A scheme for optical pulse generation using optoelectronic phase locked loops

IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308
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Volume: 03 Issue: 03 | Mar-2014, Available @ http://www.ijret.org 349
A SCHEME FOR OPTICAL PULSE GENERATION USING
OPTOELECTRONIC PHASE LOCKED LOOPS
Madhumita Bhattacharya1
1
Assistant Professor of Physics, Department of Physics, Guskara Mahavidyalaya, West Bengal
Abstract
A new scheme of optical pulse generation using multiple optoelectronic phase locked loops is proposed. The principle of the scheme
is combining three or more coherent optical waves having equal frequency separation. The coherencies of the optical waves are
attained by the use of optoelectronic phase locked loops. The repetition frequency of the generated optical pulse is equal to the
frequency of the reference microwave/ millimeter wave signal source used in the phase locked loop. The intensity profile of the
generated optical pulse and the pulse width are calculated. If four coherent optical carriers are combined, the generated optical pulse
has a typical width of 2.4 picoseconds. In the proposed scheme the central frequency of the optical pulse and the repetition frequency
can be varied. Since in this scheme, the superimposed optical waves are phase locked the associated phase noise of the optical pulse
will be low.
Keywords: optical pulse generation, optoelectronic phase locked loop, pulse repetition frequency, laser diode, photodiode
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1. INTRODUCTION
Generation of high repetition rate ultra short optical pulse is an
important topic of research due to its application in high speed
optical time division multiplexed (OTDM) communication
system and in optical signal processing [1-10]. To exploit the
terahertz bandwidth of optical fiber around 1.55 µm, different
multiplexing techniques are widely used. Wavelength division
multiplexing (WDM) and time division mutiplexing OTDM
communication system has the capability of carrying a high
data rate of 100 Gb/s over a single optical channel. It also
finds potential application as optical clocks required in optical
computing. Conventional methods of optical pulse generation
consists of gain switching, Q switching and mode locking of
semiconductor lasers. The optical pulse generated from gain
switching and Q switching of laser diodes suffer from phase
fluctuation. Mode locked lasers generate optical pulse with
repetition rate limited to 40 Ghz.
In this paper, we propose a new method of pulse generation
having short pulse width and high repetition frequency by
combining a number of coherent light waves. The coherency
of the optical waves are attained by the phase locked loops. In
this scheme, as the output lightwaves of the laser diodes are
phase locked, the phase noise of the optical waves are greatly
reduced. The phase noise of the generated optical pulse will
also be low.
1.1 System Description
The schematic circuit diagram of the proposed optical pulse
generator is shown in Fig.1. It consists of multiple
optoelectronic phases locked loops (OEPLL). A single
OEPLL consists of two laser diodes (LD1 and LD2), a
photodiode, a reference microwave signal source, a electronic
mixer and a low pass filter. The laser diodes have free running
frequencies 1f and 2f respectively. The frequency difference
between the laser diodes is maintained at a constant value,
which is equal to that of the microwave reference signal
frequency. Considering a single OEPLL, lightwaves from the
laser diodes LD1 and LD2 are heterodyned in a wideband
photodiode (PD1). At the output of the photodiode we get an
electrical signal having frequency ( 12 ff  ). The free
running frequencies of the laser diodes are so chosen that their
difference is almost equal to that of the reference microwave
signal. The signal having frequency ( 12 ff  ) and the output
of the reference oscillator are mixed in the electronic mixer.
The signal at the output of the RF mixer is low pass filtered
and is added to the bias of the laser diode LD2. In the phase
locked condition, the lightwaves from the laser diodes LD1
and LD2 are made coherent and the frequency difference (
12 ff  ) is exactly equal to that of the reference microwave
source ( rf ). By using the second OEPLL, the lightwaves
from LD2 and LD3 are phase locked and the frequency
difference ( 23 ff  ) is maintained constant. The output of
the laser diode LD2 enters the second OEPLL through the half
mirror (HM2). Thus, by using two OEPLLs, we get three
coherent lightwaves having constant frequency differences.
These three lightwaves are directed into the optical combiner
and superimposed to generate the optical pulse. The repetition
frequency of the pulse is equal to the frequency of the
reference microwave source. Similarly, the lightwaves from

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the laser diodes LD3 and LD4 (not shown in Figure 1) can be
made coherent by using the third OEPLL. As the number of
OEPLLs is increased, the number of coherent lightwaves also
increases. These multiple lightwaves are combined to generate
the optical pulse.
Optical pulse
O
Fig.1: A schematic circuit diagram of the proposed optical
pulse generator. LD: laser diodes; PD: photodiodes; X:
electronic mixer; RF: reference microwave signal source; LPF
: low pass filter; HM: half mirror ;
Electrical path:
Optical path:
2. ANALYSIS
In Fig.1, there are two OEPLLs which are assumed to be
under locked condition. Let the electric fields of the output
lightwaves from LD1, LD2 and LD3 be represented as
)](exp[ 1111   jaa (1)
)](exp[ 2222   jaa (2)
and )](exp[ 3333   jaa (3)
respectively.
As the OEPLLs are balanced, the angular frequencies satisfies
the condition
r  )()( 1223 (4)
Where r is the angular frequency of the reference
microwave signal source. The reference microwave signal
used in the OEPLLs can be obtained from a single reference
source. The reference source can be a Gunn oscillator having
a frequency of 60 GHz.
Using the scattering matrix of the half mirror [11], the
composite lightwaves that are combined can be written as
)](exp
22
)(exp
2
)(exp
2
[)(
33
3
22
2
11
1




tj
a
j
a
tj
a
jtao
(5)
These three coherent optical waves are superimposed to
generate the optical pulse. Let us consider the output optical
powers of the laser diodes are identical. aaaa  321
. So we can write eqn.(5) as
)](exp
2
1
)(exp
2
1
)([exp
2
)(
33
2211
1




tj
jtj
a
jtao
(6)
The normalized intensity of the optical pulse is calculated to
be
)]2cos(4
)cos(22
)cos(247[)(
13
23
12






t
t
ttI
r
r
rN
(7)
PD1
X
~
LPF
1
LD2
LD1
PD2X
LPF
LD3
Optical
Combiner
~
HM1
HM2
RF
RF

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The repetition frequency of the optical pulse is equal to the
frequency of the applied reference microwave signal source. If
a Gunn oscillator of frequency 60 GHz is used as the reference
source the repetition frequency is also 60 GHz. In Fig.2, the
normalized intensity of the generated optical pulse is shown as
a function of time, considering two OEPLLs. The two optical
pulse corresponds to repetition frequencies of 60 GHz and
100 GHz respectively. Fig.3 shows the intensity of a single
pulse. The optical pulse widths are calculated to be 5.2 ps and
3.2 ps for the two cases. In numerical calculations we have
assumed 0321   .
Fig.2. Variation of the normalized intensity of the generated
optical pulse as a function of time using repetition frequency
as a parameter
Fig.3. Same as in Fig.2 for a single pulse.
If we consider 3 OEPLLs, at the input of the optical combiner
there are 4 optical coherent waves. The 4 laser diodes used
have frequencies 4321 ,,  and respectively. They
satisfy the condition
r  )()()( 122334 .
In this case the normalized intensity is calculated to be
)]3cos(24
)2(4)2cos(8
)cos(22)cos(24
)cos(2815[)(
14
2413
3423
12








t
tt
tt
ttI
r
rr
rr
rN
.(8)
The variation of the normalized intensity of the generated
pulse as a function of time is shown in Fig.4. From Fig.5 , we
can notice as the number of coherent light wave are increased
the optical pulse width decreases. Again, the pulse width also
decreases for higher repetition rate pulse. The calculated pulse
width is found to be 4 ps and 2.4 ps respectively.
Fig.4. Intensity profile of the optical pulse when 3 OEPLLs
are used taking repetition frequency as a parameter

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Fig.5. Same as in Fig.4 for a single pulse
The calculated optical pulse widths for the different cases are
shown in Table-1.
Table-1
No. of
OEPLLs
Optical pulse
repetition frequency
Optical
Pulse width
1 60 GHz 5.2 ps
1 100GHz 3.2 ps
2 60 GHz 4 ps
2 100 GHz 2.4 ps
3. CONCLUSIONS
A new method of optical pulse generation using optoelectronic
phase locked loops is proposed in this paper. Three coherent
continuous lightwaves are combined to generate the optical
pulse. The coherencies of the lightwaves are maintained by the
use of optoelectronic phase locked loops. The repetition
frequency of the generated optical pulse is equal to the
frequency of the reference microwave signal source. The
intensity profile of the generated optical pulse is calculated.
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A scheme for optical pulse generation using optoelectronic phase locked loops