Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-05-08
2001-12-11
Pascal, Leslie (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06330092
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to communication links based on optical signaling, and more particularly, to an improved free space optical link having reduced background noise.
BACKGROUND OF THE INVENTION
For the purposes of the present discussion, a free space optical link is defined to be any transmission path in which data is communicated by modulating a light beam that travels through space to a detector. Free space optical links are being using with increasing frequency for the transmission of data. For example, infra-red optical ports for communicating data from a portable computer to a docking station or other computer are well known in the art. This type of link is also used in remote control units for operating television sets, VCR's, and the like.
The light entering the photodetector is the sum of the ambient background light and the modulated light beam from the transmitter. The most common background light sources are sunlight or fluorescent lighting. These background sources contribute a significant DC photocurrent to the output of the detector. Additionally, some background sources may contribute signals within the frequency range of the transmitted signal.
Detected DC background signals can cause problems in two ways. First, there is a type of noise known as shot noise which is proportional to the square root of the detected power. If the DC background signal is much larger than the data signal of interest, the shot noise from the DC background can become the dominant noise source and thus limit the minimum detectable signal power. Since the total shot noise power is related to the detection bandwidth, the problem becomes more significant at higher data rates. Furthermore, since DC background signals are often much larger than the data signal, the saturation level of a receiver with a DC coupled front end (such as a trans-impedance amplifier) must be designed to accommodate them. Because of the finite dynamic range of a receiver, this in turn can limit minimum detectable signal power.
As the data rate that these links are required to accommodate increases, problems caused by background light sources become more significant. As the bit rate increases, the power per bit decreases unless the intensity of the light source that is modulated in the transmitter can be increased. The decreased power per bit leads to an increase in the error rate unless the background light is also reduced.
There is a limit on the light intensity that can be utilized to transmit data. The transmitters of choice are lasers. Hence, the need to prevent eye damage in the event a transmitter is inadvertently aimed at the eye limits the maximum power of the light source. Accordingly, any improvements in signal to noise ratio must come from reducing the noise or increasing the dynamic range.
Two approaches have been suggested to reduce the errors caused by the background light. The first approach is based on the assumption that the background light intensity is constant in time. This approach utilizes front-end circuitry at the detector to subtract off any DC photocurrent from the front-end amplifier. A detector with capacitive coupling to the receiver circuit will also address this problem, but is rarely used since DC coupled front end circuits, such as a trans-impedance amplifier, generally provide much higher performance. This approach fails in situations in which the background light has high frequency components within the frequency band of the data signal. For example, background light from video displays may vary rapidly in time depending on the scene being displayed. Additionally, some forms of fluorescent lighting generate high frequency optical signals.
The second method utilizes the wavelength of the transmitter signal to distinguish the background light from the carrier light signal. This approach reduces the background noise by utilizing inexpensive wavelength filters made from photographic film to remove the portion of the background light that is outside the frequency band of the transmitter. However, the bandpass of the wavelength filter must still be large compared to the bandwidth of a laser to assure an economical filter design. Hence, systems based on wavelength filters are still subject to significant background light interference from sources that emit within the pass band of the filter.
Broadly, it is the object of the present invention to provide an improved optical transmission system.
It is a further object of the present invention to provide an optical transmission system, which is more resistant to background light than prior art systems.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.
SUMMARY OF THE INVENTION
The present invention is an optical link that includes a transmitter and a receiver for sending and receiving data across a free-space link or any other link in which a high level of background light may be present. The transmitter includes a source of circularly polarized light having a predetermined wavelength. The source is modulated to transmit data. The receiver includes a circuit for generating a signal indicative of the difference in intensity of left-handed circularly polarized light and right-handed circularly polarized light incident on the receiver. In one embodiment of the invention, the receiver includes a first polarization filter for blocking left-handed circularly polarized light of the predetermined wavelength and a second polarization filter for blocking right-handed circularly polarized light of the predetermined wavelength, the filters being displaced from one another. A first detector measures the intensity of light leaving the first polarization filter, and a second detector measures the intensity of light leaving the second polarization filter. The circularly polarized light source may be generated from a linearly polarized light source by passing the linearly polarized light through a ¼ waveplate. Similarly, a polarization filter for blocking light of a predetermined circular polarization may be constructed by passing the light through a ¼ waveplate and a linear polarization filter. In one embodiment of the present invention, the transmitter modulates the source of circularly polarized light by changing the handedness of the polarization of the source of circularly polarized light.
REFERENCES:
patent: 5530577 (1996-06-01), Orino
patent: 5608560 (1997-03-01), Abram
patent: 5742418 (1998-04-01), Mizutani
patent: 5777768 (1998-07-01), Korevaar
patent: 5896216 (1999-04-01), Kikushima
patent: 6057003 (2000-05-01), Dulaney
Agilent Technologie,s Inc.
Bello Agustin
Pascal Leslie
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