JPH11340920A - Linear optical repeater - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a linear optical repeater for wavelength multiplex transmission that is provided with a very small sized optical signal monitor function. SOLUTION: This linear optical repeater is provided with an optical amplifier, a light branching means 2, an optical wave branching means, a light intensity detection means, and a signal processing means, and the optical wave branching means consists of an array guide path grating filter 10. The signal processing means compares a reference value with a detected luminous intensity to monitor number of the detected values in excess of the reference value. Furthermore, the noise power is measured or stored to monitor an optical S/N. Furthermore, this linear optical repeater can controls the gain of the optical amplifier 1 so that the maximum value of each wavelength signal reaches a prescribed level and also can discriminates whether or not the maximum value is more than the reference value to detect deterioration or a fault. The linear optical repeater preferably has a configuration that the array guide path grating filter 10 and the light intensity detection means are integrated on a same printed circuit board.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a linear optical repeater for wavelength division multiplexing transmission required in a lightwave network based on wavelength division multiplexing technology.

[0002]

2. Description of the Related Art In an optical transmission system, an optical signal that has been weakened by propagating through an optical fiber is amplified as it is by a linear optical repeater without being converted into an electric signal, and is output to the next transmission optical fiber. Is done. In such a linear optical repeater, a gain control function of an optical amplifier that always keeps the output power of the optical signal constant so as not to deteriorate the quality of the optical signal is indispensable. Also, in the event that the quality of an optical signal is degraded due to some kind of failure, it is necessary to identify and restore the failed or degraded part, so the linear optical repeater monitors the quality of the optical signal. It must have functions. As a configuration of a linear optical repeater having these two functions, a configuration as shown in the conceptual diagrams of FIGS. 1 (a) and 1 (b) has been proposed.

In the configuration shown in FIG. 1A, an optical signal monitoring function 8 measures the number of channels, the optical power of each signal, the optical SN ratio, and the like.
The gain control function 9 controls the gain of the optical amplifier 1 based on the monitoring information. To realize the monitoring function 8, Otsuka, et a
l, "A High-Performance Optical Spectrum Monitor wit
h High-Speed Measuring Time for WDM Optical Networ
k "(ECOC'97, pp147-150, 1977), it is common to use an optical spectrum monitor for in-apparatus installation. The principle of this optical spectrum monitor is briefly described with reference to FIG. explain.

FIG. 2 is a diagram showing an optical system of an optical spectrum monitor. The n optical signals (n is the number of wavelength multiplexes) propagated by wavelength division multiplexing in the input optical fiber 7 are the first optical signals.
The light enters the diffraction grating 72 through the lens 71. The n wavelength division multiplexed optical signals incident on the diffraction grating 72 are diffracted at different diffraction angles for each wavelength, pass through the second lens 73, and pass through the plane mirror 74.
And is incident on the PD array 75. Accordingly, the n optical signals separated for each wavelength by the diffraction grating 72 are distributed to different positions on the PD array 75 according to the wavelength.

[0005] The PD array 75 comprises a plurality of PD elements.
The power of the optical signal that has reached each position is detected by these PD elements. By fitting data of a plurality of detected powers to a Gaussian distribution, the number of peaks, each peak power, and noise power can be known. Therefore, the number of channels is obtained from the obtained peak number,
The signal light power can be determined from the difference between each peak power and the noise power, and the optical SN ratio of each signal light can be determined from the ratio between each peak power and the noise power.

A method for performing gain control using the monitoring information thus obtained will be described below. Here, as an example, a method of gain control for keeping the average output power per channel output from the optical amplifier constant will be described with reference to FIG. First, the total power output from the optical amplifier 1 is detected by the light receiving element 82 via the optical branching means 2 and 81. At the same time, the number of channels can be detected by the above-mentioned optical spectrum monitor 83. Therefore, the total power / the number of channels is calculated by the divider 84 to obtain the average output power per channel. An error between the average output power and a reference value (corresponding to a desired output power per channel output from the optical amplifier) 85 is detected by the differential amplifier 86, and the gain of the optical amplifier 1 is set so that the error becomes zero. Control.
As a result, the average output power per channel output from the optical amplifier 1 is always kept constant.

Next, the configuration of FIG. 1B will be briefly described. As shown in FIG. 1A, the optical signal monitoring function 8 uses an in-apparatus built-in optical spectrum monitor, but the monitoring of the optical signal and the gain control of the optical amplifier 1 are completely independently performed by the gain control function 9. This differs from the case of FIG. 1 (a).
In such a configuration, in order to perform the above-described gain control in which the average output power is constant, the gain control function 9 may have a circuit for detecting the total power and the number of channels. In this configuration, the gain control of the optical amplifier 1 is not limited by the operation speed (on the order of ms) of the optical spectrum monitor, and has an advantage that high-speed gain control can be performed.

[0008] Two configurations as shown in the conceptual diagrams of FIGS. 1A and 1B have been described as the configurations of the linear optical repeaters conventionally proposed. As a function of monitoring an optical signal in the linear optical repeater, it is necessary to mount an optical spectrum monitor for incorporation in the device as described above. However, according to the above Otsuka et al, the size of the optical spectrum monitor to implement the monitoring function of the optical signal is a 170 × 220 × 67 mm ^3, is very large, a gain control function It is much larger than an optical amplifier. Therefore, when trying to realize a linear optical repeater having a function of monitoring an optical signal, there is a problem that the amount of hardware is remarkably increased, and the size of the device becomes extremely large.

[0009]

SUMMARY OF THE INVENTION An object of the present invention is to provide a linear optical repeater for wavelength division multiplexing transmission having an extremely small optical signal monitoring function.

[0010]

In order to achieve the above object, a linear optical repeater according to the present invention is provided with an optical amplifier for collectively amplifying a plurality of wavelength division multiplexed optical signals, and is disposed downstream of the optical amplifier. Optical branching means for extracting a part of the plurality of amplified optical signals, and optical demultiplexing for demultiplexing the plurality of wavelength division multiplexed optical signals extracted by the optical branching means for optical signals having different wavelengths Means, light intensity detecting means arranged at the subsequent stage of the light demultiplexing means, for individually detecting and outputting the power of the optical signal demultiplexed for each wavelength, and a detection value obtained by the light intensity detecting means And a signal processing means for performing signal processing by using an optical waveguide. The optical demultiplexing means is constituted by an arrayed waveguide grating filter.

In such a linear optical repeater of the present invention, the signal processing means includes a memory for storing a reference value,
A comparator that determines a magnitude between a reference value stored in the memory and a detection value obtained by the light intensity detection unit;
And a counter for counting the number of the detected values exceeding a reference value.

Further, in the linear optical repeater of the present invention,
A noise power measuring light intensity detecting means is disposed at a stage subsequent to an output port of the arrayed waveguide grating filter from which signal light is not output, and the signal processing means is configured to detect a light intensity based on a detection value obtained by the noise power measuring light intensity detecting means. An arithmetic processing unit for calculating noise power at each signal wavelength, and subtraction for calculating a difference between a calculated value obtained by the arithmetic processing unit and a detected value of optical signal power at each signal wavelength obtained by the light intensity detecting means. And a logarithmic arithmetic unit for performing a logarithmic operation on a value obtained by the subtractor and a converted value obtained by the arithmetic processing unit.

In the linear optical repeater according to the present invention,
The signal processing means stores a value corresponding to the noise power at each wavelength, a value read from the memory and a detection value of the optical signal power at each signal wavelength obtained by the light intensity detection means. A subtractor for calculating a difference, and a logarithmic calculator for performing a logarithmic operation on a value obtained by the subtractor and a value read from the memory may be provided.

Further, in the linear optical repeater of the present invention,
A monitoring function unit that monitors the optical signal, a maximum value selection unit that selects only the maximum value from the detection values obtained by the light intensity detection unit, and the selected maximum value is set to a predetermined level. A feedback control unit for controlling the gain of the optical amplifier may be provided.

Further, in the linear optical repeater of the present invention,
A monitoring function unit that monitors the optical signal, a maximum value selection unit that selects only the maximum value from the detection values obtained by the light intensity detection unit, and the selected maximum value becomes a predetermined level. A feedback control unit that controls the gain of the optical amplifier, and a determination processing unit that determines whether the selected maximum value is equal to or greater than a reference.

Further, in the linear optical repeater of the present invention,
The array waveguide grating filter and the light intensity detecting means may be integrated on the same substrate.

According to such a linear optical repeater of the present invention, the optical demultiplexing means is composed of an arrayed waveguide grating filter. The arrayed waveguide grating filter is extremely small, having a size of several cm or less. An optical system equivalent to the optical system shown in FIG. 2 can be realized by a single device. Therefore, a monitoring device that is much smaller than the optical spectrum monitor described above can be realized, and a compact linear optical repeater can be realized. Further, since integration with the light intensity detecting means is possible, further miniaturization can be performed, and
Since it is suitable for mass production, the cost can also be reduced.

Further, according to the linear optical repeater of the present invention, the monitoring of the optical signal and the gain control of the optical amplifier can be independently performed by skillfully combining the arrayed waveguide grating filter and the signal processing means. However, it is possible to adopt a configuration in which optical components necessary for both are shared. As a result, the required number of components can be reduced, so that further downsizing, higher reliability, and lower cost can be realized. Further, monitoring of the failure or deterioration of the optical amplifier, which is not provided in the conventional optical spectrum monitor, can be realized with an extremely simple configuration.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention will be described with reference to the drawings. FIG. 4 is a diagram showing the concept of the configuration of the linear optical repeater of the present invention. In the figure, reference numeral 1 denotes an optical amplifier that collectively amplifies a plurality of wavelength-division multiplexed optical signals, 2 denotes an optical branching unit that is disposed downstream of the optical amplifier and extracts a part of the plurality of amplified optical signals, and 3 denotes an optical amplifier. Optical demultiplexing means for demultiplexing a plurality of wavelength division multiplexed optical signals taken out by the demultiplexing means into optical signals having different wavelengths, 4 is disposed downstream of the optical demultiplexing means and demultiplexed for each wavelength. Light intensity detecting means 5 for individually detecting and outputting the power of the optical signal and signal processing means 5 for performing signal processing using the detected value obtained by the light intensity detecting means.

Embodiment 1 Embodiment 1 of the linear optical repeater according to the present invention is an example in which the number of channels is monitored. FIG. 5 is a diagram illustrating the configuration of the first embodiment. For example, eight optical signals that have been wavelength-division multiplexed and propagated in the optical fiber 7 are partially extracted by the optical coupler 2 while being multiplexed, and transmitted to the optical demultiplexing means 3 including the arrayed waveguide grating filter 10. Be guided. The operation principle of the arrayed waveguide grating filter 10 will be described below.

The arrayed waveguide grating filter 10 includes an input waveguide group 11, a first slab waveguide 12, a waveguide array 13 having different waveguide lengths by a fixed value, a second slab waveguide 14,
And an output waveguide group 15. The center of curvature of the first slab waveguide 12 is located at the center end of the waveguide of the input waveguide group 11, and the waveguide array 13 is radially arranged so that its optical axis passes through the center of curvature. The wavelength-multiplexed optical signal incident on a certain input port of the input waveguide group 11 is transmitted to the first slab waveguide.
At 12, the light is spread by diffraction and travels in the same phase in a plurality of waveguides constituting the waveguide array 13.

Since the individual waveguides of the waveguide array 13 are designed so that the waveguide lengths differ by a constant value, the phase of light at the output end of each waveguide after propagating through the waveguide array 13 is It shifts by a fixed amount. The light emitted from the output end of the waveguide array 13 in the second slab waveguide 14 is diffracted in a specific direction that satisfies the same phase condition in consideration of the phase shift. This is exactly the same operation as the optical system using the diffraction grating shown in FIG. 2 described above, and the direction of diffraction depends on the wavelength. Accordingly, since the diffracted light is coupled to different waveguide ends of the output waveguide group 15 for each wavelength, when a wavelength-multiplexed optical signal is input to a certain input port of the input waveguide group 11, it is demultiplexed for each wavelength. From the different output ports of the output waveguide group 15.

The eight optical signals λ ₁ , λ ₂ ,..., Λ ₈ demultiplexed for each wavelength from different output ports by the arrayed waveguide grating filter 10 Each of the optical powers P ₁ , P ₂ ,..., P ₈ enters the light receiving element 21 and is detected by the comparator of the channel number detection circuit 30 of the signal processing means 5.
31 determines whether each of the optical powers P ₁ , P ₂ ,..., P ₈ exceeds the reference value stored in the memory 32, and determines the presence or absence of a signal of each wavelength. Therefore, if the number of optical powers exceeding the reference value is counted by the counter 33, the number of channels can be monitored.

[Embodiment 2] Embodiment 2 of the linear optical repeater of the present invention is an example in which the optical SN ratio is monitored. FIG. 6 shows the second embodiment.
FIG. 3 is a diagram showing the configuration of FIG. This embodiment is different from the first embodiment in that the signal processing part and the arrayed waveguide grating filter 10 are different.
Is to measure the power of the spontaneous emission light (ASE), that is, the ASE power, using the empty output port of the ASE. By measuring the ASE power, each signal light power and optical SN
Monitoring of the ratio can be performed. This will be described below.

The wavelength division multiplexed optical signal in the optical fiber is repeatedly propagated by optical amplification in a linear optical repeater or the like.
At this time, the wavelength-multiplexed light to be propagated includes not only the signal light amplified as shown in FIG. 7 but also the amplified ASE having a wide spectrum width. This ASE becomes noise.
ASE and signal light passed through arrayed waveguide grating filter 10
FIG. 8 schematically shows the relationship.

At the output port O _i (i = 1, 2,..., 8) of the arrayed waveguide grating filter 10, the signal light power of wavelength λ _i and the wavelength λ
_If the ASE power near _i is detected, the wavelength λ _i
The total power P _{i of} light detected at the output port O _i corresponding to P _i is represented by P _i = Psig _i + PASE _i . Here, Psig _i and PASE _i represent the signal light power and the ASE power output from the output port O _{i of the} arrayed waveguide grating filter 10, respectively. Since P _i is obtained by measurement, it is necessary to know the ASE power PASE _i by some method in order to obtain the signal light power Psig _i .

Here, the gain deviation between channels can be ignored.
(The ASE power is almost constant in the signal light area), P
ASE_i= PASE₉Therefore, the empty output port O₉At
ASE Power PASE₉Should be measured. In this case,
P in the subtractor 42 of the signal processing means 5 _i−PASE₉If you calculate
Wavelength λ_iSignal power Psig at_iCan be calculated
Wear. Also, Psig_iCan be calculated, the logarithmic calculator 43
Optical SN ratio (= 10 log (Psig_i/ PASE_i))
can do. Reference numeral 41 denotes an arithmetic processing unit.

This embodiment is an example in the case where the gain deviation between channels can be ignored. However, when the gain deviation between channels cannot be ignored, as shown in FIG.
Two free output ports O of the arrayed waveguide grating filter 10
A method of calculating PASE _i by linear approximation using ₀ and O _{9. As} shown in FIG. 10, the center wavelength interval of the arrayed waveguide grating filter 10 is set to half of the signal light interval, and both sides of each signal light are set. And the average of the ASE powers. Of course, it is not limited to these methods,
Other methods may be used. If the ASE power in each channel can be known in advance, a light-receiving element for measuring the ASE power is unnecessary. As shown in FIG. 11, a memory 44 is provided instead, and the ASE power is stored. Alternatively, a configuration for performing subtraction and logarithmic operation is also possible.

Embodiment 3 Embodiment 3 of the linear optical repeater according to the present invention is a linear optical repeater that realizes both an optical signal monitoring function (channel number monitoring device) 8 and an optical amplifier gain control function 9 in a compact manner. This is an example of the configuration. FIG. 12 is a diagram illustrating the configuration of the third embodiment. This embodiment is different from the first embodiment in that
The difference is that a maximum value selection circuit 51 and a differential amplifier 52 are provided.
Since the optical signal monitoring function 8 is the same as the example of FIG. 5, the gain control function 9 will be described below.

The eight optical signals λ ₁ , λ ₂ ,..., Λ ₈ demultiplexed by the arrayed waveguide grating filter 10 enter the light receiving element 21 and have respective optical powers P ₁ , P ₂ ,. ., P ₈ are detected.
The maximum value selection circuit 51 selects only the maximum value of the detected P ₁ , P ₂ ,..., P ₈ . A difference between the selected maximum value and a reference value (a value corresponding to a desired output power in the optical amplifier) 53 is detected by the differential amplifier 52, and the optical amplifier 1 is controlled so that the difference becomes zero. By adopting this configuration,
The maximum power of the signal light output from the optical amplifier 1 is always kept constant.

When such a gain control method is used, it is not necessary to use monitoring information such as the number of channels for gain control, so that both the optical signal monitoring function 8 and the optical amplifier gain control function 9 can be made independent. . Further, as shown in FIG. 12, a common component (the arrayed waveguide grating filter 10 and the light receiving element 21) among the optical components required by both can be shared, and the number of required optical components is reduced by half. Reducing the number of optical components produces the advantages of miniaturization, high reliability, and low cost.

Fourth Embodiment A linear optical repeater according to a fourth embodiment of the present invention is a linear optical repeater having an optical signal monitoring function (channel number monitoring device) 8, an optical amplifier gain control function 9, and an optical amplifier deterioration failure monitoring function. It is a structural example of an optical repeater. FIG. 13 shows the fourth embodiment.
FIG. 3 is a diagram showing the configuration of FIG. This embodiment differs from the third embodiment in that deterioration or failure of the optical amplifier can be determined.
This function is useful in determining the cause of an optical signal that has deteriorated.

When the optical amplifier has deteriorated or failed, a phenomenon occurs in which the gain of the optical amplifier decreases. Therefore, when a gain control method for keeping the maximum optical power constant is adopted, the maximum power output from the optical amplifier decreases. This means that the maximum value picked up by the maximum value selection circuit decreases. Therefore, if the maximum value is compared with the reference value and it is determined that the optical amplifier has deteriorated or failed when the maximum value is lower than the reference value, the deterioration or failure of the optical amplifier can be monitored. This function can be easily realized by the optical amplifier deterioration failure judging device 61 including the comparator 62 and the memory 63 as shown in FIG.

In all the embodiments described above, the number of multiplexed wavelengths is not controlled, and the number is not limited to eight. Further, if the arrayed waveguide grating filter and the light receiving element are integrated on the same substrate, the size can be further reduced. For example, a photodiode as a light receiving element may be monolithically integrated in a semiconductor array waveguide grating filter, or a photodiode array may be hybrid-integrated on a silica-based optical waveguide circuit. Also,
When integrated, there is an advantage that not only miniaturization but also improvement in mass productivity, cost reduction, high reliability, and the like can be realized.

[0035]

As described above, according to the present invention,
Since an extremely small optical signal monitoring device using an arrayed waveguide grating filter can be realized, a small, highly mass-produced, low-cost, and highly reliable linear optical repeater can be realized. Further, according to the present invention, the array waveguide grating filter and the signal processing means are skillfully combined, and optical components necessary for both optical signal monitoring and optical amplifier gain control are shared,
The number of optical components can be halved, and further miniaturization, cost reduction, and high reliability can be achieved. Further, according to the present invention,
Monitoring of failure or deterioration of the optical amplifier can be easily realized.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a configuration of a conventional linear optical repeater.

FIG. 2 is a diagram showing an optical system of an optical spectrum monitor of a conventional linear optical repeater.

FIG. 3 is a diagram illustrating a configuration of a conventional linear optical repeater of a constant average power control type.

FIG. 4 is a diagram illustrating the concept of the basic configuration of a linear optical repeater with a monitoring function according to the present invention.

FIG. 5 is a diagram showing a configuration of a linear optical repeater with a channel number monitoring function of the present invention.

FIG. 6 is a diagram showing a configuration of a linear optical repeater with an optical SN ratio monitoring function of the present invention.

FIG. 7 is a diagram illustrating a relationship between signal light and ASE.

FIG. 8 shows signal light and A after passing through an arrayed waveguide grating filter.
It is a figure explaining the relation with SE.

FIG. 9 shows signal light and A after passing through an arrayed waveguide grating filter.
It is a figure explaining other relations with SE.

FIG. 10 is a diagram illustrating still another relationship between the signal light after passing through the arrayed waveguide grating filter and the ASE.

FIG. 11 is a diagram showing another configuration of the linear optical repeater with the optical SN ratio monitoring function of the present invention.

FIG. 12 is a diagram showing a configuration of a linear optical repeater sharing optical components of the present invention.

FIG. 13 is a diagram illustrating a configuration of a linear optical repeater with an optical amplifier deterioration failure determination function according to the present invention.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 optical amplifier 2 optical branching means 3 optical demultiplexing means 4 light intensity detecting means 5 signal processing means 6 wavelength multiplexed signal 7 optical fiber 8 monitoring function 9 gain control function 10 arrayed waveguide grating filter 11 input waveguide group 12 first slab Waveguide 13 waveguide array 14 second slab waveguide 15 output waveguide group 21 light receiving element 30 channel number detecting circuit 31 comparator 32 memory 33 counter 41 arithmetic processing unit 42 subtractor 43 logarithmic arithmetic unit 44 memory 51 maximum value selection Circuit 52 Differential amplifier 53 Reference value 61 Optical amplifier deterioration or failure determination device 62 Comparator 63 Memory 71, 73 Lens 72 Diffraction grating 74 Planar mirror 75 PD array 81 Optical branching means 82 Light receiving element 83 Optical spectrum monitor 84 Divider 85 Reference value 86 differential amplifier

Claims (7)

[Claims] 1. A linear optical repeater for amplifying a plurality of optical signals which are wavelength division multiplexed and propagate in one optical fiber, and amplifies the plurality of wavelength division multiplexed optical signals collectively. An optical amplifier, an optical branching unit disposed downstream of the optical amplifier, for extracting a part of the plurality of amplified optical signals, and converting the plurality of wavelength-division-multiplexed optical signals extracted by the optical branching unit into wavelengths. Optical demultiplexing means for demultiplexing for each different optical signal, light intensity detecting means disposed at a subsequent stage of the optical demultiplexing means, for individually detecting and outputting the power of the optical signal demultiplexed for each wavelength, and A linear optical repeater comprising signal processing means for performing signal processing using a detection value obtained by the light intensity detection means, wherein the light demultiplexing means is constituted by an arrayed waveguide grating filter. 2. A signal processing means comprising: a memory for storing a reference value; a comparator for determining a magnitude between the reference value stored in the memory and a detection value obtained by the light intensity detection means; The linear optical repeater according to claim 1, further comprising a counter that counts the number of the detected values exceeding a reference value. 3. A noise power measuring light intensity detecting means is disposed at a stage subsequent to an output port of the arrayed waveguide grating filter from which signal light is not output, and the signal processing means is controlled by the noise power measuring light intensity detecting means. An arithmetic processing unit for calculating the noise power at each signal wavelength from the obtained detection value, and a calculation value obtained by the arithmetic processing unit and a detection value of the optical signal power at each signal wavelength obtained by the light intensity detection unit. 2. The linear device according to claim 1, further comprising a subtractor for calculating a difference, and a logarithmic operation unit for performing a logarithmic operation on a value obtained by the subtractor and a calculation value obtained by the arithmetic processing unit. Optical repeater. 4. A memory in which the signal processing means stores a value corresponding to noise power at each wavelength, a value read from the memory, and an optical signal power at each signal wavelength obtained by the light intensity detecting means. 2. A subtractor for calculating a difference between the detected value and a logarithmic calculator for performing a logarithmic operation on a value obtained by the subtractor and a value read from the memory. A linear optical repeater as described. 5. A monitoring function unit for monitoring an optical signal, a signal processing unit, a maximum value selection unit for selecting only a maximum value from detection values obtained by the light intensity detection unit, and a selected maximum value. 2. The linear optical repeater according to claim 1, further comprising a feedback control unit that controls a gain of the optical amplifier so that a predetermined level is obtained. 6. A monitoring function unit for monitoring an optical signal, a maximum value selection unit for selecting only a maximum value from detection values obtained by the light intensity detection unit, and a selected maximum value being a predetermined value. 2. A feedback control unit for controlling the gain of the optical amplifier so that the level of the optical amplifier is equal to a predetermined level, and a determination processing unit for determining whether the selected maximum value is equal to or more than a reference. Linear optical repeater. 7. The linear optical repeater according to claim 1, wherein said arrayed waveguide grating filter and said light intensity detecting means are integrated on the same substrate. vessel.