JPH0356944A - Method and device for amplifying light - Google Patents

Abstract

PURPOSE:To recover a gain with a relaxation constant equal to or under a picosecond by inputting signal light which is within the amplification-enabled wavelength band of a light amplifying medium and has different wavelength from the oscillated wavelength of an optical oscillator in the light amplifying medium and obtaining the amplified light as output. CONSTITUTION:A semiconductor laser amplifier 1 having the gain in a constant wavelength band is used as the semiconductor amplifying medium, and an optical resonator constituted of two mirrors, which are (a) terminal 9 obtained by mirror-coating the end of an optical fiber and a wavelength selection mirror which reflects and returns constant light out of the light outputted from the end 17 of the fiber to the end 17 of the fiber by a lens 14 and a diffraction grating 15, is used as an outside optical resonator. Since the optical resonator has the semiconductor laser amplifier 1 functioning as a gain medium in itself, oscillation occurs by increasing an inrush current from a direct current source 30 and enhancing the amplification factor of the semiconductor laser amplifier. Thus, the relaxation time that an original state is restored from an amplification saturated state is accelerated to be equal to or under a picosecond.

Description

[Detailed Description of the Invention] <<Industrial Application Field>> The present invention relates to an optical amplification method and device used in optical communications, etc. [Prior art] Conventionally, as optical amplifiers for optical communication, semiconductor laser amplifiers using semiconductor amplification media, optical fiber Raman amplification using the nonlinear effect of optical fibers, erbium-doped fiber amplification, etc. have been expected. Research and development is actively underway. Generally, in optical amplifiers, a population inversion is created in the amplifier by pumping (corresponding to injection current in semiconductor laser amplifiers and pumping light in optical fiber amplifiers), and this population inversion realizes the optical amplification effect. There is.

In such an optical amplifier, the signal light level is lower than a certain level.
If it becomes larger than the sum level), bombing becomes insufficient and a phenomenon occurs in which the population inversion level decreases. This phenomenon is called the saturation phenomenon of output power. When this output power saturation occurs, the signal light gain decreases. This decrease in gain occurs when the signal light level is higher than the saturation level, but when the signal light is cut off, the gain is restored to the gain when there is no signal light. At this time, in conventional optical amplifiers, the time required for this gain to recover is controlled by the pumping time constant, so for example, it is several milliseconds in an erbium-doped fiber amplifier, and several milliseconds in a semiconductor laser amplifier. Requires nanoseconds and long relaxation times (G. Eisenstein et al.
at. ,” Gain recovery inse
microconductor optical pulse
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Texas1989).

(Problems to be Solved by the Invention) As mentioned above, in an optical amplifier, after the signal light is raised to a saturation level or higher, the signal light level decreases and the relaxation time required for the gain to recover is relatively long. ．． For this reason, in a system for the Gb/s band or above, the amplification factor after a continuous space becomes larger than the amplification factor after a continuous mark, resulting in a drawback that a pattern effect appears in the output light. SUMMARY OF THE INVENTION An object of the present invention is to provide an optical amplification method and apparatus that eliminates the drawback that the relaxation time required for gain recovery is long and recovers gain with a relaxation constant of less than picoseconds. (Means for Solving the Problems) In order to solve the above-mentioned problems, the optical amplification method of the present invention includes an optical amplification medium disposed in an optical resonator having an optical oscillator, which is capable of amplifying the optical amplification medium. The optical amplification method is characterized in that a signal light of a wavelength within a wavelength band and different from the oscillation wavelength of the optical oscillator is input, and the signal light is optically amplified using the optical amplification medium and output. ．． Further, one of the optical amplification devices of the present invention includes a semiconductor laser oscillator including a semiconductor amplification medium and an external optical resonator, and a signal light having a wavelength different from the oscillation wavelength of the semiconductor laser oscillator. and means for removing the signal light wavelength component from the amplified output light of the semiconductor amplification medium, and optically amplifies the signal light using the semiconductor amplification medium. It is. Another optical amplification device of the present invention includes a semiconductor amplification medium and an optical resonator having n resonant frequencies f1, f2, f3, ..., fn, and a semiconductor laser oscillator that oscillates at the resonant frequency r1. and the oscillation frequency f of the semiconductor laser oscillator.
Resonant frequencies f2, f3,... of the optical resonator different from 1
An optical amplification device characterized in that signal light having a frequency of .fn is input to the semiconductor laser oscillator and optically amplified.
(Operation) Generally, an optical amplifier performs bombing on an optical amplification medium to create a population inversion state, and uses this population inversion to amplify and output signal light.Since the speed of optical amplification is controlled by stimulated emission, A high-speed response of less than 2 seconds is possible. However, in order to keep the optical amplification factor constant even when signal light is present, it is necessary to keep the population inversion state at a constant value. It is necessary to perform pumping to the distribution at high speed.In the optical amplification method of the present invention, an optical amplifier is oscillated using an external optical resonator, and a signal light different from the oscillation wavelength of the oscillator is input to perform optical amplification. When no signal light is input to such an optical amplifier, the oscillation optical power is accumulated in the optical amplifier, so the population inversion corresponding to the oscillation optical power occurs within the optical amplifier. The density is maintained and it is in a steady state.When a signal light with a wavelength different from the oscillation wavelength is input into the optical amplifier in this state, the signal light is amplified and output.At this time, the light output from the optical amplifier The output power is the sum of the oscillation light power and the amplified output light power of the signal light.On the other hand, the rate equation (
fg III Koichi, Yajima Tatsuo, 'Quantum Electronics (Part 1)', Shokabo, 1972), the output optical power from this optical amplifier is the same as when there is no signal light to the optical amplifier. It can be easily determined that the power is equal to the oscillation optical power. This shows that the population inversion density in the steady state is the same when there is no signal light and when there is signal light.

In addition, the relaxation speed when transitioning from a steady state with signal light to a steady state without signal light is limited to the bombing speed of population inversion in conventional optical amplifiers because there is no internal oscillation light power, and is several nanoseconds. It was long. However, when the oscillation light power is inside the optical amplifier, it is relaxed by stimulated emission, making it possible to achieve a high-speed response of less than picoseconds. A first optical amplification device of the present invention includes a semiconductor laser oscillator comprising a semi-dedicated amplification medium and an external optical amplifier, and means for inputting a signal light having a wavelength different from the oscillation wavelength of the laser oscillator into the semiconductor amplification medium. It has a means for extracting signal light amplified by a semiconductor amplification medium. In the following explanation, we will explain the case where a semiconductor laser amplifier is used as the semiconductor amplification medium. At this time, if the carrier density N of the semiconductor laser amplifier (corresponding to population inversion), the optical power Sl corresponding to the oscillation wavelength of the laser oscillator, and the amplified signal light power are 82, then
These three parameters follow the following three rate equations. dN at 11 J a N (S 1 + S 2 )
' ( ' ) τ 1 Nireyu aNs1--"-'- d t f
21τ il1”” u aNs2− −”− d j
f3) τ 82 J: Injection current (bumping) τ. : Carrier life (several ns) τs1: Life of laser oscillation wavelength in the resonator (several ps>) τs2: Life of signal light in the semiconductor laser amplifier (<<
ps> a: Gain parameter A conventional semiconductor laser amplifier is not equipped with an external optical resonator, so it operates in a state where 81=0. Therefore, the carrier density N is the carrier lifetime τ. It is alleviated by
Therefore, the relaxation time required for gain recovery after gain saturation occurs is controlled by the carrier lifetime and is on the order of nanoseconds. In the optical amplifier of the present invention, oscillation light S1 exists within the semiconductor laser amplifier. In this case, the carrier relaxation time in equation (1) is determined by stimulated emission a.
N (S1+S2) and N/τ caused by carrier lifetime
．． However, in the oscillation state, the following relationship holds. aN(S1+S2)>>' (
4) τ Therefore, carrier relaxation is mainly dominated by the term due to stimulated emission, and u! The relaxation time is accelerated to less than 1/1000 times compared to the case without it, and a high-speed response of less than a bicosecond can be obtained as a relaxation time. The second optical amplification device of the present invention includes a semiconductor amplification medium and an optical resonator having optical resonance frequencies f1, f2, . . . , fn of na, and oscillates at a frequency f1 as a semiconductor laser. This semiconductor laser emits light fk (of a frequency other than f1) (
Input k=2 3...n). In this case, the rate equation for this semiconductor laser is the lifetime τ of the signal light S2 in equation (1)-<3) used for the explanation in the first invention.
. It is described by replacing τI12 with the lifetime of the signal light inside the resonator. In this case, as in the first invention, formula (1)
Among them, it is the term aN (31+32) resulting from stimulated exclusive emission that determines the relaxation time of carriers. Therefore, the time required for carrier relaxation can be less than bicosecant. In addition, in the present invention, τs2 has a lifetime of several picoseconds, which is about the same as τold. Therefore, the cavity effect of the optical resonator appears and the gain for the signal light becomes the first
It has the characteristic that it is larger than the optical amplification device of 2008, and also has an optical filter effect, which allows it to reduce the level of spontaneous emission light.

(Actual Rejection) Next, the present invention will be explained with reference to the drawings. A typical embodiment of the first optical amplification device of the present invention is shown in FIG. In this example, wavelength 1.
A semiconductor laser amplifier l having a gain in the wavelength band from 5 μm to 1.55 μm is output from the fiber end 17 using an external optical resonator, a terminal 9 whose optical fiber end is mirror-coated, a lens 14 and a diffraction grating 15. 1 of the light
．． An optical resonator is used, which is formed by two wavelength-selecting mirrors that reflect 51 μm light back to the fiber end 17. The wavelength of signal light 8 is 1.54 μm. A wavelength selective fiber coupler 2 is used as a means for inputting data to the semiconductor laser amplifier 1. This wavelength-selective fiber coupler 2 has a wavelength of 1.54 μm input from the boat 4.
This light is combined with the 1.51 μm light input from the boat 5 and output to the boat 7, and has a wavelength selection characteristic in which there is no output to the boat 6. Furthermore, a wavelength selective fiber coupler 3 is used as a means for extracting the signal light wavelength signal from the amplified output light from the semiconductor amplification medium.

This fiber coupler 3 transfers 1.54 μm light of the optical power input to the boat 13 to the boat 22.
This is a fiber coupler with wavelength selection characteristics that outputs μm light to the boat 21. Next, we will explain the operation of the optical amplifier. An injection current is applied to the semiconductor laser amplifier 1 from the DC current source 30, so that the semiconductor laser amplifier 1 has a gain in the 1.5 μm to 1.55 μm band. A terminal 9 whose optical fiber end is mirror coated, a lens 14 and a diffraction grating 15
1.51μ of the light output from the fiber end 17
The optical resonator formed by the two mirrors of the wavelength selection mirror 15 that reflects the light of m to the fiber end l7 and returns it to the fiber end l7 has a semiconductor laser amplifier l as a gain medium inside the resonator.
has. Therefore, when the amplification factor of the semiconductor laser amplifier is increased by increasing the injection current from the DC current source 30, oscillation occurs at 1.51 μm. In this example, the loss of the optical resonator is set to approximately 40 dB, so when the single pass gain of the semiconductor laser amplifier is 20 dB, the loss is 1.51 μm.
This causes oscillation at . This 1.51 μm light oscillates in a laser oscillator consisting of a terminal 9 with a mirror coated end of the optical fiber, a boat 5, a boat 7, a semiconductor laser amplifier 1, a boat 13, a boat 21, and a diffraction grating 15. ．． In addition, this oscillation light power can be adjusted to 20% by adjusting the current injected into the semiconductor laser amplifier l using a DC current source 30.
It is set to mW. 1.54, c from boat 4 to this semiconductor laser amplifier 1
-15dBm 5G b/s as Zm signal light 8
We conducted an experiment to input a signal pattern.

At this time, this signal light 8 is input from the boat 7 to the semiconductor laser amplifier l using the lens 10, and after being amplified by 20 dB corresponding to the single pass gain in the semiconductor laser amplifier l,
It was coupled to a boat 13 using a lens 11. The amplified signal light power at this time was +5 dBm. Thereafter, the output light from this semiconductor laser amplifier 1 is passed through a wavelength selective fiber coupler 3 to a signal light component of 1.54μ.
Only the light of m was outputted to the boat 22 and became the output light 16.
Figure 2 shows the temporal changes in 1.51 μm optical power, 1.54 μm signal light power, and carrier density in a semiconductor laser amplifier. When 1.54 μm light is incident, the carrier density temporarily decreases, but when the 1.51 μm light power decreases, both return to the original steady state. The relaxation time to return to this steady state was a time constant of less than ps. Therefore, the amplification factor for the signal light does not change and the 1.54 μm optical power in the semiconductor laser amplifier 10
Even when the power was increased to about mW, pattern effects due to amplification saturation did not occur. A typical embodiment of the second optical amplification device of the present invention is shown in FIG. In this embodiment, the semiconductor amplification medium has a wavelength of 1.
A semiconductor laser oscillator 101 having a length of approximately 100 μm and having a gain in a wavelength band of 5 μm to 1.55 μm is cleaved at both end faces to form a Fabriebe-type resonator, which is used as an optical resonator having n resonant frequencies.

A current is applied to this semiconductor laser oscillator 101 from a DC current source 30 to cause the semiconductor laser oscillator 101 to oscillate, and the output light level is set to +15 dBm. The output light spectrum at this time is shown in Figure 4. The oscillation output optical frequency fl from the semiconductor laser oscillator 101 (wavelength 1.53
3 μm), and in addition, f2, f3, f4, f5 corresponding to the resonant frequencies of the Fabry-Bello resonator of the semiconductor laser amplifier 101.
A small peak in the spectrum was observed, and it can be seen that oscillation occurs in an almost single longitudinal mode. This resonant frequency interval corresponds to the resonant frequency interval of the Fabry-Bello resonator determined by the length of the semi-dedicated laser oscillator 101. A signal light 40 is input to this semiconductor laser oscillator 101 using a wavelength selective fiber coupler 20. signal light 40
The wavelength is light with a wavelength of 1 and 50 μm corresponding to the peak f3 of the output optical spectrum of the semiconductor laser oscillator 101.
In addition, the wavelength selective fiber coupler combines the 1.50 μm light input from the boat 50 and the 1.50 μm light input from the boat 51.
．． It has a wavelength selection characteristic that allows light of 533 μm to be multiplexed into the boat 53 and output. Therefore, the signal light 40 coupled from the boat 53 to the semiconductor laser oscillator 101 using the lens 10 is amplified within the semi-rod laser oscillator 101 and then coupled to the boat 55 using the lens 11. After that, using the wavelength selective fiber coupler 21, the wavelength of the signal light 40 is adjusted to 1.
When the boat 57 outputs light of 5 μm, output light 41 is obtained.

The wavelength-selective fiber coupler 21 transfers 1.50 μm of the light input from the boat 55 to the boat 57.
A device having wavelength selection characteristics that outputs 33 μm light to the boat 43 is used. Also, a semiconductor laser oscillator 10L
The oscillation light output from the wavelength selective fiber coupler 2
0.21, it is output from the boat 51.56, resulting in an oscillation light output of 42.43. A 1.50 μm light is modulated with a 5 Gb/s signal pattern using an external modulator, and is input to this optical amplifier as a signal light 4o with a power of -10 dBm. When an experiment was conducted, the amplification factor of the semiconductor laser amplifier 1 for the wavelength of the signal light 4o was 20 dB, and the bandwidth was approximately 1. Therefore, the output light level of the semiconductor laser amplifier 1 is +
It was 10dBm. The semiconductor laser amplifier l at this time
Figure 5 shows how the oscillation wavelength light power, amplified signal light power, and carrier density change over time. When the signal light changes from the off state to the signal light on state and when the signal light on state changes to the off state,
Although the carrier density changed, the time to relax to a steady state was long, less than ps. In addition, the output light 41 is limited to a single band by the Fabriebe mouth resonator formed at the top of the cleavage plane 102}16, so it was possible to obtain output light with a low level of spontaneous emission. The embodiments described above are examples of the present invention, and various modifications can be made to the present invention. For example, in the optical amplification device of the first invention, the amplifiable wavelength band of the semiconductor laser amplifier is not limited to the 1.5 μm band, but may be the 1.3 μm band or the 0.8 μm band. Also,
As an external optical resonator, a Fabriebe-type resonator may be formed by using a fiber fI/A17 with a mirror coated end, or a diffraction grating may be added near the active layer to achieve wavelength selectivity. A DI”B elliptical laser with The bit rate may also be IG b/s, which is limited to 5 G b/s, or may be 10 G b/s. Also in the optical amplification device of the second invention, the amplifiable wavelength band of the semiconductor laser amplifier is 1. .5μm band, 1.3μm band or 0.8μm band may be used, and the optical resonator is not limited to a Fribelo resonator, but wavelength selectivity can be achieved by adding a diffraction grating near the active layer. It is also possible to use an optical resonator with a DFB side.Also, the wavelength selective fiber coupler 20 with one input rule can be omitted by using an optical isolator on the input side. In place of the wavelength selective fiber coupler, a diffraction grating type wavelength selective element can also be used.Furthermore, the beat rate of the input signal may be limited to 5 Gb/s, 1 Gb/s, or 10 Gb/s. /s is also fine.

(Effects of the Invention) As explained above, according to the present invention, the relaxation rate in optical amplification κ returns from a state in which the signal light level increases and amplification saturation occurs to the original state as the signal light level decreases. The relaxation time can be reduced to less than 100 seconds.

[Brief explanation of drawings]

FIG. 1 is an elliptical diagram showing a first embodiment of the present invention, FIG. 2 is a diagram showing temporal changes in optical power and carrier density in the first embodiment of the present invention, and FIG. A horizontal diagram showing the second embodiment, FIG. 4 is a diagram showing the output svector of the semiconductor laser oscillator 10l in FIG. 3, and FIG. 5 is a diagram showing the oscillation light, input light, and It is a figure showing a carrier density and a time change. DESCRIPTION OF SYMBOLS 1... Semiconductor laser amplifier, 101... Semiconductor laser oscillator, 2, 3, 20.21... Wavelength selective fiber coupler, 10, 11.14... Lens, 840...
...Signal light, 16.41...Output light, 30...DC current source.

Claims (3)

[Claims] (1) An optical amplification medium disposed in an optical resonator constituting an optical oscillator has a wavelength within the amplifiable wavelength band of the optical amplification medium,
Moreover, the optical amplification method is characterized in that a signal light having a wavelength different from the oscillation wavelength of the optical oscillator is inputted, and an amplified light of the signal light is obtained as an output. (2) a semiconductor laser oscillator comprising a semiconductor amplification medium and an external optical resonator; a means for coupling signal light having a wavelength different from the oscillation wavelength of the semiconductor laser oscillator to the semiconductor amplification medium; An optical amplification device comprising means for extracting a wavelength component of the signal light from the amplified output light, and optically amplifying the signal light using the semiconductor amplification medium. (3) Semiconductor amplification medium and n resonant frequencies f1, f2,
a semiconductor laser oscillator that oscillates at a resonant frequency f1, and a resonant frequency f2, f3,... of the optical resonator that is different from the oscillation frequency f1 of the semiconductor laser oscillator; An optical amplification device characterized in that signal light having a frequency of .fn is input to the semiconductor laser oscillator and optically amplified.