JPH036431A - Measuring apparatus for light frequency modulation characteristic - Google Patents

Abstract

PURPOSE:To enable execution of stable measurement and to lessen the number of handling processes by using a polarization retaining optical fiber which can be obtained simply, and by conducting a control. CONSTITUTION:A polarization controller 1 conducts a control so that a polarization may form an angle of 45 degrees to either one of polarization retention axes of a polarization retaining fiber 2. Besides, a polarized beam splitter 3 is provided so that a light be divided into components of X and Y axes and taken out simultaneously. Moreover, photoelectric converters 4 and 5 are connected in series so that a potential on the anode side of the converter 4 and a potential on the cathode side of the converter 5 to added up, as an electric circuit means of which an output signal is a difference between output electric signals of the converters, and an output of this difference is sent out to an output terminal 7 through an amplifier 6. The fiber 2 is provided with a piezo element 8 as a means for controlling a difference between optical paths of the X and Y axes, and a control circuit 8 controlling a voltage of the element 8 in accordance with the output of the difference between the converters 4 and 5 is connected to the element. Since there is a difference in length between the polarization retention axes X and Y of the fiber 2, interference occurs in two output lights of the splitter 3.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention is used for measuring optical communication equipment. The present invention is used to measure the modulation characteristics of a frequency-modulated optical signal. The present invention is suitable for measuring the modulation characteristics of a frequency-modulated optical signal for coherent optical communication emitted from a semiconductor laser.

[Conventional technology]

Frequency modulation is known to be superior in coherent optical communications. Moreover, for this purpose, a simple transmitting circuit can be obtained by using a frequency modulated signal obtained by directly modulating a semiconductor laser. However, the frequency modulation signal of the optical signal obtained by direct modulation of a semiconductor laser contains an amplitude modulation component as well as a frequency modulation component, so a measurement method that is less affected by the amplitude modulation component is required to measure the frequency modulation characteristics. .

Conventionally, a Matsuhatsu Enda interferometer has been used for such measurements. Figure 3 shows a configuration diagram of the measurement system. Second
The figure is a characteristic diagram showing the human power light frequency and the output light intensity.

The horizontal axis represents the optical frequency, and the vertical axis represents the optical intensity of the output light.

If a point a with a large slope of this characteristic curve is selected and input light frequency-modulated with the frequency fa at this point a as the center frequency is applied to the Matsuhatsu Enda interferometer, the input light is divided into two parts, and the two light beams are Since an optical path difference is given to cause interference and the frequency modulation component is converted into a change in intensity, two optical signals converted into changes in light intensity as shown by the solid line and broken line in FIG. 2(a) are obtained. The amplitude modulation component can be canceled by converting the two optical signals converted into light intensity into voltage signals by photoelectric conversion elements and taking the difference between the two. This is shown in FIG. 2(b).

By observing this differential electrical signal, frequency modulation response characteristics can be observed. The optical path difference changes due to changes in the environment such as temperature changes, and the point corresponding to the frequency fa is no longer a position where the slope of the characteristic curve is large.

Therefore, I installed a heater on the Matsuhatsu Enda interferometer.
The configuration is such that the optical path effectively changes according to the temperature heated by the heater, and the optical path difference changes. In order to avoid being affected by the frequency modulation component, a portion of the differential electrical signal is manually input to the control circuit through a low-pass filter, and the heater is controlled so that the differential electrical signal always has an average zero potential.

Another possible method is to use a polarization maintaining fiber. Polarization maintaining fiber maintains polarization2
There are two orthogonal axes. These are respectively referred to as the X axis and the Y axis. When light passes through the X and Y axes, it propagates at slightly different propagation times. Light is input so that the polarization of the light is at a 45 degree angle with respect to the X-axis and Y-axis of the polarization-maintaining fiber, and the X-axis and Y-axis components of the polarization of the light propagate at different propagation times. do. A polarization analyzer is placed at the optical output end of the polarization-maintaining fiber so that the polarized light is emitted at an angle of 45 degrees with respect to one polarization-maintaining axis of the polarization-maintaining fiber. This emitted light signal is converted into a voltage signal by a photoelectric conversion element.

[Problem that the invention seeks to solve]

However, the method using the Matsuhatsu Enda interferometer requires an optical waveguide equipped with a heater for stable measurement, but the guide waveguide is difficult to process and is not easily available.

Furthermore, in the method using the polarization-maintaining optical fiber, although polarization-maintaining optical fibers are easily available, the above configuration makes control impossible and stable measurement impossible. Moreover,
Since the intensity modulation component is also measured at the same time during measurement, the intensity modulation component must be removed by some method. As a way to solve this problem, the angle of incidence on the individual fibers is set to 9.
A possible method is to rotate the sensor by 0 degrees, measure it again, and extract only the frequency modulation component from the remeasurement results. However, this requires two measurements and their processing, which is very time consuming.

The present invention solves this problem by using a polarization-maintaining optical fiber that can be easily carried out by hand, and by controlling it, it enables stable measurement. The purpose is to provide

[Means for solving problems]

The present invention provides a polarized beam so that two orthogonal polarization components forming an angle of approximately 45 degrees with respect to the X-axis or Y-axis of the polarization-maintaining optical fiber can be extracted from the optical output end of the polarization-maintaining optical fiber. The present invention is characterized in that a splitter is provided, a photoelectric converter is provided at each output port from the polarization beam splitter, and an electric circuit means is provided that outputs a difference between the output electric signals of the two photoelectric converters.

Further, a means for controlling the effective optical path length of the X-axis or Y-axis of the polarization-maintaining optical fiber is provided, and this means is controlled manually in response to the difference in the output electrical signals, and particularly preferably, the time average value of the difference is zero. It is possible to provide a control circuit such that:

[Effect]

The two output optical ports of the polarization beam splitter provide intensity signals having different phases with respect to the frequency change of the human-powered light. Further, the influence of the amplitude change of the input light appears as is on the two output light ports of the polarization beam splitter. Therefore, by subtracting the signals appearing on these two light output boats, the influence of the amplitude change of the human-powered light is removed, and the intensity change with respect to frequency change is doubled. Further, a means for controlling the effective optical path length of the X-axis or Y-axis of the polarization-maintaining fiber is provided, and the means is provided with a control input corresponding to the difference in the output electrical signals, particularly preferably a time average value of the difference is zero. A control circuit such as the following can be provided.

The above method of receiving the optical outputs of the two output ports and taking the difference can also be adopted for measurement methods using polarization maintaining fibers, because the method is applicable to each of the X and Y axes of the polarization maintaining fiber. This is possible by providing a polarization beam splitter so that two orthogonal polarization components forming an angle of 45 degrees can be extracted simultaneously. This will be explained using a formula.

The input port signal is expressed by the following equation.

5(t)=Acos (2yr f 10Φ(t))(
1) Here, A is the electric field of light, f is the frequency of light, Φ(1)
is a frequency modulated signal. The X-axis and Y-axis signals of the polarization-maintaining fiber are respectively S + (t) = Acos (2πft + Φ (t))
/ -ff (2) S2(t)=Acos (2y
r f (t + r) + Φ (t + τ) / VT
(3) Here, τ is the time difference when the optical signal passes through the X-axis or the Y-axis and is output from the polarization-maintaining fiber. Since the polarization beam splitter separates and extracts the two orthogonal polarization components forming an angle of 45 degrees with respect to the X-axis or Y-axis, each 5s(t) = Acos (2rr f t+Φ(t)
)/2-ACO9(2πf(t+τ) +Φ←t+τ)/2 (4) S4(t)=Acos (2yr f t+Φ(t)
)/2 +Acos (,2rr f (t + r
) 10Φ(t+τ)/2(5) These are electric fields, so when they are detected by a light receiving element, 5s(t)2=A2/4-A2/4 xcos(2πfτ0Φ(10τ)-Φ( t)) (6) S6(t)2=A2/4 +A2/4 xcos(2πfτ0Φ(t+τ)−Φ(t)) [7]. That is, upon reception by each of the two output ports, the intensity modulation component included in A2 is also measured at the same time. However, if we take the difference between the two outputs, S s (t)2S s (t)2 A2/2CO3(2πfτ+Φ(10τ)−Φ(t))
(8) Therefore, the intensity modulation component included in the first term in equations (6) and (7) is removed. However, the second term component cannot be removed and remains even if the difference is taken, but this is sufficiently small to cause no problem.

〔Example〕

FIG. 1 is a block diagram of an apparatus according to an embodiment of the present invention. This device includes a polarization controller 1 that controls the polarization state of frequency-modulated light to be measured as input light, a polarization-maintaining fiber 2 into which the output light of the polarization controller 1 enters, and a polarization-maintaining fiber. A polarization beam splitter 3 separates and extracts the output light of the output light into two orthogonal polarization components, ie, X'-axis and Y'-axis direction components, forming an angle of 45 degrees with respect to the X-axis and Y-axis, respectively, and It includes photoelectric converters 4 and 5 that convert the intensity of the output light from the beam splitter 3 into electrical signals. The polarization controller 1 has four polarization controllers for either polarization maintaining axis of the polarization maintaining fiber 2.
Control it so that it forms an angle of 5 degrees. The feature is that the polarization beam splitter 3 is provided so as to separate the X'-axis component and the Y'-axis component of the light and take them out at the same time. This photoelectric converter 4 is used as an electric circuit means for outputting the difference between the output electric signals of the two photoelectric converters 4 and 5, so that the potential on the anode side and the potential on the cathode side of the photoelectric converter 5 are added. The differential output is sent to the output terminal 7 via the amplifier 6.

Further, a piezo element 8 is provided in the polarization maintaining fiber 2 as a means for controlling the difference in optical path length between the X axis and the Y axis.
A control circuit 9 is connected to the control circuit 9 for controlling according to the differential output.

Since there is a difference in effective length between the polarization-maintaining axis xs*i and the Y-axis of the polarization-maintaining fiber 2, such that the difference in optical propagation time is τ, the two polarization-maintaining axes of the polarization beam splitter 3 Interference occurs in the output light. Therefore, between the outputs of the two photoelectric converters 4 and 5, the second
A frequency light intensity characteristic as shown in the figure is obtained. In FIG. 2, the solid curve is the output characteristic of the photoelectric converter 4, and the broken curve is the output characteristic of the photoelectric converter 5.

In Figure 2, the slope of this curve is almost uniform at point a
If a frequency-modulated optical signal with a center frequency of frequency f6 corresponding to this point a is given to port 10 as a human-powered light, the output signals of photoelectric converters 4 and 5 will be at the frequency of this input light. According to the change, the output changes as shown in FIG. 2(a). Since this change in intensity is in opposite directions for the two photoelectric converters 4 and 5, by subtracting them, the amplitude is doubled as shown in FIG. 2(b).

In this embodiment, the optical path is effectively changed according to the pressure applied by the piezo element 8, and the above-mentioned time τ is changed. That is, the signal of the difference between the two photoelectric converters 4 and 5 is given to the control circuit 9 manually. The control circuit 9 is provided with a control circuit 9 which controls the difference signal applied to this input so that it always has zero potential on average.

Therefore, the center frequency f6 of the signal under test shown in FIG.
Even if the line changes, point a will follow the line exactly at the intersection of the solid line and the broken line.

In the above example, the piezo element 8 is used in the polarization-maintaining fiber 2 as a means for changing the optical path length difference between the X-axis and the Y-axis. The invention may similarly be practiced using various methods of varying the effective optical path difference of the axes.

The present invention will work even if the angle with respect to the polarization maintaining axis is not exactly 45 degrees but is slightly shifted.

1 2 [Effects of the Invention] As described above, it is possible to obtain a device that can measure optical frequency modulation characteristics with a simple circuit using a polarization-maintaining optical fiber that can be easily obtained. This has the effect of being simpler and more stable than using a Matsuhatsu Enda interferometer.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram of a conventional device. FIG. 2 is a waveform chart for explaining the operation of the conventional device and the device of the present invention. FIG. 3 is a configuration diagram of an apparatus according to an embodiment of the present invention. DESCRIPTION OF SYMBOLS 1...Polarization controller, 2...Polarization maintaining fiber, 3...
...Polarization beam splitter, 4.5...Photoelectric converter,
6... Amplifier, 7... Output terminal, 9... Optical path length control circuit, 11... Semiconductor laser,
12...Mirror 13...Motor. 3

Claims (1)

[Scope of Claims] 1. A polarization-maintaining optical fiber; a polarization controller that makes the light to be measured enter the polarization plane at approximately 45 degrees with respect to one polarization-maintaining axis of the polarization-maintaining optical fiber; A polarization beam splitter that splits the output light of the polarization-maintaining optical fiber into components in two polarization directions, and two photoelectric conversion units that convert the intensities of the two output lights of the polarization beam splitter into electrical signals, respectively. What is claimed is: 1. A measuring device for measuring optical frequency modulation characteristics, comprising: a detector; and a circuit for detecting the difference between the outputs of the two photoelectric converters. 2. The apparatus for measuring optical frequency modulation characteristics according to claim 1, further comprising means for controlling the propagation speed of the two polarization-maintaining axial components in the polarization-maintaining optical fiber so that the difference becomes zero.