JPS6217736A - Optical pulse width compressing and amplifying method - Google Patents

Abstract

PURPOSE:To compress a signal optical pulse width, and to amplify it without widening the pulse width, by utilizing effectively, a double refractive index difference in a fiber by an optical Kerr effect, and varying a coupling between polarized wave modes of the optical axis direction in double refraction and polarized wave holding fibers. CONSTITUTION:A high output laser beam from a high output laser 1 of a system is made incident on an optical multiplexer 3, multiplexed with a signal light from a signal use light source 10 and applied to a polarizer 4. A linearly polarized light from this polarizer 4 is made incident on an optical axis of a large refractive index of a double refraction fiber 5 of a polarized wave holding optical fiber, or in the direction orthogonal to said axis, through a lens 9. An output light from this fiber 5 is made incident on a wavelength demultiplexer 6 from the lens 9, and a high output laser beam pulse is separated from the output light of the fiber 5. Next, a signal whose pulse width has been compressed is fetched from an analyzer 7 which has been placed in the direction orthogonal to the incident signal light.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a method for compressing and simultaneously amplifying the pulse width of light source light used in communication systems using optical fibers and Senna technology using optical fibers. It is.

[Conventional technology]

Conventionally, compression of optical pulse width using a birefringent optical fiber has been achieved by a configuration as shown in FIG.

In the figure, λ is the 2/4 plate, ≠ is the polarizer, and j is the birefringent light 7.
7 is an analyzer, RI is a lens, and IO is a signal light source. The pulsed light from the signal light source is converted into circularly polarized light by the second 24 plate, and enters the Hiromu polarizer via the lens star as linearly polarized light having an angle of θ with respect to the principal axis of the birefringent optical fiber. This method uses the optical Kerr effect to change the refractive index depending on the light intensity of the incident light, combine the high and low light intensity parts to different principal axes, and use the analyzer 7 to separate the low light intensity parts. This removes shear and compresses the pulse width. The light intensity after passing through the device shown in the figure is given by a linear equation.

P t= PI, dn” (φ/!) m” (2θ
)・・・・・・・・・・・・・・・・・・ (1) Here, Po is the input intensity to the device, φ is the phase modulation amount proportional to the incident light intensity, and θ is the birefringence. 7 This is the inclination between the main axis of the eyeper or polarization maintaining fiber and the polarization plane of the incident light. The relationship between φ and incident light is given by the following equation.

2πf, t/2πLφ=7(n,
-nx)=〒X(P8-P,)・・・・・・・・・(2
)θ=tm-' (px//P, f...
・・・・・・・・・・・・・・・・・・・・・(8) Here, λ is the wavelength of light, L is the fiber length, and n≦・bow is the mutually orthogonal optical axes of the fibers. The refractive index change due to the optical Kerr effect, Px, P is the light intensity toward the optical axis, ÷ is the coefficient of the optical Kerr effect. In the conventional method, the light transmittance depends on the slope of the polarization plane of the incident light - θ, and is expressed by equation (1).
If θ is brought closer to ≠t° to increase the transmittance, Px? P,
According to equations (2) and (8), the amount of phase change φ caused by the Kerr effect becomes smaller, so there is a drawback that the intensity of the incident light must be increased accordingly in order to compress the pulse width. Furthermore, there is a drawback that input light whose pulse width has been compressed is not amplified.

[Problems to be solved by the invention and means for solving them]
In order to solve the drawback of the conventional technology that high-output pulses cannot be obtained, the present invention provides a separate light source for bombing, makes effective use of the birefringence difference in the fiber caused by the optical Kerr effect, and creates a birefringent fiber. Alternatively, the pulse width of a weak signal can be compressed by changing the coupling coefficient between polarization modes in the optical axis direction in a polarization-maintaining fiber, and at the same time the pulse width can be compressed by photon mixing or stimulated Raman scattering that occurs in the fiber. It is characterized by amplifying the generated pulse.

〔Example〕

Figure 1 shows an embodiment of the present invention; / is a high-power laser, 2 is a plate, 3 is a multiplexer, ≠ is a polarizer, j is a birefringent fiber, O is a wavelength demultiplexer, and 7 is a wavelength demultiplexer. Analyzer.

t is a signal detection element, ri is a lens, and 10 is a signal light source. For example, a YAG laser or the like is used as the high-power laser. C. Signal light made into a circularly polarized state through a plate and YAG
Laser light is used as pump light, and is made linearly polarized by a polarizer through a half mirror or dichroic mirror, and is made incident on the optical axis of a birefringent 7-taper or polarization-maintaining fiber having a high refractive index. The signal light and pump light that have passed through the fiber are separated by a wavelength demultiplexer such as a dichroic mirror, a prism, or a diffraction grating, and an analyzer extracts the axially polarized component with a large refractive index. Take it out.

The mechanism by which pulse width compression is performed by pump light will be explained. When strong pump light is present, the birefringence SS of the polarization maintaining fiber is given by the following equation as a function of the pump light power due to the optical Kerr effect of the optical fiber.

B,=B,. +7(Px-P,)・・・・・・・・・・
・・・・・・・・・・・・・・・ (1) Here -F3s
o is the initial birefringence when there is no pump light * x l't
Self-focusing coefficient, Px, P representing the Kerr effect.

are the mutually orthogonal X direction and X of the pump light power, respectively.
is the directional component. Figure 2 shows the relationship between Bs of a normal silica-based optical fiber and pump light intensity P. 6x is /-/
X/0−” From the 0 diagram where e, s, u
In the case of 0, as the pump light increases, B.

It can be seen that increases linearly. The pump light power is
Nd: io'w output can be obtained using a mode-locked Q switch with a YAG laser. At this time, the light Ke
The amount of change in B caused by the rr effect is approximately /,j'1.
It is 10-5.

FIG. 3 shows the relationship between the inter-mode coupling coefficient (hereinafter expressed as below) and BS. It is known that the inter-mode coupling coefficient of a birefringent fiber or polarization-maintaining fiber is determined by various fluctuations, and that the larger the difference in birefringence index, the smaller h becomes. Therefore, from Equation 1 (1), h can be changed by changing Bs depending on the incident pump light. That is, h is expressed as a function of the pump injection strength UP. Here, when only the hiipx component is excited, the ratio of the amount transferred to the P component is shown. In this method, the signal light and pump light to the birefringent fiber or polarization-maintaining fiber are only Px components, so B5 increases proportionally as the pump light increases, and h decreases as shown in Figure 3. Become. As the pump light becomes stronger, a phenomenon occurs in which the amount of pump light transferred to the P component of the signal light in the fiber becomes smaller. When the signal light and pump light are separated by a dichroic mirror and the Py acid component of the signal light is removed by an analyzer, when the pump light is weak, more signal light is removed by the analyzer, and as the pump light becomes stronger, the analyzer removes more signal light. Since the amount removed is small, it is possible to compress the pulse width of the signal light by narrowing the pulse width.

Next, the mechanism by which weak signals are increased will be explained. In order to amplify signal light by stimulated photon mixing, phase matching conditions must be satisfied so that part of the energy of pump light is converted into signal light by stimulated photon mixing.

The phase matching condition is as follows: k, kA is the wave vector of nine Stokes light and anti-Stokes light produced by induction≠photon mixing, and k.

is the wave vector of the pump light. Since the polarization planes of the signal light and the pump light are parallel, if the wavelength of the signal light is made to match the wavelength of the Stokes light, the energy of the pump light is converted to the signal light and the signal light is amplified. Amplification of the signal light is also possible by matching the wavelength of the signal light with the high-order Stokes light generated by the stimulated Raman effect.

Pulse width compression and amplification of this signal light are performed by generating different nonlinear phenomena within the same fiber.
The device configuration becomes simple.

〔Effect of the invention〕

As explained above, in the optical pulse width compression amplification method according to the present invention, the signal light pulse width can be compressed, and furthermore, it can be amplified without widening the pulse width by stimulated photon mixing or stimulated Raman effect. It has the advantage of being applicable as an ultrashort pulse generating light source for optical communication systems and optical sensors.

[Brief explanation of the drawing]

Fig. 1 shows an embodiment of the present invention, Fig. 1 shows the relationship between the birefringence of a normal silica-based optical fiber and the pump light intensity P, and Fig. 3 shows the relationship between the intermode coupling coefficient and the birefringence. A diagram showing the relationship between the bending and bending 5 is a diagram showing an example of a conventional pulse width compression method. /...High output laser, 2.../4 plates, 3...Multiplexer. Hiro...Polarizer! ... Birefringent fiber, 6... Wavelength demultiplexer, 7... Analyzer, Ri... Lens, 10...
- Signal light source.

Claims (1)

[Claims] The high-power laser light and the signal light are combined, the polarization state of the high-power laser light pulse is linearly polarized, and the signal light is The polarization state is input as linearly polarized light into the optical axis of a birefringent optical fiber or polarization-maintaining optical fiber with a large refractive index, or in a direction perpendicular to this, and high-power laser light is generated from the output light of the polarization-maintaining optical fiber. An optical pulse width compression amplification method characterized by separating pulses and extracting signal light with a compressed pulse width through an analyzer installed in a direction orthogonal to the incident signal light.