Coherent light generators – Particular beam control device – Optical output stabilization
Reexamination Certificate
2002-03-19
2003-12-16
Scott, Jr., Leon (Department: 2828)
Coherent light generators
Particular beam control device
Optical output stabilization
C372S064000, C372S087000, C372S029015, C372S038070, C372S106000
Reexamination Certificate
active
06665322
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention pertains to the field of optical networks employing dense wavelength division multiplexing (hereafter DWDM) and in particular to a method and apparatus for controlling the length of a laser cavity.
2. Description of the Related Art
The evolution of telecommunications networks has been such that the amount of data that can be carried by a single fiber have in general, greatly increased. Key to transporting large volumes of information over a single fiber is DWDM technology. DWDM enables the transmission of multiple “colors” or wavelengths of light over a single fiber, thereby greatly enhancing data throughput. The source for each wavelength of light is a single frequency laser, which is tuned to a precise wavelength during manufacture and/or during operation. Transmission lasers may be designed to operate a single wavelength for the duration of their useful life, or may be designed to be “tunable”, that is, their wavelength of operation may be changed from time to time.
DWDM systems typically comprise multiple separately modulated laser systems at the transmitter. These laser systems are designed or actively tuned to operate at different wavelengths.
When their emissions are combined in an optical fiber, the resulting WDM optical signal has a corresponding number of spectrally separated channels. Along the transmission link, the channels are typically collectively amplified in semiconductor amplifier systems or gain fiber, such as erbium-doped fiber and/or regular fiber, although semiconductor optical amplifiers are also used in some situations.
At the receiving end, the channels are usually separated from each other using, for example, thin film filter systems to thereby enable detection by separate detectors, such as photodiodes.
The advantage of DWDM systems is that the transmission capacity of a single fiber can be increased. Historically, only a single channel was transmitted in each optical fiber. In contrast, modern WDM systems contemplate hundreds of spectrally separated channels per fiber. This yields concomitant increases in the data rate capabilities of each fiber. Moreover, the cost per bit of data in WDM systems is typically less than comparative non-multiplexed systems. This is because optical amplification systems required along the link is shared by all of the separate wavelength channels transmitted in the fiber. With non-multiplexed systems, each channel/fiber would require its own amplification system.
However, there are challenges associated with implementing WDM systems. First, the transmitters and receivers are substantially more complex since, in addition to the laser diodes and receivers, optical components are required to combine the channels into, and separate the channels from, the WDM optical signal. Moreover, there is the danger of channel drift where the channels lose their spectral separation and overlap each other. This interferes with channel separation and demodulation at the receiving end.
The optical signal generators, e.g., the semiconductor laser systems that generate each of the optical signals corresponding to the optical channels for a fiber link, must have some provision for wavelength control. Especially in systems with center-to-center wavelength channel spacings of less than 1 nanometer (nm), the optical signal generator must have a precisely controlled carrier wavelength. Any wander impairs the demodulation of the wandering signal at the far end receiver since the wavelength is now at a wavelength different than expected by the corresponding optical signal detector, and the wandering signal can impair the demodulation of spectrally adjacent channels when their spectrums overlap each other.
In addition to wavelength stability, optical signal generators that are tunable are also desirable for a number of reasons. First, from the standpoint of manufacturing, a single system can function as the generator for any of the multiple channel wavelength slots, rather than requiring different, channel slot-specific systems to be designed, manufactured, and inventoried for each of the hundreds of wavelength slots in a given WDM system. From the standpoint of the operator, it would be desirable to have the ability to receive some wavelength assignment, then have a generator produce the optical signal carrier signal into that channel assignment on-the-fly.
For telecommunications applications involving DWDM, the wavelength range used is in what is known as the third window. The third window is the spectral region within which the attenuation exhibited by the transmission medium (commonly silica glass) is the lowest. Although loosely defined, the third window may be identified to lie in the spectral region from 1500 nm to 1650 nm. Within this window the designations “S”, “C” and “L” represent subdivisions of this spectral region. An object of transmission laser performance is therefore the capability to address the spectral region associated with S, C and L-band wavelengths. A further object of a transmission laser is that it is compliant with what is known as the “ITU grid”. The ITU grid is a defined standard covering the placement, in frequency space, of optical channels launched onto a fiber. Transmission lasers must exhibit optical specifications compatible with high performance optical transmission.
For a detailed description of the structure an optical performance requirements set on transmission lasers resort may be had to J. Gowar, “Optical Communications Systems”, Second Edition, Prentice Hall International Series in Optoelectronics, pages 257 to 487, inclusive, the contents of which are incorporated herein by reference.
It is desirable that transmission lasers (tunable or fixed) operate with a single longitudinal mode (Fabry-Perot mode) in the laser cavity, and that the primary longitudinal mode that is lasing does not change over the duration of operation of the laser. “Mode hopping”, that is, the changing of the longitudinal mode of operation, may be prevented by carefully controlling the optical length of the laser cavity.
The optical length of the laser cavity is generally a function of the effective index of refraction in the materials in the cavity, and the mechanical length of the cavity. Both of these properties are strong functions of temperature, thus temperature changes are a major source of disturbance that can cause mode hopping. Other physical phenomena that can lead to mode hopping include mechanical stress (causing length changes), vibration, changes in the material index due to aging and the like.
Another desirable feature of transmission lasers is the absolute frequency at which they operate. In order to control the absolute frequency, the optical length of the laser cavity should be controlled such that the desired absolute frequency is coincident with one of the cavity's longitudinal modes.
One method of controlling cavity length, and thereby preventing mode hopping and controlling the absolute frequency of the Fabry-Perot modes, is through active temperature control of the materials in the laser cavity. One method of controlling the temperature of DWDM semiconductor diode lasers is via a temperature sensing thermistor, a proportional integral derivative (PID) feedback control law, and a thermo-electric cooler (TEC) temperature actuator, although different choices for sensors, control laws, and actuators are clearly possible. Temperature control may be used to maintain a constant effective optical cavity length, for example laser devices using a rare earth ion (such as neodymium) doped into a crystalline host material as the active medium.
Another approach is to use a cavity that is constructed of a combination of materials, some in which the optical length increases with increasing temperature, and others in which the optical length decreases with increasing temperature. The goal is to create a cavity that has an optical length that is constant over the normal temperature range of operation, thus mode hopping does not occur, provided significant
Jr. Leon Scott
Katten Muchin Zavis & Rosenman
San Jose Systems Inc.
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