A novel technique to phase shift signal by 90° over a broad frequency range based on transversal signal processing is proposed and practically demonstrated. The proposed technique is suitable for non coherent optic implementation

Bui, L

Emami, H

Mansoori, S

Mitchell, A

Hossein Emami

Lam Anh Bui

Sana Mansoori

Arnan Mitchell

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Quadrature hybrid coupler using photonic transversal approach

RMIT Research Repository

Quadrature Hybrid Coupler Using Photonic Transversal Approach 
Hossein Emami1, Lam Anh Bui1, Sana Mansoori2 and Arnan Mitchell1
1Microelectronics and Materials Technology Centre, School of Electrical and Computer Engineering, 
RMIT University,  GPO Box 2476V, Melbourne, Victoria 3001, Australia 
2Department of Electrical and Electronics Engineering, College of Engineering, University of Bahrain 
Abstract  —  A novel technique to phase shift signal by 90 
degrees over a broad frequency range based on transversal 
signal processing is proposed and practically demonstrated. 
The proposed technique is suitable for non coherent optic 
implementation.  
I. INTRODUCTION
A Quadrature hybrid is a device that splits an RF signal 
into two equal components and imparts a 90o phase shift 
onto one of the outputs with respect to the other.  These 
devices are one of the basic components used in a variety 
of microwave circuits such as balance mixers, data 
modulators [1], phase shifters, and beam formers [2].     It 
is important that these devices provide a low-loss, equal 
power split between the two output ports and maintains a 
precisely 90 o phase shift over multiple octaves. 
Broadband quadrature hybrid coupler implementations 
using traditional microwave techniques have been 
reported [1-4]. Practical devices have moderate 
bandwidths (2-18GHz), but exhibit significant amplitude 
imbalance (>3dB). Impedance matching presents a 
significant challenge that must be overcome using multi-
resonant structures. These structures have very stringent 
fabrication tollerences and may even need trimming and 
can thus be very expensive [3,4]. 
Modern electronic warfare systems are being 
considered to cover an extended range from 2-40GHz and 
beyond.  Traditional microwave techniques may not be 
capable of realizing quardature hybrids that cover this 
extremely broad bandwidth and still maintain acceptable 
amplitude balance, phase response and practical 
fabrication tolerances. 
In recent years, microwave photonics has been 
investigated as a means of achieving ultra-broadband 
microwave signal processing.  Indeed, several microwave 
photonic quadrature hybrid implementations have been 
proposed [5,6]. These solutions employ coherent optical 
beating to transfer the optical phase into electrical phase 
[5]. Unfortunately, these systems rely on the maintenance 
of optical coherence throughout the system and thus may 
not be robust when used in a harsh defence environment. 
Another microwave photonic technique that has been 
introduced recently is the photonic transversal filter [7].  
Here, the RF signal is modulated onto several optical 
carriers (or taps) and each tap is delayed by a different 
amount before detection. By controlling the relative 
weights and delays of each tap, a number of microwave 
signal processing operations can be realized [7,8].   
We have recently proposed the use of a microwave 
photonic transversal filter to realize a broadband, high-
performance transversal filter [9].  This paper presenta a 
practical implementation of this concept using a simple 
four tap transversal filter.  A microwave photonic hybrid 
with comparable performance to traditional microwave 
implementation is demonstrated and paths for extension to 
ultra-broad bandwidths are discussed.  
II. THEORY
As discussed in [9], transversal filtering is a method by 
which a range of signal processing functions can be 
achieved.  In short, a signal is divided between a number 
of taps and each tap is delayed by a different amount.  At 
the output, all of the delayed taps are summed and 
detected.  By controlling the relative weights and delays of 
each tap, a range of filter functions can be implemented.   
Each tap in the transversal filter can be considered as a 
separate sample in the time domain with each delayed tap 
corresponding to a different point in time.  By adjusting 
the amplitude of each tap, an arbitrary impulse response 
can be formed.  The frequency response of the summation 
of these delayed taps will be the Fourier transform of the 
impulse response. Careful control the tap amplitudes 
enables a broad range of filter functions to be realized. 
It is well known that if a system has an impulse 
response of a homographic hyperbola type, the 
corresponding frequency response will have a constant 
amplitude and a frequency independent 90o degree phase 
shift.  The impulse response of such a Hyperbola is 
presented in Figure 1 and the corresponding frequency 
response is presented in Figure 2. 
For ideal system performance with infinite bandwidth, 
the impulse response has to be an infinite continuous time 
signal.  In reality, this is impractical, and thus the impulse 
response must be truncated in time. Windowing 
techniques are usually employed to reduce distortions of 
the system frequency response due to Gibes effect [9]. 
The impulse response in transversal filter is sampled by 
discrete taps which are effectively delta functions in time.  
Thus, along with being truncated, the impulse response is 
samples as shown in Fig 1.  Sufficient samples must be 
included to avoid aliasing distortion.  Ideally, appropriate 
windowing should enable synthesis of a practical filter 
with performance similar to its continuous counterpart.  
Fig 2 presents the predicted amplitude and phase of the 
frequency response of both the continuous, infinite and 
also the sampled and truncated responses shown in Fig 1. 
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2III. METHOD
To implement the transversal filter depicted in Figs 1 
and 2, we have chosen to use a wavelength division 
multiplexing (WDM) photonic technique.  Figure 3 
presents a diagram of the experimental set up.  
To demonstrate the concept of photonic RF hybrid 
coupler. Four lasers with different wavelength have been 
used to implement a four taps transversal filter using 
wavelength division multiplexing (WDM). Each laser 
goes through a polarization controller (PC). Two laser 
beams then are combined by 3-dB couplers going through 
Mach-Zehnder modulators biased at V+ and V- to 
experience a 180 degree phase difference respect to each 
other.  The outputs of the MZMs are then goes through 
Variable Optical Lengths (VOL) to make sure that both 
optical links have the same lengths.  
The two outputs are then combined and enter a length 
of dispersive fibre.  This fibre provides a different delay 
for each wavelength.  The dispersed signals are then 
amplified by an Erbium Doped Fiber Amplifier (EDFA). 
The output of EDFA is then split into two portions by a 
10% optical coupler. The 10% portion goes to an Optical 
Spectrum Analyzer (OSA) to monitor the amplitude of 
each tap and the 90% portion goes to a Photo Detector 
(PD) which is connected to the Port II of a 20GHz Vector 
Network analyzer (VNA), while port I is connected to an 
RF power splitter feeding RF inputs of MZMs. 
For 20 GHz bandwidth, the spacing of the samples of 
the impulse response must be ∆t=51.23. From this tap 
spacing it is possible to work out what wavelength spacing 
is required. Using ∆t=D∆λL, with dispersion constant 
D=17 ps/nm.km, dispersive fibre length L=2.354km, the 
wavelength separation is found to be ∆λ=1.25 nm. 
Starting from λ1=1552.2 nm for the first tap 
wavelength, the other wavelength are driven as 
λ2=1553.45 nm, λ3=1554.7 nm and λ4=1555.95 nm. To 
compensate the nonlinearity of dispersion effect, the 
actual wavelength corresponding the 2nd and 3rd taps, have 
to be practically a little different 1553.48 and 1554.67 nm 
respectively. The taps weights are 0 dB for the inner taps 
and -12.8 dB for the outer.  
To reference the system, a single carrier with 
λ=1554.075 nm is used.  This corresponds to a tap a zero 
time and can be though of as the 0o arm of the hybrid. 
IV. RESULTS
To observe the 90° phase shift, the VNA is calibrated 
setting the wavelength of LD1 to λ=1554.075 nm and the 
other laser diodes turned off.  The laser diodes were then 
all configured as described in Section III.  Figure 4 
presents the OSA trace indicating the relative wavelengths 
and amplitudes of the transversal filter.  Figure 5 presents 
the measured frequency response from the VNA. The 
simulated response of Figure 3 is also presented in Figure 
5 for comparison.  Excellent agreement between the 
measurement and theory is evident.   Amplitude variation 
< 3dB has been maintained over the range from 2.5-
17.5GHz, with negligible phase deviation from 90o. 
V. DISCUSSION
The configuration of Figure 3 could be greatly 
simplified through the use of a balanced output modulator 
(with complimentary outputs).  This would increase the 
link gain performance and reduce the component count.  
The optical combination could be achieved using 
wavelength multiplexers rather than simple 3dB couplers.  
This will greatly improve the link gain (6dB electrical per 
split).  Extension of this demonstration with these 
improvements is currently under investigation. 
The tap weights in Figure 4 vary slightly from those 
depicted in Figure 1.  This is due to the fact that different 
lasers with slightly different linewidths were used and the 
tap weight varies as the square of the total optical power.   
The results presented in Figure 5 compare favorably 
traditional microwave hybrids.  This photonic 
implementation offers the advantage that the pass band 
ripple and the bandwidth can dynamically adjusted 
changing the tap weights and/or the wavelength spacings. 
It would be desirable to extend the hybrid operation to 
2-40GHz with less than 3dB ripple.  To improve the 
bandwidth and ripple, extra taps could be introduced as 
discussed in [9].   Investigation of systems with additional 
wavelength taps is currently under investigation. 
-0.1 -0.08 -0.06 -0.04 -0.02 0 0.02 0.04 0.06 0.08 0.1
-0.1
-0.05
0
0.05
0.1
0.15
Time(nsec)
Continuous
Selected Taps a3
a1
a2
a4
∆t
Fig. 1. Hyprebolic impulse response, continuous and 
infinite; and sampled. 
0 2 4 6 8 10 12 14 16 18-15
-10
-5
0
Magnitude (dB)
Frequency(GHz)
0 2 4 6 8 10 12 14 16 18
-pi/2
0
pi/2
Phase (Radian)
Frequency(GHz)
Sampled
Continuous
Sampled
Continuous
Fig. 2.Predicted frequency response for the impulse 
responses of Fig 1: continuous and sampled. 
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3VI. CONCLUSION
A novel, photonic technique to realize a quadrature
hybrid coupler using transversal scheme has been 
practically demonstrated. This photonic implementation 
offers a dynamically adjustable bandwidth and pass-band 
ripple.  The demonstration can be readily extended to 
broader bandwidths through the introduction of extra 
wavelength channel taps. 
REFERENCES
[1] L. Fan, b. Heimer, and K. Chang, “Uniplanar hybrid 
couplers using asymmetrical coplanar strip lines,” 
Microwave Symposium Digest, 1997, IEEE MTT-S 
International, 1997, pp. 273-276. 
[2] R. T. Chen, E. R. Brown, R. S. Singh, “A compact 30 GHz 
low loss balanced hybrid coupler fabricated using 
micromachined integrated coax,” presented at Radio and 
Wireless conference, 2004 IEEE, pp. 227-230. 
 [3] J. S. Izadian, “A new 6-18 GHz, -3dB multisection hybrid 
coupler using asymmetric broadside, and edge coupled 
lines,” IEEE Int. Microwave Symp, pp.143-246 
[4] M. –H. <urgulescu, P. Leguad, E. Penard, and I. Zaquine, 
“New small 90° hybrid coupler,” Electronics Letters, vol. 
30, pp. 1289-1290, 1994. 
[5] G. A. Berenbrock and B. Schelmmer, “Active controlled 
fiber optical 90° hybrid for coherent communications,” 
IEEE Photon. Tech. Letters, vol.1, pp. 86-87, 1989. 
[6] D. Madris, B. Vidal, A. Martinez, V. Polo, J. L. Corral, and 
J. Marti, “A new optical beamforming architecture based on 
N × N optical butler matrices providing 2N beams,” 
Antennas and Propagation Society International 
Symposium,2002. IEEE, 2002. 
[7] S. Mansoori, A. Mitchell, K. Ghorbani “Photonic 
reconfigurable microwave filter with negative coefficients” 
Electronics Letters  ,vol. 40, pp 541-543, 2004. 
[8] S. Mansoori, A. Mitchell, "RF transversal filter using an 
AOTF," IEEE Photonics Technology Letters, vol. 16, 
pp 879-881, Mar (2004). 
[9] H. Emami, L. A. Bui, A. Mitchell, “Broadband 90 degree 
hybrid coupler using photonic transversal approach,” AP-
MWP 2006 
LD1
LD2
LD3
LD4
PC 
PC 
PC 
PC 
3 dB 
Optical 
Coupler
MZM 
V+
MZM 
V-
3 dB RF 
Power combiner 
 VNA Port 
VO
VO
3 dB 
Optical 
Coupler
3 dB 
Optical 
Coupler
E
D
F
A
10% 
Optical 
Coupler
PD 
VNA Port 
OSA
Dispersi
on Fiber
VNA Port II
Figure 3: Block diagram of transversal filter experimental setup 
1552 1552.5 1553 1553.5 1554 1554.5 1555 1555.5 1556
-18
-17
-16
-15
-14
-13
-12
-11
-10
-9
Wavelength(nm)
Po
w
er
(dB
)
Fig. 4. OSA Trace indicating the relative weights and 
wavelengths of each tap. 
0 2 4 6 8 10 12 14 16 18
-15
-12
-9
-6
-3
0
Magnitude (dB)
Frequency(GHz)
0 2 4 6 8 10 12 14 16 18
-pi/2
0
pi/2
Phase (Radian)
Frequency(GHz)
Simulation
Measurement
Simulation
Measurement
Fig. 5. a comparison between measurement and 
simulation result for both magnitude and phase
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Quadrature hybrid coupler using photonic transversal approach

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