High speed lasing device

The present invention relates to a lasing device for use in an optical module. The lasing device comprises a first reflector and a second reflector; a confinement layer adapted to confine current within a current-confining aperture; and an active layer between the first and second reflectors. The active layer comprises a main active region aligned with the current confining aperture and an auxiliary active region surrounding the main active region. The second reflector includes a first reflector region arranged on the current-confining aperture and a second reflector region surrounding the first reflector region. The second reflector region and the first reflector are configured to induce stimulated recombination in the auxiliary active region.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to lasing devices for use in high-speed fiber optical communication systems, and more specifically to Vertical Cavity Surface Emitting Lasers (VCSELs) with high modulation bandwidth. Moreover the present invention relates to optical interconnects including lasing devices with high modulation bandwidth. Finally, the present invention relates to a method for manufacturing high-speed lasing devices.

2. Related Art to the Invention

Lasing devices and in particular Vertical Cavity Surface Emitting Lasers commonly used in high-speed communication systems include a cavity sandwiched between two highly reflective mirrors or reflectors so as to form a resonator. The mirrors include several alternating layers of semiconductors of high and low refractive index and are doped with p-type and n-type dopants or impurities, respectively so as to form a p-n or a p-i-n diode junction. In a semiconductor laser the gain mechanism that generates the lasing is provided by light generation from the recombination of holes and electrons. The recombining holes and electrons are injected, respectively, from the p and n sides of the diode junction. In telecommunication applications, the recombination of carriers is generated by electrical pumping, i.e. by forward-biasing the diode junction. Commonly, the current in the lasing device is confined to an aperture of the laser by implanting ions into the lasing device structure everywhere except the aperture on the lasing device so as to increase the electrical resistivity of the material around the aperture. Alternatively, the current around the aperture of the lasing device can be inhibited by oxidizing the material around the aperture of the lasing device.

Semiconductor lasers are employed in telecommunication applications for building optical interconnects used in electronic devices. Such optical interconnects became in recent years widely used in electronic devices due their capability of supporting a much higher bandwidth than traditional cable interconnects. In this context, the development of optical modules for converting optical signals into electrical signals and vice versa plays a crucial role in a wide range of applications, such as mid-board applications using optical interconnects.

Semiconductor lasers, such as VCSEL, typically convey information according to two schemes. In the first scheme, the laser is maintained in a constant light-emitting state and the output intensity is modulated by means of an external modulator driven by an externally applied voltage. Since this first scheme requires a costly external apparatus, optical interconnects including VCSEL are generally directly modulated. Direct modulation involves changing the current input of the laser, or, in other words, modulating the current around the bias current so as to produce a time-dependent output in the optical intensity. Usually, the current is switched between two values, both larger than the threshold current of the device.

FIG. 7shows a lasing device4000according to the state of the art. The lasing device includes a substrate4030made of semiconductor material, a first mirror4300and a second mirror4100. The first and second mirrors4300and4100respectively include a stack of alternating semiconductor mirror layers4310,4320. The layers4310have a high refractive index while the layers4320have a low refractive index. The first mirror4300is doped with n-type dopants, while the second mirror4100is doped with p-type dopants. The lasing device4000further includes a cavity spacer4200between the first and second mirrors4300,4100. The cavity spacer4200includes a first cladding layer4230and second cladding layer4210and an active layer4220. Finally, the lasing device4000includes a current-confining region4020which defines a current-confining aperture4021. The current-confining region is formed in the second cladding layer4210immediately below the second mirror4100.

FIG. 8is a schematic drawing illustrating the working principle of the lasing device4000ofFIG. 7. In particular,FIG. 8shows the effect of direct modulation on the carrier density in the active layer of the lasing device4000. The current is switched between two values, both larger than the threshold current of the device. In the lasing device4000, the carrier density is not perfectly clamped, but swings with the injection current due to gain saturation with optical field intensity and gain reduction due to internal heating.

FIG. 8shows the distribution of the intensity of the optical field along the active layer4220. As can be seen from the dashed line plot, the intensity of the optical field is maximum in the zone of the active layer corresponding to the current-confining aperture4021. The optical field intensity is generally lower at the periphery and higher in the center of the active region. Moreover, in the peripheral regions of the active layer, optical loss is also higher than in the center of the active layer. This leads to a lower stimulated recombination rate at the periphery of the active layer. Upon switching from the high current level to the lower current level, the carrier density in the active layer4220will also switch from a high to a lower level. As can be seen fromFIG. 8, at a particular bias current above the threshold, the carrier density distribution in the active layer4220is illustrated by the dotted curve (3). Zone A of the active layer4220indicates the area where the optical gain reaches the threshold value. Outside zone A, the carrier density is not sufficient for the generated gain to reach the threshold value. In addition, within zone A, the carrier density is not constant, but is larger where the local temperature and/or local photon density are higher.

FIG. 8illustrates the particular case, in which the carrier density increases towards the center of the aperture.

At a higher bias current, both the temperature and the photon density in the active layer4220increase. Consequently, the carrier density in zone A will also increase in order to maintain the gain at the threshold value. At a higher bias current, the carrier density in the areas surrounding zone A will also increase and will become high enough to generate a gain that reaches the threshold value in a zone B surrounding zone A. This behaviour is illustrated by the dashed curve (1). Consequently, the active region of the laser where the carriers and photons are strongly coupled through stimulated recombination will expand from zone A at a lower bias to zones A and B at a higher bias current.

Upon switching from a high bias to a low bias, the carrier density in zone A of the active layer decreases at a much faster rate than the density in zone B of the active layer4220due to a stronger stimulated recombination in the areas with a higher optical field intensity. Thus, the carrier density will have two peaks at the periphery of the active layer4220as shown in the solid curve (2). These excess carriers in zone B of the active layer4220will act as a reservoir from which carriers will flow from the periphery towards the center of the active layer4220, thereby acting as a capacitance connected parallel to the active layer of the laser. This extends the fall-time of the lasing device4000and negatively affects its response to a modulating signal. Consequently, the design of common lasing devices limits the modulation bandwidth and the high speed performance of optical interconnects employing the lasing device.

More precisely, since during the high-to-low transition, the laser4000evolves towards a lower carrier density and a reduced stimulated recombination rate the laser4000slows down in adapting to the new, lower current level, thereby enhancing the effect of the excess carrier density at the periphery of the active layer on extending the fall time. Even if the optical field intensity returns to its nominal value after a current waveform is applied, the carrier density will not, thereby leading to a dynamical coupling from the past to the future causing, for instance, inter-symbol interference.

In addition, in devices wherein current confinement is obtained by using a layer of insulation oxide4020, the effective parasitic capacitance associated with the isolation oxide defining the current confining aperture is determined by the capacitance across the oxidized layer4020in series with the capacitance of the diode junction underneath the oxidized layer. If the diode is unbiased, the effective capacitance is given by the oxide layer capacitance in series with the depletion capacitance of the diode, the latest being the lowest of the two. Under forward bias the capacitance of the diode will increase while its series resistance will decrease leading to an overall increase of the effective capacitance of the structure. The maximum capacitance is only limited by the oxide capacitance, which is relatively large.

Lateral carrier spreading out from the aperture formed in the oxidized layer4020can be significant and in a steady-state, this will lead to a leakage current. The lateral carrier spreading will also provide some degree of forward bias to the outer regions of the diode structure under the isolation oxide4020, thereby leading to an increase of the effective parasitic capacitance of the device with bias.

The above described effect further limits the modulation speed of the lasing device and hence the modulation bandwidth.

In order to overcome the problems associated with direct modulation of common lasing devices, many solutions have been proposed for reducing the effect of carrier spreading out towards the periphery of the active layer. In particular, clamping of the carrier density inside the active region could be improved through reduced gain saturation and internal heating. Alternatively, proton implantation or patterned tunnel junction techniques may be used to additionally confine the carriers so as to reduce the carrier density at the periphery of the active layer.

The known techniques have, however, the disadvantage that the additional confinement is effective only if the lateral geometry of the carrier confinement features matches the transversal distribution of the optical field. Developing a device with the above mentioned design requires extensive design and is very complex and costly to realize.

SUMMARY OF INVENTION

Therefore, the problem underlying the present invention is to provide a lasing device for use in directly modulated high-speed fiber optical communication systems, which can be driven at a high-modulation speed, which has a reduced effective parasitic capacitance, and which can be fabricated in a particularly simple and cost-effective manner and at the same time allows for a faster and more reliable fiber optical communication system.

This problem is solved by providing a laser structure including, at the periphery of the active layer of the main laser, an auxiliary laser having the function of a guard laser. The guard laser will clamp the carrier density within its active region which overlaps the periphery of the active region of the main laser and further induce additional stimulated recombination at the periphery of the active region of the main laser.

The additional stimulated recombination will increase the rate at which the excess carrier density at the periphery of the active region decreases when the input current is switched down. In addition, the guard laser causes a reduction of the carrier injection in the outer region of the diode structure, thereby limiting the increase of the effective parasitic capacitance related to an oxide layer. Finally, the solution proposed in the present invention does not require using elaborated and costly techniques such as ion implantation or patterned tunnel junction.

According to a preferred embodiment the present invention provides a lasing device for use in an optical interconnect. The lasing device comprises a first reflector and a second reflector; a confinement layer adapted to confine current within a current-confining aperture, and an active layer between the first and second reflectors. The active layer comprises a main active region aligned with the current confining aperture and an auxiliary active region surrounding the main active region. The second reflector includes a first reflector region arranged on the current-confining aperture and a second reflector region surrounding the first reflector region. The second reflector region and the first reflector are configured to induce stimulated recombination in the auxiliary active region.

In this manner, although the periphery of the active layer is electrically isolated through the confinement layer and the carrier density is not clamped throughout the active layer, the excess of carriers that accumulates at the periphery of the main active region can be consumed by means of stimulated recombination induced by the second reflector region. Since the excess carriers at the periphery of the main active region are consumed by the additional stimulated recombination, the capacitive effect due to the flow of carriers from the periphery to the center of the main active region is suppressed.

According to a further advantageous development, the reflectance of second reflector region may be higher than the reflectance of the first reflector region. The higher reflectance of the second reflector region increases the intensity of the optical field at the periphery of the main active region and consequently the recombination rate of the carriers. Moreover, the higher reflectance of the second reflector region decreases the lasing threshold in the auxiliary active region. The higher recombination rate consumes the excess of carriers, thereby preventing the formation of a reservoir of carriers at the periphery of the main active region.

According to another development, the first reflector region is aligned with the current confining aperture and the area of the first reflector region is larger than the area of the current confining aperture. In particular, if the current confining aperture has a circular shape, also the first reflector region is chosen to be circular and the diameter D1of the first reflector region is larger than the diameter D0of the current confining aperture. In this case, the second reflector region is chosen to be ring-shaped with an inner diameter D2that may be equal to or larger than the diameter D1of the first reflector region.

In a further development a ratio S1/S0between the area of the first reflector region and the area of the current confining aperture may be in the range from 1.0 to 3.3. The relation between the sizes of the current confining aperture and the first reflector region allows to control the coupling between the main laser and the guard laser, and consequently the effectiveness of the guard laser on reducing the capacitive effect of the excess carriers at the periphery of the main laser.

In a lasing device according to an advantageous realization of the invention, the first reflector and the second reflector respectively include at least one layer with high refractive index and one layer with low refractive index. The layers can be easily grown by standard techniques and the so obtained reflector has an optimal reflectance of above 99.5%.

A layer of the second reflector may be fabricated so as to be thicker in the second reflector region than in the first mirror region. The thickness of a topmost reflector layer of the second reflector may be chosen to be an odd-number of a quarter wavelength in the second reflector region and zero or an even number of a quarter wavelength in the first reflector region. In this manner the first reflector region and the second reflector region can be easily defined by simply performing a further step in the manufacturing process of a conventional lasing device.

In an alternative advantageous realization of the present invention, the second reflector may include a reflector element arranged on the second reflector region and adapted to increase the reflectance of the second reflector region. Accordingly, the reflectivity of the second reflector can be selectively adjusted so as to form the first and second reflector regions by growing or mounting an additional layer on top of the second reflector, without involving any etching and masking process.

In an embodiment of the present invention, the first reflector region, the main active region and the first reflector define a main laser, and the second reflector region, the auxiliary active region and the first reflector define an auxiliary laser.

The lasing device of the present invention may further include a first cladding layer and a second cladding layer. The first and second cladding layers sandwich the active layer there-between so as to form a cavity spacer. The length of the cavity spacer may be chosen so as to correspond to an integer number of half waves at the emission wavelength. The thickness of the cavity spacer defines the resonance and the emission wavelength.

According to a further development of the present invention the active layer includes a stack of a plurality of alternating quantum wells and barriers and a vertical confinement layer on either side thereof.

The lateral confinement layer may be arranged between the first reflector and the second reflector and may include an oxidized layer of semiconductor or an ion implanted region surrounding the current-confinement aperture.

According to a further advantageous development of the present invention, the first reflector region is disk-shaped and the second reflector region is ring-shaped.

An embodiment of the present invention relates to an optical module including the lasing device described above. The above described lasing devices have a reduced carrier density at the periphery of the active region and do not suffer of carries flowing back from the periphery towards the center of the main active region upon switching from high to low driving currents. Consequently, the optical module can be driven at high modulation speeds. Optical interconnects mounting said lasing devices can therefore be used in high-speed fiber optical communication systems having an increased modulation bandwidth.

A further embodiment of the present invention provides a method for forming a lasing device for use in an optical interconnect. The method includes forming a first reflector stack on a semiconductor substrate and a confinement layer. The confinement layer is adapted to confine current within a current-confining aperture. An active layer is formed on the first reflector stack, and comprises a main active region aligned with the current confining aperture and an auxiliary active region surrounding the main active region. A second reflector stack is formed on the active layer, and in the second reflector stack a first reflector region is arranged on the current-confining aperture. A second reflector region surrounding the first reflector region is further formed, wherein the second reflector region and the first reflector are configured to induce stimulated recombination in the auxiliary active region.

The accompanying drawings are incorporated into and form a part of the specification to illustrate several embodiments of the present invention. These drawings together with the description serve to explain the principles of the invention. The drawings are only for the purpose of illustrating preferred and alternative examples on how the invention can be made and used and are not to be construed as limiting the invention to only the illustrated and described embodiments. Further, features and advantages will become apparent from the following and more particular description of the various embodiments of the invention as illustrated in the accompanying drawings, in which like reference numbers refer to like elements and wherein:

DETAILED DESCRIPTION OF THE INVENTION

In the following description, for explanatory purposes, specific details are set forth in order to provide a thorough understanding thereof. However, it may be evident that the present invention can be practiced without these specific details. Furthermore, well known structures and devices are only described in a more general form in order to facilitate the description thereof.

In the following description the expressions “mirror” and “reflector” are used to indicate the stacks of semiconductor or dielectric layers defining the resonant cavity of the laser. Similarly the expressions “first mirror region” and “first reflector region” are used in the following with the same meaning; and the expressions “second mirror region” and “second reflector region” are used in the following with the same meaning. The expression “gain region” and “cavity spacer” indicate the semiconductor layers sandwiched between the reflectors of the lasing device.

The problem underlying the present invention is based on the observation that upon switching down the driving current in direct-modulated lasing devices, the carrier density will decrease at a much faster rate at the center than at the periphery of the active layer. This is due to the fact that in the areas with higher optical field intensity, i.e. the center of the active layer, stimulated recombination is stronger. Accordingly, an excess of carriers accumulates in the periphery of the active layer, which acts as a carrier reservoir, and a lateral redistribution of carriers from the periphery towards the center of the active layer can take place. During a high-to-low transition, the laser evolves toward a lower photon density and a reduced recombination rate. This will enhance the effect of the excess carrier density at the periphery of the active layer, thereby extending the fall time of the lasing device. Moreover, lasing devices using an oxide semiconductor layer for confining the current in the device may suffer from lateral carrier spreading, resulting in a leakage current and an increase of the effective parasitic capacitance of the device with bias.

According to the present invention, the modulation of the carrier distribution at the periphery of the active layer is reduced by implementing an auxiliary laser, or guard laser, at the periphery of the main laser. The guard laser will clamp the carrier density within its active layer and consume the excess carriers at the periphery of the active layer of the main laser through stimulated recombination. In addition, the guard laser will reduce the carrier injection in the outer regions of the diode structure forming the lasing device, thereby limiting the increase of the effective parasitic capacitance related to the oxide layer.

FIG. 1illustrates a sectional view of a lasing device designed according to the principles of the present invention. The lasing device1000comprises a substrate1030of semiconductor material on top of which a first reflector or mirror1300is formed. The first reflector1300may be any semiconductor material with a high reflectivity. A cavity spacer1200is further provided on top of the first mirror1300. The cavity spacer1200includes a first cladding layer1230and a second cladding layer1210which are doped with opposite conductivity types. An active layer1220is sandwiched between the first and second cladding layers1230and1210. The active layer1220is made of an intrinsic semiconductor and is the layer in which recombination of carriers occurs when the laser is driven by electrical or optical pumping. A closer description of the gain region1200is given with reference toFIG. 4.

The lasing device1000further includes a confinement layer1020, which defines a current confining aperture1021. The confinement layer1020is a layer of oxidized semiconductor material or oxidation layer. The oxidation layer1020is generally obtained by growing a high content aluminum semiconductor layer within the lasing device1000and further oxidizing the layer. The oxidized portion is electrically non-conductive and also has a lower refractive index, which also provides some degree of confinement for the optical field generated in the active layer1220. The current confining aperture1021is a non-oxidized portion of the oxidation layer1020. The current and the optical field generated by forward biasing the lasing device1000are mostly confined within the current-confinement aperture1021.

A second reflector1100is formed on top of the current confinement layer1020. The first and second mirrors or reflectors1300and1100are doped with impurities so as to have opposite conductivity types and form a diode junction. The second mirror1100and the second cladding layer1210are p-type, while the first mirror1300as well as the first cladding layer1230are n-type. The active layer1220includes one or more layers of an intrinsic semiconductor.

According to the structure inFIG. 1, the lasing device1000is a p-i-n diode junction. However, other configurations different from the described one may be used according to the particular application of the lasing device1000. Further, in an alternative realization, the first and second mirrors1300,1100may be intrinsic semiconductors or dielectric and p-type and n-type semiconductor layers may be buried between the first and second mirror1300,1200, to provide electrical connection to the active layer1220.

The second mirror1100includes a first mirror region1140and a second mirror region1130surrounding the first mirror region1140. The reflectance of the second mirror region1130R2is selected to be higher than the reflectance R1of the first mirror region1140. The first mirror region1140is formed in the second reflector1100above the current-confining aperture1021. The area of the first mirror region1140is larger than the area of the current confining aperture1021and also partially overlaps a portion of the oxidation layer1020immediately surrounding the current confining aperture1021. However, in other preferred realizations of the present invention the first mirror region1140may be chosen to have the same size as the current-confining aperture1021, or a smaller size.

The first and second reflector regions1140,1130may be obtained by adding a top reflector1110on the top surface of the second mirror1100. The top reflector1110may be a distributed Bragg reflector made of dielectric layers of alternate high and low refractive indices, a metal reflector, or a combination thereof.

Upon forward biasing the lasing device1000holes and electrons will respectively move from the p-doped layer1210and the n-doped layer1230to the active layer1220. Recombination of holes and electrons injected into the active layer1200from the p and n sides of the p-i-n junction produces emission of light. The first and second mirrors1300and1100between which the active layer1220is sandwiched form together with the cavity spacer1200, a resonant cavity in which the light is reflected before being emitted from the lasing device1000.

The first mirror region1140, the portion of the cavity spacer1200corresponding to the area of the first mirror region1140and arranged there-below, and the first dielectric mirror1300define a main laser1400, which emits light with wavelengths depending on the energy gap of the semiconductor materials used for fabricating the lasing device1000and on the thickness of the cavity spacer. Similarly, the second mirror region1130surrounding the first mirror region1140, the portion of the cavity spacer1200below it and the first dielectric mirror1300define an auxiliary laser1500, or guard laser. The guard laser1500surrounds the portion of the lasing device1000defining the main laser1400. The portion of the active layer1220surrounding the current confining aperture1021and arranged below the second mirror region1130is isolated through the oxidation layer1020. Therefore, holes can not be injected directly from the p-type mirror into said portion of the active layer. Instead, carriers leak from the center of the active region1220, which corresponds to an active region of the main laser1400into the periphery of the active region1220, which corresponds to the active region of the auxiliary laser1500. In other words, the recombining carriers of the auxiliary laser1500are injected from the center of the active layer1220. Details of the active layers and on the different regions of the active layer will be given with reference toFIG. 5.

The carrier density in the center of the active layer1220corresponding to the first mirror region1140at and above the threshold current generally depends on the threshold gain in the active layer and thus on the reflectance of the dielectric mirrors1100and1300. Generally, the carrier density at the threshold is inversely proportional to the reflectance of the electric mirror1100and1300of the lasing device. In the lasing device of the invention, the reflectance R2of the second mirror region1130is higher than the reflectance of R1of the first mirror region1140. This configuration provides for a lower threshold carrier density of the auxiliary laser1500. A more detailed description of the design of the main and auxiliary lasers1400,1500and the corresponding active layers will be given with reference toFIG. 5.

In vertical cavity surface emitting lasers the reflectivity of the mirror1300is in the range of about 99.5 to 99.9%. This value can be lower in case of edge emitting lasers.

The details regarding the working principle of the lasing device1000also apply to all the other embodiments of the present invention as described inFIGS. 2 to 5.

The lasing device1000ofFIG. 1further includes first terminal contacts1010arranged on the top surface of the second mirror1100and second terminal contacts arranged on the bottom surface of the substrate (not shown).

FIG. 2shows a further embodiment of a lasing device2000according to the present invention. The lasing device2000includes a substrate2030, a first mirror2300, a cavity spacer2200including a first cladding layer2230and second cladding layer2210doped with opposite conductivity types, a second mirror2100and a current confinement layer2020sandwiched between the second cladding layer2210and the second mirror2100. The first mirror2300and the second mirror2100are doped so as to achieve opposite conductivity types. In particular, the first reflector2300and the first cladding layer2230are doped with n-type dopants and the second reflector2100and the second cladding layer2210are doped with p-type dopants. The current-confining layer2020includes a current-confining aperture2021.

The second mirror2100includes a first mirror region2140corresponding to the current confinement aperture2021. The area of the first mirror region2140is larger than the area of the current-confining aperture2021and may partially extend over the portion of the current confinement region2020immediately surrounding the current confining aperture2021.

In an advantageous example, the ratio between the area S1of the first mirror region2140and the area S0of the current confinement aperture2021may be about 1.4. The above value is not universal and may change depending on the particular design of the lasing device. More precisely, the optimum ratio S1/S0may be chosen based on the details of the particular realization, for instance, on the number of quantum wells and the composition of the quantum wells and barrier layer, and the actual reflectance of the first and second reflectors. The optimum ratio may be conveniently found by direct experimentation. As an example, several devices with first mirror regions having different areas S1may be processed on one substrate, such as a semiconductor wafer. The optimum design may be found by direct measurement of parameters such as the threshold current, differential efficiency and modulation bandwidth, and/or the rise and fall times of the optical signal generated when a modulated current is applied to the device. For many practical configurations commonly used in telecommunication applications the ratio S1/S0may be advantageous chosen in the range of 1.0 to 3.3.

In one particular configuration, the current confining aperture2021as well as the first mirror region2140may have a circular shape. In this case the diameters D0and D1of the current confining aperture and the first mirror region may be used as parameters. Accordingly, the ratio between the outer diameter D1of the first mirror region2140and the diameter D0of the current-confining aperture2021may be 1.2. Also in this case, the optimum ratio D1/D2depends on the details of the particular realization, for instance, on the number of quantum wells and the composition of the quantum wells and barrier layer, and the actual reflectance of the first and second reflectors. The optimum ratio may be conveniently found by direct experimentation. As an example, devices with several different D1values may be processed on one substrate. The optimum design may be found by direct measurement of parameters such as the threshold current, differential efficiency and modulation bandwidth, and/or the rise and fall times of the optical signal generated when a modulated current is applied to the device. For many practical configurations it may be advantageous to explore a range of 1.0 to 1.8 for the ratio D1/D0.

The current confinement region2020may be an oxidation layer as described with reference toFIG. 1or may be obtained by ion, for instance proton, implantation in the cladding layer2210. As an example, if the current confinement region is obtained by implantation, ions such as hydrogen ions and the like, may be implanted in the structure of the lasing device, for instance in the cladding layer2210, everywhere except the current-confining aperture2021. Ion implantation destroys the lattice structure around the aperture, thereby inhibiting the current flow through this region.

In another alternative embodiment, current confinement may be obtained by a patterned tunnel junction. Finally, first terminal contacts2010are arranged on the top surface of the mirror2100and second terminal contacts (not shown) are arranged on the bottom surface of the substrate2030.

The working principle of the lasing device2000is the same as the working principle of the lasing device1000and will not be explained again.

The first and second mirrors2300and2100are distributed Bragg reflectors (DBR) structures. The first mirror2300includes a stack of a first layer2320and a second layer2310with alternating high and low refractive indices. Each of the first and second layers2320and2310has a thickness of a quarter of the laser wavelength in the material. The layer composition of the first mirror2300according to the above design yields an intensity reflectivity in the range of 99% to 99.9%. Similarly, the second mirror2100also includes a stack of a first layer2320and a second layer2310of alternating high and low refractive indices. In a preferred realization of the present invention, the first and second mirrors2300,2100include 30 and 21 pairs of layers2310,2320, respectively. However, this configuration is not limiting and the first and second mirrors2300,2100may include any number of layers, such as 22 layers for the second DBR2100and 34 layers for the first DBR2300.

Although in the particular embodiment ofFIG. 2, the mirrors are made of distributed Bragg reflectors, the first and second mirrors of the lasing device2000may also be obtained by other means, such as using a thin metal-film arranged in a doped semiconductor layer and the like.

The second reflector2100includes a first mirror region2140and a second mirror region2130. The second mirror region2130has a higher reflectance than the first mirror region2140. This can be obtained by adding a top reflector2110on the top surface of the second mirror2100. The reflector2110may be an additional distributed Bragg reflector made of semiconductor or dielectric layers of alternate high and low refractive indices, a metal reflector, or a combination thereof.

Alternatively, the second mirror region2130can be obtained by modifying one or more of the second layers2310forming the second mirror2100by growing only part of the structure, patterning said structure and growing more layers or adding dielectric layers.

Still, in another embodiment, the second mirror region2130may be obtained by modifying one or more of the second dielectric layers2310forming the second mirror2100by converting the semiconductor layer in a similar way to the oxide aperture.

The shape of the additional reflector2110depends on the shape of the first mirror region2140and on the design of the current-confining aperture2021. More precisely, the additional reflector2110is designed so as to surround the first mirror region2140. In the embodiment ofFIG. 2, the current-confining aperture2021has a circular shape, in which case the second mirror2130and the additional reflector2110have a ring-shaped section. However, the proposed solution and the discussion thereof also apply to devices having current confining apertures, first mirror regions and second mirror regions of any shape.

The first mirror region2140, the portion of the gain region or cavity spacer2200corresponding to the area of the first mirror region2140and arranged below it, and the first mirror2300define a main laser2400, which emits light having one or several wavelengths depending on the energy gap of the semiconductor materials used for fabricating the lasing device2000and on the thickness of the cavity spacer2200. Similarly, the second mirror region2130surrounding the first mirror region2140, the portion of the cavity spacer2200below it and the first mirror2300define an auxiliary laser2500or guard laser. The guard laser2500surrounds the portion of the lasing device2000defining the main laser2400.

InFIG. 2the arrangement of the main and auxiliary lasers in the lasing device2000is illustratively indicated by the vertical dashed lines. In particular, the region between the two dashed lines identifies the main lased2400, whereas the region of the lasing device extending externally of the dashed lines identifies the auxiliary laser2500. The same illustrative delimitation is used for the embodiments ofFIGS. 1 and 3.

FIG. 3shows a sectional view of a lasing device3000according to a further embodiment of the present invention. The lasing device3000ofFIG. 3includes all the parts already described with reference toFIG. 2. In particular, the lasing device includes a first and a second mirror3300,3100, a first and a second cladding layer3230,3210, a current confinement layer3020and an active layer3220. The first mirror region3140, the portion of the cavity spacer3200corresponding to the area of the first mirror region3140and arranged there-below, and the first mirror3300define a main laser3400, which emits light with one or several wavelengths depending on the energy gap of the semiconductor materials used for fabricating the lasing device3000and on the thickness of the cavity spacer3200. Similarly, the second mirror region3130surrounding the first mirror region3140, the portion of the cavity spacer3200below it and the first mirror3300define an auxiliary laser3500or guard laser. The guard laser3500surrounds the portion of the lasing device3000defining the main laser3400.

Alternative to the embodiment ofFIG. 2, a top layer3110of the mirror3100has a modified thickness in the region corresponding to the first mirror region3140. More precisely, the second mirror3100is a DBR reflector including a stack of alternating layers of high refractive index3110and low refractive index3120. The thickness of these layers is an odd number of a quarter of the laser wavelength in the semiconductor material, said thickness providing the highest reflectance for the DBR stack. The DBR3100is terminated with a high refractive index layer3110. The thickness of this topmost dielectric layer3110is modified so as to be an even number of a quarter of the laser wavelength in the semiconductor material. The even number of a quarter wavelengths causes an anti-phase condition of the DBR reflector, thereby minimizing the reflectance of the layer stack. In this embodiment, the first mirror region3140is therefore realized by reducing the thickness of the topmost layer3110of the second DBR reflector along the region corresponding to the main laser3500.

The lasing devices1000,2000and3000according to the present invention are designed based on a stack made of Gallium Aluminum Arsenide and Gallium Arsenide (GaAlAs/GaAs) and can emit light with a wavelength in the range of approximately 650 nm to 1,300 nm. In particular, the embodiments described in the present invention emit light with a wavelength around 850 nm. However, these devices may also be designed to emit light at other wavelengths, such as 980 nanometers or 1,060 nanometers.

The lasing devices ofFIGS. 2 and 3may be vertical Cavity Surface Emitting Lasers (VCSELs). The design of these VCSELs is based on a stack made of Ga(1-x)AlxAs layers. The first and second mirrors2300,2100and3100,3300are distributed Bragg reflectors (DBRs), which use layer pairs with alternating composition to provide the desired reflectance. The cavity spacer2200,3200is also Ga(1-x)AlxAs where x is varied in order to provide desired electrical transport and confinement characteristics. The active layer2220,3220includes a number of GaAs quantum wells (not shown) with Ga(1-x)AlxAs barriers (not shown). A more detailed description of the cavity spacer2200,3200will be given with reference toFIG. 4. In an alternative embodiment, the active layer2220,3220may include strained Ga(1-x)InxAs quantum wells with Ga(1-x)AlxAs barriers. This solution is particularly advantageous in high performance VCSELs, since the Ga(1-x)InxAs semiconductor material ensures a better performance and a higher reliability.

As a concrete example, the second DBR2100,3100may be p-doped and the first DBR2300,3300may be n-doped. The substrate2030,3030may be n-doped GaAs. The cavity spacer2200,3200is designed as a separate confinement structure and may be a layer of Ga(1-x)AlxAs with graded x. As an example x may be in the range from 0.3 to 0.9. The active layer2220,3220may consist of 3 Ga(1-x)InxAs quantum wells, wherein x may be in the range from 5 to 10%, with Ga(1-x)AlxAs barriers, wherein x is in the range from about 30 to about 40%. On the p-side of the lasing device2000,3000between the second DBR mirror2100,3100and the cavity spacer2200,3200is provided an oxidation layer2020,3020. The oxidation layer2020,3020is a thin Ga(1-x)AlxAs layer with a higher x in the range from 0.96 to 0.98 and is partly oxidized from the outer periphery inwards in order to form the current-confining aperture2021,3021. The oxidized portion of the oxidation layer2020,3020is electrically non-conductive and it also has a lower refractive index which may also provide some degree of confinement for the optical field.

In an alternative embodiment, the oxidation layer2020,3020may also be arranged in the second DBR reflector2100,3100or in the cavity spacer2200,3200in a position above the active layer2220,3220. In yet a further embodiment, besides the oxidation layer2020,3020a second oxidation layer (not shown) may be provided between the first DBR mirror2300,3300and the cavity spacer2200,3200, or within the first DBR mirror2300,3300, or in the cavity spacer2200,3200in a position below the active layer2220,3220.

Commonly, the oxidation layer2020,3020and the resulting current-confining aperture2021,3021is placed in the p-doped side of the lasing device1000,2000,3000. This configuration generally gives a better current confinement as compared to placing just one aperture on the n-side. In particular, the layer is between the p-DBR2100,3100and the cavity spacer2200,3200or in one of the first pairs of the p-DBR2100,3100, closest to the spacer2200,3200. However, many other configurations such as having several apertures on the p-side or on both the p- and n-sides can be used. The various configurations aim at improving current confinement, reducing parasitic capacitance and optimizing the transverse confinement of the optical field.

In the lasing devices1000,2000,3000depicted inFIGS. 1 to 3, the area of the current-confining aperture1021,2021,3021is smaller than the area of the first mirror region1140,2140,3140defining the main laser1400,2400,3400and thus it is smaller than the inner dimension of the ring defining the second mirror region1130,2130,3130. The relation between the diameter of the current-confining aperture1021,2021,3021and the inner diameter of the guard ring1130,2130,3130dictates the coupling interaction between the main laser1400,2400,3400and the guard-ring or auxiliary laser1500,2500,3500. In order to achieve the desired effect, the portion of active layer1220,2220,3220corresponding to the main laser1400,2400,3400and the auxiliary laser1500,2500,3500and the optical fields of the main laser1400,2400,3400and of the auxiliary laser1500,2500,3500should have some degree of overlap.

Moreover, in the configuration presented, the guard-ring laser1500,2500,3500operates exploiting the carriers leaking out of the active region of the main laser.

If the inner diameter of the guard ring1130,2130,3130is made larger, the coupling is reduced and the carrier density at the periphery of the main laser may swing more with the applied modulation, while still being relatively clamped within the active region of the guard laser1500,2500,3500. Consequently, the guarding will be less effective. Also, as the inner diameter of the ring is made larger, the carrier density in the portion of the active layer1220,2220,3220of the guard laser will be effectively lower. If the carrier density is less than the threshold value, the guard laser will not lase and the guarding effect will be lost.

On the other hand, if the inner diameter is made too small, the guard ring will consume a significant fraction of the carriers injected in the device. This is effectively a loss mechanism that will reduce the efficiency of the main laser, generally leading to a degraded performance. In an extreme case, the guard laser may take over, turning off the main laser, by, for instance, consuming a too large fraction of the carriers injected into the device. The optimum ratio between the diameter of the first reflector region and the diameter of the current-confining aperture depends on the particular realization of the invention as explained herein. Also, as already explained herein, the optimum ratio may be conveniently found by direct experimentation. The optimum ratio between the outer diameter D1of the first mirror region3140and the diameter D0of the current-confining aperture3021will be in the range of 1.0 to 1.8 for many practical realizations.

The relation between the sizes of the current confining aperture, the first reflector region and the second reflector region controls the coupling between the main laser and the guard laser, thereby the effectiveness of the guard laser on reducing the capacitive effect of the excess carriers at the periphery of the main laser. In particular, for a circular current confining aperture, the coupling between the main and the guard lasers is controlled by the relation between the diameters D0, D1and D2. In a preferred configuration, the inner diameter D2of the second reflector region is equal to the diameter D1of the first reflector region. In this particular configuration, the coupling between the main and the guard lasers is controlled by the ratio between D1and D0.

For a confining aperture having a particular diameter D0, reducing the diameter D1of the first reflector region will provide a stronger coupling between the guard laser and the main laser and a more effective reduction of the capacitive effect of the excess carriers. At the same time, reducing D1will lead to a larger fraction of the carriers injected into the device being consumed by the guard laser thereby increasing the carrier loss from the main laser, leading, in turn, to a higher threshold current and lower efficiency of the main laser. As one can understand, there is a trade-off between reducing the effect of the excess carriers by the guard laser and carrier loss from the main laser. The optimum ratio between the diameter of the first reflector region and the current-confining aperture depends on the particular materials and overall design of the device. For common practical situations, the optimum ratio D1/D0is in the range of 1.0-1.8.

However, as the invention can be applied to devices realized in a wide variety of material combinations and with other specific design parameters covering relatively wide ranges, the optimum ratio may be outside this range for some particular realizations. For instance, the current confining aperture and the first mirror region may be not circular but rather have a different shape. In this case, the area S1of the first mirror region3140and the area S0of the current confinement aperture3021may be chosen as parameter. The value of the ratio S1/S0may be chosen in the range from 1.0 to 3.3, as already explained with reference toFIG. 2. Advantageously, the ratio S1/S0may be 1.4.FIG. 4schematically shows a section of a cavity spacer200used in the lasing devices1000,2000and3000. The cavity spacer200includes a first cladding layer230and a second cladding layer210. The cladding layers230,210are made of semiconductor material and are doped such as to achieve opposite conductivity types. In the particular embodiment ofFIG. 3, the first cladding layer230is doped with donors (n-type), whereas the second cladding layer210is doped with acceptors (p-type). The cavity spacer200further includes an active layer220sandwiched between the first and second cladding layers230,210.

The active layer220comprises a plurality of layers of an intrinsic semiconductor material. More precisely, the active layer220includes a stack of alternating quantum wells221and barriers222. As described above, the quantum wells221consist of a layer of Ga(1-x)InxAs, wherein x is in the range of 5% to 10%, while the barriers222include a layer of Ga(1-x)AlxAs, wherein x is in the range of ca. 30% to 40%.

AlthoughFIG. 4shows an active layer including three quantum wells221and two quantum well barriers222, this configuration should not be considered to be limiting. More precisely, the active layer200may include any number of quantum wells221and quantum well barriers222depending on the particular application of the lasing device1000,2000,3000.

The active layer200further includes two intrinsic confining layers223sandwiching the stack of quantum wells and barriers221to222. The confining layers223are arranged between the quantum wells221and the cladding layers210,230and may be graded.

FIG. 5is a schematic picture illustrating the working principle of a lasing device according to the present invention. AlthoughFIG. 5refers to the embodiment inFIG. 3, the principles introduced and described with reference toFIG. 5hold also true for the embodiments described inFIGS. 1 and 2.

FIG. 5shows in particular the carrier density in the active layer3220depending on the forward bias applied to the lasing device3000. Curve (1) shows the carrier density at a high current level corresponding to the high state of the lasing device3000. Curve (3) shows the carrier density at a lower current level, while curve (2) shows the carrier density shortly after switching the current from the higher to the lower level.

In an ideal laser at a bias above threshold, the carrier density in the active layer is clamped at the threshold carrier density. Clamping of the carrier density is a direct effect of the coupling of the optical field (photon density) and the carrier density in the active layer through stimulated recombination of carriers and stimulated emission of photons. The carrier density generates sufficient amplification of the optical field to compensate for the total optical loss of the resonator. Any increase in the carrier density above the threshold value results in a gain that exceeds the total loss and leads to a rapid increase of the photon density and the stimulated recombination rate. This process restores the threshold level by consuming the excess of carriers.

In the lasing device3000ofFIG. 5, as the current increases above the threshold value, the carrier density in the active layer3220is not perfectly clamped, but continues to increase in order to compensate for gain reduction as the temperature increases due to internal heating. Another factor that leads to an increase of the carrier density is the increase of the photon density or the intensity of the optical field generated in the lasing device3000. Any excess carrier density that would result in a gain exceeding the optical losses is effectively consumed through mainly stimulated recombination.

FIG. 5shows different zones included in the active layer3220. More precisely, holes are injected through the current-confining aperture3021from the p-type cladding layer3210into zone3221when the lasing device3000is forward biased. Zone3221indicates the active region of the main laser3400or main active region. The optical field intensity has a maximum at the center of zone3221. Therefore, the carrier density in the main active region3221is more responsive to changes in the bias current due to the stronger stimulated recombination caused by higher optical field intensity.

Zone3222surrounds zone3221and is shielded by the current-confining layer3020. Zone3222indicates the auxiliary active region. Due to the layer3020, direct injection of carriers from the cladding layer3210into the auxiliary active region3222is suppressed. Therefore, the auxiliary active region3222mainly includes carriers that leaked from the main active region3221. Since the second mirror region3130of the second mirror3100has a higher reflectivity than the first mirror region3140defining the main laser3400, the rate of stimulated recombination in the auxiliary active region3222is increased as compared to a device of a conventional design. This has the effect of limiting the increase of carrier density in the auxiliary active region3222, which corresponds to an active region of the auxiliary laser3500.

In this manner, upon switching from a high voltage value to a low voltage value, there will be no excess of carriers at the periphery of the active region of the main laser3400, thereby suppressing the lateral redistribution of carriers from the periphery towards the center of the active layer3220, which causes a capacitive effect in the laser.

The emitting wavelength of the guard laser or auxiliary laser1500,2500,3500depends on the semiconductor material of the lasing device1000,2000,3000, on the energy gap of the semiconductor in the active layer1220,2220,3220and on the thickness of the cavity spacer1200,2200,3200. The guard laser1500,2500,3500emits at a wavelength, which is very close to the wavelength of the main laser1400,2400,3400. This solution is easier to implement and more cost effective, since the same semiconductor material can be used for the main and auxiliary lasers. However, the guard laser does not need to emit light at the same wavelength as the main laser and the principles and ideas of the present invention also apply to lasing devices, wherein the guard laser emits at a different wavelength as the main laser.

Further, both the main and guard lasers1400,1500,2400,2500,3400and3500may have a multi-mode emission containing several closely spaced wavelengths. The different resonator geometry as well as carrier and photon density distributions of the two lasers may lead to different mode distributions and hence slightly different sets of emission wavelengths.

FIG. 6shows in a sectional view an optical module6000according to the present invention. The optical module6000may be an optical transceiver and may be connected to a circuit carrier, such as a printed circuit board (PCB), a connecting socket or the like, and then used in fiber optical interconnections for midboard applications or as an intra-board or inter-board module. The optical module6000includes an optically transparent carrier6100, on which may be fixed one or more integrated circuits (not shown) or any kind of surface mount component. The optically transparent carrier6100may further include one or more lasing devices1000,2000,3000.

The optical transparent carrier6100may be made of pyrex glass optically transparent for a defined wavelength, the standard communication wavelength used in midboard applications being 850 nm. However, the transparent carrier may be chosen so as to be transparent to other wavelengths according to the specific application of the optical module. Moreover, alternative to the pyrex glass, other types of optically transparent materials having other optical characteristics may also be used.

The transparent carrier6100further includes metal wirings and first electrical connection pads6030on a first surface6120of the transparent carrier6100. The first surface6120will be also indicated in the following as top surface of the transparent carrier6100. The lasing device1000,2000,3000is mounted on the first surface6120and is electrically connected to the transparent carrier6100. The lasing devices1000,2000,3000may be attached to the transparent carrier6100by any kind of known means capable of conducting current, for instance by means of solder bumps6010. The metal traces included in the transparent carrier610connect the lasing devices1000,2000,3000through the solder bump to the first electrical connection pads or terminals6030. The electrical connection terminals6030may be arranged at the periphery of the transparent carrier6100. The transparent carrier6100is mechanically attached and electrically connected to a carrier substrate6400.

Although in the particular embodiment described above the lasing devices1000,2000,3000are mounted on the first surface6120of the transparent carrier6100, they may also be mounted on any other surface of the transparent carrier6100.

During operation of the optical module6000, the lasing devices1000,2000,3000, fed by electrical signals through the carrier substrate and the optically transparent carrier, emit light through the optically transparent carrier6100towards a bottom surface of the carrier substrate6400. The emitted light may then be received by an optical coupling element (not shown) and coupled into light guiding elements such as wave guides or the like.

The carrier substrate6400is capable to handle high frequency signalling so that the optical module6000can be used for high bit rate fibre applications. Moreover, the transparent carrier6100and the carrier substrate6400are connected according to a flip-chip design.

A further embodiment of the present invention provides a method for forming a lasing device (1000,2000,3000) for use in an optical interconnect. The method refers to the lasing devices ofFIGS. 1 to 3and includes forming a first reflector stack (1300,2300,3300) on a semiconductor substrate. Subsequently, a cavity spacer (1200,2200,3200) is formed on the first reflector stack (1300,2300,3300). The cavity spacer includes a first and a second cladding layer and an active layer (1220,2220,3220) sandwiched there-between. A confinement layer (1020,2020,3020) for confining current within a current-confining aperture (1021,2021,3021) is further formed on the second cladding layer. A second reflector stack (1100,2100,3100) is formed on the active layer.

The second reflector stack (1100,2100,3100) is modified to define a first reflector region (1140,2140,3140) arranged on the current-confining aperture and a second mirror region (1130,2130,3130) surrounding the first mirror region (1130,2130,3130). In particular, the first and second mirror regions may be realized by masking the topmost surface of the second reflector (1100,2100,3100) and subsequently etching a shallow surface relief in the second reflector (1100,2100,3100). The first mirror region (1130,2130,3130) may have the same shape of the current confining aperture but its area is chosen to be larger than the area of the current confining aperture.

In the active layer (1220,2220,3220) are defined a main active region (3221) aligned with the current confining aperture and an auxiliary active region (3222) surrounding the main active region. The main active region is larger than the current confining aperture and comprises the zone of the active layer underneath the current confining aperture, into which carriers are directly injected from the p- and n-doped sides, and the region of the active layer immediately surrounding said zone.

In a particular realization of this invention the current-confining aperture may have a diameter of 8 microns. The diameter of the first reflector region may be 11 microns. In another realization, the current-confining aperture may be 10 microns and the diameter of the first reflector region may be 13 microns. The above described configurations are only particular examples and are not to be considered limitative in any way. Indeed, the dimensions of the current-confining aperture and the first reflector region may be chosen to be different than that listed above. The size and shape of the current-confining aperture and the first reflector region may depend on the design and the particular application of the lasing device. More precisely, these values may depend on other parameters of the design and on the characteristics of the materials used, such as the effective background doping of the epitaxial materials used in the cavity spacer and, in particular, the cladding layers. These characteristics are, in turn, very specific to the particular equipment and growth recipes used to produce the different layers. Accordingly, the size of the current confining aperture and of the first reflector region may deviate from the given values.

The method above describes an etching technique for modifying the second reflector (1100,2100,3100). However, alternative methods can also be used according to further realizations of the present invention. More precisely, the second mirror region (1130,2130,3130) may be formed by growing a further reflector stack on a portion of the second reflector (1100,2100,3100) so as to define a relief of the desired shape, which corresponds to the first reflector region. Alternatively, a layer of a highly reflecting material, such as a metallic layer or the like, may be mounted on the second reflector (1100,2100,3100).

The second reflector region and the first reflector formed as described above induce stimulated recombination in the auxiliary active region.

The present invention provides a lasing device with an improved response to high-to-low transitions of the bias voltage used in high-bit rate directly modulated optical interconnects.

According to the present invention, an auxiliary laser or guard laser surrounds the main laser, which burns the excess carriers surrounding the active region of the main laser and reduces therefore the capacitive effect of the excess carriers in the active region OF the main laser. This is obtained by providing a resonant cavity surrounding the main laser including reflectors that have a higher reflectivity than the reflectors of the main laser. In this manner, the intensity of the optical field in the area of the active layer surrounding the main laser can be increased so as to increase the rate of stimulated recombination at the periphery of the main laser, thereby burning the excess carriers. The solution described above with reference toFIGS. 1 to 6, allows to produce at low cost and with an easy design a lasing device and optical modules capable of supporting a high modulation bandwidth for 25 Gb/s and higher bitrates applications.