Side mode suppression and dispersion compensation analysisof a 60 GHz radio-over-fibre system based on a gain switched laser

Eamonn Martin n, Liam BarryThe Rince Institute, Dublin City University, Glasnevin, Dublin 9, Ireland

a r t i c l e i n f o

Article history:Received 12 June 2013Received in revised form19 September 2013Accepted 22 September 2013Available online 5 October 2013

Keywords:Optical fibre communicationsMillimeter-wave generationGain switchingRadio-over-fibre (RoF)Optical frequency comb

a b s t r a c t

The research and technical community have designated a band of 7 GHz between 57 and 64 GHz forshort-range wireless communications. This paper utilizes a simple and cost effective technique forgenerating a 60 GHz millimeter-wave (mm-wave) signal using an optical comb source based on a gain-switched laser (GSL). This research investigates the effects unwanted comb lines have on the overallsystem performance with 2.5 Gb/s data transmission. To do this, a programmable optical filter is used tosuppress the unwanted comb lines to varying levels. Bit-error rate (BER) measurements were carried outagainst received optical power to demonstrate the detrimental effects the unwanted comb lines have onthe modulated mm-wave signal when not sufficiently suppressed. As chromatic dispersion is a limitingfactor to the system's transmission distance, this work also investigates pre-compensation for dispersionutilizing the programmable group delay capabilities of the programmable optical filter, demonstratingthe ability to extend the transmission distance by 12 km. All experimental results obtained are reinforcedthrough simulation.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Recent years have seen several new bandwidth demandingmultimedia applications become a standard requisite in modernsociety, such as high-definition television (HDTV) and internetteleconferencing [1]. Along with the proliferation of tablet andmobile devices, this creates a major driving force for developingextremely high-speed wireless communication infrastructure[1,2]. Wireless systems operating at much higher carrier frequen-cies in the millimeter-wave (mm-wave) range have an advantageas more bandwidth is available at higher frequencies [1,3].In particular, 60 GHz has been of major interest to the researchcommunity with 7 GHz of bandwidth (57–64 GHz) contemporarilydesignated for wireless communications [4]. Despite the advan-tages of operating at 60 GHz, mm-wave signals experienceattenuation of 10–15 dB/km due to the atmospheric oxygen mak-ing it unsuitable for long range transmission ð42 kmÞ [5,6]. Thismeans that the 60 GHz band can be used exclusively for short-range communications ðo1 kmÞ [6]. When used indoors for smalldistances of wireless communications ðo50 mÞ the attenuationdue to the atmospheric oxygen becomes negligible [6]. In addition,the area of radio-over-fibre (ROF) has become increasingly populardue to the advent of utilizing fibre to extend the transmission

distance while benefiting from the inherent advantages of lowloss and large bandwidth when transmitting through opticalfibre [3,7].

Several architectures are actively being investigated to generateand distribute optical mm-waves [3]. A basic method for generat-ing optical mm-waves is to modulate a semiconductor laserdirectly with a mm-wave electrical signal. However, the methodis limited by the laser modulation bandwidth [5]. To alleviate thelimitations of the laser modulation bandwidth, the modulation canbe carried out using an external modulator. However, using anexternal modulator requires further bias control circuitry and canbe prone to bias drifting making the technique difficult to imple-ment. Another technique to generate optical mm-waves is to use aremote heterodyne receiver utilizing two optical carriers with afrequency separation equal to that of the desired mm-wave [8].The two optical carriers are subsequently transmitted over fibreand heterodyned on a high-speed photodetector to create themm-wave signal. While this technique can reduce the bandwidthof optical transmitter components required, it needs opticalsources with narrow linewidths and an optical phase locked loop(OPLL) to reduce the severe phase noise in the generated electricalsignal [9]. This increases not only the complexity of the system butalso the cost [3].

An alternative technique for optical generation of mm-waveshas been proposed in [5] using a gain-switched laser (GSL) togenerate an optical frequency comb where two comb linesseparated by the desired mm-wave frequency can be filtered off

Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/optcom

Optics Communications

0030-4018/$ - see front matter & 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.optcom.2013.09.048

n Corresponding author. Tel.: þ353 1 700 5883.E-mail address: eamonn.martin7@mail.dcu.ie (E. Martin).

Optics Communications 313 (2014) 36–41

to generate a high-quality mm-wave. In this current work,enhanced optical filtering is used to improve the suppression ofthe unwanted comb lines in [5] and an external intensity mod-ulator is used to modulate the optical mm-wave with 2.5 Gb/sdownstream data. In Section 2, the signal is transmitted throughfibre, detected and BER measurements are taken showing errorfree transmission over 25 km with a receiver sensitivity of�34 dBm. An integral part of the system set-up is the opticalfiltering used to create the mm-wave and in Section 3, the meritsof achieving good suppression of unwanted comb lines areexplored. To do this, the unwanted comb lines of the opticalfrequency comb are suppressed to varying levels using a program-mable optical filter (Finisar WaveShaper 1000 s). BER measure-ments for these different levels and various transmission lengthsare measured against received optical power. Another importantaspect of the system is the limited transmission length due to bit-walk off caused by chromatic dispersion as the data is modulatedonto the two optical comb lines separated by 60 GHz [10]. In Section4, the programmable group delay capability of the programmableoptical filter is used to not only filter out the two comb lines but alsoto pre-compensate for 12 km of dispersion. Thus the transmissiondistance is extended and the effects of dispersion are alleviated in thesystem using one multi-functional device. All experimental worksare simulated using the VPI TransmissionMaker simulation platformto validate the experimental measurements.

2. Modulated optical Mm-waves for 2.5 Gb/s downstream datausing a GSL

The set-up for this experiment is demonstrated in Fig. 1. Thefront-end transmitter utilizes a commercially available DFB laserdiode (DFB-LD) with an emission wavelength of 1541 nm at roomtemperature and a threshold current of 15 mA. This DFB-LD wasbiased at 61.72 mA and subsequently gain switched using anamplified 20 GHz RF sinusoidal with an output power from theamplifier of 19.67 dBm. The output power of the GSL is approxi-mately 5.6 dBm. The GSL is amplified using an erbium dopedamplifier (EDFA) to give a power of 16 dBm. The GSL produces theoptical frequency comb with comb lines equally separated by

20 GHz as captured on a high-resolution optical spectrum analyzer(OSA) and illustrated in Fig. 2(a). The objective is to use theprogrammable optical filter to filter out two comb lines separatedby 60 GHz. The filtered comb lines resulting in a 60 GHz mm-waveoptical signal are shown in Fig. 2(b). The suppression is excellent(46 dB) and there are no unwanted comb lines present. A polar-ization controller (PC) is used to change the polarization state ofthe optical signal before the Mach–Zehnder Modulater (MZM)where the optical signal is modulated by a 2.5 Gb/s on-off keyed(OOK) nonreturn-to-zero (NRZ) data stream.

In the back to back (BTB) case, the optical signal is detected by ahigh-speed photodetector with a 3-dB bandwidth of 70 GHz.The two comb lines beat together to generate a modulated60 GHz electrical signal. The electrical 60 GHz mm-wave is ampli-fied before being demodulated using a 60 GHz sinusoidal electricalsignal and an electrical mixer. A low pass filter (LPF) and an RFamplifier are used to reduce noise and increase the signal level forsignal analysis. The data stream is analyzed using a bit error ratetester (BERT) while a high-speed digital sampling oscilloscope isused to monitor the eye diagram and to optimize the systemduring initial set up.

The BER measurements were carried out for BTB transmission,and transmissions over 12 km, 25 km, and 37 km of standardsingle mode fibre (SSMF). The BER was plotted against thereceived optical power, measured before the EDFA in front of thephotodetector. This EDFA is required to ensure an optimum powerof �1 dBm before the photodetector. These results are presentedin Fig. 3(a). Evident from Fig. 3(a), for a BER of 10�9, the receiversensitivity for BTB transmission is �39 dBm with inset (i) of Fig. 3(a) showing a clear and open eye diagram. From these results it isclear that there is an approximate 3 dB drop in receiver sensitivitywhen the optical signal is transmitted over 12 km of fibre anda further 2 dB drop when transmitted over 25 km of fibre.The system was also tested over 37 km of fibre where the eyewas closed due to dispersion and a BER measurement was notpossible, shown by inset (ii) in Fig. 3(a). The power penalties andeye closure with increased fibre transmission are due to bit-walkoff between the two comb lines as a result of dispersion [10].To demonstrate that the system suffers due to dispersion, adispersion compensating module (DCM) was installed after the

90

10

PolarizationController

EDFA

RFAmp

20 GHzSynth

DFBLaser WaveShaper

OSA

Vbias

MZ Modulator

RFAmp

PPG

EDFA

70 GHzPhotodetector

RF Amp

LPF50

50

BERT

SCOPE60 GHzSynth

RF Amp

Fibre

Fig. 1. Experimental set-up of a gain switched laser used to create a 60 GHz mm-wave optical signal for data modulation and transmission over fibre.

E. Martin, L. Barry / Optics Communications 313 (2014) 36–41 37

37 km length of fibre. The DCM module used is a Lucent Technol-ogies DCM Type WB DK-40 which has a chromatic dispersionvalue of �681 ps/nm at 1541 nm. The DCM was able to bring thesensitivity back to �37 dBm therefore lowering the power penaltyto 2 dB, as shown in Fig. 3(a) with inset (iii) showing the openingof the eye due to the addition of the DCM. It is important to notethat �60.22 ps/nm of dispersion will remain present in the systemdue to the over compensation of the DCM. Also, as the mm-waveoptical signal is attenuated over the 37 km of fibre and the DCM,there will be a decrease in the signal-to-noise ratio (SNR) whenthis signal is amplified by the EDFA before detection.

The system was simulated using the VPI TransmissionMakersimulation platform. The set-up for the VPI simulation was similarto that of the experimental set-up in Fig. 1 with simulationparameters chosen to represent the experimental parameters ofthe gain switched laser, MZM, fibre, detector and the demodulator.To replicate the function of the programmable optical filter, it wasnecessary to use a band stop filter to remove the unwanted comblines between the required comb lines separated by 60 GHz.In conjunction with this filter, a band pass filter was used to removeremaining comb lines other than the two selected comb linesseparated by 60 GHz. The fibre used in the simulation had a groupdelay velocity (GVD) of 16 ps/nm/km and an attenuation factor of0.2 dB/km. The systemwas tested over 37 km as was carried out in

the experimental case. After transmission through 37 km of fibrethe eye diagram was closed and there was an error floor at around10�4. For the dispersion compensation measurement, fibre oflength 2 km was added to the system, after the 37 km transmissiondistance, with GVD set to 296 ps/nm/km to counter the dispersionin the system. The simulated results, in Fig. 3(b), are closelymatched to that of the experimental results, shown in Fig. 3(a).

3. Investigation of suppression of unwanted comb lines andthe effects on system performance

The second system analysis employs the programmable opticalfilter capabilities to test the system performance as a function ofthe suppression of unwanted comb lines. Using the programmableoptical filter, it is possible to suppress the unwanted comb lines ofthe optical frequency comb to varying levels. This is an importantparameter as the suppression of the unwanted comb lines will bedetermined by the filter profile used to select the comb linesseparated by 60 GHz. The three cases used to demonstrate theeffects of suppressing the unwanted comb lines were for maxsuppression (shown earlier to be 46 dB) and when the unwantedcomb lines are suppressed by 25 dB and 15 dB relative to the twoselected comb lines separated by 60 GHz. The optical frequency

194.45 194.5 194.55 194.6 194.65 194.7 194.74-90

-80

-70

-60

-50

-40

-30

-20

-10

Frequency (THz)

Pow

er (d

Bm

)

194.5 194.52 194.54 194.56 194.58 194.6 194.62 194.64-90

-80

-70

-60

-50

-40

-30

-20

-10

Frequency (THz)

Pow

er (d

Bm

)

60 GHz Separation

Fig. 2. Optical spectrum of the optical frequency comb from the gain switched laser: (a) before filtering and (b) the filtered comb spectrum with a frequency separationof 60 GHz between the two comb lines.

-46 -44 -42 -40 -38 -36 -34 -32 -30 -2811

10

9

8

7

6

5

4

(ii)

(iii)

-LO

G(B

ER

)

Received Optical Power (dBm)

(i)

-44 -43 -42 -41 -40 -39 -38 -37 -36 -35 -3411

10

9

8

7

6

5

4

-LO

G(B

ER

)

Received Optical Power (dBm)

BTB12 km25 km37 km & DCM

BTB12 km25 km37 km & DCM

Fig. 3. (a) Measured -LOG(BER) versus received optical power for transmission distances of BTB, 12 km, 25 km, and 37 km with a dispersion compensation module (DCM).Eye-diagrams inset for (i) BTB, (ii) 37 km and (iii) 37 km with a DCM transmission distances. (b) Simulated results.

E. Martin, L. Barry / Optics Communications 313 (2014) 36–4138

combwith unwanted comb lines suppressed by 15 dB is presented inFig. 4. BER measurements were carried out for BTB transmission andtransmission over 12 km. The resulting BER measurements plotted

against received optical power are demonstrated in Fig. 5(a) and (b).Evident from the results is that a reduction in suppression of theunwanted comb lines degrades the system performance. The degra-dation of the system performance results from interference ofadditional 60 GHz signals generated due to the unwanted comblines. These 60 GHz signals are not in phase with the 60 GHz signalgenerated by the main comb lines and therefore cause interferencein the down-converted data stream. In the BTB case, there is anapproximate power penalty of 3 dB (at a BER of 10�9) for asuppression of 25 dB and a power penalty of 5 dB (at a BER of10�9) when unwanted comb lines are suppressed by 15 dB. How-ever, when transmitted over 12 km of fibre the power penalty is notas severe with a 1 dB power penalty for suppression by 25 dB andonly a power penalty of 3 dB for 15 dB suppression of unwantedcomb lines. In the 12 km transmission case dispersion causes thephase of the unwanted 60 GHz signals to change such that they donot cause the same level of interference and system degradation incomparison to the BTB case.

The experiment is modelled using the VPI TransmissionMakersimulation platform. Changing the suppression variables of theband-stop and band-pass filters enables the ability to replicate theprogrammable optical filter. Simulated results for BTB transmis-sion and transmission over 12 km of fibre are demonstrated inFig. 5(c) and (d) respectively and are in excellent agreement withthe experimental results in Fig. 5(a) and (b).

194.44 194.48 194.52 194.56 194.6 194.64 194.68 194.72-90

-80

-70

-60

-50

-40

-30

-20

-10

Frequency (THz)

Pow

er (d

Bm

)

Fig. 4. Filtered optical frequency comb where the suppression of the unwantedcomb lines is set to 15 dB relative to the selected comb lines separated by 60 GHz.

-44 -43 -42 -41 -40 -39 -38 -37 -36 -35 -34 -3311

10

9

8

7

6

5

4

Max25 dB15 dB

-LO

G(B

ER

)

-LO

G(B

ER

)-L

OG

(BE

R)

-LO

G(B

ER

)

Received Optical Power (dBm) Received Optical Power (dBm)

Received Optical Power (dBm)Received Optical Power (dBm)

-41 -40 -39 -38 -37 -36 -35 -34 -33 -3211

10

9

8

7

6

5

4

Max25 dB15 dB

-44 -43 -42 -41 -40 -39 -38 -37 -3611

10

9

8

7

6

5

4

Max25 dB15 dB

-42 -41 -40 -39 -38 -3711

10

9

8

7

6

5

4

Max25 dB15 dB

Fig. 5. Measured -LOG(BER) versus received optical power for analyzing the effects of suppressing unwanted comb lines for maximum, 25 dB and 15 dB suppression over(a) BTB and (b) 12 km transmission distances. (c) and (d) show the simulated results for BTB and 12 km transmission distances respectively.

E. Martin, L. Barry / Optics Communications 313 (2014) 36–41 39

4. Dispersion pre-compensation to extend the transmissiondistance

The third system analysis utilizes the group delay capability ofthe programmable optical filter to compensate for the dispersiveeffects encountered by the system in Fig. 1. The programmableoptical filter allows a phase profile to be fitted over the spectrumto compensate for the effects of dispersion. The maximum disper-sion which can be compensated for using the programmableoptical filter is 200 ps/nm which corresponds to � 12 km of fibrewith a GVD of 16.67 ps/nm/km. Unfortunately, it was not possibleto use the programmable optical filter to compensate for dispersionusing the initial set-up in Fig. 1 as the dispersion compensationwould be before the modulation of the 60 GHz optical mm-waveand would ultimately be ineffective. Therefore, it was necessary tochange the front-end transmitter architecture while the receiverarchitecture remained unchanged.

To implement the necessary changes the EDFA and the pro-grammable optical filter were removed and replaced directly afterthe MZM in Fig. 1. This results in the modulation being carried outdirectly after the laser and over all the comb lines. The modulatedcomb lines are shown in Fig. 6(a). Subsequently, the filtering andthe dispersion compensation are carried out simultaneously by theprogrammable optical filter. The two comb lines, separated by60 GHz and modulated with a 2.5 Gb/s OOK NRZ data stream, canbe seen in Fig. 6(b). BER measurements were conducted initiallyfor BTB, 12 km and 24 km transmission distances. As evident in theresults in Fig. 7(a), the receiver sensitivity for the BTB case is�35 dBm, with less than a 1 dB power penalty for 12 km, whilethere is a significant power penalty of greater than 3 dB for the24 km case. The subsequent step was to compensate for thedispersive effects experienced by the optical signal travelling over24 km of fibre. Using the programmable group delay capability ofthe programmable optical filter brought the power penalty back toapproximately 1 dB, evident in Fig. 7(a). While the programmableoptical filter compensates for 12 km of dispersion, it slightlyattenuates the optical signal when executing the compensationand, in turn, decreases the SNR. The decrease in SNR accounts forthe � 1 dB power penalty experienced in the 24 km case using thedispersion compensation mechanism.

As with the previous schemes, the VPI TransmissionMakersimulation platform was used to perform simulated analysis.Similar changes were made to the simulation's system set-up as

194.47 194.5 194.53 194.56 194.59 194.62 194.65 194.68-90

-80

-70

-60

-50

-40

-30

-20

Pow

er (d

Bm

)

Pow

er (d

Bm

)

194.48 194.52 194.56 194.6 194.64 194.68-90

-80

-70

-60

-50

-40

-30

-20

Frequency (THz)

Frequency (THz)

Fig. 6. Optical spectrum demonstrating optical frequency comb from gain switchedlaser modulated with a 2.5 Gb/s OOK NRZ data stream; (a) before and (b) afterfiltering to generate a 60 GHz optical mm-wave signal.

-42 -40 -38 -36 -34 -32 -30 -28 -2611

10

9

8

7

6

5

4BTB12 km24 km24 km & Disp. Comp.

-LO

G(B

ER

)

-LO

G(B

ER

)

Received Optical Power (dBm)

(i)

(ii)

(iii)

-38 -37 -36 -35 -34 -33 -32 -3111

10

9

8

7

6

5

4

Received Optical Power (dBm)

BTB12 km24 km24 km & Disp. Comp.

Fig. 7. Measured -LOG(BER) versus received optical power (dBm) utilizing the programmable optical filter for dispersion compensation with eye-diagrams inset (i) 12 km,(ii) 24 km and (iii) 24 km with dispersion compensation. (b) Simulated results.

E. Martin, L. Barry / Optics Communications 313 (2014) 36–4140

were made to the experimental set-up. In addition to thosechanges 1 km of dispersion compensating fibre, with a GVD of200 ps/km, was placed directly after the optical filters to representthe programmable optical filter's dispersion compensation ability.Simulated BER measurements against received optical power forBTB transmission, and transmission over 12 km, 24 km and 24 kmwith dispersion compensation are shown in Fig. 7(b). The simu-lated results are in excellent agreement with the experimentalresults in Fig. 7(b). The results clearly show the reduction ininterference after propagation through fibre.

5. Conclusion

This paper presents the use of a GSL to create an opticalfrequency comb which generates a number of comb lines sepa-rated by the frequency of the driving frequency. Subsequentfiltering generates a highly stable 60 GHz optical mm-wave signal.The optical mm-wave is modulated using an external intensitymodulator with 2.5 Gb/s downstream data. Using this scheme, it isdemonstrated that error-free transmission with receiver sensitiv-ities as low as �34 dBm for transmission over 24 km is possible.Analysis of suppression of the unwanted comb lines has beencarried out where unwanted comb lines were not fully suppressedand their effects on the system is analyzed over BTB transmissionand transmission over 12 km. The analysis found that suppressingthe unwanted comb lines by o15 dB had a major detrimentaleffect on system performance. As chromatic dispersion is a limit-ing factor to the system, using the programmable optical filter tonot only filter but also to change the phase profile of the opticalspectrum that was explored to attempt to extend the transmissiondistance of the set-up. As per results, pre-compensation for 12 kmof dispersion using the multi functional programmable opticalfilter was possible. Overall, the techniques and analysis demon-strate an uncomplicated and cost-effective method for generatinghighly stable 60 GHz mm-waves which requires basic filtering toachieve good performance. The system is compatible with future60 GHz ROF systems which will attempt to quell the ever-increasingdemand for higher bandwidth.

Acknowledgments

This work was supported partially by the International Centrefor Graduate Education in Micro & Nano Engineering (ICGEE),Science Foundation Ireland (SFI) and the Higher Education Author-ity (HEA).

References

[1] A. Stöhr, A. Akrout, R. Buß, B. Charbonnier, F. van Dijk, A. Enard, S. Fedderwitz,D. Jäger, M. Huchard, F. Lecoche, J. Marti, R. Sambaraju, A. Steffan, A. Umbach,M. Weiß, Journal of Optical Networking 8 (5) (2009) 471, http://dx.doi.org/10.1364/JON.8.000471, URL ⟨http://jon.osa.org/abstract.cfm?URI=jon-8-5-471⟩.

[2] D. Marpaung, C. Roeloffzen, R. Heideman, A. Leinse, S. Sales, J. Capmany,Integrated microwave photonics. Laser and Photonics Review, 7 (2013)506–538. http://dx.doi.org/10.1002/lpor.201200032.

[3] R. Kashyap, J. Yao, K. Wu, X. Zhang, Techniques for photonic generation ofmillimeter-wave signals, in: 2007 Digest of the IEEE/LEOS Summer TopicalMeetings, 2007, pp. 188–189. doi:http://dx.doi.org/10.1109/LEOSST.2007.4288396⟩.

[4] P. Smulders, IEEE Communications Magazine 40 (1) (2002) 140, http://dx.doi.org/10.1109/35.978061.

[5] H. Shams, P. Anandarajah, P. Perry, L. Barry, IEEE Transactions on MicrowaveTheory and Techniques 58 (11) (2010) 3372, http://dx.doi.org/10.1109/TMTT.2010.2076930.

[6] P.F.M. Smulders, 60 GHz radio: prospects and future directions, in: Proceed-ings of the 10th IEEE Symposium on Communications and Vehicle Technology,Benelux, Nov. 2003, pp. 1–8.

[7] L. Smoczynski, M. Marciniak, A comparison of different radio over fibre systemconcepts with regard to applications in mobile internet and multimedia, in:Proceedings of the 4th International Conference on Transparent OpticalNetworks, 2002, vol. 1, 2002, pp. 211–213. doi:http://dx.doi.org/10.1109/ICTON.2002.1009550.

[8] D. Wake, C. Lima, P. Davies, IEEE Transactions on Microwave Theory andTechniques 43 (9) (1995) 2270, http://dx.doi.org/10.1109/22.414575.

[9] L. Johansson, A. Seeds, Millimetre-wave radio-over-fibre transmission using anoptical injection phase-lock loop source, in: Proceedings of the InternationalTopical Meeting on Microwave Photonics, MWP 2000, 2000, pp. 129–132./http://dx.doi.org/10.1109/MWP.2000.889804S.

[10] C. Lim, K.-L. Lee, A. Nirmalathas, D. Novak, R. Waterhouse, Impact of chromaticdispersion on 60 Ghz radio-over-fiber transmission, in: 21st Annual Meetingof the IEEE Lasers and Electro-Optics Society, 2008. LEOS 2008, 2008, pp. 89–90. doi:http://dx.doi.org/10.1109/LEOS.2008.4688502.

E. Martin, L. Barry / Optics Communications 313 (2014) 36–41 41