Optoelectronic analog-to-digital converter

Details Optoelectronic analog-to-digital converter Optoelectronic analog-to-digital converter

1998-12-23

2001-06-12

Young, Brian (Department: 2819)

Coded data generation or conversion

Analog to or from digital conversion

Using optical device,

C324S096000

Reexamination Certificate

active

06246350

ABSTRACT:

RELATED APPLICATION
The present invention is related to co-pending commonly assigned simultaneously filed application entitled “Multifunction Optoelectronic Device Structure” and is hereby incorporated herein by reference.
TECHNICAL FIELD
The present invention relates generally to analog-to-digital converters. More specifically, the present invention relates to an optoelectronic analog-to-digital converter.
BACKGROUND ART
Analog-to-digital converters are used in many applications. Known analog-to-digital converters, however, have been limited by their sampling rate. Presently, the fastest known analog-to-digital converters have sampling rates below 10Gigasamples per second (GsPs). Most analog-to-digital converters have a sampling rate below one GsPs.
Another limitation of presently known converters is that aperture jitter is typically about one psec. Aperture jitter is the amount by which the sampling window may vary. Aperture jitter lowers the sampling rate capability of the analog-to-digital converter.
Optoelectronic analog-to-digital converters are known that use the electrooptic effect in Mach-Zehnder interferometers. In these previous optoelectronic A/D converters, a plurality of waveguide Mach-Zehnder interferometers are arranged in parallel. An optical pulse input is divided in power and fed to all of the interferometers. The electrical analog input is also divided in power and fed to all of the interferometers. Each interferometer contains an electrode of different lengths, with the lengths chosen to produce a high-light “1” or a low-light “0” output from that interferometer according to the level of the analog input. The previous analog-to-digital converters have been realized using titanium-diffused lithium-niobate waveguide structures. The resolution of such converters has been limited by the small electrooptic coefficient of lithium-niobate. In such devices, the length for interaction between the analog input signal and the optical pulse is very long. Because of this, the device has a length of six centimeters. This is a very long package with respect to microelectronic applications. Another characteristic of such devices is that very high voltages must be used to power the devices. One known device, for example, uses a peak-to-peak voltage of 16.8 volts. The device has only a four bit accuracy. When based on Mach-Zehnder interferometers, the smallest voltage increment that may be sensed by the A/D converter is two volts. This is due to the limited substrate size that is presently available.
If the converter must be sensitive to low input voltages, the electrodes must be very long. Because of the length, the capacitances of these electrodes limit the electrical frequency response of the interferometers. Consequently, the maximum speed of the analog-to-digital converter is reduced.
SUMMARY OF THE INVENTION
It is, therefore, one object of the invention to provide an analog-to-digital converter capable of a higher rate of sampling than that previously known. A further object of the invention is to provide an analog-to-digital converter having a reduced package size and lower energization voltage level.
In one aspect of the invention, an analog-to-digital converter has an optical pulse source generating an optical signal having a plurality of optical pulses. An optoelectronic digitizer has a first input receiving the optical pulse chain. The digitizer has a second input for receiving the analog signal to be converted. The digitizer has a plurality of digital outputs. Each of the digital outputs corresponds to a range value. An electronic encoder receives the plurality of digital outputs. The encoder has an output in a predetermined digital format.
In a further aspect of the invention, the optoelectronic digitizer has a plurality of electrically controlled optical absorbers. The digitizer further has a traveling wave electrode coupled to the analog signal. As the optical pulses travel toward the photodetectors, the analog signal changes the absorption of the optical absorbers. The photodetectors provide an indication of the range values corresponding to the portion of the analog-to-digital signal that corresponds to the optical absorbers. The photodetectors provide an indication as to whether the voltage is above or below a given discrete value.
One advantage of the invention is that lower voltages may be used to activate the device. A further advantage of the invention is that the voltage dependent absorbers of the present invention can all be located on the same chip. This reduces the size of the analog-to-digital converter implementation.
A further advantage of the invention is that the direct digitization of wave forms such as L, X, Ku-band radar, EW signals, SHF SATCOM signals and DIRECT TV signals is possible due to high sampling rates and low aperture jitter.
Other objects and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims.


REFERENCES:
patent: 5010346 (1991-04-01), Hamilton et al.
patent: 5253309 (1993-10-01), Nazarathy et al.
patent: 5955875 (1999-09-01), Twichell et al.

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