Polarization-insensitive Semiconductor Optical Amplifiers Research Articles

Low-Polarization, Broad-Spectrum Semiconductor Optical Amplifiers.

Polarization-insensitive semiconductor optical amplifiers (SOAs) in all-optical networks can improve the signal-light quality and transmission rate. Herein, to reduce the gain sensitivity to polarization, a multi-quantum-well SOA in the 1550 nm band is designed, simulated, and developed. The active region mainly comprises the quaternary compound InGaAlAs, as differences in the potential barriers and wells of the components cause lattice mismatch. Consequently, a strained quantum well is generated, providing the SOA with gain insensitivity to the polarization state of light. In simulations, the SOA with ridge widths of 4 µm, 5 µm, and 6 µm is investigated. A 3 dB gain bandwidth of >140 nm is achieved with a 4 µm ridge width, whereas a 6 µm ridge width provides more output power and gain. The saturated output power is 150 mW (21.76 dB gain) at an input power of 0 dBm but increases to 233 mW (13.67 dB gain) at an input power of 10 dBm. The polarization sensitivity is <3 dBm at -20 dBm. This design, which achieves low polarization sensitivity, a wide gain bandwidth, and high gain, will be applicable in a wide range of fields following further optimization.

High-resolution X-ray diffraction to probe quantum dot asymmetry

The symmetry of nanostructures forming an active part of optoelectronic devices severely impacts their properties. It is crucial for the design of polarization-insensitive semiconductor optical amplifiers or sources of entangled photon pairs. We show how quantum dot symmetry can be measured, including atomic force microscopy and polarization-resolved spectroscopy. We discuss the drawbacks of the typical approaches for nanostructure symmetry detection and propose a way to determine quantum dot geometrical aspect ratio based on high-resolution X-ray diffraction. The offered method provides a non-destructive insight into crystal lattice disturbances reflecting in-plane quantum dot geometrical aspect ratio.

Polarization-insensitive fiber-to-fiber gain of semiconductor optical amplifier using closely stacked InAs/GaAs quantum dots

The gain characteristics of a semiconductor optical amplifier (SOA) that contains forty layers of closely stacked InAs/GaAs quantum dots (QDs) were studied by employing a fiber-to-fiber measurement system. The fiber-to-fiber gain values obtained for transverse-electric (TE)- and transverse-magnetic (TM)-polarized optical input signals exhibited their maxima at a wavelength of 1100 nm, which corresponds to the excited-state of the closely stacked QDs. In the wavelength range from 1060 to 1170 nm, the gain obtained for TE-polarized input signal was slightly higher than that for TM-polarized input signal and the absolute gain difference was less than 1 dB for an injection current of 80 mA. The small but non-zero difference of the gain reflects the polarization anisotropy of the optical gain provided by the closely stacked QDs. Our results suggest that closely stacked QDs are suitable for realizing polarization-insensitive SOA device with operation bandwidths of more than 100 nm.

A Polarization Insensitive Semiconductor Optical Amplifier

This letter proposes an integrated InGaAsP-InP/silicon-on-insulator (SOI) polarization insensitive semiconductor optical amplifier (SOA). The SOA consists of an SOI waveguide-based polarization diversity circuit and an InGaAsP-InP gain block. Input light with arbitrary linear polarization state will pass through the gain block twice for a roundtrip with mutually orthogonal polarization state in its forward and backward path, respectively. As such, the polarization-dependent gain (PDG) is effectively reduced. Since the returning path is not spatially overlapped with the forward path, no extra circulator is needed and the device still has two separate ports, one for input and one for output. Simulation results show that the PDG is negligibly small and the device has a broad working band of 60 nm for a PDG below 0.5 dB. This design is compatible with the mature semiconductor fabrication technology and can be monolithically integrated with other optical components.

Band-gap shrinkage calculations and analytic model for strained bulk InGaAsP

Band-gap shrinkage is an important effect in semiconductor lasers and optical amplifiers. In the former it leads to an increase in the lasing wavelength and in the latter an increase in the gain peak wavelength as the bias current is increased. The most common model used for carrier-density dependent band-gap shrinkage is a cube root dependency on carrier density, which is strictly only true for high carrier densities and low temperatures. This simple model, involves a material constant which is treated as a fitting parameter. Strained InGaAsP material is commonly used to fabricate polarization insensitive semiconductor optical amplifiers (SOAs). Most mathematical models for SOAs use the cube root bandgap shrinkage model. However, because SOAs are often operated over a wide range of drive currents and input optical powers leading to large variations in carrier density along the amplifier length, for improved model accuracy it is preferable to use band-gap shrinkage calculated from knowledge of the material bandstructure. In this letter the carrier density dependent band-gap shrinkage for strained InGaAsP is calculated by using detailed non-parabolic conduction and valence band models. The shrinkage dependency on temperature and both tensile and compressive strain is investigated and compared to the cube root model, for which it shows significant deviation. A simple power model, showing an almost square-root dependency, is derived for carrier densities in the range usually encountered in InGaAsP laser diodes and SOAs.

Input-Power and Polarization Insensitive All-Optical Wavelength Converter With Monolithically Integrated Monitor PD and Gain-Controlled SOA

This paper presents a polarization-insensitive semiconductor optical amplifier Mach-Zehnder Interferometer (SOA-MZI) wavelength converter with a wide dynamic range. Direct detection of the wavelength-converted output signal power and feedback control of the signal power input to the SOA-MZI are proposed and are confirmed to be able to reduce the variation with input signal power and polarization. The gain-controlled SOA (GC-SOA) and monitor PD used to implement the feedback control are monolithically integrated into the SOA-MZI wavelength converter. A dynamic range of over 8 dB is obtained for TE and TM polarized input signals NRZ modulated at 43 Gb/s under feedback control, 7 dB better than with no feedback.

A Self-Consistent Numerical Method for Calculation of Steady-State Characteristics of Traveling-Wave and Reflective SOAs

This paper presents a detailed numerical model of reflective and traveling-wave semiconductor optical amplifiers (SOAs), based on a self-consistent iteration method. The method is fully transparent to the input parameters and provides stable and efficient convergence of all relevant SOA variables as long as the sufficient number of previous iterations is taken into account. The model accounts for the detailed spectral and carrier density dependence of the radiative recombination rate, material gain, refractive index, and confinement factor. The analysis of unstrained bulk and strained multi-quantum well polarization insensitive SOAs based on this model provides a deep and detailed insight into the device internal state, confirming that spectral and carrier density material dependencies critically influence the modeling results.

Experimental analysis of internal optical losses in polarization-insensitive semiconductor optical amplifiers

We experimentally investigated the internal optical loss of a polarization-insensitive multiple quantum well semiconductor optical amplifier (SOA) over a wide injection current range. The precise fitting of noise figure spectra using gain spectra and transparent wavelengths enabled a direct estimation of internal optical loss inside gain bandwidth. The internal optical loss of the SOA exhibited large dependencies on injection current and the optical confinement factor of the active layer. A large difference in optical losses between polarization modes and operating wavelengths of the SOA indicates a strong influence of intervalence band absorption.

Strained InGaAsP multi-quantum-well structures for InP-based wide linewidth and polarization-insensitive semiconductor optical amplifiers

We have calculated the band structure of 1.55μm InGaAsP/InGaAsP multi-quantum-well structures using Lüttinger–Kohn Hamiltonian taking into account the strain in the quantum wells (QWs) and barriers, and the confinement in the quantum wells. Using the calculated dispersion curves and oscillator strength between the different interband transitions, we have determined the optical gain in TE and TM mode and the spontaneous amplified emission as a function of injected carrier density in devices composed of quantum wells with different thicknesses. We find that an optical gain linewidth larger than 130nm with a TE/TM polarization dependence lower than 1dB can be obtained using a three-quantum-well In0.53Ga0.47As0.96P0.04/InGaAsP active layer with quantum well thicknesses of 10, 14 and 19nm.

InAs QDs on InP: polarization insensitive SOA and non-radiative Auger processes

An efficient mechanical and electronic axial approximation of the strained 8 × 8 Hamiltonian is proposed for zinc-blende nanostructures with a cylindrical shape on (100) substrates. Vertically stacked InAs/InP columnar quantum dots (CQDs) for polarization insensitive semiconductor optical amplifier (SOA) in telecommunications applications are studied theoretically. Non-radiative Auger processes in InAs/InP quantum dots (QDs) are also investigated. It is shown that a multiband approach is necessary in both cases.

SOA Nonlinear Polarization Rotation With Linear Polarization Maintenance: Characterization and Applications

We present characterization and applications of the nonlinear polarization rotation (NPR) with linear polarization maintenance arising in polarization-insensitive semiconductor optical amplifiers (SOAs). The NPR with linear polarization maintenance is characterized in terms of probe and pump power, bias current, pump polarization, as well as wavelength. The NPR with linear polarization maintenance is then exploited for simultaneous inverted and noninverted polarization-insensitive wavelength conversions (WCs) with 2R regeneration using a single polarization beam splitter (PBS). WC of 10 Gb/s with 2R regeneration is experimentally demonstrated in the whole C-band with a power penalty reduction of 1.2 dB for the inverted WC and 0.8 dB for the noninverted WC. The NPR with linear polarization maintenance is also applied to perform on-off-keying (OOK) to polarization-shift-keying (PolSK) format conversion in conjunction with WC function, where the power fluctuation caused by the SOA cross gain modulation (XGM) is successfully mitigated using two cascaded SOAs. The power fluctuation of the converted PolSK signal is reduced to less than 0.6 dB, while the extinction ratio (ER) of polarization modulation is maintained at larger than 30 dB.

Wavelength tunable InAs/InP(1 0 0) quantum dots in 1.55-μm telecom devices

This paper reviews the growth, characterization and device applications of self-assembled InAs/InP(1 0 0) quantum dots (QDs) formed by MOVPE. The problematic As/P exchange reaction during QD growth is suppressed by the insertion of a GaAs interlayer together with optimum growth conditions. This produces QDs with continuously tunable emission over the 1.55-μm wavelength region for fiber-based telecom applications. Device quality of these QDs is proven by continuous wave lasing at room temperature from the as-cleaved facets of Fabry–Pérot narrow ridge-waveguide lasers implementing widely stacked QDs as gain medium. The low transparency current density of 6 A/cm 2 per QD layer and low loss of 4.2 cm −1 are accompanied by a 80-nm wide gain spectrum. The deeply etched QD lasers possess similar threshold current densities as the shallowly etched ones and do not deteriorate with time, revealing that device performance does not suffer from sidewall recombination. This allows the fabrication of mono-mode and more compact devices with small bending radii, as demonstrated by the operation of a QD ring laser with 40-GHz free spectral range. Unpolarized emission from the cleaved side, important for the realization of polarization insensitive semiconductor optical amplifiers, is obtained by close stacking of QDs due to vertical electronic coupling. Sharp exciton–biexciton emission from a single QD around 1.55 μm is observed with clearly resolvable peaks above 70 K, which is required for single photon sources working at liquid nitrogen temperature for fiber-based quantum cryptography systems.

Strained-Layer Quantum-Well Lasers

This tutorial article explains the two important reasons for the introduction of strain into the active region of a quantum-well laser. First, it reduces the density of states at the top of the valence band, which allows population inversion to be obtained at a lower carrier density. Second, it distorts the 3-D symmetry of the crystal lattice and matches it more closely to the 1-D symmetry of the laser beam. Together these effects greatly enhance almost all characteristics of semiconductor lasers and make possible a wide range of applications. Combinations of compressive and tensile strain can also be used, for example, to produce nonabsorbing mirrors and polarization-insensitive semiconductor optical amplifiers.

High-Speed Mode-Locked Quantum-Dot Lasers and Optical Amplifiers

Recent results on GaAs-based high-speed mode-locked quantum-dot (QD) lasers and optical amplifiers with an operation wavelength centered at 1290 nm are reviewed and their complex dependence on device and operating parameters is discussed on the basis of experimental data obtained with integrated fiber-based QD device modules. Hybrid and passive mode locking of QD lasers with repetition frequencies between 5 and 80 GHz, sub-ps pulse widths, ultralow timing jitter down to 190 fs, high output peak power beyond 1 W, and suppression of Q-switching are reported, showing the large potential of this class of devices for O-band optical fiber applications. Results on cw and dynamical characterization of QD semiconductor optical amplifiers (SOAs) are presented. QD amplifiers exhibit a close-to-ideal noise figure of 4 dB and demonstrate multiwavelength amplification of three coarse wavelength division multiplexing (CWDM) wavelengths simultaneously. Modelling of QD polarization dependence shows that it should be possible to achieve polarization insensitive SOAs using vertically coupled QD stacks. Amplification of ultrafast 80 GHz optical combs and bit-error-free data signal amplification at 40 Gb/s with QD SOAs show the potential for their application in future 100 Gb Ethernet networks.

Stacking, polarization control, and lasing of wavelength tunable (1.55 μm region) InAs/InGaAsP/InP (1 0 0) quantum dots

We report the growth, device fabrication, and performance of stacked InAs/InGaAsP/InP (1 0 0) quantum dots (QDs) in telecom lasers. Wavelength tuning of the QDs grown by metalorganic vapor-phase epitaxy (MOVPE) over the 1.55-μm region at room temperature (RT) is achieved using ultrathin GaAs interlayers underneath the QDs, which suppress As/P exchange to reduce the QD height in a controlled way. Unpolarized photoluminescence from the cleaved side, important for realization of polarization-insensitive semiconductor optical amplifiers, is obtained for closely stacked QDs due to vertical electronic coupling. The wavelength shift due to group-V-atom intermixing in the QDs during growth at higher temperatures of device structures is compensated by adjusting the GaAs interlayer thickness and V/III flow ratio during QD growth. Device quality of the QDs is highlighted by continuous-wave ground-state lasing at RT of narrow ridge-waveguide QD lasers.

Analytical Characterization of Optical Pulse Propagation in Polarization-Sensitive Semiconductor Optical Amplifiers

In this paper, we perform analytical characterization of optical pulses propagating through a polarization-sensitive semiconductor optical amplifier (SOA). We derive analytical expressions for the carrier density, gain and phase evolution along the SOA and show how these expressions prove useful in optical signal processing applications. The propagation of counter-propagating pulses as well as pulse streams across SOAs have been analysed and expressions for energy gain have been derived in all these cases. We also show that our analytical results reduce to corresponding results of polarization insensitive SOAs already published. The analytical results are in excellent agreement with detailed numerical simulations done in MATLAB using the NIMROD portal. The analytical calculations lead to significant savings with regard to simulation time and processing capacity requirements. We further prove that the energy gain difference for counter-propagating pulse streams is directly proportional to the difference delay between in them and hence can be used as a measure of the delay difference. This theoretical result agrees well with experimental results

Fabrication and characterization of 1 550 nm polarization-insensitive semiconductor optical amplifiers

A 1550 nm polarization-insensitive semiconductor optical amplifier (SOA) was fabricated with InGaAs tensile-strained bulk active region. Beam propagation method and planar wave expansion method are used to calculate the mode field profile and the mode reflectivity. For the SOA with a buried waveguide deviated 70 from the normal direction of cleaved mirrors, the thickness tolerance of the mirror is 3% for keeping the reflectivity of TE mode and TM mode less than 10−4 simultaneously. For a SOA with a cavity length of 800 μm, the polarization sensitivity of amplified spontaneous emission spectra is less than 0.5 dB at an injection current of 250 mA, the corresponding fiber-to-fiber gain is 11.9 dB at 1550 nm with a 3 dB bandwidth of 63 nm, and the saturation output power is 5.6 dBm. The noise figure shows 8.8 and 7.8 dB at 1550 and 1570 nm, respectively. For a packaged SOA with a cavity length of 1000 μm, the fiber-to-fiber gain is 15 dB at an injection current of 190 mA.

Record high saturation output power (+20 dBm) and low NF (6.0 dB) polarisation-insensitive MQW-SOA module

A polarisation-insensitive semiconductor optical amplifier module with a thin tensile-strained multiple quantum well active layer is developed. The module exhibited a record high saturation output power of +20.0 dBm and a low noise figure of 6.0 dB at 1550 nm together with a small polarisation-dependent gain of 0.6 dB.

Wavelength controlled InAs/InP quantum dots for telecom laser applications

This article reviews the recent progress in the growth and device applications of InAs/InP quantum dots (QDs) for telecom applications. Wavelength tuning of the metalorganic vapor-phase epitaxy grown single layer and stacked InAs QDs embedded in InGaAsP/InP (100) over the 1.55-μm region at room temperature (RT) is achieved using ultra-thin GaAs interlayers underneath the QDs. The GaAs interlayers, together with reduced growth temperature and V/III ratio, and extended growth interruption suppress As/P exchange to reduce the QD height in a controlled way. Device quality of the QDs is demonstrated by temperature-dependent photoluminescence (PL) measurements, revealing zero-dimensional carrier confinement and defect-free InAs QDs, and is highlighted by continuous-wave ground-state lasing at RT of narrow ridge-waveguide QD lasers, exhibiting a broad gain spectrum. Unpolarized PL from the cleaved side, important for realization of polarization insensitive semiconductor optical amplifiers, is obtained from closely stacked QDs due to vertical electronic coupling.

Receiver sensitivity improvement using polarization-insensitive semiconductor optical amplifier

We investigate a semiconductor optical amplifier (SOA) preamplifier model by suitably choosing the material and device parameters for the SOA to reduce the amplified spontaneous emission (ASE) noise at low range of 22.3 µW with 0.1-mW input power for a PIN receiver. Our proposed SOA optical preamplifier is found to be more relaxed from the optical alignment and antireflection coating, eliminates the need for optical filter, and gives large tolerance for an input light wavelength of more than 100 nm. We also investigate the receiver sensitivity for different bit rates. We show that receiver sensitivity is –69.9 dBm at a bit error rate (BER) of 4.6×10–10 for 10 Gbit/s and for the 40 Gbit/s bit rate the improved receiver sensitivity is –19.2 dBm with a PIN receiver. We also observe a sensitivity of –40.5 dBm at 40 Gbit/s with a differential phase shift keying (DPSK) receiver. Further, the impact of amplified spontaneous emission power, gain variation with input light wavelength, and optical gain for the TE & TM modes with input power for the PIN & DPSK receivers is illustrated. We show that the optical gain of the SOA is polarization independent and remains constant at 30.06 dB up to gain saturation. We also show that the DPSK receiver suffers less from ASE noise. The variation of material loss, length, and bias current for our SOA preamplifier to optimize the receiver sensitivity is further illustrated.