The lasers are classified according to their structure: FP, DFB, DBR, QW, VCSEL FP: Fabry-Perot, DFB: distributed feedback, DBR: distributed Bragg reflector, QW: quantum well, VCSEL: vertical cavity surface reflected laser.
(1) Fabry-Perot (FP) type laser diode is composed of an epitaxially grown active layer and a limiting layer on both sides of the active layer, and the resonant cavity is composed of two cleavage planes of the crystal, and the active layer may be N type, can also be P type. Due to the existence of a heterojunction barrier due to the band gap difference, electrons and holes injected into the active layer cannot be diffused and confined in a thin active layer, so that even a small current flows, it is easy to realize On the other hand, the narrow band gap active layer has a larger refractive index than the confinement layer, and the light is concentrated in a region having a large interest rate, so it is also limited to the active layer. When the electric-F forming the inverted bifurcation in the active layer transitions from the conduction band to the valence band (or impurity level), the photons are combined with the holes to emit photons, and the photons are formed in a cavity having two cleavage planes. The reciprocating reflection propagation is continuously enhanced to obtain the optical gain. When the optical gain is greater than the loss of the resonant cavity, the laser is emitted outward. The laser is essentially an stimulated-emitting optical resonant amplifier.
(2) Distributed feedback (DFB) laser diode The main difference between it and the FP type laser diode is that it has no lumped reflection of the cavity mirror, and its reflection mechanism is provided by the Bragg grating on the active area waveguide, only satisfied The aperture of the Bragg scattering principle. It is allowed to reflect back and forth in the medium, and the laser appears when the medium achieves a population inversion and the gain meets the threshold condition. This kind of reflection mechanism is a subtle feedback mechanism, hence the name distributed feedback laser diode. Due to the frequency selective function of the Bragg grating, it has very good monochromaticity and directionality; in addition, because it does not use a crystal cleavage plane as a mirror, it is easier to integrate.
(3) Distributed Bragg (DBR) reflector laser diode The difference between it and the DFB laser diode is that its periodic trench is not on the active waveguide surface, but on the passive waveguide on both sides of the active layer waveguide, this pre- A passive periodic corrugated waveguide acts as a Bragg mirror. In the spontaneous emission spectrum, only light waves near the Bragg frequency can provide effective feedback. Due to the gain characteristics of the active waveguide and the Bragg reflection of the passive periodic waveguide, only the light wave near the Bragg frequency can satisfy the oscillation condition, thereby emitting the laser.
(4) Quantum Well (QW) Laser Diodes When the thickness of the active layer is reduced to the De Broglie wavelength (λ 50 nm) or when compared with the Bohr radius (1 to 50 nm), the properties of the semiconductor are fundamental. Changes, semiconductor energy band structure, carrier mobility properties will have a new effect - quantum effect, the corresponding potential well becomes a quantum well. We call the LD with superlattice and quantum well structure a quantum well LD. Having a carrier potential well LD is called a single quantum well (SQW) LD, and a quantum well LD having n carrier potential wells and an (n+1) barrier is called a multi-precharge well (MQW) LD. The quantum well laser diode has a structure in which the active layer thickness (d) of a general double heterojunction (DH) laser diode is made tens of nanometers or less. Quantum well laser diodes have the advantages of low threshold current, high temperature operation, narrow spectral line width, and high modulation speed.
(5) Vertical cavity surface emitting laser (VCSEL) Its active region is located between two confinement layers and constitutes a double heterojunction (DH) configuration. In order to limit the injection current in the active region, the implantation current is completely confined in a circular active region by means of buried fabrication techniques. Its cavity length is buried in the longitudinal length of the DH structure, generally 5 ~ 10μm, and the two mirrors of its cavity are no longer the cleavage plane of the crystal, and its one mirror is set at the P side (key The other side of the mirror is placed on the N side (the substrate side or the light output side). It has the advantages of high luminous efficiency, extremely low work enthalpy, high temperature stability and long service life.