Title: Optical filter
Abstract: An optical filter comprises a multimode waveguide, and first and second coupling waveguides which communicate with the multimode waveguide at respective ends thereof. Filtering is achieved by the effect of self-imaging in the multimode waveguide. The filter further comprises one or more series of aperture within the multimode waveguide, the or each series being located at a longitudinal position within the multimode waveguide at which 1-to-N way intensity division of an input optical field occurs, the optical field being the lowest order transverse mode of the coupling waveguides. The (one or more) series of apertures reduces the transmission of the filter at wavelength other than those corresponding to transmission peaks of filter's transmission function, thus providing improved filtering performance compared to prior art optical filters based on self-imaging.
Patent Number: 6,973,240 Issued on 12/06/2005 to Jenkins
| Inventors:
|
Jenkins; Richard Michael (Malvern, GB)
|
| Assignee:
|
Qinetiq Limited (GB)
|
| Appl. No.:
|
492678 |
| Filed:
|
October 8, 2002 |
| PCT Filed:
|
October 8, 2002
|
| PCT NO:
|
PCT/GB02/04551
|
| 371 Date:
|
April 15, 2004
|
| 102(e) Date:
|
April 15, 2004
|
| PCT PUB.NO.:
|
WO03/036352 |
| PCT PUB. Date:
|
May 1, 2003 |
Foreign Application Priority Data
| Current U.S. Class: |
385/50; 398/48; 398/85; 385/27 |
| Intern'l Class: |
G02B 006/26 |
| Field of Search: |
385/15,27,39,50
398/48,85
|
References Cited [Referenced By]
U.S. Patent Documents
| 4693544 | Sep., 1987 | Yamasaki et al.
| |
| 5410625 | Apr., 1995 | Jenkins et al.
| |
| 5640474 | Jun., 1997 | Tayag.
| |
| 5862288 | Jan., 1999 | Tayag et al.
| |
| Foreign Patent Documents |
| 58-068713 | Apr., 1983 | JP.
| |
Other References
Jenkins et al. "Waveguide Beam Splitters and Recombiners Based on Multimode Propagation
Phenomena", Optics Letters, pp. 991-993 (1992).
Banerji et al. "Laser Resonators with Self-Imaging Waveguides", J. Opt. Soc.
Am B., pp. 2378-2380 (1997).
|
Primary Examiner: Lee; John D.
Assistant Examiner: Peace; Rhonda S.
Attorney, Agent or Firm: McDonnell Boehnen Hulbert & Berghoff LLP
Claims
1. An optical filter comprising
(a) a multimode waveguide; and
(b) first and second coupling waveguides which communicate with the multimode
waveguide at respective ends thereof and which are arranged centrally of the multimode
waveguide's transverse cross-section;
wherein the length of the multimode waveguide is such that an optical field distribution,
being lowest order transverse mode of the coupling waveguides, introduced into
the multimode waveguide via the first coupling waveguide is substantially reproduced
on the multimode waveguide's central longitudinal axis at the end of the multimode
waveguide remote from the first coupling waveguide, and coupled into the second
coupling waveguide, for radiation of a wavelength to be passed by the filter in
preference to radiation of other wavelengths by virtue of modal dispersion and
inter-modal interference within the multimode waveguide,
characterised in that the filter further comprises means presenting N apertures
at a longitudinal position within the multimode waveguide at which 1-to-N way intensity
division of said optical field occurs, the centre of each aperture being located
at a lateral position within the multimode waveguide at which a local optical intensity
maximum occurs when said division occurs.
2. A filter according to claim 1 wherein the multimode waveguide has a length
pw
22/λ, where p is a positive integer, w
2
is the width of the multimode waveguide and λ is the wavelength, within the
multimode waveguide's core layer, of radiation to be passed by the filter in preference
to radiation of other wavelengths.
3. A filter according to claim 1 wherein the coupling waveguides have a width
w
1, the multimode waveguide has a width w
2 and w
2/w
1>8.
4. A laser oscillator characterised by a filter according to claim 1.
5. An optical device comprising a radiation source and characterised by a filter
according to claim 1.
6. A filter according to claim 2 wherein the coupling waveguides have a width
w
1, the multimode waveguide has a width w
2 and w
2/w
1>8.
7. A laser oscillator characterised by a filter according to claim 2.
8. A laser oscillator characterised by a filter according to claim 3.
9. An optical device comprising a radiation source and characterised by a filter
according to claim 2.
10. An optical device comprising a radiation source and characterised by a filter
according to claim 3.
Description
The invention relates to optical filters.
In the field of optics, wavelength filtering, i.e. extraction of an optical signal
of a specific wavelength from a signal comprising a number of spectral components,
is an important function. For example, in the field of optical communication, wavelength
filtering allows a particular optical communication channel to be extracted from
a plurality of wavelength-multiplexed channels, allowing that channel to processed
further; for example it might be amplified, routed or demodulated. In the field
of optical communication, components for performing filtering and other operations
are required to be integrated with other optical devices into integrated optical
systems in which light is guided within fibre-optic or semiconductor waveguides.
Devices currently used to perform filtering within such integrated optical systems
include Bragg gratings, Fabry-Perot and Mach-Zehnder interferometers, array waveguide
gratings (AWGs) and acousto-optic filters. Such devices are complex and therefore
require a substantial amount of processing during their fabrication, as a result
of which integrated optical systems incorporating them are expensive and time-consuming
to produce. These devices are described, for example, in the book "Optical Networks—A
Practical Perspective" by R. Ramaswami and K. N. Sivarajan (Morgan Kaufmann Publishers
1998, ISBN1-55860-445-6).
Optical filters based on the effect of self-imaging in a multimode waveguide
are also known in the prior art: for example U.S. Pat. No. 5,862,288 discloses
(in FIG. 1 thereof) a filter based on the principle of 1-to-1 imaging of an input
optical field distribution over a distance L=w
2/mλ
0
within a multimode waveguide, where w is the width of the multimode waveguide and
mλ
0 is the wavelength of guided plane wave radiation, which wavelength
is passed by the filter in preference to radiation of other wavelengths. m is a
positive integer. Such filters are easily fabricated and integrated with other
optical and optoelectronic devices.
A problem associated with a filter of the latter type is that the filter's transmission
function contains a significant amount of structure between transmission peaks
associated with wavelengths mλ
0 and (m+1)λ
0,
that is, the transmission of such a filter is non-zero at wavelengths between those
which are required to be extracted from a plurality of spectral components. Such
structure degrades filtering performance and in some filtering applications is unacceptable.
It is an object of the present invention to overcome or at least ameliorate this
problem with filters based on the effect of self-imaging in a multimode waveguide.
According to a first aspect of the present invention, this object is achieved
by an optical filter comprising
- (a) a multimode waveguide, and
- (b) first and second coupling waveguides which communicate with the
multimode waveguide at respective ends thereof and which are arranged centrally
of the multimode waveguide's transverse cross-section,
wherein the length of the multimode waveguide is such that an optical field
distribution, being the lowest order transverse mode of the coupling waveguides,
introduced into the multimode waveguide via the first coupling waveguide is substantially
reproduced on the multimode waveguide's central longitudinal axis at the end of
the multimode waveguide remote from the first coupling waveguide, and coupled into
the second coupling waveguide, for radiation of a wavelength to be passed by the
filter in preference to radiation of other wavelengths by virtue of modal dispersion
and inter-modal interference within the multimode waveguide,
characterised in that the filter further comprises means presenting
N apertures at a longitudinal position within the multimode waveguide at which
1-to-N way intensity division of said optical field occurs, the centre of each
aperture being located at a lateral position within the multimode waveguide at
which a local optical intensity maximum occurs when said division occurs.
The length of the multimode waveguide may be pw
22/λ,
where p is a positive integer and w
2 is the width of the multimode waveguide.
The full width at half maximum (FWHM) of transmission peaks of the transmission
function of a filter of the invention are reduced if the length of the multimode
waveguide is increased. In addition, this provides (p-1) transmission peaks in
the transmission function of the filter between those peaks associated with wavelengths
mλ
0 and (m+1)λ
0.
Preferably, the width w
1 of the coupling waveguides and the
width w
2 are such that w
2/w
1>8. This provides
a reduced FWHM of transmission peaks of the filter's transmission function.
According to a second aspect of the invention, there is provided a laser
oscillator characterised by a filter according to the first aspect of the invention.
Such a laser oscillator has an output with spectral characteristics fixed by filter's
transmission function.
According to a third aspect of the invention, there is provided an optical
device comprising a radiation source and characterised by a filter according to
the first aspect of the invention. Such a device outputs radiation having a narrower
spectral width than that of the radiation source alone.
Embodiments of the invention will now be described, by way of example
only, with reference to the accompanying drawings in which:
FIGS. 1 and 2 show plan and perspectives views respectively of a wavelength
filter of the prior art;
FIG. 3 illustrates the spatial distribution of an optical field as a function
of distance within a portion of the filter of FIGS. 1 and 2;
FIG. 4 is graph of transmission versus wavelength of input radiation for the
filter of FIGS. 1 and 2;
FIGS. 5 to 7 are graphs of transmission versus wavelength of input radiation
for further wavelength filters of the prior art;
FIG. 8 shows a plan view of a wavelength filter of the present invention.
Referring to FIGS. 1 and 2 there are shown plan and perspective views respectively
of a prior-art wavelength filter, indicated generally by
10, which passes
radiation having a wavelength mλ
0 within the filter
10
in preference to radiation of other wavelengths. m is an integer and λ
0=1
μm. The device
10 is made by techniques familiar to those skilled
in the art of semiconductor device fabrication and is referred to a co-ordinate
system
11 which defines x- y- and z-directions. The device
10 comprises
cladding layers
14,
18 of Al
0.1Ga
0.9As which
are 2.0 μm thick, a GaAs core layer
16 which is 1.0 μm thick
and a GaAs capping layer
19 which is 0.1 μm thick. The layers
14,
16,
18,
19 are supported on a GaAs substrate
12. The
layers
14,
16,
18 form a slab waveguide which is single-moded
in the x-direction.
The device
10 has a ridge structure
20 (formed by etching) which
incorporates layers
16,
18,
19 and a portion of layer
14.
The ridge structure
20 has end regions
22,
24 of width w
1=2
μm corresponding respectively to end regions
32,
34 of the
device
10, and a central region
26 of width w
2=4 μm
corresponding to a central region
36 of the device
10. The width
w
1 of the end regions
22,
24 of the ridge structure
20
is such that optical radiation guided within those regions and having a wavelength
within the device
10 in the region of 1 μm is single-moded in both
the x- and y-directions, i.e. the end regions
32,
34 of the device
10 are single-mode waveguides. The width w
2 of the central region
26 of the ridge structure
20 is such that radiation guided within
the device
10 and having a wavelength within the device
10 in the
region of 1 μm is in general multi-moded in the y-direction, i.e. the central
region
36 of the device
10 is a multimode waveguide having ends
17,
19.
The end regions
22,
24 of the ridge structure
20 are located
centrally of the transverse cross-section of the ridge structure's central region
26 and on its central longitudinal axis
13. The central region
26
of the ridge structure
20 has a length L=w
22/λ
0=16
μm. The central region
26 of the ridge structure
20 meets the
end regions
22,
24 at xy planes
33 and
35 respectively.
The filter
10 has an entry xy plane
37 at which optical radiation
is introduced into the filter
10, and an exit xy plane
39 at which
optical radiation exits the filter
10. The lengths of the end regions
22,
24 in the z-direction may take any convenient value: the filter
10
may be integrated on a single integrated-optical chip with other components and
devices, for example amplifiers, modulators and the like.
The filter
10 operates as follows. Input optical radiation is introduced
into the layer
16 substantially in the z-direction and at the entry xy plane
37 as indicated by an arrow
40 in FIG. 2. The input optical radiation
comprises spectral components having wavelengths mλ
0 within the
filter
10 which are to be passed by the filter
10 in preference to
other spectral components of the input optical radiation. The input optical radiation
propagates in the z-direction in the end region
32 of the device
10,
substantially guided within the layer
16, as an optical field which is single-moded
in both the x- and y-directions. As the optical field enters the central region
36 of the filter
10 at the xy plane
33, each spectral component
of the input optical radiation excites a plurality of EH
i,j transverse
modes within that region, where j denotes mode index in the y-direction and is
equal to an odd integer. Thus only symmetric modes of the central region (multimode
waveguide)
36 are excited. (n
eff=3.5 for the device
10.)
A spectral component of the input optical radiation having a wavelengths λ
i
within the filter
10 has a wavelength λ
i′=λ
in
in free space, where n is the refractive index for a plane wave within the layer
16. For example the spectral component of the input optical radiation having
a wavelength λ
0=1 μm within the filter
10 has a wavelength
λ
0′=λ
0n=3.5 μm in free space.
As a result of modal dispersion and inter-modal interference within the central
region
36, the intensity distribution in the y-direction of the spectral
component of wavelength λ
0 varies with distance in the z-direction
along the central region
36 of the device
10 as shown in FIG. 3.
Referring to FIG. 3, the intensity distribution
40 in the y-direction at
the xy plane
33 of the optical field of the spectral component having wavelength
λ
0 is the EH
1,1 transverse mode of the central waveguide
region
36. As a result of modal dispersion and inter-modal interference
within the central region
36, this field distribution is substantially reproduced
at the xy plane
35 and therefore couples efficiently into the end region
34 of the device
10. For a spectral component having a wavelength
within the filter
10 other than mλ
0, where m is an integer,
the intensity distribution
40 at the xy plane
33 is not reproduced
at the xy plane
35, and hence the efficiency with which such a spectral
component is coupled into the end region
34 of the device
10 is reduced
compared to that for the spectral component λ
0. For example, a
spectral component having a wavelength λ
1>λ
0 (≠mλ
0)
within the device would require the central region
36 of the filter
10
to be of length w
22/λ
1<w
22/λ
0
in order for the EH
1,1 transverse mode distribution of that spectral
component at the xy plane
33 to be reproduced at the xy plane
35.
Similarly, a spectral component having a wavelength λ
2<λ
0
(≠mλ
0) would require the central region
36
of the filter
10 to be of length w
22/λ
2>w
22/λ
0
in order for the EH
1,1 transverse mode distribution of that spectral
component at the xy plane
33 to be reproduced at the xy plane
35.
Thus the filter
10 performs a filtering function, spectral components of
wavelength mλ
0 in the input radiation being passed in preference
to other spectral components .
Referring now to FIG. 4, transmission of the filter
10 in the region
0-2 μm is shown as a function of wavelength (within the filter
10)
of input radiation by a curve
50. The curve
50 is periodic with respect
to wavelength: wavelengths mλ
0 in the input radiation are passed
by the filter
10 with substantially 100% efficiency, where m is an integer.
Transmission peaks such as
51 of the curve
50 have a full width at
half-maximum (FWHM) of approximately 560 nm.
Another wavelength filter of the prior art has a construction like to that
of the filter
10, except that end regions
22,
24 of the ridge
structure
20 are multi-mode waveguides in the y-direction rather than single-mode
waveguides in the y-direction. In operation of the alternative device, input optical
radiation is introduced at the xy plane
37 such that only the lowest order
transverse mode is excited in the end region
32 of the device.
Referring now to FIG. 5, transmission versus input wavelength is shown
for a further prior-art wavelength filter of the invention in the region 0-2 μm
by a curve
55, the filter having construction like to that of the filter
10, except that w
2=8 μm. The curve
55 is periodic
with respect to wavelength: wavelengths mλ
0 within the input radiation
are passed with substantially 100% efficiency, where m is an integer. Transmission
peaks such as
56 have a FWHM of approximately 150 nm.
Referring now to FIG. 6, transmission versus input wavelength is shown
for a still further wavelength filter of the prior art for the region 0-2 μm
by a curve
60, the filter having construction like to that of the filter
10, except that w
2=16 μm. The curve
60 is periodic
with respect to wavelength: wavelengths mλ
0 are passed with substantially
100% efficiency, where m is an integer. Transmission peaks such as
61 have
a FWHM of approximately 34 nm.
Referring now to FIG. 7, transmission versus input wavelength is shown
for a yet further wavelength filter of the prior art in the region 0-2 μm
by a curve
65, the device having construction like to that of the filter
10, except that w
2=16 μm and L=4w
22/λ
0=64
μm. The curve
65 is periodic with respect to wavelength: wavelengths
mλ
0/4 are passed with substantially 100% efficiency, where m is
an integer. Transmission peaks such as
66 have a FWHM of approximately 34 nm.
The transmission curves
55,
60,
65 have side-lobes such
as
57,
62,
67. Such side-lobes are unacceptable in some filtering applications.
From FIGS. 4 to 7 it may be seen that the FWHM of transmission peaks of filters
of the invention such as
10 may be varied by varying the structure of the
filter so as to vary the value of w
2/w
1. The FWHM of a transmission
peak is proportional to (w
1/w
2)
2.
The number of wavelengths passed by the filter with substantially 100% efficiency
may be increased by increasing the length of the filter's central region: a filter
having a central region of length pw
22/λ
0
will pass spectral components of the input radiation having wavelengths mλ
0/p
with substantially 100% efficiency, where m and p are integers. Increasing the
length of the central region also reduces the FWHM of transmission peaks of the
transmission function of a filter of the invention.
Referring now to FIG. 8, there is shown an optical filter of the invention,
indicated generally by
300. The filter
300 has a construction and
dimensions like to that of the filter
10. The filter
300 has three
sets of apertures
315A,
315B,
315C formed by etching grooves
into the multimode waveguide
326 of the filter
300. The grooves are
of a depth such that they pass completely through the core waveguide layer of the
filter
300. The sets of apertures
315A and
315C are each located
at a distance w
22/3λ
0 from respective ends
319,
317 of the multimode waveguide
326 and each consists
of three apertures the centres of which are positioned at w
2/6, w
2/2
and 5w
2/6 from one side of the filter's multimode waveguide
326.
The set of apertures
315B is located at a distance w
22/2λ
0
from each of the ends
317,
319 of the multimode waveguide
326
and has two apertures at distances of w
2/4 and 3w
2/4 from
one side of the multimode waveguide
326. The grooves may be formed by any
suitable method of semiconductor processing, for example focused ion-beam etching.
Referring again to FIG. 3, the z-positions of the three sets of apertures
315A,
315B,
315C can be seen to coincide with z-positions
in the multimode waveguide
326 at which one-to-two and one-to-three way
splitting of input radiation of wavelength λ
0 occurs. The centre
of each aperture is located at a lateral position within the waveguide
326
at which a local maximum occurs in the intensity distribution at the corresponding z-position.
The sets of apertures
315A,
315B,
315C cause suppression
of side-lobes (such as
57,
62,
67 in FIGS. 4,
5 and
6 respectively) in the transmission function of the filter
300 thus
giving enhanced filtering performance. Side-lobes may be further suppressed by
additionally providing apertures at other z-positions within the multimode waveguide
326 where 1-to-N way splitting of an input intensity distribution occurs.
Side-lobe suppression may also be achieved in filters of the invention which have
multimode waveguides of length pw
22/λ
0,
where p is an integer, by appropriate positioning of such apertures.
Wavelength filters of the invention may modified to produce to produce
laser oscillators. For example the device
10 may be modified to provide
an optical gain element within any or all of the regions
22,
24,
26 of the ridge structure
20, and optical feedback means (e.g. mirrors
formed by cleaving) at ends of the regions
22,
24 meeting xy planes
37,
39 respectively. Such a laser oscillator has a spectral output
determined by the wavelength filter device which forms its resonator.
A filter of the invention may be combined with a radiation source to produce
an
optical device which outputs radiation having a narrower spectral width than that
of the radiation source alone.
*
