INFOCOMMUNICATIONS+18 dBm High-Power Tunable Laser Modules for
Digital Coherent Optical Communication Systems
Mitsuyoshi MIYATA*, Hirokazu TANAKA, Masao SHIBATA, Yoshiki OKA,Toshimitsu KANEKO and Hiromitsu KAWAMURA---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------With the rapid increase in communication traffic, digital coherent communication technology has spread, particularly for trunknetworks. For the application of the technology to metropolitan area networks and data center interconnection, compact andhigh-power light sources that enable high-density mounting in communication units are required. We have developed a microintegrable tunable laser assembly (Micro-ITLA) using a tunable distributed amplification chirped sampled grating distributed
reflector (TDA-CSG-DR) laser that achieves the industry’s highest output power of +18 dBm.

---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Keywords: coherent, tunable, high-power, narrow linewidth, Micro-ITLA

1. Introduction
Communication traffic is continually increasing due
to various factors, such as the dissemination of smartphones and tablet PCs, as well as the spread of cloudcomputing services. The laser module we have developed
is a continuous wave (CW) tunable light source mainly
used for optical trunk networks that support such communication demand.
Optical trunk networks use a dense wavelength division multiplexing (DWDM) system to achieve high-speed
and high-capacity communication of 10 Gbit/s x nλ (nλ:about 90). Furthermore, using a digital coherent system,
ultrahigh-speed communication of 200 Gbit/s per wavelength has recently been put into practical use.
In a coherent optical communication system, a signal
is superimposed over the optical phase. This particularsystem simultaneously detects polarization and therefore itis suitable for long-distance transmission thanks to its hightolerance for transmission line dispersion. However, since
optical interference with the local oscillator is necessary toreproduce the signal, large and complicated equipment isneeded. Thus, conventionally the use of coherent opticalcommunication was limited to long-distance communication.
It has been attempted to use this communication
system for metropolitan area networks (transmissiondistance: up to 600 km) and data center interconnection inrecent years. For such use, there are stringent requirements
for the system in terms of the size of the communicationequipment and the power consumption, and therefore lightsource modules are required to be downsized and power
efficient. In January 2016, the specifications of CFP2
analog coherent optics (CFP2-ACO),*1 a CFP2 size transceiver, were standardized(1) by the Optical Internetworking
Forum (OIF).*2 The package size is 41.5 × 107.5 × 12.4mm and the number of tunable light sources that can beinstalled is substantially limited to only one unit. The
optical output power from the light source is branched
and used for both transmission and receiving functions;
therefore, an optical output power higher than ever isrequired.

Under such circumstances, we have developed andsuccessfully mass-produced a compact tunable laser
module of the micro-integrable tunable laser assembly(Micro-ITLA)(2) that can achieve an optical output powerof +18 dBm. This paper provides an overview and thecharacteristics of the module, including a comparison withthe conventional ITLA(3) product.

2. Product Specifications andDevelopment Targets
Table 1 shows the outline dimensions and power
supply voltage of a tunable laser module standardized by
the OIF. As compared to the conventional ITLA standard,
the outline dimensions specified by the Micro-ITLA standard are less than 40% in area and less than 30% in volume(Fig. 1) and the usable power supply voltage is also set to alower level.
Table 1. Required specifications for a tunable laser module
Item

ITLA

Micro-ITLA

Unit

Max. outline
dimensions

74.0×30.5×10.5

45.0×20.0×7.5mm

Power supply
voltage+3.3-5.2+3.3-5.2 (Not used by us)
+1.8V

ITLA

Micro-ITLA

Fig. 1. Tunable laser module size comparison

30 · +18 dBm High-Power Tunable Laser Modules for Digital Coherent Optical Communication Systems

Table 2. Target specifications for Micro-ITLA development
Item

ITLA (Reference)

Micro-ITLA

Unit

Optical output
power+16+18dBm

Effective linewidth

500300kHzPowerconsumption

5.3

4.5

W

Temperature range-5 to 75°C

3. Tunable Laser Chip
The laser chip used this time is our original tunable laser
diode chip called tunable distributed amplification-chirped
sampled grating-distributed reflector (TDA-CSG-DR) laser.(4)Figure 2 shows its section diagram. The laser cavity has asingle-stripe type structure, with (1) a tunable distributed
amplification-sampled grating-distributed feedback(TDA-SG-DFB) region, i.e. a gain region, and (2) a chirped
sampled grating-distributed Bragg reflector (CSG-DBR)
region, i.e. a reflection region, optically coupled together.
In the TDA-SG-DFB region, an active waveguide having again and a passive waveguide are alternately located, andunderneath each, a sampled grating (SG) region is formed
with partial gratings periodically laid out. Current injection
to the laser electrodes located on top of the active waveguides generates periodic gain peaks in the gain spectrum.
The CSG-DBR is a DBR having SGs whose intervals areperiodically varied. The periodic reflection peaks obtained
here form a gradual envelope curve, which functions as aband reflection filter. Wavelength tuning can be performed
using heaters at the gain and reflection regions by controlling the refractive index of the waveguides right underneath them.

Fig. 2. TDA-CSG-DR laser structure

In addition, on the optical output side, a semiconductor optical amplifier (SOA) function is integrated for
optical amplification and shutter functions. In the back of
the reflection region, a backside absorber (BA) is locatedto prevent light scattering inside the package.
The most significant feature of this chip is that it caneasily obtain high optical output power. An SOA function
is integrated for some tunable laser assemblies for communication systems like ours. However, the intensity of thelight incoming to the SOA must be increased in order toefficiently obtain high optical output power because thegain of any amplifier is saturated. The TDA-SG-DFB
structure optimally amplifies the light that enters the SOA.
Furthermore, the laser cavity length including both thegain and reflection regions is as long as about 2 mm. Thisis structurally advantageous for low phase noise characteristics (narrow linewidth characteristics), which are important for the light source of coherent communication. In
terms of power consumption, we tried to suppress thermal
diffusion from the heater, and the maximum heater power
consumption is as low as 0.5 W in the whole chip.
3-1 Basic wavelength tuning principle
Figure 3 shows the gain mode generated at the gain
region and the reflection mode generated at the reflection
region as well as the resultant oscillation wavelength of the(a)CSG-DBR reflection

Oscillation

Intensity (dB)To downsize the whole laser module, it is required todownsize not only the laser device package but also its
circuit boards. We have selected smaller and lower profile
components and efficiently arranged them in two circuitboards. This allows them to be mounted to a small module
while achieving the functions equivalent to those of aconventional module. In addition, the voltage required for
driving elements has been lowered to comply with the OIF
standard.
Table 2 shows the target specifications as a MicroITLA. Conventionally, an optical output power of about+16 dBm was considered high. However, even higher
optical output power is required because the modulation
loss is increasing in line with higher functionalities andonly a single unit is used for both the transmitter and local
oscillator. We set the target to +18 dBm for product development. The target for the linewidth, or a frequency
stability indicator, was set to 300 kHz, which is requiredfor dual-polarization-16 quadrature amplitude modulation(DP-16QAM)*3 used for digital coherent optical communication at 100 Gbit/s or more. We also aimed to reduce thepower consumption and set the target to 4.5 W for driving
at a case temperature of 75°C with an optical output powerof +18 dBm.

TDA-SG-DFB gain

(b)

(c)

1540

1545

1550

1555

1560

Wavelength (nm)Fig. 3. The behavior of each function when the heater powerat the reflection region is changed

SEI TECHNICAL REVIEW · NUMBER 83 · OCTOBER 2016 · 31

changing the power of three heaters constituting theCSG-DBR. With appropriate driving conditions, the wavelength filtered by the CSG-DBR can vary widely and thewavelength range where the Vernier effect works can movesignificantly. Consequently, the whole C-band range iscovered.

4. Characteristics
4-1 Frequency tunability characteristics and optical
output power
Figure 5 shows the frequency tunability characteristics of the Micro-ITLA. The automatic frequency control(AFC) using the frequency locking mechanism provided in
the laser package has achieved good frequency stability ofless than +/-0.5 GHz within a case temperature range of -5
to 75°C.

2.0Frequency accuracy (GHz)chip. For the gain and reflection regions, the interval lengthof partial gratings L (= segment length) is designed so thateach mode spacing may slightly differ from each other, andlaser oscillation is generated at the wavelength at whichthese modes coincide with each other (Fig. 3 (a)).
Figures 3 (b) and (c) show the behavior when thepower of the phase adjustment heater slightly changed at
the reflection region. Slightly shifting the wavelength modeon the reflection region enables the coinciding wavelength
to jump to the adjacent mode at the gain region.
Tuning wavelength with a slight phase change is generally
called the Vernier effect, and this mechanism is used in almost
all single-stripe type tunable lasers available on the market.
Any wavelength can be tuned if the heater in the gain region
can be simultaneously controlled to shift the gain mode.
3-2 Full band tuning range achieved by CSG-DBR
The C-band (wavelength: 1530 to 1565 nm) mainly
used in the DWDM is a wavelength band of about 40 nm
in width; however, a tuning range of only about 20 nm canbe obtained by the aforementioned Vernier effect alone.
This is attributable to the fact that the tuning range obtainable by the Vernier effect is generally about nine times themode spacing. To fulfill the required bandwidth, a wider
mode spacing of about 5 nm is necessary, which requires ashorter segment length of 70 μm or less. (The mode
spacing is inversely proportional to the segment length.)
Should the 30μm-heater region be used in the segment, themaximum heater temperature would exceed 300°C.
To solve this problem, we made modifications to theSG-DBRs, which are normally laid out with a constantsegment length. To be specific, the reflection region wasarranged to comprise three segments of different lengths(L, L + ΔL, L - ΔL), with each segment being independently controlled by the heater. This SG-DBR, for which
the segment lengths are modulated with a constant interval,
is called chirped SG-DBR (CSG-DBR).
Figure 4 shows the reflection spectrum of theCSG-DBR and the oscillation wavelength obtained by

Tc= -5degC
Tc=+35degC
Tc=+75degC

1.0

Pset = +18dBm

0.0
-1.0
-2.0
191.0

192.0

193.0

194.0

195.0

196.0

Channel frequency (THz)

Fig. 5. Frequency characteristics

Figure 6 shows the optical output power characteristics. The automatic power control (APC) by monitoring theoptical output power has achieved good stability of lessthan +/-0.5 dB.

CSG-DBR reflection
Oscillation

Intensity (dB)

Optical power (dBm)

20.0
19.5
19.0

Pset = +18dBm

Tc= -5degC
Tc=+35degC
Tc=+75degC

18.5
18.0
17.5
17.0
16.5
16.0
191.0

1520

1540

1560

1580

192.0

193.0

194.0

195.0

196.0

Channel frequency (THz)

Wavelength (nm)Fig. 6. Optical output power characteristics
Fig. 4. CSG-DBR reflection characteristics andoscillation wavelength

32 · +18 dBm High-Power Tunable Laser Modules for Digital Coherent Optical Communication Systems

Figure 7 shows the power consumption characteristics. The power consumption was less than 4.5 W, lower
than the conventional level. The TDA-CSG-DR laser tunes
the oscillation wavelength by adjusting the phase of theTDA-SG-DFB region instead of changing the laser temperature as in the conventional way. Thus, it keeps the laser
temperature almost constant at high side, enabling low
power consumption.

Power dissipation (W)8.0

Tc=-5degC
Tc=+35degC
Tc=+75degC

7.0
6.0

Pset = +18dBm

5.0
4.0
3.0
2.0
1.0
0.0
191.0

192.0

193.0

194.0

195.0

4-2 Effective linewidth
The linewidth of the Micro-ITLA was evaluated by
the coherent detection method using an intradyne coherent
receiver (ICR). Figure 8 shows the measurement system
configuration. An external cavity laser with a linewidth of
20 kHz or less is used as a reference light source. The lightof the Micro-ITLA interferes with the reference inside theSignalIn-phaseICR

LocalRef.

Quadrature

Real-time
Oscilloscope

Fig. 8. Effective linewidth measurement system configuration
using ICR

500
Effective linewidth (kHz)We have developed a Micro-ITLA as a compact
tunable light source that enables high-density mounting inunits for digital coherent optical communication systems.
Use of the newly developed laser chip achieved the industry’s highest optical output power of +18 dBm while realizing low power consumption. In addition, an effective
linewidth of less than 300 kHz was achieved, which isrequired for large capacity communication of 100 Gbit/s ormore. We hope that our Micro-ITLA will become a key
device that can respond to the increase in communication
traffic that is expected to accelerate in the future.

Technical Terms

Fig. 7. Power consumption characteristics

ATT-20dB

5. Conclusion

196.0

Channel frequency (THz)

Micro-ITLA

ICR. The electrical output from the ICR is directly
observed using a real-time oscilloscope and the instantaneous frequency fluctuation is used to calculate the effective linewidth. Good effective linewidth characteristics ofless than 300 kHz were confirmed as shown in Fig. 9.

Pset = +18dBm

400300200＊1	CFP2-ACO (CFP2 Analog Coherent Optics): A CFP2
size transceiver having a coherent communication
function but not equipped with a digital signal
processor (DSP), which digitally processes electricalsignals.
Optical Internetworking Forum (OIF): An optical＊2	technology industry association engaged instandardization activities.
DP-16QAM (Dual-polarization-16 quadrature＊3	amplitude modulation): QAM stands for quadratureamplitude modulation. The 16-QAM is a communication
system that combines two different amplitudes in
addition to four phases mutually different by 90° andthus simultaneously transmits 16 values, or 4 bits ofinformation. The DP-16QAM is a communication
system that additionally uses orthogonal dual
polarization (DP) and thus simultaneously transmits
16-QAM signals, or a total of 8 bits of information.
References(1)	OIF-CFP2-ACO-01.0, “Implementation Agreement for CFP2- Analogue
Coherent Optics Module”(2)	OIF-Micro-ITLA-01.1, “Micro Integrable Tunable Laser Assembly
Implementation Agreement”(3)	OIF-ITLA-MSA-01.3, “Integrable Tunable Laser Assembly Multi SourceAgreement”(4)	T. Kaneko, Y. Yamauchi, K. Uesaka, and H. Shoji, “A Single-Stripe
Tunable Laser Operated at Constant Temperature Using Thermo-OpticEffect,” Microoptics Conference (MOC), G-2 (2011)100
Tc=+25degC0
191.0

192.0

193.0

194.0

195.0

196.0

Channel frequency (THz)

Fig. 9. Effective linewidth characteristics

SEI TECHNICAL REVIEW · NUMBER 83 · OCTOBER 2016 · 33

Contributors The lead author is indicated by an asterisk (*).

M. MIYATA*• ‌Lightwave Components Division, Sumitomo ElectricDevice Innovations, Inc.

H. TANAKA• ‌Lightwave Components Division, Sumitomo ElectricDevice Innovations, Inc.

M. SHIBATA• ‌Manager, Lightwave Components Division, SumitomoElectric Device Innovations, Inc.

Y. OKA• ‌Lightwave Components Division, Sumitomo ElectricDevice Innovations, Inc.

T. KANEKO• ‌Assistant General Manager, Transmission Devices
R&D Laboratories

H. KAWAMURA• ‌Group Manager, Lightwave Components Division,Sumitomo Electric Device Innovations, Inc.

34 · +18 dBm High-Power Tunable Laser Modules for Digital Coherent Optical Communication Systems