JPH03293333A - Optical fiber amplifier - Google Patents

Abstract

PURPOSE:To make the inversion distribution in an optical fiber uniform in all areas and to eliminate unnecessary gain saturation and deterioration in a noise index by inserting a rare eatch dope optical fiber into the optical path of a Fabry-Perot type laser or a ring laser. CONSTITUTION:When an optical multiplexer installed between one end face of an optical amplifier 7 and one end face of an optical fiber 1 inputs a signal light which is emitted from a signal light generation light source 3 and propagated and an exciting light emitted from the amplifier 7 to one input terminal and the other terminal, respectively in order to multiplex optically the exciting light for exciting the optical fiber 1 being the emission of a traveling-wave type optical amplifier 7 having the gain in the vicinity of absorption wavelength of the rare earth dope optical fiber 1, and the signal light, an optically multiplexed light is outputted so as to be coupled to the optical fiber 1 from one output terminal. In this case, since the optical multiplexer 4 and an optical fiber 5 are provided in the optical path, the oscillation by other wavelength than exciting wavelength is suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical fiber amplifier with high efficiency and low noise.

[Prior Art] FIG. 3 is Opt. Lett by E. Desurvire et al.

vol, 12. no, 11. This is an example of the configuration of a conventional optical fiber amplifier shown in p.889 (1987).

A conventional optical fiber amplifier will be explained below based on FIG.

In the figure, (1) is an optical fiber whose core is doped with rare earth ions, and ions such as Nd and Er are well known as materials to be doped into the core of the optical fiber (1), but quartz is used here. A case will be explained in which an E doped optical fiber having a gain near 1.5 μm, which is the minimum loss wavelength of the system optical fiber, is used. (2) is a pumping light source that generates pumping light, and the pumping light source (2) As for 5
Gas lasers, solid-state lasers that oscillate at Er absorption band wavelengths such as 14 nm, 805 nm, 980 nm, and 1480 nm;
A semiconductor laser or the like is used. (3) is a signal light generating light source, and the signal light generating light source (3) is an Er-doped optical fiber (1) having a gain of 1530r.
A semiconductor laser that oscillates at v, 1550 ns is used. (4) is a multiplexer for combining the excitation light emitted from the excitation light source (2) and the signal light emitted from the signal light generation light source (3) and inputting the combined signal to the optical fiber (1); (5) is an optical filter that passes only the signal light component of the light propagated through the optical fiber (1) and emitted, and (6) is the optical filter (5).
This is a light-receiving element for photoelectrically converting the light that has passed through the .

Next, the operation of the conventional optical fiber amplifier will be explained.

The excitation light emitted from the excitation light source (2) is transmitted to the signal light generation light source (
3), is combined with the signal light that has propagated through multiplexer a (4), and is input into the optical fiber (1). The Er-doped optical fiber (1) is a three-level amplifier, and the Er ions at the ground level absorb the excitation light and pass through the absorption band to a number Se
transition to the upper laser level with a lifetime of e.

At this time, if the pumping light power is sufficiently large, a population inversion is formed between the base level and the laser upper level, and stimulated emission occurs due to the signal light having a wavelength corresponding to the interlevel energy, and the signal light is amplified.

Here, assuming that the base level is level 1, the laser upper level is level 2, and the excitation light absorption level is level 3, as the excitation light and signal light propagate in the optical fiber (1), the first Equation 2 varies according to Eq.

dip W+A21 ρ°σp-Ip dZ R+2W+A21 (1) dIs R-A21 ρ・σS −I S dZ R+2W+A21 (2) ■s: Signal light intensity (J/cd ・see) Ip: Pumping light intensity (J/c-・see) W: Signal light intensity normalized by saturated light intensity R: Excitation rate (/5ee) A21: Spontaneous emission rate between levels 1.2 (/5ee) Z: Distance in the traveling direction ρ: Inside the optical fiber Er ion density (/crn3)σρ
; Induced absorption cross section (cd) between levels 1.3 σs 2 levels 1
．． The stimulated emission cross section (cd) between
5,1×IO-”s D-1,8X10'8, A21
-71 and the core diameter of the optical fiber (1) is 9 μm, W is the signal light power of 1.2 mW (light intensity of 2.0
kW/cj) is 1.0, and R is 1400 when the pumping light power is 100 mW. Further, the power of the pumping light for R-A21 is about 5 mW.

From this, the pumping light power input to the optical fiber (1) is 5
If it is larger than 0 mW, the right side of node 1 becomes a constant, the pump light intensity attenuates in proportion to the propagation distance Z, and the signal light intensity
Since the fractional expression on the right side of the equation is approximately 1, the propagation distance increases exponentially by 2.

Next, as the propagation distance Z increases and the pumping light power becomes about 10 mW, the fractional expression on the right side of the first equation becomes approximately 1, the pumping light intensity begins to decay exponentially, and the signal light intensity increases the rate of amplification. The pumping light power decreases and begins to attenuate when the pumping light power reaches 5 mW.

FIG. 4 schematically shows changes in pump light power and signal light power with respect to propagation distance Z.

Further, the ratio of the number of ions in level 2 to the total number of ions is approximately 1 at the input end of the optical fiber (1), and attenuates toward the output end in approximately the same manner as the excitation light intensity.

When the pumping light power is constant, the maximum gain as an amplifier is obtained when the fiber length is R-A21 according to the second equation, and at this time, the ratio of the number of ions in level 2 to the total number of ions is 0. It is 5.

A gain of 20 to 25 dB can be obtained with a fiber length of about 15 m using a pumping light power of 50 mW to 100 mW.

The light emitted from the optical fiber (1) includes, in addition to the amplified signal light, residual excitation light and spontaneous emission generated and amplified within the optical fiber (1). Light other than signal light is undesirable because it causes shot noise and beat noise during reception. From this, the light emitted from the optical fiber (1) is passed through an optical filter (5) that transmits only light near the signal light wavelength, thereby removing residual excitation light and a portion of the amplified spontaneous emission light. From this, the white signal component of the received signal photoelectrically converted by the light receiving element (6) is the third
It can be expressed as a formula.

S-R・ (e-G-PS/h-f)2 (3)
G: Optical fiber amplifier gain Ps: Signal light power input to the optical fiber amplifier (W
) f: Frequency of signal light (Hz) e: Electronic charge (C) h Niblank constant (J-8ec) R: Load resistance of light receiving element (Ω) Further, the noise component can be expressed by the fourth equation.

N-28 (G-Ps/h-f+(G 1)・77sp
・Δf+2ψG・(G−1)・η5.・Ps/h
・f + (G-1) ” 77sp2”Δf)
(4) B: Reception bandwidth (Hz) η5. : Spontaneous emission coefficient Δf of optical fiber amplifier: Bandwidth (Hz) of optical filter (5) In the fourth equation, the first term represents the shot noise of the signal light, and the second term represents the shot noise due to the amplified spontaneous emission light. Further, the third term and the fourth term correspond to the beat noise between the signal light and the amplified spontaneous emission light, and the beat noise between the amplified spontaneous emission light, respectively. If the optical fiber amplifier has a sufficient gain, the first and second terms can be ignored compared to the third and fourth terms. Also, if an optical filter (5) with a sufficiently narrow bandwidth is used, the fourth term can be ignored, so the S/N is calculated using the formula 5 (5). Also, when receiving without inserting an optical fiber amplifier, S/
N can be expressed by Equation 6, and from this, the noise figure of the optical fiber amplifier can be expressed as Equation 7.

(S/N)o-Ps/h-f・2・B (6)N
F-(S/N)/(S/N). −2・η5p (7) Furthermore, the spontaneous emission coefficient η5P is approximately the same as that of optical fiber (1).
The number of ions at level 1 in the inner metal body X and the number of ions at level 2 (1
-X) can be expressed as in the eighth equation. (Normalized with the total number of ions as 1.) 77sp-(I X)/
(1-X) X= (1-X)/ (1-2X)
(8) From this, when all E' ions are excited in the optical fiber (1) and are at level 2, that is, when X-0, ηsp takes the minimum value 1 and N
F is 3 dB, which is the theoretical limit of an optical amplifier.

However, in the conventional pumping system of optical fiber amplifiers, as mentioned above, at the input end of the optical fiber (1), almost all the ions are at level 2.
However, this proportion decreases as the fiber propagates, and only half of the fiber is excited at the fiber length where the maximum gain is obtained. Therefore, when viewed as a whole, the ratio of the number of excited ions at level 2 is about 0.8 to 0.9, and the noise figure is 3.
．． It deteriorates by 5 to 4 dB.

[Problems to be Solved by the Invention] Since the conventional optical fiber amplifier is configured as described above, the population inversion within the optical fiber (1) decreases as it moves away from the input end, resulting in gain saturation and Furthermore, there was a problem of deterioration of the noise figure.

Purpose of the Invention The present invention has been made to solve the above problem, and is to create an ideal optical fiber in which the population inversion within the optical fiber (1) is made uniform over the entire area and unnecessary gain saturation and noise figure deterioration are eliminated. The purpose is to provide a fiber amplifier.

[Means for Solving the Problems] An optical fiber amplifier according to a first invention includes a rare earth doped optical fiber, a traveling wave optical amplifier having a gain near the absorption wavelength of the rare earth doped optical fiber, and a traveling wave optical amplifier having a gain near the absorption wavelength of the rare earth doped optical fiber. A combiner installed between one end face of the traveling wave optical amplifier and one end face of the rare earth doped optical fiber in order to combine the signal light with the excitation light that excites the optical fiber, which is output from the amplifier. an optical filter installed on the other end face of the rare earth doped optical fiber that reflects the excitation light and transmits the signal light; and an optical filter installed on the other end face of the traveling wave optical amplifier that reflects the excitation light and transmits the signal light. a Fabry-Perot laser oscillator configured with an output coupler that transmits a portion of the pump light and reflects the remainder; It is characterized by comprising an automatic output controller that adjusts the gain.

Further, the optical fiber amplifier according to a second invention is installed at the input end of the rare earth-doped optical fiber in order to combine the signal light with a pumping light for pumping the rare-earth-doped optical fiber. a first multiplexer installed at the output end of the rare earth doped optical fiber to separate the signal light from the excitation light for pumping the rare earth doped optical fiber; , an output coupler that splits the pump light output from the second multiplexer, and one of the pump lights output from the output coupler enters and returns the output light to the first multiplexer. a traveling wave optical amplifier having a gain in the vicinity of the absorption wavelength of the rare earth-doped optical fiber forming a ring laser; The present invention is characterized by comprising an automatic output controller that adjusts the gain of the wave type optical amplifier.

[Operation] The optical fiber amplifier in the first invention constitutes a Fabry-Perot laser oscillator that oscillates the optical fiber at the pumping light wavelength, so the amount of current injected into the traveling wave LD amplifier is low and the laser oscillates. Until this happens, the loss in the optical fiber is large, but as the amount of current injected into the traveling wave LD amplifier becomes higher, the gain exceeds the optical fiber propagation loss and once oscillation starts, this oscillation pumping light causes population inversion inside the optical fiber. is formed, the loss decreases, and the intensity of the oscillated excitation light increases. This oscillation excitation light further forms positive feedback that creates population inversion within the optical fiber.

That is, the optical fiber acts as a saturable absorber in this laser. As a result, as the laser begins to oscillate, the loss in the optical fiber decreases, and in a steady state, a uniform and complete population inversion is formed throughout the optical fiber. This makes it possible to solve the problems of gain saturation and increase in noise figure due to the incomplete region of population inversion, and achieve high efficiency and low noise.

Further, the optical fiber amplifier in the second invention has a ring laser that oscillates the optical fiber at a pumping light wavelength, and the amount of current injected into the traveling wave LD amplifier is low and until the ring laser oscillates, the loss of the optical fiber is reduced. is large, but the amount of current injected into the traveling wave LD amplifier becomes high (the gain exceeds the optical fiber propagation loss, and once oscillation starts, this oscillation pumping light forms a population inversion inside the optical fiber. Loss is reduced and the intensity of the oscillated excitation light is increased.

This oscillation excitation light further forms a positive feedback such as population inversion within the optical fiber.

That is, the optical fiber acts as a saturable absorber in this ring laser. As a result, the loss of the optical fiber becomes almost zero along with the ring laser oscillation, and a uniform and complete population inversion is formed throughout the optical fiber.

This makes it possible to solve the problems of gain saturation and increase in noise figure due to the incomplete region of population inversion, and achieve high efficiency and low noise.

[Embodiments of the Invention] Hereinafter, an embodiment of the first and second inventions will be described based on FIG. 1 and FIG. 2.

FIG. 1 is a block diagram showing a first embodiment of the first invention, and FIG. 2 is a block diagram showing the first embodiment of the second invention.

In Figure 1, (4) is a rare earth doped optical fiber (1
) is a traveling wave optical amplifier (7) that has a gain near the absorption wavelength of
), one end face of the traveling wave optical amplifier (7) and the rare earth doped optical fiber (1) are combined in order to combine the signal light with the excitation light that excites the optical fiber (1), which is emitted from the optical fiber (1).
), the multiplexer (4) outputs the signal light generating light source (3) and transmits the signal light having a wavelength of 1.53 μm or 1.55 μm propagated through the transmission line. When a signal light of It is outputted from the output terminal so as to be coupled to the optical fiber (1).

Further, (5) is an optical filter installed on the other end face of the rare earth-doped optical fiber (1) that reflects the excitation light and transmits the signal light, and is an optical filter that reflects the excitation light and transmits the signal light. It transmits light and reflects excitation light.

(8) is an output coupler that is installed on the other end face of the traveling wave optical amplifier (7) and transmits a part of the output light and reflects the rest, and the output coupler (8) includes 10: The excitation light is reflected at a ratio of about 1. increasing the current injected into the traveling wave LD amplifier (7) with respect to the excitation light wavelength;
It operates as a laser when the gain within the Fabry-Perot cavity exceeds the loss within the Fabry-Perot cavity. (9) is an automatic output controller that receives the pumping light that has passed through the output coupler (8) and adjusts the gain of the traveling wave optical amplifier in accordance with the amount of received light; (9) detects the output power of this laser and applies feedback to the injection current of the traveling wave type LD amplifier (7) so that it becomes constant.Here, as the traveling wave type LD amplifier (7), for example, 1.
InGaAs P/In oscillating near 48am
A P-Fabrybe OLD, a strained quantum well Fabry-Perot type D that oscillates in the vicinity of 0.98 μm, and a low-reflection coating applied to both end faces are used. Further, if a resonator length of about 500 μm to 1 mm is selected, a high gain of 30 to 40 dB can be obtained.

The Fabry-Perot resonator, which can oscillate at the pumping light wavelength of the optical fiber (1), includes an output coupler (8), a traveling wave LD amplifier (7), a multiplexer (4), an optical fiber (1), and an optical fiber (1). It consists of a filter (5).

Further, in order to simplify the configuration of the Fabry-Perot resonator, it is preferable to configure an optical filter (5) with a dielectric multilayer curtain on the end face of the optical fiber (1). Furthermore, low-loss silica-based optical fibers, optical waveguides, lens systems, and mirrors are used to connect each element.

Next, the operation and effect of an embodiment according to the first invention will be explained.

For example, the absorption coefficient of optical fiber (1) is 2.5 dB/m
, the fiber length is 10 m1, and the input/output coupling efficiency of the traveling wave LD amplifier (7) is 5 dB, then the traveling wave LD amplifier (
Oscillation occurs when the gain of 7) is about 30 dB. At this time, the gain bandwidth of the traveling wave LD amplifier (7) is as wide as several tens of nanometers, but since the multiplexer (4) and optical filter (5) are in the optical path, oscillation at wavelengths other than the pumping wavelength is suppressed. be done. In a Fabry-Perot laser, the optical fiber (1) acts as a saturable absorber, so oscillation begins and when the excitation rate of Er ions by the oscillation light exceeds the spontaneous emission rate from level 2 to level 1 (> 5 mW), the loss of the optical fiber I (1) begins to decrease, and the loss in the optical fiber I (1) begins to decrease for a time approximately equivalent to the lifetime of spontaneous emission light (~1 Bmse
In c), the oscillation intensity oscillates rapidly. At this time, the increase in oscillation intensity may cause optical damage (COD) in the active layer of the traveling wave LD amplifier (7), so the
The oscillation light intensity is kept constant by an automatic output controller (9) having a response time of about ee. In this way, by inserting the optical fiber (1) into the optical path of the laser that oscillates at the pumping light wavelength, the population inversion within the optical fiber (1) is completely formed, creating an optical fiber amplifier with high efficiency pumping and a low noise figure. can be realized.

Next, an embodiment of the second invention will be described based on FIG.

In the figure, (4a) is the rare earth doped optical fiber (
A first multiplexer (4b) is installed at the input end of the rare earth-doped optical fiber (1) to combine the excitation light for pumping in step 1) and the signal light; This is a second multiplexer installed at the output end of the rare earth doped optical fiber (1) to separate the excitation light for pumping and the signal light. The first multiplexer (4a), the second
As the multiplexer (4b), a commonly used multiplexer/demultiplexer with two outputs and two manual operations can be used. Further, the first multiplexer emits the signal generating light source (3) and propagates through the transmission line at a wavelength of 1.53 as in the embodiment according to the first invention.
μm or 1.55 μm signal light to one input terminal,
In addition, the wavelength emitted from the traveling wave type LD amplifier (7) is 1.4.
When 8 μm or 0.98 μm excitation light is input to the other input terminal, combined light is output from one output terminal to the optical fiber (1) due to the difference in wavelength.

In addition, when the light propagated through the optical fiber (1) is input to one input terminal, the second multiplexer (4b) sends the pumping light to the output coupler (8) and sends the signal light to the light receiving element (6). This is what is output to.

(7) is the rare earth doped optical fiber (1) into which one of the pump lights output from the output coupler (8) is input and the output light is returned to the first multiplexer (4a) to form a ring laser. ) is a traveling wave optical amplifier that has a gain near the absorption wavelength.

The output coupler (8) receives the output light from the second multiplexer (4b), outputs a part of it to the automatic output controller (9), and outputs the rest to the traveling wave LD amplifier (7). It is a light distributor that reflects excitation light at a ratio of about 10:1. Further, an automatic output controller (9) detects the output power of the ring laser and controls the traveling wave type L so that the output power is constant.
This applies feedback to the current injected into the D amplifier (7).

Here, as the traveling wave type LD amplifier (7), for example, the first
An InGaAsP/1nP Fabry-Perot LD that oscillates at around 1.48 μm, 0.0.
Strained quantum well Fabry-Perot LD that oscillates around 98 μm
A type with low reflection coating applied to both end faces is used.

Also, if you choose a resonator length of about 500 μm to 1 mm, 3
A high gain of 0 to 40 dB can be obtained. Note that a low-loss silica-based optical fiber, a behavioral wave path, a lens diameter, or a mirror is used to connect each element.

Next, the operation and effect of an embodiment according to the second invention will be explained.

Therefore, as in the previous embodiment, the absorption coefficient of the optical fiber (1) is 2.5 dB/m, the fiber base is 10 m, and the traveling wave type LD is
If the input/output coupling efficiency of the amplifier (7) is 5 dB, oscillation occurs when the gain of the traveling wave LD amplifier (7) is about 30 dB. At this time, the gain bandwidth of the traveling wave LD amplifier (7) is several tens of nmS wide, but since the first multiplexer (4a) and the second multiplexer (4b) are in the optical path, it is possible to use wavelengths other than the pumping wavelength. oscillation is suppressed. Inside the ring laser, an optical fiber (
1) acts as a saturable absorber, so when oscillation begins and the excitation rate of E' ions by the oscillated light exceeds the natural number departure from level 2 to level 1 (>5 mW), the optical fiber (
The loss in 1) begins to decrease, and the oscillation light intensity rapidly increases in a time approximately equivalent to the lifetime of spontaneous emission light (~13g+5cc).

At this time, there is a risk that optical damage (COD) may occur in the active layer of the traveling wave LD amplifier (7) due to the increase in the intensity of the oscillated light, so the oscillated light is The intensity remains constant. In this way, by inserting the optical fiber (1) into the optical path of the ring laser that oscillates at the excitation light wavelength, the optical fiber (1)
) is completely formed, and an optical fiber amplifier with high pumping efficiency and low noise figure can be realized.

Furthermore, when pumping is performed with a constant light intensity as in the conventional example above, the rate of stimulated emission from level 2 to level 1 changes due to changes in signal light intensity, so the gain of the optical fiber amplifier changes somewhat. However, in the above example, the change in the rate of stimulated emission from level 2 to level 1 is due to the Fabry-Perot laser,
Alternatively, since it appears as a loss change in the ring laser, no change occurs due to the action of keeping the oscillation light intensity constant by the automatic output controller (9).

In each of the above embodiments, an LD amplifier is used as the traveling wave amplifier, but an amplifier using a gas or solid-state laser having a gain at the pumping light wavelength may also be used.

Further, although an Er-doped optical fiber is used as the optical fiber (1), the doping material may be a rare earth ion having a laser oscillation line such as Nd, Ho, or the like.

Furthermore, the laser transition that can be used for amplification is not limited to a three-level system, but may be a four-level system.

[Effects of the Invention] As described above, according to the present invention, a rare earth-doped optical fiber is inserted into the optical path of a Fabry-Perot laser or a ring laser that oscillates at the excitation light wavelength of the rare earth-doped optical fiber. By oscillating and keeping the oscillation light intensity constant, a uniform population inversion can be formed within the optical fiber, resulting in high efficiency, low noise, and less sensitivity to gain fluctuations due to signal light. .

[Brief explanation of drawings]

1 and 2 are configuration diagrams of an optical fiber amplifier according to an embodiment of the first and second inventions, FIG. 3 is a configuration example of a conventional optical fiber amplifier, and FIG. 4 is a configuration diagram of a conventional optical fiber amplifier. The distance from the excitation light incident end face and the excitation light in the configuration example,
FIG. 3 is a schematic diagram showing the relationship between the sizes of signal lights. In the figure, (1) is an optical fiber, (2) an excitation light source, (
3) is a signal light generating light source, (4) is a multiplexer, (4a) is a first multiplexer, (4b) is a second multiplexer, (5) is an optical filter, and (6) is a light receiver. (7) is a traveling wave type LD amplifier, (8) is an output coupler, and (9) is an automatic output controller. In addition, in the figures, the same reference numerals indicate the same or corresponding parts.

Claims (2)

[Claims] (1) An optical fiber amplifier that amplifies signal light propagating in an optical fiber, comprising: a rare earth doped optical fiber; a traveling wave optical amplifier having a gain near the absorption wavelength of the rare earth doped optical fiber; and the traveling wave amplifier. A multiplexer installed between one end face of the traveling wave optical amplifier and one end face of the rare earth doped optical fiber for multiplexing the signal light and the excitation light that excites the optical fiber, which is emitted from the optical fiber. an optical filter installed on the other end surface of the rare earth doped optical fiber that reflects the excitation light and transmits the signal light; and an optical filter installed on the other end surface of the traveling wave optical amplifier that reflects the excitation light and transmits the signal light. a Fabry-Perot laser oscillator configured with an output coupler that transmits a portion of the pump light and reflects the remainder; and a Fabry-Perot laser oscillator that receives the pump light that has passed through the output coupler and adjusts the gain of the traveling wave optical amplifier in accordance with the amount of received light. An optical fiber amplifier characterized in that it is equipped with an automatic output controller that adjusts the output power. (2) In an optical fiber amplifier that amplifies signal light propagating in an optical fiber, the rare earth doped optical fiber and the rare earth doped light are used to combine the signal light with the pumping light that pumps the rare earth doped optical fiber. a first multiplexer installed at the input end of the fiber; and a first multiplexer installed at the output end of the rare earth doped optical fiber for separating the signal light and the excitation light for pumping the rare earth doped optical fiber. 2 multiplexers; an output coupler that splits the pump light output from the second multiplexer; and one of the pump lights output from the output coupler enters and outputs the output light into the first a traveling wave optical amplifier having a gain near the absorption wavelength of the rare earth-doped optical fiber that returns to the multiplexer to form a ring laser; and a traveling wave optical amplifier that receives the other of the pump light output from the output coupler and adjusts the amount of received light. an automatic output controller that adjusts the gain of the traveling wave optical amplifier in accordance with the above.