Bulk optic wavelength division multiplexer

An optical multiplexing/demultiplexing device combines an etalon and weak diffraction grating along with temperature control to finely tune local resonant stations along the etalon to separate and/or combine a plurality of optical signals that are finely spaced in wavelength.

FIELD OF THE INVENTION 
This invention in general relates to the field of optical communications 
and in particular to wavelength division multiplexing devices for use in 
optical communications systems. 
BACKGROUND OF THE INVENTION 
Although both grating and interference filters have been used as optical 
filters for wavelength division multiplexing, neither provides for 
sufficiently high wavelength selectivity to effectively make use of the 
potential optical communication capacity inherent in the available optical 
bandwidth of fibers. For example, it is possible with relative ease to 
modulate present semiconductor laser diodes at frequencies up to 1 GHz. 
Since higher modulation rates entail excess cost penalties the modulation 
rate of 1 GHz may be adopted as typical of what will be used for a broad 
range of communication purposes in the near future. With this channel 
bandwidth, separating adjacent optical channels by a frequency difference 
much greater than this value is tantamount to wasting optical 
communication capacity. Yet 1 GHz at one micron wavelength represents a 
wavelength difference of one 0.003 nm. 
Consider a diffraction grating having 1/D=5000 lines/cm. At one micron the 
first order diffraction angle is given by 
EQU D n sin .THETA.=.lambda..THETA.=sin.sup.-1 (1/3)=0.339 radians. 
Assume further that a compact bulk optic device has dimensions of the order 
of 1 cm. Then the resolution in angle of the grating is given by, with w, 
the width of light beam,=1 cm. 
##EQU1## 
Therefore, the angular resolution of the grating is one part in 5000 or 0.2 
nm, i.e., about 100 times larger than is desired for close packed 
wavelength division multiplexing. Clearly, a grating of sufficient 
resolution must be 33 cm in size and hence bulky and prohibitively costly. 
The best interference filters have resolutions of approximately 1 nm or 
about 300 times coarser than desired. 
There are a very limited number of optical structures which provide the 
necessary selectivity. The Michelson echalon grating, the Lummer Gehrke 
plate and the Fabry-Perot etalon are well-known examples. Of these, the 
Fabry-Perot is unique in that its effective physical length is multiplied 
by the "finesse" of the etalon. That is the length over which interference 
is active is equal to the number of round trip distances the light beam 
bounces back and forth within the etalon before leaking away or being 
absorbed. The Fabry-Perot etalon is therefore a compact device having 
extraordinarily high resolution. 
Fabry-Perot resonators exhibit many resonances separated in frequency by 
the amount f where, 
##EQU2## 
denoted as the "free spectral range" , where L is the round trip distance 
in the resonator, n is the index of refraction and c is the velocity of 
light in vacuum. The half height, full bandpass of the resonator is 
defined to be equal to the free spectral range divided by the finesse. For 
example, a 1 cm thick etalon made of glass having an index of 1.5 has a 
free spectral range of 10 GHz. If the resonator finesse is made to be 
equal to 100 then the filter bandpass is equal to 100 MHz. The finesse of 
the etalon is controlled or determined by the reflectivity of the surface 
mirrors, the absorption of the internal etalon medium, diffraction losses, 
and lack of perfect parallelism of the opposing mirror surfaces. With 
care, parallel plate glass etalons may be manufactured having finesses of 
up to at least 100. 
By virtue of its high finesse, the Fabry-Perot, unlike the Michelson or 
Mach Zehnder interferometers, allows one to distinguish between a number 
of different wavelengths bands equal to the value of the finesse of the 
etalon. For example, if the Fabry-Perot finesse is 100, then in principle 
one can distinguish between any one of 100 adjacent wavelength bands. 
However, for use as a filter, one must separate adjacent channels by 3-5 
times the bandpass to achieve acceptable crosstalk levels. 
However, wavelengths separated by an integer number of the free spectral 
ranges of the etalon can not be distinguished or separated from one 
another by a (single) Fabry-Perot etalon. The presence of multiple 
resonances in effect limits the communication capacity of a single 
Fabry-Perot etalon to a single free spectral range because of this 
inability to discriminate modular the free spectral range. While at first 
sight this appears to be a disadvantage to the approach of using a 
Fabry-Perot etalon or the similar behaving ring resonators as filters, the 
ambiguity may be resolved by using, for example, more than one Fabry-Perot 
resonator in tandem, creating the effect of a much increased free spectral 
range. With multiple resonators working in vernier fashion the free 
spectral range is multiplied by the finesse of each additional resonator 
used for filtering. Thus, for example, if two resonators are used each 
having a finesse of 100 and a free spectral range of 10 and 10.1 GHz 
respectively, then the total effective free spectral range is increased 
from 10 GHz to 1000 GHz. In this case the overlap of resonances from each 
filter occurs only after 99 or 100 free spectral ranges of the two 
component filters. 
Conversely, the multiple resonances of Fabry-Perot resonators have the 
benefit not only of vernier tuning but (1) allowing the use of laser 
operating with frequency differences separated by many free spectral 
ranges (that is, the need to match laser frequencies is greatly alleviated 
for single resonator filter systems), and (2) the presence of multiple 
resonances allows one to transfer the frequency stability of a highly 
stable source to the etalon and thence electronically stabilize a laser to 
any coexisting etalon resonance. 
Since a simple Fabry-Perot etalon having a finesse of 100 can selectively 
pass one wavelength band to the exclusion of the remaining 99 wavelength 
bands, such an etalon can be used as a multiplexer/demultiplexer to 
efficiently separate or combine many wavelengths of light. One approach 
for multiplexing is to successively pass a light beam by 100 Fabry-Perot 
etalons using each etalon to separate a distinct one of the 100 
distinguishable wavelengths from the rest. Such a procedure is made 
difficult by the requirement that the light strike each etalon at 
substantially normal incidence. Clearly the manufacture and use of 100 
separate etalons for multiplexing and demultiplexing is cumbersome and 
costly both in terms of manufacturing etalons and the necessary optics and 
the associated electronics required to stabilize the wavelength of the 
etalon filters in the presence of changing ambient conditions such as 
temperature. 
What is needed is a relatively compact, rugged, and easily manufacturable 
device that provides a resolution of the order of 1 GHz and a free 
spectral range of 100 GHz, allowing approximately 100 channels to be 
multiplexed and demultiplexed. Specifically, what is implied for 
demultiplexing is that all wavelengths enter via a common single mode 
fiber and different wavelengths exit in a spatially separated format so 
that the separated wavelength components can be separately detected, sent 
to separate fibers or otherwise separately processed. Moreover, what is 
needed is a controlled method of separation such that spatial separation 
is linearly proportional to wavelength separation. However, unlike the 
diffraction grating a much higher resolution is required for close packed 
wavelength division multiplexing. It should be appreciated that, due to 
channel crosstalk considerations, the number of useful channels is 
approximately equal to the finesse divided by 3 to 5. 
It is therefore a primary object of the present invention to provide a 
wavelength division multiplexing device that satisfies these several 
requirements. 
SUMMARY OF THE INVENTION 
This invention relates to a bulk optic multiplexing/demultiplexing device 
that has the ability to separate and combine a plurality of optical 
signals that are finely spaced in wavelength and, as such, is suitable for 
use in a variety of applications in optical communications systems, 
sensing, and displays. 
In preferred embodiments, the device comprises an etalon structure 
comprising a piece of glass that may be in the form of a rectangular 
parallelepiped having two opposed lengthwise surfaces polished and coated 
to that they are substantially parallel and highly reflecting--one 
substantially 100% and slightly transparent. On the surface with the more 
highly reflective mirror there is placed a weak diffraction grating and 
the length of the grating and its corresponding mirror are shorter than 
the surface while the other partially reflective surface is slightly 
longer to provide two clear sections for coupling at opposite ends of the 
etalon. 
Optical signals are coupled into the parallelepiped near one end as a 
collimated beam directed obliquely into a section of the partially 
reflecting surface at a predetermined angle of incidence so the the beam 
strikes the grating at the same predetermined angle of incidence a number 
of times as it propagates along the length of the etalon with minimal 
loss. The weak grating is arranged to diffract a small portion of the 
signal beam within a predetermined band of wavelengths perpendicular to 
the reflecting surfaces wherever the beam strikes it along the length of 
the etalon to provide a set of equally spaced local resonator stations 
that can be fine tuned in wavelength as indicated below so that at each 
resonator station one selected wavelength can be separated out from the 
initial wavelength band diffracted by the grating. The individual signals 
emerge from the device at their corresponding resonator location by 
transmitting through the partially reflecting surface. 
The individual local resonators are tuned by controlling the local optical 
path length between the reflecting surfaces either by incorporating a 
slight wedge shape to the etalons by changing the physical length of the 
bulk glass through heating or stress or its index of refraction with 
electric fields in embodiments where the medium has electro-optic 
properties or any other such mechanism. 
Remaining signals are coupled out of the etalon by way of a waveguide and 
focusing lens that accepts collimated signals and directs them into the 
end of the waveguide.

DETAILED DESCRIPTION 
Referring now to FIG. 1, there is shown at 10 a multiplexing/demultiplexing 
device embodying the features of the present invention. Device 10 
comprises an etalon 12 that is make of a blank of high quality, low loss 
glass, such as fused silica or BK7, that is ground and polished so that 
its opposite faces, 14 and 16, are optically flat and mutually parallel. 
Flatness of a 20th of one wavelength of light and parallelism or 
controlled lack of parallelism by a small fraction of a fringe over the 
entire active surface of etalon 12 is required. These requirements are 
similar to the requirements for producing high quality, high finesse 
etalons and are achievable with present state of the art fabrication 
procedures. A weak diffraction grating 18 is manufactured on one side of 
etalon 12 so as to cause approximately one percent of the incident light 
is diffracted into the one desired first order of diffraction, the 
remaining light being speculatory reflected. Finally, highly reflective 
coatings, 20 and 22, are applied to both sides of etalon 12 except at end 
sections, 24 and 26, where light is to enter or exit the body of etalon 
12. 
In normal use, light from an input fiber 21 is directed by a collimating 
lens 30 onto entrance section 24 and enters etalon 12 at nonnormal 
incidence so that the light beam successively bounces back and forth 
between opposite reflecting surfaces of etalon 12 as well as advancing 
along it in a preselected direction. The angle of incidence of the light 
beam and the grating spacing are mutually selected so that a small portion 
(of the order of one percent as stipulated above) of the obliquely 
incident beam is diffracted to propagate perpendicularly to the surfaces 
of the etalon. Now, since both etalon surfaces are close to perfectly 
reflecting and the glass is non absorbing, the diffracted light is trapped 
by etalon 12 and can only escape via a subsequent weak diffraction process 
or via the slightly transparent mirrors. Thus, any light so 25 trapped 
bounces back and forth in etalon 12 about 100 times before escaping. 
The above light trapped in etalon 12 can interfere constructively or 
destructively with later arriving portions of the obliquely incident light 
beam diffracted so as to be trapped in etalon 12. Destructive interference 
between incident and trapped light causes the electric field build up in 
etalon 12 to be relatively small, about 0.01% of the incident light beam 
power (i.e., circulating power is down by roughly 1/finesse). However, if 
constructive interference occurs between the light circulating within 
etalon 12 and the incident light beam which "pumps" it via diffraction 
grating 18, then the power level in etalon 12 is 100 times larger (larger 
by the ratio of the finesse) than the incident light power level. Under 
such circumstances where the round trip optical path is an integer number 
of wavelengths of light, the etalon "resonates" with the incident light 
wavelength and extracts a significant fraction of light power from the 
incident beam thereby inhibiting the undiffracted passage of light past 
the resonating etalon. If no other light losses are present except light 
diffraction, then diffraction will act to remove a fraction of light from 
the forward propating beam and diffract light into the backward 
propagating direction. A more useful situation occurs, however, when one 
surface of etalon 12 is slightly transparent, since then the high electric 
field circulating within the resonant structure allows a significant 
fraction of light to exit via the slightly transparent mirror. It is this 
portion of light exiting etalon 12 via the slightly transparent mirror 
that comprises the useful output since relatively little light will exit 
via this surface unless etalon 12 resonates. 
As is readily apparent, the FIG. 1 structure contains many equivalent 
locations where light may be diffracted so as to be trapped via localized 
etalons, and these correspond to the locations having the broken arrows 
emerging from surface 16. In order for wavelength 
multiplexing/demultiplexing to occur, each successive etalon is made to 
resonate at a different wavelength. If, for example, the localized etalons 
have a finesse of 100 each successive etalon must have a length difference 
of at least 1/100 of a wavelength of light as measured in the glass. For 
all practical purposes, the thickness of etalon 12 is to be constant just 
as in a normal etalon, except for a systematic change which is accurately 
produced and controlled. Such small differences may be created by simple 
polishing procedures but can also readily be produced via imposing a 
temperature gradient to systematically control localized temperature. To 
first order, the opposite ends, 32 and 34, of the multiple etalon 
structure are held at different controlled temperatures such that a linear 
temperature gradient results and a small linear path difference is created 
between adjacent localized etalons. To provide more accuracy, localized 
sections of the etalon structure can be individually temperature 
controlled to provide a more linear controlled path difference even if 
linearity were not present in the original structure held at a single 
common temperature value, or heating electrodes can be made nonlinear to 
correct for nonlinear temperature gradients. In the case of temperature 
control, the temperature range can be adjusted to be a full free spectral 
range across the multiplexer or a fraction of a free spectral range. In 
the former case a single reference frequency or two reference frequencies 
can be used to control the temperatures present at the first and last 
localized resonators of the multiplexer so that intermediate resonators 
cover, without additional stabilization electronics, the remaining 
intermediate channel frequencies. 
In addition to temperature control, stress may also be used as a means for 
controlling the optical path length at the localized etalon stations. 
An array of photodetectors 33 including individual cells typified by 35 can 
be placed on or opposite the output face of device 10 as shown in FIG. 1 
for purposes of converting the separated light signals into electrical 
form. 
Device 10 has a number of expected uses. First of all, it can be readily 
used for long haul transmission since a single device multiplies through 
WDM by more than an order of magnitude the overall capacity of a link. 
Secondly, the device can be made to be substantially polarization 
insensitive, an attribute not necessarily held by either fiber optic 
resonators or integrated optic resonators. Thirdly, by incorporating many 
resonators into a single device, separate wavelength control of each 
resonator is not required. Thus, the electronic complexity is greatly 
reduced. Fourth, as mentioned above, the structure is simple, rugged and 
readily manufactured. Finally, the incorporation of many resonators into a 
single device makes more practical the use of such a device as a multiple 
local drop. That is, throughput coupling loss for the input/output or bus 
beam which may ordinarily be as high as perhaps 2 dB per drop, is 
effectively associated with many local drops--hence the loss per drop may 
be quite low--less than 0.2 dB for a ten resonator structure. It may 
indeed prove impossible to create either fiber or integrated optic 
resonators with as low a drop loss as may be created by this multiple 
resonator bulk optic device. 
The bus resonators can, of course, be tuned to different bandpasses by 
changing the characteristics of the diffraction grating 18 or by using 
several sections in tandem each with its own grating spacing. The grating 
resolution is of the order of 1 nm for a 1 mm diameter input beam. Thus, 
the throughput loss of a multiplexer/demultiplexer will be relatively low 
unless the wavelength (or oblique angle of incidence) is proper to 
diffract a reasonable fraction of light into a direction perpendicular to 
the etalon surfaces. The diffraction loss to the input beam will be of the 
order of 1% per diffraction grating reflection if the associated etalon is 
not resonating. 
Assuming a single channel of the multiplexer/demultiplexer is allocated to 
one subscriber, this subscriber can use different etalon orders for 
different purposes. In this case, the subscriber has a second 
demultiplexer/multiplexer device similar to the bus 
multiplexer/demultiplexer that is operated to provide coarse filtering 
action such that differing orders of the bus demultiplexer are separated 
by the subscriber's local demultiplexer. 
Suitable diffraction gratings may be constructed on the etalon surface 
either by ruling machines or by photolithography. Either procedure 
provides techniques whereby the ruling may be "blazed" so as to 
preferentially diffract light between selected desired directions thereby 
wasting less light diffracted into unwanted orders. The highly reflecting 
surfaces of etalon 12 can be created via multilayer dielectric coatings or 
via metalization. While silver reflects about 99% of 0.8 microns and so 
could easily be used to produce a at least one mirror of a resonator with 
a finesse of 100 highly reflective metallic mirrors exhibit high loss in 
transmission. Dielectric coatings are preferred at least for partially 
transparent mirrors to maximize transmission for a give level of 
reflectivity. Also if dielectric coatings are used, it would be desirable 
to create 100% reflectivity at the oblique angle of incidence of the 
exciting beam while creating a slightly transparent coating (reflectivity 
of say 98.5%) for light propagating perpendicular to etalon 12 so as to 
more readily allow for some light to escape to be used. 
FIG. 2 shows explicitly the relationship between the oblique incident 
direction of the multiplexed light beam, designated at 36, relative to the 
etalon surface and the spacing periodicity of the diffraction grating 18. 
Here also are shown the diffracted beam and the speculatory reflected beam 
which are designated at 38 and 40, respectively. For a given grating 
periodicity distance, D, the angle of incidence, .THETA., must be set so 
that sin .THETA.=.lambda./nD in order to diffract light perpendicular to 
the surface. A weak diffraction grating is created via a photolithographic 
masking procedure on photo resist and then etching the glass with a weak 
solution of HF through developed photoresist, then removing photoresist. 
FIG. 3 shows how the surface of grating 18 would appear if blazed by 
standard ruling procedures. 
FIGS. 4 and 5a-c show equivalent multiplexer/demultiplexer structures using 
electro optic media rather than glass. In these embodiments, individual 
resonator tuning may be achieved electrically rather than by thermal 
control. In addition electrical signals can now be applied to the 
resonator to modulate a cw carrier frequency present on the multiplexed 
beam. 
FIG. 4 illustrates a resonator device 50 fabricated from a crystal 52 such 
as ADP or KDP (ammonium and potassium dihydrogen phosphate respectively) 
with the "c" or optic axis pointing downwardly in the plane of the paper. 
The diffraction grating here is shown at 54, and opposite it is a 99% 
reflecting, electrically conducting mirror 55 as, for example, created by 
sputtering indium tin oxide on top or a dielectric reflector. 
Corresponding to each localized etalon station along the crystal 52 are 
100% reflective conductive electrodes 56-60, and associated with each 
electrode, 56-60, are tuning and signal voltage sources, 62-66. As before, 
an input fiber 68 is directed into an end section of crystal 52 by a 
collimating lens 70, and a focusing lens 72 directs the remaining signal 
into an output bus fiber 74 for further downstream use. With the 
arrangement of device 50, the electric field is applied in the trapped 
light beam direction. In this case, since the light paths of the etalons 
lie along the c-axis, both polarization states resonate at the same 
wavelength. 
FIGS. 5a-c show a device 80 that is fabricated of an etalon made of lithium 
niobate or lithium tantalate. With device 80, the electric field is 
applied perpendicular to the trapped beam direction rather than in its 
direction. As shown, device 80 comprises a polished crystal 81 of lithium 
niobate or lithium tantalate having on one side a grating 82 backed by a 
100% reflecting mirror 84. Facing mirror 84 is a 99% reflecting mirror 86 
to allow a small percentage of selected light to pass through it. 
The input trunk line is represented by optical fiber 88 whose output is 
collimated by lens 90 while being directed onto the surface of the grating 
82. Output is by way of branch fiber 92 which receives its input by 
focusing lens' 94. Pickoff stations by which signals can be transferred 
from trunk to station and branch and vice versa are represented by typical 
fiber 100 and its associated lens 102. 
As shown in FIGS. 5a and b, the front side of device 80 has a ground 
electrode 104 attached to it while the opposing side has a array of 
separated electrodes, 106-112, corresponding to the pickoff stations. Each 
electrode, 106-112, has associated with it a variable voltage source, 
114-120, respectively, for tuning signals through the application of an 
electric field transverse to the direction of propagation of signal 
through the bulk material 81. 
Those skilled in the art may make other embodiments without departing from 
the scope of the invention. For example, more closely spacing the 
reflecting surfaces of the etalon while at the same time widening the 
incoming beam so that its opposite edges overlap during successive 
reflections as it travels down the etalon, permits the output to exist as 
a wavelength continuum as in a spectrum analyzer rather than as discrete 
wavelengths which exist at regularly spaced locations. Also, it will be 
recognized that the devices are completely reservable in operation for 
either demultiplexing or multiplexing applications. Therefore, it is 
intended that all matter contained in the above description or shown in 
the accompanying drawings shall be interpreted as illustrative and not in 
a limiting sense.