U.S. patent application number 14/261295 was filed with the patent office on 2015-10-29 for system for controlling the chirp of an optical signal.
This patent application is currently assigned to CANON KABUSHIKI KAISHA. The applicant listed for this patent is CANON KABUSHIKI KAISHA. Invention is credited to Takefumi Ota.
Application Number | 20150309386 14/261295 |
Document ID | / |
Family ID | 54328177 |
Filed Date | 2015-10-29 |
United States Patent
Application |
20150309386 |
Kind Code |
A1 |
Ota; Takefumi |
October 29, 2015 |
SYSTEM FOR CONTROLLING THE CHIRP OF AN OPTICAL SIGNAL
Abstract
An apparatus for altering a chirp of an input light comprising
one or more grating structures and one or more electro-optical
blocks. In which a voltage is applied to the one or more
electro-optical blocks to alter a chirp of light exiting the
apparatus.
Inventors: |
Ota; Takefumi; (Tucson,
AZ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
CANON KABUSHIKI KAISHA |
Tokyo |
|
JP |
|
|
Assignee: |
CANON KABUSHIKI KAISHA
Tokyo
JP
|
Family ID: |
54328177 |
Appl. No.: |
14/261295 |
Filed: |
April 24, 2014 |
Current U.S.
Class: |
359/245 |
Current CPC
Class: |
G02F 1/035 20130101;
G02F 2201/305 20130101; G02F 2203/25 20130101 |
International
Class: |
G02F 1/35 20060101
G02F001/35 |
Claims
1. An apparatus for altering a chirp of an input light comprising:
a first grating structure positioned to receive the input light to
produce a first dispersed light; a first electro-optical block
positioned relative to the first grating structure to receive the
first dispersed light which exits the first electro-optical block
as a second dispersed light; a second grating structure positioned
relative to the first electro-optical block to receive the second
dispersed light to produce a first collinear light; a third grating
structure positioned relative to the second grating structure to
receive the first collinear light to produce a third dispersed
light; a second electro-optical block positioned relative to the
third grating structure to receive the third dispersed light which
exits the second electro-optical block as a fourth dispersed light;
a fourth grating structure positioned relative to the second
electro-optical block to receive the fourth dispersed light to
produce an output light; and wherein a voltage applied to the first
and second electro-optical blocks alters the chirp of the output
light.
2. The apparatus of claim 1 further comprising: one or more mirrors
positioned relative the second grating structure to reflect the
first collinear light into the third grating structure.
3. The apparatus of claim 1 wherein the first electro-optical block
and the second electro-optical block are the same electro-optical
block.
4. The apparatus of claim 1 wherein the first grating structure and
the fourth grating structure are the same grating structure.
5. The apparatus of claim 1 wherein the second grating structure
and the third grating structure are the same grating structure.
6. The apparatus of claim 1 wherein the first grating structure,
second grating structure third grating structure, and the fourth
grating structure are the same grating structure.
7. The apparatus of claim 1 wherein varying the voltage, varies the
chirp of the output light and does not shift a position at which
the output light exits the fourth grating structure more than an
alignment tolerance.
8. The apparatus of claim 1 wherein varying the voltage, varies the
chirp of the output light and does not shift an angle at which the
output light exits the fourth grating structure more than an
alignment tolerance.
9. The apparatus of claim 1 wherein the first grating structure,
the second grating structure, the third grating structure, and the
fourth grating structure are transmission gratings.
10. The apparatus of claim 1 further comprising one or more
apertures to limit which diffraction modes exit the apparatus.
11. The apparatus of claim 1 wherein, a group of grating structures
include the first grating structure; the second grating structure;
the third grating structure; and the fourth grating structure; and
grooves, slits, or edges of the group of grating structures are
aligned with a crystalline axis of the EO material.
12. The apparatus of claim 1 wherein, the first grating structure;
the second grating structure; the third grating structure; and the
fourth grating structure are all blazed.
13. The apparatus of claim 1 further comprising a polarizer that
receives the input light and passes polarized input light to the
first grating structure.
14. The apparatus of claim 1 wherein, a group of grating structures
include the first grating structure, the second grating structure,
the third grating structure, and the fourth grating structure; and
a group of electro-optical blocks include the first electro-optical
block and the second electro-optical block; and the group of
grating structures are formed on the group of electro-optical
blocks.
15. An apparatus for altering a chirp of an input light comprising:
a grating structure; and an electro-optical block; wherein the
input light is spatially dispersed by the grating structure a
series of four times; wherein after the input light is dispersed by
the grating structure the first time it passes through the
electro-optical block a first time; wherein after the input light
is dispersed by the grating structure the third time it passes
through the electro-optical block a second time; wherein after the
input light is dispersed a fourth time by the grating structure it
is output as chirped output light; and wherein a voltage applied to
the electro-optical block alters the chirp of the output light.
16. The apparatus of claim 15 wherein, wherein the grating
structure comprises a first grating structure and a second grating
structure.
17. The apparatus of claim 15 wherein, wherein the grating
structure comprises a first grating structure, a second grating
structure, a third grating structure, and a fourth grating
structure.
18. The apparatus of claim 15 wherein, wherein the electro-optical
block comprises a first electro-optical block and a second
electro-optical block.
Description
BACKGROUND
[0001] 1. Field of Art
[0002] Aspects of this disclosure are related to systems for
controlling the chirp of an optical signal.
[0003] 2. Description of the Related Art
[0004] In the context of the present disclosure, "chirp" refers to
the manner in which the optical frequency changes over time. Chirp
may also refer to the manner in which the optical frequency changes
over time within the period of an optical pulse. An alternative
definition of chirp is the time dependence of the instantaneous
frequency of an optical pulse.
[0005] Several methods have been used in order to control the chirp
of a pulse including: optical fibers, grating pairs, prism pairs,
chirp mirrors, and chirped fiber Bragg gratings. For example, prior
art methods have adjusted the chirp of a pulse by changing the
distance between a pair of gratings or a pair of prisms.
[0006] There are several optical techniques in which the chirp of
an optical pulse is an important parameter. When the chirp of the
optical pulse is important, an optical system is often designed
with a specific optical chirp. This is often accomplished with a
chirped fiber Bragg grating. It is often advantageous to change the
chirp of an optical pulse. This sometimes accomplished by using an
optical switch in combination with a variety chirped fiber Bragg
gratings with different chirps, or by changing the environment
(temperature, tension, voltage) of the chirped fiber Bragg grating.
Other mechanical methods of adjusting the chirp are also used, such
as changing the distance between a pair of grating.
[0007] The present techniques have problems with reliability,
speed, flexibility, and power handling. This present disclosure
aims to address these issues.
SUMMARY
[0008] Embodiments of the present disclosure provide an apparatus
for varying the chirp.
[0009] According to an aspect of the present disclosure an
apparatus for altering a chirp of an input light comprising: a
first grating structure positioned to receive the input light to
produce a first dispersed light; a first electro-optical block
positioned relative to the first grating structure to receive the
first dispersed light which exits the first electro-optical block
as a second dispersed light; a second grating structure positioned
relative to the first electro-optical block to receive the second
dispersed light to produce a first collinear light; a third grating
structure positioned relative to the second grating structure to
receive the first collinear light to produce a third dispersed
light; a second electro-optical block positioned relative to the
third grating structure to receive the third dispersed light which
exits the second electro-optical block as a fourth dispersed light;
a fourth grating structure positioned relative to the second
electro-optical block to receive the fourth dispersed light to
produce an output light. A voltage applied to the first and second
electro-optical blocks alters the chirp of the output light.
[0010] The apparatus may further comprise one or more mirrors
positioned relative the second grating structure to reflect the
first collinear light into the third grating structure. The first
electro-optical block and the second electro-optical block may be
the same electro-optical block.
[0011] The first grating structure and the fourth grating structure
maybe the same grating structure. The second grating structure and
the third grating structure maybe the same grating structure. The
first grating structure, second grating structure third grating
structure, and the fourth grating structure maybe the same grating
structure.
[0012] Varying the voltage may vary the chirp of the output light
and may not shift a position at which the output light exits the
fourth grating structure more than an alignment tolerance. Varying
the voltage may vary the chirp of the output light and may not
shift an angle at which the output light exits the fourth grating
structure more than an alignment tolerance.
[0013] The first grating structure, the second grating structure,
the third grating structure, and the fourth grating structure maybe
transmission gratings. The first grating structure, the second
grating structure, the third grating structure, and the fourth
grating structure maybe reflection gratings.
[0014] A group of grating structures includes the first grating
structure; the second grating structure; the third grating
structure; and the fourth grating structure. Grooves, slits, or
edges of the group of grating structures maybe aligned with a
crystalline axis of the EO material.
[0015] The first grating structure; the second grating structure;
the third grating structure; and the fourth grating structure maybe
all blazed. The apparatus may also include a polarizer that
receives the input light and passes polarized input light to the
first grating structure. A group of electro-optical blocks include
the first electro-optical block and the second electro-optical
block. The group of grating structures maybe formed on the group of
electro-optical blocks.
[0016] Another aspect of the present embodiment is an apparatus for
altering a chirp of an input light comprising: a grating structure;
and an electro-optical block. The input light is spatially
dispersed by the grating structure a series of four times. After
the input light is dispersed by the grating structure the first
time it passes through the electro-optical block a first time.
After the input light is dispersed by the grating structure the
third time it passes through the electro-optical block a second
time. After the input light is dispersed a fourth time by the
grating structure it is output as chirped output light. A voltage
applied to the electro-optical block alters the chirp of the output
light.
[0017] The grating structure may comprise a first grating structure
and a second grating structure. The grating structure may comprise
a first grating structure, a second grating structure, a third
grating structure, and a fourth grating structure. The
electro-optical block may comprise a first electro-optical block
and a second electro-optical block.
[0018] Further features and aspects will become apparent from the
following detailed description of exemplary embodiments with
reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] The accompanying drawings, which are incorporated in and
constitute a part of the specification, illustrate exemplary
embodiments. Similar reference numerals refer to similar parts.
[0020] FIG. 1 illustrates an apparatus for controlling chirp;
[0021] FIG. 2 illustrates a portion of an apparatus for controlling
chirp;
[0022] FIG. 3 illustrates an apparatus for controlling chirp in a
first embodiment;
[0023] FIG. 4 illustrates an apparatus for controlling chirp in a
second embodiment;
[0024] FIG. 5 illustrates an apparatus for controlling chirp in a
third embodiment; and
[0025] FIG. 6 illustrates an apparatus for controlling chirp in a
fourth embodiment.
DESCRIPTION OF THE EMBODIMENTS
[0026] Embodiments will be described below with reference to the
attached drawings.
[0027] The present disclosure is a new apparatus for controlling
the chirp of an optical pulse. FIG. 1 is an illustration of a
system 100 that produces a chirped pulse. The system 100 includes a
light source 102 that produces a broadband pulsed input light 104.
The system includes an Electro-Optical (EO) material 106. The
refractive index (n.sub.2) of the EO material 106 can be changed by
applying a voltage via electrodes 108. The voltage is applied with
a voltage source 110. The voltage source may be a DC voltage source
or a function generator. The EO material 106 includes grating
structures 112 on the input and output faces of the EO material
106.
[0028] The grating structures 112 may be transmission gratings that
are attached to the faces of the EO material 106 or placed close to
the faces of the EO material. The gratings structures may be
adjacent to the EO material 106 and index matching fluid may be
between the gratings structures 112 and the EO material. The
grating structures 112 may be manufactured as an integral part of
the EO material 106. The grating structures 112 may be made by
mechanical ruling lines upon the input and output faces of the EO
material 106. This may be done by burnishing grooves into the EO
material 106 with a mechanical tool. A thin film may be applied to
the input and output faces of the EO material and the grating
structures 112 may be formed in the thin film. The grating
structures 112 may also be formed using light exposure methods.
Chemical etching and/or ion bombardment techniques may be used to
form the gratings. The grating structures 112 may also be blazed.
The grating structures 112 may be formed as a plurality of
slits.
[0029] FIG. 2 is an illustration of the optical path that light
follows through the system 100. The grating structures 112
spatially disperse light according Equation (1).
m.lamda.d=n.sub.1 sin(.theta..sub.1)+n.sub.2 sin(.phi..sub.m)
(1)
m: Diffraction order, wherein m is an integer and is not zero i.e.
( . . . -3, -2, -1, 1, 2, 3 . . . ) d: Grating line number
[line/m]
.lamda.: Wavelength [m]
[0030] .theta..sub.i: Input light angle .phi..sub.m: Diffracted
light angle n.sub.1: Refractive index of material outside the EO
material 106 n.sub.2: Refractive index of the EO material 106
[0031] The distance l.sub.(.lamda.) that light travels from start
point 202 to end point 204 is described by equation (2).
l ( .lamda. ) = l ( .lamda. ) = l 1 + l 2 = n 2 L cos ( .phi. m ) +
n 2 L ( tan ( .phi. m ) - tan ( .phi. m c ) ) sin ( .theta. i ) + l
c ( 2 ) ##EQU00001##
l.sub.1: Distance between gratings for each wavelength l.sub.2:
Distance from 2nd grating to mirror for each wavelength L: Distance
between grating pair l.sub.c: Distance between 2nd grating and
mirror for center wavelength .lamda..sub.c .phi..sub.mc: Angle of
diffracted light for center wavelength .lamda..sub.c
[0032] When the material between the diffraction gratings 112 is an
EO material 106 the refractive index n.sub.2 can be changed.
Equation 3 describes how that change affects the distance that the
light travels.
l ( .lamda. ) = n 2 ( 1 + .DELTA. n ) L cos ( .phi. m ) n 2 L ( tan
( .phi. m ) - tan ( .phi. m c ) ) sin ( .theta. i ) + l c ( 3 )
##EQU00002##
.DELTA.: Ratio of index change of the EO material
[0033] The instantaneous chirp .kappa. added by the system may be
characterized by equation 4a below. The instantaneous chirp .kappa.
may be divided into two terms .kappa..sub.0 and
.kappa..sub..DELTA.. .kappa..sub.0 is the static instantaneous
chirp and .kappa..sub..DELTA. is the dynamic instantaneous chirp.
Another parameter of interest is the differential chirp
.DELTA..kappa. which is described in equation 4b. In which the
chirp measured between two wavelengths .lamda..sub.i and
.lamda..sub.2. Likewise, the differential chirp .DELTA..kappa. may
be divided into two terms .DELTA..kappa..sub.0 and
.DELTA..kappa..sub.A. .DELTA..kappa..sub.0 is the static
differential chirp and .DELTA..kappa..sub..DELTA. is the dynamic
differential chirp.
.kappa. = c l ( .lamda. ) .lamda. = .kappa. 0 + .kappa. .DELTA.
.kappa. 0 = cL .lamda. ( n 2 sec ( .phi. m ) + n 2 sin ( .theta. i
) ( tan ( .phi. m ) - tan ( .phi. m c ) ) ) .kappa. .DELTA. = cL
.lamda. ( n 2 .DELTA. n sec ( .phi. m ) ) .DELTA. .kappa. = c
.DELTA. l .lamda. .DELTA. .lamda. = .DELTA. l .lamda. 2 - .DELTA. l
.lamda. 2 .lamda. 2 - .lamda. 1 = .DELTA. .kappa. 0 + .DELTA.
.kappa. .DELTA. ( 4 a ) .DELTA. .kappa. 0 = cL ( n 2 ( .lamda. 2 )
sec ( .phi. m ( .lamda. 2 ) ) + n 2 ( .lamda. 2 ) sin ( .theta. i )
( tan ( .phi. m ( .lamda. 2 ) ) - tan ( .phi. m c ) ) - ( n 2 (
.lamda. 1 ) sec ( .phi. m ( .lamda. 1 ) ) + n 2 ( .lamda. 1 ) sin (
.theta. i ) ( tan ( .phi. m ( .lamda. 1 ) ) - tan ( .phi. m c ) ) )
) .DELTA. .kappa. .DELTA. = cL ( n 2 ( .lamda. 2 ) .DELTA. n (
.lamda. 2 ) sec ( .phi. m ( .lamda. 2 ) ) - n 2 ( .lamda. 1 )
.DELTA. n ( .lamda. 1 ) sec ( .phi. m ( .lamda. 1 ) ) ) ( 4 b )
##EQU00003##
[0034] When a voltage is applied to EO material the chirp is
changed, but the position and angle at which the chirped light
exits the system does not change. The gratings and mirrors are
arranged in this system such that whatever change may occur is due
to varying the voltage is within the alignment tolerance of the
chirped output light. The alignment tolerance depends upon the
numerical aperture of the lens used to output the light.
First Embodiment
[0035] FIG. 3 is an illustration of a first embodiment a system 300
that produces a chirped pulse. The system 300 includes a light
source 102 that produces a broadband pulsed input light 104. The
system also includes an EO material 106. An example of a suitable
EO material is MgO:LiNbO3. The refractive index (n.sub.2) of the EO
material 106 can be changed by applying a voltage via electrodes
108. The EO material 106 includes grating structures 112 on the
input face and the output face of the EO material 106. A first lens
302 may couple the light 104 into a first port of a circulator 304.
Light that enters the first port of the circulator 304 exits via a
second port of the circulator 304 and into a lens 306. The light
exiting lens 306 is then coupled into a lens 308. The light exiting
lens 308 then passes through a first grating structure 112a at an
incident angle .theta..sub.i. The first grating structure diffracts
the light such that light with different wavelengths reach the
second grating structure 112b at different times and at different
points in space as it passes through EO material 106.
[0036] The second grating structure 112b is arranged such that
light which is diffracted into order m is then re-diffracted into
the same order m and exits the second grating structure 112b at an
angle that is substantially similar to the incident angle
.theta..sub.i. The light exiting the grating structure 112b then
enters a lens 310. Light exiting the lens 310 is then reflected
back by a mirror 314 through components 310, 112b, 106, 112a, 308,
and 306, and into the second port of the circulator 304. The mirror
314 is positioned such that the plane of the mirror forms an angle
with the second grating structure 112b that is substantially
similar to the incident angle .theta..sub.i. The light inputted
into the second port of the circulator 304 then exits the third
port of the circulator as a chirped pulse. In which the chirp of
the pulse may be adjusted by applying a voltage to electrodes
108.
[0037] The EO material 106 may be made from MgO:LiNbO3 and it may
be 5 mm.times.5 mm.times.8 .mu.m (W.times.L.times.H). The
wavelength dependence of the refractive index of MgO:LiNbO3 can be
approximated using equations (5):
n 0 2 ( .lamda. ) = 4.8762 + 0.11554 .lamda. 2 - 0.04674 - 0.033119
.lamda. 2 n e 2 ( .lamda. ) = 4.5469 + 0.094779 .lamda. 2 - 0.04439
- 0.026721 .lamda. 2 ( 5 ) ##EQU00004##
[0038] In which n.sub.2o is the ordinary refractive index and
n.sub.2e is the extraordinary refractive index. The units of
.lamda. as used in equation (4) are in .mu.m.
[0039] The change in refractive index for MgO:LiNbO3 may be
described using equation (6).
.DELTA. n 20 = - 1 2 n 2 o 3 .gamma. 13 V D .DELTA. n 2 e = - 1 2 n
2 e 3 .gamma. 13 V D ( 6 ) ##EQU00005##
.gamma..sub.13: Electro-optic coefficient for MgO:LiNbO3 is 18 pm/V
V: The input voltage applied to the EO material 106 is -1 kV to 1
kV D: The thickness of the EO material 106 is 8 d: Grating line
number 0.900 lines/.mu.m=1 E-3/1 E-6
[0040] The center wavelength .kappa..sub.c of the input light is
1.5 .mu.m, the shape of the input pulse is sech like, and the pulse
width is 100 fs. The incident angle .theta..sub.i is 28.degree..
The EO material is arranged so that the input pulse propagates
along the ordinary axis. As input voltage is changed from -1 kV to
1 kV, the temporal pulse width is changed from 4.6 ps to 4.73
ps.
Second Embodiment
[0041] FIG. 4 is an illustration of a second embodiment a system
400 that produces a chirped pulse. The system 400 is substantially
similar to system 300. The system 400 includes a fiber coupled
light source 402 that produces a broadband pulsed input light 104.
The fiber coupled light source 402 is coupled to a first port of a
fiber coupled circulator 404. Light that enters the first port of
the circulator 404 exits via a second port of the fiber coupled
circulator 404 and into a GRIN lens 406. The light exiting the GRIN
lens 406 then enters a first optical block 408a. The first optical
block 408a has a refractive index of n.sub.1. The first optical
block 408a may be made out of silica or some other suitable optical
material.
[0042] After passing through the first optical block 408a the light
then strikes a first grating structure 112a at an incident angle
.theta..sub.i. The first grating structure 112a is between the
first optical block 408a and an EO material 106. The first grating
structure 112a may be formed on the first optical block 408a, the
EO material 106, or may be a separate item. The first grating
structure 112a diffracts the light such that light with different
wavelengths reaches a second grating structure 112b at different
times and at different points in space as it passes through EO
material 106. The second grating structure 112b may be formed on a
second optical block 408b, the EO material 106, or may be a
separate item. The second optical block 408b may have a refractive
index of n.sub.1. The second optical block 408a may be made out of
silica or some other suitable optical material. A mirror 314 may be
formed within the optical block 408a. The second optical block 408a
may take the form of a prism or a wedge. The mirror 314 may be
formed on a face of the second optical block 408a either as a thin
film or as an effective mirror due to total internal reflection.
The mirror 314 then reflects the light back through components
408b, 112b, 106, 112a, 408a, and 406, and back into the second port
of the fiber coupled circulator 404. The mirror 314 is positioned
such that the plane of the mirror forms an angle with the second
grating structure 112b that is substantially similar to the
incident angle .theta..sub.i. The light inputted into the second
port of the fiber coupled circulator 404 then exits the third port
of the fiber coupled circulator 404 as a chirped pulse. In which
the chirp of the pulse may be adjusted by applying a voltage.
[0043] In an alternative embodiment, the refractive index of the
first optical block 408a may be different from the refractive index
of the second optical block 408b. The light exiting the GRIN lens
may be collimated or set to be focused back onto the GRIN lens 406
once it has passed through all the intervening optical
components.
[0044] One advantage of the second embodiment over the first
embodiment is a reduction in the need for free space optical
alignment.
Third Embodiment
[0045] FIG. 5 is an illustration of a second embodiment, a system
500, which produces a chirped pulse. The system 500 is
substantially similar to system 300. The system 500 includes a
light source 102 that produces a broadband pulsed input light 104.
The system 500 includes a first EO material 506a and a second EO
material 506b. The EO materials 506a-b include grating structures
512a-d on the input and the output faces of the EO materials
506a-b. The light from the light source 102 is coupled by a first
lens 302 into a second lens 508a
[0046] The light exiting lens 508a then passes through a first
grating structure 512a at an incident angle .theta..sub.i. The
first grating structure 512a diffracts the light which then passes
through the first EO material 506a. After passing through the first
EO material 506a the second grating structure 512b diffracts the
light again. The second grating structure 512b is arranged such
that light with different wavelengths exit the second grating
structure 512b at an angle that are substantially parallel to each
other (collinear) but are spatially separated. The light may then
pass through a lens 510a and onto a mirrored structure 514 which
reflects the light through a third lens 510c.
[0047] The light exiting lens 510c then passes through a third
grating structure 512c. The third grating structure 512c diffracts
the light which then passes through the second EO material 506b.
After passing through the second EO material 506b the fourth
grating structure 512d diffracts the light again. The fourth
grating structure 512d is arranged such that light with different
wavelengths exit the fourth grating structure 512d at with
substantially the same angle are not spatially separated but are
separated in time. The light exiting the fourth grating structure
512d then enters a lens 508b and exits via an output lens 516. The
output light will have a chirp that can be controlled with a
voltage across the electrodes 108.
[0048] One advantage of this embodiment is the isolation of the
input from the output without having to include a circulator. This
embodiment also expands the flexibility to the chirp that may be
applied to light. The EO materials 506a-b may have different or
identical electro-optical properties. Different or identical
voltages may be applied to the EO materials or they may have the
same voltage applied to them via electrodes 108. If the EO
materials are different and/or the voltages are different then the
structure may be adjusted to minimize the different spatial
chromatic dispersions between the forward path and the return
path.
Fourth Embodiment
[0049] FIG. 6 is an illustration of a fourth embodiment, a system
600, which produces a chirped pulse. The system 600 is
substantially similar to systems 400 and 500. The system 600
includes a fiber coupled light source 102 that produces a broadband
pulsed input light 104. The fiber coupled light source 402 is
coupled to a GRIN lens 606a. The light exiting the GRIN lens 606
then enters a first optical block 608a. The system 600 includes EO
materials 506a-b. The system 600 includes grating structures
512a-d.
[0050] After passing through the first optical block 608a the light
then strikes the first grating structure 512a at an incident angle
.theta..sub.i. The first grating structure 512a is between the
first optical block 608a and the first EO material 506a. The first
grating structure 512a diffracts the light which then passes
through the first EO material 506a. The second grating structure
512b is between the first EO material 506a and the second optical
block 608b. The light is then diffracted by the second grating
structure 512b and then passes through the optical block 608b.
[0051] A mirror 614a then reflects the light into a third optical
block 608c and is then reflected by a second mirror 614b.
Alternatively, the second and third optical blocks 608b-c may take
the form of prisms or wedges. The mirrors 614a-b may be formed on a
face of the optical blocks 608b-c either as a thin film or as an
effective mirror due to total internal reflection.
[0052] The light then passes back through a third diffraction
grating 512c, a second EO material 508b, a fourth diffraction
grating 512d, a fourth optical block 608d, and a grin lens 60d
before exiting an optical fiber as a chirped optical light. In
which the chirp of the pulse may be adjusted by applying a voltage
to electrodes 108.
[0053] The first optical block 608a and the fourth optical block
608b may be combined into a single input/output optical block. The
first grating structure 512a and the fourth grating structure 512d
may be combined into a single input/output grating structure. The
second grating structure 512b and the third grating structure 512c
may be combined into a single intermediate grating structure. The
second optical block 608b and the third optical block 608b may be
combined into an intermediate reflective optical block.
[0054] An advantage of the fourth embodiment that there is no need
for a circulator and there is no free space alignment.
Fifth Embodiment
[0055] A fifth embodiment of the system may include a single
grating structure instead of multiple grating structures. Light
enters a single grating structure at an incident angle
.theta..sub.i. The light passes through a first grating structure
and then through a first EO material. A second mirror that is
parallel to the grating structure at the end of EO material may be
used to reflect the light back through for a second pass through
the EO material and through the first grating structure for a
second time. The light may then be reflected back by a first mirror
which is at an angle to the grating structure and then back through
for a third pass through the grating structure. The light then
passes through the EO material a third time before being reflected
by the second mirror a second time and passing through the EO
material a fourth time before passing though the grating structure
for the fourth time. After light passes through the grating
structure for the fourth time, the light exits the system with a
chirp that can be controlled by a voltage applied across the EO
material.
[0056] The first mirror is positioned such that when light exits
the system after it has passed though these dispersive structures
the spatial chromatic dispersion is low and the temporal chromatic
dispersion is adjustable. In an embodiment, the second mirror is
positioned such that after the light has passed through the grating
structure for the fourth time it may exit the grating structure at
substantially the same angle as the incident angle. Also, as the
light passes through the grating structure for the fourth time it
exits the grating structure at substantially the same position that
it enters the grating structure for the first time. Alternatively,
the first mirror may be positioned such that after light passes
through the grating structure for the fourth time it exits the
grating structure at a different position from where it entered the
grating structure the first time.
[0057] The first embodiment is transformed into the fifth
embodiment by cutting the system in half placing a mirror at a L/2
within the EO material and folding the second half of the system
such that it adjacent to the first half. The same transformation
may be applied to the second through fourth embodiments.
[0058] The diffraction gratings depending upon their design and the
angle of light incident upon their diffraction surfaces, spatial
disperse light over one or more diffraction modes. Apertures may be
formed in various parts of the chirping system to ensure that only
one of these diffraction modes exits the chirping system. The
mirror may act as a first aperture. The chirped light may also pass
through a second aperture after it has passed through all the
grating structures before exiting the system.
[0059] The rulings, grooves, or slits of the gratings may be
aligned with a crystalline axis of the EO materials. This may be
used to line up the polarization dependence of the gratings and the
EO materials. The system may include a polarizer between an optical
source and the gratings. This may be used minimize whatever
polarization effects there are in the system.
[0060] These systems are typically used with a pulsed input signal
104. When the light source 102 is operated in a continuous wave
mode then the chirp becomes a chirp in the phase of the output
signal. The chirp in this context refers to a wavelength dependence
of the phase retardation which is adjustable by changing the
applied voltage.
[0061] The embodiments described above make use of transmission
grating structures. The system may also be adapted to make use of
reflection grating structures without going beyond the spirit of
the present invention.
[0062] While the present invention has been described with
reference to exemplary embodiments, it is to be understood that the
invention is not limited to the disclosed exemplary embodiments.
The scope of the following claims is to be accorded the broadest
interpretation so as to encompass all modifications, equivalent
structures, and functions.
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