Digital holography experiments with degraded temporal coherence

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.

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

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

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.

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.

The reference wave phase was modulated with a sinusoidal vibrating mirror attached to a Piezoelectric Transducer (PZT), the integration was performed by a CCD, and the charge storage period of the CCD image sensor was one-quarter period of the sinusoidal phase modulation. With the frequency- synchronous detection technique, four images (four frames of interference pattern) were recorded during one period of the phase modulation. In order to obtain the optimum modulation parameter, the values of amplitude and phase of the sinusoidal phase modulation were determined by considering the measurement error caused by the additive noise contained in the detected values. The PZT oscillation was controlled by a closed loop control system based on PID controller. An ideal discrete digital sine function at 50Hz with adjustable amplitude was used to adjust the vibrating of PZT, and a digital phase shift techniques was used to adjust vibrating phase of PZT so that the phase of the modulation could reach their optimum values. The CCD detector was triggered with software at 200Hz. Based on work above a small coherent signal masked by the preponderant incoherent background with a CCD detector was obtained.

Large energetic laser facilities provide an amazing tool for bringing matter into high energy density states but achieving a good energy conversion requires to optically smooth the high-power laser beams. Optical smoothing reduces the laser spatial and temporal coherences, allowing to mitigate the development of unwanted laser–plasma instabilities (LPI). Two schemes have been mainly developed: polarization smoothing and smoothing by spectral dispersion. Here, we focus on the latter. Smoothing by spectral dispersion consists in broadening the spectrum usually through a sinusoidal phase modulation and dispersing it with an optical grating. It is usually considered that the modulation frequency should be equal to the inverse of the time delay of the grating, because it maximizes the number of uncorrelated speckles patterns at a given bandwidth. However, current optical smoothing designs are still not sufficient for mitigating LPI. Because the optical smoothing system possesses degrees of freedom, we propose to improve the optical smoothing efficiency by a better tuning of those parameters. Considering the Laser Mégajoule optical system framework, we show that an increase in the modulation frequency allows a better efficiency of the smoothing while keeping the same bandwidth. We assess the efficiency by looking at a better mitigation of the stimulated Brillouin scattering that, due to its dangerousness in hohlraums developed for inertial confinement fusion applications in the indirect drive scheme, serves as a witness instability.

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.

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

A fiber-optic sinusoidal phase modulating (SPM) interferometer was proposed for the acquisition and reconstruction of three-dimensional (3-D) surface profile. Sinusoidal phase modulation was induced by controlling the injection current of light source. The surface profile was constructed on the basis of fringe projection. Fringe patterns are vulnerable to external disturbances such as mechanical vibration and temperature fluctuation, which cause phase drift in the interference signal and decrease measuring accuracy. A closed-loop feedback phase compensation system was built. In the subsystem, the initial phase of the interference signal, which was caused by the initial optical path difference between interference arms, could be demodulated using phase generated carrier (PGC) method and counted out using coordinated rotation digital computer (CORDIC) , then a compensation voltage was generated for the PZT driver. The bias value of external disturbances superimposed on fringe patterns could be reduced to about 50 mrad, and the phase stability for interference fringes was less than 6 mrad. The feasibility for real-time profile measurement has been verified.

We present a system that measures the full-field amplitude and phase distributions of objects that vibrate with small amplitudes. The system is based on TV–holography combined with sinusoidal phase modulation, discrete vibration-phase shifts, and digital image processing. Different new algorithms, based on a linear approximation of the fringe function, are discussed. Averaging techniques, used to reduce the effects of noise sources and to increase the resolution of the system, are also introduced. For one of the algorithms, tested in combination with the averaging techniques, the amplitude threshold was approximately 1/3000 of the wavelength of the applied laser light. The amplitude resolution was of the same magnitude. The phase accuracy is amplitude dependent and was about 3° for amplitudes greater than 5 nm.

We propose a sinusoidal phase-modulating Fizeau interferometer using self-pumped conjugate wave for surface profile measurements. The interferometer is self-referencing and free from the effects of the axial movements of object surfaces. The characteristics of the interferometer are made clear through the surface measurement of a diamond-turned aluminum disk. For large-size objects, the size of the plate glass can be much smaller than that of the object. The small-size plate glass provides the accurate sinusoidal phase modulation easily. We also propose the method of absolute measurement to eliminate the undesirable phase distribution caused by aberration of the lenses. It is shown through the surface profile measurements of a mirror that the interferometer and the method of absolute measurement are useful for large- size objects.

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.

Sinusoidal phase modulation interference survey technology is an advanced interference measuring technique in international front. Through introducing the piezoelectric ceramics (PZT) of sinusoidal vibration in Michelson interferometer reference path of light ,we carry on the modulation to the reference light, which produces modulation item along with the time sine change of interference signal. The demodulation is carried on based on Fourier analysis method to obtain the phase of corresponding object to measure the interference signal. This article proposes the algorithm of Fourier analysis method to carry on the solution to the interference signal, discusses the influence of the algorithm on measuring accuracy and compares it with traditional method. Experimental result indicates the sinusoidal phase modulation interference survey technology is an effective interference measuring technique with high accuracy.

Precise delay control is of paramount importance in optical pump-probe measurements. Here, we report on a high-precision delay tracking technique for mechanical scanning measurements in a Mach-Zehnder interferometer configuration. The setup employs a 1.55-µm continuous-wave laser beam propagating along the interferometer arms. Sinusoidal phase modulation at 30 MHz, and demodulation of the interference signal at the fundamental frequency and its second harmonic, enables delay tracking with sampling rates of up to 10 MHz. At an interferometer arm length of 1 m, root-mean-square error values of the relative delay tracking below 10 attoseconds for both stationary and mechanically scanned (0.2 mm/s) operation are demonstrated. By averaging several scans, a precision of the delay determination better than 1 as is reached. We demonstrate this performance with a mechanical chopper periodically interrupting one of the interferometer arms, which opens the door to the combination of high-sensitivity lock-in detection with (sub-)attosecond-precision relative delay determination.