Recursive optical delay line filter with neutralization

A method and apparatus for generating a series of positive and negative signals from an optical signal. The optical signal is fed into a loop of optical fiber. The loop of optical fiber is provided with a first and second directional coupler. As the optical signal circulates through the optical loop, the first and second directional coupler extract a percentage of light from the optical signal, and couple the extracted light to a photodetector which converts the extracted light into electrical energy. The present invention also provides an impulse cancellation line. The impulse cancellation line is provided to neutralize the problematic initial light extracted from the optical signal in the first directional coupler and coupled to the photodetector. However, as the optical signal continues to circulate through the optical loop, the extracted light is not neutralized.

BACKGROUND

The present invention relates to a delay filter. More specifically, the present invention relates to a bipolar optical delay line filter.

2. Description of Related Art

Fiber optic delay line filters are devices that can be used for RF or microwave signal processing. The filters can be configured to perform bandpass, band-stop, apodizing, comb-selection and a variety of other filtering functions heretofore performed by conventional microwave filters.

Fiber optic delay line filters have many properties that make them attractive for high-frequency applications. The optical fibers that are used to generate the delay have negligible dispersion, almost no loss, and occupy a relatively small volume, even when the individual fibers are long. By comparison, microwave lumped-element and tapped delay line filters are lossy, have considerable dispersion, and can be quite bulky and expensive to build at the higher microwave frequencies where waveguides must be used.

A typical fiber optic delay line filter consists of a length of optical fiber, with equally-spaced signal taps distributed along its length. The signal taps are used to remove a percentage of light that propagates through the filter. The small amount of light that is removed at each tap is combined with the light extracted from other signal taps and fed into a photodetector. This photodetector converts the light into an electrical current.

The intensity of the light entering the fiber optic delay line filter is amplitude-modulated by a microwave signal. Thus, the resultant electrical current at the photodetector will be equivalent to the amplitude of the sum of a number of microwave signals, each delayed by an amount nτ, where n is the tap number and τ is the delay time (the time of propagation) between taps. If the tap strengths and delay lengths are chosen correctly, one can realize a number of different filter configurations.

The filter versatility, however, is limited by the fact that the current generated at the photodetector will always flow in one direction. Using this configuration, one cannot generate bipolar (plus and minus) signals. This is a severe restriction that limits the type of filter that can be built to a filter having a bandpass maximum at zero frequency.

To overcome this, the bipolar optical delay line filter10shown inFIG. 1has been used. Such a bipolar filter is discussed in Jose Capman, Joaquin Cascon, Jose Luis Martin, Salvador Sales, Daniel Pastor, and Javier Marti, “Synthesis of Fiber-Optic Delay Line Filters,” Journal of Lightwave Technology, Vol. 13, pp. 2003-2012 (1995).

The filter10consists of an optical splitter12for receiving and splitting an optical signal, a plurality of tap elements14to extract light from the optical signal, a summer18for combining the extracted light, a photodetector, comprised of a first and second photodiode20,22, and an amplifier24.

The optical signal enters the optical splitter12where the optical signal is split into halves, thereby generating a first optical signal and second optical signal. The first optical signal is coupled into a first delay line13and proceeds to the upper set3of tap elements14and optical delay loops16. The second optical signal is coupled into a second delay line15and proceeds to the lower set5of tap elements14and optical delay loops16. The first and second optical signals proceed through the tap elements14, which extract a percentage of light from the first and second optical signals. The light extracted by each tap element14from the first optical signal is received by the summer18and used to illuminate the first photodiode20. The light extracted by each tap element14from the second optical signal is received by the summer19and used to illuminate the second photodiode22. The first and second photodiodes20,22convert the extracted light into an electrical current.

The electrical current from the first and second photodiodes20,22is coupled to an amplifier24to generate an electrical signal that is proportional to the algebraic difference of the first and second optical signals received by summers18,19. When the amplifier24is connected to an oscilloscope or a similar device, the electrical current can be viewed. Because the first and second photodiodes20,22are used in a push-pull configuration, current can be either injected into or drawn from the amplifier24. The electrical current that results from the tap elements14in the lower set5will appear as a negative signal while the electrical energy that results from the tap elements14in the upper set3will appear as a positive signal. Thus, the first and second optical signals, though both initially positive, generate positive and negative electrical signals as they proceed through the filter10.

The filter10shown inFIG. 1also contains several optical delay loops16consisting of optical fiber. As the first and second optical signals proceed through the tap elements14, they travel through the optical loops16. The optical delay loops16delay the time it takes for the first and second optical signals to travel between the tap elements14. The delay time between tap elements14in the upper set3sets the spacing between the positive signals, while the delay time between elements14in the lower set5sets the spacing between the negative signals. In this way, adjusting the amount of fiber in the optical delay loops16can be used to adjust the spacing between the positive or negative signals.

Additionally, the second delay line15contains an extra optical delay loop17adjacent to the optical splitter12. The purpose of this optical loop17is to prevent the positive and negative signals from occurring at the same time in the time domain.

The problem with this type of bipolar optical delay line filter10is that numerous tap elements14and optical delay loops16are needed to extract light and to set the delay time between subsequent positive and negative signals. Generally speaking, the more complex the desired response from the extracted light, the greater the number of tap elements14needed to extract light from the optical signal. Furthermore, a perfect replication of any filter requires, in theory, an infinite number of tap elements to extract light. As shown inFIG. 1, there are five tap elements14in the upper set3and lower set5. This would result in five positive signals, and five negative signals. A filter with only five tap elements14in each of the upper set3and lower set5would not handle a complex response as well as a filter having an infinite number of tap elements. However, increasing the number of tap elements14in the filter10would increase the size, weight, complexity, and cost of the filter.

For a certain limited class of filters, this problem can be addressed by using a recursive delay line. A recursive delay line is a loop of fiber with a single tap element. An optical signal is injected into the recursive delay line and passes through a tap element. Each time the optical signal passes through the tap element, a small percentage of light is extracted and coupled to a photodetector. The remaining light stays in the loop and makes another trip, after which the tap element extracts more light. This process is repeated, ad infinitum, until the light remaining in the fiber asymptotically approaches zero. However, this recursive delay line can only produce unipolar pulses. To get a bipolar response, one would need two recursive loops, which increases the size, cost, and complexity of the filter. Such a technique is also discussed in Jose Capman, Joaquin Cascon, Jose Luis Martin, Salvador Sales, Daniel Pastor, and Javier Marti, “Synthesis of Fiber-Optic Delay Line Filters,” Journal of Lightwave Technology, Vol. 13, pp. 2003-2012 (1995).

SUMMARY

The present invention according to a preferred embodiment addresses the aforementioned problem by providing a bipolar optical delay line filter, which is simple to manufacture and small in size. The bipolar filter according to the present invention comprises an optical splitter. The optical splitter receives an optical signal and splits the optical signal into a first and second optical signal. The present invention also comprises an optical loop provided with two equally spaced directional couplers. The first optical signal is received by a first directional coupler, via optical fiber. A percentage of the light from the first optical signal is extracted and coupled to a first photodiode, while the remaining light in the first optical signal is injected into the optical loop. The first photodiode converts the extracted light into a series of positive signals. A second directional coupler located halfway around the optical loop receives the remaining light in the first optical signal, extracts a percentage of the light, and optically couples the extracted light to a second photodiode. The second photodiode converts the extracted light into a series of negative signals. The remaining light in the first optical signal returns to the first directional coupler where a percentage of the remaining light is again extracted and coupled to the first photodiode. In this way, the first optical signal continuously circulates through the optical loop between the first and second directional couplers ad infinitum, until the light from the first optical signal in the optical loop asymptotically approaches zero, thereby generating an infinite number of positive and negative signals.

The present invention also comprises an impulse cancellation line. The impulse cancellation line couples the second optical signal via optical fiber to the second photodiode. The second optical signal is time delayed so that it reaches the second photodiode at the same time the light initially extracted from the first optical signal in the first directional coupler reaches the first photodiode. The ratio of the signal strength between the first and second optical signals is adjusted at the optical splitter, so that the second optical signal has an amplitude equal to the amplitude of the light initially extracted and received at the first photodiode. As a result, the initial light extracted and received at the first photodiode from the first directional coupler is neutralized or canceled out by the second optical signal. However, the second optical signal is only used to remove the initial light extracted from the first optical signal by the first directional coupler. As the first optical signal continues to circulate in the optical loop, the second optical signal does not neutralize any extracted light.

DETAILED DESCRIPTION

The present invention will now be described according to a preferred embodiment with reference toFIGS. 2-5. An optical delay line filter100according to the present invention is shown schematically in FIG.2. The filter100comprises an optical splitter101having an input indicated generally at102, a first output indicated generally at103, and a second output indicated generally at105. An optical signal consisting of an intensity-modulated lightwave enters the input102and is split unequally between first and second outputs103and105, thereby generating a first optical signal and a second optical signal. The modulation can be that of any type of discrete or continuous signal. The unequal split between the first and second optical signals will be discussed later. In addition, the source (not shown) generating the first and second optical signals has a coherence length that is less than the length of the individual delay elements (discussed later). This helps avoid beating effects at the photodetector (discussed later).

The filter100also comprises an optical loop106, preferably comprising optical fiber, provided with a first and second directional coupler104,204. The optical loop106can be of a circular shape or it may be of a FIG.-8configuration as shown inFIG. 2, or such other shape as will accommodate a first and second directional coupler. The first optical signal is coupled from the first output103of the optical splitter101to first input port113(SeeFIG. 3) of the first directional coupler104via optical fiber. While in the first directional coupler104a percentage of light is extracted from the first optical signal at first output port115and coupled to a first photodiode108in a dual-balanced photodetector107. The amount of light extracted from the first optical signal is determined by the coupling coefficient (k1) of the first directional coupler104. For the directional coupler104shown inFIG. 2, increasing the coupling coefficient (k1) will couple more light into the optical loop106, while decreasing the coupling coefficient (k1) will direct more light to the first photodiode108. Those skilled in the art will realize that adjusting the coupling coefficient of a directional coupler is well known.

The remaining portion of the first optical signal enters the optical loop106through second output port116(SeeFIG. 3) and proceeds to enter the second directional coupler204through first input port213. While in the second directional coupler204a percentage of light is extracted from the remaining portion of the first optical signal at second output port216and coupled to a second photodiode109in the dual-balanced photodetector107. As shown inFIG. 2, the anode of the first photodiode108is serially connected to the cathode of the second photodiode109. The first and second photodiodes108,109convert the extracted light into an electrical current, which is received by the amplifier110. The amplifier110can then be connected to a spectrum analyzer or similar device where the electrical current can be viewed. Also, those skilled in the art will realize that any configuration in which the first and second photodiodes108,109are connected so as to generate an electrical signal that is proportional to the algebraic difference of the light extracted in the first and second directional couplers104,204would work equally as well.

The amount of light extracted from the first optical signal in the second directional coupler204is determined by the coupling coefficient (k2). For the second directional coupler204shown inFIG. 2, decreasing the coupling coefficient (k2) will couple more light into the optical loop106, while increasing the coupling coefficient (k2) will direct more light to the second photodiode109. Those skilled in the art will realize that adjusting the coupling coefficient of the directional coupler is well known. The remaining light in the first optical signal is coupled into the optical loop106through first output port215(See FIG.3). The first optical signal then continues to circulate through the optical loop106, and first and second directional couplers104,204, where a percentage of light is extracted each time the optical signal passes through the first and second directional couplers104,204. Those skilled in the art will realize that adjusting the coupling coefficient of the directional coupler is well known.

The optical delay line filter100also comprises an impulse cancellation line111as shown in FIG.2. The impulse cancellation line111is connected between the second output105of the optical splitter101and the second photodiode109. For the desired filtering characteristics to be obtained, the initial light extracted and coupled to the first photodiode108should be removed. The impulse cancellation line111couples the second optical signal, which is time delayed, to the second photodiode109so that it reaches the second photodiode109at the same time the initial light extracted from the first optical signal in the first directional coupler104reaches the first photodiode108. Also, the amplitudes of the first and second optical signals are adjusted in the optical splitter101so the amplitude of the initial light extracted from the first optical signal in the first directional coupler104is the same as the second optical signal. Adjusting the amplitudes of the first and second optical signals in the optical splitter101is well known. Because of the high common mode rejection ratio associated with the interconnected first and second photodiodes108,109, the second optical signal on the impulse cancellation line111will neutralize the initial light extracted and received at the first photodiode108. It is important to note that only the initial light extracted from the first directional coupler104is neutralized. As the first optical signal continues to circulate through the optical loop106ad infinitum, the light extracted at the first and second directional couplers104,204and coupled to the first and second photodiodes108,109is not neutralized.

Shown inFIG. 4is an exemplary time domain diagram of the extracted light from the first optical signal, when the optical signal inputted to the optical splitter101is a delta-function impulse. Of course, as discussed above, the filter of the present invention is applicable to any continuous or discrete modulation signal. However, the response of a filter can be determined for all frequencies if one knows the response to a delta-function impulse. This greatly simplifies the analysis of bipolar delay line filters. Also, as will be seen, taking the Fourier transform of the time domain response shown inFIG. 4will result in the frequency response of the filter.

The pulses denoted as an+inFIG. 4are the positive pulses received by the first photodiode108. The value of n corresponds to when the light is extracted. For example, n=2 corresponds to the light that was extracted when the delta-function impulse went through the first directional coupler104the second time, n=3 corresponds to the light that was extracted when the delta-function impulse went through the first directional coupler104the third time, etc. The pulses denoted as an−are the negative pulses received by the second photodiode109. The value of n corresponds to when the light is extracted. For example, n=1 corresponds to the light that was extracted when the delta-function impulse went through the second directional coupler204the first time, n=2 corresponds to the light that was extracted when the delta-function impulse went through the second directional coupler204the second time, etc. It is worth noting that for the positive pulses received by the first photodiode108, there is no positive pulse corresponding to n=1. This is a result of the impulse cancellation line111providing a second optical signal which is used to neutralize the initial light extracted from the first optical signal in the first directional coupler104. The energy of a given positive or negative pulse for a given value of n can be found by the following recursive relationships shown in equations 1 and 2.
an+=(k1)2×(1−k1)n−2×(1−k2)n−1, forn≧2  (1)
an−=k1k2(1−k1)n−1×(1−k2)n−1, forn≧1  (2)

The energy of the very first pulse, a1+, to reach the first photodiode108is given by equation 3:

a1+=(Initial Energy of First Optical Signal)×(1−k1)  (3)

The energy of the second optical signal should thus be adjusted to this value in order to neutralize a1+.

Furthermore, as shown inFIG. 4, the delay time between subsequent positive pulses and negative pulses is denoted as τ1and τ2, respectively. The value of τ1is determined by the time it takes the delta-function impulse to travel from the center of the first directional coupler104to the center of the second directional coupler204. The value of τ2is determined by the time it takes the delta-function impulse to travel from the center of the second directional coupler204to the center of the first directional coupler104. The output shown inFIG. 4has τ1and τ2equal to each other; however, those skilled in the art will realize that changing τ1and τ2will produce different outputs. In this way τ1and τ2can be easily adjusted by increasing or decreasing the amount of optical fiber in the optical loop106that couple the first and second directional couplers104,204together.

As discussed above, taking the Fourier transform of the time domain output that results from a delta-function impulse at the input produces the frequency response of the filter. Equation 4 shows the frequency response for a filter in which the coupling coefficients, k1and k2, of the first and second directional coupler104,204are equal, and the time delays τ1and τ2are equal. Those skilled in the art will realize that other combinations of delay lengths and coupling coefficients will lead to different responses.F⁡(f)=-(1-k)⁢k2⁢⁢exp⁡[j⁢⁢2⁢⁢π⁢⁢f⁢⁢τ](1+(1-k)⁢exp⁡[j⁢⁢2⁢⁢π⁢⁢f⁢⁢τ])⁢(2-k)(4)τ=τ1=τ2k=k1=k2f=1/τ

Shown inFIG. 5is the frequency response300of the filter according to the time domain output shown inFIG. 4, using equation 4 and k=0.3. As shown inFIG. 5, the peaks are shifted away from zero, which is an important feature of bipolar filters.