JPH1093177A - Supersaturated absorber, light amplifier amplifier employing it and optical fiber laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To effectively eliminate the effects of noise and deterioration of extinction ratio, when a light having wavelength in a specified wavelength band is handled by doping a quartz optical fiber with a specified quantity of Er in the core or on the outer circumference thereof. SOLUTION: A supersaturated absorber SA is constituted by doping a quartz optical fiber with Er in the core or on the outer circumference thereof. Dose for Er is set at 1×10<3> ppm or higher, preferably in the range of 10000-50000ppm. When Er is doped at such a dose, the so-called density extinction takes place through micro-aggregation of Er ions, and the fluorescence lifetime thereof is shortened. Consequently, the after glow time of fluorescent component is shortened after a moment of time, when one optical input signal disappears and the coefficient of absorbance is recovered quickly. Since the fluorescence lifetime must surely be shortened in the case of high-speed transmission of 10kb/s or above, the dose of Er is preferably set at 10000ppm or above.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention mainly relates to 1.55
The present invention relates to a saturable absorber exhibiting a saturable absorption characteristic for light in the μm band, an optical amplifier and an optical fiber laser using the same.

[0002]

2. Description of the Related Art Conventionally, it is known that there is a substance exhibiting a so-called saturable absorption characteristic (hereinafter referred to as a saturable absorber) whose absorption coefficient decreases when incident light becomes high in intensity. . That is, this saturable absorber is the one shown in FIG.
As shown in the figure, when the intensity of the input signal is small, it is attenuated because the absorption coefficient is large, and the intensity of the output signal is smaller than that of the input signal. Shows a non-linear characteristic in which the output signal decreases and the output signal rapidly increases from a certain point.

Conventionally, various kinds of organic dyes are used in such a supersaturated absorber in a near infrared wavelength range of about 0.6 to 1.0 μm. For example, a giant pulse in a solid-state laser such as YAG is used. Are used for a Q switch for generating a phase shift, an element for mode synchronization (phase synchronization), and the like.

[0004]

When transmitting pulse-modulated signal light, it is necessary to reduce the occurrence of code errors due to the deterioration of transmission quality. In particular, the higher the transmission speed, the higher the quality deterioration. Therefore, it is important to eliminate such deterioration factors.

[0005] Factors that degrade transmission quality include, for example, various noises, dispersion, and extinction ratio degradation.

In recent years, as a silica-based communication optical fiber, the so-called zero-dispersion wavelength has been shifted to the wavelength range of the lowest transmission loss (that is, the zero-dispersion wavelength has been shifted from the 1.3 μm band to the 1.55 μm band). A dispersion-shifted optical fiber has been provided (for example, see Japanese Patent Publication No. 3-18161), and if such a dispersion-shifted optical fiber is used,
It is possible to reduce the influence of dispersion when transmitting pulse-modulated signal light in the 1.55 μm band.

On the other hand, in order to eliminate the influence of noise, use of the above-mentioned saturable absorber can be considered.

That is, when a pulse-modulated signal light is input to the saturable absorber, the input light intensity is high during the period corresponding to the code "1", so that the absorption coefficient is reduced and the saturable absorber remains unchanged. pass. On the other hand, during the period corresponding to the code “0”, the noise component has a low intensity, and thus can be eliminated by being absorbed and attenuated by the saturable absorber.

However, in the prior art, 1.55 μm
There were no suitable materials such as organic dyes showing supersaturated absorption characteristics in the band.

The present inventors have conducted intensive studies on an optical fiber doped with Er (erbium) inside or outside the core of a silica-based optical fiber, and found that the optical fiber has a wavelength of 1.55 μm band. It has been found that this light exhibits supersaturation absorption characteristics as shown in FIG. 1 (for example, see JP-A-5-100260).

However, further studies have been made on this Er-doped optical fiber, and it has been found that the following problem arises when this optical fiber is simply used as a saturable absorber.

That is, when light having a wavelength in the 1.55 μm band enters the saturable absorber made of an optical fiber doped with Er, the 1.55 μm light excites the optical fiber. Has a lower absorption coefficient for light. And when the doping amount of Er is relatively small
In the case of (less than 1 × 10 ³ ppm), the recovery of the absorption coefficient of 1.55 μm remains low even after the light input stops, so that the transmittance remains high.

As a result, for example, for the pulse-modulated signal light in the 1.55 μm band, the “1” of the next signal light is not changed until the decrease in absorption caused by the input of “1” is completely eliminated. "1" is input, and "1",
The distinction of “0” is not clear and a code error is likely to occur.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and it is an object of the present invention to effectively eliminate the effects of noise and extinction ratio degradation when handling light having a wavelength of 1.55 μm band. And

[0015]

The present invention adopts the following constitution in order to solve the above-mentioned problems.

That is, the saturable absorber according to the first aspect is characterized in that the inside of the core or the outer periphery of the core of the silica-based optical fiber is doped with Er in an amount of 1 × 10 ³ ppm or more.

An optical amplifier according to a second aspect of the present invention includes an amplifying optical fiber for directly amplifying light based on the stimulated emission effect, and the saturable absorber according to the first aspect is connected to the amplifying optical fiber. It is characterized by having been done.

An optical fiber laser according to claim 3 is:
A positive feedback system for laser oscillation including an amplifying optical fiber for directly amplifying light based on the stimulated emission effect, wherein the saturable absorber according to claim 1 is interposed in the positive feedback system. It is characterized by.

The optical fiber according to claim 1 is 1.55 μm
When treating light having a wavelength in the band, the fluorescence lifetime is shortened while exhibiting the saturable absorption characteristic. Therefore, in the optical amplifier according to the second aspect using the saturable absorber, it is possible to effectively eliminate the effects of noise and extinction ratio deterioration of the high-speed pulse-modulated signal light. In the optical fiber laser according to the third aspect using this saturable absorber, mode locking can be achieved, and laser light having a large peak power and a narrow pulse width can be generated.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION A saturable absorber (SA) in this embodiment is constituted by doping Er in a core or an outer periphery of a core of a silica-based optical fiber.

In this example, the doping amount of Er is 1 × 10 ³ pp
m or more, preferably 10,000 ppm or more and 50,000
It is set in the range of less than ppm.

The reason is as follows.

The Er-doped optical fiber exhibits a saturable absorption characteristic as shown in FIG. 1, but when a pulse-modulated 1.55 μm band signal light is input to this optical fiber, it is changed accordingly. Fluorescence based on spontaneous emission inside (A
SE) occurs.

When the doping amount of Er is less than 1 × 10 ³ ppm, the fluorescence lifetime becomes long as shown by the broken line in FIG. Therefore, as shown in FIG. 3A, when a pulse-modulated signal light of 1.55 μm band is input for one pulse (time t ₁ ), the absorption coefficient temporarily decreases. to be excited component since the time that light is no longer (time t ₂₎ remains, delayed recovery of the absorption coefficient as shown by the broken line in FIG. 3 (b) (time t ₂ ~t _4). As a result, as shown in FIG. 4A, the next signal light "1" is generated until the fluorescence component generated due to the input of the signal light "1" completely disappears.
, The so-called extinction ratio deteriorates, and the distinction between “1” and “0” is not clear.

On the other hand, when the doping amount of Er is 1 × 10 ³
In the case of doping at ppm or more, preferably at 1000 ppm or more, so-called concentration quenching occurs due to micro-aggregation of Er ions, and the fluorescence lifetime is shortened as shown by the solid line in FIG. For this reason, the afterglow time of the fluorescent component after the time when one optical signal is no longer input (time t ₂ ) is shortened,
Recovery of the absorption coefficient as shown by the solid line in FIG. 3 (b) earlier (time t ₂ ~t _3). As a result, as shown in FIG. 4B, the transmission of the next signal light “1” starts after the transmission due to the excitation caused by the input of the signal light “1” completely disappears. The extinction ratio does not deteriorate, and the distinction between “1” and “0” becomes clear. In particular, when performing high-speed transmission of 10 Kb / s or more, it is necessary to surely shorten the fluorescence lifetime.
Is preferably not less than 10,000 ppm.
On the other hand, if the doping is 50,000 ppm or more, vitrification is inhibited in the production of a silica-based optical fiber. Therefore, from a practical standpoint, the content is preferably less than 30,000 ppm.

FIG. 5 is a diagram showing a case where an optical amplifier is formed using the above-described saturable absorber SA according to the present invention.

In the figure, AF is an amplification optical fiber for directly amplifying light based on the stimulated emission effect, P is an excitation light source such as a laser diode for pumping the amplification optical fiber AF, and W is an excitation light source from the excitation light source P. An optical coupler for multiplexing the pump light and the signal light and introducing the multiplexed light into the amplification optical fiber AF, I ₁ and I ₂ are polarization-independent isolators for preventing unnecessary end face reflection, and C ₁ and C ₂ . C ₂ is a connector for input and output of the signal light.

As the amplification optical fiber AF, for example, a predetermined amount of rare earth element such as Er, Nd or the like is doped into the core or the outer periphery of the core of a quartz optical fiber (for example, Er).
It is constituted by doping about several hundred ppm by itself.

The saturable absorber SA according to the present invention
It is, in this example, is interposed between the isolator I ₂ of the amplification optical fiber AF and the outgoing side of the.

In this optical amplifier, the pumping light (for example, wavelength: 1.48 μm band) from the pumping light source P enters the amplifying optical fiber AF via the optical coupler W, so that the optical fiber AF is pumped. . In this state, when a high-speed pulse-modulated 1.55 μm band signal light is incident on the amplification optical fiber AF via _one connector C ₁ , isolator I ₁ , and optical coupler W, the signal light is transmitted. It is amplified by the stimulated emission effect.

Then, the amplified signal light is converted to a saturable absorber S
A. When the signal light passes through the amplification optical fiber AF and the saturable absorber SA, fluorescence (ASE) is generated at the same time. However, during a period corresponding to the code “1” of the signal light, the input light intensity is reduced. Since it is large, the absorption coefficient becomes small and it passes here as it is. On the other hand, in the period corresponding to the code “0”, even if fluorescence is generated, such a noise component has a low intensity, and is thus absorbed and attenuated by the saturable absorber SA and removed. Moreover, the saturable absorber SA
Since the fluorescence generated inside has a short lifetime, the extinction ratio does not deteriorate as shown in FIG. 4 (b), and the distinction between “1” and “0” becomes clear. Can be effectively excluded.

The signal light having passed through the saturable absorber SA in this way passes through the isolator I ₂ and is emitted from the connector C ₂ .

The arrangement position of the saturable absorber SA is not limited to the configuration shown in FIG. 5 as long as it is on the output side of the signal from the amplification optical fiber AF.

FIG. 6 is a diagram showing a case where an optical fiber laser is constituted by using the above-mentioned saturable absorber according to the present invention.

In the figure, AF is an amplification optical fiber for directly amplifying light based on the stimulated emission effect, P is an excitation light source such as a laser diode for pumping the amplification optical fiber AF, and W ₁ is an excitation light source P. optical coupler, I is polarization-independent isolator for preventing unwanted edge reflection for the optical coupler, W ₂ is to extract light laser oscillation for introducing into the amplification optical fiber AF excitation light, SA
Is a saturable absorber according to the present invention. Then, a positive feedback system for laser oscillation is configured by sequentially connecting the amplification optical fiber AF, the saturable absorber SA, the isolator I, and the optical couplers W ₂ and W ₁ in a ring shape.

In this optical fiber laser, the amplification optical fiber AF uses the excitation light (wavelength: 1.48 μm) from the excitation light source P.
The fluorescence generated by pumping in the band oscillates in the positive feedback system and oscillates in the laser, and is gradually amplified.In this laser oscillation, oscillation occurs at many frequencies at the same time. Although the phase tends to fluctuate in time, the provision of the saturable absorber SA can synchronize the phases of a number of longitudinal modes simultaneously oscillating (so-called mode synchronization). Large and narrow pulse width laser light (wavelength: 1.
55 μm band).

[0037]

According to the present invention, the following effects can be obtained.

(1) The optical fiber according to claim 1 has the following features:
When light having a wavelength in the 55 μm band is handled, the fluorescence lifetime is shortened while exhibiting the saturable absorption characteristic. Therefore, the effects of noise and extinction ratio degradation can be effectively eliminated.

(2) In the optical amplifier according to the second aspect, since the saturable absorber according to the first aspect is used, it is possible to effectively eliminate the noise of the high-speed pulse-modulated signal light and the deterioration of the extinction ratio. , It is possible to reduce the occurrence of code errors.

(3) In the optical fiber laser according to the third aspect, since the saturable absorber according to the first aspect is used, mode locking can be achieved, the peak power is large, and the laser light has a narrow pulse width. Can be generated.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining a saturable absorption characteristic of a saturable absorber.

FIG. 2 is a characteristic diagram showing a difference in fluorescence lifetime depending on the amount of Er doping.

FIG. 3 is an explanatory diagram showing a temporal change of an absorption coefficient when a pulse-modulated 1.55 μm band signal light is input to a saturable absorber.

FIG. 4 is an explanatory diagram showing the presence or absence of extinction ratio deterioration based on the difference in fluorescence lifetime depending on the amount of Er doping.

FIG. 5 is a diagram in which an optical amplifier is configured using the saturable absorber of the present invention.

FIG. 6 is a diagram in which an optical fiber laser is configured using the saturable absorber of the present invention.

[Explanation of symbols]

SA ... saturable absorber, AF ... amplification optical fiber, P ... pumping light _{source, W, W 1, W 2} ... optical _{coupler, I, I 1, I 2} ... isolator.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Masataka Nakazawa 3-19-2 Nishi-Shinjuku, Shinjuku-ku, Tokyo Inside Japan Telegraph and Telephone Corporation (72) Eiichi Yamada 3-19, Nishi-Shinjuku, Shinjuku-ku, Tokyo No. 2 Nippon Telegraph and Telephone Corporation

Claims (3)

[Claims] 1. A supersaturated absorber characterized in that a silica-based optical fiber is doped with 1 × 10 ³ ppm or more of Er inside or outside the core of a core. 2. An amplifying optical fiber for directly amplifying light based on a stimulated emission effect, wherein the saturable absorber according to claim 1 is connected to the amplifying optical fiber. Optical amplifier. 3. A laser oscillation positive feedback system including an amplification optical fiber for directly amplifying light based on the stimulated emission effect, wherein the saturable absorber according to claim 1 is provided in the positive feedback system. An optical fiber laser characterized by being interposed.