Phase modulatio parallel optical delay detector for microwave agle-of-arrival measuremet with accuracy moitored Z. Cao,,* Q. Wag, R. Lu,, H.P.A. va de Boom, E. Tagdiogga, ad A.M.J. Kooe COBRA Istitute, Eidhove Uiversity of Techology, NL 56 MB Eidhove, The Netherlads School of Optoelectroic Iformatio, State Key Laboratory of Electroic Thi Films ad Itegrated Devices, Uiversity of Electroic Sciece ad Techology of Chia, Chegdu 654, Chia *Correspodig author: z.cao@tue.l Received Moth X, XXXX; revised Moth X, XXXX; accepted Moth X, XXXX; posted Moth X, XXXX (Doc. ID XXXXX); published Moth X, XXXX A ovel phase modulatio parallel optical delay detector is proposed for microwave agle-of-arrival (AOA) measuremet with accuracy moitored by usig oly oe dual-electrode Mach-Zehder modulator. A theoretical model is built up to aalyze the proposed system icludig measuremet accuracy moitorig. The spatial delay measuremet is traslated ito the phase shift betwee two replicas of a microwave sigal. Thaks to the accuracy moitorig, the phase shifts from 5 to 65 are measured with less tha 3. measuremet error. OCIS Codes: (6.565) Radio frequecy photoics, (35.4) Microwave, (.) Istrumetatio, measuremet, ad metrology. http://dx.doi.org/.364/ol.99.99999 Determiig the locatio of a microwave sigal is of great importace for retrievig the positio of objects. The parameter agle-of-arrival (AOA) or equivaletly the time differece of arrival (TDOA) is required to accurately idetify the positio. A optical approach to measure the AOA ca offer may beefits due to its itrisic features like ultra-low loss ad huge badwidth, which allows high accuracy, ad immuity to electromagetic iterfereces. Moreover, with the rapid developmet of ultra-low drive voltage electro-optical modulators (EOMs) [,] ad highspeed photo-diodes [3,4], barriers betwee electrical domai ad optical domai are gradually elimiated. Recetly some photoic approaches are proposed to measure AOAs of microwave sigals [5-8]. Some of these approaches are based o optical modulators with the advatage of the availability of mature ad commercial products. Furthermore, such kid of schemes are scalable based o itegrated optics. I Ref.[8], a serial optical delay detector usig two EOMs ad oe discrete optical delay lie is proposed for AOA measuremets. The discrete optical delay lie (fiber) betwee EOMs will itroduce uwated iterfereces from eviromet variatios (e.g. temperature). I Ref. [9], a parallel optical delay detector (PODD) is proposed based o a parallel Mach-Zehder modulator (P-MZM). Its itegrated structure ca icrease the tolerace toward eviromet variatios. Sice there are three DC-biases ad oly oe moitored parameter (power of optical carrier) i a P-MZM, the simple ad robust automatic bias cotrol (ABC) is difficult to achieve. To solve this problem, the core idea is to avoid uecessary DC-biases by replacig itesity modulatio with phase modulatio. I this letter, a ovel phase modulatio parallel optical delay detector (PM-PODD) usig oly oe dual-electrode Mach-Zehder (DE-MZM) is proposed. Because there is oly oe DC-bias i a DE- MZM, the simple ad robust ABC is achievable. Moreover, the complexity ad itrisic isertio loss of the proposed scheme are halved compared to the oe i Ref. [9]. The proposed scheme for AOA (or TDOA) measuremet is depicted i Fig.. It icludes a DFB laser, a DE-MZM, a optical otch filter (ONF), ad two optical power meters. The upper arm (U-arm) ad lower arm (L-arm) are the two arms iside DE-MZM. Bias- is the tuable phase shifter betwee U- ad L-arm. The U- ad L-arm of the DE-MZM are coected to two ateas, Ate- ad Ate-. The distace betwee Ate- ad Ate- is deoted as d. The AOA is deoted as ψ ad the correspodig TDOA ca be expressed by: τ = dcos( ψ) c where c is the light velocity i air. Properties of the electrical path betwee Ate- ad U-arm ca be differet i legth ad i impedace to the oe betwee Ate- ad L-arm. This differece ca itroduce fixed phase offsets for differet frequecies. Such phase offsets ca be easily compesated usig a look-up table. The TDOA τ will itroduce the phase shift φ betwee Ate- ad Ate- as show: ϕ = τ ω m where ωm is the agular frequecy of microwave sigal. Therefore, the task of the proposed PM-PODD scheme is to measure the phase shift φ i optical domai. The phase shift φ caused by spatial delay τ will be traslated to the phase differece of optical sidebads. The followig task is to measure this phase differece of optical sidebads by usig optical power meters. The U-arm is biased at the ull poit to suppress the optical carrier. The lightwave from the CW laser is modulated by two replicas of the microwave sigal with phase shift φ at the U- ad L-arm, of which the spectra are show i Fig. ad, () ()
τc ψ d ϕ DFB PC Ate- Bias- DE-MZM Ate- (d) (e) ONF- Power Meter Power Meter ϕ (d) (f) LO cw Bias- (e) Fig.. The priciple of AOA measuremet based o parallel optical delay detector. respectively. The output optical sigals from both U- ad L-arm with phase shift φ are the combied with a additioal phase shift θ iduced by the bias voltage applied to Bias-. The optical spectrum of the combied sigal is show i Fig.. As show i Fig. (d), the optical carrier is separated from the optical sidebads via a optical otch filter (ONF- show i Fig. ). The spectrum of filtered optical sidebads is show i Fig. (e). Now we deduce a theoretical model for the output optical power regardig the phase shift φ. The optical carrier ca be expressed as: Et () = E exp( jω where E ad ω are the amplitude of the electrical field ad the agular frequecy of optical carrier, respectively. The optical carrier is the split ito two arms (U- ad L- arm). The microwave sigal applied to the U- ad L-arm ca be expressed as: E () t = E exp( jω am m m E () t = E exp( jω t+ jϕ) bm m m The optical sigal after the DE-MZM ca be expressed as: Eout () t = Eexp( jω = [exp( jϕ) + exp( jθ)] jj ( m)exp( jω where m=πe m /V π deotes the modulatio depth. θ is a phase shift itroduced by a DC bias. This θ is equal to π for the ull poits. The high order (>d) sidebads are igored sice the received microwave power of these high order sidebads is relatively low. The expressio ca be further writte as: m (3) (4) (5) Eout() t =+ jej+ ( m)[exp( jϕ) ]exp( jωt+ jej ( m)[exp( jϕ) ]exp( jωt After the optical otch filter, the power of the upper sidebad ca be obtaied as: P+ + ( m)[exp( jϕ) ][exp( jϕ) ] 4 + ( m)[ cos( ϕ)] where J+(m) is the Bessel fuctio of first kid with regard to modulatio idex (m). Similarly, we ca obtai the power of lower sidebad as: P ( m)[exp( jϕ) ][exp( jϕ) ] 4 ( m)[ cos( ϕ)] Thus, the power of the upper ad lower sidebad ca be writte i a uified expressio: (6) (7) (8) P± ± ( m)[ cos( ϕ)] (9) It is clear that the output power is related to the phase shift φ. Sice J+ (m) is equal to J- (m), the output power of upper ad lower sidebads iduced by phase shift φ are equal. This feature will be employed for the measuremet of the two samples (output power samples of both the upper ad lower sidebads) with high robustess sice the oise is averaged. The upper ad lower sidebads do ot eed to be separated, therefore a optical otch filter
ca be used to obtai the wated results. From Eq. 9, it also idicates that the amplitudes of the sidebads are related to the modulatio idex m. The high order sidebads are egligible for low drivig power, which is the case for AOA measuremets. The value required for the AOA estimatio is the ormalized power (P), thus the value of E ad J±(m) are less iterestig. We ca obtai the expressios for the TODA (τ) ad AOA (ψ) as: P = P / P, ϕ = arccos( P ) m ( c d) τ = arccos( P ) ω, ψ = arccos τ / m () Accordig to Eq., to estimate the values of τ ad ψ, the required parameters are P ad ωm. P ca be obtaied by measurig Pm ad P. P is the measured output power with zero phase shift (φ=) ad the calibratio procedure will be detailed i the followig. Based o the measured Pm, the phase shift φ ca be estimated for a give value of ωm. Further we ca get the AOA (or TDOA) based o Eq.. If ωm is ukow, a additioal photoic scheme ca be utilized to perform a frequecy measuremet before the AOA (or TODA) measuremet. I the above discussio, we assume that the optical carrier is well suppressed, ad thus the power ad phase shift ca be fully modeled accordig to Eq.. However, both the limited extictio ratio ad the DC drift will itroduce measuremet errors. Sice the limited extictio ratio is give oce the modulator is fabricated, we emphasize o the aalysis of DC drifts iduced measuremet errors. As show i Fig. (f), the proper bias applied to the U-arm for optical carrier suppressio should itroduce π phase shift betwee the U- ad L-arm as the black arrows show i Fig. (f). The DC drift will itroduce the phase shift (θ) to the optical carrier ad the sidebads. The E-field of output optical sigal ca be expressed as: Eout () t = Eexp( jω[+ exp( jθ)] J( m) + EJ + ( m)exp( jϕ + jθ)exp( jωt+ + EJ ( m)exp( jϕ + jθ)exp( jωt () Comparig with Eq.6, the power of the sidebads is ot accurate to preset the phase shift φ with such uwated θ. Both DC drifts i the U- ad L-arm will itroduce similar effects. Thus it is of iterest to moitor the DC drift durig the measuremet. Sice the DC drift simultaeously itroduces residual leakage of the optical carrier, the power measuremet of the optical carrier ca be used to moitor the DC drift. As show i Fig., a optical otch filter (ONF- show i Fig.) is employed to deeply separate the optical carrier ad the sidebads. The separated optical carrier ca the be moitored durig the measuremet process. I a practical system, a automatic bias cotrol circuit ca be used to reduce DC drifts. Such kid of scheme is widely available sice they are previously used for stable advaced modulatio format geeratios. - DFB Bias- PC DE-MZM 549.6 55. 55.4 Wavelegth (m) LO- MHz OC CH- CH- Power- Fig. shows the proof-of-cocept experimetal setup of the AOA (or TDOA) measuremet based o the proposed PM-PODD. The optical carrier is geerated from a DFB laser at 55.6m with dbm power. It is fed ito a DE-MZM after a polarizatio cotroller (PC). Two commercial microwave sources (LO- ad LO-) are employed to drive the U- ad L-arm at a frequecy of.5ghz, respectively. A MHz siusoidal sigal geerated from LO- is set to LO- for phase sychroizatio. DE-MZM is biased at the ull of its power trasfer curve ad the optical spectrum of the combied sigal is show i Fig.. The d ad 3 rd order sidebads are observed > 35-dB ad > 45-dB lower tha the st sidebads, respectively. Thus the higher order sidebads ca be eglected. The phase differeces betwee LO- ad LO- iduced by differet electrical paths ad impedace mismatches are measured by a samplig oscilloscope (digital commuicatio aalyzer). It is the further calibrated via a look-up table. The optical output sigal is the separated by a array waveguide gratig (AWG) which acts as a optical otch filter. I geeral, ay kid of optical otch filters ca be used here if its stop-bad is arrower tha the carrier frequecy of measured microwave sigal. The chael spacig of the AWG is.5ghz ad the sigal is the separated ito three chaels. The optical carrier is i the middle chael (oted as CH-) ad two sidebads are i two eighbor chaels (oted as CH-,3). The sigal from CH- is used for the DC drift moitorig (measured at Power- ) with its spectrum show i Fig.. Optical sigals from CH-,3 are the coupled agai via a 3-dB coupler (OC) for power measuremet (at Power-). Its spectrum is show i Fig.. The measured spectra from CH-,3 with differet phase shifts are show i Fig. 3. As show i Fig. 3, the sidebads are ehaced whe the phase shift is 8. The sidebads are destructively combied oce the phase shift is show i Fig. 3. We ca clearly observe that the power of sidebads degrades whe the phase shift decreases, which coicides with Eq.9. It should be oted that the d order sidebads vary whe the phase shift chages. I our experimet, the d order sidebads are filtered out as well by the AWG as show i Fig.. Thus its ifluece will be largely LO- AWG 549.6 55. 55.4 Wavelegth (m) - CH-3 Power- 549.6 55. 55.4 Wavelegth (m) Fig.. The experimetal setup of AOA measuremet based o phase modulatio parallel optical delay detector.
Level (dbm) - -3-5 -7 - -3-3 549.6 55. 55.4 WaveLegth (m) Level (dbm) -8 549.6 55. 55.4 WaveLegth (m) Fig. 3. The measured spectrum for 8 phase shift, 9 phase shift, ad phase shift. -5-7 Level (dbm) -5-7 549.6 55. 55.4 WaveLegth (m) Measured Agle (degree) 6 8 4 8 6 4 Error Bar (degree) Measured Agle (degree) 6 8 4 8 6 4 Error Bar (degree) 4 8 6 Iitialized Phase Shift (degree) 4 8 6 Iitialized Phase Shift (degree) Fig. 4. Measured optical power (circles) ad theoretical tred (curve), Pmo=-45.3dBm; Measured phase shift (dots) ad their measuremet errors (vertical bars), Pmo=-45.3dBm; Measured phase shift (dots) ad their measuremet errors (vertical bars), Pmo=- 4.5dBm. limited. However, eve for the system usig oly oe optical otch filter, its effect is egligible sice their power is >35dB lower tha the power of st order sidebads. A calibratio to obtai the miimum output power of optical carrier is carried out. After that, the P is obtaied by gettig the maximum power of sidebads. I Fig. 3-, the optical carrier is ot completely suppressed maily due to the limited extictio ratio, sice the DC drift is mostly elimiated at the iitial calibratio stage. Two measuremets are carried out. The first measuremet is carried out just after the iitial calibratio stage. The power of the filtered optical carrier (P) is -45.3dBm. We measure the phase shift φ ad correspodig measuremet errors. The secod measuremet is after a few tes of miutes ad P icreased to.5dbm due to DC drift. The phase shift φ is measured agai with measuremet errors. The measured powers versus differet phase shifts (circles) are show i Fig. 4 for the first measuremet. The theoretical power distributio (red curve) versus phase shift is also show i Fig. 4. A accepted agreemet is obtaied. The detailed measuremet errors are show i Fig. 4 ad for both measuremets. The measuremet errors are less tha 3. withi the rage from 5 to 65 whe the filtered power is -45.3dBm. It is clear that the measuremet errors icrease whe the phase shift goes to. This is maily because of imperfect destructio of sidebads iduced by the limited extictio ratio. Comparig Fig. 4 ad, it is obvious that the measuremet errors icrease up to 7.7 whe P drifts to - 4.5dBm. It shows that the measuremet accuracy degradatio iduced by DC drifts ca be well moitored i this scheme. I this letter, a phase modulatio parallel optical delay detector for microwave agle-of-arrival measuremet with accuracy moitorig is proposed usig oly oe dualelectrode MZM. The spatial delay measuremet is traslated ito the phase shift betwee two replicas of a microwave sigal. Thaks to the accuracy moitorig, the phase shifts from 5 to 65 are measured with 3. measuremet error. With the capability of accuracy moitorig, ad robust parallel ad simple structure, the proposed scheme ca be a attractive solutio for photoic AOA measuremet. This work is supported by The Netherlads Orgaizatio for Scietific Research (NWO) uder the project grat Smart Optical-Wireless I-home Commuicatio Ifrastructure (SOWICI). Refereces. M. Jarrahi, T. H. Lee, ad D. A. B. Miller, IEEE Photo. Techol. Lett.,, 57-59(8).. H. Huag, S. R. Nuccio, Y. Yue, J. Yag, Y. Re, C. Wei, G. Yu, R. Diu, D. Parekh, C. J. Chag-Hasai, ad A. E. Willer, J. Lightwave Techol., 3, 3647-365( ). 3. E. Rouvalis, M. Chtioui, F. va Dijk, F. Lelarge, M. J. Fice, C. C. Reaud, G. Carpitero, ad A. J. Seeds, Opt. Express,, 9-95( ). 4. H. Ito, S. Kodama, Y. Muramoto, T. Furuta, T. Nagatsuma, ad T. Ishibashi, J. Sel. Top. Quatum. Electro.,, 79-77( 4). 5. R. K. Moha, Z. W. Barber, C. Harrigto, ad W. R. Babbitt, OFC/NFOEC, p. OWF3. 6. B. Vidal, M. Á. Piqueras, ad J. Martí, J. Lightwave Techol., 4, 74(6). 7. S. Pa, J. Fu, ad J. Yao, Opt. Lett., 37, 7-9(). 8. X. Zou, W. Li, W. Pa, B. Luo, L. Ya, ad J. Yao, Opt. Lett., 37, 755-757( ). 9. Z. Cao, H. P. A. va de Boom, R. Lu, Q. Wag, E. Tagdiogga, ad A. M. J. Kooe, IEEE, Photo. Techol. Lett., 5, 93-935(3)
Full Refereces []M. Jarrahi, T. H. Lee, ad D. A. B. Miller, "Widebad, Low Drivig Voltage Travelig-Wave Mach Zehder Modulator for RF Photoics", Photoics Techology Letters, IEEE, vol., pp. 57-59, 8. []H. Huag, S. R. Nuccio, Y. Yue, J. Yag, Y. Re, C. Wei, G. Yu, R. Diu, D. Parekh, C. J. Chag-Hasai, ad A. E. Willer, "Broadbad Modulatio Performace of -GHz EO Polymer MZMs", Lightwave Techology, Joural of, vol. 3, pp. 3647-365,. [3]E. Rouvalis, M. Chtioui, F. va Dijk, F. Lelarge, M. J. Fice, C. C. Reaud, G. Carpitero, ad A. J. Seeds, "7 GHz ui-travelig carrier photodiodes for IP-based photoic itegrated circuits", Opt. Express, vol., pp. 9-95,. [4]H. Ito, S. Kodama, Y. Muramoto, T. Furuta, T. Nagatsuma, ad T. Ishibashi, "High-speed ad high-output IP-IGaAs uitravelig-carrier photodiodes", Selected Topics i Quatum Electroics, IEEE Joural of, vol., pp. 79-77, 4. [5]R. K. Moha, Z. W. Barber, C. Harrigto, ad W. R. Babbitt, "Frequecy Resolved Agle ad Time Differece of Arrival Estimatio with Spatial Spectral Holography", i Proceedigs of Optical Fiber commuicatio/natioal Fiber Optic Egieers Coferece, OFC/NFOEC, p. OWF3. [6]B. Vidal, M. Á. Piqueras, ad J. Martí, "Directio-of-Arrival Estimatio of Broadbad Microwave Sigals i Phased-Array Ateas Usig Photoic Techiques", Lightwave Techology, Joural of, vol. 4, p. 74, 6. [7]S. Pa, J. Fu, ad J. Yao, "Photoic approach to the simultaeous measuremet of the frequecy, amplitude, pulse width, ad time of arrival of a microwave sigal", Opt. Lett., vol. 37, pp. 7-9,. [8]X. Zou, W. Li, W. Pa, B. Luo, L. Ya, ad J. Yao, "Photoic approach to the measuremet of time-differece-of-arrival ad agle-of-arrival of a microwave sigal", Opt. Lett., vol. 37, pp. 755-757,. [9]Z. Cao, H. P. A. va de Boom, R. Lu, Q. Wag, E. Tagdiogga, ad A. M. J. Kooe, Agle-of-Arrival Measuremet of a Microwave Sigal Usig Parallel Optical Delay Detector, Photoics Techology Letters, IEEE, vol. 5, pp. 93-935, 3.