JPH04192386A - Method and device for generating optical short-pulse - Google Patents

Abstract

PURPOSE:To offer an optical short-pulse generating device, by which a repeat period variable femto-second pulse light source is obtained, by using a semiconductor laser, a narrow-band filter, an erbium optical fiber amplifier and a fiber for compression use. CONSTITUTION:A high-repeat optical pulse train, which is obtained by the direct modulation of a semiconductor laser 15, is made to pass through a narrow-band optical filter 16, whereby the pulse train is transformed into a transformation-limited pulse train, this pulse train is amplified to a peak output capable of exciting a high-order solinton using an erbium optical amplifier 17 and an optical pulse of a narrow pulse width is obtained using a change in the waveform of an optical solinton which is propagated in an optical fiber. Thereby, a high-output and high-repeat femto-second pulse train, which reaches to a GHz or more, can be generated. When this pulse train is multiplexed, the multiplexed pulse train can be widely used for a superhigh-speed optical communication.

Description

DETAILED DESCRIPTION OF THE INVENTION "Field of Industrial Application" The present invention is directed to the generation of optical pulses from a semiconductor laser.
The present invention relates to a method and apparatus for generating short optical pulses that generate high-repetition sub-picosecond optical pulses, which have been impossible to generate conventionally, by using the pulse compression effect of optical solitons.

"Prior Art" Conventionally, a mode-locked soliton laser has been proposed as a method for generating sub-picosecond optical pulses in the 15 μm wavelength band. FIG. 10 is a diagram showing its configuration. In this figure, 1 is a mode-locked YAG laser, 2 is a mirror, and 3
is a color center crystal, 4 is a birefringent filter, 5 is a partially transmitting mirror, 6 is a lens, and 7 is a polarization-maintaining fiber.

This mode-locked soliton laser is a composite cavity laser consisting of a part including a color center crystal 3 and a birefringent filter 4, and a part including a polarization maintaining fiber 7. When the intensity of the optical pulse inside the resonator is sufficiently strong, optical solitons can be constantly generated within the polarization maintaining fiber 7. By compensating for the loss inside the resonator by the gain of the color center crystal 3, the entire composite resonator oscillates as a laser.

On the other hand, FIG. 11 is a diagram showing the configuration of a short pulse generation method using a difference frequency generation method. In this figure, 1 is a mode-locked YAG laser, 2 is a mirror, 5 is a partially transmitting mirror, and 6 is a mode-locked YAG laser.
8 is a lens, 8 is a cavity dump dye laser, 9 is a half-wave plate, 10 is a polarizer, and 11 is a nonlinear crystal. To operate this, the visible light femtosecond (f6) pulse generated from the synchronously excited dye laser 8 and the fundamental wave pulse of the mode-locked YAG laser 1 used to excite the dye laser are
Nonlinear crystals such as KT P and L iN bo s
In this method, a difference frequency light with a wavelength band of 1.5 μm is generated. In order to satisfy the phase matching condition for difference frequency light generation,
Visible light pulse and YAG
The polarization direction of the fundamental wave of the laser is adjusted, the timing of both waves is matched by a movable mirror, and the waves are incident on the nonlinear crystal through a lens 6. The difference frequency light in the 15 μM band of the wave i generated from the crystal is extracted to the outside through the partially transmitting mirror 5.

"Problems to be Solved by the Invention" By the way, among the above-mentioned short pulse generation methods, the former can generate pulses of several 10 to several 100 fs, but has the disadvantage of requiring a complicated device. was there. In addition, since it is a mode-locked laser, the pulse repetition period is fixed, and the repetition period of the mode-locked YAG laser l for color center crystal excitation is 100.
Since the frequency is around MHz, it was not possible to obtain pulses with a repetition rate of GHz or higher.

On the other hand, in the latter case, the wavelength of the output pulse can be easily varied over a wide range by changing the wavelength of the dye laser, and a high peak output can be obtained, or a device with a complicated configuration is required. It was not possible to change the pulse repetition. In addition, since difference frequency generation, which is one of the parametric optical effects, is used, in order to efficiently generate difference frequency light, both the femtosecond visible light pulse and the fundamental wave pulse of the YAG laser require high peak intensity. There were drawbacks.

This invention has been made in view of the above-mentioned circumstances, and by controlling the spectrum of a high-repetition pulse train obtained by direct modulation of a semiconductor laser, it is possible to generate picosecond optical soliton pulses with a transform limit, and to generate optical soliton pulses. The purpose of the present invention is to provide a short optical pulse generation method and device that can obtain a compact femtosecond pulse light source with a simple configuration and a variable repetition period in the wavelength band of 15 μm by performing pulse compression using the pulse compression effect of There is.

"Means for Solving the Problem" The method according to claim 1 uses a narrow band optical filter that band-limits the optical pulse generated by sinusoidally or pulse-wise changing the value of the current supplied to the semiconductor laser. The product of the pulse width and its spectral width is 0.3.
2 to 044, and then amplify the optical pulse with an erbium optical fiber amplifier so as to generate a high-order optical soliton with an amplitude of \-2 or more,
The optical pulse is coupled to a single mode optical fiber in which the wavelength of the optical pulse is in an anomalous dispersion region, and the optical pulse is obtained at a position where the pulse width of the optical pulse propagating in the optical fiber is narrower than the input optical pulse width. The method according to claim 2 is characterized in that, in the method according to claim 1, pulse compression is performed by repeating compression of the optical pulse multiple times using optical amplification and propagation of higher-order optical solitons. The method according to claim 3 is the method according to claim 1, except that instead of combining the erbium optical fiber amplifier and the optical fiber, erbium is added to the core portion at a concentration of 0.01 to 500 ppm, and The amplifier is constructed using a single mode optical fiber that has anomalous dispersion at the wavelength of the optical pulse, and the pulse width of the optical pulse propagating in this optical fiber narrows after propagating a distance of about the soliton period. The method according to claim 4 is characterized in that, in the method according to claims 1 to 3, the higher-order optical soliton propagates in the optical fiber, transmits the wavelength component generated by the soliton self-frequency software, and inputs it into the optical fiber. The apparatus according to claim 5, characterized in that an optical filter having a characteristic of removing the wavelength component of the optical pulse is placed at the output end of the pulse compression optical fiber. a semiconductor laser; a semiconductor laser driving device for obtaining a light pulse by sinusoidally or pulse-wise changing the current supplied to the semiconductor laser; a narrowband optical filter for limiting the spectral band of the light pulse; The device according to claim 6 is characterized in that it comprises an erbium optical fiber amplifier that amplifies the light emitted from the filter, and a single mode fiber that has anomalous dispersion at the wavelength of the optical pulse. The device according to claim 7, characterized in that in the configuration of claim 5 or claim 6, a plurality of combinations of erbium optical fiber amplifiers and single mode fibers are connected in series. Optical fiber amplifier and single mode optical fiber instead of wavelength 1.48.09
a semiconductor laser for pumping an erbium optical fiber in the 8 or 08 μm band; a combining means for combining the light emitted from the optical filter or the single mode fiber with the optical output of the pumping semiconductor laser; .01
The device according to claim 8, characterized in that it uses a single mode optical fiber doped at a concentration of ~500 ppm and exhibits anomalous dispersion at the wavelength of the optical pulse, and an optical filter. In addition to the structure of claim 6, the present invention is characterized in that an optical filter whose product of pulse width and spectral width satisfies a relationship of 0.32 to 044 is placed at the output end of the single mode fiber.

1. The present invention converts a highly repetitive optical pulse train obtained by direct modulation of a semiconductor laser into a transform-limited pulse train by passing it through a narrow band optical filter, and converts this into a transform-limited pulse train using an erbium optical amplifier. High-order solitons are amplified to the peak power that can be excited, and optical pulses with a narrow pulse width are obtained by changing the waveform of the optical soliton propagating in an optical fiber. As a result, the pulse repetition frequency can be varied, which was thought to be impossible with conventional techniques, and this pulsed light source can be realized by a combination of a semiconductor laser and an optical fiber.

"Embodiments" Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[First Embodiment] FIG. 1 is a configuration diagram for explaining a first embodiment of the present invention. In this figure, 12 is an electric oscillator, 13 is an electric amplifier, 14 is a comb generator, 15 is a semiconductor laser, 16 is a Fabry-Bello resonator, 17 is an erbium optical fiber amplifier, 18 is a dispersion shift fiber, and 19 is a pulse measuring device. be.

To operate this, an electrical oscillator 12)! Air tank width gauge 1
By applying the sine wave generated by 3 to the comb generator 14, an electric pulse train whose repetition period is equal to the frequency of the iE dazzling wave is obtained, and by applying this to the semiconductor laser 15 and performing gain switching, the pulse train shown in FIG. 2 is obtained. obtain. Here, the semiconductor laser 15 is a DFB laser or a DBR.
A longitudinal single mode laser such as a laser is used. Figure 2 (a
) shows the spectrum, and (b) shows the time waveform of the pulse.

As shown in the figure, a pulse with a pulse width Δτ = 24 ps is obtained, but the wavelength width Δλ is 1 due to the wavelength chirp of the semiconductor laser 15 (the oscillation wavelength changes temporally within the pulse width). This wavelength width is much wider than that required to form a pulse with the resulting pulse width, and the pulse has extra spectral components.

Here, for a pulse whose pulse width and spectral width are connected by a Fourier transform relationship (this is called a transform-limited pulse), ΔλC ΔτΔλ;□Δ2)-0.32-0.44...・・・
■There is a relationship called λ 1. In order to obtain a pulse with a higher transfer limit, the output of the semiconductor laser 15 is combined with a narrow band filter consisting of a Fabry-Perot resonator 16, thereby obtaining a pulse that satisfies the relationship of equation (2), and reducing this pulse. Further, by amplifying with the erbium optical fiber optical amplifier 17, a pulse train with high peak intensity is obtained. For details of the method for generating transform-limited pulses, please refer to Nakazawa et al., ``Optical Pulse Generation Method, Generation Device, and Transmission System'' (Japanese Patent Application No. 1-294258).

When the transform-limited pulse obtained in this way is amplified and propagated through a single mode optical fire that has an anomalous dispersion region at the pulse wavelength,
An optical soliton is generated. Since a fundamental soliton with a normalized amplitude of 1 is generated, the power of the Noculus is expressed as P, and the soliton period Zsp is expressed as. Here, λ is the wavelength, C is the speed of light, n is the nonlinear refractive index, D is the group velocity dispersion, is the pulse width, and 0 is the spot size. In this first example, the zero dispersion wavelength is 1.51 μm, the group velocity dispersion at one wavelength of 1552 μm is −2 ps/km/nil, and the spot size is 2.8.
A micrometer dispersion soft fiber 18 was used. When a pulse with a wavelength of 1.552 μm and a pulse width of 18 ps is input to this fiber 18, P71. The values of and Zsp are 4.6 mW and 64.1 Km, respectively.

And P14. When a large-amplitude soliton is propagated through a soliton, the waveform of the soliton pulse changes as the soliton pulse propagates. Here, an example of how the waveform changes with propagation is shown in FIG. This figure shows that pulse compression is possible by using waveform changes of higher-order solitons. For example, N = 2 solitons (solitons whose peak power is 4 times that of the basic soliton)
When the pulse width is injected, the pulse width is compressed to about 23% of the initial value at a distance of half the soliton period. Pulse width 18ps
, 4 for an input pulse with a peak intensity of 18.4 mW,
A 1 ps pulse is obtained. The input and output waveforms at this time are shown in FIG. Furthermore, as shown in FIG. 3, by exciting higher-order solitons, the compression ratio of the pulse can be increased and the required fiber length can be shortened. For example, when N=3 solitons are excited, the pulse width can be compressed to about 12% of the initial value at a distance of one-fourth of the soliton period. Therefore, when a 18 ps pulse is input, 2, 1
A pulse of ps is obtained.

Conversely, if the gain of the erbium optical fiber amplifier 17 is adjusted for a fiber length shorter than the soliton period so that a soliton with an appropriate amplitude is excited at the input end of the pulse compression optical fiber, the pulse width at the fiber output end will be reduced. can be made as short as possible.

[Second Embodiment By using the pulse compression method using the waveform change of an optical soliton in multiple stages as described above, an even narrower pulse train can be obtained. FIG. 5 is a configuration diagram showing a second embodiment.

In this figure, +7a, 17b, 17c are erbium optical fiber amplifiers, 18a, 18b, 1
8c is a dispersion fiber, and the other symbols are the same as in FIG. What should be noted in this embodiment is that according to formulas (1) and (2), P N is inversely proportional to the square of the pulse width, and Zsp is proportional to the square of the pulse width. That is, in order to compress a short pulse, it is necessary to input a pulse with a high peak intensity into a compression fiber, but the length of the fiber used can be short. For example, when the input pulse width is 4.1 ps in the second stage of compression, P, , from formulas ■ and ■.
= 66.7mW, Zsp = 3.2km, so N =
The pulse power and fiber length required to excite two solitons are 3546 mW and 1.6 km, respectively. At the output end of the second stage compression fiber, the pulse is 0.
．． Compressed to 95ps.

When further compressing the pulse obtained by the fiber 18b, the fundamental soliton power and soliton period at the input end of the compression fiber 18c are respectively 1.65W,
It will be 178.6m. Therefore, in order to excite N=2 solitons into the fiber 18c and perform pulse compression, the input peak power is 6.6 W.

The fiber length may be 89.3 m. At this time, a 220 fs pulse is obtained from the fiber 18c.

The obtained pulse is measured by a pulse measuring device 19.

The input peak power to the pulse compression fiber 18 increases to the order of W as the pulse is compressed.
When the pulse repetition period is IGHz, the average power at the input of the third stage fiber 18c is only 6.3 mW. This power is a value that can be easily achieved with an Er fiber amplifier.

Since it is possible to obtain an output of several tomW0mW from an Er-doped fiber amplifier, it is also possible to increase the repetition frequency of the femtosecond pulse obtained by pulse compression and further narrow the pulse width.

[Third Embodiment] When a soliton pulse is amplified using an Er-tope fiber, it is possible to integrate the optical amplifier and pulse compression optical fiber used in the first and second embodiments. E to use
The r-tope fiber has an Er-tope amount of 0.01 to 500.
It is a single mode fiber that exhibits anomalous dispersion at ppm and at the wavelength of the optical pulse. FIG. 6 is a configuration diagram showing a third embodiment. In this figure, an electrical oscillator 12)
The electrical amplifier 13, comb generator 14, semiconductor laser 15, and Fabry-Perot resonator 16 correspond to those shown in FIG. It is amplified to a peak power of , and is coupled to the soliton compression fiber 22.

The compression fiber 22 used was a fiber having a group velocity dispersion of −2 ps/km/nm′+spot size of 2.8 μm at a wavelength of 1.552 μm and containing Er of 0.5 ppm. When this fiber is pumped with a semiconductor laser having a wavelength of 1.48 μm, at a certain pumping light power, the internal loss of the fiber or the gain of Er cancels it out, resulting in an equivalently lossless fiber, compared to the first embodiment. Pulse compression can be performed with higher efficiency. Furthermore, this...
In 2, a semiconductor laser with a wavelength of 148 .mu.m was used as the excitation light source, but it goes without saying that a semiconductor laser for pumping an Erchium optical fiber with a 0.98 or 0.8 .mu.m band may also be used.

When the pumping light power of the Er fiber is further increased, the fiber 22 has a gain in a distributed constant manner. And fiber 2
The solitons propagating through the fiber have an amplification effect and can be made into short pulses with a shorter fiber length than when using a normal single mode fiber, making it possible to efficiently obtain narrow pulses. The length of the Er fiber required for pulse compression becomes shorter as the pulse width input to the fiber 22 becomes narrower, so it is necessary to increase the erbium concentration accordingly.

Furthermore, when amplifying a short pulse of ps order or less, the spectral width of the pulse is widened. waveform distortion increases. Furthermore, unless the soliton period becomes comparable to the length of the erium fiber inside the amplifier, it becomes difficult to consider the amplifier as a lumped constant. In order to avoid this, the group velocity dispersion and spot size of the Erbium fiber must be controlled so that the pulse propagates as an optical soliton even inside the optical amplifier. Even in such a case, the pulse compression method described in this embodiment is effective.

[Fourth Embodiment] In the first embodiment, a comb generator was used for the gain switch of the semiconductor laser, but this method has the disadvantage that it operates only at a specific frequency due to the nature of the comb generator. In this embodiment, as shown in FIG. 7, the semiconductor laser 15 is directly driven by the electric amplifier 13,
A pulse train is generated by switching the gain, and then pulse compression is performed using the waveform change of the soliton. Thereby, the repetition period of the short pulses obtained can be determined by the frequency of the electrical oscillator 12.

[Fifth Example] In the first to fourth examples described above, when the pulse width of the input pulse to the pulse compression fiber is narrowed and the peak intensity becomes strong, the soliton due to the high-order nonlinear optical effect is A self-frequency shift occurs, and the center wavelength of the output pulse shifts to a longer wavelength side than the input pulse. Therefore, in addition to the pulse component that has undergone self-frequency shift, spectral components of the input pulse are observed in the fiber output vector.

FIG. 8(a) shows the spectrum. The spectral components of the input pulse are unnecessary for obtaining a transform-limited pulse, and are removed by an optical filter 23. The optical filter 23 is installed at the output end of the compression fiber 1B (or fiber 22) as shown in FIG. The transmission characteristics of the filter 23 are shown in FIG. 8(b).
As shown in Figure 2, the center wavelength of the filter matches the center wavelength of the output pulse, the transmission bandwidth is wider than the transform limit wavelength width, and the input component can be removed. The spectrum after passing through this filter 23 is shown in FIG. 8(c). It can be seen from this that the pulse after compression is at the transform limit. Transform-limited pulses have the characteristic that they are less susceptible to waveform distortion when pulse compression is performed in multiple stages.

"Effects of the Invention" As explained above, according to the present invention, it is possible to generate high-output, high-repetition femtosecond pulse trains of GHz or higher, which have been considered difficult in the past, using semiconductor lasers, narrow band filters such as Fahri-Herow, and erbium optical fibers. Since it can be generated by using an amplifier and a pulse compression fiber, it has the advantage that it can be widely used in ultrahigh-speed optical communications in which pulses are multiplexed. In other words, it is possible to generate short optical pulses without using complex and large-scale techniques such as mode locking that have been used in conventional short optical pulse generation, and the repetition period of the pulse is not limited by the resonator length. It has the advantage of being able to be set to

In addition, this pulse generation method is applicable not only to optical communication but also to InC;
It can be used to evaluate the optical properties and characteristics of semiconductor materials such as aAaP.

[Brief explanation of drawings]

Figure 1 is a diagram explaining the first embodiment of the present invention, Figure 2 is a diagram showing the spectrum and waveform of a pulse generated by a semiconductor laser, and Figure 3 is a diagram explaining waveform changes in the propagation direction of a soliton. , FIG. 4 is a diagram showing waveform changes before and after pulse compression, FIG. 5 is a diagram explaining the second embodiment of the present invention, FIG. 6 is a diagram explaining the third embodiment of the present invention, and FIG. is a diagram explaining the fourth embodiment of the present invention, FIG. 8 is a diagram explaining the pulse spectrum change, FIG. 9 is a diagram explaining the fifth embodiment of the present invention, and FIG. 1O is the configuration of a soliton laser. FIG. 11 is a diagram showing the configuration of a 15 μn band short pulse generator using difference frequency generation. 1. Mode-locked YAG laser, 2. Mirror, 3
Color center crystal, 4 Birefringence filter, 5 Partially transmitting mirror, 6 Lens, 7 Polarization maintaining fiber, 8 Cavity dump dye laser, 9 Half wave plate, 10 Polarizer, 11... Nonlinear crystal, 12... ・Electric oscillator, 13
- Electrical amplifier, 14... Comb generator, 15... Semiconductor laser, 16... Fabry-Bello resonator, 17a-17c... Erbium optical fiber amplifier, 18a=18c... Dispersion soft fiber, 19...
...Pulse measuring device, 20...1.48 μm semiconductor laser, 21 Fiber coupler, 22... Distributed constant Er fiber, 23- Band pass optical filter. Figure 1 (o) (b)
Figure 2 Figure 3 Figure 4 N) L1+1

Claims (1)

[Claims] (1) A light pulse generated by changing the value of the current supplied to the semiconductor laser sinusoidally or in a pulse manner is passed through a band-limited narrowband optical filter to determine the pulse width and its spectral width. Convert the optical pulse into an optical pulse whose product is 0.32 to 0.44, and then use an erbium optical fiber amplifier to generate a high-order optical soliton whose intensity has an amplitude of N=2 or more. The optical pulse is amplified and coupled to a single mode optical fiber that has an anomalous dispersion region at the wavelength of the optical pulse, and an optical pulse is obtained at a position where the pulse width of the optical pulse propagating in the optical fiber is narrower than the input optical pulse width. A method for generating short optical pulses characterized by the following. (2) A short optical pulse generation method, characterized in that pulse compression is performed by repeating optical pulse compression using optical amplification and propagation of higher-order optical solitons as described in claim 1 a plurality of times. (3) In claim 1, instead of combining an erbium optical fiber amplifier and the optical fiber, 0.01 to 500 pp of erbium is added to the core portion.
The amplifier is constructed using a single-mode optical fiber doped with a concentration of m and has anomalous dispersion at the wavelength of the optical pulse, and the pulse width of the optical pulse propagating in this optical fiber propagates over a distance of about the soliton period. A method of generating short optical pulses characterized by using a method of generating short optical pulses. (4) In claims 1 to 3, a higher-order optical soliton propagates in an optical fiber, transmitting a wavelength component generated by the soliton self-frequency shift, and removing a wavelength component of an optical pulse input into the optical fiber. 1. A method for generating short optical pulses, which comprises placing an optical filter having a characteristic of: at the output end of an optical fiber for pulse compression. (5) A single longitudinal mode semiconductor laser with a wavelength of 1.5 μm, a semiconductor laser driver for obtaining optical pulses by sinusoidally or pulse-wise changing the current supplied to the semiconductor laser, and An optical short circuit characterized by comprising a narrow band optical filter that limits the spectral band, an erbium optical fiber amplifier that amplifies the light emitted from the optical filter, and a single mode fiber that has anomalous dispersion at the wavelength of the optical pulse. Pulse generator. (6) An optical short pulse generator according to claim 5, characterized in that a plurality of combinations of erbium optical fiber amplifiers and single mode fibers are connected in series. (7) In claim 5 or 6,
Erbium fiber amplifier and single mode fiber instead of wavelength 1.48, 0.98 or 0.8μm
a semiconductor laser for pumping an erbium optical fiber in the band; a combining means for combining the light emitted from the optical filter or the single mode fiber with the optical output of the pumping semiconductor laser; An optical short pulse generator characterized by using a single mode optical fiber doped at a concentration of 500 ppm and having anomalous dispersion at the wavelength of the optical pulse, and an optical filter. (8) In claim 5 or 6,
An optical short pulse generator characterized in that an optical filter is placed at the output end of a single mode fiber, the product of pulse width and spectral width satisfying a relationship of 0.32 to 0.44.