Broad linewidth lasers for optical fiber communication systems

Abstract: Non-linear effects in an optical fiber used for communicating AM signals at high power levels are reduced by increasing the linewidth of the pump laser output signal. The linewidth can be increased by optically broadening the laser output signal by driving an optical angle modulator with broadband electrical noise. The optical signal is then externally modulated with an AM information signal for transmission over an optical link fiber. The optical modulation can be provided using either an FM or PM optical modulator. A desired linewidth can be provided by controlling the optical modulation index during the optical modulation step and/or by controlling the bandwidth of the noise source. Other techniques for broadening a laser output signal, including modulating the signal by a periodic function such as a sine wave, or injecting spontaneous emissions into the laser cavity, are also disclosed.

Description: The present invention relates to optical fiber communication systems, and more particularly to a method and apparatus for reducing non-linear effects in an optical fiber used for communicating AM or other information signals. Optical transmission systems are currently being implemented for use in various communication applications. For example, telephone systems are now in use that utilize optical fiber technology to transmit voice and data signals over long distances. Similarly, cable television networks are available wherein optical fiber technology is used for the transmission of both analog and digital signals. In order to transmit an information signal (e.g., a television signal) over an optical fiber, a light beam ("carrier") must be modulated with the information signal. The modulated carrier is then transmitted to a receiver via the optical fiber. At high power levels, silica fibers exhibit non-linear effects due to the interaction of the local electric field across the fiber and the fiber material. These non-linear effects also depend on the length of the fiber, with a cumulative degradation of performance resulting as the length of the fiber increases. Among the non-linear effects exhibited in silica fibers at high power levels are four-wave mixing, Brillouin gain, and Raman gain. The magnitude of these interactions depend on the spectral density of the applied field. The power of the optical signal is also a factor in determining the severity of the non-linear effects. Very little effect on signal transmission is seen below a threshold power density level. Beginning at a critical power density level, power will be shifted in wavelength by the non-linear interaction between the traveling wave and the material. Since optical fibers concentrate the power into a small cross section, the large fields required to make these effects significant arise at modest absolute power levels. For long distance signal transmission, these non-linear effects constitute an upper limit on the power level that can be transmitted. See, for example, Y. Aoki, K. Tajima and I. Mito, "Input Power Limits of Single-mode Optical Fiber Due to Simulated Brillouin Scattering in Optical Communications Systems," IEEE Journal of Lightwave Technology , May 1988, pp. 710-727 and Agrawal, Govind P., "Non-Linear Fiber Optics", Academic Press 1989, ISBN 0-12-045140-9. The non-linear effects in optical fibers are particularly troublesome for the transmission of amplitude modulated ("AM") signals, such as those used in conventional analog television signal broadcasting. E-A-O 099 632 discloses an experimental set up for measuring Stimulated Brillouin Scattering and discloses that Stimulated Brillouin Scattering can be suppressed by broadening the optical linewidth by modulating the phase angle for example in square wave or sinusoidal form. It would be advantageous to provide a method and apparatus for reducing the non-linear effects in optical fiber for the transmission of information signals, such as AM vestigial sideband (VSB-AM) television signals at high power levels. According to the present invention this object is solved by a method according to claim 1. Further advantageous embodiments thereof are subject matter of claims 2 to 8. Further this object is solved by an apparatus according to claim 9, with further advantageous embodiments according to claims 10 to 14. The present invention provides such a method and apparatus, including various embodiments of broad linewidth lasers that output optical carriers for use in communicating information signals via optical fiber links. In accordance with one embodiment of the present invention, a method and apparatus are provided for reducing non-linear effects in an optical fiber used for communicating information signals, such as AM information signals, at high power levels. In the common terminology used by those skilled in the art of non-linear interactions, the terms "pump wavelength" and "signal wavelength" are used to describe the stimulated Brillouin interaction. Using this terminology, the signal laser in accordance with certain embodiments of the present invention plays the role of the pump laser. In such cases, the classical signal laser is absent since there is no desire to use the Brillouin gain, which gain would limit the achievable transmission distance and increase the system relative intensity noise from 0 Hz up to subcarriers a few times the Brillouin bandwidth. In the present invention, a laser output signal is provided. The linewidth of the laser output signal is increased to provide a broadened optical signal. The optical signal is externally modulated with an information signal (e.g., an AM signal), and coupled to an optical link fiber for transmission to a receiver. In order to increase the linewidth of the laser output signal, the laser output is optically modulated in one embodiment by broadband white electrical noise. The noise can be either frequency modulated or phase modulated on the broadened optical signal. The bandwidth of the noise and/or the optical modulation index used during the optical modulation step is controlled to provide a desired linewidth for the broadened optical signal. Apparatus in accordance with one embodiment of the present invention includes a light source for providing an optical carrier. Means are provided for optically modulating the carrier by broadband electrical noise. External modulator means, coupled to receive the optically modulated carrier, modulate the carrier with an information signal. Means are provided for coupling the information modulated carrier from the external modulator means to an optical transmission path such as an optical fiber. The optical modulating means can comprise an optical frequency modulation ("FM") modulator or an optical phase modulation ("PM") modulator. The light source can comprise a continuous wave ("CW") laser. In another embodiment of the invention, apparatus is provided for use in communicating information over an optical fiber. External modulator means, coupled to receive the broadened optical signal, modulate the broadened optical signal with an information signal. The broadened optical signal modulated with the information signal is coupled from the external modulator means to an optical transmission path. Figure 1 is a block diagram illustrating apparatus in accordance with a first embodiment of the present invention that uses broadband noise to increase linewidth ; Figure 2 is a block diagram illustrating apparatus not showing the features of claim 1 of the present invention and using a periodic function input to an external modulator to increase linewidth; Figure 3 is a block diagram of an embodiment not showing the features of claim 1 of the present invention and using a periodic function directly modulating a laser to increase linewidth; Figure 4 is a block diagram illustrating apparatus not showing the features of claim 1 of the present invention and using a spontaneous emission source in series with a laser cavity to increase linewidth; Figure 5 is a block diagram illustrating apparatus not showing the features of claim 1 of the present invention and coupling a spontaneous emission source to a laser cavity via an optical circulator; Figure 6 is a block diagram illustrating apparatus not showing the features of claim 1 of the present invention and using an optical circulator to couple a spontaneous emission source to a ring laser to increase linewidth; Figure 7 is a block diagram illustrating apparatus not showing the features of claim 1 of the present invention and utilizing a different type of ring laser; Figure 8 is a block diagram illustrating apparatus not showing the features of claim 1 of the present invention and using an optical amplifier to inject spontaneous emissions into a semiconductor laser to increase linewidth; and Figure 9 is a block diagram illustrating apparatus not showing the features of claim 1 of the present invention and inputting spontaneous emissions to a microchip laser to increase linewidth. DETAILED DESCRIPTION OF THE INVENTION In accordance with the present invention, the non-linear effects exhibited in an optical fiber at high power levels due to the spectral density of the optical signal are reduced to a level enabling the transmission of AM communication signals at relatively high power levels. Currently, a high power solid state or semiconductor laser for use in communications will produce a signal on the order of 30 milliwatts or so. The output powers of such lasers are increasing at a rapid pace, and output powers on the order of four watts or so are expected to become commercial in the future. It is expected that even higher power lasers will be available for communication purposes in the not too distant future. High power optical communication systems are advantageous in that a signal can be split into a plurality of paths (e.g., in a tree and branch distribution network). In addition, high power enables the signals to be transmitted over longer distances without the need for signal amplification. This reduces the cost of the communication system. Non-linear effects such as four-wave mixing and Brillouin gain have hampered efforts to provide a cost effective high power optical communication system for AM signals, such as television signals transmitted over a cable television network. In order to overcome the non-linear effects for relatively high power AM signals communicated over an optical fiber, the present invention increases the effective linewidth of the carrier light source (e.g., laser) to reduce the effects of fiber non-linearity. Broadening the optical linewidth reduces the spectral density of the signal, distributing the same power over a broader range. As an example, the Brillouin gain threshold is reduced by the ratio Δν B /(Δν B +Δν p ) where Δν p is the optical linewidth (i.e., the linewidth of the optical field that induces the non-linearity) and Δν B is the gain bandwidth of the Brillouin gain. For typical single mode fibers, Δν B is approximately equal to 100 MHz. For a modulated distributed feedback ("DFB") laser, the effective Δν p is on the order of 10 GHz and up. When a CW laser and an external modulator serve as the pump laser, Δν p can be as small as a few kilohertz depending on the specific source laser. Thus, a wide range of Δν p can exist, depending on the type of laser used. In practical vestigial sideband AM systems that use external modulators, approximately 95% of the optical power is concentrated within Δν p at ν o , where ν o is the optical frequency of the non-linear pump. For a typical single mode fiber having a Brillouin gain bandwidth of about 100 MHz, a laser providing a linewidth of two kHz will produce a gain Δν B /(Δν B +Δν p )≈1. For a DFB laser having a linewidth of six GHz, the Brillouin gain Δν B /(Δν B +Δν p )= .016. Thus, we see the Brillouin gain is much higher for the laser which has a two kHz linewidth. In accordance with the present invention, the output of a semiconductor or solid state laser having a narrow linewidth is broadened. For example, optical broadening can be achieved by an optical angle modulator (e.g., frequency or phase modulation) driven by broadband white electrical noise. As illustrated in the embodiment shown in Figure 1, continuous wave laser 10 produces an optical spectrum 12 having an optical frequency ν o . The narrow linewidth of the laser output signal is increased by modulating it with broadband white electrical noise input to an optical modulator 14 at terminal 16. The resultant spectrum 18 output from optical modulator 14 has a substantially increased linewidth Av. This optical signal, still centered around optical frequency ν o , serves as an optical carrier for communication of an information signal to a conventional receiver 26 over a link fiber 24. In order to modulate the optical carrier with the information signal, an external modulator 20 is provided. This modulator can comprise, for example, an electrooptic device such as a Mach Zehnder modulator. External optical modulators are well known in the art. See, e.g., S. E. Miller, T. Li, and E. A. J. Marcatili, "Research Toward Optical Fiber Transmission Systems", Proc. IEEE, Vol. 61, pp. 34-35, Dec. 1973. In the embodiment illustrated in the figure, an RF AM signal, such as an AM VSB television signal, is input to external modulator 20 via coaxial cable input terminal 22. The AM modulated optical carrier is then received by receiver 26 via the link fiber. Optical modulator 14 can comprise either a phase modulator or a frequency modulator. The linewidth of the signal output from modulator 14 is selected by controlling the bandwidth of the electrical noise source and/or the optical modulation index of the optical modulator. Optical phase modulators that can be used in connection with the present invention are commercially available. For example, the Model PM 315 modulator sold by Crystal Technology of Palo Alto, California and the Model IOC 1000 modulator sold by BT&D of Wilmington, Delaware. A difficulty in realizing a pure optical phase modulator such as modulator 14 illustrated in the embodiment of Figure 1, is that if there are any reflections the phase modulator acts as a Fabry Perot interferometer, which introduces unwanted amplitude noise, i.e., relative intensity noise (RIN). It has been recently reported that a lithium niobate modulator can achieve virtually ideal phase modulation. S. K. Korotky, et al., "High-Speed Low Power Optical Modulator with Adjustable Chirp Parameter," Integrated Photonics Research Conference , Paper TuG2, April 9-11, 1991, Monterey, California. Such a modulator can be used to increase linewidth using a broadband white electrical noise source as illustrated in Figure 1. It should now be appreciated that the present invention provides apparatus and methods for reducing the non-linear effects in link fiber by increasing the optical linewidth of the signal laser. The optical linewidth is increased by modulating the laser output with broadband white electrical noise using an optical modulator. This spreads the linewidth to reduce the effects of fiber non-linearities. Such non-linearities may include four-wave mixing, Brillouin gain and Raman gain. Various modulation formats will benefit from the broadened linewidth sources, including VSB-AM, FM, PM, and digital systems. The method and apparatus of the present invention are effective for reducing any fiber non-linear effect that depends on the optical spectral density of the pump laser. As a result, higher launch power can be accommodated.

Claims: A method for communicating optical signals over an optical fiber with reduced non-linear effects in said optical fiber, comprising the steps of: providing a laser output signal (ν o ); increasing the linewidth of said laser output signal by modulating said laser output signal to provide a broadened optical signal (Δν); externally modulating said broadened optical signal (Δν) with an information signal (RF); and coupling the modulated broadened optical signal to an optical link fiber (24) for transmission to a receiver (26); characterized in that said laser output signal is modulated with broadband white electrical noise. A method in accordance with claim 1 wherein said optical modulating step comprises frequency modulation. A method in accordance with claim 1 wherein said optical modulating step comprises phase modulation. A method in accordance with one of claims 1 to 3 comprising the further step of: controlling the bandwidth of said noise to provide a desired linewidth for the broadened optical signal. A method in accordance with one of claims 1 to 3 comprising the further step of: controlling the optical modulation index during said optical modulation step to provide a desired linewidth for the broadened optical signal. A method in accordance with claim 5 comprising the further step of: controlling the bandwidth of said noise in conjunction with said optical modulation index to provide a desired linewidth for the broadened optical signal (Δν). A method in accordance with one of claims 1 to 6 wherein said broadband white noise has a bandwidth of about 100 MHz to 300 MHz. A method in accordance with one of claims 1 to 7 wherein said information signal is an AM signal. Apparatus for communicating optical signals over an optical fiber comprising: a light source (10) for providing an optical carrier (ν o ); means for modulating (14) said carrier to provide a broadened optical carrier (Δν); external modulator means (20), coupled to receive the broadened optical carrier (Δν), for modulating said broadened carrier (Δν) with an information signal (RF); and means for coupling the broadened carrier modulated with said information signal from said external modulator means to an optical transmission path (24); characterized in that said means for modulating (14) are modulating said carrier with broadband white electrical noise. Apparatus in accordance with claim 9 wherein said optical modulating means (14) comprise an optical FM modulator. Apparatus in accordance with claim 9 wherein said optical modulating means (14) comprise an optical PM modulator. Apparatus in accordance with any of claims 9 to 11 wherein said light source (10) comprises a CW laser. Apparatus in accordance with any of claims 9 to 12 wherein said broadband white noise has a bandwidth of about 100 MHz to 300 MHz. Apparatus in accordance with any of claims 9 to 13 wherein said information signal is an AM signal.

Inventors: HUBER DAVID R, HUBER, DAVID R.
Applicants: GEN INSTRUMENT CORP, GENERAL INSTRUMENT CORPORATION OF DELAWARE, NEXTLEVEL SYSTEMS, INC.
Application year: 1992
Main classification: H04B10
IPC codes: H04B10, H01S3, H01S5, G02F1