WO2005081370A1

WO2005081370A1 - High-power short light pulse generator

Info

Publication number: WO2005081370A1
Application number: PCT/JP2005/003439
Authority: WO
Inventors: Toshiharu Sugiura; Toshiaki Bessho
Original assignee: Aisin Seiki Kabushiki Kaisha
Priority date: 2004-02-24
Filing date: 2005-02-23
Publication date: 2005-09-01
Also published as: JP2005241731A

Abstract

A high-power short light pulse generator having a high diffraction efficiency increased by a bulk diffraction grating compressor to reduce the power loss. The high-power short light pulse generator comprises a fiber expander (2) for receiving a short light pulse and outputting an expanded short light pulse, a fiber amplifier (3) for receiving the expanded short light pulse and outputting an amplified short light pulse, a compressor (4) adapted for receiving the amplified short light pulse and outputting a compressed short light pulse and using a bulk diffraction element, and polarization holding coupling means (5a to 5c). Since the fiber expander and the fiber amplifier are coupled by polarization holding coupling means, the short light pulse amplified by the fiber amplifier is linearly polarized, and thereby degradation of diffraction efficiency of the bulk diffraction grating compressor can be prevented.

Description

High power short optical pulse generator

The present invention relates to a high-power short optical pulse generator that generates an optical pulse with a high power and a narrow pulse width. Background art

From picosecond or Fuwemuto second laser, a time width (pulse width) is 1 0 ^{1 2-1} 0 ^_ ^{1 5-second} range, the average output power short manual light pulse mW level is easily generated. However, it is necessary to increase the power of short optical pulses to be applied to processing, medical treatment, and measurement. Normally, amplification techniques are used to increase the power. However, when amplifying with an optical fiber amplifier, a nonlinear phenomenon occurs when a certain threshold value is exceeded due to the small cross-sectional area of the optical waveguide. The torque waveform collapses, causing deterioration of the amplification factor and deformation of the pulse shape on the time axis. Since the peak value (peak power) of the short light pulse reached the W level, it quickly exceeded the threshold at which nonlinear phenomena occurred, making it difficult to amplify with a fiber amplifier.

Recently, a chirped pulse amplification technique for solving the threshold problem of the fiber amplifier has been developed (see, for example, Japanese Patent No. 3497735). This is a technology in which short optical pulses are stretched by a fiber stretcher before amplification to reduce the peak power, then amplified by an amplifier, and after amplification, compressed by a Balta diffraction grating compressor to restore the original pulse width. . However, in this short optical pulse generator using the conventional amplification technique, the diffraction efficiency in the Balta diffraction grating compressor was low, and the power loss was large. Therefore, even if amplification was performed to achieve high power, the amplification effect could not be sufficiently exhibited. Also, if the repetition frequency of the short optical pulse amplified by the amplifier is high, the amplification effect cannot catch up, and the amplification rate per pulse decreases.

The present invention has been created to solve such a problem. That is, an object of the present invention is to increase the diffraction efficiency in a bulk diffraction grating compressor and reduce power loss. An object of the present invention is to provide a high power short optical pulse generator. Another object of the present invention is to provide a high-power short optical pulse generator that prevents a decrease in the amplification factor of an amplifier and increases the peak power per pulse. Disclosure of the invention

A high-power short optical pulse generator according to the present invention includes: a fiber expander that receives a short optical pulse and outputs an expanded short optical pulse; and a short light that receives the expanded short optical pulse and amplifies the short optical pulse. A fiber amplifier that outputs a pulse, a compressor that uses a Park diffraction element that receives the amplified short optical pulse and outputs a compressed short optical pulse, the fiber expander, the fiber amplifier, and the compression. And polarization maintaining and connecting means for connecting the short optical pulse to a device while maintaining the polarization of the short optical pulse.

The linear polarization of the short optical pulse oscillator is high because the modules such as the fiber expander and fiber amplifier are connected by the polarization maintaining connection means, and the linear polarization of the short optical pulse amplified by the fiber amplifier is also sufficiently maintained. It is. Therefore, it is possible to prevent a reduction in diffraction efficiency of the bulk diffraction element compressor.

When the time width is compressed using a Balta diffraction element, dispersion compensation of the pulse expanded by the fiber expander is performed using the first-order diffracted light, but the reflection efficiency of the first-order folded light of the Balta diffraction element is The polarization of the incident light must be adjusted to p-polarization in order to obtain efficient reflected light, which has polarization dependence. The s-polarized component becomes zero-order diffracted light and does not contribute to dispersion compensation at all. This property makes polarization control from fiber amplifiers important.

The polarization maintaining means may include a polarization maintaining fiber and a polarization maintaining connecting means. As a result, short light pulses are not propagated in the air, so that the effects of air fluctuations can be eliminated. In addition, the fiber can be formed into a loop, and the high-power short optical pulse generator can be miniaturized.

The polarization maintaining connection means may be a polarization maintaining splice or a polarization maintaining connector. As a result, the optical axis shift is eliminated, and a high-power short optical pulse can be generated stably for a long time. The entire short optical pulse generator can be downsized. Also, By connecting with a connector, performance can be checked for each module, and maintenance is excellent.

Also, the fiber amplifier can be a rare earth element-doped polarization maintaining fiber. As a result, even if the fiber amplifier is not fixed or looped, a short pulse of linearly polarized light with a wide width is output from the fiber amplifier.

Further, the fiber stretcher can be composed of a polarization maintaining fiber. As a result, an amplified short-pulse light of linearly polarized light is output from the fiber-optic amplifier without fixing or looping the fiber stretcher.

It is possible to further include a pulse decimator between the fiber stretcher and the fiber amplifier to reduce the number of repetitions of the repeated pulses by using the short light pulse as a repeated pulse. As a result, the fiber amplifier receives and amplifies short optical pulses with pulse intervals, so that the amplification factor per pulse increases and short optical pulses with high peak power can be output.

Further, the fiber amplifier may include a preamplifier and a main amplifier. As a result, the average power, which is reduced by decimating with the pulse decimator, can be increased with the preamplifier.

Furthermore, the main amplifier may be a multimode fiber with a core diameter of 10 to 50 Xm. As a result, the threshold value at which the nonlinear phenomenon occurs increases, and short light pulses with high peak power can be received and amplified. Brief Description of Drawings

FIG. 1 is a schematic configuration diagram of a high-power short optical pulse generator showing one embodiment of the present invention.

FIG. 2 is a plan view of a pulse thinning device using a waveguide type electro-optic light modulator.

FIG. 3 is a schematic configuration diagram of the high-power short optical pulse generator according to the first embodiment.

FIG. 4 is an enlarged view of a main part of FIG. BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. In the same element The same reference numerals are given and the description is omitted.

FIG. 1 is a schematic configuration diagram of a high-power short optical pulse generator showing one embodiment of the present invention. As shown in this figure, the short light pulse generator is a fiber expander that receives a short light pulse generated from the light pulse light source 1 and outputs an expanded short light pulse.

2, a pulse decimator 6 that receives the expanded short light pulse and thins out the pulse, and a fiber amplifier that receives the expanded and thinned short light pulse and outputs an amplified short light pulse 3 And a compressor 4 using a bulk diffraction element that receives the amplified short light pulse and outputs a compressed short light pulse, and polarization maintaining and connecting means 5a, 5b, and 5c. The polarization maintaining and connecting means 5a, 5b, 5c are respectively polarization maintaining fibers 51a, 51'a; 51b, 51'b; 51c, 51'c; polarization maintaining connecting means 52a, 5 2b and 52c.

For example, the light pulse light source 1 has a time width (pulse width) of 50 to 1 ps, an average output power of 0.1 to 50 mW, an oscillation wavelength of 1.55 μΐη, and a repetition frequency of 1 to 10 OMHz A commercially available mode-locked fiber laser that generates short optical pulses can be used. It is desirable that the optical pulse light source 1 has a polarization maintaining fiber big tail that outputs a short optical pulse. The generated short optical pulse is not propagated in the air, and the fiber pigtail of the polarization maintaining fiber 51a of the polarization maintaining coupling means 5a with the fiber extender 2 can be substituted.

As the fiber stretcher 2, a dispersion compensating fiber or a chirped fiber Bragg grating can be used. The dispersion compensating fiber extends the pulse width by chromatic dispersion. Wavelength dispersion is obtained by adding waveguide dispersion to material dispersion, and can be easily controlled by appropriately selecting the material, the cut-off wavelength, and the relative refractive index difference between the core and the clad. The pulse extension rate is determined by the chromatic dispersion and the length (optical path length), and it is easy to extend the pulse extension factor 100 to 1000 times with a 10-m-long dispersion compensating fiber. Chirped fiber Bragg gratings can be formed, for example, by photolithography using a 10 cm long chirped phase mask on Ge-doped fiber. It can be extended 10 000 times with a 10 cm long chirp fiber Bragg grating. Preferably, the fiber stretcher 2 is also made of a polarization maintaining fiber. Fix the fiber stretcher or keep it in a loop You no longer need to carry it. Also, the polarization maintaining fiber 5 1 b of the polarization maintaining fiber 5 1 b of the polarization maintaining fiber 5 1 ′ a and the pulse thinning device 6 is connected to the polarization maintaining fiber 5 1 b of the fiber extension device 2. Can be substituted.

As the pulse decimator 6, an optical intensity modulator can be used. The light intensity modulator includes an electro-optic light modulator, an acousto-optic light modulator, a magneto-optic light modulator, a stress optical light modulator, and the like, but is not particularly limited. As an example, FIG. 2 shows a pulse thinning device 6 using a waveguide-type electro-optic light modulator. In this method, a voltage of a predetermined frequency is applied from a pulse generator 62 to an electrode of one arm of a Z-cut LiNb03 crystal 61 constituting a Mach-Zehnder interferometer. One end of the polarization maintaining fiber 5 l'b of the polarization maintaining coupling means 5 b is connected to the input end of the LiNb03 crystal 6 1, and the polarization maintaining fiber 5 of the polarization maintaining coupling means 5 c is connected to the output end of the LiNb03 crystal 61. One end of LC is connected with, for example, an ultraviolet curing resin. In the pulse decimator 6 shown in FIG. 2, short optical pulses having a repetition frequency of 1 to 10 OMHz from the optical pulse light source 1 are decimated and converted to a repetition frequency of 5 to 50 OHHz. When the pulses are decimated, the average power is naturally reduced by the decimation. For example, if a short optical pulse with an average power of 1 mW and a repetition frequency of 1 MHz is decimated to a repetition frequency of 5 KHz, the average power drops to 5 uW.

In the fiber amplifier 3, a preamplifier 31 and a main amplifier 32 are connected to polarization maintaining fibers 133, 33 'and polarization maintaining connection means 34.

The preamplifier 31 is, for example, for increasing the average power reduced to 5 / iW to the level of 1 mW, and can be achieved by doping the core of a single mode fiber with Er. Bombing is performed, for example, by guiding a laser beam having a wavelength of 1.48 μΐη from a laser diode to a core through a wavelength division multiplexing fiber coupler. Preferably, the single mode fiber is composed of a polarization maintaining fiber. There is no need to fix or hold the preamplifier 31 in a loop. In addition, the polarization maintaining fiber 5 l'c of the polarization maintaining coupling means 5 c to the pulse thinning device 6 and the polarization maintaining fiber 33 connected to the main amplifier 32 are connected to the polarization maintaining fiber constituting the preamplifier 31. Both ends can be substituted.

As the main amplifier 32, a multi-mode double clad with a core diameter of 10 to 50 jurn Fiber is preferred. Even if the peak power per pulse at the subsequent stage of the amplifier becomes 1 kW or more, it is possible to prevent the nonlinear phenomenon from occurring. The core has Er or Er and Yb co-doped. For the bonding, V-grooves are formed in the cladding, and so-called side pumping performed from the V-groove or cladding bombing performed using a star coupler is preferable. A lot of pumping power can be applied. As the pump light source, a multimode laser diode having an oscillation wavelength of 915 nm, 945 nm, or 975 nm can be used. Preferably, the double clad fiber is comprised of a polarization maintaining fiber. There is no need to fix the main amplifier 32 or keep it in a loop. Further, the polarization maintaining fiber 3 3 ′ connected to the preamplifier 31 can be substituted for the input end of the polarization maintaining fiber constituting the main amplifier 32.

The polarization maintaining connecting means 5 a, 5 b, 5 c and the polarization maintaining connecting means 3 4 for coupling the preamplifier 31 and the main amplifier 32 to the polarization maintaining connecting means 5 a, 5 b, 5 c A polarization maintaining splice or a polarization maintaining connector is preferred.

The compressor 4 is composed of a Balta diffraction element that does not exhibit a nonlinear phenomenon even with a short optical pulse having a high peak power amplified by the fiber amplifier 3. As the Balta diffraction element, a diffraction grating, a prism, a holographic grating, or the like can be used.

[Example 1]

As shown in FIGS. 3 and 4, the high-power short optical pulse generator according to the first embodiment includes a fiber-to-laser 1, a fiber stretcher 2, a pulse thinning device 6, a fiber amplifier 3, a compressor 4, It has polarization maintaining and connecting means 5a, 5b, 5c.

The fiber laser 1 has a time width (pulse width) of 300 to 500 fs, an average output power of 1 OmW, an oscillation wavelength of 1.56 μπι, and a repetition frequency of 40 to 50Μ.モード This is a commercially available mode-locked fiber laser that generates repetitive short optical pulses. The fiber laser 1 has a polarization-maintaining fiber big tail that outputs short optical pulses. Therefore, the generated short light pulse is not propagated in the air, and the fiber pigtail can replace the polarization maintaining fiber 5 la of the polarization maintaining coupling means 5 a with the fiber extender 2. The fiber stretcher 2 is a polarization-maintaining fiber having a wavelength dispersion of 200 ps / nm / km and a length of 100 m.The input end is the polarization-maintaining fiber 5 l'a of the polarization-maintaining coupling means 5 a, and the output end is It also serves as 5 lb of the polarization maintaining fiber of the polarization maintaining connection means 5b.

The pulse thinning device 6 is a waveguide type electro-optic light modulator shown in FIG. 2, and has a Z-cut LiNb03 crystal 61 constituting a Mach-Zehnder interferometer and a pulse generator 62. The input end of the LiNb03 crystal 6 1 has one end of the polarization maintaining fiber 5 l'b of the polarization maintaining coupling means 5b at the input end, and the output end has one end of the polarization maintaining fiber 1 51c of the polarization maintaining coupling means 5c. The parts are connected by ultraviolet curing resin. The pulse generator 62 has the function and performance of applying a voltage having a pulse width of 10 to 20 ns and a repetition frequency of 100 to 300 kHz to the LiNb03 crystal 61.

The fiber amplifier 3 has a preamplifier 31 and a main amplifier 32.

The preamplifier 31 is composed of a single-mode polarization-maintaining fiber 311 with a core length of 2.6 m and a length of 2.6 m, a laser diode 312 as a pump light source, and a wavelength-division multiplexed polarization-maintaining fiber coupler 3 1 3 have. The input end of the polarization maintaining fiber 311 is spliced (not shown) so that the polarization axis coincides with the output end of the fiber coupler 313. The incident end of the fiber coupler 313 is spliced (not shown) so that the polarization axis of the polarization maintaining fiber 5l'c of the polarization maintaining coupling means 5c for coupling to the pulse thinning device 6 coincides. The laser diode 312 generates a multimode laser beam with a wavelength of 1.48 μπι and a power of 4 W with a big tail of a fiber.

The main amplifier 32 has a core diameter of 20 μπι, a multimode polarization-maintaining double-clad fiber 321 co-doped with Er and Yb, and a length of 5 m, a laser diode array 322, and as shown in FIG. It has a V-groove coupler 323. In FIG. 4, C represents a core, CL1 represents a first clad, and CL2 represents a second clad. The laser diode array 322 generates a multimode laser beam with a wavelength of 975 nm and an output of 4 W. The laser beam generated from the laser diode array 322 is collimated by the lens 324 and is coupled from the V-groove coupler 323 to the first clad CL1 of the double clad fiber 321. The polarization maintaining connecting means 52a, 52b, 52c of the polarization maintaining connecting means 5a, 5b, 5c and the polarization maintaining connecting means 34 connecting the preamplifier 31 and the main amplifier 32 are polarization maintaining connectors.

The compressor 4 includes a pair of diffraction gratings 41 and 42.

The lens 8 is for collimating a short light pulse emitted from the emission end of the main amplifier 32.

The operation results of the apparatus of the present embodiment are described below. The pulse width of 300-500 fs was extended by fiber stretcher 2 to 200-250 ps. The repetition frequency of 40-5 OMHz was converted to 100-300 kHz by the pulse decimator 6. The short optical pulse with an average power of 1 OmW generated from the fiber laser 1 was decimated by the pulse decimator 6 to 10 to 20 W, but was amplified to 2 OmW by the preamplifier 31. The short optical pulse amplified to 2 OmW by the preamplifier 31 was amplified to 2 W by the main amplifier 32. A short optical pulse with a pulse width of 200 to 250 ps and an average power of 2 W emitted from the output end of the main amplifier 32 is compressed by the compressor 4, and a short optical pulse with a pulse width of 500 fs to 2 ps and an average power of 1 W is output. Emitted. That is, the average power of the short light pulse incident on the compressor 4 was 2 W, and the average power of the short light pulse emitted from the compressor 4 was _{1 W.} Therefore, the peak power of one pulse reached a maximum of 20 kW (= 1 W / (500 fs X 100 kHz)). Such an operation result is obtained by decimating and amplifying the light by the pulse decimator 6 and converting the short light pulse incident on the compressor 4 composed of the diffraction grating pair into linearly polarized light. INDUSTRIAL APPLICABILITY The high-power short optical pulse generator according to the present invention can be widely applied to processing, medical treatment, and measurement.

Claims

The scope of the claims

1. A high power short optical pulse generator for generating a power short optical pulse, comprising: a fiber extender that receives the short optical pulse and outputs an extended short optical pulse; A fiber amplifier that receives the pulse and outputs an amplified short optical pulse;

A compressor using a Balta diffraction element for receiving the amplified short light pulse and outputting a compressed short light pulse;

Polarization maintaining coupling means for coupling between the fiber stretcher, the fiber amplifier and the compressor while retaining the polarization of the short light pulse;

A high-power short optical pulse generator comprising:

2. The high-power short optical pulse generator according to claim 1, wherein the polarization maintaining and connecting means has a polarization maintaining fiber and a polarization maintaining and connecting means.

3. The high-power short optical pulse generator according to claim 2, wherein the polarization maintaining connection means is a polarization maintaining splice or a polarization maintaining connector.

4. The low power short optical pulse generator according to claim 1, wherein the fiber stretcher is constituted by a polarization maintaining fiber.

5. The short optical pulse is a repetitive pulse, and further comprising a pulse decimator for reducing the number of repetitions of the repetitive pulse between the fiber stretcher and the fiber amplifier. High power short light pulse generator as described.

6. The high power short optical pulse generator according to claim 5, wherein the fiber amplifier has a preamplifier and a main amplifier.

7. The high power short optical pulse generator according to claim 6, wherein the main amplifier is a multimode fiber having a core diameter of 10 to 50 Atm.