Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1999-02-08
2001-08-07
Davie, James W. (Department: 2881)
Coherent light generators
Particular active media
Semiconductor
C372S045013
Reexamination Certificate
active
06272161
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuing application of PCT Application No. PCT/RO 98/00007 filed Jun. 9, 1998 which claims priority from Romanian Patent Application No. 97-01036 filed Jun. 9, 1997.
The invention refers to diode type laser devices with an asymmetrical construction and with a reduced confinement factor having non absorbing windows.
It is known that the window catastrophic degradations and the gradual degradation of diode type laser devices, oscillators and amplifiers, in short diode lasers, represent important factors that are limiting these lasers operation at high power and high power density of the radiation that traverse the exit window. The catastrophic degradation is practically instantaneous when the power and the power density of the radiation, emitted at the mirror through the active region, overpass certain threshold values. The threshold values for the power density of the emitted radiation that passes through the active region and produce catastrophic degradation (the level for the catastrophic degradation) are in a great extent material characteristics. The catastrophic degradation level is lower for active regions obtained from materials that contain Al, for example AlGaAs, InGaAIP, or other materials, and is greater for active regions that do not contain Al, for example InGaAs, InGaAsP, or other materials. In some cases, the gradual degradation starts from mirror, having in the end, after a time period, the same effects as the catastrophic degradation, i.e. the irremediable destruction of the mirrors and of the laser. To avoid the catastrophic degradation the laser operation at power and power density levels a few time less than the catastrophic degradation level is recommendable.
The catastrophic degradation is produced by electronic states at the exit windows surface, surface states that modify the distribution of the electrical potential and the light absorption phenomena in the superficial layer at the semiconductor material—external medium interface. To remedy the effects induced by these surface states several solutions for obtaining windows for diode laser were imagined.
There are known diode lasers whereat the surface of the diode laser window, defined as the interface between the semiconductor material of the type A
3
B
5
, A
2
B
6
, or other semiconductor materials and the external medium, most frequently the surrounding air, is covered with thin layers of other materials. There are such proposals for mirror covering with different types of oxides, including the natural oxides of the semiconductor materials of the laser structure. The disadvantage of the oxide covering is that usually they do not produce the highest catastrophic degradation level. There are proposals for mirror covering with other semiconductor materials, transparent to the laser emitted radiation, for example with ZnSe. Although it produces a very high catastrophic degradation level, the disadvantage of this method is that, in order to have the highest efficiency and reliability, the deposition of other semiconductor materials need to be done in very clean conditions, for example by cleaving the mirrors in very high vacuum and by their immediately covering in this high vacuum conditions. The semiconductor material used for covering regularly is polycrystalline, the choice of the deposition conditions being such as to assure a composition close to the stoichiometric composition.
There are also known diode lasers whereat the mirror surface is covered with semiconductor materials from the same family as the semiconductor materials that form the multilayer structure of the diode laser, for example a material of the type AI
x
Ga
1-x
. As in the case of a structure obtained from layers in the Al
x
, Ga
x-1
. As system, or a material of the type in In
x
Ga
1-x
As
y
P
1-y
, in the case of structures obtained from layers in the In
x
Ga
1-x
As
y
P
1-y
, system, or other materials in other similar systems. In all cases the covering material has the energy gap higher than the energy gap of the active layer, in order to be transparent to the laser emitted radiation. In this solution the covering semiconductor material is of monocrystalline type, with the same type of crystalline structure as the materials that form the laser structure, what assure a minimum of interface states. To deposit this covering semiconductor material, in the semiconductor wafer that contains the laser structure, through narrow etching windows formed in the approximate place where the future mirror will be, the laser structure consisting of the active region and the other layers of the waveguide is etched and the new covering material is deposited instead of the etched material. The etching—deposition process can be a continuous process, for example by etching in Ga or In solution, with immediate regrowth from the same solutions of a material of the AlGaAs,InGaP, or other material type. The disadvantage of this method is that in order to etch the active region and to replace it with other semiconductor material, the entire waveguide is affected and the waveguide is interrupted at the etching—regrowth interface, to a certain distance from the exit window surface. If the etching stripe is narrow (a few micrometers) so that the radiation that quits the interrupted guide arrives at the mirror and is back reflected by the mirror toward the interrupted guide can be in a large degree captured by the interrupted guide, then the effective reflection coefficient can be sufficiently large, in order to assure the necessary feedback for the operation as an oscillator, and the losses, inside the device but external to the waveguide, can be sufficiently low. If the etching stripe is large (tens of micrometers) the losses outside the guide are relatively large. In this latter case, the divergent beam that leaves the waveguide can reach, when propagating, the surface of the closest metallic contact and be lost there, or, when it reaches the mirror surface can have a large cross section, larger than the cross section of an possible fiber to be coupled with. A narrow etching stripe, with a pronounced profile in depth, has the disadvantage that is more difficult to be obtained into practice, the regrowth processes are more difficult and the further cleaving inside of a narrow stripe is more difficult. A wide etching stripe has the disadvantage that increases the losses inside the device and the coupling losses and reduces the effective reflection coefficient.
INVENTION SUMMARY
The problem solved by this invention is the achievement of nonabsorbing windows, with a reduced number of surface states that do not interrupt essentially the propagation properties of the waveguide.
The windows for diode type laser devices according to the invention avoid the disadvantages of other known solution since:
they are obtained in asymmetric, low confinement factor structures that consists of a waveguide formed of several semiconductor layers with selected optical properties and an active region where the generation of the radiation is produced, the active region being located asymmetrically relative to the waveguide, at an extremity of the waveguide central layer, outside, at the margin or inside the waveguide central layer,
they are obtained by the partial modification of the layered diode laser structure, into a stripe placed perpendicular to the direction of propagation, by this modification a large part of the layer structure remains unaffected, and a material with an adequate crystalline structure and nonabsorbing for the radiation emitted by the laser is put in place, such that the optical properties of the waveguide are reconstructed into a great extent,
so that the radiation is propagating up to the mirror into a waveguide similar to the waveguide of the rest of the diode laser.
The diode lasers with non absorbing windows, according to the invention, have the following advantages:
the layered structure proper for the partial replacement of some layers, without an essential change of the waveguide, are l
Davie James W.
Vigil Thomas R.
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