CA2317133A1 - Tunable laser and manufacturing method therefor - Google Patents
Tunable laser and manufacturing method therefor Download PDFInfo
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- CA2317133A1 CA2317133A1 CA 2317133 CA2317133A CA2317133A1 CA 2317133 A1 CA2317133 A1 CA 2317133A1 CA 2317133 CA2317133 CA 2317133 CA 2317133 A CA2317133 A CA 2317133A CA 2317133 A1 CA2317133 A1 CA 2317133A1
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Abstract
A tunable laser light source, and manufacturing methods therefor, for emitting light over a wide range of wavelengths, with wavelength and intensity stability, that is dimensionally and thermally tolerant. One embodiment configured according to the principles of the invention includes a filter bank, with multiple diverse, independently-transmittable filter elements, which is translatable relative to a light source. Another embodiment employs filter elements having multiple cavities. A further embodiment incorporates into the filter elements temperature insensitive interference films. An embodiment of a method for manufacturing a filter bank according to the principles of the invention involves hybrid integration of individual filter elements, each having different wavelength characteristics.
Another embodiment of a manufacturing method involves selective deposition of a Distributed Bragg Reflector (DBR) on a substrate and a cavity on the DBR with a predetermined thickness. The cavity thickness is increased at selected locations to shift the cavity resonance wavelength to desired wavelengths.
Another embodiment of a manufacturing method involves selective deposition of a Distributed Bragg Reflector (DBR) on a substrate and a cavity on the DBR with a predetermined thickness. The cavity thickness is increased at selected locations to shift the cavity resonance wavelength to desired wavelengths.
Description
-'1 .._ TUNABLE LASER AND MANiJFACTURING METAOD TAEREFOR
Reference To Earlier Annlications This application claims the benefit of pending prior U.S. Provisional Patent Application Serial No. 60/099,308, filed September 3, 1998, and pending prior U.S. Provisional Patent Application Serial No. 60/099,252, filed September 4, 1998, both by Parviz Tayebati et al., and both entitled LASER WITH SETTABLE
WAVELENGTHS. The two aforementioned documents are hereby incorporated herein by reference.
Field Of The Invention The invention relates to lasers. More specifically, the invention relates to apparatuses for tuning the wavelength of a laser and manufacturing methods therefor.
Background Of The Invention Fiberoptic cables, having diameters measuring less than 0.00015 inch, can transmit multiple signals containing considerable quantities of information for hundreds of miles. The ability to carry multiple signals derives from the ability of the fiberoptic cable to "multiplex," or simultaneously transmit different light
Reference To Earlier Annlications This application claims the benefit of pending prior U.S. Provisional Patent Application Serial No. 60/099,308, filed September 3, 1998, and pending prior U.S. Provisional Patent Application Serial No. 60/099,252, filed September 4, 1998, both by Parviz Tayebati et al., and both entitled LASER WITH SETTABLE
WAVELENGTHS. The two aforementioned documents are hereby incorporated herein by reference.
Field Of The Invention The invention relates to lasers. More specifically, the invention relates to apparatuses for tuning the wavelength of a laser and manufacturing methods therefor.
Background Of The Invention Fiberoptic cables, having diameters measuring less than 0.00015 inch, can transmit multiple signals containing considerable quantities of information for hundreds of miles. The ability to carry multiple signals derives from the ability of the fiberoptic cable to "multiplex," or simultaneously transmit different light
-2-signals, each having a different wavelength. Multiplexed fiberoptic communication requires that the wavelength of the light sources introduced-into the receiving end of the cable be adjustable to any wave length in the 1300 nm to 1600 nm range.
Referring to Fig. 1, a typical laser light source includes a gain medium, or a semiconductor optical amplifier (SOA) 10. One side 12 of the SOA 10 has an antireflection (AR) coating. The other side 14 of the SOA 10 is uncoated or has a high reflection coating. Light 16 emitted from the AR-coated side 12 is trained by one or more lenses 18 onto a thin film filter 20, typically mounted on a substrate 22. The filter 20 passes light 24 in a range of wavelengths, thereby enabling a narrow linewidth or single mode laser emission. The filtered light 24 is trained by another lens 26 onto a curved mirror 28, or a lens and a flat mirror, which reflects the light 24 back into the filter 20 and the SOA 10. To ensure that out-of band light does not return into the SOA 10, with undesirable consequences, the filter 20 is positioned such that the angle of incidence 30 with respect to the projection line 32 ofthe light is not 90°, or orthogonal to the projection line 32. Out-of band light is not passed through the filter 20, but reflected away from the SOA 10.
The wavelength of the laser emission is determined by the overlap between the transmission wavelength of the filter 20 and the modes of the laser cavity 34.
In general, a problem encountered with typical film filters is the existence of temperature drifts, or gradients, in the film. Temperature drifts cause r
Referring to Fig. 1, a typical laser light source includes a gain medium, or a semiconductor optical amplifier (SOA) 10. One side 12 of the SOA 10 has an antireflection (AR) coating. The other side 14 of the SOA 10 is uncoated or has a high reflection coating. Light 16 emitted from the AR-coated side 12 is trained by one or more lenses 18 onto a thin film filter 20, typically mounted on a substrate 22. The filter 20 passes light 24 in a range of wavelengths, thereby enabling a narrow linewidth or single mode laser emission. The filtered light 24 is trained by another lens 26 onto a curved mirror 28, or a lens and a flat mirror, which reflects the light 24 back into the filter 20 and the SOA 10. To ensure that out-of band light does not return into the SOA 10, with undesirable consequences, the filter 20 is positioned such that the angle of incidence 30 with respect to the projection line 32 ofthe light is not 90°, or orthogonal to the projection line 32. Out-of band light is not passed through the filter 20, but reflected away from the SOA 10.
The wavelength of the laser emission is determined by the overlap between the transmission wavelength of the filter 20 and the modes of the laser cavity 34.
In general, a problem encountered with typical film filters is the existence of temperature drifts, or gradients, in the film. Temperature drifts cause r
-3-undesirable wavelength drifts and associated mode hopping and noise. An ideal laser light source for multiplexed communication must provide light with a stable wavelength.
To provide light at variable wavelengths, some light sources include a plurality of lasers, each emitting light at a different wavelength. However, normal wear and tear or the unavailability of a lasers at specific wavelengths can limit multiplexing potential.
Other light sources employ lasers with an angularly-adjustable filter.
Referring again to Fig. l, rotating the filter 20 changes the angle of incidence 30 between the filter 20 and projection line 32, which changes the transmission wavelength of the filter 20.
A major disadvantage of angle-tunable lasers is that tuning the transmission of wavelength of the thin film filter necessarily is accompanied by an increase in optical path length in the underlying substrate 22. This can cause undesirable wavelength and intensity instabilities absent a high degree of controlling and stabilizing the rotation angle.
Still other light sources alter emission wavelength with a filter that has a variable Fabry-Perot gap thickness along its len~h. Referring again to Fig. 1, translating the filter 20 along a plane 36 positions a portion of the filter 20 having a
To provide light at variable wavelengths, some light sources include a plurality of lasers, each emitting light at a different wavelength. However, normal wear and tear or the unavailability of a lasers at specific wavelengths can limit multiplexing potential.
Other light sources employ lasers with an angularly-adjustable filter.
Referring again to Fig. l, rotating the filter 20 changes the angle of incidence 30 between the filter 20 and projection line 32, which changes the transmission wavelength of the filter 20.
A major disadvantage of angle-tunable lasers is that tuning the transmission of wavelength of the thin film filter necessarily is accompanied by an increase in optical path length in the underlying substrate 22. This can cause undesirable wavelength and intensity instabilities absent a high degree of controlling and stabilizing the rotation angle.
Still other light sources alter emission wavelength with a filter that has a variable Fabry-Perot gap thickness along its len~h. Referring again to Fig. 1, translating the filter 20 along a plane 36 positions a portion of the filter 20 having a
-4-different thickness (not shown) in line with the projection line 32. The thickness di$'erence correspondingly alters transmission wavelength.
A major disadvantage of the filter-translating tuning approach is that, once the filter is located, it must be maintained so that it does not drift into transitions zones between portions of the filter having continuous thickness, causing wavelength drift. The effect of filter drift on wavelength variation can be minimized by increasing the projected spot size of the laser beam at the filter.
However, the filter must be long enough to cover the desired wavelength range.
Furthermore, the cost of fabricating such filters, with large wavelength variation over a few millimeters distance, as is desired for compactness, is high.
The foregoing demonstrates a need for a singular, compact, tunable light source that emits light with variable, but stable, wavelengths and stable intensity that is thermally and mechanically insensitive.
Summary Of The Invention The invention is a tunable laser light source for emitting light over a wide range of wavelengths, and manufacturing methods therefor, which is suited for multiplexed fiberoptic communication. The invention provides for emitting light with wavelength and intensity stability. The invention does not require inordinately precise positioning or maintenance. However, a filter constructed -$-according to the present invention is small and requires minimal adjustments to achieve desired wavelengths. The invention also compensates for temperature gradients occurring through the filter. The invention provides improved elements and arrangements thereof, in an apparatus and concomitant method, for the purposes described which are inexpensive, dependable and effective in accomplishing its intended purposes.
An embodiment configured according to the principles of the invention includes a filter bank, with multiple diverse filter elements, which is translatable relative to a light source. The filter bank is positioned relative to the light source such that light transmits through one of the multiple filter elements, at a desired wavelength.
Another embodiment configured according to the principles of the invention employs filter elements that have multiple cavities, for greater stability and less noise. Multiple cavity filters tolerate and discriminate between cavity mode wavelength variations.
A further embodiment configured according to principles of the invention incorporates into the filter elements temperature insensitive interference films.
This renders the filter elements thermally insensitive, thus able to transmit light with desired characteristics, regardless of thermal gradients in the filter elements.
An embodiment of a method for manufacturing a filter bank according to principles of the invention involves hybrid integration. Individual filter elements, each having different wavelength characteristics, are mounted on a transparent substrate.
Another embodiment of a method for manufacturing a filter bank according to principles of the invention involves selective deposition. A substrate has an antireflection coating on one side and a Distributed Bragg Reflector (DBR) on the other side. A cavity is mounted on the DBR having a thickness with a desired cavity resonance wavelength. The thickness of the cavity is increased at selected locations to shift the cavity resonance wavelength to desired wavelengths.
These and other features of the invention will be appreciated more readily in view of the drawings and detailed description below.
Brief Description Of The Drawings The invention is described in detail below with reference to the following drawings, throughout which similar reference characters denote corresponding features consistently, wherein:
Fig. 1 is a schematic view of a light source, including a filter;
Fig. 2 is a schematic view of an embodiment of a light source, including a filter bank, constructed according to the principles of the present invention;
a Fig. 3 is a plan view of an embodiment of a filter bank constructed according to the principles of the invention; -Fig. 4 is a plan view of another embodiment of a filter bank constructed according to the principles of the invention;
Fig. 5 is a graphical representation of a transmission performance of a single-cavity filter;
Fig. 6 is a graphical representation of a transmission performance of a multiple-cavity filter constructed according to the principles of the invention.
Fig. 7 is a right side elevational view of an embodiment of a filter bank and positioner therefor constructed according to the principles of the invention;
Fig. 8 is a plan view of another embodiment of a filter bank and positioner therefor constructed according to the principles of the invention;
Fig. 9 is a plan view of yet another embodiment of a filter bank and positioner therefor constructed according to the principles of the invention;
Fig. 10 is a cross-sectional detail view of an embodiment of a filter element adapted to be mounted on a substrate according to the principles of the invention;
Fig. 11 is a bottom view of an embodiment of a filter substrate constructed according to the principles of the invention; and _$_ Figs. 12a-a are cross-sectional detail views of an embodiment of a filter bank and elements thereof, at various construction stages, constructed according to the principles of the invention.
Detailed Description Of The Invention The invention is a light source ideally suited for multiplexed fiberoptic communication. The light source emits light over a wide range of wavelengths, with wavelength and intensity stability, that is dimensionally and thermally tolerant.
The invention also advances methods for manufacturing such a light source.
Light Source Referring now to Fig. 2, the invention includes a gain medium, or a semiconductor optical amplifier (SOA) 100. One side 102 of the SOA 100 has an antireflection (AR) coating. The other side 104 of the SOA 100 is uncoated or has a high reflection coating. Light 106 emitted from the AR-coated side 102 projects through one or more lenses 108 toward a selected filter element 110. The selected filter element I I 0 passes light within a predefined range of wavelengths, thereby enabling a narrow linewidth or single mode laser emission. Filtered light 112 is trained by another lens 114 onto a curved minor 116, or a lens and a flat mirror, which reflects the light 112 back into the filter element 110 and the SOA 100.
To ensure that out-of band light does not return into the SOA 100, with undesirable consequences, the filter element 110 is positioned such thatthe angle of incidence 118 with respect to the projection fme 120 of the light does not equal 90°, or is not orthogonal to the projection line 120. Out-of band light is not passed through the filter element 110, but reflected away from the SOA 100. The wavelength of the laser emission is determined by the overlap between the transmission wavelength of the filter element 110 and the modes of the laser cavity 122.
Referring also to Fig. 3, for lasing at any of a number of wavelengths, an embodiment configured according to the invention includes a filter bank 124.
The filter bank 124 maintains a plurality of filter elements 110, each for transmitting light at a different wavelength. The filter bank 124 is positioned relative to the light source such that light transmits through one of the filter elements 110 which corresponds to a desired wavelength. Toward that end, the filter bank 124 is translated within a plane 126 (Fig. 2) that is not orthogonal to the projection line 120 of the light 126. The plane 126 is non-orthogonal with the projection line so that the selected filter element 110 will reflect out-of band light away from the SOA 100.
The filter elements 110 may be arranged in filter bank 124 in any fashion, such as a linear one- or two-dimensional format, such as the matrix of filter elements 110 as shown in Fig. 3; or a circular array of filter elements 110 as shown in Fig. 4; or some other format more suitable to a particular-application.
Another embodiment of the filter bank 124 incorporates multiple-cavity filter elements (not shown). Multiple-cavity filter elements provide greater stability and less noise, as compared with single-cavity filter elements. Multiple-cavity filter elements tolerate and discriminate between cavity mode wavelength variations.
Multiple-cavity filter elements also provide better mode stability and less relative intensity noise.
Figs. 5 and 6, show, respectively, transmission peaks of single- and multiple-cavity filter elements and the concomitant laser cavity modes. The multiple-cavity filter element (Fig. 6) has a flatter transmission over the band pass, and better discrimination between modes that do not fall within the filter element bandwidth, than the single-cavity transmission (Fig. 5). With single-cavity filter elements, laser cavity mode shifts cause a significant change in cavity loss and lasing power. In contrast, v~~ith multiple-cavity filter elements, small variations in the wavelength of cavity modes cause significantly less change in the cavity loss, resulting in better stability. Furthermore, a multiple-cavity filter element has better wavelength discrimination because it has a sharper skirt. A sharper skirt reduces the probability of lasing at higher order modes that are out of band. For example, as shown in Fig. 5, a single-cavity filter element discriminates marginally between modes 3 and 4, causing potential multimode behavior and mode hopping between modes 2 and 3 . However, in Fig. 6, the multiple-cavity filter element distinguishes between modes 2 and 3, resulting in stable lasing at mode 2.
Yet another embodiment of the invention includes filter elements constructed from temperature-insensitive interference films (not shown). Film filters sensitive to temperature exhibit temperature drifts, or gradients, which cause undesirable wavelength drifts and associated mode hopping and noise. To overcome these gradients, the substrate on which the film is deposited is chosen such that the difference in the expansion coefficient between the substrate and the film alters the index of refraction of the layers in the film, canceling out the ef~'ects of thermal expansion. The result is a temperature insensitive interference.
Usage of such a film in a tunable laser allows high temperature stability at a specified lasing wavelength.
Although thermally stable filter elements pin the lasing wavelength, stability of the lasing wavelength depends on the temperature stability of the cavity modes as well. Accordingly, temperature insensitive films should be used in conjunction with temperature stabilized laser chips and cavities.
Referring next to Figs. 7 and 8, the invention provides for translating the filter bank 124 relative to a Light source to project light 106 through a desired filter element 110 with high mechanical stability. High stability is important to maintaining laser modal stability and reliability. In one embodiment, the filter bank 124 is mounted on a magnetic submount 126. The submount 126 is slidingly mounted on a magnetic mount 128, having a through hole 130. The magnetic mount 128 is fixed relative to a laser cavity 132 which emits light 106. The submount 126 and mount 128 are attracted to each other by magnetic force. The magnetic force enhances mechanically stability between the submount 126 and mount 128 because of correspondingly increased friction therebetween.
The mount 128 translates relative to the cavity 132, in two dimensions, within a translation plane; in Fig. 7, the translation plane extends at a right angle to the plane of the page. As described above, the translation plane is at an angle to the projection line 120 of the light 106 so as to reflect out-of band light away from the SOA (not shown). As shown in Fig. 8, the mount 128 moves relative to the cavity 132 in up, down, left and right directions. The device is intended to remain fixed in the third dimension, i.e., in and out of the page. This dimensional stability avoids drift in the filter element which may cause wavelength drift.
In one embodiment, as shown in Fig. 7, opposed screws 134 and 136 translate the mount 128 relative to the cavity 132 in a left or right direction.
Another set of screws (not shown) translate the mount 128 relative to the cavity 132 in and out of the page. The screws may be standard 080 screws, which translate the mount 128 approximately 310 microns, an ideal spacing between filter elements 110, for every full rotation. Good subrotation accuracy allows fine control in positioning of filter elements 110 in the path of the cavity. To assure that the filter bank 124 moves linearly without rotation, the screws could have flat tips 138, which would impart a more uniform force on the filter bank 124.
In another embodiment, as shown in Fig. 8, one screw 140 translates the mount 128 relative to the cavity 132 in a left or right direction. Force imparted by the screw 140 is opposed by an equal and opposite force imparted by a leaf spring 142. Another screw 144 translates the mount 128 relative to the cavity 132 in an up or down direction. Force imparted by the screw 144 is opposed by an equal and opposite force imparted by a leaf spring 146. Although leaf springs 142 and 146 are shown, the invention is not limited to leaf springs, and may employ any suitable biasing mechanism.
In yet another embodiment, as sho»~n in Fig. 9, a worm screw 148 threadingly advances a U-shaped positioner 150, relative to the cavity 132, in a lateral direction 152. The positioner I 50 maintains the filter bank 124 in the lateral direction 152. A second worm screw (not shown) and U-shaped positioner (not shown) translate the filter bank 124 relative to the positioner 150 in the transverse direction 154. The second positioner (not shown) allows the filter bank to slide in the lateral direction 152, as dictated by the positioner 150. The thickness of each positioner should be less than half the thickness of the submount 126 so that each may contact and influence position of the submount 126 independently of the other positioner. Because of strong magnetic attraction between xhe submount 126 and the mount 128, the filter bank 124 will not move significantly once positioned.
However, the positioners may be constructed from magnetic material for even better stability.
Light Source Manufacturing Method (i) Hybrid Integration One embodiment of a method for manufacturing a filter bank according to the principles of the invention involves hybrid integration. Referring to Fig.
10, a filter element 200 is constructed by, first, depositing filter films 202 of different wavelength characteristics on separate substrates 204. Each filter-substrate composite is diced to produce a plurality of filter elements 200 having sizes ranging from 250 to 400 microns. Thereafter, filter elements 200 having different wavelength characteristics are adhered to a transparent substrate 206.
Referring also to Fig. 11, preferably, the substrate 206 on which the filter films 202 are deposited include depositions of flip-chip solder 208, such as Au/SN, so as to define a lattice structure, as shown. Following deposition of the filter films, the filter-substrate-solder composite is severed along horizontally-oriented - 1$ -cut lines 210a-h and vertically-oriented cut lines 212a-h. Any configuration or number of cut lines suited for the purpose described may be-used. The resultant filter elements 200, as shown in Fig. 10, have peripheral solder rims 214 which may be mated with a transparent substrate 206.
Ideally, each filter element 200 is mounted on a silicone substrate 206.
Flip-chip pads 216 on the substrate 206 correspond to the pattern of the flip-chip solder rims 214 of the filter elements 200. The solder rims 212 and solder pads 216 are soldered together. Low temperature flip-chip bonding with Indium/tin or gold/tin based solder may be used.
The substrate 206 provides optimal transparency local to each filter element 200 with via holes 218. The via holes 218 allow passage of light without reflections. Preferably, the via holes 2 l 8 are etched following deposition of the solder pads 216.
(ii) Selective Deposition Another embodiment of a method for manufacturing a filter bank according to the principles of the invention involves selective deposition. Referring to Fig.
12a, a Distributed Bragg Reflector (DBR) 220 is deposited on the front side of a substrate 222. The DBR 220 (Fig. 12b) typically is composed of multiple, quarter-wave stack layers 224 of high and low index films with well-known characteristics.
Referring to Fig. 12c, a cavity 226, or dielectric layer, is mounted on the DBR 220. The cavity 226 has a total thickness t, with an order m, a shortest desired wavelength ~o and an index of refraction n(1), calculated as follows:
t = m~.o/2n(7~o) Referring to Fig. 12d, to shift the cavity resonance wavelength to a desired wavelength 7~'u, the cavity thickness is adjusted in local areas 228x-c by differing iterative depositions of cavity material. Each cavity material deposition has an additional thickness 8t1, calculated as follows:
8,,=m(7~,/n(~1,)-~o/n(~1o))/2 For example, when m = 3, Jgio=1520 nm and ~ ~=1520.8 nm, assuming the index of refraction does not vary with wavelength (thus no chromatic dispersion) 8,1=1.2 nm.
To achieve a desired additional thickness in a particular area, a screen 230 with a hole 232 sized according to the local areas 228a-c is positioned relative to the local area 228a-c. Additional cavity material is deposited on the local areas 228a-c as needed. Depositing small amounts of material typically takes a fraction of a second with most deposition systems.
The depositing process is repeated to generate appropriate cavities for all desired filters. Each iteration results in the deposition of material with a thickness 8~1. To avoid chromatic dispersion in the film, material must be deposited to vary nonlinearly to generate equal wavelength spacing. This may be accomplished with real-time optical monitoring of each spot as the correction layer is deposited.
Preferably, the DBR layers are deposited by electron-beam deposition which provides for high speed processing and optical thickness tolerance.
Thereafter, referring to Fig. 12e, the gap 234 and/or selective corrections 236 may be deposited with ion-beam sputtering which provides for superior accuracy, film compactness and reproduceability.
The invention is not limited to the above, but encompasses all improvements and substitutions consistent with the principles within the scope of the appended claims.
A major disadvantage of the filter-translating tuning approach is that, once the filter is located, it must be maintained so that it does not drift into transitions zones between portions of the filter having continuous thickness, causing wavelength drift. The effect of filter drift on wavelength variation can be minimized by increasing the projected spot size of the laser beam at the filter.
However, the filter must be long enough to cover the desired wavelength range.
Furthermore, the cost of fabricating such filters, with large wavelength variation over a few millimeters distance, as is desired for compactness, is high.
The foregoing demonstrates a need for a singular, compact, tunable light source that emits light with variable, but stable, wavelengths and stable intensity that is thermally and mechanically insensitive.
Summary Of The Invention The invention is a tunable laser light source for emitting light over a wide range of wavelengths, and manufacturing methods therefor, which is suited for multiplexed fiberoptic communication. The invention provides for emitting light with wavelength and intensity stability. The invention does not require inordinately precise positioning or maintenance. However, a filter constructed -$-according to the present invention is small and requires minimal adjustments to achieve desired wavelengths. The invention also compensates for temperature gradients occurring through the filter. The invention provides improved elements and arrangements thereof, in an apparatus and concomitant method, for the purposes described which are inexpensive, dependable and effective in accomplishing its intended purposes.
An embodiment configured according to the principles of the invention includes a filter bank, with multiple diverse filter elements, which is translatable relative to a light source. The filter bank is positioned relative to the light source such that light transmits through one of the multiple filter elements, at a desired wavelength.
Another embodiment configured according to the principles of the invention employs filter elements that have multiple cavities, for greater stability and less noise. Multiple cavity filters tolerate and discriminate between cavity mode wavelength variations.
A further embodiment configured according to principles of the invention incorporates into the filter elements temperature insensitive interference films.
This renders the filter elements thermally insensitive, thus able to transmit light with desired characteristics, regardless of thermal gradients in the filter elements.
An embodiment of a method for manufacturing a filter bank according to principles of the invention involves hybrid integration. Individual filter elements, each having different wavelength characteristics, are mounted on a transparent substrate.
Another embodiment of a method for manufacturing a filter bank according to principles of the invention involves selective deposition. A substrate has an antireflection coating on one side and a Distributed Bragg Reflector (DBR) on the other side. A cavity is mounted on the DBR having a thickness with a desired cavity resonance wavelength. The thickness of the cavity is increased at selected locations to shift the cavity resonance wavelength to desired wavelengths.
These and other features of the invention will be appreciated more readily in view of the drawings and detailed description below.
Brief Description Of The Drawings The invention is described in detail below with reference to the following drawings, throughout which similar reference characters denote corresponding features consistently, wherein:
Fig. 1 is a schematic view of a light source, including a filter;
Fig. 2 is a schematic view of an embodiment of a light source, including a filter bank, constructed according to the principles of the present invention;
a Fig. 3 is a plan view of an embodiment of a filter bank constructed according to the principles of the invention; -Fig. 4 is a plan view of another embodiment of a filter bank constructed according to the principles of the invention;
Fig. 5 is a graphical representation of a transmission performance of a single-cavity filter;
Fig. 6 is a graphical representation of a transmission performance of a multiple-cavity filter constructed according to the principles of the invention.
Fig. 7 is a right side elevational view of an embodiment of a filter bank and positioner therefor constructed according to the principles of the invention;
Fig. 8 is a plan view of another embodiment of a filter bank and positioner therefor constructed according to the principles of the invention;
Fig. 9 is a plan view of yet another embodiment of a filter bank and positioner therefor constructed according to the principles of the invention;
Fig. 10 is a cross-sectional detail view of an embodiment of a filter element adapted to be mounted on a substrate according to the principles of the invention;
Fig. 11 is a bottom view of an embodiment of a filter substrate constructed according to the principles of the invention; and _$_ Figs. 12a-a are cross-sectional detail views of an embodiment of a filter bank and elements thereof, at various construction stages, constructed according to the principles of the invention.
Detailed Description Of The Invention The invention is a light source ideally suited for multiplexed fiberoptic communication. The light source emits light over a wide range of wavelengths, with wavelength and intensity stability, that is dimensionally and thermally tolerant.
The invention also advances methods for manufacturing such a light source.
Light Source Referring now to Fig. 2, the invention includes a gain medium, or a semiconductor optical amplifier (SOA) 100. One side 102 of the SOA 100 has an antireflection (AR) coating. The other side 104 of the SOA 100 is uncoated or has a high reflection coating. Light 106 emitted from the AR-coated side 102 projects through one or more lenses 108 toward a selected filter element 110. The selected filter element I I 0 passes light within a predefined range of wavelengths, thereby enabling a narrow linewidth or single mode laser emission. Filtered light 112 is trained by another lens 114 onto a curved minor 116, or a lens and a flat mirror, which reflects the light 112 back into the filter element 110 and the SOA 100.
To ensure that out-of band light does not return into the SOA 100, with undesirable consequences, the filter element 110 is positioned such thatthe angle of incidence 118 with respect to the projection fme 120 of the light does not equal 90°, or is not orthogonal to the projection line 120. Out-of band light is not passed through the filter element 110, but reflected away from the SOA 100. The wavelength of the laser emission is determined by the overlap between the transmission wavelength of the filter element 110 and the modes of the laser cavity 122.
Referring also to Fig. 3, for lasing at any of a number of wavelengths, an embodiment configured according to the invention includes a filter bank 124.
The filter bank 124 maintains a plurality of filter elements 110, each for transmitting light at a different wavelength. The filter bank 124 is positioned relative to the light source such that light transmits through one of the filter elements 110 which corresponds to a desired wavelength. Toward that end, the filter bank 124 is translated within a plane 126 (Fig. 2) that is not orthogonal to the projection line 120 of the light 126. The plane 126 is non-orthogonal with the projection line so that the selected filter element 110 will reflect out-of band light away from the SOA 100.
The filter elements 110 may be arranged in filter bank 124 in any fashion, such as a linear one- or two-dimensional format, such as the matrix of filter elements 110 as shown in Fig. 3; or a circular array of filter elements 110 as shown in Fig. 4; or some other format more suitable to a particular-application.
Another embodiment of the filter bank 124 incorporates multiple-cavity filter elements (not shown). Multiple-cavity filter elements provide greater stability and less noise, as compared with single-cavity filter elements. Multiple-cavity filter elements tolerate and discriminate between cavity mode wavelength variations.
Multiple-cavity filter elements also provide better mode stability and less relative intensity noise.
Figs. 5 and 6, show, respectively, transmission peaks of single- and multiple-cavity filter elements and the concomitant laser cavity modes. The multiple-cavity filter element (Fig. 6) has a flatter transmission over the band pass, and better discrimination between modes that do not fall within the filter element bandwidth, than the single-cavity transmission (Fig. 5). With single-cavity filter elements, laser cavity mode shifts cause a significant change in cavity loss and lasing power. In contrast, v~~ith multiple-cavity filter elements, small variations in the wavelength of cavity modes cause significantly less change in the cavity loss, resulting in better stability. Furthermore, a multiple-cavity filter element has better wavelength discrimination because it has a sharper skirt. A sharper skirt reduces the probability of lasing at higher order modes that are out of band. For example, as shown in Fig. 5, a single-cavity filter element discriminates marginally between modes 3 and 4, causing potential multimode behavior and mode hopping between modes 2 and 3 . However, in Fig. 6, the multiple-cavity filter element distinguishes between modes 2 and 3, resulting in stable lasing at mode 2.
Yet another embodiment of the invention includes filter elements constructed from temperature-insensitive interference films (not shown). Film filters sensitive to temperature exhibit temperature drifts, or gradients, which cause undesirable wavelength drifts and associated mode hopping and noise. To overcome these gradients, the substrate on which the film is deposited is chosen such that the difference in the expansion coefficient between the substrate and the film alters the index of refraction of the layers in the film, canceling out the ef~'ects of thermal expansion. The result is a temperature insensitive interference.
Usage of such a film in a tunable laser allows high temperature stability at a specified lasing wavelength.
Although thermally stable filter elements pin the lasing wavelength, stability of the lasing wavelength depends on the temperature stability of the cavity modes as well. Accordingly, temperature insensitive films should be used in conjunction with temperature stabilized laser chips and cavities.
Referring next to Figs. 7 and 8, the invention provides for translating the filter bank 124 relative to a Light source to project light 106 through a desired filter element 110 with high mechanical stability. High stability is important to maintaining laser modal stability and reliability. In one embodiment, the filter bank 124 is mounted on a magnetic submount 126. The submount 126 is slidingly mounted on a magnetic mount 128, having a through hole 130. The magnetic mount 128 is fixed relative to a laser cavity 132 which emits light 106. The submount 126 and mount 128 are attracted to each other by magnetic force. The magnetic force enhances mechanically stability between the submount 126 and mount 128 because of correspondingly increased friction therebetween.
The mount 128 translates relative to the cavity 132, in two dimensions, within a translation plane; in Fig. 7, the translation plane extends at a right angle to the plane of the page. As described above, the translation plane is at an angle to the projection line 120 of the light 106 so as to reflect out-of band light away from the SOA (not shown). As shown in Fig. 8, the mount 128 moves relative to the cavity 132 in up, down, left and right directions. The device is intended to remain fixed in the third dimension, i.e., in and out of the page. This dimensional stability avoids drift in the filter element which may cause wavelength drift.
In one embodiment, as shown in Fig. 7, opposed screws 134 and 136 translate the mount 128 relative to the cavity 132 in a left or right direction.
Another set of screws (not shown) translate the mount 128 relative to the cavity 132 in and out of the page. The screws may be standard 080 screws, which translate the mount 128 approximately 310 microns, an ideal spacing between filter elements 110, for every full rotation. Good subrotation accuracy allows fine control in positioning of filter elements 110 in the path of the cavity. To assure that the filter bank 124 moves linearly without rotation, the screws could have flat tips 138, which would impart a more uniform force on the filter bank 124.
In another embodiment, as shown in Fig. 8, one screw 140 translates the mount 128 relative to the cavity 132 in a left or right direction. Force imparted by the screw 140 is opposed by an equal and opposite force imparted by a leaf spring 142. Another screw 144 translates the mount 128 relative to the cavity 132 in an up or down direction. Force imparted by the screw 144 is opposed by an equal and opposite force imparted by a leaf spring 146. Although leaf springs 142 and 146 are shown, the invention is not limited to leaf springs, and may employ any suitable biasing mechanism.
In yet another embodiment, as sho»~n in Fig. 9, a worm screw 148 threadingly advances a U-shaped positioner 150, relative to the cavity 132, in a lateral direction 152. The positioner I 50 maintains the filter bank 124 in the lateral direction 152. A second worm screw (not shown) and U-shaped positioner (not shown) translate the filter bank 124 relative to the positioner 150 in the transverse direction 154. The second positioner (not shown) allows the filter bank to slide in the lateral direction 152, as dictated by the positioner 150. The thickness of each positioner should be less than half the thickness of the submount 126 so that each may contact and influence position of the submount 126 independently of the other positioner. Because of strong magnetic attraction between xhe submount 126 and the mount 128, the filter bank 124 will not move significantly once positioned.
However, the positioners may be constructed from magnetic material for even better stability.
Light Source Manufacturing Method (i) Hybrid Integration One embodiment of a method for manufacturing a filter bank according to the principles of the invention involves hybrid integration. Referring to Fig.
10, a filter element 200 is constructed by, first, depositing filter films 202 of different wavelength characteristics on separate substrates 204. Each filter-substrate composite is diced to produce a plurality of filter elements 200 having sizes ranging from 250 to 400 microns. Thereafter, filter elements 200 having different wavelength characteristics are adhered to a transparent substrate 206.
Referring also to Fig. 11, preferably, the substrate 206 on which the filter films 202 are deposited include depositions of flip-chip solder 208, such as Au/SN, so as to define a lattice structure, as shown. Following deposition of the filter films, the filter-substrate-solder composite is severed along horizontally-oriented - 1$ -cut lines 210a-h and vertically-oriented cut lines 212a-h. Any configuration or number of cut lines suited for the purpose described may be-used. The resultant filter elements 200, as shown in Fig. 10, have peripheral solder rims 214 which may be mated with a transparent substrate 206.
Ideally, each filter element 200 is mounted on a silicone substrate 206.
Flip-chip pads 216 on the substrate 206 correspond to the pattern of the flip-chip solder rims 214 of the filter elements 200. The solder rims 212 and solder pads 216 are soldered together. Low temperature flip-chip bonding with Indium/tin or gold/tin based solder may be used.
The substrate 206 provides optimal transparency local to each filter element 200 with via holes 218. The via holes 218 allow passage of light without reflections. Preferably, the via holes 2 l 8 are etched following deposition of the solder pads 216.
(ii) Selective Deposition Another embodiment of a method for manufacturing a filter bank according to the principles of the invention involves selective deposition. Referring to Fig.
12a, a Distributed Bragg Reflector (DBR) 220 is deposited on the front side of a substrate 222. The DBR 220 (Fig. 12b) typically is composed of multiple, quarter-wave stack layers 224 of high and low index films with well-known characteristics.
Referring to Fig. 12c, a cavity 226, or dielectric layer, is mounted on the DBR 220. The cavity 226 has a total thickness t, with an order m, a shortest desired wavelength ~o and an index of refraction n(1), calculated as follows:
t = m~.o/2n(7~o) Referring to Fig. 12d, to shift the cavity resonance wavelength to a desired wavelength 7~'u, the cavity thickness is adjusted in local areas 228x-c by differing iterative depositions of cavity material. Each cavity material deposition has an additional thickness 8t1, calculated as follows:
8,,=m(7~,/n(~1,)-~o/n(~1o))/2 For example, when m = 3, Jgio=1520 nm and ~ ~=1520.8 nm, assuming the index of refraction does not vary with wavelength (thus no chromatic dispersion) 8,1=1.2 nm.
To achieve a desired additional thickness in a particular area, a screen 230 with a hole 232 sized according to the local areas 228a-c is positioned relative to the local area 228a-c. Additional cavity material is deposited on the local areas 228a-c as needed. Depositing small amounts of material typically takes a fraction of a second with most deposition systems.
The depositing process is repeated to generate appropriate cavities for all desired filters. Each iteration results in the deposition of material with a thickness 8~1. To avoid chromatic dispersion in the film, material must be deposited to vary nonlinearly to generate equal wavelength spacing. This may be accomplished with real-time optical monitoring of each spot as the correction layer is deposited.
Preferably, the DBR layers are deposited by electron-beam deposition which provides for high speed processing and optical thickness tolerance.
Thereafter, referring to Fig. 12e, the gap 234 and/or selective corrections 236 may be deposited with ion-beam sputtering which provides for superior accuracy, film compactness and reproduceability.
The invention is not limited to the above, but encompasses all improvements and substitutions consistent with the principles within the scope of the appended claims.
Claims (36)
1. A tunable laser comprising:
a filter bank including a plurality of filter elements, each of said filter elements having a distinct transmission characteristic;
said filter bank being positionable such that one of said plurality of filter elements transmits light.
a filter bank including a plurality of filter elements, each of said filter elements having a distinct transmission characteristic;
said filter bank being positionable such that one of said plurality of filter elements transmits light.
2. The tunable laser of claim 1, wherein the transmission characteristic is wavelength.
3. The tunable laser of claim 1, said plurality of filter elements describing a pattern on said filter bank selected from linear, matrix and circular patterns.
4. The tunable laser of claim 1, wherein one of said plurality of filter elements has multiple cavities.
5. The tunable laser of claim 1, wherein one of said plurality of filter elements is constructed from temperature insensitive interference films.
6. The tunable laser of claim 5, said tunable laser further comprising either or both of a temperature stabilized laser chip and a temperature stabilized cavity.
7. The tunable laser of claim 1, said tunable laser further comprising means for positioning said filter bank so that light from a light source strikes one of said plurality of filter elements at a distance from the light source.
3. The tunable laser of claim 7, wherein said filter bank is biased toward the light source.
9. The tunable laser of claim 8, wherein said filter bank is urged toward the light source by a magnetic force.
10. The tunable laser of claim 1, further comprising:
a mount fixed relative to a light source;
said filter bank including a submount slidingly mounted on said mount.
a mount fixed relative to a light source;
said filter bank including a submount slidingly mounted on said mount.
11. The tunable laser of claim 10, wherein said submount is biased against said mount.
12. The tunable laser of claim 11 wherein said submount is biased by magnetic force.
13. The tunable laser of claim 1, further comprising means for translating said filter bank in a first direction aligned with a translation plane.
14. The tunable laser of claim 13, further comprising means for translating said filter bank in a second direction aligned with the translation plane.
15. The tunable laser of claim 1, further comprising a screw threadingly translating said filter bank in a first direction aligned with a translation plane.
16. The tunable laser of claim 15, wherein said filter bank is biased against said screw.
17. The tunable laser of claim 15, further comprising a second screw threadingly translating said filter bank in a second direction-aligned with the translation plane.
18. The tunable laser of claim 17, wherein said filter bank is biased against said second screw.
19. The tunable laser of claim 1, further comprising:
a first positioner adapted to receive said filter bank; and a worm screw threadingly translating said first positioner in a first direction aligned with a translation plane.
a first positioner adapted to receive said filter bank; and a worm screw threadingly translating said first positioner in a first direction aligned with a translation plane.
20. The tunable laser of claim 19, further comprising:
a second positioner adapted to receive said first positioner; and a worm screw threadingly translating said second positioner in a second direction aligned with the translation plane.
a second positioner adapted to receive said first positioner; and a worm screw threadingly translating said second positioner in a second direction aligned with the translation plane.
21. A tunable laser comprising a filter element constructed from a temperature insensitive interference film.
22. The tunable laser of claim 21, wherein said filter element has multiple cavities.
23. The tunable laser of claim 21, said tunable laser further comprising either or both of a temperature stabilized laser chip or a temperature stabilized cavity.
24. A method for manufacturing a filter bank comprising installing a plurality of filter elements relative to each other, each of said filter elements being configured to transmit light without striking any other of said filter elements.
25. The method of claim 24, wherein said plurality of filter elements describe a pattern selected from linear, matrix and circular patterns.
26. The method of claim 24, including mounting said plurality of filter elements on a substrate.
27. The method of claim 24, further comprising:
depositing one of a plurality of distinct filter films on one of a like number of distinct substrates, defining distinct filter blocks; and cutting each of the distinct filter blocks into distinct sets of filter elements.
depositing one of a plurality of distinct filter films on one of a like number of distinct substrates, defining distinct filter blocks; and cutting each of the distinct filter blocks into distinct sets of filter elements.
28. The method of claim 27:
wherein each of the distinct substrates includes a latticework of solder on a back surface thereof, one of the plurality of distinct filter films occurring on a front surface thereof; and said cutting is aligned with the latticework such that solder occurs along peripheral edges of each of the distinct substrates, defining a solder rim;
the method including:
providing a second substrate having a soldering pad; and soldering a solder rim to the solder pad.
wherein each of the distinct substrates includes a latticework of solder on a back surface thereof, one of the plurality of distinct filter films occurring on a front surface thereof; and said cutting is aligned with the latticework such that solder occurs along peripheral edges of each of the distinct substrates, defining a solder rim;
the method including:
providing a second substrate having a soldering pad; and soldering a solder rim to the solder pad.
29. The method of claim 28, the second substrate having via holes configured to pass light to a filter element.
30. A method of claim 24, further comprising:
depositing a Distributed Bragg Reflector on the front side of a substrate;
depositing a cavity on the Distributed Bragg Reflector; and depositing additional cavity material on distinct areas of the cavity.
depositing a Distributed Bragg Reflector on the front side of a substrate;
depositing a cavity on the Distributed Bragg Reflector; and depositing additional cavity material on distinct areas of the cavity.
31. The method of claim 30, wherein the cavity has a thickness t, with an order m, a shortest desired wavelength .lambda.0 and an index of refraction n(.lambda.), defined by:
t = m.lambda.0/2n(.lambda.0)
t = m.lambda.0/2n(.lambda.0)
32. The method of claim 31, wherein the additional cavity material increases the thickness t by an additional thickness .delta.t1, which shifts a resonance wavelength to a desired wavelength .lambda.1, defined by:
.delta.0~=m(.lambda.1/n(.lambda.1/n(.lambda.1)-.lambda.0/n(.lambda.0))/2
.delta.0~=m(.lambda.1/n(.lambda.1/n(.lambda.1)-.lambda.0/n(.lambda.0))/2
33. The method of claim 30, including positioning a screen configured to focus cavity material on one of the distinct areas of the cavity.
34. The method of claim 32, including depositing cavity material iteratively with a predetermined thickness.
35. The method of claim 34, wherein the predetermined thickness corresponds with .delta.t1.
36 The method of claim 34, wherein the predetermined thickness varies non-linearly.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US38660499A | 1999-08-31 | 1999-08-31 | |
| US09/386,604 | 1999-08-31 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2317133A1 true CA2317133A1 (en) | 2001-02-28 |
Family
ID=23526296
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2317133 Abandoned CA2317133A1 (en) | 1999-08-31 | 2000-08-30 | Tunable laser and manufacturing method therefor |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2317133A1 (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US20110063720A1 (en) * | 2009-09-15 | 2011-03-17 | Christopher Lin | Optical amplifiers using switched filter devices |
-
2000
- 2000-08-30 CA CA 2317133 patent/CA2317133A1/en not_active Abandoned
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US20110063720A1 (en) * | 2009-09-15 | 2011-03-17 | Christopher Lin | Optical amplifiers using switched filter devices |
| US8294981B2 (en) * | 2009-09-15 | 2012-10-23 | Oclaro Technology Limited | Optical amplifiers using switched filter devices |
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