The present invention relates to a laser device, and in particular, though not limited to a laser device for producing light, and the invention also relates to a light signal generating device. The invention also relates to a method for producing light from a semiconductor waveguide, and to an optical resonator.
In this specification and claims unless specifically stated otherwise, the term “reflectivity” means the amplitude of the reflectivity, and the term “transmission” means the amplitude of the transmission.
Semiconductor laser devices such as optical resonators, light generators, light detectors, waveguides, amplifiers, splitters, interferometers, modulators, multiplexers and the like are commonly used in telecommunications in the transmission and reception of data by optical signals. However, due to the nature of such components, in general, it is necessary to produce all such components as discrete components, which must be subsequently assembled together. The assembly of such components is a complex and time consuming task, due to the relatively small size of the components, and furthermore, the assembly of such components requires that they be assembled with a considerable degree of precision, due to the requirement that the light guiding region of each component must be accurately aligned with the light guiding region of its next adjacent component. Misalignment of a light guiding region of one component with that of its adjacent component or components results in loss of some or all of the light signal.
It would therefore be desirable if all such components required to produce, for example, an optical signal transmitter could be integrally formed on a single semiconductor chip. However, known optical resonators and light generators such as laser devices do not lend themselves to integration with other components on a single semiconductor chip.
Optical resonators and laser light generators such as ridge waveguides comprise a light guiding region, which in the case of a light generator comprises an active region which is provided by one or more quantum wells or quantum dots. The active region is located between an upper cladding layer and a lower cladding layer. A longitudinally extending ridge which is formed in the upper cladding layer defines the lateral width of the light guiding region in the active layer. The upper cladding layer typically is doped to be p-type, and the lower cladding layer is doped to be n-type. An electrical current pumped through the active region from the upper p-type cladding layer to the lower n-type cladding layer produces light in the active region. Lasing requires optical feedback in the light guiding region. In known laser devices, for example, in Fabry-Perot cavities, optical feedback is achieved by cleaving the ends of the waveguide to form two reflective facets at the respective opposite longitudinally spaced apart ends of the light guiding region, which reflect the light back into the light guiding region. Due to this requirement to cleave the ends of an optical resonator, optical resonators must therefore be formed as discrete components.
There is therefore a need to provide a laser device, for example, an optical resonator, a light signal generating device and the like which would lend itself to being integrally formed with other optical components on a semiconductor chip.
The present invention is directed towards providing such a laser device, and the invention is also directed towards providing a light signal generating device, an optical resonator and a method for generating a light signal in a laser device.
According to the invention there is provided a laser device comprising a waveguide, a longitudinally extending light guiding region defined within the waveguide, at least two reflecting means at locations spaced apart longitudinally relative to the light guiding region and at least one of the reflecting means being located intermediate longitudinally spaced apart ends of the waveguide for partially reflecting light being guided in the light guiding region, the amplitude of the reflectivity of each reflecting means being at least 2%, and the amplitude of the combined reflectivity of the respective reflecting means is such that lasing is independent of the reflectivity of at least one of any facets in which the light guiding region may terminate.
In one embodiment of the invention at least two reflecting means are provided intermediate the longitudinally spaced apart ends of the waveguide, and the amplitude of the combined reflectivity of the respective reflecting means is such that lasing is independent of the reflectivity of any of the facets in which the light guiding region terminates.
In another embodiment of the invention the reflecting means are located intermediate the opposite longitudinally spaced apart ends of the light guiding region.
Preferably, the reflecting means are located in the waveguide.
In one embodiment of the invention the amplitude of reflectivity of each reflecting means lies in the range of 2% to 20%. Preferably, the amplitude of the reflectivity of each reflecting means lies in the range of 5% to 15%. Advantageously, the amplitude of the reflectivity of each reflecting means is approximately 10%.
In another embodiment of the invention each reflecting means extends into the light guiding region.
In a further embodiment of the invention the light guiding region comprises an active region, and each reflecting means extends into the light guiding region to a location spaced apart from the active region.
In another embodiment of the invention at least four reflecting means are provided.
Preferably, at least six reflecting means are provided.
In another embodiment of the invention the outer two of the reflecting means define a volume in the light guiding region in which lasing occurs.
In a further embodiment of the invention each reflecting means comprises a refractive index altering means for altering the refractive index of the light guiding region adjacent the location of the refractive index altering means.
In a still further embodiment of the invention each reflecting means is formed by a slot located in the waveguide for providing the partial reflection of the light in the light guiding region adjacent the slot.
Preferably, each reflecting slot extends substantially laterally of the light guiding region.
In another embodiment of the invention at least some of the reflecting slots are at least partially tilled with a reflecting medium.
In a further embodiment of the invention the reflecting medium is a metal material.
In another embodiment of the invention the waveguide is provided by a ridge waveguide having an elongated ridge extending longitudinally along the waveguide, the ridge defining the light guiding region.
In another embodiment of the invention each reflecting means is located in the ridge.
In a further embodiment of the invention each reflecting means extends into the ridge to a depth substantially similar to the depth of the ridge.
In a still further embodiment of the invention the light guiding region extends into the ridge.
In another embodiment of the invention the waveguide comprises an active layer located between an upper cladding layer and a lower cladding layer, the active layer forming the active region.
In a further embodiment of the invention the ridge is formed in the upper cladding layer and defines the lateral width of the light guiding region in the active layer.
In another embodiment of the invention each reflecting means extends into a portion of the light guiding region defined in the upper cladding layer, and terminates at a location therein spaced apart from the active layer.
In a still further embodiment of the invention the waveguide is in the form of an optical resonator.
In another embodiment of the invention the laser device is a tuneable laser device, and the light guiding region defines a first light guiding region and a second light guiding region communicating with the first light guiding region, at least two of the reflecting means being at locations spaced apart longitudinally relative to the first light guiding region to produce a first mirror loss spectrum associated with the first light guiding region with minimum peak values at respective first wavelength values, at least two of the reflecting means at locations spaced apart longitudinally relative to the second light guiding region to produce a second mirror loss spectrum associated with the second light guiding region with minimum peak values at respective second wavelength values, and a refractive index varying means for selectively varying the refractive index of at least the first light guiding region for in turn varying the first mirror loss spectrum until one of the first wavelength values is similar to one of the second wavelength values to produce light of a selected wavelength.
In a further embodiment of the invention the refractive index varying means comprises a means for injecting a first electrical current into the first light guiding region for altering the refractive index thereof. In a still further embodiment of the invention a means is provided for varying the first current for in turn varying the refractive index of the first light guiding region.
In one embodiment of the invention the refractive index varying means comprises a means for injecting a second electrical current into the second light guiding region for varying the refractive index thereof.
In a still further embodiment of the invention a means is provided for varying the second current for in turn varying the refractive index of the second light guiding region.
In a further embodiment of the invention the means for injecting the first and second currents are operable independently of each other for independently varying the refractive indices of the respective first and second light guiding regions.
Preferably, an electrical isolating means is provided for electrically isolating the first and second light guiding regions from each other.
In another embodiment of the invention the light guiding region defines a third light guiding region intermediate the first and second light guiding regions and communicating therewith, the active region extending in the third light guiding region, and the third light guiding region being adapted to be pumped with an electrical current for generating light therein.
Preferably, respective electrical isolating means are provided for substantially electrically isolating the third light guiding region from the respective first and second light guiding regions.
In another embodiment of the invention the first and second light guiding regions are passive regions.
In another embodiment of the invention the first and second light guiding regions are active regions.
In a further embodiment of the invention the active region extends into the first and second light guiding regions, and the first and second light guiding regions are adapted to be pumped with an electrical current.
In another embodiment of the invention the waveguide is a semiconductor laser.
In another embodiment of the invention the waveguide comprises a laser diode for producing light.
In a further embodiment of the invention the waveguide is adapted for receiving a pumping current.
The invention also provides a light signal generating device comprising a waveguide defining a laser device according to the invention for producing light, and an optical component integrally formed with the laser device, a light guiding region being defined in the waveguide, which forms the light guiding region of the laser device and a light guiding region of the optical component.
In one embodiment of the invention the waveguide comprises an active region located between an upper cladding layer and a lower cladding layer, the active region forming the light guiding region of the laser device and the optical component, and a longitudinally extending ridge being formed in the upper cladding layer defining the lateral width of the light guiding region of at least the laser device.
In another embodiment of the invention the ridge defines the lateral width of the light guiding region of the optical component.
In a further embodiment of the invention an electrical isolating means is provided for substantially electrically isolating the laser device from the optical component.
In a further embodiment of the invention the electrical isolating means is formed by an isolating slot extending into the ridge intermediate the laser device and the optical component.
In a still further embodiment of the invention the laser device and the optical component are integrally formed in a single piece of material.
In another embodiment of the invention the laser device and the optical component are integrally formed in a single piece of semiconductor material.
In another embodiment of the invention the optical component is selected from one or more of the following:
The invention also provides a light signal generating device comprising an elongated waveguide formed on a single piece of semiconductor material, an elongated longitudinally extending light guiding region being defined in the waveguide, the light guiding region defining a first light guiding region of a light generating device and a second light guiding region of an optical component, the second light guiding region communication with the first light guiding region for receiving light generated therein, at least two reflecting means at locations spaced apart longitudinally relative to the first light guiding region and at least one of the reflecting means being located intermediate longitudinally spaced apart ends of the waveguide for partially reflecting light in the first light guiding region, the amplitude of the reflectivity of each reflecting means being at least 2%, and the amplitude of the combined reflectivity of the respective reflecting means being such that lasing is independent of the reflectivity of at least one of any facets in which the light guiding region may terminate.
In one embodiment of the invention at least two reflecting means are provided intermediate the longitudinally spaced apart ends of the waveguide, and the amplitude of the combined reflectivity of the respective reflecting means is such that lasing in the first light guiding region is independent of the reflectivity of any of the facets in which the light guiding region may terminate.
Preferably, the respective reflecting means are located in the waveguide adjacent the first light guiding region.
In one embodiment of the invention each reflecting means extends into the first light guiding region. Preferably, the light guiding region comprises an active region, and each reflecting means extends into the first light guiding region to a location spaced apart from the active region.
In one embodiment of the invention the optical component is an optical modulator, and the waveguide adjacent the second light guiding region is adapted for receiving a control voltage signal for modulating light.
In another embodiment of the invention the optical component is an optical detector, and the waveguide adjacent the second light guiding region is adapted for producing an emf across the second light guiding region in response to detecting light.
In a further embodiment of the invention an electrical isolating means for substantially electrically isolating the first light guiding region and the second light guiding region from each other is provided.
In another embodiment of the invention the waveguide adjacent the first light guiding region defines a laser diode for producing the light.
In another embodiment of the invention the first and second light guiding regions defined in the waveguide are integrally formed on a semiconductor chip.
The invention also provides an optical resonator comprising a waveguide, a longitudinally extending light guiding region defined within the waveguide, at least two reflecting means at locations spaced apart longitudinally relative to the light guiding region and at least one of the reflecting means being located intermediate longitudinally spaced apart ends of the waveguide for partially reflecting light being guided in the light guiding region, the amplitude of the reflectivity of each reflecting means being at least 2%, and the amplitude of the combined reflectivity of the respective reflecting means being such that lasing is independent of the reflectivity of at least one of any facets in which the light guiding region may terminate.
The invention further provides a method for producing light in a waveguide of the type having a longitudinally extending light guiding region defined therein, the method comprising providing at least two reflecting means at locations spaced apart longitudinally relative to the light guiding region, and at least one of the reflecting means is located intermediate longitudinally spaced apart ends of the waveguide for partially reflecting light being guided in the light guiding region, the amplitude of the reflectivity of each reflecting means being at least 2%, and the amplitude of the combined reflectivity of the respective reflecting means being such that lasing is independent of the reflectivity of at least one of any facet in which the light guiding region may terminate.
In one embodiment of the invention the laser device is of a buried waveguide structure.
The advantages of the invention are many. By virtue of the fact that lasing is independent of at least one of any of the facets at the respective opposite longitudinally spaced apart ends of the light guiding region, the laser device according to the invention is particularly suitable for being integrally formed with at least one other optical component and in many cases two or more optical components on a semiconductor chip. Where the laser device is adapted to lase independently of both facets at the respective opposite longitudinally spaced apart ends of the light guiding region, the laser device is particularly suitable for integrally forming with two or more components on a semiconductor chip, and furthermore, components can be integrally formed with the laser device at the respective opposite ends thereof.
Additionally, by virtue of the fact that lasing is achieved independently of the end facets, cleaving of the waveguide to produce reflective end facets is no longer required. Accordingly, the waveguide does not have to be formed along the natural crystalline direction of the semiconductor material, which is essential in laser devices known heretofore, since cleaving of a semiconductor material in order to produce suitably reflective end facets can only be achieved in a direction transversely of the crystalline direction of the semiconductor material.
A further advantage relates to the yield of lasers from a semiconductor wafer. The cleaving process naturally produces a distribution of lengths for the laser structure, thus varying the optical power and wavelength of such devices. Since this step is no longer required, the overall yield from a wafer increases.
A still further advantage of the laser device according to the invention is that since lasing is independent of the reflectivity of the end facets, the distance from an end facet and the one of the reflecting means, be it a slot or otherwise, which is closest to the end facet is not critical, since the phase of reflections from the end facets do not have an effect on the lasing modes.
The invention will be more clearly understood from the following description of some preferred embodiments thereof, which are given by way of example only, with reference to the accompanying drawings, in which:
Referring to the drawings and initially to
An elongated ridge 8 is formed in the upper cladding layer 5 and extends longitudinally along the waveguide 2 and defines a longitudinally extending light guiding region 9 within which light is generated and guided. The width W of the ridge 8 is selected to define the lateral width wl of the light guiding region 9 in the active layer 7. In this embodiment of the invention the lateral width wl of the light guiding region 9 is selected to restrict the guided modes of the light to one mode. The mode is guided in the transverse direction, namely, in the Y-direction by the layered construction of the waveguide 2 and the relatively high refractive index of the active layer 7. Additionally, the ridge 8 is dimensioned to allow upper and lower portions 13 and 14 of the light guiding region 9 to extend above and below the active layer 7 into the upper and lower cladding layers 5 and 6 with the width w1 of the light guiding region 9 in the lateral X-direction being greater than its height h1 in the transverse Y-direction. In this case the light guiding region extends into the ridge 8.
An upper electrically conductive layer 10 is formed on the ridge 8, and a lower electrically conductive layer 12 is formed on the lower cladding layer 6 for facilitating pumping current into the active layer 7 through the upper and lower cladding layers 5 and 6.
A plurality of reflecting means, which in this embodiment of the invention are formed by respective lateral reflection causing slots 15 formed in and extending laterally of the ridge 8. The lateral slots 15 alter the refractive index of the light guiding region 9 for partially reflecting the guided light mode in the light guiding region 9, and are equi-spaced apart a distance d1, centre-to-centre. In this embodiment of the invention the lateral slots 15 are of length l in the longitudinal Z-direction parallel to the ridge 8 and of width Win the lateral X-direction similar to the width W of the transverse ridge 8. The lateral slots 15 extend downwardly into the ridge 8 to a depth d2, which in this embodiment of the invention is similar to the depth D of the ridge 8, and extend into the portion of the wave guiding region 9 which extends into the ridge 8. However, the lateral slots 15 do not extend into the active layer 7, and are spaced apart therefrom by a depth d3 of the upper cladding layer 5 between the active layer 7 and the ridge 8.
In this embodiment of the invention the lateral slots 15 extend into the light guiding region 9 such that there is a significant overlap of the fundamental mode intensity with each lateral slot 15 sufficient to cause a relatively large perturbation of the mode at the lateral slot 15, such that a relatively large reflectivity, 2% or greater is obtained at each lateral slot 15. Preferably, each lateral slot 15 produces a reflectivity in the range of 2% and 20%, and ideally a reflectivity of 10% approximately. The number of lateral slots 15 and their respective reflectivities is selected so that the combined reflectivity of the lateral slots is sufficient for lasing independently without the necessity for reflectivity from any other feedback elements, such as the end facets 18 and 19 at the respective opposite ends of the light guiding region 9.
By producing the laser device 1 according to the invention to lase independently of the reflectivity of the ends 18 and 19 of the waveguide 2 irrespective of whether they are reflective end facets or not, the laser device 1 according to the invention lends itself readily to integration with other optical components on a single integrated semiconductor chip.
Accordingly, referring now to
In this embodiment of the invention an optical resonator 40 within which light is generated is formed in the waveguide 31, and an optical modulator 41 is also formed in the waveguide 31 which communicates with the optical resonator 40 and modulates the light produced by the optical resonator 40 for producing light signals for use, for example, in the transmission of data by a telecommunications system. For convenience the optical modulator 41 is illustrated in block representation only. A first portion of the light guiding region 39 forms a first light guiding region 44 of the optical resonator 40, and a second portion of the light guiding region 39 forms a second light guiding region 45 of the optical modulator 41. A portion 51 of the waveguide 31 between the optical resonator 40 and the optical modulator 41 forms a light guiding region 52 for guiding light from the optical resonator 40 to the optical modulator 41. In order to minimise absorption of light in the optical modulator 41 and in the portion 51 of the waveguide 31, the portions of the laser device 30 forming the optical modulator 41 and the portion 51 are treated to increase the bandgap of the optical modulator 41 and the portion 51. Any suitable treatment to increase the bandgap may be used, for example, intermixing, resulting from subjecting the portions of the waveguide forming the optical modulator 41 and the portion 51 to a relatively high temperature for a relatively short duration. Such treatments will be well known to those skilled in the art.
The ridge 36 is dimensioned for defining the light guiding region 39 so that the light guiding region 39 is substantially similar to the light guiding region 9 of the laser device 1 with upper and lower portions 47 and 48 of the light guiding region 39 extending above and below the active layer 35 into the upper cladding layer 33 and the lower cladding layer 34, respectively. The upper portion 47 of the light guiding region 39 which extends into the upper cladding layer 33 also extends into the ridge 36.
A plurality of equi-spaced apart lateral reflecting slots 49 of length l extend into and laterally of the ridge 36 to a depth d2 substantially similar to the depth D of the ridge 36, in similar fashion as the lateral slots 15 extend into the ridge 8 of the laser device 1. The number of lateral slots 49, their combined reflectivity and the gain of the active layer 35 are selected as described with reference to the laser device 1 so that pumping of the optical resonator 40 with a suitable pumping current results in lasing in the optical resonator 40 between the outermost lateral slots 49a and 49b. Light lasing in the first light guiding region 44 is guided through the light guiding region 52 into the second light guiding region 45 of the modulator 41, where the light is modulated and transmitted through an end 50 of the light guiding region 39.
The electrically conductive layer 37 on the ridge 36 is severed at 53 in order that the optical resonator 40 can be pumped and the optical modulator 41 can be operated independently of each other. The optical resonator 40 is pumped by applying a current through the electrically conductive layers 37 and 38 adjacent the resonator 40, and the light is modulated by applying the appropriate electrical signals to the optical modulator 41 as will be well known to those skilled in the art.
In this embodiment of the invention the outermost lateral slots 49a and 49b define the optical resonator 40 and the length of the first light guiding region 44 within which lasing occurs to produce the light which is communicated into the second light guiding region 45 of the modulator 41.
Referring now to
Referring now to
The tuneable laser device 72 is a three-section tuneable laser device 72 comprising a central gain section 75 located between left and right feedback sections 76 and 77. The first light guiding region 44 forms the light guiding region for the gain section 75 and the first and second feedback sections 76 and 77. A plurality of first and second lateral slots 78 and 79 are formed in the first and second feedback sections 76 and 77, respectively. The first and second lateral slots are substantially similar to the lateral slots 15 of the laser device 1 and extend the width and almost the depth of the ridge 36 to form respective first and second optical resonators 80 and 81 within which lasing occurs when the first and second optical resonators 80 and 81 and the gain section 75 are pumped, or when the gain section 75 is pumped. The reflectivity of each of the first slots 78 is sufficient such that lasing occurs in the first feedback section 76 independently without the necessity for reflectivity from any other feedback element such as the opposite ends 82 and 83 of the light guiding region 39. Similarly the reflectivity of each of the second slots 78 is sufficient such that lasing occurs in the second feedback section 77 independently without the necessity for reflectivity from any other feedback element such as the opposite ends 82 and 83 of the light guiding region 39.
In this embodiment of the invention the first lateral slots 78 are equi-spaced apart from each other a distance dl, centre to centre, and the second lateral slots 79 are equi-spaced apart from each other a distance dr centre to centre. However, the spacing between the first lateral slots 78 is different to the spacing between the second lateral slots 79 so that the respective mirror loss spectra produced by the first and second optical resonators 80 and 81 are different. The first mirror loss spectrum which is produced by the first optical resonator 80 and the second mirror loss spectrum which is produced by the second optical resonator 81 are such that only one of the minimum peak values of each first and second mirror loss spectrum occurs at the same wavelength value.
Varying the refractive index of the portion of the first light guiding region 44 which forms the first optical resonator 80 relative to the refractive index of the portion of the first light guiding region 44 which forms the second optical resonator 81, results in variation of the wavelength at which the two minimum peak values of the respective first and second mirror loss spectra coincide, thereby facilitating tuning of the laser device 72 to produce light of the selectable wavelengths.
In this embodiment of the invention the first and second feedback sections 76 and 77 may be active or passive. If the feedback sections 76 and 77 are active, then all three sections, namely, the gain section 75 and the two feedback sections 76 and 77 are pumped independently of each other with respective pumping currents. The pumping currents for pumping the first and second feedback sections 76 and 77 are variable independently of each other. By varying the currents with which the first and second feedback sections 76 and 77 are being pumped, the refractive indices of the light guiding regions of the respective first and second feedback sections 76 and 77 are varied, thereby varying the minimum peak values of the respective first and second mirror loss spectra produced by the first and second resonators 80 and 81, for in turn varying the wavelength at which two minimum peak values, one from each of the first and second mirror loss spectra occur for in turn varying the wavelength of the light produced by the tuneable laser device 70. This is described in more detail below.
Where the first and second feedback sections 76 and 77 are passive sections, only the gain section 75 is pumped with a pumping current, and respective tuning currents, which are variable independently of each other are injected into the first and second feedback sections 76 and 77 for varying the refractive indices of the light guiding regions of the first and second feedback sections 76 and 77 for in turn varying the wavelength at which the minimum peak values of the first and second mirror loss spectra produced by the first and second resonators 80 and 81 occur, for in turn varying the wavelength of light produced by the tuneable laser device 72.
Accordingly, tuning of the tuneable laser device 72 is carried out either by independently varying the pumping currents with which the first and second feedback sections 76 and 77 are pumped when the first and second feedback sections 76 and 77 are active sections, or by independently varying the tuning currents injected into the first and second feedback sections 76 and 77 when the first and second feedback sections 76 and 77 are passive sections. This type of tuning is effectively a Vernier type tuning.
In all cases the pumping currents with which the gain section 75 and the first and second feedback sections 76 and 77 are pumped are independent of each other, and the pumping currents with which the first and second feedback sections 76 and 77 are pumped are variable independently of each other. Similarly, the pumping current with which the gain section 75 is pumped and the tuning currents with which the first and second feedback sections 76 and 77 are pumped are independent of each other, and the tuning currents with which the first and second feedback sections 76 and 77 are injected are variable independently of each other.
Whether the first and second feedback sections are operated as active or passive sections is determined based on optimising between sensitivity with which the wavelength of light can be produced, and the losses at each first and second slot. Each first and second slot produces a loss, and by pumping the first and second feedback sections 76 and 77, the losses at the first and second slots 78 and 79 can be compensated for. However, by only pumping the gain section 75, and using only tuneable currents in the first and second feedback sections 76 and 77, tuning of the laser device 70 is more sensitive than when all three sections 75, 76 and 77 are pumped.
Tuning of the tuneable laser device 72 may be discontinuous whereby the wavelength of the light produced by the tuneable laser device 72 is varied in steps or hops, or continuous whereby the wavelength of the light produced by the tuneable laser is progressively varied from the lowest wavelength of the range over which the tuneable laser device 72 is tuneable to the highest of the wavelengths of the tuneable range, or vice versa. By holding the pumping current or the tuning current, as the case may be, of one of the first and second feedback sections 76 and 77 constant, while varying the pumping current or the tuning current, as the case may be, of the other of the first and second feedback sections 76 and 77, tuning is discontinuous. By simultaneously and appropriately varying the pumping currents or the tuning currents, as the case may be, of the first and second feedback sections 76 and 77, tuning of the tuneable laser 72 is continuous.
Electrical isolating means provided by laterally extending left and right isolating slots 84 and 85 are provided in the ridge 36 between the gain section 75 and the first and second feedback sections 76 and 77, respectively, so that the gain section can be pumped independently of the first and second feedback sections 76 and 77, and so that the pumping currents or the tuning currents can be applied to the first and second feedback sections 76 and 77 independently of each other and independently of the gain section 75.
By virtue of the fact that the light of the selectable wavelength is produced by the laser device 72 independently of reflection from end facets, the tuneable laser device 72 can be formed with the optical modulator 41 and the light detector 73 or with any other optical components as one single integral unit on a single semiconductor ship.
By varying the current with which the first and second feedback sections 76 and 77 are being pumped, the wavelength at which the two peak values of the power reflectivities of the respective first and second feedback sections 76 and 77 occur is varied. By holding the pumping current of one of the first and second feedback sections 76 and 77 constant while varying the current with which the other of the first and second feedback sections 76 and 77 are being pumped results in the wavelength of the light being varied in hops or steps. However, by appropriately and simultaneously varying the pumping currents with which the first and second feedback sections 76 and 77 are being pumped can be made to result in progressive variation of the wavelength of the light produced by the laser device 72 from the lowest selectable wavelength to the highest selectable wavelength, or vice versa, as discussed above.
Referring now to
In what follows the terms “first” and “second” are used interchangeably with the terms “left” and “right”, the terms “first” and “left” corresponding, and the terms “second” and “right” corresponding.
The reflectivity and the transmitivity of each lateral reflection causing slot of the laser devices according to the invention is relatively complex. For example, in a laser device according to the invention which is similar to the tuneable laser device 72 of the light signal generating device 70 of
accounting for the reflectivity of each lateral slot in turn.
However, it has been noted that if there is a gain in the first and second feedback sections 76 and 77, the feedback reflectivities can have moduli greater than unity.
The following examples demonstrate the efficacy of a number of ridge waveguides which have been formed according to the invention, and compared with ridge waveguides not according to the invention.
In this first example it is demonstrated that by increasing the number of lateral slots a single electrical contact ridge waveguide laser structure internal reflectance resulting from the lateral slots reduces the role of the reflectivity of the end facets. The epitaxial layer structure used in this example was of similar construction to that of the laser device 1 described with reference to
The lasers can be considered as three section devices—the long cavity to the bottom (which is 0.51 of the cavity length for the six slotted device, for example), short cavity at the top (1−0.82=0.18 of the cavity length), and the region containing the slots.
The general principles of the invention can be shown using data from the zero, two and six slot devices. As all the laser devices were formed from the same bar, they have the same physical length and the same waveguide effective refractive index, ng. The wavelength spacing, in nanometres for the Fabry Perot (FP) modes in a cavity of length L is given by
For the zero slot laser device this spacing was measured to be 1.6 nm, which gives a waveguide effective refractive index of 3.50. The emission spectra below threshold from each facet for the series of lasers was measured from a wavelength of 1470 nm to 1670 nm.
For devices with zero slots and two slots, the emission spectra were qualitatively the same from each of the two facets of the laser. However, for the four or more slot devices, the emission spectra from each end facet were quite different, as can be seen from the waveforms C and D of
For the Fourier transform spectrum of the two slot laser, there is a replica of the peaks at 0.76 and 0.82 to be found at 0.18 and 0.24. This is easily explained, as the short end of the laser also will produce Fabry-Perot oscillations at frequencies corresponding to (1-L) where L is the cavity length. However, for the Fourier transform G of the six slot laser this is found not to be the case. For the six slot laser, peaks in the Fourier transform G occur at 0.51, 0.58, 0.64, 0.70, 0.76 and 0.82, and there are no corresponding 1-L peaks. This is a clear indication that the cavities at each end of the laser are not well coupled, and are semi-independent. This further indicates that there is a mirror separating the end sections. This is confirmed by considering the emission spectra from the each end of the six slot laser, namely, the spectra represented by the waveforms C and D of
Accordingly, from the above it will be clearly appreciated that the mode spacing of the multi-slot devices is not determined by the spacing of the cleaved facets but rather is determined by the slots and their separation and the separation between the slots and the facets. It has thus been clearly shown that the introduction of these slots does not perturb the modes of the Fabry-Perot cavity formed by the cleaved facets but rather form their own cavities.
The relationship between the reflectivity and the slot etch-depth is exponential as the slot approaches and enters the light guiding region without actually penetrating the active layer of quantum wells, thereby increasing feedback and maintaining threshold gain values at reasonable levels. Although a comprehensive parameter extraction has not been performed, broad agreement between experiments and simulation can be obtained when a Fresnel reflectivity of r=0.1 is assumed on entering the slot, accompanied by a 40% loss in power scattered to radiation modes. With these parameters the model captures the essential features of the experimental spectra, namely, highly asymmetric emission with respect to each facet for higher slot number, and a tendency for either end of the laser to behave independently as the number of slots is increased.
When lasing, the four, six and eight slot lasers are single mode with a side-mode-suppression-ratio greater than 30 dB for currents 1.3 Ith<I<3 Ith, where Ith is the laser threshold current. The lasing spectrum for each end of the six slotted device is shown in
This example demonstrates a laser device formed by a series of etched slots at one end of the laser device with one cleaved facet at the other end. Additionally, this example shows how an integrated power sensor can be easily and efficiently implemented in the device. A ridge type laser device 100 with the configuration indicated in
Two modes of operation are distinguished. The front section 110 is pumped and back section 111 is absorbing (mode 1) and the back section 111 pumped with front section 110 absorbing (mode 2).
When the device 100 is drive in mode 2 no lasing occurs verifying that the reflectivity of the rear facet 107 is indeed negligible. If its depth is sufficient, it has been found that even a single slot provides the necessary reflectivity.
If the two section device 100 of
The detected current also depends on whether the slots are in the front section 110 or the rear section 111 of the device. If the slot is placed in the front section 110 of the device 100, but current is collected from the rear section as was done previously, the waveform B in
In this example another laser device according to the invention in which the reflecting means is provided by slots in a Fabry-Perot optical cavity is described.
Referring to
In this case the spacing dl between the left slots 130 is 97 microns, and the spacing dr between the right slots 131 is 108 microns. In this embodiment of the invention the effective refractive index of the mode is taken as Ng of 3.5. The free spectral range (FSR) for the left mirror is 3.5 nm and the free spectral range for the right mirror is 3.2 nm. The slot length l of the respective left and right slots 130 and 131 is 1 micron, and the amplitude reflection and transmission of a single slot is assumed as 0.1 and 0.8. The left and right slots 130 and 131 are deep etched into the ridge 140 to a depth of 1.3 microns, and extend into the light guiding region. All three sections of the laser device 120, namely, the central gain section 128, and the left and right sections 127 and 129 are active, in other words, all three sections are pumped. Additionally, the three sections, namely, the central gain section 128 and the left and right sections 127 and 129 can be pumped independently of each other. Suitable electrical isolating means are provided between the three sections 127, 128 and 129 by providing suitable gaps (not shown) in the electrically conductive layer on the ridge 140.
The total length of the cavity, in other words, the total length L of the light guiding region which extends between the left and right end facets 134 and 135 is 2,345 microns.
where λ is the wavelength of the longitudinal mode. This implies that the left and right end facets 134 and 135 do not contribute to the laser operation.
Referring now to
Referring now to
Referring now to
Referring now to
Two other laser devices according to the invention, indicated generally by the reference numerals 165 and 170, are illustrated in
Referring now to
Since the principle of the invention can be applied to the laser devices of buried waveguide construction of
Slotted reflectors 177 can be made in the PBH laser 175 using etched slots that are the width of the buried optical waveguide 176, or greater than the width of the optical waveguide 176 in order to increase the reflectance. Alternatively, referring to
Additionally, where it is desired to control the free spectral range (FSR) of an optical cavity, this can be achieved by using the principle of the invention. Using the principle of the invention it is possible to create optical resonators (which may be lasers), which have an FSR corresponding to a small optical cavity. For example, a 20 micron Fabry-Perot will have a very large FSR. It is not realistic to make 20 micron long semiconductor diode lasers. However, using the principle of the invention, much longer semiconductor lasers can be made which due to the design of the slots have a FSR that is equivalent to that of a 20 micron long cavity.
This can be used to create a cavity whose FSR corresponds to wavelengths or frequencies along the International Telecommunication Union (ITU) grid. This could be used to reduce the amplified spontaneous emission (ASE) from a semiconductor optical amplifier (SOA), while ensuring that the semiconductor optical amplifier operates at the required frequencies.
Referring now to
It is also important that the length of the first and second slots, in other words, the length of the first and second slots in the direction of light propagation is relatively small, typically, less than 3 μm. This is required in order to ensure that the internal loss in the respective first and second light guiding regions is minimised, since the internal losses in the light guiding regions is substantially higher under the first and second slots than elsewhere in the first and second light guiding regions, and also as a result of the fact that the dopant concentration in semiconductor material at the bottom of a slot may be less than one-tenth of the level adjacent the dopant level adjacent the top of the ridge, and thus, it would be difficult if not impossible to create a low resistance metal contact on the ridge adjacent the bottom of the first and second slots. Thus, if the length of the slots in the direction of light propagation is increased arbitrarily, a portion of the waveguide beneath the slot will remain unpumped.
While the laser device according to the invention and the embodiments thereof which have been described with reference to
While the laser devices and laser signal generating devices described with reference to the drawings have been described as being suitable for use in the optical transmission of data signals in telecommunications applications, it will be readily apparent to those skilled in the art that the laser devices and light signal generating devices will be suitable for many other purposes, for example, in the analysis of gases and other substances, such as gas chromatography and the like. Needless to say, the laser devices and the light signal generating devices may be used for many other purposes, which effectively are unlimited.
While the laser devices described with reference to the drawings have been described in general as comprising at least two reflecting means formed by at least two reflecting slots, it is envisaged in certain cases that a single reflecting means, namely, a single slot may be used. In which case, the single reflecting slot would co-operate with one of the end facets of the waveguide, however, lasing would be independent of the other end facet.
Number | Date | Country | Kind |
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S2006/0692 | Sep 2006 | IE | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/IE07/00086 | 9/20/2007 | WO | 00 | 6/23/2009 |