Coherent light generators – Particular beam control device – Optical output stabilization
Reexamination Certificate
1999-04-12
2002-05-14
Font, Frank G. (Department: 2877)
Coherent light generators
Particular beam control device
Optical output stabilization
C372S029011, C372S038100, C372S038010, C372S038020, C372S034000, C356S454000, C356S460000, C356S506000, C356S450000
Reexamination Certificate
active
06389046
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates generally to optoelectronics and laser technology, and more specifically to methods employed in industrial and communication applications using a control laser element and laser array source for sensing and stabilizing laser array power and wavelength, without reducing array output power (i.e. lossless), and reducing drift in a wavelength stabilized laser source.
2. Description of the Prior Art
Wavelength stability in the optical output signal of the light source is necessary in many sensor systems and telecommunication systems using optical fibers. Depending upon the system, a high precision of wavelength stability, in parts per million, is required. Similarly, a high degree of stabilization is also necessary in the lasers used in fiber optic telecommunication systems. Further limiting the influence of noise and drift in these sensitive systems would enable additional traffic utilization in these systems.
Communication service providers are experiencing significant consumer demands to accommodate additional bandwidth in optically-based communications systems and the demand is ever-increasing. Today's optical communication systems and networks field rising consumer demands for e-mail, video, multimedia, data and voice-data transmission requirements across a variety of communication protocols. In the future, all indications are that the use of fiber optic networks will become even more prevalent as a preferred medium for transferring information as the marketplace for wide-band services matures. It is anticipated that additional services such as enhanced pay-per-view, video-on-demand, interactive television and gaming, image networking, video telephony, CATV, and ISDN switching services will be dependent on and be substantial to users of such systems.
Devices representative of existing technology for implementing fiber optic networks, which are known in the industry, include waveguide division multiplexers (WDMs), fiber amplifiers such as erbium doped fiber amplifiers (EDFAs), and add/drop networks. These devices, as well as other components of a fiber optic network, contribute to or are affected by power level variances in the independent channels of a fiber optic network and may also contribute to noise and drift in the system. Therefore, the maturation of economically feasible and technically satisfactory fiber optic networks for the multiple users and diverse uses previously described, is dependent upon the stabilization of power levels in the independent channels of a fiber optic network, the reduction of wavelength spacing between adjacent channels, and the reduction of drift in a stabilized optical source.
Lasers are employed in numerous applications, particularly within fiber optic networks. Often, due to constraints in design or by way of limitations of devices in the system, it is important to stabilize and control the laser output wavelength, i.e. locking the wavelength to a reference wavelength. Similarly, because a single semiconductor laser may not generate sufficient power for the system, high power semiconductor laser sources such as semiconductor laser arrays, generating high optical power, are utilized. These high power laser arrays are made of single stripes of semiconductor lasers and may also be stacked to form two dimensional laser arrays. As is often the practice, each element of a laser array may be of a different wavelength than another element. Controlling and stabilizing the wavelength and output of the arrays, while minimizing losses to or splitting of the output power is critical to system performance.
In particular, in a wavelength division multiplex communication system, there typically exists a high number of independent channels in a transmission line. These channels are independent of each other and are well-suited for multimedia and multi-data transmission and communication. Wavelength division multiplexed (WDM) communication systems have further advanced to dense wavelength division multiplexed (DWDM) systems, the DWDM systems being point-to-point systems designed to increase the capacity of installed fiber. DWDM systems currently provide up to 400 GBps capacities and beyond over a single strand of fiber, and provide transmission capabilities four to eight times those of traditional time division multiplexed (TDM) systems. DWDM systems require 0.1-20 ppm frequency stabilization over their anticipated life spans which are estimated at 25 years.
A DWDM system typically includes at least one optical amplifier having two key elements: an optical fiber that is doped with the element erbium and the amplifier. Typically a laser is employed to energize the erbium with light at a specific wavelength and the erbium thereby acts as a gain medium that amplifies the incoming optical signal. The gain of the optical amplifier is dependent upon the optical power in the signal channel. The strength of the incoming signal is desired to be optimal. If the power of the optical signal is degraded, as may occur due to phase modulations, the signal may be insufficient to meet demands of the communication system, often due to too little power resulting in not enough gain from the amplifier.
In a WDM and DWDM communication systems, many channels can be secured by narrowing the channel gaps and transmitting the signals in the same tunable range for each channel. In order to keep the gaps smaller than a change in width due to drift of wavelength of a semiconductor laser, control is necessary to reduce the influence of the drift of wavelength. In order to reduce the influence of drift of wavelength it is necessary to stabilize the wavelength absolutely or relatively.
The wavelength of light emitted from a laser source varies as a function of the operating temperature, and of the current applied to the energy source for excitation. Controlling the wavelength output in an optical multichannel system with narrow channel spacing is further complicated by the fact that laser output wavelength is influenced by other factors such as acoustic vibrations as an example.
Critical to these systems is the ability to reduce wavelength spacing between adjacent channels, and thereby increase the number of channels available to be utilized within any particular waveband. Employing a method for wavelength stabilization is necessary to compensate for the effects of temperature and current variation to obtain a reasonable degree of stabilization. Employing a method for wavelength stabilization without reducing power of the optical light sources, for a plurality of optical light sources, is also desired. Employing a method which utilizes high power optic sources and has a means for stabilizing the wavelength of the source, without dramatically reducing the output of the source, is especially desired.
A number of light source stabilization approaches are known for application to discrete laser devices. The following three patent applications referred to following are hereby incorporated by reference into the present application. For example, U.S. Pat. No. 4,842,358 discloses optical signal source stabilization using an interferometer forming optical beams through a birefringent crystal interferometer, wherein the beams have similar intensities at a desired source frequency. The difference between the intensities of each beam generates an error signal which is directed to a means for altering the drive current of the source to produce an optical output signal which minimizes the error signal.
U.S. Pat. No. 5,167,444 discloses optical signal source stabilization, wherein the optical output signal from the source is stabilized by adjusting its frequency to maintain a selected optical transmission through a Fabry-Perot interferometer. The interferometer has a “split-level” gap whereby the gap is split into two discrete portion of different widths. A photodetector associated with each portion generates an electric signal indicative of the beam intensity or power transmitted through that portion of the gap. At a
Broutin Scott L.
Plourde James K.
Stayt, Jr. John W.
Agere Systems Guardian Corp.
Font Frank G.
Rodriguez Armando
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