JP5188837B2 - Optical pulse speed control device and optical pulse speed control method - Google Patents

Description

  The present invention relates to an optical pulse speed control device and an optical pulse speed control method for controlling an optical pulse speed in an optical transmission system.

  In an optical transmission network, the transmission capacity per fiber is increased by WDM transmission. In a router where these optical fibers are terminated, transmission data is processed from light to electricity. Therefore, the optical-electric conversion processing and the electric signal processing of transmission data in the router are bottlenecks. Accordingly, devices that perform processing without converting optical signals into electricity, such as OADMs (Optical Add / Drop Multiplexers) and optical switches, have been used.

  However, these techniques remain in path control based on wavelength or static switching processing, and cannot perform flexible control such as routing for each packet, which is a unit of transmission data. In particular, since the propagation speed control of optical pulses cannot be flexibly performed, signal collision in the signal processing unit is a problem. In order to avoid this, an optical buffer for holding the optical signal without converting it into electricity is required.

  In recent years, a slow light phenomenon in which the propagation speed of an optical pulse changes has been studied for the realization of this optical buffer. Light pulse propagation speed control is realized by changing the group refractive index of the medium through which light propagates. Light pulse speed control when using a phenomenon called stimulated Brillouin scattering is used to control the signal light to be controlled. Separately, the group refractive index is controlled by making light called pump light incident on the medium.

Kwang Yong Song et al, "Observation of pulse delaying and advancement in optical fibers using stimulated Brillouin" Optics Express, Vol.13, Issue 1, pp.82-88 (January 2005) T. Yamamoto et a1, "Third- and fourth- order active dispersion compensation with a phase modulator in a terabit-per-second optical time-division multiplexed transmission", Optics Letters, Vo1. 26, Issue 9, pp.647-649

  However, in the group refractive index change technology using Brillouin scattering, it is difficult to generate delay while suppressing time waveform distortion and frequency spectrum distortion for signal light of several tens of GHz or more due to the problem of bandwidth. It has become.

  The present invention has been made in view of the above problems, and an optical pulse speed control device and optical pulse speed that generate delay while suppressing time waveform distortion and frequency spectrum distortion even for signal light of several tens of GHz or more. An object is to provide a control method.

An optical pulse velocity control apparatus according to a first invention for solving the above-mentioned problems is as follows.
A linear chirp imparting section for imparting a linear chirp to the incident light pulse;
A phase modulator that is connected to the subsequent stage of the linear chirp applying unit and changes the phase of the optical pulse to which the linear chirp is applied in the linear chirp applying unit;
A chirp compensation unit that is connected to the subsequent stage of the phase modulator and compensates for the linear chirp of the optical pulse given by the linear chirp applying unit;
An optical filter that is connected to the subsequent stage of the chirp compensation unit and restricts the optical pulse to an initial spectral width;
A phase change is applied by the phase modulator to the optical pulse to which the linear chirp is applied by the linear chirp applying unit, and a group refractive index change resulting from the phase change is used to It is characterized by controlling the speed.

An optical pulse speed control device according to a second invention for solving the above-mentioned problems is as follows.
A linear chirp imparting section for imparting a linear chirp to the incident light pulse;
A phase modulator that is connected to the subsequent stage of the linear chirp applying unit and changes the phase of the optical pulse to which the linear chirp is applied in the linear chirp applying unit;
A chirp compensation unit that is connected to the subsequent stage of the phase modulator and compensates for the linear chirp of the optical pulse given by the linear chirp application unit;
A sawtooth signal that gives a sawtooth wave-like phase change to the phase modulator such that the time change rate of the phase change is constant for one slot of the optical pulse to which the linear chirp is given by the linear chirp applying unit. With a generator,
The sawtooth wave-like phase change is given to the optical pulse to which the linear chirp is given by the linear chirp giving unit, and the group refractive index change resulting from the phase change is used. The speed of the light pulse is controlled.

An optical pulse velocity control device according to a third invention for solving the above-mentioned problems is as follows.
In the optical pulse velocity control apparatus according to the first or second invention,
An optical fiber is used for the linear chirp applying section or the chirp compensating section.

An optical pulse speed control device according to a fourth invention for solving the above-mentioned problems is as follows.
In the optical pulse velocity control apparatus according to the first or second invention,
A chirped fiber grating is used for the linear chirp applying section or the chirp compensating section.

An optical pulse speed control device according to a fifth invention for solving the above-mentioned problems is as follows.
In the optical pulse velocity control apparatus according to the first or second invention,
A Virtua 11y Imaged Phased Array (VIPA) is used for the linear chirping unit or the chirp compensation unit.

A speed control method of an optical pulse according to a sixth invention for solving the above-described problem is as follows.
Give a linear chirp to the incident light pulse,
Change the phase of the optical pulse given a linear chirp, and control the speed of the optical pulse using the group index change resulting from the phase change,
Compensating for the linear chirp of the optical pulse given a linear chirp;
The optical pulse is limited to a spectrum width at the beginning of incidence.

An optical pulse speed control method according to a seventh invention for solving the above-described problems is as follows.
Give a linear chirp to the incident light pulse,
For a slot of the optical pulse given a linear chirp, a sawtooth-like phase change is generated such that the rate of time change of the phase change is constant,
Giving the sawtooth wave-like phase change to the optical pulse given a linear chirp, and controlling the speed of the optical pulse using a group refractive index change resulting from the phase change,
Compensating for the linear chirp of the optical pulse given a linear chirp.

  According to the present invention, it is possible to generate a delay while suppressing distortion with respect to a wider-band signal as compared with a conventional optical pulse speed control technique by entering pump light. Also, compared to the conventional optical pulse speed control technology, it is not necessary to generate pump light, eliminating the need for pump light frequency control, a stable and simple structure, improving stability and economy. Also improves.

  Embodiments of an optical pulse speed control device and an optical pulse speed control method according to the present invention will be described with reference to FIGS.

FIG. 1 is a configuration diagram showing an example of an embodiment of an optical pulse velocity control device according to the present invention.
The optical pulse speed control apparatus of this embodiment is provided on an optical fiber 13 that connects a transmitter 14 that transmits signal light (optical pulse) and a receiver 15 that receives the transmitted optical pulse. Specifically, the optical pulse velocity control apparatus according to the present embodiment is configured so that a linear chirp imparting unit 10 that imparts a linear chirp to the optical pulse, and a phase of the optical pulse to which the linear chirp is imparted by the linear chirp imparting unit 10 as a signal. A phase modulator 11 that changes based on a signal given from the generator 12, a chirp compensation unit 17 that compensates for the linear chirp of the optical pulse given by the linear chirp applying unit 10, and a spectrum at which the optical pulse is initially incident The optical filter 18 is limited in width, and the linear chirping unit 10, the phase modulator 11, the chirp compensation unit 17, and the optical filter 18 are connected in this order.

  With the above configuration, the linear chirp applying unit 10 applies a linear chirp to the incident optical pulse, and the phase modulator 11 changes the phase of the optical pulse to which the linear chirp is applied, and the result of the phase change. The speed of the optical pulse is controlled by using the group refractive index change generated from the above. Then, the chirp compensation unit 17 compensates for the linear chirp of the optical pulse to which the linear chirp is given, and the optical filter 18 limits the optical pulse to the initial spectral width to compensate for the spectral change. Such control will be described in detail below.

なお、光パルスのキャリア周波数をω₀、ｔ＝０を光パルスの中心とし、Ａは実定数である。 When the optical pulse passes through the linear chirp applying unit 10, the linear chirp applying unit 10 causes the following change in the instantaneous frequency ω of the optical pulse with respect to the time axis t.

The carrier frequency of the optical pulse is ω ₀ , t = 0 is the center of the optical pulse, and A is a real constant.

  Here, the linear chirp will be described. For example, assuming that an optical pulse is obtained by modulating light at 1550 nm, the optical pulse vibrates at about 193 THz before the linear chirp is applied. When this optical pulse passes through a single mode fiber exhibiting an anomalous dispersion value at 1550 nm, that is, when the optical pulse passes through a medium providing a linear chirp, the instantaneous frequency at the front end of the transmitted optical pulse is greater than 193 THz. The instantaneous frequency at the rear end becomes smaller than 193 THz, and the change changes in proportion to the time axis t of the optical pulse (Equation 1). This is called a linear chirp.

Then, assume that the phase modulator 11 performs the following phase change φ (t), which is a cosine wave having a period R ₀ and an amplitude φ _c .

  Normally, even if this phase modulation is applied to an optical pulse that is not chirped, it does not give a cosine change to the frequency domain, but only changes in phase in the form of a cosine wave over time. However, if this phase modulation is applied to an optical pulse that has undergone linear chirping, the phase change shown in the following (Equation 3) is applied to the frequency domain.

When the time t is deleted from (Expression 1) and (Expression 2), the following expression is obtained.

  This means that a cosine-like phase change is given to the spectrum of the signal, which is equivalent to giving a phase change having frequency dependency to the optical pulse. This technique has been proposed as a dispersion compensation technique (Non-patent Document 2). Also, in the case of a sawtooth wave in Example 2, which will be described later, by changing the cosine wave function to a sawtooth wave function, a sawtooth wave-like phase change can be similarly produced on the frequency axis.

Here, a case where a sine wave is applied in the phase modulator 11 is considered. The frequency characteristic of the phase change φ (t) that occurs at this time is shown in FIG. Further, the refractive index change Δn that the optical pulse receives can be obtained from the following equation using the phase change φ (t). As shown in (Expression 4), the refractive index change Δn is determined by the phase change φ (t), and this phase change φ (t) contributes to the increase in the group refractive index.

Further, the group refractive index change [Delta] n _g of the optical pulse undergoes is determined from the following equation.

That is, as shown in FIG. 2B, the group refractive index change occurs with respect to the optical pulse around the carrier frequency ω ₀ of the optical pulse, and as a result, the optical pulse can be delayed. The group refractive index change characteristic changes depending on the waveform and amplitude of the electric signal applied to the phase modulator 11, and the speed of the optical pulse can be controlled using this characteristic.

  However, in order for the change in the group refractive index to be reflected in the optical pulse waveform, it is necessary to compensate for the linear chirp imparted by the linear chirp imparting unit 10, and further compensate for the signal light spectrum change occurring in the phase modulator 11. There is also a need. Therefore, in this embodiment, the chirp compensation unit 17 and the optical filter 18 are provided in the subsequent stage of the phase modulator 11.

  Here, an optical pulse having an optical pulse width of 25 ps and an optical pulse period of 200 ps is used, an optical fiber having a secondary dispersion value D = 15 ps / nm / km, a length L = 25 km, and a chirp compensation unit 17 are used in the linear chirping unit 10. FIG. 3 to FIG. 7 show simulation results when using an optical fiber having a secondary dispersion value D = −15 ps / nm / km, a length L = 25 km, and a Gaussian filter (FWHM = 23.5 GHz) as the optical filter. Show.

The phase modulator 11 is given a sine wave that gives a modulation index Δθ = π. The period of the sine wave is R = 200 ps. In the phase modulator 11, the modulation index Δθ is obtained from the voltage V _Pi necessary for shifting the phase by π and the amplitude V of the signal to be applied by the following formula.

  FIG. 3A shows an optical pulse incident on the optical pulse speed control device, and FIG. 3B shows an optical pulse after the optical pulse has passed through the linear chirping unit 10. The linear chirp applied by the linear chirp applying unit 10 needs to be set to such an extent that the optical pulse does not overlap with the adjacent optical pulse by the linear chirp application.

FIG. 4A shows the optical pulse waveform after passing through the phase modulator 11 and the phase shift given, and FIG. 4B shows the frequency characteristics of the phase shift given to the phase modulator 11. In FIG. 4B, f ₀ is the carrier frequency of the signal light. Then, from the characteristics of (Equation 4) and (Equation 5) and the characteristic of FIG. 4 (b), the group refractive index change characteristic shown in FIG. 5 is obtained, and the group refractive index increase having a band of 20 GHz or more centering on f ₀ Has been realized. There is a relationship of ω = 2πf.

  FIG. 6 shows an optical pulse waveform after passing through the optical filter 18. In the phase modulator 11, the optical pulse waveform after passing through the optical filter 18 when no phase shift is given is indicated by a broken line. It can be seen that the optical pulse when the phase shift is given is delayed as compared with the case where no phase shift is given in the phase modulator 11. From this result, it was proved that the speed of the light pulse can be controlled. The speed control amount can be controlled by the linear chirp application amount in the linear chirp application unit 10 and the modulation index in the phase modulator 11.

  7A to 7C show the signal light spectrum before entering the optical pulse velocity control device, the signal light spectrum after passing through the phase modulator 11, and the signal light spectrum after passing through the optical filter 18. Although the spectral width increases after passing through the phase modulator 11, it can be returned to the spectral width before entering the optical pulse velocity control device by the optical filter 18.

  A similar effect can be obtained by using a chirped fiber grating or VIPA (Virtua 11y Imaged Phased Array) capable of giving a linear chirp to an optical pulse for the linear chirping unit 10 or the chirp compensation unit 17.

FIG. 8 is a configuration diagram showing another example of the embodiment of the optical pulse velocity control device according to the present invention.
The optical pulse speed control device of this embodiment is also provided on the optical fiber 13 that connects the transmitter 14 that transmits signal light (optical pulse) and the receiver 15 that receives the transmitted optical pulse. Specifically, the optical pulse velocity control device according to the present embodiment includes a linear chirp imparting unit 10 that imparts a linear chirp to the optical pulse, and a phase of the optical pulse to which the linear chirp is imparted by the linear chirp imparting unit 10. A phase modulator 11 that changes based on a sawtooth wave signal given from the wave signal generator 16, and a chirp compensation unit 17 that compensates for the linear chirp of the optical pulse given by the linear chirp applying unit 10, The linear chirp application unit 10, the phase modulator 11, and the chirp compensation unit 17 are connected in this order.

  The sawtooth wave signal generator 16 is connected to the phase modulator 11 and generates a sawtooth wave-like phase change with a constant time change rate of the phase change with respect to one slot of the optical pulse to which the linear chirp is applied. To the phase modulator 11.

  With the above-described configuration, the linear chirp imparting unit 10 imparts a linear chirp to the incident optical pulse, and the phase modulator 11 imparts a sawtooth-like phase change to the optical pulse, resulting from the phase change result. The speed of the light pulse is controlled using the refractive index change. The chirp compensation unit 17 compensates for the linear chirp of the optical pulse to which the linear chirp is given.

In the present embodiment, the point that the linear chirp is applied to the incident optical pulse by the linear chirp imparting unit 10 is the same as in the first embodiment, but the optical pulse to which the linear chirp is applied by the phase modulator 11 A difference is that a sawtooth-like phase change is applied. When a phase change is applied to an optical pulse to which a linear chirp is given, as described in the first embodiment, a phase change having a frequency dependency is given to the optical pulse. However, as in this embodiment, When a sawtooth-like phase change is applied, the group refractive index change can be made to have a flat frequency characteristic. This is because, as can be seen from the above equation (5), since the group refractive index change [Delta] n _g is the derivative of the frequency characteristic of the refractive index, because the frequency characteristic of the group refractive index change becomes flat. Then, the optical pulse can be delayed by using the group refractive index change having such characteristics.

  Here, an optical pulse having an optical pulse width of 25 ps and an optical pulse period of 200 ps is used, an optical fiber having a secondary dispersion value D = 15 ps / nm / km, a length L = 25 km, and a chirp compensation unit 17 are used in the linear chirping unit 10. 9 to 11 show simulation results when an optical fiber having a secondary dispersion value D = −15 ps / nm / km and a length L = 25 km is used.

  The phase modulator 11 is provided with a sawtooth wave that gives a modulation index Δθ = 4π. The sawtooth period R = 200 ps.

FIG. 9A shows an optical pulse waveform after passing through the phase modulator 11 and a given phase shift, and FIG. 9B shows frequency characteristics of the phase shift given to the phase modulator 11. In FIG. 9B, f ₀ is the carrier frequency of signal light. (Equation 4) and (Expression 5), from the characteristics in FIG. 9 (b), to obtain the frequency-independent planar group refractive index change characteristics as shown in FIG. 10, above 40GHz around the f ₀ An increase in the group refractive index with a band can be realized. There is a relationship of ω = 2πf. Further, the light pulse incident on the light pulse velocity control device and the light pulse after passing through the linear chirping unit 10 are the same as those shown in FIGS. 3 (a) and 3 (b).

  FIG. 11 shows an optical pulse waveform after passing through the chirp compensation unit 17. In the phase modulator 11, the optical pulse waveform after passing through the chirp compensation unit 17 when no phase shift is given is indicated by a broken line. Compared with the case where no phase shift is given in the phase modulator 11, the optical pulse when the phase shift is given is delayed, and further, the optical pulse when the phase shift is given compared with the case of FIG. However, it can be seen that there is a delay without distortion. When a sawtooth wave-like phase shift is applied, in the phase modulator 11, the spectrum shape of the signal light does not change, so that it is not necessary to install an optical filter on the subsequent stage side. However, in order for the group refractive index change to be reflected in the optical pulse waveform, it is necessary to compensate for the linear chirp applied by the linear chirp applying unit 10, so a chirp compensating unit 17 is provided after the phase modulator 11. ing.

  A similar effect can be obtained by using a chirped fiber grating or VIPA (Virtua 11y Imaged Phased Array) capable of giving a linear chirp to an optical pulse for the linear chirping unit 10 or the chirp compensation unit 17.

  Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. In addition, various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the above embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different embodiment examples may be appropriately combined.

  The present invention has a function of realizing speed control of an optical signal and can be used as an optical buffer or a variable delay circuit.

It is a block diagram which shows an example (Example 1) of embodiment of the optical pulse rate control apparatus which concerns on this invention. FIG. 2 is a diagram for explaining an example of frequency characteristics of a phase change and a group refractive index change given to a phase modulator in the optical pulse velocity control apparatus shown in FIG. 1. It is a simulation result explaining the pulse waveform which injects into the optical pulse velocity control apparatus shown in FIG. 1, and the pulse waveform after a linear chirp provision part passed. In the optical pulse velocity control apparatus shown in FIG. 1, it is a simulation result explaining the optical pulse waveform after passing through the phase modulator and the frequency characteristic of the phase shift given to the phase modulator. 3 is a simulation result illustrating frequency characteristics of a group refractive index change given in a phase modulator in the optical pulse velocity control apparatus shown in FIG. 1. In the optical pulse speed control apparatus shown in FIG. 1, it is a simulation result explaining the optical pulse waveform after passing through the optical filter. It is a simulation result explaining the signal light spectrum which injects into the optical pulse velocity control apparatus shown in FIG. 1, the signal light spectrum after passing through the phase modulator, and the signal light spectrum after passing through the optical filter. It is a block diagram which shows another example (Example 2) of embodiment of the optical pulse velocity control apparatus which concerns on this invention. FIG. 9 is a simulation result illustrating an optical pulse waveform after passing through the phase modulator and a frequency characteristic of a phase shift given to the phase modulator in the optical pulse velocity control apparatus shown in FIG. 8. FIG. 9 is a simulation result illustrating frequency characteristics of a group refractive index change given in the phase modulator in the optical pulse velocity control apparatus shown in FIG. 8. FIG. 9 is a simulation result illustrating an optical pulse waveform after passing through the chirp compensation unit in the optical pulse velocity control apparatus shown in FIG. 8.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Linear chirp provision part 11 Phase modulator 12 Signal generator 13 Optical fiber 14 Transmitter 15 Receiver 16 Sawtooth wave signal generator 17 Chirp compensation part 18 Optical filter

Claims (7)

A linear chirp imparting section for imparting a linear chirp to the incident light pulse;
A phase modulator that is connected to the subsequent stage of the linear chirp applying unit and changes the phase of the optical pulse to which the linear chirp is applied in the linear chirp applying unit;
A chirp compensation unit that is connected to the subsequent stage of the phase modulator and compensates for the linear chirp of the optical pulse given by the linear chirp applying unit;
An optical filter that is connected to the subsequent stage of the chirp compensation unit and restricts the optical pulse to an initial spectral width;
A phase change is applied by the phase modulator to the optical pulse to which the linear chirp is applied by the linear chirp applying unit, and a group refractive index change resulting from the phase change is used to An optical pulse speed control device characterized by controlling a speed. A linear chirp imparting section for imparting a linear chirp to the incident light pulse;
A phase modulator that is connected to the subsequent stage of the linear chirp applying unit and changes the phase of the optical pulse to which the linear chirp is applied in the linear chirp applying unit;
A chirp compensation unit that is connected to the subsequent stage of the phase modulator and compensates for the linear chirp of the optical pulse given by the linear chirp application unit;
A sawtooth signal that gives a sawtooth wave-like phase change to the phase modulator such that the time change rate of the phase change is constant for one slot of the optical pulse to which the linear chirp is given by the linear chirp applying unit. With a generator,
The sawtooth wave-like phase change is given to the optical pulse to which the linear chirp is given by the linear chirp giving unit, and the group refractive index change resulting from the phase change is used. An optical pulse speed control apparatus for controlling the speed of the optical pulse. In the optical pulse velocity control device according to claim 1 or 2,
An optical pulse velocity control apparatus, wherein an optical fiber is used for the linear chirp applying section or the chirp compensating section. In the optical pulse velocity control device according to claim 1 or 2,
An optical pulse velocity control apparatus using a chirped fiber grating in the linear chirp applying section or the chirp compensating section. In the optical pulse velocity control device according to claim 1 or 2,
An optical pulse velocity control apparatus using a Virtua 11y Imaged Phased Array (VIPA) for the linear chirp applying section or the chirp compensating section. Give a linear chirp to the incident light pulse,
Change the phase of the optical pulse given a linear chirp, and control the speed of the optical pulse using the group index change resulting from the phase change,
Compensating for the linear chirp of the optical pulse given a linear chirp;
A speed control method of an optical pulse, wherein the optical pulse is limited to a spectrum width at an initial incidence. Give a linear chirp to the incident light pulse,
For a slot of the optical pulse given a linear chirp, a sawtooth-like phase change is generated such that the rate of time change of the phase change is constant,
Giving the sawtooth wave-like phase change to the optical pulse given a linear chirp, and controlling the speed of the optical pulse using a group refractive index change resulting from the phase change,
A method for controlling the speed of an optical pulse, comprising compensating for the linear chirp of the optical pulse provided with a linear chirp.

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