Wavelength Monitor Integrated CW DFB Laser Module
for DWDM Applications
by Hideyuki Nasu *, Tomohiro Takagi *, Toshikazu Mukaihara *, Takehiko Nomura *,Akihiko Kasukawa *, Mizuki Oike *2 and Takao Ninomiya *3

For signal transmission on DWDM systems, cross-talk between the nearest
channels in the wavelength region must be suppressed to maintain the required
quality of optical signals, hence good wavelength stability is strongly required of the optical signal. As an essential solution for signal sources in DWDM applications, we developed wavelength
monitor integrated CW DFB laser modules. The unique structures of our laser module were optimized for 25 GHz spacing DWDM and 50 GHz spacing DWDM, respectively. In particular, themodule for 25 GHz spacing usage was contrived to meet very tight wavelength stability requirements. This report introduces the structures and the performance of both laser modules.

ABSTRACT1.

INTRODUCTION

In dense wavelength division multiplexing (DWDM) to
enable high-capacity transmission, the emission wavelength of optical signal must be stabilized to sufficiently
suppress cross-talk, which causes a deterioration of optical signal quality. Wavelength monitor integrated distributed feedback (DFB) laser modules are promising lightsources for such DWDM applications, and an emission
wavelength of the laser module 1), 2) can be precisely stabilized to a certain fixed wavelength using a feedback circuit.
A high insertion loss is a disadvantage of the LiNbO3based external intensity modulator employed for opticalamplitude modulation in long-haul DWDM systems. Highpower continuous wave (CW) light maintained in a constant polarization state is required to obtain a large
enough extinction ratio of optical signals and increase loss
budget.
We developed such DFB laser modules supported by apolarization maintaining fiber (PMF) pigtail, achieving avery high output power of over 40 mW 3), and they havebeen widely supplied for actual applications.
Integrating a wavelength monitor into high power CW
DFB laser module led to the development of an optimum
light source for long-haul DWDM applications 4), 5). The
requirement of wavelength stability is basically dominatedby the value of wavelength spacing in DWDM systems. In
general, when a narrower wavelength spacing is*

Semiconductor R&D Center, Yokohama R&D Lab.
FA Development Dept., Production Technology Development
Center*3 FITEL-Photonics Lab.
*2

employed, tighter wavelength stability is required. For
example, the current light source for introducing 50 GHz
spacing DWDM systems must support a wavelength stability of ±20 pm or less over service life. Furthermore,
there is much tighter wavelength stability for 25 GHz spacing DWDM systems, which are promising for ultra-highcapacity transmission in next-generation systems. To
achieve a transmission rate of 10 Gbit/s per channel withthis system, wavelength stability of at least ±10 pm or lessis required. To meet the requirements for each system, wedeveloped laser modules which have the proper structures for 50 GHz spacing DWDM and 25 GHz spacing
DWDM, respectively.
In this report, the performance of fiber-coupled output is
described first. Subsequently, the structure and the performance of the wavelength monitoring function are introduced.

2.

APPEARANCE OF LASER MODULE
AND FIBER-COUPLED OUTPUT

The high power characteristics of the DFB laser module
are mainly derived from the high power performance ofthe laser diode and high coupling efficiency based onlenses. A strained layer multiple quantum well (SL-MQW)
structure has been employed in the active layer for thehigh power characteristics of laser diode. To enhance
front facet power as well, the front facet has an anti-reflection (AR) coating against the high-reflection (HR) coatingof the rear facet. Output power launched from the front
facet is coupled to the fiber pigtail through two lenses.
This realizes a high coupling efficiency of over 70 % in
mass production.

Furukawa Review, No. 23

2003

6

Wavelength Monitor Integrated CW DFB Laser Module for DWDM Applications

Figure 1 shows the appearance of the laser module,
which is based on an industry standard 14-pin butterfly
package and a wavelength monitor is integrated into it. A
stable polarization extinction ratio of 20 dB or higher isrealized by precisely polarization-controlled optics whencoupling to the PMF, in which the polarization state ismaintained on the slow axis.
Figure 2 shows typical L -I curve and optical spectrum.
The submount temperature is controlled to 28.5°C for an
output power of 20 mW to obtain a fixed ITU wavelengthof 1538.98 nm. Our high power performance enables alow operating current to be used such as 86 mA at 20
mW, and 162 mA at 40 mW.

3.

LASER MODULES FOR 50 GHZ SPACING DWDM APPLICATIONS

3.1 Module Structure
The schematic structure of the laser module is shown in
Figure 3 where Figure 3 (a) and Figure 3 (b) show top andside views, respectively. In these wavelength monitor
optics, a laser beam launched from the rear facet is collimated by a collimating lens and then is symmetricallydivided by a prism. The divided beam is coupled to a photodiode for power monitoring. The other beam is incident

on a 25 GHz spacing etalon and then the transmitted
beam is coupled to a photodiode for wavelength monitoring.
Compared to previously reported structures using a half
mirror for the beam splitter, this unique structure provides
significant advantages in terms of compactness, reducing
parts, and ease of alignment. For example, the availabilityof mounting two photodiodes to a submount is specificallyrelated to reducing parts and saving space for mounting awavelength monitor. It also provides an easy process foradjusting the coupling ratio using a prism as a beam splitter, since the alignment to adjust the coupling ratio can beachieved without rotating a prism. Furthermore, thisunique structure achieves integration of a long cavity
etalon supporting the very short period required for
DWDM application.
The wavelength monitor is mounted with a laser diodeon a base plate. An etalon has temperature dependence,but the influence can be neglected by controlling etalon
temperature. The laser modules are assembled with highly reliable soldering and laser welding without any adhesives.
3.2 Module Characteristics
Wavelength monitor photocurrent as a function of lasing
wavelength of DFB laser (i.e. wavelength discriminatorcurve) is shown in Figure 4. Laser temperature control
enables the lasing wavelength of the DFB laser to be controlled. Wavelength tuning is achieved by controlling thetemperature of the DFB laser, the temperature range of
which is between 20°C and 35°C. Fiber-coupled power iscontrolled to be 20 mW by driving an auto power control
circuit (APC). Wavelength drift from the initial set wavelength derives amplitude variations in the wavelength
monitor photocurrent on the wavelength discriminatorslope. The angle of the etalon to the incident laser beamis tuned and then fixed in the assembly process so that

Prism

Lens DFB-LD

Lens

Isolator
Lens

PD

Figure 1

Appearance of wavelength monitor integrated DFB
laser module.

Ts =28.5°C@ITU grid
CW

40

50 GHz Etalon(a) Top view

30
Normalized output power (dBm)

Fiber coupled power (mW)

50

20

10

0
0

50

10

PM fiber

SMSR>55dB

0
-10
-20
-30
-40
-50
-60
-70
1534 1535 1536 1537 1538 1539 1540 1541 1542 1543
Wavelength (nm)

100

150

200

TEC

LD forward current (mA)

Figure 2

L-I curve and optical spectrum.

(b) Side view

Figure 3

Schematic structure of 50 GHz spaced wavelength
monitor integrated DFB laser module.

Furukawa Review, No. 23

2003

7

140

7
Pf=20mW(Const.)

6120

Wavelength drift (pm)

Wavelength monitor photocurrent ( µ A)

Wavelength Monitor Integrated CW DFB Laser Module for DWDM Applications

100
80
60
40
20
0
1554.5

3
7 pm

2
1
0

-2
1555

1555.51556

1556.5

-5

25

40

55

70

Figure 6

Wavelength drift as a function of case temperature.

Another dominant factor for wavelength drift is casetemperature variation, in which radiant heat affects a slightchange in etalon temperature. Figure 6 shows wavelength
drift as a function of case temperature between -5°C and
70°C. It has a wavelength drift of 7 pm over case temperature with a fairly linear relationship.
This excellent performance promises a wavelength driftof 20 pm or less sufficiently.

50
APC
CW
25

0

-0.2 pm/mA

-25

10

Case temperature (°C)

Wavelength discriminator curve for 50 GHz spaced
wavelength monitor.

Wavelength drift (pm)4-1Wavelength (nm)Figure 4

5

-50
100

110

120

130

140

150

160

170

Operating current (mA)

Figure 5

Wavelength drift as a function of operating current.

the required ITU wavelength can be positioned on theslope. In Figure 4, dotted lines indicate 50 GHz spaced
ITU wavelengths that are located on all negative slopes.
We evaluated the wavelength-locking performance of
this laser using the wavelength monitor function and anelectronics circuit. The relationship between wavelength
drift and operating current is shown in Figure 5. The wavelength dependency upon operating current is as small as -0.2
pm/°C, which is explained with the following phenomena.
An operating current increase induces a temperature
increase of the active layer of the DFB laser. The lasing
wavelength accordingly has a blue-drift. The wavelength
monitor senses the wavelength drift. The monitor signals
are utilized for feedback to the TEC control circuit todecrease laser temperature and return the lasing wavelength to its original value. Since the wavelength monitor
is mounted on a TEC together with a DFB laser, a laser
temperature decrease accompanies a wavelength monitor
temperature decrease. Due to its temperature decrease
and the temperature dependency of the etalon, the wavelength discriminator curve has a blue-drift as operating
current increases, and a locking wavelength accordingly
has a wavelength drift. Assuming actual use in transmitters, the operating current increase is induced by laser
diode degradation over service life. With failure criteria ofa 20 % increase in operating current at the end of life(EOL) and an initial operating current of 120 mA, its
increase is 24 mA, which induces a very small wavelength
drift of -4.8 pm.

4.

LASER MODULES FOR 25 GHZ SPACING DWDM APPLICATIONS

4.1 Module Structure
25 GHz wavelength-spaced 10 Gbit/s DWDM systems
require very tight wavelength stability. According to an
ITU-T recommendation for wavelength spacing 6), a minimum wavelength stability of ±10 pm is required. To meetthis requirement, wavelength dependency upon operating
current and case temperature must be suppressed. In
terms of the module assembly for 25 GHz spaced DWDM
systems, the cavity length of a 25 GHz period etalon isexactly twice that of a 50 GHz period etalon. Since such along cavity requires a large space in which to equip it, amodule structure should be contrived.
A schematic structure of a laser module is shown in
Figure 7. Top and side views are shown in Figure 7 (a)and Figure 7 (b), respectively. Employing our uniquestructure for the wavelength monitor, a 25 GHz period
etalon was successfully equipped in an industry standard
14-pin butterfly package for the first time. To meet the
wavelength stability requirement of 10 pm or less, independent temperature control of the wavelength monitor isachieved so that operating current dependence can befundamentally eliminated and case temperature dependence can be suppressed.
4.2 Module Characteristics
Figure 8 shows a wavelength discriminator curve where
the wavelength is tuned by DFB laser temperature
between 20°C and 35°C. As the wavelength range is thesame as that of the 50 GHz spaced wavelength discriminator curve shown in Figure 4, the period in Figure 8 is as
short as a half. Dotted lines show 25 GHz spaced ITU

Furukawa Review, No. 23

2003

8

Wavelength Monitor Integrated CW DFB Laser Module for DWDM Applications

Prism

Lens DFB-LD

Lens

10

Isolator
Wavelength drift (pm)

Lens
PD

25 GHz Etalon(a) Top view

50
1.5 pm
-5

-10
-5

10

25

40

55

70

Case temperature (°C)

PM fiberFigure 9

Wavelength drift as a function of case temperature.

5

Wavelength monitor photocurrent ( µ A)

Figure 74TEC(b) Side view3

Schematic structure of 25 GHz spaced wavelength
monitor integrated DFB laser module.

60

2
1
0
-1
<±1 %

-2
-3

Pf=20 mW(Const.)

-4

50
-5
-5

10

25

40

55

70

40

Case temperature (°C)
30

Figure 10 Change of fiber-coupled power as a function of casetemperature.

20
1001534.81535.31535.81536.3

Wavelength (nm)Figure 8

Tracking error (%)

TEC

Wavelength discriminator curve for 25 GHz spaced
wavelength monitor.

wavelengths on which wavelength discriminator slopes
are positioned.
The dominant wavelength drift factor is case temperature variation. To suppress this, the temperature of thesensor should precisely correspond to the etalon temperature, regardless of case temperature variation. Optimizing
the position of the temperature sensor can suppress
dependency. Figure 9 shows wavelength drift as a function of case temperature. A very small wavelength drift of
1.5 pm is obtained over operating case temperature (-5°C
to 70°C).
The unique structure might induce a mechanical instability that causes optical axis off, since DFB laser andwavelength monitor are mounted on separate TEC. To
confirm this, the change in fiber-coupled power as a function of case temperature under the condition of a driving
APC circuit was measured as shown in Figure 10. With
optical axis off in the coupling between wavelength monitor and laser, the coupled power from the back facet to thepower monitor photodiode is varied significantly. In the
APC drive, while maintaining a constant power monitor

photocurrent, the coupling efficiency variation derives alarge operating current variation, which causes a largechange in fiber-coupled power. The measured value isless than 1 pm, which is much smaller than the common
requirement of 10 % or less for laser modules.
Therefore, it is concluded that the module structure
employed for the laser module is sufficiently stable. These
results promise a laser module with stable optics, evenwith separate temperature control of the wavelength monitor, and further meet the wavelength stability of 10 pm or
less.

5.

SUMMARY

We developed wavelength monitor integrated CW DFB
laser modules as essential signal sources for long-haul
DWDM systems. Employing our unique wavelength monitor structure, an etalon having the very short periodrequired for DWDM application was successfully equippedin an industry standard 14-pin butterfly package. We optimized the laser module structures for 25 GHz spacing and
50 GHz spacing DWDM applications, respectively. By
characterizing laser module performance, we confirmedthat a wavelength stability of ±20 pm for 50 GHz spacing
and a wavelength stability of ±10 pm for 25 GHz spacing
can be guaranteed over service life.

Furukawa Review, No. 23

2003

9

Wavelength Monitor Integrated CW DFB Laser Module for DWDM Applications

REFERENCES
1) B. Villeneuve, M. Cyr, H. B. Kim, OFC’98 (1998) 381.
2) K. Tatsuno, M. Shirai, H. Furuichi, K. Kuroguchi, N. Baba, H.
Kuwano, Y. Iwafuji, A. Murata, OFC2001 (2001) 1.
3) M. Funabashi, K. Hiraiwa, S. Koizumi, N. Yamanaka, and A.
Kasukawa, ECOC2001 (2001) 122.
4) H. Nasu, M. Oike, T. Nomura, A. Kasukawa, ECOC2001 (2001)
428.
5) H. Nasu, T. Mukaihara, T. Nomura, A. Kasukawa, M. Oike, H.
Matsuura, T. Shiba, and T. Ninomiya, OFC2002 (2002) 209.
6) ITU-T recommendation, G.692 appendix V. 5 (1998) 28.

Furukawa Review, No. 23

200310