Patent Application
20050201653
1. Technical Field of the Invention
The invention relates generally to the field of fiber optics and more particularly to electro-optical modulators.
2. Description of Related Art
Electro-optical modulators in a recent trend are moving toward conjugating smaller dimensions and lower driving voltages to enable system vendors to design their opto-electronic boards, e.g. transponders, by adopting smaller footprints, with inexpensive and low power electronic components. Conventional solutions employ three design techniques for reducing a RF driving voltage in a Lithium Niobate (“LN”) travelling wave modulator: (1) adopting a longer RF electrode, and thus resulting in a longer, instead of a shorter, chip; (2) decreasing the buffer-layer thickness, resulting in an overall worsening of the performances; and (3) decreasing the gap width between hot and ground electrodes of the CoPlanar transmission line, resulting once more in an overall worsening of the performances.
Each of these three techniques has shortcomings. First, if the RF electrode is lengthened, the chip and package dimensions in turn must be lengthened. In addition, the total electrical losses increase, thereby decreasing the electro-optical bandwidth. Thus, such a solution is not suitable for designing metro application-oriented components. Second, if the buffer-layer thickness is decreased, the electro-optical bandwidth also decreases due to the total electrical losses increasing and microwave effective refractive index being more difficult to be matched to the optic. Furthermore, the characteristic impedance of the microwave lines becomes worse, rendering more difficult to have a good electrical return loss vs. frequency profile. A good impedance matching between the driver output port and the modulator input port is desirable for a correct working of the entire transmission chain. A typical specification for the electrical return loss of the modulator (S11 parameter) is <−10 dB over the whole operating bandwidth. Third, if the gap width is decreased between hot and ground electrodes of the CoPlanar transmission line, the electro-optical bandwidth decreases due to the increasing in total electrical losses, while the characteristic impedance of the microwave lines becomes worse. A graphical diagram 100 in
Accordingly, there is a need to design a system for reducing the electrical return loss of Lithium Niobate traveling wave electro-optical modulators with low characteristic impedance.
The present invention overcomes the foregoing problems by providing an electro-optical modulator that has a microwave thin film resistor (or a lumped resistor) on an accessory connection chip located between the RF connector of the microwave input and the RF electrode of the Lithium Niobate chip. The accessory connection chip has a broadband attenuator, such as a thin film resistor, which is placed in a microstrip line to ensure that the electrical transmission reaches the Lithium Niobate chip. The insertion of the thin film resistor before the RF electrode lowers the electrical return loss, allowing a lower driving voltage design or a reduced chip length.
An electro-optical modulator comprises a modulator chip having a low impedance; and a microwave input chip, coupled to the modulator chip, having a resistor with a low impedance for increasing the total impedance at the input RF connector.
Advantageously, the present invention improves the overall performance of a modulator by designing a low impedance RF electrode and using an accessory chip with a thin film resistor, thereby producing a lower driving voltage with the same electrode length and electro-optical bandwidth or a low impedance RF electrode having a reduced length that produces the same performance in terms of driving voltage but with a larger electro-optical bandwidth relative to a standard solution. The thin film resistor in the accessory chip also facilitates the attaining of the necessary electrical return loss for low impedance RF lines.
Other structures and methods are disclosed in the detailed description below. This summary does not purport to define the invention. The invention is defined by the claims.
Referring now to
The accessory chip 240 has a broadband attenuator, such as a thin film resistor 235, for connecting a RF connector 215 to a RF line or an intermediate section to the RF electrode 280 of the Lithium Niobate chip 250. A microwave path in the microstrip line 230 could be interrupted in a relatively small section so that a conductor could be replaced by the thin resistive film 235. A typical microwave path is a 50 Ohm line, selected to match the connector impedance, while the tapered portion on the Lithium Niobate chip 250 is designed to adapt the characteristic impedance of the modulating line (about 30-36 Ohm for x-cut modulators and about 40-45 Ohm for z-cut modulators).
The microwave transition from the external RF connector 215 to an intermediate section to the RF electrode 280 can be accomplished by an accessory chip 240. A suitable external RF connector is a SMA, K, GPO type of connector or other types of connector like pin or butterfly solution, depending on the targeted frequency and selected application. The accessory chip 240 can be made of Alumina, Gallium Arsenide, Aluminum Nitride or other type of substrates commonly used for microwave applications.
A microwave line is shown as a straight microstrip line 230 for illustration purposes. One of skill in the art should recognize that other type of transmission lines (e.g., coplanar waveguide), as well as hybrid solutions where a first section is a microstrip line and a second section is a coplanar waveguide, can be practiced without departing from the spirits of the present invention. A curved path, rather than a straight microstrip line, can also be implemented.
In this embodiment, the thin film resistor 235 is selected of a few Ohm resistance, e.g. 1-10 Ohms. The low Ohm resistance of the thin film resistor 235 produces a limited effect on the electrical transmission such that the performance degradation does not exceed 0.1 dB per Ohm.
Although the driving voltage undergoes a performance degradation due to the resistive partition between the thin film resistance and the resistive load placed at the end of the RF line, this effect is negligible. Typically, a 50 mV per Ohm increase in the driving voltage is expected for driving voltages about 5V and resistive loads ranging from 20 to 30 Ohms.
To obtain the resistance values as discussed above in hundreds of micrometers in length and width, and 1-5 micrometers thick, the physical dimension of the thin film matches well with the dimension of the coplanar waveguide or microstrip line on a substrate commonly adopted for the accessory chip 240.
Table 1 shows the parameters obtained in comparing the electro-optical and microwave performance of a: standard modulator package to an electro-optical modulator with low impedance of the present invention. In particular, the electro-optical modulator with the low impedance produces a lower driving voltage with an electrode length that is the same as the standard modulator, while the total electrical losses for both the electro-optical modulator with the low impedance and the standard modulator are comparable. Alternatively, the RF electrode length can be shorten to obtain the same driving voltage as the standard modulator, which results in a device with an even better electro-optical bandwidth.
The nomenclature for the symbols adopted in Table 1 is briefly described as follows:
In addressing the decrease in a buffer-layer thickness and the decrease in the gap width between hot and ground electrodes of the CoPlanar transmission line, the total electrical losses can be reduced effectively by raising the electroplated gold electrode thickness ranging from 20 to 30 micrometers, which also has a positive effect on the microwave and optical effective refractive index matching. It is noted however, that by increasing the electrode gold thickness, the characteristic impedance could become lower, causing S11 parameter profile to exceed the required specification.
Nevertheless, the techniques in decreasing a buffer-layer thickness and the gaps between hot and ground electrodes of the CoPlanar transmission line could be used in order to design a traveling wave modulator with a low driving voltage, provided that the electroplated electrode thickness is increased in countering the increased electrical losses and an approach to contrast the mismatch between the low impedance modulator RF line and the driver amplifier is used. One way to contrast the mismatch is to place a broadband attenuator before the RF electrode. The S11 parameter profile can be adjusted to obtain the desired specification, if such element has a marginal effect on other parameters of a modulator, such as the electro-optical response. The broadband attenuator can be designed with a thin film resistor, placed on the accessory chip between the RF connector and the Lithium Niobate RF electrode.
Turning now to
The present invention provides a structure for a reduction of several millimeters, for example 5-8 millimeters, on the total chip length, which in turn reduces the total length on a device. In addition, when maintaining the same electrode length, the operating driving voltage decreases about, for example, 10-20% relative to standard solutions, that allows the adoption of lower power consumiption RF driver amplifiers. These improvements are attractive features in the design of optical modulators with metro-oriented performance, as well as local networks, long-haul networks and ultra-long-haul networks. For example, a suitable application of the present invention is the Ultra Small Form Factor LiNbO3 electro-optical modulator for a metro application.
Although the above embodiment illustrates the insertion of a resistive element on the accessory input chip as a thin film, one of ordinary skill in the art should recognize the principles in the present invention can be extended to the usage of a lumped resistance, with the same performance, provided that such an element is placed in the same position with reference to the equivalent electrical circuit, as shown in the schematic circuit of
It is apparent to one of skill in the art that other type of electro-optical modulators can be used which is not necessary a Lithium Niobate modulator or not a LiNbO3 substrate. The resistive element (i.e., the thin film resistor 235) can be placed, other than at or near the center of the microwave path, at any location along the microwave path.
Those skilled in the art can now appreciate from the foregoing description that the broad techniques of the embodiments of the present invention can be implemented in a variety of forms. Therefore, while the embodiments of this invention have been described in connection with particular examples thereof, the true scope of the embodiments of the invention should not be so limited since other modifications, whether explicitly provided for by the specification or implied by the specification, will become apparent to the skilled practitioner upon a study of the drawings, specification, and following claims.
