Development of the FBT Laser Modulewith Adhesive Bonded Structure
Maiko Ariga*, Toshio Sugaya*, Toshio Kimura*,Kazuyoshi Akizuki*2, Yasuo Nakajima*2, and Yoshihiro Arashitani*2

To get a high performance and a high reliability, we had improved theminiaturization of modules and components on the FBT laser module, and thenbonding these components with resin adhesive becomes essential to allow flexibility of its
design. However, with respect to the usage of organic resin in the sealed package, outgas
deposition in the optical components becomes a concern. Then, conducting analyticalexperiments which applied in the cases of high temperature condition over long periods orchecking the existence of uncured resin, we proved that the deposition does not influence thereliability and it becomes possible to use resin adhesive. According to this result, we havedeveloped the FBT laser modules which use the resin adhesive to bond the components, andachieved the high performances such as lower power consumption of 24% and higher optical
output power of 15%.

ABSTRACT1.

INTRODUCTION

So far, we have produced the laser modules of the FullBand Tunable (FBT) and the Distributed Feedback (DFB)
which are used in Dense Wavelength Division
Multiplexing (DWDM) 1). With the expansion of communication capacity, to get the higher performance of the FBTlaser module becomes an issue and, especially when low
power consumption and high output power are important.
As needed electric power is increasing with higher performance of the transmission equipments, the low power
consumption of mounted devices including laser modules are required to reduce the needed electric power.
Additionally, as multistage modulators are used with higher data transfer rate, higher output power of laser modules is needed to compensate insertion losses of themodulators.
To meet such demands, more appropriate optical andthermal design is needed more than ever. To achieve this,the increase of flexibility in selection of sizes or materialsof components which are used in the modules is important. Additionally, miniaturization of the laser modules isgetting more required because the transceivers or thetransponders are being miniaturized, so that the miniaturization of the components will soon become essential.
In a conventional FBT laser module, as each component is fixed with soldering or yttrium aluminum garnet* Optical Devices Department, FITEL Photonics Laboratory
R&D Div.
*2 Electronic & Telecommunication Materials Department,
FITEL Photonics Laboratory R&D Div.

(YAG) laser welding, the components such as lens andetalon, which main material is glass or quartz are mounted in a metal holder. For this reason, the components
including the metal holder are growing in size, and it limits their layouts and makes the optical path so long in its
design. Therefore, as a method of fixing components toimprove the flexibility of the design, we considered bonding of the components with resin adhesive.
Direct bonding of the components by resin adhesive
allows miniaturization of the components because metal
holders become unnecessary. In addition, conventionalsoldering requires the use of several solder materials toassemble the components, also, in descending orderaccording to their melting points. However, the bonding
with resin adhesive does not limit the order of assembling. Additionally, in YAG laser welding, materials of inner
optical bench (base) need to be metal in addition to themetal holder, however, in adhesive bonding, it is possible
to select ceramics which have high thermal conductivity
and small linear expansion coefficient. It becomes possible to draw electric circuit patterns on this non-conducting base and then to simplify an inner electrical wiring.
With such high flexibility of the design, the selection
increases for components focusing on the high performances, the high reliability and the cost.
Having such benefits mentioned above, yet resin adhesive had not been used for bonding components in semiconductor laser modules. That is because the semiconductor laser modules are sealed with full-filled nitrogengas and there is a concern about that outgassing fromthe resin adhesive may fill in the modules and influence

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Development of the FBT Laser Module with Adhesive Bonded Structure

the reliability.
Then, we designed and manufactured the FBT laser
module with adhesive bonding structure and provided
verification experiments of reliability especially focused on
influences of resin.

2.

FBT LASER MODULE WITH ADHESIVEBONDED STRUCTURE

2.1. The design of the FBT laser module with adhesive bonded structure
The FBT laser module with adhesive bonded structure isshown in Figure 1. The package is the butterfly-shaped
package which is a standard in the industry. The opticalbeam of the LD is converted into the parallel beam by collimator lens and branched by a beam splitter (BS) into
optical wavelength locker and main beam. To avoid thecoupling of the reflected beam to the LD, an isolator isused. Photodetectors (PD) for power monitoring and forwavelength monitoring and an etalon of 50GHz cycles aremounted in the wavelength locker.

LD

Lens
BS

ISO
PMF

epoxy is superior as the adhesive for the precision optics
in sealed packages; therefore, we adopt epoxy in our
product.
2.3. Collimator lensConventionally, collimator lens is mounted in metal holder
with YAG laser welding. However, we do not use metal
holder because the collimator lens is bonded with resin
adhesive. It is desirable that the shape of lens has a flat
surface for easy mounting vertically. In order to adapt, thelens is typically shaped as a square pole in its simplest
form, and also has a small size in length and width of
1 mm and depth of 0.6 mm or less according to characteristics such as beam’s diameter and focusing distance.
The adhesive is an added inorganic filler to adjust thelinear expansion coefficient. The linear expansion coefficient of materials for the components bonded is 0.5 to
0.6 ppm/K for quartz, 6.5 to 7.5 ppm/K for alumina or 4 to
5 ppm/K for aluminum nitride as compared to 100 to
150 ppm/K for epoxy, so that inorganic filler such as finesilica or fine talc is added to adjust the linear expansion
coefficient to a smaller value.
However, if size of filler is large, it may become, as it is
to be expected, a cause for the components to tilt. The
worse condition of the components to tilt the most is when
the filler is stuck under the component in the middle of its
depth. Allowable tilt angle of collimator lens adapted thistime is found to be 2 degree, according to the result of ourexperiment. Therefore, the allowable filler size x will be;
x=

PD

PD

where A is the depth of the lens. This relation is shown in
Figure 2. Since we use the lens with a depth of 0.6 mm or
less, we choose the 10 μm or less for filler size.

FBT laser module with adhesive bonded structure.

2.2. Selection of the resin adhesive
If ultraviolet (UV) curing resin is used, optical components
which required high positional accuracy, such as lens,can be fixed as aligned. However, only with UV curing,
there may be the case that the resin adhesive under thecomponents is incompletely cured because the resin
adhesive is shaded by the components, therefore wehave adopted hybrid UV cure resin adhesive which isboth UV and thermal curable. The components are fixedin their positions by UV curing right after their alignments
and then completely cured by thermal curing. This heating is necessary both for annealing after the UV curing
and curing by heating itself.
Epoxy is known as a base of resin adhesive whichmeets requirements such as its adhesive strength is high,has small dispersion and does not become weak withheat or light, and it has small shrinkage ratio and less outgas after curing 2). By contrast, acrylic adhesive is known
that it often has higher shrinkage ratio and weaker adhesive strength than epoxy, and also silicone adhesive isknown that it easily evolves outgas of low-molecularweight components with heat. With all these factors,

25
20
Filler size (µm)

Figure 1

EtalonA
sin 2°
2

15
Depth

10
Tilt angle
5

Lens

Filler
0

0

0.5

1.0

1.5

Depth of lens (mm)Figure 2

Allowable filler size to lens length.

2.4. Beam splitter
Wavelength locker consists of an etalon and PDs for
power monitoring and for monitoring the light coming
through the etalon of the wavelength filter. With the metal
holder being eliminated and with the miniaturization of theisolator, etalon and beam splitter becoming possible with

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Development of the FBT Laser Module with Adhesive Bonded Structure

Parallel

Parallel
ParallelFigure 33.

ParallelBeam splitter shape and parallelism of split beam.

CONDITION OF THE RESIN ADHESIVEAND ITS EVALUATION

3.1. Curing condition of the resin adhesive
The curing conditions with each UV and the heat of theadapted resin adhesive are confirmed by differential
scanning calorimetry (DSC). The term DSC here is a measuring method that, while the UV irradiation or the heating
is progressing by the heat sweeping method, the heatgeneration or the endotherm of the test piece is measured by measuring the temperature difference between
the test piece and the reference specimen, and the reaction such as hardening of the resin adhesive is observed.
In addition, after curing, when the resin is again irradiatedwith UV or heated up, and if the resin has no heat generation, the fact that the curing was completed with the first
condition can be confirmed and that the first condition
was sufficient.
First, to decide a tentative bonding condition, UV-DSC
under four conditions with irradiation intensities of 30, 60,
2100 and 200 mW/cm are measured. The results areshown in Figure 4. For example, it is founded that, in the2case with irradiation intensity of 100 mW/cm , the UV curing is completed within 26-second irradiation. Each irradiation intensity and the time of heat generation to complete are shown in Table 1. For this product, the irradia2
tion of 100 mW/cm × 30 sec is selected as the UV curingcondition in consideration of dispersion.

UV-DSC
200 mW/cm2

Heat value (a.u.)

the use of resin adhesive, we reviewed the optical systemand designed it as short as possible with respect to its
optical path.
We select a triple-splitting beam splitter which has two
splitting planes in parallel. Therefore, the main beam andthe split beam are definitely split in parallel and the distance between the parallel beams is mainly subjected tothe component accuracy of the beam splitter, so required
positional accuracy in the beam splitter bonding is
acceptable to be imprecise. For example, the angle ofbeam is stable even if the beam splitter is fixed on a tilt.
Therefore, tolerability for angle shifted seems to be veryhigh. Beam splitter shape and split beams are shown in
Figure 3. As just described, we could construct the wavelength locker which has very high yield ratio toward theshifts of angle and position, and high long-term reliability.

100 mW/cm2

200 mW
60 mW/cm

2

30 mW/cm2

UV irradiation time (sec)Figure 4

UV-DSC measured UV cure condition and heat value
per unit weight.

Table 1Time of heat generation completed in each irradiation
intensity

Irradiation intensity (mW/cm2)

30

60

100

200

Time (sec)

47

34

26

17

Next, for the thermal curing condition, to find out thetemperature at which the cure phenomenon occurs, theheat generation fluctuation of the resin when it is heatedis measured. The resin cured by UV is heated at 130°C for
10, 30 and 60 minutes, and then DSC measurements aretaken on the resin over 25 to 200°C. As shown in Figure
5, the heat generation temperature increases with theheating time. The heat value is 140°C or more when thecure condition is 130°C for 30 minutes and 150°C or morewhen the cure condition is 130°C for 60 minutes. This
indicates that the constituent that react on high temperature is remaining if heating time is insufficient. However,since the resin is not heated up to 150°C or more after
curing, the condition with 130°C for 60 minutes must be
considered as sufficient.
Similarly, DSC measurements are taken with the condition of 140°C and 150°C with variation of curing time, andthe heat value per unit weight is shown in Figure 6. It isfound that the curing is completed for around 10 minutes
with140°C and within 10 minutes with 150°C. According tothis, it is appropriate that the thermal curing conditions
are 60 minutes or more on 130°C and 10 minutes or moreon 140°C.

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Development of the FBT Laser Module with Adhesive Bonded Structure

2 mW

Heat value (a.u.)

Heat value (a.u.)

130°C 10min

130°C 30min

130°C 60min

Temperature (°C)

Heat value per unit weight (mJ/mg)

Figure 7

DSC measured 130°C cure condition.

100

Curingtemperature

80

130°C
140°C
150°C

60
40
20
0

0

10

20

30

40

50

60

70

Curing time (min)Figure 6First time

Second time

Temperature (°C)
Figure 52 mW

DSC measured thermal cure scheduleand heat value per unit weight.

Next, assuming the case of incomplete UV curing, theDSC measurement is taken on the resin without UV curing to see the curing condition of the resin only with heating. As a result, the heat generation is observed and curing phenomenon is confirmed without tentative UV bonding. As DSC measurement is taken again on the sametest piece, the heat generation is not observed and it was
found out that the curing can be fully completed only withheating. The result of the DSC measurement is shown in
Figure 7. According to this, it is found that the resin adhesive will be completely cured only with heating even if theresin adhesive has partially been unexposed to UV
because of shade of the components.
As described above, we examined by DSC and decided the curing condition of resin.

DSC measured thermal cure condition without UV
cure.

3.2. Analysis of outgas
The previous issue of most concern with use of resin insealed package is outgas. Amongst the concerning
issues that have come up is: after the package is sealed,
gas is generated from the curing resin with the variationof environmental temperature or irradiation of laser beam;
this gas gives negative effects to the surface of the laser
and makes the laser lifetime shorter, or the outgas deposits and solidifies as a kind of film forming on the opticalcomponents and, thus, the optical characteristics arechanged. Therefore, we analyze outgas from the resin
after curing.
A certain amount of curing resin is trapped and sealed
in a glass vial container and left out in 85°C for2000 hours. This gas in the vial container is analyzed byheadspace gas chromatography mass spectrometry(HS-GC/MS). The analysis of items covers the type of outgas and its amount. For the amount of the outgas, itssemi-quantitative value is worked out by toluene based
conversion from chromatogram detected by a mass
spectrometer. The reliability of FBT laser module needs tobe proved as high enough under temperature range of-40 to 85°C according to Telcordia Standards. However,as the outgas from the resin increases with temperature
increasing, we adopt the leaving temperature of 85°C
which is the highest temperature guaranteed, as a condition of the analysis.
According to the result of the analysis, the outgas for2000 hours is in minute amount of 0.2 to 0.3 ppm and theamount of gas in the vial container is almost saturated in
500 to 1000 hours. According to that, even though a partof constituents in the resin generate outgas, it can be said
that the outgas generation is over relatively in a short timeand does not continue for long period.
In addition, the outgas is an organic matter and notcaustic and film formability constituents is not included.
Comparing the outgas constituents with base materials, it
is found that the outgas constituents are resolution residues of the solvents or the hardener of each materials in
the resin.

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Development of the FBT Laser Module with Adhesive Bonded Structure

Absorbance (%)

3.3 Influence of LD optical illumination
The possibility of alteration of the resin or outgas generation is of concern when the chemical reaction occurs on
the resin with LD optical illumination. Therefore, absorbance of the resin in long wavelength range is measured
and its absorbance spectrum is shown in Figure 8.
As optical absorbance is measured with the wavelength
over 900 to 2000 nm, its maximum value is 0.5% with
1551 nm, the possibility of the chemical reaction or heatgeneration seems unlikely to occur because most of the
optical beams are transmissive.

can be verified the case that residues of solvent or hardener of materials which did not evaporate in the resin arepresent in large amounts. This has been done because it
became obvious that the constituents of outgas consist ofresidues of solvent and hardener of materials.
After applying the actual used resin without curing andsealing, the resin is cured in the sealed package at 130°C
for 1 hour. It can be verified that outgas which comesfrom the uncured resin and which occurs in curing in the
package gives negative effects to the surface of the laser
and makes the laser lifetime shorter, or if the optical characteristics become degraded because the gas has condensation or solidification when cooling down to lower
temperature than the curing of 130°C and makes deposits
on the surface of the optical components.
The aging test of the module which is filled with outgas
and driven on the 75°C for long terms is performed andthe output trend is shown in Figure 10. The output hardly
varies during 2000-hour aging test.

40

201551 nm: 0.5%1311 nm: 0.2%

0
800

1000

1200

1400

1600

1800

2000

20

Figure 8

Variation of output (%)

Wavelength (nm)IR absorbance spectrum of adhesive.

Conversely, the possibility that the ultraviolet rays arecontained in the optical beam from the LD is a concern.
Therefore, short wavelength spectrum of LD optical beamis measured and shown in Figure 9.
Since the UV cured wavelength of the resin is around360 nm and since the value of the LD optical beam ofabout 360 nm is smaller than threshold value with no
obvious peak observed therefore, the LD optical beam
does not likely influence the resin.

5
0
-5
-10
-15
-20

0

500

1000

1500

2000

Time (hours)
Figure 10 Output power trend in 75°C aging test of outgassed
FBT module.

In addition, the surface of LD after 2000-hour aging test
is observed. No deposit is confirmed by the observation
with a scanning electron microscope (SEM), and also nocondensing of elements such as carbon (C) on the lightemitting section is confirmed by auger electron spectroscopy. SEM images and auger map are shown in Figure 11.
According to the above, even though outgas occurs, it
has no serious influence on the reliability of LD and themodule.

3.4. Influence of the outgas from the resin to themodule performances
As the result of the analysis regarding outgas describedin the section of 3.2, we made experiments and confirmed
whether the resin has influence in practical matter use in
the module package. The reliability test for the module isperformed after the actual used resin of 10 times theamount is dropped in the package and then the module
is sealed. In this experiment, the influence of the reliability

Optical power output (dbm)15
10

Level sensitivity: -80 dBm (500~1000 nm), -60 dBm (400~500 nm, 1000~1100 nm)

-50
-55
-60
-65
-70
-75
-80
-85
-90
350

400

450

500

550

600

650

700

750

800

850

900

950

1000 1050

1100 1150 1200

Wavelength (nm)Figure 9

UV spectrum of FBT laser light.

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Development of the FBT Laser Module with Adhesive Bonded StructureLight emitting section

SEM image of Measurement site

SEM image of Measurement site

C Map

O Map

N Map

Si MapFigure 11 SEM micrographs and Auger maps of aging tested LD.

PERFORMANCES OF FBT LASER MODULE
WITH ADHESIVE BONDED STRUCTURE

According to performances of FBT laser module withadhesive bonded structure, power consumption and optical output power as conventional issues are shown.
First, power consumption is shown in Figure 12. For
example, when the optical output power is 10 mW, thepower consumption of the module with adhesive bonded
structure is 1.54 W comparing to the conventional moduleof 1.92 W and that represents a 20% reductions. Whenthe power is 40 mW, the power consumption is 2.02 W ofadhesive bonded structure comparing to 2.64 W of theconventional and that represents a 24% reductions. Thatis because, as adopting the resin adhesive technique forbonding of the components and wider options in theselection of component’s materials, it becomes possible
to use the materials with high thermal conductivity andtherefore the power consumption is reduced.
3.5

Power consumption (W)

3.0
2.5

Next, the results of the optical output power measurement are shown in Figure 13. For example, when current
value of an amplifier is 150 mA, the optical output powerof the module with adhesive bonded structure is 25.4 mW
comparing to the conventional module of 23.3 mW andthis is 9% improvement. When current value is 300 mA,
the output power is 49 mW of adhesive bonded structurecomparing to 42.8 mW of the conventional and this is
15% improvement. That is because, with miniaturizationof the components, the structure can be optimized for
optical coupling and the increase of the optical output
power becomes available.
60

Optical output power (mW)

4.

50
40
30
20
10
0

2.0

Resin adhesive bonding
YAG laser welding
0

100

200

300

400

Current value of amplifier (mA)
1.5
Figure 13 LI characteristics of FBT modules.

1.0
0.5
0

Resin adhesive bonding
YAG laser welding
0

20

40

60

80

By adopting the resin adhesive bonding structure, wehave achieved the FBT module with a high performanceof low consumption power and high output power.

Optical output power (mW)
Figure 12 Power consumption of FBT modules.

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Development of the FBT Laser Module with Adhesive Bonded Structure

4.

CONCLUSION

We have developed the FBT module which uses resin
adhesive for bonding of components to achieve components’ miniaturization and flexibility in selecting components’ materials.
Since the degradation of the module reliability by outgassing over long period storage of the resin in the package is of concern, the results of the analytical experiments, despite the slight outgas is confirmed in earlystages, reveal that the generated amount of outgas is saturated in short time and it does not continue generating,no influence is found on the long-term-characteristics ofthe FBT module and the surface of LD. Therefore we
adopted the adhesive bonding. As adhesive bonding is
adopted, the flexibilities on the module design are greatly
increased and the performances of the module are highly
improved. The power consumption is reduced by 24%
and the optical output power is improved by 15%.

REFERENCES
1) Kurobe et al., Furukawa Electric Review No. 121 (2008), 1-4.
2) N.Murata et al., J Adhesion, 59 (1996), 39-50,

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