Generation of High Stability Microwave Signal using Optoelectronic
Oscillator based on Long Fibre Delay Line
Mohamed Mousa
1
, Abdelrahman E. Afifi
2
, Mohamed Abouelatta
2
and Kamel M. Hassan
1
1
Faculty of Engineering and Technology, Future University, N 90th Street, Cairo, Egypt
2
Faculty of Engineering, Ain Shams University, Cairo, Egypt
Keywords: Optoelectronic Oscillator (OEO), Microwave Photonic Signal Generation, Fibre Delay Line.
Abstract: An optoelectronic oscillator based on long fibre delay line to generate high stable microwave signal has been
investigated and implemented experimentally. Mathematical model for this oscillator has been proposed. The
experimental results are taken for different delay line lengths (2.1 Km, 4.2 Km and 6.6 Km respectively). The
generated signal has a narrow bandwidth (less than 200 Hz) at carrier frequency 2.31 GHz and its phase noise
is less than -80 dBc/Hz at 1 KHz offset. Comparison of the experimental results and analytical ones has been
done. A critical length (Lc) concept of the used fibre delay line has been introduced as a design parameter for
the proposed optoelectronic oscillator.
1 INTRODUCTION
There are many oscillator types like LC oscillator
(Van der Pol, 1920; Van der Pol, 1934 ), cavity based
oscillator (Ishihara et al., 1980.) and atomic oscillator
(Siegman and Hagger, 1964), which provide different
degrees of stability and purity of the generated signal
at different frequency ranges. Stability of the output
signal mainly depends on using energy storage
element in the oscillator which is frequency
dependent.
Another important type of oscillators used widely
now is the electronic oscillator (based on transistors),
its stability is improved considerably using high
quality factor resonators like quartz crystals (Parzen,
1983; Halliburton et al., 1985) and dielectric cavities.
The quartz crystals resonator is the best choice, as
it gives the highest stability at room temperature. But
it has only a limited range of frequency tuning.
Microwave signals are out of this range, so it can’t be
generated using this method.
Another type of oscillator is based on use of
electric delay lines, as the delay time is equivalent to
the energy decay time. But if coaxial cable is used, it
requires high power to overcome the losses of long
cable and it will be heavy and takes a large space.
Optoelectronic oscillator is another type of
oscillators that uses optical signals to benefit from the
high performance of optical components and very low
loss and weight of optical fiber compared to coaxial
cables. It can generate microwave frequencies with
the ability of frequency tuning.
The first technique used in this field is based on
the use of long fiber delay line to stabilize the
microwave oscillator (Yao and Maleki, 1994; Yao
and Maleki, 1996a; Yao and Maleki, 1996b). Another
technique can be used by replacing the long fiber
delay line by a short fiber ring resonator (FRR),
which provides high quality factor and high stability
compared to the generated signal using the first
technique. FRR technique requires strict control on
the ring temperature (Yariv, 2000; Yariv, 2002;
Merrer et al., 2008).
New technique based on Brillouin selective side
band amplification has been introduced recently.
Brillouin oscillator doesn’t need narrow band pass
microwave filter neither microwave amplifier. On the
other hand this oscillator requires a laser source with
very narrow spectral width, high output power as well
as it requires long fiber length (Yao, 1997; Li et al.,
2013; He et al., 2016), see APPENDIX A
Each technique has its unique features. The recent
researches in this field show a phase noise as low as
–92.69 dBc/Hz at 10 kHz offset frequency from the
oscillation frequency (2.26 GHz) carrier using optical
delay line of 25.24 km and a Q factor of 2.04 × 10
9
(Correa-Mena et al., 2017), there is other technique
generate two tone signals in a range from 4 GHz to 12
GHz with phase noise about -105 dBc/Hz at 10 kHz
offset frequency from the oscillation frequency
Mousa, M., Afifi, A., Abouelatta, M. and Hassan, K.
Generation of High Stability Microwave Signal using Optoelectronic Oscillator based on Long Fibre Delay Line.
DOI: 10.5220/0006855102170222
In Proceedings of the 15th International Joint Conference on e-Business and Telecommunications (ICETE 2018) - Volume 1: DCNET, ICE-B, OPTICS, SIGMAP and WINSYS, pages 217-222
ISBN: 978-989-758-319-3
Copyright © 2018 by SCITEPRESS – Science and Technology Publications, Lda. All rights reserved
217

(9.95GHz and 10.66 GHz) (Gao et al., 2017) . In this
workk we get better phase noise -104 dBc/Hz at 10
kHz offset frequency from the oscillation frequency
(2.31 GHz) carrier using shorter optical delay line of
6.6 km and a lower Q factor of 0.019 × 10
9
.
In this paper, microwave oscillator based on the
first technique has been analysed and implemented.
The rest of this paper is organized as follows:
system model is presented in section 2; experimental
results have been investigated in section 3. The
influence of optical fiber length on the oscillator
stability is given in section 4 and finally, the
conclusion is presented in section 5.
2 SYSTEM MODEL
2.1 Main Parameters of Optoelectronic
Oscillator
The proposed system is shown in Fig.1. Electrical
signal applied to Mech-Zender modulator is driven
from the microwave filter. At the beginning this
signal is coming from noise, the loop gain is changing
till the oscillation is sustained, in this case input to the
Mech-Zender modulator will be the microwave
signal.
Light from a laser source (Po(t)) is introduced into
an electro-optic modulator; the output modulated
signal (P(t)) is passed through the fibre delay line.
Figure 1: Optoelectronic oscillator structure.
Then, the output is detected by a photodetector.
The output of the photodetector is amplified and
filtered, then applied to the Mach-Zender modulator
(Vin(t)), the electrical input voltage is related to the
optical output power from the modulator by (1).
(Chang, ed., 2007)
P(t)=αP
o
(t)[1+ηsin(π(V
in
(t)+V
B
)/V
π
))]/2 (1)
As (α) is the fractional loss of the modulator, (η) is
the extinction ratio of the modulator, (V
B
) is the bias
voltage and (V
π
) is the half wave voltage of the
modulator. Assuming the input signal is sinusoidal
wave with oscillation frequency (ω). The output
microwave signal (V
out
) is related to the modulated
signal (P(t)), the responsivity of the detector ( R), the
load resistance (R
L
), the amplifier gain (G
A
), the loss
of the fibre (α
f
), the fibre length (L) and the absolute
value of the filter transmission function (|F(ω) |)by
(2).
V
ou
t
=R.R
L
.G
A
. |F(ω) |P(t) e
-α
f
L
(2)
The quality factor of the generated signal (Q) depends
on the quality factor of the loop delay line (Q
D
), the
delay time of the fiber line (τ) and the input noise to
signal ratio (δ) of the oscillator as given by (3).
Q = Q
D
. τ /δ
(3)
The delay time offered by the fiber line depends on
the refractive index of the fiber (n), the fiber length
(L) and the speed of light in free space (c) as shown
in (4).
τ = nL /c
(4)
The quality factor of the loop delay line (Q
D
) depends
on the delay time inside fiber line (τ)and the
oscillation frequency (f), the input noise to signal
ratio (δ) which depends on the equivalent input noise
density injected into the oscillator (ρ
N
), the oscillation
power (P
Osc
) and the total gain (G
A
), as given in (5)
and (6) respectively.
Q
D
= 2πf τ
(5)
δ = ρ
N
.G
A
/P
Osc.
(6)
This technique requires a very long fibre delay line
(in the kilometre range) to satisfy high quality factor
and low phase noise, this means small mode spacing
as the free spectral range (FSR) between oscillation
modes is the inverse of the delay time in the fibre line
as described by (7). So, a microwave filter with
narrow bandwidth is required to select the oscillation
mode.
FSR = 1/τ (7)
The small FSR problem (as a result of long fibre delay
line) can be solved by using two fibre loops acts as
short and long cavities to select a single operation
mode (Smith, 1972). But this technique increases the
size and the complexity of the system.
The output of optoelectronic oscillator may be
obtained either directly as a microwave signal, or as
an optical signal. It is characterized by high stability
and low noise which is achieved by using optical
component that are characterized by high speed, high
efficiency and better performance than electrical
components.
Optoelectronic oscillator utilizes the transmission
characteristics of an electro optic modulator with the
OPTICS 2018 - International Conference on Optical Communication Systems
218

fibre delay line to convert light energy into stable
microwave signal.
The oscillation frequency is determined by the
filter characteristics and the fiber delay line length as
the oscillation frequency (f) depends on the mode
number (k) and the delay time inside fiber line (
τ
) is
given by (8).
f =k /
τ
(8)
The 3- dB bandwidth of each fiber delay line (∆f) is
given by (9).
∆f =ρ
N.
G
A
2
/(2πτ
2
p
Osc
) (9)
The phase noise (L
Osc
) at a certain frequency offset
(f
m
) is related to the quality factor (Q), the oscillation
frequency (f) and the spectral density of the amplifier
phase fluctuations (Ư
A
), and the ratio between the
phase noise using different lengths lead to different 3-
dB bandwidth and amplifier phase fluctuations are
given by (10) and (11) respectively.
L
Osc
(f
m
) = (f /(2
1.5
.Q.f
m
))
2
.(Ư
A
)
2
(10)
L
osc1
(f
m
) – L
osc2
(f
m
) = 20log[(∆f
1
)/(∆f
2
)] +
20lo
g
[(Ư
A1
)/( Ư
A2
)]
(11)
2.2 Critical Fibre Delay Line Length
The 3-dB bandwidth (∆f) of the generated signal is
inversely proportional to the optical fibre length
square as shown in (4) and (9). The equivalent input
noise density injected into the oscillator (ρ
N
) is the
sum of the thermal noise (ρ
th
), shot noise (ρ
sh
) and the
laser relative intensity noise (ρ
RIN
) as given in (12).
ρ
N
= ρ
th
+ρ
sh
+ρ
RIN
(12)
The shot noise and the relative intensity noise depend
on the photocurrent (I
ph
), the load resistance (R
L
) the
electron charge (e) and the relative intensity noise of
the laser (N
RIN
) as shown in (13) and (14)
respectively.
ρ
sh
=2eR
L
I
ph
(13)
ρ
th
=N
RIN
R
L
(I
ph
)
2
(14)
The photocurrent is related to the output modulated
signal (P(t)), the responsivity of the detector (R), the
loss coefficient of the fibre (α
f
) and the fibre length
(L) by (15)
I
ph
=RP(t)e
-α
f
L
(15)
The total gain (G
A
) is related to the amplifier gain
(G
Ao
), the loss of the fibre (α
f
) and the fibre length (L)
by (16)
G
A
=G
Ao
e
-α
f
L
(16)
The oscillation power (P
Osc
) is related to the output
oscillation voltage (V
out
) and the load resistance (R
L
)
by (17)
P
osc
=(V
out
)
2
/R
L
(17)
The previous equations show the effect of the fibre
delay line length (L) in the main parameters of the 3-
dB bandwidth (∆f). As the length increases, some of
the parameters increase and others decrease. This
leads to decrease of the rate of change of the 3-dB
bandwidth (d (∆f)/d) with increasing (L), (d (∆f)/dL)
is given by (18)
d(∆f)/dL =(G
A
)
2
.c
2
{[d(ρ
N
) /dL] L-
2ρ
N
}
/(2π.n
2
.
p
Osc
.L
3
)
(18)
The rate of change of (∆f) versus the fibre length (L)
is decaying fast as (L) exceeds a certain value. We
may define the fibre length at which the absolute
value of the rate of change of 3-dB bandwidth |d
(∆f)/dL| reaches 1 Hz/Km as a critical fibre length of
this oscillator. This critical value may be taken as a
system parameter (L
C
).
For a given equivalent input noise density injected
into the oscillator (ρ
N
), the oscillation power (P
Osc
),
the amplifier gain (G
A
) and the refractive index of the
used fibre "n".There will be a specific value for (L
C
).
The designer of this oscillator will choose the delay
fibre line length based on this value where longer
length will add costs without considerable gain in
reducing the 3- dB bandwidth (∆f).
3 EXPERIMENTAL RESULTS
Optoelectronic oscillator has been implemented using
free bias Mach-Zander modulator, microwave filter at
central frequency 2.31 GHz with three dB bandwidth
of 10 MHz using different fibre delay lengths (2.1
Km, 4.2 Km and 6.6 Km respectively).
The experimental results agree with the expected
ones for the main three parameters. First parameter is
the, frequency spectral range (FSR) which decreases
as the fiber delay line length increases (Eq. (7)).
Second parameter is the three dB bandwidth (∆f)
which decreases too as the fibre delay line length
increases (Eq. (9)). Third one is the oscillation
frequency phase noise (L
Osc
(fm)) which becomes
better with increasing the fibre length which
determines the quality factor (Q) (Eq. (10)).
The output spectrum contains several modes. The
number of these modes depends on the free spectral
range value (FSR) and the filter bandwidth. Recall
that (FSR) is proportional to the inverse of the fibre
delay line length (L).
Generation of High Stability Microwave Signal using Optoelectronic Oscillator based on Long Fibre Delay Line
219

The output microwave signal using 2.1 Km fibre
delay line with span 300 KHz is shown in Figure 2,
where Figure 3 shows the oscillation mode with span
20 KHz.
Figure 2: Output signal using 2.1 Km fibre delay line (Span
300 KHz).
Figure 3: Oscillation mode using 2.1 Km fibre delay line
(Span 20 KHz).
Using fibre length of 4.2 km, the output spectrum
is shown in Figures. 4 and 5. For optical fibre length
of 6.6 km, the output spectrum is illustrated in Figure
6.
Figure 4: Output signal using 4.2 Km fibre delay line (Span
300 KHz).
As shown in Figure 2 and Figure 4, the mode
spacing between the modes decreases from 95.2 KHz
using 2.1 Km to 47.6 KHz using 4.2 Km and the
oscillation side band suppression ratio about 25 dB.
The oscillation frequency using different lengths is
changed by a small amount as a result of the
bandwidth of the used filter and as it must be a
multiple of the (FSR).
Figure 5: Oscillation mode using 4.2 Km fibre delay line
(Span 20 KHz).
Figure 6: Oscillation mode only using 6.6 Km fibre delay
line (Span 20 KHz).
The used laser (Agilent 81940A), the modulator is
Mach-Zander modulator (JDSU: 2.5 Gb/s Bias-Free
Modulator with Integral Attenuator), the detector is
avalanche photodiode (OF3240N-MS-YT) with gain
about 24 dB, the filter is cavity band pass filter (DSC-
2310B-10M01) with 3dB bandwidth 10 MHz and the
results are taken using RF spectrum analyser up to
3GHz (R&S FSP 9k-3G).
OPTICS 2018 - International Conference on Optical Communication Systems
220

4 THE INFLUENCE OF OPTICAL
FIBER LENGTH ON THE
OSCILLATOR STABILITY
Table 1 and 2 shows the main parameters and
performance parameters, respectively of the
optoelectronic oscillators using different optical fibre
lengths.
Table 1: Oscillator parameters comparison using three
different fibre delay lines.
fibre delay
line (K
m
)
Oscillation
frequency (MHz)
Measured output
Powe
r
(dBm)
FSR
(KHz)
2.1 2311.8405 -16.4 95.2
4.2 2311.5524 -16.4 47.6
6.6 2311.2498 -17.86 30.3
Table 2: Oscillator performance parameters comparison
using three different fibre delay lines.
fibre delay
line (Km)
Measured
∆f(3dB)
(Hz)
Phasenoise
[L
Osc
(1KHz)]
(dBC/Hz)
Phasenoise
[L
Osc
(10KHz)]
(dBC/Hz)
2.1 150 -80 -100
4.2 130 -82 -101.5
6.6 120 -84 -104
The experimental results are in good agreement
with the expected ones which have been calculated
using the presented system model; whereas the length
increases the -3 dB bandwidth and the phase noise at
certain offset frequency decrease.
The measured change in -3dB bandwidth is 20 Hz
(150Hz using 2.1 Km drops to 130 Hz using 4.2 Km)
where this change drops to only 10 Hz (when
switching from 4.2 Km to 6.6 Km). This agrees with
the results given by the system model, whereas as the
fibre delay line (L) increases, the rate of change of -3
dB bandwidth change decays.
5 CONCLUSIONS
We have successfully implemented a high stability
microwave oscillator based on long fibre delay line.
System model has been presented. The expected
effect of the optical fibre length on the main
parameters has been tested and verified. Investigation
of analytical and experimental results leads to what
we call critical fibre length (L
C
) which depends on the
main specifications of the proposed oscillator. Using
fibre length (L) exceeding (L
C
) the improvement of 3-
dB bandwidth (∆f) is almost negligible. The results
based on this model are compared to the experimental
ones and a good agreement is observed. Our future
work is to use several fibre delay line lengths (seven
at least) and investigate in more depth the effect of
increasing the fibre length over the critical one.
Future work may be extended to use other techniques
based on the fibre ring resonator and Brillouin
selective side band technique in order to compare
these three techniques.
ACKNOWLEDGEMENTS
The experimental work has been done in the
laboratory of laser and optical communication at
faculty of engineering, Ain shams university. Egypt.
The authors would like to appreciate the help given
by prof. Mahmoud Ahmed and his team.
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APPENDIX A
The Brillouin threshold is given by (Li et al.., 2013):


21






(A1)
where


: the effective cross section area of the fiber
core.


: the efective length of the fiber.


: the Brillouin gain coefficient, which is given by



4πγ2
n

λ

c
ρ

v

.
1
∆ν


(A2)
where
γ: the electrostrictive coefficient.


: the refractive-index.
 : the wavelength of the laser signal.
: the light speed in free space.
ρ

: density of the scattering medium.


: velocity of the induced (acoustic) wave in the
medium
∆ν

: the spectral linewidth of the pump laser.
It's clear the effect of the spectral linewidth of the
pump laser on the threshold pump power of Brillouin
beam.
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