Lightwave Transmission Research Articles

Thermo-optic Characteristics of Micro-structured Optical Fiber Infiltrated with Mixture Liquids

We present both theoretically and experimentally the thermo-optic characteristics of micro-structured optical fiber (MOF) filled with mixed liquid. The performance of MOF depends on the efficient interaction between the fundamental mode of the transmitted light wave and the tunable thermo-optic materials in the cladding. The numerical simulation indicates that the confinement loss of MOF presents higher temperature dependence with higher air-filling ratios d/Λ, longer incident wavelength and fewer air holes in the cladding. For the 4cm liquid-filled grapefruit MOF, we demonstrate from experiments that different proportions of solutions lead to tunable temperature sensitive ranges. The insertion loss and the extinction ratio are 3~4 dB and approximate 20 dB, respectively. The proposed liquid-filling MOF will be developed as thermo-optic sensor, attenuator or optical switch with the advantages of simple structure, compact configuration and easy fabrication.

Transmission characteristics of planar optical waveguide devices on coupling interface

Lightwave transmission model for optical waveguide chip and input/output fiber array on coupling interface has been established on the basis of Fresnel Reflection Principle. Mechanism and rules for the influence on the devices’ optical performance from the features of adhesive and the geometric parameters of the interface has been analyzed and verified through experiments. Numerical calculation results have shown that with the variations in the refractive indexes of the adhesive, there is a periodical change in the return loss of the devices, namely the optical performance of the devices is related to the refractive index of the adhesive, while the experimental results have shown that the refractive-index-matching adhesive can effectively decrease the insertion loss and increase the return loss of the devices. And the insertion loss before and after adding the adhesive falls by 1.0dB and the return loss rises by 10dB averagely.

AgI-coated silver-clad stainless steel hollow waveguides for infrared lightwave transmission and their applications

We fabricated silver iodide (AgI)-coated silver hollow waveguides to transmit a wide range of infrared (IR) light. Silver-clad stainless steel pipes were used as a supporting pipe. Since this type of metallic hollow waveguide has high mechanical strength and heat resistance, it is suitable as a rigid lightwave probe for various applications such as dental or medical laser treatment, IR spectroscopy, thermal radiometry, and laser processing. Considering these applications, we estimated the hollow waveguides with different thicknesses of the AgI layer. By optimizing the AgI layer thickness according to the wavelength of propagating light, we succeeded in efficiently transmitting Er-YAG and CO(2) laser light. We also studied the optical characteristics of a wide range of incoherent light for IR spectroscopy and radiometry applications using these metallic hollow waveguides as lightwave probes.

Optimizing lightwave transmission through a nano-tip

Optical microscopy with spatial resolution below the diffraction limit is at present attracting extensive attentions. Further advancement of the near-field scanning optical microscopy (NSOM), a practical super-resolution microscopy, is mainly limited by the low transmission of optical power through the nano-meter apex. This work shows that lightwave can be efficiently delivered to a sub-100 nm apex inside a tapered metallic guiding structure. The enhanced light delivery, about 5-fold, is made possible with an adaptive optimization of the transmission via a spatial light phase-modulator. Numerical simulation shows the mechanism for the efficient light delivery to be the selective excitation of predominantly the lowest-order transverse component of standing wavevector with proper input wavefront modulation, hence favoring the transmission of lightwave in the longitudinal direction. The demonstration of such efficient focusing, to about full-width at half-maximum of a quarter wavelength, has a direct and immediate application in the improvement of the existing NSOMs.

Developing a Half-Cladding Semiconductor Photonic Device Structure for Surface Transmission of Light Waves

A control of the light-wave properties is critical issue in developing attractive photonic devices. In conventional photonic devices such as semiconductor waveguide light sources, complicated techniques were required to fabricate the high-aspect-ratio artificial periodic structures that help controlling the light-wave properties. This is because the conventional photonic devices are constructed with thick cladding layers for an optical confinement. In this paper, we propose and fabricate a half-cladding semiconductor light source as an attractive photonic device structure. We successfully demonstrated a light emission from the light source under current injection at room temperature. With a half-cladding structure, the total thickness between the device surface and the top of the active layer is on the sub-µm scale. Controlling the light-wave properties should therefore be possible with artificial surface microstructures fabricated by a low aspect ratio process sequence.

Developing a Half-Cladding Semiconductor Photonic Device Structure for Surface Transmission of Light Waves

Developing a Half-Cladding Semiconductor Photonic Device Structure for Surface Transmission of Light Waves

Chromatic Dispersion Monitoring Based on Variance of Received Optical Power

For lightwave transmission systems employing advanced modulation formats such as phase-shift keying (PSK) and quadrature-amplitude modulation (QAM), chromatic dispersion monitoring based on average variance of optical power has been demonstrated. Theoretical analyses are verified by simulations, and experiment results show that the variance of optical power at the receiver is a simple and effective statistical parameter to monitor the residual chromatic dispersion of a transmission link.

Transparency in Bragg scattering and phase conjugation

Reflectionless transmission of light waves with unitary transmittance is shown to occur in a certain class of gain-grating structures and phase-conjugation mirrors in the unstable (above-threshold) regime. Such structures are synthesized by means of the Darboux method developed in the context of supersymmetric relativistic quantum mechanics. Transparency is associated to superluminal pulse transmission.

Online Postweld Shift Measurement of Butterfly-Type Laser Module Employing High-Resolution Capacitance Displacement Measurement System

A quantitative postweld shift (PWS) correction employing a high-resolution capacitance displacement measurement system (CDMS) in laser-welded butterfly-type laser module package is demonstrated. The CDMS has a 25.4-nm resolution and enables precise online measurement of the PWS in both the horizontal and vertical directions. By applying a mechanical adjustment to compensate for the magnitude and direction of the PWS measured online by the CDMS, the results show that the fiber tip can be realigned back closer to the original optimum coupling positions. A relative coupling efficiency of the laser modules can be regained up to an average of 88% by using the CDMS. In comparison with the previous studies on the quantitative PWS compensation in laser-welded butterfly-type laser module packages, the advantages of using this novel CDMS are the high-resolution and the online measurements of the PWS in laser-welded butterfly-type laser module packages. Through an in-depth study on the precise online measurement of the PWS employing high-resolution CDMS, it is now possible to fabricate high-yield butterfly-type laser modules for use in lightwave transmission systems and utilization of many other high-yield and low-cost laser-welded photonics module packages.

Tunable Erbium-Doped Fiber Lasers Using Various Inline Fiber Filters

Several high-performance and tunable erbium-doped fiber lasers are reviewed. They are constructed by using fiber Bragg gratings (FBGs) or short-wavelength-pass filters (SWPFs) as wavelength tunable components inside the laser cavity. Broadband wavelength tuning range including C- and/or S-band was achieved, and tunable laser output with high slope efficiency, high side-mode suppression ratio was obtained. These fiber lasers can find vast applications in lightwave transmission, optical test instrument, fiber-optic gyros, spectroscopy, material processing, biophotonic imaging, and fiber sensor technologies.

Tunable Erbium-Doped Fiber Lasers Using Various Inline Fiber Filters

Several high-performance and tunable erbium-doped fiber lasers are reviewed. They are constructed by using fiber Bragg gratings (FBGs) or short-wavelength-pass filters (SWPFs) as wavelength tunable components inside the laser cavity. Broadband wavelength tuning range including C- and/or S-band was achieved, and tunable laser output with high slope efficiency, high side-mode suppression ratio was obtained. These fiber lasers can find vast applications in lightwave transmission, optical test instrument, fiber-optic gyros, spectroscopy, material processing, biophotonic imaging, and fiber sensor technologies.

Feed-forward linearisation technique for impulse radar ultra-wideband over fibre

The authors present the linearisation of impulse radar ultra-wideband (IR-UWB) transmission system over fibre. The radio frequency signal of IR-UWB is transmitted over 50 km of the single mode fibre (SMF) using external Mach–Zehnder modulator (MZM), amplified, linearised and detected by photo detector receiver. For improving the linearity and suppressing the four-wave mixing (FWM) caused by Semiconductor Optical Amplifier (SOA) and 50 km SMF, the system uses feed-forward technique. Link performance was evaluated for two IR-UWB signals to suppress the FWM. The theoretical simulation results demonstrate a distortion cancellation that is produced by SOA of better than 25 dB over 1550 nm single mode optical fibre cable. In addition, the characteristic transmission of UWB pulse radio through 50 km single mode radio over fibre (RoF) system, such as BER measurements to evaluate the performance of the UWB with respect to different laser power output level, is investigated. This technique has been used to successfully transmit indirectly modulated data using laser diode at 10 GB/s light-wave transmission system.

Ideal optical antireflection of composite films activated by spherical nanoparticles

It has been shown that a composite film activated by spherical nanoparticles can ensure ideal optical antireflection when the amplitude of the transmitted light wave is equal to the amplitude of the external wave at various angles of incidence independently on the optical properties of the underlying medium. The amplitude of the reflected wave vanishes in a wide range of optical wavelengths. It has been shown that the effect of ideal optical antireflection occurs when the effective complex refractive index of the composite film is zero.

Quantum memory for images with feedback

A spatially multimode scheme of the quantum memory for light is presented that can be considered to be a thin hologram, whose long-lived spin subsystem of a multiatomic ensemble stores image peculiarities, such as the multimode superposition and entangled quantum states, which is unattainable for ordinary holography. In the recording process, the measurement of polarization parameters of the transmitted light wave and the feedback are used for the action on an ensemble of fixed atoms (obtained, for example, by means of the laser cooling). The information capacity of a quantum hologram with a feedback will considerably exceed that of a spatially singlemode quantum memory.

Doppler effect-based fiber-optic sensor and its application in ultrasonic detection

Based on the Doppler effect of light wave transmission in optical fiber, Doppler effect-based fiber-optic (FOD) sensor possesses outstanding advantages in acquiring vibration/acoustic waves with high sensitivity. Furthermore, when shape of the FOD sensor was properly selected, its sensitivity was bonding direction-independent, namely non-directionality. In this paper, characteristics of the FOD sensor were investigated for the purpose of ultrasonic detection. A piezoelectric wafer was applied as an actuator to excite Lamb waves, a kind of ultrasonic wave, in an aluminum-alloy plate. Features of the ultrasonic wave signals, collected using a number of spiral FOD sensors with various inner diameters and outer diameters, were compared to investigate characteristics of FOD sensor. Amplitude curves of the FOD sensors were hereby obtained for the future applications in ultrasonic acquisition. The results demonstrated that sensitivity of the spiral FOD sensor with longer optical fiber length was higher than that with shorter fiber length.

A Quantitative Postweld Shift Measurement and Compensation Technique in Butterfly Laser Module Packages

A quantitative postweld shift (PWS) measurement and compensation technique employing a high-magnification camera with image capturing system (HMCICS) in butterfly-type laser module packages is investigated. The results show that the direction and magnitude of the fiber alignment shifts induced by the PWS in laser-welded butterfly-type laser module packaging can be quantitatively determined and compensated. The measured coupling powers in laser module packages after welding and compensation were clearly confirmed the measured fiber shifts that determined by the translational and rotational parameters. Therefore, the fiber shifts due to the PWS could be realigned back closer to their original optimum position after applying welding compensation, and hence the coupling powers loss due to the PWS could be regained. In comparison with previous studies of the PWS compensation by the well-known mechanical adjustment through a qualitative estimation technique, this HMCICS technique has successfully provided the mechanical adjustment through a quantitative compensation to the effect of the PWS on the fiber alignment shifts in butterfly-type laser module packages. The reliable butterfly-type laser modules with high yield used in lightwave transmission systems can be developed and fabricated.

Enhanced transmission of light through subwavelength nanoapertures by far-field multiple-beam interference

Subwavelength aperture arrays in thin metal films can enable enhanced transmission of light waves. The phenomenon relies on resonant excitation and interference of the plasmon-polariton waves on the metal surface. We show a mechanism that could provide great resonant and nonresonant transmission enhancements of the light waves passed through the apertures not by the surface waves, but by the constructive interference of diffracted waves (beams generated by the apertures) at the detector placed in the far-field zone. In contrast to other models, the mechanism depends on neither the nature of the beams (continuous waves and pulses) nor material and shape of the multiple-beam source (arrays of one- and two-dimensional subwavelength apertures, fibers, dipoles, and atoms). The Wood anomalies in transmission spectra of gratings, a long standing problem in optics, follow naturally from the interference properties of our model. The point is the prediction of the Wood anomaly in a classical Young-type two-source system. The mechanism could be interpreted as a nonquantum analog of the super-radiance emission of a subwavelength ensemble of atoms (the light power and energy scales as the number of light-sources squared, regardless of periodicity) predicted by the well-known Dicke quantum model.

Theoretical Analysis of Frequency Allocations in FDM Lightwave Transmission Systems

In long-haul frequency-division-multiplexing (FDM) lightwave transmission systems, transmission characteristics are degraded by four-wave mixing (FWM) in optical fibers. To date, equally spaced (ES), unequally spaced (US), and repeated unequally spaced (RUS) allocations have been demonstrated in FDM lightwave transmission systems. It has been already revealed theoretically and experimentally that optical power of FWM light with fFWM = fi for RUS is lower than that for ES, and a total bandwidth of signal light for RUS is comparable to that for US, where fFWM is a frequency of FWM light and /< is a frequency of signal light with a channel index i. Moreover, it has been shown that equally spaced RUS (ERUS), unequally spaced RUS (URUS), and paired RUS with combined base units have lower FWM noises than RUS with a single base unit. In this paper, paired URUS is proposed as modified URUS and theoretically analyzed. Efficiency of FWM light and a total bandwidth of signal light for paired URUS are compared with those for URUS. As a result, it is found that the efficiency of FWM light with fFWM = fi for paired URUS is lower than that for URUS, and the total bandwidth of signal light for paired URUS is slightly narrower than that for URUS. Bit error rate and power penalty for paired URUS are also lower than those for URUS at a modulation speed of 10 Gb/s. Therefore, it is concluded that paired URUS is superior to URUS in FDM lightwave transmission systems.

Bidirectional reconfigurable optical add-drop multiplexer with power compensation built-in optical amplifiers

A power-compensated bidirectional reconfigurable optical add-drop multiplexer (Bi-ROADM) based on tunable fiber Bragg gratings is proposed and demonstrated. By splicing a fiber Bragg grating (FBG) within a piece of an erbium-doped fiber, a lossless, low-cost, and simply structured Bi-ROADM is obtained. The total pumping power is 40mW with 0.6m of gain fiber for both upstream and downstream signals. The Bi-ROADM performance is evaluated using a bidirectional four-channel 50km lightwave transmission link using a 2.5Gbits/s bit rate per channel. Bit-error-rate (BER) performances show that the power penalties are 0.49 and 0.76dB at a 10−9 BER, respectively, for the 25km dropped channel and the 50km passed-through channel as compared to the back-to-back condition. An extra power penalty of only 0.2dB is observed for bidirectional transmission compared to the unidirectional transmission case. Cross-talk issues and chromatic dispersion are also studied. The induced power penalty by adjacent channel cross talk is negligible (≦0.2dB) with a narrow channel spacing of 1.0nm. The induced power penalty by intraband (homodyne) cross talk is also negligible for the cross-talk level of −30dB, corresponding to 99.9% reflectivity of the FBG. The Bi-ROADM may find important applications in bidirectional WDM networks and/or bidirectional transmission.

Numerical modeling of fiber grating

The numerical simulation of light wave transmission and reflection in fiber grating is presented. The simulation is based on three-dimensional finite difference time domain algorithm. A method that can make the faint reflected wave distinct and legible is also proposed. With this method, we exhibit light wave reflection and radiation in fiber grating and tilted fiber grating, respectively, in numerical simulation.