Wavelength conversion in a Ti:PPSMgLN channel waveguide

A periodically poled titanium (Ti)-diffusion waveguide in near-stoichiometric MgO:LiNbO₃ (SMgLN) was fabricated that exhibits a second harmonic generation (SHG) efficiency of 63%. The device shows very high resistance to photorefractive damage at room temperature. All optical wavelength conversion by difference frequency generation (DFG) has been demonstrated in a periodically poled SMgLN (PPSMgLN) with Ti-diffusion channel waveguides. The wavelength conversion efficiency was measured to be −7.3 dB with the pump power of 150 mW and the signal power of 50 mW at room temperature.     

High extinction ratio metal-insulator-semiconductor waveguide surface plasmon polariton polarizer

The attenuation characteristics of a multilayer metal clad GaAs-AlGaAs optical waveguide polarizer are theoretically investigated. The dispersion relations and field distribution of the multilayer structures are calculated for different geometrical parameters and material properties. The polarizer studied consists of a single mode finite/infinite metal clad GaAs-AlGaAs waveguide with a dielectric (SiO₂/Si₃N₄) buffer layer inserted between the metal and the waveguide.Conventionally, the TM polarized waves are found to exhibit an absorption peak at a particular buffer thickness (called critical buffer thickness).We shall show that the maximum TM absorption can be improved by a multiple factor up to 7 by choosing a buffer layer thicker than its critical value. This corresponds to an extinction ratio of 1470 dB for a polarizer length of 1 mm. Further, thicker buffers reduce the insertion losses and values as low as 0.1 dB can be obtained. The strong TM absorption in these structures is interpreted as resonant coupling of the guided mode to the lossy surface plasmon polariton supported by the thin metal film. Thicker buffer also reduces the TE losses (insertion losses) and hence increases the extinction ratio (ratio of TM to TE losses).This can be achieved by optimizing the buffer and the metal thicknesses. Another equally efficient polarizer can be designed by positioning a dielectric (same as buffer) layer (superstrate layer) above the metal film and then optimizing the buffer, metal and the superstrate thicknesses. We also show that the proposed polarizer with the superstrate layer is highly stable even when exposed to the extreme atmospheric changes.     

Design of short length and C+L-band mismatched optical coupler with waveguide weighted by the Blackman function

A mismatched optical coupler with waveguide weighted by the Blackman function is numerically investigated in the demand of short length, C+L-band, and low crosstalk. Utilizing the full factorial design, the structure parameters of coupling waveguide are obtained by beam propagation method. In the condition of crosstalk of −35 dB, the mismatched optical coupler with proper selected waveguide structure parameters is found to have a coupling length of 3.60 mm in the transmission wavelength ranges of C+L-band (1.53-1.61 μm). Obviously, the selection and design of waveguide structure are very important to satisfy the qualities of a mismatched optical coupler for the demand of short length, broad bandwidth, and low crosstalk.     

Short-length and broadband mismatched optical couplers with tapered Hamming function for C- and L-band

Mismatched optical couplers with variable widths of waveguide tapered by Hamming function are numerically investigated in the demand of short-length, broadband, and low crosstalk. We used global search algorithm and beam propagation method to seek optimal structure parameters of coupling waveguide. The coupler length is 3.6 mm within the C+L-band (1.53-1.61 μm) for variable widths of waveguide at crosstalk level of −35 dB. Comparison with constant width of waveguide, the constant width of waveguide has a coupler length of 4.4 mm and can only achieve −20 dB of crosstalk within the C-band (1.53-1.565 μm). Obviously, the waveguide with variable widths has the advantage over constant width for the demand of short-length, bandwidth, and low-crosstalk.     

Modify SRS-induced crosstalk with variety of fiber in SCM–WDM transmission links

In this paper, the SRS-induced crosstalk has been evaluated in a SCM–WDM communication links at different modulation frequencies and transmission lengths for variety of fiber. Results show that SRS-induced crosstalk dominates at low frequency. As the dispersion and effective area of fiber (A_eff) decreases, initially the crosstalk remains high and then it decreases with increase in modulation frequency. The present work shows that out of five different types of fiber, standard single mode fiber (SMF) has minimum crosstalk (−78 to −38) dB, (−55 to −33) dB and (−46 to −34) dB at modulation frequencies, transmission lengths and optical powers. Dispersion compensation fiber (DCF) has maximum crosstalk (−60 to −12) dB, (−37 to −12) dB and (−27 to −12) dB at modulation frequencies and transmission lengths.     

Er-Yb co-doped glass waveguide amplifiers using ion exchange and field-assisted annealing

Er³⁺-Yb³⁺ co-doped waveguide amplifiers fabricated using thermal two-step ion-exchange are demonstrated. K⁺-Na⁺ ion-exchange process was first carried out in pure KNO₃ molten bath, and then field-assisted annealing (FAA) was used to make the buried waveguides. The effective buried depth is estimated to be ∼3.4 μm for the buried FAA waveguides. With the use of cut-back method, the fiber-to-guide coupling loss of ∼4.38 dB, the waveguide loss of ∼2.27 dB/cm, and Er³⁺ absorption loss ∼5.7 dB were measured for a ∼1.24-cm-long waveguide. Peak relative gain of ∼7.0 dB is obtained for a ∼1.24-cm-long waveguide. The potential for the fabrication of compact optical amplifiers operating in the range of 1520-1580 nm is also demonstrated.     

Polarisation-insensitive strip-loaded waveguide for electro-optic modulators and switches

A polarisation-insensitive electro-optic (EO) waveguide consisting of a dye-doped TiO₂/SiO₂ slab and a SU-8 strip-loaded rib is designed and fabricated. By optimizing the refractive index and size of waveguide, a trade-off between polarisation-insensitive condition and large EO efficiency (optical field interaction with the EO material) is obtained. The average transmission loss of the waveguide is less than 2.0 dB/cm. A Mach-Zehnder (M-Z) interferometer intensity modulator based on this waveguide with excellent poling stability is fabricated and measured, exhibiting 7 V half-wave voltage with 1.8 cm EO interaction length and 2.7 cm total length. This strip-loaded structure is proved to be a valuable application in EO modulators and switches.     

Simulation and optimization of a polymer 2×2 multimode interference-Mach Zehnder interferometer electro-optic switch with push-pull electrodes

Structural model and design technique are proposed for a polymer 2×2 multimode interference-Mach Zehnder interferometer electro-optic (MMI-MZI EO) switch with push-pull electrodes. The electric field distribution is analyzed by the conformal transforming method and image method. To get the minimum mode loss and half-wave voltage, the parameters of the waveguide and electrodes are optimized, such as the core width, core thickness, buffer layer thickness, size of the MMI couplers and the modulating region, electrode thickness, electrode width, and electrode gap. Switching characteristics are analyzed, including the output power, crosstalk, and wavelength shift. Simulation results show that the half-wave voltage is 0.74 V, the optical 3 dB bandwidth is 12.66 GHz, and the crosstalk is less than −30 dB for the designed device.     

Study of near-infrared nonvolatile two-center holographic recording in LiNbO3:Fe:Rh crystal

The near-infrared nonvolatile holographic recording has been realized in a doubly doped LiNbO₃:Fe:Rh crystal by the traditional two-center holographic recording scheme, for the first time. The recording performance of this crystal has been investigated by recording with 633 nm red light, 752 nm red light and 799 nm near-infrared light and sensitizing with 405 nm purple light. The experimental results show that, co-doped with Fe and Rh, the near-infrared absorption and the photovoltaic coefficient of shallow trap Fe are enhanced in this LiNbO₃:Fe:Rh crystal, compared with other doubly doped LiNbO₃ crystals such as LiNbO₃:Fe:Mn. It is also found that the sensitizing light intensity affects the near-infrared recording sensitivity in a different way than two-center holographic recording with shorter wavelength, and the origin of experimental results is analyzed.     

Design and implementation of polarization filter for quantum states discriminator in optical quantum communication

In optical quantum communication, quantum state measurement is necessary. This paper proposes a new technique for realization of polarization filter based on planar lightwave circuit (PLC). This filter is used for quantum state discriminator in quantum communication and also as a Bell-state analyzer in quantum repeater. Electro-optics interferometer has been used in design and implementation of polarization filter. We use lithium niobate as a wafer material and Ti:LiNbO₃ for waveguide. Two directional couplers have been used in this device. The length and spacing of these directional couplers have been designed so that each polarization is routed in specific output. The proposed device has one input and two outputs. If polarization of the input photon is vertical, then this photon will appear in output 1, otherwise if the input photon has horizontal polarization, it appears in output 2. For vertical polarization input, the power overlaps integral (POI) shows that isolation between two outputs is 14.96 dB. As to horizontal polarization input, the isolation between two outputs is 13.8 dB. The designed polarization filter has length of 33 mm and width of 60 μm. This device is very suitable for use in integrated optics.     

Improved frequency response measurement method of half-wave voltage for phase modulator

In this paper, the improvement method of the traditional optical spectrum analyzer (OSA) is demonstrated to measure the half-wave voltage of LiNbO₃ phase modulator with low RF driving. Based on calculating the measured powers ratio of the carrier and first-order side band with optical spectrum analyzer (OSA), the phase modulation index is between 0.5 and 1.43 and the voltage of driving signal is lower than 0.796 V, the accurate characterization of the phase modulator is obtained. By using this method, the half-wave voltage of a LiNbO₃ phase modulator had been measured in the frequency range of 26 GHz. And, our experimental results showed a good agreement between the measured data and those provided by the manufacturer in 9 GHz frequency range. It is an accurate and easy method to measure the half-wave voltages of a phase modulators, especially for measuring high the frequency response.     

Dual-core leaky optical waveguide as an integrated-optic polarizer

We present a design of a dual-core leaky waveguide that can be used as an integrated-optic polarizer. The proposed polarizer works on the principle of mode filtering. The structure is characterized by two cores, namely core-1 and core-2 and a high index layer in the upper-most region, such a structure supports leaky modes. The leakage losses of the modes have been calculated by using the transfer matrix method (TMM). Single polarization operation is ensured by high differential leakage loss between fundamental TE and TM modes. We show TE-pass operation with 0.5 dB/mm loss in TE₀ mode and 13 dB/mm loss in TM₀ mode, and TM-pass operation with 0.36 dB/mm loss in TM₀ mode and 7.45 dB/mm loss in TE₀ mode. Besides single-polarization operation, single-mode operation of the structure is ensured by high leakage loss of all the other higher-order modes.     

Variational analysis of Er:Ti:LiNbO3 waveguide lasers based on Gaussian/two-half Gaussian mode field distribution

Starting with two dimensional, scalar wave equation, a variational equation was established for the fundamental TE and TM modes guided in Ti:LiNbO₃ waveguides on the basis of assuming a symmetric Gaussian mode field function in the width direction and two-half Gaussian trial functions in the depth direction. The controllable waveguide fabrication parameters, including channel width, diffusion temperature, initial Ti-strip thickness and diffusion time, dependent of fundamental mode size, effective pump area, coupling efficiency between pump and laser modes, and the coupling loss between a Ti:LiNbO₃ waveguide and a fiber were numerically calculated for Z-cut Er:Ti:LiNbO₃ channel waveguide lasers at three possible emission wavelengths 1532,1563 and 1576 nm and two possible pump wavelengths 1480 and 980 nm. The calculated results were compared with those of Gaussian/Hermite–Gaussian mode field distribution in detail.     

Broadband photoluminescence from pulsed laser deposited Er2O3 films

Photoluminescence (PL) with the bandwidth of 45 nm (1523-1568 nm at the level of 3 dB) was observed in amorphous Er₂O₃ films grown on to the quartz substrate by pulsed laser ablation of erbium oxide stoichiometric target. Optical transmission spectrum has been fitted to Swanepoel formula to determine the dispersion of refractive index and to extract resonance absorption peaks at 980 and 1535 nm. The maximum gain coefficient of 800 dB/cm at 1535 nm was estimated using McCumber theory and experimental spectrum of the resonance absorption. In 5.7 mm-long waveguide amplifier a theory predicts the spectral gain of 20 dB with 1.4 dB peak-to-peak flatness in the bandwidth of 31 nm (1532-1563 nm) when 73% of Er³⁺ ions are excited from the ground state to the ⁴I_13/2 laser level. Strong broadband PL at room temperature and inherently flat spectral gain promise Er₂O₃ films for ultra-short high-gain optical waveguide amplifiers and integrated light circuits.     

Optical waveguides in LiNbO<Subscript>3</Subscript> and stoichiometric LiNbO<Subscript>3</Subscript> crystals by proton exchange

The formation of optical planar waveguides in LiNbO₃ and stoichiometric LiNbO₃ crystals by proton exchange was reported. The prism-coupling method was used to characterize the dark-line spectroscopy at the wavelength of 633 and 1539 nm, respectively. The mode optical near-field outputs from proton-exchanged LiNbO₃ and SLN waveguides at 633 nm were presented. The mode field from stoichiometric LiNbO₃ (SLN) waveguide is lighter and more uniform than that from LiNbO₃ waveguide, which means the quality of the waveguide in SLN crystal is better than that of the LiNbO₃ waveguide. For proton-exchanged LiNbO₃ waveguides, the evolution of the refractive index profile with annealing was presented. The disorder profiles of Nb atoms in proton-exchanged LiNbO₃ waveguides were obtained by Rutherford backscattering/channeling technique. It is shown that the longer the exchange time, the larger the displacement of Nb atoms. Supported by the National Natural Science Foundation of China (Grant No. 10475052) and the Scientific Research Start-up Financing of Qufu Normal University     

Simulation and optimization of a polymer directional coupler electro-optic switch with push-pull electrodes

Structural model and design technique are proposed for a polymer directional coupler electro-optic switch with rib waveguides and push-pull electrodes, of which the electric field distribution is analyzed by the conformal transforming method and image method. In order to get the minimum mode loss and the minimum switching voltage, the parameters of the waveguide and electrode are optimized, such as the core with, core thickness, buffer layer between the core and the electrode, coupling gap between the waveguides, electrode thickness, electrode width and electrode gap. Switching Characteristics are analyzed, which include the output power, insertion loss, and crosstalk. To realize normal switching function, the fabrication error, spectrum shift, and coupling loss between a single mode fiber (SMF) and the waveguide are discussed. Simulation results show that the coupling length is 3082 μm, push-pull switching voltage is 2.14 V, insertion loss is less than 1.17 dB, and crosstalk is less than −30 dB for the designed device.     

Defect structure and optical damage resistance of Hf:Fe:LiNbO3 crystals

A series of Hf:Fe:LiNbO₃ crystals were grown by the Czochralski technique with various doping concentrations of HfO₂. Their defect structures were analyzed by the UV-visible absorption spectra and infrared absorption spectra. The optical damage resistance of Hf:Fe:LiNbO₃ crystals was measured by the photo-induced birefringence change and the transmitted light spot distortion method. The results show that the optical damage resistance ability of Hf:Fe:LiNbO₃ crystals enhances remarkably with the HfO₂ concentration increasing when the HfO₂ concentration is lower than its threshold concentration (4 mol%). However, when the HfO₂ concentration exceeds its threshold concentration, the optical damage resistance ability of the crystals returns to decrease. This unusual behavior is explained by using the photovoltaic field produced in the crystals.     

Growth and photorefractive properties of near-stoichiometric In:Fe:Cu:LiNbO3 crystals

The near-stoichiometric LiNbO₃ crystal co-doped with In₂O₃, Fe₂O₃, and CuO has been grown from a Li-rich melt (Li/Nb = 1.38, atomic ratio) by the Czochralski method in air atmosphere for the first time. The OH⁻ absorption spectra were characterized to investigate the structure defects of the crystals. The appearance of the 3506 cm⁻¹ absorption peak manifests that the composition of the grown crystal is close to the stoichiometric ratio. The photorefractive properties were also measured by the two-wave coupling experiments. The results show that the near-stoichiometric In:Fe:Cu:LiNbO₃ crystal has a larger refractive index change, higher recording sensitivity and larger two-wave coupling gain coefficient than those obtained in the congruent In:Fe:Cu:LiNbO₃ crystal under the same experimental conditions. The material of near-stoichiometric In:Fe:Cu:LiNbO₃ crystal is a promising candidate for blue photorefractive holographic recording.     

Polymer waveguide thermo-optic switches with −70 dB optical crosstalk

To reduce the crosstalk of the polymer waveguide optical switches, waveguide attenuators are integrated with the switch on the same substrate. The switch and attenuator shares a single connected electrode which is controlled by a single current source. Due to the simple structure of the integrated attenuator, the device length is reduced to 10 mm so as to provide low insertion loss of 0.8 and 1.1 dB for 1300 and 1550 nm, respectively. Further radiation of the remained optical signal on the switch-off branch is induced by the integrated attenuator so that the switching crosstalk is reduced to −70 dB with an applied electrical power of 200 mW. The low crosstalk is maintained for the environmental temperature range of −10 to 55 °C.     

Optimal design of coupling waveguide structure for adiabatic optical directional full couplers weighted by sin-square and raised-cosine functions

The optimal design of coupling waveguide structure for adiabatic optical directional full couplers (AODFC) based on weighted sin-square function (SSF) and raised-cosine function (RCF) are investigated in this paper. The coupling lengths with a crosstalk smaller than −30 dB at the operating wavelength of 1.57 μm resulted in a value of 3.0 and 2.4 mm for the SSF and RCF cases, respectively. Under the same level of crosstalk, the wavelength ranges obtained by weighted SSF and RCF could be varied about 1.33-1.70 μm and 1.27-1.70 μm with coupler lengths of 6.0 and 5.5 mm, respectively. Clearly, the AODFC weighted by RCF has the superior performances of both short coupling length and broad wavelength ranges.