Thermal Aspects of Pump-Laser Packaging

H. van Tongeren and P.J.A. Thijs Philips Optoelectronics Centre

Building WY-555, PO Box 8O.ooO 5600 JA Eindhoven, The Netherlands

ABSTRACT

A modified DIL-14 package for a pump-laser diode emit- ting at a wavelength of 1480 nm is described. By impro- ving the thermal resistance of the housing a thermal power of 1 W can be dissipated in the laser crystal. The mounting surface of the crystal assembly can be kept at 25% by a Peltier cooler operating at a current of 1 A for 65 'C ambient temperature. Under these conditions more than 60 mW is launched into the fibre pigtail.

1. INTRODUCTION

The applications of Erbium-doped fibre amplifiers in fibre- optic communication systems are growing rapidly. Pre- amplifiers may enhance the optical signal with, say, 25 dB. It has been demonstrated recently that a Distributed Feed Back laser provided with a booster amplifier can span a distance of more than 200 km at a data rate of 2.5 Gigabit per second [Ref. I]. An essential part of the amplifier is a pigtailed pumplaser operating at an optical output power of 60 mW and at a wavelength of X = 1480 nm. To obtain this high output level, the laser crystal has to be operated at an electrical power of typically P = 1 W. At this power level the laser-crystal temperature has to be stabilized by a Peltier cooler. In packaging of pump lasers it is attractive to apply the well established packa- ging methods in use for telecommunication lasers. Howe- ver, these lasers operate a much lower electrical power of typically P=50 mW. The special problem of pump-laser packaging is to keep the mounting of the laser crystal at, say, 25% at P = l W power dissipation while the tempe- rature of the outside of the package itself may be as high as 65%. For the cooler a current of Id A is available.

In this paper we report on thermal modelling and experi- mental investigations on Dual In Line (DIL) and Butterfly packages for pump lasers. The thermal model of the package is described in Sec. 2, the experimental set-up is treated in Sec. 3. The modification of the DIL-package is discussed in Sec. 4 together with the experimental results obtained. Concluding remarks are in Sec. 5.

The base plate is temperature controlled by the cold plate of a Peltier cooler, see Fig. 2. Heat is transferred from the hot plate of the cooler through an L-shaped profile made of copper and through the wall of the hou- sing to the device flange. This flange is mounted on a heat sink at ambient temperature, Tmb.

Fig. 1 The laser crystal mount. The arrangement used to couple the emission into the fibre is not shown.

wall (KOVAR)

Fig. 2 Cross section of a standard DlL-14 housing and mounted Peltier cooler. The arrows show the heat flow to the mounting flange and heat sink. Dimensions of the bottom: 20x12 mm2.

2. THERMAL MODELLING

In order to get an insight in the parameters that govern the temperatures and heat flows, a model of the laser package has been made. This section presents the details of the laser construction and the thermal model based on this set-up.

The model derived from the construction is shown in Fig. 3. It is assumed that the relation between the temperatu- re T and the heat flow P can be described by a thermal resistance R:

2.1 CO nstruction and thermal model T = P . R

The mounting of the laser crystal is shown in Fig. 1. m e crystal is soldered on a silicon submount that in turn is soldered on a carrier made of copper. Next comes a stainless steel base plate. On this plate the other parts of the device are mounted also (not shown in the figure).

The heat dissipated in the laser stripe is P,, en its tempe- rature iS T,=. Heat flows from the stripe through the submount to the carrier; the resistance is R*p. Next comes the carrier with resistance R,. The heat from the carrier goes through the base plate to the cold plate of

848 0569-5503/92/0000-0848 $3.00 01 992 IEEE

I

submount material

$ PIS

1.5~300 pm2 2.0~1000 pm2 stripe stripe

RChD WW) RChD ( K N

Fig. 3 Schematic representation of the thermal properties of a packaged laser. The symbol R is used for thermal resistance, T for tem- perature and P for thermal power.

the Peltier cooler. The resistance of the base plate is R, and the temperature of the base plate-carrier joint is T,,. Due to convection, radiation and conduction through electrical leads, heat from the housing at Tun, will be transferred to the base plate. This transfer is represented by the resistances bo, R,, en R,. For reasons of sim- plicity it is assumed here that the heat is supplied uni- formly to the whole base plate. The resistance of the base plate for convection, radiation and conduction heat is Rb,, and the temperature of the upper surface is Tw. This temperature is usually sensed and kept constant by the Peltier cooler. Because the base plate resistance for the laser heat has been taken into account already by R,, this heat flow by-passes R,,,, as shown in Fig. 3. The heat flow from the base plate to the cold plate of the cooler is not uniform. Nevertheless, it is assumed that the cold plate is at a uniform temperature Td and the hot plate of the cooler is at a uniform temperature Tm. Final- ly, the total amount of heat plus the power dissipated into the cooler pass the package resistance R, and next flow to the heat sink at Tam,.

2.2 Thermal resistances and cooler Dromrties

The resistance R, has been calculated by using a method developed gy Horikx [Ref. 21. For a submount thickness of .25 mm and a stripe-submount distance of 6 pm the calculated values are presented in the table.

I 2 1 diamond

59 48 42

17 14 12

com applications while 2.0~1000 pm2 are the typical dimensions of a pump laser stripe. Data for three diffe- rent submount materials are shown. It follows from the table that R,, is not proportional to the stripe surface. Further it can be seen that a diamond submount leads to the smallest resistance as may be expected. The ratio of the resistance of diamond and silicon is about 1.5 only, while the ratio of the thermal conductivities is about 15. The reason is that the main part of R,, is caused by the thin crystal and solder layer of 6 prn. In our devices a Si- submount is used so R,, = 17 K/W.

In calculating the value of R, it is assumed that the heat flow from the submount to carrier is uniform. For a sub- mount surface of 0.8x1.0 mm2 and a carrier thickness of 1.1 mm we find: R,=1.7 K/W. The values of the base plate resistances R,, and R , for stainless steel are: R,,= 10 K/W and Rw = 1.2 K P . In calculating Rcpl it is assumed that the heat from the carrier is uniformly ap- plied to the base plate via a 2x2 mm2 area. The calculati- on of Rw is straight forward.

Because the difference between Tmb and Tw is quite small, say 5O'C, the radiative transfer can be described by a resistance. If the absorption coefficient of the gold plated surfaces involved is taken to be (r=O.l then we find R,=30.103 K/W. For the transfer by free convecti- on &, = 1 O3 K/W is used. The resistance due to conducti- on by the two palladium bonding strips used in our devi- ce is calculated to be R, = 1700 K/W.

In calculating the resistance of the package it has been assumed that the heat flow of the hot plate of the coder to the L-shaped profile, see Fig. 1, is uniform. For the standard DIL-housing we find then R,=5.9 K/W. In a butterfly housing the heat flow from the cooler has to cross the bottom plate only, see Fig. 4. If this plate is made of CuW-material then R, = .21 K/W. The calculati- ons on R, are performed for our standard 18slement Peltier cooler with plate dimensions of 626.2 mm2.

The properties of the Peltier cooler have been derived directly from the data sheets provided by the manufactu- rer [Ref. 31. For a given current, I, through the cooler and given temperatures Tm and T,, the pump capacity and the power dissipated in the cooler are known.

waY (KOVAR)

-I- 4

Fig. 4 Cross section of a butterfly housing and mounted Peltier cooler. The arrows show the heat flow through the bottom plate that is also used for mounting on a heat sink. Dimensions of the bottom plate: 30x12 mm2.

The 15x300 pm2 stripe refers to a laser crystal for tele-

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2.3 Results

G + 30 0%

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Suppose that the power dissipated into the laser is P,=l W (this is a typical value for a pump laser) and assume that the base plate temperature is controlled at TbN=25'C. Results of the calculations on AT=T,,-T,,,,, as a function of the current through the cooler are shown in Fig. 5. The results were obtained for the standard 18- element cooler. It follows that the butterfly package is superior to the standard DIL-14 package; for a given value of I the value of AT is significantly larger for a but- te*. Further it can be seen that the butterfly is nearly perfect, only a slightly better result is obtained if R, is set to zero as indicated by the dashed curve. The calcu- lations indicate that the required ambient temperature of T, =65'C, i.e. AT=4O'C, cannot be obtained for Isd. The temperature that is really important is the stripe temperature TIM. It follows from Fig. 3 that the thermal resistance between TI, and T,, is R,,+Rur+ R,,=28.7 K/W. So for a dissipation of PI,=l W the temperature TI, is 28.7% higher then TWl. If the base plate material is changed from stainless steel to copper then R, reduces from 10 K/W to 0.7 K/W. If further the Si-subount is replaced by a diamond submount then R,, reduces from 17 K/W to 12 K/W. The result of these modifications is a reduction of the resistance between TI, and Tw to 14.4 K/W. This means that the base plate can be controlled to Tw=25+14.3'C to get the same T, as obtained without the modification. The requirement for AT now becomes AT=25.7'C instead of AT=4O'C. It follows from Fig. 5 that the butterfly just meets the requirement for I=1 A, the DIL-package does not meet the requirement at all. Moreover, it is not attractive to change the submount. It is also not attractive to change the base plate material because it serves as a basis for the fibre coupling set-up, see Ref. 4 for some details. Therefore it was decided to look for other measures to improve the package.

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-

-

I

laser

A n l b

Fig. 5 The calculated temperature difference Tun,- T,,,, as a function of the current of a stand- ard 18-element Peltier cooler, AT(I). The thermal power dissipated into the laser is P,=l w.

2.4 DQt ions for immovements

As mentioned already the aim is to improve the package construction in such a way that at an ambient temperatu- re of Tmb=65'C the base plate can be controlled at Tyr25*C for a cooler current of Is1 A and a thermal dissipation in the laser crystal of 1 W. There are two

options left to meet this goal: improving the thermal package resistance R and improving the performance of the Peltier cooler. K e effect of installing a 32-element cooler is shown in Fig. 6. It can be seen that the calcula- tions indicate that this measure is sufficient for the butter- fly package. However, the DIL-14 does not meet the requirement and therefore the R, has to be improved also. This will be discussed further in Sec. 4.

0 ' 1 0.5 1 .o 1 5

Fig. 6 The calculated temperature difference Tun,,- Tw as a function of the current of a 32- element Pettier cooler, AT(I). The thermal power dissipated into the laser is P,,= 1 W. The results AT(I) are presented up to the I- value for which AT starts to decrease

3. EXPERIMENTAL

To measure the thermal behaviour of packages the set- up shown in Fig. 7 was used. A copper heat sink is temperature controlled at Tunb. On this heat sink both 011-14 and butterfly packages can be mounted. The whole set-up is placed in a vacuum chamber to promote thermal isolation. Instead of a laser crystal and submount a small SMD-resistor, dimensions 1.5x1.5 mm2, is moun- ted in the package. It is expected that this modification has no significant influence on the experiments. All the experiments have been performed at Tmb =85'C.

Cu heat sink

I

l l l d 1 water cooled heat sink I

Fig. 7 Measuring set-up. The total set-up is placed in a vacuum chamber. The device under test is mounted on the L-shaped heat sink. This heat sink is temperature controlled by a Peltier cooler at Tmb. This temperature is measured by a thermocouple.

850

Results obtained on a butterfly housing are shown in Fig. 8 for three values of the power, P, dissipated in the SMD- resistor. The solid curves show the experimental tempe- rature differences, AT, between the ambient temperature and the base plate temperature AT=T,,,-T,,, as a functi- on of the current, I, through the standard 18-element Peltier cooler. The dashed curve shows the results of calculations. Figure 9 shows the same date but now for a standard DIL-14 package. It follows from the figures that reasonable agreement is obtained between experiment and calculations. Therefore it is concluded that the model discussed in Sec. 2 can be used for design purposes.

Tama = 05 'C A " /

80

1 70 s OI F 60 U

50

40

30

20

10

0

Fig. 8 The experimental and calculated temperatu- re difference Tamb-Tbp, as a function of the current of a standard 18-element Peltier cooler, AT(I), of a butterfly. The dashed curves show the measurements and the solid curves indicate the calculations for three different values of the power dissipa- ted.

20 1

I f A ) - Fig. 9 The experimental and calculated temperatu-

re difference T,,,-T,,, as a function of the current of a standard 18-element Peltier cooler, AT(I), of a DlL-14. The dashed cur- ves show the measured results and the solid curves indicate the results of the cal- culatins for three different values of the power dissipated.

wall (KOVAR)

Fig. 10 Cross section of a DlL-14 housing provided with a Cu-insert and mounted Peltier cooler. The arrows indicate the heat flow.

4. UP-GRADING OF THE DII -14 PACKAGF

It has been concluded already in Sec. 2 that application of a 32-element Peltier alone would not be sufficient and that an improvement of the package resistance is also needed. The cross section of a DIL-14 package is shown in Fig. 2. It turns out that the main contribution to comes from the wall of the housing and the flange. They are both made of KOVAR having a thermal conductivity, A, of A=18 K/W/m. A smaller but significant part is caused by the relatively thin L-shaped copper profile. To decrease the resistance the construction shown in Fig. 10 is used. The flange and a part of the wall of the hou- sing have been removed and replaced by a copper insert. The picture in Fig. 11 presents an overview of the construction. Calculations on this modification result in a value of R, = 1 K/W.

Fig. 11 Photograph of the improved DIL-14 housing showing the insert and the assembled hou- sing.

The performance of the modified package with a 32- element Peltier cooler is presented in Fig. 12 for 3 values of the dissipated power P. Measured date on AT(I) are indicated by the dashed lines and the solid curves repre- sent the results of the calculations. It can be seen that a I = 1 A a value of AT=40'C is obtained as required.

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a +

70

30

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0.5 1 .o 1.5

Fig. 12 The experimental and calculated temperatu- re difference T*-Tw of the modified hou- sing as a funaon of the current of a 32- element Peltier cooler, AT(I). The dashed curves show the measured results and the d i d curves indicate the results of calculati- ons for three different values of the power dissipated.

5. CONCLUS ION

The standard DlL-14 housing and the Peltier d e r in use for fibre optic communication lasers limit the perfor- mance of high power pump lasers. In order to adapt the user-friendly DlL-14 housing for pump lasers the thermal properties of the housing have been improved and a cooler with a larger capacity has been installed. These adaptations result in a pump laser delivering 60 mW out of the fibre pigtail at an ambient temperature of 65% while the laser mount assembly is kept at 25'C.

P.I. Kuindersma, W. Scheepers, J.T.M. Kluitmans, B. Teichmann and D.J. Will, 2.488 Gb/s Repeater- less Transmission over 203 km Standard Single Mode Fibre, to be published in Electron. Lett.

J.J.L. Horikx, private communication (Philips Re- search, intemal note).

MELCOR, Trenton, NJ 08648, USA. The standard cooler is type FC 0.6-18-05 (18 elements) and the cooler with the high capacity is type FC 0.6-32-05 (32 elements).

H. van Tongeren, Packaging of Long-wavelength Fibre Optic Communication Lasers, Philips J. Res., 45,243-254,1990.

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