Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2000-12-21
2004-09-14
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S2140AL, C250S227110
Reexamination Certificate
active
06791073
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to a receiver used in wireless telecommunication systems, and more particularly, relates to a receiver that has a honeycomb baffle to reduce ambient light.
BACKGROUND OF THE INVENTION
A typical optical wireless telecommunications system comprises an optical transmitter and an optical receiver, with associated electronics to modulate and demodulate data on a light beam. Often, receivers are basically telescopes.
An example of a configuration of a typical optical receiver is shown at
10
in FIG.
1
. The optical receiver
10
comprises a cylindrical receiver tube
12
with a receiver aperture
15
, (sometimes made of glass), at one end, and a system of internal mirrors
20
and
18
to collect and focus light to a small optical detection unit
22
. In this optical receiver
10
, a light beam
16
(the data communication signal) enters the receiver tube
12
at a receiver aperture
15
. The mirrors
18
and
20
focus the light beam
16
onto a small area at detection unit
22
. An example of such a traditional system is a Cassegrain telescope with detector electronics located at the focal plane.
The optical receivers used in wireless optical communication systems are often required to operate under varying background illumination conditions, i.e., at night or under direct sunlight. Most problems occur when the optical receiver is operating in direct sunlight, particularly where sunlight is entering the receiver aperture
15
at an angle close to that of the data communication signal
16
. Several potential problems are caused by this situation. First, the photodetector may be saturated with incident sunlight. Second, the optical system will be heated resulting in thermal gradients and mechanical deformation. Third, the optical system may produce unwanted images of the sun that lead to unsafe external heating or a blinding effect.
The first two problems only affect performance or internal damage of the optical receiver
10
. The third problem is a more serious concern since it relates to safety at the deployment site of the optical receiver. One situation where this problem is known to exist is in a conventional Cassegrain 2-mirror telescope, as described above in relation to FIG.
1
. In this case, when ambient light enters the receiver aperture
15
slightly off-axis, so that some light reflected from the primary mirror
20
does not subsequently strike the secondary mirror
18
, there is an image produced at the focal plane of the primary mirror. Depending on the field angle of the incident light and the particular optical design of the telescope system, non-vignetted transmission to this image can be substantial. In the case of the off-axis illumination being sunlight, an absorbing surface placed at the prime-focus image can become dangerously hot.
Conventional approaches to mitigate the problem described above include: (1) incorporating a baffle aperture that limits the field of view of the optical receiver
10
, (2) placing a band-pass filter in front of the receiver aperture
15
to reject wavelengths that are not required, or (3) only use the receiver
10
at night. The third option is clearly not acceptable for a robust communication service application. The second option is impractical in some situations due to the high cost associated with large area wavelength selective filters. Also, in the case of the communication wavelength being invisible (in the near infrared for example), the external prime-focus light can be invisible but still contain significant energy.
The baffle approach can also be impractical. If light that enters a telescope with aperture diameter of D at angles of or greater is to be rejected by the baffle, the baffle needs to be placed a distance D/tan( ) in front of the entrance aperture. For example, if D is 0.4 meters and is 5 degrees, the baffle needs to be 4.6 meters in front of the aperture. If this is accomplished by a baffle tube, the system becomes very large and unwieldy.
SUMMARY OF THE INVENTION
A baffle containing a plurality of small apertures is mounted on the receiver aperture of an optical receiver, such as a wireless optical telecommunication system receiver. The baffle attenuates ambient radiation that may interfere with the optical receiver. The baffle includes a plurality of apertures that have their axes substantially parallel to the optical axis of the receiver. The depth of the baffle assembly is smaller than a single baffle by the ratio of the small aperture diameter to the full receiver aperture diameter. Honeycomb core material is a practical embodiment of such a multi-aperture baffle.
REFERENCES:
patent: 6137535 (2000-10-01), Meyers
Bell John A.
Schuster John J.
Blakely , Sokoloff, Taylor & Zafman LLP
Le Que T.
Terabeam Corporation
LandOfFree
Optical receiver having baffle including a plurality of... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Optical receiver having baffle including a plurality of..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical receiver having baffle including a plurality of... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3191738