Optical waveguides – Temporal optical modulation within an optical waveguide – Electro-optic
Reexamination Certificate
1999-01-26
2001-03-06
Bovernick, Rodney (Department: 2874)
Optical waveguides
Temporal optical modulation within an optical waveguide
Electro-optic
C385S003000, C385S008000, C359S332000
Reexamination Certificate
active
06198854
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a Mach-Zehnder modulator which modulates the intensity of laser light in response to an electric signal for of optical communications.
2. Description of the Related Art
In a Mach-Zehnder modulator of the prior art, as reported by D. M. Adams et al. (Electronics Letters 29th February 1996, vol.32, No.5 p485-486), for example, incident signal laser light is split to pass through two optical waveguides, each optical waveguide having a multiple quantum well structure (hereinafter referred to as “MQW”) comprising an InGaAs well and an InGaAsP barrier is sandwiched by clad layers (having the same construction as that shown in
FIGS. 1A
,
1
B)
When a positive modulating voltage is applied to the MQW of one branch optical waveguide and a negative modulating voltage is applied to the MQW of the other branch optical waveguide, refractive indices of the two branch optical waveguides change in the opposite senses due to the Stark effect of quantum confinement. That is, refractive index increases in one branch optical waveguide and decreases in the other branch optical waveguide.
As a result, the phase of the signal light transmitted in the optical waveguides gains in one optical waveguide and the delays in the other optical waveguide, and intensity of the output signal light can be modulated by synthesizing, i.e., combining, these signal lights.
The Mach-Zehnder modulator of the prior art, because of two branch optical waveguides having the same construction, has a problem in having different phases in the two optical waveguides requiring application of modulating voltages of opposite phases to the two optical waveguides, thus requiring a complicated drive circuit for the modulating voltage.
SUMMARY OF THE INVENTION
Thus, it is an object of the present invention to provide a Mach-Zehnder modulator capable of intensity modulation of output signal light by using a simple drive circuit for the modulating voltage.
The present inventors have found that light can be transmitted in different phases in two optical waveguide even when the same modulating voltages are applied to the two optical waveguides by forming the MQW well layers constituting the two branch optical waveguides from layers of different thicknesses or different materials. Thus, the present invention has been completed.
The present invention provides a Mach-Zehnder modulator, which modulates the intensity of signal light with a voltage, comprising at least an input optical waveguide whereto the signal light enters, a first optical waveguide and a second optical waveguide both branching out from the input optical waveguide, a first electrode and a second electrode for applying the voltage to the first optical waveguide and the second optical waveguide, respectively, an output optical waveguide which is connected to the first optical waveguide and the second optical waveguide and gives an output signal light, all formed on a substrate, wherein refractive index of the first optical waveguide increases and refractive index of the second optical waveguide decreases as the same modulating voltage is applied to the first electrode and the second electrode, thereby modulating the signal light emerging from the output optical waveguide.
By forming the first optical waveguide and the second optical waveguide so that the refractive index of the first optical waveguide increases and the refractive index of the second optical waveguide decreases when the same modulating voltage is applied to the first electrode and the second electrode, it is made possible to modulate the intensity of the signal light without applying voltages of different phases to the first electrode and the second electrode as required in the case of the Mach-Zehnder modulator of the prior art. Thus a drive circuit for supplying the modulating voltage of the Mach-Zehnder modulator can be simplified.
Each of the first optical waveguide and the second optical waveguide preferably comprises at least a first clad layer, a multiple quantum well layer and a second clad layer formed successively on the substrate.
It is because the rate of changing the refractive index of the optical waveguide in response to the modulating voltage can be freely set by changing the multiple quantum well structure or the material thereof.
The present invention also provides a Mach-Zehnder modulator, wherein the thickness of the well layer of the multiple quantum well structure of the first optical waveguide is made greater than the thickness of the well layer of the multiple quantum well structure of the second optical waveguide.
By setting the thickness of the well layer of the MQW of the first optical waveguide greater than that of the second optical waveguide, it is made possible to increase the refractive index of the first optical waveguide and decrease the refractive index of the second optical waveguide when a predetermined modulating voltage is applied to the optical waveguides.
As a result, applying the predetermined modulating voltage causes the phase of the optical signal to gain in the first optical waveguide and the phase of the optical signal to delay in the second optical waveguide, thereby enabling intensity modulation of the output signal.
The present invention also provides a Mach-Zehnder modulator wherein the material of the well layer of the multiple quantum well structure of the first optical waveguide is selected from materials having greater band gap than the material of the well layer of the multiple quantum well of the second optical waveguide.
By forming the well layer of the multiple quantum well structure of the first optical waveguide from a material having greater band gap than the material of the well layer of the multiple quantum well structure of the second optical waveguide, it is made possible to differentiate the refractive index of the first optical waveguide and the refractive index of the second optical waveguide with the same thickness of the well layers when the predetermined modulating voltage is applied. As a result, intensity modulation of the output signal is made possible.
With the Mach-Zehnder modulator of the present invention, as will be apparent from the above description, intensity modulation of signal light can be achieved by applying modulating voltages of the same phase and therefore the drive circuit for the modulating voltage can be simplified.
The Mach-Zehnder modulator of the present invention can be easily produced particularly by differentiating the multiple quantum well structure and the composition of the material thereof, thereby altering the changing rate of the refractive index of the optical waveguide in response to the modulating voltage.
REFERENCES:
patent: 4709978 (1987-12-01), Jackel
patent: 5303079 (1994-04-01), Gnauck et al.
patent: 5502781 (1996-03-01), Li et al.
patent: 5524076 (1996-06-01), Rolland et al.
patent: 5619369 (1997-04-01), Yamamoto et al.
patent: 5652807 (1997-07-01), Fukuchi
patent: 5655034 (1997-08-01), Ishizaka et al.
patent: 6052496 (2000-04-01), O'Donnell
Adams et al., “Mach-Zehnder Modulator Integrated With A Gain-Coupled DFB Laser For 10Gbit/s, 100km NDSF Transmission At 1.55&mgr;m”, Electronics Letters, vol. 32, No. 5, Feb. 1996, pp. 485-486.
Bovernick Rodney
Kim Ellen
Leydig , Voit & Mayer, Ltd.
Mitsubishi Denki & Kabushiki Kaisha
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