An adjustable signal source with low phase noise

Abstract

The invention relates to an adjustable signal source with low phase noise, having an optical microwave phase detector (BOMPD), the optical microwave phase detector having: an intensity modulator (BIM), the intensity modulator having an optical signal input, a modulation input (I), a first output (O1) and a second output (O2); a first photodiode (PD1), the first photodiode being illuminated by light of the first output (O1) during operation; a second photodiode (PD2), the second photodiode being illuminated by light of the second output (O2) during operation; the first photodiode (PD1 ) and the second photodiode (PD2) are connected in series and a bias voltage is applied; a signal output tap is set between the first photodiode (PD1) and the second photodiode (PD2); the adjustable signal source also has a controllable DC source (N4), and an offset current is set at the tap during operation through the controllable DC source (N4), so that the symmetry operation of the optical microwave phase detector is reversed during operation through the offset current; the adjustable signal source also has a low-pass filter, and the input end of the low-pass filter is connected to the output tap; an adjustable oscillator (OSZ) is connected after the low-pass filter.

Description

Technical Field

[0001] The present invention relates to an adjustable signal source with low phase noise. Background Art

[0002] For many areas of electrical engineering it is necessary to provide signals with low phase noise. It would be particularly desirable if this signal source could be adjusted over a large frequency range with high frequency resolution.

[0003] An exemplary use is to lock a microwave oscillator to an optical clock signal from such a source.Another use is to distribute an optical clock signal over a low-loss optical fiber with low phase noise.

[0004] Various methods are known in the prior art.

[0005] For example, from the article "Sub-femtosecond synchronization of a microwave oscillator with a mode-locked erbium fiber laser" by authors Kwangyun Jung and Jungwon Kim, published in Optics Express, Volume 37, Issue 14, Page 2958, July 2012, https: / / doi.org / 10.1364 / OL.37.002958, a device is known that allows a microwave oscillator to lock to an integer harmonic of the repetition rate of an optical clock.

[0006] Furthermore, a symmetric optical microwave phase detector is known from the article “2-20 GHz ultra-low phase noise signal source using a microwave oscillator locked to a mode-locked laser” published by authors and inventors M. Bahmanian and JC Scheytt in IEEE Transactions on Microwave Theory and Techniques, pp. 1-1 (2021).

[0007] It is also known from these articles that microwave phase detectors having fiber-based Sagnac ring intensity modulators or Mach-Zehnder intensity modulators can be used for this purpose.

[0008] A common feature of all methods known to the inventors is that current system designs are only able to provide high frequency signals with integer multiples of the repetition rate of the optical signal source. This means that the output frequency results in f out =N*f rep ,in

[0009] In the methods known to the inventors so far, the symmetric modulator is biased at its odd symmetry point, ie the bias at this point results in the same intensity of the light output. The sign of the output current of the symmetric optical microwave phase detector changes at zero phase difference for all high frequency amplitudes.

[0010] A disadvantage of previously known methods is that they are only suitable for providing precisely determined (fixed) frequencies, or they are only suitable for generating precisely predetermined integer multiples.

[0011] Another disadvantage is the very complex structure. Other disadvantages include, for example, relatively high space requirements (especially for fiber-based Sagnac ring intensity modulators) and high costs. Summary of the invention

[0012] Against this background, the object of the present invention is to provide a special improvement, one object being to provide a simple, space-saving structure. Another object is to increase the frequency resolution.

[0013] This object is achieved by an adjustable signal source with low phase noise according to claim 1. Further advantageous embodiments of the invention emerge from the dependent claims, the description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The present invention will be explained in more detail below with reference to the accompanying drawings. In the drawings:

[0015] Figure 1a Schematically shows a symmetric optical microwave phase detector according to the prior art;

[0016] Figure 1b The equivalent circuit schematic diagram is shown;

[0017] Figure 2 A symmetric optical microwave phase detector according to the present invention is schematically shown; and

[0018] Figure 3a-d is a graph of measured values ​​relative to theoretical values ​​according to an embodiment of the present invention. DETAILED DESCRIPTION

[0019] The present invention will be described in more detail below with reference to the accompanying drawings. It should be noted that different aspects are described, which can be used alone or in combination. This means that any aspect can be used with different embodiments of the present invention, unless explicitly presented as a pure alternative.

[0020] Furthermore, for simplicity, only one entity will generally be referenced. Unless explicitly stated otherwise, the present invention may also have multiple affected entities. In this regard, the use of the words "a" and "an" should only be understood as an indication of the use of at least one entity in a simple embodiment.

[0021] With regard to the method described below, the individual steps of the method may be arranged and / or combined in any order, unless the context clearly states otherwise. In addition, unless otherwise clearly stated, these methods may be combined with each other.

[0022] Information with numerical values ​​should generally not be understood as exact values, but also include tolerances of + / -1% to + / -10%.

[0023] References to standards or specifications are to be understood as references to the standards or specifications applicable at the time of filing and / or (if priority is claimed) at the time of the priority application. However, this does not mean that the applicability of subsequent or replacement standards or specifications is generally excluded.

[0024] The apparatus or method is explained below with reference to the drawings.

[0025] Before explaining the present invention in detail, we first introduce the theoretical background of "interharmonic locking".

[0026] from Figure 1a The block diagram of the optoelectronic PLL circuit can be seen. The output voltage of the adjustable oscillator is sampled by the pulses of the mode-locked laser MLL. An intensity modulator BIM is used for this purpose. This intensity modulator BIM is also called a balanced intensity modulator (English: balanced intensity modulator), which stands for electro-optical converter.

[0027] The light pulse output from the intensity modulator BIM is converted into current through two photodiodes PD1, PD2. The generated current is then integrated through a loop filter and fed back to the adjustable oscillator, thus closing the controlled system.

[0028] The output current can be expressed as:

[0029]

[0030] Where i is the output current of the optical microwave phase detector BOMPD, R is the sensitivity of the photodiode, I(t) is the average light input intensity to the optical microwave phase detector BOMPD in one light cycle, ψ dc is the optical phase shift introduced by the DC electrode, V π,RF is the π-voltage of the RF electrode, v RF is the sinusoidal RF modulation voltage with amplitude V RF , the angular frequency is ω RF , the phase shift is φ(v RF )=V RF sin(ω RF t+φ).

[0031] The sinusoidal term in the first equation indicates that the RF voltage undergoes a nonlinear transformation, producing harmonics of the monotonic RF excitation.

[0032] Therefore, the BOMPD optical microwave phase detector can be modeled as a nonlinear block and a mixer. The nonlinear behavior of the optical microwave phase detector BOMPD can now be used to lock the microwave oscillator to a harmonic instead of the corresponding fundamental tone, i.e. to an even multiple of the optical reference repetition rate.

[0033] The frequency content of each node of the optical microwave phase detector BOMPD is as follows: Figure 1b ), where M is the harmonic index of the optical reference repetition rate and N is the harmonic index of the RF voltage.

[0034] The opto-PLL circuit locks when the frequency of the mixer input satisfies the following equation:

[0035]

[0036] Wherein, M and N are positive integers whose greatest common divisor is 1.

[0037] The operation of the optoelectronic PLL circuit when N=1 is referred to below as harmonic locking, and when N>1 is referred to as N-order interharmonic locking.

[0038] Assuming that the optical pulse of the mode-locked laser MLL is significantly shorter than the RF signal period, each pulse can be approximated as a DiracDelta function:

[0039]

[0040] Where I0 is the average intensity of light input, T ref =1 / f ref .

[0041] Then the equation can be expressed as α = πV RF / V πRF Rewrite it as follows:

[0042]

[0043] The characteristic function (transfer function) of a phase detector is defined as the average output current related to the phase difference between the inputs.

[0044] For N-order interharmonic locked phase detection, the period of the output current in the above equation is NT ref Therefore, the optical microwave phase detector BOMPD The characteristic function (transfer function) can be given as follows:

[0045]

[0046] Wherein, in this equation, the greatest common multiple of M and N is assumed to be 1, and the exponent M is ignored.

[0047] From this, the characteristic functions (transfer functions) of the first-order, second-order and fourth-order phase detector BOMPD can be created (corresponding to harmonic locking, second-order interharmonic locking, and fourth-order interharmonic locking, respectively):

[0048] H1(φ)=RI0sin(αsin(φ)+ψ dc )

[0049] H2(φ)=RI0sin(ψ dc )cos[αsin(φ)]

[0050]

[0051] The last two equations, H2(φ) and H4(φ), show that the characteristic function (transfer function) of the second-order and fourth-order phase detector BOMPD is related to sin(Ψ dc ) is linearly proportional, and when Ψ dc = 0. This behavior is expected because the nonlinear characteristic function (transfer function) of the phase detector BOMPD shown at the beginning is relative to v RF In dc = ±π / 2 has equal symmetry (axial symmetry) and unequal symmetry (point symmetry) at Ψ dc = 0. When using single-tone excitation, only dc = 0, generating odd harmonics only dc Even harmonics are generated at ±π / 2.

[0052] H N The magnitude of (φ) can be expressed using the Jacobi-Unger expansion as follows:

[0053] <H N (φ)>=RI0sin(ψ dc )J0(α)

[0054] Where <.> represents the average value, and J0 represents the first-order 0th-order Bessel function. The average zero value H N (φ) ensures zero crossings in the transfer function.

[0055] The following will refer to Figure 2 (and refer to Figure 1a The present invention is explained in more detail in the following description of the prior art. The same reference characters are used as much as possible.

[0056] Figure 2 An adjustable signal source with low phase noise in one application is shown. The adjustable signal source has an optical microwave phase detector BOMPD. The structure was originally provided according to the prior art ("2-20 GHz ultra-low phase noise signal source using a microwave oscillator locked to a mode-locked laser").

[0057] The microwave phase detector BOMPD initially has an intensity modulator BIM having an optical signal input, a modulation input I and a first output O1 and a second output O2. Figure 1a The intensity modulator BIM in 1b and 2 is also called a balanced intensity modulator and represents an electro-optical converter. The design of the intensity modulator BIM can be designed differently.

[0058] Furthermore, the microwave phase detector BOMPD has a first photodiode PD1 which can be illuminated by the light of the first output O1 during operation, and a second photodiode PD2 which can be illuminated by the light of the second output O2 during operation.

[0059] Compared to the prior art structures, within the scope of the invention, the first photodiode PD1 and the second photodiode PD2 connected in series are now applied with a prestress voltage as a bias during operation. The series circuit is designed to arrange a tap for outputting the signal between the first photodiode PD1 and the second photodiode PD2.

[0060] In addition, the microwave phase detector BOMPD has a controllable DC source (BOMPD zero crossing control) N4, wherein in operation the offset current can be adjusted at the tap by the DC source, so that the zero crossing of the transfer function of the BOMPD is changed, and in operation the symmetry of the optical microwave phase detector is thus reversed by the offset current. That is, unlike the prior art, the symmetrical operation of the BOMPD is intentionally not sought.

[0061] During operation, the tapped output with the offset current is fed to a low pass filter, the output of which is connected to an adjustable oscillator OSZ.

[0062] By adding an offset current using the controllable first DC source N4 (BOMPD zero crossing control), the microwave phase detector BOMPD can now also lock to non-even multiples of the optical pulse repetition rate to achieve a sign-changing output current of the microwave phase detector BOMPD.

[0063] In one embodiment of the invention, the adjustable signal source further has a controllable first DC voltage source N1 (even / odd inter-harmonic control), and the intensity modulator BIM has an input for the first DC voltage source N1.

[0064] By providing a DC voltage source, the DC voltage electrodes of the intensity modulator BIM can be biased at symmetrical equipoints so that the microwave phase detector BOMPD can also be locked to non-even multiples of the optical pulse repetition rate. Because if the modulation is symmetrically biased at this point, the intensity of one of the outputs O1, O2 will tend to the maximum value, while at the same time the intensity of the other output O2, O1 will tend to the minimum value. Therefore, the output frequency can be reliably set to in

[0065] In another embodiment of the present invention, the adjustable signal source also includes a controllable second DC voltage source N2, N3 (alternating control between even / odd harmonics), wherein a feedback output signal of the adjustable oscillator OSZ is provided to the modulation input I during operation, and the DC voltage of the second DC voltage source N2, N3 is also fed to the modulation input I during operation.

[0066] The effect of the controllable second DC voltage source (alternating control between even / odd harmonics) 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 from the oscillator OSZ.

[0067] The change of the DC voltage of the high frequency electrode of the intensity modulator BIM can lock the intensity modulator BIM at the frequency Thus, the sign change of the output current of the optical microwave phase detector BOPMD is achieved at a specific phase offset. In addition, the DC voltage of the high-frequency electrode of the intensity modulator BIM can be biased at a point where the characteristic curve is mirror-symmetrical and locked to a non-even multiple of the optical pulse repetition rate. Similarly, if the balanced modulator is biased at this point, the intensity of one of the outputs O1, O2 will tend to the maximum value, while the intensity of the other output O2, O1 will tend to the minimum value.

[0068] In another embodiment of the invention, the adjustable signal source further comprises a mode-locked laser MLL which provides an optical input signal to the optical signal input during operation.

[0069] In another embodiment of the invention, the oscillator OSZ can be set to a non-integer multiple of the optical clock repetition time. This means that in addition to being able to provide a certain non-even multiple variant, a selectable multiple can also be set.

[0070] As part of the present invention, a fiber-based Sagnac ring intensity modulator BIM can now also be used. However, it is now operated with a phase shift so that it is biased accordingly at the symmetrical straight line point.

[0071] The invention means that compact designs with low phase noise can now be realized with little complexity. In addition, the frequency resolution is improved so that it is also possible to set multiples of , where Furthermore, the phase noise is improved, i.e. reduced, compared to a purely electronic signal source.

[0072] Without limiting the generality, an optional amplifier A may also be arranged in the feedback branch.

[0073] from Figure 3a-d The characteristic curve according to this theory can be seen.

[0074] Figures 3a-3d show the BOMPD characteristic curves according to theory (dashed line) and according to measurement.

[0075] a) Harmonic locking ψ dc =0,

[0076] b)ψ dc =π / 2 at the second-order interharmonic capture,

[0077] c)ψ dc = 0, and the third-order interharmonic capture

[0078] d)ψ dc = π / 2 at the fourth-order interharmonic capture

[0079] For different RF amplitudes, α=0.8, α=1.2 or α=2.3.

[0080] The adjustable amplifier A can be used to adjust the amplitude of the oscillator signal, which results in a change in the factor α. From the characteristic curves of Figures 3a-3d, it can be seen that the characteristic curves depend on α. ​​You can also see that not all curves have zero crossings. If the characteristic curves do not have zero crossings, the control loop cannot engage. This means that by appropriately adjusting the oscillator amplitude v RF , the factor α can be adjusted so that the characteristic curve has a zero crossing, allowing inter-harmonic meshing.

[0081] Reference Numbers List

[0082] BOPMD optical microwave phase detector;

[0083] BIM intensity modulator;

[0084] I modulation input;

[0085] O1, O2 output;

[0086] PD1, PD2 photodiode;

[0087] N4 controllable DC source;

[0088] OSZ adjustable oscillator;

[0089] N1, N2, N3 controllable DC voltage source;

[0090] MLL mode-locked laser;

[0091] A Adjustable amplifier.

Claims

1. An adjustable signal source with low phase noise, having an optical microwave phase detector (BOMPD), the optical microwave phase detector having: an intensity modulator (BIM) having an optical signal input, a modulation input (I), a first output (O1) and a second output (O2); a first photodiode (PD1) which is illuminated by light of the first output (O1) during operation; a second photodiode (PD2) which is illuminated by light of the second output (O2) during operation; The first photodiode (PD1) and the second photodiode (PD2) are connected in series; characterized in that a bias voltage is applied to the first photodiode (PD1) and the second photodiode (PD2) connected in series; a signal output tap is provided between the first photodiode (PD1) and the second photodiode (PD2); The adjustable signal source also has a controllable DC source (N4), and during operation, an offset current is set at the tap by the controllable DC source (N4), so that the symmetry operation of the optical microwave phase detector is reversed by the offset current during operation; The adjustable signal source also has a low-pass filter, the input end of the low-pass filter is connected to the output tap; and an adjustable oscillator (OSZ) is connected after the low-pass filter.

2. The adjustable signal source according to claim 1, characterized in that: The adjustable signal source also has a controllable first DC voltage source (N1), wherein the intensity modulator (BIM) has an input for the first DC voltage source (N1).

3. The adjustable signal source according to claim 1 or 2, characterized in that: The adjustable signal source also has a controllable second DC voltage source (N2, N3) for providing a feedback output signal of the adjustable oscillator (OSZ) to the modulation input (I) during operation, wherein the DC voltage of the second DC voltage source (N2, N3) is fed to the modulation input (I) during operation.

4. An adjustable signal source according to any one of the preceding claims, characterized in that The adjustable signal source also has an amplifier (A) configured to interharmonically adjust the oscillator amplitude (v RF ).

5. An adjustable signal source according to any one of the preceding claims, characterized in that The adjustable signal source also has a mode-locked laser (MLL) which, during operation, provides an optical input signal to the optical signal input.

6. An adjustable signal source according to any one of the preceding claims, characterized in that The oscillator (OSZ) is set to a non-integer multiple of the optical clock repetition time.

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