KR101267879B1

KR101267879B1 - Visibility Enhanced Interferometer

Info

Publication number: KR101267879B1
Application number: KR1020100104855A
Authority: KR
Inventors: 진종한; 김재완; 강주식; 김종안
Original assignee: 한국표준과학연구원
Priority date: 2010-10-26
Filing date: 2010-10-26
Publication date: 2013-05-27
Also published as: KR20120043519A

Abstract

The present invention relates to a visibility-enhancing interferometer, and more particularly, the injection locking method is applied to a low reflectance material having a low reflectivity or a large roughness surface such as glass. In particular, the present invention relates to an interferometer that improves visibility by amplifying an optical signal reflected and reflected on a measurement surface, which is a low reflectance material, by light splitting without phase distortion, so that the amount of light with the reference plane is the same or similar.

Description

Visibility Enhanced Interferometer

Optical interferometers using a stabilized laser can be measured due to the speed of light retrospectively to the length standard. The measurement resolution can be obtained below nm, making it suitable for many applications requiring ultra-precision measurements.

Although the optical interferometer is one of the most accurate measurement techniques, it is difficult to measure a material with low reflectivity, which is commonly seen in real life by glass plates, color filters, specimens with rough surfaces, cells or molecules submerged in liquids, etc. .

For reliable measurement, the visibility of interference fringes should be good enough to analyze the received interference fringes. However, when measuring low reflectance materials, the intensity of light reflected from the measurement surface is very small, resulting in rapid visibility. Will fall out. This means that maximum visibility can be obtained when the intensity of light reflected from the reference plane and the measurement plane is the same, but low reflectance material has poor visibility due to the small reflected light.

To prevent this loss of visibility, amplify the intensity of light reflected from the measurement plane to match the intensity of light reflected from the reference plane, or adjust the intensity of light reflected from the reference plane to adjust the intensity of light reflected from the measurement plane. You can use a method to make it equal to the strength.

For example, the latter is a method of increasing the amount of reflected light by allowing more than 90% of light to travel through the measurement plane out of the light divided into the reference plane and the measurement plane. However, this method is not only troublesome to adjust the amount of light emitted from the measuring plane and the reference plane of the interferometer in accordance with the state of the measuring plane, but also the measurement itself may be impossible when it is smaller than the threshold light amount that can be measured. There is this.

The visibility enhancement interferometer of the present invention,

Improved visibility by matching the intensity of light reflected from the measurement plane to the intensity of light reflected from the reference plane by amplifying the injection light technique without changing the phase or time characteristics even at very small light levels. The purpose is to use a secondary light source.

Visibility improvement interferometer of the present invention for solving the above problems,

A reference plane reflecting unit and a measuring plane reflecting a main light source unit for irradiating short wavelength light, a light splitting unit for dividing the irradiated optical signal into two, and a light signal separated by the light splitting unit to a reference plane and a measurement plane of a measurement object, respectively An interferometer comprising a reflector and a photodetector configured to combine the optical signals reflected by the reference plane reflector and the measurement plane reflector to perform interference and observe the same, wherein either one of the reference plane reflector and the measurement plane reflector It is characterized in that the auxiliary light source unit is installed in the amplified weak reflected light to maintain a constant visibility.

The auxiliary light source unit may amplify the reflected light signal reflected from the measurement surface reflector or the reference light signal reflected from the reference plane reflector through an injection lock, and the main light source unit is a laser or an optical fiber amplifier (EDFA; Er-doped). fiber amplifier) can be used.

In addition, the auxiliary light source unit allows a feedback control to be performed so that the visibility obtained by the light detector is maintained as needed.

As described in detail above, the visibility enhancement interferometer of the present invention,

When measuring low reflectance materials with large roughness or low reflectance such as glass, the divided reflected light signal reflected from the measurement surface is amplified by the secondary light source applying the injection lock method without changing the phase information, and the reference light signal reflected from the reference plane is reflected. The same or similar makes it possible to provide an interferometer with improved visibility.

1 is a block diagram showing a visibility enhancement interferometer according to the present invention.
2 is a spectrum of light made of the injection light and the auxiliary light source.
3A-3D are photos and graphs showing basic performance tests of a visibility enhancement interferometer.

Hereinafter, the present invention will be described in detail with the accompanying drawings.

1 is a block diagram showing a visibility enhancement interferometer according to an embodiment of the present invention. As described, the visibility enhancement interferometer 10 of the present invention is separated by a main light source unit 20 for irradiating short wavelength light, a light splitting unit 30 for dividing the irradiated optical signal into two, and the light splitting unit. The reference signal reflector 40 and the measurement plane reflection unit 50 for reflecting the reflected optical signal to the measurement plane and the measurement plane of the measurement object, respectively, and the optical signal reflected from the reference plane reflection unit and the measurement plane reflection unit are combined to interfere And an auxiliary light source unit 81 for amplifying the reflected light signal reflected from the measurement surface reflection unit.

Here, the main light source unit 20 uses a DFB (Distributed feedback) laser for irradiating short wavelength light, the light splitting unit 30 uses a 2 × 2 coupler, and the photo detector unit 60 uses a photo detector (PD). do. The main light source unit 20 may select and use various light sources corresponding to the DFB laser.

In addition, the optical signal separated by the light splitter 30 is incident to the reference plane reflector 40 and the measurement plane reflector 50, respectively. In this case, the light signal incident on the measurement surface reflector 50 is incident along the incident light signal progress line 70.

The incident light signal progress line is further provided with a first circulator 71 and a second circulator 72 to proceed along the reflected light signal progress line 80 which is a direction different from when incident light is reflected from the measurement surface. To be done.

The reflected light signal processing line 80 branches from the first circulator 71 and merges in the second circulator 72. Therefore, the incident light traveling along the incident light signal progress line 70 is reflected on the measurement surface 51 of the measurement surface reflector 50, and the reflected reflected light travels back through the incident light signal progress line and then the first circulator ( At 71, the traveling direction is changed to the reflected light signal progress line 80, and the reflected light traveling along the reflected light signal progress line travels in the opposite direction to the incident light through the incident light signal progress line 70 at the second circulator 72. Let this be done.

At this time, the auxiliary light source unit 81 is further provided on the reflected light signal progress line 80 to amplify the reflected light signal reflected from the measurement surface. As such, when an auxiliary light source, which is a secondary light source, is used in addition to the main light source, injection-locked is made so that amplification is performed without changing phase information.

An embodiment of the auxiliary light source unit 81 may use an Er-doped fiber amplifier (EDFA) or a laser having the same or similar wavelength as that of the main light source, and the auxiliary light source unit may be provided to the photodetector unit 60 as necessary. By controlling the current by this, the amount of light of the amplified measurement plane reflected light signal and the reference plane 41 reference light signal is formed equally or similarly to improve visibility. At this time, the control may be made to maintain a constant visibility by the feedback control (feedback control), this control can be made automatically by the control unit, or the user can be set arbitrarily.

The circulators 71 and 72 described above can be arranged in various numbers according to the interferometer environment, and are not limited to the specific rights of the present invention. For example, in FIG. 1, two circulators may be used to form the reflector signal progress line, but in addition, one circulator may be used to return to the reflector signal progress line, or a plurality of circulators may be used. By arranging, the reflection tube signal progress line can be formed in multiple stages.

In addition, the auxiliary light source unit is installed on the measurement surface reflector side as shown to amplify the reflected light signal reflected from the measurement surface, and installed on the reference plane reflector side to amplify the reference light signal reflected from the reference plane, or measured It can be installed on both the surface reflector and the reference surface reflector side to be selectively amplified.

Example

-Visibility Enhancement Interferometer Configuration and Operation Status

As the main light source, a DFB laser (distributed feedback laser) having a center wavelength of 1542.295 nm was used. The output light amount of the main light source portion is 10 mW, and the frequency stability is about 10 ^-8 at a gate time of 1 second.

A 2 × 2 coupler was used as the light splitter.

As an auxiliary light source (secondary light source), a DFB laser having a wavelength similar to that of the main light source was used, and an injection lock was made.

PD (photo detector) was used as the photodetector.

The light emitted from the main light source portion is divided into a reference plane reflection portion and a measurement plane reflection portion by a light splitting portion (2 × 2 coupler). The incident light traveling toward the measurement plane reflecting part passes through the first circulator and the second circulator to reflect the light on the measurement plane.

The reflected light reflected from the measurement surface is incident to the DFB laser which is the secondary light source via the first circulator and amplified by the injection lock. The reflected light amplified by the DFB laser is incident to the light splitter via the second circulator.

In the light splitter, the reference plane reflected light reflected from the reference plane is also incident, so that the measurement plane reflection light and the reference plane reflection light interfere with each other, and the interference signal may be observed by the photodetector PD. In this case, when the secondary light source uses an EDFA instead of a DFB laser, the photodetector controls the current supplied to the EDFA to adjust the amount of amplified light to adjust the amount of light of the reference plane reflected light and the measurement plane reflected light equally or similarly. Improve the degree.

2 is a spectrum of light in which the injection light and the injection light of the auxiliary light source are formed.

As shown, the dotted line is a case of free oscillation before the auxiliary light source is injection-locked, and the solid line transitions so that the oscillation wavelength of the auxiliary light source matches the wavelength of the main light source while light is injected into the auxiliary light source in the main light source. is. The wavelength of the injection locked light source is 1542.295 nm, which is exactly the wavelength of the main light source.

3A to 3D are photographs and graphs showing basic performance tests of a visibility enhancement interferometer.

In the experiment, the amount of light on the measurement surface was adjusted by inserting an ND filter instead of the measurement specimen. Since the ND filter has a transmittance of 0.08 and light passes reciprocally, this can have the same effect as a specimen having a reflectance of 0.6%.

A reference mirror was attached to the PZT stage to verify the operation of the visibility enhancement interferometer.

Through this, the reference mirror was driven in the form of a ramp at a frequency of about 0.3 Hz with a size of 20 μm along the optical axis.

The generated interference signal was obtained through a photodetector, and mathematically, visibility ( V ) was obtained by Equation (1).

Where I _r is the intensity of light reflected from the reference plane (reference mirror), and I _s is the intensity of light reflected from the measurement plane (sample).

3C is a signal obtained when a reference plane (reference mirror) and a measurement plane (measurement specimen) having a reflectance of 99% are used in a general interferometer. Here, the measurement surface reflector in which the measurement surface is located has a reflectance of 0.6% by inserting an ND filter into the measurement arm, which is a component thereof, and as shown in FIG. This corresponds to only 10% of conventional visibility.

3A and 3B are measured using a Visibility Enhanced Interferometer (VEI) according to the present invention. The amount of incident light and the reflectance on the measurement surface were the same as those in FIGS. 3C and 3D.

As shown in FIG. 3A and FIG. 3B, it can be seen that the use of another visibility-enhancing interferometer according to the present invention does not significantly change the visibility even if the reflectance is drastically reduced.

In an embodiment of the present invention, an interferometer is constructed of an optical fiber for the convenience of optical alignment.

However, the length of an optical fiber varies with temperature and is even affected by pressure and humidity. Among the most influential factors, the optical fiber length changes with temperature change, and when the reference arm and the measuring arm are different, the interferometer phase may be distorted due to the unbalance of the length change. In addition, the optical fiber that is vibrated finely by floor vibration or air flow can also bring high frequency noise.

The present invention applies an injection lock technique to amplify the amount of light in the auxiliary light source.

Since phase lock is applied to only one arm (measurement surface reflection part) of two arms (reference plane reflection part and measurement plane reflection part) of the interferometer, a phase difference occurs between the injected light and the injection-locked light. Differences must be taken into account as they cause direct measurement errors or make the measurement itself impossible through interferometers in the event of large phase changes.

According to the existing data, the phase difference

Can be expressed as shown in [Equation 2].

here,

Is the frequency difference between the primary light source and the secondary light source (secondary light source),

Frequency range with injection lock,

Is a line-width enhancement factor. The linewidth enhancement factor has a value between 3 and 7 in the case of a general diode laser.

Indeed, this constant phase delay is not very important because it simply results in a longer or shorter optical path. However, time-varying phase differences can seriously affect measurement results.

The change in phase difference is proportional to the current control noise of the secondary light source (secondary light source). The following three assumptions are required to obtain this conclusion.

First, the frequency difference between the primary and secondary sources should be much smaller than the injection lockable area. In other words,

The relationship of << 1 must be established.

Secondly, the frequency of the secondary light source should be linearly proportional to the injection current, I.

The final assumption is that the temperature control of the secondary light source should be stable and stable. Here, since the temperature change is a drift form occurring for a long time, it can be ignored because it is not significantly affected when measuring a short time.

From the above Equation 2, the above three assumptions may be applied to change the phase difference as shown in [Equation 3].

here,

Is a proportionality constant.

The injection lock frequency range was about 8.3 GHz and was experimentally determined by changing the frequency of the actual main light source while checking the injection lock.

In this embodiment, since the frequency difference between the main light source and the secondary light source is 3 GHz, according to Equation 2, a certain amount of phase delay occurs. This is not an important factor that greatly affects the interferometer itself. In practice, this frequency difference can be taken very small, but in this embodiment, the frequency difference is relatively large for easy confirmation of the injection lock state.

This can be easily confirmed from the spectrum obtained from the optical spectrum analyzer as shown in FIG. It is the ratio of the frequency difference between the real primary light source and the secondary light source

The value of is 0.36, which is the first condition used to derive [Equation 3].

<< 1 is not strictly satisfied. Therefore, Equation 3 should be modified and applied as shown in Equation 4 below.

Here, the proportional constants of [Equation 3] and [Equation 4] related to the secondary light source

Was obtained at 0.57 MHz / mA or 4.5 pm / mA. This is the result of measuring the frequency shift while increasing the injection current of the secondary light source to 160 mA.

1 / cos ( Δf / Δf _L ), which is an additional term of Equation 4, was obtained as 1.07 under the present experimental conditions.

The controller for controlling the injection current of a real secondary light source typically has less than 1.5 μA of noise and 1 μA of current control resolution. Therefore, from Equation 4, the phase change with the most important time change in the interferometer is about 10 ⁻⁴ , and in terms of the length, only 0.1 nm can be obtained in consideration of the center wavelength of the light source, 1542.295 nm. The magnitude of the phase change with time actually obtained is so small that it can be ignored.

Even if the magnitude of the phase change with time is large, the phase change occurs due to the current control speed of the current controller of the secondary light source, and thus can be ignored in the use of a general interferometer. In practice, current control through analog circuits is at least a few kilohertz or higher, and an actual interferometer may have a bandwidth of several hertz to several kilohertz.

That is, since the phase difference according to the time change presented in Equations 3 and 4 corresponds to a very high frequency, it is eliminated in the use of an interferometer by hardware or software.

In conclusion, it was confirmed that the Visibility Enhancement Interferometer works well for specimens with very small reflectance. Even if the reflectance is less than 1%, it is possible to measure not only the application area with rough surface but also the reflectance itself because it can measure the surface of the existing optical interferometer.

10: visibility enhancement interferometer
20: main light source
30: light splitting part
40: reference plane reflecting unit
50: measuring surface reflector
60: light detector
70: incident light signal progress line
71: first circulator 72: second circulator
80: reflected light signal progress line
81: auxiliary light source

Claims

A reference plane reflecting unit and a measuring plane reflecting a main light source unit for irradiating short wavelength light, a light splitting unit for dividing the irradiated optical signal into two, and a light signal separated by the light splitting unit to a reference plane and a measurement plane of a measurement object, respectively In the interferometer comprising a reflector, and a light detector for observing the interference by combining the optical signal reflected from the reference surface reflector and the measurement surface reflector,
An auxiliary light source unit is installed in the measurement surface reflection unit to amplify weak reflected light without phase distortion to maintain a constant visibility.
And the auxiliary light source unit amplifies the reflected light signal reflected from the measurement surface reflector to a predetermined size through an injection lock.

delete

The method of claim 1,
And the auxiliary light source unit selects and uses any one of a laser and an Er-doped fiber amplifier (EDFA).

The method of claim 1,
And the auxiliary light source unit is configured to perform a feedback control so that the visibility obtained by the photodetector is kept constant.