OPTOELECTRONICS LETTERS Vol. 2 No. 3,15 May 2006

High power collimated diode laser stack

LIU Yuan-yuan*, FANG Gao-zhan, MA Xiao-yu, LIU Su-ping, and FENG Xiao-ming Institution of Semiconductor, Chinese Academy of Science, Beijing 100083, China

(Received 14 September 2005)

A high power collimated diode laser stack is carried out based on fast-axis collimation and stack pack- aging techniques. The module includes ten typical continuous wave (cw) bars and the total output pow- er can be up to 368W at 48.6A. Using a cylindrical lens as the collimation elements,we can make the fast-axis divergence and the slow-axis divergence are 0. 926 4 o and 8. 206 ~ respectively. The light e- mitting area is limited in a square area of 18.3 mm x 11 mm. The module has the advantage of high power density and offers a wide potential applications in pumping and material processing. CLC number_.TN248.4 Document code:A Article ID:1673-1905(2006)03-0179-03

Compared with other kinds of lasers, high power di- ode laser has many advantages such as high electro-opti- cal efficiency, compact structure,long lifetime and so on. As a laser source, diode laser is sufficient for many dif- ferent applications such as pumping of solid state lasers, direct material processing, medical applications and printing. Improved by the demands, the output power and reliability of high power diode lasers have been in- creased steadily over the recent years. Nowadays, the typical output power of the commercially available diode laser bars is about 40~50 W (cw) and the lifetime is assured to be longer than 10,000 hours [~ 21. On the oth- er hand,the disadvantage of the diode lasers is their poor beam quality. For the stripe emitting facet, the diode la- ser bar has poor space propagation characteristics. In the direction perpendicular to the plane of p-n junction which is usually called fast axis, the divergence angle is quite large and up to 100 o (measured at half intensity), while in the other direction which we called slow axis, the divergence angle is only in the range of some de- grees. The asymmetry beam quality strongly limits the application in most cases. Therefore stack technique and fast-axis collimation are necessary for most de- mands ca-5~ . In this paper with a traditional ray tracing method we have analyzed the far field distribution of a single bar and the stack before and after collimation. In our experiment, a simple and low-cost cylindrical lens is used as fast-axis beam collimation for every bar and ten collimated diode laser bars are stacked together. The maximum output power obtained at 48. 6A is 368W,the far-field divergence angles are measured to be 0. 926 40 and 8.20 ,and the emitting area is limited in 18.3 mmX 11 ram.

For the large far field beam divergence, the paraxial condition which is valid for incident angle less than 5 de- gree does not hold on in the diode laser system. Non-pa-

* E-mail: liuyy@ htoe. com, cn

raxial ray tracing method is a better way to discuss the beam transmitting property,which means that from the emitting facet of a diode laser bar a fan of equally spaced ray has been traced and each ray carries the local laser intensity. For denoting the far field distribution of the diode laser stakes, we assume a plane named receive plane which is divided into lots of segments. As shown in Fig. 1,0 is the ray determined by the assigned origin point and the centre point of each segment made with X-aixs. 80 means the emitting ray of a diode laser bar made with X-axis. The distance of receive plane and the radius of collimation lens can be specified by L and r re- spectively,and t means the offset of the bar in Y axis. There are two ways to describe the far field intensity property. One is expressed by the intensity of each ray and the relative angle 00, the other is tracing the emitting ray to one segment of the receive plane which is far a -

way from the emitting facet. All of the intensity in each segment is summed and plotted dependent on angle 0. We select the second way to analyze the far field diver- gence property of a single bar and the laser stacks before and after collimation. The distance between the emitting facet and the tip of lens is determined by the formula n X r / ( 2 X ( n - - l ) ) which is the focus of the cylindrical lens, where n is the refractive index of the lens. In Fig. 2,the real line depictes a typical measured fast-axis di- vergence property of a laser bar which shows the de- pendence of relative intensity on angle 00, while the dashed shows the same information expressed in the second way based on ray tracing simulation. The infor- mation of them is nearly consistent. Another line with dashed and dots depictes the distribution of the laser bar in the receive plane after collimation with r = 0 . 3 mm,L = 2 000 mm,and n = 1.54 also obtained by ray tracing. We can see the fast-axis divergence angles are respec- tively about 330 and 1.1 ~ Fig. 3 describes the depend- ence of the far-field distribution of the intensity on t for a single bar. We can see the strong offset of intensity in

�9 0180 �9 Optoelectron. Lett. Vol. 2 No. 3

the receive plane with little t and so we must ensure t% 0.01 mm to maintain the good collimation result. We al- so tracing the light ray of the stack including ten bars when the pitch is 1.8 mm and find that the far-field dis- tribution is nearly the same as that of a single bar. Fig. 4 shows how the offset in Y-axis of several bars have im- pact upon the far-field distribution of stack. If we sup- pose the four bars on the top of the stack have the same offset , namely , each t of them is assigned to be 0 mm, 0.01 mm and 0.015 mm respectively,the relative diver- gences are simulated to be 1.020 , 1.72 o , and 2.52 o re-

spectively. From the results of ray tracing calculation, we can conclude that the stack nearly has the same far-field divergence characteristics as a single bar before and after collimation. To stack laser the offset of each single bar in Y-axis will bring about widening of the width of di- vergent angle or even multi-peak and the fast-axis diver- gent angle can be improved up to about I o when the off-

set is controlled perfectly.

Collimation lens

Laser bar ~ ? ?

Y

L Receive plane

Fig. 1 Schematic drawing of ray tracing calculation

1.0

,--, 0.8 r~ B

0.6

�9 ~ 0.4

�9 fi 0.2

0.0 , i , J , , �9 , .

-100 -80 -60 -40 -20

Measured data . . . . Before collimation/

(L=2 000 mm) / \ ........ After cotlimation q (L:2000~/ ~, ,'

0 20 40 60 80 100

Far field divergent angle( + )

Fig, 2 Far field distribution of a single bar in fast axis

Based on the ray tracing calculation, the diode laser

stack has been implemented. The experimental set-up is shown in Fig. 5. First the single bar whose structure is the established, compressed and strained A1GaAs/A1- GaInAs/A1GaAs system was grown by metalorganicch-

2

0

-2

,-~ -4 ~ - 6

4 _ 8

-10

-12

-14

I

[ ]

i i i i

0.00 0.02 0.04 0.06 t(mm)

�9 i , +

0.08 0.10

Fig. 3 Effect of the offset in y-axis on the

far field distribution for a single bar

1.0

0.8

0.6

..~ 0.4

*fi 0.2

0.0

- - t =0 mm ......... t =0.01 mm . . . . . . . . . . . . . . t =0.015 mm

�9 , + ~ �9 , . , + �9 , �9 ,

-30 -20 -i0 0 10 20 30 Angle(~

Fig. 4 Effect of the offset in Y-axis

on the far field distribution for stack

emical vapor deposition (MOCVD). The length of one bar is 11 mm included 19 single-stripe emitters, the re- sonator length is 1 200 urn,the filling factor,i, e. the ra- tio between the optically active area to the whole area of the bar, is 30%. Each bar has its own heat sink and is soldered on the Cu cooler which acts as the p-electrode. The top n-electrode is insulated against the Cu cooler with an isolating foil and is fixed to the bar. The cw out- put power of each bar can be up to 40W. The cylindrical

lens with radius of 300 um is used as the fast-axis colli- mation element and the slow axis is out of considera-

tion. Then ten bars are assembled together with an equal pitch and the total divergent angles in two directions are respectively 0. 926 4 o and 8.20 . Fig. 6 displays the inten- sity dependence on fast divergence angle measured with the testing apparatus. The maximum optical output power of the stack is 368W obtained at 48. 6A and the emitting area is limited in 11 m m X 18.3 mm.

Using ray tracing calculation, we have analyzed the far-field Intensity distribution of a single diode laser bar and a diode laser stack. The offset of a single bar will bring strong effect on the far-field charaeter of the

LIU et el. Optoelectron. Lett. Vol. 2 No. 3 ~ 0181 �9

stack. We also have demonstrated a high power collima- ted diode laser stack based on fast-axis collimation and stack technique. In a square area of 18. 3 mm;< 11 mm, the maximum optical cw output power can be up to 368W at 48. 6A. The far-field divergence angles in the fast and slow axes are respectively 0. 926 40 and 8.2 ~ This technique can be used to get different wavelength and the stack module can be used in pumping and di- rectly material processing through farther beam sha- ping.

~ Poxwer supply ~/- Fast-axis

collimation lens

Heat s i n k

Y

Fig. 5 The collimated diode laser stack

1.0

0.8 ...... ~ .......................................................... , .....

= 0.6 .........................................................................

0.0 -10 -5 0 10

Angle( ~ )

Fig. 6

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The far field distribution of diode laser stack

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