Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2003-02-04
2004-06-08
Dang, Hung (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
Reexamination Certificate
active
06747778
ABSTRACT:
The present invention concerns a method of, and apparatus for, generating pulse width modulation (PWM) signals. More especially the invention concerns controlling temperatures of electro-optical components such as attenuators, filters and solid state lasers. For use in optical communication. Moreover, although not exclusively, the invention concerns an optical attenuator with an enhanced resolution for use in an optical communication system.
It is conventional practice to employ optical attenuators in optical communication systems for regulating and controlling the power of optical radiation propagating within the systems. Such attenuation is necessary in order to avoid saturating sensitive optical components such as detectors and optical amplifiers, as well as ensuring that optical radiation is of sufficient power not to be swamped by noise. Saturation can lead to loss of information and hence errors in communication traffic conveyed by the systems.
Conventional optical attenuators employ a number of different optical component configurations, for example they can comprise one or more of Mach-Zehnder interferometers, modulated liquid crystal shutters and dispersion effect modulators. In communication systems, it is particularly convenient to employ thermally variable optical attenuators whose optical attenuation is determined by attenuator temperature. Thus, attenuation can be selected in these thermally variable attenuators by adjusting their temperature.
Temperature adjustment is conveniently achieved by including thermoelectric elements into the variable attenuators. Such elements function by the Seebeck effect and can selectively cool or heat attenuation determining optical components incorporated within the attenuators. However, the elements often consume significant power in operation, for example 2.5 Watts corresponding to an electrical drive signal of 5 volts potential at 0.5 amps current.
Conventional optical communication systems are typically configured as a plurality of nodes interconnected by optical fibre waveguides through which communication traffic bearing. optical radiation propagates from one node to another. The nodes often comprise a considerable array of optical and electrical signal processing equipment usually arranged into equipment racks, for example conventional 19-inch racks. The equipment typically incorporates numerous examples of the aforementioned thermally variable attenuator. On account of inclusion of such examples, thermal power dissipation from the attenuators can represent a considerable thermal load in the equipment racks requiring cooling facilities, for example fans for providing cooling airflow through the racks.
The inventors have appreciated that, whereas it is not feasible to reduce thermal dissipation within the attenuators because such dissipation is dictated by fundamental characteristics of their associated thermoelectric elements, it is beneficial to reduce power dissipation within electrical driver circuits which provide power to the attenuators. It is known practice when driving thermoelectric elements to regulate the drive current using a conventional circuit comprising linear non-switching components such as series regulating bipolar power transistors driven by conventional analogue operational amplifiers. Such a circuit suffers a drawback that power dissipation within the power transistors can approach power dissipation occurring within their associated thermoelectric element. In order to address this drawback, the inventors have devised a circuit for driving a thermoelectric element of a thermally variable optical attenuator wherein the circuit employs pulse width modulation (PWM) techniques for generating a drive signal for driving the thermoelectric element, the circuit exhibiting reduced power dissipation compared to the aforementioned conventional circuit. However, the inventors have found that such PWM techniques provide insufficient resolution of attenuator temperature control when the drive signal is synthesised digitally in a known manner. Such insufficient resolution gives rise to corresponding lack of resolution of optical attenuation which creates problems in associated communication systems.
As is known a conventional PWM signal comprises a stream of repetitive pulses, each pulse having a duration t
p
and separated from neighbouring pulses thereto by a null period of duration t
n
. Thus, the pulses are repeated at a period of t
p
+t
n
and an average value V of the PWM signal is given by Equation 1 (Eq. 1):
V
=
(
A
-
B
)
·
t
p
(
t
n
+
t
p
)
+
B
Eq
.
1
where
A=signal value during the pulses; and
B=signal value during the null period.
Moreover, the pulses have a repetition frequency f
p
determined by Equation 2 (Eq. 2):
f
p
=
1
(
t
n
+
t
p
)
Eq
.
2
In contemporary pulse width modulator design, pulses are often generated by digital counter circuits operating at a clocking frequency f
clk
of a master clock. As a consequence of employing such digital circuits, the durations t
p
and t
n
can only be modified in discrete steps, the number of steps M being determinable from Equation 3 (Eq. 3):
M
=
f
clk
f
p
Eq
.
3
In order to increase the number of steps M, either f
clk
must be increased or a lower pulse repetition frequency f
p
must be accepted. In some applications, temporal variations in the value V can cause problems and thereby sets a lower limit on f
p
. A conventional approach when enhanced PWM resolution is desired is to use a higher master clock frequency f
clk
; such an approach results in greater cost associated with the digital circuits and also greater operating power dissipation therein. Moreover, there are practical limits to the frequency at which digital circuits can be clocked.
The inventors have appreciated that the number of steps M can effectively be increased by grouping the pulses into frames of F pulses where one or more pulses of each frame are made to have one step greater time duration than other pulses in the frame. Such a pulse frame technique increases the number of resolution steps to a value MF and results in a total temporal fluctuation not exceeding one step. Moreover, the inventors have appreciated that there are particular approaches to selecting the one or more pulses to be made one step greater which result in a relatively low harmonic content in a PWM signal thereby generated. Reducing harmonic content is important where the PWM signal is used to control appreciable current, for example, within an optical communication system where it is important to suppress interference between electronic assemblies resulting from PWM current surges.
The present invention has arisen in an endeavour to provide an optical attenuator and associated control circuit which provide power efficiency attributable to PWM operation but a variable attenuation of sufficient resolution for use in optical communication systems.
According to a first aspect of the invention there is provided a method of generating a pulse width modulated (PWM) signal in response to a digital demand data word which comprises a plurality of bits, the method characterised by: generating the PWM signal comprising a sequence of frames, each frame comprising a train of PWM pulses whose duty cycle is substantially governed by a plurality of the more significant bits of the demand data and in response to each of a plurality of the less significant bit of the demand data, modifying at least one of the PWM pulses, wherein the number of PWM pulses being modified and their position within the frame are selected such as to uniquely map each less significant bit onto associated PWM pulses.
The method provides the advantage that it is capable of providing a PWM signal having a reduced harmonic content in comparison to conventional methods of generating PWM signals.
Preferably the method comprises generating the PWM signal from a clock signal and is further characterised by modifying the duty cycle of the PWM pulses by an amount corresponding to a single clock cycle.
I
Brown Roger W
Butler Graham
Dennis Earl A
Leach Michael J
Vass Darren W
Dang Hung
Kirschstein et al.
Marconi Communications Limited
Martinez Joseph
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