U.S. patent application number 10/218418 was filed with the patent office on 2004-02-19 for optical channel monitoring device.
Invention is credited to Churin, Yevgeniy, Honda, Tokuyuki, Yang, Long.
Application Number | 20040032584 10/218418 |
Document ID | / |
Family ID | 31714542 |
Filed Date | 2004-02-19 |
United States Patent
Application |
20040032584 |
Kind Code |
A1 |
Honda, Tokuyuki ; et
al. |
February 19, 2004 |
OPTICAL CHANNEL MONITORING DEVICE
Abstract
An optical channel monitoring device uses a linear variable
filter (LVF) disposed in the path of a beam of light for
selectively transmitting light in a variable manner along a length
of the filter, a photodetector array positioned in the path of
light transmitted through the LVF for measuring spectral
characteristics of the transmitted light, and collimating means
disposed between the input port and the LVF for collimating said
beam of light. The device is a low-cost, compact and rugged
high-resolution spectrometer for various uses.
Inventors: |
Honda, Tokuyuki; (Sunnyvale,
CA) ; Churin, Yevgeniy; (San Jose, CA) ; Yang,
Long; (Union City, CA) |
Correspondence
Address: |
ALLEN, DYER, DOPPELT, MILBRATH & GILCHRIST P.A.
1401 CITRUS CENTER 255 SOUTH ORANGE AVENUE
P.O. BOX 3791
ORLANDO
FL
32802-3791
US
|
Family ID: |
31714542 |
Appl. No.: |
10/218418 |
Filed: |
August 15, 2002 |
Current U.S.
Class: |
356/328 ;
356/419 |
Current CPC
Class: |
G02B 6/29395 20130101;
G02B 6/4215 20130101; G01J 3/2803 20130101; H04B 10/077 20130101;
G01J 3/0208 20130101; G01J 3/0256 20130101; G02B 6/29385 20130101;
G01J 3/26 20130101; H04B 10/07955 20130101; G02B 6/29361 20130101;
G02B 6/29358 20130101; G01J 3/02 20130101 |
Class at
Publication: |
356/328 ;
356/419 |
International
Class: |
G01J 003/18; G01J
003/51 |
Claims
1. An optical channel monitoring device comprising: an input port
for launching a beam of light, a linear variable filter (LVF)
disposed in the path of the beam of light for selectively
transmitting light in a variable manner along a length of the
filter, a detector means for measuring spectral characteristics of
the light transmitted through the LVF, the detector means
comprising a photodetector array disposed in the path of light
transmitted through the LVF in a predetermined position relative to
the LVF, and collimating means disposed between the input port and
the LVF for collimating said beam of light, wherein the collimated
beam of light is incident on the LVF at a negative incidence angle
selected to optimize focusing of the transmitted light on the
photodetector array.
2. The monitoring device of claim 1 wherein the LVF has a wedge
shape and the collimated beam of light is incident on the LVF at an
angle determined according to the formula 6 Z = - Sn 2 ( 0 + )
,where Z is focus position on the photodetector array, .lambda. is
wavelength, S is wavelength slope of the LVF, n is the effective
refractive index of the LVF, .theta..sub.0 is incidence angle of
the collimated light beam on the LVF, and .DELTA..theta. is the
half divergence angle of the output light beam from LVF.
3. The monitoring device of claim 1 wherein the angle is more than
one degree.
4. The monitoring device according to claim 1 further comprising a
phase mask disposed in the light beam path between the input port
and the collimating means for modifying intensity profile of the
light beam.
5. The monitoring device of claim 1 wherein the linear variable
filter comprises at least one light transparent wedged spacer
sandwiched between two reflective layers.
6. The monitoring device of claim 1 further comprising a dispersive
element disposed in the path of the light beam between the input
port and the LVF.
7. The device of claim 6 wherein the dispersive element is a
diffraction grating.
8. The device of claim 7 further comprising a focusing lens between
the input port and the grating, wherein the grating is disposed
about the image of the input port.
9. The device of claim 1 wherein the collimating means comprises
two positive cylindrical lenses.
10. The device of claim 9 wherein the collimating means further
comprises a negative cylindrical lens.
11. The device of claim 1 further comprising a mirror disposed
between the LVF and the detector means for diverting the path of
the transmitted light and reducing an overall length of the
device.
12. The device of claim 3 wherein the angle is between 1 degree and
about 10 degrees.
Description
RELATED APPLICATIONS
[0001] None
TECHNICAL FIELD
[0002] This invention relates to optical channel monitors, and more
particularly to such devices utilizing a linearly variable filter
(LVF).
BACKGROUND ART
[0003] The evolution of optical telecommunication systems to
dynamically controlled wave-division multiplexing (WDM) networks
has created a strong demand for optical channel monitoring devices.
An optical channel monitoring device typically measures wavelength,
power, and optical signal-to-noise ratio of each wavelength
channel. It thus enables sophisticated and accurate control of the
network. Some of the typical performance requirements for the
state-of-the-art optical channel monitoring devices are wavelength
channel resolution of 0.2 nm to 0.8 nm, wavelength accuracy of 50
to 100 pm, power accuracy of 0.5 dB, and optical signal-to-noise
ratio measurement of up to 30 dB.
[0004] An optical channel-monitoring device typically consists of
an optical system, electronics, and software. The function of the
optical system resembles that of a spectrometer. That is, the
optical system decomposes the incoming signal into wavelength or
spectrum components using a dispersive element. Two types of
dispersive elements that have been widely used for this purpose are
gratings and Fabry-Perot etalons. In either case, the measurement
is rather sensitive to a change in mechanical alignment. It is
therefore a challenge to maintain required performance over long
term through severe environmental changes that are assumed in most
of the telecommunication applications.
[0005] Linear variable filter (LVF) is yet another type of
dispersive element that has been used in the field of spectroscopy.
LVF is made by depositing optical thin-film layers on a substrate
in such a way that the thickness of the films varies linearly with
position. The thickness variability is very small, of the order of
a few microns over a few inches, or even less. The filter can be
designed either as bandpass filter or high/low-cut filter. More
details about LVFs can be found e.g. in U.S. Pat. No. 6,057,925 to
Anthon, incorporated herein by reference. Spectral information of
incoming optical signal can be obtained by placing a detector array
behind the LVF (U.S. Pat. No. 5,166,755 issued to Gat, incorporated
herein by reference). This approach enables rather compact and
rugged mechanical design.
[0006] The use of the linear variable filter in optical channel
monitoring devices, however, has been hindered by packaging
problems. In particular, it has been a challenge to design a device
that has sufficiently low cross talk between wavelength channels.
The cross talk can be minimized by having a detector array that has
much larger number of pixels than the number of channels and by
making the width of the optical beam spot on the detector about as
small as the pixel width. On the other hand, the width of the
detector pixels in general decreases with the number of pixels in
order to keep the practical size of the detector element. For
example, the pixel width of the state-of-the-art 512-pixel detector
array is about 25 .mu.m. This poses a challenge in packaging.
Assuming Gaussian profile of the beam, the depth of focus that is
defined by Rayleigh range is only 0.3 mm for the beam diameter of
25 .mu.m at the wavelength of 1.55 .mu.m (B. E. A. Saleh and M. C.
Teich, "Fundamentals of Photonics," John Wiley & Sons, (New
York 1991), pp.86-87).
[0007] On the other hand, it is not practical to place LVF at close
proximity to the detector array for a number of reasons. Depositing
LVF coating on the surface of the detector array is difficult
because of the delicate surface and wiring of the detector array.
Placing a separate LVF element inside the detector package is also
problematic since it requires the removal of a window plate that is
part of a hermetic package that protects the delicate detector
surface. In addition, the need for minimizing the package size of
the device sometimes requires freedom to place LVF more than
several millimeters away from the detector array package.
[0008] In U.S. Pat. No. 6,057,925, supra, now assigned to the same
corporate assignee as the present invention, Anthon discloses the
use of micro lens array between the LVF and the detector array. The
Anthon method enables focusing of the optical beam on the detector
array while LVF is placed at an arbitrary position. However, the
introduction of the micro lens array will cause a substantial
increase in packaging cost. In addition, light scattering and/or
aberration around the boundary of each lens are potential problems
that may increase the cross talk between wavelength channels.
[0009] Accordingly, there is a need for an optical
channel-monitoring device that overcomes the above problems.
SUMMARY OF THE INVENTION
[0010] In accordance with the invention, there is provided an
optical channel monitoring device comprising: an input port for
launching a beam of light, a linear variable filter disposed in the
path of the beam of light for selectively transmitting light in a
variable manner along a length of the filter, a detector means for
measuring spectral characteristics of the light transmitted through
the LVF, the detector means comprising a photodetector array
disposed in the path of light transmitted through the LVF in a
predetermined position relative to the LVF, and collimating means
disposed between the input port and the LVF for collimating said
beam of light. The collimated beam of light is incident on the LVF
at a negative incidence angle selected to optimize focusing of the
transmitted light on the photodetector array. In an embodiment of
the invention, the LVF has a wedged layer and the collimated beam
of light is incident on the LVF at an angle .theta..sub.0
determined according to the formula 1 Z = - Sn 2 ( 0 + ) ,
[0011] where Z is focus position on the photodetector array,
.lambda. is wavelength, S is wavelength slope of the LVF, n is the
effective refractive index of the LVF, .theta..sub.0 is incidence
angle of the collimated light beam on the LVF, and .DELTA..theta.
is the half divergence angle of the output light beam from LVF.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The invention will be described and explained in more detail
by way of the following description in conjunction with the
accompanying drawings showing only typical, non-limiting
embodiments of the invention, and in which:
[0013] FIG. 1 illustrates a theoretical model of linear variable
filter (LVF),
[0014] FIG. 2 is a graph of optimum incidence angle as a function
of distance from LVF,
[0015] FIG. 3a is a top view of an embodiment of the monitoring
device of the invention,
[0016] FIG. 3b is a side view of the embodiment of FIG. 3a,
[0017] FIG. 4 is a graph showing the beam intensity profile from a
single-mode fiber as a function of the position at LVF, with or
without a phase mask,
[0018] FIGS. 5a and 5b are a top view and side view, respectively,
of another embodiment of the invention,
[0019] FIGS. 6a and 6b are a top view and side view, respectively,
of yet another embodiment of the device,
[0020] FIGS. 7a and 7b are a top view and side view, respectively,
of still another embodiment of the device,
[0021] FIG. 8a is a schematic view of an embodiment utilizing a
diffraction grating, and
[0022] FIG. 8b is a schematic view of an embodiment similar to that
of FIG. 8a.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0023] The present invention is concerned with optical channel
monitoring device that utilizes an LVF and a detector array. The
LVF has a band-pass filter design that consists of one or more
cavities. The invention is based on a finding that the focus
position of the output beam from LVF can be adjusted by the
incidence angle to LVF.
[0024] In general terms, the invention has, in a preferred
embodiment, an optical system that converts the light from the
input fiber to a nearly collimated beam and 2) a means to make the
collimated beam incident on the LVF at a tailored incidence angle
(usually more than 1 degree, typically 1 to 10 degrees) pointing
toward the thin-film side of the LVF, i.e. slanted upwards left as
schematically shown by the dotted line in FIG. 1.
[0025] The principle of the invention is explained by considering a
theoretical model of LVF as shown in FIG. 1. The LVF, generally
designated as 10 has generally a wedge shape and consists of two
reflective layers 12, 14 and a dielectric wedged spacer layer 16
with a refractive index n. In practice, the reflective layers
consist each of a number of quarter-wave dielectric layers with
alternatively variable refractive index n (high n/low n/high n . .
. ). The wedge-shaped spacer layers, typically of identical shape,
are alternated with reflective layers. The reflective layers 12, 14
have amplitude reflectance r. The wedge angle of the spacer layer
is .alpha., an angle typically much lower than 1.sup.0. The
thickness of the spacer layer at X=0 is h. For simplicity, the
coordinate position X=0 is chosen in such a way that h is an
integer times .lambda./2n. The phase shift upon reflection is
ignored since it does not change the conclusion.
[0026] The LVF is illuminated by a plane wave that has amplitude
and incidence angle of a.sub.0 and .theta..sub.0, respectively. The
plane-wave components of the output beam are depicted as a.sub.1
expi.PHI..sub.1, a.sub.2 expi.PHI..sub.2, . . . , where a.sub.N is
the amplitude and .PHI..sub.N is the phase at X=Z=0. The amplitude
and phase can approximately be written as, under the assumption
that the wedge angle and the incidence angle are relatively small,
2 a N = a 0 r 2 ( N - 1 ) ( 1 - r 2 ) 2 , ( 1 ) N = h 3 an 2 N 3 +
2 h N + C ( 2 )
[0027] where .theta..sub.N is the propagation direction of the Nth
plane-wave components and C is a constant phase that does not
depend on the propagation direction. The focus position Z can be
calculated by using the fact that the propagation of plane-wave
components introduces phase shift that is approximately quadratic
to the propagation angle (J. W. Goodman, "Fourier Optics," 2.sup.nd
ed., McGraw Hill (New York, 1996) pp. 57-58): 3 Z = - 2 2 N N 2 ( 3
)
[0028] Here, we choose the propagation direction that corresponds
to the direction of the output beam from the LVF. This means
that
.theta..sub.N=.theta..sub.0+.DELTA..theta. (4)
[0029] where .DELTA..theta. is half divergence angle of the output
beam from the LVF.
[0030] By using Eq. (2), (3), and (4) it follows that 4 Z = - h an
2 ( 0 + ) ( 5 )
[0031] Eq. (5) shows that the focus position Z has a linear
dependence on the incidence angle .theta..sub.0. Therefore, one can
adjust the focus position without additional optical elements by
simply adjusting the incidence angle to the LVF. The incidence
angle .theta..sub.0 for positive focus position Z is negative. This
means that the incident beam should be directed toward the
thin-film side of LVF, i.e. inclined left as schematically
represented by the dotted line 18. It is noted that the incidence
angle for Z=0 is non-zero. That is, the incidence angle that gives
the narrowest beam width is not zero if the detection plane is
right behind LVF. It is often convenient to characterize LVF with
wavelength slope S (i.e., wavelength shift per length). Eq. (5) can
be re-written by using the wavelength slope S as 5 Z = - Sn 2 ( 0 +
) ( 6 )
[0032] In practice, effective refractive index should be used in
place of the refractive index of the spacer layer taking into
account the fact that the reflective layers are also affected by
the incidence angle. The calculation method of the effective
refractive index for band-pass filters is explained in H. A.
Macleod, "Thin Film Optical Filters," 2.sup.nd Ed., McGraw-Hill
(New York 1986) pp. 260-265.
[0033] FIG. 2 shows an example of the optimum incidence angle that
gives focusing on a detector as a function of the distance between
the detector and an LVF. An experiment was conducted with a
band-pass LVF that had spectral width of about 0.1 nm. Experimental
results agreed with the theoretical data fairly well (FIG. 2).
Here, the optimum incidence angle was determined by the narrowest
beam width on the detector. The theoretical line was calculated by
using Eq. (6). The wavelength slope was S=2.6 nm/mm, effective
refractive index was n=1.7, and the wavelength was 1550 nm.
[0034] As already stated herein, one of the important
considerations of the present invention is an optical design that
ensures that the incidence angle on the LVF satisfies the
relationship of Eq. (6).
[0035] It will be understood by those versed in the art that the
light is treated in the LVF according to the local wavelength vs.
the position on the filter. In other words, the wavelength off
local resonance of the filter will pass through the filter, indeed
highly attenuated, with little refraction, while wavelengths in the
resonance band will be brought into a reasonable focus on
corresponding pixel(s) with little cross talk.
[0036] FIGS. 3a (top view) and 3b (side view) show one embodiment
of the device of the invention. Optical signal input is provided by
an optical fiber 20 that defines an input port. Cylindrical lenses
21 and 22 convert the light into a collimated beam that illuminates
LVF 23 at an oblique incidence angle in such a way that the beam,
while distributed over a large portion of the LVF (the portion
called "aperture") points toward the thin-film side of the LVF. The
incidence angle on the LVF is typically 1 to 7 degrees without
being limited thereto. The output beam from LVF consists of narrow
beams that correspond to channel wavelengths .lambda.1, .lambda.2,
.lambda.3, and so on. A photodetector array 24 is placed
approximately in parallel to the LVF 23 so that the propagation
distance between the LVF and the detector array is constant for all
the wavelengths. However, as explained herein, the photodetector
may be placed angularly relative to the LVF. The width of each
pixel of the detector array is typically 10 to 100 .mu.m. The
detector array may be packaged in a hermetic case, not shown. The
electric signal from the detector array may be sent to an
electronic circuit 26 for signal processing. The electronic
circuitry for this purpose is well known in the art, and is for
instance described in the Gat patent, supra. In addition, a phase
mask 28 may be placed close to the face (input port) of the input
fiber to improve the uniformity of beam intensity on the
photodetector array (I. Gur and D. Mendlovich, Opt. Commun. 145, p.
237, 1998). For wavelength around 1.55 .mu.m, for example, a simple
rectangular phase mask ("phase plate") with the phase step of .pi.
and the width of 14 .mu.m can reshape the intensity profile from a
typical single-mode fiber as shown in FIG. 4. It can be seen that
the intensity response is significantly changed (flattened)
compared to the original Gaussian shape.
[0037] When the light beam incident on LVF has a collimation error,
i.e., the beam is slightly divergent or convergent, the incidence
angle will not be uniform over the length of LVF. In this case, the
variation of incidence angle can be compensated for by making the
detector array non-parallel to the LVF. The distance between the
detector array and the LVF should be made larger on the side that
has larger (in magnitude) incidence angle so that the relationship
of Eq. (6) is satisfied over the length of LVF.
[0038] The package length of the optical system in FIGS. 3a and 3b
is largely determined by the requirement that the width of the
incident beam on LVF is larger than the length of the aperture of
the LVF (which generally corresponds to the size of the
photodetector array behind it). The width of the beam is limited by
the numerical aperture (NA) of the input fiber 20 and the focal
length of the cylindrical lens 22. Therefore, a shorter package
length requires a higher fiber NA. Alternatively, one can insert a
negative lens 28 (which may be cylindrical) between the input fiber
20 and the cylindrical lens 22 as shown in FIGS. 5a and 5b in order
to reduce the package length without increasing the fiber NA.
[0039] The focus adjustment with incidence angle enables the
placement of the detector array 24 at an arbitrary distance from
the LVF 23. This allows one to place an additional element between
the LVF and the detector without sacrificing wavelength resolution.
In one embodiment, a folding mirror 32 can be placed between the
LVF and the detector as shown in FIGS. 6a and 6b. This design can
be used to reduce the package height of the optical system.
Alternatively, a prism 34 may be used in place of a folding mirror
32. In this case, a LVF 23 may be directly deposited on a glass
prism as shown in FIGS. 7a and 7b.
[0040] The optical throughput (i.e., energy efficiency) of the
device may be improved by the use of a diffraction grating close to
the face of the input fiber. This is based on the fact that the
transmittance of the LVF is approximately equal to the ratio of the
transmitted beam width to the incident beam width. The grating
allows one to cover the length of LVF with narrower, multiple
incidence beams. FIGS. 8a and 8b show optical designs of the
present invention using a diffraction grating 36. The energy
efficiency of the device increases with the decrease of the
numerical aperture of the input light on the grating. Inserting a
positive lens 38 in front of the input fiber and placing the
grating around the image of the face of the input fiber can
decrease the numerical aperture and thus can increase the energy
efficiency (FIG. 8b). To obtain the most linear dependence of
diffraction angle versus wavelength, the central wavelength of
operating spectral range should diffract normally to the grating
surface.
[0041] It is an advantage of the invention that it offers a
relatively high accuracy of wavelength and power measurement
compared to the devices known to date.
[0042] The device offers a small package size at a relatively low
cost, and relatively high reliability due to simple design. It is
applicable to spectrometers in other fields than telecommunications
such as chemical industry, various medial applications, environment
sensing, etc.
[0043] Reference in the specification to "one embodiment" or "an
embodiment" means that a particular feature, structure, or
characteristic described in connection with the embodiment is
included in at least one embodiment of the invention. The
appearances of the phrase "in one embodiment" in various places in
the specification are not necessarily all referring to the same
embodiment.
[0044] In the foregoing specification, the invention has been
described with reference to specific embodiments thereof. It will,
however, be evident that various modifications and changes can be
made thereto without departing from the broader spirit and scope of
the invention. The specification and drawings are, accordingly, to
be regarded in an illustrative rather than a restrictive sense.
* * * * *