1 April 2007 Electro-optic tunable bandpass filter based on long-period-grating-assisted asymmetric waveguide coupling
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Abstract
We present a new electro-optic (EO) tunable bandpass filter design based on a long-period-grating-assisted asymmetric waveguide coupling mechanism. A narrow passband width of <0.2 nm and a large wavelength tuning range exceeding 30 nm can be obtained at a low driving voltage of ~16 V. This type of EO tunable filter would form key building blocks in dynamic wavelength division multiplexing (WDM) optical networks.
Lu, Li, Samarth, and Zheng: Electro-optic tunable bandpass filter based on long-period-grating-assisted asymmetric waveguide coupling

Dense wavelength division multiplexing (WDM) is becoming a leading technology in fiber-optic networks.1, 2, 3 Dynamic WDM technologies with reconfigurable channels, bandwidth, and network topologies are expected to support aggregate bandwidth and low latency requirements of both civilian and military applications such as Internet access, high-quality videoconferencing, and information acquiring and sharing in aerospace. Tunable optical filters are among the key devices in realizing the dynamic WDM networks due to their capability of providing various dynamic functions such as wavelength tunable receivers, optical channel/wavelength selections, and reconfigurations.3, 4 To achieve these functionalities, general requirements for tunable optical filters include a large wavelength tuning range covering all WDM channels, low insertion loss, narrow passband, low polarization-dependent loss (PDL), and low channel cross talk. Existing electro-optic (EO) tunable optical filter technologies include fiber Fabry-Perot (F-P) interferometers, arrayed waveguide gratings (AWG), liquid crystal F-P interferometers, Mach-Zehnder interferometers, acousto-optic filters, and fiber Bragg gratings (FBG).4, 5, 6, 7, 8, 9 While these optical filters have been employed in various optical networks, however, the major limitation is the small wavelength tuning range of a few nm, mainly due to the small EO coefficients (<90pmV) of existing materials, such as LiNbO3 and EO polymers.3, 10, 11 The small wavelength tuning range makes these devices unsuitable for broadband channel selections and reconfigurations across the whole WDM wavelength range (for example, C-band, 1530nm to 1565nm ). In this paper, we present a new EO filter structure based on a long-period-grating-assisted asymmetric waveguide coupling mechanism. An ultra-large wavelength tuning range exceeding 30nm in the C-band ( 1530nm to 1565nm ) and a narrow passband of <0.2nm can be achieved at a low driving voltage 16V with a low channel cross talk of 25dB .

The cross section and top views of the EO tunable filter are shown in Figs. 1a and 1b, respectively. The EO tunable filter consists of an input waveguide, an output waveguide, and a pair of coplanar EO tuning electrodes on top of the input waveguide. The input waveguide and the electrodes are separated by an SiO2 cladding layer. The waveguides are Ti-diffused LiNbO3 waveguides on an X-cut LiNbO3 substrate. The input and output waveguides are asymmetric so that the coupling of optical signals between the two waveguides are phase-mismatched. The input waveguide has a long-period grating to generate an addition k vector to provide phase-matched coupling for a certain wavelength. The phase-matching condition for optical coupling can be written as12, 13:

1

2πλ0neff,in2πΛ=2πλ0neff,out,
where λ0 is the specific wavelength that meets the phase-match condition, Λ is the period of the grating, and neff,in and neff,out are the effective indices of the input and output waveguide, respectively. The grating period Λ is designed to be 10μm , which is significantly larger than that of conventional quarter-wavelength Bragg gratings. From Eq. 1, the phase-matched wavelength λ0 can be expressed as:

2

λ0=(neff,inneff,out)Λ.

Fig. 1

Schematic structure of the EO tunable filter based on long-period-grating-assisted asymmetric coupling: (a) cross section; (b) top view.

040508_1_1.jpg

The tuning of phase-matched wavelength λ0 can thus be achieved by electro-optically changing the effective index of the input waveguide neff,in . As shown in Fig. 1a, the bias voltage would generate an electric field along the z direction. The EO-induced refractive index tuning can therefore be written as:

3

Δneff=12neff3γ33Ez,
where γ3331pmV is the EO coefficient of LiNbO3 along the z direction (as shown in Fig. 1). The wavelength tuning can thus be written as:

4

Δλ0=Δneff,inΛ=(12ne,eff,in3γ33Vd)Λ,
where V is the applied tuning voltage, and d is the effective separation of the top electrodes. The wavelength tuning ranges as a function of applied bias voltage for different grating periods are shown in Fig. 2 with parameters d=0.8μm and ne,eff=2.13 for LiNbO3 at the wavelength of 1.5μm . As shown in Fig. 2, for the grating period Λ10μm , an ultra-large wavelength tuning range of over 30nm can be obtained at a low tuning voltage of 16V . Such a large tuning range enhancement factor overcomes the low EO coefficient limit of the conventional EO materials and allows the EO tunable filter to cover the whole C-band.

Fig. 2

Wavelength tuning range of the EO tunable filter as a function driving voltage for different grating periods.

040508_1_2.jpg

The phase mismatch Δβ for the wavelength away from the central wavelength can be written as12, 13:

5

Δβ=2πλneff,in2πλneff,out2πΛ=2π(ΔλλΛ),
where Δλ=λλ0 is the wavelength detuning. The output power P of the phase-mismatched coupling can be written as:

6.

PP0=κ2κ2+(Δβ2)2sin2[κ2+(Δβ2)2]12L,
=κ2κ2+(πΔλλΛ)2sin2[κ2+(πΔλλΛ)2]12L,
where L is the length of the asymmetric coupling, and P and P0 are the output powers for the phase-mismatched and phase-matched coupling, respectively. Figure 3 shows the passband profiles of two adjacent 0.8-nm-spaced WDM channels with parameters κ=0.15cm1 , Λ=10μm , and L=7.5cm . As illustrated in Fig. 3, the filter shows a narrow passband of 0.2nm . The first sidelobes are located 0.15nm from the main peak, with a sidelobe suppression ratio (SLSR) of 10.5dB . For the sidelobes that are 0.8nm from the main peak, a high SLSR of > 25dB can be obtained. Since ITU-Grid WDM channels have 0.8-nm-wavelength spacing, a low cross talk of <25dB can be expected. Higher SLSR can be obtained by reducing the grating period Λ , which would increase the tuning voltage accordingly. The total insertion of the device consists of the coupling loss ( 0.5dB from Fig. 3) and the waveguide loss, which typically ranges from 0.4dBcm to 1.0dBcm at the wavelength of 1.5μm .14 The total device insertion loss is therefore less than 8dB .

Fig. 3

Simulation results of the passband profile and cross talk of the EO tunable filter.

040508_1_3.jpg

In conclusion, we present an EO tunable filter structure based on long-period-grating-assisted asymmetric waveguide coupling. This type of EO tunable filter overcomes the low EO coefficient limitation of convention materials and offers a large wavelength tuning range exceeding 30nm at a low driving voltage 16V . It is expected to enable fast wavelength selection, communication channel reconfiguration, and packet- or cell-level switching for highly dynamic WDM optical networks.

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Xuejun Lu, Miao Li, Rohit Samarth, Lei Zheng, "Electro-optic tunable bandpass filter based on long-period-grating-assisted asymmetric waveguide coupling," Optical Engineering 46(4), 040508 (1 April 2007). https://doi.org/10.1117/1.2721420
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