DE19918630A1 - Arrangements to monitor the performance of D-WDM multi-wavelength systems - Google Patents

Abstract

The device according to the invention is characterized in that the signals are supplied to a controlled wavelength demultiplexer for the purpose of channel separation. Photodetectors are allocated to said demultiplexer for detecting the signals. The signals output by said photodetectors are then available in an evaluation unit.

Description

With densely packed WDM systems (dense WDM, D-WDM) messages are sent via Light signals at different wavelengths are transmitted via just one fiber. Any wavelength is the carrier of an information signal. All channels are currently within the Wavelength range from approx. 1520 nm to 1565 nm. Further developments should Enable use of an extended wavelength range from approx. 1390 nm to 1650 nm. The channel spacing is only a few nanometers or a few hundred picometers. Of the international ITU-T working group were used to standardize this Telecommunication systems the wavelengths to be used (i.e. channels) with a Channel spacing of 100 GHz (corresponds to 0.8 nm) is recommended as standard.

At many points in this transmission system, arrangements for the continuous monitoring of all characteristic parameters are required. The ongoing monitoring allows a targeted signal regeneration or improvement, or the quick reaction to faults and failures. The most important parameters include the wavelength and the power, the wavelength drift of the lasers, and the signal-to-noise ratio in each transmission channel. Typical specification requirements for monitoring are:

• - Wavelength measurement per channel with 0.08 nm absolute accuracy and 0.01 nm resolution
• - Power measurement per channel with 0.5 dB absolute accuracy and 0.1 dB resolution
• - S / N measurement between the channels with 0.4 dB absolute accuracy, 0.1 dB
• - Repeatability and a dynamic range of at least 33 dB
• - Reliability over 1010 measuring cycles (approx. 20 years)
• - low PDL (0.1 dB max.)
• - Measurement in quasi real time

There are basically two different methods for monitoring: filter technology and the interferometer technology. Both come in conventional optical spectrum analyzers to use.  

Filter technology uses tunable narrow-band filters for wavelength selection used. There are acousto-optical filters (e.g. from Wandel & Goltermann) or piezoelectrically controlled microfilters (e.g. from Queensgate) are used, which are directly connected to a electrical size are tunable. Another variant is the lattice monochromator technology, where either the grid is rotated and the spatially resolved signal spectrum with a individual photodiode is scanned or the grid is fixed and a scanning deflecting mirror is provided in front of the exit slit of the monochromator (e.g. from Photonetics), or it will a fixed grid together with a photo diode array as a detector unit (e.g. from Yokogawa) used.

In interferometer technology, the detector signal of a Michelson interferometer is also used variable path lengths with the help of the Fourier transformation the spectrum is obtained (e.g. Fa. Hewlett Packard).

All of the conventional arrangements mentioned are not suitable, the high requirements, who are responsible for monitoring, resolution, measurement accuracy, ASE measurement and dynamics Assembly for a WDM system can be provided simultaneously and in a suitable manner meet and also the requirements for short measuring time, durability and low Space requirements.

The aim of the invention is to realize a suitable compact measuring system, the measuring times permanent and parallel monitoring of the channels of a D-WDM system in the ms range allowed by frequency, power, drift and SNR.

The object is, according to the features of claim 1, by using a special demultiplexer of D-WDM technology, combined with an array of Photodetectors realized.

The property of a phased array demultiplexer is exploited that the channel centers frequencies of such a demultiplexer variable by changing the temperature are. If you change the temperature periodically and measure it at the same time, you can The center frequencies of each separated channel can be clearly assigned.  

The filter profile of each channel of the demultiplexer is Gaussian. The output signal of the photodetector arranged at each channel output is the time average over the applied spectral function evaluated by the filter function. Knowing the evaluation function (Gaussian transmission curve over the wavelength) and the current center frequency (changing over time through temperature control), the parameters

• Central wavelength and temporal drift of the optical carrier,
• - optical performance and change over time,
• - spectral power densities outside the useful range for determining the signal-noise Distance can be determined.

The system contains no moving parts. The separation of the channels to be monitored takes place with an assembly in integrated optics. The parallel data acquisition is guaranteed short measuring times. The modules used are largely insensitive to polarization. The system has negligible repercussions. The measuring system is compact built up. Influences on the measurement accuracy due to undefined polarization directions Surveying spectra are provided by connecting a device for polarization manipulation prevented. For this come facilities that statistically determine the polarization direction change (polarization scrambler), and polarization switch used.

In Fig. 1 the basic structure of a D-WDM monitor is shown. The D-WDM spectrum to be examined is fed in at the fiber input ( 1 ). The following demultiplexer ( 2 ) separates n channels at its output, depending on how many outputs this component is specified for. Instead of the n output fibers usually present there, n photodetectors ( 4 ) are attached, each of which detects the radiation of an output channel. The electrical signals are transmitted via the n-channel ( 5 ) to the evaluation unit ( 6 ) and the display and control unit ( 7 ). A feedback ( 8 ) from the display and control unit ( 7 ) periodically changes the pass characteristic of the demultiplexer ( 2 ) via the modulation device ( 3 ). This change only affects the transmission wavelength of each individual channel.

In Fig. 2 the demultiplexer ( 2 ) is a phased array demultiplexer, which is changed in the characteristics by a substrate temperature control ( 10 ). The current temperature is measured via the integrated sensor ( 11 ), the signal ( 9 ) of which is evaluated by the display and control unit ( 7 ) and is used for sequence control.

Fig. 3 shows an arrangement in which the polarization direction is not necessarily downer of the input signal is distributed by a statistically Polarisationsscambler, so as to avoid polarization-induced measurement error.

In FIG. 4, the scrambler is replaced by a polarization switch which switches periodically and in synchronism with the data acquisition and processing between two orthogonal polarization states and the input of the phased array provides by a further polarizer a defined polarization state. This allows measurement errors due to changing polarization directions of the input signal to be eliminated.

Directory of images

Fig. 1 Basic structure of a D-WDM monitor

Fig. 2 Structure of a D-WDM monitor with phased array demultiplexer

Fig. 3 Structure of a D-WDM monitor with phased array demultiplexer with an upstream polarization scrambler

Fig. 4 Structure of a D-WDM monitor with phased array demultiplexer with an upstream polarization switch

Legend for the pictures

1

Fiber input

2nd

Demultiplexer,

3rd

Modulation device

4th

Photodetectors

5

N-channel transmission path

6

Evaluation unit

7

Display and control unit

8th

return

9

signal

10th

Substrate temperature control

11

sensor

12th

Polarization scrambler

13

Polarization switch

Claims (10)

1. Arrangement for monitoring the performance of D-WDM multi-wavelength systems, characterized in that the arrangement is implemented by a polarization manipulation device, a controllable wavelength demultiplexer for channel separation and associated photodetectors for signal detection. 2. Arrangement for monitoring the performance of D-WDM multi-wavelength systems according to claim 1, characterized in that the demultiplexer in its Pass characteristic with respect to the wavelength is changed periodically. 3. Arrangement for monitoring the performance of D-WDM multi-wavelength systems according to claim 1, characterized in that the demultiplexer preferably is designed as a phased array demultiplexer. 4. Arrangement for monitoring the performance of D-WDM multi-wavelength systems according to claim 1, characterized in that the variation of Pass characteristic z. B. by thermal modulation of the component properties is achieved. 5. Arrangement for monitoring the performance of D-WDM multi-wavelength systems according to claim 1, characterized in that at the multi-channel output of Multiplexers an equal number of photo receivers is used. 6. Arrangement for monitoring the performance of D-WDM multi-wavelength systems according to claim 1, characterized in that an array of photodiodes is used. 7. Arrangement for monitoring the performance of D-WDM multi-wavelength systems according to claim 1, characterized in that the photodiode array either is built in monolithic or hybrid construction.   8. Arrangement for monitoring the performance of D-WDM multi-wavelength systems according to claim 1, characterized in that a polarization scrambler Phased array is connected upstream. 9. Arrangement for monitoring the performance of D-WDM multi-wavelength systems according to claim 1, characterized in that a polarization switch the Phased array is connected upstream. 10. Arrangement for monitoring the performance of D-WDM multi-wavelength systems according to claim 1, characterized in that the polarization switch and the Data acquisition and processing work synchronized.