JP2002519742A - Apodization of optical filters formed on photosensitive media - Google Patents

Abstract

Abstract: An optical waveguide (50) having a photosensitive core is provided with an interference beam (38, 40) having a peak intensity (72, 74) relatively shifted along an optical axis (64) of the waveguide. By exposing the core with actinic radiation in the form (1), a diffraction grating filter is created. Each of the interfering beams has a single lobe intensity profile and the degree of spatial coherence required to obtain the desired fringe contrast between the two relatively shifted beams. The refractive index modulation of the photosensitive core matches the light illumination (interference) pattern. By relatively shifting the interference beam, the average index of refraction modulation is made uniform, thereby reducing the side lobe of the spectral sensitivity of the diffraction grating. The second exposure uses two beams, but does not use the beam interference effect, to further equalize the average refractive index.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001] The optical filter formed on the photosensitive optical medium by with the pattern in the field actinic radiation of the Invention (e.g., interference) exposure generally has a bandpass or bandstop spectral sensitivity profile. Competing requirements for refractive index changes in the medium add undesirable "structures" (e.g., sidelobes) to the sensitivity profile, which can be processed by various apodization techniques.

BACKGROUND Bragg gratings and long period gratings of the invention is an example of an optical filter by patterned exposure to actinic radiation may be formed on the photosensitive medium. Optical filters generally have a core doped with a photosensitive material, such as germanium, that can change the refractive index of the core in response to exposure to actinic radiation, and the actinic radiation is generally in the ultraviolet spectrum.
Incident light generally increases the refractive index of the exposed portion of the core in proportion to the light intensity and the length (time) of the exposure.

The required pattern writing, which controls both the coupling strength and the grating period, can be achieved by interference or mask methods. Bragg gratings have a period shorter than half the center wavelength of the spectral sensitivity and can best be obtained by intersecting and interfering two actinic beams at an angle. The long-period diffraction grating has a period that is 100 to 1000 times larger and can be drawn by a simple mask method. For example, a mask with an amplitude pattern can be made that can illuminate the fiber core with a spatially separated band of light to form a long period grating.

[0004] Irrespective of the exposure mode, the intensity profile of the incident light is transferred to a similarly shaped core refractive index profile. For example, an incident beam having a constant intensity profile according to interference or masking produces a uniform refractive index modulation and a constant average refractive index along the exposed portion of the core. However, the resulting spectral sensitivity has large side lobes on both sides of the desired band. An incident beam with a more general Gaussian intensity profile produces a refractive index modulation and average index that also becomes Gaussian. Although a Gaussian change in refractive index modulation is useful for eliminating opposing side lobes, the accompanying change in average index of refraction causes a gradual change in the effective period of the diffraction grating, which generally results in a sideband on one side of the desired band. Lobes occur.

[0005] Grating correction to remove unwanted side lobes is sometimes referred to as "apodization" because it relates to "shading" of the grating amplitude of the grating. The goal of apodization is generally to change the pulse shape (e.g., Gaussian or, more generally, increase to a peak value and then decrease) of the index modulation while maintaining a constant effective period over the length of the grating. Is to get. Many of the known techniques for apodizing optical gratings are expensive, time consuming, or difficult to perform with the required accuracy.

For example, US Pat. No. 5,367,588 to Hill et al. Discloses a non-linear phase mask over a medium to expose a photosensitive optical filter medium to a non-uniformly spaced interference pattern. Is placed. A phase mask that itself functions as a diffraction grating,
An actinic beam having a Gaussian intensity profile is split into two interference beams forming a non-uniform interference pattern. The resulting pitch change of the filter grating compensates for the change in average refractive index corresponding to the combined intensity profile of the illumination beam. Such special non-linear phase masks are expensive to make and add significant cost to the manufacture of optical filters.

[0007] US Pat. No. 5,717,799 to Robinson also corrects unwanted changes in average refractive index associated with desired changes in index modulation by changing the period of the grating. Propose to do. Suggestions for achieving this goal include drawing the grating elements individually or forming (exposure) the grating elements.
Sometimes it involves pulling the diffraction grating with different strengths from part to part. At the short period of 0.5 μm in typical Bragg gratings, individual writing of the grating elements is extremely impractical, and pulling the grating with different strengths in different parts can complicate manufacturing and reproduce. Unpredictable results can occur.

US Pat. No. 5,309,260 to Mizrahi et al. Uses continuous exposure to perform apodization of Bragg gratings. The first exposure is performed with two interfering beams having a Gaussian profile to create the desired change in refractive index modulation. A second exposure using a single beam raises the average refractive index at one end of the diffraction grating to suppress a secondary peak (fine structure) in the spectral sensitivity of the filter. However, the change in average index of refraction, which can function similarly to "chirp" and can cause undesirable temporal dispersion in the filtered signal, remains along the length of the grating.

SUMMARY OF THE INVENTION The present invention provides a sensitivity curve for an optical filter including a Bragg grating and a long-period grating by separating at least some of the refractive index modulation change from the average refractive index change along the optical axis of the filter. Determine the shape of The refractive index modulation is preferably formed by exposure to actinic radiation having a single lobe intensity profile. The same or another exposure can be used to further affect the average refractive index along the optical axis.

In one example, two actinic beams are combined to form an appropriately periodic interference pattern on a photosensitive core that is to be an optical filter. The two beams are approximately s
made from ordinary spatially coherent beams having an inc type ² intensity profile. The axes of the two beams are tilted with respect to each other to adjust the fringe spacing of the interference pattern, and are preferably located in the coaxial plane of the filter to direct the fringes in a direction transverse to the optical axis of the filter. However, the intersection of the two axes is offset from the optical axis, so that the two axes are relatively displaced along the optical axis. For an interference beam having a sinc type ² intensity profile, an axial spacing of about 0.88 FWHM (half width) is preferred.

[0011] Any such offset typically greatly reduces the fringe contrast because the interfering beams are spatially offset relative to each other at the intersection with the optical axis of the filter. Therefore, it is preferable to use a spatial filter that enhances the spatial coherence of the obtained interference beam and absorbs a required alignment difference for determining the shape of the shared beam. The resulting interference pattern is slightly shorter but retains the pulse-shaped contrast profile and the same fringe spacing. The effect of the two beams on the combined intensity profile is greatest.

By offsetting the peak intensity of the interference beam along the optical axis of the filter, the axial change in the combined intensity of the beams in the beam overlap region is reduced. The effect on the filter is to obtain a more constant average refractive index in the same region, while maintaining the desired pulse shape change in refractive index modulation in the overlap region. The fringe contrast underlying the refractive index modulation is due to the intensity difference between the two beams,
It becomes weaker toward both ends of the overlapping area. The new filter of the present invention has a flattened spectral sensitivity with reduced sidelobe structure.

Another example of the invention uses a spatial filter with an offset phase mask to produce similar spectral sensitivity. The spatial filter enhances the spatial coherence of the actinic radiation beam, which is preferably perpendicularly incident on the phase mask. Most of the light is diffracted by the phase mask to become the primary light of the opposite sign, and diverges from the phase mask as an interference beam.

However, instead of arranging a phase mask in contact with the optical filter medium to draw the diffraction grating according to the usual method, the phase mask has a peak intensity of each interference beam along the optical axis of the filter medium. And spaced apart from the filter media at a distance only. This spacing is adjusted in the same way as in the previous embodiment, so that a relatively constant combined intensity of the interference beam along the optical axis within the beam overlap range is obtained. Similarly to the previous embodiment, the refractive index modulation formed by the obtained interference pattern maintains the modulation degree change of the pulse shape.

[0015] To further improve the spectral sensitivity of the optical filter, the second exposure can be used in combination with the first exposure. Again, two beams are used simultaneously at spaced locations along the optical axis of the filter. However, the beam interval differs between the first exposure and the second exposure. The first exposure forms the desired refractive index modulation and the second exposure
The average refractive index is made uniform in cooperation with the exposure. The two beams can be generated from the same beam source, including the interference beam source of the first exposure. However, the second exposure is not used for rewriting the refractive index modulation of the filter media. During the second exposure, the spatial filter can be replaced with an amplitude mask that more clearly defines the shape of the overlapping beams, but sufficiently reduces spatial coherence to prevent fringe formation. Alternatively, the filter medium or phase mask can be finely vibrated to average out the exposure intensity of the patterned irradiation (ie, "wash out" the fringes).

The refractive index modulation of the Bragg diffraction grating is preferably drawn by an interferometer or a phase mask, and the same configuration is preferably used for the second exposure. The two exposures are cumulative and can therefore be interchanged. The refractive index modulation of the long-period diffraction grating can be drawn with a less sensitive device configuration. For example, an amplitude mask having a rectangular transmission function can be used for drawing a diffraction grating. But,
In order to adjust the average index of refraction at both ends of the grating while washing away any fringes, it is preferable to replace the amplitude mask with a phase mask that creates two relatively divergent beams.

The two views of detailed description beginning of invention, Figures 1 and 2, results are expected heretofore by exposing the photosensitive core of the optical waveguide into two interfering beams having a Gaussian intensity profile It is a graph of. The two beams create an interference pattern that varies the fringe contrast along the photosensitive core as a function of the resultant Gaussian intensity profile of the beam. In this example, the refractive index of the core, which is increased as a function of the exposure intensity, varies according to the fringe contrast of the interference pattern. Thus, the resulting refractive index modulation 10 varies in modulation along the core according to the Gaussian intensity profile of the composite beam.

For clarity, only a few of the typical refractive index modulations 10 are shown (the refractive index modulation 10 of a Bragg grating operating at an infrared wavelength of about 1550 nm is typically around 0.5 μm With periodic intervals). As desired, the change in peak-to-valley magnitude of the modulation 10 progressively decreases from the center to both sides along the photosensitive core, with a corresponding change in the average refractive index traced by the line 12. Has the undesirable consequence of changing the effective grating period, ie the optical path length of each period. Wavelengths outside of the desired band that meet the conditions for reflectivity are added, and the resulting grating may exhibit a chirp that can cause undesirable temporal dispersion of the filtered signal.

FIG. 2 is the expected spectral sensitivity of a diffraction grating having the refractive index pattern shown in FIG. The resulting spectral sensitivity has side lobes 14 containing short wavelength reflections outside the desired reflection band 16, which is sometimes referred to as adding an undesirable "structure" to the sensitivity profile.

In one or more of the various embodiments of the present invention, the present invention provides an additional means for making the average refractive index 12 uniform while maintaining the pulse shape change of the peak-to-valley magnitude of the refractive index modulation 10. Offers the freedom of Two such embodiments are shown in FIGS.

The embodiment of FIG. 3 is configured as an interferometer 20 having a laser source 22 for producing a beam 24 of actinic radiation that is temporally coherent. Laser source 22 is an excimer pump operating in the wavelength range of 200-250 nm for diffraction grating writing.
Frequency doubling—a die laser. However, other lasers and other wavelengths can be used in combination with materials sensitive to other wavelengths and power ranges. Pulsed light or continuous wave light can be used.

A cylindrical lens 26 focuses the beam 24 at a line focus 28. A spatial filter 30 near the line focus 28 diverges the high spatial frequency components of the beam 24 to enhance the spatial coherence of the beam. For details of the preferred spatial filter 30 for the inventors, see US Provisional Patent Application Ser.
No. 859, which is incorporated herein by reference. Spatial filter 30
Has a sinc ² type intensity profile. Collimator 3
2 collimates the beam 24 and a second spatial filter 34 removes side lobes from the sinc ² type intensity profile of the beam.

A beam splitter block 36 splits the beam 24 into two beams, a reflected beam 38 and a transmitted beam 40, each beam having a truncated sinc type ² intensity profile. The mirrors 42 and 44 direct the central axis 46 of the reflected beam 38 at an angle α with respect to the normal 48 of the optical waveguide 50 under construction. Mirrors 52, 54 and 56
Guides the transmitted beam 40 with an equal number of reflections, and directs the central axis 58 of the transmitted beam 40 to an equal but opposite angle β with respect to the line 48.

Cylindrical lenses 60 and 62 oriented at right angles to cylindrical lens 26 focus beams 38 and 40 toward respective line foci located in the coaxial plane of waveguide 50 (ie, the plane of FIG. 3). By doing so, the power density of the two beams 38 and 40 is increased. A beam width of about 5 to 100 μm extends along the optical axis 64 of the waveguide
Overlap with a length of about 30 mm. Energy density of incident light is almost 200mJ /
cm ² / pulse.

The waveguide 50, which may take the form of an optical fiber or a planar optical waveguide, comprises:
It has an exposed portion 66 that includes a photosensitive core surrounded by a cladding layer. An exemplary photosensitive core consists of a combination of silica and germanium, while the cladding layer consists of silica only. Hydrogenation can be used to enhance photosensitivity.

An adjustable waveguide mount 70 positions the waveguide 50 with respect to the overlapping beams 38 and 40. Unlike conventional methods, the central axes 46 and 58 of beams 38 and 40 intersect each other at a location 76 offset from optical axis 64 of waveguide 50. In other words, the central axis 46 corresponding to the peak intensity of the beams 38 and 40
And 58 are relatively offset positions 72 and 74 along the optical axis 64 of the waveguide 50.
Intersects the optical axis 64 at

Due to the alignment difference between the central axes 46 and 58 of the beams 38 and 40 where the central axes of the waveguides 50 and the respective beam central axes intersect, the resulting exposed portion 66 of the waveguide 50 In order to obtain the desired fringe contrast in the interference pattern illuminating, a high degree of spatial coherence between the beams 38 and 40 is required. A spatial filter 30 is provided to satisfy this requirement.

FIGS. 4 and 5 show that the peak intensities of the spatially coherent beams 38 and 40 are:
FIG. 3 shows the results compared to FIGS. 1 and 2 due to relative displacement along the optical axis 64. For example, FIG. 4 shows that while the required pulse shape peak-to-valley change of the index of refraction modulation 78 is maintained, the average index of refraction 80 is more constant to the extent that the beams 38 and 40 overlap and interfere. Indicates that Figure 5 shows the completed Bragg grating.
The side lobe 84 is fairly small, indicating that the desired reflection band 82 has been achieved. The steeper sidewalls 86 and 88 of the reflection band 82 also provide improved grating performance (eg, reduced crosstalk).

The central axes 46 and 58 of the two beams 38 and 40 are preferably spaced along the optical axis 64 by at least の the half-width of the beams. But s
For an inc type ² intensity profile, a spacing of approximately 0.88 times the half width appears to be optimal. If the spacing between axes 46 and 58 (ie, peak intensity) along optical axis 64 is too small, the refractive index at the center of the diffraction grating will be too high for both ends. If the spacing is too large, the index of refraction at the center of the diffraction grating will be very small relative to its ends, within which the index modulation will be drawn, and the length of overlap between beams 38 and 40 will also be short. Would be too much.

The various components of the interferometer 20 can be arranged in a variety of ways, such as adding, removing, or replacing, and even so, the single lobe in the adjusted overlapping position The result can be that the beam produces a more uniform average refractive index. For example, a sinc ² type intensity profile of the beam 24 is obtained with a particular spatial filter 30, but other single lobe beam profiles, including Gaussian beam profiles, can be used. The collimator 32 can be arranged at least after the second spatial filter 34 if necessary, or a pair of collimators 32 can be connected to the beam splitter 36.
May be placed after the. More or less mirrors can be used to orient the relative orientation of beams 38 and 40, and the intersection 76 of beam axes 46 and 58 can be used to guide waveguide 50 to mirrors 50,42,44,52,54 and 56. Can be placed before or after the optical axis 64.

An imaging optics can also be used to image the interference pattern from a desired plane onto the exposed portion 66 of the waveguide 50, whether or not to scale the interference pattern.
The desired plane retains the interference pattern that would otherwise have been formed directly on the exposed portion 66 of the waveguide 50.

Another embodiment 90 for obtaining similar results is shown in FIG. The starting point is an actinic radiation source 9 such as an excimer laser operating at a wavelength of 193 nm or 248 nm.
It is still two. Similarly, different lasers and different wavelengths may be used as appropriate for the particular application or material. A cylindrical lens 96 together with the spatial filter 100 enhances the spatial coherence of the actinic radiation beam 94. The shape of the intensity profile of beam 94 is further defined by a second spatial filter 104 that receives beam 94 from collimator 102. The mirror 106 is 9
The further shape was defined by another cylindrical lens 108 rotated by 0 ° to focus the beam toward a line focus in the axial plane of the optical waveguide 110 under construction (ie, the plane of FIG. 6). Aim beam 94. In the axial plane, beam 94 remains collimated.

A phase mask 112 supported by an adjustable mount 114 intercepts the collimated / focused beam 94 and, in the axial plane of the light guide 110, two collimated but relatively divergent beams. Split into beams 118 and 120. Preferably, the phase mask 112, which is itself a diffraction grating, has a fixed period and is arranged so that most of the incident light is first-order diffracted light having the opposite sign of the diffraction grating.
Other combinations of order light may be used, including a combination of zero order light and primary light, but two primary lights are preferred.

Near the phase mask 112, the two beams 118 and 120 overlap and interfere. However, instead of placing the phase mask 112 in contact with the exposed portion 122 of the waveguide 110, two beams 118 and 12 along the optical axis 128 of the waveguide 110 are used.
An interval is provided between the phase mask 112 and the exposed portion 122 just enough to separate the zero (preferably corresponding to the central axis) peak intensities 124 and 120. 1mm to 5
A spacing of mm is considered common, but larger or smaller spacings may be used, depending on the width of the two beams and the desired spectral sensitivity of the finished grating.

(A) The optical waveguide 110 is the same as the optical waveguide 50 of FIG. 3, and (b) the beam 94
Is the same as beam 24 in FIG. 3, (c) the diffraction angles of beams 118 and 120 are the same as tilt angles α and β in FIG. 3, and (d) peak intensity 72 along optical axis 64 in FIG.
Assuming that the same interval between the peaks 124 and 126 is provided along the optical axis 128 between the peak intensities 124 and 126, the diffraction grating illustrated in FIGS.
And 6 embodiments. The arrangement of components can be implemented with the same flexibility.

So far, the embodiments of FIGS. 3 and 6 have both been described with respect to a single exposure of the waveguides 50 and 110 for each. Although the improvements shown in FIGS. 4 and 5 are substantial, exposures that are specifically aimed at adjusting the average refractive index of the waveguide core without changing the peak-to-valley magnitude of the refractive index modulation. Additional improvements are possible. This second exposure is preferably performed with two overlapping beams, but interference fringe effects on the waveguide core are avoided.

Returning to FIG. 3, peak intensities 72 and 74 of beams 38 and 48 along optical axis 64
, The adjustable waveguide mount 70 can be moved in the direction of arrow 130. The interval at the time of the second exposure is preferably larger than the interval at the time of the first exposure. The first exposure optimizes the change in peak-to-valley magnitude of the index modulation, and the second exposure further optimizes the average index of the index modulation. In other words, only the first exposure affects the change in the peak-to-valley size of the refractive index modulation, and both the first and second exposures affect the average refractive index of the refractive index modulation.

At the time of the second exposure, the spatial coherence between the beams is weakened, so that the beam 38
Can be prevented. Spatial coherence can be reduced by replacing the spatial filter 30 with an amplitude mask having the same transmission function. Diffusion optics can be used to further reduce spatial coherence, or beams 38 and 40 can be relatively focused on to increase the effective spatial offset.

Alternatively, the interference fringes between beams 38 and 40 can be “washed out” by finely oscillating the optical waveguide in the direction of arrow 132. Any point along the optical axis 64 of the waveguide section 66 is exposed at the average intensity of the interference pattern across the plurality of fringes.

Further improvement results by the double exposure are shown in the graphs of FIGS. The change in the peak-to-valley size of the refractive index modulation 136 is greatly improved in the first exposure, and both the first and second exposures contribute to the uniformity of the average refractive index 138 over the entire range of the refractive index modulation 136. . The spectral sensitivity shown graphically in FIG. 8 indicates that the sidelobe 140 has been further reduced while maintaining the desired reflection band 142.

The embodiment of FIG. 6 can be configured to achieve equivalent results. For example, the phase mask 112 on the mount 114, which can be adjusted to change the spacing of the peak intensities 124 and 126 of the beams 118 and 120 along the optical axis 128, can be moved relatively in the direction of arrow 146. . The spacing between the peak intensities can also be changed by moving the waveguide 110 on the mount 148 as in the embodiment of FIG. The interference effect can also be avoided by weakening the spatial coherence or by flushing the fringes by finely oscillating the waveguide 110 or the phase mask 112 in the direction of the arrow 150.

Although the two exposures are referred to as first and second exposures to distinguish them from each other, the two exposures can be performed in any of the first to second or second to first order. In at least one of the two exposures, a peak intensity of 72, 74 or 124, 126
Are displaced along the optical axis 64 or 128 of the optical waveguide 50 or 110, but 2
In the other of the exposures, the peak intensity may not need to be shifted along optical axis 64 or 128, depending on the desired sidelobe suppression.

With long-period grating refractive index modulation, the grating spacing is much larger than the Bragg grating refractive index modulation, and more options are available for writing the refractive index modulation, including a digital amplitude mask. However, a second exposure, particularly using a phase mask, for simultaneously exposing the diffraction grating with the two relatively displaced beams, reduces the average index of the longer index modulation while washing away any fringes. Performance can improve performance.

[0044] Bragg gratings made in accordance with the present invention are particularly useful for communication systems. For example, Bragg gratings can be used to insert or drop specific channels, or to separate channels individually within the demultiplexing capacity. Other applications include sensors, dispersion compensators, or laser-pumped ballasts. Long-period gratings made in accordance with the present invention perform best as spectrally selective or bandstop filters to improve the operation of devices such as optical amplifiers or noise suppressors.

[Brief description of the drawings]

FIG. 1 is a graph of light-induced refractive index change as a function of position along a light grating formed by exposure to an interference pattern between two perfectly overlapping beams.

FIG. 2 is a graph of the expected spectral sensitivity of the diffraction grating of FIG. 1 according to reflectivity as a function of wavelength.

FIG. 3 is an illustration of an interferometer configured to cause two beams to interfere at spatially offset locations along the optical axis of the waveguide.

FIG. 4 is a graph of an exemplary index modulation created by exposure to two spatially offset beams.

FIG. 5 is a graph of the spectral sensitivity according to the refractive index modulation of FIG. 4;

FIG. 6 is an illustration of an optical device for creating two spatially offset beams with a phase mask offset from the illuminated waveguide.

FIG. 7 is a graph of an exemplary index modulation created by the addition of a second exposure without further interference effects between the beams.

FIG. 8 is a graph of the spectral sensitivity accompanying the refractive index modulation of FIG. 7;

DESCRIPTION OF SYMBOLS 20 Interferometer 22 Laser source 24 Laser beam 26, 60, 62 Cylindrical lens 28 Line focus 30, 34 Spatial filter 32 Collimator 36 Beam splitter 42, 44, 52, 54, 56 Mirror 50 Optical waveguide 38, 40 Interference beam 64 Optical axis 72,74 Intensity peak

──────────────────────────────────────────────────続 き Continuation of front page (81) Designated country EP (AT, BE, CH, CY, DE, DK, ES, FI, FR, GB, GR, IE, IT, LU, MC, NL, PT, SE ), AL, AM, AT, AU, AZ, BA, BB, BG, BR, BY, CA, CH, CN, CU, CZ, DE, DK, EE, ES, FI, GB, GE, GH, GM , HR, HU, ID, IL, IS, JP, KE, KG, KP, KR, KZ, LC, LK, LR, LS, LT, LU, LV, MD, MG, MK, MN, MW, MX, NO, NZ, PL, PT, RO, RU, SD, SE, SG, SI, SK, SL, TJ, TM, TR, TT, UA, UG, US, UZ, V N, YU, ZW (72) Inventor Modavis, Robert A. United States of America 14870 Painted Post Taylor Street 14 (72) Inventor Robson, Christopher Dee United States of America New York 14901 Elmira Jerusalem Hill Road 98 F Term (Reference) 2H047 LA02 LA03 PA30 TA12 2H049 AA34 AA59 AA62 AA65 2H050 AC82 AC84 AD00

Claims (60)

[Claims] 1. An optical filter formed on an optical medium, wherein the filter comprises:
A set of refractive index modulations along the optical axis corresponding to the intensity pattern of the interference beams of the plurality of actinic rays each having axes offset from one another adjusted at different locations along the optical axis. A filter comprising: 2. The interference pattern between the plurality of interference beams, wherein the refractive index modulation has an average fringe contrast along the optical axis that varies more than the average of the intensity patterns along the same portion of the optical axis. The filter according to claim 1, wherein the magnitude of the relative intensity corresponds to the magnitude of the relative intensity. 3. The optical axis wherein the plurality of interference beams are spaced apart such that the average of the interference pattern remains substantially constant over the entire overlapping range between the plurality of interference beams. 3. The filter according to claim 2, wherein the filters are displaced from each other. 4. The optical system according to claim 1, wherein the respective axes of the plurality of interference beams intersect the optical axis at positions spaced at least 1/2 of a half-value width of the plurality of beams. Item 7. The filter according to Item 1. 5. The optical system according to claim 1, wherein the respective axes of the plurality of interference beams intersect the optical axis at positions spaced apart by about 0.88 times the half width of the plurality of beams. The filter according to claim 4. 6. The filter according to claim 5, wherein the plurality of interference beams have a sinc ² type intensity profile. 7. The filter according to claim 1, wherein said respective axes of said plurality of interference beams cross each other before crossing said optical axis of said optical medium. 8. The filter according to claim 1, wherein the axes of the plurality of interference beams intersect each other after intersecting with the optical axis of the optical medium. 9. A system for making an optical filter in an optical waveguide medium, comprising: a spatial filter that expands the spatial coherence of a beam emitted from an actinic radiation source; and the spatially coherent beam is applied to the optical waveguide medium. A phase mask that converts the two interfering beams into two interfering beams that illuminate the medium within the expanded spatial coherence of the beam to create a refractive index modulation. The distance between the two interference beams along the optical waveguide, spaced apart along the optical waveguide medium, within the overlap range between the two interference beams.
A system wherein the phase mask is spaced apart from the optical waveguide medium at intervals such that the combined intensity of the interference beams is equalized. 10. The method of claim 1, wherein the peak intensities of each of the two beams are spaced along the optical waveguide medium at least one half of a half-width of the two beams. Item 10. The system according to Item 9. 11. The system of claim 9, wherein said spatial filter provides a sinc ² type intensity profile to said spatially coherent beam. 12. The optical axis of the optical waveguide medium at a position where each axis of the two interference beams is separated from each other by a distance of about 0.88 times a half width of the two interference beams. The system of claim 11, wherein the system intersects. 13. The system of claim 9, further comprising an adjustment mechanism for changing said spacing between said phase mask and said optical waveguide medium to control said peak intensity spacing along said optical waveguide medium. . 14. The optical waveguide medium according to claim 1, wherein the peak intensities of each of the two overlapping interference beams are spaced apart at different intervals along the optical waveguide medium, and within a range of overlap between the two interference beams. 14. The system of claim 13, wherein the phase mask is spaced apart from the optical waveguide by a second distance to reduce a change in average refractive index. 15. The apparatus of claim 15, further comprising means for avoiding fringe effects associated with said two interference beams overlapping at said second distance between said phase mask and said optical waveguide medium. 15. The system of claim 14. 16. The system of claim 15, wherein said means for avoiding fringe effects causes said optical waveguide to vibrate relatively finely with respect to said two overlapping interference beams. 17. The system of claim 15, wherein said means for avoiding fringe effects causes said phase mask to vibrate relatively finely with respect to said spatially coherent beam. 18. The system according to claim 15, wherein said means for avoiding fringe effects reduces the spatial coherence of said two interference beams. 19. The system of claim 9, wherein said phase mask has a constant pitch. 20. A method of forming an optical filter, the method comprising: providing an optical waveguide medium having an index of refraction that can be changed by exposure to actinic radiation; two said chemicals arranged at an angle. Orienting the beam of a line to illuminate the optical waveguide medium; and two of the angles disposed along the optical waveguide medium to form an interference pattern having a smaller change in average intensity. Positioning the optical waveguide medium at a distance from the intersection of the two beams within the spatial coherence range of the beams. 21. The step of locating the optical waveguide comprises: setting a peak intensity of each of the two beams arranged at the angle at an interval of / of a half width of the two beams. 21. The method of claim 20, including the step of separating along. 22. A common beam of the actinic radiation is arranged at two angles.
21. The method of claim 20, comprising splitting into beams of a book. 23. The method according to claim 22, including the step of spatially filtering the common beam of actinic radiation. 24. The method of claim 23, comprising providing a sinc type ² intensity profile to the shared beam. 25. The method of claim 22, wherein the step of splitting the shared beam is performed by a beam splitter. 26. The method of claim 22, wherein the step of splitting the shared beam is performed by a phase mask. 27. Different distances from the intersection of the two angled beams to further reduce the change in the average intensity imparted to the optical waveguide medium within the spatial extent of the interference pattern. 21. The method of claim 20, further comprising the step of: locating the optical waveguide medium. 28. A method of making an optical filter in an optical waveguide medium, the method comprising: a first set of overlapping respective peak intensities located at a first adjusted relative position along the optical waveguide medium. Irradiating the optical waveguide medium with a plurality of actinic beams; forming an interference pattern formed on the optical waveguide medium by the first set of overlapping beams at the first adjusted relative position; Creating a corresponding refractive index modulation in the optical waveguide medium; a second set of overlapping actinic beams with respective peak intensities located at a second adjusted relative position along the optical waveguide medium. Irradiating the optical waveguide medium; and wherein the average refractive index of the illuminated optical waveguide medium is greater over the entire range of the refractive index modulation formed on the optical waveguide medium. Adjusting the average refractive index of the illuminated optical waveguide medium with the second set of overlapping beams at the second adjusted relative position to show small changes. Features method. 29. The method of claim 28, wherein said adjusting comprises making the average refractive index substantially constant throughout the refractive index modulation range. 30. The step of irradiating the optical waveguide medium with a second set of overlapping beams comprises: adjusting a peak intensity of each of the plurality of overlapping beams along the optical waveguide medium. At least 1/2 of the half width of
29. The method of claim 28, comprising the step of: 31. The method of claim 28, further comprising the step of spatially filtering the shared beam split into the first set of overlapping beams. 32. The method of claim 28, further comprising preventing the second set of overlapping beams from altering the refractive index modulation along the optical waveguide medium. 33. An optical filter made by the method of claim 28, comprising a photosensitive waveguide medium. 34. A method of making an optical filter, the method comprising: providing an optical waveguide medium having a refractive index that can be changed by exposure to actinic radiation; disposed at a first location relative to the optical waveguide. Directing a first actinic radiation beam through a defined phase mask; diffracting the first beam into a first set of overlapping beams that produces phase modulation along an optical axis of the optical waveguide medium. Moving the phase mask relative to the optical waveguide from the first position to a second position; directing a second actinic radiation beam through the phase mask disposed at the second position. And diffracting the second beam into a second set of overlapping beams that changes the average refractive index along the optical axis of the optical waveguide medium. Wherein the Mukoto. 35. The method of claim 34, wherein the step of moving comprises moving the phase mask substantially perpendicular to the optical axis of the optical waveguide. 36. The method of claim 35, wherein the first and second sets of overlapping beams are all located on a coaxial plane of the optical waveguide. 37. The method of claim 35, wherein at the second position the phase mask is separated from the optical waveguide by at least 1 mm. 38. The method of claim 34, wherein said phase mask has a constant pitch. 39. The peak intensity of each of the second set of overlapping beams is spaced along the optical waveguide medium by at least a half of the half-width of the plurality of beams. 35. The method of claim 34, wherein: 40. The step of diffracting the second beam comprises: adjusting the average refractive index along the optical axis of the optical waveguide medium within an overlap area between the first set of overlapping beams. 35. The method of claim 34, comprising the step of substantially constant. 41. The method of claim 3, further comprising the step of spatially filtering the first actinic radiation beam to enhance spatial coherence.
4. The method according to 4. 42. The method of claim 34, further comprising preventing the second set of overlapping beams from altering the refractive index modulation along the optical axis. 43. The optical filter made by the method of claim 34, comprising a photosensitive waveguide medium. 44. The optical filter according to claim 43, wherein said optical filter is a Bragg diffraction grating. 45. The optical filter according to claim 43, wherein said optical filter is a long-period diffraction grating. 46. A method for performing apodization of an optical filter formed by refractive index modulation along an optical axis of a waveguide medium, the method comprising: a method comprising: overlapping a plurality of actinic beams with axes extending in respective propagation directions. Illuminating the waveguide medium with the plurality of overlapping beams; separating the respective axes of the plurality of overlapping beams along the optical axis of the waveguide medium; The optical axis resulting from an interference effect between the plurality of overlapping beams such that an average refractive index can be adjusted along the optical axis without substantially changing the magnitude of the refractive index modulation along the optical axis; Blocking the alteration of the refractive index modulation along. 47. The step of separating the plurality of overlapping beams along the optical axis of the waveguide medium at least one half of the half-width of the overlapping beams along the optical axis of the waveguide medium. The method of claim 46, comprising the step of separating the axes. 48. The method of claim 46, wherein the step of blocking causes the waveguide medium to vibrate relatively finely with respect to the plurality of overlapping beams.
The described method. 49. The method of claim 46, wherein the step of splitting comprises using a phase mask to split the beam. 50. The method of claim 49, wherein the step of blocking finely vibrates the phase mask. 51. The method of claim 46, wherein said step of blocking comprises reducing spatial coherence of said plurality of overlapping beams. 52. An optical filter apodized according to the method of claim 46, comprising a photosensitive waveguide medium. 53. The optical filter according to claim 52, wherein said optical filter is a Bragg diffraction grating. 54. The optical filter according to claim 52, wherein said optical filter is a long-period diffraction grating. 55. A method for apodizing an optical filter formed by refractive index modulation along an optical axis of a waveguide medium, the method comprising: applying an actinic beam to a plurality of overlapping beams illuminating the waveguide medium. Directing the actinic radiation beam through a splitting phase mask; disposing the phase mask away from the waveguide medium at a distance sufficient to separate the peak intensities of each of the plurality of overlapping beams along the optical axis. And the result of the interference effect between the plurality of overlapping beams such that the average refractive index can be adjusted along the optical axis without substantially changing the magnitude of the refractive index modulation along the optical axis. Preventing the alteration of the refractive index modulation along the optical axis from occurring. 56. The step of arranging the plurality of overlapping beams along the optical axis at an interval of at least の of a half-value width of the plurality of overlapping beams, The method of claim 55, comprising the step of separating. 57. The method of claim 55, wherein the step of blocking finely vibrates the phase mask. 58. The method of claim 55, wherein the step of blocking causes the waveguide medium to vibrate relatively finely with respect to the plurality of overlapping beams.
The described method. 59. The method of claim 55, wherein said step of blocking comprises reducing spatial coherence of said plurality of overlapping beams. 60. An optical filter apodized according to the method of claim 55, comprising a photosensitive waveguide medium.