JP7487617B2 - Optical reflection measuring device - Google Patents

Description

The present invention relates to an optical reflection measurement device that measures the reflection level of a measurement object.

Conventionally, when inspecting the reflection distribution inside an optical device, an optical reflection measuring device is used (see, for example, Non-Patent Document 1). In the optical reflection measuring device described in Non-Patent Document 1, the light output from a light source is split by an optical coupler and input to a measurement object arm having a measurement object and a reference arm having a polarization adjuster and a variable delay device. The variable delay device can change the propagation distance of light at a predetermined constant speed using a motor or the like. The light input to each arm is reflected and returned to the optical coupler, and is converted into an electrical signal by an O/E (Optical/Electronic) converter. The converted electrical signal passes through a BPF (band pass filter), a rectifier, and an LPF (low pass filter) and is input to a display, where the output level from the LPF is displayed.

At this time, when the round-trip propagation times of the light in the measurement arm and the light in the reference arm input to the O/E converter become close, the light from each arm interferes, causing a change in the intensity of the light. When the round-trip propagation times of the light reflected from each arm become equal, the intensity change due to interference is maximized. This intensity change becomes a sine wave-like intensity change with a frequency determined by the wavelength of the light source of the variable delay device and the rate of change of delay time in the variable delay device. The amplitude when the intensity change is maximized corresponds to the reflection level in the measurement arm, so by measuring the amplitude at this time, the reflection level of the measurement target can be determined.

F. Lindgren, R. Gianotti, R. Walti, R. P. Salathe, A. Haas, M. Nussberger, M. L. Schmatz, and W. Bachtold, “78-dB Shot-Noise Limited Optical Low-Coherence Reflectometry at 42-m/s Scan Speed”, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 9, NO. 12, DECEMBER 1997

However, if the delay time of the variable delayer is not changed at a predetermined constant speed, the frequency of the sinusoidal intensity change will not pass through the BPF and the reflection level will not be measured, so the variable delayer is scanned at a constant speed. However, in the optical reflection measurement device described in Non-Patent Document 1, if the polarization states of the light propagated through the arm to be measured and the light propagated through the reference arm do not match, the amplitude will decrease due to interference. Therefore, it is necessary to repeatedly scan the variable delayer while changing the polarization state with the polarization adjuster provided in the reference arm until the amplitude is maximized, which causes the problem of time-consuming measurement. In addition, if the reflection level of the object to be measured fluctuates faster than the repetition period of the scanning of the variable delayer, it becomes difficult to perform accurate measurement.

Therefore, there is a need for an optical reflection measurement device that can prevent the measurement time from increasing and accurately measure the reflection level even when the reflection level of the object being measured fluctuates.

The optical reflection measurement device of the present invention is an optical reflection measurement device that measures the reflection level of a measurement object, and is equipped with a measurement object optical path in which the measurement object is provided and through which light for the measurement object passes, a reference optical path through which light to be referenced when measuring the reflection level of the measurement object passes, a separation means into which reflected light reflected by the measurement object in the measurement object optical path and reference light that has passed through the reference optical path are input, and which separates the input reflected light and reference light into two lights having orthogonal polarization states, a conversion means for converting each of the two lights output from the separation means into an electrical signal, a combination means for combining an electrical signal that has passed through a bandpass filter for independently detecting the presence or absence of interference with each of the two electrical signals, and a Faraday rotator installed on the reference optical path that rotates the polarization direction of the light passing through by a predetermined angle, and the separation means divides the reference light approximately equally into two parts so that they are included in the two lights .

The present invention makes it possible to prevent measurement time from increasing and to accurately measure the reflection level even when the reflection level of the object being measured fluctuates.

1 is a block diagram showing an example of a configuration of an optical reflection measurement device according to a first embodiment; FIG. 1 is a block diagram showing an example of the configuration of a conventional optical reflection measuring device. FIG. 11 is a block diagram showing an example of the configuration of an optical reflection measurement device according to a second embodiment.

The following describes an embodiment of the present invention with reference to the drawings. The present invention is not limited to the following embodiment, and various modifications are possible without departing from the spirit of the present invention. In addition, in each drawing, the same reference numerals are used to denote the same or equivalent parts, and this is common throughout the entire specification.

Embodiment 1.
A description will be given of an optical reflection measurement device according to the present embodiment 1. The optical reflection measurement device according to the present embodiment 1 measures a reflection level of a measurement object when light output from a light source is reflected by the measurement object.

[Configuration of optical reflection measuring device 1]
Fig. 1 is a block diagram showing an example of the configuration of an optical reflection measuring device according to the present embodiment 1. As shown in Fig. 1, the optical reflection measuring device 1 includes a light source 11, an optical coupler 12, a variable delay device 13, a Faraday rotator 14, a mirror 15, a polarizing beam splitter 16, O/E converters 17a and 17b, band pass filters (hereinafter, appropriately referred to as "BPF") 18a and 18b, rectifiers 19a and 19b, low pass filters (hereinafter, appropriately referred to as "LPF") 20a and 20b, an adder 21, and a display 22.

The light source 11 outputs light in a preset polarization state. In the example shown in FIG. 1, the light source 11 outputs linearly polarized light parallel to the X-axis. For example, a light source with low coherence is used as the light source 11. More specifically, for example, an SLD (Super Luminescent Diode) is used as the light source 11. Note that the light source 11 is not limited to this example, and other light sources may be used, such as the spontaneous emission light of an erbium-doped fiber.

The optical coupler 12 has multiple terminals, and splits light input to one terminal into two and outputs them, and also outputs light input to the multiple terminals from one terminal. In the first embodiment, the optical coupler 12 has the light source 11, the measurement arm, the reference arm, and the polarizing beam splitter 16 connected to each of its terminals.

The optical coupler 12 splits the light input from the light source 11 into two, outputting one light to the measurement target arm side and the other light to the reference arm side. The optical coupler 12 also outputs the light input from the measurement target arm side and the light input from the reference arm side to the polarizing beam splitter 16.

The measurement object arm is provided with the measurement object 2. The measurement object arm is configured so that the input light is input to the measurement object 2, and is a measurement object optical path through which light for the measurement object 2 passes. If a reflection point exists on the measurement object 2, the light input to the measurement object 2 is reflected at the reflection point and returns to the optical coupler 12 as reflected light.

The reference arm is provided with a variable delayer 13, a Faraday rotator 14, and a mirror 15. The reference arm is a reference optical path through which light passes that is referenced when measuring the reflection level of the measurement target 2. In the reference arm, the input light passes through the variable delayer 13 and the Faraday rotator 14, is reflected by the mirror 15, passes through the Faraday rotator 14 and the variable delayer 13, and returns to the optical coupler 12 as reference light.

The variable delayer 13 delays the input light by a preset delay time and outputs it. The Faraday rotator 14 rotates the polarization direction of the input light by a preset angle and outputs it. In the first embodiment, the Faraday rotator 14 is set so that the polarization plane rotates about 90° as the light travels back and forth. The mirror 15 reflects the input light and outputs it.

The polarizing beam splitter 16 is a separation means that separates the input light into two components whose polarization states are orthogonal to each other and outputs the two components. In the present embodiment 1, the polarizing beam splitter 16 receives light from the optical coupler 12. The mounting angle of the polarizing beam splitter 16 is set so that when the light input from the optical coupler 12 is separated into two components, the output is not biased to one side.

For example, if the light output from the light source 11 is linearly polarized parallel to the X-axis, the light traveling back and forth through the reference arm from the optical coupler 12 to the polarizing beam splitter 16 will be linearly polarized parallel to the Y-axis. In this case, the mounting angle of the polarizing beam splitter 16 is set to approximately 45°, halfway between the X-axis and the Y-axis.

When using an optical coupler 12 using polarization-maintaining fiber and a polarizing beam splitter 16 designed to separate the slow and fast axes of the polarization-maintaining fiber, the optical coupler 12 and the polarizing beam splitter 16 are connected so that the angle between the slow or fast axis of the output of the optical coupler 12 and the slow or fast axis of the input of the polarizing beam splitter 16 is close to 45°.

The O/E converters 17a and 17b are conversion means that convert the input optical signal into an electrical signal and output it. The BPFs 18a and 18b extract and output a preset frequency component from the input electrical signal. The rectifiers 19a and 19b rectify the input electrical signal. The LPFs 20a and 20b remove unnecessary high-frequency components (frequency components higher than the cutoff frequency) such as noise from the input electrical signal and output it.

The adder 21 is a synthesis means that adds and outputs a plurality of input electrical signals. In the present embodiment 1, the adder 21 adds and outputs the two electrical signals output from the LPFs 20a and 20b.

The display 22 displays the output level of the input signal as a reflection level. The display 22 is also connected to the variable delay device 13 and displays the delay time set by the variable delay device 13.

[Operation of the optical reflection measuring device 1]
The operation of the optical reflection measurement device 1 according to the present embodiment 1 will be described. First, in order to facilitate understanding of the operation of the optical reflection measurement device 1 according to the present embodiment 1, the configuration and operation of a conventional optical reflection measurement device will be described.

(Configuration of a conventional optical reflection measuring device 100)
Fig. 2 is a block diagram showing an example of the configuration of a conventional optical reflection measuring device. The same reference numerals are used for the parts common to the optical reflection measuring device 1 shown in Fig. 1, and the description thereof will be omitted. As shown in Fig. 2, the optical reflection measuring device 100 includes a light source 11, an optical coupler 12, a polarization adjuster 101, a variable delay device 13, a mirror 15, an O/E converter 17, a BPF 18, a rectifier 19, an LPF 20, and a display 22.

The optical coupler 12 in Figure 2 has a light source 11, a measurement object arm, a reference arm, and an O/E converter 17 connected to its respective terminals. The optical coupler 12 splits the light input from the light source 11 into two, outputting one light to the measurement object arm in which the measurement object 2 is provided, and outputting the other light to the reference arm. The optical coupler 12 also outputs the light input from the measurement object arm side and the light input from the reference arm side to the O/E converter 17.

The reference arm is provided with a polarization adjuster 101, a variable delayer 13, and a mirror 15. The reference arm is configured so that input light passes through the polarization adjuster 101 and the variable delayer 13, is reflected by the mirror 15, and passes through the variable delayer 13 and the polarization adjuster 101 before returning. The polarization adjuster 101 adjusts the polarization state of the input light.

(Operation of the conventional optical reflection measuring device 100)
The operation of the conventional optical reflection measurement device 100 will be described with reference to Fig. 2. Light output from a light source 11 is input to an optical coupler 12. The light input to the optical coupler 12 is split into two beams, one of which is input to a measurement object arm in which a measurement object 2 is provided, and the other is input to a reference arm in which a measurement object 2 is not provided.

The light input to the measurement object arm is reflected at the reflection point of the measurement object 2 and returns to the optical coupler 12. On the other hand, the light input to the reference arm is reflected by the mirror 15 and returns to the optical coupler 12. At this time, the light output from the optical coupler 12 has its polarization state adjusted by the polarization adjuster 101 as it travels back and forth through the reference arm, and returns to the optical coupler 12 with a delay of a preset delay time by the variable delay device 13.

The light that returns to the optical coupler 12 is input to the O/E converter 17, which converts the optical signal into an electrical signal and outputs it. The electrical signal output from the O/E converter 17 passes through the BPF 18, rectifier 19, and LPF 20 and is input to the display 22. The display 22 then displays the output level according to the delay time set by the variable delay device 13.

When the round-trip propagation time of the light in the measurement arm and the round-trip propagation time in the reference arm become close, the light from each arm interferes with each other, causing a change in intensity. When the round-trip propagation times of the light from each arm become equal, the change in intensity due to interference is maximized.

(Conventional Issues)
However, if the polarization states of the light propagated through the measurement target arm and the light propagated through the reference arm do not match, the amplitude will decrease due to interference. Therefore, when measuring the output level of light from the measurement target 2, it is necessary to repeat the measurement while adjusting the polarization state with the polarization adjuster 101 so that the amplitude is maximized. Therefore, in the conventional optical reflection measurement device 100, it takes a long time to measure the amplitude. Also, if the reflection level of the measurement target 2 fluctuates, it becomes difficult to measure the amplitude accurately.

Therefore, the optical reflection measurement device 1 according to the first embodiment makes it possible to easily and accurately measure the reflection level of the measurement object 2 without adjusting the polarization state of the light propagating through the reference arm.

(Operation of the optical reflection measuring device 1)
Next, the operation of the optical reflection measurement device 1 according to the first embodiment will be described with reference to Fig. 1. Light output from a light source 11 is input to an optical coupler 12. The light input to the optical coupler 12 is branched into two beams, one of which is input to a measurement object arm in which a measurement object 2 is provided, and the other beam is input to a reference arm in which a measurement object 2 is not provided.

The light input to the measurement object arm is reflected at the reflection point of the measurement object 2 and returns to the optical coupler 12. On the other hand, the light input to the reference arm is reflected by the mirror 15 and returns to the optical coupler 12. At this time, the light output from the optical coupler 12 is delayed by a preset delay time by the variable delayer 13 as it travels back and forth through the reference arm, and returns to the optical coupler 12 with its polarization state rotated by 90° by the Faraday rotator 14.

The light returning to the optical coupler 12 is split into two orthogonal components by the polarizing beam splitter 16, one of which is input to the O/E converter 17a and the other to the O/E converter 17b. The light input to the O/E converters 17a and 17b is converted from an optical signal to an electrical signal and output.

The electrical signal output from the O/E converter 17a passes through the BPF 18a, rectifier 19a, and LPF 20a before being input to the adder 21. The electrical signal output from the O/E converter 17b passes through the BPF 18b, rectifier 19b, and LPF 20b before being input to the adder 21.

The two electrical signals input to the adder 21 are added together and input to the display 22. The display 22 then displays an output level according to the delay time set by the variable delay device 13.

Here, the polarization state of the light propagating through the reference arm is rotated 90° by the Faraday rotator 14. That is, the component of the light that propagates along the X axis from the optical coupler 12 to the mirror 15 returns along the Y axis from the mirror 15 to the optical coupler 12. Also, the component of the light that propagates along the Y axis from the optical coupler 12 to the mirror 15 returns along the X axis from the mirror 15 to the optical coupler 12.

The polarization state of the light at this time changes depending on the difference in amplitude and phase between the component propagated along the X axis and the component propagated along the Y axis, but is rotated by 90° by the Faraday rotator 14. Therefore, the change in the polarization state of the light returning to the optical coupler 12 during propagation is compensated for, and the polarization state of the light input to the polarizing beam splitter 16 is stabilized. The light input to the polarizing beam splitter 16 is separated into two orthogonal components, but in this embodiment 1, the polarizing beam splitter 16 is installed so that the light that has passed through the reference arm is not biased to only one output. Therefore, the two lights output from the polarizing beam splitter 16 are output at the same level.

In addition, the light reflected at the reflection point of the measurement object 2 and input to the polarizing beam splitter 16 is separated by the polarizing beam splitter 16 into two orthogonal components, each of which interferes with the light that has propagated through the reference arm. The polarization state of the light reflected at the reflection point of the measurement object 2 and input to the polarizing beam splitter 16 is not constant because it is affected by the characteristics of the reflection point and the birefringence during propagation.

However, in this embodiment 1, the light propagating through the measurement arm, which is separated into two orthogonal components by the polarizing beam splitter 16, interferes with the light propagating through the reference arm. Then, the electrical signals corresponding to the respective lights obtained by interference at the polarizing beam splitter 16 are added by the adder 21, and the output level according to the reflection intensity at the measurement object 2 is displayed on the display 22.

This eliminates the need to repeat measurements in which the variable delay device is scanned while adjusting the polarization adjuster, as was done in the past, making it possible to measure a stable output level regardless of the polarization state of the light reflected from the measurement target 2.

In the example shown in FIG. 1, a case has been described in which light source 11 outputs linearly polarized light parallel to the X-axis, but this is not limiting, and a light source that outputs circularly polarized light may also be used. When a light source that outputs circularly polarized light is used, the light that travels back and forth through the reference arm and enters polarized beam splitter 16 is also circularly polarized, so that polarized beam splitter 16 divides the two outputs equally regardless of the mounting angle. Therefore, when a light source 11 that outputs circularly polarized light is used, there is no need to adjust the mounting angle of polarized beam splitter 16.

In addition, if the polarization state changes between the light source 11 and the optical coupler 12 and between the optical coupler 12 and the polarizing beam splitter 16, the appropriate mounting angle of the polarizing beam splitter 16 changes. Therefore, when using optical fiber, it is necessary to fix the polarization state so that it does not change, or to use a polarization-maintaining fiber.

As described above, in the optical reflection measurement device 1 according to the first embodiment, the reflected light on the measurement target arm side and the reference light on the reference arm side are separated into two lights with orthogonal polarization states, and the two separated lights are converted into electrical signals, which are then combined and displayed as a reflection level. As a result, each of the two separated reflected lights interferes with the reference light with a stable polarization state, and each is converted into an electrical signal and combined. This prevents the measurement time from becoming too long, and allows the reflection level to be accurately measured even when the reflection level of the measurement target 2 fluctuates.

Embodiment 2.
Next, a second embodiment will be described. In a conventional optical reflection measuring device, when the delay time of a variable delay device is changed at a constant speed, the intensity of the interference light changes depending on the change speed and the frequency determined by the wavelength of the output light from the light source. Therefore, by displaying the delay time of the variable delay device and the output level from the LPF on a display, the reflection distribution within the measurement target can be measured.

Here, in conventional optical reflection measurement devices, the reflection level cannot be measured unless the delay time is changed at a constant speed, so if the reflection level of the object being measured changes, it is necessary to repeatedly scan the variable delay device. However, if the reflection level changes in a time shorter than this repetition period, it becomes impossible to measure correctly. Therefore, in this second embodiment, it becomes possible to measure the reflection level of the object being measured without changing the delay time at a constant speed.

[Configuration of the optical reflection measuring device 50]
Fig. 3 is a block diagram showing an example of the configuration of an optical reflection measurement device according to the second embodiment. As shown in Fig. 3, an optical reflection measurement device 50 includes a light source 11, a half mirror 51, a variable delay device 13, a phase modulator 52, a Faraday rotator 14, a mirror 15, a polarizing beam splitter 16, O/E converters 17a and 17b, multiplication processing units 60a and 60b, a calculator 53, a display 22, and an output terminal 54. In the second embodiment, the same reference numerals are used for the parts common to the first embodiment, and detailed explanations are omitted.

The half mirror 51 reflects a portion of the light input from the light source 11 and outputs it to the arm to be measured, while transmitting the remainder and outputting it to the reference arm. The half mirror 51 also reflects the light input from the reference arm and transmits the light input from the arm to be measured, outputting each of the lights to the polarizing beam splitter 16.

In the second embodiment, the reference arm is provided with a variable delayer 13, a phase modulator 52, a Faraday rotator 14, and a mirror 15. The reference arm is configured such that the input light passes through the variable delayer 13, the phase modulator 52, and the Faraday rotator 14, is reflected by the mirror 15, and passes through the Faraday rotator 14, the phase modulator 52, and the variable delayer 13, before returning.

The phase modulator 52 is connected to a modulation signal generator 71, and outputs the input light by phase-modulating the light with the modulation signal output from the modulation signal generator 71. The modulation signal in the second embodiment is expressed as "V _M cos ω _M t" when the angular frequency is ω _M , and the phase modulator 52 periodically modulates the light that has traveled back and forth through the reference arm with a cosine wave of angular frequency ω _M and phase modulation depth C.

The shape and input amplitude V _M of the phase modulator 52 are adjusted so that the phase modulation degree C is close to a number at which a first-kind zero-order Bessel function becomes 0, such as "2.4", "5.5", or "8.7". In the following description, a case will be described in which the phase modulator 52 is adjusted so that the phase modulation degree C becomes "5.5".

The O/E converter 17a converts the input optical signal into an electrical signal _Ia and outputs it, while the O/E converter 17b converts the input optical signal into an electrical signal _Ib and outputs it.

The multiplication processing units 60a and 60b are connected to a first reference signal generator 72 that outputs a first reference signal and a second reference signal generator 73 that outputs a second reference signal. The first reference signal is a cosine wave with an angular frequency of _5ωM and is expressed as "-(1/ _J5 (5.5)) _cos5ωMt ". The second reference signal is a cosine wave with an angular frequency of _4ωM and is expressed as "(1/ _J4 (5.5)) _cos4ωMt ".

The multiplication processing unit 60a has multipliers 61a5 and 61a4, and LPFs 62a5, 62a4, and 62a0. The multiplication processing unit 60a multiplies the electrical signal _Ia by a first reference signal in the multiplier 61a5, passes the result through the LPF 62a5, and outputs an output value _Xas . The multiplication processing unit 60a also multiplies the electrical signal _Ia by a second reference signal in the multiplier 61a4, passes the result through the LPF 62a4, and outputs an output value _Xac . The multiplication processing unit 60a also passes the electrical signal _Ia through the LPF 62a4, and outputs an output value _Xa0 .

The multiplication processing unit 60b has multipliers 61b5 and 61b4, and LPFs 62b5, 62b4, and 62b0. The multiplication processing unit 60b multiplies the electrical signal _Ib by the first reference signal in the multiplier 61b5, passes the result through the LPF 62b5, and outputs an output value _Xbs . The multiplication processing unit 60b also multiplies the electrical signal _Ib by the second reference signal in the multiplier 61b4, passes the result through the LPF 62b4, and outputs an output value _Xbc . The multiplication processing unit 60b also passes the electrical signal _Ib through the LPF 62b4, and outputs an output value _Xb0 .

In this case, the cutoff frequency of each of the LPFs 62a5, 62a4, 62a0, 62b5, 62b4 and 62b0 is set to be equal to or lower than "ω _M /2".

The calculator 53 is a synthesis means that outputs the reflection level of light based on the six output values _Xas , _Xac , _Xa0 , _Xbs , _Xbc , and _Xb0 input from the multiplication processing units 60a and 60b. The specific calculation of the calculator 53 will be described later. The reflection level output from the calculator 53 is input to the display unit 22 and the output terminal 54.

The display 22 displays the reflection level of light calculated by the calculator 53. When measuring the reflection distribution as in the first embodiment, the display 22 displays the reflection distribution based on the position in the measurement target 2 corresponding to the delay time set by the variable delay device 13 and the reflection level input from the calculator 53. When continuously observing the change in the reflection level over time at a specific position, it is advisable to adjust the delay time of the variable delay device 13 to a value corresponding to the position to be measured so that the output of the calculator 53 can be observed.

[Operation of the optical reflection measuring device 50]
Next, the operation of the optical reflection measurement device 50 according to the second embodiment will be described with reference to Fig. 3. The light output from the light source 11 is input to the half mirror 51. The light input to the half mirror 51 is reflected and transmitted, and the reflected light is input to the measurement object arm in which the measurement object 2 is provided, and the transmitted light is input to the reference arm in which the measurement object 2 is not provided.

The light input to the measurement object arm is reflected at the reflection point of the measurement object 2 and returns to the half mirror 51. On the other hand, the light input to the reference arm is reflected by the mirror 15 and returns to the half mirror 51. At this time, the light that passes through the half mirror 51 is delayed by a preset delay time by the variable delay device 13 as it travels back and forth through the reference arm, is phase modulated by the phase modulator 52, and returns to the half mirror 51 with its polarization state rotated by 90° by the Faraday rotator 14.

The light returning to the half mirror 51 is split into two orthogonal components by the polarizing beam splitter 16. One of the two beams split by the polarizing beam splitter 16 is input to the O/E converter 17a, and the other is input to the O/E converter 17b.

Here, the polarization state of the light propagating through the reference arm is rotated 90° by the Faraday rotator 14. That is, the light component that propagates along the X axis from the half mirror 51 to the mirror 15 returns along the Y axis from the mirror 15 to the half mirror 51. Also, the light component that propagates along the Y axis from the half mirror 51 to the mirror 15 returns along the X axis from the mirror 15 to the half mirror 51.

The polarization state of the light at this time changes depending on the difference in amplitude and phase between the component propagated along the X axis and the component propagated along the Y axis, but is rotated by 90° by the Faraday rotator 14. Therefore, the change in the polarization state of the light returning to the half mirror 51 during propagation is compensated for, and the polarization state of the light input to the polarizing beam splitter 16 is stabilized. The light input to the polarizing beam splitter 16 is separated into two orthogonal components, but in this embodiment 2, as in the first embodiment, the polarizing beam splitter 16 is installed so that the light that has passed through the reference arm is not biased to only one output. Therefore, the two lights output from the polarizing beam splitter 16 are output at the same level.

The light reflected at the reflection point of the measurement target 2 and input to the polarizing beam splitter 16 is split into two orthogonal components by the polarizing beam splitter 16, and each interferes with the light that has propagated through the reference arm. The interfered light is then input to the O/E converters 17a and 17b.

The light input to the O/E converter 17a is converted from an optical signal to an electrical signal _Ia and output, while the light input to the O/E converter 17b is converted from an optical signal to an electrical signal _Ib and output.

When a reflection point exists at a position within the measurement target 2 where the propagation time is the same as the round-trip propagation time in the reference arm, the electrical signals _Ia and _Ib are expressed by equations (1) and (2), respectively.
In equations (1) and (2), _Aa and _Ab indicate constant DC components regardless of the presence or absence of interference, _Ba and _Bb indicate the amplitude of the interference output, and φ is the phase difference between the two interfering lights, which indicates the phase difference of the interference output output from the O/E converters 17a and 17b.

In equations (1) and (2), the second term is expressed as the sum of components that are integer multiples of the angular frequency ω _M. The terms that are odd multiples of the angular frequency ω _M are proportional to sin φ, and the terms that are even multiples of the angular frequency ω _M are proportional to cos φ.

The electric signal _Ia output from the O/E converter 17a is input to the multiplication processing unit 60a. In the multiplication processing unit 60a, the electric signal _Ia is multiplied by the first reference signal in the multiplier 61a5, and the output value _Xas is output via the LPF 62a5. The electric signal _Ia is multiplied by the second reference signal in the multiplier 61a4, and the output value _Xac is output via the LPF 62a4. In the multiplication processing unit 60a, the electric signal _Ia passes through the LPF 62a0, and the output value _Xa0 is output.

The electric signal _Ib output from the O/E converter 17b is input to the multiplication processing unit 60b. In the multiplication processing unit 60b, the electric signal _Ib is multiplied by the first reference signal in the multiplier 61b5, and the output value _Xbs is output via the LPF 62b5. The electric signal _Ib is multiplied by the second reference signal in the multiplier 61b4, and the output value _Xbc is output via the LPF 62b4. In the multiplication processing unit 60b, the electric signal _Ib passes through the LPF 62b0, and the output value _Xb0 is output.

The output values X _as , X _ac , X _bs and X _bc output from the multiplication processing units 60a and 60b, respectively, are expressed by equations (3) to (6).

Moreover, the output values X _a0 and X _b0 are expressed by the equations (7) and (8).

The six output values _Xas , _Xac , _Xa0 , _Xbs , _Xbc , and _Xb0 output from the multiplication processing units 60a and 60b are input to the calculator 53. The calculator 53 calculates the reflection level of the object to be measured 2 based on the six input output values _Xas , _Xac , _Xa0 , _Xbs , _Xbc , and _Xb0 .

When the reflectance of the half mirror 51 is adjusted so that the level of the light that has traveled back and forth through the reference arm is greater than the level reflected by the measurement target 2, the ratio Y between the level of the light reflected at the reflection point of the measurement target 2 and the level of the light that has traveled back and forth through the reference arm is calculated based on equation (9). Since the level of the light that has traveled back and forth through the reference arm is constant, the reflection level of the measurement target 2 can be calculated by correcting the ratio of reflection and transmission of the half mirror 51, which is included in the ratio Y.

Here, in the second embodiment, the phase modulation degree C is set to 5.5. Therefore, the second terms of the output values _Xa0 and _Xb0 from the multiplication processing units 60a and 60b in the formulas (7) and (8) become "0", the terms including the unstable value "φ" disappear, and the output values _Xa0 and _Xb0 become stable values. In other words, the components that fluctuate with the phase change _φa of the light of the interferometer contained in the outputs of the LPFs 62a0 and 62b0 connected to the O/E converters 17a and 17b, respectively, are suppressed by adjusting the phase modulation degree C generated by the phase modulator 52.

By the way, the phase modulator 52 changes the phase of the light passing through it, but in many cases, it also changes the polarization state of the light. However, in this embodiment 2, the polarization state of the input light is rotated 90 degrees by the Faraday rotator 14. Therefore, the change in the polarization state of the light returning to the half mirror 51 while propagating through the phase modulator 52 is compensated for, and the polarization state of the light input to the polarizing beam splitter 16 is stabilized.

The first reference signal output from the first reference signal generator 72 is a cosine wave with an angular frequency that is an odd multiple of _ωM , and the second reference signal output from the second reference signal generator 73 is a cosine wave with an angular frequency that is an even multiple of _ωM , to operate. However, some phase modulators 52 modulate not only the phase but also the intensity of the light passing through them. In particular, many of them cause strong intensity modulation at the frequency of the input modulation signal and twice the frequency of the input modulation signal. If such a phase modulator 52 is used to make the first reference signal a cosine wave with an angular frequency that is _ωM and the second reference signal a cosine wave with an angular frequency that is twice the frequency of _ωM , intensity modulation components are mixed into the outputs of the LPFs 62a0, 62a4, 62a5, 62b0, 62b4, and 62b5 expressed by the formulas (3) to (8), causing the measurement results to fluctuate. Therefore, in the second embodiment, the first reference signal is a cosine wave with an angular frequency of _5ωM and the second reference signal is a cosine wave with an angular frequency of _4ωM , thereby suppressing the influence of the intensity modulation of the phase modulator 52 and enabling stable measurement.

The output values X _as , X _ac , X _bs and X _bc obtained by equations (3) to (6) also contain an unstable value "φ." However, these output values are stabilized by the sum-of-squares calculation by the calculator 53 described above, so that the reflection level of the object 2 can be measured.

As described above, in the optical reflection measuring device 50 according to the second embodiment, the phase modulator 52 is provided in the reference arm, and the reflection level is measured based on the frequency used for phase modulation. Therefore, regardless of the operating state of the variable delay device 13, as long as the phase modulation by the phase modulator 52 is operating, the reflection level can be measured. This makes it possible to measure the reflection level of the measurement target 2 without changing the delay time at a constant speed, as in the case of conventional optical reflection measuring devices. When recording the change over time in the reflectance at a specific position, the variable delay device 13 is stopped at a position where the delay time corresponds to the measurement target position, and the output of the calculator 53 is recorded.

Although the first and second embodiments have been described above, the present invention is not limited to the first and second embodiments described above, and various modifications and applications are possible within the scope of the gist of the present invention. For example, in the first and second embodiments, the case where the Faraday rotator 14 is provided in the reference arm has been described, but this is not limited thereto, and for example, the reference arm may be configured with other components that can stabilize the polarization state, such as a polarization-maintaining fiber.

In addition, in the first and second embodiments, a display 22 is provided to show the reflectance distribution to the operator. However, if the optical reflection measuring devices 1 and 50 constitute an automatic measuring device, the display 22 does not have to be provided. Furthermore, if the optical reflection measuring devices 1 and 50 constitute a device that shows only the reflectance, the display 22 may be configured to display only the reflection level output from the calculator 53 in the second embodiment, for example.

In the second embodiment, the phase modulator 52 is provided in the reference arm, but this is not limiting. For example, the phase modulator 52 may be provided between the half mirror 51 of the measurement object arm and the measurement object 2.

In the second embodiment, a configuration is shown that has both functions: a function to enable stable measurement even if the polarization of the interfering light changes using a polarizing beam splitter 16 or the like, and a function to enable stable measurement without scanning the variable delay device 13 at a constant speed using a phase modulator 52 or the like, but this is not limited to this example. For example, the optical reflection measuring device 50 can be simplified to have only one of the two functions. Also, when measuring only the reflectance at a specific position rather than the reflection distribution, the optical reflection measuring device 50 may be configured without the variable delay device 13.

In the first embodiment, the optical coupler 12 is used as a means for splitting light, but this is not limited to this, and for example, a half mirror 51 may be used. In the second embodiment, the half mirror 51 is used, but this is not limited to this, and for example, an optical coupler 12 may be used. In the first and second embodiments, an example is shown in which a polarizing beam splitter 16 is used, but for example, other means such as a half mirror and a polarizer may be used.

Furthermore, in the second embodiment, it has been described that multiplication processing units 60a and 60b are provided as a means for extracting frequency components that are integer multiples of the frequency that periodically changes the phase of light, but this is not limited to this, and other means such as fast Fourier transform can also be used.

1, 50, 100 Optical reflection measuring device, 2 Measurement object, 11 Light source, 12 Optical coupler, 13 Variable delay device, 14 Faraday rotator, 15 Mirror, 16 Polarizing beam splitter, 17, 17a, 17b O/E converter, 18, 18a, 18b Band pass filter, 19, 19a, 19b Rectifier, 20, 20a, 20b, 62a0, 62a4, 62a5, 62b0, 62b4, 62b5 Low pass filter, 21 Adder, 22 Display, 51 Half mirror, 52 Phase modulator, 53 Arithmetic unit, 54 Output end, 60a, 60b Multiplication processing unit, 61a4, 61a5, 61b4, 61b5 Multiplier, 71 Modulation signal generator, 72 First reference signal generator, 73 Second reference signal generator, 101 polarization adjuster.

Claims (7)

An optical reflection measurement device for measuring a reflection level of a measurement object, comprising:
a measurement object optical path through which the measurement object is provided and through which light for the measurement object passes;
a reference light path through which light passes that is referenced when measuring the reflection level of the measurement target;
a separation means for receiving reflected light reflected by the measurement object in the measurement object optical path and a reference light passing through the reference optical path, and for separating the reflected light and the reference light into two lights having orthogonal polarization states;
a conversion means for converting each of the two light beams output from the separation means into an electrical signal;
a combining means for combining the two electrical signals which have passed through band-pass filters for independently detecting the presence or absence of interference therebetween ;
A Faraday rotator is provided on the reference light path and rotates the polarization direction of the light passing through the reference light path by a preset angle;
The optical reflection measuring device according to claim 1, wherein the splitting means splits the reference light into approximately equal parts so as to be contained in the two lights . the reference light path includes a mirror that reflects the reference light,
2. The optical reflection measuring device according to claim 1, further comprising a variable retarder on the reference optical path not between the Faraday rotator and the mirror. the reference light path includes a mirror that reflects the reference light,
2. The optical reflection measuring device according to claim 1, further comprising a phase modulator on the reference optical path not between the Faraday rotator and the mirror. An optical reflection measurement device for measuring a reflection level of a measurement object, comprising:
a measurement object optical path through which the measurement object is provided and through which light for the measurement object passes;
a reference light path through which light passes that is referenced when measuring the reflection level of the measurement target;
a conversion means for receiving reflected light reflected by the measurement object in the measurement object optical path and reference light passing through the reference optical path, and converting the input reflected light and reference light into electrical signals;
a phase modulator provided in the measurement object optical path or the reference optical path, which periodically modulates a phase of the reflected light reflected by the measurement object or the reference light;
a multiplication processing unit that extracts and calculates components that are integer multiples of a frequency when the phase of the reference light is modulated. The multiplication processing unit is
5. The optical reflection measuring device according to claim 4, wherein frequency components that are even multiples and odd multiples of a frequency obtained when the phase of the reference light is modulated are extracted. The multiplication processing unit is
6. The optical reflection measuring device according to claim 4, wherein a component having a frequency three times or more higher than that when the phase of the reference light is modulated is extracted and calculated. The multiplication processing unit is
A low-pass filter is provided to cut off frequency components higher than a preset cutoff frequency,
The phase modulator comprises:
The optical reflection measuring device according to any one of claims 4 to 6, characterized in that the degree of modulation of the phase is adjusted so as to suppress components that fluctuate with the phase change of the light and are included in the output of the low-pass filter connected to the conversion means.

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