Masaki Sugiyama, Masaharu Doi, Shinji Taniguchi, Tadao Nakazawa,
and Hiroshi Onaka
Fujitsu Laboratories Ltd. 10-1 Morinosato-Wakamiya, Atsugi 243-0197, Japan
Tel:+81-46-250-8220, Fax:+81-46-248-5194

Abstract: We developed a 40 Gb/s LiNbO₃ modulator with a drive voltage of 0.9 V at 40-Gb/s . This low drive voltage was achieved with a new design concept that featured a wide-gap, long CPW electrode. The modulator makes it possible to develop compact, low-power consumption and low-cost 40-Gb/s transmitters.

Introduction
LiNbO₃ (LN) modulators[1-3] are key devices in the construction of long-haul and high-bit-rate communication systems because of their low insertion loss, low-frequency chirp, and wavelength-independent characteristics. The 2.4-Gb/s and 10-Gb/s LN modulators represent mature technology, and 40Gb/s modulators are currently being developed. Since existing 40-Gb/s modulators require a drive voltage of about 5 V for single-drive and 3 V for dual-drive, driver-amplifiers with a large output voltage are needed. This creates difficulty in producing transmitters that are cost-effective. Modulators with a low drive voltage reduce the cost, size and power consumption of transmitter modules and they thus enable 40-Gb/s systems to be installed more easily.

This paper reports on the demonstrated performance of a 40-Gb/s LN modulator with a sub-1 V drive voltage [4]. The design concept of the device and experimental results are described.

Design
An LN modulator is composed of optical waveguides and coplanar-waveguide (CPW) electrodes. The optical waveguide is designed in the form of Mach-Zehnder interferometer. When an electrical signal is applied to the CPW electrode, the phase difference between the two Mach-Zehnder arms is changed and the intensity of the output power is modulated. By matching the velocity of the microwave to that of the light, the modulation bandwidth can be increased.

Forty-Gb/s modulators require an E/O bandwidth of over 30 GHz. However, there is a tradeoff between bandwidth and drive voltage. Conventionally, CPW electrodes with a narrow gap and short interaction length have been used, resulting in a drive voltage of 5 V (single electrode) but at a reduced bandwidth [3]. We propose novel CPW electrodes that reduce both drive voltage and propagation loss in microwave signals.

Fig. 1. Schematic of dual-drive Mach-Zehnder modulator. S, L, and t indicate gap, interaction length, and thickness of electrode, respectively. (a) Top view (b) Bird’s-eye view.

Figure. 1 shows the structure of a dual-drive modulator [5]. The optical waveguide was formed by Ti-diffusion into a Z-cut LiNbO₃ substrate. We used parameters gap (S), interaction length (L), and electrode thickness (t). First, we investigated the gap dependence of microwave loss in relation to bandwidth and VpL (half-wave voltage Vp) [4]. Microwave loss per unit length was obtained from the electrical S21. The VpL was calculated from overlap integral between the electric field and optical power distribution. Using these data, we calculated the relationship between Vp and S when L was defined to keep bandwidth constant. There is a general understanding that S should be decreased to reduce drive voltage, but our results indicated that when the gap is wider, Vp can be decreased because L can be increased. We found that an electrode with a wide gap and long interaction length was more effective in decreasing the drive voltage for 40-Gb/s modulation.

Fig. 2 Map of impedance and velocity matching. The curve shows that both impedance matching and velocity matching were simultaneously achieved.

On the basis of these calculations, we designed a CPW electrode. To achieve a broad E/O bandwidth, both impedance matching with an external driving circuit and velocity matching between the light and microwave are required. Impedance matching can be obtained when the characteristic impedance of the CPW waveguide is nearly 50 W. Velocity matching can be obtained when the effective refractive index of the CPW electrode is equal to that of the optical waveguide, which is 2.15 for TM input light. We calculated the characteristic impedance (Z₀) and the effective refractive index (n_m) using a finite-element method. Figure 2 shows the relationship between electrode thickness and gap size when velocity matching and impedance matching were obtained simultaneously. We defined L to maintain an S21 bandwidth of 30 GHz and calculated the 40-Gb/s-drive-voltage shown at the top of Fig. 2. Considering manufacture on a 4-inch LiNbO₃ wafer, we defined L as 60 mm. Then S and t were set at 50 mm and 30 mm, respectively and we speculated a drive voltage of less than 1.2 V.

Fig. 3 Packaged low-drive-voltage modulator.

Experiments
We fabricated and packaged a dual-drive modulator with a novel design (Fig. 3). A polarization-maintaining fiber and a single-mode fiber were pigtailed at their respective input and output waveguides respectively. V-connectors were used to feed the electrical signal to the CPW electrodes. A connector for DC bias control is located at the side of the module.

The optical fiber to fiber insertion loss was 6 dB and the extinction ratio was 27 dB, which are suitable for practical use. Figure 4(a) shows the S-parameters and the optical response. The 6-dB bandwidth of S₂₁ was 30 GHz. Since impedance matching was being obtained, the S₁₁ was sufficiently low. An effective refractive index of 2.15 was estimated from the S₂₁ phase. This means that velocity matching can be obtained. The 3-dB bandwidth of optical response was 28 GHz. Then we measured the eye diagrams at 40 Gb/s. Figure 4(b) shows the eye diagrams for the driving signal and optical output in a PRBS 2³¹-1. Clear eye openings were obtained with a drive voltage of 0.9 V. The signals were directly supplied from a 40 Gb/s 4:1 multiplexer test set (Anritsu MP1801A) with no electrical amplifier.

Fig. 4 Characteristics of low-drive-voltage modulator. (a) Electrical S-parameters and optical response. (b)Eye diagram at 40 Gb/s.

Conclusion
We designed a low-drive-voltage 40-Gb/s LN modulator. Using a wide-gap, and long CPW electrode, we demonstrated a drive voltage of 0.9 V. This is less than half the drive voltage of conventional modulators. The technique makes it possible to develop compact low-cost 40-Gb/s transmitters. Our low drive voltage LN modulators should open up a wide range of applications for 40-G/s optical-fiber links.

References

[1] M. Doi, et al., Photonics in Switching ‘96 PThB1, pp. 172-173 (1996)

[2] K. Noguchi, et al., IPR2000 IThH2-1, pp. 148-150 (2000)

[3] M. M. Howerton, et al., IEEE Photon. Technol. Lett., Vol. 12, No. 7, pp. 792-794 (2000)

[4] M. Sugiyama, et al., OFC2002 PD FB6

[5] K. Noguchi, et al., IEICE Trans.Electron., Vol.E79-C, No.1, pp. 27-31 (1996)