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INTERFERENCE FILTERS FOR THE NEAR
ULTRA-VIOLET
R. Neilson, J. Ring
To cite this version:
JOURNAL DE PHYSIQUE Colloque C 2, supplément au no 3-4, Tome 28, mars-avril 1967, page C 2 - 270
INTERFERENCE FILTERS FOR THE NEAR ULTRA-VIOLET
R. G. T. NEILSON and J. RINGDepartment of Applied Physics, University of Hull, England
Abstract. - All-dielectric interference filters with half-widths from 10 A to 100 A have been produced for the region from 2 500
-
4 000 A. Methods of construction and the characteristics of the fiiters are reported.Résumé.
-
Des filtres interférentiels entièrement diélectriques de bande passante comprise entre 10 et 100 A ont été obtenus de 2 500 4 000 A. On expose les caractéristiques et les méthodes de construction de ces filtres.1. Introduction.
-
Very few types of interference filters exist for use in the near ultra-violet-
for wave- lengths from 2 000A
to 4 000A.
A survey shows that several manufacturers offer simple metal-dielectric types, but the passband transmission of these is rarely above 25%,
and the shape of the transmission profile is poor. One manufacturer also offers broad- band filters with good transmission and profile for the region from 2 800 to 4 000A.
However, no filters with half-widths less than 100A,
and with transmission above 30%
are available.We have been attempting to produce filters having a performance in this region superior to that of filters presently available. This paper is a report on Our progress.
2. Materials.
-
Since one of us-
J. Ring - had previously been engaged in the development of all- dielectric filters for the visible, it was decided to inves- tigate initially the performance of this type of filter in the ultraviolet.Information on suitable dielectrics is available in the literature [Il, and we have used antimony trioxide, lead fluoride and cryolite as the layer materials in Our filters, the first two (Sb203 and PbF,) as high refrac- tive index materials for the regions 3 200
A
-
4 000A
and 2 500
-
3 200A
respectively, and cryolite as the low-index material in al1 the stacks. The optical cons- tants of these materials have been found to agree well with those published.3. Production of the Filters. - a) COMPUTATION
OF FILTER PROFILES. Calculations of the theoretical
performance of many filters were done on the Univer- sity's Elliot 803 computer
-
the programme used is analytic, using the matrix method of Weinstein [2].Either dispersive or non-dispersive media may be considered, but no account is taken of absorption.
The design criteria which we have considered most important are :
(i) Squareness of the passband.
(ii) The use of minimal quantities of the materials exhibiting absorption - the high index materials.
These criteria have led us to develop the family of double-half-wave (D. H. W). filters presented in table 3. Figures 5 and 6 show the value of criterion (ii) : the profiles shown have similar passband shapes, but the filter of figure 5 has four layers more of lead fluoride, giving a passband transmission 10
%
lower in practice.b) EVAPORATION AND DEPOSITION. The filters are
prepared in an Edwards 19E7 coating-plant, by vacuum deposition on fused silica substrates, at pres- sures between and torr. The evaporation sources used for the layer materials are :
(i) An alumina crucible and tungsten heater for cryolite (Fig. lb).
(ii) A molybdenum boat for antimony trioxide. (iii) A flanged high-density alumina crucible with external tungsten heater for lead fluoride (Fig. la
I CM.
-
INTERFERENCE FLTERS FOR THE NEAR ULTRA-VIOLET C 2
-
271 and Ic). Alumina crucibles of lower density or withoutflanges were found to allow dissociation of the lead fluoride, and evaporation of free lead.
C) THE LAYER THICKNESS MONITOR. The properties of a multilayer filter depend on the optical thickness of its layers, and control of the layer thickness by observing maxima and minima of transmission during construction requires no knowledge of the refractive indices of the layer materials, so a system to record the changing transmission of the filter during deposi- tion is a versatile control system. Monitoring with light of wavelength A, turning-values of transmission occur when al1 the layers are Al4 thick optically. Use of a continuum radiation source and monochromator allows different values of À, to be chosen, and by using interchangeable monitor substrates adjacent to the filter substrates, the monitor wavelength, II, may be changed during the construction of the filter.
The above considerations led us to develop the sys- tem shown in figure 2. The monochromator is used at
fiers are of the sarne order of magnitude, the amplifiers are identical, and the ratio (control intensityjreference intensity) is displayed on the recorder. A connection between the amplifiers allows subtraction of an adjus- table fraction of the reference signal from the control signal, to give zero-suppression and scale-expansion on the recorder.
Under operating conditions, the maximum noise level observed on the chart recorder is 0.125
%
of the ratio of the signals, and the thicknesses of single evaporated quarter-wave layers are reproducible to an accuracy of+
2 . 5 and-
0 . 5%.
Due to the tuning effect of monitoring transmission through a stack of many layers, the passband of a complete filter may be positioned to withini/2 %
of the desired wavelength. d) SCANNING THE FILTERS. The finished filters are scanned at fj6 on a Unicam SP800 U. V. spectropho- tometer, the area sampled being 15 mm x 1.5 mm, resolving-power (v/Av) is 1 000 at maximum resolu- tion.OPTICAL THICKNESS MONITOR - SCHEMATIC DIAGRAM
POWER SUPPLY BEAM-SPLITTER PHOTOMULT IPLIER
DEUTERIUM I SLlT
L AMP FLTER POWER SUPPLY rn
PHOTO- MULTIPLIER-
MONOCHROMATOR
w---
- RATIOa resolving power of 40, monitor substrates being changed before the filter passband becomes narrower than the instrumental profile of the monochromator.
A reference channel has been added, to compensate
for fluctuations in lamp brightness and spectral energy distribution, and for movement of the arc within the discharge tube. This reference channel consists of a photomultiplier, filter and defining aperture, placed so that the detector (( sees » the same region of the
lamp as does the control signal detector. The signals obtained at the inputs to the cathode-follower ampli-
4. Experimental results.
-
Note : The filter profiles presented in this paper have al1 been measured at fl6 and resolving-power of 1 000.a) SIMPLE FILTERS. Tables 1, 3 present the characte- ristics of the most successful quarter-wave filters pro- duced. Figures 3, 7 show typical profiles. These are presented on a logarithmic scale of transmission, to show the suppression regions in detail. The notation used for layer structure is demonstrated by the follo- wing examples :
C 2
-
272 R. G. T. NEILSON AND J. RINGTABLE 1.
-
Properties of Reflecting StacksMaterials No of layers Transmission Minimum Min. Trans. Wavelength of (A) Width Trans. Ail @ 50
%
1
,
1
PbF,, Cryolite
1
Sbl03+
CryoliteTABLE 2.
-
Properties of Fabry-Perot Filters Structure Wavelength Central Transmission Peak Half- WidthAW1,z
Tenth-Width
AW1,io
Transmission @
first minimum (*)
INTERFERENCE FILTERS FOR THE NEAR ULTRA-VIOLET C 2
-
273TABLE 3.
-
Properties of Double-Half-Wave SystemsMaterials PbFz, Cryolite Sb20.3, Cryolite Structure 3-6 C-3-C-3-6 C-3 5-6 C-5-C-5-6 C-5 7-8 C-7-C-7-8 C-7 7-8 C-7-C-7-8 C-7 7-12 C-7-C-7-12 C-7 9-4 c-9-c-9-4 c-9 11-4 C-1 1-C-11-4 C-11 3-6 C-3-C-3-6 C-3 3-10 C-3-C-3-10 C-3 NO of layers 15 23 3 1 31 31 39 47 15 15 Passband Centre Transmission
1
Transmission @ 1 side-peaks 72%
70%
(one only) (single peak) Flat top (single peak) Flat topTABLE 4.
-
Properties of a Quadruple-Half-Wave Systemtwo 7-layer quarter-wave reflecting stacks separated 100 C. PEAK TRANS.
-
80 0b2860A 6 3 % PbF2,
Cryolite o n e @ 6 5 %
H O A
by a half-wave spacer of cryolite (c denotes cryolite). (ii) 7-4 c-7-c-7-4 c-7 is a 31-layer double-full-wave
3-2 C-3-C-5-2 C-5- c-5-2 c-5-c-3-2 c-3
ZWDO nwo A 0 0 0 3 5 0 0 3&0
WELENGTH (A) FIG. 3. FIG. 4. 18 9
-
39system, consisting of two 15-layer Fabry-Perot with
C 2
-
274 R. G. T. NEILSON AND J. RINGFIG. 5.
-
39-layer D. H. W. filter9-4c-9-c-9-4c-9 ; PbFz
+
cryolite.FIG. 6 . - 31-layer D, H. W. filter
7-8c-7-c-7-8c-7 ; PbFz
+
cryolitefull-wave spacers of cryolite, separated by a quarter wave layer of cryolite. Such double Fabry-Perot have been classified in table 3 and in the figures as double- half-wave systems, as their properties are basically those of D. H. W. systems.
The quadruple-half-wave (Q. H. W.) filter is defined as a filter composed of four Fabry-Perot separated by phasing layers. The passband shapes of Q. H. W.'s are closely related to those of D. H. W.'s.
One non-quarter-wave system develope dis shown
FIG. 7. - 39-layer Q. H. W.
3-2c-3-c-5-2c-5c-5-2c-5c-3-2c-3 PbFz cryolite
in figure 8. It is a 51-layer dichroic filter made with alternate layers of PbF, and cryolite, to transmit the 2 400-3 000
A
region and reflect longer wavelengths. The layers of the filter decrease in thickness from one side, groups of 6, 4 and finally, 5 quarter-wave layers being deposited at wavelengths spaced to give an arithmetic progression in frequency of 1 000 wave- numbers. The transmission of the « pass )) region isunsuitably low, due to absorption by the lead fluoride layers.
b) FILTER SYSTEMS. Figures 3 to 7 show that the
INTERFERENCE FILTERS FOR THE NEAR ULTRA-VIOLET C 2
-
275 ting filters are required. The techniques for rejectingthese sidebands are well understood, so we have not investigated fully blocked systems in detail.
Figure 9 shows the performance of one such system produced. The filter elements are a Fabry-Perot filter,
two reflecting-stacks and Chance 0 x 1 absorbing glass. A peak transmission of 40
%
is attained for half-width of 31A.
The suppression-ratio (peak trans- mission/transmission at wavelengthA)
has been measured as :>
400 for wavelengths longer than 3 630A
and shorter than 3 320 A.. > 1 O00 for wavelengths longer than 3 950
A,
shorter than 3 225
A.
> 2 500 from 1 900-3 225
A,
and 4 000-6 660A.
Transmission is high at wavelengths longer than 6 700 A, but below 1 900
A
is low due to absorption by the silica substrates used.Fully blocked systems may be produced with suppression matched to the incoming spectrum and to the response of suitable detectors.
c) COMMENTS ON STABILITY. Cryolite, used in these
filters because of its very low refractive index, is known to be slightly hygroscopic : lead fluoride is also. However, stacks of up to fifty-one layers have been prepared, and in general show no deterioration of spectral performance when tested after one year, if dried before testing. Filters which have been exposed to the atmosphere show deterioration of their characteristics, but full performance is restored by desiccation. This implies that these stacks will be well-behaved if sealed against moisture. Unfortuna- tely, two narrow-band stacks
-
a 10A
half-width F. P., and a 13A
half-width D. H. W. - have been found to alter their profiles on exposure. for a few hours to dry air at atmospheric pressure. It is found that these filters are restored by keeping in vacuum for a week, but adequate protection will be difficult. 5. Conclusions. - We have shown that a conside- rable variety of useful interference filters may be pro- duced for the U. V., using antirnony trioxide, lead fluoride and cryolite. The principal limitations are low transmission, due to absorption, and instability in narrow-band filters. To overcome these defects, we intend to use different materials-
sapphire (Al,O,) as the high-index material, and magnesium fluoride as the low-index material. In preparation for this, we have installed an electron-bombardment source, and have evaporated sapphire successfully. With proposed changes to the monitoring system, we should then be able to extend the work to 1 900A.
6. Acknowledgements. - We wish to thank the D. S. 1. R.
-
now the S. R. C .-
for the grant on which the work was done. We should also like to thank John Connor, Melvyn Buckton and Steven Heavens of this department for making many of these filters.Bibliographie
[l] HONCIA and KREBS, Physik, 1959,156, pp. 117-124. HONCIA and KREBS, Physik, 1961, 165, pp. 202-212. [2] WEINSTEIN, J. Opt. SOC. Amer., 1947, 37, pp. 576-581.
INTERVENTIONS
J. G. HIRSCHBERG.
