U.S. patent number 9,270,380 [Application Number 14/267,582] was granted by the patent office on 2016-02-23 for chip-based advanced modulation format transmitter.
This patent grant is currently assigned to FREEDOM PHOTONICS, LLC.. The grantee listed for this patent is Freedom Photonics, LLC.. Invention is credited to Jonathon Barton, Leif Johansson, Milan Mashanovitch.
United States Patent |
9,270,380 |
Barton , et al. |
February 23, 2016 |
Chip-based advanced modulation format transmitter
Abstract
In various embodiments, a monolithic integrated transmitter,
comprising an on-chip laser source and a modulator structure
capable of generating advanced modulation format signals based on
amplitude and phase modulation are described.
Inventors: |
Barton; Jonathon (Santa
Barbara, CA), Johansson; Leif (Goleta, CA), Mashanovitch;
Milan (Santa Barbara, CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
Freedom Photonics, LLC. |
Santa Barbara |
CA |
US |
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Assignee: |
FREEDOM PHOTONICS, LLC. (Santa
Barbara, CA)
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Family
ID: |
43220348 |
Appl.
No.: |
14/267,582 |
Filed: |
May 1, 2014 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20140356001 A1 |
Dec 4, 2014 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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13761867 |
Feb 7, 2013 |
8718486 |
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12789350 |
Mar 19, 2013 |
8401399 |
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61182022 |
May 28, 2009 |
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61182017 |
May 28, 2009 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04B
10/505 (20130101); G02B 6/125 (20130101); H04B
10/5161 (20130101); H04B 10/503 (20130101); G02B
6/2766 (20130101); H04B 10/516 (20130101); H04B
10/801 (20130101); G02F 1/2255 (20130101); G02F
1/2257 (20130101); H04B 10/615 (20130101); H04B
10/6151 (20130101); G02B 2006/12104 (20130101); G02F
1/212 (20210101); Y10T 29/49826 (20150115) |
Current International
Class: |
H04B
10/50 (20130101); H04B 10/516 (20130101); H04B
10/61 (20130101); H04B 10/80 (20130101) |
Field of
Search: |
;398/82,86,87,184,186,188 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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10-2004-0081838 |
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Sep 2004 |
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KR |
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10-2014-0077533 |
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Jun 2014 |
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KR |
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WO 95/22070 |
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Aug 1995 |
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WO |
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WO 03/079508 |
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Sep 2003 |
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WO |
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WO 2007107187 |
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Sep 2007 |
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WO |
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WO 2008/036313 |
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Mar 2008 |
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WO |
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WO 2015064225 |
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May 2015 |
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WO |
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|
Primary Examiner: Singh; Dalzid
Attorney, Agent or Firm: Knobbe Martens Olson & Bear
LLP
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation of U.S. application Ser. No.
13/761,867 filed on Feb. 7, 2013 titled "Chip-Based Advanced
Modulation Format Transmitter", which is a continuation of U.S.
application Ser. No. 12/789,350 filed on May 27, 2010 titled
"Chip-Based Advanced Modulation Format Transmitter," (now U.S. Pat.
No. 8,401,399), which claims the benefit under 35 U.S.C.
.sctn.119(e) of U.S. Provisional Application 61/182,017 filed on
May 28, 2009 titled "Chip-Based Advanced Modulation Format
Transmitter," and claims the benefit under 35 U.S.C. .sctn.119(e)
of U.S. Provisional Application 61/182,022 filed on May 28, 2009
titled "Monolithic Widely-Tunable Coherent Receiver." Each of the
above-identified applications is incorporated by reference herein
in its entirety.
Claims
What is claimed is:
1. A monolithically integrated optical transmitter comprising: at
least one substrate; a tunable laser resonator monolithically
integrated with the substrate, the laser resonator configured to
output radiation along an optical axis; a first modulator
monolithically integrated with the substrate, the first modulator
including a first input waveguide configured to receive a first
portion of the radiation output from the tunable laser resonator
and modulate at least one of intensity or phase of the first
portion of the radiation output from the tunable laser resonator; a
second modulator monolithically integrated with the substrate, the
second modulator including a second input waveguide configured to
receive a second portion of the radiation output from the tunable
laser resonator and modulate at least one of intensity or phase of
the second portion of the radiation output from the tunable laser
resonator; and an optical redirector integrated with the first or
second input waveguide, the optical redirector configured to change
the direction of propagation of optical radiation in the waveguide
it is integrated with.
2. The optical transmitter of claim 1, further comprising an
optical combiner monolithically integrated with the substrate and
configured to combine optical signals output from the first
modulator and the second modulator and generate a modulated optical
signal, the optical combiner including a third input waveguide
connected to the first modulator, a fourth input waveguide
connected to the second modulator and at least one optical
redirector integrated with the third or fourth input waveguide, the
optical redirector configured to change the direction of
propagation of optical radiation in the waveguide it is integrated
with.
3. The optical transmitter of claim 1, wherein the substrate
comprises at least one of Si, InP, InAlGaAs, InGaAsP, InGaP, GaAs
or InGaAs.
4. The optical transmitter of claim 1, wherein the optical
redirector has at least one reflective facet that is arranged at an
angle with respect to the waveguide it is integrated with.
5. The optical transmitter of claim 1, wherein the optical
redirector is configured to change the direction of propagation of
optical radiation in the waveguide it is integrated with by an
angle between approximately 90 degrees and approximately 180
degrees.
6. The optical transmitter of claim 1, wherein the optical
redirector includes a dielectric.
7. The optical transmitter of claim 1, configured to generate a
modulated optical signal with quadrature phase shift keying (QPSK)
format or quadrature amplitude modulation (QAM) format.
8. The optical transmitter of claim 1, wherein at least one of the
first and second modulators comprises a dual nested Mach-Zehnder
modulator.
9. The optical transmitter of claim 1, wherein each of the first
and the second modulators comprises at least one electrode.
10. The optical transmitter of claim 9, wherein each of the first
and the second optical modulators comprises at least four
electrodes.
11. The optical transmitter of claim 1, further comprising one or
more monitor electrodes configured to monitor power of the
modulated optical signal at the output of the first or second
optical modulator.
12. The optical transmitter of claim 11, further comprising a
feedback circuit configured to provide an input electrical signal
to the first or second optical modulators based on an output of the
one or more monitor electrodes.
13. The optical transmitter of claim 1, wherein the tunable laser
resonator comprises: a first optical path including a first
reflector; a second optical path including a second reflector; and
an active region comprising an active material, the active region
optically connected to the first and second optical paths.
14. The optical transmitter of claim 13, further comprising at
least one optical redirector configured to optically connect the
active region to the first and second optical paths.
15. An optical transmitter comprising: a substrate; an optical
source comprising: an active region comprising an active material,
the active region including a first side and a second side opposite
the first side; a first optical path including a first reflector,
the first optical path connected to the first side of the active
region; a second optical path including a second reflector, the
second optical path connected to the second side of the active
region; and at least one optical redirector disposed in the first
optical path or the second optical; a first modulator
monolithically integrated with the substrate, the first modulator
including a first input waveguide configured to receive a first
portion of the radiation output from the tunable laser resonator
along the first optical path, the first modulator configured to
modulate at least one of intensity or phase of the first portion of
the radiation output from the tunable laser resonator; and a second
modulator monolithically integrated with the substrate, the second
modulator including a second input waveguide configured to receive
a second portion of the radiation output from the tunable laser
resonator along the second optical path, the second modulator
configured to modulate at least one of intensity or phase of the
second portion of the radiation output from the tunable laser
resonator.
16. The optical transmitter of claim 15, wherein the optical
redirector is configured to change the direction of propagation of
optical radiation in the first or second optical paths by an angle
between about 90 degrees and about 180 degrees.
17. The optical transmitter of claim 15, wherein the optical
redirector comprises a reflective facet that is arranged at an
angle with respect to the first or second optical path.
18. The optical transmitter of claim 15, wherein the optical
redirector includes a dielectric.
19. The optical transmitter of claim 15, further comprising an
optical combiner monolithically integrated with the substrate, the
optical combiner configured to combine optical signals output from
the first modulator and the second modulator and generate a
modulated optical signal.
20. The optical transmitter of claim 19, wherein the optical
combiner includes a third input waveguide connected to the first
modulator, a fourth input waveguide connected to the second
modulator and at least one optical redirector integrated with the
third or fourth input waveguide, the optical redirector configured
to change the direction of propagation of optical radiation in the
waveguide it is integrated with.
Description
BACKGROUND
1. Field of the Invention
Various embodiments of the invention relate to the area of optical
communications photonic integrated circuits (PICs). In particular,
various embodiments relate to integrated optical transmitters
capable of generating multilevel optical modulation formats in
optical communications applications.
2. Description of the Related Art
As demand for higher capacity in optical networks continues to
grow, ways to increase optical network capacity with reduced
capital investment are of interest. One cost efficient solution
that allows for increased or maximum utilization of the existing
optical network infrastructure is to implement more spectrally
efficient modulation formats for increased data throughput
capacity. Advanced modulation formats such as Quadrature Amplitude
Modulation (QAM), Phase Shift Keying (PSK), and Quadrature Phase
Shift Keying (QPSK) are spectrally efficient and can improve the
efficiency of fiber Wavelength Division Multiplexing (WDM).
Modulation formats such as Quadrature Phase Shift Keying and
Quadrature Amplitude Modulation can allow for a number of data
symbols to be sent utilizing the same line rate as a lower bit-rate
On-Off keyed system. Present optical transmitters for generating
optical signals having advanced modulation formats are large-scale
monolithic photonic integrated circuits (PICs) or use hybrid
platforms. Due to their size, large-scale PICs may require low
waveguide losses, high performance optical sources and other
optical subcomponents. Moreover, refined fabrication processes and
techniques may be required to reduce defects and to improve yield
of large-scale PICs. Thus, there is a need for optical transmitters
having a reduced footprint and a tolerance for optical waveguide
losses that can be fabricated with simple integration
platforms.
SUMMARY
Systems and methods that enable an optical transmitter capable of
generating optical signals with advanced modulation formats may be
beneficial in optical networks and systems. Example embodiments
described herein have several features, no single one of which is
indispensible or solely responsible for their desirable attributes.
Without limiting the scope of the claims, some of the advantageous
features will now be summarized.
Various embodiments described herein include a compact optical
transmitter having a reduced die size. For example, the die size of
the various embodiments of the optical transmitter device described
herein can be between approximately 0.5 square mm and approximately
3 square mm. In various embodiments, the die size of the optical
transmitter device can be between approximately 1.5 square mm and
approximately 2.5 square mm. In various embodiments, the die
comprises a monolithically integrated optical transmitter device
that is included in packaging to form the device. In various
embodiments, the die can comprise a monolithically integrated
optical transmitter device that will be coupled to optical fibers
or RF/electrical connectors. The decrease in the footprint and/or
the die size of the optical transmitter device can advantageously
reduce fabrication complexity required to integrate a single
surface-ridge waveguide structure and improve yield. Various
embodiments of the optical transmitter described herein can
comprise a tunable laser resonator and an optical vector modulator.
In various embodiments, an optical vector modulator can include a
modulator arrangement capable of modulating both optical intensity
and optical phase to generate optical vector modulation. Examples
of optical vector modulation formats include QPSK modulation and
multilevel QAM modulation. An arrangement for passive modulator
bias control can be implemented in various embodiments of the
optical transmitter to adjust for the wavelength dependence of the
modulator.
In various embodiments described herein, an optical transmitter
comprising a widely tunable laser and one or more optical vector
modulators may be monolithically integrated on a single die having
a common substrate. In various embodiments, monolithic common
substrate integration can include processes and techniques that
place all the subcomponents of the device on a common substrate
through semiconductor device processing techniques (e.g.
deposition, epitaxial growth, wafer bonding, wafer fusion, etc). In
some embodiments, the optical transmitter comprising a widely
tunable laser and one or more optical vector modulators may be
integrated on a single die having a common substrate, through other
techniques such as flip-chip bonding, etc. Monolithic common
substrate integration can provide a reduction in device insertion
losses. Such tunable optical transmitter devices can allow for a
reduction in the number of components and devices required in an
optical system. Other advantages of an integrated tunable optical
transmitter can be compact die size, reduced footprint, faster
tuning mechanisms, and the lack of moving parts--which can be
desirable for applications subject to shock, vibration or
temperature variation. Integrating an optical transmitter on a
single die can offer several other advantages as well, such as
precise phase control, improved performance and stability of the
transmitter, and compact implementation. Some additional benefits
of integrating a tunable laser with an optical modulator on a
single die can be: the ability to adjust or optimize the device
performance; ability to control and optimize the bias of the
modulators for every single wavelength--(the wavelength information
is known for an integrated transmitter, but not known when a
discrete modulator is used); and ability to utilize feedback from
on-chip integrated tap signals in order to better control the
operating point of the chip.
Various embodiments, described herein include a complex optical
transmitter fabricated on a small die size. Such devices can be
fabricated using relatively simple fabrication techniques and/or
integration platforms. In various embodiments described herein,
optical interconnect losses can be reduced by reducing interconnect
length rather than by including complex low-loss optical waveguide
structures.
Various embodiments of the optical transmitter described herein
comprise a common substrate comprising a III-V material such as
Indium Phosphide and one or more epitaxial layers (InP, InGaAs,
InGaAsP, InAlGaAs etc.); a laser resonator, formed on the common
substrate in the epitaxial structure; and one or more modulator
structures comprising a plurality of arms or branches and at least
two electrodes formed on the common substrate. The one or more
modulator structures may be configured to modulate the amplitude,
the phase, or both amplitude and phase of optical radiation emitted
from the laser resonator. In various embodiments, the modulator
structures may modulate light in accordance with the principles of
optical interference. In some embodiments, the modulator structures
may be positioned external to the laser cavity and be optically
connected to the laser resonator. In various embodiments, the
various components of the optical transmitter such as waveguides,
photonic components, splitters, etc. can be formed in the same
epitaxial structure as the epitaxial structure in which the laser
is formed. In some embodiments the components of the optical
transmitter such as waveguides, photonic components, splitters,
etc. can be formed in one or more epitaxial structures that are
different from the epitaxial structure in which the laser is
formed.
In various embodiments a monolithically integrated optical
transmitter die is described. In various embodiments, the size of
the monolithically integrated optical transmitter die can be less
than approximately 3 square mm. The monolithically integrated
optical transmitter die comprises at least one monocrystalline
substrate. The monolithically integrated optical transmitter die
further comprises a tunable laser resonator monolithically
integrated with the substrate, the tunable laser resonator
comprising an output reflector and a tuning section, said tunable
laser resonator configured to emit optical radiation from the
output reflector along an optical axis, such that the wavelength of
the emitted optical radiation is tunable over a wide wavelength
range from between about 15 nm to about 100 nm, wherein the wide
wavelength range is represented by .DELTA..lamda./.lamda. and is
configured to be greater than a ratio .DELTA.n/n, wherein .lamda.
represents the wavelength of the optical radiation, .DELTA..lamda.
represents the change in the wavelength of the optical radiation, n
represents the refractive index of the tuning section, and .DELTA.n
represents the change in the refractive index of the tuning
section. The monolithically integrated optical transmitter die
further comprises a first optical vector modulator monolithically
integrated with the substrate, the first optical modulator
comprising a first optical splitter optically connected to the
laser resonator, a plurality of arms comprising at least two
electrodes and a first output coupler; and a second optical vector
modulator monolithically integrated with the substrate, said second
optical modulator comprising a second optical splitter optically
connected to the laser resonator, a plurality of arms comprising at
least two electrodes and a second output coupler. The
monolithically integrated optical transmitter die further comprises
a polarization rotator monolithically integrated with substrate,
said polarization rotator arranged at an angle between about 20 deg
and 160 deg or between about -20 deg and -160 deg with respect to
the optical axis; and an optical combiner monolithically integrated
with the substrate and configured to combine optical signals output
from the first and second optical couplers. In various embodiments,
the first and/or the second optical splitters can be disposed at a
distance of approximately less than 750 .mu.m from the output
reflector of the laser resonator as measured along the optical
axis.
In various embodiments, a method of manufacturing a monolithically
integrated optical transmitter is described. The method comprises
providing at least one monocrystalline substrate. The method
further includes monolithically integrating a tunable laser
resonator with the substrate, the tunable laser resonator
comprising an output reflector and a tuning section, said tunable
laser resonator configured to emit optical radiation from the
output reflector along an optical axis, such that the wavelength of
the emitted optical radiation is tunable over a wide wavelength
range from between about 15 nm to about 100 nm, wherein the wide
wavelength range is represented by .DELTA..lamda./.lamda. and is
configured to be greater than a ratio .DELTA.n/n, wherein .lamda.
represents the wavelength of the optical radiation, .DELTA..lamda.
represents the change in the wavelength of the optical radiation, n
represents the refractive index of the tuning section, and .DELTA.n
represents the change in the refractive index of the tuning
section. The method further includes monolithically integrating a
first optical vector modulator with the substrate, said first
optical modulator comprising a first optical splitter optically
connected to the laser resonator, a plurality of arms comprising at
least two electrodes and a first output coupler. The method can
further include monolithically integrating a second optical vector
modulator with the substrate, said second optical modulator
comprising a second optical splitter optically connected to the
laser resonator, a plurality of arms comprising at least two
electrodes and a second output coupler. The method also comprises
monolithically integrating a polarization rotator with substrate,
said polarization rotator arranged at an angle between about 20 deg
and 160 deg or between about -20 deg and -160 deg with respect to
the optical axis; and monolithically integrating an optical
combiner with the substrate, said optical combiner configured to
combine optical signals output from the first and second output
couplers. In various embodiments, the first and/or the second
optical splitters can be disposed at a distance of approximately
less than 750 .mu.m from the output reflector of the laser
resonator as measured along the optical axis. In various
embodiments, the monolithically integrated optical transmitter can
have a die size less than 3 square mm.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 schematically illustrates an embodiment of an optical
transmitter having a reduced footprint and including a laser
resonator and a pair of optical vector modulators. FIG. 1A
schematically illustrates a monolithic tunable laser assembly with
folded laser implementation.
FIG. 2 schematically illustrates an embodiment of an optical vector
modulator.
FIG. 3 schematically illustrates an embodiment of an optical vector
modulator bias control.
FIG. 4 schematically illustrates another embodiment of an optical
transmitter having a reduced footprint and including a laser
resonator and a pair of optical vector modulators.
FIG. 5 schematically illustrates another embodiment of an optical
vector modulator bias control.
FIG. 6 schematically illustrates another embodiment of an optical
transmitter having a reduced footprint and including a laser
resonator and a pair of optical vector modulators.
FIG. 7 schematically illustrates another embodiment of an optical
transmitter having a reduced footprint and including two laser
resonator and a pair of optical vector modulators.
FIG. 8A schematically illustrates top view of an integrated of the
polarization rotator.
FIG. 8B schematically illustrates a cross-sectional view of an
integrated of the polarization rotator.
FIG. 8C schematically illustrates an embodiment of a tunable
polarization rotator.
FIG. 9 schematically illustrates another embodiment of an optical
transmitter.
These and other features will now be described with reference to
the drawings summarized above. The drawings and the associated
descriptions are provided to illustrate embodiments and not to
limit the scope of the disclosure or claims. Throughout the
drawings, reference numbers may be reused to indicate
correspondence between referenced elements. In addition, where
applicable, the first one or two digits of a reference numeral for
an element can frequently indicate the figure number in which the
element first appears.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Although certain preferred embodiments and examples are disclosed
below, inventive subject matter extends beyond the specifically
disclosed embodiments to other alternative embodiments and/or uses
and to modifications and equivalents thereof. Thus, the scope of
the claims appended hereto is not limited by any of the particular
embodiments described below. For example, in any method or process
disclosed herein, the acts or operations of the method or process
may be performed in any suitable sequence and are not necessarily
limited to any particular disclosed sequence. Various operations
may be described as multiple discrete operations in turn, in a
manner that may be helpful in understanding certain embodiments;
however, the order of description should not be construed to imply
that these operations are order dependent. Additionally, the
structures, systems, and/or devices described herein may be
embodied using a variety of techniques including techniques that
may not be described herein but are known to a person having
ordinary skill in the art. For purposes of comparing various
embodiments, certain aspects and advantages of these embodiments
are described. Not necessarily all such aspects or advantages are
achieved by any particular embodiment. Thus, for example, various
embodiments may be carried out in a manner that achieves or
optimizes one advantage or group of advantages as taught herein
without necessarily achieving other aspects or advantages as may
also be taught or suggested herein. It will be understood that when
an element or component is referred to herein as being "connected"
or "coupled" to another element, it can be directly connected or
coupled to the other element or intervening elements may be present
therebetween.
FIG. 1 schematically illustrates an embodiment of an optical
transmitter device. The device comprises at least one
monocrystalline substrate 101, a laser resonator 102, one or more
optical vector modulators 106a and 106b, a polarization rotator 121
and an optical coupler 123. In various embodiments, the various
sub-components of the optical transmitter may be monolithically
integrated with the substrate 101. The optical vector modulators
106a may include an optical splitter 107 connected to the laser
resonator 102, a plurality of arms comprising at least two
modulation electrodes and an optical coupler 116. The optical
vector modulator 106b may be similar structurally similar to
optical vector modulator 106a. These and other features are further
described below.
Monocrystalline Substrate
In various embodiments, the monocrystalline substrate 101 may
comprise one or more epitaxial structures. In various embodiments,
an epitaxial structure may be formed by depositing a
monocrystalline film on a monocrystalline substrate. In various
embodiments, epitaxial films may be grown from gaseous or liquid
precursors. Because the substrate acts as a seed crystal, the
deposited film takes on a lattice structure and orientation
identical to those of the substrate. In various embodiments, the
epitaxial structure comprises InGaAsP/InGaAs or InAlGaAs layers on
either a GaAs or InP substrate grown with techniques such as MOCVD
or Molecular Beam Epitaxy (MBE) or with wafer fusion of an active
III-V material to a silicon-on-insulator (SOI) material.
Laser Resonator
As discussed above in various embodiments, the laser resonator 102
may be formed on the common substrate and/or in one or more
epitaxial structures formed on the common substrate. In various
embodiments, the one or more epitaxial structures can include
layers or stacks of layers grown, deposited or formed on the common
substrate such that the one or more layers have a lattice structure
and orientation substantially similar to the common substrate. In
various embodiments, the laser resonator 102 can include a widely
tunable laser. In various embodiments, the widely tunable laser can
comprise a lasing cavity disposed between two mirrors or reflectors
and a tuning section. The optical radiation or laser light
generated by the widely tunable laser is output from the reflector
disposed closer to the output side of the laser cavity (output
reflector) along an optical axis. In various embodiments of the
optical transmitter device the optical axis can be aligned parallel
to the crystallographic axis of the monocrystalline substrate 101
(e.g. 011 axis for an InP substrate). In the embodiment illustrated
in FIG. 1, the optical axis can be aligned parallel to the +y
axis.
In various embodiments, the wavelength of the optical radiation
emitted from the widely tunable laser can be tuned over a wide
wavelength range from between about 15 nm to about 100 nm. Without
subscribing to any particular theory, in various embodiments, the
widely tunable laser can have a relative wavelength change
(.DELTA..lamda./.lamda.) that is larger than the available relative
index tuning (.DELTA.n/n) inside the laser cavity, wherein .lamda.
represents the wavelength of the optical radiation, .DELTA..lamda.
represents the change in the wavelength of the optical radiation, n
represents the refractive index of the tuning section, and .DELTA.n
represents the change in the refractive index of the tuning
section. The widely tunable laser oscillator can be configured to
tune to any transmission wavelength in a given range, wherein the
range may be larger than the range that can be achieved by
refractive index tuning of the semiconductor material and/or the
tuning section alone. Without subscribing to any particular theory,
the wide wavelength tuning in some embodiments of the widely
tunable laser can be achieved by using the Vernier effect, in which
the two mirrors or reflectors defining the lasing cavity have
multiple reflection peaks. The lasing wavelength is then defined by
the overlap between one reflection peak of each mirror. Tuning the
index in one of the mirrors or the tuning section (e.g. by applying
a voltage to electrodes disposed on the mirrors and/or the tuning
section) can shift the wavelength of each of the many reflections,
causing a different pair of reflection peaks to come into
alignment, thus shifting the lasing wavelength further than that of
the wavelength shift of the tuned mirror.
In various embodiments, the widely tunable laser as described
herein can have a tuning range from about 15 nm to about 100 nm
around 1550 nm. In some embodiments, the laser resonator 102 can
have a tuning range that is greater than approximately 15 nm. In
certain embodiments, the tuning range may be approximately 40 nm to
100 nm. In some embodiments, the tuning range may be approximately
20 nm, approximately 25 nm, approximately 30 nm, approximately 35
nm, approximately 40 nm, approximately 45 nm, approximately 50 nm,
approximately 55 nm, approximately 60 nm, approximately 65 nm,
approximately 70 nm, approximately 75 nm, approximately 80 nm,
approximately 85 nm, approximately 90 nm, or approximately 95 nm.
In certain embodiments, the tuning range may have a value between
any of the values provided above. In some embodiments, the tuning
range may be less than approximately 15 nm or greater than
approximately 100 nm.
In various embodiments, the laser resonator 102 can include any of
a variety of widely tunable lasers such as, for example, Sampled
Grating Distributed Bragg Reflector (SGDBR) lasers, Superstructure
grating Distributed Bragg reflector, Digital Supermode Distributed
Bragg Reflector (DSDBR), Y-branch or folded tunable laser, etc.
FIG. 1A shows details of an embodiment of a folded tunable laser.
Similar to the laser resonator 102, the folded tunable laser can be
widely tunable, and its wavelength tuning range can be wider than
what can be achieved through an index change, and the modulated
signal properties such as precise amplitude, phase and chirp, are
dynamically controlled by the modulator over the wide wavelength
tuning range of the laser.
The folded tunable laser illustrated in FIG. 1A comprises a front
mirror 151, back mirror 152, a gain section 153 positioned between
the front and the back mirror or incorporated into the mirrors, and
a phase section 154, all of which are situated along a common
waveguide 155. In various embodiments, the phase section 154 can
comprise a short section of passive waveguide in which current can
be injected to change the index in order to shift the cavity mode
wavelength into alignment with the peak reflection of the mirrors.
The common waveguide 155 can be straight, or it can utilize total
internal reflection mirrors 156, or a system of bend waveguides in
order to fold the laser and make it more compact. The bend
waveguides can be made of one or all of high contrast, deep
waveguides, or lower contrast rib or ridge waveguides. The
epitaxial structure and substrate between the different components
of the folded laser can be different.
By applying an appropriate combination of currents, a light beam is
produced b the folded laser and an frequency of light within the
designed tuning range can be emitted from the laser. As discussed
above, the wavelength of the emitted light is tunable over a wider
wavelength range than is achievable by index tuning of an one
section. The wider wavelength tuning range is represented by
.DELTA..lamda./.lamda.>.DELTA.n/n, where .lamda. represents the
wavelength of the light beam, .DELTA..lamda. represents the change
or delta in the wavelength of the light beam, n represents the
refractive index of a wavelength-defining mirror section of the
laser, and .DELTA.n represents the change (or delta) in the
refractive index of a wavelength-defining mirror section of the
laser.
In some embodiments, an optical amplifier section 103 can be
integrated at an output side of the tunable laser 102. The optical
amplifier section 103 can amplify the optical radiation emitted
from the laser resonator 102 and in some embodiments, the optical
amplifier section 103 may be used to control the power generated
laser light.
Optical Splitters and Fan-Outs
In various embodiments, the optical radiation from the laser
resonator 102 can be split into two parts using an optical splitter
104. In various embodiments, the optical splitter 104 can include
without limitation a multimode interference (MMI) splitter. In
various embodiments, the optical splitter 104 can comprise at least
one input waveguide and at least two output waveguides configured
such that optical radiation propagating through the at least one
input waveguide is split between the at least two output
waveguides. In general, integrating a tunable laser with one or
more vector modulators on the same die may require mitigation of
light reflection. To this effect, in various embodiments, optical
splitters and optical couplers can comprise N inputs and M outputs
that can allow for light evacuation from the vector optical
modulators (e.g. by absorption in the substrate) when they are in
their unbiased or OFF state. In various embodiments, the number of
inputs N can be 2, 4, etc. while the number of outputs M can be 2,
3, etc. In various embodiments, the splitter 104 can split the
light either equally or unequally between the at least two output
waveguides. In some embodiments, the optical power splitting ratio
between the at least two output waveguide can be tunable.
In various embodiments, a rapid transverse fan-out of the optical
radiation propagating through the at least two output waveguides of
the splitter 104 can be achieved through the use of a plurality of
total internal reflection (TIR) mirrors 105, each TIR mirror can be
configured to the change the direction of propagation of the
optical radiation, for example, by about 90 degrees. In some
embodiments, S-bends or other optical waveguide structures may be
used to achieve transverse fan-out of the optical radiation
propagating through the at least two output waveguides of the
splitter 104.
In various embodiments, the (TIR) mirrors can also be integrally
formed on the substrate 101. In various embodiments, a TIR mirror
can comprise a high index-contrast dielectric-semiconductor
interface that allows discrete reflection of the optical mode
between two waveguides. One purpose of these structures can be to
change the direction of propagation of the optical radiation. In
some embodiments, the TIR mirror can comprise at least one
reflective facet arranged at an angle .theta. with respect to the
waveguide that is configured to change the direction of propagation
of the optical radiation by approximately 90 degrees. Consecutive
reflection by two TIR mirrors can allow a 180 degree change in
propagation direction of the optical radiation. TIR mirrors can
also allow a rapid transversal displacement of the optical
radiation, that can be advantageous to achieve a compact fan-out of
input or fan-in of output optical waveguides from the optical
splitters and optical couplers in contrast to the more commonly
used S-bends which require a gradual fan-out to maintain low
optical loss. In various embodiments, the use of TIR mirrors can
enable a reduction in the die size or the footprint of the device
since the input and output waveguides can be fanned-out or
fanned-in to achieve the desired separation between the various
sub-components in relatively less space. Furthermore, the lengths
of optical waveguides can be shortened in devices using TIR mirrors
so as to reduce optical propagation losses. Various embodiments,
comprising S-bends to fan-out or fan-in the input and output
waveguides would likely result in an increase in the die size or
the footprint of the device, since the lengths of the waveguides
with S-bends and/or the radius of curvature of the S-bends cannot
be reduced beyond a certain minimum length (e.g. in various
embodiments, S-bends can exhibit increased loss if the radius of
curvature is less than 50 microns) without increasing waveguide
losses or complicating the integration platform. Use of TIR mirrors
is thus advantageous to realize complex devices having reduced die
size and footprint by using a simple integration platform.
Nevertheless, there may be embodiments in which S-bends or other
waveguide structures may be more preferable than TIR mirrors to
achieve fan-out of input or fan-in of output optical waveguides
from the optical splitters and optical couplers.
Optical Vector Modulators
Each of the two fanned-out optical signals from the splitter 104
are input to separate optical vector modulators 106a and 106b.
Without subscribing to any particular theory, a vector optical
modulator can include an optical modulator capable of modulating
both optical intensity and optical phase of an input optical
radiation to generate optical vector modulation. Examples of
optical vector modulation formats include but are not limited to
QPSK modulation and multilevel QAM modulation.
In various embodiments, each of the optical vector modulators 106a
and 106b may comprise a multi branch structure comprising multiple
waveguides. In some embodiments, the optical vector modulators 106a
and 106b may include nested Mach-Zehnder modulator (MZM). In
various embodiments, the optical vector modulators can be
configured to have low optical transmission in their unbiased or
OFF state (i.e. when no bias voltages are applied). In some
embodiments, this could be accomplished by varying the width, the
length, and/or the optical path length of the waveguides associated
with the optical vector modulators or other methods of refractive
index variation between the branches of the optical vector
modulators.
Bias Control of Optical Vector Modulators
The operation of an optical vector modulator with a widely tunable
laser can place an increased burden on modulator bias control. In
simple optical modulator configurations (e.g. simple on-off keying
(OOK) modulators) bias control can be provided by simple
arrangements. For example, for electro-absorption modulators (EAM)
or Mach-Zehnder type modulators (MZM) configured to generate a
modulated optical signal having a simple modulation format (e.g.
OOK) bias adjustment can be made, either directly to the modulator
bias electrode (e.g. in an EAM), or via an external modulator phase
tuning pad (e.g. in a MZM). The bias adjustment can be conveniently
calibrated with wavelength of the optical input to the modulator.
In contrast, in optical vector modulators configured to generate
optical signals with complex modulation formats, there can be a
multitude of modulator bias controls. For example, in a dual
polarization nested MZM modulator for QPSK generation, there can be
up to a total of 15 phase control electrodes that may need to be
adjusted based on the wavelength of the input optical radiation.
Various active bias control schemes have been demonstrated,
however, these can be impractical for a large number of control
points. Further, active bias control schemes can lead to a small
distortion of the generated optical waveform.
An option for bias control in optical vector modulators may be to
implement passive bias control that can be combined with wavelength
calibration to achieve the required modulator stability. For
example, in optical vector modulators arranged in a Mach-Zehnder
type configuration and implemented by using MMI splitters/couplers,
the output MMI coupler can include output ports that may not be
utilized to form the output modulated optical signal. By
integrating a photo detector in the un-utilized port and partial
absorbers in all the ports of the MMI coupler, the optical power in
all the ports of the MMI coupler can be monitored. The output of
the photo detector and the partial absorbers can provide
information regarding the phase and power in various branches of
the MZ structure. In some embodiments, the partial absorber can be
a contacted passive waveguide section. Most III-V waveguide
compositions can exhibit partial absorption at 0V applied voltage.
By measuring the resulting photocurrent, a relative estimate for
the optical power in the optical waveguide can be obtained. From
the information obtained from the partial absorbers in combination
with the absolute optical power measurement in unused MMI output
port photo detector, optical power in all MMI output ports can be
estimated at any operating wavelength. This information may then be
used to provide modulator bias control. For example, in some
embodiments, a feedback circuit configured to provide an input
electrical signal based on the information obtained from the
partial absorbers and/or the photo detector in the unused MMI port
to one or more electrodes (e.g. the bias control electrode) of the
optical vector modulators.
Operation of Optical Vector Modulators
In the embodiment illustrated in FIG. 1, the input optical signal
to the optical vector modulator 106a is further split into two
parts by an optical splitter 107. In various embodiments, the
optical splitter 107 may be similar to the optical splitter 104
described above. In various embodiments, the optical splitter 107
can be disposed at a distance of approximately 750 microns or less
from the output reflector of the laser resonator 102 as measured
along the optical axis of the laser resonator 102. In various
embodiments, the optical splitter 107 can be disposed at a distance
of approximately 750 microns or less from the output reflector of
the laser resonator 102 as measured along a horizontal direction
parallel to a first edge of the die (e.g. along the y-axis). In
various embodiments, the optical splitter 107 can be disposed at a
distance of approximately 150 microns--approximately 500 microns
from the output reflector of the laser resonator 102 as measured
along the vertical direction parallel to a second edge of the die
(e.g. along the x-axis). In some embodiments, the optical splitter
107 can be disposed at a distance of approximately 250 microns or
less from the output reflector of the laser resonator 102 as
measured along the optical axis of the laser resonator 102. In yet
other embodiments, the optical splitter 107 can be disposed at a
distance of approximately 250 microns or less from the output
interface of the amplifier section 103 as measured along the
optical axis of the laser resonator 102.
Each of the two outputs from the splitter 107 is further split into
two parts by another optical splitter. For example, in the
embodiment illustrated in FIG. 1 the optical splitter 108 further
splits one of the outputs of the splitter 107. The two output
signals from the splitter 108 are fanned-out using TIR mirrors,
S-bends or other waveguide structures. Each of the fanned-out
output from splitter 108 is input to a waveguide (e.g. 140 of FIG.
1) that includes a first electrode (e.g. 109 of FIG. 1). An
electrical signal can be provided to the first electrode 109 to
modulate the optical radiation propagating through the waveguide
140. In various embodiments, the electrical signal provided to the
electrode 109 may have a bandwidth in the range of approximately 5
GHz to approximately 50 GHz. In various embodiments, a second
electrode 110 can be disposed on the waveguide 140. Electric
current may be provided to the second electrode 110 to adjust the
phase of the optical radiation propagating through the waveguide
140. In various embodiments, the electrical current provided to the
electrode 110 may have a value between about 0 mA and about 15
mA.
An optical coupler 111 (e.g. multimode interference (MMI) coupler,
evanescent coupled-mode coupler, reflection coupler, or Y-branch
coupler) unites the two optical waveguides that are disposed at the
output of the splitter 108 to form one MZM in the dual nested MZM
structure. In various embodiments, the optical coupler 111 can
include at least two input waveguides and at least one output
waveguide. The optical coupler 111 can be configured to combine
optical radiation from the at least two input waveguides either
equally or unequally and couple the combined radiation to the at
least one output waveguide. In the embodiment illustrated in FIG.
1, the optical coupler 111 includes a first and a second output
waveguide. In various embodiments, a modulator monitoring
arrangement may be formed by a partial absorber 112 disposed on the
first output waveguide and a partial absorber 113 disposed on the
second output waveguide. In various embodiments, an optical
termination detector (e.g. a photo detector) 114 may be provided to
the first output waveguide of coupler 111 to absorb radiation
propagating in the first output waveguide.
In various embodiments, the partial absorber 113 in second output
waveguide is followed by an electrode 115. Electric current may be
provided using a current driver to the electrode 115 such that the
optical phase of the optical signal propagating in the second
output waveguide may be controlled by controlling the amount of
current provided to the electrode 115. In various embodiments, the
electrical current provided to the electrode 115 may have a value
between about 0 mA and 15 mA. The optical signal propagating in the
second output waveguide of coupler 111 is combined with the output
from the second MZM in the dual nested MZM structure of optical
vector modulator 106a in a coupler (e.g. a 2.times.2 MMI coupler)
116 having at least 2 input waveguides and at least one output
waveguide. In various embodiments, the coupler 116 can couple the
combined optical signal into two output waveguides that can further
include two partial absorbers 117 and 118, a termination photo
detector 119 and an electrode 120 to which current can be provided
to control the optical phase.
In various embodiments, the output optical signal from the optical
vector modulator 106a is reflected using a TIR mirror, and coupled
to an integrated polarization rotator 121 that is configured to
rotate the polarization of the optical signal by approximately 90
degrees. In some embodiments, the polarization rotator 121 may be
configured to rotate the polarization of the optical signal by less
than or greater than 90 degrees. In various embodiments, the
polarization rotator 121 may be disposed at an angle .theta.
between about 20 degrees and 160 degrees or between about -20
degrees and -160 degrees with respect to the optical axis of the
laser resonator. In various embodiments, the polarization rotator
121 may be disposed at an angle .theta. between about 20 degrees
and 160 degrees or between about -20 degrees and -160 degrees with
respect to the crystallographic axis of the monocrystalline
substrate. The output optical signal from the polarization rotator
121 is then recombined with the output from the second optical
vector modulator 106b which is propagated in the optical waveguide
122 in the optical coupler (e.g. multimode interference (MMI)
coupler, evanescent coupled-mode coupler, reflection coupler, or
Y-branch coupler) 123. In various embodiments, the features of the
optical vector modulator 106b can be structurally and functionally
similar to the features of the optical vector modulator 106a
described above. In various embodiments, the output of the second
optical vector modulator 106b can be configured to maintain its
original input polarization. Two partial absorbers 124, 125 can be
located at the output of the coupler 123 to provide additional
signal monitoring. In some embodiments, one of the output
waveguides of the coupler 123 can be terminated in a photo detector
(126) while signal propagating in the other output waveguide of the
coupler 123 can be coupled to an external environment through an
output edge of the optical transmitter device. Facets may be
provided at the output edge of the optical transmitter device to
enable optical connection with optical fibers, planar waveguides,
other devices and systems. In some embodiments, a mode converter
127 may be disposed closer to the output edge of the device to
improve coupling efficiency.
Some Preferred Embodiments
Some preferred embodiments are described below. It is understood
that these represent a few possible embodiments out of a range of
combinations that have some similarities to the embodiment
illustrated in FIG. 1 and the sub-components described therein.
FIG. 2 schematic illustrates an embodiment of a single optical
vector modulator similar to the optical vector modulator 106a and
106b of FIG. 1. The optical vector modulator illustrated in FIG. 2
can include optical gain regions 201a and 201b and/or 202a and 202b
within the modulator structure. For example, in some embodiments,
optical amplifier sections 201 can be provided after the first
splitting stage (e.g. splitter 107 of FIG. 1). In certain
embodiments, it may be advantageous to provide an amplifying
section after the first splitting stage (e.g. splitter 107 of FIG.
1) instead of providing an amplifying section after the laser
resonator (e.g. amplifying section or region 103 of FIG. 1), since
the amplifying section 201 may be able to deliver twice the
saturated output power to the modulator sections. In some
embodiments, optical amplifying section or regions 202 may be
provided after the second splitting stage (e.g. after the optical
splitter 108 of FIG. 1). In some embodiments, if the amplifying
regions or sections in different branches of the optical vector
modulator are spaced too closely, the saturated output power may be
reduced due to heating effects. To eliminate or reduce heating
effects, it may be advantageous to rapidly fan-out the various
branches of the optical vector modulator to laterally separate the
various branches. In some embodiments, TIR mirrors can be used to
reduce the footprint of a device having such an arrangement.
FIG. 3 schematically illustrates an embodiment of a modulator bias
control system. The modulator bias control system illustrated in
FIG. 3 comprises two partial absorbers 301 and 302 that are located
at the output ports of coupler 303 which is disposed at the output
of a Mach-Zehnder structure. In various embodiments the coupler 303
can be similar to the coupler 111 or 116 of FIG. 1. The partial
absorbers 301 and 302 may be used to detect a photocurrent that is
dependent on optical power of the signal propagating in the
waveguide. The bias control system can further include an optical
termination detector 304 that is configured to absorb the radiation
propagating in one of the output ports or waveguides of the coupler
303. The fractional relation between the photocurrent of the
partial absorber 301 and the detector 304 can then be related to
the photocurrent of partial absorber 302 to determine the optical
waveguide power in the other output port or waveguide 305.
FIG. 4 schematically illustrates a second embodiment of an optical
transmitter device. Many of the features of the optical transmitter
device illustrated in FIG. 4 can be similar to the features
described with reference to FIG. 1. Similar to the embodiment shown
in FIG. 1, the device illustrated in FIG. 4 can include a single
epitaxial structure 401, a laser resonator 402, an optional optical
amplifier section 403, an optical splitter 404 (e.g. a MMI
splitter), a pair of optical vector modulators 406a and 406b and an
optical coupler 418 (e.g. a MMI coupler). The optical splitter 404
and the MMI coupler 418 can be configured to split/combine optical
radiation into either equal or unequal parts. In the illustrated
embodiments, a rapid transverse fan-out of the radiation emitted
from the two output ports or waveguides of the optical splitter 404
can be achieved through the use of multiple total internal
reflection (TIR) mirrors (405), S-bends or other waveguide
structures. In the embodiment illustrated in FIG. 4, the optical
vector modulator 406a can include an input 1.times.4 splitter (e.g.
1.times.4 MMI splitter) 407. The four output signals from the
1.times.4 splitter 407 can be fanned out using TIR mirrors (408)
and input into four optical waveguides. One or more of the four
optical waveguides in the optical vector modulator 406a can include
a first electrode (e.g. electrode 409) to which a modulation signal
can be provided to generate an optically modulated signal. In
various embodiments, the electrode 409 can be structurally and
functional similar to the electrode 109 of FIG. 1. In various
embodiments, the optical vector modulator 406a can further include
a second electrode 410 in one or more of the four optical
waveguides. An electric current can be provided to electrode 410 to
adjust the phase of the optical signal propagating in the one or
more output waveguides. In various embodiments, a 4.times.3 coupler
411 (e.g. a MMI coupler) can be provided to unite the optical
signal propagating in the four optical waveguides of optical vector
modulator 406a to obtain the modulated optical signal. A modulator
monitoring arrangement as described above with reference to FIG. 1
can be provided herein as well. The modulator monitoring
arrangement can be formed by partial absorber 412 located at the
one or more of the output ports of the 4.times.3 coupler 411 and
optical termination detectors 413 and 414 that absorb the light in
the two unused output ports or waveguides of the 4.times.3 coupler
411. In various embodiments, the partial absorber 412 in second
port or output waveguide of the 4.times.3 coupler 411 can be
followed by an electrode 415 to which an electric current can be
provided to adjust the phase of the optical signal propagating in
the waveguide. The output of the optical vector modulator 406a can
be reflected using a TIR mirror, S-bend or other waveguide
structures, and coupled to an integrated polarization rotator 416
that rotates the polarization of the optical signal by
approximately 90 degrees. The polarization rotator 416 can be
structurally and functionally similar to the polarization rotator
121 of FIG. 1. This polarization rotated optical signal can be then
combined in a 2.times.2 coupler 418 (e.g. a MMI coupler) with the
output from the second integrated optical vector modulator 406b
that is propagated through the optical waveguide 417. In various
embodiments, the optical signal propagating in the optical
waveguide 417 can maintain its original input polarization. Two
partial absorbers 419a and 419b can be located at the output of the
coupler 418 to provide additional signal monitoring capability. In
some embodiments, one of the output waveguides of the coupler 418
can be terminated in a photo detector 420, while signal propagating
in the other output waveguide of the coupler 418 can be coupled to
an external environment through an output edge of the optical
transmitter device. Facets may be provided at the output edge of
the optical transmitter device to enable optical connection with
other devices and systems. In some embodiments, a mode converter
421 may be disposed closer to the output edge of the device to
improve coupling efficiency.
FIG. 5 schematically illustrates an embodiment of a modulator bias
control system that can be implemented with the optical vector
modulator illustrated in FIG. 4. The modulator bias control
illustrated in FIG. 5 can include partial absorber 501a, 501b, 503
and photo detectors 502a and 502b As described above with reference
to FIG. 3, the output waveguide power can again be estimated by
comparing the fractional relation of the photocurrent from the
partial absorbers 501a and 501b and the detectors 502a and 502b
which can then be related to the photocurrent of middle partial
absorber 503 to determine the optical waveguide power in the output
waveguide 504. Additional modulator bias information can be
obtained by measuring the balance between the two detector
currents.
FIG. 6 shows a variation of the embodiments illustrated in FIG. 1
or FIG. 4. The illustrated device can comprise a single epitaxial
structure 601, a laser resonator 602, an optional optical amplifier
section 603 and a splitter 604 (e.g. a multimode interference (MMI)
splitter) splitting the light in two parts. As described above, TIR
mirrors 605 can be used to rapidly fan out the two output
waveguides or ports of the splitter 604. In the illustrated
embodiment, the TIR mirrors are oriented such that the direction of
propagation of the optical radiation emitted from the laser
resonator 602 is turned by about 180 degrees before being input to
the optical vector modulators 606a and 606b.
FIG. 7 shows another variation of the embodiments illustrated in
FIG. 1 or FIG. 4. The device illustrated in FIG. 7 comprises a
single epitaxial structure 701, dual laser resonators 702a and
702b, optional dual optical amplifier sections 703a and 703b, and a
2.times.2 splitter 704 (e.g. a 2.times.2 multimode interference
(MMI) splitter) for splitting the light in two parts. As described
above, two optical vector modulators 705a and 705b can be
monolithically integrated with the substrate to generate a
modulated optical signal. Such a device may be advantageously used
in optical switching applications, where a rapid transition from
one optical wavelength to a second optical wavelength may be
required. In various embodiments, the lasers 702a and 702b may be
both tuned to their designated optical wavelengths, while the
wavelength switching event takes place by turning one amplifier
(e.g. 703a) on while turning the second amplifier (e.g. 703b) off,
to switch which laser output light enters the 2.times.2 splitter
704.
In various embodiments, polarization beam splitters and rotators
can be integrated on the common substrate in the same or different
epitaxial structure as other sub-components of the integrated
optical transmitter. In various embodiments, the polarization beam
splitter elements may be formed on the common substrate, in the
same or different epitaxial structure as other components, with the
purpose of splitting the input coupled light into the TE and TM
polarized modes of the common waveguide. One embodiment of this
element can be realized by using the phenomenon of birefringence
between two modes. A polarization rotating element can convert TM
polarized light into TE polarized light and vice versa.
A polarization rotator can be formed by inducing a birefringence in
the waveguide, for example, by fabricating an asymmetric waveguide.
An asymmetric waveguide can be fabricated by etching the sidewall
of the optical waveguide at an angle. By selecting the appropriate
length of the angled etch, a halfwave plate can be formed that can
rotate linearly polarized TE or TM light by 90 degrees. Other
approaches and designs can also be used.
FIG. 8A schematically illustrates top view of an embodiment of a
polarization rotator that is integrated in the optical transmitter
device. FIG. 8B schematically illustrates a cross-sectional view of
the polarization rotator illustrated in FIG. 8A along an axis 804
parallel to the normal to the substrate 803 (e.g. parallel to the
z-axis). In one embodiment, the polarization rotator comprises an
asymmetric waveguide ridge 802 that is disposed on a substrate 803.
In various embodiments, the polarization rotator can be formed by
modifying a waveguide section 801 of the optical transmitter device
using semiconductor device processing techniques. In various
embodiments, the asymmetric waveguide ridge 802 can be formed on a
slab waveguide 805 which comprises a high-index material. In some
embodiments, the waveguide ridge 802 can have a first edge 802a
disposed at a first angle with respect to the normal to the
substrate and a second edge 802b disposed at a second angle with
respect to the normal to the substrate. In various embodiments, the
first and the second angle can be different from each other. In
various embodiments, the first angle can be approximately parallel
to the normal to the substrate as shown in FIG. 8B. The asymmetric
nature of waveguide ridge 802 results in a bi-refringent waveguide
structure.
The asymmetric waveguide structure 802 can be formed by using an
etching process. For example, in one method of fabricating the
polarization rotator, the asymmetric waveguide structure 802 is dry
etched on one side of the waveguide ridge 802 to form the edge
802a, and wet etched on the other side of the waveguide ridge 802
to form the sloping edge 802b. In some embodiments, the method can
include etching through the slab waveguide 805. Etching through the
slab waveguide 805 can be advantageous to realize a polarization
rotator structure with reduced footprint.
In one method of fabricating the polarization rotator on an InP
substrate, the sloping edge 802b can be formed by employing a wet
etch at a waveguide section oriented around 90 degrees with respect
to the laser ridge--which gives around 40-50 degrees wet etch
plane--that stops on the InGaAsP or InAlGaAs waveguide core (e.g.
slab waveguide 805) or a stop etch layer. If a different
orientation is chosen for the polarization rotator (e.g.
perpendicular to or within 20-160 degrees or -20 to -160 degrees
from the laser axis) the wet etch will align the waveguide edge to
an angle not perpendicular to the substrate. The above described
method of fabricating the polarization rotator can be a repeatable
process and can yield polarization rotators with a small
footprint.
FIG. 8C illustrates an embodiment of a tunable polarization
rotator. The tunable polarization rotator comprises an input
waveguide 810 that is connected to an optical splitter 812 having
with two output waveguides. In various embodiments, the optical
splitter 812 can be a polarization beam splitter. Electrodes 814a,
814b, 816a and 816b may be provided to the two output waveguides. A
voltage between approximately 1V to approximately -6V can be
applied to the electrodes 814a and/or 814b to change the
transmitted optical intensity. An electric current in the range of
approximately, 0 mA to approximately 15 mA may be provided to the
electrodes 816a and/or 816b to adjust the phase of the optical
radiation propagating through the output waveguides. In various
embodiments, the voltage and the current can be provided by using
an external drive circuit. The tunable polarization rotator can
further comprise a polarization rotator 818 that can be disposed in
one of the output waveguides. The polarization rotator 818 can be
similar to the various embodiments of the polarization rotators
described above. The tunable polarization rotator can further
comprise an optical coupler 820 having an output waveguide 822 and
configured to combine the optical outputs of the two output
waveguides of splitter 812. With the appropriate adjustment of
optical phase and intensity, the polarization state of the signal
in the output waveguide 822 can be tuned.
FIG. 9 shows another embodiment of the optical transmitter device.
The device illustrated in FIG. 9 can comprise at least one
monocrystalline substrate having a single epitaxial structure 901,
a laser resonator 902, an optional optical amplifier section 903, a
2.times.2 splitter 904 (e.g. a multimode interference (MMI)
splitter) for splitting the light in two parts. In this embodiment,
a second stage optical splitter 905 can be provided to further
split the optical power. The illustrated embodiment may include
four optical vector modulators 906a, 906b, 906c, 906d. The
waveguide at the output of each optical vector modulator 906a,
906b, 906c, 906d can include a pair of electrodes 907 and 908. An
electrical field can be provided to electrode 907 to control the
amplitude of the optical signal in the output waveguide while an
electrical current can be applied to electrode 908 to control the
optical phase of the optical signal propagating in the output
waveguide. By combining the output from two optical vector
modulators with the correct phase and amplitude, more complex
optical modulation formats such as 16-QAM modulation can be
generated in a practical manner. This nested arrangement can be
expanded for practical generation of even higher order modulation
formats such as 64 or 128-QAM.
In various embodiments, the various integrated optical transmitter
architectures and components can be monolithically integrated on a
common substrate with the various integrated receiver architectures
and components such as those described in U.S. Provisional App. No.
61/182,022 filed on May 28, 2009 titled "Monolithic Widely-Tunable
Coherent Receiver," which is incorporated herein by reference in
its entirety, to obtain a single transceiver device.
While the foregoing detailed description discloses several
embodiments of the present invention, it should be understood that
this disclosure is illustrative only and is not limiting of the
present invention. It should be appreciated that the specific
configurations and operations disclosed can differ from those
described above, and that the apparatus and methods described
herein can be used in contexts. Additionally, components can be
added, removed, and/or rearranged. Additionally, processing steps
may be added, removed, or reordered. A wide variety of designs and
approaches are possible.
The examples described above are merely exemplary and those skilled
in the art may now make numerous uses of, and departures from, the
above-described examples without departing from the inventive
concepts disclosed herein. Various modifications to these examples
may be readily apparent to those skilled in the art, and the
generic principles defined herein may be applied to other examples,
without departing from the spirit or scope of the novel aspects
described herein. Thus, the scope of the disclosure is not intended
to be limited to the examples shown herein but is to be accorded
the widest scope consistent with the principles and novel features
disclosed herein. The word "exemplary" is used exclusively herein
to mean "serving as an example, instance, or illustration." Any
example described herein as "exemplary" is not necessarily to be
construed as preferred or advantageous over other examples.
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