Optics: measuring and testing – By light interference
Reexamination Certificate
2001-10-05
2003-10-21
Black, Thomas G. (Department: 3663)
Optics: measuring and testing
By light interference
C356S477000, C359S238000
Reexamination Certificate
active
06636318
ABSTRACT:
TECHNICAL FIELD
This invention relates to telecommunications, and more specifically, to an improved system and method for the all optical reshaping, regeneration and retiming (“AO
3
R”) of optical signals in a data network, precluding any need for Optical-Electrical-Optical (“OEO”) conversion.
BACKGROUND OF THE INVENTION
Noise, timing jitter, and attenuation in long-haul optical line systems result in the deterioration of the transmitted signal. Consequently, one of the fundamental requirements of nodal equipment in optical networks is the capability to regenerate, reshape and retime (3R Regeneration) the optical pulses. Notwithstanding the plethora of claims by various companies to have implemented “all-optical” systems, presently retiming of the optical pulses is achieved by converting the incoming optical signal into an electrical signal. This is followed by full regeneration and retiming of the electrical signal using Application Specific Integrated Circuits (ASICs). A laser source is then modulated using this fully regenerated and retimed electrical signal. However, there are certain drawbacks to converting an optical signal into an electrical one and back again. First, electrical processing of data signals is not transparent to bit rate and is format sensitive. Thus, an OEO system could not process an arbitrary incoming data signal; the bit rate, format and coding would need to be known a priori. Second, there is a significant power loss in converting to the electrical domain, and back again therefrom to the optical domain.
As optical networks become increasingly transparent, there is thus a need to regenerate the signal without resorting to Optical-to-electrical, or OEO, conversion of the signal.
At present, signal coding in optical networks generally takes the Non-Return to Zero (NRZ) code. In such coding, the signal level of the high bit does not return to zero during a portion of the incoming signal, which consists of a series of high bits. NRZ coding is the coding of choice today in optical communications systems due to the signal bandwidth efficiency associated with it. This code has thus been used for optical line systems operating at line rates up to 10 Gb/s. However, as service line rates increase to 40 Gb/s, for reasons associated with fiber dispersion and reduction in inter-symbol interference (ISI) penalties, Return-to-Zero (RZ) coding is generally favored. In RZ coding, the signal returns to zero in each and every bit period.
Future optical networking line systems will incorporate service signals at both 10 Gb/s as well as 40 Gb/s along with their associated Forward Error Corrected (FEC) overhead. The FEC rates related to 10 Gb/s data transport include the 64/63 coding for 10 Gb/s Ethernet, the 15/14 encoding of SONET-OC192 FEC and the strong-FEC rate of 12.25 Gb/s, as well as numerous potential coding schemes yet to be developed. Effectively, to support multiple FEC—and other coding related—protocols, an optical network node must be able to process numerous line rates.
As the networks tend towards optical transparency, the nodal devices in the optical network must work with all available line rates independent of their coding. One of the fundamental functions of these devices is the capability to extract the clock from the signal wholly in the optical domain. The RF spectrum of an RZ signal reveals a strong spectral component at the line rate. Consequently, the incoming signal can be used directly to extract the clock signal. In the case of the NRZ signal format, the RF spectrum reveals no spectral component at the line rate. The RF spectrum of an ideal NRZ signal looks like a sinc function with the first zero at the line rate. As described in the Parent Applications, the fundamental problem of all-optical clock recovery from an NRZ signal is the generation of a RF spectral component at the line rate. As therein described, an NRZ/PRZ (Non-return to Zero/Pseudo Return to Zero) converter is used to generate the strong line rate frequency component by converting the incoming NRZ signal into an RZ-like signal.
As a consequence of the above, the clock recovery in these network elements must be tunable over a wide range of frequencies. What is needed therefore, is an AO
3
R system, that is truly all-optical, and that is tunable over a wide range of bit-rate frequencies and works in the carrier frequency range (wavelength range) of the modern telecommunications systems, the C and L wavelength bands.
SUMMARY OF THE INVENTION
A method and system for AO
3
R functionality is presented. The system includes an AO
2
R device followed by an AOCR clock recovery module and an AOR retiming device. The AOR retiming device takes as input a recovered clock signal extracted from the output of the AO
2
R by the AOCR clock recovery module. The output is the recovered clock signal gated by the regenerated and reshaped input signal, and a monitor circuit is used to set the optimum operations of the retiming device. In a first embodiment the output of the AOR retiming device is fed to an AOC code and wavelength conversion output stage, which returns the signal to the NRZ coding, on a service wavelength converted to match the fixed wavelength connection with the DWDM transmission system. In a second embodiment the code conversion is incorporated into the AOR retiming device, and wavelength conversion is accomplished in the AOCR clock recovery device.
Previous schemes for performing the O
3
R functionality use some level of Optical-to-electronic (OEO) conversion to generate the clock signal. The AO
3
R scheme presented here carries out all three functions in the optical domain, and returns a clean output signal using identical coding as the input, on a wavelength of choice.
A lossy component, such as an optical cross-connect switch can be placed either before the AO
3
R device or inside of it after the AO
2
R device and before the signal is split to the AOCR clock recovery and the AOR retiming devices.
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40 Gbit/s OTDM-WDM transm
Antosik Roman
Dave Bharat
Engin Doruk
Kim Kwang
Miglo Aleksandr
Alphion Corp.
Black Thomas G.
Hughes Deandra M.
Kaplan & Gilman LLP
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