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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 14, Iss. 17 — Aug. 21, 2006
  • pp: 7960–7965
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Tunable microwave photonic filter free from baseband and carrier suppression effect not requiring single sideband modulation using a Mach-Zenhder configuration

J. Mora, A. Ortigosa-Blanch, D. Pastor, and J. Capmany  »View Author Affiliations


Optics Express, Vol. 14, Issue 17, pp. 7960-7965 (2006)
http://dx.doi.org/10.1364/OE.14.007960


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Abstract

We present a full theoretical and experimental analysis of a novel all-optical microwave photonic filter combining a mode-locked fiber laser and a Mach-Zenhder structure in cascade to a 2×1 electro-optic modulator. The filter is free from the carrier suppression effect and thus it does not require single sideband modulation. Positive and negative coefficients are obtained inherently in the system and the tunability is achieved by controlling the optical path difference of the Mach-Zenhder structure.

© 2006 Optical Society of America

1. Introduction

Microwave Photonics is an area under intense research due to the various advantages of processing RF signals in the optical domain instead of using their electric counterparts. Optical fibers show low loss, high bandwidth, immunity to electromagnetic interference, tunability and reconfigurability [1–5

1. J. Capmany, B. Ortega, D. Pastor, and S. Sales, “Discrete-time optical Processing of microwave signals,” J. Lightwave Technol. 23, 702–723 (2005). [CrossRef]

]. For the last few years, different approaches have been proposed to implement high performance microwave photonic filters. Nevertheless, most of these filters still suffer from several limitations as pointed out in [1–2

1. J. Capmany, B. Ortega, D. Pastor, and S. Sales, “Discrete-time optical Processing of microwave signals,” J. Lightwave Technol. 23, 702–723 (2005). [CrossRef]

]. Carrier suppression effect (CSE) or Dispersion Fading is one example of the limitations shown by these filters reducing its operating frequency range due to the delay line dispersion. CSE can be overcome by using single sideband modulation but then more complex and costly modulation circuitry is needed. Other important issues not easy to achieve with incoherent Microwave Photonics structures are proper base-band rejection and easy tuning techniques for the RF bands filtered.

In this paper, we propose a novel filter structure. The tunable microwave photonic filter exhibits positive and negative coefficients based on the combination of an active mode-locked fiber laser that acts the optical multiwavelength source, a fiber Mach Zehnder (MZ) interferometer cascaded to a 2×1 integrated MZ electro optic modulator (EOM) and a dispersive delay line. This configuration shows some unique features as compared to other structures reported previously. Namely it is free from CSE without requiring single sideband modulation and can provide truly bandpass operation by suppressing the baseband response. In addition, it provides other advantages arising from using the active mode-locked fiber laser as anticipated in [6

6. A. Ortigosa-Blanch, J. Mora, J. Capmany, B. Ortega, and D. Pastor, “Tunable radio-frequency photonic filter based on an actively mode-locked fiber laser,” Opt. Lett. 31, 709–711 (2006). [CrossRef] [PubMed]

]: All source modes at the output of the laser have equal polarization and thus the alignment with the EOM is easier and, the output spectrum from the source is itself Gaussian apodized and therefore sidelobe suppression in the filter response is guaranteed.

2. Filter description

The topology of the filter is shown in Fig. 1. An unbalanced MZ structure is achieved by using a 2×2 coupler with a coupling factor c which is connected to a 2×1 electro-optic modulator. The optical path length mismatch for the MZ structure is Δ1 or Δτ = Δ1/vg as it will be used later. The input coupling constant of the EOM device is given by the factor a2 and the output the Y junction is characterised by a coupling constant b 2.

Fig. 1. Scheme of the RF photonic filter.

The optical samples are generated by an actively mode-locked fiber laser which has a fixed wavelength separation between adjacent optical taps given by Δλ (or Δω) as shown in (1).

E(t)=ejωot·nEn·ejnΔωtE˜(ω)=nEn·δ(ωωonΔω)
(1)

HRF(Ω)=12(12c2)·H1(Ω)·Ho(Ω)12(c1c2)·E˜(ω)2H2(Ω,ω)ejβ2LF(ωωo)Ω·
(2)

where

Ho(ω)=E˜(ω)2ejβ2LF(ωωo)Ω·
(3)

is the transfer function of the original filter and the terms H1(Ω) and H2(Ω,ω),

H1(Ω)=cos(Δα)cos(β2LFΩ22)
(4)
H2(Ω,ω)=((1+sin(Δα))cos(β2LFΩ22ΔΦ)(1sin(Δα))cos(β2LFΩ22+ΔΦ))
(5)

depend on the EOM biasing voltage Vbias through the parameter Δα = π(Vbias/Vπ) and the optical differential phase ΔΦ between the arms in the first ΔΦ, ωΔτ+θo, where θo takes into account the drift of the MZ structure.

It is clearly seen in Eq. (2) how the system shows different behaviour depending on the biasing voltage and the coupling factor c. First, we will consider the case when c = 0 (or c = 1), i.e., the input optical signal E(t) is launched either into the upper or the lower arm of the 2×1 EOM. In this case, there is a null contribution of the term H2(Ω,ω) and we obtain a typical transfer function when using a the 2×1 EOM [7

7. J. Capmany, D. Pastor, B. Ortega, J. Mora, A. Martinez, L. Pierno, and M. Varasi, “Theoretical Model and Experimental Verification of 2x1 Mach-Zehnder EOM with Dispersive Optical Fiber link Propagation,” International Topical Meeting on Microwave Photonic (Seoul, Korea, 2005), 145–148. [CrossRef]

]:

HRF(Ω)=12cos(Δα)·cos(β2LFΩ22)·Ho(Ω)
(6)

It is worth noting that a biasing voltage Vbias = 0 is required for operating into the quadrature point for the 2×1 EOM (Δα =0), providing the maximum amplitude response [7

7. J. Capmany, D. Pastor, B. Ortega, J. Mora, A. Martinez, L. Pierno, and M. Varasi, “Theoretical Model and Experimental Verification of 2x1 Mach-Zehnder EOM with Dispersive Optical Fiber link Propagation,” International Topical Meeting on Microwave Photonic (Seoul, Korea, 2005), 145–148. [CrossRef]

]. It is also shown how the CSE effect is taken into account in the second term and thus not avoided in this conventional configuration.

Secondly, we will consider the case of Vbias =Vπ /2. In that case the term H1(Ω) is null for any value of c and the transfer function of the filter becomes

HRF(Ω)=(c1c2)E˜(ω)2·cos(β2LFΩ22ΔΦ)·ejβ2LF(ωωo)Ω
(7)

From Eq. (7) it can be clearly seen how the CSE term appears together with the differential phase ΔΦn (ΔΦn = nΔωΔτ+θ0) of the MZ in the new filter topology. Then, two important effects can be withdrawn: Firstly, the cosine term is clearly a tap function weighted by the |En| terms and featuring sign polarity for the N wavelengths n = [1⋯N] given by the differential phase term ΔΦn. Hence, positive and negative optical samples are achieved along the cosine depending on the optical difference path Δl (Δτ) and the separation Δω between adjacent wavelengths. Then, the tunability of the RF filter is not only controlled through Δl, but also with the amount of dispersion β2LF. Secondly, the optical differential phase in the MZ has translated into the RF domain as a RF frequency displacement of the standard CSE. This can be seen rewriting Eq. (7) as

HRF(Ω)=(c1c2)[ejβ2LFΩ22ejθ0Ho(ΩΩo)+ejβ2LFΩ22ejθ0Ho(ΩΩo)]
(8)

We can see how the transfer function shows two terms. Both terms are identical to the initial RF filter Ho(Φ) but shifted to frequencies ± Φo, which is given by

Ωo=2π·fo=ΔW·Δτ·FSR0=Δτβ2LF
(9)

where the Free Spectral Range of the original filter is defined as FSRo =1/(β2LF·Δω). In the present configuration, tuning is achieved by changing the optical path difference (Δτ) and the two lobes shift symmetrically from the original filter bands. Then, the tuning range of the RF filter covers a frequency bandwidth up to FSRo for the case when Δτ is a integer multiple of the laser repetition rate (2π/Δω), crossing along the case of half repetition rate where the tuning is FSRo/2 It is also clear that the RF phases of both bands, consisting on the drift θo and the CSE term (1/2β2LFΦ2), only appear in the amplitude response of the filter (|HRF(Φ)|2) where there is overlapping i.e. (FSRo and FSRo/2),Hence, this configuration is free from CSE within its useful tuning range. Similarly, although our configuration is unstable in terms of the optical phase as any other containing a MZ structure, we have experimentally observed that there is no need for thermal stabilization as a change in the optical phase does not affect the amplitude response of the filter.

3. Experimental Analysis

The transversal filter was implemented using an actively mode-locked fiber laser driven by an external 10 GHz RF signal. The mode-locked fiber laser provides a multiwavelength signal that is used to obtain around 40 different carriers. Inset of Fig. 1 shows the optical signal launched into the MZ structure which correspond to a distribution of discrete optical taps with an optical frequency separation equal to the repetition rate of the laser (~ 0.08 nm for this case). The central wavelength of the multiwavelength is λ o = 1550 nm. This multi-tap source is fed to a 50/50 coupler where each one of its output arms is connected to one of the inputs of the 2×1 EOM where the optical carriers are amplitude modulated by a RF signal of frequency Φ=2πf which is generated with the LCA. The 2·1 EOM (Vπ= 14 V) was driven in Push-Pull configuration. Finally, a SMF-28 fiber of length LF = 50 km was used as the dispersive element in the filter, (15.8 ps/km nm). Figure 2 shows the experimental results of the transfer function of the filter for different configurations. Figure 2(a) shows the transfer function when c = 1, i.e., when the multi-wavelength source is launched just into one of the arms of the 2x1 EOM. The 2×1 EOM acts as a conventional modulator and the filter shows a FSRo = 15.78 GHz [2

2. J. Capmany, D. Pastor, and B. Ortega, “Microwave Signal Processing Using Optics,” Optical Fiber Conference (Annaheim, USA, 2005), Tutorial Paper OThB1.

]. Figure 2(b) shows the case for an EOM biasing voltage of 7 V. As shown in Eq. (8), the RF filter shows no baseband component and splits the original pass-band lobe at FSR0 into two side lobes when there is an optical path difference in the MZ structure. Figure 2(b) depicts the case when the optical path difference Δ1 corresponds to a delay time of 22.6 ps. For higher delays, the two lobes, Ho(Φ-Φo) and Ho(Φ+Φo), approach each other until they superimpose (measured for a delay of 50 ps). Then, when Δτ > 50 ps, the lobes cross each other up to reach the baseband and FSRo positions (as Fig. 2(a)), for a time delay of 100ps. This result is in agreement with the 10GHz repetition rate of the mode-locked laser.

Fig. 2. Transfer function of (a) the original filter and (b) for a delay time in the MZ of 22.6 ps.

Fig. 3. Frequency fo and bandwidths Δf of RF filters versus the optical delay time Δτ. Theoretical calculation (solid line) and experimental results (filled squares).
Fig. 4. Amplitude Response of (a) the base band and the RF band-pass for fo = 4.25 GHz versus the EOM biasing voltage and (b) of the RF bands for different values of fo (circles) and conventional CSE (dashed line). Inset: Detail of the transfer function of the RF filters around the first notch of CSE.

4. Conclusions

We have presented a full theoretical and experimental analysis of a novel all-optical microwave photonic filter that combines a mode-locked fiber laser, which acts as a multi-tap wavelength source and a MZ structure in cascade to a 2×1 EOM. We have theoretically presented and experimentally demonstrated several interesting properties of this novel configuration: the filter is free from the carrier suppression effect and thus it does not require single sideband modulation. It also shows positive and negative coefficients due to its own MZ structure and the tunability can be achieved by controlling the optical path difference. Other MZ configurations are currently under investigation. The present configuration can be changed by introducing different optical filters such as fiber Bragg gratings to modify the phase relationship in the MZ cavity to configure more specific responses for the positive and negative taps.

Acknowledgments

Authors acknowledge financial support from IST-2001–37435 LABELS, TEC2004–04754–01/TCM SODICO and TEC2005–08298–C02–01 ADIRA. A. Ortigosa-Blanch acknowledges financial support from the Spanish Government through the “Juan de la Cierva” Program.

References and links

1.

J. Capmany, B. Ortega, D. Pastor, and S. Sales, “Discrete-time optical Processing of microwave signals,” J. Lightwave Technol. 23, 702–723 (2005). [CrossRef]

2.

J. Capmany, D. Pastor, and B. Ortega, “Microwave Signal Processing Using Optics,” Optical Fiber Conference (Annaheim, USA, 2005), Tutorial Paper OThB1.

3.

A. Seeds, “Microwave photonics,” IEEE MTT 50, 877–887 (2002). [CrossRef]

4.

D. B. Hunter and R. A. Minasian, “Microwave optical filters using in-fiber Bragg grating arrays,” IEEE Microw. Guided Wave Lett. 6, 103–105 (1996). [CrossRef]

5.

M.E. Frankel and R.D. Esman, “Fiber-optic tunable microwave transversal filter,” IEEE Photonics Technnol. Lett. 7, 191–193 (1995). [CrossRef]

6.

A. Ortigosa-Blanch, J. Mora, J. Capmany, B. Ortega, and D. Pastor, “Tunable radio-frequency photonic filter based on an actively mode-locked fiber laser,” Opt. Lett. 31, 709–711 (2006). [CrossRef] [PubMed]

7.

J. Capmany, D. Pastor, B. Ortega, J. Mora, A. Martinez, L. Pierno, and M. Varasi, “Theoretical Model and Experimental Verification of 2x1 Mach-Zehnder EOM with Dispersive Optical Fiber link Propagation,” International Topical Meeting on Microwave Photonic (Seoul, Korea, 2005), 145–148. [CrossRef]

OCIS Codes
(060.0060) Fiber optics and optical communications : Fiber optics and optical communications
(230.0230) Optical devices : Optical devices
(230.1150) Optical devices : All-optical devices

ToC Category:
Optical Devices

History
Original Manuscript: June 2, 2006
Revised Manuscript: July 24, 2006
Manuscript Accepted: July 25, 2006
Published: August 21, 2006

Citation
José Mora, Arturo Ortigosa-Blanch, Daniel Pastor, and José Capmany, "Tunable microwave photonic filter free from baseband and carrier suppression effect not requiring single sideband modulation using a Mach-Zenhder configuration," Opt. Express 14, 7960-7965 (2006)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-14-17-7960


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References

  1. J. Capmany, B. Ortega, D. Pastor, and S. Sales, "Discrete-time optical processing of microwave signals," J. Lightwave Technol. 23, 702-723 (2005). [CrossRef]
  2. J. Capmany, D. Pastor, and B. Ortega, "Microwave signal processing using optics," Optical Fiber Conference (Annaheim, USA, 2005), Tutorial Paper OThB1.
  3. A. Seeds, "Microwave photonics," IEEE MTT 50, 877-887 (2002). [CrossRef]
  4. D. B. Hunter and R. A. Minasian, "Microwave optical filters using in-fiber Bragg grating arrays," IEEE Microw. Guided Wave Lett. 6, 103-105 (1996). [CrossRef]
  5. M. E. Frankel and R. D. Esman, "Fiber-optic tunable microwave transversal filter," IEEE Photonics Technnol. Lett. 7, 191-193 (1995). [CrossRef]
  6. A. Ortigosa-Blanch, J. Mora, J. Capmany, B. Ortega, and D. Pastor, "Tunable radio-frequency photonic filter based on an actively mode-locked fiber laser," Opt. Lett. 31, 709-711 (2006). [CrossRef] [PubMed]
  7. J. Capmany, D. Pastor, B. Ortega, J. Mora, A. Martinez, L. Pierno, and M. Varasi, "Theoretical model and experimental verification of 2x1 Mach-Zehnder EOM with dispersive optical fiber link propagation," International Topical Meeting on Microwave Photonic (Seoul, Korea, 2005), 145-148. [CrossRef]

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