JPH04242228A - Optical communication system - Google Patents

Abstract

PURPOSE:To suppress variation of amplified light due to variation of the gain ripple of the semiconductor optical amplifying element by composing one signal of two wavelength signals having an interval corresponding to the interval between the peak and bottom of the gain ripple of the semiconductor optical amplifying element. CONSTITUTION:This system has two semiconductor lasers as the light source of an optical transmitter and the two semiconductor lasers are modulated at the same time according to a signal inputted to the optical transmitter to send signal light components with wavelengths 1 and 2. Namely, even if the peak wavelength of the gain ripple of the progressive wave type semiconductor optical amplifying element shifts owing to disturbance, the intensity of the signal light beams with the wavelengths 1 and 2 is a mean amplification factor (20dB) at all times since the wavelength dispersion characteristic of the gain ripple is in a sinusoidal wave shape.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical communication system using a traveling wave semiconductor optical amplifying element to amplify a transmitted optical signal.

0002

[Prior Art] Conventionally, in optical communication systems that use semiconductor optical amplification elements to amplify optical signals that are attenuated in transmission lines,
In order to use a semiconductor optical amplification element with a high amplification factor and low gain ripple, the residual reflectance of the anti-reflection coating applied to the end face of the semiconductor optical amplification element is reduced to prevent the influence of gain ripple from affecting communication. The optical amplifier was used. The magnitude of the gain ripple is, for example, an amplification factor of 20 dB,
When the residual reflectance of the anti-reflection film on the end face is about 0.1%,
It is about 2 dB.

0003

[Problems to be Solved by the Invention] However, in a system using a semiconductor optical amplification element in the conventional usage pattern described above, when the wavelength of the input signal changes, the amplification factor changes due to the gain ripple of the amplification element. do. Conversely, when the temperature of a semiconductor optical amplification element changes, the cycle of the gain ripple changes due to changes in the wavelength dispersion characteristics of the amplification factor, and even if the wavelength of the input signal is constant, the optical amplification element The output waveform fluctuates, making it difficult to perform linear amplification.

The above situation will be explained using FIGS. 5, 6, and 7. FIG. 5 is a diagram showing an example of the amplification characteristics (wavelength dispersion characteristics of the amplification factor) of a semiconductor optical amplification element, where the horizontal axis is the wavelength and the vertical axis is the amplification factor. In this example, 3
It shows that there is a gain ripple of dB and that there is an average amplification factor of 20 dB.

FIG. 6 shows an output signal when the signal input to the semiconductor optical amplification element matches the wavelength 1 shown in FIG. 5, that is, the resonance wavelength of the gain ripple, and the amplitude at the input is This indicates that the amplitude is about 141 and is output from the optical amplification element. On the other hand, FIG. 7 shows the output signal when the wavelength of the input signal matches wavelength 2, that is, the non-resonant wavelength of the gain ripple, and if the amplitude at the input is 1, the amplitude is about 70. This indicates that the output will be output as follows.

In this way, a slight relative shift in the wavelength of the input signal with respect to the gain ripple (this can occur even when the wavelength of the input signal is constant, but due to variations in the chromatic dispersion characteristics of the amplification factor of the optical amplification element) can greatly increase the output. It can be seen that the amplitudes of the signals differ.

On the other hand, although it is theoretically possible to suppress the gain ripple of a semiconductor optical amplification element for a desired amplification factor, in the situation of a high amplification factor, an antireflection film with a low residual reflectance is It is necessary for the end face, and extremely advanced film thickness control technology and refractive index control technology are required in manufacturing the amplification element.

Therefore, in view of the above-mentioned problems, the present invention alleviates the precision of controlling the refractive index and film thickness of an antireflection film formed on the end face of a traveling wave semiconductor optical amplification element used in an optical communication system. At the same time, it is an object of the present invention to provide an optical communication system that can suppress fluctuations in an amplified signal due to gain ripples of the semiconductor optical amplifying element.

[0009]

[Means for Solving the Problems] In the optical communication system of the present invention that achieves the above object, a traveling wave type semiconductor optical amplification element is used, and an optical signal to be transmitted is transmitted through the traveling wave type semiconductor optical amplification element. It has a wavelength interval between an arbitrary resonant wavelength and an arbitrary non-resonant wavelength (which may or may not be adjacent to this resonant wavelength) in the Fabry-Perot resonance phenomenon that appears in the wavelength dispersion characteristics of the amplification factor of It is characterized in that it consists of at least two wavelengths of light, and that both of these lights are modulated by the same synchronized signal.

More specifically, the optical signal to be transmitted is transmitted at an arbitrary resonant wavelength in the Fabry-Perot resonance phenomenon that appears in the wavelength dispersion characteristic of the amplification factor of the traveling wave semiconductor optical amplification element, and at a wavelength adjacent to the resonant wavelength. The transmitted optical signal is composed of at least two wavelengths of light having a wavelength interval from either one of the two non-resonant wavelengths, or the transmitted optical signal is of the traveling wave type semiconductor optical amplification element. In the Fabry-Perot resonance phenomenon that appears in the wavelength dispersion characteristics of the amplification factor, two wavelengths that have a wavelength interval between an arbitrary resonant wavelength and one of two non-resonant wavelengths adjacent to the resonant wavelength are defined as 1.
When combined, the light beams are composed of at least a plurality of sets of wavelengths.

[0011] Furthermore, the wavelength of the light constituting the optical signal to be transmitted may be within the gain range of the traveling wave semiconductor optical amplification element, or the optical transmitter for transmitting the optical signal may The optical signal is composed of at least one semiconductor laser that oscillates light, the semiconductor laser is a longitudinal multimode laser, the optical intensity of each wavelength constituting the optical signal is equal at any given time, or the optical signal is The average amplification factor of the traveling wave type semiconductor optical amplification element at each wavelength constituting the optical signal is equal, or the traveling wave type semiconductor optical amplification element has the same average amplification factor at each wavelength constituting the optical signal, or the traveling wave type at a resonant wavelength near each wavelength constituting the optical signal and a non-resonant wavelength adjacent thereto. The differences in the amplification factors of the semiconductor optical amplification elements may be the same.

According to the above configuration, by simultaneously transmitting one signal used for optical communication as signal components of a plurality of wavelengths, the gain ripple of the traveling wave optical amplification element used in the optical communication path is reduced. The resulting fluctuations in the amplified signal can be suppressed. This is because fluctuations in the amplification factors of the signal components of each wavelength occur so as to cancel each other out, so that the amplification factor for one signal formed by combining them is kept almost constant.

[0013]

Embodiment FIGS. 1 to 4 are diagrams for explaining one embodiment of the present invention. In FIG. 1, 1 is an optical transmitter that transmits an optical signal, 2 is an optical fiber that transmits the optical signal, 3 is a traveling wave semiconductor optical amplification element, and 4 is an optical receiver. FIG. 2 is a graph showing the wavelength dispersion characteristics of the amplification factor of the semiconductor optical amplification device 3, FIG. 3 is a diagram showing the input signal of the optical signal used for communication to the semiconductor optical amplification device 3, and FIG. FIG. 4 is a diagram showing output signals (FIGS. 3(a), (b), and (c) are respectively shown in FIG. 4).
(corresponding to (a), (b), (c)). The semiconductor optical amplification device 3 has a configuration of a so-called traveling wave type semiconductor optical amplification device, and is constructed by forming an antireflection film on both end faces of a general semiconductor laser structure device (a structure having an active layer, a cladding layer, etc.). There is. This suppresses feedback due to both end faces, and high energy injection (excitation) into the amplification element 3 (for example,
(performed with bias current injection below the threshold value) is possible.

In this embodiment, if an antireflection film is formed on the end face of the optical amplification element 3 to such an extent that the wavelength dispersion characteristic of the amplification factor of the semiconductor optical amplification element 3 shown in FIG. Here, a semiconductor optical amplification element was used in which an antireflection film with a residual reflectance of about 0.4% was formed on the end face of a GaAs semiconductor laser. In this semiconductor optical amplifying element 3, approximately 2
With a gain (amplification factor) of 0 dB, the gain ripple is about 2 dB. Further, the resonator length of the semiconductor optical amplification element 3 is approximately 3
00 μm, the ripple spacing was approximately 3 Å.

In this embodiment, the configuration of the optical signal is as shown in FIG. 3, where two wavelengths (wavelength 1 and wavelength 2, shown in FIG. 3(b), (
It consists of two synchronously intensity-modulated components assigned (as shown in c)). As wavelength 1, the wavelength (resonant wavelength) 83 corresponds to one peak of the gain ripple shown in FIG.
00 Å, wavelength 2 is 8301.5 Å, which is the valley of the gain ripple adjacent to wavelength 1 (non-resonant wavelength).
was used.

As mentioned above, FIG. 3 shows an example of the configuration of an input signal composed of two wavelengths, but FIG. 3(a) shows the waveform of the input signal when focusing only on the optical intensity. . In this embodiment, the signal waveforms of the two wavelengths (8300 Å and 8301.5 Å) constituting the input signals shown in FIGS. 3(b) and 3(c) have exactly the same time waveform. If the amplitude at each wavelength 1 and 2 is 1, then the amplitude will be 2 in the signal of FIG. 3(a) in which only the light intensity is focused.

FIG. 4 shows the output waveform from the semiconductor optical amplification device 3. As mentioned above, FIG. 4(b) is the output waveform corresponding to wavelength 1 (8300 Å), and the peak of the gain ripple is Since it matched the wavelength, the intensity was approximately 126 times greater. Moreover, FIG. 4(c) shows wavelength 2 (8301.
The output waveform corresponds to 5 Å), matches the valley of the gain ripple, and has an output of about 79.4 times. As a result, the amplitude of the output waveform in FIG. 4(a), in which only the intensity is focused, is approximately 205 times larger.

In the above, even if the peak wavelength of the gain ripple of the semiconductor optical amplification element 3 is shifted due to disturbance such as heat, since the wavelength dispersion characteristic of the gain ripple shown in FIG. 2 is sinusoidal, each wavelength Although the amplification factors for the signal light components 1 and 2 vary, when attention is paid to the intensity of the signal light composed of these components, an average amplification factor (approximately 20 dB) can always be obtained. In this embodiment, the optical transmitter 1 has two semiconductor lasers whose oscillation wavelengths are stabilized as light sources, and the two semiconductor lasers are directly modulated at the same time according to the signal input to the optical transmitter 1. It was configured to transmit signal light components of wavelengths 1 and 2. Of course, instead of direct modulation, each semiconductor laser may be oscillated with a constant current and an external modulator may be used to form two signal light components modulated according to the input signal.

In this embodiment, one digital signal is transmitted at two wavelengths separated by an interval corresponding to the interval between adjacent peaks (resonant wavelength) and valleys (non-resonant wavelength) of the gain ripple of the semiconductor optical amplification element 3. Of course, if the minimum unit of the constituent wavelengths is two wavelengths separated by an interval corresponding to the interval between the peaks and troughs of the gain ripple, then this unit may be multiple. It may be composed of different wavelengths. How to select the wavelength is the semiconductor optical amplification element 3.
The interval between adjacent peaks and troughs of gain ripples may not be the same, but it may be the interval between peaks and troughs of gain ripples that are appropriately separated.

Next, a second embodiment will be explained. By using a laser that oscillates in multiple longitudinal modes (a so-called multimode laser) as a light source in the optical transmitter 1, the optical transmitter 1 can be configured more easily. In this case, the resonator length of the semiconductor laser may be adjusted so that the longitudinal mode interval of the semiconductor laser in the optical transmitter is half the ripple interval of the semiconductor optical amplification element 3. With such a configuration, by using a semiconductor laser that oscillates a large number of longitudinal modes, it is possible to eliminate the influence of gain ripple fluctuations as in the first embodiment. By using such a multi-mode laser as a light source, it becomes difficult to completely satisfy the condition of forming a set of two wavelengths as described in the first embodiment. However, if there are a very large number of longitudinal modes, the difference from the conditions of the first embodiment is at most one wavelength (the distance between adjacent peaks of gain ripple), which causes almost no problem. That is, in the first embodiment, the required number of wavelengths is 2×n (n is an integer), which is an even number, but in a multimode laser, this may not be an even number, but this hardly matters.

By the way, in order to use the present invention in a more stable situation, the average amplification factor of the traveling wave semiconductor optical amplification element 3 for the wavelength used (the wavelength of the optical signal input to the optical amplification element) must be Equal is more preferable. Furthermore, a more preferable situation would arise if the magnitude of the gain ripple near the used wavelength (difference in amplification factor between the resonant wavelength and the adjacent non-resonant wavelength) were equal.

Although the present invention has been explained above using so-called one-way communication as an example, the scope of application of the present invention is not limited to this, and is applicable to bi-directional optical communication systems, wavelength division multiplexing optical communication systems, and optical LANs. can also be applied. In short, the invention can be applied to any optical communication system that uses traveling wave semiconductor optical amplification elements to prevent optical signal attenuation.

[0023]

As explained above, according to the present invention, in an optical communication system using a traveling wave amplification element, one signal is transmitted between the peaks and troughs of the gain ripple (even adjacent ones) of the semiconductor optical amplification element. Semiconductor optical amplification device It is possible to suppress fluctuations in the amplified light due to fluctuations in the gain ripple.

[0024] Furthermore, it is possible to suppress the influence of gain ripple and use a traveling wave semiconductor optical amplification device manufactured using a technology for manufacturing an antireflection film, which is relatively easy to manufacture, with a larger amplification factor than conventional ones. I can do it.

[Brief explanation of the drawing]

FIG. 1 is a system configuration diagram for explaining an embodiment of the present invention.

FIG. 2 is a diagram showing wavelength dispersion characteristics of the amplification factor of a semiconductor optical amplification element.

FIG. 3 is a diagram showing an example of the configuration of signals used for communication.

FIG. 4 is a diagram showing a configuration example of an output signal of a semiconductor optical amplification element.

FIG. 5 is a diagram showing the wavelength dispersion characteristics of the amplification factor of the optical amplification element when explaining the problems of the conventional example.

6 is a diagram showing the state of the output signal from the optical amplification element in the optical signal of wavelength 1 in FIG. 5 when explaining the problems of the conventional example; FIG.

7 is a diagram showing the state of an output signal from an optical amplification element in an optical signal having a wavelength 2 different from wavelength 1 in FIG. 5 when explaining the problems of the conventional example; FIG.

[Explanation of symbols]

1 Optical transmitter 2 Optical fiber 3. Optical amplifier 4 Optical receiver

Claims (9)

[Claims] Claim 1: In an optical communication system using a traveling wave semiconductor optical amplification device, a transmitted optical signal is affected by the Fabry-Perot resonance phenomenon that appears in the wavelength dispersion characteristic of the amplification factor of the traveling wave semiconductor optical amplification device. It consists of at least two wavelengths of light having a wavelength interval of an arbitrary resonant wavelength and an arbitrary non-resonant wavelength, and is characterized in that these lights are both modulated by the same synchronized signal. optical communication system. 2. The optical signal to be transmitted has an arbitrary resonant wavelength in the Fabry-Perot resonance phenomenon that appears in the wavelength dispersion characteristic of the amplification factor of the traveling wave semiconductor optical amplification device, and two non-resonant wavelengths adjacent to the resonant wavelength. 2. The optical communication system according to claim 1, wherein the optical communication system is composed of at least two wavelengths of light having a wavelength interval from one of the resonant wavelengths. 3. The optical signal to be transmitted has an arbitrary resonant wavelength in the Fabry-Perot resonance phenomenon that appears in the wavelength dispersion characteristic of the amplification factor of the traveling wave semiconductor optical amplification device and two non-linear wavelengths adjacent to the resonant wavelength. Claim 2: The light beam is composed of at least a plurality of sets of wavelengths when one set includes two wavelengths having a wavelength interval from one of the resonant wavelengths.
The optical communication system described. 4. The optical communication system according to claim 1, wherein the wavelength of light constituting the transmitted optical signal is within a gain range of the traveling wave semiconductor optical amplification element. 5. The optical communication system according to claim 1, wherein the optical transmitter that transmits the optical signal is comprised of at least one semiconductor laser that oscillates light of each wavelength. 6. The optical communication system according to claim 5, wherein the semiconductor laser is a longitudinal multimode laser. 7. The optical communication system according to claim 1, wherein the optical intensity of each wavelength constituting the optical signal is equal at any given time. 8. The optical communication system according to claim 1, wherein the average amplification factor of the traveling wave semiconductor optical amplification element at each wavelength constituting the optical signal is equal. 9. A difference in amplification factors of the traveling wave semiconductor optical amplification element between a resonant wavelength and an adjacent non-resonant wavelength near each wavelength constituting the optical signal is equal.
The optical communication system described.