EP1900070A1

EP1900070A1 - Monolithic micro-lasers with stabilised polarisation

Info

Publication number: EP1900070A1
Application number: EP06776055A
Authority: EP
Inventors: Mikel Arizaleta Arteaga; Krassimir Panayotov; Hugo Thienpont
Original assignee: Vrije Universiteit Brussel VUB; Universite Libre de Bruxelles ULB
Current assignee: Vrije Universiteit Brussel VUB; Universite Libre de Bruxelles ULB
Priority date: 2005-06-20
Filing date: 2006-06-19
Publication date: 2008-03-19
Also published as: WO2006136346A1; GB0512523D0

Abstract

A Vertical-Cavity Surface-Emitting Laser has a resonant cavity (3,5) and an external cavity (6,15) formed as a monolithic structure, the external cavity being arranged to provide loss differences for different linearly polarised modes, the loss differences being arranged to provide an output with a stable polarisation state. Compared to the known polarisation stability techniques, this enables a stable polarisation output without the complexity of modification of the VCSEL structure, less sensitivity to small changes of injected current and temperature, and less need for the complex, non monolithic optical parts. Active polarisation control and stabilisation in the same device can be achieved by growing a polarisation dependent mirror on top of the VCSEL and by applying an external field to alter an optical length of the external cavity.

Description

MONOLITHIC MICRO-LASERS WITH STABILISED POLARISATION

Technical field of the invention

This invention relates to micro-lasers, such as e.g. vertical-cavity surface-emitting lasers (VCSELs) devices, to systems having such devices, and to methods of controlling such devices.

Background of the invention

Due to their wide applicability, interest in micro-lasers has significantly increased during the past years. Many types of micro-lasers are known, e.g. Vertical-Cavity Surface-Emitting Lasers (VCSELs), including types with gain or index guiding, proton-implanted types, air-post or oxide confined types, types with single or multiple quantum well or bulk material active region, types with carrier injection through the Bragg mirrors or intra-cavity, types with semiconductor or dielectric mirror(s) etc.

The quality and usability of a light source typically depends amongst others on the state of polarisation of a light source. Because VCSELs are highly symmetric semiconductor lasers that emit light in a direction perpendicular to the plane of the quantum well(s), neither waveguiding nor gain anisotropy inherently exist. As a result VCSELs suffer from poor polarisation stabilisation. Switching between two modes of linear and orthogonal oriented polarisation, called polarisation switching PS, and mode hopping between these two states is often observed and leads to mode partition noise and to degradation of the optical systems based on VCSELs when polarisation is important. As discussed by T.E. Sale in "Vertical Cavity Surface Emitting Lasers" (J.Willey & Sons, 1995), because of the cylindrical symmetry of the resonator, the surface emission perpendicular to the plane of the quantum well(s) and their polarisation-independent Bragg mirrors, VCSELs grown on (I OO)-oriented substrates have no a priori defined polarisation direction. In practice, small anisotropies arising from the electro-optic effect induced mainly by mirror doping, the elasto-optic effect due to unavoidable residual stress and from small imperfections in their symmetry, make the VCSEL have two preferred polarisation directions. The polarisation of the emitted light is almost always polarised along the [0 1 1] or the [0 -1 1] crystal axis. As a result VCSELs suffer of poor polarisation stabilisation and switching between these two modes of linear and orthogonally oriented polarisation and mode hopping between these two states is often observed, e.g. by Chang- Hasnain et al. in IEEE J. Quant. Electr. 27 (1991 ) 1402. This causes an increase of the relative intensity noise, as discussed by T. Yoshikawa et al. in IEEE Journal of Quantum Electronics 34 (1998) 1009, a reduction in the length of optical links due to an increase of the dispersion and a degradation of the optical systems based on VCSELs when the polarisation is an issue. Uncontrolled polarisation switching cannot be tolerated in a number of important applications such as optical sensing, data communication, laser absorption spectroscopy, etc.

Several methods to select and stabilise the polarisation state of light emitted by VCSELs during the fabrication process have been suggested. They are based on: anisotropic gain by growth on specific crystallographic orientation and different from [001]; anisotropic losses by side deposited reflectors, tilted etched-pillar structures or sub-wavelength diffraction gratings; anisotropic waveguiding by rectangular shaped VCSELs or with elliptical oxide aperture; local stress by elliptical aperture etched in bottom emitting lasers or trench etched next to the VCSEL; and polarisation dependent waveguiding by photonic crystal VCSELs. All these methods either require a modification of the VCSEL structure implying a complex additional processing of the VCSEL or specific orientation of the substrate. Moreover, some of the VCSEL characteristics could be degraded as a result. Furthermore, all these methods achieve polarisation stabilisation but select the polarisation of emission of the device before its fabrication, and this cannot be changed afterwards.

On the other hand several mechanisms mostly based on optical feedback from short and long external cavities have been proposed to actively control the polarisation of a VCSEL, as e.g. described by Wilkinson et al. in IEEE Photonics Technology Letters 11 (1999) 155 and by Besnard et al. in Optical Society of America 16 (1999) 1059. However in all cases, the control achieved is not stable versus small changes of injected current and temperature. Moreover, the implementation of the optical feedback based methods requires a very complex, multi-part and unstable system and precise control of the feedback ratio in order to avoid instabilities.

Periodic polarisation switching by changing the external cavity length has been presented by Panajotov et al. in Applied Physics Letters 84 (2003), p 2763-2765. It was demonstrated that by changing the external cavity length the phase of the reflected field is periodically tuned leading to a periodic modulation of the mirror losses of the two linearly polarised modes.

Summary of the invention An object of the invention is to provide improved apparatus of the micro- laser type or methods.

The invention relates to a micro-laser for outputting an illumination beam, the micro-laser having a resonant cavity and an external cavity, wherein the external cavity is adapted for stabilising a polarisation state of the illumination. The resonant cavity and the external cavity may be formed as one monolithic structure, in other words, the resonant cavity and the external cavity may form a single monolithic structure. It is an advantage of embodiments of the present invention that a more shock resistant system is obtained.

The external cavity being adapted for stabilising a polarisation state of the illumination may comprise the external cavity being arranged to provide a different loss for different polarised modes, such that the laser provides an output with a stable polarisation state. The micro-laser may be a vertical-cavity surface-emitting laser. The different polarised modes may be different linearly polarised modes. The external cavity may have an optical length and a mirror reflectivity, wherein any of the optical length or the mirror reflectivity may be adapted for stabilising a polarisation state of the illumination.

The external cavity may have a mirror layer formed on in the resonant cavity, e.g. on top of the resonant cavity. The external cavity may have a polarisation dependent mirror. The polarisation dependent mirror may comprise a sub wavelength grating.

The external cavity may have a sufficiently high mirror reflectivity, to enable sufficient gain difference between polarisation modes that the mode with higher loss does not lase at all injection currents. The reflectivity may be in the range 0.2 to 0.99.

The external cavity may have an optical length set to enable sufficient gain difference between polarisation modes that the mode with higher loss does not lase at a predetermined level of injection current. The predetermined level of injection current may be a desired level of injection current, i.e. a level of injection current preferably used.

The micro-laser may have a means, e.g. control means, for varying an optical length of the external cavity, e.g. a circuit for applying an electromagnetic field to vary an optical length of the external cavity. Alternatively, the micro-laser may have a circuit for applying an electric field to vary an optical length of the external cavity. In other words, the control means may be adapted for actively controlling the optical length of the external cavity, e.g. during operation or over life-time of the micro-laser or prior to specific applications of the micro-laser. The micro-laser may have a spatially selected optical feedback arrangement. The feedback arrangement may have an optical confinement layer on top of the external cavity.

The micro-laser may be arranged for current injection through the resonant and the external cavities. Alternatively or in addition thereto, the micro-laser also may have intermediate contacts for current injection into the resonant cavity only.

The invention also relates to a micro-laser having a resonant cavity and an external cavity, the external cavity being arranged to provide a different loss for different polarised modes, such that the laser provides an output with a stable polarisation state. The resonant cavity and the external cavity may be formed as a monolithic structure.

The invention furthermore relates to a micro-laser having a resonant cavity and an external cavity, having a means for varying the optical length of the external cavity. For example, the laser may be provided with a circuit for applying an electromagnetic or electric field to vary the optical length of the external cavity. The resonant cavity and the external cavity may be formed as a monolithic structure.

The invention also may relate to a method of stabilising a micro-laser, e.g. a vertical-cavity surface-emitting laser, having a resonant cavity and an external cavity, the method having the steps of controlling an optical length of the external cavity to maintain a window of stabilisation for a given injection current. The resonant cavity and the external cavity may be a single monolithic structure. The window of stabilisation may be a window of stabile polarisation operation of the micro-laser, i.e. wherein operation with a stabile polarisation state of the micro-laser is obtained. To maintain a window of stabilisation may comprise maintaining operation of the micro-laser with a stabile polarisation state of the micro-laser. The present invention also relates to a controller for controlling a micro- laser, the controller comprising a means for controlling an optical length of an external cavity to maintain a window of stabilisation for a given injection current. The window of stabilisation may be a window of stabile polarisation operation of the micro-laser, i.e. wherein operation with a stabile polarisation state of the micro-laser is obtained. To maintain a window of stabilisation may comprise maintaining operation of the micro-laser with a stabile polarisation state of the micro-laser.

The present invention also relates to a computer program product for performing, when executed on a computing means, a method for stabilising a micro-laser as described above, comprising controlling an optical length of the external cavity to maintain a window of stabilisation for a given injection current. To maintain a window of stabilisation may comprise maintaining operation of the micro-laser with a stabile polarisation state of the micro-laser. The present invention also relates to a machine readable data storage device storing the computer program product as described above and/or the transmission of such a computer program product over a local area or wide area telecommunications network.

Compared to the known polarisation stability techniques, embodiments of the present invention enable a stable polarisation output without the complexity of modification of the VCSEL structure, and so without degradation of the VCSEL characteristics. Furthermore, compared to known techniques based on optical feedback from non-monolithic external cavities, there is less sensitivity to small changes of injected current and temperature, and less need for the complex, non monolithic optical parts, nor the precise control of feedback ratio to avoid instabilities.

Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features of the independent claims and with features of other dependent claims as appropriate and not merely as explicitly set out in the claims.

Although there has been constant improvement, change and evolution of devices in this field, the present concepts are believed to represent substantial new and novel improvements, including departures from prior practices, resulting in the provision of more efficient, stable and reliable devices of this nature.

The above and other characteristics, features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. This description is given for the sake of example only, without limiting the scope of the invention. The reference figures quoted below refer to the attached drawings.

Brief Description of the Drawings:

How the present invention may be put into effect will now be described by way of example with reference to the appended drawings, in which:

Figs. 1 and 2 show cross section views of VCSELs according to embodiments of the invention, Fig. 3 shows a map of polarisation stable and polarisation bistable regions in the plane of injection current and external cavity length for an embodiment,

Fig. 4 shows similar plots at different temperatures, for an embodiment,

Fig. 5 shows a plot of stabilisation window vs. external cavity length for an embodiment,

Fig. 6 shows a cross section view of a VCSEL according to a further embodiment,

Fig. 7 shows a schematic overview of a system comprising a micro- laser according to embodiments of the present invention,

Fig. 8 shows a cross section view of a VCSEL according to another embodiment of the present invention,

Fig. 9 shows a map of polarisation stable and polarisation bistable regions in the plane of injection current and extended cavity length, and

Fig. 10 shows a cross section view of a VCSEL according to a further embodiment.

Fig. 11a and Fig. 11 b show a plot of the polarisation resolved output power as a function of current for a solitary VCSEL (Fig. 11 a) and for a VCSEL with optical feedback from a mirror at about 20μm (Fig. 11b), illustrating the polarisation stability for embodiments of the present invention.

Description of the Preferred Embodiments:

The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. Any reference signs in the claims shall not be construed as limiting the scope. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. Where the term "comprising" is used in the present description and claims, it does not exclude other elements or steps. Where an indefinite or definite article is used when referring to a singular noun e.g. "a" or "an", "the", this includes a plural of that noun unless something else is specifically stated.

Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein. Moreover, the terms top, bottom, over, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein.

The embodiments described are concerned with micro-lasers such as for example vertical-cavity surface-emitting lasers (VCSELs) with general performance characteristics, but can be applied to other VCSELs or other micro-lasers. For example VCSELs may be gain guided, index guided, proton implanted, air-post or oxide confined, having a single or multiple quantum well, quantum dot or bulk material active region, may comprise carrier injection through the Bragg mirrors or intracavity carrier injection, may comprise semiconductor or dielectric mirror(s) etc. In a first aspect, they show polarisation stabilisation from fabrication by introducing mirror loss differences for different linearly polarised modes. This can be achieved by an external cavity. One way of implementing this is using an extra mirror on top of the device. This can take the form of a Distributed Bragg Reflector (DBR). The embodiments also show active polarisation control by changing the optical length of the extended cavity. This can be achieved by means of an external electric field applied to the extended cavity with two extra metal contacts.

Active polarisation control and stabilisation in the same device can be achieved by growing a polarisation dependent mirror on top of the VCSEL (for example growing a sub-wavelength grating on top of a DBR) and using the above-mentioned applied external field using the electro-refractive effect. Also, these embodiments show fundamental transverse mode enhancement using spatially selected optical feedback. This can be in combination with the above mentioned features. A first embodiment of the invention, illustrated in Figs. 1 and 2, shows a novel way to achieve substantially stable polarised emission, e.g. stable polarised emission, in micro-lasers (100), such as e.g. VCSELs, over a wide range of values of injection currents, e.g. for substantially all values of the injection currents and over a wide range, preferably the whole range, of working temperatures. The stable polarised emission may e.g. be linearly polarised emission, although the invention is not limited thereto. The range of working temperature differences wherein a significant window of polarisation stabilisation may be obtained, may be, i.e. span, at least 100°, preferably at least 200⁰C, more preferably at least 300⁰C, even more preferably at least 400⁰C, still more preferably at least at least 500⁰C. The temperature range wherein this may be obtained is determined by the operational temperatures and material properties of the devices used, rather than being limited by the polarisation stabilisation properties. Polarisation stabilisation may e.g. be quantified by the polarisation mode suppression ration (PMSR), which is defined by PMSR = 10 log(P1/P2), where P1 and P2 are the powers of two polarisation modes from which a mode to be stabilised is chosen. A polarisation stabilisation may be obtained whereby PMSR is more than 25dB, preferably more than 3OdB, even more preferably more than 35dB. The external cavity is arranged to provide a different loss for different polarised modes, such that the micro-laser provides an output with a stable polarisation state. Preferably, the external cavity is a monolithic external cavity created on top of the device during its fabrication process. In other words, preferably the standard micro-laser structure is extended with an external cavity, which is constructed such that the external cavity and the micro-laser act as a single part or form a monolithic ensemble. By way of illustration, Figs. 1 and 2 depict a schematic representation of a cross-section of the micro-laser with extended cavity, e.g. VCSEL with extended cavity, which in the present examples form a monolithic part together with the resonant cavity, the invention not being limited thereto. Current injection can be performed using any suitable method, two different exemplary methods illustrated in Fig. 1 and Fig. 2. Fig.1 illustrate current injection through the two cavities, whereas Fig. 2 illustrates current injection through intermediate contacts. The micro-laser (100) device typically may comprise standard components, such as for example two contacts (1 ), also called electrodes, for providing current injection, a first mirror region (3) such as e.g. a stack of distributed Bragg reflectors (DBR), being for example a n-doped Distributed Bragg Reflector, a current confinement layer or cladding layer (4), embedding an active region (2), and a second mirror region (5) such as e.g. a stack of distributed Bragg reflectors (DBR), being for example a p- doped DBR, although the invention is not limited thereto. The first mirror region (3), the active region (2) and the second mirror regions (5) and, if present, the current confinement layer or cladding layer (4) typically are referred to as resonant cavity (20). The external cavity (30) forming a monolithic ensemble with the standard components of the micro-laser (100) comprises an extended cavity (6), typically provided on top of a mirror region of the standard components of the micro-laser (100), and an external mirror region (15), e.g. mirror layer or stack of mirror layers such as e.g. a stack of distributed Bragg reflectors (DBR), being for example p-doped DBR, on top. The mirror region composition, such as e.g. DBRs composition, as well as the current confined method used and the quantum well or quantum dot structure in the active region need not be described here in more detail since many suitable examples are well known to those skilled in VCSEL fabrication. By way of illustration, one example of a laser being a VCSEL is provided, as also described in US patent application 2003/075725. The VCSEL described in US 2003/075725 is fabricated on a semiconductor substrate, e.g. a gallium arsenide substrate. The first mirror region (3), typically a stack of distributed Bragg reflectors, comprised of a plurality of alternating layers is positioned on a surface of semiconductor substrate. The plurality of alternating layers of first stack of distributed Bragg reflectors may be formed of n-doped aluminum arsenide material and n-doped gallium aluminum arsenide material. There is next fabricated a cladding region (4) on a surface of the first stack of distributed Bragg reflectors, an active region disposed on cladding region, and a cladding region disposed on a surface of the active region. A second mirror region (5), typically a stack of distributed Bragg reflectors is positioned on a surface of cladding region (4). The second stack of distributed Bragg reflectors is formed of a plurality of alternating layers, more specifically, for example, alternating layers of a p-doped aluminum arsenide and a p-doped gallium aluminum arsenide. The second stack of distributed Bragg reflectors is followed by a one-half wavelength aluminum gallium arsenide contact layer. Contact layer (1 ) is p-doped to 10¹⁹ cm^'3 or higher. It will be obvious for the person skilled in the art that the additional components being the external cavity (6) and the top mirror (15), referred to as external mirror (15), can be added in a similar fashion. Finally, a very thin gallium arsenide cap layer is positioned on a surface of the contact layer (1 ). Cap layer is very thin, more specifically on the order of IOOangstrom thick. The cap layer is p-doped to 10¹⁹ cm^"3 or higher. The active region is typically constructed from one or more quantum wells of InGaAs, GaAs, AIGaAs, (AI)GaInP, GaInAsP or InAIGaAs, or is a bulk material active region. In practice, the thickness of the substrate is typically 150 μm compared to about 10 μm for the mirror and active regions. After processing of the VCSEL wafer, it may be cleaved from the original substrate, and mechanically coupled to a mounting substrate.

Since the two linearly polarised VCSEL modes have slightly different wavelengths the mirror losses and therefore the net gain of the two fundamental linearly polarised modes, will be different leading to polarisation stabilisation. Therefore, the practically inevitable small frequency splitting between the two fundamental linearly polarised modes of the VCSEL (in the range of 1-20 GHz in practical VCSELs) makes this phenomenon quite common across different types of VCSEL.

The external mirror (15), i.e. the top mirror (15) at the external cavity (30), has a high reflectivity such that high differences in mirror losses and consequently high differences in net gain are obtained for modes having a different polarisation state. Depending on the micro-laser properties, e.g. VCSEL parameters, the reflectivity should be in a range from 0,2 to 0,99. The reflectivity of the external mirror may be selected such that the loss difference between the modes for a certain range of external cavity length, also referred to as window of stabilisation, is so high that the gain difference variation with the injected current is not able to make the mode with higher mirror losses to lase. The latter is valid for substantially all the current values that are used. Fig. 3 shows the currents at which different modes with different polarisation states are present versus the external cavity length in a map of bistability. In this map we can distinguish three different zones: The grey zone (zone II) corresponds to the region at which the VCSEL is bistable. In the zone above the bistable region (zone III) the VCSEL is emitting in one linearly polarized mode only, which might be either the high or the low frequency fundamental transverse mode, and in the zone below the bistable region (zone I) it is emitting in the orthogonal linearly polarised mode only, which will then be either the low or high frequency fundamental transverse mode. The dashed and almost horizontal line at 0.3 mA represents the threshold current for each extended cavity length. There is a strong periodic dependence of the currents for polarisation selection on the external cavity length that becomes strongly asymmetric when increasing the external mirror reflectivity. For high enough values of external mirror reflectivity the minima of the polarisation selection currents reach threshold and polarisation stabilisation is achieved. Depending on the micro-laser properties, the reflectivity may be in the range from 0,2 to 0,99. The same selected polarisation state of the light is obtained for substantially all operational injection currents. Fig. 3 is merely an example calculated with specific VCSEL parameters to demonstrate that by using an additional monolithic extremely short external cavity (30) one can achieve polarisation control of VCSEL emission. For the specific example provided, the device has a wavelength of operation of 845nm, has a frequency splitting between two LP VCSEL modes at 5 GHz, has an external amplitude reflection coefficient of 0.5, i.e. a reflection coefficient of 0.5 for the mirror coupling out the light out of the external cavity (30) and a top mirror amplitude reflectivity of 0,997, i.e. a reflectivity of 0,997 for the mirror coupling out the light out of the resonant cavity (20). The reflectivity coefficients thereby are provided for the electric field components of the light beams. It will be clear to the person skilled in the art that the example selected is only provided by way of illustration and that similar results are obtained for other specific device parameters. The mapping as the one shown in Fig. 3 can always be carried out for any specific VCSEL design at hand.

It is important to investigate how a change of the VCSEL temperature will influence the polarisation control introduced by this technique. For a change of temperature of 100⁰C, preferably 200⁰C, more preferably 300⁰C, a significant large window for stable polarisation operation is obtained. E.g. in case of a temperature change of 300⁰C, a window for stable polarisation operation of 4nm is obtained. The range of working temperatures wherein polarisation stabilisation may be obtained, may have a lower limit of -20⁰C, preferably -100°C, more preferably -273°C and an upper limit of 100⁰C, more preferably 300⁰C, even more preferably higher, i.e. limited by the operational temperatures of the device or by the material properties for the materials constituting the device. Temperature change implies corresponding change of the refractive index and therefore a variation of the optical length of both the extended cavity (15) and the active VCSEL cavity. This will mean a horizontal displacement of the mapping along the extended cavity length axes. Fig. 4 shows how the map of bistability is shifted by temperature change of 100⁰C. In this particular case, wherein the extended cavity length is around 2μm, the shift due to temperature change is smaller than the window of stabilisation, therefore the polarisation stabilisation will be possible at any temperature within the window of stable polarisation operation (see Fig. 4). The length of the window of stable polarisation operation is equal to the difference between the stabilisation window and the shift due to temperature change. The shift due to temperature change will proportionally depend on the length of the extended cavity and if it becomes larger than the window of stabilisation the polarisation of the device will not be stable against large temperature variations. By way of example, Fig. 5 presents the size of the stabilisation window, for all currents of operation, as a function of the external cavity length. In this stabilisation window VCSEL emits in one polarisation for temperatures from -20⁰C to 80⁰C for an exemplary structure according to the present embodiment. In other words, the stabilisation window and the shift due to a temperature variation of 100⁰C is shown as a function of the extended cavity length. As can be observed from this figure, the window of polarisation stabilisation using our technique is large enough to make the VCSEL polarisation stable when temperature is varied in the range of 100 ⁰C for the case when the extended cavity is shorter than 6 μm. From Fig. 5 it can be seen that the higher flexibility for the fabrication of the extended cavity is around 7 nm, i.e. that a window is created in which stabile illumination having one polarisation is obtained. The latter is obtained in a wide temperature range and for extended cavity lengths around 1 μm. The specific optimal value of the extended cavity length is, of course, dependent on the specific VCSEL parameters and on the wavelength of emission. However, the optimisation procedure demonstrated in Fig. 5 is very general and can always be carried out for the specific VCSEL design at hand.

In such a way a monolithic VCSEL with an extended cavity constitutes a new way to achieve polarisation stabilisation from fabrication, which can easily be integrated into standard industrial VCSELs grown (see Fig. 1 ). It will just imply the growth of an external mirror, such as an external DBR on top of the VCSEL, which will add a minimum additional cost to the fabrication process, lower than in any other polarisation stabilisation mechanisms proposed so far. In a further embodiment, the present invention relates to a device for controlling the polarisation in micro-lasers, such as e.g. a VCSEL. The micro- laser (100) typically comprises a monolithic external cavity (30) created on top of the device during its fabrication process. In other words, a standard micro- laser structure is extended with an external cavity (30), which is constructed such that the external cavity (30) and the micro-laser (100) act as a single part or form a monolithic ensemble. More details about the components of the standard micro-laser can be found in the description of the first embodiment.

Furthermore, an exemplary description of the external cavity (30) is also provided in the description of the first embodiment, although the external cavity (30) does not need to be arranged to provide a different loss for differently polarised modes, such that the laser (100) provides an output with a stable polarisation, although this may be the case. In other words, in contrast or in addition to the device of the first embodiment, the present invention furthermore comprises electric controlling means for electrically controlling the polarisation of the monolithic device. The external cavity (30) of the external device may be made of material that has the electro-refractive effect, whereby the electro-refractive effect of the material of the extended cavity (6) allows to active electric control of the polarisation. The latter can be applied to the fully monolithic device. Typical materials that can be used are the same as the ones used in the VCSEL structure itself, i.e. InGaAs, GaAs, AIGaAs, (AI)GaInP, GaInAsP or InAIGaAs. The exact electro-refraction coefficient will be material dependent, e.g. for GaAs the electro-refraction coefficient typically is r₄i = -1 ,7 pm/V for wavelengths around 1 μm. Consequently, the electric field typically used for controlling a stable polarisation output of the illumination beam outputted by the micro-laser (100) depends on the exact material used. Alternatively, a means for providing an electromagnetic field to the external cavity may be used, whereby, when a magnetic field is used, selection of other polarisation states is possible. Changing the intensity of an electric field applied to the extended cavity by means of at least one additional contact 8 (see Figs. 6 and 7), the refractive index of the external cavity (30) is changed and therefore its optical length is changed too, allowing control of the polarisation of the device after its fabrication. As a second contact, one of the contacts for driving the VCSELs may be used or an additional electrical contact may be provided. The electric controlling means thus may comprise additional contacts. The electric controlling means furthermore may comprise a source for providing an electric field to the extended cavity, which may be a separate source or may be a source used for operation of the micro-laser (100).

The means for generating an electric or electromagnetic field are both examples of a means to control the optical length of the external cavity (30) according to the present invention. The latter is illustrated by way of example in a schematic overview of such a system in Fig. 7, indicating a micro-laser (100), a driving unit (102) for providing current injection to the contacts, also referred to as electrodes, and a control means (104) for controlling the optical length of the external cavity of the micro-laser (100). As described above, the control means (104) for controlling the optical length may be a means for generating an electric field or means for generating an electromagnetic field to the extended cavity. As will be clear from the above description, the micro- laser may be adapted for controlling the length of the optical length of the external cavity of the micro-laser (100), e.g. by using electro-refractive material. Furthermore, as will be described in more detail below, a spatially selected optical feedback arrangement (106) may be provided in order to enhance the fundamental transverse mode operation of the VCSEL.

The polarisation stabilisation effect may be further improved if the external mirror is polarisation-dependent (see Fig. 8). As an example a sub- wavelength grating (7) can be grown on top of the mirror in Fig. 8 as a polarisation selective element, but any other method to obtain polarisation selective mirror could be alternatively used, for example by growing a dichroic layer. A small difference (bigger than 5%) between the reflectivity of the external mirror for the two polarisation modes is enough to have a mirror losses difference between the modes large enough to change directly from one polarisation state to another polarisation state. For these devices, incorporating a polarisation selection component in the external mirror, a stabilised polarisation can be obtained in a temperature range having a lower limit being — 20⁰C, preferably -100⁰C, even more preferably -200⁰C, still more preferably -273°C and having an upper limit of 500⁰C, more preferably an upper limit defined by the limitations of the micro-laser operation based on the material properties of the materials constituting the micro-laser (100). With devices comprising a polarisation selection components in the external mirror, a significant large window for stable polarisation operation can be obtained for a temperature change up to 500°C or larger. The mapping of bistability will be channelled and the bistable region will be reduced to an almost vertical line (see Fig. 9). The border lines may be almost but not totally vertical because of the linear dependence of the wavelength of operation of the VCSEL on the injected current induced by the thermal heating. This means that depending on the length of the extended cavity the VCSEL will emit in one polarisation mode or in its orthogonal one but in any case the polarisation will be stable for any injection current. Therefore adding this extra growth process can provide e.g. a fully monolithic device that will emit stable linear polarisation at any injected current that will be actively controlled with the electric field applied in the extended cavity.

Another additional feature is a spatially selective optical feedback arrangement (106) by fabricating an optical confinement layer (9) on top of the extended mirror DBR (see Fig. 10), in order to enhance the fundamental transverse mode operation of the VCSEL. This optical confinement layer can be for example a shallow etched surface relief or a metal ring contact.

An experimental evidence that the method for polarisation stabilisation of VCSELs with an additional extended cavity is presented in Fig. 11a and Fig. 11 b. The figures show the optical power output of different components for different injection currents. In Fig. 11a the polarisation resolved output power as a function of the current for a solitary VCSEL are shown, whereas Fig. 11 b illustrates the polarisation resolved output power as a function of the current for the same VCSEL but subject to optical feedback from a mirror at about 20 μm. Curve 110 represents the high frequency mode, curve 112 represents the low frequency mode. Results are shown both for increasing injected current (solid line) and decreasing injected current (dashed line). The results for increasing injected current and decreasing injected current nearly completely coincide. It can be seen that using an external cavity results in a stable polarisation for the full operational range of injection current for the specific example. The VCSEL without external cavity shows instabilities for the differently polarised components.

All the concepts shown here are applicable to any kind of micro-laser, such as e.g. any type of VCSEL, since they are independent of the confinement method used in the fabrication, i.e. oxide confinement, proton implantation confinement or air post confinement, or of the wavelength of operation of the device. Another additional advantage of the described polarisation control in micro-lasers (100) such as e.g. VCSELs is that desirable characteristics of the micro-laser (100), such as e.g. VCSELs are maintained such as the superior Gaussian beam quality and the two dimensional array fabrication. The features described can be useful in applications such as optical systems for optical data communications, optical sensing, optical interconnections, laser absorption spectroscopy and in general in any application that requires low cost, stable and controllable polarisation optical sources. The devices described above may e.g. be used as transmitters in such optical systems.

It is an advantage of embodiments of the present invention that a laser with a stable polarisation state may be obtained.

Above has been described a micro-laser (100), e.g. VCSEL having a resonant cavity and an external cavity (6) as a monolithic structure, for polarisation stabilisation. An additional feature is the external cavity (30) having a mirror layer (15) formed on top of the resonant cavity. This enables relatively straightforward manufacture. If the external cavity (30) has a polarisation dependent mirror (5,7), then the difference in losses between polarisation modes can be enhanced. The polarisation dependent mirror (5, 7) can be a sub wavelength grating or other type. The external cavity (30) can have a sufficiently high mirror reflectivity, to enable sufficient gain difference between polarisation modes that the mode with higher loss does not lase for all injection currents.

The external cavity (30) can have an optical length set to enable sufficient gain difference between polarisation modes that the mode with higher loss does not lase at for all injection currents. A means for varying the optical length of the external cavity (30) can be provided: for example, a circuit for applying an electric or an electromagnetic field to vary an optical length of the external cavity can be used to maintain the stabilisation or to actively control the polarisation state to enable it to be switched for example, if desired.

A spatially selected optical feedback arrangement (106) such as an optical confinement layer (9) on top of the external cavity (30) can be used to enhance the fundamental transverse mode operation.

In a further aspect, the present invention also relates to a controller for stabilising a micro-laser. Such a controller (200) may be an electronic control system of a micro-laser (100), which is one example of a control system for use with a micro-laser (100) in accordance with the present invention. The controller (200) controls the overall operation of the micro-laser (100). The controller (200) may comprise a data store (202) for storing parameters for controlling the micro-laser (100). The data store (202) may comprise any suitable device for storing digital data as known to the skilled person, e.g. a register or set of registers, a memory device such as RAM, EPROM or solid state memory. The latter is also illustrated in Fig. 7. The controller (200) also may control the driving unit (102) and/or the control means (104) for controlling the optical length of the external cavity. Alternatively, the controller (200) also may be or incorporated the control means (104) for controlling the optical length of the external cavity (30).

The controller (200) may include a computing device, e.g. microprocessor, for instance it may be a micro-controller. In particular, it may include a programmable micro-laser controller, for instance a programmable digital logic device such as a Programmable Array Logic (PAL), a Programmable Logic Array, a Programmable Gate Array, especially a Field Programmable Gate Array (FPGA). One aspect of the present invention is that the micro-laser controller (200) controls the operation of the micro-laser (100) in accordance with settable parameters stored in data store (202). The controller (200) may comprise software or hardware means for controlling the operation of and polarisation stabilisation in the micro-laser (100). In accordance with embodiments of the present invention a controller (200) for a micro-laser (100) in accordance with the present invention may be programmed. Accordingly, the present invention includes a computer program product which provides the functionality of any of the methods according to the present invention when executed on a computing device. Further, the present invention includes a data carrier such as a CD-ROM or a diskette which stores the computer product in a machine readable form and which executes at least one of the methods of the invention when the program stored on the data carrier is executed on a computing device. Nowadays, such software is often offered on the Internet or a company Intranet for download, hence the present invention includes transmitting the computer product according to the present invention over a local or wide area network. The computing device may include a personal computer or a work station. The computing device may include one of a microprocessor and an FPGA.

Other arrangements for accomplishing the objectives of the methods and systems for evaluating the polarisation degree of an illumination source embodying the invention can be envisaged within the claims. It is to be understood that although preferred embodiments, specific constructions and configurations, as well as materials, have been discussed herein for devices according to the present invention, various changes or modifications in form and detail may be made without departing from the scope and spirit of this invention. For example, whereas in the above described embodiments the focus has been on devices, the invention also relates to a method for stabilising a micro-laser (100), e.g. VCSEL comprising a monolithic structure comprising a resonant cavity and an external cavity, the method comprising of controlling an optical length of the external cavity to maintain a window of stabilisation for the injection current. The controlling may be performed by controlling an electric field applied to the external cavity as to influence the optical length of the cavity by the electro-refractive effect.

Claims

Claims:

1. A micro-laser (100) for outputting an illumination beam, the micro-laser (100) having a resonant cavity (20) and an external cavity (30), wherein the external cavity (30) is adapted for stabilising a polarisation state of the illumination.

2. The micro-laser (100) of claim 1 , the external cavity (30) having an optical length and a mirror reflectivity, wherein any of the optical length or the mirror reflectivity are adapted for stabilising a polarisation state of the illumination.

3. The micro-laser (100) according to claim 1 or 2, wherein the resonant cavity (20) and the external cavity (30) are formed as one monolithic structure.

4. The micro-laser (100) of any preceding claim, wherein the external cavity (30) being adapted for stabilising a polarisation state of the illumination comprises the external cavity (30) being arranged to provide a different loss for different polarised modes, such that the micro-laser (100) provides an output with a stable polarisation state.

5. The micro-laser (100) of any preceding claim, the external cavity (30) having a mirror layer (15) formed on top of the resonant cavity (20).

6. The micro-laser (100) of any preceding claim, the external cavity (30) having a polarisation dependent mirror (5,7).

7. The micro-laser (100) of any preceding claim, the polarisation dependent mirror (5,7) comprising a sub wavelength grating.

8. The micro-laser (100) of any preceding claim, the external cavity (30) having a sufficiently high mirror reflectivity, to enable sufficient gain difference between polarisation modes that the mode with higher loss does not lase at all injection currents.

9. The micro-laser (100) of any preceding claim, the external cavity (30) having an optical length set to enable sufficient gain difference between polarisation modes that the mode with higher loss does not lase at a predetermined level of injection current.

10. The micro-laser (100) of any preceding claim, having a control means (106) to vary an optical length of the external cavity (30).

11. The micro-laser (100) of claim 10, wherein the control means (106) comprises a circuit for applying an electric field.

12. The micro-laser (100) of any preceding claim, having a spatially selected optical feedback arrangement (9).

13. The micro-laser (100) of claim 12, the feedback arrangement having an optical confinement layer (9) on top of the external cavity (6, 15).

14. The micro-laser (100) of any preceding claim, arranged for current injection through the resonant and the external cavities (6,15).

15. The micro-laser (100) of any preceding claim, having intermediate contacts for current injection into the resonant cavity only.

16. A method of stabilizing a micro-laser (100) having a resonant cavity and an external cavity (30), the method having the steps of controlling an optical length of the external cavity (30) to maintain a window of stabilisation for a given injection current.

17. A method according to claim 16, wherein to maintain a window of stabilisation comprises maintaining operation of the micro-laser (100) with a stabile polarisation state of the micro-laser.

18. A controller for controlling a micro-laser, the controller comprising a means for controlling an optical length of an external cavity (30) to maintain a window of stabilisation for a given injection current.

19. A controller according to claim 18, wherein to maintain a window of stabilisation comprises maintaining operation of the micro-laser (100) with a stabile polarisation state of the micro-laser.

20. A computer program product for performing, when executed on a computing means, a method for stabilising a micro-laser (100) as described in any of claims 16 to 17.

21. A machine readable data storage device storing the computer program product as claimed in claim 20.

22. Transmission of a computer program product according to claim 21 over a local area telecommunications network.