Yingwu Lian

Self‐pumped phase conjugation with a new mechanism in KTa1−xNbxO3:Fe crystals

A new model is proposed to explain the origin of the self‐pumped phase conjugation (SPPC) that we have observed in KTa1−xNbxO3:Fe crystals. In this model SPPC is generated by both four‐wave mixing and backscattering processes rather than by only one of them. The advantage of this kind of phase conjugator is also discussed.

PHOTOREFRACTIVE PROPERTIES AND SELF-PUMPED PHASE CONJUGATION OF TETRAGONAL FE-DOPED KTA1-XNBXO3 CRYSTAL

This letter reports the two‐wave mixing experimental results of tetragonal Fe‐doped KTa1−xNbxO3. A 7.3 cm−1 gain coefficient and 42% diffraction efficiency have been obtained. For the first time internally self‐pumped phase conjugation is being observed in this crystal. At room temperature, the conjugate reflectivity measured is 21%.

Mechanism transformation with wavelength of self-pumped phase conjugation in BaTiO 3 :Ce

We report what is to our knowledge the first observation of mechanism transformation with wavelength of self-pumped phase conjugation (SPPC) in BaTiO(3):Ce crystals in the near infrared. It was found that the SPPC mechanism changed from a backscattering and four-wave-mixing mechanism to a cat mechanism when the pump wavelength was varied from 706 to 770 nm. In addition, SPPC was obtained when the pump beam entered the crystal by the +c face, and a variation with wavelength of optical beam paths in the crystal was also observed.

Studies on formation mechanisms of self-pumped phase conjugation in BaTiO 3 :Ce crystals at wavelengths from 570 to 680 nm

Formation mechanisms of self-pumped phase conjugation (SPPC) in two BaTiO3:Ce crystals with different doping concentrations have been experimentally studied. The pump beam was from dye lasers, with an overall tunable wavelength region lying between 570 and 680 nm. Variations of the optical beam path patterns as well as reflectivity of the phase conjugators with respect to the incident angle, the incident position, and the wavelength of the pump were investigated in two configurations. One configuration corresponds to that in which the pump enters an a face of the crystal; the other to that in which the pump enters the +c face of the crystal. It was found that SPPC is sometimes established in a way similar to that of a cat phase conjugator, but usually without the formation of a clear beam path loop. Sometimes SPPC is established by a stimulated photorefractive backscattering and four-wave mixing mechanism. In these cases, however, the crystal corners as well as the scattering centers on the crystal faces usually play an important role by seeding the stimulated photorefractive backscattering processes. In other cases SPPC is established by both the cat and the stimulated photorefractive backscattering and four-wave mixing mechanisms.

Theory of a “stimulated photorefractive backscattering and four-wave mixing” self-pumped phase conjugator

Abstract We give a theoretical analysis of a “stimulated photorefractive backscattering and four-wave mixing” (SPB-FWM) self-pumped phase conjugator (SPPC) in which the generation of the phase conjugate wave relies on both stimulated photorefractive backscattering (SPB) and four-wave mixing (FWM) processes. The two pumping beams in the FWM process are, respectively, the forward-propagating beam of which the generation is seeded by the fanning effect, and the backward-propagating beam that is generated by the SPB process. Assuming that the SPB interaction is seeded by the scattering centers in the crystal, we have calculated the threshold four-wave mixing coupling strength (γ 1 l 1 ) th as a function of the SPB two-wave mixing coupling strength (γ 2 l 2 ) corresponding to the 2k grating and of the reflectivity R ′ of the scattering centers. Above threshold, we have also calculated the phase-conjugate reflectivity with a numerical method. Our calculations show that the threshold as well as the reflectivity of a SPB-FWM SPPC strongly depend on γ 2 l 2 and R ′. It is also shown that a high phase-conjugate reflectivity can be easily realised in doped barium titanate crystals which are shown to have much larger γ 1 and γ 2 than an undoped one.

Influences of iron doping on the photorefractive properties of KTa1−xNbxO3 crystals

The influences of iron doping on the photorefractive properties of KTa1−xNaxO3 (KTN) crystals were studied. The two‐wave‐mixing gain coefficients and the grating formation rates were measured in undoped and Fe‐doped KTN samples. Photorefractive parameters, such as the effective carrier number density, were determined for these samples. It was shown that the gain coefficient increases significantly with the doped Fe content, but the grating formation rate decreases with it, and that all the influences of iron doping originate mainly from the variation of the effective carrier number density.

Self-pumped phase-conjugation properties of cerium-doped BaTiO 3 crystals in the near infrared

Studies of a new Ce-doped BaTiO(3) crystal as an efficient self-pumped phase conjugator in the near infrared are presented. The internal beam production during the establishment of self-pumped phase conjugation (SPPC) is closely observed, and the corresponding mechanisms of SPPC are identified accordingly. When the incident beam is in an a-face-incidence geometric arrangement, it is observed for the first time that SPPC mechanism has a transition from stimulated photorefractive backscattering and four-wave mixing to total internal reflection when λ is varied from 706 to 733 nm. Variations of the SPPC reflectivity with the λ, incident angle, and position of the input beam have been investigated, and a high reflectivity of approximately 80% has been obtained. In addition, SPPC reflectivity of approximately 40% is obtained when the incident beam enters the crystal by the +c face, and a variation with λ of optical beam patterns in the crystal has also been observed. Qualitative explanations of the phenomenon of SPP mechanism transition with λ that will be very useful in practical applications are given.

Mechanism transition of self-pumped phase conjugation in KTa1 −xNbxO3:Fe crystals

Mechanism transitions of Self-Pumped Phase Conjugation (SPPC) with wavelength and doping concentration are observed in KTN:Fe (KTa1 −xNbxO3:Fe withx = 0.48) crystals. The SPPC mechanism in KTN: Fe (0.4 wt. %) crystal transforms from “Stimulated Photorefractive Backscattering and Four-Wave Mixing” (SPB-FWM) to “cat” (or total internal reflection) as the wavelength increases from 514.5 nm to 620 nm. SPPC at 514.5 nm is formed with the “cat” mechanism in a 0.2 wt. % doped KTN:Fe crystal, while with the SPB-FWM mechanism in a 0.4 wt. % doped one. These mechanism transitions are discussed with respect to the dependence of the backscattering gain coefficient of the crystals on wavelength and doping concentration.

Dynamics of photorefractive diffraction in KTa/sub 1-x/Nb/sub x/O/sub 3/ crystals

Although the photorefractive properties of KTaNbO/sub 3/ (KTN) crystal, one of the first identified photorefractive crystals, have been investigated in more detail in recent years, its photorefractive diffraction dynamics have been rarely studied. However, these studies might be important for understanding the mechanism of the photorefractivity including the need for a complicated photorefractive model and the influences of the internal dc electric field induced during writing with an applied field. In this presentation, our experimental investigations of photorefractive diffraction dynamics during writing, reading and erasing in a cubic KTN crystal will be reported systematically. Here, we studied not only the diffraction from the first-order grating but also the diffraction from the second-order grating.<<ETX>>

Studies of photorefractive diffraction dynamic in paraelectric KTa1−xNbxO3 crystals

The dynamics of photorefractive diffraction in paraelectric KTa1−xNbxO3 crystals during writing, reading, and erasing have been studied systematically. Experimental observations related not only to the diffraction from the first‐order photorefractive grating but also to that from the second‐order photorefractive grating, and where a writing beam with spots smaller than, as well as larger than, the entrance surface of the crystal was used. All the complicated behaviors observed have been explained qualitatively. An electron‐hole transport model and a nonequilibrium screening model have been applied, and the existence of an internal dc electric field induced during writing simultaneously by an optical field and an externally applied field has been suggested and demonstrated. The significant influence of this induced field is indicated.