High-Power CW-DFB LDs for Optical Communications
by Keishi Takaki *, Tomofumi Kise *, Kazuomi Maruyama *, Koji Hiraiwa *,Nobumitsu Yamanaka *, Masaki Funabashi * and Akihiko Kasukawa *CW-DFB Lasers can be used for WDM and CATV applications by combining
them with external modulators. For WDM application, we successfully obtained alow driving current of less than 200 mA at 40 mW from S-band to L-band by taking care of the
optical confinement of the MQW structure. On the other hand, by optimizating cavity length andcoupling coefficient, we achieved a high output power of 175 mW and a narrow linewidth of 0.8
MHz even at 100 mW for CATV applications.

ABSTRACT1.

INTRODUCTION

Wavelength Division Multiplexing (WDM) is a very effective and common system for today’s heavy traffic of optical communication. Distributed Feedback Lasers (DFBLDs), including periodical grating structures, are key
devices as signal sources due to their highly selected single emission wavelengths. Table 1 shows the classified
lists and the characteristics of DFB Lasers. In these DFB
LDs, continuous wave (CW) DFB LDs are used for long
haul applications by combining them with external modulators such as LiNbO 3 , and many characteristics aredemanded. We list up some of them below.
(1) Wide wavelength band (Conventional C-band andboth sides of S and L-band)(2) Dense channel spacing (from 100 GHz to 50 GHz
and 25 GHz)
(3) High data rate (10 Gb/s and 40 Gb/s)(4) High output power and low driving current (up to 40
mW)(5) High reliability
To broaden the band, conventional C-band and bothsides of S-band and L-band 1) can be used to increase thenumber of channels. Dense channel spacing is anothersolution to increase the number of channels and an
attempt to shift from 100 GHz to 50 GHz (twice of thenumber of channels) and 25 GHz (4 times) is expected 2).
To achieve this, the wavelength stability is important andperipheral technologies have been developed successfully. High data rates of 10 Gb/s and 40 Gb/s have beenachieved in the external modulators. Output power isincreased to 40 mW in WDM, but a higher output power(over 80 mW) is demanded for a large number of ports in
CATV applications 4)~7). CATV systems demand extremely
low noise characteristics to keep a high signal to noise
ratio (S/N ratio). To maintain a high S/N ratio after longfiber propagation, low Relative Intensity Noise (RIN) andnarrow linewidth (which shows phase noise convertible tointensity noise after fiber propagation) are key characteristics.
In this paper, we report our activities to achieve a low
driving current for WDM applications. As high power technologies expand, we are also developing higher output
power operation and narrow linewidth for CATV applications. In Section 2 we show the chip structures of DFB
LDs and their grating fabrication technique. Optimizationof coupling coefficient and cavity length to obtain a low driving current is described in Section 3. Optical confinement
optimization, especially in the L-band, is described in
Section 4. For expansion to CATV applications, efforts toachieve a narrow spectral linewidth are described in
Section 5.

2.

CHIP DESIGN AND FABRICATIONIn this section, we describe the chip structure and the key
fabrication technology. Figure 1 shows the structure of theLDs used in this experiment. These devices were 1.55 µm
InGaAsP-InP buried hetero-structure (BH) DFB LDs
grown by MOCVD. The 2 µm wide active layers of these

Table 1
DesignPhotonics & Packaging Technologies, Yokohama R & D Lab.

Modulation

Transmission

Speed

Cost

Characteristic

Distance
CW

EA

DM

Uncooled*

Classified lists and characteristics of DFB lasers.

External

Long-haul~40 Gbps

modulator

100 km~(Typ.10 Gbps)

Integrated

Metro

modulator~100 km

DirectMetro-Accessmodulation

10~100 km

High~10 Gbps

Medium~10 Gbps

MediumDirectAccess~10 Gbps

modulation~10 km(Typ.2.5 Gbps)

Low

Long-haul
Large capacity
Low-chirp
Small-package
Low-cost
Small-package
Very low-cost
Very small-size

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High-Power CW-DFB LDs for Optical Communications

LDs were strain compensated and consist of six quantumwells. The Far-Field Pattern (FFP) of these LDs is 24°(vertical) and 20° (horizontal), so these LDs can be easily
coupled with fiber. To achieve high slope efficiency andlow driving current, both facets are coated with anti-reflective (AR) and high-reflective (HR) asymmetric coatings.
The emission wavelength of DFB LD is precisely controlled by electron beam (EB) lithography and isexpressed by the equation below.

λBragg = 2dpitch × neff

(1)Here, λBragg is the emission wavelength obtained fromthe grating pitch, dpitch is the period of the grating, and neffis the effective refraction index in the waveguide layer.
The mechanical resolution of the emission wavelength inEB lithography is about 10 pm, however, the emission
wavelength has inaccuracies (~1 nm) originating from thefacet phases, so the controllability of the emission wavelength is determined by this inaccuracy. About a 240 nm
grating pitch, from which an emission wavelength of 1550
nm is obtained, was patterned by dry etching the semiconductor layers.
These LDs are packaged in standard 14-pin butterfly
modules, which include two lenses, an isolator and a thermoelectric cooler (TEC). Coupling efficiency to the polarization maintaining fiber (PMF) is as high as about 75 %.

HIGH POWER AND LOW DRIVING CURRENT
145

0.48

140

SE

0.46

Pmax

135
130

0.44

125
0.42
120
115

0.4
0.8

Figure 2

Max. output power (mW)First, we describe our development in high power operation. The cavity length (L) and the coupling coefficient (κ)
are the key design parameters for high power operation.
The coupling coefficient is a characteristic parameter of
DFB LDs, and it expresses the amount of optical feedbackin the grating layer. κ can be designed from the difference
between the refractivity of grating and waveguide layer,
the interval from grating to active layer, and the thicknessof the grating layer. Figure 2 shows the slope efficiency ofthe output power (SE) and the maximum output power asa function of the κL product. Here, the cavity length ofthese LDs is fixed as L=400 µm. As shown in this figure,SE and the maximum output power (Pmax) increase as κ

Slope efficiency (W/A)

3.

decreases. Since many characteristics such as thresholdcurrent (Ith) and SE are determined by κL, decreases of Ithand SE due to large feedback efficiency can be observedin a large κL. The typical value of κL product is considered to maintain a high side mode suppression ratio(SMSR), and we have to design the κ value for every cavity length. Power saturation seems to be mainly caused byheating around the active layers, so a large thermal conductivity, which is obtained by a long cavity, is very effective to get a higher Pmax. In these long cavities, we have tocontrol the κ to a very small value, which seems to be difficult in terms of its reproducibility. Figure 3 shows the L-I
curves of these modules for different cavity lengths. In
L=400 µm, we achieved a very high SE of 0.3 W/A and adriving current of 80 mA for a fiber coupled power (Pf) of
20 mW. A little degradation of SE can be observed with along cavity, but we also achieved a maximum Pf of 175
mW. This is the highest value for a commercial package
including two lenses, TEC, and isolator, and is reported
elsewhere 8). The spectrum of this LDM is shown in Figure
4. As shown in this figure, we can get a SMSR of morethan 50 dB. Wavelength stability is considered to beanother benefit of a long cavity. These results show that
we achieved very high slope efficiency for every cavity
length and maximum output power. Considering WDM
application, the output power range is from 10 mW to 40
mW, and we can obtain the minimum driving current witha 400 µm cavity (Table 2). On the other hand, in CATV

1

1.2

κL

1.4

1.6

κL dependence of slope efficiency and maximum output power at L=400 µm.

PMF coupled power (mW)

200
180
160
L =800 µm

140

L =600 µm

120100

L =400 µm

80
60
40
20
0
0

200

400

600

800

1000

Forward current (mA)

Figure 1

Schematic figure of distributed feedback laser diode(DFB LD) along longitudinal and lateral sections.

Figure 3

L-I curves of LDMs each having a different cavity
length.

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High-Power CW-DFB LDs for Optical Communications(a) C-band

20(b) L-band(c) L-band∆ E g (L-band)>∆ E g (C-band)∆ E g (L-band)= ∆ E g (C-band)

L=800 µ m
25°C, Pf =80mW

Intensity (dBm)0

Eg-well

SMSR=55dB-20

Eg-barrier-40∆Eg =E g-barrier - E g-well
-60
1542

1544

1546

1548

1550

1552

λ (nm)

Figure 4

Figure 5

Improvement of MQW structure in L-band.

Spectrum of the L=800 µm cavity at an output powerof 80 mW.
100
90

Characteristics of LDMs with different cavity lengths.

L

Ith

Se

Iop (40 mW)Rth[ µ m]

[mA]

[W/A]

[mA]

[K/W]

400

10

0.3

155

40

600

17

0.28

165

28

800

24

0.25

180

19

Output power (mW)

Table 2

80
70
60
50
40
30

C-band(1550nm)

20

L-band(1625nm)

10

S-band(1506nm)

0
0

50

100

150

200

250

300

Forward current (mA)applications the highest output power is 175 mW for
L=800 µm. So, we have to use an optimized cavity length
considering the applications.

4.

S-, L-BAND DFB LDS

In this section, we describe the broadening of the wavelength band, which is the most important factor forincreasing the number of channels. At present, C-band
and L-band erbium-doped fiber amplifier (EDFA), having
wavelength channel counts ranging from 80 to 160 per
direction, are used for long-distance transmission systems. Compared to C-band, the increase of threshold current density (Jth) and decrease of slope efficiency (SE) areconsidered to be factors that make for poor characteristics
in the L-band. Besides, these factors are caused by anincrease of non-radiate re-combination and a decrease ofphoton energy due to the variety of effective mass andenergy band gap 1). We optimized electrical and opticalconfinement in the active region to overcome these problems. Figure 5 is a schematic figure of the MQW structurein the L-band. The barrier band gap (Figure 5 c) was
decreased while the difference in the band gap between
well and barrier layers was kept the same as that of the Cband. As a result, optical confinement inside the active
region is increased. On the other hand, the carrier caneasily flow through the barrier layer and non-uniformity of
the carrier density along the vertical axis is suppressed.
To investigate the characteristics inside the active region,
we fabricated the broad contact lasers without grating layers. The Jth values in these broad contact lasers with thedesign shown in Figure 5 c are 880 A/cm2, and these values are as low as those of the C-band structure (Jth ~850

Figure 6

L-I curves among S-, L-band DFB LDs.

A/cm2). The characteristic temperature To, which is calculated from the temperature dependence of Ith, is observed
as the value of To = 46 K. This result shows that theseLDs are very suitable for thermally tunable laser applications.
Figure 6 shows the L-I curves of DFB LDs from S-bandto L-band based on the previous discussion. As shown inthis figure, the characteristic degradation in the L-band is
not observed clearly and we achieved a very low driving
current (<200 mA, @P o =60 mW) in all wavelength
regions. Comparing C and L-band, we observed a slightly
higher driving current in the S-band, however, there is no
problem in practical use. Finally, the median lifetime ofthese LDs is more than 73 years assuming a 20%
increase in the driving current.

5.

NARROW SPECTRAL LINEWIDTHIn this section, we describe the approach used to get anarrow spectral linewidth for the key characteristic toextend our high power CW-DFB LDs to CATV applications. In CATV applications, the number of ports increases
with output power, and an output power of over 80 mW is
examined. To maintain a high S/N ratio even in analog
propagation, a narrow linewidth of less than 1 MHz and a
low RIN level of less than -160 dB/Hz in 0~1 GHz are
required. Generally the spectral linewidth seems to be
inversely proportional to output power in low power region 9).
However, there are many reports of the spectral linewidth

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High-Power CW-DFB LDs for Optical Communications

re-broadening in the high power region 5), 10), 11), and it isconsidered to be a very difficult problem to get a narrow
linewidth in the high power region. The re-broadening is
caused by the non-uniformity of the effective refractivity
and the photon density in the cavity due to spatial holeburning (SHB). Non-uniformity of refractivity originatesfrom the non-uniformity of carrier density due to the nonuniformity of photon density. On the other hand, the photon density distribution curve is determined by the facet
phases, and these values cannot be controlled. To express
photon density, the front to rear power ratio (Pf /Pr) is considered to be a good parameter. In DFB LDs with symmetric facet coatings, linewidth re-broadening was suppressed by flattening the photon density in the phase shifted structure and multi-electrode 12). On the other hand, inthe case of high power operation with asymmetric facet
coatings, using a wide mesa is considered to be a very
effective method to suppress re-broadening 5). Here, we
achieved a very narrow linewidth by decreasing the carrier
density in a long cavity and optimizing κ and Pf/Pr to get ahighly uniform photon density. Figure 7 shows the spectral
linewidth as a function of output power and cavity length.
Here, κL and Pf/Pr are fixed as 1.2 and 15, respectively,
for the effects of SHB be suppressed as much as possible. We achieved the minimum linewidth of 0.5 MHz at 40mW and 0.8 MHz, and even at 100 mW for a 800 µm cavity. In addition, re-broadening is suppressed in a long cavi-

2
L=400 µ m
1.5

∆ν (MHz)

L=600 µ m
1

L=800 µ m

0.5

30

60

90

120

Pf (mW)

Figure 7

6.

CONCLUSION

Finally, we summarize this paper. We developed the highpower CW-DFB LDs for optical communications. We took
an approach to get low driving current from C-band to S
and L-band for WDM systems. Considering optical andelectronic confinement in optimizing the MQW structure,
we achieved a very low driving current of less than 200
mA at a facet output power of 60 mW between S and Lband. To extend these LDs to CATV applications, wedeveloped very high output power and low noise CW-DFB
LDs. Concretely, using a long cavity and optimizing thecoupling coefficient we achieved a high power of 175 mW
and a very narrow linewidth of 0.8 MHz, even at 100 mW.
These values are the highest values for commercial packages and these LDMs have possible new applications in
the future.

ACKNOWLEDGEMENT

00ty. Thus, we demonstrated that a narrow linewidth in highpower operation could be achieved by means of a longcavity and optimized κL and Pf/Pr. Figure 8 shows RIN
data. We also successfully achieved a very low RIN of 160 dB/Hz in 0~1 GHz. As a result, we achieved a very
low noise characteristic that is sufficient to operate as a
CATV signal source. Finally, we show the results of a reliability test in Figure 9. The figure shows that the increaseof driving current as a function of aging hour in autopower-control (APC) mode. We tested under an ambient
temperature of 35°C and a chip output power of 120 mW
constant (which is considered Pf >80 mW). As shown, theincrease of driving current is less than 1 % after 2000
hours of testing. These results show that the LD chips
have a median lifetime of over 25 years and no problems
in practical use.

Spectral linewidth versus output power for LD with acavity length of 400 µm~800 µm

The authors would like to thank the members of Photonics& Packaging Technologies, Yokohama R & D
Laboratories for their helpful support in all processes.
They also would like to thank Fitel Optical Component
Division for its helpful support in the packaging process.

-140

20

15

-150

∆ Iop (%)

RIN (dB/Hz)

-145

-155

5

-160
0

-165
-170

-5
0

250

500

750

0

1000

Frequency (MHz)Figure 8

10

RIN data in 0~1 GHz. In all ranges, RIN<-160 dB/Hz
was achieved.

500

1000

1500

2000

Time (h)Figure 9

Changing rate of operating current under 35°C and
120 mW APC condition for 2000 hours.

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High-Power CW-DFB LDs for Optical Communications

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