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Fiber chromatic dispersion measurement by using a novel RF spectrum phase detection method based on DSP

Bin Li a ,Fengguang Luo b ,?,Ming Tian b ,Lu Shi b

Wuhan National Laboratory for Optoeletronics,Wuhan 430074,China

b National Engineering Laboratory for Next Generation Internet Acess System,School of Optoeletronics Science and Engineering,Huazhong University of Science and technology,Wuhan 430074,China

a b s t r a c t

a r t i c l e i n f o Article history:

Received 24June 2011

Accepted 24February 2012

Available online 10March 2012

In this paper,a novel chromatic dispersion (CD)measuring method based on a novel RF spectrum phase detection technique is proposed,this is performed using in-band tone monitoring RF,electrically down-converted to direct current (DC)or a low intermediate-frequency (IF)of less than 1MHz through electronic mixing with local oscillator (LO).We theoretically and experimentally demonstrated the CD measurement for CD components (etc.?ber).The method can support any CD range measuring with good accuracy through adjusting RF frequency.

1.Introduction

Fiber dispersion originates from combined effects of ?ber material/waveguide dispersion and cause the linear optical pulse broadening effects through the wavelength-dependent group time delay.In the literature,many dispersion measurement methods have been demon-strated.They can be roughly classi ?ed into two main categories:noin-terferometric and interferometric.The time of ?ight technique and the modulation phase-shift (MPS)technique are the most well-known nointerferometric methods [1–4].Some recent schemes using super-contiuum pulse light sources for broadband measurement are also under this category [5–7].The interferometric category also includes many subcategories.Temporal interferomatry methods and various spectral interferometry methods with continuous-wave (CW)nar-rowband or broadband lights are the most well-known approaches [8–10].Some special techniques based on the cavity resonance effects like the self-seeding laser oscillation method [11–14]can also be listed under this category.In general the nointerferomatric methods will have the advantages of setup simplicity and measurement stability,while the interfermatric methods have the main advantages of higher sensitivity.

Among the above-mentioned approaches,the standard MPS tech-nique is probably the most commonly used method due to its sim-plicity.However,since it will typically require the precise phase measurement of the ratio-frequency (RF)modulation signals through an expensive RF network analyzer,the equipment cost is higher.In the present work,we propose and demonstrate a new method of ?ber

chromatic dispersion measurement based on a novel RF spectrum phase detection technique.In this method,does not require high-speed components,and using digital signal processing (DSP)to disposal measured results with the proposed arithmetic,the accuracy of mea-surement system is advanced while OSNR not add.2.Principle of CD measurement

The model of this technique is depicted in Fig.1.this technique is based on electrical mixing using an I-Q con ?guration.

The incoming signal E DSB (t )is a modulated dual sideband (DSB)signal,two single sideband (SSB)tunable band-pass optical ?lter is used to distill upper and lower sideband of the incoming signal,that the upper sideband signal and lower sideband signal are respectively given by:E L t eT?α

????I 0p cos ω0t eTtβ????

I 0p cos ω0?ωd eTt tφL eTe1TE U t eT?α

????I 0p cos ω0t eTtβ????

I 0p cos ω0tωd eTt tφU eT

e2T

After the photodetector and the electrical mixing,the in-phase and in-quadrature received signals I I ,Q for the LSB and USB are given by:I IU ?E U t eTj j 2

?H I t eTe3TI QU ?E U t eTj j 2?H Q t eTe4TI IL ?E L t eTj j 2?H I t eT

e5TI QL ?E L t eTj j 2

?H Q t eT

e6T

Optics Communications 285(2012)3249–3253

?Corresponding author.

E-mail address:fgluo@https://www.wendangku.net/doc/8a13286738.html, (F.

Luo).

10.1016/j.optcom.2012.02.085

Contents lists available at SciVerse ScienceDirect

Optics Communications

j o u r na l h o me p a g e :ww w.e l s e v i e r.c o m /l oc a te /op t c o m

where H I(t)and H Q(t)are the electrical mixer transfer functions for the in-phase and in-quadrature signals I I,Q,respectively given by:

H I teT?cosωT ttφck

eTe7T

H Q teT?cosωT ttφ

ck tπ2

e8T

whereωT is the RF modulation frequency of the LO signal and theφck is its relative phase to the data clock.

WhenωT=ωd,the detected signals I IU,I IL,I QL,I QU after LPF are determined from Eq(1.1)–(1.4)

I IL?αβI0

cosφ

tφck

eTe9T

I QL?

αβI0

cosφLtφckt

e10T

I IU?

αβI0

cosφ

tφck

eTe11T

I QU?

αβI0

cosφ

tφckt

e12T

The relative phase of the carrier and each sideband for the LSB and

USB(φL andφU)are given by:

φL?Arctg

I QL

?φcke13T

Electrical

mixer

Fig.1.principle of the CD measurement based on RF spectrum phase detection using electrical mixing.

Mixer Phase Shift

Fig.2.Experimental setup for chromatic dispersion measurement based on optic-electronic signal processing.

3250 B.Li et al./Optics Communications285(2012)3249–3253

USB-1

LSB-1

USB-1

USB-2

LSB-2

USB-1

USB-2

LSB-1LSB-2

LSB-1

USB-1a

Fig.3.the optical spectrum of USB and LSB after TOFs;(a):RF =2GHz,the lower TOF outputs;(b):RF =2GHz,the upper TOF outputs;(c)RF =5GHz,the lower TOF outputs;(d)RF =5GHz,the upper TOF outputs;Carrier:the continuous wave light spectrum;LSB-1:the ?rst order lower signal band;LSB-2:the second order lower signal band;USB-1:the ?rst order upper signal band;USB-2:the second order upper signal band;.

Fig.4.RF =2GHz,electrical signal spectrum of four LPF outputs;(a):the output of the I IU -LPF ;(b):the output of the I QU -LPF ;(c):the output of the I IL -LPF ;(d):the output of the I QL -LPF .

3251

B.Li et al./Optics Communications 285(2012)3249–3253

φU ?Arctg I QU

?φck

e14T

It is noted that the phase difference Δφis independent of LO signal's phase φck ,and Δφis given by:Δφ?φU ?φL ?Arctg I QU IU ?Arctg

I QL

e15T

The GVD is then given by:GVD ?

2πc Δφ

λ2ωT 2

e16T3.Experimental implementation

Experimental demonstration was performed using the setup of Fig.2.The transmitter comprises a tunable laser (TL)operating at 1549.715nm with 10MHz linewidth,The ?ber under test (FUT)comprises single mode ?ber (G.652)and dispersion compensation ?ber (DCF).An erbuim-doped ?ber ampli ?er (EDFA)was used to compensate the ?ber loss.At the receiver,both tunable optical ?lters (TOF)have same 3dB-bandwidth of 12.5GHz,in this experiment,the center frequency of upper TOF is set at 193.458THz,and the low-er's at 193.442THz,the optical spectrum of upper signal band (USB)and lower signal band (LSB)after TOFs is shown in Fig.3.The optical signal of upper and lower band were respectively detected by analog photonic-detector with 3dB bandwith of 8GHz,the output RF elec-trical signal was split into I and Q channels for mixing with local os-cillator (LO),and I channel LO signal has π2phase difference with Q channel LO signal.In this experiment we used tunable RF source that outputs 2GHz,3GHz and 5GHz RF signals respectively.Four low pass ?lters (LPF)have same 3dB-cutoff frequency of 1.92MHz,the outputs of four LPFs is shown in Fig.4.The analog-digital-converters (ADC)with 14bit-width and 50MHz sample rate were used to sampling the output analog intermediate frequency (IF)sig-nals and converting to digital signals,signals processing was per-formed in digital signal processor (DSP).

4.Experimental results

In this experimental,the OSNR was varied with a variable noise loading stage using an ASE source.The OSNR was maintained at a 20dB level for all CD measurements.The CD measurement was tested without introducing any DGD.In condition of RF 2GHz,the positive chromatic dispersion was added to the signal using four spools of SMF of 20,50,100,150km corresponding,respectively,to 335,837,1675and 2513ps/nm.The negative chromatic dispersion was added to the signal using four spools of DCF of 2.4,6,12,18km corre-sponding,respectively,to ?335,?840,?1680,?2513ps/nm.Fig.5shows the experimental results of the RF of 2GHz.

M e a s u r e d C D v a l u e p s /n m

Actual CD ps / nm a

Actual CD ps / nm

M e a s u r e d p h a s e Δ r a d

?Fig.5.(a)The measured value of Δφcorresponding to variable CD (RF=2GHz).(b)The measured CD corresponding to actual CD (RF=2GHz).

M e a s u r e d p h a s e Δ r a d

M e a s u r e d C D v a l u e p s /n m

Actual CD ps/nm

?Fig.6.(a)The measured value of Δφcorresponding to variable CD (RF=3GHz).(b)The measured CD corresponding to actual CD (RF=3GHz).

Actual CD ps /nm

-0.6

-0.4-0.200.20.40.6

OSNR=20dB RF=5GHz

-670

-335-167.5-6733.5134251.25586.25

M e a s u r e d p h a s e Δ r a d

M e a s u r e d C D v a l u e p s /n m

?Fig.7.(a)The measured value of Δφcorresponding to variable CD (RF=5GHz).(b)The measured CD corresponding to actual CD (RF=5GHz).

3252 B.Li et al./Optics Communications 285(2012)3249–3253

In condition of RF3GHz,the positive chromatic dispersion was added to the signal using three spools of SMF of20,40,50km corresponding,respectively,to335,670,838ps/nm.The negative chromatic dispersion was added to the signal using three spools of DCF of2.4,4.8,6km corresponding,respectively,to?335,?670,?838ps/nm.Fig.6shows the experimental results of the RF of 3GHz.

In condition of RF5GHz,the positive chromatic dispersion was added to the signal using twenty spools of SMF of2to40km with step of2km corresponding,respectively,to33.5to670ps/nm with step of33.5ps/nm.The negative chromatic dispersion was added to the signal using twenty spools of DCF of0.24to4.8km with step of 0.24km corresponding,respectively,to?33.5to?670ps/nm with step of?33.5ps/nm.Fig.5shows the experimental results of the RF of5GHz(Fig.7).

5.Conclusion

In this paper,we demonstrated a novel CD measuring method based on a novel RF spectrum phase detection technique,this mea-suring method proves to be?exible in CD range through adjusting RF https://www.wendangku.net/doc/8a13286738.html,pare to some methods used in Agilent8603A and 8603B,the method is very cost effective and measuring process is more fast,so that the results is affected less by environment(etc. PMD,temperature,humidity,or noise).The precision can be im-proved by DSP that operating amending arithmetic to heighten OSNR.In condition of2GHz RF,The resolution reaches5ps/nm,it is more sensitive with increasing RF value and corresponding to shrink-ing measurement range.This CD measuring method will form a pow-erful solution for satisfying the requirement of CD management in high-speed optical link building.

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