An improved stepped etalon comprises a transparent body having a stepped surface. Adjacent step lands are separated from each other by a transition region which includes a curved, waved, or otherwise varied step wall such that the average height of the stepped surface does not change abruptly in the transition region from the height of one land to another, but instead varies gradually according to the particular shape of the step wall. In an alternative embodiment, the step transition is formed using a grey-scale or half-tone patterning in which the average height gradually varies across the transition region. The non-planar transition region reduces the amount of coherent interference caused by the step transition thereby reducing the dead spot behind the step transition portions where interference prevents accurate measurements of light transmission from being made.

TECHNICAL FIELD
 This invention is related to an improved multi-wavelength stepped etalon.
 BACKGROUND OF THE INVENTION
 In many applications, it is necessary to accurately determine the
 wavelength(s) of light incident on a suitable detector. A widely used type
 of detector includes an etalon to filter specific frequencies of light. An
 etalon is a type of interference filter in which the intensity of
 transmitted light is dependent on its wavelength. In a conventional
 design, an etalon is comprised of two partially reflective parallel
 surfaces a distance d apart and separated by a material with an index of
 refraction r. When collimated light having a wavelength .lambda. is passed
 through the etalon, some of the light is reflected from the surfaces. The
 multiply reflected light beams interfere, either constructively or
 destructively, with each other, and thus alter the overall intensity of
 the light which passes through the etalon. Maximum transmission occurs
 when twice the distance between the reflective surfaces is an integral
 number of wavelengths .lambda. in the etalon. In other words,
 2d*r/.lambda.=x, where x is an integer.
 Often, it is desirable to provide a sensor which is sensitive to, and can
 discriminate among, several different frequencies of incident light at the
 same time. Such a sensor is particularly useful for spectrographic
 analysis. Although several discrete etalons can be utilized for this
 purpose, in some implementations, a stepped etalon is used instead. In
 this arrangement, one or both active surfaces of the etalon are stepped so
 that each step on the etalon provides a region of different thickness. By
 adjusting the thicknesses appropriately, each step can be configured to
 pass different frequencies of light. Stepped spectrographic etalon
 arrangements of this type are shown in U.S. Pat. No. 4,822,998 to Yokota
 et al. and U.S. Pat. No. 5,144,498 to Vincent.
 A newly developed application requires a specifically configured stepped
 etalon to tune the output frequency of a laser. For fiber optic
 communications in particular, accurate tuning of the communication lasers
 is necessary to permit adjacent transmission channels to be closely
 spaced, often at wavelengths differing by only 0.4 nanometers or less. For
 such closely spaced channels, a laser's wavelength must be tuned to the
 assigned channel with an accuracy of +/-0.1 nanometers or less. Although
 only a single wavelength of light needs to be detected to tune such a
 laser, at these high accuracies, thermal variations in the thickness of an
 etalon and slight variations in the angle of applied light from normal to
 the etalon surface can shift the light transfer function an unacceptable
 degree.
 According to the new application, the nominal thickness of the etalon can
 be chosen so that the periodicity of the etalon filter roughly matches the
 periodicity of a data communication channel spacing, i.e., 1500.12,
 1550.52 nm for a system with a channel separation of substantially 0.4 nm.
 Two or more steps are formed on one side of the etalon. The step size is
 selected to be a fraction of the channel separation, on the order of 0.1
 nm, and is substantially optimized so that a peak or trough in the
 transmission curve in the region of one step overlaps a steep portion of
 the transmission curve for one or more other steps. In this manner, as
 thermal changes in the etalon shift the transmission curve for one step
 beyond the desired range, the curve for a second step is shifted into the
 desired frequency. By selecting a particular step according to a measured
 temperature and etalon calibration information, and measuring the
 intensity of laser light transmitted through the selected step of the
 etalon, a feedback signal is provided which can be used to adjust the
 output wavelength of the laser. Similarly, different steps can be selected
 to compensate for tolerance errors in the angle of light incident to the
 etalon.
 With reference to FIGS. 1a and 1b, in both views of stepped etalon
 configurations, the stepped etalon 10 has partially reflective coatings
 11a, 11b and is positioned adjacent an appropriately configured array of
 photodetectors 14a, 14b, where each detector is aligned with a
 corresponding etalon step land 12a, 12b. When a beam of light 16 is
 directed onto the etalon 10, the intensity of the output signal attributed
 to each detector 14a, 14b indicates the intensity of light passing through
 the etalon in the region of the corresponding step, therefore providing a
 measure of the intensity of incident light, with the particular
 frequencies determined by the thickness of the etalon in that region.
 A significant drawback to a conventional stepped etalon is the interference
 caused by the abrupt transition between the lands of adjacent steps. When
 no step is present, the intensity within a collimated light beam
 transmitted through an etalon has the same intensity pattern as the
 incident beam, typically gaussian-like as shown in FIG. 1c. However, when
 an abrupt step is present, the incident and resonant light is diffracted
 by the step wall 18, producing interference within the transmitted beam
 along the z-axis (perpendicular to the step edge). The resulting fringe
 pattern is illustrated in FIG. 1d. The result of the diffraction is that
 in the vicinity of the step, there is substantial angular dispersion of
 the light which reduces the quality of the transmission function resulting
 in reduced signal amplitude, broadened peaks, as well as reduced ability
 to differentiate small changes in the frequency of the input light.
 Such a reduction in wavelength discrimination is illustrated in FIG. 1e for
 a two step etalon. Curves A1-A5 are measured on a first step A and curves
 B1-B5 are measured on a second step B. Curves A1 and B1 represent
 positions distant from the step wall. The remaining curves A2-A5 and B2-B5
 are measurements made at locations progressively closer to the step wall.
 The input signal is provided by a temperature tuned laser and therefore
 increases in temperature represent increases in input signal wavelength.
 As indicated, the peaks and troughs for curves close to the step
 transition are lower and less defined than those measured far from the
 step transition, indicating that near the step transition, it is harder to
 discriminate between wavelengths that are close to each other.
 The effect of the interference and overall reduction in etalon quality
 associated with abrupt steps creates a "dead spot" behind and near the
 step edge in which accurate intensity readings are compromised. Thus,
 there are portions of the etalon where a detector cannot be placed due to
 the reduced quality of the transmitted beam.
 For example, experiments using an etalon with a thickness of approximately
 2 mm and a step height of approximately 0.2 um reveal a "dead spot"
 approximately 600 to 800 um wide directly behind the step. Since input
 beam widths of between 0.5 to 5.0 mm are common, a significant portion of
 the transmitted beam will not have high quality etalon transmission
 characteristics and thus will not be suitable for detection. This reduces
 the available optical power for measurement and lowers the
 power-per-detector. Since a minimum signal-to-noise ratio is required for
 reliable measurements, decreasing the power-per-detector thus can decrease
 the accuracy of the detector and the stability of equipment which is
 adjusted according to the etalon measurements. The interference also
 limits the number of possible steps which can be placed on an etalon of a
 given size.
 Although the size of the etalon can be increased to provide more area
 within each step land which is distant from the edge, this is often an
 undesirable solution. First, the detector array is commonly formed on an
 integrated circuit which may not be as easily increased in size without a
 relatively large increase in production cost. Second, the width of the
 input light beam itself may not be variable and increasing the etalon
 width will introduce the additional problems of directing the beam to the
 desired portion of the etalon.
 SUMMARY OF THE INVENTION
 According to the invention, a stepped etalon is formed with a transition
 region between the lands of adjacent steps in the form of a non-planar
 wall arranged so that the points of transition between the adjacent lands
 vary transversely along a lateral axis of the etalon. In particular, the
 step wall is curved, waved, or otherwise varied so that the average height
 of the stepped surface does not change abruptly in the transition region
 from the height of one land to another, but instead varies gradually
 according to the particular shape of the step wall. In an alternative
 embodiment, the step transition is formed using a grey-scale or half-tone
 patterning in which the average height gradually varies across the
 transition region. While the modified step will still produce local
 interference, the varied shape of the step wall produces varied
 interference patterns which are subject to a lesser degree of constructive
 combination than is present with a flat wall and may advantageously
 produce destructive combinations which reduce the net interference when
 measured a distance from the etalon. An etalon according to the invention
 can easily be made using conventional fabrication techniques and without
 additional processing steps.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S):
 Turning to FIGS. 2a and 2b, there is shown a stepped etalon 20 according to
 the invention. The etalon 20 is comprised of a body 22 having a first
 substantially flat side 24 and an opposing stepped second side 26. Both
 sides are covered with a partially reflecting surface (not shown) and the
 body is comprised of a material, such as SiO.sub.2, which is transparent
 to at least a predetermined range of wavelengths.
 The stepped side 26 has a first step land 28 and a second step land 30
 separated by a non-planar wall 32. The lands 28, 30 are substantially flat
 and parallel to the first side 24 and distant from it a respective first
 and second distance d.sub.1, d.sub.2. The wall 32 extends laterally along
 the x-axis across the etalon 20 and has points of transition (i.e., drop
 points) that vary transversely along the z-axis within a transition region
 34 to produce a "wavy" or otherwise curved step wall.
 The wavy step wall 32 breaks up the coherent diffraction pattern present in
 a conventional straight wall to reduce the net interference caused by the
 step as viewed a distance from the etalon. In other words, as viewed from
 a distance, the transition between the two lands 28, 30 is not as abrupt
 as a straight wall, but instead the average distance from the first side
 24 to the second side 26 varies gradually on the average across the
 transition region, where the particular average height depends on the
 specific shape of the step wall 32. In FIGS. 2a and 2b, the transition
 point between the two lands 28, 30 varies along a triangle or saw-tooth
 curve. In a particular etalon embodiment having a step height d.sub.1
 -d.sub.2 of approximately 170.+-.15 mn, a saw-tooth curve having an
 amplitude of between 100 and 300 um and a period of approximately 100 um
 has been found to reduce the net interference caused by the step
 transition. Of course, other scales can be used, as will be apparent to
 one of skill in the art.
 A wide variety of other step wall shapes can be used as well. FIG. 3
 illustrates a smoothly oscillating curve 36, which may lie along a
 sinusoid or other curve. In one embodiment, the curve varies according to
 the square of the sine of the lateral position. For the example etalon
 having a step height of 170.+-.15 nm, a preferred step wall lies along a
 curve z A*sin.sup.2 (.pi.x/(0.20+0.20x)), where A is the amplitude of the
 curve and is preferably between 100 and 300 um.
 FIG. 4 is an illustration of a step wall that lies along a square wave 38.
 In a preferred configuration for the example etalon, such a square wave
 has a period of approximately 10 um and an amplitude of between about 100
 and 300 um. Alternatively, the curve may also vary in a generally random
 manner in either or both of the period and amplitude, as illustrated in
 FIG. 5. Other boundaries for the wall can also be used to break up the
 interference patterns, such as various fractal or fractal-like curves (not
 shown).
 According to a second embodiment of the invention, illustrated in FIG. 6a,
 the transition region 34 is comprised of a plurality of elevations 40
 which have surfaces that are above the second land 30, and preferably are
 substantially the same height as the first land 28 and a plurality of
 areas 41 having surfaces that are substantially level with the second land
 30. The elevations 40 can be considered as analogous to pixels arranged on
 a grid 42, shown overlaid on etalon 20 in FIG. 6a for illustrative
 purposes. The elevations 40 are arranged to produce an average height in
 the transition region which is between the first and second distances.
 Such a configuration can easily be produced by selective etching of the
 surface 26 of the etalon to create the second land 30 and the areas 41,
 which etching is controlled by a photoresist deposited in the transition
 region in a manner similar to grey scale or half-tone printing techniques.
 In a preferred embodiment, the elevations 40 are arranged such that the
 total area covered by the elevation "pixels" 40 decreases transversely
 (along the z-axis) across the transition region from the border 44 with
 the first land 28 to the border 46 with the second land 30. The aggregate
 result is a "fuzzy" transition between the two lands 28, 30 illustrated in
 FIG. 6b. It should be noted that the borders 44, 46 of the transition
 region 34 need not be linear, but instead can vary in a manner similar to
 that of the wall in the transition region illustrated in FIGS. 2-5. FIG.
 6c is an illustration of a grey-scale transition having a border 44 which
 is a random curve. Other variations are also possible. While square
 elevation pixels 40 are illustrated, the pixels can be of any shape, which
 shape can vary among the various elevation pixels 40.
 While the invention has been particularly shown and described with
 reference to preferred embodiments thereof, it will be understood by those
 skilled in the art that various changes in form and details may be made
 therein without departing from the spirit and scope of the invention. For
 example, steps according to the invention may be formed on both sides of
 the etalon.