JPH0687105B2 - Optical pulse compressor - Google Patents

Description

The present invention relates to an optical pulse compression device for obtaining high-power ultrashort pulses.

[Prior Art] The method of pulse compression has long been used for waveform processing (compression, expansion) of ultrasonic pulses using a dispersion delay line. The optical pulse compression is also the same in principle, and an optical pulse having a linear frequency change (frequency increases with time) is made incident on a medium having anomalous dispersion and is compressed. The method and apparatus consist of three important elements as shown in FIG. The first element is a stable, narrow and high-power optical pulse light source 1, which is usually a mode-locking dye laser or a CPM ring dye laser. The high power pulses from these lasers are incident on the second element, the single mode optical fiber 2. Light intensity readily 1 MW / cm chirp optical Kerr effect is a third-order nonlinear effect since ² could be more than is to be changed temporally frequency so generated along the envelope of the optical pulses in a single-mode fiber The pulses are available at the fiber output. This is called the self-phase modulation effect. This chirping pulse is compressed using the abnormal dispersion medium 3 which is the third element. That is, due to the anomalous dispersion, the propagation speed of the low frequency part located in the front part of the pulse becomes slow, and the propagation speed of the high frequency part located in the rear part of the pulse becomes fast. As a result, the light pulse is compressed.

In the optical pulse compression methods reported up to now, the elements of the high-power pulse light source 1 and the single-mode optical fiber 2 are the same, but the elements of the anomalous dispersion medium 3 are shown in FIGS.
Various methods as shown in the figure have been proposed. Figure 9 shows Na
It is composed of a vapor cell 3A and a total reflection mirror 3A ', and a method of utilizing anomalous dispersion accompanying resonance of Na vapor, FIG. 8 is a method of using two diffraction gratings 3B arranged in parallel,
FIG. 9 shows a method using a combination of two prisms 3C. The method shown in Fig. 9 is performed by H. Nakatsuka and D. Grischkowsky in 1981 in Optics Lettres Vol.6 Page.13-15
“Recompression of optical pulses broadened b
y passage through optical fibers ". Since this method uses the resonance of Na, it has a drawback that the wavelength for pulse compression is limited to around 0.58 μm.
The method shown in the figure is the most general method and has been proposed by E. Treacy (“Optical pulse compression with d
iffraction gratings, "IEEE, J.QE, QE-5, Page 454-45
8,1969), 10 femtosecond (10 × 10 ^-15 seconds) pulse is possible. However, since the diffraction efficiency cannot be so high, there are drawbacks such as low output intensity and disorder of the pulse tail. The method of Fig. 11 is Zs, B.
Proposed by OR and B.RA′CZ (“Group veloc
ity Dispersion in Prisms and Its application
to Pulse Compression and Traveling-Wave Ex
citaion, "Opt.Commun, 54 volumes, page 165-170, 1985) Anomalous dispersion can be created using a simple prism.
However, since the wavelength dependence of the prism refractive index is small, it is necessary to lengthen the distance between the prisms 3C, 3C to several tens of meters, and there is a drawback that the anomalous dispersion of infrared light is small and the application range is limited.

[Problems to be Solved by the Invention] Any of the anomalous dispersion media in the above-described conventional example is diffracted,
Due to absorption and insertion loss, the average energy of the compressed optical pulse has the drawback of being attenuated by about 20% of the input.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the problem of optical loss, which has been inevitable in the conventional medium that forms anomalous dispersion, and to provide an optical pulse compressor capable of obtaining a high-power ultrashort optical pulse.

[Means for Solving the Problems] In order to achieve such an object, the optical pulse compression apparatus of the present invention uses an optical pulse light source for generating a high output optical pulse and an incident high output optical pulse for the optical pulse. A single mode optical fiber for an incident wave to positively chirp in the propagation process of
The degenerate four-wave mixing medium for converting the light emitted from the single-mode optical fiber for the incident wave into a negatively chirped optical pulse and reflecting it, and the high-power optical pulse reflected by the degenerate four-wave mixing medium A single-mode optical fiber for reflected waves for compressing in the propagation process of an optical pulse, and a semi-transmissive mirror for extracting the optical pulse from the optical path provided in the optical path of the high-power optical pulse. And

[Operation] The present invention uses degenerate four-wave mixing to obtain ultrashort pulses that are amplified with high gain. That is, paying attention to the non-linear amplification effect of the degenerate four-wave mixing, the positive chirp pulse to the degenerate four-wave mixing medium is made to enter the single-mode optical fiber again as a negative chirp pulse with a high output, and a degenerate pulse with a high output is obtained.

Excitation lights Pf and Pb having electric field intensities Ef and Eb are incident on the degenerate four-wave mixing medium 4 in the respective reflection directions. When an input optical pulse Pin with an electric field intensity Ein is made incident there from another direction, degenerate four-wave mixing occurs, and a reflected pulse Pr with an electric field intensity Er generated by a nonlinear interaction is generated in the opposite direction to the input optical pulse. . Er is expressed by the following equation, where χ ⁽³⁾ is the third-order nonlinear susceptibility of the degenerate four-wave mixing medium 4.

Er = −i ▲ E ^* _in ▼ (Z = 0) exp (iδ) tan | B | L
(1) However, B = (12πω / nc) χ ⁽³⁾ EfEb (2), δ is the phase change of B, * is the complex conjugate, ω is the angular frequency of light, n
Is the refractive index and L is the optical path length in the degenerate four-wave mixing medium.

From equation (1), the reflected wave is a complex conjugate of the input, and its amplification degree G is You can see that it is given by. Therefore, when | B | L = π / 4, G
= 1 and the reflected pulse can be generated with the same magnitude as the incident pulse. Electric field strengths Ef and Eb of two excitation lights
By controlling L and L, an amplified pulse of inverted chirp is obtained.

According to the present invention, since the inverted chirp pulse can be generated by using the degenerate four-wave mixing, there is an advantage that this pulse can be highly efficiently compressed by the normal dispersion of the single mode optical fiber. Further, degenerate four-wave mixing can amplify the optical pulse together with the reversal of the chirp, so that the average energy of the compressed pulse can be increased.

Embodiments Embodiments of the present invention will be described below with reference to the drawings.

FIG. 2 is a diagram for explaining the first embodiment of the present invention, in which 1 is an optical pulse generator (optical pulse light source), 2 is a pulse positively chirped by the self-phase modulation effect (frequency changes with time). Single-mode optical fiber for producing increasing pulses, 4 is a degenerate 4-wave mixing medium, and 5 is a semi-transmissive mirror for guiding reflected light from the 4-wave mixing medium to the single-mode optical fiber 2B. The single mode optical fiber 2B is a single mode optical fiber for compressing the negative chirp pulse created by the four-wave mixing medium 4 by using the normal dispersion of the fiber.

In order to operate this, first, the optical pulse from the optical pulse generator 1 is passed through the single mode optical fiber 2 for the incident wave, a positive chirp pulse is obtained, and the positive chirp pulse is made incident on the degenerate four-wave mixing medium 4. In the degenerate four-wave mixing medium 4, a positive input chirp pulse is converted into a negative chirp pulse and reflected. (The specific configuration of degenerate four-wave mixing will be described later.) The output is reflected by the semi-transmissive mirror 5 and enters the single-mode optical fiber 2B for reflected waves, where the pulse is compressed.

FIGS. 3A and 3B show how the optical pulse is compressed.
The input pulse of the reflected wave single mode optical fiber 2B is a negative chirp pulse having a high frequency in the portion before the pulse and a low frequency in the portion after the pulse. The wavelength dependence of the speed of the optical pulse in the single mode optical fiber changes as shown in FIG. That is, a region having a wavelength shorter than the zero-dispersion wavelength (1.32 μm) is a normal dispersion region having a short wavelength, that is, a component having a high frequency has a low velocity and a component having a low frequency has a high velocity. Therefore, the negative chirp pulse is compressed as shown in FIG. 3 (B). Although the degree of compression depends on the inclination of the chirp, it can be compressed to about 1/10. The optical pulse is not compressed in the wavelength region longer than the zero dispersion wavelength. Therefore, this method is effective only in the normal dispersion region, but there is no problem because ultrashort pulses can be generated at wavelengths shorter than 1.32 μm.

FIG. 4 shows a second embodiment of the present invention. In this embodiment, the incident wave single mode optical fiber 2 is also used as the reflected wave optical fiber. In this case, the semitransparent mirror 5 is provided between the optical pulse light source 1 and the optical fiber 2. The operation of this embodiment is similar to that of the first embodiment.

5 and 6 show the configuration of degenerate four-wave mixing. To generate degenerate four-wave mixing, it is necessary to use two pump beams Pf and Pb. The generation source of the excitation lights Pf and Pb is the laser 6, and the wavelength is set to match the wavelength of the pulse light source 1. The operation may be continuous wave or pulse. FIG. 5 shows an example in which the excitation light Pf is reflected by the reflecting mirror 7 to produce Pb. In FIG. 6, two excitation lights Pf and Pb are produced from one laser light by the semi-transmissive mirror 5B.

FIG. 7 shows another method of generating excitation light. The optical pulse light source 1 is used as the excitation light source, which has a great advantage in practical use. Through the semi-transmissive mirror 5, half of the pulse is used as an excitation pulse for degenerate four-wave mixing. The other half excites the single mode optical fiber 2 and produces a positive chirp pulse. The excitation pulse creates a negative chirp as soon as the pulse enters the four-wave mixing line. This pulse again passes through the single mode optical fiber 2 and is reflected by the semi-transmissive mirror 5 to produce a compressed pulse.

In the formula (3), when | B | L becomes close to π / 2, 10 ² ~ 1
It is easy to get a gain of 0 ³ . According to J. Marburger, chirp inversion in degenerate four-wave mixing is
Let p, the length of the medium be L, and the velocity be υ (Appl.Phys.Lett., Vol.32, P.372 (19
78 “Optical pulse integration and chirp rever
sal in degenerate four-wave mixing). Therefore,
To invert a pulse of about 10 psec by chirp, L is
It must be 1 mm or less. | B | L with CS ₂ as the medium
The pumping power I at the reflectance of 1 is Pc = nc ³ / 24πω ²
If χ ⁽³⁾ , it can be obtained from the relationship of 1 | B | L4πIL / kPc (4) in the above paper. k is the wave number. CS ₂
If Pc35kW L0.5mm is set at a wavelength of 1 μm, then I
It can be calculated as 5.5 MW / cm ² . Since this kind of power density can be easily created, the degenerate four-wave mixing can create an inverted chirp pulse. Since the compressed pulse obtained by the method of FIG. 7 has been amplified, a further ultrashort pulse can be obtained by further inputting this pulse and performing this compression amplification in multiple stages.

Although CS ₂ was used as the degenerate four-wave mixed medium, semiconductor vapor-doped glass obtained by doping Na vapor, As ₂ S ₂ chalcogenide glass, borosilicate glass with CdSSe, etc. may also be used as the degenerate four-wave mixed medium. it can. Also, since the interaction time of degenerate four-wave mixing can be controlled by turning on and off the pumping light temporally, not only the inversion of the chirp but also the width of the chirp can be controlled, and the compression of high-power linear chirp pulse can be controlled. It is possible.

[Advantages of the Invention] As described above, since the inversion chirp pulse can be generated by using the degenerate four-wave mixing, there is an advantage that this pulse can be highly efficiently compressed by the normal dispersion of the single mode. Further, degenerate four-wave mixing can amplify the optical pulse together with the reversal of the chirp, so that there is an advantage that the average energy of the compressed pulse can be increased. This gain is expressed by equation (1)
Since it can be determined by the magnitudes Ef, Eb, and L of the excitation electric field as can be seen in (3), its control is easy. Also,
Since the interaction time of degenerate four-wave mixing can be controlled by turning the excitation light on and off temporally, not only the inversion of the chirp but also the width of the chirp can be controlled, enabling the compression of high-power linear chirp pulses. There are advantages.

[Brief description of drawings]

FIG. 1 is a diagram for explaining the operation of degenerate four-wave mixing which is the basic principle of the present invention, FIG. 2 is a configuration diagram of the first embodiment of the present invention, and FIGS. 3 (A) and 3 (B) are respectively FIG. 4 is a diagram showing a normal dispersion condition and a compression operation for compressing a negative chirp pulse, FIGS. 4 to 7 are diagrams showing the configuration of another embodiment of the present invention, and FIG. The basic configuration of the pulse compression device and FIGS. 9 to 11 are configuration diagrams of various conventional compression devices, respectively. 1 ... Optical pulse light source, 2A, 2B ... Single-mode optical fiber, 3 ... Anomalous dispersion medium, 4 ... Degenerate four-wave mixing medium, 5,5B ... Format transmission mirror, 6 ... Degenerate four-wave mixing Light source for generation.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Takashi Nakajima Inventor Takashi Nakajima, Tokai-mura, Ibaraki Prefecture 162 Shirahoji, Shirane, Nippon Telegraph and Telephone Corporation Ibaraki Telecommunications Research Institute (72) Inventor Yoshiyuki Aomi Naka, Ibaraki Prefecture 162, Shirahone, Shirahata, Tokai-mura, Gunma, Ibaraki Telecommunications Research Institute, Nippon Telegraph and Telephone Corporation

Claims (4)

[Claims] 1. An optical pulse light source for generating a high output optical pulse, a single mode optical fiber for an incident wave for positively chirping the incident high output optical pulse in a propagation process of the optical pulse, and the incident Degeneration for converting the output light from the single mode optical fiber for waves into a negatively chirped optical pulse and reflecting it 4
Wave mixing medium, a single-mode optical fiber for reflected waves for compressing the high-power optical pulse reflected by the degenerate four-wave mixing medium in the propagation process of the optical pulse, and in the optical path of the high-power optical pulse And a semi-transmissive mirror for extracting the optical pulse from the optical path. 2. The optical pulse according to claim 1, wherein the semi-transmissive mirror is inserted between the single-mode optical fiber for incident wave and the degenerate four-wave mixing medium. Compressor. 3. The single mode optical fiber for the incident wave and the single mode optical fiber for the reflected wave are one optical fiber, wherein the semi-transmissive mirror is the optical pulse light source and the light. The optical pulse compression device according to claim 1, wherein the optical pulse compression device is inserted between fibers. 4. The degenerate four-wave mixed medium is CS ₂ , Na vapor, As ₂ S ₃
CdSSe on chalcogenide glass and borosilicate glass
The optical pulse compression device according to any one of claims 1 to 3, wherein the optical pulse compression device is one of semiconductor-doped glasses doped with the above.