Coded data generation or conversion – Analog to or from digital conversion – Using optical device
Reexamination Certificate
1999-06-30
2001-09-18
Tokar, Michael (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Using optical device,
C341S155000
Reexamination Certificate
active
06292119
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to an optical analog-to-digital converter and, more particularly, to an optical analog-to-digital converter that employs a downward-folding successive approximation conversion approach incorporating optical subtraction in successive stages, where saturable absorbers are used as the optical subtractors in one embodiment.
2. Discussion of the Related Art
Advances in signal processing technology, including the need for greater processing speeds, increased channel bandwidths and improved transmission reliability, has resulted in a steadily growing focus on the optical domain and the vast potential that lies therein with respect to these parameters. However, optical technology, as compared to electrical and radio frequency based technology, lacks the necessary technical sophistication in many areas. Particularly, the desire for high-speed, large-bandwidth processing devices employing digital optics has been hampered by the lack of many basic optical devices and technologies readily available in the RF domain.
Optical analog-to-digital (A/D) converters are one such device that has not heretofore met basic design requirements. Current digital optical systems rely on digital conversion in the electrical/RF domain. This requires conversions back and forth between the optical and RF domains that are slower, have more loss and are noisier than conversions in the optical domain only. An optical device that converts an optical analog signal to an optical digital signal with little or no reliance on RF technology would eliminate system complexity and provide for high speed and large bandwidth processing advances.
An especially difficult obstacle to optical analog-to-digital conversion is the need for optical subtraction. Adding optical signal intensity is relatively straight-forward and well understood, but the process of subtracting a specific amount of optical power from an optical signal is comparatively more difficult. U.S. patent application Ser. No. 09/133,138, filed Aug. 11, 1998, titled “Upward-Folding Successive-Approximation Optical Analog-To-Digital Converter and Method of Performing Conversion” and assigned to the assignee of this application, is directed to an optical A/D converter that provides an analog-to-digital conversion within the optical domain without the need to subtract optical signals. However, various applications may require an optical A/D converter that performs the conversion by employing optical subtraction.
What is needed is an optical A/D converter that employs subtraction of an optical intensity from an optical signal in an efficient and cost effective manner.
SUMMARY OF THE INVENTION
In accordance with the teachings of the present invention, an optical analog-to-digital converter is disclosed that makes use of a downward-folding successive approximation conversion scheme that employs subtraction of optical signals. A pulsed optical analog signal to be converted is applied as an input to each of a plurality of converter channels, where each channel outputs one of the bits B
1
-B
N
of the digital output of the converter. The input signal to each channel is sent to a thresholding device that determines whether the intensity of the signal is greater than or less than a predetermined threshold value. The first channel thresholding device compares the input signal to a threshold value that is one-half of a known maximum intensity. Subsequent channel thresholding devices compare the input signal to a threshold value that is one-half of the intensity used in the previous channel in a downward-folding scheme. If the intensity of the input signal is greater than the threshold value in a particular channel, then that threshold value is subtracted from the input signal in each of the successive channels before being applied to the thresholding device in that channel. Therefore, for each channel m, the input signal passes through m−
1
subtraction elements before reaching the thresholding device in that channel. The pulse intensity at the thresholding device in each channel is thus reduced by L
m
, where L
m
=(B
1
*½+B
2
*¼+. . . +B
m−1
*½
(m−1)
), and is in the range from 0.0 to ½
(m−1)
. The last thresholding device, which has a threshold value equal to ½
m
, produces the least significant bit.
The general implementation of the analog-to-digital converter of the invention includes various components that perform this general downward-folding successive approximation scheme. The converter can operate completely in the optical domain, or in a combination of the optical domain and the electrical domain. The subtraction elements can be saturable absorbers or electro-absorption modulators in a particular design. Additionally, control signals from a particular channel to each of the successive channels to determine whether subtraction will occur can be provided by an optically controlled switch, such as a saturable absorber, or an electrically controlled switch, such as an electro-absorption modulators.
Additional objects, advantages, and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.
REFERENCES:
patent: 4901275 (1990-02-01), Hardie et al.
patent: 5264849 (1993-11-01), Kondoh et al.
patent: 5675428 (1997-10-01), Henmi
Carillo, Jr. Juan C.
Ferguson Bruce A.
Fields Richard A.
Kintis Mark
Kunkee Elizabeth T.
Jeanglaude Jean B.
Tokar Michael
TRW Inc.
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