JP2007178681A - Multi-wavelength light source - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To eliminate the deterioration in S/N of super-continuum light by reducing the noise component of a multi-wavelength light source used for wavelength multiplex transmission. <P>SOLUTION: Heretofore, when a short pulse light source (10) is formed into multi-wavelengths by using an optical amplifier (11) and a nonlinear optical medium (14), the noise component (ASE light component) generated thus far by the optical amplifier (11) is reduced by using a nonlinear optical effect of an extra-ordinary dispersion optical waveguide (12) interposed between the optical amplifier (11) and the nonlinear optical medium (14). In making a light pulse signal including the noise component incident on the extra-ordinary dispersion optical waveguide (12), the incident peak power thereof is so controlled as to be made greater than the fundamental soliton power and to be less than twice as large as the fundamental soliton power. The fiber length of the extra-ordinary dispersion optical waveguide (12) is so set as to be made longer than a soliton period. The fluctuating component (noise) of the spectrum width of the light power signal is cut off by a band-pass filter (13). <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a multi-wavelength light source used for wavelength multiplexing transmission or the like.

  As communication traffic increases, wavelength division multiplexing is applied to increase transmission capacity in an optical communication system. In order to perform wavelength-division multiplexing (WDM) transmission, light sources corresponding to the number of wavelengths are required. For this reason, the cost is the same as the number of light sources, and precise adjustment of the wavelength of the light sources is required. Therefore, multi-wavelength light sources that collectively generate multi-wavelength light from a single light source have been studied. Multi-wavelength light sources that generate multi-wavelength light using super-continuum (SC) light generation technology have been reported as multi-wavelength light sources (for example, Patent Document 1 and Patent Document 2). FIG. 4 shows a configuration of the conventional multi-wavelength light source and a spectrum diagram (101, 102, 103 in the figure) for explaining the operation of the multi-wavelength light source.

  As shown in FIG. 4, the short pulse light source 30 generates an optical pulse signal composed of a longitudinal mode having a frequency interval equal to the repetition frequency. When this optical pulse signal is amplified by an optical amplifier 31 and is incident on a nonlinear optical fiber (optical fiber having a nonlinear optical effect) 32 with a strong intensity, the optical pulse signal is transmitted through the nonlinear optical fiber 32 while the optical spectrum is mutually amplified. Due to this nonlinear optical effect, new optical spectra are generated on the short wavelength side and the long wavelength side of the optical spectrum of the original optical pulse signal. When the optical pulse signal further propagates through the nonlinear optical fiber 32, new optical spectra are generated on the shorter wavelength side and the longer wavelength side of the generated optical spectrum. By repeating this, while propagating through the nonlinear optical fiber 32, the incident optical spectrum becomes a broadband optical spectrum (103 in FIG. 4). This technology is called supercontinuum light generation technology. By extracting each longitudinal mode of this multi-wavelength light source composed of supercontinuum light with an optical filter such as an arrayed waveguide grating (AWG), continuous light in a single longitudinal mode can be obtained. Used as a light source for wavelength multiplexing transmission.

Japanese Patent Application Laid-Open No. 8-29815 “White Ultrashort Pulse Light Source” Japanese Patent Application No. 2005-201327 “Multi-carrier light source” Japanese Patent Application Laid-Open No. 11-284261 “Low Noise Optical Amplifier”

  The optical amplifier 31 is required to increase the light intensity of the optical pulse signal to the light intensity necessary for causing the nonlinear optical effect in the nonlinear optical fiber 32 and expanding the spectrum width. However, the amplified optical pulse signal and spontaneous emission light (ASE light) amplified with a wide spectral width are mixed in the output of the optical amplifier 31 (104 in FIG. 4). Since the ASE light is randomly generated in the optical amplifier 31 and interferes with the signal light, the S / N (signal-to-noise ratio) of the amplified optical pulse signal is deteriorated. As shown in the spectrum diagram 105 in FIG. 4, the ASE light component is also superimposed on the finally generated supercontinuum light, leading to the degradation of the S / N of the supercontinuum light.

  Therefore, as shown in FIG. 5, a bandpass optical filter 33 is inserted between the optical amplifier 31 and the nonlinear optical fiber 32 to remove the ASE light component outside the spectrum band of the optical pulse signal (patent). Literature 2, Patent Literature 3). By doing so, as shown in the spectrum diagrams 107 and 108 in FIG. 5, the S / N degradation of the supercontinuum light due to the ASE light component outside the spectrum band of the optical pulse signal can be suppressed.

  However, the ASE light component in the spectral band of the optical pulse signal still remains, which limits the S / N of supercontinuum light.

  An object of the present invention is to eliminate the S / N degradation of supercontinuum light due to the ASE light of the optical amplifier by removing the ASE light component of the optical amplifier within the spectrum band of the optical pulse signal. is there.

    FIG. 1 shows a basic configuration of the multi-wavelength light source of the present invention. The multi-wavelength light source of the present invention includes a short pulse light source 10 as an optical pulse signal generating means for generating an optical pulse signal configured in a longitudinal mode with a frequency interval equal to a repetition frequency, and an optical pulse generated from the optical pulse light source 10 Compared to a conventional pixel in a multi-wavelength light source having an optical amplifier 11 that amplifies the optical power of a signal to a predetermined value and a nonlinear optical medium 14 that expands the spectral width of the optical pulse signal amplified by the optical amplifier 11. The optical amplifier 11 is provided with an anomalous dispersion optical waveguide 12 and a bandpass optical filter 13 at the subsequent stage, and the length of the anomalous dispersion optical waveguide 12 and the incident peak power of the optical pulse signal are set as follows: . Thereby, the ASE light of the optical amplifier 11 can be removed, and the S / N of the generated multi-wavelength light can be improved. The principle will be described below.

  In FIG. 1, reference numerals 1 to 4 denote time waveforms of optical pulse signals, reference numerals 6 to 8 denote spectral waveforms (envelopes) corresponding to the optical pulse signals 1 to 4, and reference numeral 9 denotes generated supercontinuum light. The spectrum waveform (envelope) of (multi-wavelength light) is shown. An optical pulse signal 1 generated from the short pulse light source 10 is amplified by an optical amplifier 11 including an intensity noise component due to ASE light, and becomes an optical pulse signal 2. When the optical pulse signal 2 including the intensity noise component is incident on the anomalous dispersion optical waveguide 12, the gain of the optical amplifier 11 is set so that the incident peak power is larger than the basic soliton power and less than twice the basic soliton power. To control. The fiber length of the anomalous dispersion optical waveguide 12 is set to be longer than the soliton period.

Here, the chromatic dispersion is D (s / m ² ), the effective core area of the optical waveguide is A _eff (m ² ), the nonlinear refractive index is n ₂ (m ² / W), and the pulse width of the optical pulse signal (1 / E half-value time width) is T ₀ (sec), the wavelength is λ (m), and the speed of light C (m / sec).
The basic soliton power P ₀ (W) is expressed by the following equation (1).

P ₀ = λ ³ A _eff D / (4π ² Cn ₂ T ₀ ² ) (1)
The basic soliton period Z ₀ (m) is expressed by the following equation (2).

Z ₀ = π ² CT ₀ ² / (λ ² D) (2)

When an optical pulse signal is incident on the anomalous dispersion optical waveguide 12 longer than the soliton period Z _0, chirping by self-phase modulation (SPM) which is a nonlinear optical effect of the anomalous dispersion optical waveguide 12 is performed. ) And chirping due to dispersion are in the opposite direction, so that both cancel each other and a basic soliton is generated. Here, since the optical pulse signal 2 having a peak power larger than the basic soliton power P ₀ is incident on the anomalous dispersion optical waveguide 12, the chirping effect by self-phase modulation becomes larger, and the optical pulse signal 2 is pulse-compressed. As a result, the optical pulse signal 3 is generated, and a spectrum broadening like a spectrum waveform 7 is generated.

  As described above, the intensity fluctuation (ASE light component) of the input optical pulse signal is converted into the fluctuation of the spectrum width by the self-phase modulation (spectrum waveforms 6 and 7). At this time, since the incident peak power is close to the basic soliton power, the central portion of the spectrum converges to a shape close to the basic soliton. Therefore, by setting the transmission band of the bandpass optical filter 13 at the center of this spectrum and cutting off the fluctuation component (noise) of the spectrum width of the optical power signal, as shown in the optical pulse signal 4 and its spectrum 8, The intensity noise component (ASE light component) converted into the fluctuation of the spectrum width can be removed.

At this time, the transmission bandwidth δν of the bandpass optical filter 13 is δν = 0.44 / T ₀ (when the waveform of the optical pulse is a Gaussian distribution).
δν = 0.31 / T ₀ (when optical pulse waveform is set to sec ² )
Is set to a value satisfying the above relationship, the optical pulse output from the bandpass optical filter 13 becomes a transform limit pulse (conversion limit pulse: a pulse having the shortest pulse width obtained from a given spectrum width), and the ASE light The original optical pulse waveform having no components can be effectively restored.

  According to the configuration of the present invention, the noise component due to the ASE light of the optical amplifier 11 is removed as described above, and the S / N of the optical pulse signal after passing through the bandpass optical filter 13 is improved. Therefore, there is no noise component due to the optical amplifier 11 in the optical pulse signal passing through the nonlinear optical medium 14, and the S / N of the generated supercontinuum light is greatly improved as shown in the spectrum 9 of FIG. The individual longitudinal modes are also optical signals that do not contain noise.

  At this time, the peak power of the optical pulse input to the nonlinear optical medium 14 needs to be larger than the peak power of the input optical pulse necessary for the spectral width expansion in the nonlinear optical medium 14. Therefore, the peak power of the optical pulse signal output from the anomalous dispersion optical waveguide 12 is larger than the peak power of the input optical pulse necessary for expanding the spectrum width in the nonlinear optical medium 14, larger than the basic soliton power, and basic soliton. It is necessary to set the gain of the optical amplifier 11 so as to be less than twice the power.

  Furthermore, if the peak power of the optical pulse input to the nonlinear optical medium 14 is too large, the nonlinear optical effect occurs too much, and a noise component due to the nonlinear optical effect is superimposed on the generated supercontinuum light. Therefore, it is necessary to set the peak power of the optical pulse input to the nonlinear optical medium 14 to an appropriate value. For this reason, the light intensity control means 15 is added so that the peak power of the optical pulse input to the nonlinear optical medium 14 is larger than the peak power of the input optical pulse necessary for expanding the spectrum width, and the nonlinear optical effect occurs. It is effective to control the light intensity control means 15 so that the S / N of the supercontinuum light is less than or equal to the peak power at which it deteriorates.

  Note that a technique for removing intensity noise by the anomalous dispersion optical waveguide 12 and the bandpass optical filter 13 is invented in Patent Document 3.

  Further, in order to control the incident peak power and outgoing peak power of the optical pulse signal incident on the anomalous dispersion optical waveguide 12 to the set values as described above, the output optical pulse signal of the optical amplifier 11 or the output of the anomalous dispersion optical waveguide 12 A gain control means (not shown) for monitoring the optical power of the optical pulse signal or the output optical pulse signal of the bandpass optical filter and adjusting the gain of the optical amplifier 11 is preferably connected to the optical amplifier 11.

  As described above, the multi-wavelength light source of the present invention is a conventional optical amplifier and a non-linear optical medium that eliminates noise components due to ASE that have occurred when a short pulse light source is multi-wavelength using an optical amplifier and a non-linear optical medium. Since the non-linear optical effect of the anomalous dispersion optical waveguide interposed between the two is reduced, the noise component due to ASE added by the amplifier is removed with a relatively simple configuration, and a large number of outputs are output. There is an effect that the S / N of the wavelength light can be improved.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings.
(First embodiment)
FIG. 2 shows a first embodiment of the multi-wavelength light source of the present invention. The multi-wavelength light source includes a short pulse light source 20 that generates an optical pulse signal configured in a longitudinal mode with a frequency interval equal to a repetition frequency, an optical fiber amplifier 21 that uses a rare earth-doped optical fiber, and the like, and a fiber length that is an optical pulse. An anomalous dispersion optical fiber 22 having a characteristic that the average chromatic dispersion becomes anomalous dispersion longer than the soliton period for the signal, a bandpass optical filter 23 having a predetermined transmission band including the center wavelength of the optical pulse signal, and an optical pulse The nonlinear optical fiber 24 that expands the spectrum width of the signal is cascaded in this order. Here, the peak power of the optical pulse signal incident on the anomalous dispersion optical fiber 22 is larger than the basic soliton power and less than twice the basic soliton power, and the optical pulse signal is propagated through the non-linear optical fiber 24 at the subsequent stage. The gain of the optical fiber amplifier 21 is set so as to be equal to or higher than the optical peak power necessary for expanding the spectrum width due to the effect.

  Further, since the dispersion value of a normal single mode fiber is about 16 ps / nm / km near a wavelength of 1550 nm, a normal single mode optical fiber can be used as the anomalous dispersion optical fiber 22.

The band-pass optical filter 23 is an optical filter having a transmission characteristic that transmits the spectral band of the optical pulse signal from the short pulse light source 20 and cuts out the fluctuation component (noise) of the spectral width converted from the intensity noise component. For example, a Fabry-Perot etalon filter or the like can be used. The center of the transmission band of the band-pass optical filter 23 is the center wavelength of the optical pulse signal 5, and the transmission bandwidth is 0.44 / T ₀ or 0.31 / T ₀ (T ₀ is the pulse of the optical pulse signal 1). Width), the output light of the bandpass optical filter 23 becomes a transform limit pulse, and an amplified optical pulse signal 4 in which the original optical pulse waveform without the influence of the ASE light component is restored can be obtained.

  The short pulse light source 20 is a pulse using a device using a mode-locked semiconductor laser or a single-wavelength CW light (laser light / carrier wave generated continuously at the same level) with phase dispersion. Applicable devices can be applied.

  As the nonlinear optical fiber 24, various optical fibers can be used, and an optical fiber having normal dispersion characteristics, an optical fiber having anomalous dispersion characteristics, an optical fiber having a dispersion flatness characteristic, and the like can be used. In order to generate the optical fiber, it is preferable to use an optical fiber whose chromatic dispersion decreases from anomalous dispersion to normal dispersion in the longitudinal direction.

  The operation principle of the circuit of the present embodiment has been described in the section [Means for Solving the Problems], and will not be repeated.

  S / N improvement of optical pulse signal by simulation when the center wavelength of optical pulse signal is 1567 nm, pulse width is 5 psec, and single mode fiber (dispersion is 16 ps / nm / km) is used as anomalous dispersion optical fiber 22 As a result of examining the effect, it was confirmed that the S / N of the optical pulse signal after passing through the band-pass optical filter 23 was improved as compared with that immediately before the single mode fiber 22.

(Second Embodiment)
FIG. 3 shows a second embodiment of the multi-wavelength light source of the present invention. In the second embodiment, a variable optical attenuator 25 is installed as a light intensity control means in the preceding stage of the nonlinear optical fiber 24 of the first embodiment. With this variable optical attenuator 25, the optical pulse signal exceeds the optical peak power necessary for expanding the spectrum width due to the nonlinear optical effect while propagating through the subsequent nonlinear optical fiber 24, and the nonlinear optical effect is excessive and super The peak power of the optical pulse signal input to the nonlinear optical fiber 24 can be controlled so that the S / N of the continuum light is less than the peak power at which the S / N is degraded.

(Other embodiments)
In the above, the preferred embodiment of the present invention has been described by way of example. However, the embodiment of the present invention is not limited to the above-described example. Various modifications such as replacement with equivalents, change, addition, increase / decrease in number, and change in shape design are all included in the embodiments of the present invention.

  The multi-wavelength light source of the present invention can be used not only as a WDM (wavelength multiplexing) communication light source but also as a light source for evaluating, calibrating, monitoring and tracking optical components and optical devices for next-generation optical communication. It can be expected to greatly contribute to the development of capacity WDM communication.

It is a figure which shows the structure of the multiwavelength light source of this invention, and the optical spectrum of each area. It is a figure which shows the structure in 1st Embodiment of the multiwavelength light source of this invention, and the optical spectrum of each area. It is a figure which shows the structure in 2nd Embodiment of the multiwavelength light source of this invention, and the optical spectrum of each area. It is a figure which shows the structure of the conventional 1st multiwavelength light source, and the optical spectrum of each area. It is a figure which shows the structure of the conventional 2nd multiple wavelength light source, and the optical spectrum of each area.

Explanation of symbols

1-4 Time waveform of optical pulse signal 6-8 Spectral waveform (envelope) corresponding to optical pulse signal 1-4
9 Spectral waveform (envelope) of generated supercontinuum light (multi-wavelength light)
DESCRIPTION OF SYMBOLS 10 Short pulse light source 11 Optical amplifier 12 Anomalous dispersion optical waveguide 13 Bandpass optical filter 14 Nonlinear optical medium 15 Light intensity control means 20 Short pulse light source 21 Optical fiber amplifier 22 Anomalous dispersion optical fiber 23 Bandpass optical filter 24 Nonlinear optical fiber 25 Variable Optical attenuator 30 Short pulse light source 31 Optical amplifier 32 Non-linear optical fiber 33 Band pass optical filter 101 to 102 Ideal spectrum waveform (envelope)
103 Spectral waveform (envelope) of ideal supercontinuum light (multi-wavelength light)
104, 106, 107 Actual spectrum waveform (envelope)
105, 108 Spectral waveform (envelope) of supercontinuum light (multi-wavelength light) actually generated

Claims (5)

An optical pulse signal generating means for repeatedly generating an optical pulse signal, an optical amplifier for amplifying the optical power of the optical pulse signal generated from the optical pulse signal generating means, and a spectral width of the optical pulse signal amplified by the optical amplifier. In a multi-wavelength light source including an expanding nonlinear optical medium, between the optical amplifier and the nonlinear optical medium,
An anomalous dispersion optical waveguide having a characteristic that the waveguide length is longer than the soliton period for the optical pulse signal and the average chromatic dispersion becomes anomalous dispersion;
A bandpass optical filter having a predetermined transmission band including the center wavelength of the optical pulse signal passing through the anomalous dispersion optical waveguide, and cutting off a fluctuation component (noise component) of the spectral width of the optical pulse signal;
The incident peak power of the optical pulse signal incident on the anomalous dispersion optical waveguide is larger than the basic soliton power for the optical pulse signal in the anomalous dispersion optical waveguide and less than or equal to twice the basic soliton power, and the anomalous dispersion light The gain of the optical amplifier is set so that the peak power of the optical pulse signal output from the waveguide is larger than the peak power of the input optical pulse necessary for the spectral width expansion in the nonlinear optical medium. Multi-wavelength light source. In front of the nonlinear optical medium, the peak power of the optical pulse signal input to the nonlinear optical medium is larger than the peak power of the input optical pulse necessary for expanding the spectral width and is generated due to excessive nonlinear optical effects. 2. The multi-wavelength light source according to claim 1, further comprising a light intensity control means for controlling the S / N of the wavelength light so as not to deteriorate. The transmission band δν of the bandpass optical filter is δν = 0.44 / T ₀ (when the waveform of the optical pulse signal is Gaussian distribution), or δν = 0.31 / T ₀ (the waveform of the optical pulse signal is sec ² The multi-wavelength according to claim 1 or 2, characterized in that it is set to a value satisfying the relationship (where T ₀ is the pulse width of the optical pulse signal generated from the optical pulse signal generating means). light source. The basic soliton power P ₀ (W) and the basic soliton period Z ₀ (m) are the chromatic dispersion of the anomalous dispersion optical waveguide is D (s / m ² ), the effective core area is A _eff (m ² ), The nonlinear refractive index is n ₂ (m ² / W), the pulse width of the optical pulse signal (1 / e half-value time width) is T ₀ (sec), the wavelength is λ (m), and the light velocity C (m / sec). When
P ₀ = λ ³ A _eff D / (4π ² Cn ₂ T ₀ ² )
Z ₀ = π ² CT ₀ ² / (λ ² D)
The multiwavelength light source according to claim 1, wherein the multiwavelength light source is given by: 4. The multi-wavelength according to claim 1, wherein the optical pulse signal generating means is a short pulse light source that generates a short optical pulse composed of a longitudinal mode having a frequency interval equal to a repetition frequency. light source.

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