Asymmetric Multiple Quantum Well Research Articles

Improved performance of GaN-based blue laser diodes using asymmetric multiple quantum wells without the first quantum barrier layer.

An asymmetric multiple quantum well (MQW) without the first quantum barrier layer is designed, and its effect on the device performance of GaN-based blue LDs has been studied experimentally and theoretically. It is found that compared with LD using symmetrical multiple quantum well, device performance is improved significantly by using asymmetric MQW, i.e. having a smaller threshold current density, a higher output optical power and a larger slope efficiency. The threshold current density decreases from 1.28 kA/cm2 to 0.86 kA/cm2, meanwhile, the optical power increases from 1.77 W to 2.52 W, and the slope efficiency increases from 1.15 W/A to 1.49 W/A. The electroluminescence characteristics below the threshold current demonstrate that asymmetric MQW is more homogeneous due to the suppressed strain and piezoelectric field. Furthermore, theoretical calculation demonstrates that the enhancement of electron injection ratio and reduction in optical loss are another reason for the improvement of device performance, which is attributed to a smaller electron potential barrier and a more concentrated optical field distribution in the asymmetric structure, respectively. The new structure design with asymmetric MQW is concise for epitaxial growth, and it would also be a good possible choice for GaN-based LDs with other lasing wavelengths.

Effect of doping profile on the output power of broadly tuneable InGaAsP/InP asymmetric multiple quantum well lasers: finite element method simulations and experimental results

Broadly tuneable InGaAsP/InP asymmetric multiple quantum well (AMQW) ridge waveguide laser diodes suffer from low output power. Theoretical and experimental studies to understand the effect of doping profile on the laser output power and to solve the output power problem without affecting the tuning range of the laser are reported. A commercially available partial differential equation solver, FlexPDE, was used to solve the main equations for flow of current and hence simulate the AMQW devices. The simulations, which were validated with published data, showed poor current injection efficiency to the centre of the active region. The simulations identified the ridge structure as the main reason for this poor current injection efficiency. Since the ridge structure is an essential part of this type of laser diode, effort was put forth to reduce the effect of the ridge structure on the current injection efficiency by optimising the doping profile of the device. Both simulation and experimental data showed that extending the doping profile closer to the active region would improve significantly the current injection efficiency consequently, the output power without affecting the tuning range of the device. Over all, the Flex PDE solver showed potential to handle laser diode design problem.

Analysis of broadly tunable coupled-cavity semiconductor lasers

We report on the spectral properties for above-threshold operation of broadly tunable, asymmetric multiple quantum well (AMQW), coupled-cavity InGaAsP/InP semiconductor diode lasers. We developed a traveling wave model to understand the mode selection that the lasers exhibit. We find that a weak, short external cavity (SXC) can be used to obtain single frequency operation on each longitudinal mode over the ∼100 nm tuning range of the uncoated AMQW coupled-cavity lasers. We measured the spectral properties of AMQW coupled-cavity lasers with and without an SXC. In a synthesized optical coherent optical tomography application, the use of an SXC with an AMQW coupled-cavity laser reduces the coherence length and hence enhances the performance of the AMQW coupled-cavity laser for optical coherence tomography applications.

Gain flattening coatings for improved spectral performance of asymmetric multiple quantum well lasers

A gain flattening coating was designed and fabricated to enhance the wavelength tuning for asymmetric multiple quantum well (AMQW) lasers. After coating, a nonlasing gap in the middle range of the lasing wavelengths, which might exist for AMQW lasers that are operated without an external cavity, was overcome and the total lasing range was increased. With the coating, the tuning range of an AMQW laser, as measured without an external cavity, was increased to 85 nm from 70 nm.

Coupled cavity InGaAsP/InP laser for synthetic optical coherence tomography and other applications

The authors describe widely tunable coupled cavity semiconductor lasers with sub-microsecond switching times between modes over the operating range of ∼100 nm. With appropriate modulation of injection currents and time averaging of the output, these devices provide short coherence lengths and can be an excellent source for synthesised optical coherence tomography (OCT). The depth resolution was found to be ∼15 μm for a 100 nm wavelength tuning range centred at 1580 nm. High-output power and brightness together with a short coherence length confers on these asymmetric multiple quantum well (AMQW) C 3 laser some advantages over conventional sources for OCT. Also, a rapid wavelength switching capability allows the AMQW C 3 lasers to be used in real-time OCT and other applications needing wavelength agile sources.

Characterization of asymmetric GaN/InGaN multiple quantum well

AbstractThe microstructure and electronic structure of the asymmetric GaN/InGaN multiple quantum well (MQW) light emitting diodes are characterized by transmission electron microscope (TEM), cathodoluminescence (CL) and capacitance‐voltage (C‐V) measurements. In TEM images, the asymmetric structure of InGaN/GaN MQW are observed as designed with different width of QWs and barriers, and indium content in QWs as well. In high‐resolution TEM images, the InGaN quantum wells and GaN barriers show the same lattice constant, and some degradation can be seen at the bottom of each InGaN QW. There are two emission peaks, 450 and 530 nm in CL spectrum, similar to the ones in electroluminescence spectrum. The double emission peaks are assigned to the irradiative recombination in different InGaN QWs. From the C‐V data, the apparent carrier concentration in the asymmetric MQW is calculated. There are five obvious peaks which are well explained by an energy band scheme of InGaN/GaN MQW. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Design Considerations for Asymmetric Multiple Quantum Well Broad Spectral Width Superluminescent Diodes

We investigated the design of broad spectral width super luminescent diodes (SLDs) with asymmetric multiple-quantum-well (AMQW) active regions. The design is based on theoretical modeling, which starts with calculation of the gain. Two InGaAsP/InP AMQW SLD structures centered at 1.3 mum were grown and processed. Both gave a broad spectral width of >80 nm when operated at the transition carrier density (TCD). Structure 1 was designed based on gain simulations only and was found to be able to provide broad spectral width only at low injection currents. Thus, Structure 1 gives low output power. Based on this experience, structure 2 was designed to achieve a broad gain at a higher TCD than Structure 1. Experimental results show that structure 2 provided a reasonable power and broad spectral width at the TCD. Thus, we conclude that, compared with normal design criterion which depends on the gain spectrum only, the design of an AMQW SLD has to include a relatively high TCD to obtain reasonable output power. The SLDs were ridge waveguide structures with seven degree tilted-stripes. Use of double section waveguides could further broaden the SLD spectrum at the red side by >10 nm.

Bias polarity dependence of carrier injection into high-energy states in asymmetric GaAs/AlAs multiple-quantum wells embedded in an n-i-n diode

We investigated the electric field dependence of photoluminescence (PL) spectra from a GaAs/AlAs asymmetric multiple-quantum well (AMQW) embedded in an n-i-n diode. The bias polarity dependence of the PL spectra caused by the difference of the subband resonances was observed. In the forward bias voltage regime, PL from the narrower QW was observed due to carrier injection by an X state in the thick AlAs barrier. The PL intensity strictly depends on the carrier injection caused by the Γ-X-Γ and the LO-phonon-assisted transfers.

Broadly tuneable, short external cavity diode laser for optical coherence tomography

The application of a broadly tuneable asymmetric multiple-quantum-well (AMQW) laser is demonstrated in an external cavity for both time-domain and Fourier-domain optical coherence tomography (OCT). The AMQW laser was custom designed and fabricated to have a large tuning range when operated with no facet coatings in a diffractive optical element (DOE) short external cavity (SXC). The depth resolution of Fourier domain OCT was found to be as short as 13 µm in air with the 117 nm tuning range (full-width-half-maximum of 80 nm) of the AMQW DOE SXC source. For time domain OCT, synthesised OCT with the AMQW sources was investigated. It was found that the depth resolution is set by the tuning range of the AMQW DOE SXC laser. The depth resolution can be improved as 117 nm is not the limit to the tuning range of AMQW lasers.

On the Evolution of Carrier Distribution and Wavelength Switching in Asymmetric Multiple Quantum-Well Lasers

The evolution of the carrier distribution with current in two different asymmetric multiple quantum laser structures was studied experimentally through measurements of electroluminescence (EL). The EL was measured through the substrates of lasers and provided information about the carrier distributions. The carrier concentration was observed to increase with current both below and above threshold, presumably owing to the change of threshold conditions. The switch of the lasing wavelength from long to short wavelength was explained by inhomogeneous broadening of the gain of the wells and by incomplete clamping of the carrier concentration above threshold, as inferred from the measured EL and gain spectra. The role of thermal effects was investigated by comparing the laser performance under continuous-wave and pulsed operation

Gain Characteristics of Asymmetric Multiple Quantum-Well Lasers

The spectra and gain characteristics were investigated for an asymmetric multiple quantum-well (AMQW) laser with two wide and two narrow QWs. It is reported that the laser initially lased on a long wavelength corresponding to the lowest energy transition of the wide well then switched to a short wavelength that was red-shifted more than 50 nm off the lowest energy transition of the narrow well. Correspondingly, the gain spectrum is much narrower than predicted by a free carrier gain theory. This large red wavelength shift or narrowing of the gain spectrum could not be explained by the present gain theories, while it could be explained satisfactorily by the measured gain spectra of QW lasers. This indicates that an accurate gain theory is required to predict the performance of AMQW lasers.

Frequency response measurements for mirror image asymmetric multiple quantum-well lasers

A comparison of measurements of the frequency response characteristics of mirror image asymmetric multiple quantum-well (AMQW) lasers is presented in this paper. Conclusions on the carrier transport were drawn from these measurements, which are significant for the design of both AMQW and standard devices. The effective resonance frequency trend with current derived from these measurements shows no significant difference for several mirror image structures. It is thought that this is because of a very fast inter-well transport time in these devices. There was also no significant difference in the temperature dependence of the effective resonance frequency for these mirror image structures. This indicates that temperature-dependent transport mechanisms such as carrier capture and escape have little effect on the resonance frequency of these structures. These trends provide support for an analytic equation for the resonance frequency of AMQW structures that is independent of well position and carrier transport times. These results are important for designing AMQW lasers for high-speed applications.

A rate equation model of asymmetric multiple quantum-well lasers

This paper gives a description of a rate equation model to simulate the steady-state and frequency response characteristics of asymmetric multiple quantum well (AMQW) lasers using several different methods. AMQW lasers consist of several quantum wells of different widths or compositions designed to achieve a broad net gain profile for improved wavelength tuning. The position of the quantum wells relative to the p-side of the device plays an important role in the steady-state characteristics due to a significant nonuniform carrier distribution across the active region. Simulations of the carrier distribution and the onset of simultaneous lasing for certain structures are demonstrated using this model. Appropriate values for the distributed material loss and tunneling times were determined here by fitting the model output to measurements of the transition cavity length for a number of AMQW lasers. As well, comparison of different calculation methods demonstrates it is more appropriate to use a numerical analysis of the laser rate equations rather than analytic equations when modeling the frequency response of these structures. However, an analytic equation for the resonance frequency was found that fit well with numerical simulations of several different AMQW laser structures.

Sci‐PM Fri ‐ 01: Application of broadly tunable asymmetric multiple‐quantum‐well (AMQW) lasers to optical coherence tomography (OCT)

Optical coherence tomography (OCT) requires a broad‐band light source for time domain OCT or a broadly tunable light source for Fourier domain OCT. People use an LED or an SLD for time domain OCT and use a swept laser for Fourier domain OCT. Asymmetric multiple‐quantum‐well (AMQW) lasers have superiority in obtaining a broad and flat gain spectrum, thus it should be possible to achieve both broadband and high‐power operation with AMQW lasers. We show in this work that with a DOE applied short external cavity system, an AMQW laser could work as both a tunable laser for Fourier domain OCT and as a broad‐band light source for time domain OCT. The former application is straight‐forward. We put our new idea in the latter application. If one uses a Fourier domain approach and integrates in time the output of the detector while the wavelength of the source is tuned rapidly, the tunable light source will appear as an incoherent light source with a depth resolution that is inversely proportional to the spectral width that the laser is tuned over. Therefore the depth resolution can be increased by broader spectral width. With our AMQW laser, the maximum tuning range is as broad as 100 nm, with a corresponding depth resolution of 10.6 μm in air with center wavelength of 1.55 μm. The maximum depth that could be measured depends on the spacing of the longitudinal modes of the laser and is 550 μm for a laser with mode‐spacing of 1 nm.

How to restrain Auger recombination predominance in the threshold of asymmetric bi-quantum-well lasers

Both radiative and nonradiative processes which occur in the active region of GaInAs–GaInAsP–InP asymmetric multiple quantum-well (AMQW) heterolasers with two quantum wells of different width (4 and 9nm) are described. Several possible processes of non-radiative Auger recombination which affect the temperature sensitivity of the lasing threshold are analyzed and the temperature dependencies of the investigated processes are presented. For the above-mentioned AMQW heterostructure, it is shown that the influence of the Auger recombination processes on the temperature behaviour of the lasing threshold can be restrained by operation at temperatures lower than 340K and the cavity losses which do not exceed 60cm−1.

비대칭 다중 양자우물 레이저 다이오드에서 모드이득의 이론 및 실험적 분석

InGaAsP/InP 비대치 다중 양자우물 구조에서 모드 이득의 이론 및 실험적 해석을 통해 광대역 이득특성을 확인하였고, 여러가지 선폭증가 함수를 적용하여 비대칭 선폭증가 함수가 가장 실험치에 근접한 모델임을 확인하였다. 광대역 이득 대역폭을 얻기 위한 활성층 구조를 설계하여 RWG형 FP-LD를 제작하였고, 공진기 길이가 400 <TEX>$\mu\textrm{m}$</TEX>에서 발진개시 전류는 36 ㎃, 주입전류가 33 ㎃에서 -1 ㏈ 이득폭은 45 nm, -3 ㏈ 폭은 80 nm을 얻을 수 있었다. Wide- and flat-gain laser diodes were designed and fabricated from asymmetric multiple quantum well (AMQW) structures which consist of three compressively strained InGaAsP wells of different thicknesses. For a 400 <TEX>${\mu}{\textrm}{m}$</TEX>-long lasers with as-cleaved facets, -1 ㏈ and -3 ㏈ gain bandwidth were 45 nm and 80 nm, respectively. For this AMQW structure, calculated gain spectra with various line broadening functions were compared with experimental results. We confirmed the calculated gain spectra using an asymmetric line broadening function were in good agreement with the measured data.

Sensitive absorption spectroscopy by use of an asymmetric multiple-quantum-well diode laser in an external cavity

We have developed a broadly tunable diode laser system by employing custom-designed asymmetric multiple-quantum-well (AMQW) InGaAsP lasers in an external cavity configuration. Feedback is provided by a diffractive optical element with high coupling efficiency and wavelength selectivity, allowing for single-mode tuning of the output wavelength by varying the external cavity length. This tunable laser system was used experimentally to perform absorption spectroscopy on weak CO2 lines over a broad wavelength region in the near infrared. An experimental tuning range of 80 nm has been observed for a laser cavity length of 600 micron, which is double the tuning range found with conventional, uncoated quantum-well lasers. We achieved a detection sensitivity of 5 x 10(-6) at 95% confidence over the wavelength range of 1.54-1.62 micron by employing a second-harmonic detection technique. The theoretical predictions of a broad gain profile from an ambipolar rate equation model are found to correspond to the experimentally observed increased tunability of the uncoated AMQW lasers.

Experimental and theoretical analysis of the carrier distribution in asymmetric multiple quantum-well InGaAsP lasers

The authors present an experimental and theoretical analysis of the carrier distribution in multiple quantum-well (MQW) lasers and the effect of this carrier distribution on the gain of wells at different locations in the active region. An experimental technique using mirror image asymmetric multiple quantum-well (AMQW) lasers is described which provides quantitative information on the degree to which the carrier distribution affects the gain of quantum wells (QWs) in the active region. A gain model for AMQW lasers is developed and used to explain some important characteristics of AMQW devices. A rate equation model is presented which incorporates the effects of fields across the p-i-n junction active region. The model is able to predict experimental results measured from thirteen AMQW laser structures to within experimental uncertainty.

Critical design parameters for engineering broadly tunable asymmetric multiple-quantum-well lasers

Asymmetric multiple-quantum-well (AMQW) lasers are MQW lasers with QW's of varying thickness and/or composition in a single active region. AMQW lasers can, if designed properly, exhibit broad spectral tuning ranges more than twice as large as those of conventional MQW lasers. In the paper, it is shown experimentally and theoretically that AMQW lasers only exhibit broad wavelength tuning ranges near a specific cavity length defined as the transition cavity length. The transition cavity length is a critical design parameter for engineering broadly tunable AMQW Lasers.

Effect of barrier thickness on the carrier distribution in asymmetric multiple-quantum-well InGaAsP lasers

Four asymmetric multiple-quantum-well (AMQW) laser structures have been grown and tested. The structures were designed to study the effect of the thickness of the barriers on the distribution of carriers amongst the quantum wells by comparing the transition cavity lengths (TCL) of mirror image AMQW lasers. The TCL method provides a quantitative measure of the degree to which the uneven carrier distribution affects the net gain of wells owing to the position of the well in the active region. We experimentally demonstrate that reducing the thickness of the barrier layers from 100 to 50 /spl Aring/ results in a significantly more uniform carrier distribution. The thickness of the barriers is thus shown to be an important design parameter for MQW lasers.