Optical cavities for optical devices

Details Optical cavities for optical devices Optical cavities for optical devices

2001-02-06

2004-12-21

Robinson, Mark A. (Department: 2872)

Optical: systems and elements

Light interference

Produced by coating or lamina

C359S260000, C257S098000

Reexamination Certificate

active

06833958

ABSTRACT:

FIELD OF THE INVENTION
The present disclosure relates to optical cavities for optical devices. More particularly, the disclosure relates to optical cavities for semiconductor and/or dielectric optical devices incorporating a focusing reflector.
BACKGROUND OF THE INVENTION
Semiconductor fabrication techniques have enabled the construction of miniaturized optical devices. Two examples of such devices are semiconductor lasers, e.g., vertical cavity surface emitting lasers (VCSELs), and semiconductor optical filters. Through these techniques, optical devices can be constructed having dimensions on the order of only a few microns. Applications for such devices are many and include optical communications as well as the construction of optical circuits.
Semiconductor lasers and filters comprise optical cavities through which light passes before being emitted from the devices. Such optical cavities normally include highly reflective, flat mirrors positioned at opposed ends of the cavities that reflect light back and forth within the cavity. The cavities often include an air gap positioned between the mirrors and, in the case of semiconductor lasers, a gain medium that increases the intensity of the light.
In early designs, semiconductor lasers and filters were only capable of emitting fixed frequencies of optical radiation. More recent semiconductor lasers and filters have been constructed with displaceable mirrors to provide for frequency tuning. Displacement of a mirror at an end of an optical cavity changes the relative spacing of the mirrors and therefore the length of the cavity. As is known in the art, adjustment of the cavity length alters the frequency at which the laser or filter emits radiation.
Optical cavities formed with flat mirrors present significant disadvantages. For instance, flat mirror optical cavities are highly susceptible to losses due to misalignment of the mirrors. This misalignment can be magnified when one or both of the mirrors is displaced during tuning. In addition, even where the mirrors are aligned correctly, diffraction losses can occur. To reduce such losses, recent semiconductor lasers and filters have been constructed with a concave, semispherical mirror at one end of the optical cavity. With such a configuration, light is reflected back on itself within the device cavity to prevent the light from escaping.
FIGS. 1 and 2
illustrate an example prior art semiconductor laser
100
and filter
200
, respectively. As indicated in
FIG. 1
, the semiconductor laser
100
comprises an optical cavity
102
. At one end of the cavity
102
is a first mirror
104
and at the other end of the cavity is a second mirror
106
. Below the second mirror
106
is a substrate
108
constructed of a semiconductor material. Formed on the substrate
108
is a first current injection layer
110
that is used to provide current to the laser
100
during operation. Disposed within the optical cavity
102
is an active region
112
that is responsible for generating the light that is emitted out of the laser
100
. In contact with the active region
112
is a second current injection layer
114
that, like the first current injection layer
110
, is used to provide current to the laser
100
. Formed on top of the second current injection layer
114
are support posts
116
that, together with support tethers
118
, suspend the first mirror
104
above the active region
112
. Normally formed on the support tethers
118
are tuning electrodes
120
that are used to deliver voltage to the first mirror
104
that displaces it when the laser
100
is tuned. As is evident from
FIG. 1
, the first mirror
104
is arranged in a concave, semispherical orientation such that light incident on the first mirror is focused inwardly on itself to prevent diffraction losses.
FIG. 2
illustrates the semiconductor filter
200
. As is apparent from this figure, the semiconductor filter
200
is similar in construction to the semiconductor laser
100
shown in FIG.
1
. Accordingly, the filter
200
comprises an optical cavity
202
that is defined by a first mirror
204
and a second mirror
206
. In addition, the semiconductor filter
200
includes a substrate
208
, first tuning electrode
210
, support posts
212
, support tethers
214
, and second tuning electrodes
216
. Accordingly, the semiconductor filter
200
primarily differs from the semiconductor laser
100
of
FIG. 1
in the omission of the active region
112
.
Although capable of providing for reduced losses, optical cavities having a concave, semispherical mirror are difficult to manufacture. As is known in the art, it is difficult to form a precise concave surface on a very small scale (e.g., 10 &mgr;m in diameter) through present semiconductor fabrication techniques. Accordingly, it can be appreciated that it would be desirable to have a tunable, low-loss optical cavity for semiconductor lasers and filters that does not require a concave, semispherical mirror.
SUMMARY OF THE INVENTION
The present disclosure relates to an optical cavity, comprising a first non-concave reflector positioned at a first end of the optical cavity and a second non-concave reflector positioned at a second end of the optical cavity that receives and reflects light reflected from the first non-concave reflector. The first non-concave reflector is configured to focus light that reflects off of the reflector back upon itself to avoid diffraction losses from the optical cavity.
In one embodiment of the invention, the first non-concave reflector includes a layer of material that has a thickness that varies as a function of radial distance out from an axial center of the layer. By way of example, the outer layer can include a substantially convex, semispherical outer surface and a substantially planar inner surface.
In another embodiment of the invention, the first non-concave reflector includes a layer of material that has an index of refraction that varies as a function of radial distance out from an axial center of the layer.
The features and advantages of the invention will become apparent upon reading the following specification, when taken in conjunction with the accompanying drawings.


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