Optical: systems and elements – Optical demodulator
Reexamination Certificate
1999-06-23
2001-08-07
Lee, John D. (Department: 2874)
Optical: systems and elements
Optical demodulator
C356S477000
Reexamination Certificate
active
06271959
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to signal demodulation in optical communication networks and more particularly to the demodulation of an optical signal by interferometry.
BACKGROUND OF THE INVENTION
In a typical fiber optic transmission network, information is conveyed in the form of optical channel signals generated by light sources such as lasers or light-emitting diodes. In order to transmit the information optically in a network, a payload signal is used to modulate the source input current of a particular light source so as to impress the optical output of that source with the content of the information to be transmitted. For example, digital information can be transposed onto an optical channel signal by modulating the associated source current between two fixed amplitude levels such that the light source produces an ON-OFF keyed (OOK) optical output operating at two distinct optical intensity levels. These different intensities in the optical output can then be used for representing binary data according to the form of encoding used.
In order to monitor the transmission of information, it is well known to administrate and maintain the optical channels by using a small portion of the available channel bandwidth in the fiber to transmit a low frequency, low amplitude overhead signal. This overhead signal is typically superimposed onto the payload information signal (hereinafter referred to as the payload signal) before the latter is applied to modulate a light source. As a result of the modulation, the light source produces an optical channel signal carrying both the payload and overhead information for transmission in a particular optical channel.
In current systems, the overhead modulation is carried out at a fixed modulation depth which is typically 5% to 10% of the payload signal amplitude. As an example, for a 20 mA payload signal the overhead signal may be set to ±0.5 mA (1 mA peak-to-peak) for a modulation depth of 5%. This high level of modulation depth is required to ensure proper demodulation and recovery of the overhead information at a suitable signal-to-noise ratio (SNR). However, the presence of an overhead signal modulated with a high modulation depth reduces the signal-to-noise ratio (SNR) of the payload signal and results in a substantial degradation of the system's performance. For systems which necessitate the transmission of overhead information, it would be desirable to transmit overhead signals at a lower modulation depth for improving payload SNRs and minimize the performance penalty.
In addition to the need to reduce the modulation depth at which overhead signals are modulated, it would also be desirable to have available a larger overhead bandwidth for transmitting more overhead information to improve management and control flexibility. Currently however, this need cannot be effectively addressed without resulting in a substantial penalty in performance. A larger overhead bandwidth will interfere more with the payload signal content, particularly in systems such as wavelength division multiplexed (WDM) networks for example, in which the payload signal necessitates a large bandwidth to accommodate multiple payload signals operating at different bit rates. With a larger bandwidth, the payload signal may overlap with an overhead signal, more particularly in the low frequency range where the frequency content of the overhead signal is typically located. This would cause a degradation of the payload signal SNR and seriously affect the system's performance. Therefore, it is also desirable to have an overhead signal which can carry additional information without seriously impeding the system's performance.
Presently, in order to retrieve an overhead signal from an optical channel signal, the optical channel signal must first be demodulated and converted into an electric form before any manipulation of the overhead information can be carried out. The optoelectrical conversion and demodulation of the entire optical channel signal can become quite expensive, particularly in transit terminals where only the overhead information is required for control and management. As optical technology evolves toward all optical networking, there will be a need to monitor optical signals at various points for fault detection and performance monitoring applications. A low cost, high SNR gain technique for recovering the overhead information without having to do a complete optical channel demodulation (payload and overhead) would also be desirable in the control of such networks.
SUMMARY OF THE INVENTION
The present invention addresses these issues and to this end provides a methodology and apparatus to mitigate the present limitations in this art.
The present invention provides a novel optical frequency demodulation technique for extracting overhead information from an optical channel signal carrying both payload and overhead information. The present invention makes use of interferometry in an optical frequency demodulator to efficiently extract the overhead information from the optical channel signal without complete signal demodulation.
In a preferred embodiment, the optical frequency demodulator is comprised of a tuned asymmetrical Mach-Zehnder interferometer (AMZI) for extracting the overhead information by frequency discrimination, a balanced detector pair for converting the overhead information extracted into an electrical signal and a low pass filter for eliminating high frequency components from the recovered overhead information signal.
The AMZI is designed to receive a suitable amount of optical channel signal power and operates to frequency discriminate from it the overhead information. In order to do this, the tuned AMZI operates to introduce ripples of a high frequency into the unwanted payload FM output signal component present in the optical channel signal such that the interference caused by the payload component can be subsequently eliminated in the low pass filter and the overhead information retrieved.
The optical frequency demodulating method and apparatus of the present invention can preferably be used to extract any overhead signal transmitted at frequencies where thermal chirp is induced. The overhead extraction can be made from any optical signal exhibiting large payload induced frequency variations which spread over a plurality of AMZI interference ripples. In a preferred embodiment of the invention, the overhead extraction can also be made in connection with payload signals which do not introduce any frequency variations in the optical signal. The overhead and payload signals can be of a wide range of frequencies but are typically respectively of a low frequency nature and a high frequency nature. Without loss of generality, these signals are hereinafter referred to as low frequency overhead signals and high frequency payload signals.
Preferably, the AMZI tuning is carried out with a feedback loop located between the AMZI and the low pass filter.
For optimal overhead recovery, the feedback loop also operates to compensate for ambiant temperature variations which may occur within the optical frequency demodulator.
Advantageously, the use of thermal characteristics of conventional laser sources provides an efficient amplitude modulation (AM) to frequency modulation (FM) conversion of the overhead information. With this higher AM to FM conversion efficiency, overhead information may be modulated at a lower modulation depth to improve the payload signal-to-noise ratio (SNR) and enhance performance.
Another advantage of the present invention is that the use of an AMZI provides a high slope for FM discrimination of the overhead information. The AMZI also advantageously exhibits a wavelength to photo-current response formed of a repetition of plus and minus “peaks” and “valleys” which introduce the necessary high frequency ripples (hereinafter referred to as interference ripples) into the unwanted payload FM output signal component present in the optical channel signal such that the interference caused by the p
Kim Ki-hong
Richardson Bruce
Lee John D.
Nortel Networks Limited
LandOfFree
Method and apparatus for optical frequency demodulation of... 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 demodulation of..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for optical frequency demodulation of... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2504150