Adjacent Wavelength Channels Research Articles

Reconfigurable multichannel amplitude equalizer based on cascaded silicon photonic microrings

A compact on-chip reconfigurable multichannel amplitude equalizer based on cascaded elliptical microrings is proposed and demonstrated experimentally. With the optimized structure of the elliptical microring with adiabatically varied radii/widths, the average excess loss for each channel in the initialized state is measured to be less than 0.5 dB, while the attenuation dynamic range can be over 20 dB. Flexible tunability through the overlapping of the resonance peaks of adjacent wavelength-channels enables even higher attenuation dynamic ranges up to 50 dB. Leveraging the thermo-optic effect and fine wavelength-tuning linearity, precise tuning of the resonance peak can be implemented, enabling dynamic power equalization of each wavelength-channel in wavelength-division-multiplexing (WDM) systems and optical frequency combs. The proposed architecture exhibits excellent scalability, which can facilitate the development of long-haul optical transport networks and high-capacity neuromorphic computing systems, while improving the overall performance of optical signals in WDM-related systems.

Cosine apodization of dual-resonance all-fiber acousto-optic tunable filters.

We experimentally demonstrate a novel cosine apodization technique for dual-resonance all-fiber acoustic-optic tunable filter. The technique is based on a hybrid control of input acoustic polarization state and circumferential fiber twist. We will show that intrinsic sidelobe spectra occurring between dual filtering bands are successfully suppressed through our approach, which will be also theoretically confirmed via our analytical and numerical studies. The results illustrate that the spectral positions of each resonance are tuned linearly and continuously by the fiber twist, and that overall sidelobe spectra between two resonances are suppressed regardless of fiber twist angle. The proposed scheme is useful to minimize cross talk between adjacent wavelength channels in optical sensor systems. We highlight that our approach is directly applicable to low-noise matched filtering.

Compressive sensing in a photonic link with optical integration

In this Letter, we present a novel structure to realize photonics-assisted compressive sensing (CS) with optical integration. In the system, a spectrally sparse signal modulates a multiwavelength continuous-wave light and then is mixed with a random sequence in optical domain. The optical signal passes through a length of dispersive fiber, the dispersion amount of which is set to ensure that the group delay between the adjacent wavelength channels is equal to the bit duration of the applied random sequence. As a result, the detected signal is a delay-and-sum version of the randomly mixed signal, which is equivalent to the function of integration required in CS. A proof-of-concept experiment with four wavelengths, corresponding to a compression factor of 4, is demonstrated. More simulation results are also given to show the potential of the technique.

Novel architecture of WDM/OCDMA-PON based on SSFBG and wavelength re-modulation technology

A novel architecture of wavelength-division multiplexing/optical code division multiplexing access-passive optical network (WDM/OCDMA-PON) based on superstructure fiber Bragg grating (SSFBG) and wavelength re-modulation technology is proposed. In this scheme, WDM is overlaid on OCDMA channel in a single network by virtue of a kind of SSFBG, and the total capacity of hybrid PON can be extended by regulating the transmission power reasonably. Re-modulation technology is also a good method to save wavelength-specific components at the optical network unit (ONU) and cost of wavelength management on the customer side. In simulation system, 1.25 Gb/s up/downstream data are transported with good performance. In addition the crosstalk penalties from adjacent wavelength channels (with the same OC) are found to be negligible in upstream and downstream transmissions.

Chromatic Dispersion Monitoring for High-Speed WDM Systems Using Two-Photon Absorption in a Semiconductor Microcavity

This paper presents a theoretical and experimental investigation into the use of a two-photon absorption (TPA) photodetector for use in chromatic dispersion (CD) monitoring in high-speed, wavelength division multiplexing network. In order to overcome the inefficiency associated with the nonlinear optical-to-electrical TPA process, a microcavity structure is employed. An interesting feature of such a solution is the fact that the microcavity enhances only a narrow wavelength range determined by device design and angle at which the signal enters the device. Thus, a single device can be used to monitor a number of different wavelength channels without the need for additional external filters. When using a nonlinear photodetector, the photocurrent generated for Gaussian pulses is inversely related to the pulse width. However, when using a microcavity structure, the cavity bandwidth also needs to be considered, as does the shape of the optical pulses incident on the device. Simulation results are presented for a variety of cavity bandwidths, pulse shapes and durations, and spacing between adjacent wavelength channels. These results are verified experimental using a microcavity with a bandwidth of 260 GHz (2.1 nm) at normal incident angle, with the incident signal comprising of two wavelength channels separated by 1.25 THz (10 nm), each operating at an aggregate data rate of 160 Gb/s. The results demonstrate the applicability of the presented technique to monitor accumulated dispersion fluctuations in a range of 3 ps/nm for 160 Gb/s return-to-zero data channel.

Chromatic Dispersion Monitoring for High-Speed WDM Systems Using Two-Photon Absorption in a Semiconductor Microcavity

This paper presents a theoretical and experimental investigation into the use of a two-photon absorption (TPA) photodetector for use in chromatic dispersion (CD) monitoring in high-speed, WDM network. In order to overcome the inefficiency associated with the nonlinear optical-to-electrical TPA process, a microcavity structure is employed. An interesting feature of such a solution is the fact that the microcavity enhances only a narrow wavelength range determined by device design and angle at which the signal enters the device. Thus, a single device can be used to monitor a number of different wavelength channels without the need for additional external filters. When using a nonlinear photodetector, the photocurrent generated for Gaussian pulses is inversely related to the pulsewidth. However, when using a microcavity structure, the cavity bandwidth also needs to be considered, as does the shape of the optical pulses incident on the device. Simulation results are presented for a variety of cavity bandwidths, pulse shapes and durations, and spacing between adjacent wavelength channels. These results are verified experimental using a microcavity with a bandwidth of 260 GHz (2.1 nm) at normal incident angle, with the incident signal comprising of two wavelength channels separated by 1.25 THz (10 nm), each operating at an aggregate data rate of 160 Gb/s. The results demonstrate the applicability of the presented technique to monitor accumulated dispersion fluctuations in a range of 3 ps/nm for 160 Gb/s RZ data channel.

A simple approach to deriving an electron energy distribution from an incoherent Thomson scattering spectrum

A clear picture is given to illustrate how the electron energy distribution in a plasma can be directly related to an incoherent Thomson scattering spectrum when the electron density distribution is spherically symmetric in velocity space. Based on this relationship, a simple approach is outlined to derive the electron energy distribution unambiguously. The method involves plotting the differences in Thomson scattering intensities between adjacent wavelength channels as a function of the square of the wavelength shift. Thomson scattering spectra obtained from microwave-induced helium plasmas sustained at atmospheric pressure at forward powers of 100 and 350 W are used to derive electron energy distributions through this procedure. Significant deviations from a Maxwellian distribution were found under both conditions over the entire wavelength span covered in the experiment, corresponding to an electron energy range of 0.1–6.6 eV. Compared with a Maxwellian energy distribution, the portion of the electrons with energies between 2 and 6 eV appear to be shifted toward both lower and higher energy values in these plasmas. The ratios of the Thomson scattering intensity at the wavelength channel farthest from the center to the corresponding Maxwellian value indicate that the total number of electrons with energies larger than 6.6 eV is at least 1.8 and 3.1 times higher than that predicted by local thermodynamic equilibrium for plasmas at 100 and 350 W, respectively. In contrast to a Maxwellian distribution, electrons in the microwave-induced plasmas are not most concentrated at the center of velocity space, but reach a maximum in a spherical layer at a distance of approximately 4×10 7 cm s −1 from the space center. In addition, for both plasma power levels, the bulk of the electron velocity distribution function is slightly compressed and the maximum value is shifted toward lower velocity values compared with a Maxwellian distribution.

On-the-Fly Fluorescence Lifetime Detection in Liquid Chromatography with Data Collected Simultaneously at Multiple Emission Wavelengths

A novel liquid chromatography (LC) fluorescence detector simultaneously generates fluorescence decay curves at multiple emission wavelengths. The fourth harmonic (266 nm) excitation light from a pulsed Nd:YAG laser is focused into a standard LC flow cell. A small spectrograph disperses the fluorescence. Fiber-optic delay lines positioned in the exit focal plane time the arrival of photons at the photomultiplier tube detector. Four wavelength channels with 50 ns channel spacing were used in this study. The intensity-decay time waveform is averaged with a digital storage oscilloscope. Temporal overlap between adjacent wavelength channels is removed during the post-processing of the data. System performance was characterized for chromatographic elution of polycyclic aromatic hydrocarbons (PAHs). Detection limits are 1.6 ppb for naphthalene and 0.14 ppb for fluorene. Resolution of co-eluting 1,4-dimethyl-naphthalene and anthracene components on the basis of differences in their lifetimes and spectral distributions is demonstrated.

Impact of thermal drifts in LED spectrally sliced WDM systems

In LED spectrally sliced wavelength division multiplexed (WDM) systems the optical crosstalk caused by spectral overlapping of adjacent wavelength channels is known to depend on the source spectrum. However, the LED output is temperature dependent and therefore the crosstalk performance of such a system may be expected to vary with operating temperature drifts of the optical source. These effects are theoretically explored to estimate the subsequent performance degradation due to optical crosstalk. Analytical expressions developed indicate that the optical crosstalk noise-to-signal ratio for all spectrally sliced wavelength channels has very little dependence on the LED operating temperature. However, the optical power coupled into a spectrally sliced channel is strongly dependent on the source operating temperature which can cause a depletion of optical power levels within channels placed at the extremes of the source emission spectrum. Hence, the LED source may be employed without any temperature control when precautions are taken in the case of channels located at the tail-ends of the source emission spectrum.

Narrow spectral width surface emitting LED for long wavelength multiplexing applications

We discuss a novel surface emitting InGaAsP LED with a relatively narrow spectrum obtained by the integral growth of a 2.6 μm thick semiconductor absorbing, or filtering, layer on a conventional double heterostructure device. The full width at half maximum for the filtered LED with peakpower at \lambda_{0} = 1.29 \mu m is \Delta\lambda = 850 A (63 meV),compared with an unfiltered LED half width of \Delta\lambda = 1300 A (97 meV). Half width measurements were made at a drive current density of 10 kA/cm2. The optimization of the filtered LED is considered, and it is found that a substantial reduction of the power in the short wavelength spectral tail can be obtained without significantly decreasing the total LED output power. For these reasons, the filtered LED has potential applications for low cost wavelength division multiplexing systems where a low overlap of spectral output between adjacent wavelength channels is required. In addition, the narrow spectrum may prove useful in moderate bit rate (> 100 Mbits/s) multimode transmission systems where the output from broad spectrum, conventional surface emitting LED's leads to unacceptably high levels of signal dispersion.