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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 23 — Nov. 5, 2012
  • pp: 25636–25643
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Digital joint compensation of IMD3 and XMD in broadband channelized RF photonic link

Xiaojun Xie, Yitang Dai, Kun Xu, Jian Niu, Ruixin Wang, Li Yan, Yuefeng Ji, and Jintong Lin  »View Author Affiliations


Optics Express, Vol. 20, Issue 23, pp. 25636-25643 (2012)
http://dx.doi.org/10.1364/OE.20.025636


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Abstract

Based on forward distortion information acquisition and post digital signal processing (DSP), we propose and demonstrate a novel scheme to effectively suppress the third-order intermodulation distortion (IMD3) and cross-modulation distortion (XMD) in a channelized RF photonic link. The simultaneous distortion compensation capacity is studied numerically, and suppression of XMD and IMD3 by about 28 dB and 25 dB, respectively, is achieved experimentally. The scheme principle and the digital compensation procedure are discussed, which shows a simple hardware implementation and algorithm.

© 2012 OSA

1. Introduction

The frequency channelization of broadband radio-frequency (RF) signals relieves the bandwidth limitation of high-resolution analog-to-digital converters (ADCs) and the data volume challenge of downstream digital signal process (DSP), which is a key functionality in both commercial and military microwave applications. Recently the RF photonics has made enormous strides since it owns the advantages such as large RF bandwidths, continuous spectral coverage, enhanced signal processing capabilities, and size, weight, and power (SWaP) benefits. Several innovative photonic-assisted RF channelization schemes have been demonstrated [1

1. S. T. Winnall, A. C. Lindsay, M. W. Austin, J. Canning, and A. Mitchell, “A microwave channelizer and spectroscope based on an integrated optical Bragg-grating Fabry-Perot and integrated hybrid Fresnel lens system,” IEEE Trans. Microw. Theory Tech. 54(2), 868–872 (2006). [CrossRef]

5

5. X. Xie, Y. Dai, K. Xu, J. Niu, R. Wang, L. Yan, and J. Lin, “Broadband photonic RF channelization based on coherent optical frequency combs and I/Q demodulators,” IEEE Photon. J. 4(4), 1196–1202 (2012). [CrossRef]

]. In an optical channelized receiver, the wideband RF signal is up-converted to an optical signal, divided into multiple frequency bands, and then extracted individually. The down-conversion into intermediate frequency (IF) or baseband is required for digitalization and further processing.

The link linearity or dynamic range is a key to achieve high fidelity analog link in each channel, which challenges on the design of the whole channelized receiver. Linearity in RF photonic links is frequently limited by the modulator response. In a conventional narrow-band link where the third-order inter modulation distortion (IMD3) dominates, the linearization has been demonstrated by several designs, such as electronic pre-distortion [6

6. V. Magoon and B. Jalali, “Electronic linearization and bias control for externally modulated fiber optic link,” in IEEE International Topical Meeting on Microwave Photonics, paper 145–147 (2000).

, 7

7. R. B. Childs and V. A. O’Byrne, “Multichannel AM video transmission using a high-power Nd: YAG laser and linearized external modulator,” IEEE J. Sel. Areas Comm. 8(7), 1369–1376 (1990). [CrossRef]

] or feed-forward compensation [8

8. R. M. De Ridder and S. K. Korotky, “Feedforward compensation of integrated optic modulator distortion,” in Optical Fiber Communication Conference and Exposition, Technical Digest (CD) (Optical Society of America, 1990), paper WH5. http://www.opticsinfobase.org/abstract.cfm?URI=OFC-1990-WH5

], cascaded or parallel electro-optic modulators [9

9. H. Skeie and R. V. Johnson, “Linearization of electro-optic modulators by a cascade coupling of phase modulating electrodes,” Proc. SPIE 1583, 153–164 (1991). [CrossRef]

, 10

10. J. L. Brooks, G. S. Maurer, and R. A. Becker, “Implementation and evaluation of a dual parallel linearization system for AM-SCM video transmission,” J. Lightwave Technol. 11(1), 34–41 (1993). [CrossRef]

], and post digital signal compensation [11

11. Q. Lv, K. Xu, Y. Dai, Y. Li, J. Wu, and J. Lin, “I/Q intensity-demodulation analog photonic link based on polarization modulator,” Opt. Lett. 36(23), 4602–4604 (2011). [CrossRef] [PubMed]

, 12

12. T. R. Clark and M. L. Dennis, “Coherent optical phase modulation link,” IEEE Photon. Technol. Lett. 19(16), 1206–1208 (2007). [CrossRef]

], etc. However, in a channelized RF photonic link where the input RF signal is broadband with multiple frequency components, a component in one channel is not only distorted by IMD3, but also impacted by all other frequency components of the input RF signal, which is referred as cross-modulation distortion (XMD) and has been demonstrated the same order as IMD3 in a multi-component RF photonic link. Obviously both nonlinearities are demanded to be minimized in a high-quality optical channelized receiver, which however has been achieved in few reports. In [13

13. A. Agarwal, T. Banwell, P. Toliver, and T. K. Woodward, “Predistortion compensation of nonlinearities in channelized RF photonic links using a dual-port optical modulator,” IEEE Photon. Technol. Lett. 23(1), 24–26 (2011). [CrossRef]

] XMD is suppressed by predistortion, where the dynamic range is still limited by IMD3. In [14

14. T. Banwell, A. Agarwal, P. Toliver, and T. K. Woodward, “Compensation of cross-gain modulation in filtered multi-channel optical signal processing applications,” in Optical Fiber Communication Conference and Exposition, Technical Digest (CD) (Optical Society of America, 2010), paper OWW5. http://www.opticsinfobase.org/abstract.cfm?URI=OFC-2010-OWW5

], IMD3 and XMD are mitigated through post digital distortion compensation. However the entire output signals of all channels are recorded and synchronized to compensate the link nonlinearity and reconstruct the original signal, which increases the configuration complexity largely and requires heavy computation in DSP module.

In this paper, we propose a novel linearization scheme to suppress both XMD and IMD3 in a coherent optical channelized receiver. The distortions information is firstly acquired by hardware, and then fed forward to a DSP unit where the XMD and IMD3 are extracted to linearize the received signal. The nonlinear transfer function of the broadband channelized RF photonic link, as well as the XMD and IMD3 compensation functions, is presented. The proposed scheme is studied numerically, where the important practical concerns, as well as the multi-component capacity, are discussed. Experimentally, simultaneous suppression of XMD and IMD3 by about 28 dB and 25 dB, respectively, is achieved.

2. Operation principle

The proposed linearization technique for the general optical channelization is shown in Fig. 1
Fig. 1 Optical channelization and the proposed simultaneous XMD and IMD3 suppression scheme.
. The output of a continuous wave (CW) laser is modulated by a wideband RF signal consisting of multiple frequency components centered at ωk with amplitude of vk(t) and phase of φk(t):

V(t)=kvk(t)sin[ωkt+φk(t)].
(1)

In order to minimize the XMD, the first term in Eq. (2) is acquired by directly detecting the output of the upper branch of the 1 × 2 MZM through a photo detector (PD). Under the designed bias condition (the lower branch is carrier-suppressed), all the even order sidebands are present at the upper output port, and the received signal after narrow-baseband filtering (the bandwidth is less than the minimum beating frequency of all RF components), IXMDC(t), has been proved [13

13. A. Agarwal, T. Banwell, P. Toliver, and T. K. Woodward, “Predistortion compensation of nonlinearities in channelized RF photonic links using a dual-port optical modulator,” IEEE Photon. Technol. Lett. 23(1), 24–26 (2011). [CrossRef]

] to reflect the XMD information since

IXMDC(t){pJ0[βvp(t)]}2.
(3)

SkL(t)=S˜k(t)/IIMDC(t)=Sk(t)IIMDC(t)IXMDC.
(6)

3. Simulation and experiment

As a proof of concept, one channel of an optical channelized receiver is tested. The experiment setup is illustrated in Fig. 2
Fig. 2 Experimental setup. WS: waveshaper; DL: delay line.
. A CW light (Koheras AdjustiK Benchtop Fiber Laser) with wavelength of 1550 nm, linewidth <1 kHz, and power of 17 dBm is divided into two paths through an optical coupler. In the lower path (the LO path), a microwave tone with frequency of 15.05 GHz and power of 18 dBm is fed into an MZM that is biased under carrier suppression condition. By a WaveShaper (Finisar 4000S; the bandwidth is 15 GHz), the + 1st order sideband is filtered out as the LO. In the upper path (the signal path), the CW light is modulated through a polarization modulator (PolM). The PolM is a special phase modulator where transverse electric (TE) mode and transverse magnetic (TM) mode get the opposite but equal-depth phase modulation [15

15. J. D. Bull, N. A. F. Jaeger, H. Kato, M. Fairburn, A. Reid, and P. Ghanipour, “40-GHz electro-optic polarization modulator for fiber optic communications systems,” Proc. SPIE 5577, 133–143 (2004). [CrossRef]

]. The polarization state of the incident CW light is rotated at 45° to one principal axis of the PolM using a polarization controller (PC). Following the PolM, a second PC and a polarization beam splitter (PBS) are used to convert the polarization modulation to two intensity modulation outputs. When the second PC is adjusted so that the principal axis of the PBS is at 45° to that of the PolM, the PolM followed by PBS is equivalent to a 1 × 2 MZM (Vπ is about 8.8V). Both the PolM and the MZM could be stabilized by proper bias controls. The multi-component RF signal is emulated by two dual-tone RF signals:
V(t)=a1cos(2πδ1t)cos(2πf1t)+a2cos(2πδ2t)cos(2πf2t),
(7)
where f1 / f2, α1 / α2, and 2δ1 / 2δ2 are the center frequencies, amplitudes, and frequency intervals of the two dual-tone RF signals, respectively. The dual-tone RF signal centered at f1 is the fundamental signal that will be received in the tested channel, while the other one (centered at f2) is the out-of-channel signal. The carrier-suppressed output is filtered by a second WaveShaper (with bandwidth of 15 GHz), and the + 1st order sideband is mixed with the LO and received by a BPD, as the coherently-down-converted RF component [Sk(t)]. The other output of the PBS is received directly by a PD, as the XMD information signal [IXMDC(t)]. Outputs from both BPD and PD are sent to a real-time sampling oscilloscope (LeCroy WavePro 7400A with 8-bit resolution). An offline MATLAB program is used to achieve the numerical linearization. Polarization controllers are used in order to optimize the polarization states within each fiber link, which could be stabilized by polarization-maintaining devices. Note that though the coherent down-conversion is used, both the signal and the LO path use the same CW light source, and the impact of the laser phase noise could be voided by matching the lengths of the signal and LO fibers. Meanwhile, no transmission is involved, and the chromatic dispersion and the polarization mode dispersion are ignored in our scheme.

The setup is studied numerically. The fundamental dual-tone signal is at 14.999 GHz and 15.001 GHz, spaced by 2δ1 = 2 MHz, while the out-of-channel dual-tone signal is at 11.000 GHz and 11.005 GHz, spaced by 2δ2 = 5 MHz. When α1 and α2 are both 0.15Vπ, the time-domain waveform and spectrum of the coherent downconverted electric signal without any compensation are shown in Fig. 3(a)
Fig. 3 A numerical example for the proposed digital distortions compensation. (a) The time domain waveform and (b) the spectrum of the coherently-down-converted electrical signal without distortion compensation; (c) the spectrum of the down-converted signal with only XMD compensation and (d) XMD + IMD3 compensation.
and 3(b), respectively. One can observe that the fundamental signal is distorted by both IMD3 and XMD which offset from fIF by ± 3δ1 and ± 2δ2, respectively [13

13. A. Agarwal, T. Banwell, P. Toliver, and T. K. Woodward, “Predistortion compensation of nonlinearities in channelized RF photonic links using a dual-port optical modulator,” IEEE Photon. Technol. Lett. 23(1), 24–26 (2011). [CrossRef]

]. Figure 3(c) shows the signal when only the XMD is compensated [i.e. S˜k(t) by Eq. (4)], which is stilled distorted by IMD3. However, both XMD and IMD3 are minimized by Eq. (6), and the original fundamental signal is recovered effectively as shown in Fig. 3(d).

Thirdly, the XMD compensation by Eq. (4) requires the synchronization between the downconversion and the XMD information path. Profile rather than the phase synchronization is demanded. Obviously the timing error would result in a less XMD suppression, which is shown in Fig. 4(d). The simulation shows that a larger individual RF component bandwidth requires a more precise timing. In our experiment, a piece of optical fiber is used to match the time delay difference of the two paths, and the time delay difference is less than 0.1 ns, which is sufficient for our case [the bandwidths of the two dual-tone RF signals (i.e., their frequency internals) are 2 MHz and 5 MHz, respectively]. Note that the required precision could always be obtained by soft-synchronization in DSP, since the XMD is suppressed digitally in our scheme, which is especially useful when the channel number is increased.

The theory from Eq. (2) shows no limit on the channel number. The simultaneous distortions suppression for more RF components is also tested by simulation. Figure 5
Fig. 5 The simulated simultaneous XMD and IMD3 compensation for ten-component broadband RF signal. (a) without compensation; (b) with compensation
shows the compensation capacity when the input broadband RF signal contains ten components, each of which is a dual-tone signal. The RF carriers range randomly from 5 GHz to 15 GHz, and the bandwidth of each component (i.e. the frequency interval of each dual-tone signal) is from 1 MHz to 10 MHz. The 1-MHz component is received and distortion compensated. The simulation shows good XMD and IMD3 suppression under multi-component input.

The above compensation scheme is demonstrated experimentally. The frequencies of the tones are the same as those in the simulation. Firstly, the power of the fundamental dual-tone signal is fixed at 15 dBm, while the power of the out-of-channel dual-tone signal is variable. The powers of the received fundamental signal, IMD3 sidebands and XMD sidebands are plotted in Fig. 6(a)
Fig. 6 The powers of the received fundamental signal, XMD sidebands and IMD3 sidebands with increased power of (a) the input out-of-channel signal and (b) the input fundamental signal, before and after the distortions compensation.
. One can observe that as the power of the out-of-channel signal increases, the power (in dBm) of the XMD sidebands increases with the slope of two, while the powers of the fundamental signal and IMD3 do not change almost, which agrees with the analysis [14

14. T. Banwell, A. Agarwal, P. Toliver, and T. K. Woodward, “Compensation of cross-gain modulation in filtered multi-channel optical signal processing applications,” in Optical Fiber Communication Conference and Exposition, Technical Digest (CD) (Optical Society of America, 2010), paper OWW5. http://www.opticsinfobase.org/abstract.cfm?URI=OFC-2010-OWW5

]. Under each out-of-channel signal power, the XMD and IMD3 are compensated based on Eq. (6), and the measured residual distortions are plotted in Fig. 6(a). The XMD and IMD3 are suppressed by about 28 dB and 25 dB, respectively.

Secondly, when the power of the out-of-channel signal is fixed at 15 dBm, and that of the fundamental signal is variable, the powers of the received signal and both distortions are also measured and shown in Fig. 6(b). As expected, with increase of the fundamental signal power, the powers of the XMD sidebands and IMD3 sidebands increase with the slope of one and three, respectively. By the proposed compensation technique, the XMD and IMD3 are suppressed by about 30 dB and 27 dB, respectively, as shown in Fig. 6(b).

As a particular example, when the powers of the fundamental and out-of-channel dual-tone signals are 15 and 16 dBm, respectively, the spectrum of the measured signal as well as both distortions is plotted in Fig. 7
Fig. 7 The received spectrum of fundamental signal before (the blue line) and after (the red line) the simultaneous distortions compensation.
, before and after compensation.

4. Conclusion

In conclusion, based on forward distortion information acquisition and post digital distortion compensation, we have theoretically analyzed and experimentally demonstrated a novel linearization scheme for a coherent channelized RF photonic link. Important practical factors that would show impact on the capacity were discussed numerically. Greatly suppressed traditional IMD3 and XMD from all other channels were demonstrated. Improved dynamic range is expected by the simple hardware implementation and digital processing.

Acknowledgments

This work was supported in part by 863 Program (2011AA010306, 2011AA010305), National 973 Program (2012CB315705) NSFC Program (61107058, 61120106001, and 61271042) and Beijing Excellent Doctoral Thesis Project under Grant YB20101001301.

References and links

1.

S. T. Winnall, A. C. Lindsay, M. W. Austin, J. Canning, and A. Mitchell, “A microwave channelizer and spectroscope based on an integrated optical Bragg-grating Fabry-Perot and integrated hybrid Fresnel lens system,” IEEE Trans. Microw. Theory Tech. 54(2), 868–872 (2006). [CrossRef]

2.

W. Wang, R. L. Davis, T. J. Jung, R. Lodenkamper, L. J. Lembo, J. C. Brock, and M. C. Wu, “Characterization of a coherent optical RF channelizer based on a diffraction grating,” IEEE Trans. Microw. Theory Tech. 49(10), 1996–2001 (2001). [CrossRef]

3.

C. S. Brès, S. Zlatanovic, A. O. J. Wiberg, J. R. Adleman, C. K. Huynh, E. W. Jacobs, J. M. Kvavle, and S. Radic, “Parametric photonic channelized RF receiver,” IEEE Photon. Technol. Lett. 23(6), 344–346 (2011). [CrossRef]

4.

X. Xie, Y. Dai, Y. Ji, K. Xu, Y. Li, J. Wu, and J. Lin, “Broadband photonic radio-frequency channelization based on a 39-GHz optical frequency comb,” IEEE Photon. Technol. Lett. 24(8), 661–663 (2012). [CrossRef]

5.

X. Xie, Y. Dai, K. Xu, J. Niu, R. Wang, L. Yan, and J. Lin, “Broadband photonic RF channelization based on coherent optical frequency combs and I/Q demodulators,” IEEE Photon. J. 4(4), 1196–1202 (2012). [CrossRef]

6.

V. Magoon and B. Jalali, “Electronic linearization and bias control for externally modulated fiber optic link,” in IEEE International Topical Meeting on Microwave Photonics, paper 145–147 (2000).

7.

R. B. Childs and V. A. O’Byrne, “Multichannel AM video transmission using a high-power Nd: YAG laser and linearized external modulator,” IEEE J. Sel. Areas Comm. 8(7), 1369–1376 (1990). [CrossRef]

8.

R. M. De Ridder and S. K. Korotky, “Feedforward compensation of integrated optic modulator distortion,” in Optical Fiber Communication Conference and Exposition, Technical Digest (CD) (Optical Society of America, 1990), paper WH5. http://www.opticsinfobase.org/abstract.cfm?URI=OFC-1990-WH5

9.

H. Skeie and R. V. Johnson, “Linearization of electro-optic modulators by a cascade coupling of phase modulating electrodes,” Proc. SPIE 1583, 153–164 (1991). [CrossRef]

10.

J. L. Brooks, G. S. Maurer, and R. A. Becker, “Implementation and evaluation of a dual parallel linearization system for AM-SCM video transmission,” J. Lightwave Technol. 11(1), 34–41 (1993). [CrossRef]

11.

Q. Lv, K. Xu, Y. Dai, Y. Li, J. Wu, and J. Lin, “I/Q intensity-demodulation analog photonic link based on polarization modulator,” Opt. Lett. 36(23), 4602–4604 (2011). [CrossRef] [PubMed]

12.

T. R. Clark and M. L. Dennis, “Coherent optical phase modulation link,” IEEE Photon. Technol. Lett. 19(16), 1206–1208 (2007). [CrossRef]

13.

A. Agarwal, T. Banwell, P. Toliver, and T. K. Woodward, “Predistortion compensation of nonlinearities in channelized RF photonic links using a dual-port optical modulator,” IEEE Photon. Technol. Lett. 23(1), 24–26 (2011). [CrossRef]

14.

T. Banwell, A. Agarwal, P. Toliver, and T. K. Woodward, “Compensation of cross-gain modulation in filtered multi-channel optical signal processing applications,” in Optical Fiber Communication Conference and Exposition, Technical Digest (CD) (Optical Society of America, 2010), paper OWW5. http://www.opticsinfobase.org/abstract.cfm?URI=OFC-2010-OWW5

15.

J. D. Bull, N. A. F. Jaeger, H. Kato, M. Fairburn, A. Reid, and P. Ghanipour, “40-GHz electro-optic polarization modulator for fiber optic communications systems,” Proc. SPIE 5577, 133–143 (2004). [CrossRef]

OCIS Codes
(060.2360) Fiber optics and optical communications : Fiber optics links and subsystems
(060.5625) Fiber optics and optical communications : Radio frequency photonics

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: July 24, 2012
Revised Manuscript: September 27, 2012
Manuscript Accepted: October 18, 2012
Published: October 29, 2012

Citation
Xiaojun Xie, Yitang Dai, Kun Xu, Jian Niu, Ruixin Wang, Li Yan, Yuefeng Ji, and Jintong Lin, "Digital joint compensation of IMD3 and XMD in broadband channelized RF photonic link," Opt. Express 20, 25636-25643 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-23-25636


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References

  1. S. T. Winnall, A. C. Lindsay, M. W. Austin, J. Canning, and A. Mitchell, “A microwave channelizer and spectroscope based on an integrated optical Bragg-grating Fabry-Perot and integrated hybrid Fresnel lens system,” IEEE Trans. Microw. Theory Tech.54(2), 868–872 (2006). [CrossRef]
  2. W. Wang, R. L. Davis, T. J. Jung, R. Lodenkamper, L. J. Lembo, J. C. Brock, and M. C. Wu, “Characterization of a coherent optical RF channelizer based on a diffraction grating,” IEEE Trans. Microw. Theory Tech.49(10), 1996–2001 (2001). [CrossRef]
  3. C. S. Brès, S. Zlatanovic, A. O. J. Wiberg, J. R. Adleman, C. K. Huynh, E. W. Jacobs, J. M. Kvavle, and S. Radic, “Parametric photonic channelized RF receiver,” IEEE Photon. Technol. Lett.23(6), 344–346 (2011). [CrossRef]
  4. X. Xie, Y. Dai, Y. Ji, K. Xu, Y. Li, J. Wu, and J. Lin, “Broadband photonic radio-frequency channelization based on a 39-GHz optical frequency comb,” IEEE Photon. Technol. Lett.24(8), 661–663 (2012). [CrossRef]
  5. X. Xie, Y. Dai, K. Xu, J. Niu, R. Wang, L. Yan, and J. Lin, “Broadband photonic RF channelization based on coherent optical frequency combs and I/Q demodulators,” IEEE Photon. J.4(4), 1196–1202 (2012). [CrossRef]
  6. V. Magoon and B. Jalali, “Electronic linearization and bias control for externally modulated fiber optic link,” in IEEE International Topical Meeting on Microwave Photonics, paper 145–147 (2000).
  7. R. B. Childs and V. A. O’Byrne, “Multichannel AM video transmission using a high-power Nd: YAG laser and linearized external modulator,” IEEE J. Sel. Areas Comm.8(7), 1369–1376 (1990). [CrossRef]
  8. R. M. De Ridder and S. K. Korotky, “Feedforward compensation of integrated optic modulator distortion,” in Optical Fiber Communication Conference and Exposition, Technical Digest (CD) (Optical Society of America, 1990), paper WH5. http://www.opticsinfobase.org/abstract.cfm?URI=OFC-1990-WH5
  9. H. Skeie and R. V. Johnson, “Linearization of electro-optic modulators by a cascade coupling of phase modulating electrodes,” Proc. SPIE1583, 153–164 (1991). [CrossRef]
  10. J. L. Brooks, G. S. Maurer, and R. A. Becker, “Implementation and evaluation of a dual parallel linearization system for AM-SCM video transmission,” J. Lightwave Technol.11(1), 34–41 (1993). [CrossRef]
  11. Q. Lv, K. Xu, Y. Dai, Y. Li, J. Wu, and J. Lin, “I/Q intensity-demodulation analog photonic link based on polarization modulator,” Opt. Lett.36(23), 4602–4604 (2011). [CrossRef] [PubMed]
  12. T. R. Clark and M. L. Dennis, “Coherent optical phase modulation link,” IEEE Photon. Technol. Lett.19(16), 1206–1208 (2007). [CrossRef]
  13. A. Agarwal, T. Banwell, P. Toliver, and T. K. Woodward, “Predistortion compensation of nonlinearities in channelized RF photonic links using a dual-port optical modulator,” IEEE Photon. Technol. Lett.23(1), 24–26 (2011). [CrossRef]
  14. T. Banwell, A. Agarwal, P. Toliver, and T. K. Woodward, “Compensation of cross-gain modulation in filtered multi-channel optical signal processing applications,” in Optical Fiber Communication Conference and Exposition, Technical Digest (CD) (Optical Society of America, 2010), paper OWW5. http://www.opticsinfobase.org/abstract.cfm?URI=OFC-2010-OWW5
  15. J. D. Bull, N. A. F. Jaeger, H. Kato, M. Fairburn, A. Reid, and P. Ghanipour, “40-GHz electro-optic polarization modulator for fiber optic communications systems,” Proc. SPIE5577, 133–143 (2004). [CrossRef]

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