Optical waveguides – With optical coupler – Plural
Reexamination Certificate
1998-12-01
2001-11-20
Font, Frank G. (Department: 2877)
Optical waveguides
With optical coupler
Plural
C385S003000, C359S199200, C359S199200, C359S199200, C359S333000
Reexamination Certificate
active
06321000
ABSTRACT:
This invention relates to the equalisation of optical signals in an optical communications system and in particular but not exclusively to the equalisation of wavelength division multiplexed optical signals.
BACKGROUND TO THE INVENTION
The control of optical power levels in optical communications systems is critical in obtaining optimum performance since the power level should 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 such as 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.
The inventor has disclosed in U.S. Pat. No. 5,513,029 a method of monitoring component power levels in WDM transmission using orthogonal low frequency dither signals and controlling component signal power to maintain optimum performance.
It is also known from GB2314714A that an imbalance of component signal powers in a WDM transmission is likely to occur at an optical amplifier stage, as used to boost signal power at stages in a long distance transmission, utilising optical amplifiers such as erbium doped fibre amplifiers. Such amplifiers have a non-uniform gain characteristic as a function of wavelength which is variable in dependence on the amplifier gain, this change in gain characteristic consequent on change of gain being commonly referred to as dynamic gain tilt.
There is therefore a need to provide optical filtering which is adaptive and which can be used in conjunction with optical amplifiers, or otherwise, in order to maintain a preferred spectral profile of an optical signal.
It is known from Huang et al, IEEE Photonics Technical Letters, September 1996 pp 1243-1245, to provide an acousto-optic tunable filter for dynamic equalization of channel powers. A disadvantage of such a method is that the filters suffer from polarisation sensitivity and severe channel cross talk.
It is also known from Gobel et al, IEEE Photonics Technology Letters, March 1996, pp 446 to 448, to provide a WDM power level compensator in which demultiplexed channels are subject to power control in respective erbium doped waveguides. A disadvantage of this arrangement is that significant distortion of the modulated optical signal occurs.
It is also known from Madsen et al, IEEE Journal of Lightwave Technology, March 1996, pp 437 to 447, to provide fixed (non-adaptive) filters using a sequence of concatenated Mach-Zehnder interferometers in a planar waveguide structure. Such structures require lengths which are difficult to fit onto a single planar waveguide structure and which have an inherent high insertion loss.
Parallel structures on planar waveguides are known from Dragone, IEEE Photon Technology Letters, September 1991, pp 812 to 815, which provide non-adaptive filtering with output at a single wavelength.
It is also disclosed by S. Day in co-pending application U.S. Ser. No. 08/997,752 to provide a variable optical attenuator by means of localised heating of a waveguide.
Yamada et al, Electron Letters 1995, 31, pp 360 to 361, discloses a multiplexer using planar waveguide technology and in which a waveguide array is provided with heating strips for each waveguide in order to compensate for phase errors occurring during fabrication. After such compensation, light components passing through the arrayed waveguides are delayed by respective amounts which differ by a constant phase difference between adjacent waveguides to that recombination in a star coupler at the output of the waveguides is dispersive in wavelength to provide separation of the WDM channels, this arrangement thereby being termed an arrayed waveguide grating.
The assignee's co-pending application U.S. Ser. No. 09/158,684, discloses an optical equaliser for a wavelength division multiplexed optical signal in an optical communications system utilises an array of parallel waveguides using planar waveguide technology. Waveguides having a range of different lengths have transmission controlled respectively in amplitude and/or phase in accordance with parameters calculated from a Fourier transform of an input frequency characteristic. The equaliser has application to optical systems having line amplifiers where fibre amplifiers result in gain tilt, the equaliser allowing gain tilt to be corrected.
This equaliser utilises a parallel filtering technique in which a complex filtering function is provided through a multi-branch interferometer. Incoming light is directed into a 1:M optical splitter and divided between a number of parallel waveguides with different optical path lengths then subsequently recombined at an M:1 combiner. The multiplicity of different path lengths can be thought of as generating a suite of periodic parallel filtering functions. In this device more waveguides generally result in finer spectral detail to be synthesised, and better flattening of spectra. The relative optical phases of each path can be adjusted to control the phase and amplitude of the periodic filtering functions, so that complex filter shapes can be synthesised.
However, 1:M splitters and combiners are increasingly difficult to build, for large values of M, in integrated waveguide technology. This impacts the yield and cost of the manufacturing process for such devices significantly. Also, the average insertion loss of the device increases with number of waveguides, because power is necessarily lost at the 1:M combiner; furthermore the loss is concentrated in a single device, which has a greater impact on optical system SNR (Signal to Noise Ratio) than distributed losses.
Furthermore, the relative phases of all the periodic functions must be controlled, even in situations where reduced spectral detail is required—i.e. it is not possible to use just half the device. Therefore, complexity, scalability and the localised losses and their affect on SNR are significant problems with this device.
There remains a need to provide an improved optical equaliser, particularly for use in the context of correcting gain tilt in optical amplification stages of a communications system.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an easily scalable optical equalisation to at least partially compensate for the effects of gain tilt in optical amplifiers.
It is a further object of the present invention to provide an adaptive optical equaliser using planar waveguide technology, without the fabrication problems of prior art devices.
According to a first aspect of the present invention there is provided an optical equaliser comprising a plurality of serially concatenated filter elements the phase and modulation depth of which can be individually tuned, the filter elements being tunable in order to generate a desired optical equalisation function.
Preferably, each filter element is a three arm interferometer.
Preferably, the filter elements are fabricated utilising integrated waveguide technology.
According to a second aspect of the present invention there is provided a method of optical equalisation of an optical signal, for use in an optical communications system, comprising the steps of:
filtering the optical signal transmitted by said optical element in the optical communications system through a plurality of serially concatenated, variable modulation depth and variable phase, periodic filters elements, and tuning the filter elements in order to generate a desired optical equalisation function.
Preferably, the individual filter elements are distributed at remote locations within a device the optical output of which is to be equalised.
Preferably, individual filter elements can be added to the equaliser to improve the gain flattened produced by the equaliser.
According to a further aspect of the present invention there is provided an optical amplifier including an amplifying element and an optical equaliser comprising a plurality of seria
Font Frank G.
Lauchman Layla
Lee, Mann, Smith McWilliams, Sweeney and Ohlson
Nortel Networks Limited
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