JPH055912A - Multistage optical amplifying device - Google Patents

Abstract

PURPOSE:To improve the gain and S/N by driving an optical switch arranged on the output side of each optical amplifier synchronously with a signal light pulse to remove natural emission light from the output light of the optical amplifier on the time base synchronously with the signal pulse. CONSTITUTION:When a signal light pulse 35 is made incident on an optical amplifier 13 in the first stage, output light 36 where natual emission light is superposed on the amplified signal light pulse 35 is outputted from the optical amplifier 13 and is made incident on an optical switch 31 in the first stage. A current (or voltage) signal 37 synchronized with the signal light pulse 35 is applied to the optical switch 31 from a driving device 33. The optical switch 31 allows the light to pass through only in the period of existence of the signal in the signal 37 but intercepts the light in the other period. Consequently, signal light 38 where almost all of the natural emission light in the period of the absence of the signal light pulse 35 is removed is outputted from the optical switch 31. With respect to output light of optical amplifiers 14 in second and following stages, the level of natural emission light is reduced by time gating due to optical switches 32 in the same manner.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement of a high gain multistage optical amplifier used for optical communication and the like.

[0002]

2. Description of the Related Art FIG. 2 shows an example of a conventional optical amplifier. In the figure, 1 is an excitation light source, 2 is a multiplexer, 3 is a rare earth-doped optical fiber (hereinafter referred to as RDF), and 4 is. It is a duplexer. In the above configuration, the signal light pulse (frequency ν _i )
5 is excitation light (frequency ν _p ) 6 emitted from the excitation light source 2
Is combined by the multiplexer 2 and is incident on the RDF 3. At this time, the level of the rare earth element, which is an additive to the optical fiber, is excited (photoexcited) by the pumping light 6, and the population inversion of the level of hν _i corresponding to the signal light pulse 5 is thereby generated. The light pulse 5 is amplified by stimulated emission. The amplified signal light pulse is separated from the pumping light 6 by the demultiplexer 4 and output as the signal light pulse 7.

FIG. 3 shows another example of a conventional optical amplifier. In the figure, 8 is a laser diode (hereinafter referred to as LD) optical amplification element, and 9 is a current source. In the above configuration, the signal light pulse 10 is incident on the LD optical amplification element 8, and the LD optical amplification element 8 is supplied with a direct current 1 from the current source 9.
1 is injected (current excitation), and the carrier distribution becomes an inverted distribution by this, and stimulated emission occurs and is amplified and output as a signal light pulse 12.

In general, the optical amplifier shown in FIG. 2 is called a rare earth-doped optical fiber (RDF) optical amplifier, and the optical amplifier shown in FIG. 3 is called a laser diode (LD) optical amplifier. In the output light, in addition to the amplified signal light pulse, spontaneous emission light (Amplif
ied Spontaneous Emission (ASE) exists. Since the population inversion within the RDF or LD optical amplification element is consumed even for the generation and amplification of the spontaneous emission light, the maximum gain thereof is limited, and the amplification factor (gain) per one stage of the optical amplifier is limited to the upper limit. There was a problem that.

Therefore, conventionally, a multistage optical amplifying device has been proposed in which a high gain is obtained by connecting a plurality of optical amplifiers in series.

FIG. 4 shows an example of a conventional multi-stage optical amplifier. In the figure, 13 and 14 are optical amplifiers of the first and second stages, and 15 and 16 are optical amplifiers of the first and second stages. It is a bandpass filter. FIG. 5 shows the spectrum and optical power of the optical signal in each part of the apparatus of FIG. 4, and the operation will be described below according to this.

In the above structure, the first stage optical amplifier 13
When a signal light pulse 17 having a spectrum and optical power as shown in FIG.
7 is amplified and output as described above. At this time, since spontaneous emission light exists in the output light 18 in addition to the signal light pulse amplified as described above, the optical amplifier 13 in the first stage
As shown in FIG. 5B, the output light 18 has a broad spectrum of the spontaneous emission light and the bright line spectrum of the input signal light pulse 17 superimposed on it. The difference G1 in optical power from FIG. 5A shows the gain of the optical amplifier 13 in the first stage.

When the output light 18 of the first-stage optical amplifier 13 including the above-mentioned spontaneous emission light is directly incident on the second-stage and subsequent optical amplifiers, most of the population inversion of the second-stage and subsequent optical amplifiers is almost the same. It is taken away by the amplification of the spontaneous emission light, the amplification factor for the signal light pulse is significantly reduced, and finally the amplification is stopped. This is equivalent to the fact that when the length of the gain medium (RDF, LD optical amplification element, etc.) is increased in the one-stage optical amplifier, the spontaneous emission light is increased and the maximum gain is saturated. It shows that the amplification factor is not improved only by connecting the amplifiers in series. Therefore, in order to improve the amplification factor of the optical amplifier with the multi-stage configuration, it is necessary to remove as much spontaneous emission light as possible between the optical amplifiers.

In the apparatus of FIG. 4, narrow band optical bandpass filters (hereinafter, simply referred to as optical filters) 15 and 16 for transmitting only the spectrum of the signal light pulse are arranged at the output sides of the optical amplifiers 13 and 14, respectively. As a result, the spontaneous emission light was removed. That is, the output light 1 of the optical amplifier 13
By passing through the optical filter 15, 8 has a spectral characteristic in which a part of unnecessary spontaneous emission light is removed as shown in FIG. 5 (c).

Signal light pulse 1 including a part of the spontaneous emission light
When 9 is incident on the optical amplifier 14 of the second stage, the signal light pulse 19 is amplified and output by the amount by which the spontaneous emission light is reduced. At this time, the output light 2 of the second-stage optical amplifier 14
Since the spontaneous emission light is also present in 0 as in the above case, the spectrum of the output light 20 becomes as shown in FIG. 5 (d), but by passing through the optical filter 16, the spectrum of FIG. As shown in, the signal light 21 having a spectral characteristic in which a part of unnecessary spontaneous emission light is removed is output.

Also, when the optical amplifiers are connected in multiple stages, rather than the two stages of the apparatus of FIG. 4, the same operation as described above is performed.

[0012]

However, the above configuration has a problem that the spontaneous emission light cannot be completely removed, so that the amplification factor is saturated. Further, the above-mentioned configuration has the following problems when the repetition cycle of the signal light pulse is sufficiently longer than the pulse width, that is, when the so-called duty ratio is small.

As described above, the gain medium of the optical amplifier is CW.
Since it is driven (excited) by light or a direct current, spontaneous emission light becomes CW light that does not depend on time. When a signal light pulse with a small duty ratio is incident on this optical amplifier and is optically amplified, the output light is a signal light pulse amplified and a spontaneous emission light which is CW light is superimposed.

FIG. 6 (a) shows the input light before amplification, and FIG.
(b) shows the output light after amplification, and it can be seen that there is always an unnecessary spontaneous emission light component 22 in the output light, although its level is low. In this case, even if the pass band of the optical filter is equal to the spectrum of the signal light pulse, it corresponds to the ratio of the spectral width of the spontaneous emission light to the pass band width of the optical filter (for example, about several tenths). Only the level of spontaneous emission light was lowered, and the spontaneous emission light component 22 between the signal light pulses, that is, the spontaneous emission light component 22 between the signal light pulses could not be essentially removed sufficiently. For example, according to Masuda and Takada, "Two-stage amplification characteristics of Er-doped fiber amplifier" (1990 IEICE Spring National Convention Lecture Collection B-980), the improvement is only about 5 dB.

As described above, in the optical amplification of the signal light pulse having a small duty ratio, unnecessary spontaneous emission light always exists on the time axis, and therefore the gain of the optical amplifier is not saturated by the amplification of the signal light pulse, but the spontaneous emission light. However, there is a problem that the spontaneous emission light has a larger average optical power, so that the SN ratio is significantly deteriorated.

In view of the above-mentioned conventional problems, the present invention effectively removes spontaneous emission light from the output light of each optical amplifier,
It is an object of the present invention to provide a multi-stage optical amplifier which can improve the gain and SN ratio for signal light pulses.

[0017]

In order to achieve the above object, the present invention provides a multi-stage optical amplifying device comprising a signal light pulse as an input light and a plurality of optical amplifiers connected in series. A multi-stage optical amplifying device in which an optical switch is arranged on the output side of an optical amplifier, and a driving device for driving the optical switch in synchronization with the signal light pulse is provided, and the signal light pulse is used as input light. In a multi-stage optical amplification device in which a plurality of optical amplifiers are connected in series, an optical switch and an optical bandpass filter are arranged on the output side of each optical amplifier, and the optical switch is driven in synchronization with a signal light pulse. A multi-stage optical amplification device provided with a drive device,
As a third aspect, in a multi-stage optical amplification device in which a signal light pulse is used as input light and a plurality of optical amplifiers are connected in series,
An optical switch is arranged on the output side of each optical amplifier, a driving device for driving the optical switch in synchronization with signal light pulses is provided, and a polarizer is arranged on the input side or output side of each optical amplifier. An optical switch and an optical bandpass filter are disposed on the output side of each optical amplifier in a multi-stage optical amplifying device in which a signal light pulse is used as input light and a plurality of optical amplifiers are connected in series. Then, we propose a multi-stage optical amplifying device in which a driving device for driving the optical switch in synchronization with the signal light pulse is provided and a polarizer is arranged on the input side or the output side of each optical amplifier.

[0018]

According to the first aspect of the present invention, the optical signal amplified and output by the one optical amplifier is incident on the optical switch.
Since the optical switch transmits light only during the period corresponding to the signal light pulse and blocks the light during the other period, the optical signal in which the spontaneous emission light is almost removed in the period without the signal light pulse is the next stage light. It is incident on the amplifier. Further, according to claim 2, an optical signal in which the spontaneous emission light is almost removed in the same period as the signal light pulse is not present and the spontaneous emission light component is removed on the wavelength axis by the optical bandpass filter is obtained. It is incident on the next stage optical amplifier. Further, according to claim 3, in the same manner as described above, almost all the spontaneous emission light in the period without the signal light pulse is removed, and the spontaneous emission light component orthogonal to the polarization direction of the signal light pulse is removed by the polarizer. The optical signal is incident on the optical amplifier at the next stage.
Further, according to claim 4, in the same manner as above, almost all the spontaneous emission light in the period without the signal light pulse is removed, and the spontaneous emission light component is removed on the wavelength axis by the optical bandpass filter, and the polarization The optical signal from which the spontaneous emission light component orthogonal to the polarization direction of the signal light pulse is removed by the child enters the optical amplifier of the next stage.

[0019]

1 shows a first embodiment of a multistage optical amplifier of the present invention. In the figure, the same components as those of the conventional example are designated by the same reference numerals. That is, 13 and 14 are first-stage and second-stage optical amplifiers, 31 and 32 are first-stage and second-stage optical switches, and 33 and 34 are first-stage and second-stage driving devices.

The optical switches 31 and 32 have a function of arbitrarily transmitting (on) or blocking (off) light, and are arranged on the output side of the optical amplifiers 13 and 14, respectively. A laser diode (LD) optical switch or an optical modulator having no polarization dependence can be used as the optical switches 31 and 32.

The LD optical switch is a switch utilizing the population inversion distribution due to current injection like the above-mentioned LD optical amplifier. When current is injected, the carrier distribution becomes population inversion distribution so that light is transmitted, but current is injected. Otherwise it will block the light. Further, as the optical modulator, an optical modulator using an electro-optical effect material such as LiNbO ₃ can be used. This light modulator changes (modulates) its light transmittance according to an applied electric field (voltage), and thereby transmits or blocks light. In general, the modulation characteristics of this optical modulator have polarization dependency, but there are some that operate at the same voltage in both TM and TE modes and have no polarization dependency (reference: RA Steinber).
get al., Appl.Opt., 16,2166, (1977), or Tanizawa et al.,
In 1987, Shinbun University Semiconductor / Material 354), this polarization-independent type can be used in this embodiment. Since this polarization-independent optical modulator can modulate the light transmittance by the electric field regardless of the polarization state of the incident signal light, it is possible to switch spontaneous emission light having all polarization components.

The drive devices 33 and 34 have a function of driving the optical switches 31 and 32 in synchronization with the signal light pulse, respectively, and apply a pulsed current (or voltage) signal synchronized with the signal light pulse to the optical switch 31. And 32.

There are various means for synchronizing the signal light pulse and the current (or voltage) signal, but for example, a part of the signal light pulse input to the optical amplifier is taken out by a demultiplexer or the like to perform light detection. A photoelectric signal is converted into a timing signal by a device or the like, and a current (or voltage) signal is generated based on this to generate a current (or voltage) signal synchronized with the signal light pulse. In this case, the generation timing of the current (or voltage) signal is slightly delayed from the signal light pulse input to the optical amplifier, but the output light of the optical amplifier, that is, the signal light pulse input to the optical switch is also delayed. (In particular, when the RDF optical amplifier is used), the timing can be substantially synchronized.

As the optical amplifiers 13 and 14, either the RDF optical amplifier or the LD optical amplifier described in the conventional example may be used.

FIG. 7 shows a signal waveform in each part of the apparatus, and the operation will be described based on this.

When a signal light pulse 35 as shown in FIG. 7 (a) enters the first stage optical amplifier 13, the optical amplifier 13
From the output, output light 36 in which spontaneous emission light is superimposed on the amplified signal light pulse as shown in FIG. 7B is output and enters the first-stage optical switch 31. Here, the optical switch 3
A current (or voltage) signal 37 synchronized with the signal light pulse as shown in FIG. 7 (c) is injected (or applied) from the drive device 33 to the optical switch 1, and the optical switch 31 outputs a current (or voltage) signal. Light is transmitted only when there is a signal in 37, and light is blocked while there is no signal. Therefore, as shown in FIG. 7 (d), the signal light 3 from which the spontaneous emission light is almost removed during the period without the signal light pulse is output from the first-stage optical switch 31.
8 is output.

At this time, assuming that the pulse width (≧ pulse width of the signal light pulse) of the current (or voltage) signal 37 is t and the repetition period (= repetition period of the signal light pulse) is T, the removal rate of spontaneous emission light is shown. (= Transmittance) R is expressed as R = t / T. That is, when time gating by an optical switch is used, spontaneous emission light can be reduced to t / T times. For example, assuming that the frequency of the signal light pulse 35 is 1 MHz (repetition period T = 1 μsec), the pulse width is ˜50 psec, and the pulse width of the current (or voltage) signal 37 is t = 100 psec, the transmission average power of spontaneous emission light is It will be lowered by 40 dB.

As described above, according to this embodiment, the spontaneous emission light can be made sufficiently smaller than in the case where the conventional optical filter is used, so that the deterioration of the gain in the next-stage optical amplifier due to the spontaneous emission light can be improved. , SN ratio can be improved. It should be noted that the level of spontaneous emission light can be similarly reduced for the output light of the second and subsequent optical amplifiers by time gating by an optical switch.

As described above, the timing signal is created from the signal light pulse, and the current (or voltage) is based on this.
When a signal is generated, a current (or voltage) signal is also generated only when there is a signal light pulse, and extra spontaneous emission light is not output to the next stage, especially when the mark ratio is small. However, a clock source corresponding to the repetition period of the signal light pulse may be built in the driving device to constantly generate a current (or voltage) signal synchronized with the signal light pulse. ..

FIG. 8 shows a second embodiment of the present invention, in which the optical bandpass filter used in the conventional example is added to the first embodiment. That is, in the figure, 15 and 16 are narrow band optical bandpass filters (hereinafter, simply referred to as optical filters) that transmit only the spectrum of the signal light pulse, and are arranged on the output side of the optical switches 31 and 32, respectively. ing.

According to this embodiment, the optical switches 31 and 3 are used.
The effect of removing spontaneous emission light on the time axis by 2 and the effect of removing spontaneous emission light on the wavelength axis by the optical filters 15 and 16 can be used, and the spontaneous emission light can be removed more effectively. The other configurations and operations are the same as those in the first embodiment.

Further, in this embodiment, the optical filters 15 and 1 are used.
Although 6 is arranged on the output side of the optical switches 31 and 32, respectively, it may be arranged on the input side of the optical switches 31 and 32. Further, as the optical amplifiers 13 and 14, either the RDF optical amplifier or the LD optical amplifier described in the conventional example may be used. Further, as the optical switches 31 and 32, either an LD optical switch or an optical modulator may be used.

Actually, an LD optical switch and an optical filter are arranged on the output side of an RDF optical amplifier using an optical fiber doped with Er (erbium) as a rare earth element, and a multi-stage optical amplifying device combining these two stages is used. Signal pulse with a small duty ratio (wavelength 1.53 μm, frequency 2 MH
z, pulse width 10 psec, peak power 1.5 mW), the peak light power was 24.5 mW.
And 90 mW, high peak power lights of 35 W and 105 W were obtained, respectively. This value is 12 dB higher and 6 dB higher than when the spontaneous emission light was not removed at all and when only the removal by the optical filter was performed, and the effectiveness of the action of removing the spontaneous emission light on the time axis is improved. Was shown.

FIG. 9 shows a third embodiment of the present invention, in which a polarizer is added to the first embodiment. That is, in the figure, 41 and 42 are known polarizers,
They are arranged between the optical amplifier 13 and the optical switch 31 and between the optical amplifier 14 and the optical switch 32, respectively.

According to this embodiment, the spontaneous emission light which is orthogonal to the polarization direction of the signal light pulse can be removed, so that the amount of transmission can be reduced to 1/2 (-3 dB) as compared with the first embodiment. it can. The other configurations and operations are the same as those in the first embodiment.

In the present embodiment, the polarizers 41 and 42 are arranged between the optical amplifier 13 and the optical switch 31 and between the optical amplifier 14 and the optical switch 32, respectively. It may be placed in. Further, as the optical amplifiers 13 and 14, either the RDF optical amplifier or the LD optical amplifier described in the conventional example may be used. Also,
As the optical switches 31 and 32, either an LD optical switch or an optical modulator may be used.

Further, as the optical switches 31 and 32, L
When an optical modulator using an electro-optic effect material such as iNbO ₃ is used, it may be of a type whose light transmission characteristics depend on the polarization direction of incident light. That is, the optical switches 31 and 3
When an optical modulator having polarization dependency is used as 2, if the polarization direction of the signal light, the transmission polarization direction of the polarizers 41 and 42, and the modulation polarization direction of the optical switches 31 and 32 are set to the same direction, Since the spontaneous emission light other than the polarization direction modulated by the switches 31 and 32 is removed by the polarizers 41 and 42, the spontaneous emission light is removed on the time axis as in the case of the first embodiment.

FIG. 10 shows a fourth embodiment of the present invention, in which the optical bandpass filter used in the conventional example and the polarizer used in the third embodiment are added to the first embodiment. Show things. That is, in the figure, 15 and 16 are optical filters, 41 and 42 are polarizers, the optical filters 15 and 16 are on the output side of the optical switches 31 and 32, and the polarizers 41 and 42 are on the optical amplifier 13 respectively. And the optical switch 31 and between the optical amplifier 14 and the optical switch 32.

According to this embodiment, the optical filters 15 and 1 are
6 can remove the spontaneous emission light on the wavelength axis, and the polarizers 41 and 42 can remove the spontaneous emission light orthogonal to the polarization direction of the signal light pulse. Therefore, the spontaneous emission light can be removed very effectively. .. The other configurations and operations are the same as those in the first embodiment.

Further, in this embodiment, the optical filters 15 and 1 are used.
Although 6 is arranged on the output side of the optical switches 31 and 32, respectively, it may be arranged on the input side of the optical switches 31 and 32. In the present embodiment, the polarizers 41 and 42 are provided between the optical amplifier 13 and the optical switch 31, respectively, and the optical amplifier 14 is provided.
The optical switch 31 is disposed between the optical switch 31 and the optical switch 32.
And 32 may be arranged on the output side. Also, the optical amplifier 1
3 and 14, RDF optical amplifier described in the conventional example,
Any of the LD optical amplifiers may be used. Further, as the optical switches 31 and 32, either an LD optical switch or an optical modulator may be used.

[0041]

As described above, according to claim 1 of the present invention, an optical switch is arranged on the output side of each optical amplifier, and a driving device for driving the optical switch in synchronization with a signal light pulse is provided. Since it is provided, the spontaneous emission light can be removed from the output light of the optical amplifier on the time axis in synchronization with the signal light pulse, and the level of the spontaneous emission light can be reduced in the entire region of the spectrum. Even in a signal light pulse having a small ratio, the ratio of the signal light component and the spontaneous emission light component can be made large, and therefore, the saturation of the gain due to the spontaneous emission light can be prevented, and the gain and the SN
The ratio can be improved.

According to a second aspect of the present invention, an optical switch and an optical bandpass filter are arranged on the output side of each optical amplifier, and a driving device for driving the optical switch in synchronization with a signal light pulse is provided. Since it is provided, the spontaneous emission light can be removed from the output light of the optical amplifier on the time axis in synchronization with the signal light pulse, and can be removed on the wavelength axis. The ratio can be further improved.

According to claim 3 of the present invention, an optical switch is arranged on the output side of each optical amplifier, and a drive device for driving the optical switch in synchronization with the signal light pulse is provided, and each optical amplifier is provided. Since the polarizer is arranged on the input side or the output side of, the spontaneous emission light can be removed from the output light of the optical amplifier on the time axis in synchronization with the signal light pulse.
Spontaneous emission light orthogonal to the polarization direction of the signal light pulse can be removed, and therefore the gain and the signal-to-noise ratio for the signal light pulse can be further improved.

According to a fourth aspect of the present invention, an optical switch and an optical bandpass filter are arranged on the output side of each optical amplifier, and a drive device for driving the optical switch in synchronization with a signal light pulse is provided. In addition, since a polarizer is placed on the input side or output side of each optical amplifier, spontaneous emission light can be removed from the output light of the optical amplifier on the time axis in synchronization with the signal light pulse, and on the wavelength axis. And the spontaneous emission light orthogonal to the polarization direction of the signal light pulse can be removed. Therefore, the gain and SN ratio for the signal light pulse can be further improved.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing a first embodiment of a multistage optical amplifier of the present invention.

FIG. 2 is a configuration diagram showing an example of a conventional optical amplifier.

FIG. 3 is a configuration diagram showing another example of a conventional optical amplifier.

FIG. 4 is a configuration diagram showing an example of a conventional multistage optical amplifier.

5 is a graph showing the spectrum and optical power of an optical signal in each part of the apparatus of FIG.

6 is a waveform diagram showing input light and output light in the device of FIG.

7 is a signal waveform diagram in each part of the apparatus of FIG.

FIG. 8 is a configuration diagram showing a second embodiment of the multistage optical amplifier of the present invention.

FIG. 9 is a configuration diagram showing a third embodiment of the multistage optical amplifier of the present invention.

FIG. 10 is a configuration diagram showing a fourth embodiment of the multistage optical amplifier of the present invention.

[Explanation of symbols]

13, 14 ... Optical amplifier, 15, 16 ... Optical bandpass filter, 31, 32 ... Optical switch, 33, 34 ... Driving device, 41, 42 ... Polarizer.

Claims (1)

Claim: What is claimed is: 1. In a multi-stage optical amplifying device in which a signal light pulse is used as input light and a plurality of optical amplifiers are connected in series, an optical switch is arranged on the output side of each optical amplifier, and A multistage optical amplifying device comprising a driving device for driving the optical switch in synchronization with a signal light pulse. 2. A multistage optical amplifying device comprising a signal light pulse as input light and a plurality of optical amplifiers connected in series, wherein an optical switch and an optical bandpass filter are arranged on the output side of each optical amplifier, and A multi-stage optical amplification device comprising a drive device for driving an optical switch in synchronization with a signal light pulse. 3. In a multi-stage optical amplifying device in which a plurality of optical amplifiers are connected in series using a signal light pulse as input light, an optical switch is arranged on the output side of each optical amplifier, and the optical switch is used as the signal light pulse. A multistage optical amplifying device, characterized in that a driving device that is driven in synchronization with is provided, and a polarizer is arranged on the input side or the output side of each optical amplifier. 4. A multistage optical amplifying device comprising a signal light pulse as input light and a plurality of optical amplifiers connected in series, wherein an optical switch and an optical bandpass filter are arranged on the output side of each optical amplifier, A multistage optical amplifying device comprising a driving device for driving a switch in synchronization with a signal light pulse, and a polarizer being arranged on an input side or an output side of each optical amplifier.