Coherent light generators – Particular beam control device – Optical output stabilization
Reexamination Certificate
2002-11-13
2004-04-06
Scott, Jr., Leon (Department: 2828)
Coherent light generators
Particular beam control device
Optical output stabilization
C372S098000, C372S020000, C372S029010, C372S038100
Reexamination Certificate
active
06717967
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to optical devices and more particularly concerns periodic filters and sources.
BACKGROUND OF THE INVENTION
Wavelength Division Multiplexed (WDM) communication systems offer a high data transmission capacity by allowing multiple laser sources to transmit many high-speed data channels simultaneously over a single fiber, where each channel is transmitted at a unique optical frequency (or wavelength). In order to regularize the frequencies of the channels across telecommunication systems, the industry has adopted a standard which specifies that the nominal optical frequency of every channel should be at an integer multiple or submultiple of 100 GHz. On this uniform frequency grid, typical channel frequencies are therefore 193.100 THz, 193.200 THz, 193.300 THz etc. The frequency of these channels must typically be accurate within 2.5 GHz or 1.25 GHz or even better for correct system operations. During recent years, pressure to put more channels in the same fiber created a need for closer spacing at 50 GHz, 25 GHz, 12.5 GHz and so forth, with an accompanying increase of accuracy.
For a number of reasons, semiconductor lasers currently used in telecommunication systems do not intrinsically generate frequencies that are accurate or stable enough to be used alone in such a frequency grid system, whether they are narrowly or widely tunable lasers. First, current fabrication technologies do not allow to build lasers with a sufficiently accurate relationship between the frequency tuning signal and the actual frequency. Second, the frequency of the laser varies significantly with environmental factors or operating conditions such as injection current or temperature. Third, even if all other parameters are kept constant, the frequency of a laser tends to drift with aging. All these factors can easily detune a laser frequency beyond the accepted limit during its lifetime, and, if used alone, make it unsuitable for operation in a high performance telecommunication system.
Various means have been devised to stabilize the frequency of semiconductor lasers to a predetermined value with a sufficient accuracy. Many of those use an optical frequency reference filter that is sufficiently accurate and stable for telecommunication applications. This reference filter is used to compare the frequency of the laser with the desired predetermined value and generate an error signal which is fed back to the laser to correct its frequency. Once the feedback system is operational and the laser is frequency-locked, the stability of the reference filter is transferred to the laser.
Different optical reference filters have been used in the past to stabilize semiconductor lasers. Some atomic or molecular gases, for instance, exhibit absorption lines in the optical frequency range of telecommunication networks. The frequency of these absorption lines is determined by quantum mechanical laws and are generally extremely precise and stable with respect to environmental factors. These can therefore be considered as absolute reference filters since their accuracy does not depend on a factory calibration. Furthermore, the width of the absorption lines is very narrow, which allows for very sensitive frequency drift detection. Once properly frequency-locked to an absorption line, a laser can display frequency accuracy and stability orders of magnitude better than is required for current telecommunication systems. However, an important drawback of using a gas as a frequency reference is that the absorption lines that serve as references are not evenly spaced, do not occur at exact multiples or submultiples of 100 GHz, and are not present over the whole telecommunication bands.
Various types of optical interferometers or resonators can also be used as optical references to stabilize semiconductor lasers. Devices such as Fabry-Perot etalons or Mach-Zehnder or Michelson interferometers can easily be constructed and integrated into a laser transmitter for the purpose of frequency locking (hence the common name Wavelength Locker). These can be fabricated so that they display a periodic frequency response over a wide range of frequencies depending on the materials used. For instance, the spacing of transmission peaks can be tuned to be near 100 GHz, 50 GHz or whatever spacing is required for telecommunication applications. One drawback of these resonators or interferometric devices is that the accuracy of their frequency response is not absolute, that is, it is not intrinsic to the device but rather depends on their fabrication and installation processes. Further, their frequency response can change with external conditions such as mechanical stresses, temperature and aging. Although very good progresses have been made in constructing and packaging resonators or interferometers that have adequate stability performance for current telecommunication systems, these technologies may not be sufficient for the higher level of accuracy required for very closely spaced frequency grids of the future Dense Wavelength Division Multiplexing (DWDM) systems.
It would be advantageous for telecommunication systems and various kinds of optical instruments to use a device which provides an optical filter displaying a set of evenly spaced transmission peaks over a broad frequency spectrum such as a resonator or an interferometer, but whose frequency response can be known with the accuracy and stability inherent to atomic or molecular gas references. Indeed, DWDM transmitters could use such an absolute periodic reference for internal frequency alignment of the laser on finely spaced ITU sub-channels. Optical monitoring systems could even more be in need of such a calibration-free, low maintenance absolute frequency reference since they must act as a reliable watchdog over a number of channels. Furthermore, optical spectrum measurement instruments and widely tunable laser sources could use this absolute periodic spectrum to calibrate themselves over a wide range of frequencies.
Combining both periodic filters and absolute reference filters into a single apparatus is one step that can be taken to benefit from the properties of both devices. Additional devices and methods can then optionally be added to these optical devices in order to transfer the accuracy of the absolute reference filter to the periodic filter, thereby achieving an absolutely calibrated periodic filter. Such a system could effectively be used as an absolute, calibration-free periodic filter or wavelength locker if the following characteristics are present: a) the periodic filter frequency response is continuously calibrated and stabilized relative to the absolute reference filter; b) the calibration and stabilization procedures are completely automatic and c) the user is able to interrogate the periodic filter without disturbing or being disturbed by the stabilization system.
The general concept of combining an absolute reference filter with a periodic filter to obtain an extended high precision periodic reference is already known in the art. A number of applications of this concept have been previously disclosed in the scientific literature, patent applications and commercial products. These implementations solve some of the problems related to the realization of the absolutely calibrated periodic filter described before, but they still possess some significant drawbacks which are described below, and none presents all the characteristics of a absolute periodic filter that could transparently replace current periodic filters or wavelength lockers.
The C2H2-EX product family from Wavelength Reference, Mulino, Oreg., is one example of a passive (non-tunable) product combining a acetylene gas cell (absolute reference filter) with an etalon (periodic filter) which generates a comb of periodic transmission peaks.
FIGS. 1A and 1B
(PRIOR ART) show two particular implementations of the general principle behind this product family. In both cases, the etalon (also identified as optical artifact generator) is placed in series with the g
Cliche Jean-François
Latrasse Christine
Têtu Michel
Zarka Alain
Dicos Technologies Inc.
Fogg and Associates LLC
Jr. Leon Scott
Ryan Laura A.
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