JPS6361132A - Single mode light testing circuit - Google Patents

Abstract

PURPOSE:To simplify circuit constitution and to enhance mounting efficiency, by making it possible to perform the alignment of monitor light and the emission/reception of pulse light by one four-terminal directional coupler satisfying a predetermined condition. CONSTITUTION:This test circuit is constituted of a four-terminal directional coupler having the first terminal inputting and outputting light signals of wavelengths lambda1, lambda2, the second terminal outputting and inputting the light signals of the wavelengths lambda1, lambda2, the third terminal coupling with the first terminal and the light signal of the wavelength lambda1 and having a signal level monitor apparatus connected thereto and the fourth terminal coupling with the second terminal and the light signal of a wavelength lambda3 and having a light pulse tester connected thereto. Now, when the length of the coupling part of the four-terminal directional coupler is set to (l), the perfect coupling length to an arbitrary wavelength lambda is set to L(lambda) and coupling efficiency is set to eta(lambda), these variables are given by the relation of formula I. The length (lambda) of the coupling part setting the coupling efficiency eta(lambda1) of the wavelength lambda1 to almost 100%, the coupling efficiency eta(lambda2) of the wavelength lambda2 to almost 0% and the coupling efficiency of the wavelength lambda3 to eta(0%<=eta<=100%) is determined by the integer values (m), (n), (p) satisfying the relation of formula II or III.

Description

Detailed Description of the Invention (Technical Field of the Invention) The present invention relates to a single mode optical transmission system for monitoring the power level of a transmitted optical signal in a single mode optical transmission system and searching for a fault in an optical fiber transmission line while the line is in continuity. This relates to optical test circuits.

(Prior art and its problems) FIG. 1 is a diagram for explaining the configuration of an optical transmission system using this type of optical test circuit, with the left side being the station side and the right side being the subscriber side. 1 and 9 are electrical signals (downstream), 2 and 12 are electrical to optical converters, 3 is optical signals (downstream), and 18 is optical signals (
4 and 7 are optical multiplexers/demultiplexers, 5 is an optical test circuit, 6 is an optical fiber transmission line, 8 and 13 are optical-to-electrical converters, 10 is a terminal device, 11 and 14 are electrical signals (upstream), 15 is a signal level monitor device, 16 is an optical pulse tester, and 17 is pulsed light. 0 Electrical signals (downstream) such as telephone, facsimile, image, etc. at the office side are converted to wavelength λ1 by an electric-to-optical converter 2. It is converted into an optical signal (downlink) 3. This optical signal (downlink) 3 passes through an optical multiplexer/demultiplexer 4 and an optical test circuit 5, and is sent to an optical fiber transmission line 6. The optical signal (downstream) 3 propagated through the optical fiber transmission line 6 passes through an optical multiplexer/demultiplexer 7 on the subscriber side, is converted into an electrical signal (downstream) 9 by an optical-to-electrical converter 13, and finally phone.

The information is output from the terminal device 10 as facsimile and image information.

On the other hand, telephone, facsimile, and image information from the subscriber side is first input as an electrical signal (upstream) 11 to an electrical-to-optical signal converter 12, and is converted into an optical signal (upstream) 18 with wavelength λ2.
is converted to The optical signal (upstream) 18 is sent to the optical fiber transmission line 6 via the optical multiplexer/demultiplexer 7. This optical signal (
Up) 18 is an optical test circuit 5 and an optical multiplexer/demultiplexer 4 on the station side.
The electrical signal (upstream) 1 is transmitted by the optical-to-electrical converter 8.
4, and various information from the subscriber side is sent to the station side.

The signal level monitor device 15 is coupled to the optical test circuit 5, and constantly monitors the optical power level of the optical signal 3 with wavelength λ1, and usually sets an optical power level of several percent of the optical signal (downstream) as the monitor level. It takes. The optical pulse tester 16 sends out pulsed light 17 having a wavelength λ to the optical fiber transmission line 6, and constantly searches for a fault point on the transmission line. In this way, the optical test circuit 5 has the functions of monitoring the optical power level of the optical signal (downlink) 3 and branching and adding optical signals for searching for fault points in the optical fiber transmission line, and is useful for maintenance of the optical transmission network and It is an essential component to ensure conversion properties.

FIG. 2 shows a schematic diagram of the configuration of an optical test circuit for two-wavelength bidirectional communication for explaining the basic functions of the optical test circuit 5. Boat A is coupled to the optical multiplexer/demultiplexer 4, boat B is coupled to the optical fiber transmission line 6, boat C is coupled to the signal level monitor device 15, and boat D is coupled to the optical pulse tester 16. The optical test circuit 5 includes an optical branch circuit I for sending a part of the optical power of the optical signal (downstream) 3 to the signal level monitor device 15 and an optical combiner branch circuit for searching for a fault point in the optical fiber transmission line 6. ■
Consists of. Fault point search for optical fiber transmission line is performed using baud)C
The test light pulse from the optical pulse tester 16 is connected to an optical combining/branching circuit including a wavelength selection element such as an interference film filter.
This is possible by transmitting the scattered light into the optical fiber transmission line 6 through the optical fiber transmission line 6 and observing the power distribution of the scattered light along the longitudinal direction of this optical fiber transmission line.

At this time, the wavelength of the pulsed light needs to be set to a different wavelength for the upstream and downstream optical signals in order to search for a fault point without disconnecting the communication line.

Figure 3 shows the intensity distribution of the backscattered light in the longitudinal direction of the optical fiber transmission line observed through the optical combiner/branching circuit ■. In the figure, before the failure occurs, the intensity distribution of the backscattered light is shown by the solid line.
For example, when a failure occurs at point A, the intensity of backscattered light after point A decreases to the level shown by the broken line, making it possible to identify the point of failure. The following is an outline of conventional bulk type and waveguide type optical test circuits that operate according to the principle shown in FIG.

FIG. 4 shows a schematic diagram of a conventional bulk type optical test circuit. 1
9 is an optical fiber collimator with a lens, 20 is a reflective surface for extracting monitor light, 21 is a reflective surface for optical pulse input/output for searching for fault points, and 22 is a glass block. The bulk type optical test circuit shown in FIG. 4 requires complicated operations such as high-precision polishing of the glass block 22, attaching lenses to optical fibers, and forming reflective surfaces 20 and 22 on the polished surface of the glass block, as well as these individual components. Since sophisticated technology is required to fix the components while maintaining their mutual positional relationship with high precision, a decrease in productivity is inevitable, and the optical test circuit itself becomes extremely expensive. Also,
Since the basic configuration is made up of individual elements such as lenses and glass blocks with mutually different refractive indexes, there is a loss due to Fresnel reflection of light when propagating between the individual elements, and a coupling loss between optical fibers due to the configuration with a lens system. is considered to be an essential problem and unavoidable.

Figure 5 shows a conventional waveguide type optical test circuit, where 23 is an optical waveguide;
24 is a reflection plate for extracting a part of the optical signal power, 25 is an interference film filter having a wavelength selection function, 26 is an optical fiber, and 27 is an St substrate. This waveguide optical test circuit is S
After depositing a quartz-based waveguide film on the T-substrate 27, unnecessary parts are removed using photolithography technology to form the leading waveguide 23. Since the technology is suitable for mass production, inexpensive optical circuits can be manufactured. It is.

In addition, in this waveguide type optical test circuit, the optical path between each boat that couples with the optical fiber 26 is determined at the time of forming the optical waveguide, so the complicated and sophisticated optical axis alignment required in the bulk type optical test circuit is not required. As mentioned above, this waveguide type optical test circuit is superior to conventional bulk type optical test circuits in terms of mounting and assembly efficiency.
Guide wave path 2 for inserting interference film filter 25 and reflection plate 24
In principle, radiation loss of light is unavoidable in the gap provided in the waveguide 3 and in the part of the waveguide branch that does not have a waveguide structure. The loss in this gap is approximately 3
dB, about 1 dB for a single mode optical waveguide, and as optical fiber transmission lines become longer, it is necessary to further reduce the loss of optical circuits.

(Object of the Invention) The object of the present invention is to provide an optical signal monitor and fault point search that is easy to assemble and has low loss, solving problems related to the complexity of element mounting work and insertion loss characteristics required in conventional optical test circuit assembly. The objective is to provide an optical test circuit that can be applied to

(Structure and characteristics of the invention) The present invention will be explained in detail below.

The present invention is a single-mode optical test whose main feature is to perform optical path separation into three wavelengths using one four-terminal directional coupler consisting of two adjacent single-mode optical waveguides formed on a substrate. It is a circuit.

In conventional technology, the separation of optical signals and optical pulses with different wavelengths, or the extraction of a portion of the optical signal power, was performed using individual elements such as interference film filters, and the associated complicated work of mounting the elements was unavoidable. According to the present invention, optical paths can be separated for each wavelength using only the directional coupling portion formed on the substrate, so there is no need for element mounting work. Further, it is possible to construct a low-loss optical test circuit without the increase in loss associated with element mounting and the unavoidable loss inherent in conventional optical test circuits.

(Principle of the Invention) FIG. 6 is a diagram for explaining the operating principle of the directional coupling unit that forms the basis of the present invention, and 29 and 30 are single mode optical waveguides. The directional coupler consists of two parallel single-mode optical waveguides 29 and 30 arranged close to each other on the order of a few μI. The light propagating through gradually transfers its energy to the other optical waveguide. The waveguide length required for complete energy transfer is the perfect coupling length L(λ)
The coupling efficiency η(λ) between the two parallel single-mode leading waveguides constituting the directional coupler is the total length of the coupling portion l.

It is determined by the wavelength λ, etc., and is given by the following formula.

For example, two wavelengths λ1. λ2 is single mode optical waveguide 3
0, the demultiplexing conditions for separating these lights are such that the length l of the coupling part is an even or odd multiple of the complete coupling length λI) and L(λ2) for each wavelength. Just set it like this. That is, wavelengths λ1 and λf can be separated by integer values m and n that satisfy the following equation.

1 = (2m+1) ・L(λ +) =2n
−L(λ 2 ) (1) or /=2nL(λz) = (2m+1)L(λI)
(2) For example, in the case of (11), the coupling efficiency η(λ,) at the directional coupling part for light with wavelength λ1 is 100%, and the wavelength λ
The coupling efficiency η(λt) of the light of wavelength λ1 is 0%, the light of wavelength λ1 propagates through the single mode optical waveguide 29 shown in FIG. 6, and the light of wavelength λ2 propagates through the single mode optical waveguide 30.

A design example in which three wavelengths are multiplexed and demultiplexed using one directional coupler will be described below.

FIG. 7 is a cross-sectional view of a directional coupling section made of a silica-based optical waveguide used in the design, in which 31 is a core section, 32 is a cladding section, and 33 is a substrate. In the design example described below, the refractive index n of the core part 31 of the directional coupling part shown in FIG. 7 is 1.46.
4. The refractive index of the cladding portion 32 was 1.460°, and the shape of the core portion was 10 μm×10 μm.

Figure 8 shows the perfect coupling length λ for the two waveguide spacing S.
) is shown using wavelength as a parameter.

In the figure, as a design example, the wavelengths are set to 1, 2 μm, 1.3 μre, and 1.55 μI, which are used practically. For example, when the optical waveguide spacing is set to 2 μm, the complete coupling length is 1.2 pre, approximately 2.9 m, and 1.3 μs.
Approximately 2.4m.

It is 1.8n at 1.55 μm. Thus, it can be seen that the complete bond length shows different values depending on each wavelength.

Figure 9 shows an example of calculating the coupling efficiency η(λ) for the three wavelengths when the coupling length l of the directional coupling part is changed when the optical waveguide spacing S in Figure 7 is set to 2 μm. Some 0
From the figure, since the period of coupling efficiency with respect to the coupling length determined by the complete coupling length L(λ) for each wavelength is different, by selecting the coupling length, the coupling efficiency of the three wavelengths η(λ)
It can be seen that can be arbitrarily set with one directional coupler.

On the other hand, in an optical test circuit, the coupling characteristics of a directional coupler for three wavelengths are 100% for the wavelength for pulse testing, 0% for the optical signal (upper? J), and 0% for the optical signal (upper? For downlink), it is necessary to set it to several percent. In response to such a request, for example, as shown in FIG.
14. When set to hn, the coupling efficiency is 100% for a wavelength of 1.2 μm, the coupling efficiency is 0% for a wavelength of 1.3 μm, and the coupling efficiency is 0% for a wavelength of 1.2 μm.
At 55 μm, the coupling efficiency is 5%.

(Example) Figure 10 shows a first example of the present invention in which the optical waveguide is a quartz-based waveguide manufactured by the fire deposition method and photolithography technology, which has high manufacturing controllability, and the dotted line in the figure indicates an optical waveguide embedded in the cladding. For the four-terminal directional coupler shown in FIG. 10, for example, in the waveguide configuration described above, the wavelength λ of the downstream signal is set to 1.55 μm, the wavelength λ2 of the upstream signal is set to 1.3 μm, and the wavelength of the pulsed light is set to 1.55 μm. If λ3 is set to 1.2 μm, it functions as an optical test circuit. According to this embodiment, since it is possible to multiplex and demultiplex three wavelengths using only the leading wave circuit formed on the board, there is no need for the mounting work required for conventional bulk type optical test circuits, and the reliability of the circuit is improved. Sexuality also improves.

Also, in FIG. 10, the monitor output level of the downstream signal is ! ! , = (2m-1) ・L(λ+)=2nL(λ
z)=p-L(λ3)+-5in-'(fi)L(
λ, )π or 1 = (2m-1) ・L(λυ=2n-L(7g
)=p! ,,(λ3)−−5in−′(fi)L(λ
3) By setting the coupling length l of the directional coupler to the coupling length determined by the integer values m, n, and p that satisfy the relationship π,
It can be arbitrarily selected within the range of ~100%.

In the above embodiments, a quartz-based leading waveguide was described as an example, but Li
Optical waveguides made of NbO5-based or GaAs-based materials are also applicable.

FIG. 11 shows a second embodiment of the present invention,
34 and 35 are optical fibers, 36 is a light emitting element, 37 is a light receiving element, 38 is a monitoring light receiving element, and 39 is an optical multiplexing/demultiplexing circuit section. In this figure, the optical multiplexing/demultiplexing circuit 4 shown in FIG. 1 is simultaneously formed on the same substrate as the optical test circuit, so that the functions can be combined.

(Effects of the Invention) As explained above, conventionally, an optical test circuit was composed of two circuits: an optical signal monitoring branch circuit and a fault point searching branch circuit, whereas the present invention consists of one four-channel branch circuit. The circuit configuration has been greatly simplified because the terminal directional coupler can extract the monitor light and send and receive pulsed light. As a result, it has become possible to significantly improve packaging efficiency compared to conventional circuits that have a structure of aggregating individual elements.

In addition, since the opposing terminals of the optical test circuit are coupled using only one single mode optical waveguide, it is possible to significantly improve loss characteristics and suppress increase in loss due to element mounting compared to conventional ones. It became. Since the optical test circuit manufacturing technology according to the present invention is supported by photolithography technology that is highly suitable for mass production, it greatly contributes to the economical construction of optical communication networks.

[Brief explanation of drawings]

Figure 1 is a block diagram showing a configuration example of an optical transmission system using an optical test circuit, Figure 2 is a configuration diagram of an optical test circuit, Figure 3 is a schematic diagram showing an example of an optical line fault position monitor, and Figure 4 is a conventional FIG. 5 is a perspective view showing an example of a configuration of a conventional waveguide type optical test circuit; FIG. 6 is a directional coupling section configuration for explaining the function of a directional coupler. A schematic diagram showing an example, Fig. 7 is a cross-sectional view of the coupling part of a directional coupler, Fig. 8 is a characteristic diagram showing the relationship between the optical waveguide spacing and perfect coupling length, and Fig. 9 shows the relationship between the coupling length and coupling efficiency. FIG. 10 is a perspective view showing a first embodiment of the present invention, and FIG. 11 is a perspective view showing a second embodiment of the present invention equipped with an optical multiplexing/demultiplexing circuit. DESCRIPTION OF SYMBOLS 1. Electrical signal (downlink), 2... Electro-optical converter, 3... Optical signal (downlink), 4... Optical multiplexer/demultiplexer, 5
... Optical test circuit, 6. Optical fiber transmission line, 7.
... Optical multiplexer/demultiplexer, 8... Optical-electrical converter, 9...
Electrical signal (downstream), 10...Terminal device, 11...
Electrical signal (up), 12... Electrical-optical converter, 13
...Optical-electrical converter, 14... Electrical signal (up)
, 15... Signal level monitor device, 16... Optical pulse tester, 17... Pulse light, 18... Optical signal (up), 19... Optical fiber with lens, 20...
- Reflective surface for monitor light extraction, 21... Reflective surface for optical pulse input/output, 22... Glass block, 23...
Leading waveguide, 24... Reflection plate, 26... Interference film filter, 27... Optical fiber, 2nd... Si substrate,
29... Single mode leading waveguide, 30... Single mode leading waveguide, 31... Core part, 32... Clad part, 33... Substrate, 34... Optical fiber, 35
...Optical fiber, 36...Light emitting element, 37...
・Light-receiving element, 38... Light-receiving element for monitor, 39-
...Optical multiplexing/demultiplexing circuit section.

Claims (1)

[Claims] (1) A first terminal that inputs and outputs optical signals with wavelengths λ_1 and λ_2, a second terminal that outputs and inputs optical signals with wavelengths λ_1 and λ_2, and the first terminal and light with wavelength λ_1. a third terminal to which a signal is coupled and to which a signal level monitor device is connected; and a fourth terminal to which an optical signal of wavelength λ_3 is coupled to the second terminal and to which an optical pulse tester is connected. A single mode optical test circuit consisting of a 4-terminal directional coupler, where the coupling length of the 4-terminal directional coupler is l, the complete coupling length for an arbitrary wavelength λ is L(λ), and the 4-terminal directional coupler is When the coupling efficiency of the device is η(λ), these variables are η(λ
)=sin^2(πl/2L(λ)), and in this case, the coupling efficiency η(λ_1) for the wavelength λ_1 is approximately 100%, the coupling efficiency η(λ_2) for the wavelength λ_2 is approximately 0%, Wavelength λ_3
The condition for the coupling length l that makes the coupling efficiency η (0%≦η≦100%) is l=(2m-1)・L(λ_1)=2n・L(λ_2)
=p・L(λ_3)+[2/π]−sin＾−＾1(√
η) L(λ_3) or l=(2m-1)・L(λ_1)=2n・L(λ_2)
=p・L(λ_3)−[2/π]−sin＾−＾1(√
Integer values m, n, p that satisfy the relationship η)L(λ_3)
A single mode optical test circuit characterized in that the coupling length l is determined by .