Optoelectronic pll

Abstract

The invention relates to an optoelectronic PLL.

Description

[0001] Our reference number: UPA 47753 P DE

[0002] Registration number: Follow-up registration to DE 102025 107680.4

[0003] -1-

[0004] Optoelectronic PLL

[0005] The invention relates to an optoelectronic PLL.

[0006] background

[0007] Many technical fields require highly stable periodic signals. "High stability" here refers particularly to phase stability with respect to temperature and manufacturing tolerances. Often, the frequency of this highly stable periodic signal must also be finely adjustable over a wide range.

[0008] There are various approaches based on the current state of the art.

[0009] For example, the generation of high frequency by means of pulse superposition and direct detection of the new pulse train by means of a photodiode is known from the article "Optical-fiber pulse rate multiplier for ultralow phase-noise signal generation" by the authors Haboucha et al. , published in Optics Letters 36.18 (Sept. 2011), page 3654, doi: 10.1364 / ol.36.003654 .

[0010] Furthermore, the article "Improved signal-to-noise ratio of 10 GHz microwave signals generated with a mode-filtered femtosecond laser frequency comb" by the authors SA

[0011] Diddams et al., published in Optics Express 17.5 (Feb. 2009), page 3331, doi:

[0012] 10.1364 / oe.17.003331, the generation of high frequency by multiplying the repetition rate of the optical pulse train with a Fabry-Perot resonator and the direct detection of the new pulse train with a photodiode is known. Our reference: UPA 47753 P DE

[0013] Registration number: Follow-up registration to DE 102025 107680.4

[0014] -2-

[0015] However, these approaches have the disadvantage that it is very difficult to generate a sinusoidal high-frequency signal with high frequency resolution and low noise. After photodetection, the electronic signal exhibits many noise frequencies, i.e., harmonics and possibly subharmonic frequencies.

[0016] To generate a pure sine wave signal, these interfering frequencies must be filtered out using a tunable filter or a switchable set of bandpass filters (filter bank).

[0017] To achieve high frequency resolution at high frequencies, this filter must have an extremely high quality factor and at the same time be broadband tunable - requirements that contradict each other.

[0018] This either increases costs or, in the worst case, a filter with sufficient interference suppression at a specific frequency resolution and output frequency is technically impossible.

[0019] Similarly, the locking of a microwave oscillator to integer multiples of the repetition rate of the optical pulse trains of a mode-locked laser (MLL) using a symmetrical optical microwave phase detector with a fiber Sagnac loop intensity modulator is known from the article "Subfemtosecond synchronization of microwave oscillators with mode-locked Er-fiber lasers" by the authors K. Jung and J. Kim, published in Optics Letters 37.14 (July 2012), page 2958, doi: 10.1364 / ol.37.002958.

[0020] From this article and also from other articles, such as "Femtosecond synchronization of radio frequency signals with optical pulse trains" by J. Kim, FX Kärtner, and MH Perrott, published in Optics Letters 29.17 (Sept. 2004), page 2076, doi:

[0021] 10.1364 / ol.29.002076, as well as articles with the inventors as authors / co-authors "Octave-Band Microwave Frequency Synthesizer Using Mode-Locked Laser as a Reference", Our reference: UPA 47753 P DE

[0022] Registration number: Follow-up registration to DE 102025 107680.4

[0023] -3-

[0024] Published in: 2019 International Topical Meeting on Microwave Photonics (MWP). I IEEE, Oct. 2019, pages 1-4, doi: 10.1109 / MWP.2019.8892046 and "Ultra Low Phase Noise and Ultra Wide-Band Frequency Synthesizer Using an Optical Clock Source", published in 2020 IEEE MTT-S International Microwave Symposium (IMS), Aug. 2020, pages 1283-1286, doi:

[0025] 10.1109 / IMS30576.2020.9224118 the snapping of a microwave oscillator to the harmonics of the envelope of the optical pulse trains of a mode-locked laser (MLL) using a symmetrical optical microwave phase detector with a Mach-Zehnder intensity modulator.

[0026] Furthermore, the article "Locking of microwave oscillators on the interharmonics of mode-locked laser signals," published in Opt. Express 30.5 (Feb. 2022), pages 7763-7771, doi: 10.1364 / OE.451894, url: [url missing in original text], is listed among the articles with the inventors as authors / co-authors.

[0027]

[0028] :.cfm?URI oe-30-5-7763 and "Noise Processes and Nonlinear Mechanisms in Optoelectronic Phase-Locked Loop Using a Balanced Optical Microwave Phase Detector", published in IEEE Transactions on Microwave Theory and Techniques 70.10 (2022), pages 4422-4435, doi: 10.1109 / TMTT.2022.3197621, discloses the locking of a microwave oscillator onto the interharmonics of the envelope of the optical pulse train of a mode-locked laser (MLL) using a balanced optical microwave phase detector with a Mach-Zehnder intensity modulator.

[0029] These approaches have the advantage of simultaneously achieving high frequency resolution, wide tunability, and a pure sine wave signal with low interference. However, a disadvantage is that the transmission characteristics of the BOMPD (Balanced Optical Microwave Phase Detector) with fiber Sagnac loop or Mach-Zehnder modulator (MZM) are very sensitive to manufacturing tolerances, aging, and temperature fluctuations. Our reference: UPA 47753 P DE

[0030] Registration number: Follow-up registration to DE 102025107680.4

[0031] -4-

[0032] This is because the wavelength of the optical pulse train is typically small, i.e., in the infrared range (about 1 to 2 pm), so the optoelectronic phase detection process in the fiber Sagnac loop or MZM, which relies on the interference of two infrared pulse signals, is very sensitive to even the slightest tolerances in geometry due to manufacturing tolerances, thermal expansion due to temperature changes, and aging effects, leading to changes in the phase detection transfer characteristic of the BOMPD. Over time, this change in the transfer characteristic results in a phase drift of the VCO relative to the optical pulse train.

[0033] Such phase drift prohibits the use of these approaches for applications where a predictable phase of the sinusoidal output signal is required with respect to the phase of the envelope of the optical pulse train.

[0034] Another disadvantage of all the aforementioned approaches is that a large number of optical components are required, which leads to larger dimensions and higher costs.

[0035] However, the high number of optical components required also leads to increased sensitivity to interference, such as vibrations, which further impairs the output signal quality.

[0036] Task

[0037] Against this background, one objective of the invention is to provide an improvement. In particular, one goal is to provide a simple, space-saving design. Another objective is to improve the frequency resolution.

[0038] Brief description of the invention

[0039] The problem is solved by an optoelectronic PLL according to claim 1 or claim 3. Further advantageous embodiments of the invention are the subject of the respective dependent claims, the description, and the figures. Our reference: UPA 47753 P DE

[0040] Registration number: Follow-up registration to DE 102025 107680.4

[0041] -5-

[0042] Brief description of the characters

[0043] The invention is explained in more detail below with reference to the figures. These show:

[0044] Fig. 1 shows a first embodiment of the invention,

[0045] Fig. 2 shows a second embodiment of the invention.

[0046] Fig. 3a-3b Output sequence at the photodiode in the time or frequency domain, Fig. 3c-3d Filtered output sequence at the photodiode in the time or frequency domain (first sub-band), and

[0047] Fig. 3e-3f Filtered output sequence at the photodiode in the time or frequency domain (second sub-band)

[0048] Detailed description of the invention

[0049] The invention will now be described in more detail with reference to the figures. It should be noted that different aspects are described, each of which can be used individually or in combination. That is, each aspect can be used with different embodiments of the invention, unless explicitly presented as a pure alternative.

[0050] Furthermore, for the sake of simplicity, reference will generally be made to only one entity at a time. Unless explicitly stated otherwise, the invention may also include several of the entities concerned. Therefore, the use of the words "a", "an", and "one" should only be understood as an indication that at least one entity is used in a simple embodiment.

[0051] Where procedures are described below, the individual steps of a procedure can be arranged and / or combined in any order, provided that the Our reference: UPA 47753 P DE

[0052] Registration number: Follow-up registration to DE 102025 107680.4

[0053] -6-

[0054] The context does not explicitly indicate anything to the contrary. Furthermore, the procedures are combinable with each other – unless expressly indicated otherwise.

[0055] Information with numerical values ​​should generally not be understood as exact values, but also include a tolerance of + / - 1% to + / - 10%.

[0056] References to standards or specifications are to be understood as references to standards or specifications that are valid or were valid at the time of filing and / or – insofar as priority is claimed – at the time of the priority filing. However, this does not imply a general exclusion of applicability to subsequent or superseding standards or specifications.

[0057] With reference to the figures, a device or a corresponding method will be explained below.

[0058] The following terms will be used:

[0059] • Desired output frequency f ou t,

[0060] • Reference frequency f re f,

[0061] • Minimum desired output frequency f M in,

[0062] • Maximum desired output frequency f Max ,

[0063] • Frequency step size Af as the minimum difference between two successive desired output frequencies,

[0064] • Frequency resolution as the number of available desired output frequencies within a fixed frequency band. High frequency resolution therefore means a small frequency step size.

[0065] • Interference frequency f s is any frequency component that does not correspond to the desired output frequency f ou t corresponds to

[0066] • Offset frequency fo as a fixed frequency, Our reference: UPA 47753 P DE

[0067] Registration number: Follow-up registration to DE 102025 107680.4

[0068] -7-

[0069] • For the desired output frequency f ou t holds that f out = fo + N x Af, where N is an integer. Likewise, the desired output frequency is limited to f. M in — fout — f Min •

[0070] The invention enables the derivation of a high-frequency, largely tunable sinusoidal signal from an optical pulse train, while providing excellent phase stability of the output signal with respect to temperature and manufacturing tolerances. Furthermore, the invention also offers a tunable sinusoidal signal with high frequency resolution, low interference frequencies, and low phase noise. In addition, the number of optical components is minimized to reduce size, cost, and sensitivity to vibrations.

[0071] A sinusoidal signal means that the output signal is a pure sinusoidal signal with only minor spurious frequency components.

[0072] The frequencies of the sinusoidal signal are equal to an integer multiple of the repetition frequency of the optical pulse train.

[0073] Therefore, the optical pulse train has a low repetition frequency to enable a high frequency resolution for the sine signal.

[0074] Wide tunability of the sinusoidal signal means that the desired signal frequency of the sinusoidal signal can be tuned within a wide frequency band, e.g., up to several octaves.

[0075] Figures 1 and 2 show two different embodiments of the invention. Each of these may have further embodiments.

[0076] With reference to Figure 1, a first embodiment will be explained. Our reference: UPA 47753 P DE

[0077] Registration number: Follow-up registration to DE 102025 107680.4

[0078] -8-

[0079] Figure 1 shows an optoelectronic PLL comprising a laser MLL, which generates an optical pulse train, and a photodiode PD. The laser is, for example, a mode-locked laser (MLL).

[0080] During operation, the MLL laser emits pulsed light with a pulse frequency f re f on the photodiode PD, wherein the photodiode PD generates an electrical signal.

[0081] During operation, the electrical signal is fed to a frequency divider FD and a first filter unit TF / SFB. The division ratio of the frequency divider FD can be any ratio, but in particular an λ:integer division ratio.

[0082] A mixer MX is arranged downstream of the first filter unit TF / SFB, with the output signal of the mixer MX being fed to a second filter unit BPF.

[0083] The output signal of the frequency divider FD and the output signal of the second filter device BPF is fed to a stage PD / PFD acting as a phase detector.

[0084] Furthermore, the output signal of the stage PD / PFD, which acts as a phase detector, is fed to a third filter unit LF in order to control an adjustable oscillator TO, whereby the desired output signal f ou The t of the adjustable oscillator TO is fed back to the mixer MX.

[0085] The invention enables offset detent adjustment at the MLL repetition rate, thereby reducing the available frequencies to f ou t = (N ± ratio) xf refThe inventive arrangement of an electronic phase detection system with a mixer phase detector (PD) or phase / frequency detector (PFD) instead of an electro-optical phase detection system using a BOMPD (Balanced Optical Microwave Phase Detector) significantly reduces the sensitivity of the phase at the desired frequency to temperature fluctuations. Our reference: UPA 47753 P DE

[0086] Registration number: Follow-up registration to DE 102025 107680.4

[0087] -9-

[0088] Likewise, a low frequency step size Af is enabled, which at a ratio of 1:4 is equal to half the repetition rate of the MLL, i.e., Af = 0.5f ref. Furthermore, the use of an electronic PLL enables the suppression of interference pulses. This provides, for example, a frequency resolution improved by a factor of 2 compared to a scenario with harmonic detent.

[0089] The optical microwave phase detector present in previous approaches, which, for example, includes a fiber Sagnac loop or a Mach-Zehnder intensity modulator with photodetectors, can be replaced by a novel optical microwave phase detector comprising a photodiode PD, a tunable or switchable bandpass filter TF / SFB and an electronic mixer MX, a bandpass filter BPF corresponding to the divided reference frequency and a phase detector PD or phase / frequency detector PFD.

[0090] The MLL signal is first transformed into the electrical domain using a photodiode PD and then filtered with a tunable or switchable bandpass filter TF / SFB. The output signal of the tunable or switchable bandpass filter TF / SFB is applied to a first input of the electronic mixer MX. The output signal f ou The t signal of the tunable oscillator TO is applied to the second input of the electronic mixer MX. The tunable oscillator signal f ou The signal t is converted down to an intermediate frequency (IF) corresponding to the division ratio in the mixer MX. The down-converted signal is then phase-locked to the division ratio generated by the clock divider FD via the phase detector PD or the phase / frequency detector PFD.

[0091] In embodiments of this configuration, the frequency divider FD may also provide a 1:4 division, wherein the second filter device BPF is a bandpass filter with a center frequency corresponding to the output frequency of the frequency divider FD. Our reference: UPA 47753 P DE

[0092] Registration number: Follow-up registration to DE 102025 107680.4

[0093] -10-

[0094] The use of a tunable or switchable filter TF / SFB reduces the nonlinearity in an electronic mixer MX. The output signal of a mixer MX contains hundreds to thousands of frequencies, whereas the tunable oscillator should only lock onto one of these frequencies. The loop dynamics and phase noise of the PLL are improved when the signal-to-noise ratio (SNR) at the output of the mixer MX is maximized. The SNR depends on the power of the desired frequency at the output of the photodiode PD, to which the tunable oscillator TO locks. On the other hand, the input signal to the mixer MX should preferably not be too high in amplitude, as the mixer MX could otherwise become saturated and thus nonlinear. Ideally, the tunable or switchable filter TF / SFB would filter out only the frequency to which the tunable oscillator TO is intended to lock.

[0095] For high frequency resolution (low Af) at a high desired frequency f ou However, this would require a tunable or switchable TF / SFB filter with a high Q factor, which is very expensive or technically impossible. Furthermore, it would exhibit high insertion loss, thus increasing the noise in the system.

[0096] Therefore, the invention allows the use of a tunable or switchable filter TF / SFB, which filters out not a single frequency, but a subband from the frequency spectrum at the output of the photodiode PD. This enables both high power for the frequency to which the tunable oscillator TO is to lock, and saturation of the mixer MX due to a significantly smaller number of frequencies at the input of the mixer MX. Figures 3a-3b show, by way of example, the output signal of the photodiode PD when driven with an MLL signal in both the time (Fig. 3a) and frequency (Fig. 3b) domains. Figures 3c to 3f then show filtering corresponding to a lower subband (Fig. 3c, 3d) and an upper subband (Fig. 3e, 3f), respectively. Our reference: UPA 47753 P DE

[0097] Registration number: Follow-up registration to DE 102025 107680.4

[0098] -11-

[0099] This enables high power for the desired frequency to which the tunable oscillator TO is to lock, and thus a high SNR, so that the phase noise of the PLL output signal f ou t is small.

[0100] With reference to Figure 2, a second embodiment will first be explained.

[0101] Figure 2 shows an optoelectronic PLL comprising a laser MLL and a photodiode PD. The laser is, for example, a mode-locked laser.

[0102] During operation, the MLL laser emits light f re f on the photodiode PD, wherein the photodiode PD generates an electrical signal.

[0103] During operation, the electrical signal is routed to a first filter unit TF / SFB.

[0104] A mixer MX is arranged downstream of the first filter unit TF / SFB, with the output signal of the mixer MX being fed to a second filter unit LF.

[0105] The output signal of the second filter device LF is fed to a stage acting as a phase detector - analogous to the phase detector PD / PFD as described in Figure 1.

[0106] Furthermore, the output signal of the PD / PFD stage, which acts as a phase detector, controls an adjustable oscillator TO, where the output signal f ou The t of the adjustable oscillator TO is fed back to the mixer MX.

[0107] The invention thus enables locking onto the harmonics of the MLL repetition rate. By using electronic phase detection with a mixer instead of electro-optical phase detection via a BOMPD (Balanced Optical Microwave Phase Detector), the sensitivity of the phase at the desired frequency to temperature fluctuations can be significantly reduced. Our reference: UPA 47753 P DE

[0108] Registration number: Follow-up registration to DE 102025 107680.4

[0109] -12-

[0110] Likewise, a low frequency step size Af is enabled, corresponding to the repetition rate of the MLL. Furthermore, the use of an electronic PLL allows for the suppression of interference pulses. The presented design also prevents mixer saturation at high optical pulse power. Additionally, the design allows for an improvement of the mixer's input signal to enable high phase detector gain and low phase noise.

[0111] The optical microwave phase detector present in previous approaches, which, for example, includes a fiber Sagnac loop or a Mach-Zehnder intensity modulator with photodetectors, can be replaced by a novel optical microwave phase detector comprising a photodiode PD, a tunable or switchable bandpass filter TF / SFB and an electronic mixer MX.

[0112] The MLL signal is first transformed into the electrical domain using a photodetector PD and then filtered with a tunable or switchable bandpass filter TF / SFB. The output signal of the tunable or switchable bandpass filter TF / SFB is applied to a first input of the electronic mixer MX. The output signal f ouThe t signal of the tunable oscillator TO is applied to the second input of the electronic mixer MX. The mixer MX acts as a phase detector between the tunable oscillator signal and the frequencies of the output signal of the tunable or switchable bandpass filter.

[0113] The use of a tunable or switchable TF / SFB filter reduces the nonlinearity in an electronic phase detector. The output signal of a phase detector contains hundreds to thousands of frequencies, whereas the tunable oscillator is intended to lock onto only one of these frequencies. The loop dynamics and phase noise of the PLL are improved when the phase detector gain at the output of the phase detector is maximized. The phase detector gain depends on the power at the desired frequency at the output of the photodiode PD, to which the... Our reference: UPA 47753 P DE

[0114] Registration number: Follow-up registration to DE 102025 107680.4

[0115] -13-

[0116] The tunable oscillator TO locks into place. On the other hand, the input signal for mixer MX should preferably not have too high an amplitude, as mixer MX could otherwise become saturated and then non-linear. Ideally, the tunable or switchable filter TF / SFB would only filter out the frequency at which the tunable oscillator TO is to lock.

[0117] For high frequency resolution (low Af) at a high desired frequency f ou However, this would require a tunable or switchable filter with an extremely high Q factor, which is very expensive or technically impossible. Furthermore, it would exhibit high insertion loss, thus increasing the noise in the system.

[0118] Therefore, the invention can employ a tunable or switchable filter TF / SFB, which filters out not a single frequency, but a subband from the frequency spectrum at the output of the photodiode PD. This enables both high power for the frequency to which the tunable oscillator TO is to lock, and saturation of the mixer MX due to a significantly smaller number of frequencies at the mixer MX's input. Furthermore, filtering a subband instead of a single frequency ensures that the filter specification is physically feasible. This allows, on the one hand, high power for the desired output frequency f. ou t, to which the adjustable oscillator TO should lock, and thus a phase detector gain that reduces the phase noise of the PLL output signal f ou t reduced.

[0119] Figures 3a-3b show, as an example, the output signal of the photodiode PD when subjected to an MLL signal in both the time (Fig. 3a) and frequency (Fig. 3b) domains. Figures 3c to 3f then show filtering corresponding to a lower subband (Fig. 3c, 3d) and an upper subband (Fig. 3e, 3f).

[0120] By means of the tunable or switchable bandpass filter TF / SFB upstream of the mixer MX according to the invention, it is possible to increase the phase detector gain without driving the mixer MX into the nonlinear operating range, resulting in higher Our reference: UPA 47753 P DE

[0121] Registration number: Follow-up registration to DE 102025 107680.4

[0122] -14-

[0123] Phase detector gain and thus lower phase noise of the desired signal f ou t enables.

[0124] Furthermore, the tunable or switchable bandpass filter TF / SFB before the mixer MX enables high frequency resolution at high desired frequencies f. ou t in combination with a high phase detector gain, without driving the mixer MX into the nonlinear operating range.

[0125] The Q factor of the tunable or switchable bandpass filter TF / SFB before the mixer MX does not need to be excessively high and still allows for high frequency resolution at high desired frequencies f. ou t in combination with a high phase detector gain, which enables low phase noise. This reduces the cost of the TF / SFB filter.

[0126] The tunable or switchable bandpass filter TF / SFB upstream of the mixer MX allows for lower linearity requirements for the mixer MX. This reduces the cost of the mixer MX.

[0127] Since fiber Sagnac loops and Mach Zehnder modulators are no longer needed for phase detection, manufacturing tolerances, aging resistance, and thermal expansion no longer play a decisive role.

[0128] In the embodiments according to the invention, phase detection takes place in the electronic domain after the envelope detection of the optical signal, i.e., at signal frequencies in the microwave range (wavelengths in the millimeter or centimeter range). This makes the transmission characteristic of the phase detector significantly more robust against manufacturing tolerances, thermal expansion, and aging effects. As a result, the precision of the phase of the desired signal is considerably improved. Our reference: UPA 47753 P DE

[0129] Registration number: Follow-up registration to DE 102025 107680.4

[0130] -15-

[0131] In all configurations, it can also be provided that the first filter unit TF / SFB has an adjustable filter.

[0132] Likewise, in all configurations, the first filter unit TF / SFB can have a switchable filter bank.

[0133] Without limiting the generality, in all configurations the stage acting as a phase detector (PD / PFD) can include an additional mixer.

[0134] Furthermore, the third filter unit LF can have a loop filter.

Claims

Our reference number: UPA 47753 P DE Registration number: Follow-up registration to DE 102025 107680.4 -16- Claims 1. Featuring optoelectronic PLL • A laser (MLL) with a pulsed output signal, and a photodiode (PD), wherein the laser (MLL) emits light (f) during operation re f) shines onto the photodiode (PD), whereby the photodiode (PD) generates an electrical signal, • wherein the electrical signal during operation is directed to a frequency divider (FD) and a first filter device (TF / SFB), • wherein a mixer (MX) is arranged downstream of the first filter device (TF / SFB), wherein the output signal of the mixer (MX) is fed to a second filter device (BPF), • wherein the output signal of the frequency divider (FD) and the output signal of the second filter device (BPF) are fed to a stage acting as a phase detector (PD / PFD), • wherein the output signal of the stage acting as a phase detector (PD / PFD) is fed to a third filter device (LF) in order to control an adjustable oscillator (TO), wherein the output signal (f ou t) of the adjustable oscillator (TO) is fed back to the mixer (MX).

2. Optoelectronic PLL according to claim 1, characterized in that the frequency divider (FD) provides a 1:4 division, wherein the second filter device (BPF) is a bandpass filter with a center frequency corresponding to the output frequency of the frequency divider (FD).

3. Optoelectronic PLL according to one of the preceding claims, characterized in that the third filter device (LF) comprises a loop filter.

4. Featuring optoelectronic PLLs. Our reference: UPA 47753 P DE Registration number: Follow-up registration to DE 102025 107680.4 -17- • A laser (MLL) and a photodiode (PD), wherein the laser (MLL) emits light (f) during operation re f) shines onto the photodiode (PD), whereby the photodiode (PD) generates an electrical signal, • wherein the electrical signal during operation leads to a first filter device (TF / SFB), • wherein a mixer (MX) is arranged downstream of the first filter device (TF / SFB), wherein the output signal of the mixer (MX) is fed to a second filter device (LF), • wherein the output signal of the second filter device (LF) is fed to a stage acting as a phase detector (PD / PFD), • wherein the output signal of the stage acting as a phase detector (PD / PFD) controls an adjustable oscillator (TO), wherein the output signal (f ou t) of the adjustable oscillator (TO) is fed back to the mixer (MX).

5. Optoelectronic PLL according to any one of the preceding claims 1 to 4, characterized in that the first filter device (TF / SFB) has an adjustable filter.

6. Optoelectronic PLL according to any one of the preceding claims 1 to 4, characterized in that the first filter device (TF / SFB) has a switchable filter bank.

7. Optoelectronic PLL according to one of the preceding claims, characterized in that the stage acting as a phase detector (PD / PFD) has a further mixer.

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