Wavelength selective optical coupler

An optical coupler for a fiber optic communication system, which couples one band of wavelengths out of the fiber while allowing others to be carried further. This allows wavelength division multiplexing of different signals within a single fiber. The preferred embodiment of the invention is comprised of a waveguide having aperiodic corrugations in one wall and a Fabry-Perot type resonator enclosing the waveguide around the corrugations with its axis transverse thereto.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a frequency selective optical coupler which is 
particularly useful in fiber optic waveguide multiplex communication 
systems. 
2. Description of the Prior Art 
Fiber optic communication systems have been developed to the point where 
they appear to be able to provide reliable long distance communication 
channels. While use of such systems considerably reduces the cable size 
and weight for a given bandwidth, generally only a single light beam is 
transmitted via each specific fiber, since there has been no satisfactory 
structure for segregating the various wavelengths of light which might 
carry different signals on a single fiber. One known device, described in 
U.S. Pat. No. 3,905,676 issued Sept. 16, 1975, employs uniform periodic 
diffraction gratings to provide selective coupling of a particular 
wavelength band of light exiting a fiber. 
SUMMARY OF THE INVENTION 
The present invention is directed to an optical coupler which provides 
greater control of wavelength selectivity and permits a single 
light-waveguide to be used as the carrier medium for a plurality of 
different signals in a wavelength-division multiplex transmission system. 
In such a transmission system, signals of different frequencies can pass 
in opposite directions in the same optical waveguide. Clearly the utility 
of optical waveguides is substantially expanded, allowing a greater number 
of signals to be transmitted, and allowing conversion of the waveguide 
into a bidirectional transmission path. 
The invention, in general, is a wavelength selective optical coupler 
comprising a waveguide having corrugations in one wall including a varying 
dimension, and a Fabry-Perot type resonator enclosing the waveguide around 
the corrugations with its axis transverse thereto. 
The corrugations in the waveguide, which are preferably blazed and 
aperiodic, serve to couple light out of the waveguide into the resonator 
at a wavelength-dependent angle. The resonator cannot be excited, however, 
if the coupling angle and the wavelength do not correspond to a resonance 
of the structure. In that case, optical radiation entering the resonator 
on a bound mode of the waveguide is not coupled into the resonator and 
passes further along the waveguide. 
On the other hand, if the wavelength and coupling angle are appropriate to 
excite a resonator excitation mode, the radiation is coupled into the 
resonator. 
The resonator is made very leaky in comparison to the waveguide so that 
power coupled into the resonator leaks out very much faster than the power 
at that wavelength carried in the portions of the waveguide external to 
the resonator. The power leaking out of the resonator is available for 
optical detection or other purposes. 
It has been found that the wavelength band over which the resonator is 
excited can be very much narrower than the wavelength bands that do not 
excite it. The power at a very narrow band of wavelengths can thus be 
stripped out of the waveguide by the present structure. 
A better understanding of the invention will be obtained by reference to 
the more detailed description below, and to the following drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a planar optical waveguide 1 which may form part of or be 
coupled to an optical fiber. Corrugations 2 are formed in a wall of the 
waveguide. Enclosing the waveguide around the corrugations is a cavity 
resonator 3 of the Fabry-Perot type, in which the axis of the resonator is 
transverse to that of the waveguide. 
The resonator is comprised of reflectors 4 and 5, one of which, shown as 
reflector 4, on the side adjacent the corrugations, is made leaky to 
optical radiation such that power which is coupled into the resonator 
leaks out faster than the power remaining in the waveguide. 
Various forms of corrugations may be useful in various alternative designs. 
Accordingly, the corrugations should be considered in their more generic 
sense as diffraction gratings, particularly blazed gratings which are 
gratings with specially shaped teeth. A description of such gratings may 
be found in the publication THE BELL SYSTEM TECHNICAL JOURNAL, Volume 56, 
March 77, No. 3, pages 329-353. Blazed corrugations can be used to couple 
unequal amounts of power out on the two sides of the waveguide. For 
example, the loss at nonresonant wavelengths can be reduced by coupling 
more of the power out of the waveguide on the side having the highest 
reflectivity Fabry-Perot reflector. 
In operation, it may be seen that optical power at two example frequencies 
L.sub.1 and L.sub.2 enter the corrugation portion of the waveguide. The 
corrugations, being predetermined to match the wavelength and provide an 
output coupling angle for one of the signals, for instance at wavelength 
L.sub.1, being appropriate to excite a resonator mode at the wavelength 
L.sub.1. The radiation at that wavelength is coupled into the resonator. 
The signal at a single wavelength L.sub.1 is further coupled out through 
the leaky reflector wall 4, which signal can be detected or transmitted 
further. 
Accordingly, it will be seen that a single wavelength (or band of) signal 
is coupled out of the optical waveguide. Yet the signal L.sub.2 is 
unaffected and continues to pass further along the waveguide. 
As shown in FIG. 2, the corrugations in the waveguide are made aperiodic, 
with a gradually diminishing period for example, such that a band of 
signals of controlled optical bandwidth could be coupled out. For the same 
purpose, the width of the waveguide can be varied along the length of the 
coupler, as shown in FIG. 3. These couplers can also act as 
wavelength-selective filters of controllable properties. FIG. 4 shows 
another variation using a tandem arrangement of corrugations of different 
periods for coupling a plurality of wavelengths sequentially out of the 
waveguide. 
FIG. 5 is a graph of relative power plotted against wavelength for the 
coupler of FIG. 1. Curve 6 depicts the power coupled out of the coupler, 
while curve 7 depicts the power which is transmitted further down the 
waveguide. Curve 8 depicts the reflected power within the resonator. The 
wavelength of the power output is clearly of extremely narrow band, 
centered at 5940.0 angstroms. 
To illustrate the degree of signal coupling which may be obtained, consider 
FIG. 6 in which the loss co-efficient of a very wide corrugated dielectric 
planar optical waveguide near mode cutoff which operation mode is not 
preferred, but will suffice for example purposes. This relationship was 
determined to exist where the waveguide width which was 100 microns, the 
corrugation depth was 0.2 microns, the corrugation period was 440 
nanometers. This structure had no external resonator, but employed the 
Fresnel reflections at the waveguide walls themselves as the Fabry-Perot 
mirrors. The peaks in output response occur at approximately wavelengths 
of 881, 883, 885 and 887 nanometers. The relationship of FIG. 6 is based 
on the output radiation propagating nearly at the critical angle in the 
waveguide, resulting in a relatively high Q. The loss co-efficient at the 
maximum of the first peak has about 140 times its value at the minimum 
after the first peak. If the waveguide were of a length equal to the 
inverse of the peak loss co-efficient, the power coupled out at the first 
resonance would be 19.5 dB greater than the power coupled at the first 
minimum. 
A thick, high-refractive-index structure operated near cutoff is not 
suitable for a practical output coupler. The loss co-efficients are 
believed to be too small, the output signal travels at an inconvenient 
angle, and the power comes out both sides of the waveguide in 
wavelength-dependent proportions. 
Considering the TE modes only, one of the groups of bound modes in the 
structure of FIG. 1 consists of modes whose field amplitudes are 
non-cyclic in the Z direction in the dielectric of the resonator. The 
modes of this group (called "bound" modes) are similar to the modes of 
dielectric slab of the waveguide in the absence of the resonator. If the 
effective width of the waveguide for modes of this group is less than the 
separation of the resonator reflectors, from the waveguide walls, the 
propagation constants of the group match closely the bound-mode 
propagation constants of the free slab. Reflection of power in bound modes 
propagating into the coupler is thus small. The modes of the second group 
(called "resonator" modes) have cyclic field distribution in the Z 
direction in the regions of the resonator dielectric. These modes are 
analogous to the radiation modes of the free slab waveguide, but because 
of the reflectors, they are bound in the resonator. 
When the reflectors are perfect, this group consists of a finite discrete 
set of modes. If the reflectors are imperfect, however, the discrete modes 
of this set broaden into sections of a continuum of leaky modes. 
In one example of the invention, the width of the waveguide was 5 microns 
and the width of the resonator from reflector to reflector was 20 microns. 
The wavelength for resonance was 880 nanometers, and coupling occurred in 
the lowest-order bound mode and the highest-order resonant mode. The 
linewidth of the wavelength-selective coupler was about 0.3 nanometers, 
while rejection of power at a wavelength further than 1 nanometer from the 
resonant wavelength was greater than 20 dB. This rejection corresponds to 
40 dB of signal power rejection after detection because of the 
power-voltage conversion that occurs within an optical detector. At 
resonance 90% of the optical power leaves the coupler, whereas off 
resonance more than 99% is transmitted. 
The free spectral range is greater than 20 nanometers so that this coupler 
would in principle provide the possibility of demultiplexing 20 channels 
with less than 40 dB crosstalk. 
It should be noted, however, that the linewidth, crosstalk, rejection and 
other properties of such a device can be varied considerably by altering 
the device parameters. The aforenoted example provides but a general idea 
of the properties of the wavelength selective resonant couplers of the 
inventive type. 
The present invention could be used in pulse code modulation systems using 
various wavelengths for various channels. While for analog television 
systems, the crosstalk may well be excessive, isolation can be obtained by 
employing radio frequency carrier signals at different wavelengths on each 
of the channels. 
The present coupler can also be used in duplex transmission systems. The 
upstream channel and the downstream channel can be sent at different 
wavelengths. The coupler can be used to remove the received power from 
fiber optical waveguide, while allowing the transmitted power to pass 
through it, with little loss in both directions. The present coupler can 
also be used as an input coupler for wavelength multiplexing. 
Clearly the inventive device offers a novel component which allows 
considerable expansion of the utility of optical communication systems. 
A person skilled in the art understanding this invention may now conceive 
of various alternatives or embodiments using the principles described. All 
are considered within the sphere and scope of the invention as defined in 
the appended claims.