JPH0829815A - White ultrashort pulse light source - Google Patents

Abstract

PURPOSE:To provide a light source for outputting >=1 kinds of ultrahigh-speed light pulse trains having a desired central wavelength and spectra and outputting chirp control pulses which have desired repetition and pulse width, are stable in wavelength with all channels and are controlled in chirping without having excess spectrum spread. CONSTITUTION:Exciting light pulse is made incident on an optical nonlinear waveguide 3 exhibiting a three-dimensional nonlinear optical effect to induce the white light pulses spread in spectra. The chirping included in the white light pulses induced in the nonlinear optical waveguide 3 is at least partially offset by a chirp control element 4. The components of the previously determined central wavelength and spectral width are extracted by a wavelength demultiplexing element 5 from the output light of this chirp control element 4.

Description

Detailed Description of the Invention

[0001]

The present invention is used in optical transmission or optical signal processing. In particular, it relates to a white ultrashort pulse light source that generates one or more ultrashort optical pulse trains having a desired center wavelength and a spectral width in a wide wavelength range.

[0002]

2. Description of the Related Art Optical time division multiplexing (OT) is a technique for dramatically increasing the communication capacity in future optical communication transmission systems.
The DM) transmission system and the wavelength division multiplexing (WDM) transmission system are under study.

The OTDM transmission system is a system that uses ultrashort optical pulses and multiplexes up to about the reciprocal of the pulse width on the time axis. As a light source, it is only necessary to obtain an ultrashort optical pulse train of a relatively high speed at one wavelength, and a gain switching method and a mode locking method for a semiconductor laser (LD), a mode locking method for an Er fiber laser, etc. have been studied.

On the other hand, in the WDM transmission system, a large number of signal lights having different wavelengths (optical carrier frequencies) are multiplexed / demultiplexed by an optical multiplexer / demultiplexer, so that the capacity that can be transmitted by one optical fiber is determined by the number of wavelengths There is an advantage that the number of channels can be increased.
However, since the optical fiber has a limited wavelength range with low loss and low dispersion, dense wavelength multiplexing technology is required to increase the number of channels, and high accuracy (accuracy) for the wavelength of the light source used. And stability is required. However, since the current semiconductor laser light sources have large variations in oscillation wavelength and large temperature changes, in conventional multi-channel WDM transmission experiments, semiconductor lasers selected according to the number of wavelengths are prepared and the operating voltage and temperature are controlled respectively. Therefore, the desired wavelength was obtained. Therefore, the conventional WDM transmission system has a problem in terms of cost, stability, and reliability in addition to the need to select the light source and the extremely large control system.

Recently, an array type distributed feedback semiconductor laser (DFB-LD) capable of obtaining continuous wave oscillation output of multiple wavelengths with one chip is described in Reference 1: CEZAH et al., "1.5 μm compressive-strained m".
ultiquantum-well 20 wavelength distributed-feedbac
k laser arrays ", Electron. Lett., 1992, 28, pp.824-8
25, a mode-locked Er fiber (ML-EDF) laser in which pulse trains of four different wavelengths simultaneously oscillate is described in Reference 2: Takara, Kawanishi, Saruwatari, Schlager, "Multiwaveleng.
th birefringent-cavitymode-locked fiber laser ", El
ectron. Lett., 1992, 28, pp.2274-2275.

Array type DFB-LD described in Document 1
In, a continuous wave oscillation output of 20 wavelengths is obtained by integrating a plurality of DFB-LD waveguides in which the pitch of the diffraction grating that determines the oscillation wavelength is changed little by little. However, this array type DFB-LD still has unsolved problems peculiar to semiconductors, that is, the problem that the oscillation wavelength largely changes with temperature and current. The problem arises that the coupling with the array) is extremely difficult.

On the other hand, the multi-wavelength ML-E described in Document 2
The DF has a ring resonator configuration using a polarization-maintaining optical fiber, and is capable of extracting a high-speed multiwavelength short optical pulse train from the optical fiber. A configuration example of the multi-wavelength ML-EDF is shown in FIG. In this configuration example, the Er fiber amplifier 101 and the modulator 102 are connected to the polarization maintaining optical fiber 1
By inserting the birefringent medium 104 and the polarizer 105 in the mode-locked laser resonator connected by 03 so that the resonator length varies depending on the wavelength, multi-wavelength mode-locked oscillation is possible. Since the output of the multi-wavelength ML-EDF is taken out from the optical fiber, there is an advantage that an optical fiber type optical functional device can be easily connected. Therefore, there is an advantage that the WDM system can be simply constructed and the OTDM multiplex system utilizing the short pulse property can be used together.

However, in the multi-wavelength ML-EDF laser, the wavelength range in which oscillation is possible is limited by the gain bandwidth of the laser medium, and it is necessary to set the wavelength interval relatively wide so that mode competition does not occur. It was not possible to obtain a sufficient number of WDM channels. Further, since the pulse width is comparatively large at several tens of ps, sufficient characteristics have not been realized even for OTDM.

[0009]

In order to construct a future ultra large capacity optical transmission system, it is desired to use the OTDM technology and the WDM technology together. A laser light source that achieves this is desired to output one or more kinds of ultrafast optical pulse trains having a desired center wavelength and spectrum, and also have a desired repetition and pulse width. Also, wavelength stability is required for all channels, and a chirp controlled chirp control pulse with no extra spectral broadening is desired. Here, the chirp control pulse is a Fourier transform limited (TL) chirpless pulse having a minimum spectrum width in a given time waveform, or a linear shape having a desired chirping amount (wavelength change amount per unit time). It is a chirp pulse.

The present invention outputs a white ultrashort pulse light source satisfying the above conditions, that is, one or more kinds of ultrafast optical pulse trains having a desired center wavelength and a spectrum, and further, a desired repetition and pulse width. It is intended to provide a light source which has a stable wavelength for all channels and outputs a controlled chirp control pulse of chirping without extra spectral broadening.

[0011]

A white ultrashort pulse light source according to the present invention comprises an optical nonlinear waveguide exhibiting a third-order nonlinear optical effect, and an excitation optical pulse incident on the optical nonlinear waveguide, and its optical nonlinear guide A pumping light source that induces a white light pulse whose spectrum is broadened on both the long wavelength side and the short wavelength side of the pumping light pulse in the waveguide, and a white light pulse induced in the optical nonlinear waveguide. A chirp control means having a group velocity dispersion characteristic that at least partially cancels the included chirping, and a wavelength demultiplexing means for extracting a component having a predetermined center wavelength and spectrum width from the output light of the chirp control means. It is characterized by having.

The pumping light pulse is preferably a short light pulse whose spectrum is substantially Fourier transform limited. In addition, the spectrum width of the white light pulse is 10 times wider than the width which spreads due to the self-phase modulation effect of the optical nonlinear waveguide.
It is desirable to be more than twice as large.

The wavelength demultiplexing means can include means for extracting one or more components with a spectral width wider or narrower than the spectrum of the pumping light pulse.

It is preferable that the chirping control means includes a dispersion medium having a group velocity dispersion amount having a value of a different sign which is substantially the reciprocal of the amount of chirp contained in the white light pulse in a predetermined wavelength range. An optical waveguide having a positive or negative dispersion characteristic in a predetermined wavelength range can be used as the chirping control means.

[0015]

When an ultra-short optical pulse having a nearly Fourier transform limit (TL) is transmitted through an optical waveguide having a third-order nonlinear optical effect, when a certain input level is exceeded, the self-wavelength on both sides of the wavelength of the pumping optical pulse is increased. An ultra-wide band white light pulse (Superc
ontinuum (hereinafter also referred to as “SC”) occurs. This ultra-wide band white light pulse has a chirp characteristic determined by the dispersion characteristic of the third-order optical nonlinear waveguide, and the chirp control means having an appropriate group velocity dispersion characteristic makes the chirping amount near the desired wavelength zero. Alternatively, it is controlled to a desired positive or negative magnitude. Therefore, the optical pulse cut out by the wavelength dispersion means becomes a chirpless or linear chirp pulse having a desired center wavelength, and when the chirping is zero and the spectrum width is wider than the input spectrum width,
An output pulse narrower than the input pulse width can be obtained. Also,
If it is wider than the spectral line width, an output pulse narrower than the input pulse width is obtained. The pulse width, the shape of the pulse, and the number of channels can be selected by appropriately selecting the band width and transmission characteristics of the optical filter to be cut out.

[0016]

FIG. 1 is a block diagram showing a white ultrashort pulse light source according to an embodiment of the present invention. This white ultra-short pulse light source is provided with an SC generator 1, and this SC generator 1 makes an optical nonlinear waveguide 3 exhibiting a third-order nonlinear optical effect and an excitation light pulse enter the optical nonlinear waveguide 3. The optical nonlinear waveguide 3
And a pumping short optical pulse light source 2 for inducing a white light pulse whose spectrum is broadened to both the long wavelength side and the short wavelength side with respect to the pumping optical pulse. The white ultrashort pulse light source also has a chirp control element 4 having a group velocity dispersion characteristic for at least partially canceling the chirping contained in the white light pulse induced in the optical nonlinear waveguide 3, and the chirp control element 4 And a wavelength demultiplexing element 5 for extracting a component having a predetermined center wavelength and a spectral width from the output light.

The short optical pulse light source 2 for excitation generates a short optical pulse for excitation of about several ps and makes it enter the optical nonlinear waveguide 3. In the optical nonlinear waveguide 3, this short optical pulse causes
Self-phase modulation, which is a third-order nonlinear optical effect
modulation: SPM, cross-phase modulation
modulation: XPM), four-wave mixing, stimulated Raman scattering, etc. occur simultaneously, and a white light pulse (SC) having an ultra-wide band spectrum over a wide wavelength range is obtained. This is shown in FIG. For example, the optical nonlinear waveguide 3 has a length of 3
Using an optical fiber of km, this has a peak power of about 2W,
When excited by a short light pulse having a pulse width of about 3 ps, the spectral width of the white light pulse generated reaches 200 nm.
This corresponds to 50 times the spread width of 4 nm due to only self-phase modulation. At this time, the white light pulse has a chirp characteristic that is determined by the dispersion characteristic of the optical nonlinear waveguide 3. However, if the white pulse is limited to a spectral range equal to the spectral width of the incident pulse, the pulse width is almost the incident pulse width at any wavelength. Is maintained at a size (about several ps).

The chirp control element 4 converts the white light pulse into a chirp control pulse having a desired chirp. By extracting a specific wavelength component from the chirp control pulse by the wavelength demultiplexing element 5, it is possible to extract an ultrashort pulse having a desired chirp at an arbitrary center frequency. When the controlled chirping is zero and the spectrum width of the wavelength component to be cut out is wider than the input spectrum width, an output pulse having a pulse width narrower than the input pulse width is obtained. Further, when the spectrum width is narrower than the input spectrum width, an output pulse having a wider pulse width than the input pulse width can be obtained.

FIG. 1 shows an example in which a WDM (Wavelength-division Multiplexing) filter with one input and multiple outputs is used as the wavelength demultiplexing element 5, but a dielectric multilayer film bandpass filter or the like may be used. In the case of a WDM filter, it is possible to obtain chirp control pulses of multiple wavelengths (multiple channels) at an appropriate wavelength interval.

A band attenuating filter for adjusting the output level of the pumping light wavelength to the output level of another wavelength may be inserted into the output terminal of the SC generator 1 as required.

3 and 4 are diagrams for explaining the spectrum of an ultra-wide band white light pulse due to the third-order nonlinear optical effect. FIG. 3 shows the dispersion characteristics of the optical nonlinear waveguide, and FIG. 4 shows the optical nonlinear optical waveguide. The group delay characteristic of the generated white light pulse is shown. This group delay characteristic is given by the wavelength integration of chromatic dispersion. For example, an ordinary optical fiber exhibits the chromatic dispersion characteristics as shown in FIG. 3A. The sign is negative (normal dispersion) on the short wavelength side and the sign is positive (abnormal) on the long wavelength side across the zero dispersion wavelength. Dispersion). In this case, the group delay characteristic has a downward convex curve as shown in FIG. 3 (b) and 3 (c) show the dispersion characteristics of a special optical fiber whose chromatic dispersion has a substantially constant negative value or positive value in the wavelength region shown in FIG. As in b) and (c), linear chirping (a positive or negative slope depending on the sign of the variance value) has a substantially constant slope over a wide range. Therefore, the generated white light pulse has a blue shift chirping (a negative slope) in which the wavelength becomes shorter with time in the case of normal dispersion (a negative value (b)), and an abnormal dispersion (a positive value (c). In the case of (1), the wavelength becomes red shift chirping (inclination is positive) with time.

FIG. 5 is a diagram for explaining the principle of chirp control. Here, in the case of complete compensation, an ultra-wide band white light pulse having a red shift chirping characteristic will be described as an example.

The output of the SC generator is, as shown in FIG.
The change in wavelength with time has a substantially linear positive slope (for example, 10 Anm / ps). Again, A ^-1 (p
s / nm) If the white light pulse is input to a chirp control element that has the same group delay amount as A ⁻¹ and the opposite group sign amount of the white light pulse represented by Is completely compensated, and the output white light pulse becomes a straight line having a zero slope as shown in FIG. Here, if a white light pulse is cut _out with a wavelength dispersion element having a band width wider than the spectral width of the excitation light pulse, the pulse width Δt _out is given by the Fourier transform of the band width,
The pulse is compressed by the excitation light pulse width. In this case, as shown in FIG. 5, the chirping is compensated over a wide range of wavelengths, so that an ultrashort optical pulse compressed at any wavelength within this range can be obtained. Therefore, by using a 1-input multi-output WDM wavelength filter or the like, it is possible to simultaneously obtain a multi-channel ultrashort pulse train having a plurality of center wavelengths. As is clear from this principle, the chirp amount (nm / ps) of the white light pulse can be controlled to a desired value by properly selecting the group delay amount and the sign of the chirp control element. The chirp pulse can be used even when the wavelength of the light source is largely shifted from the zero-dispersion wavelength of the transmission line optical fiber in order to suppress generation of four-wave mixing (FWM).

FIG. 6 shows a specific embodiment. here,
An optical nonlinear waveguide exhibiting a third-order nonlinear optical effect and a chirp control means having group velocity dispersion characteristics for at least partially canceling the chirping contained in the white light pulse induced in the optical nonlinear waveguide An arrayed waveguide type periodic optical wavelength filter 15 is used as a wavelength demultiplexing means for extracting components of a predetermined center wavelength and spectrum width from the output light of the optical fiber 14 using the fibers 13 and 14. For details of the arrayed waveguide type periodic optical wavelength filter 15, see, for example, Document 3: Hiroshi Takahashi, Yoshinori Hibino, Isao.
Nishi, "Polarization-insensitive arrayed-waveguide
grating wavelength multiplexer on silicon ", Optics
Lett., Vol.17, pp.499-501.

A short optical pulse train for excitation having a wavelength λ ₀ is input to an optical fiber 13 for generating a white light pulse, and an ultra-wide band white light pulse train having chirping is generated. This white light pulse train is input to the optical fiber 14 for chirp control, and after its chirping is completely compensated, a plurality of different wavelengths are cut out by the periodic optical wavelength filter 15 and each corresponds to the Fourier transform of the bandwidth. As a result, a multichannel ultrashort pulse train having a pulse width is obtained. The center wavelength of the obtained multi-wavelength ultra-short pulse train is determined by the wavelength characteristics of the periodic optical wavelength filter 15, and its value is continuously adjusted by finely adjusting the relative optical path difference of a large number of optical paths forming the optical filter. Can be changed. Furthermore, by adjusting the bandwidth of the periodic optical wavelength filter 15,
It is possible to generate an ultrashort pulse satisfying the TL characteristic with an arbitrary pulse width. As an example, if a bandpass filter having a Lorentz type transmission characteristic with a band width of 3 nm is used, it is possible to obtain an ultrashort pulse of about 300 fs at 1550 nm.

FIGS. 7, 8 and 9 are diagrams for explaining the operation when an optical fiber is used for the optical nonlinear waveguide and the chirp control means, respectively. Here, FIG. 7 shows a red shift chirp pulse, FIG. 8 shows a blue shift chirp pulse, and FIG. 9 shows an example of chirp control (compensation) for a white light pulse having both of them.

In FIG. 7, as the group delay characteristics of the chirp controlling optical fiber, an example thereof is that the dispersion wavelength is on the wavelength side sufficiently longer than the wavelength area for chirp compensation, and the group delay characteristics are in the wavelength area for chirp compensation. Use one that can be approximated by a straight line. Similarly, in FIG. 8, a wavelength region where the zero dispersion wavelength is sufficiently shorter than the wavelength region in which chirp compensation is performed is used. Further, in FIG. 9, the group delay characteristic of the white light pulse is zero in a downwardly convex curve including the zero dispersion wavelength, and has both blue and red chirping characteristics. In this case, two different optical fibers whose zero dispersion wavelength is on the sufficiently long wavelength side or the sufficiently short wavelength side may be used as shown in the figure, depending on the wavelength region for chirp compensation. As an example,
In the case of FIG. 7 and FIG. 8, assuming that the dispersion of the optical fiber for generating the white light pulse is about −1 ps / nm / km at 1550 nm and the fiber length is 3 km, the blue shift of the white light pulse generated with 1550 nm as the excitation wavelength is obtained. The inclination of chirping is about -0.3 to -0.5 ps / nm,
If a 1.3 μm zero dispersion optical fiber (dispersion 17 ps / nm / km at 1550 nm) is used for 120 to 180 m, the chirping can be compensated. In addition,
As is clear from the above description, the chirp pulse having an arbitrary magnitude can be obtained by selecting the sign of the dispersion value and the fiber length of the chirp control optical fiber.

[0028]

As described above, the white ultrashort pulse light source of the present invention outputs one or more kinds of ultrafast optical pulse trains having a desired center wavelength, and has a desired repetition rate and bandwidth (pulse width). ) Is provided.
In addition, in principle, it is possible to obtain a chirped controlled optical pulse train that has a stable center wavelength in channels of all wavelengths and has no extra spectral spread. Therefore, by using the white ultra-short pulse light source of the present invention, it becomes possible to use the WDM technology in addition to the OTDM technology, and the future 100 Gbit / s to 1 Tbit / s ultra-high-speed wavelength division multiplex transmission method and ultra-high-speed optical signal processing. It has a great effect when used for.

[Brief description of drawings]

FIG. 1 is a block diagram showing a white ultrashort pulse light source according to an embodiment of the present invention.

FIG. 2 is a diagram showing respective spectra of a pumping light pulse and a white light pulse induced in an optical nonlinear waveguide.

FIG. 3 is a diagram illustrating a spectrum of an ultra-wide band white light pulse due to a third-order nonlinear optical effect, and a diagram showing dispersion characteristics of an optical nonlinear waveguide.

FIG. 4 is a diagram for explaining a spectrum of an ultra-wide band white light pulse due to a third-order nonlinear optical effect, and is a diagram showing a group delay characteristic of a white light pulse generated in an optical nonlinear optical waveguide.

FIG. 5 is a diagram illustrating the principle of chirp control.

FIG. 6 is a diagram showing a specific example.

FIG. 7 is a diagram illustrating chirp compensation in the case of a red shift chirp pulse.

FIG. 8 is a diagram illustrating chirp compensation in the case of a blue shift chirp pulse.

FIG. 9 is a diagram illustrating chirp compensation of a white light pulse having both red and blue shift chirp pulses.

FIG. 10 is a diagram showing a conventional multi-wavelength ML-EDF.

[Explanation of symbols]

 1 SC generation unit 2 Short optical pulse light source for pumping 3 Optical nonlinear waveguide 4 Chirp control element 5 Wavelength dispersive element 13, 14 Optical fiber 15 Periodic optical wavelength filter 101 Er fiber amplifier 102 Modulator 103 Polarization-maintaining optical fiber 104 Complex Refractive medium 105 Polarizer

Claims (7)

[Claims] 1. An optical nonlinear waveguide exhibiting a third-order nonlinear optical effect, and a pumping light pulse is incident on the optical nonlinear waveguide, and the wavelength of the pumping light pulse is long within the optical nonlinear waveguide. Pump light source for inducing a white light pulse having a spectrum broadened on both the side and the short wavelength side, and a group velocity for at least partially canceling the chirping contained in the white light pulse induced in the optical nonlinear waveguide. A white ultrashort pulse light source comprising a chirp control means having a dispersion characteristic and a wavelength demultiplexing means for extracting a component having a predetermined center wavelength and spectrum width from the output light of the chirp control means. 2. The white ultrashort pulse light source according to claim 1, wherein the excitation light pulse is a short light pulse whose spectrum is given substantially by Fourier transform limitation. 3. The white ultrashort pulse light source according to claim 1, wherein the white light pulse has a spectral width 10 times or more wider than a width thereof widened by a self-phase modulation effect of the optical nonlinear waveguide. 4. The white ultrashort pulse light source according to claim 1, wherein said wavelength demultiplexing means includes means for extracting one or more components with a spectrum width wider than the spectrum of said pumping light pulse. 5. The white ultrashort pulse light source according to claim 1, wherein said wavelength demultiplexing means includes means for extracting one or more components with a spectral width narrower than the spectrum of said pumping light pulse. 6. The chirping control means includes a dispersion medium having a group velocity dispersion amount having a value of a different sign which is approximately the reciprocal of the chirp amount included in the white light pulse in a predetermined wavelength range. The described white ultrashort pulse light source. 7. The white ultrashort pulse light source according to claim 1, wherein the chirping control means includes an optical waveguide having a positive or negative dispersion characteristic in a predetermined wavelength range.