Electro-optic modulator with enhanced bias stability

Details Electro-optic modulator with enhanced bias stability Electro-optic modulator with enhanced bias stability

2000-03-08

2002-09-10

Mai, Huy (Department: 2873)

Optical: systems and elements

Optical modulator

Light wave temporal modulation

C359S322000, C385S002000, C385S003000, C385S008000

Reexamination Certificate

active

06449080

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to improvements in electro-optic modulators. In particular, the present invention relates to methods and apparatus for reducing bias point sensitivity to ambient temperature and applied RF in an electro-optic modulator.
BACKGROUND OF THE INVENTION
Electro-optic modulators are typically biased with a DC voltage to set the quiescent phase difference between the two optical paths and to establish the operating point on the intensity-voltage curve about which modulation is induced. The bias point of electro-optic modulators is a function of the ambient temperature and the applied RF. As the ambient temperature and the applied RF changes, the desired bias point changes. The sensitivity of the bias point to ambient temperature and to applied RF can cause an increase in the bit error rate in digital communication systems.
FIG. 1
illustrates a schematic diagram of a prior art asymmetric co-planar waveguide (ACPW) Mach-Zehnder Interferometric (MZI) modulator indicating the field lines and thermal stress from the comers of the electrodes. Asymmetric RF electrodes are used to produce chirped optical signals. The modulator
100
includes an electro-optic substrate
102
with two waveguides
104
,
104
′ diffused in the substrate
102
. Crystal axes for x-cut lithium niobate are shown. A buffer layer
106
is formed on top of the substrate
102
and the two waveguides
104
,
104
′. An asymmetric co-planar waveguide (ACPW) electrode structure
108
is formed on top of the buffer layer
106
. The electrode structure
108
includes a ground electrode
110
and a RF electrode
112
.
Electric field lines
114
are illustrated for the electrode structure
108
. The path of the electric field lines to waveguide
104
is significantly longer than the path of the electric field lines to waveguide
104
′. Therefore, the modulation experienced by waveguide
104
is significantly weaker than the modulation experienced by waveguide
104
′. The imbalance in modulation generates chirp, which can be desirable for some communication systems.
Asymmetric co-planar waveguide modulators have particularly strong bias point sensitivity to temperature. The bias point sensitivity results from a mismatch in thermal-expansion coefficients between the metal forming the electrodes, which is typically gold, and the electro-optic substrate, which is typically lithium niobate. The mismatch results in thermal stress
116
in the substrate
102
that is localized near the bottom comers of the electrodes as illustrated in FIG.
1
. This “thermal stress” is a mechanical stress that is a function of temperature. The thermal stress
116
generates a piezoelectric voltage that is experienced by waveguide
104
′.
The relatively wide ground electrode
110
causes significantly more thermal stress than the RF electrode
112
because it has a larger amount of strain accumulated across the width of the electrode and, therefore, generates a higher piezoelectric voltage compared with the RF electrode
112
. The difference in the piezoelectric voltages experienced by waveguides results in a significant phase change that shifts the bias point of the modulator
100
as ambient temperature is increased.
Asymmetric co-planar waveguide modulators also have bias point sensitivity to the applied RF because of the “skin-effect.” The RF electrode
112
is significantly smaller in cross section than the ground electrode
110
, and therefore introduces more RF attenuation than the ground electrode
110
. The lost RF energy is dissipated as heat, which causes a rise in temperature in the waveguides. Since the ground electrode
110
is a more effective heat sink than the RF electrode
112
, a temperature differential may be created between the waveguides
104
,
104
′. The temperature differential shifts the bias point because the waveguides
104
,
104
′ experience different magnitudes of thermal stress and because the optical refractive index of the substrate
102
changes as a function of temperature.
Some prior art electro-optic modulator designs use electrode structures that reduce bias point sensitivity to the applied RF signal. For example, “LiNbO
3
Mach-Zehnder Modulators with Fixed Negative Chirp,” IEEE Photonics Technology Letters, Vol. 8, October 1996, pp. 1319-1321, describes various designs for x-cut lithium niobate chirped-modulator that reduce bias point sensitivity to the applied RF signal. Two of these prior art designs are illustrated below in FIG.
2
and FIG.
3
.
FIG. 2
illustrates a schematic diagram of a prior art three electrode co-planar-waveguide Mach-Zehnder Interferometric modulator
130
having asymmetric gaps
132
that introduce chirp, yet reduce bias sensitivity to applied RF. The modulator
130
includes an electro-optic substrate
102
with two waveguides
104
,
104
′ diffused in the substrate
102
. Crystal axes for x-cut lithium niobate are shown. A buffer layer
106
is formed on top of the substrate
102
and the two waveguides
104
,
104
′. Two ground electrodes
110
are formed on top of the buffer layer
106
. A RF electrode
112
is formed on top of the buffer layer and it is asymmetrically positioned between the two ground electrodes
110
.
FIG. 3
illustrates a schematic diagram of a prior art three electrode co-planar-waveguide Mach-Zehnder Interferometric modulator
140
having asymmetric waveguide locations that introduce chirp, yet reduce bias sensitivity to applied RF. The modulator
140
includes an electro-optic substrate
102
with two waveguides
104
,
104
′ diffused in the substrate
102
. Crystal axes for x-cut lithium niobate are shown. A buffer layer
106
is formed on top of the substrate
102
and the two waveguides
104
,
104
′. Two ground electrodes
110
are formed on top of the buffer layer
106
so that they are asymmetrically positioned relative to the waveguides
104
,
104
′. A RF electrode
112
is formed on top of the buffer layer and it is symmetrically positioned between the two ground electrodes
110
.
Although some prior art electrode structures for electro-optic modulator reduce the bias point sensitivity to applied RF, they do not reduce the bias point sensitivity to ambient temperature. This is because they do not relieve or compensate for stresses caused by thermal expansion resulting from changes in the ambient temperature.
SUMMARY OF THE INVENTION
The present invention relates to electro-optic modulators with reduced bias point sensitivity to ambient temperature and to applied RF. The modulators may be chirped or zero-chirp modulators. The invention is particularly useful for electro-optic modulators that have asymmetric co-planar waveguide electrode structures, which have relatively strong bias point sensitivity to ambient temperature and applied RF.
An electro-optic modulator of the present invention reduces the bias point sensitivity to ambient temperature and to applied RF by reducing the net phase shift caused by changes in the ambient temperature and by the applied RF field. Specifically, in one embodiment, an electro-optic modulator according to the present invention reduces the net phase shift by reducing the piezoelectric voltage experienced by one of the waveguides relative to the other waveguide. In another embodiment, an electro-optic modulator according to the present invention reduces the net phase shift by substantially matching the thermal stresses experienced by the waveguides and thus by causing the piezoelectric voltage experienced by one waveguide to be similar the piezoelectric voltage experienced by the other waveguide.
A discovery of the present invention is that bias point sensitivity in electro-optic modulators to both ambient temperature and to applied RF can be reduced by positioning the waveguides relative to the electrodes so that the thermal expansion proximate to one waveguide is similar to the thermal expansion proximate to the other waveguide. In one embodiment, an electro-optic modulator of the present inv

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