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Multiple Wavelength RZ Pulse Generation

IP.com Disclosure Number: IPCOM000019762D
Original Publication Date: 2003-Oct-25
Included in the Prior Art Database: 2003-Oct-25
Document File: 3 page(s) / 139K

Publishing Venue

Siemens

Related People

Juergen Carstens: CONTACT

Abstract

In transmission systems with a channel date rate of 40 Gbits/s and higher, the RZ (return-to-zero) data format has many advantages compared to the NRZ (nonreturn-to-zero) data format. However, the RZ pulse generator is more expensive than the NRZ pulse generator. In today's transmission systems, for each channel a RZ pulse source is needed. The RZ pulse source can be realised by either using a special RZ laser (for instance a mode locked fiber ring laser) or alternatively by modulating a CW (Continuous Wave) laser using a gating device (for instance an electroabsorption modulator). Both solutions are not cost effective. A tunable, stable RZ laser is not commercially available up to now. Mode locked fiber ring lasers for instance can be tuned in frequency, but do not have a stable operating point at all frequencies. The electroabsorption modulator can convert a CW signal into a RZ signal, though its working frequency range is limited. A further disadvantage is the dependency of the pulse quality at the output on the frequency. The idea is to feed a pulsed pump signal to a spectral inverter creating a switching window for all input signals. The spectral inverter converts a CW signal to a RZ signal (fig. 1). The wavelength of the RZ signals can be tuned by tuning the CW inputs. Several techniques can be used as spectral inverter: a periodically poled lithium niobate, four-wave mixing in highly nonlinear fiber, etc.

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© SIEMENS AG 2003 file: 2003J13236.doc page: 1

Multiple Wavelength RZ Pulse Generation

Idea: Sander Jansen, NL-Bunnik; Stefan Spaelter, DE-Muenchen

In transmission systems with a channel date rate of 40 Gbits/s and higher, the RZ (return-to-zero) data format has many advantages compared to the NRZ (nonreturn-to-zero) data format. However, the RZ pulse generator is more expensive than the NRZ pulse generator. In today's transmission systems, for each channel a RZ pulse source is needed. The RZ pulse source can be realised by either using a special RZ laser (for instance a mode locked fiber ring laser) or alternatively by modulating a CW (Continuous Wave) laser using a gating device (for instance an electroabsorption modulator). Both solutions are not cost effective. A tunable, stable RZ laser is not commercially available up to now. Mode locked fiber ring lasers for instance can be tuned in frequency, but do not have a stable operating point at all frequencies. The electroabsorption modulator can convert a CW signal into a RZ signal, though its working frequency range is limited. A further disadvantage is the dependency of the pulse quality at the output on the frequency.

The idea is to feed a pulsed pump signal to a spectral inverter creating a switching window for all input signals. The spectral inverter converts a CW signal to a RZ signal (fig. 1). The wavelength of the RZ signals can be tuned by tuning the CW inputs. Several techniques can be used as spectral inverter: a periodically poled lithium niobate, four-wave mixing in highly nonlinear fiber, etc.

For successful RZ pulse generation, the data channels need to be aligned inside the spectral inverter. Therefore the data sources have to be timed with the main clock. When the transmitter is not integrated on a chip, phase variations due to for instance temperature changes might occur. Variations in phase disturb the alignment of the transmitter. To monitor this variations a phase detector can be installed at the output of the spectral inverter. When the phase of a certain channel "X" varies, the phase detector will notice that. A control circuit can now re-align channel "X" by adjust its optical or electrical delay. It is possible to use one detector for detecting the phase of all channels when each delay is wobbled by a specific frequency f_x and the detector of the error signal (e.g. the power at f_x) is frequency specific. Hence an inexpensive phase detector can be realised.

A possible setup is shown in the figure 2. A main clock working at a single frequency feeds the RZ pulse generator and times the data sources. The RZ pulses are used as a pump source for the spectral inverter (SI). The CW lasers are modulated and mixed up via array waveguide grating (AWG). The NRZ data signals and the RZ pump signal are mixed up via coupler and then fed into the spectral inverter. Here th...