QIU Bo-cang, MARTIN Hai HU, WANG Wei-min, LIU Wen-bin, BAI Xue

(1.Research Institute of Tsinghua University in Shenzhen,Shenzhen 518057,China;2.Shenzhen Raybow Optoelectronics Corp,Shenzhen 518055,China;3.Guangdong Provincial Key Laboratory of Optomechatronics,Shenzhen 518057,China)

Abstract: In this paper, a high efficiency and high reliability 915 nm semiconductor laser is designed and fabricated, which is a key component of the fiber lasers. In order to maximize the electro-optic conversion efficiency of the device, a double asymmetric large-cavity waveguide structure is adopted in the design, and the quantum well structure, waveguide structure, doping, and device structure are systematically optimized. Device simulations show that the device′s maximum electro-optical conversion efficiency reaches 67% at an ambient temperature of 25 ℃. The material is grown by Metal Organic Chemical Vapor Deposition(MOCVD), and a laser chip having a light emitting region width of 95 μm and a cavity length of 4.8 mm is prepared. Tests show that the efficiency of the packaged device and other parameter indices have reached the advanced level of similar devices in the world. In the case that the threshold current is 1 A at room temperature, the slope efficiency is 1.18 W/A, the maximum electro-optic conversion efficiency is 66.5%, the output power is 12 W, and the electro-optical conversion efficiency reaches 64.3%. It can be seen that the test results are in good agreement with that of the device theory simulation. After approximately 6 000 hours of long-life accelerated testing, the device power does not attenuate, indicating that the produced high-power 915 nm laser chip has very high reliability.

Key words: semiconductor laser;electro-optical conversion efficiency;brightness;cavity surface catastrophic power

1 Introduction

引 言

The performance of ytterbium-doped fiber laser(YDFL) has improved dramatically over the past 20 years and its output power has increased at an average rate of 170% per year. In 2009, Stile reported that IPG used large-mode single-mode fiber(large mode area:LMA) by IPG to obtain 10 000 watts of output power[1]. Due to the outstanding characteristics of fiber lasers, including output power, beam quality, easy system integration with industrial equipment systems, laser industrial processing equipment with fiber lasers as its core components has been widely used in various industrial manufacturing processes[2-3]. YDFL has a very broad absorption spectrum (approximately 900-977 nm), and its pump source wavelength is usually set at about 915 nm[4]. Although high-power 915 nm semiconductor lasers are quite mature, industrial and academic circles are still working hard to continuously improve the parameters and the performance of the devices, including beam quality, polarization characteristics, output power, and electro-optical conversion efficiency[5]. Muritaetal. from Hamamatsu Photonics Japan reported high-electron-to-optical conversion efficiency of 976 nm(96 μm) laser in 2013. The device has a maximum electro-optic conversion efficiency of 68% (corresponding to an output power of 6.3 W) at a test environment of 20 ℃, and the efficiency is reduced to 61% at an output power of 15 W[6]. Crump et al. from FBI Research Institute Germany reported that the 96 μm-wide laser had a lifetime test of more than 4 000 hours at 20 W output power[7]. Literature[8-9]reported that the efficiency of a 976 nm device at room temperature at 10 W output power reached 65%. In addition to output power and efficiency, device reliability is also a very important design consideration. Although the reliability of the device is related to many factors, including junction temperature, current density, and optical power density, the optical power density has a more direct effect on the reliability of the device, especially the reliability related to the facet failure. In order to improve the reliability of the device operation, several methods are used, such as low power density design[10], non-absorption window facet technology[11]. In this paper, for the requirements of high power and high electro-optic conversion efficiency and high reliability, we adopt the design concept of double asymmetric large optical cavity[7,12-13]and that of low power density, and systematically optimize the device′s material structure, including the waveguide structure, doping profile, device cavity length, facet reflectance,etc. to improve the device efficiency. Tests have shown that a device with a width of 95 μm and a cavity length of 4.8 mm can reliably operate at 12 W with a threshold current of approximately 1 A, a slope efficiency of 1.18 W/A. When the output power is 12 W, the corresponding electro-optic conversion efficiency is 64.3%, while the highest conversion efficiency is 66.5%(corresponding to 8.9 W of output power). The device′s energy conversion efficiency and other major parameter levels have reached the level of international devices of the same type.

摻有稀土元素鐿的光纖激光器(YDFL)性能在過去20余年來取得了驚人的提升，其輸出功率的提升速率平均每年達到170%，2009年，Stile報告了IPG公司采用大模場單模光纖(large mode area,LMA)獲得了一萬瓦的輸出功率[1]。由于光纖激光的優(yōu)異特性，包括：輸出功率、光束質量、易于和工業(yè)設備系統(tǒng)進行系統(tǒng)集成，以光纖激光器為其核心部件的激光工業(yè)加工設備已經大規(guī)模應用于各種工業(yè)制造過程中[2-3]。YDFL的吸收譜極為寬廣(大約900～977 nm)，其泵浦源的波長通常選在915 nm左右[4]。雖然高功率915 nm 半導體激光已經相當成熟，然而工業(yè)與學術界仍在不斷努力，以期持續(xù)改進器件的參數(shù)性能指標，包括：光束質量、偏振特性、輸出功率以及電光轉換效率等[5]。2013年，日本Hamamatsu Photonics公司的Morita等人報導了高電光轉換效率的976 nm寬條(96 μm)激光，器件在20℃測試環(huán)境下，最大電光轉換效率為68%(對應于輸出功率6.3 W)，在輸出功率為15 W時，效率降低到61%[6]。2009年，德國FBI研究所的Crump等人報導了96 μm條寬的激光器在20 W輸出功率時壽命測試超過4 000 h[7]。文獻[8-9]報導了976 nm器件在室溫下10 W輸出功率時效率達到65%。除了輸出功率與效率，器件的可靠性也是非常重要的設計考慮。盡管器件的可靠性與多種因素有關，包括結溫、電流密度以及光功率密度等，但光功率密度對器件的可靠性特別是與腔面失效有關的可靠性影響更為直接。為了改善器件運行的可靠性，人們采用若干方法，如低功率密度設計[10]、無吸收腔面技術等[11]。針對高功率以及高電光轉換效率以及高可靠性這一要求，本文采用了雙非對稱大光腔結構[7,12-13]、低功率密度設計理念，并系統(tǒng)地優(yōu)化了器件的材料結構，包括波導結構、摻雜分布、器件腔長、腔面反射率等來提高器件效率。實驗測試表明，95 μm條寬、4.8 mm腔長的器件可以可靠地工作在12 W，閾值電流大約為1 A，斜率效率為1.18 W/A，輸出功率為12 W時所對應的電光轉換效率為64.3%，而最高轉換效率則為66.5%(對應于輸出功率8.9 W)。器件的能量轉換效率以及其它主要參數(shù)水平達到了國際同類器件的水平。

2 Epitaxial structure design

外延結構設計

Semiconductor epitaxial structure design involves the selection of quantum wells, waveguide design, and doping optimization. For 915-nm lasers, the quantum wells can use 5-8 nm compressive strain material In(x)GaAs, and the content of In component needs to be adjusted according to the thickness of the used quantum well to ensure that the lasing wavelength is around 915 nm. In addition to the quantum well material, the height of the barrier has a significant influence on the performance of the device because the barrier height affects the internal quantum efficiency and temperature characteristics of the device. In order to examine the exact relationship between the barrier height and the quantum efficiency within the device, a numerical model of internal quantum efficiency based on traveling wave amplification is developed. This model considers the photon flux density at different locations in the resonator, the carrier concentration in the quantum wells, thermionic emission,etc., to calculate the number of radiatively recombined carriers among. Fig.1 shows the relationship between the internal quantum efficiency and the barrier Al content based on our traveling wave amplification model. It can be found from the calculation that for the 915 nm high-power laser, the internal quantum efficiency reaches 99% when the content of the Al component in the Al(x)GaAs material exceeds 0.23. Based on the above calculation, the quantum well structure we selected is In0.10GaAs/Al0.23GaAs.

半導體外延結構設計涉及到量子阱的選取、波導設計以及摻雜優(yōu)化等。對于915 nm激光來說，量子阱可選用5～8 nm的壓應變材料In(x)GaAs材料，其中In組份含量需要根據(jù)所采用的量子阱厚度做必要的調整，以保證激射波長為915 nm 左右。除量子阱材料外，勢壘的高度對器件的性能影響至關重要，因為勢壘高度影響器件的內量子效率以及溫度特性。為了考察勢壘高度與器件內量子效率之間的精確關系，我們發(fā)展出一種基于行波放大的內量子效率計算模型，模型中考慮了在諧振腔中不同位置的光子通量密度、量子阱內的載流子濃度、熱電子發(fā)射等，從而計算出受激復合的載流子占注入載流子的比例。該計算模型是我們自己首次提出的一種計算方法，尚未查閱到類似的文獻。圖1是根據(jù)我們的行波放大模型計算的內量子效率與勢壘Al組份含量之間的關系。由計算可見，對于915 nm 高功率激光來說，當Al(x)GaAs材料中Al組份含量超過0.23時，內量子效率達到99%?；谏鲜鲇嬎悖覀冞x取的量子阱結構為In0.10GaAs/Al0.23GaAs。

Fig.1 Relationship between internal quantum efficiency and Al component content of quantum well barrier materials 圖1 內量子效率與量子阱勢壘材料Al組份含量之間的關系

After the quantum well material is determined, the material structure design should be performed according to actual application requirements, and its main purpose is to find a waveguide structure that satisfies the laser operating parameters(such as a suitable quantum well confinement factor). This is because the material structure of the semiconductor laser is essentially a typical one-dimensional waveguide structure,i.e., a low refractive index under clad layer, a high refractive index waveguide core, and a low refractive index upper cladding layer in the material growth direction in turn. In this way, light waves can be transmitted in a guided wave manner in the growth plane. The waveguide structure generally has two different design ideas: one is a traditional narrow-waveguide single-mode design[10,14], and the other is a popular large-cavity structure design in high-power lasers. The advantage of the large-cavity structure is that the optical loss can be designed to be very low because of the small overlap of the light field and the material doped region, so that free carrier absorption(FCA)[15]and inter-valence band absorption(IVBA)[16]can be minimized. The waveguide design calculates the mode field distribution of light in the waveguide in the vertical direction(i.e., growth direction) according to the refractive index distribution of the given epitaxial structure. Then, the parameters related to the device performance including the quantum well confinement factor Г, far field distribution, normalized power density,etc. are calculated by the mode field distribution. The mode field distribution in the optical waveguide needs to solve the following Helmholtz equation derived from the Maxwell electromagnetic equations:

量子阱材料確定后，需要根據(jù)實際應用需求進行材料結構設計，其主要目的是找出滿足激光工作參數(shù)(如合適的量子阱限制因子)的波導結構。這是因為半導體激光器的材料結構本質上是一典型的一維波導結構，即在材料生長方向上，先后是低折射率的下包層，高折射率的波導核以及低折射率的上包層。這樣光波可以在生長面內以導波的方式進行傳播。波導結構一般有兩種不同的設計思路：其一為傳統(tǒng)的窄波導單模設計[10,14]，其二是在高功率激光中頗為流行的大光腔結構設計。大光腔結構的優(yōu)點是光損耗可以被設計得非常低，因為光場和材料摻雜區(qū)域重疊很小，如此一來，自由載流子吸收(FCA)[15]以及價帶間的空穴躍遷引起的吸收(IVBA)[16]可以被降低到最小程度。波導設計是根據(jù)給定的外延結構的折射率分布計算光在波導中垂直方向(亦即生長方向)的模場分布，然后通過模場分布計算出與器件性能相關的參數(shù)包括量子阱限制因子Г、遠場分布、歸一化的功率密度等。光波導中的模場分布需要求解從麥克斯韋爾電磁方程組推導而來的下述亥爾姆霍爾茲方程：

(1)

This equation is actually an eigenvalue equation, which means that the effective refractive index of the waveguide mode can only take a specific discrete value, and each eigenvalue(effective refractive index value) corresponds to a mode field distribution. For a single mode waveguide, only the fundamental mode exists, and correspondingly, there are multiple waveguide modes for the multimode waveguide.

這一方程式實際上是一本征值方程，意味著波導模式的有效折射率只能取特定的分立值，每一個本征值(有效折射率值)對應一個模場分布。對于單模波導來說，只存在基模，與此相對應，多模波導存在多個波導模式。

(2)

The threshold current of chip can be expressed as:

而芯片的閾值電流可以表示為：

(3)

WhereNthis the carrier concentration in the quantum well when the threshold condition is reached,eis the electron charge amount;A,B,Care non-radiative recombination coefficient, spontaneous emission coefficient, and Auger recombination coefficient, respectively. By comparing with a large number of experimental test data, we fit theA,B,Ccoefficient values at 915 nm to approximately 5×10-7s-1, 7×10-10cm3/s, 1×10-30cm6/s, respectively.d,w,Lare the quantum well thickness, the width of the chip current injection region, and the cavity length, respectively, andηis the carrier quantum well implantation efficiency, which can be further expressed asη=1-ηe,ηeis the probability that carriers escape the quantum well. The following equation solves A in equation (3):

式中，Nth為到達閾值條件時量子阱內的載流子濃度，e為電子電荷量，A、B、C分別為非輻射復合、自發(fā)輻射以及俄歇復合系數(shù)。通過與大量實驗測試數(shù)據(jù)比較， 我們擬合出915 nm 時的A、B、C系數(shù)取值分別大約為5×10-7s-1、7×10-10cm3/s、1×10-30cm6/s。d、w、L分別為量子阱厚度、芯片電流注入區(qū)寬度以及腔長，η為載流子的量子阱注入效率，可以進一步表示為η=1-ηe，ηe為載流子逃逸出量子阱的幾率。式(3)中的Nth需要求解下方程：

Γg(Nth)=α-ln(RARH)/(2L) .

(4)

Equation (4) describes the device reaching the threshold condition when the gain and loss obtained are balanced and the photons go back and forth within the cavity, whereg(Nth) is the material gain of the quantum well. The calculation can be referred to Ref.[17].αis the cavity loss,RAandRHare the reflectivity of the two facets. The left side of equation (4) is the mode gain of the light, and the first term on the right side of the equation is the internal loss(we will then see the dependence of the loss on material doping). The second term is loss related to the mirror loss.

方程(4)意味著當光子在腔內往返一周時所獲得的增益與損耗達到平衡時，器件達到閾值條件，式中，g(Nth)是量子阱的材料增益，其計算可以參考文獻[17]，α為腔內光損耗，RA、RH分別為兩個腔面鍍膜的反射率。式(4)等式左邊為光的模式增益，而等式右邊第一項為腔內光損耗(隨后我們將會看到損耗與材料摻雜之間的依賴關系)，第二項為與腔面透光有關的損耗。

The divergence angle of the beam, that is, the far field distribution and the near field distribution constitute a Fourier transform, and it is also necessary to consider the refraction of light at the interface between the semiconductor and the air. Fig.2-Fig.4 shows the relationship among quantum well confinement factor, beam divergence angle(full width at half maximum(FWHM)), normalized power density and the total waveguide thickness(the total thickness of the SCH layer) in the case of symmetric waveguides where the aluminum component of the waveguide layer is 0.06, 0.09, and 0.12 respectively higher than the aluminum component of the cladding layer. In this paper, the normalized power density refers to the peak power density corresponding to a 100 μm wide active area chip output power of 10 W. It can be seen from the figure that all three parameters obtain the highest value at a certain SCH thickness and after the highest point, the above parameters decrease with the increase of the SCH thickness. The reason for the existence of the maximum value of the quantum well confinement factor Г is that, as the thickness of the SCH increases, the light-restricting ability of the waveguide increases, and the mode field value of the waveguide at the quantum well increases. After reaching the highest point, the thickness of the SCH increases further. As a result, the mode field size increases, eventually resulting in a decrease in the field value of the mode at the quantum well. It should be pointed out that, unlike low-power communication laser chips, the design of high-power laser chips mainly considers whether the highest possible output power can be obtained, and the actual output power is limited by the highest power density that the bulk material and the facet coating can withstand. Therefore, in a high-power semiconductor laser design, the normalized power density value should be reduced as much as possible. In this paper, the normalized power density value is about 10 MW/cm2.

光束發(fā)散角亦即遠場分布與近場分布構成傅里葉變換關系，同時還需要考慮到光在半導體與空氣界面處的光折射。圖2～圖4為在對稱波導情形時，波導層的鋁組份分別比包層鋁組份高0.06、0.09和0.12三種情況下的量子阱限制因子、光束發(fā)散角 (半高全寬值：FWHM)、以及歸一化功率密度與波導總厚度(SCH層總厚度)之間的關系。在本論文中，歸一化功率密度是指100 μm條寬的有源區(qū)芯片輸出功率為10 W時所對應的峰值功率密度。由圖可見，3個參量均在某一SCH厚度處取得最高值，過了最高點后，上述參量隨SCH厚度的增加而減小。量子阱限制因子Г存在最大值的原因在于：隨著SCH厚度的增加，波導對光的限制能力隨之增強，在量子阱處的波導模場值增大，當達到最高點后，SCH厚度的進一步增加，導致了模場尺寸的增大，從而導致了量子阱處的模式的場值的減小。需要指出的是，不同于低功率通訊激光芯片，高功率激光芯片設計的主要考慮在于獲得盡可能高的輸出功率，而實際的輸出功率受芯片材料以及腔面鍍膜材料所能承受的最高功率密度限制，所以實際上在高功率半導體激光設計中，應盡可能降低歸一化的功率密度值。在我們的設計中我們取歸一化功率密度值為10 MW/cm2左右。

Fig.2 Quantum well confinement factor versus total SCH thickness 圖2 量子阱限制因子與SCH總厚度之間的關系

Fig.3 Beam divergence(FWHM) versus total SCH thickness 圖3 光束發(fā)散角(FWHM)與SCH厚度之間的關系

Fig.4 Normalized power density versus total SCH thickness 圖4 歸一化的功率密度與SCH厚度之間的關系

It should be noted that Fig.2-Fig.4 are calculated for symmetric waveguides. In order to increase the efficiency of the device as much as possible, we adopt a dual asymmetrical waveguide design based on the calculation of symmetric waveguides, that is:1)the aluminum composition of the P-type AlGaAs material is higher than that of the N-type AlGaAs material; 2)the quantum well is placed asymmetrically in the SCH waveguide as shown in Fig.5. Fig.5-Fig.6 show the calculated waveguide near-field and far-field distributions. From the near-field distribution, it can be seen that about 72% of the light field is on the side of the N-type region. The asymmetrical distribution of the light field allows us to simultaneously reduce internal losses and electrical resistance to a minimum. This is because considering light absorption, the cross-section of electrons is only about half of that of holes, and the mobility of electrons in Al(x)GaAs(whenx<0.45) is much higher than that of holes.

需要指出的是，圖2～圖4是針對對稱波導進行計算的。為了盡可能提高器件的效率，我們在對稱波導計算的基礎上，采用了雙非對稱波導設計：即(1)P型AlGaAs材料的鋁組份比N型AlGaAs材料的鋁組份要高一些；(2)量子阱在SCH波導中是非對稱的放置，如圖5所示。圖5～圖6給出了計算的波導近場與遠場分布。從近場分布可以看出，大約72%的光場處于N型區(qū)域一邊。光場的非對稱分布使得我們能夠同時將腔內損耗以及芯片電阻降低到最小程度。這是因為，電子對光的吸收截面大約只有空穴對光的吸收截面的一半左右，另外，電子在Al(x)GaAs材料中(當x<0.45)的遷移率要遠遠高于空穴的遷移率。

Fig.5 Double unsymmetrical waveguide structure(line 1) and corresponding light field distribution(line 2) 圖5 雙非對稱波導結構(線1)以及對應的光場分布(線2)

Fig.6 Profile of far-field distribution 圖6 遠場分布計算結果

After optimization of the waveguide, doping optimization should be considered because doping not only affects the series resistance of the chip but also affects the optical loss of the laser resonator. For shallowly etched waveguide, the scattering loss of the light at the material interface is negligible, so that the internal optical loss is completely determined by the absorption of free carriers and that of holes in the valence band transition. This means that as long as electrons and holes exist within the scope of the light field, absorption by the above mechanism will occur. Thus, the optical loss of the semiconductor laser is composed of three parts:one is the light absorption caused by electrons and holes in the quantum well, because the carrier concentration in the quantum well generally exceeds 1×1018cm-3during normal operation of the device; the second is the electron-induced absorption of light in the n-type region; the third is the light absorption caused by holes in the p-type doped region. Therefore, the total light loss and the resistance per unit area of the material and doping distribution can be expressed as:

波導優(yōu)化完畢之后，隨后的設計考慮是摻雜優(yōu)化，因為摻雜不僅影響芯片的串聯(lián)電阻，而且也影響激光諧振腔的光損耗。在波導為淺刻蝕情形下，光在材料界面處的散射損耗可以忽略不計，從而腔內的光損耗完全取決于自由載流子吸收以及空穴在價帶間躍遷引起的吸收，這就意味著在光場所及的范圍內，只要存在電子與空穴，上述機理引起的吸收就會發(fā)生?；谏鲜隼碛?，半導體激光器的光損耗由三部分組成：其一為量子阱內的電子與空穴引起的光吸收，因為在器件正常工作時，量子阱內的載流子濃度一般會超過1×1018cm-3；其二為n型區(qū)域的電子引起的光吸收；其三為p型摻雜區(qū)域內的空穴引起的光吸收。所以，光的總損耗以及材料的單位面積電阻與摻雜分布可表示為：

(5)

(6)

WhereΓis quantum well confinement factor,Nthis the carrier concentration in the quantum well when the device reaches the threshold,I(x)(the dimension is cm-1) is a normalized light field distribution whose integral over the entire growth direction is 1, indicating that the intensity distribution of the light field in the material growth direction, which is proportional to the square of the field strengthE, and the field strength can be obtained by solving the aforementioned equation (1).Cfc,Civba(in cm2) are the absorption cross-sections of the free carriers electrons and holes, respectively,Nd,Naare the doping concentration distributions of donor and acceptor atoms,μe,μhare the mobility of electrons and holes in the material,eis the electron charge. In equation (5), the first term indicates light absorption due to carriers in the quantum well, and the second and third terms are the optical losses in the n-type doped region and the p-type doped region, respectively. In equation (6), the first and second terms are the resistances of the n-type region and the p-type region, respectively. The actual calculations show that for a 915 nm high-power laser structure using a large optical cavity, the light loss caused by the doping of the epitaxial layer can be reduced to below 0.2/cm through doping optimization, and the resistance per unit area of the device can be reduced to below 6.0×10-5Ω·cm2.

式中，Γ為量子阱限制因子，Nth為器件達到閾值時量子阱內的載流子濃度，I(x)(量綱為cm-1)是歸一化的光場分布，其在整個生長方向上的積分為1，表示光場在材料生長方向上的強度分布，與場強E的平方成正比，而場強可以由求解前述的方程(1)而得到。Cfc、Civba(單位為cm2)分別為自由載流子電子與空穴的吸收截面，Nd、Na分別為施主與受主原子的摻雜濃度分布，μe、μh分別為電子與空穴在材料中的遷移率，e為電子電荷量。在(5)式中，第一項表示量子阱內的載流子引起的光吸收，第二與第三項分別為n型摻雜區(qū)域與p型摻雜區(qū)域內的光損耗。在式(6)中，第一與第二項分別是n型區(qū)域與p型區(qū)域的電阻。實際計算表明，對于采用大光腔的915 nm高功率激光結構，經過摻雜優(yōu)化，外延層摻雜引起的光損耗α可以減小到0.2/cm以下，同時器件的單位面積電阻可以降低到6.0×10-5Ω·cm2以下。

Appropriate cavity length for the required output power is to be calculated by device design. Careful consideration has been given to thermal issues during packaging for cavity length calculations. A three-dimensional numerical analysis method is adopted to analyze the heat dissipation typically used for packaging of 915 nm fiber laser pump modules to calculate the thermal resistance of the entire package module. The calculation assumes that the chip′s electrical injection region has a width of 95 μm and is soldered to the AlN heat sink with a thickness of 350 μm(this type of package is called COS) in the form of P-face down. The AlN heat sink is then welded on a 2 mm thick copper block. Equations (7) and (8) give the relationship of the thermal resistance of the package module varies with the length of the laser cavity and the corresponding change in the junction temperature, respectively:

器件設計主要計算對于要求的輸出功率時所需要的合適腔長。腔長計算時需要仔細考慮封裝時的熱問題。我們采用三維數(shù)值分析方法分析了典型的用于915 nm 光纖激光泵浦模塊熱封裝的熱場分布，從中計算出整個封裝模塊的熱阻。在計算中，假定芯片的電注入區(qū)寬度為95 μm并以P面朝下的形式被焊接在厚度為350 μm的AlN熱沉上(這一封裝形式被稱之為COS)，隨后AlN熱沉再被焊接在2 mm厚的銅塊上。式(7)與式(8)分別給出了上述封裝模塊的熱阻隨激光器腔長的變化關系以及對應的結溫變化：

(7)

ΔT=RtQ,

(8)

whereLis the cavity length of the laser which is given in centimeters. The dimension of the thermal resistance is K/W,Qis the thermal power(in Watt) generated by the chip. ΔTis the chip junction temperature rises, which is given in K. After the high-power laser is fabricated, the facets are coated as required. The reflectivity of the rear facet coating is usually greater than 95%. The reflectivity of the front facet coating(i.e., the light exiting facet) is optimized based on the epitaxial structure, device cavity length, and light output power. This is because the reflectance affects both the device′s threshold current and the external quantum efficiency. Using the self-developed semiconductor laser design software, the dependences of device′s electro-optical conversion efficiency(WPE) and the output power on the device cavity length and front-facet reflectivity at an injected current of 12 A and at room temperature at 25 ℃ are calculated(see Fig.7-Fig.8). In this calculation model, the calculations to be performed include:discrete energy levels in quantum well structures and their corresponding quantum wells, material gain and spontaneous emission coefficient, optical waveguide and its quantum well limiting factor, and heat dissipation of packaged modules and corresponding thermal resistance, light-current-voltage(L-I-V) characteristics,etc. From the simulation results, it can be seen that the electro-optical conversion efficiency at a cavity length of 4.8 mm reaches 67% when the AR film reflectivity is about 1%, and the output power exceeds 13 W. In the case of the fixed front-facet reflectance, due to the interaction between the parameters of mirror loss, electrical resistance and thermal resistance, the output power will decreases as the cavity length increases, while the electro-optical conversion efficiency increases as the cavity length increases.

式中，L為激光器的腔長，單位為厘米，熱阻的量綱為K/W，Q為芯片產生的熱功率，單位為瓦，ΔT為芯片結溫升高量，單位為K。高功率激光器制作完畢后， 需要根據(jù)需要對腔面進行鍍膜處理，后腔面的反射率通常大于95%，而前腔面(即出光面)的反射率需要根據(jù)外延結構、器件腔長以及出光功率大小進行優(yōu)化。這是因為腔面反射率會同時影響器件的閾值電流以及外量子效率。利用我們自己研發(fā)出的半導體激光器設計軟件，我們計算了器件在25 ℃室溫下，當注入電流為12 A時的電光轉換效率(WPE)以及輸出功率與前腔面抗反射膜(antireflection coating)反射率和腔長之間的關系(見圖7，圖8)。在我們的計算模型中，所需要進行的計算包括：量子阱結構及其對應的量子阱內的分立能級計算，材料增益以及自發(fā)輻射系數(shù)計算、光波導及其量子阱限制因子計算、封裝模塊的熱分析及其對應的熱阻計算，光—電流—電壓(L-I-V)特性計算等。由仿真計算可以看出，腔長為4.8 mm 時的電光轉換效率在AR膜反射率為1% 左右時達到67%，而輸出功率超過13 W。在相同AR膜反射率情況下，因為腔面光提取效率、電阻以及熱阻等參量之間的相互作用，導致輸出功率隨腔長增加而減小，而電光轉換效率卻隨腔長增加而增大。

Fig.7 Relationship between output power, cavity length and reflectivity of antireflection(AR) layer 圖7 輸出功率與腔長以及AR膜反射率之間的關系

Fig.8 Relationship between electro-optical conversion efficiency, cavity length, and reflectivity of antireflection(AR) layer 圖8 電光轉換效率與腔長以及AR膜反射率之間的關系

3 Material growth,process production and testing

材料生長,器件制作與測試

The epitaxial material is grown by metal organic vapor deposition(MOCVD). The quantum well is an In0.10GaAs with a thickness of about 7 nm and the barrier is an Al0.23GaAs material. The thickness of the entire SCH layer is 1.5 μm. The device process begins with the definition of the waveguide by optical exposure, followed by electrical contact windows, P-side metallization, wafer thinning, N-side metallization, rapid annealing, facet passivation and coating etc. In order to enable the device to operate as a continuous wave at high currents, we soldered the device on a 350 μm thick AlN ceramic with P side down, and 30 gold wires are welded on the N side. Fig.9 is a photo-current(LI) characteristic curve and a corresponding electro-optical conversion efficiency curve measured at a continuous current mode(CW) at a temperature of 20 ℃, which consists of the proportion of the threshold current in the overall injected current, the Joule heating, and the thermal, optical, and electrical interactions. It can be seen that, at 20 ℃, the COS threshold current is about 1 A, the slope efficiency is about 1.18 W/A, and the operating current required to reach 12 W output power is 11.5 A. The highest electro-optic conversion efficiency is 66.5%. When the output power is 12 W, the electro-optical conversion efficiency is reduced to 64.3%. The above test results are in good agreement with the simulation in Fig.7-Fig.8, which shows that this design software can accurately predict the performance of the device. In order to find the facet damage threshold of the device, a destructive test on the device under high current is conducted. The test conditions are 20 ℃ quasi-continuous-wave(QCW) current mode and 40 ℃ continuous current mode. In the QCW test, the pulse width is 1 ms and the repetition rate is 1 KHz. During the test, the current increase continuously until the sudden loss of power due to facet damage. The test results are shown in Fig.10. As can be seen from Fig.10, the facet COD power value is 23.8 W under the QCW condition, and the facet damage threshold power becomes 18.1 W during the CW test. Fig.11 shows the relationship between the divergence angle of the beam and the injected current when the energy contained in the beam is 95% of the total energy. It can be seen from the figure that in the process of increasing the current from 2 A to 12 A, the far field in the vertical direction remains basically constant. However, the horizontal far field increases with the increase of the current. This change is related to various mechanisms, including the thermal lens effect and the spatial hole burning effect of carriers. Since the reliability of lasers is critical for industrial applications, we have performed life testing of COS devices under over-current conditions[18]. The test conditions are as follows:temperature 35 ℃, 14 A constant current. The output power is 14 W under this condition. Fig.12 shows the recording of the power of the eight COS devices over time under the above test conditions. It is clear that the power of the device did not change during the life test of about 6 000 hours, indicating that our device works reliably even in high temperature(35 ℃) and high current(14 A) and high power(14 W).

Fig.9 Photocurrent curve and electro-optical conversion efficiency curve of 915 nm single-tube COS module at 20 ℃ 圖9 915 nm單管COS模塊在20 ℃下的光—電流曲線與電光轉換效率曲線

Fig.10 Cavity damage power tested under quasi-continuous(QCW) and continuous(CW) conditions 圖10 在準持續(xù)(QCW)以及持續(xù)(CW)條件下測試的腔面損傷功率

Fig.11 Beam divergence angle corresponding to 95% of the total energy 圖11 95%能量所對應的光束發(fā)散角值

Fig.12 Life test record(device initial power 14 W, test temperature 35 ℃, current 14 A)(colour figures is availabe in electro-version) 圖12 壽命測試記錄(器件初始功率為14 W，測試溫度為35 ℃，電流為14 A)(見彩圖)

外延材料用金屬有機氣相沉積(MOCVD)生長而成。量子阱為7 nm左右的In0.10GaAs，而勢壘為Al0.23GaAs材料，整個SCH層的厚度為1.5 μm。器件工藝以光學曝光以定義波導為起點，隨后有電接觸窗口、P面金屬化、晶片減薄、N面金屬化、快速退火以及解理鈍化與腔面鍍膜等工序。為了使得器件能夠在高電流下以連續(xù)波形式工作，我們將器件以P面朝下的方式焊接在350 μm厚的AlN陶瓷上，隨后在N面打上30根金絲線。圖9為在20 ℃溫度下以持續(xù)電流方式(CW)測試的光—電流(L-I)特性曲線以及對應的電光轉換效率曲線，該典型效率曲線是由閾值電流在整體注入電流中的比例、焦耳熱以及熱、光、電相互作用構成的。由其可見，COS在20 ℃測試環(huán)境下，閾值電流大約為1 A，斜率效率大約為1.18 W/A，而達到12 W輸出功率時所需要的工作電流為11.5 A。電光轉換效率最高值為66.5%，當輸出功率為12 W時，電光轉換效率減小到為64.3%，上述測試結果與圖7、圖8的模擬結果非常吻合，說明我們的設計軟件能夠準確預測器件的性能。為了找出器件的腔面損傷閾值，我們對器件在高電流下進行了破壞性測試，測試條件分別為20 ℃準持續(xù)(QCW)電流模式以及40 ℃持續(xù)電流模式。在QCW測試時，脈寬為1 ms，重復頻率為1 kHz。在測試過程中，電流持續(xù)增加，直到因腔面損傷而功率急劇衰減為止。測試結果如圖10所示。由圖10可見，在QCW條件下，腔面COD功率值為23.8 W，而在CW測試時，腔面損傷閾值功率則變?yōu)?8.1 W。圖11為光束所含能量為總能量的95%時所對應的光束發(fā)散角與注入電流的關系，由圖11可見，垂直方向的遠場在電流從2 A增到12 A過程中，基本維持恒定，而水平遠場則隨電流的增大而增大，這一變化與多種機理有關，包括熱透鏡效應以及載流子的空間燒孔效應。鑒于激光器的可靠性對工業(yè)應用來說至關重要，我們對COS器件進行了過電流條件下的壽命測試[18]，測試條件為：溫度35 ℃，14 A恒定電流。在該條件下，輸出功率為14 W。圖12為8個COS器件在上述測試條件下的功率隨時間變化的記錄，很顯然在大約6 000 h的壽命測試過程中，器件的功率沒有變化，說明我們的器件即使在高溫環(huán)境(35 ℃)以及高電流(14 A)和高功率(14 W)時依然能夠可靠工作。

4 Conclusion

結 論

This paper briefly describes the design, fabrication, and testing results of high power 915 nm semiconductor laser manufactured by Shenzhen Raybow Optoelectronics Co., Ltd. The test results show that the electro-optical conversion efficiency of the developed device is up to 66.5%, and the electro-optical conversion efficiency can still reach 64.3% when the output power is 12 W. This efficiency has reached the advanced level of similar devices in the world. Under the accelerated life test conditions of 35 ℃ and 14 A injection current(corresponding to output power of 14 W), the output power of the device remained stable during the test period of about 6 000 hours, indicating that the reliability of the device is satisfactory.

本文簡要敘述了瑞波光電子公司的高功率915 nm 半導體激光的設計、制作與測試結果。測試結果表明，研發(fā)的器件電光轉換效率最高達66.5%，輸出功率為12 W時的電光轉換效率仍然達到64.3%，這一效率指標達到國際同類器件的先進水平。在35 ℃、14 A注入電流(對應輸出功率14 W)的加速壽命測試條件下，器件的輸出功率在大約6 000 h的測試過程中保持穩(wěn)定，說明器件的可靠性令人滿意。