DE19816612A1 - Monitor for testing performance of dense wavelength division multiplex multiple wavelength communication system - Google Patents

Abstract

The monitor allows tests on the system operating at wavelengths from 1520 nm to 1565 nm, with channel separation of 100 GHz, or about 0.8 nm.- DETAILED DESCRIPTION - The monitor provides a small, tuned, narrowband bandpass filter. This is done by realizing a monochromator with a ladder grating in a single-lens Littrow arrangement, with a high angle of incidence and wide line separation. The combination of a rotating prism with a fixed grating is used, monitored by an auxiliary laser source

Description

In densely packed WDM systems (dense WDM, DWDM) messages are sent via Light signals at different wavelengths are transmitted via just one fiber. Each Wavelength is the carrier of an information signal. All channels are within the Wavelength range from approx. 1520 nm to 1565 nm. The channel spacing is a few Nanometers or a few hundred picometers. From the international ITU-T working group were used to standardize these telecommunications systems using wavelengths (= channels) with a channel spacing of 100 GHz (≈ 0.8 nm) recommended as standard.

At many points in this transmission system, arrangements for continuous monitoring of the entire system are required. The object of the invention is to carry out the necessary monitoring of all characteristic parameters of a multi-wavelength (WDM) system. The most important parameters include the wavelength and the power of all channels, the monitoring of the line width and the wavelength drift of the lasers, as well as 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 10 ¹⁰ measuring cycles (approx. 20 years)
• - low PDL (0.1 dB max.)
• - Small size z. B. 20 × 17 × 5 cm³.

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

With filter technology, tunable narrow-band are used for wavelength selection Filter used. There are acousto-optical filters (e.g. from Wandel & Goltermann) or Piezoelectrically controlled microfilters (e.g. from Queensgate) are used, which are directly above an electrical variable can be tuned. Another variant is the Lattice monochromator technology, in which either the lattice is rotated and spatially resolved signal spectrum is scanned with a single photodiode or the grating is fixed and a scanning deflection mirror in front of the exit slit of the monochrome projector is provided (e.g. from Photonetics) or there is a fixed grid together with a line of photodiodes used as a detector unit (e.g. Yokogawa).

In interferometer technology, the detector signal of a Michelson Interferometers with variable path lengths using the Fourier transform Spectrum won (Hewlett Packard).

All of the conventional arrangements mentioned are unsuitable, the high ones Requirements regarding resolution, measuring accuracy, ASE measurement and dynamics be placed on a monitoring module for a WDM system, simultaneously and in suitably and also to meet the requirements for a short measuring time, Longevity and small space requirements.

The invention therefore relates to an arrangement and a method for rapid and high-precision monitoring of the entire spectrum in WDM transmission systems.

According to the invention, this requirement is met by an arrangement which is a narrowband and tunable bandpass filter for the WDM range. This can be achieved, for example, by means of a special grating spectrometer in which an Echelle grating in a Littrow arrangement ( 1 ) according to FIG. 1 is operated with a large angle of incidence, a large line spacing and high order. This means that the light paths for incoming and outgoing light are almost identical. The advantages of the Echelles in Littrow arrangement are the higher resolution and dispersion compared to ordinary gratings and the possible optimal adaptation of the measurable spectral range to the WDM wavelength range of z. B. to see 1565 nm.

In addition, high efficiency is achieved without PDL (polarization-dependent losses) since the beams hit the grating surfaces perpendicularly and illuminate a large grating length at a high angle of incidence with a small beam diameter. Additional space can be saved, for example, by using a small concave mirror ( 2 ) and a long, narrow grille.

The basic equation applies to a general grid

mλ = d (sinα + sinβ) (I)

where m is the order, d is the line spacing and α, β is the angle of incidence and angle of exit. Since the angle of incidence and the angle of exit of an Echelle in a Littrow arrangement are almost identical, this simplifies:

mλ = 2 d sinα (II).

Typical blaze angles α are 63 ° or 76 °. Available lattice constants are 31.6 / 79/316 / mm, ie the lattice constant d is 31.6 / 12.7 / 3.16 µm. The free spectral range F is restricted by the high order m.

Wavelengths outside the WDM range are suppressed by a dielectric pre-filter ( 3 ) as a bandpass. The filter then passes z. B. only the approximately 50 nm wide WDM-Be rich. An order of up to 30 can thus be achieved. If the angle of incidence α is constant, the output beam must be deflected. If, on the other hand, the condition α = β is maintained, a deflection unit must tilt both beams, or the grating is tilted by this angle. As with any spectral apparatus, the theoretical resolution is calculated from the number of lines times the order. In order to be able to use the resolution, the gap ( 4 ) in front of the photodetector ( 5 ) for obtaining the measurement signal ( 15 ) must be dimensioned correspondingly narrow.

As shown in Fig. 1, the proposed application can be realized by the following structure, for example. The light of the WDM system to be analyzed emerges from a single mode fiber ( 6 ) and is reduced in the aperture by a lens ( 7 ). A plane mirror ( 8 ) deflects the light path, so that small external dimensions of the arrangement arise. A dielectric bandpass filter ( 3 ) only allows the wavelength range to be measured to pass. Since it is run through a second time on the way back to the detector, it suppresses all wavelengths outside the WDM range with double quality. This avoids ambiguities due to the high order. The concave mirror ( 2 ) forms a parallel beam ( 9 ) with a diameter of approx. 12 mm and directs it onto the Echelle grating ( 1 ) via a mirrored prism on four sides. A motor ( 10 ) sets the prism ( 11 ) in a constant rotary movement with about 1 rev / s.

Mechanically moving parts should work for many years designed device can be avoided, but with modern brushless motors self-lubricating plain bearings that have hardly any axle loads to cope with suitable for decades of trouble-free operation.

The entrance and exit angles used by rotating the prism vary e.g. B. in the area 76 ° ± 2.75 °, whereby four measurements of the entire spectrum are possible per revolution.

Since the incident and exit angles are identical, the same light path is used on the return path of the light beam. At the focal point, which is slightly offset from the input fiber, there is a photodetector ( 5 ) with a slit diaphragm ( 4 ). A narrow frequency band of e.g. B. 0.025 nm passes the exit slit of the arrangement.

This has the following advantages: Focusing elements can be used twice become. By operating at high order, Echelle have higher resolution and dispersion than ordinary grids. The spectral range can be examined on the narrow one WDM wavelength range can be tailored. It becomes high efficiency without Polarization dependency achieved because the rays hit the grating surface perpendicularly. At high angle of incidence can have a long grating length with a small beam diameter be illuminated. A concave mirror is not required, so a long grating with high resolution can be used. The grid width can be narrow, so that a robust and space-saving construction is possible. Going through the dielectric filter, its original quality is doubled.  

Furthermore, the position of the prism ( 11 ) is detected with very high precision by an auxiliary laser ( 12 ). The focused beam of the auxiliary laser ( 12 ) is directed onto the same surface and the reflected beam is fed to a position sensor ( 13 ). This position sensor consists, for example, of an incremental scale with a pitch of 20 µm (glass / mirrored) and a linear photodiode arranged behind it. With a focal length of 300 mm as well, the 5.5 ° change in angle is converted into a location offset of 29 mm. The position signal ( 16 ) therefore contains 1450 light-dark transitions per measurement. The WDM spectrum therefore has a spectral width of z. B. 35 nm every 0.024 nm exact support points that do not depend on the constant speed of the motor ( 10 ).

The following requirements for good performance of this arrangement should apply to the Realization should be considered: The measuring rate should only be chosen as high as necessary (4 / s) become.

The time for a scan should be chosen as long as possible (e.g. 8-sided prism with ½ U / s), so that the photodiode and the amplifier have little noise due to low cut-off frequencies and high dynamics can be achieved. Within a scan can be very large Level differences occur, but the gain cannot be switched in between so that an AD converter with at least 16 bit (48 dB optical dynamic) or two AD converters with different preamplification should run in parallel. The The focal length of the concave mirror or lens should be chosen as large as possible, so that the dispersion of the grating (approx. 5.5 ° to 35 nm) fuses a wide range in the Focal plane results and the slit width does not have to be too small.

Claims (7)

1. Arrangement for monitoring the performance of DWDM multi-wavelength systems, characterized in that a narrow-band and tunable bandpass filter for the WDM range is realized by an arrangement with an Echelle grating in a Littrow arrangement with a large angle of incidence and a large line spacing. 2. arrangement for monitoring the performance of DWDM multi-wavelength systems, characterized in that, for example, the combination of a rotating Prism with a fixed grating is used. 3. arrangement for monitoring the performance of DWDM multi-wavelength systems, characterized in that a higher order to increase the resolution is used. 4. arrangement for monitoring the performance of DWDM multi-wavelength systems, characterized in that, for example, by using an Echelle Grid with ruled-grating an almost vertical incidence on the grille is realized to avoid polarization-dependent reflections. 5. arrangement for monitoring the performance of DWDM multi-wavelength systems, characterized in that, for example, by using an auxiliary laser Scanning of the rotating object to obtain a synchronous signal for the wavelengths assignment of the output signal of the arrangement is obtained. 6. arrangement for monitoring the performance of DWDM multi-wavelength systems, characterized in that, for example, the extraction of a Position signal of the rotating object is realized by a position sensor, which for Example of a linear photodiode and one in front of it Incremental scale exists. 7. arrangement for monitoring the performance of DWDM multi-wavelength systems, characterized in that, for example, a dielectric pre-filter for Suppression of wavelengths outside the measuring range is used, which due to the double iteration doubles its effective quality.