JPH0240537A - Apparatus for measuring change in refractive index - Google Patents

Abstract

PURPOSE:To prevent decrease in resolution due to the use of a compact optical system by sweeping the frequency of a light source with the formation of an interference waveform, and taking out the interference waveform on a time axis. CONSTITUTION:A current is injected into a DBR (turnable distributed Bragg- reflector semiconductor laser) 1 as a reference laser apparatus from the outside. The current is repeatedly swept with a sawtooth wave generator 2. The oscillating frequency of the output light of the DBR 1 is transformed into the sawtooth wave. The output light from the DBR 1 is transformed into parallel light through a lens 3. Thereafter, the light is inputted into a Fabry-Perot interferometers 5. As the interferometers 5, two semitransparent mirrors are arranged so that the mirror surfaces are in parallel. The gap between the interferrometers is filled with a medium wherein change in refractive index is measured. The output light from the DBR 1 is inputted. The waveform of the light that is transmitted through the interferometers 5 is detected with a photodetector 6 as a waveform detecting means.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a refractive index change measuring device for measuring the refractive index change of any liquid or gas.

As an apparatus for measuring changes in refractive index with high precision, the apparatus described on page 106 of "Wave Optics" by Hiroshi Kubota (published by Iwanami Shoten) is conventionally known. This device inputs light emitted from a light source that is spread out to some extent spatially into a Fabry-Perot interferometer, which has two semi-transparent mirrors arranged in parallel and filled with a medium to be measured. This method measures the change in refractive index by changing the radius of
(see figure). In this configuration, the direction of the m-th maximum strength is 2
It is given by ndcostpm=mat. Here, n is the refractive index of the medium to be measured, λ is the wavelength of the light source, and d is the mirror spacing of the Fabry-Perot interferometer. Instep due to the change Δn in n. The amount of movement is ΔK. Then, from the above equation, the mouth.

It is calculated as follows. From this equation, the change Δp in the radius of the interference ring at the focal point of the lens having the focal length f placed behind the interferometer emission section is given by: Therefore, by measuring Δp, Δn
can be calculated.

(Problems to be Solved by the Invention) Since the above configuration uses a spatially spread light source, it is necessary to use a Fabry-Perot interferometer consisting of a large semi-transparent mirror, and the interferometer output light Since it is necessary to provide an interference ring observation tube with a length corresponding to the focal length of the lens for condensing the light, there is a problem that the overall device becomes larger.

In addition, the reflectance of the semi-transparent mirror that makes up the interferometer is angularly dependent, and the angle of incidence of the light incident on the interferometer is distributed over a wide range, so there is an angular range in which the finesse of the interferometer decreases. However, there was also the problem that the resolution decreased in this range. Furthermore, it is also conceivable that the interference ring at the focal plane of the lens becomes blurred due to wavefront aberration of the lens that condenses the interferometer output light, thereby reducing the resolution.

The purpose of the present invention is to eliminate such conventional problems,
It is an object of the present invention to provide a refractive index change measuring device that does not cause a decrease in resolution due to an optical system used in an outer mold.

(Means for Solving the Problems) In order to achieve the above object, in the present invention, a reference laser device whose oscillation frequency is swept in accordance with an external input signal and a reference laser device are arranged so that their mirror surfaces are parallel to each other, A Fabry-Perot interferometer consisting of two semi-transparent mirrors with a gap between them for filling a medium whose refractive index change is to be measured, and which receives the light emitted from the reference laser device as input; and the Fabry-Perot interferometer. It is characterized by comprising a waveform detection means for detecting the transmitted light waveform.

(Function) In the present invention, the light incident on the interferometer is a point light source. When generating an interference waveform, instead of sweeping the incident angle, the frequency of the light source is swept, and the interference waveform is depicted on the time axis. Therefore, since it is not necessary to spatially spread the light incident on the interferometer, the semi-transparent mirror forming the interferometer only needs to have a size approximately equal to the beam diameter of the light beam. Furthermore, since the collimating lens for the light source, the interferometer, and the photodetector for observing interference waveforms can be arranged in this order in close contact with each other, the length of the device in the optical axis direction can be significantly shortened.

In addition, the light rays entering the interferometer are incident from a point light source and do not have an angular distribution like the light rays from a diffuse light source, and the present invention does not use a spatial interference waveform, resulting in aberrations in the lens. Since this is not a problem, it is possible to prevent a decrease in resolution.

(Example) Hereinafter, the present invention will be described in detail with reference to Examples. FIG. 1 is a block diagram of an embodiment of the present invention.

The current injected into the 1.55 pm wavelength tunable distributed Bragg reflection semiconductor laser (hereinafter referred to as DBR) 1 is swept by a sawtooth wave generator 2 at a repetition frequency of 500 Hz.

The oscillation frequency of the light emitted from the DBRI changes in a sawtooth waveform due to such an injection current. The structure and characteristics of DBRI are detailed in the paper by Uchide et al. on page 403 of Electronics Letters published in 1987.

The light emitted from the DBRI is converted into parallel light by a lens 3, and then enters a Fabry-Perot interferometer 5. At this time,
By making the incident light perpendicular to the Fabry-Perot interferometer, the Fabry-Perot interferometer can be downsized to a size equivalent to the beam diameter. The Fabry-Perot interferometer 5 has a reflectance of 95%.
It consists of two semi-transparent mirrors facing each other with a gap in between.
It is arranged in a fluid tank 4 through which a fluid, which is a medium to be measured, flows. A portion of the wall of the fluid tank 4 that intersects with the optical path is coated with an anti-reflection coating. The output light waveform of the Fabry-Perot interferometer 5 is converted into an electrical signal by a photodetector 6, and then displayed on an oscilloscope 7. When the refractive index of the medium to be measured changes, the position of the peak of the interference waveform on the oscilloscope 7 moves to the left or right. That is, the time interval between the peaks of the emitted light changes. The amount of movement is DB
The frequency sweep width of RI can be converted into the amount of movement δf on the optical frequency axis. On the other hand, since the Fries vector range F of the Furi-Perot interference Ht 5 is (C is the speed of light in vacuum, n is the refractive index of the medium to be measured, and ■ is the distance between mirror surfaces), the change ΔF in F due to the amount of change in refractive index Δn is ΔF Δn It is given by F n . Since the left side is given by δf/fo, where fo is the optical frequency that gives the observed resonance peak, the amount of change in the refractive index can be calculated from the above conversion result.

In this embodiment, the mirror spacing of the Fabry-Perot interferometer 5 is fixed, but by increasing it, the resolution can be increased. Furthermore, although a 1.55 pm DBR was used as the light source, any wavelength can be used as long as the oscillation frequency can be swept by an external signal, and the type of light source is not limited to semiconductor lasers.

(Effects of the Invention) As described above, it has become possible to downsize and increase the resolution of the conventional refractive index change measuring device.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an embodiment of the present invention, and FIG. 2 is a block diagram of a conventional refractive index change measuring device. In the figure, 1... 1.55 pm band wavelength tunable distributed Bragg reflection semiconductor laser, 2... Sawtooth wave generator, 3... Lens, 4... Fluid tank, 5... Fabry-Perot interferometer, 6
...Photodetector, 7...Oscilloscope.

Claims (1)

[Claims] A reference laser device whose oscillation frequency is swept in response to an external input signal is placed so that its mirror surfaces are parallel to each other, and an air gap is placed in between to fill the medium whose refractive index change is to be measured. A refraction device comprising a Fabry-Perot interferometer which is made up of two semi-transparent mirrors and which inputs the light emitted from the reference laser device, and a waveform detection means which detects the waveform of the transmitted light of the Fabry-Perot interferometer. Rate change measuring device.