Development of ITLA Using a Full-Band Tunable Laser
Koji Horikawa *, Atsushi Yamamoto *, Takashi Osada *,Hiroyuki Koshi *, Yoshitaka Yafuso *, and Tatsuro Kurobe *A laser control module complying with the integrable tunable laser assembly(ITLA) standards has been developed. A DFB laser array that covers either theC- or L-band in wavelength is incorporated in this module, enabling precise wavelength control
by using a wavelength locker integrated with the laser. Also, by controlling each the SOA, DFB
laser and two independent TECs, the optical output intensity and wavelength can be tuned.
Moreover, dithering functions have been implemented including FM for SBS suppression andAM for signal discrimination. In this paper, the configuration of this module is described andmain characteristics including wavelength stability are presented.

ABSTRACT1.

INTRODUCTION

With the explosive growth in the Internet traffic in recent
years, capacity expansion of dense wavelength-divisionmultiplexing (DWDM) systems is advancing, creating aneed for implementation of flexible networks that canreconfigure themselves depending on traffic situations,and for cost reduction. For this reason, conventional distribution feedback (DFB) lasers with fixed wavelengths are
rapidly being replaced with broadband wavelength tunable lasers that permit lasing wavelength tuning withease. Figure 1 shows the outline of a DWDM communication system. DWDM systems require, since multiple optical signals with different wavelengths are simultaneouslytransmitted, light sources that are stabilized in both wavelength and optical output. And since fixed wavelength
light sources were used conventionally, it was necessary
to keep so many light sources as spares for system main-

tenance that the cost became a problem. But, by using
light sources of wavelength tunable laser that permit arbitrary wavelength tuning, reduction of the system operation and maintenance costs can be achieved. Moreover,light sources of wavelength tunable laser allow for reconfiguration of wavelength allocation depending on situation.
To satisfy such system requirements, wavelength tunable lasers based on various principles have been proposed as shown in Table 1 1)~5). However, they are complicated in their wavelength tuning method, raising problemsof operation characteristics under external environmentchanges and deterioration with age. On the other hand,taking much interest in the long-term stability of thermallywavelength tunable DFB lasers, we have been developing
laser modules that incorporate, on a single substrate,
arrayed DFB lasers with different lasing wavelengths,
S-shaped waveguides and optical couplers 6)~9).
Table 1λ１…λnBack-upLight source

λ１
λ２

λ１
λ２
…

λn

Literature No.

…

…

…

λn

Typical wavelength tuning technologies.
General description

1)

SG-DBR-LD

2)

SSG-DBR-LD

3)

MEMS-based compact external cavity LD

4)External cavity LD using temperature-controlled etalon filter

5)

Surface-emitting LD using length-controlled MEMS cavity

Add
λ１
λ２
…

Figure 1λ１…λn…

Tunable laserλnOutline of DWDM communication system.

* FITEL-Photonics Lab., R&D Div.

The OIF (Optical Internetworking Forum) has established OIF-ITLA (Integrable Tunable Laser Assembly)-MSA(Multi-Source Agreement) as standard specifications, inorder to improve the compatibility of wavelength tunable
lasers with various configurations and formalities as mentioned above. Table 2 shows main specifications of ITLAMSA.

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Development of ITLA Using a Full-Band Tunable Laser

Table 2ITLA-MSA specifications (Long Haul / 50-GHz channel spacing).
Item

Specification

Frequency error

ITU Grid ±2.5GHz

Optical output

10 ~ 13 dBm

Power consumption

6.6 W or lessDitherOptional (AM/FM)

Linewidth

10 MHz or less

RIN

-135 dB/Hz (10 MHz ~ 10 GHz)

SMSR

40 dB or more

Size

74.0 x 30.5 x 10.5 mm3Communication IF

RS232CPower supply voltage-5.2 V / +3.3 V

Here we have developed an ITLA with a control circuitboard complying with the MSA, that integrates a full-band
wavelength tunable laser covering the whole wavelength
range of either the C- or L-band. The ITLA has achieved,by each controlling the semiconductor optical amplifier(SOA), DFB laser and two thermoelectric coolers (TECs),
precise control of optical output intensity and wavelength.
Moreover, two kinds of dithering functions have been
implemented as options, including frequency modulation(FM) for stimulated Brillouin scattering (SBS) suppression
and amplitude modulation for signal discrimination. In thispaper, the configuration of this ITLA is described andmain characteristics including optical characteristics andpower consumption are presented.

2.

CONFIGURATION OF ITLA

and noise has been implemented on a substrate measuring 74 mm x 30.5 mm with a height of 10.5 mm, whereby
the circuit is used to control the full-band tunable laser
module. Below will be described the configuration of
ITLA.
2.1 Full-Band Tunable Laser Module
Figure 3 shows a schematic configuration of the full-bandtunable laser module. The laser chip incorporated consists of a DFB laser array with different lasing wavelengths, an optical coupler that combines optical outputs,
and an SOA that compensates for the loss due to theoptical coupler. The DFB laser array has 12 lasing elements, and the lasing wavelength of each element has aspacing of about 3.5 nm. By thermally tuning the wavelength of a selected DFB laser corresponding to thedesired ITU grid, over a temperature range between 10 ~50°C, it is possible to cover all the full-band wavelength
grids. In addition to the laser chip mentioned above, awavelength locker is incorporated in the laser module tosuppress laser’s wavelength drift due to temporal change.
The wavelength locker comprises two photo diodes (PDs)and an etalon filter, whereby one of the PD monitors optical output, and the other monitors the light that passes
through the etalon to be used for wavelength control. The
curve shown in Figure 4 is known as the wavelength discrimination curve, and shows the relationship between
lasing wavelength and wavelength monitor current. Using
the slope of the wavelength discrimination curve, thewavelength shift is detected as a change in photodiodecurrent. Also, the laser module has two TECs, allowing
the temperature of the laser chip and etalon to be controlled independently. In the conventional structure in

Figure 2 shows the ITLA developed here. A control circuitthat is designed taking account of power consumption

TEC

PD

DFB Array

Full-band tunable laser module
TEC

DFB laser
SOA
Optical coupler

PMF（Polarization
maintaining ﬁber）

PD

30.5 mmFigure 374.0 mm

EtalonConfiguration of full-band tunable laser.

350

Iwm（µA）

300

10.5 mm

75%

250
200

50%

150

25%

100
50
0
1543.71544.71545.71546.71547.7

Wavelength（nm）

Figure 4
Figure 2

Appearance of ITLA.

Wavelength discrimination characteristics with
25-GHz spacing.

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Development of ITLA Using a Full-Band Tunable Laser

which a single TEC is used for controlling laser temperature, changes in etalon temperature tend to cause wavelength drift, making control and manufacturing of themodule difficult. In the module structure presented here,
the use of two independently controllable TECs based on
Peltier element enables enhancement of wavelength controllability and relaxing of manufacturing tolerances.
2.2 Control Circuit Block
Figure 5 shows the control circuit block diagram of the
ITLA. In the temperature control circuit for the two independent TECs, the first control circuit controls the temperature of the etalon to suppress wavelength drift, while thesecond controls the temperature of the DFB laser to makethe temperature correspond to a predetermined wavelength. By means of the two temperature control circuits,
the wavelength can be controlled with high stability andhigh precision. In the DFB laser current control circuit andthe DFB wavelength channel switching circuit, the laser ofan arbitrary wavelength channel is driven based on thepreset data stored in the memory on the circuit board.
Since the SOA current control circuit controls the SOA
current based on the monitored value of optical output
intensity, the intensity can be changed independently of
wavelength control. Also, functions conforming to MSA
have been implemented on this ITLA, which carry out
diverse command processing and communication of various alarms such as wavelength, temperature, opticalintensity, etc. to higher-level devices. Moreover, the builtin CPU allows version upgrade of its firmware, without
impacting the wavelength and optical output.
In terms of SBS suppression FM, any suitable signals
are generated in the module, which are then superim-ITLA
DFB Laser module

TEC

DFB Laser

ＳＯＡ
EtalonＰＤ

TEC
TEC

posed on the DFB current. As for signal discrimination
AM, the TxTrace circuit in the ITLA converts the TxTracesignal that is input from higher-level devices via connector
to suitable signals, and subsequently superimposes themon the SOA current. One of the features of this ITLA is
that, since this structure allows independent control of the
DFB and SOA currents, the dithering functions can becontrolled without being interfered from the other,enabling simultaneous control.

3.

Main characteristics of the ITLA developed here will bepresented in this Section. A laser module with a wavelength locker adapted to 50-GHz C band was mainly used
for the evaluation. Below will be described the wavelength
tunable range and wavelength stability, followed by performance evaluation including power consumption, noisecharacteristics such as linewidth and relative intensity
noise (RIN). Lastly, evaluation results of dithering functions will be summarized.
3.1

Wavelength Tunable Range and Wavelength
Stability
ITLAs are required to provide performance equivalent tothat of conventional fixed wavelength DFB lasers, so thata variety of specifications are set out in the MSA withregard to optical characteristics 10). The ITLA developed
here has achieved, to make it adequate for 50-GHz spacing DWDM systems, extremely precise wavelength control
by controlling the temperatures of the laser and etalon.
Figure 6 shows optical spectrum traces, where two lasers
were used to lock the wavelength to arbitrary wavelength
channels. From the Figure it can be seen that this ITLA isadequate for both C-band (1530 nm ~ 1565 nm) andL-band (1565 nm ~ 1610 nm). Also, the side-mode suppression ratio (SMSR) is 40 dB or better-- amply goodresult. Moreover, it is possible to make this ITLA adequate
for 25-GHz spacing DWDM systems, by changing thelaser to the one having wavelength discrimination characteristics shown in Figure 4.

ＰＤ

10
TEC−ATC

DFB−ACCTEC−ATC（AGC）
Dither CTRL
（FM）

Dither CTRL（AM）

C ont r ol l er

ＣＰＵ

RS232

Non−volatilememoryTxTrace signal

Nomalized intensity（dB）

Channel selectSOA−APC（ACC）

CHARACTERISTICS EVALUATION

0
-10
-20
-30
-40
-50
-60
1510

1530

1570

1590

1610

Wavelength（nm）

Host module
Figure 6
Figure 51550

Lasing spectra at C band and L band.

Block diagram of ITLA.

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Development of ITLA Using a Full-Band Tunable Laser

2.5
2
1.5
Frequency error（GHz）

two independent TECs, low maximum power consumption of 4.2 W or less. Figure 10 shows the power consumption at each wavelength at ambient temperatures of
75°C and -5°C, both of which are assumed to cause highpower consumption. It has been confirmed that the power
consumption is 4.2 W or less at each wavelength channel.
‑120

1

ch=1ch，Isoa=103mA，Idfb=150mA‑130
RIN（dB/Hz）

Figure 7 shows the error in targeted frequency corresponding to optical output changes ranging from 7 dBm
to 13 dBm for every wavelength channel. It has been confirmed that, to whatever wavelength channel the target isset, the frequency error is ±0.5 GHz or lower at any optical intensity, showing that the wavelength is maintained at
a high level of stability. In addition, it has been shown thatthe wavelength stability is sufficiently within ±2.5 GHz, i.e.,
ITLA-MSA specification.

‑140
‑150
‑160
T=‑5℃
T=25℃
T=75℃

‑170

0.5

‑180

0

0

5

10

-0.5
-1

Figure 9

-1.5

20

25

Relative intensity noise characteristics.

df [GHz]̲7 dBm
df [GHz]̲10 dBm
df [GHz]̲13 dBm

-2
-2.5

15

Frequency [GHz]

0

10

20

30

40

50

60

70

80

90

7

Wavelength channel

Frequency stability.

3.2 Performance Characteristics
In order to realize long-haul transmission, it is necessary
to satisfy MSA standards with respect to noise characteristics such as spectrum linewidth and RIN 10). Figures 8
and 9 show the measured linewidth at each wavelengthand the measured RIN at each wavelength, in different
ambient temperatures, respectively. In all measurement
results, virtually no changes in noise due to differing
ambient temperatures were observed, thus the low-noisecharacteristics suitable for long-haul transmission havebeen confirmed.
The MSA stipulates, with the aim of being incorporated
in 300-pin transponders, not only transmission characteristics but also the size and power consumption 10). This
full-band tunable laser module has achieved, by using

10
T=‑5℃

9

T=25℃

Linewidth（MHz）

8

T=75℃

7
6
5
4
3
2
1
0
191.00

192.00193.00

194.00195.00

196.00Freqency（THz）

Figure 8

Linewidth characteristics.

197.00

Dissipation power（W）

6

Figure 75432
T= ‑5℃
T= 75℃

10
191.00

192.00193.00

194.00195.00

196.00

197.00Frequency（THz）

Figure 10 Power consumption.

3.3 Dithering Function Characteristics
Since light reflection due to SBS has a significant influence on communication systems such that it limits thetransmissible optical intensity, suppression of such reflection is essential. Frequency modulation is one of thecountermeasures to suppress SBS, so that, in this ITLA,
control of optical wavelength is carried out by modulating
the DFB laser driving current using specified frequency
and amplitude patterns. Figure 11 shows the measured
reflection loss vs. optical output intensity, in which theoptical output intensity is measured using an opticalpower meter, after passing through a single-mode fiber 30
km in length. It can be seen that, with FM dithering off, thereflection loss increases as the optical output intensity
increases, while with dithering on, reflection loss due to
SBS remains low showing virtually no change irrespectiveof the level of the optical output intensity. Thus, it has
been shown that the frequency modulation is functioning
effectively.

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Reﬂection（dB）

Development of ITLA Using a Full-Band Tunable Laser
0

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FM Dither oﬀ
FM Dither on

‑15
‑20
‑25
‑30

5

6

7

8

9

10

11

12

13

14

15

LD setting（dBm）

Figure 11 Reflection loss characteristics with a 30-km SMF.

Amplitude modulation for signal discrimination is thefunction in which an arbitrary discrimination signal
TxTrace sent from the higher-level device is used to control the optical output intensity in order for the receiver todiscriminate the transmitter. In this ITLA, control of optical
output intensity is achieved by converting the TxTrace signal from the higher-level device into suitable signals, andsubsequently superimposes them on the SOA current.

4.

CONCLUSIONSAn ITLA incorporating a full-band tunable laser compliant
with MSA has been developed. This ITLA covers the entire
range of either the C- or L-band, achieving wavelength
locking for DWDM systems of 50-GHz spacing. It is also
applicable to 25-GHz spacing systems by changing thelaser used. Since this ITLA offers multi-function performance including dithering function incorporated as an
option, it is expected to make a significant contribution to
performance enhancement, downsizing and cost reduction of DWDM systems.

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