JP2005241732A - Optical pulse amplification apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical pulse amplification apparatus provided with a light source capable of making a wavelength band used for excitation variable even though having a simple structure similar to conventional one. <P>SOLUTION: The optical pulse amplification apparatus is provided with the light source generating a laser pulse sequence whose intensity is changed in time by modulating intensity of a laser beam pulse sequence and outputting the laser beam pulse sequence as a optical soliton sequence whose frequency is changed by using a soliton self-frequency shift effect. The optical pulse amplification apparatus is further provided with an optical fiber for Raman amplification converting light outputted from the light source into excited light, an optical coupler inputting the optical soliton sequence to the optical fiber for Raman amplification, an input section inputting a pulse beam to be amplified to the optical fiber for Raman amplification and an output section outputting amplified light from the optical fiber for the Raman amplification and the excited light and the pulse beam to be amplified are going in directions reverse to each other and the pulse beam to be amplified collides with a plurality of pulse beams contained in the excited light. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an optical pulse amplifying apparatus that amplifies an optical pulse signal to be amplified by excitation with an optical pulse for Raman excitation when passing through an optical fiber.

  In large-capacity optical communications, signals are distributed over a wide wavelength range, so an ultra-broadband optical amplifier is desired. In an optical communication wavelength region, when an erbium-doped fiber amplifier is used, it is known that light can be amplified in a band of 1530 to 1570 nm, but it is difficult to use in a band beyond that. As means for solving this problem, means using the stimulated Raman effect is known. This inputs light of an appropriate wavelength into the optical fiber as pumping light, thereby causing a stimulated Raman effect and amplifying the signal slightly longer wavelength from the pumping light (about 100 nm for silica-based fibers). It is to do. However, in general, the gain band due to this effect is narrow. Even in the case of the optical pulse amplification device using the stimulated Raman effect in the silica fiber, the gain band is narrow, and therefore, a wideband amplification may be performed using a large number of light sources having different wavelengths as excitation light.

  Non-Patent Document 1 reports the optical amplifier shown in FIG. In this optical amplifier, the light from the wavelength sweep type pulse laser light source is incident on the optical fiber from the opposite direction to the optical signal to be amplified, and the light is amplified by the stimulated Raman effect by the excitation light. Is to do. Here, a mechanism is adopted in which a femtosecond pulse having a constant amplitude is input to a nonlinear fiber, and a spectrum component in a specific wavelength band is extracted after the spectrum has spread. After that, by passing through the fiber and chirping to widen the pulse, the interaction due to the Raman effect that occurs in the excitation light function is suppressed.

  Furthermore, Non-Patent Document 2 reports the optical amplifier shown in FIG. In this optical amplifier, light from light sources of various emission wavelengths are combined, and pumping light is optical fiber (for example: optical fiber mainly composed of tellurium oxide) from the opposite direction to the optical signal to be amplified. The light is amplified by the stimulated Raman effect by the excitation light.

  Non-Patent Document 3 reports the optical amplifier shown in FIG. In this optical amplifier, the light emission times of light sources having a plurality of emission wavelengths are controlled so that light pulses of different wavelengths are sequentially incident on the optical fiber in which stimulated Raman occurs from the opposite direction to the optical signal to be amplified. Thus, optical amplification is performed by the stimulated Raman effect.

J.W.Nicholson,. Fini, J. C. Boutiller, J. Bromage, and K. Bar, “A Swept-wavelength Raman pump with 69 MHz repetition rate”, PD46-1, Optical Society of America (2003) A. Mori, H Masuda, K. Shikano, and M. Shimizu, “Ultra-Wide-Band Tellurite-Based Fiber Raman Amplifier”, vol. 21, No.5, 1300-1306, JOURNAL OF LIGHT WAVE THECHNOLOGY, 2003. L. F. Mollenauer, A. R. Grant, and P. V. Mamyshev, “Time-division multiplexing of pump wavelengths to achieve ultrabroadband, flat, backward-pumped Raman gain”, OPTICS LETTERS, Vol. 27, No. 8, pp. 592-594 2002.

  As described above, a light source having a broad emission wavelength band is required for an ultra-broadband optical amplifier. However, in the case where a light source in which a large number of lasers having different wavelengths are multiplexed as in the conventional case, there is a problem that a non-uniform gain is seen in the spectrum due to the stimulated Raman effect. Further, with the excitation means that electrically changes the wavelength as shown in FIG. 15, it is difficult to change the wavelength at high speed, and there is a limit to the power of the obtained excitation light. Further, the apparatus shown in FIG. 13 is difficult to use because of its complicated structure.

  Since light from such a light source is used for excitation of the stimulated Raman effect, it is desirable that its intensity be sufficiently large. However, the configuration of such a conventional light source is not simple. The present invention proposes an optical pulse amplification device provided with a light source capable of making the wavelength band used for excitation variable while having a simple configuration as in the prior art.

  In the present invention, the excitation light is incident on a fiber (for example, silica fiber or tellurium oxide optical fiber) that induces stimulated Raman from the opposite direction to the optical signal to be amplified, and the wavelength bandwidth of the excitation light is electrically reduced. By adjusting to, the wavelength bandwidth to be amplified can be easily adjusted.

  First, the soliton self-frequency shift effect, which is the backbone of the light source used in the present invention, will be described using the configuration of FIG. The mode-locked fiber laser (MLFL) 2 receives a signal of, for example, 10 GHz from the high-frequency signal source 1 and oscillates an optical pulse train having a repetition frequency of 10 GHz. This optical pulse train is amplified by an optical amplifier 3 using an optical fiber. This amplification degree is also variable and is used to adjust the peak intensity of the optical pulse train. The polarization direction of the amplified light is selected by the polarization adjuster 4, passes through the photonic crystal fiber (PCF) 5, and is sent to the measuring device 6.

FIG. 2 shows the spectrum of the output light when the average intensity of the input pulse having a constant amplitude to the PCF is gradually changed from 8 mW to 1070 mW in the apparatus of FIG. The spectra shown in FIGS. 2 (a), (b), (c), and (d) are the spectra when the average power of the optical pulse at the time of incidence on the PCF 5 is 8, 810, 950, and 1070 mW, respectively. Show. As can be seen from this graph, a spectral component of the hyperbolic secant function sech ² that shifts to the longer wavelength side as the incident intensity increases is generated. This is known to be a soliton, and this phenomenon is known to be due to the soliton self-frequency shift effect.

  FIG. 3 shows an example of such a soliton. FIG. 3A shows a spectral characteristic, and FIG. 3B shows an autocorrelation waveform of the soliton pulse. As can be seen from this figure, solitons have a broad spectrum and therefore their pulse width is very short. FIG. 4 is an example showing the relationship between the intensity of incident light and the wavelength. From this example, it can be seen that as the intensity increases, the value shifted to the longer wavelength side increases.

  FIG. 5 is a diagram showing the pulse width (white square) with respect to the wavelength and the spectral width (black circle) with respect to the wavelength, and it can be seen that as the amount of wavelength conversion increases, the pulse width decreases and the spectrum width increases. It can also be seen that the optical pulse is as short as several hundred femtoseconds even when the center wavelength of the soliton changes by about 120 nm.

  By the above method, the wavelength conversion of the pulse is performed with a high conversion efficiency of 40% or more. When the output light is observed using an optical detector and an oscilloscope, it can be seen that two pulses that are separated in time repeat as shown in inset of FIG. Here, for the component that was not converted in the front and the wavelength-converted pulse in the rear, these intervals further depend on the wavelength change of the (delayed) soliton. FIG. 6 is an example showing the group delay versus wavelength characteristics of solitons. As can be seen from this figure, the delay time increases as the wavelength conversion value increases.

  The excitation light source used in the present invention applies amplitude fluctuation to each pulse of the pulse train at a constant time interval, amplifies this, and then inputs it to the nonlinear fiber. For this reason, it can be seen that due to the soliton self-frequency shift, the amount of change in wavelength of each optical pulse differs (based on FIG. 4), and the time output from the fiber exit also varies. When the intensity modulation is periodic, it can be easily seen that the soliton pulse periodically varies corresponding to the modulation signal. For example, in the optical pulse train having a repetition frequency of 10 GHz shown in FIG. 6, the energy of the optical pulse is periodically changed from 65 pJ (average power: 0.65 W) to 0.81 pJ (average power: 0.81 W) and input to the fiber. The wavelength of the output pulse from the fiber output surface varies periodically between 1570 nm and 1587 nm, or the output time (based on the incident wave pulse) for about 15 psec (20 psec to 35 psec). I can see you going.

Next, the concept of the excitation light source of the optical pulse amplifier of the present invention is shown in FIG.
1) First, in this device. An optical pulse train from a light source that periodically generates optical pulses is input to an optical modulator.
2) Next, intensity modulation is performed so that the intensity of the input optical pulse changes gradually and so that each optical pulse changes periodically. This modulation generates optical pulse trains having different amplitudes.
3) This optical pulse train is amplified and input to the nonlinear fiber. Instead of a nonlinear fiber, a nonlinear optical crystal or semiconductor that causes a Raman self-frequency shift can be used.
4) Since the incident light component and the optical soliton component are output from the nonlinear optical waveguide 14, the optical soliton component is selected and output by the filter 15. The filter cuts the input wavelength component without soliton shift. This can be realized by using, for example, a fiber type Bragg grating. In addition, when the amplitude of the input pulse is increased and the frequency shift amount is increased, the intensity fluctuation of the output pulse also increases. Therefore, a pulse having a constant amplitude is generated by suppressing the dependency of the intensity of the output pulse on the wavelength shift amount. In order to achieve this, it is desirable to use a filter 15 having a wavelength-dependent transmission characteristic. This can be realized by using, for example, a long-period fiber Bragg grating shown in FIG. However, the wavelength that changes due to the soliton self-frequency shift effect is also different, and it is obvious that even if the wavelength of the pulse output from the nonlinear fiber is different, it can be used as the pumping light.

  In the optical modulator that modulates the above optical pulse train, the input optical pulse train is modulated by a sine function having a frequency lower than the repetition frequency, or a periodic signal having a predetermined shape such as a sawtooth wave or a staircase wave. The optical pulse train having the amplitude distributed according to the modulation signal can be output periodically. From the difference in amplitude, the wavelength of the optical pulse train changes due to the soliton self-frequency shift effect. For example, if the repetition frequency of the input light pulse is f0 and the modulation frequency is fm, two light pulses of the same wavelength are generated every fm seconds. In particular, in order to change the wavelength of the optical pulse monotonously with respect to time, it is desirable to synchronize the repetition signal of the optical pulse train and the modulation signal. However, it is clear that even in the asynchronous case, a change in the wavelength of the optical pulse can be seen. Obviously, it can be observed even if K is a natural number and repeated every K / fm seconds. In particular, when the ratio between f0 and fm is large (fo / fm >> 1), excitation light that appears to change the wavelength of the optical pulse continuously with modulation is obtained. The repetition frequency of the pulse train to be amplified is independent of f0 and fm.

  When the wavelength of the optical pulse is converted by the soliton self-frequency shift effect as described above, the wavelength of the optical pulse is converted depending on the intensity of the optical pulse or the length of the nonlinear optical fiber.

  For this reason, in the optical pulse amplification device of the present invention, as a light source, a laser generator that generates a laser light pulse train and light that generates a laser pulse train that changes in intensity temporally by modulating the intensity of the laser light pulse train described above. A modulator, means for amplifying, and an optical transmission medium that generates optical solitons by entering a modulated laser light pulse train and has solitons of different wavelengths (or frequencies) depending on the intensity of the incident pulses; A light source that outputs an optical pulse from the optical transmission medium is used. Further, a Raman amplification optical fiber that amplifies an optical signal using output light from the light source as excitation light, and an optical coupler that inputs output light from the optical transmission medium to the Raman amplification optical fiber; An input unit that inputs pulsed light to be amplified to the Raman amplification optical fiber, and an output unit that outputs the amplified light from the Raman amplification optical fiber. The pulsed light travels in the Raman amplification optical fiber in opposite directions, and the amplified pulsed light is a plurality of pulsed light included in the pumping light in the Raman amplification optical fiber. To collide with.

  In another optical pulse amplification device of the present invention, the modulation signal applied to the optical modulator is a periodic signal, and the period is a time required for the amplified pulse light to pass through the Raman amplification optical fiber. It should be less than 4 times.

  Embodiments of the present invention will be described below in detail with reference to the drawings. In the following description, devices having the same function or similar functions are denoted by the same reference numerals unless there is a special reason. It should be noted that the following examples are for illustrative purposes and are not limited to the following examples.

  In the excitation light source of the present invention, as shown in FIG. 7, an optical pulse train from the light source 11 that periodically generates optical pulses is input to the intensity modulator 12, and the input optical pulses are gradually generated in the optical pulse train. As it changes, each light pulse is subjected to a periodically changing intensity modulation. By this modulation, optical pulse trains having different amplitudes can be generated. This optical pulse train is amplified and input to the nonlinear optical waveguide 14 using a nonlinear optical fiber. Here, since the input optical pulses have different amplitudes, the wavelengths that change due to the soliton self-frequency shift effect are different, and the wavelengths of the pulses output from the nonlinear fiber are also different. Further, since the amplitude of the optical pulse output from the nonlinear fiber is generally not constant, the optical pulse is passed through the filter 15 having saturation characteristics in order to suppress variations in the amplitude.

  In general, since the light output from the light source in FIG. 7 is polarized light, it is easy to increase the intensity by combining polarized light orthogonal thereto. FIG. 8 shows a configuration for that purpose. The output light from the intensity modulator 12 is branched and amplified. The amplification in this case is desirably performed at a higher degree of amplification than the amplification in the case of FIG. 7 in order to recover the intensity attenuated at the branch. In addition, since the stimulated Raman effect depends on the polarization of the pumping light, one optical pulse amplified to eliminate the polarization dependence is passed through the polarization adjuster 17 to obtain a polarization orthogonal to the other optical pulse, The signal is multiplexed by the multiplexer 16. Moreover, if it is set as the structure passed through a filter after combining, the number of the filters to be used can also be reduced. The multiplexer 16 is a multiplexer that uses the property of transmitting one polarized light and reflecting the polarized light orthogonal thereto, which is already well known.

  Next, an optical pulse amplifier using the stimulated Raman effect will be described with reference to FIG. In this figure, an input signal is output through an optical fiber 23 for Raman amplification by an induced Raman effect and an optical coupler 22. In addition, the pumping light is obtained by shaping the light from the wavelength-tunable optical pulse generator 20 using the soliton self-frequency shift effect using the pulse width (or spectrum) shaping means 21 and then shaping the optical pulse. When the light enters the optical path in the reverse direction from the optical coupler and passes through the optical fiber for Raman amplification, a stimulated Raman gain is performed. As this optical coupler 22, in addition to a normal optical coupler, an optical saculator shown in FIG. 11A can also be used. FIG. 12 shows an example of pulse width (or spectrum) shaping means. These can be configured by using a long-period fiber Bragg grating and a high dispersion type optical fiber. By using a long-period fiber Bragg grating, the wavelength dependency of the intensity of the wavelength tunable optical pulse generator can be eliminated, and the intensity can be made uniform or have a predetermined wavelength distribution. Since the pulse width from the light source is usually several hundred femtoseconds, it is desirable to widen the pulse width by passing it through a fiber having dispersion as shown in FIG. 12 in order to widen the pulse width. As a result, it is possible to increase the time of overlapping the input signal and the excitation pulse propagating in the opposite direction in the fiber, and to increase the energy transfer of the signal pulse light from the excitation pulse, so-called Raman gain high rate.

  In order to increase the so-called Raman gain high factor and to eliminate the polarization dependence of the stimulated Raman gain, the pumping light source can be realized in a series configuration as shown in FIG.

  When the input signal is composed of a multi-wavelength pulse train and the spectrum width is wide, it is desirable that the spectrum width of the wavelength-variable pumping light is the same as that of the input signal or wide so as to include it. Such a setting can be realized by adjusting the modulation degree of the light intensity modulator shown in FIG. 7 or FIG. 8, the bias voltage for adjusting the phase of light by the light intensity modulator, or the gain of the fiber amplifier. it can.

  In general, the band in which the stimulated Raman gain is obtained is on the long wavelength side of the pumping light by the wavelength determined by the medium. For this reason, it is necessary to design an excitation light source in consideration thereof. For example, when a silica-based optical fiber is used, a Raman gain is obtained on the long wavelength side of about 100 nm from the excitation wavelength, and thus a light source based on the above-described soliton self-frequency shift effect, in which the wavelength is variable in the 1550 to 1600 mm band, is used. In some cases, a stimulated Raman gain is obtained in the 1650-1700 nm band.

The input signal assumed in this optical amplification is an optical pulse train. In this amplifying apparatus, when the repetition frequency of the excitation optical pulse train is lowered, the intensity of the amplified optical pulse is changed, so that the repetition frequency of the excitation optical pulse train is desirably sufficiently high. This is because the stimulated Raman effect is likely to occur at a position in the optical fiber immediately after the excitation light pulse passes, but the excitation state is relaxed if time passes after the excitation light pulse passes. This is because the degree of amplification decreases accordingly. When the length of the Raman gain fiber is L, the transit time of the fiber in the signal light is Ln _g / c (n _g light group Index, c is the speed of light). If the modulation frequency of the modulator is fm, two excitation pulses having the same wavelength are generated every 1 / fm seconds. The collision between the excitation pulse of the signal light and the opposite direction, in order to be a collision between the excitation pulse all _{wavelengths, Ln g / c> 1 /} (4fm) it is necessary to satisfy the, Ln _g It is desirable to satisfy the condition of / c >> 1 / (4fm).

  In the above, the optical pulse amplification device using the excitation light source using the soliton self-frequency shift effect is used. However, the bandwidth occupied by the excitation light pulse increases by increasing the luminance of the pulse light source in the excitation light source. . It is easy to adjust the luminance of the pulsed light source by performing electrical control. Therefore, the bandwidth of the optical pulse amplifier can be easily increased electrically. In this way, an optical pulse amplifier that can electrically control the bandwidth can be easily realized.

It is a block diagram of the apparatus using the soliton self-frequency shift effect. It is a figure which shows the soliton self-frequency shift effect. (A) of the soliton shows the spectral characteristics, and (b) shows the autocorrelation waveform. It is a figure which shows the characteristic of the wavelength depending on the intensity | strength of incident light. It is a figure which shows the pulse width (white circle) with respect to a wavelength, and the spectrum width (black circle) with respect to a wavelength. It is a figure which shows the characteristic of group delay versus wavelength of solitons. It is a block diagram which shows the light source of this invention. It is a block diagram which shows the light source of this invention. It is a block diagram which shows the Raman amplification apparatus using the light source of this invention. It is a block diagram which shows the Raman amplification apparatus using the light source of this invention. It is a conceptual diagram which shows the optical circulator (a) used as an optical coupler, and an optical coupler (b). It is a block diagram which shows a pulse width (or spectrum) shaping means. It is a block diagram which shows the example of the wavelength scanning type | mold optical pulse light source used for the 1st conventional Raman amplification. It is a block diagram which shows the example of the 2nd conventional optical pulse Raman amplifier. It is a block diagram which shows the example of the 3rd conventional optical pulse Raman amplifier.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Signal source 2 Mode-locked fiber laser 3 Optical amplifier 4 Polarization regulator 5 Photonic crystal fiber (PCF)
6 Measuring instrument 11 Optical pulse light source 12 Optical modulator 13 Optical amplifier 14 Non-linear optical waveguide 15 Filter 16 Multiplexer 17 Polarization adjuster 20 Optical pulse generator 21 Pulse width shaping means 22 Optical coupler 23 Optical fiber for Raman amplification

Claims (4)

Light source
A laser generator for generating a laser light pulse train;
An optical modulator that generates a laser pulse train that changes in intensity with time by modulating the intensity of the laser light pulse train;
An optical transmission medium that generates optical solitons by entering a modulated laser light pulse train, and generates solitons having different wavelengths (or frequencies) depending on the intensity of the incident pulses;
A light source that outputs an optical pulse from the optical transmission medium,
An optical fiber for Raman amplification that amplifies an optical signal using output light from the light source as excitation light;
An optical coupler for inputting output light from the optical transmission medium to the Raman amplification optical fiber;
An input unit for inputting pulsed light to be amplified to the Raman amplification optical fiber;
An output unit for outputting the light amplified from the Raman amplification optical fiber,
The pumping light and the amplified pulsed light travel in opposite directions in the Raman amplification optical fiber,
The pulsed light to be amplified collides with a plurality of pulsed light included in pumping light in the Raman amplification optical fiber.   The modulation signal applied to the optical modulator is a periodic signal, and the period thereof is not more than four times the time required for the amplified pulse light to pass through the optical fiber for Raman amplification. The optical pulse amplification device described.   2. An optical pulse amplifying device further comprising a fiber Bragg grating for adjusting an intensity distribution of an optical spectrum of output light from the optical pulse amplifying device in addition to the optical pulse amplifying device according to claim 1.   2. The optical pulse amplification device according to claim 1, further comprising means for adjusting a pulse width of output light from the optical pulse amplification device by using dispersion of an optical fiber. .

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