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In this paper, we report the design and realization of 2-D focus scanning of the reflected beam by exploiting the frequency dispersion characteristic of reflectarrays operating at terahertz (THz) frequencies. In the first design, we choose four focal points on a circular path at four different frequencies ranging from 0.225 to 0.3 THz and 2-D beam scanning in the focal plane is achieved as frequency increases. To increase the scanning area, we design a dual-band reflectarray and 1-D scanning is achieved in each of the operating bands with different focal positions along the direction orthogonal to the scanning. The proposed reflectarrays are fabricated and their 2-D focus-scanning abilities are measured and there are good agreements between the simulation results and the measurements. The proposed designs demonstrate and expand further the potential applications of reflectarrays in fast THz imaging and detection. [ABSTRACT FROM AUTHOR]

Er3+-doping effect on electro-optic property of LiNbO3 crystal has been studied. A series of congruent LiNbO3 crystals homogenously doped with different Er3+ concentrations up to 2 mol% in melt were grown. Unclamped electro-optic coefficients r 13 and r 33 of these crystals were measured by Mach–Zehnder interferometry. The results show that both r 13 and r 33 reveal little dependence on Er3+ doping level. r13 hardly changes in the studied Er3+ doping level range 0–2 mol% within the experimental error of 3%. r33 reveals a degradation tendency with a rise in the Er3+ doping level, but the degradation is no more than 5% in the Er3+ doping level range studied. The small effect is consistent with previous result that the performance of an electro-optic device based on Ti4+-diffused Er3+-doped LiNbO3 waveguide is not influenced noticeably by Er3+-doping. A qualitative, comprehensible explanation is given for the small effect. [ABSTRACT FROM PUBLISHER]

We demonstrate electro-optic long period grating in Ti:LiNbO3 strip waveguide embedded in a planar waveguide. The device was fabricated starting from a Z-cut congruent plate with a technological process in sequence of planar waveguide, strip waveguide, SiO2 buffer layer, and periodical electrode. The rejection band of the device with a grating pitch 675 \mu \textm is observed at the 1.1–1.3- \mu \textm wavelength regime with a maximum contrast of 27 dB under the 360 V driving voltage. It is found that the band red-shifts linearly with the voltage increase. The shift is $\sim 160$ nm as the voltage is increased from 100 to 500 V and lasts 1 min. Accompanying the shift, the contrast reduces. The shift is attributed to different photorefraction effects in planar and strip waveguides. Higher than 360 V, the contrast decreases due to photorefraction effect and over-coupling phenomenon. [ABSTRACT FROM PUBLISHER]

Multimode near-stoichiometric (NS) Ti:LiNbO3 planar waveguide was fabricated by Ti-diffusion followed by post-Li-rich vapor transport equilibration. The crystalline phase, composition and guided modes in the planar waveguide were characterized. The refractive index profile in the waveguide is constructed from the measured mode indices using the inverse Wentzel–Krames–Brillouin method and correlated with the Ti profile obtained from secondary ion mass spectroscopy analysis. The results show that the LiNbO3 phase dominates in the waveguide layer and the waveguide has an NS composition. The index increase has a linear relation to Ti-concentration for both cases of ordinary and extraordinary rays, and the relationship is considerably different from those of some congruent or NS bulk materials or waveguides. A comparison of various congruent and NS bulk materials or waveguides allows to conclude that the relationship changes from congruent to NS composition, from one fabrication method to another and from bulk material to waveguide configuration. [ABSTRACT FROM PUBLISHER]

We report Sc3+-doped Ti:LiNbO3 (Ti:Sc:LN) strip waveguide fabricated by Ti-diffusion following homogeneous Sc3+-diffusion-doping in a Z-cut congruent substrate. We show that Sc3+-doping has little contribution to the substrate index and the Li2O out-diffusion was effectively suppressed. The refractive index increases in the waveguide layer are mainly contributed from the Ti4+ dopants. The waveguide well supports both transverse electric (TE) and magnetic (TM) modes, is single-mode at 1.5-μm wavelength, and has a loss of 1.4 dB/cm for the TE mode and 1.8 dB/cm for the TM mode. A secondary ion mass spectrometry study shows that the Sc3+-profile covers 80% ordinary refractive index profile and almost 100% extraordinary refractive index profile, and the 1/e Sc3+ concentration is above the threshold of photorefractive effect. Further, two-beam hologram recording experimental results verify the optical-damage-resistant feature of the waveguide. Highlights are given for fabrication of an optical-damage-resistant Ti:Sc:LN waveguide. [ABSTRACT FROM PUBLISHER]

Er3+ upconversion (UC) fluorescent characteristics of ErNbO4 phosphor were studied for temperature sensing purposes. This letter shows that the ErNbO4 phosphor emits more UC fluorescence than the Er3+-doped LiNbO3 single-crystal and Er3NbO7 phosphor. Both energy-transfer UC and excited state absorption play a role in the UC emission. Both the peak and integrated UC intensities decrease with the raised temperature, and the 530, 560, and 670 nm UC intensities reduce by \(\sim 26\) %, 42%, and 34%, respectively, with the temperature rise of only 57 °C from 21 °C, implying that the phosphor is applicable to temperature sensing based upon either the 560- or 670-nm emission. The feasibility to realize the temperature sensing is discussed from the application requirements of smartness and low-cost, suggesting that not only the single 560- or 670-nm band, but the combined 530- and 560-nm bands can be also utilized. [ABSTRACT FROM PUBLISHER]

Multimode Ti4+-diffused LiNbO3 planar waveguide doped with Sc3+ ions was fabricated by codiffusion of stacked Sc2O3 and Ti-metal thin film coated onto Z-cut congruent LiNbO3 substrate at 1060 °C in wet O2. The Ti4+-concentration was profiled by secondary ion mass spectrometry. The refractive index profile is constructed from measured mode index, and correlated with the Ti4+ profile. Other two related issues including the contribution of Sc3+-doping to substrate refractive index and Li2O out-diffusion were also studied. The results show that the Sc3+-doping has little contribution to the substrate index and the Li2O out-diffusion was effectively suppressed. The index change and Ti 4+-concentration follow an exponential relationship with a power index 0.54/0.79 for the ordinary/extraordinary ray. The relationship is similar to that of conventional Ti:LiNbO3 waveguide because of little contribution of Sc3+-doping to the substrate index and effective suppression for Li2O out-diffusion. Some considerations for fabricating an optical-damage-resistant Ti:Sc:LiNbO3 waveguide are given. [ABSTRACT FROM PUBLISHER]

We report a Ti:Er:LiNbO3 strip waveguide with high diffusion-doped surface Er3+ concentration. The waveguide was fabricated with a technological process in sequence of preparation of noncongruent, Li-deficient LiNbO3 substrate by performing Li-poor vapor transport equilibration treatment on a congruent Z-cut LiNbO3 plate, diffusion of 40-nm-thick Er metal film, and fabrication of 8-μm-wide Ti-diffused strip waveguide. The waveguide retains the LiNbO3 phase and shows the waveguiding characteristics similar to the conventional Ti:LiNbO3 waveguide. Secondary ion mass spectrometry study shows that the Er3+ diffusion reservoir was exhausted and the profile is Gaussian with a surface concentration two times larger than that of the conventional Ti:Er:LiNbO3 waveguide. The waveguide shows stable 1547-nm small-signal enhancement under the 1480-nm pumping without serious optical damage observed, and a 5-dB signal enhancement is obtained for the available coupled pump power of only 90 mW. A saturated net gain as much as 5 dB/cm is predicted theoretically. [ABSTRACT FROM PUBLISHER]

We report optical-damage-resistant Ti:Er:LiNbO3 strip waveguide with high diffusion-doped surface Er^3+ concentration. The waveguide was fabricated starting from a commercial X-cut congruent LiNbO 3 plate with a two-step technological process in sequence of simultaneous work of Er^3+ diffusion doping and Li-poor vapor transport equilibration treatment, and fabrication of 6-μm-wide Ti-diffused strip waveguide (Z-propagation). The waveguide retains still the LiNbO3 phase and has the waveguiding characteristics similar to the conventional Ti:LiNbO3 waveguide except with a larger loss due to the imperfection of waveguide. Secondary ion mass spectrometry study shows that the Er^3+ diffusion reservoir was exhausted and the profile is the desired Gaussian-type with a surface concentration 1.0 mol%, which is about two times larger than the value of conventional Ti:Er:LiNbO3 amplifier. Further optical characterization shows that the waveguide shows stable 1547 nm small-signal gain under the 980 nm pumping without serious photorefractive effect observed. An unsaturated gain 1.7 dB/cm is obtained for the available coupled pump power of 160 mW. With increased pump power, optimized Er ^3+ diffusion condition and degraded loss figure, a higher gain is expected. [ABSTRACT FROM PUBLISHER]

A three-segment structure L-band wavelength tunable multi-quantum-well Fabry–Pérot (FP) laser is realized and characterized. Wavelength-tuning is achieved by current injection into the unpumped segments. Depending on the pumping schemes, the lasing wavelength undergoes either red or blue shift. Red-shifting of the lasing wavelength is caused by band-gap shrinkage effect because of Joule heating. Blue-shifting of the lasing wavelength is mainly caused by band-gap filling effect. At room temperature, a tuning range of \sim12.8~nm is obtained. The tunable FP laser diode with simple architecture has potential applications in L-band optical communication systems. [ABSTRACT FROM PUBLISHER]

Refractive index in special optical-damage-resistant MgO-diffused near-stoichiometric (NS) Ti:LiNbO3 (Ti:LN) waveguide structure that a number of single-mode strip waveguides are embedded in a planar waveguide is profiled on the basis of the measured mode field profile of strip waveguide. In the NS strip waveguide, the Ti^4 +-induced extraordinary index increase studied follows a sum of two error functions in the width direction and a sum of two Gaussian functions in the depth direction. In the NS planar waveguide, the index increase follows a Gaussian function. On the basis of the established index profile model, the mode field profiles were calculated using variational method and compared with the measured data. The results show that the calculated mode sizes are in good agreement with the measured. In addition, the profile characteristics of Ti^4 +-concentration in both the strip and planar waveguides were analyzed by secondary ion mass spectrometry and are correlated with the refractive index increase. The result shows that the profile characteristics of the index increase and Ti^4 +-concentration are similar for both the strip and planar waveguides and there exists a linear relationship with a slope of \sim2.0\,\times\,10^- 3(mol\\%)^- 1. [ABSTRACT FROM PUBLISHER]

We have measured the polarized visible and near-infrared, and unpolarized mid-infrared (2.7 \mum) emission spectra of photorefractive-damage-resistant locally Er-Mg-doped near-stoichiometric (NS) Ti:Mg:Er:LiNbO3 strip waveguide, fabricated on an X-cut initially congruent LiNbO3 substrate in sequence by local Er doping in air, Mg-Ti pre-diffusion in wet O2 and post Li-rich vapor transport equilibration treatment. From the measured emission spectra, the emission and absorption cross section spectra were calculated based upon McCumber theory. The spectroscopic features are discussed in comparison with the spectra recorded from the area outside the waveguide, and with the previously reported results of bulk-doped NS Er:Mg:LiNbO3 crystals and congruent Er:LiNbO3 bulk material or Ti:Er:LiNbO3 waveguide structure. The results show that the spectra of the NS waveguide are traditional and reveal small differences from those spectra outside the waveguide in spectral shape, polarization dependence, as well as cross section values. In contrast, the cross section values of the NS waveguide show considerable differences from those of bulk-doped NS material and congruent bulk material or waveguide structure. The 552, 673, 996, and 1531 nm emission lifetimes of the Er^3+ ions outside the waveguide were also measured, and found to be comparable to the results of the bulk-doped NS crystal and the congruent bulk material or waveguide structure. [ABSTRACT FROM PUBLISHER]

We report that the polarization property of \Er^3+ transition in \LiNbO3 waveguide or bulk material is closely related to the incident direction of excitation beam relative to the optical axis of the crystal. It is found that the 1.5-\mu\m transition is highly \sigma- (\pi-) polarized as the excitation direction is perpendicular (parallel) to the optical axis, and the 0.67-\mu\m transition reveals a reversed effect. Incident perpendicularly (parallel), the transition is highly \pi- (\sigma-) polarized. The effect is found to be independent of the polarization state and wavelength of the excitation light. It is attributed to the selective Er site excitation. [ABSTRACT FROM PUBLISHER]

A special structure of a strip Ti : \LiNbO3 waveguide embedded in a planar Ti : \LiNbO3 waveguide is proposed to fabricate a long-period waveguide grating (LPWG). Its applicability is demonstrated by fabricating a long-period grating on the waveguide surface using usual photoresist. We show that an LPWG device based on such a special structure is thermally stable. [ABSTRACT FROM PUBLISHER]