JPH07113578B2 - Method and apparatus for measuring linearity of optical power detector - Google Patents

Description

TECHNICAL FIELD The present invention relates to an optical power detection element and an optical power detector using the optical power detection element, which can measure linearity of sensitivity in a wide range of levels with high accuracy. The present invention relates to a linearity measuring method and device for a power detector.

(Prior Art) Linearity of sensitivity in a conventional optical power detector (linearity of optical input power to the optical power detector vs. electrical output of the optical power detector: hereinafter referred to simply as linearity For the measurement of (referred to), a so-called white light source, which is mainly a visible light region and has a non-coherent and wide generation wavelength region, was used. Especially when measuring the linearity of the optical power detector in a high level region, a tungsten lamp can obtain a sufficient measurement output level when the optical spectrum width is wide and the light beam diameter is large, which is sufficient. It was common to use a white light source such as a halogen balance.

However, semiconductor optical power detectors used mainly in the field of optical communication in recent years have a large wavelength dependency of detection sensitivity, and the wavelength dependency of linearity cannot be ignored. White light sources cannot be used. In order to obtain the light of the required wavelength, when the spectrum is narrowed with a spectrometer with a narrow bandwidth, the white light source has a low energy density per unit wavelength, so the high output light required for linearity measurement especially in the high level saturation region. I can't get energy. Further, since the white light source has a low energy density per unit area, it has a low coupling efficiency with respect to an optical fiber having a small diameter, so that an extremely low output can be obtained. For example, the output light of a halogen lamp with an output of 50 W has a core diameter of 50 μm and NA (numerical aperture)
When guided into a 0.2 fiber, its output is 850 wavelengths.
When the wavelength is 10 nm and the spectrum width is 10 nm, it is only 100 nW or less.

Further, as described above, the conventional technique is premised on a light source which is incoherent and has a large bright spot area, and is essentially not suitable for coupling to a small-diameter medium such as an optical fiber.

Next, details of the prior art will be described with reference to FIGS. 6 and 7. FIG. 6 is a diagram showing a conventional measurement configuration, and FIG. 7 is a diagram showing an example of the shape of the aperture.

In FIG. 6, the white light emitted from the white light source 51 is converted into parallel light in the collimator 52, given a desired attenuation in the variable attenuator 53, and then added to the variable aperture 54. The light passing through the variable aperture 54 is condensed by the condenser 55 and added to the measured light power detector 56.

Next, the structure and function of the aperture will be described with reference to FIG. In FIG. 7 (a1), the aperture plate 60 is provided with an aperture A61 and an aperture B62. If the areas of both apertures are equal and the luminous flux density is uniform over the entire area including both apertures, as shown in FIG. 7 (a1), the amount of light that passes through those apertures when they are opened at the same time As shown in FIG. 7 (a2), when one of the apertures is blocked by the light-shielding piece 63, the amount of light passing through the aperture has a two-to-one relationship.

Further, in FIG. 7B, the aperture disk 64 is provided with a circular aperture C65 and semicircular apertures D66 and E67. The area of aperture D66 and aperture E67 is
It is half the area of aperture C65. That is, aperture D
66 The amount of light passing through E67 is half the amount of light passing through aperture C65. The aperture disc 64 is provided so as to rotate about its center.

Next, a procedure for measuring the linearity of the conventional optical power detector will be described.

(A) Open apertures A61 and B62, or place aperture C65 on the light beam.

(B) The variable attenuator 53 is adjusted so that the detection output of the measured optical power detector 56 has a desired value.

At this time, the optical power passing through the aperture is P ₁ .

(C) Either APACHA A61 or B62 is closed,
Put aperture D66 or E67 on the luminous flux. The amount of light that reaches the measured optical power detector 56 is half of that in step (a). Let this optical power be P ₂ .

(D) The decrease amount of the detection output of the measured optical power detector 56 with respect to the detection output of step (b) is measured.

As described above, the amount of change in the output of the measured optical power detector 56 is obtained with respect to the change in the amount of light passing through the aperture 2 to 1, and the ratio of the two is calculated as follows.
The non-linear rate γ can be calculated.

γ = {(I ₂ / I ₁ ) / (P ₂ / P ₁ )}-1 where P ₂ / P ₁ : incident light intensity ratio I ₂ ,, I ₁ : P ₂ , P ₁ detector output Current, where P ₂ / P ₁ = 2. Therefore, γ = {(I ₂ / I ₁ ) / 2} -1.

In FIGS. 7 (a1) and (a2), two apertures A6
Optical power detector output current for 1 and B62 respectively
When there is a slight difference between I _A and I _B , that is, when there is a difference in the amount of light passing through each aperture A and B, the output current of the optical power detector when both apertures are opened simultaneously
In comparison with I _{A + B} , γ = {(I ₂ / I ₁ ) / (P ₂ / P ₁ )}-1 = {I _{A + B} / I _A ) / (P _A / P _B ) / I _{a} -1 = {I a +} B / (P a + P B)} - 1 as had sought nonlinear rate.

Further, in the example of FIG. 7 (b), the aperture disc 64 is rotated about the center to rotate the aperture C65 of the unit light amount.
And the apertures D66 and E67 with 1/2 light intensity operate at the same position, the apertures A61 and B shown in FIG.
Compared with 62, it is advantageous in the uniformity of the luminous flux distribution. The detection output corresponding to aperture C65 is I _C , the detection output corresponding to aperture D66 is I _D , and the detection output corresponding to aperture E67 is I _E
Then, when there is a difference between I _D and I _E , γ = {I _C / (I _D + I _E )} − 1 and the non-linear rate was obtained.

(Problems to be Solved by the Invention) That is, the related art has the following problems (a) to (d).

(B) As described above, in order to accurately measure the linearity of the semiconductor optical power detector over a wide dynamic range, a white light source cannot be used due to the influence of wavelength characteristics and insufficient power.

(B) Further, in the configuration of FIG. 6, no matter which aperture is used for measurement in FIG. 7 (a) or (b), simple addition of light fluxes emitted from the same light source and having different optical paths on the way is accurately established. The condition is that the light source must be sufficiently incoherent, and the above relationship cannot be established with a light source with high coherence such as a gas laser or a semiconductor laser.

(C) In any of the aperture shapes shown in FIGS. 7 (a) and 7 (b), the beam position on the light-receiving surface changes when the aperture condition changes, and the beam position on the light-receiving surface changes. It is inevitable to be affected by the uniformity of the sensitivity of the light receiving surface of the power detector. For elements such as Ge photodiodes, which have large nonuniformity in sensitivity, this may cause a decrease in measurement accuracy.

(D) Furthermore, the two apertures A in FIG. 7 (a)
It is impossible for the light fluxes from B and B, or the light fluxes from both apertures D and E in FIG. 7 (b) to be focused at the same position via the same optical path, which is the same as that of the small-diameter optical fiber. In this connection, the fiber excitation condition from each part cannot be made the same, which is not suitable for the measurement of the optical power detector connected to the fiber and used.

In other words, the problem to be solved in the present application is to enable linearity measurement with a narrow spectrum width and high optical power like laser light, which has been difficult with the prior art, and to enable accurate measurement. Is.

(Means for Solving Problems) The present invention solves the above problems, and further enables the optical power detector connected to the fiber to measure at a sufficiently high level. The aim is to obtain a sex measuring method and device.

In that case, according to the first invention, (1) two light sources that are close to each other in the oscillation wavelength and do not interfere with each other are prepared, and (2) the light fluxes emitted by the two light sources are combined to be used as a measured optical power detector. And (3) adjusting the output lights of the two light sources to be equal to each other by the output power adjusting means provided in at least one of the two light sources, ) After the adjusting step, each of the steps of detecting the luminous flux from any one of the two light sources and the combined luminous flux from the two light sources by the measured optical power detector, The output value of the measured optical power detector when only one of the luminous fluxes is added and the output value of the measured optical power detector when both of the luminous fluxes are combined and added To measure linearity It was.

In addition, in the second invention, (a) a laser light source having a high energy density is used as a light source in order to improve the coupling efficiency with respect to the small-diameter optical fiber and expand the measurable level range. In order to eliminate the error factor caused by the coherence, two independent light sources with wavelength intervals that make the correlation between them sufficiently low are provided corresponding to the required measurement accuracy. A means for independently controlling the output and a means for independently opening and closing the emission of each light source are provided, and (d) a combining means capable of emitting the outputs of the two light sources on the same optical path is provided. In order for the scene from the light source to be effectively equal on the light receiving surface of the measured optical power detector,
A means for adjusting the output of the light source (equivalent light quantity adjustment between two light sources) was provided.

(Embodiment) FIG. 1 shows an embodiment of the linearity measuring method for the optical power detector of the first invention.

In FIG. 1, 1 and 2 are first and second light sources, respectively, which are configured to stably output a laser beam having a high energy density in order to increase the incidence efficiency to a small diameter optical fiber. Where the first and second light sources (1,2) are
Those are selected so that they do not interfere with each other, and the linearity measurement results are close to each other such that the wavelength characteristics of the measured optical power detector 7 are not affected. Reference numeral 8 is an output power adjusting means for adjusting the output light from the first and second light sources (1, 2) to be at the same level at the input end of the optical power detector 7 to be measured. It may be on either the first side or the second light source 2 side, but in the embodiment, it was prepared on the first light source 1 side. Reference numeral 9 denotes an optical combining / optical path means, which combines the respective output lights from the first light source 1 and the second light source 2 through the optical output power adjusting means 8 and sets the combined light flux to a desired level. Then, it is guided to the measured optical power detector 7 with good matching by an optical fiber or the like. By combining the light beams from two laser light sources with different wavelengths in this way, the coherence of the laser light can be increased compared to the case where the light beams are split into two optical paths from one coherent laser light source, and the desired level is reached and then the light beams are combined again. It is possible to reduce the measurement error caused by the property.

By the way, the measured optical power detector 7 is roughly classified into the following four types.

(B) Single detector element (b) Power meter based on direct detection method (c) Power meter based on optical chopper method (d) Power meter based on light source modulation method (where the power meter is a detection element that detects at least light, and a detection element) 5 is composed of an amplifier that amplifies the output signal of and the indicator that indicates the amplified signal level.A general configuration example is shown in Fig. 5. In this example, the measured optical power detector 7 It has a zero adjustment circuit for adjusting the offset error and a data processor for causing the indicator to indicate the output signal of the detection element into a desired data format and outputting it to the outside by GP-IB or the like. In the above (a) to (c), the response speed of (b) is significantly different depending on the internal circuit type and the measurement range. Therefore, when measuring the linearity of this type of power meter, a measurement time longer than this response speed is required. Since both (c) and (d) are measured by modulating at a specific frequency, a long measurement time is required for measuring the linearity of this type of power meter compared to a specific modulation cycle. .

Hereinafter, a measuring method according to an embodiment in which the above power meter is the measured power detector 7 will be described. In addition, it is necessary to consider the above measurement time in the measurement.

(A) First, as shown in FIG. 1, the two light sources (1,
2) is prepared, and the light beams from these light sources are combined into a single light beam and guided to the measured optical power detector 7.

(B) Only the light flux from the second light source 2 is guided to the measured optical power detector 7, and the output of the measured optical power detector 7 at that time is measured. Let the measured value be P _b . In order to guide only the light flux from the second light source to the measured optical power detector 7,
In the figure, even if the first light source is turned off, an optical switch may be provided toward the light combining / optical path means 9 so that only the light flux is selected from the second light source 2. Further, in order to measure the output of the measured optical power detector 7, the instruction of the measured optical power detector 7 may be read as in the configuration example of FIG. GP-IB
You may take it out and read it.

(C) Only the light flux from the first light source 1 is guided to the measured optical power detector 7, and the output of the measured optical power detector 7 at that time is equal to the value P _b measured in (b) above ( The output power adjusting means 8 adjusts so that P _a ).

(D) The light fluxes from the first and second light sources (1, 2) are combined and guided to the measured optical power detector 7, and the output of the measured optical power detector 7 at that time is measured. The value at that time
Let P _c .

(E) As in the previous equation described in the prior art, the non-linearity is obtained as follows.

γ = {(P _c / P _a ) / (P _a + P _b ) / P _a } -1 = (P _c / 2P _a ) -1 FIG. 2 shows the linearity measuring device of the optical power detector of the present invention. An example is shown.

In FIG. 2, reference numerals 1 and 2 denote first and second light sources, respectively, which are configured to output stable laser light.
That is, in this example, the first and second light sources (1, 2) each branch the output light of the LD (abbreviation of laser diode) light source (1A, 2A) by the branch circuit (1B, 2B) Is the output light of the light source (1, 2), the other one detects the optical power with the photodetector (1C, 2C) and converts it to a DC signal. Based on this DC signal, the drive circuit (1D, 2D) The LD light source (1A, 2A) is driven to emit stable light. Where the first and second LDs
The light sources (1A, 2A) are selected so that they do not interfere with each other and the linearity measurement result has a wavelength interval that is not affected by the wavelength characteristics of the measured optical power detector. Three
Is an output light power adjusting means, and includes first and second light sources (1,
It is a means for adjusting the output light from 2) so that it is at the same level at the input end of the measured optical power detector 7, and it may be provided on either the first light source 1 side or the second light source 2 side. In the embodiment, the first side is provided on both light source sides so that fine adjustment can be performed.
And the 2nd light intensity regulator (3A, 3B) was prepared. Reference numeral 4 is a switching means, which is a first optical switch 4A and a second optical switch.
It is composed of 4B and is switched on / off in the optical paths from the first and second light sources (1, 2), respectively.
Reference numeral 5 denotes a light combining means, which combines the output lights from the first light source 1 and the second light source 2 and outputs the combined light. As the light synthesizing means 5, for example, a cube prism in which the influence of polarization fluctuation of each light source is small and each surface is coated with an antireflection coating is used. An optical path means 6 guides the output light from the light combining means 5 to the measured optical power detector with good matching. The variable optical attenuator 6A in the optical path means 6 is for setting the incident power to the measured optical power detector 7, that is, the measurement point for linearity measurement. The beam diameter adjusting means 6B is for adjusting the diameter of the light beam in accordance with the diameters of the optical fiber excitation means 6C, the optical fiber 6D, etc. for guiding the light to the measured optical power detector 7. A lens is used as the beam diameter adjusting means 6B,
There is a zoom lens or a combination of lenses that can change the beam diameter, and the operation is to output incident parallel light by changing only the beam diameter as parallel light. As long as the beam diameters match, the variable optical attenuator 6A or the beam diameter adjusting means 6 may directly enter the measured optical power detector 7 for measurement, if necessary. The optical fiber excitation means 6C is for exciting the optical fiber 6D by receiving the parallel light from the beam diameter adjusting means 6B, a lens for narrowing the parallel light and making it enter the optical fiber 6D, and a mode for limiting the propagation mode of light. There are means such as filters, or a combination of both. Alternatively, the light may be directly incident on the measured optical power detector 7 from the optical fiber excitation means 6C. The optical fiber 6D is for guiding the output light of the optical fiber excitation means 6C to the measured optical power detector 7.

By the way, there are four types of optical power detectors 7 to be measured as shown in the first invention, and in the measurement, the second invention needs to consider the measurement time as in the first invention.

The operation of the embodiment in which the power meter to be measured is the above power meter will be described below.

(1) Turn off the first and second optical switches (4A and 4B), and confirm that the measured optical power detector 7 indicates zero. Measured optical power detector 7 if zero is not specified
Adjust. (For example, in the measured optical power detector 7 as shown in FIG. 5, the offset error of the measured optical power detector 7 is adjusted to zero by adjusting the zero adjustment circuit.) (2) First light Switch 4A ON, second optical switch 4B
After turning off, only the light from the first light source 1 is incident on the measured optical power detector 7, and the light amount adjuster 3A is adjusted so that the instruction of the measured optical power detector 7 becomes a predetermined level. At this time, the variable optical attenuator 6A is set in consideration of the desired operating range of the measured optical power detector 7.

For example, when it is desired to measure from the maximum level that can be detected by the measured optical power detector 7 to the minimum level, the attenuation amount of the optical variable attenuator 6A is set to the minimum side, and the measured optical power detector 7 indicates 1/2 of the maximum detectable level P _L
The light amount adjuster 3A is adjusted so that the value P _A0 becomes.

(3) First optical switch 4A is turned off, second optical switch 4B
Is turned on, only the light from the second light source 2 is incident on the measured optical power detector 7, and the amount of light is adjusted so that the measured optical power detector 7 indicates the same level as the level measured in (2). Adjust adjuster 3B.

If the level at this time is P _B0 , P _B0 = P _A0 .

(4) Turn on both the first and second optical switches (4A and 4B)
And read the instruction of the measured optical power detector 7. This value
_Assuming P _C0 , the nonlinearity γ ₀ is γ ₀ = {(P _C0 / P _A0 ) / (P _C0 + P _B0 ) / P _A0 } −1 = {P _C0 / (P _B0 + P _A0 )} − 1 = ( P _C0 / 2P _A0 ) -1 Here, in order for the simple addition P _A0 + P _B0 or the measurement of P _C0 to be established, the light source has no coherence. For example, if P _A0 and P _B0 are created by dividing a light source of a single wavelength having coherence into two optical paths, and P _A0 and P _B0 are combined to create P _A0 + P _B0 , the vector sum results and simple addition does not hold. In order to eliminate this drawback, this embodiment is characterized in that a first light source and a second light source having different wavelengths are provided, and the sum from these two light sources is also determined by the photosynthesis means 5.

(5) In this state, the variable optical attenuator 6A is changed and set to the next operating point of the measured optical power detector 7, and the nonlinearity is obtained in the same manner as in the above (1) to (4).

Similarly, the variable optical attenuator 6A is sequentially changed to obtain nonlinearity.

There are two methods for measuring linearity over a wide level range.

(5-1) 2 ^N method As shown in FIG. 3 (a), this is the addition of equal light amounts (P _A0
This is a method of measuring linearity over a level range of 2 ^N (or 2 ^−N ) by sequentially accumulating twice (or 1/2 times) by + P _B0 ) at each measurement step. In this case an optical variable attenuator
As 6A, it is only necessary to be able to change the step twice (or 1/2 times) with the required accuracy and to change it over the required level range. As a configuration of the variable optical attenuator 6A, an attenuator that can be set in steps of about twice and a continuously variable attenuator for fine adjustment are practically required as shown in FIG. 3 (b).

(5-2) Arbitrary step method As shown in FIG. 4 (a), this is a method of performing linearity measurement by equal light amount addition at arbitrary level intervals for each measurement step irrespective of measurement points. Is. However, it is not possible to directly obtain the cumulative linearity over a wide level range. However, since the step interval of the variable optical attenuator 6A does not require precision, it is a simple method for determining whether the linearity is good or bad. The configuration of the variable optical attenuator 6A at this time may be that shown in FIG. 4 (b).

In addition, the fine adjustment of the optical level performed by the variable optical attenuator 6A
It can also be performed by the first and second drive circuits (1D, 2D) in the figure. In this case, the first and second driving circuits (1D, 2D) have a variable and stable reference voltage inside, and the reference voltage and the first and second photodetectors (1
C, 2C) are compared with each other, and the first and second LD light sources (1A, 2A) are driven so that the difference between the two voltages becomes zero, and the output light level of the light source is controlled. With such a configuration, the light level can be finely adjusted by changing the reference voltage.

Although the case where the measured optical power detector 7 is mainly a power meter has been described above, the configuration of the measured optical power detector 7 is the same as that of the power meter when measuring the linearity of the detection element as shown in FIG. Then, it is preferable to replace the detection element to be measured. Further, in FIG. 5, the output current _ID of the detection element to be measured may be directly read by an ammeter. At this time, the non-linearity γ ₀ of the measured detection element is as follows from the above equation.

γ ₀ = {(I _C0 / I _A0 ) / (I _C0 + I _B0 ) / I _A0 } −1 where I _C0 , I _A0 and I _B0 are P _C0 , P _A0 and P _A0 in (1) to (4) above. It is the value of I _D corresponding to P _B0 .

Not only the semiconductor laser but also a coherent coherent light source such as a cass laser can be used as the light source of the first and second inventions.

(Effects of the Invention) The present application is provided with two laser light sources having different wavelengths, the light amounts from the respective light sources are set to be equal, and the linearity can be measured by adding them in equal light amounts. ) It is possible to perform measurement over a wide dynamic range up to a high power region, and prevent measurement errors due to interference that occurs when adding equal amounts of light.

(2) The influence of the uniformity of sensitivity on the light receiving surface of the detector can be reduced by the configuration in which equal light amounts are added without using a mechanical aperture and the light is guided to the measured optical power detector by the same optical path. Since the fiber excitation condition can be stably maintained, accurate measurement can be performed.

(3) The output power of the light source can be adjusted by using the power detector to be measured, and it is not necessary to prepare another standard power detector, so that the operability is good.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an embodiment according to the first invention of the present application, FIG. 2 is a diagram showing a configuration of an embodiment according to the second invention,
FIG. 3 (a) is a diagram showing an example of setting the variable optical attenuator in the 2 ^N method, FIG. 3 (b) is a diagram showing a configuration example of the variable optical attenuator in the 2 ^N method, and FIG. FIG. 5A is a diagram showing an example of setting an optical variable attenuator in the arbitrary step method, FIG. 4B is a diagram showing a configuration example of the optical variable attenuator in the arbitrary step method, and FIG. 5 is a measured optical power. Diagram showing a configuration example of the detector,
FIG. 6 is a diagram showing a configuration of a conventional example, and (a1) and (a of FIG.
2) and FIG. 7 (b) are diagrams showing the aspect of the aperture in the conventional example. In the figure, 1 and 2 are first and second light sources, 1A and 2A are first and second LD light sources, 1B and 2B are first and second optical branch circuits, and 1C and 2C are first and second optical branch circuits. A second photodetector, 1D and 2D are first and second drive circuits, 3 is output light power adjusting means,
3A and 3B are first and second light amount adjusters, 4 is switching means, 4A and 4B are first and second optical switches, 5 is light combining means, 6 is optical path means, 6A is a variable optical attenuator, and 6B. Is a beam diameter adjusting means, 6C is an optical fiber excitation means, 6D is an optical fiber,
7 is a measured optical power detector, 8 is output power adjusting means,
9 is photosynthesis / optical path means, 51 is a white light source, 52 is a collimator, 53 is a variable attenuator, 54 is a variable aperture, 55 is a condenser,
56 is an optical power detector to be measured, 60 is an aperture plate, 61 is an aperture A, 62 is an aperture B, 63 is a light-shielding piece, 64 is an aperture disk, 65 is an aperture C, 66 is an aperture D, and 67 is an aperture E.
Indicates.

Claims (2)

[Claims] 1. A step of preparing two light sources (1, 2) having oscillating wavelengths close to each other and not interfering with each other, synthesizing light fluxes emitted by the two light sources, and synthesizing the synthesized light in a measured optical power detector. The step of configuring the optical path (9) to guide the generated light flux, and the output light adjusting means (8) provided in at least one of the two light sources cause the respective output lights of the two light sources to be mutually After the adjusting step, the luminous flux from any one of the two light sources and the combined luminous flux from the two light sources are respectively detected by the measured optical power detector. The output value of the measured optical power detector when only one of the luminous fluxes is added and the output of the measured optical power detector when both luminous fluxes are combined and added. Optical power detection from the value Measuring instrument linearity. 2. A first light source (1), a second light source (2) having an oscillation wavelength close to the first light source, and not interfering with the first light source, Optical combining means (5) for combining and outputting the light beams emitted from the first and second light sources on the same optical path; an optical path means (6) for guiding the output of the optical combining means to a measured optical power detector; Or, at least one of the second light sources is provided with an optical power adjusting means (3) for adjusting the optical powers of both light sources to be equal, and only one of the output lights of the first and second light sources is provided. Switching means for switching to output to the optical path means and switching to output combined light of the output light of the first and second light sources to the optical path means, and only the light flux of any one of the light sources The output value of the optical power detector to be measured when And linearity of the optical power detector, which is characterized by measuring the linearity of the optical power detector from the output value of the measured optical power detector when the light fluxes of the second light source are combined and added. apparatus.