TECHNICAL FIELD

The present disclosure relates to optical communications using a spin vertical-cavity surfaceemitting laser (VCSEL). The disclosure presents a laser apparatus that comprises the spin VCSEL and a light source for optically pumping the spin VCSEL. The laser apparatus may be used for optical communication.

BACKGROUND

Optical communication technology for short-haul optical communication, for example in data centers, is mainly based on intensity modulation of laser light in fully-integrated VCSELs or in some cases (e.g., in some hyperscale data centers) based on external modulators. These conventional optical devices are used for converting information carriers in the electrical domain into the optical domain.

However, these conventional optical devices reach either fundamental physical limits when it comes to the modulation bandwidth (e.g., in the case of VCSELs), or are bulky and costly (e.g., in the case of external modulators). Hence, new concepts for ultrafast short-range communication are desired.

SUMMARY

This disclosure accordingly aims to improve the conventional optical devices used for converting information from the electrical domain into the optical domain. An objective is to achieve an increased modulation bandwidth, which goes beyond that of a conventional intensity-modulated VCSEL. Another objective is to make a fully-integrated and compact solution possible, in contrast to external modulators and conventional spin VCSELs. Another objective is to enable a room-temperature operation, while using existing (processing) technologies.

These and other objectives are achieved by the concept of this disclosure as described in the independent claims. Advantageous implementations are further defined in the dependent claims. A first aspect of this disclosure provides a laser apparatus comprising: a spin VCSEL; and a light source for optically pumping the spin VCSEL using light with a non-zero circular polarization degree (CPD); wherein the spin VCSEL is configured to receive an electrical current signal comprising an amplitude modulation, wherein the spin VCSEL is configured to output a first laser signal when being pumped with the light with the non-zero circular polarization degree and when receiving the electrical current signal, the first laser signal comprising a circular polarization modulation, and wherein the circular polarization modulation of the first laser signal correlates with the amplitude modulation of the electrical current signal.

A spin VCSELs is a VCSEL with a majority of, for example, spin-up or spin-down charge carriers during recombination and generation of its laser signal. Due to the conservation of the angular momentum, the emitted laser signal carries a spin angular momentum as well, e.g., it is circularly polarized (to be more precise, the CPD may evolve in time).

The bandwidth of a conventional intensity-modulated VCSEL is limited by the dynamics, which govern the interactions between the carriers in the active region of the VCSEL and the photons, which are created there whenever carriers recombine and generate laser light, in other words, the carrier-photon dynamics. Spin dynamics describe the temporal evolution of spin states of carriers and photons in the active region of the spin VCSEL, at least in the case of a majority and minority of spin states (due to the optical pumping with the light with the nonzero CPD. The speed of the spin dynamics of the carriers and photons in spin VCSEL (e.g., a cavity of the spin VCSEL) is limited by usually very short spin-relaxation times for the spin- polarized charge carriers and the optical birefringence in the laser cavity. Compared to the carrier-photon dynamics in the conventional VCSEL, the spin dynamics in the spin VCSEL may be much faster, as they may occur on much shorter time scales. Typically, carrier-photon dynamics and spin dynamics do not influence each other. The concept of this disclosure enables a coupling of these two systems together via the optical pumping.

Optical pumping comprises a process, in which light is used to excite (or "pump") electrons from a lower energy level to a higher one in the active region of, in this case, the spin VCSEL. In this disclosure, the optical pumping is used in connection with circularly polarized light, in order to create the majority of, for example, spin-up charge carriers in the active region of the spin VCSEL by virtue of angular-momentum conservation (i.e., spin-polarized carrier creation).

The spin VCSEL emits the first laser signal. The CPD of the first laser signal may oscillate between different values and can be modulated and used for information transfer. Thus, the first laser signal comprises the circular polarization modulation. The modulation bandwidth for this can be much higher than for a conventional intensity-modulated VCSEL. Thus, the present disclosure enables a significant increase of the modulation bandwidth.

This disclosure also makes external modulators and modulated optical spin injection with external lasers obsolete. A fully-integrated, compact system is possible with the concept of this disclosure. Notably, the concept of this disclosure may be based on only existing (processing) technologies, which work at room temperature.

In an implementation form of the first aspect, the amplitude modulation of the electrical current signal comprises at least a higher current amplitude and a lower current amplitude.

Thus, information may be encoded into the electrical current signal, and may be transferred to the first laser signal by the spin VCSEL.

In an implementation form of the first aspect, a change of the current amplitude of the electrical current signal correlates with a change of a CPD of the first laser signal.

The change of the CPD may be used to encode information in the first laser signal, wherein this information may correspond to the information encoded into the electrical current signal by means of the changing current amplitude.

In an implementation form of the first aspect, the first laser signal further comprises an intensity modulation that correlates with the amplitude modulation of the electrical current signal.

In an implementation form of the first aspect, the laser apparatus further comprises: one or more optical components configured to convert the first laser signal into a second laser signal comprising an intensity modulation, wherein the intensity modulation of the second laser signal correlates with the circular polarization modulation of the first laser signal. The second laser signal enables the use of a standard receiver. The receiver may convert the second laser signal back into an electrical signal, for example, using a photodiode with a high enough bandwidth.

In an implementation form of the first aspect, the laser apparatus further comprises an optical fiber configured to receive the first laser signal or the second laser signal for transmitting it to a receiver.

The first laser signal may be directly coupled into the optical fiber. Thus, no optical components for converting the first laser signal are required at the laser apparatus of the first aspect.

In an implementation form of the first aspect, the laser apparatus further comprises a receiver configured to receive the first laser signal or the second laser signal, and to convert the first laser signal or the second laser signal into an electrical signal.

The receiver may be a conventional receiver, as it may be used in a communication systems that includes a conventional intensity-modulated VCSEL.

In an implementation form of the first aspect, the spin VCSEL and the light source for optical pumping are integrated.

Thus, a fully integrated solution can be provided.

In an implementation form of the first aspect, the light source for optical pumping comprises a DC-current-operated VCSEL and a quarter- wave plate configured to convert an output laser light of the DC-current-operated VCSEL into the light with the non-zero circular polarization degree.

In an implementation form of the first aspect, the spin VCSEL is stacked on the DC-current- operated VCSEL with the quarter-wave plate arranged in between.

This enables a very compact design of the laser apparatus of the first aspect. In an implementation form of the first aspect, the laser apparatus further comprises: one or more further spin VCSELs; wherein each further spin VCSEL is configured to receive a respective electrical current signal comprising an amplitude modulation, wherein each further spin VCSEL is configured to output a respective laser signal when also being pumped with the light with the non-zero circular polarization degree and when receiving the respective electrical current signal, the respective laser signal comprising a circular polarization modulation, and wherein the circular polarization modulation of each respective laser signal correlates with the amplitude modulation of the corresponding respective electrical current signal.

Therefore, a laser apparatus with one light source for optical pumping and multiple spin VCSELs, to generate multiple laser signals, may be realized.

In an implementation form of the first aspect, the light source is configured to output the light with the non-zero circular polarization degree, and the apparatus further comprises two or more polarization-maintaining optical fibers configured to transmit the light with the non-zero circular polarization degree from the light source to, respectively, the spin VCSEL and the one or more further spin VCSELs.

In an implementation form of the first aspect, the light source is configured to output light without circular polarization degree; and the apparatus further comprises polarization optics arranged in the light path from the light source for optical pumping to, respectively, the spin VCSEL and the one or more further spin VCSELs, in order to generate the light with the nonzero circular polarization degree.

In an implementation form of the first aspect, at least two of the spin VCSELs are configured to output laser signals of different wavelengths.

A second aspect of this disclosure provides a method for operating a laser apparatus comprising a spin VCSEL, and a light source for optical pumping, the method comprising: optically pumping the spin VCSEL with light having a non-zero circular polarization degree; and providing an electrical current signal comprising an amplitude modulation to the spin VCSEL; wherein the spin VCSEL outputs a first laser signal comprising a circular polarization modulation, and wherein the circular polarization modulation of the first laser signal correlates with the amplitude modulation of the electrical current signal. The method of the second aspect may have implementation forms according to the implementation forms of the laser apparatus of the first aspect. The method may operate a laser apparatus according to any implementation form of the first aspect. The method of the second aspect achieves the same advantages as described above for the laser apparatus of the first aspect.

In summary, an idea of this disclosure is to couple carrier-photon dynamics to spin dynamics in a VCSEL. Typically, these two are not coupled, hence, no interaction between the two systems occurs conventionally. However, according to the concept of this disclosure, the two are directly coupled to each other, when the additional optical pumping with, for example, circularly polarized light is applied. This allows for direct electrical modulation of the first laser signal via amplitude modulations of the injected current signal - similar to a conventional VCSEL. However, due to the faster spin dynamics, this increases the bandwidth, with which the first laser signal can be modulated, going beyond the modulation bandwidth of a conventional intensity-modulated VCSEL. The concept of this disclosure presupposes that the resonant frequency of spin dynamics is increased to the required frequency regime by implementing birefringence in the resonator.

It has to be noted that certain devices, elements, units and means described in the present disclosure could be implemented in the software or hardware elements or any kind of combination thereof. All steps which are performed by the various entities described in the present application as well as the functionalities described to be performed by the various entities are intended to mean that the respective entity is adapted to or configured to perform the respective steps and functionalities. Even if, in the following description of specific embodiments, a specific functionality or step to be performed by external entities is not reflected in the description of a specific detailed element of that entity which performs that specific step or functionality, it should be clear for a skilled person that these methods and functionalities could be implemented in respective software or hardware elements, or any kind of combination thereof.

BRIEF DESCRIPTION OF DRAWINGS

The above described aspects and implementation forms will be explained in the following description of specific embodiments in relation to the enclosed drawings (input and output signals are only for illustrative purposes, and do not necessarily correspond to a physical field or signal), in which

FIG. 1 shows a laser apparatus according to this disclosure.

FIG. 2 shows an exemplary laser apparatus according to this disclosure with two integrated VCSELs.

FIG. 3 illustrates an exemplary conversion of a polarization modulation into an intensity modulation.

FIG. 4 shows an exemplary laser apparatus according to this disclosure, wherein the first laser signal of the spin VCSEL is converted into the second laser signal.

FIG. 5 shows an exemplary laser apparatus according to this disclosure without conversion of the first laser signal of the spin VCSEL into the second laser signal.

FIG. 6 shows an exemplary laser apparatus according to this disclosure with a separated light source for optical pumping and using polarization-maintaining fibers.

FIG. 7 shows an exemplary laser apparatus according to this disclosure with a separated light source for optical pumping and using standard fibers.

FIG. 8 shows a method for operating a laser apparatus, according to this disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows a laser apparatus 100 according to this disclosure. The laser apparatus 100 is suitable for optical communications, in particular, communications using a modulated laser signal to transmit information. The laser signal can be transmitted to a receiver, which may also be part of the laser apparatus 100.

The laser apparatus 100 comprises at least one spin VCSEL 101 and at least one light source 102 for optically pumping the spin VCSEL 101. According to this disclosure, the spin VCSEL 101 is optically pumped using light 103 with a non-zero CPD.

In addition, the spin VCSEL 101 is configured to receive an electrical current signal 104 comprising an amplitude modulation. The current amplitude modulation may comprise one or more time periods of a higher current amplitude and one or more time periods of a lower current amplitude. In this way, information may be encoded into the electrical current signal 104. When the spin VCSEL 101 is pumped with the light 103 with the non-zero CPD, and when the spin VCSEL 101 receives the electrical current signal 104, the spin VCSEL 101 is configured to output a first laser signal 105. The first laser signal 105 comprises a circular polarization modulation, which correlates with the amplitude modulation of the electrical current signal 104. In particular, a change of the current amplitude of the electrical current signal 104 may correlate with a change of a CPD of the first laser signal 105. In addition, the first laser signal 105 may also comprise an intensity modulation that correlates with the amplitude modulation of the electrical current signal 104. Information encoded into the electrical current signal 104 can be encoded into the first laser signal 105 in this way, wherein a modulation bandwidth is increased over a conventional intensity-modulated VCSEL.

The laser apparatus 100 according to this disclosure can comprise, or consist of, only the abovedescribed transmitter-side components of the laser apparatus 100. However, the laser apparatus 100 may further comprise one or more receiver-side components, for example, the laser apparatus 100 may further comprise a receiver configured to receive the first laser signal 105, and to convert the first laser signal 105 back into an electrical signal.

FIG. 2 shows an exemplary laser apparatus 100 according to this disclosure, which builds on the laser apparatus 100 shown in FIG. 1. Same elements are labelled with the same reference signs.

The laser apparatus 100 of FIG. 2 has two integrated VCSELs, the spin VCSEL 101 and a DC- current-operated VCSEL as the light source 102 for the optical pumping. The laser apparatus 100 also comprises a quarter- wave plate 201, which is configured to convert an output laser light of the DC-current-operated VCSEL, acting as the light source 102, into the light 103 with the non-zero CPD. As shown in FIG. 2, the spin VCSEL 101 can be stacked on the DC-current- operated VCSEL 102, with the quarter-wave plate 201 arranged in between the two VCSELs, which may achieve a fully-integrated laser apparatus 100. In detail, an operation of the laser apparatus 100 may be as follows.

Circularly polarized laser light 103 may be used for the optical spin pumping of the spin VCSEL 101. In the exemplary embodiment of FIG. 2, this is realized with the VCSEL 1 (the DC- current-operated VCSEL acting as the light source 102), which is continuously operated with an injected DC current. The output light of the VCSEL 1 may be converted into circularly polarized light with the quarter- wave plate 201. Notably, in an alternative implementation, an edge-emitting laser could be used, or the laser light could be transported via an optical fiber to the spin VCSEL 101. Simultaneous optical pumping of multiple spin VCSELs 101 using one light source 102 is also conceivable.

The spin VCSEL 101 is operated in the same way as a conventional intensity-modulated VCSEL. That is, the amplitude of the injected electrical current signal 104 may correspond to ‘0’s and ‘l’s (e.g., in a simple non-return to zero (NRZ) modulation scheme, higher schemes are also possible).

The CW circularly polarized laser light 103 couples intensity and spin dynamics in the spin VCSEL 101 (referred to as VCSEL 2 in FIG. 2). How this works is explained by the so-called spin-flip model, e.g., in ‘M. Lindemann, G. Xu, T. Pusch, R. Michalzik, M.R. Hofmann, I. Zutic, and N.C. Gerhardt, Jllrafasl spin-lasers". Nature, 568, 212 (2019)’. Notably, not only the intensity of the first laser signal 105, but also the CPD of the first laser signal 105 changes according to the injected electrical current signal 104. For example, higher CPD for higher injected electrical current (logic ‘ 1’) or lower CPD for lower injected electrical current (logic ‘0’).

Using one or more passive optical elements 202, the polarization modulations can be converted into intensity modulations on the side of the spin VCSEL 101 (transmitter side of the laser apparatus 100). For instance, one or more polarizers, and/or Wollaston prisms, or the like, may be used for this purpose, so that standard fibers and detectors on an optional receiver side of the laser apparatus 100 can be used. Another approach may be to use the intensity modulation coupled to the polarization dynamics directly for the signal detection. In this case, no additional polarization selective elements 202 would be necessary for the detection.

One possible application scenario of the laser apparatus 100 of FIG. 2 is short-haul optical communication in data centers, e.g., for server-to-server communication. The laser apparatus 100 may in this case comprise three main components: the spin VCSEL 101 at the transmitter side, an optical fiber for transmission, and a receiver at the receiver side, e.g., a fast photodiode.

At the transmitter side, with reference to FIG. 2, the VCSEL 1 as the light source 101 is used to couple carrier and spin dynamics in VCSEL 2. As an example, this may be achieved with a CW optical pumping scheme. Since the spin is an angular momentum, this pumping scheme uses circularly polarized light. For this, the light from VCSEL 1, which is linearly polarized due to inherent properties of VCSEL, may be converted into circularly polarized light using the quarter-wave plate 201 sandwiched between VCSEL 1 and VCSEL 2 (the spin VCSEL 101). VCSEL 2 is used for converting information encoded in the electrical current signal 104 (high and low current and, potentially, higher modulation schemes) into the first laser signal 105. Due to the function of the spin VCSEL 101, this is manifested as modulations of the polarization, for example, modulations of the CPD.

Since the direct detection of the modulation of the polarization, e.g. the CPD, may not be straightforward, and may require the use of polarization-maintaining optical fibers, these polarization modulations may also be converted into intensity modulations, for instance, by the passive optical elements 202. The passive optical elements 202 may comprise a combination of a quarter- wave plate and a linear polarizer, as schematically shown in FIG. 3.

As shown in FIG. 3, the incoming first laser signal 105 with the polarization modulations (i.e., the output of the spin VCSEL 101 shown in FIG. 2) passes through a quarter-wave plate 303 first, which is configured to convert circular polarization into a linear one, and then through a linear polarizer 304, which is configured to only allow passing linearly polarized light, which is aligned with the direction of the polarizer 304. Notably, this is only one possible implementation, and other implementations are conceivable. The output second laser light 301 may then be fed into an optical fiber 302. Since only intensity modulations are now considered, standard optical fibers (non-pol arization maintaining ones) may be used transmit the second laser signal 301 from the transmitter side to the receiver side. At the receiver side, the second laser signal 301 can be converted back into an electrical signal, e.g., with a photodiode with high enough bandwidth. Notably, the detection scheme may be conceptually the same as for conventional intensity-modulated VCSELs.

FIG. 4 shows an exemplary laser apparatus 100 according to this disclosure with such a conversion of the first laser signal 105 of the spin VCSEL 101 into the second laser signal 301. The laser apparatus 100 of FIG. 4 builds on that of FIG. 2. Same elements are labelled with the same reference signs. Like shown in FIG. 2, the VCSEL 1 (the light source 102; alternatively, e.g., an edge-emitting laser) is used to pump the VCSEL 2 (the spin VCSEL 101), for instance, continuously with circularly polarized light, in order to couple the carrier-photon dynamics to the spin dynamics in the active region of the VCSEL 2. Since the VCSEL 1 emits linearly polarized light, the quarter- wave plate 201 is used to convert this into circularly polarized light. Alternative implementations could use an edge-emitting laser or the laser light could be transported via an optical fiber to the spin VCSEL 101.

At the spin VCSEL 101, due to the coupling of the carrier-photon dynamics and the spin dynamics via the optical pumping with the light 103 with the non-zero circular polarization degree, intensity modulations of the injected current signal 104 are converted into polarization modulations of the emitted laser light 105 These polarization modulations have a higher bandwidth than conventional intensity-modulated VCSELs, because of the fast spin dynamics. The polarization modulations are converted into intensity modulations of the second laser signal 301, before the second laser signal 301 is coupled into a fiber 302 for optical transmission. Via the optical fiber 302, the second laser signal 301 may be transmitted to a receiver. This is analogous to a short-haul optical communication system with conventional VCSELs.

FIG. 5 shows an exemplary laser apparatus according to this disclosure without conversion of the first laser signal 105 of the spin VCSEL 101 into the second laser signal 301. The laser apparatus of FIG. 5 builds on that shown in FIG. 1. Same elements are labelled with the same reference signs.

The exemplary laser apparatus 100 of FIG. 5 is similar to the one shown in FIG. 4 and the one in FIG. 2, but without the passive optical elements 202, which are used to convert polarization modulations into intensity modulations. Since these optical elements 202 are missing, the polarization modulations may be directly fed into an optical fiber 302. However, the coupling of the carrier-photon dynamics and the spin dynamics not only allows to make use of the higher bandwidth of the spin dynamics/polarization modulations, but the coupling also increases the bandwidth of photon dynamics/the intensity modulation of the emitted laser signal 105. Therefore, this can be used directly without conversion of polarization modulations into intensity modulations. An advantage is that the laser apparatus 100 is simpler and, hence, easier to manufacture. The laser apparatus 100 shown in FIG. 5 also allows for a fully-integrated transmitter system, which incorporates, amongst others, two lasers: the pump light source 102 and the spin VCSEL 101.

FIG. 6 shows an exemplary laser apparatus 100 according to this disclosure with a separated light source 102 for optical pumping and with polarization-maintaining fibers 602. The laser apparatus 100 of FIG. 6 builds on that of FIG. 1. Same elements are labelled with the same reference signs.

In FIG. 6, the light source 102 for pumping (in this example again the VCSEL 1, but could also be an edge-emitting laser or alternative) is physically separated from the spin VCSEL 101 and a further spin VCSELs 601 (VCSEL 2 and 3, respectively) using the polarization-maintaining fibers to provide the light 103 with the non-zero CPD from the light source (102) to, respectively, the spin VCSELs 101 and the further spin VCSEL 601. More than one further spin VCSEL 601 may be included in the laser apparatus 101. That is, the light source 102 can be used to optically pump more than one spin VCSEL 101, 601. The spin VCSELs 101, 601 are operated as above in the laser apparatus 100 of FIG. 2 and 4. Each further spin VCSEL 601 produces a respective laser signal when also being pumped with the light 103 with the non-zero CPD and when receiving the respective electrical current signal 604. The respective laser signal comprises a circular polarization modulation that correlates with the amplitude modulation of the corresponding respective electrical current signal 604, but may be converted into a respective intensity-modulated laser signal 603, as for the spin VCSEL 101 in this example. However, this conversion is not required, like for the spin VCSEL 101 as described above. The spin VCSEL 101 and the further spin VCSEL 601 may operate at the same wavelength, or at different wavelengths if an off-resonant pumping scheme is employed.

FIG. 7 shows an exemplary laser apparatus 100 according to this disclosure with a separated light source 101 for optical pumping and with standard fibers. The laser apparatus 100 of FIG. 7 builds on that of FIG. 1. Same elements are labelled with the same reference signs.

Again, the pump light source 101 is physically separated from the spin VCSELs 101, 601 (VCSEL 2 and 3), like in FIG. 6, but now using standard fibers. That is, compared to FIG. 6, there is no polarization optics after the pump light source 102. Rather, polarization optics 701 are put right in front of the spin VCSELs 101, 601. Hence, the light source 102 is configured to output light without CPD, and the polarization optics 701 are arranged in the light path from the light source 102 to, respectively, the spin VCSEL 101 and the further spin VCSEL 601, in order to generate the light 103 with the non-zero CPD. An advantage is that the standard optical fibers can be used.

Notably, the laser apparatus 100 of each figured can further be simplified, if a laser diode with direct circularly polarized emission is used, e.g. with chiral cavities.

FIG. 8 shows a method 800 for operating a laser apparatus 100 according to this disclosure. As described above, the laser apparatus 100 comprises a spin VCSEL 101 and a light source 102 for optical pumping the spin VCSEL 101.

The method 800 comprises a step 801 of optically pumping the spin VCSEL 101 with light 103 having a non-zero circular polarization degree, wherein the light source 102 is used. The method 800 further comprises a step 802 of providing an electrical current signal 104 comprising an amplitude modulation to the spin VCSEL 101. The spin VCSEL 101 then outputs 803 a first laser signal 105 comprising a circular polarization modulation, wherein the circular polarization modulation of the first laser signal 105 correlates with the amplitude modulation of the electrical current signal 104.

With the solution of this disclosure, the following main advantages can be achieved:

• Higher bandwidth of polarization modulations in a spin VCSEL 101, to be used for data transfer in optical short-haul communication, compared to standard intensity-modulated VCSELs. The polarization modulations show higher bandwidth than conventional intensity modulations, due to a higher resonance frequency for the spin-dynamics. This resonance frequency can be controlled via birefringence in the resonator. The bandwidth of the intensity modulations is also increased, similar to the polarization modulations, if and when intensity and spin dynamics are coupled via (here: optical) spin injection.

• Electrical control of the polarization modulations in the spin VCSEL 101 : the modulation of the polarization (more precisely: modulation of the CPD) is feasible via the amplitude of the injected current signal 104 into the spin VCSEL 101. In other words, the carrier-spin-photon coupling leads to a modulation of the CPD via the amplitude modulation of the injected electrical current into the spin VCSEL 101. • Standard contact technology for the electrical contacts of the spin VCSEL 101 and roomtemperature operations are possible. Neither a new contact technology is required, nor a low temperature for spin-polarized current injection.

• Full system integration is possible. Only a standard CW-laser with circular polarization for continuous pumping is used, in order to achieve the carrier-spin-photon coupling.

• Higher-order modulation schemes are possible. For instance, PAM4 has been demonstrated in simulations.

• Simulations indicate that the coupling of the carrier spin and photon dynamics also increase bandwidth of the conventional intensity modulations. Thus, conventional detection schemes are feasible.

• The polarization modulations can be converted into intensity modulations on the transmitter side with passive optical elements 202 (polarizers, Wollaston prisms etc., depending on the exact implementation), so that standard fibers and detectors on the receiver side can be used.

• Potentially lower energy-per-bit ratio and consequently potentially lower power consumption due to a larger bandwidth for comparable electrical power consumption.

The present disclosure has been described in conjunction with various embodiments as examples as well as implementations. However, other variations can be understood and effected by those persons skilled in the art and practicing the claimed matter, from the studies of the drawings, this disclosure and the independent claims. In the claims as well as in the description the word “comprising” does not exclude other elements or steps and the indefinite article “a” or “an” does not exclude a plurality. A single element or other unit may fulfill the functions of several entities or items recited in the claims. The mere fact that certain measures are recited in the mutual different dependent claims does not indicate that a combination of these measures cannot be used in an advantageous implementation.