JP2563697B2 - Fiber laser equipment - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fiber laser device capable of stably generating highly repetitive ultrashort pulses.

[0002]

2. Description of the Related Art In recent years, many attempts have been made to generate ultrashort optical pulses using a laser device, particularly a passive mode-locked laser (hereinafter referred to as a fiber laser device) using a rare earth-doped optical fiber as a laser medium. ing.

FIG. 3 is a diagram showing the configuration of a conventional fiber laser device. As shown in the figure, this fiber laser device is roughly composed of a first main loop LO1, a second main loop LO2, and a 3 dB optical coupler C1 coupling these main loops LO1 and LO2 to each other.

The first main loop LO1 is an optical fiber doped with a rare earth element that emits a laser beam (hereinafter, referred to as "optical fiber").
A rare earth-doped optical fiber) F1, an excitation light source P that emits light that excites a rare earth element in the rare earth-doped optical fiber F1 (hereinafter referred to as excitation light), an optical coupler C2 that combines the excitation lights, and an optical soliton switch. It comprises an optical fiber F2 for use with a laser beam, a polarization controller PC1 for controlling the wavefront of the laser beam, and a single mode optical fiber MF1 for coupling these elements. In addition, the second
The main loop LO2 includes an optical coupler C3 for extracting an output, an optical isolator IS that is a non-reciprocal device that passes only light in the forward direction (direction indicated by an arrow in the figure), a polarization controller PC2, and each of these elements. It consists of a coupling single mode optical fiber MF2.

Next, the first main loop LO1 will be described in more detail. The first main loop LO1 is called a non-linear amplification loop mirror, which is a modification of the non-linear optical loop mirror. As shown in FIG. 4, the nonlinear optical loop mirror is an apparatus in which two ports D and E on one end side of an optical coupler C having different branching ratios are connected by a single mode optical fiber MF3 to form an optical fiber loop LO3. is there. In the nonlinear optical loop mirror having such a configuration, the light that has entered the optical coupler C from the input port A includes light that passes through the port D and rotates clockwise around the loop LO3, and light that passes through the port E and rotates around the loop LO3 counterclockwise. It is divided into

At this time, when the intensity of the light incident on the input port A of the optical coupler C is weak, 3 is applied to any light rotating in the clockwise / counterclockwise direction in the optical fiber MF3.
Since the following non-linear optical effect, the self-phase modulation effect, does not work, the light that rotates clockwise in the loop LO3 and the loop LO3
There is no phase difference with the light that rotates 3 counterclockwise. In such a linear phenomenon region, the intensity of light appearing at the port B has a value proportional to the incident intensity, and the proportional constant is determined by the branching ratio of the optical coupler C. That is, when the branching ratio of the optical coupler C is α: 1-α and the intensity of the light incident on the port A of the optical coupler is I ^O , the intensity I of the light appearing at the port B is expressed by the following equation. I = (4α ² −4α + 1) · I _O (1) As is clear from the above equation, for example, as an optical coupler C, 3
When using a dB optical coupler (α = 0.5), the intensity I
= 0 (no light appears at port B).

On the other hand, when the intensity of the light incident on the optical coupler C is strong enough to generate the self-phase modulation effect in the optical fiber MF3 in the loop LO3, the light rotating clockwise in the loop LO3 and the loop LO3 are reversed. There is a phase difference with the clockwise light. As a result, the intensity of the light that appears at port B is greater than when the self-phase modulation effect does not occur, and increases non-linearly with respect to the incident intensity. In this case, the intensity I of the light appearing at the port B is expressed by the following equation. I = I _O (2α ² −2α + 1) + 2I _O α (α−1) COS {2πn ₂ L (1-2α) I _O / λ} (2) where n ₂ is the nonlinear refractive index and L is the fiber loop. , Λ is the wavelength of the incident light.

Such a non-linear optical loop mirror is suitable for generating an ultrashort pulse because the incident light from the port A hardly appears at the port B for a weak input and the incident light becomes incident. This is because light appears at port B. Therefore, when a nonlinear optical loop mirror is incorporated in the resonator of a laser, it behaves the same as an ultrafast saturable absorber.

FIG. 5 is a diagram showing the structure of a non-linear amplification loop mirror. The difference between this non-linear amplification loop mirror and the non-linear optical loop mirror is that, as shown in the figure, a laser amplification medium G such as a rare earth-doped optical fiber is used. Is loop L
It is located at the end of O3 (close to port C or close to port D), and the optical coupler C has a branching ratio of exactly 1: 1.

When the intensity of the light incident on the optical coupler C is weak, the self-phase modulation effect does not occur in the optical fiber MF3 in the loop LO3 even if the amplifying medium G is placed, as described above. There is no phase difference between the light rotating clockwise and the light rotating counterclockwise. Therefore, when the intensity of the incident light is weak, the nonlinear amplification loop mirror operates in the linear region, most of the light that has passed through the loop is returned to the port A, and almost no light appears at the port B.

On the other hand, when the intensity of the light incident on the optical coupler C is high, the light is further amplified by the amplification medium G, so that the self-phase modulation effect occurs in the optical fiber MF3. At this time, since the amplification medium G is located at the end of the loop LO3 (close to the port C or the port D), the amplification medium G is positioned between the light rotating clockwise in the loop LO3 and the light rotating counterclockwise. A phase difference occurs, and the light incident from port A is switched to port B (light appears at port B). The phase difference between the two lights in this case is given by the following equation. δφ _C −δφ _OC = πI ₀ n ₂ L (g−1) / λ (3) where δφ _C is the phase change amount of the light that rotates the loop clockwise, and δφ _OC is the light that rotates the loop counterclockwise. the amount of phase change, the intensity of I _O is the light incident on the optical coupler C, n ₂ is the nonlinear refractive index, L is the length of the fiber, g is the gain, lambda is the wavelength of the incident light. As can be seen from this equation, the larger the gain g and the longer the fiber length L, the larger the phase difference, and when it becomes π, maximum switching occurs.
The non-linear amplification loop mirror is characterized in that it can switch the optical pulse even when the intensity of the light incident on the optical coupler is relatively weak compared to the above-mentioned non-linear optical loop mirror.

The conventional fiber laser device shown in FIG. 3 has a first main loop L which is a nonlinear amplification loop mirror.
A second main loop LO2 in which an optical isolator IS is inserted is connected to O1. Thus, the first main loop LO1 is mode-locked by the non-linear amplification loop mirror effect to generate a short pulse.

[0013]

By the way, in the above-mentioned conventional fiber laser device, although a short light pulse can be easily realized, there is no device for controlling the repetition of the light pulse, and therefore, it is shown in FIG. As described above, the optical pulses are randomly repeated, and there is a drawback that a stable repeating optical pulse cannot be obtained.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a fiber laser device which can stably realize periodic high repetition of ultrashort optical pulses.

[0015]

In order to solve the above problems, the present invention provides a rare-earth-doped optical fiber as a laser medium, a pumping light source for pumping the rare-earth-doped optical fiber, and an output from the pumping light source. First light including a multiplexing optical coupler for multiplexing the pumping light to be generated, a soliton generating optical fiber for generating an optical sorington, and a first polarization controller for aligning the polarization planes of the light A second loop including a fiber loop, an output optical coupler for outputting laser light, an optical isolator for limiting the traveling direction of the light to one direction, and a second polarization controller for aligning the polarization plane of the light Optical fiber loop, and the first
And a 3 dB optical coupler for optically coupling the second optical fiber loop with the second optical fiber loop, a third optical fiber loop including an optical path length adjuster for adjusting an optical path length, A branching light that is inserted into the first optical fiber loop or the second optical fiber loop and branches the light into the third optical fiber loop and the first or second optical fiber loop. A coupler is added, and the optical path lengths around the third optical fiber loop and the branching optical coupler are the same as the first optical fiber loop, the second optical fiber loop, and the 3 dB light. It is characterized in that it is set to an arbitrary integer fraction of the length of the laser resonator comprising a coupler.

[0016]

According to the above structure, the repetition of the ultrashort optical pulse can be increased to an integral multiple of the repetition (see FIG. 6) determined by the length of the resonator of the laser, and a stable high repetition ultrashort optical pulse can be obtained. Can be obtained.

[0017]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing the configuration of a fiber laser device according to an embodiment of the present invention. In this figure, parts that are the same as the parts shown in FIG. 3 are given the same reference numerals, and descriptions thereof will be omitted. The fiber laser device of this example is largely different from the above-mentioned conventional fiber laser device in that, in addition to the above-mentioned conventional structure, a branch coupler C4, an optical path length varying device K, and a sub-loop including an optical fiber SF connecting them. The point is that LO4 is provided. The branch coupler C4 is an optical isolator IS.
And the polarization controller PC2, whereby the second main loop LO2 and the sub-loop LO4 are coupled.

Here, what is important is that the sub-loop LO is
4 is set to an arbitrary integer (n) fraction of the length of the resonator of the laser constituted by the first main loop LO1 and the second main loop LO2. Therefore, it is important to finely adjust the length of the sub-loop LO4, which is performed by the optical path length changer K inserted in the loop. The rare earth-doped optical fiber F1 is arranged toward the end of the first main loop LO1.

In the above structure, pulse oscillation occurs as follows. First, the light emitted from the excitation light source P enters the rare earth-doped optical fiber F1 via the multiplexing optical coupler C2 and excites the rare earth element. The polarization controllers PC1 and PC2 are inserted in order to match the polarization of the light rotating around the first main loop LO1 which is a non-linear amplification loop mirror and the light rotating around the second main loop LO2 including the optical isolator IS.

When the intensity of the light entering the first main loop LO1 from the input port A is weak, it is due to the above-mentioned reason.
Light does not appear at port B, but returns to port A. Since this light travels in the second main loop LO2 in the direction opposite to the optical isolator IS, it is removed by the optical isolator IS.

On the other hand, when the intensity of the light incident on the first main loop LO1 is high, the light that has passed through the first main loop LO1 appears at the port B, and the light inside the second main loop LO2 passes through the optical isolator IS. Go forward. This light again enters the first main loop LO1 and reappears at port B. By this switching (light appears on B), mode synchronization is applied and a short optical pulse is generated. The optical short pulse generated in this way is output from the optical coupler C3.

The light pulse rotating clockwise in the second main loop LO2 is split into two lights when passing through the branch optical coupler C4. Of the split lights, one of the lights goes to the second main loop LO2 as it is, and the other light goes to the sub-loop LO2.
It is incident on 4. The light incident on the sub-loop LO4 goes around the loop once and then enters the second main loop LO2 again.

Here, the length of the sub-loop LO4 is the length of the main loop (resonator), that is, the first main loop LO1.
Of the length of the second main loop LO2 and 1 / n (n
Is set to an arbitrary integer), so that if the length of the main loop is L and the speed of the optical pulse is V, the main loop LO
In 1 and LO2, the optical pulse passing through the sub loop LO4 is delayed by L / (nV) time from the optical pulse not passing through the sub loop LO4. The optical pulse divided into two in this way is the main loop LO1, LO2.
When passing through the branch optical coupler C4 again for one round, it is divided again into two, and when n rounds are passed, the main loop LO1,
This coincides with the optical pulse having the repetition frequency determined by LO2.

Since the laser device having the above structure is a passive mode-locking rare earth-doped optical fiber laser, the degree of optical pulse waveform shaping can be adjusted by appropriately adjusting the group velocity dispersion of the optical fiber F2 for an optical soliton switch. Can be varied (optical soliton effect) and in this way shorter pulses can be generated.

The optical soliton is a stable pulse generated by balancing the spread of the pulse width due to the chromatic dispersion of the optical fiber and the compression of the pulse width due to the self-phase modulation effect. It has the property of propagating without changing. That is, by making the group velocity dispersion of the optical fiber F2 negative, an optical soliton is generated and a stable ultrashort pulse in which the optical pulse does not spread can be obtained. For example, the peak intensity required to make N = 1 standard solitons is given by: P _{N = 1} = 0.776 · (λ ³ / π ² cn ₂ ) · (| D | / τ ² ) · πw ² (4) where D is the group velocity dispersion at the wavelength λ of the optical fiber,
c is the speed of light, τ is the pulse width, and w is the spot size of the optical fiber.

For example, group velocity dispersion D = -24 ps / km
/ Nm, pulse width τ = 1.1 ps, spot size w = 4 μm, and wavelength λ = 1.56 μm, the peak intensity required to make a standard soliton is about 30 W.
Since the intensity of this degree can be easily obtained in the laser device, switching by the optical soliton can be realized.

In the fiber laser device of this example,
Since the optical soliton propagates through the resonator, the energy generated per pulse is constant. It can be seen from the equation (4) that the energy P _{N = 1} τ per pulse is proportional to | D | / τ. When the excitation intensity is constant, the absolute value of group velocity dispersion | D of the optical fiber F2 for the optical soliton switch | D
When | is made small, the pulse width τ is necessarily made small so as to make the energy constant. For this reason,
By reducing | D |, femtosecond pulses can be generated.

At this time, if there is no sub-loop LO4,
The femtosecond pulse is randomly generated, but the sub loop L
By inserting O4, the repetition of the femtosecond pulse can be made constant as shown in FIG. Here, this repetition corresponds to an integral multiple of the repetition determined by the length of the laser cavity (ratio of the length of the sub-loop LO4 to the length of the laser cavity (integer n)).

According to the above configuration, it is possible to stably realize periodic high repetition of ultrashort optical pulses.

[0030]

As described above, in the fiber laser device of the present invention, the optical path lengths between the third optical fiber loop and the branching optical coupler are set to the first optical fiber loop and the second optical fiber loop. Optical fiber loop, and 3d above
Since it is set to an arbitrary integer fraction of the length of the laser cavity composed of the B optical coupler, the repetition of the ultrashort optical pulse is
The number of repetitions (see FIG. 6) determined by the length of the laser cavity can be increased to an integral multiple, and a stable highly repetitive ultrashort optical pulse can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of a fiber laser device according to an embodiment of the present invention.

FIG. 2 is a diagram showing an example of repetition of output light pulses in the same example.

FIG. 3 is a diagram showing a configuration of a conventional fiber laser device.

FIG. 4 is a diagram showing a configuration of a conventional nonlinear optical loop mirror.

FIG. 5 is a diagram showing a configuration of a conventional nonlinear amplification loop mirror.

FIG. 6 is a diagram showing an example of repetition of output light pulses in a conventional fiber laser device.

[Explanation of symbols]

 F1 rare-earth doped optical fiber P pumping light source C2 optical coupler for multiplexing F2 soliton generation optical fiber PC1 polarization controller (first polarization controller) LO1 first main loop (first optical fiber loop) C3 output Optical coupler IS Optical isolator PC2 Polarization controller (second polarization controller) LO2 Second main loop (second optical fiber loop) C1 3 dB Optical coupler K Optical path length variator LO4 Sub loop (third optical fiber loop) ) C4 branch coupler (branching optical coupler)

Claims (1)

(57) [Claims] 1. A rare-earth-doped optical fiber as a laser medium, a pumping light source for pumping the rare-earth-doped optical fiber, a multiplexing optical coupler for multiplexing pumping light output from the pumping light source, A soliton generating optical fiber for generating an optical sorington, a first optical fiber loop including a first polarization controller for aligning the polarization planes of light, and an output optical coupler for outputting laser light, A second optical fiber loop including an optical isolator for limiting the traveling direction of light to one direction, and a second polarization controller for aligning the polarization plane of the light, the first optical fiber loop and the first optical fiber loop. A fiber laser device having a 3 dB optical coupler that optically couples the second optical fiber loop with a third light including an optical path length adjuster for adjusting the optical path length. Fiber loop, and a branch for being inserted into the first optical fiber loop or the second optical fiber loop and for branching light into the third optical fiber loop and the first or second optical fiber loop. An optical path coupler is added, and an optical path length around the third optical fiber loop and the branching optical coupler is the first optical fiber loop,
A fiber laser device, wherein the second optical fiber loop and the 3 dB optical coupler are set to an arbitrary integer fraction of a laser resonator length.