Low Phase Noise Tunable Signal Source

Abstract

The present invention relates to a low phase noise adjustable signal source, comprising: an optical microwave phase detector (BOMPD), the optical microwave phase detector having: an intensity modulator (BIM) including an optical signal input, a modulation input (I) and a first output (O1) and a second output (O2); a first photodiode (PD1) capable of irradiating light of the first output (O1) in operation; and a second photodiode (PD2) capable of irradiating light of the second output (O2) in operation, the first photodiode (PD1) and the second photodiode (PD2) being biased in operation. a tap for the tap signal is arranged between the first photodiode (PD1) and the second photodiode (PD2); a controllable direct current source (N4) is further provided, an offset current is adjustable in operation by the first direct current source (N4) at the tap, whereby the symmetry of the optical microwave phase detector is eliminated in operation by the offset current; the tap together with any offset current present is guided to a low pass filter, and the low pass filtered tap signal is provided to a tunable oscillator (OSZ). [Diagram 2]

Description

[Technical field]

[0001] The present invention relates to a low phase noise adjustable signal source. [Background technology]

[0002] In many areas of electrical engineering, it is necessary to provide signals with low phase noise. It is particularly desirable if the signal source is tunable over a wide frequency range with high frequency resolution. An exemplary use would be to lock a microwave oscillator to an optical clock signal from such a source. Another use would be to distribute an optical clock signal over low phase noise, low loss fibers. Various techniques are known from the prior art. For example, from the paper “Optics Letters”, Vol. 37, no. 14, p. 2958, Jul 2022, https: / / doi.org / 10.1364 / OL.37.002958, by Kwangyun Jung and Jungwon Kim, a device is known that makes it possible to lock a microwave oscillator to an integer harmonic of the repetition rate of an optical clock generator. Furthermore, a balanced optical microwave phase detector is known from non-patent document 2, a paper published in IEEE Transactions on Microw. Theory Tech, pp. 1-1 (2021) by M. Bahmanian and JC Cheytt. It is also known from these papers that microwave phase detectors equipped with fiber-based Sagnac loop intensity modulators or Mach-Zehnder intensity modulators can be used for this purpose. What all approaches known to the inventors so far have in common is that current system designs are only capable of providing high frequency signals that are integer multiples of the repetition rate of the optical signal source, i.e., the output frequency is fout=N*frep, where N∈N. In a method previously known to the inventors, a balanced modulator is biased at its odd symmetrical points, i.e. the bias at these points results in equal optical output intensities, and the sign of the output current of a balanced optical microwave phase detector is then reversed when the phase difference of all high frequency amplitudes is zero. A disadvantage of hitherto known approaches is that they are only suitable either for providing precisely determined (fixed) frequencies or for generating only precisely pre-defined integer multiples. Moreover, it is disadvantageous that some of the structures have a very complicated structure. Yet another drawback is, for example, the relatively large space requirement and the high cost, which may be considered especially in the case of fiber-based Sagnac loop intensity modulators. [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] Kwangyun Jung, Jungwon Kim, “Subfemtosecond synchronization of microwaVe oscillators with mode-locked Er-fiber lasers”, Optics Letters, Vol. 37, no. 14, p. 2958, Jul 2012, https: / / doi.org / 10.1364 / OL.37.002958 [Non-Patent Document 2] M. Bahmanian, J.C. Scheytt, “A 2-20-GHz ultralow phase noise signal source using a microwaVe oscillator locked to a mode-locked laser,” IEEE Transactions on Microw.Theory Tech.pp.1-1(2021) Summary of the Invention [Problem to be solved by the invention]

[0004] Against this background, the object of the present invention is to provide an improvement, in particular to provide an uncomplicated and space-saving structure, another aim being to improve the frequency resolution. Brief description of the invention The above problem is solved by a low phase noise adjustable signal source according to claim 1. Further advantageous embodiments of the invention are the subject of the dependent claims, the description and the figures. [Brief description of the drawings]

[0005] The present invention will now be described in detail with reference to the drawings, in which: [Figure 1a] FIG. 1 shows a schematic diagram of a balanced optical microwave phase detector according to the prior art; [Figure 1b] FIG. 2 is a schematic block equivalent circuit diagram of this embodiment. [Diagram 2] 1 shows a schematic diagram of a balanced optical microwave phase detector according to the present invention; [Figure 3a] 1 is a graph of measured values ​​according to an embodiment of the present invention relative to theoretical values. [Figure 3b] 1 is a graph of measured values ​​according to an embodiment of the present invention relative to theoretical values. [Figure 3c] 1 is a graph of measured values ​​according to an embodiment of the present invention relative to theoretical values. [Figure 3d] 1 is a graph of measured values ​​according to an embodiment of the present invention relative to theoretical values. Detailed Description of the Invention

[0006] The invention will now be described in more detail with reference to the figures, in which it is noted that different aspects are described which can be used either alone or in combination, i.e. each aspect can be used in different embodiments of the invention, unless expressly indicated as merely alternatives. Moreover, for simplicity in the following, generally, only entities are always referred to. Unless explicitly stated, the invention may also include several of the respective entities. To that extent, the use of the singular terms "ein", "eine" and "eines" should only be understood as indicating that at least one entity is used in a single embodiment. Insofar as methods are described below, the individual steps of the methods can be arranged and / or combined in any order, unless the context explicitly dictates otherwise. Furthermore, unless otherwise stated, these methods can be combined with each other. Information containing numerical values ​​should not normally be understood as exact values ​​but also include a tolerance of + / -1% to + / -10%. References to standards or specifications are to be understood as references to the standards or specifications that are / were in force at the time of filing the application and / or to which priority is claimed, i.e. at the time of the priority application, but are not to be understood as a general exclusion of applicability to subsequent or alternative standards or specifications.

[0007] The apparatus and method will now be described with reference to the drawings. Before describing the present invention in detail, we first explain the theoretical background of "Interharmonic Locking". In Figure 1a we can see the block diagram of an optoelectronic PLL circuit. The output voltage of a tunable oscillator is sampled with the pulses of a mode-locked laser MLL. For this purpose an intensity modulator BIM is used. This intensity modulator BIM, also called balanced intensity modulator (English: Balanced intensity modulator), is an electrical-optical converter.

[0008] In that case, the light pulse emitted by the intensity modulator BIM is converted into a current via two photodiodes PD1, PD2. The resulting current is then integrated by a loop filter and then fed back to a tunable oscillator, so that the controlled system is closed. The output current can be expressed as:

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[0009] The optical microwave phase detector BOMPD can therefore be modeled as a nonlinear block and a frequency mixer, where the nonlinear behavior of the optical microwave phase detector BOMPD can be used to lock a microwave oscillator to a harmonic instead of the respective fundamental (Grundton), i.e. to an even multiple of the repetition rate of the optical reference. The frequency content of the various nodes of the optical microwave phase detector BOMPD is shown in Fig. 1b), where M is the harmonic index of the repetition rate of the optical reference and N is the harmonic index of the RF voltage. The optoelectronic PLL circuit locks when the frequency at the mixer input satisfies the following equation:

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[0010] In the following, the operation of the optoelectronic PLL circuit is called harmonic locking when N=1 and Nth order interharmonic locking when N>1. Assuming that the optical pulses of the mode-locked laser MLL are much shorter than the RF signal period, we can approximate each pulse as a Dirac delta function,

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[0011] In the case of N-th order interharmonic locking phase detection, the output power in the above formula has a period of NTref. Therefore, the characteristic function (transfer function) H of the optical microwave phase detector BOMPD is N (φ) can be shown as follows:

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[0012] From this, the characteristic functions (transfer functions) of the first, second and fourth order phase detectors BOMPD (corresponding to harmonic locking, second order interharmonic locking and fourth order interharmonic locking) can be constructed.

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[0013] H N The value of (φ) can be expressed using the Jacobi-Unger expansion as

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[0014] In Fig. 2, one use of a low phase noise tunable signal source is shown. The tunable signal source has an optical microwave phase detector BOMPD. This structure is originally presented as in the prior art ("A 2-20-ghz ultralow phase noise signal source using a microwave oscillator locked to a mode-locked laser"). In that case, the microwave phase detector BOMPD firstly comprises an intensity modulator BIM, which includes an optical signal input, a modulation input I, a first output O1 and a second output O2. The intensity modulator BIM in Fig. 1a / 1b and Fig. 2 is also called a balanced intensity modulator and is an electro-optical converter. The form of the intensity modulator BIM can be designed in various ways.

[0015] Furthermore, the microwave phase detector BOMPD has a first photodiode PD1 capable of emitting light of a first output O1 during operation, and a second photodiode PD2 capable of emitting light of a second output O2 during operation. In this case, unlike the prior art structure, within the scope of the present invention, a first photodiode PD1 and a second photodiode PD2 connected in series are biased during operation, and the series circuit has a structure in which a tap for a tap signal is arranged between the first photodiode PD1 and the second photodiode PD2.

[0016] Furthermore, the microwave phase detector BOMPD has a controllable DC power supply (BOMPD zero crossing control) N4, so that during operation an offset current can be adjusted at the tap by the DC power supply, thereby changing the zero crossing of the transfer function of the BOMPD, so that the symmetry of the optical microwave phase detector during operation is broken by the offset current, i.e., unlike the prior art, a balanced operation of the BOMPD is not intentionally aimed for. In operation, the output of the tap, along with any offset currents that may be present, is routed to a low pass filter and the low pass filtered tap signal is provided to the tunable oscillator OSZ.

[0017] By adding an offset current by a controllable DC power supply N4 (BOMPD zero crossing control), it becomes possible to lock the microwave phase detector BOMPD to a non-even multiple of the optical pulse repetition rate in order to achieve a sign swap of the output current of the microwave phase detector BOMPD.

[0018] In one embodiment of the invention, the adjustable signal source further comprises a controllable DC voltage source N1 (even / odd interharmonic control), and the intensity modulator BIM has an input for the DC voltage source N1. By providing a DC voltage source, the DC voltage electrodes of the intensity modulator BIM can be biased at the same point of symmetry (Gleichpunkt), which also allows the microwave phase detector BOMPD to lock onto a non-even multiple of the optical pulse repetition rate. Because if the balanced modulator is biased at this point, the intensity of one of the outputs O1, O2 tends to be maximum, while at the same time the other output O2, O1 tends to be minimum. Therefore, in that case, it is necessary to ensure that the output frequency

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[0019] In another embodiment of the present invention, the adjustable signal source further comprises a controllable second DC voltage source N2, N3 (alternative even / odd interharmonic control), and in operation the fed back output signal of the adjustable oscillator OSZ is supplied to the modulation input I, and the DC voltage of the second DC voltage source N2, N3 is also supplied to the modulation input I in operation. The function of the controllable second DC voltage source (alternative even / odd interharmonic control) N2, N3 is that the DC component, i.e. the average level, of the signal of the oscillator OSZ is adjustable. This can be achieved, for example, by adding a DC voltage to the signal of the oscillator OSZ.

[0020] The change in the DC voltage of the high-frequency electrode of the intensity modulator BIM is

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[0021] In another embodiment of the present invention, the tunable signal source further comprises a mode-locked laser MLL which, in operation, provides an optical input signal to the optical signal input. In another embodiment of the invention, the oscillator OSZ can be adjusted to a non-integer multiple of the optical clock repetition time, i.e. to a selectable multiple, in addition to variants that can only provide specific non-even multiples. It is therefore possible within the scope of the invention to use a fiber-based Sagnac loop intensity modulator BIM, but then operating it with a phase shift in order to bias it accordingly at the symmetrical point of the line.

[0022] The present invention allows for a compact design with low phase noise with little complexity. In addition, the frequency resolution is improved, thereby

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[0023] The characteristic curves according to this theory can be seen in Figures 3a to 3d. In that case, Figs. 3a to 3d are For different RF amplitudes of α=0.8, α=1.2 or α=2.3 a) Harmonic locking when Ψdc=0, b) Second-order interharmonic locking for Ψdc=π / 2, c) Third order interharmonic locking for Ψdc=0; d) 4th interharmonic locking when Ψdc=π / 2 The theoretical (dotted line) and measured BOMPD characteristic curves are shown.

[0024] The amplitude of the oscillator signal can be adjusted by means of an adjustable amplifier A, which leads to a change in the coefficient α. As can be seen in the characteristic curves of Fig. 3a-3d, the characteristic curve depends on α. ​​It can further be seen that not all curves have a zero crossing. Therefore, if the characteristic curve does not have a zero crossing, the control loop cannot lock. This means that by appropriately adjusting the oscillator amplitude VRF, the coefficient α can be adjusted so that the characteristic curve has a zero crossing, which allows interharmonic locking.

[0025] Reference Code List BOPMD Optical Microwave Phase Detector BIM Intensity Modulator I Modulation input O1, O2 output PD1, PD2 photodiodes N4 Controllable DC power supply OSZ Tunable Oscillator N1, N2, N3 Controllable DC voltage sources MLL mode-locked laser A Adjustable Amplifier

Claims

1. 1. A low phase noise tunable signal source, comprising: An optical microwave phase detector (BOMPD), comprising: an intensity modulator (BIM) comprising an optical signal input, a modulation input (I) and a first output (O1) and a second output (O2); a first photodiode (PD1) capable of emitting light of said first output (O1) in operation; a second photodiode (PD2) capable of emitting light of said second output (O2) in operation, the first photodiode (PD1) and the second photodiode (PD2) are connected in series so as to be biased during operation; A tap for a tap signal is arranged between the first photodiode (PD1) and the second photodiode (PD2), Further comprising a controllable direct current source (N4), an offset current is adjustable in operation by the first DC current source (N4) at the tap, whereby the symmetry of the optical microwave phase detector is broken in operation by the offset current; the taps are led to a low-pass filter together with any offset currents present, The low pass filtered tap signal is provided to a tunable oscillator (OSZ); Low phase noise adjustable signal source.

2. 2. The adjustable signal source of claim 1, further comprising a controllable second DC voltage source (N1), said intensity modulator (BIM) having an input for said second DC voltage source (N1).

3. 3. The adjustable signal source according to claim 1 or 2, further comprising a controllable second DC voltage source (N2, N3), wherein in operation, a fed back output signal of the adjustable oscillator (OSZ) is supplied to the modulation input (I) and a DC voltage of the second DC voltage source (N2, N3) is also supplied to the modulation input (I) in operation.

4. 10. The adjustable signal source of any one of the preceding claims, further comprising an amplifier (A), the amplifier (A) configured to adjust the oscillation amplitude (VRF) to enable interharmonic locking.

5. 10. A tunable signal source according to any one of the preceding claims, further comprising a mode-locked laser (MLL) which, in operation, provides an optical input signal to the optical signal input.

6. A tunable signal source according to any one of the preceding claims, characterised in that the oscillator (OSZ) is tunable to a non-integer multiple of the optical clock repetition time.

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