Dense Wavelength Division Multiplexing Applications Research Articles

Metallic layered VSe2 saturable absorber based single- and dual-wavelength ultrafast fiber laser

Vanadium selenide (VSe2) is a special two-dimensional (2D) transition metal dichalcogenide (TMD) material with metallic properties similar to graphene, which has been proved to have saturable absorption properties and used in mode-locked fiber lasers. As an excellent saturable absorber (SA) material, VSe2 has advantage of ultrafast carrier recovery time and broadband optical response range. However, the potential application of VSe2 in the field of ultrafast photonics has not been fully developed so far. In this work, the as-prepared VSe2 nanosheets were deposited on tapered fiber by optical deposition to obtain VSe2 SA. By inserting VSe2 SA into an Er-doped fiber laser, the mode-locked pulses with a central wavelength of 1574.4 nm can be obtained. The pulse width of the laser is measured to be ∼680 fs which is narrowest in the reported VSe2 SA-based lasers. In addition, the unique dual-wavelength mode-locked pulse is also obtained by further adjusting intracavity dispersion and polarization. This kind of dual-wavelength pulse is valuable in high order harmonic generation enhancement, terahertz wave generation, dense wavelength division multiplexing, dual-frequency comb and other applications. Our results show that metallic VSe2 is of great potential value in ultrafast fiber laser.

Electro-optical tunable interleaver in hybrid silicon and lithium niobate thin films.

The interleaver was one of the key devices in dense wavelength division multiplexing (DWDM) applications. In this study, an interleaver with an asymmetrical Mach-Zehnder interferometer structure was designed, fabricated, and characterized in hybrid silicon and lithium niobate thin films (Si-LNOI). The interleaver based on Si-LNOI could be fabricated by mature processing technology of Si photonic, and it was capable of the electro-optical (E-O) tuning function by using the E-O effect of LN. In the range of 1530-1620 nm, the interleaver achieved a channel spacing of 55 GHz and an extinction ratio of 12-28 dB. Due to the large refractive index of Si, the Si loading strip waveguide based on Si-LNOI had a compact optical mode area, which allowed a small electrode gap to improve the E-O modulation efficiency of the interleaver. For an E-O interaction length of 1 mm, the E-O modulation efficiency was 26 pm/V. The interleaver will have potential applications in DWDM systems, optical switches, and filters.

High resolution, high channel count silicon arrayed waveguide grating router on-chip.

A 32×32 100 GHz silicon photonic integrated arrayed waveguide grating router (AWGR) is experimentally demonstrated for dense wavelength division multiplexing (DWDM) applications. The dimension of the AWGR is 2.57 mm×1.09 mm with a core size of 1.31 mm×0.64 mm. It exhibits 6.07 dB maximum channel loss non-uniformity with -1.66 dB best-case insertion loss and average channel crosstalk of -15.74 dB. In addition, in the case of 25 Gb/s signals, the device successfully realizes high-speed data routing. The AWG router provides clear optical eye diagrams and low power penalty at bit-error-rates of 10-9.

Magnetically Tunable Micro-Ring Resonators for Massive Magneto-Optical Modulation in Dense Wavelength Division Multiplexing Systems.

We demonstrate, numerically, a new concept for on-chip magneto-optical (MO) modulation in dense wavelength division multiplexing (DWDM) applications. Our idea uses materials and mechanisms that are compatible with current silicon-on-insulator fabrication and CMOS technologies for monolithic integration. The physics behind our idea stems in the exploitation of the enhanced MO activity of a micro-ring, made of cerium substituted yttrium iron garnet (Ce:YIG) material, to actively manipulate the resonance wavelengths of an adjacent micro-ring resonator (MRR) of silicon (Si). This active manipulation of the latter MO-MRR structure is used to modulate the optical signal traveling through a side-coupled Si bus waveguide. Moreover, by proper tailoring multiple MO-MRRs (side-coupled to the single Si bus waveguide) to match wavelength channels in DWDM across the entire C-band optical communications spectrum, we extend our proposal to massive and dynamic MO modulation in DWDM applications. Significantly, we noticed that the active MO shifting of the resonant wavelength (used for MO modulation here) can be used for improvements in the spectrum utilization efficiency in future elastic optical networks (EONs).

High-Efficiency Quantum Dot Lasers as Comb Sources for DWDM Applications

The trend of data center transceivers is to increase bandwidth while simultaneously decreasing their size, power consumption, and cost. While data center links have previously relied on vertical-cavity surface-emitting lasers (VCSELs) or in-plane lasers using coarse wavelength division multiplexing (WDM) to encode data, recently, dense WDM (DWDM) has moved to the forefront for next-generation links. Several approaches exist as light sources for DWDM links; DFB arrays, nonlinear microcombs, and semiconductor mode-locked lasers (MLLs). This paper focuses on quantum dot MLLs (QDMLLs), which currently offer the best efficiency, simplicity, and size. The efficiency of optical combs generated by QDMLLs is analyzed in depth in this study.

Comparative spectral tuning and fluctuation analysis of an all-fiber Mach–Zehnder interferometer and micro Mach–Zehnder interferometer

This paper presents a comparative spectral analysis for tuning all-fiber directional coupler-based Mach–Zehnder Interferometer (MZI) and all-fiber inline Micro Mach–Zehnder Interferometer (MMZI) configurations. The analytical background is necessary to estimate the change in the differential path length for tuning the channel (peak) wavelengths and inter-channel spacing (i.e. free spectral range) of the MZI. We briefly describe the method used to adjust the spectral response experimentally. The spectral tuning of all-fiber MZI and MMZI has been successfully implemented in different applications of fiber optic communication and sensing (Ramachandran et al (2014 IEEE Sens. J. 15 1270–4; 2017 Opt. Fiber Technol. 36 195–8), Kumar and Ramachandran (2013 Opt. Commun. 289 92–96; 2014 Opt. Laser Technol. 63 144–7)). The spectral tuning of the MMZI is based on the interference length. Compared to MZI, miniaturized MMZI does not deal with any spectral fluctuation when employed in dense wavelength division multiplexing (DWDM) application and a sensing scheme. Research during the past year has surveyed the different MZI structures from an application perspective to infer that MMZI might be the most suitable configuration for sensing applications, and MZI may prove to be more useful in DWDM optical networks.

Magneto-optical micro-ring resonators for dynamic tuning of add/drop channels in dense wavelength division multiplexing applications.

We numerically demonstrate an all-dielectric approach for magnetically tunable add/drop of optical channels in dense wavelength division multiplexing applications. Our concept comprises a micro-ring resonator, with an inner magneto-optical disk, side-coupled to two waveguides. The simulation results, obtained within the ITU-T G.694.1 recommendation, indicate high performance add/drop of odd and even optical channels (along the entire C-band) by flipping the intrinsic magnetization of the disk. Since the simulations were performed with CMOS-compatible materials, it is hoped that the structure proposed here can be integrated into future ultrafast optical communication networks.

Design and optimization of unidirectional emitting multi-wavelength InAs/GaAs quantum dot microring lasers on silicon

We proposed a silicon-based monolithic integrated multi-wavelength InAs/GaAs quantum dot microring laser array with radially coupled waveguides. To achieve the stable multi-wavelength lasing of the laser array in the 1.3 μm band, the three-dimensional finite-difference time-domain method is used to numerically optimize structure parameters of the microring laser with a connected III–V waveguide. The results show that the microring laser can realize a stable mode TE50,1 with an outer-wall radius of 3.5 μm, a microring width of 1.0 μm, a cladding thickness of 1.50 μm, an etching depth of 5.255 μm and a waveguide width of 0.5 μm. The mode wavelength is 1302.43 nm with a quality factor of 20,093.6. The optical coupling efficiency from the laser to the waveguide is about 47.8%. Moreover, mode wavelengths can be adjusted by the microring radius. When the microring width is 1.0 μm, changing the outer-wall radius from 2.7 to 3.9 μm with an interval of 0.2 μm, the mode wavelength ranges from 1289.29 to 1307.28 nm with a step of about 3.00 nm. It is feasible to achieve multi-wavelength laser arrays for monolithic silicon integration, which facilitates the preparation of silicon-based III–V multi-wavelength integrated light sources for dense wavelength division multiplexing applications.

Chirped-apodized Thue-Morse quasiperiodic structures for multichannel optical dispersion compensation

Nowadays, optical data transmission systems are significantly involved in telecommunication networks. Particularly, hundreds of data channels can be sent concurrently over a single optical fiber through dense wavelength division multiplexing (DWDM) and optical time division multiplexing technologies. Nevertheless, dispersion imposes main limitations for having high capacity systems with high transmission rates. This paper suggests a new dispersion compensating scheme by introducing one-dimensional chirped and apodized Thue-Morse quasiperiodic structures. Numerical investigations of the dispersion features of the proposed structure is presented. According to the acquired relatively smooth and negative slope group delay (GD) along with corresponding negative constant group velocity dispersion (GVD) of the reflection bands, it would be possible to apply the suggested structure in an efficient way in DWDM applications for multiband dispersion compensation in optical communication systems. The designed multilayer structure with short physical length and ultrahigh negative dispersion coefficient and with the free spectral range (FSR) of about 0.8 nm meets the International Telecommunication Union-Telecommunication Standardization (ITU-T) Sector requirements. Besides, the impact of main structural design parameters on the spectral features of the suggested system are presented in this paper.

Analysis of supermode properties in three-core homogeneous strongly coupled multicore fiber

We have investigated the supermode properties, namely, mode effective refractive index (RI) (neff), effective area (Aeff), and chromatic dispersion (CD), in three-core homogeneous strongly coupled multicore fibers (SC-MCFs) for different core RI profiles (step index and RI dip) and in different possible core arrangements (triangular and linear). Further, the impacts of fiber parameters, namely, core radius (a), relative RI difference between core and cladding (Δ), and core pitch (Λ), on the differential group delay (DGD) between different supermodes have also been analyzed for all the considered configurations of SC-MCFs. The analysis presented has been done numerically using the FemSIM simulation platform. It has been observed that core arrangement has significant impact on the levels of neff, Aeff, CD, and DGD, and it also affects the degeneracy of the supermodes in a three-core homogeneous SC-MCF. Further, there exist certain core pitches in which the Aeff values of fundamental supermodes in different core layouts are equal. Furthermore, there exists a certain relative RI difference between the core and cladding (Δ) values in which DGD values in all the considered SC-MCF configurations are equal. On the other hand, over a range of Δ values, DGD in a linear layout is flattened compared to a triangular layout. Incorporating the RI dip structure in the cores of SC-MCFs affects the CD levels significantly. Therefore, by careful control of the fiber parameters and core arrangement, large mode effective area and low and flattened dispersion SC-MCF can be designed that may be suitable in dense wavelength division multiplexing application.

2D-Photonic Crystal based Demultiplexer for WDM Systems – A Review

Abstract Photonic crystals (PCs) have an excellent ability to confine and control the manipulation of light. PCs are periodic or aperiodic arrangement of dielectric, metallo dielectric or super conductor micro and nanostructure materials. The periodicity of refractive index distribution in one, two and three dimensions affects the propagation of electromagnetic waves inside the PC structure. As a result of periodicity, photonic band gap (PBG) exists in a specific wavelength of PC structure and reflects the beam of light through it. This is an active part to design optical devices with a specified wavelength range. The PBG of periodic and aperiodic structures is calculated by plane wave expansion method. The wavelength division multiplexing (WDM) is an emerging technology in optical field. The multiplexer and demultiplexer are playing a prominent role for transmission and reception of light signals in WDM systems. This paper investigates and enumerates the two-dimensional (2D) PC square lattice structure-based WDM demultiplexers using quasi-square ring resonator (QSRR). The periodic array of inner rods located inside the QSRR whose shapes and refractive index justify the resonant wavelength of the channels. The channels are dropped at output ports with different resonant wavelengths by altering the size and refractive index of cavity. Typically, the resonant wavelength of the channel is shifted to higher wavelength while increasing the dielectric strength of the QSRR. The dielectric strength of the QSRR is altered by varying the inner rod radius which reflects the size of the air gap. The demultiplexer is composed of bus waveguide, scatterer rod, and reflector rod whose dimensions are also responsible to enhance or alter the performance of the systems. The finite difference time domain method is utilized to analyse the functional parameters of the structure such as the resonant wavelength, transmission efficiency, Q factor, spectral width, channel spacing and crosstalk. The impact of performance parameters is investigated through different shape of inner core and refractive index of QSRR. The observed resonant wavelength of the channels (1,525–1,610 nm) of the demultiplexers is falling under WDM systems and overall size is minimized to 434.16 μm2. The average channel spacing is about 0.75 nm, spectral width is 0.76 nm, Q factor is 2,036 and crosstalk between the channels is −30 dB. The significance of the attempt is to reduce the crosstalk which is attained by grouping odd and even numbers of channels separately. Finally, four-channel demultiplexer is designed by altering the refractive index of the inner rods (i. e. inner core) linearly and the performance parameters are investigated. Then, the point-to-point network is designed to incorporate and evaluate the performance of the demultiplexer. The observed maximum travelling distance, Bit Error Rate (BER) and receiver sensitivity of the network are 85 km, 10−9 and −16 dBm, respectively. The device could be incorporated in photonic integrated circuits for dense WDM applications.

Ring resonator based polarization diversity WDM receiver.

A ring resonator based 4 channel wavelength division multiplexing (WDM) receiver with polarization diversity is demonstrated at 10 Gb/s per channel. By forming a waveguide loop between the two output ports of a polarization splitter-rotator (PSR), the input signals in the quasi-transverse-electric (quasi-TE) and the quasi-transverse-magnetic (quasi-TM) polarizations can be demultiplexed by the same set of ring resonator filters, thus reducing the number of required channel control circuits by half compared to methods which process the two polarizations individually. Large signal measurement results indicate that the design can tolerate a signal delay of up to 30% of the unit interval (UI) between the two polarizations, which implies that compensating for manufacturing variability with optical delay lines on chip is not necessary for a robust operation. The inter-channel crosstalk is found negligible down to 0.4nm (50 GHz) spacing, at which point the adjacent channel isolation is 17 dB, proving the design's compatibility for dense WDM application.

6-mW Single-Mode High-Speed 1550-nm Wafer-Fused VCSELs for DWDM Application

This paper presents data on wafer-fused 1550-nm vertical-cavity surface-emitting lasers (VCSELs) based on the active region and distributed Bragg reflectors (DBRs) grown by molecular beam epitaxy. VCSELs with a tunnel junction aperture diameter of 8 μm show lasing at a threshold current density j th <; 3 kA/cm 2 , an output optical power of ~4 mW, and a -3 dB bandwidth of approximately 7 GHz at a 10-mA bias current. The devices demonstrate single-mode continuous wave operation with the transverse side-mode suppression ratio (SMSR) varying in the range of 40-45 dB up to roll-over currents. The increase in mirror losses due to the etching of the top DBR makes the output optical power increase to 6 mW and causes the wallplug efficiency value to reach 20%, but SMSR remains in the range of 40-45 dB. This also makes it possible to reduce both the photon lifetime and, as a result, the effect of damping and increase the modulation bandwidth to 9 GHz. The observed open and clear eye diagrams indicate that non-return-to-zero operation is possible at bit rates of up to 30 Gbps without equalization or forward error correction. The high-output optical power and modulation performance pave the way for the dense wavelength division multiplexing application of wafer-fused 1550-nm VCSELs.

Investigation on modified quasi-square PCRR based demultiplexer for WDM applications

A nano periodic two dimensional photonic crystal (2DPC) structure is proposed to design Ring Resonator based eight channels demultiplexer for wavelength division multiplexing applications. The proposed demultiplexer is composed of linear bus waveguide, modified quasi square ring resonator and L bend waveguide. The channel selection is done by altering the size of the ring resonator. The finite difference time domain and plane wave expansion methods are utilized to calculate the normalized transmission and photonic band gap of the structure. The proposed demultiplexer is dropped eight channels that are centred at 1606.8, 1608.8, 1610.4, 1612.4, 1613.9, 1615.9, 1617.7 and 1619.3 nm. The average transmission efficiency, Q factor, and line spacing are 90 %, 1960 and 1.8 nm, respectively. The miniaturized size of this device could be implemented effectively for photonic integrated circuits for dense wavelength division multiplexing applications.

Size- and current-density-controlled tunable wavelength in gan-based leds for potential dense wavelength-division multiplexing application

A micro-LED array device with hybrid pixel sizes from 30 μm~60 μm is designed and fabricated. A wavelength tunable range of 20 nm is obtained, which shows the device’s potential as a tunable light source to enable DWDM technology for visible light communication applications. For example, superimposing the luminescent spectra of μLEDs by different combinations, a series of spectra and light intensity values can be obtained. In addition, the FWHM of the complex spectrum broadens from 16 nm to 45 nm. Considered from the aspect of general lighting, a broader FWHM can increase lighting quality. The tunable peak wavelength is controlled by micro-LED size and input current density. Underlying mechanisms are discussed. The structure parameters of the arrayed-μLED device can be further optimized to satisfy requirements on communications and general lighting.

Design and Analysis of a Novel 25 GHz Interleaver for DWDM Applications with Two Ring Configurations

We present a novel scheme of an excellent flat-top 25 GHz optical interleaver based on two ring configurations. And the Advanced Systems Analysis Program (ASAP) optical modeling software has been utilized for the interleaver design. The optical path difference for interference and the phase shift are provided by the interferometer with two birefringent crystals and dual-ring arrangement. The proposed structure exhibits the passband utilization of more than 90% and the channel isolation greater than 95 dB within the C-band. Furthermore, we improve the dispersion performance by employingλ/6 wave plates as birefringent compensators for interleavers. The research results illustrate that our proposed scheme with compensator can improve the dispersion of more than 85.8%. Comparing the performance with the previous studies of optical interleavers with birefringent crystal and ring structure, the proposed system can achieve an excellent 25 GHz multichannel filter for dense wavelength division multiplexing (DWDM) transmission systems.

Polarization-independent metal-dielectric grating with improved bandwidth

A polarization-independent reflective grating with improved bandwidth is presented based on the mixed metal-dielectric grating. The novel grating is composed of the substrate, the metal slab, two dielectric layers, and the covering layer. The grating parameters are optimized for the special duty cycle of 0.6 at an incident wavelength of 1550 nm used in dense wavelength division multiplexing (DWDM). With the designed grating, high efficiency can be diffracted into the reflective –1st order for both TE and TM polarizations, where the polarization-independence is exhibited. Most importantly, the efficiency of more than 90% can be achieved within the incident wavelength bandwidth of 1262–1686 nm, where the bandwidth is improved greatly compared with the reported surface-relief single-layer grating. The reflective grating can have advantages of high efficiency, polarization-independence, and wide bandwidth, which should be useful for DWDM applications.

Design of CMOS compatible and compact, thermally-compensated electro-optic modulator based on off-axis microring resonator for dense wavelength division multiplexing applications.

In this work, we propose and demonstrate the performance of silicon-on-insulator (SOI) off-axis microring resonator (MRR) as electro-optic modulator (EOM). Adding an extra off-axis inner-ring in conventional microring structure provides control to compensate thermal effects on EOM. It is shown that dynamically controlled bias-voltage applied to the outer ring has the potency to quell the thermal effects over a wide range of temperature. Thus, besides the appositely biased conventional microring, off-axis inner microring with pre-emphasized electrical input message signal enables our proposed structure suitable for high data-rate dense wavelength division multiplexing scheme of optical communication within a very compact device size.

Tunable filter based on cavity electro-optic modulation for DWDM applications

In this paper the authors have demonstrated the feasibility and the realization of a wavelength tunable filter with an electro-optic modulation effect realized on diffused Ti:LiNbO3 waveguide. The two reflectors of the cavity consist in two Bragg׳s mirrors of identical period Λ=1.81μm and whose total number N=140. Both mirrors have been engraved using the Focused Ion Beam technique up to the depths l1=l2=2.7μm separated by a distance of lc=500μm. We obtained thanks to electro-optic modulation effect a tunable shift whose value is 0.67nm in a free spectral range FSR=1.59nm for a voltage variation up to 60V. This designed and realized probe will find many DWDM (Dense Wavelength Division Multiplexing) applications.

Low-loss and low-crosstalk 8 × 8 silicon nanowire AWG routers fabricated with CMOS technology.

Low-loss and low-crosstalk 8 × 8 arrayed waveguide grating (AWG) routers based on silicon nanowire waveguides are reported. A comparative study of the measurement results of the 3.2 nm-channel-spacing AWGs with three different designs is performed to evaluate the effect of each optimal technique, showing that a comprehensive optimization technique is more effective to improve the device performance than a single optimization. Based on the comprehensive optimal design, we further design and experimentally demonstrate a new 8-channel 0.8 nm-channel-spacing silicon AWG router for dense wavelength division multiplexing (DWDM) application with 130 nm CMOS technology. The AWG router with a channel spacing of 3.2 nm (resp. 0.8 nm) exhibits low insertion loss of 2.32 dB (resp. 2.92 dB) and low crosstalk of -20.5~-24.5 dB (resp. -16.9~-17.8 dB). In addition, sophisticated measurements are presented including all-input transmission testing and high-speed WDM system demonstrations for these routers. The functionality of the Si nanowire AWG as a router is characterized and a good cyclic rotation property is demonstrated. Moreover, we test the optical eye diagrams and bit-error-rates (BER) of the de-multiplexed signal when the multi-wavelength high-speed signals are launched into the AWG routers in a system experiment. Clear optical eye diagrams and low power penalty from the system point of view are achieved thanks to the low crosstalk of the AWG devices.