MXPA97010293A - Periodic optical filters from mach-zehn - Google Patents

Abstract

The present invention relates to a Mach-Zehnder filter with a very aperiodic transfer function which includes a sharply defined wide pass band, the filter can be used together with optical amplifiers.

Description

PERIODIC OPTICAL FILTERS OF I1 CH-ZEHNDER BACKGROUND OF THE INVENTION The present invention relates to selective wavelength devices that can be used in opt-Lcos communication systems. The fiber optic communication systems use selective wavelength devices for various purposes such as, for example, to send light from different wavelengths to different destinations or as optical filters that perrm + in which the light in one The desired band of wavelengths pass over the communication channel to the +? ernpo by removing or attenuating the light at wavelengths outside the desired band. The selective wavelength lengths must meet the requirements in demand for use in practical communication systems. The devices must be able to separate the wavelengths that differ from each other only by some nanornetros. The selective wavelength device must be reliably and durably environmentally stable. Also, the selective wavelength device must operate with a relatively low loss of optical energy, that is, the device must not dissipate substantial amounts of the optical energy supplied at the same time in the desired wavelength bands.

Mach + Zehnder M + erferornets have been used as selective wavelength devices in optical communication systems. As illustrated in Figure 1, a conventional Hach-7ehnder interface includes a pair of fibers F-1 and F ?. The fibers are coupled to each other in a first coupler-Cl and a second coupler C2. The couplers are arranged to transfer light from one fiber to another. As explained hereinafter, the couplers may be so-called tapered couplers overlapped where narrow, elongated portions of the fibers are closely juxtaposed with each other within an external matrix or coating. The couplers can be couplers of 3dB, arranged to transfer approximately half of the optical energy supplied in one fiber to the other fiber. The fibers Fl and F2 have regions of phase change with different optical path lengths disposed between the couplers. In this way, the optical path length over the phase change region in the fiber Fl is different from the optical path length over the phase change region in the fiber F2. As used in this description, the term "optical path length" is a measure of the time required for light at a given wavelength and at a given propagation time to pass through the fiber from one end to the other. The difference of the optical path length has been provided by making the phase change region of one fiber physically longer than the other, by making the two fibers Fl and F2 with propagation constants dLferrent that the light phase velocity den The fibers of the two fibers are different, or both. The fibers can be provided with different propagation constants making the fibers with different propagation index profiles. Where the fibers are "pitch index" fibers, incorporating a core having a relatively high refractive index and a relatively low refractive mdLe coating covering the core, the two fibers may have cores of different refractive indices, core diameters d? fer-ent.es, different coating refractive indices or some combination of these. In spite of the particular mechanism used to produce optical path length difference, the single-stage Mach-Zehnder filter illustrated in Fig. 1 will direct light supplied through input 1 either to output 3 or to output 4 depending on the wavelength of the light. A typical single-stage Mach-Zehnder filter has a substantially periodic transfer function that refers to the proportion of light directed to a particular output port at the wavelength of the light. That is to say, the amount of light that appears in any particular output port varies repetitively as the wavelength of the light varies. A typical transfer function for a single-stage Mach-Zehnder device is illustrated in Figure 2. It includes a series of step bands alternating 5 and slots b. In the wavelengths within the pass bands, a substantial portion of the light supplied through port 1 is present in port 3.; in ls wavelengths in the slots ñ, little or no light supplied through port 1 reaches the port.] "The transfer function is periodic in that the pass bands and slots are presented back to regular substantial intervals on the axis of the wavelength. Although several features can be achieved by coupling several Mach-Zehnder devices in series, or by making each device with more than two optical path lengths, further improvements would be desired. In particular, there are needs for optical filters that will substantially pass all the light within a single relatively broad band of wavelengths, commonly referred to as a "pass band" and that would sharply attenuate the wavelengths of light being just outside the wavelength. step band. This need arises particularly in connection with optical amplifiers. An optical amplifier is a device that adds energy to an optical signal. It mainly binds to compensate for the loss of energy in transmission through longer optical fibers. A form of an optical amplifier is known as an erbium-contaminated fiber amplifier (EDFA). The EDFA includes a fiber optic length formed of specific glass materials containing the erbium element. The light beam of optical input, at a wavelength used for signal transmission, passes in the fiber along with light at another shorter wavelength referred to as a "pump" light. The energy of the pump is absorbed and stored in the fiber, so that the signal light beam passes through the fiber, this energy is released and incorporated into the signal light beam. The fiber amplifiers contaminated with erbium can be used with wavelengths in an operating band centered at approximately 1.55 micrometers. Commonly, the band of useful operation of the amplifier is approximately 30 nm (0.U3 rnicrornet ro) wide or nm. In this way, the band of useful operation of the amplifier can cover wavelengths of about 1.53 millimeters to about 1.56 millimeters. This operating band is wide enough to allow simultaneous amplification of several different light beams at slightly different wavelengths. Unfortunately, the EDFA also provides some illumination to light at wavelengths slightly outside its useful operating band. In other words, the EDFA gain curve does not have a sharp cut at the edges of the operating band. In this way, where the input signal incorporates spurious components or "noise" at wavelengths slightly outside the useful operating band, likewise these spurious components will be amplified to a certain degree. In addition, the amplifier itself can introduce noise at wavelengths that are slightly outside the operating band. In both cases, the amplified noise passes downstream in the system and degrades the operation of the system. In addition, the optical energy converted from the fiber to amplify the noise is not available to amplify the desired serial. Thus, there is a substantial need for a simple filter that can be applied to the input or output of an EDFA to suppress signals that are generally outside the desired EDFA band of operation, but that will pass substantially all of the lengths wave within the desired band of operation without substantially attenuating them. In particular, there is a need for filters that can pass-wavelengths from about 1,549 to about 1,565 millimeters to the same time also suppressing signals with wavelengths of about 1,525 to about 1,545 micrometers. There are corresponding needs for optical filters with wide pass-bands and acute attenuation of wavelengths 1 and generally outside a desired passband for use with other types of optical amplifiers and for use with other devices. There are also needs for the inverse type of filter, that is, a filter that will suppress light at wavelengths within a wide band, but that will provide essentially no attenuated light path that is slightly outside of that band.

BRIEF DESCRIPTION OF THE INVENTION The present invention is dedicated to these needs. One aspect of the present invention provides an iNter-ferornetpco Mach-Zehnder device including an input port, an output port, an input end coupler and an output end coupler. The device further includes first and second optical paths that extend between the couplers. The input coupler-end is adapted to direct light applied at the input port to the first and second optical paths while the output end coupler is adapted to combine light in the first and second optical paths and directs the combined light to the port of exit. The trajectories have optical path lengths li and I2 respectively between the couplers. At least one of the path lengths varies non-linearly with the wavelength X of light that passes through this path. The variation of the optical path lengths with the wavelength is selected so that in the vicinity of an operating wavelength), the transfer function refers to the proportion of light supplied through the port of transmission. The input that appears in the output port is substantially aperiodic and the transfer function includes a relatively wide pass band or slot for which the value of the transfer function is almost a minimum or almost one maximum over a Relatively wide range of wavelengths that encompass AO-L main passband or slot constitutes the widest passband or slot of the interference function in the neighborhood of > or »Ma preferred blernente, the main pass band or slot has a maximum average width? at least about twice the maximum average width of the pass band or ranun, adjacent near masses. The required issues of variation in lengths of the optical path can be established mathematically as d2 (Dl) > B dl2 where: 1 is (li-12); A is a maximum change speed in the path length with the wavelength and B is a minimum curvature of the path length with respect to the wavelength. In this way, the preferred devices according to this aspect of the invention have a slower speed in the path length with respect to the wavelength, but a high curvature of the path length with respect to the wavelength. Preferably, A is approximately ir / A or less and B is approximately 5 / (?) 2 or more, where? is the width of the pass band or slot. Very preferred, at the wavelength of operation, A zero. The optical paths can be constituted by fibers or by other waveguides. In a particularly preferred arrangement, the first and second opthal paths are constituted by first and second fibers, and the fibers extend through the couplers. A portion of a fiber extending beyond the input coupler constitutes the input port, while the portion of the same fiber, or other fiber, extends beyond the output coupler and constitutes the exit port. Automatically, one or more additional fibers can form the input and output ports, and these additional fibers can be coupled to first and second fibers in the input end and output end couplers. Most preferred, the couplers are tapered overdraft couplers, wherein each fiber includes a tapered coupling region and the tapered coupling regions of the fibers are juxtaposed with each other. The couplers further include an overcoating which surrounds the tapered coupling regions of the fibers. Preferably, the device includes an integral housing with the overcovers of the couplers, the housing surrounding the fibers between the couplers. As described, for example, in a U.S. Patent. commonly assigned 5,295,205, the stable, monolithic Mach-Zehnder device can be formed by placing the fibers through the hole of a glass tube, heating the glass tube, and crushing the tube in the fibers, and also heating and stretching the tube and fibers in two separate locations to form the couplers. Most preferred, the couplers are arranged to direct substantially equal portions of the light applied through the input port through each of the two paths, although an unequal division can also be used as described below. Other aspects of the present invention provide optical systems incorporating selected Mach-Zehnder-wave length and wave devices having a sharply defined passband as described above in series with an optical-amplifier or other device, wherein the band of the Mach-Zehnder device is substantially aligned with the operating band of the amplifier or other device. As discussed later, the filter improves the signal to noise ratio of the system. Other objects, features and advantages of the present invention will be more apparent, by means of the detailed description of the preferred embodiments presented below, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic diagram of a conventional Mach-Zehnder optical device. Figure 2 is a graph illustrating the periodic variation in the transfer function that refers to the transmission of light at wavelength in the device illustrated in Figure 1. Figure 3 is a diagrammatic view of a Mach-Zehnder device in accordance with one embodiment of the present invention. Figure 4 is a graph illustrating the transfer function of the device of Figure 3. Figure 5 is a diagrammatic view of an amplifier system according to another embodiment of the invention. Figure 6 is a graph illustrating certain transfer functions associated with the system of Figure 5. Figure 7 is a view similar to Figure 3, but illustrates a device in accordance with yet another embodiment of the invention. Figure 8 is a graph illustrating a transfer function for a system in accordance with another embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED MODALITIES A Mach-Zehnder internet device in accordance with one embodiment of the invention includes a first fiber 1!) And a second fiber 12. Fibers 10 and 1? they are pitch index fibers. In this way, the fiber 10 includes a core 10a shown in divided lines in Figure 3 and a liner 10b covering the core. Fiber 1? it includes a similar nucleus and reverse. The fibers 10 and 12 can be formed from conventional materials such as silica glasses with contaminants or additives such as germnan and fluorine to adjust the refractive indices of the cores and coatings to the desired values. The fibers are disposed within a tubular-integral glass housing 14. The housing 14 and fibers 10 and 1 are elongate and narrow to form a first overcoating coupler 16 and a second overcoating coupler 18. The first overcoating coupler includes a narrow coupling region 20 of fiber 10 extending from side to side with a tapered, narrow engagement region 22 of the fiber 12. These coupling regions are encased in an overcoat 24 integral with the housing 14. The second overcoating coupler 18 includes similar tapered coupling regions 26 and 28 and overcoating 30. the couplers and housing can be manufactured by a method as described in the US Pat. D, 29 !. , 205, The description of which is incorporated herein by reference. Briefly, as described in the '205 patent, said structure can be formed by placing the two fibers side by side on the tube 14, assuming the housing, heating and softening the tube and crushing the tube in the fibers. The procedure also includes heating the tube and fibers to a sufficient degree to ablate the fibers in the locations that are to form the couplers. The tube and fibers are stretched as a unit at the location of each coupling to lengthen-and constrict the tube and fibers imul- ately. The two fibers 10 and 12 can have the same physical length z between the couplers 16 and 18. In this way, the fiber 10 has a central portion 32 defining a first path between the two couplers, while the fiber 12 has a central portion 34 defining a path between the two couplers, both paths having the same physical length. Both fibers extend through the couplers to end regions disposed beyond the couplers. A first end region the first fiber 10 provides a first input port 36, while the second end region 38 of the same fiber provides an output port at the opposite end of the device. The fiber end regions forming ports 36 and 38 are separated in the conventional manner to provide l /? a suitable end to couple-to other fibers in the optical system. The extreme regions 40 and 4? of the second fiber provide other ports. However, these additional ports end up fusing the ends of the fiber to form extremes of anti-reflexion. As described, for example, in the U.S. Patent. No. 4,979,972, the disclosure of which is also incorporated herein by reference, the anti-reflective finish can be formed by heating and pulling the ends of the fiber to separate it and further heating the end of the fiber to cause the glass form a round, ball-like end face having a diameter equal to or slightly less than the original outer diameter of the fiber coating. The two fibers have different optical properties. In particular, the load pattern in the propagation constants of the two fibers with wavelength is selected so that the proportion of light supplied to tr-birds from the input port 36 that appears at the output port 38 it varies with the wavelength of the light applied in accordance with a function of periodic interference. That is, the transfer function that refers to the proportion of light appearing at the output port 38 at the wavelength includes the features discussed above, such as a passband or slot at a preselected wavelength scale. . In general, for a Mach-Zehndor- metric device, in which the light is evenly distributed between the two paths, the proportion 1 of light is applied in an input port that appears on an output port. wavelength function: sepD / o I = cos2 cael 0 (1) where l is the difference in optical path length between the two trajectories. The relationship between I and \ is referred to herein as the "transfer function" of the device. For the device of Figure 3, where both trajectories have the same physical length z between the couplers: apDb or I = cos2 c a e 2 0 (2) where: 1 is the proportion of the light applied in the entrance port 36 that appears in the exit port 38; ß is the difference between the propagation constant ßi of the first path 32 between the couplers and the propagation constant 2 of the second path 34 between the couplers. "Propagation constant" is a measure of the phase velocity of light over a trajectory. For the propagation of light on a path defined by a fiber, the propagation constant varies with the wavelength? of the light. The variation of ß with is commonly referred to as? Ot? A or time. Under some conditions, light at a single wavelength can propagate through a single fiber in several different transmission modes, having different values of ß. However, the preferred embodiments of the present invention use fibers with relatively small nucle diameters so that light can be propagated in only an individual mode, so that light at any wavelength has only one constant of fi propagation The value of ß to which any value-given of X depends on factor-is such as the relation between the dLarnetro ele core of the fiber y? and the refractive indexes ni and r.2 «read the nucleus and reverse in the fiber. The refractive indexes themselves may depend, to some degree, on X. If ß varies linearly with X, then the transfer function I will have the periodic characteristic illustrated in figure 2. As can be seen in figure 2, The period of the transfer function is constant; Each one of its peaks has essentially the same width. In contrast, the devices according to the preferred embodiments of the present invention have transfer functions that are substantially aperiodic in wavelength. An aperiodic transfer function, as shown in FIG. 4, has a main band 50 centered on an operating wavelength Xo. Band 50 represents a group of wavelengths within which a relatively high proportion of the light applied at the port of entry appears at the port of departure, (T >; 0.5) and therefore the band 50 is referred to as a "step axis band". Adjacent bands 52 and 54 are groups the wavelengths to which a low proportion of the light applied at the input port reaches the output port (K0.5), and therefore these adjacent bands are referred to as "Slots" The bands 56 and 58 represent other pass bands. In the vicinity of), the widths of the bands change substantially with the wavelength length. The main step band 50 has a width Substorcially greater use than the width of the other bands. As used in this description with reference to a pitch or groove band, the term "width" means the width presented as the maximum total width or total width "FU1HM". The limiting wavelengths of a band become the wavelengths on either side of the center of the band at which T is halfway between the minimum or maximum value in the band in question and the minimum or maximum value in the band. next adjacent band. For example, the main passband 50 has wavelengths Xi and lower 2-and higher, while slot 52 has wavelengths 52 and limiting i. The width of a band is the difference between its limiting wavelengths. In this way, the width X of the main pass band 50 is (2 -?). The degree to which the transfer function is aperiodic can be presented as the ratio between the width of the main band, with the largest width, and the width of the next adjacent band.

Preferably, this relationship is at least initially optimized. To provide a substantially aperiodic ele ransference function with a broad band or wide band centered on X or t (D) it should be as small as possible. , and smaller than a value A which represents a maximum change velocity in the path length with length of on < the. On the other hand,? J2 (DI) elebe ser- grande < 112 and more than a value II that represents a second minimum derivative or "curvature" of the path length length with respect to the wavelength. For a pass band or X-width groove, and typical fibers for which it is reasonable to approximate the variation of l with parabolic horn, it should be approximately 6% / X or less, and preferably approximately 4%. X or less, while B must be approximately 3 / (?) 2 or more, and desirably approximately 5 / (X) 2 or more. For a device as shown in Fi 3, where both trajectories have equal length z, and both trajectories have substantially uniform propagation constants over the entire path length z between the couplers, 1 - ßz. The characteristics of the time delay (ß against X) for common fibers are known, and therefore the axis values can be calculated'-, for any fiber part. Another way to present the rules for the selection of fi re charac- ters for the case where both trajectories have z-length is as follows. First, to pr-oveer * a main step band or X-width slot, P aed (Db) o- g a Db (l2) - Db (l2) DI e di 0 (3) Substituting this value for z in equation (2) produces e a ou e p c; Db au [= eos2 e -? e2Dlc; eJ (Dh) au e <; eú e e di u (4) To provide the maximum variation in the period of the transfer function, the expression dC ... l / d? it must be-augmented to the maximum, where [...] r-represents the argument in brackets of the cosine in equation (4). Through differentiation, e u e < j2 (Db) ud [...] P e di2 u 1 - - Db eu di 2D1 e sed (Db) o2 ue ?: aee di 0 ü (5) By inspection in equation 5, this expression can increase maximally increasing the product of ß and the second derivative of ß with respect to the wavelength in the vicinity of the band < 1e step or desired slot, and minimizing the first derivative ele ß with respect to the wavelength in the vicinity of the desired Panela or slot. Generalizing to the case of equal or unequal trajectory axis lengths zi and 2, e where zi is the length of a trajectory a and Z2 is the length of the other path ia: d (i) / d X- ~ -Zid (ß?) / eJ -Z2d (ß2) / X (6) This expression should be as small as possible, while d2 (1) / d X2"Zl ej2 (ßt) / d? 2 - Z2 j2 (ß2) / d ? 2 (7) should be as large as possible The terms ej2 (ß1) / q 2 and ej2 (ß2) d X2 are proportional to the dispersion in the fibers constituting the trajectories. The trajectories, weighted by the trajectory lengths, zi and Z2, should be as different as possible for maximum magnification.The terrestrial devices as previously discussed can be used in conjunction with optical amplifiers. Optical amplifier according to another embodiment of the present invention includes a first amplifier having a long section 60 of contaminated fiber a erbium and an optical coupler or combiner 62 connected to an end shaft fiber 60. Fl combiner 62 has a input port bl to receive the input of light signals. The combiner 62 also has a port 63 connected to a source of pump radiation at approximately 1.48μm wavelength, i.e., fior example, the laser * diode axis operating at said wavelength. The opposite end of the fiber 60 is connected to a filter or selective device of conventional wavelength such as a Mach-Zehnder filter * normal newspaper adapted to block the radiation axis pumping, but it passes radiation at wavelengths in the band of operation of the amplifier, in the neighborhood of 1.55jjrn. The output 65 of this filter constitutes the output connection of the amplifier. Such fiber amplifiers are well known in the art. They are described, for example, in Palais, Fiber Optic Communicat ons, third edition, pp. 162-163 (Prentice-Hall, Tnc. 1992), the description of which is incorporated herein by reference. The amplifier provides a gain or energy ratio off at the output 65 to power at an input 61 that is substantially greater than one for a relatively wide range of wavelengths centered at approximately 1.55μrn. A typical curve 70 of the transfer function that refers to the gain of the optical amplifier at the wavelength is illustrated in Figure 6. As shown, the gain curve has a maximum average width of Ul,. The maximum gain can be many orders of magnitude, that is, up to approximately 40 dB.

")") As shown in Fig. 5, one or more ptc filters as shown above, each with a main bandwidth of width U50, are connected seriously with the amplifier. In this way, the first filter of the type discussed above can be connected in series with the input port so that the output port 30 of the filter is connected to the input shaft port of the amplifier, while the input port 36 of a second The filter as discussed above can be connected directly to the port of séllela 65 ejel filtro. The composite device has a sharper, sharper defined gain curve 72 which is the product of the transfer functions I of the filter with the gain transfer function of the amplifier. The composite device provides essentially all the gain provided by the amplifier on its own at frequencies within the pass bands U50 of the filter or filters, but provides an acute cutoff for the frequencies outside this scale. In this way, where the input signal incorporates noise or unwanted signals at frequencies slightly outside the passband, eg, the W50 filter, the filters substantially eliminate the unwanted signals. Also, where a filter is disposed over the filter input port, the unwanted signals are eliminated before they can absorb energy in the amplifier. Accordingly, the composite filter and the amplifier provide superior performance when used in an optical communication system employing wavelengths within the < Step 1 Use the filter Use In addition, the pass band of the Mitro U50 includes a wavelength scale that is broad enough to accommodate several wavelengths, modulated amplitude axis, or wavelength division or wavelength signal. For service with erbium-contaminated fiber amplifiers, the main passband desirably has a central wavelength of about 1.55 icro-inches and a width of at least about 10 inches, preferably between about 10 and about 40 meters. The composite gain function (Figure 6) achieved by multiplying the transference function of the filter (Figure 4) by the gain transfer function 70 of the Lmplifier arc rapidly decreases to wavelengths very far away from the central wavelength Xo of the Main step axis band. Where the filter is connected in series with an optical amplifier, the characteristics of the filter at wavelengths well outside the gain bandwidth U a 'of the amplifier are essentially negligible. In this manner, the aperiodic transfer function of the filter alone (of Figure 4) includes regions 57 and 59, at remote wavelengths of the center frequency of the main passband, where the interference function rapidly fluctuates as a wavelength function. This region of the filter transfer function is essentially eliminated by the gain characteristics of the amplifier. In other words, the transfer function of the filter depends only on the acute axis of the central peak cut in the gain transfer function of the filter. More generally, the apictive Mach-Zehnder filter in accordance with preferred embodiments of the present invention can be combined with another device because it has a relatively broad pass axis band, where the other device is effective for removing wavelengths in regions which fluctuate rapidly from the curve of the transfer function of the photometer, but the other device has a relatively wide band spanning the main passage band of the aperitif Mach-Zehnder filter. The aperiodic filter in this way provides a sharper cut for the pass band of the other device. Numerous variations and combinations of the features discussed above can be used without departing from the present invention. For example, on the filter of figure 3, the port 42 formed by * the second fiber can be configured as a port of sállela instead of, or in addition to, the output port 38. The transmission transfer function for the output port 42 is simply the reverse of the transmission transfer function for port 38. In this way, the main band of the transfer function on port 42 must be a slot instead of a pass band. It is not essential that the fibers that form the optical paths extend beyond the couplers,? h or the input and output ports are consituted by the fibers, ie they form the paths between the couplers. For example, the device illustrated in Figure 7 has a fiber lll which defines the input port 136 and the output port 138. Other fibers 110 and 112 are coupled to the fiber 111 in the input coupler 116 and the dial coupling 118. The couplers are arranged so that the light provided in the fiber 111 through the input port 136 is coupled to the fibers 110 and 112 in substantially equal proportions and in turn by the The light of these fibers is recombined into the fiber 111. In this arrangement, that portion of the fiber 111 extending beyond the couplers carries no appreciable portion of the light, and may be omitted. Multi-fiber couplers are known in the art and are described, for example, in the E.U.A. commonly assigned 5,351,325, the description of which is incorporated herein by ferenc ia r. As illustrated in Figure 7, the fibers need not be uniform in composition. In this way, the fiber 110 can be of a uniform composition while the fiber 112 may include sections 113 and 115 which have the same composition and configuration as fiber 110 and another section 117 of different optical properties. Since portions 113 and 115 do not influence L, only the length of section 117, and the corresponding length of fiber 110, need to be considered in computation of the transfer function. A composite fiber can be used -also on the 2-fiber device as illustrated in FIG. 3. The use of composite fibers in an intra-ferruginal device of Mach-Zehnder-is described, for example, in the application US Patent commonly Uilliarn 3. Identification card entitled Mach-Zehnder inter ferornetrie Devices ith Cornposite Fibers, filed on the same date with the present, the axes of which is incorporated herein by reference. Although the filter has been described by means of reference to the paths formed by fiber optic waveguides, other waveguides can be used. For example, the filter can be formed using flat waveguides. In addition, the couplers do not need to be tapered couplers overcast; other coupler shaft shapes may be used, such as polished fiber and cast fiber couplers. In addition, the couplers do not need to be 3dB couplers; you can use couplers that provide distribution of unequal energy between the trajectories. This proposal can be used to reduce the size or depth of modulation of the transmitted signal. The filter transfer function may include more than one wide pass band or slot. As shown in Figure 8, the transfer function may include a wide slot 140 centered at central wavelengths X140, in addition to the main pass band 150 centered at the main wavelength Xiso- This transfer function can be achieved if the (D / d X is zero or nearly zero at both wavelengths X140? Iso, and if d2p) / d X2 is relatively large at both said wavelengths, Similarly, the device can have tree or other bands. relatively large where these conditions are met at different wavelengths. In this arrangement, the wide range of bands to pl can be observed as the main band discussed above. A filter with relatively wide slots on opposite sides of a main passband can be used as a selectable wavelength filter, as in a wavelength division multiplexer system, and can also be used in conjunction with a device such as a Optical amplifier as I was before.

EXAMPLE 1 A Mach-Zehnder shaft filter substantially in accordance with Figure 3 is formed of a first pass fiber having a core radius of 3.80 micrometers and (n) 0.35%. The second fibers used in the filter have a core radius of 0.82 micrometers and (delta n) - 1.80%. As used in the present, the term "delta n" means (ni - n2) / n ?, where ni is the refractive index of the core and n2 is the refractive index of the coating. The physical path length between the input and output couplers is 23 cm. The transmission spectrum is as illustrated in Figure 4. Xi is approximately 1.53μm, while X2 is approximately l "57μ? N. Xo is approximately 1.55μrn.

Claims (8)

1. NOVELTY OF THE INVENTION CLAIMS 1. - An inter-peripheral device of Mach-Zehnder comprising an input port, an output port, an input end coupler and an output end coupler, and first and second optical paths extending between said couplers, said end couplers input being adapted to direct light applied at said input port to said first and second optical paths, said output shaft coupler-end being adapted to combine light in said first and second optical paths and direct the combined light to said output port, These trajectories having optical path lengths Li and I2, respectively, between said couplers, said device also has a transfer function that refers to the light axis proportion supplied through said first input port appearing in said port. output that is substantially aperiodic, said transfer function includes a band of or relatively large main or slot for which a value of said transfer function is almost a minimum or a maximum on a relatively wide scale axis wavelengths encompassing an operating wavelength Xo, said main passband or slot that constitutes the widest pass-band or slot of said transfer function in the neighborhood of Xo, so one gives it a longitude and rationale that varies in a non-linear manner. of wave e le uz that happens at ravos ele di cha t rayectona e so that to Xo, d (D l) £ A di eJ2 (Dl) > B ell2 where: 1 is (li - I2); A is approximately 4? R / X; and B is approximately 5 / (X) 2, and X is the maximum average width of the main pass band or slot.
2. 2. A device according to claim 1, further characterized by a? O, d (Dl) = 0 di 3 .- A device according to claim 1, characterized aelernas because said first and second trajectories have physical lengths zi and Z2, respectively, and said first and second trajectories have substantially uniform propagation constants ßi and ß2. respectively, where 1 = ß? z? -ß2Z2. 4. A device according to claim 1, further characterized in that said first and second optical paths are constituted by first and second fibers, said first and second fibers extending through said couplers. 5. A device according to claim 4, further characterized in that portions of said fibers extend beyond said couplers and constitute said ports. 6. A device according to claim 5, further characterized in that said couplers are tapered couplers overcoated, each of said fibers including a region of tapered coupling, said tapered coupling regions of said fibers being juxtaposed with each other in said couplers, each of said couplers further includes an overcoating surrounding said tapered engagement regions of said fibers. A device according to claim 6, further characterized in that it comprises integral housing with said overcoats of said couplers, said housing surrounding said fibers between said couplings. 8. A device according to claim 1, further characterized in that said couplers provide substantial coupling of 3dB of light between said first and second trajectories at X = Xo. 9. A device according to claim 8, further characterized in that said couplers provide substantially achromatic coupling on a scale of wavelengths encompassing Xo-10. A device according to claim 9, further characterized by having a width The maximum half of said main passage strip or groove is at least approximately double the maximum average width of the adjacent pass or adjacent groove. 11. A device according to claim 9further characterized in that the value of said transfer function is within about 20% of its minimum or maximum in said main passband or groove over a width of at least about 0.015 .o. 12. An optical system comprising an optical device having a transfer function device with a relatively broad device pass band connected in series with a Mach-Zehnder device according to claim 1, said Mach device. -Zehnder that has a main pass band aligned with this pass band of the device. .- A system according to claim 12, further characterized in that said optical device is an optical amplifier. 14.- A Mach-Zehnder interferometric device comprising an input port, an output port, an input end coupler and an output end coupler, and first and second optical paths <which extend between said couplers, said input end couplers being adapted to direct light applied at said input port to said second and second optical paths, said sill-end coupler being adapted to combine light in said first and second paths optical and direct the combined light to said exit port, said trajectories having optical path lengths li and I2, respectively, between said collectors, wherein at least one of said path lengths varies non-linearly with the wavelength X of light passing through said path in the vicinity of an operating wavelength Xo, said device has a transfer function which refers to the proportion of light supplied through said first input port which appears in said exit port which is substantially aperiodic, said transfer function includes a pass band pri relatively large ncipal or slot for which a value of said transfer function is almost minimum or a maximum on a relatively wide range of wavelengths encompassing an operating wavelength Xo, said main passband or slot which constitutes the broad pass band or groove of said transfer function in the neighborhood of? o, said main pass band or groove having a maximum average width of at least about twice the maximum average width of the pass band. or adjacent slot nearby. 15. A device according to claim 14, further characterized in that the value of the transfer function is about 20% of its minimum or maximum in said main passband or slot over a width of at least approximately 1%. 0.015 \ o. 16. An optical system comprising an optical device having a transfer function of the optical device with a pass band of the device connected in series with a Mach-Zehnder interfero et al filter having a ran- ner function? A non-periodic filter of the filter that defines a larger main pass band than the other pass bands of said transfer function, said main pass band of said filter transfer function being encompassed within said pass band of the device and that it has a more sharp cut than said pass band of the device. 17. A system in accordance with the claim 16, further characterized in that said aperiodic transfer function of the filter includes a fluctuating region in which a value of the filter transfer function rapidly fluctuates between maximum and minimum values as the length of the transmitted light wave changes, and where said transfer function of the optical device has relatively low values for wavelengths in said fluctuating region of said filter transfer function. 18. A system according to claim 17, further characterized in that said device is an optical amplifier.