JPH0620987Y2 - Optical spectrum measuring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to an optical spectrum measuring apparatus suitable for use in spectrum analysis of output light of a semiconductor laser.

<Prior art example> As a means for obtaining the spectrum of the output light of a light emitting element such as a semiconductor laser, an optical spectrum measuring apparatus that does not require a local oscillation light source has been proposed. The structure of such an optical spectrum measuring apparatus is shown in FIG. In FIG. 4, the light to be measured is split into two by the optical splitter 1. One of the two split lights is shifted in frequency by the acousto-optic modulator 2, delayed by the other delay element 3, and combined by the optical combiner 4. The combined light is converted into an electric signal by the light receiving section 5, and the spectrum thereof is analyzed by the frequency analyzer 6. The light that has passed through the delay element 3 acts as local oscillation light. If the delay time of the delay element 3 is sufficiently longer than the coherence time of the measuring light, there is no correlation between the light passing through the two optical paths, and the statistical properties of the noise at this time become equal, so the optical combiner 4 The spectrum of the light under measurement can be obtained from the power spectrum of. Such an optical spectrum measuring device is disclosed in, for example, a journal published in 1980 in the Technical Report OQE80-50, "A new high resolution measuring method of the oscillation spectrum of a semiconductor laser (delayed self-heterodyne method)".
It is described as.

<Problems to be Solved by the Invention> However, such an optical spectrum measuring device has the following problems. The characteristics of the elements constituting the device, particularly the frequency shifter, vary greatly depending on the wavelength of the incident light, and cannot be shared with light of different wavelengths. Therefore, in order to measure the spectrum with high accuracy, a measuring device corresponding to each wavelength is required for each wavelength, and there is a drawback that a plurality of measuring devices must be prepared.

<Object of the Invention> An object of the present invention is to provide an optical spectrum measuring device capable of measuring spectra of a plurality of wavelengths with a single device.

<Means for Solving the Problems> In order to solve the above problems, the present invention proposes an optical branching device for branching the measured light, a delay element for delaying one of the output lights of the optical branching device, and the optical An optical demultiplexer that receives the output light of the other side of the branching device and demultiplexes the light according to the wavelength, a plurality of frequency shifters to which the output light of this optical demultiplexer is input, and the output light of these frequency shifters An optical multiplexer that multiplexes the optical multiplexer, an optical combiner to which the output light of the optical multiplexer and the delay element is input, and a light receiving unit that receives the output light of the optical combiner and converts the light into an electrical signal, , And a frequency analyzer to which the output of the light receiving unit is input.

<Operation> Light is demultiplexed by the wavelength by the optical demultiplexer, input to a plurality of frequency shifters corresponding to the demultiplexed wavelengths, the frequencies are shifted, and then multiplexed by the optical multiplexer. Since the elements other than the frequency shifter can be shared, it is possible to measure the spectra of light of a plurality of wavelengths with a single device.

<Embodiment> FIG. 1 shows an embodiment of the optical spectrum measuring apparatus according to the present invention. The same elements as those in FIG. 4 are designated by the same reference numerals,
The description is omitted. In FIG. 1, the light to be measured is split into two by an optical splitter 1. One of the branched lights is demultiplexed by the optical demultiplexer 10 according to the wavelength. In this embodiment, 1.
The light is demultiplexed into two lights of 3 μm band and 1.5 μm band.
The demultiplexed 1.3 μm band light and 1.5 μm band light are input through the lens 11 to the acousto-optic modulators 12 and 13 which are frequency shifters adjusted to the wavelengths of the respective lights.
These acousto-optic modulators 12 and 13 are oscillators 1 respectively.
It is driven by 4 and 15 and shifts incident light by 140 MHz.
A value that is sufficiently larger than the width of the spectrum of the measured light is selected as this shift amount. The output light of the acousto-optic modulators 12 and 13 is input to the optical fiber that is the optical path via the lens 11,
The signals are input to the optical multiplexer 16 and multiplexed. The other output light of the optical branching device 1 is input to the delay element 3. The delay element 3 is composed of a single mode optical fiber having a length of 2 km and delays light by about 10 μs. This delay amount is set to a time sufficiently longer than the coherence time of the measured light.
The output lights of the delay element 3 and the optical multiplexer 16 are input to the optical combiner 4 and combined. The intensity of the output light of the optical combiner 4 is converted into an electric signal by the light receiving section 5, and the spectrum is measured by the frequency analyzer 6. As the optical demultiplexer 10 and the optical multiplexer 16, for example, "single-mode fusion type demultiplexer / multiplexer" sold by Nippon Mining Co., Ltd. can be used.

FIG. 2 shows another embodiment of the present invention. The optical spectrum measuring device shown in FIG. 1 is characterized in that an extremely stable local oscillation light source is not required, but if the polarization planes of the light input to the optical combiner 4 do not match, the light receiving unit 5 There is a drawback that the intensity of the interference light that is input decreases and the S / N ratio of the signal deteriorates. This embodiment is an improvement over these drawbacks. The same elements as those in the embodiment of FIG. 1 are designated by the same reference numerals, and the description thereof will be omitted. In FIG. 2, reference numerals 20 and 21 denote polarization plane controllers, which input the output lights of the optical multiplexer 16 and the delay element 3, respectively, and change their polarization planes. The output lights from the polarization plane controllers 20 and 21 are combined by the optical combiner / splitter 22 and split again. One of the branched lights is converted into an electric signal by the light receiving section 5 as in the embodiment of FIG. 1 and its spectrum is analyzed by the frequency analyzer 6. The other output light from the optical combiner / splitter 22 is input to the light receiving unit 23. The light receiving unit 23 is composed of a deflection prism and a photodiode, and receives only a specific polarized component of the input light and converts it into an electric signal. This electric signal is sent to the control unit 2
4 is input to the polarization plane controllers 20, 21 so that the input signal from the light receiving unit 23 has an extreme value.
To control. By doing this, the optical merging / branching device 22
Since the polarization planes of the two input lights coincide with each other, the intensity of the signal becomes strong, and the measurement with a good S / N ratio can be performed.

FIG. 3 shows an example of the configuration of the polarization plane controllers 20 and 21. In FIG. 3, reference numeral 30 is a twisted fiber polarization element composed of a loop of a single mode optical fiber, and two twisted fiber polarization elements are connected in series. When the optical fiber is wound into a small diameter, stress is applied to the inside of the optical fiber, so that the anisotropy of the refractive index is caused by the photoelastic effect, and the same effect as the wave plate can be provided. A fixture 32 is fixed to the twist fiber polarization element 30. Reference numeral 31 is a stepping motor, the shaft of which is connected to the fixture 32. As can be seen from FIG. 3A viewed from the X direction, when the stepping motor 31 rotates, the twist fiber polarization element 30 rotates together with the fixture 32 to change the polarization plane of the incident light. If the control unit 24 controls the stepping motor 31 so that the output of the light receiving unit 23 takes the extreme value, the polarization planes of the input light of the optical merging / branching device 22 can be matched. The means for changing the polarization plane of the light, not only the twisting fiber polarization element, element utilizing pressure birefringence generated when a pressure is applied to the fiber, the lithium niobate (L i _N b _{O 3)} An element that uses such an electro-optic crystal, an element that rotates a phase compensation plate such as a λ / 4 wavelength plate, an element that uses the Faraday effect by the magnetic field of a single mode fiber, and the like can be used.

In addition, in this embodiment, the optical branching device 10 is used to obtain 1.3 μm.
And 1.5 μm band into two wavelength bands, but may branch into three or more wavelength bands, 1.3 μm and 1.5 μm.
The wavelength is not limited to m and may be branched to any wavelength.

Further, although the light input to the acousto-optic modulators 12 and 13 is branched, the light input to the delay element 3 may also be branched.

Further, although the polarization plane controllers 20 and 21 are inserted after the optical multiplexer 16 and the delay element 3 in the embodiment of FIG. 2, they may be inserted before.

Further, although the polarization plane controllers 20 and 21 are installed on both of the optical paths branched by the optical branching device 1, they may be installed on only one of them. In this case, the control unit 24 controls the polarization plane controllers 20 and 21 so that the power spectrum of the frequency components shifted by the acousto-optic modulators 12 and 13 of the light receiving unit 5 becomes maximum.

Further, although the acousto-optic modulator is used as the frequency shifter, a frequency shifter having another structure, for example, an element having a structure for diffracting by applying a high frequency voltage may be used. The point is that it can shift the required frequency.

<Effect of Device> As described above in detail with reference to the embodiments, in this device, the light input to the frequency shifter is branched according to its wavelength and then combined again. Therefore, it is possible to obtain the spectra of light having a plurality of wavelengths with one device. This has the effect of eliminating the need to prepare a plurality of measuring devices corresponding to the respective wavelengths.

[Brief description of drawings]

1 and 2 are block diagrams showing an embodiment of an optical spectrum measuring apparatus according to the present invention, FIG. 3 is a block diagram showing a configuration of a polarization plane controller, and FIG. 4 is a conventional optical spectrum measuring apparatus. It is a block diagram which shows a structure. 1 ... Optical branching device, 2, 12, 13 ... Acousto-optic modulator,
3 ... Delay element, 4 ... Optical combiner, 5, 23 ... Light receiving part, 6 ... Frequency analyzer, 10 ... Optical demultiplexer, 16 ...
Optical multiplexer, 20, 21 ... Polarization plane controller, 22 ...
... Optical merging / branching device, 24 ... Control unit, 30 ... Twist fiber polarization element, 31 ... Stepping motor, 32 ...
…Fixture.

Claims (1)

[Scope of utility model registration request] 1. An optical branching device for branching a light to be measured into two, a delay element for delaying one of the output lights of the optical branching device, and the other output light of the optical branching device are inputted, and their wavelengths are inputted. An optical demultiplexer that demultiplexes the incident light, a plurality of frequency shifters to which the output light of this optical demultiplexer is input, an optical multiplexer that receives the output light of these multiple frequency shifters, and this optical multiplexer. An optical combiner to which the output light of the wave multiplexer and the output light of the delay element are input, and a light receiving unit to which the output light of the optical combiner is input,
An optical spectrum measuring apparatus, comprising: a frequency analyzer to which the output of the light receiving section is input.