Coherent light generators – Particular component circuitry – For driving or controlling laser
Reexamination Certificate
1999-09-10
2001-10-16
Font, Frank G. (Department: 2877)
Coherent light generators
Particular component circuitry
For driving or controlling laser
C372S036000, C372S026000, C356S221000, C356S222000
Reexamination Certificate
active
06304586
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention generally relates to optical multimeters and more particularly to signal generating portions thereof.
2. Description of the Related Art
The telecommunications network serving the United States and the rest of the world is presently evolving from analog to digital transmission with ever increasing bandwidth requirements. Fiber optic cable has proved to be a valuable tool, replacing copper cable in nearly every application from large trunks to subscriber distribution plants. Fiber optic cable is capable of carrying much more information than copper with lower attenuation.
The T-1 standards committee ANSI has provided a draft document, “ANSI T1.105-1988”, dated Mar. 10, 1988, which sets forth specifications for rate and format of signals which are to be used in optical interfaces. The provided specifications detail the Synchronous Optical Network (SONET) standard. SONET defines a hierarchy of multiplexing levels and standard protocols which allow efficient use of the wide bandwidth of fiber optic cable, while providing a means to merge lower level DS0 and DS1 signals into a common medium. In essence, SONET established a uniform standardization transmission and signaling scheme, which provided a synchronous transmission format that is compatible with all current and anticipated signal hierarchies. Because of the nature of fiber optics, expansion of bandwidth is easily accomplished.
Currently this expansion of bandwidth is being accomplished by what is known as “wavelength division multiplexing” (WDM), in which separate subscriber/data sessions may be handled concurrently on a single optic fiber by means of modulation of each of those subscriber datastreams on different portions of the light spectrum. WDM is therefore the optical equivalent of frequency division multiplexing (FDM). Current implementations of WDM involve as many as 128 semiconductor lasers each lasing at a specific center frequency within the range of 1525-1575 nm. Each subscriber datastream is optically modulated onto the output beam of a corresponding semiconductor laser. The modulated information from each of the semiconductor lasers is combined onto a single optic fiber for transmission. The data structure of a basic SONET signal at a typical data rate of 51.84 Mbps, a.k.a. an STS-1 signal, has 9 rows of 90 columns of 8 bit bytes at 125 &mgr;s frame period. The first three columns of bytes in the SONET signal are termed the transport overhead (TOH) bytes that are used for various control purposes. The remaining 87 columns of bytes constitute the STS-1 synchronous payload envelope (SPE). As this digital signal is passed across a SONET network, it will be subject at various intervals to amplification by, for example, Erbium doped amplifiers and coherency correction by, for example, optical circulators with coupled Bragg filters. At each node in the network, e.g. central office or remote terminal, optical transceivers mounted on fiber line cards are provided. On the transmit side, a framer permits SONET framing, pointer generation and scrambling for transmission of data from a bank of lasers and associated drivers, with each laser radiating at a different wavelength. On the receive side, the incoming signals are detected by photodetectors separated into channels, framed and decoded.
As more and more optical signal equipment (transmitting, receiving, amplification, coherence and switching) is being designed and utilized, a need has arisen for optical multimeters, e.g. signal generators and detectors, which can be used to test the various components of an optical, e.g. SONET, network. What is needed is a tunable optical signal generator that does not require the complex control systems relied on by prior art devices. Those control systems utilize closed loop feedback of wavelength or position to select the output wavelength of the optical signal generator. As a result they are expensive and exhibit a large form factor.
SUMMARY OF THE INVENTION
The present invention provides an optical signal generator for use in calibrating and testing optical components. The optical signal generator utilizes a tunable laser in conjunction with a novel modulation circuit to output an analog/digital optical signal. The modulator circuit combines a low frequency, closed loop power control with a separate digital modulator. The output beam of the tunable laser may be modulated over a wide range of frequencies, duty cycles and amplitudes. Circuitry is also provided for modulation of an analog signal onto the output.
In an embodiment of the invention a modulation circuit for a tunable laser is disclosed. The modulation circuit includes: a current source, a setpoint module, an error integrator, a control unit, and a first and second switch. The current source has a control input and a source. The current level at the source is determined by the level of an error signal applied to the control input. The setpoint module has an output. The setpoint module generates a setpoint signal at the output. The error integrator has an optical input, a setpoint input, and an output, coupled to, respectively, the output of the gain medium, the output of said setpoint module and the control input of said current source. The error integrator supplies, at its output, the error signal at a level corresponding to an integration of the error between the energy level of the output beam and the setpoint signal. The first switch couples and uncouples the source of the current source with the control input of the gain medium in, respectively, a closed state and an open state. The second switch couples and uncouples the output of the setpoint module and the setpoint input of the error integrator in, respectively, the closed state and the open state. The control unit couples to the first switch and the second switch to substantially synchronously switch both the first switch and the second switch between the open state and the closed state to modulate the output beam.
In another embodiment of the invention a method for modulating a tunable laser with a gain medium is disclosed. The method for modulating comprises the acts of:
supplying a setpoint signal;
generating a control signal alternating between an on state and an off state;
supplying, during the on state, the applied current to the gain medium at a level corresponding to an integration of an error between the energy of the output beam and the setpoint signal; and
holding, during the off state, the applied current at substantially a prior level of the applied current during the on state.
Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.
REFERENCES:
patent: 4332469 (1982-06-01), Wendland
patent: 4771431 (1988-09-01), Nakazawa et al.
patent: 5319668 (1994-06-01), Lueke
patent: 5347527 (1994-09-01), Favre et al.
patent: 5970078 (1999-10-01), Walker
Carney Robert A.
Pease John
Cary Charles C.
Cary & Kelly LLP
Font Frank G.
New Focus Inc.
Rodriguez Armando
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