JPH0789147B2 - Distance measuring device - Google Patents

Description

The present invention relates to an interferometric distance measuring device using two types of light having different wavelengths.

(Prior Art) Various methods have been proposed for distance measuring devices using light. One of them is an interferometry method using two types of light having different wavelengths. FIG. 4 shows an optical system of an example of a conventional distance measuring device by an interferometry method using a semiconductor laser as a light source.

This distance measuring device has two semiconductor lasers 20a and 20b. Of the laser light of wavelengths λ ₁ and λ ₂ (λ ₁ ≠ λ ₂ ) emitted from the two semiconductor lasers 20 a and 20 b, the laser light of wavelength λ ₁ passes through the λ / 2 plate 21 and is used as a polarization separation prism.
The laser beam having the wavelength λ ₂ is incident on the polarization separation prism 22 after being reflected by the mirror 23. The laser light of wavelengths λ ₁ and λ ₂ combined by the polarization separation prism 22 is divided into _{two by} the half mirror 24, one of which is irradiated on the object to be measured, and the other is incident on the polarization separation prism 25 as reference light. It The reflected light reflected by the measurement object is incident on the polarization separation prism 25 through the λ / 2 plate 26.

The reflected light and the reference light that have entered the polarization separation prism 25 are
The light of wavelength λ _{1 and} the light of wavelength λ ₂ are respectively demultiplexed, the reflected light of wavelength λ ₁ interferes with the reference light, and the reflected light of wavelength λ ₂ interferes with the reference light. Interfering light of wavelength λ ₁ and wavelength λ ₂
The interference light of the photodiode 27 and the photodiode 28
Are converted into electric signals respectively. Both electric signals are input to the phase difference meter 29. The phase difference meter 29 measures the phase difference between the interference light of wavelength λ _{1 and} the interference light of wavelength λ ₂ .

The optical path of the reflected light is longer than the optical path of the reference light by an amount corresponding to twice the distance from the distance measuring device to the measurement object. As a result, between the reflected light and the reference light, wavelengths λ ₁ and λ ₂
A phase difference occurs for each of the. The phase difference for light of wavelength λ ₁ is θ ₁ , and the phase difference for light of wavelength λ ₂ is θ
₂ and the optical path difference between the reflected light and the reference light is L,
The following relational expression holds.

Since the value on the left side of equation (1) can be obtained with the phase difference meter 29,
The optical path difference L is obtained based on the equation (1).

(Problems to be Solved by the Invention) In the distance measuring device of FIG. 4, two semiconductor lasers are required to obtain two light beams having different wavelengths. Further, a λ / 2 plate or a polarization separation prism is required as a demultiplexing means for causing the reflected light from the measurement object to interfere with the reference light from the semiconductor laser that emitted the reflected light.
Since the conventional distance measuring device requires a large number of light emitting elements and optical components as described above, the structure is complicated and the cost is high, and the integration is very difficult. A Fabry-Perot type semiconductor laser is usually used as the light emitting element. Since the Fabry-Perot type semiconductor laser is apt to change the oscillation wavelength due to the temperature change and the like, the conventional distance measuring device using this has a difficulty in measuring accuracy.

The present invention has been made in view of such a current situation,
The object is to provide a distance measuring device having a simple structure, a small number of parts, and easy integration.

(Means for Solving the Problem) A distance measuring device of the present invention is a distance measuring device by an interferometry method using two types of light having different wavelengths, and is provided as a light source of the two types of light, Laser oscillation means that simultaneously oscillates in TE mode and TM mode with different oscillation wavelengths, branching means that branches the two kinds of light from the laser oscillation means into a plurality of optical paths, and branching means branches into one optical path. Irradiation means for irradiating the measurement object with the two types of light, light receiving means for receiving reflected light of the two types of light emitted by the irradiation means from the measurement object, and the other optical path by the branching means. The phase difference between the TE mode light and the TM mode light is detected from the two kinds of light branched into two and the two kinds of light from the light receiving means, and the detected TE mode light is detected. Light and each of the TM mode light By detecting the difference of the difference, and a phase difference detecting means for measuring the distance to the measurement object, it is possible to achieve the above object by its.

The distance measuring device of the present invention is preferably a laser generating means that simultaneously oscillates in a TE mode and a TM mode whose oscillation wavelengths are different from each other, and the light emitted from the laser generating means and the reference light and the reference light. Means for splitting the light into TE and TM mode light contained in the reflected light from the object to be measured and TM mode light and TM mode light contained in the reference light, respectively. And means for obtaining TM mode interference light, means for detecting the TE mode interference light and the TM mode interference light, and the TE mode interference light and the TM mode interference light based on the signal output from the detection means Means for detecting the phase difference between the two are provided.

Function The present invention is provided with the branching means, which allows the laser oscillating means
Divide light of various types into multiple optical paths. The two kinds of light branched to one optical path by the branching means are applied to the measurement object by the irradiation means. The reflected light from the measurement object of the two types of light emitted by the irradiation means is received by the light receiving means. On the other hand, the phase difference detecting means detects each phase difference for each of the TE mode light and the TM mode light from the two kinds of light branched to the other optical path by the branching means and the two kinds of light from the light receiving means. Then, the distance to the measurement object is measured by detecting the difference in each phase difference between the detected TE mode light and the detected TM mode light.

(Examples) The present invention will be described below with reference to Examples.

FIG. 1 shows an optical system of an embodiment of the present invention. In this embodiment, the optical system is premised on integration, and optical components are connected by optical waveguides. Semiconductor laser 1
Is a TE with a predetermined injection current value and different oscillation wavelengths.
(Transverse-Electric) mode and TM (Transverse-Electric)
Distributed feedback type (DFB) that oscillates simultaneously in Magnetic mode
It is a laser. The DFB laser used as the semiconductor laser 1 exhibits current-output characteristics as shown in FIG. 2, for example. The DFB laser also has the advantage that the fluctuation of the oscillation wavelength due to changes in the environment such as temperature is small compared to the Fabry-Perot type laser used in the conventional distance measuring device. The semiconductor laser 1 is not limited to the DFB laser, and may be any laser capable of simultaneously oscillating in the TE mode and the TM mode, and may be, for example, a distributed reflection (DBR) laser.

Normally, in distributed feedback (DFB) laser devices, transitions occur between TE and TM polarization modes (both distributed feedback modes) depending on the operating temperature, and both TE and TM modes oscillate simultaneously. There is no. However, the oscillation probability of each polarization mode can be controlled by changing the mirror loss on the end face of the element. FIG. 5 shows the mirror loss dependence of the oscillation probability of each polarization mode with a constant element length. This mirror loss can be changed by changing the state of the end face coat.

As shown in Fig. 5, when the mirror loss is set to about 250 cm ^-1 as shown by the arrow, the oscillation probabilities of TE mode and TM mode are almost equal, and both TE mode and TM mode oscillate simultaneously. Can be made.

FIG. 6 shows the actual operating temperature dependence of the oscillation wavelength of the laser device. As shown in FIG. 6, the operating temperature is 30-35.
Within the temperature range of ° C, both TE and TM modes with different oscillation wavelengths oscillate simultaneously. As shown in FIG. 6, the wavelengths used are TE mode and TM mode oscillated lights, and oscillated lights having slightly different wavelengths are used.

Here, TE mode and TM mode are defined as the TE mode, which is the polarization component in which the electric field component changes in the direction perpendicular to the bonding surface of each layer in the laser element, and is parallel to the bonding surface. The polarization component whose electric field component changes in various directions
It is defined as a mode.

The light of two types of modes emitted from the semiconductor laser 1 is
The Y branch 2 of the optical waveguide splits the emitted light and the reference light. The emitted light is focused into parallel rays by the collimator lens 3 and is applied to the measurement object. The reference light is the first
It is incident on the TE-TM mode splitter 4. The reflected light generated by irradiating the measurement object with the emitted light is focused into parallel rays by the collimator lens 5 and is incident on the second TE-TM mode splitter 6.

The first TE-TM mode splitter 4 converts the incident reference light
Split into TE wave and TM wave. The second TE-TM mode splitter 6 splits the incident reflected light into a TE wave and a TM wave.
TE-TM mode splitter 4, 6 of reference light after demultiplexing
The E wave and the TE wave of the reflected light are combined in the Y branch 7 of the optical waveguide to cause interference, and become TE wave interference light and enter the photodiode 8. Similarly, the TM wave of the reference light and the TM wave of the reflected light also combine at the Y branch 9 of the optical waveguide to cause interference, and
The wave interference light is incident on the photodiode 10. The photodiodes 8 and 10 respectively convert the TE wave interference light and the TM wave interference light into electric signals and input the electric signals to the phase difference meter 11. The phase difference meter 11 measures the phase difference between the TE wave interference light and the TM wave interference light based on the input electric signal.

A phase difference corresponding to the optical path difference between the reflected light and the reference light occurs. Therefore, the TE wave of the reflected light demultiplexed by the second TE-TM mode splitter 6 has a phase difference according to the optical path difference with the TE wave of the reference light. Similarly, the TM wave of the reflected light and the TM wave of the reference light have a phase difference corresponding to the optical path difference.

When the TE wave of the reference light and the TE wave of the reflected light interfere with each other, the TE wave interference light has a phase θ ₁ corresponding to the phase difference between the two. Similarly, the TM wave of the reference light and the TM wave of the reflected light interfere with each other to generate TM wave interference light having a phase difference θ ₂ corresponding to the phase difference between the _two . The phase difference (θ ₁ −θ ₂ ) between these two interference lights is detected by the phase difference meter 11.
As described above, the optical path difference can be obtained from the phase difference (θ ₁ −θ ₂ ). Thus, the distance from the distance measuring device to the measuring object is measured.

FIG. 3 is a perspective view of an example of an optical integrated circuit in which the distance measuring device shown in FIG. 1 is integrated.

The substrate 12 is made of LiNbO ₃ . Optical waveguide 13
Are formed by partially selectively thermally diffusing Ti into the substrate 12 using a photolithography technique.
The TE-TM mode splitters 4, 6 are loaded with aluminum 14 for inducing a large negative dielectric constant in the optical waveguide portion 13b on the optical waveguide portion 13b among the optical waveguide portions 13a, 13b which are close and parallel to each other. It is formed by doing. In the TE-TM mode splitters 4 and 6, the loading of aluminum 14 reduces the effective refractive index of the optical waveguide portion 13b for the TM wave, so that the power transfer rate between the optical waveguide portions 13a and 13b is extremely high. Get smaller. As a result, the TM wave is
Thus, the TE wave travels through the unloaded optical waveguide portion 13a and the TE wave travels through the aluminum 14 loaded optical waveguide portion 13b.

The semiconductor laser 1, collimator lenses 3, 5 and photodiodes 8, 10 are attached to the substrate 12 as described above. The phase meter 11 and the connections between it and the photodiodes 8, 10 are not shown in FIG. The operation of the optical integrated circuit of FIG. 3 is similar to that of the distance measuring device of FIG.

The above embodiment is an example of an optical integrated circuit or an optical system based on the optical integrated circuit, but the present invention can be applied to a case where no integration is performed.

(Effect of the Invention) In the distance measuring device of the present invention, since the required light can be obtained by a single laser generating means, not only the number of laser generating means itself but also the number of other optical components required are greatly increased. And the structure is simplified. Therefore, the distance measuring device of the present invention is very easy to integrate. When the distance measuring device is integrated, it is less likely to be affected by vibration and the like, so that stable alignment between the constituent elements is guaranteed and a great effect can be obtained in stabilizing the measurement. In addition, since the configuration is simplified, the measurement accuracy is improved. Since the DFB laser or the DBR laser is used as the laser generating means of the distance measuring device of the present invention, the oscillation wavelength is changed depending on the environment such as temperature as compared with the Fabry-Perot type semiconductor laser used in the conventional distance measuring device. Since it does not change easily, stable measurement results can be obtained.

Further, the distance measurement according to the present invention is realized by using a single optical path including two kinds of light irradiation paths from the irradiation means to the measurement object and a reflection path of light from the measurement object to the light receiving means. To be done. Therefore, as compared with the case where a configuration in which a plurality of optical paths including a light irradiation path and a reflection path are used is assumed, the configuration can be simplified and the number of parts can be reduced, and further, integration can be achieved. It will be possible.

[Brief description of drawings]

FIG. 1 is a diagram showing an optical system of an embodiment of the present invention, FIG. 2 is a characteristic diagram of a DFB laser used in the embodiment of FIG. 1,
FIG. 3 is a perspective view of an optical integrated circuit in which the embodiment of FIG. 1 is integrated, FIG. 4 is a view showing an optical system of a conventional example, and FIG. 5 is a mirror loss-oscillation probability relationship between TE mode and TM mode. Showing the figure,
FIG. 6 is a diagram showing the relationship between the operating temperature and the oscillation wavelength in the TE mode and the TM mode. 1 ... Semiconductor laser, 2 ... Y branch, 4 ... First TE-TM
Mode splitter, 6 …… Second TE-TM mode splitter, 7,9 …… Y branch, 8,10 …… Photodiode, 11 ……
Phase difference meter.

Claims (1)

[Claims] 1. A distance measuring device by interferometry using two types of light having different wavelengths, which are provided as light sources of the two types of light and simultaneously oscillate in a TE mode and a TM mode having different oscillation wavelengths. A laser oscillating means comprising a distributed feedback or distributed reflection type laser element, a branching means for branching the two types of light from the laser oscillating means into a plurality of optical paths, and a branching means for branching into one optical path. An irradiation unit that irradiates the measurement object with the two types of light, a light reception unit that receives reflected light of the two types of light emitted by the irradiation unit from the measurement target, and the other by the branching unit. The phase difference between the TE mode light and the TM mode light is detected from the two types of light branched into the optical path and the two types of light from the light receiving means, and the detected TE mode is detected. Light and the TM mode light By detecting the difference between the phase differences, the distance measuring device and a phase difference detection means for measuring a distance to the measurement object.