JPS64836B2 - - Google Patents

Description

Detailed Description of the Invention [Technical Field] The present invention uses stimulated Raman scattering and the optical Kerr effect in the optical fiber to generate a nonlinear refractive index in the optical fiber, which is balanced with the dispersion in the optical fiber. Wavelength tunable optical soliton (soliton, rising wave)
The present invention relates to a method and apparatus for generating laser light to obtain ultrashort optical pulses with a small pulse width.

[Prior art]

Research on solitons in optical fibers began about 10 years ago. Initially, a theoretical study was reported in which the nonlinear optical effect (Kerr effect) in the optical fiber and the dispersion effect in the medium were balanced, and the envelope of the optical pulse became a soliton. after that,
Optical solitons have been observed using an F ²⁺ color center laser with a wavelength of 1.55 μm excited by an Nd ³⁺ :YAG laser.

FIG. 1 shows the pulse waveform after propagation through the optical fiber.

1A shows how conventional optical soliton pulses are generated.
This will be explained with reference to FIGS. 1E to 1E. Figures 1A to 1E
The figure shows the waveform of a light pulse propagated through an optical fiber and extracted, with time on the horizontal axis and pulse output measured by the autocorrelation method on the vertical axis. According to this, it can be seen that from Figure 1A to Figure 1E, the fiber input P _io increases from 0.3W, 1.2W, 5.0W, 11.4W, and 22.5W, and the output width narrows. . This shows that the incident optical pulse becomes a soliton.

However, this method simply uses a powerful wavelength of 1.55μ
This is simply a matter of injecting m light pulses into the optical fiber to balance the dispersion and nonlinearity in the optical fiber. In other words, this shows that it is impossible to generate such a soliton pulse without a light source in the 1.55 μm band. Furthermore, this method has the disadvantage that it is not possible to generate wavelength-tunable optical soliton pulses in the 1.5 μm band.

〔the purpose〕

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide an oscillation method that solves the above-mentioned drawbacks and enables laser oscillation of optical soliton pulses in the wavelength band of 1.5 μm, and an apparatus for implementing such a method.

Another object of the present invention is to provide an oscillation method that can vary the wavelength of laser oscillation of optical soliton pulses in the 1.5 μm wavelength band, and an apparatus for implementing such a method.

[Structure of the invention]

In order to achieve such an objective, the present invention uses excitation light that generates ultrashort optical pulses in the wavelength band of 1.0 to 1.4 μm to generate light in the 1.5 μm band through the stimulated Raman scattering effect in the optical fiber. Furthermore, by balancing the nonlinear refractive index generated by the strong electric field with the dispersion effect in the 1.5 μm band in the optical fiber, laser oscillation of tunable optical soliton pulses in the 1.5 μm wavelength band is made possible.

The method of the present invention generates excitation light in the form of ultrashort optical pulses in the wavelength band of 1.0 to 1.4 μm, and guides the excitation light into a silica-based single mode optical fiber with a wavelength of 1.5 μm.
By generating stimulated Raman scattered light in the wavelength band, the nonlinear refractive index due to the Kerr effect generated in the optical fiber by the stimulated Raman scattered light is balanced with the dispersion in the optical fiber in the wavelength band of 1.5 μm, and the wavelength is determined by
It is characterized by laser oscillation of optical soliton pulses in the 1.5 μm band.

Here, the optical pulse of the excitation light is generated in time synchronization with the pulse of the stimulated Raman scattered light, and the length of the resonator when generating the stimulated Raman scattered light in the optical fiber is made variable so that the wavelength is 1.5 μm. It is preferable to perform stimulated Raman oscillation with a variable wavelength in the band.

The device of the present invention includes a laser light source that generates excitation light in the form of ultrashort optical pulses in the wavelength band of 1 μm to 1.4 μm, a silica-based single mode optical fiber, and a silica-based single mode optical fiber disposed at one end of the optical fiber to emit the excitation light. a first reflecting mirror that allows the excitation light to pass through and reflects light in the wavelength band of 1.5 to 1.6 μm; and a second reflecting mirror that reflects the excitation light and the light in the wavelength band of 1.5 to 1.6 μm; Nonlinear refraction obtained by guiding the optical fiber through a reflecting mirror to generate stimulated Raman scattered light with a wavelength of 1.5 μm in the optical fiber, and by the optical Kerr effect generated in the optical fiber by the stimulated Raman scattered light. 1.5 μm by balancing the ratio and the dispersion in the optical fiber in the 1.5 μm band.
It is characterized by generating ultra-short optical pulses by generating band optical solitons.

Here, the laser light source is a mode-locked laser, and the pulse of the stimulated Raman scattered light in the optical fiber is time-synchronized with the optical pulse of the excitation light, and the position of the second reflecting mirror in the optical axis direction is determined. By making it variable, it is preferable to generate stimulated Raman oscillation with a variable wavelength in the 1.5 μm band.

[Example] The present invention will be described in detail below with reference to the drawings.

One embodiment of the invention is shown in FIG. Here, 1
is a light source that generates an excitation ultrashort light pulse with a wavelength of 1.0 to 1.4 μm, 2 is a laser mirror, which transmits the excitation ultrashort light with a wavelength of 1.0 to 1.4 μm from light source 1,
Reflects light in the wavelength band of 1.5 to 1.6 μm, that is, optical soliton oscillation light. Note that an optical isolator may be inserted between the light source 1 and the laser mirror 2 in order to eliminate instability of the light source 1 caused by coupling of the two. 3 is the optical fiber coupling lens, 4 is the length L
This is a silica-based single mode optical fiber. 5 is a laser mirror that reflects light in the wavelength band of 1.5 to 1.6 μm and excitation light, and can be moved in the optical axis direction as shown by the arrow. Here, optical fiber 4
Let the distance between the output end of the movable mirror 5 and the movable mirror 5 be l.

In operation, first, excitation light in the form of ultrashort optical pulses from a light source 1 is guided to an optical fiber 4 through a mirror 2. It is assumed that the repetition period of the ultrashort optical pulses from the light source 1 is Δt sec as shown in FIG. 3A. When such an ultrashort optical pulse enters the optical fiber 4 as shown in FIG. 3B, stimulated Raman scattering occurs, and the scattered light is reflected by the movable mirror 5 and returns to the input end side of the optical fiber 4. At this time, if the Raman pulse propagates overlapping with the excitation pulse again, the Raman pulse in the optical fiber 4 is amplified and eventually starts laser oscillation. When the Raman wavelength is λ, the group velocity in the optical fiber 4 is Vg (λ), and the speed of light in vacuum is c, the following equation can be obtained from the above-mentioned synchronization conditions.

T=2L/Vg(λ)+2l/c (1) T=mΔt (m is an integer) (2) Now, let us consider the case where the movable mirror 5 is moved by Δl. In this case, since T does not change, in order for stimulated Raman oscillation to occur, the group velocity Vg (λ) must be changed, and in this case, such a change must be compensated for by dispersion. Therefore, for Raman oscillation, it is necessary to satisfy T=2L/Vg(λ+Δλ)+2(l+Δl)/c (3), similar to equation (1). However, the wavelength λ is λ+
It is assumed that there is a Raman gain even when Δλ. From equations (1) and (3), 1/Vg(λ)-1/Vg(λ+Δλ)=1/Lc(Δl)(
4), and if the total variance is σ _t (λ), then Equation (4)
The left side of can be expressed as σ _t (λ)・Δλ, so
σ _t (λ) becomes σ _t (λ)=1/Lc(Δl/Δλ) (5). That is, the present invention makes it possible to realize a wavelength-tunable fiber Brahman laser. This means that it is possible to form wavelength-tunable optical soliton pulses. That is, when the pumping light input is increased and the pulse of the stimulated Raman laser light becomes larger, the nonlinear refractive index becomes larger and eventually balances with the dispersion of the optical fiber to form a soliton. Also,
From equation (5), if Δλ is measured when Δl is changed, the dispersion σ _t (λ) of the optical fiber can be measured.

As the excitation light source 1, the output power is 5W or more,
It is desirable that the optical pulse width be 50 ps or less and the wavelength be one that generates a large stimulated Raman scattering gain in the 1.5 μm band. Here, in order for the first Stokes light to be in the 1.5 μm band, a light source in the 1.4 μm band is required, and in that case, a Ho ³⁺ laser or an F ²⁺ color center laser is suitable. The second Stokes beam is 1.5μm
In order to become a band, the wavelength must be 1.32μm or
A 1.34 μm Nd ³⁺ :YAG laser is preferred. In that case, since the output is strong, a large gain can be expected even with the second Stokes light.

Generally, to generate a rapid optical Kerr effect, an ultrashort optical pulse of several tens of p sec or less is required.
This causes a self-phase modulation effect, and optical solitons are produced in balance with dispersion. At this time, stimulated Raman transition (in silica-based optical fiber
When this laser is oscillated at a center of about 460 cm ^-1 ), the optical Kerr effect associated with the third-order Raman nonlinear refractive index becomes smaller. However, a normal nonlinear refractive index that is not originally a Raman type exists at the Raman wavelength, and there is also an optical Kerr effect. Therefore, optical soliton shapes are possible. If you want to incorporate a Raman nonlinear refractive index, you can shift the oscillation wavelength of the Raman laser from the center. This can be easily realized in the present invention since the wavelength is tunable.

FIG. 4 shows the expected relationship between excitation input and output pulse width. Here, Raman laser oscillation is started from a certain threshold while increasing the excitation input,
As the excitation input is further increased, the width of the output optical pulse begins to decrease rapidly. From this, it can be said that the optical soliton is oscillating. As the excitation input is further increased, Figures 1D and 1E
As shown in the figure, multiple solitons begin to occur.
In high-speed optical communications, it is advantageous to use a single soliton generation region.

Furthermore, in the present invention, by moving the movable mirror 5 shown in FIG. 2, a wavelength-tunable optical soliton laser can be oscillated. The situation is shown in FIG. The horizontal axis in FIG. 5 is the oscillation wavelength, and the vertical axis is the amount of movement Δl of the movable mirror 5. According to this, it can be seen that as Δl increases, the oscillation wavelength shifts to the shorter wavelength side.

〔effect〕

As explained above, according to the present invention, by oscillating a time-synchronized optical fiber Brahman laser in the wavelength band of 1.5 μm, the nonlinear refractive index (Kerr effect) and dispersion in the optical fiber are balanced, and light Soliton laser oscillation can be performed. This has the advantage of being able to generate high-power ultrashort optical pulses in the 1.5 μm wavelength band. Furthermore, according to the present invention, it is also possible to generate wavelength-tunable optical soliton pulses, which has been difficult until now.

[Brief explanation of the drawing]

1A to 1E are explanatory diagrams of how conventional optical soliton pulses are generated, FIG. 2 is a configuration diagram showing an embodiment of the present invention, and FIGS. 3A and 3B are
4 is a diagram showing the relationship between the excitation input and the pulse width of the output soliton according to the present invention, and FIG. 5 is a diagram showing the relationship between the wavelength tuning amount and the moving amount of the movable mirror according to the present invention. FIG. DESCRIPTION OF SYMBOLS 1...Ultrashort optical pulse generator, 2...Laser mirror, 3...Lens for optical fiber coupling, 4...Optical fiber for generating optical soliton, 5...Movable laser mirror for wavelength tuning.

Claims (1)

[Claims] 1. Excitation light in the form of ultrashort optical pulses in the wavelength band of 1.0 to 1.4 μm is generated, and the excitation light is guided to a silica-based single mode optical fiber to generate stimulated Raman scattered light in the wavelength band of 1.5 μm. The nonlinear refractive index due to the Kerr effect generated in the optical fiber by the stimulated Raman scattered light is balanced with the dispersion in the optical fiber in the 1.5 μm wavelength band, and an optical soliton pulse is generated in the 1.5 μm wavelength band. An oscillation method for an optical soliton laser characterized by performing laser oscillation. 2. In the method for oscillating an optical soliton laser according to claim 1, the optical pulse of the excitation light is generated in time synchronization with the pulse of the stimulated Raman scattering light, and stimulated Raman scattering in the optical fiber is generated. By making the actual resonator length variable, the wavelength
An optical soliton laser oscillation method characterized by performing stimulated Raman oscillation with a wavelength variable in the 1.5 μm band. 3. A laser light source that generates excitation light in the form of ultrashort optical pulses in the wavelength band of 1 μm to 1.4 μm, a silica-based single mode optical fiber, and a laser light source disposed at one end of the optical fiber that allows the excitation light to pass through and 1.5~
A first reflecting mirror that reflects light in a 1.6 μm band; and a second reflecting mirror that reflects the excitation light and light in a wavelength band of 1.5 to 1.6 μm; stimulated Raman scattered light with a wavelength of 1.5 μm is generated in the optical fiber, and the nonlinear refractive index obtained by the optical Kerr effect generated in the optical fiber by the stimulated Raman scattered light and the nonlinear refractive index in the 1.5 μm band are An oscillation device for an optical soliton laser, characterized in that an ultrashort optical pulse is obtained by generating an optical soliton in the 1.5 μm band by balancing the dispersion in the optical fiber. 4. In the optical soliton laser oscillation device according to claim 3, the laser light source is a mode-locked laser, and the pulse of stimulated Raman scattered light in the optical fiber and the optical pulse of the excitation light are time-synchronized. An oscillation device for an optical soliton laser, characterized in that the position of the second reflecting mirror in the optical axis direction is made variable to generate stimulated Raman oscillation with a variable wavelength in the 1.5 μm band.