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Photonic Generation of Frequency-Coded MicrowaveSignals Using a Polarization Modulator and

Equivalent-Chirped Sampled Bragg GratingsPeng Xiang1,2, Xiaoping Zheng1, Hanyi Zhang1, Yuquan Li2, Rong Wang2

1 State Key Laboratory on Integrated Optics, Department of Electronic Engineering, Tsinghua University, Beijing 100084, China2 Institute of Communication Engineering, PLA University of Science and Technology, Nanjing 210007, China

sunshinetim@126.com.cn

Abstract-A novel approach to photonic generation offrequency-coded microwave signals based on optical pulseshaping using the cost efficient sampled Bragg grating techniqueis proposed. The generation of binary frequency-codedmicrowave pulse signals with center frequency of 23GHz isdemonstrated via simulation.

I. INTRODUCTIONPhotonic generation of microwave signals is of great

research interest in the last few decades for applications such

as radio-over-fiber networks, phased array antenna, wireless

communications, and modern radar systems. Various

techniques to generate microwave signals in the optical

domain have been proposed, which can take the advantage of

the high frequency and broad bandwidth offered by modern

optics [1]. Among the existing techniques, optical pulse

shaping followed by frequency-to-time mapping has beendemonstrated to be a promising technique to generate

microwave signals, in which a temporal microwave pulse with

a shape identical to the shaped optical pulse spectrum can be

generated [2]. In the optical pulse shaping system, a fiber-

optic device such as fiber Bragg gratings (FBGs) can be used

to shape the optical spectrum of a short optical pulse. Optical

pulse shaping using FBG filters offers the advantages such as

lower loss, better stability and higher potential for integration.

FBG filters can be fabricated using the phase mask scanning

method, where different chirped phase masks are usually

required to fabricate FBG filters with different chirp profile.

On the other hand, chirped FBG filter response can be

achieved equivalently within the -1st-order channel ofsampled Bragg gratings (SBGs) by using the equivalent-chip

technique, where only one uniform phase mask is required [3].

In this paper, a novel approach to generate microwave

frequency-coded signals based on optical pulse shaping is

proposed, where the FBG pulse shapers are designed based on

equivalent-chirp technique. In the fabrication of the designed

pulse shapers, only one uniform phase mask is required, which

leads to enhanced flexibility and reduced cost. Simulation

results show that frequency-chipped microwave signals with

center frequency of 23GHz can be generated in optical domain.

The frequency chirped microwave signals are then encoded by

a input binary data and can be decoded through pulse

compression, where the pulse compression ratio of 57 is

achieved.

II. OPERATION PRINCIPLEFig. 1 shows the schematic diagram of the proposed

system. In the proposed system, an optical pulse train from ashort pulse laser source (SPLS) is launched into a polarizationmodulator (PolM) followed by a polarization beam splitter(PBS). When a binary digital data stream is applied to thePolM, the optical pulse train is split into two branches at theoutputs of the PBS. In each branch, the optical pulses are sentto pulse shaper1 and pulse shaper2 respectively. Each pulseshaper consists of two superimposed equivalent chirpedsampled Bragg gratings (ECSBGs) with opposite chirp rates.The two pulse shapers are designed to have chirped free

spectral range (FSR) in their -1st-order channel with inverselychirped FSR. The optical pulses from the two branched arespectrally shaped by pulse shaper1 and pulse shaper2respectively in their -1st-order channel, and then combinedand sent to a dispersive element (DE) to perform frequency-to-time mapping. Therefore microwave frequency chirpedsignals with different chirp rates are generated afterphotodetector (PD) detection. The generated microwavesignals are frequency-coded since the chirp rates of thegenerated microwave signals are encoded by the binary datastream applied to the PolM.

The key devices in the proposed microwave signal

generation system are the pulse shapers, which consists of two

superimposed ECSBGs as shown in Fig. 2(a). The ECSBG isa specially designed SBG with linear chirped sampling

periodPas shown in Fig. 2(b), which can be equivalent to an

SBG with chirp in the grating period [4]. By superimposing

two ECSBGs of opposite chirped rate into the fiber with a

longitudinal offset d , a pulse shaper with a chirped FSR can

be obtained in its -1st-order channel. A tunable optical filter

(TOF) is used after the pulse shaper to select the required -1st-

order channel. These pulse shapers are Fabry-Perot-like filters

with equivalent cavity length ( )l varies with respects to thewavelength, which shares functional similarity with the

chirped FSR filter in [5], however since the equivalent chirp

978-1-4673-0677-5/12/$31.00 2012 IEEE

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technique is used here, only a single uniform phase mask is

required in the fabrication of the pulse shapers.

Two pulse shapers in the proposed system shown in Fig. 1.

are designed by superimposing two ECSBGs of an identical

grating length ofL = 2cm with a longitudinal offset of d=

2.4cm. And the first sampling period 0P and the chirp

constant as described in [4] are taken the same value as

200m and 0.013 for the two pulse shapers uniformly, except

the superimposing sequence of the ECSBGs in pulse shaper1

and pulse shaper2 is exchanged to achieve an opposite FSR

chirp rate. Fig. 3. shows the -1st-order channel reflection

spectrum response of the two pulse shapers, which are

computed using the standard transfer-matrix method [6].

The PolM is another key component in the proposed system.

When a linearly polarized incident light is aligned with an

angle of 45 to one principal axis of the PolM, the

polarization state of the output lightwave will change between

two orthogonal linear polarization driven by the applied

binary digital data. A PBS is cascaded to the output of the

PolM with one of its principal axes aligned with an angel of

45 to that of the PolM. Therefore, when a linearly polarizedoptical pulse train is launched into the PolM with an angle of

45 to the principal axis of the PolM, the optical pulse train

will be split into two branches and switched to two outputs of

the PBS under the control of the binary data signal, thanks to

the polarization modulation at the PolM.

III. SIMULATION MODEL AND RESULTSA simulation model was built using the commercial

software package Virtual Photonic Incorporation (VPI)

Transmission Maker as shown in Fig. 4. The SPLS generates

Gaussian pulse train with a pulsewidth of 550fs and a

repetition rate of 125MHz, and the center wavelength is set to

1551nm to match the center wavelength of the FBG pulse

shapers. A 60km single mode fiber (SMF) is used as DE, and

the bandwidth of the PD and TOF are set to 70GHz and

Pulse shaper2

Pulse shaper1

Generated

signal

Digital data

Optical Pulse train

PDTOFPBSPolMSPLS DEOC

00 1 10

PC1 PC2

1

Figure 1. Schematic diagram of the proposed frequency-coded microwave signal generation system. (SPLS: short pulselaser source; PolM: polarization modulator; PBS: polarization beam splitter; PC: polarization controller; OC: optical

coupler; TOF: tunable optical filter; DE: dispersive element; PD: photodetector.)

(a)

(b)

Figure 2. Schematic diagram of (a) the superimposed

ECSBGs pulse shaper and (b) the ECSBG

Figure 3. The designed -1st-order channel reflection

spectrum of (a) pulse shpaper1 and (b) pulseshaper2.

(b)

Re

flec

tion

(dB)

Wavelength (nm)

0

-5

-10

-15

-20

-251550.8 1551.0 1551.2 1551.4 1551.6

Wavelength (nm)

Re

flec

tion

(dB)

0

-5

-10

-15

-20

-251550.8

1551.0 1551.21551.4 1551.6

(a)

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200GHz respectively. The bit rate of the driving digital data

applied to the PolM is set to 125Mb/s, which is synchronized

with the optical pulse train from SPLS. And the bit rate can be

easily adjusted by changing the repetition rate of the SPLS.

The generated microwave signals through pulse shaper1

and pulse shaper2 are shown respectively in Fig. 5(a) and (b),and their instantaneous frequencies within their main

pulsewidth are also plotted.

As seen from Fig. 5(a), the generated microwave signal is

frequency-chirped within the main pulsewidth with a

nonlinear chirp rate, which stems from the nonlinear chirp

rate of the ECSBG in its -1st-order channel. The

instantaneous microwave carrier frequency increases across

the pulse with time, and the central frequency is estimated to

be 23GHz. The full-width at half-maximum (FWHM) of the

signal envelope is 860ps. As seen from Fig. 5(b), the

microwave signal generated through pulse shaper2 is also

frequency-chirped within the main pulsewidth, yet theinstantaneous microwave carrier frequency decreases across

the pulse with time, and the central frequency is estimated to

be 23GHz. The FWHM of the signal envelope is 810ps.

The generated frequency-chirped microwave signals are

encoded by the digital data applied to the system at the PolM.

In order to recover the digital data, the chirped microwave

signals need to be compressed through matched filtering,

Time (ps)

Norma

lize

dAmp

litude

Instan

taneous

frequency

(GHz

)

(a)

1.0

0.8

0.6

0.4

0.2

0.0

-600 -400 0 600400200

40

30

20

10

-200

Time (ps)

Norma

lize

dAmp

litude

Instan

taneous

frequency

(GHz

)

(b)

1.0

0.8

0.6

0.4

0.2

0.0-600 -400 0 600400200

40

30

20

10

-200

Figure 5. Simulation results. Time-domain waveformand instantaneous frequency of the generatedmicrowave signal: (a)through pulse shaper1. (b)through pulse shaper2.

Norma

lize

dAmp

litude

(Au

tocorrec

ting

)

Norma

lize

dAmp

litude

(Cross-corre

lation

)

(a)

Time (ps)

1.0

0.8

0.6

0.4

0.2

0.0-1200 -800 -400 0 800400

1.0

0.8

0.6

0.4

0.2

0.0

Time (ps)(b)

Norma

lize

dAmp

litude

(Cross-corre

lation

)

Norma

lize

dAmp

litude

(Au

tocorrec

ting

)

1.0

0.8

0.6

0.4

0.2

0.0

-1200 -800 -400 0 1200800400

1.0

0.8

0.6

0.4

0.2

0.0

Figure 6. Autocorrelation waveforms of the generated signal(a) through pulse shaper1 and (b) through pulse shaper2, withthe cross-correlation waveform of these two generated signalshown in both Fig. 6 (a) and (b).

SPLS EDFA

PC1 PolM PC2 PBSPulse shaper1

Pulse shaper2

OCTOF

SMF

PD

Figure 4. Simulation model of the proposed system.

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which is mathematically equivalent to autocorrelation. Fig.

6(a) and (b) shows the autocorrelation waveforms of the

generated microwave signals in Fig. 5(a) and (b) respectively.

And their cross-correlation waveform is also plotted in Fig. 6.

As can be seen from the results, both of the autocorrelation

waveforms have a peak, which are obviously higher than that

of their cross-correlation waveform. Therefore, after matched

filtering, the digital data can be recovered by a data recover

module with an optimum decision threshold and clock

recovery timing. The FWHM of the autocorrelation

waveforms shown in Fig. 6(a) and (b) are 15ps and 14ps

respectively, so that the pulse compression ratios of the two

microwave signal are estimated to be 57.3 and 57.9

respectively.

It is found in the simulation that the pulse compression ratio

of the generated microwave signals are mainly decided by the

effective bandwidth of the pulse shaper, which is limited by

the bandwidth of the -1st-order channel of the ECSBGs. Inorder to improve the signal compression ratio, the sampling

period of ECSBG needs to be reduced by minimize the UV

beam size in the FBG fabrication platform. On the other hand,

the pulse compression ratio is also affected by the spectral

bandwidth of the Gaussian pulse train from the SPLS. In

order to achieve large signal compression ratio, the 3-dB

spectral bandwidth of the ultra-short pulse laser source should

be large enough to cover the -1st-order channel of the FBG

pulse shapers. Additionally, the offset dbetween the two

superimposed ECSBG of the pulse shapers also affects the

pulse compression ratio of the generated signal, but the

influence is insignificant, and the relationship between d and

signal compression ratio is not directly proportional. From thesimulation results, it is found that when Ld 2.1< thecompression ratio increases slowly with the increase ofd .

However when Ld 2.1> , the compression ratio begins todecrease. Therefore an appropriate offset d is required to

maximize the compression ratio. The simulation results also

show that the center frequency of the generated microwave

signals increases with the increasing ofd , which can be

explained by the same principle used in [5]. Therefore

largerd is favored for the generation of microwave signals

with high center frequency.

The bit rate used in the simulation was 125Mb/s, but it can

be adjusted by changing the reception rate of the SPLS. The

upper limit of the bit rate is decided by the temporal durationof the generated signal pulse. To avoid the waveform

interference, the bit period, i.e. the reciprocal of the bit rate,

should be larger than the time-domain duration of the

generated signal pulse. The temporal duration of the

generated microwave signals can be reduce by using DE with

smaller dispersion, but the amount of dispersion have to be

large enough to meet the requirement of frequency-to-time

mapping as described in [1].

IV. CONCLUSIONA novel approach to photonically generate frequency-coded

microwave signals based on optical pulse shaping and

frequency-to-time mapping is proposed and demonstrated via

simulation. The key advantage of the proposed approach isthat the equivalent-chip method are used to design the FBG

pulse shapers, which leads to reduced cost and enhanced

flexibility. The proposed approach is simple and compact,

which can find applications in wireless communications and

modern radar systems.

ACKNOWLEDGMENT

This work is supported by National Nature Science

Foundation of China (NSFC) under grant No. 61032005.

REFERENCES

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