Elsevier

Optics Communications

Volume 284, Issue 21, 1 October 2011, Pages 5144-5147
Optics Communications

Analysis of polarization-independent tunable optical comb filter by cascading MZI and phase modulating Sagnac loop

https://doi.org/10.1016/j.optcom.2011.07.028Get rights and content

Abstract

A novel scheme of tunable optical comb filter constructed by cascading a polarization-swapping MZI and a phase modulating Sagnac loop is proposed. The key features of this comb filter design are its polarization-independence and high speed in wavelength tuning. The wavelength tunability can be readily achieved by incorporating an electro-optic phase modulator into the Sagnac loop. In addition, the filter can be made amplitude adjustable as well. The filter will be desirable for multi-wavelength fiber lasers, multi-channel optical signal processing and optical WDM systems.

Highlights

► A polarization-independent ultra-fast tunable optical comb filter is proposed. ► It consists of a polarization-swapping MZI and a phase modulating Sagnac loop. ► The MZI and Sagnac loop combination ensures the filter polarization-independent. ► Introducing a phase modulator to Sagnac loop enables high-speed wavelength tuning.

Introduction

There has been considerable interest in recent years in the development of optical comb filters. Potential applications for these filters range from multi-wavelength fiber laser [1], [2], multi-channel optical signal processing and optical WDM systems. Up to date, several techniques have been developed for tuning the central wavelength of filters [3], [4]. However these tuning techniques based on electro-mechanical adjustment or heating may cause the variation of the polarization state of the light, resulting in the degradation of the spectral characteristics (e.g., decrease of the visibility, increase of the insertion loss, and so forth). In addition, the tuning speed of these methods is typically very slow and the response time is limited to ms. Two innovative schemes, one introducing a phase modulator (PM) into a Lyot birefringence fiber filter [5], [6], and another employing a semiconductor optical amplifier (SOA) [7], were proposed to achieve high speed tuning. Nevertheless these two schemes, using either polarizer [5], [6], or polarization controller [7], are input polarization dependent and will suffer from signal fluctuations when used in conventional randomly polarized transmission systems. Lee et al. demonstrated a polarization-independent tunable fiber comb filter by using both polarization-diversity Sagnac loop and rotating waveplate technique [8]. Also fiber birefringence filters based on Sagnac configuration have been proposed for an optical comb filter [9], [10], [11].

Here we propose a modified scheme of optical comb filter that is constructed by cascading a polarization-swapping MZI and a phase modulating Sagnac loop and that can achieve both high-speed tuning and polarization-independent. The polarization-swapping MZI introduces the polarization exchange between the two orthogonal polarizations by connecting the birefringent fiber with a 90° offset. The birefringent phase modulator introduces the phase shift difference between the two polarization components of lightwave signals traveling both clockwise (CW) and counter clockwise (CCW) inside the Sagnac loop. Changing the phase difference will tune the spectral characteristics. Since it is electrically tuned and no moving mechanic part is involved, it can operate at high speed that is limited by the speed of phase modulator. This device may be constructed in optical fiber or Planar Light Circuit (PLC) structures. In the case of PLC, all the components including the polarization rotators and phase modulator could be fabricated in various material systems such as InGaAs, Si, and Ti-diffused Lithium Niobate. In this letter the discussion will be mainly focus on the analysis of the structures' spectral response, rather than optical fiber or Planar Light Circuit (PLC) structures' detailed design.

Section snippets

Analysis and results

Fig. 1 shows the configuration of the comb filter built on a polarization-swapping MZI and a phase modulating Sagnac loop. It consists of one birefringent electro-optic (EO) phase modulator, two 2 × 2 high birefringent fiber directional couplers (HBF-DC), and three high birefringent fiber (HBF) sections. The thick lines inside the loop represent the HBF sections. The phase modulator is employed to change phase difference between the two polarization components of both CW and CCW lightwave signals

Discussions

With low-loss polarization-maintaining fusion splicing, we can obtain precise axis alignment for the required θ1, θ2, θ3 and θ4. Compared with the mechanically or thermally tuned comb filters reported in previously studies, our filter is electrically tuned through an EO phase modulator which can readily achieve the ultra-fast tuning, and could be more stable since there is not any mechanically rotating component. Also the time-response is only dependent on the phase modulator, and this has been

Conclusions

We proposed and analyzed a tunable optical comb filter based on a 90° offset polarization-swapping MZI and a high speed phase modulating Sagnac loop. It can achieve both polarization-independent and ultra-fast tuning. In addition, amplitude adjustment/attenuation is possible. The ultra-fast wavelength tuning was achieved by incorporating a high-speed electro-optic phase modulator into the Sagnac loop. The proposed device can be used as high-speed wavelength routers as well as tunable filter

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