JPH06283798A - Fiber laser and optical fiber amplifier - Google Patents

Abstract

PURPOSE:To efficiently oscillate 1.9mum band light by using a fluoride fiber whose core contains thulium as a gain medium and using a pumping beam which has a specific wavelength. CONSTITUTION:A pumping beam, which is oscillated by a color center laser 2 and have a wavelength range of 1.55-1.65mum, are reflected by a dichroic mirror 3 to be applied on the one edge of an optical fiber 1 through a condenser lens 4 and are permitted to enter a laser resonator. Thulium ions are present in the core area of the fluoride optical fiber 1, the thulium ions which are at <3>H6 level are bumped once to a level of <3>H4 by the pumping beam and are returned to the <3>H6 level. At that time, light which have a wavelength of 1.9mum band and have energy that equals to the energy difference between the <3>H4 level and the <3>H6 level is emitted. The beam is emitted from the other edge of the optical fiber 1, reflected by the mirror 5 to be directed to the fiber 1 again and a beam with wavelength of 1.9mum band is emitted. Thus, the high- power and highly efficient fiber laser with a wavelength of 1.9mum band is provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fiber laser and an optical fiber amplifier, and a technique effective when applied to a fiber laser and an optical fiber amplifier using a rare earth element, in particular, an optical fiber doped with thulium as a core, as a gain medium. It is about.

[0002]

2. Description of the Related Art FIG. 1 is an energy level diagram of thulium ions in a fluoride glass. Energy values are shown on the right side of each energy level in FIG. 1, and energy values are shown on the left side of each energy level in FIG. The level name, the number given to the arrow is absorbed (corresponding to the upward arrow in FIG. 1) or released (corresponding to the downward arrow in FIG. 1) when the transition of each arrow occurs. The respective wavelengths of the emitted light are shown. However, the unit of energy is represented by 1 / cm (corresponding to K (Kaiser) in spectroscopic analysis) based on the wave number unit, and the name of energy level is Russell-Saunders notation (reference document) :
G. Herzberg, Translated by Takeo Hori, "Atomic Spectra and Atomic Structures" Maruzen Co., Ltd.). The multiplicity and the subscripts with the subscripts represent the total angular momentum, respectively. The ³ H ₆ level is a level having a width in which the degenerated level is split and spread due to the Stark effect generated by the crystal electric field.

In a fluoride fiber in which thulium (Tm) is added to the core, the ³ H ₄ ^-3 H ₆ transition of the thulium ion in the core (the energy of the thulium ion is ³ H ₄ is shown in FIG. 1).
This means to move from the level to the ³ H ₆ level, and this notation will be used hereinafter. ) Has been found,
The wavelength of light generated by this transition is in the 1.9 μm band (1.6
.About.2.0 .mu.m wavelength range). Conventionally, the energy of the thulium ion in the ³ H ₆ level has a wavelength of 0.67μ.
A method of exciting to the ³ F ₃ level with m pumping light (hereinafter, 0.67).
μm excitation) or ³ F with excitation light of 0.79 μm wavelength
_In the method of exciting to _four levels (hereinafter referred to as 0.79 μm excitation), oscillation of light in the 1.9 μm band was confirmed. However, when pumping light of these wavelengths is used, oscillation of light having a wavelength of 0.8 μm (hereinafter referred to as 0.8 μm oscillation) due to ³ F ₄ − ³ H ₆ transition easily occurs, and this is It is difficult to oscillate the light in the 1.9 μm band with high efficiency in order to suppress the oscillation of the light in the 1.9 μm band.

[0004] Therefore, in the conventional 1.9μm band oscillation, by optimizing the wavelength dependence of the reflectance and transmittance of a fiber laser cavity ^{_{mirrors, 3 F 4 - 3 H 6}} 0 due to the transition. Suppresses 8 μm oscillation, and at the same time as 1.9 μm band oscillation, light of wavelength 1.47 μm band caused by ³ F ₄ − ³ H ₄ transition or wavelength 2.3 μm caused by ³ F ₄ − ³ H ₅ transition
By oscillating the light in the band, the efficiency of the 1.9 μm band oscillation was improved.

[0005]

However, in the above conventional fiber laser and optical fiber amplifier,
The excitation light having a wavelength of 0.67 μm or 0.79 μm was excited from the ³ H ₆ level to the ³ F ₃ level or ³ F ₄ level to obtain oscillated light in the 1.9 μm band. Since the wavelength difference between the laser light and the oscillation light is large, there is a problem that the oscillation efficiency of the laser light having a wavelength in the 1.9 μm band is not so high. For example, according to the report of RM Percival, etc., when the wavelength of pumping light is 0.79 μm, the slope efficiency is 3
It is about 8%. (Electronics letters 1992 vol.28 N
o.10 pp1866-1868).

Therefore, the present invention has been made to solve the above problems, and the object of the present invention is 1.9 μm.
An object of the present invention is to provide a fiber laser and an optical fiber amplifier that efficiently oscillate light in the m band.

The above and other objects and novel features of the present invention will become apparent from the description of this specification and the accompanying drawings.

[0008]

In order to achieve the above object, the present invention provides a fiber laser and an optical fiber amplifier in which a fluoride fiber having thulium (Tm) added to a core is used as a gain medium and excitation light is used as a light source. .55 to 1.75
This is characterized in that light having a wavelength of μm is used, and the wavelength of the excitation light is different from that of the conventional method.

[0009]

According to the above-mentioned means, the excitation light in the 1.55 μm band (wavelength range of 1.55-1.75 μm) due to the ³ H ₆ ^-3 H ₄ transition in FIG. by exciting the thulium trivalent ions in the core, the energy of Tm ions ³ H ₄ - so ³ H ₆ are directly excited to ³ H ₄ level on a level of the transition, 0.67 .mu.m excitation and 0 Suppresses the 0.8 μm oscillation that occurred in the case of 0.79 μm excitation, and 1.
Wavelength of 1.47 μm band or 2.3 at the same time as 9 μm band oscillation
There is no need to cause oscillation in the μm band. Further, the wavelength difference between the excitation light and the oscillation light becomes small.

As a result, it is possible to provide a fiber laser and an optical fiber amplifier which efficiently oscillate light in the 1.9 μm band.

[0011]

Embodiments of the present invention will now be described in detail with reference to the drawings. In all the drawings for explaining the embodiments, those having the same function are given the same name and the same reference numeral, and the repeated description thereof will be omitted.

(First Embodiment) FIG. 2 shows a first embodiment according to the present invention.
2 is a diagram showing the configuration of the fiber laser of FIG. 1, 1 is a thulium-doped fluoride optical fiber as a gain medium, 2 is a NaCl color center laser as an excitation light source, 3 is a dichroic mirror, 4 is a condenser lens, and 5 is a mirror Is. One end of the optical fiber 1 has a mirror 5 having a reflectance of 90% or more.
And the laser resonator has a mirror 5 and one end surface of the optical fiber 1 on the side of the condenser lens 4 (reflectance is about 4%).
It consists of and. As the excitation light source 2 that oscillates light in the 1.55 μm band, a semiconductor laser or an Er-doped fiber laser (with an oscillation wavelength range of 1.52 to 1.58) can be used.

The operation of the fiber laser of the first embodiment will be briefly described below. The color center laser 2 oscillates excitation light in the wavelength range of 1.50 to 1.65 μm, and this excitation light is reflected by the dichroic mirror 3 and enters the one end surface of the optical fiber 1 through the condenser lens 4. Enters the inside of the laser resonator. Thulium ions are present in the core region of the fluoride optical fiber 1, and the thulium ions in the ³ H ₆ level are once excited to the ³ H ₄ level by this excitation light, but again ³ H ₆ Return to the level. At this time, ³ H ₄ level and ³
Light having an energy equal to the energy difference from the H ₆ level, that is, light having a wavelength of 1.9 μm band is emitted. The light thus generated is emitted from the other end of the optical fiber 1, is reflected by the mirror 5 having a reflectance of 90% or more, and is incident on the optical fiber 1 again. It emits light in the 1.9 μm band. The emitted light reaches one end surface of the optical fiber 1 on the side of the condenser lens 4, a part of which is transmitted through this end surface, and the rest is reflected by this end surface. Of the light in the 1.9 μm wavelength band that has passed through this end face and emitted from the optical fiber 1, only the light not in the 1.55 μm wavelength band passes through the dichroic mirror 3 and is emitted to the outside of the fiber laser. The light reflected by the one end surface of the optical fiber 1 on the side of the condenser lens 4 again emits light in the 1.9 μm wavelength band by the same mechanism as described above in the optical fiber 1. In this way, laser light having a constant wavelength and a uniform phase is oscillated to the outside of the fiber laser in a chain reaction.

As can be seen from the above description, the first embodiment
The following effects can be obtained with the fiber laser.

That is, by exciting the thulium trivalent ion in the core of the fluoride fiber with excitation light in the 1.55 μm band due to the ³ H ₆ ^-3 H ₄ transition in FIG.
Energy of the ions is ³ H ₄ - is a level above the ³ H ₆ transition ³ H
Since it is directly excited to the _four levels, 0.67 μm excitation and 0.7
Suppresses the 0.8 μm oscillation that occurred in the case of 9 μm excitation, and at the same time as the 1.9 μm band oscillation, the wavelength is 1.47 μm.
It is not necessary to cause oscillation in the band or 2.3 μm band. Further, the wavelength difference between the excitation light and the oscillation light becomes small.

Therefore, the fiber laser of the first embodiment can oscillate light in the 1.9 μm band with high efficiency.

In the following, it will be explained that the above-mentioned effects were quantitatively confirmed by experiments.

FIG. 3 shows the wavelength of 1.
The dependence of the oscillation light output of 9 μm on the excitation light intensity is shown with the wavelength of the excitation light as a parameter. Tm used as gain medium
The core diameter, cutoff wavelength, thulium concentration, and fiber length of the doped fluoride fiber are 11 μm and 2.1 μ, respectively.
m, 2000 ppm, 9 m.

When the pumping light wavelength is 1.55 μm, the threshold value is 37 mW, the slope efficiency is 56%, and the pumping light wavelength is 1.6.
In case of 0 μm, the threshold is 31 mW and the slope efficiency is 7
The threshold is 24 when 3% and the excitation light wavelength is 1.65 μm.
The mW and slope efficiency were 74%. Excitation light wavelength 1.
Maximum output of 5 at 60 μm and excitation light intensity of 104 mW
3 mW was obtained. As described above, the effect of the present Example 1 was quantitatively confirmed.

(Second Embodiment) FIG. 4 shows a second embodiment according to the present invention.
FIG. 3 is a diagram showing the configuration of the 1.9 μm band tunable fiber laser of FIG. The 1.9 μm band wavelength tunable fiber laser of the second embodiment has the same basic configuration as that of the first embodiment shown in FIG. 2, except that a focusing lens 4 for collimation is separately provided in the resonator. A dielectric multilayer narrow band filter 6 as a wavelength selection element is arranged to tune the oscillation wavelength.
A birefringent filter or a prism can be used as the wavelength selection element. The operation is basically the same as that of the first embodiment, but only the light having a specific wavelength out of the light in the 1.9 μm band generated by the fiber laser passes through the dielectric multilayer film narrow band filter 6. However, all other wavelengths of light are reflected by the dielectric multilayer film narrow band filter 6.

Here, by changing the angle between the surface of the dielectric multilayer narrow band filter 6 and the laser light incident thereon, the wavelength of the light passing through the dielectric multilayer narrow band filter 6, that is, the oscillation wavelength, can be set. Can be changed. For example, in the second embodiment, by using the dielectric multilayer narrow band filter 6 having the transmission characteristics as shown in FIG. 5, laser light having a wavelength in the range of 1.892 to 1.928 μm is oscillated. It was confirmed.

Since the wavelength of the fluorescence spectrum of the thulium-doped fiber is spread in the range of 1.88 to 1.98 μm, if a wavelength selecting element operating in this wavelength range is used, a desired wavelength within this wavelength range is obtained. It is possible to select and transmit only light having a wavelength and oscillate as laser light.

As can be seen from the above description, the second embodiment
It is needless to say that the 1.9 μm band tunable fiber laser having the above configuration can obviously obtain the same effect as that of the first embodiment. Particularly, in the second embodiment, the wavelength of the laser light oscillated outside the fiber laser can be arbitrarily set within the wavelength range of 1.88 to 1.98 μm of the fluorescence spectrum.

(Third Embodiment) FIG. 6 shows a third embodiment according to the present invention.
7 is a diagram showing the configuration of the Q-switch operation type fiber laser of FIG. 7, and 7 is a mechanical chopper. Q of the third embodiment
The basic configuration of a switch operation type fiber laser is shown in FIG.
Similar to that of the first embodiment shown in FIG.
Similar to the second embodiment, a focusing lens 4 for collimation is separately provided.
And a mechanical chopper 7 as a Q switch shutter. An acousto-optical element, an electro-optical element, or the like can be used as the Q-switch shutter. The operation is basically the same as that of the first embodiment,
During the time when the thulium ion is in the excited state of ³ H ₄ (several milliseconds), the shutter of the mechanical chopper 7 is closed so that the resonator is not configured,
The thulium ion in the optical fiber 1 is continuously excited to accumulate energy. The shutter is suddenly opened from this state, and the resonator is configured for the same time as the time when the shutter is in the closed state. After that, by repeating this operation, the time pulse width is small and the output is large.
A μm band light pulse is generated at regular time intervals.

As can be seen from the above description, according to the Q-switch operation type fiber laser of the third embodiment,
Obviously, the same effect as that of the first embodiment can be obtained.

(Fourth Embodiment) FIG. 7 shows a fourth embodiment according to the present invention.
FIG. 3 is a diagram showing the configuration of the 1.9 μm band optical fiber amplifier of FIG. 1, and the basic configuration is the same as that of the first embodiment shown in FIG. 2. In the 1.9 μm band optical fiber amplifier of the fourth embodiment, the signal light enters the thulium-doped fluoride optical fiber 1 from the side of the thulium-doped fluoride optical fiber 1 that does not face the condenser lens 4. There is. Therefore, the operation is basically the same as that of the first embodiment, but the optical fiber 1 newly emits light by the signal light in addition to the pumping light. Instead of the dichroic mirror 3, 1.55 / 1.9 is used as a multiplexing element for multiplexing the signal light and the emission light generated by the optical fiber 1.
A μm WDM optical fiber coupler can also be used.

FIG. 8 is a graph showing the dependence of the 1.9 μm band gain on the pumping light intensity in the 1.9 μm band optical fiber amplifier of the fourth embodiment. As a signal light source, the oscillation wavelength is 1.87.
A Tm-doped fluoride fiber laser of μm was used as an excitation light source, and an Er-doped fiber laser of oscillation wavelength 1.57 μm was used. When pumping light is not incident, 21 dB of signal light is lost due to basic absorption of Tm ions. However, when the pumping light intensity is 39 mW or higher, the net gain reaches a positive value.
A net gain of 18db was obtained at 0mW.

As can be seen from the above description, the fourth embodiment
According to the 1.9 μm band optical fiber amplifier, the following effects can be obtained.

That is, for the same reason as described in the first embodiment, the optical fiber amplifier of the fourth embodiment is 1.9.
Amplification of light in the μm band can be performed with high efficiency.

Although the present invention has been specifically described based on the embodiments, it is needless to say that the present invention is not limited to the embodiments and various modifications can be made without departing from the scope of the invention. For example, in the above-mentioned embodiment, the wavelength of the excitation light for exciting the energy of thulium ion to the ³ H ₄ level is 1.55, 1.57, 1.60, 1.6.
Although 5 μm of excitation light was used, the present invention is not limited to this, and any light having a wavelength in the range of 1.55 to 1.75 μm may be used.

This is because the measured value of the absorption spectrum of the Tm ion-doped fiber laser is 1.55-1.75 μm.
It is considered that light having a wavelength in the range is absorbed by Tm ions sufficiently efficiently. In addition, as the wavelength of the excitation light, light having a wavelength of 1.7 μm is optimal.

Although the present invention has been specifically described based on the embodiments, it is needless to say that the present invention is not limited to the above embodiments and various modifications can be made without departing from the scope of the invention. Absent.

[0033]

As described above, according to the present invention, a high-output and high-efficiency fiber laser and a high-efficiency optical fiber amplifier having a wavelength of 1.9 μm band can be realized.

Further, since the operating wavelength range of the fiber laser and the optical fiber amplifier according to the present invention is in the vicinity of the 1.9 μm band, the minimum loss value (wavelength of the silica-based fiber for the light of the 1.9 μm band is the wavelength. When a laser medium having a minimum loss value at 1.55 μm and a loss value sufficiently lower than 0.154 db / km) is developed in the future, by combining the fiber laser and the optical fiber amplifier according to the present invention, It becomes possible to construct an ultra long distance transmission system.

Since the light in the 1.9 μm band is also the absorption band of water, the fiber laser and optical fiber amplifier according to the present invention are expected to be applied to sensors and medical fields.

[Brief description of drawings]

FIG. 1 is a diagram showing energy levels of trivalent trivalent ions in a fluoride glass, transitions between energy states, and wavelengths of light emitted or absorbed by the transitions.

FIG. 2 is a diagram showing a schematic configuration of a fiber laser according to a first embodiment of the present invention,

FIG. 3 is a graph showing the dependence of the output of the fiber laser of Example 1 on the excitation light intensity, using the excitation light wavelength as a parameter;

FIG. 4 is a diagram showing a schematic configuration of a wavelength tunable fiber laser according to a second embodiment of the present invention,

5 is a graph showing transmission characteristics of the dielectric multilayer film narrow band filter 6 of FIG.

FIG. 6 is a diagram showing a schematic configuration of a Q-switch operating fiber laser according to a third embodiment of the present invention,

FIG. 7 is a diagram showing a schematic configuration of an optical fiber amplifier according to a fourth embodiment of the present invention,

FIG. 8 is a graph showing the pumping light intensity dependency of the signal light gain by the optical fiber amplifier of the fourth embodiment.

[Explanation of symbols]

1 ... Thulium-doped fluoride fiber, 2 ... Color center laser, 3 ... Dichroic mirror, 4 ... Condensing lens, 5 ... Mirror, 6 ... Dielectric multilayer narrow band filter, 7
… Mechanical chopper.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tomonori Sugawa 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation

Claims (2)

[Claims] 1. In a fiber laser using an optical fiber having a core doped with a rare earth element as a gain medium, thulium is added to the core of the optical fiber, and a wavelength of 1.30 is used as pumping light.
A fiber laser characterized by using light of 55 to 1.75 μm. 2. An optical fiber amplifier for amplifying a signal light by combining an optical fiber having a core to which a rare earth element is added as a gain medium, injecting the pumping light and the signal light into the medium, and amplifying the signal light. An optical fiber amplifier characterized in that thulium is added to, and light having a wavelength of 1.55 to 1.75 μm is used as pumping light.