Separate Confinement Heterostructure Layer Research Articles

Optimal waveguide structure for low-threshold InGaN/GaN-based photonic-crystal surface-emitting lasers

We analyzed the optimal waveguide structure of two types of InGaN-based photonic crystal surface-emitting lasers (PCSELs) to suppress the coupling with leaky modes via mode simulations. To minimize the threshold material gain (gth), we calculated the confinement factor and quality factor of PCSELs with varying waveguide layer thicknesses in the separate confinement heterostructure (SCH) layer. The optical mode intensity profile revealed the coupling between the fundamental mode of SCH and parasitic leaky modes in the cladding layer or substrate as the primary root cause of the low-quality factor and high threshold gain of PCSELs. The asymmetric nature of the SCH structure yielded the optimal waveguide structure to be dependent on the position of the air holes. With a proper waveguide thickness and air hole depth, the optimized threshold modal gain of PCSELs with the n-side air holes can be less than 30 cm−1.

Gallium Arsenide Photonic Integrated Circuit Platform for Tunable Laser Applications

An active-passive integration technique for operation near a wavelength of 1030 nm has been developed on a gallium arsenide (GaAs) photonic integrated circuit platform. The technique leverages quantum wells (QWs) that are slightly offset vertically from the center of the waveguide, and selectively removed prior to upper cladding regrowth to form active and passive regions. The active region consists of indium gallium arsenide (InGaAs) QWs, gallium arsenide phosphide (GaAsP) barriers, GaAs separate confinement heterostructure layers, and aluminum gallium arsenide (AlGaAs) cladding. Fabry Perot lasers with various widths were fabricated and characterized, exhibiting high injection efficiency of 98.8%, internal active loss of 3.44 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , and internal passive loss of 4.05 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> for 3 μm wide waveguides. The 3 μm, 4 μm, and 5 μm wide lasers demonstrated greater than 50 mW output power at 100 mA continuous wave (CW) current and threshold current as low as 9 mA. 20 μm wide broad area lasers demonstrated 240 mW output power, 35.2 mA threshold current under CW operation, and low threshold current density of 94 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for 2 mm long lasers. Additionally, these devices exhibit transparency current density of 85 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and good thermal characteristics with T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> = 205 K, and T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">η</sub> = 577K.

A Compact High-Efficient Equivalent Circuit Model of Multi-Quantum-Well Vertical-Cavity Surface-Emitting Lasers for High-Speed Interconnects

Due to their low power consumption, high modulation speed, and low cost, vertical-cavity surface-emitting lasers (VCSEL) dominate short-reach data communications as the light source. In this paper, we propose a compact equivalent circuit model with noise effects for high-speed multi-quantum-well (MQW) VCSELs. The model comprehensively accounts for the carrier and photons dynamisms of a MQW structure, which includes separate confinement heterostructure (SCH) layers, barrier (B) layers, and quantum well (QW) layers. The proposed model is generalized to various VCSEL designs and accommodates a flexible number of quantum wells. Experimental validation of the model is performed at 25 Gb/s with a self-wire-bonded 850 nm VCSEL.

Effect of separate confinement hetero-structure layer on tunnel injection transistor laser-based transmitter for high-speed optical communication networks

Transistor Laser has already established as a promising candidate for high speed optical interconnects and optical telecommunication networks. The present work is focused on the performance of multiple quantum wells (MQW) based transistor laser by incorporating tunnel injection phenomenon and it’s performance by varying the width of the separate confinement heterostructure layer. The virtual state (VS) is assumed as a path for the movement of injected charge carriers from bulk to nanostructures. We have estimated the various characteristics such as base threshold current, light power output, and optical modulation response by considering various physical parameters such as diffusion time, gain compression factor, optical confinement factor, photon lifetime, tunneling time and diffusion length, as a function of separate confinement hetero-structure (SCH) layer thickness. It is found that with increasing thickness of the SCH layer, base threshold current reduces, the light output power increases, but the optical modulation response remains almost constant, which ensures the potentiality of the proposed structure for high speed opto-electronic transmitter with low power consumption.

Design and analysis of InN − In0.25Ga0.75N single quantum well laser for short distance communication wavelength

A single quantum well semiconductor laser based on wurtzite-nitride is designed and analyzed for short distance communication wavelength (at around 1300 nm). The laser structure has 12 Å well layer of InN, 15 Å barrier layer of In0.25Ga0.75N, and 54 Å separate confinement heterostructure layer of GaN. To calculate the electronic characteristics of the structure, a self-consistent method is used where Hamiltonian with effective mass approximation is solved for conduction band while six-bands Hamiltonian matrix with k · p formalism including the polarization effect, valence-band mixing effect, and strain effect is solved for valence band. The interband optical transition elements, optical gain, differential gain, radiative current density, spontaneous emission rate, and threshold characteristics have been calculated. The wave function overlap integral is found to be 45.93% for TE-polarized structure. Also, the spontaneous emission rate is found to be 6.57 × 1027 s − 1 cm − 3 eV − 1 at 1288.21 nm with the carrier density of 5 × 1019 cm − 3. Furthermore, the radiative current density and the radiative recombination rate are found to be 121.92 A cm − 2 and 6.35 × 1027 s − 1 cm − 3, respectively, while the TE-polarized optical gain of the structure is 3872.1 cm − 1 at 1301.7 nm.

Laser Arrays With 25-GHz Channel Spacing Fabricated by Combining SAG and REC Techniques

Multiwavelength laser arrays with 25-GHz channel spacing have been fabricated by upper separate confinement heterostructure (SCH) layer selective area growth (SAG) technique. The reconstruction equivalent chirp (REC) technique is used to introduce an equivalent phase shift into the grating structure, so that a high single longitude mode yield of lasers can be ensured. Because different emission wavelengths are realized through thickness modulation of the upper SCH layer by SAG, the sampling period in the REC technique is the same for all the lasers in an array, which may alleviate the effects of the facet phase on the emission wavelength. The fabricated laser arrays have less than 0.051 nm standard deviation of wavelength deviations. Only conventional photolithography process is needed, helping to lower the fabrication cost of the laser arrays.

Performance improvement by enhancing the well-barrier hole burning in a quantum well semiconductor optical amplifier

In this paper, we demonstrated a novel physical mechanism based on the well-barrier hole burning enhancement in a quantum well (QW) semiconductor optical amplifier (SOA) to improve the operation performance. To completely characterize the physical mechanism, a complicated theoretical model by combining QW band structure calculation with SOA’s dynamic model was constructed, in which the carrier transport, interband effects and intraband effects were all taken into account. The simulated results showed optimizing the thickness of the separate confinement heterostructure (SCH) layer can effectively enhance the well-barrier hole burning, further enhance the nonlinear effects in SOA and reduce the carrier recovery time. At the optimal thickness, the SCH layer can store enough carrier numbers, and simultaneously the stored carriers can also be fast and effectively injected into the QWs.

Fabrication of Low-Cost Multiwavelength Laser Arrays for OLTs in WDM-PONs by Combining the SAG and BIG Techniques

We present the fabrication of a low-cost multiwavelength laser array monolithically integrated with a passive optical combiner for optical line terminals (OLTs) in wavelength-division multiplexing (WDM) passive optical networks (PONs). By combining the upper separate confinement heterostructure layer selective area growth technique and the bundle integrated guide technique, both multiwavelength emission with highly uniform spacing and low-loss passive-waveguide material can be obtained in a single metal–organic chemical vapor deposition (MOCVD) growth step, which greatly simplifies the fabrication of the device. A prototype laser array, which has four distributed feedback (DFB) laser elements and a multimode interference coupler as combiner, is successfully fabricated. The shallow ridge structure of the DFB lasers and the deep ridge structure of the passive waveguides are obtained by a single dry etching step, which further eases the device fabrication. The properties of the device are measured and discussed. The results indicate that our method is promising for fabricating cost-effective OLT light sources for WDM-PONs.

The effects of temperature dependent recombination rates on performance of InGaN/GaN blue superluminescent light emitting diodes

The effects of temperature dependent radiative and nonradiative recombination (Shockley–Read–Hall, spontaneous radiative, and Auger coefficients) on the spectral and power characteristics of a blue multiple quantum well (MQW) superluminescent light emitting diode (SLD or SLED) have been studied. The study is based on the rate equations model, where three rate equations corresponding to MQW active region, separate confinement heterostructure (SCH) layer, and spectral density of optical power are solved self-consistently with no k-selection energy dependent gain and quasi-Fermi level functions at steady state. We have taken into account the temperature effects on Shockley–Read–Hall (SRH), spontaneous radiative, and Auger recombination in the rate equations and have investigated the effects of temperature rising from 300K to 375K at a fixed current density. We examine this procedure for a moderate current density and interpret the spectral radiation power and light output power diagrams. The investigation reveals that the main loss due to temperature is related to Auger coefficient.

Optimization of Hybrid Silicon Lasers for High-Speed Direct Modulation

We report the optimization of hybrid silicon lasers for high-speed direct modulations by studying the small- and large-signal modulation responses based on the simple carrier transport model in this paper. The theoretical model matches well with published experimental data as the same structure parameters are used. To investigate the strong heating effect in hybrid lasers, we apply all of the carrier diffusion/capture and thermionic escape lifetimes to be temperature dependent in simulations. The frequency response of the small-signal analysis shows that the modulation bandwidth is much more sensitive to the temperature and the separate confinement heterostructure (SCH) thickness compared with the other limiting factors, such as the confinement factor, the interlayer thickness, etc. The largest modulation bandwidth decreases from 6.2 to 2.2 GHz under an injection current of 100 mA and an SCH thickness of 80 nm when the temperature rises from 300 to 350 K. The modulation speed will be greatly improved with a thinner SCH layer, particularly the p-type SCH layer, which helps shorten the diffusion lifetime. Furthermore, eye diagrams are also calculated under different bit rates, SCH thicknesses, and temperatures, respectively. With the increase of the temperature or SCH thickness, the quality of eye diagrams becomes worse. It shows that the modulation frequency can reach 10 Gb/s. We believe that this paper can serve as a guideline for the optimization of next-generation high-speed modulated hybrid silicon lasers.

Multichannel DFB Laser Arrays Fabricated by Upper SCH Layer SAG Technique

Multiwavelength laser arrays (MWLAs) fabricated by a novel upper separate confinement heterostructure (SCH) layer selective area growth (SAG) technique are studied in comparison with laser arrays fabricated by the conventional multiquantum well (MQW) SAG technique. For MWLAs by the MQW SAG, the range of the laser emission of the array is smaller than the span of the PL wavelengths of the MQW active layers. In addition, the material quality of the MQW layer degrades greatly as the width of the SAG strips is increased. Thus, the MWLAs fabricated by the method have scattered L-I characters and large fluctuation of wavelength spacings. In contrast, the MWLAs fabricated by the novel SAG method have much better wavelength spacing uniformity than those by MQW SAG, which is comparable to or better than the MWLAs fabricated by E-beam direct writing. Furthermore, the upper SCH layer SAG technique is suitable for the fabrication of MWLAs with wavelength interval, which is a challenge even for E-beam technique. In the SCH layer SAG, the MQW layer is untouched, and the same is true for all the laser elements in an array. The metal organic vapor phase epitaxies ability to control the material properties with high accuracy can be fully utilized for the fabrication of MWLAs. Combined with partially gain-coupled distributed feedback structures, the technique is promising for the fabrication of low-cost, high-quality MWLAs.

A modified SAG technique for the fabrication of DWDM DFB laser arrays with highly uniform wavelength spacings

A modified selective area growth (SAG) technique, in which the effective index of only the upper separate confinement heterostructure (SCH) layer are modulated to obtain different emission wavelengths, is reported for the fabrication of dense wavelength division multiplexing (DWDM) multi-wavelength laser arrays (MWLAs). InP based 1.5 μm distributed feedback (DFB) laser arrays with 0.8 nm, 0.42 nm, and 0.19 nm channel separations are demonstrated, all showing highly uniform wavelength spacings. The standard deviation of the distribution of the wavelength residues with respect to the corresponding linear fitting values is 0.0672 nm, which is a lot smaller than those of the MWLAs fabricated by other techniques including electron beam lithography. These results indicate that our SAG technique which needs only a simple procedure is promising for the fabrication of low cost DWDM MWLAs.

Effect of Temperature on Optical and Electronic Properties of InGaP/InGaAlP Multiple Quantum Wells

The optical and electronic properties in an InGaP/InGaAIP multiple quantum well (MQW) grown by using molecular-beam epitaxy utilizing the digital alloy technique were investigated through temperature-dependent photoluminescence (PL) measurements and numerical calculations. The high-resolution transmission electron microscopy images showed that the sample clearly displayed the InGaP wells and the InGaAIP barriers and separate confinement heterostructure layers. The PL measurements at various temperatures were performed to investigate the interband transitions of the InGaP/InGaAIP MQW. The electronic subband energies and the wavefunctions in the InGaP/InGaAIP MQW at several temperatures were determined by using a finite element method employing the standard 8-band k x p Lagrangian. The numerical results for optical interband transition energies from the ground state electron subband to the ground state heavy-hole subband of the InGaP/InGaAIP MQW at various temperatures were in reasonable agreement with the excitonic transition energies observed in the PL measurements.

Analysis of Main Degradation in Lasers With p-/n-Type InP Buried Layers Using OBIC Technique

The main degradation of the t <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sup> deterioration property (second-stage degradation) in lasers during constant-power aging is investigated by using the optical beam-induced current technique. We clarify that defects that cause this second-stage degradation diffuse mainly from around the regrown interface of the p-type InP cladding layer above the active layer to the active layer via the separate confinement heterostructure layer.

Analytical and visual modeling of InGaN/GaN single quantum well laser based on rate equations

An analytical, visual and open source model based on solving the rate equations for InGaN/GaN single quantum well (QW) lasers has been carried out. In the numerical computations, the fourth-order Runge–Kutta method has been used for solving the differential rate equations. The rate equations which have been considered in this simulation include the two level rate equations for the well and separate confinement heterostructure (SCH) layers. We present a new and inexpensive modeling method with analytical, visual and open source capabilities to investigate and comprehend the QW laser characteristics such as time behavior of carriers in SCHs and QW, photon density, output power and gain, and also the output power versus current which presents the threshold current of the laser. The characteristics of the QW lasers, which include laser time response ( P– t), turn-on delay time of lasing and output power–current ( P– I) characteristic and related features such as threshold current and slope efficiency have been investigated. Our model accurately computes the P– t and P– I characteristics such as turn-on delay time, threshold current and slope efficiency, and also illustrates the effects of parameters such as the injection current and geometry.

Relative Intensity Noise for Self-Assembled III-Nitrides Quantum-Dot Lasers

Rate-equation model with a four-level system is used to study relative intensity noise (RIN) in III-nitrides quantum-dot (QD) lasers. These levels are: the ground- and excited-states in the QD, the wetting layer (WL) and separate confinement heterostructure (SCH) layers. The most possible relaxation paths and carrier transport are considered in two types of QD structures: GaN/AlxGa1-xN/AlN and InxGa1-xN/ In0.04Ga0.96N /GaN. Effect of: QD and WL compositions, QD sizes, doped and undoped active regions is studied. The RIN is shown to be reduced with higher Al content in the WL, while an increased is shown for InGaN QD structures. RIN decreases with size reduction. Increased doping until 12 acceptors/QD reduced RIN also. The article presents some promising patents on Quantum-Dot Lasers.

Full $C$-Band 10-Gb/s 40-km SMF Transmission of InGaAlAs Electroabsorption Modulator

This paper describes the 10-Gb/s, full C-band (1530 to 1565 nm) operation of 1.55-m electroabsorption modulators (EAMs) designed to eliminate the cost of a back up light source for a 10-Gb/s dense wavelength division multiplexing (DWDM) system. We employ InGaAlAs as the material for the EAM multiple-quantum well (MQW), and design it to achieve a sufficient extinction ratio and a low chirp () parameter over a wide wavelength range. A ridge waveguide buried in benzocyclobutene (BCB) is also introduced for the EAM waveguide. We design the width of the ridge waveguide and the thickness of a core layer that includes an MQW and separate confinement hetero-structure (SCH) layers by calculating the EAM capacitance. 6-well and 12-well based EAMs are fabricated to allow us to compare the wavelength dependence of their static/dynamic extinction ratios (SER/DER), and their small-signal electrical to optical (E/O) responses. We confirm that the 12-well based EAM provides a sufficient DER over the C band. We also determine that the wavelength detuning ( ) between an input light wavelength and an EA absorption peak is 100 nm at an input wavelength of 1530 nm, and evaluate the wavelength dependence of the chirp parameter, and the 40-km SMF transmission characteristics such as eye diagrams and the power penalty (PP). These experiments were conducted using a wavelength tunable laser while maintaining the input power of the EAM at about 13 dBm and changing the input wavelength over the C band. The EAM was also driven with a constant modulation voltage swing (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pp</sub> ) of 2.0 V while changing only the EAM bias voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> ). We also measured the input power dependence of the chirp parameter. These results show that the 12-well based InGaAlAs EAM is a promising candidate for an optical modulator that can be used over the C band in a low cost 10-Gb/s optical network system.

An Equivalent Circuit Model for Analyzing Separate Confinement Heterostructure Quantum Well Laser Diodes Including Chirp and Carrier Transport Effects

In this article, we derive a new SPICE equivalent-circuit model, using the rate equations for a separate confinement heterostructure quantum well laser. In this model, based on physical principles, the frequency chirping phenomenon has been incorporated by the introduction of an exact carrier-dependent linewidth enhancement factor for quantum well lasers. The effects of different separate confinement heterostructure layer lengths and carrier transport effects on the transient and modulation responses are also analyzed through this new model.

Novel effects in extremely broadband semiconductor optical amplifiers with non-identical quantum wells

An extremely broad emission spectrum is obtained for semiconductor optical amplifiers with multiple quantum wells fabricated on the substrate. The spectral width is nearly 400 nm (1200–1600 nm), which covers the entire usable bandwidth of an optical fiber. Broadband characteristics allow observing three novel effects: (i) the bi-directional guided effect of lasing mode in a bent waveguide of semiconductor optical amplifiers, (ii) the optical switching effect in one semiconductor optical amplifier for optical communication band, and (iii) the effect of separate confinement heterostructure layer thickness.

Effect of crystal thickness of separate confinement heterostructure layers on 1.55‐μm loss‐coupled dfb laser characteristics

AbstractWe investigated the effect of the crystal thickness of the separate confinement heterostructure (SCH) guiding layers on the characteristics (threshold current, quantum efficiency) of 1550‐nm loss‐coupled Distributed Feedback laser diode fabricated with a simple step growth having an automatically buried absorptive grating InAsP layer. By increasing the thickness of SCH crystal layers from 40‐nm to 80‐nm in both below and above the multi‐quantum well region, we could achieve lower threshold currents both in 25°C and 85°C with increased ratio of quantum efficiencies between two temperatures. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)