Abstract:
A method and apparatus is provided for generating optical pulses with an electro-optic amplitude modulator. The modulator includes first and second waveguides that form an optical interferometer. At least the first waveguide includes an electro-optic material such as lithium niobate and an electrode extending along a portion thereof. Input and output optical waveguides are respectively coupled to input and output junctions of the interferometer. A voltage source biases the electrode such that a modulation switching curve arises that generates two optical pulses over a complete voltage cycle.

Description:
TECHNICAL FIELD 
     This invention relates generally to amplitude modulators and more particularly to an electro-optic modulator for generating solitons from a continuous wave signal. 
     BACKGROUND OF THE INVENTION 
     Long distance optical transmission using optical amplifiers can provide greater bandwidth at lower cost than that using electronic regeneration. Erbium doped optical fiber amplifiers can easily handle several channels simultaneously, and do so with low crosstalk. For long distance transmission, it is necessary to use a transmission mode which is resistant to the various dispersive effects of the fiber. In au optical fiber transmission path, the optical fiber&#39;s chromatic dispersion, acting by itself, attempts to broaden pulse signals in time. The fiber&#39;s index, which also depends on the intensity of light, acting by itself through the process of self phase modulation, always serves to broaden the pulse&#39;s frequency spectrum. Thus, for long distance transmission, an optical signal which is resistant to the various dispersive effects of the optical fiber can result in an increase in the spacing between optical amplifiers in the optical transmission path. 
     Under certain conditions such as, for example, zero loss or loss periodically compensated by optical gain, a soliton is nondispersive in the time domain. Thus, the waveshape of a soliton is independent of the distance that it travels along an optical fiber. In addition, a soliton is also nondispersive in the frequency domain. Thus, for a range of soliton pulse widths, such as 50-80 ps for a data rate of 2.5 G b/s, and fiber group delay dispersion parameters of approximately 0.7-2 ps/nm/km, the distance that a soliton can be transmitted before serious dispersive effects occur is typically 500 km or greater. 
     Creation of soliton pulses is dependent upon proper launch and transmission characteristics such as pulse power, pulse width, center frequency, and fiber dispersion. Of particular concern for the present purposes, creation of solitons require the generation of temporally narrow pulses, typically on the order of 1-10 picoseconds. These characteristics of solitons are well known to those skilled in the art and will not be discussed further herein. For additional information concerning soliton generation and soliton transmission, see Optical Fiber Telecommunications II, ed. S. E. Miller et al., p.90 et seq. (Academic Press 1988). 
     One device for generating solitons consists of a high speed amplitude modulator such as an electro-optic waveguide modulator. One class of electro-optic modulators are made of ferroelectric materials, such as z-cut lithium niobate (LiNbO 3 ) or lithium tantalate (LiTaO 3 ). These modulators convert an applied voltage to an optical signal. Typically, an electric pulse is used to generate an optical pulse. Lithium niobate modulators are commonly employed because they offer high speed, a high extinction ratio, and a controllable (or zero) chirp. However, one problem with such modulators is that it is difficult to generate extremely narrow electrical pulses that can be translated into optical pulses of sufficiently narrow temporal width to form solitons. 
     Therefore, it is desirable to provide an electro-optic amplitude modulator with an electric signal that allows the modulator to generate temporally narrow optical pulses. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a method and apparatus is provided for generating optical pulses with an electro-optic amplitude modulator. The modulator includes first and second waveguides that form an optical interferometer. At least the first waveguide includes an electro-optic material such as lithium niobate and an electrode extending along a portion thereof. Input and output optical waveguides are respectively coupled to input and output junctions of the interferometer. A voltage source biases the electrode such that a modulation switching curve arises that generates two optical pulses over a complete voltage cycle. 
     In accordance with another aspect of the invention, a method is provided for generating optical pulses with an electro-optic amplitude modulator having a pair of waveguides and at least one pair of electrodes for controlling a refractive index of at least one of the waveguides. In particular, a cw optical signal is received at an input waveguide of the modulator. At least one electrical pulse is applied to the electrode pair to modulate the cw optical signal so that an edge of the electrical pulse yields an optical pulse at an output waveguide of the modulator. The optical pulse may have a temporal width substantially equal to the temporal width of the edge of the electrical pulse. 
     In contrast to known biasing arrangements in which an electrical pulse was required to produce an optical pulse, the present invention advantageously produces an optical pulse upon a change in voltage levels. Since it is relatively easy to produce sharp voltage transitions (as opposed to narrow electrical pulses), the invention is capable of produces extremely narrow optical pulses, such as solitons, for example. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a known lithium niobate amplitude modulator. 
     FIG. 2 shows a known modulation switching curve for the modulator shown in FIG.  1 . 
     FIG.  3 ( a ) shows a modulation switching curve in accordance with the present invention and 
     FIG.  3 ( b ) shows a complete voltage cycle applied to the modulator and the resulting optical signal levels corresponding thereto. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, there is illustrated an example of a known lithium niobate (LiNbO 3 ) high-speed amplitude modulator for modulating an optical signal with an electrical signal to form a soliton. It should be noted that the present invention is applicable to a wide variety of electro-optic amplitude modulators and that the modulator of FIG. 1 is shown for illustrative purposes only. As shown, an electro-optic material substrate  20  such as lithium niobate (LiNbO 3 ) or the like, which can convert an electrical potential into optical phase shifts, includes an optical waveguide  22 . The waveguide  22  may be formed, for example, by diffusing titanium (Ti) into the substrate. Alternatively, the waveguide  22  may be formed in the substrate by a proton exchange process. The optical waveguide  22  is constructed to include two parallel paths  26  and  28  positioned between two optical Y junctions  30  and  32 , which are respectively coupled to two end sections  23  and  25  of waveguide  22 . The LiNbO 3  substrate, including the optical Y junctions, the parallel paths and the end sections, supports an SiO 2  buffer layer which forms a common ground plane and at least one pair of electrodes. The ground plane and the electrodes can be electroplated onto the buffer layer and may be formed from aluminum, silver, gold or the like. One pair of electrodes can comprise a ground plane  40  and an elongated electrode  36  positioned over optical waveguide  28 . Electrode  36  can extend along the waveguide  28  for a distance of approximately 1 cm. Longer or shorter lengths can be chosen depending on the desired bandwidth. If another pair of electrodes is employed, it can compromise a ground plane  38  and an elongated electrode  34  positioned over optical waveguide  26 . Electrode  34  can extend along the waveguide for a distance comparable to the length of electrode  36 . A common ground plane  33  can be included to cooperate with electrodes  34  and  36 . The assemblage of the LiNbO 3  substrate, the optical Y junctions and associated optical waveguides, and the set of electrodes is one manifestation of an interferometer normally identified as a Y junction Mach-Zehnder interferometer. The specific example of a double pair of electrodes to provide one set of electrodes is applicable to z-cut LiNbO 3 , which is a commonly used crystal orientation. For x-cut LiNbO 3 , a single pair of electrodes can be used in place of the double pair of electrodes. 
     In a Y junction interferometer, a change in the index of refraction of one or both waveguides  26  and  28 , which is directly proportional to the voltages applied to the individual pairs of electrodes, causes an optical signal in the waveguides  26  and  28  to experience an optical phase shift. It is this optical phase shift which causes the optical signal to undergo an amplitude change. In operation, optical energy in the form of a continuous wave (cw) of optical energy from, for example, a laser via a single mode waveguide, is directed into end section  23  of waveguide  22 , where it is divided into two equal optical signals by Y junction  30 . At this instant, an electrical signal having a specific waveshape is applied to the pair of electrodes  36  and  40 . If a second pair of electrodes is employed, an electrical signal having a phase which is 180 degrees out of phase with the first signal is applied to the second pair of electrodes  34  and  38 . The electrical signal applied to the first pair of electrodes causes a change in the index of refraction of the waveguide  28 . (If the second pair of electrodes is employed, the electrical signal applied to the second pair of electrodes causes a change in the index of refraction of the waveguides  26 ). The second Y junction  32  combines the two signals from the waveguides  26  and  28  into a single signal which causes an amplitude change to the optical signals in the waveguide  25 . This signal advances along the end section  25  of waveguide  22  to an outgoing single mode fiber  42 . 
     FIG. 2 shows a typical modulation switching curve for the modulator shown in FIG.  1 . Normalized optical output power is shown on the ordinate and voltage is shown on the abscissa. The electrode pair or pairs is normally biased so that a pulse in the electrical domain is translated into a pulse in the optical domain. That is, an electrical bit of “1” (represented by maximum voltage) is translated into an optical bit of “1” (represented by maximum optical output power). Likewise, an electrical bit of “0” (represented by minimum voltage) is translated into an optical bit of “0” (represented by minimum optical output power). As FIG. 2 shows, an optical bit of “1” will yield an optical bit of “0” when the voltage changes by one-quarter of a complete voltage cycle (i.e., from V a  to V b  in FIG.  2 ). As a consequence, only a quarter of the voltage cycle is employed to generate the optical bits. 
     In accordance with the present invention, the lithium niobate amplitude modulator is biased in such a way that a change in voltage level (from “1” to “0” or visa versa) is translated into an optical bit of “1” and a constant voltage level is translated into an optical bit of “0.” This is accomplished by initially biasing the modulator at a voltage that produces a maximum optical power output. In other words, the voltage bias is initially placed at a value that would translate into an optical “1” in the known arrangement shown in FIG.  2 . 
     FIG.  3 ( a ) shows a modulation switching curve in accordance with the present invention in which the modulator is initially biased at V 1 , which is intermediate to voltages V c  and V d  defining the lower and upper limits of the voltage applied to the modulator. FIG.  3 ( b ) shows a complete cycle of the voltage (curve  30 ) as it is applied to the modulator (left-most side of FIG.  3 ( b )) and the corresponding optical signal levels that are produced (right-most side of FIG.  3 ( b )). In FIG.  3 ( b ) time is indicated on the ordinate. As the applied voltage is changed from V c  to V d  along curve  30  during the time interval between t 0  and t 1 , the optical output power changes in accordance with the modulation switching curve shown in FIG.  3 ( a ). That is, the change in voltage from V c  to V d  is translated into optical pulse  32  shown on the rightmost portion of FIG.  3 ( b ). Pulse  32  corresponds to an optical bit of “1.” 
     Next, the voltage remains constant at V d  for a prescribed time interval between t 1  and t 2 , producing an optical bit of “0.” During the time interval between t 2  and t 3  the voltages changes from V d  to V c , yielding a second optical pulse  34 . Once again, the voltage remains constant (at the level V c ) for the time interval between t 3  and t 4 , producing an optical bit of “0.” 
     FIG.  3 ( b ) shows that over the course of a complete voltage cycle, which occurs between time t 0  and t 4 , two optical pulses or bits are produced. In contrast, known biasing arrangements such as discussed in connection with FIG. 2 generate one pulse over a quarter of a voltage cycle. The present invention thus allows more refined control over the generation of optical bits. Another advantage of the inventive biasing arrangement is that a change in voltage level (i.e., the edge of the voltage pulse defined between times t 0  and t 1  in FIG.  3 ( b )) produces an optical pulse, whereas in the prior arrangement an electrical pulse was required to produce an optical pulse. This is advantageous because it is easier to produce a sharp voltage transition than it is to produce a narrow electrical pulse. The present invention is therefore capable of producing extremely narrow optical pulses, thus facilitating the generation of solitons, which require such narrow optical pulses.