Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
1999-11-24
2001-11-20
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S239000, C359S279000, C359S325000, C385S027000
Reexamination Certificate
active
06320692
ABSTRACT:
The present invention relates to a method of modulating an optical signal, and an optical emitter with external modulation.
The present invention also relates to a system for controlling the operating point of an optical modulator used in CATV systems.
The distribution of television signals in CATV (community antenna television) systems can be carried out by means of optical fibres. The optical signal can be modulated in a direct way, by acting on the optical source, normally a laser, or in an indirect way, by means of an optical modulator.
An optical modulator which can be used for the amplitude modulation of an optical signal, with a modulating signal at radio frequency (RF) having very high frequencies (such as the carriers of television channels, which conventionally range from 40 to 860 MHz), consists, for example, of a device based on an interferometer of the Mach-Zehnder type, constructed on lithium niobate (LiNbO
3
). The 40-860 MHz band is considered to be flat within ±0.5 dB. The first channel is at 40 MHz and the last channel is at 860 MHz.
A required characteristic of an external modulator is linearity of modulation, which is very important, in particular, for use in CATV systems with transmission of the analog type, consisting of numerous television channels, for example from 40 to 80 channels.
The electro-optical characteristic, in other words the output optical power as a function of the input radio-frequency voltage, of modulators of the type mentioned is typically non-linear. To limit the signal distortion, it is desirable to make the modulator working in the vicinity of a section of the characteristic which is as linear as possible.
For this purpose, a radio-frequency modulating signal is applied to an RF electrode of an electro-optical modulator of the Mach-Zehnder interferometer type, and a continuous bias voltage, which determines the operating point of the modulator, is applied to the same electrode or to a second electrode.
An example of a modulator of this type is marketed by the applicant under the symbol PIR PIM1510.
The modulating signal, applied to the RF input, consists, for example, of the set of modulated carriers of the television channels to be distributed to the users.
In the case of a Mach-Zehnder modulator, the form of the characteristic approximates to a sinusoid, and it is advantageous for the modulator to operate in the vicinity of the inflection point of the sinusoid, with an applied voltage VQ which is that of the operating point.
The modulation characteristic of the Mach-Zehnder modulator, with respect to the operating point, can be expressed by the relation:
Pu=KZ
sin &bgr;
where
Pu is the output optical power,
KZ is a coefficient which depends on the characteristic of the Mach-Zehnder modulator, p
1
&bgr;=&pgr;Vm/V&pgr; is the modulation index of the modulating signals, expressed in radians,
Vm is the modulation voltage,
V&pgr; is a constant.
This characteristic with a sinusoidal form is identified by two values:
the voltage V&pgr; (shown in FIG.
3
), which represents the variation of voltage to be applied to the RF input to change the optical power from the maximum to the minimum value;
the voltage VQ which represents the voltage to be applied to the bias electrode, to make the operating point correspond to the inflection point of the characteristic which has a sinusoidal form, that is it, has odd symmetry. In this case, the even-order distortions are cancelled and the odd-order distortions take on a clearly defined value.
For example, in the case of a Mach-Zehnder modulator of the PIR PIM1510 type, produced by the applicant, the aforesaid voltages may have the following values:
V&pgr;=
4.3 V
and
VQ=
0.7 V.
The value of the biasing point voltage VQ is not constant, but varies with time, for example as a result of the accumulation of static charges in the LiNbO
3
, and also with temperature.
The biasing voltage must therefore be adjusted continually, using as information, for example, the presence and amplitude of the even-order distortions, in other words the second-order intermodulation products, indicated in a general way as CSO (composite second order). The total level of the CSO must be kept suitably low to obtain good quality in the signals distributed to the users, for example more than 65 dB below the level of the vision carrier of each channel. Even when the modulator is working at the aforesaid biasing point, which minimizes the even-order distortions, the non-linearity of the input-output characteristic causes residual odd-order distortions, principally of the third order, which are manifested in intermodulation products or CTB (composite triple beats) and cross modulation (XMOD), which degrade the quality of the signals reaching the users. This is because the quality of the television channels with amplitude modulation of the carrier is strongly affected by the presence of the aforesaid intermodulation products.
To limit these distortions to some extent, it is desirable to select a depth of modulation which is not too great for the modulating signals (the term “depth of modulation” denoting the maximum value, expressed as a percentage, of the modulation index &bgr;, for example approximately 3.5% or 4% per channel, in order to operate as near as possible to the linear portion of the characteristic.
In order to limit the distortion introduced by the non-linearity of the modulator characteristic, the maximum percentage utilization of the modulator (relative to the percentage modulation) is normally approximately 40%. This is because the maximum number of channels which can be applied, allowing for the aforesaid requirements in terms of noise characteristics, is calculated by assuming that the various carriers, being non-correlated, are added statistically in quadrature; the sum of the channels is therefore considered in terms of power. Consequently, in order not to exceed the aforesaid percentage, the maximum number of channels that can be applied to the modulator is approximately 100.
To reduce the residual distortions, and in particular the third-order distortions, has been suggested of pre-distorting the modulating signals by means of non-linear elements, so that the pre-distortion is compensated by the subsequent distortion introduced by the modulator. For this purpose, it is possible to connect before the modulator a distortion circuit, having an input-output characteristic for radio-frequency signals which is the inverse function of the input-output characteristic of the modulator. This method is disclosed, for example, in the article by M. Nazarathy et al., “Progress in Externally Modulated AM CATV Transmission Systems”, published in the Journal of Lightwave Technology, vol. 11, No. 1, 01/93, pp. 82-104.
Therefore, when a suitable depth of modulation has been selected for the modulating signals, the modulator must be provided with suitable circuits which enable two functions to be performed:
a) application of the bias voltage and its maintenance at the required value, to make the modulator operate at the inflection point of the sinusoidal characteristic;
b) linearization of the characteristic of the modulator, by means of pre-distortion circuits.
It is possible to apply, together with the bias voltage, a sinusoidal signal, called “pilot tone”, having a predetermined amplitude and frequency (f
1
) (generally much lower than the minimum frequency of the radio-frequency signal band), and to detect, by means of a suitable circuit, the presence in the output optical signal of the even-order harmonics of the pilot tone (for example, the second harmonic,
2
f
1
). The bias voltage is adjusted in such a way as to minimize the value of this second harmonic.
This method provides adequate stabilization of the bias point, corresponding to CSOs of the order of 65-70 dB.
It requires a large depth of modulation for the pilot tone (approximately 10%, according to the experiments conducted by the applicant), so that disturbances associated with the noises, especially that which is intrins
Epps Georgia
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Lucas Michael A.
Pirelli Cavi E Sistemi S.p.A.
LandOfFree
Biasing system for an optical modulator with double output does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Biasing system for an optical modulator with double output, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Biasing system for an optical modulator with double output will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2590907