LASER

Abstract

A laser comprising a photonic component comprising a gain medium; and a waveguide platform comprising a Distributed Bragg Reflector, DBR, section. The photonic component is optically coupled to the waveguide platform. One or more thermal heaters are positioned at the DBR section of the waveguide platform, and/or at a phase section of the waveguide platform located between the gain medium and the DBR section.

Claims

1. A laser comprising: a photonic component comprising a gain medium; and a waveguide platform comprising a Distributed Bragg Reflector, DBR, section, wherein the photonic component is optically coupled to the waveguide platform, and wherein one or more thermal heaters are positioned at the DBR section of the waveguide platform, and/or at a phase section of the waveguide platform located between the gain medium and the DBR section.

2. The laser of claim 1, wherein the waveguide platform is a silicon on insulator, SOI or a silicon nitride platform.

3. The laser of claim 1 or claim 2, wherein the photonic component comprises a Reflection Semiconductor Optical Amplifier, RSOA or a III-V compound semiconductor gain chip.

4. The laser of any preceding claim wherein the DBR section comprises an optical mirror configured to selectively reflect light having a wavelength within a predetermined range of wavelengths.

5. The laser of any preceding claim, wherein the photonic component is a III-V semiconductor.

6. The laser of any preceding claim, wherein the phase section of the waveguide platform is thermally isolated from the DBR section.

7. The laser of any preceding claim, wherein the phase section of the waveguide platform is thermally isolated from the DBR section of the waveguide platform by a thermal isolation space between the phase section and the DBR section.

8. The laser of any preceding claim, wherein the one or more heaters comprise metal or heavy doped silicon.

9. The laser of any preceding claim, wherein a first heater is positioned on the phase section of the waveguide platform and a second heater is positioned on the DBR section of the waveguide platform.

10. The laser of any preceding claim, wherein the one or more heaters and the photonic component are configured to receive power from one or more power sources.

11. The laser of any preceding claim, wherein the one or more heaters are positioned on a ridge of the waveguide platform.

12. The laser of any preceding claim, wherein the one or more heaters are positioned adjacent to a ridge of the waveguide platform, and extend in a longitudinal direction parallel to the ridge of the waveguide platform.

13. The laser of any preceding claim, comprising a SiO.sub.2 layer between the one or more heaters and the waveguide platform.

14. A method of characterizing a laser according to any preceding claim, wherein the laser comprises a DBR heater positioned at the DBR section, the method comprising: determining an optimal DBR heater power value to be supplied to the DBR heater, wherein determining the optimal DBR heater power value comprises: providing power to the photonic component; providing power to the phase heater; monitoring the output power of the laser as power provided to the DBR heater is increased; and selecting the optimal DBR heater power value based on the monitored output power of the laser.

15. The method of claim 14, wherein the selected optimal DBR heater power value corresponds to a local maximum of the monitored output power of the laser as the power supplied to the DBR heater is increased.

16. The method of claim 14 or claim 15, wherein the laser comprises a phase heater positioned at the phase section, the method comprising: determining an optimal phase heater power value to be supplied to the phase heater, wherein determining the optimal phase heater power comprises: providing power to the photonic component; providing power to the DBR value at the selected optimal DBR heater power value; monitoring the output power of the laser as power provided to the phase heater is increased; and selecting the optimal phase heater power value based on the monitored output power value of the laser.

17. The method of claim 16, wherein the selected optimal phase heater power value corresponds to a local minimum of the monitored output power of the laser as the power supplied to the phase heater is increased.

18. The method of claim 16 or claim 17, further comprising operating the laser using the determined optimal DBR heater power value and the determined optimal phase heater power value.

19. A spectrometer comprising: a plurality of lasers according to any of claims 1-13; an optical manipulation region comprising an optical multiplexer, the optical manipulation region being optically coupled to each of the plurality of devices; and an optical output for light originating from the plurality of lasers.

20. A method of characterizing a spectrometer according to claim 19, wherein each laser in the spectrometer comprises a DBR heater positioned at the DBR section, the method comprising: determining an optimal DBR heater power value to be supplied to the DBR heater of a first laser of the plurality of lasers, wherein determining the optimal DBR heater power value for the first laser comprises: (i) providing power to the photonic component of the first laser; (ii) monitoring the output power of the optical multiplexer as power provided to the DBR heater of the first laser is increased; (iii) selecting the optimal DBR heater power value of the first laser, wherein the selected optimal DBR heater power value corresponds to a maximum output power of the optical multiplexer as the power supplied to the DBR heater of the first laser is increased; and determining an optimal DBR heater power value to be supplied to the DBR heater of each of the remaining lasers of the plurality of lasers by performing steps (i)-(iii) for each of the remaining lasers in turn.

21. The method of claim 20 wherein characterizing the spectrometer includes aligning the output wavelength of each laser source with the optical multiplexer pass band peak.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0072] Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

[0073] FIG. 1 is a schematic of a laser according to an embodiment of the present invention;

[0074] FIG. 2A shows a perspective view of a laser according to an embodiment of the present invention;

[0075] FIG. 2B shows a cross-sectional view of a laser according to an embodiment of the present invention;

[0076] FIG. 3 is a perspective view of a laser according to an embodiment of the present invention;

[0077] FIG. 4 is a schematic of a laser according to an embodiment of the present invention;

[0078] FIG. 5A and FIG. 5B are graphs showing the DBR grating power reflection spectrum of a laser;

[0079] FIG. 6 includes graphs showing the effect of increasing the power of a DBR section heater on laser wavelength and laser output power;

[0080] FIG. 7 includes graphs showing the effect of increasing the power of a phase section heater on laser wavelength and laser output power;

[0081] FIG. 8 is a schematic of a system 60 for self-characterizing a DBR laser;

[0082] FIG. 9 is a flow diagram for a laser self-characterization method;

[0083] FIG. 10 is a schematic of a system for self-characterizing a spectrometer including a plurality of DBR lasers;

[0084] FIG. 11 is a graph of an optical wavelength multiplexer channel pass band profile;

[0085] FIG. 12 is a graph of the power spectrum of an optical multiplexer with respect to DBR heater power; and

[0086] FIG. 13 is a graph of the power spectrum of an optical multiplexer with respect to selected DBR heater power values.

DETAILED DESCRIPTION

[0087] The detailed description set forth below in connection with the appended drawings is intended as a description of exemplary embodiments of a laser and a method for operating a laser provided in accordance with the present invention and is not intended to represent the only forms in which the present invention may be constructed or utilized.

[0088] FIG. 1 shows a laser 10 comprising a silicon-on-insulator chip 12, and with three sections; a gain section 14, a phase section 16 and a Distributed Bragg Reflector (DBR) section 18. The phase section 16 is located at a position between the gain section 14 and the DBR section 18. The phase section 16 and DBR section 18 are formed on a silicon waveguide platform 20 having a ridge. In the embodiments shown, the waveguide platform 20 is a rib waveguide. The waveguide platform may be a ridge waveguide. The gain section 14 comprises a gain medium and waveguide ridge 22.

[0089] In FIG. 1, gain section 14 is a III-V gain chip within a mounting cavity of the silicon-on-insulator chip 12, wherein the gain chip is flipped on the silicon-on-insulator chip 12. As shown in FIG. 1, an Si edge coupler 24 couples the waveguide ride 22 of the III-V gain chip with the phase section 16 of the silicon waveguide 20 by an overlap between the gain chip 14 and the Si edge coupler 24. The III-V gain chip length (in a direction parallel to the silicon waveguide 20) may be approximately 700 μm and the length (in a direction parallel to the silicon waveguide 20) of the overlap between the gain chip 14 and the Si edge coupler 24 may be approximately 50 μm. The length (in a direction parallel to the silicon waveguide 20) of the phase section 16 of the Si waveguide may be approximately 50 μm, for example. The length (in a direction parallel to the silicon waveguide 20) of the DBR section 18 may be approximately 1000 μm.

[0090] The phase section 16 may be thermally isolated from the DBR section 18 of the silicon waveguide 20 by a thermal isolation space 26. The length (in a direction parallel to the silicon waveguide 20) of the thermal isolation space 26 may be approximately 30 μm, for example.

[0091] FIG. 2A is a perspective view of laser 10′, which may be an example of laser 10. A heater 28 is positioned above (with respect to a silicon substrate of the silicon-on-insulator chip 12) a ridge of the silicon waveguide 20 at the DBR section 18. For completeness, a heater may also be positioned above the ridge of the silicon waveguide 20 at the phase section 16. As best show in FIG. 2B, which is a cross-sectional view of the laser 10′ shown in FIG. 2A, an oxide layer 30 (in this example, a silicon oxide layer) is positioned between the ridge of the silicon waveguide 20 and the heater 28, such that the heater 28 is spaced from the ridge of the silicon waveguide 20 by a gap g. The heater 28 may comprise metal, such as titanium nitride, TiN, for example. Alternatively, the heater 28 may be a heavy doped (p+ or n+) silicon heater. The thickness of the heater 28 (i.e. in a direction parallel to the thickness of gap g) may be approximately 200 nm, with a width of approximately 3.0 μm, for example. Gap g thickness may be between 50 nm and 500 nm, for example.

[0092] FIG. 3 is a perspective view of laser 10″, which may be an example of laser 10. Laser 10″ is similar to laser 10′, except that there are two heaters 28a, 28b positioned at the DBR section 18 (two heaters may also be positioned at the phase section 16), and the heaters 28a, 28b are not positioned above the ridge of the silicon waveguide 20, but are instead positioned adjacent to the ridge of the silicon waveguide 20. In particular, heaters 28a and 28b are positioned on either side of the ridge of the silicon waveguide 20 (in a slab of the waveguide), spaced from the ridge of the silicon waveguide 20 by a distance d. The heaters 28a, 28b extend longitudinally in a direction parallel to the length of the silicon waveguide 20. The heaters 28a, 28b may comprise metal, such as TiN, for example. Alternatively, the heaters 28a, 28b may be heavy doped p+ or n+ silicon heaters.

[0093] When the heater(s) is a heavy doped silicon heater, the heater may have a doping level of 8×10.sup.19 cm.sup.−3.

[0094] The structure of the lasers 10′, 10″ may provide a fast thermal response time of approximately ˜100 microseconds.

[0095] FIG. 4 is a schematic of a laser 10 (such as laser 10′ or 10″) according to an embodiment of the invention. As shown, a first heater is positioned at the phase section, and a second heater is positioned at the DBR section. Power is supplied to the heaters, and the gain section.

[0096] FIG. 5A and FIG. 5B are graphs showing the silicon waveguide DBR grating power reflection spectrum, when the DBR length is 1000 μm, and with a coupling constant, kappa, of 6.688 cm.sup.−1.

[0097] The external cavity DBR laser longitudinal mode space is given by:

[00001]$\Delta {\lambda }_m=\frac{{\lambda }^2}{2\left[n_{\mathrm{eg}}{L}_g+n_{\mathrm{ep}}{L}_p+n_{\mathrm{edbr}}{L}_{\mathrm{effdbr}}\right]}$

where λ is the wavelength of light, L.sub.9 is the gain section length, n.sub.eg is the group refractive index of the gain section, L.sub.p is the phase section length (including the overlapping region and the thermal isolation region), n.sub.ep is the group refractive index of the phase section, L.sub.effdbr is the DBR section effective length, and n.sub.edbr is the group refractive index of the DBR section. For example, for λ=1550 nm, L.sub.g=700 μm, n.sub.eg=3.305055 (group index), L.sub.p=130 μm, n.sub.ep=3.617098 (group index), L.sub.effdbr=700 μm, n.sub.edbr=3.617098 (group index), then Δλ.sub.m=0.226 nm.

[0098] FIG. 6 includes graph 44 showing the effect of increasing the power supplied to a heater on the DBR section 18 of laser 10 in a DBR heater temperature scanning procedure, when the laser gain is biased at the operating current, and a heater at the phase section 16 is biased at a specific power. In particular, as shown in graph 44, when the DBR heater power is increased at a constant power step value, the laser wavelength increases. This increase in wavelength with temperature is not linear; there is a periodic “hopping up” or jump of the wavelength at certain heater power values, i.e. at certain “hopping points” 48. In other words, at the “hopping points”, the difference in wavelength between a pair of adjacent discrete power measurements at the hopping point is greater than the difference in wavelength between pairs of adjacent discrete power measurements away from the hopping point. The difference between the two wavelengths at the hopping point 48 is equivalent to the DBR laser longitudinal mode interval, Δλ.sub.m. As shown in the graph 46 of FIG. 6, the laser output power (e.g. as measured from DBR output facet) reaches a maximum at the hopping points 48.

[0099] FIG. 7 includes graph 50 showing the effect of increasing the power supplied to a heater on the phase section 16 of laser 10 in a phase heater temperature scanning procedure, when the laser gain is biased at the operating current, and the heater at the DBR section at one of the hopping points 48 described above in relation to FIG. 6. As shown in graph 50, the laser wavelength increases within a mode hopping range (i.e. between adjacent hopping points 49). At each hopping point 49, the laser wavelength drops back to the original wavelength. In this way, with increasing phase heater power, the wavelength periodically changes in a saw tooth-profile. The difference between the two wavelengths at each hopping point 49 is equal to the DBR laser longitudinal mode interval, Δλ.sub.m. As shown in graph 52 of FIG. 7, the laser output power (e.g. as measured from DBR output facet) reaches a maximum value at the hopping points 49. The SMSR is optimized at a position away from the hopping points 49, e.g. at the minimum values of the laser output power, plotted with respect to phase heater power.

[0100] FIG. 8 shows a system 60 for self-characterizing a DBR laser, such as laser 10. A phase heater 62 is positioned at the phase section 16 of the laser, and a DBR heater 64 is positioned at the DBR section 18 of the laser. The phase heater 62, DBR heater 64 and gain section 14 are powered by one or more power sources (ADC) 66. Temperature sensors 68 are positioned near or adjacent to the phase section 16 and DBR section 18 to detect the temperature of the phase section and DBR section. The system 60 also comprises a controller 72 (e.g. a microcontroller unit, MCU) and a monitor photodiode detector (MPD) 74. An optical splitter 76 splits the output from laser 10 such that some (e.g. 3%) of the power is directed to the MPD.

[0101] The controller 72 is connected to the temperature sensors 68, the MPD and the one or more power sources 66, in order to implement self-characterization of laser 10. In particular, the controller 72 acquires the DBR laser output power data (e.g. using MPD 74 and/or temperature sensors 68) during a DBR and phase thermal tuning process (also referred to as DBR and phase heater temperature scanning processes, respectively), finds the mode hopping points of the laser output power and the corresponding DBR heater 64 and phase heater 62 power values, and sets the DBR heater 64 and phase heater 62 power values based on the mode hopping points of the laser output power and the corresponding DBR heater 64 and phase heater 62 power values. System 60 can therefore find preferred, or optimal, operating points for the gain section 14, the phase section 16, and the DBR section 18. This DBR laser self-characterization method is described in further detail below with respect to FIG. 9.

[0102] FIG. 9 is a flowchart of a laser self-characterization method. First, a DBR heater temperature scanning procedure (S101-S104) is performed. In particular, at S101, power is provided to the gain section 14 (e.g. by power source 66) at an operating current (which may be predefined, e.g. by application specification). At S102, power is provided to the phase heater 62 at an arbitrary power value (which may be zero, or a low power compared to the power provided to the gain section, for example). At S103, the power provided to the DBR heater 64 is increased (e.g. at a constant, discrete step value). As the power provided to the DBR heater 64 is increased in this step wise manner, the output power of the laser 10 is monitored, e.g. using MPD 74. A plurality of measurements (e.g. 10-20) of the output power of the laser may be sampled for each DBR heater power value in order to remove noise. The laser wavelength may also be monitored using techniques known in the art. As described above in relation to FIG. 6, with increasing DBR heater power, the laser output power cyclically reaches a maximum value at the hopping points 48, with the laser output power falling between these hopping points 48. This is an intrinsic characteristic of the laser. Thus, at S104, in order to maximize performance (e.g. output power) of the laser, the values of DBR heater power corresponding to the maximum value of laser output power are determined and recorded. These recorded DBR heater values are preferred, or optimal, power values for the DBR heater 64, corresponding to laser output power local extreme points. Any of these recorded DBR heater values may be applied to the DBR heater 54 to maximize laser output performance. These local extremes of the laser output power are selected for laser self-characterization as they are computationally simpler to find compared to other approaches.

[0103] Next, after the DBR heater temperature scanning procedure, a phase heater temperature scanning procedure (S105-S109) is performed. At S105, power is provided to the gain section 14 at the operating current (e.g. the power provided may be maintained between the DBR heater temperature scanning procedure and the phase heater temperature scanning procedure, or may be paused then reapplied). At S106, power is provided to the DBR heater 64 at one of the recorded DBR heater values (i.e. one of the DBR heater values for which the laser output power is maximized). At S107, the power provided to the phase heater 62 is increased (at a constant, discrete step value). As the power provided to the phase heater 62 is increased in this step wise manner, the output power of the laser 10 is monitored, e.g., using MPD 74. A plurality of measurements (e.g. 10-20) of the output power of the laser may be sampled for each phase heater power value in order to remove noise. The laser wavelength may also be monitored. As described above in relation to FIG. 7, with increasing phase heater power, the laser output cyclically reaches a maximum value at the hopping point 49, with the laser output power falling between these hopping points 49. The SMSR is optimized at a position away from the hopping points 49, e.g. at the minimum values of the laser output power, plotted with respect to phase heater power. Thus, at S.108, in order to maximise the SMSR of the laser, the values of the phase heater power corresponding to the minimum values of laser output power are recorded. In practice, a phase heater power value corresponding to a minimum value of laser output power may be calculated by dividing the sum of the phase heater power values corresponding to two adjacent maximum values of laser output power, by 2. The recorded phase heater power values are preferred, or optimal, power values for the phase heater 62 in order to maximize the SMSR ratio of the laser. Any of the recorded phase heater values may be applied to the phase heater 62 to maximize the SMSR of the laser. Preferably, the lowest power value of the recorded phase heater values may be chosen.

[0104] Therefore, this self-characterization method allows for optimized DBR heater and phase heater bias point values to be determined, without human intervention. This reduces the time and labour costs required, e.g. compared to manual testing.

[0105] Optionally, the method may further comprise, at S109, operating the laser by providing power at the operating current to the gain section, providing power to the phase heater 62 at one of the determined optimized phase heater values, and providing power to the DBR heater 64 at one of the determined DBR heater values.

[0106] FIG. 10 is a schematic of a system 80 for self-characterizing a spectrometer 82. Spectrometer 82 comprises a plurality of lasers 10a, 10b . . . 10n (e.g. laser 10 described above). Each of the lasers 10a, 10b, 10n, and in particular each of the outputs of the lasers, are optically coupled to an optical manipulation region comprising an optical wavelength multiplexer, MUX, 84. The output of the MUX 84 is coupled to an optical outlet 86, such as an optical port. Power is independently supplied to each of the lasers 10a, 10b, 10n, and in particular each photonic component, DBR heater, and phase heater of each laser, via one or more power sources, ADC, 88. The power supply to the lasers 10a, 10b, 10n is controlled by a controller (e.g. microcontroller unit 90). The system 80 also comprises a monitor photodiode detector (MPD) 92 and a transimpedance amplifier, TIA, 94. An optical splitter 96 splits the output from MUX 84 such that some (e.g. 3%) of the power is directed to the MPD 92, instead of to optical outlet 86. The controller 90 is connected to the TIA 94 and is configured to self-characterize the spectrometer 82, so that the maximum output power is output from the optical outlet (and align the wavelength of light output from the lasers to that output from the MUX). In addition, or as an alternative, to the optical splitter 96, some or all of the light output from the MUX 84 may be reflected to a receiver, which can align the DBR laser source wavelength with the MUX pass band peak, as set out below.

[0107] Graph 100 of FIG. 11 is an optical wavelength multiplexer channel pass band profile, e.g. an example MUX transmission profile is plotted against wavelength of output light. As shown in graph 100, for a bandwidth of 0.87 nm, there is a channel loss variation of 0.5 dB. The MUX output power spectrum is a function of DBR heater power, H.sub.i, (e.g. the power supplied to each DBR heater in each of lasers 10a, 10b, . . . 10n). In particular, the MUX output power spectrum (P.sub.RPS) is defined as:

P.sub.RPS(H.sub.i)=P.sub.DBR(H.sub.i).Math.MUX.sub.PB(λ.sub.i)

where λ.sub.i=f(H.sub.i), P.sub.DBR(H.sub.i) is the DBR laser output power spectrum, MUX.sub.PB(λ.sub.i) is the optical wavelength multiplexer each channel pass band spectrum, and i is the DBR heater power increasing step number.

[0108] Returning to FIG. 10, system 80 is configured to tune the wavelength of light output from the optical outlet 86 of the spectrometer 82, e.g. to align the DBR laser source wavelength with the MUX pass band peak.

[0109] In order to do this, the wavelength of light with the minimum loss has to be found. As mentioned above in relation to S104, for each laser, a plurality of DBR heater power values may be recorded and they may each be preferred, or optimal, power values for the DBR heater in order to maximize the performance of that laser and characterize that laser automatically. However, some of these DBR heater power values may shift the wavelength of light output from that laser such that it is not aligned with the MUX pass band peak (thus resulting in less power being output from MUX 84).

[0110] Therefore, in order to select the optimal DBR heater power value for each laser when coupled to the MUX 84 in spectrometer 82, the DBR heater scanning procedure must be repeated for each laser 10a, 10b, 10n. Therefore, for each laser 10a, 10b, 10n in turn, the following DBR heater scanning procedure is performed. First, power is provided to the photonic component (e.g. the Reflection Semiconductor Optical Amplifier, RSOA) of a first laser 10a at the operating current (which may be predefined, e.g. by application specification), in turn. The power supplied to the phase heater of the first laser 10a is set to 0. Next, the power provided to the DBR heater 64 of the first laser 10a is increased (e.g. at a constant, discrete step value). The values of the power provided to the DBR heater 64 in this step-wise process may be the plurality of recorded DBR heater values which were found in the DBR heater scanning process described above in relation to S104 of FIG. 9, or they may be arbitrarily chosen power values (e.g. a constant discrete step value). As the power provided to the DBR heater 64 of laser 10a is increased, the output power of the MUX 84 is monitored, e.g. using MPD 74 and/or TIA 94. A plurality of measurements (e.g. 10-20) of the output power of the MUX 84 may be sampled for each DBR heater power value in order to remove noise. The output wavelength may also be monitored. An example graph 102 of MUX output power plotted against DBR heater power for a laser in a spectrometer (such as spectrometer 82) is shown in FIG. 12.

[0111] In order to maximize output power of the MUX, the value of DBR heater power corresponding to the maximum value of MUX output power is determined and recorded. This recorded DBR heater value is a preferred, or optimal, power value for the DBR heater 64 of laser 10a, in order to maximize MUX output power of the spectrometer 82.

[0112] Optionally, in order to reduce power consumption of the tuning process, only the plurality of recorded DBR heater values which were found in the DBR heater scanning process described above in relation to S104 of FIG. 9 are used as the values of the power provided to the DBR heater. As such, only the DBR heater power preferred values which are related to local extreme laser output power are selected. In this way, less DBR heater power values are used to perform the DBR heater power scanning procedures. For example, if three DBR heater power values have been selected during the DBR heater power scanning procedure, only three laser output values from the MUX outlet port will be recorded. Example results are shown plot 104 of FIG. 13 (with the three DBR heater power values corresponding to the three maximum “hopping points” 48 in FIG. 6). The DBR heater power value that corresponds to the maximum optical output power is selected as the preferred DBR heater power value. This is an alternative approach to align DBR laser wavelength with MUX passband peak wavelength which uses less processing power than scanning arbitrarily chosen DBR heater power values.

[0113] The DBR heater scanning process is then repeated for each of the other lasers 10b, 10n in turn. The recorded phase heater value as determined and recorded in S108 of the method of FIG. 9 may be used as the optimal phase heater power value for each respective phase heater of the lasers 10a, 10b, 10n. The pairs of optimal DBR heater power value and phase heater power value determined as set out above may then be recorded and used as the best operating points for the respective laser, in spectrometer 82.

[0114] While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention. All references referred to above are hereby incorporated by reference.

[0115] For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.

[0116] Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

[0117] It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example +/−10%.