Optical waveguides – With optical coupler – Plural
Reexamination Certificate
2001-10-19
2004-08-10
Ullah, Akm Enayet (Department: 2874)
Optical waveguides
With optical coupler
Plural
C398S118000
Reexamination Certificate
active
06775432
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
(Not Applicable)
STATEMENT RE: FEDERALLY SPONSORED RESEARCH/DEVELOPMENT
(Not Applicable)
BACKGROUND OF THE INVENTION
This invention relates to demultiplexing, routing and multiplexing of a number of different optical wavelength channels. The demultiplexer provides a basis for robust designs of multi-channel optical communication receiver, spectrum analyzer and router. The demultiplexer is operationally bi-directional providing a means for multiplexing.
DESCRIPTION OF THE PRIOR ART
Wavelength division multiplexing is a very important function in optical communication. To increase the system bandwidth, it is common to propagate a number of wavelength separated channels over the same carrier (such as an optical fiber or waveguide), and the objective of a wavelength division demultiplexer is to separate the channels in such a way that the signals can be routed to individual destinations. In general, an intensity or phase modulated optical beam of a certain wavelength constitutes a channel in optical communication. The objective of a wavelength division multiplexer is to combine the channels at different wavelengths from separate sources so that they can simultaneously be propagated over a common carrier. For this application, we use the term WDM to denote both demultiplexing and multiplexing of channels of different wavelengths. We use the term DWDM to denote WDM devices which operate on a large number of very closely spaced typically (100 GHz for ITU-T DWDM grid) wavelength channels.
WDM function requires wavelength selective devices such as prisms, gratings, interference filters and waveguides, which can be found in the literature as prior art. The drawback of all previous approaches is that when the channel separation becomes small, such as ~0.8 nm (~\100 GHz) around 1550 nm center wavelength, and the number of channels are large (>8), the DWDM devices become bulky and have tight dimensional tolerances, which affect manufacturability and reliability of operation. Majority of the techniques are temperature and stress sensitive, and some of the approaches have large differential loss among DWDM channels and large cross talk between nearby channels.
It is therefore an object of the present invention to provide an apparatus and associated methods to combine a wavelength separation element such as a grating with a series of internal reflection surfaces which are positioned within a range of angles near critical angle in a way that channels separate out one at a time by becoming transmissive at the internal reflection surfaces, and the remaining channels propagate to the next internal reflection surface with insignificant loss following a total internal reflection from the previous surface. This results in physical separation of the optical channels. The process is bi-directional, and propagation in the other direction combines a number of wavelength separated channels into a single beam resulting in multiplexing. The separated channels can be imaged into individual carriers such as fibers, when the apparatus works as a router or a demultiplexer.
In one feature of the invention, a number of internal reflecting surfaces at near critical angle is provided in a single optical element, which is configured as a slab of a low-loss optical material. The signal to be demultiplexed propagates through the slab by bouncing back and forth between the top and bottom surfaces of the slab. The incidence angle at the first bounce is designed such that it exceeds the critical angle for total internal reflection for all but one wavelength channel, which becomes partially transmitted out of the optical slab. A critical feature of the present invention is that the incidence angle of the optical beam at the top and bottom slab surfaces is gradually reduced from bounce to bounce by providing a small inclination angle between the top and the bottom faces of the slab. The incidence angle of the beam progressively reduces from bounce to bounce in a way that one wavelength channel at a time emerges from one of the slab surfaces by switching from being totally internally reflective to partially transmissive.
In another embodiment of the invention, a grating disperses the input optical beam such that the beams of different wavelengths propagate at different angles leaving the grating. The beams then enter the slab and are incident at slightly different angles on the first surface. The geometry is designed such that incidence angle of either the longest or the shortest wavelength channel falls below the critical angle, and is partially transmitted out of the slab waveguide. The remaining beams propagate to the other surface, which is inclined with respect to the first surface with the sign (positive or negative) such that the incidence angle of all the wavelength channels is reduced at this surface by the fixed “slab inclination angle”. The “slab inclination angle” is designed such that the incidence angle for only one other wavelength channel falls below the critical angle, and the second wavelength is partially transmitted out of the optical slab. The remaining beams propagate to the first surface, the incidence angle is again decreased by the “slab inclination angle”, and a third wavelength channel escapes the slab through transmission. The process continues until all channels exit the slab at various physical locations and are therefore separated. We use the term SIR ports to denote the regions on the slab top and bottom surfaces which act as ingress/exit points for wavelength channels and where the internally reflected beam touches the slab surface.
In another feature of the invention, other wavelength separating devices such as prisms can be used to replace the grating.
In another feature of the invention, the input beam to the optical slab is polarized such that the partial transmission at the slab faces near critical angle is maximized (polarization parallel to the plane of incidence for example).
In another feature of the invention, the input beam comes from an input optical fiber and is collimated using a lens prior to its incidence on the grating surface.
In another feature of the invention, the physical separation from the channels on either the top or the bottom face of the slab can be linearly increased with the slab thickness.
In another feature of the invention, the transmitted angle is very sensitive to the incidence angle at near the critical angle, which amplifies the angular separation imposed on the channels by the grating or another dispersive element. The exploitation of this phenomenon which aids in channel separation is a key feature of this invention. This provides a means for easier blocking of unwanted channels using baffles, which leads to low cross talk among nearby channels.
In another feature of the invention, the optical slab can be made longer than the minimum required to provide additional internal reflection ports which can accommodate variations in manufacturability and operating conditions. In some cases, a designated channel skips a slab internal reflection port by not being incident at less than the critical angle, and emerges from the next slab internal reflection port.
In another feature of the invention, the optical slab can be fabricated with much higher precision than is required in the present invention for slab inclination angle, the flatness of the slab top and bottom surfaces, the optical quality of the surfaces, and propagation loss through the slab.
In another feature of the invention, the input and the exit faces of the slab can be antireflection coated for the wavelength channels.
In another feature of the invention, the top and bottom surfaces of the optical slab may be protected from nearby coupling optics and detectors using mechanical means.
In another feature of the invention, the top and bottom faces of the optical slab may have a lower index optical layer of material which preserve the low loss for the totally internally reflecting beams inside the optical slab, allow transmission of the separated channel
Basu Santanu
Stetina Brunda Garred & Brucker
Ullah Akm Enayet
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