount is divided into two parts. The middle gold pillar is used for heat dissipation, and the gold pillars on both sides serve as mechanical support. The structure improves the heat dissipation of the laser chip and reduces the slow axis far field divergence angle, which increases the linear brightness of the laser by about 14% as a current rating of 14 A[2]. In 2013, Joachim Piprek proposed a method of adding a pedestal between chip and heat sink. This method improved the microscopic heat conduction path of the chip and mitigated the slow axis far field blooming[3]. In 2015, Martin Winterfeldt proposed an approach to improve laser beam quality by limiting accumulation of lateral carriers, which reduced the increase rate of BPP with self-heating of the laser by 35%, and allowed BPP <2 mm·mrad to be maintained to 7 W optical output[4]. In this paper, a novel laser adiabatic package method was proposed, which mitigates thermal lens effect caused by the nonuniform refractive index, reduces far field slow axis divergence angle and improves beam quality[4].

2 Thermal Lens Effect

Electro-optical conversion efficiency of high-power semiconductor lasers is nearly to 1/2 . The electrical energy which doesn’t convert into laser mainly dissipates in the form of thermal. Main source of thermal is Joule heat, nonradiative recombination heat generation, radiation absorption, spontaneous radiance, and Ohmic contact heat generation,etc. The heat conduction follows the equation when lasers work:

(1)

hereQandKdenote thermal power density and thermal conductivity,Tis the temperature,xaxial,yaxial, andzaxial position represents slow axis (parallel to the PN junction) direction, fast axis (perpendicular to the PN junction) direction, and lase (along resonator) direction respectively. When laser operates in lasing mode, thermal convection between device and heat sink and air is ignored, then the sum of temperature variation inxaxial,yaxial, andzaxial is the total device thermal power variation[5-6].

The refractive index change of material is related to carrier concentration and temperature:

(2)

According to gradient refractive index theory, for an active layer with high refractive index in the central region and low refractive index in the edge region, the influence of thermal lens effect is equivalent to the addition of a convex lens on waveguide, which plays a role of self-focusing.The focal length of the equivalent convex lens is considered as the focal length of thermal lens, and it can be described as

(3)

Heren0is the initial refractive index,αreflects the non-uniformity of refractive index distribution,αrepresents the non-uniformity of the temperature distribution, which can be obtained from temperature distribution curve andlis the thickness of self-focusing lens. The aperture angle Δθcan be regarded as divergence angle variation approximately. According to geometric optics, the slow axis divergence angle’s increase can be quantitatively described as:

(4)

Herefis focal length of equivalent thermal lens,wstands for the active region’s vertical width. Then we can calculate laser thermal lens focal length and the increment of slow axis divergence angle quantitatively under different temperature working conditions[9-10].

When working current is small, most of the carriers concentrate in the center of strip, which reduce the refractive index. Refractive index in the central region will be smaller than that in the edge region, which is called the anti-waveguide effect. The light field limiting capability in central region will decrease and leads to far field blooming.On the one hand, with the gradual increase of working current, the broad-area semiconductor LDs, higher-order lateral mode(such as Hermit-Gaussian beam, Laguerre-Gaussian beam,etc.) will be excited. On the other hand, heat generated by Joule heat and non-radiative recombination,etc. causes higher temperature in central area. The refractive index guiding mechanism in the horizontal direction of the material in the active region is weak, and the divergence angle and beam width in the slow axis direction vary greatly with the rise of the chip temperature, which are easily affected by thermal lens effect. And beam quality in slow axis deteriorates severely with the increase of temperature. Compared with normal packaged chip which directly contact with heat sink, an air gap outside the p-contact is implemented between chip and heat sink[11-12]. When the contact width between the chip and the heat sink is smaller than chip width in lateral direction (slow axis direction), there will be an air gap between chip and heat sink, which is called adiabatic package. The schematic diagram of normal package and adiabatic package is shown in Fig.1. The outward heat flow from the center to edge causes the decrease of temperature and refractive index in normal package. In adiabatic package method, the addition of air gap forces almost all heat to flow vertically through the convex platform to the bottom heat sink. The heat flows inward from center to edge, making the chip center heat conduction better than edge and the temperature decreases faster. As a result, the temperature of the whole laser chip becomes more uniform in lateral direction. Adiabatic package is expected to weaken thermal lens effect and improve beam quality in lateral.

Fig.1 Schematic diagram of normal package(a) and adiabatic package(b)

3 Simulation Analysis

In this paper, we use Finite Element Analysis software ANSYS 18.0 to simulate semiconductor laser heat distribution. SolidWorks is used to build the

three-dimensional models of semiconductor laser, transition heat sink and solder. Then the models are imported into ANSYS 18.0. We divide the grids, set initial conditions and boundary conditions,apply loads. ANSYS 18.0 performs thermal analysis based on heat transfer equation, mainly based on Fourier’s law

(5)

Tab.1 Related material parameters

In the model, the active layer and solder are the key parts, which play a decisive role in the accuracy of calculation. The mesh of active region should be intensive to guarantee accuracy. The BeO ceramic transition heat sink’s mesh can use a coarse type to avoid over-dense meshing. In order to simplify the mathematical model, we take the following approximations：

(1) In practical application, the bottom of the transition heat sink contacts with the external refrigeratory. The transition heat sink can be seen as having a constant temperature of 25 ℃. Chip’s convection and thermal radiation with air are neglected.

(2) The semiconductor laser we simulate is of asymmetric structure.Flipping can reduce the thermal resistance and enhance the heat dissipation. Suppose the heat source is applied to the surface of P-plane.

(3) The effect of solder on waste heat is neglected.

(4) The change of thermal conductivity with temperature is neglected.

The paper analyzed the heat generation of laser in lateral (slow axis direction) direction with contact widthWof 100, 200, 300, 400, 500 μm. Through simulation, the temperature distribution under different contact widths and thermal power are obtained. With thermal power of 1.1 W, temperature distribution under contact widthW=100 μm is shown in Fig.2. The highest temperature locates at the center of active region, which is corresponding to the analysis in Section 1. Temperature distribution gradually decreases from center to edge in lasing surface. Temperature distribution data in slow axis are shown in Fig.3. When the device with contact widthW=500 μm, the working temperature rises from 25.69 ℃ to 27.09 ℃ from edge to center, the temperature has changed by 1.4 ℃. When the device with contact widthW=100 μm, the working temperature rises from 27.07 ℃ to 27.33 ℃ from edge to center, which has changed by only 0.26 ℃. Implement of air gap leads to a slightly increased junction temperature. As the contact width decreases, temperature uniformity of the chip along slow axis becomes better. And the uniformity of the refractive index distribution is positively correlated with the uniformity of the temperature distribution. According to Eq.(3) and Eq.(4), the uniformity of refractive index distribution is improved, and the focal length of equivalent thermal lens becomes larger, hence slow axis divergence angle becomes smaller.

Fig.2 Temperature distribution change with the contact widthW=100 μm

Fig.3 Temperature distribution of active region along slow axis at different contact widthsW

4 Experiments

The epitaxial structure of the 808 nm In0.08Ga0.78-Al0.14As quantum well Fabry-Perot cavity laser used in this experiment is shown in Fig.4. The cavity length is 900 μm, and the period is 500 μm. Indium solder is uesd to sinter and encapsulate. BeO ceramic is selected for heat sink metarial and plate thick gold to achieve good contact between chip and heat sink.

Adiabatic package structure is shown in Fig.5[13-14]. A scratch-off machine is used to modify the existing BeO ceramics and leave the protruding strips with width of 500, 400, 300, 200, 150 and 120 μm at the ceramic heat sink center to control the contact width between chip and heat sink. In Fig.5(b), W represents for contact width. Normal package structure is shown in Fig.5(c). Fig.6 is the front end view of package experiment diagram with adiabatic packaged chip withW=200 μm and normal packaged chip. The indium solder is coated on the transition heat sink boss to realize different contact widths. It can be seen in Fig.6 that there exist air gaps with different widths between chip and heat sink. The chip is electrically connected to heat sink by gold wire bonding, and the number of gold wires can meet the current requirement during normal operations.The broad-area semiconductor laser is of asymmetric structure, the thickness of N-type waveguide layer is larger than that of P-type. Heat generation area of the laser is mainly concentrated in the active region, which is close to the P-plane. Flipchip backpacking method was adopted in this experment. Fineplacet Mounter is used for adiabatic package.

Fig.4 Schematic diagram of the laser epitaxial structure

Fig.5 (a) Three dimensional view of adiabatic package structure. (b) The chip size of adiabatic package structure. (c) Lateral view of normal package structure.

Fig.6 Front end view of package experiment diagram with contact widthW=200 μm(a) and normal package(b)

Near field spot of semiconductor lasers is difficult to capture, thus far field characteristics are measured to describe beam quality. Beam quality of Gauss beams remains constant after lens transformation. Therefore in practical measurement, lens is usually implemented in propagation path to transform beam waist. It is convenient to deduce initial beam parameters by measuring the transformed beam parameters. The measurement methods that are used commonly include knife-edge method, sleeve method and CCD image acquisition and analysis method. The knife-edge method and sleeve method are easily affected by human factors in the actual measurement process, but the CCD image acquisition and analysis method usually contains less measurement errors which is mainly about camera noise, resolution error and lens spher error[15-16], and the error is less than 5% totally, which is within the acceptable range. The fast axis divergence angle is too large to be collected, so it is necessary to collimate fast axis light and compress the fast axis light beam with cylindrical lens. To prevent excessive output power from reaching the saturation of CCD image surface and burning it, attenuator and filter are added to optical path to ensure accuracy of measurement. Measuring light path is shown in Fig.7.

Fig.7 Optical path of laser beam quality measurement

After collimated by lenses, the beam will be under a measurable distribution state. By moving the position of CCD, spot image at different positions can be acquired, then the coordinates of each moving position and the corresponding spot size are recorded. According to ISO11146 standard, in order to ensure accuracy, at least 10 different positions should be measured. The numerical calculation software is used to fit the transmission beam equation and obtain the beam parameters.

5 Results and Discussions

Different from the heat flows from the center to edge in normal package, there is an air gap between the chip and the bottom heat sink in the adiabatic package. The air gap force nearly all heat flows to bottom heat sink through the convex platform, which enhances thermal conductivity of the laser chip in the central area.As the thermal conductivity of air is poor with only 0.02 W/(cm·K), and the thermal conductivity of BeO ceramic is 2.6 W/(cm·K). When laser is working, the central area generates more heat, and adiabatic package can improve the temperature uniformity in lateral and improve slow axis beam quality greatly.

5.1 Slow Axis Divergence Angle, BPP and M2 in Adiabatic Package and Normal Package

Fig.8 is the numerical fitting curve of slow axis divergence angle, BPP andM2of normal packaged chip and adiabatic packaged chip withW=120 μm at different working currentI0. As it can be seen from Fig.8, when the working currentI0rises from 0.4 A to 1.4 A, the slow axis divergence angle of adiabatic packaged chip rises from 2.9° to 4.9° with the variation of 2.0°, and BPP (Beam parameter product) rises from 5.2 mm·mrad to 8.5 mm·mrad with the variation of 3.3 mm·mrad. For normal packaged chip, when the working currentI0rises from 0.4 A to 1.4 A, the slow axis divergence angle rises from 5.7° to 8.4° with the variation of 2.7°, and BPP rises from 10.2 mm·mrad to 14.9 mm·mrad, and its variation is 4.5 mm·mrad. Thus, it can be concluded that slow axis divergence angle an BPP are reduced substantially while using adiabatic package method.

M2, the beam quality factor, also called the diffraction limit factor, is the ratio of actual BPP and ideal Gaussian BPP, which can describe the essential characteristics of light beam more comprehensive[17-18]. As it can be seen from Fig.8(c), for adiabatic packaged chip, when the working currentI0increases from 0.4 A to 1.4 A, the beam quality factorM2varies from 5.0 to 8.3 with the variation of 3.3. For normal packaged chip, when working currentI0increases from 0.4 A to 1.4 A, beam quality factorM2varies from 9.0 to 14.4 with the variation of 4.5. The larger theM2is, the more scattered the beam and the worse the beam quality will be. The adiabatic packaged chip is of better collimation, better focus and lower diffraction degree in slow axis of the far field. Moreover, with the increase of working currentI0, the variation ofM2becomes smaller, indicating the deterioration degree of beam quality is weakened.

As the working current increases, the chip temperature also increases accordingly, resulting in high non-uniformity and variations in slow axis temperature distribution. As a result, the focal length of thermal lens decreases, slow axis divergence extent increases, and beam quality deteriorates. Heat of normal packaged chip flows outward from center to edge, resulting in the decrease of temperature and refractive index. After implementing the air gap, the vertical heat that flow at the edge of the chip is blocked, and the heat flow is reversed, making almost all the heat flows to the bottom through convex platform of heat sink directly. In addition, the transverse heat flow makes the thermal conductivity in center better than that in the edge, homogenizing the temperature of the entire laser chip. Furthermore, it keeps the refractive index substantially constant until the edge, which weakens the thermal lens effect and improves the characteristics of the slow axis divergence. It can be seen from the experimental results that slow axis divergence angle has been reduced by about 40% by implementing adiabatic package and became more stable as the current changes. The corresponding BPP and beam quality factorM2can also be reduced by approximately 40%.

Fig.8 Numerical fitting curve of divergence angle(a), BPP(b) andM2(c)of slow axis with working current of adiabatic packaged chip and normal packaged chip.

5.2 Divergence Angle of Slow Axis, BPP and M2 of Adiabatic Packaged Chip with Different Working Currents

Fig.9 is the numerical fitting curve of slow axis divergence angle, BPP andM2of adiabatic packaged chip withW=120, 150, 200, 300, 400, 500 μm at different working currentI0.With the increase of working current, slow axis divergence angle, BPP andM2all show an increasing trend, indicating beam quality is deteriorated.When working current is small(near threshold), thermal lens effect caused by thermal power is not enough to offset the weakening of the optical field limitation caused by central carriers increment, and anti-waveguide effect is dominant.When the working current continues to increase, the refractive index guiding mechanisms dominate in the optical field limitation, and the thermal lens effect enhances obviously. The focal length of the equivalent thermal lens is shortened and the far-field divergence angle of the slow axis increased, and the beam quality is deteriorated.

Fig.9 Numerical fitting curve of slow axis divergence angle(a), BPP(b) andM2(c) of adiabatic packaged chip withW=120, 150, 200, 300, 400, 500 μm at different working currentI0.

5.3 Slow Axis Divergence Angle of Adiabatic Packaged Chip with Different Contact Widths

When working currentI0=1.0 A, the far field spot pattern of normal packaged chip and adiabatic packaged chip with different contact widths is shown in Fig.10. As contact width increases, the spot size also increases at the same time. Normal packaged chip has the largest spot size and the most dispersed outgoing beam. When the working current continues to increase, a multi-filament phenomenon occurs, and the beam quality is deteriorated severely.

Fig.11 sketches the relationship of slow axis divergence angles of adiabatic packaged lasers with contact widthsWof 120, 150, 200, 300, 400, 500 μm at different working currents. It can be depicted from Fig.11 that the slow axis divergence angle decreases with the decrease of the contact width, especially at high working current. WhenI0=0.6 A, the slow axis divergence angle of laser withW=120 μm is 3.85°, while the slow axis divergence angle of laser withW=500 μm is 5.68°, which is about 32% lower. BPP is 5.9 mm·mrad ofW=120 μm, while BPP is 9.9 mm·mrad withW=500 μm, which decreases by about 40%. Beam quality factorM2ofW=120 μm packaged chip is 5.7, while that ofW=500 μm packaged chip is 9.6. The slow axis divergence angle ofW=120 μm packaged chip is 4.91° atI0=1.4 A, while it is 8.01° withW=500 μm laser, which decreases by about 37%. The corresponding BPP andM2decrease 33% and 30% respectively.

With the decrease of the contact width of chip and the transition heat sink in slow axis direction, the width of the air gap increases. Heat conduction increases through central convex platform, and the transverse temperature distribution in active region becomes more uniform as the Fig.2 and Fig.3 show. The refractive index becomes more uniform, and thermal lens effect is mitigated. And slow axis divergence angle decreases correspondingly, which is consistent with the ANSYS 18.0 simulation analysis in previous Section 2. With the decrease of contact width, the slope of fitting curve of slow axis divergence angle, BPP andM2decreases correspondingly. The conclusion can be reached that the beam parameters mentioned above become more stable with the increase of working current. It is found that the maximum bearable working current is slightly reduced with the decrease of contact width. In practical applications, design should be considered comprehensively according to specific conditions.

Fig.10 Far field pattern of adiabatic packaged chip and normal packaged chip with different contact widths. (a)W=120 μm. (b)W=150 μm. (c)W=200 μm. (d)W=300 μm. (e)W=400 μm. (f)W=500 μm. (g) Normal package.

Fig.11 Variation of slow axis divergence angle with different contact widths

5.4 Photoelectric Characteristics of Adiabatic Packaged Chip with Different Contact Widths

Fig.12 shows theP-Icharacteristic curve of the laser chip with different contact widths. When working currentI0is small, there is subtle difference among the adiabatic packaged chips with different contact widths. As the working current increases, the difference begins to increase. When working currentI0=0.4 A, its output powerP=0.15 W of the chip withW=120 μm, and itsP=0.23 W of the chip withW=500 μm. The difference is 0.08 W. When working currentI0=1.4 A, its output powerP=1.29 W of the chip withW=120 μm, and itsP=1.5 W of the chip withW=500 μm. The difference is 0.21 W,Pis reduced by about 14%.

Fig.12P-Icharacteristic curves of the laser chip with different contact widths

Fig.13 shows thePth-Icharacteristic curve of the laser chip with different contact widths. As the thermal conductivity of air is poor of only 0.02 W/(cm·K), and the thermal conductivity of BeO ceramic is 2.6 W/(cm·K). The introduction of air gap will reduce the heat dissipation capability of the laser COS to a certain degree. Thus thermal powerPthincreases, and electro-optical conversion efficiencyηdecreases correspondingly. When working currentI0is small, there is little difference among the adiabatic packaged chips with different contact widths. As the working current increases, the differences gradually increase.WhenI0=1.4 A, for an adiabatic packaged laser chip with contact widthW=500 μm, its thermal powerPth=1.15 W, the electro-optical conversion efficiencyη=56.6%.And itsPth=1.40 W of the chip withW=120 μm, and itsη=47.9%, the electro-optical conversion efficiencyηreduces by 8.7%. The effect is within acceptable range. The experimental results are consistent with the simulation results in the Section 2.

Fig.13Pth-Icharacteristic curves of the laser chip with different contact widths

6 Conclusion

In this paper, the causes of thermal lens effect and its influence on beam quality are analyzed. The method of adiabatic package is proposed and analyzed theoretically. Based on thermal distribution numerical simulation of ANSYS18.0, the thermal lens condition of laser working is analyzed. In this paper, the thermal lens effect and the characteristic of 808 nm broad-area semiconductor lasers using adiabatic package method are theoretically and experimentally studied. Charge coupled device (CCD) image acquisition and analysis method are adopted to measure beam quality.

The simulation results indicate that when thermal power is 1.1 W and the contact width decreases from 500 μm to 100 μm, the working temperature difference is reduced from 1.4 ℃ to 0.26 ℃ in slow axis direction from sides to the center. Temperature uniformity along slow axis is improved, so does the refractive index distribution uniformity. The focal length of the equivalent thermal lens increases, and the slow axis divergence angle decreases. The experimental results show that when working currentI0is 1.4 A, the slow axis divergence angle decreases by about 40%, and it changes more steadily with current. The corresponding BPP andM2also decrease, which proves the improvement of beam quality. The introduction of the air gap in the adiabatic package leads to a decrease in heat dissipation capability. For adiabatic packaged chip with contact widthW=120 μm, output powerPreduces by 14%, electro-optical conversion efficiencyηreduces by 8.7%. The contact widthWshould be selected according to practical applications. The adiabatic package method has important guiding significance for improving slow axis beam quality of 808 nm broad-area semiconductor lasers.