JP3727529B2 - Multi-wavelength generator - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a multi-wavelength generator capable of generating optical signals having a large number of wavelengths necessary for wavelength multiplexing transmission.
[0002]
[Prior art]
Conventionally, in an optical transmission system of a wavelength division multiplexing transmission system, a plurality of light sources having a uniform optical output level at a constant wavelength interval are required.
[0003]
As a multi-wavelength generation method for generating a large number of wavelengths at once, a number of methods such as a method using an ultrashort light pulse and a method using a nonlinear effect of an optical fiber have been studied.
[0004]
In these multi-wavelength generation methods, it is necessary to generate high output light in order to generate a pulse train having a pulse width on the order of picoseconds or to obtain a nonlinear effect.
[0005]
In addition, in order to flatten the optical output level of a large number of generated wavelengths, an optical filter having reverse characteristics is inserted.
[0006]
[Problems to be solved by the invention]
However, if a large number of wavelengths are to be generated at a low cost, a simpler method is required, but there is no example using such a simple method in the prior art.
[0007]
In addition, the insertion of an optical filter or the like for flattening the optical output level decreases the output of each generated wavelength, so that the optical output is reduced as much as possible.
[0008]
Accordingly, an object of the present invention is to provide a multi-wavelength generator capable of generating an optical signal having a large number of wavelengths necessary for wavelength division multiplexing with an inexpensive configuration by using a simple combination technique of sinusoidal modulation. There is to do.
[0009]
[Means for Solving the Problems]
The present invention is a multi-wavelength generator that generates optical signals of a plurality of wavelengths from an optical signal of a constant wavelength, and is generated by a single light source that generates an optical signal of a single wavelength, and the single light source. A splitting unit for splitting the single-wavelength optical signal into two to generate split light; an amplitude adjusting unit for creating a modulation signal having sine waves or cosine waves having the same frequency but different amplitudes; Angle modulation means for creating each divided light having a different modulation index by performing angle modulation on each of the divided divided lights using different modulation signals, and each divided light having a different modulation index The multi-wavelength generator is configured by including a multiplexing unit that outputs a large number of optical signals having different wavelengths and having a predetermined output level by combining the polarization planes of the laser beams orthogonally.
[0010]
Here, the multiplexing means may arrange the polarization type optical coupler at a multiplexing position where the polarization planes of the divided lights are orthogonalized and multiplexed.
[0011]
The angle modulation means is constituted by a Mach-Zehnder type phase modulator, and the plane of polarization is rotated 90 degrees on one optical path immediately before the multiplexing position that passes through the Mach-Zehnder type phase modulator and is multiplexed by the multiplexing means. A polarization rotator or a polarization rotator that rotates the polarization plane by 45 degrees may be disposed on both optical paths immediately before the multiplexing position, and the polarization may be multiplexed with the polarization planes of the respective divided lights orthogonal to each other. .
[0012]
The different modulation index values are set in such a combination that the output levels of the respective divided lights to be combined are complemented and the optical signals of the respective wavelengths after the combination obtain a predetermined output level.
[0013]
Combinations of different modulation indices are
[1.45 and 3.0], [1.85 and 4.0], [3.2 and 4.8], [3.4 and 5.2], [4.8 and 6.8], [5.2 and 6.8], [6.8 and 8.6], [8.6 and 10.1], [10.1 and 11.8], [11.5 and 13.1], [13.1 and 14.8], [14.8 and 16.5], [16.5 and 18.1], [18.3 and 19.9], or [19.9 and 21.5] The modulation index may be set in the vicinity of these values.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0015]
[First example]
First, a first embodiment of the present invention will be described with reference to FIGS.
[0016]
(Overview)
First, an outline of the present invention will be described.
[0017]
As a method of combining simple sine wave modulation,
By performing angle modulation (including frequency modulation and phase modulation) on the carrier frequency fc from a single light source using a sine wave or cosine wave signal having a constant frequency fm, the carrier frequency fc of the light source is centered. In addition, a large number of optical signals having different wavelengths n times the frequency fm can be output. However, the output level of the optical signal of each wavelength is represented by the first type n-order Bessel function, and the levels are not uniform.
[0018]
Therefore, in the present invention, first, the output of a light source that emits light at a fixed single wavelength is divided into two, and different modulations are performed using the sine wave or cosine wave modulation signal at the same frequency for each divided light. By performing angle modulation of the index and dividing the angle-modulated split light of different modulation indices by making the planes of polarization of the light orthogonal and matching them as the respective power sums, a large number having different wavelengths and a predetermined output level The optical signal is output.
[0019]
In this case, the value of the modulation index is set to a combination that complements the output level of each divided light to be combined, and that the optical signal of each wavelength after combining obtains a predetermined output level.
[0020]
The reason for dividing into two parts is that when the divided lights are combined, they cannot be combined at an angle such as 45 °, and the planes of polarization need to be orthogonal.
[0021]
In addition, the reason why the polarization planes of the split lights are orthogonal is the sum of power because the polarizations do not interfere with each other when orthogonal, but there is a frequency where the phase state of each electric field is different by π when not orthogonal, This is because it is canceled out and the power sum cannot be constructed.
[0022]
(Concrete example)
Hereinafter, a specific example will be described.
[0023]
FIG. 1 shows a configuration example of a multi-wavelength generator according to the present invention.
[0024]
Reference numeral 1 denotes a light source that emits light at a single wavelength (linearly polarized wave). For example, a signal having a carrier frequency fc = 200 THz is generated.
[0025]
Reference numerals 2 and 3 denote polarization maintaining fibers as polarization type optical fibers connected to the light source 1 and partially divided into two.
[0026]
In this case, as a means for dividing the light into two, it is possible to easily divide the light by arranging a known light separation element such as a 3 dB light splitter or a half mirror on the optical path.
[0027]
Reference numerals 4 and 5 denote LN (= LiNbO ₃ ) angle modulators that perform angle modulation such as frequency modulation and phase modulation. The LN angle modulators 4 and 5 are connected to the polarization maintaining fibers 2 and 3.
[0028]
Reference numeral 6 denotes a polarization type optical coupler having a function of coupling the split light beams transmitted from the polarization maintaining fibers 2 and 3 so that their polarization planes are orthogonal to each other.
[0029]
Reference numeral 10 denotes an oscillator that oscillates at a predetermined frequency fm (for example, fm = 10 GHz).
[0030]
Reference numerals 11 and 12 are amplitude adjusters for adjusting the amplitude of the signal having the frequency fm. The amplitude adjusters 11 and 12 are configured by ordinary amplifiers or the like, and their outputs are input to the angle modulators 4 and 5, respectively.
[0031]
As a result, the LN angle modulators 4 and 5 perform angle modulation on each of the divided light beams using the modulation signals having different amplitudes output from the amplitude adjusters 11 and 12, respectively, and thereby generate the modulation index. The divided lights having different mf are created.
[0032]
The modulation index is normally defined as mf = (maximum deviation of the carrier frequency fc) / (modulation frequency fm). As an example, the modulation index mf = 52 GHz / 10 GHz = 5.2.
[0033]
Then, the angle-modulated split lights having different modulation indexes are coupled to the polarization type optical coupler 6 through the polarization maintaining fibers 2 and 3 with their polarization planes orthogonal to each other. By simply adding the powers of the combined light, a large number of optical signals having different wavelengths and a predetermined output level are output.
[0034]
Next, the operation principle will be described.
[0035]
FIG. 2 is a configuration example when angle modulation is performed on the LN angle modulator 4 side of FIG.
[0036]
FIG. 2A shows a configuration example when the frequency fm = 10 GHz and the modulation index mf = 5.2. FIG. 2B shows calculated values of the optical spectrum characteristics at that time.
[0037]
In the case of mf = 5.2, the optical spectrum has a sparse optical spectrum characteristic in which the optical output of the 0th order (that is, carrier frequency fc) and second order (that is, fc + 2fm, fc-2fm) is suppressed. .
[0038]
FIG. 3 shows a configuration example when angle modulation is performed on the LN angle modulator 4 side of FIG.
[0039]
FIG. 3A shows a configuration example when the frequency fm = 10 GHz and the modulation index mf = 6.8. FIG. 3B shows calculated values of the optical spectrum characteristics at that time.
[0040]
The optical output of the first order (that is, fc + fm, fc−fm) and the third order (that is, fc + 3fm, fc−3fm) is suppressed.
[0041]
Therefore, by combining the powers of the optical signals of both FIG. 2B and FIG. 3B, the output levels of the optical signals are complemented to obtain a flat optical spectrum characteristic of a desired level or higher. It becomes possible.
[0042]
FIG. 4 shows the spectral characteristics (calculated values) of the output light when the combination of the modulation index mf is mf = 5.2 and mf = 6.8.
[0043]
Waves whose output level variation for each wavelength falls within about 3 dB (that is, waves that are present in about half or more of the maximum value in relative intensity, in this example of FIG. 4, the output level is about 0.08 or more in relative intensity) 13 waves (the number of wavelengths) can be employed.
[0044]
The multi-wavelength optical signal output from the polarization optical coupler 6 includes various signals having different wavelengths such as f ₁ = fc ± fm, f ₂ = fc ± 2fm,..., F _n = fc ± nfm. Is output.
[0045]
In this example, the frequency modulation is taken as an example of the angle modulation, but the same operation and effect are obtained in the case of phase modulation.
[0046]
[Second example]
Next, a second embodiment of the present invention will be described with reference to FIG. The description of the same parts as those in the first example described above is omitted, and the same reference numerals are given.
[0047]
In this example, a Mach-Zehnder type phase modulator is used as a means for performing angle modulation, and polarization rotation is performed to rotate the polarization plane by a predetermined angle on one or both optical paths before the combined position of the split light. This is an example when the child 20 is arranged.
[0048]
In FIG. 5, a Mach-Zehnder type LN (= LiNbO ₃ ) modulator 30 widely used in an optical transmission system is used as the angle modulator.
[0049]
This Mach-Zehnder type LN modulator 30 includes an optical waveguide 40 that is partially branched by a bifurcated structure, and angle modulators 15 and 16 provided on each of the branched optical waveguides 40. The
[0050]
A polarization rotator 20 that rotates the plane of polarization by 90 degrees is disposed on one optical path before the multiplexing position on the output side of the angle modulation unit 15 of the Mach-Zehnder LN modulator 30. As this polarization rotator 20, a half-wave plate made of thin film polyimide, a Faraday rotator, or the like can be applied.
[0051]
The separated light having a different modulation index by the angle modulation unit 15 of the Mach-Zehnder type LN modulator 30 and subjected to angle modulation passes through the polarization rotator 20 disposed immediately before the multiplexing position, so that the polarization direction of the polarization plane is changed. be changed. As a result, at the multiplexing position of the two split lights, the lights are multiplexed in a state where the polarization planes of the separated lights are orthogonal to each other.
[0052]
Therefore, in this example, it is not necessary to provide the polarization type optical coupler 6 used in the first example described above at the multiplexing position, and the configuration can be simplified.
[0053]
As the polarization rotator 20, an optical element that rotates the plane of polarization by 45 degrees may be used. In this case, the polarization rotator 20 is disposed on both optical paths of the angle modulation units 15 and 16 before the combined position.
[0054]
[Third example]
Next, a third embodiment of the present invention will be described with reference to FIGS.
[0055]
In this example, an example of combinations of modulation indexes is shown.
[0056]
The combination of the modulation indexes is selected such that when the demultiplexed lights are combined, the optical output levels of each other are complemented, and the optical signals of the respective wavelengths after the multiplexing obtain a predetermined output level.
[0057]
FIG. 6 shows the amplitude characteristics (power) with respect to the modulation index of each order represented by the Bessel function, and is an example of a combination in which the peaks and valleys of the amplitude can be compensated.
[0058]
FIG. 7 shows the spectral characteristics of the optical signal of each wavelength output from the multi-wavelength generator when modulated with the combination shown in FIG.
[0059]
FIG. 7A shows a combination example of the modulation index mf = 1.85 and 4.0. Seven waves can be adopted as the number of wavelengths within a variation of about 3 dB.
[0060]
Similarly, in the case of the combination of mf = 3.2 and 4.8 in FIG. 7B, 9 waves, and in the case of the combination of mf = 8.6 and 10.1 in FIG. 7C, 19 waves. In the case of the combination of mf = 11.5 and 13.1 in FIG. 7D, 27 waves can be employed.
[0061]
Other examples of combinations of modulation index mf include
[1.45 and 3.0], [3.4 and 5.2], [4.8 and 6.8], [5.2 and 6.8], [6.8 and 8.6], [10.1 and 11.8], [13.1 and 14.8], [14.8 and 16.5], [16.5 and 18.1], [18.3 and 19.9], Alternatively, [19.9 and 21.5] is an appropriate combination example, and by setting a modulation index in the vicinity of these values, it is possible to complement each other and make output levels equal to or higher than a predetermined value.
[0062]
【The invention's effect】
As described above, according to the present invention, the output of a light source that emits light at a single wavelength is divided into two, and a sine wave or cosine wave modulation signal having the same frequency is used for each divided light. Since angle modulation with different modulation indexes is performed and the polarization planes of both lights with different modulation indexes are made orthogonal to each other, it becomes possible to generate a large number of wavelengths from one light source. An optical filter for flattening the spectrum is no longer necessary, and it is possible to increase the number of wavelengths by making the best use of the output of the original light source. In addition to being able to output optical signals having a large number of wavelengths while maintaining the output level, it is possible to realize a low-cost multi-wavelength generator with a simple configuration that combines sinusoidal modulation.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration of a multi-wavelength generator according to a first embodiment of the present invention.
FIG. 2A is a configuration diagram for explaining a process of angle modulation by a simple sine wave (modulation index mf = 5.2), and FIG. 2B is a characteristic diagram showing optical spectrum characteristics;
FIG. 3A is a configuration diagram for explaining a process of angle modulation by a simple sine wave (modulation index mf = 6.8), and FIG. 3B is a characteristic diagram showing optical spectrum characteristics.
FIG. 4 is a characteristic diagram showing a spectrum of combined light with a modulation index mf = 5.2 and 6.8.
FIG. 5 is a block diagram showing a configuration of a multi-wavelength generator according to a second embodiment of the present invention.
FIG. 6 is an explanatory diagram showing a combination example of modulation indexes, which is the third embodiment of the present invention.
FIG. 7 is a characteristic diagram showing examples of optical spectra respectively corresponding to various combinations of modulation indexes.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Light source 2 Polarization maintaining fiber 3 Polarization maintaining fiber 4 LN angle modulator 5 LN angle modulator 6 Polarization type optical coupler 10 Oscillator 11 Amplitude adjuster 12 Amplitude adjuster 15 Angle modulator 16 Angle modulator 20 Polarization rotation Child 30 Mach-Zehnder type LN modulator 40 Optical waveguide

Claims (5)

A multi-wavelength generator that generates optical signals of a number of wavelengths from an optical signal of a fixed wavelength,
A single light source that generates a single wavelength optical signal;
Splitting means for splitting the optical signal of the single wavelength generated by the single light source into two to generate split light;
Amplitude adjusting means for creating modulated signals consisting of sine waves or cosine waves having the same frequency and different amplitudes;
Angle modulation means for creating each of the divided lights having different modulation indexes by performing angle modulation on each of the divided divided lights using the modulation signals having different amplitudes;
And a multiplexing means for outputting a large number of optical signals having different wavelengths and having a predetermined output level by orthogonally combining the polarization planes of the divided lights having different modulation indexes. Multi-wavelength generator. 2. The multi-wavelength generating apparatus according to claim 1, wherein the multiplexing unit includes a polarization type optical coupler disposed at a multiplexing position where the polarization planes of the divided lights are orthogonalized and multiplexed. The angle modulation means is constituted by a Mach-Zehnder type phase modulator,
A polarization rotator that rotates the plane of polarization by 90 degrees on one optical path immediately before the multiplexing position that passes through the Mach-Zehnder type phase modulator and is multiplexed by the multiplexing means, or just before the multiplexing position Place a polarization rotator that rotates the plane of polarization by 45 degrees on both optical paths,
2. The multi-wavelength generator according to claim 1, wherein the multiplexed light is multiplexed in a state where the polarization planes of the respective divided lights are orthogonal to each other. The modulation index values different from each other are set to a combination that complements the output level of each divided light to be combined, and obtains a predetermined output level for each optical signal after the combination. Item 4. The multiwavelength generator according to any one of Items 1 to 3. The combination of the different modulation indices is
[1.45 and 3.0], [1.85 and 4.0], [3.2 and 4.8], [3.4 and 5.2], [4.8 and 6.8], [5.2 and 6.8], [6.8 and 8.6], [8.6 and 10.1], [10.1 and 11.8], [11.5 and 13.1], [13.1 and 14.8], [14.8 and 16.5], [16.5 and 18.1], [18.3 and 19.9], or [19.9 and 21.5] The multi-wavelength generator according to claim 4, wherein a modulation index is set in the vicinity of these values.

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