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The field of the invention relates to communication systems and more particularly to optical communication systems. The use of lasers for the transmission of information in communication systems is known.
Such use has typically been limited to amplitude or phase modulated systems that in use often attain a speed of 1. Laser systems in current use for communications e. The ability of a laser to produce a stable optical carrier frequency is inherent in the resonant cavity used in the generation of laser signals.
A resonant cavity of a laser is modified duobinary signaling proteins to amplify optical signals of a desired frequency and attenuate signals of an undesired frequency. The cavity amplifies desired frequencies through use of modified duobinary signaling proteins laser cavity dimensioned in one-quarter wavelength increments. The closer the cavity dimensions are to a desired tolerance, the narrower the range of frequencies within which the laser will operate the linewidth.
The narrower the linewidth, the less inherent phase noise will be transmitted in a laser signal. Further, the narrower the linewidth, the more power is focused into a desired center frequency. While amplitude or intensity modulation is the predominant mode of modified duobinary signaling proteins transmission, modified duobinary signaling proteins progress has been made by using a combination of AM modulation and multiplexing.
For example, systems have been proposed using subcarrier multiplexing SCM of the laser beam as an alternative e. Because a laser is essentially a resonant cavity, it tends to resist changes in frequency.
Direct amplitude modulation by current injection has been the preferred method used in prior systems, thus modulating the laser beam at its source. This has proven only partially successful because of a tendency of the resonant cavity to exclude frequencies outside a low bandwidth regime. Direct AM modulation is typically limited to an information block of less than 1. Because of the importance of optical communications systems a need exists for improved methods of optical modulation.
A method and apparatus are provided for modulating a coherent light source with a digital signal. The method includes the steps of splitting the digital signal into a first and a second copy and mixing modified duobinary signaling proteins first copy with a clock subcarrier signal to provide a first modulation component.
The method further includes the steps of inverting the second copy and delaying the clock signal by one-hundred eighty degrees. The method also includes the steps of mixing the inverted second copy with the delayed clock signal to provide a second modulation component and summing the first and second modulation components. The method also includes a means of extracting the embedded clock subcarrier signal at the receiving end for use modified duobinary signaling proteins heterodyning the received information signal block back to a base band.
As shown, a transmitter 56 located on a first end of the transmission system receives an information signal e. While it has been found that the transceiver i.
Creating a symmetric waveform removes the DC bias component of the signal. This is accomplished in the modified duobinary signaling proteins 56 by splitting the incoming low-pass LP filtered and amplified signal into first and second copies, inverting the first copy, mixing the second copy with a clock signal from a VCO 26, mixing the inverted first copy with a degree modified duobinary signaling proteins version of the clock signal and summing the mixed second copy and clock with the mixed inverted first copy and delayed clock.
The LP filtering and amplification encompasses the entire frequency range from DC up to the fundamental frequency of the data signal. No upper harmonics from modified duobinary signaling proteins data signal remain after the LP filtering and amplification, thus narrowing the required optical fiber spectral bandwidth necessary for transmission. The transmitter accomplishes compression by low-pass filtering and thereby eliminating the fundamental frequency of the digital pulses in the information signal.
Where the information signal is in the radio frequency range under a comparable radio frequency rf format, the device 12 may be a unity-gain amplifier. LP filtering may be modified duobinary signaling proteins to reduce unwanted harmonics and extraneous high-frequency noise. For example in the case of a OC signal, low-pass filtering may be performed with a cut-off frequency of no more than 10 MHz above that of the data frequency of MHz. From the low-pass filter 16, the shaped, filtered and amplified information signal is processed in a signal divider 18 into two signals.
An appropriate amplification factor e. Following signal division, one output of the signal divider 18 is inverter in an modified duobinary signaling proteins Since the output of the previously discussed signal divider modified duobinary signaling proteins is either a positive value or zero, the result of the inversion in the inverted 20 is a series of negative-going pulses. Following inversion of one of the divided signals, the two signals are frequency upshifted within multipliers 24, 28 using a clock subcarrier signal from a voltage controlled oscillator VCO While the positive, component signal is multiplied directly in a first multiplier 28, the inverted divided signal is multiplied by a phase delayed version of the clock subcarrier signal from the VCO The VCO 26 may be any tunable, but stable high frequency reference e.
For example, phase-locked loop PLLs frequency multipliers are known which use integer dividers in a feedback path and a VCO in a feedforward path to generate stable multiples of a base clock frequency. Alternatively, fractional frequency dividers using sigma-delta modulation of the feedback divider may be used for the generation of sub-integer multiples of the base frequency. Multiplication within the multiplier 28 of the positive, divided signal from the divider 18 by the clock signal results in the output of a positive-going pulse whenever the information signal is at a high level.
The multiplied positive pulse is transferred to a summer The phase delay introduced within the phase delay device 22 is substantially equal to degrees. The phase delay device 22 may be any appropriate phase-shifting device e. Since the phase delayed clock signal is offset by modified duobinary signaling proteins, the phase delayed clock pulses are non-coincident i.
Further, since the phase delayed clock pulses are multiplied by a negative-going information signal, the output of the second multiplier 24 is a series of negative-going pulses. When each positive-going pulse from the first multiplier 28 is summed with a delayed negative-going pulse from the second multiplier 24, the result is an approximation of one cycle of a sine wave. The net result of the summation is that whenever a positive value is present on the input of the divider 18, a sine wave at the clock frequency of the VCO 26 is present at the output of the summer 30 and is ultimately applied to the laser modulator Whenever a zero value is present on the input to the divider 18, a zero value is applied to the modulator For example, in the case of an OC signal with a MHz data clock a minimum clock subcarrier frequency of 2.
The laser modulator 36 may be any electro-optical device capable of modulating an optical signal with a controlling electrical signal through refractive synchronization. For example lithium niobate LiNbO 3 or gallium arsenide GaAs crystals are known to have such characteristics. SuitaDle devices for use with the modulator 36 include those devices with a suitable radio frequency RF interface and internal laser source or optical interface for an external laser source e.
Optical refractive synchronization is a process whereby a controlling clock subcarrier and information signal are used to modulate in optical signal. Fundamentally, the process of refraction is the change of energy, direction or speed of a light beam which is propagating through a medium.
In a first case, the change in direction may be a continuous bending of the light beam and of the subsequent change of the speed of light in the medium which is referred to as the index of refraction of the material.
In a second case, there is an abrupt change in the index, polarity, or phase of the medium which directs the energy out of the medium, or changes the ability of light to pass through the medium, thereby absorbing the energy. This second case is the technique employed in optical refractive synchronization. The light is passed through a crystal which has the refractive index changed abruptly, by imposition of an electric field, by passing a controlling signal through the crystal, which then causes the crystal to allow the light to pass through, or be absorbed in the crystal at the frequency of the controlling signal.
This optical modulation is performed, therefore, through control of the refraction and polarization signal of the optical crystal. By applying the output of the summer 30 through the amplifier 32 to the modulator 36, the coherent light output of the laser 34 is effectively modulated by the sampled information signal.
The modulated laser beam may then be transmitted to a distant receiver 58 over an intervening fiber optic cable Within the receiver 58, the modulated laser beam is first detected in an appropriate photonic detector e.
Following detection, the detected signal may be amplified within an amplifier 42 and applied to another splitter Within the splitter 44, the detected amplified signal may be divided into a pair of identical signals. The signals at this point are still modulated by the original information signal and the clock signal from the VCO The first of the pair of identical signals is applied to a notch filter The notch filter 46 functions to recover the clock signal provided by the VCO The notch filter 46 does this by passing frequencies within the notch i.
The notch filter would be expected to have an appropriate bandwidth tailored to the application e. Once isolated, the recovered clock subcarrier signal is amplified in an amplifier 48 and mixed with the second of the pair of identical signals in a mixer Mixing the recovered subcarrier clock signal with the second signal in the modified duobinary signaling proteins 50 effectively recovers the information signal. For example, mixing two signals together provides sum and difference signals.
Mixing the recovered subcarrier clock with the detected signal containing the subcarrier clock with a superimposed information signal results in a summation signal i. Since the summation signal lies at a relatively high frequency, it may be easily attenuated. Following shaping, the signal may be converted back to an original format. A specific embodiment of a method and apparatus for modulating a coherent light beam according to the present invention has been described for the purpose of illustrating the manner in which the invention is made and used.
It should be understood that the implementation of other variations and modifications of the invention and its various aspects will be apparent to one skilled in the art, and that the invention is modified duobinary signaling proteins limited by the specific embodiments described. Therefore, it is contemplated to cover the present invention any and all modifications, variations, or equivalents that fall within the true spirit and scope of the basic underlying principles disclosed and claimed herein.
The method includes the steps of splitting the digital signal into a first and a second copy and mixing the first copy with a clock signal to provide a first modulation component.
What is claimed is: A method of modulating modified duobinary signaling proteins coherent light source with a digital signal, such method comprising the steps of: The method of modulating as in claim 1 further comprising defining the digital signal as being one of a synchronous transport signal STS-n or an optical carrier level n OC-n signal.
The method of modulating as in claim 2 further comprising low-pass filtering the STS-n or OC-n signal modified duobinary signaling proteins advance of splitting. The method of modulating as in claim 3 wherein the step of low-pass filtering further comprises providing a filter cut-off frequency at substantially MHz above a bit rate of the STS-n or OC-n signal. The method of modulating as in claim 5 further comprising defining the clock subcarrier frequency at substantially 2.
The method of modulating as in claim 1 further comprising defining the coherent light source as a laser beam. The method of modulating as in claim 7 further comprising modulating the laser beam with the summed first and second modulation components.
The method of modulating as in claim 8 wherein the step of modulating the laser beam further comprises refractively synchronizing the laser beam with the summed first and second modulation components. The method of modulating as in claim 9 wherein the step of refractively synchronizing the laser beam further comprises routing the laser beam through a lithium niobate crystal.
The method of modulating as in claim 9 wherein the modified duobinary signaling proteins of refractively synchronizing the laser beam further comprises applying the summed first and second modulation components to an RF input of the lithium niobate crystal.
Apparatus for modulating a coherent light source with a digital signal, such apparatus comprising: The apparatus for modulating as in claim 12 further comprising means for providing the digital signal under one of a synchronous transport signal, level n, STS-n format or an optical carrier level n OC-n signal format.
The apparatus for modulating as in claim 13 further comprising means for low-pass filtering the STS or OC-n signal in advance of splitting. The apparatus modified duobinary signaling proteins modulating as in claim 14 wherein the means for low-pass filtering modified duobinary signaling proteins comprises means for providing a filter cut-off frequency at substantially 10 MHz above a data frequency of the STS-n or OC-n signal. The apparatus for modulating as modified duobinary signaling proteins claim 16 further comprising means for defining the clock frequency at substantially 2.
The apparatus for modulating as in claim 17 further modified duobinary signaling proteins means for modulating the laser beam with the summed first and second modulation components. The apparatus for modulating as modified duobinary signaling proteins claim 18 wherein the means for modulating the laser beam further comprises means for refractively synchronizing the laser beam with the summed first modified duobinary signaling proteins second modulation components.