Coherent light generators – Particular resonant cavity
Reexamination Certificate
1999-04-20
2001-11-06
Font, Frank G. (Department: 2877)
Coherent light generators
Particular resonant cavity
C372S043010, C372S044010, C372S045013, C372S049010, C372S050121, C372S049010, C372S054000, C372S064000, C372S075000, C369S121000
Reexamination Certificate
active
06314122
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to optical detection apparatus and, more particularly, to near-field optical apparatus, such as optical storage apparatus, optical microscopes and optical receivers.
BACKGROUND OF THE INVENTION
In the context of optical storage and optical microscopes, U.S. Pat. No. 5,265,617 granted on Apr. 27, 1997 to L. C. Hopkins et al. (hereinafter referred to as Hopkins) describes near-field optical detection apparatus that includes a semiconductor laser having a non-uniform emission face (e.g., output facet) configured so that at least 50% (preferably 90-95% of the total radiation emission from the output facet is emission from a first region having a width w<&lgr;
s
, where &lgr;
s
is the center wavelength of the laser emission at the output facet. Illustratively, the emission face has a coating thereon, and an aperture, recess, protrusion or other structural feature (hereinafter referred to as a feature) is formed in the coating. Alternatively, as pointed out at column
7
, line
59
-
67
, the feature may be formed directly in an (uncoated) output facet. Depending on the relative reflectivity of the feature compared to the surrounding areas of the facet, the first region may be located either at the feature or in the surrounding area. Hopkins contemplates embodiments with both edge-emitting semiconductor lasers (e.g., ridge waveguide lasers) and surface-emitting semiconductor lasers (e.g., VCSELs). This patent is incorporated herein by reference.
As noted at column
6
, lines
55
-
57
of Hopkins, it is typically desirable to position the feature (e.g., a recess in the output facet coating) substantially in the center of the active region of the laser. In the case of edge-emitting ridge waveguide lasers, this desideratum means that the feature is typically positioned at the center of the cross-section of the waveguide.
The utilization of such a laser to read information contained at the data sites (i.e., bit marks) of a storage medium is described by Hopkins at column
10
, lines
32
-
67
. Briefly, variations in the reflection properties of the data sites result in variations in at least one laser parameter (e.g., laser optical output intensity, and/or laser terminal voltage). In an exemplary embodiment, these variations in the radiation reflected by the data sites back into the laser result in corresponding variations in the intensity of radiation emitted from the back facet of the laser, which can be detected in conventional fashion by means of a photodetector. Alternatively, the Hopkins optical storage apparatus may be operated in a transmission mode as shown and described with reference to
FIG. 11
therein.
The Hopkins laser and optical storage apparatus do not, however, rely on the laser operating in multiple transverse modes, and in particular, do not exploit optical filaments formed by the coupling of such transverse modes to one another.
With respect to solid state lasers that include a planar optical waveguide, the term filament as used herein means an intracavity, in-plane (i.e., in the plane of the waveguide) intensity distribution of the lasing radiation that exhibits a meandering (e.g., sinusoidal) pattern of nodes and peaks that weaves from one side of the waveguide cross-section to another (or from the top to the bottom of the waveguide cross-section) along the longitudinal axis of the laser. A few prior art lasers have exhibited such filaments; e.g., 0.98 &mgr;m pump laser diodes and 1.3 &mgr;m buried heterostructure (BH) laser diodes investigated by Ohkubo et al.,
Jpn. J Appl. Phys
., Vol. 35, pp. L34-L36 (1996) and Schemmann et al.,
Appl. Phys. Lett
., Vol. 66, No. 8, pp. 920-922 (1995), both of which are incorporated herein by reference. But prior art workers have considered filamentation in these lasers to be undesirable because the maximum useful output power is limited by the lateral beam deflection that occurs when the filament forms. In addition, the authors did not appreciate the way such filaments might be used in optical detectors.
On the other hand, in the context of optical receivers for use, for example, in optical communication systems, information (e.g., data, voice, video) carried on a laser beam is typically detected by a p-i-n photodiode. In such devices absorption of the laser beam in the i-region generates electron-hole pairs. Under reverse bias electrons and holes swept out of the i-region generate a photocurrent that corresponds to the intensity of the laser beam (e.g., to the intensity of the pulses in a digital system). The maximum speed (i.e., data rate in a digital system) at which the system can operate is determined, in part, by how fast carriers can be swept out of the i-region of the photodiode. Typically the maximum speed is on the order of a few 10s of GHz.
There is a need in the optical communications systems industry, however, for systems and hence photodetectors that can operate at much higher speeds; e.g., on the order of 100s of GHz.
SUMMARY OF THE INVENTION
In accordance with one aspect of the invention, optical detection apparatus comprises a solid state laser having an intracavity optical waveguide that supports a lasing filament and operates in one of two states: (1) in a first state, the filament is fixed in one of two orientations, and (2) in a second state, the filament may be induced to switch or oscillate between the two orientations, or it may be suppressed altogether. At least one feature is located on the output facet off-center with respect to the waveguide cross-section in order to provide a preferred orientation for the filament. In one embodiment, a single feature is located off-center and optical radiation of one intensity is injected through the facet feature, thereby causing the laser to operate in one of its states, whereas injected radiation of another intensity causes the laser to operate in the other of its states. The injected radiation may be radiation emitted by the laser that is reflected by an object (e.g., a bit mark in an optical storage medium or a region of an integrated circuit being viewed under an optical microscope) proximate the facet feature (e.g., in the laser nearfield). Alternatively, the injected radiation may originate from a transmitter in an optical communication system. In either case, any change in the state of the laser is detected as change in a parameter of the laser (e.g., a change in its impedance that is detected as a corresponding change in its terminal voltage).
In an alternative embodiment, based on a differential mode of detection, a pair of facet features are located on opposite sides of the center of the waveguide's cross-section. When optical radiation is injected essentially simultaneously into each of the features, the laser again operates in one of two possible states: (1) in a first state, the filament is stable if the two injected signals have substantially different intensities, and (2) in a second state, the filament switches (or oscillates) between different orientations, or it may be suppressed altogether. As with the first embodiment, a parameter of the laser has different values in these two states, which enables radiation signals of different intensities to be detected and differentiated from one another. Also similar to the first embodiment, the injected radiation may come from a reflecting object, or it may originate from a transmitter in an optical communication system.
REFERENCES:
patent: 5625617 (1997-04-01), Hopkins et al.
E. Betzig et al., Near-field magneto-optics . . . , Appl. Phys. Lett., vol. 61, No. 2, pp. 142-144 (Jul. 1992).
E. Betzig et al., Fiber laser probe . . . , Appl. Phys. Lett., vol. 63, No. 26, pp. 3550-3552 (Dec. 1993).
M. F. C. Schemmann et al., Kink power . . . , Appl. Phys. Lett., vol. 66, No. 8, pp. 920-922 (Feb. 1995).
M. Ohkubo et al., Experimental Study of Beam Steering . . . , Jpn. J. Appl. Phys., vol. 35, pp. L34-L36 (Jan. 1996).
Font Frank G.
Lucent Technologies - Inc.
Rodriguez Armando
Urbano Michael J.
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