Gain Gratings Research Articles

Single-Frequency Continuous-Wave Self-Sweeping Fiber Laser Based on Separated Gain and Absorption Dynamics Gratings

A new cavity scheme for a self-sweeping fiber laser with separated gain and absorption dynamics gratings is presented. The scheme is experimentally realized in an Er-doped ring fiber laser generating in the continuous-wave (CW) regime near the wavelength of 1604 nm. Switching between single longitudinal mode stabilization and wavelength self-sweeping regimes is demonstrated by controlling the intracavity losses in the laser. The pump power and optimization of the intracavity losses made it possible to demonstrate record sweeping range of 2.6 nm in a single-frequency self-sweeping regime in the telecommunication L-band.

原子介质中的近共振增益光栅

原子介质中的近共振增益光栅

One- and two-dimensional electromagnetically induced gratings in an Er3+ - doped yttrium aluminum garnet crystal

A coherently prepared Er3+-doped yttrium aluminum garnet (YAG) crystal with a four-level ionic configuration is exploited for realizing one-dimensional (1D) and two-dimensional (2D) electromagnetically induced gratings (EIGs). Owing to the probe gain induced by the incoherent pump, the diffraction efficiency of the crystal grating, especially the first-order diffraction, can be significantly improved via increasing the incoherent pumping rate or decreasing the probe detuning. The enhancement of the grating diffraction efficiency originates from the interference between the gain and phase gratings. It is also demonstrated that the diffraction of the crystal grating can be dynamically controlled via tuning the intensity and detuning of the standing-wave driving field or the concentration of Er3+ ion. More importantly, the probe energy of the diffraction side lobes around the central principle maximum is comparable to that of the first-order diffraction field for small driving intensity or large driving detuning. Our scheme may provide a possibility for the active all-optical control of optical switching, routing and storage in fiber communication wavelengths.

Spectral characterization technique of self-organized distributed feedback in a self-sweeping fiber laser.

We report on spectral characterization technique of self-organized dynamical distributed feedback (DDFB) in a self-sweeping Yb-doped fiber laser. The DDFB is originated from gain and refraction index gratings formed (recorded by standing waves) in the laser's active medium and dynamically changes during frequency self-sweeping operation. Dynamic nature of the feedback requires fast characterization (reading) of corresponding reflection spectrum. The reading process can be separated from the recording one in time due to sufficiently long characteristic time of the dynamical gratings. The DDFB spectra are measured during off-state of the self-sweeping laser with a tunable probe radiation near the dynamical grating reflection maximum. The spectra reconstructed in a range of ~1 GHz from a sequence of shorter probe wave scans have narrow sharp peak with width of ~50 MHz and reflectivity of about 0.1%. A good agreement between theory and experimental results is demonstrated.

High-resolution analysis of the wavelength stopping effect in a self-sweeping fiber laser

We experimentally investigate for the first time, to the best of our knowledge, wavelength stopping, as well as slow reverse sweeping regimes in a self-sweeping fiber laser with a high temporal and spectral resolution, with a heterodyne technique. The experiments have shown that reverse sweeping, in fact, is a drifting series of short normal scans consisting of 5–7 pulses and there is no drift of central wavelength in the case of wavelength stopping. An effective spectral width of the mode set of the investigated fiber laser can be estimated as ~0.2 and ~30 MHz in the case of normal and reverse sweeping, respectively. An influence of absorption as well as gain and refractive index dynamical gratings on the wavelength stopping effect is also experimentally demonstrated.

Control of electromagnetically induced grating by surface plasmon and tunneling in a hybrid quantum dot–metal nanoparticle system

We investigate the electromagnetically induced grating (EIG) in a quantum dot–metal nanoparticle (QD-MNP) hybrid system. The EIG can be controlled and improved by the surface plasmon effect and the interdot tunneling effect between quantum dots. By manipulating the tunneling effect and the QD-MNP distance, not only the first-order diffraction intensity of the grating can be efficiently enhanced, but also the EIG can be switched from the absorption grating to the gain grating. Almost two times the first-order diffraction efficiencies can be achieved in the gain gratings compared with the absorption gratings.

Narrow-Strip Single-Longitudinal-Mode Laser Based on Periodic Anodes Defined by i-Line Lithography

A low-cost regrowth-free narrow-strip single-longitudinal-mode diode laser, based on periodic anodes defined only by i-line lithography is proposed. With the periodic anodes separated by surface insulated grooves acting as a 44th-order gain grating, a high gain contrast is achieved without introducing effective index coupling. The CW output power of the device without facet coatings is up to 100.9 mW/facet at 980.63 nm in single-longitudinal-mode operation with a single facet having a power conversion efficiency of up to 18.6% and a slope efficiency of 0.53 W/A. The highest side-mode suppression ratio is 45 dB with a linewidth below 889 MHz, at 350 mA. The beam parameter product, M ${\,}^{2}$ , in the slow axis is 1.85, achieving a near diffraction-limited beam quality. A novel method for easy surface gratings fabrication is first presented, wherein, 4-μm-width periodic surface grooves, acting as waveguide ridges, are defined once by i-line lithography only without regrowth, nanoscale gratings fabrication, or facet coating. Owing to the low-cost fabrication methods and good performances, our single-longitudinal-mode devices can be used for widespread practical applications.

基于增益光栅的单频自调Q激光器单频稳定性研究

基于增益光栅的单频自调Q激光器单频稳定性研究

基于增益光栅的注入锁定单频自调Q激光器

基于增益光栅的注入锁定单频自调Q激光器

Index and gain gratings in Nd-YVO4 - Applications to speckle vibrometry and photoacoustic detection

We perform wave mixing and dynamic holography by exploiting gain saturation in diode pumped laser media like Nd-YVO4. Such crystals offer the possibility of performing adaptive gain interferometers with a sub ms response time, typically between 100- 200μs. This temporal response is of great interest for applications such as speckle interferometry and acousto-optic detection for tissue imaging in biophotonics. In this work we operate in new conditions of high sensitivity with a linear response due to a refractive index contribution in the gain media. It originates from the wavelength mismatch of about 0.1 nm between the incident Nd-YAG probe laser and laser transition in diode pumped Nd-YVO4 amplifier. This permits the recording of two types of volume holograms respectively a gain and a phase volume holograms. It is the index grating which provides a linear response to incident low phase modulation at frequencies higher than 10 kHz. Experimental results confirm this behavior for adaptive interferometry and speckle vibrometry. In addition, the fast response makes the two wave mixing interaction in Nd-YVO4 very interesting for the optical detection of large bandwith photoacoustic waves excited by a pulsed ns Q-Switch laser at 532 nm. Temporal photoacoustic signals and images of small objects behind a scattering media will be shown.

Coherent control of light-pulse propagation in a Raman induced grating

We study light-pulse propagation in a dynamically controllable periodic structure (grating) resulting from Raman interaction of a weak probe pulse with a standing-wave pump and a second control laser field in N-type four-level atomic media. The grating is induced due to periodic spatial modulation of the Raman gain in a standing pump field (Raman gain grating). We show that it is possible to control both the probe pulse amplitude and the group velocity of the pulse from subluminal to superluminal by varying the pump or control field. Such a grating is of interest for all-optical switches and transistors.

Retrodirective tracking of a moving target using phase conjugate light generated in a Fabry–Pérot Nd:YAG laser

This report describes the retrodirective tracking of a target moving at a constant speed using a Fabry–Pérot laser resonator-type self-starting phase conjugator. Three-dimensional pointing ability was confirmed with this system: the generated phase conjugate light can focus at a target position. To track the moving targets, target-reflected light is Doppler-shifted, which causes wash-out of the gain grating. To address this problem, numerical analysis and experiments showed that shortening the pulse width is an efficient means of eliminating the gain grating wash-out effect. For the measured target speeds of 0–80 mm/s, the phase conjugate light output is unaffected when the pulse width of 18 ns was used, in contrast to the case of 400 μs pulse width, where the phase conjugate light output rapidly decreased as the target speed approached 10 mm/s. Numerical simulation revealed that phase conjugate light can even be generated for fast-moving targets such as space debris in low-earth-orbit with pulses of several tens of picoseconds. Unlike conventional mechanical gimbal steering methods, this system is applicable for wireless energy transfer to rapidly moving objects.

Refractive index and gain grating in Nd:YVO4: application to speckle vibrometry and photoacoustic detection.

Probing local displacements on a scattering surface can be achieved using an adaptive interferometer. The photorefractive-crystal-based interferometer is popular, but alternatives exist such as an adaptive gain interferometer. Such setups take advantage of the nonlinear phenomena in laser media. Because of the gain saturation, it is possible to write a gain hologram and a refractive index hologram to achieve an adaptive interferometer with a linear response. In addition, laser-media-based setups have a fast response time (≤100 μs), which makes them interesting for applications such as detection of photoacoustic waves in living biological samples.

Half-spectral unidirectional invisibility in non-Hermitian periodic optical structures.

The phenomenon of half-spectral unidirectional invisibility is introduced for one-dimensional periodic optical structures with tailored real and imaginary refractive index distributions in a non PT-symmetric configuration. The effect refers to the property in which the optical medium appears to be invisible, both in reflection and transmission, below the Bragg frequency when probed from one side and above the Bragg frequency when probed from the opposite side. Half-spectral invisibility is obtained by a combination of in-phase index and gain gratings whose spatial envelopes are related to each other by a Hilbert transform.

Self-Q-switching of a loop laser cavity with a self-pumped four-wave phase-conjugate mirror in an active laser medium

Self-Q-switching of a loop laser cavity with a self-pumped four-wave phase-conjugate mirror in an active laser medium with the use of a Faraday isolator or a passive Q-switch is studied theoretically. It is found that, in contrast to passive Q-switching of an ordinary cavity, a passive Q-switch in a loop phase-conjugate cavity serves as an auxiliary unit that increases the self-Q-switching efficiency by gain gratings in the active laser medium, since it decreases the difference in the intensities of waves that write a phase-conjugate mirror in the active medium, which is similar to the use of a Faraday isolator. However, the passive Q-switch operates alternating the highly efficient writing of a phase-conjugate mirror in the closed state with the highly efficient generation of a giant laser pulse in the open state, which stabilizes the period of repetitive laser pulses.

Holographic self-Q-switching of Nd:YAG all-solid-state lasers with Cr:YAG saturable absorber

The study of the generation characteristics of Nd:YAG lasers with multiloop self-organizing phase conjugate cavities using a passive Q-switch based on a Cr:YAG crystal is carried out. It is shown that Q-switching of the phase conjugate cavity is caused not only by absorption saturation of the passive Q-switch, but also by modulation of diffraction efficiency of holographic saturable gain gratings in the Nd:YAG active laser medium.

Bragg holography in active semiconductor microcavities

High-speed dynamic holography is studied numerically and experimentally in broad-area InGaAs/InP multiple-quantum-well asymmetric Fabry–Perot microcavities with optically pumped gain gratings operating near the lasing threshold. Two numerical approximation methods are developed to predict Bragg diffraction effects in these active microcavities. The Cavity Raman–Nath method assumes multipass Raman–Nath diffraction, while the Cavity Bragg method assumes an effective medium. Both methods predict that Raman–Nath diffraction in a micrometer-thick cavity resonator can exhibit Bragg-like selectivity for angles near the Bragg angle. The angular and spectral selectivities are studied experimentally in optically pumped free-carrier gratings using a tunable 1.55 μm laser in a nondegenerate four-wave mixing configuration. The device operates at gigahertz speeds with a transient diffraction efficiency near 40%.

Generation regimes of a pulsed Nd:YAG laser with transverse LED pumping and multiloop self-pumped phase-conjugate cavity

The generation regimes in a pulsed Nd:YAG laser with transverse LED pumping and multiloop self-pumped phase-conjugate cavity on the gain gratings are studied. The differential efficiency of laser is 27% in the free-running regime at a pulse energy of up to 1 J and quality parameter M2 of no greater than 1.5. The pulse energy under passive Q-switching is no less than 60% of the pulse energy in the free-running regime at the same beam quality. The generation of the narrow-band radiation is demonstrated. A generation band of no greater than 1.2 GHz corresponds to the primary single-frequency high-power laser pulse in the free-running mode under conditions for self-Q-switching on the gain gratings. When additional elements (F2−:LiF and Cr4+:YAG crystals) are introduced in the optical scheme of the phase-conjugate cavity, similar narrowband single-mode generation is observed in the passive Q-switching regime as a pulse train or monopulse. The laser pulse power is up to 2 MW at a pulse duration of 20 ns.

PT-symmetric microring laser-absorber.

The lasing and coherent perfect absorption (CPA) properties of PT-symmetric microrings with mixed index and gain gratings, externally coupled to a bus waveguide, are theoretically investigated. For a complex grating at the PT-symmetry breaking point, perfect unidirectional (either clockwise or counterclockwise) laser emission can be realized, but the grating does not discriminate longitudinal modes and CPA can not be simultaneously achieved. Above the grating the PT-symmetry breaking point, single-mode emission, and simultaneous CPA can be obtained, with unbalanced and controllable excitation of clockwise and counterclockwise modes in the ring.

All-solid-state Nd:YAG lasers with self-pumped multiwave-mixing phase conjugate cavities

Several configurations of the laser self-pumped phase-conjugate cavities with multiwave mixing directly in the laser medium based on a single Nd:YAG laser element pumped by multikilowatt laser diode 2D stacks are experimentally studied and compared. In the laser configurations with six- and eight-wave mixing there are generated a high-intensive self-Q-switched 200-ns laser pulse followed by low-intensive free-running lasing. An increase of the number of waves taking part in multiwave mixing from six to eight leads to increase in the laser maximum output energy and optical-to-optical efficiency from 0.55 up to 1.1 J and from 8.3 up to 18% respectively. In the laser configuration with ten-wave mixing the energy characteristic is similar to the same in the laser configuration with eight-wave mixing, but the temporal and spectral characteristics are different from the same in the laser configurations with six- and eight-wave mixing. In last laser configuration not only the first pulse, but also all the train of pulses is generated in high-intensive TEM00 diffraction-limited (M 2 = 1.3---1.5) single-frequency self-Q-switched mode via writing and erasing the gain gratings by the intracavity radiation.