Phase-matching Curves Research Articles

High-sensitive refractive index sensor based on the long-period gratings inscribed in the tapered fiber at dispersion turning point

We demonstrated the fabrication of long-period fiber gratings (LPFGs) in the tapered single-mode fiber by CO2 laser. The phase-matching curves of the LPFGs inscribed in the tapered fiber with different waist diameters have been studied theoretically and experimentally. When the waist diameter of the tapered fiber decreased from 90 μm to 22 μm, the slope of the phase-matching curve changed dramatically. The refractive index sensitivity can be increased by inscribing the LPFGs in the tapered fiber around the dispersion turning point. A maximum refractive index sensitivity of 21702.78 nm/RIU has been achieved in the range of 1.445–1.457 for the LPFG inscribed in the tapered fiber with a waist diameter of 50 μm. This kind of LPFGs inscribing in the tapered fiber may be a good candidate for the development of refractive index based chemical and biological sensors.

Dispersion-Oriented Inverse Design of Photonic-Crystal Fiber for Four-Wave Mixing Application

In this paper, we demonstrate the application of a deep learning neural network (DNN) in the dispersion-oriented inverse design of photonic-crystal fiber (PCF) for the fine-tuning of four-wave mixing (FWM). The empirical formula of PCF dispersion is applied instead of numerical simulation to generate a large dataset of phase-matching curves of various PCF designs, which significantly improves the accuracy of the DNN prediction. The accuracies of DNNs’ predicted PCF structure parameters are all above 95%. The simulations of the DNN-predicted PCFs structure demonstrate that the FWM wavelength has an average numerical mean square error (MAE) of 1.92 nm from the design target. With the help of DNN, we designed and fabricated a specific PCF for wavelength conversion via FWM from 1064 nm to 770 nm for biomedical imaging applications. Pumped by a microchip laser at 1064 nm, the signal wavelength is measured at 770.2 nm.

Long-period gratings on lithium-niobate waveguides: phase-matching turning points for ultra-broadband mode conversion

We analyze a polymer long-period grating (LPG) formed on the surface of a z-cut lithium-niobate (LN) few-mode waveguide for TM00–TM10 mode conversion. We identify and explore the turning points along the phase-matching curves of the LPGs for the achievement of ultra-broadband mode conversion. We fabricate a number of gratings with annealed proton-exchanged LN waveguides and our experimental results agree well with the theoretical results. A typical fabricated grating operating near the turning point provides a mode extinction ratio larger than 20 dB from 1550 nm to a wavelength longer than 1700 nm, and another one provides a mode extinction ratio larger than 10 dB from 1464 nm to a wavelength longer than 1700 nm. Our gratings show good thermal stability. The structure of the LPG on LN waveguide could be applied to the realization of ultra-broadband high-speed mode switches for mode-multiplexing applications.

Pr:LGSB as a new nonlinear optical crystal: Czochralski growth and optical characterization

Herein, we report on the Czochralski growth and optical characterization of Pr:LGSB as a new nonlinear optical (NLO) and laser crystal. Single crystals of Pr:LGSB were grown by the Czochralski method, for the first time to the best of our knowledge, from the starting melt compositions La 0.656 Pr 0.022 Gd 0.572 Sc 2.75 (BO 3 ) 4 and La 0.651 Pr 0.027 Gd 0.572 Sc 2.75 (BO 3 ) 4 . The crystal growth parameters were optimized and two Pr:LGSB crystals with good optical quality were obtained. The chemical compositions of the grown crystals were determined to be La 0.750 Pr 0.020 Gd 0.450 Sc 2.780 (BO 3 ) 4 and La 0.743 Pr 0.025 Gd 0.452 Sc 2.780 (BO 3 ) 4 corresponding to a Pr 3+ ions concentration in the crystals of 2.0 at% and 2.5 at%, respectively. The spectroscopic properties were investigated and the obtained results show that the 2.5 at% Pr:LGSB is a promising laser crystal emitting in the orange spectral range at ~ 603 nm. The refractive indices were measured and the phase-matching curves for type I and type II SHG were determined. The walk-off angle, the external angular acceptance, and the nonlinear coefficient d 11 for type I SHG of 603 nm fundamental wavelength were also evaluated. This work shows that the 2.5 at% Pr:LGSB crystal could be a new promising candidate for obtaining laser sources in the UV range (~ 301.5 nm) in SFD configuration. • Pr:LGSB crystal were grown by the Czochralski method for the first time. • Spectroscopic and NLO properties of the 2.5 at% Pr:LGSB crystal are presented. • 2.5 at% Pr:LGSB has promising properties for laser emission in the orange domain. • Phase-matching curves for type I and type II SHG are determined. • Preliminary prospects for SFD in the UV domain were analyzed.

Design of an ultra-broadband optical filter based on a local micro-structured long period fiber grating near PMTP.

This paper presents a local micro-structured long period fiber grating (LMS-LPFG) ultra-broadband optical filter based on the wide bandwidth near the phase-matching turning point (PMTP). The structure of LMS-LPFG is obtained by dividing a LPFG into two parts of equal length and reducing the cladding radius of the second LPFG. At this time, the LMS-LPFG can be regarded as a cascade of two equal-length LPFGs with different resonance wavelengths. The cladding mode and grating period are determined to make the first LPFG work in the double-peak resonance state, and the second LPFG operates near PMTP. It is found that the transmission spectra of the two LPFGs can be superimposed to form a wide loss bandwidth. Then the cladding radii of the second LPFG and grating structure parameters are designed based on coupled-mode theory. First, the grating period corresponding to the operating wavelength is determined from the phase-matching curve of LMS-LPFG. Then, the radius of the second LPFG with a designated grating period is selected to make LPFG 2 work in PMTP by reducing its cladding radius. In addition, the grating lengths of the two LPFGs are determined by maximizing the loss of the LMS-LPFG's transmission spectrum. Finally, the two LPFGs are cascaded into a LMS-LPFG, and the optical transmission spectrum of the LMS-LPFG is calculated by the transfer matrix method. Simulation results show that the bandwidth of the transmission spectrum can reach 380 nm. In addition, the flexibility of design for the operating wave band is discussed and confirmed, and can meet different actual requirements of optical communication.

Analysis of broadband mid-infrared optical parametric amplification based on LiGaS2, LiGaSe2, LiInS2, and LiInSe2 crystals

Optical parametric amplification (OPA) is a powerful technique for generating mid-IR ultrashort pulses. Determination of proper OPA conditions is based on the study of phase-matching curves. In this paper, we provide this study for L i G a S 2 , L i G a S e 2 , L i I n S 2 , and L i I n S e 2 non-oxide crystals in a type I arrangement. Phase-matching characteristics and gain spectra were calculated when pumped at wavelengths of 1.03, 1.064, 1.94, and 2.1 µm. By pumping few-millimeter-long Li-based crystals at 10 G W / c m 2 pump intensity, the signal gain spectrum bandwidths from a few tens of nanometers to a few thousands of nanometers can be obtained. All these crystals can display a broad bandwidth signal amplification, allowing the generation of broadband mid-IR ultrashort pulses.

Generation of 116 mW output power at 461 nm in a periodically poled lithium niobate waveguide

We demonstrate a second harmonic generation (SHG) of 116 mW at 461 nm in a periodically poled lithium niobate waveguide when the power of the 922 nm fundamental light is coupled into the waveguide was 350 mW. The waveguide is 12.5 μm wide, 12.0 μm thick, 22 mm long, and has a 1 mm long slab window at the output facet of the waveguide. The temperature acceptance bandwidth of the phase-matching curve of the SHG is approximately 0.5 °C. The SHG system demonstrates good beam quality and is reliable for cold atom experiments, including research on optical lattice clocks.

Leaky-mode long-period grating on a lithium-niobate-on-insulator waveguide

Lithium niobate on insulator (LNOI) is a promising platform for the construction of high-density integrated photonics circuits. In this paper, we propose compact long-period gratings (LPGs) formed on LNOI rib waveguides for achieving light coupling from the guide mode to the leaky mode. Our LPGs are polymer gratings coated on the surfaces of LNOI waveguides and can be designed to achieve strong mode coupling at specific resonance wavelengths for the TE-like and the TM-like polarization. By means of a numerical analysis, we find that the LPGs exhibit turning points in the phase-matching curves, which promises the realization of broadband wavelength filters and polarizers with such grating structures on the LNOI platform. To experimentally verify the theoretical results and demonstrate the LPG characteristics, we fabricate a number of LPG filters on LNOI rib waveguides, which include two broadband LPGs operating near the turning points: a TE-pass polarizer with a 20 dB extinction ratio over a bandwidth of 270 nm from 1430 to 1700 nm and a TM-pass polarizer with a 20 dB extinction ratio over a bandwidth of 90 nm from 1500 to 1590 nm. The insertion losses for the passing polarization are smaller than 2 dB in the C + L band. Our LPGs can serve as building blocks for the development of a wide range of integrated LNOI devices for application in on-chip signal processing.

Growth and characterization of 3.5 at.% Nd:LGSB bifunctional crystal

The performances of 3.5 at.% Nd:LGSB bifunctional laser and nonlinear optical (NLO) crystal were investigated. A high-quality single crystal with incongruent melting, having dimensions of about 12 mm in diameter and 35 mm in length, was grown along the c-axis by the Czochralski technique. The phase-matching curve for type I second harmonic generation (SHG) was determined. Efficient laser emission at 1062 nm, along the phase-matching direction for SHG of 1062 nm radiation, was obtained under the pump at 807 nm with a fiber-coupled laser diode. The Nd:LGSB laser operated with a high slope efficiency of ηsa = 0.56 (with respect to the absorbed pump power), delivering the highest output power of 2.1 W for 4.04 W of absorbed pump power. Preliminary SFD experiments resulted in achieving a maximum green output power of 48 mW at 3.94 W absorbed pump power.

Millimetre-wave range optical properties of BIBO

We present the thorough studies of optical properties of BiB3O6 (BIBO) crystal in the millimeter-wave (subterahertz) range. We observe a large birefringence Δn = nZ -nX = 1.5 and the values of absorption coefficients of all three axes to be less than 0.5 cm−1 at the frequency of 0.3 THz. The difference from visible range in angle ϕ between the dielectric axis z and crystallophysical axis X is found to be more than 6°. The simulated phase-matching curves in the xz plane of the crystal show the optimal value of the angle θ to be around 25.5°±1° for an efficient millimeter-wave generation under the pump of 1064 nm laser radiation.

Potential of Sub-THz-Wave Generation in Li2B4O7 Nonlinear Crystal at Room and Cryogenic Temperatures

Due to their high optical damage threshold, borate crystals can be used for the efficient nonlinear down-conversion of terawatt laser radiation into the terahertz (THz) frequency range of the electromagnetic spectrum. In this work, we carried out a thorough study of the terahertz optical properties of the lithium tetraborate crystal (Li2B4O7; LB4) at 295 and 77 K. Approximating the terahertz refractive index in the form of Sellmeier’s equations, we assessed the possibility of converting the radiation of widespread high-power laser sources with wavelengths of 1064 and 800 nm, as well as their second and third harmonics, into the THz range. It was found that four out of eight types of three-wave mixing processes are possible. The conditions for collinear phase matching were fulfilled only for the o − e → o type of interaction, while cooling the crystal to 77 K did not practically affect the phase-matching curves. However, a noticeable increase of birefringence in the THz range with cooling (from 0.12 to 0.16) led to an increase in the coherence length for o − o → e and e − e → e types of interaction, which are potentially attractive for the down-conversion of ultrashort laser pulses.

Temperature tuning of BaGa4Se7 optical parametric oscillator

The temperature tuning of BaGa4Se7 (BGSe) was demonstrated for the first time, to the best of our knowledge. When the temperature of BGSe (56.3°,0°) was raised from 30°C to 140°C, the idler light under type I raised from 3637 nm to 3989 nm, the tunable range reached 352 nm, and Δλ2/ΔT reached 3.20 nm/°C. We calculated the phase matching curve of BGSe when ∅ and T took different values. The relationship between θ and Δλ2/ΔT was obtained by fixing ∅ at 0°. The maximum Δλ2/ΔT and its corresponding (θ, ∅) phase matching type were reported under different fixed λ2 (3 µm, 3.2 µm,…, 5 µm).

A highly-sensitive temperature sensor based on GeO2–SiO2 long period fiber grating

A highly-sensitive long period grating temperature sensor has been introduced here, which is composed of GeO2 in the core and SiO2 in the cladding. Due to high thermo-optic coefficient contrast of the core and the cladding, the proposed sensor provides high temperature sensitivity of S = Δλ/ΔT = −2.3871 nm/°C in the temperature range of 71 °C to 406 °C. The grating period and the length of the sensor are 445.4 nm and ∼25 mm, respectively and the operating wavelength range is λ∼1.48–2.28 μm. The proposed sensor has been investigated considering material dispersion for the core and the cladding at different temperatures. Also, the conditions required for coupling of the fundamental core mode to the higher order cladding modes with respect to lattice constant have been regarded. The phase matching curves has been considered in the design of the proposed sensor and the structure has been optimized to present the maximum sensitivity, achievable in the regarded temperature range. The presented optical temperature sensor is feasible with low cost which benefits the high sensitivity, large detection range, and large operating wavelength range and it can find potential applications in the measurement of temperature in toxic, unreachable, and far distant environments such as in temperature control of lasers, and in oil or gas industries.

β-BBO, LBO, AND KTP Nonlinear Crystals as Sources of Millimeter-Wave Radiation

The refractive indices and the absorption coefficients of β-BBO, LBO, and KTP nonlinear optical crystals have been measured in the wavelength range of 0.5–3 mm at temperatures of 298 and 78 K. The absorption coefficients of all crystals at a wavelength of 1 mm do not exceed 3.5 cm–1 and fall with cooling. The anisotropy of the absorption coefficients of crystals increases significantly with decreasing wavelength ( 2 mm, it almost disappears. Upon cooling, the absolute values of the absorption coefficients fall below 0.2 cm–1. Based on the measured refractive indices, the collinear phase-matching curves are simulated numerically. The theoretical feasibility of IR (1.064 μm) laser radiation conversion into the millimeter-wave range is demonstrated.

High diffraction order cladding modes of helical long-period gratings inscribed by CO2 laser.

The helical long-period gratings (HLPGs) with resonance at high diffraction order are fabricated in single-mode fiber using a CO2 laser. A series of HLPGs with different pitches are fabricated, and the phase-matching curves of the HLPGs with first and second diffraction orders are presented based on the experimental results. The temperature, surrounding refractive index (SRI), and torsion-sensing characteristics of the HLPGs with different diffraction orders have been investigated experimentally. The maximum torsion sensitivity of resonance at the second diffraction order is about 0.228 nm/(rad/m), which is twice as high as that of the first diffraction order cladding mode. The HLPG offers great potential to perform simultaneous multiparameter measurement due to the resonance dips at different diffraction orders having quite different sensitivities to temperature, SRI, and torsion.

Long period grating in a liquid crystal waveguide

This paper introduces, for the first time to the best of our knowledge we theoretically report the long period grating (LPG) in the liquid crystal (LC) layer of planar optical waveguide. LPG has the ability to couple the light between core and cladding modes of waveguide. LPG can be used for the development of band rejection filter, gain flattening in erbium-doped fiber amplifiers (EDFA) and for sensing applications. We have shown the realization of proposed LPG in temperature sensing. First, we theoretically analyzed the propagation of light through a 6-layer planar optical waveguide by analyzing its modal field profiles and effective index profile of fundamental mode TE0 and two higher order modes TE1 and TE2. Then we investigated in details the phase-matching curves of the long period grating, which supervise the association between the grating period and the resonance wavelength. Using phase matching curve to choose grating period, we have studied the transmission spectrum of the PLG which display evident elimination of bands at individual wavelengths, noted as the resonance wavelengths. High sensitivity of LC to the environmental temperature makes it a good choice for temperature sensing. The performance of LPG has been studied for the temperature ranging from 306 K to 329 K. The novelty of this structure is the realization of LPG in the LC, which is a key component of display devices. Hence, our structure can provide freedom to use this in variety of pragmatic applications.

Ultra-short embedded long-period waveguide grating for broadband mode conversion

We propose a novel embedded long-period waveguide grating (LPWG) for the implementation of a broadband mode conversion. The proposed LPWG is formed in the inner of waveguide where the two coupled modes have the largest field overlap and, hence, features an ultra-short length and a wide bandwidth. Besides, the bandwidth of the proposed LPWG is further increased by manipulating its phase-matching curve. Our proof-of-principle device, fabricated with polymer for the conversion between the E11 and E21 modes, contains only two periods and achieves a mode conversion efficiency larger than 98.2% (99.2%) for the x (y) polarization over the C + L band. The proposed LPWG could find applications as an ultra-compact and broadband mode converter in mode-division-multiplexing system.

Titanium Dioxide Film Coated Excessively Tilted Fiber Grating for Ultra-Sensitive Refractive Index Sensor

In this paper, we report a sensitive refractive index sensor based on the titanium dioxide (TiO $ _2$ ) film coated excessively tilted fiber grating (Ex-TFG) operating around the dispersion tuning point. The effects of the TiO $ _2$ film on the cladding mode field, transmission, and sensing characteristics of the Ex-TFG are outlined. It is found that the TiO $ _2$ film induces a two-step mode transition process for both TE/TM $ _{0,l}$ and HE/EH $ _{v,l}$ ( $ v=1,2,\ldots$ ) cladding modes. During the first transition, the same optimized film thickness can be obtained for TE $ _{0,l}$ , HE $ _{1,l}$ , and EH $ _{2,l-1}$ cladding modes. The results show that the sensitivity can be optimized based on the combined effects of the intermodal separation, initial wavelength separation, and the variation in the phase matching curve (i.e., $ \partial \Lambda / \partial \lambda$ ). The improved sensitivities are up to $ 32261.2\;\text{nm}/\text{RIU}$ and $ 23183.3\;\text{nm}/\text{RIU}$ by tracking dual resonance band corresponding to the s-polarized TE $ _{0,10}$ and p-polarized EH $ _{2,9}$ cladding modes, respectively, in the low refractive index region. For the single resonance corresponding to the s-polarized HE $ _{1,9}$ cladding mode, the sensitivity as high as $ 25860.0\;\text{nm}/\text{RIU}$ is achieved. The sensitivity is significantly enhanced by a factor of $ 13.2 \sim 18.1$ and $ 3.7 \sim 5.2$ times as compared with the uncoated Ex-TFG and classical TiO $ _2$ film coated long period fiber grating with reduced fiber diameter, respectively. The sensitivity can be further improved, which makes the film coated Ex-TFG very attractive for the refractive index sensing.

Anisotropy of nonlinear optical properties in monoclinic SmxY1-xCa4O(BO3)3 crystals

The angular noncritical phase matching (A-NCPM) second-harmonic-generation (SHG) properties of SmxY1-xCOB (x = 0, 0.44, 0.55 and 1) nonlinear optical (NLO) crystals were investigated. The type-I A-NCPM SHG wavelengths along the Y and Z axes were found to be tunable in the ranges of 725–900 nm and 826–1116 nm, respectively, by adjusting the ratio of the rare-earth elements Sm and Y in SmxY1-xCOB crystals. Taking advantages of the fundamental laser pulses generated from a mode-locked Ti:Sapphire laser and a long Y-cut Sm0.44Y0.55COB crystal sample, the single-pass SHG average output power of ∼436 mW at 397 nm was obtained with a conversion efficiency of 41.5%. In addition, the SHG phase-matching (PM) curve and the effective NLO coefficients (deff) of SmCOB crystal for 1064 nm laser were studied, where the optimal type-I SHG PM direction in principal planes was determined to exist in the XY principal plane i.e. (90°, 52.4°), rather than the ZX plane i.e. (θº, 180°), which was different with those of TmCOB (32.5°, 180°), YCOB (30.7°, 180°), TbCOB (22.6°, 180°), GdCOB (19.7°, 180°) and LaCOB (15.1°, 180°) crystals in the same crystal family.

INVITED] Design of turn around point long period fiber grating sensor with Au-nanoparticle self monolayer

In this paper studies on the design and fabrication of a long period fiber grating (LPFG) with a self mono layer of gold nanoparticle (AuNP) has been presented. Refractive index (RI) sensitivity of a dispersed cladding mode (DCM) near turn around point (TAP) of its phase matching curve (PMC) has been investigated with and also without AuNP coated LPFG. The typical role played by the intermediate layer of AuNP on the effective index and thus on the sensitivity of the cladding mode to the surrounding RI has also been explored by carrying out coupled mode analysis of the requisite multilayer waveguide. Deposition of AuNP enhanced the sensitivity by more than a factor of 2. Measured sensitivity was found to be ∼3928 nm/refractive index unit (RIU) in the range of 1.3333–1.3428.