U.S. patent application number 09/944126 was filed with the patent office on 2002-07-18 for system and method for fabricating components of precise optical path length.
Invention is credited to Chen, Gang Paul, Eyal, Avishay, Kewitsch, Anthony S., Leyva, Victor, Rakuljic, George A..
Application Number | 20020093662 09/944126 |
Document ID | / |
Family ID | 26924395 |
Filed Date | 2002-07-18 |
United States Patent
Application |
20020093662 |
Kind Code |
A1 |
Chen, Gang Paul ; et
al. |
July 18, 2002 |
System and method for fabricating components of precise optical
path length
Abstract
Optical components, particularly microoptic glass components
used in synthesizing birefringence in filter systems based on
polarization interferometer techniques, are fabricated using
systems and methods which provide accurate frequency periodicity
measurements. These measurements are derived from differential
delays induced by in-process glass elements between beam components
in a polarization interferometer unit and from progressive
wavelength scanning across a wavelength band of interest. The
consequent sinusoidal output variation has peak to peak spacings
which are measured to provide frequency periodicity values from
which precise length corrections for the optical elements can be
calculated.
Inventors: |
Chen, Gang Paul; (Monterey
Park, CA) ; Eyal, Avishay; (Pasadena, CA) ;
Kewitsch, Anthony S.; (Santa Monica, CA) ; Leyva,
Victor; (Pasadena, CA) ; Rakuljic, George A.;
(Santa Monica, CA) |
Correspondence
Address: |
JONES, TULLAR & COOPER, P.C.
P.O. BOX 2266 EADS STATION
ARLINGTON
VA
22202
|
Family ID: |
26924395 |
Appl. No.: |
09/944126 |
Filed: |
September 4, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09944126 |
Sep 4, 2001 |
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09898469 |
Jul 5, 2001 |
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60230618 |
Sep 5, 2000 |
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Current U.S.
Class: |
356/491 |
Current CPC
Class: |
G02B 6/2766 20130101;
G02B 6/29302 20130101; G02B 6/272 20130101; G02B 6/2773 20130101;
G02B 6/29358 20130101 |
Class at
Publication: |
356/491 |
International
Class: |
G01B 009/02 |
Claims
We claim:
1. A system for enabling manufacture of elements of precise optical
path length to levels of precision needed for DWDM filtering
applications, comprising: an adjustable monochromatic optical
wavelength source providing a test beam output; a scanning control
coupled to the source to vary the test beam output sequentially
through incrementally varying wavelengths; an optical delay line
system coupled to receive the test beam from the source and
including at least one input beam displacer for receiving the test
beam and propagating two beams in parallel along a first and second
delay path, at least one output beam displacer for recombining
beams from the first and second delay paths into a single,
co-propagating path, the two delay paths each being adapted to
receive one or more glass elements, each of different optical path
length, and an optical transmission analysis system receiving
recombined beams from the delay line system and providing an output
representing the frequency periodicity of the recombined optical
beams, the frequency periodicity resulting from the interference of
beams traveling along the different optical delay paths.
2. A system as set forth in claim 1 above, wherein the transmission
analysis output is a sinusoidal variation having a frequency
periodicity measurable by the peak to peak spacing in the sinusoid
output.
3. A system as set forth in claim 1 above, wherein the system also
includes a data processor in communication with the transmission
analysis system for measuring the peak to peak frequency spacing in
the sinusoidal output, mathematically extracting the frequency
period, and calculating length corrections needed in the optical
elements.
4. A system as set forth in claim 1 above, wherein the adjustable
wavelength source comprises a tunable laser, a polarization
scrambler in the test beam path to the optical delay line system,
an optical power meter monitoring the test beam power, and the data
processor comprises scanning control algorithms to interrelate the
outputs from the power meter and the transmission analysis system
to provide a least square fit deviation of frequency periodicity
from the measured output.
5. A system as set forth in claim 4 above, wherein the delay line
system includes adjustable glass element support stages in each of
the delay paths for selectively interposing glass elements relative
to the individual delay paths, and wherein the system further
includes a broadband light source positioned to illuminate the
transmission analyzer through the glass elements, so that the
elements can be adjusted for precisely normal incidence angle in
relation to output from the optical transmission analysis system,
and wherein the transmission analysis system comprises a spectrum
analyzer.
6. A system as set forth in claim 3 above, wherein the scanning
control operates the wavelength source at successive wavelength
steps in the range of 1520 to 1570 nm.
7. A method of providing sets of interrelated glass elements for
use in DWDM applications, wherein the sets of elements provide an
athermal interferometric function with a selected frequency
periodicity, comprising the steps of: calculating preliminary
target values for at least two different glasses which establish
the athermal condition and the frequency periodicity desired for
the interferometric function; initially fabricating lengths of
glass elements of the different materials to slightly greater
dimensions than the target lengths; generating a sinusoidally
varying transmission response using the initially fabricated glass
elements in different delay paths; calculating a temperature
compensated frequency periodicity from the sinusoidally varying
transmission function; and adjusting the lengths of sets of glass
elements to final target lengths having selected frequency
periodicity and substantially athermal response at a center
wavelength.
8. The method as set forth in claim 7 above, wherein the steps of
generating a sinusoidally varying frequency response comprises the
steps of directing a series of beams varying incrementally in
wavelength through the initially fabricated lengths, combining beam
components from the different delay paths, and measuring the
amplitude responses from the different incrementally varying
wavelength beams.
9. The method as set forth in claim 8 above wherein the step of
calculating the frequency periodicity comprises calculating the
peak to peak spacing in the sinusoidally varying frequency
response.
10. The method as set forth in claim 8 above, wherein the varying
wavelengths are substantially centered on a selected central
operating wavelength, and wherein the method further comprises the
steps of testing temperature response at the central wavelength of
the adjusted lengths of glass elements under interferometric
conditions and adjusting the lengths of the glass elements to the
final target lengths.
11. The method as set forth in claim 7 above, wherein the lengths
of glass elements in the different delay paths comprise one glass
element in one delay path and two glass elements in the other delay
path.
12. The method as set forth in claim 7 above, wherein there are
individual glass elements of different indices of refraction in the
two delay paths.
13. The method as set forth in claim 7 above, wherein the glass
elements for DWDM applications are microoptic elements and the
initially fabricated elements are of substantially greater
cross-section than the microoptic elements, and further including
the steps of forming at least a pair of microoptic elements from
the initially fabricated elements; employing such microoptic
elements in the delay paths in to generate a sinusoidally varying
transmission response, measuring the temperature dependence at a
center wavelength, calculating final target lengths needed for
frequency periodicity and athermal characteristics, measuring the
frequency periodicity of the initially fabricated elements,
calculating the correction needed for establishing the final target
lengths, removing material from the initially fabricated elements
to establish the final target lengths, and dividing such elements
into microoptic elements of the final target lengths.
14. The method of fabricating microoptic elements of precise
optical path lengths for use in optical communication systems
employing differential delays in interferometric processes in which
frequency periodicity and stable temperature response at a center
wavelength must be precise, comprising the steps of: fabricating at
least two different types of precursor optical elements that can
each be formed into multiple microoptic elements, the precursor
elements having preliminary thicknesses which are oversized
relative to values calculated on the basis of nominal index of
refraction, thermal expansion coefficient and thermooptic
properties for each precursor optical element; separating
individual precursor microoptic elements from each precursor
optical element; employing precursor microoptic elements of at
least two different types of delay elements in a multi-frequency
interferometric measurement under varying temperature conditions;
calculating final target thicknesses needed for stable temperature
response and removing corrective amounts of material from each of
the precursor optical elements to provide final thicknesses needed
for selected frequency periodicity and stable temperature
response.
15. The method as set forth in claim 14 above, including the added
step of subdividing the corrected precursor optical elements to
multiple microoptic elements of the selected thicknesses and
precise optical path lengths.
16. The method as set forth in claim 15 above, wherein the step of
removing corrective amounts of material comprises the additional
items of measuring the frequency periodicity derived by a
differential delay between a first of the precursor optical
elements and a known media such as air, calculating the thickness
reduction needed to achieve the final target thickness for that
optical element, affecting such reduction to provide a corrected
first optical element, measuring the frequency periodicity derived
by differential delay between the corrected first optical element
and the second precursor optical element, calculating the thickness
reduction needed to achieve the final target thickness needed for
that optical element, and effecting such reduction to provide a
corrected second optical element.
17. A method of measuring the optical path lengths of optical
elements for use in an interferometric application comprising the
steps of: establishing a first target frequency periodicity of a
first optical element; separating an input optical beam into two
beam paths, delaying the optical signal in the first beam path by
inserting the first optical element, while separately delaying the
optical signal in the second beam path, and interferometrically
re-combining the two beam paths into a combined beam path;
determining the first initial frequency periodicity of the combined
beam path carried by a single polarization of the interfering
optical signals; adjusting the length of the first optical element
to provide a first final frequency periodicity within a closer
tolerance to the first target frequency period than that of the
first initial frequency periodicity.
18. A method in accordance with claim 17 comprising the additional
steps of: establishing a second target frequency periodicity;
inserting the second optical element into the second beam path;
determining the second initial frequency periodicity at the
combined beam path of the interfering optical signals at the
combined beam path; adjusting the length of the second optical
element to provide a second final frequency periodicity closer to
the second target frequency period than that of the second initial
frequency periodicity.
19. The method as set forth in claim 18, wherein the optical
elements are further adjusted by the steps of: determining the
temperature dependence of the second final frequency periodicity,
and comparing the measured temperature dependence with the target
temperature dependence, using the measured temperature dependence
of the second final frequency periodicity, determine a new first
target frequency periodicity with more precisely athermal
characteristics.
20. The method as set forth in claim 19, wherein the target
temperature dependence exhibits less than 2 GHz of shift over the
operating temperature range.
21. The method as set forth in claim 18, wherein the first target
frequency periodicity is determined from calculations using the
thermal expansion coefficients, thermo-optic coefficients, indices
of refraction, and from the second target frequency
periodicity.
22. The method as set forth in claim 19, wherein the second target
frequency periodicity is 25.000 GHz, 50.000 GHz, 66.667 GHz,
100.000 GHz, or 200.000 GHZ.
23. The method as set forth in claim 18 above, wherein the
frequency periodicity is determined by the step of transmitting
optical signals across a range of incrementally different
wavelengths, and measuring the optical transmission response to
determine the frequency periodicity.
24. The method as set forth in claim 23 above, wherein the step of
measuring the amplitude response comprises analyzing the sinusoidal
response using a least squares fit, wherein the primary fit
parameters are frequency periodicity and phase.
25. The method as set forth in claim 17 above, wherein the optical
elements comprise glass blanks having substantially greater
cross-sectional areas than are intended for individual elements in
an operative systems, and wherein the glass blanks after finishing
to the target frequency periodicities are segmented longitudinally
in cross section to provide a multitude of microoptic elements of
substantially identical target frequency periodicity.
26. A method of providing shared glass elements for use in DWDM
applications, wherein the shared elements are to provide an
athermal interferometric function with a selected frequency
periodicity, comprising the steps of: calculating target values for
two different glasses which establish the athermal condition and
desired frequency periodicity; providing glasses of the different
materials that are initially fabricated to slightly greater lengths
than the target lengths; determining the sinusoidal transmission
variation resulting from the differential delay of one beam through
a first glass element in a first delay path in relation to a
different media in the other delay path; adjusting the length of
the first glass element to a close approximation of the target
length; using a second glass element having a predetermined initial
length greater than a second target length in the second delay path
while retaining the first glass element in the first delay path;
determining the interferometric frequency variations resulting from
differential delay of the beams in the different glass elements in
the first and second paths; adjusting the second glass element to a
close approximation of the target length.
27. The method of providing optical elements of precise path length
and optical delay characteristics for use in DWDM interleaving
components operating at selected frequency periodicity within
selected frequency bands, comprising the steps of: generating a
variable wavelength beam of monochromatic light in the frequency
band of interest; varying the beam wavelength sequentially to
provide a test beam that varies with time incrementally in
frequency across the frequency band of interest; inputting the test
beam to a polarization interferometer as a beam which is split into
two substantially parallel beams of orthogonal polarization,
directed through separate delay paths and then recombined into a
single output beam producing a wavelength dependent, intensity
modulated output; interposing in one delay path a first glass delay
element of slightly greater thickness than the calculated target
thickness, the calculation based on the indices of refraction,
thermal expansion coefficients, and thermooptic coefficients of the
glasses, to provide both the correct frequency periodicity and low
drift of the center wavelength with temperature when combined with
one or more additional glass elements in an interleaving component;
employing a different delay media in the other delay path;
measuring the amplitude output at each wavelength from the
polarization interferometer; applying a least squares fit
calculation to the measured amplitude output, to extract measured
frequency period and phase from the fit; using the measured
frequency period to ascertain the needed reduction in thickness of
the first glass element to provide an element with frequency period
accurate to within a given frequency period tolerance; reducing the
thickness of the first glass element to the target thickness until
the desired frequency accuracy is achieved; interposing a second
glass element of typical index of refraction, thermal expansion and
thermooptic coefficient which is of an initial thickness slightly
greater than the calculated target thickness needed for the
athermal interleaver stage in the second delay path, while
maintaining the first glass element in the other parallel delay
path; repeating the sequence of steps of inputting beams
sequentially at incrementally varying wavelengths, employing the
polarization interferometer to derive wavelength dependent,
intensity modulated beams, calculating the frequency periodicity
and determining the thickness reduction needed in the second glass
element; reducing the thickness of the second glass element to the
target thickness to establish the target periodicity to within the
frequency period tolerance; and repeating the same steps as for the
second element for any additional glass elements.
28. A method in accordance with claim 27 above, wherein the process
is preceded by: fabricating a first and second glass element to a
first and second oversize thickness; testing the interferometric
characteristics of the glass element pair thus provided by
measuring the center frequency offset while cycling the temperature
of the unit over the desired operating temperature range; and
calculating, based on the measured center frequency offset, the
target frequency periodicity of the first and second glass
element.
29. The method as set forth in claim 27 above, further including
the steps of preliminarily adjusting the tilt and tip of each glass
element in its delay path to be normal to the input beam, by
maximizing the frequency periodicity as measured by the amplitude
output.
30. The method as set forth in claim 27 above, wherein the initial
glass elements are blanks approximately 100 .mu.m above the target
lengths and are of 2 inches or greater in diameter, wherein they
are ground and polished to the target thickness by polishing both
faces concurrently to achieve a high level of parallelism and low
transmitted wavefront distortion, after being formed to the target
and wherein the blanks are subdivided into microoptic elements with
substantially identical optical characteristics and like
thicknesses.
Description
REFERENCES TO RELATED APPLICATIONS
[0001] This application relies for priority on U.S. provisional
application No. 60/230,618 filed on Sep. 5, 2000 and entitled
"System and Method for Fabrication Components of Precise Optical
Path Length", P. Chen et al, and U.S. application Ser. No.
09/898,469 filed Jul. 6, 2001 by A. Eyal et al for "Interleaver
Filters Employing Non-Birefringent Elements".
FIELD OF THE INVENTION
[0002] This invention relates to methods and systems for the
fabrication of optical elements of precise length and more
particularly to fabrication of elements having optical path lengths
exact enough to be used in applications for optical communications
which use optical interferometry.
BACKGROUND OF THE INVENTION
[0003] There have long been needs for exactness in mechanical and
optical devices and systems, and these needs have heretofore been
met by a variety of techniques, from mechanical to optical.
Examples of the latter are found in a 1922 publication of the
Department of Commerce, entitled "Interference Methods for
Standardizing and Testing Precision Gage Blocks" by C. G. Peters
and H. S. Boyd, which describes light wave interference methods
which are not subject to the "appreciable errors" found with
"micrometer-microscopes" and "contact instruments". The authors
describe optical interference approaches imparting about an order
of magnitude improvement, e.g. from 0.25 to 0.025 microns. While
directed to the calibration of gage blocks, this article
nonetheless evidences what even today must be acknowledged as an
ingenious optical approach to ascertaining the dimensions, flatness
and parallelism of gage surfaces.
[0004] In modern telecommunication systems, however, dimensional
measurements are embedded in a number of other factors which arise
from the way in which optical elements are used. In
telecommunications systems using device wavelength division
multiplexing (DWDM), for example, polarization interferometry that
requires precise differential delays between different beams is
used in generating a required filter function. Interleavers
employing these relationships are described in U.S. application
Ser. No. 09/898,469 referenced above to provide athermal operation
within individual stages of multi-stage multiplexers and
demultiplexers.
[0005] The optical path length of an optical component through
which light traverses is dependent not only on distance but also on
intrinsic properties, such as the index of refraction, of the
components. Modern optical systems must meet such demanding
specifications that optical path length has become an important
consideration. The fabrication of interleaving optical filters for
DWDM using athermal delay line interferometers requires strict
control of the optical path lengths of the glass elements. This
control is necessary to meet the tight tolerances on absolute
channel frequencies for DWDM applications. Traditional physical
path length measurements such as mechanical or non-contact
thickness probes with sub-micron accuracies are thus not adequate
because the optical path length depends on both physical thickness
and the absolute index of refraction of the medium. Also, for
individual glass melts the index of refraction of typical optical
glasses varies by 10.sup.-5 to 10.sup.-4 from a nominal or target
value despite best production methods. This variation introduces
substantial uncertainty in optical path length even if the physical
thickness of the glass is known exactly. A particular additional
requirement is that the optical path length of any "glass window"
must be accurately measured at a chosen wavelength of operation
(e.g., 1550 nm) to account for material dispersion.
SUMMARY OF THE INVENTION
[0006] Methods and systems for fabricating optical elements such as
microoptic elements used in introducing differential delays in DWDM
interleavers, use a number of different measurements of frequency
periodicity at successive evolutionary processing steps leading to
final sizing. The method and system provide precise frequency
periodicities, with optical elements being so interrelated as to be
acceptably athermal at a chosen frequency. The optical elements
thus form the basis for a desired transmission spectrum for a DWDM
interleaver. Frequency periodicity is synthesized by measuring
output amplitudes derived from differential delays of a test beam
at a plurality of incrementally varying wavelengths in the
wavelength range of interest, using polarization interferometers to
introduce a filter function.
[0007] To fabricate to interleaver precision, the optical frequency
response of the interleaver must be characterized to sub-GHz in
terms of accuracy. The first step in the characterization process
is to measure the temperature dependence of the glass, and then
calculate to high accuracy the physical lengths needed for both
athermal operation and desired frequency periodicity. A first
optical element or window is then ground and polished to the
desired frequency periodicity given by a first calculation. For
example, consider the case of a 50 GHz interleaver with a 100 GHz
free spectral range (FSR). The first window is fabricated from the
higher index glass of a pair of glasses to be used and is polished
to within about 0.04 GHz of the target value to ensure good
temperature insensitivity of the final interleaver. This glass
element is left in the optical frequency measurement system, and
the second window is then ground, polished and repeatedly measured
by placing into the second arm of the optical path length
measurement system until the frequency periodicity of the combined
two glass interleaver is 100.00+-0.03 GHz. A like process is
utilized to fabricate time delay elements for other FSR's, such as
25 GHz to 200 GHz.
[0008] In accordance with the invention, the frequency periodicity
is ascertained during different steps using the differential delays
introduced between different optical elements or one optical
element and air in the delay paths of a polarization
interferometer. An input test beam is propagated through both delay
paths, but varied incrementally in wavelength through a selected
range of wavelengths. This results in derivation of a sinusoidal
variation from which peak to peak spacings determinative of
frequency period can be calculated so that optical path length
correction can be computed to a degree of accuracy dependent on the
state of dimensional refinement of the element. By starting with
precursor elements large enough in transverse area for multiple
microoptic elements, and using the given oversize in thickness in
the precursors, removal of thickness to final dimension can suffice
for all microoptic elements at the same time.
[0009] To achieve these tolerances, the measurement system
described herein meets the extremely high accuracy standards
implicitly required for the measurement of optical path length.
During the final polishing process of the glass window blanks, the
optical path length is periodically measured until the target value
is achieved. This measurement technique enables conventional
polishing techniques to achieve the desired thicknesses.
[0010] Precise optical path length measurements have been achieved,
accurate to better than 10 ppm. That is, a glass element with a
free spectral range of 100.000 GHz can be fabricated to have a
period accurate to better than 1 MHz. The parameter that is
directly measured is the optical frequency response of the
interleaver in about the 1500 to 1600 nm wavelength range but the
determinative result is the establishment of optical path
length.
[0011] A measurement system in accordance with the invention
employs a tunable laser, controlled by a data processor to scan a
selected wavelength range in equal, small increments, to generate
wavelength varying test beams. The beams are directed through a
differential delay system using polarization interferometry to
generate a wavelength dependent output. This is received at a
spectrum analyzer which stores the sinusoidally varying amplitude
readings from the different wavelengths for analysis.
[0012] The data processor receives the data and employs a least
squares fit program to analyze the sinusoidal variations and
ultimately derives the length correction needed for an optical
element. The optical measurement apparatus for introducing
differential delays in microoptic elements, which area used in
testing temperature dependence is in the form of a single stage
interleaver. Measurement apparatus for large precursor elements
incorporates stages which can be adjusted in two dimensional to
position the optical element. Additionally, using illumination
directed from a broadband light source through the optical element
onto the spectrum analyzer, the tilt and tip orientation of the
optical element can be optimized before differential delay readings
are made. The laser beam power is advantageously monitored by a
power meter coupled to provide measurement signals to the data
processor for use in equalizing readings derived during scanning.
Also a polarization scrambler is preferably employed in the beam
path where polarization dependence in the interferometer may affect
readings, by assuring that there is no dominant polarization.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] A better understanding of the invention may be made by
reference to the following description, taken in conjunction with
the accompanying drawings, in which:
[0014] FIG. 1 is a block diagram of an optical measurement system
in accordance with the invention that includes a top view of a
polarization interferometer optic;
[0015] FIG. 2 is a side view of the optical layout of the
polarization interferometer optical path length measurement system
of FIG. 1;
[0016] FIG. 3 is a top view indicating the location of positioning
elements for placement of glass elements into a polarization
interferometer;
[0017] FIG. 4 is a process flow chart depicting steps in
determining glass temperature characteristics to high accuracy;
[0018] FIG. 5 is a process flow chart depicting steps in
fabricating glass elements to high accuracy in frequency optical
path length;
[0019] FIGS. 6A and 6B are graphs presenting the temperature
stability of the center frequency of fabricated using the process
outlined in FIGS. 4 and 5;
[0020] FIG. 7 is a graph illustrating the center frequency
temperature dependence achieved with two and three glass designs;
and
[0021] FIG. 8 is a graph illustrating the temperature dependence of
the response of an interleaver fabricated in accordance with the
process depicted in FIGS. 4 and 5.
DETAILED DESCRIPTION OF THE INVENTION
[0022] Extremely tight fabrication tolerances must be maintained
between the differential optical paths of a system such as an
advanced state of the art interleaver (e.g. Ser. No. 09/898,469
supra) to both provide athermal operation and to achieve the
desired frequency periodicity. The differential optical path length
between the two arms must be kept constant to a high level of
precision (1 part in 10.sup.4) to maintain a constant period of
100.00 or 200.00 GHz, for example. The approximate optical
frequency is 193.000 THz, and the absolute frequency of each
channel should be aligned to the ITU wavelength grid to better than
1 GHz. The deviation of the frequency period from the target value,
for a typical 80 channel C band application, should be below 50
MHz. Practically, there is no glass material available with optical
path length temperature dependence matched to the necessary level
to that dependence exhibited by air. Individual glasses have a
relatively large dependence of the optical path length on
temperature. Therefore, a second glass material is necessary to
compensate for the temperature dependence introduced by the first
glass, and a third glass material can also be used for further
improvement. Furthermore, it is advantageous to place glasses of
substantially equal length in both arms. This eliminates the
dependence of the interleaver's frequency response on ambient air
conditions, which can be significant for 50 GHz or denser
interleavers. The length of glass is selected to give the correct
optical path length difference. The two glasses also should be
matched in their optical path length temperature dependence. 50 and
100 GHz interleavers include glass elements cascaded from length L,
such as 2L and 4L in a stage, where L is typically about 5 mm. The
tolerances of L are on the submicron level, requiring precise
thickness or optical path length control during the polishing
process.
[0023] Some considerations based on the theory of the polarization
interferometer are desirable to understand the degree of exactitude
needed in optic components for optical interferometer systems. The
transmission frequency response in terms of optical power of a
single interleaver stage is given by:
T=sin .sup.2(.phi.), (1)
[0024] where the phase for an N glass interleaver is given by: 1 =
2 f c ( ( n 1 - n air ) L 1 - ( n 2 - n air ) L 2 - - ( n N - n air
) L N ) . ( 2 )
[0025] The frequency dependence of the phase for an N glass
interleaver is: 2 f = 2 c ( ( n 1 - n air ) L 1 - ( n 2 - n air ) L
2 - - ( n N - n air ) L N ) + 2 f c ( n 1 f L 1 - n 2 f L 2 - - n N
f L N ) ( 3 )
[0026] In the following, we will focus specifically a two glass
interleaver design; however, similar analysis applies for the case
of three or more glass types. For this case of two glasses the
temperature dependence of the phase is: 3 T = 2 c ( ( n 1 - n air )
L 1 T - ( n 2 - n air ) L 2 T ) + 2 f c ( n 1 T L 1 - n 2 T L 2 ) (
4 )
[0027] The frequency periodicity of the total interleaver is: 4 f =
c ( ( n 1 - n air ) L 1 - ( n 2 - n air ) L 2 ) + f ( n 1 f L 1 - n
2 f L 2 ) ( 5 )
[0028] For example, for a 50 GHz interleaver, .DELTA.f=100.00 GHz,
and for a 100 GHz interleaver, .DELTA.f=200.00 GHz. The frequency
periodicity of individual glasses 1 and 2 under the condition of
temperature compensation is: 5 f 1 = f ( 1 - f 1 T f 2 T ) ( 6 ) f
2 = f ( f 2 T f 1 T - 1 ) ( 7 )
[0029] By choosing L.sub.1=L.sub.2, the temperature dependence of
air factors out of the interleaver frequency response. If this
condition were not met, a 6 mm element of air would contribute a
frequency shift of approximately 3 GHz/10.degree. C. This effect
also depends on whether the interleaver operates under constant
pressure or constant volume conditions after it is sealed and
packaged. Therefore, for highly stable telecom applications,
insensitivity to inner atmosphere (e.g., air pressure or
temperature) is necessary.
Measurement System
[0030] A functional diagram of a measurement system used to
characterize the glass elements is illustrated in FIG. 1. The
measurement system comprises a number of operative elements: a
computer 10 with frequency period analysis software, a tunable
laser 12 operated in a wavelength scanning mode by the computer 10
using scanning control algorithms, a polarization scrambler 14, an
optical measurement bench 16, a power meter 18 to which the laser
beam can be directed after the bench 16 at a junction 19 comprising
a switch, fiber or splitter, and an optical spectrum analyzer 20.
the power meter 18 and the spectrum analyzer 20 provide signals to
the computer 10 for use in normalizing the readings. The computer
10 is interfaced to the tunable laser 12 using a standard bus such
as GPIB, serial, or parallel, and the optical measurement bench 16
provides communication between the tunable laser 12, optical power
meter 18 and optical spectrum analyzer 20, through switches or
optical fibers. A separate, broadband light source 22 provides
light into the laser beam path via a junction 24 which may comprise
a beam combiner on an optical switch.
[0031] The optical measurement bench 16 consists of a series of
polarization beam splitters, 1/2 waveplates and polarizers, as seen
in both FIGS. 1 and 2, corresponding in substantial part to the
basic elements of the interleaver. Beam displacers, fabricated from
birefringent crystals, can be used for both the polarization beam
splitters and polarizers. In general terms, an incoming beam from
the laser 12 is split in the optical measurement bench 16 into two
polarized beams which are laterally displaced from one another but
exit the crystal along parallel paths. One outgoing beam from the
displacer is e polarized, the other is o polarized, where the
orientations of the e and o polarizations are dictated by the
crystal orientation of the beam displacer. The parallel beams are
directed through differential delay paths which are used in the
testing of different glasses or relationships before the beams are
recombined for the remainder of the measurement.
[0032] Several optical layouts achieve the desired optical
measurement response. This example (FIGS. 1 and 2) utilizes an
input collimator 30, a polarizer 32 (Coming Polarcor.TM. for
example) oriented at 45.degree. to the horizontal, a first
horizontal beam displacer 34, and a true zero order 1/2 waveplate
35 to rotate the polarization of the displaced e and o beams by
90.degree. before one or more glass elements to be measured for
optical path length. The elements, called windows or pucks when in
multi-element size, or microoptics elements if they are sized for
use in an interleaver, may be used singly or in combinations in the
measurement bench 16. in this example, two windows, 36, 37
(designated Glasses 1 and 2) are depicted as in positions they
occupy (individually or concurrently) intercepting the beam paths
in the delay segments. An optional window 38, shown in dotted lines
in FIGS. 1 and 2, is Glass 3 which may be measured also if a three
glass stage is to be fabricated. A second horizontal beam displacer
40 follows the windows 36, 37 to recombine the two beams, an output
polarizer 42 oriented at -45.degree. to the horizontal, and an
output collimator 44. All optics are antireflection coated in the
1500 to 1600 nm window, and the zero order waveplate is here
designed for 1550 nm operation. Note that this optical layout
closely resembles a single stage of the actual microoptic
interleaver described in U.S. patent application Ser. No.
09/898,494 by Eyal et al., but on a larger scale to allow the
precursor window blanks to be tested before dicing into microoptic
windows.
[0033] The input optical beam is here, however, not dependent on
the state of polarization of the laser 12 output. The polarization
scrambler 14 rotates the input beam polarization repeatedly during
the duration in which each beam of different wavelength is
transmitted. The power meter 18 samples the beam amplitude and is
operated separately from the optical spectrum analyzer 20 after
recombination, so that the computer 10 can normalize the readings
from the spectrum analyzer 20.
[0034] This arrangement provides a relatively simple yet precise
means of sampling the wavelength scanning test beams. If the laser
output is uniformly polarized, the components may be oriented
suitably to that reference. If the beam has an arbitrary state of
polarization, the input beam may be split into upper and lower
pairs of orthogonal polarizations, to provide a polarization
independent output as in the Eyal et al application.
[0035] The input horizontal beam displacer 34 separates the input
beams into e and o polarized beams, horizontally spaced by about
0.7 mm. The e and o beams may pass through one or more different
glass elements 36, 37, etc., depending upon the measurement to be
performed. Upon propagating through the glass elements, 36, 37,
etc., the two beams acquire a relative phase shift between one
another, because of the different indices of refraction of the
glasses, and next enter a 1/2 waveplate 35 oriented at 45.degree.
to convert the o to e and the e to o polarizations. The two beams
are then recombined into a single output beam by the output
horizontal beam displacer 40. The waveplate ensures that the
optical path lengths traveled by the two beams (left and right)
through the displacers 34, 40 are equal. The lengths of the input
and output beam displacers 34, 40 are also precisely matched to
ensure that the split beams are recombined into a single spot, and
to precisely match the net distance each beam travels within the
two displacers. Any residual path-length mismatch is precisely
measured and mathematically corrected for in the data processing
performed during the measurements.
[0036] This optical system is connected to the lightwave
measurement system (FIG. 1) which scans and processes the
wavelength response of the polarization interferometer. An Agilent
81641 tunable laser mainframe 12 provides a test beam to the
interferometer input, and the interferometer output is measured by
the single channel optical power meter 18 to record the transmitted
optical power in transmission on the computer 10. The laser 12 is
scanned from 1520 to 1570 nm with a 0.01 nm step size for a total
of 5000 points. The signals measured by the lightwave measurement
system constitute a sinusoidal amplitude response varying with
optical frequency. The data is least-squares fit by the computer 10
and period analysis software to a sinusoid, the two fit parameters
being the optical frequency period and the phase to determine the
frequency period to a few parts in 10,000. A 50 GHz interleaver,
for example, requires an optical frequency period of 100.00 GHz.
The control of the laser wavelength scanning, data acquisition, and
least-squares curve fitting is performed by commercial software,
for example, LabVIEW from National Instruments, Inc..
[0037] The optical path length of the glass windows 36, 37, 38
depends partly on the angle of incidence of the beams with respect
to the windows. Normal incidence corresponds to the shortest
optical path length. To ensure that the one or more glass windows
are positioned exactly normal to the optical beams (within a few
arcmins), an Agilent 71452B optical spectrum analyzer functioning
with the broadband light source 22 in the communication link may be
used to monitor the spectral response of the polarization
interferometer in real time as the tilt and tip of glass windows
36, 37 are adjusted.
[0038] Upon inserting only the first piece of glass, e.g. 36, the
tilt and tip are to be adjusted until the frequency period of the
interleaver is minimized, which minimizes the optical path length
in the glass. FIG. 3 illustrates the apparatus used to achieve
orientational adjustment of the glass windows during the
measurement process, with commercial micropositioning tables 47, 48
being employed that are adjustable in two angular directions,
specifically in tilt and tip. Such micropositioning tables are
available from Newport Corp. and other optical equipment suppliers.
For the second piece of glass, the tilt and tip are also adjusted
until the frequency period of the interleaver is maximized or
minimized, depending on the glass type, index of refraction and its
length. Additional pieces of glass are to be inserted using a
similar procedure. Those angles (1b, 2b, 3b) which are to be
maintained close to normal incidence are indicated as small squares
in FIGS. 1 and 2. If the angles are not 90 degrees, then the
optical path length measurement is incorrect. The beam displacers
and waveplates are epoxied in place to maintain mechanical
stability during the measurements, and the input and output
collimators are welded to the optical bench. The polarization
scrambler 14 is typically installed in-line with the tunable laser
output to ensure that the state of polarization is scrambled or
depolarized at the input to the measurement system 16.
Characterization of Temperature Dependence of Glasses
[0039] FIG. 4 is a flow chart outlining the steps required to
characterize glass delay line elements to the level -needed to
temperature compensate interleavers while also achieving the
precise frequency periodicity. The flow chart of FIG. 5 depicts
processing steps used after this characterization has been
completed.
[0040] This example applies to a two glass design, but the method
can be readily extended to a three or more glass design using
Equations 2-3 above. These delay line elements are to be processed
as large glass "pucks" or plane parallel windows, which are
subsequently diced into microoptic elements. The first step is to
select suitable glasses which compensate for one another's
temperature dependence. This is based initially on the thermo-optic
and thermal expansion contributions to the temperature dependence,
as determined from the vendor specifications. These published
parameters, which are not adequately precise for present purposes
are input into Equations 6 and 7 to provide the target frequency
periodicity of each glass. The glass pucks are then ground and
polished to be at some predetermined frequency above the target
frequency. Typically, these frequencies are chosen to correspond to
pucks each 100 ums thicker than the predicted target thicknesses
(each about 9 mm thick). A microoptic element from each puck is
then diced and used to build, in effect a one stage interleaver,
from which the temperature dependence of the center frequency is
measured. These measurements enable the residual temperature
dependence of the pair to be calculated, from which the errors in
the published specifications can be calculated and corrected for.
Each glass melt has slightly different index of refraction and
thermal characteristics, so that in general this thermal
characterization process should be repeated for each glass melt,
and used in calculating the residual temperature dependence.
Method to Fabricate Glass Elements of Precise Optical Path
Length
[0041] FIG. 5 illustrates the method by which glass elements are
fabricated to a precise optical path length, after the measurements
have been made which characterize the temperature dependence of the
glasses. The input signal to the optical measurement bench 16 is
generated by the tunable laser 12 which scans the wavelength region
of interest (C or L band). The polished window blanks e.g. 36, 37
are first ground and polished to a thickness slightly over the
target value. Conventional thickness measurement techniques are
used up to this point. Next, the optical thicknesses of the parts
are determined using the measurement system described herein and
the following sequence. First, one oversize glass element e.g. 36,
is placed in the optical measurement bench 16 and aligned. Next,
the amount of material to be removed is calculated, and further
material is removed. This step is typically a final mechanical
polishing step which can be carried out commercially to a high
degree of precision once the absolute value of material to be
removed is known. Alternatively, processes such as reactive ion
etching or chemical etching can be utilized to remove the small
amount of material during this final process step.
Magneto-rheological polishing is an alternate technique which
allows precise figuring of both the flatness and optical thickness
of individual polished windows. When the first glass element 36 has
been polished to the correct optical path length, the second glass
element is inserted, and it is polished until the second target
frequency periodicity is achieved. This process may be continued if
more than two glass elements are used in the design. Note that the
polishing may be simultaneously conducted on a large number of
relatively large glass "pucks" of identical thickness. This
provides the advantage of batch processing because thousands of
microoptic elements are produced during each production run.
[0042] The windows are ground on a double sided ring lap using
aluminum oxide slurry, and subsequently are polished on a similar
double sided ring lap using cerium oxide slurry. Both these double
sided machines optimally utilize pitch polishing rather than pad
polishing. Alternately, a conventional double sided polishing
machine using polyurethane pads, for example, may be suitable for
the lapping and polishing operations. In either approach, the
double sided polishing has the inherent advantage that the flatness
errors of both surfaces are in general complementary. As a
consequence, the transmitted wavefront distortion of these plane
parallel windows is inherently low, which is important to maintain
consistent optical path length across the entire window. These
windows are fabricated to provide a transmitted wavefront
distortion of better than .lambda./3 to .lambda./10 (where
.lambda.=633 nm) across the 2 inch diameter substrate.
[0043] Processing of large diameter parts provides several
advantages; namely, excellent surface flatness, transmitted
wavefront distortion, parallelism of polished surfaces, and batch
processing. The optical path length or physical thickness of the
parts can be measured during the polishing stage to determine how
much material should be removed. The removal rates are a well
characterized part of the process (e.g., um per hour). This ensures
that the glass elements are fabricated to the correct thickness to
guarantee temperature insensitivity and to achieve the correct
interleaver frequency response.
[0044] After these frequency targets are achieved, the glass is
diced into a large number of identical microoptic windows. Upon
dicing these large windows into microoptic delay line elements of,
for example, 2.6.times.2.6 mm cross section, the residual power
contribution to the flatness, which scales as the square of the
diameter of the part, results in a transmitted wavefront distortion
of less than .lambda./300 across the individual parts. Note,
however, that this level of wavefront distortion is in practice not
measureable.
[0045] In practice, this procedure gives extremely good temperature
stability of the center wavelength. FIGS. 6A and 6B illustrate some
typical dependencies of the center frequency with temperature, for
50.000 and 25.000 GHz interleavers, respectively. Note that the
frequency drift varies approximately quadratically with center
frequency within the passband. The linear dependence has been
effectively nulled. The total shift with temperature is typically
less than 2 GHz over the -5 to 65.degree. C. operating temperature
range for this group of interleavers.
[0046] By adding a third glass element, additional design
flexibility is obtained. The residual quadratic temperature
dependence can then be nulled, leaving only a cubic dependence.
FIG. 7 illustrates the residual temperature dependence for a two
and three glass design. FIG. 8 depicts the resulting transmission
spectrum of an interleaver using the process described herein to
fabricate a 50 GHz interleaver of precise period and low center
frequency drift with temperature. The measured transmissions at -5,
+5, +25, +45, and +65 degrees C. are overlaid for comparison.
[0047] Systems and methods in accordance with the invention enable
noncontact measurement of optical path length in terms of the
thickness of optical windows to an accuracy of 100 nanometers.
Further it is amenable to use in high production processes since
large precursor blanks can be dimensioned together to provide a
multiplicity of individual microoptic elements. While particularly
suited for meeting the critical requirements of optical
communication system, such as interleavers, these systems and
methods are applicable wherever comparable requirements exist.
* * * * *