Post on 22-Mar-2023
All-optical clock division at 40 GHz using a semiconductor amplifier
nonlinear interferometer
R. J. Manning, I. D. Phillips, A. D. Ellis, A. E. Kelly, A. J. Poustie, K.J. Blow
BT Laboratories,
Martlesham Heath,
Ipswich,
Suffolk
IP5 3RE
UK
Tel: +44 1473 645362
Fax: +44 1473 646885
email: bob.manning@bt.com
Abstract
We demonstrate all-optical clock division of a 40 GHz pulse train, using a semiconductor
nonlinear interferometer with feedback. Blocks of pulses are also output if operating
conditions are chosen appropriately. We also observed the dynamical evolution of the
clock divided train.
Introduction
Semiconductor optical amplifiers (SOAs) have become widely used in optical
communications research for high –speed switching, in particular demultiplexing [1,2].
The so-called ‘TOAD’ topology [3] has become widely used in this context, and the use
of SOAs in a loop mirrorand has also formed the basis of demonstrations of more
sophisticated all-optical logic functionality, such as an all-optical memory [4], a shift
register with inverter [5], and a half adder [6]. We have recently reported spontaneous all-
optical clock division at 10GHz and 20 GHz using a ‘TOAD’ arrangement with optical
feedback, and have also shown that this unit can also act as an all-optical shift register
with inverter at the same repetition rates[7], where the output consists of alternate blocks
of ‘ones’ (pulses), and ‘zeros’. Here we show that the spontaneous clock division
phenomenon is scaleable to 40 GHz repetition rates, using a derivative of the TOAD
topology, namely a ‘UNI’ (Ultrafast Nonlinear Interferometer) [8]. We also
experimentally follow the evolution of the clock divided output with number of
circulations in the loop.
Experimental
A schematic of the experimental arrangement is shown in Figure 1. A similar
arrangement has been used recently to demonstrate a 10 Gbit/s all-optical memory [9].
The pulse source used was a 10 GHz external cavity modelocked semiconductor laser
(ECMLL), which produced 3.5 ps pulses (FWHM) at a wavelength of 1550nm. These
pulses were passively multiplexed in optical fibre to a give a 40 GHz pulse stream that
was amplified using an Erbium doped fibre amplifier (EDFA) and input to the UNI as
probe pulses. The unswitched probe pulses from the UNI passed through ~1 km of
dispersion shifted fibre (having a dispersion zero at a wavelength of ~1550nm), an
acoustic-optic modulator (AOM) (usually transmitting) and an EDFA. After
amplification, these pulses were fed back to the UNI as switching pulses.
The UNI is essentially a Mach-Zehnder interferometer, which is made using a single fibre
for both arms by exploiting polarisation diversity. In these experiments, the UNI was
used in a counter-propagating configuration similar to that described in references [10,11].
Probe pulses input to the UNI were launched at 45° to the axes of polarisation
maintaining (PM) fibre and split into orthogonally polarised pulse pairs. The pulse pair,
separated by 15ps after propagation through 7 m of fibre due to polarisation mode
dispersion (PMD), and was input to a polarisation insensitive SOA with a mean power of
~ 3 dBm. After the SOA the pulse pair was launched at - 45° into a second 7m length of
PM fibre, so the pulse pair suffered a delay of the opposite sense and therefore
recombined. The resultant pulse passed through a fibre polariser P. Switching pulses were
input via the 3dB coupler with a mean power of ~ 7dBm and were counter-propagating to
the probe pulse pair. The switching pulse causes a change in gain and hence refractive
index of the SOA and this affects the relative phase of the pulse pair. If the phase
difference is π then a polarisation rotation of 90° occurs at the polariser of the UNI, and
full switching occurs. The SOA used here had a gain recovery time of 80 ps at a current
bias of 400 mA, and an alpha parameter of ~9 at 1550nm [12]. The SOA was 1 mm long,
which meant that only one pulse pair was present in the SOA at any one time. This was
important, since it meant one pulse pair was affected by only one switching pulse.
Results and Discussion
Depending on the arrival time of the switching pulses with respect to the probe pulses, we
were able to observe either block behaviour or clock division [Error: Reference source
not found,10]. The driving electronics for the AOM allowed us to start a sequence and
allow it to run for up to 185 ms, or ~35,000 circulations. We were able to observe eye
diagrams for everyany individual round trip of the system, and hence time resolve the
clock division evolution. Figure 2 shows the output pulse train after ~20 circulations,
where clock division is virtually complete. Figure 3 is a 3D plot showing the input
40GHz pulse train and its observed evolution as it circulated in the shift register. The
clock division can be seen to be complete after ~100µs (i.e. 20 circulations). The
evolution shows that clock division occurs over a fairly small number of circulations.
Existing theory [13] would suggest that, for an input pulse train of ~105 pulses, such as we
have here for a feedback path of ~1km, the number of circulations required for clock
division to evolve would be of the same order. However, further modelling shows that if
there is even a small amplitude modulation (~4%) on one channel, the number of
circulations required is drastically reduced. The 40 GHz input used does have some small
amplitude modulation, and it is believed that this is responsible for the very rapid
evolution we observe here.
Further experiments using a 40 GHz fibre ring laser source were performed in which the
1 km length of fibre was removed. Here there was no amplitude modulation on the
channels and clock division was again observed (the evolution was not monitored). We
were also able to observe blocks of 40 GHz pulses as output, as first shown by Hall et al.
[9], by delaying the arrival time of the switching pulses by half a bit period [Error:
Reference source not found]. The block output is shown in Figure 4. The block period of
~300ns corresponded to the time taken to traverse the feedback path, which was mostly
made up of fibre for the EDFA.
Conclusions
We have shown that a UNI combined with optical feedback can be used as an all-optical
clock divider or an all-optical shift register at 40 GHz. The principles of operation
described in reference [7] still apply at these higher repetition rates.
10 GHzECMLL∆t=3.5 ps
λ= 1550 nm
SOA
Lightwave
Converter
3dB
3dB
Filter
Ultrafast NonlinearInterferometer (UNI)
Isolator
P
7 m PM Fibre
PMD=15ps
7 m PM Fibre
PMD=15ps
45°Splice
Variable Delay
Variable Delay
EDFA
~1 kmdispersion
shifted fibre
10-40GHz
Mux
Sampling‘scope
AOM1
0.
100.
100.
100.
200.0
14
25
37
49
61
71
84
94
0
0.2
0.4
0.6
0.8
1
0.
100.
100.
100.
200.
Time(ps)
Circulation
time (us)
Pulse Amp.
1 References
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Demultiplexing of 80 to 10 Gb/s Signals with Monolithic Integrated High Performance Mach-
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