JP2008311629A - Ultrashort light pulse amplifying method, ultrashort light pulse amplifying device, and broadband comb generator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an ultrashort light pulse amplifying method and an ultrashort light pulse amplifying device capable of surely providing high output by eliminating a phenomenon wherein output is varied on a device basis by using both a mode synchronization fiber laser and an optical fiber amplifier; and a broadband comb generator. <P>SOLUTION: This ultrashort light pulse amplifying method and this ultrashort light pulse amplifying device are characterized in that an optical fiber amplifier having a mode synchronization fiber laser, a single mode fiber and an amplification medium fiber is used, and the fiber length of the single mode fiber or the amplification medium fiber is selected to maximize the average output of ultrashort pulses from the optical fiber amplifier. The broadband comb generator includes the high-output ultrashort light pulse amplifying device, and a high nonlinear fiber connected to the output side of the high-output ultrashort light pulse amplifying device, and converting wavelength to broadband. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an ultrashort optical pulse amplification method and an ultrashort optical pulse amplifier that amplify the output of a mode-locked fiber laser with an optical fiber amplifier, and a broadband comb generator that generates an optical comb suitable for optical frequency measurement.

  Research on erbium-doped optical fiber amplifiers (hereinafter referred to as EDFAs) was started in 1987 to realize a long-distance optical transmission system for optical high-speed communication, and in 1989, the realization of EDFA by LD pumping was in full swing. . In 1990, ultrashort optical pulse amplification by EDFA was reported, and about 1992, a mode-locked fiber laser using EDFA was reported. On the other hand, around 2000, optical frequency measurement using a broadband optical frequency comb using a mode-locked fiber laser and a photonic crystal fiber (hereinafter referred to as PCF) began to attract attention. The optical comb was broadened by EDFA, highly nonlinear fiber (hereinafter referred to as HNLF) and PCF, and optical frequency measurement was also realized. Since the output of a mode-locked fiber laser is generally limited to several mW and cannot be widened as it is, in these studies, an ultrashort optical pulse is amplified by EDFA.

R. J. Mears, et al., Electron. Lett. 23, 1026 (1987) E. Desurvire, et al., Opt. Lett. 12, 888 (1987) M. Nakazawa, et al., Electron. Lett. 25 199 (1989) Y. Kimura, et al., Electron. Lett. 25 1656 (1989) K. Suzuki, et al., “Subpicosecond soliton amplification and transmission using Er3 + -doped fibers pumped by InGaAsP laser diodes,” Opt. Lett. 15,865-867 (1989). M. Nakazawa, et al., “Femtosecond erbium-doped optical fiber amplifier,” Appl. Phys. Lett. 57, 653-655 (1990). K. Tamura, et al., “Self-starting additive pulse mode-locked erbium fiber laser,” Electron. Lett. 28, 2226-2228 (1992). N. Nakazawa, et al., “Continuum suppressed, uniformly repetitive 136 fs pulse generation from an erbium-doped fiber laser with nonlinear polarisation rotation,” Electron. Lett. 29, 1327-1329 (1993). D. J. Jones, et al., “Carrier-envelope phase control of femtosecond mode-locked lasers and direct optical frequency synthesis,” Science vol. 288, 635 (2000). F. Tauser, et al., “Amplified femtosecond pulses from an Er: fiber system: Nonlinear pulse shortening and self-referencing detection of the carrier-envelope phase evolution,” Optics Express 11, 594 (2003). F.-L. Hong, et al., “Broad-spectrum frequency comb generation and Carrier envelope offset frequency measurement by second-harmonic generation of a mode-locked fiber laser,” Opt. Lett 28, 1516-1518 (2003). T. R. Schibli, et al., “Frequency metrology with a turnkey all-fiber system,” Opt. Lett. 29, 2467-2469 (2004). H. Inaba, et al., “Long-term measurements of optical frequencies using a simple, robust and low-noise fiber based frequency comb,” Opt. Express 14,5223-5231 (2006), J. W. Nicholson, et al., “High power, single mode, all-fiber source of femtosecond pulses at 1550 nm and its use in supercontinuum generation,” Opt. Express 12,3025-3033 (2004).

  The present inventors have produced many mode-locked fiber lasers and optical fiber amplifiers, and have generated broadband optical combs exceeding one octave with various highly nonlinear fibers. Under such circumstances, even though the pumping power and input power are the same, the output from the optical fiber amplifier has been plagued by a phenomenon that is not constant for each device. In addition, in a device that happens to have only a low output, spectrum expansion by a highly nonlinear fiber may not be sufficiently obtained. According to the knowledge of the present inventors, it is clear that the cause of the phenomenon that the output from the optical fiber amplifier is not constant for each device is that the chirp amount of the incident pulse to the optical fiber amplifier is not constant. Became.

  An object of the present invention is to use a mode-locked fiber laser and an optical fiber amplifier in combination, eliminate the phenomenon that the output is not constant for each device, and an ultrashort optical pulse amplifying method and an ultrashort optical pulse amplifying device that can reliably obtain a high output And a broadband optical comb generator.

In the present invention, means for solving the above-described problems are as follows.
(1) Using a mode-locked fiber laser, a single-mode fiber, and an optical fiber amplifier having an amplification medium fiber, the single-mode fiber or the single-mode fiber or the optical fiber amplifier so that the average output of ultrashort optical pulses from the optical fiber amplifier is maximized. A method for amplifying an ultrashort optical pulse, comprising selecting a fiber length of the amplification medium fiber.
(2) a mode-locked fiber laser, a single-mode fiber connected to the output side of the mode-locked fiber laser, and an optical fiber amplifier having an amplification medium fiber connected to the output side of the single-mode fiber, The ultrashort optical pulse amplifying apparatus, wherein the fiber length of the single mode fiber is selected so that the average output of the ultrashort optical pulse from the optical fiber amplifier is maximized.
(3) a mode-locked fiber laser, a single-mode fiber connected to the output side of the mode-locked fiber laser, and an optical fiber amplifier having an amplification medium fiber connected to the output side of the single-mode fiber, An ultrashort optical pulse amplifying apparatus characterized in that the fiber length of the amplification medium fiber is selected so that the average output of the ultrashort optical pulse from the optical fiber amplifier is maximized.
(4) A broadband optical comb including the ultrashort optical pulse amplifying device according to (2) or (3) above and a highly nonlinear fiber connected to the output side of the ultrashort optical pulse amplifying device to broaden the wavelength. Generator.

  According to the high output ultrashort optical pulse amplifier of the present invention, an ultrashort optical pulse with a high output can be obtained, or the same output can be obtained with a low excitation power. In addition, according to the broadband optical comb generator of the present invention, the amplified ultrashort optical pulse output from the high output ultrashort optical pulse amplifier is widened with a highly nonlinear fiber, so that the wavelength of 1.5 μm band is increased. A broadband optical comb having one or more octaves as a center can be easily and optimally generated.

A first embodiment of the present invention will be described with reference to FIGS.
FIG. 1 is a diagram showing the relationship between the output of the optical fiber amplifier and the pulse width when the chirp amount of the incident pulse is changed. In the figure, white squares and black squares are the output of the optical fiber amplifier and the width of the incident light pulse, respectively, and the time of the shortest pulse is zero.
Since the single mode fiber (SMF) has anomalous dispersion in the 1.5 μm wavelength band, the chirp amount of the incident pulse can be changed by changing the SMF length between the fiber laser and the optical fiber amplifier. When the output from an EDFA having normal dispersion in the wavelength 1.5 μm band is observed, there is an incident chirp amount at which the power is maximum in a region where the incident pulse is slightly chirped to the anomalous dispersion side. The increase in power is accompanied by a spectrum expansion. By adjusting the chirp amount of the incident light pulse in this way, a high output ultrashort light pulse amplifier can be obtained. Alternatively, the same output can be obtained with low excitation power.

FIG. 2 is a diagram showing the configuration of the high-power ultrashort optical pulse amplifier according to the invention of this embodiment.
In the figure, a WDM coupler (Wavelength Division Multiplexing coupler) is a wavelength division multiplex coupler, an EDF (Erbium Doped Fiber) is an erbium-doped fiber, and an SMF (Single Mode Fiber) is a single mode fiber.

In FIG. 2, the ring on the left side of the figure constitutes the mode-locked fiber laser 1, the optical fiber amplifier 3 is constituted between the WDM coupler and the WDM coupler on the right side of the figure, and the mode-locked fiber laser 1 and the optical fiber amplifier 3 are shown (AB) Are connected by several optical elements based on the single mode fiber 2. Here, although the WDM coupler on the left side of the figure B exists for the optical fiber amplifier 3, it is in front of the erbium fiber where the amplification actually occurs, and therefore a part of the single mode fiber 2 for the amplified optical pulse. It has become. The mode-locked fiber laser 1 outputs an ultrashort optical pulse train having a wavelength of 1.5 μm. This laser uses a mode-locking mechanism that utilizes nonlinear polarization rotation, and details regarding this are described in Non-Patent Document 6 and Non-Patent Document 7. Here, what is described in these documents is improved, and adjustment for optimizing the mode-locking operation is made possible by adding a polarization controller.
Further, the optical fiber amplifier 3 is pumped by a semiconductor laser through a WDM coupler (wavelength division multiplexing coupler) from both directions (either one direction is acceptable) of the EDF (erbium-doped fiber) constituting the optical fiber amplifier 3. . The EDF (erbium-doped fiber) used here has an absorption coefficient of about 20 dB / m at 1530 nm and a dispersion value of about −30 ps / m / km. A semiconductor laser (excitation laser) having a wavelength of 980 nm (1480 nm is acceptable) and an output of 200 to 500 mW was used.
The length of the SMF (single mode fiber) 2 connected between the mode-locked fiber laser 1 and the optical fiber amplifier 3 is adjusted, and the chirp amount of the incident pulse is selected so that the output from the optical fiber amplifier 3 is maximized. As a result, a high-output ultrashort light pulse can be generated.

Next, a second embodiment of the present invention will be described with reference to FIGS.
FIG. 3 is a diagram showing the configuration of the broadband optical comb generator according to the invention of this embodiment.
As shown in the figure, the broadband optical comb generator of the present invention includes a mode-locked fiber laser 1, an SMF (single mode fiber) 2, optical fiber amplifiers 3 (1), 3 (2), and a highly nonlinear fiber 4 (1 ) And 4 (2). When the broadband optical comb generator is used for optical frequency measurement or the like, it is necessary to detect an “offset signal” and a “beat signal with the laser to be measured”. Therefore, in order to obtain these signals optimally, it is desirable to divide the output of the mode-locked fiber laser 1 into two and independently amplify them by the optical fiber amplifiers 3 (1) and 3 (2). Further, when an optical pulse having a large peak intensity is incident on the highly nonlinear fibers 4 (1) and 4 (2), the spectrum is expanded by a strong nonlinear optical effect, and a broadband optical comb is output.
The broadband optical comb generator of the present invention includes a high-power supercombination comprising a mode-locked fiber laser 1, an SMF (single mode fiber) 2, and optical fiber amplifiers 3 (1) and 3 (2) shown in FIG. The output of the short optical pulse amplifier is used as part of the configuration of the broadband optical comb generator. In FIG. 3, the chirp amount of the incident pulse is changed by changing the single mode fiber length of the SMF (single mode fiber) 2. Outputs from the two optical fiber amplifiers 3 (1) and 3 (2) are used to obtain an offset signal and a beat signal, respectively. Details regarding this are described in Non-Patent Document 13, for example.

  As shown in FIG. 3, the mode-locked fiber laser 1 outputs an optical comb having a wavelength of 1.5 μm, and the output optical comb is divided into two, each of which is a single mode fiber length of an SMF (single mode fiber) 2. After the adjustment of the chirp amount of the incident pulse by changing, the signals are input to the optical fiber amplifiers 3 (1) and 3 (2), respectively. After the optical comb amplified by the optical fiber amplifiers 3 (1) and 3 (2) is once again subjected to dispersion adjustment (it is widely known that these adjustments are necessary), the highly nonlinear fiber 4 (1), 4 (2) is used to broaden the band (wavelength of 1-2 μm). This is a general optical frequency measurement technique using a mode-locked fiber laser and an optical fiber amplifier, and details are described in several papers. In this embodiment, the output of about 4 mW from the mode-locked fiber laser 1 is amplified to about 220 mW by the optical fiber amplifiers 3 (1) and 3 (2), respectively, and the optical fiber amplifiers 3 (1) and 3 (2) The respective outputs are incident on the highly nonlinear fibers 4 (1) and 4 (2), respectively. When the above dispersion adjustment is not performed, the output may be reduced to 150 mW at the minimum.

Next, as described in paragraph 0010 and the like, the length of the SMF (single mode fiber) 2 connected between the mode-locked fiber laser 1 and the optical fiber amplifier 3 is adjusted, and the chirp amount of the incident pulse is changed to the optical fiber amplifier 3. By selecting so that the output from can be maximized, a high-power ultrashort light pulse can be generated.
By the way, generally, in the 1.5 μm wavelength band, the single mode fiber has anomalous dispersion in which the refractive index increases as the wavelength becomes longer, and the erbium-doped optical fiber that is an amplification medium fiber included in the optical fiber amplifier has a longer wavelength. Has normal dispersion with decreasing refractive index.
The optimum single mode fiber length is determined by the balance with the length of the amplifying medium fiber in the optical amplifier, and there are many optimum combinations.
Therefore, in order to maximize the average output of the ultrashort optical pulse from the optical fiber amplifier when amplifying the ultrashort optical pulse, a method of selecting by adjusting the single mode fiber length while fixing the length of the amplification medium fiber It is also possible to adjust the amplification medium fiber length while fixing the single mode fiber length.

It is a figure which shows the output and pulse width of an optical fiber amplifier when changing the chirp amount of an incident pulse. It is a figure which shows the structure of the high output ultrashort optical pulse amplifier based on invention of 1st Embodiment. It is a figure which shows the structure of the broadband optical comb generator based on invention of 2nd Embodiment.

Explanation of symbols

1 Mode-locked fiber laser 2 Single mode fiber 3, 3 (1), 3 (2) Optical fiber amplifier 4 (1), 4 (2) Highly nonlinear fiber WDM coupler Wavelength division multiplexing coupler EDF Erbium-doped fiber SMF Single mode fiber

Claims (4)

  Using a mode-locked fiber laser, a single-mode fiber, and an optical fiber amplifier having an amplification medium fiber, the single-mode fiber or the amplification medium so that the average output of ultrashort optical pulses from the optical fiber amplifier is maximized A method for amplifying an ultrashort optical pulse, wherein the fiber length of the fiber is selected.   A mode-locked fiber laser; a single-mode fiber connected to the output side of the mode-locked fiber laser; and an optical fiber amplifier having an amplification medium fiber connected to the output side of the single-mode fiber. The fiber length is selected so that the average output of the ultrashort optical pulse from the optical fiber amplifier is maximized.   A mode-locked fiber laser; a single-mode fiber connected to the output side of the mode-locked fiber laser; and an optical fiber amplifier having an amplifying medium fiber connected to the output side of the single-mode fiber; The fiber length is selected so that the average output of the ultrashort optical pulse from the optical fiber amplifier is maximized.   4. A broadband optical comb generator comprising: the ultrashort optical pulse amplifier according to claim 2; and a highly nonlinear fiber connected to the output side of the ultrashort optical pulse amplifier for widening the wavelength.

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