TECHNICAL FIELD

The present invention relates to a device for driving an electro-optical modulator. The invention also relates to a system, which includes both the device and the electro -optical modulator. This system can be used in an optical transmitter. Accordingly, the invention also envisages a transmitter that employs the device and system, respectively. In particular, the device and system of the invention allow improving the performance of the electro-optical modulator, and thus of the transmitter.

BACKGROUND

Transmitters for high speed optical communications are basically implemented by cascading three blocks as shown in FIG. 7. The first block is a digital source. The second block is a driver amplifier, which is used to increase the power level of the electrical signal provided by the source. The third block is an electro-optical modulator, which transduces the electrical signal to an optical signal to be transferred in a fiber.

In most conventional transmitters, the electro-optical modulator (for instance implemented as an Electro-absorption Modulated Laser (EML)) requires a DC voltage. Thus, particularly output DC coupled driver amplifiers are of interest, in order to avoid the use of bias-tees between the driver amplifier and the electro-optical modulator, because they are very area consuming and limit the integration capability. However, a nominal DC current flowing in the electro-optical modulator is usually different from the one required by the driver amplifier. Accordingly, one challenge is that the driver amplifier has to be able to absorb DC current from another path, which is not directly connected to the electro-optical modulator.

Another challenge is that the adopted driver amplifier should show a matched output impedance, since then the electro-optical modulator is less susceptible to signal degradation. This is, because the matched output impedance of the driver amplifier absorbs reflections coming from the interconnection between driver amplifier and electro -optical modulator, such as bonding wires and bonding pads. This aspect is actually of high importance to achieve optical communications with high speed and high data rate. In conclusion, driver amplifiers with high performance, which contribute for achieving very high speed and highly integrated transmitters for optical communication, have the following properties:

1. A DC coupled output.

2. A DC bias current independent of the DC coupled output.

3. A matched output impedance (with the electro-optical modulator).

A conventional solution to realize a driver amplifier, with the properties highlighted above, bases on a distributed amplifier having separately biased sections. A block-diagram thereof is shown in FIG. 8. The solution includes a distributed amplifier having a not integrated RF coil. This allows a path different from the electro-optical modulator for the DC bias current of the distributed amplifier. Moreover, due to the high impedance of the RF coil, the performance of the distributed amplifier is not affected.

SUMMARY

In view of the above-mentioned challenges and disadvantages, the present invention aims to improve conventional driver amplifiers. The present invention has thereby the object to provide a device for driving an electro-optical modulator with improved performance. In particular, the device should allow a high integration level and an unlimited low frequency cut-off. Further, the device should not show voltage spikes, and should be able to operate with only a single DC supplier. To this end, the device should not include any inductors in the biasing network.

The object of the present invention is achieved by the solution provided in the enclosed independent claims. Advantageous implementations of the present invention are further defined in the dependent claims.

The main idea of the invention is a distributed current source added to a distributed amplifier. Thus, the invention implements a DC coupled output distributed amplifier with a DC bias current, which is not dependent on the DC bias current of the electro-optical modulator. A first aspect of the invention provides a device for driving an electro-optical modulator, the device comprising a distributed amplifier and a distributed current source, wherein the distributed current source has a DC input for a supply voltage and has M DC outputs connected to the distributed amplifier, being a natural number with M > l, and the distributed amplifier has a Radio Frequency (RF) input, a RF output for connecting to the electro-optical modulator, and at least M DC inputs connected to the M DC outputs of the distributed current source.

A distributed amplifier is an amplifier with several amplifier stages, which are connected together to form a transmission line with gain. The input of each stage (but the first stage) is the output of a previous stage. Thus, the gain on the transmission line is the sum of the gains of the individual stages. The bandwidth of the distributed amplifier is the bandwidth of each of the stages. The main difference to a cascaded-stage amplifier is that in the latter the input of each stage is the original signal provided to the amplifier.

In the device of the first aspect, the M DC inputs of the distributed amplifier may be connected one-by-one to the M DC outputs. That is, each of the M DC inputs may be connected to a corresponding one of the M DC outputs of the distributed current source.

The device of the first aspect may be an integrated circuit, for example a Monolithic Microwave Integrated Circuit (MMIC).

The device of the first aspect combines a (DC coupled output) distributed amplifier and a distributed current source, which combination allows a DC bias current for the distributed amplifier to be independent of the DC bias current of the electro-optical modulator. Moreover, the distributed nature of the distributed current source allows minimizing intrinsic parasitic capacitances of the distributed current source, thus not affecting the high frequency response of the distributed amplifier. As a consequence, the device allows maximizing the performance of the electro-optical modulator it is connected to, and thus of a transmitter for optical communications including device and electro-optical modulator.

In particular, the device of the first aspect has the following advantages over conventional driver amplifiers: 1. High integration level: The device of the first aspect can be easily implemented, for instance, in MMIC technologies.

2. Unlimited low frequency cut-off: The device of the first aspect does not have a low frequency cut-off, because it does not include any inductors in the biasing network.

3. Absence of voltage spikes: The device of the first aspect does not generate any voltage spikes, even when the distributed amplifier is pinched-off abruptly. This is, because the distributed current source does not store any energy, as e.g. an RF coil would (or does in the conventional driver amplifier shown in FIG. 8).

4. Single DC supplier: The device of the first aspect can be implemented with a single DC voltage supply.

In an implementation form of the first aspect, the distributed current source includes M DC current sources connected in parallel, each DC current source being connected to the DC input and to one of the M DC outputs.

The M DC current sources provide the distributed nature of the current source, and allow a DC bias current for the distributed amplifier, which is independent of the DC bias current of the electro-optical modulator, to which the device is connected.

In a further implementation form of the first aspect, the DC current sources are transistors, and for each transistor, an output terminal of the transistor is connected to the DC input, and an input terminal of the transistor is connected to one of the DC outputs and to a control terminal of the transistor.

The M transistors provide a solution that can be easily integrated, for instance, in MMIC technologies.

In a further implementation form of the first aspect, the transistors are field effect transistors or bipolar junction transistors.

In a further implementation form of the first aspect, for each transistor, the input terminal of the transistor is directly connected to the control terminal of the transistor.

This direct connection provides a particularly easy implementation of the device. In a further implementation form of the first aspect, for each transistor, the input terminal of the transistor is connected to the control terminal of the transistor through a constant voltage source.

Thus, the input and control terminals of the transistors can advantageously be kept at a constant and well defined (and controllable) voltages.

In a further implementation form of the first aspect, the distributed amplifier has at least one further DC input for a supply voltage, and the DC input of the distributed current source and the further DC input of the distributed amplifier are connected to each other.

Thus, the DC inputs of distributed current source and amplifier, respectively, may be used for connecting to the same supply voltage source. Accordingly, only one DC voltage supply is needed to operate the device. This can even be shared with the electro-optical modulator.

In a further implementation form of the first aspect, the distributed amplifier has at least one further DC input for a supply voltage, and the DC input of the distributed current source and the further DC input of the distributed amplifier are not connected to each other.

That means, the DC inputs of distributed current source and amplifier are separate from another. In other words, they are individual DC inputs, and are not associated to each other in any way. Accordingly, the DC inputs of distributed current source and amplifier may be used for connecting to separate supply voltage sources. Therefore, different DC voltage supplies may be used to operate the device. They can be different from the DC voltage supply of the electro- optical modulator.

In a further implementation form of the first aspect, the distributed amplifier includes N transistors connected in parallel, N being a natural number with N ³ M, and includes an output transmission line connected between the further DC input and the RF output, and output terminals of the N transistor and the M DC outputs of the distributed current source, respectively, are connected to the output transmission line.

The N transistors provide the different amplifier stages of the distributed amplifier, and cause the gain on the output transmission line. In a further implementation form of the first aspect, the distributed amplifier includes an input transmission line connected between the RF input and a reference voltage port, and control terminals of the N transistor are connected to the input transmission line.

In a further implementation form of the first aspect, the N transistors are field effect transistors or bipolar junction transistors.

A second aspect of the invention provides a system including a device according to the first aspect and any of its implementation forms, and an electro-optical modulator having an RF input connected to the RF output of the device.

In an implementation form of the second aspect, the electro-optical modulator has a DC input for a supply voltage, and the DC input of the electro-optical modulator is connected to the DC input of the distributed current source.

Accordingly, all advantages and effects of the device of the first aspect can be achieved with the system of the second aspect.

It has to be noted that all devices, elements, units and means described in the present application 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 can 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 of the present invention will be explained in the following description of specific embodiments in relation to the enclosed drawings, in which

FIG. 1 shows a device according to an embodiment of the invention.

FIG. 2 shows a system according to an embodiment of the invention.

FIG. 3 shows a device according to an embodiment of the invention.

FIG. 4 shows a device according to an embodiment of the invention.

FIG. 5 shows a device according to an embodiment of the invention.

FIG. 6 shows a device according to an embodiment of the invention.

FIG. 7 shows an implementation of a conventional transmitter.

FIG. 8 shows a conventional driver amplifier.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows a device 100 according to an embodiment of the invention. The device 100 is configured to drive an electro-optical modulator 110, which is not part of the device 100. The device 100 includes a distributed current source 102 and a distributed amplifier 101, and is thus configured as a driver amplifier for the electro-optical modulator 110.

The distributed current source 102 has a DC input 103 for connecting a supply voltage, which is here labelled VDD. The supply voltage VDD may also be connected to the electro-optical modulator 110. Further, the distributed current source 102 has M DC outputs 104, which are connected to the distributed amplifier 101. M is a natural number with M > 1. The distributed amplifier 101 has accordingly at least M DC inputs 107, which are connected to the M DC outputs 104 of the distributed current source 102. In FIG. 1 the connections between the DC outputs 104 and DC inputs 107 are labelled il, i2...iM. The distributed amplifier 101 has further a Radio Frequency (RF) input 105, which may be connected to and configured to receive an electrical signal from a digital source. Further, the distributed amplifier 101 has a RF output 106, which is configured to connect to the electro-optical modulator 110.

FIG. 2 shows a system 200 according to an embodiment of the invention, which includes a device 100 and an electro-optical modulator 1 10. Thereby, the device 100 of FIG. 2 builds on the device 100 of FIG. 1 , wherein the same elements are provided with the same reference signs and function likewise. The device 100 of FIG. 2 may also be provided individually (i.e. without the electro-optical modulator 110). The device 100 of FIG. 2 is advantageously an integrated circuit, for example a MMIC. That means, the distributed current source 102 and the distributed amplifier 101 are integrated with another, for instance, in MMIC technologies.

The RF input 105 of the distributed amplifier receives an input voltage (labelled Vin), which may be a signal from a source, and the RF output 106 outputs a voltage (labelled Vout), which is the amplified input voltage, to an RF input 201 of the electro -optical modulator 110. The electro-optical modulator 110 has further a DC input 202 configured to connect to a supply voltage. Advantageously, in order to use only one supply voltage, here VDD, the DC input 202 of the electro-optical modulator 110 is connected to the DC input 103 of the device 100, particularly of at least of the distributed current source 102. The electro-optical modulator 110 may be an EML.

FIG. 3 shows a device 100 according to an embodiment of the invention, which builds on the devices 100 shown in FIG. 1 and FIG. 2, respectively. Again the same elements are provided with the same reference signs and function likewise.

The distributed amplifier 101 of the device 100 shown in FIG. 3 includes includes N transistors 302 connected in parallel. A is a natural number with N³M. The N transistors 302 are labelled Ql, Q2 ... QN, and are here indicated as being field effect transistors (FETs). However, the may also be bipolar junction transistors (BJTs). Further, the distributed amplifier 101 includes an output transmission line 303. The output transmission line 303 is connected between a further DC input 301 of the distributed amplifier 101, which is configured to connect to a supply voltage (here VDD), and the RF output 106 for providing Vout to the electro-optical modulator 110. The output terminals D of the N transistor 302 (‘drain’ for FETs and‘collector’ for BJTs) and the M DC outputs 104 of the distributed current source 102, respectively, are connected to the output transmission line 303.

The distributed amplifier 101 further includes an input transmission line 304, which is connected between the RF input 105 for receiving Vin and a reference voltage port 305, which is for connecting to a reference voltage (here labelled VGG). The reference voltage may also be ground. The control terminals G of the N transistor 302 (‘gate’ for FETs and‘base’ for BJTs) are connected to the input transmission line 304. The input terminals S of the N transistors 302 (‘source’ for FETs and‘emitter’ for BJTs) may be grounded.

Further, the distributed amplifier 101 includes two resistors labelled RG and RD, wherein the resistor RD is connected between the DC input 301 and the output transmission line 303, and the resistor RG is connected between the input transmission line 304 and the reference voltage port 305. Accordingly, a voltage drop occurs over the resistor RG and the resistor RD, respectively. Notably, the device 100 may even include the two DC voltage sources VGG and VDD, these voltage sources may be separate from the device 100.

Further, the distributed amplifier 101 of the device 100 shown in FIG. 3 includes N+ 2 inductors labelled FG1, FG2 ... FGN+l and FD1, FD2 ... FDN+l . The inductors FD1, FD2 ... FDN+l may be connected in series on the output transmission line 303. The output terminal D of each transistor 302 may be connected to the output transmission line 303 in-between two of these inductors. The inductors FG1, FG2 ... FGN+l are connected in series on the input transmission line 304. The control terminal G of each transistor 302 is connected to the input transmission line 304 in-between two of these inductors.

Conventional distributed amplifiers, which are oriented to improve the performance of the distributed amplifier itself, are also suitable to be used in the proposed solution of the invention. The transistors composing the distributed amplifier can be of FET type or BJT type. Further in FIG. 3, the distributed current source 102 is composed of Af DC current sources 300, here labelled II, 12 ... IM. That is, the distributed current source 102 includes M DC current sources 300, which are connected in parallel. Thereby, each DC current source 300 may be connected to the DC input 103 (here thereby connected to VDD, like the distributed amplifier

101 and the electro-optical modulator 110, respectively), and to one of the M DC outputs 104 of the distributed current source 102. The DC outputs 104 of the distributed current source

102 may be connected to M output terminals D of the N transistors 302 composing the distributed amplifier 101.

As it is possible to note from Error! Reference source not found., the DC bias current of the distributed amplifier 101 is the sum of the current flowing in the electro-optical modulator 110 and the distributed current source 102, thus simultaneously optimizing the bias current of the electro-optical modulator 110 and of the distributed amplifier 101, even if they are different in level. Moreover, the matched output impedance of the distributed amplifier 101 is not affected by the high impedance shown by the DC current sources 300 composing the distributed current source 102.

FIG. 4 shows a device 100 according to an embodiment of the invention, which builds on the device 100 shown in FIG. 3. The same elements are again provided with the same reference signs and function likewise. The device 100 shown in FIG. 4 is the easiest way to achieve full integration, for example, in MMIC technologies.

The device 100 of FIG. 4 is again composed of at least the distributed amplifier 101 and the distributed current source 102. The distributed current source 102 again includes DC current sources, each one implemented by means of a transistor 400. The transistors 400 are labelled Tl, T2 ... TM. In FIG. 4 the transistors 400 are realized by FETs, however, they can also be BJTs.

Each of the M transistors 400 has an output terminal D (‘drain’ for FETs and‘collector’ for BJTs), which is connected to the DC input 103 of the distributed current source 102. Further, each transistor 400 has an input terminal S (‘source’ for FETs and‘emitter’ for BJTs), which is connected to one of the M DC outputs 104 and to a control terminal G (‘gate’ for FETs and ‘base’ for BJTs) of the same transistor 400. That is, each one of the transistors 400 has its control and input terminals G and S connected to another. A size of each of the M transistors 400 of the distributed current source 102 may be selected to obtain the desired currents labelled II, 12 ... IM in FIG. 3. Each of these currents flows out of one DC output 104 of the distributed current source 102 and into one DC input 107 of the distributed amplifier 101.

FIG. 5 shows a device 100 according to an embodiment of the invention, which builds on FIG. 3. The same elements are again provided with the same reference signs and function likewise.

In FIG. 5, each of the transistors 400 of the distributed current source 102 has its control terminal G and input terminal S connected through a constant voltage source 500. That is, the control terminals G and input terminals S of the transistors 400 are not directly connected as in FIG. 4, but are kept to a constant voltage value through the constant voltage sources 500 labelled as VGS1, VGS2 ... VGSM. Notably, the direct connections in FIG. 4 can be regarded as a particular implementation of the indirect connections in FIG. 5, where VGS1 = VGS2 = ... = VGSM = 0V.

FIG. 6 shows a device 100 according to an embodiment of the invention, which builds on FIG. 3. The same elements are again provided with the same reference signs and function likewise.

In FIG. 6 a DC voltage source of the distributed current source 102 is labelled VDD2 and is not the same as a DC voltage source used for the distributed amplifier 101, which is labelled VDD3 and/or respectively a DC voltage source used for the electro-optical modulator 110, which is labelled VDD1. That is, in contrast to FIG. 4, where the DC input 103 of the distributed current source 102 and the further DC input 301 of the distributed amplifier 102 are connected to each other and to the common voltage source VDD, in FIG. 6 the DC input 103 of the distributed current source 102 and a further DC input 600 of the distributed amplifier 101 are not connected to each other. That is, they are separate DC inputs and are connected to different voltage sources. Notably, the single voltage source of FIG. 4 may be regarded as a particular implementation of the voltage sources in FIG. 6, where VDD1 = VDD2 = VDD3 = VDD.

The features of the devices 100 shown in FIG. 5 and FIG. 6, respectively, may also be combined. That is, a device 100 according to an embodiment of the invention may have a constant voltage source 500 for connecting the control and input terminals G and S of a transistor 400, and may have different voltage sources for supplying the distributed current source 102, the distributed amplifier 101 and the electro-optical modulator 110, respectively.

The device 100 and system 200 shown in the previous figures, respectively, can be used in an optical transmitters for high speed optical communications. That is, the invention relates also to such a transmitter comprising the device 100 or the device 100 and the electro-optical modulator 110. Such a transmitter according to an embodiment of the invention is implemented in principle like the conventional transmitter shown in FIG. 7. That is the transmitter of the invention may further include a digital source. The device 100 may be connected to this source in order to receive an electrical signal via its RF input 105 from the source. The device 100 is configured to increase the power level of the received electrical signal, and to output the amplifier signal. The electro-optical modulator 110 may be connected to the device 100 via the RF output 106 and the RF input 201. The electro-optical modulator 110 may be configured to transduce the electrical signal received from the device 100 to an optical signal, which may be further transferred to a fibre or the like.

The invention 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 invention, 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.