JP4604754B2 - Variable dispersion compensator - Google Patents

Description

  The present invention relates to a tunable dispersion compensator, and more particularly to an optical dispersion compensator suitable for application to an optical transmission system using an optical fiber and a system employing an optical transmission method using wavelength multiplexing.

  In recent years, research and practical application of a long-distance optical transmission system using an optical amplifier as a repeater has been active. In particular, in order to support multimedia services centered on the Internet, it is effective to increase the capacity by WDM (Wavelength Division Multiplex) that multiplexes a plurality of signal lights having different wavelengths onto one optical fiber as a communication transmission path. It is considered a technology. In such a long-distance optical transmission system, the transmission speed and transmission distance are greatly limited by the phenomenon of chromatic dispersion of the optical fiber. Chromatic dispersion is a phenomenon in which light having different wavelengths propagates through optical fibers at different speeds. Since the optical spectrum of the optical signal modulated at high speed includes different wavelength components, these components reach the receiver at different times due to the influence of chromatic dispersion when propagating through the optical fiber. As a result, the optical signal waveform after fiber transmission is distorted. In order to suppress such waveform deterioration due to dispersion, a technique called dispersion compensation is important. Dispersion compensation cancels the chromatic dispersion characteristics of an optical fiber by placing an optical element with chromatic dispersion characteristics opposite to that of the optical fiber used in the transmission line in an optical transmitter, receiver, or repeater. This is a technique to prevent deterioration. As such an optical element, that is, a dispersion compensator, a device having inverse dispersion characteristics such as a dispersion compensation fiber and an optical fiber grating has been studied and put into practical use.

  The dispersion tolerance indicates a range of residual dispersion (a total amount of dispersion by the transmission line fiber and the dispersion compensator) that satisfies a certain standard of transmission quality. Since the dispersion tolerance decreases in inverse proportion to the square of the bit rate of the optical signal, the dispersion compensation technique becomes more important as the transmission speed increases. For example, in a 10 Gbit / s transmission system, the dispersion tolerance of an optical signal is about 1000 ps / nm, and considering that the dispersion amount of a single mode fiber is about 17 ps / nm / km, a dispersion compensation technique is not used. Only about 60 km can be transmitted. Furthermore, the dispersion tolerance in 40 Gbit / s transmission is about 1/16 of 60 ps / nm, which corresponds to about 4 km of single mode fiber. Currently, the transmission distance of trunk optical fiber transmission using an optical repeater is about several tens to several thousand km, but it is necessary to change the dispersion amount of the dispersion compensator according to the transmission distance. For example, in a 10 Gbit / s transmission system, a dispersion compensator with a fixed compensation amount is prepared in advance in steps of about 100 ps to several hundreds ps in consideration of dispersion tolerance, and the amount of compensation is determined during installation according to the transmission distance and installed. The method of doing has been taken. A typical example of the dispersion compensator in this case is a method using a dispersion compensating fiber having chromatic dispersion having a sign opposite to that of the transmission line. Next, in a 40 Gbit / s transmission system, it is considered that a dispersion compensator capable of changing the compensation dispersion amount in steps of about 10 ps to several tens of ps is necessary. In addition, in this case, the change in the amount of chromatic dispersion due to the temperature of the transmission line fiber cannot be ignored. For this reason, a dispersion compensator capable of variably controlling the dispersion amount is required.

Japanese Patent Laid-Open No. 10-221658

  However, these conventional dispersion compensators also have various problems. When a fixed amount of dispersion compensation is performed, a dispersion compensating fiber requires a long compensating fiber extending from several kilometers to several hundred kilometers, so that a storage space for the fiber is increased. In addition, an extra optical amplifier may be required to compensate for the loss of the dispersion compensating fiber. Further, the dispersion compensating fiber generally has a small mode field diameter, which causes a large optical fiber nonlinear effect and may cause distortion of the transmission waveform.

  In the case of an optical fiber grating, since there is a ripple with respect to the wavelength on the transmission characteristics and chromatic dispersion characteristics, the compensation characteristics change greatly with respect to slight wavelength changes. Therefore, it is known that the transmission characteristic when used for dispersion compensation is inferior to that of a dispersion compensation fiber. Also, due to manufacturing problems, it is difficult to produce a product with a large amount of dispersion or wavelength band, and a product with a narrow band needs to stabilize temperature and wavelength. Also, in principle, dispersion compensation fiber can not change the dispersion amount continuously, and realizes variable dispersion compensation that continuously changes the dispersion amount according to the change of the dispersion amount of the transmission line. Is difficult.

  In the case of an optical fiber grating, as a method for realizing continuous variable dispersion compensation, for example, a method of generating a chirped grating by creating a temperature gradient in the longitudinal direction of the optical fiber grating and performing dispersion compensation transmission has been reported. ing. In this case, a variable amount of dispersion compensation can be performed by controlling the temperature gradient. However, in this method, it is difficult to obtain a uniform temperature gradient, and there is a problem that dispersion compensation with sufficient performance cannot be performed such as ripples in chromatic dispersion, and there is a problem in practicality. In addition, there is a known example in which dispersion compensation is performed by providing a temperature gradient with a plurality of small heaters, for example, Patent Document 1, but a structure with fine processing or a complicated control method is required. An object of the present invention is to provide a tunable dispersion compensator which has a wide band, a small dispersion ripple, and a small loss and loss ripple, which solves the above problems.

The object is to incline the second plane of the plate-like etalon having the first and second planes facing each other and the reflection surface of the mirror having the reflection surface by an angle Δθ, so that the second plane of the etalon and the mirror A first collimator for injecting a light beam from between the reflecting surfaces of the etalon and reflecting the light beam alternately between the etalon and the mirror, reflecting the natural number N times by the etalon, A second collimator for emitting and receiving a light beam from between the plane and the reflecting surface of the mirror is provided, the width of the mirror is m, and the effective beam diameter when reflecting the kth time by the mirror is ω _k When
m ≧ Σω _k
When the incident angle in the etalon is Θ, the position of the second collimator is the origin, and the position where the loss is smallest when the second plane of the etalon is virtually total reflection mirror is the origin. there horizontally moved a distance _{x S} on the side away from the first collimator from the distance _{x S} is greater than 0, can be achieved by less than 4NL tan (Θ).

  Further, the second plane of the plate-like etalon having the first and second planes facing each other and the reflection plane of the mirror having the reflection plane are inclined at an angle Δθ, and the second plane of the etalon is A first collimator for injecting a light beam from between the reflecting surfaces of the mirrors, reflecting the light beam alternately between the etalon and the mirror, reflecting the natural number N times by the etalon, and then the second etalon of the etalon. This can be achieved by having a second collimator for emitting and receiving a light beam from between the plane of the mirror and the reflecting surface of the mirror, and reducing the incident angle of the light beam to the etalon for each reflection from the etalon.

According to the present invention, a variable dispersion compensator having a low loss and a good characteristic of low loss ripple was obtained. Thereby, dispersion compensation can be performed for each signal light at the time of wavelength multiplexing transmission, and the transmission distance in the transmission system can be extended.
Furthermore, dispersion compensation can be performed collectively for high-order dispersion. By using the dispersion compensator of the present invention, a simple and inexpensive optical communication system having excellent transmission characteristics can be constructed.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The etalon will be explained.
FIG. 1 shows the structure of the etalon. The etalon is obtained by coating the reflecting films 11 and 12 on both surfaces of the flat plate 10 which is accurately parallel. As the reflective film, a highly reflective metal film such as gold or silver, or a dielectric multilayer film is used. In particular, an etalon ideally having a reflectance of 100% on one side is called a GT etalon after the names of the proposers Gires and Tournois. However, in practice, it is difficult to realize a reflectance of 100%, and therefore a reflective film having a reflectance of about 90% or more may be used. In addition, the other reflectance does not need to be so high when used for dispersion compensation, and does not exceed 90% as will be shown later with specific numerical values. Such a GT etalon is called an all-pass filter because the transmittance is constant with respect to the wavelength. However, the phase (group delay time) has wavelength dependency. The group delay time τ at this time is expressed by the following equation.

ここで、ｒは振幅反射率、ωは光の角周波数、ΔＴは平行平面板を１往復することによって生じる光学的な距離を示す。波長分散Ｄは、群遅延時間τを波長で微分したものである。

Here, r represents the amplitude reflectance, ω represents the angular frequency of light, and ΔT represents the optical distance generated by one round trip of the plane parallel plate. The chromatic dispersion D is obtained by differentiating the group delay time τ with respect to the wavelength.

However, it is important for a dispersion compensator in a high-speed signal, particularly in a 40 Gbit / s long-distance optical transmission system, to have a wide effective band (40 GHz or more), a large variable range, and a small dispersion ripple. Therefore, in order to obtain desired characteristics as a dispersion compensator, a method of reflecting a plurality of times using a mirror as shown in FIG. 2 is effective. In the configuration of FIG.
The mirrors (20, 21) are arranged in parallel or slightly at an angle to the etalon (10, 11, 12). As will be shown later with specific numerical values, if this angle is about 1 degree or less, the effect of expanding the effective band according to the present invention can be obtained. Here, the mirrors (20, 21) are configured by coating the mirror substrate 20 with a reflective film 21 having a high reflectance. The light emitted from the collimator 30 is reflected by the etalon (10, 11, 12) and the mirror (20, 21) alternately and enters the collimator 31. A collimator is an optical component that radiates light propagating through an optical fiber into space as a parallel beam. The temperature of the etalon (10, 11, 12) can be controlled by the temperature change element 40. At this time, in order to make the heat distribution of the etalon uniform, a heat transfer agent 41 is interposed between the temperature change element 40 and the etalon (10, 11, 12). A heat transfer sheet or thermal grease is used as the heat transfer agent.

ここで、ΔＴ_ｉはエタロンの各反射において平行平面板を１往復することによって生じる光学的な距離である。さらに、入射角度を考慮するとΔＴ_ｉは以下の式で表される。

ここで、ｃは光速、Θ_ｉはエタロン中での入射角、ｎは屈折率、Ｌはエタロン反射膜間隔である。エタロン中での入射角Θ_ｉとエタロン入射角θ_iの関係は数式（５）で表される。 The group delay characteristic will be described with reference to FIG. When the angle between the mirror (20, 21) and the etalon (10, 11, 12) is Δθ, the incident angle θ with respect to the etalon.
_The light incident at ₀ is reflected i times from the mirror and then enters the etalon again at an angle of θ _i = θ ₀ + 2iΔθ. If the total number of reflections of the mirror is k, the group delay time τ _{total of the} entire compensation unit can be obtained by summing the etalon group delay time τ _i at each reflection (k + 1) times.

Here, ΔT _i is an optical distance generated by one round trip of the plane parallel plate in each reflection of the etalon. Further, ΔT _i is expressed by the following equation in consideration of the incident angle.

Here, c is the speed of light, Θ _i is the incident angle in the etalon, n is the refractive index, and L is the etalon reflecting film interval. The relationship between the incident angle Θ _i in the etalon and the etalon incident angle θ _i is expressed by Equation (5).

FIG. 4 shows a method of fixing the etalon (10, 11, 12), mirror (20, 21), and collimators 30, 31 by controlling with the optical component fixing member 200. Here, the optical component fixing member 200 is realized using a metal or glass having a low coefficient of thermal expansion. The optical component fixing member 200 is hollow and can be created by processing using an ultrasonic drill or the like. FIG. 5 is a top view of the optical component fixing member 200, and FIG. As shown in FIG. 5, the hollow hole of the optical component fixing member 200 has a shape that combines an elliptical hole for passing the beam and two round holes for fixing the collimators 30 and 31. It has become. Further, as shown in FIG. 6, the upper part of the component fixing member 200 is obliquely polished, so that the angle Δθ between the etalon (10, 11, 12) and the mirror (20, 21) is precisely controlled and fixed. be able to. Also, the collimator fixing hole can be precisely processed in the same manner, so that the incident angle θ ₀ from the collimator 30 to the etalon (10, 11, 12) can be precisely controlled and fixed. These optical components are fixed using an optical component adhesive or the like.

A variable method using temperature control will be described with reference to FIG. The tunable dispersion compensator 100 has an input port 110 and an output port 120. The tunable dispersion compensator 100 includes two compensators, a plus-side tunable dispersion compensator 130 and a minus-side tunable dispersion compensator 140. Each compensator has a configuration in which the etalon (10, 11, 12) and the mirror (20, 21) shown in FIG. 2 are opposed diagonally or in parallel, and the laser beam emitted from the collimator 30 is obliquely reflected a plurality of times. Realized. As shown in the figure, the plus-side tunable dispersion compensator is a linear function with a positive slope in the relationship between dispersion and wavelength, and the minus-side tunable dispersion compensator has a negative slope. When the temperature of the etalon is changed, the resonance wavelength changes due to thermal expansion of the etalon substrate. For this reason, the dispersion characteristic shown in FIG. 7 shifts in the wavelength direction. At this time, it is desirable that the thermal expansion coefficient of the etalon substrate is made of a glass member having a coefficient of 10 ⁻⁴ or less and 10 ⁻⁶ or more in terms of controlling the resonance wavelength. For example, under the conditions evaluated this time, when a borosilicate optical glass widely used as an optical glass having a coefficient of thermal expansion of 0.87 × 10 ⁻⁵ / ° C. is used as a substrate, 100 GHz (about the wavelength of light). 0.8 nm) A temperature change of 60 ° C. is required to shift the dispersion characteristics. As a specific numerical value will be shown later, the tunable dispersion compensator needs a wavelength shift of about 10 GHz or more in one stage, so the temperature control range is preferably 5 ° C. or more. Here, the plus-side variable dispersion 130 and the minus-side variable dispersion compensator 140 are configured to be able to independently control the temperature. At this time, the dispersion characteristic of the entire tunable dispersion compensator is the sum of the two compensators. If the overlap between the straight line part with positive slope and the straight line part with negative slope is large, the flat part on the upper side will be wide and low shape (upper right of FIG. 7). Conversely, if the overlap is small, the flat part on the upper side will be Narrow and high shape (lower right in FIG. 7). Thus, the amount of dispersion can be changed by applying temperature control to the plus side and minus side variable dispersion compensators.

  In the tunable dispersion compensator according to the present invention, light is incident on the etalon at an angle, and after reciprocating between the mirror and the mirror several times, light is received by the collimator. Therefore, excessive loss and loss ripple are caused by each reflection at the etalon. . This phenomenon will be described using figures and mathematical expressions.

  Main examples of chromatic dispersion and loss characteristics obtained by experiments are shown in FIGS. In both cases, the horizontal position of the collimator is different. In the experiment, Δθ≈0 and the number of total reflections k = 4. 8 and 9, there is not much difference in the wavelength dependence of dispersion, but it can be seen that the wavelength dependence of loss is greatly different. First, the state of the dispersion characteristics shown in FIGS. 8 and 9 will be described. This period is called Free Spectral Range (FSR), and is 100 GHz (about 0.8 nm) in this figure. Next, the state of the loss characteristic in FIGS. 8 and 9 will be described. In FIG. 8, there are two peaks of equal height in the FSR range, or the maximum loss is minimized. In FIG. 9, the loss characteristic is a state where there is a flat portion with little loss (hereinafter referred to as a top flat state), but there are two large drops in the FSR range.

と表される。ここで、Θはエタロン中での入射角、λは光の波長である。また、よく知られた近軸光線近似Θ＜＜１を適用できる場合のコリメータ結合効率の関係から、反射戻り光の、振幅Ｂ_１，Ｂ_２，Ｂ_３…は以下のように示される。

ただし、ここでδθはコリメータと反射光との角度ずれ、ω_１，ω_２は二つのコリメータのビームウェストの半径、結合効率ηは、

で示される。結合効率ηの式中のκとω_１（ｚ）は、さらに

と表される。これらの式より、エタロン１回反射時の戻り光全振幅Ａ_ｒは、最終的に次式で表される。

この時、振幅反射率ｒ₁とＢ_１，Ｂ_２，…との関係であるが、図１１に示すように、ｒ₁ ^２
が（３―√５）／２、ｒ₁が６１．８％より大きい場合は、Ｂ_１＞Ｂ_２、小さい場合は、Ｂ_２＞Ｂ_１となる。コリメータの結合効率は水平位置および波長の関数であるため、受光側コリメータの水平位置を変化させると波長に対する結合効率も変化し、損失および損失リップルの特性が変化する。特にｒ₁の大きさによってエタロン出射後の強度分布が変わるため、反射率に応じた最適な状態となるよう受光側コリメータの水平位置を決定する必要がある。以上の考察より、コリメータ水平位置が損失リップル最小となる最適な条件で、かつエタロン入射角θをなるべく小さくすることで、損失および損失リップルの抑制が可能となる。
発明者は、この考え方を発展させ、本発明によるミラーとエタロンを対向させた構成で複数回反射したときの数式を導いた。すなわち、エタロンの多重反射を纏めて扱わずに、１つ１つの光線に分けて考え、その各々における結合効率を計算し、その合計をとる。これを図１２を用いて説明する。尚、簡単のため、ミラー角度Δθは０とする。 This phenomenon will be described using mathematical expressions. FIG. 10 shows multiple reflections in the etalon of the tunable dispersion compensator according to the present invention. As shown in this figure, incident light having an amplitude A ₁ generates rays having amplitudes B ₁ , B ₂ , B ₃ ... By multiple reflection in an etalon. The phase shift of light in B ₁ and B ₂ , B ₂ and B ₃ ,..., That is, the phase shift δ of light that makes one round trip in the etalon is

It is expressed. Here, Θ is the incident angle in the etalon, and λ is the wavelength of light. From the relationship of the collimator coupling efficiency when the well-known paraxial ray approximation Θ << 1 can be applied, the amplitudes B ₁ , B ₂ , B ₃ ... Of the reflected return light are expressed as follows.

Where δθ is the angular deviation between the collimator and the reflected light, ω ₁ and ω ₂ are the beam waist radii of the two collimators, and the coupling efficiency η is

Indicated by Κ and ω ₁ (z) in the equation of coupling efficiency η

It is expressed. From these equations, the return light full amplitude A _r of the reflection one etalon is ultimately expressed in the following equation.

At this time, the amplitude reflectance r ₁ and _B 1, _{B 2,} is a relationship ... and, as shown in FIG. 11, r ₁ ²
When (3−√5) / 2 and r ₁ is larger than 61.8%, B ₁ > B ₂ , and when r ₁ is smaller, B ₂ > B ₁ . Since the coupling efficiency of the collimator is a function of the horizontal position and wavelength, when the horizontal position of the light receiving side collimator is changed, the coupling efficiency with respect to the wavelength also changes, and the characteristics of loss and loss ripple change. In particular, since the intensity distribution after etalon emission varies depending on the size of r ₁ , it is necessary to determine the horizontal position of the light receiving side collimator so as to obtain an optimum state according to the reflectance. From the above consideration, it is possible to suppress the loss and the loss ripple by reducing the etalon incident angle θ as much as possible under the optimal condition that the horizontal position of the collimator is the minimum loss ripple.
The inventor has developed this idea and derived a mathematical formula when the mirror and the etalon according to the present invention are reflected a plurality of times. In other words, the multiple reflections of the etalon are not treated together, but are considered separately for each light ray, the coupling efficiency in each of them is calculated, and the sum is taken. This will be described with reference to FIG. For simplicity, the mirror angle Δθ is 0.

クロネッカーδ関数、

を用いると、

Kronecker δ function,

Using

Thus, the calculated | required loss and the dependence of a wavelength are shown to FIGS. In Figure 13, the horizontal position _{x 0} of the collimator, was varied from 0 to 1.0 mm. From this figure, when gradually increasing the x _0, a maximum coupling efficiency is poor (the minimum loss is not a 0 dB) from the top flat state, loss characteristic shifts to a state having a peak in two equal height, further It can be seen that the loss characteristic is shifted to a state having one indented peak.

Next, FIG. 14 shows the behavior of the loss characteristic when the etalon incident angle θ is changed. At this time, x _{0 is} also changed at the same time, and the state where the loss ripple is minimized is selected and shown so that the maximum loss is minimized (the state having two peaks of equal height). As a result, the optimal horizontal position x ₀ of the collimator chosen, by as small as possible etalon incident angle theta, it was found to be significantly reduced loss ripple.

の関係式で表されることになる。さらに、損失リップルを最低にする最適な受光側コリメータ水平位置ｘ_Ｓが存在し、このために受光側コリメータ位置は、低反射エタロンによるビーム広がりを考慮し、受光部でのビーム重心位置になるよう水平にずらす。例えば、ｒ_１＝２５％程度の時は、この位置は近似的に
［水平ずれ量］≒２ Ｎ Ｌ ｔａｎ Θ
と表すことができる。さらに反射率の低い場合も考慮すると、水平ずれ量はその倍の量の４ＮＬ ｔａｎ Θ までとればよいと言える。この水平位置の関係を図１５に示す。図１５に示すように、出射側コリメータから出射された光が反射膜とミラー面との間を最短光学距離で反射して来た第一反射光の光軸に対して受光側コリメータの中心を合せると、先に述べたように振幅反射率ｒ_１≦６１．８％の場合には強度が最も強い光の光軸からずれ、損失劣化および損失リップルを発生する。そこで、この劣化を抑制するために第一反射光の光軸に対して受光側コリメータの中心を出射側コリメータから遠ざかる方向に所定の距離ｘ_Ｓだけ変位させる。この距離ｘ_Ｓは、図１５で示すように、０から４ＮＬ ｔａｎ Θ程度までの範囲がよいと考えられる。本発明による可変分散補償器の構成と構成要素である光部品の位置関係とミラーの長さとの関係を図１６に示す。ここで、出射側コリメータから出射されたビームのミラーとの各反射における実効的なビーム径をω_ｌとする。このビーム径ω_ｌをミラーとの全ての反射する分だけ足し合わせた長さよりミラーの長さｍが短い場合、ビームが重なることになる。その場合、エタロンでの反射回数が異なるビームが受光側コリメータに同時に入射することになるため、群遅延時間は数式（３）で示した関係にはならない。よって、この状態を避けるため、ミラーの長さｍはΣω_ｋと同じ（最もビーム同士を近づけた場合）、あるいは少しマージンをとってΣω_ｋよりも長くしなければならない。さらに言えば、損失および損失リップルを抑制するためには、先に述べたように出射側コリメータからエタロンへの入射角をなるべく小さくする方がよいので、ミラーの長さｍをなるべく小さく、すなわちΣω_ｋと同じとすることが好ましい。
From the examination results using the above mathematical formulas, important points leading to the present invention were derived. That is, in order to reduce the loss and the loss ripple, it is important to take the incident angle θ as small as possible. Specifically, the distance between the mirror and the etalon may be taken by (1) reducing the mirror size to such an extent that adjacent beams do not overlap, (2) approximately equal to the collimator operating distance, or less. When this is expressed by a mathematical expression, it is assumed that the minimum loss can be obtained when the two collimators have an operating distance of 1 _WD , and the average distance between the mirror and the etalon is h (where the average distance is from both end points of the mirror). When the distance between the first collimator and the etalon is a ₁ , the distance between the second collimator and the etalon is a ₂ , and the number of reflections at the etalon is N ,

It is expressed by the relational expression. Furthermore, there is an optimum light receiving side collimator horizontal position x _S to the loss ripple in the lowest, the light receiving side collimator position for this is to consider the beam spread due to low reflection etalon, the beam center of gravity of the light receiving portion Shift horizontally. For example, when r ₁ = about 25%, this position is approximately [horizontal deviation amount] ≈2 N L tan Θ
It can be expressed as. Further, considering the case where the reflectance is low, it can be said that the horizontal shift amount should be up to 4NL tan Θ, which is twice that amount. The relationship between the horizontal positions is shown in FIG. As shown in FIG. 15, the center of the light receiving side collimator with respect to the optical axis of the first reflected light which the light emitted from the emission-side collimator came reflected between the reflecting film and the mirror surface in the shortest optical distance In combination, as described above, when the amplitude reflectivity r ₁ ≦ 61.8%, the intensity is shifted from the optical axis of the strongest light, and the loss deterioration and the ripple are generated. Therefore, displacing a predetermined distance x _S away the center of the light receiving side collimator from the emission-side collimator with respect to the optical axis of the first reflected light in order to suppress this degradation. As shown in FIG. 15, the distance x _S is considered to be in a range from 0 to about 4NL tan Θ. FIG. 16 shows the configuration of the tunable dispersion compensator according to the present invention and the relationship between the positional relationship of the optical components as the components and the mirror length. Here, the effective beam diameter in each reflection with the mirror of the beam radiate | emitted from the output side collimator is set to (omega) _l . When the length m of the mirror is shorter than the length obtained by adding the beam diameter ω ₁ to the amount reflected by the mirror, the beams overlap each other. In this case, beams having different numbers of reflections from the etalon are incident on the light receiving side collimator at the same time. Therefore, the group delay time does not have the relationship expressed by Equation (3). Therefore, in order to avoid this state, the length m of the mirror must be the same as Σω _k (when the beams are closest to each other) or longer than Σω _k with a little margin. Furthermore, in order to suppress loss and loss ripple, as described above, it is better to reduce the incident angle from the output side collimator to the etalon as much as possible. Therefore, the mirror length m is as small as possible, that is, Σω It is preferable to be the same as _k .

  Further, as shown in FIG. 12, the light beam which is repeatedly reflected by the mirror and the etalon is asymmetric in the horizontal direction due to the influence of etalon multiple reflection. Such asymmetry causes a reduction in coupling efficiency when receiving light with a collimator. Since it is known that such an asymmetric light beam can be corrected by an aspheric lens system, the coupling efficiency can be improved by adding an optical system using an aspheric lens.

In the examination so far, the case where the etalon and the mirror are parallel is described, but the case where the mirror angle Δθ is not 0 is also described. In this case, the smaller the etalon incident angle θ _i , the smaller the influence of the beam spread by the etalon. Therefore, when the incident angle of the light beam emitted from the collimator is first increased and decreased by reflection with the mirror, the beam spread at the collimator that receives light at the end can be reduced. Therefore, it can be seen that the loss can be reduced by making the light incident in the direction shown in FIG. When the mirror angle Δθ is set to about 0.08 °, it is confirmed by experiments that the loss is reduced by about 1 dB when the beam direction is shown in FIG.

The etalon shown in FIG. 1 has a single cavity structure having one resonance structure surrounded by the reflection film 11 and the reflection film 12, and this is a multi-cavity etalon having a plurality of resonance structures shown in FIG. A dispersion compensator may be configured instead of the above. The multi-cavity etalon shown in FIG. 18 has a structure in which flat plates 10 and reflective films 11 are alternately stacked, and the degree of freedom increases compared to a single-cavity etalon by changing each reflectance. Desired dispersion characteristics can be realized. Regarding the temperature control of the multi-cavity etalon, a structure in which one set of heat transfer agent 41 and temperature change element 40 are bonded to the reflective film 12 is preferable in terms of reducing the number of components. However, from the viewpoint of finely controlling the temperature, as shown in FIG. 18, M sets of heat transfer agents 41 and temperature change elements 40 are bonded to each plane plate 10 for the number M of layers of the multicavity etalon. The structure is good. In this figure, the temperature control element is in contact with the end portion of the flat plate 10. More specifically, it is preferable that the temperature control element is in contact with the flat plate 10 so as to surround it.
As described above, according to the present invention, a variable dispersion compensator having a wide band, low dispersion ripple (or low group delay ripple), a large amount of variable dispersion, and good characteristics with little change in the center wavelength is obtained.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram for explaining an embodiment of the present invention and shows an etalon that is a basic element of a tunable dispersion compensator according to the present invention. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram for explaining an embodiment of the present invention and shows a first configuration of a tunable dispersion compensator according to the present invention. FIG. 3 is a diagram for explaining one embodiment of the present invention, and is a diagram for explaining in detail a first configuration of a tunable dispersion compensator according to the present invention. It is a figure for demonstrating one Example of this invention, Comprising: The figure which showed the 1st structure of the variable dispersion compensator in this invention in three dimensions. It is a figure for demonstrating one Example of this invention, Comprising: The top view of the optical component fixing member in the 1st structure of the variable dispersion compensator in this invention. It is a figure for demonstrating one Example of this invention, Comprising: The figure which shows the structure of the hollow hole of the optical component fixing member in the 1st structure of the variable dispersion compensator in this invention. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram for explaining an embodiment of the present invention, and is a diagram illustrating the principle and configuration of a tunable dispersion compensator according to the present invention. It is a figure for demonstrating one Example of this invention, Comprising: The figure which shows the 1st relationship with respect to the wavelength of the dispersion | distribution and loss of the positive side variable dispersion compensation part of the variable dispersion compensator in this invention. It is a figure for demonstrating one Example of this invention, Comprising: The figure which shows the 2nd relationship with respect to the wavelength of the dispersion | distribution and loss of the positive side variable dispersion compensation part of the variable dispersion compensator in this invention. It is a figure for demonstrating one Example of this invention, Comprising: The figure which shows the multiple reflection in the etalon of the variable dispersion compensator in this invention. BRIEF DESCRIPTION OF THE DRAWINGS It is a figure for demonstrating one Example of this invention, Comprising: a is a figure which shows the light intensity by the etalon multiple reflection of the variable dispersion compensator in this invention. b) is a diagram showing a relationship with the etalon one-side reflectance r ₁ . It is a figure for demonstrating one Example of this invention, Comprising: The figure which shows the multiple reflection in the mirror and etalon which the variable dispersion compensator in this invention opposes. It is a figure for demonstrating one Example of this invention, Comprising: The figure which shows the relationship with respect to the wavelength when changing the horizontal position of a collimator with the variable dispersion compensator in this invention. It is a figure for demonstrating one Example of this invention, Comprising: The figure which shows the effect by which the ripple with respect to the wavelength of the loss of the variable dispersion compensator in this invention was suppressed. FIG. 3 is a diagram for explaining an embodiment of the present invention, and is a diagram illustrating details of the principle of a tunable dispersion compensator according to the present invention. It is a figure for demonstrating one Example of this invention, Comprising: The figure which shows the positional relationship of the structure of the variable dispersion compensator in this invention, and the optical component which is a component. It is a figure for demonstrating one Example of this invention, Comprising: The figure which shows the 2nd structure of the variable dispersion compensator in this invention. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram for explaining an embodiment of the present invention, and is a diagram showing a multi-cavity etalon that is a basic element of a tunable dispersion compensator according to the present invention.

Explanation of symbols

10: plane plate,
20 ... mirror (substrate),
11, 12, 21 ... reflective film,
30, 31 ... Collimator,
40 ... temperature change element,
41 ... Heat transfer agent,
100: variable dispersion compensator,
110: Input port,
120 ... output port,
130 ... plus-side variable dispersion compensator,
140 ... negative side variable dispersion compensator,
200: Optical component fixing member.

Claims (6)

A plate-like etalon having a first plane and a second plane facing the first plane;
A first collimator in which the second plane and the reflection surface of the mirror having a reflection surface are inclined at an angle Δθ and a light beam is incident between the second plane and the reflection surface;
The light beam is alternately reflected between the etalon and the mirror, reflected by the etalon a natural number N times, and then the light beam is emitted from between the second plane and the reflecting surface, A second collimator for receiving the emitted light;
A temperature change element that controls the temperature of the etalon,
When the width of the mirror is m and the effective beam diameter at the k-th reflection by the mirror is ω _k , the width of the mirror is m ≧ Σω _k
It is expressed by the relational expression of
When the incident angle in the etalon is Θ and the thickness of the etalon is L, the position of the second collimator is such that the light emitted from the first collimator is between the second plane and the mirror surface. Is fixed on the side away from the first collimator by a distance of 2NL tan (Θ) from the position where the optical axis of the first reflected light that has been reflected at the shortest optical distance comes to the center of the second collimator,
The variable dispersion compensator according to claim 1, wherein the reflectance of the second plane is 61.8% or less.   The variable dispersion compensator according to claim 1, wherein the etalon is a multi-cavity etalon.   The variable dispersion compensator according to claim 1, wherein the angle Δθ is 1 degree or less.   The variable dispersion compensator according to claim 1, wherein the collimator is replaced with an aspheric lens to correct a beam intensity distribution. The variable dispersion compensator according to claim 1, wherein a temperature control range of the temperature adjusting means is 5 ° C or more and 60 ° C or less . The variable dispersion compensator according to claim 1, wherein the thermal expansion coefficient of the substrate constituting the etalon is characterized that you have made of glass member 10 ^-4 10 ^-6.

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