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

-0.2

0

0.2

0.4

0.6

0.8

1

0 50 100 150

Time(ps)

Pu

lse

Am

p.

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-

Zehnder Interferometer’, IEEE Phot. Technol. Lett, 1998 10, pp165-167

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