Phase noise issues in femtosecond lasers

In this paper we report the results of extensive research on phase modulation (PM) and amplitude modulation (AM) noise in linear bipolar junction transistor (BJT) amplifiers. BJT amplifiers exhibit 1/f PM and AM noise about a carrier signal that is much larger than the amplifier's thermal noise at those frequencies in the absence of the carrier signal. Our work shows that the 1/f PM noise of a BJT based amplifier is accompanied by 1/f AM noise which can be higher, lower, or nearly equal depending on the circuit implementation. The 1/f AM and PM noise in BJTs is primarily the result of 1/f fluctuations in transistor current, transistor capacitance, circuit supply voltages, circuit impedances, and circuit configuration. We discuss the theory and present experimental data in reference to common emitter amplifiers, but the analysis can be applied to other configurations as well. This study provides the functional dependence of 1/f AM and PM noise on transistor parameters, circuit parameters, and signal frequency, thereby laying the groundwork for a comprehensive theory of 1/f AM and PM noise in BJT amplifiers. We show that in many cases the 1/f PM and AM noise can be reduced below the thermal noise of the amplifier.

A practical implementation of a portable secondary standard for phase modulation (PM) and amplitude modulation (AM) noise at 5, 10, and 100 MHz is described. The accuracy of the standard for both PM and AM noise is +or-0.14 dB, and the temperature coefficient is less than 0.02 dB/K. The noise floor of the standard for PM noise measurements is less than -190 dBC rel to 1 rad/sup 2//Hz at 5, 10, and 100 MHz. The noise floor for AM measurements depends on the configuration. A calibrated level of PM and AM noise of approximately-130+or-0.2 dB rel to 1 rad/sup 2//Hz is used to evaluate the accuracy versus Fourier frequency. This standard can also be used as an alternative to the normal method of the PM/AM detector for PM/AM measurements. >

We show that a high-resolution software package can be adapted to calculate the amplitude modulation (AM) and phase modulation (PM) noise performance of Si Bipolar Junction Transistor (BJT) amplifiers. Our simulations predict that the PM noise induced by bias current noise can be greatly suppressed by operating at special bias conditions. Furthermore, a dc balance amplifier circuit is introduced to stabilize the base-to-collector voltage. The total sensitivity of PM noise to power supply noise using dc balance architecture can be reduced by 40 dB compared to basic amplifier architecture. This is experimentally verified.

A portable amplitude-modulation (AM) and phase-modulation (PM) noise standard has been developed for carrier frequencies of 10.6, 21.2 and 42.4 GHz. The noise level from the carrier 1/2 S/sub /spl phi//(f) (phase noise) or 1/2 S/sub a/(f) (amplitude noise) is -122.6 dBc/Hz at 10.6 GHz, -119.6 dBc/Hz at 21.2 GHz and -116.3 dBc/Hz at 42.4 GHz. For Fourier frequencies of 1 kHz to 10 MHz the added noise is flat with an uncertainty of /spl plusmn/0.25 dB. The uncertainty of the noise level for Fourier frequencies of 1 kHz to 100 MHz is /spl plusmn/0.5 dB. The residual PM noise floor between the signal and reference outputs at 100 MHz is less than -182 dBc/Hz.

When an RF carrier is amplified by a GaAs MESFET, amplitude modulation (AM) and phase modulation (PM) noises are imposed on the carrier. This is generally believed to be caused by transposition to the carrier frequency of the low-frequency flicker noise generated by the FET. The cross correlation between the AM and PM noises and the low-frequency noise observed on the drain of the FET is measured. While the AM noise and the low-frequency noise on the drain of the FET exhibit a high degree of correlation, the PM noise and the low-frequency drain noise are not highly correlated. The latter result may explain the limited success of oscillator phase noise reduction methods which rely on the existence of a large cross correlation between the PM and low-frequency noises. >

In this paper, we numerically demonstrate the advantage of utilizing continuous amplitude and phase modulation in super-oscillation focusing lens design. Numerical results show that compared with simple binary amplitude modulation, continuous amplitude and phase modulation can greatly improve the super-oscillation focusing performance by increasing the central lobe intensity and the ratio of its energy to the total energy, reducing the sidelobe intensity, and substantially extending the field of view. Our study also reveals the role of phase distribution in reducing the spatial frequency bandwidth of the super-oscillation optical field on the focal plane. Based on continuous amplitude and binary phase modulation, a lens was designed with double layer metal slit array for wavelength of 4.6 µm. COMSOL is used to carry out the 2D simulation. The lens focal length is 40.18λ and the focal spot FWHM is 0.308λ. Two largest sidelobes are located right next to the central lobe with intensity about 40% of the central lobe intensity. Except for the two sidelobes, other sidelobes have intensity less than 25% of the central lobe intensity, which leads to a clear field of view on the whole focal plane.

Recent evidence is re-shaping the view of primary auditory cortex (A1) from a unisensory area to one more involved in dynamically integrating multisensory- and task-related information. We found A1 single- (SU) and multiple-unit (MU) activity correlated with macaques's choices in an amplitude modulation (AM) discrimination task. Animals were trained to discriminate AM-noise from unmodulated noise by releasing a lever for AM-noise and holding down the lever for unmodulated noise. Activity for identical stimuli was compared between trials where the animals reported AM and trials where they did not. We found 47.4% of MUs and 22.8% of SUs significantly increased firing shortly before the animal's behavioral response to report AM, when compared to the equivalent time period on trials where AM was not reported. Activity was also linked to lever-release in a different task context, suggesting A1 modulation by somatosensory, or efference-copy, input. When spikes were counted only during the stimulus, 19.6% of MUs and 13.8% of SUs increased firing rate when animals reported AM compared to when they did not, suggesting an attentional effect, or that A1 activity can be used by higher decision areas, or such areas provide feedback to A1. Activity associated with AM reporting was correlated with a unit's AM sensitivity, suggesting AM sensitive neurons’ involvement in task performance. A1 neurons’ phase-locking to AM correlated more weakly (compared to firing rate) with the animals’ report of AM, suggesting a preferential role for rate-codes in A1 for this AM discrimination task.

Dramatic advances in millimeter and submillimeter wave receivers in recent years have resulted from the development of superconductor insulator superconductor (SIS) mixers, which now offer unsurpassed performance from 70 GHz to 1 THz. To increase the sensitivity of the receivers further at these frequencies, one needs to use dual-polarized and balanced receivers. When both the polarizations are received simultaneously, there is a square root of two improvement in the signal to noise ratio (SNR). Balanced mixers improve the sensitivity of receivers by suppressing local oscillator (LO) amplitude modulation (AM) noise and rejecting LO thermal noise. This thesis describes the design, fabrication and performance of mixers and components for low noise dual-polarized and balanced receivers at millimeter and submillimeter wavelengths both in quasi-optical and waveguide configurations. The quasi-optical receiver utilizes a novel cross-slot antenna on a silicon hyperhemispherical lens, two junction tuning circuits, niobium trilayer junctions, and an IF circuit containing a lumped element 180 degree hybrid. The antenna has four feed points, two for each polarization; and each feed point is coupled to a two-junction SIS mixer. For dual polarization operation, the mixer is mounted in such a way that a single LO can pump the junctions for both the polarizations. For the balanced receiver, the LO and the RF signals are coupled to the mixer in orthogonal polarizations using a wire-grid polarizer. For waveguide dual polarization receivers, a moderately broadband septum ortho-mode transducer (OMT) is designed and experimental results are presented at millimeter wavelengths. Broadband finline OMTs are investigated for possible use at millimeter wavelengths, and experimental results of a finline OMT is presented at X-band.

Radio frequency (RF) receivers are common in many modern communications and radar systems, and they suffer from many performance degradation factors due to hardware limitations. Among all performance degradation contributors, phase noise and time jitter are particularly troublesome since they cause random errors which are difficult to compensate. The local oscillator in the receiver front end is a major contributor of phase noise, while the analog-to-digital converter (ADC) introduces time jitter. It is desired to know the accumulated effect of individual phase noise sources and time jitter. The total effect of all phase noise and jitter can be represented by an accumulated phase noise term at the ADC's output, called total phase noise (TPN) in this brief. The focus of this work is on measuring and modeling TPN in the RF receiver by applying optimization techniques. In contrast to traditional phase noise measurement that typically requires a high-quality tunable downconverter, a digital approach using the data captured directly by the RF receiver is proposed. In addition, iterative optimization-based TPN spectral model fitting and statistic modeling are introduced. The model is examined using the measured TPN. It is confirmed that the RF receiver TPN can be viewed as a wide-sense stationary zero-mean Gaussian process with certain spectral profile.

Acoustic holography underpins the number of modern acoustics applications. The usage of multi-focal acoustic holograms with phased array transducers (PATs) in ultrasonic tactile displays and acoustic levitation are increasing, and achieving a high-quality hologram are of significant interest. Inspired by the latest optical hologram optimizer, we developed a novel acoustic hologram optimizer for PATs called Diff-PAT [T. Fushimi, K. Yamamoto, and Y. Ochiai, “Acoustic hologram optimisation using automatic differentiation”]. Diff-PAT is based on a gradient-descent algorithm and automatic differentiation. The performance of Diff-PAT was numerically evaluated using three array configurations and it achieved superior accuracy over the conventional optimizers. For example, when two focal points are generated with 196 ultrasonic (40 kHz) transducers; the state of art optimizer (with phase and amplitude modulation) based on Eigensolver has an average error of 4.07 % where Diff-PAT (with only phase modulation) achieve average error of 0.0174%. These results show that amplitude and phase modulation may not be necessary for PATs to be successful. Furthermore, we demonstrate the versatility of Diff-PAT by applying it to binary acoustic holograms, simultaneous modulation of acoustic amplitude and phase at the target plane, and phase plates. [Work supported by Pixie Dust Technologies, Inc., Diff-PAT is patent pending (JP2020-167367A).]

This paper deals with mechanical fault monitoring in induction machines from stator current measurements. The considered faults lead to amplitude and/or phase modulations of the measured stator current. The different faults can be characterized by their time-frequency signatures via the Wigner distribution. The time-frequency representations apply to complex signals that may be obtained through the Hubert transform of the real measured signal. In case of high frequency modulations, it can not be taken advantage of the time frequency signatures. This study proposes an alternative complex signal representation for modulated stator currents. The so-called space vector is obtained through the Concordia transform. From three stator current measurements, the Concordia transform builds a complex vector which conveniently carries the information about phase and amplitude modulations. This paper applies and compares the Wigner distribution computed with the Hilbert and Concordia transforms in case of simulated and experimental signals with various modulation frequency ranges.

The opto-electronic oscillator (OEO) has emerged in recent years as an excellent low-noise source that rivals the best RF oscillators over broad offset frequencies. The main sources of noise in an OEO are the laser and RF modulator, photo detector, loop amplifier, and the long fiber that is needed for high Q. Recent studies have shown that even by using state-of-the-art components and a low-loss long fiber, the phase-modulated (PM) noise of these OEOs fails to meet the theoretical value at offset frequencies close to carrier, from a few hertz to 1 kHz. The main cause for this shortfall is vibration effects on the optical fiber. External, environmental vibration causes mechanical distortions in the fiber that induce time-delay (phase) fluctuations. The spool onto which the fiber is wound is primarily responsible for imparting these vibration-induced delay fluctuations to the fiber and thus diminishing the performance of the OEO. In this paper, we compare the vibration-induced phase fluctuations of a 3 km optical fiber wound on spools made of four materials-metal, ceramic, plastic, and foam-covered plastic. We investigate fiber-on-spool winding and mounting techniques that reduce vibration susceptibility. We present residual PM measurements that compare the vibration sensitivity of an optical fiber wound on these different materials.

The harmonic mode locking (ML), i.e. the modulation of resonator losses at a frequency that is an integer multiple N of the fundamental mode spacing, has been a technique intensively studied for achieving minimum pulse duration in low-gain actively mode locked continuos wave (CW) Nd:YAG laser (see refs. [1, 2] and references therein). The experiment we have performed concerns the first demonstration of the harmonic ML on an high-gain QS Nd:YAG laser, operated at 10 Hz repetition rate. To avoid the generation of several pulses per round-trip we implemented the ML by simultaneously operating two phase-locked acousto-optic modulators (AOM) with N=3 (radio frequency: RF=150 MHz) and N=4 (Rf=200 MHz), placed at 1/3 and 1/4 of the resonator length, respectively. By operating a stable cavity in QS ML regime we observed that the short buildup time allowed by the QS was not sufficient to let the ML to evolve to the stationary regime, so that the generated pulses resulted surrounded by a≈1 ns long pedestal and satellites exist placed at 1/3 and 1/4 of the cavity length. To increment the build-up time we realised a simple kind of prelasing by slightly rotating the QS quarter-wave plate and letting the laser starting to oscillate in free running (FR) ML regime. When the first FR spike was developed, a photodiode triggered the QS to allow the seeding pulse to be amplified up to gain saturation. The amplified ML train contains 4-5 pulses whose duration, measured by SH scanning autocorrelation, was =100 ps FWHM. A typical autocorrelation trace is given in Fig. 1. No satellites were present in this regime. With the laser operated with 80% output coupling, the maximum pulse-train energy, set by the damage threshold of resonator optics, was ≈10 mJ. The shot-to-shot energy fluctuations at the fundamental wavelength were below 5%, while below 8% those at the SH, indicating the high energy and temporal stability's of the generated pulses.

A new microcomputer based air temperature measurement system is presented. An accurate temperature measurement is derived from the measurement of sound velocity by using an ultrasonic time-of-flight (TOF) technique. The study proposes a novel algorithm that combines both amplitude modulation (AM) and phase modulation (PM) to get the TOF measurement. The proposed system uses the AM and PM envelope square waveform (APESW) to reduce the error caused by inertia delay. The APESW ultrasonic driving waveform causes an envelope zero and phase inversion phenomenon in the relative waveform of the receiver. To accurately achieve a TOF measurement, the phase inversion phenomenon was used to sufficiently identify the measurement pulse in the received waveform. Additionally, a counter clock technique was combined to compute the phase shifts of the last incomplete cycle for TOF. The presented system can obtain 0.1% TOF resolution for the period corresponding to the 40 kHz frequency ultrasonic wave. Consequently, with the integration of a humidity compensation algorithm, a highly accurate and high resolution temperature measurement can be achieved using the accurate TOF measurement. Experimental results indicate that the combined standard uncertainty of the temperature measurement is approximately 0.39 degrees C. The main advantages of this system are high resolution measurements, narrow bandwidth requirements, and ease of implementation.

This paper presents a comprehensive analysis of submillimeter wave generation through optical four-wave mixing (FWM) in an injection-locked semiconductor laser. The probe wave introduces pump-probe difference frequency amplitude modulation (AM) and phase modulation (PM) of the slave laser injection locked to the pump wave. The AM and PM indexes of the locked laser output lightwave modulated at the submillimeter wave frequency have been calculated. The submillimeter wave power generated through this technique has also been calculated theoretically. The analysis predicts a submillimeter wave power of -3.7 dBm at 300-GHz frequency for a free-running slave laser output power of 30 mW. In the presence of amplitude modulation of the probe wave, the same modulation is transferred to the submillimeter wave. The suppression of output lightwave amplitude noise relative to the pump, probe, or free-running slave laser amplitude noise has also been estimated in this analysis. At a submillimeter wave frequency of 300 GHz and a probe power equal to the pump power, typical amplitude noise reduction occurs by 7 dB.