Abstract

The characteristics of RF parasitic modes and the methods to suppress leakage phenomena in LiNbO3 optical modulators were studied. The dominant zero-cutoff CBCPW modes and several undesired parasitic modes were analyzed with two-dimensional FEM. The effect of parasitic modes on high frequency RF power transmission characteristics were simulated and experimented in the respects of LiNbO3 wafer thickness, the kind of material contacting the back surface of the modulator chip, the gap and width of the CPW electrodes. An appropriate RF electrode geometry, to minimize coupling efficiency between co-planar waveguide and substrate mode, is presented. Experimental results proved that the approaches made in this work are effective for broadening of modulation bandwidth.

© 2004 Optical Society of America

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References

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  1. R. A. Becker, "Traveling-wave electro-optic modulator with maximum bandwidth product," Appl. Phys. Lett. 45, 1168-1170 (1984).
    [CrossRef]
  2. G. K. Gopalakrishnan, W. K. Burns, and C. H. Bulmer, "Electrical loss mechanism in traveling wave LiNbO3 Optical Modulator," Electron. Lett. 28, 207-209 (1992).
    [CrossRef]
  3. Jajid riaziat, Reza Majidi-Ahy, and I-Juang Feng, "Propagation modes and dispersion characteristics of coplanar waveguides," IEEE Trans. Microwave Theory and Technol. 38, 245-251 (1990).
    [CrossRef]
  4. Xiang Zhang, and Tanroku Miyoshi, "Optimum design of coplanar waveguide for LiNbO3 optical modulator," IEEE Trans. Microwave Theory Technol. 43, 523-528 (1995)
    [CrossRef]
  5. Jeng-Wen Huang, and Ching-Kuang C. Tzuang, "Mode-coupling-avoidance of shielded conductor-backed coplanar waveguide using dielectric lines compensation," IEEE MTT-S Digest, 149-152 (1994).
  6. Rangaraj Madabhushi, Yukio Uematsu, Mitsuhiro Kitamura, "Wide-band Ti: LiNbO3 optical modulators with reduced microwave attenuation," ECOC 1997, 2, 29-32 (1997).

Appl. Phys. Lett. (1)

R. A. Becker, "Traveling-wave electro-optic modulator with maximum bandwidth product," Appl. Phys. Lett. 45, 1168-1170 (1984).
[CrossRef]

ECOC 1997 (1)

Rangaraj Madabhushi, Yukio Uematsu, Mitsuhiro Kitamura, "Wide-band Ti: LiNbO3 optical modulators with reduced microwave attenuation," ECOC 1997, 2, 29-32 (1997).

Electron. Lett. (1)

G. K. Gopalakrishnan, W. K. Burns, and C. H. Bulmer, "Electrical loss mechanism in traveling wave LiNbO3 Optical Modulator," Electron. Lett. 28, 207-209 (1992).
[CrossRef]

IEEE MTT-S Digest (1)

Jeng-Wen Huang, and Ching-Kuang C. Tzuang, "Mode-coupling-avoidance of shielded conductor-backed coplanar waveguide using dielectric lines compensation," IEEE MTT-S Digest, 149-152 (1994).

IEEE Trans. Microwave Theory and Technol (1)

Jajid riaziat, Reza Majidi-Ahy, and I-Juang Feng, "Propagation modes and dispersion characteristics of coplanar waveguides," IEEE Trans. Microwave Theory and Technol. 38, 245-251 (1990).
[CrossRef]

IEEE Trans. Microwave Theory Technol. (1)

Xiang Zhang, and Tanroku Miyoshi, "Optimum design of coplanar waveguide for LiNbO3 optical modulator," IEEE Trans. Microwave Theory Technol. 43, 523-528 (1995)
[CrossRef]

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Figures (11)

Fig. 1.
Fig. 1.

Schematic diagram of CPW electrodes in a LiNbO3 traveling wave optical modulator; (a) top view and (b) cross-section view in a interaction region

Fig. 2.
Fig. 2.

Physical structure for analysis of propagation characteristics in a launch section.

Fig. 3.
Fig. 3.

plot of CBCPW and parasitic modes Ez at 50GHz and z=b-0.05 mm (a=1mm, b=0.5mm, W=0.2mm, S=0.3mm, h=2mm)

Fig. 4.
Fig. 4.

Effective relative permittivity of the parasitic modes as a function of frequency (b=0.5mm, W=0.2mm, S=0.3mm, h=7mm)

Fig. 5.
Fig. 5.

S21 of the fabricated CPW electrode to show leaky modes; W=0.15 mm, S=0.3 mm, and b=0.5 mm.

Fig. 6.
Fig. 6.

Parasitic mode characteristics in multi-layered waveguide structure; (a) physical structure for analysis, (b) dispersion curve

Fig. 7.
Fig. 7.

The transmission characteristics of the CPW electrodes for the conductor backed (circled line) and the glass backed (solid line) structures; W=0.15 mm, S=0.3 mm, b=0.5 mm, and c=0.5 mm.

Fig. 8.
Fig. 8.

The overlap integral of the CBCPW and parasitic modes Ezm0, b=0.5 mm, h=2mm, f=50GHz.

Fig. 9.
Fig. 9.

S21 measurement results of the fabricated CPW electrodes with different dimensions at the RF launch region while keeping the dimension of the other area identical; εc=4, b=0.5 mm, and c=0.5 mm in the structure shown in Fig. 4. (a).

Fig. 10.
Fig. 10.

S21 measurement results of the CPW electrodes fabricated on LiNbO3 substrates with different thickness, b; W=0.25 mm, S=0.5 mm, c=0.5 mm, and εc=4.

Fig. 11.
Fig. 11.

Packaged samples and measured s-parameter results with different structure; (a) type A (b) type B (c) type C

Equations (2)

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T 2 + β 2 E ( x , z ) = 0
E = ( z ̂ Ψ z + y ̂ Ψ y ) e j β y for E z mode , and E = ( x ̂ Ψ x + y ̂ Ψ y ) e j β y for E x mode

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