Patent Application
20090067783
Tuneable dispersion compensation may be achieved via a concatenation of optical “all-pass” filters such as Gires-Tournois Etalons, waveguide Mach Zehnder interferometers and micro-ring devices.
So called “all-pass” filters have the characteristic that they substantially pass through all input light but alter the group delay as a complex Lorentzian-like function of wavelength or optical frequency. In the case of an all-pass etalon, the strength of the response is governed by the reflectivity of the partially transmissive mirror, the other mirror being 100% reflective. The greater the reflectivity of the partial mirror the sharper the group delay response and vice versa. The coarse bulk distance between the mirrors sets the free-spectral range of the etalon—that is to say the repeat frequency of the etalon response. The fine distance between the mirrors sets the absolute frequency position and this may be tuned by very small changes in the mirror gap. Etalons are commonly fabricated via a thickness of glass block onto which the mirrors are deposited, whereby the free-spectral range is defined by the bulk thickness and the fine tuning is achieved by changing the glass temperature and relying on its thermal expansion coefficient.
An example of the optical response of a commercially available 4-stage concatenated Gires-Tournois etalon dispersion compensator is shown in
The etalon dispersion compensator can only give quasi-linear group delay over a limited wavelength range. Beyond a certain point the characteristic diverges from this linear approximation, and then it repeats the profile again at a period equal to the free-spectral range of the etalons.
Commercially available devices require complex factory calibration data, typically supplied in the form of a lookup table. The calibration data indicates each etalon temperature for each required dispersion setting.
Such a scheme only allows quantised dispersion values and is normally optimised to derive a linear group delay versus wavelength with minimum group delay ripple. Consequently, the dispersion setting is only as good as the initial calibration and a residual transmission penalty is likely unless the set-point is exactly that required. Furthermore, should the setting requirement change (e.g. cable repair), active intervention is necessary to re-tune the operation. In addition, wavelength accuracy of the transmitter with respect to the receiver is paramount to achieve a meaningful dispersion setting defined in the device calibration.
It has also been found that tuning for linear group delay with minimum group delay ripple is not necessarily the optimum operating point to achieve best transmission Q or BER. Often a residual penalty is seen when compared to idealised dispersion compensating fibre.
In the present invention, a method of controlling a tuneable optical dispersion compensating device to act on an optical signal by automatically controlling a plurality of dispersion control settings of the device in a systematic way using feedback, thereby to adapt freely the optical group delay for the optical signal within a predetermined wavelength range including that of the optical signal. The tuneable optical dispersion compensating device may be positioned at any point with an optical transmission line, but preferably is placed within a transmitter or receiver, or both.
Preferably, the tuneable optical dispersion compensating device is a concatenated series of optical all-pass filters, more preferably etalon filters, waveguide Mach Zehnder interferometers or micro-rings.
Where etalon filters are used, preferably the dispersion control settings are the etalon temperature for each respective etalon filter. Preferably, the method includes an initial step of selecting an etalon temperature for each etalon based on a required dispersion, using standard calibration data for the device, which typically assumes a linear group delay. More complex group delay profiles are automatically discovered over time, which profiles would not be included in the standard calibration data.
Preferably, the dispersion control settings are controlled in dependence on a bit error rate (BER) measured at a receiver.
Preferably, the etalons are tuned to act over a specific wavelength range including that of the optical signal to be compensated.
The plurality of dispersion control settings may be varied sequentially or in parallel. Preferably, the method utilises a dither algorithm to adjust the dispersion control parameters. Alternatively, dispersion control settings may be adjusted in parallel by use of more sophisticated algorithms such as a “Nelder-Mead simplex” algorithm.
Examples of the present invention will now be described in detail with reference to the accompanying drawings, in which:
BER measurement is now generally available as a by-product of FEC in transponder design, and control loops may be designed to utilise this information in order to optimise transmission. As shown in
In an optical transmission system 30, incoming data is processed by an FEC encoder 31 and then a transmitter 32 modulates this pre-coded data onto an optical carrier. Typically the optical transmission system is a WDM system containing a number of individual transmitter and receiver pairs, whereby each pair has its own individual tuneable dispersion compensation element. For the sake of description only a single wavelength is considered. The optical data signal enters an optical transmission line 33 which is typically made up of a concatenation of optical booster amplifiers 34 interconnected via significant lengths of optical fibre 35. The net received signal will be subject to dispersion distortion imparted by the optical fibre 35 which effectively linearly delays or advances the signals spectral components with increasing wavelength—that is to say, depending on the sign of the dispersion imparted. A tuneable etalon dispersion compensator 36 is controlled using BER feedback to subsequently impart an opposite sign of dispersion to cancel that of the transmission system. Finally, a receiver 37 demodulates the optical signal and is followed by an FEC decoder 38 which detects and aims to correct transmission errors. By use of the FEC error detection mechanism, dispersion compensator control parameters, for example etalon temperature, may be optimally adjusted via a control setting circuit 39 to achieve a minimum BER. It is possible that in some circumstances a tuneable optical dispersion compensator may be additionally required at the transmitter end to enable a form of dispersive pre-distortion to be applied for enhanced transmission performance (not shown). For the specific control of the etalon temperatures it is normal to use either heaters or more preferably thermoelectric coolers (TEC) which may be driven by a control current. Such devices are typically integrated into the component together with a form of temperature measurement based on thermocouples, thermistors or resistive temperature devices.
A simplified example of a tuneable etalon device 40 is shown in
A power amplifier 46 is used to significantly amplify the error voltage seen between its inputs and this is applied to drive current through the TEC 42. Depending on the sign of the error voltage the direction of the current is reversed causing the TEC 42 to either heat or cool for each respective current direction. The TEC 42 acts as a heat pump and removes thermal energy from one surface and emanates this energy through the opposite surface. Thus the temperature of the etalon 41 may be approximately stabilized given environmental changes and be simply controlled by an adjustment of the set temperature voltage.
In the present invention, the etalon temperatures are not set and maintained according to the detailed calibration data supplied by the manufacturer, which data inherently assumes that a pseudo-linear group delay is required for each desired dispersion setting. Instead, the temperature of each tuneable etalon is initially set according to the dispersion required (based on the supplied calibration data) and is subsequently modified in a continuous or periodic manner using a feedback control loop which allows the individual etalon temperatures to vary outside the supplied calibration settings for the group in order to reduce BER at the receiver and thereby improve transmission performance.
Typically BER is derived from an error counter register at the receiver that may periodically be read and re-set. A simplistic control algorithm may take this error counter reading as a basis of transmission performance. By periodically monitoring this BER and then modifying one of the etalon wavelength offset tuning parameters (such as the etalon temperature), the algorithm can be crafted to try and improve the BER using a classical dither algorithm. For instance:
In the above example, the etalon temperature is adjusted for each individual etalon in the concatenated series in a sequential manner to achieve an on-going optimisation of the BER performance. In an alternative arrangement, the etalon temperatures, or other control parameters, may be adjusted in parallel by use of more sophisticated algorithms such as the “Nelder-Mead simplex algorithm”.
The scheme is adaptive and can compensate for system ageing or repair, and environmental temperature effects on the equipment or transmission fibre.
As shown in
Although the example described above is based upon a tuneable etalon device, the present invention can also be used with other tuneable dispersion compensators such as waveguide Mach Zehnder interferometers and micro-rings. In a micro-ring device, which is fabricated on a waveguide substrate (eg. silica glass on silicon), miniature waveguide rings are closely coupled to a signal waveguide. The micro-rings effectively resonate with a repeating wavelength characteristic that is similar to an all-pass etalon with the circumference substantially setting the repeat period. By adjusting the localised temperature of the micro-ring, with a heater for example, again the expansion coefficient may be utilised to minutely alter the ring circumference and this will in turn be reflected as a slight wavelength movement of the group delay characteristic. The depth of the Lorentzian-like group delay characteristic is a characteristic of the coupling distance between the transmission waveguide and the ring, stronger coupling leading to a stronger response. Thus several concatenated micro-rings with appropriate coupling factors can be tuned to a quasi-linear group delay response in exactly the same way as a group of all-pass etalons.
The above example uses a FEC BER feedback control algorithm. However, alternative measures of received signal quality could be used instead of BER. For example, it is possible to detect a rectified peak RF voltage of an optical signal and adjust the dispersion control settings using feedback to maximise this detected RF voltage. This is useful for RZ signals which give rise to a pulse with a clearly defined (sharp) peak when the dispersion compensation is optimised.
