JPH03185402A - Optical frequency filter - Google Patents

Abstract

PURPOSE:To offset a change in refractive index by a temp. change and to allow spectral measurement with high accuracy by integrally joining an optical medium and a refractive index control member which generates thermal strains and thermal stresses by the temp. change. CONSTITUTION:An etalon material 2 of the optical frequency filter is formed by joining and integrating, for example, a thermal glass as the optical medium 1 and, for example, quartz glass as the refractive index control member 3. Both the medium and the member have end faces 2-1, 2-2 parallel with each other. A material which generate the different thermal strains and thermal stresses at the time of the temp. change and can offset the change i the refractive index by the action of the optical medium 1 and the control member 3 on each other is selected for the refractive index control member 3. As a result, the change in the frequency of the transmitted light of the etalon material by the temp. change is eliminated and the spectral measurement with the high accuracy is executed without requiring an intricate temp. stabilizer.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical frequency filter that passes only light having a specific frequency band.

[Prior Art and Its Section N] Optical frequency filters are important as devices that perform light absorption/emission phenomena as demultiplexing elements in wL measurement and optical frequency multiplex communication.

Conventional typical optical frequency filters are Fabry-Perot filters.
It is called an etalon and uses the phenomenon of multiple reflection of light on two reflective surfaces of a highly parallel flat plate. FIG. 6 is a diagram explaining the principle, and the symbol!
is the etalon material. The refractive index of etalon material 2 is n,
Assuming that the thickness is Q, a part of the light of wavelength λ that is incident from one point O on the incident end surface 1-1 of the etalon material 2 is the light ray OCA.
.

The light passes through the etalon material 2, but a portion of the light is reflected inside the etalon material 2, producing multiple reflected light such as the light ray 0CBAAt.

The phase difference δ between two outgoing beams with an optical path difference of one round trip is:
It is calculated by the following equation using the refraction and refraction angles θ in the etalon material 2.

δ−4trn(lcosθ/λ ・(1)
Therefore, the condition for maximum transmittance is δ=2−π (2), where Peng is an arbitrary integer.
is shown. Here, if the speed of light is C1 and the optical frequency is ν, then from the relationship of C−νλ (3), the −th transmitted light frequency C is given by the following equation.

vm =s C/2n Q cosθ ・
-(4) FIG. 7 is a graph showing the transmission characteristics of the Fabry-Perot etalon calculated by taking into account the reflectance R towards the entrance end face 1-1 and the exit end face 1-2 of the etalon material 2, The horizontal axis shows the frequency of light, and the vertical axis shows the transmittance. (M
, Born and E, 11o1r, Pr1ncip
lesof 0ptics, 3d Ed, New Yo
rk, Pergason, 1965. Chap.

7) This theoretical characteristic has been confirmed by many experiments, and the Fabry-Perot etalon is widely used as an excellent optical frequency filter.

However, the transmitted light frequency sheath of the Fabry-Perot etalon has the disadvantage that it is easily affected by temperature changes and requires highly accurate temperature stabilization. That is, when calculating the influence of the change in temperature T from the above equation (4), −(dν lI/dT)(l/ν −)=(1/n)(
θn/θT)+(1/12)(θUθT) −(
5), and further considering quartz, which is commonly used as etalon material 2, (1/n)(θn
/θT)#7XlO-”(1#ri(θQ/θT)45
It is about X1G-'.

Now, the optical frequency ν haze that we are interested in is 2X10”H2 (
If λ = 1.5 μs + l equivalent), then dν/dT = 1
．． It becomes 4X10@Hz. That is, a temperature increase of 1'C causes the transmitted light frequency to change by as much as 1.4 GHz. For this reason, for example, in order to maintain the stability of the transmitted light frequency below IMIIz, temperature stability of 1/1000° C. or below is required.

The present invention has been made in view of the above problems, and it is an object of the present invention to provide an optical frequency filter that has a small change in the frequency of transmitted light with respect to temperature changes and operates stably with only simple temperature control.

[Means for Solving the Problems] The optical frequency filter according to claim 1 of the present invention has an incident end face and an outgoing end face parallel to each other, and has an optical medium whose refractive index changes with temperature change. The solution is to integrally bond a refractive index adjusting member that generates thermal strain and thermal stress to offset the change in the refractive index of the optical medium.

Moreover, the optical frequency filter according to claim 2 of the present invention has a solution in that the outer peripheral surface of an optical medium is coated with a refractive index adjusting member.

[Function] By integrating the optical medium and the refractive index adjusting member, the optical frequency filter of the present invention generates thermal strain and thermal stress that are different from those produced by the optical medium alone, thereby reducing the etalon caused by temperature changes. Change in refractive index of material (1/n) (θn/θT)
The main feature of this technology is to offset the effects of heat, and it differs greatly from conventional technology in that it eliminates the need for high-precision temperature control.

The present invention will be explained in detail below. FIG. 1 is a schematic diagram illustrating the principle of an example of the optical frequency filter of the present invention,
Reference numeral 2 is an etalon material. This etalon material 2 is
The optical medium 1 and the refractive index adjusting member 3 are integrally bonded together. The optical medium 1 and the refractive index adjusting member 3 both have two end surfaces parallel to each other, and are made of a material that allows light to propagate, and are capable of experiencing different thermal strains and thermal stresses when the temperature of the etalon material 2 changes. The refractive index of the etalon material 2 changes to O. These are bonded to each other with a natural length Q6 at a reference temperature T0, and are integrated so that the direction of light incidence coincides with the bonding surface.

Here, the linear expansion coefficient of the optical medium l and the refractive index adjusting member 3 is α□α3, the Young's modulus is E and E s, the cross-sectional area of the joint surface is 81S3, the refractive index of the etalon material 2 is n, and the Poisson's ratio is Let it be μ. In the integral structure as shown in FIG. 1, the optical medium l and the refractive index adjusting member 3 exert a force F on each other at a temperature of 'ro+'r, and the length Q is extended as a unit. At this time, the optical medium l and the refractive index adjusting member 3
If the average linear expansion coefficient of is α, then the following equation can be obtained by simple calculation.

F=(α1αs)T/(1/S1L+1/5sEs)
・” (6) αmi(1/12)(θQ/θT) −(α, S, E, +α3S3E3)/(SIEl+53
E3) ... (7) However, when the force acting on the etalon material 2 is α, > α1, it is a tension force, and when α, α1, it is a compressive force.

At this time, the temperature change in the transmitted light frequency of the etalon material 2, including the effect of thermal stress F/S, is expressed by the following equation.

(1/νm) (dνs/dT)・(1#ri(#Q/
θT)+(1/n)(θn/θT)+(1/n)(θn
/θσ) (θσ/θT) (8) Here, the first term on the right side of equation (8) is the average linear expansion coefficient α given by equation (7) above. The second term on the right side is a term due to temperature change in the refractive index of the optical medium l, and has a value specific to the material. The third term on the right side is stress σ-F/S.

This is the change in refractive index due to The photoelastic constant of the optical medium l is P
ll+P l! Then, (1/n)(θn/θa)(n'/2E+) (P
t t-# (P lt "P + t))...(9
), and from equation (6) above, (θσ/θT) = F/S, T ・(α1α*)/S+(1/S+E++l/5sE3)
...(10) Therefore, (1/n) (θn/θσ) (θσ/θT) The purpose of the present invention is to design the value on the right side of equation (8) to be 0.

That is, (P, -μ(p,,+t't*))・0
...(12).

Once the optical medium l and the refractive index adjustment member 3 are determined, all values other than the cross-sectional area S , , S s are determined, so by determining St and Ss so as to satisfy the above equation (12), The etalon material 2, that is, the optical layer wavenumber filter with no change in transmitted light frequency (θshi/θT−〇) is obtained.

Specific examples of such combinations of materials are listed below.

Athermal glass is used in telescopes and laser oscillation devices loaded onto artificial satellites.
ss) is (θσ/θT)・−(n−1)α, ・
...Medium that satisfies (13), ATCI, ATF
2. ATF4. Devices with names such as FCDIO have been developed. (Both manufactured by Hoya Glass Co., Ltd.) In such glasses, the temperature coefficient of refractive index is negative, so the stress refractive index and positive coefficient of linear expansion cancel each other out, and the optical path length of the etalon material as a whole increases. Temperature change can be reduced to zero. Substituting the above formula (13) into formula (12), S, E, /S
When solving for ,E, we get S,E,/S,E.

・(n-1-n'A/2)+(as/α+)(n'
A/2-n) ・” (14)Tap, A=P+t-μ(p,,+P,)
...(15). To give a specific numerical example, if we choose ascidian glass ATC1 as the optical medium l and quartz as the refractive index adjusting member 3, then U + = 1.1XIO-''[deg-'], a 5 = 5
Xlo-'[deg-']n=1.5. El=E3=7
X10” [N/f11”], PII=0.11゜P,,
=0.23. By substituting the physical property value of μ=0.2, St/
S*=o, t'r...(
16) is obtained.

[Example 1 (Example 1)] Fig. 2 is a specific configuration diagram of an embodiment of the present invention in the above design example, in which a refractive index adjusting member 3 with a radius r and an optical medium l with a radius ri is located at the center. A concentric cylindrical etalon material 2 is used as a covering material. At this time, r, /r, = = 2.61+
1/(S,/S) is obtained. Therefore, when the transmitted beam diameter is 1111, it is sufficient to set r+=0.5 mg and ra=1.3 mm.

In general, in order to realize such a structure, L' teS IE r / S s E is added to the above equation (14).
It suffices if the s'' value is positive. For that purpose, the linear expansion coefficient α3 of the refractive index adjusting member needs to be small to some extent.

(Example 2) The refractive index adjusting member does not necessarily need to be made of a single material, and may be made of two or more materials.

FIG. 3 shows a second embodiment of the present invention,
A refractive index adjusting member 3-1, 3-2.・
...Multilayer coating with 3-N.

The cross-sectional area of each of the refractive index adjusting member layers 3...
3-1+S 3-1o...5s-yl, linear expansion coefficient α, -3. α.

hα, -〇, Guyang's modulus El-1+E3-2+"'E3
-n, the average linear expansion coefficient α of the refractive index adjusting member layers 3-1...3-N is given by the following equation.

Similarly, the average Young's modulus E is j=l j=1, where the cross-sectional area S3 of the refractive index adjusting member layer is S, ・Σ S
s, -J (19) If defined as follows, equation (12) or equation (14) also holds true for a general multilayer structure (N layers). Therefore, for example, liquid crystal plastic with a negative coefficient of linear expansion (α=-6XIO-@[deg-'], E=1.5
By methods such as using x 1010 (N/s+'') with a thin silicone rubber layer, integrated layers with average coefficients of linear expansion ranging from positive to negative can be obtained at will.

(Embodiment 3) FIG. 4 shows a third embodiment of the present invention. In FIG. 4, reference numeral 4 denotes a refractive index adjusting member piece having a large Young's modulus and a small or negative coefficient of linear expansion.

This refractive index adjusting member piece 4 is made of, for example, fibers (Kevlar fiber, carbon fiber, etc.) or powder (ceramics, etc.), and is integrated so as to cover the optical medium 1 with a binder 5 made of a plastic material or a metal material. ing.

Next, a method of manufacturing each of the optical frequency filters of Examples 1 to 3 described above will be described.

The manufacturing process of the optical frequency filter according to the first embodiment of the present invention shown in FIG. 2 is simplified and shown in FIG.

A starting round rod 6 made of optical medium 1 and a refractive index adjusting member 3
A starting hollow pipe 7 consisting of the following is prepared in advance. Next, the starting round rod 6 is inserted into the starting hollow pipe 7, heated by a burner 8 to soften and integrate them, and is stretched to a desired diameter while monitoring the outer diameter with an external diameter measuring meter (not shown). etalon material 2.

After stretching it to a predetermined finished diameter, it is cut to a predetermined length, and the end faces 2-1 and 2-2 (not shown) are polished, respectively.
A reflective film may be processed if necessary.

Further, in order to manufacture an optical frequency filter having a plurality of coating layers as shown in FIG. 3, the above process may be repeated. At this time, it goes without saying that a process at an appropriate temperature is used for products that require a low-temperature process, such as liquid crystal plastics and silicone rubber.

Alternatively, if the refractive index adjusting member is made of ceramics such as alumina or silicon nitride, it can be integrated by plasma spraying.

[Effects of the Invention] As explained above, since the optical frequency filter according to claim 1 of the present invention integrates an optical medium and a refractive index adjusting member, temperature changes in the refractive index of the optical medium can be controlled by By offsetting thermal strain and stress strain of the refractive index adjusting member, it is possible to eliminate changes in the frequency of light transmitted through the etalon material due to temperature changes.

Such an optical frequency filter does not require a high-precision temperature stabilization device, enables spectroscopic measurements with good reproducibility, and has the advantage of being able to demultiplex optical frequency multiplexed signals with a simple configuration. There is.

Furthermore, since the optical frequency filter according to claim 2 of the present invention is formed by coating the outer peripheral surface of an optical medium with a refractive index adjusting member, it is not only easy to downsize, but also suitable for various optical integrated circuits. This has the advantage that it can be easily joined to optical fibers used in other applications.

[Brief explanation of drawings]

FIG. 1 is a schematic configuration diagram of the optical frequency filter according to claim 1 of the present invention, and FIGS. 2, 3, and 4 are all embodiments of the optical frequency filter according to claim 2 of the present invention. FIG. 5 is a schematic diagram showing the manufacturing method of the optical frequency filter according to claim 2 of the present invention, FIG. 6 is a diagram illustrating the operating principle of a conventional Fabry-Perot etalon, and FIG. is a graph showing the transmission characteristics of the Fabry-Perot etalon shown in FIG. 6. 1... Optical medium, 2... Etalon material 2-1.2-2... Etalon reflective surface, 3... Refractive index adjusting member, 4... Refractive index adjusting member piece, 5... Binder .

Claims (1)

[Claims] (1) A refractive index that generates thermal strain and thermal stress due to temperature changes in an optical medium whose entrance end face and exit end face are parallel to each other and whose refractive index changes with temperature changes, canceling out the change in the refractive index of the optical medium. 2. The optical frequency filter according to claim 1, wherein the optical frequency filter is formed by integrally bonding an adjustment member. (2) The optical frequency filter according to claim 1, characterized in that the outer peripheral surface of an optical medium is coated with a refractive index adjustment member.