JPS59170802A - Optical fiber, method for manufacturing and connecting optical fiber, detector for its fracture, and optical communication device - Google Patents

Abstract

PURPOSE:To connect optical fibers with extremely small loss by providing a thin metallic film around the outer circumference of a core where light propagates or the outer circumference of a clad. CONSTITUTION:The long-sized core 1 which is made of quartz glass fiber has a 50mum core diameter is covered coaxially with a thin metallic film layer 5 which is made of nickel alloy and has 30mum thickness. The wall surface of the core 1 is finished specularly as the thin metallic film layer 5, so a total reflecting surface is formed regardless of the angle of light incidence. Therefore, incident light is reflected totally by the wall surface of the core 1 to travel straight toward then output terminal, so no light leaks out of the optical fiber.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical fiber, a method for connecting the fiber, a method for manufacturing the fiber, a breakage detection device for the fiber, and an optical communication device.

Furthermore, in this specification, communication or power supply using light is also referred to as optical communication, and such fiber is referred to as optical fiber.

A conventional optical fiber will be explained using FIGS. 1 and 2.

FIG. 1 is a partial sectional view of a conventional optical fiber taken in the axial direction (vertical), and FIG. 2 is a sectional view of the same fiber taken in a direction perpendicular to the axial direction (horizontal).

Reference numeral 1 in the figure is the part through which the optical signal propagates, and is called the core.

2 is a cladding concentrically surrounding core 1;
The refractive index of the cladding 2 is approximately 1% higher than that of the cladding 2.

Currently, graded silica fibers are the most popular type of optical fiber for communications, and those with a core diameter of 50 μm and kerat diameter of 125 μm are being standardized internationally. In addition, since the bare optical fiber (hereinafter referred to as core wire 3) is brittle, plastics such as silicone, nylon, and polyethylene (hereinafter referred to as coating or cover) are used to strengthen the optical fiber, improve transmission characteristics, and make it easier to handle. )4. Note that optical fibers include quartz fibers, multicomponent fibers, plastic clad fibers, plastic fibers, etc., and some have not only primary coatings but also secondary coatings and tertiary coatings.

However, conventional optical fibers have the following drawbacks.

(i) As shown by the arrow in Figure 1, the incident light propagates while being reflected on the wall surface of the core 1, but due to insufficient finishing accuracy of the wall surface of the core 1 and stress acting locally, the incident light is distorted. As a result, a portion of the incident light leaks into the crack 2, making long-distance transmission difficult.

(ii) Arc discharge methods and the like have been studied for connecting optical fibers, but these methods have the drawback of requiring large-sized equipment and being difficult for amateurs.

(iii) During welding and bloodless surgery, powerful laser light from a high-output laser propagates within the optical fiber, so how well the heat generated at the input and output ends is absorbed determines the lifespan of the optical fiber and of the entire device. However, there is still no sufficient solution.

(iv) Furthermore, if the optical fiber that propagates such strong light breaks during use, it is necessary for safety to immediately stop the oscillation of the high-output laser device.

However, optical fiber breakage detection devices (which separate and measure the reflected light from the optical fiber) are large and expensive, so they are not widely used.

The present invention eliminates these drawbacks, and provides an optical fiber with a simple structure and extremely low loss, and an easy-to-connect optical fiber connection method. Further, the present invention provides a manufacturing method for optical fibers that is excellent in mass productivity, a fracture detection device for the fibers, and an optical communication device using the fibers.

An optical fiber according to an embodiment of the present invention is shown in Figs. 3 and 4 below.
This will be explained using figures.

FIG. 3 is a longitudinal cross-sectional view of an optical fiber according to an embodiment of the present invention, and FIG. 4 is a cross-sectional view thereof.

In the figure, 1 is an elongated core made of quartz glass fiber with a core diameter of 50 μm, and 5 is a 30 μm thick metal thin film layer made of nickel alloy that concentrically surrounds the core 1 (hereinafter referred to as M
(referred to as TL). Arrows indicate the propagation state of light. In this case, since the wall surface of the core 1 has a mirror-like surface due to the MTL 5, it forms a total reflection surface regardless of the incident angle of light. Therefore, the incident light is totally reflected on the wall surface of the core 1, travels toward the output end, and does not leak out of the optical fiber.

By doing so, a thinner optical fiber with excellent propagation characteristics can be easily obtained without forming a cladding, and a multi-core optical cable can be manufactured easily.
Furthermore, since the wall surface of the core 1 is isolated from the outside world by the MTL 5, it is chemically stable, and radiant heat from the outside world can also be shielded, so that distortion and deterioration due to heat can be reduced.

In addition, the optical fibers of this embodiment are arranged horizontally, and these MTLs are
By soldering the MTLs 5, an optical fiber sheet can be easily obtained, and each MTL 5 can be kept at the same potential. Furthermore, the MTL5 of this optical fiber can be connected to a conductive part of a printed wiring board and used as part of an electric circuit, and both the fixing of the optical fiber and the electrical connection can be performed with a single optical fiber. Furthermore, optical communications can also be performed using this optical fiber.

Next, an optical fiber according to another embodiment of the present invention will be described with reference to FIGS. 5, 6, 7, and 8.

FIG. 5 is a longitudinal cross-sectional view of an optical fiber according to another embodiment of the present invention, and FIG. 6 is a cross-sectional view thereof.

In the figure, 1 is a core made of acrylic fiber, 5 is an MTL made of a chromium alloy, and 4 is a cover made of silicone resin. By doing this, there is no need for a clad around the outer periphery of the core 1, and moreover, it is reinforced by the MTL 5 and the cover 4. Furthermore, since the MTL 5 is interposed between the cover 4 and the core 1, there is no possibility that they will interact with each other to cause chemical changes.

Further, FIG. 7 is a longitudinal cross-sectional view of an optical fiber according to another embodiment of the present invention, and FIG. 8 is a cross-sectional view thereof.

In the figure, 1 indicates a core made of quartz fiber with a high refractive index, and 2
A core 1 made of quartz glass having a low refractive index has a concentric cladding, 5 an MTL made of a silver alloy, and 4 a cover made of nylon. In this case, almost all of the incident light is reflected by the wall surface of the core 1, and the light leaked to the cladding 2 is reflected by the MTL 5 and blocked, and does not reach the cover 4. Therefore, cover 4 due to leakage light to cover 4
There is no deterioration.

Next, an optical fiber according to another embodiment of the present invention will be explained with reference to FIGS. 9 and 10.

FIG. 9 is a longitudinal cross-sectional view of an optical fiber according to another embodiment of the present invention, and FIG. 10 is a cross-sectional view thereof.

In the figure, 1 is a core made of quartz fiber, 5 is an MTL made of an aluminum thin film, 4 is a cover made of silicon,
6 is an MTL made of nickel.

Note that the gap between MTL5 and MTL6 is such that the cover 4 made of silicone resin has a high frequency impedance of 75Ω.
The thickness is adjusted.

By doing so, it is possible to propagate light through the core 1 and also to transmit high frequency currents (signals) through the MTLs 5 and 6.

Further, direct current and alternating current power (small power) can also be transmitted via the MTLs 5 and 6. Furthermore, the MTL may have more than two layers.

As is clear from the above four embodiments, the provision of the MTL 5 can improve and stabilize the propagation characteristics. Further, the cover 4 may be made of polyethylene and the cover 4 may also be used as an optical path separate from the core 1.

Next, an application example of the optical fiber of the present invention will be explained using FIG. 11.

FIG. 11 is a longitudinal sectional view showing an example of application of the optical fiber shown in FIGS. 7 and 8, but the same applies to optical fibers of other embodiments.

In the figure, 3 is the core wire, 5 is the MTL, 4 is the cover, and 6a is the core wire 3
The cooler 6a is made of copper and has a large number of concentric blades, and the cooler 6a is connected to the MTL 5 by solder 7.

Next, this operation will be explained.

Light emitted from a high-output laser (not shown) propagates within the core wire 3 while being repeatedly reflected.

At this time, heat is generated at the output end due to mismatching. However, the MTL 5 immediately absorbs the heat and efficiently transfers it to the cooler 6a connected to the output end. Therefore, by air cooling the cooler 6a, the temperature rise of the core wire 3 near the output end is significantly reduced compared to the conventional case, and the life of the optical fiber can be extended.

Although this application example shows an example of an optical fiber having a cladding 2, it is not limited to this, and it is also possible to use an optical fiber without a cladding, as shown in FIGS. 3 and 4, and the cooling effect is better when there is no cladding. is large. However, in this case, since the core is thin, the cooler 6a is set to MTL in order to strengthen the core.
It is preferable that the cover 5 is connected to the cover 5 and also fixed to the cover 4 using metal fittings or the like.

Next, the light propagation characteristics of an optical fiber that is an embodiment of the present invention will be explained using FIGS. 12 and 13.

FIG. 12 is a cross-sectional view showing a conventional optical fiber bent at a right angle, and FIG. 13 is a cross-sectional view showing an optical fiber according to an embodiment of the present invention bent at a right angle. In the figure, 1 is the core, 2 is the cladding, 4 is the cover, and 5 is the MTL.

As is clear from Figure 12, in conventional optical fibers, when core 1 is bent at a steep angle, the incident angle becomes small even if cladding 2 is provided, and only a small amount of light is reflected on the wall of core 1. Most of the light becomes refracted rays and leaks into the cladding 2, turning into heat. As a result, the amount of attenuation due to bending increases, making it impossible to transmit optical signals and rendering the fiber useless as an optical fiber.

On the other hand, in the optical fiber of this embodiment shown in FIG. 13, the wall surface of the core 1 is mirror-like, so total reflection occurs regardless of the incident angle, and the attenuation due to bending is almost zero. This enables installation in winding conditions. It also has the advantage of being almost unaffected by stress and strain. This is due to total internal reflection based on the refractive index of the core itself and MT
This is because the reflection of the mirror effect by L exerts a synergistic effect.

In the above five embodiments, examples were shown in which a single metal thin film layer (MTL) was formed, but the invention is not limited to this. Al) with a thickness of 20 μm
Next, copper (Cu), which is compatible with solder, is formed outside the Al layer as an MTL with a thickness of 20 μm by an ion plating method. By doing this, the reflectivity of the core is improved (improved propagation characteristics) and the solder connection accuracy is improved (simplified connection work).
This can be achieved at the same time. Further, as described above, the structure may have more than two layers instead of two layers. In addition, metal thin film forming materials include chromium (Cr), nickel (Ni), aluminum (Al), copper (Cu), iron (Fe), and zinc (Zn).
Such. Alternatively, alloys of these metals and other metals may be used.
Furthermore, other metals or conductive materials may be used. The material to be selected depends on the wavelength of the light to be propagated to the core, the band, the length of the transmission path, and the usage of MTL (for example, whether it will be used as a high-frequency signal transmission path, a power supply path, or only for solder connection). However, what is important here is the wavelength, band, and MTL of the light. There is a correlation between the type of material, especially the type of metal thin film material that forms the mirror surface on the core wall, and the amount of attenuation of light propagating within the core, so the material is determined based on the wavelength, band, and transmission distance of the light. However, the thickness must also be determined.

For example, an optical fiber whose MTL is made of copper and has a thickness of 10 μm has better propagation characteristics for far-infrared light than other materials, but the propagation characteristics for infrared to visible light are poor and are significantly attenuated.

Next, an optical fiber breakage detection device which is an embodiment of the present invention will be described with reference to FIG.

FIG. 14 is a block diagram of an optical fiber breakage detection device which is an embodiment of the present invention.

In the figure, 1 is a core with a core diameter of 50 μm, 5 is an MTL with a thickness of 20 μm, 4 is a cover made of silicon, 7 is a broken part, and 8
Reference numeral denotes a high-output laser beam oscillator (hereinafter referred to as a laser), 9 a power supply section for oscillating the laser 8, 10 a control section for cutting off power supply from the power supply section 9, 11 a battery, and 12 a resistor.

Next, the operation of this device will be explained.

The laser 8 oscillates when power is supplied from the power supply section 9, and sends laser light into the core 1 in the direction of the arrow. At this time, a load was applied to the middle of core 1, which had not broken so much, and
Assume that the core 1 is broken and a fractured portion 7 is generated. At this time,
Since the MTL 5 provided on the outer periphery of the core 1 is also thin, it breaks together. Therefore, the electrical closed circuit previously formed by the battery 11, resistor 12, and MTL 5 is interrupted by the breakage portion 7 and becomes an open circuit, so that no current flows. The control section 10 detects this change in current and immediately cuts off the power supply from the power supply section 9 to stop the oscillation of the laser 8, thereby preventing a disaster caused by a breakage of the optical cable.

As described above, according to this embodiment, it is possible to easily detect a break in the core, which is invisible from the outside, and therefore it is possible to easily and accurately detect whether or not the optical fiber is broken.

Next, an optical communication device using an optical fiber, which is an embodiment of the present invention, will be explained with reference to FIG.

FIG. 15 is a block diagram of an optical communication device using an optical fiber, which is an embodiment of the present invention.

In the figure, 1 is a core made of quartz-based glass fiber, 4 is a cover made of silicon, 5 is an MTL made of nickel with a thickness of 50 μm, and 6b is an outer conductor made of copper wire knitted in a net shape. 13 is a semiconductor laser oscillator, 14 is a modulator, 1
5 is a light receiving element, 16 is a demodulator, 17 is a high frequency transmitter, 1
8 is a receiver. Note that the MTL 5 and the outer conductor 6b constitute a coaxial cable with a high frequency impedance of 75Ω. In addition, semiconductor laser oscillators (GaAs LEDs)
)13 and MTL5 are connected by solder.

Next, the operation of this device will be explained.

The light emitted from the semiconductor laser 13 is modulated by the modulator 14, and is propagated to the light receiving element 15 while being repeatedly totally reflected on the wall surface of the core 1. The light receiving element 15 converts this light into an electrical signal and sends it to the demodulator 16 for demodulation and reproduction into the original signal. In this way, optical communication can be performed via the core 1. Also, from the transmitter 17
Center frequency 350MH sent to L5 and outer conductor 6b
The high frequency signal of
8 and demodulated.

At this time, since the frequency of the high-frequency signal is high, the skin effect is strong,
Communication is possible with thin film MTL5.

In this way, one optical fiber cable can perform both optical communication and high frequency communication completely independently, which is extremely economical. Also, optical communication is normally used,
High frequency communication may be performed when the optical communication device fails. Since the MTL 5 and the external conductor 6b are insulated from each other, the DC current is supplied to the light receiving element 15, the demodulator 16, or the relay station optical amplifier (not shown), etc. It can also be used as a power supply line.

In this example, an example using a coaxial optical cable is shown, but the invention is not limited to this. Two or more optical fibers shown in Figs. 3 and 4 are insulated from each other as shown in Fig. 21. They may be surrounded by a cover 4 and connected to a power source 11a to supply power, or may be used as high frequency communication lines. In addition, one side of the coaxial cable may be connected via ground instead of being coaxial. Furthermore, in the above embodiment, the outer conductor of the coaxial cable was constructed using a reticulated copper wire, but it is not limited to this. It may also be formed by

Next, an optical fiber connection method according to an embodiment of the present invention will be explained with reference to FIGS. 16 to 18.

16 to 18 are connection process diagrams of an optical fiber connection method according to an embodiment of the present invention.

In the figure, 1a and 1b are cores to be connected, 5a and 5b are two-layer MTLs consisting of an aluminum thin film with a thickness of 10 μm on the inside and a thin copper film with a thickness of 20 μm on the outside, 4 is a cover made of nylon, and 19 is a It's solder.

First, as shown in FIG. 16, the cut surfaces of the cores 1a and 1b are mirror-cut using a cutter (not shown) in order to reduce connection loss. Next, insert both cores 1a into the V groove of the jig (not shown).
1b is fixed as shown in FIG. 17 by moving both cores 1a and 1b as shown by arrows in FIG. 16 so that their axes coincide. Next, solder 19 from the outer periphery of MTL5a, 5b.
Both MTLs 5a and 5b are glued together using

By doing so, the connection can be made without requiring equipment such as arc discharge, and both cores 1a and 1b can be easily separated by melting the solder 19 if necessary. Also, since MTLs 5a and 5b are of a two-layer type, the aluminum thin film becomes a mirror surface for the cores 1a and 1b, and the solder 1
9, the outer copper thin film is exposed, allowing for easy and reliable solder connection. Also, both MTLs 5a and 5b are soldered 1
Since the optical fibers connected in this way are also electrically connected by 9, the presence or absence of breakage of the optical fibers connected in this manner can be easily detected by the breakage detection device of the embodiment described above.

In addition, the heat associated with light transmission loss due to the connection point is MTL5a
, 5b and the solder 19, and is radiated and transmitted to the outside. However, if this is not sufficient, a cooler (not shown), for example the 11th
You can connect a cooler as shown in the figure.

Next, an embodiment of the optical fiber manufacturing method of the present invention will be described using FIG. 19.

FIG. 19 is a manufacturing process diagram showing an embodiment of the optical fiber manufacturing method of the present invention.

In the figure, 1 is a core made of a quartz fiber with a core diameter of 50 μm, 5 is an MTL made of a nickel vapor-deposited thin film, 20 is molten nickel, 22 is a heater, and 21 is nickel vapor. These vapor deposition devices are arranged in a large vacuum container (not shown).

First, a current is applied to the heater 22 to melt the nickel and create the molten nickel 20. Nickel vapor 21 is evaporated from this molten nickel. At this time, by moving the core 1 from right to left in the figure while rotating it in the direction of the arrow, a nickel vapor-deposited thin film (MTL) is uniformly distributed over the entire outer circumference of the core 1.
)5 can be formed. Furthermore, although this embodiment shows an example in which the core 1 is rotated for vapor deposition, the present invention is not limited to this, and the apparatus or both may be rotated in opposite directions.

Moreover, FIG. 20 is a manufacturing process diagram showing another embodiment of the optical fiber manufacturing method of the present invention.

In the figure, 23 and 24 are filaments for electron beam generation, 25 and 2
6 is an ingot made of aluminum alloy, 27 is a vacuum pump, 28 is aluminum alloy molecules discharged from the ingots 25 and 26, and 29 is a sputtering device. Although two sets of filaments 23, 24 and ingots 25, 26 are shown in FIG. 20, four sets including filaments 23, 24 and ingots 25, 26 are arranged at approximately equal intervals, that is, every 90 degrees, in the sputtering device 29. filaments and ingots are arranged. Therefore, a substantially uniform aluminum alloy thin film (MTL) 5 is formed on the entire outer periphery of the core 1 passing through the sputtering device 29.

The core 1 is now slowly moved 1 from top to bottom and wound up on a winding bobile (not shown) provided at the bottom. At this time, since the moving direction of core 1 coincides with the direction of gravity, core 1
Since no unreasonable stress is applied to the optical fiber, an optical fiber with less distortion can be obtained.

The sputtering device 29, a supply bobbin (not shown) for supplying the core 1, and a take-up bobbin are further housed in a large vacuum container (not shown).

By doing this, the core 1 does not need to be rotated as in the previous embodiment, and the MTL 5 is uniform over the entire outer circumference of the core 1.
It has excellent mass productivity and uniformity.

In addition, in the above example, the vapor deposition method and the sputtering method were given as examples, but the method is not limited to these, and electroless plating method,
The metal thin film may be formed by a plating method, an ion plating method, a spraying method, or the like.

Further, although quartz fiber is used for the core, plastic fibers made of polyethylene, polypropylene, acrylic, etc. can also be used. Further, in order to form a multilayer metal thin film layer, the ingots and the like of the above-described apparatus may be changed and the same steps may be continuously performed.

Furthermore, the optical fiber thus obtained can be applied to optical computers and optical ICs. It is also effective for gastroscopy.

As described above, according to the present invention, the following effects can be achieved.

(1) According to the optical fiber of the present invention, propagation characteristics can be greatly improved, bending loss can be reduced, connection with connected devices can be extremely easily made, and disconnection can be easily performed when necessary. can.

(2) According to the optical fiber breakage detection device of the present invention, the presence or absence of breakage in the middle of an optical cable can be detected very easily and simply, and disasters and the like can be prevented.

(3) According to the optical communication device of the present invention, optical communication and electrical communication, or optical communication and power transmission, etc. can be performed simultaneously and independently with a single optical fiber, thereby achieving effective use of optical fiber cables. be able to.

(4) By using the optical fiber connection method of the present invention, optical fibers can be directly connected to light emitting diodes, semiconductor laser elements, light receiving elements, etc., and optical fibers can be connected extremely easily and reliably with a single soldering iron. be able to. Furthermore, according to the connection method of the present invention, the heat generated by the connection loss generated at the core connection point can be quickly transmitted to the outside, so that the life of the optical fiber can be extended.

(5) According to the optical fiber manufacturing method of the present invention, a thin metal film can be uniformly formed on the entire outer periphery of the core or cladding, facilitating mass production. Further, multilayering of metal thin film layers becomes easy.

In the manufacturing method shown in FIG. 20, an example is shown in which the core 1 is moved from the top to the bottom, but it may be configured to be moved from the bottom to the top. In addition, MTL5 is added to the outer peripheral surface of core 1.
An example was shown in which MTL is formed on the outer peripheral surface of the cladding.
5 may be formed as shown in the above embodiment. In addition, the core and conductor are external CVD method and internal CVD method.
D method (MCVD method), and V method, which is widely used in Japan.
Glass fibers formed by any of the AD methods may be used, and even plastic fibers may be used.

[Brief explanation of drawings]

Figure 1 is a partial cross-sectional view of a conventional optical fiber taken from the vertical direction.
FIG. 2 is a cross-sectional view of the optical fiber, FIG. 3 is a vertical cross-sectional view of an optical fiber according to an embodiment of the present invention, FIG. 4 is a cross-sectional view of the optical fiber, and FIG.
7 and 9 are longitudinal cross-sectional views of optical fibers according to other embodiments of the present invention, FIGS. 6, 8, and 10 are cross-sectional views of the optical fibers, and FIG. 11 is a cross-sectional view of optical fibers according to other embodiments of the present invention. FIG. 12 is a cross-sectional view showing the light propagation characteristics of the conventional optical fiber, FIG. 13 is a cross-sectional view showing the light propagation characteristics of the optical fiber of the present invention, and FIG. 14 15 is a block diagram of an optical fiber breakage detection device which is an embodiment of the present invention, FIG. 15 is a block diagram of an optical communication device which is an embodiment of the present invention, and FIG.
Fig. 1 is a block diagram of the main parts showing another example of the same device, Figs. 16, 17, and 18 are connection process diagrams showing an embodiment of the optical fiber connection method of the present invention, and Fig. 19 is a diagram of the main parts of the device. FIG. 20 is a process diagram showing one embodiment of the optical fiber manufacturing method of the invention, and FIG. 20 is a process diagram showing another embodiment of the optical fiber manufacturing method of the invention. 1.1a, 1b------core, 2------------clad, 3---------
−−−−−−−− Core wire, 4−−−−−−−−−−−− Cover (sheathing), 5, 5
a, 51, 6---MTL (metal thin film layer), 6---
-------------Outer conductor, 6a------
--------Cooler, 7---------Broken part, 8---------High power laser beam generator (laser), 9 −−−−−−−−−−−−−Power supply section,
10---------Control unit, 11---Battery, 12------Resistance, 13------
- semiconductor laser oscillator, 14------- modulator,
15-----Photo-receiving element, 16-----Demodulator, 17--
-High frequency transmitter, 18------- Receiver, 19-
--- Solder, 20 --- Melted nickel, 21 ---
Nickel steam, 22------ heater, 23, 24---
--Filament, 25, 26-----Ingot.

Claims (5)

[Claims] (1) An optical fiber characterized in that a metal thin film layer is provided on the outer periphery of the core or the outer periphery of the clad through which light propagates. (2) After an optical fiber with a metal thin film layer provided on the outer periphery or cladding of the core through which light propagates is brought into contact with a metal part of a connected device, the metal thin film layer of the optical fiber and the metal part of the connected device are brought into contact. An optical fiber connection method characterized by soldering and connecting. (3) A method for producing an optical fiber, which comprises forming a metal thin film layer on the outer periphery of a translucent glass fiber or plastic fiber using a sputtering method, a vapor deposition method, or a plating method. (4) Measuring the resistance value between the metal thin film layer at one end of an optical fiber having a metal thin film layer on the outer periphery of the core or the outer periphery of the cladding through which light propagates and the metal thin film layer at the other end of the optical fiber. An optical fiber breakage detection device, characterized in that it is configured to detect the presence or absence of breakage of the optical fiber. (5) An optical cable having a metal thin film layer on the outer periphery of the core or the outer periphery of the clad through which light propagates, a light emitting part for applying light from one end of the optical cable, and a light emitting part for receiving the light at the other end of the optical cable. 1. An optical communication device comprising: a light receiving section; and configured to transmit signals or supply power via a metal thin film layer of the optical cable.