# Patent

## Title
Monolithic tunable lasers and reflectors

## Abstract

A wavelength tunable device for operating on at least a portion of energy propagating through a waveguide is disclosed. The wavelength tunable device includes an upper cladding and a lower cladding having a core substantially disposed there between and suitable for optically coupling to the waveguide, a pattern of nanostructures positioned substantially on the upper cladding distal to the core so as to define a reflectivity for energy propagating through the waveguide, and, a movable membrane aligned with the pattern of nanostructures so as to at least partially define a gap there between. This gap may be selectively controlled upon actuation of the movable membrane so as to cause a corresponding change in the reflectivity.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a CON of application Ser. No. 10/463,473 Jun. 17, 2003 Pat. No. 6,920,272, which claims the benefit of application No. 60/417,230 Oct. 09, 2002.


FIELD OF THE INVENTION
The present invention relates generally to waveguides, and particularly to monolithic tunable lasers and reflectors.
BACKGROUND OF THE INVENTION
In the field of optical networking, telecommunications, optical applications and photonics it is highly desirable to continually enhance device performance and reduce fabrication, packaging and assembly costs. Accordingly, multi-functional photonic components or photonic components exhibiting enhanced functionality are highly desirable.
Super-grating distributed Bragg reflector tunable lasers and sampled/chirped grating distributed Bragg reflector tunable lasers both usually require special fabrication techniques to make the distributed Bragg reflector gratings and usually require tuning through carrier injection. Current and temperature tuned distributed Bragg reflector tunable lasers and current and temperature tuned fixed distributed feedback/distributed Bragg reflector lasers usually have very small tuning ranges and difficult are to maintain.
Therefore, the need exists to have a monolithic tunable laser providing a larger tunable range and standard fabrication techniques.
SUMMARY OF THE INVENTION
A wavelength tunable device for operating on at least a portion of energy propagating through a waveguide is disclosed. The device includes an upper cladding and a lower cladding having a core substantially disposed there between and suitable for being optically coupled to the waveguide, a pattern of nanostructures positioned substantially on the upper cladding distal to the core so as to define a reflectivity for energy propagating through the waveguide, and a movable membrane aligned with the pattern of nanostructures so as to at least partially define a gap there between. The gap may be selectively controlled upon actuation of said movable membrane so as to cause a corresponding change in said reflectivity.
A monolithic tunable optical energy source suitable for emitting energy having at least one wavelength is also disclosed. The source includes a gain portion suitable for amplifying the energy to be emitted, at least a first reflector suitable for substantially reflecting the at least one wavelength including an upper cladding and a lower cladding having a core substantially disposed there between; a pattern of nanostructures positioned substantially on the upper cladding distal to said core so as to define a reflectivity for propagating energy; and a movable membrane aligned with the pattern of nanostructures so as to at least partially define a gap there between, wherein, the gap may be selectively controlled upon actuation of the movable membrane so as to cause a corresponding change in the reflectivity, and a waveguide portion substantially optically coupling the first reflector with the gain portion.
A waveguide module suitable for interacting with input energy propagation utilizing a wavelength tunable device is also disclosed. The waveguide module includes a waveguide wavelength demultiplexer suitable for dividing the energy propagation into parts, each part comprising approximately an equal wavelength portion of the energy propagation, and a plurality of reflectors suitable for interacting with the divided energy propagation, each reflector comprising an upper cladding and a lower cladding having a core substantially disposed there between and suitable for being optically coupled to the waveguide wavelength demultiplexer, a pattern of nanostructures positioned substantially on the upper cladding distal to said core so as to define a reflectivity for the energy propagating through the waveguide wavelength demultiplexer, and a movable membrane aligned with the pattern of nanostructures so as to at least partially define a gap there between, wherein the gap may be selectively controlled upon actuation of said movable membrane so as to cause a corresponding change in said reflectivity, thereby determining the add/drop characteristics of operating wavelength portion.



BRIEF DESCRIPTION OF THE FIGURES
Understanding of the present invention will be facilitated by consideration of the following detailed description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which like numerals refer to like parts:
FIG. 1 illustrates a block representation of a tunable integrated Bragg reflector;
FIG. 2 illustrates a block representation of a monolithic tunable laser incorporating the tunable distributed Bragg reflector shown in FIG. 1; and,
FIG. 3 illustrates a block representation of a waveguide add/drop module utilizing a tunable integrated Bragg reflector shown in FIG. 1.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
It is to be understood that the figures and descriptions of the present invention have been simplified to illustrate elements that are relevant for a clear understanding of the present invention, while eliminating, for the purpose of clarity, many other elements found in typical photonic components and methods of manufacturing the same. Those of ordinary skill in the art will recognize that other elements and/or steps are desirable and/or required in implementing the present invention. However, because such elements and steps are well known in the art, and because they do not facilitate a better understanding of the present invention, a discussion of such elements and steps is not provided herein. The disclosure herein is directed to all such variations and modifications to such elements and methods known to those skilled in the art.
Active devices are devices that operate on signals such as creating emissions, filtering transmissions, balancing transmissions, splitting transmissions, and adding or dropping transmissions, for example. Passive devices are devices which are a transmission medium such a planar waveguides and fibers, for example.
Referring now to FIG. 1, there is shown a tunable integrated Bragg reflector 100. Tunable integrated Bragg reflector 100 may include lower cladding layer 140 and upper cladding layer 120 and a core 130 therein between. A pattern of subwavelength elements, such as nanoelements and nanostructures 110, may be formed in a surface of upper cladding 120 substantially distal to core 130. A mechanically controllable membrane 150 may be placed near the pattern of nanostructures 110 with a gap 160 formed there between.
Pattern of nanostructures 110 may include multiple nanostructures 170 each having an element width FG and element height DG. Pattern of nanostructures 110 may have a period of nanostructures 170, XG. The filling ratio of pattern of nanostructures 110, denoted FG/XG, is the ratio of the width of a nanostructure FG to the overall period. Filling ratio, FG/XG, may determine the operating wavelength of device 10, as would be evident to one possessing an ordinary skill in the pertinent arts.
Pattern of nanostructures 110 may be formed into or onto upper cladding 120 using any suitable process for replicating, such as a lithographic process. For example, nanoimprint lithography consistent with that disclosed in U.S. Pat. No. 5,772,905, entitled NANOIMPRINT LITHOGRAPHY, the entire disclosure of which is hereby incorporated by reference as if being set forth in its entirety herein may be used. This patent teaches a lithographic method for creating ultra-fine nanostructure, such as sub 25 nm, patterns in a thin film coated on a surface. For purposes of completeness, a mold having at least one protruding feature may be pressed into the thin film applied to upper cladding. The at least one protruding feature in the mold creates at least one corresponding recess in the thin film. After replicating, the mold may be removed from the film, and the thin film processed such that the thin film in the at least one recess may be removed, thereby exposing an underlying pattern or set of devices. Thus, the patterns in the mold are replicated in the thin film, and then the patterns replicated into the thin film are transferred into the upper cladding 120 using a method known to those possessing an ordinary skill in the pertinent arts, such as reactive ion etching (RIE) or plasma etching, for example. Of course, any suitable method for forming a structure into or onto an operable surface, such as of upper cladding 120, may be utilized though, such as photolithography, holographic lithography, e-beam lithography, for example. Upper cladding 120 may take the form of InP, GaAs, or SiO2 with a thin film of InGaAs, InGaAsP, AlGaAs, or Si forming pattern of nanostructures 110.
As will be recognized by those possessing ordinary skill in the pertinent arts, various patterns may be nanoimprinted onto upper cladding 120. These patterns may serve various optical or photonic functions. Such patterns may take the form of holes, strips, trenches or pillars, for example, all of which may have a common period or not, and may be of various heights and widths. The strips may be of the form of rectangular grooves, for example, or alternatively triangular or semicircular grooves. Similarly, pillars, basically the inverse of holes, may be patterned. The pillars may be patterned with a common period in both axes or alternatively by varying the period in one or both axes. The pillars may be shaped in the form of, for example, elevated steps, rounded semi-circles, or triangles. The pillars may also be shaped with one conic in one axis and another conic in the other.
According to an aspect of the present invention, an underlying one-dimensional (1-D) pattern of nanostructures 110, preferably formed of materials of having different reflective indices, may be formed on upper cladding 120. This 1-D pattern may be of the form of trenches, for example. According to an aspect of the present invention, two-dimensional (2-D) pattern of nanostructures 110, preferably formed of materials having different refractive indices, may be formed on upper cladding 120. This 2-D pattern may be of the form of pillars, for example.
Upper cladding 120, in combination, with lower cladding 140 envelops core 130. Upper cladding 120 may be substantially InP, GaAs, or SiO2, for example. Lower cladding 140 may be substantially InP, GaAs, or SiO2 for example. Core 130 may be substantially InGaAs or SiN.
Mechanically controllable membrane 150, such as a microelectromechanical system (MEMS) for example, may be placed in close proximity to pattern of nanostructures 110 with gap 160 substantially there between. MEMS are integrated micro devices or systems combining electrical and mechanical components, fabricated using integrated circuit processing techniques and may range in size from micrometers to millimeters. These systems may sense, control and actuate on the micro scale, and may function individually or in arrays to generate effects on the macro scale. The use of MEMS is known those possessing an ordinary skill in the pertinent arts.
In brief, a MEMS may include a base and a deflector. The base and deflector may be made from materials as is known to those possessing and ordinary skill in the pertinent arts, such as for example, InP, GaAs, SiN, Si, or SiO2. The MEMS may operate wherein an application of energy to the MEMS causes a longitudinal deflection of the deflector with respect to the base. The longitudinal displacement of the deflector from the base is proportional to the energy applied to the MEMS.
Gap 160 may be created substantially between mechanically controlled membrane 150 and pattern of nanostructures 110. Gap 160 may include a material such as air or nitrogen or may be a vacuum, for example. The size of gap 160 may be in the range 0.1 um to 1 um, such as 0.3 um for example, which is the distance in the longitudinal direction between mechanically controlled membrane 150 and pattern of nanostructures 110.
Controlling the size of gap 160 by electro-mechanically actuating the deflector causes longitudinal displacement of the deflector with respect to the base. This control creates a tunable distributed Bragg reflector suitable for tunable wavelength selection.
Referring now to FIG. 2, there is shown a monolithic tunable laser 200. Monolithic tunable laser 200 includes a gain portion 210, a waveguide portion 220, a first reflector 230 and a second reflector 240. Gain portion 210 may be substantially optically coupled to first reflector 230 by waveguide portion 220. Second reflector 240 may be optically coupled to gain portion 210 distal to waveguide portion 220. Portion 210, 220, 230, 240 may be monolithically formed on a common substrate, such as InP, GaAs, or Si for example.
Gain portion 210, including for example a gain region, may include a Type III-V compound semiconductor, such as for example InP or GaAs. The performance and use of gain materials is known to those possessing an ordinary skill in the pertinent arts. Briefly, gain portion 210 may provide an area and configuration for population inversion and stimulated emission. Gain portion 210 operates to maintain more excited or pumped atoms in higher energy levels than atoms existing in the lower energy levels.
Waveguide portion 220, including for example a waveguide region, may be adapted to optically couple gain portion 210 to first reflector 230. The use of waveguides for optical coupling is known to those possessing an ordinary skill in the pertinent arts.
First reflector 230 may be designed to be tuned thereby selecting a desired stimulated emission of device 200. First reflector 230 may be a tunable integrated Bragg reflector 100 (FIG. 1), for example. First reflector 230 provides an optical feedback device by directing propagating energy back through gain portion 210.
Second reflector 240 may be adapted to provide simultaneous reflection and transmission by providing a small amount of transmission on the order of approximately 1% of the impinging radiation. Substantially the remainder of the impinging radiation may be reflected back through gain portion 210. Second reflector 240 may be designed as a tunable integrated Bragg reflector 100 (FIG. 1), for example, or alternatively may be designed to provide broadband reflection/transmission characteristics, thereby allowing first reflector 230 to be substantially determinative of the operating emitting wavelength.
In operation, gain portion 210 may be energized thereby exciting atoms from their lower state to one of several higher states. Energy, photons for example, of a wavelength selected by at least first reflector 230, and additionally, by second reflector 240, radiating between first reflector 230 and second reflector 240 pass through this population inverted portion thereby causing a stimulated emission. Second reflector 240 transmits a portion of the radiation incident upon it, thereby creating an emission of the wavelength selected by at least first reflector 230, and additionally, by second reflector 240.
Referring now to FIG. 3, there is shown a waveguide add/drop module 300 utilizing tunable distributed Bragg reflector 100. The waveguide add/drop module 300 includes an optical fiber 310, circulator 320 for input and output coupling, a waveguide wavelength demultiplexer 330 and an array or plurality 340 of tunable distributed Bragg reflectors 100.
Circulator 320 may have a number of ports identified in a specific sequence. As in known to those possessing an ordinary skill in the pertinent arts, circulator 320 operates by substantially outputting energy input through one port through the next port in the sequence. For example, light of a certain wavelength enters circulator 320 through port x and exits through port x+1, while light of another wavelength enters through port x+2 and exits through x+3. For example, a circulator disclosed in U.S. Pat. No. 4,650,289, entitled OPTICAL CIRCULATOR, the entire disclosure of which is hereby incorporated by reference as if being set forth in its entirety herein may be used.
Waveguide wavelength demultiplexer 330 may be used to separate the incoming input signal into constituent parts for use in add/drop module 300. A multi-channel-input signal may be demultiplexed, separated spatially into different waveguide branches based on wavelength, for example. For example, if the incoming signal has a wavelength range λ, the demultiplexer may separate the signal into 6 equally sized branches, as may be seen in FIG. 3, each branch including signal of wavelength range λ/6. Demultiplexer 330 may take the form of an arrayed waveguide grating or echelle grating, for example. Such an arrayed waveguide grating or echelle grating combines and splits optical signals of different wavelengths utilizing a number of arrayed channel waveguides that act together like a diffraction grating offering high wavelength resolution and attaining narrow wavelength channel spacing. After being demultiplexed, each channel propagating a portion of the overall wavelength range may be aligned with one tunable distributed Bragg reflector 100 filter in an array 340 of electrically tunable narrow-band waveguide distributed Bragg reflector 100 filters.
Array 340 of tunable distributed Bragg reflectors 100 may include individual tunable distributed Bragg reflectors 100 as shown in FIG. 1 and discussed hereinabove. Each tunable distributed Bragg reflector 100 may operate as a tunable narrow-band reflective mirror and a tunable notch filter. When energy propagation reaches tunable distributed Bragg reflector 100, by controlling mechanically controllable membrane 150 aligned in one of the tunable distributed Bragg reflector 100, such as a MEMs or other suitable device, each may be configured according to whether the channel is desired to be added or dropped. For example, if a channel desiring to be dropped 350 is received at a filter 100, that filter 100 may be configured so as to pass this channel's signal, as a notch filter, for example. On the other hand, if the channel contains a signal desired to continue to propagate 360, i.e. not to be dropped, the filter will be configured so as to reflect this channel's signal, a narrow-band reflective mirror. Additionally, if a signal is desired to be added corresponding in wavelength with a signal to be dropped 370, or a previously substantially unused wavelength 380, this signal may be added by passing through the corresponding filter used to drop a portion of the signal. For either adding a previously unused wavelength or for adding a previously dropped wavelength, filter 100 may be configured so as to pass this signal to be added, as a notch filter, for example. In the case of adding a signal corresponding in wavelength to a signal to be dropped, filter 100 would already be configured to pass the wavelength in order to effectuate the signal drop discussed hereinabove. When the signal reaches filter 100, since filter 100 may be configured as a notch filter suitable to pass the signal, the signal may be transmitted through filter 100, thereby entering the system and passing through to the waveguide wavelength demultiplexer 330.
Wavelengths reflected or added at array 340 of tunable distributed Bragg reflectors 100 propagate through waveguide wavelength demultiplexer 330. Waveguide wavelength demultiplexer 330 operates to combine this returning energy back into a single energy propagation. This combined energy propagation propagates through to circulator 320 and is outputted through fiber 310.
Further, if an electrically tunable narrow-band waveguide distributed Bragg reflector 100 mirror/filter operates as a variable optical attenuator or variable optical reflector, then the above waveguide add/drop module 300 may be utilized as a dynamic gain equalization filter. Dynamic gain equalization may be necessary due to effects resulting from increasing bandwidth causing channel powers to become unbalanced. Non-uniformity of channel powers arises from non-linear effects such as Raman scattering in a communicative fiber and the cumulative effects of cascaded optical amplifiers. Further, in large systems, these effects may be pronounced. If the channel power imbalance is not mitigated, overall system performance may be degraded and service reliability may be reduced. Dynamic equalization eliminates gain tilt, gain shape changes, and accumulated spectral ripple that occurs due to dynamic changes in optical networks. It permits longer distance, higher bandwidth and light-path flexibility in optical transmission links with less frequent O-E-O regeneration.
Operatively, for example, the above waveguide add/drop module 300 may be configured, instead of substantially transmitting or reflecting the incoming signal as described hereinabove, to partially transmit and reflect the signal. By so doing, filter 100 may gain equalize the overall signal substantially equating the signal in each band.
As would be known to those possessing an ordinary skill in the pertinent arts, filter 100 may have a defined pass-band and an edge of the pass-band. In order to gain equalize, filter 100 may be set to pass a wavelength slightly offset from the wavelength propagating as described in the add/drop discussion, thereby utilizing the edge of the band as a partially transmitting/reflecting filter. Slight tuning of the offsets may be utilized to modify the amount of reflected signal, thereby being suitable for use in equalizing the signal reflected from filter 100 in each pass band. The amount of offset for a given pass band may be modified according to the incoming signal characteristics, varying the reflectance in a pass band as described herein, thereby adding a dynamic feature to the gain equalization.
Those of ordinary skill in the art will recognize that many modifications and variations of the present invention may be implemented without departing from the spirit or scope of the invention. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

## Claims

1. A device comprising:
a waveguide comprising a core and a first cladding adjacent the core;
a plurality of nanostructures on the first cladding of the waveguide; and
a membrane positioned relative to said plurality of nanostructures so that a surface of the membrane faces said plurality of nanostructures,
wherein a distance between said membrane and said plurality of nanostructures is variable in accordance with actuation of said membrane, and the device is a wavelength tunable active device configured to operate on energy propagating through the waveguide.

2. The device of claim 1, wherein said nanostructures comprise a periodic structure.

3. The device of claim 1, wherein said nanostructures are formed substantially from at least one of Si, InP, and GaAs.

4. The device of claim 1, wherein said nanostructures comprise at least one pattern selected from the group consisting of holes, strips, trenches and pillars.

5. The device of claim 4, wherein said pattern has a common period.

6. The device of claim 4, wherein said pattern is one-dimensional.

7. The device of claim 4, wherein said pattern is two-dimensional.

8. The device of claim 1, wherein the nanostructures are formed of materials wherein a refractive index of a pattern of said nanostructures is greater than the refractive index of said first cladding.

9. The device of claim 1, wherein the first cladding is formed substantially of at least one of SiO2 and InP.

10. The device of claim 1, wherein the core is formed substantially of at least one of SiN and InGaAs.

11. The device of claim 1, wherein said membrane is a microelectromechanical system.

12. The device of claim 1, wherein said membrane is formed of substantially at least one of the group consisting of SiN, Si and SiO2.

13. The device of claim 1, wherein the distance is within the range 0.01 um to 1 um.

14. The device of claim 1, wherein the distance is approximately 0.3 um.

15. The device of claim 1, wherein the distance is controllable by electro-mechanically actuating said membrane thereby providing for wavelength selection.

16. The device of claim 1, wherein the waveguide further comprises a second cladding adjacent to the core of the waveguide.

17. The device of claim 16, wherein the second cladding is formed substantially of at least one of SiO2 and InP.

18. The device of claim 1, wherein the first cladding is formed from a first material and the plurality of nanostructures are formed from a second material that is different from the first material.

19. A system, comprising:
the device of claim 1; and
a gain portion optically coupled to the device of claim 1 during operation of the system,
wherein the system is a laser.

20. The system of claim 19, wherein the gain portion is optically coupled to the device of claim 1 via a waveguide portion.

21. The system of claim 19, wherein the device of claim 1 is configured to reflect radiation from the gain portion to provide optical feedback.

22. A system, comprising:
a waveguide wavelength demultiplexer;
an optical fiber coupled to the waveguide wavelength demultiplexer; and
one or more of the devices of claim 1 configured to receive radiation from the waveguide wavelength demultiplexer,
wherein the system is an add/drop module and the one or more of the devices of claim 1 are operable as tunable reflectors for the add/drop module.

23. A wavelength tunable active device for operating on energy propagating through a waveguide, comprising:
a plurality of nanostructures on an upper cladding of the waveguide adjacent to a core of the waveguide; and,
a membrane at least partially aligned with multiple ones of said plurality of nanostructures, wherein the at least partial alignment defines a distance between said membrane and the multiple ones of said plurality of nanostructures, and wherein the distance is variable in accordance with actuation of said membrane,
wherein said nanostructures comprise at least one pattern selected from the group consisting of holes, strips, trenches and pillars, and wherein said pattern is two-dimensional.

24. A wavelength tunable active device for operating on energy propagating through a waveguide, comprising:
a plurality of nanostructures on an upper cladding of the waveguide adjacent to a core of the waveguide; and,
a membrane at least partially aligned with multiple ones of said plurality of nanostructures, wherein the at least partial alignment defines a distance between said membrane and the multiple ones of said plurality of nanostructures, and wherein the distance is variable in accordance with actuation of said membrane,
wherein the nanostructures are formed of materials wherein the refractive index of a pattern of said nanostructures is greater than the refractive index of said upper cladding.