Self-mixing Laser Doppler Velocimetry Research Articles

Self-mixing thinly sliced ruby laser for laser Doppler velocimetry with high optical sensitivity

In self-mixing laser Doppler velocimetry (LDV), the motion of a moving target is observed by using intensity-modulated laser light detected by a simple photodetector. Here, the self-mixing laser output modulation takes place, reflecting the pronounced effective loss modulation index, which is proportional to the fluorescence-to-photon lifetime ratio. The fluorescence lifetime of a ruby laser is extremely long, so if a ruby crystal can be used as a laser light source for a self-mixing LDV system, high-sensitivity LDV measurements can be performed with it. We describe a method for velocimetry of moving targets using self-mixing LDV in which a CW oscillating ruby laser is the light source. The oscillation mechanism of the thin-slice ruby laser with a large fluorescence-to-photon lifetime ratio, which is suitable for LDV measurements, is clarified and the results of highly sensitive LDV measurements are presented, featuring nonlinear dynamics observed associated with the self-mixing velocimetry experiment. The measurement accuracy is clarified by measuring the rotating disc with various conditions using self-mixing LDV.

Self-mixing interference in a thin-slice solid-state laser with few feedback photons per observation period

Formation of a peculiar lasing pattern leading to a TEM00-like mode accompanied by a nonzero bright outer-ring emission (namely, a modified TEM00 mode) was observed in a 300-\mu m-thick LiNdP4O12 (LNP) laser with reflective flat end mirrors that was pumped with a laser diode whose focused spot size was larger than the lasing beam spot size determined by the thermal lens effect. The photon lifetimes of the laser oscillations were found to be greatly shortened to \tau_{p} = 6.74 ps as compared with those in pure TEM00 mode operation, while the fluorescence lifetime was \tau = 130 \mu s. The formation of the peculiar transverse mode was theoretically treated in terms of the pump-intensity-dependent thermal lens effect, which yields the lasing beam spot sizes in the resultant optical cavity. In addition, transverse spatial hole burning of population inversions due to the preceding transverse eigenmode intensity and the associated anti-guidance of the pure TEM00 field leading to formation of the TEM00 with an emergent outer ring emission was shown to appear with increasing pump power through the electric lens (i.e., population lens) effect, in which almost all of the population inversion contributes to lasing with a pronounced effective modal gain. Self-mixing laser Doppler velocimetry (LDV) experiments and numerical simulations revealed that spontaneous emission factors in the peculiar mode oscillations were greatly decreased in comparison with those in pure TEM00 mode operation. The present thin-slice solid-state laser possessing a large fluorescence-to-photon lifetime and decreased spontaneous emission factor enabled us to detect LDV signals in the regime of few feedback photons per observation period.

Coherent laser detection of the femtowatt-level frequency-shifted optical feedback based on a DFB fiber laser.

The theoretical basis and experimental realization of an all-fiber self-mixing laser Doppler velocimetry based on frequency-shifted feedback in a distributed feedback (DFB) fiber laser are presented, which employs a pair of fiber-coupled acousto-optic modulators to adjust the modulation intensity and frequency of the laser self-mixing effect. Moreover, the minimum optical feedback intensity for the velocity signal successfully measured by the interferometer is 5.12 fW, corresponding to 0.16 photons per Doppler cycle. The results demonstrate that the proposed scheme can adapt to the non-contact measurement requirements of the wide-range speed and weak feedback level in the complex environment.

All-fiber self-mixing laser Doppler velocimetry with much less than 0.1 pW optical feedback based on adjustable gain.

The all-fiber self-mixing laser Doppler velocimetry with adjustable gain is experimentally investigated based on a distributed Bragg reflector fiber laser. In the measurement system, the modulation gain of the injected light in the laser cavity is adjusted by a pair of fiber-coupled acousto-optic modulators (AOMs) in the external cavity, which can change the intensity and frequency of the self-mixing modulation effect. Simultaneously, the minimum feedback intensity from the target to the laser for successful measurements is 0.063 pW. Thus, the all-fiber laser velocimetry can adapt to the detection of ultraweak optical feedback and wide-range velocity measurements in various complex scenes.

Self-mixing thin-slice solid-state laser Doppler velocimetry with much less than one feedback photon per Doppler cycle.

The drastic shortening of a photon lifetime as compared with normal TEM00 operations has been shown to be associated with the formation of annular mode operations in a thin-slice Nd:GdVO4 laser with tilted laser diode end pumping. The 15 dB enhancement of the signal-to-noise ratio, owing to the shortened photon lifetime, has been demonstrated in the self-mixing laser Doppler velocimetry experiment in comparison with the TEM00 operations, where the minimum intensity feedback rate from a target to the laser for successful measurements was estimated to be -123 dB, which corresponds to 0.007 photon per Doppler cycle.

Long-haul self-mixing interference and remote sensing of a distant moving target with a thin-slice solid-state laser

An effective long-haul self-mixing interference effect has been observed in a thin-slice LiNdP₄O₁₂ (LNP) laser due to Doppler-shifted optical feedback from a distant target. The narrow spectral linewidth of the LNP laser, which was evaluated to be 16 kHz by heterodyne measurements, led to successful self-mixing laser Doppler velocimetry and vibrometry of targets placed 2.5 km away from the laser through single-mode optical fiber access.

Analysis of molecular dynamics of colloidal particles in transported dilute samples by self-mixing laser Doppler velocimetry

Colloidal particles in a liquid medium are transported with constant velocity, and dynamic light scattering experiments are performed on the samples by self-mixing laser Doppler velocimetry. The power spectrum of the modulated wave induced by the motion of the colloidal particles cannot be described by the well-known formula for flowing Brownian motion systems, i.e., a combination of Doppler shift, diffusion, and translation. Rather, the power spectrum was found to be described by the q-Gaussian distribution function. The molecular mechanism resulting in this anomalous line shape of the power spectrum is attributed to the anomalous molecular dynamics of colloidal particles in transported dilute samples, which satisfy a nonlinear Langevin equation.

Self-mixing laser Doppler velocimetry with a dual-polarization Yb:Er glass laser

Self-mixing laser Doppler velocimetry using a dual-polarization Yb:Er-doped phosphate glass laser is investigated. The two orthogonally polarized eigenstates emitted simultaneously by the laser oscillator are employed to perform a heterodyne detection, allowing a complete characterization of the lateral sidebands generated via the optical feedback around the beating peak. Two different experimental set-ups have been implemented: (i) direct feedback on one of the two modes or (ii) crosstalk between the two modes via a non-reciprocal Faraday element. The results are analysed and applications to velocity measurements are discussed. Finally, some considerations of the noise correlated in phase and in opposite phase on the two polarization states are also discussed.

Ultrahigh-sensitivity self-mixing laser Doppler velocimetry with laser-diode-pumped microchip LiNdP/sub 4/O/sub 12/ lasers

A self-mixing laser Doppler velocimetry (LDV) has been demonstrated with laser-diode-pumped microcavity LiNdP/sub 4/O/sub 12/ lasers. The feedback signal required for a usable measurement without using photon-correlation techniques was <1 photon per Doppler-beat cycle. An enhancement of LDV signals by resonant excitations of relaxation oscillations has been performed.

Two-dimensional vector LDV using laser diode frequency change and self-mixing effect

A new method of a laser Doppler velocimetry (LDV) for two-dimensional velocity measurement is described. Using a laser diode as a light source, the two velocity components of a light scattering object are simultaneously calculated from the beat frequencies in differential and self-mixing LDVs. By the laser diode frequency change due to the modulation of the injection current, the frequency offsets for the net Doppler beat signals are introduced. Both the magnitude and direction of the velocity components can be measured by the proposed method.