Control for periodic optical filter

Details Control for periodic optical filter Control for periodic optical filter

1999-12-29

2002-02-12

Hellner, Mark (Department: 3662)

Optical: systems and elements

Optical amplifier

Correction of deleterious effects

C359S337200, C359S199200

Reexamination Certificate

active

06347006

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a method and apparatus for determining control signals for a periodic optical filter, and a system incorporating the same. Such a filter can be used for optimisation of the gain profile of an amplifier, particularly although not exclusively for use in optical communication systems.
BACKGROUND TO THE INVENTION
The control of optical power levels in optical communications systems is critical in obtaining optimum performance. The power level needs to be sufficient to establish a signal to noise ratio which will provide an acceptable bit error rate but without the power level exceeding a level at which limiting factors (e.g. the onset of non-linear effects) result in degradation of the signal. In wavelength division multiplexed (WDM) transmission, it is desirable to maintain each of the power levels of the individual wavelength components at substantially the same level.
FIG. 1
illustrates a typical WDM transmission system, in which optical signals are transmitted from the multiplexer
10
to the demultiplexer
12
via optical fibre
14
. The individual wavelength components for each channel are generated by the transmitters
16
(Tx) and sent to the receivers
18
(Rx). In order to ensure that optical power is maintained within each of the transmitted channels, one or more line amplifiers
20
are located along the optical fibre transmission path to compensate for power losses in the transmission system.
A typical line amplifier
20
comprises two EDFA (Erbium Doped Filter Amplifier) amplifying elements
22
,
22
′ separated by one or more filters
24
.
The gain of the EDFA (and hence the output
28
from line amplifier) depends upon both the optical power in the transmitted input signal
26
and the optical power from the pump laser (not shown). As
FIG. 2
illustrates, the shape of the gain (the gain profile) of an EDFA changes with the gain of the EDFA. The gain profile may also be affected by temperature, age and other effects. In order to maintain each of the power levels of the individual wavelength components at substantially the same level, it is desirable to have a flat gain profile over the wavelength range of the transmitted channels. A fixed filter cannot flatten the profile of the amplifier for all gain conditions. It is therefore desirable to provide an adaptive filter for the line amplifier to provide compensation for (i.e. to flatten) the varying gain profiles.
The article “Tunable Gain Equalisation Using a Mach-Zehnder Optical Filter in Multistage Fibre amplifiers” (Reference IEEE Photonics Technology Letters, Vol. 3. No. Aug. 8, 1991, Pg718; Kyo Inoue, Toshimi Kominato, and Hiromu Toba) indicates how a tunable signal gain equalisation may be demonstrated using a waveguide type Mach-Zehnder (MZ) optical filter, such that by adjusting the MZ transmittance with the external control current, tuneable gain equalisation may be achieved at the output of each of the amplifier stages. Further, EP794,599 discloses a gain equaliser which includes a plurality of periodic optical filters for equalising the gain of an optical amplifier. This application suggests that the wavelength, phase and amplitude (attenuation) of the transparency characteristics of the filters may be adjusted to allow the optical SNR (Signal to Noise Ratio) in the transmission system to be equalised.
Neither of the above documents discloses the control strategies appropriate for such tunable filters.
As current system designs are approaching the limit of what it is possible to achieve with fixed filters, there is an increasing requirement for a device that will equalise the optical powers in the transmission system channels, and compensate for any non-flat losses in the system.
Co-pending U.S. application Ser. No. 09/1361,950 (incorporated herein by reference) describes a method of generating control signals for a plurality of periodic optical filters for equalising the gain of an optical amplifier. The method includes the step of determining an error profile based upon the amplifier output gain profile and a predetermined target output gain profile. A number of techniques exist to measure the output gain profile, i.e. to provide a measure of the optical power across the spectrum of interest. For instance, an optical spectrum analyser could be utilised, although this has the disadvantage that it is an expensive, frequently inaccurate, and a relatively fragile component.
Periodic filters are well known in the art, and typically refer to an optical filter having a response that is approximately period across the wavelength range of interest. Such filters typically have a sinusoidal response, although the term may of course refer to filters having differently shaped responses e.g. saw tooth or square wave.
Alternatively, the profile may be determined from a received pilot carrier signal transmitted with the optical signal. Such a carrier signal is typically referred to as Analogue Maintenance, an example of which is described in GB 2,260,046. In such systems, an update of the spectrum is performed regularly every several seconds or so. This response time is relatively slow compared with the time frame in which the optical signals may be transmitted. Additionally, the technique requires a considerable amount of hardware and software to process the raw data into a spectrum.
It is an object of the present invention to provide a method and an apparatus for determining a control signal for a periodic filter that substantially addresses at least one of the problems of the prior art.
SUMMARY OF THE INVENTION
In a first aspect, the present invention provides an apparatus for generating a control signal for at least one periodic filter arranged to filter the optical profile of a signal, at least one of the phase and the amplitude of the filter being tunable, the apparatus comprising: an optical waveguide for receiving a portion of said signal, optical filtering means connected to said waveguide for filtering the received portion, and arranged to act as a first and second periodic filter, each having substantially the same period as the filter to be controlled, the phase between the periods of the first and second filters being a non-integer multiple of &pgr;, and optical power measuring means arranged to measure the optical power of the signal transmitted by said optical filtering means.
By taking power measurements of each of the signals passed by two similar periodic filters that are not in phase or in antiphase, control signals for the controllable periodic filter may be directly derived. Alternatively, such power measurements can be further processed to be placed in the appropriate format for providing control signals, e.g. the power levels may be treated as two vectors, and the magnitude and phase of the resultant vector formed from the power levels (which correspond to the desired movement of the controllable filter) can be determined.
Preferably, said optical filtering means comprises a single tunable optical filter.
Alternatively, said optical filtering means comprises two periodic filters.
The apparatus may further comprise at least one optical splitter having an input and at least two outputs, said input being connected to said waveguide and said two periodic filters each being connected to a respective output. Consequently, it is possible for the power measurement to be taken simultaneously from each of the respective filter configurations. If the filter profiles are of similar amplitude and if the splitter acts to uniformly split any input optical signal into two components, then any subsequent calculations that may be required to derive the control signal can be mathematically simpler.
Preferably, said phase is substantially &pgr;/2. If the wave functions relating to the periods of the pair of periodic filters are orthogonal, then device sensitivity is maximised for any given equipment configuration. This will be apparent to a skilled person by considering the fact that the resulting power measurements will now relate to perpendicular vect

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