JP4002091B2 - Wavelength multiplexing / demultiplexing device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a wavelength multiplexing / demultiplexing element, and more particularly to a wavelength multiplexing / demultiplexing element used in the fields of optical communication and optical measurement.
[0002]
[Prior art]
2. Description of the Related Art In recent years, in an optical communication system using an optical fiber, it is difficult to meet a user's request in transmission using a single wavelength as the amount of information increases. Therefore, wavelength multiplexing transmission has been developed, which has already been implemented, by combining a plurality of intensity-modulated lights having different wavelengths into wavelength multiplexed light, and transmitting the wavelength multiplexed light through a single optical fiber to increase transmission capacity. Has been.
[0003]
In such a transmission system, for the purpose of transmitting more signals with a single optical fiber, an attempt is made to realize higher transmission density by narrowing the interval between wavelengths on which signals are carried. ing.
Conventionally, in such a transmission system, as shown in FIG. 10, for example, by using a multilayer filter that transmits light of wavelength λ ₁ and reflects light of wavelength λ ₂ , wavelength λ ₁ and wavelength λ ₂ are used. There is known a method for combining the light beams.
[0004]
However, in the case of this method, it is necessary to reduce the reflection bandwidth of the multilayer filter as the wavelength interval used for signal transmission becomes narrower. However, the narrower the bandwidth, the more narrow the multilayer film. There is a problem that it is difficult to create a filter with a high yield.
As a method for solving such a problem, there is a method using an optical multiplexing / demultiplexing device as shown in FIG.
[0005]
This optical multiplexing / demultiplexing element is assembled with, for example, components described later, and includes one incident port P ₀ and two exit ports P ₁ and P ₂ .
In this optical multiplexing / demultiplexing element, when a signal obtained by wavelength multiplexing wavelengths of λ ₁ , λ ₂ , λ ₃ , λ ₄ ,... Λ _n−1 , λ _n is incident from the incident port P ₀ , wavelength lambda ₁ from the port _{P 1, λ 3, ... λ} n-1 of the signal is emitted, from the other output port P ₂ wavelengths _{λ 2, λ 4, ... λ} n signal is emitted. In addition, the incident port and the output port are reversed, and a signal in which wavelengths of λ ₁ , λ ₃ ,... Λ _n-1 are wavelength-multiplexed is input from _one output port P ₁ , and λ ₂ , λ ₄ , ... Λ _n wavelength-multiplexed signals are incident from the other output port P ₂ , so that both signals are combined and λ ₁ , λ ₂ , λ ₃ , λ ₄ ,... Λ _n−1 , λ A signal in which the wavelength of _n is wavelength-multiplexed can be obtained from the incident port P ₀ .
[0006]
This wavelength multiplexing / demultiplexing element is composed of a single birefringent crystal plate (to be described later), two polarization separating / combining means such as a polarizing beam splitter, and two prisms. An example of the arrangement is as shown in FIG. That is, one birefringent crystal plate 3 is arranged in the center, and the first polarization separation / combination means 1A and the first total reflection mirror 20A are arranged on the incident port P ₀ side. Further, on the exit ports P ₁ and P ₂ side, the second polarization separation / combination means 1B and the second all-polarization means 1B and the first mirror 20A are in a point-symmetrical position with respect to the first polarization separation / combination means 1A. Each of the reflection mirrors 20B is arranged.
[0007]
In this wavelength multiplexing / demultiplexing element, when a signal (incident light) in which wavelengths of λ ₁ , λ ₂ , λ ₃ , λ ₄ ,... Λ _n−1 , λ _n are multiplexed is incident from the incident port P ₀ , The incident light is first separated into polarized light (1) and polarized light (2) in a polarization state orthogonal to each other by the first polarization separation / combination means 1A.
Each polarized light enters the birefringent crystal plate 3 and propagates through the birefringent crystal. At that time, each polarized light is separated into an ordinary ray and an extraordinary ray of the birefringent crystal. Then, the respective polarized lights emitted from the birefringent crystal plate 3 are synthesized by the second polarization separation / combination means 1B.
[0008]
In this case, the wavelength multiplexed light having the wavelengths λ ₁ , λ ₃ ,... Λ _n-1 is output from the output port P ₁ after being combined by the second polarization separation / combination means 1B, and λ ₂ , Wavelength multiplexed light with wavelengths λ ₄ ,... λ _n needs to be emitted from the emission port P ₂ .
The necessary conditions for this are as follows.
[0009]
First, in the case of wavelength multiplexed light of wavelengths λ ₁ , λ ₃ ... Λ _n−1 , as shown in FIG. 13, polarized light that is emitted from the birefringent crystal plate 3 and incident on the second polarization separation / combination means 1B. The polarization plane of (1) and the polarization plane of polarized light (2) reflected by the second mirror 20B and incident on the second polarization separation / combination means 1B are orthogonal to each other. For this purpose, the polarization states of the polarized light (1) and the polarized light (2) separated into orthogonal polarization states by the first polarization separation / combination means 1A are both before incident on the birefringent crystal plate 3. Even after the light is emitted, it does not change.
[0010]
In the case of wavelength multiplexed light of wavelengths λ ₂ , λ ₄ ,... Λ _n , the polarized light (3) separated by the first polarization separation / combination means 1A is birefringent crystal plate as shown in FIG. The polarization plane of the polarized light (4) is rotated by 90 ° before and after passing through the birefringent crystal 3 and the polarization plane of the polarized light (4) is also rotated by 90 ° in the process of passing through the birefringent crystal plate.
By the way, when light propagates in the birefringent crystal, it is separated into an ordinary ray and an extraordinary ray, and when it is emitted from the birefringent crystal, it is emitted with a phase different from the original phase (phase before incidence). To do. That is, the polarization state of the incident light to the birefringent crystal and the polarization state of the outgoing light therefrom are different.
[0011]
The wavelength multiplexing / demultiplexing element described above exhibits the multiplexing / demultiplexing function by utilizing the fact that the phase difference between the ordinary ray and the extraordinary ray in the birefringent crystal is different when the wavelength of the incident light is different. The above phase difference depends on the ordinary ray refractive index of the birefringent crystal, the extraordinary ray refractive index of the birefringent crystal, and the thickness of the birefringent crystal.
Based on the above points, the wavelength lambda _1, lambda _3, and ... λ _n-1 of the wavelength-multiplexed light, the wavelength _{λ 2, λ 4, ... λ} n respective wavelength multiplexed light is, shown in FIGS. 13 and 14 The type and thickness of the birefringent crystal plate 3 are selected so as to obtain a polarized state.
[0012]
As a result, the wavelength _{λ 1, λ 3, ... λ} n-1 of the wavelength multiplexing is emitted from the emitting port P _1, the wavelength lambda _2, lambda _4, the wavelength multiplexed light ... lambda _n is emitted from the emission port P _2.
[0013]
[Problems to be solved by the invention]
However, the above-described conventional wavelength multiplexing / demultiplexing device has the following problems. That is, when the temperature of the use environment fluctuates, the refractive index and thickness of the incorporated birefringent crystal may also fluctuate, and thereby the phase difference between ordinary rays and extraordinary rays also changes. When such a situation occurs, the profile of the emission waveform from the wavelength multiplexing / demultiplexing element changes.
[0014]
For example, in the case of a wavelength multiplexing / demultiplexing element assembled using a birefringent crystal made of rutile (TiO ₂ ), the wavelength characteristic has temperature dependence as shown in FIG. 15, and the output port P ₁ , P ₂ has a wavelength that varies with temperature.
The present invention solves the above-mentioned problems in the conventional wavelength multiplexing / demultiplexing device incorporating a birefringent crystal plate, and has a novel structure that does not cause wavelength variation of emitted light even when the environmental temperature changes. An object is to provide a wavelength multiplexing / demultiplexing device.
[0015]
[Means for Solving the Problems]
In order to achieve the above object, in the present invention, a first polarization separation / combination means for separating incident light into polarized light in polarization states orthogonal to each other, and two types of birefringence on which the polarized light enters. A wavelength multiplexing / demultiplexing device in which a birefringent crystal plate formed by laminating crystals and a second polarization separation / combination means for synthesizing light emitted from the birefringent crystal plate are arranged in the optical path in this order. And
The ratio (γ) between the thickness (La) of one birefringent crystal (a) and the thickness (Lb) of the other birefringent crystal (b) in the birefringent crystal plate is expressed by the following formula:
[0016]
[Expression 2]
. . . . . (1)
. . . . . (2)
[0017]
_{(However, αa, na o, na e} , naT o, naT e , respectively, in the birefringent crystal (a), the linear expansion coefficient, the ordinary ray refractive index, the extraordinary ray refractive index, the temperature dependence coefficient of the ordinary refractive index represents the temperature dependence coefficient of the extraordinary refractive index, _{also, αb, nb o, nb e} , nbT o, nbT e , respectively, in the birefringent crystal (b), the linear expansion coefficient, the ordinary ray refractive index, the extraordinary ray Refractive index, temperature dependence coefficient of ordinary ray refractive index, temperature dependence coefficient of extraordinary ray refractive index)
A wavelength multiplexing / demultiplexing device characterized by having a value indicated by
[0018]
Specifically, the crystal axes of the two types of birefringent crystals are both arranged parallel to the incident surface of each birefringent crystal and are inclined by 45 ° with respect to the polarization state of the incident polarized light. Thus, when the value of the δ expression is positive, the axes of the ordinary ray and the extraordinary ray in the two types of birefringent crystals are arranged in parallel with each other, and when the value of the δ expression is negative. Provides a wavelength multiplexing / demultiplexing element in which the axis of the ordinary ray and the axis of the extraordinary ray in the two types of birefringent crystals are arranged orthogonal to each other.
[0019]
In this case, the first and second polarization separation / combination means are both polarization beam splitters, or the first and second polarization separation / combination means are two mutually parallel total reflection surfaces. And two opposing entrance / exit surfaces parallel to each other and one polarization separation / combination unit parallel to the total reflection surface, or the first polarization separation / combination means is made of a birefringent crystal. The second polarization separation / combination means is polarization separation / combination means in which birefringent crystals, dove prisms, and birefringent crystals are arranged in this order. Next, the process by which the above mathematical formula is derived will be described.
The phase difference between the ordinary ray and extraordinary ray received when light passes through the birefringent crystal changes depending on the linear expansion coefficient of the crystal and the temperature dependence coefficient of the refractive index because the ambient environmental temperature changes. Normally, it is difficult to eliminate the temperature dependence with only one type of crystal, but the temperature characteristics of each other can be offset by combining two crystals of different materials in a certain thickness ratio.
First, a case is assumed where the axes of ordinary rays and extraordinary rays of the two birefringent crystals a and b are arranged in parallel to each other.
At this time, the phase difference Δa generated when the light passes through the birefringent crystal a is (x-axis direction−y-axis direction).
[Equation 3]
――――― (1)
It is. Here, λ is the wavelength of light, T is temperature, and T _c is room temperature.
Similarly, the phase difference Δb that occurs when light passes through the birefringent crystal b is (also x-axis direction−y-axis direction).
[Expression 4]
――――― (2)
When light passes through the birefringent crystals a and b, the phase difference Δab = Δa + Δb, so
――― Obtain (3).
In order not to change the characteristics when the temperature changes, the phase difference Δab does not change when the temperature changes.
There is a need. That is, [Equation 6]
-(4)
It is desirable that
Differentiating equation (3) :
――― (5)
In order for equation (5) to be 0,
――― (6)
Because it is necessary to be
-(7)
However, since the temperature dependence coefficient and the linear expansion coefficient of the refractive index in the optical crystal are both small values of about 10 ^-5 to 10 ^-6 , the term by the product of these two values is compared to the other terms. Can be ignored.
Therefore, as a condition for temperature independence,
-(7)
Is obtained. Equation (7) is a case where the axes of the ordinary rays and the axes of extraordinary rays of the two birefringent crystals a and b are arranged in parallel. Independence cannot be achieved. This is because the result of Expression (7) may be negative.
In this case, the normal ray axis and the extraordinary ray axis of the two birefringent crystals a and b are set to be perpendicular to each other. When the arrangement is changed to such an arrangement, the sign of Δb in the equation (2) is reversed.
-(8)
Get.
In either case, the ratio of the lengths of the two crystals for making temperature independence is :
-(9)
If [Formula 13]
-(10)
Can be expressed as
[0020]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows constituent elements related to Example A of the wavelength multiplexing / demultiplexing element of the present invention and their arrangement relation.
This wavelength multiplexing / demultiplexing A uses a birefringent crystal plate 3 in which two kinds of birefringent crystals a and b, which will be described later, are arranged in parallel to each other, and prisms 2A and 2B are used instead of the mirrors 20A and 20B. Except for this, the structure is the same as that of the conventional wavelength multiplexing / demultiplexing element described in FIG.
[0021]
Light incident from the input port P ₁ of the wavelength mux A, for example, in the first polarization separating and synthesizing means 1A is a polarizing beam splitter is split into mutually orthogonal polarization states.
One polarized light is totally reflected by the first prism 2A and then enters the birefringent crystal plate 3 composed of the birefringent crystal (a) and the birefringent crystal (b). It passes through an optical path L ₁ leading to the polarization separation / combination means 1B.
[0022]
Other polarization is first passed through the birefringent crystal plate 3 described above, through the optical path L ₂ reaching the second polarized light separating and synthesizing means 1B described above then totally reflected by the second prism 2B.
Then, the two polarized lights are synthesized by the second polarization separation / combination means 1B, and the wavelength multiplexed light of the wavelengths λ ₁ , λ ₃ ,... Λ _n-1 is emitted from the emission port P ₁ , and the wavelengths λ ₂ , λ ₄ are emitted. , ... wavelength-multiplexed light of lambda _n is emitted from the emission port P _2.
[0023]
The greatest feature of the wavelength multiplexing / demultiplexing element A is that the birefringent crystal plate 3 is composed of two birefringent crystals (a) and (b), and between them, the expressions (1) and (2 ) Is designed to hold.
That is, by using two birefringent crystals, even if the refractive index and thickness of one birefringent crystal change with temperature fluctuation, for example, both of them can be compensated by the change of the other birefringent crystal. Based on the design philosophy of suppressing the fluctuation of the phase difference between the ordinary ray and the extraordinary ray after passing through. The conditions for this are the expressions (1) and (2).
[0024]
Here, the fractional expression (2) is calculated as either positive or negative depending on the type and combination of the birefringent crystals (a) and (b) to be used.
In this case, when the calculated value is a positive value, as shown in FIG. 2, the axes of the ordinary refractive indices na _o and nb _{o in} the birefringent crystal (a) and the birefringent crystal (b), respectively. The birefringent crystal (a) and the birefringent crystal (b) are arranged in parallel so that the axes of the extraordinary ray refractive indexes na _e and nb _e are in the same direction.
[0025]
Further, when the calculated value is a negative value, as shown in FIG. 3, each of the ordinary refractive index na _o at birefringent crystal (a) and a birefringent crystal (b), the axis of nb _o, The birefringent crystal (a) and the birefringent crystal (b) are arranged in parallel so that the axes of the extraordinary ray refractive indexes na _e and nb _e are orthogonal to each other.
Examples of the birefringent crystal used in the present invention include YVO ₄ , LiNbO ₃ , α-BaB ₂ O ₄ , rutile (TiO ₂ ), quartz (SiO ₂ ), and calcite (CaCO ₃ ).
[0026]
Any two types of these materials are selected as birefringent crystals (a) and (b), and the values of the fractional expressions are calculated based on their physical property values. Realize the placement. At this time, if the thickness of one birefringent crystal is set to a certain value (La), the thickness (Lb) of the other birefringent crystal is based on the formula (1):
Lb = γ · La (3)
Should be set.
[0027]
By doing in this way, the wavelength multiplexing / demultiplexing element to be assembled exhibits a transmittance characteristic in which temperature dependency is suppressed.
As for the parallel arrangement of the birefringent crystal (a) and the birefringent crystal (b), both the incident surface and the exit surface are parallel to each other, and the crystal axes of the respective birefringent crystals are both included. -It is preferable that it is arrange | positioned in the state inclined 45 degrees with respect to the incident polarized light, and is arrange | positioned on an output surface. This is because the transmittance can be maximized at the transmission wavelength of each port suitable for the configuration of the wavelength multiplexing / demultiplexing element by adopting such an arrangement mode.
[0028]
Further, when an antireflection film is formed at least in a region through which light passes among the surfaces of the first and second polarization separation / combination means, the first and second prisms, and the two types of birefringent crystals, This is preferable because loss of light energy based on light reflection at the interface when light passes can be minimized.
Next, FIG. 4 shows another example B of the wavelength multiplexing / demultiplexing element of the present invention.
[0029]
In this wavelength multiplexing / demultiplexing element B, the birefringent crystal plate 3 is composed of two types of birefringent crystals (a) and (b), which is the same as the wavelength multiplexing / demultiplexing element A described above. The polarized light separating / combining means 1A and the second polarized light separating / combining means 1B themselves have a prism function.
That is, the first polarization separation / combination means 1A includes total reflection surfaces 1a ₁ and 1a ₂ parallel to each other, an entrance surface 1a ₃ and an output surface 1a ₄ that are also parallel to each other, and the total reflection surface 1a. _A polarization separating / synthesizing means 1a ₅ is provided which is arranged in parallel with ₁ and 1a ₂ , for example, by disposing a dielectric multilayer film, which is a so-called deformable polarization beam splitter.
[0030]
The second polarization separation / combination means 1B has the same structure as the first polarization separation / combination means 1A.
These polarized light separating / combining means can be produced, for example, as follows. This will be described with reference to FIG.
First, a transparent substrate 11 is prepared (FIG. 5A). Then, a dielectric multilayer film 12 is formed on one surface of the transparent substrate 11 by, for example, a vapor deposition method (FIG. 5B). Next, as shown in FIG. 5C, the same transparent substrate 11 is stuck on the dielectric multilayer film 12 to form a laminate 13.
[0031]
Next, as shown in FIG. 5D, the laminate 13 is cut in the thickness direction to cut out the precursor 14 of the polarization separation / combination means. At this time, the cut surfaces of the precursor 14 are cut so as to be parallel to each other.
Next, after the cut surface of the precursor 14 is polished (FIG. 5E), an antireflection film is formed on the polished surface to form a polarization separation / synthesis means.
[0032]
In the case of the wavelength multiplexing / demultiplexing element B shown in FIG. 4, the wavelength multiplexed light from the incident port P ₀ is incident on the first polarization separation / combination means 1A from the incident surface 1a ₃ , and the polarization separation / combination unit 1a _5. Are separated into polarized light in orthogonal polarization states.
One polarization is emitted from the emitting surface 1a ₄ then totally reflected by the total reflection surface 1a ₂ birefringent crystal (a), (b) sequentially passes through the second polarized light separating and from the incident surface 1b ₃ The light enters the combining unit 1B and reaches the polarization separation / combination unit 1b ₅ .
[0033]
The other polarized light exits from the exit surface 1a _{4 of} the first polarization separation / combination means 1A, passes through the birefringent crystals (a) and (b) in sequence, and then enters the second polarization separation / combination from the entrance surface 1b ₃ . The light enters the combining means 1B. Then, the light is totally reflected by the reflection surface 1b ₁ and reaches the polarization separation / combination unit 1b ₅ .
Of the two polarizations synthesized by the polarization separation / synthesis unit 1b ₅ , the wavelength multiplexed light of a certain wavelength group is totally reflected by the total reflection surface 1b ₂ , then passes through the emission surface 1b ₄ and is emitted from the emission port P ₁ . The wavelength multiplexed light of the other wavelength group passes through the exit surface 1b ₄ and exits from the exit port P ₂ .
[0034]
Since this wavelength multiplexing / demultiplexing element B has a structure in which an optical component such as a reflecting prism can be omitted, it is useful for manufacturing a low-cost optical multiplexing / demultiplexing module.
Next, still another example C of the wavelength multiplexing / demultiplexing element of the present invention is shown in FIGS. Here, FIG. 6 is a plan view showing the configuration of the wavelength multiplexing / demultiplexing element C, and FIG. 7 is a side view. FIG. 8 is a schematic diagram showing the polarization state of the passing light at the positions d _{1 to} d ₆ shown in FIG. 6 or FIG.
[0035]
In this wavelength multiplexing / demultiplexing element C, the first polarization separation / combination means 1A is composed of a birefringent crystal c ₁ , and the second polarization separation / synthesis means 1B is composed of a birefringence crystal c ₂ , a dove prism 4 and a double prism 4. The refractive crystals c ₃ are arranged in this order. The birefringent crystals (a) and (b) arranged between the first polarized light separating / combining means 1A and the second polarized light separating / combining means 1B are the relations of the expressions (1) and (2). Meet.
[0036]
In the case of this wavelength multiplexing / demultiplexing element C, the wavelength multiplexed light that enters the birefringent crystal c ₁ from the incident port P ₀ and is in the polarization state indicated by the position d ₁ in FIG. 8 is separated into an ordinary ray and an extraordinary ray. The Its propagation direction is as shown in FIG. 6, each of the polarization state is as shown in position d ₂ of FIG.
Each of these ordinary rays and extraordinary rays passes through the birefringent crystal plate 3 according to the present invention, and at the time of emission, the polarization state changes as indicated by a position d ₃ in FIG.
[0037]
These polarized lights are incident on the birefringent crystal c ₂ in the second polarization separation / combination means 1B, and are separated again into ordinary rays and extraordinary rays in the polarization state as shown by the position d ₄ in FIG. .
Then, only two ordinary rays in the dove prism 4 are switched and incident on the birefringent crystal c ₃ as shown in FIG. 6, and the extraordinary rays are directly reflected from the birefringent crystal c ₂ as shown in FIG. Incident on the refractive crystal c ₃ . Therefore, the polarization state of these lights is the same as the polarization state at the position d ₄ , as indicated by the position d ₅ in FIG.
[0038]
Then, an ordinary ray and an extraordinary ray are synthesized by the birefringent crystal c ₃ , one of which is emitted from the exit port P ₁ and the other is emitted from the exit port P ₂ . The polarization state at this time is as shown at position d ₆ in FIG.
In the case of the wavelength multiplexing / demultiplexing element C, the temperature dependence of the transmittance characteristic is suppressed as in the case of the wavelength multiplexing / demultiplexing element A. In addition, the wavelength multiplexing / demultiplexing element C is not provided with a polarizing beam splitter manufactured using an adhesive on the optical path as in the case of the wavelength multiplexing / demultiplexing element A, so that the optical power of high output can be increased. Also has the advantage of being able to respond.
[0039]
【Example】
Example 1
As the birefringent crystals (a) and (b), YVO ₄ and LiNbO ₃ were selected, and the wavelength multiplexing / demultiplexing device A shown in FIG. 1 was assembled.
Here, the characteristic values of each birefringent crystal are as follows.
[0040]
YVO4:
Linear expansion coefficient (αa): 4.43 × 10 −6 (1 / ° C.)
Ordinary refractive index (nao):. 1 94473
Extraordinary refractive index (nae):. 2 14861
Temperature dependence coefficient of ordinary light refractive index (naTo): 8.5 × 10 −6 (1 / ° C.)
Temperature dependence coefficient of extraordinary ray refractive index (naTe): 3.0 × 10 −6 (1 / ° C.)
LiNbO3:
Linear expansion coefficient (αb): 14.8 × 10 −6 (1 / ° C.)
Ordinary ray refractive index (nbo): 2.1131
Extraordinary ray refractive index (nbe): 2.13806
Temperature dependence coefficient of ordinary light refractive index (nbTo): 9.76 × 10 −6 (1 / ° C.)
Temperature dependence coefficient of extraordinary ray refractive index (nbTe): 3.80 × 10 −6 (1 / ° C.)
Substituting the above numerical values into equation (2), the value of the fractional expression becomes a positive value. Therefore, both crystals were arranged in parallel so that the axes of ordinary rays of YVO4 and LiNbO3 coincided.
[0041]
The thickness (La) of YVO ₄ was set to 14.935 mm. The thickness (Lb) of LiNbO ₃ was set to 1.912 mm based on the formula (3).
In the wavelength multiplexing / demultiplexing element A thus assembled, the cycle of the maximum transmittance value of each port is about 100 GHz.
With the environmental temperature changed at 0 ° C., 20 ° C., 40 ° C., and 60 ° C., the transmittance at the output port P ₁ and the output port P ₂ at each temperature was measured for this wavelength multiplexing / demultiplexing device A. The result is shown in FIG.
[0042]
Although the total reflection prism is used in this embodiment, a total reflection mirror may be used instead.
As is clear from FIG. 9, the output waveform from each output port is independent of temperature, and the wavelength shift due to temperature does not occur at all and is kept constant.
[0043]
Example 2 A wavelength multiplexing / demultiplexing device A was assembled using rutile (TiO2) instead of YVO4. Here, the physical properties of rutile (TiO2) are as follows. Rutile (TiO2): linear expansion coefficient (αa): 0.714 × 10 −5 (1 / ° C.) ordinary ray refractive index (nao): 2.45319 extraordinary ray refractive index (nae): 2.70930 ordinary ray refractive index Temperature dependence coefficient (naTo): 4.0 × 10 −5 (1 / ° C.) Temperature dependence coefficient of extraordinary ray refractive index (naTe): 9 × 10 −5 (1 / ° C.) The value of 2) is a negative value.
[0044]
Therefore, both crystals were arranged in parallel so that the axes of the ordinary rays of rutile and LiNbO ₃ were orthogonal to each other. The rutile thickness was 7.8315 mm, and the LiNbO ₃ thickness was 11.303 mm.
The wavelength multiplexing / demultiplexing element A in this case also has a temperature-independent output waveform from each output port.
[0045]
【The invention's effect】
As is clear from the above description, the wavelength multiplexing / demultiplexing device of the present invention uses two types of birefringent crystals, and the thicknesses of the birefringent crystals satisfy the formulas (1) and (2). Therefore, even if there is a change in temperature, the change in the phase difference between the ordinary ray and the extraordinary ray is suppressed, and the emission waveform becomes temperature-independent.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing a configuration of a wavelength multiplexing / demultiplexing device example A of the present invention.
FIG. 2 is a perspective view showing an arrangement state of a birefringent crystal (a) and a birefringent crystal (b) when δ is a positive value.
FIG. 3 is a perspective view showing an arrangement state of a birefringent crystal (a) and a birefringent crystal (b) when δ is a negative value.
FIG. 4 is a schematic diagram showing a configuration of a wavelength multiplexing / demultiplexing device example B of the present invention.
FIG. 5 is a process diagram when creating a polarization separating / synthesizing unit incorporated in the wavelength multiplexing / demultiplexing element example B;
6 is a plan view showing a configuration of a wavelength multiplexing / demultiplexing device example C according to the present invention. FIG.
FIG. 7 is a side view showing the configuration of a wavelength multiplexing / demultiplexing device example C of the present invention.
8 is a schematic diagram showing a polarization state of polarized light passing through positions d _{1 to} d ₆ in the wavelength multiplexing / demultiplexing element C. FIG.
9 is a graph showing transmittance characteristics of the wavelength multiplexing / demultiplexing element according to Example 1. FIG.
FIG. 10 is a schematic view showing an example of a conventional wavelength multiplexing / demultiplexing element.
FIG. 11 is a schematic view showing a conventional wavelength multiplexing / demultiplexing device incorporating a birefringent crystal.
12 is a schematic diagram showing a configuration of the wavelength multiplexing / demultiplexing element in FIG. 11. FIG.
13 is a schematic diagram showing polarization separation and synthesis of wavelength multiplexed light of wavelengths λ ₁ , λ ₃ ,... Λ _{n−1 in} the wavelength multiplexing / demultiplexing device of FIG.
14 is a schematic diagram illustrating polarization separation and synthesis of wavelength multiplexed light of wavelengths λ ₂ , λ ₄ ,... Λ _{n in} the wavelength multiplexing / demultiplexing device of FIG.
15 is a graph showing transmittance characteristics of the wavelength multiplexing / demultiplexing device in FIG. 11. FIG.
[Explanation of symbols]
a, b, c ₁ , c ₂ , c ₃ birefringent crystal 1A first polarized light separating / combining means 1B second polarized light separating / combining means 2A, 2B total reflection prism 3 birefringent crystal plate 4 dove prism 1a ₁ , 1a ₂ , ₁ b ₁ , ₁ b ₂ total reflection surfaces 1 a ₃ , 1 b ₃ entrance surfaces 1 a ₄ , 1 b ₄ exit surfaces 1 a ₅ , 1 b ₅ polarization separation / combination unit

Claims (4)

A first polarization separation / combination means for separating incident light into polarized light in polarization states orthogonal to each other; a birefringent crystal plate formed by stacking two types of birefringent crystals on which the polarized light is incident;
A wavelength multiplexing / demultiplexing device in which the second polarization separation / combination means for synthesizing the outgoing light from the birefringent crystal plate is disposed in the optical path in this order,
The ratio (γ) between the thickness (La) of one birefringent crystal (a) and the thickness (Lb) of the other birefringent crystal (b) in the birefringent crystal plate is expressed by the following formula:
(However, αa, nao, nae, naTo, naTe are the linear expansion coefficient, ordinary ray refractive index, extraordinary ray refractive index, ordinary ray refractive index temperature dependence factor, extraordinary ray refraction, respectively, in the birefringent crystal (a). Represents the temperature dependence coefficient of the refractive index, and αb, nbo, nbe, nbTo, and nbTe are the linear expansion coefficient, ordinary ray refractive index, extraordinary ray refractive index, and ordinary ray refractive index of the birefringent crystal (b), respectively. temperature dependency coefficient, and Tsu name to the value indicated by the representative of the temperature dependence coefficient of the extraordinary ray refractive index),
The crystal axes of the two types of birefringent crystals are both arranged parallel to the incident surface of each birefringent crystal and inclined by 45 ° with respect to the polarization state of the incident polarized light. When the value of δ is positive, the axis of ordinary ray and the axis of extraordinary ray in the two types of birefringent crystals are both arranged in parallel with each other, and when the value of the δ expression is negative, the two types The wavelength multiplexing / demultiplexing device according to claim 1, wherein the axis of the ordinary ray and the axis of the extraordinary ray in the birefringent crystal are orthogonal to each other . 2. The wavelength multiplexing / demultiplexing element according to claim 1, wherein each of the first and second polarization separation / combination means is a polarization beam splitter. The first and second polarization separation / combination means comprise two opposing total reflection surfaces, two opposite parallel input / output surfaces, and one polarization separation / parallel to the total reflection surface. The wavelength multiplexing / demultiplexing device according to claim 1, further comprising a combining unit. The first polarization separating and synthesizing means comprises a birefringent crystal, the second polarized light separating and synthesizing means, the wavelength of Claim 1 formed by arranging a birefringent crystal and Dobupurizumu the birefringent crystal in this order Multiplexing / demultiplexing element.