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From the background information supplied by the inventors, news correspondents obtained the following quote: "Chirp controllable Mach-Zehnder optical modulators comprising LiNbO.sub.3 (LN-MZ modulators) are in wide use as a component of transmitters for optical communication systems. In recent years, there have been active efforts at development of semiconductor-based Mach-Zehnder optical modulators (semiconductor MZ modulators). A semiconductor MZ modulator can be formed on a semiconductor substrate, such as InP, GaAs, etc., which is used for a semiconductor laser serving as light source. This allows a light source and an optical modulator to be formed on one substrate. The change in refractive index caused by the quantum-confined Stark effect (QCSE) when an electric field is applied to a quantum well structure is larger than the change in refractive index caused by the electrooptical effect of LiNbO.sub.3. Thus, the semiconductor MZ modulators are useful for developing small-sized modulators and reducing the driving voltage.
"It has been proposed that a Mach-Zehnder optical modulator in which the core layers of the two arms have different thicknesses can operate under either negative or zero chirp conditions. When the absolute value of the bias voltage applied to the arm with the thinner core layer is set to be larger, negative chirp operation can be accomplished. On the contrary, when the absolute value of the bias voltage applied to the arm with the thinner core layer is set to be lower, zero chirp operation can be accomplished."
Supplementing the background information on this patent, VerticalNews reporters also obtained the inventor's summary information for this patent: "Semiconductor MZ modulators are advantageous over LN-MZ modulators in terms of size reduction and low voltage operation, but they are disadvantageous against LN-MZ modulators in terms of chirp control. The refractive index changes nearly linearly with the applied voltage in the case of the electrooptical effect of LiNbO.sub.3, but the refractive index changes nonlinearly, i.e. quadratically, in the case of QCSE. In addition, the optical loss changes only slightly with a change in applied voltage in the case of the electrooptical effect of LiNbO.sub.3, but the optical loss increases with an increasing applied voltage in the case of QCSE.
"Modulation having a parameter a of 0 (zero chirp modulation) or modulation having the parameter a of about -0.7 (negative chirp modulation) is required to the MZ modulator for optical communications. In the devices based on the prior art described above, the two arms must have core layers with different thicknesses to drive one semiconductor MZ modulator with both zero chirp modulation and negative chirp modulation. Accordingly, complicated production steps are required.
"In a semiconductor MZ modulator in which the optical waveguides constituting the two arms have the same layered structure, zero chirp modulation can be performed by causing the same phase shift in the two arms. Specifically, this is achieved when the amplitudes and the center voltages of the applied modulation signals are the same for the two arms. When the center voltages of the applied signals are the same for the two arms, the same signal losses take place for the optical signals propagating in the two arms. This serves to ensure good extinction characteristics.
"Negative chirp modulation can be performed by causing different phase shifts in the two arms. Different phase shifts can be caused by applying modulation signals of different amplitudes to the two arms. For this technique, however, two driver circuits are separately required for zero chirp modulation and negative chirp modulation. Different phase shifts can also be caused by using two arms with different lengths. For this technique, however, both of zero chirp modulation and negative chirp modulation cannot be performed in one modulation device.
"Optical semiconductor devices that can easily perform both zero chirp modulation and negative chirp modulation are expected.
"According to an aspect of the invention, an optical semiconductor device including:
"an optical demultiplexer having a first and a second input ports and a first and a second output ports wherein intensity of an optical signal that is output from the first output port and an optical signal that is output from the second output port are equal to each other when an optical signal is introduced into the first input port whereas intensity of an optical signal that is output from the first output port is larger than intensity of an optical signal that is output from the second output port when an optical signal is introduced into the second input port;
"an optical multiplexer having a third and a fourth input ports and a third and a fourth output ports;
"a first optical waveguide of semiconductor to connect the first output port to the third input port;
"a second optical waveguide of semiconductor to connect the second output port to the fourth input port;
"a first modulation electrode to apply an electric field to the first optical waveguide so as to cause changes in the optical path length of the first optical waveguide; and
"a second modulation electrode to apply an electric field to the second optical waveguide so as to cause changes in the optical path length of the second optical waveguide.
"The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims.
"It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed."
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Keywords for this news article include: Electronics,
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OCTOBER 31, 2014
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