Optical: systems and elements – Holographic system or element – With optical waveguide
Patent
1997-02-07
2000-02-15
Spyrou, Cassandra
Optical: systems and elements
Holographic system or element
With optical waveguide
359566, 359572, 372 6, 372102, 385 37, G03H 100, H01S 330
Patent
active
060259396
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates (a) to broad band continuous laser tuning, (b) to automatic adaptive means of tuning the output wavelength of a laser to a desired offset from a signal at an arbitrary wavelength provided as a reference, and (c) to fiber-optics communications network architectures that use such lasers. These lasers can be used to generate arrays of downstream wavelengths and to automatically maintain the required hierarchical relationship between such an array of downstream wavelengths and their upstream counterparts in a wavelength division multiplex optical fiber communication system.
BACKGROUND OF THE INVENTION
A need has long existed for lasers that can be tuned to any wavelength within a given band. Until the present invention, it has only been possible to tune lasers, for example diode lasers operating in the 1.3 micron band or the 1.55 micron band, that are the focus of experimentation for optical fiber communication systems, over a range of one-thousandth of a micron, that is to say a range of perhaps a nanometer. Since such optical fiber communication systems typically employ a multiplicity of individual channels, each separated from its neighboring channels by at least a nanometer, a multichannel system has required a multiplicity of different lasers, each one fabricated to operate at one of the required wavelengths. Three types of lasers have been used in such applications. These are: the distributed feedback (DFB) laser, the Fabry-Perot (FP) laser, and the Distributed Bragg Reflector (DBR) laser. In the DFB laser, an internal periodic feedback structure establishes the wavelength of operation. In the Fabry-Perot (FP) laser, the two facets of the diode, the rear and the front or emitting surface, are cleaved to establish the dimensions of the structure such that a primary longitudinal mode of resonance will exist at the desired wavelength. In the DBR laser, a periodic feedback structure external to the laser diode is used to establish the operating wavelength. Hybrid types of lasers also have been proposed for such uses. The hybrid lasers use various combinations of these principles to establish the operating wavelength.
A problem with Fabry-Perot types of lasers is that their structure supports a multiplicity of resonant frequencies, so that these lasers output a band of individual wavelengths. Such lasers cannot therefore be used in wideband applications, since a single, narrow linewidth output is needed for wideband applications.
The other laser diode types previously mentioned can be fabricated to produce essentially single line (single-wavelength) output that is suitably narrow, but in order to change their output wavelength, the spacing of their periodic structures must be changed. Where an internal periodic structure is involved, the cost of tailoring that structure to a specific wavelength is quite high. Therefore, present manufacturing practice is to design the structure for a nominal wavelength, typically at the center of a desired band, and to take advantage of the manufacturing tolerances to produce a range of laser diodes with individual output wavelengths at various points centered about that band center. Some of those wavelengths are the ones desired, so those lasers can be marketed. However, most are not, so the effective yields are low and the prices correspondingly high.
This fact has stimulated interest in laser diodes that employ external wavelength control structures such as feedback gratings. However, such designs as have been suggested have had inherently narrow tunability or, at best, have been discontinuous, with small regions of smooth tunability alternating with regions of instability.
The problem with those designs is that they fail to provide a means of adjusting cavity length in proportion to a change in the periodicity of the feedback structure. An illustration of such a design is contained in U.S. Pat. No. 4,786,132. The apparatus of that patent proposes to tune the output of a laser diode by incorporating into the effective laser
REFERENCES:
patent: 4986623 (1991-01-01), Sorin
patent: 5058977 (1991-10-01), Sorin
patent: 5134620 (1992-07-01), Huber
patent: 5724164 (1998-03-01), Lowenhar et al.
Hulland Burton Louis
Lowenhar Herman Leonard
Kol OHR Corporation
Schuberg Darren E.
Spyrou Cassandra
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