JPH07260684A - Accurate reflectivity measurement method and instrument - Google Patents

Abstract

PURPOSE:To provide an accurate reflectivity measurement method and instrument by accurately measuring a sample with a high reflectivity. CONSTITUTION:Light from a light source 1 is branched into two light waves of a measurement wave (s) and a reference wave (r) and is chopped so that the light waves are in opposite phases at a frequency f1. Further, light for reference is chopped at a frequency f2 which is different from the frequency f1, the light is transmitted through a reflection mirror 17 with 90% reflection factor, and the transmitted light is received by one zero-point adjustment light reception element 19. The value is adjusted to be zero by light attenuators 13 and 14, light reflected by the reflection mirror is received by one light reception element through a standard sample and a measurement sample, and then a signal with f1 as a signal component and that with f2 as a reference signal are measured. The signals are processed by a division operator.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is a reflection for highly accurately evaluating the reflectance of a DBR (Bragg reflection) mirror or the like which constitutes a semiconductor surface emitting laser expected to be applied to an optical communication / optical information processing system. A rate measuring method and a measuring instrument.

[0002]

2. Description of the Related Art In an optical system, a conventional method for determining the reflectance of an optical component such as a mirror is a method using a spectrophotometer shown in FIG. The reflectance measuring system using this spectrophotometer is intended to check the reflectance of the sample by alternately switching the reference optical path and the measuring optical path and inserting a measurement sample into the measuring optical path. That is, in FIG. 2, the light from the wavelength tunable light source 1 is alternately switched to the reference optical path R and the measurement optical path S by the rotating mirror 2 whose reflectance is repeated at 1 and 0. The light passing through the respective optical paths R and S is again received by one light receiving element 3 and converted into an electric signal, and then the signal processing system 4 obtains the transmittance and the reflectance. In this case, the measurement sample 5 is inserted into the measurement optical path S.

In such a spectrophotometer, it is necessary to calibrate the difference in light intensity depending on the wavelength and the optical path before the measurement. That is, as shown in FIG. 3, the electrical signal converted by the light receiving element without the sample during calibration is sampled by the gate signal synchronized with the rotating mirror 2.
The sampling signal at this time is stored in the RAM of the signal processing system 4. And at the time of actual measurement, this signal is R
It is read from the AM and compared with a preset reference value, and the difference is fed back to the dynode applied voltage of the photomultiplier 3. Thereby,
When there is no sample after calibration, as shown in the middle part of FIG. 3, the sampled signal always has a constant value regardless of the difference in the optical path and the wavelength. Next, when the sample 5 is put in, the signal from the optical path S changes due to changes in the transmittance and the reflectance depending on the wavelength. And finally, the transmittance and reflectance are
It is obtained as the ratio of the sampled signals r, s from the respective optical paths R, S.

[0004]

In the reflectance measurement by the above-mentioned spectrophotometer, the reflectance is 0% to 100%.
There is no problem with the evaluation of the sample that changes significantly at. However, it is impossible to measure a semiconductor DBR mirror having a reflectance extremely close to 100% with an accuracy of about 0.01%. This means that, when considering the signal and noise on the frequency axis in the measurement, the noise component becomes a large value by existing in any frequency band and integrating, that is, the influence of excess noise from the light source becomes large. The problem of receiving, moreover, when comparing parts that are close to 100%, it is very sensitive to noise due to high-precision measurement, or in the past it was the whole wavelength before measurement. On the other hand, it is necessary to calibrate, and there is a problem that it takes time and requires a complicated processing system.

An object of the present invention is to provide a high-accuracy reflectance measuring method and measuring instrument capable of highly accurately measuring a high-reflectance component in order to solve the above problems.

[0006]

Means for Solving the Problems The present invention that achieves the above-mentioned object is as follows. (1) The light from a light source is split into two light waves, a measurement light and a reference light, and the two light waves are opposite in phase with each other at a frequency f _1. Then, the reference light is chopped at a frequency f ₂ different from f _1, and then the two light waves are incident on a reflecting mirror, and most of the two light waves are reflected and at the same time transmitted through a part of each. Then, the two light waves transmitted through the reflecting mirror are received by one light receiving element for zero point adjustment, the zero point is adjusted by the optical attenuator inserted in the optical path so that the received light signal becomes zero, and the light is reflected by the reflecting mirror. The generated two light waves are vertically incident on the measurement sample and the standard sample, respectively, and the reflected light from the measurement sample and the standard sample is received by one light receiving element, and the component of the frequency f ₁ is measured as a signal component, and as a reference signal. component of the frequency f ₂ It is characterized in that the reflectance is measured by measuring and processing the signal component and the reference signal with a division calculator, and (2) the light from the light source is split into two light waves of the measurement light and the reference light. 1 optical branching means and the two light waves are chopped at a frequency f ₁ so as to have mutually opposite phases, and the reference light is f
A chopping means for chopping at a frequency f ₂ different from ₁ , a second light branching means for branching the two light waves into two at the same ratio, and one of the two light waves passing through the second light branching means Zero-point adjusting means for receiving light by one zero-point adjusting light-receiving element and performing zero-point adjustment by an optical attenuator inserted in the optical path so that the light-receiving signal becomes zero, and the other of the second optical branching means passing through the second optical branching means. Light incident means for vertically injecting two light waves into the measurement sample and the standard sample, respectively,
The reflected light from the measurement sample and the standard sample is received by one light receiving element, the component of frequency f ₁ is measured as a signal component, the component of frequency f ₂ as a reference signal is measured by the light receiving means, and the signal component and the reference are referred to. It is characterized in that it has an arithmetic means for calculating the reflectance by processing the signal with a division arithmetic unit.

[0007]

[Operation] The measurement light and the reference light all pass through the same optical component,
Moreover, by measuring the difference from the standard sample by the reverse-phase chopping with the frequency f _{1, the} sample with high reflectance is hardly affected by noise, and the narrow band detection of f ₁ and f ₂ is performed. Since a certain lock-in method is used, it is not easily affected by excess noise of the light source, and since the difference between the reference light and the measurement light is constantly monitored, the zero point reference is extremely accurate. Since the reference light is chopped at the frequency f ₂ , by monitoring the signal, calibration for all wavelengths before measurement is unnecessary.

[0008]

EXAMPLE An example of the present invention will now be described with reference to FIG. In FIG. 1, as the light source 1 and the light output unit,
There is a tunable semiconductor laser fiber module,
Further, via a polarization controller 10, it is split into a reference light r and a measurement light s by a fiber coupler. Incidentally, this branch is performed by a fiber coupling or a beam splitter. The branched reference light r and measurement light s are collimated by the collimators 11a and 11b. Next, the parallel lights r and s are both modulated by the first chopper 12a as shown in the enlarged view of FIG. 1 at frequencies f ₁ of opposite phases, and only the reference light r is further received by the second chopper 12b. Intensity modulation of frequency f ₂ different from f ₁ is performed.

Next, there is a first attenuator 13a and a second attenuator 13b, which receives an output from a lock-in amplifier 20 which will be described later and calibrates the difference in light intensity between the reference light and the measurement light due to an optical path or the like. It is for. First
The polarization beam splitter (PSB) 14 raises the polarization purity, and the polarization beam splitter (PSB) 14 passes through the second PSB 15 and the first λ / 4 wave plate 16 to
The 0% reflection mirror 17 reflects and partially transmits the two light waves of the reference light r and the measurement light s.

The light transmitted through the 90% reflection mirror 17 is narrow-band detected only in the f ₁ frequency component by the lock-in amplifier 20 via the condenser lens 18 and the light receiving element 19 so that the signal becomes zero. The attenuators 13a and 13b are adjusted. That is, the difference in the optical path between the reference light r and the measurement light s is calibrated.

The light reflected by the 90% reflection mirror 17 is linearly polarized by the reciprocal movement of the light by the first λ / 4 wavelength plate 16.
Rotated and returned to the 2nd PBS and reflected to the left,
The light is emitted to the λ / 4 wavelength plate 21, reflected by the standard sample 22 having a reflectance of 99.96%, and the measurement sample 23, further returned from the second λ / 4 wavelength plate 21 and the second PSB 15, and passed through the condenser lens 24. It reaches the light receiving element 25. In this case 90
The% mirror shows an example of partial transmission and residual reflection. With respect to the intensity of light received by the light receiving element 25, only the respective frequency components are narrow-band detected by the lock-in amplifier 26 for f ₁ and the lock-in amplifier 27 for f ₂ and processed by the division calculator 28. This division calculator 28 reflects only the standard sample 22 without the measurement sample 23.
A conversion factor, which is the reciprocal of the ratio of f ₁ and f ₂ by the lock-in amplifiers 26 and 27 based on reflection close to%, is calculated, and is multiplied by the ratio of f ₁ and f ₂ when the measurement sample 23 is present. The reflectance is required. The conversion coefficient is the reciprocal of the output of the division calculator 28 before the measurement sample 23 is attached.

In this embodiment, the following effects are obtained. (1) Except for the sample, the reference light r and the measurement light s all pass through the same element, and since the differential method using the anti-phase chopper is adopted, only the difference in reflectance of the sample is accurate. Will be extracted. For example, when measuring 99% of the sample,
Since the conventional method measures 99% as it is, if there is an error of 1% due to some influence, the measurement error will be about 1% as it is. Because of the measurement, the fluctuation of the error of 1% is 1% when the error is 100%, which is 0.01% with respect to the reflectance, which is extremely strong against the fluctuation. (2) Since the narrow band detection method limited to f ₁ and f ₂ is adopted, it is possible to significantly remove excess noise such as 1 / f noise of the light source, and it is possible to perform measurement with extremely high sensitivity. it can. (3) Part of the light is extracted from the 90% reflecting mirror and the zero point is adjusted, so that the standard of the standard sample can be set accurately. (4) Since the reference signal can be constantly monitored by double-chopping the reference signal, it is not necessary to calibrate for all wavelengths, which is required only by obtaining the conversion coefficient before measurement and setting it. At this time, crosstalk between the components of the frequencies f ₁ and f ₂ can be considered, but f ₂ is higher than f ₁ ,
Moreover, crosstalk can be largely prevented by setting f ₂ so as not to be equal to an integral multiple of f ₁ . Further, even if there is some crosstalk, there is no problem if the signal of the lock-in amplifier 1 is set to 0 after that.

[0013]

As described above, according to the present invention, it is possible to measure reflectance with extremely high precision even for a sample with high reflectance.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an embodiment of the present invention.

FIG. 2 is a configuration diagram of a conventional example.

FIG. 3 is an explanatory diagram of a conventional measurement principle.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 light source 11a, 11b fiber collimator 12a, 12b chopper 13a, 13b attenuator 14, 15 polarization beam splitter 16, 21 λ / 4 wavelength plate 17 90% reflection mirror 18, 24 condenser lens 19, 25 light receiving element 20, 26, 27 Lock-in amplifier 22 Standard sample 23 Measurement sample 28 Division calculator

Claims (6)

[Claims] 1. A light from a light source is split into two light waves, a measurement light and a reference light, and the two light waves are chopped so as to have mutually opposite phases at a frequency f ₁ , and the reference light has a frequency f different from f _1. After chopping at ₂ , the two light waves are made incident on a reflecting mirror, most of the two light waves are reflected, and at the same time, a part of each is transmitted, and the two light waves transmitted by the reflecting mirror are used for one zero point adjustment. The light is received by the light receiving element, the zero point is adjusted by the optical attenuator inserted in the optical path so that the received light signal becomes zero, and the two light waves reflected by the reflecting mirror are vertically incident on the measurement sample and the standard sample, respectively. One light receiving element receives reflected light from the measurement sample and the standard sample, measures a component of frequency f ₁ as a signal component, measures a component of frequency f ₂ as a reference signal, and measures the signal component and the reference signal. Processing with division calculator A high-accuracy reflectance measuring method, characterized in that the reflectance is measured by measuring the reflectance. 2. A first light splitting means for splitting light from a light source into two light waves of a measurement light and a reference light, and the two light waves having a frequency f.
Chopping to opposite phases at _1, and chopping means for chopping further reference light f ₁ frequency different f _2, a second light branching means for branching into two respectively the two light waves at the same rate , One of the two light waves that has passed through the second optical branching means is received by one zero-point adjusting light receiving element, and the zero point is adjusted by an optical attenuator inserted in the optical path so that the received light signal becomes zero. Point adjusting means, light incidence means for vertically injecting the other two light waves that have passed through the second light splitting means into the measurement sample and the standard sample, respectively, and one reflected light from the measurement sample and the standard sample is received. The element receives and measures the component of frequency f ₁ as a signal component, measures the component of frequency f ₂ as a reference signal, and measures the reflectance by processing the signal component and the reference signal by a division calculator. Calculate Precision reflectance measuring instrument and having an arithmetic unit. 3. A means for chopping the two light waves of the measurement light and the reference light so as to have mutually opposite phases at the frequency f ₁ is a hole intersecting with one optical path of the two light waves and a hole intersecting with the other optical path. The high-precision reflectance measuring device according to claim 2, wherein the plate is a rotatable plate body that is arranged so as to be alternately in an opposite phase and is opened inside and outside. 4. The light incident means includes a polarization beam splitter that bends two light waves from the second light branching means at right angles toward the measurement sample and the standard sample and transmits reflected light from the measurement sample and the standard sample. And λ / disposed between the polarization beam splitter and the measurement sample and the standard sample.
The high-precision reflectance measuring device according to claim 2, further comprising a four-wave plate. 5. The high-precision reflectance measuring instrument according to claim 2, wherein the second light splitting means is a reflecting mirror that partially transmits light. 6. The measurement light from the light source and the reference light are branched.
Collimator that converts two light waves into parallel light, and this collimator
2 light waves from the frequency f₁First chopping at
Only the reference light from the chopper and the first chopper is frequency
f₂The second chopper to chop at
Zero adjustment for the two light waves that passed through the first and second choppers
Optical attenuator to perform and 2 from this optical attenuator
A first polarization beam splitter for increasing the purity of the light wave
The two light waves from this first polarization beam splitter are made to go straight.
After returning straight, bend the reflected light at a right angle, and after bending at a right angle
Second polarization beam splitter that straightens the return reflected light of
And the straight output from this second polarization beam splitter
90 ° reflection mirror where light is incident through the first λ / 4 wave plate
And a receiver that receives the two transmitted light waves of this 90 ° reflection mirror.
The optical element and the frequency of the two light waves received by this light receiving element
Number f₁The lock-in amplifier 1 for detecting the component,
° The second reflected beam from the reflection mirror
The second λ incident on the measurement sample and the standard sample via the shutter
/ 4 wave plate and return reflection from the measurement sample and standard sample
Receiving the incident light through the second beam splitter
The two light waves of the element and this light receiving element are₁Detect at
Lock-in amplifier 2 and frequency f ₂Lock to detect at
The output of the in-amp 3 and the lock-in amps 2 and 3
And a division arithmetic unit for performing arithmetic processing.
The high precision reflectance measuring instrument according to claim 2.