Development of Full-Band Tunable Laser
by Tatsuro Kurobe *, Tatsuya Kimoto *, Kengo Muranushi *, Toshikazu Mukaihara *, Maiko Ariga *,Nozomu Matsuo * and Akihiko Kasukawa *2

A novel laser module has been developed comprising a tunable laser that is
indispensable as a signal light source for increasing the capacity of DWDM
systems. DFB laser array known for its wavelength stability in principle has been employed forthe wavelength tuning scheme. The module includes a low-noise semiconductor amplifier
enabling high-power operation. This paper describes the structure, operating principle andoptical characteristics of this wavelength tunable laser, and also presents the structure,
characteristics and reliability of the laser module with a built-in wavelength locker.

ABSTRACT1.

INTRODUCTION

The increasing capacity of dense wavelength-divisionmultiplexing (DWDM) communication systems absolutely
requires, in order to realize reconfigurable networks
adaptable to traffic conditions and to reduce the inventory
cost of equipping backup transmitters, full-band tunable
lasers with tunable lasing wavelengths. Figure 1 shows an
optical communication network in which various opticaltransmitters are used depending on application. Whereas
light sources with a fixed wavelength have been usedconventionally, full-band tunable lasers combined withmodulators are recently mounted in transponders andline cards for medium- to long-range applications, thereby
rapidly replacing conventional fixed-wavelength lasers.
Application areas of the new laser have been expandedfrom long-range areas to the medium- to short-range
areas such as the metro and access networks.
In order to meet such market requirements, a variety oftunable lasers have been proposed so far, including
lasers using mechanical movable mechanism, lasers withexternal cavity and lasers with distributed Bragg reflector.
But the wavelength tuning methods are complicated for
all these lasers, raising problems in terms of operating
performance against environmental changes as well as
characteristic changes due to deterioration with age.
Moreover, since their wavelength tuning mechanisms are
significantly different from that of thermally tuned distributed feedback (DFB) lasers conventionally used in opticalcommunications, use of sophisticated control circuit isindispensable. On the other hand, wavelength selectablelasers 1)~4) have been proposed as a wide wavelength tunable laser based on the DFB laser technology with provenreliability and established track record. The laser consistsof an array of DFB laser elements of different lasing wavelengths that is integrated with an optical coupler and asemiconductor optical amplifier (SOA), whereby a wide
range of wavelength tuning can be realized by means oflaser element switching and thermal wavelength tuning.
Since it is based on the same operation principle as forthe DFB laser, it has good long-term reliability, and it
allows for using a simple feedback circuit to easily control
the wavelength and optical output. In this development,
we have expanded the wavelength tuning range of thiswavelength selectable laser to enable full-band wavelength tenability, and improved its fundamental characteristics such as optical output and noise characteristics.
Furthermore, we have developed a laser module with abuilt-in wavelength locker, which is capable of wavelengthtuning and controlling over the entire range of the C bandand L band, respectively.

2.

INTEGRATED FULL-BAND TUNABLE
LASER CHIP

A tunable laser for transponder application is required toprovide a tunable wavelength range of 35 nm or more

LFT TranponderLong Haul

SFT Tranponder

METRO
LAN

XFP, X2, XENPAK

Access* FITEL Photonics Lab., R&D Div.
*2 Yokohama Lab., R&D Div.

Figure 1

Wavelength-division multiplexing optical network andoptical transmitter.

Furukawa Review, No. 33

2008

1

Development of Full-Band Tunable Laserand a fiber-coupled output of 20 mW or more. The tunable wavelength range of thermally tuned DFB laser elements that constitute the laser developed here is determined by the power consumption and the operating temperature range specified from the standpoint of reliability,
and is typically about 4 nm. With a view to covering thefull band, the laser was designed to be such that, taking
manufacturing variation into account, twelve DFB laser
elements having different wavelengths at a wavelength
spacing of 3.45 nm were integrated. From the standpointof lasing wavelength control, a phase shift-type diffraction
grating was employed. This design has achieved a wavelength tuning range of 38 nm by means of changing theoperating temperature between 10℃ and 50℃. Figure 2
shows a photomicrograph of the integrated laser elements manufactured here. In addition to the DFB laser
array, an optical coupler to combine the output from every
DFB laser element is integrated together with an opticalamplifier that allows compensating for the loss in the optical coupler. The chip size is 500 μm × 2600 μm, and thelengths of the DFB laser element and the optical amplifierare 600 μm and 900 μm, respectively. Bent waveguides
and antireflection coating were provided at the end facet
to suppress the reflection from there.
As for the manufacturing process, a quantum well structure to be used as the emitting region of DFB laser andthe amplifying region of SOA was grown on an InP substrate, followed by formation of diffraction gratings byusing an electron beam lithography system and dry etching equipment. Then, etching and regrowth processes
were carried out to form the passive waveguide structure
and the current blocking layer for current confinement, as
well as the cladding layer that serves for current injection

Figure 2

Optical integrated chip of full-band tunable laser
showing DFB laser array, coupler and SOA.

REDUCTION OF SPECTRUM
LINEWIDTH

The tunable laser is required to have characteristics
equivalent to those of the DFB lasers conventionally used,
including, when application to long-haul transmission isintended, low-noise characteristics shown by such indexes as spectrum linewidth and relative intensity noise(RIN). In the case of the integrated laser developed here,
return light that is reflected to the DFB laser from the laser
element itself and from the outside should be taken into
account. Thus, we have optimized the structure of theintegrated laser to achieve low-noise characteristics 5).
Figure 5 shows a model of possible reflection points within the laser chip, including those at the joint between thepassive and active elements, the end facet of coupler andthe end facet of the SOA. Analysis results of the lasing
spectrum have shown that the return light from the end
facet of the SOA influences the noise characteristics,
resulting in periodic increases in the spectrum width

Nomalized Intensity（dB）

PMF coupled power（mW）

3.

10

50
40
10℃

30
20
50℃

10
0

and light guiding. Finally, electrodes for current injection
were formed and the laser elements were electrically separated to complete the laser chip. Layer growth was carried out using metal organic chemical vapor deposition(MOCVD), and methane hydrocarbon-based dry etching
was used for etching. Figure 3 shows the optical fibercoupled output characteristics of the integrated laserchip.
The DFB laser is driven at a constant current of 150 mA,
and the optical output is controlled by adjusting the SOA
current. To tune the wavelength, the laser is operated at an
operating temperature between 10℃ and 50℃. Any DFB
laser element can output, even when driven at an operating temperature of 50℃, an optical power of 30 mW or
more, which is sufficient for applications to be combinedwith external modulators. Figure 4 shows the superimposed lasing spectrum. It has been confirmed that both
the C band and L band integrated laser chips show a highside-mode suppression ratio (SMSR) of 40 dB or more
and good optical signal-to-noise ratio (OSNR).

0
‑10
‑20
‑30
‑40
‑50
‑60
1510

0

100

200

1530

1550

1570

1590

1610

Wavelength（nm）

300

SOA current（mA）

Figure 4
Figure 3

Optical output characteristics.

Superimposed lasing spectrum of the C-band and Lband lasers.

Furukawa Review, No. 33

2008



Development of Full-Band Tunable Laser

R facetR MMI（a）Top view

R MMI

DFB laser

Figure 5

R joint

R facet

SOA

50

5

40

4

30

3

20

2

10

1

0

100

0
300

200

SOA current dependency of optical output and spectrum linewidth.

Tld=25℃, Isoa=200 mA, Idfb=150 mA‑130

Rout=5.7×10‑3

RIN（dB/Hz）

Linewidth（MHz）

6

‑120

Rout=1.0×10‑4

6
4
2

‑140
‑150
‑160
‑170

120

140

160

180

200

DFB laser current（mA）

Figure 6

Figure 7

Rout=1.1×10‑5

860

SOA current（mA）

Typical model of reflected return light in optical integrated chip.

0100

In actual DWDM systems, the wavelength channels arearrayed with a wavelength spacing of 50 GHz, so that the
wavelength accuracy should be kept within a range of±2.5 GHz including deterioration with age. To realize
such a precise range of wavelength control, it is necessary to install a wavelength locker in the tunable laser
module. In general, however, laser modules with a built-in
wavelength locker reported so far were relatively large insize.
Accordingly, we have developed a new laser modulewith a built-in wavelength locker that can lock the fullband wavelength channels 6). Figure 9 shows a schematicof the laser module with built-in wavelength locker. The
package is a 26-pin butterfly package, the industry standard, making it adapted to high-density mounting in the
recent optical communication systems. The module
accommodates a beam splitter for collimated beam
branching, an etalon for 50-GHz channel spacing (possibly 25-GHz in future), and two photodetectors for wavelength and power monitoring. Figure 10 shows the wavelength discrimination curve using a 25-GHz spacing etalon. Two sets of Peltier elements are used to independently control the temperatures for laser wavelength tuning
and for wavelength locking, so that by means of feedback0

Waveguide +MMI coupler（b）Cross‑section view

10

LASER MODULE WITH BUILT-IN
WAVELENGTH LOCKER

Linewidth（MHz）

R joint

4.

Output power（mW）

depending on the laser driving conditions. This is causedby the fact that the reflected return light is amplified again
in the SOA, giving disturbance to stabilized operation of
the DFB laser.
We have optimized the structure of the SOA end facet tominimize the effects of the return light. Figure 6 shows thedependency of the spectrum linewidth on the DFB laser
driving current for the lasers with different end facet reflectivity. As can be seen, while the spectrum linewidth
exceeds 10 MHz at a reflectivity of 5.7 × 10-3 dependingon the DFB current, oscillatory behavior of the linewidth issuppressed at a reflectivity of 1.1 × 10-5, achieving a
superior value of 2 MHz or lower. Figure 7 shows the SOA
current dependency of the light output and the spectrum
linewidth at an operating temperature of 25℃. It can beseen that spectrum linewidths of 2 MHz or lower and optical outputs of 50 mW or higher have been achieved for all
DFB lasers. Moreover, as shown in Figure 8, the effect ofspontaneous emission of the SOA on the spectrum linewidth is minimal, together with a low RIN of -145 dB/Hz.
Thus, as the result of optimization through modeling of
the spectrum linewidth for the integrated laser, the wavelength selectable light source using semiconductor amplifier with maximized output and narrowest spectrum linewidth has been realized.

Dependency of spectrum linewidth on facet reflection
coefficient and DFB current.

‑180

0

5

10

15

20

Frequency（GHz）

Figure 8

Relative intensity noise (RIN) characteristics.

Furukawa Review, No. 33

2008



Development of Full-Band Tunable Laser

control based on this curve, it is possible to control the
wavelength very precisely.
Not only initial characteristics but also long-term reliability is important for laser modules for optical communications. Figures 11 and 12 show the changes in output
power and wavelength, respectively, after high-temperature shelf tests at 85℃ for 2000 hours. It can be seen thatthe power change and the wavelength change are very
precisely controlled at 10 % or less and 5 pm or less,respectively. An assembly process using YAG laser-based
welding and soldering has enabled us to manufacturesuch a laser module with built-in locker that is robust, low

LD Lens

ISO

Etalon5.

PD
PMF

CONCLUSION

A tunable laser module that is indispensable as a signal
light source for increasing the capacity of DWDM systems
has been developed. DFB laser array known for its wavelength stability in principle has been employed as the
wavelength tuning scheme, and the long-held problem ofnoise in semiconductor amplifier has been solved to real-

PD

Figure 9

in deterioration with age and highly reliable. Figure 13
shows the result of consumption power evaluation of thismodule including the control circuit. Even under the most
severe condition of external temperature at 75℃, thepower consumption is 3 W or less for the laser portion, or
5 W or less including the control circuit. While it is recommended that, in view of installation on transponders, thepower consumption of a laser with control circuit shouldbe 6.6 W or less 7), the results here fully satisfy this criterion.
Thus it has been shown that the laser module with builtin wavelength locker developed here is tunable over the
full-band wavelengths, low in noise and is highly reliable,
thereby meeting the requirements for optical transmittersto be used in transponders and the like.

  Laser module with built-in wavelength locker.
20

Iwm（µA）

300

75％

250

50％

200

25％

150
100
50
0
1543.71544.71545.71546.71547.7

Locked wavelength shift（pm）

350

wavelength（nm）

15
10

0
‑5
‑10
‑15
‑20

Figure 10 Wavelength discrimination curve with 25-GHz wavelength spacing.

< ±5 pm

5

0

500

1000

1500

2000

Time（hr）

Figure 12 Results of high-temperature shelf test: change inwavelength.

20
15

8.0
< ±10%

Power dissipation（W）

5
0
‑5
‑10
‑15
‑20

7.0

6.6 W

Tc=75 ℃
Max 4.8 W

6.0
5.0
4.0
3.0
2.0
1.0
0.0

0

500

1000

1500

2000

Time（hr）

Figure 11 Results of high-temperature shelf test: change in
optical output.

19
1.
90
19
2.
30
19
2.
75
19
3.
30
19
3.
75
19
4.
10
19
4.
55
19
5.
00
19
5.
40
19
5.
85
19
6.
25

Pf change（％）

10Frequency（THz）

Figure 13 Power consumption of module (control circuit included).

Furukawa Review, No. 33

2008



Development of Full-Band Tunable Laserize lower noise characteristics, achieving a wide wavelength tuning range covering the entire bandwidth of the
C and L bands, as well as high power operation.
Moreover, a compact laser module with built-in wavelength locker that is compliant to the size of industry standard has been manufactured, thereby realizing highly reliable control of wavelength and optical output. The fullband tunable laser developed here has high reliability,
and is very promising as a light source for application to
the developing DWDM optical communications.

REFERENCES
1) M. Bouda et al.; Proc., OFC 2000, TuL1, pp. 178-180.
2) H. Oohashi et al.; Tech. Dig., IPRM ‘2001, Nara, FBI-2, pp. 575-578.
3) K. Kudo et al.; IEEE Photonics Technology Letters, vol. 12, pp.242244, 2000.
4) T. Kurobe et al.; Tech. Dig.,IPRM2003, ThB1, pp. 339-341, 2003
5) T. Kimoto et al.; Proc., ISLC2004, SaA6, pp.149-150, 2004.
6) T. Mukaihara et al.; Proc., ECOC 2003, We.4.P.81, pp. 718-719,
Sept. 2003.
7) OIF-ITLA-MSA-01.1; Integrable Tunable Laser Assembly Multi
Source Agreement

Furukawa Review, No. 33

2008