1.55 µm high speed low chirp electroabsorption
modulated laser arrays based on SAG scheme
Yuanbing Cheng, 1 Qi Jie Wang, 1,* and Jiaoqing Pan2
1
Photonics Center of Excellence, OPTIMUS, School of Electrical & Electronic Engineering, Nanyang Technological
University, 50 Nanyang Ave., 639798, Singapore
2
Key Laboratory of Semiconductor Materials, Institute of Semiconductors, Chinese Academy of Sciences, Beijing,
100083, China
*
[email protected]
Abstract: We demonstrate a cost-effective 1.55 µm low chirp 4 × 25 Gbit/s
electroabsorption modulated laser (EML) array with 0.8 nm channel
spacing by varying ridge width of the lasers and using selective area growth
(SAG) integration scheme. The devices for all the 4 channels within the
EML array show uniform threshold currents around 18 mA and high
SMSRs over 45 dB. The output optical power of each channel is about 9
mW at an injection current of 100 mA. The typical chirp value of single
EML measured by a fiber resonance method varied from 2.2 to −4 as the
bias voltage was increased from 0 V to 2.5 V. These results show that the
EML array is a suitable light source for 100 Gbit/s optical transmissions.
©2014 Optical Society of America
OCIS codes: (250.0250) Optoelectronics; (250.5300) Photonic integrated circuits; (250.5960)
Semiconductor lasers; (140.3290) Laser arrays
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1. Introduction
To cope with the demand for huge data capabilities in Internet and associated data-driven
applications, 100-Gb/s transmission technology has been widely considered as a strong
candidate for next-generation terabit per second communication networks [1]. The
Electroabsorption Modulated Laser (EML) has attracted much attention due to its
compactness, low packaging cost, low driving voltage and high stability [2–7]. Monolithically
integrated EML arrays are promising light sources for modern reconfigurable dense
wavelength division multiplexing (DWDM) systems. The devices can potentially reduce
optical network costs with simplified optical alignment and packaging processes. Compared
with the monolithically integrated distributed feedback (DFB) laser arrays and an
electroabsorption modulator (EAM) which operates as the wavelength selectable light sources
[8], the EML arrays can handle much higher data capabilities and the device performance of
different channels in EML arrays can be optimized separately. There have been several
reports on EML arrays that operate more than 40 Gbit/s over the past years. Compact
monolithically integrated light sources based on 1.310 µm 4 × 25 Gbit/s and 4 × 40 Gbit/s
electroabsorption modulated DFB laser array were demonstrated for future long distance data
communication system, respectively [9–11]. A 4 × 1 optical mulitiplexer was used to reduce
the chip size. A 1.55 µm 4 × 10 Gbit/s electroabsorption modulated distributed Bragg
reflector (DBR) laser array was also proposed for better control of the output wavelength due
to the inherent wavelength tunability of DBR laser [12,13].
Although much progress is made for integrated light sources as mentioned above, it is
expected that integrated light sources will be cost-effective and they will demonstrate
improved performance of both the laser and the modulator simultaneously with relative
simple fabrication processes for practical applications. To fabricate EML arrays, one of the
key issues is that different wavelength channel have to be defined side by side on one chip
and the lasing wavelength between neighboring channels must have a uniform wavelength
space. Up to now, electron-beam lithography (EBL) is the most widely used techniques to
fabricate gratings with different periods [9–11]. However, drawbacks such as high costs, time
consumption, low yields, and complex manufacture processes make this method not the best
choice for mass production. As shown in earlier reports [14], the variation in the ridge width
would lead to a slight difference in the effective index and then in the lasing wavelength. And
this method was used to fabricate multi-wavelength laser arrays with the same grating period
[15,16].Such width-varying DFB waveguides can be made by standard photolithography and
holographic lithography.
Another key issue is the wavelength compatibility between the DFB and the EAM. The
DFB lasing wavelength is longer than the absorption-peak wavelength of the EAM. The
wavelength detuning between the DFB lasing wavelength and the EAM absorption-peak
wavelength can greatly affect the devices performances such as extinction ratio, output power
and 3dB bandwidth. Among the many ways of laser-modulator monolithic integration
explored [2–13, 17,18], selective area growth (SAG) has large degree of freedom in bandgap
energy control and almost 100% optical coupling achievable between the components. This
method allows definition of different regions of MQW bandgaps on a single masked substrate
with a one-step growth process. Unlike the butt–joint integration scheme, no partial removal
of the active region by selective etching and no different active material by regrowth are
necessary for the SAG method. Thus, the fabrication complexity of the EMLs is significantly
reduced by using SAG and no postgrowth annealing such as quantum-well intermixing is
required. In this work, selective area growth (SAG) is employed to grow an active layer in the
regions of the laser and the modulator. The ridge width varying techniques are used to
monolithically integrate a 4 × 25 Gbit/s EML array. As a result, the fabrication process of the
integrated device is significantly simplified. Our method has great potential for the fabrication
of low-cost, high quality integrated multi-wavelength laser arrays.
2. Device design and fabrication
Each EML of the array is composed of a 250 µm long DFB laser and a 170 µm long EAM.
Different active regions of laser and EAM at 1552 nm and 1495 nm respectively, have been
obtained by the SAG method. The laser array is designed to have a frequency spacing of 100
GHz (0.8 nm) which is obtained by slightly changing the ridge waveguide width according to:
2neff Λ =mλB
Where the 𝜦𝜦 is the grating period, 𝜆𝜆 B is the Bragg wavelength determined by the hologram
lithography, n eff is the effective refractive index in waveguide structure and m is grating order
which is 1 here for the first order grating. The EML arrays have a typically 2.0 to 3.5 µmwide ridge waveguide structures to achieve transverse fundamental mode operation. The
transverse fundamental mode operation is necessary for EML with single longitudinal mode
output because high order transverse modes lead to power kinks, side-mode-suppression
reduction or multimode lasing. The effective refractive index of the ridge waveguide as a
function of the waveguide width was calculated by using beam propagation method (BPM).
For all of the four-channel EML in the array, waveguide width are designed as 2 µm, 2.35
µm, 2.6 µm and 3.1 µm, which corresponds to the calculated effective index of 3.21, 3.2116,
3.2133 and 3.2149, respectively.
The designed EML array was fabricated using the similar process in [2]. Device
fabrication started with deposition of 200 nm thick SiO 2 dielectric films on the S-doped (100)
InP substrates by plasma-enhanced chemical vapor deposition (PECVD). Masks were
patterned along the [011] direction by conventional photolithography. Then, an n-InP buffer
layer and an eight-pair InGaAsP/InGaAsP MQWs structure were selectively grown on the
patterned substrates by ultra-low-pressure MOCVD (30 mbar, 655°C). The MQWs consist of
un-doped 8-nm-thick 0.7% compressive strain InGaAsP wells separated by 9-nm-thick 0.3%
tensile strain In 0.85 Ga 0.15 As 0.42 P 0.58 barriers, sandwiched between 100-nm-thick latticematched In 0.78 Ga 0.22 As 0.47 P 0.53 optical confinement layers. It is well known that the principal
mechanism of selectively grown MOCVD lies in the lateral gas phase diffusion of group-III
precursors. At ultra-low pressures, the reagent particles can easily diffuse out of the stagnation
gas phase layer. The measured growth enhancement rate between the stripes is 20% which fits
well with the lateral gas phase diffusion model. A first-order grating with period of 241.7 nm
is partially formed on the laser section by conventional holographic exposure followed by
chemical etching. The exposure and dry-etching processes were controlled to achieve a
grating duty factor of approximately 50% for an optimum coupling coefficient. To complete
the epitaxial structure, a p-type InP cladding layer and p + -InGaAs contact layer were
successively grown.
To ensure a small series resistance of the DFB laser and a low capacitance of the EAM, a
single reverse-mesa-ridge structure was formed by conventional photolithographic technology
and etching along the [011] direction. RIE was first employed to remove the p + -InGaAs
contact layer during the ridge etching. After that, the p-type InP cladding layer was etched by
using selective wet chemical etching in HCl:H 2 O solution. The etching processes were
controlled to achieve the low loss reverse-mesa-ridge. The EA modulator section was further
processed into deep mesa ridge waveguide structure by the second RIE step to reduce the
junction capacitance of the device. Electrical isolation between the laser and the modulator
were realized by forming 50-μm-wide trench between them and adopting He+ implantation in
the trench .Three steps of He+ implantation were used for a flat ion distribution with doses of
1 × 1014, 8 × 1013, 5 × 1013 cm−2 and at energies of 180kev, 100kev,80kev respectively. An
isolation resistance greater than 100 kΩ was obtained. BCB was used under the bonding pad
to reduce the parasitic electrode pad capacitance of EAM. The ridge waveguide was well
protected and passivated. Thus low capacitance of the modulator and low leakage currents
were obtained. Then, patterned Ti/Au p-electrode was formed on top of the planarized wafer.
Au/Ge/Ni n-electrode was evaporated onto the backside of the device after thinning the wafer
down to about 100μm. Finally, the wafer was cleaved into device chips, and the facet of the
EA modulator and DFB laser were antireflection (AR) and high reflection (HR) coated with
dielectric layers deposited by PECVD, respectively. A 40-GHz vector network analyzer and a
calibrated receiver were applied for a high-speed measurement.
3. Characteristics of the device
The device was mounted on a copper submount using an optimized Au 80 Sn 20 solder. The
copper sub-mount was placed on a thermoelectric cooler (TEC) to control the chip
temperature under continuous-wave (CW) operation. During the measurement, the EMLs
were discretely operated. The serial resistance of the fabricated lasers is 4 ± 0.1Ω. Figure 1
shows the measured light output power versus injection current curve of a typical chip of
EML array at the temperature of 25°C. A threshold current as low as 18 mA and an output
power of about 9mW at 100mA was achieved for each EML channel.
Fig. 1. Typical light output power versus current curve of a typical chip of EML array at the
temperature of 25°C under CW condition.
The output light from the EML array was collected by a lensed single mode fibre and
coupled to an optical spectrum analyzer (OSA) for spectrum analyse. Figure 2 shows the
recorded spectra for all 4 channel lasers at the injection current of around 65 mA. The lasing
wavelengths were around 1551.80 nm, 1552.56 nm, 1553.28 nm, and 1554.04 nm, which had
a slight shift of around ± 0.1 nm from the design due to lithography resolutions. The measured
single-mode suppression ratios (SMSRs) are larger than 40 dB and the average value is 45
dB, which exhibits good single longitudinal mode property in the DFB laser array. The
measured
Fig. 2. Typical lasing spectrum of the EML chip at a driving current of 65mA.
lasing wavelengths appear to have good linearity. After linear fitting, the residual wavelength
changes only from −0.004 to 0.012 nm, this is relatively quite small. Supposing that the
gratings are fabricated by other advanced fabrication methods such as EBL, the deviation of
period must be controlled within a very small value of 0.124 nm, corresponding to the
tolerance of 0.8 nm. As we know, it is quite challenge to obtain space of grating pitches less
than 1nm with typical EBL. We also measure the output optical spectrum of the EML under
different operation injection current. The wavelength shift is about 0.012 nm/mA. Thus, the
EML array can be finely tuned by changing the injection current of each channels and the
required injection current range is about 10 mA to make the lasing wavelengths exactly linear.
Fig. 3. Measured extinction behavior of the integrated light source with 170µm EA modulator
section using an integrating sphere.
The divergence angles from the EAM output are 40.2 ° × 34.6° in the vertical and
horizontal, respectively; the coupling efficiency to single mode fiber reached 42% in the
experiment. In Fig. 3, the dc extinction ratio of all the four EML channels measured using an
integrating sphere is plotted as a function of the reverse bias applied to the EAM. The
extinction ratio at 5V reverse bias is estimated to be approximately 15 dB. When coupled to
single mode fiber the dc extinction ratio at 5V reverse bias is more than 35dB. By adopting a
deep ridge waveguide and planar electrode structures combined with buried BCB, the
capacitance of the EAM is reduced to 0.22 pF. The measured relative E/O response of a
typical EML channel
Fig. 4. Measured E/O responses of a typical EML (channel 2) at various reverse bias voltages.
Fig. 5. Measured reverse bias voltage dependence of the α parameter for a typical EML
(channel 2).
(channel 2) is shown in Fig. 4, which has been calibrated to account for the frequency
response of the photo detector and high frequency probe. The measured 3dB bandwidth is
more than 18.5 GHz (~24 GHz by fitting the experimental result) for all the four channels,
which are sufficiently large for 25 Gbit/s NRZ signal operation. The intrinsic bandwidth of
the EML will be larger if the resonance is removed by optimizing the design of the submount.
Wavelength chirping which is defined as the ratio of the increments of the real and
imaginary parts of the EAM complex refractive index is important to the transmission
characteristics of an EAM. The wavelength chirping is estimated from the small signal
parameter measurements using a fiber resonance method proposed by Devaux et al.
[19]. Figure 5 shows the reverse bias voltage dependence of the α parameter for EML channel
2. The α parameter varies from 2.2 to −4, as the reverse bias voltage increases from 0V to
2.5V. A zero chirp parameter was achieved at a bias voltage of 1.58 V. The transfer
characteristic of the device offers potential for getting a low dispersion penalty.
4. Discussions and conclusions
A low chirp 4 × 25 Gbit/s EML array with 0.8 nm channel spacing has been fabricated by
varying the laser ridge width and SAG. The grating was fabricated only by common
holographic exposure and an additional micrometer-level standard photolithography, which
results in a low cost. The device shows uniform threshold current around 18 mA and high
SMSRs over 45 dB. The optical output power of each channel is about 9 mW at an injection
current of 100 mA.The chirp values of the EML chip was measured using a fiber resonance
method. The chirp varied from 2.2 to −4 as the bias voltage was increased from 0 V to 2.5 V
respectively. The transfer characteristic of the device indicates potential for transmission with
a low dispersion penalty. The experiment results show that our technique is a simple and
powerful tool for the fabrication of low-cost, high quality multi-wavelength laser arrays.
Acknowledgments
This work is partially supported by the Ministry of Education, Singapore (MOE2011-T2-2147).