Experimental and theoretical study of turning mirrors and beam splitters with optimized waveguide structures

Integrated waveguide turning mirrors (IWTM) based on total internal reflection are investigated theoretically and experimentally. The effects of the mirror surface displacement, the rotation, and the roughness are studied. The loss due to the various spectral components of the roughness is comprehensively analysed. A wide waveguide turning mirror is more influenced by the low-frequency component, which is the major component of the actually fabricated mirror surface, resulting in high radiation loss of an IWTM with long taper, contrary to expectation. The excess radiation loss is about 0.8 dB/mirror and 1.0 dB/mirror for quasi-TE and quasi-TM modes, respectively, in the optimum design, which could be reduced by further minimizing the displacement error of the mirror position. We also designed and fabricated a low-loss beam splitter with modal interference region, the measured excess losses being about 1.2 dB and 1.8 dB for quasi-TE and quasi-TM modes, respectively, which are believed to be the lowest in this kind of beam splitter.     

Scattering losses from planar waveguides with material inhomogeneity

We consider the propagation of guided waves in a planar waveguide that has a finite-length segment of inhomogeneous media. Except for the inhomogeneous segment, which varies in length from 0 to 160λ, the waveguide is piecewise homogeneous everywhere. The inhomogeneity is modelled by two-dimensional random permittivity fluctuations that are numerically generated from an assumed Gaussian correlation function.

In 2D, the Maxwell equations are solved in the frequency domain for both TE and TM polarization by using modal expansion methods, perfectly matched absorbing boundary layers and the R-matrix transfer matrix algorithm. The guided waves are excited by a Gaussian beam incident on the waveguide aperture. For various waveguide design parameters, numerical results are given for waveguide power loss per unit length of waveguide inhomogeneity. The power loss curves are calculated as the average from numerous realizations of the random permittivity and the coefficient of variation is also given.     

Second-harmonic generation in periodically poled nonlinear polymer waveguides

We demonstrate quasi-phase-matched (QPM) second-harmonic generation (SHG) at the optical communication wavelengths with side-chain polymer waveguides. A ridge waveguide structure is designed to support fundamental mode guiding at both the pump and the second harmonics, leading to a high field overlap integral of the guided modes. The nonlinearity contrast in the +/0 type QPM waveguide is maximized under a QPM poling electrode width of nearly half the coherence length. Using these configurations, we record a normalized SHG efficiency of 2.2% W(-1) cm(-2) in the polymer waveguide.     

1.5μm质子交换铌酸锂光波导TE0模偏振器

在１．５μｍ光波长，首次研制出质子交换铌酸锂光波导ＴＥ０模偏振器。器件由嵌在Ｔｉ扩散波导之间的一段质子交换波导构成，器件长度为２ｍｍ。实验测得，偏振器的消光比和带尾纤插入损耗分别为４２ｄＢ和４．３ｄＢ。     

Wideband wavelength conversion using double-pass cascaded χ:χ interaction in lossy waveguides

We report the wavelength conversion based on double-pass cascaded nonlinear interaction (χ⁽²⁾:χ⁽²⁾) of sum and difference frequency generation in quasi-phase matched lithium niobate waveguides and compare it with double-pass cascaded second harmonic generation and difference frequency generation with and without waveguide loss. It is shown that the efficiency decreases considerably even for the low-loss waveguide compared to the lossless one especially for long waveguides and to achieve the higher efficiency for the same length, the amount of the extra power to compensate the loss increases. Also, an increased detuning of pump wavelength is proposed to flatten the response with a small efficiency penalty. The detuning- and loss-compensating pump powers can be found using the design diagrams in which the criteria for the design of waveguide length and the assignment of pumps power to obtain the desired efficiency, ripple and bandwidth are presented assuming a 75-nm pump wavelength difference.     

Design and Evaluation of an N:N Optical Coupler Using an Integrated Waveguide Mirror

We proposed and fabricated an N:N optical coupler using an integrated waveguide mirror. Optical circuits can be miniaturized with small bending by reflecting mirrors. The parabolic curve was applied to the mirror shape because it was necessary to collimate the reflected beam so as to distribute the input light to the output waveguides. We fabricated the 8:8 optical coupler using silica waveguide. The chip size was about 7mm x 15mm. The excess loss of the device was about 8 dB larger than simulation results calculated using beam propagation method. The dependence on the wavelength and polarization was about 0.4 and 0.15 dB, respectively. The excess loss was mainly attributed to the mirror tilt and will be reduced by optimizing the etching conditions to decrease the mirror tilt angle.     

A wide-band polymeric electro-optic modulator array based on unidirectional coupling between multi-mode waveguide array and a vertical configured dumping planar waveguide

A polymeric wide-band electro-optic (EO) modulator array based on unidirectional mode-coupling between the guiding multi-mode waveguide array and a vertical configured planar dumping waveguide was developed. A low insertion loss of <1.7 dB was obtained due to asymmetrically designed guiding waveguide and the dumping waveguide. A modulation depth of 91% was achieved with the device length of 3.5 cm at a low driving voltage of 3.2 V. The employment of the dumping planar waveguide not only provides an efficient way to achieve the unidirectional coupling mechanism, but also effectively confines the light coupled from the guiding waveguides within the planar dumping waveguide. The combination of unidirectional coupling and light confinement effects ensures low cross talk of <22 dB between adjacent guiding waveguides. Since the proposed modulator is based on the unidirectional coupling mechanism, it is intrinsically wide-band. A wide-band operation from 1308 to 1324 nm was experimentally obtained. The unidirectional mode-coupling-based wide-band modulation principle can be readily applied to other wavelengths, such as 1330 and 1550 nm ranges.     

High gain double-pass L-band EDFA with dispersion compensation as feedback loop

We demonstrate an efficient double-pass L-band erbium-doped fiber amplifier (EDFA) incorporating chirped fiber Bragg grating (CFBG). The amplifier structure exploits the characteristics of CFBG to reflect the amplified signal back into the gain medium, filter out the recycled forward amplified spontaneous emission and block the residual 1480 nm pump power. The amplifier configuration has high gain and low noise figures as compared to double-pass EDFA using broadband mirror. The demonstrated amplifier has gain of more than 48 dB and low noise figure of less than 4 dB at low input signal power of −40 dBm.     

Source abundances of ultra heavy elements derived from UHCRE measurements

A total of 205 tracks have been located, measured, and positively identified as originating from Ultra Heavy (Z ≥ 65) cosmic ray ions with energies over 2 GeV/amu in the 10 UHCRE plastic track detector (mainly Lexan polycarbonate) stacks studied by our Group. About 40 values of reduced etch rate S have been obtained along each of these tracks. A method based on determining the gradient of S, together with calibration in accelerators, is used to determine the charge of each ion resulting in one of such tracks to obtain the charge spectrum of the recorded Ultra Heavy ions. The abundance ratio of ions with 87 ≤ Z ≤ 100, to those with 74 ≤ Z ≤ 86, as well as that of ions with 81 ≤ Z ≤ 86 to those with 74 ≤ Z ≤ 80 are calculated at 0.016 and 0.32, respectively, which agree with the values obtained from measurements in the HEAO-3 and Ariel-6 experiments. The abundance ratio of ions with 70 ≤ Z ≤ 73 to those with 74 ≤ Z ≤ 86 is also calculated, but its value (0.074) did not seem to be significant because of our detectors' low registration efficiency in the charge range 70 ≤ Z ≤ 73. A computer program developed by our Group, based on the Leaky Box cosmic ray propagation model, has been used to determine the source abundances of cosmic ray nuclei with Z ≥ 65 inferred from the abundances measured in the UHCRE. It appeared that r-process synthesized elements were overabundant compared to the Solar System abundances, as predicted by other authors.     

Using a grating structure for phase compensation in achieving an efficient round-trip optical parametric process in periodically poled lithium niobate with an incomplete quasi-phase-matching period

An incomplete period of the last quasi-phase-matching (QPM) segment in a periodically poled nonlinear crystal represents a key problem in implementing a nonlinear wavelength-conversion device with round-trip or oscillating waves of interaction. Such a segment at the crystal end implies broken QPM periodicity for the reflected waves of interaction. We demonstrate numerically that with a grating structure at the crystal end for the reflecting signal the wavelength-dependent phase shift at Bragg reflection can compensate for the phase mismatch of the reflected signal, idler, and pump of an optical parametric process. Therefore, by slightly shifting the signal wavelength we can maintain the phase-matching condition with any length of the last QPM segment. It is shown that with a grating structure the conversion efficiency of the round-trip optical parametric process can always be optimized.     

Second-harmonic generation with a high-index-clad waveguide

Theoretical and experimental analyses of second-harmonic generation (SHG) with a high-index-clad waveguide are reported. It was found that confinement of the propagation modes and the overlap between the fields of fundamental and second-harmonic waves could be increased in this waveguide. This structure was achieved in an x-cut MgO:LiNbO (3) quasi-phase-matched (QPM) waveguide by use of Nb(2)O(5) as a cladding layer. With the QPM SHG device, harmonic blue light of 5.5 mW at the 434-nm wavelength was generated with a normalized conversion efficiency of 1200%/W cm(2).     

Double-pass erbium-doped zirconia fiber amplifier for wide-band and flat-gain operations

The double-pass erbium-doped zirconia fiber amplifier (EDZFA) is proposed and demonstrated to provide a wide-band amplification as well as flat-gain operation in both the C- and L-band regions using only a single-gain medium. The proposed amplifier utilizes an erbium-doped zirconia fiber (EDZF) with erbium ion concentration of 2800 ppm as a gain medium. The medium is fabricated in a ternary glass host, zirconia-yttria-aluminum codoped silica fiber through solution doping technique along with modified chemical vapor deposition (MCVD). Compared to a single-pass operation, the double-pass EDZFA shows a better gain performance. At input signal power of 0 dBm and the optimum EDZF length of 2 m, a flat gain of around 16 dB is achieved by the proposed double-pass amplifier with gain variation of approximately 2.5 dB throughout the wavelength range from 1530 to 1590 nm. However, the noise figure of the double-pass amplifier is slightly higher than that of the single-pass due to inefficient population inversion at the input part of the amplifier.     

Quasi-group-velocity matching using integrated-optic structures

We propose a device to compensate for group-velocity mismatch (GVM) effects that limit the efficiency-bandwidth product in nonlinear frequency-mixing devices. Integrated wavelength-dependent delay lines are introduced periodically in a waveguide containing a series of quasi-phase-matching (QPM) gratings. Appropriate choice of the time delays can compensate for GVM. We have demonstrated a two-stage device in a periodically poled lithium niobate waveguide. Two approximately 150-fs-long pulses generated 6 ps apart by second-harmonic generation in two QPM gratings were resynchronized by a fixed delay line, and their relative phase was fine controlled by temperature tuning. This technique, which can be iterated to more than two segments, permits optical frequency mixers with a higher efficiency-bandwidth product than would be possible in a single grating short enough to avoid GVM effects.     

Scattering losses from planar waveguides with material inhomogeneity

Abstract

We consider the propagation of guided waves in a planar waveguide that has a finite-length segment of inhomogeneous media. Except for the inhomogeneous segment, which varies in length from 0 to 160λ, the waveguide is piecewise homogeneous everywhere. The inhomogeneity is modelled by two-dimensional random permittivity fluctuations that are numerically generated from an assumed Gaussian correlation function.

In 2D, the Maxwell equations are solved in the frequency domain for both TE and TM polarization by using modal expansion methods, perfectly matched absorbing boundary layers and the R -matrix transfer matrix algorithm. The guided waves are excited by a Gaussian beam incident on the waveguide aperture. For various waveguide design parameters, numerical results are given for waveguide power loss per unit length of waveguide inhomogeneity. The power loss curves are calculated as the average from numerous realizations of the random permittivity and the coefficient of variation is also given.     

反射镜双程放大对类氖锗软X射线激光的输出影响研究

纳秒量级激光驱动的类氖锗软X射线激光具有出光稳定和输出强度大等优点,在诸多领域拥有很好的应用价值. 研究了通过在光路中增加一块多层膜反射镜实现双程放大对出光特性的影响. 通过估算和实验,证实了增加反射镜双程放大确实能够提高类氖锗软X射线激光的输出强度,对于24mm的双程放大长度,总输出强度会增加约2.3倍. 但同时这种模式也带来了光束质量的下降,对应用研究并非完全有益. 关键词： 软X射线激光 反射镜双程放大 增益饱和     

Enhanced structure of a double-pass erbium-doped fiber amplifier for multiple wavelength amplifications

We demonstrate an enhanced double-pass erbium-doped fiber amplifier for multichannel amplification. The multichannel selection is formed by combining a demultiplexer and a multiplexer together in the fiber-loop mirror. The structure is able to filter out the amplified spontaneous emission that saturates the amplifier gain in the small signal regime. The maximum average gain of 47.2 dB is obtained with a gain enhancement of 12.8 dB at a ?50-dBm signal power per channel. The noise figure penalty is almost negligible for a signal power per channel of less than ?15 dBm.     

Multiwaveguide implantable probe for light delivery to sets of distributed brain targets

Optical fibers are commonly inserted into living tissues such as the brain in order to deliver light to deep targets for neuroscientific and neuroengineering applications such as optogenetics, in which light is used to activate or silence neurons expressing specific photosensitive proteins. However, an optical fiber is limited to delivering light to a single target within the three-dimensional structure of the brain. We here demonstrate a multiwaveguide probe capable of independently delivering light to multiple targets along the probe axis, thus enabling versatile optical control of sets of distributed brain targets. The 1.45-cm-long probe is microfabricated in the form of a 360-μm-wide array of 12 parallel silicon oxynitride (SiON) multimode waveguides clad with SiO(2) and coated with aluminum; probes of custom dimensions are easily created as well. The waveguide array accepts light from a set of sources at the input end and guides the light down each waveguide to an aluminum corner mirror that efficiently deflects light away from the probe axis. Light losses at each stage are small (input coupling loss, 0.4 ± 0.3 dB; bend loss, negligible; propagation loss, 3.1 ± 1 dB/cm using the outscattering method and 3.2 ± 0.4 dB/cm using the cutback method; corner mirror loss, 1.5 ± 0.4 dB); a waveguide coupled, for example, to a 5 mW source will deliver over 1.5 mW to a target at a depth of 1 cm.     

Metallic waveguide mirrors in polymer film waveguides

A technology for the fabrication of metallic waveguide mirrors is developed. Plane and curved waveguide mirrors, the latter acting in the same way as cylindrical lenses, are realized in benzocyclobutene (BCB) film waveguides. The waveguide mirror structure is dry-etched into the BCB film waveguide. To enhance the reflectivity of the waveguide mirrors, the waveguide edge is metallized. The BCB film waveguide mirrors are characterized with respect to waveguide attenuation and mirror reflectivity. The waveguide attenuation of the processed BCB waveguide is 0.5 dB/cm. Ag-coated BCB waveguide mirrors show a reflectivity of 71%. The efficiency of total internal reflection (TIR, i.e. in the case without metallization) at the dry-etched waveguide edge is 74%. As an application of the BCB waveguide mirrors a hybrid integrated optical module for Fourier-optical transverse mode selection in broad area lasers (BAL) is proposed. Received: 16 May 2001 / Published online: 23 October 2001     

Tunable Optical Add/Drop Multiplexers Using Cascaded Mach Zehnder Coupler

A tunable add/drop filter based on Cascaded Mach Zehnder (CMZ) coupler using SiO₂/silicon oxinitride (SiON) is presented with its mathematical model. The increase of filtered width (i.e., free spectral response) and lowering of crosstalk for this filter are optimized using its mathematical model with truncated binomial coupler distribution. The number of wavelength channels with 100 GHz (0.8 nm) channel spacing for 5-stage (M = 9, r = 2) truncated binomial CMZ filter with index contrast ∼5% at -20 dB crosstalk and the bending loss of 0.1 dB per MZ section are obtained as ∼37 (free spectral range of 31 nm). It is seen that if during the fabrication process, waveguide core width w is increased or decreased by 0.1 μm (in percentage terms ∼±6.6%), the crosstalk is slightly increased by ∼7%. Thermal tuning for wavelength channels is achieved by thin film heater with low power thermooptic delay line structure which reduces the heating power by ∼1.58 times in comparison to the conventional structure.     

Efficient 340-nm light generation by a ridge-type waveguide in a first-order periodically poled MgO:LiNbO3

Quasi-phase-matched (QPM) UV second-harmonic generation (SHG) in a periodically poled MgO:LiNbO3 waveguide is presented. A ridge-type waveguide with high nonlinearity and strong resistance to photorefractive damage was achieved by use of an ultraprecision machining technique. By use of this waveguide in 1.4-microm periodically poled MgO:LiNbO3, a first-order QPM SHG device for 340-nm UV radiation was demonstrated. In a single-pass configuration, continuous-wave 22.4-mW UV light was generated for a fundamental power of 81 mW, corresponding to a normalized conversion efficiency of 340%/W.