>> The Basics of Quantum Wells Lasers
Regular double heterostructure (DH) semiconductor lasers have an active region of 0.1 to 0.2um thick. Since the 1980s, lasers with very thin active regions, quantum well lasers, were being developed in many research laboratories.
A quantum well laser is a laser diode in which the active region of the device is so narrow that quantum confinement occurs. The wavelength of the light emitted by a quantum well laser is determined by the thickness of the active region rather than just the bandgap of the material from which it is constructed. This means that much shorter wavelengths can be obtained from quantum well lasers than from conventional laser diodes using a particular semiconductor material. The efficiency of a quantum well laser is also greater than a conventional laser diode due to the stepwise form of its density of states function.
Quantum well lasers have active regions of about 100 Å thick, which restricts the motion of the carriers (electrons and holes) in a direction normal to the well. This results in a set of discrete energy levels and the density of states is modified to a two-dimensional-like density of states. This modification of the density states results in several improvements in lasers characteristics such as lower threshold current, higher efficiency, and higher modulation bandwidth and lower CW and dynamic spectral width. All of these improvements were first predicted theoretically and then demonstrated experimentally.
Quantum well lasers require fewer electrons and holes to reach threshold than conventional double heterostructure lasers. A well-designed quantum well laser can have an exceedingly low threshold current.
Moreover, since quantum efficiency (photons-out per electrons-in) is largely limited by optical absorption by the electrons and holes, very high quantum efficiencies can be achieved with the quantum well laser.
To compensate for the reduction in active layer thickness, a small number of identical quantum wells are often used. This is called a multi-quantum well laser.
The development of InGaAsP quantum well lasers was made possible by the development of MOCVD and GSMBE growth techniques. The transmission electron micrograph (TEM) of a multiple Quantum Well laser structure is shown in the following figure. It shows five InGaAs quantum wells grown over n-InP substrate. The well thickness is 70 Å, and the wells are separated by barrier layers of InGaAsP λ = 1.1 um. Multiquantum well lasers with threshold current densities of 600 A/cm2 have been fabricated.
The schematic of a Multi-Quantum-Well Buried Heterostructure laser is shown in the following figure. The laser has a Multi-Quantum-Well (MQW) active region and it utilizes Fe doped InP semi-insulating layers for current confinement and optical confinement.
The light versus current characteristics of a MQW BH laser are shown in the following figure. The laser emits near 1.55 um.
The MQW lasers have lower threshold currents than regular Double Heterostructure (DH) lasers. Also the two-dimensional-like density of states of the QW lasers makes the transparency current density of these lasers significantly lower than that for regular DH lasers. This allows the fabrication of very low-threshold (Ith 1 mA) lasers using high-reflectivity coatings.
The optical gain (g) of a laser at a current density J is given by
where a is the gain constant and J0 is the transparency current density. The cavity loss α is given by
where αc is the free carrier loss, L is the length of the optical cavity and R1 and R2 are the reflectivity of the two facets.
At threshold, gain equals loss; hence, it follows from the two equations that the threshold current density (Jth) is given by
Thus, for a laser with high-reflectivity facet coatings (R1 R2 1) and with low loss (αc ~ 0), Jth J0. For a QW laser, J0 ~ 50 A/cm2 and for a DH laser, J0 ~ 700 A/cm2; hence, it is possible to get much lower threshold current using QW laser as the active region.
The light versus current characteristics of a QW lasers with high-reflectivity coatings on both facets are shown in the following figure. The threshold current at room temperature is ~ 1.1 mA. The laser is 170 um long and has 90% and 70% reflective coating at the facets. Such low-threshold lasers are important for array applications.
Recently, QW lasers were fabricated that have higher modulation bandwidth than regular DH lasers. The current confinement and optical confinement in this laser are carried out using MOCVD grown Fe doped InP lasers. The laser structure is then further modified by using a small contact pad and etching channels around the active region mesa. These modifications are designed to reduce the capacitance of the laser structure. A 3-dB bandwidth of 25 GHz is obtained.
>> What is Transparency Current Density?
The transparency current density represents a fundamental limit to achieving the lowest lasing threshold for semiconductor lasers in general.
The current needed for lasing is composed of two parts: the first part being the current needed for maintaining the electron density at the optical transparency level, and beyond that a second part to attain the necessary gain to overcome all the losses in the laser cavity. It can be argued (and can actually be demonstrated experimentally) that a laser cavity can be designed such that the losses are minimal, but this can only reduce the second part of the threshold current while the first part, that responsible for optical transparency, is unaffected.
The key to building an ultralow threshold laser is thus to design a laser cavity with a very low loss, with a material that has the lowest transparency current density. A single quantum well structure is one that possesses both of these qualities and, when combined with high reflectivity coatings to minimize mirror loss, results in some of the lowest lasing threshold currents achieved to date.
>> Strained Quantum-Well Lasers
Quantum well lasers have also been fabricated using an active layer whose lattice constant differs slightly from that of the substrate and cladding layers. Such lasers are known as strained quantum-well lasers.
Over the last few years, strained quantum well lasers have been extensively investigated all over the world. They show many desirable properties such as
- A very low-threshold current density
- A lower linewidth than regular Multi-Quantum-Well (MQW) lasers both under continuous wave (CW) operation and under modulation
The origin of the improved device performance lies in the band-structure changes induced by the mismatch-induced strain. Strain splits the heavy-hole and the light-hole valence bands at the Τ point of the Brillouin zone where the bands gap is minimum in direct band-gap semiconductors.
Two material systems have been widely used for strained quantum well lasers
- InGaAs grown over InP by the MOCVD or the CBE growth technique
- InGaAs grown over GaAs by the MOCVD or the MBE growth technique
The first material system is of importance for low-chirp semiconductor laser for lightwave system applications. The second material system has been used to fabricate high-power lasers emitting near 0.98um, a wavelength of interest for pumping erbium-doped fiber amplifiers (EDFA).