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Optics Communications 266 (2006) 240–248

Tunable microwave photonic notch filter using a dual-wavelengthfiber laser with phase modulation

D. Liu a,*, N.Q. Ngo a, G. Ning b, P. Shum b, S.C. Tjin a

a Photonics Research Centre, School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue,

Singapore 639798b Network Technology Research Centre, School of Electrical and Electronic Engineering, 50 Nanyang Drive, Research TechnoPlaza, 4th Storey,

XFrontiers Block, Singapore 637553

Received 18 June 2005; received in revised form 16 March 2006; accepted 16 March 2006

Abstract

A novel tunable microwave photonic notch filter using a phase-modulated dual-wavelength fiber laser is presented. A stabledual-wavelength erbium-doped fiber laser with a linear cavity is formed by a polarization-maintaining uniform fiber Bragg grating(PM-FBG) and a polarization maintaining linearly chirped fiber Bragg grating (PM-LCFBG), both of which were fabricated on ahigh-birefringence (Hi-Bi) fiber. It is found that a stable room-temperature dual-wavelength operation can be achieved due to the pres-ence of two reflection peaks arising from the orthogonal states of polarization (SOP) of the PM-FBG. Experimental results show stabledual-wavelength lasing operation with a wavelength separation of �0.36 nm and a large optical signal-to-noise ratio (OSNR) of over40 dB under room temperature. The dual-wavelength fiber laser is combined with a phase modulator and a segment of single-mode fiber(SMF) as a dispersive device to form a tunable microwave photonic notch filter. By stretching the PM-FBG to tune the wavelengthseparation of the dual-wavelength fiber laser, a tunable microwave photonic notch filter with various free spectral ranges (FSRs) anda rejection ratio greater than 35 dB was developed.� 2006 Elsevier B.V. All rights reserved.

PACS: 42.55.+W; 42.55.Wd

Keywords: Dual-wavelength; Erbium-doped fiber laser; Fiber Bragg grating; High birefringence fiber; Notch filter; Microwave photonics

1. Introduction

All-optical processing of microwave and millimeter-wavesignals provides such advantages as large time-bandwidthproducts, insensitivity to electromagnetic interference, andlight weight. Therefore, a number of microwave photonicfilters have been reported in the literature [1–4]. By incorpo-rating a fiber Bragg grating (FBG) into the filter, e.g., [1,2],the filter response can be tuned. A laser array with an FBGwas used in Ref. [1] to realize a tunable notch filter, and a

0030-4018/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.optcom.2006.03.026

* Corresponding author. Tel.: +65 6790 5363; fax: +65 6792 6894.E-mail address: liuduan@pmail.ntu.edu.sg (D. Liu).

fiber Mach–Zehnder section with a linearly chirped fiberBragg grating (LCFBG) was reported in Ref. [2] to achievea microwave filter with tunable notch frequency. However,most reported approaches are based on laser arrays to over-come the coherent operation of the filter [3,4], and are there-fore costly.

We propose a novel continuously tunable microwavephotonic notch filter by exploiting a dual-wavelength fiberlaser with tunable wavelength spacing. The dual-wavelength fiber laser has a linear laser cavity, whichconsists of an 8-m long polarization-maintaining erbium-doped fiber (PM-EDF), a uniform FBG with a sectionfor strain modulation and a LCFBG, both of which werefabricated on the Hi-Bi fiber. The dual-wavelength is

D. Liu et al. / Optics Communications 266 (2006) 240–248 241

combined with a phase modulator followed by a segmentof SMF as a dispersive device to construct the microwavephotonic notch filter. By stretching the PM-FBG to tunethe wavelength separation of the dual-wavelength fiberlaser, a tunable microwave photonic notch filter with vari-ous FSRs and a rejection ratio greater than 35 dB wasdeveloped.

2. Proposed system

The experimental setup of the proposed tunable micro-wave photonic notch filter based on a tunable dual-wave-length fiber laser is depicted in Fig. 1. Pump light from a980 nm laser diode is coupled into the laser cavitythrough a 980/1550 nm wavelength division multiplexing(WDM) coupler. The linear laser cavity consists of an8-m long PM-EDF, a uniform FBG with a section forstrain modulation and a LCFBG, both of which were fab-ricated on the Hi-Bi fiber [5]. The PM-EDF with a bire-fringence of 2.4 · 10�4 at 1550 nm and a concentrationof erbium ions of 3 · 1024 m�3 is spliced (instead of usingconnectors to reduce the cavity loss) to the PM-FBG andthe PM-LCFBG. The Hi-Bi fiber, with a beat length of4 mm at 1550 nm, was hydrogen-loaded for two weeksto enhance the photosensitivity before grating fabrication.The PM-FBG was fabricated with a uniform phase mask(with a grating pitch of 1.045 lm), and has two reflectionpeaks around 1545 nm. The 60-mm long PM-LCFBG wasfabricated with a phase mask (with a chirp rate of2.25 nm/cm), and has a stopband of about 17 nm in thetransmission mode, which is centred at 1545.5 nm andhas a reflectivity of �90%. Instead of using a bulk dielec-tric mirror, the PM-LCFBG used here can be easilyspliced with other fiber-optic components, thus makingthe laser cavity an all-fiber structure. As the linear lasercavity is formed by the PM-FBG and PM-LCFBG, thelasing wavelengths are defined by the reflection peaks of

PM-ED980/1550 nm

980 nm pump

WDMCoupler

PM-FBG

Pmo

PD

Nean

50 km SMF

Applied strain

Fig. 1. Experiment setup of the proposed tu

the PM-FBG. It is worth noting that, except for theWDM coupler, the whole laser cavity is all polarization-maintaining (PM). The polarization state of the lightwithin the laser cavity is thus maintained, resulting in astable lasing operation from the laser’s output port underroom temperature conditions. The two output lasingwavelengths from the dual-wavelength fiber laser areamplified by an erbium-doped fiber amplifier (EDFA),and the amplified laser light from a 10/90 fiber coupleris monitored by an optical spectrum analyzer (OSA).

A polarization controller (PC) is used to optimize theSOP of the amplified laser light before it is fed into a phasemodulator. After the light is modulated by the 10 GHzphase modulator, it goes through a 50 km SMF as a disper-sive device to eliminate the basement resonance of thenotch filter and introduce a time delay to the optical signal.As the dual-wavelength fiber laser source, which cangenerate two lasing wavelengths with tunable wavelengthseperation, is used as the optical source, a two-taps trans-versal microwave notch filter with tunable FSR is achieved.The two lasing wavelengths from the tunable dual-wavelength fiber laser source are uncorrelated with eachother, thus the microwave notch filter is free from coherentlimitation (i.e. the filter is operating in the incoherentregime). The output from the SMF is then fed into a12 GHz photodetector (PD) which is followed by a vectornetwork analyzer. The transfer function of the tunablemicrowave photonic notch filter can be approximated as:

HðxmÞ / sinpDðk1 þ k2Þ2f 2

mL4 c

!cos Dsxm=2ð Þj j ð1Þ

where fmðfm ¼ xm2pÞ denotes the modulating frequency of the

microwave signal, D and L represent the chromatic dis-persion parameter (D = 17 ps/nm/km here) and the lengthof the SMF (L = 50 km here) respectively, Ds is the timedelay due to the 50 km SMF, k1 and k2 are the two lasing

F

PM-LCFBG

hase dulator

tworkalyzer

PC

EDFA

90%

10%

Coupler

OSA

nable microwave photonic notch filter.

Fig. 2. Amplified laser output spectrum monitored by the OSA: (a) without stretching the PM-FBG and the wavelength separation is 0.36 nm, (b) whenthe wavelength separation is reduced to 0.22 nm with axial strain applied to the PM-FBG, (c) when stretching the PM-FBG further, the wavelengthseparation is reduced to 0.129 nm and (d) Repeated scan of the dual-wavelength fiber laser’s output spectrum for half an hour when the wavelengthseparation is 0.129 nm.

242 D. Liu et al. / Optics Communications 266 (2006) 240–248

Fig. 2 (continued)

D. Liu et al. / Optics Communications 266 (2006) 240–248 243

wavelengths (k2 > k1) of the dual-wavelength fiber laser, andc is the speed of light in vacuum. The time interval or sam-pling period Ds of the notch filter can be expressed as

Ds ¼ DLDk ð2Þ

where Dk = k2 � k1 is the wavelength separation of the twolasing lines from the dual-wavelength fiber laser. The FSRof the microwave notch filter is given by

FSR ¼ 1=Ds ð3Þ

Thus, the microwave notch filter’s FSR can be tunable bychanging the wavelength separation Dk (and hence Ds) ofthe dual-wavelength fiber laser.

3. Experiment results and discussion

Fig. 2(a) shows the amplified laser output spectrummonitored by the OSA when no strain was applied tothe PM-FBG, and the wavelength separation between thetwo lasing lines is �0.36 nm, which can be determined from

Fig. 3. Measured RF spectrum of the dual-wavelength fiber laser using a 45 GHz photodetector.

244 D. Liu et al. / Optics Communications 266 (2006) 240–248

[5,6]. Due to the homogeneous broadening of the erbium-doped fiber, it is normally difficult to achieve a stabledual-wavelength operation with wavelength separationsmaller than 0.5 nm under room temperature conditions

Fig. 4. RF spectrum used to determine the fiber laser’s linewidth using

[7]. However, in our design, due to the enhanced polariza-tion hole burning (PHB) and spectral hole burning (SHB)in the saturated PM-EDF, when stretching the PM-FBGfurther, stable room-temperature dual-wavelength laser

the optical delay self-heterodyne technique (Agilent ESA E4407B).

D. Liu et al. / Optics Communications 266 (2006) 240–248 245

output with a wavelength separation of as small as�0.129 nm can still be obtained as shown in Fig. 2(c) and(d). When the axial strain was applied to the PM-FBG,the birefringence of the Hi-Bi fiber would be reduced dueto the deformation of the fiber core [8], thus a dual-wave-length lasing output with a smaller wavelength separationwere achieved [6]. All these dual-wavelength lasing lineswith different wavelength separations have an OSNR ofover 40 dB. Repeated scan of the dual-wavelength lasingoperation at a 60 s interval over half an hour is shown inFig. 2(d). The output of the dual-wavelength fiber laser isvery stable as can be seen from the repeated scan inFig. 2(d) for over half an hour. Furthermore, Fig. 3 showsthe measured radio frequency (RF) spectrum of the dual-

Fig. 5. Simulation results of the microwave notch filter according to itsDk = k2 � k1 = 1553.909–1553.78 nm = 0.129 nm; (b) Dk = k2 � k1 = 1554.76–

wavelength fiber laser with a 45 GHz photodetector andan electrical spectrum analyzer (ESA). The beat signalshows that the dual-wavelength lasing operation has onlytwo longitudinal modes as there are no sidebands in themeasured RF spectrum [9]. The 3-dB linewidth of eachlasing wavelength of the dual-wavelength fiber laser wasmeasured with the optical delayed self-heterodyne tech-nique with over 80 km SMF to provide the time delay.The displayed �3 dB linewidth (2Dt) is about 5 kHz asshown in Fig. 4, which yields a laser linewidth (Dt) of2.5 KHz [10]. As we increased the applied axial strain,the wavelength separation between the two generatedlasing wavelengths decreases monotonously. The maxi-mum tuning range of the wavelength separation Dk is from

transfer function (Eq. (1)) at two different wavelength seperation: (a)1554.54 nm = 0.22 nm.

0 1 2 3 4 5 6 7 8 9 10-70

-60

-50

-40

-30

-20

-10

0

Frequency (GHz)

No

rma

lize

dre

spo

nse

(dB

)

FSR/2=4.73GHz

(a)

0 1 2 3 4 5 6 7 8 9 10-70

-60

-50

-40

-30

-20

-10

0

Frequency (GHz)

No

rma

lize

dre

spo

nse

(dB

)

FSR=4.592GHz

(b)

(c) 0 1 2 3 4 5 6 7 8 9 10

-60

-50

-40

-30

-20

-10

0

Frequency (GHz)

No

rma

lize

dre

spo

nse

(dB

)

FSR=3.175 GHz

Fig. 6. Measured filter responses of the microwave notch filter with a vector network analyzer for different wavelength separation Dk of the two outputlasing lines from the dual-wavelength fiber laser: (a) Dk = 0.36 nm, (b) Dk = 0.22 nm, and (c) Dk = 0.129 nm.

246 D. Liu et al. / Optics Communications 266 (2006) 240–248

Fig. 7. Simulation results (solid line) of the relationships among the notch filter’s time delay Ds, applied axial strain e to the PM-FBG within the lasercavity and the wavelength separation Dk of the dual-wavelength fiber laser. Measured experimental results of the Ds and e are shown in h and D,respectively.

D. Liu et al. / Optics Communications 266 (2006) 240–248 247

0.36 nm down to as small as 0.05 nm. It is worth notingthat the laser cavity consists of all PM components exceptfor the WDM coupler, thus the polarization state of thefiber laser is well maintained.

Using Eqs. (1) and (2), the simulation results of themicrowave notch filter’s frequency response at twodifferent lasing wavelength seperations (Dk = k2 � k1 =1553.909–1553.78 nm = 0.129 nm and Dk = k2 � k1 =1544.76–1544.54 nm = 0.22 nm) are shown in Fig. 5.And the corresponding measured frequency responses ofthe microwave notch filter with different FSRs are shownin Fig. 6(a)–(c) when we tuned the dual-wavelength fiberlaser’s output wavelength separation. From Figs. 5 and6, we can see that the simulation results are consistentwith the experiment results. The notch rejection ratiosof the microwave filter at three different FSRs are allgreater than 35 dB (see Fig. 6(a)–(c)). The wavelength sep-arations between the two laser lines are 0.36 nm, 0.22 nmand 0.129 nm as shown in Fig. 2(a)–(c), respectively, andthe corresponding measured filter responses with threedifferent FSRs of 9.46 GHz, 4.952 GHz, and 3.175 GHz,when different strain was applied to the PM-FBG areshown in Fig. 6(a)–(c), respectively. The low frequencypart of Fig. 6(c) is not very clear due to the weakresponse of the 12 GHz photodetector from 0–2 GHz.The simulation results (solid lines) of the relationshipsamong the notch filter’s time interval Ds (using Eq. (2)),the applied strain e (see equations in Ref. [5]), and thefiber laser’s wavelength separation Dk are shown inFig. 7. The experimental results shown in Fig. 7 (whereh represents the measured notch filter’s time interval Ds

(or 1/FSR) and D represents the axial strain applied tothe PM-FBG) clearly show that they agree very well withthe simulation results. When the applied strain to thePM-FBG increases, the birefringence of the grating willdecrease and this results in a reduction of the wavelengthseparation of the dual-wavelength fiber laser is alsoreduced [8]. According to Eqs. (2) and (3), the smallerthe wavelength separation Dk, the smaller the microwavefilter’s time interval Ds will be and this results in a largerFSR of the notch filter. Thus, a tunable microwave pho-tonic notch filter with tunable FSR can be achieved with atunable wavelength-spacing dual-wavelength fiber laser.

The proposed scheme has certain advantages comparedwith some earlier approaches. First, it has a simpler struc-ture with less optical components. Second, it does notrequire two or more tunable distributed feedback (DFB)lasers which are costly. Another advantage of thisapproach is that the wavelength separation of the dual-wavelength fiber laser can be easily tuned by stretchingthe PM-FBG; therefore a tunable microwave notch filterwith tunable FSR can be achieved.

4. Conclusion

A novel tunable microwave photonic notch filter using adual-wavelength fiber laser has been presented. The linearcavity of the dual-wavelength fiber laser consists of an8-m long PM-EDF, a uniform FBG with a section of strainmodulation and a LCFBG, both of which were fabricated onthe Hi-Bi fiber. Stable room-temperature dual-wavelengthlasing operation with tunable wavelength separation has

248 D. Liu et al. / Optics Communications 266 (2006) 240–248

been achieved. The output of the dual-wavelength fiber lasergoes into a phase modulator followed by a dispersive deviceto form a tunable microwave notch filter. The FSR of thetunable microwave notch filter can be tuned from3.175 GHz to 9.46 GHz by changing the separation betweentwo lasing wavelengths of the dual-wavelength fiber laser,and the rejection ration of the notch is greater than 35 dB.

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