Hybrid Optical Beam-Former in Receiver Mode

doi:10.4236/opj.2011.13022

Paper Menu >>

Journal Menu >>

Optics and Photonics Journal, 2011, 1, 130-136

doi:10.4236/opj.2011.13022 Published Online September 2011 (http://www.SciRP.org/journal/opj)

Hybrid Optical Beam-Former in Receiver Mode

Pham Q. Thai, Arokiaswami Alphones

Nanyang Technological University, Singapore City, Singapore

E-mail: [email protected]

Received July 6, 2011; revised August 10, 2011; accepted August 22, 2011

Abstract

In this paper, an optical beam-former in receiving mode has been proposed and experimentally demonstrated.

The requirement in system’s hardware has been dramatically reduced using a hybrid approach between dis-

persive and non-dispersive delay. The proposed system is capable of supporting RF signals from L-band to

X-band, with large coverage and strong robustness against the grating’s group delay ripples.

Keywords: Chirped Fiber Grating, Group Delay Ripple, Microwave Photonics, Optical Phased Array

Antennas

1. Introduction

Despite having many advantages such as better directivity,

higher gain, and beam-steering, traditional electrical con-

trolled phase array antennas face serious problems

working with large bandwidth RF signals at high fre-

quencies. One of the most difficult obstacles to overcome

is the “beam-squint” phenomenon. The beam direction of

the array changes across the RF signal’s bandwidth. For

applications with large bandwidth and high frequency

requirements, optical beam-forming is a promising can-

didate. Optical beam-formers inherit advantages charac-

teristics such as large bandwidth, compactness, light-

weight, and immunity to electromagnetic interference.

More importantly, they can operate squint-free with wide-

band, high frequency signals, thus avoiding the most

significant drawback of the traditional electrical beam-

formers.

Optical beamformers using chirped grating and optical

delay lines are among the most prominent approaches for

photonic beamforming [1]. However, in the former ap-

proach, the supported array size is limited since each

element requires one tunable laser [2]. In the later ap-

proach, the length of the delay device results in many

obstacles [3]. In order to combine the advantages of both

approaches, optical beamforming systems using a com-

bination of dispersive and non-dispersive delays have

been proposed [4-7]. In [8], we have shown a hybrid

approach between dispersive and non-dispersive delay for

optical beam-former in transmitting mode. The novel

system employing that method has dramatically reduced

hardware requirements.

In this paper, the receiving schematic following our

hybrid approach is presented. In our proposed receiver,

the number of tunable laser is reduced by more than three

times. The number of tunable optical delay line is reduced

by two times. The required time delay for both devices is

also greatly reduced. Experimental and simulation results

have shown that the proposed beam-former can support

signals with wide RF bandwidth from L-band to X-band.

There were only slight distortions in the simulated radia-

tion patterns in comparison with the ideal radiation pat-

terns.

In [9], preliminary simulations and measurements of

the proposed system were reported. In this article, many

more details about the simulation radiation patterns and

measurement results have been disclosed and discussed,

especially about the distortions in radiation patterns. The

system in this article has been improved from the previous

system discussed in [10]. In [10], the combination of

optical signals required intensive fine-tuning to ensure no

interference between signals modulated at the same op-

tical wavelength. In this paper, the problem has been

overcome using polarization beam combiners.

Since the proposed system use chirped grating, the ef-

fect of group delay ripples (GDR) has also been addressed.

As studied in [11,12], the GDR has a strong negative

impact on the performance of optical beam-former using

chirped grating in transmission operation. The study in

[12] also suggested that grating may not be suitable for

receiving operation. However, simulations have shown

that our proposed hybrid system is more robust against

GDR.

The paper is organized as described. The proposed

P. Q. THAI ET AL.131

beam-former is presented in Section 2. Experimental and

simulation results of the proposed system are shown and

discussed in Section 3. Finally, conclusions are drawn in

Section 4.

2. Principles

In order to form the beam toward a particular direction θ

in transmitting operation, an amount of time delay t is

required between two consecutive elements of a phased

array antenna:





sintd c



 (1)

where d is the spacing between elements and c is the speed

of light. Because of the additive or destructive combina-

tion of the transmitted signals from the elements, the main

beam in far field would point at the desired direction.

In receiving operations, the above concept is also ap-

plied. It is assumed that the receiving signal approaches at

the angle θ. The delay between the received signals of two

consecutive elements is t. By properly delaying the re-

ceived signal at each element, the beam-former can pro-

vide the strongest accumulated output signal. In a sense,

delaying the received signal can be considered as pointing

the main beam toward the desired direction.

The proposed beam-former in receiving mode is shown

in Figure 1. The signal’s delay at each element is also

noted in that figure. The purpose of the beam-former is to

correctly delay the signals, thus obtaining the strongest

output signal after the photodiode.

The working principle has been discussed in [9]. How-

ever, for better clarity, a brief explanation is provided here.

Firstly, the received signal from each element after the

low noise amplifier is modulated with an optical signal

through an electro-optic modulator. The first and third

laser sources are tunable lasers, while the second and

fourth laser sources are fixed wavelength lasers. The

modulated optical signal for each modulator is noted in

Figure 1.

Couplers are used to combine the modulated signals

into several branches. Optical delay lines are used to delay

the signal as shown in Figure 1. All optical delay lines are

non-dispersive devices. The delay signals from the bran-

ches are then combined and passed through the multi-

Figure 1. Hybrid optical beamformer in receiving mode.

P. Q. THAI ET AL.

132

channel chirped fiber grating (MCFG). The reflectivity

and group delay characteristic of the MCFG can be re-

ferred to in [11]. In a sense, the MCFG can be considered

as a cascade of several nearly identical chirped gratings.

Each optical wavelength falls into one channel of the

MCFG. By tuning the wavelength, λ1 is delayed by 4t in

comparison with λ2, and λ3 is delayed by 2t in comparison

with λ4.

In order to avoid interferences, the optical signals with

the same wavelength have to be of different polarizations.

At the inputs of the polarizing beam splitter (PBS), the

two input signals at the same wavelength are turned or-

thogonal via polarization controllers. The output signal

after the PBS has two polarizations and can still be proc-

essed with the photo detector in the later stage.

As a result, all the input signals have same amount of 7t

delay in time. The accumulated output signal after the

photodiode should be the strongest possible signal. In

other words, by tuning the optical delay lines and the

tunable lasers, the receiver has turned its main beam to-

ward the signal’s incoming direction.

Only four channels of the MCFG are used in this

schematic. Since the MCFG has around 50 channels, it is

possible to increase the supported array size by adding

similar delay branches before the MCFG. In Table 1, the

comparison between hardware’s requirements of the hy-

brid system versus the typical systems is shown.

For optical beamformers using chirped grating, each

element in the array requires one expensive tunable laser,

which makes the system impractical. Optical beamformer

using only optical delay lines is more cost-effective.

However, the long delay requirement causes many dis-

advantages [13].

Our proposed approach, which utilizes a combination

of both chirped grating and optical delay lines, has several

advantages. The hybrid approach has been able to dra-

matically reduce the number of required tunable lasers

and optical delay lines, as seen in Tabl e 1. Moreover, the

required delay for both the grating and delay line is

greatly reduced. The proposed hybrid approach has been

able to subdue the most prominent disadvantages of op-

tical beamformers using chirped grating and optical delay

line.

On the other hand, the scalability of our proposed sys-

tem is limited. In receiving mode, combining modulated

optical signals at the same wavelength may cause inter-

ferences. Polarization controller have been used to ensure

that optical signal at the same wavelength have different

polarization. As a result, each optical carrier signal can

only be used two times. Therefore, although applying the

same concept as in [8], the number of laser sources has to

be increased in receiving operation.

3. Experimental and Simulation Results

In order to test the hybrid system, experiments have been

conducted. Since there were not enough devices, impro-

vising was made. Combining optical signal of different

wavelengths does not distort the signal [14]. With proper

polarizing, combining optical signal of the same wave-

lengths also does not cause distortion [15]. The MCFG

helps prevent beatings [2]. On the other hand, the stability,

precision, linearity, and the supported amount of time

delay are among the most important parameters of a

beamformer. Therefore, the set-up in Figure 2 was used

to measure the delay capability in time domain of the

proposed hybrid optical beamformer scheme. Each

branch of the schematic in Figure 1 was measured at a

time. For the branch with optical delay line, an optical

delay was inserted between the modulator and the MCFG.

In the experiment, the wavelengths for four sources

were 1547.503, 1548.312, 1549.112 and 1549.917 nm,

respectively. The tuning step was 0.001 nm. The optical

signal occupied four separated channels of the MCFG.

The grating was a MCFG from Teraxion, modeled as the

Clear Spectrum DCX D061983.

The photodiode was use to convert the signal back into

electrical domain. The output RF signals were then

compared with the reference input RF signals. The tuning

ranges of the tunable sources were from 1547.373 to

1547.643 nm for the first wavelength and from 1548.982

to 1549.252 nm for the third wavelength. The optical

delay lines were tuned accordingly to the absolute delay

between the wavelengths as explained in the previous

section. The resulting delays between the RF output sig-

nals after the photodiodes at the outputs are shown in

Table 1. Comparison between systems’ requirements in receiving mode.

Number of optical

delay lines

Maximum time delay

from delay line

Number of

tunable lasers

Maximum time delay

from chirped grating

Optical beamformer using chirped grating

(4/8/16 elements) 0/0/0 0/0/0 3/7/15 3t/7t/15t

Optical beamformer using optical delay line

(4/8/16 elements) 3/7/15 3t/7t/15t 0/0/0 0/0/0

Optical beamformer using the hybrid method

(4/8/16 elements) 1/3/7 t/2t/3t 1/2/4 2t/4t/8t

P. Q. THAI ET AL.

133

Figure 2. Experiment setup.

Figures 3 and 4.

As seen in those figures, the time delays are mostly

linear. The results indicate that the tuning devices in the

system, such as the laser sources and the optical lines, can

be programmed to linearly and continuously tune.

The amount of delay stayed mostly unchanged at dif-

ferent RF signal frequencies, as shown in Fi gures 5 and 6.

Considering the optical system as a black box, those re-

sults indicated that the system is able to support signals

for L, S, C, and X-band arrays. The measured group de-

lays of the two set of elements are also nearly the same

since the MCFG channels have similar characteristics.

The results have suggested that the hybrid optical beam-

former system is flexible.

The measured data was imported into Matlab to simu-

late the radiation pattern. The simulated arrays were 8-

element arrays with half-wavelength spacing at 8 GHz

and 2 GHz. Using the measured data, the first array was

able to steer from broadside to 58˚, while the second array

was able to steer from broadside to 17˚. For the 8-element

array, the MCFG is only needed to provide 4t in time

delay. A typical schematic using chirped grating requires

the grating to provide 7t. As a result, given the same

amount of delay from the MCFG, a typical beam-former

using chirped grating can only steer the beam toward 29˚

for 8 GHz RF signals and toward 10˚ for 2 GHz RF signal,

respectively.

In Fi gures 7 and 8, the simulated radiation pattern were

compared with the ideal radiation patterns, where the

there is no GDR and no amplitude fluctuations. The effect

of mutual coupling was calculated in all cases. The time

delay could be fine-tuned to correct the beam direction.

There was less than 0.2˚ and less than 0.5˚ in main beam

direction error for the 2 GHz and 8 GHz arrays, respec-

tively. Both the beam width and the sidelobe level are

affected by the signal’s amplitude. In the system, each

signal passing through one optical delay line suffered 1

dB loss. As a result, the main beam gain was decreased by

1.89 dB, and the sidelobe level was increased by 0.3 dB.

In the worst case with additional amplitude fluctuations

1547.5 1547.521547.54 1547.56 1547.581547.61547.62 1547.641547.66

100

150

200

250

Wavelength (nm)

Delay (ps)

2nd element

3rd element

4th el em ent

Figure 3. Time delays at the elements in the upper branch.

1548.9515491549.051549.11549.151549.2 1549.251549.3

100

150

200

250

W a velengt h (nm )

Delay (ps)

2nd el em ent

3rd element

4t h elem ent

Figure 4. Time delays at the elements in the lower branch.

2345678

-40

-30

-20

-10

RF s i gn al frequency (G Hz )

Delay (ps)

1547. 47 3 nm

1547. 49 3 nm

1547. 51 3 nm

1547. 53 3 nm

Figure 5. Time delay at different frequencies while tuning

the first tunable laser.

from all devices, the main beam gain was decreased by

2.57 dB and the sidelobe level was increased by 0.6 dB.

The simulated pattern’s 3dB beam width is increased by

P. Q. THAI ET AL.

134

2345678

-40

-30

-20

-10

RF si gnal frequency (GHz )

Delay (ps)

1549.082 nm

1549.103 nm

1549.122 nm

1549.142 nm

Figure 6. Time delay at different frequencies while tuning

the second tunable laser.

-80 -60 -40 -20020 40 60 80

-35

-30

-25

-20

-15

-10

-5

St eering angle ( D egree)

Gain pattern (dB)

Ideal case

Best case

Worst case

Figure 7. Comparison between the simulated and the ideal

normalized radiation patterns for 8 GHz signals at 55˚.

-80 -60 -40 -20020 40 60 80

-35

-30

-25

-20

-15

-10

-5

St e ering a ngle (Degr ee)

Gain pattern (dB)

Ideal c ase

Best case

Worst case

Figure 8. Comparison between the simulated and the ideal

normalized radiation patterns for 2 GHz signals at 17˚.

about 0.17˚.

As analyzed in [11,12], the fluctuations in group delay

characteristic of the grating force many limitations on the

optical beamformer using chirped grating. Since the

proposed beamformer uses a chirped grating, its robust-

ness against the GDR error should be tested. The main

beam direction errors of the proposed system were com-

pared with which of a typical optical beamformer using

chirped grating.

In Figure 9, the errors in the hybrid system stay mostly

the same for different array sizes. In comparison, the

errors of the typical system quickly increase. The reason

for that relates to the way the chirped grating is used. In a

typical beamformer, each element is directly controlled

via the delay of the grating, while the hybrid method only

uses two channels of the grating to control the array. The

less dependence on dispersive delay also reduces the

negative effect of the group delay ripples.

In receiving, the quality and the signal’s impulse re-

sponse also have a higher priority. Previous report sug-

gested that optical beamformers using chirped grating do

not perform well in reception [12]. However, the system

in [12] did not use the hybrid approach. The proposed

system, on the other hand, is robust against GDR.

Using the same condition as in [12], linear frequency

modulated (LFM) signals are considered. The details

about LFM can be found in [16]. In short, this particular

type of signal is highly recommended for its resolution

and detection range. The radar systems using those signals

place a strong emphasis on the quality of received signal’s

impulse response [17,18]. The change in the peak’s posi-

tion of the impulse response may cause false distance’s

estimation, while the rise in sidelobe level may conceal

targets.

The fluctuation in group delay causes symmetrical re-

23 4 56 7 8

x 10

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

RF ( G Hz)

Mean v al ue of error (degree)

4 elem e nt s

8 elem e nt s

16 elements

4 elem e nt s (hy b ri d)

8 elem e nt s (hy b ri d)

16 elements (hybrid)

Figure 9. Main beam direction error at different RF signal

frequencies and different array sizes.

P. Q. THAI ET AL.135

sponses of the received signal around the non-error signal.

This phenomenon, called “paired echo”, is caused by

dispersion error as studied in [18]. Depending on the

disturbance’s period, the 3 dB width of the main lobe may

increase or new sidelobes may arise in the impulse re-

sponse.

Preliminary study was done in [9]. Further analysis has

shown that the negative effect of GDR is nearly non-

existent. In the hybrid system, only two wavelengths were

tuned across a small region in the grating. Furthermore,

only a part of the total delay was from the grating. As a

result, the influence from GDR was minimized. Although

the echo was presented at several different positions in

time domain, the displacement of them was greatly re-

duced to a few picoseconds, while the LFM pulse duration

is usually in microseconds. Since the scaling is so dis-

tinctive, the negative effects such as distortions from

additive and subtractive combinations are negligible. The

study in [12] also agreed that small ripples do not reduce

the system performance. However, the system in [12]

could not achieve this goal since every element was under

the influence from GDR.

For example, a simple case of LFM has been consid-

ered. In Figure 10, the impulse response of a LFM with

bandwidth of 1 GHz and pulse duration of 10 μs with and

without GDR are shown. The centre frequency was cho-

sen as 7.5 GHz. As seen in the figure, the two cases were

almost undistinguishable. All the important factors such

as the 3 dB width of the main lobe, the main lobe position,

the number of sidelobes, and the sidelobe level are mostly

unchanged. The change in main lobe level across the

signal bandwidth was only around 0.015 dB.

4. Conclusions

In this paper, an optical beam-former in receiving mode

Figure 10. Impulse response of the LFM signal under the

influence of GDR.

using a hybrid approach between dispersive and non-

dispersive delay has been introduced. The support RF

signal covers a huge bandwidth from L-band to X-band.

In addition, the proposed beam-former has provided such

promising capabilities using incredibly less requirement

in optical hardware and time delay. Analysis about the

effect of GDR has also indicated that the proposed sys-

tem is more robust against GDR.

5. References

[1] N. A. Riza, Selected Papers on Photonic Control Systems

for Phased Array Antennas, 694 SPIE Optical Engineer-

ing Press, 1997.

[2] D. B. Hunter, M. E. Parker and J. L. Dexter, “Demonstra-

tion of a Continuously Variable True-Time Delay Beam-

former Using a Multichannel Chirped Fiber Grating,”

IEEE Transactions on Microwave Theory and Tech-

niques, Vol. 54, No. 2, 2006, pp. 861-867.

doi:10.1109/TMTT.2005.863056

[3] M. Y. Frankel, P. J. Matthews and R. D. Esman, “Two-

Dimensional Fiber-Optic Control of a True Time-Steered

Array Transmitter,” IEEE Transactions on Microwave

Theory and Techniques, Vol. 44, No. 12, 1996, pp. 2696-

2702. doi:10.1109/22.554632

[4] N. A. Riza and N. Madamopoulos, “Photonic Time Delay

Beamforming Architectures Using Polarization Switching

Arrays,” In: D. R. Pape, Ed., Advances in Optical Infor-

mation Processing VII, Proceedings of the Society of

Photo-Optical Instrumentation Engineers (Spie), SPIE—

International Society Optical Engineering, Bellingham,

1996, pp. 186-197.

[5] N. A. Riza and N. Madamopoulos, “Phased-Array An-

tenna, Maximum-Compression, Reversible Photonic

Beam Former with Ternary Designs and Multiple Wave-

lengths,” Applied Optics, Vol. 36, No. 5, 1997, pp. 983-

996. doi:10.1364/AO.36.000983

[6] N. A. Riza, M. A. Arain and S. A. Khan, “Hybrid Ana-

log-Digital Variable Fiber-Optic Delay Line,” Journal of

Lightwave Technology, Vol. 22, No. 2, 2004, pp. 619-

624. doi:10.1109/JLT.2004.824383

[7] B. M. Jung and H. P. Yao, “A Two-Dimensional Optical

True Time-Delay Beamformer Consisting of a Fiber

Bragg Grating Prism and Switch-Based Fiber-Optic De-

lay Lines,” IEEE Photonics Technology Letters, Vol. 21,

No. 9-12, 2009, pp. 627-629.

doi:10.1109/LPT.2009.2015275

[8] P. Q. Thai, A. Alphones and D. R. Lim, “A Novel Sim-

plified Dual Beam-Former Using Multichannel Chirped

Fiber Grating and Tunable Optical Delay Lines,” Journal

of Lightwave Technology, Vol. 26, No. 13-16, 2008, pp.

2629-2634. doi:10.1109/JLT.2008.927152

[9] P. Q. Thai and A. Alphones, “Hybrid Optical Beamform-

ers and Their Robustness Against Group Delay Ripples

Errors,” IEEE Topical Meeting on Microwave Photonics

(MWP), Montreal, 5-9 October 2010, pp. 276-279.

[10] A. Alphones and T. P. Quang, “Hybrid Approach for

P. Q. THAI ET AL.

136

Optical Beamforming for Phased Array,” Microwave

Conference Proceedings (APMC), Asia-Pacific 2010,

Singapore City, 7-10 December 2010, pp. 311-317.

[11] P. Q. Thai, A. Alphones and D. R. Lim, “Limitations by

Group Delay Ripple on Optical Beam-Forming With

Chirped Fiber Grating,” Journal of Lightwave Technol-

ogy, Vol. 27, No. 24, 2009, pp. 5619-5625.

doi:10.1109/JLT.2009.2032786

[12] R. Rotman, O. Raz and M. Tur, “Requirements for True

Time Delay Imaging Systems with Photonic Compo-

nents,” IEEE International Symposium on Phased Array

Systems and Technology 2003, New York, 14-17 October

2003, pp. 193-198.

[13] R. Soref, “Optical Dispersion Technique for Time-Delay

Beam Steering,” Applied Optics, Vol. 31, No. 35, 1992,

pp. 7395-7397. doi:10.1364/AO.31.007395

[14] J. Capmany, B. Ortega and D. Pastor, “A Tutorial on

Microwave Photonic Filters,” Journal of Lightwave Tech-

nology, Vol. 24, No. 1, 2006, pp. 201-229.

doi:10.1109/JLT.2005.860478

[15] J. Q. Zhou, S. Aditya, P. Shum, L. Xia and B. P. Parhusip,

“Wide-Range Continuously Tunable Microwave Pho-

Tonic Filter Using High-Birefringence Linearly Chirped

Fiber Bragg Grating and Polarization Beamsplitters,” Op-

tical Engineering, Vol. 48, No. 1, 2009, Article ID:

010502.

[16] M. Skolnik, “Radar Handbook,” 3rd Edition, McGraw-

Hill Professional, Boston, 2008.

[17] A. Kawalec, C. Lesnik, W. Komomiczak, W. Czamecki

and J. Pietrasiniski, “Influence of the Displacement Effect

on Compressed LFM Signal Parameters,” In: C. A. Breb-

bia and G. M. Carlomagno, Eds., Computational Methods

and Experimental Measurements XIII, Wit Transactions

on Modelling and Simulation, 2007, pp. 769-778.

[18] S. Darlington, W. J. Albersheim and A. C. Price, “The

Theory and Design of Chirp Radars,” The Bell System

Technical Journal, Vol. 39, No. 4, 1960, pp. 745-802.