Optical: systems and elements – Optical amplifier – Particular active medium
Reexamination Certificate
2001-06-07
2002-11-19
Hellner, Mark (Department: 3663)
Optical: systems and elements
Optical amplifier
Particular active medium
C359S337000
Reexamination Certificate
active
06483635
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the manipulation of light in optical communication networks and fiber optic systems. Particularly, the present invention relates to elements disposed within the light stream of a fiber optic communications network, such as a DWDM network. More particularly, the present invention relates to anti-reflective structures for use in semiconductor lasers and amplifiers.
2. Related Art
Optical communication systems are a vital part of today's communication networks. In a typical optical communication system, information-containing optical signals are transmitted along an optical fiber. Lasers are important components of optical communication systems for producing the information-containing signals. Information is modulated upon light signals emitted from the laser into the optical fiber. Initially, optical fibers were used to carry only a single data channel at one wavelength in one direction. Technology, such as wavelength division multiplexing (WDM), has increased the amount of information transferred over a single optical fiber communication system by multiplexing several signals on a single fiber. WDM allows the launching and retrieving of multiple data channels in and out of an optical fiber. Each data channel is transmitted in a unique wavelength region. The WDM wavelength regions are typically hundreds of nanometers apart.
An improvement on WDM is dense wavelength division multiplexing (DWDM), which allows channels of information to be propagated at different wavelength regions that can be spaced spectrally at a set wavelength or frequency distance apart from one another. For DWDM, information channels may propagate at industry standard spacings that may be on the order of one or two nanometers apart from one another. Typically, DWDM systems operate with fine wavelength separations from one laser to the next of about 0.8 to 1.6 nanometers (nm). Thus, for DWDM, lasers must operate at precisely determined wavelengths to avoid interference with other channels.
DWDM lasers and many communications lasers typically comprise a semiconductor lasing medium contained within an optical cavity. Such lasers are frequently referred to as diode lasers, semiconductor lasers, or laser diodes. A laser operates through stimulated emission, whereby light is emitted from excited atoms or molecules of the lasing medium. In operation of the laser, the lasing medium is disposed in an optical cavity of the laser that is situated between two reflectors. Light intensity is amplified as the photons oscillate back and forth between the reflectors, which stimulates further photon emission in the lasing medium. For this reason, the lasing medium is sometimes referred to as a gain medium. By making at least one of the reflectors partially transmissive, monochromatic radiation is emitted when the light passes through the reflector. Monochromatic radiation is typically radiation containing a narrow wavelength region of light. This wavelength region of light may appear as one visible color, for example, red, blue, green, or yellow, etc., or it may be invisible, for example, ultraviolet or infrared light. Infrared light is typical of DWDM optical networks.
Fabry-Perot diode lasers are a type of laser in which the cavity reflectors can be formed by the boundary surfaces of a semiconductor material itself. The boundary surfaces are typically referred to as front and rear facets, wherein the front facet is the port of primary light emission. Thus between these mirrored facets lies a section of semiconductor material that comprises a waveguided gain medium. The optical cavity is formed between the front and rear facets. Typically, the rear facet strongly reflects light resonating within the cavity. The front facet partly reflects the light resonating within the cavity and partly transmits the light at a laser wavelength region. The laser wavelength region is the wavelength region of light emitted from the laser; it is defined, at least in part, by the length of the cavity. The light reflected back from the front and rear facets sustains oscillation by interacting with excited atoms to drop electrons from the upper level to the ground state, thereby emitting photons. Various parameters associated with the optical cavity can be adjusted to alter the wavelength characteristics of the emitted light, such as varying the spacing between the front and rear facets. For example, the cavity may be so short as to only allow the laser to operate in ‘single mode’ and thereby only emit light at a single wavelength position; the length of this cavity can be varied to “tune” this spectral position.
While Fabry-Perot lasers are inexpensive to produce, their output is not optimized for many applications. They typically operate in a multi-mode state wherein a broad spectral range is emitted. This undesirable characteristic arises from the energy reflection that occurs at the facet surfaces. The reflection is due to the refractive index of the semiconductor lasing medium being high relative to that of an adjacent medium such as air. The difference in refractive index produces a strong, innate reflection in the laser. This reflection, which defines one of the cavity mirrors and is responsible for the basic operation of the laser, is called Fresnel reflection. The lack of monochromacity of the energy output is undesirable for many optical networking applications. The plurality of radiation patterns, or longitudinal modes, that occur within the optical cavity of a Fabry-Perot laser are linked to this lack of monochromacity.
If the output of the laser is not controlled, or is “free running,” multiple longitudinal modes will be seen in the output. In a typical free-running Fabry-Perot diode laser deployed in an access network employing the synchronous optical network (SONET) standard, the energy output (intensity) is often situated in the 1310 nm window. Typically, the longitudinal modes are spaced approximately 0.6 nm apart and are distributed over many nanometers, approximately 30. Additionally, the spectral output is subject to drift with temperature and other influences. Consequently, the output is unsuitable for dense wavelength division multiplexing, because of spectral overlap of adjacent DWDM signals. The output is also not well suited for high-speed fiber optic communications at wavelengths that are displaced from the optical fibers zero chromatic dispersion point, because of signal degradation due to chromatic dispersion.
By addressing the innate facet reflections and/or their influence on the longitudinal modes of a Fabry-Perot diode laser, attempts have been made to improve the quality of laser output. The reflections can be either redirected or suppressed so that the natural modes are eliminated or minimized. Free from these reflections, a number of techniques are available to stabilize the laser so that it emits light at a single, or at least tightly confined, wavelength position.
To overcome the above problems, semiconductor distributed feedback (DFB) lasers are frequently used in DWDM communications networks as an alternative to free running Fabry-Perot lasers. DFB lasers utilize wavelength-selective feedback to make certain modes in the optical cavity of the laser oscillate more strongly than others, thus causing the laser to output light in a tightly confined wavelength region. DFB lasers attempt to achieve precise wavelength control, thereby allowing lasers to be used with DWDM systems. DFB lasers typically include a semiconductor gain medium, as discussed above, but with an added feature that controls the output wavelength of the laser. The semiconductor gain medium of a DFB laser is imparted with a periodic (repeating) structure that limits the emitted light to a tightly defined wavelength region. The periodic structure may be a corrugated structure or an undulation in refractive index. The structure is designed so that light of a particular wavelength will be constructively reflected back and forth to achieve lasing. L
Cirrex Corp.
Hellner Mark
King & Spalding
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