Asymmetric Multiple-quantum-well Lasers Research Articles

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.

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.

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.

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.

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.

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.

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

Four asymmetric multiple-quantum-well (AMQW) laser structures have been grown and tested. The structures demonstrate that carriers are not evenly distributed across the active region of a MQW laser. Wells at the p-side of the active region are preferentially pumped indicating there are more carriers at the p-side of the active region than at the n-side. The structures also demonstrate that decreasing the height of the barriers reduces this effect and results in a more even carrier distribution. Thus, well position and barrier height are shown to be important design parameters for AMQW and conventional MQW lasers.