Referring now to FIG. 1, a portion of a Mach Zehnder interferometer type modulator, with three coplanar strip electrodes 10, 12 and 14 is shown, having only a single drive voltage. This externally modulated system has a Z-cut LiNbO3 substrate, which requires a lower drive voltage than is generally required for X-axis or X-cut crystal orientation. The Z-cut LiNbO3 substrate has an electro-optical effect, which provides a broadband low drive voltage modulator. The electrodes are shown to be disposed over waveguides 15 and 17.
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 signal. As the ambient temperature and the applied RF signal changes, the desired bias point changes. The sensitivity of the bias point to ambient temperature and to the applied RF signal can cause an increase in the bit error rate in digital communication systems. Conventional single-drive Z-cut modulators are known to exhibit a large bias shift with temperature change, potentially reducing the useable lifetime of the modulator, if the bias voltage approaches or reaches a voltage “rail” or limit.
X-cut lithium niobate modulators with Asymmetric Co-Planar Waveguide (ACPW) electrodes, and single-drive z-cut modulators with symmetric Co-Planar Waveguide (CPW) electrodes, 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 lithium niobate for Z-cut modulators. The mismatch results in thermal stress in the substrate that is localized near the bottom of the electrodes. Some components of this stress are significant near the corners of the electrodes, while other components are significant directly underneath the middle of the electrode. The relative importance of these stress components depends on the crystal cut. This “thermal stress” is a mechanical stress that is a function of temperature. The thermal stress generates an unwanted piezoelectric voltage that is experienced by the waveguides.
In conventional Z-cut modulators, a relatively wide ground electrode is required, which causes significantly more thermal stress than the RF or hot electrode, because there is more strain accumulated across the width of the ground electrode thereby generating a higher piezoelectric voltage compared with the RF electrode. The difference in the piezoelectric voltages experienced by waveguides results in a significant phase change that shifts the bias point of the modulator as ambient temperature is increased.
These modulators also have bias point sensitivity to the applied RF because of the “skin-effect.” The RF electrode is significantly smaller in cross section than the ground electrode and therefore introduces more RF attenuation than the ground electrode. The lost RF energy is dissipated as heat, which causes a rise in temperature in the waveguides. Since the wider ground electrode is a more effective heat sink than the RF electrode, a temperature differential may occur between the waveguides. The temperature differential shifts the bias point because the waveguides experience different magnitudes of thermal stress and because the optical refractive index of the substrate 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, U.S. Pat. No. 6,449,080 incorporated herein by reference, in the name of Kissa et al. issued Sep. 10, 2002 and assigned to JDS Uniphase Corporation, having common inventorship and ownership with this instant invention, discloses a slotted electrode design which mitigates some of the problems described heretofore, with X-cut electrodes.
It is an object of this invention to provide a different solution, which significantly reduces bias drift over temperature, more especially for Z-cut electrodes, but not limited thereto.
There is yet another aspect which an embodiment of this invention addresses:
In prior art modulators, DC voltages are sometimes combined with RF voltages by use of a “bias Tee”, and applied to the RF hot electrode together. The DC voltage is utilized to steer the quiescent operating point of the modulator in a desired manner. Unfortunately, the “bias Tee” can introduce RF signal loss at high frequencies, especially for 40 Gb/sec modulation rates. It is therefore sometimes desirable to be able to steer the operating point of the modulator, without the use of a “bias Tee”. For example U.S. Pat. No. 5,214,724 in the names of Seino et al. issued May 25, 1993 and assigned to Fujitsu Limited, teaches that the bias point can be steered with use of a third electrode. At DC voltage this third electrode operates similar to a ground electrode. Notwithstanding, the '724 patent does not teach a solution for integrating this third electrode into a ground electrode suitable for operating at high RF frequencies.
It is an object of an embodiment of this invention to provide such a solution by use of capacitive bridging between slotted ground electrodes.