JPH01274002A - Laser length measuring apparatus - Google Patents

Abstract

PURPOSE:To achieve a higher measuring accuracy with a displacement made measurable, by detecting variations with a gas cell for a laser beam of a semiconductor laser element to determine an oscillation wavelength in real time. CONSTITUTION:Another beam divided with a beam splitter 9 is received 10 with a gas cell 8 varying in intensity thereof according to a change in a wavelength of a laser element to obtain an output signal. A semiconductor laser element 1 is of a near infrared range type and the cell 8 is used in a near infrared range. Then, a level detector is built in the element 1, light source level signals outputted from these parts and an output signal of a light receiver are adjusted in level with respective amplifiers 11-1 and 11-2 and then, a specific voltage (v) is calculated with a divider 12. In other words, a reference ratio voltage vc for the center of selection characteristic linearity of the cell 8 is subtracted with a subtractor 13 from the ratio voltage (v) outputted from the divider 12. A value thus obtained is inputted into a current control circuit 14 to control an injection power thereby enabling the stabilizing of the oscillation wavelength.

Description

[Detailed Description of the Invention] [Industrial Field of Application] This invention relates to a laser length measuring device, and more specifically, in an old interference type length measuring device using a semiconductor laser element as a light source, it is possible to use This eliminates measurement errors.

[Prior Art] Optical length measuring instruments utilize the interference phenomenon of laser light to measure minute displacements on the order of wavelengths.

Fig. 3 is a principle diagram of a length measuring device using a Twyman-Green type interferometer, in which a laser beam from a laser light source 1 is split into a measuring beam m in the direction of a measuring object 5 by a beam splitter 2. The reference beam r is split into a reference beam r perpendicular to the reference beam r. The measurement beam is reflected back by a recursive measurement mirror 4 attached to the object to be measured, and is reflected by a beam splitter in the right-angled direction r. Thank you for this!
K (The beam is reflected by the retroreflective reference mirror 3 fixed at a fixed position, passes through the beam splitter, and is superimposed with the measurement beam L to produce interference fringes.
When the object to be measured moves in the direction of the optical axis
A pulse signal 6a having a pinch interval of is obtained. By counting this number and multiplying it by λ/2, the amount of displacement of the object to be measured is measured.

In the beginning, gas laser tubes such as helium and neon were used as light sources, but recently there has been a trend to use small, low-power semiconductor laser elements in their place. However, the ``1'' conductor laser element has the disadvantage that the oscillation wavelength fluctuates significantly compared to the gas laser tube, and when applied to a length measuring device with a constant wavelength, the measurement error is large and accurate data cannot be returned. Since the oscillation wavelength of a semiconductor laser is influenced by the operating dioptric power and the injection current, the wavelength is stabilized by controlling these factors. In the temperature controller 7 shown in FIG. 3, a conductor laser element 1 is fixed to a base 7a having good thermal conductivity, its temperature is detected by a sensor 7b, and the temperature is controlled by a Peltier element 7d by a temperature control circuit 7C. The current is increased by 5 to maintain the furnace chamber temperature at Tt.

On the other hand, regarding the injection current, there is a method of detecting fluctuations in the oscillation wavelength by some method and controlling the i' and IE input main currents based on this to keep the oscillation wavelength constant. As a method for detecting wavelength fluctuations, a method is known that utilizes the selective characteristics of spectral absorption lines of certain metal gases, such as rubidium. FIGS. 4(a) and 4(b) illustrate the structure and characteristics of a gas cell 8 using rubidium, in which 1 volume of rubidium gas 8b is poured into a suitable container 8a. Rubidium is within the oscillation wavelength region of the near-infrared 1'-conductor laser element r.
It has an absorption line near 0 nm, and the beam transmitted through this gas cell exhibits extremely sharp linear selection characteristics centered around the wavelength λ0 of the absorption line, as shown in Figure (b). The invention of controlling the injection current using this gas cell has been filed by the inventor of the present invention as a utility model registration application No. 83-135 titled "Light source device for semiconductor laser length measuring device".

In this idea, a gas cell that detects fluctuations in the oscillation wavelength of the light source is installed in the light source and a control unit in a separate housing, an optical path is formed between them by an optical fiber, and a temperature control method is used in combination to adjust the oscillation wavelength from both. It was designed to stabilize the

[Problem to be solved] In the invention related to the utility model registration application described in L, the temperature controller for wavelength stabilization is designed to handle extremely small temperature changes,
For example, a device that faithfully controls the temperature with an accuracy on the order of 0.001' C is required, and the base, Peltier element, etc. are inevitably large. However, a certain amount of time delay is unavoidable in temperature control, and high Accurate wavelength control is difficult. Furthermore, the miniaturization of the device is not yet sufficient due to the above-mentioned components.

In this invention, the idea is changed from the above, and attention is paid to the fact that even if the wavelength fluctuates, if the accurate value is known, an accurate displacement M can be calculated, and a certain amount of fluctuation is allowed rather than stabilization of the wavelength. This method uses calculations to calculate the ever-changing wavelength in real time from the signal of the beam transmitted through the gas cell. At the same time, it is possible to significantly reduce the accuracy of temperature control, for example on the order of 0.01°C. The object of the present invention is to provide a laser length measuring device which has a temperature controller that is more compact than the conventional device described above.

[Means for Solving the Problems] This invention uses a conductive laser element as a light source, and divides the laser beam of the light source into a measurement beam and a reference beam 11,
A laser length measuring device with a 1-interval meter method that measures the displacement flt of an object to be measured by counting the number of interference fringes formed by a reference beam and a reflected beam of a measurement beam from a measurement mirror provided on the object to be measured, The interferometer is configured by a pre-beam splitter that splits the laser beam from the light source, one of the split laser beams, a gas cell that selects the wavelength of the other laser beam, and transmits the other laser beam, and a gas cell that selects the wavelength of the other laser beam and transmits it. It is equipped with a photoreceiver that receives the transmitted beam, and an arithmetic circuit that calculates the oscillation wavelength from changes in the intensity of the output signal of the photoreceiver.
By multiplying the oscillation wavelength data obtained by the arithmetic circuit by the number of interference fringes counted by the above interferometer,
It is composed of a multiplier that outputs the amount of displacement of the object to be measured.

The two arithmetic circuits include a divider that calculates the ratio voltage V of the output signal of the light receiver with respect to the light source level signal output from the output level detector provided in the light source, and a reference for the center of the selection characteristic line of the gas cell. The ratio voltage between the light source level signal and the output signal of the optical receiver at the wavelength λc is set as the reference ratio voltage vc, and the reference ratio voltage vc is subtracted from the ratio voltage V output from the divider with respect to the varying oscillation wavelength λ(v) of the light source. a subtracter that multiplies the output voltage (v-vc) of the subtracter by the slope coefficient k of the selection characteristic straight line of the gas cell, and a coefficient multiplier that multiplies the output voltage (v-vc) of the subtracter by the slope coefficient k of the selection characteristic straight line of the gas cell, and the output voltage k (v-vc) of the coefficient multiplier is multiplied by the reference wavelength λc. It consists of an adder that adds .

[Function] In the laser length measuring device of the present invention having the above configuration,
A laser beam from a light source passes through a gas cell, and the intensity of the output changes as the wavelength changes due to its selective characteristic straight line. An arithmetic circuit calculates the oscillation wavelength that fluctuates based on the signal whose intensity has changed in real time, and multiplies this by the number of interference fringes counted by the interferometer to determine the exact amount of displacement of the object to be measured.

Here, regarding the calculation by the above calculation circuit, FIG.
) and (b). The intensity of the laser beam transmitted through the gas cell has the selection characteristic shown in Figure (a). For example, the light receiving voltage ya on the left side of this characteristic curve,
Take the straight line between yb and exchange the vertical and horizontal axes to create the figure (
b). The slope coefficient of the straight line in figure (b) is 1
If the light receiving voltage at the center point of the straight line is vc, and the wavelength with respect to vc is the reference wavelength λc, then the oscillation wavelength λ(
v) is expressed by the following formula.

λ(v) =k(v-vc)+λc...(1
) Here, k=(λa-λb)/(va-vb)=”(2)
vc = (va + vb)/2””
(3) λc=(λa+λb)/2...
(4) In the above, since the voltage V changes not only due to fluctuations in the oscillation wavelength but also due to fluctuations in the output level of the light source, the influence of level fluctuations is eliminated by calculating the voltage relative to the output level of the light source using a divider. Voltage V in each of the above equations
shall mean this specific voltage. Next, from the selection characteristics of the gas cell, Vc+λc and k in Equation 42 can be known in advance by measurement, and calculations are performed using these in accordance with Equation (1).

That is, (v-vc) is changed by a subtracter, multiplied by a coefficient multiplier, and λc is added by an adder to obtain λ(
v) is required.

In the following, we used the straight line part on the left side of the characteristic curve in Figure 1(a), but the same calculation formula with different signs of the coefficients can also be obtained using the straight line part on the right side, so The calculation is performed by one of the following methods, taking into consideration the variation range of the oscillation wavelength of the laser element r.

[Embodiment] FIG. 2 shows a block configuration of an embodiment of the laser length measuring device according to the present invention. In the figure %tl
, the conductive laser element 1 is provided with a temperature controller 7 to stabilize the temperature. However, in this case, since fluctuations in the oscillation wavelength are allowed to some extent, a small device that can be controlled on the order of, for example, 0°01°C may be used. Next, the laser beam from the laser element is made into a parallel beam by the collimator 1a and split by the pre-beam splitter 9. Manpower is used for the calculation.

The other beam split by the front beam splitter 9 changes its intensity in accordance with the wavelength fluctuation of the laser element by the gas cell 8, and is received by the light receiver 10 and converted into an output signal. Here, it is assumed that a conductor laser element with a wavelength in the near-infrared region is used, and a rubidium gas cell having an absorption line within this wavelength region is used. Next, the laser element has a built-in level detector, and the light source level signal output from this and the output signal of the photoreceiver are transmitted to amplifiers 11-1.1!, respectively. After the level is appropriately adjusted by -2, the specific voltage V is calculated by the divider 12, and then the calculation according to the above formula: % formula % (1) is performed. That is, by subtracting 7Si13, the reference ratio voltage V with respect to the center of the selection characteristic line of the gas cell
C is subtracted from the specific voltage V output by the divider. Since the obtained (v-vc) is the variation amount of the specific voltage V with respect to the reference specific voltage VC, this can be manually input to the current control circuit 14 to control the current injected into the light source and stabilize the oscillation wavelength to some extent. It is measured. However, this stabilization does not necessarily require high precision, and may be one that allows variation within the range of the gas cell selection straight line. The analog data (v-vc) is then digitized by an A/I converter 15, multiplied by the slope coefficient of the selection characteristic straight line in a coefficient multiplier 16, and further added with a reference wavelength λc by an adder z7. The oscillation wavelength λ(v) is determined. On the other hand, a pulse signal based on the displacement mx of the object to be measured outputted from the light receiver 6 of the interferometer is counted by the pulse counter 18 and transferred to the multiplier 119, where the oscillation wavelength λ( v) is multiplied by a value of 1/2, and data on the displacement mx of the measurement object is output to the terminal 20.

In the above, the resolution of the pulse signal was assumed to be λ/2, but depending on how the interferometer is configured, λ/2 may be further subdivided to have a resolution of a fraction of that. We prefer multiplication of fractions of λ.

[Effects of the Invention] As is clear from the above explanation, the laser length measuring device according to the present invention uses a gas cell to detect fluctuations in the laser beam of a semiconductor laser element whose oscillation wavelength fluctuates and determine the oscillation wavelength. It is calculated in real time and can accurately measure the amount of displacement by multiplying the number of pulses of interference fringes produced by the interferometer by this oscillation wavelength, and it allows for wavelength fluctuations to a certain extent.
Compared to the conventional method of keeping the oscillation wavelength constant by controlling the lH degree and injection current, it is possible to follow wavelength fluctuations at high speed and with high precision. Furthermore, the temperature controller can be made even smaller than before, and there are significant effects that contribute to improving the measurement accuracy and downsizing of the length measuring device using the semiconductor laser element.

[Brief explanation of the drawing]

1(a) and (b) are diagrams illustrating the calculation formula for the selection characteristic of the gas cell in the calculation circuit of the laser length measurement device according to the present invention, and FIG. 2 is an embodiment of the laser length measurement device according to the invention. Fig. 3 is an explanatory diagram of the principle of a laser length measuring device that uses a semiconductor laser element as a light source, fluctuations in oscillation wavelength, and countermeasures against it, and Fig. 4 is an explanation of the structure and selection characteristics of the gas cell. be. 1... Semiconductor laser element F, 2... Beam splinter, 3... Reference mirror, 4... Measurement mirror, 5... Measurement object, 6... Light receiver, 6a.
...Pulse signal, 7. Temperature controller, 8. Gas cell, 9. Front beam splitter, 10.
...Receiver, I! ...Amplifier, 12...
Divider, 13... Subtractor, 14... Current control circuit, I5... A/D converter, 16... Coefficient multiplier, t7... Adder, 18... Pulse counter ,! 9... Multiplier, 20... Terminal.

Claims (2)

[Claims] (1) A semiconductor laser element is used as a light source, the laser beam of the light source is divided into a measurement beam and a reference beam, and interference fringes are formed between the measurement beam reflected from a measurement mirror provided on the measurement object and the reference beam. In an interferometer-type length measuring instrument that measures the amount of displacement of an object by counting the number of objects, a pre-beam splitter splits a laser beam from the light source, and one of the split laser beams measures the amount of displacement of the object. The interferometer includes a gas cell that selects the wavelength of the other split laser beam and transmits it, a light receiver that receives the transmitted beam that has passed through the gas cell, and a light receiver that receives the transmitted laser beam. an arithmetic circuit for determining the oscillation wavelength, and a multiplier that multiplies the oscillation wavelength data obtained by the arithmetic circuit by the number of interference fringes counted by the interferometer and outputs the amount of displacement of the object to be measured. 1. A laser length measuring device comprising: (2) a divider for calculating the ratio voltage v of the output signal of the light receiver with respect to the light source level signal output by the output level detector provided in the light source; and a reference wavelength λc at the center of the selection characteristic straight line of the gas cell. The ratio voltage between the light source level signal and the output signal of the light receiver at is set as a reference ratio voltage vc,
a subtracter that subtracts the reference specific voltage vc from the specific voltage v output from the divider with respect to the varying oscillation wavelength λ(v) of the light source; The arithmetic circuit is configured by a coefficient multiplier that multiplies a slope coefficient k of a cell selection characteristic straight line, and an adder that adds the reference wavelength λc to the output voltage k (v-vc) of the coefficient multiplier. The laser length measuring device according to item 1.