Ring cavity laser

Details Ring cavity laser Ring cavity laser

1999-07-09

2001-08-28

Davie, James W. (Department: 2874)

Coherent light generators

Particular resonant cavity

Folded cavity

Reexamination Certificate

active

06282226

ABSTRACT:

BACKGROUND OF THE INVENTION
1.Field of the Invention
The present invention relates to a laser of a ring cavity (or resonator) type which is usable for optical interconnection, parallel data processing, large-capacity parallel optical transmission and so forth, and particularly to a laser, such as a semiconductor laser, having a three-dimensional ring cavity.
2.Description of the Related Background Art
Semiconductor lasers are well known as light sources which are usable in optical communication and optical recording, and various types thereof have been developed. Further, in recent years, there have been developed opto-electronic integrated circuits in which optical functional devices, such as a semiconductor laser, a photodetector, a modulator and an optical switch, are arranged on a common substrate, and the integration of arrays of semiconductor lasers suitably usable for parallel processing has also been studied. In respect of those integrated circuits, functional improvement of the semiconductor laser is strongly required, and especially a low-threshold semiconductor laser is a key device.
With an approach to such a low-threshold semiconductor laser, a so-called micro-cavity structure is known, whose cavity length is reduced to about a wavelength of light to increase a coupling rate of spontaneous radiation light to its oscillation mode. Devices as illustrated in
FIGS. 1
,
2
A and
2
B are known, as examples of such a laser. The device of
FIG. 1
is a surface emitting semiconductor laser in which distributed Bragg reflectors
753
and
755
are provided above and under an active layer
751
, respectively, a current can be injected into the active layer
751
through upper and lower electrodes
758
and
759
, and its cavity is constructed perpendicular to the substrate
757
.
The devices of
FIGS. 2A and 2B
are disc-type semiconductor lasers in which there are arranged discs
873
and
893
having diameters of about several microns and including circular and hexagonal active layers
875
and
895
above substrates
871
and
891
, respectively in which and light is totally reflected at peripheral surfaces of those discs
873
and
893
, which thus provide ring cavities in planes parallel to the substrates
871
and
891
, respectively.
The surface emitting semiconductor laser as illustrated in
FIG. 1
, however, needs distributed Bragg reflectors with high reflectivity. When a GaAs substrate is used as a substrate, an AlAs/(Al)GaAs multi-layer mirror is ordinarily used as the reflector. In this case, however, more than twenty pairs of AlAs/(Al)GaAs are needed to obtain a sufficiently high reflectivity, and hence it takes much time to grow those pairs on a wafer. Further, when an InP substrate is used, it is necessary to etch the substrate with an active layer and the like grown thereon to form a cylindrical hole in the substrate and to deposit an SiO
2
/Si or Al
2
O
3
/Si multi-layer in the hole by vacuum evaporation or sputtering. The process is hence complicated. In addition, it is difficult to further lower the reduced threshold since light propagating in directions parallel or slanting to the substrate is only weakly coupled to the cavity mode.
On the other hand, in the case of the disc-type semiconductor lasers, the circular disc is ordinarily formed by dry-etching a predetermined portion after the crystalline growth. Therefore, it is difficult to precisely form the circular periphery of the disc's side, and the side face of the active layer is likely to be damaged. In addition, it is also difficult in this case to further lower the reduced threshold since light propagating in directions parallel or slanting to the substrate is not strongly coupled to the cavity mode.
Regarding the case of
FIG. 2B
, there is indeed a method of forming the hexagonal structure by a selective growth using the face-orientation dependence of crystalline growth speed, and this method can improve the flatness of the side face, compared with dry etching. Also in this case, however, it is likewise difficult to further lower the reduced threshold because light propagating in directions parallel or slanting to the substrate is not strongly coupled to the cavity mode.
Furthermore, in semiconductor lasers which have thus far been developed, oscillation basically occurs in a linearly-polarized light mode, and a semiconductor laser capable of oscillating in a circularly-polarized or elliptically-polarized light mode has not yet been put into practical use.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a ring cavity laser which needs no or only one multi-layer mirror having a high reflectivity, is suitable for a micro-cavity structure, can be readily fabricated, improves a coupling rate of spontaneous radiation light to an oscillation mode, can achieve a low threshold, and is capable of oscillating even with circularly-polarized or elliptically-polarized light.
The present invention is generally directed to a ring cavity laser which has a polyhedral structure contoured by a plurality of planes, includes a ring cavity having reflective faces formed by the planes of the polyhedral structure, and an active medium, and is constructed such that there exists a three-dimensional light path (i.e., one which is not contained in a single plane) among light paths of the ring cavity through which light pumped in the active medium travels when this light starts at a point on one reflective face, is reflected by each reflective face and returns to the starting point. This fundamental structure is capable of increasing a degree of light confinement, enhancing a coupling rate of spontaneous light to its cavity mode, and effecting oscillation even with circularly-polarized or elliptically-polarized light. This leads to a ring cavity laser which can be readily fabricated by using a selective etching method, a selective growth method and the like (which will be described later), and lower its threshold. Since the laser can oscillate with circularly-polarized or elliptically-polarized light, a light source having a strong resistivity to noise, such as returning light, can be built in optical communication systems, for example, when the laser is used as such a light source.
Specifically, the following structures can be adopted based on the above fundamental structure.
The ring cavity can be constructed such that light is totally reflected at at least one reflective point of the plural reflective points in its three-dimensional light path. The ring cavity can be constructed such that light is totally reflected at all the reflective points in its three-dimensional light path. A cavity with little cavity loss can thus be built. Where light is totally reflected at all the reflective points, a cavity with little cavity loss can be constructed without using any multi-layer mirrors.
The polyhedral structure can be a tetrahedron or specifically a regular tetrahedron. More specifically, the polyhedral structure can be formed of a semiconductor of zinc-blende-type crystalline structure. Such a structure can be easily fabricated by using known semiconductor processing techniques.
The semiconductor tetrahedron can include three {
111
}B faces, three {
111
}A faces, or three {
110
} faces. Those configurations can be readily fabricated by a selective growth method using a face-orientation dependence, or a selective etching method using a face-orientation dependence. Accordingly, a polyhedron having flat reflective faces can be easily obtained. Further, a minute tetrahedron can be fabricated.
The polyhedron or tetrahedron can be constructed by performing the selective growth on a substrate having a principal plane of a (
111
)A face. Semiconductor faces of ({overscore (
1
)}
11
)B, (
1
{overscore (
1
)}
1
)B and (
11
{overscore (
1
)})B faces appear during the selective growth. Alternatively, the polyhedron or tetrahedron can be constructed by performing the selective growth on a substrate having a principal plane of a (
111
)A face. Semiconductor faces of (
011
),

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