(12)
`
`United States Patent
`Erdogan et al.
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 7,123.416 B1
`*Oct. 17, 2006
`
`USOO7123416B1
`
`4/1996 Kumar et al.
`5,512,131 A
`8/1997 Scobey et al.
`5,656,138 A
`1/1998 Erdogan et al.
`5,712,715 A
`5,828,489 A * 10/1998 Johnson et al. ............. 359/487
`5,900,160 A
`5/1999 Whitesides et al.
`6,518,168 B1
`2/2003 Clem et al.
`6,623,803 B1
`9/2003 Krivokapic
`6,649.208 B1
`11/2003 Rodgers
`6,704,130 B1
`3/2004 Ford et al.
`6,809,859 B1
`10/2004 Erdogan et al.
`2005/0110999 A1
`5/2005 Erd
`tal.
`rdogan et a
`
`OTHER PUBLICATIONS
`-- - 9
`& 8
`Becker, J., “Ion-Beam Sputtering.” Handbook of Optical Properties,
`vol. 1. Thin Films for Optical Coatings, Ed. By R.E. Hummel and
`K.H. Guenther, Chapter 7, pp. 189-211. (CRC Press, Boca Raton,
`Macleod, H. Angus, “Thin-Film Optical Filters,” 3" Ed., Institute of
`Physi ysics (2001).
`
`1995).
`
`(Continued)
`Primary Examiner Favez, G. Assaf
`y
`y
`74). A
`y, Ag
`Fi
`L
`Sandler PC
`ttorney,
`ent, Or Firin—LOWenStein Sandler
`
`(57)
`
`ABSTRACT
`
`(54) METHOD OF MAKING HIGH
`PERFORMANCE OPTICAL EDGE AND
`NOTCH FILTERS AND RESULTING
`PRODUCTS
`
`(75) Inventors: Turan Erdogan, Spencerport, NY (US);
`Joseph T. Foss, Rochester, NY (US);
`Ligang Wang, Rochester, NY (US)
`
`(73) Assignee: Semrock, Inc., Rochester, NY (US)
`(*) Notice:
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`This patent is Subject to a terminal dis-
`claimer
`
`(21) Appl. No.: 11/248,456
`
`y x- - -
`
`9
`
`(22) Filed:
`
`Oct. 11, 2005
`
`Related U.S. Application Data
`(63) Continuation-in-part of application No. 10/840,134,
`filed on Mav 6, 2004
`ed on May
`s
`(60) Provisional application No. 60/637,697, filed on Dec.
`21, 2004, provisional application No. 60/468,245,
`filed on May 6, 2003.
`(51) Int. Cl.
`(2006.01)
`GO2B 5/28
`(2006.01)
`GO2B I/O
`(52) U.S. Cl. ...................... 359,589. 350/588, 350/580
`s
`s 359/5 87
`(58) Field of Classification Search ................
`Sosso.
`359/588, 580, 587
`S
`lication file f
`let
`h hist
`s
`ee appl1cauon Ille Ior complete searcn n1story.
`References Cited
`U.S. PATENT DOCUMENTS
`
`(56)
`
`3, 1979 Wei et al.
`4,142,958 A
`4,793.908 A 12/1988 Scott et al.
`5, 112,127 A
`5, 1992 Carrabba et al.
`
`
`
`& G
`
`99
`
`High performance optical edge and notch filters and meth
`ods of making the same are disclosed. The multi-layer,
`thin-film optical edge filters have an edge steepness greater
`than about 0.8% as measured by dividing (a) the edge width
`than about U.8% as measure
`1V1d1ng (a) the edge W1Clt
`from the 50% transmission wavelength to the optical density
`6 (ODS) wavelength by (b) the 50% transmission wave
`length. The optical edge filters also have an average trans
`mission above about 95%. The notch filters exhibit a block
`ing of OD>6, very high transmission (>90%) outside the
`notch(es), and a narrow notch bandwidth comparable to that
`of holographic notch filters. The methods for making Such
`filters accurately determine when deposition of each layer of
`the filter should terminate.
`
`O
`
`53 Claims, 18 Drawing Sheets
`
`Notch filter
`coatin
`9
`
`Substrate
`
`Multi-notch
`filter coating
`
`Substrate
`
`WP filter
`Coating
`
`O High-index Material
`Low-Index Material
`
`O High-index Material
`low-index Material
`
`MATERION
`PGR2019-00017
`EX. 2003-001
`
`

`

`US 7,123.416 B1
`Page 2
`
`OTHER PUBLICATIONS
`Macleod, H. Angus, "Turning value monitoring of narrow-band
`all-dielectric thin-film optical filters,” Optica Acta, vol. 19, pp. 1-28
`(1972).
`Press, W.H., et al., The Levenberg-Marquardt method implemented
`under the name “mrqmin()". Numerical Recipes in C. The Art of
`Scientific Computing, 2" ed., Chapter 15, pp. 683-688 (1995).
`Martin, P.J. et al., “Ion-beam-assisted deposition of thin films.”
`Applied Optics, vol. 22, No. 1, pp. 178-184 (1983).
`“Interference Filters,” Melles Griot, pp. 13.25-13.29.
`J.M.E. Harper, “Ion Beam Deposition.” In Thin Film Processes, Ed.
`by J.L. Vossen and W. Kern, pp. 175-206 (Academic Press, New
`York, 1978).
`
`U.J. Gibson, “Ion-Beam Processing of Optical Thin Films,” in
`Physics of Thin Films, vol. 13, Ed. by G. Hass and M.H. Fancombe,
`pp. 109-150 (Academic Press, New York, 1978).
`J.M.E. Harper et al., “Modification of Thin Film Properties by Ion
`Bombardment During Deposition,” in Ion Bombardment Modifica
`tion of Surfaces, Ed. by O. Auciello and R. Kelly, from Beam
`Modification of Materials, vol. 1, pp. 127-162 (Elsevier,
`Amsterdam, 1984).
`W.H. Press et al., Numerical Recipes, “Numerical Recipes in C: The
`Art of Scientific Computing.” 2" ed., Cambridge University Press,
`Cambridge, Chapter 15.7, pp. 699-706 (1995).
`* cited by examiner
`
`MATERION
`PGR2019-00017
`EX. 2003-002
`
`

`

`U.S. Patent
`
`Oct. 17, 2006
`
`Sheet 1 of 18
`
`US 7,123.416 B1
`
`Fig. 1A
`
`T (transmission)
`
`LWP Filter
`
`^T
`
`X (wavelength)
`
`Fig. 1B
`
`T (transmission)
`
`SWP Filter
`
`MATERION
`PGR2019-00017
`EX. 2003-003
`
`

`

`U.S. Patent
`
`Oct. 17, 2006
`
`Sheet 2 of 18
`
`US 7,123.416 B1
`
`Fig. 1C
`
`
`
`MATERION
`PGR2019-00017
`EX. 2003-004
`
`

`

`U.S. Patent
`
`Oct. 17, 2006
`
`Sheet 3 of 18
`
`US 7,123.416 B1
`
`Fig. 2
`
`20
`
`23
`
`23A
`
`(I T
`
`- 7–
`25
`
`21
`
`2 - 2
`22
`22A
`
`24
`
`(y
`-
`
`Fig. 3
`
`
`
`30
`
`33A
`33A
`33A
`33A
`33A
`31
`
`MATERION
`PGR2019-00017
`EX. 2003-005
`
`

`

`U.S. Patent
`
`Oct. 17, 2006
`
`Sheet 4 of 18
`
`US 7,123.416 B1
`
`Fig. 4
`
`402
`
`
`
`
`
`
`
`
`
`401
`
`
`
`
`
`Signal
`ay Detector
`Detector
`Filtet - 411 B
`
`413
`
`Deposition
`On Beatn
`
`Reference
`Detector
`
`412
`
`
`
`MATERION
`PGR2019-00017
`EX. 2003-006
`
`

`

`U.S. Patent
`
`Oct. 17, 2006
`
`Sheet 5 of 18
`
`US 7,123.416 B1
`
`503
`
`- 502
`
`Determine a
`
`Calculate TVS d
`at a series of
`
`
`
`
`
`- 501
`inputs.
`date
`
`504
`Determine
`which layers
`to monitor 8.
`which to time
`
`Start deposition
`
`505
`
`Begin depositing
`thin film layer i
`
`506
`
`Calculate
`expected
`deposition time t,
`
`507
`
`
`
`508
`
`No
`
`509
`
`Deposit until
`expected
`deposition time
`expires
`
`510
`1
`
`
`
`
`
`
`
`Monitored
`Layer?
`
`Yes
`
`Measure T vs t
`during deposition
`
`511
`
`Generate 2D array
`of data files based
`on TVs data
`
`l
`Tws
`for
`
`dii,
`
`
`
`fort. "At
`
`> 1 T, vs d
`extreturn?
`
`Scale I."
`Scale T'a', T?ar
`
`Calculate RMS
`error between
`T, and T vs t
`
`Determine j, k that
`yield min RMS
`error and use t
`
`Extrapolate T.
`and terminate
`layer using t
`
`Configure
`machine layer i+1
`and Continue ...
`
`521
`
`522
`
`Stop deposition
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`MATERION
`PGR2019-00017
`EX. 2003-007
`
`

`

`U.S. Patent
`
`Oct. 17, 2006
`
`Sheet 6 of 18
`
`US 7,123.416 B1
`
`604
`
`Calculate f. f.
`f, for each layer
`at wavelength a
`
`
`
`
`
`Determine
`which layers
`to monitor 8
`which to time
`
`Start deposition
`
`Begin depositing
`thin film layer i
`
`Calculate
`expected
`deposition time t,
`
`
`
`
`
`
`
`
`
`
`
`Monitored
`Layer?
`
`Yes
`
`
`
`Measure T vs t
`during deposition
`
`Fit Tvst to
`T vs t and thus
`calculate rater
`
`Calculate best
`rate r using
`rolling average
`
`Using f
`extrapolate T vs t
`and terminate the
`layer
`
`Configure
`machine layer i+1
`and Continue ...
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`603
`
`602
`
`601
`
`Determine a
`
`Calculate TVs d
`at a series of A.
`
`inputs:
`design & rate
`estimates
`
`605
`
`606
`
`607
`
`608
`
`
`
`610
`
`609
`
`
`
`Deposit until
`expected
`No
`deposition time
`expires
`
`612
`
`613
`
`614
`
`615
`
`617
`
`Stop deposition
`
`MATERION
`PGR2019-00017
`EX. 2003-008
`
`

`

`U.S. Patent
`
`Oct. 17, 2006
`
`Sheet 7 of 18
`
`US 7,123.416 B1
`
`
`
`Notch filter
`Coating
`
`Substrate
`
`O High-index Material
`Low-Index Material
`
`Multi-notch
`filter coating
`
`Substrate
`
`LWP filter
`Coating
`
`OHigh-lindex Material
`O Low-index Material
`
`MATERION
`PGR2019-00017
`EX. 2003-009
`
`

`

`U.S. Patent
`
`Oct. 17, 2006
`
`Sheet 8 of 18
`
`US 7,123.416 B1
`
`Fig. 9A
`
`
`
`Fic. 9B
`9
`
`T
`
`< 0.5% of W.
`(- 3 nm at 532)
`
`Fig. 9C
`
`
`
`S.
`
`i
`
`s
`
`O
`
`A.
`
`> 90%
`
`- 3% of Al (p)
`- 4% of A (s)
`(16-21 nm at 532)
`
`- p polarization
`a
`Spolarization
`
`Al
`
`Wavelength
`
`MATERION
`PGR2019-00017
`EX. 2003-0010
`
`

`

`U.S. Patent
`
`Oct. 17, 2006
`
`Sheet 9 of 18
`
`US 7,123.416 B1
`
`Fig. 10 - LWP
`
`100 r=
`
`"AAAAYAVAAAAAAAA th Aaaayaasa
`
`p
`
`90 -
`
`s 70------- - ---.
`5 60
`h
`i? 50---
`E
`--1001, 1002
`
`samersmagaa
`
`------aa-eeee-eranese-spars-sur-a-w
`
`R. R. M.
`
`30- -
`20 - tea-re- - Design -100
`- Measured
`1002
`10 -- wwwakas.
`- - Laser Line -o
`45
`so so too
`isso
`too
`750
`Wavelength (nm)
`
`Fig.11 - LWP
`
`
`
`e t
`
`o
`CD
`O
`s
`O
`S
`P
`
`-110
`- Design
`- - - Measured - 192
`1101 - Laser Line --1 103
`
`P
`
`52
`
`525
`
`-
`
`|\
`| |
`540
`535
`530
`Wavelength (nm)
`
`545
`
`550
`
`MATERION
`PGR2019-00017
`EX. 2003-0011
`
`

`

`U.S. Patent
`
`Oct. 17, 2006
`
`Sheet 10 of 18
`
`US 7,123.416 B1
`
`Fig. 12 - SWP
`
`1201
`
`100m
`--- -- Desian -120
`--- H 9
`1202
`90
`am Measured
`m. Laser Line
`
`Fig. 13- SWP
`8 - Wraneaemorrain-a-sanageaissanayeva
`
`7----
`
`instrument Noise Limit
`6 -- a-------- rare
`1301
`
`-
`
`
`
`4 -
`
`- Measured
`1 - - Laser Line
`o
`
`530
`520
`Wavelength (nm)
`
`
`
`500
`
`51O
`
`MATERION
`PGR2019-00017
`EX. 2003-0012
`
`

`

`U.S. Patent
`
`Oct. 17, 2006
`
`Sheet 11 of 18
`
`US 7,123.416 B1
`
`Fig. 14 (633 nm single-notch filter example
`Transmission spectrum (%)
`1401401
`
`s
`: 9
`.
`
`E.
`t
`
`i
`
`}
`
`470
`
`510
`
`550
`
`590
`
`630
`
`670
`
`10
`
`750
`
`790
`
`S30
`
`87.
`
`Wave enth TT
`
`Fig. 15
`
`
`
`4
`
`
`
`1501,1502
`
`1502
`
`470
`
`510
`
`550
`
`590
`
`710
`670
`630
`Wavelength (nm)
`
`750
`
`790
`
`830
`
`so
`
`MATERION
`PGR2019-00017
`EX. 2003-0013
`
`

`

`U.S. Patent
`
`Oct. 17, 2006
`
`Sheet 12 of 18
`
`US 7,123.416 B1
`
`Fig. 16 Triple-notch filter (triple-notch in a single coating)
`Transmission Spectrum (%)
`
`Measured
`
`;
`
`: sy
`9
`
`:
`
`i
`
`:
`
`i
`
`470
`
`500
`
`530
`
`560
`
`S50
`S2O
`590
`Wavelength inn
`
`680
`
`7 10
`
`740
`
`770
`
`Fic. 17
`9
`
`o
`Optical Density (OD) Spectrum
`
`8
`
`
`
`O
`
`... 1/U
`
`.
`
`-
`
`
`
`702 ||
`u Measured
`a a sign
`1701
`
`1701,1702
`
`: 3
`
`5
`
`t
`
`:
`:
`
`:
`
`i
`
`:
`|
`I
`:
`
`:
`
`470
`5oo
`530
`560
`59
`620
`650
`680
`710
`40
`770
`- Waventh (nm) -
`
`MATERION
`PGR2019-00017
`EX. 2003-0014
`
`

`

`U.S. Patent
`
`Oct. 17, 2006
`
`Sheet 13 of 18
`
`US 7,123.416 B1
`
`Fig. 18 Triple-notch filter example 2 (dual-notch plus LWP)
`
`Transmission spectrum (%)
`
`| \
`
`1802
`
`1802 - Measured
`Design
`
`:
`
`:
`
`1801
`
`1801
`
`
`
`10o
`
`80
`
`70
`
`60
`
`SO
`
`40
`
`30
`
`20
`
`10
`
`e-48
`380 400. 420 440 46
`
`480 500 520 540 560 580 600 620 640 660 so 700 72
`Ways lefth nin
`
`Fig. 19
`
`Optical Density (OD) spectrum
`
`7
`
`5.6
`
`4.9
`
`4.2
`
`35
`
`2.8
`
`2.1
`
`14
`
`0.7
`
`mu e sign
`
`1902
`
`1901
`
`1901, 1902
`
`to
`
`901
`
`|
`
`0
`
`is
`
`a si
`
`a so is so so so it
`go is
`Wave ength int.
`
`is
`
`I
`
`MATERION
`PGR2019-00017
`EX. 2003-0015
`
`

`

`U.S. Patent
`
`Oct. 17, 2006
`
`Sheet 14 of 18
`
`US 7,123.416 B1
`
`Fig. 20 Triple-notch filter example 2- Continued)
`
`Transmission spectra of Side 1 and Side 2 coatings (%)
`
`
`
`5 O
`
`1. D La - notch coating
`to Side 2: WP coating
`
`;
`380 400 4 20 440 460 480 500 520 540. 560 580 600 62O 64 O 660 680 700 720
`Wavelength (nrt
`
`MATERION
`PGR2019-00017
`EX. 2003-0016
`
`

`

`U.S. Patent
`
`Oct. 17, 2006
`
`Sheet 15 of 18
`
`US 7,123.416 B1
`
`Fig. 21 45 degree single-notch filter example
`
`Transmission spectrum (%)
`21 O1
`21 O2
`10 ... '...... se:Egg FIRESSES
`'hi St. Shi'it's
`i is
`Si Eggs: SE y f /
`...
`I
`''''',
`
`2103
`
`O
`
`
`
`s
`
`- 2101,2102
`
`- Arafaga Planaro's 21 O2
`a - - 8 Poiriar
`21 O3
`. . . . . . p Polarizatian
`
`Wavetangth (na)
`
`68
`
`MATERION
`PGR2019-00017
`EX. 2003-0017
`
`

`

`U.S. Patent
`
`Oct. 17, 2006
`
`Sheet 16 of 18
`
`US 7,123.416 B1
`
`Fig. 22 45 degree single-notch filter example continued
`Optical Density (OD) spectrum
`
`s
`
`4
`
`3
`
`22O1
`- Average Polarization 2202
`- - - s Polarization
`
`
`
`- - - - - - - p Polarization
`
`2203
`
`2 2 O 3
`
`Wavelength (nm)
`
`MATERION
`PGR2019-00017
`EX. 2003-0018
`
`

`

`U.S. Patent
`
`Oct. 17, 2006
`
`Sheet 17 of 18
`
`US 7,123.416 B1
`
`Fig. 23 Ouadruple-notch filter
`
`Transmission spectrum (%)
`
`
`
`2301
`
`2301
`
`2301
`
`2301
`
`MATERION
`PGR2019-00017
`EX. 2003-0019
`
`

`

`U.S. Patent
`
`Oct. 17, 2006
`
`Sheet 18 of 18
`
`US 7,123.416 B1
`
`Fig. 24 Quadruple-notch filter
`
`Optical Density (OD) spectrum
`
`6
`
`3.
`2
`
`go
`s
`s
`a
`C
`
`2402
`
`
`
`24O1
`24O2 2401
`2402
`
`i
`
`380 400 42O 440 460 480 500 520 540 560 580 600 620 640 660 680 700 720
`Wavelength (nm)
`-
`
`O
`
`MATERION
`PGR2019-00017
`EX. 2003-0020
`
`

`

`US 7,123,416 B1
`
`1.
`METHOD OF MAKING HGH
`PERFORMANCE OPTICAL EDGE AND
`NOTCH FILTERS AND RESULTING
`PRODUCTS
`
`CROSS REFERENCE TO RELATED
`APPLICATIONS
`
`This application claims the benefit of U.S. Provisional
`Application No. 60/637,697, titled High Performance Thin
`Film Notch Filters, filed on Dec. 21, 2004 by Turan Erdogan,
`Joseph T. Foss, and Ligang Wang, and is a continuation-in
`part of prior U.S. patent application Ser. No. 10/840,134,
`filed May 6, 2004, which claims the benefit of U.S. Provi
`sional Application Ser. No. 60/468,245, filed May 6, 2003.
`The entire disclosures of U.S. Provisional Application No.
`60/637,697, U.S. patent application Ser. No. 10/840,134,
`and U.S. Provisional Application No. 60/468,245 are hereby
`incorporated herein by reference.
`
`10
`
`15
`
`FIELD OF INVENTION
`
`This invention relates to methods of making optical edge
`filters and optical notch filters and also relates to the result
`ing improved filters.
`
`25
`
`BACKGROUND OF THE INVENTION
`
`2
`light with high transmission on both sides of the narrow
`blocking range. Because lasers emit a very Small, but
`non-zero, bandwidth of light, an ideal notch filter blocks
`light at wavelengths within this bandwidth (O-(BW/2)) to
`(W--(BW/2))) with no ripple and perfectly steep (vertical)
`transition edges, as shown in FIG. 1C. The ideal notch filter
`passes light at wavelengths longer than the blocking band
`(wd (2+(BW/2))) and passes light at wavelengths shorter
`than the blocking band (0-(0,-(BW/2))). A realistic notch
`filter does not have complete transmission outside of the
`blocking band (O-(BW/2)) to ( --(BW/2))), does not
`completely block radiation within the blocking band, and
`has non-vertical transition edges, thereby changing from
`blocking to transmission over a small range of wavelengths,
`as shown in FIG. 1D. Accordingly, the steepness of the
`edges, the transmission amount outside of the blocking
`band, and the blocking effectiveness within the blocking
`band are important parameters of notch filters in many
`applications.
`Edge filters and notch filters are particularly useful in
`optical measurement and analysis systems that use laser
`light to excite a sample at one wavelength (or a small band
`of wavelengths) w and measure or view an optical response
`of the excited sample at other wavelengths. The excitation
`light
`is delivered to the sample by an excitation light path,
`and the optical response of the sample is delivered to the eye
`or measuring instrument by a collection path. Edge filters
`can be used to block spurious light from the excitation path.
`Edge filters and/or notch filters can be used to block exci
`tation light from entry into the collection path. The steeper
`the filter edge(s), the more effectively spurious signals are
`blocked. In the case of both edge filters and notch filters, the
`lower the transmission loss, the more light from the sample
`reaches the measuring instrument.
`Raman spectroscopy is one such optical analysis system.
`It is based on the fact that when molecular material is
`irradiated with high intensity light of a given wavelength (or
`series of wavelengths) w, a small portion of the incident
`light scattered by the material will be shifted in wavelength
`above and below ... This Raman shifting is attributed to the
`interaction of the light with resonant molecular structures
`within the material, and the spectral distribution of the
`Raman-shifted light provides a spectral “fingerprint char
`acteristic of the composition of the material. As a practical
`example, a Raman probe can identify the contents of a bottle
`without opening the bottle.
`FIG. 2 is a simplified schematic diagram of a Raman
`probe 20. In essence, the probe 20 comprises an optical
`excitation path 22, and a collection path 23. These paths
`advantageously comprise optical fiber. In operation, excita
`tion light W from a laser 24 passes through the fiber path 22
`and one or more edge filters or a narrowband laser-line filter
`22A to illuminate a portion of the sample 21 with high
`intensity light. The edge filter(s)/laser-line filter 22A act(s)
`to block light outside of
`from the sample 21. Light
`scattered from the sample 21 passes through a notch filter (or
`one or more edge filters) 23A and then through fiber col
`lection path 23 to a spectral analyzer 25 where the “finger
`print’ of the sample is determined.
`The light scattered from the sample 21 is a mixture of
`unshifted scattered excitation light
`Rayleigh scattering)
`and Raman-shifted light at wavelengths longer and shorter
`than
`. The scattered excitation light
`would not only
`Swamp the analyzer, it would also excite spurious Raman
`scattering in a collection fiber. Thus the unshifted excitation
`light a should be removed from the collection path. This
`can be accomplished by disposing a notch filter (or one or
`
`A. Optical Edge Filters, Optical Notch Filters, and Their
`US
`Optical edge filters and thin-film notch filters are impor
`tant components in systems for optical measurement and
`analysis including Raman spectroscopy and fluorescence
`spectroscopy. Optical edge filters and/or notch filters are
`used in Such systems to block unwanted light that would
`otherwise constitute or generate spurious optical signals and
`Swamp the signals to be detected and analyzed.
`Optical edge filters block unwanted light having wave
`lengths above or, alternatively, below a chosen “transition'
`wavelength w, while transmitting light on the unblocked
`side of W. Edge filters which transmit optical wavelengths
`longer than ware called long-wave-pass filters (LWP fil
`ters), and edge filters which transmit wavelengths shorter
`than
`are short-wave-pass or SWP filters.
`Referring to the drawings, FIGS. 1A and 1B schemati
`cally illustrate the spectral transmission of idealized long
`wave-pass and short-wave-pass filters respectively. As can
`be seen from FIG. 1A, a LWP filter blocks light with
`wavelengths below w and transmits light with wavelengths
`above W. As shown in FIG. 1B, a SWP filter transmits light
`with wavelengths below ... and blocks light with wave
`lengths above W. W., is the wavelength at which the filter
`“transitions' from blocking to transmission, or vice versa.
`While an ideal edge filter has a precise transition wave
`length w represented by a vertical line at W, real edge filters
`change from blocking to transmission over a small range of
`wavelengths and are more accurately represented by a
`non-vertical but steeply sloped line near . Similarly, while
`an ideal edge filter transmits all light in the transmission
`region (transmission T=1), real filters invariably block a
`small portion of the light to be transmitted (T-1). The
`steepness of the line and the proportion of the light trans
`mitted are important parameters in many applications.
`Turning now to FIGS. 1C and 1D, the spectral transmis
`sion of an ideal and a realistic notch filter are illustrated
`respectively. Notch filters block a specific and narrow range
`of wavelengths (ideally a single laser "line' w) and pass
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`more edge filters) 23A between the sample 21 and the
`collection fiber 23, the notch filter (or edge filter(s)) 23A
`blocking the unshifted scattered excitation light w.
`Edge filters and notch filters also are useful in fluores
`cence spectroscopy. Here, laser excitation light w is used to
`excite longer wavelength emissions from fluorescent mark
`ers. The markers can be fluorescent atoms chemically
`bonded to a biological molecule to track the molecule in a
`body or cell. Edge filters may be used to reject spurious light
`from an excitation path and to reject excitation light from a
`collection path. Notch filters may be used to reject excitation
`light from the collection path.
`In the case of edge filters, it should now be clear that the
`steeper the filter slope at the transition wavelength w, the
`greater the amount of spurious light that can be filtered out.
`In addition, the steeper the slope, the greater the amount of
`shifted light from the sample that will reach the analyzer.
`Similarly, higher levels of transmission of the shifted light
`through the filters provide more light for analysis. Higher
`edge filter blocking provides better rejection of the laser
`excitation light from the spectrum analyzer, thus decreasing
`the noise and improving both specificity and sensitivity of
`the measurement. Higher edge-filter transmission enables
`the maximum signal to reach the analyzer, further improving
`the signal-to-noise ratio and hence the measurement or
`image fidelity. A steeper filter edge also permits shifts to be
`resolved much closer to the excitation wavelength, thus
`increasing the amount of information from the measure
`ment.
`In the case of notch filters, the steeper the edges of the
`notch filter at the laser wavelength w, the greater the amount
`ofunshifted excitation light
`that can be filtered out before
`reaching an analyzer. Similarly, the higher the levels of
`transmission outside of the blocking band, the more infor
`mation there is for measurement.
`B. Edge Filter and Notch Filter Structure and Conven
`tional Fabrication
`FIG. 3 is a simplified schematic illustration of an optical
`filter 30, which may be either an edge filter or a notch filter.
`The optical filter 30 comprises a transparent substrate 31
`having a flat major Surface 32 Supporting many thin coatings
`33A, 33B. The thickness of the coatings is exaggerated and
`the number is reduced for purposes of illustration. Coatings
`33A and 33B are typically alternating and of different
`respective materials chosen to present markedly different
`indices of refraction (index contrast). The coating indices
`and thicknesses are chosen and dimensioned to filter imping
`ing light by interference effects in a desired manner. Spe
`cifically, if a light beam 34 impinges on the filter, a first
`wavelength portion 34T of a beam is transmitted and a
`second wavelength portion 34R is reflected and thus rejected
`by the filter. What is transmitted and what is reflected
`depends on the precise thicknesses and indices of the thin
`coatings.
`Two basic types of thin-film edge filters and thin-film
`notch filters exist: those based on “soft coatings' and those
`based on “hard coatings, both of which are typically
`manufactured by an evaporation technique (either thermal
`evaporation or electron-beam evaporation). Hard coating
`filters, however, may also be manufactured by non-evapo
`rative techniques such as ion-beam Sputtering.
`Soft coatings imply literally what the name Suggests-they
`are physically soft and can be readily Scratched or damaged.
`They are fairly porous, which also means they tend to be
`hygroscopic (absorb water vapor) leading to dynamic
`changes in the film index and hence the resulting filter
`spectrum in correlation to local humidity. There are two
`
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`main reasons soft coatings are used. First, an advantageous
`larger index contrast can be realized with soft coatings. (The
`index contrast is the relative difference between the index of
`refraction of the low-index material and that of the high
`index material.) For example, many high-performance soft
`coated filters are made using Sodium aluminum fluoride
`(“cryolite'), with a chemical composition of NaAlF and an
`index of about 1.35 for visible wavelengths, and zinc sulfide,
`with a chemical composition of ZnS and an index of about
`2.35. The second reason for using these materials is that the
`evaporation process can be controlled well for these mate
`rials, largely because they have relatively low melting
`temperatures. Hence it is possible to maintain fairly accurate
`control over the layer thicknesses even for filter structures
`with many tens of layers and perhaps even up to 100 layers.
`As described above, edge filter performance is measured by
`edge steepness, depth of blocking, and high transmission
`with low ripple. A larger index contrast and a larger number
`of layers both yield more steepness and more blocking. High
`transmission with low ripple is improved with more layers
`and higher layer thickness accuracy. For these reasons the
`highest performance conventional thin-film edge filters have
`been made with Soft-coating technology.
`Hard coatings are made with tougher materials (generally
`oxides), and result from “energetic' deposition processes, in
`which energy is explicitly supplied to the film itself during
`the deposition process. This is accomplished with a beam of
`ions impinging directly on the coating Surface. The ion
`bombardment acts to “hammer the atoms into place in a
`more dense, less porous film structure. Such processes are
`usually called ion-assisted deposition (IAD) processes.
`High-performance edge filters have been made with ion
`assisted electron-beam evaporation. Typically the index con
`trast available with hard-coating (oxide) thin-film materials
`is not as high as that of the soft-coating materials, and
`consequently more layers must be deposited to achieve a
`comparable level of performance. This problem, coupled
`with the more difficult to control deposition rates and overall
`processes of high-melting-temperature oxides, leads to
`much more stringent requirements on the layer-thickness
`control techniques to achieve a reasonable level of layer
`thickness accuracy for good edge steepness and high, low
`ripple transmission.
`For the best filters, some kind of “optical monitoring
`(direct measurement of filter transmission or reflection dur
`ing deposition) is necessary to determine when to terminate
`the deposition of each layer. Optical monitoring can be
`performed on the actual filters of interest or on “witness
`pieces often positioned in the center of the deposition
`chamber. There are three basic types of optical monitoring
`algorithms. The first is often called “drop-chip' monitoring,
`and is based on measuring the transmission (or reflection)
`vs. time through a new witness piece for each new layer.
`Since the theoretical transmission vs. time can be calculated
`accurately for each layer deposited on a blank piece of glass,
`then a good comparison between the measured and theory
`curves can be made independent of the history of the
`deposition (thickness errors in previous layers). This tech
`nique is accurate and useful for layers of arbitrary thickness,
`but it is cumbersome, especially for filters comprised of at
`least many 10's of layers.
`The second type of monitoring is called “turning-point'
`monitoring, and is used for depositing layers that are pre
`cisely a quarter of a wavelength in thickness (or multiples
`thereof). The technique is based on the fact that the trans
`mission vs. time reaches a turning point (or extremum) at
`each multiple of a quarter wave of thickness, so an algorithm
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`is developed to cut layers precisely at the turning points. The
`elegant feature of this method is that there is inherent
`compensation for layer thickness errors from previous lay
`ers, so long as one adheres to the rule of cutting exactly at
`turning points. It thus works extremely well even for very
`thick coatings with even hundreds layers (it is the basis for
`manufacturing very high-performance filters for DWDM
`telecom applications, which can have as many as 200–400
`quarter-wave layers).
`The third type of monitoring is called “level monitoring.”
`and is applicable for non-quarter-wave thick layers. Moni
`toring can be done through the actual filters or through
`witness piece(s). The concept is to cut layers at predeter
`mined transmission levels, based on a calculated prediction
`of transmission vs. time for the entire structure. However,
`because Small layer errors lead to large variations in the
`absolute transmission values, one must instead rely on
`cutting at the correct transmission level relative to the local
`maximum and minimum values. Hence the method works
`well only for non-quarter-wave thick layers that are more
`than a half-wave thick, so that there is both a maximum and
`a minimum transmission value in the transmission vs. time
`curve for that layer. Even in this case, this method does not
`contain inherent compensation for errors in the thickness of
`previously deposited layers, and thus is not as forgiving as
`the turning-point method. However, to obtain an edge filter
`with high transmission and low ripple requires primarily
`non-quarter-wave thick layers, and hence turning-point
`monitoring is not applicable for edge filters.
`Besides thin-film filters, the other predominant type of
`30
`optical filter used for the applications described herein is the
`volume holographic filter. These filters accomplish blocking
`of unwanted excitation light with a “notch' of very low
`transmission over a relatively narrow bandwidth, and hence
`are often called “holographic notch filters.” The non-trans
`mitted light is diffracted at an acute angle relative to the
`direction of the transmitted light. The holograms are
`exposed and developed in a thick gelatinous film that is
`typically sandwiched between two glass Substrates. Because
`the film can be relatively thick, allowing a very large number
`of fringes in the holographic grating, Such filters can achieve
`a narrow notch bandwidth with accordingly steep edges.
`A need in the art exists for an improved method of making
`optical edge filters and notch filters and for improved edge
`filters and notch filters having increased edge steepness and
`increased transmission.
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`6
`The methods for making these edge and notch filters
`accurately determine when deposition of each layer of a
`filter should terminate. The methods include calculating
`theoretical transmission data for a layer of the filter and
`calculating an expected deposition duration for the layer.
`The methods also include measuring transmission through
`the layer during deposition for a period less than the
`expected deposition duration. When the measuring period
`elapses, a new deposition duration is calculated based upon
`the theoretical transmission data and the measured transmis
`sion data, thereby providing an accurate deposition duration
`for the layer.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The advantages, nature and various additional features of
`the invention will appear more fully upon consideration of
`the illustrative embodiments now to be described in detail in
`connection with the accompanying drawings. In the draw
`1ngS.
`FIGS. 1A and 1B are schematic graphical illustrations
`showing the spectral transmission of long-wave-pass and
`short-wave-pass optical edge filters, respectively;
`FIGS. 1C and 1D are schematic graphical illustrations
`showing the spectral transmission of an ideal and realistic
`notch filter, respectively;
`FIG. 2 is a schematic diagram of a conventional Raman
`probe;
`FIG. 3 is a schematic drawing illustrating the structure of
`a conventional optical filters;
`FIG. 4 is a schematic diagram of apparatus useful in
`making an optical edge filters and notch filters in accordance
`with an embodiment of the invention;
`FIG. 5 is a process flow illustrating the process of
`manufacturing a long-wave-pass filter in accordance with an
`embodiment of the invention;
`FIG. 6 is a process flow illustrating the process of
`manufacturing a short-wave-pass filter in accordance with
`an embodiment of the invention;
`FIG. 7 illustrates a first structure of a notch filter in
`accordance with an embodiment of the invention;
`FIG. 8 illustrates a second structure of a notch filter in
`accordance with an embodiment of the invention;
`FIGS. 9A to 9C illustrate transmission through a notch
`filter at a 45 degree angle of incidence;
`FIGS. 10 and 11 are transmission and optical density
`spectra, respectively, of an LWP filter fabricated in accor
`dance with an embodiment of the invention;
`FIGS. 12 and 13 are transmission and optical density
`spectra, respectively, of an SWP filter fabricated in accor
`dance with an embodiment of the invention;
`FIGS. 14 and 15 illustrate transmission and optical den
`sity spectra, respectively, of a 633 nm single-notch filter
`fabricated in a