The present invention in general relates to semiconductor light emitting devices and in particular to methods of altering the spatial emission patterns of such devices.
A laser diode is a laser where the active medium is a semiconductor p-n junction similar to that found in an edge-emitting light emitting diode or a super luminscent diode. In a laser diode, the semiconductor crystal is fashioned into a laminar rectangle which is very thin in one direction and rectangular in the other two. The top of the crystal is n-doped, and the bottom is p-doped, resulting in a large, flat p-n or p-i-n junction. The two ends of the crystal are cleaved so as to form perfectly smooth, parallel edges; two reflective parallel edges are called a Fabry-Perot cavity. Photons emitted in precisely the right direction will be reflected several times from each end face before they are emitted. Each time it passes through the cavity, the light is amplified by stimulated emission and losses occur through light scattering or absorbtion. Hence, if there is more amplification than loss, the diode begins to “lase”. Fabry-Perot laser diodes incorporating discrete refractive index perturbations have been shown to operate in a single longitudinal mode over a wide temperature range, see for example, EP 1 214 763 in the name of Trinity College Dublin and PCT/IE/04/00091 of Eblana Photonics Limited (a copy of which is included in appendix 1 of this application) the contents of which are incorporated herein explicitly and by reference. So called “slotted lasers”, which achieve single longitudinal mode emission by means of optical feedback resulting from the formation of slot features along the laser cavity, are also disclosed in Irish Patent No S82521 (National University of Ireland, Cork), the contents of which are incorporated by reference. The type of laser diode just described is called a double heterostructure laser diode or quantom well laser diode if the gain medium consists of quantom wells.
For ease of description the term “slot” will be taken to include a slot etched, or otherwise formed, in a part of the laser cavity as well as any other form of discrete refractive index perturbation which has the effect of modifying optical feedback within the cavity. Exemplary refractive index perturbation means are disclosed in the prior art cited above.
A known problem with these prior art light emitting devices (and laser diodes in general) is that their far-field emission patterns are elliptical and astigmatic in nature. This is the angle of the laser emission cone is different in the directions perpendicular and parallel to the p-n junction plane. This leads to the well known effect called astigmatism where the focal points and divergence of the emission is different in the two perpendicular planes, the impact of which is that the laser emission cannot be properly collimated or brought to a focus using simple lenses. It will be appreciated, in contrast to for example a circular non-astigmatic far-field emission pattern, that astigmatic emission presents significant focussing difficulties. Optical focussing solutions (for example aspherical lenses) are available which correct for the elliptical pattern. However, these solutions are generally extremely complex and expensive.
Jing-Kaung Chen and SiChen Lee, “AlGaAs/GaAs Visible Ridge Waveguide Laser with Multicavity Structure”, IEEE Journal of Quantum Electronics. Vol. QE-23, No. 23, No. 8, August 1987 discloses a ridge waveguide laser exhibiting fundamental transverse mode operation with output power more than 13 mW under pulsed operation.
U.S. Pat. No. 4,783,788 discloses semiconductor lasers which operate in the fundamental lateral and transverse mode.
WO02/31863 discloses a ridge waveguide device having a defect defining region in which the width of the ridge is greater in the defect defining region than in adjoining regions of the ridge.
US2002/0085604 discloses a laser diode whose output is largely single mode.
Another solution which has been used in the prior art to ameliorate the ellipticality of laser diode far-field emission patterns is that of burying the layer heterostructures within another lower index semiconductor material. In these devices, a layer of low bandgap material is sandwiched between two high bandgap layers. One commonly-used pair of materials is GaAs with AlGaAs. The process for burying the laser heterostructures is however extremely complex and requires at least one re-growth stage (a single mode laser requires at least two re-growth stages). A further disadvantage is an effective reduction in power output due to a reduction in the laser cross sectional area and hence volume. However, an advantage of a buried heterostructure laser is its greater efficiency of converting electrical energy to light energy since the region where free electrons and holes exist simultaneously—the “active” region-is now also more confined to the thin middle layer. This means that many more of the electron-hole pairs can contribute to amplification-not so many are left out in the poorly amplifying periphery. In addition, light is reflected from the heterojunction; hence, the light is confined to the region where the amplification takes place.
Accordingly, it is an object of this invention to provide for an improved method of providing a substantially circular far-field emission pattern from a light emitting device. Another object is to provide a method which does not require re-growth. A further object of the invention is to provide a method which can be used easily with other mode engineering techniques to provide simple wavelength circular emissions without re-growth.
In the present invention, one or more (refractive) index perturbations substantially aligned to effect the electromagnetic field distribution in one direction in a light emitting device may be used to alter the electromagnetic field distributions along another direction within the device. For example, slots placed in a longitudinal series in a laser to effect the longitudinal electromagnetic field distribution with suitable selection of the slots (index perturbations), the far-field emission pattern may also be altered to a desired configuration (for example a circular pattern). Previously, the main purpose of introducing(refractive) index perturbations was to modify the emission wavelength spectrum of laser diodes. The present application ameliorates the ellipticality of laser diode far-field emission patterns without burying the layer heterostructures within another lower index semiconductor material.
The present invention provides for the introduction of index perturbations into a laser device to modify its far-field emission pattern without altering the spectral characteristics of a laser as well, i.e. the distributions in these other directions may be independently modified without affecting the spectral characteristics of the device in question. This is because it is primarily the positions of the interfaces associated with the slot features along the cavity, which influence the spectral characteristics of a laser diode, while it is a combination of the shape, length and/or depth of the perturbations which determine how the lateral and transverse electromagnetic field distributions within the device will be modified. Thus it is possible to use a pattern of index perturbation principally arranged along one direction to modify electromagnetic field distributions in one or both of the other two orthogonal directions, while having either a negligible or even a profound effect on the spectral characteristics of the device. Although, it will be appreciated that modifying the spectral characteristics may alter the far-field emission pattern.
One embodiment of the invention provides a method of modifying the electromagnetic field distributions in one or more directions in a laser/light emitter by using a pattern of index perturbations principally arranged in one direction.
Another embodiment provides a way of modifying the field distribution in one direction in a laser light emitter using a pattern of index perturbations principally arranged along another direction. A further embodiment provides a method of modifying the far-field emission pattern independently of the spectral characteristics using a set of index perturbations arranged principally in one direction.
Another embodiment provides a method of manufacturing a semiconductor device for emitting light in a first direction comprising the step of creating at least one index perturbation in the semiconductor device aligned in a direction substantially transverse to the first direction to achieve a desired spatial distribution of the emission. The at least one index perturbation may comprise a pattern of index perturbations. Suitably, the semiconductor device comprises a laser, for example a ridge waveguide laser. More particularly, the semiconductor device may be a slotted laser, in which case the index perturbation may be provides as a slot. One or more of the following: slot depth, slot length and slot shape may be suitably selected to contribute to the desired emission pattern.
The index perturbation may be provided by one or the following or a combination thereof: introduction of a dopant, etching and ion implantation.
A further embodiment of the invention provides a method of modifying the field distribution in one direction in a laser light emitter using a pattern of index perturbations principally arranged along another direction.
A further embodiment of the invention provides a method of modifying the far-field emission pattern of a semiconductor light emitting device independently of the spectral characteristics using a set of index perturbations arranged principally in one direction.
Still a further embodiment provides a semiconductor light emitting device comprising a longitudinal active region for producing the light and one or more effective refractive index perturbations disposed along the longitudinal axis and aligned transverse thereto, wherein at least one of the refractive index perturbations is dimensioned to effect a desired emission pattern from the active region. The device may be a laser, for example a ridge waveguide laser. If the laser comprises a ridge, the at least one perturbation may be formed by a slot defined in the ridge. In which case, the length, depth and or shape of the slot may be selected to contribute to a desired far-field emission pattern. Similarly, the at least one effective refractive index perturbation may be formed by one or more indentations defined in the side of the ridge of the laser or more generally by indentations defined in the ridge. The semiconductor light emitting device may also be an LED, for example an edge-emitting LED.
A still further embodiment of the invention provides a method of manufacturing a laser comprising the steps of: (1) forming a laser cavity with a lasing medium, the laser cavity defining a longitudinally extending optical path and having a facet at either end, and (2) forming a plurality of perturbations into the laser cavity, wherein the shape and/or dimensions of the plurality of perturbations are selected to provide a desired far-field emission pattern for the laser. The cavity may be formed with a longitudinally extending ridge with at least one perturbation provided by etching a slot in the ridge and wherein at least one of the following:
a) slot depth,
b) slot width, and
c) slot shape,
may be selected to contribute to the desired emission pattern. Similarly, the perturbations may be provided by etching one or more indentations in the side of the ridge.
Yet another embodiment of the invention provides a ridge laser device having a cavity defining a longitudinal axis of the device, the laser being adapted to provide light having a far field emission pattern and wherein, a plurality of index perturbations are provided along the longitudinal cavity, the position of the index perturbations within the cavity effecting a spectral tuning of the laser, the dimension of the index perturbations being selected to effect a modification of the far field emission pattern.
A still further embodiment provides a method of tuning a ridge laser, the method including the steps of providing a first pattern of perturbations to define the spectral emission characteristics of the laser and a second pattern of perturbations to define the spatial emission characteristics of the laser.
A further embodiment provides a ridge wave-guide laser having a substantially non-astigamtic emission pattern. The invention extends to playback devices incorporating such a laser, for example optical disc playback devices. Exemplary optical playback devices would include DVD or CD players. Similarly, the invention extends to recording devices incorporating such a laser, for example optical disc recording devices. Exemplary optical recording devices would include DVD or CD recorders. The invention also extends to display devices comprising such a ridge wave-guide laser.
A further embodiment provides an edge-emitting LED having a substantially non-astigamtic emission pattern. The invention also extends to a light coupler or photo amplifier comprising such an edge-emitting LED
This technique allows for the manufacture of laser diodes with low emission astigmatism, which simplifies the coupling of the emitted light into waveguides and optical fibres. This improved coupling in turns allows for laser diode components which consume less power by being operated less aggressively and consequently for laser driver circuits which consume less power. It also allows for the possibility of lasers with higher powers before the onset of catastrophic optical facet damage.
The technique has application in the manufacture of light emitting devices for sensing and recording systems, e.g. DVD players where a circular and non-astigmatic emission pattern would be preferred. The technique also has application in the field of displays, where it is generally preferred that the pattern caused by individual light devices is non-elliptical and non-astigmatic in nature.
The invention will now be described in greater detail with reference to the following drawings in which:
The invention is based on the principle explained below of how an index perturbation (or pattern thereof) may be used to modify the electromagnetic field distribution within a semiconductor light-emitting device, such as a edge-emitting LED or laser diode. Desirably such an index perturbation is located along one direction of the device such as the longitudinal direction (but aligned in a transverse direction). This is in addition to the conventional uses for such index perturbations, for example in the case of the laser diode, to the spectral characteristics of the device. In fact, as will be explained below, the two aspects may be controlled independently. It should noted that while the concept which forms the basis of this invention is illustrated using a slotted laser having rectangular slot features, that the concept may be applied to other types of light emitting devices, e.g. LEDs, where the electromagnetic field distribution may also be altered by introducing perturbations. Moreover, effects similar to those which will be described here are attainable using index perturbations of other shapes. For examples of such index perturbations see International Patent Application No. PCT/IE04/00091, the entire contents of which are incorporated herein by reference.
In fact, there may be several advantages to using the described perturbations of PCT patent Application No. PCT/IE04/00091 and in particular that the slot lengths may readily be alterable to attain a particular spatial emission pattern without significantly altering the spectral characteristics, thus providing for relatively simple independent control of the spatial emission pattern and spectral characteristics.
However, for ease of explanation, the invention will now be described with reference to a conventional slotted laser with rectangular slots. While the description of the present invention which follows refers primarily to the case where the perturbations are defined by slots etched along the device it will be appreciated by a person skilled in the art that the teaching of the invention is equally applicable to other forms of perturbations (for example modifying the refractive index profile by employing doping or ion implantation methods).
Slotted lasers including those of the present invention may be used to obtain a desired spectral characteristic, e.g. a fundamental frequency from the laser. Moreover, the desired spectral characteristic may be defined by the single mode suppression ratio of the device. For most single frequency applications, a Side Mode Suppression Ratio of 30 dB or more is desired and in some a ratio of 35 dB or more is required. Devices having these figures may be readily fabricated using methods of the present invention.
The mechanism where by slotted lasers achieve their single mode performance may be exemplarily described as follows:
It is well known that the free spectral range of a laser is given by
and as such is in effect determined by the cavity length, L, of the device, where, Δλ, is the free spectral range, λ, is the free space wavelength of the light and, neff, is the effective index of the optical mode in the laser cavity.
However, it is observed that by placing reflective interfaces in the laser cavity at intervals separated L/N it is possible to enhance approximately every Nth Fabry Perot mode, where L is again the cavity length of the laser and N is an integer. This is essentially what occurs in a slotted laser, except for the fact that when a rectangular slot feature is etched into a laser cavity, two reflective interfaces are created simultaneously.
In order for rectangular slots to be efficient at modifying the mirror spectrum of a laser diode, the length of the slot must generally obey the following relation.
Adherence to this criterion ensures constructive interfere between reflections from both sides of the index perturbations. It will be appreciated that in the context of the present invention, the term ‘slot length’ refers to the distance between the longitudinal slot faces in the device material, i.e. slot length is measured along the direction of light propagation. Now consider the configuration (pattern) of slots shown in
However, in accordance with the teachings of the present invention it will be understood that slot patterns with different slot lengths will have different effects on the transverse electromagnetic field distribution within the laser diode, and thus can used to tailor the transverse far-field emission profile. The basis of this effect is as follows. As the far-field emission profile is the Fourier transform of the near-field intensity profile, the larger the extent of the near-field profile the narrower the far-field emission profile will be. Now consider
Experimental Data
The slot patterns which produced the perpendicular and parallel farfield profiles shown in
The
In contrast
However, a pattern of index perturbation arranged principally along one direction could be designed to accomplish this task. In the exemplary case shown, it was not necessary as modifying the transverse (perpendicular) far field was enough to remove the astigmatism from the laser emission pattern.
The present invention provides a method for making reliable, easily manufacturable and hence relatively inexpensive light emitting devices having a desired far-field emission pattern. Such devices have instant application in the field of fibre-optic telecommunications both in transmission lasers and also semiconductor optical-amplifiers since it may be used to improve coupling efficiency between the semiconductor waveguide and the input and output optical fibres. The devices also have application in laser display systems. It will be appreciated that the availability of devices with a desired far-field emission pattern (e.g. circular) has knock-on effects to other parts of systems and includes benefits such as improved coupling efficiency, reduced power, less complex optics.
Devices described herein also have instant application in laser playback and recording devices, where to date complicated methods have been employed to overcome the elliptical nature of the laser beams.
It should be noted that many other applications of the method and devices are considered possible and the relevant applications are not limited to those specifically recited above. Also, the present invention may be embodied in other specific forms. The embodiments described above are to be considered in all respects as illustrative and not restrictive in any manner.
A copy of PCT/IE/04/00091 is included below and incorporated specifically herein as the devices and/or slot patterns described may be advantageously employed in the context of the present invention. Similarly, the methods of manufacture described may also be employed. Thus the scope of the present invention extends to include the use of these devices/slot patterns and methods in conjunction with this invention. It will be appreciated that the reference numerals and drawings referred to below are contained in the drawings for Appendix 1, which are appended to the drawings of the present invention but marked as the drawings for appendix 1.
The present invention relates a semiconductor laser, in particular such a laser which operates with substantially single longitudinal mode emission.
Achieving single mode emission by introducing perturbations at prescribed positions along the length of a device is known, see EP 1 214 763 (Trinity College Dublin) the contents of which are incorporated herein by reference. So called “slotted lasers”, which achieve single longitudinal mode emission by means of optical feedback resulting from the etching of slot features along the laser cavity, are also disclosed in Irish Patent No S82521 (National University of Ireland, Cork).
In general terms, the perturbations may be caused by any index altering means which modifies the refractive index profile of the waveguide to an appropriate degree to manipulate optical feedback and hence the spectral content of the device. While the description of the present invention which follows refers primarily to the case where the perturbations are defined by slots etched along the device it will be appreciated by a person skilled in the art that the teaching of the invention is equally applicable to other forms of perturbations (for example modifying the refractive index profile by employing doping or ion implantation methods).
The term ‘slot length’ (designated Lslot in
The mechanism where by slotted lasers achieve their single mode performance may be described as follows:
It is well known that the free spectral range of a laser is given by
and as such is in effect determined by the cavity length, L, of the device. Where, Δλ, is the free spectral range, λ, is the free space wavelength of the light and, neff, is the effective index of the optical mode in the laser cavity. However it is observed that by placing reflective interfaces in the laser cavity at intervals separated L/N it is possible to enhance approximately every Nth Fabry Perot mode. Where L is again the cavity length of the laser and N is an integer. This is essentially what occurs in a slotted laser, except for the fact that when a rectangular slot feature is etched into a laser cavity, two reflective interfaces are created simultaneously. What is important to note here is that each of the reflective interfaces created provides a similar amount of optical feedback. It is also important to realise that the length of the etched slot features must be kept reasonably small (typically <3 μm). The principal reasons for this are the following: Firstly, the internal loss in the waveguide beneath slots is substantially higher that elsewhere in the cavity. Secondly, since the dopant concentration in the semiconductor material below the bottom of a slot may be less than one tenth of that in the cap layer it is impossible to create a low resistance metal contact on this material. This means that if the length of a slot feature is increased arbitrarily, then a portion of material beneath the slot will remain unpumped.
In order to accurately specify the emission wavelength of a device it is necessary to be able to position all the edges of the slot features relative to each other with an accuracy that is inversely proportional to the distance between them. This can be understood by recognising that the standing wave conditions in a long cavity device are less effected by a fixed change in the length of the cavity, Δx, than the standing wave conditions in a short cavity device. (It is noted that since the facets of a device provide a significant amount of optical feedback, the positioning of these interfaces with respect to the slot features is important). As typical slotted lasers incorporate etched features, the lengths of which are less than an order of magnitude greater than the wavelength of the optical field in the laser cavity. Also given that the two interfaces of a given conventional slot feature provide a significant amount of optical feedback, then it can be appreciated that the emission wavelengths, or more precisely the mirror loss spectra of such devices, are extremely sensitive to errors in the distance between the interfaces of such a feature. The emission wavelength of a slotted laser is thus critically dependent on length of the slot features themselves. The process of accurately realising a slot feature of a given length is therefore also important.
The most important factor in determining the accuracy with which a slot feature can be implemented is the choice of lithographic technique used. This varies between ±10-20 nm for e-beam systems to ±100-200 nm optical lithography systems. Beyond the accuracy of the lithographic system itself, the procedure of realising a rectangular slot feature of a certain length is also severely hampered by the bias associated with etching process (the offset due to process bias is designated Opb in
As discussed above there are considerable difficulties in accurately specifying the emission wavelength of slotted lasers. It is an object of the present invention to address these difficulties.
It is a further object of the invention to provide manufacturing method, which addresses the problems, associated with processing bias and the resulting effect on slot positioning.
It is a still further object to provide a substantially single mode laser whose performance is less temperature dependent.
It is another object of the invention to provide a method of enhancing the free spectral range of a laser and to provide a laser having improved free spectral range.
The present invention provides a laser emitting light of substantially a single wavelength, comprising a lasing cavity with a lasing medium and primary optical feedback means in the form of a facet at either end of the cavity, the laser cavity defining a longitudinally extending optical path; and secondary optical feedback means formed by one or more effective refractive index perturbations in the lasing cavity, each perturbation defining two interfaces; wherein for at least one perturbation, only one of the two interfaces contributes to optical feedback along the optical path.
Preferably the laser comprises a ridge and at least one effective refractive index perturbation is formed by a slot defined in the ridge. In a preferred embodiment each perturbation comprises a slot formed along the ridge.
It is preferred that the contributing interface of each perturbation is substantially planar and substantially perpendicular to the longitudinally extending optical path.
Preferably at least one slot comprises a first face which is substantially planar and substantially perpendicular to the longitudinally extending optical path and a second face which is non-perpendicular to the optical path and is preferably substantially stepped, curved or angled with respect to the first face. Such slot design minimises or prevents destructive interference between interfaces.
According to the invention only the interfaces which are substantially perpendicular to the optical path contribute to optical feedback within the device, with feedback from non-perpendicular interfaces being suppressed thus improving performance characteristics of the laser.
In an alternative embodiment a laser comprising a ridge has at least one effective refractive index perturbation is formed by one or more indentations defined in the side of the ridge. Suitably each perturbation may be formed by indentations defined in the ridge.
Typically a series of effective refractive index perturbations may be employed wherein the spacing between adjacent contributing interfaces is a uniform number of quarter material wavelengths. One or more additional series of effective refractive index perturbations may be overlaid with a first series of perturbations. Such series of perturbations result in devices with a larger effective free spectral range.
In a further embodiment two or more slots are of different length (while the spacing between adjacent contributing faces is a uniform number of quarter wavelengths). The effect of such ‘chirped’ slots is that the contributing faces can result in constructive interference of the optical feedback within the cavity whereas the non-contributing faces do not since the lengths of individual slots are different from each other.
The present invention also relates to a method of manufacturing a laser comprising the steps of: (1) forming a laser cavity with a lasing medium, the laser cavity defining a longitudinally extending optical path and having a facet at either end, and (2) forming optical feedback means by introducing a plurality of perturbations into the laser cavity, each perturbation defining two longitudinal interfaces; characterised in that, the longitudinal interfaces of at least one perturbation are adapted such that only one interface contributes to optical feedback along the longitudinally extending optical path. That is to say that, for at least one perturbation, only one interface contributes to optical feedback along the longitudinally extending optical path.
Preferably the cavity is formed with a longitudinally extending ridge and at least one perturbation is provided by etching a slot in the ridge.
Preferably at least one slot is formed with a first face which is substantially planar and substantially perpendicular to the longitudinally extending optical path and a second face which is non-perpendicular to the optical path and is preferably substantially curved, stepped or angled with respect to the first face. Alternatively perturbations may be provided by etching one or more indentations in the side of the ridge.
The method of the invention improves processing tolerances and enhances temperature characteristics of the resultant laser as shall be described further below.
The invention also provides a method of enhancing the free spectral range of a laser device comprising forming a series of effective refractive index perturbations along the optical path wherein the spacing between adjacent contributing interfaces is a uniform number of quarter material wavelengths.
The invention is described in further detail below with reference to the accompanying drawings in which:
a-e) shows side profile views of a device at various at stages in the fabrication process.
Known slotted lasers suffer from the problems discussed above. These problems stem from the fact that prior art slot patterns form pairs of contributing interfaces (or interfaces providing feedback) separated by very small distances (typically <3 μm) as illustrated in
Where the length of a slot is constrained by the factors outlined above, any change in slot length will have profound effects on the mirror loss spectrum and thus emission wavelength of the device. As a result of the present invention this is no longer a problem since the spectral selectivity of the slot features is now no longer dependent upon the size of the slots themselves. The only dimensions which remain critical are the distances between those interfaces which provide a significant amount of optical feedback. Since these dimensions are typically more than an order of magnitude greater than the length of the slots themselves the accuracy with which these features have to be positioned is also relaxed by more than an order of magnitude.
Considering
The invention is based on the premise that structural features (such as slots, doped regions or the like) can be used to modify the effective refractive index profile of a device. (The effective refractive index is obtained by summing the products of the refractive index in a particular region of the laser cavity and the fraction of the optical intensity which is present in that region, and dividing this value by the integral over the spatial extent of the optical field.) Such structural features cause perturbation of the refractive index profile within the device, thus influencing performance characteristics. In other words, the faces of a slot etched in the ridge of a laser such as that shown in
Different types of etched features, which fulfill the requirement of providing only one contributing interface, are discussed below. Also discussed below are example patterns of such features that enable single longitudinal emission at a specified wavelength over an extended temperature range. It is noted that the patterns and their constituent etched features can be used interchangeably to achieve the desired spectral content.
As previously mentioned each slot pattern has two distinct design elements associated with it, the first is shape of etched slot features the second is positions of these features with respect to one another and the facets of the laser cavity. In general any slot configuration in which optical feedback from one of the slot interfaces is suppressed may be employed in the present invention. For the purpose of the invention therefore a slot should produce a refractive index profile such as that shown in
Preferably each slot defines a first interface (or contributing interface) which is substantially planar and substantially perpendicular to the longitudinally extending optical path and a second interface (or feedback suppressing interface) which is substantially curved or angled with respect to the first interface. The contributing interface acts in the usual manner to provide optical feedback to L/Nth modes while the suppressing interface is designed to avoid adding to the optical feedback within the laser cavity. Having the second interface curved or angled with respect to the first reduces the amount of optical feedback it can provide to any particular longitudinal mode for two reasons. Firstly, light which interacts with a curved or angled interface is more likely (than light interacting with a planar interface aligned perpendicular to its direction of propagation) to be scattered out of the laser cavity. Secondly, light which is reflected back into lasing mode from different parts of such an interface will not be in phase, thus the optical feedback it provides will be distributed over a wavelength range which encompasses a number of longitudinal modes of the laser cavity thus diluting its impact in determining spectral content.
Specifying the emission wavelength of a laser diode, by etching features discussed above can be achieved by placing the interfaces which provide the bulk of the optical feedback, i.e. the straight interfaces which are perpendicular to the direction of light propagation, at distances from one another that correspond to multiples of half the free space emission wavelength divided by effective refractive index of the lasing mode. At this juncture it is worth defining λm which is the wavelength of light in the laser cavity, this is also known as the material wavelength. The material wavelength is related to the free space wavelength, λ, by the following equation
The problem of achieving single longitudinal mode laser emission at a specified wavelength over a particular temperature range is also addressed by the present invention. In order to do this it is necessary to discriminate against enough of the longitudinal modes of cavity to cope with changes in the laser's gain spectrum that occur over the temperature interval in question. Once the number of longitudinal modes (of the unperturbed structure) which must be discriminated against for a particular application is determined the appropriate slot pattern can be determined. For the most basic type of slot patterns, i.e. those in which all contributing interfaces providing the bulk of the optical feedback are separated by the same distance, the effective free spectral range, Δλeff, can be calculated from the formula
(where d is the distance between the contributing interfaces of the slot features, L is the cavity length, and Δλ is the free spectral range of the Fabry Perot cavity).
The two aspects of spectral selectivity discussed thus far i.e. the ability to specify the wavelength and the extent of the effective free spectral range, are clearly evident in
where dG/dT is the rate at which the gain peak tunes with temperature). In this case it is possible to achieve single longitudinal mode emission over a temperature interval of about 80° C.
The slot pattern (
A number of laser diode devices incorporating various configurations of tapered slot features were fabricated. These devices were fabricated using standard processing techniques. The steps used in the manufacture of the devices, whose characteristics are detailed here, were as follows.
Steps 2, 3, 4, 5 and 6 respectively are illustrated in
The slot patterns which were incorporated into the fabricated devices were designed to demonstrate two principal aspects of the invention. Namely the ability to fabricate single longitudinal mode laser diodes which emit at a stipulated wavelength, and the ability to manipulate the mirror loss spectrum of a laser diode so as to allow a laser emitting in a single longitudinal to operate over a predetermined temperature range without suffering from mode hops. It is noted that the data below was obtained on prototype samples, which were fabricated at the same time, and that the samples used in this these experiments had a high reflectivity coating applied to one facet, and a low reflectivity coating applied to the other facet.
First the task of achieving single longitudinal mode operation over a predetermined temperature range is considered. The device was designed to lase in a single longitudinal at λ=1.585 μm, given an operating temperature 20° C. The measured lasing wavelength turned out to be 1.577 μm (operating at a temperature 20° C.). The difference between the design wavelength and experimentally measured wavelength was attributed to the fact that the effective index of the guided mode was not known to a high enough accuracy at design time. The design of the tapered slot features, which were incorporated into the first set of devices, is shown in
Next, the ability to specify the emission wavelength of individual laser diode devices is considered. Two devices were designed to lase in a single longitudinal mode, the first at λ=1.550 μm and the second at λ=1.545 μm. In practice the emission of the first device was at 1.544 μm (
Turning to
As with other embodiments, this embodiment of the invention has the advantage of facilitating greater manufacturing tolerances. From a manufacturing perspective the critical tolerances are reduced from the placement of two faces per slot (as is the case in prior art devices, as shown for example in
Number | Date | Country | Kind |
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04104274.8 | Sep 2004 | EP | regional |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP2005/054374 | 9/5/2005 | WO | 00 | 3/10/2008 |