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Instantaneous and Controlled Excitation of the Spatial Modes in Planar Processed Multi-Mode Waveguides Obtained by Lithographically Defined Lenses in the Waveguide Core

IP.com Disclosure Number: IPCOM000105380D
Original Publication Date: 1993-Jul-01
Included in the Prior Art Database: 2005-Mar-19
Document File: 4 page(s) / 159K

Publishing Venue

IBM

Related People

Olsen, CM: AUTHOR

Abstract

In optical chip interconnects using multi-mode waveguides it is of great importance to minimize the modal noise induced bit-error-rate (BER) floor. This BER floor declines as the number of waveguide modes increases. Thus, it is of advantage to use waveguides that support as many modes as possible. The difficult part is to ensure in a practical and inexpensive fashion that all of these modes are actually being excited. One of the requirements for excitation of all the waveguide modes is that the numerical aperture (NA) of the light source is equal to or greater than the waveguide NA. This is usually not the case because planar multi-mode guides often have a NA which is much larger than the lateral NA of the laser source.

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Instantaneous and Controlled Excitation of the Spatial Modes in Planar Processed Multi-Mode Waveguides Obtained by Lithographically Defined Lenses in the Waveguide Core

      In optical chip interconnects using multi-mode waveguides it is
of great importance to minimize the modal noise induced
bit-error-rate (BER) floor.  This BER floor declines as the number of
waveguide modes increases.  Thus, it is of advantage to use
waveguides that support as many modes as possible.  The difficult
part is to ensure in a practical and inexpensive fashion that all of
these modes are actually being excited.  One of the requirements for
excitation of all the waveguide modes is that the numerical aperture
(NA) of the light source is equal to or greater than the waveguide
NA.  This is usually not the case because planar multi-mode guides
often have a NA which is much larger than the lateral NA of the laser
source.

      Lenses are often used to adjust the beam divergence in free
space optics.  We are proposing to integrate lenses in the waveguide
core by utilization of lithographical techniques.  This may be
achieved in planar waveguides fabricated in materials whose chemical
structure can be appropriately changed within well defined
geometrical areas by selective exposure (e.g. through a mask or by
beam steering) with a radiation source (e.g. UV light, X-rays,
E-beam).  For example, planar epoxy waveguides (without lenses) have
been fabricated [*]  by adding a photo synthesizer to the material.
Upon exposure with UV light through a mask and subsequent baking the
unexposed regions can be conveniently removed with an appropriately
chosen developer.  Thus, incorporating a lens in the waveguide core
is just a matter of designing a lens in the mask.  In this fashion
the lens and the channeled waveguide can be lithographically defined
and subsequently developed simultaneously in the same processing
step.  Fig. 1 shows the basic idea of a waveguide with an integrated
lens.  The refractive index of the lens, n sub 3, is smaller than the
waveguide core index, n sub 1.  w is the width of the waveguide and f
sub 0, f sub 1 and f sub 2 are the shapes of the three lens
interfaces.  Also shown is a light ray to illustrate the ability of
the lens to spread the light i.e. to excite the waveguide modes.
Fig. 1  Proposed planar waveguide configuration with integrated lens.

      When using lenses it is always important to position the light
source in a well defined distance from the lens.  This distance
depends on the focal length of the lens.  In the example in Fig. 1
the laser source is located at the focal point of the initial tapered
waveguide input end such that only the lowest order lateral mode is
excited (i.e., the lateral light rays are parallel to the waveguide
axis).  The idea is, when the light enters the lens, that a beam with
a NA equal to the NA of the lens is produced on the output of the
lens.  Thus, the lens should be design...