Optical Fiber Technology 18 (2012) 242–246

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Optical Fiber Technology

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Frequency up-conversion of optical microwaves for multichannel opticalmicrowave system on a WDM network

Myunghun Shin a,⇑, Prem Kumar b

a School of Electronics, Telecommunications and Computer Engineering, Korea Aerospace University, Goyang City, Gyeonggi-do 412-791, South Koreab Center for Photonic Communication and Computing, ECE Dept., Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, United States

a r t i c l e i n f o a b s t r a c t

Article history:Received 28 March 2012Revised 21 May 2012Available online 5 July 2012

Keywords:All-optical frequency up-conversionOptical carrier suppression modulationOptical millimeter generationOptoelectronic oscillatorOptical microwavesWavelength division multiplexing

1068-5200/$ - see front matter � 2012 Elsevier Inc. Ahttp://dx.doi.org/10.1016/j.yofte.2012.06.002

⇑ Corresponding author. Fax: +82 2 3159 9257.E-mail address: mhshin@kau.ac.kr (M. Shin).

We propose a multichannel optical microwave system employing a frequency up-converting optoelec-tronic oscillator (FU-OEO) [FU-OEO: frequency up-converting optoelectronic oscillator] as a low-phasenoise local oscillator (LO) and a multichannel frequency up-converter. Employing the FU-OEO, we dem-onstrated an optical microwave system capable of 16 optical microwave links in the C-band on a WDMnetwork; the generated optical microwaves were distributed to their designated remote stations accord-ing to the channel wavelength. When the FU-OEO was used as the system LO, the phase noise of the opti-cal microwaves was under �80 dBc/Hz at a 10 kHz offset from a 20 GHz carrier frequency. As a frequencyup-converter, the FU-OEO simultaneously up-converted all optical data channels at a 1.25 Gbps data ratefor optical microwaves in the 20 GHz band of an optical carrier suppression mode having almost 100%modulation depth. The overall system performance was verified by measuring the bit error rates (BER)of the data recovered from optical microwaves received through single-mode fibers. The measuredBER indicated that the system can transmit over 50 km with a power penalty of less than 1 dB. Thismethod can be useful for high-frequency applications where the generation and transmission of opticalmicrowaves are greatly restricted by optical or electrical bandwidths.

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1. Introduction

Analog optical links or radio-over-fiber systems can deliver dataat multi-gigabit rates in the microwave band over long distancesthrough optical fibers. Using optical approaches for generatingoptical microwaves [1–6], the downstream system can be easilybuilt to reduce construction cost. Optical microwave systems thatdistribute individual data channels to their respective remotestations need to generate optical microwaves of multiple channelsand assign every channel to each remote station. Systems for con-trolling the network using wavelength division multiplexing(WDM) were demonstrated to generate optical microwaves inmultiple channels with an external modulator. However, due tothe complexity of this method, the modulation depths of the gen-erated optical microwaves were under 70% [6]. A simpler methodfor generating optical microwaves with a clean eye pattern isneeded to improve the system performance.

Previously, we developed a simpler method for generating asingle-channel optical microwave. By adding an optical modula-tion function on an optical local oscillator (LO), we experimentallydemonstrated that the optical signal at a 1.25 Gbps data rate in the

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baseband can be up-converted to the optical microwave region inthe 20 GHz band via a frequency-doubling optoelectronic oscillator[4,5] or a frequency up-converting optoelectronic oscillator(FU-OEO). The use of the FU-OEO in optical microwave systemshas several advantages. This OEO is a self-starting LO with lowphase noise [7,8], and the output carrier frequency of the opticalmicrowave is twice the oscillating frequency at which the electriccomponents operate in the circuits. The OEO also performs fre-quency up-conversion, transforming input optical digital signalsinto output optical microwaves. The generated optical microwavesare in the optical carrier suppression (OCS) mode, which showsbetter performance in transmission through single-mode fibers(SMF) as compared to the conventional mode of the optical dou-ble-side band (DSB) [4]. In particular, the transmission distanceof optical microwaves is limited by the dispersion penalty in theSMF, and the dispersion penalty of OCS is half that of optical DSBmode [6,9]. The most interesting feature of the FU-OEO is that allthese functionalities are fulfilled by only one optical modulator,which is very cost effective in millimeter wave applications. Thequality of system (QoS) using the FU-OEO depends on the noiseproperties of the LO and the modulation characteristics of multi-channel up-conversion by the FU-OEO. However, the overallperformance of such systems, including the modulation depth,has not been reported yet.

M. Shin, P. Kumar / Optical Fiber Technology 18 (2012) 242–246 243

In this paper, we show that the FU-OEO can simultaneously up-convert multiple optical data channels at a 1.25 Gbps data ratewith optical microwaves in the 20 GHz band. The generated opticalmicrowaves are in the OCS mode with a modulation depth of al-most 100%. Employing the FU-OEO, we developed an optical micro-wave system on the WDM network to distribute opticalmicrowaves to their designated remote stations by channel wave-length. Using measured bit-error rates of the recovered data fromthe received optical microwaves as a QoS, we demonstrate thatsystem performance is capable of transmission over 50 km.

2. Multichannel optical microwave system on WDM network

The proposed optical microwave system is shown in Fig. 1a. Atthe central office, 16 optical channels for input are connected tothe FU-OEO through a WDM. Each channel has a data rate of1.25 Gbps. The FU-OEO is a self-starting internal loop, shown inFig 1a, which operates partly optically and partly electrically, asshown in Fig. 1b. In the optical part of the loop, a Mach–Zehndermodulator (MZM) generates 10 GHz optical pulses of 1310 nmlight. These optical pulses are sent to 10 GHz optical receivers over

Fig. 1. (a) Proposed optical microwave transmission system on a WDM network. The sysremote stations on a WDM network. (b) Frequency up-converting optoelectronic oscillafilter, E-AMP: electrical amplifier, CF: channel filters (1310/1550 nm).

3 km and 6.8 km. In the electrical part of the loop, the opticalreceivers convert the optical pulses into a 10 GHz electrical signalthat drives the Mach–Zehnder modulator (MZM), which in turnrepetitively generates optical pulses.

The MZM inside the FU-OEO is set to an operating voltage thatis not only at the quadrature point of the 1310 nm light but also atthe null point of the 1550 nm light in the middle of the C-band[8,10]. The external optical channels in the C-band are connectedfor input to and output from the MZM through the channel filters(CF), combining and splitting the 1310 nm and 1550 nm channels.In this condition, the FU-OEO oscillates with the internal 1310 nmlight source at 10 GHz as the LO of the system. The external opticalinputs in the C-band are modulated at 20 GHz in the OCS mode sothat the 16 optical channels in the 1.25 Gbps baseband are up-con-verted to optical microwaves in the 20 GHz band.

In wireless communication systems, the number of microwavechannels (bandwidths & carriers) limits the number of subscribersin the service area. Dividing the service area with remote stationsoperating remote antennas, the number of service subscribers,using the same bandwidth on the same carrier, can be increasedor the service area can be widen as many as remote stations can

tem consists of a central office including a FU-OEO, WDM network controllers, andtor. 10G Rx: 10 GHz optical receiver, PS: phase shifter, E-BPF: electrical band-pass

Fig. 2. Experimental setup for the multichannel up-conversion of 1.25 Gbps optical data in the 20 GHz optical microwave band and transmission system. k1, k8, k16:1544.28 nm, 1549.85 nm, and 1556.27 nm respectively, MZM: LiNbO3 Mach–Zehnder modulator, WDM: wavelength division multiplexer, EDFA: erbium-doped fiberamplifier, FU-OEO: frequency up-converting optoelectronic oscillator.

244 M. Shin, P. Kumar / Optical Fiber Technology 18 (2012) 242–246

be linked from a central office. The optical microwaves, simulta-neously up-converted in the same carrier frequency by the singleFU-OEO at the central office of Fig. 1, can be easily assigned to theirrespective remote stations by channel wavelength. At the remotestations, the delivered optical microwaves are received and trans-mitted through antennas. In this system, the central office may in-clude internal subsystems producing optical data channels. Unlikethe other optical microwave systems, if the polarization and powerof the external optical data channels are controlled, the system ofFig. 1 can also use the optical data channels of the outer WDM net-work (OC-24) as the input to the FU-OEO without additional datarepeats. In this way, distribution networking from the central officeto remote stations can be realized by the WDM method.

3. Experimental setup and results

In order to emulate a system capable of 16 optical microwavelinks in the C-band on a WDM network, as shown in Fig. 2, we usedthree optical channels of 1544.28 nm, 1549.85 nm, and1556.27 nm as channels 1, 8, and 16, respectively. With WDM1,the MZM modulated the intensity of these three channels with a1.25 Gbps data rate. Optical fibers of different lengths betweenWDM2 and WDM3 de-correlated the bit-stream patterns for thechannels. These optical digital signals in the baseband were simul-taneously up-converted to optical microwave signals in the 20 GHzband through the FU-OEO. Erbium-doped fiber amplifiers (EDFAs)

Fig. 3. 1.25 Gbps optical data signals in the baseband. (a) Electrical spectra for optical

were employed to compensate for the insertion losses of theWDM filters. The combination of WDM3 and WDM4 directs eachoptical microwave channel to its individual remote station inaccordance with the wavelength, as shown in Fig. 1a. In the setupof Fig. 2, we used passive arrayed-waveguide grating WDM mod-ules and manually assigned the optical microwave channels tothe remote stations in Fig. 1. Active optical switching of WDMcan be used for the better networking.

For the remote stations in Fig. 1a, we configured a data recoverymodule at a distance of 1 m (back-to-back), 25.25 km, and 50.5 kmin order to measure the bit-error-rates and the optical or electricaleye patterns of the received optical microwaves. The data recoverymodule restored the 1.25 Gbps electrical data stream using thedown-converter and the 10 GHz electrical output of the FU-OEO,shown in Fig. 2 [4]. Since the FU-OEO is the sole LO in the setup(Fig. 2), the electrical output of the FU-OEO was used as the10 GHz reference to the pattern generator and error detector. Thisoutput also triggered the communication signal analyzer. We notethat the time delay of the optical microwaves through the 50.5 kmSMF (in Fig. 2) to the reference is about 0.25 ms, which is a verylong time for a 1.25 Gbps data stream in the 20 GHz microwaveband. For the setup of Fig. 2, the experiments were conducted afterat least 1 h warming-up for stable operation and the experimentalresults were characterized with a HP 8565/HP 861444 (for theelectrical/optical spectra), Agilent ParBERT 81250 (for the bit errorratio) and Agilent 86100 (for eye patterns).

channels of 1544, 1550, and 1556 nm wavelength, and (b) Measured eye patterns.

Fig. 4. Measured electrical spectra for the output of the FU-OEO. (a) 10 GHz electrical reference and 20 GHz optical microwaves for the channel wavelength, and evolution ofthe spectra after transmission at the optical channels of (b) 1544, (c) 1550, and (d) 1556 nm wavelength.

Fig. 5. Measured bit-error-rates at the data recovery module for the optical channels of (a) 1544, (b) 1550, and (c) 1556 nm wavelength, and (d) eye-pattern evolution of the1550 nm optical channel for transmission distance of a SMF.

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Fig. 3 shows the 1.25 Gbps optical data signals in the baseband:(a) the spectra in the baseband for each optical data channel takenat (1) of the setup in Fig. 2 and (b) the typical eye pattern of a1.25 Gbps optical data channel. The LO operation and frequencyup-conversion of the FU-OEO were shown in Fig. 4. In Fig. 4a, themeasured phase noise of the FU-OEO was below �100 dBc/Hz ata 10 kHz offset from the 10 GHz reference and below �80 dBc/Hzat a 10 kHz offset from the 20 GHz carrier of the optical micro-waves for all channels. These low levels of phase noise and the sta-ble operation as the system LO during the experiments areattributed to the dual loop configuration of the FU-OEO [4,8].Fig. 4b–d shows the spectra in the 20 GHz band for each opticalmicrowave channel taken at (2) of the setup in Fig. 2 after trans-mission. This shows that as the transmission distance (the amountof dispersion) increases, the spectra are gradually degraded so as tolose their sideband [9]. The evolution in the spectra shows the up-conversion characteristics of the FU-OEO for transmission over aSMF. In Fig. 2, the performance of our system depends on the mod-ulation characteristics of up-conversion and the noise properties ofthe LO (i.e., the FU-OEO). Thus, we measured the bit error rates(BERs) as the QoS, representing the comprehensive performanceof the generated optical microwaves. Additional EDFA and passiveoptical attenuator were used as a pre-amplifier. By changing theattenuation level, the error rates were measured for the optical in-put power to the data recovery module. Fig. 5a–c shows the mea-sured BERs at transmission distances of the SMF for 1 m (back-to-back), 25.25 km, and 50.5 km, respectively. The sensitivity isdependent not only on the gain and noise at the receiver (includingthe receiver amplifier and the down converter of Fig. 2) but also onthe optical gain and loss at the transmitter consisting of FU-OEO,EDFAs, WDMs and fibers. Thus, the wavelength dependency inoptical modulations, gain of EDFAs, and loss of WDM filters resultsin the difference of the power penalties for the channels. The error-rate curves show that the maximum power penalty for the QoSwas less than 1 dB at a distance of 50.5 km for all channels evenwith the time delay of 0.25 ms. This implies that the FU-OEOworked very well as a system LO and that the proposed systemis capable of applications in typical metropolitan areas.

Fig. 5d shows the eye patterns of the optical microwaves in the20 GHz band and the restored 1.25 Gbps electrical data streams foreach transmission distance. Though the rising and falling edges ofthe optical microwave patterns are degraded for transmission, themodulation depths are almost 100% so that the clean eye patternsof the 1.25 Gbps data streams are obtained over a distance of50.5 km. The transmission distance limited by a dispersion penaltyof �3 dB was estimated to be about 94 km for the OCS signals at a1.25 Gbps data rate [5].

4. Conclusions

We proposed a frequency up-converting optoelectronic oscilla-tor (FU-OEO) as a low-phase noise LO and as an up-convertor for

optical microwaves in multiple channels. A multichannel opticalmicrowave system was demonstrated using this FU-OEO on awavelength division multiplexing network. This system is able tosimultaneously up-convert 16 optical data channels at a 1.25 Gbpsdata rate for optical microwaves in the 20 GHz band and deliveroptical microwaves to their appropriate destinations on theWDM network according to channel wavelength. The FU-OEO gen-erates the optical microwaves in an optical carrier suppressionmode with a modulation depth of almost 100%. The phase noiseof the optical microwaves was under �80 dBc/Hz at a 10 kHz offsetfrom the 20 GHz carrier frequency, and accordingly, the maximumpower penalty of the QoS for transmission at distances of up to50.5 km was less than 1 dB. In this paper, we demonstrated thesystem in a 20 GHz band based on the available optical and electri-cal components. Using the latest high-speed components, thismethod should be useful for high-frequency applications in milli-meter wave bands or in applications using carrier frequencies be-yond 100 GHz, where the generation and transmission of opticalmicrowaves are greatly restricted by optical or electricalbandwidths.

Acknowledgments

This work was supported in part by the US National ScienceFoundation (Grant No. ECS-0401251) and a KAU Research grant(No. 2011-01-002).

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