JP2001320115A - Laser amplifier, laser amplifying method, laser amplifying device and laser oscillator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a laser amplifier, a laser amplifying method, a laser amplifying device and a laser oscillator wherein pumping by a semiconductor laser is enabled and high efficiency, small size, long life and highly stable operation can be simultaneously realized, in a laser amplifier into which rare earth element for forming self-terminal system transition is added, and to provide a laser amplifying method, a laser amplifying device and a laser oscillator. SOLUTION: This laser amplifier is achieved by using a semiconductor laser light source having two properly selected wavelength as a pumping light source. For the pumping light source, a first semiconductor laser pumping light source for exciting ions from a ground level up to a laser lower level or an energy level higher than the laser lower level, and a second semiconductor laser pumping light source which has wavelength different from the wavelength of the first pumping light source and excites ions from the laser lower level up to a laser upper level, are used.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a laser amplifier and a laser oscillator using a medium to which a rare earth element is added as a gain medium and using a semiconductor laser as an excitation light source, and a laser amplification method and a laser amplification apparatus using the same.

[0002]

2. Description of the Related Art A laser device using a crystal and glass medium to which a rare earth element is added as an active medium, that is, a laser oscillator, a laser amplifier and the like are widely applied to the information communication industry and the machine industry. High power solid state lasers for metal working and fiber optic amplifiers in fiber optic communication systems are typical examples.

[0003] The efficiency, size, lifetime and mechanical stability of laser amplifiers and laser oscillators are mainly determined by the pump light source. The semiconductor laser as the excitation light source is superior in these respects to the solid-state laser and the fiber laser.
For this reason, in consideration of the practicality of the apparatus, a method using a semiconductor laser (laser diode: LD) as an excitation light source (LD excitation) is preferable. Solid state lasers and fiber lasers as excitation light sources have many disadvantages over semiconductor lasers in various respects. Particularly disadvantageous is that, besides being disadvantageous in terms of efficiency, size, mechanical stability and life as compared with LDs, there are few passive optical components in the wavelength band of 1.05 μm. For example, an optical isolator with low loss and high isolation has not been put to practical use.

In such a laser device, since energy is supplied to rare-earth ions by light excitation, selection of the wavelength of the excitation light source is particularly important for high-efficiency operation of the device. However, depending on the type of rare earth ions, the optimum excitation wavelength, that is, the absorption wavelength band of the ions, does not match well with the wavelength of the semiconductor laser, and it becomes difficult to excite the LD with a single wavelength semiconductor laser. May need to be used. In particular, when an ion forming a self-terminating system whose laser lower level lifetime is longer than the laser upper level lifetime is used as a gain medium, the excitation wavelength is more limited as shown in the following example. Becomes more difficult.

As an example, the case of Tm (thulium) ions in a fluoride glass will be described. When a rare earth element such as Tm is added to a medium such as a fluoride glass, it is ionized in the medium to become Tm ions. FIG. 12 is an energy level diagram showing a conventional thulium fiber amplifier pumping method. At the same time, the conventional excitation wavelength is clearly shown in FIG. FIG. 13 is a graph showing an ASE spectrum (Amplified Spontaneous Emission: amplified spontaneous emission) when the transition shown in FIG. 12 occurs. As shown in FIG. 12, in a fiber amplifier in which Tm ions are added to a core, an optical amplification in a wavelength band of 1.47 μm is set by setting an excitation wavelength to 1.04 to 1.07 μm (hereinafter, referred to as a 1.05 μm band). ⁽³ F ₄ → ³ H ₄ transition) can be realized. In the figure, the transition is clearly indicated as a. At this time, FIG.
An ASE spectrum as shown in FIG. For more information, see “IEEE Journal of Quantum Electronics.
Electronics) Vol. 31, p. 1880, 1995
Year ", Japanese Patent Application No. 11-156745 and" Optics Letters, Vol. 24, 1684.
, 1999 ".

[0006] In such a fiber amplifier, as shown in FIG.
Ground level absorption ( ³ H ₆ → ³ H ₅ ) of the ion, followed by non-radiative transition (not shown) and excited state absorption ( ³ H ₄
→ ³ F ₂ or ³ F ₄ → ¹ G ₄₎ cause, by a transition of 2 ^steps, 3 _F 4 ^- to form a population inversion between the 3 _{H 4} level. The reason why this technique is effective is that the ground level absorption spectrum and the excited level absorption spectrum of Tm ions overlap at a wavelength of about 1.05 μm.
It can be excited by a single excitation light of m.

[0007]

However, in the above-described Tm-doped fiber amplifier, it is difficult to achieve 1.05 μm band pumping with a semiconductor laser. The reason is that the oscillation of the laser beam in the 1.05 μm band in the semiconductor laser has been reported at the research paper level, but an apparatus capable of realizing an output power level for practical use, for example, a transverse single mode output of about 500 mW, This is because both research level and commercial products do not yet exist. For example, see "Applied Physics Le
tters), Vol. 69, p. 248, 1996 ", at present, the output of a semiconductor laser having a wavelength of 1.06 μm is only about 200 mW at the research paper level.

For this reason, in a known example of a Tm-doped fiber amplifier, as a pumping light source in the 1.05 μm band, for example, Ld such as Nd: YAG, Nd: YLF, Yb: YAG, etc.
D-pumped solid-state lasers and LD-pumped fiber lasers such as Yb-doped fiber lasers have been used.

On the other hand, the pumping light source other than 1.05μm band, for ^example, 3 _{F 4} level directly excited to 0.79μm band (wavelength 0.77～0.80Myuemu, in FIG. 12 transitions b) and ³
0.67μm band to excite the F ₂ level (wavelength 0.64～
The excitation of 0.68 μm, transition c) in FIG. 12, etc. can be LD-excited.
Letters (Electronics Letters) Vol. 25, 166
0 page, as shown in 1989 ", increases the ion number density of laser low level ^{(3 H} ₄₎ is not able to maintain a population inversion in the steady state, a high efficiency operation is impossible. This is because the Tm ion has a laser lower level lifetime of about 10
a msec, laser high level life ⁽³ _{F 4} life = 1.
This is because it is longer than 3 msec). Such a system is called a self-terminating system. Such a self-terminating system includes Er (erbium) and H in addition to Tm in rare earth elements.
o (holmium) and so on.

In a laser amplifier and a laser oscillator using a rare earth element forming a self-terminating system, they play a role of exciting ions from the ground level to the lower level of the laser or higher for high efficiency operation. Excitation light and excitation light that excite ions from the lower laser level to the upper laser level and form a population inversion are essentially required.
As described above, the excitation light in the 1.05 μm band at Tm is:
Although these two roles are realized simultaneously, excitation by LD is impossible.

As shown in the above example, when a medium to which ions forming a self-terminating system are added is used as a gain medium, it is difficult to excite an LD because the excitation wavelength is limited.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and is directed to a laser amplifier, a laser amplification method, a laser amplification apparatus, and a laser oscillator to which a rare earth element which forms a self-terminating system transition is added. It is an object of the present invention to provide a laser amplifier, a laser amplification method, a laser amplification device, and a laser oscillator that can realize high efficiency, small size, long life, and high stability operation at the same time.

[0013]

According to the laser amplifier of the present invention, a medium to which a rare earth element is added is used as a gain medium, and two of the energy levels of the rare earth ions in the medium whose energy is higher than the ground level. A laser amplifier that uses a stimulated emission transition between energy levels to form a self-terminating system transition in which the stimulated emission transition has a longer laser lower level lifetime than the laser upper level lifetime of the two energy levels. A first excitation light source for exciting ions from a ground level to the laser lower level or an energy level higher than the laser, and a wavelength different from the wavelength of the first excitation light; A second excitation light source for exciting ions from a lower level to an upper level of the laser, and at least one of the first excitation light source and the second excitation light source is configured by a semiconductor laser. Characteristic To.

In the laser amplifier of the present invention, fluorozirconate glass can be used as a medium to which the rare earth element is added.

Further, in the laser amplifier of the present invention,
As an example, the rare earth ion is thulium (Tm ³⁺ )
Having a wavelength of 1.53 to 1.90 μm and a wavelength of 0.1 μm.
77 ~ 0.80μm, wavelength 0.64 ~ 0.68μ
m, a light source of the first pumping light having any one of the three wavelength ranges, and a wavelength of 1.35 to 1.46.
μm of a second excitation light source. Further, in the laser amplifier of the present invention, the medium is preferably in the form of an optical fiber.

A laser amplification method according to the present invention is characterized in that a plurality of laser amplifiers including the above-described laser amplifier are arranged in series or in parallel to broaden the gain.

A laser amplifier according to the present invention is characterized in that a plurality of laser amplifiers including the aforementioned laser amplifier are arranged in series or in parallel.

In the laser oscillator according to the present invention, a medium to which a rare earth element is added is used as a gain medium, and an induction between two energy levels having higher energy than the ground level among the energy levels of the rare earth ions in the medium. A laser oscillator that uses an emission transition to form a self-terminated transition in which the stimulated emission transition has a longer laser lower level lifetime than the laser upper level lifetime of the two energy levels, and the base level shifts from the ground level. A first excitation light source for exciting ions to a laser lower level or an energy level higher than the laser, and having a wavelength different from the wavelength of the first excitation light; A second excitation light source for exciting ions to an upper level, wherein at least one of the first excitation light source and the second excitation light source is constituted by a semiconductor laser.

In the laser oscillator of the present invention, fluorozirconate glass can be used as a medium to which the rare earth element is added.

Further, in the laser oscillator of the present invention,
As an example, the rare earth ion is thulium (Tm ³⁺ )
Having a wavelength of 1.53 to 1.90 μm and a wavelength of 0.1 μm.
77 ~ 0.80μm, wavelength 0.64 ~ 0.68μ
m, a light source of the first pumping light having any one of the three wavelength ranges, and a wavelength of 1.35 to 1.46.
μm of a second excitation light source.

In the present invention, a semiconductor laser light source having two appropriately selected wavelengths is used as an excitation light source in a laser amplifier and a laser oscillator to which a rare earth element forming a self-terminating transition is added.

First, the function of the two excitation lights in the present invention will be described. The first excitation light excites ions from the ground level to an energy level below or above the laser level. The first excitation light has a role to efficiently excite ions to an energy level group involved in stimulated emission transition, that is, an upper laser level and a lower laser level. Here, the first excitation light does not necessarily need to excite ions to an energy level higher than the upper laser level. When only the first pumping light is irradiated, since the energy level is a self-terminating system, the ion number density at the lower level of the laser increases, and a steady population inversion is not formed.

Next, ions are excited from the lower laser level to the upper laser level by the second excitation light. Thereby, a population inversion is formed between the desired energy levels, and the laser amplification operation in the stimulated emission transition is achieved.

As the first pumping light, a wavelength matching the ground level absorption transition to the lower level of the laser or an energy level higher than the laser may be selected, and the number of options is significantly increased as compared with the case of single wavelength pumping. Excitation by a semiconductor laser is also possible. As the second pumping light, a wavelength corresponding to an energy gap between the upper laser level and the lower laser level may be selected. This can be achieved by selecting a light source having a wavelength slightly shorter than the focused stimulated emission transition wavelength (about 0.02 to 0.10 μm). If the stimulated emission transition can be realized by a semiconductor laser,
The second pump light can also be realized by a semiconductor laser.

In the pump configuration of the present invention, when the wavelength of the first pump light is set to correspond to the energy gap between the ground level and the laser lower level, the energy conversion efficiency, ie, the slope, Efficiency is maximized. The reason is that the energy loss due to the non-radiative transition is usually the main cause of lowering the energy conversion efficiency. However, when the wavelength of the pump light is set as described above, the energy loss due to the non-radiative transition is extremely small. It is because it becomes less. A simple estimate ignoring the width of each energy level shows that for a 1.05 μm pump in a Tm-doped fiber amplifier, the theoretical maximum ηs of the slope efficiency of the fiber amplifier is ηs =
1.05 / 1.46 / 2 = 36%. But the first
When the wavelength of the excitation light is 1.56 μm and the wavelength of the second excitation light is 1.46 μm, the slope efficiency reaches 50%. This is more pronounced in the case of a laser oscillator, and the theoretical slope efficiency at an excitation power of about 5 times the threshold is 1.0
In the case of 5 μm excitation, ηs = 73%, but 1.42 μm.
In the case of m excitation + 1.56 μm excitation, ηs = 97%. Therefore, in the present invention, it is also possible to provide high operation efficiency of the laser device.

Next, the gain medium will be described. Here, the present invention will be described by taking Tm as an example of the rare earth element added to the gain medium. However, the rare earth element that can be used in the present invention is a laser upper level having two energy levels higher than the ground level. In addition, the stimulated emission transition between these two levels can be used as the lower level of the laser. The two levels form a self-terminating transition, and the existing semiconductor laser wavelength is considered in consideration of the energy level of ions. It is only required that at least one of the first pumping light and the second pumping light matching the above can be selected, and is not limited to Tm.

The medium to which the rare earth element is added may be any medium that is used as a medium for ordinary solid-state lasers or fiber lasers, but is generally vitreous. For example,
Examples thereof include quartz, phosphate glass, borate glass, germanium glass, tellurite glass, and fluorozirconate glass. Among them, fluorozirconate glass has the lowest phonon energy,
Even a transition with a narrow energy difference does not cause non-radiative transition, and energy can be extracted as light by radiative transition, which is preferable. Further, when the medium is in the form of a fiber, a gain can be obtained depending on the length. Therefore, the medium is preferably in the form of a fiber.

As described above, the laser amplifier according to the present invention has two light sources that oscillate the first and second pump lights, so that the semiconductor laser can be pumped.
Therefore, a light source such as a solid-state laser or fiber laser,
It is possible to eliminate problems caused by using the light source as an excitation light source and to realize high efficiency. The present invention can be configured as a laser oscillator by adding a normal resonator structure in addition to the laser amplifier. Furthermore,
By connecting a plurality of the laser amplifiers in series or in parallel, it is possible to configure a laser amplifying device having a widened gain.

[0029]

Embodiments of the present invention will be specifically described below with reference to the accompanying drawings. First, a first embodiment of the present invention will be described. In this embodiment, a fluoride fiber doped with thulium (Tm) and amplifying fiber, as emitted light ^{transition, 3} _{F 4} → 3 H ₄ transition (1.4
(7 μm band).

FIG. 1 is an energy level diagram showing a pumping method of the laser amplifier according to the present embodiment. As shown in FIG. 1, the wavelength of the first pumping light is (1) laser lower level ( ³ H ₄ ) pumping wavelength: 1.53 to 1.90 μm (transition a), (2) ³ H ₅ level Excitation wavelength: 1.10-1.25
μm (not shown), (3) ³ F ₄ level excitation wavelength: 0.7
There are options such as 7 to 0.80 μm (transition b) and (4) ³ F ₂ level excitation wavelength: 0.64 to 0.68 μm (transition c). 1),
(3) and (4). The wavelength of the second excitation light is ³ H ₄
→ ³ F ₄ 1.35~1.46μ that match the excitation level absorption
It is sufficient if the wavelength is within the wavelength range of m and can be realized by a semiconductor laser.

For example, in the case of transition a, the wavelength of the first pumping light may be set to have a photon energy corresponding to the energy gap between the ground level and the ³ H ₄ level. This ground level absorption transition has a peak at a wavelength of 1.65 μm, and has a tail in a wavelength range of 1.53 μm to 1.90 μm. In this embodiment, the first excitation light source is a semiconductor laser having a wavelength of 1.56 μm and a maximum output of 100 mW. The reason for this is that it is easy to manufacture a vertical multi-mode semiconductor laser, and passive components such as an optical coupler or an isolator at this wavelength have already been developed and there is no hindrance to introduction. However, within the above wavelength range, essentially the same operation is possible.

The wavelength of the second pump light is lower than the laser lower level.
A wavelength corresponding to the energy gap between ³ H ₄ and the upper laser level ³ F ₄ is selected. This transition is called excitation level absorption, and detailed data of the wavelength has not been publicly known, and thus the present inventors measured it. As a result, the wavelength of 1.4
An excitation level absorption spectrum having a peak near 1 μm and extending in the range of 1.35 to 1.46 μm was obtained. The wavelength of the second pumping light may be selected within this wavelength range. Fabrication of a semiconductor laser in the above wavelength band is easy. In the present embodiment, a vertical multi-mode LD having a wavelength of 1.42 μm near the peak of the excitation level absorption and a maximum output of 100 mW is used as the second excitation light source.

FIG. 2 is a block diagram showing the configuration of the laser amplifier according to this embodiment. The amplification fiber 1 is based on fluorozirconate glass and has a thulium concentration of 2
000 ppm, core diameter 2.0 μm, fiber length 20
m. On the input side of the amplification fiber 1, wavelength multiplex couplers 3 and 7 are arranged. WDM couplers 3 and 7
Are connected to the first excitation light source 2 and the second excitation light source 6, respectively. An isolator 4 is disposed at a signal input port on the input side of the wavelength multiplexing coupler 7, and an isolator 5 is disposed at an output port of the amplification fiber 1. In FIG. 2, arrows shown in the isolators 4 and 5 indicate directions in which light can pass through the isolators 4 and 5.

Next, the operation of the laser amplifier according to this embodiment will be described. The signal light 8 before amplification passes through the isolator 4 of the signal input port, passes through the wavelength multiplexing couplers 7 and 3, and is introduced into the amplification fiber 1. On the other hand, the pumping lights output from the first pumping light source 2 and the second pumping light source 6 are introduced into the amplification fiber 1 via the wavelength multiplexing couplers 3 and 7, respectively. The signal light 8 is amplified in the amplification fiber 1, passes through the isolator 5, and is output as an amplified signal 9. The isolator 4 and the isolator 5 suppress unwanted laser oscillation due to return light.

FIG. 3 shows the ASE in the laser amplifier of FIG.
It is a graph which shows a spectrum. As shown in FIG.
When the first excitation light (peak 15) was not incident and only the second excitation light (peak 13) was incident, no ASE occurred around 1.470 μm (spectrum 10). However, when both the first excitation light and the second excitation light are incident, a spectrum 12 is obtained. The spectrum 12 has a peak 14 at a wavelength of 1.470 μm. Peak 14 extends from 1.450 μm to 1.490 μm,
Therefore, an ASE band similar to the ASE spectrum in the case of 1.05 μm single wavelength excitation (see FIG. 13) was obtained.

The ASE spectrum can be controlled by adjusting the output of the first pumping light and / or the second pumping light. When the output of the first pumping light is 20 mW and the output of the second pumping light is 10 mW, spectrum 11 is obtained. The peak position is 1.485 μm, which is shifted to a longer wavelength side compared to the spectrum 12. The reason can be explained as follows. When the transition intensity due to the first excitation light is stronger than the transition intensity due to the second excitation light, the number of ions in the lower laser level ³ H ₄ increases, and the population inversion ratio in the fiber decreases.
Therefore, the ASE spectrum shifts to the longer wavelength side. This is because if we consider the ³ H ₄ level as a virtual ground level,
In the erbium-doped fiber amplifier, the mechanism is the same as the phenomenon in which the decrease in the population inversion ratio causes a long-wave shift in the gain band (eg, see Japanese Patent Application No. 11-156745).
In FIG. 2, the excitation light is incident from the input side of the device, but it has been confirmed that the effect of the present invention does not depend on the incident direction of the excitation light.

FIG. 4 is a graph showing the measurement results of the signal wavelength dependence of the gain in the laser amplifier of this embodiment.
In this measurement, a wavelength variable semiconductor laser is used as a signal light source, and the wavelength is changed from 1.44 μm to 1.55 μm. The output is -30 dBm. The wavelength of the saturation signal is 1.500 μm, and the output is −10 dBm. As a result, the maximum gain is about 30 at a signal wavelength of 1.470 μm.
dB and a noise figure of about 5 dB are obtained (black circles). At this time, the first pumping light output is 50 mW, and the second pumping light output is 70 mW.
mW. In the case of 1.05 μm pumping, it can be seen that pumping power of 200 mW to 300 mW is required to obtain the same level of gain, but higher efficiency is obtained.
The output of the first pumping light is 100 mW, and the output of the second pumping light is 7
When it is set to 0 mW, a long wavelength shift (open circle) of the gain can be realized in the same manner as the long wavelength shift of the ASE spectrum. In this case, the maximum gain decreases, but this can be compensated for by increasing the length of the fiber.

Next, a second embodiment of the present invention will be described. FIG. 5 is a block diagram showing the configuration of the laser oscillator according to the second embodiment of the present invention. The amplification fiber 1, the first pumping light source 2, and the second pumping light source 6 are the same as those in the first embodiment. In FIG. 5, the same components as those in the first embodiment are denoted by the same reference numerals. . On the input side of the amplification fiber 1, a rear mirror 20 that is non-reflective for the first excitation light wavelength and the second excitation light wavelength and totally reflected for an oscillation wavelength band (1.47 μm) is installed. On the side,
An output mirror 16 that partially reflects light in the oscillation wavelength band is provided. A dichroic mirror 2 that transmits the first excitation light and reflects the second excitation light is provided on the input side of the rear mirror 20.
1, a lens 17 and a first excitation light source 2 are provided on the input side. The rear mirror 20, the dichroic mirror 21, the lens 17, and the first excitation light source 2
They are arranged on the same straight line. The input side of the dichroic mirror 21 is provided with the lens 18 and the second excitation light source 6.
Is also provided. The lens 18 and the second excitation light source 6
The second excitation light output from the second excitation light source is
, Is reflected by the dichroic mirror 21, and is incident on the amplification fiber 1 via the rear mirror 20.

Next, the operation of the laser oscillator according to this embodiment will be described. The first output from the first excitation light source 2
The excitation light is once emitted into free space,
The light enters the amplification fiber 1 via the dichroic mirror 21 and the rear mirror 20. The second pumping light output from the second pumping light source 6 is once radiated into free space, and then enters the amplifying fiber 1 via the lens 18, the dichroic mirror 21, and the rear mirror 20. A laser having a wavelength of 1.47 μm is oscillated and amplified in the amplification fiber 1, and is output from the output mirror 16.

FIG. 6 is a graph showing a laser oscillation spectrum in this embodiment. With the laser oscillator of the present embodiment, when the pumping power is 170 mW, the wavelength is 1.4.
Laser oscillation of 7 μm and maximum output of 50 mW can be obtained. Slope efficiency is 35%, light / light conversion efficiency is 30
%. The output can be further increased by optimizing the fiber length. Further, the wavelength of the first excitation light source is set to 0.79 μm (transition b shown in FIG. 1) or 0.68 μm.
(Transition c shown in FIG. 1) can obtain the same gain characteristics.

Next, a tunable thulium-doped fiber laser oscillator according to a third embodiment of the present invention will be described. FIG. 7 is a block diagram showing the configuration of the wavelength-tunable thulium-doped fiber laser oscillator of the present embodiment. The amplification fiber 1, the first pumping light source 2, and the second pumping light source 6 are the same as those in the first and second embodiments. In FIG. 7, the same components as those in the second embodiment are denoted by the same reference numerals. It is attached. The configuration on the input side of the amplification fiber 1 is the same as in the second embodiment. In the third embodiment, a collimator lens 23 and a diffraction grating 22 are provided on the output end side of the amplification fiber 1.

Next, the operation of the third embodiment will be described. The first excitation light output from the first excitation light source 2 is:
After being once radiated into free space, it is incident on the amplification fiber 1 via the lens 17, the dichroic mirror 21 and the rear mirror 20. The second excitation light output from the second excitation light source 6 is once radiated into free space,
8. The light enters the amplification fiber 1 via the dichroic mirror 21 and the rear mirror 20. In the amplification fiber 1,
A laser having a wavelength of 1.47 μm is oscillated and amplified, collimated by a collimating lens 23, diffracted by a diffraction grating 22, and output as fiber laser oscillation light 19.

FIG. 8 is a graph showing a laser wavelength tuning oscillation spectrum in this embodiment. In this embodiment, laser oscillation of any wavelength in the range of 1.46 μm to 1.48 μm can be realized. 1.47μ wavelength
m, laser oscillation with a maximum output of 10 mW, excitation power 1
It can be obtained at 70 mW.

Next, a fourth embodiment of the present invention will be described. FIG. 9 is a block diagram illustrating the configuration of the laser amplification device according to the present embodiment. The laser amplifying device according to the present embodiment is configured by connecting two laser amplifiers (see FIG. 2) according to the first embodiment in series. As a result, the length of the Tm-doped fiber is increased, and the gain band of the Tm-doped fiber amplifier is increased by 1.49 μm from the normal 1.47 μm band.
It has shifted to the long wave side to the m band.

In the laser amplifying device shown in FIG.
Laser amplifiers 24a and 24b are arranged in series,
It has a step configuration. The laser amplifier 24a is arranged on the input side with respect to the laser amplifier 24b. The amplification fibers 1a and 1b are made of fluorozirconate glass as a base material, have a thulium concentration of 2000 ppm, and have a core diameter of 2.
The optical fiber is 0 μm and has a fiber length of 20 m. In the laser amplifying apparatus of the present embodiment, the total fiber length is 40 m by connecting the amplification fibers 1a and 1b in series.

In the laser amplifier 24a, an amplification fiber 1a is provided, and on the input side of the amplification fiber 1a,
Wavelength multiplexing couplers 3a and 7a are provided. The wavelength multiplex couplers 3a and 7a are connected to the first pump light source 2a and the second pump light source 6a, respectively. Wavelength multiplex coupler 7
Further, an isolator 4 is provided at a signal input port on the input side of “a”, and an isolator 5a is provided at an output port of the amplification fiber 1. A laser amplifier 24b is connected to the output side of the isolator 5a. The first pumping light source 2a and the second pumping light source 6a introduce the first pumping light and the second pumping light from the input side into the amplification fiber 1a, respectively.

In the laser amplifier 24b, the wavelength multiplexing couplers 7b and 3b and the amplifying fiber 1
b, a wavelength division multiplexing coupler 7c and an isolator 5b are provided and connected in series. Wavelength multiplexing couplers 7b and 3
The second pumping light source 6b and the first pumping light source 2b are connected to b, respectively, and the second pumping light source 6c is connected to the wavelength multiplexing coupler 7c. First excitation light source 2b and second excitation light source 6b
The first pump light and the second pump light are introduced into the amplification fiber 1b from the input side, respectively, and the second pump light source 6c is a backward pump light source that introduces the second pump light into the amplification fiber 1b from the output side. It is. First excitation light sources 2a and 2b
Are the same as the first pumping light source 2 in the first embodiment, and the second pumping light sources 6a to 6c are the same as the second pumping light source 6 in the first embodiment. The isolators 4, 5a, and 5b suppress unwanted laser oscillation due to return light. The arrows shown in the isolators 4, 5a, and 5b in FIG.
The direction in which light can pass through the isolators 4, 5a and 5b is shown.

Next, the operation of the laser amplifying device of this embodiment will be described. As shown in FIG. 9, the signal light 8 before amplification is
Is input to the laser amplifier 24a, passes through the isolator 4, passes through the wavelength multiplexing couplers 7a and 3a, and is introduced into the amplification fiber 1a. On the other hand, the second excitation light source 6a
The pumping light output from the first pumping light source 2a is introduced into the amplification fiber 1a via the wavelength multiplexing couplers 7a and 3a, respectively. After being amplified in the amplification fiber 1a, the signal light 8 passes through the isolator 5a, and
b.

The light output from the laser amplifier 24a is
In the laser amplifier 24b, the light passes through the wavelength multiplexing couplers 7b and 3b and is introduced into the amplification fiber 1b. on the other hand,
The pumping light output from the second pumping light source 6b and the first pumping light source 2b is introduced into the amplification fiber 1b via the wavelength multiplexing couplers 7b and 3b, respectively. Further, the second excitation light source 6c
The pumping light output from is supplied to the amplification fiber 1c via the wavelength multiplexing coupler 7c. The aforementioned laser amplifier 2
The light output from 4a is amplified in the amplification fiber 1b, passes through the isolator 5b, and is output as an amplified signal 9.

FIG. 10 is a graph showing the measurement results of the wavelength dependence of the gain and noise figure of the amplified signal 9 in the laser amplifier shown in FIG. In this measurement, a wavelength-division multiplexed signal composed of 16 waves arranged at equal intervals in the wavelength range of 1.476 μm to 1.509 μm as a saturation signal, and a wavelength-variable semiconductor laser light having an output of −20 dBm as a small signal. And input to the laser amplifier. The wavelength multiplexed signal has an output of each wave of -15 dBm / ch,
The total output of all input signals is -3 dBm. At this time, the output of the first excitation light source 2a is 12 mW, the output of the first excitation light source 2b is 1.1 mW, and the output of the second excitation light source 6a is 93 mW.
W, the output of the second excitation light source 6b is 107 mW, and the output of the second excitation light source 6c is 267 mW. The gain and the noise figure are measured by changing the wavelength of the small signal. As a result of the measurement, the maximum value of the gain was obtained when the signal wavelength was 1.490 μm. At this time, the gain was about 26 dB and the noise figure was about 6.
It becomes 5 dB. The wavelength is 1.477 to 1.507.
When in the range of μm, a gain of 20 dB or more can be obtained.

FIG. 11 is a graph showing the dependence of the output and the optical conversion efficiency of the laser amplifying device of this embodiment on the total pump power. The total pump power is the sum of the respective outputs of the first pump light sources 2a and 2b and the second pump light sources 6a, 6b and 6c. The light conversion efficiency is a value obtained by dividing the output of the laser amplifier by the total pump power. At this time, both the output and the optical conversion efficiency of the laser amplifying device monotonically increase with respect to the total pump power, and reach the maximum value when the total pump power is maximum. At this time, the light conversion efficiency reaches a maximum of 29%. This value is 1.
The efficiency is higher than in the case where excitation is performed at 05 μm.

The present invention provides the above-described Tm ion by appropriately selecting the rare-earth ion and the wavelength and intensity of the first excitation light source and the wavelength and intensity of the second excitation light source which can realize a desired stimulated emission transition. It is also effective for other transitions between excited levels and other rare earth ions.

[0053]

As described above, according to the present invention,
A laser amplifier doped with ions forming a self-terminating system,
A semiconductor laser can be excited in a laser amplifier and a laser oscillator, and a laser amplifier, a laser amplifier, and a laser oscillator with high efficiency, small size, long life, and high stability can be obtained. Further, according to the present invention, a laser amplifier whose gain peak wavelength is shifted by a long wavelength and a laser oscillator whose oscillation wavelength is shifted by a long wavelength can be realized. For this reason, by connecting a fiber amplifier that does not shift the gain peak and a shifted fiber amplifier in series or in parallel, a laser amplifier having a wide amplification wavelength band can be realized, and wavelength multiplexing communication capable of coping with large capacity. Can be used for

[Brief description of the drawings]

FIG. 1 is an energy level diagram showing a pumping method of a laser amplifier according to a first embodiment of the present invention.

FIG. 2 is a block diagram illustrating a configuration of a laser amplifier according to the present embodiment.

FIG. 3 is a graph showing an amplified spontaneous emission light (ASE) spectrum in the laser amplifier of the present embodiment.

FIG. 4 is a graph showing measurement results of signal wavelength dependence of gain in the laser amplifier of the present embodiment.

FIG. 5 is a block diagram illustrating a configuration of a laser oscillator according to a second embodiment of the present invention.

FIG. 6 is a graph illustrating an oscillation spectrum of the laser amplifier according to the present embodiment.

FIG. 7 is a block diagram showing a configuration of a tunable thulium fiber laser oscillator according to a third embodiment of the present invention.

FIG. 8 is a graph showing a wavelength tuning curve of the wavelength tunable thulium fiber laser oscillator in the present embodiment.

FIG. 9 is a block diagram illustrating a configuration of a laser amplifier according to a fourth embodiment of the present invention.

FIG. 10 is a graph showing the measurement results of the wavelength dependence of gain and noise figure in the laser amplifying device of the present embodiment.

FIG. 11 is a graph showing the dependence of the output and the optical conversion efficiency of the laser amplification device of the present embodiment on the total pump power.

FIG. 12 is an energy level diagram showing a conventional thulium fiber amplifier pumping method.

FIG. 13 is a graph showing a fluorescence spectrum (ASE spectrum) by a conventional thulium excitation method.

[Explanation of symbols]

1, 1a, 1b; amplification fiber 2, 2a, 2b; first excitation light source 3, 3a, 3b; wavelength multiplexing coupler 4: isolator 5, 5a, 5b; isolator 6, 6a, 6b, 6c; second excitation light source 7 , 7a, 7b, 7c; wavelength multiplexing coupler 8; signal light 9; amplified signal 10;
ASE spectrum when only m) is irradiated (dashed line) 11; ASE spectrum when the first excitation light is strong and the excitation configuration according to the present invention is adopted in thulium (dotted line). 12; ASE spectrum (solid line) when the first pumping light and the second pumping light have the same intensity, and the pumping configuration according to the present invention is adopted in thulium. 13; second pumping light (wavelength 1.42 μm) 14; 1.47 μm band ASE 15; first pumping light (wavelength 1.56 μm) 16; output mirror 17; lens 18; lens 19; fiber laser oscillation light 20; 21; dichroic mirror 22; diffraction grating 23; collimating lenses 24a, 24b; laser amplifier

Claims (10)

[Claims] 1. A medium to which a rare earth element is added is used as a gain medium, and among the energy levels of rare earth ions in the medium,
A self-terminating system using a stimulated emission transition between two energy levels having an energy higher than the ground level, wherein the stimulated emission transition has a lower laser level lifetime than the upper laser level lifetime of the two energy levels. A laser amplifier for forming a transition, wherein a first excitation light source for exciting ions from a ground level to an energy level higher than or lower than the laser, and a wavelength of the first excitation light; Has a different wavelength, and comprises a second excitation light source for exciting ions from the lower laser level to the upper laser level, at least one of the first excitation light source and the second excitation light source A laser amplifier comprising a semiconductor laser. 2. The medium to which the rare earth element is added,
The laser amplifier according to claim 1, wherein a fluorozirconate glass is used. 3. The method according to claim 1, wherein the rare earth ion is thulium (T
m ³⁺ ), and any one of three wavelength ranges of 1.53 to 1.90 μm, 0.77 to 0.80 μm, and 0.64 to 0.68 μm. 3. The laser amplifier according to claim 1, further comprising: a first excitation light source having the first excitation light source; and a second excitation light source having a wavelength of 1.35 to 1.46 μm. 4. The laser amplifier according to claim 1, wherein the medium has an optical fiber shape. 5. A laser amplification method, comprising: arranging a plurality of laser amplifiers including the laser amplifier according to claim 1 in series or in parallel to widen the gain. 6. A laser amplifier comprising a plurality of laser amplifiers including the laser amplifier according to claim 1 arranged in series or in parallel. 7. A medium to which a rare earth element has been added is used as a gain medium, and among the energy levels of rare earth ions in the medium,
A self-terminating system using a stimulated emission transition between two energy levels having an energy higher than the ground level, wherein the stimulated emission transition has a lower laser level lifetime than the upper laser level lifetime of the two energy levels. A laser oscillator for forming a transition, wherein a first excitation light source for exciting ions from a ground level to the laser lower level or an energy level higher than the lower level, and a wavelength of the first excitation light. Has a different wavelength, and comprises a second excitation light source for exciting ions from the lower laser level to the upper laser level, at least one of the first excitation light source and the second excitation light source Is a laser oscillator comprising a semiconductor laser. 8. The medium to which the rare earth element is added,
The laser oscillator according to claim 7, wherein a fluorozirconate glass is used. 9. The method according to claim 1, wherein the rare earth ion is thulium (T
m ³⁺ ), and any one of three wavelength ranges of 1.53 to 1.90 μm, 0.77 to 0.80 μm, and 0.64 to 0.68 μm. The laser oscillator according to claim 7, further comprising: a first excitation light source having the first excitation light source; and a second excitation light source having a wavelength of 1.35 to 1.46 μm. 10. The laser oscillator according to claim 7, wherein the medium has an optical fiber shape.