A transversal phase-shifter using multiple MZMs is demonstrated. The device exhibits continuously variable phase-shift exceeding 360° at 2 GHz and amplitude uniformity within 3 dB over 0.2-2 GHz. The device stability and practicality are discussed

Bui, L

Chio, T

Mitchell, A

RMIT Research Repository

Demonstration of a wide band RF photonic transversal 
phase-shifter 
 
Lam Anh Bui and Arnan Mitchell 
Australian Photonics CRC, RMIT University, GPO Box 2476V, Melbourne VIC 3001, Australia.  
l.bui@ieee.org 
 
Tan-Huat Chio 
DSO National Laboratories, S(118230) Singapore. 
ctanhuat@dso.org.sg 
 
Abstract: A transversal phase-shifter using multiple MZMs is demonstrated. The device exhibits 
continuously variable phase-shift exceeding 360° at 2 GHz and amplitude uniformity within 3 dB 
over 0.2-2 GHz. The device stability and practicality are discussed. 
ã2004 Optical Society of America 
OCIS codes: (060.2340) Fiber optics components; (350.4010) Microwaves 
1. Introduction 
Modern military RF and microwave systems and subsystems are required to process signals with significant 
bandwidth in order to maximise probability of intercept in an unpredictable environment. This demand has created 
much research interest in the area of microwave photonics [1]. In phased array antenna applications, broadband 
beam forming requires phasing systems to exhibit true time delay behaviour. Several techniques have been 
proposed over the years to achieve this. Examples are switched delay lines [2], dispersive fibre prism [3] and fibre 
Bragg grating approaches [4]. 
Even though TTD techniques can result in very broad system bandwidth, they tend to be expensive and 
complex which limit their use in practical applications. Other approaches have therefore been conceived. These 
approaches approximate true time delay behaviour over an operating frequency at the expense of behaviour outside 
the band. Examples of such device are the reflection type phase-shifter [5], the varactor loaded transmission line 
phase-shifter [6], the vector sum phase-shifter [7] and recently the transversal phase-shifter [8]. 
In [8], the transversal phase-shifter (TPS) has been proposed and analysed theoretically. In this paper, we 
present a practical demonstration of the TPS device. A particular implementation of the TPS is described in Section 
2. Experimental results are then compared with the theoretical prediction. Section 3 discusses various aspects of the 
TPS implementations covering the system stability, controlling scheme and the practicality of the device. Future 
research direction is then drawn from these discussions. Conclusions are finally given in Section 4. 
2. Demonstration of an RF photonic transversal phase-shifter 
A seven tap TPS, which permits 360° phase shifting range at 2 GHz and has less than 1 dB amplitude variation over 
the entire 0.2 to 2 GHz band has been previously proposed and theoretically studied in [8]. In the current 
investigation, a five tap TPS is demonstrated practically. 
The experimental setup is presented in Fig. 1. The TPS comprises of 5 taps, which each is implemented as a 
simple RF photonic link using a Mach Zehnder modulator (MZM). Separate lasers are employed to avoid 
undesirable coherent interference [9]. The input RF signal is split equally using a 1:8 active power splitter. Unused 
output ports of the power splitter are terminated in 50 W to avoid reflections. The taps are combined optically in a 
cascade of 50:50 fibre couplers. The time delay increment between taps is calculated to be 180 ps using an iterative 
numerical optimisation procedure. Using an RF network analyser, the required path lengths of these taps are 
matched within 0.5mm by carefully trimming the fibre. 
The TPS tap weighting is calculated for various time delays using the method proposed in [8]. Table 1 presents 
the values of the tap’s weights for some nominal values of the time delays. Delays have been chosen to illustrate the 
ability of the TPS to implement time delay in between the delays of the independent taps. The theoretical phase and 
magnitude responses of the TPS with the nominal time delays are presented as solid lines in Fig. 2 a) and b) 
respectively. 
© 2004 OSA/OFC 2004
Using a vector network analyser to measure the RF response, the modulator bias is varied until a right tap 
weight is achieved. In addition to positive values, negative values of tap weight are obtained by utilising the 
negative slope of the modulator transfer function as illustrated in the inset of Fig. 1. The measured phase and 
magnitude responses of the TPS for time translation of Table 1 are presented as points in Fig. 2 a) and b) 
respectively. 
The measured amplitude variation is within 3 dB over the entire band from 0.2-2 GHz and more than 360° 
phase-shifting range is available at 2 GHz. There is excellent agreement between the calculated and measured phase 
responses, and although having some deviations, the magnitude response traces well with the prediction. It is 
believed that the discrepancy between the measured and predicted responses could be caused by the variation in 
tap’s weights due to the modulator bias drift.  
3. Discussions 
Although the RF photonic link gain depends on the polarisation of the optical carriers, it is not a source of 
instability in the system demonstrated in Section 2. The biggest source of instability in this system is the modulator 
bias drift, which results in an excess variation of the magnitude response on top of the TPS theoretical response. 
This problem could be avoided if modulator bias controls were employed. It is also of interest to note that 
modulator bias control can be programmed to detect and lock into a given modulation slope therefore allows the 
automation of the TPS control [10]. 
It is expected that varying the bias of the MZM as a mean of changing TPS’ weights will result in reduction of 
the TPS dynamic range due to increasing power of the harmonic components of the RF signal. In particular, the 
second harmonic component of RF signal is maximised when the MZM is biased to maximum or minimum. This 
can significantly affect signal with bandwidth broader than an octave. It is therefore desirable to always bias the 
modulator at quadrature. Having fixed the bias at one of the quadrature points, the tap weights could be changed by 
varying the carrier powers. This can be achieved in two ways: varying the laser powers or employing variable 
optical attenuators. Negative weighting could then be obtained by changing bias to a quadrature point in the 
negative slope of the MZM transfer function. 
 
LASER 1
LASER 2
LASER 3
LASER 4
LASER 5
PHASE-SHIFTER
CONTROLLER
1:8 POWER
DIVIDER
OPTICAL
COMBINER
RF VECTOR
NETWORK ANALYSER
1
VARY BIAS
TO VARY SLOPE
MZM TRANSFER FUNCTION
OPTICAL FIBRE
DELAY LINES
OPTICAL LINK
RF LINK
DC LINK
POLARISATION
CONTROLLER
MZM 1
MZM 2
MZM 3
MZM 4
MZM 5
 
Fig. 1. Transversal phase-shifter demonstration setup 
Table 1. List of coefficients for generating time delays. The first column represents the deviation from the delay of 
the central tap (t = 360 ps). Note that these delays are not integer multiples of the tap delays. 
Dt(ps) C0 C1 C2 C3 C4 
-252 0.515467 0.743515 -0.200601 0.10827 -0.0690464 
-100.8 -0.206133 0.70204 0.56445 -0.204039 0.123238 
0 -0.0395802 0.0398948 0.96 0.0398948 -0.0395802 
151.2 0.084906 -0.115864 0.216527 0.923175 -0.0958654 
252 -0.0690464 0.10827 -0.200601 0.743515 0.515467 
© 2004 OSA/OFC 2004
The TPS performance depends on the method used to weight the taps. It is therefore anticipated that significant 
research should be directed toward the algorithm used to select these tap weights. Numerical optimisation could be 
useful for this task. 
It will also be important to minimise the complexity of the system. The requirement for an array of modulators 
could be met using a monolithic modulator array and optical combiners to minimise packaging requirements. 
Alternatively, WDM approaches using a single modulator could be considered. 
4. Conclusions 
An implementation of the transversal phase-shifter using multiple Mach Zehnder modulators is demonstrated. The 
device exhibits continuously variable phase-shift, which is linear with frequency over the 0.2-2 GHz band. The 
continuously variable phase shifting range is greater than 360° at 2 GHz and the measured magnitude response is 
uniform within 3 dB across the entire operating band. There is good agreement between measurement and 
prediction; hence the transversal phase shifter concept is validated. The major source of instability in this TPS 
implementation is the tap weight variation due to the modulator bias drift. This issue can be avoided by employing 
bias control circuits. Simplified implementation of the TPS using WDM techniques is currently under investigation. 
5. References 
1. P. J. Matthews and P. D. Biernacki, "Photonic signal processing for microwave applications," 1999 IEEE MTT-S International Microwave 
Symposium Digest, 3, 877-880 (1999). 
2. A. Ouacha, B. Carlegrim and M. Alfredson, "Ultra broadband MMIC 5-bit time shifter," 1999 Asia Pacific Microwave Conference. 
APMC'99, 2, 242-245 (1999). 
3. R. D. Esman, M. Y. Frankel, J. L. Dexter, L. Goldberg, M. G. Parent, D. Stilwell and D. G. Cooper, "Fiber-optic prism true time-delay 
antenna feed," IEEE Photonics Technol Lett. 5(11), 1347-1349 (1993). 
4. J. L. Corral, J. Marti, S. Regidor, J. M. Foster, R. Laming and M. J. Cole, "Continuously variable true time-delay optical feeder for phased-
array antenna employing chirped fiber grating," IEEE Trans. on Microwave Theory and Tech. 45(2), 1531-1536 (1997). 
5. S. Shin, V. R. Snyder and E. Niver, "360-degree linear analog phase shifter design using tunable short-circuit terminated combline filters," 
Microwave Symposium Digest, 2001 IEEE MTT-S International, 1, 303-306 (2001). 
6. F. Ellinger, H. Jackel and W. Bachtold, "Varactor-loaded transmission-line phase shifter at C-band using lumped elements," IEEE Trans. 
on Microwave Theory and Tech. 51(4), 1135-1140 (2003). 
7. L. A. Bui, A. Mitchell, K. Ghorbani and T. H. Chio, "Wide-band RF Photonic Vector Sum Phase-Shifter," Electron. Lett. 39(6), 536-537 
(2003). 
8. L. A. Bui, A. Mitchell, W. G. Slade and T. H. Chio, "Wide-band Variable Transversal Phase-shifter," Microwave Symposium Digest, 2003 
IEEE MTT-S International, 2, 1291-1294 (2003). 
9. A. Arie and M. Tur, "The Effects of Polarization Control on the Transfer Function and the Phased Induced Intensity Noise of a Fiber -Optic 
Recirculating Delay Line," J. Lightwave Technol. 6(10), 1566-1574 (1988). 
10. E. Ackerman, H. Roussell and C. Cox, "Bias controller for external modulators in fiber -optic systems," Lightwave, 175-182 (2001). 
 
 
a) 
 
b) 
Fig. 2. Response of the transversal phase-shifter. a) Phase response b) Magnitude response. Measurements are 
indicated as points and predictions are as solid lines. Responses are normalised to the centre tap response. 
© 2004 OSA/OFC 2004


Demonstration of wide band RF photonic transversal phase-shifter

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