1. Field of the Invention
The present invention relates to a multi-wavelength laser diode, more particularly, which can improve a reflective layer structure in order to achieve a high reflectivity in the entire visible light range.
2. Description of the Related Art
Laser diodes are used as an optical pickup for various storages in order to produce light in response to application of electric current.
A laser diode needs a high reflectivity mirror or reflective face on one end of an oscillating layer in order to realize high power laser beams, in which producing such a reflective face requires coating a high reflectivity thin film. Various studies have been made so far in order to produce such a high reflective thin film.
FIG. 1 is a perspective view of a conventional laser diode, FIG. 2 is a cross-sectional view taken along the line II-II in FIG. 1, and FIG. 3 is another cross-sectional view taken along the line III-III in FIG. 1.
Referring to FIGS. 1 to 3, a laser diode 10 includes an oscillating structure 12 for generating light, p- and n-metal layers 26 and 28 coated on the top and underside of the oscillating structure 12, respectively, a reflective layer 30 of a high reflectivity formed on one end of the oscillating structure 12 and an anti-reflective layer 40 formed on the other end of the oscillating structure 12 opposite to the reflective layer 30.
The oscillating structure 12 includes a lower cladding layer 16, an active layer 18, an upper cladding layer 20 and an upper capping layer 24 formed in their order on a semiconductor substrate 14. The upper cladding layer 20 has a ridge 20a formed in a central upper portion thereof so that inactive regions 22 such as current blocking layers are formed at both sides of the ridge 22a. 
When a forward voltage is applied to the p- and n-metal layers 26 and 28 of the laser diode 10 having the above oscillating structure 12, electrons and holes are introduced along vertical routes into the active layer 18 where the electrons and holes are combined to emit a laser beam, i.e., photons corresponding to the energy band gap of the active layer 18.
The reflective layer 30 is formed on one face of the oscillating structure 12 adjacent to one end of the ridge 22a and the anti-reflective layer 40 is formed on another face of the oscillating structure opposite to the reflective layer 30 so that a cavity resonator formed between the reflective and anti-reflective layers 30 and 40 can generate laser oscillation.
The reflective layer 30 is made of dielectric material, and includes at least 7 pairs of Al2O3 and Si3N4 layers which are deposited one atop another to raise reflectivity.
However, a process for forming the reflective layer 30 of multilayer dielectric material spends a long time period while producing poor reliability. Although the multilayer dielectric layer can achieve excellent reflectivity with respect to a single wavelength, there is a critical drawback in that it cannot achieve a uniform reflectivity with respect to a multi-wavelength band.
That is, the thickness of each dielectric layer is determined by λ/4n, in which λ is the wavelength of a laser beam to be reflected, which is emitted from an oscillating layer including the active layer 18 and the upper and lower cladding layers 16 and 20, and n is the reflectivity of corresponding dielectric material.
Reflectivity examples of conventional dielectric reflective layers with respect to wavelengths are shown in FIGS. 4 and 5, in which FIG. 4 illustrates a reflectivity curve of a dielectric reflective layer with respect to wavelengths where a corresponding laser beam has a wavelength of 715 nm, and FIG. 5 illustrates a reflectivity curve of a dielectric reflective layer with respect to wavelengths where a corresponding laser beam has a wavelength of 780 nm.
The graph shown in FIG. 4 has the highest reflectivity of about 92% at 715 nm. However, it is seen that the reflectivity significantly drops at a wavelength range of 650 nm or less. The graph shown in FIG. 5 also has the highest reflectivity of about 92%. It can be observed, however, that the reflectivity also significantly drops at a wavelength range of 700 nm or less.
Accordingly, such a reflective layer based upon dielectric material can have a high reflectivity only in a specific wavelength range, but cannot maintain the high reflectivity in a wide wavelength range.