Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-01-07
2001-03-06
Chan, Jason (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S199200, C359S249000
Reexamination Certificate
active
06198560
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the operation of optical modulators and in particular, although not exclusively, to the operation of a Mach Zehnder type optical modulator in an optical telecommunications network operating with a synchronous digital hierarchy (SDH) format.
BACKGROUND OF THE INVENTION
In an optical telecommunications network data is transmitted in the form of pulses of light, in which a pulse of light represents a logic state “1” and the absence of a pulse, or a pulse of different magnitude, represents a logic state “0”. In low data rate systems it is known to produce the pulses of light by driving a solid state laser using a signal which is related to the data to be transmitted. Lasers, however, can “chirp”; that is they no longer produce pulses of light of a single wavelength. This spreading of the wavelength of light results in dispersion of the pulses as they travel along optical fibres which can ultimately limit the operating frequency and/or range of the optical network.
To overcome the problems associated with laser “chirp” it is known, when operating at high data rate, to run the laser continuously and externally modulate the continuous light output using an optical modulator. The optical modulator applies variable attenuation to light passing through it, the amount of attenuation being dependent upon an electrical signal applied to a control input. One example of such a modulator is a lithium niobate Mach Zehnder modulator which has an optical transfer response (that is optical attenuation versus the voltage applied to the control input) which is approximately sinusoidal in shape. In an optical telecommunications network in which digital data is being transmitted it is often required that the data fully modulates the light output, that is light should pass substantially unattenuated for a logic state “1” and be completely attenuated for a logic state “0” or vice versa. To achieve such modulation requires the modulator to be operated at voltages which correspond to the maxima and minima in the optical transfer response.
In practice whilst the shape of the modulator's transfer response may be known its relative position along the voltage axis drifts resulting in distortion of the transmitted signal. As described in U.S. patent Ser. No. 5,400,417 such drift can be compensated for by applying a DC bias voltage v
bias
to a second input of the modulator to maintain the modulator's transfer response at the desired voltage position. Control circuitry is provided which monitors the optical output of the modulator to determine by how much the modulator's transfer response has drifted and the DC bias voltage is adjusted accordingly to maintain the modulator's transfer response at the desired voltage position. Whilst such an arrangement is found to track any drift in the modulator's response, a problem can arise if the transfer response drifts by an amount which would require a bias voltage which exceeds the maximum bias voltage available as set by the power supply of the control circuitry or by constraints in the modulator.
When the modulator's response drifts by an amount such that the bias voltage required for correct operation exceeds the available bias voltage range it is necessary to reset the system.
For an optical modulator which has a transfer response which is cyclic it has been proposed in U.S. Pat. No. 5,003,624 to reset the bias voltage by an amount corresponding to one cycle of the response. In this way the modulator is reset to a corresponding portion of its transfer response. However with such a system a loss of the transmitted data will occur during the period that the voltage is adjusted. In the case of some transmission systems, for example the analogue transmission of television signals, this may be acceptable. In digital telecommunication networks however, such as those operating using a synchronous digital hierarchy (SDH) format, there are strict constraints on performance. In particular in high bit rate systems, such as would typically use external optical modulators of this type, resetting the system could corrupt unacceptably large quantities of data.
The present invention addresses the technical problem of resetting an optical modulator without causing appreciable disruption to the data and in particular concerns the problem of resetting a Mach Zehnder optical modulator which is used in an optical telecommunications network operating with a synchronous digital hierarchy (SDH) format.
SUMMARY OF THE INVENTION
According to a first aspect of the invention there is provided a method of operating an optical modulator of a type which has an optical attenuation versus voltage response which is cyclic, the optical modulator including an input to which the application of a bias voltage moves the modulator's response along the voltage axis, the method comprising:
generating a bias voltage which comprises the sum of a first and a second voltage component, wherein the first voltage component is fixed during normal operation and is selected to correspond to the expected bias voltage required for maintaining the modulator's response at a desired voltage position;
detecting the position of the modulator's response and adjusting the second voltage component to maintain the response at the desired voltage position; the method further comprising:
detecting when the bias voltage approaches the limit of the available bias voltage range;
adjusting the first voltage component until it is equal to the sum of the bias voltage and a voltage corresponding to one or more half cycles of the modulator's response; and
setting the second voltage component to zero.
Advantageously the method further comprises adjusting the first voltage component slowly enough for the second voltage component to track the change and thereby maintain the modulator's response at the desired voltage position until the second voltage component is set to zero.
A particular advantage of using a bias voltage which comprises a first voltage component which is fixed and a second voltage component is that the bias voltage can be reset with minimal disruption to the modulator's operation by adjusting the first voltage component to the desired reset bias voltage and then setting the second voltage component to zero. This is because during the period in which the first voltage component is adjusted the operation of the modulator is unaffected as the second voltage component tracks the change, thereby maintaining the modulator's response at the desired voltage position. The time taken to adjust the first voltage component is thus irrelevant provided that the modulator's drift is sufficiently slow that a reset will not be required for a relatively long time compared with the operation of the method.
Setting the second voltage component to zero thus determines the effective reset time (i.e., the time during which any disruption could occur) and this can be achieved almost instantaneously and is limited only by the slew rate of the generator used to generate the second voltage component.
Preferably the method comprises resetting the bias voltage by an amount which corresponds to an even number of half cycles of the modulator's response such that the modulator will be reset to an equivalent position on a different cycle of the response. The modulator will then continue to operate without disruption to the data applied to the modulator.
Alternatively the method comprises resetting the bias voltage by an amount which corresponds to an odd number of half cycles and inverting data applied to the modulator. Although the data needs to be inverted such a method is found to be advantageous as it enables a faster reset since the second voltage component can be set to zero from a smaller voltage.
When the modulator is operated with a data format which includes overhead capacity the method advantageously comprises setting the second voltage component to zero during the transmission of the overhead capacity to minimise disrup
Callan Paul A
Claringburn Harry R
Chan Jason
Kirschstein et al.
Marconi Communications Limited
Sedighian M. R.
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