Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-05-13
2001-07-17
Pascal, Leslie (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S199200, C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06262822
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to a monitor for interrogating a plurality of wavelengths corresponding to a plurality of channels or inter-channel wavelengths in an optical communications system and for providing information relating to optical energy at wavelengths, particularly signal to noise ratios.
BACKGROUND OF THE INVENTION
To increase the capacity of communications systems, wavelength division multiplexing systems have been developed. Such systems transmit signals on a plurality of wavelengths. At the receiver, or at intermediate points for channel monitoring, it is frequently desirable or necessary to select a particular wavelength signal from a group of several wavelengths. The strength of each channel can then be measured, channels can be selectively amplified, or problems such as missing channels or excessive signal to noise (S/N) can be detected.
Wavelength calibration is typically performed by comparing a reference signal to the transmitted or received signal. Reference signals may include a stable wavelength laser source or coherent beat or other means. As disclosed in U.S. Pat. No. 5,283,845 issued to JDS Fitel Inc. in 1994, a known broadband source is provided through a wavelength monitor and control circuit to adjust a tunable etalon filter spacing. However these referencing techniques require more complicated input transmission devices which generate known frequencies and more complicated detecting circuits for performing wavelength comparisons. To monitor a plurality of channels through a single tunable filter assembly providing a known wavelength source to tune the filter for each channel would be difficult, cumbersome and expensive.
In a prior art system disclosed in U.S. Pat. No. 5,646,762 issued to Lucent Technologies, a reference signal is provided by a separate source into a tunable sampling filter. Example reference sources include a semiconductor laser, a fiber ring laser or a fiber grating laser. However, the filter calibration, and the resulting sampled channel information are only as accurate as the known wavelength of the reference signal. These stable wavelength lasers are expensive, and subject to a degree of error.
It is necessary in large multiple channel systems to be able to monitor the signals precisely. It is desired to provide a simplified referencing system for identifying multiple channels without impeding signal transmission. It is also important to know the signal to channel noise ratio to properly assess signal quality. This capability does not exist in the prior art for monitoring multiple channel systems.
SUMMARY OF THE INVENTION
By passing a multi-channel optical signal first through a tunable band pass filter to sequentially isolate a small portion of the signal, for example a single channel, or the interchannel spacing, and then passing that small portion through a fixed etalon which has a periodic response selected to match the channel or interchannel spacing, each channel or the interchannel spacing can be examined individually for signal strength. If the portion of the signal is passed from the tunable bandpass filter to a fixed etalon having a response to match inter-channel spacing, specific interchannel wavelengths can be examined to determine noise levels. The combination of a tunable bandpass filter with a filter having a periodic response provides a monitor which is able to isolate and measure a broad range of channels in a multiple channel system and to determine where noise is occurring and to measure its amplitude between channels.
In accordance with the invention there is provided, a monitor for monitoring a plurality of optical input channels and for monitoring energy between adjacent channels, each channel being spaced apart from an adjacent channel by predetermined inter-channel spacing, each of the plurality of channels having a different central wavelength, said monitor comprising:
a tunable bandpass filter having an input end and an output end, the filter having a passband selected to pass a bandwidth less than the predetermined channel spacing while substantially blocking wavelengths outside the passband, the filter being tunable to sequentially pass wavelengths corresponding to each of the plurality of channels, and the wavelengths in the inter-channel spaces;
an output terminal optically coupled to the output end of the tunable filter for receiving light that has propagated through the tunable filter;
first detector means for detecting a characteristic of the light at the output terminal;
a second filter having a periodic response optically coupled to the output end of the tunable filter for receiving light that has propagated through the tunable filter, the second filter being dimensioned, in accordance with the predetermined channel spacing, to pass central wavelengths in the inter-channel spaces and to substantially block central wavelengths of light corresponding to the plurality of channels; and
second detector means, for detecting a characteristic of light that passes through the second filter.
In accordance with the invention there is provided, a monitor for monitoring a plurality of optical communications channels and for monitoring inter-channel bands corresponding to wavelengths of light between said optical communications channels, comprising:
a tunable filter, tunable to pass each of the communications channels and inter-channel bands, one at a time while substantially blocking other of said channels and inter-channel bands;
a fixed etalon coupled to receive light that has passed through the tunable filter, the fixed etalon having a free spectral range that will substantially transmit wavelengths of light in the inter-channel bands while substantially blocking wavelengths of light corresponding to said communications channels; and,
and sensor means for detecting light that has propagated through the fixed etalon.
In accordance with the invention a monitor is provided for monitoring a plurality of input channels and for monitoring energy between channels, each channel being spaced apart from an adjacent channel by predetermined inter-channel spacing, each of the plurality of channels having a different central wavelength, said monitor comprising:
a tunable bandpass filter for scanning and sequentially transmitting wavelengths in a range including the plurality of channels;
a first periodic filter for blocking channel wavelengths from the response transmitted through the bandpass filter, and allowing inter-channel wavelengths from the response transmitted through the bandpass filter to pass;
a second periodic filter for blocking inter-channel wavelengths from the response transmitted through the bandpass filter, and allowing channel wavelengths from the response transmitted through the bandpass filter to pass;
a reference circuit for identifying at least one wavelength during the bandpass filter scan;
detector means for detecting the responses from the first and second periodic filters and the reference circuit;
means for comparing the response from the reference circuit to the responses of the periodic filters for providing a channel identification to each response during the scan.
In accordance with a preferred embodiment of the invention the monitor further includes a means for determining the wavelength at which the tunable filter is tuned. In a preferred embodiment the means comprises a reference circuit comprising a reference light source, an optical element having a known wavelength response, and a detector to detect the response of the reference circuit, such that when the reference light is directed through the tunable filter, the wavelength response is detected when the tunable filter is tuned at the reference wavelength. Preferably, the reference light is directed through a second path of the tunable filter so that the reference light does not interfere with the signal light.
The present invention allows inter channels noise to be measured with a wider bandwidth tunable filter, easing manufacturing requirements.
Further advantages will be apparent to persons
Colbourne Paul
Obhi Jasvinder
JDS Fitel Inc.
Nguyen Chau M.
Pascal Leslie
Teitelbaum Neil
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