A two-photon absorption microcavity-based technique for monitoring in-band optical signal-to-noise ratio (OSNR) in nonreturn-to-zero phase-shift-keying systems is presented. Experiments using a 10-Gb/s differential phase-shift-keying system showed that accurate measurements ( 1 dB) were possible for OSNRs in excess of 20 dB

Anandarajah, Prince M.

Barry, Liam P.

Bondarczuk, Krzysztof

Bradley, Ann Louise

Dexter, Karl J.

Donegan, John Francis

Guo, Wei Hua

Lynch, Michael

O'Dowd, John

Reid, Douglas A.

Shi, Kai

English

Name not available

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 22, NO. 5, MARCH 1, 2010 275Two-Photon-Absorption-Based OSNR Monitor forNRZ-PSK Transmission SystemsDouglas A. Reid, Krzysztof Bondarczuk, Karl J. Dexter, Student Member, IEEE, Kai Shi,Prince M. Anandarajah, Member, IEEE, Liam P. Barry, Member, IEEE, Wei-Hua Guo, John O’Dowd,Michael Lynch, A. Louise Bradley, and John F. Donegan, Member, IEEEAbstract—A two-photon absorption microcavity-based tech-nique for monitoring in-band optical signal-to-noise ratio (OSNR)in nonreturn-to-zero phase-shift-keying systems is presented. Ex-periments using a 10-Gb/s differential phase-shift-keying systemshowed that accurate measurements ( 1 dB) were possible forOSNRs in excess of 20 dB.Index Terms—Optical performance monitoring, opticalsignal-to-noise ratio (OSNR) measurement, phase-shift keying(PSK), two-photon absorption (TPA).I. INTRODUCTIONR ECENTLY, phase-shift-keying (PSK) modulation for-mats have become popular in optical transmissionsystems because of their sensitivity improvement over standardon–off keying (OOK) and their potential for greater spec-tral efficiency [1]. In such systems, it remains important tomonitor the optical signal-to-noise ratio (OSNR) throughoutthe network. It has been shown that the introduction of am-plified spontaneous emission (ASE) noise near the start of atransmission link has a larger effect on PSK than equivalentOOK systems because it results in detrimental nonlinear phasenoise as it copropagates with the signal [2]. OSNR is oftenmeasured spectrally by comparing the optical power withinand between channels. This out-of-band measurement becomesdifficult to use in high density wavelength-division-multiplexed(WDM) systems where the spectral efficiency is high, and forsystems where optical filtering renders an in-band noise levelwhich is significantly different from that found out-of-band. Toovercome this, several techniques have been recently proposedManuscript received July 27, 2009; revised October 22, 2009; accepted De-cember 02, 2009. First published January 12, 2010; current version publishedFebruary 03, 2010. This work was conducted under the framework of the IN-SPIRE programme, funded by the Irish Government’s Programme for Researchin Third Level Institutions, Cycle 4, National Development Plan 2007–2013and by the SFI funded CSET, Centre for Telecommunications Value Driven Re-search (CTVR).D. A. Reid, K. Bondarczuk, K. J. Dexter, K. Shi, P. M. Anandarajah,and L. P. Barry are with the Research Institute for Networks and Com-munications Engineering, Dublin City University, Dublin, Ireland (e-mail:douglas.reid@dcu.ie; krzysztof@eeng.dcu.ie; karl.dexter2@mail.dcu.ie;kaishi@eeng.dcu.ie; anandara@eeng.dcu.ie; liam.barry@dcu.ie).W.-H. Guo, J. O’Dowd, M. Lynch, A. L. Bradley, and J. F. Donegan arewith the Semiconductor Photonics Group and CTVR, School of Physics,Trinity College Dublin, Dublin, Ireland (e-mail: guow@tcd.ie; jodowd@tcd.ie;lynchmi@tcd.ie; louise.bradley@tcd.ie; jdonegan@tcd.ie).Color versions of one or more of the figures in this letter are available onlineat http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/LPT.2009.2038718including the orthogonal delayed-homodyne method [3], thepolarization nulling method [4], and polarization diversity tech-nique [5]. These techniques show great promise, however, theydo have some draw-backs. The orthogonal delayed-homodynemethod requires a high-frequency receiver and radio-frequency(RF) spectrum analyzer, two relatively expensive components.In contrast, [4] and [5] have low component costs but rely onthe assumption that the ASE is completely unpolarized; thisassumption has been shown to generate significant errors insystems with polarized noise [6]. In this letter, we present asimple technique for monitoring OSNR in nonreturn-to-zero(NRZ)-PSK systems based on two-photon absorption (TPA) ina semiconductor microcavity.II. PRINCIPLETPA is a nonlinear absorption process which occurs whenincident photons have less energy than the semiconductorbandgap but greater than half the band-gap, .This means that the absorption of two photons results in thegeneration of a single electron–hole pair. The resulting gener-ated photocurrent is proportional to the incident electric fieldto the power of 4. This means that the TPA photocurrent isa function of the peak power and duty cycle of the incidentoptical signal. This fact has been used to implement optical au-tocorrelators [7], and for the monitoring of dispersion [8]. Morerecently, it was shown that the TPA photocurrent is a functionof the OSNR [9]. This can be understood if we consider theelectric field of a quasi-monochromatic signal and that ofspontaneous emission . These can be represented as(1)(2)where and are independent random variables withGaussian probability density functions (zero mean andvariance). Because these electric fields have quite differentprobability density functions when we calculate their momentswe find(3)(4)(5)(6)1041-1135/$26.00 © 2010 IEEEAuthorized licensed use limited to: DUBLIN CITY UNIVERSITY. Downloaded on February 19,2010 at 10:38:36 EST from IEEE Xplore.  Restrictions apply. 276 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 22, NO. 5, MARCH 1, 2010Fig. 1. Experimental setup for 10-Gb/s NRZ-DPSK OSNR monitor.From these expressions and since(7)(8)where is the optical power and is the TPA photocurrentit follows that(9)(10)The result is that spontaneous emission generates twice theTPA photocurrent as a coherent continuous-wave (CW) signalof the same average power. This can be intuitively understoodif you consider the higher peak-to-average-power ratio of asignal dominated by intensity noise when compared to a noisefree signal. If both noise and a modulated signal are incidenton the detector, the total time averaged photocurrent can beexpressed as(11)where is the average optical power. is related to the OSNR(in decibels) bynm (12)where is the noise equivalent bandwidth. is the signalseffective duty cycle given by(13)and and are coefficients relating to the single and TPAefficiencies, respectively. This means that if the effective dutycycle is known, then the TPA photocurrent can be used to findthe OSNR. In many cases, the exact duty cycle is not knownso the TPA photocurrent cannot be used to uniquely definethe OSNR. In addition, for the special case of NRZ-OOK, if, then the TPA photocurrent is independent of theOSNR. However, in NRZ-PSK systems where no intensitymodulation is present [for transmitters using phase modula-tors (PMs)], the duty cycle is known to be 1 and, therefore,can be eliminated from (11). For NRZ-PSK transmittersusing Mach–Zehnder modulators, the short dips in intensity atthe transitions results in a duty cycle less than one; however,knowledge of the RF amplifier and modulator rise-times canalso be used to accurately calculate which can be used tosolve (11). Because of the high speed of the TPA process, thismeans that the in-band ONSR can be monitored independentlyof the data rate. Rearranging (11) and setting equal to unitygives an expression for the OSNR(14)Unfortunately, dispersion will cause a small reduction in , re-sulting in an underestimate of the OSNR. However, for shortreaches and longer dispersion-compensated systems, this effectis small. Assuming the dominant error is in the photocurrentmeasurement, the uncertainty in the measured OSNR value isgiven by(15)where is the variance in the photocurrent measurement.III. EXPERIMENTThe setup shown in Fig. 1 was used to demonstrate theTPA OSNR monitoring technique experimentally. An externalcavity laser (ECL) at 1558.7 nm was used in conjunction witha LiNbO optical PM and a 10-Gb/s PRBS data source(PPG) to form a 10-Gb/s NRZ differential phase-shift-keying(NRZ-DPSK) transmitter. A cascade of two erbium-dopedfiber amplifiers (EDFAs) followed by a 1-nm filter and avariable optical attenuator (VOA) was then used to introducea known amount of band-limited ASE to the signal, therebydegrading the OSNR. The resulting signal was passed to theOSNR monitor, and a DPSK receiver for eye measurements.An OSA was used for verification of the OSNR. The OSNRmonitor was formed by a GaAs microcavity TPA photodetector[10] followed by a picoammeter (PA). A second VOA wasused to ensure that optical power onto the TPA photodetectorremained at a constant average power of 5-dBm independentof the OSNR. The polarization of the PSK signal was alignedAuthorized licensed use limited to: DUBLIN CITY UNIVERSITY. Downloaded on February 19,2010 at 10:38:36 EST from IEEE Xplore.  Restrictions apply. REID et al.: TPA-BASED OSNR MONITOR FOR NRZ-PSK TRANSMISSION SYSTEMS 277Fig. 2. Measured photocurrent from TPA microcavity photodiode as a functionof signal OSNR (0.1-nm bandwidth), for a constant optical power of 5 dBm.Fitted curve using (11). Inset: Received eye diagrams of NRZ-DPSK signal forvarious in-band OSNRs: (a) 21.1 dB; (b) 18.5 dB; (c) 11 dB; (d) 6.3 dB.Fig. 3. Measured OSNR as a function of actual OSNR. Error bars calculatedusing equation (7) for  10-pA photocurrent uncertainty.to maximize the TPA photocurrent, while the ASE was leftunpolarized.Fig. 2 shows the variation of the measured photocurrent asthe OSNR was varied along with a fitted curve given by (11).Since ASE generates twice as much TPA photocurrent as a CWsignal with the same average optical power, the total measuredphotocurrent is seen to increase by 500 pA as the OSNR is de-creased from 30 to 6 dB while the average optical power is keptconstant. Calibration measurements gave the values for andof A/W and A/W , respectively.The OSNR values calculated by (14) are shown in Fig. 3; theresults have good agreement allowing 1-dB measurements forOSNRs in excess of 20 dB for a photocurrent measurement withan error of 10 pA.To highlight the effect of this OSNR reduction on systemperformance, Fig. 2 insets (a) and (b) show received eye-dia-grams following a DPSK demodulator [implemented using a1-bit delay interferometer (DI)] for a selection of OSNRs. Asexpected, the eye is seen to close with the addition of ASE,which would result in a significant increase in bit-error rate. TheTPA microcavity used had a resonance enhanced bandwidth of1.2 nm centered at 1558.7 nm which can be tuned by 5 nm withadjustment of the incident optical angle. While this is nonop-timal for this application, the cavity mirrors could potentially bedesigned to accommodate a smaller channel bandwidth therebyincreasing the enhancement of the TPA process. This wouldallow the monitoring of a single WDM channel. One drawbackof TPA is its polarization dependence [11]; however, this couldbe overcome with a polarization diversity configuration.IV. CONCLUSIONWe have presented a novel technique for the monitoring ofOSNR in NRZ-PSK transmission systems using TPA in a semi-conductor microcavity. Experiments using a 10-Gb/s DPSKsystem show that monitoring of OSNRs up to and above 20 dBwould be possible with less then 1-dB error. The technique hasthe advantages of simplicity, low cost, and ease of integrationand in principle is insensitive to data rate.REFERENCES[1] W. A. Atia and R. S. Bondurant, “Demonstration of return-to-zero sig-naling in both OOK and DPSK formats to improve receiver sensitivityin an optically preamplified receiver,” in Proc. Lasers and Electro-Op-tics Society 12th Annual Meeting (LEOS ’99), 1999, pp. 226–227.[2] H. Kim and A. H. Gnauck, “Experimental investigation of the perfor-mance limitation of DPSK systems due to nonlinear phase noise,” IEEEPhoton. Technol. Lett., vol. 15, no. 2, pp. 320–322, Feb. 2003.[3] C. J. Youn, K. J. Park, J. H. Lee, and Y. C. Chung, “OSNR monitoringtechnique based on orthogonal delayed-homodyne method,” IEEEPhoton. Technol. Lett., vol. 14, no. 10, pp. 1469–1471, Oct. 2002.[4] J. H. Lee, D. K. Jung, C. H. Kim, and Y. C. Chung, “OSNR monitoringtechnique using polarization-nulling method,” IEEE Photon. Technol.Lett., vol. 13, no. 1, pp. 88–90, Jan. 2001.[5] T. B. Anderson, K. Clarke, S. D. Dods, and M. Bakaul, “Robust, lowcost, in-band optical signal to noise monitoring using polarization di-versity,” in Proc. Optical Fiber Communication and the National FiberOptic Engineers Conf., Mar. 2007, pp. 1–3.[6] M. D. Feuer, “Measurement of OSNR in the presence of partially po-larized ASE,” IEEE Photon. Technol. Lett., vol. 17, no. 2, pp. 435–437,Feb. 2005.[7] Y. Takagi, T. Kobayashi, K. Yoshihara, and S. Imamura, “Multiple- andsingle-shot autocorrelator based on two-photon conductivity in semi-conductors,” Opt. Lett., vol. 17, no. 9, pp. 658–660, 1992.[8] S. Wielandy, M. Fishteyn, and B. Zhu, “Optical performance moni-toring using nonlinear detection,” J. Lightw. Technol., vol. 22, no. 3,pp. 784–793, Mar. 2004.[9] W. H. Guo, J. O’Dowd, M. Lynch, A. L. Bradley, J. F. Donegan, L.P. Barry, and D. C. Kilper, “Two-photon absorption generated by opti-cally amplified signals,” Electron. Lett., vol. 44, no. 18, pp. 1087–1088,Aug. 2008.[10] H. Folliot, M. Lynch, L. P. Barry, A. L. Bradley, L. A. Dunbar, J.Hegarty, J. F. Donegan, J. S. Roberts, and G. Hill, “Two-photon absorp-tion photocurrent enhancement in bulk AlGaAs semiconductor micro-cavities,” Appl. Phys. Lett., vol. 80, pp. 1328–1330, 2002.[11] J. O’ Dowd, W. H. Guo, E. Flood, M. Lynch, A. L. Bradley, L. P.Barry, and J. F. Donegan, “Polarization dependence of a GaAs-basedtwo-photon absorption microcavity photodetector,” Opt. Express, vol.16, pp. 17682–17688, 2008.Authorized licensed use limited to: DUBLIN CITY UNIVERSITY. Downloaded on February 19,2010 at 10:38:36 EST from IEEE Xplore.  Restrictions apply. 

Two-photon-absorption-based OSNR monitor for NRZ-PSK transmission systems

D.A. Reid

K. Bondarczuk

K.J. Dexter

Kai Shi, 

P.M. Anandarajah

L.P. Barry

Wei-Hua Guo, 

J. O'Dowd

M. Lynch

A.L. Bradley

J.F. Donegan

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Two-Photon-Absorption-Based OSNR Monitor for NRZ-PSK Transmission Systems

DCU Online Research Access Service

Demonstration of return-to-zero signaling in both OOK and DPSK formats to improve receiver sensitivity in an optically preampliﬁed receiver,” in

Experimental investigation of the performancelimitationofDPSKsystemsduetononlinearphasenoise,”IEEE Photon.

Hegarty,J.F.Donegan,J.S.Roberts,andG.Hill,“Two-photonabsorption photocurrent enhancement in bulk AlGaAs semiconductor microcavities,”

Measurement of OSNR in the presence of partially polarized

Optical performance monitoring using nonlinear detection,”

OSNR monitoring technique based on orthogonal delayed-homodyne method,”

OSNR monitoring technique using polarization-nulling method,”

Polarization dependence of a GaAs-based two-photon absorption microcavity photodetector,”

Robust, low cost, in-band optical signal to noise monitoring using polarization diversity,” in

Two-photon absorption generated by opticallyampliﬁedsignals,”Electron.Lett.,vol.44,no.18,pp.1087–1088,

Y.Takagi,T.Kobayashi,K.Yoshihara,andS.Imamura,“Multiple-and single-shot autocorrelator based on two-photon conductivity in semiconductors,”

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Two-photon-absorption-based OSNR monitor for NRZ-PSK transmission systems

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