JPH0145751B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical amplifier suitable for optical communication systems using wavelength-multiplexed optical signals. In particular, it relates to an optical amplifier that directly amplifies a wavelength-multiplexed optical signal without converting it into an electrical signal.

Conventionally, an amplifier for repeating wavelength-multiplexed optical signals uses a demultiplexer to demultiplex signal light that has been multiplexed with N wavelengths using carrier waves with N different wavelengths into N optical beams, and then splits the signal light into N optical beams using a demultiplexer. Amplification is performed using individual elements. In particular, in practical use, light is detected once, converted to an electrical signal, identified and reproduced, and then an optical signal is generated using a laser oscillator again.
is configured to combine two light beams. Therefore, the configuration becomes complicated, and as the multiplex order N increases, the scale of the repeater increases.

A method has also been considered in which each optical beam split into N beams is directly amplified by one optical amplifier without performing photoelectric conversion, but in this case as well, a splitter and N optical amplifiers are used. , a multiplexer is required, which complicates the configuration, and the noise unavoidably generated from each optical amplifier is sent out to the transmission line through the multiplexer, deteriorating the signal-to-noise ratio of the transmission system and reducing the number of relays. This has the disadvantage that the spacing becomes short and a practical design cannot be obtained.

The present invention is an amplifier that can directly amplify an optical signal without performing opto-electrical conversion, and is a compact amplifier that can perform the amplification action with one common element for wavelength-multiplexed signal light. The present invention also aims to provide an amplifier with a simple configuration.

The present invention uses two or more reflecting surfaces disposed so that the reflected light repeatedly passes through the same optical path, and an amplification medium provided between the reflecting surfaces and supplied with energy from the outside. The gain frequency width is set to be larger than the longitudinal mode frequency interval between the reflecting surfaces, and the wavelengths of the plurality of carrier waves of the incident wavelength-multiplexed signal light match one of the resonant wavelengths of this amplifier, and , the wavelength width of each incident light beam is set to be smaller than a certain wavelength width of amplification gain in the resonant mode of the amplifier.

A detailed explanation will be given below using the drawings of the embodiment.

FIG. 1 is a basic structural diagram of an apparatus according to an embodiment of the present invention. Reference numeral 1 denotes a single current injection type semiconductor laser amplification element, into which a bias current is injected from a terminal 2. This element 1 has two opposing reflective surfaces 3,
4, and light enters from one side and exits from the other as shown by the arrow. The reflective surfaces 3 and 4 act as half mirrors.

When the current injection type semiconductor laser amplification element 1 receives a forward current from the terminal 2, it starts laser oscillation when the current value exceeds a certain value. In a bias state lower than this oscillation limit, it becomes a Fabry-Perot resonant optical linear amplifier with gain versus frequency characteristics as shown by the solid line in FIG. 2a. FIG. 2a shows frequency on the horizontal axis and gain on the vertical axis. Resonance points appear at each frequency (approximately 110 GHz, wavelength approximately 2.7 Å in this example) indicated by f _p in the figure. This frequency f _p is determined by the period of the longitudinal mode in which the light travels back and forth between the reflecting surfaces 3 and 4. When the peaks of the resonance points are connected, a frequency distribution as shown by the broken line in FIG. 2a is obtained. Each resonance point has a small frequency width.

In the frequency distribution shown by this broken line, for the two points where the gain is 1/2 of the point where the gain is maximum,
Given that frequency spacing f _w , in this example approximately
It can take more than 4000GHz (approximately less than 100Å). Also, for one resonance point, its gain half width f _s
Considering this, it will be about 2 to 5 GHz. maximum gain G
can get 20~30dB.

For an amplifier with such characteristics, each carrier wavelength (f ₁ in frequency expression,
f ₂ ,...f _N ) are set to match the frequency of this resonance point. In the above example, the gain half-width
If f _w is 4000 GHz and the resonance point spacing f _p is 110 GHz, nearly 40 resonance points are obtained within this gain half width f _w , so the multiplex number N can be selected up to about 40.

That is, when wavelength-multiplexed signal lights whose carrier frequencies are f ₁ , f ₂ , ... f _N that match the resonant frequency of the amplifier are input to the resonant optical amplifier 1, each light is directly amplified and the other lights are The light passes through the reflective surface 4 and is taken out. Input frequency f ₁ , f ₂ , ... f _N
do not necessarily have to be next to each other,
It doesn't matter if it's random.

In this way, a single amplifier directly amplifies all the multiplexed signal lights of f ₁ , f ₂ , ...f _N , so the noise generated from the amplifier and sent out to the transmission path is transmitted to the individual amplifiers. 1/N compared to the method that amplifies by
Therefore, the relay interval can be expanded.

The amplification gain saturates as the input signal optical power increases, but when the gain decreases by 3 dB due to saturation, the saturated output is -10 in a normal semiconductor laser amplifier.
The value is ~-5dBm. In the present invention, in which signals of N frequencies are multiplexed and amplified, the amplifier output allowed for each frequency of light is 1/N of the above-mentioned saturation output value. For repeaters, this is not a major drawback.

In addition to the above, various types of lasers such as gas, solid, and dye lasers that perform axial multimode oscillation above a threshold can be used as amplifier elements in which the present invention can be carried out by operating them below the threshold. A similar wavelength multiplexing optical amplifier can be constructed.

As a further embodiment of the present invention, three or four resonators as shown in FIG. 3 or 4 are used as a resonator for constructing a resonant type amplification element using a semiconductor gain medium or other medium other than the Fabry-Perot type. reflective surface 10
Alternatively, a ring-shaped resonator with 11 can be used. In this example as well, a biased medium is placed between each reflective surface 10 or 11.

In addition, in FIG. 5, as a device integrated in a semiconductor, an amplification gain is provided between the input waveguide 12 and the output waveguide 14, and the length of one circumference is shown in order to couple the waveguides 12 and 14. A configuration in which the directional coupling waveguide 13 with L is arranged can be used. In this case, the resonance frequency interval is Δν=c/nL, where n is the refractive index and c is the speed of light. The amplification center frequency is Δν·N (N is an integer).

As described above, by using one resonant optical amplifier whose gain width is larger than the resonant frequency interval, it is possible to simultaneously amplify multiplexed signal light having a large number of frequencies that match the resonant frequency. Especially small size,
A semiconductor laser amplifier is a high-gain, low-Q resonator.
By doing this, a high-performance optical repeater for wavelength division multiplexing transmission can be easily constructed.

[Brief explanation of drawings]

FIG. 1 is a basic structural diagram of an apparatus according to an embodiment of the present invention. Second
The figure shows a characteristic diagram and a frequency allocation diagram to explain the operating principle. 3 and 4 are structural diagrams of another embodiment of the device of the present invention. FIG. 5 is an explanatory diagram of an applied example that operates on the same principle. 1...Amplification medium (semiconductor laser element), 2...
Bias current supply terminal, 3, 4...reflection surface.

Claims (1)

[Claims] 1. Two or more reflecting surfaces arranged so that the reflected light repeatedly passes through the same optical path, and an amplification medium provided between the reflecting surfaces and supplied with energy from the outside. , the gain frequency width is set to be larger than the longitudinal mode frequency interval determined by the interval between the reflecting surfaces, and the resonant type is configured to amplify and extract the incident light incident on the amplification medium. In an optical amplifier, the incident light is a wavelength-multiplexed signal light, the wavelength of a plurality of carrier waves of the wavelength-multiplexed signal light matches one of the resonant wavelengths of the amplifier, and the wavelength width of each incident light is equal to the wavelength width of the wavelength-multiplexed signal light. A wavelength division multiplexing optical amplifier characterized in that the amplification gain in the resonant mode of is set to be smaller than a certain wavelength width. 2. The wavelength multiplexing optical amplifier according to claim 1, wherein the number of reflecting surfaces is two, and the amplification medium is a semiconductor laser element to which a bias current lower than the oscillation limit is applied.