JP2742513B2 - Optical fiber amplifier and optical fiber transmission system using it - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber amplifier and an optical fiber transmission system using the optical fiber amplifier. More specifically, the present invention relates to an optical fiber amplifier and an optical fiber transmission system in which optical fiber amplifiers including fibers doped with rare earth ions are cascaded in multiple stages.

[0002]

2. Description of the Related Art In recent years, with the advent of an erbium-doped optical fiber amplifier capable of directly amplifying light in the 1550 nm band as it is, an analog optical fiber transmission system using the same has been developed.

FIG. 20 shows a configuration of a 1550 nm wavelength analog optical fiber transmission system using a conventional optical fiber amplifier. This system comprises a DFB semiconductor laser 110 having a wavelength of 1550 nm and an erbium-doped optical fiber amplifier 12 pumped by a semiconductor laser having a wavelength of 1480 nm.
0, 1 × 10 optical fiber splitter 130, 1550 nm
Band dispersion-shifted optical fiber 140 and a light receiving element (In
GaAs photodiode or avalanche photodiode) 150 is provided.

[0004] The optical output intensity of the DFB semiconductor laser 110 is modulated according to an analog signal of AM-FDM or FM-FDM. The signal light emitted from the DFB semiconductor laser 110 is amplified by the optical fiber amplifier 120. Thereafter, the signal light is split by the optical fiber splitter 130, transmitted through the 10 km dispersion shift optical fiber 140, and received by the light receiving element 150.

[0005] Erbium-doped optical fiber amplifier 120
Has a high gain in the 1550 nm wavelength band, so that it can be used as a booster amplifier to construct an all-optical distribution system that converts everything from the headend to the user's home.
Video distribution service of over 100 channels and high-definition TV
It is also possible to provide a video distribution service and the like.

[0006]

In an optical fiber transmission system using the above optical fiber amplifier, an AM-FDM
It is known that when performing analog transmission such as FM and FM-FDM, distortion of signal light (particularly, distortion component based on second-order nonlinearity called CSO; Composite Second Order) increases when passing through an optical fiber amplifier. (For example, IEE Photonics Technology Letters
1991, pp. 945-947).

If the intensity of the signal light output from the semiconductor laser is to be analog-modulated, not only the intensity of the signal light but also the frequency is modulated (chirping phenomenon).
The signal light whose frequency has been modulated in this way,
When the gain is amplified by an optical fiber amplifier having wavelength dependency, the frequency fluctuation of the signal light is converted into the intensity fluctuation of the signal light, so that the intensity of the optical signal shifts from a desired level. . This is because the gain (gain) of the optical fiber amplifier has a large wavelength dependence (gain tilt) in the wavelength band (for example, the 1550 nm band) of the signal light. This is caused by shifting from

In particular, when optical fiber amplifiers are cascaded in multiple stages, the distortion in the case of single-stage connection overlaps,
It is thought that the distortion increases more.

The present invention has been made in view of the above circumstances, and an object of the present invention is to increase distortion regardless of the number of stages even when a plurality of optical fiber amplifiers are cascaded. Another object of the present invention is to provide an optical fiber amplifying device and an optical fiber transmission system in which distortion is suppressed to about the same level as the original signal light.

[0010]

An optical fiber amplifier according to the present invention is an optical fiber amplifier in which a plurality of optical fiber amplifiers are connected in cascade, each of the plurality of optical fiber amplifiers being doped with rare earth ions. Optical fiber,
An excitation light source that emits excitation light that excites the optical fiber,
The intensity of the signal light input to each optical fiber amplifier, the value of the distortion component of the signal light output from each optical fiber amplifier, the distortion component of the signal light input to each optical fiber amplifier It is set so as to be equal to the value, and when the intensity of the signal light input to each optical fiber amplifier increases, the distortion component of the signal light output from each optical fiber amplifier increases, Thereby, the above object is achieved.

[0011] The apparatus further comprises a variable light intensity adjuster connected to at least an input side of each of the plurality of optical fiber amplifiers, wherein the variable light intensity adjusters adjust the intensity of the signal light input to each optical fiber amplifier. It may be configured to adjust.

In one embodiment, the signal light is a laser light obtained by modulating a distributed feedback semiconductor laser having an oscillation wavelength of 1550 nm band with an analog signal current, and the optical fiber is provided with the rare earth ion. As erbium. In an embodiment, each of the plurality of optical fiber amplifiers includes a wavelength combiner between the optical fiber and the pumping light source for optically coupling the pumping light to the fiber.

Another optical fiber amplifier of the present invention is an optical fiber amplifier in which a plurality of optical fiber amplifiers are connected in cascade, each of the plurality of optical fiber amplifiers includes an optical fiber doped with rare earth ions. And an excitation light source that emits excitation light that excites the optical fiber,
The optical fiber is formed of a fiber in which a derivative of a wavelength dependence curve of an absorption cross section and a wavelength dependence curve of an emission cross section becomes substantially zero at a predetermined wavelength, and the wavelength of the signal light is the predetermined wavelength. , And the above object is achieved.

An optical fiber transmission system according to the present invention comprises: an optical fiber transmission line including any one of the optical fiber amplifying devices; a signal light emitted in response to an analog signal current; A signal light generating device to be input to, and a light receiving device that converts the signal light into an electric signal, comprising, after propagating the signal light emitted from the signal light generating device in the optical fiber transmission line, The light is received by the light receiving device, whereby the object is achieved.

Still another optical fiber amplifier according to the present invention is an optical fiber amplifier in which a plurality of optical fiber amplifiers are cascaded, each of the plurality of optical fiber amplifiers being an optical fiber doped with rare earth ions. And a pump light source that emits pump light that pumps the optical fiber, and the signal light input to each of the plurality of optical fiber amplifiers has substantially the same power; The wavelength of the signal light is in a wavelength band where the absolute value of the gain tilt with respect to the modulated light of the optical fiber amplifier is 0.15 dB / nm or less, thereby achieving the above object.

Still another optical fiber amplifier according to the present invention is an optical fiber amplifier in which a plurality of optical fiber amplifiers are cascaded, wherein each of the plurality of optical fiber amplifiers is an optical fiber doped with rare earth ions. And an excitation light source that emits excitation light that excites the optical fiber, wherein the signal light input to at least one optical fiber amplifier of the plurality of optical fiber amplifiers is another optical fiber amplifier. And the wavelength of the signal light is such that the absolute value of the gain tilt for the modulated light of the optical fiber amplifier is 0.3.
It is in a wavelength band of less than dB / nm, thereby achieving the above object.

In one embodiment, the optical fibers of the plurality of optical fiber amplifiers have substantially the same length, the same ion concentration, and are pumped by pump light having substantially the same power. You.

At least one of the plurality of optical fiber amplifiers may have a different length or ion concentration from the optical fibers of the other optical fiber amplifiers. The power of the pump light applied to at least one optical fiber of the plurality of optical fiber amplifiers may be different from the power of the pump light applied to the optical fibers of the other optical fiber amplifiers.

In one embodiment, as the signal light, 15
The optical fiber amplifier is provided with a semiconductor laser that emits a laser beam having a wavelength in the 50 nm band.
(Erbium) is added.

A semiconductor laser emitting a laser beam having a wavelength of 1300 nm band as the signal light;
The optical fiber amplifier may be doped with Pr (praseodymium) as a rare earth element.

Still another optical fiber transmission system according to the present invention is a signal semiconductor laser for emitting a signal light intensity-modulated by a multi-channel analog electric signal, and the signal light emitted from the signal semiconductor laser. The optical fiber amplifying device for amplifying the optical signal, an optical fiber for transmitting the signal light amplified by the optical fiber amplifying device, and a light receiving device for converting the signal light transmitted through the optical fiber into an electric signal. And an apparatus for achieving the above object.

[0022]

The present invention focuses on the relationship between the intensity and wavelength of signal light and the change in signal light distortion (CSO) caused by the optical fiber amplifier.
A configuration in which distortion of signal light is not accumulated is adopted.

When the present inventors appropriately select the intensity of the signal light, the signal light distortion (CS
O) did not change. Furthermore, even if the distortion (CSO) of the signal light emitted from the signal semiconductor laser does not change, the optical fiber amplifier itself may have distortion. In this case, the distortion increases when the optical fiber amplifiers are connected in cascade. I figured out what to do.

According to the present invention, the distortion (CSO) of the optical fiber amplifier itself is set to zero by setting the intensity of the signal light input to the optical fiber amplifier to a predetermined level based on the relationship between the intensity of the signal light and the distortion. To As a result, in an optical fiber amplifier in which a plurality of optical fiber amplifiers are cascaded, an increase in distortion due to each optical fiber amplifier is prevented.

CSO is expressed by the product of gain tilt and chirping. In another optical fiber amplifier of the present invention, the gain tilt is substantially reduced by setting the wavelength of the signal light to a wavelength at which the differential coefficient of the optical absorption cross section and the light emission cross section of the optical fiber becomes practically zero. To zero.

The inventor has made clear by calculation that the absolute value of the gain tilt can be reduced to a predetermined level or less by setting the wavelength of the signal light within a predetermined range. In another optical fiber amplifier of the present invention, the wavelength of the signal light is set within a predetermined range based on the above calculation result, and the absolute value of the gain tilt is set to a level required by the standard or less. Note that the relationship between the wavelength of the signal light and the gain tilt differs when the intensity of the signal light input to each optical fiber amplifier is made constant and when at least one intensity is made different from the other intensity. Based on such a difference, the wavelength range of the signal light can be appropriately set in a wider range.

[0027]

【Example】

(Embodiment 1) First, referring to FIG.
The principle of the aspect will be described.

The graph of FIG. 1 shows how the distortion of the signal light amplified by one optical fiber amplifier changes according to the signal light intensity (signal light input intensity). The signal light intensity dependency of the distortion as shown in the graph of FIG. 1 occurs when the gain peak wavelength of the optical fiber amplifier shifts to the longer wavelength side with an increase in the signal light input intensity (theoretical considerations are as follows). For example, IEE Photonics Technology Letters 1993 41
Oya et al. On page 4-416).

The distortion characteristic (signal light input intensity dependency) shown in the graph of FIG. 1 greatly depends on the phase difference between intensity modulation and phase modulation at the time of signal light source modulation, as theoretically predicted. However, we have experimentally confirmed that the phase difference between the intensity modulation and the phase modulation during modulation has a constant phase relationship in a DFB type semiconductor laser used as a signal light source for analog transmission. I have. This is because the DFB semiconductor laser used for analog transmission is designed to have a low κL structure in which the coupling between the diffraction grating and the laser light is weak in order to suppress the effect of hole burning in the axial direction.

At each of the points A, B, C, D and E in the graph of FIG. 1, the distortion (CSO _LD ) of the semiconductor laser,
The distortion caused by the optical fiber amplifier (CSO _Amp ) and the distortion measured after amplification by the optical fiber amplifier (CSO _Amp )
_{LD + Amp} ) is represented by a vector considering the phase and the magnitude as schematically shown on the right side of FIG. Where CSO <sub>LD
+ Amp</sub> is obtained by vectorwise combining CSO _LD and CSO _Amp . The CSO _{L D} and CSO _{LD + Amp,} respectively, can be quantitatively measured directly by converting the optical signal into an electric signal, CSO _Amp is an amount not be measured directly.

At point C in the figure, the distortion CSO _{LD + Amp} appears to be improved at first glance compared to the distortion CSO _LD of the signal light semiconductor laser, but the distortion CSO _Amp generated by the optical fiber amplifier is actually It is not 0. At point B, distortion characteristic C
The magnitude of SO _{LD + Amp} is the value CS of the original semiconductor laser.
Although the distortion generated in the optical fiber amplifier appears to be 0 because it is equal to the size of O _LD, the CSO _Amp is actually C
The distortion CSO _{Amp which} is represented by a vector having a magnitude twice that of the direction opposite to the SO _LD and generated by the optical fiber amplifier is not zero. Point D is a point where CSO _Amp becomes 0. CSO
_Amp is uniquely determined by the wavelength of the signal light irrespective of the magnitude of the distortion amount or chirp amount (degree of frequency modulation) of the semiconductor laser as the signal light source. The remaining point A,
At point E, CSO _{LD + Amp} is always larger than the strained CSO _LD of the original semiconductor laser. Therefore, in the configuration of a plurality of cascade-connected optical fiber amplifiers, CSO _Amp = 0 only in the vicinity of the operating point of D, so that the distortion CSO _LD of the original semiconductor laser does not depend on the number of stages of the optical fiber amplifier. The same distortion characteristics as described above can be obtained. Other operating points A,
Each of B, C, and E depends on the number of stages of the optical fiber amplifier, and generally has a distortion characteristic degraded from the distortion characteristic of the original semiconductor laser.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 2 schematically shows the configuration of an optical fiber amplifier 20 used in the optical fiber amplifier according to the present invention. This optical fiber amplifier 20 is used, for example, in a multi-distribution analog optical transmission system in the 1550 nm wavelength band.

The optical fiber amplifier 20 shown in FIG. 2 comprises an optical fiber (length 50 m) 5 doped with erbium and a wavelength 1
Excitation semiconductor laser 4 that emits 480 nm band excitation light
And 7. 1550nm wavelength signal light 1
Is introduced into the optical fiber 5 via the optical isolator 2 and is amplified while propagating through the optical fiber 5. This amplification is performed when the erbium-doped optical fiber 5 has a wavelength of 1480 n.
This is caused by having a gain for the signal light because it is excited by the pump light of m. The amount of input light is 6 dBm
At this time, an optical output intensity of 18 dBm is obtained.

The first pumping light for pumping the erbium-doped optical fiber 5 is introduced from the pumping semiconductor laser 4 to the input section of the optical fiber 5 via the wavelength coupler (combiner) 2. The second pumping light emitted from the pumping semiconductor laser 7 is introduced into the output section of the optical fiber 5 via the wavelength coupler 6. The two pump lights propagate in the optical fiber 5 in opposite directions. The signal light output from the wavelength coupler 6 is output as output light 9 via the optical isolator 8.

The amount of erbium added to the optical fiber 5 used in this embodiment is 3000 ppm, and the amount of aluminum added is 8000 ppm. The signal light amplified by the erbium-doped optical fiber 5 passes through the wavelength multiplexer 6 and the optical isolator 8, and is then input to an optical fiber amplifier (not shown) located on the right side.

FIG. 3 is a block diagram showing a simple example of a system including the optical fiber amplifier 20 of FIG. Using this system, the optimum input signal light intensity of the optical fiber amplifier 20 was determined. Here, the “optimal input signal light intensity” means that the distortion given to the input light by one optical fiber amplifier 20 becomes substantially zero, and as a result, the optical fiber amplifier 2
It means the intensity of the input light at which the distortion of the signal light does not change before and after zero. That is, the “optimal input signal light intensity” is the same as that in FIG.
Of the input light corresponding to the point D of FIG.

A variable optical attenuator 60-1 for adjusting the light intensity in order to obtain an accurate optimum input signal light intensity.
And 60-2 were inserted before and after the optical fiber amplifier 20. The signal light emitted from the signal light semiconductor laser 10 is analog-modulated by controlling a current for driving the signal light semiconductor laser 10. With the state of the analog modulation kept constant, the intensity of the signal light input to the optical fiber amplifier 20 is changed by the optical attenuator 60-1. At this time, the intensity of the light output from the optical fiber amplifier 20 also changes. The optical attenuator 60- is compensated for this change so that the intensity of the signal light input to the light receiving element 50 becomes constant.
Adjust 2. This is because the measurement conditions are the same because the distortion generated in the light receiving element 50 has the input light intensity dependency.

FIG. 4 is a graph showing the input light intensity dependency of the optimum input signal light intensity. As the wavelength of the signal light shifts to the longer wavelength side, the “optimum input signal light intensity” increases. This is because a gain saturation phenomenon occurs inside the erbium-doped optical fiber with an increase in signal light intensity, and as a result, the gain peak shifts to a longer wavelength side.

When the wavelength of the signal light and the intensity of the signal light satisfy the relationship shown in the graph of FIG.
The value of the distortion before and after the input does not depend on the magnitude of the distortion or the amount of chirping of the semiconductor laser for signal light,
It was the same. More specifically, even when the frequency of the modulation signal of the semiconductor laser for signal light is changed at a frequency of 1 GHz or less, the number of carriers of the modulation signal is changed from two channels to a maximum of four.
When changing to two channels, the optical fiber amplifier 20
The values of the distortion before and after input were the same. From this, it can be seen that the optical fiber amplifier 20 of the present embodiment does not generate distortion during amplification by adjusting the relationship between the signal light intensity and the input signal intensity. The relationship between the optimum input signal light intensity and the signal light wavelength shown in the graph of FIG. 4 is obtained under the conditions of the present embodiment. Needless to say, it changes depending on the composition of the optical fiber and the like.

In this embodiment, the optical fiber has a wavelength of 1
It was excited by 480 nm excitation light,
Excitation light having a wavelength in the 0 nm band may be used. In addition, even if an optical fiber containing a rare earth element other than erbium (neodymium, praseodymium, thulium, ytterbium, dysprodium) and other impurity elements such as aluminum, fluorine and phosphorus is used, an appropriate excitation light wavelength can be obtained. The same effect can be obtained by selecting the signal light wavelength.

(Embodiment 2) Next, another optical fiber amplifier according to the present invention will be described.

First, reference is made to FIG. The illustrated optical fiber amplifier includes five optical fiber amplifiers 20-1 and 20-2.
Devices 0-2, 20-3, 20-4, and 20-5 are cascaded. Optical fiber amplifier (20-1 to 2
0-5) correspond to the optical fiber amplifier 2 of the first embodiment.
It has a configuration similar to that of 0. Between the optical fiber amplifiers (20-1 to 20-5), variable optical attenuators (variable light intensity adjusters) 60-1, 60-2, 60-
3, 60-4, 60-5 and 60-6 have been inserted. The signal light from the signal light semiconductor laser 10 is input to the input section of the optical fiber amplifier. From the output section of the optical fiber amplifier, signal light is output to the light receiving device 50. In this embodiment, the DFB emitting signal light having a wavelength of 1560.3 nm is used as the semiconductor laser 10 for signal light.
Type semiconductor laser is used. The wavelength of the signal light is 156
Since it is 0.3 nm, the optimum input light intensity is 3 dBm as can be seen from the graph of FIG. Therefore, in this embodiment, each optical fiber amplifier (20-1 to 20-5)
Is adjusted to 3 dBm.

FIG. 6 shows how the distortion of the signal light changes each time the light passes through a plurality of optical fiber amplifiers, for each intensity of the signal light input to each optical fiber amplifier. The intensity of the signal light is -10 dBm, -5 dBm, 0
dBm, 3 dBm, and 6 dBm. This corresponds to the operating points A, B, C, D and E in FIG. In the graph of FIG. 6, ● corresponds to the operating point D, ■ corresponds to the operating point C, ◆ corresponds to the operating points B and E, and 、 corresponds to the operating point A. FIG.
As is clear from the graph, at the operating point D (the intensity of the signal light: 3 dBm), the distortion of the signal light does not change even after passing through a plurality of optical fiber amplifiers, and finally only the smallest distortion occurs.

When an experiment for obtaining the graph of FIG. 6 is performed, the analog modulation of the semiconductor laser 10 is always kept constant, and the intensity of the signal light input to the light receiving element 50 is always kept at a constant level. Maintained. This is the semiconductor laser 1
This is to eliminate the influence of the distortion characteristics of the light receiving element 50 and 0. In addition, each optical attenuator (60-1 to 60)
By adjusting -6), the intensity of the signal light input to each of the optical fiber amplifiers (20-1 to 20-5) was made equal.

Operating points A (−10 dBm input), B (−5d
For Bm input) and E (6 dBm input), as described with reference to FIG. 1, the value of the distortion generated in the optical fiber amplifier is not zero. Therefore, as the number of stages of the optical fiber amplifier 20 increased, the distortion of the signal light accumulated and increased. At the operating point C (0 dBm input), after passing through the first-stage optical fiber amplifier 20-1, a value better than the distortion caused by the semiconductor laser 10 was obtained. Was done. Fifth-stage optical fiber amplifier 2
After 0-5, 10 dB higher than the distortion caused by the semiconductor laser
The above values were also bad.

From these results, according to the present invention, in a system in which a plurality of optical fiber amplifiers are cascaded, an optical signal can be amplified and transmitted without increasing distortion.

(Embodiment 3) Still another optical fiber amplifier according to the present invention will be described.

The schematic configuration of the optical fiber amplifier used in this optical fiber amplifier is the same as that described in the first embodiment (FIG. 2). However, in this example, the addition amount of erbium was 3000 ppm, and the addition amount of germanium dioxide was 3
An erbium-doped optical fiber having a length of 000 ppm and a length of 50 m is used.

FIG. 7 shows a wavelength dependence curve of an absorption cross section and a wavelength dependence of an emission cross section of the erbium-doped optical fiber used in this embodiment. FIG.
In the graph, the wavelength dependence curve of the absorption cross section is indicated by a thin solid line, and the wavelength dependence curve of the absorption cross section is indicated by a thick solid line. According to the erbium-doped optical fiber of this embodiment, when the wavelength is about 1550 nm, the derivative of the wavelength dependence curve of the absorption cross section and the derivative of the wavelength dependence curve of the emission cross section both become zero. In this way, the wavelength of the signal light when the derivative of both wavelength-dependent curves is substantially zero,
It will be referred to as “optimal wavelength λopt”. The optical output from one optical fiber amplifier having the optical fiber is:
It was 15 dBm when the input signal light intensity was 3 dBm.

Using the system configuration shown in FIG. 3, the distortion characteristics of the optical fiber amplifier of this embodiment were evaluated. In the evaluation, the DFB semiconductor laser 10 having an oscillation wavelength of 1550 nm was analog-modulated under a constant condition. The intensity of the signal light input to the optical fiber amplifier is adjusted by the optical attenuator 60.
Performed by -1. On the other hand, the intensity of the signal light input to the light receiving element 50 was maintained at a constant level by the optical attenuator 60-2.

The measured distortion is applied to the optical fiber amplifier 20.
Regardless of the intensity of the input signal light, the distortion was the same as that caused by the semiconductor laser at any input signal light intensity.

In the embodiment of the present invention, an optical fiber having a composition of erbium addition of 3000 ppm and germanium dioxide of 3000 ppm was used, but other rare earth elements (neodymium, praseodymium,
An optical fiber containing thulium, yttrium, dysplasia) and other impurity elements such as aluminum, fluorine, and phosphorus may be used. Even with such an optical fiber,
By appropriately selecting the composition ratio, it is possible to obtain an “optimum wavelength λopt” in which the values of the derivative of the wavelength dependence curve of the absorption and emission cross-sections are both 0. By using the signal light having the optimum wavelength λopt, effects similar to those of the present embodiment can be obtained in optical fibers other than the optical fiber of the present embodiment.

(Embodiment 4) Next, an analog optical fiber transmission system according to the present invention will be described. FIG. 8 is a block diagram of the optical fiber transmission system.

This optical fiber transmission system comprises a semiconductor laser 10 and a plurality of cascaded optical fiber amplifiers 2.
0-1, 20-2, and 20-3, 1 × 16 couplers 30-1, 30-2, and 30-3 provided between optical fiber amplifiers, single mode optical fiber 40, and light receiving element 50.

A plurality of optical fiber amplifiers 20-1 and 20-
Each of 2 and 20-3 is an optical fiber (not shown) to which Er ions, one of rare earth ions, are added.
And excitation light (wavelength 1480 nm) for exciting the optical fiber.
A semiconductor laser (not shown) that emits a band.
The signal light is first amplified by the optical fiber amplifier 20-1, then branched by the 1 × 16 coupler 30-1, and further divided by the optical fiber amplifiers 20-2 and 20-3, the 1 × 16 coupler 30-2 and At 30-3, the signal is multi-divided by repeating the amplification branch. The fiber 40 is a 1550 nm dispersion-shifted optical fiber having a length of 10 km.

The semiconductor laser 10 has an oscillation wavelength 156
A 0.3 nm DFB semiconductor laser is used, and its output is set to 3 dBm. The modulation of the semiconductor laser 10 is
The test was performed using a 12-channel NTSC television signal. The modulation degree of the semiconductor laser per channel is 12
%.

The intensity of the signal light input to each of the optical fiber amplifiers 20-1, 20-2, and 20-3 is changed from the amplified output intensity of 18 dBm to the loss due to the branch of the optical fiber splitter and the excess loss ( It is set to be 3 dBm by subtracting a total loss of 15 dB). The intensity (3 dBm) of this signal light matches the “optimal input signal intensity” for the oscillation wavelength of the semiconductor laser 10 (FIG. 4).
reference).

FIG. 9 shows the distortion of the optical signal in each stage of the cascaded optical fiber amplifier. An extremely good distortion (CSO) of -65 dBc was obtained irrespective of the number of stages of the optical fiber amplifier.

(Embodiment 5) Still another optical fiber amplifier according to the present invention will be described.

FIG. 10 schematically shows the optical fiber amplifier. The present optical fiber amplifier has a wavelength of 155.
Used for a multi-distribution analog optical transmission system in the 0 nm band. This optical fiber amplifier comprises a plurality of cascaded optical fiber amplifiers 12, 13, 14, and 15, and a 1 × 8 coupler 16, 17, provided between the optical fiber amplifiers.
18 and 19 are provided. In the optical fiber amplifier shown in FIG. 10, since the optical fiber amplifiers are cascaded in four stages, about 4000 distributions are possible.

A plurality of optical fiber amplifiers 12, 13, 14
And 15 each have the configuration shown in FIG. The signal light is first amplified by the optical fiber amplifier 12, then branched by the 1 × 8 coupler 16, and further divided by the optical fiber amplifiers 13, 14, and 15, and the 1 × 8 coupler 1
At 7, 18 and 19, the multi-partition is performed by repeating the amplification branch. The optical fiber amplifiers 12, 13, 14, and 15 have the same configuration. Also, each fiber amplifier 12,
Power P of the signal light input to 13, 14, or 15
1, P2, P3 or P4 are all substantially the same.

Each optical fiber amplifier of this embodiment uses two pumping light sources (bidirectional pumping configuration) in order to obtain low-noise signal light with high output. The power of the excitation light is 10
0 mW. The length of the Er-doped optical fiber in the optical fiber amplifier is 59 m, and the Er ion concentration is 240 pp.
m. In order to reduce the gain tilt, 8000 ppm of Al is co-doped into the Er-doped optical fiber.

The inventors performed a theoretical analysis on the relationship between the gain tilt characteristics and the secondary distortion characteristics of such an optical fiber amplifier, and determined the wavelength band of signal light in which low distortion characteristics were obtained.

Hereinafter, this will be described.

The gain tilt characteristics of an optical fiber amplifier are obtained by changing the wavelength of continuous light,
What is different for the above high-frequency modulated light is different. The gain tilt for the modulated light is expressed by the following equation.

Gtilt = ∫Γ _s [(dσ _e / dν) N ₂₀ − (dσ _a / dν) N ₁₀ ] dz (1) As the light receiving device, an InGaAs photodiode or an avalanche photodiode is usually used. .

Here, Γ _s is the overlap integral of the signal light and the core of the Er-doped fiber, σ _e and σ _a are the light emission and absorption cross sections for the signal light wavelength, ν is the signal light frequency, N ₂₀ and N
Numeral ₁₀ is the ion density in the steady state of the upper and lower levels of Er ions. At this time, the secondary intermodulation distortion generated in the optical fiber amplifier is given by the product of gain tilt and chirping.

IM2 = Gtilt * Δν (2) If the gain tilt or chirping is small, the second-order distortion is small.

FIG. 11 shows a calculation result of the wavelength dependence of the gain tilt of the optical fiber amplifier of FIG.

It is assumed that the input signal light power to each of the optical fiber amplifiers 12 to 15 is the same, and is 6 dBm and 7 dB.
Calculated for m. Further, FIG.
Modulation degree 11.3% / ch by analog signal of M11ch
Generated in optical fiber amplifier when modulated by
The calculation result of the wavelength dependence of O strain is shown. The number of composites was 3, and the chirping per channel was 1.8 GHz.

From FIGS. 11 and 12, the CSO distortion increases as the absolute value of the gain tilt increases, but the wavelength at which the gain tilt becomes zero (input signal light power 6 dBm
At 1558 nm, and 1561.8 nm at 7 dBm input signal light power), no CSO distortion occurs. In AM video light transmission, a value of −60 dBc is required as a specification value of CSO.

FIG. 12 shows that the wavelength range of the signal light satisfying CSO <−60 dBc is 1556.3-1561.5 nm when the input signal light power is 6 dBm, and the input signal light power is 7
It can be seen that in the case of dBm, it is 1557.5-1563.5 nm. It can be seen from FIG. 11 that these wavelength ranges correspond to the wavelength ranges in which the absolute value of the gain tilt is 0.15 dB / nm or less. In other words, it is clear that a low CSO distortion characteristic of −60 dBc or less can be realized by setting the signal light wavelength to a wavelength range in which the absolute value of the gain tilt is 0.15 dB / nm or less.

(Embodiment 6) Next, another optical fiber transmission system according to the present invention will be described.

The optical fiber amplifier 27 shown in FIG.
0 has the same configuration as the optical fiber amplifier. The signal light emitted from the signal DFB laser 28 is amplified by an optical fiber amplifier, and is input to the light receiving device 30 via the transmission optical fiber 29. As the signal light semiconductor laser 28, a DFB laser that emits laser light having a wavelength of 1562 nm was used. For the semiconductor laser 28,
A bias was applied to a threshold current of +54 mA, and modulation was performed using an analog signal of AM11ch. The degree of modulation was 11.3% / ch. The chirping at this time was almost independent of the modulation frequency and was 1.8 GHz per channel.

The power of the signal light input to each of the optical fiber amplifiers 12-15 of the optical fiber amplifier 27 (from P 1 to P
4) was set to 7 dBm. CS of DFB laser at this time
O distortion and CSO after amplification by each optical fiber amplifier
FIG. 14 shows the distortion characteristics.

The CSO distortion of the DFB laser used in this embodiment is -64 dBc. Even when the DFB laser is amplified by four optical fiber amplifiers, the CSO distortion hardly deteriorates.
Low distortion analog transmission of -60 dBc or less was realized.
This is because, at the wavelength of 1562 nm, the gain tilt of the optical fiber amplifier is close to zero and the CSO distortion is as small as -82 dBc as shown in the calculation results of FIGS.

(Embodiment 7) FIG. 15 shows a configuration diagram of still another optical fiber amplifier according to the present invention. The difference from the optical fiber amplifier shown in FIG. 10 is that the input signal light power P1 to the first-stage optical fiber amplifier 12 is different from the input signal light power P2- to the second-fourth-stage optical fiber amplifier 13-15. P
That is, it is smaller than 4. When the input signal light power to the first-stage optical fiber amplifier is reduced, the bias current of the signal semiconductor laser can be reduced, thereby reducing chirping, and thus reducing the CSO distortion generated by the interaction between gain tilt and chirping. can do.

FIG. 16 shows a calculation result of the wavelength dependence of the gain tilt of the present optical fiber amplifier. The input signal light power to the first-stage optical fiber amplifier was 3 dBm, and the input signal light power to the second-fourth-stage optical fiber amplifier was the same. Calculations were made for the cases of 6 dBm and 7 dBm. FIG. 17 shows a calculation result of the wavelength dependence of the CSO distortion generated in the optical fiber amplifier when the signal light is modulated by the analog signal of AM 11ch at a modulation factor of 11.3% / ch. The number of composites was 3, and chirping per channel was 900 MHz.

According to FIG. 17, the wavelength range in which the CSO specification value of -60 dBc or less for AM video light transmission can be achieved is 1555-1562 nm when the input signal light power is 6 dBm, and 1555.3-1563.2n when the input signal light power is 7 dBm.
m. FIG. 16 shows that these wavelength ranges correspond to the wavelength ranges in which the absolute value of the gain tilt is 0.3 dB / nm or less. That is, in this embodiment, the absolute value of the gain tilt is 0.3 dB
It can be seen that a CSO strain characteristic as low as −60 dBc or less can be realized by setting the wavelength range to / nm or less. Also,
A comparison between FIG. 12 and FIG. 17 shows that the wavelength at which CSO distortion does not occur can be controlled by changing the input signal light power to one of the optical fiber amplifiers connected in multiple stages.

(Eighth Embodiment) Next, still another optical fiber transmission system according to the present invention will be described.

FIG. 18 shows the configuration of this optical fiber transmission system. In FIG. 18, reference numeral 31 denotes the optical fiber amplifier shown in FIG.
m DFB laser for signal, 29 is an optical fiber for transmission,
50 is a light receiving device. The DFB laser 28 was biased at a threshold current of +27 mA to perform modulation with an analog signal of AM11ch. The degree of modulation is 11.3% / ch. The chirping at this time is the sixth because the bias current is small.
It was smaller than the analog optical transmission device of the example of Example 1 and was 900 MHz per channel. The input signal light power P1 to the first-stage optical fiber amplifier 12 is 3 dBm,
The input signal light power P2-P4 to the second-fourth-stage optical fiber amplifier 13-15 was set to 7 dBm. DFB at this time
FIG. 19 shows the characteristics of the CSO distortion of the laser and the CSO distortion after amplification by each optical fiber amplifier. The CSO distortion of the DFB laser used in this example is -65 dBc,
As the signal was amplified by the optical fiber amplifier, the CSO distortion decreased, and analog transmission with a very low distortion of -80 dBc, which is almost the detection limit, was realized. From the calculation result of FIG. 17, the CSO of the optical fiber amplifier at the wavelength of 1562 nm
Although the distortion is -65 dBc, which is the same level as the CSO distortion of the DFB laser, the phases are opposite to each other, so that they cancel each other out to obtain a low distortion characteristic.

In the optical fiber amplifiers and the analog optical transmission systems of Examples 7 and 8, the input signal light power to one of the optical fiber amplifiers connected in multiple stages was changed. The same effect can be obtained by changing the length, Er ion concentration or excitation light power. Shortening the Er ion-doped fiber, lowering the Er ion concentration, or increasing the pumping light power has the effect of reducing the NF of the optical fiber amplifier. The noise characteristics of an optical fiber amplifier in which optical fiber amplifiers are connected in multiple stages are dominated by the NF of the first stage optical fiber amplifier. Low noise analog optical transmission was realized.

In the above-described embodiment, Er is used as the rare earth element and the oscillation wavelength of the semiconductor laser is in the 1550 nm band. However, the same applies to the case where Pr is used as the rare earth element and the oscillation wavelength of the semiconductor laser is in the 1300 nm band. The effect was obtained.

[0084]

According to the optical fiber amplifier of the present invention,
It is possible to suppress the distortion component that occurs when the frequency fluctuation of the optical signal is converted into the intensity fluctuation due to the wavelength dependency of the gain of the optical fiber amplifier, and the distortion characteristics deteriorate even in the configuration of the cascaded optical fiber amplifier. do not do.

Further, according to the optical fiber transmission system of the present invention, an optical fiber amplifier and a multi-stage optical fiber splitter in which signal light from a semiconductor laser analog-modulated with a multi-channel video signal or the like is cascaded in multiple stages. And low-distortion, multi-distribution analog optical transmission through an optical fiber. This transmission method uses optical CATV or FTTH (F
It can be used in multi-distribution systems such as iber to the home) and has great practical significance.

According to another optical fiber amplifier of the present invention, by setting the signal light wavelength to a wavelength band where the gain tilt with respect to the modulated light is small, the second-order distortion generated by the interaction between the gain tilt and the chirping is small. An optical fiber amplifier can be realized, and multi-distribution or long-distance analog optical transmission having low distortion characteristics can be performed.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining the principle of the present invention.

FIG. 2 is a diagram showing a configuration of an optical fiber amplifier used in an optical fiber amplifier according to the present invention.

FIG. 3 is a block diagram illustrating a system including the optical fiber amplifier of FIG. 2;

FIG. 4 is a graph showing the signal light wavelength dependence of the optimum input signal light intensity.

FIG. 5 is a block diagram illustrating a configuration of an optical amplifier according to the present invention.

FIG. 6 is a graph showing the distortion included in each output of a plurality of cascaded optical fiber amplifiers.

FIG. 7 is a graph showing the wavelength dependence of the absorption cross section and the emission cross section of the optical fiber.

FIG. 8 is a diagram showing a configuration of an optical fiber transmission system according to the present invention.

FIG. 9 is a graph showing distortion of the optical fiber transmission system of FIG.

FIG. 10 is a diagram showing a configuration of another optical fiber amplifier according to the present invention.

11 is a graph showing a calculation result of wavelength dependence of gain tilt of the optical fiber amplifier of FIG.

12 is a graph showing a calculation result of wavelength dependence of CSO distortion of the optical fiber amplifier of FIG.

FIG. 13 is a diagram showing a configuration of an analog optical transmission device according to the present invention.

14 is a diagram illustrating CSO distortion of the analog optical transmission device of FIG.

FIG. 15 is a diagram showing another optical fiber amplifier according to the present invention.

16 is a graph showing a calculation result of wavelength dependence of gain tilt of the optical fiber amplifier of FIG.

17 is a graph showing a calculation result of wavelength dependence of CSO distortion of the optical fiber amplifier of FIG.

FIG. 18 is a diagram showing a configuration of another analog optical transmission device according to the present invention.

19 is a diagram illustrating CSO distortion of the analog optical transmission device of FIG.

FIG. 20 is a diagram illustrating a conventional optical fiber amplifier.

[Description of Signs] 1 signal light 2, 8 optical isolator 3, 6 multiplexer 5 optical fiber 4, 7 pumping semiconductor laser 9 output light 10 signal light semiconductor laser 12, 13, 14, 15 optical fiber amplifier 16, 17, 18, 19 1 × 8 coupler 20 Optical fiber amplifier 28 Semiconductor laser for signal light 29 Optical fiber 30-1, 30-2, 30-3 1 × 16 coupler 50 Light receiving element 60-1, 60-2, 60- 3,60-4,60-5
60-6 Variable ATT

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H01S 3/10 H01S 3/07

Claims (15)

(57) [Claims] An optical fiber amplifier in which a plurality of optical fiber amplifiers are connected in cascade, wherein each of the plurality of optical fiber amplifiers includes an optical fiber doped with rare earth ions, and an excitation pump for exciting the optical fiber. And a pump light source that emits light. The intensity of the signal light input to each optical fiber amplifier is determined by the value of the distortion component of the signal light output from each optical fiber amplifier. The distortion component of the signal light output from each optical fiber amplifier increases so as to become equal to the value of the distortion component of the signal light to be input and when the intensity of the signal light input to each optical fiber amplifier increases. as is set, the optical fiber amplifier. 2. The apparatus according to claim 1, further comprising a variable light intensity adjuster connected to at least an input side of each of the plurality of optical fiber amplifiers, wherein the variable light intensity adjuster is configured to control the signal input to each optical fiber amplifier. 2. The optical fiber amplifier according to claim 1, wherein the optical intensity is adjusted. 3. The signal light has an oscillation wavelength of 1550 nm.
The optical fiber according to claim 1, wherein the optical fiber is a laser beam obtained by modulating a band distributed feedback semiconductor laser with an analog signal current, and wherein the optical fiber is doped with erbium as the rare earth ion. 4. Amplifying device. 4. The optical fiber amplifier according to claim 1, wherein each of the plurality of optical fiber amplifiers includes a wavelength combiner between the optical fiber and the pumping light source for optically coupling the pumping light to the fiber. Optical fiber amplifier. 5. An optical fiber amplifier in which a plurality of optical fiber amplifiers are connected in cascade, wherein each of the plurality of optical fiber amplifiers includes an optical fiber doped with rare earth ions, and an excitation pump for exciting the optical fiber. An excitation light source that emits light, wherein the optical fiber is a fiber in which a derivative of a wavelength dependence curve of an absorption cross section and a wavelength dependence curve of an emission cross section becomes substantially zero at a predetermined wavelength. An optical fiber amplifying device , wherein the wavelength of the input signal light is set to the predetermined wavelength. 6. An optical fiber transmission line including the optical fiber amplifying device according to claim 1, and a signal light generator for emitting a signal light in response to an analog signal current and inputting the signal light to the optical fiber transmission line. And a light receiving device for converting the signal light into an electric signal. The light received by the light receiving device after the signal light emitted from the signal light generating device propagates in the optical fiber transmission line. Fiber transmission system. 7. An optical fiber transmission line including the optical fiber amplifying device according to claim 5, and a signal light generator for emitting a signal light in response to an analog signal current and inputting the signal light to the optical fiber transmission line. And a light receiving device for converting the signal light into an electric signal. The light received by the light receiving device after the signal light emitted from the signal light generating device propagates in the optical fiber transmission line. Fiber transmission system. 8. An optical fiber amplifier in which a plurality of optical fiber amplifiers are connected in cascade, wherein each of the plurality of optical fiber amplifiers includes an optical fiber doped with rare earth ions, and an excitation pump for exciting the optical fiber. An excitation light source that emits light,
Wherein the signal light input to each of the plurality of optical fiber amplifiers has substantially the same power, and the wavelength of the signal light is a gain tilt with respect to the modulated light of the optical fiber amplifier. An optical fiber amplifier in a wavelength band in which the absolute value of is less than 0.15 dB / nm. 9. An optical fiber amplifier in which a plurality of optical fiber amplifiers are connected in cascade, each of the plurality of optical fiber amplifiers includes an optical fiber doped with rare earth ions, and an excitation pump for exciting the optical fiber. An excitation light source that emits light,
Wherein the signal light input to at least one optical fiber amplifier of the plurality of optical fiber amplifiers has a power different from the power of the signal light input to the other optical fiber amplifiers. An optical fiber amplifying device wherein the wavelength of the signal light is in a wavelength band in which the absolute value of gain tilt with respect to the modulated light of the optical fiber amplifying device is 0.3 dB / nm or less. 10. The optical fibers of the plurality of optical fiber amplifiers have substantially the same length and the same ion concentration, and are pumped with pump light having substantially the same power. 10. The optical fiber amplifier according to 8 or 9. 11. The light according to claim 8, wherein at least one optical fiber of the plurality of optical fiber amplifiers has a different length or ion concentration from the optical fibers of the other optical fiber amplifiers. Fiber amplifier. 12. The power of pumping light applied to at least one optical fiber of the plurality of optical fiber amplifiers,
The optical fiber amplifier according to claim 8, wherein the power of the pump light is different from the power of the pump light applied to the optical fiber of another optical fiber amplifier. 13. The optical fiber amplifier according to claim 8, further comprising a semiconductor laser that emits a laser beam having a wavelength in a 1550 nm band as the signal light, and wherein the optical fiber amplifier is doped with Er (erbium) as a rare earth element. An optical fiber amplifying device according to any one of the above. 14. The optical fiber amplifier according to claim 8, further comprising a semiconductor laser that emits a laser beam having a wavelength in a 1300 nm band as the signal light, and wherein the optical fiber amplifier is doped with Pr (praseodymium) as a rare earth element. Any one
An optical fiber amplifying device according to the above item. 15. A signal semiconductor laser for emitting a signal light intensity-modulated by a multi-channel analog electric signal, and an amplifier for amplifying the signal light emitted from the signal semiconductor laser. An optical fiber amplifier according to any one of the above, an optical fiber for transmitting the signal light amplified by the optical fiber amplifier, and a light receiver for converting the signal light transmitted through the optical fiber into an electric signal An optical fiber transmission system comprising: a device;