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Schematic of BOM-PD. EDFA: erbium-doped fibre amplifier; FR: Faraday rotator; QWP: quarter waveplate; DRO: dielectric-resonator oscillator; VOA: variable optical attenuator. All fibres after the polarizer are polarization maintaining. The inset shows the phase error θe between the microwave signal and the pulse train in the optical domain.
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We demonstrate an amplitude-to-phase (AM-PM) conversion coefficient for a balanced optical-microwave phase detector (BOM-PD) of 0.001 rad, corresponding to AM-PM induced phase noise 60 dB below the single-sideband relative intensity noise of the laser. This enables us to generate 8 GHz microwave signals from a commercial Er-fibre comb with a single...
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Citations
... Various techniques have been demonstrated in the literature to experimentally analyze [28], suppress [38], and characterize the pointing and power-to-phase conversion effects [39,40]. An amplitude fluctuation-based data selection has been shown to be effective in eliminating powerto-phase errors for measurements over several km, which are strongly affected by atmospheric turbulence [41]. ...
Simultaneous distance measurements on two or more optical wavelengths enable dispersion-based correction of deviations that result from insufficient knowledge of the refractive index along the signal propagation path. We demonstrate a supercontinuum-based approach for highly accurate distance measurements suitable for such an inline refractivity compensation. The distance is estimated from the phase delay observations on the intermode beats. We use a supercontinuum (SC) coherently broadened from a 780 nm frequency comb and spanning the spectral range of 570-970 nm. Experiments are performed on the 590 and 890 nm wavelength bands filtered from the SC spectrum. Results show distance measurements with standard deviations of around 0.01 mm at 50 m, and a distance-dependent component below 0.2 ppm on the individual spectral bands. Distance residuals compared to a reference interferometer are on the order of 0.1 ppm for displacements up to 50 m. Controlled pressure-induced refractivity variations are created over a length of 15 m, resulting in an optical path length change of 0.4 mm. Using the two-color method, we demonstrate refractivity-corrected distance measurement with a standard deviation of around 0.08 mm for a 60 s averaging window. The current experimental configuration can be easily extended to distance measurements on more than two wavelengths. The results highlight its potential for practical long-distance measurements through inline refractivity compensation.
... Photodetection of ultrashort pulses to generate phase-and frequency-stable microwave signals places extremely stringent demands on the optical-to-electrical converter in terms of power handling and nonlinearity. Concerns over photodetection fidelity has led to the development of ways to extract the timing information of ultrashort pulses without direct detection of optical pulses [31,32], where a microwave oscillator is phase locked to the optical pulse train through an electro-optic timing comparison. With this method, excellent results have been achieved in synchronizing an optical pulse train with a microwave oscillator [33], with residual noise comparable to the best optical-to-microwave conversion with direct photodetection [19,34]. ...
Electrical signals derived from optical sources have achieved record-low levels of phase noise, and have demonstrated the highest frequency stability yet achieved in the microwave domain. Attaining such ultrastable phase and frequency performance requires high-fidelity optical-to-electrical conversion, typically performed via a high-speed photodiode. This paper reviews characteristics of the direct photodetection of optical pulses for the intent of generating high power, low phase noise microwave signals from optical sources. The two most popular types of photodiode detectors used for low noise microwave generation are discussed in terms of electrical pulse characteristics, achievable microwave power, and photodetector nonlinearities. Noise sources inherent to photodetection, such as shot noise, flicker noise, and photocarrier scattering are reviewed, and their impact on microwave phase fidelity is discussed. General guidelines for attaining the lowest noise possible from photodetection that balances power saturation, optical amplification, and amplitude-to-phase conversion, are also presented.
... There are two optical-microwave timing detection concepts: direct conversion of the timing error into intensity imbalance [16][17][18]22] and synchronous detection using half of the repetition rate as the modulation signal [19][20][21]. The former method, which is also collectively called as the electro-optic sampling-based timing detection (EOS-TD) method, has demonstrated subfemtosecond optical-microwave timing synchronization performance [17,22] and high amplitude-to-phase suppression ratio [23] by introducing balanced detection. This approach has been actively used for a variety of applications such as low-noise microwave generation [24,25], long-distance timing transfer [26,27], comb stabilization [6,28], time-of-flight sensing [29], laser-microwave synchronization for UED [30], and fiber link stabilization for FELs [31]. ...
Precise and stable synchronization between an optical frequency comb (femtosecond mode-locked laser oscillator or microresonator-based comb) and a microwave oscillator is important for various fields including telecommu- nication, radio astronomy, metrology, and ultrafast X-ray and electron science. Timing detection and synchro- nization using electro-optic sampling with an interferometer has been actively used for low-noise microwave generation, long-distance timing transfer, comb stabilization, time-of-flight sensing, and laser-microwave syn- chronization for ultrafast science facilities. Despite its outstanding performance, there has been a discrepancy in synchronization performance of more than 10 dB between the projected shot-noise-limited noise floor and the measured residual noise floor. In this work, we demonstrate the shot-noise-limited performance of an electro-optic timing detector-based comb-microwave synchronization, which enabled an unprecedented residual phase noise floor of −174.5 dBc∕Hz at 8 GHz carrier frequency (i.e., 53 zs∕Hz1∕2 timing noise floor), integrated rms timing jitter of 88 as (1 Hz to 1 MHz), rms timing drift of 319 as over 12 h, and frequency instability of 3.6 × 10−20 over 10,000 s averaging time. We identified that bandpass filtering of the microwave signal and optical pulse repetition-rate multiplication are critical for achieving this performance.
... The amplitude to phase conversion coefficient is defined as the phase change between the input and RF output of the device ∆ϕ, divided by the fractional power fluctuation ∆P/P Lessing et al. 2013;Yue et al. 2017;Taylor et al. 2011;Zhang et al. 2012). Two ways can be used to obtain the AM-PM coefficient. ...
... Using Fourier analysis, the phase information of a certain frequency tone can be extracted from the electrical impulse response of a photodiode to examine phase fluctuations that occur due to the increasing the optical power. Similarly, the amplitude noise (∆P/P) can be obtained by comparing the RF output power fluctuation of the certain frequency tone with the average power (Lessing et al. 2013;Yue et al. 2017;Taylor et al. 2011;Zhang et al. 2012). The second method is based on the frequency-domain response under continuous-wave illumination ). ...
... It is noted that the null of the AM-PM coefficient appears at the typical incident optical intensity where the slope of phase change curve changes from positive to negative. The behavior of the powerdependent AM-PM coefficient is consistent with the experimental reports Lessing et al. 2013;(Yue et al. 2017;Taylor et al. 2011;Zhang et al. 2012), which proves the reliability of our simulation. ...
The phase noise known as the amplitude to phase conversion (AM–PM) in modified uni-traveling carrier photodetector (MUTC-PD) is severely power-dependent. To achieve low AM–PM operation, applying proper injection optical power will be of crucial importance. For this purpose, the characterization of the phase change between the 60-GHz sinusoidal input and RF output of a MUTC-PD is analyzed under different incident optical power. Moreover, the equation of the phase change is deduced by applying an equivalent circuit model. The results demonstrate that the power-dependent nonlinear transit time plays a key role in the phase change. Besides, the absolute phase change is observed smallest when the differential capacitance equals to zero in a typical optical power. To the best of our knowledge, this is the first demonstration of the relationship between the differential capacitance and the phase change. When the differential capacitance equals to zero, the total capacitance of the MUTC exhibits a valley which could be a measurable feature of the optimum optical power for low phase noise operation.
... On the other hand, since mode-locked lasers can provide an ultrashort pulse train with ultralow timing jitter, such an "optically assisted" phase detection can provide much higher resolution for microwave phase discrimination. In the past decades, several kinds of optical-microwave phase detectors have been demonstrated [44][45][46][47][48][49][50][51][52]. Figure 5 shows the architecture of a free-space-coupled balanced optical-microwave phase detector (BOMPD), which was first proposed in Refs. [44,45] and improved in Refs. ...
... In all these schemes, the phase error signal is encoded in the mode-locked laser's baseband power change; thus, a balanced detection is necessary to remove the laser's high average power. Special attention must be paid to suppress the AM-PM noise conversion, as discussed in Ref. [52]. ...
... Vol. 5, No. 12 / December 2018 / Optica stabilization by [47,61,[81][82][83], the synchronization between the microwave reference and master laser, master laser to klystron, linear accelerators, and bunch compressor by [44][45][46][47][48][49][50][51][52]61,88,91,92], master laser to injector laser, seed laser and seed oscillator of the probe laser by [34,35,38,39,47,61,87,90], probe laser seed oscillator to probe laser output by [62][63][64][65][66], and x-ray timing characterization by [58][59][60][61]. ...
Ultra-precise timing has become a prerequisite for many modern large-scale scientific instruments, and timing precision is a crucial enabling factor to achieve the ultimate goals of those instruments. Here, we review the recent progress in timing technologies, including timing characterization methods among different kinds of sources (optical lasers, microwaves and x-ray pulses), large-scale free-space timing synchronization, and fiber-based timing synchronization. Technical and fundamental limitations of fiber-based timing systems are also discussed to provide future directions.
... Existing solutions to the AM-to-PM problem in high-speed photodetectors can be separated into two general categories. One is to avoid direct detection of the ultrashort pulses, and instead synchronize a microwave oscillator to the optical pulse train [20][21][22]. This can be accomplished by way of a Sagnac interferometer containing an electro-optic modulator that compares the optical pulse time of arrival to the zero-crossings of the microwave oscillator. ...
... This can be accomplished by way of a Sagnac interferometer containing an electro-optic modulator that compares the optical pulse time of arrival to the zero-crossings of the microwave oscillator. While this technique has demonstrated AM rejection as large as 60 dB [21], the output frequency is limited to the microwave oscillator's capture range, and it hasn't demonstrated the extremely low phase noise capabilities of direct optical pulse detection [23]. The other solution is to directly detect the optical pulses, but operate at a low AM-to-PM value either by operating at a "null" point where the coefficient is near zero [10,17,24] or by using a photodiode with sufficiently low AM-to-PM coefficient [25] in combination with a low RIN oscillator. ...
Analog photonic links require high-fidelity, high-speed optical-to-electrical conversion for applications such as radio-over-fiber, synchronization at kilometer-scale facilities, and low-noise electronic signal generation. Photodetector nonlinearity is a particularly vexing problem, causing signal distortion and excess noise, especially in systems utilizing ultrashort optical pulses. Here we show that photodetectors designed for high power handling and high linearity can perform optical-to-electrical conversion of ultrashort optical pulses with unprecedented linearity over a large photocurrent range. We also corroborate and expand upon the physical understanding of how the broadband, complex impedance of the circuit following the photodiode modifies the linearity - in some cases quite significantly. By externally manipulating the circuit impedance, we extend the detector’s linear range to higher photocurrents, with over 50 dB rejection of amplitude-to-phase conversion for photocurrents up to 40 mA. This represents a 1000-fold improvement over state-of-the-art photodiodes and significantly extends the attainable microwave power by a factor of four. As such, we eliminate the long-standing requirement in ultrashort pulse detection of precise tuning of the photodiode’s operating parameters to coincide with a nonlinearity minimum. These results should also apply more generally to reduce nonlinear distortion in a range of other microwave photonics applications.
... By coherently combining the microwave signals generated by a dual photodetector system operating at the "magic point," Zhang et al. demonstrated APC rejection of more than 24 dB over a large range of incident optical power [23]. Additionally, as a common noise cancellation technique, a balanced optical-tomicrowave phase detector also shows less sensitivity to APC and is used to generate ultralow noise microwave signals [24]. However, none of the previous technical methods use active compensation. ...
... To improve the sensitivity, the modulation frequency should be chosen at the place where the microwave signal has lower phase noise and fewer spurious spurs. Fortunately, changing the frequency will not influence the value of the measured APC coefficient because of its frequency independence [24]. Under such considerations, we chose a modulation frequency of 21.2 kHz. ...
Microwave signals generated by photodetection of the pulse train of an optical frequency comb locked to an ultrastable laser have ultrahigh spectral purity and frequency stability. The amplitude-to-phase noise conversion (APC) that occurs in the photodetection, however, degrades the phase noise of the generated microwave signal over the range from tens of hertz to several kilohertz. Here we demonstrate active APC compensation by precisely tuning the bias voltage applied to a photodetector using the APC coefficient measured by comparing with a reference photodetector. In this scheme, the APC was dramatically reduced and a long-term steady rejection of the laser relative intensity noise of more than 60 dB was achieved.
... Existing solutions to the AM-to-PM problem in high-speed photodetectors can be separated into two general categories. One is to avoid direct detection of the ultrashort pulses, and instead synchronize a microwave oscillator to the optical pulse train [20,21,22]. This can be accomplished by way of a Sagnac interferometer containing an electro-optic modulator that compares the optical pulse time of arrival to the zero-crossings of the microwave oscillator. ...
... This can be accomplished by way of a Sagnac interferometer containing an electro-optic modulator that compares the optical pulse time of arrival to the zero-crossings of the microwave oscillator. While this technique has demonstrated AM rejection as large as 60 dB [21], the output frequency is limited to the microwave oscillator's capture range, and it hasn't demonstrated the extremely low phase noise capabilities of direct optical pulse detection [23]. The other solution is to directly detect the optical pulses, but operate at an AM-to-PM "null" point where the coefficient is near zero [10,17,24]. ...
Analog photonic links require high-fidelity, high-speed optical-to-electrical conversion for applications such as radio-over-fiber, synchronization at kilometer-scale facilities, and low-noise electronic signal generation. Photodetector nonlinearity is a particularly vexing problem, causing signal distortion and excess noise, especially in systems utilizing ultrashort optical pulses. Here we show that photodetectors designed for high power handling and high linearity can perform optical-to-electrical conversion of ultrashort optical pulses with unprecedented linearity over a large photocurrent range. We also show that the broadband, complex impedance of the circuit following the photodiode modifies the linearity significantly. By externally manipulating the circuit impedance, we extend the detector's linear range to higher photocurrents, with over 50 dB rejection of amplitude-to-phase conversion for photocurrents up to 40 mA. This represents a 1000-fold improvement over state-of-the-art photodiodes and significantly extends the attainable microwave power by a factor of four. As such, we eliminate the long-standing requirement in ultrashort pulse detection of precise tuning of the photodiode's operating parameters (average photocurrent, bias voltage or temperature) to coincide with a nonlinearity minimum. These results should also apply more generally to reduce nonlinear distortion in a range of other microwave photonics applications.
... The location of the phase modulator is carefully selected, so that the counter-propagating pulses experience opposite phase modulation and thus the phase error between the microwave signal and the optical pulses manifests itself as the amplitude imbalance of the Sagnac interferometer's two outputs, which could be detected by a balanced photodetector (BPD). Later, an improved implementation with a much lower phase noise floor, referred to fiber-loop optical-microwave phase detector (FLOM-PD), was achieved using a fiber Sagnac loop [29]- [31]. A nonreciprocal quarter-wave bias unit is inserted to induce a constant π/2 phase difference between the counter-propagating pulses in the Sagnac loop, which however causes slow phase drift because of the temperature-dependent birefringence change in the quarter-wave plate and therefore limits the long-term stability of the entire system. ...
... As can be seen, even very small optical power difference between the two inputs of the BPD causes non-negligible additional phase noise especially in a PLL with high feedback gain. Actually, as early as 2013, the residual phase noise of the FLOM-PD originated from the RIN of the optical pulses was suppressed by fine tuning of the optical power using two optical tunable attenuators in front of the BPD [31]. In our scheme, fine tuning of optical power could be achieved simply by rotating the PC for adjusting the polarization direction of Ex and Ey. ...
We propose and demonstrate a compact optical- microwave phase detector taking advantage of polarization modulation. The proposed polarization modulator-based phase detector (PolM-PD) shows 460-attosecond residual timing jitter integrated from 1 Hz to 100 kHz when synchronizing an 8-GHz dielectric resonator oscillator to a free-running 250-MHz mode-locked Erbium-doped fiber laser. The absolute single-sideband (SSB) phase noise of the locked 8-GHz DRO is -138 dBc/Hz (-165 dBc/Hz) at an offset frequency of 10 kHz (10 MHz). The proposed PolM-PD has a compact structure implemented by all commercially-available components. Because no fiber loop and delay line are used, the proposed PolM-PD can be potentially integrated on a chip.
... Compared with the fibercoupled approach [37,64], this new design can provide a >10× improvement for long-term timing stability. Besides, in contrast to other BOMPDs [65,66], its output error signal is insensitive to optical input power fluctuations. ...
We present a timing synchronization system that can synchronize optical and microwave signals with attosecond-level precision across kilometer distances. With this technique, the next-generation photon science facilities like X-ray free-electron lasers and intense laser beamlines can be enabled to observe ultra-fast dynamics in atoms, molecules and condensed matter taking place on an attosecond time scale. We discuss some key technologies including master-laser jitter characterization, local one-color laser synchronization, remote two-color laser synchronization, and analyze technical noise contributions in the system. A 4.7-km laser-microwave network with 950-attosecond timing jitter is realized over tens of hours of continuous operation. Finally, an integrated balanced optical cross correlator is introduced. With the same input power level, the required operational power for each timing link is significantly reduced.
