JPS6257280B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical amplifier having an automatic frequency control function.

Conventionally, an optical amplifier is an active fabric amplifier consisting of an input side mirror _R1 , an output side mirror _R2 , and an amplification medium sandwiched between them, as shown in Fig. 1.
A Perot resonator was used. This optical amplifier has two operating modes. One is to set the pumping level of the amplification medium below the lasing threshold of this active Fabry-Perot resonator and use it as a linear amplifier. In this case,
As shown in FIG. 2, it is most efficient to perform amplification by matching the resonance mode frequency ₀ of the active Fabry-Perot resonator with the frequency of the incident signal light. If the frequency of the incident signal light differs greatly from the Fabry-Perot resonance frequency of the optical amplifier, no amplification can be obtained as shown in the figure, and on the contrary, the signal light will be attenuated. In the second mode of operation, the pumping level of the amplification medium is set above the laser oscillation threshold of this active Fabry-Perot resonator, and the amplifier is used as an injection-locked amplifier. In this case as well, it is efficient to perform amplification by making the self-oscillation frequency of the injection-locked amplifier substantially the same as the frequency of the incident signal light, as shown in FIG. If the incident signal optical frequency and the self-sustained oscillation frequency of the amplifier are separated by more than the injection locking width _BL , the synchronization will be lost and signal amplification will no longer be performed.

Conventional devices of this kind were used with open gates without control signal gates as shown in Figure 1, so the oscillation frequency of the optical amplifier (resonant mode frequency) may vary depending on temperature, pumping fluctuations, etc. Therefore, stable amplification characteristics could not be obtained. For example, in the case of a GaAs semiconductor laser amplifier, its oscillation frequency or resonance mode frequency shifts by 22 GHz per 1°C of temperature change.
The 3 dB reduction bandwidth Δv1/2 when the GaAs semiconductor laser amplifier is used as a linear amplifier and the locking width B _L when used as an injection-locked amplifier are both about 2 to 3 GHz. Therefore, in order to obtain stable amplification characteristics, it was necessary to control the temperature to about 0.01°C. Further, even if stabilization is performed by such temperature control, it is not possible to cope with fluctuations in the incident signal frequency. Therefore, there has been a need for an optical amplifier that can automatically perform frequency control in response to fluctuations in the incident signal frequency and amplifier frequency.

In order to solve these drawbacks of optical amplifiers, the present invention provides an optical amplifier with a mechanism that automatically tunes the oscillation frequency and resonant mode frequency (hereinafter simply referred to as optical amplifier frequency) of the optical amplifier to the incident signal frequency. The drawings will now be described in detail.

FIG. 4 shows the relationship between the output optical phase of the optical amplifier and the incident signal optical frequency, which is the operating principle of the present invention. In both the case of the linear amplifier operation mode shown in FIG. 4a and the injection-locked amplifier operation mode shown in FIG. 4b, if the input signal optical frequency and the optical amplifier frequency match, the output optical phase is It becomes completely in phase with the input optical phase. On the other hand, when the input signal optical frequency shifts to a higher frequency side than the optical amplifier frequency, the phase is delayed (negative value), and conversely, when the input signal optical frequency shifts to a lower frequency side than the optical amplifier frequency, teeth,
Phase advances (positive value). Therefore, automatic frequency control becomes possible by detecting this amount of phase shift and feeding it back to the optical amplifier as an error signal.

FIG. 5 shows an embodiment of the present invention, in which 4 is a half mirror for splitting input signal light, 5 is a half mirror for output signal light branching, 6 is an optical amplifier, and 7 is a wavelength that provides a phase shift of π/2. Board 8 is a multiplexer for two optical signals, and 9 is a photodetector. The operating principle will be explained below. One part of the input signal light and one part of the output signal light are taken out by a half mirror, and the two optical signals are combined so that the phase of the output signal light is advanced by π/2. It is desirable that both optical signal powers be equal. In this case, the output current of the photodetector 9 is given by i=DP _s (1−sinθ) (1). D is a constant of the optical detector, P _s is the power of each optical signal, and θ is the relative phase difference shown in FIG. Therefore, when θ is fluctuating around 0, an error signal i _s =DP _s θ approximately proportional to θ, excluding the DC component, can be extracted as the optical detector output. If this error signal is fed back to a part that can change the optical amplifier frequency,
Automatic frequency control becomes possible. For example, the frequency of a semiconductor laser amplifier can be changed by injection current. Increasing the injection current, in a linear amplifier operating below the oscillation threshold,
The carrier density increases and the resonant mode frequency shifts to the high frequency side, and in an injection-locked amplifier operating above the oscillation threshold, the active layer temperature increases and the oscillation frequency shifts to the low frequency side. Therefore, for a linear amplifier, use the configuration shown in Figure 4, and for an injection-locked amplifier, use a phase shift of -π/2 instead of π/2 to amplify the photodetector output current and merge it with the injection current of the semiconductor laser amplifier. For example, the frequency of the incident signal light and the frequency of the semiconductor laser amplifier can be automatically matched. The present invention can also be applied to optical amplifiers other than semiconductor laser amplifiers. For example, in an amplifier using an external mirror, the error signal may be fed back to a PZT element or the like attached to the external mirror.

FIG. 6 shows another embodiment of the present invention, in which 10 is an n-GaAs substrate, 11 is a p-GaAs active layer, 12 is a p-GaAs layer, 13 is an n-GaAs layer, and 14 is a p-side A transparent electrode 15 is an n-side electrode that constitutes a GaAs homojunction laser. 4, 5, and 8 are the half mirror and optical multiplexer illustrated in FIG. The operating principle will be explained below. Similar to the embodiment shown in FIG. 5, the phases of the incident signal light and the output signal light are compared, and an error light signal corresponding to the frequency mismatch is applied to the region 13. Since the pn junctions 10, 11, and 12 are forward biased, active layer 1
A carrier is injected into 1, causing population inversion and becoming an amplification medium. On the other hand, npn of 10, 11, 12, 13
The junction portion forms a phototransistor, and has a structure in which an error optical signal is detected by a pn junction 12-13, and an amplified photocurrent flows into the active layer 11. The principle of operation will be explained using FIG. Here, P ₁ represents the potential before light irradiation, and P ₂ represents the potential during light irradiation. The error optical signal is 1
It is absorbed by the depletion layer formed in the pn junction of 2-13, generating electron-hole pairs. Holes flow into the p-GaAs layer by the depletion layer electric field, and the electrostatic effect lowers the height of the potential barrier to electrons in this region. As a result, electrons flow from n-GaAs on the right side to p-GaAs, and carriers are injected into the active layer. As explained above, in the embodiment shown in FIG. 6, the function of detecting and amplifying the error optical signal is realized by the phototransistor formed in the semiconductor laser. FIG. 6 shows the basic configuration of the present invention. In order to perform more efficient operation, a heterojunction is used to improve the confinement effect of light and carriers in the active layer 11, and the light detection The present invention includes a structure in which a window layer 13 is provided by a heterojunction in order to improve quantum efficiency. Also, semiconductor materials
The same effect can be expected by using InP/In GaAsP other than AlGaAs.

In the embodiment of the present invention, the error optical signal extraction unit 4,
There are no particular limitations on how to configure 5, 7, and 8.
A bulk type half mirror, a phase plate, and a multiplexer may be used, or all of these may be composed of a single mode optical fiber optic dielectric waveguide, or,
It is also possible to form it with a semiconductor optical waveguide and to configure it with a semiconductor laser as well. However, it is desirable to minimize the optical path difference between the two optical signals. This is because even if the phase difference between the two optical signals is set to π/2 or -π/2, it is expected that the optical length difference between the two beams will change due to temperature fluctuations and the like, causing an error in the phase difference. For example, if the error optical signal extraction circuit is
Assume that it is composed of a SiO ₂ waveguide and that the difference between both arms is within 1 mm. For an optical signal with a wavelength of 1 μm,
The temperature change that causes the phase difference to fluctuate by 10% (π/2±π/20) from the set value of π/2 is 2.3°C. If the difference between both arms can be kept within 100 μm, temperature changes up to 23°C can be suppressed to 10% fluctuations. When both arms are composed of semiconductor optical waveguides, the temperature coefficient of the refractive index is
Since it is about one order of magnitude larger than SiO ₂ , the allowable limit is 2.3°C with a difference of 100 μm between both arms.

As explained above, according to the present invention, it is possible to realize a linear amplifier in which the Fabry-Perot resonant mode frequency automatically follows the incident signal optical frequency, and an injection-locked amplifier in which the oscillation frequency automatically follows the incident signal optical frequency. Since such optical amplifiers are compact and provide stable amplification characteristics, they are expected to be applied to optical repeaters.

[Brief explanation of the drawing]

Figure 1 shows an example of the configuration of a conventional optical amplifier, Figure 2 shows the relationship between the amplification gain of a linear amplifier and the incident signal optical frequency, and Figure 3 shows the relationship between the amplification gain and the incident signal optical frequency of an injection-locked amplifier. Diagram showing. 4a and 4b show the relative phase θ of the output signal light with respect to the input signal light versus the difference between the input signal light frequency and the optical amplifier frequency for a linear amplifier and an injection-locked amplifier, and FIG. 5 shows an embodiment of the present invention. An example configuration, FIG. 6 shows another embodiment of the present invention, and FIG. 7 shows the operating principle of a phototransistor. 1 is an input mirror, 2 is an output mirror, 3 is an amplification medium, 4 is a half mirror for splitting input signal light, 5 is a half mirror for splitting output signal light, 6 is an optical amplifier, 7
is a phase shifter, 8 is an optical combiner, 9 is a photodetector, 10 is an n-GaAs substrate, 11 is a p-GaAs active layer, 12 is a p-GaAs layer, 13 is an n-GaAs layer, 14 is a p-side transparent layer The electrode 15 is an n-side electrode.

Claims (1)

[Scope of Claims] 1. In an optical amplifier that amplifies and outputs an incident signal light, a first branching means for branching a part of the input signal light, a second branching means for branching a part of the output signal light, It has a multiplexer that multiplexes the outputs of each branching means directly or via a phase shifter, and a photodetector that converts the output of the multiplexer into an electrical signal, and the phase shifter is the multiplexer of the multiplexer. The optical path difference is provided so that the output optical power is proportional to the phase difference between the respective incident lights of the multiplexer, and the output of the optical detector is fed back to the optical amplifier to differentiate between the incident signal optical frequency and the optical amplifier resonant mode frequency. AFC optical amplifier characterized by matching. 2. The optical amplifier is a semiconductor laser, the photodetector is a phototransistor integrated on the same substrate as the semiconductor laser, and the semiconductor laser is activated by the interference light power irradiated to the phototransistor. 2. The AFC optical amplifier according to claim 1, wherein the current injected into the layer is controlled.