Optical modulator having Mach-Zender arrangement and method to generate modulated optical signal using the same
A semiconductor optical modulator with the Mach-Zender type is disclosed. The optical modulator of the invention cab driven by a single phase signal and reduce the chirping of the modulated light. Two waveguides of the Mach-Zender modulator each including an active layer showing the exciton resonance in the refractive index are connected with a resistor. The driving signal is applied to one of the waveguides, while, the signal is applied to the other waveguide through the resistor where the other waveguide is grounded through a resistor. Adjusting the resistance of two resistors and the amplitude of the applied signal, the Mach-Zender modulator shows a substantial modulation degree with substantially no chirping characteristic.
1. A semiconductor optical modulator with a Mach-Zender type having a semiconductor substrate, an optical de-coupler, an optical coupler, and first and second waveguides each provided on said semiconductor substrate and optically coupled with said optical de-coupler and said optical coupler, said optical modulator further comprising:
a first resistor connecting said first waveguide and said second waveguide; and
a second resistor for grounding said second waveguide, wherein said first and second resistors are connected in series,
wherein said first waveguide is provided with a driving signal superposed with a first bias voltage, and said second waveguide is biased by a second bias voltage generated by a resistive divider constituted of said first and second resistors.
2. The semiconductor optical modulator of claim 1 ,
wherein said first waveguide includes a multi-quantum well structure and said second waveguide includes a multi-quantum well structure whose layer arrangement is same with a layer arrangement of said multi-quantum sell structure of said first waveguide,
wherein said first waveguide is biased in a region where a refractive index of said first waveguide shows negative dependence on said first bias voltage and said second waveguide is biased in another region where a refractive index of said second waveguide shows positive dependence on said second bias voltage.
3. The semiconductor optical modulator of claim 2 ,
wherein said multi-quantum well structure of said first and second waveguides has a five-layer asymmetric coupled quantum well structure.
4. The semiconductor optical modulator of claim 3 ,
wherein said five-layer asymmetric coupled quantum well structure includes at least three well layers, two of which has a total thickness substantially equal to a thickness of a rest well layer.
5. The semiconductor optical modulator of claim 2 ,
wherein said multi-quantum well structure of said first and second waveguides shows a peak in said refractive index of said first and second waveguides against said bias voltage,
wherein said first bias voltage is set in a negative slope side with respect to said peak and said second bias voltage is set in a positive slope side with respect to said peak.
6. The semiconductor optical modulator of claim 1 ,
wherein one of said first and second waveguides provides an additional electrode to be supplied with a static bias voltage to shift a phase of light propagating in said one of said first and second waveguides providing said extra waveguide.
7. The semiconductor optical modulator of claim 6 ,
wherein other of said first and second waveguides not providing said additional electrode provides another electrode to shift a phase of light propagating in said another one of said first and second waveguides.
8. The semiconductor optical modulator of claim 1 ,
wherein at least said first resistor is integrally formed on said semiconductor substrate.
9. The semiconductor optical modulator of claim 8 ,
wherein said first resistor is a thin film resistor integrally formed on said semiconductor substrate.
10. A method to generate a modulated light by a semiconductor optical modulator with a Mach-Zender type having an optical de-coupler, an optical coupler, and first and second waveguides each connected to said optical de-coupler in an end and connected to said optical coupler in another end, said method comprising steps of:
generating first and second light each de-coupled by said optical de-coupler from incident light, said first light being provided to said first waveguide and said second light being provided to said second waveguide;
applying a first bias voltage to said first waveguide to generate third light and a second bias voltage different from said first bias voltage to said second waveguide to generate fourth light;
generating output light by coupling said third light with said fourth light by said optical coupler,
wherein said second bias voltage is generated from said first bias voltage by a resistive divider constituted of a first resistor connecting said first waveguide to said second waveguide and a second resistor grounding said second waveguide.
11. The method of claim 10 ,
wherein said first bias voltage is set in a range where a refractive index of said first waveguide shows negative dependence on said first bias voltage, and said second bias voltage is set in another range where a refractive index of said second waveguide shows positive dependence on said second bias voltage.
12. The method of claim 10 ,
wherein said optical modulator further provides a electrode on at least one of said first and second waveguides to shift a phase of one of said third and fourth light propagating in one of said first and second waveguides providing said electrode,
wherein said method further comprises a step of applying an extra bias voltage to said electrode.