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Optical beamforming networks employing phase modulation and direct detection
Optics Communications 284 (2011) 2695–2699
Contents lists available at ScienceDirect
Optics Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / o p t c o m
Optical beamforming networks employing phase modulation and direct detection Xiaoxiao Xue ⁎, Xiaoping Zheng, Hanyi Zhang, Bingkun Zhou State Key Laboratory on Integrated Optoelectronics, Tsinghua National Laboratory for Information Science and Technology (TNList), Department of Electronic Engineering, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 18 October 2010 Received in revised form 19 February 2011 Accepted 21 February 2011 Available online 9 March 2011 Keywords: True-time delay Phase modulation Direct detection Periodic dispersive device
a b s t r a c t We propose a novel dispersion-based optical beamforming network scheme employing phase modulation and direct detection. Optical phase modulators have the advantages of simple-structure, low loss and absence of bias. Dispersion-induced phase-to-intensity conversion is utilized to facilitate direct detection. A structure of wideband dispersive device (WDD) cascaded with periodic dispersive device (PDD) is introduced to enhance the system flexibility, so that the delay adjustability and RF response can be properly designed respectively by choosing appropriate dispersions of the WDD and PDD. A concept-proof system with a wideband chirped fiber grating (CFG) as the WDD and two multiband CFGs (MCFG1 and MCFG2) as the PDD separately is built to demonstrate the basic idea. The delay tuning range is 0–1.8 ns with increment of 164.2 ps. The passband center is 30 GHz for MCFG1 and 20 GHz for MCFG2, and the fractional bandwidth is 51.8%. The shot-noise-limited spurious-free dynamic range is also analyzed and measured to be 105.7 dB ⋅ Hz2/3 when the average photocurrent is 2.7 mA. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Photonic true time delay (TTD) techniques have attracted great interests for their potential in microwave phased array antennas [1,2]. The advantages of a photonic beam-former include light weight, immunity to electromagnetic interference, ultra-wide instantaneous bandwidth, and no-dispersion over multiple microwave bands. Various configurations have been proposed for photonic TTD in the past decades, of which the optical dispersion based method is considered to be one of the most elegant solutions for its simplicity and high flexibility [3–7]. To the best of our knowledge, all the dispersion based TTD schemes reported employed optical intensity modulation (IM). For large-bandwidth and high-linearity applications, LiNbO3 Mach– Zehnder interferometer modulators (MZMs) are the most common choice. The fabrication of MZMs is complicated and the device usually has bias voltage drift problem. A complex active feedback circuit is needed to control the device bias for stable operation [8,9]. In this paper, we propose a novel photonic TTD scheme using optical phase modulation (PM). LiNbO3 optical phase modulators have the advantages of simple structure, low loss, and absence of bias [8,9]. Heterodyne detection is generally needed for PM, which enhances the system complexity. We propose a direct detection scheme (DD) in our system utilizing the dispersion-induced PM-to-IM conversion [10]. However, when ordinary wideband dispersive devices (WDDs) ⁎ Corresponding author. E-mail addresses: [email protected] (X. Xue), [email protected] (X. Zheng). 0030-4018/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2011.02.062
are used as in IM-based schemes, the time delay adjustability and the frequency response of PM-to-IM conversion are not independent to each other since they are both determined by the same optical dispersion. This yields an inflexible system which is not widely applicable. So we introduce a periodical dispersive device (PDD) to adjust the system passband while the time delays induced by the WDD are retained. Finally, a passband and delay configurable photonic TTD system based on optical PM-DD has been constructed which demonstrates our basic idea.
2. Theoretical principle The photonic beam-former based on PM-DD is shown in Fig. 1. The lights from N tunable lasers are combined and then phase-modulated by the RF input signal. The optical signals then pass through two dispersive devices: WDD and PDD. The WDD has a bandwidth typically larger than 10 nm, and the dispersion is nearly uniform (e.g., chirped fiber grating, high-dispersion fiber). The PDD has much narrower passbands typically less than 1 nm, and the multiple channels have identical group delay properties (e.g., multi-channel chirped fiber grating, Fabry–Pérot filter). The optical wavelengths are set within the channels of the WDD cascaded with PDD and aligned to the channel centers. The optical time delays can be controlled by switching the wavelengths in different channels. The optical signals after dispersion are demultiplexed after amplification and detected by photodetectors to feed antenna elements. The wavelength demultiplexer can be implemented as an 1 × N optical coupler and N tunable optical filters.
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Fig. 2. Experimental setup of dispersion-based photonic true-time delay line employing phase modulation and direct detection.
conversion point which has the maximum bandwidth, the PDD should be chosen to meet the condition DPDD =
1 −DWDD ; 2 2fRF
ð4Þ
and the 3-dB RF bandwidth Δf =
Fig. 1. Schematic diagram of dispersion-based optical beam-beaming networks using phase modulation and direct modulation.
Optical PM-to-IM conversion occurs due to the combined dispersion of WDD and PDD. The photodetected RF amplitude varies with the electrical frequency. Under small-signal approximation, the RF signal after photodetection can be expressed as h i 2 is ð fRF Þ = mId sin πðDWDD + DPDD ÞfRF
pffiffiffi pffiffiffi pffiffiffi 3−1 6− 2 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi = fRF : 2 2 DWDD + DPDD
ð5Þ
The fractional bandwidth is approximately 51.8% and thereby fulfilling the requirement of wideband antennas [11]. It is worth to be noted that the function of PDD can be implemented instead by the in-band dispersion of the optical tunable filters which compose the optical wavelength demultiplexer. Then continuously variable true-time delay can be achieved by tuning the optical filters and lasers continuously.
ð1Þ
× cosf2πfRF ½t−τ 0 −DWDD ðνi −ν0 Þg where fRF denotes the RF frequency, Id the average photocurrent, m the modulation index, DWDD/DPDD dispersion of WDD/PDD, τ0 time delay at optical reference frequency ν0, νi optical frequency aligned to the ith channel center. The factor m is defined as m = πVRF/Vπ where VRF is the RF voltage and Vπ is the modulator half-wave voltage. For a specified system delay requirement in practical implementation, the value of DWDD is determined by several factors including the WDD bandwidth, the laser tunable range and the operating wavelength range of the optical amplifier. Larger DWDD leads to smaller laser tunable range, but higher wavelength stability is needed and the crosstalk gets more serious when the adjacent wavelengths are too close. On the other hand, smaller DWDD leads to larger laser tunable range which may exceed the operating range of related devices. For a specified DWDD, the delay tuning increment can be expressed as Δτ = DWDD × Δν
ð2Þ
and the tuning range τmax = ðN−1ÞΔτ;
ð3Þ
where Δν is the separation of adjacent channels, and N is the total channel number. Usually the dispersion of PDD should be designed together with that of the WDD to enable maximum PM-to-IM conversion at the center of the system bandwidth. Corresponding to the first maximum
Fig. 3. Reflectivity and group delay of (a) CFG and (b) MCFGs (red: MCFG1 and blue: MCFG2).
X. Xue et al. / Optics Communications 284 (2011) 2695–2699
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The intermodulation distortion of PM-to-IM conversion has been fully addressed in [12]. The third-order intermodulation (IM3) amplitude can be expressed as
jiIM3 j =
i 1 3 3h 2 m sin πðDWDD + DPDD ÞfRF ; 2
ð6Þ
At the passband center, the sine terms in (1) and (6) equal zero, the output third-order intercept point is given by — 2 is
OIP3
2
= Id :
ð7Þ
To investigate the dynamic range, we assume shot noise as the limitation. The spurious-free dynamic range (SFDR) is then 0 — 12 = 3 i2 Id 2 = 3 B s OIP3 C SFDR = @ — = ; A 2qB i2sn
ð8Þ
where q is the electron charge and B is the receiver bandwidth. For the standard conditions of Id = 1 mA and B = 1 Hz, we find SFDR = 103 dB · Hz2/3. 3. Experimental results The experimental setup is shown in Fig. 2. We used a wideband chirped fiber grating as the WDD of which the reflectivity and group delay are shown in Fig. 3 (a). The bandwidth is 20 nm and the dispersion is 102.6 ps/nm. To demonstrate the RF passband configurability, we used two multi-channel chirped fiber gratings with different dispersion as the PDD separately. The MCFG was first introduced in [13]. Multiple channels with identical characteristics are created using sampled grating technique. The reflectivity and group
Fig. 4. Reflectivity and group delay of (a) CFG + MCFG1 and (b) CFG + MCFG2.
Fig. 5. RF phase delay for (a) CFG + MCFG1 and (b) CFG + MCFG2.
delay of the MCFGs we used are shown in Fig. 3 (b). MCFG1 has a channel width of 1 nm with spacing 1.6 nm and an in-band dispersion of −30 ps/nm, while MCFG2 has the same reflectivity but a different dispersion of 50 ps/nm.
Fig. 6. RF transmission for (a) CFG + MCFG1 and (b) CFG + MCFG2.
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Fig. 8. Eye diagrams for (a) IM-DD and (b) PM-DD.
Fig. 7. Spurious-free dynamic range of the PM-DD scheme.
The combined reflectivity and dispersion of CFG with MCFGs are shown in Fig. 4. There are totally 12 channels corresponding to 12 delay states. The unwrapped microwave phases with different delays are shown in Fig. 5. Clearly linear phase-frequency responses are obtained which implies true time delay property. The delay step increment is 164.2 ps and the tuning range is 1.8 ns. Each delay state is fine tuned by fine tuning the optical wavelength so that the deviation between the actual and ideal delays is within ± 1 ps. The measured RF transmission is shown in Fig. 6. The total dispersion of CFG combined with MCFG1 is 70 ps/nm, leading to an effective PM-to-IM conversion around 30 GHz as shown in Fig. 6 (a). The measured −3-dB bandwidth is 10.4 GHz. The deviation between the experimental and theoretical results when fRF N 30 GHz is mainly due to the response degradation of the modulator and the photodetector. In Fig. 6 (b), the first maximum PM-to-IM conversion occurs at 20 GHz for CFG cascaded with MCFG2 which has a total dispersion of 150 ps/nm. The measured − 3-dB bandwidth is 10 GHz which is in good agreement with the theoretical prediction of 10.3 GHz. The two configurations with CFG + MCFG1 or CFG + MCFG2 have the same delay tunability but different passbands. This demonstrates our idea that the RF transmission and time delay can be properly designed respectively by choosing the appropriate dispersion of WDD and PDD. We also measured the SFDR of the system with CFG + MCFG2, and the results are shown in Fig. 7. The optical phase modulator was driven by a two-tone signal with frequencies of 19.999 GHz and 20.001 GHz. The input microwave power was increased from 5.5 dBm to 15.5 dBm with increment of 0.5 dB, and the output fundamental and third-order intermodulation components are measured by a RF power spectral analyzer. The average photocurrent is 2.7 mA. The measured SFDR is 105.7 dB · Hz2/3, which is in good agreement with the theoretical result 105.9 dB · Hz2/3. In practical implementation of large-scale phase-array antennas, optical amplifiers are essential to compensate the optical loss due to optical splitters and other devices. We inserted a 20-dB attenuator cascaded with an erbium-doped fiber amplifier (EDFA) after the modulator to emulate the real cases. We compared the system performances
of the PM-DD and the IM-DD schemes by transmitting a 20-GHz 625Mbps nonreturn-to-zero (NRZ) on-off-keying (OOK) signal. The MCFG2 was used in the PM-DD test. It is worth to be noted that the same structure of PDD cascaded with WDD can also be used in IM-DD systems to mitigate the dispersion-induced microwave power degradation [14]. We used another MCFG (MCFG3) which has the same reflectivity as the MCFG2 and the opposite dispersion to the CFG in the IM-DD test. The phase modulator we used is from the CONVEGA Corporation, and the intensity modulator from the AVANEX Corporation. Both devices can operate up to 40 GHz. The experimental parameters are listed in Table 1. The insertion loss of the phase modulator is 1 dB lower than that of the intensity modulator. The intensity modulator
Table 1 Experimental parameters.
Modulator insertion loss (dB) Half-wave voltage (V) DC bias (rad) RF modulation power (dBm) EDFA gain (dB) Optical power before PD (dBm)
PM-DD
IM-DD
2.7 5.0 N/A 12 17.7 −3
3.7 5.7 π/2 12 21.3 −3
Fig. 9. Impacts of the bias drift on the system performance of the IM-DD scheme. (a) Optical power, (b) RF power level and (c) carrier-to-noise ratio (noise bandwidth 1 Hz).
X. Xue et al. / Optics Communications 284 (2011) 2695–2699
was biased at the quadrature point; by taking account of the 3-dB loss due to bias, the optical power after the intensity modulator was 4 dB lower than that after the phase modulator. The gain of the EDFA was adjusted that the optical power before the photodetector was −3 dBm for both IM and PM. The measured RF carrier-to-noise ratio (CNR) was 107.1 dB and 115.8 dB for IM-DD and PM-DD (noise bandwidth 1 Hz), respectively. The eye diagrams are shown in Fig. 8. The signal-to-noise ratio (S/N) was 4.7 and 9.3 for IM-DD and PM-DD, respectively. Compared to IM-DD, the system performance with PMDD benefitted from lower amplification-induced noise due to the lower loss of the phase modulator. We also measured the impacts of the bias drift on the system performance of the IM-DD scheme, and the results are shown in Fig. 9. It is found that the best CNR occurs at 2.2 V which deviates by 12%Vπ from the quadrature point (1.5 V). This is due to the variation of the amplification-induced noise as the bias varies. The S/N is 5.0 at the bias 2.2 V, and degrades to 3.2 and 2.9 at 0.5 V and 3 V, respectively. Investigations of the bias stability of LiNbO3 intensity modulators show that the long-term bias drift may be up to several volts [15], thus active feedback circuits are essential to guarantee the proper operation of the modulator. In comparison, the PM-DD scheme has no such bias drift problem, giving rise to a more reliable and simple system. 4. Conclusion We have proposed a novel photonic dispersion-based TTD scheme using the PM-DD method. Compared to MZM optical intensity modulators, phase modulators have the advantages of simple-structure, low loss and absence of bias. Dispersion-induced PM-to-IM conversion is utilized to facilitate direct detection. Besides of WDD which induces true time delay, we also introduced a PDD to adjust the passband for a
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specified system demand. The delay property and the RF response can be properly designed respectively by choosing the appropriate dispersion of WDD and PDD. This gives us a flexible structure which can be more easily implemented by current-available devices. The SFDR has also been analyzed and measured to be 105.7 dB · Hz2/3. Acknowledgements This work is supported in part by National Nature Science Foundation of China (NSFC) under grant No. 60736003, 863 Project under grant No. 2009AA01Z222 and 2007AA01Z256, 973 Project under grant No. 2006CB302805 and Project iCHIP financed by Italian Ministry of Foreign Affairs. References [1] Istvan Frigyes, A.J. Seeds, IEEE Trans. Microwave Theory Tech. 43 (1995) 2378. [2] Wille Ng, Andrew A. Walston, Gregory L. Tangonan, Jar Juch Lee, Irwin L. Newberg, Norman Bernstein, J. Lightwave Technol. 9 (1991) 1124. [3] R.D. Esman, M.J. Monsma, J.L. Dexter, D.G. Cooper, Electron. Lett 28 (1992) 1905. [4] Budi Juswardy, Feng Xiao, Kamal Alameh, Opt. Express 17 (2009) 4773. [5] Bo Zhou, Xiaoping Zheng, Xianbin Yu, Hanyi Zhang, Yili Guo, Bingkun Zhou, Photon. Technol. Lett. 20 (2008) 733. [6] Harish Subbaraman, Maggie Yihong Chen, Ray T. Chen, J. Lightwave Technol. 26 (2008) 2803. [7] Sebastien Blais, Jianping Yao, J. Lightwave Technol. 27 (2009) 1147. [8] Ed L. Wooten, Karl M. Kissa, Alfredo Yi-Yan, et al., IEEE J. Sel. Top. Quant. Electron. 6 (2000) 69. [9] G.L. Li, P.K.L. Yu, J. Lightwave Technol. 21 (2003) 2010. [10] A.R. Chraplyvy, R.W. Tkach, L.L. Luhl, R.C. Alferness, Electron. Lett 22 (1986) 409. [11] Robert J. Mailloux, Phased Array Antenna Handbook, Second EditionArtech House, 2005. [12] Chi Hao, Xihua Zou, Jianping Yao, J. Lightwave Technol. 27 (2009) 511. [13] M. Ibsen, M.K. Durkin, M.J. Cole, R.I. Laming, IEEE Photon. Technol. Lett. 10 (1998) 5. [14] Xue Xiaoxiao, Zheng Xiaoping, Zhang Hanyi, Zhou Binkun, Proc. MWP, 2010, paper WE4-11. [15] Steven K. Korotky, John J. Veselka, J. Lightwave Technol. 14 (1996) 2687.
Contents lists available at ScienceDirect
Optics Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / o p t c o m
Optical beamforming networks employing phase modulation and direct detection Xiaoxiao Xue ⁎, Xiaoping Zheng, Hanyi Zhang, Bingkun Zhou State Key Laboratory on Integrated Optoelectronics, Tsinghua National Laboratory for Information Science and Technology (TNList), Department of Electronic Engineering, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 18 October 2010 Received in revised form 19 February 2011 Accepted 21 February 2011 Available online 9 March 2011 Keywords: True-time delay Phase modulation Direct detection Periodic dispersive device
a b s t r a c t We propose a novel dispersion-based optical beamforming network scheme employing phase modulation and direct detection. Optical phase modulators have the advantages of simple-structure, low loss and absence of bias. Dispersion-induced phase-to-intensity conversion is utilized to facilitate direct detection. A structure of wideband dispersive device (WDD) cascaded with periodic dispersive device (PDD) is introduced to enhance the system flexibility, so that the delay adjustability and RF response can be properly designed respectively by choosing appropriate dispersions of the WDD and PDD. A concept-proof system with a wideband chirped fiber grating (CFG) as the WDD and two multiband CFGs (MCFG1 and MCFG2) as the PDD separately is built to demonstrate the basic idea. The delay tuning range is 0–1.8 ns with increment of 164.2 ps. The passband center is 30 GHz for MCFG1 and 20 GHz for MCFG2, and the fractional bandwidth is 51.8%. The shot-noise-limited spurious-free dynamic range is also analyzed and measured to be 105.7 dB ⋅ Hz2/3 when the average photocurrent is 2.7 mA. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Photonic true time delay (TTD) techniques have attracted great interests for their potential in microwave phased array antennas [1,2]. The advantages of a photonic beam-former include light weight, immunity to electromagnetic interference, ultra-wide instantaneous bandwidth, and no-dispersion over multiple microwave bands. Various configurations have been proposed for photonic TTD in the past decades, of which the optical dispersion based method is considered to be one of the most elegant solutions for its simplicity and high flexibility [3–7]. To the best of our knowledge, all the dispersion based TTD schemes reported employed optical intensity modulation (IM). For large-bandwidth and high-linearity applications, LiNbO3 Mach– Zehnder interferometer modulators (MZMs) are the most common choice. The fabrication of MZMs is complicated and the device usually has bias voltage drift problem. A complex active feedback circuit is needed to control the device bias for stable operation [8,9]. In this paper, we propose a novel photonic TTD scheme using optical phase modulation (PM). LiNbO3 optical phase modulators have the advantages of simple structure, low loss, and absence of bias [8,9]. Heterodyne detection is generally needed for PM, which enhances the system complexity. We propose a direct detection scheme (DD) in our system utilizing the dispersion-induced PM-to-IM conversion [10]. However, when ordinary wideband dispersive devices (WDDs) ⁎ Corresponding author. E-mail addresses: [email protected] (X. Xue), [email protected] (X. Zheng). 0030-4018/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2011.02.062
are used as in IM-based schemes, the time delay adjustability and the frequency response of PM-to-IM conversion are not independent to each other since they are both determined by the same optical dispersion. This yields an inflexible system which is not widely applicable. So we introduce a periodical dispersive device (PDD) to adjust the system passband while the time delays induced by the WDD are retained. Finally, a passband and delay configurable photonic TTD system based on optical PM-DD has been constructed which demonstrates our basic idea.
2. Theoretical principle The photonic beam-former based on PM-DD is shown in Fig. 1. The lights from N tunable lasers are combined and then phase-modulated by the RF input signal. The optical signals then pass through two dispersive devices: WDD and PDD. The WDD has a bandwidth typically larger than 10 nm, and the dispersion is nearly uniform (e.g., chirped fiber grating, high-dispersion fiber). The PDD has much narrower passbands typically less than 1 nm, and the multiple channels have identical group delay properties (e.g., multi-channel chirped fiber grating, Fabry–Pérot filter). The optical wavelengths are set within the channels of the WDD cascaded with PDD and aligned to the channel centers. The optical time delays can be controlled by switching the wavelengths in different channels. The optical signals after dispersion are demultiplexed after amplification and detected by photodetectors to feed antenna elements. The wavelength demultiplexer can be implemented as an 1 × N optical coupler and N tunable optical filters.
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Fig. 2. Experimental setup of dispersion-based photonic true-time delay line employing phase modulation and direct detection.
conversion point which has the maximum bandwidth, the PDD should be chosen to meet the condition DPDD =
1 −DWDD ; 2 2fRF
ð4Þ
and the 3-dB RF bandwidth Δf =
Fig. 1. Schematic diagram of dispersion-based optical beam-beaming networks using phase modulation and direct modulation.
Optical PM-to-IM conversion occurs due to the combined dispersion of WDD and PDD. The photodetected RF amplitude varies with the electrical frequency. Under small-signal approximation, the RF signal after photodetection can be expressed as h i 2 is ð fRF Þ = mId sin πðDWDD + DPDD ÞfRF
pffiffiffi pffiffiffi pffiffiffi 3−1 6− 2 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi = fRF : 2 2 DWDD + DPDD
ð5Þ
The fractional bandwidth is approximately 51.8% and thereby fulfilling the requirement of wideband antennas [11]. It is worth to be noted that the function of PDD can be implemented instead by the in-band dispersion of the optical tunable filters which compose the optical wavelength demultiplexer. Then continuously variable true-time delay can be achieved by tuning the optical filters and lasers continuously.
ð1Þ
× cosf2πfRF ½t−τ 0 −DWDD ðνi −ν0 Þg where fRF denotes the RF frequency, Id the average photocurrent, m the modulation index, DWDD/DPDD dispersion of WDD/PDD, τ0 time delay at optical reference frequency ν0, νi optical frequency aligned to the ith channel center. The factor m is defined as m = πVRF/Vπ where VRF is the RF voltage and Vπ is the modulator half-wave voltage. For a specified system delay requirement in practical implementation, the value of DWDD is determined by several factors including the WDD bandwidth, the laser tunable range and the operating wavelength range of the optical amplifier. Larger DWDD leads to smaller laser tunable range, but higher wavelength stability is needed and the crosstalk gets more serious when the adjacent wavelengths are too close. On the other hand, smaller DWDD leads to larger laser tunable range which may exceed the operating range of related devices. For a specified DWDD, the delay tuning increment can be expressed as Δτ = DWDD × Δν
ð2Þ
and the tuning range τmax = ðN−1ÞΔτ;
ð3Þ
where Δν is the separation of adjacent channels, and N is the total channel number. Usually the dispersion of PDD should be designed together with that of the WDD to enable maximum PM-to-IM conversion at the center of the system bandwidth. Corresponding to the first maximum
Fig. 3. Reflectivity and group delay of (a) CFG and (b) MCFGs (red: MCFG1 and blue: MCFG2).
X. Xue et al. / Optics Communications 284 (2011) 2695–2699
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The intermodulation distortion of PM-to-IM conversion has been fully addressed in [12]. The third-order intermodulation (IM3) amplitude can be expressed as
jiIM3 j =
i 1 3 3h 2 m sin πðDWDD + DPDD ÞfRF ; 2
ð6Þ
At the passband center, the sine terms in (1) and (6) equal zero, the output third-order intercept point is given by — 2 is
OIP3
2
= Id :
ð7Þ
To investigate the dynamic range, we assume shot noise as the limitation. The spurious-free dynamic range (SFDR) is then 0 — 12 = 3 i2 Id 2 = 3 B s OIP3 C SFDR = @ — = ; A 2qB i2sn
ð8Þ
where q is the electron charge and B is the receiver bandwidth. For the standard conditions of Id = 1 mA and B = 1 Hz, we find SFDR = 103 dB · Hz2/3. 3. Experimental results The experimental setup is shown in Fig. 2. We used a wideband chirped fiber grating as the WDD of which the reflectivity and group delay are shown in Fig. 3 (a). The bandwidth is 20 nm and the dispersion is 102.6 ps/nm. To demonstrate the RF passband configurability, we used two multi-channel chirped fiber gratings with different dispersion as the PDD separately. The MCFG was first introduced in [13]. Multiple channels with identical characteristics are created using sampled grating technique. The reflectivity and group
Fig. 4. Reflectivity and group delay of (a) CFG + MCFG1 and (b) CFG + MCFG2.
Fig. 5. RF phase delay for (a) CFG + MCFG1 and (b) CFG + MCFG2.
delay of the MCFGs we used are shown in Fig. 3 (b). MCFG1 has a channel width of 1 nm with spacing 1.6 nm and an in-band dispersion of −30 ps/nm, while MCFG2 has the same reflectivity but a different dispersion of 50 ps/nm.
Fig. 6. RF transmission for (a) CFG + MCFG1 and (b) CFG + MCFG2.
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Fig. 8. Eye diagrams for (a) IM-DD and (b) PM-DD.
Fig. 7. Spurious-free dynamic range of the PM-DD scheme.
The combined reflectivity and dispersion of CFG with MCFGs are shown in Fig. 4. There are totally 12 channels corresponding to 12 delay states. The unwrapped microwave phases with different delays are shown in Fig. 5. Clearly linear phase-frequency responses are obtained which implies true time delay property. The delay step increment is 164.2 ps and the tuning range is 1.8 ns. Each delay state is fine tuned by fine tuning the optical wavelength so that the deviation between the actual and ideal delays is within ± 1 ps. The measured RF transmission is shown in Fig. 6. The total dispersion of CFG combined with MCFG1 is 70 ps/nm, leading to an effective PM-to-IM conversion around 30 GHz as shown in Fig. 6 (a). The measured −3-dB bandwidth is 10.4 GHz. The deviation between the experimental and theoretical results when fRF N 30 GHz is mainly due to the response degradation of the modulator and the photodetector. In Fig. 6 (b), the first maximum PM-to-IM conversion occurs at 20 GHz for CFG cascaded with MCFG2 which has a total dispersion of 150 ps/nm. The measured − 3-dB bandwidth is 10 GHz which is in good agreement with the theoretical prediction of 10.3 GHz. The two configurations with CFG + MCFG1 or CFG + MCFG2 have the same delay tunability but different passbands. This demonstrates our idea that the RF transmission and time delay can be properly designed respectively by choosing the appropriate dispersion of WDD and PDD. We also measured the SFDR of the system with CFG + MCFG2, and the results are shown in Fig. 7. The optical phase modulator was driven by a two-tone signal with frequencies of 19.999 GHz and 20.001 GHz. The input microwave power was increased from 5.5 dBm to 15.5 dBm with increment of 0.5 dB, and the output fundamental and third-order intermodulation components are measured by a RF power spectral analyzer. The average photocurrent is 2.7 mA. The measured SFDR is 105.7 dB · Hz2/3, which is in good agreement with the theoretical result 105.9 dB · Hz2/3. In practical implementation of large-scale phase-array antennas, optical amplifiers are essential to compensate the optical loss due to optical splitters and other devices. We inserted a 20-dB attenuator cascaded with an erbium-doped fiber amplifier (EDFA) after the modulator to emulate the real cases. We compared the system performances
of the PM-DD and the IM-DD schemes by transmitting a 20-GHz 625Mbps nonreturn-to-zero (NRZ) on-off-keying (OOK) signal. The MCFG2 was used in the PM-DD test. It is worth to be noted that the same structure of PDD cascaded with WDD can also be used in IM-DD systems to mitigate the dispersion-induced microwave power degradation [14]. We used another MCFG (MCFG3) which has the same reflectivity as the MCFG2 and the opposite dispersion to the CFG in the IM-DD test. The phase modulator we used is from the CONVEGA Corporation, and the intensity modulator from the AVANEX Corporation. Both devices can operate up to 40 GHz. The experimental parameters are listed in Table 1. The insertion loss of the phase modulator is 1 dB lower than that of the intensity modulator. The intensity modulator
Table 1 Experimental parameters.
Modulator insertion loss (dB) Half-wave voltage (V) DC bias (rad) RF modulation power (dBm) EDFA gain (dB) Optical power before PD (dBm)
PM-DD
IM-DD
2.7 5.0 N/A 12 17.7 −3
3.7 5.7 π/2 12 21.3 −3
Fig. 9. Impacts of the bias drift on the system performance of the IM-DD scheme. (a) Optical power, (b) RF power level and (c) carrier-to-noise ratio (noise bandwidth 1 Hz).
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was biased at the quadrature point; by taking account of the 3-dB loss due to bias, the optical power after the intensity modulator was 4 dB lower than that after the phase modulator. The gain of the EDFA was adjusted that the optical power before the photodetector was −3 dBm for both IM and PM. The measured RF carrier-to-noise ratio (CNR) was 107.1 dB and 115.8 dB for IM-DD and PM-DD (noise bandwidth 1 Hz), respectively. The eye diagrams are shown in Fig. 8. The signal-to-noise ratio (S/N) was 4.7 and 9.3 for IM-DD and PM-DD, respectively. Compared to IM-DD, the system performance with PMDD benefitted from lower amplification-induced noise due to the lower loss of the phase modulator. We also measured the impacts of the bias drift on the system performance of the IM-DD scheme, and the results are shown in Fig. 9. It is found that the best CNR occurs at 2.2 V which deviates by 12%Vπ from the quadrature point (1.5 V). This is due to the variation of the amplification-induced noise as the bias varies. The S/N is 5.0 at the bias 2.2 V, and degrades to 3.2 and 2.9 at 0.5 V and 3 V, respectively. Investigations of the bias stability of LiNbO3 intensity modulators show that the long-term bias drift may be up to several volts [15], thus active feedback circuits are essential to guarantee the proper operation of the modulator. In comparison, the PM-DD scheme has no such bias drift problem, giving rise to a more reliable and simple system. 4. Conclusion We have proposed a novel photonic dispersion-based TTD scheme using the PM-DD method. Compared to MZM optical intensity modulators, phase modulators have the advantages of simple-structure, low loss and absence of bias. Dispersion-induced PM-to-IM conversion is utilized to facilitate direct detection. Besides of WDD which induces true time delay, we also introduced a PDD to adjust the passband for a
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specified system demand. The delay property and the RF response can be properly designed respectively by choosing the appropriate dispersion of WDD and PDD. This gives us a flexible structure which can be more easily implemented by current-available devices. The SFDR has also been analyzed and measured to be 105.7 dB · Hz2/3. Acknowledgements This work is supported in part by National Nature Science Foundation of China (NSFC) under grant No. 60736003, 863 Project under grant No. 2009AA01Z222 and 2007AA01Z256, 973 Project under grant No. 2006CB302805 and Project iCHIP financed by Italian Ministry of Foreign Affairs. References [1] Istvan Frigyes, A.J. Seeds, IEEE Trans. Microwave Theory Tech. 43 (1995) 2378. [2] Wille Ng, Andrew A. Walston, Gregory L. Tangonan, Jar Juch Lee, Irwin L. Newberg, Norman Bernstein, J. Lightwave Technol. 9 (1991) 1124. [3] R.D. Esman, M.J. Monsma, J.L. Dexter, D.G. Cooper, Electron. Lett 28 (1992) 1905. [4] Budi Juswardy, Feng Xiao, Kamal Alameh, Opt. Express 17 (2009) 4773. [5] Bo Zhou, Xiaoping Zheng, Xianbin Yu, Hanyi Zhang, Yili Guo, Bingkun Zhou, Photon. Technol. Lett. 20 (2008) 733. [6] Harish Subbaraman, Maggie Yihong Chen, Ray T. Chen, J. Lightwave Technol. 26 (2008) 2803. [7] Sebastien Blais, Jianping Yao, J. Lightwave Technol. 27 (2009) 1147. [8] Ed L. Wooten, Karl M. Kissa, Alfredo Yi-Yan, et al., IEEE J. Sel. Top. Quant. Electron. 6 (2000) 69. [9] G.L. Li, P.K.L. Yu, J. Lightwave Technol. 21 (2003) 2010. [10] A.R. Chraplyvy, R.W. Tkach, L.L. Luhl, R.C. Alferness, Electron. Lett 22 (1986) 409. [11] Robert J. Mailloux, Phased Array Antenna Handbook, Second EditionArtech House, 2005. [12] Chi Hao, Xihua Zou, Jianping Yao, J. Lightwave Technol. 27 (2009) 511. [13] M. Ibsen, M.K. Durkin, M.J. Cole, R.I. Laming, IEEE Photon. Technol. Lett. 10 (1998) 5. [14] Xue Xiaoxiao, Zheng Xiaoping, Zhang Hanyi, Zhou Binkun, Proc. MWP, 2010, paper WE4-11. [15] Steven K. Korotky, John J. Veselka, J. Lightwave Technol. 14 (1996) 2687.