Coherent light generators – Particular beam control device – Optical output stabilization
Reexamination Certificate
2001-12-13
2004-04-20
Ip, Paul (Department: 2828)
Coherent light generators
Particular beam control device
Optical output stabilization
C372S020000
Reexamination Certificate
active
06724789
ABSTRACT:
BACKGROUND OF THE INVENTION
The present application is generally related to optical communication and signal generation devices, systems, and methods. In one embodiment, the invention provides a multiple channel fiberoptic light source, particularly for use in dense wavelength division multiplexed systems (DWDM).
Optical telecommunications networks would benefit from increased bandwidth to handle both the current and projected communications traffic. Many optical networks use a single laser transmitter with time division multiplexing techniques to transmit separate data streams. The advantageous properties of optical fiber allow quite significant data rates to be transferred using such single laser systems. Nonetheless, to allow even greater data rates to be transmitted, the current market trend is toward systems that use many different wavelengths. Simultaneous transmission of these different wavelength signals can significantly increase data capacity of a fiberoptic communication link. This approach is known generally as wavelength division multiplexing (“WDM”).
WDM systems have significantly increased the capacity of each fiberoptic communication link within the system. As with many successes, still further improvements would be desirable. To increase the number of channels or distinct frequencies transmitted across an optical fiber, it has been proposed to decrease the frequency separation between adjacent channels so as to more densely transmit data within a spectral range. Such dense wavelength division multiplexing (“DWDM”) optical communications networks show great promise in providing increased data communication capabilities and bandwidth. Unfortunately, as the adjacent channels get closer and closer in spacing, the likelihood of error (from signal frequency instability, cross-talk, and the like) increases.
The reliability and accuracy of data transmissions using DWDM systems are particularly sensitive to the stability and reliability of the light generating structures. A variety of techniques have been used for individually controlling each laser which generates the light for a particular channel within a DWDM system. While the cost of individual laser diodes has dramatically decreased, the price of a multiple wavelength (channel) light generation devices, together with control systems for accurately maintaining stable and tightly packed channel frequencies, can be quite expensive.
In work related to the present invention, it has recently been proposed to flexibly combine a plurality of tunable laser diodes together so as to form a reconfigurable multi-channel transmitter for DWDM optical communications. U.S. patent application Ser. No. 09/610,312 filed on Jul. 5, 2000 (assigned to the assignee of the present invention) describes this advantageously reconfigurable multi-channel transmitter in detail, and the full disclosure of that application is incorporated herein by reference.
While the recently proposed reconfigurable multi-channel transmitter represents a significant advancement in the art, still further refinements would be desirable. In general, it would be beneficial to provide improved structures, systems, and methods for generating light for use in optical signal systems. In particular, it would be beneficial to provide improved light sources for fiberoptic communications. It would be particularly beneficial to provide improved combinations and/or arrangements of components for generating optical signaling light, ideally resulting in simple structures and providing highly reliable and stable light at a plurality of tightly controlled channel frequencies for a reasonable cost.
BRIEF SUMMARY OF THE INVENTION
The present invention generally provides improved structures and systems for generating light for use in optical communications systems, together with related methods for their fabrication and/or use. In one embodiment, the invention provides an optical light source for use in wavelength division multiplexed and dense wavelength division multiplexed systems. In some embodiments, a reference gas may be contained by a hermetically sealed enclosure of the device itself, which is also used to protect lasers or other light generating structures. Laser injection current control and thermal management systems may together provide feedback control of the light generating lasers and/or a frequency grid generator such as an etalon. Electronic control over a number of separate lasers (and the grid) may be effected using a multiplexed digital controller.
In a first aspect, the invention provides a light generating structure comprising a sealed enclosure with a light source disposed within the enclosure. A waveguide has an entrance disposed within the enclosure in an optical path from the light source. A reference fluid is contained by the enclosure itself. The reference fluid imposes a reference characteristic on light from the light source.
The waveguide may comprise an optical fiber, for example, which can transport light outside the enclosure. In many embodiments, the reference characteristics will comprise a reference wavelength. The reference fluid will often comprise a reference gas, and may absorb light energy at the reference wavelength. The light source will often comprise a frequency adjustable laser, and a sensor may couple the light (with the reference characteristic imposed thereon) to the frequency adjustable laser along a feedback control circuit so as to frequency stabilize the laser by use of the reference wavelength.
Advantageously, a plurality of lasers can be disposed within the enclosure, with the reference gas imposing the reference characteristic upon light traveling from each laser. The waveguide will often comprise an optical fiber, the waveguide entrance comprising a first end of the optical fiber, and the optical fiber extending outside the enclosure to a second optical fiber end. Additional optical fibers may similarly be provided for the additional lasers for transmission of light from the lasers out of the enclosure.
The use of (for example) a hermetically sealed enclosure which protects the lasers as a reference gas cell significantly facilitates the miniaturization of light sources having reference gas stabilized frequencies. Fiberoptic enclosures are often quite small, and accurate detection of the reference wavelength may be easier if the light travels a significant distance through the reference gas. Hence, a length of the optical path through the reference fluid within the enclosure will often be greater than a length of the enclosure, the optical path typically comprising a folded optical path reflected by at least one reflecting element within the enclosure. This allows, for example, the reference gas to sufficiently absorb energy along a characteristic wavelength for easy and accurate wavelength control, without having to resort to a large, separately sealed gas cell disposed within the enclosure.
In another aspect, the invention provides a fiberoptic source comprising a sealed enclosure, with a first variable-frequency laser disposed within the enclosure. An optical fiber has an entrance disposed within the enclosure in an optical path from the laser. The fiber extends out from the enclosure, and a reference gas is contained by the enclosure. The reference gas tags light from the laser with a reference frequency. Feedback control circuitry couples the tagged light to the laser so as to control a frequency of the light in response to the reference frequency.
The reference gas typically tags the light via light absorption, which occurs at a characteristic frequency or frequencies of the (often narrow) absorption lines of the gas. In many embodiments, a plurality of frequency-adjustable lasers will be provided, each laser producing an additional light signal having a signal frequency. The signal frequencies may be adjusted in response to the reference frequency. A frequency adjustable etalon may be disposed in an optical path from at least one of the lasers. The etalon may generate interference fringes at a plurality of discrete
Ip Paul
Nguyen Phillip
SRI - International
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