Abstract
A waveguide/diode is manufactured by taking into account the effect that losses have on a multi-mode optical signal as it transits through the waveguide/diode. In particular, the loss effects that are caused by higher order modes in the optical signal as it passes back and forth through the cross charge region of a PN junction are considered. The consequent stretching of the cross coupling distance for the optical signal is then evaluated to minimize the required length for the waveguide/diode.
Claims
1. A device for switching an optical signal from one pathway to another, the optical signal having a fundamental (first order) mode and higher order modes, the device comprising: an optical waveguide/diode switch having a first end and a second end with a length L therebetween and a dimension in a cross-section along its length L in the order of the optical signal wavelength, wherein the waveguide/diode defines an axis and includes a PN junction having a space charge region with a depletion width w.sub.d, wherein the PN junction is centered on the axis and extends along the length L, and wherein the waveguide/diode has an index of refraction n accounting for a plasma dispersion effect in the PN junction and losses due to absorption effects in the waveguide/diode, n=(n.sub.o+n.sub.o)+i(.sub.o+.sub.o), where n varies along the length L of the waveguide/diode to establish a distance-dependent, propagation interference distance .sub.c for the optical signal; and a voltage source connected with the PN junction to provide a base voltage (V=V.sub.base) with a consequent distance-dependent, propagation interference distance .sub.c for the optical signal, and further wherein the voltage source selectively provides a switching voltage V.sub. (V=V.sub.base+V.sub.) to establish a distance-dependent, propagation interference distance .sub.c for the optical signal with a change in the distance-dependent interference length equal to .sub.c(.sub.c=.sub.c.sub.c) for the optical signal to switch the optical signal from one pathway to another at the length L in the waveguide/diode where .sub.c.sub.c.
2. The device of claim 1 wherein the waveguide/diode is a reverse bias diode.
3. The device of claim 1 wherein the distance-dependent, propagation interference distance .sub.c results from a modal interference between the fundamental (first order) mode and a second order mode.
4. The device of claim 1 wherein the propagation interference distance .sub.c for a multi-mode optical signal is expressed as .sub.c=.sub.cn+.sub.cn+.sub.c, wherein .sub.cn is a constant set by the physical characteristics of the waveguide/diode, .sub.cn is a consequence of phase considerations in the index of refraction n, and .sub.c is a consequence of loss considerations in the index of refraction n.
5. The device of claim 4 wherein the index of refraction varies along the cross-section dimension of the waveguide/diode due to free electron and hole distributions variations around the PN junction.
6. A method for manufacturing a device to switch an optical signal from one pathway to another, the optical signal having a fundamental (first order) mode and higher order modes, the method comprising the steps of: providing an optical waveguide/diode, wherein the waveguide/diode defines an axis and has a PN junction oriented on the axis to create a propagation interference distance .sub.c for the optical signal; determining a distance-dependent change, .sub.c, in the propagation interference distance .sub.c for the waveguide/diode, wherein .sub.c equals the difference between the interference distance .sub.c established for the waveguide/diode when a first voltage is applied across the PN junction, and an interference distance .sub.c established for the waveguide/diode when a second voltage is applied across the PN junction (.sub.c=.sub.c.sub.c); and establishing a length L for the waveguide/diode between a first end and a second end thereof, to switch the optical signal from one pathway to another at the length L, where .sub.c.sub.c.
7. The method of claim 6 wherein the waveguide/diode has a dimension in a cross-section along its length L in the order of the optical signal wavelength.
8. The method of claim 6 where in the PN junction has a space charge region with a depletion width w.sub.d, wherein the PN junction is centered on the axis and extends along the length L, and wherein the index of refraction n accounts for a plasma dispersion effect in the PN junction and losses due to absorption effects in the waveguide/diode.
9. The method of claim 8 wherein n=(n.sub.o+n.sub.o)+i(.sub.o+.sub.o), and n varies along the length L of the waveguide/diode to establish the distance-dependent, propagation interference distance .sub.c for the optical signal.
10. The method of claim 9 wherein the propagation interference distance .sub.c for a multi-mode optical signal is .sub.c=.sub.cn+.sub.cn+.sub.c wherein .sub.cn is a constant that is set by the physical characteristics of the waveguide/diode, .sub.cn is a consequence of phase considerations in the index of refraction n, and .sub.c is a consequence of loss considerations in the index of refraction n.
11. The method of claim 6 wherein the waveguide/diode is a reverse bias diode.
12. The method of claim 6 wherein the first voltage is a base voltage (V=V.sub.base) and the second voltage is a switching voltage V.sub. (V=V.sub.base+V.sub.).
13. The method of claim 6 wherein the distance-dependent, propagation interference distance .sub.c results from a modal interference between the fundamental (first order) mode and a second order mode.
14. A method for manufacturing a device to switch an optical signal from one pathway to another, the optical signal having a fundamental (first order) mode and higher order modes, the method comprising the steps of: providing an optical waveguide/diode having a first end and a second end, wherein the waveguide/diode defines an axis and includes a PN junction having a space charge region with a depletion width w.sub.d, wherein the PN junction is centered on the axis, and wherein the waveguide/diode has an index of refraction n accounting for a plasma dispersion effect in the PN junction and losses due to absorption effects in the waveguide/diode, with a consequent distance-dependent propagation interference distance .sub.c for transit of the optical signal along the axis through the waveguide/diode; connecting a voltage source with the PN junction to provide a base voltage (V=V.sub.base) with a consequent distance-dependent, propagation interference distance .sub.c for the optical signal, and further to selectively provide a switching voltage V.sub. (V=V.sub.base+V.sub.) to establish a distance-dependent, propagation interference distance .sub.c for the optical signal, with a change in the distance-dependent interference distance equal to .sub.c (.sub.c=.sub.c.sub.c); and establishing a length L between the first end and the second end of the waveguide/diode to switch the optical signal from one pathway to another at the length L, where .sub.c.sub.c.
15. The method of claim 14 wherein n=(n.sub.o+n.sub.o)+i(.sub.o+.sub.o), and n varies along the length L of the waveguide/diode to establish the distance-dependent, propagation interference distance .sub.c for the optical signal.
16. The method of claim 15 wherein the propagation interference distance .sub.c for a multi-mode optical signal is .sub.c=.sub.cn+.sub.cn+.sub.c wherein .sub.cn is a constant that is set by the physical characteristics of the waveguide/diode, .sub.cn is a consequence of phase considerations in the index of refraction n, and .sub.c is a consequence of loss considerations in the index of refraction n.
17. The method of claim 14 wherein the waveguide/diode has a dimension in a cross-section along its length L in the order of the optical signal wavelength.
18. The method of claim 14 wherein the waveguide/diode is a reverse bias diode.
19. The method of claim 14 wherein the distance-dependent, propagation interference distance .sub.c results from a modal interference between the fundamental (first order) mode and a second order mode.
20. The method of 14 wherein the index of refraction varies along the cross-section dimension of the waveguide/diode due to free electron and hole distributions variations around the PN junction.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which:
(2) FIG. 1 is a perspective view of a waveguide/diode in accordance with the present invention;
(3) FIG. 2 is cross-section view of the waveguide/diode as seen along the line 2-2 in FIG. 1, showing a profile of the PN junction of the present invention;
(4) FIG. 3 is a cross-section view of the waveguide/diode as seen along the line 3-3 in FIG. 1 showing the difference in beam paths between a light beam that is influenced by a switching voltage V.sub. (dashed line) and one that is not influenced (solid line);
(5) FIG. 4 is a perspective view of an alternate embodiment of the present invention;
(6) FIG. 5 is a cross-section view of the waveguide/diode as seen along the line 5-5 in FIG. 4;
(7) FIG. 6A is a graphical comparison showing exemplary energy amplitude levels of the first (fundamental) mode and the second order mode in an optical signal over a cross-section dimension of the waveguide/diode as seen along the line 2-2 in FIG. 1;
(8) FIG. 6B is a graphical comparison showing exemplary energy amplitude levels in the first (fundamental) mode and the second order mode of the optical signal shown in FIG. 6A after one cycle of the optical signal;
(9) FIG. 6C is a depiction of the effect of the combined energy amplitude levels shown in FIG. 6A;
(10) FIG. 6D is a depiction of the effect of the combined energy amplitude levels shown in FIG. 6B;
(11) FIG. 7 is a representative graph showing changes in the interference length .sub.c of an optical signal relative to waveguide/diode length under the influence of a change in voltage bias from the voltage source; and
(12) FIG. 8 is a graph showing the cumulative change in the interference length .sub.c of an optical signal as a function of waveguide length caused by a change in voltage bias as indicated in FIG. 7.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
(13) Referring initially to FIG. 1, a waveguide/diode in accordance with the present invention is shown and is generally designated 10. Preferably, the waveguide/diode 10 is made of a semiconductor material, such as silicon. Also, as shown, the waveguide/diode 10 preferably has an elongated body portion that extends through a length L from a first end 12 to a second end 14, and it defines a central axis 16. Further, two input optical waveguides 18a and 18b are attached to the first end 12 of the waveguide/diode 10, and a pair of output optical waveguides 20a and 20b are attached to the second end 14 of the waveguide/diode 10.
(14) By referring to FIG. 2, it will be appreciated that the waveguide/diode 10 includes a P-type region 22 and an N-type region 24, with a cross charge region 26 that is located between them. Functionally, for purposes of the present invention, the P-type region 22 and the N-type region 24 of the waveguide/diode 10, together with the cross charge region 26, present a typical PN profile that is like any PN junction known in the pertinent art. Further, like any diode known in the pertinent art, the waveguide/diode 10 of the present invention includes an anode 28 and a cathode 30 that are respectively connected with a voltage source 32. For the present invention, because the anode 28 (positive) is connected to the N-type region 24 of the waveguide/diode 10, and the cathode 30 (negative) is connected to the P-type region 22, the waveguide/diode 10 is reverse biased.
(15) Still referring to FIG. 2, it is to be appreciated that the voltage source 32 will generate a base voltage V.sub.base which establishes the reverse bias for the waveguide/diode 10. In addition to V.sub.base, the voltage source 32 will also provide a switching voltage V.sub. that is necessary for an operation of the present invention. The importance here is that, as shown in FIG. 2, with only V.sub.base applied, the depletion width w.sub.d of the cross charge region 26 will be different from the depletion width w.sub. that results when the switching voltage V.sub. V.sub. is applied. The consequence here is that as the switching voltage V.sub. changes the depletion width between w.sub.d (dashed lines) and w.sub. (solid lines), the effective index of refraction n of the waveguide/diode 10 having the cross charge region 26 will also be changed.
(16) In another aspect of the present invention, it is an important feature that the two input optical waveguides 18a and 18b be eccentrically attached to the first end 12 of the waveguide/diode 10. This attachment should be made at a predetermined location that is at an offset distance d.sub.offset from the central axis 16. Specifically, this is done to create higher order modes (e.g. in particular, a second order mode) for optical signals as they transit the length L of waveguide/diode 10. As best seen in FIG. 3, the purpose of creating a higher order mode for an optical signal is to have it proceed through the waveguide/diode 10 on a sinusoidal wave path 36/36 having a mode propagation interference length .sub.c, rather than along a straight path as would be the case for an optical signal having only a single, fundamental mode.
(17) For an operation of the present invention, an optical signal enters the waveguide/diode 10 from the input optical waveguide 18a. The signal can then be directed from the waveguide/diode 10 onto either the output optical waveguide 20a or the output optical waveguide 20b simply by applying, or withholding, the switching voltage V.sub.. Functionally, this happens because V.sub. causes the depletion width w.sub.d of the cross charge region 26 to change. Consequently, the effective index of refraction n of the waveguide/diode 10 having the cross charge region 26 will also change. In turn, as the optical signal transits the length L of the waveguide/diode 10 back and forth through the cross charge region 26 in the plane 34, the second order mode propagation interference distance, .sub.c, of the optical signal also changes by an increment of .sub.c as shown in FIG. 3. As all this happens, .sub.c is cumulative for each time the wave path 36/36 of the optical signal passes through the cross charge region 26. Accordingly, instead of following an unaltered wave path 36, the summation of .sub.c increases along the wave path 36 of the optical signal as it progresses through the waveguide/diode 10. The consequence for the wave path 36 of the optical signal is that it can be changed to a wave path 36 for directing the optical signal from one output optical waveguide 20a onto the other output optical waveguide 20b, or vice versa. Mathematical expressions to support this consequence are satisfied when L and N are selected such that switching occurs when L=N.sub.c and .sub.c(N1) .sub.c, where N is a positive real number greater than 10.
(18) Referring now to FIG. 4 an alternate embodiment for the waveguide/diode of the present invention is shown and is generally designated 40. As shown, the waveguide/diode 40 is structurally similar to the waveguide/electrode 10 disclosed above. Both waveguide/diodes 10 and 40 have a similar purpose, and they function similarly. They differ from each other in the nature of the materials used for their manufacture and their consequent electrical characteristics. In detail, the distinctive characteristics of the waveguide/diode 40 will be best appreciated with reference to FIG. 5.
(19) With reference to FIG. 5 it is to be appreciated that the P-type region 22 and the N-type region 24 are made of different semiconductor materials. In particular, the difference is characterized by the fact that the regions 22 and 24 each exhibit a different plasma dispersion effect. Nevertheless, the P-type region 22 and the N-type region 24 need to be somehow bonded, or joined, together.
(20) As shown in FIG. 5, the present invention envisions the use of an oxide layer 42 for the purpose of joining the regions 22 and 24 together. For example, the present invention envisions a PN junction wherein the N-type region 24 is made of a Multiple-quantum-well (MQM) material such as Indium-Gallium-Arsenide-Phosphide (InGaAsP). In combination with this N-type region 24, the P-type region 22 is envisioned to be silicon (Si), and the oxide layer 42 is silica (SiO.sub.2). Other combinations of materials are, or course, possible. In each combination, however, it is important that the two materials are different, and that they will, accordingly, have different plasma dispersion effects.
(21) A consequence of the waveguide/diode 40 is that the N-type region 24 will exhibit an N depletion region 44, and the P-type region 22 will exhibit a P depletion region 46. Together these regions 44 and 46 function similarly to the space charge region 26 of the waveguide/diode 10. In a variation for the alternate embodiment of the waveguide/diode 40, the present invention envisions in a different embodiment, an elimination of the oxide layer 42. In this case, the present invention envisions that the N-type region 24 and the P-type region 22 will be grown together.
(22) In another embodiment of the present invention the current waveguide/diode can also be realized in a structure similar to that shown in FIG. 1 and FIG. 2. In this case, the PN junction waveguide/diode is made of a P-type region 22 from a first semiconductor material (e.g. poly-silicon), an N-type region 24 from a second semiconductor material (e.g. InGaAsP), and a buffer oxide material (e.g. silica) between the P-type semiconductor and the N-type semiconductor.
(23) FIGS. 6A-D are to be considered collectively as they all pertain to the same multi-mode optical signal during its transit through the waveguide/diode 10. The import here, however, is not so much on distance traveled but rather on the back-and-forth changes in the location of energy in the waveguide/diode 10. In particular, these changes are most important relative to the effect caused by the cross charge region 26 of a PN junction in the waveguide/diode 10. With this in mind, the disclosure below regarding FIGS. 6A-D is directed to energy amplitudes of the various modes in a multi-mode optical signal.
(24) FIG. 6A depicts an energy amplitude 50 for the fundamental mode of a multi-mode optical signal, together with an energy amplitude 52 for the higher order modes (primarily second order) of the optical signal. In FIG. 6A, the energy amplitudes 50 and 52 are shown in a same cross-section 54 of the waveguide/diode 10 with their respective relationship to the cross charge region 26 of a PN junction. It is important to note in FIG. 6A that the energy amplitude 50 of the fundamental mode is less affected by the cross charge region 26 (i.e. plasma dispersion effect) than is the energy amplitude 52 of the higher order modes. Stated differently, the higher order modes are more attenuated.
(25) FIG. 6B depicts the optical signal at a subsequent cross-section 56, after the optical signal has completed a cycle. At the cross-section 56 it is noted that the energy amplitude 50 of the optical signal is substantially unchanged and is similar to the previous energy amplitude 50 profile it had at cross-section 54 (FIG. 6A). On the other hand, unlike the fundamental mode, the higher order modes have actively interacted with the cross charge region 26. Accordingly, the profile of the energy amplitude 52 for the higher order modes has changed. The consequence here is two-fold. For one, because the energy amplitudes 50 and 52 for the optical signal are cumulative, the combined energy amplitude 58 for the optical signal effectively follows the frequency of the fundamental mode. For another, due to its increased interaction with the cross charge region 26, the higher order modes experience greater losses.
(26) With reference to FIGS. 6C and 6D, it will be seen that the combined (total) energy 58 in an optical signal stays relatively constant in amplitude, but changes location in the waveguide/diode 10. Specifically, FIG. 6C shows the combined (total) energy amplitude 58 of the optical signal at cross section 54, and FIG. 6D shows the combined (total) energy amplitude 58 of the optical signal at cross-section 56. Thus, the optical signal is shown to effectively go back and forth from one cross-section (e.g. cross-section 54) to another (e.g. cross-section 56), and vice versa. As it does so, it transits through the cross charge region 26 of the waveguide/diode 10. With this in mind it is important to note that, from an energy perspective, the fundamental mode dominates and remains constant. The higher order modes, however, are more attenuated and thereby introduce continuously increasing losses.
(27) Referring now to FIG. 7, the distance-dependent, propagation interference changes, .sub.c, are shown relative to the waveguide length L. In particular, these changes .sub.c are shown as a consequence in the bias voltage V. Of importance here is the fact that .sub.c itself changes at different rates, depending on the bias voltage V. The result of this is that the total .sub.c for a given length of waveguide/diode 10 is an integration of .sub.c along the given length, .sub.c.
(28) By way of example, consider the change in propagation interference distance .sub.c(350) at the point 60 on the waveguide/diode 10, i.e. when an optical signal has traveled 350 m along the length L of the waveguide/diode 10. As indicated in FIG. 7, at the point 60, .sub.c(350) will have an approximated value of 0.8473 under the influence of a bias voltage of zero volts. On the other hand, .sub.c(350) will have a value of 0.8478 when the waveguide/diode 10 is under the influence of a bias voltage of 0.6 volts. The difference here produces a change in the interference distance .sub.c(350) of approximately 0.0005 m. Now consider the propagation interference distance .sub.c(750) at point 62 on the waveguide/diode 10. At this time the same optical signal has traveled another 400 m along the length L of the waveguide/diode 10 to the point 62. Again, depending of the voltage bias, at the point 62, .sub.c(750) will have a value of 0.8481 under the influence of a bias voltage of zero volts. On the other hand, .sub.c(750) will have a value of 0.8491 under the influence of a bias voltage of 0.6 volts. The difference here produces a change in the interference distance .sub.c(750) of approximately 0.0010 m. Note: .sub.c(750) is approximately twice the interference distance .sub.c(750).
(29) With the consequences of FIG. 7 in mind, FIG. 8 shows the integrated effect. Again by way of example, FIG. 8 shows that .sub.c results in a variation of .sub.c through a length L of the waveguide/diode 10 that is substantially a parabolic curve 64. This is so because .sub.c increases dramatically as the length L increases. Specifically, the values given in FIG. 8 are for a waveguide/diode 10 having a length L=1000 m, with voltage changes between V.sub.base=0 volts and a switching voltage V.sub.=0.6 volts. The result is an overall .sub.c that is equal to approximately 0.85 m. Compare these values with those of FIG. 7 for a waveguide/diode 10 having an index of refraction n that creates a distance-dependent, propagation interference distance .sub.c. Specifically, for the example given, it is shown that the waveguide/diode 10 will switch a multi-mode optical signal from one pathway to another when .sub.c=.sub.c.
(30) While the particular Improved Reverse Bias Modulating Waveguide/Diode as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.
