Coherent light generators – Particular resonant cavity – Distributed feedback
Reexamination Certificate
2002-03-26
2004-11-16
Wong, Don (Department: 2828)
Coherent light generators
Particular resonant cavity
Distributed feedback
C372S093000
Reexamination Certificate
active
06819701
ABSTRACT:
FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Available
REFERENCE TO MICROFICHE APPENDIX
Not Available
TECHNICAL FIELD OF INVENTION
This invention relates to light-emitting devices, or more specifically to a class of semiconductor light emitting diodes known as super-luminescent light emitting diodes, which are constructed as multilayered structures having vertical cavities that, on the minimum, comprise a substrate base, a light reflecting mirror structure, which is typically a quarterwave mirror stack assembly, a double-heterostructure (LED) “Light Emitting Diode”, and a window emitter-layer.
BACKGROUND OF THE INVENTION
Side-emitting “Light Emitting Diodes” (LEDs) are well known semiconductor light emitting devices in which, electrical current (i.e., electrical pumping) is made to pass through a diode junction to produce light emissions within an active layer of semiconductor material, which is located within the p-n junction of the previously mentioned diode itself. At least one facet of a side-emitting LED device is coated with an anti-reflective material, which will cause light emissions to exit the coated facet. This is to be contrasted with a side-emitting light emitting diode laser, where stimulated emission of light is also made to occur within the light emitting diode's junction. Stimulated emission occurs when the electrically pumped fundamental light already created within the light emitting diode's double-heterojunction is made to optically stimulate the double-heterojunction's active semiconductor layer or layers, which are also normally located between the side emitting diode's two contra-opposed light reflecting crystal facets.
Wherein, repeated reflections of light are made to oscillate through the diode junction's active semiconductor layer or layers, back and forth, between the diode's previously mentioned contra-opposed light reflecting crystal facets, causing a coherent laser beam to emerge. The resulting laser beam usually has a very narrow spectral width (i.e., meaning monochromatic). Non-laser light emitting diodes that operate at a relatively higher power over other LEDs, while having a relatively broad spectral width are within a third category of devices called super-luminescent light-emitting diodes. There is a need for these devices, when they are used in fiber optic systems having a requirement for low Raleigh backscattering, such as in fiber optic gyroscopes or devices needing low modal noise. Commercially available super-luminescent light-emitting diodes typically emit light at powers as high as “4” to “6” mW (i.e., milliwatts).
However, when the power in these devices is increased above “1” to “2” mW, the frequency spectrum is substantially narrowed. Driving devices with contra-positioned edge-emitting facets to higher powers may eventually cause lasing, in spite of the presence of the anti-reflective coating on the previously mentioned facets, since even the best anti-reflective coating will reflect some proportion of the light impinging on it, and lasing will eventually occur if the power is increased to a high enough level. The lasing threshold for pulsed diode operation increases with decreased facet reflectivity. The only successful high-power anti-reflective coated super-luminescent diodes were made by dynamically monitoring the pulsed laser threshold during the coating process. For this reason, the anti-reflective coatings in super-luminescent light-emitting diodes have to be carefully controlled to permit operation at higher power levels. When a super-luminescent diode having one or both facets coated with an anti-reflective material is operated at a high enough current, the spectral content of the output light may still cover a desirably broad band of wavelengths.
However, above a certain power level the device operates more and more like a laser and its output spectrum is characterized by narrow modal lines spread over a relatively broad band. In this lasing mode of operation, the device is said to operate with a high degree of Fabry-Perot modulation, the characteristic laser-cavity modulation that is undesirable for applications like the fiber optic gyroscope. These applications require very low Raleigh back-scattering noise, which can only be obtained with a low coherence length and a wide spectral width. As the power of a side-emitting super-luminescent light emitting diode is increased and its spectral width is consequently decreased, the coherence length of light from the device is increased. The coherence length is another measure of the spectral purity of light, and is inversely proportional to spectral width. As the spectral width becomes narrower, the coherence length increases.
Moreover, if the edge-emitting device operates with a large degree of Fabry-Perot modulation and moves into a lasing mode, the coherence length is inversely proportional to the spectral width of the individual modal-lines within the intensity-wavelength characteristics of the device. Thus, the coherence length for the lasing mode of operation is several orders of magnitude larger than the coherence length for a super-luminescent diode. The requirement for a light-emitting device with low coherence length and relatively high power is simply not attainable with presently available super-luminescent diodes using antireflective coatings to suppress lasing. The cross-referenced U.S. Pat. No. 4,634,928 proposes one technique for the suppression of lasing in a light-emitting device. That approach employs means within the semiconductor structure for producing a non-uniform gain profile along the active layer of the device. The non-uniformity of the gain profile results in a broadening of the frequency spectrum of emitted light. As the power is increased, the spectral width increases even more, permitting the output of relatively high powers while maintaining a broad spectral width.
Some years ago, D. R. Scifres et al. reported in the IEEE Journal of Quantum Electronics, QE-14, 223 (1978), that he experimented with a different type of structure that showed promise as a super-luminescent diode. Conventionally, a side-emitting semiconductor laser is constructed to laze in a direction normal to the crystalline cleavage plane along which the facets are formed. These researchers constructed a laser at an angle inclined to the normal direction, such that light propagating at an internal angle of zero, i.e. parallel to the longitudinal direction of the laser, would impinge on the facets at a small angle to the perpendicular. The Scifres et al. cavity structure was of the “gain-guided” type of optical cavity.
Moreover, all light-emitting semiconductors emit light from a diode junction to which power is supplied electrically from a contact stripe formed on the device. If a narrow electrical contact is employed to supply the current, lasing action is typically limited to a correspondingly narrow region, with the lateral waveguide boundary defined by the gain profile, i.e. with no intentional refractive index profile built into the structure. This process is generally referred to as gain guiding. The Scifres et al. side-emitting device was run in a pulsed mode and, although super-luminescence was observed, a large proportion of the output was due to lasing.
Moreover, there was an observed tendency at higher currents for the internal beam angle to move toward zero, which minimizes reflectivity losses at the facets and pushes the device more strongly into lasing operation. It will be appreciated from the foregoing that there is still a need for a super-luminescent diode with the characteristics of high power, large spectral width, and low Fabry-Perot modulation. Specifically, the requirement is for a device operable at powers more than “10” mW (i.e., milliwatts), a spectral half-width of at least “50” angstroms, and at most 10% Fabry-Perot modulation. The present invention meets or exceeds these requirements without difficulty. The present invention has a redefined Fabry-Perot modulation neutralizing vertical folded cavity design. Inspired
Kirkpatrick & Lockhart LLP
Vy Hung Tran
Wong Don
LandOfFree
Super-luminescent folded cavity light emitting diode does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Super-luminescent folded cavity light emitting diode, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Super-luminescent folded cavity light emitting diode will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3296041