JPH02230221A - optical bias shifter - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical functional device, and particularly to an optical bias shifter that lowers a low signal light level in a signal component and increases the signal-to-noise ratio of the signal. [Prior technology] In optical communication, there are intensity modulation methods that add ON and OFF signals by changing the intensity of light, and coherent light that uses optical coherency to add ON and OFF signals to the frequency and phase of light. Frequency modulation and phase modulation methods are used for communication.

In intensity modulation, a semiconductor laser is used as a signal light source, and by adding a signal current to its input current, the intensity of the output light of the semiconductor laser is modulated. Intensity-modulated laser light is transmitted through an optical fiber and directly restored to an electrical signal by a receiver. In order to increase the modulation speed of the light intensity of a semiconductor laser, it is necessary to lower the capacitance and resistance of the semiconductor laser, and to reduce the carrier density fluctuation in the amplification and attenuation process of carriers (electrons, holes) injected into the semiconductor laser and excitation photons. It is necessary to increase the relaxation oscillation frequency caused by the tracking time lag. The former is improved by changing the structure. The latter has the following characteristics. The relaxation oscillation frequency fr is proportional to the square root of the optical output Pa.
Because of this large light output! ．． can be expensive. but,
In order to improve the S/N of intensity modulation, it is necessary to turn on the light intensity.
In order to make the OFF state clearer, it was necessary to input and modulate the signal current from around the oscillation threshold current value, where the optical output is almost zero. Currently, a bias point is set near this threshold and used. In coherent optical communication, a signal is placed on the frequency and phase of light, and the signal light transmitted through an optical fiber is interfered with the DC light provided in the receiving system.
At this time, a frequency or phase signal component appears as an intensity change in the form 2EsE-, which is the product of the electric field Es of the signal light and the electric field E- of the DC light, and this is converted into an electric signal by the optical receiver. Therefore, the greater the electric field strength E- of the DC light is, the greater the intensity conversion of the signal component becomes, and there is an advantage that the S/N ratio is improved over the above-mentioned intensity modulation method. In addition, optical repeaters using optical amplifiers. Since there is no need to convert the signal light into an electrical signal, the configuration is extremely simple. [Problems to be solved by the invention] However, when attempting to modulate at a higher speed or increase the S/N ratio, the following problems occurred. In the intensity modulation method,
Conventionally, there was no element or method for lowering the DC light output, and in the end,
A bias was applied to the low optical output near the threshold current, and modulation was applied from there. For this reason, the high relaxation oscillation frequency in the high power range was limited by the low relaxation oscillation frequency in the low power range, and the modulation speed could not be increased much. Also,
In a coherent optical communication system, when the electric field E- of the DC interference light is increased in order to increase the output of the interference signal, the intensity IE-1" of this DC light itself increases, and the dynamic range of the amplifier in the light receiving system deteriorates. In addition, in optical repeaters using optical amplifiers, when no signal light enters, the spontaneous emission light emitted by the optical amplifier itself becomes a noise source and lowers the S/N. [Means for solving the problem] In order to use only the high relaxation oscillation range with the intensity modulation method and obtain a high-to-low power ratio of the signal output light, the optical output bias point at low signal output should be set to a semiconductor. Just set the laser to a high output point, and set the high signal output point to an even higher output point.Then, with respect to this output signal, reduce the bias of the low signal output, and reduce the difference between high and low signal outputs and the low signal output. In other words, it is sufficient to increase the ratio of the DC bias light output and the duty ratio.Also, in coherent optical communication, similarly, the output of the DC light intensity is reduced, and the output ratio of the interference output and the DC intensity is reduced. In other words, the duty ratio can be increased.The above means that in the case of intensity modulation method, the modulated optical output is This is achieved by inserting a saturated absorber that is set to have an absorption saturation region at high optical power levels.

In optical repeaters, the signal-to-noise ratio is improved by lowering the spontaneous emission component of the optical amplifier. [Operation] Figure 1 shows the relationship between the input light and the output light to the saturable absorber when a saturable absorber is used. The operating principle is explained below. In a saturable absorber, the absorption coefficient is large due to the sufficient absorption factor acting on weak optical input, and there is almost no output light. However, as the optical input increases, there is a shortage of absorption factors, so the absorption coefficient of light decreases and output light appears. This is a manifestation of absorption saturation, and the stronger the optical input, the more the output optical intensity increases as shown in the superlinear curve 1-1 in the figure. Now, O with DC bypass light intensity 11
When the N-OFF signal light 2 is input to this saturable absorber, the low signal level 11 is largely absorbed, and the optical output intensity 12 becomes small. On the other hand, at high signal levels,
Absorption is reduced and the output light level remains high to obtain an overall output light of 3. The ratio of the signal modulation component 14 of the output light 3 and the DC bias light component 12 is improved relative to the ratio of the signal modulation component 13 of the input light and the DC bias light component 11. At this time, the ratio of the modulated input light to the output light is much larger than the absorption attenuation of the DC component because the high-level absorption coefficient is small. [Example] Example 1 will be explained with reference to Figures 1 and 2. FIG. 2 is a schematic diagram of an optical communication system according to an embodiment of the present invention. The current signal from the signal current generator 31 is added to the DC bias to intensity-modulate the optical output of the semiconductor laser 32, and after transmitting it through the optical fiber 34, it is restored to an electrical signal by the optical receiver 35, and then converted into an electrical signal by the amplifier 36. Amplify the signal output. This embodiment is a system in which a saturable absorber 33 is inserted into such an intensity modulation type optical communication system. This saturable absorber 33 was fabricated by providing a non-reflection coating 37 of SiO on a Fabry-Bello type semiconductor laser having an oscillation wavelength of 1.57 μm. The oscillation wavelength of the semiconductor laser is 1.55 μm, and a signal from the signal generation source 31 is added with a DC bias applied.
Figure 1 shows the relationship between the optical input from the semiconductor laser 32 to the saturable absorber 33 and the optical output from the saturable absorber 33 under the condition that the current injected into the saturable absorber 33 is O. It is. The DC bias component 11 of the input optical signal 2 is set to 1 so that it is slightly lower than the threshold value of the input/output characteristic 1-1 of the saturable absorber.
It is set to mW. The signal component 13 of the input light is in the saturable region of the bathing force characteristic 1-1. 2mW as signal component
Add equivalent. As a result, the DC component 12 of the output optical signal 3 from the saturable absorber is 50 μW, and the signal component is ~1
．． It became 1mW. When a signal 2' with a small DC component of the input light is input, by adding an injection current to the saturable absorber 33. The saturable absorption characteristics can be shifted as shown in the curve ml-2. This makes it possible to match the saturable absorption point to the intensity of the DC input light and prevent signal component deterioration. here,
Characteristic 1-2 is the injection current value. This is the input and output light intensity characteristics of the saturable absorber 33 when 2 mA is applied. Furthermore, in the configuration shown in FIG. 2, it is easy to monolithically bond the optical bias shifter using a distributed feedback semiconductor laser for the semiconductor laser 32 and omit the anti-reflection coating on the input side of the optical bias shifter. Example 2 The second example will be explained using FIG. Figure 4 shows the basic configuration of a coherent optical communication system in which the optical bias shifter of the present invention is introduced. The current signal from the signal current generator 31 is added to the DC bias to modulate the frequency of the optical output of the semiconductor laser 32. This modulated signal light is transmitted via the optical fiber 34 through the optical isolator 38a, and is transmitted to the optical fiber 34 of the receiving system. The coupler 39 causes interference with the DC output light of the local oscillation semiconductor laser 40 that has passed through the optical isolator 38b. In the interference light, the signal intensity is superimposed on the direct current intensity of the locally oscillated light as a signal light component interfering with the signal light. This interference signal light is applied to the Fabry-Perot laser with anti-reflection coating 3.
A saturable absorber 33 which is an optical bias shifter provided with 7
Figure 4 shows the characteristics of the outgoing light incident on the optical bias shifter 33 at this time. For the interference signal light 4, set the DC component to the saturable absorption point of the saturable absorption characteristic 1-1. As in the first embodiment, the direct current component of the output light 5 is compressed. Embodiment 3 FIG. 5 shows, as a third embodiment, the configuration of an optical system when the optical bias shifter of the present invention is used in an optical repeater. The configuration of this embodiment is such that a saturable absorber 33 as an optical bias shifter for waveform shaping is inserted into an optical repeater using an optical amplifier 41 and an optical filter 42. The intensity of the signal light propagated through the optical fiber 34 is amplified by the optical amplifier 41. Since the optical amplifier 41 also contains a large amount of spontaneous emission light, the filter 42 cuts off the spontaneous emission light of wavelengths unrelated to the signal light. Furthermore, by inserting a saturable absorber 33, which is an optical bias shifter, it is also possible to cut out the spontaneous emission light of the optical amplifier that has passed through the filter. The operation is shown in Figure 6. The temporal change 6 of the output light from the optical filter 42 includes spontaneous emission light noise from the optical amplifier at low signal levels. Using this as the input/output optical characteristic, curve 1
-2, the output waveform 7 absorbs low-level noise and is shaped to increase the S/N. The optical filter 42 and the optical bias shifter 33 may be replaced, or the noise component of the spontaneous emission light of the optical amplifier can be considerably suppressed by omitting the optical filter and using only the optical bias shifter. [Effects of the Invention] According to the present invention, it is possible to change the amount of DC bias of light intensity, so it is possible to remove DC components and suppress noise included in low signal levels, which is useful for optical communication and optical information processing systems. The S/N ratio can be increased.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing input/output optical characteristics and signal light bias settings of the present invention, FIGS. 2 and 3 are schematic diagrams showing the basic configuration of an optical communication system according to an embodiment of the present invention, and FIG. The figure is a characteristic diagram showing the relationship between input and output light when the present invention is implemented in a coherent optical communication system, Figure 5 is a schematic diagram showing the basic configuration when the present invention is implemented in an optical repeater, and Figure 6 is a diagram showing the basic configuration when the present invention is implemented in an optical repeater. It is a characteristic diagram showing the relationship between input and output light when implemented in an optical repeater. 1-1... Input/output optical characteristics when bias current is zero, 1-2... Input/output characteristics when bias current is injected, 2.
...Input signal when bias current is zero, 2'...Input signal when bias current is injected, 4...Input signal to optical bias shifter in frequency modulation coherent interference system, 5
... Output signal, 6... Input signal to optical bias shifter in optical repeater, 7... Output signal, 11... DC component of input signal, 12... DC component of output signal, 13
... Signal component of input signal, 14... Signal component of output signal, 31... Signal current source, 32... Semiconductor laser light source, 33... Optical bias shifter 34... Optical fiber, 35 ... Light receiver, 36... Electric signal amplifier, 37... Anti-reflection coating film, 38... Optical isolator, 39... Optical coupler, 40... Local oscillation laser light source, 41... Optical amplifier, 42... Optical filter b^Jinkoji 1) to Fengku Bongkou Rout

Claims (1)

[Claims] 1. In a saturable absorber having a region where the absorption coefficient decreases with respect to the amount of incident light, when signal light having a high light amount level and a low light amount level is input as the incident light, the absorption coefficient decreases with respect to the low light amount level. An optical bias shifter characterized in that the amount of incident light at the saturation point of the saturable absorber is set so that the absorption region of the saturable absorber corresponds to a high light amount level. 2. An optical bias shifter characterized by being able to adjust the saturation absorption point for input light by injecting a current. 3. An optical system that uses an optical bias shifter that inserts a saturable absorber into the optical path and increases the ratio of the intensity modulation component and the DC component of the signal light. 4. Optical repeater using optical amplifier and saturable absorber.