JP4678236B2 - Optical property measuring device - Google Patents

Description

  The present invention relates to an optical characteristic measuring apparatus for obtaining optical characteristics of an object to be measured, in particular, a transfer function matrix (for example, Jones matrix) of the object to be measured. The present invention relates to an optical characteristic measuring apparatus capable of measuring optical characteristics by one wavelength sweep.

  An optical characteristic measuring device is an optical characteristic (for example, insertion loss, reflectance, transmittance, polarization dependence) of an object to be measured (for example, an optical element, an optical apparatus, a test apparatus / measuring apparatus for the optical element or the optical apparatus). , Chromatic dispersion, polarization mode dispersion, etc.), specifically, the transfer function matrix (for example, Jones matrix) of the object to be measured is obtained by measurement, and the optical characteristics of the object to be measured are determined from this transfer function. Only the necessary optical properties are obtained in a lump.

  In order to obtain the transfer function matrix by measurement, the signal light having the frequency fs is incident on the measurement target, and the signal light (transmitted light or reflected light) output from the measurement target is combined with the reference light (frequency fr). Cause interference. Then, the interference signal is received by the light receiving unit, and the amplitude and phase of the interference signal are measured (so-called heterodyne detection) (see, for example, Patent Documents 1 to 3).

  The signal light generally enters two types of linearly polarized light whose polarization planes are orthogonal, for example, s-polarized light and p-polarized light, into the object to be measured. Further, in order to obtain a transfer function in a predetermined measurement wavelength range, the wavelength of the light source is swept.

  As a measuring method, there are a case where the wavelength is swept with s-polarized light and then swept again with p-polarized light, and a case where s-polarized light and p-polarized light are simultaneously incident on the measurement object and measured with one wavelength sweep. . When the measurement is performed by one wavelength sweep, the measurement time can be shortened and the measurement can be performed accurately without an error caused by reproducibility (for example, wavelength reproducibility) in the first and second wavelength sweeps.

  However, since s-polarized light and p-polarized light are simultaneously incident on the measurement target, it is necessary to separate the interference signal between the s-polarized light and the reference light and the interference signal between the p-polarized light and the reference light. For the separation, a method of separating the s-polarized interference signal and the p-polarized interference signal in the time domain by setting different optical path length differences (for example, refer to Patent Document 2), s-polarized interference signal and p There is a method (for example, refer to Patent Document 3) in which each polarization interference signal is intensity-modulated at different frequencies and separated from the difference in modulation frequency for intensity modulation.

JP 2002-243585 A US Pat. No. 6,376,830 JP 2004-20567 A

  However, since the interference signal based on s-polarized light and the interference signal based on p-polarized light are separated, there is a problem that the interference optical system becomes complicated.

  Accordingly, an object of the present invention is to realize an optical characteristic measuring apparatus capable of measuring optical characteristics by a single wavelength sweep even with a simple interference optical system.

The invention described in claim 1
In an optical property measuring device that measures the optical properties of a measurement object,
A polarization mode converter that periodically converts the polarization state of the wavelength-swept laser light into a first polarization and a second polarization orthogonal to the first polarization;
An interference optical system in which light from this polarization mode converter is branched into two and multiplexed in different optical paths to output interference light, and the object to be measured is installed in one optical path;
An interference light branching unit that branches the interference light from the interference optical system into a third polarization and a fourth polarization whose polarization state is orthogonal to the third polarization;
A sampling unit that samples each of the third and fourth polarized light beams branched by the interference light branching unit based on a period of the polarization mode converter ;
The polarization mode converter is characterized in that the period for converting the polarization state is no longer than the frequency of the interference signal .

The invention according to claim 2 is the invention according to claim 1,
The polarization mode converter is characterized by periodically converting the polarization state of laser light into s-polarized light and p-polarized light.
The invention according to claim 3 is the invention according to claim 1,
The sampling part
A first light receiving unit that receives the third polarized light branched by the interference light branching unit;
A second light receiving portion for receiving the fourth polarized light branched by the interference light branching portion;
First and second sampling and holding circuits for sampling the output from the first light receiving unit;
And a third sampling and holding circuit for sampling the output from the second light receiving section.
The invention according to claim 4 is the invention according to claim 3 ,
A low-pass filter or a band-pass filter is provided after the first to fourth sampling and holding circuits.
The invention according to claim 5 is the invention according to claim 1,
The interference light branching unit is a polarization beam splitter.
The invention according to claim 6 is the invention according to claim 5 ,
The interference optical system is characterized in that the polarization plane of the light of the other branched optical path is incident at an angle of 45 ° with respect to the optical axis of the polarization beam splitter.
The invention according to claim 7 is the invention according to claim 1,
The interference optical system is a spatial light type interference system .

The present invention has the following effects.
According to the first to seventh aspects, the polarization mode converter alternately converts the wavelength-swept laser light into light having two polarization states orthogonal to each other at a predetermined cycle and outputs the light. Then, the interference light branching unit splits the interference light obtained by combining one light passing through the object to be measured and the other light not passing through into the light in an orthogonal polarization state, and each of the branched light is sampled. Sample at a predetermined period. Thereby, the interference signal related to the first polarization incident on the measurement target and the interference signal related to the second polarization incident on the measurement target are separated. Therefore, there is no need to change the optical path or the modulation frequency for each polarization state of the light incident on the measurement target, and even with a simple interference optical system, the optical characteristics are measured with a single wavelength sweep. be able to.

According to the fourth aspect , since the low-pass filter or the band-pass filter removes high-frequency noise due to sampling, the optical characteristics can be obtained with high accuracy.

Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 is a block diagram showing an embodiment of the present invention. In FIG. 1, a wavelength tunable light source 1 continuously outputs laser light while performing wavelength sweeping at a predetermined wavelength sweeping speed. The polarization mode converter 2 periodically converts the polarization state of the output light from the wavelength tunable light source 1 into s-polarized light (first polarized light) and p-polarized light (second polarized light).

  The branching unit 3 is, for example, a non-polarizing beam splitter, a half mirror, an optical coupler, etc., and branches the light from the polarization mode converter 2 into two. One light branched by the branching unit 3 is used as signal light, and the other light is used as reference light.

  The multiplexing unit 4 is, for example, a non-polarizing beam splitter, a half mirror, an optical coupler, and the like, and multiplexes the signal light branched by the branching unit 3 and the reference light. The optical path OP <b> 1 is a path through which the signal light branched by the branching unit 3 is transmitted to the multiplexing unit 4. The optical path OP <b> 2 is a path through which the reference light branched by the branching unit 3 is transmitted to the multiplexing unit 4. The optical path OP1 and the optical path OP2 have different optical path lengths. For example, (optical path length of optical path OP1 (signal light side)) <(optical path length of optical path OP2 (reference light side)).

  The object to be measured 5 is installed in the optical path OP1, and the signal light from the branching unit 3 enters. Accordingly, the optical path length of the optical path OP1 includes the optical path of the measurement target 5. Further, the polarization state of each of the s-polarized light and p-polarized light incident on the measurement target 5 changes depending on the optical characteristics of the measurement target 5 and is emitted.

  That is, there are outgoing s-polarized light and outgoing p-polarized light for incident s-polarized light, and outgoing s-polarized light and outgoing p-polarized light for incident p-polarized light. The incident s-polarized light is s-polarized light incident on the measurement target 5, and the outgoing s-polarized light is s-polarized light emitted from the measurement target 5. Similarly, incident p-polarized light and outgoing p-polarized light are p-polarized light that enters and exits the measurement target 5.

  Accordingly, when the transfer function matrix of the input / output characteristics of the measurement target 5 is represented by a Jones matrix of 2 rows and 2 columns, it is represented by the following equation (1).

Here, T ₁₁ represents the relationship of the outgoing s-polarized light to the incident s-polarized light, T ₂₁ represents the relationship of the outgoing p-polarized light to the incident s-polarized light, and T ₁₂ represents the relationship of the outgoing s-polarized light to the incident p-polarized light. , T ₂₂ represents the relationship of the outgoing p-polarized light to the incident p-polarized light. That is, in _Txy , x represents the polarization state on the exit side (x = 1 is s-polarized light, x = 2 is p-polarized light), y is the polarization state on the incident side (y = 1 is s-polarized light, y = 2) Represents p-polarized light).

Therefore, the outgoing light of the object 5 to be measured is T ₁₁ and T ₂₁ combined when the incident signal light is s-polarized light, and T ₁₂ and T ₂₂ are combined when the incident signal light is p-polarized light. It becomes light.

  Note that the Mach-Zehnder type interference optical system 100 is formed by the branching unit 3, the multiplexing unit 4, the measurement target 5, and the optical paths OP1 and OP2.

  The polarization beam splitter 6 is an interference light branching unit, and branches the interference light combined by the interference optical system 100 into s-polarized light (third polarized light) and p-polarized light (fourth polarized light). The first light receiving unit 7 receives the branched s-polarized light. The second light receiving unit 8 receives the branched p-polarized light.

  The optical path OP2 is arranged so that the polarization plane of the reference light is inclined by 45 ° with respect to the optical axis of the polarization beam splitter 6. For example, a polarization plane rotating means, for example, a λ / 2 wavelength plate (not shown) is provided on the optical path OP2, and the optical axis of the wavelength plate is installed at an angle of 22.5 [°] with respect to the polarization plane of the reference light. .

  The first sampling hold (hereinafter abbreviated as S / H) circuit 9 and the second S / H circuit 10 receive an output signal from the light receiving unit 7. The third S / H circuit 11 and the fourth S / H circuit 12 receive an output signal from the light receiving unit 8. The light receiving units 7 and 8 and the S / H circuits 9 to 12 form a sampling unit, and each of the s-polarized light and the p-polarized light branched by the polarization beam splitter 6 is based on the period of the polarization mode converter 2. Sampling.

The operation of such an apparatus will be described.
As an example, the wavelength sweep speed of the tunable light source 1 is 10 [nm / s], the optical path length difference between the optical path OP1 of the signal light passing through the measurement target 5 and the optical path OP2 of the reference light is 10 [m], and the polarization mode The conversion cycle of the converter 2 will be described as 20 [MHz]. Therefore, the frequency of the interference light (beat signal) measured by the light receiving units 7 and 8 is about 40 [kHz].

  Laser light from the wavelength tunable light source 1 enters the polarization mode converter 2. Then, the polarization mode converter 2 periodically converts the polarization state of the laser light (20 [MHz]) and periodically outputs light in two orthogonal linear polarization states (s-polarized light and p-polarized light). . Here, FIG. 2 is a diagram illustrating the output light of the polarization mode converter 2. In FIG. 2, the horizontal axis is the time axis, and the vertical axis is the optical power. Further, (a) in FIG. 2 is an output of s-polarized light from the polarization mode converter 2, and (b) is an output of p-polarized light. As shown in FIG. 2, s-polarized light and p-polarized light are alternately output in a cycle of 20 [MHz] (interval of 0.05 [μs]).

  Then, the branching unit 3 splits the output light from the polarization mode converter 2 into signal light and reference light, outputs the signal light to the measurement target 5 on the optical path OP1, and sends the reference light to the optical path OP2. Output. Then, the signal light from the measurement target 5 and the reference light that has passed through the other optical path OP2 are combined and interfered by the combining unit 5. Further, the combined interference light is split into s-polarized light and p-polarized light by the polarization beam splitter 6. Note that the reference light is incident on the optical axis of the polarization beam splitter 6 with the polarization plane inclined by 45 [°], so that the reference light also enters the light receiving units 7 and 8.

The light receiving unit 7 receives the branched s-polarized light, and the light receiving unit 8 receives the branched p-polarized light. That is, the light receiving unit 7 receives the interference light between the signal light subjected to the action of T ₁₁ or T ₁₂ and the reference light. In the light receiving unit 8, interference light between the signal light and the reference light subjected to the action of T ₂₁ or T ₂₂ is input.

  Further, the light receiving unit 7 outputs an output signal corresponding to the optical power of the interference light to the S / H circuits 9 and 10. Similarly, the light receiving unit 8 outputs an output signal corresponding to the optical power of the interference light to the S / H circuits 11 and 12.

  Then, the S / H circuits 9 to 12 separate the incident s-polarized light and the incident p-polarized light, and obtain a Jones matrix by a calculation unit (not shown) in the subsequent stage.

Next, the operation of the S / H circuits 9 to 12 will be described in detail.
The S / H circuits 9 and 11 synchronously sample (sample) the output signals of the light receiving units 7 and 8, and the S / H circuits 10 and 12 synchronously sample and hold. Here, FIG. 3 is a diagram showing operations of the light receiving unit 7 and the S / H circuits 9 and 10, and the horizontal axis is a time axis. Further, (a) in FIG. 3 is the incident light to the light receiving unit 7, and the vertical axis is the optical power. (B) is the sampling timing of the S / H circuit 9, and (c) is the sampling timing of the S / H circuit 10.

  As shown in FIG. 3, the light receiving unit 7 receives the outgoing s-polarized interference light with respect to the incident s-polarized light and the outgoing s-polarized interference light with respect to the incident p-polarized light every 0.05 [μs]. Outputs interference signals according to power.

  Each of the S / H circuits 9 and 10 includes an interference signal for incident s-polarized light having a period (10 [MHz]) that is half the conversion period (20 [MHz]) of the polarization mode converter 2; An interference signal for incident p-polarized light is input.

  Similarly, the light receiving unit 8 also receives the outgoing p-polarized interference light with respect to the incident s-polarized light and the outgoing p-polarized interference light with respect to the incident p-polarized light every 0.05 [μs], and interferes according to the optical power of the interference light. The signal is output to the S / H circuits 11 and 12.

  Further, each of the S / H circuits 9 to 12 samples at a period of 10 [MHz] (that is, at intervals of 0.1 [μs]). The S / H circuits 9 and 11 and the S / H circuits 10 and 12 sample alternately at intervals of 0.05 [μs]. That is, as shown in FIG. 3, the S / H circuits 9 and 11 sample at t0, t2, t4, t6, t8, t10..., And the S / H circuits 10 and 12 perform t1, t3, t5, t7. , T9, t11... The time interval between times t0 and t11 is 0.05 [μs].

As a result, only the interference signal of the outgoing s-polarized light with respect to the incident s-polarized light, that is, the signal affected by T _{11 in} the Jones matrix is extracted from the sampled data of the S / H circuit 9.

Similarly, the sampled data of each of the S / H circuits 10 to 12 includes an output s-polarized interference signal (T ₁₂ ) for incident p-polarized light, an output p-polarized interference signal (T ₂₁ ) for incident s-polarized light, and an incident p An interference signal (T ₂₂ ) of the outgoing p-polarized light with respect to the polarized light is extracted.

  Then, as described above, the calculation means (not shown) subsequent to each S / H circuit 9-12 obtains each element of the Jones matrix from the amplitude and phase of the interference signal from the S / H circuits 9-12. The optical characteristic of the measurement object 5 is obtained from the Jones matrix.

  In this way, the polarization mode converter 2 outputs light in two orthogonal linearly polarized states (s-polarized light and p-polarized light) at a predetermined cycle. The polarization beam splitter 6 splits the interference light obtained by combining the reference light and the signal light from the measurement target into s-polarized light and p-polarized light, and the light receiving units 7 and 8 receive the light. Further, the S / H hold circuits 9 to 12 sample the interference signal at a predetermined cycle (1/2 of the conversion speed of the polarization mode converter 2). This separates the interference signal for incident s-polarized light from the interference signal for incident p-polarized light. Therefore, it is not necessary to change the optical path by incident s-polarized light or incident p-polarized light, or to change the modulation frequency for intensity modulation, and even with a simple interference optical system 100, optical characteristics are measured with a single wavelength sweep. be able to.

The present invention is not limited to this, and may be as shown below.
1 shows a configuration in which the polarization mode converter 2 periodically converts the polarization state of laser light that is wavelength-swept into s-polarized light (first polarized light) and p-polarized light (second polarized light) that are linearly polarized light. However, it is not necessary to convert with linearly polarized light, and it is sufficient that the polarization states of the first polarized light and the second polarized light are orthogonal.

  A configuration is shown in which a polarization beam splitter 6 is used for the interference light branching unit, and the interference light from the interference optical system 100 is branched into s-polarized light (third polarized light) and p-polarized light (fourth polarized light) that are linearly polarized light. However, it does not have to be branched by linearly polarized light, and it is sufficient that the polarization states of the third polarized light and the fourth polarized light are orthogonal. The polarization state of the first polarization and the third polarization may not be the same.

  In addition, a low-pass filter or a band-pass filter is provided after the S / H circuits 9 to 12, that is, between the S / H circuits 9 to 12 and arithmetic means (not shown) to remove high-frequency noise due to sampling of 10 [MHz]. May be. Specifically, it is preferable to pass a signal of less than 10 [MHz] with a low-pass filter. Alternatively, a signal only in the vicinity of 40 [kHz] may be passed using a bandpass filter. Thereby, an optical characteristic can be calculated | required accurately.

  Further, although the optical path length difference between the optical path OP1 and the optical path OP2 is different, the optical path length difference between the optical path OP1 and the optical path OP2 may be the same. In this case, each of the low-pass filter and the band-pass filter may pass a signal near the DC component.

  Further, the synchronization between the period of conversion by the polarization mode converter 2 and the sampling timing of the S / H circuits 9 to 12 may be synchronized with a synchronization circuit (not shown).

  Moreover, although the polarization mode converter 2 showed the structure which converts a polarization state with a 20 [MHz] period, the conversion speed should just be higher than the frequency of an interference signal (beat signal). That is, the frequency of the interference signal is determined by the wavelength sweep speed of the wavelength tunable light source 1 and the optical path length difference between the optical paths OP1 and OP2. Therefore, the conversion speed of the polarization mode converter 2 may be made lower than the frequency of the interference signal in consideration of the wavelength sweep speed and the optical path length difference. Of course, the wavelength sweep speed and the optical path length difference may be values other than 10 [nm / s] and 10 [m].

  The interference optical system 100 may be a spatial light type interferometer. Specifically, the spatial light is converted from the polarization mode converter 2 into parallel light, branched by the branching unit 3, and the signal light is incident on the measurement target 5. Further, the reference light may be folded back with a mirror or the like to obtain a predetermined optical path length difference. A condensing unit is preferably provided between the polarizing beam splitter 6 and the light receiving units 7 and 8 so that the light receiving units 7 and 8 collect the light. Thus, by using a spatial light type interferometer, the optical system can be miniaturized and is resistant to vibration.

  Further, although the interference optical system 100 is configured to use a Mach-Zehnder interferometer, any two-beam interferometer may be used.

It is the block diagram which showed one Example of this invention. FIG. 6 is a diagram for explaining the operation of the polarization mode converter 2. FIG. 6 is a diagram for explaining operations of a light receiving unit 7 and S / H circuits 9 and 10.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Light source 2 Polarization mode converter 5 Object to be measured 6 Polarization beam splitter 7, 8 Light-receiving part 9-12 S / H circuit 100 Interference optical system

Claims (7)

In an optical property measuring device that measures the optical properties of a measurement object,
A polarization mode converter that periodically converts the polarization state of the wavelength-swept laser light into a first polarization and a second polarization orthogonal to the first polarization;
An interference optical system in which light from this polarization mode converter is branched into two and multiplexed in different optical paths to output interference light, and the object to be measured is installed in one optical path;
An interference light branching unit that branches the interference light from the interference optical system into a third polarization and a fourth polarization whose polarization state is orthogonal to the third polarization;
A sampling unit that samples each of the third and fourth polarized light beams branched by the interference light branching unit based on a period of the polarization mode converter ;
The polarization mode converter has an optical property measuring apparatus characterized in that the period for converting the polarization state is no longer than the frequency of the interference signal .   2. The optical property measuring apparatus according to claim 1, wherein the polarization mode converter periodically converts the polarization state of the laser light into s-polarized light and p-polarized light. The sampling part
A first light receiving unit that receives the third polarized light branched by the interference light branching unit;
A second light receiving portion for receiving the fourth polarized light branched by the interference light branching portion;
First and second sampling and holding circuits for sampling the output from the first light receiving unit;
Third and fourth sampling and holding circuits for sampling the output from the second light receiving unit;
The optical characteristic measuring apparatus according to claim 1, wherein: 4. The optical characteristic measuring apparatus according to claim 3, wherein a low-pass filter or a band-pass filter is provided after the first to fourth sampling and holding circuits. The optical characteristic measuring apparatus according to claim 1, wherein the interference light branching unit is a polarization beam splitter. 6. The optical characteristic measuring apparatus according to claim 5, wherein the interference optical system is incident with the polarization plane of the light of the other branched optical path inclined by 45 degrees with respect to the optical axis of the polarization beam splitter. 2. The optical characteristic measuring apparatus according to claim 1, wherein the interference optical system is a spatial light type interference system.

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