InP-based high-speed transponder

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OFC 2014 © OSA 2014
InP-Based High-Speed Transponder
R A Griffin
Oclaro Technology, Caswell Science and Technology Park, Towcester, NN12 8EQ, UK
[email protected]
Abstract: A new generation of line-side pluggable transponders and transceivers capable of
flexible 100 and 200 Gb/s transmission will be underpinned by developments in InP PICs, which
offer high performance, compact footprint and low power dissipation.
OCIS codes: (250.3140) Integrated optoelectronic circuits; (060.1660) Coherent communications
1. Coherent Hardware
Digital coherent technology has become the de-facto standard for high capacity optical communication, exploiting
the combination of high-order modulation formats, coherent reception with local-oscillator laser, and CMOS digital
signal processing to achieve robust 100 Gb/s long-haul transmission. Up to the present time, most network
equipment manufacturers (NEMs) have realized coherent transmission hardware using a combination of discrete
optical components, aided by standardization of component function and form by the OIF [1]. With continued
advances in CMOS technology and the maturing of coherent optical component technology, there is increasing
focus on reduction in footprint, power dissipation and cost of the coherent hardware to enable more widespread
deployment, particularly in very cost-sensitive metro application.
Following a similar path to the evolution of 10 Gb/s line-side transmission over the past decade, InP-based
coherent optical components are well placed to balance demanding performance requirements, size reduction and
suitability for high-volume manufacturing. InP photonic integrated circuits (PICs) can provide the compact
footprint and low power dissipation needed for evolving requirements, and ease production through volume
semiconductor processing, early test and screening throughout production, and reduced assembly steps to full
packaging. Currently there is considerable activity in refining InP PIC designs, validating system performance, and
development of miniaturized packaging for hardware integration.
2. Transponders for Flexible Transmission
Functionality of a transponder capable of flexible coherent transmission is shown schematically in Fig. 1. High
speed electronics performing framing, high gain forward error correction (FEC), digital signal processing (DSP),
digital-analog and analog-digital conversion (DAC, ADC) may be integrated within a single CMOS chip. On the
optical line side, a dual-polarization (DP) in-phase and quadrature (I&Q) optical modulator arrangement is
employed for transmission, fed with 4 electrical data streams [1]. An integrated coherent receiver (ICR) containing
polarization management, optical hybrid mixers and photodiode/TIAs provides I&Q outputs for each of 2
orthogonal polarizations [1]. Separate tunable lasers (TLs) are shown for the transmitter and local-oscillator (LO),
however, a single laser with split may be used for both functions at the expense of wavelength flexibility.
While the first generation of coherent hardware was based on DP-QPSK modulation with binary signals feeding
the DP I&Q modulator, the inclusion of DACs and associated signal processing on the transmit side provides great
flexibility for operation. Using linear rather than limiting drivers, Nyquist pulse shaping can be employed at the
transmitter to tightly constrain the optical spectrum, allowing close spacing of DWDM channels. In addition to
QPSK, 16-QAM (or higher) signaling can be employed using the same transmit and receive optical hardware [2].
Operating at the same baud-rate, 16-QAM provides 200 Gb/s transmission with the same spectral width, at the
expense of higher OSNR requirements compared to QPSK.
The CFP module format is an attractive option for provision of the flexible transponder functionality of Fig. 1 in
a pluggable module, with a face area of 82x14 mm2 and depth 145mm. The CFP dimensions dictate that the
coherent optical components employed require footprints significantly smaller than those standardized by the OIF.
Further, a total maximum power dissipation specification of 32W places a premium on low power dissipation
components. Even allowing for reduction in power dissipation for the DSP through optimization of architecture and
reduced CMOS feature size, it can be expected that the available power dissipation for the opto-electronic
functionality of the transponder will be < 12W, requiring considerable advances on current technology.
An alternative transponder approach which has increasing support separates opto-electronic functionality into a
pluggable module, with DSP and other electronics residing on the host board. This approach has the advantages for
NEMs of cost-effective module standardization while retaining the IP and know-how derived from heavy
investment in DSP development. With a CFP2 format as the vehicle for the opto-electronics, dimensions are
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TxDATA
DSP
DACs
FEC
FRAMER
42x12x1088 mm3. Since maximum
m
pow
wer dissipation for the CFP2 is
i 12W, power requirements on
o the optical
componentts for both CFP
P and CFP2 aree similar; compponent vendorss can apply a common
c
opticaal sub-system design
d
to realize both
b
options in a cost-effectivve way.
6.6  2.6 mm
m 2
T
TL
ASIC
DP-I&Q
Modulator
3.11  0.5 mm2
ADCs
DSP
TL
FEC
RxDATA
FRAMER
CO
ONTROL ELECTRONICS
3.3  1.2 mm
m 2
Integrrated
Cohe
erent
Rece
eiver
Fig. 1. Traansponder functio
onality for flexibble digital coherent transmissionn. Also shown arre InP PICs provviding the requissite set
of opto-elecctronic building blocks: tunable laser, DP-I&Q modulator
m
and optical
o
hybrid recceiver.
3. InP Du
ual-Polarizatio
on I&Q Modu
ulator
Compared to lithium niob
bate modulatorrs – which havve been used alm
most exclusiveely for coherennt hardware
deploymennt – InP-based modulators cann realize dramaatic reduction in
i chip footprinnt. InP I&Q modulators
m
havee
already beeen deployed co
ommercially foor DQPSK trannsmission at 400 Gb/s [3], and suitability of the
t fundamentaal
characterisstic of these bu
uilding blocks for
f coherent appplications has been established through sysstem simulationn [4].
Figure 2a illustrates
i
a hig
gh quality 16-Q
QAM constellaation simulated with realistic InP
I MQW elecctro-refractive
effects incoorporated into the model.
Potentiially one of thee larger contributors to powerr dissipation off the coherent opto-electronic
o
cs is from 4 electrical
drivers whhich provide intterfaces betweeen the low-leveel DAC outputts and each MZ
ZM. A critical performance metric
m
is hence onn-off switching
g voltage V; reeduction in V translates
t
to loower power dissipation since drivers are opeerated
at lower avverage RF outp
put powers, andd since less gaiin is required too boost the DA
AC output. Cloosely coupled to
t
design of V for MZM elements is realiization of sufficcient electro-opptic bandwidthh for high Baudd rate modulatiion.
As shown in Fig. 2b, a traavelling-wave design fabricaated in InGaAsP
P with an N+ platform
p
providdes 3dB EO
bandwidth for an MZM chip
c
~ 20GHz; exploiting InG
GaAlAs MQW epitaxy achievves V for this design < 2.5V [4].
Further im
mprovement to the
t MZM figurre-of-merit mayy be achieved using a segmennted electrode design on a seemiinsulating (SI) platform [5];
[ results show
wn in Fig. 2b demonstrate
d
im
mproved bandw
width in Fig. 2bb, while V wass
further redduced.
Although 3dB bandw
width has traditiionally been a key metric for performance, the availabilityy of DAC
functionaliity allows pre-eemphasis of traansmit electricaal signals whicch can efficienttly compensatee for reduced EO
E
bandwidth. For CFP2+ architecture
a
whhere there will be significant separation betw
ween modulatoor and DACs, the
t
f
respoonse shown is insignificant compared
c
to coombined RF intterconnect lossses. In
difference in modulator frequency
n small chip arrea may be morre important thhan 3dB bandw
width. Fig 2c shows DP-I&Q
Q
this scenarrio, lowest V in
modulatorss fabricated on
n a 3” InP wafeer, which can yield ~ 150 chipps.
a)
b)
EO Magnitude (dB)
3
c)
0
‐3
‐6
N+
‐9
S
SI
‐12
0
5
10 15 20 25 30 35 40
Frequency (GHz)
Fig. 2. a)) Simulated 16-Q
QAM constellation from InP model; b) Measuredd EO response for
f InP MZM varriants; c) Fabricaated
3”InP wafer of DP-I&Q moddulators
4. InP Traansmitter
InP MZMss are well suiteed to laser co-packaging due to
t their small size
s and the facct that the waveeguide spot sizze is
more closeely matched to a laser waveguuide than a stanndard fiber corre. Co-packagiing can providee reduced packkage
footprint while
w
allowing separate optim
mization and scrreening of laseer and modulatoor chips. Figuure 3a shows a
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prototype Tx package combining monolithic tunable laser, dual I&Q InP MZM, and micro-optics to perform
polarization management. The DSDBR lasers we employ are monolithic devices fabricated on 3” InP wafers. They
achieve full-band tuning with a combination of rear phase sampled DBR grating together with multi-contact front
DBR grating for super-mode selection. The use of InAlGaAs multiple quantum well active material with bulk
InGaAsP tuning regions, together with design optimization has been shown to be suitable for coherent applications,
providing +16dBm fiber power, and < 200kHz Lorentzian linewidth with a power dissipation < 3W [6]. Integrated
monitor and control elements on the modulator PIC allow for precise control of critical MZM bias points.
Performance of the Tx prototype for a demanding digital coherent application was evaluated, using high speed 8bit DACs and linear drivers to generate 28 GBaud DP-16QAM signals. After noise loading, received BER was
derived from an optical modulation analyzer sampling at 80 GS/s, using custom processing algorithms. Due to the
fundamental increase in OSNR required relative to QPSK, 16-QAM will be deployed in conjunction with high
coding gain FEC capable of correcting BERs ~ 0.01; measurements in this region are shown in Fig. 2b. BER
performance of the InP-based Tx matched that measured with an alternative implementation combining a discrete
tunable laser and lithium niobate modulator.
-1
b)
-2
Optical Power Spectrum (5dB/div)
log(BER)
a)
-3
-4
-5
-6
Frequency (10GHz/div)
14
15
16
17
18
19
20
21
22
23
24
25
OSNR (dB 0.1nm)
Fig. 3: a) Photo of prototype transmitter co-packaging tunable laser, dual I&Q modulator and micro-optics; b) measured BER
results for Tx with 28GBaud DP-16QAM. Inset shows measured optical spectrum from the transmitter
5. InP Integrated Coherent Receiver
a)
0.2
OE Magnitude (dB)
Responsivity (A/W)
An InP platform similar to that used for modulators has a number of advantages for realization of the ICR. Deeply
etched ridge waveguides provide strong lateral confinement of the optical mode, allowing tight bends, MMI
couplers, and waveguide cross-overs to be integrated in a very compact fashion. A major advantage compared to
alternative passive planar waveguide materials is that the 4 photodetectors (PDs) required for each polarization may
be integrated directly on the InP PIC, simplifying assembly, improving responsivity, and allowing extensive chip
characterization and screening throughout the production process. The wealth of data gathered from large numbers
of fabricated devices aids design refinement to optimize performance. Results in Fig. 4a illustrate high responsivity
and good matching for 4 PDs on a receive PIC like that shown in Fig. 1. Excellent bandwidth of the waveguide PD
elements is shown in Fig. 4b.
0.15
0.1
PD1
PD2
PD3
PD4
0.05
0
1525
1535
1545
1555
Wavelength (nm)
1565
b)
3
0
‐3
‐5C
25C
‐6
90C
‐9
0
10
20
30
40
50
Frequency (GHz)
Fig. 4. a) Resposivity for 4 PDs on a hybrid receiver PIC; b) PD frequency response over temperature
6. Conclusion
Adoption of InP PICs to perform the coherent opto-electronic interface can enable a new generation of high
performance, low power dissipation line-side pluggable transponders and transceivers capable of flexible 100 and
200 Gb/s transmission. The technology is set to further widen the deployment of digital coherent hardware.
7. References
[1] http://www.oiforum.com/public/documents/OIF-FD-100G-DWDM-01.0.pdf
[2] X. Zhou, “DSP for high spectral efficiency 400G transmission”, paper Tu.1.E.3, ECOC 2013.
[3] C. F. Clarke, R. A. Griffin and T. C. Goodall, “Highly integrated DQPSK modules for 40 Gb/s transmission”, paper NWD3, OFC 2009.
[4] R.A. Griffin, S.K. Jones, N. Whitbread, S.C. Heck and L.N Langley, “InP Mach–Zehnder Modulator Platform for 10/40/100/200-Gb/s
Operation”, to be published IEEE J Sel. Topics Quantum Electron., 2013.
[5] R. G. Walker, “High-speed III-V semiconductor intensity modulators”, IEEE J. Quantum Electron, Vol. 27, pp. 654-667, 1991.

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