U.S. patent application number 14/103666 was filed with the patent office on 2015-07-30 for optical waveguide terminators with doped waveguides.
The applicant listed for this patent is ACACIA COMMUNICATIONS INC.. Invention is credited to Long CHEN.
Application Number | 20150212271 14/103666 |
Document ID | / |
Family ID | 53678862 |
Filed Date | 2015-07-30 |
United States Patent
Application |
20150212271 |
Kind Code |
A1 |
CHEN; Long |
July 30, 2015 |
OPTICAL WAVEGUIDE TERMINATORS WITH DOPED WAVEGUIDES
Abstract
Disclosed herein are methods, structures, apparatus and devices
for the termination of unused waveguide ports in planar photonic
integrated circuits with doped waveguides such that free-carrier
absorption therein may advantageously absorb any undesired optical
power resulting in a significant reduction of stray light and
resulting reflections.
Inventors: |
CHEN; Long; (MAYNARD,
MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ACACIA COMMUNICATIONS INC. |
Maynard |
MA |
US |
|
|
Family ID: |
53678862 |
Appl. No.: |
14/103666 |
Filed: |
December 11, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61735710 |
Dec 11, 2012 |
|
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Current U.S.
Class: |
385/14 ;
438/24 |
Current CPC
Class: |
G02B 6/243 20130101;
G02B 6/122 20130101; G02F 1/2257 20130101; G02F 2001/212 20130101;
G02F 2201/08 20130101 |
International
Class: |
G02B 6/24 20060101
G02B006/24; G02B 6/134 20060101 G02B006/134 |
Claims
1. A method for enhancing the operational characteristics of a
photonic integrated circuit, said photonic integrated circuit
including: one or more used ports and one or more unused ports;
said method comprising the step of: terminating one or more unused
ports of the photonic integrated with a doped waveguide such that
internal reflections and stray light is reduced when light
traverses the one or more used ports.
2. The method according to claim 1 further comprising the step of:
modifying a used port of the photonic integrated circuit such that
the photonic integrated circuit includes an additional unused port;
and terminating the additional port with a doped waveguide.
3. The method according to claim 1 wherein said doped waveguide
terminator is n-doped having an electron concentration of 1E19-5E20
and exhibits a length of 20 micrometers to 1.5 mm.
4. The method according to claim 1 wherein said doped waveguide
terminator includes both doped and undoped portions.
5. The method according to claim 1 wherein said doped waveguide is
doped at different concentrations at different parts of the
waveguide.
6. An optical waveguide terminator comprising: a length of doped
waveguide being optically connected to an unused port of a photonic
integrated circuit.
7. The terminator of claim 6 wherein said doped waveguide
terminator is n-doped having an electron concentration of 1E19-5E20
and exhibits a length of 20 micrometers to 1.5 mm.
8. The terminator of claim 7 which includes both doped and undoped
portions.
9. The terminator of claim 8 wherein said doped waveguide is doped
at different concentrations at different parts of the
waveguide.
10. A method for enhancing the operational characteristics of a
photonic integrated circuit, said photonic integrated circuit
including one or more used ports, said method comprising the step
of: modifying a used port of the photonic integrated circuit such
that the photonic integrated circuit included an unused port; and
terminating the unused port of the photonic integrated with a doped
waveguide.
11. The method of claim 10 wherein said doped waveguide terminator
is n-doped having an electron concentration of 1E19-5E20 and
exhibits a length of 20 micrometers to 1.5 mm.
12. The terminator of claim 11 which includes both doped and
undoped portions.
13. The terminator of claim 12 wherein said doped waveguide is
doped at different concentrations at different parts of the
waveguide.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Patent Application Ser. No. 61/735,710 filed Dec. 11, 2012 which is
incorporated by reference in its entirety as if set forth at length
herein.
TECHNICAL FIELD
[0002] This disclosure relates generally optical communications and
planar photonic integrated circuits. More particularly, this
disclosure pertains to techniques, methods, apparatus, structures
and materials pertaining to the termination of unused waveguide
ports in planar photonic integrated circuits with doped
waveguides.
BACKGROUND
[0003] Contemporary optical communications and other photonic
systems make extensive use of photonic integrated circuits.
Accordingly, techniques, methods, apparatus and structures that
improve operational characteristic of such photonic circuits would
represent a welcome addition to the art.
SUMMARY
[0004] An advance in the art is made according to an aspect of the
present disclosure directed to techniques, methods, apparatus,
structures and materials that enhance the operational
characteristics of planar photonic integrated circuits by
terminating unused waveguide ports with doped waveguides.
[0005] Advantageously compared to other waveguide termination
techniques known in the art, doped waveguide termination of unused
ports according to the present disclosure significantly reduces
stray light and reflections in the photonic circuits.
BRIEF DESCRIPTION OF THE DRAWING
[0006] A more complete understanding of the present disclosure may
be realized by reference to the accompanying drawings in which:
[0007] FIGS. 1(a) and 1(b) show a schematic illustrations of a
optical components that may generate reflections/stray light
including (a) a polarization filter including a directional coupler
when TM port is not property terminated, and (b) a Mach-Zehnder
interferometer employed as an optical modulator in which .about.50%
of average optical power becomes stray light during modulation;
and
[0008] FIG. 2 shows a schematic illustration of using a doped
waveguide to terminate unused waveguide port(s) wherein the
free-carrier absorption in the doped waveguide gradually reduces
the optical power level without causing reflection or stray light
according to the present disclosure.
DETAILED DESCRIPTION
[0009] The following merely illustrates the principles of the
disclosure. It will thus be appreciated that those skilled in the
art will be able to devise various arrangements which, although not
explicitly described or shown herein, embody the principles of the
disclosure and are included within its spirit and scope. More
particularly, while numerous specific details are set forth, it is
understood that embodiments of the disclosure may be practiced
without these specific details and in other instances, well-known
circuits, structures and techniques have not be shown in order not
to obscure the understanding of this disclosure.
[0010] Furthermore, all examples and conditional language recited
herein are principally intended expressly to be only for
pedagogical purposes to aid the reader in understanding the
principles of the disclosure and the concepts contributed by the
inventor(s) to furthering the art, and are to be construed as being
without limitation to such specifically recited examples and
conditions.
[0011] Moreover, all statements herein reciting principles,
aspects, and embodiments of the disclosure, as well as specific
examples thereof, are intended to encompass both structural and
functional equivalents thereof. Additionally, it is intended that
such equivalents include both currently-known equivalents as well
as equivalents developed in the future, i.e., any elements
developed that perform the same function, regardless of
structure.
[0012] Thus, for example, it will be appreciated by those skilled
in the art that the diagrams herein represent conceptual views of
illustrative structures embodying the principles of the
invention.
[0013] In addition, it will be appreciated by those skilled in art
that any flow charts, flow diagrams, state transition diagrams,
pseudocode, and the like represent various processes which may be
substantially represented in computer readable medium and so
executed by a computer or processor, whether or not such computer
or processor is explicitly shown.
[0014] In the claims hereof any element expressed as a means for
performing a specified function is intended to encompass any way of
performing that function including, for example, a) a combination
of circuit elements which performs that function or b) software in
any form, including, therefore, firmware, microcode or the like,
combined with appropriate circuitry for executing that software to
perform the function. The invention as defined by such claims
resides in the fact that the functionalities provided by the
various recited means are combined and brought together in the
manner which the claims call for. Applicant thus regards any means
which can provide those functionalities as equivalent as those
shown herein. Finally, and unless otherwise explicitly specified
herein, the drawings are not drawn to scale.
[0015] Thus, for example, it will be appreciated by those skilled
in the art that the diagrams herein represent conceptual views of
illustrative structures embodying the principles of the
disclosure.
[0016] By way of some additional background, we begin my noting
that it is known for the operation of microwave circuits, unused
microwave ports may be terminated thereby avoiding undesirable back
reflections. Generally--and according to the present disclosure--a
similar concept is now applied to photonic circuits.
[0017] Compounding the problem however, is the fact that in
addition to any optical reflections, stray light in photonic
circuits is also problematic. More particularly, unlike free space
configurations in which stray-light diffracts away rapidly, stray
light often remains in photonic integrated circuits because
vertical stacks of materials provide optical confinement.
[0018] As those skilled in the art will appreciate when different
functional photonic elements are closely integrated together into
one or more photonic integrated circuits--and different optical
power levels are involved--high optical isolation is required and
stray light should be minimized throughout the circuits.
[0019] For example, an integrated circuit including a transmitter
and a receiver might have a laser input approaching 15 dBm, and a
receiver part to measure another signal with a power level of -35
dBm (for example, an optical power monitor with 5% tap of the
received signal). In this example, an optical isolation of more
than 50 dB is thus required.
[0020] Unfortunately, many optical components generate reflections
and/or stray light. With reference now to FIGS. 1(a) and 1(b),
there is shown two such examples. FIG. 1(a) shows a schematic
example of a polarization filter based on a directional coupler. As
depicted therein, most of the transverse electric (TE) signal
continues along a through port. All of the undesired orthogonal
transverse magnetic (TM) light and some of the TE signal are shown
as being filtered out to another port. As may be appreciated--with
such a configuration - some of light traversing the structure will
be reflected back, and some will become stay light.
[0021] FIG. 1(b) shows in schematic form an illustration of an
exemplary Mach-Zehnder interferometer (MZI). As is known in the
art, such (MZIs) are oftentimes used in optical modulators. Here
one or both arms of the MZI are adjusted, and an optical signal at
an output port is modulated. For a non-return-to-zero on-off-keying
modulation (NRZ-OOK), in average only 50% of the optical power is
delivered to the output port, and the remaining 50% becomes stray
light in the circuit.
[0022] According to an aspect of the present disclosure, such
infirmity may advantageously be avoided if a 2.times.2 optical
combiner replaces the 2.times.1 optical combiner and the unused
port of the 2.times.2 combiner is properly terminated according to
the present disclosure.
[0023] There exist several techniques for terminating unused
waveguide ports. However these techniques principally reduce
optical reflections. Accordingly, most of the undesired light is
still converted to stray light, which is problematic for photonic
integrated circuits.
[0024] For example, one technique uses relatively long waveguides
to terminate the unused ports. As light propagates along the
waveguides, it gradually diminishes due to the propagation loss of
the waveguides. However, in many cases the propagation loss is
predominately optical scattering loss, which converts the optical
signal mostly to stray light. Another technique uses a waveguide
inverse taper with reducing waveguide width. Here the optical mode
gradually loses confinement and light is diffracted into the
claddings surrounding the waveguide, again becoming stray light.
Yet another technique routes the unused ports to the edges of the
photonic chip and the undesired light is sent off the chip.
However, such routing might become difficult in many circuits
having a high level of integration and therefore a large number of
"internal" circuits.
[0025] According to an aspect of the present disclosure,
intentionally doped waveguides are used to terminate any unused
waveguide ports in a photonic integrated circuit--or other photonic
structure--such that undesired light is absorbed by the free
carrier absorption without causing additional reflections or stray
light, as was the case with prior art existing techniques. In a
preferred embodiment, any undesired light is completely absorbed by
the free carrier absorption and additional reflections and stray
light is eliminated. In certain embodiments, unused ports are added
to a photonic structure and then terminated according to the
present disclosure such that reflections and stray light are
reduced or eliminated, and the structure's overall performance is
enhanced.
[0026] As used herein, an unused waveguide port in a photonic
integrated circuit is one that is either unused, or open-ended, and
the doped waveguides that form the terminators are intentionally
doped.
[0027] These concepts of the present disclosure are illustrated
schematically in FIG. 2. Therein an unused waveguide port is
connected to a waveguide that has been intentionally doped. This
doping adds free carriers such as electrons or holes to the
waveguide, which induces optical absorption through free-carrier
absorptions. As light propagates along the doped waveguide, the
power level exponentially drops and can be suppressed to a
sufficiently low level with enough propagation length.
[0028] Advantageously, because the suppression mechanism according
to the present disclosure is based on absorption rather than
scattering, no significant stray light will be generated. Also,
unless the doping level in the absorber region is extremely high,
the difference in refractive indices between the undoped region and
the doped region is small enough to avoid significant optical
reflection at the interface.
[0029] We may now provide a further example of a device and/or
structure according to the present disclosure. Here we give an
example as based on a silicon photonic integrated circuit. In doped
silicon, the changes in the refractive index and the absorption
coefficient can be written as
.DELTA.n=-8.8.times.10.sup.-22.DELTA.N-8.5.times.10.sup.-18(.DELTA.P).su-
p.0.8,
.DELTA..alpha.=8.5.times.10.sup.-18.DELTA.N+6.0.times.10.sup.-18.DELTA.P-
,
respectively, where N and P are the concentrations of free
electrons and holes, in cm.sup.-3. For example, if an n-doped
region with an electron concentration of 1E19 is used as the
waveguide terminator, assuming a confinement factor of close to 1,
the absorption coefficient is 85 cm.sup.-1 or about 370 dB/cm. So
an absorber with 1.5 mm length is sufficient to produce an
attenuation of more than 55 dB. The change in the refractive index
is about -8.8E-3, which corresponds to a reflection level of only
-58 dB, negligible for most applications. For an even higher doping
level of 1E20, the absorption coefficient becomes 3700 dB/cm (an
absorber length of merely 150 micrometers produces an attenuation
of more than 55 dB), and the reflection level increases to -38 dB,
which might still be acceptable for many applications.
[0030] At this point, those skilled in the art will readily
appreciate that while the methods, techniques and structures
according to the present disclosure have been described with
respect to particular implementations and/or embodiments, those
skilled in the art will recognize that the disclosure is not so
limited. For example, the termination waveguide may be partially
doped and partially undoped, or doped to different types or levels
in different segments. Additional transition pieces such as
waveguide width tapers or transitions between waveguides of
different thickness/etch depths may be added between the unused
waveguide port and the doped waveguide absorber. The doped
waveguide can be routed in a straight pattern, or in spiral
patterns to reduce its footprint. The end of the doped waveguide
absorber can be either an abrupt end or a gradual taper, without
affecting the device performance. Accordingly, the scope of the
disclosure should only be limited by the claims appended
hereto.
* * * * *