Optics: measuring and testing – By light interference – Having partially reflecting plates in series
Reexamination Certificate
2001-03-17
2003-08-26
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
Having partially reflecting plates in series
C372S032000
Reexamination Certificate
active
06611341
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The invention generally relates to dense wavelength division multiplexers (DWDM) and in particular to a technique for locking transmission wavelengths of individual lasers of the DWDM.
II. Description of the Related Art
A DWDM is a device for simultaneously transmitting a set of discrete information channels over a single fiber optic transmission line. A conventional fiber optic transmission line is capable of reliably transmitting signals within a bandwidth of 1280 to 1625 nanometers (nm), the “low loss” region for silica fiber. Within that overall bandwidth, the International Telecommunications Union (ITU) has defined various transmission bands and specified certain transmission channel protocols for use within each transmission band. One example of a transmission band is the ITU “C” band, which extends 40 nm from 1525 nm to 1565 nm. Within the C band, specific transmission channel protocols of 40, 80, or 160 discrete channels are defined and, for each protocol, the ITU has defined a grid of transmission wavelengths, with each line corresponding to an acceptable transmission wavelength. The protocols have been defined to ensure that all DWDM transmission and reception equipment are fabricated to operate at the same wavelengths. For the 40-channel protocol, the corresponding ITU grid has 40 lines with channel spacing of 0.8 nm; for the 80-channel protocol, the corresponding ITU grid has 80 lines with channel spacing of 0.4 nm; and so forth. Additional protocols have been proposed, including 320 channel and 640 channel protocols. Maximum theoretical transmission frequencies for the various ITU protocols are as follows: 100 GHz for the 40 channel protocol; 50 GHz for the 80 channel protocol; 25 GHz for the 160 channel protocol; 12.5 GHz for the 320 channel protocol; and 6.25 GHz for the 640 channel protocol. Closer channel spacing necessitates a lower modulation rate since channel spacing must be larger than the modulation frequency. High frequency modulation requires suitable optic fibers, as well as appropriate transmission and receiving equipment. Current state-of-the-art DWDMs typically employ a 40 channel ITU protocol but transmit at 2.5 GHz, well below the theoretical maximum. Other exemplary ITU transmission bands are the S- and L-bands.
To simultaneously transmit the set of channels on a fiber optic cable, a conventional DWDM employs a set of the individual distributed feedback (DFB) lasers, with one DFB laser per channel and with the DFB configured to transmit.
FIG. 1
illustrates a DWDM
100
having forty individual DFB lasers
102
for transmitting optical signals via a single optic fiber
104
. An optical multiplexer
106
couples signals received from the individual DFBs via a set of intermediate optic fibers
107
into output optic fiber
104
. Each DFB laser transmits at a different wavelength of the 40-channel ITU C band. This enables forty separate channels of information to be transmitted via the single optical fiber
104
to a de-multiplexer (not shown) provided at the far end of the optical fiber.
To permit the DWDM to transmit forty separate channels simultaneously, each individual DFB must be tuned to a single ITU transmission channel wavelength. A DFB laser can be tuned only within a narrow wavelength band, typically about 2 nm in width. Hence, for the 40-channel protocol of the ITU C band having 0.8 nm transmission line spacing, the typical DFB can only be tuned to one of a few adjacent lines out of the total of 40 lines of the ITU grid. Traditionally each individual DFB laser is manually calibrated at the factory to emit at a corresponding one of the ITU transmission lines. This is achieved by adjusting the laser operating temperature and current to obtain the desired wavelength.
The laser is then, in some implementations, locked to the target wavelength by routing the output beam from each DFB laser through a corresponding manually tunable etalon. (The etalons are not shown in
FIG. 1.
) A manually tunable etalon is an optical device that produces a periodically-varying transmission spectrum as a function of laser wavelength. By tilting the etalon relative to the DFB laser beam path, a transmission peak of the etalon can be made coincident with the target ITU channel. The wavelength of an etalon transmission peak is calibrated to one of the ITU transmission lines by manually adjusting the angle of the etalon while monitoring the wavelength output from the etalon using an optical wavelength analyzer. The angle of the etalon is adjusted until the output wavelength is properly aligned with one of the ITU transmission lines, then the etalon is mounted in place in an attempt to lock the output wavelength of etalon to the selected ITU transmission line. This is a difficult and time-consuming process requiring skilled technicians resulting in the alignment of the etalon transmission peaks with on several ITU transmission lines. In addition, if transmission over a certain range of ITU transmission lines is desired, multiple tunable lasers are required to ensure adequate coverage. Such an arrangement is costly and does not provide rapid switching between ITU transmission lines. Furthermore, mechanical or thermal drift of the etalon over time often moves the transmission peak away from the target ITU channel, which requires recalibration.
Once the DFB lasers of a single DWDM are properly aligned with the ITU grid, the DWDM may then be used for transmitting signals over a fiber optic line, such as for transmitting digital data over computer networks or for transmitting television signals from a television network to one of its affiliates. A single DWDM must be provided for use with each fiber optic line employed for DWDM transmissions and hence a single customer installation, such as a television broadcast center, may require large numbers of DWDMs. If one of the DFB lasers within a DWDM drifts from its corresponding ITU transmission line or otherwise malfunctions, the entire DWDM typically needs to be replaced to permit the malfunctioning DWDM to be returned to the factory to be re-calibrated or otherwise fixed. As a result, the cost of maintaining a set of DWDMs is often substantial. To help remedy this problem, some DWDMs are provided with an additional widely tunable laser (WTL), which can be tuned separately to any one of the ITU grid lines. Hence, if one of the DFB lasers malfunctions, the single WTL can be tuned to the corresponding transmission wavelength of the DFB to thereby permit the DWDM to continue to operate. Additional WTLs can be supplied with a DWDM to accommodate the failure of two or more DFB channels, and such “sparing” is a major advantage a WTL over a DFB. However, conventional WTLs cannot simply and accurately be tuned to any target ITU channel at a customer installation and must be calibrated at the factory for operation at a specific channel.
Another problem associated with employing DFB lasers within DWDMs is that, because each DFB laser can only be tuned within a narrow range of about 2 nm, each DFB laser can only be calibrated to one of a few adjacent ITU transmission wavelength lines. It is sometimes desirable to configure the DWDM to use many lasers for transmitting at a single ITU transmission line to provide more bandwidth on that channel. When using DFB lasers, no more than two or three of the lasers can be calibrated to a single ITU transmission line. Hence, in some DWDMs, WTLs are used exclusively instead of DFB lasers, thus permitting any of the lasers to be manually calibrated at the customers installation to transmit on any of the ITU transmission lines. Although the use of WTLs remedies many of the problems associated with using DFB lasers, WTLs are difficult and expensive to fabricate and initially calibrate, and are susceptible to wavelength drift requiring frequent recalibration at the customers installation by trained technicians and hence necessitating high overall installation and maintenance costs.
Thus, whether using DFB lasers or WTLs within a DWDM, significan
Kukkonen, III Carl A.
SpectraSensors, Inc.
Turner Samuel A.
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