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Compact optical modulator

This application claims priority of Provisional Application Ser. No. ______, filed on Jan. 9, 2007, which is incorporated herein by reference.

This invention relates generally to the field optical communications and in particular to an optical modulator for creating a high-speed optical data signal.

Next generation Ethernet is likely to have a data rate around 100 Gb/s. One possibility to accomplish 100-Gb/s transmission is the use of a multiplex of parallel lower-speed channels. However, the parallel approach typically has a low spectral efficiency, requires temporal de-skewing among channels, and has a large footprint or consumes significant chip real-estate. Another possibility is the use of a single 100-Gb/s serial channel. With a multi-level modulation format, such as differential quadrature phase-shift keying (DQPSK), in which the data is encoded using four different phase levels, the serial approach can have a high spectral efficiency, and there is no need for de-skewing. Many existing 10-Gb/s systems use pluggable transceivers. It would be highly desirable to make a 100-Gb/s multi-level modulator that is small enough to fit in a pluggable transceiver.

DQPSK modulators demonstrated to date have been too large for a pluggable transceiver because they employ phase modulators based on GaAs or LiNbO3. A reported GaAs DQPSK modulator was 52 mm long, R. Griffin, R. Johnstone, R. Walker, S. Wadsworth, A. Carter, and M. Wale, “Integrated DQPSK transmitter for dispersion-tolerant and dispersion-managed DWDM transmission,” Optical Fiber Communication Conference, paper FP6, 2003, and a reported LiNbO3 DQPSK modulator was more than 43 mm long, K. Higuma, S. Mori, T. Kawanishi, and M. Izutsu, “A bias condition monitor technique for the nested Mach-Zehnder modulator,” IEICE Electronics Express, vol. 3, pp. 238-242, 2006. These modulators use a traditional DQPSK modulator design consisting of a nested pair of Mach-Zehnder modulators.

The existing modulators are so long because of the relatively weak electro-optic effect in GaAs and LiNbO3. A solution is to make this design in InP, which has a much stronger electro-optic effect in the C-band by using the quantum-confined Stark effect. However, despite an increased electro-refractive effect, a phase shifter in InP with a reasonable Vπ is still quite long, 0.5, L. Zhang, J. Sinsky, D. Van Thourhout, N. Sauer, L. Stulz, A. Adamiecki, and S. Chandrasekhar, “Low-voltage high-speed traveling wave InGaAsP—InP phase modulator,” IEEE Photon. Technol. Lett., vol. 16, pp. 1831-1833, August 2004 to 4 mm, H. N. Klein, H. Chen, D. Hoffmann, S. Staroske, A. G. Steffan, and K.-O. Velthaus, “1.55 μm Mach-Zehnder modulators on InP for optical 40/80 Gbit/s transmission networks,” Integrated Photonics Research M, paper TuA2.4, 2006, and so requires a traveling-wave structure. A traveling-wave structure in InP is highly demanding to fabricate.

Thus there is a need for a new approach to making a DQPSK modulator.

I have developed—according to the present invention—a highly compact DQPSK modulator design. The design is so compact because it uses the electro-absorption (EA) effect rather than the electro-refraction effect, and because it requires only one interferometer.

The modulator consists of an at least three-arm interferometer with EA modulators (EAMs) in at least two of the at least three arms. This device can be made fully integrated in a semiconductor material such as InP.

A more complete understanding of the present invention may be realized by reference to the accompanying drawings in which:

FIG. 1 is a schematic of a prior art DQPSK modulator;

FIG. 2 is a schematic of an embodiment of the present invention;

FIG. 3 is are schematics and photographs of an optical device according to the present invention; FIG. 3a is a schematic of the optical modulator, stretched vertically for clarity; FIG. 3b is a cross-section of the waveguide structure in the EAM; FIG. 3c is a photograph of the actual modulator (the size is 1.5 mm×0.25 mm); and FIG. 3d is a zoomed-in photograph of the left-hand star coupler;

FIG. 4 is a series of diagrams explaining how the present invention works;