Wavelength modulated laser for minimizing effects of Rayleigh backscattering

[0002] This invention relates generally to lasers, and more specifically to communication devices using semiconductor lasers for optical communication.

[0003] Cable television systems typically include a headend section for receiving satellite signals and demodulating the signals to baseband. The baseband signal is then converted to an optical signal for transmission from the headend section over fiber optic cable. Optical transmitters are distributed throughout the cable system for splitting and transmitting optical signals, and optical receivers are provided for receiving the optical signals and converting them to radio frequency (RF) signals that are further transmitted along branches of the system over coaxial cable rather than fiber optic cable. Taps are situated along the coaxial cable to tap off the cable signals to subscribers of the system.

[0004] In conventional communication systems, such as cable television systems, the fiber optic cable is characterized by small material variations or inhomogeneities that can cause reflection of the optical signal in the reverse direction, i.e., a Rayleigh backscattered signal. The backscattered signal then encounters further fiber variations which cause generation of yet another reflected signal in the forward direction. This signal is referred to as a double Rayleigh backscattered signal. As a result, two signals, the initial forward optical signal and the double Rayleigh backscattered signal, travel in the forward direction and are received by the optical receiver. This generates interference and intensity beats in the receiver, which can cause erroneous demodulation.

[0005] Thus, what is needed is a way to reduce interference generated by Rayleigh backscattering in an optical communication system.

[0006] FIG. 1 is side view of a distributed feedback laser that is wavelength modulated in accordance with the present invention.

[0007] FIG. 2 is a signal diagram depicting a mode of operation in which the laser of FIG. 1 can operate in accordance with the present invention.

[0008] FIG. 3 is a signal diagram depicting a modulating waveform for modulating the laser of FIG. 1 in accordance with the present invention.

[0009] FIG. 4 is a side view of a communication system including a forward signal, a Rayleigh backscattered signal, and a double Rayleigh backscattered signal in accordance with the present invention.

[0010] FIG. 1 is a side view of a semiconductor laser 100, such as a distributed feedback (DFB) laser diode, that is wavelength modulated by application of heat to minimize the effect of Rayleigh backscattering in a communication system, such as a cable television system, including the laser 100. Preferably, the laser 100 and corresponding thermal modulation apparatus is included within an optical transmitter (not shown) for transmitting optical signals generated by the laser 100.

[0011] The laser 100 itself is configured similar to conventional DFB lasers and includes a first terminal 105 for receiving a direct current (DC) bias current and a second terminal 110 for receiving data, preferably in the form of a radio frequency (RF) signal. Left and right facets 130, 135 partially enclose a cavity 140 that is bounded from beneath by a fiber grating 115 extending the length of the cavity 140. The cavity 140 can be, for instance, approximately 300 micrometers in length and about 1—3 micrometers in width.

[0012] In accordance with the present invention, a heater 125 is physically coupled to a surface of the semiconductor laser 100 for heating the semiconductor material, thereby varying the optical wavelength of the optical signal that is output by the laser 100. The heater 125 can be, for example, a chip resistor having a ceramic substrate plated with a resistive material, e.g., titanium and/or platinum. The resistor/heater 125 can substantially run the length of the laser 100, as shown. Alternatively, a smaller heater 125 could be centrally located on the surface of the laser 100 or even skewed to one side or the other.

[0013] The resistor is electrically coupled to a modulated current source 120 for driving the resistor with a continuous signal, such as a sinusoidal, sawtooth, or triangular waveform. Preferably, the current source 120 produces a triangular waveform, as will be explained in greater detail below.

[0014] The signal generated by the modulated current source 120 causes the resistor to produce varying amounts of heat over time, resulting in the continuous changing of temperature within the cavity 140. As a result, the optical wavelength of the emitted optical signal varies with time. This conveniently minimizes the effect of Rayleigh backscattering upon signals received by an optical receiver (not shown) within the communication system.

[0015] Referring next to FIG. 2, a signal diagram of reflected power versus launch power of the laser 100 is shown. Region 205 above line 200 depicts a region in which the launch power is great enough to cause the onset of problematic stimulated Brillouin scattering (SBS). Region 210 represents an operational mode in which the laser 100 is operated in the linear regime below the SBS threshold 200 (Doren and Rezin, does this terminology sound correct?). In lower power applications, such as 1310 nanometer laser applications for use in cable television systems, the effects of SBS are minimal. However, even in lower power applications, Rayleigh backscattering can create problems in the communication system. As a result, the laser 100 according to the present invention is particularly useful for minimizing effects of Rayleigh backscattering in lower power applications in which the laser is operated below the SBS threshold, i.e., at powers low enough so that SBS has not become problematic, although problems associated with Rayleigh backscattering can be reduced above the SBS threshold as well.

[0016] Referring next to FIG. 3, a triangular waveform 300 that can be generated by the modulated current source 120 (FIG. 1) is illustrated. As the amplitude of the waveform 300 changes, the current with which the resistor/heater is driven also changes, thereby varying the temperature of the laser 100. It will be appreciated that variations in the amplitude of the waveform 300 therefore vary the optical wavelength of the optical signal produced by the laser 100. For instance, the optical wavelength of the laser output is different when the resistor is driven at time 305 than when driven at time 310.

[0017] FIG. 4 is a diagram illustrating optical signals that are processed within an optical communication system, such as a cable television system 400 including an optical transmitter 405, an optical receiver 410, and a communication medium 415, such as a fiber optic cable, coupling the receiver 410 and the transmitter 405. The optical transmitter 405 includes the wavelength modulated laser 100 that generates the forward optical signal 420, which essentially has a different optical wavelength at each point in time. Due to material variations in the fiber and other inhomogeneities, the forward signal 420 can be reflected to generate a reflected signal 425 in the reverse direction, i.e., a Rayleigh backscattered signal 425. Further variations and inhomogeneities in the fiber can result in this reverse signal being reflected yet again to generate a second forward optical signal, i.e., a double Rayleigh backscattered signal 430. By the time that this double Rayleigh backscattered signal 430 reaches the optical receiver 410, the optical wavelength of the original forward signal 420 will have been varied such that the forward optical signal 420 and the double Rayleigh backscattered signal 430 have different optical wavelengths, eliminating the intensity beats in the receiver 410 and the interference that can result when optical signals are generated using prior art methods.

[0018] In accordance with the present invention, the current source 120 (FIG. 1) can drive the heater 125 with any continuous waveform. In the context of Rayleigh backscattering, however, generation of a triangle waveform 300 (FIG. 3) by the current source 120 is preferred because the double Rayleigh backscattered signal 430 follows the forward optical signal 420 relatively closely in time. As a result, the gently sloping peaks and valleys of a sinusoidal signal increase the risk that the optical wavelength of the double Rayleigh backscattered signal 430 will be repeated in the forward optical signal 420. The sharper peaks and valleys of the triangular waveform 300, on the other hand, create a much smaller region 315 (FIG. 3) in which this could occur.

[0019] In summary, the communication system as described above includes an optical transmitter having a wavelength modulated semiconductor laser, the cavity of which is exposed to variations in temperature over time to produce an output signal having an optical wavelength that varies with time. The forward optical signal produced by the laser is received by an optical receiver coupled to the transmitter by a communication medium such as fiber optic cable. Inhomogeneities in the fiber of the cable can create reflections, known as Rayleigh backscattering, that can result in a second, reflected optical signal in the forward direction, and this reflected forward signal is also received by the optical receiver.

[0020] In prior art systems, the optical wavelength of the initial forward signal is equivalent to the optical wavelength of the reflected, forward signal, which causes interference at the receiver. Conversely, the communication system according to the present invention provides, as a result of the wavelength modulated laser, an initial forward signal that has an optical wavelength different from that of any reflected, forward signals. Therefore, reception and processing of the optical signals provided to the optical receiver can be performed in a more reliable manner.