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WO2002006860A1 - Reseau de diffraction a perte par polarisation reduite - Google Patents

Reseau de diffraction a perte par polarisation reduite Download PDF

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Publication number
WO2002006860A1
WO2002006860A1 PCT/US2001/022229 US0122229W WO0206860A1 WO 2002006860 A1 WO2002006860 A1 WO 2002006860A1 US 0122229 W US0122229 W US 0122229W WO 0206860 A1 WO0206860 A1 WO 0206860A1
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WO
WIPO (PCT)
Prior art keywords
diffraction grating
optical signal
faces
substrate
reflective
Prior art date
Application number
PCT/US2001/022229
Other languages
English (en)
Inventor
Larry Fabiny
Tony Sarto
Lilac Muller
Kenneth Edmund Arnett
Kristofer Stefan Jo Pister
Original Assignee
Network Photonics, Inc.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from US09/615,300 external-priority patent/US6449096B1/en
Priority claimed from US09/669,758 external-priority patent/US6517734B1/en
Application filed by Network Photonics, Inc. filed Critical Network Photonics, Inc.
Priority to AU2001280555A priority Critical patent/AU2001280555A1/en
Publication of WO2002006860A1 publication Critical patent/WO2002006860A1/fr

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Classifications

    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B5/00—Optical elements other than lenses
    • G02B5/18—Diffraction gratings
    • G02B5/1861—Reflection gratings characterised by their structure, e.g. step profile, contours of substrate or grooves, pitch variations, materials
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B5/00—Optical elements other than lenses
    • G02B5/18—Diffraction gratings
    • G02B5/1809—Diffraction gratings with pitch less than or comparable to the wavelength

Definitions

  • This application relates generally to a method and apparatus for diffracting light, and more specifically to a diffraction grating useful in various applications, such as optical telecommunications, that require high diffraction efficiency in multiple polarization orientations.
  • DWDM dense wavelength division multiplexing
  • DWDM dense wavelength division multiplexing
  • DWDM multiple wavelengths of light simultaneously transport information through a single optical fiber. Each wavelength operates as an individual channel carrying a stream of data. The carrying capacity of a fiber is multiplied by the number of DWDM channels used.
  • DWDM systems using up to 80 channels are available from multiple manufacturers, with more promised in the future.
  • Optical wavelength routing functions often use demultiplexing of a light stream into its many individual wavelengths, which are then optically directed along different paths.
  • different wavelength signals may then be multiplexed into a common pathway.
  • the optical signals are routed between the common and individual optical pathways by a combination of dispersion and focusing mechanisms.
  • the focusing mechanism forms discrete images of the common pathway in each wavelength of the different optical signals and the dispersion mechanism relatively displaces the images along a focal line by amounts that vary with the signal wavelength.
  • phased arrays and reflective diffraction gratings may be used to perform the dispersing functions. While phased arrays are adequate when the number of channels carrying different wavelength signals is small, reflective diffraction gratings are generally preferable when large numbers of channels are used. However, reflective diffraction gratings tend to exhibit greater polarization sensitivity and since the polarization of optical signals often fluctuates in optical commumcation systems, this sensitivity may result in large variations in transmission efficiency. Loss of information is possible unless compensating amplification of the signals is used to maintain adequate signal-to-noise ratios.
  • polarization sensitivity may generally be mitigated by increasing the grating pitch of the reflective grating, limitations on the desired wavelength dispersion for signals at optical telecommunication wavelengths preclude an increase in grating pitch sufficient to achieve high diffraction efficiency in all polarization directions.
  • Embodiments of the present invention provide such a diffraction grating, achieving high diffraction efficiency in all polarization states when used for diffraction of an optical signal' at telecommunications wavelengths.
  • the diffraction grating in such embodiments includes a plurality of spaced triangular protrusions on a substrate in which reflective faces are blazed at angles ⁇ b that are substantially different from the Littrow condition.
  • the diffraction grating is configured to diffract an optical signal of wavelength ⁇ . It has a substrate and a plurality of reflective faces oriented at respective blaze angles ⁇ b spaced along the substrate surface at a grating density lid.
  • Each of these reflective faces is supported by a support wall that is connected with the substrate surface such that the optical signal is reflected essentially only off the reflective faces and not off the support walls. Since the optical signal is reflected off the reflective faces but not the support walls, the diffraction efficiency of certain polarization states is improved.
  • the support walls are connected substantially normal with the surface of the substrate and in other embodiments they are connected at an obtuse angle with the substrate.
  • the blaze angles are preferably within the range 50° ⁇ ⁇ ⁇ 70° and more preferably within the range 50° ⁇ ⁇ b ⁇ 60°.
  • the density at which the reflective faces are spaced along the substrate is preferably between 700 and 1100 faces/mm and more preferably between 800 and 1000 faces/mm.
  • the reflective faces are equally spaced along the surface of the substrate at density lid between 800 and 1000 faces/mm without exposing the surface of the substrate, with each of the blaze angles ⁇ substantially equal to 54.0°.
  • the reflective faces are equally spaced along the surface of the substrate at density lid between 800 and 1000 faces/mm such that a portion of the surface of the substrate is exposed between each such reflective face, with each of the blaze angles ⁇ b substantially equal to 55.8°.
  • the support walls preferably have an altitude between 1200 and 1400 nm, more preferably substantially equal to 1310 nm.
  • the diffraction grating is configured to be used to diffract the optical signal in an interference order higher than first order.
  • the reflective faces are spaced at a grating density of approximately 450 faces/mm, with blaze angles substantially equal to 55.8°.
  • a portion of the substrate may be exposed between subsequent reflective faces, defining trenches between the faces.
  • each trench has a width between 0.50 and 0.70 ⁇ m.
  • the height of the support walls defines a groove depth, which in one embodiment is between 2300 and 2500 ⁇ m.
  • Embodiments of the invention are also directed to a diffraction grating for diffracting an optical signal with a polarization-dependent loss less than 0.4 dB when the wavelength of the optical signal is between 1500 and 1600 nm.
  • the grating is configured to diffract an optical signal having a wavelength between 1530 and 1565 nm with a polarization-dependent loss less than 0.04 dB.
  • the diffraction grating is fabricated by forming two sets of parallel trenches in a crystal surface, one made with a crystalline-independent etching technique and the other made with a crystalline-dependent chemical etchant.
  • the intersection of the two sets of trenches removes material from the crystal surface to produce an etched crystal surface that can be coated with a reflective material to form the diffraction grating or can be used as a master for batch fabrication of diffraction gratings.
  • the first set of parallel trenches is initially formed perpendicularly from a surface of a silicon wafer. This set of trenches is then filled with a sacrificial material that also coats the surface of the wafer. The sacrificial material is subsequently patterned lithographically to expose the underlying wafer, with the crystalline- dependent chemical etchant being applied at the exposed portions. The deposited sacrificial material acts as an etch stop to the chemical etchant.
  • Appropriate techniques for forming the first set of parallel trenches include reactive ion etching, deep reactive ion etching, and ion milling.
  • Appropriate crystalline-dependent chemical etchants that preferentially stop etches along [111] orientations include KOH, hydrazine, and ethylene diamine pyrocatechol.
  • plurality of parallel trenches are formed in a crystal surface with a crystalline-independent technique.
  • Sacrificial material is deposited in each of the plurality of trenches, with some of the sacrificial material also being deposited on the crystal surface.
  • the excess sacrificial material is removed from the crystal surface, such as by chemical and mechanical polishing (CMP).
  • CMP chemical and mechanical polishing
  • the crystal surface is exposed to a crystalline-dependent etchant.
  • the resulting structure may be used for fabrication of the diffraction grating. Alternatively, the remaining sacrificial material may be removed from the structure before finalizing the grating fabrication.
  • Fig. 1(a) illustrates a right-apex-angle diffraction grating
  • Fig. 1(b) illustrates the shape of a diffraction grating according to a full- sawtooth embodiment of the invention
  • Fig. 1(c) illustrates a the shape of a diffraction grating according to a truncated-sawtooth embodiment of the invention
  • Fig. 2 shows results of numerical simulations of diffraction efficiency profiles for a full-sawtooth embodiment in S and P polarization configurations
  • Fig. 3 shows results of numerical simulations of diffraction efficiency profiles for a truncated-sawtooth embodiment in S and P polarization configurations
  • Fig. 4(a) shows numerical results for the efficiency in S and P polarization configurations for a full-sawtooth grating as a function of blaze angle
  • Fig. 4(b) shows numerical results for the efficiency in S and P polarization configurations for a truncated sawtooth grating as a function of blaze angle
  • Fig. 5 shows the variation in optimal blaze angle as a function of groove density for a full-sawtooth diffraction grating
  • Fig. 6 shows the variation in efficiency in S and P polarization configurations for a full-sawtooth grating as a function of the angle of incidence of the optical signal
  • Fig. 7(a) shows results of numerical simulations of diffraction efficiency for a truncated-sawtooth embodiment in an S polarization configuration as a function of triangle groove height
  • Fig. 7(b) shows results of numerical simulations of diffraction efficiency for a truncated-sawtooth embodiment in a P polarization configuration as a function of triangle groove height
  • Fig. 8 shows the variation in efficiency in S and P polarization configurations for a full-sawtooth grating at optimal blaze angle as a function of groove density
  • Fig. 9 shows the effect on efficiency in S and P polarization configurations of allowing the support walls to connect with the substrate non-normally
  • Fig. 10(a) shows results of numerical simulations of diffraction efficiency for a truncated-sawtooth embodiment appropriate for use in second order in an S polarization configuration as a function of trench width;
  • Fig. 10(b) shows results of numerical simulations of diffraction efficiency for a truncated-sawtooth embodiment appropriate for use in second order in a P polarization configuration as a function of trench width;
  • Fig. 11 shows results of numerical simulations of diffraction efficiency in S and P polarizations as a function of trench width for a truncated-sawtooth embodiment with a facet angle of 55.8° appropriate for use in second order;
  • Fig. 12 shows results of numerical simulations for the average polarization efficiency as a function of wavelength and groove depth for a truncated-sawtooth embodiment with a facet angle of 55.8° appropriate for use in second order;
  • Fig. 13 shows results of numerical simulations for the polarization-dependent loss as a function of wavelength and groove depth for a truncated-sawtooth embodiment with a facet angle of 55.8° appropriate for use in second order
  • Fig. 14 shows results of numerical simulations for both the average efficiency and polarization-dependent loss as a function of wavelength for a truncated-sawtooth embodiment with a facet angle of 55.8° appropriate for use in second order
  • Fig. 14 shows results of numerical simulations for both the average efficiency and polarization-dependent loss as a function of wavelength for a truncated-sawtooth embodiment with a facet angle of 55.8° appropriate for use in second order
  • Fig. 13 shows results of numerical simulations for the polarization-dependent loss as a function of wavelength and groove depth for a truncated-sawtooth embodiment with a facet angle of 55.8° appropriate for use in second order
  • Fig. 14 shows results of numerical simulations for both the average efficiency and
  • part (a) shows a crystal surface with a normal that defines an angle ⁇ with respect to the [110] crystallographic direction; part (b) shows the formation of a first set of trenches using a crystalline-independent etching technique; part (c) shows the result of the depositing sacrificial material on the etched crystal surface; part (d) shows exposure of the underlying crystal surface at specific locations; part (e) shows the result of applying a crystalline-dependent chemical etchant at the exposed locations; and part (f) shows the resulting structure after dissolving the sacrificial material and agitating; and
  • Fig. 16 shows the steps performed in another embodiment to fabricate the diffraction grating: part (a) shows a crystal surface with a normal that defines an angle ⁇ with respect to the [110] crystallographic direction; part (b) shows the formation of a first set of trenches using a crystalline-independent etching technique; part (c) shows the result of depositing sacrificial material on the etched crystal surface; part (d) shows the result of chemical and mechanical polishing to remove the sacrificial material from the crystal surface; part (e) shows the result of subsequently exposing the crystal to a crystalline-dependent chemical etchant; and part (f) shows the resulting structure after removal of the remaining sacrificial material.
  • Embodiments of the invention can thus be used with a wavelength router to achieve the goals of optical networking systems.
  • optical wavelength router accepts light having a plurality of spectral bands at an input port and selectively directs subsets of the spectral bands to desired ones of a plurality of output ports. Light entering the wavelength router from the input port forms a diverging beam, which includes the different spectral bands.
  • the beam is collimated, such as by a lens, and directed to a diffraction grating that disperses the light so that collimated beams at different wavelengths are directed at different angles.
  • the high efficiency achieved by the diffraction grating in multiple polarization states translates directly into improved efficiency in operation of the wavelength router.
  • Other uses for the diffraction grating where high efficiency is desirable in multiple polarization states will be similarly evident to those of skill in the art.
  • Demultiplexing of an optical signal that contains a plurality of signals at different wavelengths may be accomplished with a diffraction grating with appropriately sized and shaped diffraction grooves.
  • a demultiplexing diffraction grating is illustrated in Fig. 1(a).
  • the manner in which incident light will be distributed among the various orders of interference depends on the shape and orientation of the groove sides and on the relation of wavelength to groove separation.
  • d ⁇ ⁇ diffraction effects predominate in controlling the intensity distribution among orders, but when d > ⁇ , optical reflection from the sides of the grooves is more strongly involved.
  • Diffraction gratings 100 are manufactured classically with the use of a ruling engine by burnishing grooves with a diamond stylus in a substrate 120 or holographically with the use of interference fringes generated at the intersection of two laser beams.
  • Current efforts may extend the operational wavelength range for optical telecommunications applications by about 30 mn on either end of the 1530 - 1570 range. It is further possible to "blaze" a grating by ruling its grooves to produce multiple reflective faces 112 that reflect a large fraction of the incoming light of suitably short wavelengths in one general direction.
  • a blazed grating has been understood to refer to one in which the grooves of the diffraction grating are controlled so that the reflective faces 112 form one side of right-apex triangles 110, inclined to the substrate surface with an acute blaze angle ⁇ b . Obtuse apex angles up to ⁇ 110° are sometimes present in blazed holographic gratings.
  • the diffraction efficiency for an S polarization state (also described as a TM polarization state), in which the electric field is polarized orthogonal to the grating grooves, is >90%.
  • Fig. 1(b) illustrates a first set of embodiments of the invention (referred to herein as the "full-sawtooth" embodiments), which provide a high diffraction efficiency for optical signals at telecommunications wavelengths in both S and P polarization states.
  • the grating of these embodiments may be used in first order.
  • the diffraction grating 150 includes multiple reflective faces 162 formed in a substrate 170, each inclined at blaze angle ⁇ b to the substrate surface. Each reflective face 162 is supported by a support wall 164 that is connected substantially normal with the substrate 170.
  • the diffraction grating 150 has a sawtooth configuration formed in the substrate 170 from multiple right-base triangular protrusions.
  • each of the reflective faces 162 is equally spaced along the surface of the substrate 170, with each reflective face 162 extending through a full spacing period.
  • the full-sawtooth embodiment is characterized by the absence of exposure of the substrate 170 at the base of the triangles 210 to incident light — the support wall 164 supporting each reflective face 162 is also connected to the adjacent reflective face.
  • the reflective faces 162 are not equally spaced.
  • High reflectivity of the reflective faces 162 is achieved in one embodiment by coating the diffraction grating 150 with gold.
  • different reflective coatings, such as aluminum, are used.
  • Fig. 1(b) shows the grating to be configured on a flat substrate 170, the invention more generally includes the use of curved substrates.
  • the support wall 164 is connected non-normally with the substrate 170, preferably forming an obtuse angle so that the characteristic of limiting reflection of incident light essentially only off the reflective faces 162 and not off the support walls 164 is maintained (see discussion below with regards to Fig. 9).
  • the blaze angle ⁇ b at which the reflective faces 162 are inclined to the substrate surface is preferably in the range 50 - 70°, most preferably in the range 50 - 60°.
  • a grating density l/d 900 faces/mm, which is a suitable value for the 1530 - 1570 nm wavelength used for optical telecommunications signals, this is preferably ⁇ b 54.0° .
  • FIG. 1(c) A second set of embodiments of the invention (referred to herein as the "truncated-sawtooth" embodiments) is illustrated in Fig. 1(c).
  • the diffraction grating 200 includes a plurality of reflective faces 212 each oriented at blaze angle ⁇ b with respect to a surface of the substrate 220.
  • Each such reflective face 212 is supported by a support wall 214 that is substantially normally connected with the surface of the substrate.
  • the diffraction grating 200 has a configuration that uses multiple right-base triangles 210, thereby sharing the advantage of the full-sawtooth configuration in which the normal orientation of the support walls 214 mitigates boundary effects for P- polarized light as the electric field of the light passes the apex of one triangle 210 and reflects off an adjacent reflective surface 212.
  • the support walls 214 connect with the substrate 170 non-normally, preferably forming an obmse angle so that the characteristic of limiting reflection of incident light essentially only off the reflective faces 162 and not off the support walls 164 is maintained.
  • Such truncated-sawtooth embodiments may be characterized by recognizing that each reflective face has an extent such that its orthogonal projection on the substrate is less than the average separation between the reflective faces. Thus, a trench is defined between each support wall and the reflective face subsequent to that support wall.
  • each of the reflective faces 212 is equally spaced along the surface of the substrate 220. In alternative embodiments such spacing may be irregular. Also, Fig. 1(c) shows each reflective face 212 extending through substantially half of the spacing period, although other fractions of the spacing period may also be used.
  • this optimal blaze angle corresponds to the angle at which the product of diffraction efficiencies in the S and P polarization configurations is maximized, as discussed in the context of Fig. 4(b) below.
  • the substrate 220 is shown to be flat only for illustrative purposes. More generally, the invention includes the use of a curved substrate. Additionally, various reflective materials may be used and may be differently applied in various embodiments. For example, in one embodiment, the entire diffraction grating 200 is coated with gold. In alternative embodiments, different reflective coatings, such as aluminum, are used. 4. Diffraction Efficiency of the Specific Embodiments
  • the maximum efficiency is expected at the blaze wavelength ⁇ b with a 50% reduction at 0.7 ⁇ b and 1.8 ⁇ b .
  • the range in wavelengths 1530 - 1570 nm (i.e.
  • Fig. 2 shows the results of calculations of a diffraction efficiency profile in both the S and P polarization configurations for the diffraction grating shown in Fig. 1(b) over a wavelength range of 1530 - 1570 nm.
  • Fig. 3 shows the diffraction grating shown in Fig. 1(c).
  • G-Solver solid lines
  • PC-Grate dashed lines
  • the diffraction efficiency exceeds 85% for both S and P polarizations for both the illustrated full-sawtooth and truncated-sawtooth configurations, with approximately less than a ⁇ 2% variation over the wavelength range for any given polarization.
  • the explicit comparison of the two numerical packages in Figs. 2 and 3 highlights their close agreement, with differences no greater than about 3%.
  • the invention produces a high diffraction efficiency that is substantially independent of polarization.
  • that polarization-independent efficiency is 90 ⁇ 4 %.
  • Fig. 4(a) and Fig. 4(b) an illustration is made of how the numerically calculated diffraction efficiency is used to determine the optimal blaze angle.
  • Fig. 4(a) shows the variation in efficiency for both the S and P polarization configurations as a function of blaze angle as calculated with the G-Solver package.
  • this particular incident angle ⁇ is approximately the angle dictated by the Littrow condition.
  • the optimal blaze angle is the angle where the curves for the S and P polarization intersect, i.e.
  • Fig. 4(b) shows the efficiencies for S and P polarizations for a truncated-sawtooth embodiment with triangles having an altitude equal to 80% of the maximum possible altitude.
  • Results are summarized in Fig. 5 of the optimal blaze angle ⁇ for a grating in the full-sawtooth embodiment.
  • the calculations were again performed using the G-Solver package and show how the optimal blaze angle varies as a function of the grating density 1/d.
  • Fig. 6 The effect of moving the incident angle away from the Littrow condition is illustrated in Fig. 6.
  • the results of calculations for a full-sawtooth grating using the G-Solver package are shown.
  • similar results are also obtained when a truncated-sawtooth grating is used with its optimal blaze angle.
  • Figs. 7(a) and 7(b) The variation in efficiency for the truncated-sawtooth embodiments for the S and P polarization states is shown respectively in Figs. 7(a) and 7(b) as a function of the height of support wall 214.
  • the S-polarization efficiency has two local maxima and the P-polarization efficiency exhibits asymptotic behavior.
  • the preferred triangle altitude for the truncated-sawtooth configuration is at the second peak in the S-polarization efficiency, i.e. near 1310 nm.
  • Fig. 8 shows that the diffraction efficiency of both S and P polarization states is also dependent on the grating density 1/d.
  • the calculations were performed with the G- Solver package for a full-sawtooth grating with optimal blaze angle ⁇ b .
  • the incident optical signal had wavelength ⁇ and was incident at the angle defined by the Littrow condition.
  • the grating density 1/d be between 700 and 1100 faces/mm. More preferably, the grating density is between 800 and 1000 faces/mm.
  • the back angle when the angle at which the support wall connects with the substrate (the "back angle"), is acute, the efficiency of the P-polarization configuration decreases sharply.
  • the calculations were performed with the G-Solver package for a full-sawtooth grating having a grating density lid of 900 faces/mm and a blaze angle ⁇ b of 54.0°.
  • a decrease of the back angle from 90° results directly in a significant decrease in P-polarization diffraction efficiency.
  • a decrease of the angle by about 10° causes an efficiency decrease of about 10%.
  • the S-polarization diffraction efficiency shows some improvement in the region between about 82 and 84°, it too drops off for more acute angles.
  • the back angle is obtuse, the diffraction efficiency is expected to be the same as it is for a right back angle since the reduction in boundary effects for P-polarized light is maintained. Accordingly, it is preferred that the back angle be approximately > 90°.
  • the grating embodiments described above are appropriate for use in at least the first order of interference. More generaly, however, the resolving power of the grating is mN, where m is the order of interference and N is the number of grooves in the grating.
  • a grating is provided for use in nonunity orders of interference m by decreasing the line density to 1/m times the line density used for first order.
  • m 2
  • a line density of lid between 350 and 550 faces/mm is appropriate for optical telecommunications wavelengths.
  • Fig. 10(b) A similar dependence for the P-polarization diffraction efficiency in second order is shown in Fig. 10(b).
  • an optimal blaze angle is found to be approximately 55.8°.
  • Fig. 12 summarizes the average of diffraction efficiency results for the S and P polarizations, simultaneously illustrating the dependence on the signal wavelength ⁇ and the groove depth t.
  • the average efficiency may be maintained above 90%) over a wavelength range between 1530 and 1560 nm for groove depths up to 2400 nm and at least as low as 2300 nm.
  • the average efficiency may be maintained above 94% for groove depths between 2300 and 2400 nm.
  • Polarization-dependent loss results from the fact that the efficiency of the diffraction grating depends on the polarization state of the incident light.
  • ⁇ and ⁇ respectively denote the permittivity and permeability of the medium.
  • ⁇ 4 ⁇ so that the coefficient relating the intensity and squared electric field is unity.
  • the diffraction efficiency is governed by independent efficiency coefficients E in the orthogonal polarization directions such that the electric field E' of the signal reflected by the grating is
  • Fig. 13 shows the degree of loss attributable to this polarization dependence in decibels as a function both of signal wavelength and trench depth.
  • the polarization-dependent loss is less than 0.4 decibels for a groove depth t ⁇ 2600 nm.
  • a trench in the profile at t 2500 nm where the polarization-dependent loss is everywhere less than about 0.04 decibels.
  • Fig. 14 provides a comparison of the diffraction efficiency and polarization- dependent loss over a wavelength range of 1500 - 1600 nm for two different configurations of the diffraction grating.
  • each of the different embodiments may be preferable for different applications.
  • it is possible to maintain average efficiency over 85% over the wavelength range ⁇ 1530 - 1570 nm while simultaneously keeping the polarization-dependent loss under about 0.08 dB.
  • FIG. 15 A process in accordance with embodiments of the present invention for fabricating diffraction gratings with the characteristics described above is illustrated in Fig. 15.
  • the process combines both crystalline-independent and crystalline-dependent etching techniques while exploiting the crystalline characteristics of appropriate materials.
  • Techniques that rely solely on crystalline-dependent etching techniques such as described in U.S. Pat. No. 4,330,175, filed July 17, 1979 by Fujii et al, which is herein incorporated by reference for all purposes, can only produce grating profiles narrowly limited by the crystallographic structure of the material used.
  • the combination of crystalline-independent and crystalline-dependent techniques in accordance with the present invention permit the grating pitch and angle to be varied independently.
  • the process begins with a suitable crystal surface 500 having a surface normal inclined at a tilt angle ⁇ from an etch direction, such as from the [110] direction when the crystal surface is a silicon surface, where the notation [ / 7c/] is used to denote the usual Miller indices.
  • the crystal surface is a wafer.
  • the tilt angle ⁇ of the starting crystal surface 500 is chosen to be complementary to the desired blaze angle ⁇ b of the final diffraction grating.
  • the tilt angle is in the approximate range 34° ⁇ ⁇ ⁇ 36°.
  • Silicon is one material with crystallographic properties that permit such tilt angles and can be used with the combined crystalline-independent and crystalline-dependent etching process of the invention, although the use of any alternative material with suitable crystalline structure is also within the scope of the invention.
  • III-V or II- VI semiconductor materials such as GaAs, InP, or ZnSe may be used instead of a Group-IV semiconductor material.
  • a series of vertical trenches 510 are etched into the crystal surface 500, as shown in Fig. 15(b).
  • the spacing of the trenches corresponds to the spacing of reflective faces in the resulting diffraction grating; accordingly, equally spaced vertical trenches 510 are etched for those embodiments in which a diffraction grating having equally spaced reflective faces is produced.
  • the etching technique used to produce the vertical trenches 510 is a crystalline-independent technique, in the sense that its etch activity is irrespective of the crystalline structure of the starting crystal surface 500.
  • One appropriate technique for producing the vertical trenches 510 is to identify the trench locations lithographically and then apply vertical reactive ion etching ("RIE"), a technique in which positively charged ions are accelerated towards the crystal surface 500.
  • Alternative crystalline-independent techniques also suitable for etching the vertical trenches 510 include ion milling and deep RIE ("DRIE"). Both the vertical RIE and ion-milling techniques produce vertical trenches 510 with substantially straight walls. DRIE produces a trench with a characteristic inwards scalloping of the walls. Use of DRIE in this step results in a diffraction grating where the support walls 164 include this inwards scalloping feature; since the side walls are not optically important for the grating, such scalloping is not objectionable.
  • the etched crystal surface is subsequently coated with a sacrificial layer 515 as shown in Fig. 15(c).
  • the sacrificial layer 515 is deposited so that it fills the bottoms of the vertical trenches 510, where it is used to prevent excessive etching of the crystal surface in the crystalline-dependent etch step described below (Fig. 15(e)).
  • a sacrificial layer 515 is deposited so that it fills the bottoms of the vertical trenches 510, where it is used to prevent excessive etching of the crystal surface in the crystalline-dependent etch step described below (Fig. 15(e)).
  • voids do not significantly affect the process adversely provided there is sufficient material at the bottom of the trenches to terminate the crystalline-dependent etch step (Fig. 15(e)).
  • Appropriate materials for the sacrificial layer include oxides or nitrides such as SiO 2 or Si 3 N .
  • the sacrificial layer 515 is subsequently patterned by etching it at locations that define the extent of the diffraction gratings reflective faces.
  • the etched portions 518 of the sacrificial layer 515 are formed adjacent to the vertical trenches 510.
  • the depth of the vertical trenches 510 formed during the crystalline- independent etch (Fig. 15(b)) and the positions of the etched portions 518 of the sacrificial layer 515 are constrained so that they define parallel segments inclined with respect to the crystal surface.
  • the specific portions of the sacrificial layer 515 are etched with any appropriate lithographic and etching technique, for example by using photoresist and RIE.
  • Fig. 15(e) the effect of applying a crystalline-dependent etch through the patterned sacrificial layer 515 is shown.
  • inclined trenches 520 are formed in the crystal.
  • a suitable crystalline-dependent chemical etchant is KOH, which has an etching ratio between the [110] and [111] orientations that exceeds 600.
  • the structure of a material such as silicon includes many crystallographic planes, leading to a concern that there may be interference with other of such planes when the crystalline-dependent etch is applied.
  • the inventors have recognized, however, that with the particular crystallographic structure of silicon and similarly structured materials, the etching can be limited to the desired [110] orientations.
  • the inclined trenches 520 are etched with negligible etching in undesirable directions.
  • Alternative crystalline-dependent chemical etchants that may also be used with a silicon crystal in accordance with the invention include hydrazine and ethylene diamine pyrocatechol.
  • the activity of the chemical etchant is stopped when it encounters the sacrificial layer 515 deposited within the vertical trenches 510.
  • the crystal surface 500 is exposed to an etchant that dissolves the sacrificial layer and is agitated to release the remaining sacrificial layer portions and unattached pieces of the surface that may be discarded.
  • the profile of the resulting micromachined crystal surface 500' is shown in Fig. 15(f).
  • This micromachined crystal surface 500' may be used directly to produce the diffraction grating or may be used as a master for replication of diffraction gratings.
  • the mesas 525 that result from the crystalline-independent etching step produce the truncated sawtooth profile shown in Fig. 1(c).
  • the resulting diffraction grating (which has a profile inverted from the micromachined crystal surface 500') has right-base triangles 160 that are truncated at the apex as a result of the mesas 525 in the master. Provided the size of this apical truncation is not too large, there are no significant adverse effects on the optical properties of the diffraction grating; the size of these mesas can be limited as desired by increasing the aspect ratio for the vertical trenches 510.
  • the grating is coated with a reflective overlay, such as gold, which may be adhered to the grating with an adhesion layer such as titanium or chrome.
  • a suitable starting crystal surface 500 having a surface normal inclined at tilt angle ⁇ from the [110] direction.
  • a series of vertical trenches 540 are etched into the crystal surface 500 with a crystalline-independent technique such as RIE [Fig. 16(b)].
  • a sacrificial material 545 such as SiO 2 or Si 3 N 4 , is subsequently deposited so as to fill the trenches [Fig. 16(c)].
  • the top of the sacrificial layer 545 overlying the crystal surface 500 is removed substantially completely after the deposition step, producing the structure shown in Fig. 16(d).
  • One technique that may be used to remove the material in this way is chemical and mechanical polishing (CMP). As can be seen in Fig. 16(d), this still leaves the material deposited within the vertical trenches 510 to act as an etch stop.
  • CMP chemical and mechanical polishing
  • Application of the chemical crystalline-dependent etch as shown in Fig. 16(e) acts as before to etch preferentially along [110] orientations and not along [111] orientations. Termination of the crystalline-dependent etch by the deposited sacrificial material 545 thus produces the inclined surfaces.
  • Subsequent removal of the remaining sacrificial material 545 to produce the micromachined surface 500" shown in Fig. 16(f) is optional. Either the surface shown in Fig. 16(e) or the surface shown in Fig. 16(f) may be used as described above to complete production of the diffraction grating.
  • the sacrificial layer 515 is not deposited uniformly over the crystal surface 500; instead, such material is deposited within each of the trenches at a sufficient depth to act as an etch stop when the crystalline-dependent chemical etch is applied.
  • the crystal surface 500 is replaced with a silicon-on-insulator ("SOI") layered structure, in which an insulator layer (e.g., an oxide layer) lies intermediate between underlying bulk silicon and an overlying epitaxial silicon layer.
  • SOI silicon-on-insulator
  • the crystalline-independent etch of the vertical trenches 510 is performed down to the insulator layer. As before, the vertical trenches are protected by filling them with sacrificial material, and the process is otherwise performed as previously described. This embodiment simplifies etching the vertical trenches 510 to substantially equal depths.
  • a crystalline-dependent chemical etchant produces an atomically smooth surface along the inclined trench 520, a feature that is beneficial to the optical characteristics of the diffraction grating because the reflective faces 112 are formed on this surface.
  • Other techniques to form the reflective faces such as ion-beam etching (as described in, e.g., U.S. Pat. No. 5,279,924, filed July 1, 1992 by Sakai et al, which is incorporated herein by reference for all purposes) fail to produce atomically smooth surfaces so that the optical characteristics of the diffraction grating are poorer.
  • ion- beam etching is an undesirable curvature of the grating profile at the intersection of the support walls and reflective faces.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Diffracting Gratings Or Hologram Optical Elements (AREA)

Abstract

L'invention concerne un réseau de diffraction (150) permettant d'obtenir une efficacité élevée de diffraction à toutes les polarisations pour des signaux optiques à des longueurs d'ondes qui sont celles de télécommunications. Le réseau de diffraction comprend un substrat (170) et plusieurs faces réflectives (162) orientées selon des angles de blaze respectifs espacés le long de la surface du substrat, les angles de blaze étant sensiblement différents de la condition de Littrow. Chaque surface réflective est supportée par une paroi (164) reliée sensiblement à la surface du substrat. On peut utiliser le réseau à des ordres d'interférence plus grands que le premier ordre et ce réseau peut diffracter des signaux à faibles pertes par polarisation avec une efficacité élevée sur une polarisation moyennée.
PCT/US2001/022229 2000-07-13 2001-07-11 Reseau de diffraction a perte par polarisation reduite WO2002006860A1 (fr)

Priority Applications (1)

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AU2001280555A AU2001280555A1 (en) 2000-07-13 2001-07-11 Diffraction grating with reduced polarization-dependent loss

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Application Number Priority Date Filing Date Title
US09/615,300 US6449096B1 (en) 2000-07-13 2000-07-13 Diffraction grating with reduced polarization-dependent loss
US09/615,300 2000-07-13
US09/669,758 2000-09-26
US09/669,758 US6517734B1 (en) 2000-07-13 2000-09-26 Grating fabrication process using combined crystalline-dependent and crystalline-independent etching
US09/748,687 2000-12-21
US09/748,687 US6490089B1 (en) 2000-07-13 2000-12-21 Diffraction grating with reduced polarization-dependent loss

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WO2001095540A3 (fr) * 2000-06-02 2003-02-27 Lightchip Inc Dispositif et procede de surveillance de la qualite optique dans un reseau de communication optique
US6597491B2 (en) 2000-08-01 2003-07-22 Cheetah Omni, Llc Micromechanical optical switch
US6795605B1 (en) 2000-08-01 2004-09-21 Cheetah Omni, Llc Micromechanical optical switch
US6844974B1 (en) 2001-02-02 2005-01-18 Cheetah Omni, Llc Variable blazed grating
US6943925B1 (en) 2001-02-02 2005-09-13 Cheetah Omni, Llc Optical logic gate based optical router
US7145704B1 (en) 2003-11-25 2006-12-05 Cheetah Omni, Llc Optical logic gate based optical router
US7339714B1 (en) 2001-02-02 2008-03-04 Cheetah Omni, Llc Variable blazed grating based signal processing
US7429983B2 (en) 2005-11-01 2008-09-30 Cheetah Omni, Llc Packet-based digital display system
WO2010017979A1 (fr) * 2008-08-13 2010-02-18 Schott Ag Procédé et appareillage pour la fabrication d'un objet structuré, ainsi qu'objet structuré
JP2015121605A (ja) * 2013-12-20 2015-07-02 キヤノン株式会社 回折格子および回折格子の製造方法
CN115248468A (zh) * 2022-08-22 2022-10-28 之江实验室 一种高色散高衍射效率的折射反射混合型光栅
CN115824061A (zh) * 2023-02-14 2023-03-21 中国科学院长春光学精密机械与物理研究所 一种基于利特罗衍射的光栅位移测量装置及方法
US11976270B2 (en) 2019-05-15 2024-05-07 Massachusetts Institute Of Technology Optimization of circular single stranded DNA using M13 phage

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JP2000089011A (ja) * 1998-09-10 2000-03-31 Cymer Inc 回折格子とその製造方法

Patent Citations (1)

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JP2000089011A (ja) * 1998-09-10 2000-03-31 Cymer Inc 回折格子とその製造方法

Cited By (24)

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WO2001095540A3 (fr) * 2000-06-02 2003-02-27 Lightchip Inc Dispositif et procede de surveillance de la qualite optique dans un reseau de communication optique
EP1312181A2 (fr) * 2000-06-02 2003-05-21 Lightchip, Inc. Dispositif et procede de surveillance de la qualite optique dans un reseau de communication optique
US6577786B1 (en) 2000-06-02 2003-06-10 Digital Lightwave, Inc. Device and method for optical performance monitoring in an optical communications network
US6950225B2 (en) 2000-08-01 2005-09-27 Cheetah Omni, Llc Micromechanical optical switch
US7142347B2 (en) 2000-08-01 2006-11-28 Cheetah Omni, Llc Method and system for processing photonic systems using semiconductor devices
US6654157B2 (en) 2000-08-01 2003-11-25 Che Tah Omni, Llc Micromechanical optical switch
US6795605B1 (en) 2000-08-01 2004-09-21 Cheetah Omni, Llc Micromechanical optical switch
US6611366B2 (en) 2000-08-01 2003-08-26 Cheetah Omni, Llc Micromechanical optical switch
US6597491B2 (en) 2000-08-01 2003-07-22 Cheetah Omni, Llc Micromechanical optical switch
US6859301B1 (en) 2000-08-01 2005-02-22 Cheetah Omni, Llc Micromechanical optical switch
US7339714B1 (en) 2001-02-02 2008-03-04 Cheetah Omni, Llc Variable blazed grating based signal processing
US6900919B1 (en) 2001-02-02 2005-05-31 Cheetah Omni, Llc Variable blazed grating
US6847479B1 (en) 2001-02-02 2005-01-25 Cheetah Omni, Llc Variable blazed grating
US6972886B2 (en) 2001-02-02 2005-12-06 Cheetah Omni, Llc Variable blazed grating
US6844974B1 (en) 2001-02-02 2005-01-18 Cheetah Omni, Llc Variable blazed grating
US6943925B1 (en) 2001-02-02 2005-09-13 Cheetah Omni, Llc Optical logic gate based optical router
US7145704B1 (en) 2003-11-25 2006-12-05 Cheetah Omni, Llc Optical logic gate based optical router
US7429983B2 (en) 2005-11-01 2008-09-30 Cheetah Omni, Llc Packet-based digital display system
US8379061B2 (en) 2005-11-01 2013-02-19 Gopala Solutions Limited Liability Company Packet-based digital display system
WO2010017979A1 (fr) * 2008-08-13 2010-02-18 Schott Ag Procédé et appareillage pour la fabrication d'un objet structuré, ainsi qu'objet structuré
JP2015121605A (ja) * 2013-12-20 2015-07-02 キヤノン株式会社 回折格子および回折格子の製造方法
US11976270B2 (en) 2019-05-15 2024-05-07 Massachusetts Institute Of Technology Optimization of circular single stranded DNA using M13 phage
CN115248468A (zh) * 2022-08-22 2022-10-28 之江实验室 一种高色散高衍射效率的折射反射混合型光栅
CN115824061A (zh) * 2023-02-14 2023-03-21 中国科学院长春光学精密机械与物理研究所 一种基于利特罗衍射的光栅位移测量装置及方法

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