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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 24 — Nov. 21, 2011
  • pp: 24712–24717
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Instantaneous microwave frequency measurement using optical carrier suppression based DC power monitoring

Songnian Fu, Ming Tang, and Perry Shum  »View Author Affiliations


Optics Express, Vol. 19, Issue 24, pp. 24712-24717 (2011)
http://dx.doi.org/10.1364/OE.19.024712


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Abstract

A novel photonic-assisted technique for instantaneous microwave frequency measurement is proposed using two cascaded Mach-Zehnder modulators (MZMs) biased at the transmission null point. Then, the microwave frequency can be estimated by monitoring direct current (DC) optical power. Moreover, the measurement range and the measurement resolution can be optimized by setting the time delay between optical and electrical link and optical dispersion, respectively. The approach is theoretically investigated and experimentally verified with a measurement range of 8 GHz and a measurement error of less than ± 0.15 GHz.

© 2011 OSA

1. Introduction

It is in strong demand that an electronic warfare (EW) system is capable of instantaneous frequency measurement (IFM) for fast threat warning and communications interception. A typical IFM system can identify the dominant frequency of an unknown microwave signal in nearly real time, which leads to high possibility of interception (POI). The measured frequency information is generally necessary for instructing the following countermeasure systems or matching a concentrated receiver that performs the information reception and analysis. However, the conventional electrical IFM approaches suffer from bulky implementation, high attenuation, and limited bandwidth [1

1. H. Gruciiala and A. Slowik, “The complex signals instantaneous frequency measurement using multichannel IFM systems,” in Proceedings of 15th International Conference on Microwaves, Radar and Wireless Communications, 1, 210–213 (2004).

]. Therefore, photonic technique is expected to be introduced in high-performance IFM systems thanks to its unique advantages such as wide band, potential parallel processing capability, and immunity to electromagnetic interference (EMI) [2

2. J. Capmany and D. Novak, “Microwave photonics combines two worlds,” Nat. Photonics 1(6), 319–330 (2007). [CrossRef]

]. According to their operation principles, those photonic-assisted IFM systems can generally be divided into three categories, i.e., frequency-to-time mapping, frequency-to-space mapping, and frequency-to-power mapping [3

3. J. Niu, S. Fu, K. Xu, J. Zhou, S. Aditya, J. Wu, P. Shum, and J. T. Lin, “Instantaneous microwave frequency measurement based on amplified fiber-optic recirculating delay loop and broadband incoherent light source,” J. Lightwave Technol. 29(1), 78–84 (2011). [CrossRef]

]. Recently, photonic-assisted IFM systems based on frequency-to-power mapping have been verified as a promising solution with a large frequency measurement range (>10 GHz) and a high resolution (<200 MHz), although the measurement is accurate for single carrier frequency signal only [4

4. L. V. T. Nguyen and D. B. Hunter, “A photonic technique for microwave frequency measurement,” IEEE Photon. Technol. Lett. 18(10), 1188–1190 (2006). [CrossRef]

10

10. X. H. Zou, W. Pan, B. Luo, and L. Yan, “Full-scale phase demodulation approach for photonic instantaneous frequency measurement,” Opt. Lett. 35(16), 2747–2749 (2010). [CrossRef]

]. In those techniques, microwave frequency is estimated according to a unique relationship, named as the amplitude comparison function (ACF), between the frequency of the microwave signal and the ratio of two dispersion-induced RF power-fading functions. Thus, high-speed photodetectors (PDs) are commonly used to cover the whole measurement range. A novel method has been proposed to relax the setup complexity by monitoring the direct-current (DC) power [11

11. N. Sarkhosh, H. Emami, L. Bui, and A. Mitchell, “Reduced cost photonic instantaneous frequency measurement system,” IEEE Photon. Technol. Lett. 20(18), 1521–1523 (2008). [CrossRef]

]. However, the calibration is time-consumed and the measurement range is limited to 3 GHz only. Further simple schemes have been reported by monitoring the optical power directly [12

12. H. Chi, X. Zou, and J. Yao, “An approach to the measurement of microwave frequency based on optical power monitoring,” IEEE Photon. Technol. Lett. 20(14), 1249–1251 (2008). [CrossRef]

,13

13. X. Zou, H. Chi, and J. Yao, “Microwave frequency measurement based on optical power monitoring using a complementary optical filter pair,” IEEE Trans. Microw. Theory Tech. 57(2), 505–511 (2009). [CrossRef]

]. A measurement range of 2-24 GHz is experimentally obtained with a measurement error of less than ±0.2 GHz, in case the stability of complementary optical filter pair using polarization-maintaining fiber (PMF) is well solved. In this paper, a novel photonic-assisted IFM approach is proposed based on frequency-to-optical power mapping. A proof-of concept experiment is performed to measure frequencies from 0.5 to 8 GHz with a measurement error less than ±0.15 GHz. Since the microwave frequency can be estimated by simply monitoring the DC optical power, the proposed approach is easier to implement by using low-frequency components at lower cost. Our proposed approach also features independent adjustment of the measurement range and the measurement resolution.

2. Operation Principle

The schematic setup of our proposed IFM approach is shown in Fig. 1
Fig. 1 Schematic setup for the proposed IFM system.
. The key observation is that a tunable optical delay line (TODL) is inserted between the two cascaded MZMs both of which are biased at the minimum transmission point (i.e. both MZMs are operated under carrier-suppressed mode). A laser diode (LD) provides the required optical carrier. The input unknown microwave signal is split by a 3-dB microwave power divider into two portions, one of which is delayed by a fixed microwave delay line (FMDL). The two portions of microwave signal are then applied onto the cascaded MZMs, respectively. Finally, the output optical signal from MZM2 is monitored by an optical power meter. The electric field of the optical carrier emitted from the LD with power P0 and angular frequency ω0 can be expressed as E(t)=P0ejω0t. Likewise, the input microwave signal with amplitude V and angular frequency Ω can be denoted as v(t)=Vcos(Ωt). Thus the microwave signals which are applied onto the two cascaded MZMs are respectively given by
v1(t)=V2cos(Ωt)
(1)
v2(t)=αV2cos[Ω(tτ1)]
(2)
where α and τ1 are the attenuation and the induced time delay of the FMDL, respectively. Since MZM1 is biased at the minimum transmission point, the output of MZM1 is
E1(t)=L1E(t)sin[πv1(t)2Vπ1]=L1E(t)sin[β1cos(Ωt)]
(3)
where β1=πV/(22Vπ1), L1, and Vπ1 are the modulation index, insertion loss, and half-wave voltage of MZM1, respectively. Assuming that the total optical time delay including the TODL between the two MZMs is τ2, we can obtain the incident signal of MZM2 by E1(tτ2). Accordingly, the output of MZM2 which is also biased at the minimum transmission point can be described as
E2(t)=L2E1(tτ2)sin[πv2(t)2Vπ2]=L2E1(tτ2)sin{β2cos[Ω(tτ1)]}
(4)
where β2=παV/(22Vπ2),L2, and Vπ2 are the modulation index, insertion loss, and half-wave voltage of MZM2, respectively. Supposing small signal modulation at both MZMs (i.e. β1<<1andβ2<<1), Eq. (4) can be approximated to

E2(t)=L1L2P0ejω0(tτ2)sin{β1cos[Ω(tτ2)]}sin{β2cos[Ω(tτ1)]}L1L2P0J1(β1)J1(β2)×{ej(ω0+2Ω)tjΩ(τ1+τ2)jω0τ2+ej(ω02Ω)t+jΩ(τ1+τ2)jω0τ2+2cos[Ω(τ2τ1)]ejω0(tτ2)}
(5)

It can be clearly seen from Eq. (5) that the output of MZM2 approximately contains three dominant frequency components which are located at ω0±2Ω and ω0, respectively. Finally, we can obtain the monitored optical power by an optical power meter as follows.
Pout=2L1L2P0J12(β1)J12(β2)[2+cos(4πfΔτ)]
(6)
where Δτ=|τ2τ1|, and f=Ω/2π. It is evident that the monitored optical power varies with the input microwave frequency by Eq. (6).

3. Experimental Setup

Figure 2
Fig. 2 Experimental setup of the proposed IFM approach.
shows a typical experimental setup for the proposed photonic-assisted IFM approach. Two distributed feedback (DFB) laser diodes (LDs) are used to obtain two mapping functions at 1550 nm and 1551nm, respectively. After combination by a wavelength-division multiplexer (WDM), the continuous wave (CW) lights are modulated by a LiNbO3 Mach-Zehnder modulator (MZM 1, Avanex SD-10) biased at the transmission null point with a portion of unknown microwave signals from a vector network analyzer (VNA, Anritsu 3769C). An electrical bias controller is used to lock the operation points of MZM and ensure a stable operation over time and environmental conditions. Then, the modulated carriers are sent to a wavelength selective switch (Finisar WaveShaper [14

14. M. A. F. Roelens, S. Frisken, J. A. Bolger, D. Abakoumov, G. Baxter, S. Poole, and B. J. Eggleton, “Dispersion trimming in a reconfigurable wavelength selective switch,” J. Lightwave Technol. 26(1), 73–78 (2008). [CrossRef]

] 4000S), whose dispersion-bandwidth product is approximately 80 ps. This means that dispersions of up to ± 80 ps/nm can be comfortably generated over an optical bandwidth of 1 nm. As a result, the definition of τ2 in Eq. (6) can be modified by adding the delay of WaveShaper. Moreover, the WaveShaper will introduce a certain differential group delay between two optical wavelengths due to its dispersive characteristics. Then, the delayed carriers is re-modulated by MZM2 (Avanex SD-10) biased at the transmission null point with another portion of unknown microwave signals after properly delayed using a length of co-axial cable. The twice-modulated signals at different wavelengths are de-multiplexed by another WDM and monitored by optical power meters (Anritsu MA9331A optical sensor head plus MU931001A mainframe), respectively. The optical sensor head has a 3dB bandwidth of 100 KHz, permitting us monitor DC optical power easily. The measured results are recorded for subsequent data processing. Finally, the microwave frequency is estimated based on a look-up table established in the calibration stage.

As mentioned above, the WaveShaper introduces a wavelength-dependent group delay. It will accordingly lead to a delay difference between Δτ1 and Δτ2 in Eq. (6), where Δτ1 and Δτ2 corresponding to the two wavelengths, respectively. We assume that Δτ2>Δτ1, and define ΔΓ=Δτ2Δτ1 here. On the other hand, the difference between the modulation characteristics of an MZM at different operating wavelengths can be neglected due to the small wavelength spacing of 1 nm. The optical power from the two LDs is experimentally tuned to be identical 0 dBm. Therefore, the optical powers recorded by the optical power meter, at the two wavelengths, can be expressed as,

Pout1=2L1L2P0J12(β1)J12(β2){2+cos[4πf(Δτ2ΔΓ)]}
(7)
Pout2=2L1L2P0J12(β1)J12(β2)[2+cos(4πfΔτ2)]
(8)

The ratio of those measured powers, referred to the commonly defined ACF, can be derived

ACF=Pout2Pout1=2+cos(4πfΔτ2)2+cos[4πf(Δτ2ΔΓ)]
(9)

4. Experimental results

Based on the experimental setup shown in Fig. 2, microwave frequency measurement is carried out in order to verify the proposed approach. First, by properly choose the delay between the optical and electrical link, Δτ2 is set to be 30 ps. Meanwhile, we properly set WaveShaper to introduce a dispersion of 20 ps/nm. Considering that the wavelength spacing between two LDs is 1 nm, ΔΓ=20psis satisfied in our IFM system. According to Eq. (9), we store a look-up table with a frequency resolution of 1 MHz. Then, the microwave frequency can be estimated from such look-up table. Figure 4 (a)
Fig. 4 Measurement results. (a) Measured ACF and theoretical ACF against frequency. (b) Estimated frequency versus the input microwave frequency.
summarizes the measured ACF, which agree well with the calculated ACF. The ACF monotonically decreases as long as the frequency does not exceed the notch point, which is located at around 8 GHz. Therefore, the measurement range of our proposed IFM system is limited to 8 GHz, based on the measured ACF. Next, the estimated frequencies with respect to the real input frequencies are shown in Fig. 4 (b). It is worth noting that the measured results are in a good agreement with the real values over the frequency range of 0.5-8 GHz. Meanwhile, the measurement errors are within ± 0.15 GHz over the whole frequency measurement range, as shown in Fig. 5
Fig. 5 Measurement error as a function of input frequency
.

5. Conclusion

A novel photonic-assisted approach for microwave frequency measurement has been proposed, theoretically investigated, and experimentally verified. Besides the benefits from the assistance of photonic technique, our approach possessed its own advantages such as low-cost by replacing commonly used photodetectors (PDs) with optical power meters, independent optimization of measurement range and measurement resolution, provided the bias drift of MZM and the power fluctuation of laser source is well solved. The proposed IFM system was experimentally verified with the measurement error less than ±0.15 GHz over a frequency range of 0.5-8 GHz. The approach is oriented for future EW systems but it can be extended to any applications which require fast microwave frequency measurements over a broad frequency range.

References and links

1.

H. Gruciiala and A. Slowik, “The complex signals instantaneous frequency measurement using multichannel IFM systems,” in Proceedings of 15th International Conference on Microwaves, Radar and Wireless Communications, 1, 210–213 (2004).

2.

J. Capmany and D. Novak, “Microwave photonics combines two worlds,” Nat. Photonics 1(6), 319–330 (2007). [CrossRef]

3.

J. Niu, S. Fu, K. Xu, J. Zhou, S. Aditya, J. Wu, P. Shum, and J. T. Lin, “Instantaneous microwave frequency measurement based on amplified fiber-optic recirculating delay loop and broadband incoherent light source,” J. Lightwave Technol. 29(1), 78–84 (2011). [CrossRef]

4.

L. V. T. Nguyen and D. B. Hunter, “A photonic technique for microwave frequency measurement,” IEEE Photon. Technol. Lett. 18(10), 1188–1190 (2006). [CrossRef]

5.

X. H. Zou and J. Yao, “An optical approach to microwave frequency measurement with adjustable measurement range and resolution,” IEEE Photon. Technol. Lett. 20(23), 1989–1991 (2008). [CrossRef]

6.

J. Li, S. Fu, K. Xu, J. Q. Zhou, P. Shum, J. Wu, and J. Lin, “Photonic-assisted microwave frequency measurement with higher resolution and tunable range,” Opt. Lett. 34(6), 743–745 (2009). [CrossRef] [PubMed]

7.

L. A. Bui, M. D. Pelusi, T. D. Vo, N. Sarkhosh, H. Emami, B. J. Eggleton, and A. Mitchell, “Instantaneous frequency measurement system using optical mixing in highly nonlinear fiber,” Opt. Express 17(25), 22983–22991 (2009). [CrossRef] [PubMed]

8.

M. V. Drummond, P. Monteiro, and R. N. Nogueira, “Photonic RF instantaneous frequency measurement system by means of a polarizatio-ndomain interferometer,” Opt. Express 17(7), 5433–5438 (2009). [CrossRef] [PubMed]

9.

J. Zhou, S. Aditya, P. Shum, and J. Yao, “Instantaneous Microwave Frequency Measurement Using a Photonic Microwave Filter with an Infinite Impulse Response,” IEEE Photon. Technol. Lett. 22(10), 682–684 (2010). [CrossRef]

10.

X. H. Zou, W. Pan, B. Luo, and L. Yan, “Full-scale phase demodulation approach for photonic instantaneous frequency measurement,” Opt. Lett. 35(16), 2747–2749 (2010). [CrossRef]

11.

N. Sarkhosh, H. Emami, L. Bui, and A. Mitchell, “Reduced cost photonic instantaneous frequency measurement system,” IEEE Photon. Technol. Lett. 20(18), 1521–1523 (2008). [CrossRef]

12.

H. Chi, X. Zou, and J. Yao, “An approach to the measurement of microwave frequency based on optical power monitoring,” IEEE Photon. Technol. Lett. 20(14), 1249–1251 (2008). [CrossRef]

13.

X. Zou, H. Chi, and J. Yao, “Microwave frequency measurement based on optical power monitoring using a complementary optical filter pair,” IEEE Trans. Microw. Theory Tech. 57(2), 505–511 (2009). [CrossRef]

14.

M. A. F. Roelens, S. Frisken, J. A. Bolger, D. Abakoumov, G. Baxter, S. Poole, and B. J. Eggleton, “Dispersion trimming in a reconfigurable wavelength selective switch,” J. Lightwave Technol. 26(1), 73–78 (2008). [CrossRef]

OCIS Codes
(070.6020) Fourier optics and signal processing : Continuous optical signal processing
(350.4010) Other areas of optics : Microwaves
(060.5625) Fiber optics and optical communications : Radio frequency photonics

ToC Category:
Instrumentation, Measurement, and Metrology

History
Original Manuscript: August 19, 2011
Revised Manuscript: October 7, 2011
Manuscript Accepted: October 16, 2011
Published: November 17, 2011

Citation
Songnian Fu, Ming Tang, and Perry Shum, "Instantaneous microwave frequency measurement using optical carrier suppression based DC power monitoring," Opt. Express 19, 24712-24717 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-24-24712


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References

  1. H. Gruciiala and A. Slowik, “The complex signals instantaneous frequency measurement using multichannel IFM systems,” in Proceedings of 15th International Conference on Microwaves, Radar and Wireless Communications, 1, 210–213 (2004).
  2. J. Capmany and D. Novak, “Microwave photonics combines two worlds,” Nat. Photonics1(6), 319–330 (2007). [CrossRef]
  3. J. Niu, S. Fu, K. Xu, J. Zhou, S. Aditya, J. Wu, P. Shum, and J. T. Lin, “Instantaneous microwave frequency measurement based on amplified fiber-optic recirculating delay loop and broadband incoherent light source,” J. Lightwave Technol.29(1), 78–84 (2011). [CrossRef]
  4. L. V. T. Nguyen and D. B. Hunter, “A photonic technique for microwave frequency measurement,” IEEE Photon. Technol. Lett.18(10), 1188–1190 (2006). [CrossRef]
  5. X. H. Zou and J. Yao, “An optical approach to microwave frequency measurement with adjustable measurement range and resolution,” IEEE Photon. Technol. Lett.20(23), 1989–1991 (2008). [CrossRef]
  6. J. Li, S. Fu, K. Xu, J. Q. Zhou, P. Shum, J. Wu, and J. Lin, “Photonic-assisted microwave frequency measurement with higher resolution and tunable range,” Opt. Lett.34(6), 743–745 (2009). [CrossRef] [PubMed]
  7. L. A. Bui, M. D. Pelusi, T. D. Vo, N. Sarkhosh, H. Emami, B. J. Eggleton, and A. Mitchell, “Instantaneous frequency measurement system using optical mixing in highly nonlinear fiber,” Opt. Express17(25), 22983–22991 (2009). [CrossRef] [PubMed]
  8. M. V. Drummond, P. Monteiro, and R. N. Nogueira, “Photonic RF instantaneous frequency measurement system by means of a polarizatio-ndomain interferometer,” Opt. Express17(7), 5433–5438 (2009). [CrossRef] [PubMed]
  9. J. Zhou, S. Aditya, P. Shum, and J. Yao, “Instantaneous Microwave Frequency Measurement Using a Photonic Microwave Filter with an Infinite Impulse Response,” IEEE Photon. Technol. Lett.22(10), 682–684 (2010). [CrossRef]
  10. X. H. Zou, W. Pan, B. Luo, and L. Yan, “Full-scale phase demodulation approach for photonic instantaneous frequency measurement,” Opt. Lett.35(16), 2747–2749 (2010). [CrossRef]
  11. N. Sarkhosh, H. Emami, L. Bui, and A. Mitchell, “Reduced cost photonic instantaneous frequency measurement system,” IEEE Photon. Technol. Lett.20(18), 1521–1523 (2008). [CrossRef]
  12. H. Chi, X. Zou, and J. Yao, “An approach to the measurement of microwave frequency based on optical power monitoring,” IEEE Photon. Technol. Lett.20(14), 1249–1251 (2008). [CrossRef]
  13. X. Zou, H. Chi, and J. Yao, “Microwave frequency measurement based on optical power monitoring using a complementary optical filter pair,” IEEE Trans. Microw. Theory Tech.57(2), 505–511 (2009). [CrossRef]
  14. M. A. F. Roelens, S. Frisken, J. A. Bolger, D. Abakoumov, G. Baxter, S. Poole, and B. J. Eggleton, “Dispersion trimming in a reconfigurable wavelength selective switch,” J. Lightwave Technol.26(1), 73–78 (2008). [CrossRef]

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