US20080075469A1 - Optical transmitter with sbs suppression - Google Patents
Optical transmitter with sbs suppression Download PDFInfo
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- US20080075469A1 US20080075469A1 US11/566,118 US56611806A US2008075469A1 US 20080075469 A1 US20080075469 A1 US 20080075469A1 US 56611806 A US56611806 A US 56611806A US 2008075469 A1 US2008075469 A1 US 2008075469A1
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/50—Transmitters
- H04B10/501—Structural aspects
- H04B10/503—Laser transmitters
- H04B10/505—Laser transmitters using external modulation
- H04B10/5059—Laser transmitters using external modulation using a feed-forward signal generated by analysing the optical or electrical input
- H04B10/50597—Laser transmitters using external modulation using a feed-forward signal generated by analysing the optical or electrical input to control the phase of the modulating signal
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/25—Arrangements specific to fibre transmission
- H04B10/2507—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion
- H04B10/2537—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion due to scattering processes, e.g. Raman or Brillouin scattering
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/50—Transmitters
- H04B10/501—Structural aspects
- H04B10/503—Laser transmitters
- H04B10/505—Laser transmitters using external modulation
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/50—Transmitters
- H04B10/501—Structural aspects
- H04B10/503—Laser transmitters
- H04B10/505—Laser transmitters using external modulation
- H04B10/5059—Laser transmitters using external modulation using a feed-forward signal generated by analysing the optical or electrical input
- H04B10/50593—Laser transmitters using external modulation using a feed-forward signal generated by analysing the optical or electrical input to control the modulating signal amplitude including amplitude distortion
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S2301/00—Functional characteristics
- H01S2301/03—Suppression of nonlinear conversion, e.g. specific design to suppress for example stimulated brillouin scattering [SBS], mainly in optical fibres in combination with multimode pumping
Definitions
- FIG. 7 illustrates a 500 kHz span of a frequency spectra generated by a commercially available CATV receiver that received an optical signal that was generated by an optical transmitter according to the present invention with SBS suppression compensation.
- the first output 114 of the SBS suppression signal generator 112 is electrically connected to an input 118 of the laser bias power supply 106 .
- the transmitter 100 also includes an optical modulator 120 that is optically coupled to the output 104 of the laser 102 .
- the optical modulator 120 modulates the optical signal that is generated by the laser 102 .
- the present invention is described with embodiments that use external modulation. However, the present invention can be practiced with any type of modulation.
- the signal processor 132 generates a modified SBS suppression signal that includes a signal that is a mathematical transform of the SBS suppression signal.
- the mathematical transform is chosen to cause vector cancellation of the SBS suppression signal.
- the mathematical transform is chosen to produce a pre-distortion that compensates for modulation non-linearities caused by the laser 102 and/or the modulator 120 .
- the signal processor 132 generates a modified SBS suppression signal that includes a signal having a harmonic of the SBS suppression signal.
- FIG. 3 shows an embodiment of an integrated optical transmitter subassembly 300 that can be used to generate an optical signal with SBS suppression according to the present invention.
- the transmitter subassembly 300 is an opto-electronic integrated circuit.
- the transmitter subassembly 300 comprises discrete components.
- the payload modulation signal has SBS suppression resulting from the increased line width produced by the SBS suppression signal.
- the payload modulation signal also has reduced AM noise because the optical signal is gain modulated by the modified SBS suppression signal.
- the payload modulation signal has a relatively high signal-to-noise ratio because the full modulation depth of the EA diode 316 is available for the payload.
Abstract
Description
- Optical fiber communication systems are lightwave systems that employ optical fibers to transmit information. Optical fiber communications systems include optical transmitters, optical receivers, and transmission media that propagate information between the optical transmitters and the optical receivers. An optical transmitter for an optical fiber communication system includes an optical source, such as a semiconductor laser, that generates an optical signal and an optical modulator that modulates the optical signal with data or voice information. The modulated optical signal is transmitted through a transmission media, such as an optical fiber, to an optical receiver. The optical receiver detects the transmitted optical signal and processes the optical signal into an electronic waveform that contains the data or voice information.
- Optical fiber communication systems are now widely deployed. Recently, relatively new communications services, such as the Internet, high-speed data links, video services, wireless services and CATV, have resulted in a dramatic increase in the need for higher information data rates. The aggregate data throughput rate of a communication system can be increased either by increasing the bandwidth of an individual data channel or by increasing the number of data channels.
- In addition, many optical fiber communication systems today are being built to transmit data over long distances with high data rates. Moreover, such systems are currently being built to transmit data and voice information over these longer distances without employing repeaters in order to reduce the capital and operating costs associated with transmitting data. In order to achieve these higher data rates and longer transmission distances, optical signals having relatively narrow line widths must be transmitted at relatively high intensity.
- The noise detected at the receiver increases as the bandwidth of an individual channel is increased. The amount of optical power in the transmitted carrier signal must be increased to maintain a sufficient signal-to-noise ratio at the receiver as the bandwidth of an individual channel is increased. However, the propagation distance that can be achieved using a carrier signal with increased power in a narrow line width is severely limited by a physical effect known as stimulated Brillouin scattering (SBS).
- Stimulated Brillouin scattering is a stimulated scattering process that converts a forward traveling optical wave into a backward traveling optical wave that is shifted in frequency relative to the forward traveling optical wave. Backward scattering occurs within optical fibers because of coupling between acoustic phonons created by vibrational excitation of acoustic modes in the optical fiber material itself and by the incident photons of the optical signal.
- The acoustic phonons and photons generate transient gratings that produce backward scattering and frequency shifting of the incident optical signals. The frequency shifting is typically between about 10-100 MHz for commonly used optical communication fibers. Stimulated Brillouin scattering also causes multiple frequency shifts. In addition, SBS can permanently damage the optical fiber if the optical propagating power is sufficiently high.
- The transmission quality of optical signals having relatively high intensity and narrow line width can be improved by reducing the effects of SBS. The increase in the transmission quality can allow data and voice service providers to increase the optical signal power level, and therefore, increase the possible propagation distance of their communication links between repeaters. Consequently, reducing the effects of SBS can reduce the cost per bit to transmit data and voice information.
- This invention is described with particularity in the detailed description. The above and further advantages of this invention may be better understood by referring to the following description in conjunction with the accompanying drawings, in which like numerals indicate like structural elements and features in various figures. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.
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FIG. 1 illustrates a block diagram of an optical transmitter that suppresses SBS by increasing the bandwidth of the optical signal and by reducing amplitude modulation noise according to the present invention. -
FIG. 2 illustrates a block diagram of an embodiment of a SBS suppression signal generator that generates a SBS suppression signal that increases the line width of the optical signal according to the present invention. -
FIG. 3 illustrates an embodiment of an integrated optical transmitter subassembly that can be used to generate an optical signal with SBS suppression according to the present invention. -
FIG. 4 illustrates another embodiment of an integrated optical transmitter subassembly that can be used to generate an optical signal with SBS suppression according to the present invention. -
FIG. 5 illustrates an embodiment of a fiber laser optical transmitter module that can be used to generate an optical signal with SBS suppression according to the present invention. -
FIG. 6 illustrates a 500 kHz span of a frequency spectra generated by a commercially available CATV receiver that received an optical signal that was generated by an optical transmitter according to the present invention without SBS suppression compensation. -
FIG. 7 illustrates a 500 kHz span of a frequency spectra generated by a commercially available CATV receiver that received an optical signal that was generated by an optical transmitter according to the present invention with SBS suppression compensation. - Optical fibers used for communications exhibit stimulated Brillouin scattering (SBS) at optical signal power levels that are as low as ˜1 mW in some optical fibers.
- The threshold optical power that causes SBS can be expressed by the following equation:
-
/hd th ≅21(α/GB) - where the parameter ax represents absorption in the optical fiber and the parameter GB represents the peak gain, which is approximately 5×1031 11 m/W for narrow-bandwidth signals used for communications. The peak gain decreases as the incident optical signal bandwidth increases. For example, an optical fiber having an effective area of 50 μm2, and having an absorption coefficient α≅0.2 dB/km, will exhibit a threshold optical power level which causes SBS that is approximately 2.4 mW for an optical fiber length that is approximately 20 km.
- Optical power levels that exceed the threshold optical power will cause the SBS to rapidly rise until the SBS limits the power that can be transmitted through the optical fiber. When the SBS limits the power that can be transmitted through the optical fiber, the power transmitted forward through the optical fiber will become nearly independent of the power of the incident optical signal.
- One method of suppressing SBS in optical fibers is to vary the phase angle of the optical signal with time in order to impose recurrent phase deviations that suppress SBS. Another method of suppressing SBS is to increase the effective optical source line width by using a carrier waveform that has multiple frequencies. For example, a single optical source can be configured to generate two longitudinal optical modes with slightly different wavelengths that produce a beat frequency that increases the effective optical source line width.
- Another method of suppressing SBS is to increase the line width of the optical source by modulating the broadened optical signal with a noise source. The noise source can be frequency or phase modulated. The bandwidth of the noise and/or the optical modulation index is controlled to provide a desired line width for the broadened optical signal. For example, one method of suppressing SBS generates white noise and extracts a component of the white noise having a predetermined frequency band. The component of white noise is then superimposed on the bias current of the optical source to widen the line width of the optical source.
- Another method of suppressing SBS is to tune the optical source with a dither signal in order to increase the line width of the optical source. For example, one method of suppressing SBS uses a resonant cavity distributed feedback (DFB) laser with an external modulator to superimpose a dither signal onto the optical signal.
- This method is undesirable because it requires a relatively complex and expensive laser.
- These methods of increasing the line width of the optical source can have the undesirable effect of generating residual signals or noise in the frequency band of interest or generating residual signals or noise as intermodulation frequency products. These undesirable residual signals or noise can be reduced or eliminated by applying a dither signal to the laser bias drive signal that has a frequency which is outside of the frequency band of interest. However, modern broadband optical communication systems typically require the use of a microwave frequency dither signal in order to reduce these residual signals or noise. Including such a microwave dither signal generator in the communication system is undesirable because it can significantly increase the overall cost of the system and can also cause undesirable electromagnetic interference.
- The methods and apparatus of the present invention suppress SBS in optical fiber transmission systems by increasing the line width of the optical signal and by reducing amplitude modulation (AM) noise. Suppression of stimulated Brillouin scattering is achieved by generating a SBS suppression signal that can be a single coherent dither tone or any combination of signals that increases the optical signal line width, such as noise, pseudorandom noise sequences or other line width increasing techniques. The SBS suppression signal can have a bandwidth that is within or outside the data transmission bandwidth. Reducing the AM noise is achieved by modulating a cancellation signal.
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FIG. 1 illustrates a block diagram of anoptical transmitter 100 that suppresses SBS by increasing optical signal bandwidth according to the present invention and by reducing amplitude modulation noise. The term “optical transmitter” is defined herein to mean all devices (sources, modulators, amplifiers and processors) that prepare an optical signal for transmission through a transmission media. Thetransmitter 100 includes alaser 102 that generates an optical signal at anoutput 104. Thelaser 102 can be any type of laser that generates the appropriate optical signal and that is responsive to an electrical bias control signal. - The
optical transmitter 100 includes a laserbias power supply 106 having anoutput 108 that is electrically coupled to abias input 110 of thelaser 102. The laserbias power supply 106 generates a bias signal at theoutput 108 that biases thelaser 102 at the appropriate operating point so that it emits the optical signal at the desired wavelength and at the desired power level. In one embodiment, a detector (not shown) monitors thelaser 102 and generates a feedback signal that is used to change the bias signal that is generated by the laserbias power supply 106 in order to control the wavelength and/or power of the optical signal that is generated by thelaser 102. - The
optical transmitter 100 also includes a SBSsuppression signal generator 112 that generates a SBS suppression signal at a first 114 and asecond output 116. The SBSsuppression signal generator 112 generates a signal that broadens the line width of the optical signal that is generated by thelaser 102. The SBS suppression signal generator according to the present invention can generate any type of signal that broadens the line width of an optical signal. There are numerous types of signals that are known to broaden the line width of an optical signal. For example, the SBS suppression signal can comprise a wide range of frequency spectra that can include random noise, pseudo random noise, and discrete tones. - The SBS
suppression signal generator 112 can be any type of signal generator that generates a signal that broadens the line width of the optical signal that is generated by thelaser 102. For example, the SBSsuppression signal generator 112 can include a random noise source, a deterministic noise source, such as a pseudorandom sequence source, a dither signal generator, or a coherent signal sources. In one embodiment, the SBSsuppression signal generator 112 includes a filter (not shown) that limits the bandwidth of the SBS suppression signal. In one embodiment, the SBSsuppression signal generator 112 includes a modulator (not shown) that generates a modulation signal that broadens the line width of the optical signal. An exemplarily embodiment of the SBSsuppression signal generator 112 is described in connection withFIG. 2 . - The
first output 114 of the SBSsuppression signal generator 112 is electrically connected to aninput 118 of the laserbias power supply 106. Thetransmitter 100 also includes anoptical modulator 120 that is optically coupled to theoutput 104 of thelaser 102. Theoptical modulator 120 modulates the optical signal that is generated by thelaser 102. The present invention is described with embodiments that use external modulation. However, the present invention can be practiced with any type of modulation. - The
optical modulator 120 can be any type of optical modulator that is responsive to an electrical modulation signal. In one embodiment, theoptical modulator 120 is an electro-absorption modulator. In another embodiment, theoptical modulator 120 is a Mach Zehnder-type interferometric modulator. In still another embodiment, theoptical modulator 120 is an optical gain modulator, such as a semiconductor optical amplifier (SOA) or other gain modulating device. - A
modulation signal generator 122 generates a payload modulation signal at anoutput 124. In some embodiments, themodulation signal generator 122 also generates a DC bias offset voltage that biases theoptical modulator 120 at the appropriate operating point. In the embodiment illustrated inFIG. 1 , anoutput 126 of a separate DC biasvoltage power supply 128 is electrically coupled to abias input 130 of theoptical modulator 120. - The
optical transmitter 100 also includes asignal processor 132 that has aninput 134 that is electrically coupled to theoutput 116 of the SBSsuppression signal generator 112. Thesignal processor 132 processes the SBS suppression signal that is generated by the SBSsuppression signal generator 112 and produces a modified SBS suppression signal at anoutput 136. The modified SBS suppression signal is a signal that is used to reduce or suppress amplitude modulation (AM) noise caused by the SBS suppression signal that is used to spread the line width of the optical signal generated by thelaser 102. The modified SBS suppression signal, however, maintains the desired line spreading in the optical signal. The term “AM noise” is defined herein to mean undesirable amplitude modulation or residual signals. - A
summation circuit 140 is used to combine the payload modulation signal that is generated by themodulation signal generator 122 with the modified SBS suppression signal that is generated by thesignal processor 132. In the embodiment shown inFIG. 1 , theoutput 124 of themodulation signal generator 122 is electrically coupled to afirst input 142 of thesummation circuit 140. Theoutput 136 of thesignal processor 132 is electrically coupled to asecond input 144 of thesummation circuit 140. Anoutput 146 of thesummation circuit 140 is electrically coupled to aninput 148 of themodulator 120. In some embodiments (not shown), the processed SBS suppression signal and the payload modulation signal are electrically coupled to different inputs (not shown) of theoptical modulator 120. In other embodiments, the processed SBS suppression signal and the payload modulation signal are electrically coupled to separate gain or loss elements as shown inFIG. 3 . - In operation, the laser
bias power supply 106 generates a laser bias signal that is sufficient to cause thelaser 102 to generate the optical signal with the desired wavelength and power level. The SBSsuppression signal generator 112 generates a SBS suppression signal that increases the bandwidth of the optical signal that is generated by thelaser 102. For example, the SBS suppression signal can be a bandwidth limited noise signal, a discrete tone, or a combination of noise signals and discrete tones. In one embodiment, aphase shifter 138 shifts the phase of the SBS suppression signal. - The laser bias signal is combined with the SBS suppression signal and then the combined signal is coupled to the
input 110 of thelaser 102. The combined laser bias signal and SBS suppression signal drives thelaser 102 to generate an optical signal having increased bandwidth at theoutput 104. - The
signal processor 132 processes the SBS suppression signal and generates a modified SBS suppression signal at theoutput 136. In one embodiment, thephase shifter 138 applies a phase shift to the SBS suppression signal and thesignal processor 132 processes the phase-shifted SBS suppression signal and generates the modified SBS suppression signal at theoutput 136. In one embodiment, thesignal processor 132 shifts the phase of the SBS suppression signal. Phase shifting can be used to achieve vector cancellation that compensates for an imperfect match of the phase response between the optical signal that is generated by thelaser 102 and the optical signal modulated by themodulator 120. - In one embodiment, the modified SBS suppression signal has a signal bandwidth that is within the frequency range of the payload modulation signal. In another embodiment, the modified SBS suppression signal has a signal bandwidth that is outside the payload modulation signal bandwidth.
- In one embodiment, the modified SBS suppression signal includes a cancellation signal that reduces or substantially cancels AM noise in the optical modulation signal that is caused by the SBS suppression signal. For example, in one embodiment, the modified SBS suppression signal is a complementary SBS suppression signal. In this embodiment, the
signal processor 132 generates a modified SBS suppression signal that includes an inverted replica of the SBS suppression signal. The term “replica” is defined herein to mean a substantially exact or approximate copy of the signal that causes approximate vector cancellation of the SBS suppression signal. - In another embodiment, the
signal processor 132 generates a modified SBS suppression signal that includes an inverted replica of the SBS suppression signal that is phase shifted relative to the SBS suppression signal that is used to drive thelaser 102. In yet another embodiment, thesignal processor 132 generates a modified SBS suppression signal that includes an inverted replica of the SBS suppression signal that is scaled in amplitude relative to the SBS suppression signal that is used to drive thelaser 102. - In other embodiments, the
signal processor 132 generates a modified SBS suppression signal that includes a signal that is a mathematical transform of the SBS suppression signal. In some embodiments, the mathematical transform is chosen to cause vector cancellation of the SBS suppression signal. In other embodiments, the mathematical transform is chosen to produce a pre-distortion that compensates for modulation non-linearities caused by thelaser 102 and/or themodulator 120. In yet other embodiments, thesignal processor 132 generates a modified SBS suppression signal that includes a signal having a harmonic of the SBS suppression signal. - The
summation circuit 140 combines the payload modulation signal with the modified SBS suppression signal and applies the combined signal to theinput 148 of themodulator 120. Theoptical modulator 120 modulates the optical signal that is generated by thelaser 102 with the combined signal. The AM noise portion of the SBS suppression signal is reduced or substantially canceled in the resulting modulated optical signal. - Experimental results have been obtained by using a SBS
suppression signal generator 112 that includes a thermally generated noise signal. These experimental results indicate that greater than 10 dB of AM noise suppression is achievable for noise that is bandwidth limited to 20 kHz. Experimental results have also been obtained by using a SBSsuppression signal generator 112 that includes a discrete electrical tone generator. These experimental results indicate that 20 dB of AM noise suppression is achievable when injecting a single 9 kHz tone. The experimental results show that the AM noise suppression is sufficient to reduce the AM noise detected at an optical receiver to negligible levels for the applied signal power levels that are required to produce the desired frequency spreading. -
FIG. 2 illustrates a block diagram of an embodiment of a SBSsuppression signal generator 200 that generates a SBS suppression signal that increases the line width of the optical signal according to the present invention. The SBSsuppression signal generator 200 includes anoise source 202 that generates a noise signal at anoutput 204. In the embodiment shown, thenoise source 202 is a Zener diode noise source. However, numerous other noise sources can be used. In other embodiments, a signal generator (not shown) is used to generate a single or multi-frequency waveform that is used to produce the SBS suppression signal. In these embodiments, the SBSsuppression signal generator 200 can produce waveforms, such as sinusoidal, ramp, or other waveforms that are used to generate the SBS suppression signal. - The
output 204 of thenoise source 202 is capacitively coupled to aninput 206 of avoltage amplifier 208. Thevoltage amplifier 208 amplifies the noise signal to a signal level that is appropriate for SBS suppression. Anoutput 210 of thevoltage amplifier 208 is electrically coupled to aninput 212 of a low-pass filter 214. The low-pass filter 214 passes a portion of the noise signal that has frequency components below a certain cut-off frequency of the low-pass filter 214. Anoutput 216 of the low-pass filter 214 is electrically coupled to aninput 218 of a high-pass filter 220. The high-pass filter 220 passes a portion of the low-pass filtered noise signal that has frequency components above a cut-off frequency of the high-pass filter 220. - Experimental data are shown in
FIGS. 6 and 7 for an embodiment of the SBS suppression signal generator 112 (FIG. 1 ) where thelow pass filter 214 comprises a 50 kHz fourth-order Butterworth filter and the high-pass filter 220 comprises a second-order Butterworth filter that is tuned to 1 kHz. -
FIG. 3 shows an embodiment of an integratedoptical transmitter subassembly 300 that can be used to generate an optical signal with SBS suppression according to the present invention. In one embodiment, thetransmitter subassembly 300 is an opto-electronic integrated circuit. In another embodiment, thetransmitter subassembly 300 comprises discrete components. - Referring to
FIG. 1 andFIG. 3 , thetransmitter subassembly 300 includes alaser diode 302 that generates an optical signal at anoutput 304 with the appropriate wavelength and power level. Aninput 308 of thelaser diode 302 is electrically coupled to theoutput 108 of the laserbias power supply 106 which is driven by theoutput 114 of the SBSsuppression signal generator 112. - In one embodiment, the
transmitter subassembly 300 includes a backfacet diode detector 306 that generates a feedback signal at anoutput 307. The feedback signal can be used to change the bias signal that is generated by the laserbias power supply 106 in order to control the wavelength and/or power of the optical signal that is generated by thelaser 302. - The
transmitter subassembly 300 also includes aSOA diode 310, such as a Fabry Perot or traveling wave semiconductor amplifier. TheSOA diode 310 is optically coupled to theoutput 304 of thelaser diode 302. Aninput 312 of theSOA diode 310 is electrically coupled to theoutput 136 of thesignal processor 132 and the output of an SOA bias power (not shown inFIG. 1 ) that is used to bias theSOA diode 310 at the desired operating point. TheSOA diode 310 is an in-line optical amplifier that generates an optical signal at anoutput 314 that is gain modulated by the SBS suppression signal. - The
transmitter subassembly 300 also includes an electro-absorption (EA)diode 316 that is optically coupled to theoutput 314 of theSOA diode 310. Aninput 318 of theEA diode 316 is electrically coupled to theoutput 124 of themodulation signal generator 122 and to theoutput 126 of the modulationbias power supply 128. - The operation of the
transmitter subassembly 300 is similar to the operation of theoptical transmitter 100 that was described in connection withFIG. 1 . Referring toFIG. 1 andFIG. 3 , the laserbias power supply 106 generates a laser bias signal that is sufficient to cause thelaser diode 302 to generate the optical signal with the desired wavelength and power level. The SBSsuppression signal generator 112 generates a SBS suppression signal that increases the bandwidth of the optical signal that is generated by thelaser diode 302. The laser bias signal and the SBS suppression signal are applied to theinput 308 of thelaser diode 302. Thelaser diode 302 generates an optical signal having increased bandwidth at theoutput 304. - The
signal processor 132 processes the SBS suppression signal and generates a modified SBS suppression signal at theoutput 136. In one embodiment, the modified SBS suppression signal includes a cancellation signal that reduces or substantially cancels undesirable AM noise in the optical modulation signal that is caused by the SBS suppression signal as described in connection withFIG. 1 . Theoutput 136 of thesignal processor 132 is applied to theinput 312 of theSOA diode 310. A SOA bias power supply (not shown) generates a bias signal that biases theSOA diode 310 at an operating point that produces the desired gain modulation. The bias signal is applied to theinput 312 of theSOA diode 310. TheSOA diode 310 generates an amplified optical signal at theoutput 314 that is gain modulated by the modified SBS suppression signal. - The
EA diode 316 is biased by the modulation bias signal that is generated at theoutput 126 of the modulationbias power supply 128. TheEA diode 316 modulates the amplified optical signal that is gain modulated by the modified SBS suppression signal with the modulation payload signal that is generated by themodulation signal generator 122. TheEA diode 316 generates a payload modulation signal at anoutput 320. - The payload modulation signal has SBS suppression resulting from the increased line width produced by the SBS suppression signal. The payload modulation signal also has reduced AM noise because the optical signal is gain modulated by the modified SBS suppression signal. In addition, the payload modulation signal has a relatively high signal-to-noise ratio because the full modulation depth of the
EA diode 316 is available for the payload. -
FIG. 4 illustrates another embodiment of an integratedoptical transmitter subassembly 350 that can be used to generate an optical signal with SBS suppression according to the present invention. The integratedoptical transmitter subassembly 350 ofFIG. 4 can be less expensive to manufacture compared with the integratedoptical transmitter subassembly 300 ofFIG. 3 . Thetransmitter subassembly 350 is similar to thetransmitter subassembly 300 that was described in connection withFIG. 3 . However, theoptical transmitter subassembly 350 does not include theSOA diode 310 shown inFIG. 3 . Instead, theEA diode 316 of thetransmitter subassembly 350 is electrically coupled to theoutput 136 of the signal processor 132 (FIG. 1 ), theoutput 124 of the modulation signal generator 122 (FIG. 1 ), and to theoutput 126 of the modulation bias power supply 128 (FIG. 1 ). - The operation of the
transmitter subassembly 350 is similar to the operation of thetransmitter subassembly 300 that was described in connection withFIG. 3 . - Referring to
FIG. 1 ,FIG. 3 andFIG. 4 , the laserbias power supply 106 generates a laser bias signal that is sufficient to cause thelaser diode 302 to generate the optical signal with the desired wavelength and power level. The SBSsuppression signal generator 112 generates a SBS suppression signal that increases the bandwidth of the optical signal generated by thelaser diode 302. The laser bias signal and the SBS suppression signal are applied to theinput 308 of thelaser diode 302. Thelaser diode 302 generates an optical signal having increased bandwidth at theoutput 304. - The
signal processor 132 processes the SBS suppression signal and generates a modified SBS suppression signal at theoutput 136. In one embodiment, the modified SBS suppression signal includes a cancellation signal that reduces or substantially cancels amplitude modulation noise in the optical modulation signal that is caused by SBS suppression signal as described herein. The output of thesignal processor 132 is applied to theinput 318 of theEA diode 316. - The
EA diode 316 is biased by the signal that is generated at theoutput 126 of the modulationbias power supply 128. TheEA diode 316 modulates the optical signal that is generated by thelaser diode 302 with the modified SBS suppression signal that is generated by thesignal processor 132 and with the modulation payload signal that is generated by themodulation signal generator 122. TheEA diode 316 generates a payload modulation signal at anoutput 320. - The payload modulation signal has SBS suppression resulting from the increased line width produced by the SBS suppression signal. The payload modulation signal also has reduced AM noise because the optical signal is modulated by the modified SBS suppression signal. However, the payload modulation signal has a somewhat lower signal-to-noise ratio compared with the payload modulation signal that is generated by the
transmitter subassembly 300 ofFIG. 3 because some of the modulation depth of theEA diode 316 is used to modulate the modified SBS suppression signal. -
FIG. 5 illustrates an exemplary embodiment of a fiber laseroptical transmitter subassembly 400 that can be used to generate an optical signal with SBS suppression according to the present invention. Thetransmitter subassembly 400 is similar to thetransmitter subassembly 300 that was described in connection withFIG. 3 . However, thetransmitter subassembly 400 includes afiber laser 402. There are numerous types of fiber lasers that are well known in the art. Although a Fabry Perot-type fiber laser is shown inFIG. 5 , any type of fiber laser can be used with thetransmitter subassembly 400 of the present invention. - Referring to
FIG. 1 andFIG. 5 , thefiber laser 402 includes amodulation input 404 that is electrically coupled to theoutput 114 of the SBSsuppression signal generator 112. A laser bias power supply (not shown) generates a laser bias signal that is used to bias thefiber laser 402 at an operating point that causes thefiber laser 402 to generate an optical signal having the desired wavelength and power level. - The
transmitter module 400 also includes adiscrete SOA 406, such as a Fabry Perot or traveling wave semiconductor amplifier. Anoptical input 408 of theSOA 406 is optically coupled to the output of thefiber laser 402. Anelectrical input 410 of theSOA 406 is electrically coupled to theoutput 136 of thesignal processor 132 and the output of an SOA bias power (not shown inFIG. 1 ) that is used to bias theSOA 406 at the desired operating point. TheSOA 406 is an in-line optical amplifier that generates an optical signal at an output 41 2 that is gain modulated by the SBS suppression signal. - The
transmitter module 400 also includes an electro-optic modulator 414. Any type of electro-optic modulator can be used in thetransmitter module 400. For example, in one embodiment, themodulator 414 is an electro-absorption modulator. In another embodiment, themodulator 414 is a Mach-Zehnder interferometric modulator. Theoutput 412 of theSOA 406 is optically coupled to aninput 416 of theoptical modulator 414. Aninput 418 of themodulator 414 is electrically coupled to theoutput 124 of themodulation signal generator 122 and to theoutput 126 of the modulationbias power supply 128. - The operation of the
transmitter module 400 is similar to the operation of thetransmitter subassembly 300 that was described in connection withFIG. 3 . Referring toFIG. 1 andFIG. 5 , thefiber laser 402 generates an optical signal with the desired wavelength and power level. The SBSsuppression signal generator 112 generates a SBS suppression signal. The SBS suppression signal is then applied to themodulation input 404 of thefiber laser 402. The SBS suppression signal increases the bandwidth of the optical signal. - The
signal processor 132 processes the SBS suppression signal and generates a modified SBS suppression signal at theoutput 136. In one embodiment, the modified SBS suppression signal includes a cancellation signal that reduces or substantially cancels AM noise in the optical modulation signal that is caused by SBS suppression signal as described herein. - The
output 136 of thesignal processor 132 is applied to theinput 410 of theSOA 406. A SOA bias power supply (not shown) generates a bias signal that biases theSOA 406 at an operating point that produces the desired gain modulation. The bias signal is applied to theinput 410 of theSOA 406. TheSOA 406 generates an amplified optical signal at the output 41 2 that is gain modulated by the modified SBS suppression signal. - The
modulator 414 is biased by the signal that is generated at theoutput 126 of the modulationbias power supply 128. Themodulator 414 modulates the amplified optical signal that is gain modulated by the modified SBS suppression signal with the modulation payload signal that is generated by themodulation signal generator 122. Themodulator 414 generates a payload modulation signal at anoutput 420. - The payload modulation signal has SBS suppression resulting from the increased line width produced by the SBS suppression signal. The payload modulation signal also has reduced AM noise because the optical signal is modulated by the modified SBS suppression signal.
-
FIG. 6 illustrates a 500 kHz span of afrequency spectra 450 generated by a commercially available CATV receiver that received an optical signal that was generated by an optical transmitter according to the present invention without SBS suppression compensation. Referring toFIG. 1 andFIG. 3 , the SBSsuppression signal generator 112 generated the SBS suppression signal from amplified filtered noise produced by a 50 kHz noise source. The SBS suppression signal was applied to thelaser 302. Themodulation signal generator 122 generated a single 300 MHz carrier tone modulation signal and applied the modulation signal to theEA diode 316. However, thesignal processor 132 was not activated and did not apply a modified SBS suppression signal to theSOA 310. Therefore, the SBS suppression was not active in the transmitter. - The optical power of the received signal was about −1 dBm. The
frequency spectra 450 illustrated inFIG. 6 shows AM noise as amplitude modulated sidebands. Specifically, the 50 kHz noise signal is clearly visible more than 10 dB above the noise floor on either side of the carrier for the illustrated 500 kHz span.FIG. 7 illustrates the ability to suppress the AM noise caused by the SBS suppression signal in the optical signal. -
FIG. 7 illustrates a 500 kHz span of afrequency spectra 500 generated by a commercially available CATV receiver that received an optical signal that was generated by an optical transmitter according to the present invention with SBS suppression compensation. The optical transmitter that generated the optical signal was identical to the transmitter that generated the optical signal described in connection withFIG. 6 , but the signal processor was active in the transmitter. Therefore, the SBS suppression compensation was active in the transmitter. - Referring to
FIG. 1 andFIG. 3 , the SBSsuppression signal generator 112 generated the SBS suppression signal from amplified filtered noise produced by a 50 kHz noise source with the same level of noise used to generate the SBS suppression signal that was described in connection withFIG. 6 . The SBS suppression signal was applied to thelaser 302. Themodulation signal generator 122 generated a single 300 MHz carrier tone modulation signal and applied the modulation signal to theEA diode 316. Thesignal processor 132 was activated so as to apply a modified SBS suppression signal to theSOA 310. Therefore, the SBS suppression was activated in the transmitter. - The optical power of the received signal was about −1 dBm. The
frequency spectra 500 illustrated inFIG. 7 illustrates the same 300 MHz carrier with the same level of noise. However, applying the modified SBS suppression signal to theSOA 310 substantially eliminated the AM noise. - While the invention has been particularly shown and described with reference to specific preferred embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined herein.
Claims (21)
Priority Applications (1)
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US11/566,118 US7349637B1 (en) | 2003-02-11 | 2006-12-01 | Optical transmitter with SBS suppression |
Applications Claiming Priority (2)
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US10/248,710 US7146110B2 (en) | 2003-02-11 | 2003-02-11 | Optical transmitter with SBS suppression |
US11/566,118 US7349637B1 (en) | 2003-02-11 | 2006-12-01 | Optical transmitter with SBS suppression |
Related Parent Applications (1)
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US10/248,710 Continuation US7146110B2 (en) | 2003-02-11 | 2003-02-11 | Optical transmitter with SBS suppression |
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US11/566,118 Expired - Fee Related US7349637B1 (en) | 2003-02-11 | 2006-12-01 | Optical transmitter with SBS suppression |
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US (2) | US7146110B2 (en) |
EP (1) | EP1597845B1 (en) |
JP (1) | JP2006517772A (en) |
AT (1) | ATE373352T1 (en) |
DE (1) | DE602004008886T2 (en) |
DK (1) | DK1597845T3 (en) |
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US20130101290A1 (en) * | 2010-04-21 | 2013-04-25 | Dublin City University | Method and apparatus to overcome linewidth problems in fast reconfigurable networks |
US20120163833A1 (en) * | 2010-12-22 | 2012-06-28 | General Instrument Corporation | System and method for reducing mutual leakage between distributed feedback laser and electro-absorption modulator in integrated electro-absorption modulated laser |
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CN108173596A (en) * | 2012-08-23 | 2018-06-15 | 菲尼萨公司 | For the integration laser and modulator transmitter of CATV applications |
Also Published As
Publication number | Publication date |
---|---|
EP1597845A1 (en) | 2005-11-23 |
US7146110B2 (en) | 2006-12-05 |
DE602004008886T2 (en) | 2008-06-19 |
DE602004008886D1 (en) | 2007-10-25 |
WO2004073214A1 (en) | 2004-08-26 |
US7349637B1 (en) | 2008-03-25 |
EP1597845B1 (en) | 2007-09-12 |
DK1597845T3 (en) | 2008-01-14 |
ATE373352T1 (en) | 2007-09-15 |
JP2006517772A (en) | 2006-07-27 |
US20040156643A1 (en) | 2004-08-12 |
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