Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-10-27
2001-10-16
Chan, Jason (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C372S029021, C372S031000
Reexamination Certificate
active
06304356
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an optical transmitter comprising a semiconductor laser intended to transmit an optical signal, fed with an AC current called modulating current resulting from the modulation of at least one carrier by an analog or digital signal, and with a DC bias current regulated on the basis of a measurement of the light emitted by the laser.
The invention applies to optical links using a laser modulated by a plurality of carriers forming a frequency multiplex.
An optical transmitter as defined in the opening paragraph above is known from U.S. Pat. No. 4,611,352. According to this document, the DC bias current is adjusted to a threshold level thanks to a regulation system which compensates the variations due to temperature and manufacturing tolerance.
SUMMARY OF THE INVENTION
It is an object of the invention to reduce the impulse noise that is likely to affect an optical link in which a semiconductor laser is used.
Transmitters using a semiconductor laser are rendered imperfect by noise signals which may have various origins. The noise in the received signal is more often than not stationary such as is the case with noise of thermal origin or noise resulting from the granular nature of electricity or light (or quantum noise). However, in certain cases the signal may be rendered imperfect by a noise formed by short pulses whose amplitude and frequency are random. This impulse noise is particularly generated when the laser is modulated by a signal that has sufficiently intense peaks for the current passing through the laser at times to adopt a value lower than the value of the threshold current of the laser, or also when the laser is faintly modulated in which case the impulse noise is caused by the back-scattering of the light injected into the optical fiber; the laser in effect becomes unstable when it is exposed to the back-scattered light and this instability results in the appearance of impulse noise affecting the optical power transmitted by the laser.
In the case of a plurality of signals forming a frequency multiplex, the number and/or amplitude of the multiplexed signals may vary in unforeseen manner in the course of time, the multiplex being, for example, formed by signals transmitted by a certain number of users who are connected or disconnected at instants that cannot be foreseen, so that the number of channels forming the multiplex may vary between zero and a large number. The laser is slightly modulated when a small number of users are active simultaneously, the signal then presents impulse noise.
For solving this problem, the transmitter according to the invention includes means for automatically adjusting the point of operation of the laser as a function of the amplitude of the modulating signal, so that the value of the current inside the laser is, on the one hand, under any circumstances, higher than the minimum value necessary to ensure the absence of impulse noise which could occur in case the peak of a strong signal is limited by the laser and, on the other hand, sufficiently low to ensure a sufficient modulation depth at the level of the laser when the modulating signal has a low amplitude.
In a first embodiment, said means are means for modifying the regulated value of the DC bias current as a function of the value of the modulating signal.
Preferably, said means comprise a peak detector detecting the peak value of the modulating signal, and a regulation device for regulating the transmitted average optical power which is slaved to the voltage delivered by the peak detector, which regulation device for regulating the transmitted average optical power advantageously comprising a transistor used in the variable conductance mode is inserted in a branch of a resistor bridge.
Thus, the device is both effective and simple.
A method frequently used for combatting the impulse noise owing to the back-scattering in the fiber consists of modulating the laser with the aid of a signal that has a sufficient amplitude. In these conditions, the wavelength of the light emitted by the laser in effect varies with the timing of the modulation and, consequently, the laser is exposed to a back-scattered light whose wavelength is continually slightly different from the transmitted wavelength. The result is that the laser is less sensitive to back-scattered light. Therefore, it is known to add to the transmit signal a non-modulated sinusoidal wave (or dummy carrier) having for its aim to prevent the modulation depth of the optical signal dropping below a minimum value which is sufficient to avoid the appearance of impulse noise. This method, however, presents a certain number of disadvantages:
the dummy carrier limits the modulation depth available for the transmit signals and produces, via intermodulation with the payload carriers, beats which degrade the quality of the received signal and form error sources in a digital link,
the presence of the dummy carrier forms an unwanted load for the laser and aggravates the phenomena of peak limiting and impulse noise caused by the peak limiting.
To avoid these disadvantages, a transmitter comprising a generator of an auxiliary carrier further includes means for slaving the amplitude of this auxiliary carrier to the peak value of said modulating signal.
REFERENCES:
patent: 4611352 (1986-09-01), Fujito et al.
patent: 4864649 (1989-09-01), Tajima et al.
patent: 5402433 (1995-03-01), Stiscia
patent: 5488621 (1996-01-01), Slawson et al.
patent: 5638390 (1997-06-01), Gilliland et al.
Dubos Laurent
Dumont Francois
Chan Jason
Leung Christina Y.
Piotrowski Daniel J.
U.S. Philips Corporation
LandOfFree
Optical transmitter for reducing impulse noise in a fiber... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Optical transmitter for reducing impulse noise in a fiber..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical transmitter for reducing impulse noise in a fiber... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2604123