Note: Descriptions are shown in the official language in which they were submitted.
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INTEGRATED WAVELENGTH TUNABLE SINGLE AND TWO-
STAGE ALL-OPTICAL WAVELENGTH CONVERTER
This invention is made with Government support under Grant No.
N00014-96-6014, awarded by the Office of Naval Research. Grant No.
9896283, awarded by the National Science Foundation and Grant No.
F49620-98-1-0399, awarded by the Department of Air Force. The
Government has certain rights in this invention.
Related Applications
The present application is related to U.S. Provisional Patent
Application, serial no. 60/156,459, filed on Sept. 28, 1999.
Background of the Invention
1. Field of the Invention
The invention relates to a method and apparatus for integrated
wavelength tunable single and two-stage all-optical wavelength converter.
2. Description of the Prior Art
The current usage of optical components and lasers has made
communications and data transfer more efficient and more cost effective. The
use of semiconductor lasers has made the fabrication and packaging of optical
sources more cost effective, as well as reducing the size of the overall
device.
However, the requirements for communications and data transfer
systems have also increased. Widely tunable lasers are essential
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components for a wide variety of wavelength-division multiplexing (WDM)
and packet switching network architectures. They can be used as replacement
sources in long haul dense WDM communication systems or for wavelength
routing in access networks. They are also important devices for next
generation phased array radar systems that use true-time delay beam steering.
There is a need in such systems for stable monolithic integrated optical
frequency converters, but until now none have been available.
Brief Summary of the Invention
The invention is an apparatus comprising a semiconductor
heterostructure, a tunable laser fabricated in the semiconductor
heterostructure
and an interferometer having an input coupled to the output of the tunable
laser. The interferometer is monolithically fabricated with the tunable laser
in
the semiconductor heterostructure.
In the illustrated embodiment the tunable laser is a distributed Bragg
reflector laser, although the invention contemplates any type of semiconductor
laser now known or later devised. The laser also comprises a buried ridge
stripe waveguide laser. The buried ridge stripe waveguide laser comprises two
sampled grating DBR mirrors, a gain section and a phase section.
The interferometer has a semiconductor optical amplifier coupled in
each its arms. The apparatus further comprises a cross-gain semiconductor
optical amplifier converter coupled to the interferometer. The semiconductor
optical amplifier coupled in each arm is biased so that an optical path length
difference between the two arms is in antiphase which results in
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destructive interference. The semiconductor optical amplifier is polarization
insensitive.
In one embodiment the apparatus has an input to which an input signal,
~,I, is coupled and a coupler. The polarization insensitive semiconductor
optical amplifier has an output coupled to the coupler. The output of the
tunable laser is coupled to the coupler. The polarization insensitive
semiconductor optical amplifier is used as a gain controller for the
semiconductor optical amplifiers in the interferometer to allow wavelength
conversion over a larger range of input signal powers.
A dense wavelength division multiplexing communication system with
multiple channels is coupled to the output of the interferometer so that the
tu;~able laser can be used to convert between any two of the multiple
channels.
The interferometer further comprises a multimode interference coupler
characterized by a wavelength insensitive splitting ratio coupled to the input
of
the interferometer.
The heterostructure substrate comprises a low bandgap waveguide
layer and thinner mufti-quantum well active regions disposed above the low
bandgap waveguide layer. The heterostructure substrate has nonabsorbing
passive elements formed therein by selectively removing the quantum wells
regions above the waveguide layer to allow formation of active and passive
sections in the waveguide layer without having to perform a butt joint
regrowth.
In one embodiment an input signal, ~,I, is coupled thereto and the
apparatus further comprises a distributed feedback laser having an output to
modulate the input signal, ~,I. A semiconductor optical amplifier
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has an output and an input coupled to the input signal, ~,I, and to the output
of
the distributed feedback laser. A notch filter has an output and an input
coupled to the output of the semiconductor optical amplifier. An input of the
interferometer is coupled to the output of the notch filter, so that the input
signal, 7~I, is converted to a desired wavelength via cross phase modulation.
A
comb filter has its input coupled to the output of the interferometer. The
semiconductor optical amplifier has an input coupled to the input signal, ~,~,
and is polarization insensitive. The interferometer is operated at a fixed
polarization of an intermediate wavelength. The apparatus further comprises a
distributed feedback laser having an output to modulate the input signal, ~,L.
A
semiconductor optical amplifier has an input coupled to the input signal, ~.I,
and to the output of the distributed feedback laser. A notch filter has an
input
coupled to the output of the semiconductor optical amplifier. The input of the
interferometer is coupled to the output of the notch filter, so that the input
signal, 7~I, is converted to a desired wavelength via counter propagating
cross
gain modulation. A comb filter has an input coupled to the output of the
interferometer.
The invention is also characterized as a method of fabricating an
integrated optical device comprising providing a base structure comprised in
turn of a cap layer, a multiquantum well layer disposed beneath the cap layer,
a first waveguide layer disposed beneath the multiquantum well layer, and a
heterostructure waveguide layer disposed beneath the first waveguide layer.
The cap layer and multiquantum well layer are selectively removed to define a
passive section. An MOCVD layer is regrown on the passive section and the
remaining portions of the base structure. Optical structures are
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then selectively formed in the MOCVD layer, the passive section and
remaining portions of the base structure.
The step of selectively forming optical structures in the MOCVD layer,
the passive section and remaining portions of the base structure comprises
forming an active optical device in the remaining portions of the base
structure, or more particularly a laser or an optical grating.
The step of selectively forming optical structures in the MOCVD layer,
the passive section and remaining portions of the base structure also
comprises
forming a passive optical device in the passive section, such as a spot size
converter.
In the illustrated embodiment, the step of selectively forming optical
structures in the MOCVD layer, the passive section and remaining portions of
the base structure comprises forming a tunable laser and at least two
semiconductor optical amplifiers in the remaining portions of the base
structure, an interferometer in the passive section and a waveguide circuit
coupling the laser, at least two semiconductor optical amplifiers, and
interferometer into an optical circuit to form an at least partially
integrated
tunable wavelength converter.
Although the invention has been described as a method of steps for the
sake of grammatical ease, it is to be expressly understood that the invention
is
not to be limited by the illustrated embodiment under the construction of 35
USC 112, but is to be defined by the full scope of the claims without
limitation
to the illustrated embodiments. The invention can be better visualized by
turning now to the following drawings wherein like elements are referenced
by like numerals.
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Brief Description of the Drawings
Fig. 1 is a diagrammatic perspective view of a photonic chip in which a
single stage wavelength converter has been fabricated.
Fig. 2 is a block diagram of the elements of a photonic two stage
wavelength converter in which non-integrated components are used.
Fig. 3 is a diagrammatic perspective view of a photonic chip in which a
two stage wavelength converter has been fabricated in an integrated manner.
Figs. 4a - 4i(4) are simplified cross-sectional diagrams, which illustrate
the method by which the optical devices of the invention are fabricated in an
imegrated fashion.
The invention and its various embodiments can now be better
understood by turning to the following detailed description of the preferred
embodiments which are presented as illustrated examples of the invention
defined in the claims. It is expressly understood that the invention as
defined
by the claims may be broader than the illustrated embodiments described
below.
Detailed Description of the Preferred Embodiments
This invention is a device and method for performing an all optical
wavelength conversion using a tunable laser 10 integrated with single stage
and two-stage Mach-Zehnder interferometer converter configurations 12 and
14 respectively best depicted in Figs. 1 and 3. One aspect of this
implementation is integration of a widely tunable sampled grating distributed
Bragg reflector (DBR) laser 10 that can be vernier tuned over more than 40
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nm and is optically isolated from the wavelength converter sections 12 and 14
due to the DBR mirror section in laser 10. This isolation overcomes
fundamental limitations of previous attempts to integrate these devices. See
also copending U.S. Patent Application serial no. , entitled
"Tunable Laser Source with Integrated Optical Modulator," claiming priority
to Provisional Patent Application serial no. 60/152,432 filed on Sept. 2,
1999,
which are both incorporated herein by reference.
The single-stage wavelength converter 12 in Fig. 1 comprises a Mach-
Zehnder interferometer 12 combined with semiconductor optical amplifiers
(SOAs) 16 and 18 in each arm 20 and 22 of interferometer 12. The two-stage
converter 14 in Fig. 3 is comprised of a cross-gain semiconductor optical
amplifier converter followed by the Mach-Zehnder interferometer based
converter 16, 18, 20, 22. In the interferometer converter section 16, 18, 20,
22, pumped light from the tunable laser 10 is split evenly between the two
arms 20 and 22 of the interferometer 12, 14. The input signal 7~; is amplified
by semiconductor optical amplifier 38, combined with the output of tunable
laser 10 in a coupler 40 and fed to semiconductor optical amplifier 18 in arm
22 of interferometer 12. The optical power fed into semiconductor optical
amplifier 18 modifies the transfer function through amplifier 18 resulting in
an
amplified output signal at ~,; and a,~,,. ~,; can then be filtered out by a
conventional off chip optical filter (not shown).
Semiconductor optical amplifier 16 is provided in the opposing arm 20
of interferometer 12 to adjust optical path lengths between arms 20 and 22.
Amplifiers 16 and 18 can be biased so that the optical path length difference
between the two arms 20 and 22 is in antiphase resulting in
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destructive interference at the output waveguide 24. The input signal, ~,;, is
coupled into a single arm 22 of the interferometer 12, 14. When the input
light, ~,;, is in the high power state, it changes the phase difference
between the
two arms 20 and 22 and allows light from the pumped beam, ~,,",, to be
transmitted.
This method transfers the modulation on the input data signal, 7~;, to the
pumped light, 7,~,,, from the tunable laser 10 which can be performed with or
without logical bit inversion by selectively operating on the appropriate
slope
of the transfer curve of semiconductor optical amplifier 18. The input beam,
~,;, can be filtered out at the output 24 allowing the converted light to be
transmitted.
A monolithic tunable wavelength converter 11 has advantages over an
implementation based upon discrete components in that it eliminates two fiber
pigtails that increase the noise figure due to additional insertion loss and
packaging expense. The tunable nature of this implementation also allows one
device to be used to optically convert between any two channels in a dense
wavelength division multiplexing (DWDM) communication system as
opposed to a separate untunable device for each channel.
A feature of the implementation of Figs. 2 and 3 is the use of an
internal wavelength between stages to avoid the need for fast tunable filters
and the relaxation of the need for polarization insensitive converters, since
the
input internal wavelength at which one stage, tunable converter of Fig. 1
operates can be at a fixed polarization state and the second converter stage
16,
18, 20, 22 can be fabricated using polarization sensitive waveguide
technology.
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There are a number of methods in general which are used to implement
wavelength conversion of Fig. 1, e.g. four-wave mixing, cross gain
modulation (XGM), and cross phase modulation (XPM). Cross phase
modulation in an interferometer 12, 14 employing semiconductor optical
amplifiers 16, 18 is considered to be the leading method at this time due to
the
conversion efficiency, extinction ratio enhancement, and low chirp. It is very
attractive to incorporate laser 10 providing the continuous wave light on the
same chip 26 as the interferometer 12, 14 due to the elimination of two
optical
fiber pigtails and the similarity in the fabrication processes required to
produce
both devices.
It is important that the integrated continuous wave source be
insensitive to the back reflections amplified by the semiconductor optical
amplifier's 16, 18 in the interferometer 12, 14. To fulfill this requirement a
DBR laser 10 should be chosen as the continuous wave source due to the
inherent isolation properties of the laser mirrors. An added benefit of the
DBR
or sampled-grating-distributed-Bragg-reflector (SGDBR) laser 10, is their
ability to be electrically tuned to cover several wavelength channels.
In the preferred embodiment the device is comprised of a SGDBR
laser 10 coupled to a Mach-Zender interferometer 12, 14 with a polarization
insensitive semiconductor optical amplifier 16, 18 located in each the arms 20
and 22 respectively. A schematic of the device is shown in Fig. 1. The laser
10 is a 2 ~,m wide buried ridge stripe (BRS) waveguide device that is
comprised of four separate elements. These separate elements include two
sampled grating DBR mirrors and sections for gain and phase control. By
controlling the injection current into the sections for gain and
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phase control, lasers 10 of this type can be made to tune over more than 40 nm
with continuous wavelength coverage.
At the output 28 of the front mirror section 30, the laser waveguide 32
is coupled into a 3 dB multimode interference coupler 34 (chosen for its
wavelength insensitive splitting ratio) that forms the input of the Mach-
Zender
interferometer 12, 14. The input signal, ~,;, is coupled from an optical fiber
(not shown) to a waveguide 36 on the integrated optic chip 26. A spot size
converter 126 can be used to enhance the efficiency of this coupling. A key
feature of this geometry is that the input signal, ~,;, may be passed through
a
polarization insensitive semiconductor optical amplifier 38 before being
combined in another 3-dB coupler 40 with the continuous wave light from
tunable laser 10. This front end semiconductor optical amplifier 38 allows
wavelength conversion in the second stage to be performed over a larger range
of input signal powers, since it can be used as a gain control element.
As shown in copending U.S. Patent Application serial no.
entitled "Tunable Laser Source with Integrated Optical
Modulator, " the transverse device structure of optical chip 26 is comprised
of
a thick low bandgap waveguide layer with mufti-quantum well active regions
placed above it. The thick low bandgap waveguide layer is necessary for good
carrier-induced index change in the tuning sections. Nonabsorbing passive
elements are formed by selectively removing the quantum wells from on top
of the waveguide layer. The use of the offset quantum wells allows the
formation of active and passive sections in a single waveguide layer without
having to perform a butt joint regrowth. This, allows the device to be
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fabricated with only two metal organic chemical vapor deposition (MOCVD)
growth steps.
A key advantage of the monolithic wavelength converter is that it can
be fabricated using many of the steps already required for tunable lasers 10,
making it relatively easy to integrate on chip 26. There are eight main steps
in
the fabrication procedure for the wavelength converter with the integrated
SGDBR laser 10 as shown in Figs.4a-4f. In the first step as shown in Fig. 4a,
a base structure, generally denoted by reference numeral 100, is grown using
near atmospheric metalorganic chemical vapor deposition (MOCVD) with
tertiarybutlyphosphine and tertiarybutylarsine for the group V sources. In the
illustrated embodiment a 0.16 ~m Zn doped InP cap layer 102 is disposed on a
strained multiquantum well active region 104. A thin 100 Si doped InP layer
106 is disposed between multiquantum well active region 104 and a 0.35~m
InGaAsP main waveguide or layer 108 characterized by an bandgap, Eg =
0.885eV. Below InGaAsP waveguide 108 is a O.S~m Si doped InP layer 110.
Two 0.10 ~,m InGaAsP waveguides or layers 112 and 114 characterized by an
bandgap, Eg = 1.127 eV sandwich a O.S~m Si doped InP layer 116. Finally,
there is a basal InP substrate or layer 118.
Passive sections in the waveguide layer 108 of chip 26 are formed by
selectively etching off the cap layer and then quantum well layer 104 as shown
in Fig. 4b. The sectional view of Fig. 4b is in the direction of light
propagation. The gratings in laser 10 are formed for the laser mirrors using a
dry etch process in region 120 shown in Fig. 4b. Region 120 is where active
:i::vices will be fabricated while region 122 is where passive devices will be
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fabricated. Regions 120 and 122 are covered by a thick MOVCD regrown
layer 124 of InP as shown in Fig. 4c.
It is also possible to integrate an optical spot size converter 126 at this
point into the waveguide layer 108 by performing a diffusion limited etch to
taper the thickness of waveguide layer 108 as shown in Fig. 4d before those
regions which in which the facets of the laser will be formed.
The DBR mirrors are then formed by opening a window through cap
layer 108 and quantum well layer 104. A grating structure 128 is then formed
into waveguide layer 108 in chip 26 using selectively reactive ion etching in
methane-hydrogen-argon (MHA) as shown in Fig. 4e.
A ridge is patterned into structure 100 using reactive ion etching in
methane-hydrogen-argon (MHA) into active section 120, grating section 128,
passive section 122 or spot size converter 126 as shown in cross sectional
transverse side view taken perpendicularly across the direction of light
propagation as shown in Figs. 4f(1) - 4f(4) as would be seen through sectional
lines 1 - 1 to 4 - 4 of Fig. 4e respectively. A wet etch (Br:Methanol) is used
to
r~~nove the damaged layer from the reactive ion etch (RIE). In another
MOCVD step as shown in Fig. 4g, a 3 - 4 ~,m p-InP upper cladding layer 130
and a 100nm InGaAs contact layer 132 are regrown yielding the structures
shown in longitudinal view or in the plane of the direction of light
propagation
as depicted in Fig. 4g.
Isolation between the adjacent laser 10 section and between the
semiconductor optical amplifier's 16, 18, 38 is achieved by adding a contact
layer 133 and etching off the InGaAs layer 132 and performing a deep proton
(H+) implant 134 as shown in Fig. 4h. The proton implant is also
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used to limit the area of the parasitic p-n InP junction 136 shown in Fig.
4i(1)
surrounding the buried ridge stripe 138 and to lower the loss in the passive
waveguide regions 122 by compensating the Zn acceptor atoms in these areas
140. Active section 120, grating section 128, passive section 122 and spot
size
converter 126 are shown in cross sectional transverse side view taken
perpendicularly across the direction of light propagation as shown in Figs.
4i( 1 ) - 4i(4) as would be seen through sectional lines 1 - 1 to 4 - 4 of
Fig. 4h
respectively. In the final steps, the sample is lapped to 100 pm thick, and a
backside contact (not shown) is deposited before cleaving and mounting.
There are several additional considerations that exist for tunable
wavelength converter 11, 12, 14 over fixed wavelength converters. It is
important to filter out the original signal and amplified spontaneous emission
from the converted wavelength that is present at the output of the device. In
a
fixed wavelength converter the filters can be easily defined to pass only the
new wavelength signal. In a tunable device the output wavelength can vary,
so a comb filter can be used on the output 24 to pass the desired wavelengths
only. Unfortunately, the original wavelength will also pass through the comb
filter, so an additional filter is needed to block the original wavelength.
This
limits the flexibility of the tunable wavelength converter 11 as it can now no
longer convert to the same wavelength as the input and the filters need to be
specified for a given input wavelength.
A more flexible implementation is illustrated in Fig. 2 where
wavelength conversion is performed in two stages. Figs. 2 and 3 show the
device as a combination of an integrated device and off chip components, but
the scope of the invention expressly contemplates that all
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components of Fig. 2 could be integrally fabricated on chip 26 using the above
processes. Fig. 3 depicts the preferred embodiment of a fully integrated
device. The first stage converts the signal to an out of band wavelength using
distributed feedback (DFB) laser 48 using a cross gain modulation wavelength
conversion technique which is then converted to the desired wavelength via
cross phase modulation in the tunable wavelength converter 11. There are
several advantages to this implementation. The range of usable input signal
powers is increased dramatically compared to a single stage cross phase
modulation conversion as the output power of the intermediate wavelength can
now be controlled in the first wavelength conversion process. Using only a
notch filter 44 and a comb filter 42, as illustrated in Fig. 2, any wavelength
channel can be converted to any other wavelength channel without adjusting
the filters 42 and 44. Conversion to a fixed internal wavelength also allows a
choice of only wavelength up- or down-conversion for any input wavelength,
~,;, allowing the tunable wavelength converter to be optimized for converting
from a specific wavelength instead of having to accept any wavelength.
Another advantage is the relaxation of the need for polarization insensitivity
in
the second stage tunable wavelength converter 11 by using a polarization
insensitive semiconductor optical amplifier 38 in the first stage and
preserving
the polarization of the intermediate wavelength when coupling to the second
stage. Not having to be polarization insensitive greatly simplifies the active
region growth and improves the tunable laser performance.
The general approach illustrated in Fig. 2 can also be implemented in
a monolithic device by performing the first stage conversion using counter
propagating cross gain modulation within an semiconductor
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optical amplifier 38 integrated on chip 26. A diagram illustrating the layout
of
such an integrated device is shown in Fig. 3. In this case, the input
wavelength travels in the opposite direction to the intermediate wavelength
and the output wavelength This implementation eliminates the need for the
intermediate wavelength filter, however it requires full polarization
insensitivity in all the semiconductor optical amplifier's 16, 18 and 38 on
the
chip 26. Fig 2 is a block diagram in which non-integrated components are used,
namely filters which cannot be easily integrated monolithically. Fig. 3 is a
monolithic version of an analogous optical circuit to that shown in Fig. 2. If
one
wanted to describe the operation of Fig 3 as a block diagram, it would that
shown
in Fig 2 except the input signal, ~,;, would be injected after SOA 38,
however,
and is sent towards SOA 38 (i.e. in the opposite direction of the arrows in
Fig. 2).
In the integrated case in Fig. 3, 1510nm pass filter 44 is no longer necessary
because the input signal does not need to be blocked from reaching the tunable
wavelength converter stage 11.
Many alterations and modifications may be made by those having
ordinary skill in the art without departing from the spirit and scope of the
invention. Therefore, it must be understood that the illustrated embodiment
has been set forth only for the purposes of example and that it should not be
taken as limiting the invention as defined by the following claims. For
example, notwithstanding the fact that the elements of a claim are set forth
below in a certain combination, it must be expressly understood that the
invention includes other combinations of fewer, more or different elements,
which are disclosed in above even when not initially claimed in such
combinations.
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The words used in this specification to describe the invention and its
various embodiments are to be understood not only in the sense of their
commonly defined meanings, but to include by special definition in this
specification structure, material or acts beyond the scope of the commonly
defined meanings. Thus if an element can be understood in the context of this
specification as including more than one meaning, then its use in a claim must
be understood as being generic to all possible meanings supported by the
specification and by the word itself.
The definitions of the words or elements of the following claims are,
therefore, defined in this specification to include not only the combination
of
elements which are literally set forth, but all equivalent structure, material
or
acts for performing substantially the same function in substantially the same
way to obtain substantially the same result. In this sense it is therefore
contemplated that an equivalent substitution of two or more elements may be
made for any one of the elements in the claims below or that a single element
may be substituted for two or more elements in a claim. Although elements
may be described above as acting in certain combinations and even initially
claimed as such, it is to be expressly understood that one or more elements
from a claimed combination can in some cases be excised from the
combination and that the claimed combination may be directed to a
subcombination or variation of a subcombination.
Insubstantial changes from the claimed subject matter as viewed by a
person with ordinary skill in the art, now known or later devised, are
expressly
contemplated as being equivalently within the scope of the claims. Therefore,
obvious substitutions now or later known to one with ordinary skill in the art
are defined to be within the scope of the defined elements.
The claims are thus to be understood to include what is specifically
illustrated and described above, what is conceptionally equivalent, what can
be
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ohviously substituted and also what essentially incorporates the essential
idea
of the invention.
17
