Quantum Dash Semiconductor Optical Amplifiers Research Articles

Quantum-Dash Semiconductor Optical Amplifier for Millimeter-Wave over Fibre Wireless Fronthaul Systems

This paper demonstrates a five-layer InAs/InP quantum-dash semiconductor optical amplifier (QDash-SOA), which will be integrated into microwave-photonic on-chip devices for millimeter-wave (mmWave) over fibre wireless networking systems. A thorough investigation of the QDash-SOA is conducted regarding its communication performance at different temperatures, bias currents, and input powers. The investigation shows a fibre-to-fibre (FtF) small-signal gain of 18.79 dB and a noise figure of 6.3 dB. In a common application with a 300 mA bias current and 25 °C temperature, the peak FtF gain is located at 1507.8 nm, which is 17.68 dB, with 3 dB gain bandwidth of 56.6 nm. Furthermore, the QDash-SOA is verified in a mmWave radio-over-fibre link with QAM (32 Gb/s 64-QAM 4-GBaud) and OFDM (250 MHz 64-QAM) signals. The average error vector magnitude of the QAM and OFDM signals after a 2 m wireless link could be as low as 8.29% and 6.78%, respectively. These findings highlight the QDash-SOA’s potential as a key amplifying component in future integrated microwave-photonic on-chip devices.

Performance Investigations of InAs/InP Quantum-Dash Semiconductor Optical Amplifiers with Different Numbers of Dash Layers.

We present here a performance comparison of quantum-dash (Qdash) semiconductor amplifiers (SOAs) with three, five, eight, and twelve InAs dash layers grown on InP substrates. Other than the number of Qdash layers, the structures were identical. The eight-layer Qdash SOA gave the highest amplified spontaneous emission power (4.3 dBm) and chip gain (26.4 dB) at 1550 nm, with a 300 mA CW bias current and at 25 °C temperature, while SOAs with fewer Qdash layers (for example, three-layer Qdash SOA), had a wider ASE bandwidth (90 nm) and larger 3 dB gain saturated output power (18.2 dBm) in a shorter wavelength range. The noise figure (NF) of the SOAs increased nearly linearly with the number of Qdash layers. The longest gain peak wavelength of 1570 nm was observed for the 12-layer Qdash SOA. The most balanced performance was obtained with a five-layer Qdash SOA, with a 25.4 dB small-signal chip gain, 15.2 dBm 3 dB output saturated power, and 5.7 dB NF at 1532 nm, 300 mA and 25 °C. These results are better than those of quantum well SOAs reported in a recent review paper. The high performance of InAs/InP Qdash SOAs with different Qdash layers shown in this paper could be important for many applications with distinct requirements under uncooled scenarios.

Dynamic power and chirp measurements of a quantum dash semiconductor optical amplifier amplified picosecond pulses using a linear pulse characterization technique

Quantum Dash (QDash) SOAs are of interest as an alternative to quantum dot SOAs, since they have some dot-like properties and are easier to fabricate such as to operate in the 1550 nm region, albeit with longer gain recovery times in the range of hundreds of picoseconds. QDash-SOAs are promising core component is optical subsystems for applications in next generation coherent communications and optical signal processing, which often involve pulse amplification. It is of interest to measure both the dynamic power, phase and associated chirp and thereby the spectrum of typical QDash-SOA amplified pulses. Non-linear measurement techniques are appropriate for pulsewidths less than 5 ps but have low sensitivity for wider pulsewidths often encountered in practice. This paper describes the use of a modified linear pulse characterization technique to measure the dynamic power and phase of the identical pulses of a QDash-SOA amplified 20 GHz repetition rate 19 ps pulsewidth pulse train. The pulse chirp and power from which the pulse chirp and pulse train power spectrum is calculated. The amplified pulse structure is strongly dependent on the amplifier gain and the input pulse shape and energy.

40 Gb/s NRZ-DQPSK data wavelength conversion with amplitude regeneration using four-wave mixing in a quantum dash semiconductor optical amplifier

Differential quadrature phase shift keying (DQPSK) modulation is attractive in high-speed optical communications because of its resistance to fiber nonlinearities and more efficient use of fiber bandwidth compared to conventional intensity modulation schemes. Because of its wavelength conversion ability and phase preservation, semiconductor optical amplifier (SOA) fourwave mixing (FWM) has attracted much attention. We experimentally study wavelength conversion of 40 Gbit/s (20 Gbaud) non-return-to-zero (NRZ)-DQPSK data using FWM in a quantum dash SOA with 20 dB gain and 5 dBm output saturation power. Q factor improvement and eye diagram reshaping is shown for up to 3 nm pump-probe detuning and is superior to that reported for a higher gain bulk SOA.

Gain and refractive index dynamics in p-doped InAs quantum dash semiconductor optical amplifiers

The ultrafast carrier dynamics in a p-doped dash-in-a-well structure at 1.5 μm is experimentally investigated. An analysis of the timescales related to carrier relaxation and escape processes as well as the “dynamical” linewidth enhancement factor is presented and compared with results obtained from similar un-doped materials. Intentional p-doping of the active region results in an enhancement of the intermediate timescale of the gain dynamics associated with phonon-assisted electron capture and a reduction of the α-factor due to increased differential gain.

Broadband gain-clamped linear quantum dash optical amplifiers

The linear optical gain of gain-clamped quantum dash semiconductor optical amplifiers (GCSOAs) has been investigated using the rate equation model. The gain spectrum of GCSOA for different wavelength detuning and different doping has been studied. Our analysis shows that the linear gain can be increased as the laser wavelength is detuned to high wavelength where the peak of the optical gain, which is found at wavelengths below the ground state wavelength, is shifted to lower wavelength as the laser wavelength is increased. We find that doping the dashes by either N-type or P-type enhances the linear optical gain and shifts the gain peak to lower wavelength. Moreover, we found that GCSOA with lightly N-type doping demonstrates large separation between the laser and the amplifier wavelength. Also we find that small inhomogeneous line broadening enhances the linear gain peak, shifts the gain peak to wavelength lower than the GS wavelength and widens the gain spectrum.

Efficiency of four-wave mixing of doped closely spaced energy states quantum dash semiconductor optical amplifiers

A detailed analysis of the efficiency of four-wave mixing (FWM) is performed for a closely spaced energy state quantum dash semiconductor optical amplifier (SOA). The analysis takes into account the effect of all discrete energy states, the gain dispersion of the quantum dash, and the effect of doping on the FWM characteristics. Our analysis reveals that large FWM efficiency can be obtained when the energy of the pump signal is equal to the first excited state. We find that at low-applied current doping the dashes by p -type concentration enhances the FWM efficiency. While at high applied current, we find that doping the SOA with p -type concentration slightly enhances the FWM efficiency of the ground state and the first excited state and lowers the FWM efficiency of the higher excited states. Also we find that doping the dashes by n -type concentration slightly modifies the FWM efficiency spectrum.

Suppression of slow gain recovery in ultralong quantum-dash semiconductor optical amplifier emitting at 1.55 μm

Gain dynamics in two quantum-dash semiconductor optical amplifiers of different lengths emitting in the 1.55 μm region are investigated experimentally and compared. It is shown that slow gain recovery due to total carrier relaxation is totally suppressed in the ultralong amplifier. Consequently, the 10%–90% gain recovery time is drastically reduced from about 40 ps (short sample) to 10 ps (long one).

Cross-gain modulation in closely spaced energy states quantum dash semiconductor optical amplifiers

A detailed small-signal analysis of cross-gain modulation is performed for closely spaced energy state quantum dash (QDsh) semiconductor optical amplifier (SOA). The analysis takes into account the carrier transition in all electron and hole states, the gain dispersion of the active layer, the effect of the energy detuning between the probe and pump signals and the effect of doping on the characteristics of cross-gain wavelength conversion. Our analysis reveals that broadband conversion efficiency can be obtained in QDsh SOA when the energy of the pump signal is at − 20 meV below the ground state. Also we find that large 3 dB bandwidth can be achieved when the energy of the pump and the probe signals is at + 30 meV. Our analysis shows that doping the dashes with P-type concentration can enhance the efficiency and the intrinsic 3 dB bandwidth of cross-gain wavelength conversion.

Low-Current N-Type Quantum Dash Semiconductor Optical Amplifiers

In this letter, we propose the use of light N-type doping to enhance the optical gain of the quantum dash semiconductor optical amplifier (SOA) at low applied current. We find that doping the dashes with light donor concentration increases the unsaturated optical gain and widens the optical gain spectrum range at low applied current. Also, our calculation reveals that the use of N-type doping is associated with a reduction in the slope efficiency and a decrease in the transparency current, which makes N-type quantum dash SOA attractive for low power dissipation applications.

Variable optical delays at 1.55 μm using fast light in an InAs/InP quantum dash based semiconductor optical amplifier

Room temperature optical variable delays are demonstrated using InAs/InP quantum dash semiconductor optical amplifiers (SOAs) at 1.55 μm. A microwave frequency modulated single optical beam allows to demonstrate fast light through the achievement of maximum optical delays of 136 ps at 250 MHz and ∼55 ps at 2 GHz by means of electrical control of the SOA bias current. The group index variation is attributed to enhanced nearly degenerate four wave mixing of this material system as well as population pulsation.

Ultrafast gain and index dynamics of quantum dash structures emitting at 1.55μm

The authors systematically characterize the ultrafast gain and index recovery of a quantum dash semiconductor optical amplifier after it has amplified a strong femtosecond pulse. The results show a recovery dominated by a fast time constant of 1.4ps with an ultimate recovery taking place on a 150ps time scale. The results are distinctly different from the recovery of quantum dot amplifiers and reflect the special density of states of the quantum-wire-like dashes.

Gain and noise saturation of wide-band InAs-InP quantum dash optical amplifiers: model and experiments

We present a theoretical model for gain and noise saturation in quantum dash (QDash) semiconductor optical amplifiers. The model is based on the density matrix formalism and addresses static saturation spectra. The calculations are confirmed by a series of experiments which highlight the unique properties of these amplifiers. We demonstrate a high gain, a wide bandwidth, and high saturation power. The saturation spectrum is shown to be asymmetric, emphasizing saturation at short wavelength. The asymmetry stems from the high energy tail of the density of state function in those quantum wire (QWire) like gain media as well as from the interactions with the wetting layer.

Numerical Modeling of the Emission Characteristics of Semiconductor Quantum Dash Materials for Lasers and Optical Amplifiers

This paper deals with the simulation of the emission characteristics of self-assembled semiconductor quantum dash (QDash) active materials, characterized by high length-to-width and width-to-height ratios of the dash size and by a wide spreading of the dash dimensions. This significant size fluctuation requires to compute numerically the corresponding energy distribution of the electron and hole confined states. Furthermore, due to the long dash length, it is necessary to take into account the many longitudinal confined states that contribute to the emission spectrum. To implement a model that does not require excessive computation time, some simplifying assumptions have been introduced and validated numerically. Starting from good knowledge of the dash size, material composition, and optical waveguide dimensions, we have been able to simulate the amplified spontaneous emission and gain spectra of a quantum dash semiconductor optical amplifier with a good quantitative agreement with the measured data. As an application example, the model is used to predict the gain properties of different QDash ensembles having various size distributions.

Broad-band wavelength conversion based on cross-gain modulation and four-wave mixing in InAs-InP quantum-dash semiconductor optical amplifiers operating at 1550 nm

Wavelength conversion based on four-wave mixing (FWM) and cross-gain modulation (XGM) is experimentally demonstrated for the first time in a 1550-nm InAs-InP quantum-dash semiconductor optical amplifier. Continuous-wave FWM with a symmetric conversion efficiency dependence on detuning direction and FWM mediated short-pulse wavelength conversion are demonstrated. Using XGM, we have successfully implemented short-pulse wavelength conversion over 10 THz and error-free data conversion of a 2.5-Gb/s data sequence over 7.5 THz. The pulsed XGM experiments suggest that adjacent regions within an inhomogeneously broadened gain spectrum are partially coupled which increases the operational bandwidth, but at the expense of speed.