Note: Descriptions are shown in the official language in which they were submitted.
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S P E C I F I C A T I 0 N
Title of the Invention
Interference filter and method of making the same
Field of the Invention
The present invention relates to a variable wavelength
type interference optical filter of narrow band, used in optical
communication, optical measurement, optical information
processing, or the like, and more particularly to a variable
wavelength type optical filter not depending on the polarization
states capable of varying the transmission wavelength continuous-
ly, and a method of making the same.
Background of the Prior Art
In a conventional optical filter of narrow band, as a
variable wavelength type filter designed to vary the transmission
wavelength continuously, a narrow band optical filter possessing
a dielectric multiple layer film is used. As such optical fil-
ter, for example, a high refractive index material such as Ti02
ZnS and a low refractive index material such as Si02 are applied
on a substrate alternately in a multiple layer coating at an
optical thickness of .1/4 wavelength exactly. Herein, supposing
the transmission wavelength to be ~, and the refractive index of
each layer to be n, the optical thickness is expressed as ~,/4n.
By forming such multiple layer coating on the substrate, an
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interference optical filter of narrow band is realized.
In this state, the wavelength of the transmission light
through the interference optical filter is defined by the optical
thickness to be a specific wavelength. However, by varying the
angle of incident light into the interference optical filter, the
transmission wavelength may be varied continuously. Fig. 10
shows the state of emitting light from a light source not shown,
to an optical filter 102 of narrow band through an optical fiber
100 and a collimating lens 101. The transmission light of the
optical filter 102 is focused by a focusing lens 103, and enters
an optical fiber 104. By varying the incident angle of the light
beam into the optical filter 102 by a small angle D from the
vertical state shown by solid line in the drawing, for example,
in a range of about 0 to 10°, the transmission wavelength ~, can
be changed continuously as shown in Fig. 11. At this time, the
full width at half maximum(F.W.H.M.) is not changed so much as
shown in Fig. 12.
However, as shown in Fig. 11, the transmission center
wavelength may be different depending on the polarized states (S
wave, P wave), and the difference is too large to be ignored when
the inclination angle is set, for example, at 10° or more.
Moreover, the transmission band width (full width at half maxi-
mum) of the interference optical filter also differs individually
depending on the polarized states (S wave, P wave). And it
changes significantly depending on the inclination angle 8 in P
wave as shown in Fig. 12. The transmissibility is also lowered
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depending on the inclination angle 0, which was a defect.
Thus, when continuously changing the transmission
wavelength by using a conventional interference optical filter,
there were many limits in use, and, disadvantageous.
Summary of the Invention
The present invention is devised in the light of such
conventional problems. Hence a primary object of the present
invention is to present a variable wavelength interference opti-
cal filter of narrow band capable of changing the transmission
wavelength continuously without varying the inclination of the
interference optical filter, not depending on the polarized wave,
and a method of manufacturing the same.
The variable wavelength interference optical filter of
the present invention is able to vary the wavelength of trans-
mission light continuously in a specific wavelength range. In the
optical filter comprises a substrate composed of a substance for
transmitting light in the specific wavelength range, and a multi-
ple layer evaporation substance film formed on the substrate,
being composed of substance of high transmissiblity for transmit-
ting light at wavelength in the specific wavelength range. And
the apical thickness of the multiple layer evaporation substance
film is composed so as to change continuously along a specific
direction of the substrate, and the wavelength of the transmis-
sion light is changed by varying the incident position of light
to the optical filter along the specific direction of the sub-
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strata.
In the invention, by varying the light irradiation posi-
tion linearly without changing the inclination of the optical
filter, the transmission wavelength can be changed continuously.
The transmission wavelength differs only with the incident posi-
tion, and the transmissiblity is not changed by the polarization
states so that the same transmissiblity may be obtained in P wave
and S wave. Furthermore, the full width at half maximum, the
selective characteristic of the light is not changed depending on
the light incident position, but remains at a specific value,
which is an excellent effect. That is, not depending on the
plane of polarization, the full width at half maximum and light
transmissiblity are kept constant, and only the wavelength can be
changed continuously.
Brief Description of the Drawings
Fig. 1 (a) is a sectional view showing the constitution
of a variable wavelength type interference optical filter in
single cavity structure according to a first embodiment of the
invention, Fig. 1 (b) is a graph showing the change of transmis-
siblity on the x-axis, and Fig. 1 (c) is a magnified sectional
view of a circular part of FIG.1 (a).
Fig. 2 is a graph showing the changes of transmission
wavelength, full width at half maximum, and transmissiblity,
relative to the incident position of the embodiment.
Fig. 3 is a graph showing the selective characteristic of
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the wavelength relative to the number of cavity layers.
Fig. 4 is a schematic diagram showing the state of use of
variable wavelength type interference optical filter according to
the embodiment.
Fig. 5 (a) is a sectional view showing the constitution
of a wavelength variable type interference optical filter
according to a second embodiment of the invention, Fig. 5 (b) is
its front view, and Fig. 5 (c) is a graph showing the changes of
its transmissibility.
Fig. 6 is a graph showing the changes of transmission
wavelength, full width at half maximum and transmissiblity rela-
tive to the rotational angle of the variable wavelength type
interference optical filter according to the second embodiment of
the invention.
Fig. 7 is a schematic diagram showing a state of use of
the variable wavelength type interference optical filter accord-
ing to the second embodiment of the invention.
Fig. 8 (a) is a front view showing a combined state of
two circular optical filters according to the second embodiment,
and Fig. 8 (b) is a front view showing an example of use composed
by adhering four different variable wavelength type interference
optical filters.
rotary dome.
Fig. 10 is a schematic diagram showing a state of use
when varying the transmission wavelength in the conventional
interference filter.
Fig. 11 is a graph showing changes of wavelength relative
to the incident light angle of the conventional interference
filter.
Fig. 12 is a graph showing changes of full width at half
maximum by varying the incident angle in the conventional inter-
ference filter.
Detailed Description of the Preferred Embodiments
Fig. 1 is the diagram showing the constitution of the
variable wavelength type interference optical filter of polariza-
tion states independent type according to the first embodiment of
the invention. The variable wavelength type interference optical
filter 1 in the embodiment is composed by evaporating multiple
layers on a substrate 2 such as glass and silicon. The substrate
2 is composed of a material high in the transmissiblity of light
in the range of the wavelength to be used, and a dielectric
material or a semiconductor may be used. In the embodiment,
quartz glass is used. Above the substrate 2, a dielectric mate-
rial high in transmissiblity of light at the wavelength to be
used, a semiconductor, and other substances are formed in multi-
ple layers by evaporation to form a multiple layer film 3.
Herein, the multiple layer film 3 is composed of, as shown in the
s:
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drawing, a lower multiple layer film 31, a cavity layer 32, and
an upper multiple layer film 33. On the lower surface of the
substrate 2, an anti-reflection film 4 is formed by evaporation.
The anti-reflection film 4 is, for example, a two-layer film of
Si02 and Ti02, but either one may be used.
Herein, the substance used as evaporation material for
the multiple layer film 3 and the anti-reflection film 4 is, for
example, Sip2 (refractive index n = 1.46), Ti02 (n = 2.15), Si (n
- 3.46), A12(33, Si2N4, MgF, etc. In this embodiment, the multi-
ple layer films 31 and 33 are formed by alternately laminating
and evaporating a low refractive film and a high refractive film.
The film thickness d, transmission wavelength ~,, and refractive
index n are defined in the following formula.
~.=4nd ~ ~ ~ ( 1 )
That is, the optical thickness of each layer of the lower and
upper multiple layer 31 and 33 is ~,/4. The cavity layer 32 is
composed of the low or high refractive film same as that of the
lower and upper multiple layers 31 and 33, and the thickness of
the cavity layer do is two times of that of the low or high
relative film of the multiple layer 31 and 33, namely ~,=2ndc. By
alternately laminating the low refractive film and high refrac-
tive film in this way, the full width at half maximum (F.W.H.M.)
of the peak of transmissiblity is lowered.
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1 according to the embodiment, having the relation of the trans-
mission wavelength and film thickness as shown in the formula
(1), is composed with the substrate 2 which is a slender plate,
and the optical thickness of the multiple layer film 3 is varied
continuously to differ the transmission wavelength ~,. The trans-
mission wavelength of the variable wavelength type interference
optical filter 1 is set at ~,a to ~,C ( ~,a < ~,c ) , corresponding to
the incident position x of the light, and the transmission wave-
length of the position at its central position (x = xb) is ~,b.
The upper and lower multiple layer films 31, 33 are composed by
alternately laminating a first evaporation substance film with
first refractive index nl, and a second evaporation substance
film with lower second refractive index n2. That is, As the
circular part of Fig. 1 (a) is shown in a magnified view in Fig.
1 (c), the film thickness of the individual multiple layer films
is changed continuously. In Fig. 1 (c), the low refractive film
of the lower multiple layer film 31 is supposed to be 31L, the
high refractive film to be 31H, the high refractive film of the
upper multiple layer film 33 to be 33H, and the low refractive
film to be 33L. Relative to the transmission wavelength ~,a at
the end xa on the x-axis of the interference optical filter 1 in
Fig. 1 (a), it is set so that formula (1) may be established in
the low refractive film and high refractive film, respectively.
Similarly, relative to the transmission wavelengths ~,b, ~,C at
positions xb, xc, the film thickness is set so that formula (1)
may be established at individual wavelengths ~.b, ~.c. The film
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thickness between them is also set so that the wavelength may
change linearly. Therefore, each film thickness of the multiple
layer film 3 changes continuously from position xa to xc on the
x-axis, and the film thickness increases toward the positive
direction of the x-axis.
For example, supposing ~,a to be 1540 nm, ~,c to be 1560
nm, and ~.b to be 1550 nm, by alternately laminating T102 of which
first refractive index (high refractive index) nl is 2.15, and
Si02 of which second refractive index (low refractive index) n2
is 1.46, the upper and lower low refractive films 31L, 33L have
the film thickness d of 263.7 nm at the left end (x = xa), and
film thickness of 267.1 nm at the right end (x = xc). The film
thickness of the upper and lower high refractive films 31H, 33H
is 179.1 nm at x = xa, and 181.4 nm at x = xc. If using Si of
which refractive index n is 3.46 as high refractive films 31H,
33H, the film thickness of high refractive films 31H, 33H is
111.7 nm at x = xa and 112.7 nm at x = xc.
Herein, the change of the film thickness in the x-axis
direction is expressed by the function of x, d(x), the wavelength
~. by the function of x, 7~(x), and the refractive index is the
variable of x, n(x), and hence their relation is expressed in
formulas (2), (3), where x0 is an arbitrary position, for exam-
ple, position of x = xa, and A is a constant.
7~(x)=4n(x)d(x) ~~~ (2)
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~,l,x)=~(x0)+EA(x-xB)k ... (3)
k=1
In this embodiment, the optical thickness is controlled
by controlling the film thickness of the upper and lower multiple
layer films 31, 33 and the cavity layer 32. But it is also possi-
ble to compose an optical filter by varying the refractive index
along the x-axis direction of the substrate while the film thick-
ness is unchanged. It is not necessary to change the optical
thickness continuously on a straight line, but the film thickness
or optical thickness such as refractive index may be varied along
an arbitrary line of the substrate.
In a filter of single cavity structure with center wave-
length of 1550 nm, by alternately laminating Si02 as low refrac-
tive films 31L, 33L, and Ti02 as high refractive films 31H, 33H
on the substrate 2 of quartz glass, when 32 or more layers are
formed by laminating upper and lower multiple layer films 31 and
33, a narrow band filter with full width at half. maximum
(F.W.H.M.) of 1 nm is formed. Or, in a filter of single cavity
structure by laminating Si02 and Si on. the substrate 2 of quartz
~",.. glass, when the upper and the lower multiple layer films 31 and
33 are combined to laminate 24 layers, a narrow band filter with
full width at half maximum of l nm is realized. Thus, the great
er the difference in the refractive index between the high re
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fractive film and low refractive film, a narrow band filter is
realized by the smaller number of layers.
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Fig. 2 shows changes of transmission wavelength ~, rela-
tive to the incident position x of light in the variable wave-
length type interference optical filter in the embodiment, and
changes of full width at half maximum, and transmissibility. If
composed to vary linearly the transmission wavelength ~, by chang-
ing the incident position x in formula (2), the half width at
half maximum and transmissiblity do not change corresponding to
the position of the x-axis, but it shows a state of constant
value. There is also no difference in the polarization states of
incident light, and a narrow band optical filter not depending on
the polarization states is realized.
The foregoing embodiment relates to the variable
wavelength type interference optical filter of single cavity
structure, but a filter may be also composed in a mufti-cavity
structure by using plural cavity layers. For example, another
cavity layer may be formed on the upper multiple layer film 33 in
the single cavity structure of the embodiment, and another multi-
ple layer film may be formed thereon to compose a variable wave-
length type interference optical filter of double cavity struc-
ture. In this case, as shown in Fig. 3, the selectivity of the
wavelength can be enhanced even by decreasing the number of films
of the multiple layer film. In the diagram, m shows the changes
of the number of cavities and wavelength selective characteris-
tics in the case of using multiple layer films of the same num-
ber. As clear from the diagram, the greater the number of cavi-
ties, the higher becomes the wavelength selective characteristic.
In such variable wavelength type interference optical
filter 1, as shown in Fig. 4, by emitting light, for example,
white light to one end of the optical filter 1, it is used to
separate the spectrum components of the desired wavelength. In
Fig. 4, by emitting light to the interference optical filter 1
through an optical fiber 11 and a collimating lens 12, the light
transmitted through a focusing lens 13 and an optical fiber 14 is
received. By moving the optical filter ~. in the direction of x-
axis, the selected wavelength A is varied. As shown in the
drawing, by slightly inclining the variable wavelength type
interference optical filter 1, the light reflected by the front
face of the interfErence optical filter 1 will not be directly
reflected to the light source side, and adverse effects on the
light source side can be avoided. This inclination angle should
be out of polarization dependent range, for example, 10° or less.
In the embodiment, the filter is a rectangular plate
filter, but a circular filter may be also composed. Fig. 5 shows
a disk-shaped variable wavelength type interference optical
filter 41 forming a multiple layer film 43 and an anti-reflection
film 44 on a disk-shaped substrate 42. In this case, too, in
proportion to the position of the x-axis direction in Fig. 5, it
is composed so that each film thickness changes continuously.
Thus, as shown in Fig. 6 (a) to (c), the transmission wavelength
is proportional to cos A, expressing the position in the relation
of angle A centered on the origin of the x-axis. Therefore, when
the circular filter 41 is rotated, it changes continuously as
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shown in the diagram. In this case, too, same as in the forego-
ing embodiment, the full width at half maximum and transmissibli-
ty are constant, regardless of the rotational angle (cos 0 or 0).
Fig. 7 is a perspective view showing the state of use of
such circular variable waveleng~h type interference optical
filter. In the diagram, same as in the example of use in Fig. 4,
the light entering through the optical fiber 11 is put into the
interference optical filter 41 as parallel light through the
collimating lens 12. The transmitted light is received by the
optical fiber 14 through the focusing lens 13. Herein, the
interference optical filter 41 is rotated in the direction of
arrow by rotating means not shown herein, for example, a motor, a
crank type knob and a reduction gear, or the like. As a result,
the wavelength is selected depending on the angle of rotation.
In this case, the wavelength linearized in proportion to cos 8,
not rotational angle 8, is selected. Therefore, by preliminarily
measuring the angle of rotation and selective wavelength, a
desired wavelength can be selected.
In this way, by forming the variable wavelength type
interference optical filter in a disk form, and continuously
varying the thickness of the multiple layer film 43 by the posi-
tion in the x-axis direction as shown in Fig. a (a), the wave-
length range.can be selected.by rotating the optical filter by a
half circle, that is, in a range of 180°. Therefore, the other
optical filter can be composed by gluing together two semicircu-
lar variable wavelength type interference optical filters ~lA and
- 13 -
41B differing in the selective wavelength range as shown in Fig.
8 (a). In this case, by mutually varying the selective wave-
length, a wavelength selective characteristic in a wide range is
achieved. Or, as shown in Fig. 8 (b), it is also possible to
composed by using a further multiplicity of variable wavelength
type interference optical filters 41A to 41D.
In the circular variable wavelength type interference
optical filter, the film thickness of the multiple layer evapo-
rated substance film is changed continuously by the position in
the x-axis direction, but it is also possible to compose to vary
the film thickness continuously corresponding to the angle 0. In
this case, the wavelength characteristic depending on the rota-
tional angle 8 is obtained.
A manufacturing method of filters shown in the
embodiments is described below. In the embodiments, the film
thickness of the multiple layer film 3 of the filter 1 must be
changed continuously depending on the position of the x=axis.
The manufacturing method for composing the filter having such
film thickness is described below while referring to Fig. 9. In
Fig. 9 (a), a vacuum vessel 51 for vacuum evaporation contains an
evaporation substance 52 as evaporation source such as Si02, Ti02
or Si in the foregoing embodiments in its bottom. In the upper
part of the vacuum vessel 51, a parabolic rotary dome 53,opened
in the lower side is provided. The rotary dome 53 is rotated at a
constant speed in the direction of arrow. A circular substrate 2
is glued to a specific radial position of its inner surface.
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f
This substrate 2 is not mounted along the inner surface
of the rotary dome 53, but is disposed at an inclination of a spe-
cific angle a from the inner surface of the rotary dome 53 as
shown in Fig. 9 (b). Furthermore, to make the film thickness
uniform, the evaporation substance 52 is disposed in the center
of the vacuum vessel 51, but in this embodiment its position is
at a remote place from the center at a specific distance L.
Thus, the film thickness can be controlled by an evaporation
temperature or evaporation time, and moreover by properly setting
the angle a and position L of the substance 52, the film thick-
ness change can be controlled. Thus, the high refractive film
and the low refractive film are evaporated alternately. In this
way, the distance from the evaporation substance 52 varies con-
tinuously depending on the vertical direction position (x-axis)
of the substrate as shown in Fig. 1, and therefore the film
thickness changes continuously corresponding to the distance of
the evaporation source or the like, by evaporation for a specific
time. By sequentially changing over the evaporation substance 52
to Si02, Ti02, etc. in this way, a filter possessing evaporation
substances in multiple layer film is composed. In this case,
directly, the circular variable wavelength type interference
optical filter can be composed as shown in Fig. 5, and by cutting
off in a rectangular form along the x-axis in Fig. 5, a variable
wavelength type interference optical filter in a rectangular
plate form shown in the first embodiment can be constituted.
In this embodiment, the desired film thickness is
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211~'~~~~
obtained by mounting the substrate on the inner surface of the
rotary dome 53, but when the substrate is disposed at a specific
inclination angle from the same film thickness surface equal in
the distance from the substance as the evaporation source, it is
possible to constitute by sequentially laminating and evaporating
evaporation substances of high refractive index and low refrac-
Live index. In this case, the film thickness distribution can be
controlled by varying the inclination angle a and distance from
the evaporation source.
Also in this embodiment, the substrate is mounted on the
rotary dome 53 at the inclination, and the evaporation source 52
is located at the position remote from the center of the vacuum
vessel 51 by the specific distance L, but it may be achieved only
by either inclining the substrate when mounting, or by locating
the evaporation substance 52 away from the center of the vacuum
vessel 51 by a specific distance L. In this case, too, by prop-
erly setting either the inclination angle a or the distance L
from the evaporation source, the film thickness distribution can
be controlled.
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