JP6248031B2 - Optical amplifier, optical amplification system, wavelength converter, optical amplification method, and optical communication system - Google Patents

Description

  The present invention relates to an optical amplifier, an optical amplification system, a wavelength converter, an optical amplification method, and an optical communication system using the same.

  In optical communications, optical amplifiers are indispensable. In the current optical communication system, an EDFA (Erbium-Doped Fiber Amplifier), a Raman amplifier, or a Raman amplification system is put to practical use as an optical amplifier or an optical amplification system in the optical communication band.

  On the other hand, an optical parametric amplifier (OPA) that utilizes a nonlinear effect in an optical fiber for optical amplification as disclosed in Patent Document 1 has not been put into practical use. Major factors that are not practical are the narrow amplification band and the non-flat gain spectrum. OPA is also used as a wavelength converter. Further, as an optical amplifier using a nonlinear effect in an optical fiber, there is a phase sensitive optical amplifier (PSA).

JP 2008-89781 A International Application PCT / JP2012 / 055596

  As a method for flattening the gain spectrum of OPA, there is a method using two pump lights. Here, the wavelengths of the two pump lights are set to both the short wavelength side and the long wavelength side, respectively, which are substantially symmetrical wavelengths around the zero dispersion wavelength of the optical fiber to be amplified. However, the configuration using two pump lights may not be a practical configuration because the cost increases. Therefore, although this specification mainly discusses OPA using only one pump light, the present invention is not limited to this.

  The typical gain spectrum of an OPA that uses only one pump beam has the smallest gain at the pump beam wavelength, and maximum values for both the long wavelength side and the short wavelength side that are several to tens of nm away from the pump beam wavelength. It is a shape with Unlike the gain spectrum of EDFA or Raman amplifier, the flatness of the gain spectrum of OPA as described above is low and is not practical.

  In order to make OPA or PSA a practical optical amplifier, it is preferable to have flatness of the gain spectrum such that the difference between the maximum gain and the minimum gain in the wavelength band to be amplified is within 1 dB.

  In the patent document 2, the inventor of the present invention, in order to realize the flatness and wide bandwidth of the gain spectrum, the signal light and the pump light are input and one or more relative phase shifters are inserted. An optical amplifier comprising an optical fiber is disclosed.

  The present invention has been made in view of the above, and provides an optical amplifier, an optical amplification system, a wavelength converter, an optical amplification method, and an optical communication system that can more suitably realize gain spectrum flatness and broadband characteristics. The purpose is to do.

In order to solve the above-described problems and achieve the object, an optical amplifier according to the present invention includes an amplification optical fiber and pump light for amplifying signal light incident on the amplification optical fiber. An optical amplifier having a pump light source for supplying to an optical fiber, wherein the amplification optical fiber is divided into a plurality of amplification stages by a relative phase shifter that changes a relative phase, and a front stage side of the relative phase shifter And a nonlinear constant γ and length L of the amplification optical fiber of the amplification stage and the amplification of the amplification stage so that the relative phase regarding the light output from the amplification optical fiber of the amplification stage connected to The product γPL with the input power P of the pump light to the optical fiber is set, and the relative phase shifter changes the relative phase to a value smaller than 0.5π and outputs it to the amplification stage connected to the rear stage side You It is characterized in.
Here, the relative phase φrel is Δk = ksignal defined by the signal light phase φsignal, the idler light phase φidler, the pump light phase φpump, the signal light wavenumber ksignal, the idler wavenumber kidler, and the pumplight wavenumber kpump. + kidler-2kpump is an amount described by the following equation using the propagation distance z of each light in the amplification optical fiber .
φrel = Δk z + φsignal + φidler-2φpump

  The optical amplifier according to the present invention is characterized in that, in the above-mentioned invention, the relative phase shift amount of a certain relative phase shifter in the subsequent stage is smaller than the relative phase shift amount of a certain relative phase shifter among the relative phase shifters. And

  The optical amplifier according to the present invention is characterized in that, in the above-mentioned invention, the relative phase shift amount of the relative phase shifter decreases toward the subsequent stage.

  In the optical amplifier according to the present invention, the total loss from the first-stage amplification optical fiber to the (N−1) -th relative phase shifter is X [dB] in the above-described invention. The relative phase shift amount of the relative phase shifter is smaller by X [dB] than the relative phase shift amount of the first relative phase shifter.

  In the optical amplifier according to the present invention, in the above invention, the γPL of the amplification optical fiber of the amplification stage connected to the rear stage side is set so as to compensate for a relative phase shift caused by the loss of the relative phase shifter. It is characterized by being.

  In the optical amplifier according to the present invention, in the above invention, the length of the amplification optical fiber of the amplification stage connected to the rear stage is longer than the length of the amplification optical fiber of the amplification stage connected to the front stage. It is characterized by.

  The optical amplifier according to the present invention is characterized in that, in the above-mentioned invention, the length of the amplification optical fiber of the amplification stage becomes longer toward the subsequent stage.

  In the optical amplifier according to the present invention, the total loss from the first-stage amplification optical fiber to the (N−1) -th relative phase shifter is X [dB] in the above-described invention. The length of the amplification optical fiber is X [dB] longer than the length of the first amplification optical fiber.

  In the optical amplifier according to the present invention, in the above invention, the nonlinear constant of the amplification optical fiber of the amplification stage connected to the rear stage is larger than the nonlinear constant of the amplification optical fiber of the amplification stage connected to the front stage. It is characterized by that.

  The optical amplifier according to the present invention is characterized in that, in the above invention, the nonlinear constant of the amplification optical fiber of the amplification stage increases toward the subsequent stage.

  In the optical amplifier according to the present invention, the total loss from the first-stage amplification optical fiber to the (N−1) -th relative phase shifter is X [dB] in the above-described invention. The nonlinear constant of the amplification optical fiber is X [dB] larger than the nonlinear constant of the amplification optical fiber in the first stage.

  In the optical amplifier according to the present invention, the nonlinear phase shift amount of the pump light generated in the amplification optical fiber of each amplification stage in the above invention, and each of the stages in the same stage as the amplification optical fiber of each amplification stage, It is characterized in that ½ of the relative phase shift amount of the relative phase shifter coincides with an error of 10%.

The optical amplifier according to the present invention is the optical amplifier according to the present invention, wherein the relative phase shifters have the same relative phase shift amount and have different losses, and the amplification optical fibers of the amplification stages have optical characteristics. At least the nonlinear constant and the loss are the same, and the length of the amplification optical fiber at the Nth stage when viewed from the light input side is defined by the following equation.
P _{first stage} × L _{first stage} = P _{N stage} × L _{N stage}
Here, the P _{first stage} is the power of the pump light input to the _first stage amplification optical fiber, the L _{first stage} is the length of the first stage amplification optical fiber, and the _{PN stage} is the N stage amplification optical fiber. L _{N stage} is the length of the N-stage amplification optical fiber.

  The optical amplifier according to the present invention is characterized in that in the above invention, the length of the amplification optical fiber of the amplification stage is set to an effective length.

  In the optical amplifier according to the present invention, in the above invention, at least one of the relative phase shifters is configured as a relative phase shifter module including a pigtail fiber for inputting / outputting light to / from the relative phase shifter, The dispersion of the pigtail fiber is characterized in that it is not less than −2 [ps / nm / km] and not more than 2 [ps / nm / km] in the entire operating wavelength.

  An optical amplification system according to the present invention includes the optical amplifier according to the present invention.

  An optical communication system according to the present invention includes the optical amplifier according to the present invention.

  A wavelength converter according to the present invention includes the optical amplifier according to the above invention.

An optical amplification method according to the present invention is an optical amplification method for supplying pump light for amplifying signal light incident on an amplification optical fiber to the amplification optical fiber, the amplification optical fiber being a relative The relative phase shifter that changes the phase is divided into a plurality of amplification stages, and the relative phase related to the light output from the amplification optical fiber of the amplification stage connected to the front stage side of the relative phase shifter is larger than 0.5π. As described above, the product γPL of the nonlinear constant γ and the length L of the amplification optical fiber of the amplification stage and the input power P of the pump light to the amplification optical fiber of the amplification stage is set, and the relative phase shifter is The relative phase is changed to a value smaller than 0.5π and output to the amplification stage connected to the rear stage side.
Here, the relative phase φrel is Δk = ksignal defined by the signal light phase φsignal, the idler light phase φidler, the pump light phase φpump, the signal light wavenumber ksignal, the idler wavenumber kidler, and the pumplight wavenumber kpump. + kidler-2kpump is an amount described by the following equation using the propagation distance z of each light in the amplification optical fiber .
φrel = Δk z + φsignal + φidler-2φpump

  According to the present invention, there is an effect that the flatness of the gain spectrum and the broadband property can be realized more suitably.

FIG. 1 is a diagram illustrating light input to and output from an amplification optical fiber. FIG. 2 is a diagram illustrating light input to and output from the amplification optical fiber. FIG. 3 is a configuration diagram in which a relative phase shifter is inserted into an amplification optical fiber that is an amplification medium. FIG. 4 is a diagram illustrating a configuration example of an optical amplifier. FIG. 5 is a spectrum of output signal light power when there is no relative phase shifter. FIG. 6 is a fluctuation waveform diagram of the signal light power with respect to the length of the amplification optical fiber when there is no relative phase shifter. FIG. 7 is a fluctuation waveform diagram of the relative phase with respect to the length of the amplification optical fiber when there is no relative phase shifter. FIG. 8 shows a spectrum of output signal light power when a lossless relative phase shifter is inserted. FIG. 9 is a fluctuation waveform diagram of the signal light power with respect to the length of the amplification optical fiber when a lossless relative phase shifter is inserted. FIG. 10 is a fluctuation waveform diagram of the relative phase with respect to the length of the amplification optical fiber when a lossless relative phase shifter is inserted. FIG. 11 is a spectrum of output signal light power when a relative phase shifter having a loss is inserted. FIG. 12 is a fluctuation waveform diagram of the signal light power with respect to the length of the amplification optical fiber when a relative phase shifter having a loss is inserted. FIG. 13 is a fluctuation waveform diagram of the relative phase with respect to the length of the amplification optical fiber when a relative phase shifter having a loss is inserted. FIG. 14 is a spectrum of output signal light power when a relative phase shifter having a loss is inserted and the length of the amplification optical fiber is adjusted in consideration of the loss. FIG. 15 is a fluctuation waveform diagram of the signal light power with respect to the length of the amplification optical fiber when a relative phase shifter having a loss is inserted and the length of the amplification optical fiber is adjusted in consideration of the loss. is there. FIG. 16 is a fluctuation waveform diagram of the relative phase with respect to the length of the amplifying optical fiber when a relative phase shifter having a loss is inserted and the length of the amplifying optical fiber is adjusted in consideration of the loss. . FIG. 17 is a diagram showing an example of the reflection phase and reflection loss spectrum of a reflective dielectric multilayer filter that can be used as a relative phase shifter. FIG. 18A is a schematic diagram of the appearance of a relative phase shifter using a reflective dielectric filter. FIG. 18B is a schematic diagram illustrating a configuration example of a relative phase shifter using a reflective dielectric filter. FIG. 19 is a schematic cutaway cross-sectional view showing a configuration example of a relative phase filter module using a reflective dielectric multilayer filter. FIG. 20 is a diagram illustrating an example of a configuration of a dielectric multilayer filter. FIG. 21 is a diagram illustrating an example of a measurement system for a dielectric multilayer filter. FIG. 22 is a diagram showing another example of the configuration of the dielectric multilayer filter. FIG. 23 is a diagram illustrating another example of the measurement system for the dielectric multilayer filter. FIG. 24 is a diagram showing the configuration of the experimental system. FIG. 25 is a diagram showing the gain spectrum of the optical amplifier measured in the experimental system of FIG.

  Hereinafter, embodiments of an optical amplifier, an optical amplification system, a wavelength converter, an optical amplification method, and an optical communication system according to the present invention will be described in detail with reference to the drawings. Note that the present invention is not limited to the embodiments. Moreover, in each drawing, the same code | symbol is attached | subjected suitably to the same or corresponding element.

  In the configuration of the optical amplifier disclosed in Patent Document 2, amplification optical fibers having the same optical characteristics and the same length and relative phase shifters having the same relative phase shift amount and no optical loss (loss) are alternately used. When connected, the gain spectrum of the signal light is the flattest and broadband.

  However, if there is a loss in the relative phase shifter or the amplification optical fiber, or a loss occurs in the fusion splicing between the amplification optical fibers, ideal quasi-phase matching cannot be obtained, and the gain of the signal light The flatness and band of the spectrum or idler wavelength conversion efficiency spectrum may be lost.

  On the other hand, in the embodiment of the present invention described below, even when there is a loss in an optical device such as a relative phase shifter to be used, an optical fiber for amplification, or a fusion splicing portion, a more suitable pseudo phase matching Thus, it is possible to realize a gain spectrum of signal light or a wavelength conversion efficiency spectrum of idler light having more preferable flatness and broadband characteristics. For example, the flatness and the wavelength band realized by the embodiment of the present invention can coincide with the calculated values of the flatness and the wavelength band when there is no loss as described above with a deviation amount of 20% or less. Here, the flatness can be expressed, for example, by a deviation between the maximum value and the minimum value of the gain within a predetermined wavelength band.

  Hereinafter, OPA indicates the following state. FIG. 1 is a diagram illustrating light input to and output from an amplification optical fiber in OPA. Pump light and signal light that is light to be amplified are input to an amplification optical fiber 1 that is an amplification medium. In the amplification optical fiber 1, idler light is generated due to the nonlinear effect of pump light and signal light. The wavelength λidler [nm] of the idler light has the following relationship with the wavelength λpump [nm] of the pump light and the wavelength λsignal [nm] of the signal light.

    1 / λidler = 2 / λpump-1 / λsignal

  Moreover, PSA in this specification refers to the following state. FIG. 2 is a diagram showing light input to and output from the amplification optical fiber in the PSA. In addition to the pump light and the signal light, idler light having a power 1/10 to 10 times that of the signal light is input to the amplification optical fiber 1. At the output of the amplification optical fiber 1, pump light, amplified signal light, and amplified idler light are output. The wavelength of this idler light is determined by the following relationship, like the idler light of the OPA.

    1 / λidler = 2 / λpump-1 / λsignal

  In order to make the gain spectrum waveform of OPA and PSA flat and wide in the wavelength region, one or more relative phase shifters for shifting the relative phase are inserted into an amplification optical fiber as an amplification medium. FIG. 3 is a configuration diagram in which a relative phase shifter 2 is inserted into an amplification optical fiber 1 that is an amplification medium. The amplification optical fiber 1 is divided into a plurality of amplification stages by a relative phase shifter 2. In this way, the configuration in which the relative phase shifter 2 is inserted into the amplification optical fiber 1 is referred to as an optical amplifying body 100. In the optical amplifying body 100, the amplification optical fiber 1 and the relative phase shifter 2 are alternately connected. In FIG. 3, light is input from, for example, the amplification optical fiber 1 on the right side of the drawing. Further, in this specification, when viewed from the light input side, a combination of the amplification optical fiber 1 of one amplification stage and the following two relative phase shifters 2 is considered as one stage. The optical amplifying body 100 in FIG. 3 has a three-stage configuration. Note that the amplification optical fiber 1 on the light output side (for example, the left side of the drawing) is not included in the number of stages. Further, the three-stage configuration is an example, and the number of stages of the optical amplifying body is not particularly limited.

  Here, the relative phase φrel is an amount described by the following equation using the phase φsignal [radian] of the signal light, the phase φidler [radian] of the idler light, and the phase φpump [radian] of the pump light.

φrel = Δk z + φsignal + φidler-2φpump [radian]

The relative phase shifter 2 shifts the relative phase φrel by an appropriate amount according to the power of the input pump light, the dispersion characteristic of the amplification optical fiber 1 and the like. The length and dispersion of the amplification optical fiber 1 are appropriately set according to the required gain spectrum waveform. Here, Δk = ksignal + kidler−2kpump. ksignal, kidler and kpump are wave numbers of each light. z is the propagation distance of each light in the amplification optical fiber 1.

  The installation of the relative phase shifter 2 realizes gain spectrum flatness that cannot be obtained without inserting the relative phase shifter into the amplification optical fiber 1. At the same time, a noise figure (NF: Noise Figure) lower than that without the relative phase shifter is obtained.

  As an embodiment of the present invention, FIG. 4 shows a configuration example of an optical fiber amplifier operating as OPA or PSA. For example, a relative phase shifter 2 is inserted into the amplification optical fiber 1 of the optical fiber amplifier 200 as shown in FIG. 3, and the amplification optical fiber 1 and the relative phase shifter 2 are alternately connected. During OPA operation, pump light supplied from a pump light source (not shown) and signal light are combined by the optical coupler 3. During the PSA operation, the pump light, the signal light, and the idler light are combined by the optical coupler 3. The combined lights are input to the amplification optical fiber 1. The signal light is amplified by the nonlinear effect in the amplification optical fiber 1. The optical band-pass filter 4 that selectively transmits the signal light extracts the amplified signal light from the light output from the amplification optical fiber 1 and realizes a function as an optical amplifier.

  Here, the optical coupler 3 may be a WDM coupler or a C / L coupler. The optical bandpass filter 4 can also be replaced with a WDM coupler or a C / L coupler. Here, the C band is, for example, a wavelength band of 1530 nm to 1565 nm. The L band is, for example, a wavelength band of 1565 nm to 1620 nm. The C / L coupler is an optical coupler having a function of combining both bands using a low-pass filter or a high-pass filter.

Furthermore, if the nonlinear constant of the amplification optical fiber 1 is 10 [1 / W / km] or more as measured by the XPM method (Cross Phase Modulation Method), the fiber length required for OPA or PSA operation is 1 km. Shorter and easier to implement. As for the chromatic dispersion characteristics of the amplification optical fiber 1, if the zero dispersion is within 10 nm before and after the pump light wavelength λpump [nm] and the absolute value of the dispersion slope is 0.05 [ps / nm ² / km] or less, amplification is performed. The bandwidth becomes wide, and the function as an amplifier is enhanced. Alternatively, even if the chromatic dispersion of the amplification optical fiber 1 is in the range of 0.0 [ps / nm / km] ± 1.0 [ps / nm / km] in the wavelength band to be amplified, the amplification optical fiber 1 is As in the case of the chromatic dispersion characteristic, the amplification band becomes wide, and the function as an optical amplifier is enhanced.

  Note that quasi phase matching is realized under the condition that the following expression (1) is satisfied, and the gain spectrum of the signal light is most preferably flat and broadband.

γ _{N stage} × P _{N stage} × L _{N stage} = φ _{N stage} / 2 ・ ・ ・ (1)

In the above formula (1), the L _{N -th stage} [km] represents the length of the N-th amplification optical fiber 1 from the light input side in the configuration of the optical amplifying body 100 shown in FIG. Here, N is a positive integer, and the maximum value of N is the number of stages constituting the optical amplifying body 100. φ _{N stage} [radian] represents the amount of phase shift in the relative phase shifter 2 of the N stage. _{PN stage} [W] represents the pump light power input to the amplification optical fiber 1 of the N stage. γ _{N stage} [1 / W / km] represents a nonlinear constant of the amplification optical fiber 1 in the N stage. Here, γ _{N stage} × _{PN stage} × L _{N stage} represents the nonlinear phase shift amount [radian] received by the pump light input to the amplification optical fiber 1 of the N stage. In the embodiment of the present invention, if the condition of the above formula (1) is satisfied, the nonlinear constant or length of the amplification optical fiber 1 or the phase shift amount of the relative phase shifter 2 at each stage, Any physical quantity may be changed. Alternatively, even if there is a loss in the amplification optical fiber 1 or the relative phase shifter 2, any of the nonlinear constant and length of the amplification optical fiber 1 or the phase shift amount of the relative phase shifter 2 at each stage The physical quantity may be changed to satisfy the condition of the above formula (1).

If the relative phase shifter shifts only the phase of the pump light and does not shift the phase of the signal light and idler light, the phase shift amounts φ _{pump_N stage} [radian] and γ _{N stage} × P of the pump light at each stage _{N-th stage} × L _{N-th stage} is the same value. This is because the phase shift amount φ _{N stage} [radian] of the relative phase shifter at each stage is twice the phase shift amount φ _{pump_N stage} [radian] of the pump light at each stage.

  For example, if the phase shift amount of the relative phase shifter at each stage is the same, and the optical characteristics (at least the nonlinear constant and loss) of the amplification optical fiber at the amplification stage at each stage are the same, the amplification for each stage If the length of the optical fiber is appropriately adjusted and set, quasi-phase matching is realized, and a suitably flat and wide-band gain spectrum can be obtained. Such adjustment is a situation where the loss in the amplification optical fiber is small such that the propagation loss of each amplification optical fiber is 3 dB / km or less and the length of each amplification optical fiber is 200 m or less, and Although the relative phase shift amounts of the relative phase shifters at the respective stages are the same, the present invention can be suitably applied in a situation where the relative phase shifters at the respective stages have different losses.

That is, the above adjustment is performed so as to satisfy the following conditions.
P _{first stage} × L _{first stage} = P _{N stage} × L _{N stage}
Here, P _{first stage} : pump light power input to the first stage optical fiber from the light input side
L _{first stage} : Length of optical fiber for amplification in the first stage from the light input side
P _{N stage} : Pump optical power input to the N stage amplification optical fiber from the light input side
L _{N-th stage} : length of optical fiber for amplification of N-th stage from light input side Here, the subscript “first stage” may be “M-th stage” (M is an integer different from N) ).

When the above conditions are satisfied, the nonlinear phase shift amount of the pump light in the amplification optical fibers at each stage is the same. Here, the non-linear phase shift amount is an amount calculated by γ × P _{first stage} × L _{first stage} or γ × _{PN stage} × L _{N stage} . Note that γ is a nonlinear constant [1 / W / km] of the amplification fiber. If the length _of the _{LN stage} is within 10% of the length defined by the above formula, gain flatness can be obtained.

  Here, as the pump light propagates to the subsequent stage, it receives a loss due to a relative phase shifter, an amplification optical fiber, a fusion splicing, and the like, so that its power decreases. Therefore, when using an amplification optical fiber having the same optical characteristics at each stage, in order to make the nonlinear phase shift amount constant in the amplification optical fiber at each stage, the length of the amplification optical fiber at the subsequent stage is set to be longer. Lengthen. In this case, in the optical amplifying body 100 of FIG. 3, the length of the certain amplification optical fiber 1 in the subsequent stage is longer than the length of the certain amplification optical fiber 1. Further, the length of the amplification optical fiber 1 may be increased toward the subsequent stage over a plurality of stages. Furthermore, when the total loss from the first-stage amplification optical fiber 1 to the (N−1) -th relative phase shifter 2 is X [dB], the length of the N-th amplification optical fiber 1 is The length of the amplification optical fiber 1 in the first stage may be longer than X [dB].

  Further, in order to satisfy the condition of Expression (1), instead of adjusting the length of the amplification optical fiber at each stage, amplification optical fibers having different nonlinear constants at each stage may be used. In this case, the nonlinear constant may be adjusted by the same method as the length adjustment. In this case, the nonlinear constant of a certain subsequent amplification optical fiber is larger than the nonlinear constant of a certain amplification optical fiber. Further, the nonlinear constant of the amplification optical fiber may increase toward the subsequent stage over a plurality of stages. Furthermore, when the total loss from the first-stage amplification optical fiber to the (N−1) -th relative phase shifter is X [dB], the first-stage amplification optical fiber is the first-stage amplification optical fiber. An optical fiber having a nonlinear constant X [dB] larger than that of the optical fiber may be applied.

Further, the phase shift amount of the relative phase shifter at each stage is calculated as γ × P _{first stage} × L _{first stage} or γ × P _{N stage} × L _{N stage with} respect to the amplification optical fiber 1 of each stage. It is preferably about twice the amount (non-linear phase shift amount) [radian] (or a value within 10% of the error). When the relative phase is shifted by shifting the phase of the pump light, an appropriate pump phase shift amount of the N-th relative phase shifter is an amount calculated by γ × _{PN stage} × L _{N stage} [ radian] (or a value within 10% of the error).

Also, when the length of the amplification optical fiber at each stage is the same and the phase shift amount of the relative phase shifter is set appropriately,
φ _{first stage} / P _{first stage} = φ _{N stage} / P _{N stage}
If the phase shift amount of the relative phase shifter at each stage is adjusted so as to hold, pseudo phase matching is realized, and the gain spectrum of the signal light is preferably flat and broadband.
Here, the subscript “first stage” may be “M stage” (M is an integer different from N).

  Also in this case, as the pump light propagates to the subsequent stage, the power of the pump light decreases because of loss due to the relative phase shifter, the amplification optical fiber, the fusion splicing, and the like. Therefore, in order to make the above equation hold, the relative phase shift amount is increased in the later relative phase shifter. In this case, the relative phase shift amount of a later relative phase shifter is smaller than the relative phase shift amount of a certain relative phase shifter. Further, the relative phase shift amount of the relative phase shifter may decrease toward the subsequent stage over a plurality of stages. Furthermore, when the total loss from the first-stage amplification optical fiber to the (N−1) -th relative phase shifter is X [dB], the relative phase shift amount of the N-th relative phase shifter is The relative phase shift amount of the relative phase shifter may be smaller by X [dB].

  Hereinafter, the results of numerical simulation based on the configuration of the optical fiber amplifier 200 operating as the OPA having the configuration of FIG. 4 will be described. First, the characteristics of OPA when there is no relative phase shifter, and an optical fiber for amplification that has no loss using an optical fiber for amplification having the same length as described in Patent Document 2, an optical fiber for amplification Shows the characteristics of OPA when inserted periodically. Next, the OPA characteristics when the lengths of the amplification optical fibers remain equal to each other when there is a loss in the relative phase shifter are shown. Finally, when there is a loss in the relative phase shifter, the characteristics of OPA when the length of the amplification optical fiber is adjusted are shown, and the effect of the adjustment is clarified. In addition, when the nonlinear constant of the amplification optical fiber or the relative phase shift amount of the relative phase shifter is adjusted instead of the length of the amplification optical fiber, the result is similar to the case of adjusting the length of the amplification optical fiber. Is obtained.

The characteristics of the amplification optical fiber used are as follows. Dispersion: -0.2719 [ps / nm / km], Dispersion slope: 0.02 [ps / nm ² / km], 4th order dispersion: -0.00025 [ps / nm ³ / km], Nonlinear constant: 12 [1 / W / km], propagation loss: 0.8 [dB / km], zero dispersion wavelength: 1565 [nm], length: 200 [m].
For the pump light, the wavelength was 1566.0 [nm], and the power was 1.5 [W] (31.76 [dBm]). For the signal light, the power was 0.01 [mW] (-20 [dBm]).

(Calculation Example 1)
First, as Calculation Example 1, the behavior of signal light when parametric amplification is generated by inputting pump light and signal light to the amplification optical fiber 1 without using a relative phase shifter is shown. FIG. 5 is a spectrum of output signal light power when there is no relative phase shifter. FIG. 6 is a fluctuation waveform diagram of the signal light power with respect to the length of the amplification optical fiber when there is no relative phase shifter. FIG. 7 is a fluctuation waveform diagram of the relative phase with respect to the length of the amplification optical fiber when there is no relative phase shifter. 6 and 7, the wavelengths of the signal light are 1530 [nm], 1540 [nm], 1550 [nm], and 1560 [nm]. When the signal light power of −20 [dBm] is subtracted from the output signal light power spectrum of FIG. 5, a gain spectrum (unit: [dB]) is obtained. The same applies to the subsequent FIGS.

(Calculation Example 2)
Next, as calculation example 2, optical parametric amplification characteristics when a total of three relative phase shifters without loss are inserted into the amplification optical fiber 1 with a period of 50 m are shown. As the relative phase shifter 2, one that shifts the relative phase by 0.573π (0.2865 × π) was used. 0.2865π is an amount obtained by the product γPL [radian] of the nonlinear constant, length, and input pump power of the first-stage amplification optical fiber.

  FIG. 8 is a spectrum of output signal light power when a lossless relative phase shifter is inserted. FIG. 9 is a fluctuation waveform diagram of the signal light power with respect to the length of the amplification optical fiber when a lossless relative phase shifter is inserted. FIG. 10 is a fluctuation waveform diagram of the relative phase with respect to the length of the amplification optical fiber when a lossless relative phase shifter is inserted.

  Compared with the case where the relative phase shifter of Calculation Example 1 is not inserted, when the relative phase shifter without loss of Calculation Example 2 is inserted, the output power (gain) increases, for example, as shown in FIG. The wavelength range where the gain is flat with respect to the wavelength extends from 1530 nm to 1600 nm. Further, in FIG. 9, the absence of signal wavelength dependency regarding the development of the output power in the direction of the optical fiber length supports that the output power (gain) spectrum is flat in FIG. Further, as shown in FIG. 10, the relative phase shift periodically oscillates around the matching phase of 0.5π, and it can be seen that the pseudo phase matching as disclosed in Patent Document 2 is realized. .

(Calculation Example 3)
Next, as Calculation Example 3, as in Calculation Example 2, optical parametric amplification characteristics when a total of three relative phase shifters 2 are inserted in a 50-m cycle into the amplification optical fiber 1 are shown. However, the relative phase shifter 2 shifts the relative phase by 0.573π (0.2865 × π) as in the calculation example 2, but unlike the calculation example 2, 0.87 [dB], Those having a loss of 1.24 [dB] and 1.47 [dB] were used.

  FIG. 11 is a spectrum of output signal light power when a relative phase shifter having a loss is inserted. FIG. 12 is a fluctuation waveform diagram of the signal light power with respect to the length of the amplification optical fiber when a relative phase shifter having a loss is inserted. FIG. 13 is a fluctuation waveform diagram of the relative phase with respect to the length of the amplification optical fiber when a relative phase shifter having a loss is inserted.

  In the case of calculation example 3 in which a relative phase shifter having a loss is inserted, as shown in FIG. 11, in the output power (gain) spectrum, the flatness decreases and the gain deviation increases to 2-3 [dB]. In addition, the gain of the signal light was also reduced by 10 [dB] or more compared to the case of using the relative phase shifter without loss in Calculation Example 2. As can be seen from FIG. 12, there was almost no gain particularly in the fourth-stage amplification optical fiber. This is probably because the fourth-stage amplification optical fiber does not include the value of 0.5π, which is the matching phase, as shown in FIG.

(Calculation Example 4)
Next, as calculation example 4, similarly to calculation example 3, optical parametric amplification characteristics when a total of three relative phase shifters 2 are inserted into the amplification optical fiber 1 are shown. The relative phase shifter 2 shifts the relative phase by 0.573π (0.2865 × π) and has losses of 0.87 [dB], 1.24 [dB], and 1.47 [dB] in order from the light input side. Was used. However, unlike the calculation example 3, the length of the amplification optical fiber 1 at each stage was adjusted and set so as to satisfy the P _{first stage} × L _{first stage} = _{PN stage} × L _{N stage} . Specifically, the length of the amplification optical fiber 1 at each stage was set to 50 [m], 61.1 [m], 81.3 [m], and 114 [m] from the light input side. The total length of the total length of the amplification optical fibers 1 at each stage is 306.3 [m].

The length of each amplification optical fiber 1 in the second and subsequent stages was calculated as follows.
The second stage is 50 [m] × 10 ^ (0.87 [dB] / 10), the third stage is 50 [m] × 10 ^ ((0.87 [dB] +1.24 [dB]) / 10), 4 The stage is 50 [m] × 10 ^ ((0.87 [dB] +1.24 [dB] +1.47 [dB]) / 10).
Here, for convenience, the loss of the amplification optical fiber 1 was calculated as zero. Actually, there is a loss in the amplification optical fiber, but when the length of the first amplification optical fiber 1 is 50 [m] as in this simulation, the loss due to the amplification optical fiber 1 is 0.04 [dB. ] Small enough to be treated as almost zero. Even in the subsequent stages, the loss due to the amplification optical fiber 1 is so small that it is not necessary to consider it.

  FIG. 14 is a spectrum of output signal light power when a relative phase shifter having a loss is inserted and the length of the amplification optical fiber is adjusted in consideration of the loss. FIG. 15 is a fluctuation waveform diagram of the signal light power with respect to the length of the amplification optical fiber when a relative phase shifter having a loss is inserted and the length of the amplification optical fiber is adjusted in consideration of the loss. is there. FIG. 16 is a fluctuation waveform diagram of the relative phase with respect to the length of the amplifying optical fiber when a relative phase shifter having a loss is inserted and the length of the amplifying optical fiber is adjusted in consideration of the loss. .

  From the spectrum of the signal light power shown in FIG. 14, it can be seen that the signal light is amplified to approximately −3 [dBm] with a flat characteristic from 1530 [nm] to 1600 [nm]. The flat spectral region is almost the same as that in calculation example 2 when a lossless relative phase shifter is inserted with a period of 50 m. The development of the power (gain) of the signal light shown in FIG. 15 is almost independent of wavelength. The relative phase shown in FIG. 16 includes a value of 0.5π at each stage, and it can be said that pseudo phase matching is realized.

  As shown in FIG. 15, the amplification light of the amplification stage is set so that the relative phase regarding the light output from the amplification optical fiber of the amplification stage connected to the previous stage of a certain relative phase shifter is larger than 0.5π. A product γPL of the nonlinear constant γ of the fiber, the length L and the input power P of the pump light to the amplification optical fiber of the amplification stage is set, and the relative phase shifter sets the relative phase to a value smaller than 0.5π. It is preferable to change and output to the amplification stage connected to the rear stage side.

As described above, when the amplification optical fiber having the same optical characteristics and the relative phase shifter having loss and the same relative phase shift amount are used, the length of the N-stage amplification optical fiber is set to the _first P _stage. Good optical parametric amplification characteristics when set with the formula of × L _{first stage} = _{PN stage} × L _{N stage} . Such a good characteristic is similarly applied when the optical fiber amplifier 200 according to the embodiment is applied to a phase-sensitive optical parametric amplifier that simultaneously inputs wavelength conversion by four-wave mixing and idler light whose phase is adjusted. As a result, wavelength conversion efficiency and flatness of signal amplification characteristics are realized.

  The present invention is not limited to the above embodiment. According to the present invention, in an optical amplifier including an optical amplifying body in which an amplification optical fiber and a relative phase shifter are alternately connected, the relative phase [radian] changed by each relative phase shifter changes in value across 0.5π. (If the relative phase value is 0.5π or less (or more), it changes to 0.5π or more (or less)) Amplification connected immediately before each relative phase shifter (on the light input side) For which the product γPL [radian] of the nonlinear constant γ [1 / W / km] and length L [km] of the optical fiber for optical use and the input power P [W] of the pump light to the optical fiber for amplification is set If it is an amplifier, the output (gain) spectrum of the amplified signal light can be flattened over a wide band. At this time, the relative phase shift amounts of the relative phase shifters may be different from each other. Further, 1/2 of the relative phase shift amount [radian] of each relative phase shifter is the nonlinear constant γ [1 / W / km] and length L of the amplification optical fiber immediately before the relative phase shifter, and the amplification optical fiber. It is more desirable if it is equal to the product γPL [radian] with the input power P [W] of the pump light to the.

  Even if half of the relative phase shift amount [radian] of each relative phase shifter is shifted from the value of γPL [radian] by an error of about 10%, the same effect can be obtained. If the loss of the amplification optical fiber is not negligible, half of the relative phase shift amount may be set to γPLeff [radian] using the effective fiber length Leff considering the loss of the amplification optical fiber. Here, Leff is defined by the following equation.

L：ファイバ長[1/km]、α:光ファイバのロス[1/km]

L: Fiber length [1 / km], α: Optical fiber loss [1 / km]

  The effective fiber length Leff may be substituted for L in all of the above equations.

  By the way, a module using an all-pass filter can be used as the relative phase shifter. FIG. 17 is a diagram showing an example of the reflection phase and reflection loss spectrum of a reflective dielectric multilayer filter that can be used as a relative phase shifter. When pump light having a wavelength near 1566 nm is reflected by this filter, the phase of the pump light is shifted. On the other hand, signal light or idler light with a wavelength of 266 nm or more away from 1566 [nm] does not change by more than 2 degrees (π / 90 [radian]) even when reflected by this filter, and is almost constant. It is.

FIG. 18A is a schematic diagram of the appearance of a relative phase shifter using a reflective dielectric filter. FIG. 18B is a schematic diagram illustrating a configuration example of a relative phase shifter using a reflective dielectric filter. The relative phase shifter module 10 shown in FIG. 18B includes two pigtail fibers 15 and 16 which are also shown in FIG. 18A. The pigtail fibers 15 and 16 are fusion-spliced with the amplification optical fiber 1. “X” indicates a fusion splice point. Here, for example, a zero dispersion optical fiber having a dispersion in the range of −2 to 2 [ps / nm / km] is used for the pigtail fibers 15 and 16 for inputting / outputting light to / from the reflective dielectric filter. preferable. The range of the dispersion value is preferably satisfied over the entire wavelength range (operating wavelength range) in which the signal light, the pump light, and the idler light can be arranged. For example, this wavelength range is from 1530 [nm] to 1620 [nm]. A highly nonlinear fiber may be applied to the pigtail fiber. For example, this highly nonlinear fiber has a zero dispersion wavelength near 1565 [nm], a dispersion slope of 0.02 [ps / nm ² / km], a nonlinear constant of 12 [1 / W / km], and a propagation loss of 0.8 [ dB / km] and NA is 0.15.

  FIG. 19 is a schematic cutaway sectional view showing a configuration example of a relative phase shifter module using a reflective dielectric multilayer filter. The relative phase shifter module 10 includes a cylindrical housing 11, a ferrule 12 inserted into the housing 11, two pigtail fibers 15 and 16 inserted into the ferrule 12, and a housing 11. A lens 13 and a dielectric multilayer filter 14 are provided. The light input from the outside and output from the pigtail fiber 15 is collimated by the lens 13, reflected by the dielectric multilayer filter 14 which is an all-pass filter, input to the other pigtail fiber 16, and output to the outside. The ferrule 12, the lens 13, and the dielectric multilayer filter 14 are fixed to the housing 11. In fixing, an adhesive may be used or welding using a YAG laser may be used. The two pigtail fibers 15 and 16 inserted into the ferrule 12 may be inserted into the ferrule 12 so as to be separated from each other as shown in FIG. 19, or may be inserted into the ferrule 12 so as to be in contact with each other. . Furthermore, the two pigtail fibers 15 and 16 inserted into the ferrule 12 may be arranged in parallel, or may be arranged so that the distance between them decreases as the lens 13 is approached.

  The return loss of the relative phase shifter module in FIG. 19 is desirably −30 dB or less. The module transmission loss is preferably as small as possible, but is preferably 2 dB or less, for example.

  The glass substrate surface of the reflective dielectric multilayer filter 14 of FIG. 19 is on the lens 13 side or on the opposite side to the lens 13 depending on the configuration of the dielectric multilayer filter 14. The lens 13 may be a SELFOC (registered trademark) lens.

  FIG. 20 is a diagram illustrating an example of the configuration of the dielectric multilayer filter 14. The dielectric multilayer filter 14 is formed by first forming a multilayer band-pass filter film 14b on a glass substrate 14a, and then forming a multilayer total reflection mirror film 14c thereon, whereby the wavelength of the pump light is increased. Only a reflection type all-pass filter capable of changing the phase can be formed. When the dielectric multilayer filter 14 configured as shown in FIG. 20 is modularized, the total reflection mirror film 14 c must be directed to the opposite side of the lens 13. Therefore, the incident light to the dielectric multilayer filter 14 is reflected by the multilayer total reflection mirror film 14c after passing through the glass substrate 14a, and is again output through the glass substrate 14a.

  As shown in FIG. 21, the dielectric multilayer filter 14 having the configuration of FIG. 20 receives monochromatic light L1 from the monochromatic light source 20 for film thickness measurement from the glass substrate 14a side of the dielectric multilayer filter 14 during film formation. Then, by measuring the power of the transmitted light L2 with the monochromatic light power measuring device 30, it can be manufactured by adjusting the film thickness. However, since the total reflection mirror film 14c reflects the monochromatic light L1 for film thickness measurement, if the total reflection mirror film 14c exists, the power of the transmitted light L2 is remarkably reduced, and the film thickness measurement becomes impossible. Therefore, the band-pass filter film 14b is formed first, and then the film forming material is sputtered under film formation time control so that the total reflection mirror film 14c is formed without measuring the film thickness. Is preferred.

  In the relative phase shifter module 10 shown in FIG. 19, it is desirable that the film-forming surface of the dielectric multilayer filter is directed to the lens 13 side. For this purpose, when forming a multilayer film, first, the total reflection mirror film 14c must be formed on the glass substrate, and then the band-pass filter film 14b must be formed. An example of the configuration of the dielectric multilayer filter 14A fabricated in this way is shown in FIG. At this time, since the monochromatic light for film thickness measurement is reflected on the upper side of the glass substrate, a measuring instrument for the film thickness measurement light may be installed at a position where the light above the glass substrate is reflected. The configuration of the measurement system at this time is shown in FIG. At this time, the monochromatic light power measuring device 30 measures the reflected light L3 that is the reflected light of the monochromatic light L1 by the dielectric multilayer filter 14A.

  When the dielectric multilayer filter that operates as a relative phase shifter for pump light is a transmission type, the relative phase shifter module preferably has a module configuration in which the filter is installed between two opposing lenses. As the pigtail fiber at this time, it is preferable to use a zero dispersion optical fiber having a dispersion in the range of −2 to 2 [ps / nm / km]. This dispersion value is preferably satisfied in the entire wavelength range in which the signal light, the pump light, and the idler light are arranged.

  In order to investigate the characteristics of optical amplification or wavelength conversion using an optical amplifier having a configuration in which amplification optical fibers and relative phase shifters are alternately connected, an experiment was performed with the configuration of an experimental system 1000 shown in FIG.

  The pump light is generated by a TLS (tunable laser source) 71, a PC (polarization controller) 73, a PM (phase modulator) 74, an EYDFA (Erbium Ytterbium Doped Fiber Amplifier) 77, and a BPF (Band pass filter) 78. The source was used. The TLS 71 outputs CW (continuous wave) light without using coherence control. The TLS 71 inputs output light to the PM 74. At this time, phase modulation can be performed efficiently if the polarization axis of PM 74 and the polarization axis of the input light coincide. In order to realize this, the polarization of the input light was adjusted using the PC 73. At this time, the polarization was adjusted so that the output light power of PM74 was maximized. This is because the polarizer is included in the PM 74, so that the power of the output light is maximized when the polarization axis of the light incident on the PM 74 and the polarization axis of the polarizer coincide. In order to increase the degree of freedom of polarization adjustment, a PC 73 is installed after the PM 74. The output light of the PC 73 was amplified by the EYDFA 77 to generate pump light.

  A white noise source 75 having a bandwidth of 1.2 GHz was used as a signal source for driving the PM 74. The white noise is amplified to about 27 dBm by the broadband RF amplifier 76, and the PM 74 is driven. As a result, the pump light can be phase-modulated with a wide band and high intensity. This suppresses reflected light caused by SBS (stimulated Brillouin scattering) generated in the optical amplifying body 300. In addition to the method shown in FIG. 24, there is a method using a Fabry-Perot LD (laser diode) as a method for generating light that is phase-modulated with a wide band and high intensity.

  TLS72 and PC73 were used for generation of signal light. The TLS 72 outputs CW light without using coherence control. In order to maximize the OPA gain, the PSA gain, or the wavelength conversion efficiency, it is necessary to make the polarization of the signal light coincide with the polarization of the pump light, so the PC 73 is arranged.

  The pump light and the signal light were combined via a 17 dB coupler 79. Here, the pump light passes through the transmission port of the 17 dB coupler 79 and the signal light passes through the -17 dB port. In the experiment, power of 30 dBm or more is used as the pump light in most cases, so it is desirable to avoid loss as much as possible.

  If the output light of the optical amplifier 300 is directly input to the OSA (optical spectrum analyzer) 81, the input power may exceed the input upper limit of the OSA 81. Therefore, an optical attenuator (ATT) 80 is installed immediately before the OSA 81, OSA81 was protected.

The power of pump light input to the optical amplifying body 300 was set to 31.7 [dBm] (1.5 [W]), and the wavelength was set to 1561.90 [nm]. The power of idler light was set to -20 [dBm]. The characteristics of the highly nonlinear fiber used as the optical fiber for amplification in the optical amplifier 300 are as follows: the zero dispersion wavelength is 1567.4 [nm], the dispersion slope is 0.02 [ps / nm ² / km], and the fourth-order dispersion is −0.00025 [ps]. / nm ³ / km], the nonlinear constant was 12 [1 / W / km], and the propagation loss was 0.8 [dB / km]. The length of the optical fiber for amplification was 50 [m] for the first stage, 62 [m] for the second stage, and 82 [m] for the third stage. As the relative phase shifter, an all-pass filter using a dielectric multilayer filter having the characteristic wavelength shown in FIG. 17 was used.

  FIG. 25 is a diagram showing a gain spectrum of the optical amplifying body 300 measured in the experimental system of FIG. The gain at each stage and the gain of a highly nonlinear fiber of the same length without the relative phase shifter is shown. For example, “1st HNLF out” in the legend indicates the gain spectrum of the first stage of the optical amplifier 300. “WO phase shifter” shows the gain spectrum of a highly nonlinear fiber of the same length without the relative phase shifter. A dotted line, a broken line, a one-dot chain line, or a two-dot chain line is a simulation result under each of the above conditions.

  From FIG. 25, it can be seen that, in the optical amplifying body 300, by increasing the number of stages, the gain is increased while the flatness and the band are maintained. It can also be seen that flatness can be obtained by inserting a relative phase shifter.

  By the way, the design of the zero dispersion wavelength of the amplification optical fiber and the wavelength of the pump light is an important parameter for appropriately operating the quasi phase matching. For example, the quasi phase matching can be appropriately realized by controlling the zero dispersion wavelength by controlling the temperature or tension of the amplification optical fiber.

  Similar to Patent Document 2, the optical amplifier according to the above embodiment may be installed in the front stage of the EDFA or in the rear stage of the optical amplification system using the Raman effect to constitute the optical amplification system. Such an optical amplification system is an optical amplification system with low noise and high output in the entire system due to the low noise characteristics of the optical amplifier according to the embodiment.

  As in Patent Document 2, it is also possible to construct an optical communication system using the optical amplifier according to the above embodiment. This makes it possible to extend the transmission distance of an optical communication system using a conventional EDFA as an optical amplifier, reduce transmission power, and reduce power consumption.

  The present invention is not limited to the above embodiment. What was comprised combining each component mentioned above suitably is also contained in this invention. Further effects and modifications can be easily derived by those skilled in the art. Therefore, the broader aspect of the present invention is not limited to the above-described embodiment, and various modifications can be made.

  As described above, the present invention is suitable for use in optical communication.

DESCRIPTION OF SYMBOLS 1 Optical fiber for amplification 2 Relative phase shifter 3 Optical coupler 4 Optical band pass filter 10 Relative phase shifter module 11 Case 12 Ferrule 13 Lens 14, 14A Dielectric multilayer filter 14a Glass substrate 14b Band pass filter film 14c Total reflection mirror film 15, 16 Pigtail fiber 20 Monochromatic light source 30 Monochromatic optical power measuring instrument 71, 72 TLS
73 PC
74 PM
75 White noise source 76 Broadband RF amplifier 77 EYDFA
78 BPF
79 17 dB coupler 80 Optical attenuator 81 OSA
100, 300 Optical amplifier 200 Optical fiber amplifier 1000 Experimental system L1 Monochromatic light L2 Transmitted light L3 Reflected light

Claims (24)

An optical fiber for amplification;
A pump light source for supplying pump light for amplifying signal light incident on the amplification optical fiber to the amplification optical fiber,
The amplification optical fiber is divided into a plurality of amplification stages by a relative phase shifter that shifts only the phase of the pump light and changes the relative phase without shifting the phase of the signal light and idler light,
The nonlinear constant γ and length of the amplification optical fiber of the amplification stage so that the relative phase related to the light output from the amplification optical fiber of the amplification stage connected to the upstream side of the relative phase shifter is larger than 0.5π. The product γPL of L and the input power P of the pump light to the amplification optical fiber of the amplification stage is set,
The relative phase shifter changes the relative phase to a value smaller than 0.5π,
Output to the amplification stage connected to the back side,
An optical amplifier characterized in that a relative phase shift amount of a certain relative phase shifter in a subsequent stage is smaller than a relative phase shift amount of a certain relative phase shifter among the relative phase shifters.
Here, the relative phase φrel is Δk = ksignal defined by the signal light phase φsignal, the idler light phase φidler, the pump light phase φpump, the signal light wavenumber ksignal, the idler wavenumber kidler, and the pumplight wavenumber kpump. + kidler-2kpump is an amount described by the following equation using the propagation distance z of each light in the amplification optical fiber .
φrel = Δk z + φsignal + φidler-2φpump   2. The optical amplifier according to claim 1, wherein a relative phase shift amount of the relative phase shifter decreases toward a subsequent stage. An optical fiber for amplification;
A pump light source for supplying pump light for amplifying signal light incident on the amplification optical fiber to the amplification optical fiber,
The amplification optical fiber is divided into a plurality of amplification stages by a relative phase shifter that shifts only the phase of the pump light and changes the relative phase without shifting the phase of the signal light and idler light,
The nonlinear constant γ and length of the amplification optical fiber of the amplification stage so that the relative phase related to the light output from the amplification optical fiber of the amplification stage connected to the upstream side of the relative phase shifter is larger than 0.5π. The product γPL of L and the input power P of the pump light to the amplification optical fiber of the amplification stage is set,
The relative phase shifter changes the relative phase to a value smaller than 0.5π,
Output to the amplification stage connected to the back side,
When the total loss from the first-stage amplification optical fiber to the (N−1) -th relative phase shifter is X [dB], the relative phase shift amount of the N-th relative phase shifter is An optical amplifier characterized by being smaller than the relative phase shift amount of the relative phase shifter by X [dB].
Here, the relative phase φrel is Δk = ksignal defined by the signal light phase φsignal, the idler light phase φidler, the pump light phase φpump, the signal light wavenumber ksignal, the idler wavenumber kidler, and the pumplight wavenumber kpump. + kidler-2kpump is an amount described by the following equation using the propagation distance z of each light in the amplification optical fiber .
φrel = Δk z + φsignal + φidler-2φpump   When the total loss from the first-stage amplification optical fiber to the (N−1) -th relative phase shifter is X [dB], the relative phase shift amount of the N-th relative phase shifter is The optical amplifier according to claim 1, wherein the relative phase shift amount is smaller by X [dB] than the relative phase shift amount of the relative phase shifter. An optical fiber for amplification;
A pump light source for supplying pump light for amplifying signal light incident on the amplification optical fiber to the amplification optical fiber,
The amplification optical fiber is divided into a plurality of amplification stages by a relative phase shifter that shifts only the phase of the pump light and changes the relative phase without shifting the phase of the signal light and idler light,
The nonlinear constant γ and length of the amplification optical fiber of the amplification stage so that the relative phase related to the light output from the amplification optical fiber of the amplification stage connected to the upstream side of the relative phase shifter is larger than 0.5π. The product γPL of L and the input power P of the pump light to the amplification optical fiber of the amplification stage is set,
The relative phase shifter changes the relative phase to a value smaller than 0.5π,
Output to the amplification stage connected to the back side,
The γPL of the amplification optical fiber of the amplification stage connected to the rear stage side is set so as to compensate for a relative phase shift caused by the loss of the relative phase shifter,
An optical amplifier characterized in that the length of the amplification optical fiber of the amplification stage connected to the rear stage is longer than the length of the amplification optical fiber of the amplification stage connected to the front stage.
Here, the relative phase φrel is Δk = ksignal defined by the signal light phase φsignal, the idler light phase φidler, the pump light phase φpump, the signal light wavenumber ksignal, the idler wavenumber kidler, and the pumplight wavenumber kpump. + kidler-2kpump is an amount described by the following equation using the propagation distance z of each light in the amplification optical fiber .
φrel = Δk z + φsignal + φidler-2φpump An optical fiber for amplification;
A pump light source for supplying pump light for amplifying signal light incident on the amplification optical fiber to the amplification optical fiber,
The amplification optical fiber is divided into a plurality of amplification stages by a relative phase shifter that shifts only the phase of the pump light and changes the relative phase without shifting the phase of the signal light and idler light,
The nonlinear constant γ and length of the amplification optical fiber of the amplification stage so that the relative phase related to the light output from the amplification optical fiber of the amplification stage connected to the upstream side of the relative phase shifter is larger than 0.5π. The product γPL of L and the input power P of the pump light to the amplification optical fiber of the amplification stage is set,
The relative phase shifter changes the relative phase to a value smaller than 0.5π,
Output to the amplification stage connected to the back side,
The γPL of the amplification optical fiber of the amplification stage connected to the rear stage side is set so as to compensate for a relative phase shift caused by the loss of the relative phase shifter,
An optical amplifier characterized in that the length of the amplification optical fiber in the amplification stage becomes longer toward the subsequent stage.
Here, the relative phase φrel is Δk = ksignal defined by the signal light phase φsignal, the idler light phase φidler, the pump light phase φpump, the signal light wavenumber ksignal, the idler wavenumber kidler, and the pumplight wavenumber kpump. + kidler-2kpump is an amount described by the following equation using the propagation distance z of each light in the amplification optical fiber .
φrel = Δk z + φsignal + φidler-2φpump   6. The optical amplifier according to claim 5, wherein the length of the amplification optical fiber of the amplification stage becomes longer toward the subsequent stage. An optical fiber for amplification;
A pump light source for supplying pump light for amplifying signal light incident on the amplification optical fiber to the amplification optical fiber,
The amplification optical fiber is divided into a plurality of amplification stages by a relative phase shifter that shifts only the phase of the pump light and changes the relative phase without shifting the phase of the signal light and idler light,
The nonlinear constant γ and length of the amplification optical fiber of the amplification stage so that the relative phase related to the light output from the amplification optical fiber of the amplification stage connected to the upstream side of the relative phase shifter is larger than 0.5π. The product γPL of L and the input power P of the pump light to the amplification optical fiber of the amplification stage is set,
The relative phase shifter changes the relative phase to a value smaller than 0.5π,
Output to the amplification stage connected to the back side,
The γPL of the amplification optical fiber of the amplification stage connected to the rear stage side is set so as to compensate for a relative phase shift caused by the loss of the relative phase shifter,
When the total loss from the first-stage amplification optical fiber to the (N−1) -th relative phase shifter is X [dB], the length of the N-th stage amplification optical fiber is the first-stage amplification fiber. An optical amplifier characterized by being X [dB] longer than the length of the amplification optical fiber.
Here, the relative phase φrel is Δk = ksignal defined by the signal light phase φsignal, the idler light phase φidler, the pump light phase φpump, the signal light wavenumber ksignal, the idler wavenumber kidler, and the pumplight wavenumber kpump. + kidler-2kpump is an amount described by the following equation using the propagation distance z of each light in the amplification optical fiber .
φrel = Δk z + φsignal + φidler-2φpump   When the total loss from the first-stage amplification optical fiber to the (N−1) -th relative phase shifter is X [dB], the length of the N-th stage amplification optical fiber is the first-stage amplification fiber. The optical amplifier according to claim 5, wherein the optical amplifier is longer than the length of the amplification optical fiber by X [dB]. An optical fiber for amplification;
A pump light source for supplying pump light for amplifying signal light incident on the amplification optical fiber to the amplification optical fiber,
The amplification optical fiber is divided into a plurality of amplification stages by a relative phase shifter that shifts only the phase of the pump light and changes the relative phase without shifting the phase of the signal light and idler light,
The nonlinear constant γ and length of the amplification optical fiber of the amplification stage so that the relative phase related to the light output from the amplification optical fiber of the amplification stage connected to the upstream side of the relative phase shifter is larger than 0.5π. The product γPL of L and the input power P of the pump light to the amplification optical fiber of the amplification stage is set,
The relative phase shifter changes the relative phase to a value smaller than 0.5π,
Output to the amplification stage connected to the back side,
The γPL of the amplification optical fiber of the amplification stage connected to the rear stage side is set so as to compensate for a relative phase shift caused by the loss of the relative phase shifter,
An optical amplifier characterized in that a nonlinear constant of the amplification optical fiber of the amplification stage connected to the rear stage side is larger than a nonlinear constant of the amplification optical fiber of the amplification stage connected to the front stage side.
Here, the relative phase φrel is Δk = ksignal defined by the signal light phase φsignal, the idler light phase φidler, the pump light phase φpump, the signal light wavenumber ksignal, the idler wavenumber kidler, and the pumplight wavenumber kpump. + kidler-2kpump is an amount described by the following equation using the propagation distance z of each light in the amplification optical fiber .
φrel = Δk z + φsignal + φidler-2φpump   The optical amplifier according to claim 10, wherein a nonlinear constant of the amplification optical fiber of the amplification stage increases toward the subsequent stage.   When the total loss from the first-stage amplification optical fiber to the (N−1) -th relative phase shifter is X [dB], the nonlinear constant of the N-th stage amplification optical fiber is the first-stage amplification optical fiber. 12. The optical amplifier according to claim 10, wherein X [dB] is larger than a nonlinear constant of the amplification optical fiber. An optical fiber for amplification;
A pump light source for supplying pump light for amplifying signal light incident on the amplification optical fiber to the amplification optical fiber,
The amplification optical fiber is divided into a plurality of amplification stages by a relative phase shifter that shifts only the phase of the pump light and changes the relative phase without shifting the phase of the signal light and idler light,
The nonlinear constant γ and length of the amplification optical fiber of the amplification stage so that the relative phase related to the light output from the amplification optical fiber of the amplification stage connected to the upstream side of the relative phase shifter is larger than 0.5π. The product γPL of L and the input power P of the pump light to the amplification optical fiber of the amplification stage is set,
The relative phase shifter changes the relative phase to a value smaller than 0.5π,
Output to the amplification stage connected to the back side,
A non-linear phase shift amount of the pump light generated in the amplification optical fiber of each amplification stage and a relative phase shift amount of each relative phase shifter in the same stage as the amplification optical fiber of each amplification stage Are matched within a range of 10% error.
Here, the relative phase φrel is Δk = ksignal defined by the signal light phase φsignal, the idler light phase φidler, the pump light phase φpump, the signal light wavenumber ksignal, the idler wavenumber kidler, and the pumplight wavenumber kpump. + kidler-2kpump is an amount described by the following equation using the propagation distance z of each light in the amplification optical fiber .
φrel = Δk z + φsignal + φidler-2φpump   A non-linear phase shift amount of the pump light generated in the amplification optical fiber of each amplification stage and a relative phase shift amount of each relative phase shifter in the same stage as the amplification optical fiber of each amplification stage The optical amplifier according to any one of claims 1 to 12, wherein the values coincide with each other within an error range of 10%. An optical fiber for amplification;
A pump light source for supplying pump light for amplifying signal light incident on the amplification optical fiber to the amplification optical fiber,
The amplification optical fiber is divided into a plurality of amplification stages by a relative phase shifter that shifts only the phase of the pump light and changes the relative phase without shifting the phase of the signal light and idler light,
The nonlinear constant γ and length of the amplification optical fiber of the amplification stage so that the relative phase related to the light output from the amplification optical fiber of the amplification stage connected to the upstream side of the relative phase shifter is larger than 0.5π. The product γPL of L and the input power P of the pump light to the amplification optical fiber of the amplification stage is set,
The relative phase shifter changes the relative phase to a value smaller than 0.5π,
Output to the amplification stage connected to the back side,
The γPL of the amplification optical fiber of the amplification stage connected to the rear stage side is set so as to compensate for a relative phase shift caused by the loss of the relative phase shifter,
A non-linear phase shift amount of the pump light generated in the amplification optical fiber of each amplification stage and a relative phase shift amount of each relative phase shifter in the same stage as the amplification optical fiber of each amplification stage Are matched within a range of 10% error.
Here, the relative phase φrel is Δk = ksignal defined by the signal light phase φsignal, the idler light phase φidler, the pump light phase φpump, the signal light wavenumber ksignal, the idler wavenumber kidler, and the pumplight wavenumber kpump. + kidler-2kpump is an amount described by the following equation using the propagation distance z of each light in the amplification optical fiber .
φrel = Δk z + φsignal + φidler-2φpump An optical fiber for amplification;
A pump light source for supplying pump light for amplifying signal light incident on the amplification optical fiber to the amplification optical fiber,
The amplification optical fiber is divided into a plurality of amplification stages by a relative phase shifter that shifts only the phase of the pump light and changes the relative phase without shifting the phase of the signal light and idler light,
The nonlinear constant γ and length of the amplification optical fiber of the amplification stage so that the relative phase related to the light output from the amplification optical fiber of the amplification stage connected to the upstream side of the relative phase shifter is larger than 0.5π. The product γPL of L and the input power P of the pump light to the amplification optical fiber of the amplification stage is set,
The relative phase shifter changes the relative phase to a value smaller than 0.5π,
Output to the amplification stage connected to the back side,
The relative phase shifters have the same relative phase shift amount and have different losses, and the amplification optical fibers of the amplification stages have at least the same nonlinear constant and loss as optical characteristics, The length of the amplification optical fiber at the Nth stage as viewed from the input side of the optical amplifier is defined by the following equation.
P _{first stage} × L _{first stage} = P _{N stage} × L _{N stage}
Here, the P _{first stage} is the power of the pump light input to the _first stage amplification optical fiber, the L _{first stage} is the length of the first stage amplification optical fiber, and the _{PN stage} is the N stage amplification optical fiber. L _{N stage} is the length of the N-stage amplification optical fiber.
Further, the relative phase φrel is defined by the signal light phase φsignal, the idler light phase φidler, the pump light phase φpump, the signal light wavenumber ksignal, the idler light wavenumber kidler, and the pump light wavenumber kpump Δk = ksignal + kidler-2kpump is an amount described by the following equation using the propagation distance z of each light in the amplification optical fiber .
φrel = Δk z + φsignal + φidler-2φpump   The optical amplifier according to claim 1, wherein the setting is performed by replacing the length of the amplification optical fiber of the amplification stage with an effective length.   At least one of the relative phase shifters is configured as a relative phase shifter module including a pigtail fiber for inputting and outputting light to and from the relative phase shifter. The optical amplifier according to claim 1, wherein the optical amplifier is −2 [ps / nm / km] or more and 2 [ps / nm / km] or less. An optical fiber for amplification;
A pump light source for supplying pump light for amplifying signal light incident on the amplification optical fiber to the amplification optical fiber,
The amplification optical fiber is divided into a plurality of amplification stages by a relative phase shifter that shifts only the phase of the pump light and changes the relative phase without shifting the phase of the signal light and idler light,
The nonlinear constant γ and length of the amplification optical fiber of the amplification stage so that the relative phase related to the light output from the amplification optical fiber of the amplification stage connected to the upstream side of the relative phase shifter is larger than 0.5π. The product γPL of L and the input power P of the pump light to the amplification optical fiber of the amplification stage is set,
The relative phase shifter changes the relative phase to a value smaller than 0.5π,
Output to the amplification stage connected to the back side,
At least one of the relative phase shifters is configured as a relative phase shifter module including a pigtail fiber for inputting / outputting light to / from the relative phase shifter, and the dispersion of the pigtail fiber is in the entire operating wavelength range. -2 [ps / nm / km] or more and 2 [ps / nm / km] or less.
Here, the relative phase φrel is Δk = ksignal defined by the signal light phase φsignal, the idler light phase φidler, the pump light phase φpump, the signal light wavenumber ksignal, the idler wavenumber kidler, and the pumplight wavenumber kpump. + kidler-2kpump is an amount described by the following equation using the propagation distance z of each light in the amplification optical fiber .
φrel = Δk z + φsignal + φidler-2φpump An optical fiber for amplification;
A pump light source for supplying pump light for amplifying signal light incident on the amplification optical fiber to the amplification optical fiber,
The amplification optical fiber is divided into a plurality of amplification stages by a relative phase shifter that shifts only the phase of the pump light and changes the relative phase without shifting the phase of the signal light and idler light,
The nonlinear constant γ and length of the amplification optical fiber of the amplification stage so that the relative phase related to the light output from the amplification optical fiber of the amplification stage connected to the upstream side of the relative phase shifter is larger than 0.5π. The product γPL of L and the input power P of the pump light to the amplification optical fiber of the amplification stage is set,
The relative phase shifter changes the relative phase to a value smaller than 0.5π,
Output to the amplification stage connected to the back side,
The γPL of the amplification optical fiber of the amplification stage connected to the rear stage side is set so as to compensate for a relative phase shift caused by the loss of the relative phase shifter,
At least one of the relative phase shifters is configured as a relative phase shifter module including a pigtail fiber for inputting / outputting light to / from the relative phase shifter, and the dispersion of the pigtail fiber is in the entire operating wavelength range. -2 [ps / nm / km] or more and 2 [ps / nm / km] or less.
Here, the relative phase φrel is Δk = ksignal defined by the signal light phase φsignal, the idler light phase φidler, the pump light phase φpump, the signal light wavenumber ksignal, the idler wavenumber kidler, and the pumplight wavenumber kpump. + kidler-2kpump is an amount described by the following equation using the propagation distance z of each light in the amplification optical fiber .
φrel = Δk z + φsignal + φidler-2φpump   An optical amplification system comprising the optical amplifier according to claim 1.   An optical communication system comprising the optical amplifier according to claim 1.   A wavelength converter comprising the optical amplifier according to claim 1. An optical amplification method for supplying pump light for amplifying signal light incident on an amplification optical fiber to the amplification optical fiber,
The amplification optical fiber is divided into a plurality of amplification stages by a relative phase shifter that shifts only the phase of the pump light and changes the relative phase without shifting the phase of the signal light and idler light,
The nonlinear constant γ and length of the amplification optical fiber of the amplification stage so that the relative phase related to the light output from the amplification optical fiber of the amplification stage connected to the upstream side of the relative phase shifter is larger than 0.5π. The product γPL of L and the input power P of the pump light to the amplification optical fiber of the amplification stage is set,
The relative phase shifter changes the relative phase to a value smaller than 0.5π,
Output to the amplification stage connected to the back side,
An optical amplification method characterized in that a relative phase shift amount of a certain relative phase shifter in a subsequent stage is smaller than a relative phase shift amount of a certain relative phase shifter among the relative phase shifters.
Here, the relative phase φrel is Δk = ksignal defined by the signal light phase φsignal, the idler light phase φidler, the pump light phase φpump, the signal light wavenumber ksignal, the idler wavenumber kidler, and the pumplight wavenumber kpump. + kidler-2kpump is an amount described by the following equation using the propagation distance z of each light in the amplification optical fiber .
φrel = Δk z + φsignal + φidler-2φpump

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