Terahertz Optical Asymmetric Demultiplexer Research Articles

Photonic time-division multiplexing (OTDM) using ultrashort picosecond pulses in a terahertz optical asymmetric demultiplexer (TOAD)

The performance of a terahertz optical asymmetric demultiplexer (TOAD) operating with an ordinary fiber and with a DDF and DIF (dispersion decreasing and increasing fiber) configurations, for three lengths of fiber ( ξ=π/2,2π and 5π) and using soliton and quasi-soliton laser profiles for the control pulse, was studied. The numerical simulations show that the increase of the fiber length leads to the decrease of the power for the first and second demultiplexed pulses and leads to a broadening of these pulses, with the exception of the TOAD operating with the DDF fiber. For the TOAD operating with a basic telecommunication fiber one see that the increase of the power of the control power lead to a strong compression of the demultiplexed pulse. Operating the TOAD using a DDF fiber one can say that the control power necessary to demultiplex the signal pulse is always lower compared with the TOAD with the normal telecommunication fiber. This is a strong suggestion that the use of the DDF fiber will allow the use of less control power. Our simulations considering the TOAD operating with a DDF and DIF with a linear profile conclude that it is possible to operate the TOAD with lower control power using a DDF fiber setup. For this device the demultiplexed pulses will present a compression on time duration and will be insensitive to the time profile of the control pulse. We also did simulations with the TOAD operating with DDF in four different profiles: hyperbolic, exponential, linear and Gaussian. For all the profiles the increase of the length of the fiber also decreases the pump power of the three first peaks for the soliton and quasi-soliton regimes. The first critical power is always lower for the quasi-soliton regime compared to the soliton regime for all profiles under consideration and all lengths of the TOAD under consideration. It was also observed that for all the profiles and lengths of fiber one has pulse compression for the switched pulse. For the ξ=2π fiber with the hyperbolic profile, both soliton and quasi-soliton profiles present the lowest critical power and the highest compression factors for all the considered profiles.

A novel fast optical switch based on two cascaded Terahertz Optical Asymmetric Demultiplexers (TOAD)

A novel optical switch based on cascading two terahertz optical asymmetric demultiplexers (TOAD) is presented. By utilizing the sharp edge of the asymmetric TOAD switching window profile, two TOAD switching windows are overlapped to produce a narrower aggregate switching window, not limited by the pulse propagation time in the SOA of the TOAD. Simulations of the cascaded TOAD switching window show relatively constant window amplitude for different window sizes. Experimental results on cascading two TOADs, each with a switching window of 8ps, but with the SOA on opposite sides of the fiber loop, show a minimum switching window of 2.7ps.

Experimental study on the regeneration capability of the terahertz optical asymmetric demultiplexer

All-optical regeneration capability of the terahertz optical asymmetric demultiplexer (TOAD) is experimentally explored. A continuous wave light is injected into the TOAD structure to reduce the pattern dependent effect caused by the data pulse injected into the control port. The bit error rate (BER) of the regenerated output at 10 Gb/s is measured with different amounts of input data timing jitter and switching window size. Experimental results show that optical regeneration using the TOAD has a strong tolerance to the timing jitter of the incident data signal. Open eye diagram and BERs below 10 −9 have been successfully obtained for input timing jitter value of 17 ps.

Monolithic integration of an all-optical Mach-Zehnder demultiplexer using an asymmetric twin-waveguide structure

We demonstrate monolithic integration of a twin-waveguide Mach-Zehnder terahertz optical asymmetric demultiplexer (MZ-TOAD). The InGaAsP-InP device operates at 1.55 /spl mu/m wavelength and has two semiconductor optical amplifiers in the asymmetric MZ interferometer configuration with a single control input and a built-in switching delay. A 28-ps full-width at half-maximum (FWHM) switching window with high contrast ratio for a range of control powers is obtained using a counter-propagating optical signal and control pulses.

Optical time-division multiplexing using picosecond solitons in a terahertz optical asymmetric demultiplexer

The performance of a terahertz optical asymmetric demultiplexer (TOAD) operating with an ordinary fiber and with a dispersion decreasing fiber (DDF) and dispersion increasing fiber (DIF) configurations, for three length of fiber ( ξ=π/2, 2π and 5π) and using soliton and quasi-soliton laser profile for the control pulse, was studied. The numerical simulations shows that the increase of the fiber length lead to the decrease of the power for the first and second demultiplexed pulses and lead to a broadening of these pulses, with exception to the TOAD operating with the DDF fiber. For the TOAD operating with a basic telecommunication fiber one see that the increase of the power of the control power lead to a strong compression of the demultiplexed pulse. One can also conclude that operating the TOAD with one soliton period ( ξ=π/2) one has compression of the demultiplexed pulse. For higher length (four and 10 soliton periods) one start having broadening of the switched pulses. Operating the TOAD using a DDF fiber one can say that the control power ( P CTL1) necessary to demultiplex the signal pulse is always lower compared with the TOAD with the normal telecommunication fiber. This is a strong suggestion that the use of the DDF fiber will allow the use of less control power. One can also say that P CTL1 is always higher for the soliton profile compared with the quasi-soliton profile. It was also observed that the compression factor for the demultiplexed pulse ( C 1) is between 2 and 3 for all the lengths of the TOAD and considering the soliton and quasi-soliton profile for the control pulse. When operating the TOAD using a DIF fiber it was observed that P CTL1 for the DIF TOAD is always lower compared with the TOAD with the normal telecommunication fiber (with exception to the ξ=5π fiber in the soliton profile), however the power values are higher when compared with the DDF TOAD. It was also observed that the compression factor C 1 is showing compression ( C 1∼2.2) for shorter length of fiber ξ=π/2 considering the soliton and quasi-soliton, profile for the control pulse. For higher lengths of fiber the compression factor is showing broadening of the demultiplexed pulse (with exception to the ξ=5π fiber in the soliton profile). Our simulations considering the TOAD operating with a DDF and DIF with a linear profile conclude that it is possible to operate the TOAD with lower control power using a DDF fiber setup. For this device the demultiplexed pulses will present a compression on time duration and will be insensitive to the time profile of the control pulse.

Contrast ratio optimisation in terahertz optical asymmetric demultiplexers

Numerical simulations are undertaken to optimise the contrast ratios of terahertz optical asymmetric demultiplexer (TOAD) devices subject to frequency-detuned signal and control picosecond optical pulses. Optimum control pulse widths and pulse energies as well as optimum small-signal gains of semiconductor optical amplifiers (SOAs) are identified for achieving maximum contrast ratios in TOAD devices.

Enhanced TOAD performance by negative frequency-detuned signal and control picosecond optical pulses

A numerical investigation has been undertaken of the operating characteristics of Terahertz Optical Asymmetric Demultiplexer (TOAD) devices subject to frequency-detuned signal and control picosecond optical pulses, taking into account the wavelength dependence of the differential gain coefficient and carrier density linewidth enhancement factor. It is shown that, by shifting the signal pulses toward lower frequencies compared with that of control pulses, the contrast ratio can be significantly enhanced and the amplitude modulation can simultaneously be considerably decreased. The switching energy demonstrates small changes over a broad frequency detuning range. The simulations show excellent agreement with published experimental measurements. Furthermore, two operational modes for the TOAD configuration are identified.

Analysis of TOAD switching characteristics considering the gain and phase response of a semiconductor optical amplifier to control pulses

In this paper, we analyze the switching characteristics of a terahertz optical asymmetric demultiplexer (TOAD) considering the gain and phase response of a semiconductor optical amplifier (SOA) to control pulses. The saturation depth of the SOA depends on the energy of the control pulse when the width of the control pulse is smaller. However, when the control-pulse width is increased to a certain extent, the saturation depth of the SOA reaches a steady value, which is governed by the power of the control pulse rather than its energy. To assure a perfect switching window of the TOAD, a large control pulse width should be avoided. © 2000 John Wiley & Sons, Inc. Microwave Opt Technol Lett 24: 374–377, 2000.

Analysis of the Switching Characteristics of TOAD

The dynamic gain responses of the semiconductor optical amplifier (SOA) in a terahertz optical asymmetric demultiplexer (TOAD) to intense control pulses with different input energies and pulse widths are investigated. The influences on the switching characteristics of TOAD for different control pulse energies, widths, loop time asymmetry, and time delays between the signal pulse and control pulse are studied and analyzed in detail. The theoretical analysis coincides well with the experimental result.

Ultrafast optical packet switching using low optical pulse energies in a self-synchronization scheme

A detailed numerical treatment is undertaken to investigate the operating characteristics of a self-synchronization scheme based on a modified terahertz optical asymmetric demultiplexer (TOAD) device for ultrafast packet switching. The simulations agree with published experimental measurements. It is shown that input picosecond optical pulse energies as low as /spl sim/100 fJ can be utilized to achieve a larger than 10 dB contrast ratio of the first to the remaining pulses of a transmitted signal packet. The utilization of low input pulse energies in the scheme also provides significant advantages including insensitivity of contrast ratio to pulse energies and pulse widths as well as distortionless extracted pulse shapes. Means for achieving a maximum contrast ratio are identified.

A simple packet-level clock extraction scheme using a terahertz optical asymmetric demultiplexer

The authors experimentally demonstrate simple packet-level clock extraction using a modified terahertz optical asymmetric demultiplexer without wavelength-division-multiplexed couplers in the loop mirror. For a 10-Gb/s optical packet composed of identical pulses, we have obtained more than 10-dB contrast ratio between the packet-level clock pulse and the other pulses.

All-optical burst-mode clock extraction based on the thresholding operation of a modified terahertz optical asymmetric demultiplexer

To implement all-optical burst-mode clock extraction we adopt a modified terahertz optical asymmetric demultiplexer (MTOAD). The transmittance and reflectance of the MTOAD depend on the input intensity. For the MTOAD, two levels of pulse intensity can be chosen in such a way that while the pulses with similar intensity are reflected for both strong and weak pulses, only the strong pulse transmits. The device is useful, for example, for bit-level clock extraction from a packet, where strong and weak intensity pulses are assigned to `1' and `0', respectively. When the input optical signal power is fixed to −1.6 dBm and the intensity ratio between `1' and `0' is varied in the range of 0.2–0.5, the extinction ratio (ER) at the transmitted port is more than 10 dB and a clock amplitude jitter (CAJ) of the bit-level clock at the reflected port is less than 14%. Inversely, when the input power is varied in the range of −6–−1 dBm with fixed intensity ratio of 0.3, more than 11 dB of ER and less than 15% of CAJ are obtained.

An analysis of signal-to-noise ratio and design parameters of a terahertz optical asymmetric demultiplexer

Analysis of the signal-to-noise ratio (SNR) as a function of the component parameters in a terahertz optical asymmetric demultiplexer (TOAD) can help optimize its performance as a demultiplexer or as a router in an optical time division multiplexed network. The analysis presented here accounts for crosstalk due to deviation from a perfect 3 dB splitting ratio in the TOAD's 2/spl times/2 coupler, and the degradation of SNR due to fluctuations in the control pulse energy. The spontaneous emission noise output of the semiconductor optical amplifier (SOA) contained in the TOAD is simulated under different degrees of saturation. The analysis indicates that leakage due to the asymmetric placement of the SOA contributes significantly to the crosstalk. To achieve a SNR of 21.5 dB or higher in a 100 Gb/s system, one must use a 2/spl times/2 coupler that deviates less than 1% from a perfect 3 dB splitting ratio. Also, control pulse energy fluctuations must be less than 10% for a 20-GHz bandwidth electronic receiver. Novel crosstalk-free designs are proposed and analyzed which meet the stringent requirements of current TOAD devices. A significant enhancement in SNR is predicted when the SOA is operated near the optimal saturation point.

Analysis of the characteristics of TOADs subject to frequency-detuned control and signal picosecond pulses

A detailed numerical investigation has been undertaken of the dynamic responses of semiconductor optical amplifiers (SOAs) and hence the operating characteristics of terahertz optical asymmetric demultiplexers (TOADs) devices subject to frequency-detuned picosecond control and signal pulses, taking into account both gain saturation and nonlinear gain compression in the SOA. It is shown that the output behavior of TOADs exhibits significant differences depending on whether the signal and control pulses are at higher relative frequencies (positive detuning) or lower relative frequencies (negative detuning). The analysis indicates that negative frequency detuning can be used to achieve an enhanced output power in the switched region and a reduced width of switching window. Calculations are shown to be in good agreement with available experimental results.

A novel OTDM frame synchronization scheme based on a terahertz optical asymmetric demultiplexer with feedback

A novel optical time-division multiplexing frame synchronization scheme based on a terahertz optical asymmetric demultiplexer (TOAD) and an electrical clock extraction circuit with feedback is presented and simulated. This scheme is characterized by both the ultrahigh speed of a TOAD and the high clock fidelity of the electrical clock extraction circuit. The simulation shows that this scheme can perform frame synchronization very quickly, and a very low timing jitter can be obtained in the recovered frame clock.

Theory of ultrahigh-speed clock extraction with phase lock loop based on a terahertz optical asymmetric demultiplexer

A novel ultrahigh-speed clock extraction scheme using optical phase lock loop based on a terahertz optical asymmetric demultiplexer (TOAD) is proposed and analyzed theoretically. It is shown that our proposed scheme can operate up to 1000 Gb/s and extremely low timing jitter can also be obtained (for the same loop noise bandwidth, the RMS timing jitter is only 1/17 of the FWM-type PLL at the bit rate 16×6.3 GHz Gb/s).

Demonstration of pulse frequency division multiplexing using terahertz optical asymmetric demultiplexer

A new method of pulse frequency division multiplexing (PFDM) to increase the capacity of local fiber-optic communications networks heavily relies on optical signal processing using an all-optical AND gate. In this work, we report a conceptual demonstration of the PFDM method using a terahertz optical asymmetric demultiplexer as a fast (picosecond switching time), low switching energy (as low as 125 fJ) optical AND gate for the optical signal processing. This creates a real possibility for the practical realization of the PFDM method.

Clock and frame synchronization recovery based on a terahertz optical asymmetric demultiplexer

A novel phase-lock type clock recovery technique based on a terahertz optical asymmetric demultiplexer (TOAD) is proposed. The technique is compatible with the conventional time-division multiplexing (TDM) timing techniques but does not require the ultrahigh-speed optical detector nor ultrafast electrical components. Further, it can readily be used to achieve frame synchronization, upon which a complete self-timing demultiplexer can be demonstrated.

Single-shot optical sampling oscilloscope for ultrafast optical waveforms

We present an all-optical technique for measuring single-shot optical pulse phenomena using a feed-forward pulse replicator and a compact, nonlinear loop mirror called the terahertz optical asymmetric demultiplexer (TOAD). By exploiting the fast nonlinearity of a semiconductor optical amplifier (SOA) placed asymmetrically within the loop, a sampling window on the order of a few picoseconds can be used to detect features in low-energy (<pJ) optical waveforms. We successfully demonstrate a sampling system with 10-ps resolution capable of sampling single-shot optical waveforms up to 160 ps in duration. The sampled real-time pulsewidth agrees well with standard, long-range space autocorrelation.

Influence of fast gain depletion on the dynamic response of TOAD's

The operating characteristics of the terahertz optical asymmetric demultiplexer (TOAD) devices subject to picosecond control pulses have been investigated taking into account gain saturation and nonlinear gain compression, due to carrier heating and spectral holeburning. It is shown that when the control pulse width is less than /spl sim/2 ps, fast gain depletion process appears in the time resolved gain dynamics of the SOA. The fast depletion process leads to an oscillating output power structure in the switched region. The oscillatory structure is found to persist for over 1 ps and sets a limit on the practical utilization of these devices.