JPH0817259B2 - Optical amplifier - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical amplifier using an optical gain medium made of a semiconductor in a system using light as a signal for optical communication, optical information processing and the like.

2. Description of the Related Art Communication and information processing using light as a signal, such as optical communication and optical computing, are expected to be put into practical use because light is high speed and is not easily affected by electromagnetic induction.
However, the signal light is attenuated due to absorption and scattering in the transmission line, and thus needs to be amplified during the transmission. Optical amplification using a semiconductor laser amplifier is considered as one means of this optical amplification (for example, Electronics Letters, Vol. 23, No. 20, 105).
2-1053).

FIG. 4 shows an example of a conventional optical amplifier. The 401 is a Fabry-Perot type semiconductor laser, and the carrier density for laser oscillation changes, so that gain saturation and optical length change of the resonator occur, and the optical output vs. optical input characteristic is non-linear.

Reference numeral 402 denotes a bias current supplied to the semiconductor laser 401, which is lower than I _th and is set so that the active layer of the semiconductor laser 401 has a gain with respect to the optical input 403. Therefore, the semiconductor laser does not generate its own laser oscillation. The wavelength of the optical input 403 is usually selected to be the wavelength at which the gain of the semiconductor laser is almost maximum. Reference numeral 404 denotes a component including an optical isolator and a lens, which collects the light input 403 and makes it incident on the active layer of the semiconductor laser 401. In such a state, the optical input 403 is about 10 in the semiconductor laser 401.
It is amplified to an intensity of about 100 times and extracted as an optical output 405.

Problems to be Solved by the Invention However, according to the configuration described above, recombination of electrons and holes occurs in the semiconductor laser 401 for optical amplification. Therefore, the electron and hole densities decrease as the photon density inside the semiconductor laser 401 increases, which induces the following two problems. That is, the first problem is that the gain saturates as the optical input increases, as indicated by the curve 1 shown in the optical output-optical input characteristic of FIG. Next, the second problem is that the semiconductor laser 401 has an increased refractive index due to a decrease in the density of holes and electrons.
That is, the optical length of the resonator changes and the characteristic becomes non-linear as shown by the curves 2 and 3 in FIG. This is because the light that reciprocates in the resonator strengthens or weakens each other due to the change in the optical length.

However, as for the characteristic of the amplifier, it is desired that the relationship of the output with respect to the input is linear. Therefore, the non-linear response as shown in FIG. 5 is a big problem in the characteristic of the optical amplifier.

Means for Solving the Problems In order to solve the above problems, the present invention provides an optical amplifier using a semiconductor laser by setting a bias current larger than a threshold current to cause laser oscillation. The main purpose is to fix the density of holes and electrons in the active layer of a semiconductor laser regardless of the intensity of light input from the outside. Means for injecting light into the semiconductor laser, and an optical input for selectively blocking laser light generated by the semiconductor laser by oscillation peculiar to the semiconductor laser and for unilaterally entering the semiconductor laser from the outside of the semiconductor laser. And a means for selectively transmitting only the optical output obtained by amplification inside the laser.

Action According to the above-mentioned structure, since the density of holes and electrons does not change due to the change of the light input, the gain and the optical length of the resonator can be maintained constant, so that the linearity of the light output vs. light input characteristics is maintained. Be done.

Embodiment FIG. 1 shows a block diagram of an embodiment according to the present invention. 101 is a semiconductor laser having a threshold current of I _th . A bias current having a value higher than I _th is supplied to the current source 102. Therefore, the laser light 104 having the wavelength λ ₁ is emitted from the active layer 103 in a lateral electric field state (hereinafter, referred to as TE mode). Now, in this state, the optical input 106 of wavelength λ ₂ is injected from the outside. 107 and 108 are optical isolators, respectively. It is a lens that prevents the light input 106 from being injected into the active layer 103 and that the light emitted from the semiconductor laser 101 travels in the direction opposite to that of the light input 104. From the end surface on the right side of the semiconductor laser 101, the laser light 104 and the amplified light 109, that is, the optical input 106 is the active layer 103
Two types of light, that is, the light that is amplified and output in the inside, are emitted. These lights are converted into parallel rays by the lens 110 and enter the wavelength filter 105. The wavelength filter 105 is made to pass the wavelength λ ₂ and not the wavelength λ ₁ . Therefore, only the amplifier 109 is selectively output from the wavelength filter 105. It should be noted that λ ₂ is not equal to λ ₁ and is set within the wavelength range in which the semiconductor laser 101 has a gain.

Now, according to the above-mentioned configuration, the present inventors, when the photon density of the optical input in the resonator of the semiconductor laser is below a certain value,
It was confirmed experimentally and theoretically that the hole and electron densities of the active layer 103 are kept constant regardless of the value of the light input 106. The mechanism will be described below.

2A and 2B show the relationship between the photon density of the laser light in the resonator and the photon density of the light input in the resonator, and the active layer 10 respectively.
The relationship between the electron density in 3 and the photon density of the optical input in the cavity is shown. When the optical input is zero, the semiconductor laser oscillates in TE mode. At this time, the photon density of the laser light in the resonator is
Let it be P ₀ . N _{th in} FIG. 2B is the threshold carrier density. The carrier density is fixed to N _th when the supplied current is larger than I _th . Then inject the optical input 106,
When the photon density of the optical input in the resonator is increased, optical amplification and laser oscillation for the optical input coexist. However, since the bias current is constant, the total gain is constant. Therefore, when the photon density of the optical input increases, the optical amplification becomes active, and the gain that can be used for laser oscillation decreases. Therefore, the relationship is as shown in FIG. 2A, and when the photon density of the light input is P _i , the photon density of the laser light becomes almost zero. However, when the photon density of the optical input is smaller than P _i , the laser oscillation is maintained as shown in FIG. 2A, so the electron density is kept at N _th as shown in FIG. 2B. Be done. This is the main point of the present invention. Since the electron density is kept constant, the gain is kept constant and the optical length is kept constant. Figure 3A,
B indicates the transmittance with respect to the intensity of the optical input from the outside, that is, the ratio of the intensity of the amplified light with respect to the optical input, and the intensity of the amplified light. E _i is the value of the optical input when the photon density of the optical input in the resonator becomes P _i . Third when the optical input is below E _i
The transmittance is kept constant as shown in FIG. Therefore, the relationship between the amplified light and the light input is linear as shown in FIG. 3B. Therefore, an optical amplifier capable of linear response can be constructed under the condition that an optical input of E _i or less is incident.

In this embodiment, the polarization direction of the optical input is not specified, and the amplified light and the laser light are separated by utilizing the difference in wavelength from the laser light. If the transverse magnetic field mode is set inside, amplified light and laser light can be easily separated by using a polarizer instead of the wavelength filter 105. In this case, the wavelengths of the two lights may be the same.

EFFECTS OF THE INVENTION According to the present invention, an optical amplifier capable of linearly amplifying the intensity of signal light can be easily configured. Also, since the concentration of electrons and holes is set higher than in the conventional example,
They can shorten their life and are suitable for high speed operation.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of the optical amplifier of the present invention, and FIG. 2A shows the relationship between the photon density of laser light in the resonator of the embodiment of the present invention and the photon density of the optical input in the resonator. Shown, second
FIG. B is a diagram showing the relationship between the electron density in the active layer and the photon density of the optical input in the resonator of the embodiment according to the present invention, and FIG. 3A is the transmittance of the optical input and the optical input of the embodiment according to the present invention. FIG. 3B is a diagram showing the relationship between the amplified light and the optical input of the embodiment according to the present invention, FIG. 4 is a configuration diagram of a conventional amplifier, and FIG.
The figure shows the relationship between the optical output and the optical input in the conventional example. 101 ... Semiconductor laser, 102 ... Current source, 103 ... Active layer, 104 ... Laser light, 105 ... Wavelength filter, 106 ...
… Optical input, 107 …… Optical isolator, 108 …… Lens, 10
9 …… Amplified light, 110 …… Lens.

Claims (1)

[Claims] 1. A semiconductor laser, a current source for supplying a bias current equal to or more than a threshold current to the semiconductor laser, a means for unilaterally entering light from the outside into the semiconductor laser, and an oscillation peculiar to the semiconductor laser. Thus, only the optical output obtained by amplifying inside the semiconductor laser the optical input that selectively cuts off the laser light generated by the semiconductor laser and unilaterally enters from the outside of the semiconductor laser is provided. An optical amplifier comprising: a means for selectively transmitting light.