ITU Grid Research Articles

Eight-channel C-Band demultiplexer based on one-dimensional photonic crystals defected with lithium niobate

We propose an eight-channel Dense Wavelength Division Demultiplexer (DWDM) device based on one-dimensional photonic crystal (1-D PC) structures defected with lithium niobate (LiNbO3) thin films. The demultiplexer operates in the C-band window (1460–1560 nm), which belongs to the ITU grid for DWDM applications. The desired output wavelength is achieved by applying the proper external electric field to the LiNbO3 layer at various temperatures. The transfer matrix method (TMM) is used to simulate the transmittance spectrum. Efficient output channels with transmittances of about 90% are attained. An excellent quality factor of 30919 and a very small line width of 0.05 nm are exceptional values acquired by the output channels. A very small channel spacing of 0.182 nm is attained without compromising the crosstalk, which reaches a minimum value of −34 dB. The astounding characteristics of the eight channels reveal that the proposed demultiplexer could be a potentially efficient device for DWDM networks.

Demultiplexers for DWDM applications using one-dimensional planar binary photonic crystals defected with ZnSxSe1-x ternary alloys

We propose a structure comprising a defective one-dimensional planar binary photonic crystal (PC) to realize a demultiplexing operation for dense wavelength division multiplexer (DWDM) transmission systems. The unit cell of the crystal consists of ZnSe and ZnS thin films, and the defective layer is ZnSxSe1 − x, (0 ≤ x ≤ 1). Transmittance for the structure was numerically investigated using the transfer matrix method. The selecting mechanism of the output wavelength was based on selecting the appropriate sulfur concentration (x value) in the defective layer of the PC. We used six different sulfur concentrations to demultiplex six different output channels. The line width of the output channels was about 0.13 nm with a quality factor of 12,199. The average crosstalk between the adjacent transmitted channels was about −22.16 dB. The overall dimension of the proposed demultiplexer was about 108.4 μm, which practically makes the demultiplexer easily integrated into photonic circuits. Our results show that the proposed structure has all of the required characteristics to demultiplex the C-band signals with a 0.16 nm separation, which corresponds to the ITU grid for DWDM.

Investigation and analysis of inter-satellite- optical wireless communication system based on dense-wavelength-division multiple-access

Optical wireless communication offers high data rates applications due to license-free and wide-bandwidth access techniques and cost-effective implementations. Recently, the world introduced to a revolutionary technology called Inter-satellite optical communication that aims to establish transmission among satellites. However, transmitting pointing errors is the main issue in creating an inter-satellite link, which leads turbulences in the connection. This article aims to study the implementation of an optical wireless communication system in satellite communication. In this work, we investigate and analysis of data rate of 16-channel in inter-satellite optical wireless communication systems (Is-OWC) based on dense-wavelength-division multiple-access (DWDM) with standard downlink channel spacing DWDM (ITU grid specification). Computer simulations are carried using Opti-system over different wavelengths between two satellites transmit 160 Gbps over 7000 km. This evaluation performance is based on eye diagrams, BER, optical spectrum and Q-Factor with minimum input power under turbulences of transmitter pointing error angle

Investigation and analysis of inter-satellite- optical wireless communication system based on dense-wavelength-division multiple-access

Optical wireless communication offers high data rates applications due to license-free and wide-bandwidth access techniques and cost-effective implementations. Recently, the world introduced to a revolutionary technology called Inter-satellite optical communication that aims to establish transmission among satellites. However, transmitting pointing errors is the main issue in creating an inter-satellite link, which leads turbulences in the connection. This article aims to study the implementation of an optical wireless communication system in satellite communication. In this work, we investigate and analysis of data rate of 16-channel in inter-satellite optical wireless communication systems (Is-OWC) based on dense-wavelength-division multiple-access (DWDM) with standard downlink channel spacing DWDM (ITU grid specification). Computer simulations are carried using Opti-system over different wavelengths between two satellites transmit 160 Gbps over 7000 km. This evaluation performance is based on eye diagrams, BER, optical spectrum and Q-Factor with minimum input power under turbulences of transmitter pointing error angle

InAs/InP Quantum Dash Semiconductor Coherent Comb Lasers and their Applications in Optical Networks

We report on the design, growth, and fabrication of InAs/InP quantum dash (QD) gain materials and their use in lasers for optical network applications. A noise performance comparison between QD and quantum well (QW) Fabry–Perot (F-P) lasers has been made. By using the QD gain material we have successfully developed and assembled C-band coherent comb laser (CCL) modules with an electrical fast feedback loop control system to ensure a targeted mode frequency spacing. The frequency spacing was maintained within ±100 ppm and the operation wavelengths locked on the desired ITU grid within 0.01 nm over a period of several months. We also investigated a 25-GHz C-band QD CCL with an external cavity self-injection feedback locking (SIFL) system to reduce the optical linewidth of each individual channel to below 200 kHz in the wavelength range from 1537.55 nm to 1545.14 nm. The RF mode beating signal 3-dB bandwidth was also reduced from 9 kHz to approximately 500 Hz with this SIFL system. These QD CCLs with ultra-low relative intensity noise (RIN), ultra-narrow optical linewidth, and ultra-low timing jitter are excellent laser sources for multi-terabit optical networks. Using a 34.2 GHz QD CCL we demonstrate 10.8 Tbit/s (16QAM 48 × 28 GBaud PDM) coherent data transmission over 100 km of standard single mode fiber (SSMF) and 5.4 Tbit/s (PAM-4 48 × 28 GBaud PDM) aggregate data transmission capacity over 25 km of SSMF with error-free operation.

Widely Tunable Electro-Absorption Modulated V-Cavity Laser

We report a widely tunable V-cavity laser monolithically integrated with an electro-absorption modulator (EAM). The device shows a 41-nm wavelength tunability covering 51 channels at 100 GHz spacing on the ITU grid, with a side-mode suppression ratio (SMSR) higher than 47 dB. Error-free transmission over 50 km standard single-mode fiber at 10 Gb/s is demonstrated at all wavelengths over the tuning range. The integrated EAM exhibits a dynamic extinction ratio higher than 12.5 dB. The bit-error-rate measurements show a power penalty less than 3 dB for 50-km transmission. The results demonstrate that this ultra-compact and regrowth-free electro-absorption modulated tunable laser can be a cost-effective transmitter solution for DWDM metropolitan and access networks.

Toward a reconfigurable quantum network enabled by a broadband entangled source

We present a proof-of-principle experimental demonstration of a reconfigurable entanglement distribution scheme utilizing a poled fiber-based source of broadband polarization-entangled photon pairs and dense wavelength-division multiplexing. A large bandwidth (> 90 nm, centered at 1555 nm) and highly spectrally correlated nature of the entangled source can be exploited to allow for the generation of more than 25 frequency-conjugate entangled pairs when aligned to the standard 200 GHz ITU grid. In this work, three frequency-conjugate entangled pairs are used to demonstrate quantum key distribution, with the wavelength-selective switching done manually. The entangled pairs are delivered over 40 km of actual fiber, and an estimated secure key rate of up to 20 bits/s per bi-party is obtained.

Integrated Reconfigurable Silicon Photonics Switch Matrix in IRIS Project: Technological Achievements and Experimental Results

This paper reports the performances of a silicon photonics optical switch matrix fabricated by using large-scale three-dimensional (3-D) integration. The wavelength selective optical switch consists of a photonic integrated circuit (PIC), with 1398 circuit elements, interconnected in a 3-D stack with its control electronic integrated circuit (EIC). Each PIC element can be trimmed or reconfigured by using metallic heaters. The EIC is designed to drive the heaters and to read the signal of monitor photodiodes integrated into the PIC. Small footprint and high energy efficiency are achieved in the PIC and the EIC. Automatic wavelength alignment of the optical circuits in the PIC to the ITU grid and fine temperature tuning of each photonic element to optimize the switch insertion losses are obtained by an optimization routine. A fully packaged switch with input/output fibers is tested both for optical and electrical characteristics as well as for the system performances. Fiber-to-fiber insertion losses of about 20 dB and channel isolation of −35 dB are achieved. Bit error rate characteristics at 25 Gb/s are evaluated. Perspective applications of the optical switch in optical transport and intra-data center networks are discussed.

Multiple-DWDM-channel heralded single-photon source based on a periodically poled lithium niobate waveguide.

We report on the experimental realization of a multiple-DWDM-channel heralded single-photon source in a periodically poled lithium niobate waveguide. Our single photon at the telecom wavelength covers more than 40 channels of the ITU grid. All channels have virtually identical efficiencies, and the multi-photon emission probability is reduced by a factor up to more than 150 compared to a Poissonian light source. Together with the all-fiber structure, all these advantages make our heralded single-photon source suitable for real-world quantum networks. The implementation with a 50 MHz pulsed laser provides a data rate compatible with current quantum communication systems, while being able to be pumped at higher repetition rates.

Reconfigurable DP-16QAM/QPSK transponders for the transmission of coherent 200 Gb/s carriers in a flexgrid super-channel arrangement

The next generation of Dense Wavelength Division Multiplexing (DWDM) networks are likely to use Flexgrid arrangement, providing operators with additional flexibility when assigning spectrum compared to traditional DWDM networks using the 50GHz ITU grid. Flexgrid breaks the spectrum up into small (typically 12.5GHz) slots, but its key feature is that contiguous slots can be joined together to form arbitrary sized blocks of spectrum. This additional flexibility will allow faster transponders that utilize high spectral efficiency modulation techniques, like multi-level m-QAM schemes. From the use of these new spectrum efficient modulation formats and finer control over spectrum allocations, a key benefit that Flexgrid offers to the network operators is that their DWDM networks can carry more traffic with optimized spectral efficiency. High speed technology became essential for realizing greater network capacity and enabling network operators to meet the increasing bandwidth challenges from new generation superfast devices, services and applications. Coherent technologies beyond 100G are now focusing on higher level modulation formats and multiple sub-carriers/channels, using super-channels to achieve Terabit transmission.

Fully Switchable Multi-Wavelength Fiber Lasers Based on Random Distributed Feedback for Sensors Interrogation

In this paper, the experimental study and characterization of a novel real-time switchable multi-wavelength fiber laser has been carried out. Two different gain materials, such as a 50-km SMF and a 2.5-km DCF fibers were characterized and compared, respectively. The MWFL can generate any wavelength combination with an emission lines distance of 50, 100, and 200 GHz fitting the ITU grid specifications. By using both Er-doped fiber and Raman amplification, a ~30-nm-wide lasing window at the C band can be utilized to create up to 30 different lasing wavelengths into the ITU Grid, that can be switched automatically and in realtime when desired. Utilization of such a laser for versatile interrogation of different sensing networks is also shown.

Athermal Colorless C-Band Optical Transmitter System for Passive Optical Networks

This paper reports an uncooled transmitter system using a digital super-mode (DS) distributed Bragg reflector (DBR) tunable laser, which is able to act as an athermal, wavelength agnostic transmitter suitable for wavelength division multiplexed (WDM) passive optical network (PON) applications. An open-loop laser current control algorithm is designed to compensate autonomously for wavelength drift, thus allowing constant operating wavelength to be achieved regardless of ambient temperature. An improved wavelength accuracy of ±3 GHz is achieved when using low bandwidth feedback from the central office using information from a centralized shared wavelength locker. The entire laser start-up, channel selection and subsequent wavelength control is autonomous and has been implemented on micro-controllers and field programmable gate arrays. We demonstrate a three channel WDM-PON system comprising an uncooled packaged DS-DBR laser in the presence of two neighboring interfering channels. Error free transmission over 40 km single mode fiber of 10 Gb/s externally modulated NRZ data, is achieved for each of 48 C-band channels on the 100 GHz ITU grid. Successful athermal operation is demonstrated by sweeping the ambient temperature of the laser from 15 to 70 °C with a maximum wavelength deviation for any channel of no more than 0.1 nm.

Wavelength tunable single longitudinal mode fiber laser pinned to 25 GHz spacing

ABSTRACTA wavelength tunable single longitudinal mode (SLM) fiber laser is demonstrated with a high power Er‐Yb codoped fiber amplifier (EDFA) used as gain medium. A fiber Fabry‐Perot filter and a bandpass filter ensure single wavelength lasing. To realize SLM operation, a section of unpumped EDF in a Sagnac loop is adopted as a saturable absorber, which results in a self‐induced FBG (frequency band gap) with an ultranarrow bandwidth. The proposed laser is very stable and in SLM operation. The signal to noise ratio is measured about 65 dB. The output laser has a tuning range over 30 nm from 1535 to1565 nm. The tuning step of the proposed laser is 0.20 nm pinned to the ITU grid. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:2404–2406, 2014

Ten-channel InP-based large-scale photonic integrated transmitter fabricated by SAG technology

Abstract A 10-channel InP-based large-scale photonic integrated transmitter was fabricated by selective area growth (SAG) technology combined with butt-joint regrowth (BJR) technology. The SAG technology was utilized to fabricate the electroabsorption modulated distributed feedback (DFB) laser (EML) arrays at the same time. The design of coplanar electrodes for electroabsorption modulator (EAM) was used for the flip-chip bonding package. The lasing wavelength of DFB laser could be tuned by the integrated micro-heater to match the ITU grids, which only needs one electrode pad. The average output power of each channel is 250 μW with an injection current of 200 mA. The static extinction ratios of the EAMs for 10 channels tested are ranged from 15 to 27 dB with a reverse bias of 6 V. The frequencies of 3 dB bandwidth of the chip for each channel are around 14 GHz. The novel design and simple fabrication process show its enormous potential in reducing the cost of large-scale photonic integrated circuit (LS-PIC) transmitter with high chip yields.

Transmission of High-Baud PDM-64QAM Signals

In this paper we demonstrate high-baud generation and transmission of PDM-64QAM signals. The 64QAM signal is generated using one single in-phase/quadrature (I/Q) modulator driven with eight-level signals synthesized by all binary electrical inputs. With hard-decision forward error correction (HD-FEC), we first demonstrate 11.2-GBd and 22.4-GBd PDM-64QAM transmissions over 320-km standard single mode fiber (SSMF). Then we specifically introduce a long memory filter (LMF) and prove its effectiveness in reflection compensation both theoretically and experimentally. With the aid of soft-decision FEC (SD-FEC), optical pre-shaping, and LMF, we transmit a 41.4-GBd PDM-64QAM signal over 1200-km SSMF. Finally, using per-channel 41.6-GBd PDM-64QAM with SD-FEC, we successfully demonstrate 50-GHz-spaced, 8 × 499-Gb/s transmission over 720-km SSMF. This is the first demonstration of 400-Gb/s/ch WDM transmission on the 50-GHz ITU grid that uses a per-channel single-carrier scheme.

Simultaneous Demultiplexing and Demodulation of 10 Gbit/s RZ-DPSK Signal using Arrayed Waveguide Gratings

We propose and demonstrate simultaneous demultiplexing and demodulation of wave length division multiplexed (WDM) return-to-zero differential phase-shift keying (RZ-DPSK) signal by a standard Arrayed Waveguide Gratings (AWG) at 10 Gbit/s data rate using ITU grid of 100 GHz channel spacing. Simulation results show error free transmission in a distance of 25 km with negligible power penalty and improved receiver sensitivity. The propose scheme is cost effective in term of component counts, hence reduces the deployment cost significantly.

Stabilized Wide-Band Wavelength Conversion Enabled by CW-Triggered Supercontinuum

We experimentally demonstrate a wide-band wavelength converter enabled by continuous-wave (CW) triggered supercontinuum (SC) generated from a picosecond pulse source. In the presence of the trigger, the pulse-to-pulse SC stability considerably improved, leading to error-free wavelength conversion operations spanning from 1510 to 1615 nm, which covers the S-, C-, and L-bands of the ITU grids. In addition, the CW-triggered SC spectrum is significantly enhanced to over 300 nm measured at 20 dB below the peak spectral power (from ~1400 to 1700 nm and beyond). Our scheme thus provides a low pump power and wide-band solution for fiber-based optical wavelength conversion. With its better CW source, e.g., lower relative intensity noise and longer coherence length, the present scheme has the potential to be further applied to phase-modulated signals.

Simultaneous four channel wavelength conversion of 50Gbps CSRZ–DPSK signals in S and C bands using HNLF without additional pump signals

Optical wavelength conversion is expected to be an important technique for future advanced dense wavelength division multiplexing systems. It enhances wavelength routing capabilities, improves network reconfigurability and eliminating the problem associated with wavelength reuse in network. Here, simultaneous 50Gbps four channel wavelength conversion is established in S and C bands of ITU grid using four wave mixing (FWM) technique in high nonlinear fiber (HNLF) without additional pump signals. Since the four channel wavelength conversion is to be performed, the frequency spacing between the pairs of signal in S and C bands should be maintained in order to avoid the signal degradation by the effect of higher order FWM. Thereby the best frequency spacing between the pairs of signals in S and C bands is estimated to maintain good BER over the wavelength converted signals of both bands. So the selected frequency spacing between the pairs fulfills the freedom of selecting any frequency spacing within a pair of wavelengths in S and C band signals. It is also shown that CSRZ–DPSK modulated input signal enhances the BER of wavelength converted signals over the RZ–DPSK. In addition to this, uniform wavelength conversion over a wide bandwidth with a reduced length of HNLF is achieved and also the best power range is estimated to obtain good conversion efficiency.

Transmission of 10-Gb/s Directly Modulated RSOA Signals in Single-Fiber Loopback WDM PONs

We demonstrate the transmission of 10-Gb/s upstream signals over 20-km fiber using a 1.3-GHz-bandwidth reflective semiconductor optical amplifier and a 40-ps delay interferometer (DI) in a loopback configured wavelength-division-multiplexed (WDM) passive optical network. We show that a single DI can be used in a set-and-forget mode to equalize 34 WDM channels anchored at the 100-GHz spaced ITU grid, without any help of postdetection electronic processing.

A Liquid Crystal Tunable Wavelength-Interleaved Isolator With Flat Spectral Response

A liquid crystal tunable bidirectional isolator is proposed. A wave plate is employed to realize a wavelength interleaving function. If it works as a full wave plate, the corresponding light only may pass along a specific direction while the opposite way is isolated. However, for wavelengths that the light experiences a half wave plate, the isolator's passing direction is reversed. A liquid crystal cell is utilized to tune the isolator so that both the isolated wavelength and direction could be reconfigured. A 100-GHz channel-interleaved tunable bidirectional isolator is demonstrated with 30 dB isolation in both directions. The spectral response at ITU grids are further flattened by using a wave plate stacking technique.