Bandwidth tuning of hybrid liquid-crystal Šolc filters based on an optical cancelling technique

We demonstrate that in addition to their role in tuning the wavelength of an N-stage hybrid liquid-crystal Šolc filter, liquid-crystal cells can also be used to vary the transmission bandwidth of such filter around any of the tuned wavelength. This bandwidth tuning is based on the variation of the number of stages by what we call here an “optical cancelling technique”. This is achieved by varying the birefringence of the liquid-crystal cells whose optical path difference switches between two particular values. We show that for a 10-stage filter and at λ_i = 1.532 μm, the calculated 3-dB bandwidth varies from 2.6 to 11.8 nm when the number of “optically-cancelled” hybrid plates increases from 0 to 8. During the tuning process, the contrast ratio remains equal to that of the equivalent classical Šolc filter.     

Fast and wide-range continuously tunable Šolc-type filter based on periodically poled LiNbO3

We present a fast and wide-range continuously tunable Šolc-type filter based on periodically poled LiNbO₃ (PPLN) in this paper. The filter is formed in PPLN by applying a biased dc electric field along the y-axis, and the tuning of a transmitted central wavelength is realized by applying another dc electric field along the z-axis. The numerical results demonstrate that the tuning range covers as much as 16 nm, and the dependence of the transmitted central wavelength shift on the control electric field, shows a nearly linear relation with a tuning rate of 0.95 kV/mm per nm. PACS 42.65.-k; 78.20.Jq; 42.79.Ci     

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.     

Widely phase-matched tunable difference-frequency generation in periodically poled LiNbO3 crystal

We report on the development of a laser source in the mid-infrared spectral region based on difference-frequency generation (DFG) in a periodically poled LiNbO₃ (PPLN) crystal. Continuously tunable coherent radiation from 2.75 to 4.78 μm was produced by optical parametric interaction between a diode-pumped monolithic continuous-wave (CW) Nd:YAG laser operating at 1.064 μm and a CW Ti:Sapphire laser tunable from 767 to 871 nm. Temperature-dependent quasi-phase-matched DFG wavelength acceptance bandwidth was studied and characterized. An empiric formula is given to estimate the phase-matched wavelength acceptance bandwidth as a function of the crystal temperature at Λ = 22.5 μm. A large frequency scan of 128 cm⁻¹ (about 78 cm⁻¹ above 1 μW) near 4.2 μm was achieved. The whole absorption spectrum of the P and R branches of the ν₃ band of atmospheric carbon dioxide has been recorded with a single phase-matched frequency scan.     

Tolerance and tuning properties of the optical parametric processes using periodically poled RbTiOAsO4

The maximal tolerance parameters of poling period and phase-matching, temperature in second harmonic generation (SHG) using periodically poled RbTiOAsO₄(PPRTA) as a function of the fundamental wavelength are investigated theoretically. The tolerance of the poling period ΔΛ of PPRTA is found larger than that of PPLN and PPKTP when the fundamental wavelength is beyond 2 μm, which reaches its maximum ΔΛ_max for PPRTA at a fundamental wavelength of 2.7433 μm. However, the tolerance for the phase-matching temperature ΔT of PPRTA is found smaller than that of PPLN and PPKTP with an exception that PPRTA has a larger tolerance of the temperature or a larger temperature phase-matching bandwidth at fundamental wavelength of 2.2474 μm, where the maximum of ΔT(ΔT_max) is obtained. Furthermore, the tuning characteristics of the optical parametric processes using PPRTA for collinear quasi-phase-matching (CQPM) is analyzed. The combination of temperature tuning and poling period tuning enables a quasi-continuous wavelength tuning range of 1493.2-1593.7 nm for the signal and 3201.8-3699.2 nm for the idler, where poling period of 39 μm, 39.5 μm and 40 μm and a temperature between 20 and 120° have been employed in the corresponding theoretical analysis.     

Design of tunable asymmetric directional coupler filter using periodically segmented Ti:LiNbO3 waveguides

A tunable wavelength filter is designed using Ti:LiNbO₃ asymmetric directional coupler. One arm of the directional coupler is made of continuous waveguide while the other arm is periodic segmented. This segmentation reduces the lithographic steps and hence the fabrication cost required for such asymmetric coupler based filter. The propagation constant of periodical segmented and continuous waveguides are analysed using Effective-Index-based Matrix Method. The tunability is achieved using electro-optic property of LiNbO₃.     

基于光纤环形腔激光器的可调谐全光波长转换器的研究

在光纤环形腔激光器中引入周期极化反转铌酸锂(PPLN)光波导，用该激光器产生的连续光作为抽运光和控制光，使其与外加的信号光发生非线性效应实现可调谐波长转换.介绍了基于准相位匹配的PPLN光波导中的和频与差频级联型全光波长转换器的基本原理.对抽运光、信号光、控制光以及转换光的光功率随着PPLN光波导的变化进行了模拟.还对转换效率随着转换光波长的变化进行了数值计算.实验验证了该波长转换器的可调谐性. 关键词： 周期极化反转铌酸锂 和频与差频 可调谐全光波长转换 光纤环形腔激光器     

Few‐cycle,broadband, mid‐infrared optical parametric oscillator pumped by a 20‐fs Ti:sapphire laser

A few‐cycle, broadband, singly‐resonant optical parametric oscillator (OPO) for the mid‐infrared based on MgO‐doped periodically‐poled LiNbO₃ (MgO:PPLN), synchronously pumped by a 20‐fs Ti:sapphire laser is reported. By using crystal interaction lengths as short as 250 µm, and careful dispersion management of input pump pulses and the OPO resonator, near‐transform‐limited, few‐cycle idler pulses tunable across the mid‐infrared have been generated, with as few as 3.7 optical cycles at 2682 nm. The OPO can be continuously tuned over 2179‐3732 nm (4589‐2680 cm^‐1) by cavity delay tuning, providing up to 33 mW of output power at 3723 nm. The idler spectra exhibit stable broadband profiles with bandwidths spanning over 422 nm (FWHM) recorded at 3732 nm. The effect of crystal length on spectral bandwidth and pulse duration is investigated at a fixed wavelength, confirming near‐transform‐limited idler pulses for all grating interaction lengths. By locking the repetition frequency of the pump laser to a radio‐frequency reference, and without active stabilization of the OPO cavity length, an idler power stability better than 1.6% rms over >2.75 hours is obtained when operating at maximum output power, in excellent spatial beam quality with TEM₀₀ mode profile. Photograph shows a multigrating MgO:PPLN crystal used as a nonlinear gain medium in the few‐cycle femtosecond mid‐IR OPO. The visible light is the result of non‐phase‐matched sum‐frequency mixing between the interacting beams.     

Electro-optic tuning of a periodically poled LiNbO3 optical parametric oscillator and mixing its output waves to generate mid-IR tunable from 9.4 to 10.5 μm

We demonstrate electro-optic spectral tuning in a pulsed periodically poled LiNbO₃ (PPLN) optical parametric oscillator (OPO) in ns regime. A 3-cm-long LiNbO₃ crystal is segmented in three equal sections; the outer sections are periodically poled. The center segment is of single domain whose refractive index is changed by electro-optic effect. Applying voltage from 0 to −5000 V, the OPO signal and idler waves are tuned from 1.932 to 1.912 μm and 2.368 to 2.40 μm, respectively. The signal and idler waves obtained are difference-frequency-mixed in a 10 mm long AgGaS₂ crystal to produce mid-IR tunable from ∼10.5 to ∼9.4 μm, which matches the tuning range of a CO₂ laser.     

In‐band pumped Ti:Tm:LiNbO3 waveguide amplifier and low threshold laser

The fabrication by diffusion doping and a detailed optical investigation of a Ti:Tm:LiNbO₃ waveguide amplifier and of a Fabry‐Pérot type Ti:Tm:LiNbO₃ laser are reported. Both devices are in‐band pumped by a laser diode at 1650 nm. The wave‐guide amplifier shows broad‐band optical gain in the wavelength range 1750 nm < λ < 1900 nm. The laser emits at 1890 nm, the longest emission wavelength of a Tm:LiNbO₃ laser reported so far; also 1850 nm emission could be demonstrated. Laser threshold (1890 nm) is at 4 mW coupled pump power; the slope efficiency is ∼13.3%. Properties and potential of both devices are analyzed by extensive modeling.     

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.     

White light interferometry test and analysis of LiNbO3 polarizer

White light interferometer can be used to measure the amplitude extinction ratio (ER) of polarizer and coupling distribution in fiber. A LiNbO₃ polarizer coupled with a polarization maintaining fiber and a silica planar waveguide at the two ends was measured using white light interferometer. According to the principles of optical coherence domain polarimeter (OCDP) technique, the test scheme is analyzed and presented to measure the ER of LiNbO₃ polarizer with its apparatus proposed correspondingly. By analyzing the interference intensity, both the ER of LiNbO₃ polarizer and its coupling crosstalk with optical fiber and waveguide are obtained. The results illustrate that the ER of a 5 mm-long LiNbO₃ polarizer is 71 dB and the crosstalk of the coupling points are around 40 dB. The results have good agreement with analysis.     

Simultaneous wavelength conversion and pulse compression exploiting cascaded second-order nonlinear processes in LiNbO3 waveguides

Simultaneous wavelength conversion and pulse compression are proposed and demonstrated exploiting cascaded second-order nonlinear processes in periodically domain-inverted LiNbO₃ waveguides. The influences of initial pulse widths and waveguide length on the conversion efficiency and converted pulse compression are theoretically analyzed. Tunable wavelength conversion is performed for the signal pulse with the temporal width of 7.5 ps and repetition rate of 40 GHz. Conversion efficiency of more than −24 dB is obtained for 35-nm conversion span under average signal power of 10 dBm when a CW control wave is adopted.     

Experimental observation of tunable wavelength down- and up-conversions of ultra-short pulses in a periodically poled LiNbO3 waveguide

All-optical tunable wavelength conversion of ps-pulses is proposed and experimentally demonstrated by use of cascaded sum- and difference frequency generation (cSFG/DFG) in a periodically poled LiNbO₃ (PPLN) waveguide. The amplified spontaneous emission (ASE) noise is effectively suppressed by employing two tunable filters. As a result, tunable wavelength down- and up-conversions are simultaneously observed. The temporal evolutions of the signal, pump, control and idler waves propagating along the PPLN waveguide are simulated. From the temporal waveforms and optical spectra in the simulations, it is interesting to find that continuous wave (CW) pump and control are evolved into optical pulses during the cSFG/DFG nonlinear interactions. Moreover, some new sidebands in the output pump spectrum are observed both in the simulations and experiments.     

Optical waveguide with homogeneous refractive index profile in LiNbO3 by double-low-energy Oxygen ion implantation

We report on the formation of the planar waveguide by 550 keV O ion followed by 250 keV O ion implantation in lithium niobate (LiNbO₃), at fluences of 6 × 10¹⁴ ions/cm² and 3 × 10¹⁴ ions/cm², respectively. The Rutherford backscattering/channeling spectra have shown the atomic displacements in the damage region before and after annealing. A broad and nearly homogeneous damage layer has been formed by double-energy ion implantation after annealing. Both the dark mode spectra and the data of refractive index profile verified that the extraordinary refractive index was enhanced in the ion implanted region of LiNbO₃. A homogeneous near-field intensity profile was obtained by double-low-energy ion implantation. There is a reasonable agreement between the simulated modal intensity profile and the experimental data. The estimated propagation loss is about 0.5 dB/cm.     

Defect modes tuning of one-dimensional photonic crystals with lithium niobate and silver material defect

In this paper, a novel PCs defect mode is designed by inserting a Ag/LiNbO₃/Ag sandwich structure into periodically stacked TiO₂/MgF₂ dielectric superlattice. The band gap calculation is conducted using transfer matrix method. An emergence of defect mode around 525 nm is demonstrated. By applying an external voltage on this defect mode, remarkable wavelength tuning can be achieved. At normal incidence, a tuning bandwidth up to 70 nm is obtained by moderate change of external voltage from −250 to 250 V. This feature can be employed to fabricate practical tunable filters in visible region.     

High speed optical sampling covering C band and L band using CPPLN waveguide

Two means of high speed optical sampling covering C band and L band based on sum frequency generation (SFG) in chirped periodically poled LiNbO₃(CPPLN) waveguide are studied in this paper. For the first method, arbitrary waveband inside C and L band can be sampled by tuning pump wavelength and the bandwidth is controllable by choosing suitable waveguide length and chirped coefficient of CPPLN waveguide. For another, the broad bandwidth covering C band and L band can be obtained with a fixed pump. The numerical simulations based on coupled-wave equations are carried out. The NRZ sequences of 10 Gb/s and 640 Gb/s transmission rate are sampled in simulation. Distinct eye diagrams and quality factors (Q) are obtained by software-synchronized algorithm. The results show that the two means of the optical sampling both can cover C band and L band. SFG in CPPLN waveguide used for optical sampling system has a broad bandwidth and more flexible selectivity.     

High repetition rate mid-infrared generation with singly resonant optical parametric oscillator using multi-grating periodically poled MgO:LiNbO3

A high repetition rate mid-infrared singly resonant optical parametric oscillator (OPO) using MgO-doped multi-grating periodically poled LiNbO₃ (MgO:PPLN) is demonstrated. A 1064 nm Q-switched Nd:YVO₄ laser at 10 kHz repetition rate and pulse width of 17.8 ns was used to pump the OPO. The period of the quasi-phase matched (QPM) grating in the multi-grating MgO:PPLN chip varied from 25.5 to 31.5 μm in steps of 0.5 μm. This corresponds to the generation of a signal beam from 1.37 to 1.64 μm and an idler beam from 3.0 to 4.8 μm, respectively. A maximum signal power of 250 mW and idler power of 140 mW has been obtained with an input pump beam of power 1.92 W, for a grating period of 30.5 μm. A maximum optic-optic conversion efficiency of 20% and 7.4% in the idler has been observed. It has been observed that the output power increases as the period of the grating increases.     

Ytterbium femtosecond fiber laser without dispersion compensation tunable from 1015 nm to 1050 nm

We report on a widely tunable ytterbium fs-fiber laser without dispersion compensation. The all-normal dispersion laser contains a spectral filter for wavelength tuning and for generating additional amplitude modulation to support the nonlinear polarization evolution as mode-locking mechanism. By tilting the interference filter the center wavelength of the laser can be tuned from 1015 nm to 1050 nm with a pulse energy up to 2.0 nJ. The pulses can be dechirped externally to 108 fs.     

Design of thermo-optic tunable optical filter based on Si/Air DBR and polymer Fabry–Perot microcavity in SOI

An integrated tunable optical filter (TOF) based on thermo-optic effect in silicon on insulator (SOI) rib waveguide is designed and simulated. The device is comprised of two high refractivity contrast Si/Air stacks, functioning as high reflectivity of DBRs (distributed Bragg reflectors) and separating by a variable refractive index polymer Fabry–Perot (F–P) cavity. The designed device exhibits Q = 24077, FWHM = 0.065 nm and finesse = 566. Wavelength tuning is achieved through thermal modulation of refractive variation of the cavity. As the cavity polymer is heated, the refractive index of the cavity decreases. When the temperature of cavity polymer changes within 105, the central wavelength gets a continuous 35 nm shift from 1530 nm to 1565 nm, which can operate the whole C-band in the WDM (wavelength division multiplexing) networks. Moreover, by calculating, the tuning sensitivity is about 0.33 nm/°C. Owing to the compact size and excellent characteristics of integration, the proposed component has a promising utilization in spectroscopy and optical communication.