Highly Reliable 40-mW 25-GHz × 20-ch Thermally Tunable DFB
Laser Module, Integrated with Wavelength Monitor
by Tatsuya Kimoto *, Tatsushi Shinagawa *, Toshikazu Mukaihara *, Hideyuki Nasu *,Shuichi Tamura *, Takehiko Numura * and Akihiko Kasukawa *To suppress crosstalk between adjacent channels in communications systems
based on dense wavelength-division multiplexing (DWDM) requires a laser
module that incorporates a wavelength monitor capable of high-precision locking on the channelof the desired wavelength. We have developed a distributed feedback (DFB) laser module with aPMF output of 40 mW, integrated with a wavelength monitor for DWDM with 25-GHz spacing. Bymeans of a highly efficient and highly reliable DFB laser diode and an optimized thermal design,
it has been possible to realize thermally tunable operation, and, under variations in laser diodetemperature of approximately 40°C, a tunable range of 4 nm or more and a maximum power
consumption of 4 W or less. High-temperature storage tests and aging tests showed that despitea structure that is more complex than conventional DFB laser modules, these modules exhibit
higher reliability, making them applicable to DWDM systems.

ABSTRACT1.

INTRODUCTION

With increased use of the Internet and of broadband
access, the growth in the volume of information on trunklines is accelerating. In WDM systems, the following threemethods are available to increase transmission volumes:
1) increasing the bit rate per channel, from 2.5 to 10 to40 Gbps;
2) broadening the bands of wavelengths used, fromC-band to the S-, C- and L-bands; and3) narrowing channel spacing, from 100 to 50 to 25
GHz.
Of these the third, which supports most C-band systems
with EDFAs and existing equipment, promises to allow a
comparatively low-cost means of increasing informationtransmission volumes. In the past we have reported
on DFB laser modules integrated with wavelengthmonitors 1)~5).
When the number of channels exceeds 100, however,
it is necessary to keep so many DFB laser modules asspares for system maintenance that the cost becomes aproblem. Device vendors also have to maintain enormous
inventories. Tunable laser light sources offer one solution
to these problems of cost and inventory. Not only this, butthe lasing wavelength of such tunable light sources canbe remote controlled, showing promise in applications in
the next generation of WDM systems, in which dynamic
wavelength allocation is carried out. Thermally tunable*Semiconductor R&D Center, Yokohama Lab.

lasers using the DFB laser diodes developed in this work
have single-mode lasing characteristics superior to thoseof most other tunable lasers, and have a long track recordin the optical transmission field promising high reliability.
Against this background, this paper reports on athermally tunable DFB laser module integrated with awavelength monitor capable of high-precision wavelength
locking at 25-GHz spacing, including long-term reliability.

2.

LASER MODULE STRUCTURE

Figure 1 shows a schematic view of the tunable DFB
laser module with an integrated wavelength monitor for
25-GHz spacing. The structure of the integrated monitor
is as follows. The light from the back facet of DFB laser
diode is passed through a collimator to produce parallelrays, which are divided in two beams by a prism. One ofthese beams is used as an optical output power monitor
directly coupled to a photodiode (PD), and the other iscoupled to a PD via an etalon with 25-GHz spacing foruse as a wavelength monitor. Using a prism to divide thelight from the back facet makes possible savings in spacein comparison with the conventional structure using a half
mirror, allowing a wavelength monitor structure for 25-GHz
spacing and a DFB laser diode to be accommodated ina standard 14-pin butterfly package. It was also possible
to reduce the component count and simplify the opticalalignment. The module is extremely strong, since allcomponents are YAG welded or soldered. Thus, as will be

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Highly Reliable 40-mW 25-GHz × 20-ch Thermally Tunable DFB Laser Module, Integrated with Wavelength Monitor

EtalonPrismLens

DFB-LD60Lens
TLD = 5°C
2545

PMF-coupled power (mW)

50PMF
PD

4030

20

10Figure 1

Schematic view of tunable DFB laser modulewith integrated wavelength monitor.

00100

50

150

200

250300Forward current (mA)

Figure 2

described in Section 5 below, the module is resistant tothermal and mechanical shock and to moisture, offeringhigh reliability.
Another feature is that it has two thermoelectric coolers(TECs) for compatibility with DWDM systems of 25-GHz
spacing, allowing the temperatures of the laser and theoptical filter (etalon) to be controlled independently. As
will be described in Section 4, this structure enables
all wavelength channels to be locked almost exactly at
the mid-point of the negative slope of the wavelength
discriminator curve. In addition the high-precision etalon
temperature regulating capability means that wavelength
drift can be made extremely low against both casetemperature and operating current.

L-I characteristics at selected laser diodetemperatures.

30
TLD = 5~45°C, Pf =40 mW

Intensity (dBm)

10-10
SMSR > 50dB
-30-50-70
1536

1538

1540

1542

1544

1546

Wavelength (nm)3.

L-I CHARACTERISTICS AND OPTICAL
SPECTRUM

When wavelength tuning is accomplished by controlling
the temperature of the laser diode, it is very important toachieve low operating current and high output as a meansof assuring reliability during high-temperature operation,and by optimizing the coupling coefficient of the grating κand the cavity length L of the DFB laser diode from thestandpoint of output and temperature characteristics, it ispossible to realize low operating current and high output
at high temperatures.
Figure 2 shows the light output vs. current (L-I)
characteristics at selected laser diode temperatures (5,25 and 45°C). Even at a temperature as high as 45°C,
the deterioration in slope efficiency is slight, and a PMF
output of 60 mW or more can be obtained. The operating
current for 40 mW is less than 175 mA. This order oflow operating current is important in terms of laser
diode reliability and reduced power consumption. Since
operation at outputs as high as 40 mW is possible these
diodes are also applicable to amp-less systems in metrosystem WDM.
Figure 3 shows optical spectra at laser diodetemperatures T LD of from 5 to 40°C, when optical
output P f is 40 mW. By varying diode temperature byapproximately 40°C, tunable range of 4 nm or more wasachieved, and in all temperature ranges the side mode

Figure 3

Optical spectra at selected laser diode temperatures(TLD = 5~40°C, Pf = 40 mW).

suppression ratio (SMSR) was greater than 50 dB. Other
characteristics were also satisfactory for all temperature
ranges, i.e., linewidth greater than 5 MHz and relative
intensity noise (RIN) better than -150 dB. These results
confirm that this tunable laser is applicable to 10- and40-Gbps trunk and metro DWDM systems.

4.

WAVELENGTH MONITOR
CHARACTERISTICS

4.1 Wavelength Discriminator Curve
The curve shown in Figure 4 is known as the wavelength
discriminator curve, and shows the relationship between
lasing wavelength λ and wavelength monitor current Iwm .
Measurement was carried out at an etalon temperature
Tf of 30°C (constant), with fiber output current held to 40
mW by automatic power control (APC) at constant powermonitor current, with laser diode temperature TLD varied
between 5 and 45°C. Using the slope of the wavelength
discriminator curve, the wavelength shift can be detected
as a change in photodiode current. Since this module
has an integrated etalon of 25-GHz spacing, the ITU gridfor all 20 channels at 25-GHz spacing coincides almostFurukawa Review, No. 24

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Highly Reliable 40-mW 25-GHz × 20-ch Thermally Tunable DFB Laser Module, Integrated with Wavelength Monitor

Wavelength monitor current Iwm ( µ m)

4.2 Wavelength Drift Characteristics
For DWDM systems with a channel spacing of 25 GHz
and a bit rate of 10 Gbps, the wavelength stability required
for a laser module with integrated wavelength monitor
is extremely high, with a permissible wavelength driftof no more than ±10 pm 6). To evaluate the wavelengthdrift characteristics, measurements were made using
the wavelength monitor and a simple external analog
feedback control circuit with the wavelength actuallylocked. With respect to the drift characteristics an average
was taken over time and the fluctuation due to the control
circuit is not included.
Figure 5 shows the wavelength drift characteristics
versus case temperature when the wavelength is locked
to an ITU grid with a Pf of 20 mW. Over a range of casetemperatures Tc from -5 to 70°C, wavelength drift wasextremely low--less than ±1 pm. The factor in wavelength
drift that relates to case temperature is due to the effectof the heat radiated from the case and received by
the etalon, so that, even though etalon temperature iscontrolled by the TEC, its temperature changes by minute

5.
170

ITU grid
Pf = 20 mW, APC

13050 %

9050
1539.51540.51541.51542.51543.5

Wavelength λ (nm)Figure 4

Wavelength discriminator curve.

RELIABILITY TESTSWith regard to thermally tunable DFB lasers, that laser
diodes themselves have a long track record and arerecognized as a highly reliable technology. Care must,however, be taken with respect to reliability when the laser
diodes are operated at high temperature. We have carriedout tests of laser diode reliability at various temperature
conditions and operating conditions. Omitting the details
for the present, when diode life is defined as an increasein operating current of 20 % from the initial value, thewear-out failure rate found by linear extrapolation for
operating conditions of fiber output power equivalent to101055

Wavelength drift (pm)

Wavelength drift (pm)amounts. This minute temperature change is suppressedby optimizing the positional relationship between theetalon and the thermistors that act as temperaturesensors. In this way even if the case temperaturechanges, the temperatures of the thermistors and theetalon agree and wavelength drift is suppressed, yielding
the low levels of drift described above.
Figure 6 shows the dependence of wavelength drift on
operating current. The wavelength drift due to changes
in operating current is extremely low--less than ±0.2
pm. When operating current increases due to aging of
the laser diode, diode temperature increases and thewavelength shift to the longer side. When wavelength
monitoring is in effect diode temperature is reduced
as operating current increases to keep the wavelength
constant. The temperature control mechanism can beunderstood from Figure 6. In the conventional structurein which the laser diode and wavelength monitor
are mounted on a single TEC, control of laser diodetemperature causes changes in etalon temperature,
which becomes a factor in wavelength drift. In the module
presented here, however, the use of two TECs enables
the temperatures of the laser diode and the etalon to becontrolled independently, achieving an extremely low drift
and allowing application to DWDM systems with 25-GHzspacing.

0-5-100020406080

24-5-10-202660

Wavelength drift vs. case temperature.

10012014022160

Operating current (mA)

Case temperature (°C)

Figure 580

Laser diode temperature (°C)

exactly with the mid-point of the negative slope. The slopeof the wavelength discriminator curve of the ITU grid issteep, at approximately 14 %/GHz, and the wavelength
drift when the wavelength monitor photodiode deteriorates
and Iwm fluctuates is slight.

Figure 6

Wavelength drift vs. operating current.

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3Highly Reliable 40-mW 25-GHz × 20-ch Thermally Tunable DFB Laser Module, Integrated with Wavelength Monitor

410Power consumption (mW)

Wavelength drift (pm)Tcase = 70°C, T f = 30°C, Iop = 200 mA
50-5PTEC2 (wavelength monitor)
PTEC total

210-100500

1000

1500

2000

PTEC1 (laser diode)

30Figure 7

Figure 9

Wavelength drift during high-temperature storage.

10Wavelength drift (pm)105152025Laser diode temperature TLD (°C)

Time (hours)

Power consumption of thermoelectric coolers(at Tc = 70°C, Tf = 30°C, Iop = 200 mA).

than the ±10 pm, which is the range permissible for
10-Gbps DWDM systems with 25-GHz spacing.

56.

0-5-10

0500

1000

1500

2000

Time (hours)

Figure 8

Wavelength drift during aging test (at Pf = 20 mW,
Tc = 80°C, Tf = 30°C and TLD = 10°C).

40 mW and T LD of 40°C, was less than 500 FITs after
20 years, confirming high reliability at high-temperatureoperation.
In the module with integrated wavelength monitor, it is
the optical axis displacement from the back facet of thelaser diode to the prism, etalon, wavelength monitor andphotodiode that gives rise to wavelength drift. We carriedout a high-temperature storage test for 2000 hours at an
ambient temperature of 85°C. Figure 7 shows the results.
An initial fluctuation can be discerned but saturates atless than ±5 pm. This module is constructed using highly
reliable YAG welding and soldering, and it was confirmedthat the two-TEC structure, though more complex than in
the past, was more resistant to optical axis displacement.
And since tuning is effected by temperature control, it
can be anticipated that there will be a much higher loadon the TECs than with the fixed-wavelength module of thepast. It is therefore possible that there could be increased
optical axis displacement due to strain in the TECs. To
evaluate these aspects an aging test was carried out for2000 hours at conditions designed to increase the load on
the TECs: Pf = 20 mW, Tc = 80°C, Tf = 30°C and TLD = 10°C. The results are shown in Figure 8. Wavelength drift isapparent at the initial stages, but saturates at less than ±3pm
From these results it was possible to confirm thatwavelength drift including long-term reliability, was less

POWER CONSUMPTION

Since an extremely large number of laser modules areused in DWDM systems, reducing the power consumptionof the modules plays an important role in reducing theconsumption of the system as a whole. Since the modules
presented here use two TECs, it was to be expected thatthey would be power-hungry, but when tuned to shorter
wavelengths, for example, the wavelength monitor is kept
constant at a comparatively high etalon temperature T fof around 30°C, and only the laser part is controlled toaround 0°C, so that the amount of heat absorbed by thetwo TECs is different. Thus by designing each TEC tobe optimally suited to its heat absorption, it is possible toreduce the power consumption. Figure 9 shows the power
consumption of the TECs at laser diode temperaturesof 0 to 25°C at operating conditions that would place ahigh load on the TECs, namely Tc = 70°C, Tf = 30°C, andoperating current Iop of 200 mA. For TLD = 0°C, a low total
TEC power consumption--4 W or less--was achieved, andsince power consumption was below 4 W at TLD = 0°C,
even in the case of a tunable range of 4 nm, the specified
temperature can be set at the comparatively low rangeof 0~40°C, holding promise of high reliability for the laser
diode.

7.

CONCLUSIONA CW-DFB laser light source with integrated wavelength
monitor has been developed for DWDM systems with25-GHz spacing. Wavelength drift, which is the mostimportant characteristics with respect to DWDM systemapplicability, was evaluated in terms of variation intemperature, variation in operating current, and longterm reliability. By means of a structure that makes useof two TECs to control the temperatures of the laser
and wavelength monitor sections separately, together
with optimized thermal design, it has been possible toFurukawa Review, No. 242003

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Highly Reliable 40-mW 25-GHz × 20-ch Thermally Tunable DFB Laser Module, Integrated with Wavelength Monitor

obtain highly precise temperature control of the etalon,
and thereby achieve a wavelength drift with respect tocase temperature of ±1 pm and with respect to operating
current of ±0.2 pm.
High-temperature storage tests and aging tests
were also carried out, and although there were initial
fluctuations in wavelength drift, it was confirmed that thesesaturated to ±5 pm, and wavelength drift including longterm reliability was less than ±10 pm, demonstrating thatthese laser modules are applicable to DWDM systems
with 25-GHz spacing.

REFERENCES
1) H. Nasu et al.: ECOC2001 (2001) 428.
2) H. Nasu et al.: OFC2002 (2002) 209.
3) T. Mukaihara et al.: ECOC2002 (2002) P2.7
4) Takagi et al.: Proc. General Conference of IEICE 2002,C-4-44. (in Japanese)
5) H. Nasu et al.: IEEE Photon. Technol. Lett., 15 (2003), 293.
6) ITU-T recommendation, G.692 V.5, (1998) 28.

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