NL2030075B1

NL2030075B1 - Method and system for determining half-wave voltage of intensity modulator

Info

Publication number: NL2030075B1
Application number: NL2030075A
Authority: NL
Inventors: Ciaran Berrevoets Remon; Middelburg Thomas; Alexander Slater Joshua
Original assignee: Univ Delft Tech
Priority date: 2021-12-08
Filing date: 2021-12-08
Publication date: 2023-06-22
Also published as: WO2023106921A1

Abstract

The present disclosure relates to a method for determining the half-wave voltage of an intensity modulator that is conﬁgured to modulate laser light based on input on a radiofrequency input port and a bias port, comprising: - supplying the light ﬁeld to the intensity modulator; - modulating the supplied light ﬁeld by applying a periodic signal with an estimated peak-to-peak voltage to the radiofrequency input port of the intensity modulator and applying a bias voltage to the bias port of the intensity modulator and/or to the periodic signal at the radiofrequency input of the intensity modulator; - measuring the modulated light ﬁeld coming from the intensity modulator and determining from the measured modulated light ﬁeld the average light intensity; and - determining the half-wave voltage of the intensity modulator based on the estimated peak-to-peak voltage of the periodic signal and the determined average light intensity of the modulated light ﬁeld.

Description

METHOD AND SYSTEM FOR DETERMINING HALF-WAVE VOLTAGE OF INTENSITY

MODULATOR

Field of the invention

The present disclosure relates to a method, system and device for determining the half- wave voltage of an intensity modulator that is configured to modulate laser light based on input on a radiofrequency input port and a bias port, a quantum communication network comprising such a system, and a computer program configured to execute the method.

Background of the invention

Quantum information systems are data processing systems that use a quantum system, €.g. a qubit, as an information carrier. In conventional data processing systems the basic unit of information 15 bits that either have the value “0” or “1°. In contrast, the basic unit of information in quantum information systems are qubits, wherein a qubit may be a two-state quantum mechanical system. The special property of a qubit is that it can be in either “0°, “1°, or a superposition of both states simultaneously. One example of a quantum information system wherein qubits are used is a

Quantum Key Distribution (QKD) system.

QKD systems allow two or more users at different locations to securely generate cryptographic keys by at least partly making use of the special property of the qubits. The first proposal for a QKD system (BB84) was done by C.H. Bennett and G. Brassard, described in the article "Quantum cryptography: Public key distribution and com tossing", Proceedings of IEEE

International Conference on Computers, Systems and Signal Processing, volume 175, page 8. New

York, 1984. An advantage of using a QKD system is that, at least in theory, the key is even secure in case an eavesdropper is present in the system.

Often QKD systems are based on lasers and field programmable gate arrays to generate time-bin qubits. Time bin qubits may be formed by a coherent superposition of two independent temporal modes of light field. Time-bin encoding is especially suitable for single-mode optical fibre propagation and compatible with already existing fibre networks. Hence, the formation of time bin qubits in QKD systems is an essential element in the development of practical QKD implementations. QKD protocols such as the above referred BB84 protocol. the coherent one-way (COW) QKD protocol and other QKD protocols such as described in the article by Vagniluca et al,

Efficient time-bin encoding for practical high-dimensional quantum key distribution, physical review applied 14, 014051 (2020), may use a train of phase coherent temporal modes by intensity modulation of the output of a continuous-wave (CW) laser and subsequent attenuation. As explained earlier, measures should be taken to ensure indistinguishability of the time-bin light pulses.

The requirement for QKD and other optical quantum information systems is that qubits have well-prepared states (e.g. fidelity) and well prepared intensities, which necessitates a very fine calibration and recalibration of the intensity modulators that are used to create the time-bin light pulses. In particular, there is a need for accurately measuring the half-wave voltage of the intensity modulator. In the context of the present invention, the half-wave voltage is the voltage required to tum the intensity modulator from minimum to maximum transmission (or vice versa).

Summary of the disclosure

In a first aspect, the disclosure relates to a method of determining the half-wave voltage of an intensity modulator that is configured to modulate a light field, such as laser light, based on input on a radiofrequency input port and a bias port. The method may comprise: supplying the light field to the intensity modulator, modulating the supplied light field by applying a periodic signal with an estimated peak-to-peak voltage to the radiofrequency input port of the intensity modulator and applying a bias voltage to the bias port of the intensity modulator and/or to the periodic signal at the radiofrequency input of the intensity modulator, measuring the modulated light field coming from the intensity modulator and determining from the measured modulated light field the average light intensity, and determining the half-wave voltage of the intensity modulator based on the estimated peak-to-peak voltage of the periodic signal and the determined average light intensity of the modulated light field.

In an embodiment the step of determining the half-wave voltage further comprises changing the bias voltage applied to the bias input and/or to the periodic signal, wherein changing the bias voltage results in a phase shift in a periodic transmission response of the intensity modulator, and determining that the estimated peak-to-peak voltage of the periodic signal is the half-wave voltage of the intensity modulator in case the determined average light intensity remains substantially constant when changing the bias voltage.

In an embodiment the method further comprises changing the estimated peak-to-peak voltage of the periodic signal to at least one further estimated peak-to-peak voltage in case the determined average light intensity changes when changing the bias voltage.

In an embodiment the method further comprises repeating the changing of the further estimated peak-to-peak voltage of the periodic signal to an even further estimated peak-to-peak voltage of the periodic signal until a peak-to-peak voltage level has been reached at which a change ofthe determined average light intensity during the changing of the bias voltage is minimal.

In an embodiment the method further comprises splitting the light field that is coming out of the intensity modulator with a beam splitter.

In an embodiment the periodic signal is a square wave.

In an embodiment the method further comprises determining when at least one of peak values of the estimated peak-to peak-voltage and peak values of the further estimated peak-to-peak voltage of the periodic signal are in antiphase over a transmission response of the intensity modulator, and determining that at least one of the estimated peak-to peak-voltage and the further estimated peak-to-peak voltage of the periodic signal is the half-wave voltage in case the peak values of the periodic signal are in antiphase over the transmission response.

In an embodiment the determination of at least one of the peak values of the estimated peak-to peak-voltage and the peak values of the further estimated peak-to-peak voltage of the periodic signal being in antiphase is based on the determined average light intensity remaining constant when changing the bias voltage.

In an embodiment the changing of the bias voltage comprises sweeping the bias voltage over a predetermined range.

In an embodiment the average light intensity of the laser light is executed by a photodetector unit that is configured to generate a detector signal representative of the average light intensity, wherein the photodetector unit optionally comprises a photodiode and a voltage measurement device that is connected to the photodiode.

In an embodiment a frequency of the periodic signal is in the range of 50 Mhz — | Ghz, preferably in the range of 100 Mhz — 500 Mhz, and most preferably is 200 Mhz.

In an embodiment the method further comprises calibrating the intensity modulator based on the determined half-wave voltage.

In a further aspect the disclosure relates to a system for determining a half-wave voltage of an intensity modulator. The system may comprise a laser unit for sending a light field, an intensity modulator comprising a radiofrequency input port and a bias port, wherein the intensity modulator is configured to modulate incoming light field from the laser unit based on input received at the radiofrequency input port and the bias port, a periodic signal generator for applying a periodic signal to the radiofrequency input port of the intensity modulator, a DC input for applying a bias voltage to the bias port of the intensity modulator and/or to a periodic signal at the radiofrequency input of the intensity modulator, a controller that is connected to the periodic signal generator and is configured to set an estimated peak-to-peak voltage of the periodic signal from the periodic signal generator, and a measurement device for determining an average light intensity of the modulated light field coming from the intensity modulator, wherein the controller is configured to determine the half-wave voltage of the intensity modulator based on the estimated peak-to-peak voltage of the periodic signal and the average light intensity determined by the measurement device.

In an embodiment the controller is configured to change a bias voltage of the DC input resulting in a phase shift in a periodic transmission response of the intensity modulator, and wherein the estimated peak-to-peak voltage of the periodic signal that is determined as the half- wave voltage in case the determined average light intensity remains substantially constant when changing the bias voltage.

In an embodiment the controller is configured to change the estimated peak-to-peak voltage of the periodic signal to a further estimated peak-to-peak voltage in case the determined average light intensity changes when changing the bias voltage.

In an embodiment the controller is further configured to repeat the changing of the estimated peak-to-peak voltage of the periodic signal until a peak-to-peak voltage of the periodic signal has been reached at which a change of the determined average light intensity during the changing of the bias voltage is minimal.

In an embodiment the system further comprises a beam splitter positioned between the intensity modulator and the measurement device for splitting the modulated light field into two beams, wherein preferably a first beam is directed into a quantum channel and a second beam is directed at the measurement device.

In an embodiment the periodic signal is a square wave.

In an embodiment the controller is configured to determine that at least one of peak values of the estimated peak-to-peak voltage and peak values of the further estimated peak-to-peak voltage of the periodic signal are in antiphase over a transmission response of the intensity modulator for determining that at least one of the estimated peak-to-peak voltage and the further estimated peak-to-peak voltage of the periodic signal is the half-wave voltage.

In an embodiment the determination of at least one of the peak values of the estimated peak-to-peak voltage and the peak values of the further estimated peak-to-peak voltage of the periodic signal being in antiphase is based on the determined average light intensity remaining constant when changing the bias voltage.

In an embodiment the measurement device comprises a photodetector unit that is configured to generate a detector signal representative of the average light intensity, wherein the photodetector unit optionally comprises a photodiode and a voltage measurement device that is connected to the photodiode.

In an embodiment a frequency of the periodic signal is in the range of 50 Mhz — 1 Ghz. preferably in the range of 100 Mhz — 500 Mhz, and most preferably is 200 Mhz.

In an embodiment the laser unit, the periodic signal generator, the current input, the controller and the measurement device are arranged in a single housing.

In a further aspect the disclosure relates to a quantum communication network comprising a system according to any one foregoing embodiments. 5 In a further aspect the disclosure relates to a device for measuring the half-wave voltage of an intensity modulator. The device may comprise a periodic signal generator for applying a periodic signal to a radiofrequency input port of the intensity modulator, a controller that is connected to the periodic signal generator and 1s configured to set an estimated peak-to-peak voltage of the periodic signal from the periodic signal generator, and a measurement device for determining an average light intensity of a modulated laser light coming from the intensity modulator, wherein the controller is configured to determine the half-wave voltage of the intensity modulator based on the estimated peak-to-peak voltage of the periodic signal and the average light intensity determined by the measurement device.

In a further aspect the disclosure relates to a computer program configured to for executing the method steps according to any one of the foregoing embodiments.

Detailed description of exemplary embodiments

Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. Exemplary embodiments are described below with reference to the figures.

Figure | shows an example of a QKD system comprising a system for determining the half-wave voltage according to the invention;

Figure 2 shows an example of a system for determining the half-wave voltage according to the invention

Figure 3 shows a detailed example of a system for determining the half-wave voltage according to the invention;

Figures 4A-C show examples of graphs with characteristics of the method according to the invention;

Figures 5A-C show examples of graphs with characteristics of the method according to the invention;

Figure 6 shows an example of a method for determining the half-wave voltage of an intensity modulator; and

Figure 7 shows a detailed example of a method for determining the half-wave voltage of an intensity modulator.

General

It is to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present disclosure will be determined by the appended claims.

Furthermore, unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Still, certain elements are defined below for the sake of clarity and ease of reference.

Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limit of that range and any other stated or intervening value in that stated range, is encompassed within the disclosure. The upper and lower limits of these smaller ranges may independently be included in the smaller ranges and are also encompassed within the disclosure, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the disclosure.

It is noted that, as used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. It is further noted that the claims may be drafted to exclude any optional element. As such, this statement is intended to serve as antecedent basis for use of such exclusive terminology as “solely,” “only” and the like in connection with the recitation of claim elements, or use of a “negative” limitation.

As will be apparent to those of skill in the art upon reading this disclosure, each of the individual exemplifying embodiments described and illustrated herein has discrete components and features which may be readily separated from or combined with the features of any of the other several exemplifying embodiments. Any recited method can be carried out in the order of events recited or in any other order which is logically possible.

Detailed description

Figure 1 depicts an example of a QKD system comprising a device for measuring the half- wave voltage of according to an embodiment of the disclosure. The system 100 comprises a first qubit module 102 and a receiver node 110. First qubit module 102 comprises a laser unit 114, in the illustrated example a distributed feedback laser, for supplying a continuous light field. After the laser unit 114 an isolator 115 is positioned for stabilising the laser light. After the isolator the laser light goes to a first intensity modulator IM1 116 that is configured to pulse the light field into time bins. Afterwards a second intensity modulator IM2 120 changes the intensity of the qubits to create adequate decoy states. The intensity modulators IM1 116 and IM2 120 are driven by pulse generators PG 122, wherein the pulse generators PG 122 receive qubit state information, such as time bins, intensity and phase, from a field programmable gate array FPGA 124. FPGA 124 is also connected to a COMP 125. Phase modulator PM 126 is configured to add a phase to (a part of) the laser light. After PM 126 beam splitter BS 117 is positioned to split the laser light coming from

PM 126. One arm coming from BS 117 is going to measurement device MD 119 that is configured to calibrate the intensity modulators IMI 116 and IM2 120. MD 119 is further explained in figure 2. A second arm coming from BS 117 is going to a Variable Optical Attenuator VOA 130 that is configured to attenuate the laser light down to the suitable intensity. After the VOA 130 a final isolator ISO 132 is positioned to prevent outside light from entering the first qubit module 102.

The qubit is then sent from first qubit module 102 over optical fiber 108 to receiver node 110.

The qubit that is sent over optical fiber 108 is received at a qubit measurement device 139, wherein a qubit projection measurement can be performed on the qubit. This qubit information corresponding to the projection measurement is then sent to a local computer COMP 129 for example for the creation of a secret key.

Figure 2 shows system 200 that can be part of the QKD system that is depicted 1n figure 1.

Laser 214 is configured to generate a continuous wave and is directed to intensity modulator 216.

Connected to intensity modulator 216 is periodic signal generator 240 and DC input 242. Periodic signal generator 240 may be a Field Programmable Gate Array (FPGA) or a function generator.

However, it is clear to the skilled person that other devices suitable for generating a periodic signal can also be used as a periodic signal generator. Periodic signal generator 240 is connected to the radiofrequency input port 241 of the intensity modulator 216. DC input 242 is connected to the bias input port 243 of the intensity modulator 216.

Intensity modulator 216 1s configured during normal operation of the quantum communication network to “carve out” time-bin optical modes, comprising early and/or late time bins, to define a photonic qubit. After being modulated by intensity modulator 216 the laser light goes through beam splitter 117 and one arm of the laser light is sent to measurement device 219. In measurement device 219 the half-wave voltage of intensity modulator 216 may be determined.

Figure 3 shows a specific embodiment of a system 300 for measuring the half-wave voltage of intensity modulator 316. Laser 314 is configured to generate a continuous wave and is directed to intensity modulator 316. Connected to intensity modulator 316 is FPGA 344. Arranged in between intensity modulator 316 and FPGA 344 is a Digital to Analog Converter (DAC) 346.

DAC 346 is connected to radiofrequency input port 341 of intensity modulator 316. DC input 342 provides the DC voltage to the intensity modulator and is connected to bias input port 343 of the intensity modulator 316.

Laser light exiting from intensity modulator 316 may fall on beam splitter 317. Beam splitter 317 can be a 50/50-beam splitter and can be positioned such that half of the photons that incident on beam splitter 317 are transmitted to measurement device 319 and half of the photons that incident on beam splitter 317 are reflected to rest of the QKD system. Altematively, beam splitter 317 may be positioned such that half of the photons that incident on beam splitter 317 are reflected to measurement device 319 and half of the photons that incident on beam splitter 317 are transmitted to rest of the QKD or optical quantum information processing system. An advantage of beam splitter 317 is that the half-wave voltage can be determined without the need to change the set-up of the system. It is clear to the skilled person that it is not essential that a 50/50-beam splitter is used. It is important that at least a fraction is reflected to measurement device 319. A beam splitter that reflects and transmits in different ratios, such as 70/30 or 30/70, can also be used for the present invention.

Measurement device 319 comprises, in the illustrated embodiment, photodiode 348.

Photodiode 348 is a device which converts the photons incident on the photodiode to an electrical current. Photodiode 348 therefore produces a voltage that is proportional to the light intensity of the laser light that is incident on the photodiode. In this illustrated embodiment multimeter 350 is connected to the photodiode 348. In this way, a voltage that is proportional to the light intensity of the laser light emitted by intensity modulator 316 can be effectively measured by multimeter 350.

It is clear to the skilled person that any device which is suitable for measuring the light intensity of the laser light is suitable for the present concept.

Photodiode 348 may be configured to measure the peaks of the light intensity that correspond to the peaks of the periodic signal that is supplied to the radiofrequency input port 341 of the intensity modulator 316. Photodiode 348 may alternatively be configured to only measure the average light intensity of laser light emitted by intensity modulator 316. A photodiode 348 that directly measures the average light intensity of laser light emitted by intensity modulator 316 has as its advantage that the determination of the estimated peak-to-peak voltage of the periodic signal being the half-wave voltage of intensity modulator 316 can directly be based on the determined average light intensity.

Figure 4A shows a graph of characteristics of system 300 of figure 3. On the X-axis the voltage Vrrapplied to the radiofrequency input port 341 is shown, while on the Y-axis the measured voltage Vep of the photodiode 348 is shown. As the voltage Ven of photodiode 348 is a result of photons of the laser light coming from intensity modulator 316 incident on photodiode 348, voltage Vp is proportional to the light intensity coming from intensity modulator 316.

Transmission response 452 shows the amount of voltage Vep that is measured by photodiode 348 as aresult of the applied voltage Vrs on radiofrequency input port 341 of intensity modulator 316.

Lowest point 453 on transmission response 452 denotes the lowest amount of light intensity of the laser light coming out of intensity modulator 316. Highest point 455 on transmission response 452 denotes the highest amount of light intensity of the laser light coming out of intensity modulator 316. The voltage that is needed to move from lowest point 453 to highest point 455 on transmission response 452 is half-wave voltage Vz.

FPGA 344 may set a periodic signal with an estimated peak-to-peak voltage on radiofrequency input port 341 of intensity modulator 316. The FPGA 344 is configured to set an estimated peak-to-peak voltage Vp of the periodic signal, wherein the peak-to-peak voltage Vy, is the minimum to maximum amount of voltage of the periodic signal. In figure 4A the peak-to-peak voltage Vpp of the periodic signal is shown in relation to the transmission response 452. The lowest value of the peak-to-peak voltage Vy, corresponds to first point 454 on transmission response 452.

The highest value of the peak-to-peak voltage Vy, corresponds to second point 456 on transmission response 452. The periodic signal will set the intensity modulator to emit laser light that is alternating between an intensity of light that corresponds to first point 454 and an intensity of light that corresponds to second point 456. These intensities of light are measured as a voltage Vep by

I5 photodiode 348. The average voltage Vas 15 the average voltage of a voltage Vep measured by photodiode 348 corresponding to first voltage of first point 454 and a second voltage of second point 456. The Vy, of first point 454 and second point 456 in figure 4A is Vi. The average voltage

Vue may be measured by multimeter 350.

In figure 4B the same transmission response 452 from intensity modulator 316 is shown.

The Vp, of the periodic signal in figure 4B 1s the same as in figure 4A. Preferably. the periodic signal in figures 4A and 4B is identical. Figure 4B differs from figure 4A in that the bias voltage applied to the bias input port 343 of intensity modulator 316 has been changed. The change of the bias voltage results in a phase shift in the periodic transmission response. The periodic transmission response can also be understood as the periodic transmission curve. The phase shift in the periodic transmission response can also be understood as a shift in the operation point in the periodic transmission response or periodic transmission curve. In an embodiment the DC input 342 has set a different bias voltage on DC input 343 Alternatively or additionally, a bias voltage is added as a DC offset to the periodic signal instead of to the DC input. In any case the periodic signal changes relative to the transmission response of the intensity modulator. The lowest value of the peak-to-peak voltage Vy, now corresponds to third point 458 on transmission response 452.

The highest value of the peak-to-peak voltage Vy, now corresponds to fourth point 460 on transmission response 452. The average voltage Va, is the average voltage of a voltage Vp measured by photodiode 348 corresponding to a third voltage of third point 458 and a fourth voltage of fourth point 460. The Vase of third point 458 and fourth point 460 in figure 4B is Vs.

Due to the changing of the bias voltage, third point 458 and fourth point 460 are positioned higher on the transmission response 452. Therefore, the Vave V2 of figure 4B is a higher value than Varg Vi of figure 4A.

By sweeping the bias voltage over a predetermined range, the average voltage 462, shown in figure 4C, may have a sine-like behavior when setting the average voltage Va, against the bias voltage Vrias. This sine-like behavior of average voltage 462 is caused by the fact that the peak-to- peak voltage Vyp of the periodic signal is not equal to the half-wave voltage V. This has as its consequence that the points corresponding to the peak-to-peak values of the periodic signal are not in antiphase in relative to the transmission response 452 when the bias voltage is changed. FPGA 344 may determine that the peak-to-peak voltage of the periodic signal is not the half-wave voltage

V; of intensity modulator 316 when the average voltage Vay has a sine-like behavior. FPGA 344 may determine that the peak-to-peak voltage of the periodic signal is not the half-wave voltage Vz of intensity modulator 316 when the average voltage Va 18 not constant.

Figure 5A shows the same transmission response 552 of intensity modulator 316 as transmission response 452 shown in figures 4A and 4B. A different between figures SA-B and figures 4A-B is that the peak-to-peak voltage Vy, of the periodic signal has a higher value in figures SA-B than in figures 4A-B. The lowest value of the peak-to-peak voltage Vy; now corresponds to fifth point 564 on transmission response 552. The highest value of the peak-to-peak voltage Vy now corresponds to sixth point 566 on transmission response 552. Fifth point 564 also corresponds to the lowest value of transmission response 552, while sixth point 566 corresponds to the highest value of transmission response 552. This means that the peak-to-peak voltage Vp of the periodic signal is equal to the half-wave voltage Vz. The average voltage Va. 18 in this case also the average of transmission response 552, and its value is Vs.

In figure 5B the same transmission response 552 from intensity modulator 316 is shown.

The Vy; of the periodic signal in figure 5B is the same as in figure 5A. Preferably, the periodic signal in figures SA and 5B is identical. Figure 5B differs from figure 3A in that the bias voltage that is applied to the bias input port 343 of intensity modulator 316 has been changed, resulting in a phase shift in the periodic transmission response. In an embodiment the FPGA 344 has set a different bias voltage on bias input port 343. The lowest value of the peak-to-peak voltage Vyp now corresponds to seventh point 368 on transmission response 552. The highest value of the peak-to- peak voltage Vy, now corresponds to eighth point 570 on transmission response 552. The Ve of seventh point 558 and eighth point 570 in figure 5B is V3. As the peak-to-peak value of the periodic signal is equal to the half-wave voltage V;, a change of the bias voltage does not result in a change in the average voltage Vas. Therefore, the Vas Vs of figure 3B is equal to Vave Vi of figure SA.

The average voltage Vavg remains constant when the bias voltage is changed, because fifth point 564 and sixth point 566 are in antiphase to cach other. Fifth point 564 and sixth point 566 being in antiphase means that the Vrs value between fifth point 564 and sixth point 566 is half of the Vre value needed for a full period of the transmission response 552 of intensity modulator 316.

Due to the fifth points 564 and sixth point 566 being in antiphase, a change in the bias voltage results in a change in measured voltage Vep of fifth point 564 that is equal to and opposite in sign compared to a change in measured voltage Vp of sixth point 566. This assures that the average voltage Vie remains constant when the bias voltage is changed.

The present disclosure makes use of the fact that the periodic signal is changed relative to the transmission response of the intensity modulator. It is clear for the skilled person that a bias voltage can be applied either to the transmission response or to periodic signal to achieve this relative change.

By sweeping the bias voltage of the periodic signal, the average voltage 572, shown in figure 5C, may behave as a flat line when setting the average voltage Va. against the bias voltage

Vias. This flat behavior of average voltage 572 is caused by the fact that the peak-to-peak voltage

Vp of the periodic signal is equal to the half-wave voltage Vz. This has as its consequence that the points corresponding to the peak-to-peak values of the periodic signal are in antiphase in relative to the transmission response 452 when the bias voltage is changed. A user may determine that the peak-to-peak voltage of the periodic signal is the half-wave voltage V of intensity modulator 316 when the average voltage Va, has a flat behavior. The user may determine that the peak-to-peak voltage of the periodic signal is the half-wave voltage Vz of intensity modulator 316 when the average voltage Va, remains constant. It is clear to the skilled person that the determination of the

Vag having a flat behavior or remaining constant may also be done automatically, for example by a controller or FPGA 344.

Figure 6 shows a method according to the disclosure. In step 680 the laser light is supplied by a laser unit. In step 681 the laser light is modulated by an intensity modulator. The modulation of the laser light provided in step 680 is based on the applied periodic signal to the radiofrequency input port of the intensity modulator. The periodic signal applied to the intensity modulator may be a square wave. An advantage of applying a square wave to the radiofrequency input port is that the only light intensities measured by the measurement device are the peaks of the square wave. This simplifies the determination of the average light intensity. The average light intensity of the modulated laser light is determined in step 682. The determination of the average light intensity can be executed by a photodetector receiving the modulated laser light and/or a voltage measurement device measuring the voltage of the photodiode. The photodetector may be a photodiode. A voltage coming from the photodiode may be representative of the light intensity from the modulated laser light. Alternatively, the average light intensity may be determined by a controller comprising a calculating unit for calculating the average of the light intensity. In step 683 the half-wave voltage of the intensity modulator may be determined. This can be done by changing the bias voltage applied to the bias input port of the intensity modulator. The determination of the half-wave voltage of the intensity modulator is further illustrated in figure 7.

Step 784 in figure 7 comprises estimating the peak-to-peak voltage of the periodic signal.

The peak-to-peak voltage of the periodic signal may be set by a controller that is connected to the periodic signal generator. In step 785 the periodic signal with the estimated peak-to-peak voltage is applied to the intensity modulator, thereby modulating the laser light coming into the intensity modulator. In step 786 the average light intensity of the modulated laser light is determined. This average light intensity may be represented by a voltage from a photodetector, for example a photodiode. In step 788 the bias voltage that is applied to a bias input port of the intensity modulator is changed.

In an embodiment the changing of the bias voltage comprises sweeping the bias voltage over a predetermined range. An advantage of sweeping the bias voltage is that the bias voltage cannot be accidentally changed with exactly the half-wave voltage, resulting in an incorrect determination of the half-wave voltage. Therefore, sweeping the bias voltage prevents incorrect determination of the half-wave voltage.

In step 789 the average light intensity is measured again by the measurement device. The change in average light intensity measured in step 786 and step 789 is determined. Subsequently, in step 790, it is determined if the average light intensity measured has remained constant. This may be determined by evaluating the change in average light intensity from step 789. In case of the change being determined as zero, it is determined in step 790 that the average light intensity is constant. In case the change is non-zero, it is determined in step 790 that the average light intensity is not constant. In an embodiment according to the disclosure the change being zero or non-zero may be determined by the change being below or above an acceptability threshold, respectively.

If in step 790 it 1s determined that the average light intensity is constant, then in step 791 it is determined that the estimated peak-to-peak voltage is the half-wave voltage of the intensity modulator. Quantum information processing system 100 can be calibrated based on the determined half-wave voltage. If in step 790 it is determined that the average light intensity is not constant when changing the bias voltage, a new estimated peak-to-peak voltage is set. Afterwards, steps 785,786, 788, 789 and 790 are then repeated.

An advantage of the present disclosure is that the half-wave voltage of an intensity modulator is accurately measured in a simple way. The average light intensity can easily be determined by known measurement devices, thereby reducing costs of the present disclosure.

Another advantage is that the present disclosure can be applied to an intensity modulator in a

QKD-system or quantum information processing system without the need to change the set-up of the system.

The peak-to-peak voltage may increase during each successive estimate 784. The peak-to- peak voltage may start at a value of 0 V. In this way it is assured that the peak-to-peak voltage that is determined as the half-wave voltage is not for example three times the true half-wave voltage.

The present disclosure is by no means limited to the above described preferred embodiments thereof. The rights sought are defined by the following claims within the scope of which many modifications can be envisaged.

Claims

CONCLUSIESCONCLUSIONS

1. Werkwijze voor het vaststellen van de halfgolfspanning (Engels: “half-wave voltage”) van een intensiteitsmodulator die is ingericht om een lichtveld, zoals een laserlicht, te moduleren, gebaseerd op invoer op een radiofrequentie-invoerpoort en een voorspanningspoort, omvattende: - het voorzien van het lichtveld aan de intensiteitsmodulator; - het moduleren van het voorziene lichtveld door het toepassen van een periodiek signaal met een geschat piek-tot-piek spanning op de radiofrequentie-invoerpoort van de intensiteitsmodulator en het toepassen van een voorspanning (“bias voltage™) op de voorspanningspoort van de intensiteitsmodulator en/of op het periodieke signaal bij de radiofrequentie-invoer van de intensiteitsmodulator; - het meten van het gemoduleerde lichtveld dat van de intensiteitsmodulator komt en het vaststellen van de gemiddelde lichtintensiteit van het gemeten gemoduleerde lichtveld; en - het vaststellen van de halfgolfspanning van de intensiteitsmodulator gebaseerd op het geschatte piek-tot-piek spanning en de vastgestelde gemiddelde lichtintensiteit van het gemoduleerde lichtveld.A method of determining the half-wave voltage of an intensity modulator configured to modulate a light field, such as a laser light, based on inputs to a radio frequency input port and a biasing port, comprising: - providing the light field to the intensity modulator; - modulating the projected light field by applying a periodic signal with an estimated peak-to-peak voltage to the radio frequency input port of the intensity modulator and applying a bias voltage™ to the bias voltage port of the intensity modulator and /or to the periodic signal at the radio frequency input of the intensity modulator; - measuring the modulated light field coming from the intensity modulator and determining the average light intensity of the measured modulated light field; and - determining the half-wave voltage of the intensity modulator based on the estimated peak-to-peak voltage and the determined average light intensity of the modulated light field.

2. Werkwijze volgens conclusie 1, waarbij de stap van het vaststellen van de halfgolfspanning verder omvat: - het aanpassen van de voorspanning die is toegepast op de voorspanningsinvoer en/of op het periodiek signaal, waarbij het aanpassen van de voorspanning resulteert in een faseverschuiving in een periodieke transmissierespons van de intensiteitsmodulator; en - het vaststellen dat de geschatte piek-tot-piek spanning van het periodieke signaal de halfgolfspanning van de intensiteitsmodulator is in het geval de vastgestelde gemiddelde lichtintensiteit in hoofdzaak constant blijft bij het veranderen van de voorspanning.The method of claim 1, wherein the step of determining the half-wave voltage further comprises: - adjusting the bias voltage applied to the bias voltage input and/or to the periodic signal, the adjustment of the bias voltage resulting in a phase shift in a periodic transmission response of the intensity modulator; and - determining that the estimated peak-to-peak voltage of the periodic signal is the half-wave voltage of the intensity modulator in case the determined average light intensity remains substantially constant when changing the bias voltage.

3. Werkwijze volgens conclusie 2, verder omvattende: - het veranderen van de geschatte piek-tot-piek spanning van het periodieke signaal naar ten minste een verder geschatte piek-tot-piek spanning in het geval de vastgestelde gemiddelde lichtintensiteit verandert bij het veranderen van de voorspanning. 4, Werkwijze volgens conclusie 3, verder omvattende:A method according to claim 2, further comprising: - changing the estimated peak-to-peak voltage of the periodic signal to at least a further estimated peak-to-peak voltage in case the determined average light intensity changes upon changing of the preload. The method of claim 3, further comprising:

- het herhalen van het veranderen van de geschatte piek-tot-piek spanning van het periodiek signaal totdat een piek-tot-piek spanningsniveau is bereikt waarop een verandering van de vastgestelde gemiddelde lichtintensiteit gedurende het veranderen van de voorspanning minimaal is.- repeating the changing of the estimated peak-to-peak voltage of the periodic signal until a peak-to-peak voltage level is reached at which a change in the determined average light intensity during the changing of the bias voltage is minimal.

5. Werkwijze volgens één van de voorgaande conclusies, verder omvattende: - het met een bundelsplitser opsplitsen van het Lichtveld dat uit de intensiteitsmodulator komt .5. A method according to any one of the preceding claims, further comprising: - splitting the Light field emerging from the intensity modulator with a beam splitter.

6. Werkwijze volgens één van de voorgaande conclusies, waarbij het periodieke signaal een blokgolf is.A method according to any one of the preceding claims, wherein the periodic signal is a square wave.

7. Werkwijze volgens één van de voorgaande conclusies, verder omvattende: - het vaststellen wanneer ten minst één van piekwaarden van de geschatte piek-tot-piek spanning en piekwaarden van de verder geschatte piek-tot-piek spanning van het periodiek signaal in tegenfase zijn ten opzichte van een transmissierespons van de intensiteitsmodulator; - het vaststellen dat ten minste één van de geschatte piek-tot-piek spanning en de verder geschatte piek-tot-piek spanning van het periodieke signaal de halfgolfspanning is in het geval de piekwaardes van het periodiek signaal in tegenfase zijn ten opzichte van de transmissierespons.A method according to any one of the preceding claims, further comprising: - determining when at least one of peak values of the estimated peak-to-peak voltage and peak values of the further estimated peak-to-peak voltage of the periodic signal are in phase opposition relative to a transmission response of the intensity modulator; - determining that at least one of the estimated peak-to-peak voltage and the further estimated peak-to-peak voltage of the periodic signal is the half-wave voltage in case the peak values of the periodic signal are in opposition to the transmission response .

8. Werkwijze volgens conclusie 7. wanneer afhankelijk van ten minste conclusie 2, waarbij het vaststellen dat ten minste één van de piekwaardes van de geschatte piek-tot-piek spanning en de piekwaardes van de verder geschatte piek-tot-piek spanning van het periodieke signaal in tegenfase is, is gebaseerd op het constant blijven van de vastgestelde gemiddelde lichtintensiteit bij het veranderen van de voorspanning.The method of claim 7 when dependent on at least claim 2, wherein determining that at least one of the peak values of the estimated peak-to-peak voltage and the peak values of the further estimated peak-to-peak voltage of the periodic signal is out of phase is based on the determined average light intensity remaining constant when changing the bias voltage.

9. Werkwijze volgens één van de conclusies 2-8, waarbij het veranderen van de voorspanning het doorlopen (Engels: “sweeping”) van de voorspanning over een vooraf bepaald bereik omvat.The method of any one of claims 2 to 8, wherein changing the bias voltage comprises sweeping the bias voltage over a predetermined range.

10. Werkwijze volgens één van de voorgaande conclusies, waarbij het vaststellen van de gemiddelde lichtintensiteit van het laserlicht is uitgevoerd door een fotodetectoreenheid die is ingericht om een detectiesignaal dat representatief is voor de gemiddelde lichtintensiteit te genereren, waarbij de fotodetectoreenheid optioneel een fotodiode en een spanningsmeetinrichting die is verbonden met de fotodiode omvat.A method according to any one of the preceding claims, wherein the determination of the average light intensity of the laser light is performed by a photodetector unit arranged to generate a detection signal representative of the average light intensity, the photodetector unit optionally comprising a photodiode and a voltage measuring device. connected to the photodiode.

11. Werkwijze volgens één van de voorgaande conclusies, waarbij een freguentie van het periodieke signaal in bereik van 50 Mhz — 1 Ghz ligt, bij voorkeur in het bereik van 100 Mhz — 500 Mhz ligt, en met de meeste voorkeur 200 Mhz is.A method according to any one of the preceding claims, wherein a frequency of the periodic signal is in the range of 50 MHz - 1 Ghz, preferably in the range of 100 MHz - 500 MHz, and most preferably is 200 MHz.

12. Werkwijze volgens één van de voorgaande conclusies, verder omvattende: - het kalibreren van de intensiteitsmodulator op basis van de vastgestelde halfgolfspanning.A method according to any one of the preceding claims, further comprising: - calibrating the intensity modulator on the basis of the determined half-wave voltage.

13. Systeem voor het vaststellen van de halfgolfspanning van een intensiteitsmodulator, het systeem omvattende: - een lasereenheid voor het verzenden van een lichtveld; - een intensiteitsmodulator omvattende een radiofrequentie-invoerpoort en een voorspanningspoort, waarbij de intensiteitsmodulator is ingericht om een inkomend lichtveld van de lasereenheid te moduleren op basis van invoer die is ontvangen bij de radiofrequentie-invoerpoort en de voorspanningspoort; - een periodieksignaalgenerator voor het toepassen van een periodiek signaal aan de radiofrequentie-invoerpoort van de intensiteitsmodulator; - een gelijkspanningsinvoer voor het toepassen van een voorspanning aan de voorspanningspoort van de intensiteitsmodulator en/of aan een periodiek signaal op de radiofrequentie-invoer van de intensiteitsmodulator; - een besturingseenheid die is verbonden met de periodieksignaalgenerator en is ingericht om een geschatte piek-tot-piek spanning van het periodiek signaal van de periodiek signaalgenerator in te stellen; en - een meetinrichting voor het vaststellen van een gemiddelde lichtintensiteit van het gemoduleerde lichtveld dat van de intensiteitsmodulator komt, waarbij de besturingseenheid is ingericht om de halfgolfspanning van de intensiteitsmodulator vast te stellen op basis van de geschatte piek-tot-piek spanning van het periodieke signaal en de gemiddelde lichtintensiteit die is vastgesteld door de meetinrichting.13. System for determining the half-wave voltage of an intensity modulator, the system comprising: - a laser unit for transmitting a light field; - an intensity modulator comprising a radio frequency input port and a biasing port, the intensity modulator configured to modulate an incoming light field of the laser unit based on input received at the radio frequency input port and the biasing port; - a periodic signal generator for applying a periodic signal to the radio frequency input port of the intensity modulator; - a DC voltage input for applying a bias voltage to the bias voltage gate of the intensity modulator and/or to a periodic signal at the radio frequency input of the intensity modulator; - a control unit connected to the periodic signal generator and adapted to set an estimated peak-to-peak voltage of the periodic signal from the periodic signal generator; and - a measuring device for determining an average light intensity of the modulated light field coming from the intensity modulator, the control unit being adapted to determine the half-wave voltage of the intensity modulator on the basis of the estimated peak-to-peak voltage of the periodic signal and the average light intensity determined by the metering device.

14. Systeem volgens conclusie 13, waarbij de besturingseenheid is ingericht om een voorspanning van de gelijkspanningsinvoer te veranderen resulterend in een faseverschuiving in een periodieke transmissierespons van de intensiteitsmodulator, en waarbij de geschatte piek-tot-piek spanning van het periodieke signaal is vastgesteld als de halfgolfspanning in het geval de vastgestelde gemiddelde lichtintensiteit in hoofdzaak constant blijft bij het veranderen van de voorspanning.The system of claim 13, wherein the controller is configured to change a DC input bias voltage resulting in a phase shift in a periodic transmission response of the intensity modulator, and the estimated peak-to-peak voltage of the periodic signal is determined as the half-wave voltage in case the determined average light intensity remains substantially constant when changing the bias voltage.

15. Systeem volgens conclusie 14, waarbij de besturingseenheid is ingericht om de geschatte piek-tot-piek spanning van het periodieke signaal te veranderen naar ten minste een verder geschatte piek-tot-piek spanning in het geval de vastgestelde gemiddelde lichtintensiteit verandert bij het veranderen van de voorspanning.The system of claim 14, wherein the control unit is arranged to change the estimated peak-to-peak voltage of the periodic signal to at least a further estimated peak-to-peak voltage in case the determined average light intensity changes when changing of the preload.

16. Systeem volgens conclusie 15, waarbij de besturingseenheid verder is ingericht om het veranderen van de geschatte piek-tot-piek spanning van het periodiek signaal te herhalen totdat een piek-tot-piek spanningsniveau is bereikt waarop een verandering van de vastgestelde gemiddelde lichtintensiteit gedurende het veranderen van de voorspanning minimaal is.The system of claim 15, wherein the controller is further adapted to repeat changing the estimated peak-to-peak voltage of the periodic signal until a peak-to-peak voltage level is reached at which a change in the determined average light intensity during changing the preload is minimal.

17. Systeem volgens één van de voorgaande conclusies, waarbij het systeem verder een bundelsplitser die is gepositioneerd tussen de intensiteitsmodulator en de meetinrichting omvat voor het in twee bundels splitsen van het gemoduleerde lichtveld, waarbij bij voorkeur een eerste bundel gericht is in een kwantumkanaal en een tweede bundel is gericht naar de meetinrichting.A system according to any one of the preceding claims, wherein the system further comprises a beam splitter positioned between the intensity modulator and the measuring device for splitting the modulated light field into two beams, preferably with a first beam directed into a quantum channel and a second beam is directed to the metering device.

18. Systeem volgens één van de voorgaande conclusies, waarbij het periodieke signaal een blekgolf is.A system according to any one of the preceding claims, wherein the periodic signal is a pale wave.

19. Systeem volgens één van de voorgaande conclusies. waarbij de besturingseenheid is ingericht om vast te stellen dat ten minst één van piekwaarden van de geschatte piek-tot- piek spanning en piekwaarden van de verder geschatte piek-tot-piek spanning van het periodiek signaal in tegenfase zijn ten opzichte van een transmissierespons van de intensiteitsmodulator voor het vaststellen dat ten minste één van de geschatte piek-tot-piek spanning en de verder geschatte piek-tot-piek spanning van het periodieke signaal de haltgolfspanning is.19. System according to one of the preceding claims. wherein the control unit is arranged to determine that at least one of peak values of the estimated peak-to-peak voltage and peak values of the further estimated peak-to-peak voltage of the periodic signal are in antiphase with respect to a transmission response of the intensity modulator for determining that at least one of the estimated peak-to-peak voltage and the further estimated peak-to-peak voltage of the periodic signal is the half-wave voltage.

20. Systeem volgens conclusie 19, wanneer afhankelijk van ten minste conclusie 14, waarbij het vaststellen dat ten minste één van de piekwaardes van de geschatte piek-tot-piek spanning en de piekwaardes van de verder geschatte piek-tot-piek spanning van het periodieke signaal in tegenfase is, is gebaseerd op het constant blijven van de vastgestelde gemiddelde lichtintensiteit bij het veranderen van de voorspanning.The system of claim 19 when dependent on at least claim 14, wherein determining that at least one of the peak values of the estimated peak-to-peak voltage and the peak values of the further estimated peak-to-peak voltage of the periodic signal is out of phase is based on the determined average light intensity remaining constant when changing the bias voltage.

21. Systeem volgens één van de conclusies 14 - 20, waarbij het veranderen van de voorspanning het doorlopen van de voorspanning over een vooraf bepaald bereik omvat. The system of any one of claims 14 to 20, wherein changing the bias voltage comprises cycling the bias voltage over a predetermined range.

22, Systeem volgens één van de voorgaande conclusies, waarbij de meetinrichting een fotodetectoreenheid omvat die is ingericht om een detectorsignaal te genereren dat representatief is voor de gemiddelde lichtintensiteit, waarbij de fotodetectoreenheid optioneel een fotodiode en een spanningsmeetinrichting die is verbonden met de fotodiode omvat.A system according to any one of the preceding claims, wherein the measuring device comprises a photodetector unit arranged to generate a detector signal representative of the average light intensity, the photodetector unit optionally comprising a photodiode and a voltage measuring device connected to the photodiode.

23. Systeem volgens één van de voorgaande conclusies, waarbij een frequentie van het periodieke signaal in bereik van 50 Mhz — 1 Ghz ligt, bij voorkeur in het bereik van 100 Mhz — 500 Mhz ligt, en met de meeste voorkeur 200 Mhz is. A system according to any one of the preceding claims, wherein a frequency of the periodic signal is in the range of 50 MHz - 1 GHz, preferably in the range of 100 MHz - 500 MHz, and most preferably is 200 MHz.

24, Systeem volgens één van de voorgaande conclusies, waarbij de lasereenheid, de periodieksignaalgenerator, de stroominvoer, de besturingseenheid en de meetinrichting zijn voorzien in een enkele behuizing.A system according to any one of the preceding claims, wherein the laser unit, periodic signal generator, power input, control unit and measuring device are provided in a single housing.

25. Kwantumcommunicatienetwerk omvattende een systeem volgens één van de conclusies 13-24.A quantum communication network comprising a system according to any one of claims 13-24.

26. Inrichting voor het meten van de halfgolfspanning van een intensiteitsmodulator, omvattende: - een periodieksignaalgenerator voor het toepassen van een periodiek signaal op de radiofrequentie-invoerpoort van de intensiteitsmodulator; - een besturingseenheid die is verbonden met de periodiek signaalgenerator en is ingericht om een geschatte piek-tot-piek spanning van het periodiek signaal van de periodiek signaalgenerator in te stellen; en - een meetinrichting voor het vaststellen van een gemiddelde lichtintensiteit van het gemoduleerde lichtveld dat van de intensiteitsmodulator komt, waarbij de besturingseenheid is ingericht om de halfgolfspanning van de intensiteitsmodulator vast te stellen op basis van de geschatte piek-tot-piek spanning van het periodieke signaal en de gemiddelde lichtintensiteit die is vastgesteld door de meetinrichting.26. Apparatus for measuring the half-wave voltage of an intensity modulator, comprising: - a periodic signal generator for applying a periodic signal to the radio frequency input port of the intensity modulator; - a control unit connected to the periodic signal generator and arranged to set an estimated peak-to-peak voltage of the periodic signal from the periodic signal generator; and - a measuring device for determining an average light intensity of the modulated light field coming from the intensity modulator, the control unit being adapted to determine the half-wave voltage of the intensity modulator on the basis of the estimated peak-to-peak voltage of the periodic signal and the average light intensity determined by the metering device.

27. Computerprogramma dat is ingericht voor het uitvoeren van de werkwijzestappen volgens één van de conclusies 1-12 uit te voeren.A computer program adapted to perform the method steps of any one of claims 1-12.