JP5933165B2 - Optical delay line and manufacturing method thereof - Google Patents

Description

  The present invention relates to an optical delay line, and more particularly to an optical delay line composed of a microfiber coil having a size in which a fundamental mode of an optical signal can propagate along the delay line with relatively low loss.

  This application claims its benefit based on US Provisional Application No. 61 / 153,722, filed on Feb. 19, 2009, and 61 / 156,565, filed on Mar. 2, 2009. Which is incorporated herein by reference.

  The optical delay line or buffer is a key element of future optical circuits for optical signal processing in applications such as communications and computer use. Conventionally, an optical delay line consists of one or more optical fiber sections, in which case the length of the fiber determines the delay introduced into the propagated signal. For example, a standard optical fiber approximately 20 meters (m) long provides a 100 nanosecond (ns) delay in the light pulse. In order to fit this type of delay device in a compact container, the 20 m fiber is coiled to fit a relatively small container. In a sense, the underlying dimensions of the container are limited by optical fiber bending loss, which increases with decreasing coil radius. For example, for a 20 meter long fiber, a box-shaped container that is several cubic centimeters in size is required to minimize the effects of bending loss. These dimensions are a problem for the unremitting efforts to miniaturize optical components.

In contrast to the delay line fiber-based, microspheres, or integrated optical components such as micro-ring resonator is, the delay amount of the same extent in the component size in the range of a few millimeters from a few tens of microns (i.e. hundreds Nanosecond). Although the amount of delay introduced by the within a desired range, the product of constraints on time / bandwidth of the delay of the microresonator is limited to a pulse of about 1MHz the corresponding bandwidth (i.e., the number 100ns delay , And only 0.00001 nanometers (nm)). This bandwidth is too small for these microstructured resonators to be considered as a realistic optical buffer for commercial systems.

Thus, while having a larger bandwidth than known microresonator device, but the loss or the need for an optical delay element is smaller than the conventional fiber delay lines without causing reliability problems still remaining Yes.

The optical fiber delay device, which is one of the optical delay lines as a key element of an optical circuit for optical signal processing in applications such as communication and computer use, or a buffer device, is 100 nanoseconds (ns). A 20m fiber is required to introduce the delay, and a container of several cubic centimeters is required to minimize the effects of bending loss in order to put this in a compact container. These dimensions become a problem in order to make it easier. Microspheres is another method, or integrated optical components such as micro-ring resonator, the delay amount of the same extent in the component size in the range of a few millimeters from a few tens of microns (i.e., a few hundred nanoseconds) leads to a , bandwidth delay the microresonator corresponding is about 1 MHz, too small a light shock absorber for commercial systems. Thus, while having a larger bandwidth than known microresonator device, loss or still remains a need for an optical delay element is smaller than the conventional optical fiber delay lines without causing reliability problems Yes.

(Often micro fiber) A fiber relates to an optical delay line formed from coils, in which case, the diameter of the optical fiber is larger than the wavelength of signals propagating, and the radius of the coil, the optical fiber Considering the diameter, it is chosen to limit the propagation loss by minimizing the coupling between adjacent turns of the coil.

ここで β＝（２πｎ）／λ、ｎは光ファイバの屈折率、λは伝播する光信号の波長である。 In one embodiment, the optical delay line includes an optical fiber of radius r (2r> λ, where λ is the wavelength of the propagating optical signal) wound around a central core rod having a radius R. The core rod at the center may be removed if a coil is formed. The fiber is wound with or without spacing between adjacent turns, and the mode in which the dimensional difference between the fiber diameter and wavelength propagates along one turn is coupled to the adjacent turns. To suppress that. It has been found that the intensity of the mode at the interface between the central rod and coil is minimized when the radius of the optical fiber satisfies the following conditions:

Here, β = (2πn) / λ, n is the refractive index of the optical fiber, and λ is the wavelength of the propagating optical signal.

Examples of the present invention relates to a device for linking the advantages of a conventional optical fiber delay line benefits optical microresonator of (low-loss wide-band) (low loss compact). At the same time, the apparatus of the present invention is a drawback in the form of a delay element according to the prior art, it does not exhibit any of unnecessarily narrow bandwidth of large size or the optical microresonator, undesirable optical fiber delay line. In one exemplary embodiment, silica microfibers , each having a diameter (2r) in the range of about 5 μm to about 100 μm, are wound around a rod having a diameter (2R) in the range of about 100 μm to about 10 mm. After fiber coil manufacture, strain that may be introduced by the winding process is relaxed by heat treatment. Also, as described above, if desired, the core core rod may be removed after the coil is formed.

  The low loss fiber optic coil delay line of the present invention can be used in many applications such as optical attitude control devices, amplifiers, sensors, and the like. The relatively small size and robust characteristics of the elements of the present invention enable applications associated with optical circuit based devices. Indeed, the present invention may be configured as a tunable optical delay line having various optical fiber coils that are controlled by a switch to add to or disconnect from the optical signal path.

Other and further properties and applications of the microfiber coil delay element of the present invention will become apparent during the following series of discussions and by reference to the accompanying drawings.

FIG. 2 illustrates an exemplary fiber optic coil delay line formed in accordance with the present invention. FIG. 3 illustrates exemplary mode field strengths of three sets of adjacent turns T-1, T-2, and T-3 of a fiber optic coil delay line according to the present invention. It is a graph which shows the relationship between the radius R of the center part core stick | rod expressed using the geodesic curve which has a length and the coordinate of a radial direction, and the radius r of a microfiber . FIG. 4A shows the influence of the core radius on the mode field strength for the microfiber embodiment of the present invention, and FIG. 4A shows a configuration according to the prior art having a mode field arranged near the center of the microfiber . 4 (b) shows an embodiment of the present invention. The same microfiber radius as shown in FIG. 4 (a) is used, but with a small bending radius R sufficient to deviate the mode field intensity from the region where coupling to adjacent turns can occur. Figure 3 illustrates the relationship between r and R for two further microfiber embodiments of the present invention. The diagram in FIG. 5A relates to values r = 2.5 μm and R = 50 μm, and the diagram in FIG. 5B relates to values r = 12 μm and R = 5000 μm. Connected to the system configuration or a plurality of discrete fine fibers coils disconnected, it illustrates a typical use of the optical micro fiber coil of the present invention in a tunable configuration. Figure 6 illustrates another embodiment of a tunable optical microfiber coil delay line according to the present invention, in which separate portions of a continuous coil are connected to or disconnected from the system.

FIG. 1 illustrates an exemplary optical microfiber delay line 10 formed in accordance with the present invention. The delay line 10 includes an optical microfiber portion 12 wound around a core rod 14 at the center. For the purposes of the present invention, an “optical microfiber ” (hereinafter referred to as “microfiber”) has little or no border between the central “core” region and the cladding layer surrounding it. , Defined as an optical fiber having a diameter in the range of about 5-100 μm used to propagate the fundamental mode of the optical signal. For the purposes of the present invention, the diameter of the fine fibers 12 is required to be larger than the wavelength of the signal propagating. For example, when used with a communication signal having a wavelength of 1550 nm, the diameter of the microfiber needs to be about 2 μm or more. As is apparent, different constraints on the radius of the microfiber apply for different wavelength systems. In addition, it may be desirable to use a conventional optical fiber (or a fiber of the same dimensions as a conventional optical fiber) instead of a microfiber in the delay device of the present invention. Thus, most broadly, the present invention is directed to the formation of a coiled optical fiber delay line having a fiber radius r controlled to minimize losses and a coil radius R. Thus, the following discussion regarding implementation with “ microfiber 12” is considered to be exemplary only.

  As in the case of conventional optical delay elements, the length L of the coil 10 determines the degree of delay time introduced. Indeed, for a given central core rod of diameter 2R, the introduced delay is increased by increasing the number of turns T of the fiber 12 wound around the central core rod 14. It should be understood that when using different diameter central core bars, different numbers of turns are used to provide the same time delay interval.

  Propagation loss along the coil 10 is minimized when the fundamental mode of the incident optical signal propagating along a winding of the coil does not interact with the central core rod 14 or an adjacent winding of the coil 10. Thus, by limiting the effect of physical contact between the central core bar and the coil, as well as adjacent windings of the coil, the scattering and coupling of the optical signal between the turns is minimized, and the optical fiber Significantly reduces the presence of bending loss along the coil. In accordance with the present invention, it has been found that by confining the mode field strength of the propagating signal to the optical fiber region removed from these problematic contact points, the loss of the propagating optical signal is minimized. Yes.

ここで、β＝（２πｎ）／λ、ｎは微小光ファイバ１２の屈折率、λは伝播する光信号の波長である。 In particular, by controlling the relationship between the radius R of radius r and center core bar 14 of the micro fiber 12, in particular, low loss is achieved when the following relationship is satisfied.

Here, β = (2πn) / λ, n is the refractive index of the minute optical fiber 12, and λ is the wavelength of the propagating optical signal.

FIG. 2 shows typical mode field intensities for a set of three adjacent turns T-1, T-2, and T-3 selected from a microfiber coil formed in accordance with the present invention. The boundary surface A defines a portion where the microfiber 12 contacts the center core rod 14 when each winding T is wound around the center core rod. The boundary surface B defines the contact position between the windings T-3 and T-2, and the boundary surface C defines the contact position between the windings T-2 and T-1. As clearly shown in FIG. 2, due to the relationship between r and R as outlined above, the mode field intensity is offset from the interfaces A, B, and C, and instead the coil 10 It is confined to the outer periphery of each winding T. The strength of the outer periphery of this winding T has little, if any, interaction with the central core rod 14, or just below and above the winding T (referred to as windings T-1 and T + 1, respectively). The desired low loss condition is achieved for the delay line of the present invention.

ここでｒおよびＲは上に規定される光ファイバおよび中心部コア棒の半径、λは動作波長、Ｈ_ｍ（ｘ）はエルミートの多項式、Ａｉ（ｘ）はエアリー関数、ｔ_ｎはエアリー関数（ｔ_０＝２．３３８、ｔ_１＝４．０８８、ｔ_２＝５．５２、．．．）の根、ｎは光ファイバの屈折率、β_ｍｎは以下の式で与えられるようにモード（ｍ，ｎ）の伝播定数である。

The relationship between the radius r of the microfiber and the radius R of the coil is such that the ground mode of the bent optical fiber and the nearby higher-order modes are located on the outer portion of the surface of the microfiber as shown in FIG. It was developed from the understanding that it is observed as a mode propagating in the vicinity of. Local coordinates near this geodesic line are shown as a longitudinal coordinate s and a transverse coordinate x and y. Using known short wavelength scalar diffraction theory, a simple asymptotic solution is derived for the propagation modes with lateral quantum numbers m and n as follows:

Where r and R are the radii of the optical fiber and central core rod defined above, λ is the operating wavelength, H _m (x) is Hermitian polynomial, Ai (x) is the Airy function, and t _n is the Airy function ( t ₀ = 2.338, t ₁ = 4.088, t ₂ = 5.52,...), n is the refractive index of the optical fiber, and β _mn is the mode (m , N).

For the sake of the current discussion, it is assumed that the coil is made of uniform elements, ie the radii r and R are constant. Thus, both r and R are independent of the longitudinal coordinate, s. However, the scope of the invention is not so limited, and the fiber optic coil has a non-uniform fiber radius r (•), non-uniform coil radius R (•), or a combination of the two. Can be obtained against. For fundamental mode propagation that is m, n˜1, the propagation signal shows strong localization near the geodesic line s, and the relationship between r and R is as shown above. Indeed, for optical communication system applications, eg, phase diversity systems utilizing delays where n = 1.5, λ = 1.5 μm, the relationship between r and R (microns) is approximated by:

The r, and the relationship between R is required to shift the mode intensity from the interface is illustrated in detail in FIG. 4, it is illustrates a representative portion of one "winding" of the micro fiber coil . FIG. 4 (a) shows a case where r and R do not satisfy the required relationship, and FIG. 4 (b) shows a configuration using the same microfiber radius r but using a coil radius R that satisfies the required relationship. Show. In particular, FIG. 4 (a) with a radius r = 5.5 [mu] m, which illustrates a portion of the radius R = a 5000 .mu.m (not shown) micro-fiber 12 wound around a central core rod. Using the relationship of equation (1), r and the expression (R / β ² ) ^1/3 are relatively equal values (the latter calculated values are about 5 for n = 1.5, λ = 1.5 μm). .5567), indicating the same level of magnitude, and that the inequality of equation (1) is not satisfied. As a result, the propagating fundamental mode remains at the center point C of the fiber with the mode field intensity uniformly distributed across the fiber diameter as shown, thus causing high losses when the signal propagates through the coil.

In contrast, the configuration of FIG. 4 (b) illustrates an embodiment of the present invention using the same microfiber with a radius of 5.5 μm as shown in FIG. 4 (a). Here, the mode field intensity is clearly shown as deviating from the center of the fiber to the outer peripheral region of the microfiber by using a smaller bend radius, in this case R = 500 μm. As discussed above, the modal field peak intensity shift (if any) will not only contact / couple the center core rod but also signal contact / combination from one winding to the adjacent winding. Restrict. In accordance with the present invention, the microfiber coil is utilized as a delay line that retains the desired low-loss quality by confining the mode field to the outer portion of the microfiber , although it is relatively small (compared to conventional fibers).

FIGS. 5 (a) and 5 (b) illustrate a typical configuration of a low loss microfiber formed in accordance with the present invention. In each case, the peak intensity of the mode field is shifted from the central region of the microfiber . FIG. 5A shows a configuration in which the radius r of the microfiber is 2.5 μm and the radius R of the coil is 50 μm. FIG. 5B shows a configuration in which the radius r of the microfiber is 12 μm and the radius R of the coil is 5000 μm. In each case, it is clearly shown that the mode field is confined to a geodesic line located in the outer peripheral part of the winding.

As mentioned above, smaller size of the micro fiber coil delay line of the present invention, it is compact, allows it to be used light gyroscopes, sensors, amplifiers, etc. in various applications others similar To. In fact, the small size of the delay line of the present invention allows it to be incorporated into various types of integrated optical systems and subsystems that utilize optical delay lines such as optical buffers. Furthermore, the configuration of the present invention may take the form of a pacing delay line, in which case, in order to adjust the overall length of the delay line fiber, the time period of the delay in switching the different parts of the micro fiber coil Controlled by switching and connecting to or disconnecting from the configuration.

FIG. 6 illustrates the use of the fiber optic coil of the present invention in a typical tunable configuration. In this configuration, a plurality of separated optical fiber coils 10-1, 10-2, 10-3, 10-4, and 10-5 having different overall lengths are coupled to a typical optical integrated circuit 30 one by one. The A set of switches in circuit 30 (not shown) is used to control which of the separate coils are coupled to the delay line formed in circuit 30. FIG. 7 illustrates another configuration of a tunable fiber optic coil, in which case a single coil 10 is used. In this _{arrangement, 10 a, 10 b, 10} c, 10 d, and a separate portion of the coil 10, shown as 10 _e, is controlled by a switching signal associated are used to determine the total length of the delay line. When a particular part of the coil 10 is turned “off”, the switching arrangement of FIG. 7 directs the propagating optical signal in a waveguide direction that bypasses the particular part of the coil 10 and directs the signal to the next part to be used. Join. In the particular configuration of FIG. 7, each portion 10 _i introduces a 25 ns delay in the propagating optical signal. In this particular configuration, the switch is configured so that the 10 _a , 10 _b , and 10 _c portions are used for the delay structure, and the 10 _d and 10 _e portions are bypassed. In this case, a total delay of 75 ns is generated. The ability to dynamically control the connection of each part of the coil 10 makes it possible to create an adjustable delay relatively easily.

コイルの遅延時間ｔはコイルの長さに比例し、以下の式から計算される。

ここでｎは光ファイバの屈折率、Ｌはコイルの全長、Ｒはコイルの半径、ｒは光ファイバの半径、ｃは真空中での光の速度である。上に述べられたように、１．５μｍの波長λ、ｎの値１．５についてｒとＲとの間の関係は（ミクロンで計算するとき）ｒ≧０．７Ｒ^１／３の形を取ることが出来、体積Ｖを以下のように表すことが出来る。

コイルの体積、あるいは連結されたコイルの組の体積は光ファイバの半径ｒを減少させると急速に減少するということが、この関係から明らかである。しかし、半径ｒがどれだけ小さくなるかについては実際的な限界がある。図５から明らかなように、比較的小さな半径（例えば２．５μｍ以下）では、モードフィールド強度はファイバの他の部分への物理的な接触が避けられなくなる部分へ入り始め、したがって伝播する信号のパワー損失を増大させる。コイルの半径をｒ＝２．５μｍと仮定すると、その結果コイルの体積Ｖはその値が０．４Ｔに近くなり、ここで体積、および時間はそれぞれ立方ｍｍ、およびナノ秒で測定される。したがって、全遅延時間１００ｎｓを与える複数の微小ファイバからなる本発明の実施例は１ｍｍ×７ｍｍ×７ｍｍ、あるいは４９ｍｍ^３の寸法の箱に収められる。 The dimensions of a tunable optical delay line having a specific delay time are defined as follows: Volume V occupied by a typical micro fiber coil is determined from the following relationship.

The coil delay time t is proportional to the coil length and is calculated from the following equation.

Here, n is the refractive index of the optical fiber, L is the total length of the coil, R is the radius of the coil, r is the radius of the optical fiber, and c is the speed of light in vacuum. As stated above, the relationship between r and R for a wavelength λ of 1.5 μm and a value of n of 1.5 (when calculated in microns) takes the form r ≧ 0.7R ^1/3. The volume V can be expressed as follows:

It is clear from this relationship that the volume of the coil, or the volume of the set of connected coils, decreases rapidly when the radius r of the optical fiber is decreased. However, there is a practical limit on how small the radius r can be. As can be seen from FIG. 5, at relatively small radii (e.g. 2.5 μm or less), the mode field intensity begins to enter parts where physical contact to other parts of the fiber is unavoidable, and thus the propagation Increase power loss. Assuming the radius of the coil is r = 2.5 μm, the resulting coil volume V is close to 0.4T, where the volume and time are measured in cubic mm and nanoseconds, respectively. Therefore, an embodiment of the present invention consisting of a plurality of microfibers giving a total delay time of 100 ns is housed in a box with dimensions of 1 mm × 7 mm × 7 mm, or 49 mm ³ .

If one or both of the optical fiber radius r and the coil radius R are not uniform, the criterion for determining R (•) as a function of r (•) is derived from the analysis of equation (1). Specifically, this relationship is expressed as follows for all values of r (•).

  Although the invention has been particularly described and shown by reference to specific embodiments, various changes in form and detail may be found in the spirit of the invention, as defined by the claims appended hereto, and It should be understood by those skilled in the art that this may be done without departing from the scope.

DESCRIPTION OF SYMBOLS 10 Coil 12 Microfiber 14 Center part core rod 30 Optical integrated circuit

Claims (8)

An optical delay line including an optical microfiber having a diameter 2r larger than the wavelength λ of the propagating optical signal,
The optical microfiber is wound around a coil of length L associated with a predetermined time delay t,

Is a radius of curvature R selected to satisfy the relationship: where β = (2πn) / λ, where n is the refractive index of the optical microfiber, and the relationship between r and R is An optical delay line that confines a propagating optical mode in an outer peripheral region of the optical microfiber, suppresses coupling of a mode propagating along one winding of the coil to adjacent windings, and limits propagation loss. The length L of the generated optical microfiber coil is

The optical delay line of claim 1, wherein the optical delay line is selected to provide a predetermined time delay t that satisfies the relationship: where c is the speed of light in a vacuum. The delay line further comprises:
The optical delay line according to claim 1, comprising a central core rod having a radius R, wherein the optical microfiber is wound around the central core rod. 2. The optical delay line according to claim 1, wherein the n is 1.5, the λ is 1.5 μm, and the r is about 0.7R ^1/3 .   The optical delay line of claim 1, wherein the optical microfiber has a non-uniform radius.   The optical delay line of claim 3, wherein the central core bar has a non-uniform radius. An optical delay line including a plurality of optical microfibers having a diameter 2r larger than the wavelength λ of the propagating optical signal,
Each optical microfiber is wound around a coil of length L associated with a predetermined time delay t, and each coil is

, Where β = (2πn) / λ, where n is the refractive index of each optical microfiber, and the relationship between r and R is An optical delay line that confines the propagating optical mode in a peripheral region away from the center of each optical microfiber. A method of manufacturing an optical delay line exhibiting a predetermined time delay t,
a) providing an optical microfiber having a radius r larger than the wavelength λ of the propagating optical signal;
b)

Selecting a core rod with a radius R that satisfies the relationship: β = (2πn) / λ, where n is the refractive index of the optical microfiber,
c) winding the optical microfiber around the core rod to generate a plurality of N turns sufficient to generate the time delay t.

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