JPH02287329A - Optical amplifier - Google Patents

Abstract

PURPOSE:To improve SN ratio and stability by having a feedback optical path for light to return a part of light output by an antiphase to an input side. CONSTITUTION:This amplifier has the feedback optical path 10 for light to return a part of the light output in the antiphase to the input side. Namely, an optical system to return the light from the output side of the optical amplifier to the input side, i.e. a half mirror 13 and a mirror 12 are provided on the outside of a gain medium 11. The sum of the length l of the feedback optical path 10 of light and the optical path length nL of the gain medium 11 is determined by equation l+nL=mlambda/2 (m: odd number). In the equation, L is the length of the gain medium 11; n is the refractive index in the gain medium 11; lambda is the gain peak wavelength of the gain medium 11. Natural release light is suppressed in this way and the SN ratio and frequency stability are improved in this way.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to an optical amplifier.

[Conventional technology]

Conventionally, optical amplifiers that utilize stimulated amplification using population inversion,
□It is used for laser light amplification in dye lasers and solid-state lasers, and more recently, semiconductor optical amplifiers that directly amplify optical signals without performing optical-to-electrical conversion are also being researched in long-distance optical communication systems. . In particular, semiconductor optical amplifiers have been actively researched in recent years because they are small, operate with low power, and have the potential to provide high amplification factors.

[Problem to be solved by the invention]

However, the conventional optical amplifier described above consists only of an optical gain medium and does not have an optical feedback circuit, and problems still remain in terms of S/N ratio and stability. In particular, an optical amplifier that amplifies weak signals that have been attenuated by long-distance transmission, such as an optical repeater in a long-distance optical communication system, is required to have a particularly high signal-to-noise ratio. However, in terms of signal-to-noise ratio, compared to electromagnetic wave amplifiers in the microwave band such as masers, it amplifies light, which is electromagnetic waves, at a much higher frequency, so in principle the transition probability of spontaneous emission increases, so this spontaneous emission Light mixes in as noise, leading to deterioration of the S/N ratio. At the same time, the contamination of this spontaneous emission light causes frequency fluctuations and gain fluctuations in the output light, and is a factor that deteriorates the stability of the optical amplifier.

The purpose of the present invention is to eliminate the drawbacks of conventional optical amplifiers, and to provide an optical amplifier that can be fully applied to the field of optical amplifiers that require high signal-to-noise ratio and stability, such as optical repeaters in long-distance optical communication systems. Our goal is to provide the following.

[Means to solve the problem]

As a means to solve the above problem, in an optical amplifier that inputs light into a medium that has an optical gain due to population inversion and extracts the amplified optical output from the output side, a part of the optical output is output in reverse phase. The configuration includes a return optical path for returning light to the input side.

Returning a part of the optical output to the input side with an opposite phase can be realized by using a configuration in which the sum of the length l of the optical return path of the light and the optical path length nL of the gain medium is related by the following equation.

1 + n L=mλ/2 (m: odd number) Here, is the length of the gain medium, n is the refractive index within the gain medium, and λ is the gain peak wavelength of the gain medium.

[Effect]

In the optical amplifier of the present invention, the principle of a so-called negative feedback amplifier in an electronic circuit that returns part of the output to the input side in an opposite phase is applied to improve the S/N ratio and frequency stability. As a method for applying negative feedback to light, as shown in the example in Figure 1, an optical system that returns light from the output side of a normal optical amplifier to the input side (in the case of this figure, half mirror 13 and mirror 1 are used)
2) is provided outside the gain medium. That is, if the length of the gain medium 11 is the length of the gain medium 11, the refractive index inside it is n, and the length of the optical feedback optical path 10 is l, then in order to perform negative II* return on the gain medium that has a gain peak at the wavelength λ, is the length g of the return optical path so that the following relationship holds true:
All you have to do is decide.

1 + n L=mλ/2 (m: odd number) - (1) By doing this, negative feedback is applied to light having a wavelength near the gain peak that satisfies the above condition.

In this case, it is desirable that negative feedback be applied to light in all wavelength ranges in which the optical amplifier has a gain. Since the following relationship holds between the total feedback optical path length J2+nL of light and the wavelength range λ to which negative feedback can be applied, it is desirable that the gain medium that applies negative feedback first has a gain as narrow as possible. .

, RL/n<λ2/Δλ (2) where 1 is the length of the optical return path, L is the length of the gain medium, n is the refractive index inside it, and λ is the wavelength of the gain peak. be.

Figures 2 and 3 show the differences between the conventional optical non-feedback amplifier (Figure 2A) and the pre-negative feedback amplifier of the present invention (Figure 3A), and the non-feedback amplifier in the electronic circuit (Figure 2B). and a negative feedback amplifier (FIG. 3B). In the non-feedback amplifiers shown in Figure 2A and B, the input/output relationship is expressed as in Figure 2C and D, but in contrast, in the amplifier with negative feedback, it is shown in Figure 3B. In the case of electronic circuits, the input/output relationship is as shown in Figure 3, as is well known, and the SN
Ratio and frequency specialization are improved. On the other hand, in the case of the optical amplifier shown in FIG. 3A, as with electronic circuits, it can be expected that the stability of the S/N ratio and gain can be improved by applying negative feedback, but in the case of the optical amplifier, in equation (1), If m is an odd number, the negative feedback will be harsh, and if m is an even number, it will be positive feedback and may cause oscillation, so as shown in Figure 3C, negative feedback is applied in the entire gain range. is desirable. By doing so, spontaneous emission light is suppressed, and the S/N ratio is improved when an optical signal near the gain peak wavelength is amplified.

〔Example〕

FIG. 4 shows an embodiment of an optical amplifier using a semiconductor, in which the gain medium 21 is a DC-PBH semiconductor laser with an element length of 30 μm and an active layer with a wavelength of 1.6 μm. A semiconductor optical amplifier with a reflective coating was used. The light emitted from the output side end face of the semiconductor optical amplifier is split into two by an optical fiber coupler 23, one as an output, and the other enters a negative feedback optical path consisting of an optical fiber 25, passes through an optical isolator 24, and returns to the semiconductor light. It is returned to the input side of the amplifier. In this case, by adjusting the length (approximately 100 μm) of the optical fiber 25 of the negative feedback optical path, negative feedback can be applied to light having a wavelength near the gain peak of the amplifier. Actually optical fiber 2
By changing the distance between the isolator 5 and the isolator 24 using a piezo element, the spontaneous emission light can be adjusted to be minimized. Here, an anti-reflection coating is applied to the end face of a normal semiconductor laser and used as a gain medium, so a particularly narrow band gain medium is not used. However, even with this configuration, the S/N ratio can be improved by about 2 dB (7) compared to the case where no prior negative feedback is performed. In order to further improve the S/N ratio, it is necessary to make the amplifier gain narrower, and it is necessary to make the active layer a quantum well structure, use a semiconductor doped with a rare earth such as Eu, or use filter characteristics. There is also a method of using an amplifier coupled to a waveguide having .

〔Effect of the invention〕

In the optical amplifier shown in the embodiment of the present invention, a fiber-to-fiber gain of 17 dB was obtained when a current of 200 mA was injected.
Conventional non-feedback optical amplifier (fiber-to-fiber gain 16 to 18
dB) was obtained. On the other hand, regarding gain fluctuations, while conventional non-feedback optical amplifiers have gain fluctuations of about 3 dB, the optical amplifier according to the embodiment of the present invention suppresses the gain fluctuations to within 0.5 dB. Also S
The N ratio is also improved by about 2 dB over conventional optical amplifiers, and therefore it is fully possible to apply the optical amplifier of the present invention to repeaters in long-distance, large-capacity optical communication systems. This suggests that it can be used as a substitute for conventional optical-to-electrical conversion type optical repeaters. Additionally, optical repeaters that use optical amplifiers have fewer parts than conventional optical-to-electrical converter repeaters, and the probability of failure is much lower with optical repeaters that use optical amplifiers. Therefore, there is a great advantage in applying it to locations where failure and repair are difficult, such as transoceanic submarine optical cable repeaters.Furthermore, if the optical amplifier according to the present invention is used as an optical amplifier for solid-state lasers or dye lasers, laser light output with a higher coherency trend can be achieved. can be obtained, and can be used as a light source with excellent monochromaticity and focusing properties.

[Brief explanation of the drawing]

FIG. 1 is a conceptual diagram of the optical amplifier of the present invention, and FIGS. 2 and 3 are diagrams for explaining the operation of the optical amplifier. FIG. 4 is a diagram showing an embodiment of the present invention. In the figure, 11...Medium having optical gain due to population inversion, 12...Mirror 13...Half mirror 2
1... Semiconductor optical amplifier, 23... Optical fiber coupler, 24... Optical isolator, 25... Optical fiber

Claims (1)

[Claims] An optical amplifier that inputs light into a medium having an optical gain due to population inversion and extracts an amplified optical output from the output side, which is equipped with a return optical path for returning part of the optical output to the input side with an opposite phase. An optical amplifier featuring