Second-generation holographic grating technology

Transcription

1 Second-generation holographic grating technology Dr. Erik Wilkinson University of Colorado

2 Outline Challenges of UV instrumentation Holographic gratings in general Aberration-control theory in a viewgraph First-generation holographic technology Second-generation holographic technology What SGHG technology might do for NHST Where should research be done

3 The technical challenges of UV instrumentation One goal of NHST is 10 HST/COS sensitivity ~ 5.8 meter diameter HST at Lyman a But component efficiencies at UV wavelengths are low Optical Coatings: ~90% for 2000Å<l<3000Å (Al/MgF 2 ) ~85% for 1150Å<l<2000Å (Al/MgF 2 ) ~65% for 1050Å< l <1150Å (Al/LiF) 5-40% for l<1050å (SiC, B 4 C) Detectors: 5-40% for l<2000 Å (KBr, CsI, RdBr, NaBr, CsTe) 2 improvement in DQE fi NHST becomes 10 HST/COS w/ 4 meter mirror The obvious strategy has been to use fewer optical elements improves instrument sensitivity For example, HUT, FUSE, COS degrades image quality, especially for off-axis sources Holographic grating technology has enabled new scientific missions, because it has allowed instrument designers to minimize the number of optical elements.

4 Holographic Gratings in Review Photoresist is deposited onto grating substrate Pattern is recorded in photoresist Grating is then chemically etched to produce the diffractive structure Advantages include: Very low in-plane scatter <2x10-5 /Å. Net efficiencies are improving, ~50% The efficiency is more uniform Lower risk fabrication More flexible aberration-control

5 A few words on aberration-control Grating theory is generally based on applying Fermat s Principle to the light path function of a diffractive system. The exact expression of F is F = [( x -x) 2 + ( y - w) 2 + ( z - l) 2 ] [( x'-x ) 2 + ( y'-w) 2 + ( z'-l) 2 ] nml d Expand F into a power series in terms of w and l, the horizontal & vertical position on the grating. F = F wf w 2 F l2 F w 3 F wl2 F where F ijk = M ijk ( r,a,r',b) The advantage of this formulation is that each F ijk term is associated with a specific aberration F 100 is the grating equation F 200 is spectral focus F 020 is spatial focus F 300 is coma F 120 slit curvature 4th order is spherical aberration (x,w,l) (x,y,z ) (x,y,z)

6 Classic Analogs First holographic gratings were classic analogs parallel grooves Just replacements for mechanically ruled, parallel groove gratings yawn. Aberration control was only through controlling the geometry, i.e. Rowland circle, Wadsworth, or toroidal Rowland circle.

7 First Generation Holographic Gratings Now assume rulings defined by the interference pattern of two coherent, stigmatic laser sources. The nth groove is then defined by. nl 0 = [ CP - DP ] -[ CO - DO ] The groove pattern is then defined by the interference pattern generated by the laser sources and the substrate. This allows the designer to zero out aberrations inherent in the design by introducing equal and opposite aberrations with the groove pattern. ( ) + ml ( ) F ijk = M ijk r,a,r',b H ijk r c,g,r d,d l 0 FGHG designs have 4 unknowns (r c, r d, g, d) & thus 4 aberration terms can be zeroed out. First Generation Holographic Grating Noda, Namioka, & Seya, Geometric theory of the grating, J. Opt. Soc. Am. 64, (1974).

8 Second-generation holographic technology The groove pattern is set by the interference pattern generated using at least 1 aberrated wave-front (i.e. non planar or spherical) SGHG designs provide more degrees of freedom for controlling aberrations.

9 The aberration coefficients Aberration Coefficient Geometric & Holographic Components Aberration These terms controlled by FGHG SGHGs can control higher order terms F 00 M 00 = r + r ' Basic Light Path F 10 F 20 F 02 F 30 F 12 Auxiliary Equations H 00 = r c - r d M 10 = -sina - sinb H 10 = -sing + sind M 20 = cos2 a r H 20 = cos2 g r c + cos2 b r ' - cos2 d r d - 1 cosa + cosb R Grating Equation ( ) Spectral Focus ( ) - 1 cosg - cosd R M 02 = 1 r cosa + cosb r ' r H 02 = cosg - cosd r c r d r ( ) Astigmatism ( ) Ê M 30 = T( r,a) ˆ Á Ë r sina + Ê T( r',b) ˆ Á sinb Ë r' Ê H 30 = T ( r c,g) ˆ Ê Á sing - T ( r d,d) ˆ Á sind - 2 A 2 ( 10) C K Ë Ë R C sinh C + 2 A 2 ( 10) D K D sinh D 1 r c Ê M 12 = S( r,a) ˆ Á Ë r sina + Ê S( r',b) ˆ Á sinb Ë r' r d H 12 = sing È 1 Ê - r ˆ C Á ' cosg Í - sing È 1 Ê - r ˆ D Á ' cosd Í + 2 ( A 10 ) r C ' Î r C ' Ë r C r r D ' Î r D ' Ë r D r r C ( B 01 ) C V C sinh C - 2 ( A 10 ) 1 r D ( B 01 ) D V C sinh D 2 ( ) = cos2 a T r,a r - cosa R ( ) C = - A 10 cosg A C q C cosh C R 2 S( r,a) = 1 r - cosa ( B r 01 ) C = 1 B C q C Type I Coma Slit Curvature (Type II Coma) Note: 1. Aberrations scale with 1/(F/#), so fast systems exhibit larger aberrations. 2. NHST will use fast optics. 3. Aberration control of some sort will be needed to maintain image quality.

10 SGHG technology has real potential The increased number of degrees of freedom available will provide enhanced aberrations control. Better resolution, astigmatism control over a wider range of operational parameters e.g. field of view, band-pass May allow simpler optics to be used in place of aspheric optics SGHG technology has been demonstrated in a limited number of cases (Duban et al., Grange & Laget, Namioka & Kioke) Much of FGHG technology is directly transferable to the development of SGHGs. Techniques exist to create groove profiles (sinusoidal, psuedo-laminar, laminar, and triangular) Efficiency has steadily been improving and will continue The lithography industry is indirectly supporting holographic gratings (photoresists, laser technology, and etching techniques) SGHG performance capabilities are largely unexplored, especially for astronomical applications.

11 A Potential Application We have designed a FGHG system for wide-field spectroscopic imaging, but the performance is limited by incomplete aberration-control. This design could be adapted to meet NHST goals. FGHG instrument l/dl~500 l~ å f.o.v =0.5 Off-axis Gregorian telescope Minimum # of optics Slit wheel for point source, long-slit, and multi-object spectroscopy Possibly replace elliptical secondary with a spherical secondary (lower technical and programmatic risk) Camera at n=0 could provide simultaneous imaging and spectroscopy of the same field. Compound lens or tertiary could correct image quality over a wide field. Primary Focus/ Slit Parabolic Primary 0 Order Focus Elliptical Grating Diffracted Focus

12 Conclusions The capabilities of SGHGs are largely unexplored, because there has yet to be a general study of their uses. There is every reason to believe they will work based on the few existing examples There is real potential for enabling very capable instruments for NHST SGHG technology could simplify optical substrates, reducing cost and risk SGHG technology is low risk. The technology to create a SGHG is in aligning the recording set-up and is comparatively low-tech (improving the diffractive facets is ongoing work) What is industry doing? Jobin-Yvon is increasing fabrication capacity they see a long-term future. Industry is providing significant R&D for photoresists and recording lasers to support the lithography industry for m-chip fabrication. What steps should NASA take in the near future? Study the uses of aberration-corrected gratings FGHG development has paid-off handsomely, now is the time to take the next step It is unlikely that industry will do this no need Large format gratings will be needed