Optics: measuring and testing – By light interference – Using fiber or waveguide interferometer
Reexamination Certificate
1999-09-08
2001-01-23
Kim, Robert (Department: 2877)
Optics: measuring and testing
By light interference
Using fiber or waveguide interferometer
C356S478000
Reexamination Certificate
active
06178001
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the characterization of communication laser sources, and more particularly to the optical frequency modulation characterization of communication laser sources by interferometry.
BACKGROUND OF THE INVENTION
In a typical fiber optic transmission network, optical frequency modulation (FM) characteristics of communication laser sources such as laser diodes must be accurately determined in order to define the limits within which information can be reliably modulated and exchanged between optical terminals in the network. The optical frequency behavior of a particular laser source under modulation is determinative of the ability of that laser source to transmit information. When a laser source is directly modulated with an input current signal, the laser source output exhibits a frequency deviation (&Dgr;f) per milliAmpere which varies as a function of the laser input modulation frequency. This FM characteristic is commonly referred to as the laser chirp. For most optical systems, an accurate characterization of laser chirping is essential to ensure transmission accuracy and improve performance. For example, in wavelength division multiplex (WDM) networks, laser chirp measurements are used to fine tune the laser operating wavelengths for an optimal use of the available optical bandwidth thereby enhancing transmission performance.
Presently, laser chirp is measured with specialized equipment that typically includes sophisticated network or optical spectrum analyzers. For network applications using multiple laser sources, the use of this equipment is not cost-effective. In a typical WDM network for example, many laser sources are used in different transmitting terminals which necessitates the duplication of the chirp measuring equipment used for each transmitting location. Duplicating this equipment for each transmitting location in the network may very rapidly prove to have a major impact on the cost associated with chirp characterization.
There may be situations where it is practical to use the same equipment for laser sources in different locations. In these cases, it would also be desirable to have chirp-measuring equipment that is simple and more portable. This is due to the fact that the chirp measurement equipment currently used is heavy and bulky which makes it difficult to transport between transmitting sites.
SUMMARY OF THE INVENTION
The present invention addresses these issues and to this end provides a methodology and apparatus to avoid the present limitations in this art.
The present invention provides a cost-effective apparatus and method for optical frequency modulation (FM) characterization of a laser source and provide a measure of laser chirp by using inexpensive equipment that is readily available. The invention makes use of interferometry to efficiently measure the chirp of a laser source in a simple manner without resorting to expensive and bulky equipment such as optical network or optical spectrum analyzers.
According to a preferred embodiment, the chirp characteristic of a laser source can be measured by injecting an input signal at various modulation frequencies into the laser source under test and passing the optical signal generated by the laser source into an asymmetrical Mach Zehnder interferometer (AMZI). For each modulation frequency used, the laser input signal induces an optical frequency deviation in he optical signal generated which is converted by the AMZI into cyclical variations in optical power of the optical signal. Because the AMZI exhibits a cyclical wavelength to photo-current response, a number of high frequency ripples (hereinafter referred to as interfering ripples) are introduced in the optical signal. According to the invention, the interference ripples introduced therein are counted over a half modulation period of the laser input signal to provide a measure of the approximate chirp amount on each modulated frequency.
In order to determine the number of interfering ripples brought about by the AMZI, the optical signal is converted into an electrical form which is representative of the interfering ripples introduced. The electrical signal is then subsequently applied to an oscilloscope for an estimation of the number of ripples within a half modulation period of the laser input signal.
For optimal chirp measurement, the AMZI is preferably designed with a short free spectral range (FSR) so as to introduce a large number of interference ripples within a half modulation period. According to the invention, the AMZI FSR is used as a measuring unit and defines the degree of accuracy with which chirp measurements are made.
Advantageously, the chirp measurement apparatus used by the present invention is simple and more portable. In particular, the interferometry circuit used for converting variations in frequency in the laser output signal into variations in optical power can be easily transported or alternatively duplicated and integrated in multiple transmitting sites at a reasonable cost. Further, the invention advantageously uses a low-cost oscilloscope which is also more portable than the optical network or optical spectrum analyzers used in conventional chirp measurement systems.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.
REFERENCES:
patent: 5515166 (1996-05-01), Mori et al.
Kim Robert
Nortel Networks Limited
LandOfFree
Method and apparatus for optical frequency modulation... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for optical frequency modulation..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for optical frequency modulation... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2452569