Research of optical coherence tomography microscope based on CCD detector

Simultaneous measurement of the Henry factor of 14 longitudinal modes is implemented using a novel method based on sinusoidal phase modulation, without using any filter. Excellent agreement with fiber transfer function technique is achieved.

High-power narrow linewidth fiber lasers are extensively applied in coherent detection and power-spectrum beam combination etc. The suppressing method of stimulated Brillouin scattering is analyzed. And the theory of sinusoidal phase modulation is analyzed also. A single frequency laser is broadened to become a 2.9 GHz linewidth seed by sinusoidal phase modulation technology. The power of the seed is then boosted from 50 mW to 780 W through a three-stage power amplifiber configuration. Central wavelength and linewidth of the laser are 1064.34 nm and 2.9 GHz respectively, with an optical-optical efficiency being 79%. And the beam quality is Mx2 =1.44 and My2 =1.43. The output powers before and after phase modulation are compared with each other. And the reason why output power increases is analyzed. The stimulated Brillouin scattering threshold is promoted by added longitudinal mode. Finally, the output power is promoted after phase modulation, so that the output power of this laser is only limited by the pump power. If the pump power is increased, the higher output power of narrow linewidth fiber laser will be achieved.

We present experimental results achieved by a method of direct conversion of the relative changes of the measurement optical path of Michelson interferometer to relative changes of the resonant optical-frequency of Fabry-Perot (F.-P.) resonator. We developed the method as a testing process for verification of scale-linearity of Michelson interferometer with total resolution 0,3 nm. The method consists of a mechanical coupled shift of the corner cube mirror of the interferometer measurement arm with one of the mirrors of F.-P. resonator. A piezoelectric transducer (PZT) with approximately 10 microns elongation drives that mechanical shift. An external tunable laser source at 633 nm wavelength provides identification of one of the resonant optical frequency of F.-P. resonator by the frequency locking mechanism with synchronous detection technique in the servo loop feedback. Because definition of the meter unit is based on iodine stabilized He-Ne laser, then the optical frequency of the locked tunable laser is frequency compared with HeNeI2 laser by the heterodyne optical mixing. A fast high-resolution counter counts the resultant radio-frequency signal as a product of the optical mixing. Measured frequency values and values of interference phase acquired by the interferometer are simultaneously sampled step by step for each elongation position of PZT element. The experimental data achieved by F.-P. resonator shows uncertainty of the relative distance change better than 0,01 nm. We verified the scale-linearity of Michelson interferometer to ±1,0 nm limit.

The simultaneous photometric and spectroscopic observations of the RR Lyrae variables in the globular cluster, M3, published in Jurcsik et al. (2017, Paper I) made it possible to perform Baade–Wesselink (BW) analysis of a large sample of Blazhko stars for the first time. The BW distances of Blazhko stars turned out to be unreliable, as significantly different distances were obtained for the stars of the Blazhko sample and also for the same star in different modulation phases. Even the results of small modulation–amplitude Blazhko stars may be doubtful. This result warns that the application of the BW method to Blazhko stars is not trustworthy. Keeping the distance fixed for each Blazhko star in each modulation phase, a significant difference between the spectroscopic and the photometric radius (Rsp, Rph) variations is detected. The phase and amplitude variations of Rsp follow the changes of the light curve during the Blazhko cycle but the Rph curve seems to be not (or only marginally) affected by the modulation. The asynchronous behaviour of Rsp and Rph supports the interpretation of the Blazhko effect as a depth-dependent phenomenon, as the spectroscopic radius variation reflects the radial displacement of the line-forming region high in the atmosphere, while the photospheric radius variation is derived from the information of the observed visual-band light emitted mostly by the lower photosphere. The stability of Rph may be interpreted as a strong argument against the non-radial-mode explanation of the Blazhko phenomenon.

Sinusoidal phase-modulating laser diode interferometer insensitive to the intensity modulation of the light source

A pulse-rate-tunable, highly extinctive, ultra-highspeed electrooptic pulse pattern generator has been developed. The optical short pulse generation is based on sinusoidal electrooptic phase modulation and linear chirp compensation using a dispersive medium. Filtering the nonlinear chirp components generated by sinusoidal phase modulation drastically improves the pulse extinction, and makes nearly background-free picosecond pulsation over a wide pulse-rate range even when the group delay dispersion value is fixed.

Resonant frequency doubling of periodically phase-modulated single-frequency fiber laser is investigated as a method for power scaling of visible fiber lasers. Sinusoidal phase modulation is applied to generate few-frequency lasers at 1064 nm in the proof of principle experiments. By adjusting the modulation frequency to match the free spectral range of a doubling cavity, a resonant enhancement condition can be achieved and a near 30 W 532 nm laser is generated with a maximum conversion efficiency above 80%. The indistinguishable conversion efficiencies between the single-frequency and few-frequency cases prove the feasibility of the approach. Interesting spectral evolvement of the phase-modulated laser in second-harmonic generation is analyzed theoretically and observed in the experiment.

A simplified algorithm for digital fringe analysis in two-wave interferometry with sinusoidal phase modulation

A theoretical analysis of the measurement accuracy in sinusoidal phase modulating (SPM) interferometry is presented. The measurement accuracy is dependent on multiplicative and additive noise. The characteristics of SPM interferometry in the presence of this noise are made clear. Theoretical results show clearly that SPM interferometry has a high measurement accuracy of the order of 1 nm.

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).]

Akhmediev breather signatures from dispersive propagation of a periodically phase-modulated continuous wave

Relationships are developed between internal-wave vertical velocity and acoustic phase. For source–receiver separations of many internal-wave wavelengths, it is shown that the mapping of internal-wave energy onto the phase of a single acoustic path is highly directional. Internal waves propagating perpendicular to the acoustic path produce the greatest phase modulation, and modulation of acoustic phase is reduced in all other geometries. For the omnidirectional internal-wave spectrum proposed by Garrett and Munk, the acoustic phase-modulation power spectrum is proportional to ω−3 and j−3, where ω is the internal-wave radian frequency and j is the internal-wave vertical-mode number. The first-mode internal wave contributes more than half of the acoustic phase-modulation power, and in this sense, the first-mode internal wave dominates the acoustic phase spectrum.

Traditionally alpha and beta oscillations have been viewed a passive phenomenon, being active only when our minds are idling. Empirical evidence over the last two decades challenges this classic view and suggests that alpha/beta oscillations play a much more active role during cognitive processes. In the human brain, alpha oscillations influence processing of large neural ensembles by regulating the strength of synchronization within these ensembles and by providing discrete time windows for firing. With EEG recordings we can observe these two mechanisms via modulations of the signal strength (i.e., power) and modulations of phase, respectively. This chapter focuses on these two signal properties of alpha/beta oscillations and reviews studies that investigated the relevance of alpha power and phase modulations in tasks involving perception and memory processes. These studies reveal a reduction in alpha/beta power when a stimulus is being perceived or mentally replayed. Furthermore, alpha phase reflects rhythmical sampling of stimulus information during perception, and the replay of that information during retrieval. Based on these findings, we argue that alpha oscillations play an active role in representing information-rich content in the brain. Importantly, alpha/beta oscillations perform this function regardless of whether the information is being presented externally, as is the case in perception, or whether it is being generated internally, as is the case in memory retrieval.

We report on the operation and performance of a novel electrooptic polarization scrambler that is capable of generating any desired combination of optical phase and polarization modulation. The scrambler comprises a dual-drive Mach-Zehnder-type intensity modulator on lithium niobate and an external microoptic polarization splitter which combines the two complementary outputs of the integrated-optic intensity modulator in orthogonal polarization states. The modulation indexes of the sinusoidal phase and polarization modulation in the output light can be varied independently of each other via the amplitudes and phases of the two drive voltages to the Mach-Zehnder modulator. The scrambler has a 3-dB modulation bandwidth of 14 GHz and operates in the 1.5-1.6-/spl mu/m wavelength range with an overall fiber-to-fiber insertion loss of less than 5 dB. At 5 GHz modulation frequency and electric drive powers between 390-2030 mW, we demonstrate phase-chirped and chirp-free depolarization with a residual degree of polarization of less than 0.02 and continuously variable phase modulation to 2.3-rad peak phase deviation.

We review the evolution of modulation formats designed for direct detection, from on-off keying intensity modulation (IM) of a single polarization to three-dimensional formats where the intensity of both polarizations and the interpolarization phase are all independently modulated. We present the transmitter and receiver architectures capable of modulating an increasing number of dimensions, all orthogonal to each other. We detail the digital signal processing for Stokes vector receivers capable of recovering all three dimensions of a lightwave via self-beating direct detection, performing simultaneously polarization derotation and mitigation of intersymbol interference. We also analyze two different transmitter architectures modulating all three dimensions where the interpolarization phase is modulated over two levels using either a direct phase modulator (DPM) or a binary phase modulator, also called a BPSK modulator. We study the impact of increasing the bandwidth efficiency from 4 to 5 b/symbol by adding two level modulation of the interpolarization phase of a dual-polarization 4-level pulse amplitude modulation (PAM4) signal on the signal quality of the PAM4 IM on the polarization bearing the phase modulator by modeling the optical fields generated by both architectures. Assuming equal quality of the driving signals to the PAM4 and phase modulator, we show that a DPM minimizes the impact on PAM4 for short reach direct detect systems operating close to the zero dispersion wavelength where chromatic dispersion is negligible.