Electro-optical PLL

Abstract

Electro-optical PLL (1), comprising • an input (I) for an electrical input signal (f CW , f Ramp ), • a first Mach-Zehnder modulator (MZM1), • a first optoelectronic converter device (PD1) and a second optoelectronic converter device (PD2), • a filter (LF), • a first optical power divider (SP1), • wherein the electrical input signal is fed to the first Mach-Zehnder modulator (MZM1) during operation, • wherein a first optical output signal of the first Mach-Zehnder modulator (MZM1) is supplied to the first optoelectronic converter device (PD1) during operation, and • wherein a second optical output signal of the first Mach-Zehnder modulator (MZM1) is supplied to the second optoelectronic converter device (PD2) during operation, • wherein the first optoelectronic converter device (PD1) and the second optoelectronic converter device (PD2) are connected in series, • wherein an electrical signal is tapped between the first optoelectronic converter device (PD1) and the second optoelectronic converter device (PD2), which is fed to the filter (LF) during operation, • wherein the electrical output signal of the filter (LF) is fed to a voltage-controlled optical oscillator (EO) VCO ) in frequency controls, • wherein on the output side of the voltage-controlled optical oscillator (EO) VCO ) the first optical power divider (SP1) is arranged, wherein in operation a part ((1-c)Ψ out,1 ) is fed back as an optical input signal to the first Mach-Zehnder modulator (MZM1), • wherein the first Mach-Zehnder modulator (MZM1) is operated at the maximum point or at the minimum point of the first Mach-Zehnder modulator (MZM1).

Description

background

[0001] Phase-locked loops (PLLs) are required in many areas of modern technology. The requirements for phase-locked loops vary.

[0002] However, newer applications in particular require phase-locked loops with a wide bandwidth but the narrowest possible spectral characteristics. This requirement exists, at least if a transmission channel controlled by the PLL is to be as efficient as possible.

[0003] Examples include radar systems with optical clock distribution and radio over fiber (RoF) systems, which require optical signals with the narrowest possible spectral characteristics to utilize the subsequent wireless channel as efficiently as possible. FMCW radar systems with optical clock distribution still require ramps that are as linear as possible.

[0004] From the publication "Fast optical frequency sweeping using voltage controlled oscillator driven single sideband modulation combined with injection locking" by the authors J. Wang, D. Chen, H. Cai, F. Wei, R. Qu, published in Opt. Express 23, pages 7038-7043 (2015), it is known to generate such signals using a frequency-stabilized electro-optical modulator.

[0005] From the publication "A 77 GHz 90 nm CMOS Transceiver for FMCW Radar Applications" by the authors T. Mitomo, N. Ono, H. Hoshino, Y. Yoshihara, O. Watanabe, and I. Seto, published in IEEE Journal of Solid-State Circuits, vol. 45, no. 4, pages 928-937, April 2010, doi: 10.1109 / JSSC.2010.2040234, a system is known that uses an electronic PLL to generate an electrical FMCW ramp. In this system, at least a fraction of the carrier and at least a fraction of the ramp frequency are multiplied in an electrical mixer and, after passing through a phase frequency detector (PFD), charge pump (CP), and loop filter (LF), converted into a control signal of a voltage-controlled oscillator. The electrical output signal of the voltage-controlled oscillator is then the linearized FMCW ramp at the desired frequency.

[0006] However, the system presented there does not provide a narrowband optical output signal. Furthermore, an external mixer is required to multiply the carrier and ramp. This external mixer not only increases power dissipation but also the complexity of the system.

[0007] The publication "Optoelectronic oscillator for photonic systems" by XS Yao and L. Maleki, published in the IEEE Journal of Quantum Electronics, vol. 32, no. 7, pages 1141-1149, July 1996, doi: 10.1109 / 3.517013, describes an electro-optical oscillator that, among other things, generates an optical output signal. Without external circuitry, however, this oscillator is not capable of generating different frequencies.

[0008] From the publication "A 2-20-GHZ Ultralow Phase Noise Signal Source Using a Microwave Oscillator Locked to a Mode-Locked Laser" by authors M. Bahmanian and J.C. Scheytt, published in IEEE Transactions on Microwave Theory and Techniques, vol. 69, no. 3, pages 1635-1645, March 2021, doi: 10.1109 / TMTT.2020.3047647, an electro-optical PLL is known that uses an optical MLL signal as a reference signal. This system uses a balanced optical microwave phase detector (BOMPD) in which an MZM mixes the electrical output signal of the PLL with the optical MLL input signal. Furthermore, the CP was replaced by two diodes. However, the electrical output signal can only be locked to multiples of the repetition frequency of the MLL. In particular, the system shown there has only one electrical output signal. Furthermore, only multiples of the MLL's repetition frequency can be generated. Furthermore, MLLs are very expensive to purchase and have a short lifespan.

[0009] Furthermore, a PLL circuit with a single oscillator is known from the article "Locking of microwave oscillators on the interharmonics of mode-locked laser signals" by Meysam Bahmanian; Christian Kress; and J. Christoph Scheytt, published in Optics Express, Vol. 30, February 28, 2022, No. 5, pp. 7763–7771. In particular, the system shown there has only an electrical output signal. Furthermore, MLLs are very expensive to purchase and have a short lifetime.

[0010] From the publication "Fast optical frequency sweeping using voltage-controlled oscillator driven single sideband modulation combined with injection locking" by J. Wang, D. Chen, H. Cai, F. Wei, and R. Qu, published in Opt. Express 23, pages 7038-7043 (2015), a system for generating optical FMCW ramps is known. In this system, an electrical FMCW ramp is modulated onto an optical signal using an electro-optical modulator. However, in this system—similar to the system from the article "Optoelectronic oscillator for photonic systems," a nonlinearity in the transfer characteristic of the voltage-controlled oscillator is necessary to exploit the maximum dynamic range.

[0011] Based on this, it is an object of the invention to provide a circuit which allows to provide a highly stable electro-optical PLL with a narrowband output frequency.

[0012] The problem is solved by an electro-optical PLL according to one of the independent claims. Advantageous embodiments are the subject of the dependent claims, the description, and the figures.

[0013] The invention is explained in more detail below with reference to the figures, which show: Fig. 1 is an exemplary block diagram of an electro-optical PLL according to an embodiment of the invention, and Fig. 2 an exemplary block diagram of an electro-optical PLL according to another embodiment of the invention.

[0014] The invention will be described in more detail below with reference to the figures. It should be noted that various aspects are described, each of which can be used individually or in combination. This means that any aspect can be used with different embodiments of the invention unless explicitly presented as a mere alternative.

[0015] Furthermore, for the sake of simplicity, reference will generally only be made to one entity in the following. Unless explicitly stated, the invention may also comprise several of the entities in question. Therefore, the use of the words "a," "an," and "another" is to be understood merely as an indication that at least one entity is used in a simple embodiment.

[0016] Where procedures are described below, the individual steps of a procedure can be arranged and / or combined in any order, unless the context explicitly indicates otherwise. Furthermore, the procedures can be combined with one another unless expressly indicated otherwise.

[0017] Numerical values ​​are generally not to be understood as exact values, but also include a tolerance of + / - 1% to + / - 10%.

[0018] References to standards or specifications are to be understood as references to standards or specifications that are / were in effect at the time of the application and / or, if priority is claimed, at the time of the priority application. However, this does not imply a general exclusion of applicability to subsequent or replacing standards or specifications.

[0019] Where multimode interferometers are referred to below, the designations of the shape are to be understood merely as an indication that this is the desired configuration with regard to a specific effect. If a multimode interferometer can, in principle, achieve more, but is, for example, triggered by suitable wiring / control to achieve the effect according to the designation, this is of course encompassed by the spirit and claim of the invention.

[0020] In Fig. Figure 1 shows an exemplary electro-optical PLL 1. This exemplary electro-optical PLL 1 has an input I for an electrical input signal. For the purposes of further understanding, the input signal is initially unimportant.

[0021] Furthermore, the exemplary electro-optical PLL 1 comprises a first Mach-Zehnder modulator MZM1, a first optoelectronic converter device PD1 and a second optoelectronic converter device PD2, a filter LF, and a first optical power splitter SP1.

[0022] In operation, the electrical input signal is supplied to the first Mach-Zehnder modulator MZM1, wherein a first optical output signal of the first Mach-Zehnder modulator MZM1 is supplied during operation to the first optoelectronic converter device PD1, wherein a second optical output signal of the first Mach-Zehnder modulator MZM1 is supplied during operation to the second optoelectronic converter device PD2.

[0023] The first optoelectronic converter device PD1 and the second optoelectronic converter device PD2 are connected in series, wherein an electrical signal is tapped between the first optoelectronic converter device PD1 and the second optoelectronic converter device PD2, which electrical signal is fed to the filter LF during operation.

[0024] The electrical output signal of the filter LF in turn controls a voltage-controlled optical oscillator EO VCO in frequency, where on the output side of the voltage controlled optical oscillator EO VCO the first optical power splitter SP1 is arranged, wherein in operation a part (1-c)Ψ out,1 is fed back to the first Mach-Zehnder modulator MZM1 as an optical input signal, and wherein the first Mach-Zehnder modulator MZM1 is operated at the maximum point or the minimum point of the first Mach-Zehnder modulator MZM1.

[0025] In a further embodiment of the invention, which is exemplified in Fig. As shown in Figure 2, the electro-optical PLL 1 has an input I for an electrical input signal. For further understanding, the input signal is initially unimportant.

[0026] Furthermore, the exemplary electro-optical PLL 1 comprises a first Mach-Zehnder modulator MZM1 and a second Mach-Zehnder modulator MZM2, a first optoelectronic converter device PD1 and a second optoelectronic converter device PD2, a filter LF, and a first optical power splitter SP1 and a second optical power splitter SP2.

[0027] In operation, the electrical input signal is supplied to the first Mach-Zehnder modulator MZM1 and the second Mach-Zehnder modulator MZM2, wherein a first optical output signal of the first Mach-Zehnder modulator MZM1 is supplied to the first optoelectronic converter device PD1, and wherein a second optical output signal of the second Mach-Zehnder modulator MZM2 is supplied to the second optoelectronic converter device PD2.

[0028] Again, the first optoelectronic converter device PD1 and the second optoelectronic converter device PD2 are connected in series, wherein an electrical signal is tapped between the first optoelectronic converter device PD1 and the second optoelectronic converter device PD2, which electrical signal is fed to the filter LF during operation.

[0029] The electrical output signal of the filter LF controls a voltage-controlled optical oscillator EO VCOin frequency, where on the output side of the voltage controlled optical oscillator EO VCO the first optical power splitter SP1 is arranged, wherein in operation a part (1-c)Ψ out,1 as an optical input signal to the first Mach-Zehnder modulator MZM1, with the output side of the voltage-controlled optical oscillator EO VCO the second optical power splitter SP2 is arranged, wherein in operation a part (1-d)Ψ out,2 is fed back as an optical input signal to the second Mach-Zehnder modulator MZM2, whereby now both the first Mach-Zehnder modulator MZM1 and the second Mach-Zehnder modulator MZM2 are each operated at the maximum point or the minimum point of the first Mach-Zehnder modulator MZM1.

[0030] In other words, Fig. 1 a so-called single-ended case of an electro-optical PLL 1, while Fig.2 shows a differential case of an electro-optical PLL 1.

[0031] In both cases, an (additive) input signal is multiplied in the Mach-Zehnder modulator MZM1, MZM2. The input signal of the electro-optical modulators MZM1, MZM2 is a fraction of the optical output signal. The multiplied electrical input signal is then applied to this optical signal.

[0032] If a Mach-Zehnder modulator MZM1, MZM2 is operated at the maximum point or at the minimum point, an advantageous operating point results, since around these points a Mach-Zehnder modulator MZM1, MZM2 works almost as a square law mixer.

[0033] The photodiodes PD1 and PD2 now generate a current proportional to the phase and frequency difference. The electrical filter LF filters out unwanted harmonics and provides the control voltage for the electro-optical oscillator EO. VCO, which changes its frequency based on the control voltage. The oscillator can be locked.

[0034] In embodiments of the invention, the electrical input signal f CW , f Ramp at input I a superimposed signal from a first electrical signal f CW and another electrical signal f Ramp . In particular, the electro-optical PLL has an input stage ES in which a first signal f CW with another electrical signal f Rampis superimposed. Such an input stage ES can in particular have a power combiner. In particular, the input stage ES is selected from a group comprising coupled transmission lines, branch-line couplers, Lange couplers, Y-couplers, Wilkinson power divider, hybrid couplers, hybrid ring couplers, Bethe-hole directional couplers, Riblet short-slot couplers, Schwinger reversed-phase couplers, Moreno crossed-guide couplers, magic tees, transformers, resistive tees, and 6 dB resistive bridge hybrids, but without being limited to these.

[0035] In embodiments of the invention, the first electrical signal (f CW ) a carrier signal and the further electrical signal (f Ramp ) selected from a group comprising a ramp signal and a triangular signal, a sine signal.

[0036] According to a further embodiment of the invention, the electrical filter LF is designed as a low-pass or band-pass filter.

[0037] That is, for example, f CW as a carrier signal or a fraction thereof and an optional ramp signal or a fraction of a ramp signal f Rampin a summer, e.g., a Wilkinson power combiner. This added signal is then fed to an electro-optical modulator MZM1 (single-ended case) or two electro-optical modulators MZM1, MZM2 (differential case). The added signals are multiplied by the non-linear transfer function of the input signals – intensity changes – of the electro-optical modulators MZM1, MZM2. The input signal of the electro-optical modulators MZM1, MZM2 is a fraction of the optical output signal. The multiplied electrical input signal is then applied to this optical signal at input I of the electro-optical modulators MZM1, MZM2. The photodiodes PD1, PD2 now generate a current proportional to the phase and frequency difference. The electrical filter LF filters out unwanted harmonics and provides the control voltage for the electro-optical oscillator EO. VCO, which changes its frequency based on the control voltage. This makes it possible to use the electro-optical oscillator EO VCO on the product of the signals f CW and f Ramp to lure. It should be noted that f Ramp does not necessarily have to be a ramp (e.g. RoF system).

[0038] The invention thus enables an optical output signal to be converted to the sum of two electrical input signals f CW , f Ramp is lured.

[0039] The efficient design of the invention eliminates the need for an electrical mixer, which would increase the power loss and the complexity of the system.

[0040] In addition, the invention makes it possible to compensate for nonlinearities of an arbitrarily detunable oscillator with optical output in order to generate FMCW ramps.

[0041] It should be noted that the invention is not limited to two sources of electrical input signals f CW , fRamp is limited.

[0042] Rather, a multitude of sources can be fed in. This allows the generation of arbitrarily complex optical pulse shapes whose spectral properties can only be determined by the electrical filter LF, the electro-optical modulators MZM1, MZM2 and the electro-optical oscillators EO. VCO depend.

[0043] Thus, by means of the invention, any complex optical signals, e.g. FMCW (frequency modulated continuous wave) ramp signals, can be generated without the need for an additional electrical mixer and / or an electrical frequency stabilization unit.

[0044] Such signals are used, for example, in frequency-modulated continuous wave radar systems (FMCW radar).

[0045] In such a system, frequency and phase stabilization is part of the control loop. This allows, for example, optical FMCW ramps for radar systems with optical clock distribution to be linearized or the channel bandwidth in RoF systems to be reduced.

[0046] The above-described embodiments can thus be used in particular in a radar application, wherein the bandwidth of the electrical filter LF is in particular 7 GHz or smaller, in particular 5 GHz or smaller, in particular 250 MHz or smaller, in particular 200 MHz or smaller, in particular 76 MHz or smaller.

[0047] In general, however, the embodiments described above can also be used in a communications application, wherein the bandwidth of the electrical filter LF corresponds in particular to the bandwidth of one of the so-called ISM bands, see in particular ITU Radio Regulations Articles, Edition of 2020.

Claims

[1] Electro-optical PLL (1), comprising • an input (I) for an electrical input signal (f CW , f Ramp ), • a first Mach-Zehnder modulator (MZM1), • a first optoelectronic converter device (PD1) and a second optoelectronic converter device (PD2), • a filter (LF), • a first optical power splitter (SP1), • wherein the electrical input signal is fed to the first Mach-Zehnder modulator (MZM1) during operation, • wherein a first optical output signal of the first Mach-Zehnder modulator (MZM1) is supplied to the first optoelectronic converter device (PD1) during operation, and • wherein a second optical output signal of the first Mach-Zehnder modulator (MZM1) is supplied to the second optoelectronic converter device (PD2) during operation, • wherein the first optoelectronic converter device (PD1) and the second optoelectronic converter device (PD2) are connected in series, • wherein an electrical signal is tapped between the first optoelectronic converter device (PD1) and the second optoelectronic converter device (PD2), which electrical signal is fed to the filter (LF) during operation, • the electrical output signal of the filter (LF) being fed to a voltage-controlled optical oscillator (EO VCO ) in frequency, • where on the output side of the voltage controlled optical oscillator (EO VCO ) the first optical power splitter (SP1) is arranged, wherein during operation a part ((1-c)Ψ out,1 ) is fed back to the first Mach-Zehnder modulator (MZM1) as an optical input signal, • wherein the first Mach-Zehnder modulator (MZM1) is operated at the maximum point or at the minimum point of the first Mach-Zehnder modulator (MZM1). [2] Electro-optical PLL (1), comprising • an input (I) for an electrical input signal (f CW , f Ramp ), • a first Mach-Zehnder modulator (MZM1) and a second Mach-Zehnder modulator (MZM2), • a first optoelectronic converter device (PD1) and a second optoelectronic converter device (PD2), • a filter (LF), • a first optical power splitter (SP1) and a second optical power splitter (SP2), • wherein the electrical input signal is supplied to the first Mach-Zehnder modulator (MZM1) and the second Mach-Zehnder modulator (MZM2) during operation, • wherein a first optical output signal of the first Mach-Zehnder modulator (MZM1) is supplied to the first optoelectronic converter device (PD1) during operation, and • wherein a second optical output signal of the second Mach-Zehnder modulator (MZM2) is supplied to the second optoelectronic converter device (PD2) during operation, • wherein the first optoelectronic converter device (PD1) and the second optoelectronic converter device (PD2) are connected in series, • wherein an electrical signal is tapped between the first optoelectronic converter device (PD1) and the second optoelectronic converter device (PD2), which electrical signal is fed to the filter (LF) during operation, • the electrical output signal of the filter (LF) drives a voltage-controlled optical oscillator (EO VCO ) in frequency, • where on the output side of the voltage controlled optical oscillator (EOVCO ) the first optical power splitter (SP1) is arranged, wherein during operation a part ((1-c)Ψ out,1 ) is fed back to the first Mach-Zehnder modulator (MZM1) as an optical input signal, • where on the output side of the voltage controlled optical oscillator (EO VCO ) the second optical power splitter (SP2) is arranged, wherein during operation a part ((1-d)Ψ out,2 ) is fed back to the second Mach-Zehnder modulator (MZM2) as an optical input signal, • wherein the first Mach-Zehnder modulator (MZM1) and the second Mach-Zehnder modulator (MZM2) are each operated at the maximum point or at the minimum point of the first Mach-Zehnder modulator (MZM1). [3] Electro-optical PLL (1) according to claim 1 or 2, characterized by that the electrical input signal (f CW , f Ramp ) at the input (I) a superimposed signal from a first electrical signal (f CW) and another electrical signal (f Ramp ) is. [4] Electro-optical modulator (1) according to claim 3, further comprising an input stage (ES) in which a first signal (f CW ) with another electrical signal (f Ramp ) is superimposed. [5] Electro-optical PLL (1) according to claim 4, characterized by that the input stage (ES) has a power combiner. [6] Electro-optical PLL (1) according to claim 5, characterized by that the input stage (ES) is selected from a group comprising Coupled transmission lines, Branch-line coupler, Lange coupler, Y-coupler, Wilkinson power divider, Hybrid coupler, Hybrid ring coupler, Bethe-hole directional coupler, Riblet short-slot coupler, Schwinger reversed-phase coupler, Moreno crossed-guide coupler, Magic tee, transformer, Resistive tee, 6 dB resistive bridge hybrid. [7] Electro-optical PLL (1) according to one of claims 3 to 6, characterized bythat the first electrical signal (f CW ) a carrier signal and the further electrical signal (f Ramp ), selected from a group comprising a ramp signal and a triangular signal, is a sine signal. [8] Electro-optical PLL (1) according to one of the preceding claims, characterized by that the filter (LF) is designed as a low-pass or band-pass filter. [9] Use of an electro-optical PLL (1) according to one of the preceding claims in a radar application, wherein the bandwidth of the filter is in particular 7 GHz or less, in particular 5 GHz or less, in particular 250 MHz or less, in particular 200 MHz or less, in particular 76 MHz or less. [10] Use of an electro-optical PLL (1) according to one of the preceding claims 1 to 8 in a communication application, wherein the bandwidth of the filter corresponds in particular to the bandwidth of one of the ISM bands.

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