Symmetric Mach-Zehnder Research Articles

Demultiplexer for optical orthogonal frequency-division multiplexing using an optical fast-Fourier-transform circuit

An integrated-optic device for demultiplexing optical orthogonal frequency-division-multiplexed signals is described that consists of an optical fast-Fourier-transform circuit and optical gates. The fast-Fourier-transform circuit is composed of symmetrical Mach-Zehnder interferometers that are suitable for precisely adjusting filter characteristics. This device can process four or more channels with a relatively simple configuration and reduced size. An optical demultiplexer for four subcarrier channels is realized and used to demultiplex 4 x 10 Gbit/s signals.

Advanced quantum dot and photonic crystal technologies for integrated nanophotonic circuits

Two types of nanophotonic technologies—two-dimensional photonic crystal (2D PC) slab waveguides (WGs) and quantum dots (QDs)—were developed for key photonic device structures in the future. For an ultrafast digital photonic network, an ultrasmall and ultrafast symmetrical Mach–Zehnder (SMZ)-type all-optical switch (PC-SMZ) and an optical flip–flop device (PC-FF) have been developed. To realize these devices, one method is to develop a selective-area molecular beam epitaxial growth QD technique by employing a metal mask method. Another method is to establish a new design method, i.e., topology optimization of the 2DPC WG with a wide and flat bandwidth, high transmittance, and low reflectivity. We also fabricated an optical microcavity in a photonic crystal slab embedded with GaAs QDs by droplet epitaxy. The Purcell effect on the exciton emission of GaAs QDs was confirmed by microphotoluminescence and lifetime measurements.

Calculation of crosstalk in the 8×10 Gbit/s OTDM system using SMZ switch

Optical time division multiplexing is an emerging and promising alternative for future high-speed photonic networks because of its ability to accommodate higher bit rate and flexible bandwidth. Symmetric Mach–Zehnder (SMZ) have been found to be the most suitable switching element than all the available de-multiplexing switches because of compact size, thermal stability, and low-power operation. In this paper, we simulate eight-channel OTDM systems (all-channel multiplexer and de-multiplexer (DEMUX)) with a Mach–Zehnder modulator and SMZ de-multiplexer to investigate the impact of control signal power and to calculate the crosstalk.

Performance of a 4×10 Gbit/s optical time domain multiplexed system using SMZ switching

Optical time division multiplexing (OTDM) is emerging and promising alternative for future high-speed photonic networks because of its ability to accommodate higher bit rate and flexible bandwidth. Among other factors the performance of an OTDM system largely depends upon the switching characteristics of a de-multiplexer (DEMUX). Symmetric Mach–Zehnder (SMZ) have been found to be most suitable than all the available de-multiplexing switches because of compact size, thermal stability, and low power operation. In this paper, we simulate four-channel OTDM systems (all channel multiplexer (MUX) and DEMUX) with a Mach–Zehnder modulator and SMZ DEMUX to investigate the impact of signal power, pulse width and control signal power on BER.

Optical-Nonlinearity-Induced Phase Shift via Selective-Area Grown InAs Quantum Dots in a Photonic Crystal Waveguide

The selective-area growth (SAG) of InAs quantum dots (QDs) in a GaAs two-dimensional photonic crystal waveguide (2DPC-WG) slab was performed by a metal-mask molecular beam epitaxy (MBE) method, and phase shifts induced by an optical nonlinearity (ONL) of the QD were characterized by two-color pump and probe measurements. The phase shift is caused by the refractive index change (Δn) in the PC-WG, while the Δn is induced by the absorption saturation as a third-order ONL effect of the QD. For the samples with different SAG-QD lengths, it was confirmed that the phase shift is proportional to the length of the SAG area and to the pump energy. For application to the PC/QD-based symmetrical Mach–Zehnder (SMZ)-type all-optical switch, that is, PC-SMZ, a bright prospect for the design criterion was obtained such that the 180° phase shift, necessary for the PC-SMZ, can be achieved using the four-stacked-layer-QD WG with a 250 µm length at a pump energy of 100 fJ if a slow light with a group velocity of 0.05c (c is the light velocity in vacuum) is used.

Polarization independent thermally tunable erbium-doped fiber amplifier gain equalizer using a cascaded Mach-Zehnder coupler

A thermally tunable erbium-doped fiber amplifier (EDFA) gain equalizer filter based on compact point symmetric cascaded Mach-Zehnder (CMZ) coupler is presented with its mathematical model and is found to be polarization dependent due to stress anisotropy caused by local heating for thermo-optic phase change from its mathematical analysis. A thermo-optic delay line structure with a stress releasing groove is proposed and designed for the reduction of polarization dependent characteristics of the high index contrast point symmetric delay line structure of the device. It is found from thermal analysis by using an implicit finite difference method that temperature gradients of the proposed structure, which mainly causes the release of stress anisotropy, is approximately nine times more than that of the conventional structure. It is also seen that the EDFA gain equalized spectrum by using the point symmetric CMZ device based on the proposed structure is almost polarization independent.

All-optical flip flop based on a symmetric Mach-Zehnder switch with a feedback loop and multiple forward set/reset signals

A novel all-optical set/reset flip flop (AOFF) based on a symmetric Mach-Zehnder switch with a feedback loop and multiple forward set/reset signals is presented. The proposed flip flop has a fast response, a flat output gain, and a short switching-on interval of a few hundreds of picoseconds regardless of the associated feedback-loop delay. It is shown that a high on/off constrast ratio at the AOFF output is achieved above 20 dB.

Photonic crystal and quantum dot technologies for all-optical switch and logic device

Nano-photonic technologies of GaAs-based two-dimensional photonic crystal (2DPC) slab waveguides (WGs) and InAs-based quantum dots (QDs) are reviewed for a symmetrical Mach–Zehnder (SMZ) type, ultra-small and ultra-fast all-optical switch (PC-SMZ) and logic device. As the first phase, ultra-fast (∼ps) and ultra-low energy (∼100 fJ) switching has been demonstrated using a chip 600 μm×300 μm in size. The second phase is to create a PC-SMZ-based ultra-fast photonic logic switch with a latch function for a future ultra-fast photonic digital processor. One of the priority subjects is to establish a new design method, i.e., topology optimization (TO) method of 2DPC-WGs with wide/flat bandwidth, high transmittance and low reflectivity. Another one is to develop selective-area-grown, high-density and highly uniform InAs QDs with large optical nonlinearity (ONL) by using a metal-mask (MM) molecular beam epitaxy (MBE) growth method. Recent results regarding these two subjects encourage us to reach the final goal.

Waveguided SPR sensor using a Mach-Zehnder interferometer with variable power splitting ratio

This paper proposes and analyses the possibility of using variable power splitting Mach-Zehnder interferometer to increase the sensitivity of an integrated optics surface plasmon resonator (SPR) sensor. As the SPR sensors produces changes to both the phase and attenuation of the propagating light, this configuration is adjustable to utilize the full range of changes provided by the sensed material. Sensor sensitivity and working range can be increased by optimizing the power entering each branch of the Mach-Zehnder interferometer. Analysis of a specific SPR waveguide shows a 20% increase in sensitivity as compared to the single channel and to the conventional symmetric Mach-Zehnder setup. A very general analytic study of two configurations: variable power splitting sensor and single-channel sensor is presented, providing a decision tool as to which one would furnish the best results. As an additional outcome of our analysis we revealed that for large attenuation values in a waveguide sensor, the variable Mach-Zehnder interferometer is a preferred scheme even for a case where the sensing process causes only variations in amplitude of the optical carrier. The method is applicable to any perturbation mechanism that induces phase and amplitude changes on a waveguide.

BER performance analysis of 100 and 200 Gbit∕s all-optical OTDM node using symmetric Mach–Zehnder switches

In future high-speed self-routing photonic networks based on all-optical time division multiplexing (OTDM) it is highly desirable to carry out packet switching, clock recovery and demultiplexing in the optical domain in order to avoid the bottleneck owing to the optoelectronics conversion. We propose a self-routing OTDM node structure composed of an all-optical router and a demultiplexer based on the symmetric Mach–Zehnder (SMZ) for high bit rate OTDM packets. Here, we investigate both numerically and by means of simulation the noise and crosstalk characteristics of the SMZ and the bit error rate (BER) performance of the proposed OTDM node. For BER of 10−9 and the total bit rates of 100 and 200 Gbit/s the power penalty incurred is about 2 and 2.5 dB, respectively compared with a 2.5 Gbit/s back-to-back (B-B) system.

Comparison of 40GHz optical demultiplexers using SMZ switch and EA modulator in 160Gbit/s-500km OTDM transmission

We compare two types of optical demultiplexers, one with a symmetric Mach-Zehnder (SMZ) switch and the other with an electro-absorption modulator (EAM), when used for 160Gbit/s (4 × 40Gbit/s) OTDM transmission over 500km. By combining the SMZ switch with a time-domain optical Fourier transformation technique, we succeeded in obtaining slightly better bit-error rate performance than with the EAM.

Thermooptic switch based on photonic-crystal line-defect waveguides

A thermooptic switch with a symmetric Mach-Zehnder interferometer structure was demonstrated with silicon photonic-crystal-slab line-defect waveguides. The device size was as small as 160×65 μm. The optic switch operated at a wavelength of 1550 nm, and an extinction ratio greater than 30 dB was obtained by 120-mW heating (switching) power over the wavelength range of 15 nm. The switching on-off response times were about 120 μs.

Fabrication and characterization of photonic crystal-based symmetric Mach-Zehnder (PC-SMZ) structures based on GaAs membrane slab waveguides

We have experimentally demonstrated two- dimensional (2-D) photonic crystal (PC) waveguides (WGs) such as straight, bent, Y-branch, directional coupler (DC), and quantum dots (QDs)-based nonlinear optical (NLO) WGs using a GaAs air-bridge membrane with triangular-lattice line-defect WGs. These WGs are necessary for a 2-D PC-based symmetric Mach-Zehnder (PC-SMZ) type ultrafast all-optical switch. Experimental results on the clear observation of the optical interference at the Y-junction DC, nonlinearity-induced phase shift at the NLO waveguide, and transmission spectra in the total PC-SMZ WG configuration will be addressed, as well as measured results on each WG element.

Design, fabrication, and characterization of a two-dimensional photonic-crystal symmetric Mach-Zehnder interferometer for optical integrated circuits

We report on the design, fabrication, and optical characterization of a GaAs two-dimensional photonic-crystal-based symmetric Mach-Zehnder interferometer (PC-SMZ) aimed at all-optical switches in planar photonic integrated systems. The directional couplers that constitute the PC-SMZ interferometer prevent the occurrence of ring resonance which degrades the interference, and allow control pulses for the switching operation to be received. We show 50%- and 100%-power couplings from the transmission characteristics of the directional couplers, thus demonstrating their operation as 3-dB dividers in the PC-SMZ. We also show experimentally wavelength selectivity between signal and control pulses with a separation of ∼7nm. These results indicate that the structure is promising as an all-optical switch when a nonlinear material such as a quantum dot is introduced into both arms of the interferometer.

Simulation of 1×2 OTDM router employing symmetric Mach–Zehnder switches

In high-speed all-optical time division multiplexed (OTDM) routers it is desirable to carry out data routing, switching, clock recovery and synchronisation in the optical domain in order to avoid the bottleneck due to optoelectronics conversion. The authors propose an optical switch based on all-optical symmetric Mach–Zehnder (SMZ) switching and investigate its characteristics. The proposed switch is to be used as a building block for a simple 1×2 OTDM router for asynchronous OTDM packet routing, where clock recovery, address recognition and payload routing are all carried out in the optical domain. Simulation and numerical results demonstrate that clock recovery, address recognition and payload routing are possible with small amounts of crosstalk. Also presented are simulation results for bit error rate (BER) performance for the 1×2 router. For a BER of 10−9 the receiver sensitivity is −26 dB compared with baseline detection without a router of −38 dB. The proposed router displays great potential for use in ultra-high-speed OTDM networks.

High-precision optical interference in Mach-Zehnder-type photonic crystal waveguide

Excellent optical interference was experimentally demonstrated in the near infrared region using asymmetric Mach-Zehnder (MZ) type GaAs-based two-dimensional photonic crystal (2DPC) slab waveguides with directional couplers (DCs). As one of two MZ arm lengths changed in units of the lattice constant, the output intensities exhibited sinusoidal curves in excellent agreement with coupled-mode theory. In another experiment where the DCs were operated by two incident optical beams with externally controlled phase's difference, a sinusoidal change was observed also in output intensities according to the theory of the DC. These results were obtained by virtue of excellent nano-fabrication of the 2DPC structures and pave the way to successful operation of a PC-based ultra-small symmetrical MZ (SMZ) all-optical switch.

Simultaneous demultiplexing and clock recovery for 160-gb/s OTDM signal using a symmetric Mach-Zehnder switch in electrooptic feedback loop

We demonstrated simultaneous demultiplexing and clock recovery for a 160-Gb/s optical time-division multiplexing (OTDM) signal through a fiber transmission line by using a symmetric Mach-Zehnder switch and a mode-locked laser diode in an electrooptic feedback loop. Moreover, synchronous optical sampling of the OTDM signal with the recovered clock was also demonstrated.

Similar role of waveguide bends in photonic crystal circuits and disordered defects in coupled cavity waveguides: An intrinsic problem in realizing photonic crystal circuits

We investigate, experimentally and theoretically, the influence of disordered defects on the transmission properties of optical delay lines based on coupled cavity waveguides (CCW's). Also, the modification of transmission for a line-defect waveguide in a two-dimensional (2D) photonic crystal (PC) upon bending is examined by numerical simulation. We reveal that the effect of waveguide bending on the transmission properties of PC circuits is very similar to that of disordered defects on the transmission properties of CCW's. It is shown that the broad impurity band of a one-dimensional CCW will evolve into sharp defect modes upon increasing the disorder. Using a symmetric Mach-Zehnder structure formed in a 2D PC as an example, we show that a PC circuit containing multiple bends and branches of different properties generally gives rise to sharp resonant modes whose linewidth is inversely proportional to the size of the PC circuit. This might be an intrinsic problem in realizing PC circuits.

Modeling Chirp and Phase Inversion in Wavelength Converters based on Symmetrical MZI-SOAs for use in All-Optical Networks

All-optical wavelength conversion based on multi-section semiconductor optical amplifiers (SOAs) in a symmetrical Mach-Zehnder interferometer (SMZI) is modeled for use in optical networks. It incorporates an enhanced SOA model that is implemented using the time domain transfer matrix approach and hence the overall numerical model determines simultaneously the wavelength and gain parameters for the wavelength converter. The overall model accurately predicts the optimal conditions for the SMZI arrangement in order to achieve the best results for the chirp, the phase inversion and the converted probe signal power. It is also demonstrated that large chirp and mismatch of the phase inversion reduces the eye opening ratio (EOR) which can seriously affect the performance of the wavelength converter to be used as a sub-system component in all-optical networks.

Synchronous optical demultiplexing and sampling of 80-Gb/s OTDM signals by optically recovered clock using mode-locked laser diode and symmetric Mach-Zehnder switch

A demonstration of optical demultiplexing of an 80-Gb/s signal through a fiber transmission line by using an injection mode-locked laser diode (LD) to perform all-optical subharmonic clock recovery to control a symmetric Mach-Zehnder demultiplexer is described. Moreover, simultaneous optical sampling by the recovered clock was also demonstrated.